diff options
Diffstat (limited to 'old/66493-h')
220 files changed, 0 insertions, 28909 deletions
diff --git a/old/66493-h/66493-h.htm b/old/66493-h/66493-h.htm deleted file mode 100644 index 11ba9fd..0000000 --- a/old/66493-h/66493-h.htm +++ /dev/null @@ -1,28909 +0,0 @@ -<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" - "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> -<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en"> - <head> - <meta http-equiv="Content-Type" content="text/html;charset=utf-8" /> - <meta http-equiv="Content-Style-Type" content="text/css" /> - <title> - The Ancient Volcanoes of Great Britain: Volume II, by Sir Archibald Geikie—A Project Gutenberg eBook - </title> - <link rel="coverpage" href="images/cover_epub.jpg" /> - <style type="text/css"> - -body { - margin-left: 10%; - margin-right: 10%; -} - -p { - margin-top: .51em; - text-align: justify; - margin-bottom: .49em; -} - -hr { - width: 33%; - margin-top: 2em; - margin-bottom: 2em; - margin-left: 33.5%; - margin-right: 33.5%; - clear: both; -} - -hr.tb {width: 45%; margin-left: 27.5%; margin-right: 27.5%;} -hr.chap {width: 65%; margin-left: 17.5%; margin-right: 17.5%;} -@media print { hr.chap {display: none; visibility: hidden;} } - -div.chapter {page-break-before: always;} -h2.nobreak {page-break-before: avoid;} - -table { - margin-left: auto; - margin-right: auto; - border-collapse: collapse; -} -.tblcont {width: 80%;} -.tblcont td:hover {background-color: #f5f5f5;} -td .hanging {text-indent:-2em; margin-left:2em; text-align: justify;} - -.pagenum { - position: absolute; - left: 92%; - font-size: smaller; - text-align: right; - font-style: normal; - font-weight: normal; - font-variant: normal; -} /* page numbers */ - -.bdb {border-bottom: 1px solid;} -.bdb2 {border-bottom: 4px double;} -.bdt {border-top: 1px solid;} -.bdl {border-left: 1px solid;} -.bdr {border-right: 1px solid;} - -.tdl {text-align: left;} -.tdc {text-align: center;} -.tdr {text-align: right;} -.smcap {font-variant: small-caps;} -.allsmcap {font-variant: small-caps; text-transform: lowercase;} -.justify {text-align: justify;} - -h1, h2, h3, .caption2, .caption3 {font-weight: bold; text-align: center; text-indent:0;} -h1 {font-size:2.00em; margin-top: 1.5em;} -h2 {font-size:1.50em; margin-top: 1.0em;} -h3 {font-size:1.25em; margin-top: 1.0em;} -.caption2 {font-size:1.50em; text-indent:0; margin-top: 1.0em;} -.caption2nb {font-size:1.50em; text-align: center; text-indent:0; margin-top: 1.0em;} -.caption3 {font-size:1.25em; text-indent:0; margin-top: 1.0em;} -.caption3nb {font-size:1.25em; text-align: center; text-indent:0; margin-top: 1.0em;} -.caption4 {font-size:1.125em; text-align: center; text-indent:0; margin-top: 1.0em;} -.pmt2 {margin-top: 2em;} -.pmb2 {margin-bottom: 2em;} -.pmt4 {margin-top: 4em;} -.pmb4 {margin-bottom: 4em;} -.vsmall {font-size: 0.5em;} -.smaller {font-size: 0.8em;} -.blockquot p {margin-left: 4em; margin-right: 2em; text-indent: -2em; text-align: justify;} -.vtop {vertical-align: top;} -.vbot {vertical-align: bottom;} - -sub, sup {font-size: 0.75em;} - -/* Images */ - -img { - max-width: 100%; - height: auto; -} - -.figcaption { - font-size: 0.75em; -} - -.figcenter { - margin: auto; - text-align: center; - page-break-inside: avoid; - max-width: 100%; -} - -.figleft { - float: left; - clear: left; - margin-left: 0; - margin-bottom: 1em; - margin-top: 1em; - margin-right: 1em; - padding: 0; - text-align: center; - page-break-inside: avoid; - max-width: 100%; -} -/* comment out next line and uncomment the following one for floating figleft on ebookmaker output */ -.x-ebookmaker .figleft {float: none; text-align: center; margin-right: 0;} -/* .x-ebookmaker .figleft {float: left;} */ - -.figright { - float: right; - clear: right; - margin-left: 1em; - margin-bottom: 1em; - margin-top: 1em; - margin-right: 0; - padding: 0; - text-align: center; - page-break-inside: avoid; - max-width: 100%; -} -/* comment out next line and uncomment the following one for floating figright on ebookmaker output */ -.x-ebookmaker .figright {float: none; text-align: center; margin-left: 0;} -/* .x-ebookmaker .figright {float: right;} */ - -/* Footnotes */ -.footnote {margin-left: 10%; margin-right: 10%; font-size: 0.9em;} - -.footnote .label {position: absolute; right: 84%; text-align: right;} - -.fnanchor { - vertical-align: super; - font-size: .8em; - text-decoration: - none; -} - -.antiqua {font-family: "Old English Text MT", "Lucida Calligraphy", "Perpetua", "Times New Roman", - Arial, Helvetica, sans-serif; font-size: 1.25em;} - -/* Transcriber's notes */ -.transnote {background-color: #E6E6FA; - color: black; - font-size:smaller; - padding:0.5em; - margin-bottom:5em; - font-family:sans-serif, serif; } - - </style> - </head> -<body> -<p style='text-align:center; font-size:1.2em; font-weight:bold'>The Project Gutenberg eBook of The Ancient Volcanoes of Great Britain, Volume II (of 2), by Archibald Geikie</p> -<div style='display:block; margin:1em 0'> -This eBook is for the use of anyone anywhere in the United States and -most other parts of the world at no cost and with almost no restrictions -whatsoever. You may copy it, give it away or re-use it under the terms -of the Project Gutenberg License included with this eBook or online -at <a href="https://www.gutenberg.org">www.gutenberg.org</a>. If you -are not located in the United States, you will have to check the laws of the -country where you are located before using this eBook. -</div> - -<p style='display:block; margin-top:1em; margin-bottom:1em; margin-left:2em; text-indent:-2em'>Title: The Ancient Volcanoes of Great Britain, Volume II (of 2)</p> -<p style='display:block; margin-top:1em; margin-bottom:0; margin-left:2em; text-indent:-2em'>Author: Archibald Geikie</p> -<p style='display:block; text-indent:0; margin:1em 0'>Release Date: January 4, 2022 [eBook #66493]</p> -<p style='display:block; text-indent:0; margin:1em 0'>Language: English</p> - <p style='display:block; margin-top:1em; margin-bottom:0; margin-left:2em; text-indent:-2em; text-align:left'>Produced by: T Cosmas, MWS and the Online Distributed Proofreading Team at https://www.pgdp.net (This file was produced from images generously made available by The Internet Archive)</p> -<div style='margin-top:2em; margin-bottom:4em'>*** START OF THE PROJECT GUTENBERG EBOOK THE ANCIENT VOLCANOES OF GREAT BRITAIN, VOLUME II (OF 2) ***</div> - - - - -<div class="figcenter" id="cover" style="width: 203px;"> - <img src="images/cover.png" width="203" height="330" alt="The Ancient Volcanoes of Great Britain (Vol 2), by Sir Archibald Geikie" /> -</div> - -<p><span class="pagenum" id="Page_i">- i -</span></p> - - - - -<h1>THE ANCIENT VOLCANOES OF GREAT BRITAIN</h1> - -<p><span class="pagenum" id="Page_ii">- ii -</span></p> - - - -<div class="pmt4 pmb4 figcenter" id="logo" style="width: 103px;"> - <img src="images/logo.png" width="103" height="37" alt="" /> -</div> - -<p><span class="pagenum" id="Page_iii">- iii -</span></p> - - -<h1> -<span class="vsmall">THE</span><br /> - -ANCIENT VOLCANOES<br /> - -<span class="vsmall">OF</span><br /> - -GREAT BRITAIN</h1> - -<p class="tdc">BY</p> - -<h2>SIR ARCHIBALD GEIKIE, F.R.S.</h2> - -<p class="tdc"><span class="smcap">D.C.L. Oxf., D. Sc. Camb., Dubl.; LL.D. St. And., Edinb.</span><br /> -<br /> -DIRECTOR-GENERAL OF THE GEOLOGICAL SURVEY OF GREAT BRITAIN AND IRELAND; -CORRESPONDENT OF THE INSTITUTE OF FRANCE; -OF THE ACADEMIES OF BERLIN, VIENNA, MUNICH, TURIN, BELGIUM, STOCKHOLM, GÖTTINGEN, NEW YORK; OF THE -IMPERIAL MINERALOGICAL SOCIETY AND SOCIETY OF NATURALISTS, ST. PETERSBURG; NATURAL HISTORY -SOCIETY, MOSCOW; SCIENTIFIC SOCIETY, CHRISTIANIA; AMERICAN PHILOSOPHICAL SOCIETY; OF THE -GEOLOGICAL SOCIETIES OF LONDON, FRANCE, BELGIUM, STOCKHOLM, ETC.<br /> -<br /> -WITH SEVEN MAPS AND NUMEROUS ILLUSTRATIONS<br /> -<br /> -<br /> -IN TWO VOLUMES<br /> -<br /> -VOL. II</p> - -<div class="pmt2 tdc antiqua">London</div> - -<p class="pmb4 tdc">MACMILLAN AND CO., <span class="smcap">Limited</span>.<br /> - -NEW YORK: THE MACMILLAN COMPANY.<br /> - -1897<br /> - -<i>All rights reserved</i> -</p> - - -<hr class="chap x-ebookmaker-drop" /> - - -<p><span class="pagenum" id="Page_iv">- iv -</span></p> - - -<div class="chapter"> -<p><span class="pagenum" id="Page_v">- v -</span></p> - -<h2 class="nobreak" id="CONTENTS">CONTENTS</h2> -</div> - -<table summary="TOC"> -<tr> - <td colspan="2"> - <p class="caption2">CHAPTER XXIX</p> - <p class="tdc smcap">The Carboniferous Volcanoes of England</p> - </td> -</tr> -<tr> - <td></td> - <td class="tdr smaller">PAGE</td> -</tr> -<tr class="tblcont"> - <td class="tdl">The North of England: Dykes, Great Whin Sill—The Derbyshire - Toadstones—The Isle of Man—East Somerset—Devonshire</td> - <td class="vbot tdr"><a href="#CHAPTER_XXIX">1</a></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">CHAPTER XXX</p> - <p class="tdc smcap">The Carboniferous Volcanoes of Ireland</p> - </td> -</tr> -<tr class="tblcont"> - <td class="tdl">King's County—The Limerick Basin—The Volcanic Breccias of - Doubtful Age in County Cork</td> - <td class="vbot tdr"><a href="#CHAPTER_XXX">37</a></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">BOOK VII</p> - <p class="caption2">THE PERMIAN VOLCANOES</p> - <p class="caption2">CHAPTER XXXI</p> - <p class="tdc smcap">The Permian Volcanoes of Scotland</p> - </td> -</tr> -<tr class="tblcont"> - <td class="tdl">Geographical Changes at the Close of the Carboniferous Period—Land - and Inland-Seas of Permian time—General Characteristics and - Nature of the Materials erupted—Structure of the several - Volcanic Districts: 1. Ayrshire, Nithsdale, Annandale; 2. - Basin of the Firth of Forth</td> - <td class="vbot tdr"><a href="#CHAPTER_XXXI">53</a></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">CHAPTER XXXII</p> - <p class="tdc smcap">Permian Volcanoes of England</p> - </td> -</tr> -<tr> - <td class="tdl">The Devonshire Centre—Eruptive Rocks of the Midland - Coal-fields</td> - <td class="vbot tdr"><a href="#CHAPTER_XXXII">94</a></td> -</tr> -<tr> - <td colspan="2"> - <p><span class="pagenum" id="Page_vi">- vi -</span></p> - <p class="caption2">BOOK VIII</p> - <p class="caption3">THE VOLCANOES OF TERTIARY TIME</p> - <p class="caption2">CHAPTER XXXIII</p> - </td> -</tr> -<tr class="tblcont"> - <td class="tdl">Vast lapse of time between the close of the Palæozoic and beginning - of the Tertiary Volcanic Eruptions—Prolonged Volcanic - Quiescence—Progress of Investigation among the Tertiary - Volcanic Series of Britain</td> - <td class="vbot tdr"><a href="#CHAPTER_XXXIII">107</a></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">CHAPTER XXXIV</p> - <p class="tdc smcap">The System of Dykes in the Tertiary Volcanic Series</p> - </td> -</tr> -<tr class="tblcont"> - <td class="tdl">Geographical Distribution—Two Types of Protrusion—Nature of - component Rocks—Hade—Breadth—Interruptions of Lateral - Continuity—Length—Persistence of Mineral Characters</td> - <td class="vbot tdr"><a href="#CHAPTER_XXXIV">118</a></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">CHAPTER XXXV</p> - <p class="tdc"><span class="smcap">The System of Dykes</span>—<i>continued</i></p> - </td> -</tr> -<tr class="tblcont"> - <td class="tdl">Direction—Termination upward—Known vertical extension—Evidence - as to the movement of the Molten Rock in the - Fissures—Branches and Veins—Connection of Dykes with - Intrusive Sheets—Intersection of Dykes—Dykes of more than - one infilling—Contact metamorphism of the Dykes—Relation of - the Dykes to the Geological Structure of the Districts which - they traverse—Data for estimating the Geological Age of the - Dykes—Origin and History of the Dykes</td> - <td class="vbot tdr"><a href="#CHAPTER_XXXV">145</a></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">CHAPTER XXXVI</p> - <p class="tdc smcap">The Plateaux</p> - </td> -</tr> -<tr class="tblcont"> - <td class="tdl">Nature and Arrangement of the Rocks: 1. Lavas.—Basalts, Dolerites, - Andesites—Structure of the Lavas in the Field—2. Fragmental - Rocks.—Agglomerates, Conglomerates, and Breccias—Tuffs and - their accompaniments</td> - <td class="vbot tdr"><a href="#CHAPTER_XXXVI">181</a></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">CHAPTER XXXVII</p> - </td> -</tr> -<tr class="tblcont"> - <td class="tdl smcap">The Several Basalt-Plateaux and their Geological History—Antrim, - Mull, Morven and Ardnamurchan</td> - <td class="vbot tdr"><a href="#CHAPTER_XXXVII">199</a></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">CHAPTER XXXVIII</p> - </td> -</tr> -<tr class="tblcont"> - <td class="tdl">The Basalt-Plateau of the Parish of Small Isles—Rivers of the - Volcanic Period</td> - <td class="vbot tdr"><a href="#CHAPTER_XXXVIII">215</a></td> -</tr> -<tr> - <td colspan="2"> - <p><span class="pagenum" id="Page_vii">- vii -</span></p> - <p class="caption2">CHAPTER XXXIX</p> - </td> -</tr> -<tr class="tblcont"> - <td class="tdl smcap">The Basalt-Plateaux of Skye and of the Faroe Isles</td> - <td class="vbot tdr"><a href="#CHAPTER_XXXIX">249</a></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">CHAPTER XL</p> - </td> -</tr> -<tr class="tblcont"> - <td class="tdl smcap">The Modern Volcanoes of Iceland as illustrative of the Tertiary - Volcanic History of North-Western Europe</td> - <td class="vbot tdr"><a href="#CHAPTER_XL">260</a></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">CHAPTER XLI</p> - <p class="tdc smcap">The Eruptive Vents of the Basalt-Plateaux</p> - </td> -</tr> -<tr class="tblcont"> - <td class="tdl">Vents filled with Basalt or other Lava-form Rock—Vents filled - with Agglomerate</td> - <td class="vbot tdr"><a href="#CHAPTER_XLI">270</a></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">CHAPTER XLII</p> - </td> -</tr> -<tr> - <td class="tdl">The Basic Sills of the Basalt-Plateaux</td> - <td class="vbot tdr"><a href="#CHAPTER_XLII">298</a></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">CHAPTER XLIII</p> - <p class="tdc smcap">The Bosses and Sheets of Gabbro</p> - </td> -</tr> -<tr class="tblcont"> - <td class="tdl">Petrography of the Rocks—Relations of the Gabbros to the other - members of the Volcanic series—Description of the Gabbro - districts—Skye</td> - <td class="vbot tdr"><a href="#CHAPTER_XLIII">327</a></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">CHAPTER XLIV</p> - </td> -</tr> -<tr class="tblcont"> - <td class="tdl">The Bosses and Sheets of Gabbro in the Districts of Rum, - Ardnamurchan, Mull, St. Kilda and North-East Ireland. - History of the Gabbro Intrusions</td> - <td class="vbot tdr"><a href="#CHAPTER_XLIV">349</a></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">CHAPTER XLV</p> - <p class="tdc smcap">The Acid Rocks</p> - </td> -</tr> -<tr class="tblcont"> - <td class="tdl">Their Petrography—Their Stratigraphical Position and its - Analogies in Central France</td> - <td class="vbot tdr"><a href="#CHAPTER_XLV">364</a></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">CHAPTER XLVI</p> - </td> -</tr> -<tr class="tblcont"> - <td class="tdl">Types of Structure in the Acid Rocks—Bosses</td> - <td class="vbot tdr"><a href="#CHAPTER_XLVI">378</a></td> -</tr> -<tr> - <td colspan="2"> - <p><span class="pagenum" id="Page_viii">- viii -</span></p> - <p class="caption2">CHAPTER XLVII</p> - </td> -</tr> -<tr class="tblcont"> - <td class="tdl smcap">The Acid Bosses of Mull, Small Isles, St. Kilda, Arran, and the North-East<br /> - of Ireland</td> - <td class="vbot tdr"><a href="#CHAPTER_XLVII">395</a></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">CHAPTER XLVIII</p> - </td> -</tr> -<tr class="tblcont"> - <td class="tdl smcap">The Acid Sills, Dykes and Veins</td> - <td class="vbot tdr"><a href="#CHAPTER_XLVIII">430</a></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">CHAPTER XLIX</p> - </td> -</tr> -<tr class="tblcont"> - <td class="tdl smcap">The Subsidences and Dislocations of the Plateaux</td> - <td class="vbot tdr"><a href="#CHAPTER_XLIX">447</a></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">CHAPTER L</p> - </td> -</tr> -<tr class="tblcont"> - <td class="tdl smcap">Effects of Denudation</td> - <td class="vbot tdr"><a href="#CHAPTER_L">455</a></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">CHAPTER LI</p> - </td> -</tr> -<tr class="tblcont"> - <td class="tdl smcap">Summary and General Deductions</td> - <td class="vbot tdr"><a href="#CHAPTER_LI">466</a></td> -</tr> -</table> - - - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_ix">- ix -</span></p> - -<h2 class="nobreak" id="LIST_OF_ILLUSTRATIONS">LIST OF ILLUSTRATIONS</h2> -</div> - - -<table class="tblcont" summary="LOI"> -<tr> - <td class="tdl smaller">FIG.</td> - <td></td> - <td class="tdr smaller">PAGE</td> -</tr> -<tr> - <td class="tdr vtop">176.</td> - <td><p class="hanging">Section from the great Limestone escarpment on the west to - the Millstone Grit hills east of Teesdale</p></td> - <td class="tdr"><a href="#v2fig176">4</a></td> -</tr> -<tr> - <td class="tdr vtop">177.</td> - <td><p class="hanging">Sections of the Carboniferous Limestone series of Northumberland showing the variations in the position of the Whin Sill. By Messrs. Topley and Lebour</p></td> - <td class="tdr"><a href="#v2fig177">6</a></td> -</tr> -<tr> - <td class="tdr vtop">178.</td> - <td><p class="hanging">View of two volcanic necks in the Carboniferous Limestone series, at Grange Mill, five miles west of Matlock Bath, from the north</p></td> - <td class="tdr"><a href="#v2fig178">14</a></td> -</tr> -<tr> - <td class="tdr vtop">179.</td> - <td><p class="hanging">Plan of necks and bedded tuff at Grange Mill, five miles west of Matlock Bath</p></td> - <td class="tdr"><a href="#v2fig179">15</a></td> -</tr> -<tr> - <td class="tdr vtop">180.</td> - <td><p class="hanging">Section across the smaller volcanic neck and the stratified tuff in Carboniferous Limestone, Grange Mill</p></td> - <td class="tdr"><a href="#v2fig180">15</a></td> -</tr> -<tr> - <td class="tdr vtop">181.</td> - <td><p class="hanging">Section of vesicular and amygdaloidal diabase resting on Carboniferous Limestone, Peak Forest Limeworks, Great Rocks Quarry</p></td> - <td class="tdr"><a href="#v2fig181">19</a></td> -</tr> -<tr> - <td class="tdr vtop">182.</td> - <td><p class="hanging">View of the superposition of Carboniferous Limestone upon toadstone, Raven's Tor, Millersdale (length about 100 feet)</p></td> - <td class="tdr"><a href="#v2fig182">19</a></td> -</tr> -<tr> - <td class="tdr vtop">183.</td> - <td><p class="hanging">Section at lime-kiln, south of Viaduct, Millersdale Station</p></td> - <td class="tdr"><a href="#v2fig183">20</a></td> -</tr> -<tr> - <td class="tdr vtop">184.</td> - <td><p class="hanging">Limestones passing under stratified tuffs, Poyll Vaaish, Isle of Man</p></td> - <td class="tdr"><a href="#v2fig184">24</a></td> -</tr> -<tr> - <td class="tdr vtop">185.</td> - <td><p class="hanging">Section of tuff, showing intercalations of black impure chert, west of Closenychollagh Point, near Castletown, Isle of Man</p></td> - <td class="tdr"><a href="#v2fig185">25</a></td> -</tr> -<tr> - <td class="tdr vtop">186.</td> - <td><p class="hanging">Section of intercalated dark limestone, shale and chert in the tuff south of Poyll Vaaish Bay, Isle of Man</p></td> - <td class="tdr"><a href="#v2fig186">26</a></td> -</tr> -<tr> - <td class="tdr vtop">187.</td> - <td><p class="hanging">Section of part of a volcanic neck on shore to the south-east of Poyll Vaaish Bay, Isle of Man</p></td> - <td class="tdr"><a href="#v2fig187">29</a></td> -</tr> -<tr> - <td class="tdr vtop">188.</td> - <td><p class="hanging">Section of successive discharges and disturbances within a volcanic vent. Scarlet Point, Isle of Man</p></td> - <td class="tdr"><a href="#v2fig188">29</a></td> -</tr> -<tr> - <td class="tdr vtop">189.</td> - <td><p class="hanging">Section of dyke and sill in the tuffs west of Scarlet Point, Isle of Man</p></td> - <td class="tdr"><a href="#v2fig189">30</a></td> -</tr> -<tr> - <td class="tdr vtop">190.</td> - <td><p class="hanging">Section on south side of vesicular sill west of Scarlet Point</p></td> - <td class="tdr"><a href="#v2fig190">31</a></td> -</tr> -<tr> - <td class="tdr vtop">191.</td> - <td><p class="hanging">Bands of vesicles in the same sill</p></td> - <td class="tdr"><a href="#v2fig191">31</a></td> -</tr> -<tr> - <td class="tdr vtop">192.</td> - <td><p class="hanging">Croghan Hill, King's County, from S.S.W.</p></td> - <td class="tdr"><a href="#v2fig192">38</a></td> -</tr> -<tr> - <td class="tdr vtop">193.</td> - <td><p class="hanging">Section in quarry on roadside east of Limerick, close to viaduct of the Limerick and Erris Railway</p></td> - <td class="tdr"><a href="#v2fig193">44</a></td> -</tr> -<tr> - <td class="tdr vtop">194.</td> - <td><p class="hanging">Section of the volcanic escarpment, east of Shehan's Cross-roads, south of Limerick</p></td> - <td class="tdr"><a href="#v2fig194">45</a></td> -</tr> -<tr> - <td class="tdr vtop">195.</td> - <td><p class="hanging">View of Derk Hill, a volcanic neck on the south side of the Limerick basin</p></td> - <td class="tdr"><a href="#v2fig195">47</a></td> -</tr> -<tr> - <td class="tdr vtop">196.</td> - <td><p class="hanging">Section across the Limerick volcanic basin</p></td> - <td class="tdr"><a href="#v2fig196">48</a></td> -</tr> -<tr> - <td class="tdr vtop">197.</td> - <td><p class="hanging">Section of a bed of volcanic breccia in the Carboniferous Slate; White Bull Head, County Cork</p></td> - <td class="tdr"><a href="#v2fig197">50</a></td> -</tr> -<tr> - <td class="tdr vtop">198.</td> - <td><p class="hanging">Volcanic breccia invading and enclosing Carboniferous slate, White Bull Head</p></td> - <td class="tdr"><a href="#v2fig198">50</a></td> -</tr> -<tr> - <td class="tdr vtop">199.</td> - <td><p class="hanging">General section across the Permian basin of Ayrshire -<span class="pagenum" id="Page_x">- x -</span></p></td> - <td class="tdr"><a href="#v2fig199">59</a></td> -</tr> -<tr> - <td class="tdr vtop">200.</td> - <td><p class="hanging">Section of lavas, east side of Mauchline Hill</p></td> - <td class="tdr"><a href="#v2fig200">60</a></td> -</tr> -<tr> - <td class="tdr vtop">201.</td> - <td><p class="hanging">Section of the top of the volcanic series near Eastside Cottage, Carron Water, Nithsdale</p></td> - <td class="tdr"><a href="#v2fig201">60</a></td> -</tr> -<tr> - <td class="tdr vtop">202.</td> - <td><p class="hanging">Section of two outliers of the Permian volcanic series at the foot of Windyhill Burn, Water of Ae, Dumfriesshire</p></td> - <td class="tdr"><a href="#v2fig202">61</a></td> -</tr> -<tr> - <td class="tdr vtop">203.</td> - <td><p class="hanging">The Green Hill, Waterside, Dalmellington, from the south; a tuff-neck of Permian age</p></td> - <td class="tdr"><a href="#v2fig203">62</a></td> -</tr> -<tr> - <td class="tdr vtop">204.</td> - <td><p class="hanging">Patna Hill from the Doon Bridge, Ayrshire; a tuff-neck of Permian age</p></td> - <td class="tdr"><a href="#v2fig204">63</a></td> -</tr> -<tr> - <td class="tdr vtop">205.</td> - <td><p class="hanging">Ground plans of Permian volcanic vents from the Ayrshire Coal-field. On the scale of six inches to a mile</p></td> - <td class="tdr"><a href="#v2fig205">64</a></td> -</tr> -<tr> - <td class="tdr vtop">206.</td> - <td><p class="hanging">Section of sills traversing the Permian volcanic series. River Ayr, Ballochmyle</p></td> - <td class="tdr"><a href="#v2fig206">66</a></td> -</tr> -<tr> - <td class="tdr vtop">207.</td> - <td><p class="hanging">Section showing the relations of the later rocks of Arthur Seat</p></td> - <td class="tdr"><a href="#v2fig207">68</a></td> -</tr> -<tr> - <td class="tdr vtop">208.</td> - <td><p class="hanging">Section in brooks between Bonny town and Baldastard, Largo</p></td> - <td class="tdr"><a href="#v2fig208">70</a></td> -</tr> -<tr> - <td class="tdr vtop">209.</td> - <td><p class="hanging">View of Largo Law from the east</p></td> - <td class="tdr"><a href="#v2fig209">71</a></td> -</tr> -<tr> - <td class="tdr vtop">210.</td> - <td><p class="hanging">View of small neck in Calciferous Sandstones, on the shore, three miles east from St. Andrews</p></td> - <td class="tdr"><a href="#v2fig210">72</a></td> -</tr> -<tr> - <td class="tdr vtop">211.</td> - <td><p class="hanging">Ground-plan of Permian volcanic vents</p></td> - <td class="tdr"><a href="#v2fig211">73</a></td> -</tr> -<tr> - <td class="tdr vtop">212.</td> - <td><p class="hanging">Small neck in Calciferous Sandstones a little east from the "Rock and Spindle," two and a half miles east from St. Andrews</p></td> - <td class="tdr"><a href="#v2fig212">74</a></td> -</tr> -<tr> - <td class="tdr vtop">213.</td> - <td><p class="hanging">Plan of volcanic necks at Kellie Law, East of Fife, on the scale of three inches to one mile</p></td> - <td class="tdr"><a href="#v2fig213">75</a></td> -</tr> -<tr> - <td class="tdr vtop">214.</td> - <td><p class="hanging">Plan of the craters in Volcanello, Lipari Islands</p></td> - <td class="tdr"><a href="#v2fig214">75</a></td> -</tr> -<tr> - <td class="tdr vtop">215.</td> - <td><p class="hanging">Section of the strata at the edge of the volcanic vent on the east side of Elie Harbour</p></td> - <td class="tdr"><a href="#v2fig215">76</a></td> -</tr> -<tr> - <td class="tdr vtop">216.</td> - <td><p class="hanging">Agglomerate of neck on shore at Ardross, two miles east from Elie</p></td> - <td class="tdr"><a href="#v2fig216">77</a></td> -</tr> -<tr> - <td class="tdr vtop">217.</td> - <td><p class="hanging">Ground-plan of volcanic neck, Elie Harbour, showing circular disposition of the stratification</p></td> - <td class="tdr"><a href="#v2fig217">80</a></td> -</tr> -<tr> - <td class="tdr vtop">218.</td> - <td><p class="hanging">Section across the great vent of Kincraig, Elie, on a true scale, vertical and horizontal, of six inches to a mile</p></td> - <td class="tdr"><a href="#v2fig218">81</a></td> -</tr> -<tr> - <td class="tdr vtop">219.</td> - <td><p class="hanging">Dyke in volcanic neck, on the beach, St. Monans</p></td> - <td class="tdr"><a href="#v2fig219">82</a></td> -</tr> -<tr> - <td class="tdr vtop">220.</td> - <td><p class="hanging">Section of part of crater rim, Island of Volcano</p></td> - <td class="tdr"><a href="#v2fig220">83</a></td> -</tr> -<tr> - <td class="tdr vtop">221.</td> - <td><p class="hanging">Dyke rising through the agglomerate of a volcanic vent; Kincraig, Elie</p></td> - <td class="tdr"><a href="#v2fig221">84</a></td> -</tr> -<tr> - <td class="tdr vtop">222.</td> - <td><p class="hanging">Radiating columnar dyke in the tuff of a volcanic vent. Rock and Spindle, two and a half miles east from St. Andrews</p></td> - <td class="tdr"><a href="#v2fig222">86</a></td> -</tr> -<tr> - <td class="tdr vtop">223.</td> - <td><p class="hanging">View of part of the shore front of the great vent at Kincraig, looking westward, with the columnar basalt in front</p></td> - <td class="tdr"><a href="#v2fig223">88</a></td> -</tr> -<tr> - <td class="tdr vtop">224.</td> - <td><p class="hanging">Plan of volcanic neck on beach near St. Monans</p></td> - <td class="tdr"><a href="#v2fig224">89</a></td> -</tr> -<tr> - <td class="tdr vtop">225.</td> - <td><p class="hanging">Columnar basalt in the neck of Kincraig, Elie, seen from the west</p></td> - <td class="tdr"><a href="#v2fig225">90</a></td> -</tr> -<tr> - <td class="tdr vtop">226.</td> - <td><p class="hanging">Section across Largo Law</p></td> - <td class="tdr"><a href="#v2fig226">91</a></td> -</tr> -<tr> - <td class="tdr vtop">227.</td> - <td><p class="hanging">Vein of "white-trap" cutting black carbonaceous shales, a little west from St. Monans Church</p></td> - <td class="tdr"><a href="#v2fig227">92</a></td> -</tr> -<tr> - <td class="tdr vtop">228.</td> - <td><p class="hanging">Section at Belvedere, S. W. of Exeter</p></td> - <td class="tdr"><a href="#v2fig228">97</a></td> -</tr> -<tr> - <td class="tdr vtop">229.</td> - <td><p class="hanging">Diagram to show the unconformability and overlap of the Permian rocks in the Crediton Valley</p></td> - <td class="tdr"><a href="#v2fig229">97</a></td> -</tr> -<tr> - <td class="tdr vtop">230.</td> - <td><p class="hanging">Section of the volcanic series at Kellerton, Devonshire</p></td> - <td class="tdr"><a href="#v2fig230">98</a></td> -</tr> -<tr> - <td class="tdr vtop">231.</td> - <td><p class="hanging">Section of agglomerate overlain with sandstone and andesite, Posbury, Crediton</p></td> - <td class="tdr"><a href="#v2fig231">99</a></td> -</tr> -<tr> - <td class="tdr vtop">232.</td> - <td><p class="hanging">Diagrammatic section across Titterstone Clee Hill</p></td> - <td class="tdr"><a href="#v2fig232">102</a></td> -</tr> -<tr> - <td class="tdr vtop">233.</td> - <td><p class="hanging">Dyke on the south-east coast of the Island of Mull</p></td> - <td class="tdr"><a href="#v2fig233">119</a></td> -</tr> -<tr> - <td class="tdr vtop">234.</td> - <td><p class="hanging">Fissure left by the weathering out of a dyke</p></td> - <td class="tdr"><a href="#v2fig234">120</a></td> -</tr> -<tr> - <td class="tdr vtop">235.</td> - <td><p class="hanging">Plan of basalt-veins with selvages of black basalt-glass, east side of Beinn Tighe, Isle of Eigg -<span class="pagenum" id="Page_xi">- xi -</span></p></td> - <td class="tdr"><a href="#v2fig235">126</a></td> -</tr> -<tr> - <td class="tdr vtop">236.</td> - <td><p class="hanging">Arrangement of lines of amygdales in a dyke, Strathmore, Skye</p></td> - <td class="tdr"><a href="#v2fig236">130</a></td> -</tr> -<tr> - <td class="tdr vtop">237.</td> - <td><p class="hanging">Systems of joints in the dykes</p></td> - <td class="tdr"><a href="#v2fig237">132</a></td> -</tr> -<tr> - <td class="tdr vtop">238.</td> - <td><p class="hanging">Section of cylindrical vein or dyke, cutting the bedded lavas, east side of Fuglö, Faroe Islands</p></td> - <td class="tdr"><a href="#v2fig238">133</a></td> -</tr> -<tr> - <td class="tdr vtop">239.</td> - <td><p class="hanging">Joint-structures in the central vitreous portion of the Eskdale Dyke (B. N. Peach)</p></td> - <td class="tdr"><a href="#v2fig239">133</a></td> -</tr> -<tr> - <td class="tdr vtop">240.</td> - <td><p class="hanging">Microscopic structure of the vitreous part of the Eskdale Dyke</p></td> - <td class="tdr"><a href="#v2fig240">136</a></td> -</tr> -<tr> - <td class="tdr vtop">241.</td> - <td><p class="hanging">Section along the line of the Cleveland Dyke at Cliff Ridge, Guisbrough (G. Barrow), scale, 12 inches to 1 mile</p></td> - <td class="tdr"><a href="#v2fig241">147</a></td> -</tr> -<tr> - <td class="tdr vtop">242.</td> - <td><p class="hanging">Section along the course of the Cleveland Dyke, at the head of Lonsdale, Yorkshire (G. Barrow, in the <i>Memoirs of the Geol. Survey</i>, Geology of Cleveland, p. 61)</p></td> - <td class="tdr"><a href="#v2fig242">148</a></td> -</tr> -<tr> - <td class="tdr vtop">243.</td> - <td><p class="hanging">Section across the extreme upper limit of Cleveland Dyke, on the scale of 20 feet to one inch (Mr. G. Barrow)</p></td> - <td class="tdr"><a href="#v2fig243">149</a></td> -</tr> -<tr> - <td class="tdr vtop">244.</td> - <td><p class="hanging">Upper limit of Cleveland Dyke in quarry near Cockfield (after Mr. Teall)</p></td> - <td class="tdr"><a href="#v2fig244">149</a></td> -</tr> -<tr> - <td class="tdr vtop">245.</td> - <td><p class="hanging">Section along the course of the Cleveland Dyke across the Cross Fell escarpment (scale of one inch to one mile)</p></td> - <td class="tdr"><a href="#v2fig245">150</a></td> -</tr> -<tr> - <td class="tdr vtop">246.</td> - <td><p class="hanging">Branching portion of the great dyke near Hawick (length about one mile)</p></td> - <td class="tdr"><a href="#v2fig246">153</a></td> -</tr> -<tr> - <td class="tdr vtop">247.</td> - <td><p class="hanging">Branching dyke at foot of Glen Artney (length about four miles)</p></td> - <td class="tdr"><a href="#v2fig247">153</a></td> -</tr> -<tr> - <td class="tdr vtop">248.</td> - <td><p class="hanging">Basic veins traversing Secondary limestone and sandstone on the coast cliffs, Aidnamurchan</p></td> - <td class="tdr"><a href="#v2fig248">155</a></td> -</tr> -<tr> - <td class="tdr vtop">249.</td> - <td><p class="hanging">Section showing the connection of a Dyke with an Intrusive Sheet, Point of Suisnish, Skye</p></td> - <td class="tdr"><a href="#v2fig249">156</a></td> -</tr> -<tr> - <td class="tdr vtop">250.</td> - <td><p class="hanging">Section to show the connection of a Dyke with an Intrusive Sheet, Stirlingshire Coal-field</p></td> - <td class="tdr"><a href="#v2fig250">157</a></td> -</tr> -<tr> - <td class="tdr vtop">251.</td> - <td><p class="hanging">Intersection of dykes in bedded basalt, Calliach Point, Mull</p></td> - <td class="tdr"><a href="#v2fig251">158</a></td> -</tr> -<tr> - <td class="tdr vtop">252.</td> - <td><p class="hanging">Basalt veins traversing bedded dolerites, Kildonan, Eigg</p></td> - <td class="tdr"><a href="#v2fig252">159</a></td> -</tr> -<tr> - <td class="tdr vtop">253.</td> - <td><p class="hanging">Ground-plan of intersecting dykes in Lias limestone, Shore, Harrabol, East of Broadford, Skye</p></td> - <td class="tdr"><a href="#v2fig253">159</a></td> -</tr> -<tr> - <td class="tdr vtop">254.</td> - <td><p class="hanging">Compound dyke, Market Stance, Broadford, Skye</p></td> - <td class="tdr"><a href="#v2fig254">162</a></td> -</tr> -<tr> - <td class="tdr vtop">255.</td> - <td><p class="hanging">Section of coal rendered columnar by intrusive basalt, shore, Saltcoats, Ayrshire</p></td> - <td class="tdr"><a href="#v2fig255">164</a></td> -</tr> -<tr> - <td class="tdr vtop">256.</td> - <td><p class="hanging">Dolerite dyke with marginal bands of "white trap," in black shale, Lower Lias, Pabba</p></td> - <td class="tdr"><a href="#v2fig256">166</a></td> -</tr> -<tr> - <td class="tdr vtop">257.</td> - <td><p class="hanging">Map of the chief dykes between Lochs Ridden and Striven (C. T. Clough, Geological Survey, Sheet 29)</p></td> - <td class="tdr"><a href="#v2fig257">170</a></td> -</tr> -<tr> - <td class="tdr vtop">258.</td> - <td><p class="hanging">Basalt-veins traversing granophyre, St. Kilda</p></td> - <td class="tdr"><a href="#v2fig258">173</a></td> -</tr> -<tr> - <td class="tdr vtop">259.</td> - <td><p class="hanging">Section of scoriaceous and prismatic basalt, Camas Tharbernish, north shore of Canna Island</p></td> - <td class="tdr"><a href="#v2fig259">187</a></td> -</tr> -<tr> - <td class="tdr vtop">260.</td> - <td><p class="hanging">Banded amygdaloidal basalt showing layers of elongated and steeply inclined vesicles, Macleod's Maidens, Skye</p></td> - <td class="tdr"><a href="#v2fig260">191</a></td> -</tr> -<tr> - <td class="tdr vtop">261.</td> - <td><p class="hanging">Termination of basalt-beds, Carsaig, Mull</p></td> - <td class="tdr"><a href="#v2fig261">193</a></td> -</tr> -<tr> - <td class="tdr vtop">262.</td> - <td><p class="hanging">Breccia and blocks of mica-schist, quartzite, etc., lying between bedded basalts, Isle of Mull</p></td> - <td class="tdr"><a href="#v2fig262">197</a></td> -</tr> -<tr> - <td class="tdr vtop">263.</td> - <td><p class="hanging">Section of Knocklayd, an outlier of the Antrim basalt-plateau lying on Chalk</p></td> - <td class="tdr"><a href="#v2fig263">202</a></td> -</tr> -<tr> - <td class="tdr vtop">264.</td> - <td><p class="hanging">Diagram-Section of the Antrim Plateau</p></td> - <td class="tdr"><a href="#v2fig264">203</a></td> -</tr> -<tr> - <td class="tdr vtop">265.</td> - <td><p class="hanging">View Of Basalt escarpment, Giant's Causeway, with the Amphitheatre and Chimneys. (From a photograph by Mr. R. Welch)</p></td> - <td class="tdr"><a href="#v2fig265">207</a></td> -</tr> -<tr> - <td class="tdr vtop">266.</td> - <td><p class="hanging">Basalt-capping on the top of Ben Iadain, Morven</p></td> - <td class="tdr"><a href="#v2fig266">209</a></td> -</tr> -<tr> - <td class="tdr vtop">266<i>a</i>.</td> - <td><p class="hanging">View of the south side of Staffa, showing the bedded and columnar structure of the basalt -<span class="pagenum" id="Page_xii">- xii -</span></p></td> - <td class="tdr"><a href="#v2fig266a">210</a></td> -</tr> -<tr> - <td class="tdr vtop">267.</td> - <td><p class="hanging">View of Rum from the harbour of Canna</p></td> - <td class="tdr"><a href="#v2fig267">216</a></td> -</tr> -<tr> - <td class="tdr vtop">268.</td> - <td><p class="hanging">Section of the cliffs below Compass Hill, Isle of Canna</p></td> - <td class="tdr"><a href="#v2fig268">218</a></td> -</tr> -<tr> - <td class="tdr vtop">269.</td> - <td><p class="hanging">Lava cutting out conglomerate and shale. Shore below Canna House</p></td> - <td class="tdr"><a href="#v2fig269">224</a></td> -</tr> -<tr> - <td class="tdr vtop">270.</td> - <td><p class="hanging">Section of shales and tuffs, with a coniferous stump lying between two basalt-sheets, Cùl nam Marbh, Canna</p></td> - <td class="tdr"><a href="#v2fig270">225</a></td> -</tr> -<tr> - <td class="tdr vtop">271.</td> - <td><p class="hanging">Dùn Mòr, Sanday. (From a photograph by Miss Thom)</p></td> - <td class="tdr"><a href="#v2fig271">226</a></td> -</tr> -<tr> - <td class="tdr vtop">272.</td> - <td><p class="hanging">View of the Dùn Beag, Sanday, seen from the south. (From a Photograph by Miss Thom)</p></td> - <td class="tdr"><a href="#v2fig272">230</a></td> -</tr> -<tr> - <td class="tdr vtop">273.</td> - <td><p class="hanging">View of Dùn Beag, Sanday, from the north. The island of Rum in the distance. (From a Photograph by Miss Thom)</p></td> - <td class="tdr"><a href="#v2fig273">231</a></td> -</tr> -<tr> - <td class="tdr vtop">274.</td> - <td><p class="hanging">Section of eastern front of Dùn Beag</p></td> - <td class="tdr"><a href="#v2fig274">232</a></td> -</tr> -<tr> - <td class="tdr vtop">275.</td> - <td><p class="hanging">Enlarged section on the western side of Dùn Beag</p></td> - <td class="tdr"><a href="#v2fig275">233</a></td> -</tr> -<tr> - <td class="tdr vtop">276.</td> - <td><p class="hanging">Geological map of the Island of Eigg</p></td> - <td class="tdr"><a href="#v2fig276">235</a></td> -</tr> -<tr> - <td class="tdr vtop">277.</td> - <td><p class="hanging">Section of the geological structure of the Island of Eigg</p></td> - <td class="tdr"><a href="#v2fig277">236</a></td> -</tr> -<tr> - <td class="tdr vtop">278.</td> - <td><p class="hanging">View of the Scuir of Eigg from the east</p></td> - <td class="tdr"><a href="#v2fig278">237</a></td> -</tr> -<tr> - <td class="tdr vtop">279.</td> - <td><p class="hanging">Natural section at the cliff of Bideann Boidheach, north-west end of the Scuir of Eigg</p></td> - <td class="tdr"><a href="#v2fig279">239</a></td> -</tr> -<tr> - <td class="tdr vtop">280.</td> - <td><p class="hanging">View of the Scuir of Eigg from the south</p></td> - <td class="tdr"><a href="#v2fig280">242</a></td> -</tr> -<tr> - <td class="tdr vtop">281.</td> - <td><p class="hanging">View of the Scuir of Eigg from the south-west of the Loch a Bhealaich, showing the bedded character of the mass</p></td> - <td class="tdr"><a href="#v2fig281">243</a></td> -</tr> -<tr> - <td class="tdr vtop">282.</td> - <td><p class="hanging">Section at the base of the Scuir of Eigg (east end)</p></td> - <td class="tdr"><a href="#v2fig282">244</a></td> -</tr> -<tr> - <td class="tdr vtop">283.</td> - <td><p class="hanging">Terraced hills of basalt plateau (Macleod's Tables), Skye</p></td> - <td class="tdr"><a href="#v2fig283">250</a></td> -</tr> -<tr> - <td class="tdr vtop">284.</td> - <td><p class="hanging">"Macleod's Maidens" and part of basalt cliffs of Skye</p></td> - <td class="tdr"><a href="#v2fig284">251</a></td> -</tr> -<tr> - <td class="tdr vtop">285.</td> - <td><p class="hanging">Intercalated group of strata between Basalts, An Ceannaich, western side of Skye</p></td> - <td class="tdr"><a href="#v2fig285">252</a></td> -</tr> -<tr> - <td class="tdr vtop">286.</td> - <td><p class="hanging">Escarpment of Plateau-basalts, Cliffs of Talisker, Skye</p></td> - <td class="tdr"><a href="#v2fig286">253</a></td> -</tr> -<tr> - <td class="tdr vtop">287.</td> - <td><p class="hanging">Section of the largest of Macleod's Maidens</p></td> - <td class="tdr"><a href="#v2fig287">254</a></td> -</tr> -<tr> - <td class="tdr vtop">288.</td> - <td><p class="hanging">Dying out of lava-beds, east side of Sandö, Faroe Isles</p></td> - <td class="tdr"><a href="#v2fig288">257</a></td> -</tr> -<tr> - <td class="tdr vtop">289.</td> - <td><p class="hanging">Lenticular lavas, western front of Hestö, Faroe Isles</p></td> - <td class="tdr"><a href="#v2fig289">257</a></td> -</tr> -<tr> - <td class="tdr vtop">290.</td> - <td><p class="hanging">Lenticular lavas, east side of Svinö, Faroe Isles</p></td> - <td class="tdr"><a href="#v2fig290">258</a></td> -</tr> -<tr> - <td class="tdr vtop">291.</td> - <td><p class="hanging">Section at Frodbonyp, Suderö, Faroe</p></td> - <td class="tdr"><a href="#v2fig291">258</a></td> -</tr> -<tr> - <td class="tdr vtop">292.</td> - <td><p class="hanging">Fissure (gjá) in a lava-field, Iceland. (From a photograph by Dr. Tempest Anderson)</p></td> - <td class="tdr"><a href="#v2fig292">262</a></td> -</tr> -<tr> - <td class="tdr vtop">293.</td> - <td><p class="hanging">Cones on the great Laki fissure, Iceland. (From a photograph by Dr. Tempest Anderson)</p></td> - <td class="tdr"><a href="#v2fig293">263</a></td> -</tr> -<tr> - <td class="tdr vtop">293<i>a</i>.</td> - <td><p class="hanging">Plan of small craters along the line of great Laki fissure, Iceland. (After Mr. Helland, reduced)</p></td> - <td class="tdr"><a href="#v2fig293a">264</a></td> -</tr> -<tr> - <td class="tdr vtop">294.</td> - <td><p class="hanging">Slemish, a volcanic neck or vent on the Antrim plateau, seen from the north</p></td> - <td class="tdr"><a href="#v2fig294">272</a></td> -</tr> -<tr> - <td class="tdr vtop">295.</td> - <td><p class="hanging">Section of volcanic vent at Carnmony Hill (E. Hull)</p></td> - <td class="tdr"><a href="#v2fig295">272</a></td> -</tr> -<tr> - <td class="tdr vtop">296.</td> - <td><p class="hanging">Section of the east side of Scawt Hill, near Glenarm</p></td> - <td class="tdr"><a href="#v2fig296">273</a></td> -</tr> -<tr> - <td class="tdr vtop">297.</td> - <td><p class="hanging">Section of Neck of basalt, Bendoo, Ballintoy</p></td> - <td class="tdr"><a href="#v2fig297">273</a></td> -</tr> -<tr> - <td class="tdr vtop">298.</td> - <td><p class="hanging">Volcanic Neck of dolerite near Cushendall</p></td> - <td class="tdr"><a href="#v2fig298">274</a></td> -</tr> -<tr> - <td class="tdr vtop">299.</td> - <td><p class="hanging">Section of Volcanic Neck at 'S Airde Beinne, near Tobermory, Mull</p></td> - <td class="tdr"><a href="#v2fig299">274</a></td> -</tr> -<tr> - <td class="tdr vtop">300.</td> - <td><p class="hanging">Interior of the Volcanic Neck of 'S Airde Beinne, near Tobermory, Mull</p></td> - <td class="tdr"><a href="#v2fig300">275</a></td> -</tr> -<tr> - <td class="tdr vtop">301.</td> - <td><p class="hanging">Diagram to show the probable relation of the Neck at Carrick-a-raide, Antrim, to an adjacent group of tuffs</p></td> - <td class="tdr"><a href="#v2fig301">277</a></td> -</tr> -<tr> - <td class="tdr vtop">302.</td> - <td><p class="hanging">Section of agglomerate Neck at Maclean's Nose, Ardnamurchan</p></td> - <td class="tdr"><a href="#v2fig302">279</a></td> -</tr> -<tr> - <td class="tdr vtop">303.</td> - <td><p class="hanging">Diagram to show the probable relations of the rocks on the southern flank of Beinn Dearg Bheag</p></td> - <td class="tdr"><a href="#v2fig303">282</a></td> -</tr> -<tr> - <td class="tdr vtop">304.</td> - <td><p class="hanging">Section of Volcanic Vent and connected lavas and tuffs, Scorr, Camas Garbh, Portree Bay, Skye -<span class="pagenum" id="Page_xiii">- xiii -</span></p></td> - <td class="tdr"><a href="#v2fig304">284</a></td> -</tr> -<tr> - <td class="tdr vtop">305.</td> - <td><p class="hanging">Section of the Volcanic Series at Ach na Hannait, south of Portree, Skye</p></td> - <td class="tdr"><a href="#v2fig305">288</a></td> -</tr> -<tr> - <td class="tdr vtop">306.</td> - <td><p class="hanging">View of part of a Volcanic Neck at the eastern end of the island of Canna. (From a photograph by Miss Thom)</p></td> - <td class="tdr"><a href="#v2fig306">289</a></td> -</tr> -<tr> - <td class="tdr vtop">307.</td> - <td><p class="hanging">Columnar Basalt invading agglomerate of Volcanic Vent, Coroghon Mòr, Isle of Canna. (Height above 20 feet)</p></td> - <td class="tdr"><a href="#v2fig307">291</a></td> -</tr> -<tr> - <td class="tdr vtop">308.</td> - <td><p class="hanging">Columnar Basalt invading Volcanic conglomerate, north side of Alman Islet, Canna</p></td> - <td class="tdr"><a href="#v2fig308">291</a></td> -</tr> -<tr> - <td class="tdr vtop">309.</td> - <td><p class="hanging">View of neck-like mass of breccia, Brochel, Raasay</p></td> - <td class="tdr"><a href="#v2fig309">292</a></td> -</tr> -<tr> - <td class="tdr vtop">310.</td> - <td><p class="hanging">View of Volcanic Neck piercing and overlain by the Plateau-Basalts, Stromö, entrance of Vaagöfjord, Faroe Islands. (From a photograph by Colonel Evans)</p></td> - <td class="tdr"><a href="#v2fig310">294</a></td> -</tr> -<tr> - <td class="tdr vtop">311.</td> - <td><p class="hanging">Section of the same Neck as that shown in <a href="#v2fig310">Fig. 310</a></p></td> - <td class="tdr"><a href="#v2fig311">295</a></td> -</tr> -<tr> - <td class="tdr vtop">312.</td> - <td><p class="hanging">Volcanic Neck close to that shown in Figs. <a href="#v2fig310">310</a> and <a href="#v2fig311">311</a></p></td> - <td class="tdr"><a href="#v2fig312">296</a></td> -</tr> -<tr> - <td class="tdr vtop">313.</td> - <td><p class="hanging">Section of wall of another Neck of agglomerate in the same group with those represented in Figs. <a href="#v2fig310">310</a>, <a href="#v2fig311">311</a>, and <a href="#v2fig312">312</a></p></td> - <td class="tdr"><a href="#v2fig313">296</a></td> -</tr> -<tr> - <td class="tdr vtop">314.</td> - <td><p class="hanging">View of "Segregation-Veins" in a dolerite sill, Portrush, Antrim</p></td> - <td class="tdr"><a href="#v2fig314">300</a></td> -</tr> -<tr> - <td class="tdr vtop">315.</td> - <td><p class="hanging">View of Fair Head, from the east, showing the main upper sill and a thinner sheet cropping out along the talus slope</p></td> - <td class="tdr"><a href="#v2fig315">301</a></td> -</tr> -<tr> - <td class="tdr vtop">316.</td> - <td><p class="hanging">View of Fair Head from the shore. (From a photograph by Mr. R. Welch)</p></td> - <td class="tdr"><a href="#v2fig316">302</a></td> -</tr> -<tr> - <td class="tdr vtop">317.</td> - <td><p class="hanging">Section at Farragandoo cliff, west end of Fair Head, showing the rapid splitting up and dying out of an Intrusive Sheet</p></td> - <td class="tdr"><a href="#v2fig317">304</a></td> -</tr> -<tr> - <td class="tdr vtop">318.</td> - <td><p class="hanging">View of the Trotternish Coast, showing the position of the band of Sills</p></td> - <td class="tdr"><a href="#v2fig318">305</a></td> -</tr> -<tr> - <td class="tdr vtop">319.</td> - <td><p class="hanging">Columnar Sill intrusive in Jurassic Strata east of Kilmartin, Trotternish, Skye</p></td> - <td class="tdr"><a href="#v2fig319">306</a></td> -</tr> -<tr> - <td class="tdr vtop">320.</td> - <td><p class="hanging">View of the northern precipice (500 feet high) of the largest of the Shiant Isles. (From a photograph by Colonel Evans)</p></td> - <td class="tdr"><a href="#v2fig320">308</a></td> -</tr> -<tr> - <td class="tdr vtop">321.</td> - <td><p class="hanging">Section of thin Intrusive Sheets and Veins in carbonaceous shales lying among the Plateau-basalts, cliffs north of Ach na Hannait, between Portree Bay and Loch Sligachan</p></td> - <td class="tdr"><a href="#v2fig321">311</a></td> -</tr> -<tr> - <td class="tdr vtop">322.</td> - <td><p class="hanging">Upper part of Sill, Moonen Bay, Waternish, Skye, showing the divergence of veins</p></td> - <td class="tdr"><a href="#v2fig322">313</a></td> -</tr> -<tr> - <td class="tdr vtop">323.</td> - <td><p class="hanging">Section of the base of the Basalt-plateau with sill and dykes, Sound of Soa, Skye</p></td> - <td class="tdr"><a href="#v2fig323">314</a></td> -</tr> -<tr> - <td class="tdr vtop">324.</td> - <td><p class="hanging">Section of Dolerite Sill cut by another sill, both being traversed by dykes, Rudh' an Iasgaich, western side of Sleat, Skye</p></td> - <td class="tdr"><a href="#v2fig324">316</a></td> -</tr> -<tr> - <td class="tdr vtop">325.</td> - <td><p class="hanging">Section to show Bedded and Intrusive Sheets, Eigg</p></td> - <td class="tdr"><a href="#v2fig325">318</a></td> -</tr> -<tr> - <td class="tdr vtop">326.</td> - <td><p class="hanging">Ground plan of Sills at Ben Hiant, Ardnamurchan</p></td> - <td class="tdr"><a href="#v2fig326">321</a></td> -</tr> -<tr> - <td class="tdr vtop">327.</td> - <td><p class="hanging">Section of two Sills in schistose grits, west end of Beinn na h-Urchrach, Ardnamurchan</p></td> - <td class="tdr"><a href="#v2fig327">322</a></td> -</tr> -<tr> - <td class="tdr vtop">328.</td> - <td><p class="hanging">Sill traversing bedded Basalts, cliffs of Stromö, at entrance of Vaagöfjord</p></td> - <td class="tdr"><a href="#v2fig328">323</a></td> -</tr> -<tr> - <td class="tdr vtop">329.</td> - <td><p class="hanging">View of the same Sill seen from the channel opposite the island of Kolter</p></td> - <td class="tdr"><a href="#v2fig329">324</a></td> -</tr> -<tr> - <td class="tdr vtop">330.</td> - <td><p class="hanging">Granulitic and coarsely foliated Gabbro traversed by later veins of felspathic Gabbro, Druim an Eidhne, Cuillin Hills, Skye</p></td> - <td class="tdr"><a href="#v2fig330">331</a></td> -</tr> -<tr> - <td class="tdr vtop">331.</td> - <td><p class="hanging">Scuir na Gillean, Cuillin Hills, showing the characteristic craggy forms of the Gabbro. (From a photograph by Mr. Abraham, Keswick)</p></td> - <td class="tdr"><a href="#v2fig331">335</a></td> -</tr> -<tr> - <td class="tdr vtop">332.</td> - <td><p class="hanging">Section across Glen Brittle, to show the general relations of the Bedded Basalts and the Gabbros</p></td> - <td class="tdr"><a href="#v2fig332">336</a></td> -</tr> -<tr> - <td class="tdr vtop">333.</td> - <td><p class="hanging">View of the crest of the Cuillin Hills, showing the weathering of the Gabbro along its joints and of a compound basic dyke which rises through it. (From a photograph by Mr. Abraham, Keswick)</p></td> - <td class="tdr"><a href="#v2fig333">338</a></td> -</tr> -<tr> - <td class="tdr vtop">334.</td> - <td><p class="hanging">Section across the Coire Uaigneich, Skye</p></td> - <td class="tdr"><a href="#v2fig334">341</a></td> -</tr> -<tr> - <td class="tdr vtop">335.</td> - <td><p class="hanging">Banded and puckered gabbro, Druim an Eidhne, Glen Sligachan, Skye</p></td> - <td class="tdr"><a href="#v2fig335">342</a></td> -</tr> -<tr> - <td class="tdr vtop">336.</td> - <td><p class="hanging">Banded structure in the Gabbro, from the ridge of Druim an Eidhne, between Loch Coruisk and Glen Sligachan -<span class="pagenum" id="Page_xiv">- xiv -</span></p></td> - <td class="tdr"><a href="#v2fig336">343</a></td> -</tr> -<tr> - <td class="tdr vtop">337.</td> - <td><p class="hanging">Banded and doubly folded Gabbro, Druim an Eidhne, 10 feet broad</p></td> - <td class="tdr"><a href="#v2fig337">345</a></td> -</tr> -<tr> - <td class="tdr vtop">338.</td> - <td><p class="hanging">Sketch of banded structure in the Gabbros of the hills at the head of Loch Scavaig</p></td> - <td class="tdr"><a href="#v2fig338">347</a></td> -</tr> -<tr> - <td class="tdr vtop">339.</td> - <td><p class="hanging">Outline of the hills of the Island of Rum, sketched from near the Isle of Eigg</p></td> - <td class="tdr"><a href="#v2fig339">350</a></td> -</tr> -<tr> - <td class="tdr vtop">340.</td> - <td><p class="hanging">View of Allival, Rum, sketched from the base of the north-east side of the cone</p></td> - <td class="tdr"><a href="#v2fig340">352</a></td> -</tr> -<tr> - <td class="tdr vtop">341.</td> - <td><p class="hanging">Section of foliated Gabbros in the Tertiary volcanic series of Allival, Rum</p></td> - <td class="tdr"><a href="#v2fig341">353</a></td> -</tr> -<tr> - <td class="tdr vtop">342.</td> - <td><p class="hanging">Altered Plateau-Basalts invaded by Gabbro, and with a Dyke of prismatic Basalt cutting both rocks, north slope of Ben Buy, Mull</p></td> - <td class="tdr"><a href="#v2fig342">357</a></td> -</tr> -<tr> - <td class="tdr vtop">343.</td> - <td><p class="hanging">Theoretical representation of the structure of one of the Gabbro bosses of the Inner Hebrides</p></td> - <td class="tdr"><a href="#v2fig343">362</a></td> -</tr> -<tr> - <td class="tdr vtop">344.</td> - <td><p class="hanging">Section through the Puy de la Goutte and Puy de Chopine</p></td> - <td class="tdr"><a href="#v2fig344">374</a></td> -</tr> -<tr> - <td class="tdr vtop">345.</td> - <td><p class="hanging">View of the Huche Pointue and Huche Platte west of Le Pertuis</p></td> - <td class="tdr"><a href="#v2fig345">376</a></td> -</tr> -<tr> - <td class="tdr vtop">346.</td> - <td><p class="hanging">View of Glamich, 2537 feet, Glen Sligachan. (From a photograph by R. J. A. Berry, M.D., lent by the Scottish Mountaineering Club)</p></td> - <td class="tdr"><a href="#v2fig346">380</a></td> -</tr> -<tr> - <td class="tdr vtop">347.</td> - <td><p class="hanging">Section across the north slope of Beinn an Dubhaich, Skye</p></td> - <td class="tdr"><a href="#v2fig347">383</a></td> -</tr> -<tr> - <td class="tdr vtop">348.</td> - <td><p class="hanging">Section from Beinn Dearg to Beinn an Dubhaich, Skye</p></td> - <td class="tdr"><a href="#v2fig348">385</a></td> -</tr> -<tr> - <td class="tdr vtop">349.</td> - <td><p class="hanging">Section at north end of Beinn na Cro, Skye</p></td> - <td class="tdr"><a href="#v2fig349">388</a></td> -</tr> -<tr> - <td class="tdr vtop">350.</td> - <td><p class="hanging">Ground-plan of basic dyke in Cambrian limestones truncated by granophyre which encloses large blocks of the dyke, Torrin, Skye</p></td> - <td class="tdr"><a href="#v2fig350">393</a></td> -</tr> -<tr> - <td class="tdr vtop">351.</td> - <td><p class="hanging">Section on south side of Beinn an Dubhaich, Skye, showing the truncation of a basalt-dyke</p></td> - <td class="tdr"><a href="#v2fig351">394</a></td> -</tr> -<tr> - <td class="tdr vtop">352.</td> - <td><p class="hanging">View of the hills on the south side of the head of Loch na Keal, showing the junction of the Granophyre and the bedded basalts</p></td> - <td class="tdr"><a href="#v2fig352">396</a></td> -</tr> -<tr> - <td class="tdr vtop">353.</td> - <td><p class="hanging">Section on south side of Cruach Tòrr an Lochain, Mull</p></td> - <td class="tdr"><a href="#v2fig353">398</a></td> -</tr> -<tr> - <td class="tdr vtop">354.</td> - <td><p class="hanging">Section at head of Allt na Searmoin, Mull</p></td> - <td class="tdr"><a href="#v2fig354">398</a></td> -</tr> -<tr> - <td class="tdr vtop">355.</td> - <td><p class="hanging">Section on south side of Beinn Fhada, Mull</p></td> - <td class="tdr"><a href="#v2fig355">399</a></td> -</tr> -<tr> - <td class="tdr vtop">356.</td> - <td><p class="hanging">Section to south of Loch na Dà iridh, Mull</p></td> - <td class="tdr"><a href="#v2fig356">400</a></td> -</tr> -<tr> - <td class="tdr vtop">357.</td> - <td><p class="hanging">Section of junction of south side of Loch Ba' Granophyre boss, with the bedded basalts, Mull</p></td> - <td class="tdr"><a href="#v2fig357">401</a></td> -</tr> -<tr> - <td class="tdr vtop">358.</td> - <td><p class="hanging">Mass of dark gabbro about two feet in diameter traversed by pale veins of Granophyre, lying on north slope of Creag na h-Iolaire, Mull</p></td> - <td class="tdr"><a href="#v2fig358">402</a></td> -</tr> -<tr> - <td class="tdr vtop">359.</td> - <td><p class="hanging">Section at Creag na h-Iolaire, Glen More, Mull, showing basalts and gabbros resting on and pierced by Granophyre</p></td> - <td class="tdr"><a href="#v2fig359">402</a></td> -</tr> -<tr> - <td class="tdr vtop">360.</td> - <td><p class="hanging">Section on north side of Orval, Rum</p></td> - <td class="tdr"><a href="#v2fig360">404</a></td> -</tr> -<tr> - <td class="tdr vtop">361.</td> - <td><p class="hanging">Junction of Quartz-porphyry (Microgranite) and basic rocks, south-east side of Orval, Rum</p></td> - <td class="tdr"><a href="#v2fig361">404</a></td> -</tr> -<tr> - <td class="tdr vtop">362.</td> - <td><p class="hanging">Junction of Granophyre and gabbro, north side of St. Kilda</p></td> - <td class="tdr"><a href="#v2fig362">410</a></td> -</tr> -<tr> - <td class="tdr vtop">363.</td> - <td><p class="hanging">Veins of Granophyre traversing gabbro and splitting up into thin threads, north side of St. Kilda</p></td> - <td class="tdr"><a href="#v2fig363">411</a></td> -</tr> -<tr> - <td class="tdr vtop">364.</td> - <td><p class="hanging">Pale Granophyre injected into dark basalt, South Bay, St. Kilda</p></td> - <td class="tdr"><a href="#v2fig364">412</a></td> -</tr> -<tr> - <td class="tdr vtop">365.</td> - <td><p class="hanging">Veins of Granophyre traversing a fine-grained gabbro and scarcely entering a coarse-grained sheet, west side of Rueval, St. Kilda</p></td> - <td class="tdr"><a href="#v2fig365">413</a></td> -</tr> -<tr> - <td class="tdr vtop">366.</td> - <td><p class="hanging">View of sills and veins of pale Granophyre traversing sheets of gabbro, west side of St. Kilda. (From a photograph by Colonel Evans)</p></td> - <td class="tdr"><a href="#v2fig366">414</a></td> -</tr> -<tr> - <td class="tdr vtop">367.</td> - <td><p class="hanging">Section of the sea-cliff below Conacher, St. Kilda, showing basic dykes in Granophyre</p></td> - <td class="tdr"><a href="#v2fig367">417</a></td> -</tr> -<tr> - <td class="tdr vtop">368.</td> - <td><p class="hanging">Triple basic dyke, sea-cliff, east side of St. Kilda</p></td> - <td class="tdr"><a href="#v2fig368">417</a></td> -</tr> -<tr> - <td class="tdr vtop">369.</td> - <td><p class="hanging">Jointed structure of the Granite near the top of Goatfell, Arran. (From a photograph by Mr. W. Douglas, lent by the Scottish Mountaineering Club)</p></td> - <td class="tdr"><a href="#v2fig369">419</a></td> -</tr> -<tr> - <td class="tdr vtop">370.</td> - <td><p class="hanging">Intrusive Rhyolite in the lower basalt group of Antrim, Templepatrick</p></td> - <td class="tdr"><a href="#v2fig370">427</a></td> -</tr> -<tr> - <td class="tdr vtop">371.</td> - <td><p class="hanging">Section across the southern slope of Carnearny Hill, Antrim -<span class="pagenum" id="Page_xv">- xv -</span></p></td> - <td class="tdr"><a href="#v2fig371">427</a></td> -</tr> -<tr> - <td class="tdr vtop">372.</td> - <td><p class="hanging">Section across the Granophyre Sills at Loch a' Mhullaich, above Skulamus, Skye</p></td> - <td class="tdr"><a href="#v2fig372">433</a></td> -</tr> -<tr> - <td class="tdr vtop">373.</td> - <td><p class="hanging">Section to show the connection of a sill of Granophyre with its probable funnel of supply, Raasay</p></td> - <td class="tdr"><a href="#v2fig373">436</a></td> -</tr> -<tr> - <td class="tdr vtop">374.</td> - <td><p class="hanging">Granophyre sill resting on Lower Lias shales with a dyke of basalt passing laterally into a sill, Suisnish Point, Isle of Raasay</p></td> - <td class="tdr"><a href="#v2fig374">436</a></td> -</tr> -<tr> - <td class="tdr vtop">375.</td> - <td><p class="hanging">Weathered surface of spherulitic Granophyre from dyke in banded gabbros, Druim an Eidhne, Meall Dearg, Glen Sligachan, Skye. Natural size</p></td> - <td class="tdr"><a href="#v2fig375">438</a></td> -</tr> -<tr> - <td class="tdr vtop">376.</td> - <td><p class="hanging">Plan of portion of the ridge north of Druim an Eidhne, Glen Sligachan, Skye, showing three dykes issuing from a mass of Granophyre</p></td> - <td class="tdr"><a href="#v2fig376">439</a></td> -</tr> -<tr> - <td class="tdr vtop">377.</td> - <td><p class="hanging">Weathered surface of spherulitic Granophyre, from dyke in banded gabbros, Druim an Eidhne, Meall Dearg, Glen Sligachan, Skye. Natural size</p></td> - <td class="tdr"><a href="#v2fig377">440</a></td> -</tr> -<tr> - <td class="tdr vtop">378.</td> - <td><p class="hanging">Plan of pale Granophyric dyke, with spherulitic and flow-structure, cutting and enclosing dark gabbro, Druim an Eidhne</p></td> - <td class="tdr"><a href="#v2fig378">441</a></td> -</tr> -<tr> - <td class="tdr vtop">379.</td> - <td><p class="hanging">Dyke (six to ten feet broad) proceeding from a large body of Granophyre and traversing gabbro, from the same locality as Figs. <a href="#v2fig375">375</a> and <a href="#v2fig377">377</a></p></td> - <td class="tdr"><a href="#v2fig379">442</a></td> -</tr> -<tr> - <td class="tdr vtop">380.</td> - <td><p class="hanging">Section of intruded veins of various acid rocks above River Clachaig, Mull</p></td> - <td class="tdr"><a href="#v2fig380">443</a></td> -</tr> -<tr> - <td class="tdr vtop">381.</td> - <td><p class="hanging">Pitchstone vein traversing the bedded basalts, Rudh an Tangairt, Eigg</p></td> - <td class="tdr"><a href="#v2fig381">445</a></td> -</tr> -<tr> - <td class="tdr vtop">382.</td> - <td><p class="hanging">Reversed fault on the eastern side of Svinö, Faroe Isles</p></td> - <td class="tdr"><a href="#v2fig382">454</a></td> -</tr> -<tr> - <td class="tdr vtop">383.</td> - <td><p class="hanging">Reversed fault on the north-east headland of Sandö, Faroe Isle</p></td> - <td class="tdr"><a href="#v2fig383">454</a></td> -</tr> -</table> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<h2 class="nobreak" id="MAPS">MAPS</h2> -</div> - - -<table summary="maps"> -<tr> - <td class="tdr">V.</td> - <td class="tdl">Map of the Permian volcanic districts of Scotland</td> - <td class="tdr"><a href="#v2map5"><i>To face p.</i> 106</a></td> -</tr> -<tr> - <td class="tdr">VI.</td> - <td class="tdl">Map of the Tertiary volcanic region of the West of Scotland</td> - <td class="tdr"><a href="#v2map6"><i>To face p.</i> 296</a></td> -</tr> -<tr> - <td class="tdr">VII.</td> - <td class="tdl">Map of the Tertiary volcanic district of the North-East of Ireland</td> - <td class="tdr"><a href="#v2map7"><i>To face p.</i> 446</a></td> -</tr> -</table> - - -<hr class="chap x-ebookmaker-drop" /> - -<p><span class="pagenum" id="Page_1">- 1 -</span></p> - -<div class="chapter"> -<p><span class="pagenum" id="Page_xvi">- xvi -</span></p> - -<h2 class="nobreak" id="CHAPTER_XXIX">CHAPTER XXIX<br /> - -<span class="smaller">THE CARBONIFEROUS VOLCANOES OF ENGLAND</span></h2> -</div> - -<div class="blockquot"> - -<p>The North of England: Dykes, The Great Whin Sill—The Derbyshire Toadstones—The -Isle of Man—East Somerset—Devonshire</p> - -</div> - -<h3>1. THE NORTH OF ENGLAND</h3> - -<p>The volcanic intercalations which diversify the Lower Carboniferous formations -of Southern Scotland extend but a short way across the English -Border, and although, over the moors and hills of the north of Cumberland -and Northumberland, the Carboniferous sandstones, limestones and shales -are well exposed, they present no continuation of either the plateau or puy-eruptions -which play so prominent a part in the geology of Roxburghshire -and Dumfriesshire. This deficiency is all the more noticeable seeing that -the Carboniferous system is exposed down to its very base, in the deep dales -of the North of England. Had any truly interstratified volcanic material -existed in the system there, it could hardly fail to have been detected.</p> - -<p>But while contemporaneous volcanic rocks are absent, the northern -English counties contain many intrusive masses of dolerite, diabase, andesite -or other eruptive rocks, which may be found traversing all the subdivisions -of the Carboniferous system. These eruptive materials have taken two -forms: in some cases they rise as Dykes, in others they appear as Sills.</p> - -<p><span class="smcap">Dykes.</span>—With regard to the dykes, some are probably much later than -the Carboniferous period, and consequently will be more appropriately considered -in Chapters xxxiv. and xxxv. The great Cleveland dyke, for example, -which runs across the Carboniferous, Permian, Triassic and Jurassic formations, -is probably referable to the Older Tertiary volcanic period. One dyke known -as the Hett Dyke, has been plausibly claimed as possibly of Carboniferous -age. It runs in a W.S.W. direction from the Magnesian Limestone escarpment -at Quarrington Hill, a few miles to the east of Durham, through -the great Coal-field, across the Millstone Grit and Carboniferous Limestone, -disappearing near Middleton in Teesdale. Its total length is thus about -23 miles. It varies in breadth from about 6 to about 15 feet, and appears -to increase in dimensions as it goes westward.<a id="FNanchor_1" href="#Footnote_1" class="fnanchor">[1]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_1" href="#FNanchor_1" class="label">[1]</a> Sedgwick, <i>Trans. Geol. Soc.</i> 2nd series, iii. part 1 (1826-28), p. 63; <i>Trans. Cambridge Phil. Soc.</i> -ii. (1822), p. 21. Sir J. Lowthian Bell, <i>Proc. Roy. Soc.</i> xxiii. (1875), p. 543.</p> - -</div> - -<p><span class="pagenum" id="Page_2">- 2 -</span></p> - -<p>The age of this dyke cannot at present be satisfactorily fixed. It must -be later than the Coal-measures through which it rises. Sedgwick long ago -pointed out that though it reaches the escarpment of the Magnesian Limestone, -it does not cut it; yet it is found in coal-mining to traverse the -Coal-measures underlying the Limestone. He was accordingly inclined -to believe it to be of older date than the Magnesian Limestone. At its -western extremity it approaches close to the Great Whin Sill of Teesdale, -though no absolute connection between the two has been established. Mr. -Teall, however, has called attention to the similarity between the microscopic -structure of the rock forming the Hett Dyke and that of the mass of -the Whin Sill, and he is strongly inclined to regard them as belonging to -the same period of intrusion.<a id="FNanchor_2" href="#Footnote_2" class="fnanchor">[2]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_2" href="#FNanchor_2" class="label">[2]</a> <i>Quart. Journ. Geol. Soc.</i> xl. (1884), p. 230.</p> - -</div> - -<p>It is especially worthy of remark that in the course of its nearly -rectilinear course across the Durham Coal-field, the Hett Dyke, where it -crosses the Wear, is flanked on the north at a distance of a little more than -two miles by a second parallel dyke of nearly identical composition. -Between the two dykes, during mining operations, a sill about 20 feet thick -has been met with, lying between two well-known coal-seams at a depth of -about 60 fathoms from the surface, and extending over an area of at least -15 acres.<a id="FNanchor_3" href="#Footnote_3" class="fnanchor">[3]</a> Microscopic examination of this sill by Mr. Teall proved that -the rock presents the closest resemblance to that of the Hett Dyke.<a id="FNanchor_4" href="#Footnote_4" class="fnanchor">[4]</a> In -this case, it may be regarded as probable that the two dykes and the intermediate -sill form one related series of intrusions, and the conjecture that the -Hett Dyke may be connected with the Whin Sill thus receives corroboration. -The age of the Whin Sill itself will be discussed a few pages further on.</p> - -<div class="footnote"> - -<p><a id="Footnote_3" href="#FNanchor_3" class="label">[3]</a> Sir Lowthian Bell, <i>Proc. Roy. Soc.</i> xxiii. (1875), p. 544.</p> - -<p><a id="Footnote_4" href="#FNanchor_4" class="label">[4]</a> <i>Quart. Journ. Geol. Soc.</i> xl. (1884), p. 230.</p> - -</div> - -<p>Of the other dykes which may possibly be coeval with the Hett Dyke -we may specially note those which follow the same W.S.W. trend, for that -strike differs from the general W.N.W. direction of most of the dykes. -Two conspicuous examples of the south-westerly trend may be seen, one -near Morpeth, the other north of Bellingham. The former dyke, as regards -microscopic structure, is more nearly related to the majority of the series -in the North of England. But that north of Bellingham (High Green) -presents affinities both in structure and composition with the Hett Dyke,<a id="FNanchor_5" href="#Footnote_5" class="fnanchor">[5]</a> -and may perhaps belong to the same period of intrusion.</p> - -<div class="footnote"> - -<p><a id="Footnote_5" href="#FNanchor_5" class="label">[5]</a> Mr. Teall, <i>op. cit.</i> p. 244. <i>Quart. Journ. Geol. Soc.</i> xxxix. (1884), p. 656, and <i>Proc. Geol. -Assoc.</i> (1886). See also Prof. Lebour, <i>Geology of Northumberland and Durham</i>, chap. xi.</p> - -</div> - -<p><span class="smcap">The Great Whin Sill.</span>—The geologist who, after making himself acquainted -with the abundant sills among the Carboniferous rocks in the -centre of Scotland, finds his way into Northumberland, meets there with -geological features that have become familiar to him further north. The -sea-cliffs of Bamborough and Dunstanborough, the rocky islets of Farne, the -long lines of brown crag and green slope that strike inland through the -Kyloe Hills and wind across the cultivated lowlands and the moorlands -beyond, remind him at every turn of the scenery in the basin of the Forth. -<span class="pagenum" id="Page_3">- 3 -</span> -But not until he has traced these ridges for many miles southwards and -found their component rocks to form there an almost continuous sheet does -he realize that nothing of the kind among the Scottish Carboniferous rocks -can be compared for extent to this display in the North of England.<a id="FNanchor_6" href="#Footnote_6" class="fnanchor">[6]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_6" href="#FNanchor_6" class="label">[6]</a> The Whin Sill has been the subject of much discussion, and a good deal of geological literature -has been devoted to its consideration. The writings of Trevelyan, Sedgwick, W. Hutton, Phillips -and Tate are especially deserving of recognition. The intrusive character of the Sill, maintained -by some of these writers, was finally established by the mapping of the Geological Survey, and was -discussed and illustrated by Messrs. W. Topley and G. A. Lebour in a paper in the 33rd volume -of the <i>Quart. Journ. Geol. Soc.</i> (1877), in which references to the earlier observers will be found. -See also Prof. Lebour's <i>Outlines of the Geology of Northumberland</i>, 2nd edit. (1886), p. 92. The -petrography of the Whin Sill is fully treated by Mr. Teall in <i>Quart. Journ. Geol. Soc.</i> xl. (1884), -p. 640, where a bibliography of the subject is also given.</p> - -</div> - -<p>From the furthest skerries of the Farne Islands southwards to Burton -Fell on the great Pennine escarpment, a distance in a straight line of about -80 miles, this intrusive sheet may be traced in the Carboniferous Limestone -series (Map I.). There are intervals where its continuity cannot be actually -followed at the surface, but that it really runs unbroken from one end to the -other underground cannot be doubted by any one who has examined the region. -This singular feature in the geology and scenery of the North of England is -known locally as the Great Whin Sill.<a id="FNanchor_7" href="#Footnote_7" class="fnanchor">[7]</a> From the rocky islets and castle-crowned -crags of the coast-line it maintains its characteristic topography, -structure and composition throughout its long course in the interior. So -regularly parallel with the sedimentary strata does it appear to lie, that it -was formerly regarded by many observers as a true lava-sheet, poured out -upon the sea-floor over which the limestones and shales were laid down. -But its really intrusive character has now been clearly demonstrated. Not -a vestige of any tuff has been detected associated with it, nor does it ever -present the usual characters of a true lava-stream.<a id="FNanchor_8" href="#Footnote_8" class="fnanchor">[8]</a> Its internal structure -and the wonderful uniformity in its character mark it out as a typical -intrusive sheet.</p> - -<div class="footnote"> - -<p><a id="Footnote_7" href="#FNanchor_7" class="label">[7]</a> "Whin" is a common term in Scotland and the North of England for any hard kind of -stone, especially such as can be used for making and mending roads. "Sill" denotes a flat course -or bed of stone, and was evidently applied to this intrusive sheet from its persistent flat-bedded -position and its prominence among the other gently inclined strata among which it lies. It is from -this example in the North of England that the word "sill" has passed into geological literature.</p> - -<p><a id="Footnote_8" href="#FNanchor_8" class="label">[8]</a> On the coast at Bamborough and the Harkess Rocks the usual petrographical characters of -the Whin Sill are exchanged for those of fine-grained amygdaloidal diabases arranged in distinct -sheets, which in their upper parts are highly vesicular and show ropy surfaces—peculiarities -suggestive of true lava-streams. But according to Professor Lebour the rocks are intrusive into -limestone and shale (<i>Geology of Northumberland and Durham</i>, p. 98). Mr. Teall has expressed -the suspicion that these rocks must have consolidated under conditions somewhat different from -those which characterized the normal Whin Sill (<i>Quart. Journ. Geol. Soc.</i> xl. p. 643). They -seem to be the only parts of the sill which present features that might possibly indicate superficial -outflow.</p> - -</div> - -<p>Among the manifestations of the subterranean intrusion of igneous rocks -in the British Isles the Great Whin Sill, next after the Dalradian sills of -Scotland, is the most extensive. Its striking continuity for so great -a distance, and the absence around it of any other trace of igneous action, -save a few dykes, place it in marked contrast to the ordinary type of -Carboniferous sills. The occasional gaps on its line of outcrop in the -<span class="pagenum" id="Page_4">- 4 -</span> -northern part of its course do not really affect our impression of the -persistence of the sheet. They not improbably indicate merely that in -its protrusion it had a wavy irregular limit, which in the progress of -denudation has occasionally been not yet reached. For mile after mile -the sill has been mapped by the Geological Survey in lines of crag across -the moorlands, and as a conspicuous band among the limestones and shales -that form the steep front of the Pennine escarpment, where it has long been -known in the fine sections exposed among the gullies by which that noble -rock-face has been furrowed.</p> - -<div class="figcenter" id="v2fig176" style="width: 508px;"> - <img src="images/v2fig176.png" width="508" height="77" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 176.</span>—Section from the great Limestone escarpment on the west to the Millstone Grit hills -east of Teesdale.<br /> - -1. Silurian strata; 2. Carboniferous Limestone series; 3. The Great Whin Sill, which gradually rises to higher -stratigraphical position as it goes westward; 4. Millstone Grit.</div> -</div> - -<p>Along its main outcrop, the sill dips gently eastwards below the portion -of the Carboniferous Limestone series which overlies it. But so slight are -the inclinations, so gentle the undulations of the rocks in this part of the -country, that far to the east of that outcrop the sill has been laid bare by -the streams which in the larger dales have cut their way through the overlying -cake of Carboniferous strata down to the Silurian platform on which -they rest (<a href="#v2fig176">Fig. 176</a>). Among these inland revelations of the eastward continuation -of the sill under Carboniferous Limestone strata, the most striking -and best known are those which have been made by the River Tees, and of -which the famous waterfalls of the High Force and Cauldron Snout are the -most picturesque features. The distance of the remotest of these denuded -outcrops or "inliers" from the main escarpment is not less than 20 -miles.</p> - -<p>It is not possible to form an accurate estimate of the total underground -area of the Whin Sill. In the southern half of the district, south of the -line of the Roman Wall, where, the inclination of the strata being generally -low, the same stratigraphical horizons are exposed by denudation far to the -east of the main outcrops of the rocks, we know that the sill must have a -subterranean extent of more than 400 square miles. Yet this is probably -only a small part of the total area over which the molten material was -injected. In the northern part of the district, the Carboniferous Limestone -series is not exposed over so broad a stretch of country, and denudation has -not there revealed the eastward extension of the sill. But there is no reason -to suppose the sheet to be less continuous and massive there. We must -remember also that the present escarpment has been produced by denudation, -and that the intrusive sheet must have once extended westwards beyond its -present limits at the surface. If, therefore, we were to state broadly that -the Great Whin Sill has been intruded into the Carboniferous Limestone -<span class="pagenum" id="Page_5">- 5 -</span> -series over an area of 1000 square miles we should probably be still below -the truth.</p> - -<p>The rock composing this vast intrusive sheet is a dolerite or diabase, -which maintains throughout its wide extent a remarkable uniformity of -petrographical characters. In this and other respects it illustrates the -typical features of sills. Thus it is coarsest in texture where it is thickest, -and somewhat finer in grain towards its upper and lower surfaces than in -the centre. Among the coarser varieties the component crystals of augite -are not infrequently an inch in length and occur in irregular patches.<a id="FNanchor_9" href="#Footnote_9" class="fnanchor">[9]</a> -Occasional amygdaloidal portions are observable, but these are not more -marked than those to be found in the "whin-dykes" of the same region.<a id="FNanchor_10" href="#Footnote_10" class="fnanchor">[10]</a> -The amygdaloidal and vesicular fine-grained rocks of the Bamborough district -may possibly be quite distinct from the main body of the Whin Sill.</p> - -<div class="footnote"> - -<p><a id="Footnote_9" href="#FNanchor_9" class="label">[9]</a> Sedgwick, <i>Cambridge Phil. Trans.</i> ii. p. 166. Mr. Teall, <i>Quart. Journ. Geol. Soc.</i> xl. -p. 643.</p> - -<p><a id="Footnote_10" href="#FNanchor_10" class="label">[10]</a> Messrs. Topley and Lebour, <i>Quart. Journ. Geol. Soc.</i> xxxiii. p. 418.</p> - -</div> - -<p>Under the microscope the rock is seen to consist essentially of the -usual minerals—plagioclase, augite and titaniferous magnetic iron-ore. An -ophitic intergrowth of the augite and felspar is observable, likewise a certain -quantity of micropegmatite which plays the part of groundmass between -the interstices of the lath-shaped felspars. Full details of the characteristics -of the component minerals and their arrangement are given by Mr. Teall in -the paper already cited.</p> - -<p>The main body of the sill is a sheet which sometimes diminishes to less -than 20 feet in thickness and sometimes expands to 150 feet, but averages -from 80 to 100 feet. It occasionally divides, as near Great Bavington, -where it appears at the surface in two distinct beds separated by an -intervening group of limestones and shales. Occasionally, as at Elf's -Hill Quarry, it gives out branches which send strings into the adjacent -limestone.<a id="FNanchor_11" href="#Footnote_11" class="fnanchor">[11]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_11" href="#FNanchor_11" class="label">[11]</a> Messrs. Topley and Lebour, <i>op. cit.</i> <a href="#Page_413">p. 413</a>.</p> - -</div> - -<p>Although in most natural sections it seems to lie quite parallel with -the strata above and below, yet a number of examples of its actual intrusion -have been observed. When traced across the country, it is found not to -remain on a definite horizon, but to pass transgressively across considerable -thicknesses of strata. Its variations in this respect are well shown in the -accompanying table of comparative sections constructed by Messrs. Topley -and Lebour.<a id="FNanchor_12" href="#Footnote_12" class="fnanchor">[12]</a> It will be seen that while at Harlow Hill the sill is found -overlying the Great Limestone of Alston Moor, at Rugley, five miles off it -lies about 1000 feet lower down, far below the position of the Tyne-bottom -Limestone. Still farther north, however, the sill west of Holy Island is -said to lie 800 feet above the Great Limestone and to come among the -higher beds of the Carboniferous Limestone series.<a id="FNanchor_13" href="#Footnote_13" class="fnanchor">[13]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_12" href="#FNanchor_12" class="label">[12]</a> <i>Op. cit.</i> plate xviii.</p> - -<p><a id="Footnote_13" href="#FNanchor_13" class="label">[13]</a> <i>Op. cit.</i> <a href="#Page_414">p. 414</a>.</p> - -</div> - -<p>The Whin Sill appears generally to thicken in an easterly or north-easterly -direction. There are further indications that it was intruded from -east to west. Thus, at Shepherd's Gap, on the Great Roman Wall, the -<span class="pagenum" id="Page_6">- 6 -</span> -dolerite, coming evidently from an easterly quarter, has broken up and -thrust itself beneath a bed of limestone. Again, when the sill bifurcates -the branches unite towards the east or north-east.<a id="FNanchor_14" href="#Footnote_14" class="fnanchor">[14]</a> The sill can be proved -to thin away to the west from Teesdale to the Pennine escarpment, and in -Weardale the "Little Whin Sill" diminishes from 20 feet, till in three -miles it disappears.<a id="FNanchor_15" href="#Footnote_15" class="fnanchor">[15]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_14" href="#FNanchor_14" class="label">[14]</a> <i>Op. cit.</i> <a href="#Page_415">p. 415</a>.</p> - -<p><a id="Footnote_15" href="#FNanchor_15" class="label">[15]</a> <i>Op. cit.</i> <a href="#Page_419">p. 419</a>.</p> - -</div> - -<div class="figcenter" id="v2fig177" style="width: 658px;"> - <img src="images/v2fig177.png" width="658" height="401" alt="" /> - <div class="figcaption"> - -<div class="tdr smaller"> -<i>Walker & Bontall sc.</i><br /> -</div> - -<span class="smcap">Fig. 177.</span>—Sections of the Carboniferous Limestone series of Northumberland showing the variations in the position of the Whin Sill. -By Messrs. Topley and Lebour.</div> -</div> - -<p><span class="pagenum" id="Page_7">- 7 -</span></p> - -<p>The strata in contact with the Whin Sill, both above and below, have -been more or less altered. Sandstones have been least affected; shales -have suffered most, passing into a kind of porcellanite, with development of -garnet and other minerals.<a id="FNanchor_16" href="#Footnote_16" class="fnanchor">[16]</a> Limestone often shows only slight traces of -change, though here and there it has become crystalline.</p> - -<div class="footnote"> - -<p><a id="Footnote_16" href="#FNanchor_16" class="label">[16]</a> Mr. Teall, <i>op. cit.</i> xxxix. (1884), p. 642, and authors cited by him.</p> - -</div> - -<p>No trace of any boss or neck has been detected in the whole region -which might be supposed to mark a funnel of ascent for the material of -the Whin Sill. The Hett Dyke and the High Green Dyke, already noticed, -may, however, have been possibly connected with the injection of this great -intrusive sheet. No other visible mass of igneous rock in the region has -been even plausibly conjectured to indicate a point or line of emission for -the sill.</p> - -<p>It is certainly singular that in so wide a territory, where the whole -succession of strata has been so admirably laid bare by denudation in -thousands of natural sections, and where, moreover, much additional information -has been obtained from lead-mining as to the nature of the rocks below -ground, not a single vestige of tuff, agglomerate or interstratified lava has -been up to the present time recorded, unless the Harkess rocks already -alluded to can be so regarded.</p> - -<p>Judging, however, from the analogy of the other districts of igneous -rocks in Britain, we can hardly resist the conclusion that the Great Whin -Sill is essentially a manifestation of volcanic action, that it was connected -with the uprise of basic lava in volcanic orifices, and that the subterranean -energy may quite probably have succeeded in reaching the surface and ejecting -there both lavas and tuffs.</p> - -<p>It appears to be certain that any vents which existed cannot have lain -to the west of the present escarpment of the sill, for no trace of them can -be found there piercing the Carboniferous or older formations. They -must have lain somewhere to the east in the area now overspread with -Millstone Grit and Coal-measures, or still farther east in the tract now -concealed under the North Sea. The evidence of the sill itself, as we have -seen, corroborates this view of the probable situation of the centre of -disturbance.</p> - -<p>The question of the geological age of the sill is one of considerable -difficulty, to which no confident answer can be given.<a id="FNanchor_17" href="#Footnote_17" class="fnanchor">[17]</a> The injection of the -diabase must obviously be considerably later than the highest strata through -which it has risen; that is, it must be younger than some of the higher -members of the Carboniferous Limestone series. But here our positive -evidence fails.</p> - -<div class="footnote"> - -<p><a id="Footnote_17" href="#FNanchor_17" class="label">[17]</a> See Messrs. Topley and Lebour, <i>op. cit.</i> <a href="#Page_418">p. 418</a>.</p> - -</div> - -<p>The Sill is traversed by the same faults which disrupt the surrounding -Carboniferous rocks. It is therefore of older date than these -dislocations. Its striking general parallelism with the shales and limestones -<span class="pagenum" id="Page_8">- 8 -</span> -probably proves that it was intruded before the rocks were much -disturbed from their original horizontal position. But the manner in which -the intrusive rock has been thrust into and has involved the shales and limestones -seems to indicate that these strata had already become consolidated -and lay under the pressure of a great thickness of superincumbent -Carboniferous strata.</p> - -<p>In the absence of all certainty on the subject it seems most natural to -place the Whin Sill provisionally among the Carboniferous volcanic series -with which petrographically and structurally it has so much in common. -In Scotland the puy-eruptions continued till the time of the Coal-measures. -If, before the close of the Carboniferous period, volcanic vents were opened -somewhere to the east of the coal-fields of Northumberland and Durham, -they might be accompanied with basic sills injected into the Carboniferous -Limestone series, which was then lying still approximately horizontal under -a thickness of from 3500 to 5000 feet of Carboniferous sedimentary deposits. -These still undiscovered volcanoes seem to have been endowed with even more -energy than those of Central and Southern Scotland, at least nowhere else -among the Carboniferous records of Britain is there such a colossal manifestation -of subterranean intrusion as the Great Whin Sill.</p> - - -<h3>2. THE DERBYSHIRE TOADSTONES</h3> - -<p>In the absence of any certain evidence that the Whin Sill belongs to -the Carboniferous period, we must advance southward into the very heart -of England before any clear vestiges can be found of contemporaneous -volcanic eruptions among the members of the Carboniferous system. After -quitting the lavas and tuffs of Roxburghshire and their brief continuations -across the English border, we do not again meet with any truly bedded -volcanic rocks in that system until we reach the middle of Derbyshire. In -this picturesque district, famous for its lead-mines and its mineral waters, -a feebly developed but interesting group of intercalated lavas, locally -called "toadstones," has long been known. There is thus a space of some -150 miles across which, though the formations are there so fully developed -and so abundantly trenched by valleys from the top to the bottom of the -system, no volcanic vents nor any trace of Carboniferous volcanic ejections -has yet been found. On the other hand, after the district of the "toadstones" -is passed, the Carboniferous rocks are again destitute of any volcanic intercalations -across the centre and south-west of England and over Wales, until -after a space of about 150 miles they reappear in Somerset.</p> - -<p>The volcanic group of Derbyshire thus stands out entirely isolated. -Lying in the Carboniferous Limestone, where that formation is typically -developed, it presents an admirable example of a thoroughly marine phase -of volcanic action (Map I.).</p> - -<p>One of the most prominent features in the geology of the centre of -England is the broad anticlinal fold which brings up the lower portion of -the Carboniferous system to form the long ridge of the Pennine chain that runs -<span class="pagenum" id="Page_9">- 9 -</span> -from Yorkshire to the Midland plain, and separates the eastern from the -western coal-fields. This fold widens southwards until not only the Millstone -Grit and Yoredale rocks, but the underlying Mountain Limestone is laid bare. -A broad limestone district is thus exposed in the very heart of the country, -ranging as a green fertile undulating tableland, deeply cut by winding -valleys, which expose admirable sections of the strata, but nowhere reach -the base of the system. The total visible depth of the limestone series is -computed to be about 1500 feet; the Yoredale shales and limestones may -be 500 feet more; so that the calcareous formations in which the volcanic -phenomena are exhibited reach a thickness of at least 2000 feet.</p> - -<p>It is not yet definitely known through what vertical extent of this -thickness of sedimentary material the volcanic platforms extend, but where -most fully developed they perhaps range through 1000 feet, lying chiefly in -the Carboniferous Limestone, but apparently in at least one locality extending -up into the lower division of the Yoredale group. The area within which -they can be studied corresponds nearly with that in which the limestone -forms the surface of the country, or a district measuring about 20 miles -from north to south, with an extreme breadth of 10 miles in an east and -west direction.</p> - -<p>A special historical interest belongs to the Derbyshire "toadstones."<a id="FNanchor_18" href="#Footnote_18" class="fnanchor">[18]</a> -They furnished Whitehurst with material for his speculations, and were -believed by him to be as truly igneous rocks as the lava which flows from -Hecla, Vesuvius or Etna. But he thought that they had been introduced -among the strata and "did not overflow the surface of the earth, according -to the usual operations of volcanoes."<a id="FNanchor_19" href="#Footnote_19" class="fnanchor">[19]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_18" href="#FNanchor_18" class="label">[18]</a> This word has by some writers been supposed to be corrupted from <i>tod-stein</i>, dead-stone, in -allusion to the dying out of the lead veins there; by others the name has been thought to -be derived from the peculiar green speckled aspect of much of the rock, resembling the back -of a toad.</p> - -<p><a id="Footnote_19" href="#FNanchor_19" class="label">[19]</a> <i>An Enquiry into the Original State and Formation of the Earth</i>, 1778, Appendix, -pp. 149, <i>et seq.</i></p> - -</div> - -<p>His views were published as far back as 1778, three years after -Hutton read the first outline of his theory of the earth and made known -his observations regarding the igneous origin of whinstones.<a id="FNanchor_20" href="#Footnote_20" class="fnanchor">[20]</a> The first -detailed account of the Derbyshire eruptive rocks was that given by -Fairey,<a id="FNanchor_21" href="#Footnote_21" class="fnanchor">[21]</a> which has served as the basis of all subsequent descriptions. -Conybeare, in particular, prepared a succinct narrative from Fairey's -more diffuse statements, and thus placed clearly before geologists the -nature and distribution of these volcanic intercalations.<a id="FNanchor_22" href="#Footnote_22" class="fnanchor">[22]</a> Subsequently -the district was mapped by De la Beche and the officers of the -Geological Survey, and the areas occupied by the several outcrops of -igneous rock could then be readily seen.<a id="FNanchor_23" href="#Footnote_23" class="fnanchor">[23]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_20" href="#FNanchor_20" class="label">[20]</a> <i>Trans. Roy. Soc. Edin.</i> i. p. 275, <i>et seq.</i> Hutton specially mentions the toadstone of Derbyshire -as one of the rocks produced by fusion, p. 277.</p> - -<p><a id="Footnote_21" href="#FNanchor_21" class="label">[21]</a> <i>General View of the Agriculture and Minerals of Derbyshire</i> (1811).</p> - -<p><a id="Footnote_22" href="#FNanchor_22" class="label">[22]</a> <i>Outlines of the Geology of England and Wales</i> (1822), p. 448.</p> - -<p><a id="Footnote_23" href="#FNanchor_23" class="label">[23]</a> See Sheets 71 N.W., 72 N.E., 81 N.E. and S.E. and 82 S.W. of the Geological Survey of -England and Wales.</p> - -</div> - -<p><span class="pagenum" id="Page_10">- 10 -</span></p> - -<p>Though the "toadstones" were believed to form definite platforms -among the limestone strata, and thus to be capable of being used as -reliable horizons in the mineral fields of Derbyshire, they appear to -have been generally regarded as intrusive sheets like the Whin Sill of -the north. Thus De la Beche in his <i>Manual of Geology</i>, giving a -summary of what was known at the time regarding intercalated igneous -rocks, remarks with regard to the Derbyshire toadstones that they may -from all analogy be considered to have been injected among the limestones -which would be easily separated by the force of the intruded igneous -material.<a id="FNanchor_24" href="#Footnote_24" class="fnanchor">[24]</a> But the same observer, after his experience among the ancient -volcanic rocks of Devonshire, came fully to recognize the proofs of contemporaneous -outflow among the Derbyshire toadstones. In his subsequently -published <i>Geological Observer</i>, he described the toadstones as submarine -lavas that had been poured out over the floor of the sea in which the -Carboniferous Limestone was deposited, and had been afterwards covered up -under fresh deposits of limestone.<a id="FNanchor_25" href="#Footnote_25" class="fnanchor">[25]</a> It is remarkable, however, that he -specially comments on the absence, as he believed, of any contemporaneously -ejected ashes and lapilli, such as occur in Devonshire. That true tuffs or -volcanic ashes are associated with the toadstones was noticed by Jukes -in 1861,<a id="FNanchor_26" href="#Footnote_26" class="fnanchor">[26]</a> and afterwards by the Geological Survey.<a id="FNanchor_27" href="#Footnote_27" class="fnanchor">[27]</a> Since that time -geologists have generally recognized these Derbyshire igneous rocks as truly -contemporaneous intercalations. But very little has recently been written -on the structure of the district, our information regarding it being still -based mainly on the early observations of Fairey and the mapping of the -Geological Survey.</p> - -<div class="footnote"> - -<p><a id="Footnote_24" href="#FNanchor_24" class="label">[24]</a> <i>Manual</i>, 3rd edit. 1833, p. 462.</p> - -<p><a id="Footnote_25" href="#FNanchor_25" class="label">[25]</a> <i>Geological Observer</i> (1851), pp. 642-645.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_26" href="#FNanchor_26" class="label">[26]</a> <i>Student's Manual of Geology</i>, 2nd edit. (1863), p. 523. For a general <i>résumé</i> of the proofs of -contemporaneity furnished by the toadstones, see "The Geology of North Derbyshire," by Messrs. -A. H. Green and A. Strahan (<i>Memoirs of the Geological Survey</i>, 2nd edit. (1887), p. 123).</p> - -<p><a id="Footnote_27" href="#FNanchor_27" class="label">[27]</a> In the first edition of the <i>Memoir on the Geology of North Derbyshire</i>, published in 1859, the -authors of which were Messrs. A. H. Green, C. le Neve Foster and J. R. Dakyns.</p> - -</div> - -<p>The subject, however, has now been resumed by Mr. H. Arnold Bemrose, -who in 1894, after a prolonged study of the petrography of the rocks, -communicated the results of his researches to the Geological Society.<a id="FNanchor_28" href="#Footnote_28" class="fnanchor">[28]</a> In -his excellent paper, to which I shall immediately make fuller reference, he -mentions the localities at which lava-form and fragmental rocks may be -observed, but does not enter on the discussion of the geological structure of -the region or of the history of the volcanic eruptions. Before the announcement -of his paper, hearing that I proposed to make for the first time a rapid -traverse of the toadstone district, for the purpose of acquainting myself with -the rocks on the ground, he kindly offered to conduct me over it. My chief -object, besides that of seeing the general nature of the volcanic phenomena -of the region, was to examine more particularly the areas of the volcanic -fragmental rocks, with the view of discovering whether among them some -remains might not be found of the actual vents of discharge. In this search -I was entirely successful. Aided by Mr. Bemrose's intimate knowledge of -<span class="pagenum" id="Page_11">- 11 -</span> -the ground, I was enabled to visit in rapid succession those tracts which -seemed most likely to furnish the required evidence, and in a few days was -fortunate enough to obtain proofs of six or seven distinct vents, ranging -from the extreme northern to the furthest southern boundary of the volcanic -district. Mr. Bemrose has undertaken to continue the investigation, and will, -I trust, work out the detailed stratigraphy of the Carboniferous Limestone so -as eventually to furnish an exhaustive narrative of the whole volcanic history -of Derbyshire. Meanwhile no adequate account of the area can be given. -But I will here state all the essential facts which up to the present time -have been ascertained.</p> - -<div class="footnote"> - -<p><a id="Footnote_28" href="#FNanchor_28" class="label">[28]</a> <i>Quart. Journ. Geol. Soc.</i> vol. l. (1894), p. 603.</p> - -</div> - -<p>1. <span class="allsmcap">THE ROCKS ERUPTED.</span>—Mr. Allport has described the microscopic -character of some of the toadstones,<a id="FNanchor_29" href="#Footnote_29" class="fnanchor">[29]</a> and further details have been -supplied by Mr. Teall.<a id="FNanchor_30" href="#Footnote_30" class="fnanchor">[30]</a> The fullest account of the subject, however, is -that given by Mr. Bemrose in the paper above referred to. This observer -distinguishes the lava-form from the fragmental rocks, and gives the -minute characters of each series. He does not, however, separate true -interstratified lavas from injected sills, nor the bedded tuffs from the coarse -agglomerates which fill up the vents. These distinctions are obviously -required in order that the true nature and sequence of the materials in the -volcanic eruptions may be traced, and that the phenomena exhibited in -Derbyshire may be brought into comparison with those found in other -Carboniferous districts. But to establish them satisfactorily the whole -region must be carefully re-examined and even to some extent re-mapped.</p> - -<div class="footnote"> - -<p><a id="Footnote_29" href="#FNanchor_29" class="label">[29]</a> <i>Quart. Journ. Geol. Soc.</i> xxx. (1874), p. 529.</p> - -<p><a id="Footnote_30" href="#FNanchor_30" class="label">[30]</a> <i>British Petrography</i>, p. 209.</p> - -</div> - -<p>The lavas (including, in the meantime, sheets which there can be little -doubt are sills) show three main types of minute structure and composition, -which are discriminated by Mr. Bemrose as—(<i>a</i>) Olivine-dolerites; these, -the most abundant of the series, consist of augite in grains, olivine in -idiomorphic crystals, plagioclase giving lath-shaped and tabular sections, -and magnetite or ilmenite in rods and grains; (<i>b</i>) Ophitic olivine-dolerites, -consisting of augite in ophitic plates forming the groundmass, in which are -imbedded idiomorphic olivine, plagioclase (often giving large lath-shaped -sections and magnetite or ilmenite); (<i>c</i>) Olivine-basalts; these rocks are distinguished -by containing crystals of augite and olivine in a groundmass of -small felspar-laths, granular augite and magnetite or ilmenite, with very -little interstitial matter. They have been noticed only in two of the outcrops -of toadstone.</p> - -<p>The fragmental rocks have been shown by Mr. Bemrose to cover a -much more extensive space than had been previously supposed. He has -found them to be distinguished by an abundance of lapilli varying from -minute fragments up to pieces about the size of a pea, and composed of a -material that differs in structure from the dolerites and basalts with which -the tuffs are associated. These lapilli consist largely of a glassy base more -or less altered, which is generally finely vesicular and encloses abundant -skeleton crystals and crystallites. The tuffs thus very closely resemble -<span class="pagenum" id="Page_12">- 12 -</span> -some of the Carboniferous basic tuffs of Fife, already referred to (<a href="../../66492/66492-h/66492-h.htm#Page_422">vol. i. p. 422</a>), -and like these they include abundant blocks of dolerite and basalt.</p> - -<p>2. <span class="allsmcap">GEOLOGICAL STRUCTURE OF THE TOADSTONE DISTRICT.</span>—As the -volcanic rocks of Derbyshire lie among the Carboniferous Limestones of a -broad anticlinal dome, they are only exposed where these limestones have -been sufficiently denuded, and as the base of the limestones is nowhere laid -bare, the lowest parts of the volcanic series may be concealed. Over the -tract where the toadstones can be examined they appear as bands -regularly intercalated with the limestones, but varying in thickness in the -course of their outcrops. As they are prone to decay, they usually form -smooth grassy slopes between the limestone scarps, though isolated blocks -of the dull brown igneous rocks may often be seen protruding from the -surface. Now and then a harder bed of toadstone caps a hill, and thus -forms a prominent feature in the landscape, but as a rule these igneous -bands play no distinguishing part in the scenery, and are indeed less -conspicuous than the white escarpments of limestone which overlie them.</p> - -<p>It was the opinion of the older geologists that three distinct platforms -of toadstone extend without break throughout the district, and subdivide the -limestones into four portions. But this opinion does not seem to have been -based on good evidence either of sequence or of continuity. Various facts -were brought forward by the officers of the Geological Survey to show that -the supposed persistence of the three platforms of toadstone did not really -exist, but that these sheets of igneous material are found at different spots -on very different horizons, and are of limited horizontal range.<a id="FNanchor_31" href="#Footnote_31" class="fnanchor">[31]</a> So far as -my own limited observations go, they entirely corroborate this view. There -can be little doubt, I think, that the identity of certain outcrops of toadstone -has been assumed, and the assumption has been carried throughout -the district. The truth is that the number of successive platforms on -which igneous materials appear will never be satisfactorily determined until -the stratigraphy of the Derbyshire Carboniferous Limestone is worked out -in detail. When the successive members of this great calcareous formation -have been identified by lithological and palæontological characters over the -district, it will be easy to allocate each outcrop of toadstone to its true -geological horizon. When this labour has been completed, it will probably -be found that instead of three, there have been many discharges of volcanic -material during the deposition of the limestone series; that these have -proceeded from numerous small vents, and that they are all of comparatively -restricted horizontal extent. Such a detailed examination will -also determine how far the toadstones include veritable sills, and on what -horizons these intrusive sheets have been injected.</p> - -<div class="footnote"> - -<p><a id="Footnote_31" href="#FNanchor_31" class="label">[31]</a> <i>Geol. Surv. Mem. on North Derbyshire</i>, by Messrs. Green and Strahan (1887), p. 104.</p> - -</div> - -<p>In the meantime, we know that the lowest visible bands of toadstone -are underlain by several hundred feet of limestone, thus proving that the -earliest known volcanic explosions took place over the floor of the -Carboniferous Limestone sea, after at least 700 or 800 feet of calcareous -sediment had accumulated there. The latest traces of volcanic activity are -<span class="pagenum" id="Page_13">- 13 -</span> -found in a part of the Yoredale group of shales and limestones which form -the uppermost member of the Carboniferous Limestone of this region. But -it is not quite clear whether the vesicular diabase found there is interstratified -or intrusive. Certainly no contemporaneous tuffs have yet been -found among the Yoredale rocks, nor in any higher subdivision of the -Carboniferous system, though coarse agglomerates marking the position of -vents do traverse the Yoredale group at Kniveton.</p> - -<p>It may be remarked that in the district over which the toadstones can -be seen, two areas are recognizable, in each of which the exposures of the -igneous rocks are numerous, while between them lies an intervening tract -wherein there is hardly any visible outcrop of these rocks. The northern -and much the more extensive area stretches from Castleton to Sheldon, -while the southern spreads from Winster to Kniveton. This distribution -not improbably points to the original position of the vents, and indicates a -northern more numerous group of volcanic orifices, and a southern tract -where the vents were fewer, or at least spread their discharges over a more -limited space.</p> - -<p>3. <span class="allsmcap">THE VENTS.</span>—It had always appeared to me singular that, in ground -so deeply trenched by valleys as the toadstone district of Derbyshire, no -trace had been recognized of any bosses or necks from which these volcanic -sheets might have been erupted. It is true that in mining operations -masses of toadstone had been penetrated to a considerable depth without -their bottom being reached, and the suggestion had been made that in such -cases a shaft may actually have been sunk on one of the vents through -which the toadstone came up.<a id="FNanchor_32" href="#Footnote_32" class="fnanchor">[32]</a> One instance in particular was cited where, -at Black Hillock, on Tideswell Moor, close to Peak Forest Village, a mass -of toadstone was not cut through, though pierced to a depth of 100 -fathoms. In that neighbourhood, however, several of the sheets of eruptive -material are probably sills, and the shaft at Black Hillock may have been -sunk upon the pipe or vein that supplied one or more of these intrusive -sheets.</p> - -<div class="footnote"> - -<p><a id="Footnote_32" href="#FNanchor_32" class="label">[32]</a> <i>Geol. Surv. Mem. on North Derbyshire</i>, p. 134.</p> - -</div> - -<p>It was therefore with no little interest that I detected a series of -vents at four separate localities, viz. Castleton, Grange Mill, Hopton, and -Kniveton Wood. I have no doubt that a more extended search will bring -others to light. Those observed by me are all filled with coarse agglomerate, -the blocks in which are mostly composed of different lavas, sometimes with -the addition of blocks of limestone, while the matrix consists mainly of -lapilli of basic devitrified glass.</p> - -<p>The most typical examples form a group of two, possibly three, vents -which rise into two isolated, smooth, grassy dome-shaped hills at Grange -Mill, five miles west from Matlock Bath.<a id="FNanchor_33" href="#Footnote_33" class="fnanchor">[33]</a> In external form and colour, -these eminences present a contrast to the scarped slopes of limestone around -them. They at once recall the contours of many of the volcanic necks in -Central Scotland. On examination it is found that the material composing -<span class="pagenum" id="Page_14">- 14 -</span> -them is a dull green agglomerate, the matrix of which is a compact substance -weathering spheroidally, and full of small lapilli of minutely vesicular -diabase. The larger stones consist, for the most part, of various vesicular -dolerites or diabases, together with some pieces of limestone and occasionally -large blocks of the latter rock, altered into a saccharoid condition. Two -dykes of dolerite or basalt traverse the margin of the larger vent.</p> - -<div class="footnote"> - -<p><a id="Footnote_33" href="#FNanchor_33" class="label">[33]</a> This is Mr. Bemrose's outcrop, No. 46, <i>op. cit.</i> p. 633.</p> - -</div> - -<p>The steep sides of these agglomerate domes rise from the low ground -around them to a height of 100 to 180 feet, their summits being a little -more than 900 feet above the sea. The smaller neck is nearly circular, -and measures about 1000 feet in diameter. The larger mass is less regular -in shape, and is prolonged into such a bulge on the south-east as to suggest -that its prolongation in that direction may really mark the position of a -third and much smaller vent contiguous to it. The longer diameter of the -larger mass is 2300 and the shorter 1300 feet.</p> - -<div class="figcenter" id="v2fig178" style="width: 507px;"> - <img src="images/v2fig178.png" width="507" height="316" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 178.</span>—View of two volcanic necks in the Carboniferous Limestone series, at Grange Mill, five miles - west of Matlock Bath, from the north.</div> -</div> - -<p>On the south and west sides, the surrounding limestone can be traced up -to within a few feet of the edge of the agglomerate, and its strata are there -found to be much jumbled and broken, while their texture is rather more -crystalline than usual, though not saccharoid. The two necks are separated -by a narrow valley in which no rock is visible. Their opposite declivities -meet at the bottom of this hollow, and are so definitely marked off that, -even in the absence of proof that they are disjoined by intervening limestone, -there can be little hesitation in regarding each hill as marking a -distinct vent. -<span class="pagenum" id="Page_15">- 15 -</span> -A wider valley extends along the eastern base of the necks, and slopes -upward on its east side until it is crowned by a long escarpment of limestone, -which reaches a height of 1000 feet above the sea, or about 100 feet -above the valley from which it rises. Unfortunately, the bottom and slopes -of this depression are thickly covered with soil, but at one or two places -debris of fine tuff may be observed, and at the northern and southern ends -of the hollow well-bedded green and reddish tuff appears, dipping gently -below the limestone escarpment. This band of volcanic detritus evidently -underlies the limestone, and forms most of the gentle slope on the east side -of the valley. It may be from 70 to 100 feet thick. That it was discharged -from one or both of the necks seems tolerably clear. Its material -resembles that forming the matrix of the agglomerate. The general -arrangement of the rocks at this interesting locality is represented in Fig. -179, which is reduced from my survey on the scale of six inches to a mile. -A section across the smaller vent would show the structure represented in -<a href="#v2fig180">Fig. 180</a>.</p> - -<div class="figcenter" id="v2fig179" style="width: 313px;"> - <img src="images/v2fig179.png" width="313" height="289" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 179.</span>—Plan of necks and bedded tuff at Grange Mill, five miles west of Matlock Bath.</div> -</div> - -<div class="figcenter" id="v2fig180" style="width: 333px;"> - <img src="images/v2fig180.png" width="333" height="101" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 180.</span>—Section across the smaller volcanic neck and the stratified tuff in Carboniferous Limestone, - Grange Mill.<br /> - 1. Limestone; 2. Stratified tuff intercalated among the limestones; 3. Agglomerate.</div> -</div> - -<p><span class="pagenum" id="Page_16">- 16 -</span></p> - -<p>This group of vents lies in the southern of the two tracts of the volcanic -district. In the northern tract a mass of agglomerate pierces the base of -the limestone escarpment about a quarter of a mile west from the entrance to -the Peak Cavern at Castleton.<a id="FNanchor_34" href="#Footnote_34" class="fnanchor">[34]</a> It is rudely semicircular in area, stretching -down the slope until its northern extension is lost under the lower ground. -The agglomerate is not well exposed, but it can be seen to be a green, -granular crumbling rock, made up in great part of minutely vesicular -lapilli, enclosing blocks of various diabases two feet long or more. From the -abrupt way in which this agglomerate rises through the limestone, there can -be little doubt that it marks the position of one of the volcanic vents of the -time. As it stands on the extreme northern verge of the limestone area, the -ground further north being covered with the Yoredale rocks and Millstone -Grit, it is the most northerly of the whole volcanic district.</p> - -<div class="footnote"> - -<p><a id="Footnote_34" href="#FNanchor_34" class="label">[34]</a> This is outcrop No. 1 of Mr. Bemrose's paper, p. 625.</p> - -</div> - -<p>Along the southern margin of the limestone country a group of agglomerate -masses probably marks another chain of vents. These are specially -interesting, inasmuch as they abut on the Yoredale series, and may thus be -looked upon as among the latest of the volcanic chimneys. One of them is seen -at Hopton,<a id="FNanchor_35" href="#Footnote_35" class="fnanchor">[35]</a> where along the side of the road a good section is exposed of coarse -tumultuous agglomerate, having a dull green matrix, composed of green, brown, -and black, minutely cellular, basic, devitrified, glassy lapilli, showing under the -microscope abundant microlites and crystals or calcareous pseudomorphs of -olivine, augite, and felspar, and much magnetite dust. Through this matrix -are distributed blocks of slaggy basalt and dolerite. An interesting feature -of this mass is the occurrence in it of some veins, two or three inches broad, -of a compact black porphyritic basalt. I did not trace the relations of this -agglomerate to the stratified rocks around it. But its internal structure -and composition mark it out as a true neck. It extends, according to the -Geological Survey map, for about half a mile along the edge of the limestone, -and is represented as being separated by two faults from the Yoredale -series immediately to the south. So long as the belief is entertained that -the toadstones are contemporaneous outflows of lava lying on certain -definite horizons, far below the summit of the limestones, the position of the -Hopton agglomerate is only explicable on the assumption of some dislocation -by which the Yoredale shales have been brought down against it. But when -we realize that the rock is an unstratified agglomerate, probably marking -the place of a volcanic vent, and therefore rising transgressively through -the surrounding strata, the necessity for a fault is removed, or if a fault -is inserted its existence should be justified on other evidence than the -relations of the igneous rock to the surrounding strata.</p> - -<div class="footnote"> - -<p><a id="Footnote_35" href="#FNanchor_35" class="label">[35]</a> <i>Geol. Surv. Mem. North Derbyshire</i>, p. 24. This is outcrop No. 53 of Mr. Bemrose's paper, -p. 635.</p> - -</div> - -<p>Four miles to the south-west of Hopton, on the slope of the hill at -Kniveton Wood, another remarkable mass of agglomerate forms a rounded -ridge between the two forks of a small stream.<a id="FNanchor_36" href="#Footnote_36" class="fnanchor">[36]</a> Its granular matrix, like -that of the other necks, consists of lapilli of minutely vesicular basic glassy -<span class="pagenum" id="Page_17">- 17 -</span> -lava or pumice, and encloses large and small rounded blocks of finely -cellular basalt and pieces of limestone. The rock is unstratified, and in all -respects resembles that of ordinary Carboniferous necks in Scotland. Its -relations to the Yoredale rocks are laid bare in the channels of the -streamlets. There the shales and thin limestones may be seen much broken -and plicated, their curved and fractured ends striking directly at the agglomerate. -They may be traced to within a yard of the agglomerate. On the -Geological Survey map the igneous rock is represented as bounded by two -parallel faults. But I hardly think that this explanation suffices to account -for the relations of the rocks and their remarkable boundary-line, which -seems to me to be undoubtedly the wall of a volcanic vent. To the east of -the streams, another mass of agglomerate may mark another neck, while to -the north, a third detached area of the same kind of rock, rising among the -limestones, may be regarded as likewise a distinct mass. At this locality, -therefore, there are two, possibly three, vents. One of these, from the way -in which it cuts across the Yoredale shales and limestones, is to be assigned -to a time later than the older part of the Yoredale series, and thus, like the -Hopton mass, it indicates that in the south of the volcanic area eruptions did -not cease with the close of the deposition of the thick limestones, but were -prolonged even into the time of the Yoredale rocks.</p> - -<div class="footnote"> - -<p><a id="Footnote_36" href="#FNanchor_36" class="label">[36]</a> Outcrop No. 56, p. 638 of Mr. Bemrose's paper.</p> - -</div> - -<p>A further proof of the late age of these southern patches of volcanic -material is shown by two bands of vesicular toadstone in the Yoredale -series, a little south from the village of Kniveton. These rocks are -traced on the Survey Map, and are shown in a diagram in the Memoir, -where their position is sought to be explained by a system of parallel faulting.<a id="FNanchor_37" href="#Footnote_37" class="fnanchor">[37]</a> -I was able to trace the actual contact of the western band with the -strata underneath it, and satisfied myself that there is no fault at the -junction. The igneous material is regularly bedded with the Yoredale shales -and limestones. Either, therefore, these bands are intercalated lava-streams -or intrusive sills. If mere vesicular structure were enough to distinguish -true outflowing lavas, then there could be no doubt about these Kniveton -rocks. But this structure is found in so many Carboniferous sills, -particularly in those thin sheets which have been injected into coals -and black shales, that its presence is far from decisive. The vesicles in the -Kniveton rocks are small and pea-like, tolerably uniform in size and shape, -and crowded together. They are thus not at all like the irregular cavities -in the ordinary cellular and scoriaceous lavas of the toadstone series.</p> - -<div class="footnote"> - -<p><a id="Footnote_37" href="#FNanchor_37" class="label">[37]</a> <i>Op. cit.</i> <a href="#Page_87">p. 87</a>.</p> - -</div> - -<p>Whether or not the question of their true relations be ever satisfactorily -settled, these Kniveton bands are certainly younger than the lower portion -of the Yoredale group. Their evidence thus agrees with that of the southern -agglomerates in showing that the volcanic activity of this region was continued -even after the thick calcareous masses of the Carboniferous Limestone -series had ceased to be deposited.</p> - -<p>Besides the six necks to which I have referred, a rock in Ember Lane, -above Bonsall, probably belongs to another vent.<a id="FNanchor_38" href="#Footnote_38" class="fnanchor">[38]</a> It is particularly interesting -<span class="pagenum" id="Page_18">- 18 -</span> -from the great preponderance of limestone fragments in it. The volcanic -explosions at this locality broke up the already solidified limestones on the -floor of the Carboniferous Limestone sea, and strewed them around, mingled -with volcanic blocks and dust of the prevailing type.</p> - -<div class="footnote"> - -<p><a id="Footnote_38" href="#FNanchor_38" class="label">[38]</a> This is outcrop No. 39 of Mr. Bemrose's paper, p. 632.</p> - -</div> - -<p>When the district has been more carefully searched, other centres of -eruption will no doubt be discovered. It may then be possible to depict -the distribution of the active vents, and to connect with them the outflow of -the bedded lavas. So far as I have been able to ascertain, there are no -necks of dolerite or basalt, though, as I have shown, dykes or veins of molten -rock are occasionally to be found in the agglomerates of the necks.</p> - -<p>4. <span class="allsmcap">THE LAVAS AND TUFFS.</span>—I have referred to the opinion of De la -Beche that the toadstones of Derbyshire were poured out as lava-streams -without any accompanying fragmentary discharges, and to the correction of -this opinion by the subsequent observations of Jukes and of the Geological -Survey. But though the existence of interbedded tuffs has long been known, -it was not until Mr. Bemrose's more careful scrutiny that the relative importance -of the tuffs among the lavas was first indicated. He has shown that -a number of the bands mapped as "toadstone" are tuffs, and he has discovered -other bands of tuff which have not yet been placed on any published map.</p> - -<p>In examining the outcrops of the various toadstones of Derbyshire -we learn that some of them are lavas without tuffs, probably including a -number of bands, which are really sills; that others are formed of both -lavas and tuffs, and that a third type shows only bedded tuff. Each of -these developments will deserve separate description. But before entering -into details, we may take note of the varying thicknesses of the different -toadstones which have been determined by observation at the surface or by -measurement underneath in mining operations. In some cases a distinct -band of toadstone, separated by many feet or yards of limestone from the -next band, and therefore serving to mark a separate volcanic discharge, may -not exceed a yard or two in total thickness, and from that minimum may -swell out to 100 feet. The majority of the bands probably range between -50 and 100 feet in thickness. In one exceptional case at Snitterton, a mass -of "blackstone" is said to have been proved to be 240 feet thick, but -this rock may not improbably have been a sill.<a id="FNanchor_39" href="#Footnote_39" class="fnanchor">[39]</a> The true contemporaneous -intercalations seem to be generally less than 100 feet in thickness.</p> - -<div class="footnote"> - -<p><a id="Footnote_39" href="#FNanchor_39" class="label">[39]</a> A difference is made by the mining community between "toadstone" and what is called -"blackstone." The former name appears to be restricted to the amygdaloidal green and generally -more or less decayed lavas; the latter, so far as I can learn, is applied to the dark, more -solid and crystalline rocks. If this distinction be well founded the one name may perhaps serve -to mark the open cellular lavas, the other the more compact, dark, and heavy intrusive sheets.</p> - -</div> - -<div class="figright" id="v2fig181" style="width: 227px;"> - <img src="images/v2fig181.png" width="227" height="170" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 181.</span>—Section of vesicular and amygdaloidal - diabase resting on Carboniferous limestone, - Peak Forest Limeworks, Great Rocks Quarry.<br /> - 1. Limestone with a surface dissolved into cauldron-like - hollows; 2. Rotten yellow and brown clay resulting - from decomposition of toadstone and white clay - from the solution of the limestone—sometimes three - or four feet thick; 3. Toadstone, a diabase with - highly slaggy base.</div> -</div> - -<p>(<i>a</i>) Lavas without Tuffs.—Examples occur of sheets of toadstone which -consist entirely of contemporaneously ejected diabase, basalt or dolerite. This -rock is then dull green or brown in colour, more or less earthy in texture, -and irregularly amygdaloidal. The vesicles are extremely varied in size, -form and distribution, sometimes expanding until the rock becomes a slaggy -mass. A central more solid portion between a scoriaceous bottom and top -<span class="pagenum" id="Page_19">- 19 -</span> -may sometimes be observed, as at the Great Rocks Quarry, Peak Forest -Limeworks (<a href="#v2fig181">Fig. 181</a>). In this, as in other examples, a remarkably -hummocky and uneven surface of limestone -lies below the igneous band, -the calcareous rock presenting knobs -and ridges, separated by cauldron-shaped -cavities and clefts, some of -which are several yards deep. These -inequalities are filled in and covered -over with a soft yellow and brown -clay, varying up to three or four feet -thickness, and passing upwards into -the more solid toadstone. There can -hardly be any doubt that this singularly -uneven limestone surface is due -to the solvent action of water lying -between the limestone and the somewhat -impervious toadstone above, and -that the clay represents partly the -insoluble residue of the calcareous rock, but chiefly the result of the action -of the infiltrating water on the bottom of the igneous band.<a id="FNanchor_40" href="#Footnote_40" class="fnanchor">[40]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_40" href="#FNanchor_40" class="label">[40]</a> <i>Geological Survey Memoir on North Derbyshire</i>, p. 20 and footnote.</p> - -</div> - - -<p>Junctions of the upper surfaces of the lava-sheets with the overlying -limestone show that the igneous material sometimes assumed hummocky -forms, which the calcareous deposits gradually overspread and covered.<a id="FNanchor_41" href="#Footnote_41" class="fnanchor">[41]</a> A -good example of this kind may be observed by the roadside at the foot of -Raven's Tor, Millersdale. As shown in the subjoined figure, the limestone -has here been worn into a cave, the floor of which is formed by the toadstone. -The latter rock, of the usual dull green, slaggy and amygdaloidal -character, is covered immediately by the limestone, but I did not observe -any fragments of the toadstone, nor any trace of ashy materials in the overlying -calcareous strata. This section shows that after the outflow of the -lava, the sedimentation of the limestone was quietly resumed, and the igneous -interruption was entirely buried.</p> - -<div class="footnote"> - -<p><a id="Footnote_41" href="#FNanchor_41" class="label">[41]</a> Compare De la Beche, <i>Geological Observer</i>, pp. 559, 560, and <i>North Derbyshire Memoir</i>, p. 123.</p> - -</div> - -<div class="figcenter" id="v2fig182" style="width: 418px;"> - <img src="images/v2fig182.png" width="418" height="91" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 182.</span>—View of the superposition of Carboniferous limestone upon toadstone, Raven's Tor, - Millersdale (length about 100 feet).<br /> - 1. Toadstone; 2. Limestone; <i>f</i>, Fault.</div> -</div> - -<p><span class="pagenum" id="Page_20">- 20 -</span></p> - -<p>In some cases there is evidence of more than one outflow of lava in the -same band of toadstone. Jukes believed that each band "was the result, not -of one simultaneous ejection of igneous matter, but of several, proceeding -from different foci uniting together to form one band," and he found that -near Buxton, two solid beds of toadstone could be seen to have proceeded -from opposite quarters towards each other without overlapping.<a id="FNanchor_42" href="#Footnote_42" class="fnanchor">[42]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_42" href="#FNanchor_42" class="label">[42]</a> <i>Student's Manual of Geology</i>, 2d edit. (1862), p. 523.</p> - -</div> - -<p>In Millersdale the authors of the <i>Geological Survey Memoir on North -Derbyshire</i> observed that a band of toadstone about 100 feet thick showed -six distinct divisions, which they were disposed to regard as marking so -many separate beds.<a id="FNanchor_43" href="#Footnote_43" class="fnanchor">[43]</a> In Tideswell Dale, on the west side of the valley, -immediately to the south of the old toadstone quarry, two bands of toadstone -are seen to be separated by a few yards of limestone.</p> - -<div class="footnote"> - -<p><a id="Footnote_43" href="#FNanchor_43" class="label">[43]</a> <i>Op. cit.</i> p. 19.</p> - -</div> - -<p>(<i>b</i>) Lavas with Tuffs.—It will probably be found that in many, if not in -most cases, the outflow of lava was preceded, accompanied or followed by -fragmental discharges. As far back as 1861, Jukes noticed that a toadstone -band, about 50 feet thick, near Buxton consisted of two solid beds of lava -"with beds of purple and green ash, greatly decomposed into clay, both above -and below each bed and between the two."<a id="FNanchor_44" href="#Footnote_44" class="fnanchor">[44]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_44" href="#FNanchor_44" class="label">[44]</a> <i>Op. cit.</i> p. 523>.</p> - -</div> - -<div class="figleft" id="v2fig183" style="width: 119px;"> - <img src="images/v2fig183.png" width="119" height="226" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 183.</span>—Section at lime-kiln, - south of Viaduct, Millersdale Station.</div> -</div> - - -<p>An interesting section, showing this intercalation of the two kinds of -material is exposed at the lime-kilns beyond the southern end of the railway -viaduct at Millersdale Station. Over a mass of solid -blue limestone (1 in <a href="#v2fig183">Fig. 183</a>) lies a band of bright -yellow and brown clay (2), varying from six inches to -two feet in thickness. This may be compared with -the clay found above the limestone at Peak Forest -(<a href="#v2fig181">Fig. 181</a>). But it is probably a layer of highly -decomposed tuff. It is succeeded by a thin band of -greenish limestone (3) containing an admixture of fine -volcanic detritus, and partially cut out by an irregular -bed, four to eight feet thick, of a highly slaggy, greenish, -decomposing, spheroidal and amygdaloidal diabase (4). -This unmistakable lava-sheet is followed by a bed of -green granular tuff (5), which in some places reaches -a thickness of three feet, but rapidly dies out. Over a -space several yards in breadth, the succeeding strata -are concealed, and the next visible rock is a dark, -compact dolerite which weathers spheroidally (6).</p> - -<p>(<i>c</i>) Tuffs without Lavas.—Mr. Bemrose has shown that some of the -bands of toadstone consist entirely of bedded tuff. In these cases, so far -as the present visible outcrops allow us to judge, no outflow of lava accompanied -the eruption of fragmentary materials. But that the ejection of these -materials was not the result of a sudden spasmodic explosion, but of a -continued series of discharges varying in duration and intensity, is indicated -by the well-bedded character of the tuff and the alternation of finer and -<span class="pagenum" id="Page_21">- 21 -</span> -coarser layers. Large blocks of lava, two feet or more in diameter, may -mark some of the more vigorous paroxysms of the vents, while the usual fine -granular nature of the tuff may point to the prevailing uniformity and less -violent character of the eruptions. Bands of tuff 70 feet or more in thickness, -without the intercalation of any limestone or other non-volcanic intercalation, -point to episodes of such continued volcanic activity that the ordinary -sedimentation of the sea-bottom was interrupted, or at least masked, by the -abundant fall of dust and stones.</p> - -<p>One of the best exposures of such intercalations of bedded tuffs was -pointed out to me by Mr. Bemrose, immediately to the east of the village of -Litton. The matrix is crowded with the usual minutely vesicular glassy -lapilli, and encloses fragments of diabase of all sizes, up to blocks more than -a foot in diameter. The rock is well stratified, and the layers of coarse -and fine detritus pass beneath a group of limestone beds. The actual -junction is concealed under the roadway, but only two or three feet of rock -cannot be seen. The lowest visible layer of limestone is nodular and contains -decayed bluish fragments which may be volcanic lapilli. Immediately above -the lower limestones the calcareous bands become richly fossiliferous. Some -of their layers consist mainly of large bunches of coral; others are crowded -with cup-corals, or are made up mainly of crinoids with abundant brachiopods, -polyzoa, lamellibranchs, gasteropods and occasional fish-teeth. This remarkable -profusion of marine life is interesting inasmuch as it succeeds -immediately the band of volcanic ash.</p> - -<p>Another well-marked zone of tuff, with no traceable accompaniment of -lava, has already been referred to as connected with the Grangemill vents. -In this case also, the limestone that lies directly upon the volcanic material -is rather impure and nodular in character. The tuff itself is well bedded, -perhaps from 70 to 100 feet thick and dips underneath an overlying series -of marine limestones.</p> - -<p>I did not observe thin partings of tuff and disseminated volcanic lapilli -among the limestones, such as are so marked in the Lower Carboniferous -formations of West Lothian, and in the Limerick basin, to be described in the -following chapter. But a diligent search might discover examples of them, -and thus prove that, besides the more prolonged and continuous eruptions -that produced the thick bands of tuff, there were occasional feeble and intermittent -explosions during the accumulation of the thick sheets of limestone. -Some of the layers of "red clay" observed in shafts sunk for mining purposes -may perhaps represent such spasmodic discharges of fine fragmental material.</p> - -<p>5. <span class="allsmcap">THE SILLS.</span>—No attempt has yet been made to determine whether and -to what extent the toadstone bands include true intrusive sheets. My own -brief examination of the ground does not warrant me in making any positive -statement on this subject. I can hardly doubt, however, that some, perhaps -not a few, of the toadstone bands are really sills. In the accounts of these -rocks contained in the mining records a distinction, as already remarked, -appears to have been generally drawn between "toadstone" and "blackstone." -The latter term is applied to the black, fresh, more coarsely crystalline, and -<span class="pagenum" id="Page_22">- 22 -</span> -generally non-amygdaloidal rocks, which, so far as I have been able to -examine them, have the general external and many of the internal characters -of the Carboniferous sills of Central Scotland. At Snitterton near Matlock -one of these "blackstones," as already mentioned, is said to have been -found to be 240 feet thick.<a id="FNanchor_45" href="#Footnote_45" class="fnanchor">[45]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_45" href="#FNanchor_45" class="label">[45]</a> <i>North Derbyshire Memoir</i>, p. 23.</p> - -</div> - -<p>It is stated that the toadstones, though subject to great variations in thickness, -are never seen to cut across the limestones.<a id="FNanchor_46" href="#Footnote_46" class="fnanchor">[46]</a> But I suspect that proofs -of intrusion and transgression will be found when diligently sought for. It -appeared to me that the dark, compact, crystalline dolerite, which was formerly -quarried in the middle of Tideswell Dale, may be separated from the vesicular -toadstone of that valley, which is undoubtedly a true lava-flow, and that it -does not always occupy the same horizon there, being sometimes below and -sometimes above the amygdaloid. Where it rests on a band of red clay the -latter rock has been made columnar to a depth of nine feet.<a id="FNanchor_47" href="#Footnote_47" class="fnanchor">[47]</a> Alteration of -this kind is very rare among the Carboniferous bedded lavas, but is by -no means infrequent in the case of sills. But the most important proof of -alteration which I have myself observed occurs at Dale Farm near the village -of Peak Forest, where the limestone above a coarsely crystalline dolerite has -been converted into a white saccharoid marble for about two yards from the -junction.</p> - -<div class="footnote"> - -<p><a id="Footnote_46" href="#FNanchor_46" class="label">[46]</a> <i>Op. cit.</i> <a href="#Page_123">p. 123</a>.</p> - -<p><a id="Footnote_47" href="#FNanchor_47" class="label">[47]</a> J. M. Mello, <i>Quart. Journ. Geol. Soc.</i> vol. xxvi. (1871), p. 701.</p> - -</div> - - -<h3>3. THE ISLE OF MAN</h3> - -<p>Rising from the middle of the Irish Sea, within sight of each of the -three kingdoms, with a history and associations so distinct, yet so intimately -linked with those of the rest of Britain, this interesting island presents in -its geological structure features that connect it alike with England, Scotland -and Ireland, while at the same time it retains a marked individuality -in regard to some of the rocks that form its framework. Its great central -ridge of grits and slates, which still rises 2000 feet above the sea in the -summit of Snaefell, must have formed a tract of dry land in Carboniferous -time, until it sank under sea-level, and was buried beneath the Carboniferous -and later formations. Along the southern margin of this ancient -land, a relic of the floor of the Carboniferous sea has been preserved in a -small basin of Carboniferous Limestone which covers about seven or eight -square miles. This remnant has a special interest in geological history, -for it has preserved the records of a series of volcanic eruptions which took -place contemporaneously with the deposition of the Carboniferous Limestone.</p> - -<p>The geology of the Isle of Man was sketched in outline by J. F. -Berger,<a id="FNanchor_48" href="#Footnote_48" class="fnanchor">[48]</a> J. Macculloch,<a id="FNanchor_49" href="#Footnote_49" class="fnanchor">[49]</a> and J. S. Henslow,<a id="FNanchor_50" href="#Footnote_50" class="fnanchor">[50]</a> and was afterwards more fully -illustrated by J. G. Cumming.<a id="FNanchor_51" href="#Footnote_51" class="fnanchor">[51]</a> To the last-named observer we owe the -<span class="pagenum" id="Page_23">- 23 -</span> -recognition of true intercalated volcanic rocks among the calcareous -formations of the southern end of the island. These rocks have subsequently -been studied in greater detail by a number of geologists. An -excellent general account of them was published in 1874 by Mr. John -Horne, of the Geological Survey.<a id="FNanchor_52" href="#Footnote_52" class="fnanchor">[52]</a> A few years later some further observations -on them were prepared by J. Clifton Ward.<a id="FNanchor_53" href="#Footnote_53" class="fnanchor">[53]</a> More recently -their petrography has been studied by Messrs. E. Dickson, P. Holland and -F. Rutley,<a id="FNanchor_54" href="#Footnote_54" class="fnanchor">[54]</a> and in more detail by Mr. B. Hobson.<a id="FNanchor_55" href="#Footnote_55" class="fnanchor">[55]</a> To some of the -observations of these writers reference will be made in the succeeding -pages. During the progress of the Geological Survey in the Isle of Man, -the rocks in question have been mapped in detail by Mr. A. Strahan -and Mr. G. W. Lamplugh, and I have had an opportunity of examining -the coast-sections with the last-named geologist. The following description -of these sections is taken mainly from my field note-book. The full details -will appear in the official <i>Memoirs</i>.</p> - -<div class="footnote"> - -<p><a id="Footnote_48" href="#FNanchor_48" class="label">[48]</a> <i>Trans. Geol. Soc.</i> 1st ser. vol. ii. (1814), p. 29.</p> - -<p><a id="Footnote_49" href="#FNanchor_49" class="label">[49]</a> <i>Western Islands of Scotland</i> (1819), vol. ii. p. 571.</p> - -<p><a id="Footnote_50" href="#FNanchor_50" class="label">[50]</a> <i>Trans. Geol. Soc.</i> 1st ser. vol. v. (1821), p. 482.</p> - -<p><a id="Footnote_51" href="#FNanchor_51" class="label">[51]</a> <i>The Isle of Man</i> (1848), chap. x.</p> - -<p><a id="Footnote_52" href="#FNanchor_52" class="label">[52]</a> <i>Trans. Geol. Soc. Edin.</i> ii. (1874), p. 332.</p> - -<p><a id="Footnote_53" href="#FNanchor_53" class="label">[53]</a> <i>Geol. Mag.</i> 1880, p. 4.</p> - -<p><a id="Footnote_54" href="#FNanchor_54" class="label">[54]</a> <i>Proc. Liverpool Geol. Soc.</i> vol. vi. (1888-89), p. 123.</p> - -<p><a id="Footnote_55" href="#FNanchor_55" class="label">[55]</a> <i>Quart. Journ. Geol. Soc.</i> xlvii. (1891), p. 432. This paper was reprinted with additions -and corrections in <i>Yn Lioar Manninagh</i>, Douglas, Isle of Man, vol. i. No. 10, April 1892.</p> - -</div> - -<p>It may be remarked at the outset that the last outcrop of the plateau-lavas -of the Solway basin occurs only 60 miles from the south end of the -Isle of Man, at the foot of the hills of Galloway, the blue outline of which -can be seen from that island. The distance from the Manx volcanoes to -the nearest of the puys of Liddesdale is about 100 miles. Though the -fragment which has been left of the ejections is too small to warrant any -confident parallelism, there appears to be reason to believe that, alike in -geological age and in manner of activity, the Manx volcanoes may be -classed with the type of the puys.</p> - -<p>The Carboniferous strata of the Isle of Man lie in a small trough at -the south end of the island. The lowest members of the series consist of -red conglomerates and sandstones, which pass upward into dark limestones -full of the characteristic fossils of the Carboniferous Limestone. As the -bottom of the basin is on the whole inclined seawards, the highest strata -occur along the extreme southern coast. It is there that the volcanic -rocks are displayed. They occupy a narrow strip less than two miles in -length, which is almost entirely confined to the range of cliffs and the -ledges of the foreshore. Yet though thus extremely limited in area, they -have been so admirably dissected along the coast, that they furnish a -singularly ample body of evidence bearing on the history of Carboniferous -volcanic action.</p> - -<p>Unfortunately the bottom of the volcanic group is nowhere visible. At -the east or lower end of the series, exposed on the shore, an agglomerate -with its dykes appears to truncate the Castletown Limestones. No trace -of any tuff has been noticed among these lower limestones. We may infer -that the volcanic activity began after they were deposited. The highest -accessible portions of the volcanic group, as Mr. Horne showed, are clearly -<span class="pagenum" id="Page_24">- 24 -</span> -exposed on the coast at Poyll Vaaish, intercalated in and overlying the -dark limestones of that locality (<a href="#v2fig184">Fig. 184</a>), which have been assigned, from -their fossil contents, to the upper part of the Carboniferous Limestone -series.<a id="FNanchor_56" href="#Footnote_56" class="fnanchor">[56]</a> The Manx volcanoes may therefore be regarded as having probably -been in eruption during the later portion -of the Carboniferous Limestone period.</p> - -<div class="footnote"> - -<p><a id="Footnote_56" href="#FNanchor_56" class="label">[56]</a> R. Etheridge jun., in Mr. Horne's paper above cited.</p> - -</div> - -<div class="figleft" id="v2fig184" style="width: 183px;"> - <img src="images/v2fig184.png" width="183" height="147" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 184.</span>—Limestones passing under stratified - tuffs, Poyll Vaaish, Isle of Man.</div> -</div> - -<p>Owing to irregularities of inclination, -the thickness of the volcanic group can -only be approximately estimated. It is -probably not less than 200 or 300 feet. -But as merely the edge of the group lies -on the land, the volcanic rocks may reach -a considerably greater extent and thickness -under the sea.</p> - -<p>The volcanic materials consist mainly -of bedded tuffs, but include also several -necks of agglomerate and a number of dykes and sills. So far as I -have observed, they comprise no true lava-streams.<a id="FNanchor_57" href="#Footnote_57" class="fnanchor">[57]</a> These Manx tuffs -present many of the familiar features of those belonging to the puy-eruptions -of Central Scotland, but with some peculiarities worthy of -attention. They are on the whole distinctly bedded, and as their -inclination is generally in a westerly direction, an ascending order can -be traced in them from the eastern end of the section to the highest -parts of the group associated with the Poyll Vaaish limestones. Their -colour is the usual dull yellowish-green, varying slightly in tint with -changes in the texture of the materials, the palest bands consisting of the -finest dust or volcanic mud. Great differences in the size of their fragmentary -constituents may be observed in successive beds, coarse and fine -bands rapidly alternating, with no admixture of non-volcanic sediment, -though occasional layers of fine ash or mudstone, showing distinct current-bedding, -may be noticed.</p> - -<div class="footnote"> - -<p><a id="Footnote_57" href="#FNanchor_57" class="label">[57]</a> The occurrence of intercalated lavas has been described in this series, but, as I shall show -in the sequel, they are probably intrusive masses.</p> - -</div> - -<p>Pauses in the succession of eruptions are marked by the intercalation -of seams of limestone or groups of limestone, shale and black impure chert. -Such interstratifications are sometimes curiously local and interrupted. -They may be observed to die out rapidly, thereby allowing the tuff above -and below them to unite into one continuous mass. They seem to have -been accumulated in hollows of the tuff during somewhat prolonged intervals -of volcanic quiescence, and to have been suddenly brought to an end -by a renewal of the eruptions. There are some four or five such intercalated -groups of calcareous strata in the thick series of tuffs, and we may -regard them as marking the chief pauses in the continuity or energy of the -volcanic explosions.</p> - -<p>An attentive examination of these interpolated sedimentary deposits -<span class="pagenum" id="Page_25">- 25 -</span> -affords some interesting information as to the submarine conditions in -which the eruptions took place. The intercalations, sometimes 12 feet or -more in thickness, consist mainly of dark limestones, enclosing the usual -Carboniferous Limestone fossils; black shales, sometimes showing very -fragmentary and much macerated remains of ferns and other land-plants; -and black impure argillaceous chert or flint, arranged in bands interposed -between the other strata, and also in detached lumps and strings. The dark -flaggy limestones and black shales may be paralleled lithologically with -those of Castletown and Poyll Vaaish. Indeed, there seems to be little -doubt that they represent the contemporaneous type of marine sediment -that was gathering on the sea-floor outside the volcanic area, and which -during intervals of quiescence or feeble eruptivity spread more or less continuously -into that area. The thick mass of tuff must thus have been strictly -contemporaneous with a group of calcareous muddy and siliceous deposits -which gathered over the bottom beyond the limits of the showers of ashes.</p> - -<div class="figcenter" id="v2fig185" style="width: 384px;"> - <img src="images/v2fig185.png" width="384" height="169" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 185.</span>—Section of tuff, showing intercalations of black impure chert, west of Closenychollagh - Point, near Castletown, Isle of Man.</div> -</div> - -<p>One of the most singular features of these sedimentary intercalations -is the occurrence of the black cherty material. It may generally be -observed best developed at the bottom and top of each group of included -strata. Looking at the lumps of this substance scattered through the -adjoining tuffs, we might at first take them for ejected fragments, and such -no doubt may have been the derivation of some of them. But further -examination will show that, as a rule, they are of a concretionary nature, -and were formed <i>in situ</i> contemporaneously with or subsequent to the -deposition of the tuffs. The accompanying section (<a href="#v2fig185">Fig. 185</a>) represents -the manner in which the chert is distributed through two or three square -yards of tuff overlying one of the calcareous groups. The material has been -segregated not only into lumps, but into veins and bands, which, though on -the whole parallel with the general stratification-planes of the deposits, sometimes -run irregularly in tongues or strings across these planes, as shown in -<a href="#v2fig186">Fig. 186</a>, where the dark chert band which overlies the limestones and -shales sends a tongue upwards for several inches into the overlying tuff.</p> - -<p><span class="pagenum" id="Page_26">- 26 -</span></p> - -<p>That these interstratified calcareous and muddy strata were laid down -in water of some considerable depth may be inferred from their general -lithological characters. The dark carbonaceous aspect of the limestones -points to the probable intermingling of much decayed vegetation with the -remains of the calcareous organisms of which these strata chiefly consist. -The thin unimportant bands or partings -of dark shale show that only the finest -muddy sediment reached the quiet depths -in which the strata were deposited, while -the macerated fern-fragments suggest a -long flotation and ultimate entombment -of terrestrial vegetation borne seawards -from some neighbouring land.</p> - -<div class="figleft" id="v2fig186" style="width: 202px;"> - <img src="images/v2fig186.png" width="202" height="130" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 186.</span>—Section of intercalated dark limestone, - shale and chert in the tuff south of Poyll Vaaish Bay, Isle of Man.<br /><br /> - 1. Limestones and shales; 2. Chert; 3. Tuff.</div> -</div> - -<p>The cherty bands and nodules, like -the flints of the chalk, bear their testimony -to the quiet character of the sedimentation -in rather deep water beyond the -limits within which the sediment from the -land was mainly accumulated on the sea-bottom. The origin of these siliceous -parts of the series of deposits has still to be investigated. Whether or not -they are to be referred to organic causes like chalk-flints, and the radiolarian -cherts of the Lower Silurian system, they furnish a fresh example of the -remarkable association of such siliceous material with volcanic phenomena, -which has now been observed in many widely separated areas all over the -world.</p> - -<p>If we next turn to the stratification of the tuffs, we obtain further -evidence of undisturbed conditions of deposition on the sea-floor. The -bedding of these volcanic masses, though distinct, appears for the most part -to be due rather to the eruption and settlement of alternately finer and -coarser detritus than to any marked drifting and rearrangement of these -materials by current-action into different layers. Throughout the series of -tuffs, indeed, there is, on the whole, a notable absence of any structure suggestive -of strong currents or of wave-action in the dispersal and reassortment -of the volcanic detritus. The ashes and stones were discharged in -such a way as to gather irregularly over the sea-floor into ridges and -hollows. There does not seem to have been sufficient movement in the -bottom water to level down these inequalities of surface, for we find that -they remained long enough to allow twelve feet or more of calcareous and -siliceous ooze to gather in the hollows, while the intervening ridges still -stood uneffaced until buried under the next fall of ashes. At rare intervals -some transient current or deeper wave may have reached the bottom and -spread out the volcanic detritus lying there. Such exceptional disturbances -of the still water are not improbably indicated by occasional well-defined -stratification, and even by distinct false-bedding, in certain finer layers of -tuff.</p> - -<p>The materials of the tuffs are remarkably uniform in character and -<span class="pagenum" id="Page_27">- 27 -</span> -conspicuously volcanic in origin. With the exception of occasional blocks -of limestone, which range up to masses several feet, and occasionally several -yards, in diameter, the dust, lapilli and included stones consist entirely of -fragmentary basic lava, so persistent in its lithological features that we may -regard its slightly different varieties as merely marking different conditions -of the same rock. The accumulation of pumiceous ash in this southern -coast of the Isle of Man is one of the most remarkable in Britain. As Mr. -Hobson has well shown, the matrix of this tuff consists of irregular lapilli, -representing what may have been various conditions of solidification in one -original volcanic magma. This magma he has described as an "augite-porphyrite" -or olivine-basalt. Some of the lapilli, as he noted, consist of a -pumice "crowded with vesicles which occupy more space than the solid -part"; others show nearly as many vesicles, but the glass is made brown -by the number of its fine dust-like inclusions; a third type presents the -cells and cell-walls in nearly equal proportions. The same observer found -that where the substance is most cellular the vesicles, fairly uniform in -size, measure about a tenth of a millimetre in longest diameter.</p> - -<p>An interesting feature of the tuffs is the abundant occurrence of loose -felspar crystals throughout the whole group up to the highest visible strata. -These crystals, sometimes nearly an inch in length, appear conspicuously as -white spots on weathered surfaces of the rock. They are so much decayed, -however, that it is difficult to extract them entire. On the most cursory -inspection they are observed to enclose blebs of a greenish substance like -the material that fills up the vesicles in the pumiceous fragments and in -the pieces of cellular lava.</p> - -<p>I have not ascertained the original source of these scattered felspars. -In one of the dykes on the north side of the agglomerate at Scarlet Point, -as was pointed out by Mr. Hobson, large crystals of plagioclase occur in the -melaphyre, but the felspars in the tuffs and agglomerates differ so much -from these that we cannot suppose them to have come from the explosion -of such a rock. I failed to detect any other mineral in detached crystals in -the tuffs, but a more diligent search might reveal such, and afford some -grounds for speculating on the probable nature of the magma from the -explosion of which the scattered crystals were derived. It is at least certain -that this magma must have included a large proportion of plagioclase -crystals.</p> - -<p>Between the lapilli and the minute pumice-dust that constitute the -matrix of this tuff much calcite may be detected. Though this mineral -may have been partly derived from the decay of the felspar in the lava-fragments, -I believe that it is mainly to be attributed to the intermingling -of fine calcareous ooze with the ash accumulated on the sea-floor. A more -remarkable association of the same kind will be described in later pages -from King's County in Ireland. That abundant calcareous organisms -peopled the sea in which the Manx Carboniferous volcanoes were active is -shown by the contemporaneously deposited limestones. The tuffs themselves -are occasionally fossiliferous. Species of <i>Spirifer</i>, <i>Productus</i> and -<span class="pagenum" id="Page_28">- 28 -</span> -other brachiopods, together with broken stems of encrinites, may be found -in them, and doubtless the diffused calcite, though now crystalline, as -in the limestones, and showing no organic structure, owes its presence -to the detritus of once living organisms.</p> - -<p>The stones imbedded in the tuff consist almost exclusively of slightly -different varieties of the same pale, always vesicular rock, and sometimes -pass into a coarse slag. They vary up to six feet or more in length. In -many cases, they appear to have been derived from the disruption of already -solidified lava, for their vesicles are not elongated or arranged with reference -to the form of the block, but have been broken across and appear in section -on the outer surface. In other instances, however, the cavities are large -and irregular in the centre of the block, while on the outside they are -smaller and are drawn out round the rudely spherical shape of the mass, as -in true volcanic bombs.</p> - -<p>The limestone fragments enclosed in the tuff include pieces of the dark -carbonaceous and of the pale encrinal varieties. In no case did I observe -any sensible alteration of these fragments. They seem to have been derived -from material disrupted and ejected during the opening of successive vents, -and not to have been exposed for any considerable time to the metamorphic -influence of volcanic heat and vapours.</p> - -<p>Narrow though the strip of volcanic material is along the south coast of -the Isle of Man, it has fortunately preserved for us some of the vents from -which the tuffs were ejected. A group of these vents, three or four in -number, may be traced along the shore in a general W.N.W. and E.S.E. -line from Scarlet Point for rather more than a mile. Their margins are in -some places exceedingly well defined. The most striking example of this -feature occurs in the most westerly vent, where a neck of remarkably coarse -volcanic agglomerate rises vertically through well-bedded, westerly-dipping -tuff (<a href="#v2fig187">Fig. 187</a>). In other portions of their boundaries no sharp line can -be drawn between the material filling the vent and that of the surrounding -tuffs. Hence it is difficult to define precisely the form and size of the vents. -I am inclined to believe from this indefiniteness of outline, and from the -remarkable structure of the dykes, to which I shall afterwards refer, that -the presently visible parts of these necks must lie close to the mouths of -the original vents, if indeed they do not actually contain parts of the -craters and of their surrounding walls.</p> - -<p>The materials that have filled up the eruptive vents consist chiefly of -agglomerate, but partly also of intrusive portions of vesicular lava. The -agglomerate is composed of similar materials to the tuffs. Its matrix -shows the same extraordinarily abundant fine greenish-grey basic pumiceous -lapilli, with the same kind of plentiful loose felspar-crystals. The large -blocks of lava, too, resemble in composition and structure those of the bedded -tuffs, but greatly exceed them in size and abundance.</p> - -<p>Besides the fragments of vesicular lava, there occur also occasional blocks -of limestone. Some of these are several yards in length. Messrs. Strahan and -Lamplugh have mapped a large mass of limestone at the Scarlet vent, which, -<span class="pagenum" id="Page_29">- 29 -</span> -so far as can be observed, lies in the agglomerate—a large cake of white -limestone with pebbles of quartz, which has probably been broken off from -some underlying bed and carried up in the chimney of the volcano.</p> - -<p>As a rule the agglomerate is a tumultuous, unstratified mass. But in -many places it shows lines of bedding and, as already stated, passes outward -into ordinary bedded tuff, the number and size of the ejected blocks rapidly -diminishing. Where this transition occurs we seem to see a remnant of -the base of the actual volcanic cone. Thus, in the most westerly vent -already cited, while the wall of the vent has been laid bare on the side next -the sea, so that the agglomerate on the beach descends vertically through -the surrounding bedded tuffs, on the western side the cliffs have preserved a -portion of the material that accumulated outside the orifice (<a href="#v2fig187">Fig. 187</a>). -In this section we observe that the coarse agglomerate which fills up -the main part of the vent has been left with a hummocky, uneven surface, -and that a subsequent and perhaps feebler eruption of finer material has -covered over these inequalities, and has extended to the left above the fine -tuffs through which the agglomerate has been drilled.</p> - -<div class="figcenter" id="v2fig187" style="width: 344px;"> - <img src="images/v2fig187.png" width="344" height="138" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 187.</span>—Section of part of a volcanic neck on shore to the south-east of Poyll Vaaish Bay, - Isle of Man.</div> -</div> - -<div class="figcenter" id="v2fig188" style="width: 358px;"> - <img src="images/v2fig188.png" width="358" height="121" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 188.</span>—Section of successive discharges and disturbances within a volcanic vent. Scarlet Point, - Isle of Man.</div> -</div> - - -<p>Again, in the largest of the vents, that near Scarlet Point, still -clearer proof of successive eruptions and dislocations within a volcanic -chimney may be noticed. At one point the accompanying section (Fig. -188) has been laid bare by the waves. The oldest accumulation is a fine -green granular tuff (<i>a</i>), rudely and faintly arranged in layers inclined at -high angles, like the fine materials in many of the vents of the basin of the -Firth of Forth. This peculiar stratification, due not to the assortment of -<span class="pagenum" id="Page_30">- 30 -</span> -materials in water, but to the deposition of coarser and finer detritus by successive -explosions, and to subsequent slipping or tilting, is a characteristic -feature of the detritus which has filled up ancient volcanic funnels. A later -explosion from some adjacent part of the same vent has given rise to the -discharge of a coarse agglomerate (<i>b</i>), which with blocks sometimes six feet -long, overspreads the earlier material. A third detrital accumulation in the -same vent, consisting of a firm brecciated tuff (<i>c</i>) with much calcite in its -matrix, has been brought down by a slip (<i>f</i>) which cuts across both of the -previous deposits. A broad dyke (<i>d</i>) of vesicular diabase (augite-porphyry) -traverses the vent, and is probably later than any of the other rocks in the -section.</p> - -<p>I will conclude this account of the Manx Carboniferous volcanic rocks -with a brief reference to the intrusive masses which form a prominent -feature of the coast-line. From the picturesque headland of Scarlet Point -the broad dyke which forms that promontory may be traced for some -distance westwards. Several other parallel dykes run in the same direction -which, it will be observed, is also that of the chain of vents. It might be -said that the vents are, as it were, strung together by a line of dykes. -These eruptive masses traverse both the agglomerates and the bedded tuffs. -They probably belong, therefore, to a comparatively late part of the volcanic -history. That they are truly intrusive and not lava-flows is, I think, -clearly shown by their vertical walls which descend through the surrounding -rocks, and by the greater closeness of their texture, as well as the diminution -in the size of their vesicles along the contact surfaces. But it must be -admitted that in their remarkably developed vesicular structure they look -more like streams of lava than ordinary dykes.</p> - -<p>It is this structure which gives to these dykes their peculiar interest. -Bands of vesicles, from an inch or less to several inches in breadth, run -along the dykes parallel to the outer walls. Unlike the familiar rows of -little amygdaloidal cells in ordinary basalt dykes, such as those of the -Tertiary series in Scotland, these vesicles, though small and pea-like in the -narrower bands towards the margins of the dykes, became so large, numerous, -and irregular in the broader and more central bands, that the rock passes -there into a rough slag.</p> - -<div class="figleft" id="v2fig189" style="width: 229px;"> - <img src="images/v2fig189.png" width="229" height="103" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 189.</span>—Section of dyke and sill in the tuffs west - of Scarlet Point, Isle of Man.</div> -</div> - -<p>While the intrusive material has for the most part risen in the form of -dykes, in one part of the coast-section, -a little to the west of Scarlet -Point, it has been injected as a sill -among the bedded tuffs.<a id="FNanchor_58" href="#Footnote_58" class="fnanchor">[58]</a> A section -taken at this locality gives the -structure represented in <a href="#v2fig189">Fig. 189</a>. -On the north side of the great dyke, -the strata of tuff which dip under -it, roll over and support an outlying -sheet of the same material. The slaggy structure of parts of this sill give -<span class="pagenum" id="Page_31">- 31 -</span> -it some resemblance to a true lava-flow. But it is the same structure -which can be seen in the dykes, while the closer grain along the contact-surface -further connects it with these intrusions.</p> - -<div class="footnote"> - -<p><a id="Footnote_58" href="#FNanchor_58" class="label">[58]</a> It is this sheet which has been described as a lava-stream.</p> - -</div> - -<table summary="images"> -<tr> - <td id="v2fig190" class="tdc" style="width: 45%;"><img src="images/v2fig190.png" width="159" height="148" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 190.</span>—Section on south side - of vesicular sill<br />west of Scarlet Point.</div> - </td> - <td id="v2fig191" class="tdc" style="width: 45%;"><img src="images/v2fig191.png" width="141" height="196" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 191.</span>—Bands of vesicles in - the same sill.</div> - </td> -</tr> -</table> - -<p>There is, however, a peculiarity about the development of the vesicular -structure in this sill which I have not observed anywhere else. If we -examine the southern side of the crag near its -eastern end we observe that the successive bands -of vesicles are arranged in the same direction -as the surface of contact with the underlying -tuffs, precisely as they are ranged in dykes -parallel to the bounding walls. So far the structure -is quite normal. But, moving a few yards -westwards, we find that the bands begin to curve, -and, instead of following the contact surface, strike -it first obliquely and then at right angles, until -we have the structure shown in <a href="../../66492/66492-h/66492-h.htm#v1fig191">Fig. 191</a>. The -bands here vary from less than an inch to more -than a foot in breadth, and where broadest assume -a slaggy texture. I sought in vain for any evidence of subsequent disturbance -such as might have truncated these parallel rows of vesicles and -pushed the rock bodily over the tuffs. The perfect parallelism of the bands -with the surface of the tuff at the east end, and -the absence of all trace of a thrust-plane at the -base of the sill, seem to show that, though the -rows of vesicles were undoubtedly at first arranged -parallel to the surfaces between which the intrusion -took place, the mass, before completely consolidating -and coming to rest, was ruptured, and a -portion of it was driven onwards at right angles -to its previous line of movement.</p> - -<p>A consideration of the singularly slag-like -structure of the injected masses in the tuffs and -agglomerates leads to the conclusion that though -what we now see of these rocks did not actually -flow out at the sea-bottom in streams of lava, it -was intruded so close to the surface that the -imprisoned vapours had opportunity to expand, as in superficial outflows.<a id="FNanchor_59" href="#Footnote_59" class="fnanchor">[59]</a> -This inference is in accord with that derived from an examination of the -necks, wherein we find evidence of the probable survival of parts of the -actual craters and volcanic cones.</p> - -<div class="footnote"> - -<p><a id="Footnote_59" href="#FNanchor_59" class="label">[59]</a> As illustrative of the occurrence of the vesicular structure in superficial intrusions, I -may again cite the dyke which cuts the ash of the outer crater-wall of the Puy de Pariou -in Auvergne. The andesite of this dyke is in places as vesicular as the lava-stream -with which it was doubtless connected, but the vesicles have been flattened and drawn out -parallel to the walls of the dyke. In this instance it is quite certain that there could never -have been any great depth of detrital material above the fissure into which the material of the -dyke was injected (see <a href="../../66492/66492-h/66492-h.htm#Page_66">vol. i. p. 66</a>).</p> - -</div> - -<p><span class="pagenum" id="Page_32">- 32 -</span></p> - -<p>As the records of the earliest eruptions during the Carboniferous Limestone -period in the district of the Isle of Man are concealed, so also those -of the last of the series lie under the sea. Where the highest visible tuffs -overlie the Poyll Vaaish limestones they show no change in the nature of -the materials ejected, or in the energy of eruption. They lie so abruptly -on the dark calcareous deposits as to show that a considerable pause in -volcanic activity was followed by a violent explosion. The same abundant -grey-green pumice, the same kind of loose crystals of felspar, the same type -of lava-blocks and bombs as had characterized the foregoing eruptions -remained as marked at the end. But the further volcanic records cannot -be perused, and we are left to speculate whether the coast-sections reveal -almost the whole chronicle, or if they merely lay before us the early -chapters of a great volcanic history of which the main records lie buried -under the waves of the Irish Sea.</p> - - -<h3>4. EAST SOMERSET</h3> - -<p>Various limited outcrops of igneous rocks have long been known to -occur in the eastern part of Somerset. The largest of these lies in the -midst of the Old Red Sandstone, on the crest of the axis of the Mendip -Hills, between Downhead and Beacon Hill. Smaller patches occur in -the Carboniferous Limestone near Wrington Warren, on the north side of -Middle Hope, on Worle Hill and at Uphill. These rocks have been mapped -as intrusive, though some of them have been described as conglomeratic -or as volcanic breccias. While some of the masses are probably intrusive, -others appear to be truly contemporaneous with the deposition of the -Carboniferous Limestone. The highly vesicular basalt of Middle Hope -looks much more like a superficial lava than an intrusion. Mr. Aveline -gave a section showing three alternations of limestone and "igneous rock" at -Middle Hope. A recent examination of that coast-line by Mr. A. Strahan -shows that there are undoubted tuffs interstratified with the calcareous -strata. There is thus proof that one or more small volcanic vents were -in eruption on the floor of the Carboniferous Limestone sea in the neighbourhood -of Weston-super-Mare.<a id="FNanchor_60" href="#Footnote_60" class="fnanchor">[60]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_60" href="#FNanchor_60" class="label">[60]</a> See <i>Geological Survey Memoir</i> "On East Somerset," by H. B. Woodward, 1876, and authorities -there cited. Mr. Aveline's section above referred to will be found on <a href="#Page_22">p. 22</a>.</p> - -</div> - - -<h3>5. DEVONSHIRE</h3> - -<p>The change from the typical Old Red Sandstone of South Wales to the -Devonian system of Devonshire, to which I have already referred, is hardly -more striking than the contrast between the Carboniferous formations of -these two areas.<a id="FNanchor_61" href="#Footnote_61" class="fnanchor">[61]</a> The well-marked threefold subdivisions of Carboniferous -Limestone, Millstone Grit and Coal-measures, so persistent throughout -Britain, and nowhere more typically developed than in South Wales, are -<span class="pagenum" id="Page_33">- 33 -</span> -replaced in a distance of less than forty miles by the peculiar "Culm-measures" -of Devonshire—a series of black shales, grey sandstones and -thin limestones and lenticular seams of impure coal (culm), which are -not only singularly unlike in original characters to the ordinary Carboniferous -formations, but have been made still more unlike by the extensive -and severe cleavage to which the Palæozoic rocks of Devon and Cornwall -have been subjected. That these Culm-measures are truly Carboniferous -is made abundantly clear by their fossil contents, though it has not yet -been possible to determine how far they include representatives of the great -stratigraphical subdivisions in other parts of the country.</p> - -<div class="footnote"> - -<p><a id="Footnote_61" href="#FNanchor_61" class="label">[61]</a> In the centre of England numerous outlying areas of igneous rocks are found in the -Carboniferous Limestone, Millstone Grit and Coal-measures. These will be considered by -themselves in Chap. xxxii.</p> - -</div> - -<p>It is to De la Beche that geology owes the first intimation of the -occurrence of interstratified igneous rocks in the Carboniferous series of -Devonshire. As far back as the year 1834, in his singularly suggestive -treatise, <i>Researches in Theoretical Geology</i>, this eminent geologist expressed -his opinion that not only were the "trappean" bands regularly -intercalated in the sedimentary series and continuously traceable with the -general stratification, but that they occurred at various localities in such -a manner as to raise the suspicion that these points may mark some of -the centres of eruption. He particularly cited the example of Brent Tor -as a remarkable volcanic-looking hill, composed in part of a conglomerate -"having every appearance of volcanic cinders."<a id="FNanchor_62" href="#Footnote_62" class="fnanchor">[62]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_62" href="#FNanchor_62" class="label">[62]</a> <i>Op. Cit.</i> <a href="#Page_384">p. 384</a>.</p> - -</div> - -<p>In his subsequently published <i>Report on the Geology of Cornwall, -Devonshire and West Somerset</i>, De la Beche dwelt in more detail on the -results of his study of these rocks, which he had traced out on the ground -and expressed upon the maps of the Ordnance Geological Survey.<a id="FNanchor_63" href="#Footnote_63" class="fnanchor">[63]</a> Hardly -any additions have since been made to our knowledge of the field-relations -of the rocks. It is to the maps and Report of De la Beche that we must -turn for nearly all the published information on the subject. I shall -therefore give here a summary of what can be gathered from these -publications.</p> - -<div class="footnote"> - -<p><a id="Footnote_63" href="#FNanchor_63" class="label">[63]</a> Sheets 22, 23, 24, 25, 30, 31, 32 and 33.</p> - -</div> - -<p>In tracing the limits of the Culm-measures, De la Beche found that -no well-defined line could be drawn between these strata and the "grauwacke" -or Devonian formations underneath. The Carboniferous series lies -in a great trough, of which the axis runs nearly east and west, so that the -lowest members of the series rise along the northern and southern margins. -But De la Beche was struck with one remarkable contrast between the two -opposite sides of the trough—a contrast which marks the Devonian as well -as the Carboniferous formations of this region. On the south side an -abundant and persistent group of intercalated bands of igneous, or as he -called them, "trappean," materials can be followed along the whole line of -boundary, while no such group occurs on the north side. He found these -bands to be lenticular, traceable sometimes for a number of miles, then -dying out and reappearing on the same or other horizons. He mapped -them the whole way from Boscastle on the west to near Exeter on the -east, and found that though the individual sheets might be short, the -<span class="pagenum" id="Page_34">- 34 -</span> -trappean zone was continuous as far as the southern margin of the -Carboniferous series could be seen, except where it had been broken -through by the great granitic mass of Dartmoor. He ascertained that -the intercalated trappean rocks are not confined to the Culm-measures, -but occur also in the contiguous portions of the "grauwacke" or Devonian -system.</p> - -<p>But further, he clearly recognized that the bands of igneous material -which he mapped included both "greenstones," together with other varieties -of massive eruptive rocks, and also volcanic ash or tuff, though he did not -attempt to separate these out upon the maps, but contented himself with -representing them all under the same colour. He admitted that some -doubt might be entertained as to the age of the greenstones, for some of -them might be intrusive and therefore later than the sedimentary deposits -between which they lie. But he contended that there could be no uncertainty -with regard to the trappean ash or tuff, which being regularly -interstratified in the Carboniferous series, must be contemporaneous with it. -He pointed out that many of the greenstones, as well as fragments in the -conglomerates or ashes, were highly vesicular and must originally have been -in the condition of pumice.</p> - -<p>As an illustration of the centres of eruption from which these materials -were ejected, De la Beche drew special attention once more to the conspicuous -eminence of Brent Tor and the rocks in its neighbourhood. His -remarks on this subject are well worthy of being quoted—"The idea that -in the vicinity of Brent Tor a volcano has been in action, producing effects -similar to those produced by active volcanoes, forcibly presents itself. -That this volcano projected ashes, which, falling into adjacent water, -became interstratified with the mud, silt and sand there depositing, seems -probable. That greenstones and other solid trappean rocks constituted the -lavas of that period and locality, here and there intermingled with the ash, -appears also a reasonable hypothesis. Upon the whole there seems as -good evidence as could be expected that to the north and north-west of -Tavistock, ash, cinders and liquid melted rocks were ejected and became -intermingled with mud, silt and sand during this ancient geological epoch, -corresponding with the phenomena exhibited in connection with volcanoes -of the present day, more particularly when they adjoin or are situated in -the sea, or other waters where ejected ashes, cinders and lava can be intermingled -with ordinary mud, silt and sand."<a id="FNanchor_64" href="#Footnote_64" class="fnanchor">[64]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_64" href="#FNanchor_64" class="label">[64]</a> <i>Op. cit.</i> <a href="#Page_122">p. 122</a>.</p> - -</div> - -<p>It remains for some future observer to fill up the outlines thus sketched -by De la Beche, by tracing the respective areas of lavas and tuffs, distinguishing -the various petrographical types, separating the intrusive from the -interstratified sheets, identifying the necks and bosses that may mark -centres of eruption, and expressing these various details upon maps on a -sufficiently large scale.</p> - -<p>A serious difficulty in this research arises from the effect of the profound -alteration which has been produced on the igneous rocks by the cleavage of -<span class="pagenum" id="Page_35">- 35 -</span> -the region. Many of the "greenstones" have been so cleaved as to become -slaty or almost schistose. De la Beche recognized this change and wrote -of the "schistose trappean ash." A result of this metamorphism has been -to impart to rocks originally massive the same fissile structure as the -adjacent slates possess; and in this condition it is often hardly possible to -distinguish between "greenstone" and fine-grained "ash." There can indeed -be little doubt that among these Carboniferous volcanic rocks, as we have -seen to be the case with those of the Devonian system in the same region, -many lavas or sills have been mapped as tuffs.</p> - -<p>The chief additions to our knowledge of the Carboniferous volcanic -group of Devonshire since the time of De la Beche have been made by Mr. -F. Rutley, Mr. W. A. Ussher and General M'Mahon. Mr. Rutley<a id="FNanchor_65" href="#Footnote_65" class="fnanchor">[65]</a> has -endeavoured to trace the respective areas occupied by the different varieties -of volcanic rocks in the district around Brent Tor, near Tavistock, and to -show the probable connection of the successive bands of lavas and tuffs -with a central vent of discharge situated at that hill. He believes that -these bands occur on four different horizons in the sedimentary series. He -has studied the microscopic structure of the rocks, which in his view include -"amphibolites, gabbros, basalts, pitchstones and schistose ashes, or clastic -rocks of a doubtful nature."<a id="FNanchor_66" href="#Footnote_66" class="fnanchor">[66]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_65" href="#FNanchor_65" class="label">[65]</a> "The Eruptive Rocks of Brent Tor and its Neighbourhood," <i>Mem. Geol. Surv.</i> 1878. "On the -Schistose Volcanic Rocks occurring on the west of Dartmoor, with some Notes on the Structure -of the Brent Tor Volcano," <i>Quart. Journ. Geol. Soc.</i> xxxvi. (1880), p. 286.</p> - -<p><a id="Footnote_66" href="#FNanchor_66" class="label">[66]</a> "The Eruptive Rocks of Brent Tor," p. 45.</p> - -</div> - -<p>Mr. Ussher has re-mapped the tract of Culm-measures on the east side of -the Dartmoor granite, besides visiting some of the other areas outside of the -granite mass. While confirming the general accuracy of De la Beche's -survey, he has been able to improve the mapping by inserting more detail, -separating especially the tuffs from the "greenstones." The latter have been -found by him to be mostly dolerites, some of which, from their parallelism -the bands of tuff, may be in his opinion contemporaneous lavas, though -the majority of them are evidently intrusive. The tuffs are regularly interstratified -among the Culm-measures, their most important band in this district -having an average breadth of about 100 yards, and being traceable for -at least two miles, possibly considerably further.<a id="FNanchor_67" href="#Footnote_67" class="fnanchor">[67]</a> In going over this tract -with Mr. Ussher I was led to regard many of the sheets of diabase (dolerite) -or gabbro as true sills and bosses. Most of them occur as short lenticular -or oval patches tolerably numerous, but not traceable for more than a short -distance, though a connection may often exist which cannot be detected by -the scanty evidence on the surface. One sheet which has been followed by -Mr. Ussher from Combe to beyond Ashton, a distance of nearly two miles, -presents in the centre a somewhat coarsely crystalline texture which rapidly -gives way to a much closer grain, and the rock then becomes highly vesicular. -It is overlain with dark Culm-shales and bands of fine shaly tuff, passing -upward into a granular tuff. Some layers of this tuff assume a finely -<span class="pagenum" id="Page_36">- 36 -</span> -foliated appearance by the development of pale leek-green folia, which show -slickensided surfaces parallel with the bedding. The rock then presents one -of the usual appearances of schalstein. This structure seems obviously due -to mechanical movement along the planes of stratification.</p> - -<div class="footnote"> - -<p><a id="Footnote_67" href="#FNanchor_67" class="label">[67]</a> "The British Culm-measures," <i>Proc. Somerset Archæol. and Nat. His. Soc.</i> xxxviii. (1892), -p. 161.</p> - -</div> - -<p>Bands of black chert and cherty shale are interpolated among the tuffs, -which also contain here and there nodular lumps of similar black impure -earthy chert—an interesting association like that alluded to as occurring in -the Carboniferous volcanic series of the Isle of Man, and like the occurrence -of the radiolarian cherts with the Lower Silurian volcanic series already -described.<a id="FNanchor_68" href="#Footnote_68" class="fnanchor">[68]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_68" href="#FNanchor_68" class="label">[68]</a> Cherts containing numerous species of radiolaria have recently been found by Dr. Hinde -and Mr. Howard Fox to form an important part of the Lower Culm-measures of Devonshire, -<i>Quart. Journ. Geol. Soc.</i> vol. li. (1895), p. 609.</p> - -</div> - -<p>The volcanic belt in the valley of the Teign can be followed for about -two miles. It is undoubtedly interstratified among the dark Culm-measures, -which are distinctly seen dipping under and overlying it.</p> - -<p>General M'Mahon has recently shown what may be done by careful -and detailed examination of the ground broadly sketched in by De la Beche. -He chose for study a strip of "greenstone" shown on the Geological Survey -Map to extend for about three and a half miles along the north-west margin of -the Dartmoor granite. He has found that what is represented under one wash -of colour on that map includes both tuffs and lavas. The tuffs, in spite of -the alteration which they appear to have undergone from the proximity of -the great granite mass, are found by microscopic investigation to be made -up of fine volcanic dust containing minute lapilli of various lavas. Sometimes -as many as six or seven different kinds of lava may be represented in -the same microscopic slide. These include felsitic or rhyolitic and trachytic -rocks together with fragments of dark glassy lava full of magnetite dust. -With the tuffs are intercalated sheets of felsite and trachyte. In the -same district coarse volcanic agglomerate occur, made up of blocks of different -lavas and pieces of different sedimentary rocks.<a id="FNanchor_69" href="#Footnote_69" class="fnanchor">[69]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_69" href="#FNanchor_69" class="label">[69]</a> <i>Quart. Journ. Geol. Soc.</i> vol. l. (1894), p. 338.</p> - -</div> - -<p>These observations are of special interest, inasmuch as they point to the -eruption of a much more acid series of volcanic lavas and tuffs than had -previously been known to exist in the Culm-measures. Until the ground -has been more accurately mapped, it is impossible to say whether these -rocks are older or younger than those that lie around Brent Tor, a few -miles to the south-west. General M'Mahon has noted the presence of -more basic eruptive rocks in the same district. He specially cites the -occurrence of mica-diorite, of basaltic lavas altered into a serpentinous mass, -and of a dolerite which may possibly mark the actual vent of the old Brent -Tor volcano. His observations on the influence of the Dartmoor granite in -inducing new mineral rearrangements in the igneous rocks of the Culm-measure -series are full of interest.</p> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_37">- 37 -</span></p> - -<h2 class="nobreak" id="CHAPTER_XXX">CHAPTER XXX<br /> - -<span class="smaller">THE CARBONIFEROUS VOLCANOES OF IRELAND</span></h2> -</div> - -<div class="blockquot"> - -<p>King's County—The Limerick Basin—The Volcanic Breccias of Doubtful Age in -County Cork.</p> -</div> - - -<p>Although the Carboniferous system spreads over by far the larger part of -the surface of Ireland, and is laid bare in many thousands of natural and -artificial sections, it displays undoubtedly contemporaneous igneous rocks, so -far as at present known, at only one locality—the region around Limerick. -A second district, however, lies in King's County, where some vents occur -which may be of Carboniferous age, and of which a description will be given -in the following pages. That the relics of volcanic action should be so few, -while the exposures of the Carboniferous formations are so numerous and -so completely disclose the geological history of the whole system, must be -regarded as good evidence that while volcanoes abounded and continued -long active in Scotland and in parts of the Centre and South-west of England, -they hardly appeared at all in Ireland. It is worthy of remark, also, that -the Irish eruptions belong to the time of the Carboniferous Limestone—a -period distinguished by volcanic activity in Scotland and England—that -the nature of the materials erupted bears a close resemblance to that of the -lavas and tuffs of the sister island, and that the manner of their eruption -finds a close counterpart in the Puy-eruptions, already described.</p> - - -<h3>1. KING'S COUNTY</h3> - -<p>In the progress of the Geological Survey several small tracts of "greenstone -ash" and "greenstone" were mapped within an area of a few square -miles lying to the north of Philipstown. These igneous rocks were shown to -form Croghan Hill, which, rising into a conical eminence 769 feet above the -sea, and some 450 feet above the general level of the great limestone plain -around it, forms the only conspicuous feature in the landscape for many -miles. In the maps and their accompanying Explanations, the "greenstones" -are treated as intrusive masses, but the "greenstone ash" or breccia appears -to have been regarded as interstratified in the Carboniferous Limestone, -<span class="pagenum" id="Page_38">- 38 -</span> -though the admission is made that "from the scanty exposures of the rocks -and the total absence of any connected section, it has been found impossible -to arrive at any definite conclusion as to the relations existing between -these traps and ashes with regard to each other or to the surrounding -limestone."<a id="FNanchor_70" href="#Footnote_70" class="fnanchor">[70]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_70" href="#FNanchor_70" class="label">[70]</a> See Sheets 109 and 110 of the Geological Survey of Ireland and Explanation to accompany -Sheets 98, 99, 108 and 109, by F. J. Foote and J. O'Kelly (1865), pp. 7-18.</p> - -</div> - -<p>In the course of a brief visit to this locality I did not succeed in -obtaining any certain proof of the age of the igneous rocks, but I found -their structures to be more varied and interesting than would be inferred -from the way in which they have been mapped, and I came to the conclusion -that the strong balance of probability was in favour of regarding -them as of the age of the Carboniferous Limestone.</p> - -<div class="figcenter" id="v2fig192" style="width: 430px;"> - <img src="images/v2fig192.png" width="430" height="173" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 192.</span>—Croghan Hill, King's County, from S.S.W.</div> -</div> - -<p>The first and most important fact to be announced regarding the -district is that it includes a group of volcanic necks which rise through the -Carboniferous Limestones. The chief of these forms Croghan Hill. It is -nearly circular in ground-plan, and measures about 4000 feet in diameter -from the limestone on one side to that on the other. It rises with steep -grassy slopes out of the plain, the naked rock projecting here and there in -crags and low cliffs. Its general outward resemblance to the Carboniferous -necks of Scotland strikes the eye of the geologist as he approaches it -(<a href="#v2fig192">Fig. 192</a>).</p> - -<p>But Croghan Hill, though the chief, is not the only vent of the district. -It forms the centre round which a group of subsidiary vents has been -opened. These form smaller and lower eminences, the most distant being -one and a half miles E.S.E. from the summit of Croghan Hill, and measuring -approximately 1200 feet in its longest and 800 feet in its shortest diameter.</p> - -<p>That the igneous materials of these necks really break through the -limestones may be clearly seen in several sections. Thus by the roadside -at Gorteen, on the south-western side of Croghan Hill, the limestones have -been thrown into a highly inclined position, dipping towards the east at -60° or more, and their truncated ends abut against the side of the neck. -Again, on the eastern side of the same hill the limestones have been much -<span class="pagenum" id="Page_39">- 39 -</span> -disturbed close to the margin of the neck, sometimes dipping towards the -volcanic centre, and sometimes striking at it. Among these strata a small -neck of breccia, of which only a few square yards are visible, rises close to -the edge of the bog that covers the adjacent part of the great plain.</p> - -<p>The material which chiefly forms these necks is one of the most -remarkable breccias anywhere to be found in the volcanic records of the -British Isles. The first feature noticeable in it is the pumiceous character -of its component fragments. These consist of a pale bluish-grey basic -pumice, and are generally about the size of a hazel-nut, but descend to -mere microscopic dust, while sometimes exceeding a foot in length. They -are angular, subangular and rounded. Occasionally they stand out as -hollow shells on weathered surfaces, and in one instance I noted that the -vesicles were flattened and drawn out parallel to the surfaces of the shell, -as if deformed by gyration, like a true bomb.</p> - -<p>The breccia remains singularly uniform in character throughout all -the necks. Its basic pumice presents much resemblance to that so characteristic -of the Carboniferous necks of Scotland, Derbyshire and the Isle of -Man. The abundant vesicles are generally spherical, and as they have -been filled with calcite or chlorite, they look like small seeds scattered -through a grey paste. Though I broke hundreds of the lapilli, I did -not notice among them any volcanic rock other than this pumice. I am -not aware of any other neck so homogeneously filled up with one type of -pyroclastic material, and certainly there is no other example known in the -British Isles of so large and uniform a mass of fragmentary pumice.</p> - -<p>Limestone fragments are not uncommon in this breccia. They resemble -the strata around the vents. Pieces of the adjacent cherts may -also be observed. In one or two cases, the limestone fragments were found -by me to have an exceptionally crystalline texture, which may possibly -indicate a certain degree of marmarosis, but on the whole there is little -trace of alteration.</p> - -<p>The fragments of pumice in the breccia are bound together by a cement of -calcite. In fact the rock is, so to speak, saturated with calcareous material, -which, besides filling up the interstices between the lapilli, has permeated -the pumice and filled up such of its vesicles as are not occupied by some -chloritic infiltration.</p> - -<p>I did not observe unmistakable evidence that any part of the breccia -is stratified and intercalated among the limestones, nor any vestige of -ashy material in these limestones. But it is possible that traces of such -interstratification may occur in the low ground to the north-west of -Croghan Hill, which I did not examine.</p> - -<p>In only two places did I notice even a semblance of the intercalation -of limestone in the breccia. One of these is at Gorteen, where a band of -limestone strata a few feet thick is underlain and overlain by breccia. But -though the superposition of the layers of finely stratified dark limestone and -chert on the breccia is well seen and thoroughly defined, no lapilli or ashy -material are to be seen in the limestone. Detached pieces of similar limestone -<span class="pagenum" id="Page_40">- 40 -</span> -and chert occur in the breccia. The band of stratified rock, if <i>in -situ</i>, may be a tongue projecting from the wall into the body of the neck, -like some instances already cited from Scotland, but more probably it is -really a large included mass lying within the vent itself. The breccia here -as elsewhere is entirely without any trace of stratification. The second -locality occurs at the most easterly neck north of Coole House, where the -limestones, rapidly undulating, seem at last to plunge below the breccia, -which shows a series of parallel divisional planes suggestive of bedding. -But these may be only joint-structures, for there is no stratification of the -component materials of the rock.</p> - -<p>In the necks, and also through the limestone surrounding them, masses -of eruptive rock have been intruded as irregular bosses and veins. The -material of these intrusions presents little variety, and, so far as I could -note, gives no indication of the successive protrusion of progressively -different lava. It varies from a deep blue-black fine-grained basalt to a -dolerite where the plagioclase is distinct. Some portions, however, are -more basic and pass into limburgite. Externally there is nothing worthy -of special remark in these rocks unless it be their prevalent amygdaloidal -structure. The amygdales, generally of calcite, vary from small pea-like -forms in the basalts up to kernels half an inch long or more in the dolerites. -From a microscopic examination Mr. Watts found that some of the basalts -have a base of felspar and augite rich in brown mica, and that their porphyritic -felspars enclose idiomorphic crystals of augite.</p> - -<p>Perhaps the most noticeable feature in these later parts of the volcanic -series is the occurrence in them at one locality in Croghan Demesne of -lumps of a highly crystalline material quite distinct from the surrounding -rock. These enclosures vary from an inch or two to a foot or more in -diameter. They must be regarded as blocks which have been carried up -in the ascent of the basic lava. Their composition has been ascertained by -Mr. Watts from microscopic examination to be somewhat singular. One -specimen "contains relics of garnets, surrounded by rings of kelyphite, -imbedded in a mosaic of felspar, with a mineral which may possibly be -idocrase." Another specimen from the same locality (south-east from Gorteen) -"contains the relics of garnets preserved as kelyphite, set in a matrix -of quartz-grains, much strained, and containing a profusion of crystals of -greenish-yellow or red sillimanite. This appears to be a metamorphic -rock, and may be a fragment of some sediment enclosed in the igneous -rocks."<a id="FNanchor_71" href="#Footnote_71" class="fnanchor">[71]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_71" href="#FNanchor_71" class="label">[71]</a> <i>Guide to the Collections of Rocks and Fossils belonging to the Geological Survey, in the Museum -of Science and Art, Dublin</i> (1895), pp. 38, 39.</p> - -</div> - -<p>As regards the history of volcanic action in Britain one of the chief -points of interest connected with these Irish breccias and lavas relates -to their geological age. As no proof has been produced that any portion -of them is contemporaneously interstratified in the Carboniferous Limestone -which surrounds them, we cannot definitely affirm that the volcanic -eruptions which they record took place during the accumulation of that -<span class="pagenum" id="Page_41">- 41 -</span> -formation. The vents must, of course, be later than that portion of the -limestone which they pierce. But the evidence seems to me to be on -the whole most favourable to the view that they are of Carboniferous -Limestone age, for the following reasons:—</p> - -<p>1. The breccias of Croghan Hill do not present a resemblance to any -of those belonging to the Tertiary volcanic series in Antrim or the Inner -Hebrides. The possibility of their being of Tertiary age may therefore be -dismissed from consideration.</p> - -<p>2. There are no known Permian volcanic rocks in Ireland. Nor does -the Croghan Hill breccia show any resemblance to the ordinary material of -the breccias in the Permian necks of Scotland. It is thus not likely to be -of Permian age.</p> - -<p>3. The peculiar basic pumice of these Croghan Hill vents has many -points in common with the palagonite fragments so abundant among the -volcanic breccias and tuffs of Carboniferous age in Scotland, Derbyshire, -and the Isle of Man, and which occurs also among the Carboniferous tuffs -of the Limerick basin. It differs from the general type of the material in -its pale colour, in its uniformity of character, in its calcareous cement, and -above all in its vast preponderance over all the other materials in the -breccia.</p> - -<p>4. The saturation of the Croghan Hill breccia with calcite is a singular -feature in the composition of the rock. Had the vents been opened long -subsequent to the deposition of the Carboniferous Limestone, it is difficult -to understand how this calcite could have been introduced. Mere percolation -of meteoric water from the adjacent limestone does not seem -adequate to account for the scale and thoroughness of the permeation. -But if the vents were opened on the floor of the Carboniferous Limestone -sea, it is intelligible that much fine calcareous silt should have found its -way down among the interstices of the breccia and into the pores of the -pumice which, being caked together within the vent, did not all float away -when the sea gained access to the volcanic funnel. The effect of subsequent -percolation would doubtless be to carry the lime into still unfilled -crevices, and to impart to the cement a crystalline structure similar to -that which has been developed in the ordinary limestones.</p> - - -<h3>2. THE LIMERICK BASIN</h3> - -<p>About 70 miles to the south-west of the area just described lies the most -compact, and, for its size, one of the most varied and complete, of all the -Carboniferous volcanic districts of Britain (Map I.). It takes the form of an -oval basin in the Carboniferous Limestone series near the town of Limerick, -about twelve miles long from east to west and six miles broad from north to -south. Round this basin the volcanic rocks extend as a rim about a mile -broad. A portion of a second or inner rim, marking a second and higher -volcanic group, partially encloses a patch of Millstone Grit or Coal-measures, -which lies in the heart of the limestone basin. (See the section in <a href="#v2fig196">Fig. 196</a>.)</p> - -<p><span class="pagenum" id="Page_42">- 42 -</span></p> - -<p>But it is evident that, as the denuded edges of the volcanic sheets emerge -at the surface all round the basin, the present area over which these rocks -extend must be considerably less than that which they originally covered. -Some indication of their greater extension is supplied by outliers of the -bedded lavas and tuffs, as well as by bosses which doubtless indicate the -position of some of the eruptive vents. The distance between the furthest -remaining patches is 24 miles. The original tract over which the volcanic -materials were spread cannot have been less than 24 miles long by 10 -miles broad. If we assume its area to have been between 250 and 300 -square miles we shall probably be under the truth.</p> - -<p>This volcanic centre made its appearance on the floor of the Carboniferous -Sea in the same district which had witnessed the eruptions of Upper Old Red -Sandstone time. The two visible vents that crown the Knockfeerina and -Ballinleeny anticlines (Chapter xxii.), are only some ten miles distant, and -there may be others of the same age even under the Limerick basin. This -district thus supplies another instance of that recurrence of volcanic energy -in the same area, after a longer or shorter geological interval, which stands -out as a conspicuous feature in the history of volcanic action in Britain. -That a prolonged interval elapsed between the extinction of the Old Red -Sandstone volcanoes and the outbreak of their successors during the -accumulation of the Carboniferous limestone series, may be inferred from -the thickness of strata which separate their respective tuffs. From the -published sections of the Geological Survey there would appear to be about -500 feet of Old Red Sandstone above the volcanic series of that formation. -Then comes the Lower Limestone shale, which is computed to be about the -same thickness. From the scarcity of observable dip among the Lower -Limestones and their variable inclination, it is not easy to form any -satisfactory estimate of the depth of this group up to the base of the -volcanic series. It may be as much as 800 feet,<a id="FNanchor_72" href="#Footnote_72" class="fnanchor">[72]</a> and if so there would -thus intervene a mass of sedimentary material nearly 2000 feet in -thickness between the two volcanic platforms. Throughout this thick -accumulation of stratified deposits no trace of contemporaneous volcanic -activity has been detected. From the descriptions published more than -thirty years ago by Jukes and his colleagues in the Geological Survey of -Ireland, geologists learnt how full and interesting are the proofs of great -volcanic activity contemporaneous with the deposition of the Carboniferous -Limestone series in the Limerick district.<a id="FNanchor_73" href="#Footnote_73" class="fnanchor">[73]</a> Nowhere, indeed, is the evidence -<span class="pagenum" id="Page_43">- 43 -</span> -more complete for the occurrence of a long succession of volcanic eruptions -during a definite period of geological time. The officers of the Survey -showed that two epochs of activity during the older part of the Carboniferous -period were each marked by a group of tuffs and lavas, while -the interval of quiescence between them is represented by a thousand feet -of limestone. The same observers likewise mapped outside the volcanic -ring a number of eruptive bosses, which they regarded as probably marking -some of the actual vents of that time.</p> - -<div class="footnote"> - -<p><a id="Footnote_72" href="#FNanchor_72" class="label">[72]</a> This is the thickness given in the Explanation to Sheet 144 of the Geological Survey of -Ireland, p. 8. A still greater thickness is claimed in Explanation to Sheet 154, p. 8.</p> - -<p><a id="Footnote_73" href="#FNanchor_73" class="label">[73]</a> See especially Explanations of Sheets 143, 144, 153 and 154, Geol. Surv. Ireland (1860, -1861). The geology of the district had been previously noticed by earlier observers, to whose -writings reference is made on p. 26 of the Explanation of Sheet 144. See also Jas. Apjohn, -<i>Journ. Geol. Soc. Dublin</i>, vol. i. (1832), p. 24; Prof. Hull, <i>Geol. Mag. for 1874</i>, p. 205. Jukes -(<i>Student's Manual of Geology</i>, 2nd edit. 1862, p. 325) gave subsequently an excellent epitome -of the volcanic history. The microscopic structure of some of the Limerick volcanic rocks has -been described by Mr. Allport, <i>Quart. Journ. Geol. Soc.</i> vol. xxx. (1874), p. 552, and by Prof. -Hull, <i>Geol. Mag. for 1873</i>, p. 153. See also Mr. Watts' account of these rocks in the <i>Guide to -the Collections of Rocks and Fossils</i> (Dublin, 1895), p. 93.</p> - -</div> - -<p>The lower volcanic group, which forms a complete ring round the -Upper Limestones of the Limerick basin, is estimated to reach a thickness of -1000 feet in some parts of its course.<a id="FNanchor_74" href="#Footnote_74" class="fnanchor">[74]</a> Its base appears to coincide -generally with the upward termination of the Lower Limestone group of -this district, though here and there small patches of volcanic rocks in that -group have been regarded as interstratified and contemporaneous bands.<a id="FNanchor_75" href="#Footnote_75" class="fnanchor">[75]</a> -It consists of a series of lavas and tuffs, the alternations and rapid incoming -and dying out of which were well made out by the Geological Survey.</p> - -<div class="footnote"> - -<p><a id="Footnote_74" href="#FNanchor_74" class="label">[74]</a> Explanation of Sheet 144, p. 27.</p> - -<p><a id="Footnote_75" href="#FNanchor_75" class="label">[75]</a> Some of them, however, have characters that rather seem to place them with the intrusive -materials of the district, and therefore not necessarily earlier than the bedded lavas and tuffs. -The boundary line of the volcanic series is not consistently followed along the same horizon on -the Survey maps. Thus to the east of Caherconlish, a strip of the Upper Limestone is inserted -below the base of the tuffs for a distance of about four miles. Unless a different horizon has -been in some places taken for the boundary between the two groups of limestones, it would -appear that the eruptions had not extended over the north and north-east of the district until -some time after the deposition of the Upper Limestone had begun. The division between the -two limestone groups is taken at a set of chert-bands, but as these are not constant it is sometimes -difficult to draw a satisfactory line of division.</p> - -</div> - -<p><i>Tuffs.</i>—The base of the volcanic series is generally formed by a band of -tuff sometimes as much as 350 feet thick,<a id="FNanchor_76" href="#Footnote_76" class="fnanchor">[76]</a> which may be traced nearly -continuously round the basin as well as in detached outliers even as far as -Carrigogunnel overlooking the alluvial plain of the Shannon. The manner -in which the bottom of this tuff is interstratified with the limestone below -it may be instructively examined in many quarries around the town of -Limerick. Striking evidence is there supplied that the first eruptions were -comparatively feeble and spasmodic, and were separated by intervals of -longer and shorter duration, during which the limestone with its fragmentary -organisms was deposited, little or no volcanic detritus falling at that -time. Yet even in some of the limestones the microscope reveals fine -broken needles of felspar, representing doubtless the finest ejected dust.<a id="FNanchor_77" href="#Footnote_77" class="fnanchor">[77]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_76" href="#FNanchor_76" class="label">[76]</a> Explanation of Sheet 154, p. 21.</p> - -<p><a id="Footnote_77" href="#FNanchor_77" class="label">[77]</a> For the details of the microscopic structure of the Limerick volcanic rocks I am mainly -indebted to the examination of them made for me by my Survey colleague, Mr. W. W. Watts.</p> - -</div> - -<p>As an illustration of the way in which the volcanic and organic detritus -alternated over the sea-floor, the following section from a quarry in the -townland of Loch Gur on the southern side of the basin is here given:<a id="FNanchor_78" href="#Footnote_78" class="fnanchor">[78]</a>—</p> - -<div class="footnote"> - -<p><a id="Footnote_78" href="#FNanchor_78" class="label">[78]</a> Explanation of Sheet 154, pp. 21, 22.</p> - -</div> - -<p><span class="pagenum" id="Page_44">- 44 -</span></p> - -<table summary="data"> -<tr> - <td class="tdl">Cherty limestone more than</td> - <td class="tdr">20</td> - <td>feet</td> - <td class="tdr">0</td> - <td>in.</td> -</tr><tr> - <td class="tdl">Decomposed green tuff</td> - <td class="tdr">2</td> - <td class="tdc">"</td> - <td class="tdr">6</td> - <td class="tdc">"</td> -</tr><tr> - <td class="tdl">Bluish-green, calcareous laminated tuff</td> - <td class="tdr">4</td> - <td class="tdc">"</td> - <td class="tdr">0</td> - <td class="tdc">"</td> -</tr><tr> - <td class="tdl">Limestone, slightly ashy</td> - <td class="tdr">1</td> - <td class="tdc">"</td> - <td class="tdr">8</td> - <td class="tdc">"</td> -</tr><tr> - <td class="tdl">Green tuff</td> - <td class="tdr">0</td> - <td class="tdc">"</td> - <td class="tdr">2</td> - <td class="tdc">"</td> -</tr><tr> - <td class="tdl">Fine-grained decomposed tuff</td> - <td class="tdr">0</td> - <td class="tdc">"</td> - <td class="tdr">4</td> - <td class="tdc">"</td> -</tr><tr> - <td class="tdl">Green tuff, obliquely laminated</td> - <td class="tdr">1</td> - <td class="tdc">"</td> - <td class="tdr">7</td> - <td class="tdc">"</td> -</tr><tr> - <td class="tdl">Fine laminated tuff</td> - <td class="tdr">0</td> - <td class="tdc">"</td> - <td class="tdr">8</td> - <td class="tdc">"</td> -</tr><tr> - <td class="tdl">Green compact tuff</td> - <td class="tdr">1</td> - <td class="tdc">"</td> - <td class="tdr">8</td> - <td class="tdc">"</td> -</tr><tr> - <td class="tdl">Obliquely laminated shaly tuff</td> - <td class="tdr">0</td> - <td class="tdc">"</td> - <td class="tdr">10</td> - <td class="tdc">"</td> -</tr><tr> - <td class="tdl">Concretionary ashy limestone</td> - <td class="tdr">1</td> - <td class="tdc">"</td> - <td class="tdr">4</td> - <td class="tdc">"</td> -</tr><tr> - <td class="tdl">Compact ashy limestone</td> - <td class="tdr">2</td> - <td class="tdc">"</td> - <td class="tdr">0</td> - <td class="tdc">"</td> -</tr><tr> - <td class="tdl">Green shaly tuff, much weathered</td> - <td class="tdr">0</td> - <td class="tdc">"</td> - <td class="tdr">5</td> - <td class="tdc">"</td> -</tr><tr> - <td class="tdl">Ashy limestone</td> - <td class="tdr">0</td> - <td class="tdc">"</td> - <td class="tdr">7</td> - <td class="tdc">"</td> -</tr><tr> - <td class="tdl">Compact green tuff more than</td> - <td class="bdb tdr">4</td> - <td class="bdb tdc">"</td> - <td class="bdb tdr">0</td> - <td class="bdb tdc">"</td> -</tr><tr> - <td></td> - <td class="tdr">41</td> - <td>feet</td> - <td class="tdr">9</td> - <td>in.</td> -</tr> -</table> - -<p>The tuffs which in the southern part of the basin underlie the less basic -lavas differ in some respects from those which further north are associated -with the Upper Limestones. They are green, sometimes dull purplish-red, -finely granular rocks, made up in large part of andesitic debris. They are -full of loose felspar crystals, minute, somewhat rounded and subangular -lapilli of andesite or some less basic lava, together with bits of grit and -baked shale. Though generally much decomposed, they are sometimes -compact enough to be used for building-stone. Under the microscope these -tuffs are seen to abound in andesite-lapilli, with a few pieces of felsitic rocks -enclosed in an opaque base, through which are scattered broken felspars and -occasional vesicular lapilli.</p> - -<div class="figright" id="v2fig193" style="width: 114px;"> - <img src="images/v2fig193.png" width="114" height="215" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 193.</span>—Section in - quarry on roadside east - of Limerick close to - viaduct of the Limerick - and Erris Railway.<br /><br /> - 1. Limestone; 2. Calcareous - tuff; 3. Ashy limestone - or calcareous tuff.</div> -</div> - - -<p>The tuffs around Limerick, interbedded with the -Black (Upper) Limestone, are distinguished by a scarcity -of andesitic debris, by their persistent dull greenish-grey -colour, and more particularly by the abundance of minute -lapilli and larger fragments of an epidote-green, finely -vesicular, easily sectile basic pumice. Under the microscope -much of this material is found to be an altered -basic glass of the nature of palagonite. These tuffs are -in evident relation with the more basic lavas that -accompany them. The manner in which they alternate -with the black limestone shows that the conditions -for the eruption of this more basic detritus continued -to be very similar to those that existed when the andesitic -tuffs were ejected. As a good illustration of this -feature the accompanying section (<a href="#v2fig193">Fig. 193</a>) is given -from a quarry on the side of the high-road between -Limerick and Annacotty. The total depth of strata -here represented is about 15 feet. The black limestone -at the bottom is a tolerably pure calcareous rock. It -is divided into bands by thin partings of a fine greenish -calcareous tuff, each marking a brief discharge of ashes from some neighbouring -<span class="pagenum" id="Page_45">- 45 -</span> -vent. Half-way up the succession of strata, the ashy material -rapidly increases until it usurps the place of the limestone, though its calcareous -composition shows that the accumulation of calcareous sediment -had not been entirely suspended during the eruption of ash.</p> - -<p>Among these tuffs I have noticed fragments of fine, dark, flinty felsite, -grit and other rocks. The stones are for the most part small, but vary up -to blocks occasionally a foot in diameter.</p> - -<p><i>Lavas.</i>—The lavas occur in numerous sheets, sometimes separated by thin -partings or thicker beds of tuff and volcanic conglomerate. On the northern -rim of the basin Mr. G. H. Kinahan has described the volcanic series east -of Shehan's Cross-roads as composed of six zones of tuff, each bed varying -from about 50 to 250 feet in thickness, alternating with as many sheets of -lava ranging from 27 to 180 feet in thickness, the total depth of tuff being -estimated at nearly 500 feet and that of the lavas at about 800 feet.<a id="FNanchor_79" href="#Footnote_79" class="fnanchor">[79]</a> -Some of these tuffs are coarse conglomerates or agglomerates, with blocks of -lava occasionally 10 feet long.</p> - -<div class="footnote"> - -<p><a id="Footnote_79" href="#FNanchor_79" class="label">[79]</a> Explanation of Sheet 144, p. 28.</p> - -</div> - -<p>Some of the lavas in the lower volcanic group are andesites quite like -those of the plateau series in the Carboniferous system of Scotland. -Externally they appear as dull reddish-brown or purplish-red compact rocks, -with abundant porphyritic felspars scattered through the fine-grained base. -They are generally much decomposed, showing on a fresh fracture pseudomorphs -of chlorite, hæmatite and calcite after some of the minerals, with -abundant hæmatitic staining through the body of the rock. Amygdaloidal -structure is commonly developed.</p> - -<p>These andesites, when examined microscopically, were found by Mr. -Watts to present the characteristic base of minute felspar-laths with -magnetite and enstatite, and with porphyritic crystals, often large, of zoned -plagioclase, as well as of ilmenite and hæmatite.</p> - -<div class="figcenter" id="v2fig194" style="width: 336px;"> - <img src="images/v2fig194.png" width="336" height="91" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 194.</span>—Section of the volcanic escarpment, - east of Shehan's Cross-roads, south of Limerick.<br /><br /> - 1. Limestone; 2 2. Tuffs; 3 3. Lavas.</div> -</div> - -<p>But besides the andesites there occur also, and, so far as I have observed, -in larger number, sheets of true basalt. This rock is typically black, -exceedingly close-grained in the central portion of each sheet, but becoming -highly slaggy and vesicular along the upper and lower parts. Under the -microscope it is found to contain granular augite and magnetite, set in -a more or less devitrified glass, with microlites of felspar, porphyritic -plagioclase, serpentinized olivine, and some well-marked augite. These -rocks form distinct escarpments along the northern rim of the basin as in -<span class="pagenum" id="Page_46">- 46 -</span> -the foregoing section east from Shehan's Cross-roads (<a href="#v2fig194">Fig. 194</a>). From the -summit of this ridge, which is about 600 feet above the sea, the eye looks -northward over the plain, across which low outliers of the volcanic series -are scattered, and southwards across the basin to the corresponding line of -volcanic heights forming the southern rim.</p> - -<p>The upper volcanic group has been estimated by the officers of the -Geological Survey to lie about 1000 feet higher in the Carboniferous -system than the lower, the intervening strata consisting of the Upper -Limestone.<a id="FNanchor_80" href="#Footnote_80" class="fnanchor">[80]</a> It is possible that the interval is greater in some parts of the -district than in others, and if so, the difference may be due either to greater -local accumulation of volcanic materials, or to local prolongation of the -eruptions into higher stratigraphical horizons. The outcrop of the upper -volcanic band forms about half of a ring round the little cup of Millstone -Grit or Coal-measures which lies within the volcanic basin. On the north-west -side of the cup the volcanic rocks disappear. Hence the upper band -has a much more restricted area than the lower. But if the tuffs -immediately around Limerick are assigned to the upper group, its extent -will be proportionately increased. There can be little doubt, however, that -neither in thickness nor in superficial area did the lavas and tuffs of the -second group equal those of the first. The volcanic energy was gradually -dying out.</p> - -<div class="footnote"> - -<p><a id="Footnote_80" href="#FNanchor_80" class="label">[80]</a> Explanation of Sheet 154, p. 24.</p> - -</div> - -<p>The lavas of the second period are characteristic dull, black, compact -basalts, like those of the first period, becoming here and there strongly -amygdaloidal, and being occasionally separated by slaggy or conglomeratic -partings. But they include also certain rocks wherein the felspar diminishes -in quantity, while augite and olivine become conspicuous, together with a -little enstatite. The augite occurs in large porphyritic forms, as well as of -medium size and in small prisms. The olivine, as usual, is now in the -condition of serpentine. These rocks are more basic than the ordinary -basalts, containing only 38·66 per cent of silica, and thus approaching the -limburgites. With these basic lavas are associated dull green tuffs and -conglomerates, made up largely of basalt-debris, together with abundant -pieces of finely vesicular basic pumice and lapilli of a palagonitic material.</p> - -<p>The manner in which the lavas and tuffs have alternated with each -other, and also with the limestones, is well seen on Nicker Hill above Pallas -Grean.<a id="FNanchor_81" href="#Footnote_81" class="fnanchor">[81]</a> The Survey sections show eight sheets of lava, separated by six -bands of tuff and eight intercalations of limestone, the whole passing under -the Coal-measures.</p> - -<div class="footnote"> - -<p><a id="Footnote_81" href="#FNanchor_81" class="label">[81]</a> See Explanation of Sheet 144, p. 30, where a description with detailed map and sections of -this ground will be found.</p> - -</div> - -<p>The upper volcanic group may be as much as 600 or 800 feet thick. -It appears to have been left, at the close of the eruptions, with a very -uneven surface, some portions being so low as to be overspread with the -Upper Limestones, other parts so high as not to be covered until the Coal-measure -shales and flagstones came to be deposited.<a id="FNanchor_82" href="#Footnote_82" class="fnanchor">[82]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_82" href="#FNanchor_82" class="label">[82]</a> Explanation of Sheet 154, pp. 24, 35.</p> - -</div> - -<p><span class="pagenum" id="Page_47">- 47 -</span></p> - -<p><i>Vents.</i>—All round the edges of the Limerick basin, where the escarpments -of the volcanic groups, rising abruptly above the plain, show that these rocks -once extended beyond their present limits, the progress of denudation has revealed -a number of bosses which, as above stated, Jukes and his associates -looked upon as marking some of the vents from which the lavas and tuffs were -erupted. Especially striking is the line of these vents along the southern -margin. The rocks now filling them present some unusual and rather -anomalous features. They are decidedly more acid than the lavas of the -basin, some of them even containing free quartz. Mr. Watts remarks that -"though they have a good deal in common with the trachytes, they are -crystalline throughout. They are red granite-looking rocks, which are made -up chiefly of stumpy idiomorphic prisms of felspar which is mainly orthoclase. -Some plagioclase also occurs, and the two felspars are imbedded in -interstitial quartz. A trace of hornblende or mica is frequently present, -and the rocks contain about 65 per cent of silica." These characters are -specially observable in the necks furthest removed from the basin, which -may possibly have been connected with the andesitic outflows. Nearer to -the basin the necks "contain about 60 per cent of silica, seldom show any -interstitial quartz, and stand between trachytes and porphyrites, some -perhaps being bostonites."<a id="FNanchor_83" href="#Footnote_83" class="fnanchor">[83]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_83" href="#FNanchor_83" class="label">[83]</a> <i>Guide to the Collections of Rocks, etc., Geol. Survey, Ireland</i>, p. 93, Dublin 1895.</p> - -</div> - -<div class="figcenter" id="v2fig195" style="width: 488px;"> - <img src="images/v2fig195.png" width="488" height="311" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 195.</span>—View of Derk Hill, a volcanic neck on the south side of the Limerick basin.</div> -</div> - -<p>A geologist, familiar with the Carboniferous and Permian necks of -Scotland, has no hesitation in confirming the surmise of Jukes and his -colleagues that the cones and domes around the Limerick basin mark -the sites of eruptive vents. On the south side of the basin, at least nine -such necks rise into view, partly from among the lavas and tuffs, but -<span class="pagenum" id="Page_48">- 48 -</span> -chiefly through the limestones that emerge from -below these volcanic sheets. One of the most conspicuous -of them, Derk Hill (<a href="#v2fig195">Fig. 195</a>), rises to a -height of 781 feet above the sea, and comes through -the bedded andesites, as represented in <a href="#v2fig196">Fig. 196</a>, -which gives, in diagrammatic form, the general -structure of the Limerick volcanic basin. Around -the northern side of the basin a smaller number of -necks has been observed, consisting of similar acid -rocks.</p> - -<div class="figcenter" id="v2fig196" style="width: 756px;"> - <img src="images/v2fig196.png" width="756" height="97" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 196.</span>—Section across the Limerick volcanic basin.<br /><br /> - - 1. Lower limestone; 2. Lower series of lavas and tuffs; 3. Middle and Upper Limestone; 4. Upper series of lavas and tuffs; 5 5. Two volcanic necks; 6. Millstone Grit series.</div> -</div> - - -<p>A few of the necks appear to be filled with -volcanic agglomerate. Here and there detached -patches of fragmental volcanic material have been -shown on the Survey maps, and referred to in the -Explanations, as if they were outliers of the bedded -tuffs; though in some cases the coarseness of their -materials and the want of any distinct bedding, -together with the absence of any indication of their -relation to the nearest limestones, have evidently -offered considerable difficulty in their mapping. -One of the best examples occurs about two miles -to the south-east of the village of Oola. The -boundaries of this patch, as put on the map, are -confessed to be "entirely speculative." It was only -seen on the side of the railway where it appeared -as "a very coarse brecciated purple ash."<a id="FNanchor_84" href="#Footnote_84" class="fnanchor">[84]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_84" href="#FNanchor_84" class="label">[84]</a> Explanation of Sheet 154, p. 25.</p> - -</div> - -<p>On comparing the maps of the Limerick basin -with those of the Carboniferous districts of Scotland, -the main difference will probably be acknowledged -to be the absence of any recognizable sills in the -Irish ground. That no sills actually occur, I am -not prepared to affirm. Indeed some of the more -acid rocks, both outside the basin and among the -rocks of the older volcanic group, appeared to me -during my traverses of the ground to have much of -the character of sills. A more critical examination -of the area would not improbably detect some truly -intrusive sheets which have hitherto been mapped -among the interstratified lavas. Some appear to -exist among the surrounding Lower Limestones.</p> - -<p>An intrusive mass, like a sill or dyke, is represented -on the Geological Survey Map as traversing -the Coal-measures in the inner basin south of -Ballybrood. But as the strata are on end along -its southern margin, it may possibly be only a -<span class="pagenum" id="Page_49">- 49 -</span> -portion of the upper volcanic series which has been thrown into its present -position by one or more faults.<a id="FNanchor_85" href="#Footnote_85" class="fnanchor">[85]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_85" href="#FNanchor_85" class="label">[85]</a> Sheet 154 and Explanation to the same, p. 24.</p> - -</div> - - -<h3>3. THE VOLCANIC BRECCIAS OF DOUBTFUL AGE IN COUNTY CORK</h3> - -<p>In the south-western headlands of Ireland, from Bear Island to Dursey -Island, various igneous rocks have been traced on the maps of the Geological -Survey. They have been described as consisting of "greenstone," "felstone," -and "ash" or "breccia," and as including both interstratified and intrusive -masses.<a id="FNanchor_86" href="#Footnote_86" class="fnanchor">[86]</a> If contemporaneous with the strata in which they occur, they -would prove the existence of a group of volcanic rocks in the Carboniferous -slate, or lowest division of the Carboniferous system. After an examination -of the coast-line I came to the conclusion that while there is undoubtedly -evidence of former volcanic activity in this part of Ireland, no proof has -been obtained that the eruptions occurred in the Carboniferous period. The -felsites and dolerites appeared to me to be all intrusive, the former having -certainly been injected before the terrestrial movements that have disturbed -the rocks, for some of them share very markedly in the cleavage of the -region. The dolerites and diabases, on the other hand, so far as I observed, -are not cleaved, and are thus probably of later date.</p> - -<div class="footnote"> - -<p><a id="Footnote_86" href="#FNanchor_86" class="label">[86]</a> See Sheets 197 and 198 of the Geological Survey of Ireland, and the Explanation of these -Sheets by Messrs. Jukes, Kinahan, Wilson, and O'Kelly, 1860.</p> - -</div> - -<p>The most interesting rocks are undoubtedly the "ash" and "breccia," -for they are obviously of volcanic as distinguished from plutonic origin. -On the coast north of White Bull Head, a bed of volcanic breccia -may be seen made up of rounded and angular fragments of different sandstones, -shales and limestones, with pieces of felsite and andesite wrapped up -in a dull-grey fine-grained sandy felspathic matrix. The rock weathers -with a rough or rugged surface, owing to the dropping out of the more -decomposable stones. This bed, about five feet thick, runs with the bedding -of the strata around it, and like these dips S.S.W. at an angle of 70°. If no -other evidence were obtainable, this breccia would be naturally set down as -a truly interstratified deposit of volcanic detritus. A short distance from -it, a second, rather thicker band of similar material occurs, specially -distinguished by its abundant worn crystals of hornblende, sometimes three -inches in diameter, as well as large crystals of muscovite. These minerals -are not unknown elsewhere in volcanic agglomerates. The occurrence -of lumps of augite in the vents of Upper Old Red Sandstone age in -Caithness has been already alluded to, and a still larger series of ejected -minerals will be shown in a later chapter to characterize the younger necks -of Central Scotland.</p> - -<p>In parts of its course, this second band appears to run so perfectly -parallel with the bedding of the strata between which it lies that the -observer would readily believe it to be a part of the same series of -deposits, and might therefore regard it as affording good evidence of -<span class="pagenum" id="Page_50">- 50 -</span> -volcanic action contemporaneous with the formation of these deposits. -A transverse section of the bed, where thus apparently conformable, is -shown in <a href="#v2fig197">Fig. 197</a>.</p> - -<div class="figcenter" id="v2fig197" style="width: 288px;"> - <img src="images/v2fig197.png" width="288" height="139" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 197.</span>—Section of a bed of Volcanic Breccia in the Carboniferous Slate; White Bull Head, - County Cork.<br /><br /> - 1 1. Sandstones and shales; 2. Breccia.</div> -</div> - -<div class="figcenter" id="v2fig198" style="width: 430px;"> - <img src="images/v2fig198.png" width="430" height="227" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 198.</span>—Volcanic Breccia invading and enclosing Carboniferous Slate, White Bull Head.</div> -</div> - -<p>Further examination, however,reveals that this seemingly regular sequence -is entirely deceptive. At various points the breccia abruptly truncates the -sandstones, and involves large pieces of them, as shown in <a href="#v2fig198">Fig. 198</a> A. At -other places, the lower side of the breccia, or what would be its base if it -were a regular bed, cuts out the strata and sends veins into them (B). And -the same structure is visible, on its upper side, or what would be its -top (C).</p> - -<p>It is clear that these highly-inclined bands of breccia are not contemporaneous -with the deposition of the Carboniferous Slate, but have -been introduced into their position at some time subsequent not only -to the deposition, but to the disturbance and elevation of the strata. The -peninsula of White Bull Head is crossed by several other similar bands. -On Black Bull Head, also, together with abundant felsitic and doleritic -intrusions, a similar breccia or agglomerate is to be seen. In some parts it -is compact in texture with spheroidal flinty lumps, and weathers somewhat -<span class="pagenum" id="Page_51">- 51 -</span> -like a nodular felsite. This variety ends off rather abruptly to the -north, but swells out southward, and then runs out into a high, narrow headland, -in which it contains asbestos, as well as rounded crystals of hornblende. -It has here disrupted the shales and sandstones, and near the junction is -largely composed of fragments of them, the strata themselves being jumbled, -bent, and broken up.</p> - -<p>The only semblance of a neck-like mass of this volcanic fragmental -material occurs on White Bull Head, where one of the bands expands about -the centre of the ridge, and is there full of large blocks of grey sandstone. -The breccia appears to have filled fissures which have been opened -between the bedding planes of the highly tilted strata, giving rise to long -narrow dyke-like intercalations. We have seen that among the Carboniferous -volcanic phenomena such dyke-like masses of agglomerate occasionally -present themselves in the vents both of the plateaux and the puys.</p> - -<p>In one or two places I noticed what may be traces of cleavage in the -breccia. The rock is not one that would yield easily to the rearrangements -required for the production of this structure, and the doubtful cleavage may -be deceptive. If we are justified in regarding the introduction of this -volcanic material as having necessarily taken place after the tilting of -the strata, we may not unreasonably infer further that the eruptions could -only have been effected at no great distance from the surface. But the -Carboniferous Slate in which these agglomerates lie is the lowest member -of the Carboniferous system. As there is no known unconformability -throughout this system in the south of Ireland, the whole of the rest of the -pile of Carboniferous strata, amounting to a depth of several thousand feet, -once probably extended over this region. It must, therefore, have been not -only after the plication, but after extensive denudation of the formations that -the fissures were filled with agglomerate. These geological changes no doubt -occupied a vast period of time. While, therefore, no positive evidence has -yet been gathered to fix the age of these volcanic eruptions of the south-west -of Ireland, it is tolerably clear that they cannot be assigned to the -Carboniferous period, but must belong to some later volcanic epoch. They -may be of Permian age, perhaps even as late as the Tertiary volcanic series.</p> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_53">- 53 -</span></p> - -<h2 class="nobreak" id="BOOK_VII">BOOK VII<br /> - -<span class="smaller">THE PERMIAN VOLCANOES</span></h2> -</div> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<h2 class="nobreak" id="CHAPTER_XXXI">CHAPTER XXXI<br /> - -<span class="smaller">THE PERMIAN VOLCANOES OF SCOTLAND</span></h2> -</div> - -<div class="blockquot"> - -<p>Geographical Changes at the Close of the Carboniferous Period—Land- and Inland-Seas -of Permian time—General Characteristics and Nature of the Materials erupted—Structure -of the several Volcanic Districts: 1. Ayrshire, Nithsdale, Annandale; 2. -Basin of the Firth of Forth.</p> -</div> - - -<p>The close of the Carboniferous portion of the geological record in Britain is -marked by another of those great gaps which so seriously affect the continuity -of geological history. No transitional formation, such as in other -countries marks the gradation from the Carboniferous into the succeeding -period, has been definitely recognized in this country. The highest Carboniferous -strata are here separated from all younger deposits by an unconformability, -indicating the lapse of vast periods of time whereof, within the -British area, no chronicle has been preserved.</p> - -<p>When we pass from the Carboniferous system to that which comes next -to it in order of time, we soon become sensible that great changes in -geography, betokening an immense interval, took place between them. The -prolonged subsidence during which the Coal-measures were accumulated, not -only carried down below sea-level all the tracts over which the Carboniferous -system was deposited, but possibly submerged the last of the islets, -which, like those of Charnwood Forest, had survived so many geological -changes. Eventually, however, and after what may have been a vast -period of quiescence, underground movements began anew, and the tracts of -Coal-measures were unequally ridged up into land. The topography thus -produced appears to have resulted in the formation of a series of inland -seas somewhat like those of the Old Red Sandstone, but probably less in -area and in depth. In these basins the water seems to have been on the -whole unfavourable to life, for the red sand and mud deposited in them are -<span class="pagenum" id="Page_54">- 54 -</span> -generally unfossiliferous, though, when the conditions became more suitable, -calcareous or dolomitic sediment accumulated on the bottom, to form what -is now known as the "Magnesian Limestone," and muddy sediment was -deposited which is now the "Marl Slate." In these less ferruginous strata, -betokening a less noxious condition of water, various marine organisms are -met with.<a id="FNanchor_87" href="#Footnote_87" class="fnanchor">[87]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_87" href="#FNanchor_87" class="label">[87]</a> In some recent borings around Hartlepool the Magnesian Limestone has been found to be -interstratified with thick bands of gypsum and anhydrite, and to be overlain by more than 250 feet -of the latter substance. Nothing could show more forcibly the exceedingly saline and insalubrious -character of the Permian lakes or inland seas.</p> - -</div> - -<p>The vegetation of the land surrounding these basins was still essentially -Palæozoic in character. It presented a general resemblance to that -of Carboniferous time, but with some notable differences. The jungles of -<i>Sigillaria</i> seem to have disappeared, while on the other hand, conifers -increased in number and variety. The sediments of the water-basins have -handed down only a scanty remnant of the animal life of the time. Along -the sandy shores walked various amphibians which have left their footprints -on the sand. A few genera of ganoid fishes have been found in some of -the shales, and a comparatively poor assemblage of crinoids and molluscs -has been obtained from the Magnesian Limestone. To the geological period -distinguished by these geographical and biological characters the name of -Permian is assigned.</p> - -<p>In his survey of the progress of volcanic history in the area of Britain, -the geologist finds that the long period of quiescence indicated by the deposition -of the Coal-measures, and probably also by the unconformability between -the Coal-measures and the Permian formations, was at length terminated by -a renewed volcanic outbreak, but on a singularly diminished scale and for a -comparatively brief period of time. Whether, had the Permo-Carboniferous -strata which connect the Coal-measures with the Permian formations on the -Continent been found in this country, they would have filled up the gap in -the geological record, and would have supplied any trace of contemporaneous -volcanic action, cannot even be surmised. All that we know is that, after a -vast interval, and during the deposition of the breccias and red sandstones -which unconformably overlie the Coal-measures, a few scattered groups of -little volcanoes appeared in the area of the British Isles.</p> - -<p>It is unfortunate that in those districts where these volcanic relics have -been preserved, the stratigraphical record is singularly imperfect, and that on -the eastern side of England, where this record is tolerably complete, there are -no intercalated volcanic rocks. The latter occur in tracts where the strata -are almost wholly destitute of fossils, and where therefore no palæontological -evidence is available definitely to fix the geological age of the eruptions. -Nevertheless there is usually ample proof that the strata in question are -much later than the Coal-measures, while their geological position and lithological -characters link them with the undoubted Permian series of the north-east -of England. They may, however, belong to a comparatively late part -of the Permian period, if indeed some of them may not be referable to the -succeeding or Triassic period.</p> - -<p><span class="pagenum" id="Page_55">- 55 -</span></p> - -<p>The comparatively feeble and short-lived volcanoes now to be described -are found in two regions wide apart from each other. The more important -of these lies in the south-west and centre of Scotland. A second group -rose in Devonshire. It is possible that a third group appeared between -these two regions, somewhere in the midlands. The evidence for the -history of each area will be given in a separate section in the following -pages.</p> - - -<h3>i. <span class="smcap">GENERAL CHARACTERISTICS—NATURE OF MATERIALS ERUPTED</span></h3> - -<p>The chief district for the display of volcanic eruptions that may be -assigned to the Permian period lies in the centre of Ayrshire and the -valleys of the Nith and Annan. But, for reasons stated below, I shall -include within the same volcanic province a large part of the eastern half -of the basin of the Firth of Forth (see Map V.).</p> - -<p>Unfortunately the interesting volcanic rocks now to be considered have -suffered severely from the effects of denudation. They have been entirely -removed from wide tracts over which they almost certainly once extended. -But this enormous waste has not been wholly without compensations. -The lavas and tuffs ejected at the surface, and once widely spread over it, -during the deposition of the red sandstones, have been reduced to merely a -few detached fragments. But, on the other hand, their removal as a superficial -covering has revealed the vents of discharge to an extent unequalled -in any older geological system, even among the puys of the Carboniferous -period. The Permian rocks, escaping the effects of those great earth-movements -which dislocated, plicated and buried the older Palæozoic -systems of deposits, still remain for the most part approximately horizontal -or only gently inclined. They have thus been more liable to complete -removal from wide tracts of country than older formations which have been -protected by having large portions of their mass carried down by extensive -faults and synclinal folds, and by being buried under later sedimentary -accumulations. We ought not, therefore, to judge of the extent of the -volcanic discharges during Permian time merely from the small patches of -lava and tuff which have survived in one or two districts, but rather from -the number, size and distribution of the vents which the work of denudation -has laid bare.</p> - -<p>The evidence for the geological age of the volcanic series now to be -described is less direct and obvious than most of that with which I have -been hitherto dealing. It consists of two kinds. (<i>a</i>) In the first of these -comes the series of lavas and tuffs just referred to as regularly interstratified -with the red sandstones, which, on the grounds given in the next -paragraph, it is agreed to regard as Permian. (<i>b</i>) Connected with these -rocks are necks which obviously served as vents for the discharge of the -volcanic materials. They pierce not only the Coal-measures, but even parts -of the overlying bedded lavas. So far there is not much room for difference of -opinion; but as we recede northward from Ayrshire and Nithsdale, where the -<span class="pagenum" id="Page_56">- 56 -</span> -intercalation of the volcanic series in the red sandstones is well displayed, -we enter extensive tracts where these interstratified rocks have disappeared -and only the necks remain. All that can be positively asserted regarding -the age of these necks is that they must be later than the rocks which they -pierce. But we may inferentially connect them with the interstratified -lavas and tuffs by showing that they can be followed continuously outward -from the latter as one prolonged group, having the same distribution, -structure and composition, and that here and there they rise through the -very highest part of the Coal-measures. It is by reasoning of this kind -that I include, as not improbably relics of Permian volcanoes, a large number -of vents scattered over the centre of Scotland, in the East of Fife.</p> - -<p>The red sandstones among which the volcanic series is intercalated cover -several detached areas in Ayrshire and Dumfriesshire. Lithologically they -present a close resemblance to the Penrith sandstone and breccias of Cumberland, -the Permian age of which is generally admitted. They lie unconformably -sometimes on Lower and Upper Silurian rocks, sometimes on -the lower parts of the Carboniferous system, and sometimes on the red -sandstones which form the highest subdivision of that system. They are -thus not only younger than the latest Carboniferous strata, but are separated -from them by the interval represented by the unconformability. On these -grounds they are naturally looked upon as not older than the Permian period. -The only palæontological evidence yet obtained from them in Scotland is -that furnished by the well-known footprints of Annandale, which indicate the -existence of early forms of amphibians or reptiles during the time of the -deposition of the red sand. The precise zoological grade of these animals, -however, has never yet been determined, so that they furnish little help -towards fixing the stratigraphical position of the red rocks in which the -footprints occur.</p> - -<p>The stratigraphical relations of the red sandstones of Ayrshire and -Nithsdale were discussed by Murchison, Binney and Harkness.<a id="FNanchor_88" href="#Footnote_88" class="fnanchor">[88]</a> These -observers noticed certain igneous rocks near the base of the sandstones, to -which, however, as being supposed intrusive masses, they did not attach -importance. They regarded the volcanic tuffs of the same district as -ordinary breccias, which they classed with those of Dumfries and Cumberland, -though Binney noticed the resemblance of their cementing paste to that of -volcanic tuff, and in the end was doubtful whether to regard the igneous rocks -as intrusive or interstratified.</p> - -<div class="footnote"> - -<p><a id="Footnote_88" href="#FNanchor_88" class="label">[88]</a> See Murchison's <i>Siluria</i>, 4th edit. p. 331; <i>Quart. Journ. Geol. Soc.</i> vol. vii. (1851), p. 163, -note; vol. xii. (1856), p. 267; Binney, <i>ibid.</i> vol. xii. (1856), p. 138; vol. xviii. (1862), p. 437; -Harkness, <i>ibid.</i> vol. xii. (1856), p. 262.</p> - -</div> - -<p>In the year 1862, on visiting the sections in the River Ayr, I recognized -the breccia as a true volcanic tuff. During the following years, while mapping -the district for the Geological Survey, I established the existence of a series -of contemporaneous lavas and tuffs at the base of the Permian basin of -Ayrshire, and of numerous necks marking the vents from which these materials -had been erupted. An account of these observations was published in the -<span class="pagenum" id="Page_57">- 57 -</span> -year 1866.<a id="FNanchor_89" href="#Footnote_89" class="fnanchor">[89]</a> Since that time the progress of the Survey has extended the -detailed mapping into Nithsdale and Annandale, but without adding any new -facts of importance to the evidence furnished by the Ayrshire tract.<a id="FNanchor_90" href="#Footnote_90" class="fnanchor">[90]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_89" href="#FNanchor_89" class="label">[89]</a> <i>Geol. Mag. for 1866</i>, p. 243; and Murchison's <i>Siluria</i>, 4th edit. (1867), p. 332.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_90" href="#FNanchor_90" class="label">[90]</a> The rocks are shown in Sheets 9, 14 and 15 of the Geological Survey of Scotland, to which, -and their accompanying Explanations, reference is made. The Ayrshire basin was mapped by me, -the necks in the Dalmellington ground by Mr. James Geikie, the Nithsdale area by Mr. R. L. Jack, -Mr. H. Skae and myself.</p> - -</div> - -<p>The materials erupted by the Scottish Permian volcanoes display a very -limited petrographical range, contrasting strongly in this respect with the -ejections of all the previous geological periods. They consist of lavas generally -more or less basic, and often much decayed at the surface; and of agglomerates -and tuffs derived from the explosion of the same lavas.</p> - -<p>The lavas are dull reddish or purplish-grey to brown or almost black rocks; -sometimes compact and porphyritic, but more usually strongly amygdaloidal, -the vesicles have been filled up with calcite, zeolites or other infiltration. -The porphyritic minerals are in large measure dull red earthy pseudomorphs -of hæmatite, in many cases after olivine. These rocks have not yet been -fully studied in regard to their composition and microscopic structure. A few -slides, prepared from specimens collected in Ayrshire and Nithsdale, examined -by Dr. Hatch, were found to present remarkably basic characters. One from -Mauchline Hill is a picrite, composed chiefly of olivine and augite, with a -little striped felspar. Others from the Thornhill basin in Dumfriesshire show -an absence of olivine, and sometimes even of augite. The rock of Morton -Castle consists of large crystals of augite and numerous grains of magnetite -in a felspathic groundmass full of magnetite. Around Thornhill are -magnetite-felspar rocks, composed sometimes of granular magnetite with -interstitial felspar. Throughout all the rocks there has been a prevalent -oxidation of the magnetite, with a consequent reddening of the masses.</p> - -<p>The pyroclastic materials consist of unstratified agglomerates and tuffs, -generally found in necks, and of stratified tuffs, which more or less mingled -with non-volcanic material, especially red sandstone, are intercalated among -the bedded lavas or overlie them, and pass upward into the ordinary -Permian red sandstones.</p> - -<p>The agglomerates, though sometimes coarse, never contain such large -blocks as are to be seen among the older Palæozoic volcanic groups. Their -composition bears reference to that of the bedded lavas associated with them, -pieces of the various basalts, andesites, etc., which constitute these lavas -being recognizable, together with others, especially a green, finely-vesicular, -palagonitic substance, which has not been detected among the sheets of lava. -In general the agglomerates contain more matrix than blocks, and pass -readily into gravelly tuffs. A series of specimens collected by me from necks -which pierce the Dalmellington coal-field has been sliced and examined -under the microscope by Mr. Watts, who finds it to consist of basic -tuffs, in which the lapilli include various types of olivine-basalt, sometimes -glassy, sometimes palagonitic, and occasionally holocrystalline, also pieces of -grit, shale and limestone. In one case a crinoid joint detached from its -<span class="pagenum" id="Page_58">- 58 -</span> -matrix was noticed. A specimen from Patna Hill consists of "a clear -irregularly cracked aggregate of carbonates and quartz with hornblende, and -its structure reminds one of that of olivine. The hornblende is in small -irregular patches surrounded by the clear mineral, and is probably a replacement -of a pyroxene, perhaps diallage." If this stone was once an olivine -nodule, the agglomerate might in this respect be compared with some of the -tuffs of the Eifel so well known for their lumps of olivine.</p> - -<p>The stratified tuffs are generally more or less gravelly deposits, composed -of lapilli varying in size from mere grains up to pea-like fragments, but with -numerous larger stones and occasional blocks of still greater dimensions. -They often pass into a tough dull compact mudstone. In colour they are -greenish or reddish. They have been largely derived from the explosion -of lavas generally similar to those of which fragments occur in the agglomerates. -They often contain non-volcanic detritus, derived from the blowing up -of the rocks through which the vents were opened. Occasionally they -include also various minerals such as pyrope, black mica, sanidine, augite, -and others which appear to have been ejected as loose and often broken -crystals. This character is more fully described in regard to its occurrence -among the necks of the east part of Fife.</p> - -<p>The intrusive rocks, probably referable to the same volcanic period, consist -chiefly of dolerites and basalts which occur as dykes, sills and bosses, -and are more particularly developed in the south-west of Ayrshire.</p> - - -<h3>ii. <span class="allsmcap">GEOLOGICAL STRUCTURE OF THE VOLCANIC DISTRICTS</span></h3> - -<p class="tdc">1. Ayrshire, Nithsdale and Annandale</p> - -<p>(1) <i>Interstratified Lavas and Tuffs.</i>—It will be convenient to consider -first the volcanic chronicle as it has been preserved in the south-west and -south of Scotland, where the existence of Permian volcanoes in Britain was -first recognized. The volcanic rocks in the middle of the Ayrshire coal-field -rise from under a central basin of red sandstone, which they completely -enclose. Their outcrop at the surface varies up to about a mile or rather -more in breadth, and forms a pear-shaped ring, measuring about nine miles -across at its greatest width (Map V.).<a id="FNanchor_91" href="#Footnote_91" class="fnanchor">[91]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_91" href="#FNanchor_91" class="label">[91]</a> Mr. Gunn has recently detected among the newest red sandstones of Arran a small patch of -volcanic rocks which may be of this age. Mr. A. Macconochie has also found what may be traces -of a similar volcanic band below the Permian sandstones of Loch Ryan, in Wigtonshire.</p> - -</div> - -<p>This volcanic ring runs as a tract of higher ground encircling the hollow -in which the Permian red sandstones lie, and forming a marked chain of -heights above the Carboniferous country around. It is built up of a -succession of sheets of different lavas, with occasional partings of tuff or -volcanic breccia, which present their escarpments towards the coal-field outside, -and dip gently into the basin under the inner trough of brick-red sandstones. -Good sections of the rocks are exposed in the ravines of the River Ayr, -particularly at Ballochmyle, in the Dippol Burn near Auchinleck House, and -in the railway cutting near Mossgiel.</p> - -<p><span class="pagenum" id="Page_59">- 59 -</span></p> - -<p>That these are true lava-flows, and not intrusive sills, is sufficiently -obvious from their general outward lithological aspect, -some of them being essentially sheets of slag and -scoriæ. Their upper surfaces may be found with a -fine indurated red sand wrapping round the scoriform -lumps and protuberances, and filling in the rents and -interspaces, as in the case of the Old Red Sandstone -lavas already referred to. As an example of these -characteristics, I may cite the section represented in -<a href="#v2fig200">Fig. 200</a>. At the bottom lies a red highly ferruginous -and coarsely amygdaloidal basalt (<i>a</i>). Over it comes -a volcanic conglomerate three feet thick, made up of -balls of vesicular lava like that below, wrapped in -a brick-red sandy matrix (<i>b</i>). Lenticular bands of -sandstone without blocks occur in the conglomerate, -and others lie in hollows of its upper surface (<i>c</i>). This -intercalation of detrital material is followed by another -basic lava (<i>d</i>), about six feet thick, highly amygdaloidal -in its lower and upper parts, more compact in the -centre. The amygdales and joints are largely filled -with calcite. The slaggy bottom has caught up and -now encloses some of the red sand of the deposit below. -Another lava from three to six feet thick next appears -(<i>e</i>), which is remarkable for its slaggy structure, and -is so decomposed that it crumbles away. Like the -others it is dull-red and ferruginous and full of calcite. -It must have been at the time of its outflow a sheet -of rough slag that cracked into open fissures. That -it was poured out under water is again shown in the -same interesting way just referred to, by the red sand -which has been washed into the interspaces between -the clinkers and has filled up the fissures, in which -it is stratified horizontally between the walls. Above -this band, and perhaps passing into it as its slaggy -base, lies another more compact lava (<i>f</i>) like the lower -sheets.</p> - -<div class="figcenter" id="v2fig199" style="width: 709px;"> - <img src="images/v2fig199.png" width="709" height="83" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 199.</span>—General section across the Permian basin of Ayrshire.<br /><br /> - 1. Highest group of the Coal-measures; 2. Volcanic tuffs and ashy brick-red sandstones; 3. Lavas with interstratified tuffs and brick-red sandstones; 4. Brick-red - Permian sandstones; 5, 5. Necks of volcanic agglomerate; 6. Boss of dolerite.</div> -</div> - -<p>Throughout the series of lavas, as indicated in -the foregoing section, traces of the pauses that elapsed -between the separate outflows may be seen in the -form either of layers of red sandstone or of tuff and -volcanic breccia. Here and there, under the platform -of bedded lavas, the brick-red sandstone is full of -fragments of slag and fine volcanic dust. But the -most abundant accumulation of such detritus is to -be seen at the top of the volcanic series, where it -contains the records of the closing phases of eruption. Thick beds of -<span class="pagenum" id="Page_60">- 60 -</span> -tuff and volcanic breccia occur there, interleaved with seams of red sandstone, -like the chief mass of that rock, into which they gradually pass -upward. Yet, even among the sandstones above the main body of tuff, -occasional nests of volcanic lapilli, and even large -bomb-like lumps of slag, point to intermittent explosions -before the volcanoes became finally extinct -and were buried under the thick mass of red Permian -sandstone.</p> - -<div class="figleft" id="v2fig200" style="width: 140px;"> - <img src="images/v2fig200.png" width="140" height="199" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 200.</span>—Section of lavas - east side of Mauchline Hill.</div> -</div> - -<div class="figleft" id="v2fig201" style="width: 239px;"> - <img src="images/v2fig201.png" width="239" height="144" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 201.</span>—Section of the top of the volcanic series - near Eastside Cottage, Carron Water, Nithsdale.</div> -</div> - -<p>There is good reason to believe that both the -volcanic sheets and the red sandstones overlying -them, instead of being restricted to an area of only -about 30 square miles, once stretched over the lowlands -of Ayrshire; and not only so, but that they -ran down Nithsdale, and extended into several of -its tributary valleys, if indeed, they were not continuous -across into the valley of the Annan.<a id="FNanchor_92" href="#Footnote_92" class="fnanchor">[92]</a> Traces -of the lavas and tuffs are to be found at intervals -over the area here indicated. The most important -display of them, next to their development in Ayrshire, occurs in the -vale of the Nith at Thornhill, whence they extend continuously up the -floor of the Carron Valley for six miles. They form here, as in Ayrshire, a -band at the base of the brick-red sandstones, and consist mainly of bedded -lavas with the basic characters above referred to. These lavas, however, -are followed here by a much thicker -development of fragmental volcanic -materials. Abundant volcanic detritus -is diffused through the overlying -sandstones, sometimes as a -gravelly intermixture, sometimes in -large slaggy blocks or bombs, and -sometimes in intercalated layers of -tuff, while an occasional sheet of one -of the dull red lavas may also be -detected. The final dying-out of -the volcanic energy in a series of intermittent explosions, while the ordinary -red sandy sediment was accumulating, is here also admirably chronicled. -As an illustration of these features the accompanying section is given -(<a href="#v2fig201">Fig. 201</a>). The last of the lavas (<i>a</i>) presents an uneven surface -against which the various kinds of detritus have been laid down. First -comes a coarse volcanic breccia (<i>b</i>) made up of angular and subangular -blocks of different lavas imbedded in a matrix of red ashy sand. -This deposit is succeeded by a band of dull red tufaceous sandstone, -evidently formed of ordinary red sandy sediment, into which a quantity of -volcanic dust and lapilli fell at the time of its accumulation. Some of the -ejected blocks which lie inclosed in the finer sediment are upwards of a -<span class="pagenum" id="Page_61">- 61 -</span> -foot in length. A more vigorous discharge of fragmental material is shown -by the next bed (<i>d</i>), which consists of a coarse nodular tuff, mingled with a -little red sandstone and crowded with blocks of the usual lavas. Beyond -the locality of this section these tuffs are found to pass up insensibly into -the ordinary Permian sandstone.</p> - -<div class="footnote"> - -<p><a id="Footnote_92" href="#FNanchor_92" class="label">[92]</a> See <i>Memoirs of Geol. Surv. Scotland</i>, Sheet 15 (1871), p. 35; Sheet 9 (1877), p. 31.</p> - -</div> - -<div class="figcenter" id="v2fig202" style="width: 390px;"> - <img src="images/v2fig202.png" width="390" height="99" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 202.</span>—Section of two outliers of the Permian volcanic series at the foot of Windyhill Burn, - Water of Ae, Dumfriesshire.</div> -</div> - -<p>But we can detect the edges of yet more distant streams of lava emerging -from under the red sandstones and breccias to the east of the Nith. -On the farther side of the Silurian ridge that forms the eastern boundary -of the Nith valley, above which it rises some 700 or 800 feet, there is -preserved at the bottom of the valley of the Capel Water, which flows into -Annandale, another small outlier of a similar volcanic band. Three miles -to the south-east of it two little fragments of the volcanic group lie on the -sides of a small tributary of the Water of Ae. Since these may serve as a -good illustration of the extent to which denudation has reduced the area -of the Permian volcanic series, a section of the locality is here given (Fig. -202). The general foundation rocks of the country are the Silurian greywackes -and shales in highly inclined and contorted positions (<i>a</i>). Each -outlier has, as its basement material, a volcanic breccia (<i>bb</i>) in which, together -with the usual lava-fragments, are mingled pieces of the surrounding Silurian -strata. In the smaller outlier lying to the north-east, this detrital layer -is only about one foot thick. It is overlain by a slaggy amygdaloid of the -usual character (<i>cc</i>), which in the lower outlier is covered with boulder clay -(<i>d</i>). There can be little doubt that these detached fragments were once -united in a continuous sheet of lava which filled the valley of the Water of -Ae and that of its tributary. That the lava stretched down the Ae valley -for some distance is proved by the occurrence of another outlier of it two -miles below.</p> - -<p>But there is still additional evidence for the wide extension of these -volcanic sheets. It appears to be certain that they stretch far to the eastward, -under the Permian sandstones of the Lochmaben basin of Annandale, -for breccias largely made up of pieces of the bedded lavas are found close to -the northern edge of the basin on the west side of the River Annan. To this -remarkable adherence of the lavas and tuffs to the bottom of the Permian -valleys I shall afterwards more specially refer.</p> - -<p>The thickness of the whole volcanic group cannot be very accurately -determined. It reaches a maximum in the Ayrshire basin, where, at its -greatest, it probably does not exceed 500 feet, but is generally much less; -<span class="pagenum" id="Page_62">- 62 -</span> -while in the Nithsdale and Annandale ground the detached and much -denuded areas show a still thinner development.</p> - -<div class="figcenter" id="v2fig203" style="width: 491px;"> - <img src="images/v2fig203.png" width="491" height="193" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 203.</span>—The Green Hill, Waterside, Dalmellington, from the south; a tuff-neck of Permian age.</div> -</div> - - -<p>(2) <i>Vents.</i>—One of the most interesting features in this south-western -district of Scotland is the admirable way in which the volcanic vents of -Permian time have been preserved. Their connection with the lavas and -tuffs can there be so clearly traced that they serve as a guide in the interpretation -of other groups of vents in districts where no such connection now -remains. In Ayrshire, the lower part of the Permian volcanic band is -pierced by several small necks of agglomerate. There cannot, I think, be -any doubt that these necks mark the positions of some of the vents from -which the later eruptions took place. Immediately beyond them necks of -precisely similar character rise through the upper division of the Coal-measures. -There can be as little hesitation in placing these also among -the Permian vents. And thus step by step we are led away from the -central lavas, through groups of necks preserving still the same features, -external and internal, and rising indifferently through rocks of any -geological age from the Coal-measures backward. Thus, although if we -began the investigation at the outer limits of the chain of necks, we might -well hesitate as to their age, yet, when we can fix their geological position -in one central area, we are, I think, justified in classing, as parts of one -geologically synchronous series, all the connected groups that retain the -same general characteristics. It is to denudation that we owe their having -been laid bare to view; but at the same time, denudation has removed the -sheet of ejected materials which may have originally connected most of -these vents together.</p> - -<p>In this regard, it is most instructive to follow the vents south-eastwards -from the Ayrshire basin into Nithsdale for a distance of some eighteen miles. -If we traced them down that valley to Sanquhar, without meeting with any -vestige of superficial outflows to mark their stratigraphical position, we -might possibly hesitate whether the age of those which are so far removed -from the evidence that would fix it should not be left in doubt. But if we -continued our traverse only a few hundred yards farther, we should find -<span class="pagenum" id="Page_63">- 63 -</span> -some fragmentary outliers of the Permian lavas capping the Upper Coal-measures; -and if we merely crossed from the Nith into the tributary valley -of the Carron Water, we should see preserved in that deep hollow a great -series of Permian lavas, tuffs and agglomerates. It is only by a happy -accident that here and there these superficial volcanic accumulations have -not been swept away. There was probably never any great thickness of -them, but they no doubt covered most, if not all, of the district within -which the vents are found.</p> - -<p>The Permian necks are, on the whole, smaller than those of the Carboniferous -period. The largest of them in the Ayrshire and Nithsdale region -do not exceed 4000 feet in longest diameter; the great majority are much -less in size, while the smallest measure 20 yards, or even less. Those of Fife, -to be afterwards described, exhibit a wider range of dimensions, and have -the special advantage of being exposed in plan along the shore.</p> - -<div class="figcenter" id="v2fig204" style="width: 370px;"> - <img src="images/v2fig204.png" width="370" height="212" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 204.</span>—Patna Hill from the Doon Bridge, Ayrshire; a tuff-neck of Permian age.</div> -</div> - -<p>These necks, from their number and shapes, form a marked feature in -the scenery. They generally rise as prominent, rounded, dome-shaped, or -conical hills, which, as the rock comes close to the surface, remain permanently -covered with grass (Figs. <a href="#v2fig203">203</a> and <a href="#v2fig204">204</a>). Such smooth green -puys are conspicuous in the heart of Ayrshire, and likewise further south in -the Dalmellington coal-field, where some of them are locally known as -"Green Hill," from their verdant slopes in contrast to the browner vegetation -of the poorer soil around them (<a href="#v2fig203">Fig. 203</a>).</p> - -<p>As in those of older geological periods, the necks of this series are, -for the most part, irregularly circular or oval in ground-plan, but sometimes, -like those of the Carboniferous system, they take curious oblong -shapes, and occasionally look as if two vents had coalesced (<a href="#v2fig205">Fig. 205</a>). -Here and there also the material of the vents has consolidated between the -walls of a fissure or the planes of the strata, so as to appear rather as a -dyke than as a neck. Descending, as usual, vertically through the rocks -which they pierce, the necks have the form of vertical columns of -volcanic material, ending at the surface in grassy rounded hillocks or hills.</p> - -<p><span class="pagenum" id="Page_64">- 64 -</span></p> - -<p>In almost all cases, the necks of the Ayrshire region consist of a gravelly -tuff or agglomerate, reddish or greenish in colour, made up of blocks of such -lavas as form the bedded sheets, together with fragments of the stratified -rocks through which the chimneys have been blown out. Thus, in some -of the necks, pieces of black shale are abundant, as at Patna. In other cases, -there are proofs of the derivation of the stones from much greater depths, -as in the Green Hill of Waterside, where fragments of fine greywacke are -not infrequent, probably derived from the Silurian formations which lie deep -beneath the Carboniferous and Old Red Sandstone series.</p> - -<p>The fragmentary material of the necks is generally unstratified, but a -rude stratification may sometimes be noticed, the dip being irregularly -inward at high angles towards the middle of the vent. This structure, best -seen in the vents of the Fife coast, as will be shown in the sequel, may be -detected in some of the necks of the Dalmellington district.</p> - -<div class="figcenter" id="v2fig205" style="width: 402px;"> - <img src="images/v2fig205.png" width="402" height="280" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 205.</span>—Ground plans of Permian volcanic vents from the Ayrshire Coal-field. On the scale of - six inches to a mile. - -<div class="blockquot"> - -<p>1. Neck half a mile north-west from Dalmellington; 2. Neck at Auchengee, four miles north-east from Patna; 3. -Neck at head of Drumbowie Burn, five and a half miles due north from Dalmellington; 4. Patna Hill, 853 feet -above sea-level (for outline of this hill see the preceding Fig.); 5. Neck on Kiers Hill (1005 feet above the -sea), two miles south from Patna, with lava adhering to part of the wall.</p> -</div></div> -</div> - - -<p>Occasionally some form of molten rock has risen in the funnel, and has -partially or wholly removed or concealed the agglomerate. This feature is -especially noticeable among the necks that pierce the Dalmellington coal-field. -Portions of basic lavas traverse the agglomerate or intervene between -it and the surrounding strata. These have probably in most cases been -forced up the wall of the funnel, while here and there sills run outward -from the necks into the surrounding Coal-measures. Sometimes a thin sheet -of lava, adhering to the wall of a funnel, may be the remnant of a mass of -rock that once filled up the orifice. In one of the necks of the Muirkirk -Coal-field, which was pierced by a mine driven through it from side to side, -<span class="pagenum" id="Page_65">- 65 -</span> -fingers and sheets of "white trap," or highly altered basalt, were found to run -out from the neck into the surrounding strata.<a id="FNanchor_93" href="#Footnote_93" class="fnanchor">[93]</a> Dark heavy basalt, or some -still more basic rock, has here and there filled up a vent. As so many of -the necks rise through the coal-fields, opportunities are afforded of studying -the effects of volcanic action upon the coal-seams, which for some distance -from them have been destroyed.</p> - -<div class="footnote"> - -<p><a id="Footnote_93" href="#FNanchor_93" class="label">[93]</a> Explanation of Sheet 23, Geol. Surv. Scotland, p. 39.</p> - -</div> - -<p>Another feature, which can be recognized from the information obtained -in mining operations, is that, in the great majority of instances, no connection -is traceable between the positions of the vents and such lines of dislocation -as can be detected at the surface or in the underground workings. Some -vents, indeed, have evidently had their positions determined by lines of fault, -as, for instance, that of the Green Hill below Dalmellington. Yet in the -same neighbourhood a number of other examples may be found where the -volcanic funnels seem to have avoided faults, though these exist close -to them.</p> - -<p>In this south-western district of Scotland upwards of sixty distinct vents -have been mapped in the course of the Geological Survey. They run from -the north of Ayrshire to the foot of the Southern Uplands, and descend for -some distance the vale of the Nith. The area over which they are distributed -measures roughly about forty miles from north-west to south-east, and at -its greatest breadth twenty miles from south-west to north-east. Within -this tract the vents are scattered somewhat sporadically in groups, sometimes -numbering twenty necks in a space of sixteen square miles, as in -the remarkable district of Dalmellington.</p> - -<p>In considering their distribution we cannot but be impressed by the -striking manner in which these necks keep to the valleys and low grounds. -I have already alluded to this characteristic, as shown by the volcanoes of -the Old Red Sandstone and Carboniferous periods. But it is displayed by -the Permian volcanoes in a still more astonishing way. Beginning at the -northern end of the long chain of necks in the West of Scotland, we find a -row of them on the plains fronting the volcanic plateau of the Ardrossan, -Dunlop and Stewarton Hills. Thence we may follow them, as single individuals -or in small groups, across the broad lowland of Ayrshire, southward -to the very base of the great chain of the Southern Uplands. There, a -cluster of some two dozen of them may be seen rising out of the Carboniferous -rocks on the low grounds, but they abruptly cease close to the base -of the hills; not one has been detected on the adjacent Silurian heights. -Moreover, if we turn into the valleys that lead away from the great -Ayrshire plain to the interior, we find necks of the same character in -these depressions. They ascend the valley of Muirkirk, and may be met -with even at its very head, near the base of the Hagshaw Hills. Again, -on the floor of the remarkable transverse valley trenched by the Nith across -the Southern Uplands, Permian necks pierce the Coal-measures, while the -outlying fragments of bedded lava show that these vents flooded the bottom -of that valley with molten rock. Turning out of Nithsdale into the long -<span class="pagenum" id="Page_66">- 66 -</span> -narrow glen of the Carron Water, we observe its floor and sides to be -covered with the sheets of lava and tuff already noticed. And so travelling -onward from the vale of the Nith into that of the Capel Water, thence into -the Water of Ae and across into the great strath of Annandale, we may -detect, if not actual vents, at least the beds of lava and layers of volcanic -detritus that were ejected from them.</p> - -<p>All along these valleys, which were already valleys in Carboniferous time, -traces of the volcanic activity of this epoch may be detected. But, so far -as I am aware, in not a single case has any vent been observed to have been -opened on the high surrounding ridges. There has obviously been a -determining cause why the volcanic orifices should have kept to the plains -and the main valleys with their tributaries, and should have avoided the -hills which rise now to heights of 500 to 1000 feet or more above the -bottoms of the valleys that traverse them. It might be said that the -valleys follow lines of fracture, and that the vents have been opened along -these lines. But my colleagues in the Geological Survey, as well as myself, -have failed, in most cases, to find any evidence of such dislocations among -the rocks that form the surface of the country, while it is sometimes possible -to prove that they really do not exist there.</p> - -<p>Though only a few scattered patches of the Permian bedded lavas and -tuffs have been preserved, enough is left to indicate that the vents were -active only in the early part of the period represented by the Scottish -Permian red sandstones, for it is entirely in the lower part of these strata -that volcanic rocks occur. The eruptions gradually ceased, and the sheets -of ejected material, probably also the volcanic cones, were buried under at -least several hundred feet of red sandstone. Whether or not any portion -of the erupted material was for a time built up above the level of the water, -there seems to be no question that the vents were, on the whole, subaqueous.</p> - -<div class="figcenter" id="v2fig206" style="width: 472px;"> - <img src="images/v2fig206.png" width="472" height="64" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 206.</span>—Section of sills traversing the Permian volcanic series. River Ayr, Ballochmyle.<br /><br /> - - <i>a</i>, Coal-measures; <i>b</i> <i>b</i>, Basic lavas; <i>c</i> <i>c</i>, Brick-red sandstones with tuff; <i>d</i>, Red tuff and volcanic breccia; - <i>e</i> <i>e</i>, Dolerite sills.</div> -</div> - -<p>3. <i>Sills.</i>—The phenomena of sills and dykes are less clearly developed -among the Permian volcanic rocks of the Ayrshire basin than among those -of older formations. In the section exposed in the course of the River -Ayr at Howford Bridge, a coarsely crystalline dolerite which extends for -nearly 300 yards up the stream, cuts the Permian lavas, of which it -encloses patches as well as pieces of sandstone. At the contact, the rock -becomes fine-grained (<a href="#v2fig206">Fig. 206</a>). Through the coarsely crystalline material -run long parallel "segregation veins" of a paler, more acid substance, as -among the Carboniferous sills. Similar rocks are well seen in the Dippol -Burn near Auchinleck House.</p> - -<p>Passing outward into the Coal-measures, we encounter a much larger -<span class="pagenum" id="Page_67">- 67 -</span> -display of similar intrusive sheets. The best district for the study of these -sills lies around Dalmellington. The Coal-measures are there traversed by -many intrusions, which have produced great destruction among the coal-seams. -Some of the rocks are extremely basic, including a beautiful picrite -like that of Inchcolm (Letham Hill, near Waterside). The age of these -sills must be later than the Coal-measures into which they have been -injected. Some of them are obviously connected with the agglomerate-necks, -and the whole or the greater number should thus probably be assigned to -the Permian period.<a id="FNanchor_94" href="#Footnote_94" class="fnanchor">[94]</a> The phenomena of intrusion presented by these rocks -reproduce the appearances already described in connection with the basic -intrusive sheets of Carboniferous age.</p> - -<div class="footnote"> - -<p><a id="Footnote_94" href="#FNanchor_94" class="label">[94]</a> Explanation of Sheet 14, Geol. Surv. Scotland, p. 22.</p> - -</div> - - -<p class="tdc">2. Basin of the Firth of Forth</p> - -<p>The other district of Southern Scotland, where traces of volcanic action -later in age than the Coal-measures may be observed, lies in the basin of -the Firth of Forth (Map V.). They include no bedded lavas, and only -at one locality do any relics of a covering of stratified tuffs overspread the -Carboniferous formations. The evidence for the old volcanoes consists -almost entirely of necks of tuff, which mark the position of vents of eruption.</p> - -<p>(1) <i>Vents.</i>—On the south side of the estuary of the Forth there is only -one neck which may be plausibly placed in this series. It forms the upper -part of Arthur Seat, at Edinburgh. This hill has already been cited as -consisting of two distinct portions. The lower, built up of bedded tuffs, -basalts and andesites, forms part of the Midlothian volcanic plateau of Carboniferous -time. The vent from which these materials were ejected must -lie at some little distance, and its site has not been certainly ascertained. -The upper part of the hill is formed of a distinct group of rocks which has -now to be described.</p> - -<p>The geological structure of Arthur Seat has long been well known. It -served as a theme for discussion in the Neptunist and Plutonist controversy, -and was often referred to in the various mineralogical or geognostical -writings of the time. The first thorough examination of it as a relic of -ancient volcanic action was that of Charles Maclaren, published in 1839.<a id="FNanchor_95" href="#Footnote_95" class="fnanchor">[95]</a> -This author clearly recognized the later age and unconformable position of -the coarse mass of agglomerate pierced by the basalt of the apex, and -pointed out the evidence of the upheaval and denudation of the older -volcanic series during a long interval of repose before the latest eruptions -took place. Subsequently Edward Forbes suggested that the upper part of -the hill might be of Tertiary age.<a id="FNanchor_96" href="#Footnote_96" class="fnanchor">[96]</a> Thereafter I mapped the ground in -<span class="pagenum" id="Page_68">- 68 -</span> -detail for the Geological Survey, entirely confirming the observations of -Maclaren.<a id="FNanchor_97" href="#Footnote_97" class="fnanchor">[97]</a> In the end it seemed to me that the interval between the two -epochs of volcanic activity might not be so great as Forbes had supposed; -and after tracing the Permian vents of Ayrshire, I came to the conclusion -that the younger unconformable agglomerate of Arthur Seat was not improbably -Permian.</p> - -<div class="footnote"> - -<p><a id="Footnote_95" href="#FNanchor_95" class="label">[95]</a> <i>Geology of Fife and the Lothians</i>, p. 34. In a reprint of this work, published in 1866, the venerable -author briefly remarked in a footnote that he no longer believed in the second period of volcanic -activity. This view was adopted in 1875 by Professor Judd, <i>Quart. Journ. Geol. Soc.</i> xxxi. p. 131. -For the reasons stated in the text I believe Maclaren's original explanation of the structure of the -hill to be correct.</p> - -<p><a id="Footnote_96" href="#FNanchor_96" class="label">[96]</a> Forbes never published his views regarding Arthur Seat, but expounded them to his class, -and explained them in diagrams, some of which are preserved in the Edinburgh Museum of Science -and Art, in association with the specimens which he collected from the hill.</p> - -<p><a id="Footnote_97" href="#FNanchor_97" class="label">[97]</a> Sheet 32, Geol. Survey of Scotland and descriptive Memoir. See also <i>Rep. Brit. Assoc.</i> 1867, -address Geol. Sect., and Murchison's <i>Siluria</i>, 4th edit. p. 331.</p> - -</div> - -<p>The older volcanic series of this hill has been broken through by the -agglomerate which occupies a true neck, and is abruptly marked off from all -the rocks older than itself. There is no trace of any of the older lavas or -tuffs thickening towards this vent. On the contrary they are completely -truncated by it, and their outcrops on the north side reappear from under -the agglomerate on the south side. Their escarpments are wrapped round -by the agglomerate which likewise fills the head of the hollow that had -been previously worn by denudation out of the stratified deposits between -the oldest lavas. There is thus a violent unconformability between the -later and the older volcanic rocks of Arthur Seat.</p> - -<p>The length of time indicated by this stratigraphical break must be great. -There is no known discordance in the Carboniferous system of the Lothians, -yet the Coal-measures, Millstone Grit, Carboniferous Limestone series and -much of the Calciferous Sandstones were stripped from this hill before the -eruption of the agglomerate. It will be shown in the sequel that a nearly -similar amount of denudation preceded some of the probably Permian -eruptions of Fife.</p> - -<p>The agglomerate contains abundant fragments of the older volcanic -series. Its matrix is a dull red gravelly detritus, crowded with blocks -of all sizes up to a yard or more in diameter. It is pierced by a -column or plug of basalt, which sends veins into it, and rises to the apex of -the hill. A beautiful olivine-basalt forms the lateral mass of the Lion's -Haunch, which rests on the agglomerate.</p> - -<div class="figcenter" id="v2fig207" style="width: 481px;"> - <img src="images/v2fig207.png" width="481" height="87" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 207.</span>—Section showing the relations of the later rocks of Arthur Seat.<br /><br /> - -<div class="blockquot"> - -<p>1. Grey and reddish sandstones and shales (Calciferous Sandstones); 2. The lava of the Long Row: the oldest of -the Carboniferous volcanic series; 3. Tuffs of the Dry Dam; 4. Columnar basalts overlying the tuffs; 5. -Andesite lavas of the eastern half of Arthur Seat; 6. Sill of Heriot Mount; 7. Sill of Salisbury Crags; 8. Sill -of the Dasses. These complete the Lower Carboniferous volcanic series (compare <a href="../../66492/66492-h/66492-h.htm#v1fig112">Fig. 112</a>). 9. White sandstones -and black shales, upper division of the Calciferous Sandstones; 10. Younger volcanic agglomerate -resting on the denuded ends of the older volcanic series; 11. Basalt of the summit sending veins into the -agglomerate; 12. Basalt of the Lion's Haunch.</p> -</div></div> -</div> - - -<p>In general characters the agglomerate of Arthur Seat resembles that of -some of the younger vents of Fife which pierce the Coal-measures and are -connected with tuffs that lie unconformably on the Carboniferous Limestone. -On these various grounds I think that it may be reasonably assigned to the -same geological period.</p> - -<p><span class="pagenum" id="Page_69">- 69 -</span></p> - -<p>That a new vent should be opened, after the lapse of one or more -geological periods, on or near the site of more ancient volcanic orifices is an -incident of which, as we have seen, the geological history of the British -Isles furnishes a number of examples. It will be remembered that little -more than a mile to the south of Arthur Seat lies the great vent of the -Braid Hills, which in the time of the Lower Old Red Sandstone gave forth -such a huge pile of lavas and tuffs. Volcanic energy thereafter entirely -died away, and in this district was succeeded by a prolonged period of -quiescence, during which the Lower Old Red Sandstone was upraised and -extensively denuded, while the Upper Old Red Sandstone was deposited. At -length, in the immediate neighbourhood, from one or more vents, the exact -site of which is not certainly known, the older lavas and tuffs of Arthur -Seat, Calton Hill and Craiglockhart Hill were erupted. Again, after another -vast interval, a new volcano appeared, and the agglomerate and younger -basalts of Arthur Seat were ejected from it. This is one of the most -striking examples in this country of the remarkable persistence of volcanic -energy in the same locality.</p> - -<p>There is no evidence at Arthur Seat itself to fix the geological date of -the last volcanic activity of the hill. If the group of younger rocks stood -alone, with no other trace of post-Carboniferous eruptions in the surrounding -district, a plausible conjecture as to its age would not be easily offered. -But in reality it is not a solitary example of such rocks; for within sight, -on the opposite side of the Firth of Forth, its counterparts may be seen. -To the description of these numerous and clearer illustrations I now -proceed.</p> - -<p>The East of Fife is remarkable for a large assemblage of volcanic vents, -which, unlike those in Ayrshire and Nithsdale, stand alone, their superficial -ejections having been removed by denudation, and no connection being traceable -between them and any Permian sandstones. The vents filled up with agglomerate -and pierced with plugs and veins of basalt, rise through the Carboniferous -rocks, but have left no record for precisely defining their geological age. On -the one hand, it is quite certain that in this district volcanic eruptions took -place during the earlier half of the Carboniferous period. To the north of -Largo, and still more distinctly to the north-east of Leven, sections occur to -show the contemporaneous outpouring of volcanic rocks during the time of -the Carboniferous Limestone. The Leven section, seen in a ravine a little -to the north-east of the town, is specially important. It presents a succession -of red and green fine sandy tuffs, interstratified with fire-clays and -sandstones, and containing a zone of basalt in the centre. These rocks lie -not far from the top of the Carboniferous Limestone series.</p> - -<p>On the other hand, there is equally clear proof of far later eruptions. -From St. Andrews to Elie a chain of necks may be traced, having the same -general characters, and piercing alike the Calciferous Sandstones, and -the older part of the Carboniferous Limestone series. That these -vents must in many cases be long posterior to the rocks among which they -rise, is indicated by some curious and interesting kinds of evidence. -<span class="pagenum" id="Page_70">- 70 -</span> -They are often replete with angular fragments of shale, sandstone -and limestone, of precisely the same mineral characters as the surrounding -strata, and containing the same organic remains in an identical state of -fossilization. It is clear that these strata must have had very much their -present lithological aspect before the vents were opened through them. -Again, the necks may often be observed to rise among much contorted -strata, as, for example, along the crest of a sharp anticlinal arch, or across a -synclinal basin. The Carboniferous rocks must thus have been considerably -plicated before the time of the volcanic eruptions. In the next place, the -vents often occur on lines of dislocation without being affected thereby. -They must be posterior, however, not only to these dislocations, but also to -much subsequent denudation, inasmuch as their materials overspread the -rocks on each side of a fault without displacement. Hence we conclude -with confidence, that a great deal of volcanic activity in the East of Fife -must have been posterior to most, if not all, of the Carboniferous period.</p> - -<div class="figcenter" id="v2fig208" style="width: 524px;"> - <img src="images/v2fig208.png" width="524" height="104" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 208.</span>—Section in brooks between Bonnytown and Baldastard, Largo.<br /><br /> - <i>a</i>, Sandstone shales and coals of Carboniferous Limestone series; <i>b</i>, unconformable tuff.</div> -</div> - - -<p>In the neighbourhood of Largo, further important evidence is presented, -confirming and extending this conclusion. The highest member of the Upper -Coal-measures, consisting of various red sandstones, with red and purple -clays, shales, thin coals and ironstones, is prolonged from the Fife coal-field -in a tongue which extends eastward beyond the village of Lower Largo. It -is well displayed on the shore, where every bed may be followed in succession -along the beach for a space of nearly two miles. Two volcanic necks, presenting -the same features as those which pierce the older portions of the -Carboniferous system to the east, rise through these red rocks. We are -thus carried not only beyond the time of the Carboniferous Limestone, but -beyond the close of the very latest stage of the Carboniferous period in -Central Scotland. Connected with these and other vents farther north, there -is a large area of tuff which has been thrown out upon the faulted and -greatly denuded Carboniferous rocks. It may be traced passing from the -red Upper Coal-measures across the large fault which here separates that -formation from the Carboniferous Limestone, and extending inland athwart -different horizons of the latter series. Outlying fragmentary cakes of it may -be seen resting on the upturned edges of the sandstones, shales and coal-seams, -even at a distance of some miles towards the north-west, proving that -the fragmentary materials discharged from the vents spread over a considerable -area. The accompanying section (<a href="#v2fig208">Fig. 208</a>) may serve as an illustration -of the relation between this sheet of bedded tuff and the underlying rocks.</p> - -<p><span class="pagenum" id="Page_71">- 71 -</span></p> - -<p>Though interstratified volcanic rocks occur in the Carboniferous system -of the East of Fife, no connection has been traced between them and any of -the vents now referred to. While none of these vents can be proved to be of -Carboniferous age, it is of course possible that such may be the true date of -some of them. Others, nevertheless, and probably much the largest number, -judged from the data just given, may be regarded as probably post-Carboniferous. -Those which happen to rise through the uppermost Coal-measures -do not appear to be distinguishable by any essential characters from those -which pierce indifferently the Carboniferous Limestone series and Calciferous -Sandstones of the East of Fife. They seem to be all one connected aggregate, -resembling each other alike in their external characters, internal -structure and component materials, and the limit of their age must be -determined by the geological horizon of the youngest formation which they -traverse. By this process of reasoning I reach the conclusion that this -remarkable series of old volcanoes in the East of Scotland not improbably -dates from the same time as that of Ayrshire and Nithsdale, already -described.</p> - -<div class="figcenter" id="v2fig209" style="width: 541px;"> - <img src="images/v2fig209.png" width="541" height="195" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 209.</span>—View of Largo Law from the east (the crag on the left, at the base of the cone, is a - portion of a basalt-stream. See <a href="#v2fig226">Fig. 226</a>).</div> -</div> - -<p>Some idea of the importance and interest of the volcanic area of Eastern -Fife may be gathered from the fact that in a space of about 70 square miles -no fewer than 60 necks may be counted, and others are probably concealed -below the drift-deposits which cover so much of the interior of the country. -The area of this remarkable display extends from St. Andrews Bay and the -Vale of the Eden southwards to the coast of the Firth of Forth between -Lundin Links and St. Monans. All over the inland tract the necks form -more or less marked eminences, of which the largest are conspicuous landmarks -from the southern side of the Firth. But the distinguishing -characteristic of the area is the display of the necks along the coast, where, -in a series of natural dissections, their form, composition, internal structure -and relations to the surrounding rocks have been laid open in such clearness -and variety as have been met with in the volcanic records of no other geological -period within the compass of these islands. As this district thus -possesses a singular interest and value for the study of volcanic vents, I -shall enter in some detail into the description of the sections so admirably -laid bare.</p> - -<p><span class="pagenum" id="Page_72">- 72 -</span></p> - -<div class="figcenter" id="v2fig210" style="width: 775px;"> - <img src="images/v2fig210.png" width="775" height="481" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 210.</span>—View of small neck in Calciferous Sandstones, on the shore, three miles east from St. Andrews.<br /><br /> - (This illustration, likewise Figs. <a href="#v2fig212">212</a>, <a href="#v2fig216">216</a>, <a href="#v2fig219">219</a>, <a href="#v2fig221">221</a>, <a href="#v2fig222">222</a>, <a href="#v2fig225">225</a> and <a href="#v2fig227">227</a> are from photographs taken for the Geological Survey by Mr. R. Lunn.)</div> -</div> - -<p><span class="pagenum" id="Page_73">- 73 -</span></p> - -<p>As in Ayrshire, the necks in the East of Fife generally rise as isolated -conical or dome-shaped hills, with smooth grassy slopes, but where a dyke -or boss of basalt occurs in them, it usually stands out as a crag or knoll. -Largo Law (<a href="#v2fig209">Fig. 209</a>) may be taken as a singularly perfect example of the -cone-shaped neck. This hill, however, comprises more than one vent. The -mass of tuff of which it consists probably includes at least three distinct -funnels of discharge, and surrounding it there still remains a good deal of the -fragmental material that gathered around these vents and is now seen to lie -unconformably upon the Carboniferous formations (<a href="#v2fig208">Fig. 208</a>). There must -be a total area of not much less than four square miles over which tuff -occupies the surface of the ground.</p> - -<p>While the Fife necks possess the great advantage of having been laid -bare by the sea, their frequent small size on the coast allows their whole -area to be examined. As illustrations of these little vents, two plates are -here given from the coast-line to the east of St. Andrews, where a number -of small necks of agglomerate have been planted among the plicated Calciferous -Sandstones. In <a href="#v2fig210">Fig. 210</a> the abrupt truncation of the sandstones -by the volcanic rock is well shown. The strata on the right have been -broken through, and the sea has indented a small gully along the wall of -the old volcanic funnel. The sandstones in front, however, still adhere -firmly to the agglomerate, which rises above them as a rugged mass of rock.</p> - -<p>In <a href="#v2fig212">Fig. 212</a> the edge of the vent can be traced partly in section and -partly in plan for about half of its circumference. On the right hand, the -actual wall of the funnel is visible where the false-bedded sandstones are -sharply cut off by the agglomerate. In front the strata appear in plan on -the beach, and their ledges can be seen to the left striking at the margin of -the neck.</p> - -<div class="figcenter" id="v2fig211" style="width: 476px;"> - <img src="images/v2fig211.png" width="476" height="181" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 211.</span>—Ground-plan of Permian volcanic vents.</div> -</div> - -<p><span class="pagenum" id="Page_74">- 74 -</span></p> - -<div class="figcenter" id="v2fig212" style="width: 746px;"> - <img src="images/v2fig212.png" width="746" height="488" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 212.</span>—Small neck in Calciferous Sandstones a little east from the "Rock and Spindle," two and a half miles east from St. Andrews.</div> -</div> - -<p><span class="pagenum" id="Page_75">- 75 -</span></p> - -<p>The shape of the Fife vents is, as usual, generally circular or oval; but -is subject to considerable irregularity. The coast-section between Largo and -St. Monans exposes many ground-plans of them, and permits their irregularities -to be closely examined. The accompanying figure (<a href="#v2fig211">Fig. 211</a>) exhibits some -characteristic forms. Eccentricities of outline no doubt arose from the -irregular way in which the rocks yielded to the forces of explosion during the -piercing of a volcanic orifice. This is often well shown by the veins and -nests of tuff or agglomerate which have been forced into the rents or -sinuosities of the orifices. In other cases, however, it is probable that, as -among the Ayrshire necks, and those of Carboniferous age already cited, what -appears now as one volcanic neck was the result of a shifting of the actual -funnel of discharge, so that the neck really represents several closely adjacent -vents. The case of Largo Law has been already noticed. The necks at -Kellie Law (<a href="#v2fig213">Fig. 213</a>) show clearly the same structure, the Law itself -(1) probably consisting of two contiguous vents, while -a third (2) forms a smaller cone immediately to the -east. Such a slight lateral displacement of the vent -has been noticed at many Tertiary and recent volcanic -orifices. In the island or peninsula of Volcanello, -for example, three craters indicate successive -shiftings of the vent, the most perfect of them marking -the latest and diminishing phase of volcanic -activity (<a href="#v2fig214">Fig. 214</a>, compare <a href="../../66492/66492-h/66492-h.htm#v1fig29">Fig. 29, vol. i., p. 70</a>).</p> - -<div class="figcenter" id="v2fig213" style="width: 371px;"> - <img src="images/v2fig213.png" width="371" height="281" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 213.</span>—Plan of volcanic necks at Kellie Law, east of Fife, on the scale of three inches to one mile.<br /><br /> - 1, Kellie Law (tuff); 2, Carnbee Law (tuff); 3, 4, 5, small tuff necks; B B, basalt dykes and bosses; <i>c</i> <i>c</i>, coal-seams; - <i>l</i>, limestone; <i>f</i>, fault. The arrows mark the dip of the strata through which the necks have been drilled.</div> -</div> - -<div class="figright" id="v2fig214" style="width: 128px;"> - <img src="images/v2fig214.png" width="128" height="134" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 214.</span>—Plan of the craters -in Volcanello, Lipari Islands.</div> -</div> - -<p>The Fife necks vary from only a few yards up -to perhaps 4000 feet in diameter. One of the -smallest and most completely exposed occurs on the shore at Newark Castle, -near St. Monans. It measures only 60 yards in length by about 37 yards -in breadth. A ground-plan of it is given in Fig 224. Still smaller is -<span class="pagenum" id="Page_76">- 76 -</span> -the neck at Buddo Ness, on the coast east of St. Andrews, which measures -only 20 yards across.</p> - -<p>From the way in which the vents have been dissected by the sea along -the Fife coast, the geologist is enabled to study in minute detail the -effects of the volcanic operations upon the strata through which the -funnels have been drilled. Considerable variation may be observed in the -nature and amount of change. -Sometimes the orifice has been -made without any noticeable -alteration of the sandstones, -shales and limestones, which -retain their dip and strike up -to the very wall of the chimney. -Usually there is more or less -jumbling and crushing of the -stratification, and often a considerable amount of induration. As a typical -example of these effects I give a section from the margin of the neck of tuff -on the east side of Elie Harbour (<a href="#v2fig215">Fig. 215</a>). Here the sandstones and shales -(<i>a</i>) have been doubled over and dragged down against the tuff (<i>b</i>). They -have likewise been hardened into a kind of quartzite, and this alteration -extends for about 20 to 30 feet from the edge of the neck.</p> - -<div class="figright" id="v2fig215" style="width: 260px;"> - <img src="images/v2fig215.png" width="260" height="103" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 215.</span>—Section of the strata at the edge of the volcanic - vent on the east side of Elie Harbour.</div> -</div> - -<p>The material which has filled up the vents is almost entirely fragmental, -varying from a coarse agglomerate to a fine volcanic tuff. Some minor necks -have been completely or in great part filled with angular debris of the ordinary -rocks of the neighbourhood. In the western neck on the Largo shore, for -example, which rises through the red rocks of the Upper Coal-measures, the -material consists largely of fragments of red sandstone, clay and shale. -Between Elie and St. Monans, some of the necks are filled almost wholly with -debris of black shale and encrinal limestone.</p> - -<p>There does not appear to be any relation between the diameter of a -funnel and the size of the blocks that now fill it. Some of the larger necks, -for example, consist of comparatively fine tuff. The Buddo Ness, on the -other hand, though so small a vent, is packed with blocks of shale six feet -long, while the sandstone through which the orifice has been drilled passes, as -usual, into quartzite for several yards away from the edge. As an example -of the general aspect presented by one of the coarse agglomerates in the -necks of the Fife coast, a view is given in <a href="#v2fig216">Fig. 216</a> of a portion of the neck -at Ardross, about two miles east from Elie. This thoroughly volcanic -accumulation is here shown to consist of blocks of all sizes heaped together -without any definite arrangement.</p> - -<p><span class="pagenum" id="Page_77">- 77 -</span></p> - -<div class="figcenter" id="v2fig216" style="width: 762px;"> - <img src="images/v2fig216.png" width="762" height="490" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 216.</span>—Agglomerate of neck on shore at Ardross, two miles east from Elie.</div> -</div> - -<p><span class="pagenum" id="Page_78">- 78 -</span></p> - -<p>Since the first stage in the history of the vents has been the perforation -of the solid crust by explosion, and the consequent production of debris from -the disrupted rocks, we may hope to detect underneath the pile of thoroughly -volcanic ejections, traces of the first explosions. I have been much struck -with the fact that in the East of Fife such traces may frequently be found -here and there within the outer border of the vents. At Largo, and -again between Elie and St. Monans, it may be noticed that the mass of -material adhering to the wall of a neck, exposed in ground-plan upon the -beach, often consists largely, or even wholly, of debris of sandstone, shale and -limestone, while the central and chief mass is made up of green tuff or -agglomerate, with occasional pieces of the surrounding stratified rocks -scattered through it. It seems probable, therefore, that the sections of these -Fife necks, laid bare on the present shore, do not lie far below the original -crater-bottoms.</p> - -<p>Some light might be expected to be thrown upon the phenomena in an -active volcanic chimney by the condition of the fragments of recognizable -sedimentary rocks imbedded in the ejected debris which has filled up the -orifice. But the assistance from this source is neither so full nor so reliable -as could be wished. In some of the Fife vents, indeed, the fragments of -shale, sandstone and other sedimentary strata are so unchanged that they -cannot on a fresh fracture be distinguished from the adjacent parent strata. -The <i>Spirifers</i>, <i>Lingulæ</i>, crinoids, cyprid-cases, ganoid scales and other -fossils are often as fresh and perfect in the fragments of rock imbedded -in tuff as they are in the rock <i>in situ</i>. In some cases, however, distinct, -and occasionally even extreme, metamorphism may be detected, varying in -intensity from mere induration to the production of a crystalline texture. -The amount of alteration has depended not merely upon the heat of the -volcanic vent, but also in great measure upon the susceptibility of the fragments -to undergo change and the duration of their exposure to it.</p> - -<p>Dr. Heddle has computed the temperature to which fragments of shale, -etc., in tuff-necks of the Fife coast have been subjected. He found that the -bituminous shales have lost all their illuminants, and of organic matter have -retained only some black carbonaceous particles; that the encrinal limestones -have become granular and crystalline; that the sandstones present -themselves as quartzite, and that black carbonaceous clays show every stage -of a passage into Lydian-stone. He inferred from the slight depth to which -the alteration has penetrated the larger calcareous fragments, that the heat -to which they were exposed must have been but of short continuance. As -the result of his experiments, he concluded that the temperature at which -the fragments were finally ejected from the volcanic vents probably lay -between 660° and 900° Fahr.<a id="FNanchor_98" href="#Footnote_98" class="fnanchor">[98]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_98" href="#FNanchor_98" class="label">[98]</a> <i>Trans. Roy. Soc. Edin.</i> vol. xxviii. p. 487.</p> - -</div> - -<p>It may be perhaps legitimate to infer that, while the fragments that fell -back into the volcanic funnel, or which were detached from the sides of the -vent, after having been exposed for some time to intense heat under considerable -pressure, would suffer more or less metamorphism, those, on the -other hand, which were discharged by the æriform explosions from the cool -upper crust, on the first outburst of a vent, would not exhibit any trace of -such a change. Where, therefore, we meet with a neck full of fragments of -unaltered stratified rocks, we may suppose it to have been that of a short-lived -volcano; where, on the other hand, the fragments are few and much -altered, they may mark the site of a vent which continued longer active. -<span class="pagenum" id="Page_79">- 79 -</span> -The metamorphism of the fragmentary contents of a volcanic funnel by the -action of ascending vapours has already been described in the case of one of -the vents of the Carboniferous plateaux (<a href="../../66492/66492-h/66492-h.htm#Page_404">vol. i. p. 404</a>).</p> - -<p>One of the most curious and puzzling features in the contents of the tuff -necks of the Fife coast is the occurrence there of crystals and fragments of -minerals, often of considerable size, which do not bear evidence of having-been -formed <i>in situ</i>, but have undoubtedly been ejected with the other -detritus. Dr. Heddle has noticed the fact, and has described some of the -minerals which occur in this way. The following list comprises the species -which he and I have found:—</p> - -<p class="smaller" style="padding-left: 4em;"> -Hornblende, in rounded fragments of a glassy black cleavable variety.<br /> -Augite, sometimes in small crystals, elsewhere in rounded fragments of - an augitic glass.<br /> -Orthoclase (Sanidine), abundant in worn twin crystals in the tuffs of - the east of Fife.<br /> -Plagioclase.<br /> -Biotite.<br /> -Pyrope, in the tuffs (and more rarely in the basalts) of Elie.<br /> -Nigrine, common in some of the dykes, more rarely in the tuffs of Elie.<br /> -Saponite, Delessite and other decomposition products.<br /> -Semi-opal, one specimen found in the later (Permian?) agglomerate of - Arthur's Seat.<br /> -Asphalt, abundant at Kincraig, near Elie.<br /> -Fragments of wood, with structure well preserved, may be included here. -</p> - -<p>Dr. Heddle has described from the neck of tuff at Kinkell, near St. -Andrews, large twin crystals of a glassy orthoclase, which are invariably -much worn, and preserve only rudely the form of crystals. He justly -remarks that they have no connection with drusy cavity, exfiltration vein, -or with any other mineral, and look as if a portion of their substance has -been dissolved away. Internally, however, they are quite fresh and brilliant -in lustre, though sometimes much fissured.<a id="FNanchor_99" href="#Footnote_99" class="fnanchor">[99]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_99" href="#FNanchor_99" class="label">[99]</a> <i>Trans. Roy. Soc. Edin.</i> vol. xxviii. p. 223.</p> - -</div> - -<p>The tuffs at Elie are full of similar crystals. I obtained from one of -the necks east of that village a specimen which measures 4 inches in -length, 3<sup>1</sup>/<sub>2</sub> in breadth, and 2<sup>1</sup>/<sub>4</sub> in thickness, and weighs about 2 lbs. It is, -however, a well-striated felspar. From the same tuff I procured an orthoclase -twin in the Carlsbad form. All the felspar pieces, though fresh and -brilliant internally, have the same rounded and abraded external appearance.</p> - -<p>The fragments of hornblende form a characteristic feature in several of -the Elie dykes (to be afterwards described), and in the neighbourhood of -these intrusive rocks occur more sparingly in the tuff. It is a glossy-black -cleavable mineral, in rounded pieces of all sizes, up to that of a small egg. -Dr. Heddle obtained a cleavage angle of 124° 19', and found on analysis -that the mineral was hornblende.<a id="FNanchor_100" href="#Footnote_100" class="fnanchor">[100]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_100" href="#FNanchor_100" class="label">[100]</a> <i>Op. cit.</i> xxviii. p. 522.</p> - -</div> - -<p>Augite occurs sparingly in two forms among the rocks. I have obtained -small crystals from the red agglomerate on the south side of Arthur Seat, -recalling in their general appearance those of Somma. Lumps of an augitic -glass have been found by Dr. Heddle, sometimes as large as a pigeon's egg, -in two of the dykes at Elie, and in the tuff at the Kinkell neck, near St. -<span class="pagenum" id="Page_80">- 80 -</span> -Andrews. He observed the same substance at the Giant's Causeway, both -in the basalt and scattered through one of the interstratified beds of red -bole. Much larger rounded masses of a similar augitic glass, but with a -distinct trace of cleavage, have already been referred to as occurring in a -volcanic vent of Upper Old Red Sandstone age at John o' Groat's House.<a id="FNanchor_101" href="#Footnote_101" class="fnanchor">[101]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_101" href="#FNanchor_101" class="label">[101]</a> <i>Op. cit.</i> xxviii. pp. 481 <i>et seq.</i>, and <i>ante</i>, <a href="../../66492/66492-h/66492-h.htm#Page_352">vol. i. p. 352</a>.</p> - -</div> - -<p>Biotite is not a rare mineral in some tuffs. It may be obtained in -Lower Carboniferous tuffs of Dunbar, in plates nearly an inch broad; but -the largest specimen I have obtained is one from the same Elie vent which -yielded the large felspar fragment. It measures 2<sup>1</sup>/<sub>2</sub> × 2 × <sup>1</sup>/<sub>2</sub> inches. These -mica tables, like the other minerals, are abraded specimens.</p> - -<p>That these various minerals were ejected as fragments, and have not -been formed <i>in situ</i>, is the conclusion forced upon the observer who examines -carefully their mode of occurrence. Some of them were carried up to the -surface by liquid volcanic mud, and appear in dykes of that material like -plums in a cake. But even there they present the same evidence of attrition. -They assuredly have not been formed in the dykes any more than in the -surrounding tuff. In both cases they are extraneous objects which have -been accidentally involved in the volcanic rocks. Dr. Heddle remarks that -the occurrence of the worn pieces of orthoclase in the tuff is an enigma to -him. I have been as unable to frame any satisfactory explanation of it.<a id="FNanchor_102" href="#Footnote_102" class="fnanchor">[102]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_102" href="#FNanchor_102" class="label">[102]</a> Occasionally the crystals can be matched in some lava-form rock of the same volcanic area; -but many of them have no such counterparts. See <a href="../../66492/66492-h/66492-h.htm#Page_62">vol. i. p. 62 and <i>note</i></a>.</p> - -</div> - -<div class="figleft" id="v2fig217" style="width: 186px;"> - <img src="images/v2fig217.png" width="186" height="143" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 217.</span>—Ground-plan of volcanic - neck, Elie Harbour, showing circular - deposition of the stratification.<br /><br /> - T, Tuff of the neck, the arrows showing its - inward dip; B B, Dykes; S, Sandstones - and shales, through which the neck has - been opened.</div> -</div> - - -<p>It might have been thought that within the throat of a volcano, if in -any circumstances, loose materials should have taken an indefinite amorphous -aggregation. And, as has been shown in the foregoing chapters, this is -usually the case where the materials are coarse and the vent small. Oblong -blocks are found stuck on end, while small and large are all mixed confusedly -together. But in numerous cases where the -tuff is more gravelly in texture, and sometimes -even where it is coarse, traces of stratification -may be observed. Layers of coarse and fine -material succeed each other, as they are seen -to do among the ordinary interstratified tuffs. -The stratification is usually at high angles -of inclination, often vertical. So distinctly -do the lines of deposit appear amid the -confused and jumbled masses, that an observer -may be tempted to explain the problem -by supposing the tuff to belong not to a -neck, but to an interbedded deposit which -has somehow been broken up by dislocations. -That the stratification, however, belongs to the original volcanic vents themselves -is made exceedingly clear by some of the coast-sections in the East of -Fife. On both sides of Elie, examples occur in which a distinct circular disposition -of the bedding can be traced corresponding to the general form of the -<span class="pagenum" id="Page_81">- 81 -</span> -neck. The accompanying ground-plan (<a href="#v2fig217">Fig. 217</a>) represents this structure as -seen in the neck which forms the headland at Elie harbour. Alternations of -coarse and fine tuff with bands of coarse agglomerate, -dipping at angles of 60° and upwards, may be traced -round about half of the circle. The incomplete part -may have been destroyed by the formation of another -contiguous neck immediately to the east. To the -west of Earlsferry another large, but also imperfect, -circle may be traced in one of the shore necks. A -quarter of a mile farther west rises the great cliff-line -of Kincraig, where a large neck has been cut -open into a range of precipices 200 feet high, as well -as into a tide-washed platform more than half a mile -long. The inward dip and high angles of the tuff -are admirably laid bare along that portion of the -coast-line. The section in which almost every bed -can be seen, and where, therefore, there is no need -for hypothetical restoration, is as shown in <a href="#v2fig218">Fig. 218</a>.</p> - -<p>I have already referred to the frequently abundant -pieces of stratified tuff, found as ejected blocks -in vents filled with tuff, and to the derivation of -these blocks from tuff originally deposited within the -crater. There can, I think, be little hesitation in -regarding the stratification of these Fife vents as a -record of successive deposits of volcanic detritus inside -the vents. The general dip inwards from the outer -rim of the vent strikingly recalls that of some modern -volcanoes. By way of illustration, I give here a -section of part of the outer rim of the crater of the -Island of Volcano, sketched by myself in the year -1870 while ascending the mountain from the north -side (<a href="#v2fig220">Fig. 220</a>). The crater wall at this point consists -of two distinct parts—an older tuff (<i>a</i>), which -may have been in great measure cleared out of the -crater before the ejection of the newer tuff (<i>b</i>). The -latter lies on the outer slope of the cone at the usual -angle of 30°. It folds over the crest of the rim, -and dips down to the flat tuff-covered crater -bottom, at an angle of 37°. These are its natural angles of repose.</p> - -<div class="figcenter" id="v2fig218" style="width: 579px;"> - <img src="images/v2fig218.png" width="579" height="109" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 218.</span>—Section across the great vent of Kincraig, Elie, on a true scale, vertical and horizontal, of six inches to a mile.<br /><br /> - 1, Sandstones, shale, etc., of Lower Carboniferous age, plunging down toward the neck T; B, columnar basalt, shown also in Figs. <a href="#v2fig223">223</a> and <a href="#v2fig225">225</a>.</div> -</div> - -<p><span class="pagenum" id="Page_82">- 82 -</span></p> - -<div class="figcenter" id="v2fig219" style="width: 508px;"> - <img src="images/v2fig219.png" width="508" height="690" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 219.</span>—Dyke in volcanic neck, on the beach, St. Monans.</div> -</div> - -<p><span class="pagenum" id="Page_83">- 83 -</span></p> - -<div class="figright" id="v2fig220" style="width: 293px;"> - <img src="images/v2fig220.png" width="293" height="115" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 220.</span>—Section of part of crater rim, Island of Volcano.</div> -</div> - -<p>Applying modern analogies of this kind, I have been led to conclude -that the stratification so conspicuous in the tuff of the vents in the east of -Fife and in the Carboniferous series of the Lothians belongs to the interior -of the crater and the upper part of the volcanic funnel.<a id="FNanchor_103" href="#Footnote_103" class="fnanchor">[103]</a> These stratified -tuffs, on this view of their origin, must be regarded as remains of the beds -of dust and stones which gathered within the crater and volcanic orifice, -and which, on the cessation of volcanic action, sometimes remained in their -original position, or were -dislocated and slipped down -into the cavity beneath. -That the tuffs consolidated -on slopes, perhaps quite as -steep as those of Volcano, -is now and then indicated -by an interesting structure. -The larger stones imbedded -in the layers of tuff may be observed to have on their fronts in one direction -a small heap of coarse gravelly debris, while fine tuff is heaped up -against their opposite side. This arrangement doubtless points to deposit -on a slope of loose debris, from which the larger blocks protruded so -as to arrest the smaller stones, and allow the fine dust to gather behind.</p> - -<div class="footnote"> - -<p><a id="Footnote_103" href="#FNanchor_103" class="label">[103]</a> Further illustrations of this characteristic structure of some vents will be found in the account -of the Tertiary vents of the Faroe Isles in Chapter xli. See also the remarks in the introductory -chapters, <a href="../../66492/66492-h/66492-h.htm#Page_63">vol. i. p. 63</a>.</p> - -</div> - -<p>If the inference be correct, that the stratification here described belongs -to the old craters or the upper parts of the funnels, it furnishes additional -evidence of the wide interval of time that elapsed between the deposition of -the Carboniferous strata and the outbreak of these vents. During that -interval prolonged denudation reduced the upturned Carboniferous Limestone -series to nearly its present form of surface, and any materials discharged -from the vents over the surrounding ground would obviously lie with a -violent unconformability on the rocks below.</p> - -<p>The frequent great disturbance in the bedding of the tuff within the -vents may be connected with some kind of collapse, subsidence or shrinkage -of the materials in the funnel below. That a movement of this nature did -take place is shown by the remarkable bending down of the strata round -the margins of the vents, which has been already described.</p> - -<p>The minor vents for the most part contain only fragmentary materials; -but those of larger size usually present masses of lava in some characteristic -forms. In not a few cases, the lava has risen in the central pipe and has -hardened there into a column of solid rock. Subsequent denudation, by -removing most of the cone, has left the top of this thick column projecting -as a round knoll upon the hill-top. Arthur Seat presents a good example -of this structure. Where the denudation has not proceeded so far, we may -still meet with a remnant of the cake of lava which sometimes overflowed -the bottom of a crater. The summit of Largo Law affords indications of -this arrangement, the cone of tuff being there capped with basalt, evidently -the product of successive streams, which welling out irregularly covered the -crater bottom with hummocks and hollows (<a href="#v2fig226">Fig. 226</a>). The knolls are -beautifully columnar, and sometimes show a divergent arrangement of the -prisms.</p> - -<p><span class="pagenum" id="Page_84">- 84 -</span></p> - -<div class="figcenter" id="v2fig221" style="width: 504px;"> - <img src="images/v2fig221.png" width="504" height="679" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 221.</span>—Dyke rising through the agglomerate of a volcanic vent; Kincraig, Elie.</div> -</div> - -<p><span class="pagenum" id="Page_85">- 85 -</span></p> - -<p>But the most frequent form assumed by the lava in the necks is that -of veins or dykes running as wall-like bands through the tuff or agglomerate. -Many admirable examples may be cited from the shore between Largo and -St. Monans. Two illustrations of them are given in Figs. <a href="#v2fig219">219</a> and <a href="#v2fig221">221</a>. -In <a href="#v2fig219">Fig. 219</a> the dyke is about four feet broad, and is seen to present the -common transverse jointing as it pursues its way through the tuff. White -veins of calcite along its margin serve to define its limits. Its position -in reference to the general body of the neck is shown in the ground-plan -<a href="#v2fig224">Fig. 224</a>. The second instance (<a href="#v2fig221">Fig. 221</a>) is that of a dyke of basalt -only one foot wide, which runs like a wall up the agglomerate of the -Kincraig neck near Elie. It is seen at the bottom of the cliff projecting -from the agglomerate; but higher up it has decayed, leaving its fissure as a -gaping chasm. Here the general character of the pyroclastic material is -well brought out. One or two large blocks may be seen imbedded in it, the -largest lying above where the dyke bends away to the left.</p> - -<p>The intruded masses vary in breadth from mere threadlike veins up -to dykes several yards in breadth, which sometimes expand into large -irregular lumps. They generally consist of some form of basalt; now and -then, as at Ruddon Point, near Elie, they are amygdaloidal; and it may be -observed among them, as among dykes in general, that where the amygdaloidal -texture is developed, it is apt to occur most markedly in the central -part of the vein, the amygdales running there in one or more lines parallel -with the general trend of the mass.</p> - -<p>That the basalt of these veins and dykes was sometimes injected in an -extremely liquid condition is shown by its frequently exceedingly close -homogeneous texture. Within the neck on the shore to the west of Largo, -the basalt assumes in places an almost flinty character, which here and -there passes into a thin external varnish of basalt-glass. A farther indication -of the liquidity of the original rock seems to be furnished by the great -number of included extraneous fragments here and there to be observed in -the basalt.</p> - -<p>But besides basalt, other materials may more rarely be detected assuming -the form of dykes or veins within the necks. Thus, at the Largo neck just -referred to, strings of an exceedingly horny quartz-felsite accompany the -basalt—a remarkable conjunction of acid and basic rock within the same -volcanic chimney. To the east of Elie some dykes, which stand out prominently -on the beach from a platform worn by the sea in a neck, consist of -an extremely compact volcanic mudstone, stuck full of the worn twin -crystals of orthoclase and pieces of hornblende and biotite already noticed. -So like is this rock to one of the decomposing basalts that its true fragmental -nature may easily escape notice, and it might be classed confidently -as a somewhat decayed basalt. A considerable amount of a similar fine -compact mudstone is to be seen round the edges of some of the Elie vents. -This material must have been injected into open fissures, where it solidified. -There is further evidence of the presence of "mud-lava" in some of the -vents of East Fife, where these orifices contain a remarkable compact -volcanic sandstone, composed of the usual detritus, but weathering into spheroidal -crusts, so as externally to be readily mistaken for some form of basalt.</p> - -<p><span class="pagenum" id="Page_86">- 86 -</span></p> - -<div class="figcenter" id="v2fig222" style="width: 490px;"> - <img src="images/v2fig222.png" width="490" height="640" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 222.</span>—Radiating columnar dyke in the tuff of a volcanic vent, Rock and Spindle, two and a half - miles east from St. Andrews.</div> -</div> - -<p><span class="pagenum" id="Page_87">- 87 -</span></p> - -<p>A columnar arrangement may often be observed among the basalt -dykes. When the vein or dyke is vertical, the columns of course seem -piled in horizontal layers one above the other. The exposed side of the -dyke then reveals a wall of rock, seemingly built up of hexagonal or polygonal, -neatly fitting blocks of masonry, as in the Lower Carboniferous vent -of the Binn of Burntisland (Figs. <a href="../../66492/66492-h/66492-h.htm#v1fig166">166</a>, <a href="../../66492/66492-h/66492-h.htm#v1fig166">168</a>). An inclination of the dyke from -the vertical throws up the columns to a proportional departure from the horizontal. -Sometimes a beautiful fan-shaped grouping of the prisms has taken -place. Of this structure the Rock and Spindle, near St. Andrews, presents -a familiar example (<a href="#v2fig222">Fig. 222</a>). Much more striking, however, though less -known, is the magnificent basalt mass of Kincraig, to the west of Elie, where -the columns sweep from summit to base of the cliff, a height of fully -150 feet, like the Orgues d'Expailly, near Le Puy in Auvergne. The -general position of this basalt in the vent is represented in the section -(B, <a href="#v2fig218">Fig. 218</a>). The curvature of the basalt is shown in <a href="#v2fig223">Fig. 223</a>, which is -taken from the Elie side looking westward, beyond the intrusions, to the -picturesque cliffs of tuff. The details of the cliff are given in <a href="#v2fig225">Fig. 225</a>.</p> - -<p>That many of the dykes served as lines of escape for the basalt to the -outer slopes of the cones is highly probable, though denudation has usually -destroyed the proofs of such an outflow. A distinct radiation of the dykes -from the centre of a neck is still sometimes traceable. This structure is -most marked on the south cone of Largo Law, where a number of hard ribs -of basalt project from the slopes of the hill. Their general trend is such -that if prolonged they would meet somewhere in the centre of the cone. -On the south-east side of the hill a minor eminence, termed the Craig -Rock, stands out prominently (<a href="#v2fig209">Fig. 209</a>). It is oblong in shape, and, -like the dykes, points towards the centre of the cone. It consists of a compact -columnar basalt, the columns converging from the sides towards the -top of the ridge. It looks like the fragment of a lava-current which flowed -down a gully on the outer slope of the cone (B' in <a href="#v2fig226">Fig. 226</a>).</p> - -<p>Veins of basalt are not confined to the necks, but may be seen running -across the surrounding rocks. The shore at St. Monans furnishes some -instructive examples of this character. As the veins thin away from the -main mass of basalt they become more close-grained and lighter in colour, -and when they enter dark shales or other carbonaceous rocks they pass, as -usual, into the white earthy clay-like "white-trap." The influence of -carbonaceous strata in thus altering basic dykes and sills may be instructively -studied along the shore of the East of Fife. A good instance -occurs near St. Monans Church (<a href="#v2fig227">Fig. 227</a>), where a vein of "white-trap" -traverses black shales which have been somewhat jumbled.</p> - -<p><span class="pagenum" id="Page_88">- 88 -</span></p> - -<div class="figcenter" id="v2fig223" style="width: 676px;"> - <img src="images/v2fig223.png" width="676" height="395" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 223.</span>—View of part of the shore front of the great vent at Kincraig, looking westward, with the columnar basalt in front.</div> -</div> - -<p><span class="pagenum" id="Page_89">- 89 -</span></p> - -<p>In a modern volcano no opportunity is afforded of examining the contact -of the erupted material with the rocks through which the vent has been -opened. But in the basin of the Firth of Forth, within the area now under -description, a numerous series of coast-sections lays bare this relation in the -most satisfactory manner. The superincumbent cones of tuff have been -swept away, and we can examine, as it were, the very roots of the old -volcanoes. The margin of a neck or volcanic vent is thus found to be -almost always sharply defined. The rocks through which the funnel has -been drilled have been cut across, as if a huge auger had been sunk through -them. This is well displayed in the beautifully perfect neck already cited -at Newark Castle, near St. Monans -(<a href="#v2fig224">Fig. 224</a>). The strata through which -this neck rises consist of shales, sandstones, -thin coal and encrinal limestones, -dipping in a westerly direction -at angles ranging from 25° to 60°. -At the south end of the neck they -are sharply truncated, as if by a fault. -Elsewhere they are much jumbled, -slender vein-like portions of the tuff -being insinuated among the projecting -strata. A large vertical bed of -sandstone, 24 yards long by 7 yards -broad, stands up as a sinuous reef -on the east side of the vent (<i>s</i>). It -is a portion of some of the surrounding -strata, but, so far as can be seen -at the surface, is entirely surrounded -with agglomerate. Here and there -the shales have been excessively -crumpled, and at the north end have -been invaded by a vein of basalt -which, where it runs through them, assumes the usual clay-like character. -The strata have been blown out, and their place has been occupied by a -corresponding mass of volcanic agglomerate. But their remaining truncated -edges round the margin of the orifice have undergone comparatively little -alteration. In some places they have been hardened, but their usual -texture and structure remain unaffected.</p> - -<div class="figright" id="v2fig224" style="width: 207px;"> - <img src="images/v2fig224.png" width="207" height="267" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 224.</span>—Plan of volcanic neck on beach - near St. Monans.<br /><br /> - T, Neck of tuff enclosing a mass of sandstone (<i>s</i>), and - piercing sandstones and shales With beds of limestone, - (<i>l</i> <i>l</i>), and a thin seam of coal (<i>c</i>); B, Basalt - "white-trap" dyke. The arrows show the dip of - the strata.</div> -</div> - - -<p>In a few examples, the progress of denudation has not advanced so far -that the cone cannot still be partially made out amidst its surrounding -masses of tuff. One of the most interesting of these is Largo Law, of which -an outline has been given in <a href="#v2fig209">Fig. 209</a>. The accompanying section (<a href="#v2fig226">Fig. 226</a>) -represents what appears to me to be the structure of this hill. Each of the -two now conjoined cones was probably in succession the vent of the volcano. -The southern and rather lower eminence, as already mentioned, is traversed -by rib-like dykes of basalt, which point towards its top, where there is a -bed of the same rock underlying a capping of tuff. On its eastern declivity -lies the basalt stream already described (<a href="#Page_87">p. 87</a>). The higher cone is surmounted -by a cake of basalt which, as I have above suggested, may have -solidified at the bottom of the latest crater. Of course all trace of the -crater has disappeared, but the general conical form of the volcanic mass -remains. Doubtless, still more of the old volcano would have been removed -by denudation but for the protection afforded to the tuff by the intrusion of -the basalt. The upper dotted lines in the figure are inserted merely to -indicate hypothetically how the cone may originally have stood. On the -west side the sheets of tuff which were thrown out over the surrounding -country have been almost entirely removed, but on the east and south they -still cover an extensive area. (See <a href="#v2fig208">Fig. 208</a>).</p> - -<p><span class="pagenum" id="Page_90">- 90 -</span></p> - -<div class="figcenter" id="v2fig225" style="width: 491px;"> - <img src="images/v2fig225.png" width="491" height="730" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 225.</span>—Columnar basalt in the neck of Kincraig, Elie, seen from the west.</div> -</div> - -<p><span class="pagenum" id="Page_91">- 91 -</span></p> - -<div class="figcenter" id="v2fig226" style="width: 522px;"> - <img src="images/v2fig226.png" width="522" height="137" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 226.</span>—Section across Largo Law.<br /><br /> - <i>l</i> <i>l</i>, Lower Carboniferous strata; <i>t</i>, tuff of cones; <i>t'</i>, tuff of plain beyond the cones; B B, basalt ascending vents - and sending out veins: B', basalt which has probably flowed out at the surface. The dotted lines are - suggestive of the original outline of the hill.</div> -</div> - - -<p>(2) <i>Sills.</i>—In the Clyde coal-field and in the basin of the Firth of -Forth, among the vast number of sills which there traverse the Carboniferous -formations, it is possible that some belong to the Permian volcanic period (see -<a href="../../66492/66492-h/66492-h.htm#Page_474">vol. i. p. 474</a>). Where the sheets have been intruded along horizons that lie -below the upper stratigraphical limit of the puy eruptions, they may not unnaturally -be held to belong to these manifestations of volcanic energy, though -it is obviously quite conceivable that some of them may be of much later date. -But where they lie above the highest platforms of Carboniferous lavas and -tuffs, they may be assigned to a younger volcanic period. We know as yet -of only two such periods after the deposition of the Carboniferous Limestone -series in Scotland—Permian and older Tertiary. Unless, therefore, these -higher sills formed part of some other display of subterranean activity -which is not known to have culminated in eruptions at the surface, they -must be looked upon as probably either Permian or Tertiary.</p> - -<p>In the great coal-field of Stirlingshire and Lanarkshire, among the -large sills that break into the Millstone Grit and the Coal-measures, -one lies entirely in the Coal-measures, and covers about six square miles -of ground, stretching from near Caldercruix Station, a little east of Airdrie, -to near Kirk of Shotts, a distance of about four miles. A group of -smaller sheets, possibly connected with the larger mass, runs for four -miles further west to beyond New Monkland. Another chain of sills, which -may also be part of the same great intrusion, extends from the Cant Hills, -near the Kirk of Shotts, for more than eight miles in a north-easterly -direction. The largest mass in this chain stretches from Blackridge, west -of Bathgate, for upwards of three miles, covering an area of about three -square miles and terminating on the north at the line of dislocation -which has been followed by one of the east and west dykes. Another -large sill, which appears nearly two miles further east on the north side -of that dyke, lies on a lower stratigraphical horizon, for it cuts the -Carboniferous Limestone series, and does not reach the top of the Millstone -Grit. This sill is cut through by two of the later dykes.</p> - -<p><span class="pagenum" id="Page_92">- 92 -</span></p> - -<div class="figcenter" id="v2fig227" style="width: 752px;"> - <img src="images/v2fig227.png" width="752" height="501" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 227.</span>—Vein of "white-trap" cutting black carbonaceous shales, a little west from St. Monans Church.</div> -</div> - -<p><span class="pagenum" id="Page_93">- 93 -</span></p> - -<p>That these great intrusions took place later than the deposition of the -Coal-measures is obvious. There is no satisfactory evidence to enable us to -decide to which of the two post-Carboniferous volcanic periods they may -with most probability be assigned. As one of them is distinctly cut by -dykes that have been referred to the Tertiary series, it might be plausibly -argued that it at least is of pre-Tertiary date, and therefore probably -Permian. On the other hand, as will be shown in a later chapter, some -portion of the sills appears to be connected with the younger or Tertiary -dykes. This problem must for the present remain unsolved.</p> - -<p>In Ayrshire where, as already described, basic sills traverse the Permian -volcanic series, other large intrusive sheets are found around the Permian -basin. On the north side an important group of them, passing through the -Coal-measures into the Carboniferous Limestone series, runs from Troon -eastward for more than eight miles to beyond Craigie. On the south side a -much more extensive series may be traced from the River Ayr southwards -into the Dalmellington coal-field, and thence north-eastwards in a wide -semicircular sweep into the coal-field of New Cumnock and Airds Moss. -That some of these sills proceed from the Permian necks has been definitely -ascertained, and this fact has been already alluded to in connection with the -vents. I have little doubt that the great majority, if not the whole, of -these intrusive sheets are to be referred to the Permian period.</p> - -<p>Some of the sills must be later than a part of the Permian volcanic -eruptions, for they are found in at least three places intercalated in the zone -of lavas and tuffs. But no instance has been observed of their traversing -the basin of Permian sandstone which overlies that zone, though a few dykes, -possibly of Tertiary age, do cut this sandstone.</p> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_94">- 94 -</span></p> - -<h2 class="nobreak" id="CHAPTER_XXXII">CHAPTER XXXII<br /> - -<span class="smaller">PERMIAN VOLCANOES OF ENGLAND</span></h2> -</div> - -<div class="blockquot"> -<p>The Devonshire Centre—Eruptive Rocks of the Midland Coal-fields.</p> -</div> - -<p>From the south of Scotland we need to pass to the extreme south-west of -England before we again encounter a group of volcanic rocks which may be -referred with some confidence to the Permian period. An interesting group -of lavas and tuffs has been preserved in some of the valleys over a limited -area in the east of Devonshire. The Midland coal-fields, however, are -traversed by a series of basic eruptive rocks which are younger than the -Coal-measures, and may possibly be Permian. Their mode of occurrence, -and the arguments regarding their geological age, will be given in the -present chapter.</p> - - -<h3>1. DEVONSHIRE</h3> - -<p>The counties of Devon and Cornwall furnish one of the most striking -examples to be met with in Britain of the persistence of volcanic action -over a limited area through a long succession of geological periods. The -extensive eruptions in Devonian time were followed after a long interval by -a diminished series in the Carboniferous period. But the subterranean -energy was not then wholly exhausted, for it showed itself on a feeble -scale in at least one limited tract of the same region during the Permian -period. Thus throughout the later half of Palæozoic time the extreme south-west -of England continued to be a theatre of volcanic action.</p> - -<p>The geological age of the igneous rocks now to be referred to depends -upon the particular place in the geological record to which we assign the -remarkable breccias and sandstones with which they are associated. By -many geologists who have been unable to recognize any true break in the -red rocks from their base up to the bottom of the Lias, these strata have -been grouped as one great series referable to the "New Red Sandstone" or -Trias. This is the classification adopted on the one-inch maps of the -Geological Survey. On the other hand, various able observers have pointed -out the close resemblance of the coarse and fine breccias at the bottom of -<span class="pagenum" id="Page_95">- 95 -</span> -the series to recognized Permian deposits in the centre of England and to -parts of the typical Rothliegende of Germany. I need only refer to the -strongly expressed views of Murchison, in which, as he stated in his -<i>Siluria</i>, he "entirely agreed with Conybeare and Buckland, who, after a -journey in Germany in 1816, distinctly identified the Heavytree conglomerate, -near Exeter, with the Rothliegende of the Germans."<a id="FNanchor_104" href="#Footnote_104" class="fnanchor">[104]</a> In the -absence of any fossil evidence, we have only lithological characters and -sequence to guide us, and though the known facts hardly warrant a very -positive opinion, my inclination is to regard these red Devonshire breccias -as probably Permian, and to follow Murchison in looking upon their -associated igneous masses as furnishing additional reason for assigning them -to that particular geological platform.<a id="FNanchor_105" href="#Footnote_105" class="fnanchor">[105]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_104" href="#FNanchor_104" class="label">[104]</a> <i>Siluria</i>, 4th edit. (1867), p. 333. See also Berger, <i>Trans. Geol. Soc.</i> vol. i. (1811), pp. -98-102; Conybeare and Phillips, <i>Geology of England and Wales</i>, p. 313, footnote; De la Beche, -<i>Report on the Geology of Cornwall, Devon and West Somerset</i> (1839), chap. vii. p. 193. Messrs. -Hull and Irving (<i>Quart. Journ. Geol. Soc.</i> vol. xlviii. 1892, pp. 60, 68) have more recently -discussed the subject, and follow the view of Murchison.</p> - -<p><a id="Footnote_105" href="#FNanchor_105" class="label">[105]</a> Murchison cogently argued that as no signs of volcanic activity were known in the Trias, but -were abundant in the Permian system, the Devonshire rocks might be regarded as appertaining to -the older series, <i>op. cit.</i> Triassic volcanic rocks, however, are now well known on the Continent.</p> - -</div> - -<p>No proper account has yet been written of the volcanic group which I -now propose to describe.<a id="FNanchor_106" href="#Footnote_106" class="fnanchor">[106]</a> De la Beche was, I think, the first to recognize -the true volcanic nature of the rocks and their contemporaneous interstratification -in the red sandstone series.<a id="FNanchor_107" href="#Footnote_107" class="fnanchor">[107]</a> As traced by him on the -Geological Survey maps, these rocks lie at or near the base of the red -sedimentary deposits, resting sometimes directly on the Culm-measures, -sometimes on an intervening layer of red strata. He found them in three -separate districts in the neighbourhood of Exeter, the most northerly lying -near Tiverton, the central extending from Kellerton for a few miles up the -Yeo Valley, beyond Crediton, and the third stretching from the City of -Exeter to Pen Hill, about five miles to the south-west. He recognized the -amygdaloids as slaggy lavas, and saw that the volcanic breccias and tuffs -are interleaved with the sandstones. With regard to the probable vents -from which these materials were ejected, he thought that the chief centre of -activity lay at Kellerton Park, while in other localities he believed the -bosses of igneous rock "to descend in mass downwards, as if filling up some -crater or fissure through which these rocks had been vomited."<a id="FNanchor_108" href="#Footnote_108" class="fnanchor">[108]</a> He speaks -also of "quartziferous porphyries" occurring among them, a statement -which, if petrographically accurate, would suggest the uprise of a later more -acid lava in some of the vents.</p> - -<div class="footnote"> - -<p><a id="Footnote_106" href="#FNanchor_106" class="label">[106]</a> An outline of some of their characters will be found in a paper by Mr. W. Vicary in <i>Trans. -Devonshire Assoc.</i> 1865, vol. i. part iv. p. 43.</p> - -<p><a id="Footnote_107" href="#FNanchor_107" class="label">[107]</a> See his "Report" cited in the note above. De la Beche quotes J. J. Conybeare as pointing out -the intimate connection of these igneous and stratified rocks (<i>Annals of Philosophy</i>, 2nd series, vol. -ii. (1821) p. 165); but this author wrote at the time of the Plutonist and Neptunist controversy, -and does not commit himself to any distinct expression of opinion on the subject.</p> - - -<p><a id="Footnote_108" href="#FNanchor_108" class="label">[108]</a> Report, p. 201.</p> - -</div> - -<p>More recently the ground has been revised by Mr. W. A. E. Ussher of the -Geological Survey, who has ascertained that the volcanic rocks appear in -<span class="pagenum" id="Page_96">- 96 -</span> -many more places than those where they were noted on the older maps, and -likewise extend for some miles further to the north and west.</p> - -<p>It now appears that in the central and chief district the lavas can be -followed westward from Spray Down near Kellerton to Greenslade near -North Tawton, a distance of about twenty-one miles. Their most northerly -outcrop is at Thorn above Loxbere in the Tiverton district, and their most -southerly visible portion passes under the Cretaceous rocks of Pen Hill. -The distance between these extreme points is likewise about twenty-one -miles. The whole display of volcanic phenomena is comprised within an -area of less than 400 square miles.</p> - -<p>One of the most obvious features in this volcanic tract is the way in -which the erupted materials lie along the lines of hollow or valley in which -the red rocks were deposited. This is most distinctly exhibited in the central -district. Here a belt of breccias and sandstones, varying from one to three -and a half miles in breadth, runs for about five and twenty miles westward -in a depression of the Culm-measures. At intervals, the lavas which lie -near the base of the red rocks crop out along the margin of the belt -throughout most of its extent. But they do not spread out over the older -rocks, and they have evidently been erupted from orifices situated along the -line of the valley. It is another example of the relation between the trend -of hollows and the outbreak of volcanic vents, which I have referred to as -so strikingly displayed in the distribution of the Permian volcanic rocks of -south-western Scotland.</p> - -<p>The volcanic materials of the Devonshire Permian district consist -mainly of lavas, but include also red sandy and gravelly tuffs. The -whole volcanic group is remarkably thin, never attaining even the limited -development of the Ayrshire series. No adequate petrographical investigation -of these rocks has yet been made. Externally, as seen in the quarries -and lanes, the lavas present the closest resemblance to those of the Permian -basins of Ayrshire and Nithsdale. They show considerable differences of -texture even within the same mass, some portions being dull, fine-grained -purplish-red rocks, with scattered pseudomorphs of hæmatite and a few -porphyritic felspars, other parts passing into an exceedingly coarse amygdaloid -or slaggy pumice. Dr. Hatch, after a microscopical examination of -a small collection of specimens, found that while most are olivine-basalts, -containing ferruginous pseudomorphs after olivine (Raddon Court, Pocombe, -and near Budlake), others are true andesites (Ide, Kellerton Park) and even -mica-trachytes (Copplestone, near Knowle Hill).<a id="FNanchor_109" href="#Footnote_109" class="fnanchor">[109]</a> As already remarked, -some of the older writers mention the existence of quartz-porphyries.<a id="FNanchor_110" href="#Footnote_110" class="fnanchor">[110]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_109" href="#FNanchor_109" class="label">[109]</a> <i>Geol. Mag.</i> 1892, p. 250. The rocks have been more recently described by Mr. B. Hobson, -<i>Quart. Jour. Geol. Soc.</i> vol. xlviii. (1892), p. 502. The rock of Kellerton Park is called by Mr. -Hobson "mica-augite-andesite," and he gives a chemical analysis of it by Mr. E. Haworth, <i>op. cit.</i> -p. 507. Mr. Watts has lately found one of the orthoclase rocks to be rich in olivine.</p> - -<p><a id="Footnote_110" href="#FNanchor_110" class="label">[110]</a> See De la Beche, <i>Report</i>, pp. 203, 204. My colleague, Mr. Ussher, found close to the Thurlestone -outlier of conglomerate near Kingsbridge, Devonshire, a small boss of quartz-porphyry -which rises through and alters the Devonian rocks. The actual junction of this mass with -the conglomerate is not seen, nor have any fragments of the porphyry been noticed among the -pebbles.</p> - -<p>Mr. Ussher informs me that in the quarry the visible exposure of the acid rock is surrounded -an covered by mica-porphyrite, probably andesite.</p> - -</div> - -<p><span class="pagenum" id="Page_97">- 97 -</span></p> - -<p>The geographical conditions in which the red rocks of Devonshire -accumulated were those so characteristic of the Permian and Trias -formations throughout Britain. The red sandstones and sandy marls -gathered in inland basins, where the water seems to have become too -saline and bitter to support animal life. The strata are consequently -singularly devoid of organic remains. The climate was probably arid, -and the absence or scarcity of traces of terrestrial vegetation indicates -that the land around the water-basins stretched in wide sandy and rocky -wastes. In the dry atmosphere and under the influence of rapid radiation -the cliffs and crags of Culm-measures would disintegrate into angular rubbish, -and this material, slipping into the lakes or washed down by occasional rain-storms, -forms now the breccias that constitute so typical a feature in the -Permian system.</p> - -<div class="figcenter" id="v2fig228" style="width: 352px;"> - <img src="images/v2fig228.png" width="352" height="97" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 228.</span>—Section at Belvedere, S.W. of Exeter.<br /><br /> - <i>a</i>, Culm-measures; <i>b</i>, breccia and marls; <i>c</i>, lavas; <i>d</i>, red pebbly sandstones.</div> -</div> - -<div class="figcenter" id="v2fig229" style="width: 331px;"> - <img src="images/v2fig229.png" width="331" height="107" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 229.</span>—Diagram to show the unconformability and overlap of the Permian rocks - in the Crediton Valley.<br /><br /> - <i>a</i>, Culm-measures; <i>b</i>, breccias and sandstones; <i>c</i>, lava-group; <i>d</i>, breccias with fragments of lava - passing up into sandstones and marls (<i>e</i>).</div> -</div> - -<p>It was while this geographical type continued in the South-west of -England that the volcanic eruptions took place which we are now considering. -De la Beche correctly referred these eruptions to the early part of the red -sandstone series. A brief examination of the ground suffices to show that -although, as he pointed out, the volcanic rocks lie towards the base of that -series, as shown in <a href="#v2fig228">Fig. 228</a>, they do not all occupy the same platform. That -in some cases the lavas lie directly on the Culm-measures, while in others -they are separated from these strata by 100 feet or more of red sandstones -and breccias (<a href="#v2fig229">Fig. 229</a>), would not in itself be proof of any difference of -age or stratigraphical position in the igneous rocks, for the floor on which -the Permian formations were here laid down can be shown to have been -<span class="pagenum" id="Page_98">- 98 -</span> -singularly uneven. Prominent hills of Culm grit, several hundred feet -high, rose above the basins in which the earliest Permian sediments were -deposited, and these eminences were gradually submerged and buried under -the detritus.</p> - -<p>But that the volcanic zone includes in some places more than one outflow -of lava with layers of sandstone, breccia and tuff between the successive -sheets may be proved in different parts of the district. Thus the two conspicuous -hills at Kellerton are composed of several sheets of highly slaggy -lava, separated by breccia, and a third much thinner sheet lies above these, -intercalated in a mass of breccia, sandstone and sandy tuff (<a href="#v2fig230">Fig. 230</a>). -Again, at Budlake the sandstones and fine breccias include a thin band of -vesicular lava, while farther to the east they are interrupted by a higher -and thicker zone of similar material.</p> - -<div class="figcenter" id="v2fig230" style="width: 353px;"> - <img src="images/v2fig230.png" width="353" height="102" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 230.</span>—Section of the volcanic series at Kellerton, Devonshire.<br /><br /> - <i>a</i>, Breccias and sandstones; <i>b</i>, lavas.</div> -</div> - - -<p>These igneous sheets can be shown by many interesting sections to have -been poured out contemporaneously with the deposit of the sedimentary -material among which they occur. At Crabtree, for instance, near Kellerton, -the uppermost lava is a thin sheet of highly slaggy texture, which rests -immediately on the gravelly red sandstone and catches up parts of it, while -the pebbles include fragments of some of the andesites below. The dark -lavas are occasionally traversed by veins of fine hard sandstone, which -descending from above, like those in the Old Red Sandstone and Permian -lavas of Scotland, have been produced by the silting or drifting of fine sand -into cracks in the lava, before the igneous material was entirely buried. -These features are well exposed in the high ridge of the Belvedere near -Exeter (<a href="#v2fig228">Fig. 228</a>), where, over a thin and inconstant band of red breccia -and marl which rests on the upturned ends of the Culm-measures, a band of -dull-red andesite may be seen. This rock, partly compact and partly highly -amygdaloidal, is in some portions full of irregular fissures and cavities filled -with sandstone.</p> - -<p>Nowhere among the Palæozoic volcanic rocks of Britain are more -remarkable examples of the slaggy structure to be found than in these -Devonshire lavas of probably Permian age. I would especially cite the -rock of Knowle Farm, a few miles to the west of Crediton, as in part a mere -spongy pumice, blocks of which would originally have floated in water.</p> - -<p>One of the best sections in the district for the exemplification of the -internal structures of these lavas is that in the large quarry at the top of -Posbury Hill. On the west side of this quarry the rock is tolerably compact, -<span class="pagenum" id="Page_99">- 99 -</span> -but contains vesicles and irregular steam-holes. On the east side it -passes upward and laterally into a coarse agglomerate of its own fragments, -and in its mass it encloses similar agglomerate. No sharp passage can be -traced between the two rocks. So far as I could judge, it seemed to me -that the lava had broken up as it moved along, possibly -shattered by coming in contact with water. The -agglomerate is overlain by some reddish ashy sandstone, -which fills up the interstices between the slags, -and is immediately covered by a bed of lilac andesite, -marking another distinct outflow.</p> - -<div class="figright" id="v2fig231" style="width: 136px;"> - <img src="images/v2fig231.png" width="136" height="193" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 231.</span>—Section of agglomerate - overlain with - sandstone and andesite, - Posbury, Crediton.</div> -</div> - - -<p>As in Ayrshire, the lavas of Devonshire are not -accompanied by any thick accumulation of tuff. The -fragmentary discharges consisted in both areas of fine -dust and gravelly detritus of small lapilli, which were -not ejected in such quantities as entirely to conceal -the ordinary non-volcanic sediment of the water-basin. -The dust and cinders mingled with the red -sand and angular scree-material, so that we now see a -group of red, somewhat ashy sandstones and breccias. -Among the component fragments of the breccias, -a considerable variety of igneous material may be observed. While the -most of the non-volcanic stones may have been derived by ordinary processes -of weathering from rocks exposed at the surface, it is by no means improbable -that some of them, including even pieces of Culm grit, killas and baked -slate, may have been ejected from volcanic vents.<a id="FNanchor_111" href="#Footnote_111" class="fnanchor">[111]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_111" href="#FNanchor_111" class="label">[111]</a> On the composition of the Devonshire breccias see Mr. R. N. Worth, <i>Quart. Journ. Geol. Soc.</i> -vol. xlvi. (1890), p. 69. This author has adopted the view that the granite of Dartmoor represents -the neck of a great volcano from which these later volcanic materials were ejected. But all the -evidence seems to me in favour of numerous small vents situated not far from the outcrops of the -lavas, as stated in the text. See Mr. B. Hobson, <i>Quart. Journ. Geol. Soc.</i> vol. xlviii. (1892), -p. 498. The Dartmoor granite is later than the surrounding Carboniferous rocks, but no good -evidence has been obtained to connect it with the Permian volcanic phenomena of Devonshire.</p> - -</div> - -<p>Taking the volcanic rocks of this district as a whole, I regard them as -the mere edges of sheets that have flowed from vents which not improbably -lie concealed somewhere along the centres of these old Permian valleys. -No visible necks have been described from any part of the area, and though -I have not examined the whole of it, nothing of that nature was detected by -me either in the Crediton Valley or between Silverton and the Exeter -neighbourhood. The Tiverton district, which has not yet been searched, -appears to be the only tract where any chance remains of finding some of -the vents.</p> - -<p>No indication of any sills has been met with among the Devonshire -Permian rocks. None of the lavas which I have seen have the internal -characters of true sills, while in the field their association with the sandstones -and breccias in no observed case points to intrusion.</p> - -<p>Though much remains to be done in this region before an adequate -account can be given of the interesting series of eruptions which concludes -<span class="pagenum" id="Page_100">- 100 -</span> -the long volcanic history of the South-west of England, enough is known to -indicate the general character of the phenomena. The eruptions were on -even a feebler scale than those of the Permian period in Scotland, but they -seem to have resembled them in their general character. Small puy-like -vents were opened, from which dark scoriaceous lavas and showers of -gravelly tuff and stones were discharged over the floor of the inland sea or -lake-basin in which the red sandstones and breccias were accumulated. -These outflows and explosions took place too, as in Scotland, towards the -beginning of the deposition of the red strata, and entirely ceased long before -that deposition came to an end. In each area the eruptions mark the -close of Palæozoic volcanic activity in Britain. The varied and recurrent -volcanic episodes which distinguished each successive geological period from -the Archæan onwards now definitely terminate, not to be resumed until -after the passing of the whole of the vast cycle of Mesozoic ages.</p> - - -<h3>2. ERUPTIVE ROCKS IN THE MIDLAND COAL-FIELDS</h3> - -<p>Between the thick and thoroughly marine development of the Carboniferous -Limestone in Derbyshire and in South Wales, there lies the region, -already referred to, wherein both the Carboniferous Limestone and Millstone -Grit die out against what must have been a ridge of land or group of islands -that stretched in a general east and west direction from the high grounds -of Wales through Shropshire, Staffordshire and Leicestershire. On the -slopes of this ridge the limestone is gradually overlapped by the Millstone -Grit, and both are in turn overlapped by the Coal-measures, which are then -found lying immediately on the more ancient rocks of the region—Cambrian -or pre-Cambrian, Silurian and Old Red Sandstone. The gradual subsidence -that led to the deposit of several thousand feet of Carboniferous strata over -the regions to north and south, before the beginning of the Coal-measure -period, does not seem to have sensibly affected the persistence of this old -terrestrial surface, which probably lay on an axis of upward movement, so -that, amidst the surrounding depression, its position above water was on the -whole maintained. But there are indications that the inequality of movement -in this part of the earth's crust was of much older date than the -Carboniferous period. The Old Red Sandstone is conformably continuous -below the base of the Carboniferous system, and in Wales is estimated to be -some 10,000 feet thick. No break has yet been detected in this vast -accumulation of sedimentary material, though it is highly probable that some -such unconformability must exist in it as that between the Scottish Lower -Old Red Sandstone, which passes down into the Upper Silurian shales, and -Upper Old Red Sandstone, which graduates upward into the base of the -Carboniferous formations. But even if such a break should be discovered, -it will not account for the position of the Coal-measures on Cambrian or -even perhaps older rocks. It is hardly conceivable that, had these rocks -been covered with a full development of Old Red Sandstone, they could have -<span class="pagenum" id="Page_101">- 101 -</span> -been stripped of it by denudation before the deposition of the Coal-measures. -It seems much more probable that the discrepancy in the terrestrial movements -had commenced in Old Red Sandstone time, and that these ridges -of ancient Palæozoic rocks never sank below the waters in which the vast -thickness of red sandstones, marls and conglomerates was laid down.<a id="FNanchor_112" href="#Footnote_112" class="fnanchor">[112]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_112" href="#FNanchor_112" class="label">[112]</a> See a discussion of this subject in Jukes' Preface to his <i>South Staffordshire Coal-field</i>.</p> - -</div> - -<p>But apart from the question of its antiquity, this tract of persistent -land has a special interest in the history of volcanic action in Britain, for it -was the scene of some remarkable protrusions of eruptive material which -took place after a part, and possibly after the whole, of the Coal-measures -were accumulated. The date of these protrusions cannot be fixed with -greater precision; but there can be no doubt that they belong to one of the -later volcanic periods in the geological history of Britain, and the account -of them is therefore included in the present Chapter of this work.</p> - -<p>In the English Midlands south of Stafford, over a tract of country about -700 square miles in extent, stretching from Birmingham on the east, across -the vale of the Severn, to the uplands of Shropshire on the west, the Coal-measures, -partly isolated into outliers by denudation and partly separated -by overlying younger formations, are pierced by masses of intrusive igneous -rocks. Many of these masses have long been familiar to geologists. Those, -for example, of the Clee Hills of Shropshire, and the Rowley, Barrow and -Pouk Hills of Staffordshire and Worcestershire, have been frequently described, -their relations to the surrounding strata have been minutely sought -out, their composition has been chemically determined, and their microscopic -structure has been investigated. But they have been studied rather as -individual masses of local importance. No attempt has yet been made to -ascertain how far they are capable of being grouped together as one connected -series, linked with each other in chemical and mineralogical characters, -and containing a definite record in the volcanic history of the country. -This is a task which, it is to be hoped, some competent inquirer will before -long undertake.</p> - -<p>In the meantime it is only possible to review here the already published -information, and to gather from it what may at present be surmised to have -been the history of these later eruptions of the Midlands.</p> - -<p>The areas where the igneous rocks now to be described are exhibited -may be conveniently placed in the following five groups:—1st, Titterstone -Clee Hill; 2nd, Brown Clee Hill; 3rd, The Forest of Wyre Coal-field; 4th, -The Coalbrookdale Coal-field; and 5th, The South Staffordshire Coal-field.</p> - -<p>1. <i>The Titterstone Clee Hill</i> forms a ridge about seven miles long and a -mile and a quarter broad, running in a north-easterly direction over the Old -Red Sandstone uplands of the south of Shropshire. The ground rises gradually -towards the south-west, until it reaches there a height of 1754 feet -(<a href="#v2fig232">Fig. 232</a>). On the north-western side of the ridge, the last vanishing -representative of the Carboniferous Limestone can be seen to be overlapped -the Millstone Grit, which, as it is traced towards the south-west, is in -turn overlapped by the Coal-measures, and these, about 400 feet thick, then -<span class="pagenum" id="Page_102">- 102 -</span> -rest immediately on the Old Red Sandstone. Two sheets of columnar -olivine-dolerite, possibly originally connected, lie as cakes on the summit -and eastern slope of the ridge, and cover in all a space of about a square -mile and a half. The larger sheet, which varies from 60 to 180 feet in -thickness, overlies the Coal-measures, and the coals of the Cornbrook coal-field -have been worked underneath it. The smaller mass, which may -be 300 feet in thickness, forms the summit of the ridge. On its eastern -side it reposes on Coal-measures, which are there much disturbed; but on -the west side, where it forms a bold capping to the escarpment, it is underlain -at once by the Old Red Sandstone. There cannot be any doubt that -these masses of eruptive material are sills, which have been injected into the -Carboniferous strata, and partly between these strata and the Old Red -Sandstone. One or more dykes of eruptive rock have been met with in -mining, and the coal on approaching them undergoes alteration.<a id="FNanchor_113" href="#Footnote_113" class="fnanchor">[113]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_113" href="#FNanchor_113" class="label">[113]</a> See J. R. Wright, <i>Trans. Geol. Soc.</i> (2nd ser.) iii. (1832), p. 487. Titterstone Clee Hill is -shown on Sheet 55 N.E. and N.W. of the Geological Survey, and in Horizontal Sections, Sheets -33 and 36, from which <a href="#v2fig232">Fig. 232</a> is reduced. The microscopic structure of the dolerite has been described -by Mr. Allport, <i>Geol. Mag.</i> 1870, p. 159; <i>Quart. Journ. Geol. Soc.</i> xxx. (1874), p. 550.</p> - -</div> - -<div class="figcenter" id="v2fig232" style="width: 454px;"> - <img src="images/v2fig232.png" width="454" height="100" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 232.</span>—Diagrammatic section across Titterstone Clee Hill.<br /><br /> - 1. Old Red Sandstone; 2. Carboniferous Limestone; 3. Millstone Grit; 4. Coal-measures; 5 5. Columnar olivine-dolerite.</div> -</div> - -<p>2. <i>Brown Clee Hill</i> consists of two outliers of Coal-measures, each about -a mile long, placed on the summit of a broad ridge of Old Red Sandstone, -and rising to a height of 1800 feet above the sea. Both of the outliers is -capped with a cake of dolerite, and a third smaller patch of the same -material lies on the southern outlier between the cappings. Neither at this -locality nor around Titterstone Clee have any eruptive rocks been observed -rising through the older strata. It is evident that in both cases the orifices -or fissures up which the molten material rose lie concealed under the surviving -cakes of dolerite.<a id="FNanchor_114" href="#Footnote_114" class="fnanchor">[114]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_114" href="#FNanchor_114" class="label">[114]</a> Brown Clee Hill is mapped in Sheet 61 S.W. of the Geological Survey, and its structure is -shown in Sheet 36 of the Horizontal Sections.</p> - -</div> - -<p>3. <i>Forest of Wyre Coal-field.</i>—On both sides of this extensive tract of -Coal-measures, the strata near the base of the series are traversed by sills -or dykes of olivine-dolerite like that of the Clee Hills. The sandstones in -contact with the eruptive rock have been indurated. In this district, also, -the evidence shows that the sheets are intrusive, and later than the portion -of the Coal-measures there visible.<a id="FNanchor_115" href="#Footnote_115" class="fnanchor">[115]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_115" href="#FNanchor_115" class="label">[115]</a> This district is represented in Sheets 55 N.E. and 61 S.E. of the Geological Survey. The -microscopic structure of the larger mass on the west side of the coal-field, and the variations in the -minute structure of the intrusion which forms a long ridge on the east side, are described by Mr. -Allport, <i>Quart. Journ. Geol. Soc.</i> xxx. pp. 550, 551.</p> - -</div> - -<p><span class="pagenum" id="Page_103">- 103 -</span></p> - -<p>4. <i>Coalbrookdale Coal-field.</i>—In this interesting district a sill of rather -finely crystalline olivine-dolerite, which is estimated to be nearly 200 feet -thick, is traceable from near Little Wenlock for three miles to the north, -intercalated between the Carboniferous Limestone and the Silurian rocks -underneath. It appears to underlie the western part of the Coal-field, for it -is exposed by denudation in several valleys between Little Wenlock and -Great Dawley. Owing to the thinning out of the Carboniferous Limestone -in an easterly direction, the sill gradually comes to have the Millstone Grit on -its upper surface, and at one point is represented on the Geological Survey -map as even intruded into the Coal-measures. Here again we have an -intrusive sheet of later date than at least the earlier part of the Coal-measures, -and no evidence of any superficial outflow of volcanic material.<a id="FNanchor_116" href="#Footnote_116" class="fnanchor">[116]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_116" href="#FNanchor_116" class="label">[116]</a> The Coalbrookdale coal-field has been described by Sir Joseph Prestwich, <i>Trans. Geol. Soc.</i> (2) -v. p. 428; and Prof. E. Hull, <i>Quart. Jour. Geol. Soc.</i> xxxiii. (1877), p. 629. The minute structure of -the sill at Little Wenlock is referred to by Mr. Allport, <i>op. cit.</i> p. 550. The ground is mapped on -Sheet 61 N.E. of the Geological Survey, and its structure is shown on Sheet 54 of the Horizontal -Sections.</p> - -</div> - -<p>5. <i>South Staffordshire Coal-field.</i>—This district, in respect to its igneous -intercalations, has been much more fully examined and described than any -of the others. It forms the subject of an exceedingly able memoir by Jukes, -who carefully studied its geology and delineated it on the maps and sections -of the Geological Survey. Since his time the rocks have been studied -microscopically, but no material facts regarding the stratigraphy have been -obtained in addition to those which he patiently collected and generalized -upon.<a id="FNanchor_117" href="#Footnote_117" class="fnanchor">[117]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_117" href="#FNanchor_117" class="label">[117]</a> Jukes, "South Staffordshire Coal-field," <i>Mem. Geol. Surv.</i> 2nd edit. (1859). The area is -embraced in Sheet 62 N.W. and S.W. of the Geological Survey, and is illustrated in Sheets 23, -24 and 25 of the Horizontal Sections.</p> - -</div> - -<p>This coal-field is above 20 miles long and 5 miles broad. Its strata rest -unconformably on Upper Silurian strata, which, as part of the ancient ridge -or island already referred to, project here and there from amidst the Coal-measures. -The boundaries of the field on the east and west sides are chiefly -made by faults which bring down Permian and Triassic formations against -the Carboniferous strata.</p> - -<p>Throughout this coal-field sheets of igneous rock are abundant. -In the detailed account of them given by Jukes in his admirable essay -on the South Staffordshire Coal-field,<a id="FNanchor_118" href="#Footnote_118" class="fnanchor">[118]</a> he distinguished two kinds of -igneous material—"basalt," which comes out at the surface, and sometimes -overlies the Coal-measures in large cakes like that of the Rowley -Hills, which extends for two miles in one direction and more than a mile -in another; and "greenstone," which burrows among the coal-bearing -strata, and gives off dykes and veins of "white rock-trap." There does not -appear, however, to be any essential difference in composition, age or origin -between these contrasted kinds of igneous material. They not improbably -all belong to one series of extrusions, their distinctions being due rather to -<span class="pagenum" id="Page_104">- 104 -</span> -the conditions under which they were erupted, and in particular to their -comparative thickness, and the influence of adjacent coals and carbonaceous -shales upon them.</p> - -<div class="footnote"> - -<p><a id="Footnote_118" href="#FNanchor_118" class="label">[118]</a> <i>Op. cit.</i> <a href="#Page_117">p. 117</a>.</p> - -</div> - -<p>The igneous rocks seen at the surface in this district form a series -of well-marked eminences. Of these the largest extends as a ridge from -Dudley to beyond Rowley Regis, a distance of more than two miles. To the -west of this tract, a number of small patches of the same material crop out -at the surface, the most important forming Barrow Hill. Six miles farther -north another group of similar patches may be seen. Of these the largest -occurs at Wednesfield, but the most noted forms the Pouk Hill, which has -long been noted for the beauty of its columnar structure.</p> - -<p>The sheets of "greenstone" met with in the coal-field are more numerous -and extensive than the detached areas of more compact rock visible above -ground, a single sheet being sometimes traceable in the coal-workings for -two miles in one direction.</p> - -<p>The eruptive rocks of this district, when examined in their freshest form, -consist of well-preserved olivine-dolerite. An examination of the "greenstone" -and the "white rock-trap," which runs in fingers and threads through -the coal, shows that these are really the same dolerite which has undergone -alteration, the ferruginous silicates having especially been decomposed.<a id="FNanchor_119" href="#Footnote_119" class="fnanchor">[119]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_119" href="#FNanchor_119" class="label">[119]</a> Allport, <i>Quart. Journ. Geol. Soc.</i> xxx. (1874), p. 547. Chemical analysis also shows the identity -of the rocks and the nature of the alteration of the "white rock." See Jukes, "South Staffordshire -Coal-field," pp. 117, 118.</p> - -</div> - -<p>The sills of greenish decomposed material that have been injected amongst -and alter the coals, vary from 15 feet to 80 or 90 feet in thickness. The -largest of the dolerite cakes on the surface, that of the Rowley Hills, is -somewhat irregular in its thickness, but may reach as much as 100 feet.</p> - -<p>That nearly the whole of the igneous material is intrusive is admitted -by all observers who have studied the ground. The manner in which the -"basalts" and "greenstones" send out veins into the Coal-measures shows -conclusively that they have been injected into the strata. The only rock -about which some doubt has been expressed is that of the Rowley Hills, -which Jukes was disposed, though not without some hesitation, to consider -as part of an actual lava-stream. He based this inference chiefly on the -occurrence, immediately under the dolerite, of what he looked upon as a -"trappean breccia or brecciated ash, containing rounded and angular fragments -of igneous rock lying in a brown rather ferruginous paste, that looks -like the debris of a basaltic rock."<a id="FNanchor_120" href="#Footnote_120" class="fnanchor">[120]</a> This breccia he regarded as belonging -to and passing into the Coal-measures, and he was thus inclined to regard -the dolerite as a lava of Coal-measure age.</p> - -<div class="footnote"> - -<p><a id="Footnote_120" href="#FNanchor_120" class="label">[120]</a> <i>Op. cit.</i> <a href="#Page_119">p. 119</a>.</p> - -</div> - -<p>It is possible, however, that the "trappean breccia" may be of the same -nature as the "uncompressed balls of basalt bedded in a mass of decomposed -basalt or basaltic 'wacke' or clay"<a id="FNanchor_121" href="#Footnote_121" class="fnanchor">[121]</a>—that is, a decayed contact -layer of the eruptive rock. But if it be regarded as the fragmental accompaniment -of a lava-stream, it can hardly belong to the Coal-measures. If -<span class="pagenum" id="Page_105">- 105 -</span> -the dolerite had been a lava of that age, it ought to be found lying conformably -on the Coal-measures. But this it does not appear to do. Making -every allowance for the way in which an advancing current of lava might -plough up soft sediment on the bottom of the sea or of a lake, we can hardly -thus account for the very uneven surface of Coal-measures on which the -sheet of igneous rock rests. If the Rowley rock be looked upon as a lava -which flowed out at the surface, it must, I think, be assigned to a time subsequent -to that of the Coal-measures, when these strata had been upraised -and had suffered some amount of denudation. I confess, however, that the -petrographical characters of the rock, the alteration of the coals which have -been worked underneath it, and the abundant veins of "white rock" which -there traverse the seams, induce me to regard this rock as forming no -exception to the general rule in the Midlands, but as having been intruded -as a sill, now laid bare by denudation. Its fresher condition may arise -from its thickness, or from some other circumstance which has not been -ascertained.</p> - -<div class="footnote"> - -<p><a id="Footnote_121" href="#FNanchor_121" class="label">[121]</a> <i>Op. cit.</i> <a href="#Page_126">p. 126</a>.</p> - -</div> - -<p>We have now to consider the probable geological date of the various -intrusions of basic igneous material which can be traced over so wide an -area in the centre of England. In discussing the subject, Jukes pointed out -that in the surrounding district "no igneous rocks of any kind are found in -any formation newer than the Coal-measures."<a id="FNanchor_122" href="#Footnote_122" class="fnanchor">[122]</a> This statement is, with the -exception of one locality, undoubtedly true.<a id="FNanchor_123" href="#Footnote_123" class="fnanchor">[123]</a> But on any view there must -have been a long interval of time between the formation of the highest strata -of the South Staffordshire coal-field and that of the lowest Permian deposits -of the district. It is quite conceivable, though at present incapable of proof, -that the extravasation of eruptive material took place after the close of the -Carboniferous period and during the earlier part of the Permian period.</p> - -<div class="footnote"> - -<p><a id="Footnote_122" href="#FNanchor_122" class="label">[122]</a> <i>Op. cit.</i> <a href="#Page_131">p. 131</a>.</p> - -<p><a id="Footnote_123" href="#FNanchor_123" class="label">[123]</a> See note on next page.</p> - -</div> - -<p>Jukes further shows that "at whatever period these igneous rocks were -produced, they were all existent before the production of the faults and dislocations -that have traversed the Coal-measures, and before any great -denudation had been effected on the country." This argument may be -readily granted. But, so far as we know, many, if not most, of the faults -traverse also the surrounding Permian and Triassic rocks, so that igneous -masses protruded during those periods would be affected by the same -dislocations.</p> - -<p>When we consider the history of Palæozoic time in this country, and -especially the proof, obtainable everywhere else in Britain, that volcanic energy -became quiescent during the accumulation of the Coal-measures, we may well -demand better evidence than has hitherto been forthcoming that any portion of -the dolerites of the Midlands is of Carboniferous age. It is important to -notice that though the dolerite sills and veins are so abundant in the South -Staffordshire coal-field, coming even in many places up to the present surface -of the ground, no single case has been observed where they rise into the -Permian rocks that overlie the Coal-measures unconformably. It is difficult -to believe that, had these intrusions taken place after the deposition of the -<span class="pagenum" id="Page_106">- 106 -</span> -younger formation, they should not be found penetrating it.<a id="FNanchor_124" href="#Footnote_124" class="fnanchor">[124]</a> It seems -almost certain that they must be of an age intermediate between the Coal-measures -of South Staffordshire and the surrounding breccias and sandstones -of the Permian series. And as there is clear evidence of contemporaneous -volcanic action in the lowest part of the Permian system to the north in -Scotland and to the south in Devonshire, the inference seems not unreasonable -that these intrusive basalts of the Midlands are most probably of -Permian age.</p> - -<div class="footnote"> - -<p><a id="Footnote_124" href="#FNanchor_124" class="label">[124]</a> Only one instance is known where in Staffordshire any igneous rock has been intruded into -rocks younger than the Coal-measures (Allport, <i>Quart. Journ. Geol. Soc.</i> vol. xxx. p. 551; Sheet -72 S. W. of the Geological Survey, and Horizontal Sections, Sheet 57). It forms a dyke which has -been traced near Norton Bridge, Swinnerton and Butterton, running for 8 miles in a N.N.W. -direction, and rising through Permian, Bunter and Keuper strata. It is a highly basic olivine-basalt, -and is unquestionably a dyke. Mr. J. Kirkby, who has recently mapped and described it -(<i>Trans. North Staffordshire Naturalists' Field-Club</i>, xxviii. (1894), p. 129), suggests that it may be -connected with the igneous rocks of the South Staffordshire coal-field. But of this idea there is no -evidence. The last point to which the dyke has been traced is some five-and-twenty miles from -the nearest known portion of the dolerites of the coal-field. I have little doubt that this dyke is -really an outlying member of the great system of Tertiary dykes described in Book VIII. of the -present work.</p> - -</div> - -<p>No trace of vents has been met with in the Coal-measures of the Midland -district or among the surrounding older rocks, nor any proof that the -abundant sills and veins were connected with the eruption of volcanic -materials at the surface. Nevertheless, from the analogy of the structure -of these intrusive sheets to that of the sills in such volcanic districts as the -southern half of Scotland, we may well believe that they were connected -here and there with eruptive vents, and thus that besides the northern and -southern districts of Permian volcanoes, there rose a central group among -the lagoons of the heart of England. Though no vestige of any such -group has been detected, we must remember that a large portion of the -Midlands is overspread with Permian and Triassic deposits, and that -much more igneous rock may be concealed than appears at the surface. -Possibly there may be buried under these younger sheets of red sandstone -and marl, lavas and tuffs with their connected vents, such as may be seen -where the Permian volcanic series has been laid bare by denudation in -Ayrshire and Devonshire. In this respect it would be interesting to make -a thorough examination of the Permian breccias of the district, with the -view of discovering whether, though the volcanic rocks <i>in situ</i> may still lie -covered up, fragments of them may not be found in these deposits.</p> - - -<table id="v2map5" style="border: #000 1px;" summary="Map V"> -<tr> - <td class="tdl vsmall" colspan="2">TO ACCOMPANY SIR ARCHIBALD GEIKIE'S "ANCIENT VOLCANOES OF BRITAIN"</td> - <td class="tdr vsmall">Map V.</td> -</tr> -<tr> - <td colspan="3"><a href="images/v2map5lg.png"><img src="images/v2map5.png" width="567" height="632" alt="" /></a></td> -</tr> -<tr> - <td style="width: 33%;" class="tdl vsmall">The Edinburgh Geographical Institute</td> - <td style="width: 33%;" class="tdc vsmall">Copyright</td> - <td style="width: 33%;" class="tdr vsmall">J. G. Bartholomew</td> -</tr> -<tr> - <td colspan="3" class="tdc smaller" style="padding-top: 0.5em;">MAP OF THE PERMIAN VOLCANIC DISTRICTS OF SCOTLAND<br /> - Click on map to view larger sized.</td> -</tr> -</table> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_107">- 107 -</span></p> - -<h2 class="nobreak" id="BOOK_VIII">BOOK VIII<br /> - -<span>THE VOLCANOES OF TERTIARY TIME</span></h2> - -<h2 class="nobreak" id="CHAPTER_XXXIII">CHAPTER XXXIII</h2> -</div> - -<div class="blockquot"> - -<p>Vast lapse of time between the close of the Palæozoic and beginning of the Tertiary -Volcanic Eruptions—Prolonged Volcanic Quiescence—Progress of Investigation -among the Tertiary Volcanic Series of Britain.</p> -</div> - - -<p>From the evidence which has been led in the foregoing chapters it is clear -that during the later stages of the Palæozoic period there was a gradual -enfeeblement of volcanic vigour over the area of the British Isles. When -the last puys of the Permian series became extinct a remarkable volcanic -quiescence settled down on the region. This interval of rest lasted -throughout the whole of the long succession of the Mesozoic ages. Though -the geological record of this section of geological time is singularly complete -in Britain, not a single vestige has yet been found in it of any contemporaneous -eruption. And what is true of this country is, on the whole, true -of the entire European continent. With some trifling exceptions there were -no volcanoes in Europe, so far as we know, during the enormous lapse of -time between the last of the Palæozoic and the earliest of the Tertiary -eruptions.</p> - -<p>When the geologist attempts to form an estimate of the chronological -value of this interval of time he is soon lost in bewilderment over its -obvious vastness, and the impossibility of discovering any standards of -measurement by which to reckon its duration. On the one hand, he sees -that it lasted long enough to admit of the gradual elaboration of many -thousands of feet of various sedimentary deposits, which, from their remarkable -diversities of character, were evidently accumulated, on the whole, with -extreme slowness and amidst many geographical vicissitudes. On the other -hand, he perceives that the interval sufficed to bring about an entire change -in the fauna and flora of the globe. Indeed, the more he investigates the -details of this biological transformation, the more he is impressed with the -length of time that it must have required. For it is not merely one -complete change, but a multifold succession of changes. The stratigraphical -<span class="pagenum" id="Page_108">- 108 -</span> -records of the long array of geological periods over which it was spread -show that the biological evolution advanced through a vast series of species, -genera and orders which one by one appeared and disappeared.</p> - -<p>The ages that elapsed between the final dying out of the Palæozoic -volcanoes and the outburst of those of Tertiary time were so protracted that -many revolutions of the geography of Europe were comprised within them. -Land and sea changed places again and again. First came the singular -topography of the Trias, which prolonged and accentuated the characteristics -of the closing Palæozoic ages. Next arose the more genial climate and -more varied geography of the Jurassic period, when comparatively shallow -seas overspread the site of most of the European continent, and tracts of old -land stretched away to the west and north. Another crowded succession -of changes in the disposition of land and sea filled the long Cretaceous -period, at the close of which a more rapid and complete transformation in -European geography took place.</p> - -<p>Yet during all these transitions and vicissitudes, so far as we know, -volcanic energy remained quiescent throughout Western Europe. It was -not until some time after the great terrestrial movements that raised so -much of the Cretaceous sea-floor into land, and laid the foundations of the -modern continent, that the subterranean fires once more awoke to vigorous -action.</p> - -<p>The renewal of eruptions in the early ages of Tertiary time was as widespread -as it was energetic. Over many regions of the European continent -volcanoes broke out either in new areas or on old sites. For the most part -they appeared as scattered puys or as Vesuvian vents, generally not of the -first magnitude, like those of Central France, Hungary, Würtemberg and -Italy. But in the north-west they assumed more colossal proportions, and -took the form of fissure-eruptions by which many thousands of square miles -of country were deluged with lava. From the South of Antrim all along the -West of Scotland to the north of the Inner Hebrides remains of these basalt-floods -form striking features in the existing scenery. The same kind of rocks -reappear in the Faroe Islands and in Iceland, so that an enormous tract -of North-western Europe, much of it now submerged under the sea, was the -scene of activity of the Tertiary volcanoes. In entering, therefore, upon a -consideration of the British Tertiary volcanic rocks, we are brought face to -face with the records of the most stupendous succession of volcanic phenomena -in the whole geological history of Europe. Fortunately these records -have been fully preserved in the British Isles, so that ample materials -remain there for the elucidation of this last and most marvellous of all the -volcanic epochs in the evolution of the continent.</p> - -<p>As the remains of the Tertiary series of volcanic eruptions are the -youngest of all the volcanic records of Britain, they are naturally the -freshest and most abundantly preserved. They consequently reveal with -singular clearness multitudes of volcanic phenomena that are less distinctly -recognizable, or not to be found at all, among the Palæozoic systems. Hence -they will be discussed in greater detail in the following chapters.</p> - -<p><span class="pagenum" id="Page_109">- 109 -</span></p> - -<p>As a consequence of their greater freshness and wider extent, and largely -also because of the way in which they have been exposed along many leagues -of picturesque sea-cliffs in the North of Ireland and the West of Scotland, -they attracted attention at an earlier time than the less obvious volcanic -memorials of older ages. The gradual development of opinion regarding the -nature and history of volcanic rocks is thus in no small measure bound up -with the progress of observation and inference in regard to the Tertiary -volcanic series. I shall therefore begin this narrative by offering a rapid -sketch of the history of inquiry respecting the Tertiary volcanic areas of -the British Isles.</p> - -<p>The basaltic cliffs of Antrim and the Inner Hebrides had attracted the -notice of passing travellers, and their striking scenery had become more or -less familiar to the reading public, before any attention was paid to their -remarkable geological structure and history. In particular, the wonders of -the Giant's Causeway and the Antrim coast had already begun to draw -pilgrims, even from distant countries, at a time when geology had not -come into existence. The scientific tourist of those days who might care to -look at rocks was, in most cases, a mineralogist, for whom their structural -relations and origin were subjects that lay outside of the range of his knowledge -or habits of thought. In the year 1772 Sir Joseph Banks, together -with Solander and a party, visited Staffa and brought back the earliest -account of the marvels of that isle as they appeared to the sober eyes of -science. His narrative was communicated to Pennant, together with a -number of drawings of the cliffs and of Fingal's Cave. These were inserted -by that geographer in his <i>Second Tour</i>, published in 1774, and from their -careful measurements of the basaltic pillars and their delineation of the -basaltic structure, are of special interest in the history of volcanic geology.</p> - -<p>An intelligent appreciation of some of the geological interest of the -region is to be found in the writings of Whitehurst,<a id="FNanchor_125" href="#Footnote_125" class="fnanchor">[125]</a> who gave a good -account of the basalt-cliffs of Antrim, and regarded the basaltic rocks as the -results of successive outflows of lava from some centre now submerged -beneath the Atlantic. More important are the observations contained in -two letters of Abraham Mills.<a id="FNanchor_126" href="#Footnote_126" class="fnanchor">[126]</a> This writer had been struck with the dykes -on the north coast of Ireland, and was led to examine also those in some of -the nearer Scottish islands. He believed them to be of truly volcanic origin, -and spoke of them as veins of lava. A few years later, Faujas St. Fond -made his well-known pilgrimage to the Western Isles. Familiar with the -volcanic rocks of Central France, he at once recognized the volcanic origin -of the basalts of Mull, Staffa and the adjoining islands.<a id="FNanchor_127" href="#Footnote_127" class="fnanchor">[127]</a> His account of -the journey, published in Paris in 1797, may be taken as the beginning of -the voluminous geological literature which has since gathered round the -subject. Three years afterwards (1800) appeared Jameson's <i>Outline of the -Mineralogy of the Scottish Isles</i>. Fresh from the teaching of Werner at -<span class="pagenum" id="Page_110">- 110 -</span> -Freiberg, the future distinguished Professor of Natural History in the -Edinburgh University naturally saw everything in the peculiar Wernerian -light. He gave the first detailed enumeration of some of the eruptive -rocks of the Hebrides, but of course ridiculed the idea of their igneous -origin. Having heard of a reported "crater of a volcano" near Portree, he -ironically expressed a hope that "there may be still sufficient heat to revive -the spirits of some forlorn fire-philosopher, as he wanders through this cold, -bleak country."<a id="FNanchor_128" href="#Footnote_128" class="fnanchor">[128]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_125" href="#FNanchor_125" class="label">[125]</a> <i>Inquiry into the Original State and Formation of the Earth</i>, 2nd edit. 1786.</p> - -<p><a id="Footnote_126" href="#FNanchor_126" class="label">[126]</a> <i>Philosophical Transactions for 1790.</i></p> - -<p><a id="Footnote_127" href="#FNanchor_127" class="label">[127]</a> <i>Voyage en Angleterre, en Écosse et aux ÃŽles Hébrides.</i> Paris, 1797.</p> - -<p><a id="Footnote_128" href="#FNanchor_128" class="label">[128]</a> It will be shown in a later chapter that there is a remarkably perfect volcanic vent near -Portree, but the supposed crater referred to by Jameson was probably some little corry among -the sheets of basalt.</p> - -</div> - -<p>The advent of Jameson to Edinburgh gave a fresh impetus to the -warfare of the Plutonists and Neptunists, for he brought to the ranks -of the latter a mineralogical skill such as none of their Scottish opponents -could boast. The igneous origin of basalt, which the Plutonists stoutly -maintained, was as strongly denied by the other side. For some years one -of the most telling arguments against the followers of Hutton was derived -from the alleged occurrence of fossil shells in the basalt of the north coast -of Ireland. Kirwan<a id="FNanchor_129" href="#Footnote_129" class="fnanchor">[129]</a> quoted with evident satisfaction Richardson's observation -of "shells in the basalts of Ballycastle," and Richardson<a id="FNanchor_130" href="#Footnote_130" class="fnanchor">[130]</a> himself, -though the true explanation, that the supposed basalt is only Lias shale -altered by basalt, had been stated in 1802 by Playfair,<a id="FNanchor_131" href="#Footnote_131" class="fnanchor">[131]</a> continued for ten -years afterwards to reiterate his belief in the aqueous origin of basalt. -Thus the Tertiary volcanic rocks furnished effective weapons to the combatants -on both sides. The dispute regarding the black fossiliferous rocks -of Portrush had the effect of drawing special attention to the geology of the -North of Ireland. Among the more noted geologists who were led to examine -them, particular reference must be made to Conybeare and Buckland, who, -in the year 1813, studied the interesting coast-sections of Antrim. The -report of their observations gives an excellent summary of the arguments -for the truly igneous origin of basalt, and a statement of opinion in favour -of the view that the bedded basalts are the products of submarine volcanoes. -Berger also about the same time described in fuller detail the geology of the -Antrim district, and showed the rocks of the basalt-plateau to be younger -than the Chalk. He likewise made a study of the basalt-dykes of the North -of Ireland, and was the first to point out their prevalent north-westerly -direction. The memoirs of these geologists<a id="FNanchor_132" href="#Footnote_132" class="fnanchor">[132]</a> may justly be regarded, to -quote the words of Portlock, as "the first effectual step made in Irish -<span class="pagenum" id="Page_111">- 111 -</span> -geology." Portlock's own description is still the most complete summary -of the geology of that interesting region.<a id="FNanchor_133" href="#Footnote_133" class="fnanchor">[133]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_129" href="#FNanchor_129" class="label">[129]</a> <i>Geological Essays</i>, 1799, p. 252, <i>footnote</i>.</p> - -<p><a id="Footnote_130" href="#FNanchor_130" class="label">[130]</a> Richardson lived on the Antrim coast, and had daily opportunities of examining the admirable -rock-sections there exposed. It was he who found the shells in supposed basalt, and led -the geologists of his day astray on this subject. He made a clever but irrelevant reply to Playfair's -plain statement of facts (<i>Trans. Roy. Irish Acad.</i> vol. ix. 1803, p. 481). His elaborate -attack on "the Volcanic Theory" will be found in <i>Trans. Roy. Irish Acad.</i> vol. x. (1806), pp. 35-107. -Though lively enough as a specimen of controversial writing, it forms, when seriously -considered, rather a melancholy chapter in geological literature.</p> - -<p><a id="Footnote_131" href="#FNanchor_131" class="label">[131]</a> <i>Illustrations of the Huttonian Theory</i>, § 252.</p> - -<p><a id="Footnote_132" href="#FNanchor_132" class="label">[132]</a> They are contained in the third volume of the <i>Transactions of the Geological Society</i>.</p> - -<p><a id="Footnote_133" href="#FNanchor_133" class="label">[133]</a> "Report on the Geology of the County of Londonderry and parts of Tyrone and Fermanagh," -<i>Mem. Geol. Survey</i>, 1843.</p> - -</div> - -<p>While such advances were being made in the knowledge of the structure -of the volcanic rocks of the North of Ireland, the geologist had already -appeared who was the first to attempt a systematic examination of the -Western Islands, and whose published descriptions are still a chief source -of information regarding the geology of this extensive region. Dr. Macculloch -seems to have made his first explorations among the Hebrides some -time previous to the year 1814, for in that year he published some remarks -on specimens from that district transmitted to the Geological Society.<a id="FNanchor_134" href="#Footnote_134" class="fnanchor">[134]</a> For -several years in succession he devoted himself with great energy and -enthusiasm to the self-imposed task of geologically examining and mapping -in a generalized way all the islands that lie to the westward of Scotland, -from the remote St. Kilda even as far as the Isle of Man. From time to -time, notices of parts of his work were given in the <i>Transactions of the -Geological Society</i>. But eventually in 1819 he embodied the whole in his -<i>Description of the Western Islands of Scotland, including the Isle of Man</i>.</p> - -<div class="footnote"> - -<p><a id="Footnote_134" href="#FNanchor_134" class="label">[134]</a> <i>Trans. Geol. Soc.</i> vol. ii. 1814.</p> - -</div> - -<p>This great classic marks a notable epoch in British geology. Properly to -estimate its value, we should try to realize what was the state of the science -in this country at the time of its appearance. So laborious a collection of -facts, and so courageous a resolution to avoid theorizing about them, gave to -his volumes an altogether unique character. His descriptions were at once -adopted as part of the familiar literature of geology. His sections and -sketches were reproduced in endless treatises and text-books. Few single -works of descriptive geology have ever done so much to advance the progress -of the science in this country. With regard to the special subject of -the present memoir, Macculloch showed that the basalts and other eruptive -rocks of the Inner Hebrides pierce and overlie the Secondary strata of these -islands, and must therefore be of younger date. But though he distinguished -the three great series of "trap-rocks," "syenites" and "hypersthene-rocks" -or "augite-rocks," and indicated approximately their respective areas, he did -not attempt to unravel their relations to each other. Nor did he venture -upon any speculations as to the probable conditions under which these rocks -were produced. He claimed that those who might follow him would find a -great deal which he had not described, but little that he had not examined. -Subsequent observers have noted many important facts, of which, had he -observed them, he would at once have seen the meaning, and which he certainly -would not have passed over in silence. But as a first broad outline -of the subject, Macculloch's work possesses a great value, which is not -lessened by the subsequent discovery of details that escaped his notice, and -of important geological relations which he failed to detect.</p> - -<p>It has already been pointed out that some of the earliest and ablest -observations among the volcanic rocks of this country, especially in Scotland, -were made by foreigners. Students who had repaired from abroad to Edinburgh -<span class="pagenum" id="Page_112">- 112 -</span> -for education sometimes caught the geological enthusiasm, then so -marked in that city, and made numerous journeys through the country in -search of further knowledge of Scottish rocks and minerals. In other -instances, geologists of established reputation, attracted by the interest -which the published accounts of the geology of Scotland had excited, were -led to visit the country and to record their impressions of its rock-structure. -Of the first class of observers the two most noted were Ami Boué and L. A. -Necker; of the second, special acknowledgment is due to Faujas St. Fond -and to Von Oyenhausen and Von Dechen.</p> - -<p>The labours of Boué<a id="FNanchor_135" href="#Footnote_135" class="fnanchor">[135]</a> have already been referred to in connection with -the literature of the Scottish Old Red Sandstone (<a href="../../66492/66492-h/66492-h.htm#Page_269">vol. i. p. 269</a>). In his -treatment of the Tertiary Volcanic series of Scotland he appears to have -relied mainly on the then recently published volumes of Macculloch.</p> - -<div class="footnote"> - -<p><a id="Footnote_135" href="#FNanchor_135" class="label">[135]</a> <i>Essai géologique sur l'Écosse.</i> Paris, 1820.</p> - -</div> - -<p>L. A. Necker, as the grandson of the illustrious De Saussure, had strong -claims on the friendly assistance of the School of Geology at Edinburgh when -he went thither in 1806, at the age of twenty, to prosecute his studies. -He was equally well received by the Plutonists and Neptunists, and devoted -some time to the exploration of the geology not only of the Lowlands, but -of the Highlands and the Inner Hebrides. Most of his observations appear -to have been made in the year 1807, but it was not until fourteen years -afterwards that he published the account of them.<a id="FNanchor_136" href="#Footnote_136" class="fnanchor">[136]</a> The geological part of -this work must be admitted to be somewhat disappointing. The author's -caution not to commit himself to either side of the geological controversy -then waging makes his descriptions and explanations rather colourless. He -adds little to what was previously known. Even as regards the origin of -the basalts of the Western Islands, he could not make up his mind whether -or not to regard them as volcanic, but contented himself by referring them -to "the trappean formation." Yet these islands had so fascinated him that -eventually he returned to them as his adopted home, passed the last twenty -years of his life among them, and died and was buried there. Besides his -<i>Voyage</i>, he published in French an account of the dykes of the Island of -Arran.<a id="FNanchor_137" href="#Footnote_137" class="fnanchor">[137]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_136" href="#FNanchor_136" class="label">[136]</a> <i>Voyage en Écosse et aux ÃŽles Hébrides.</i> See also biographical notice of L. A. Necker, by -Principal J. D. Forbes, <i>Proc. Roy. Soc. Edin.</i> v. (1862), p. 53.</p> - -<p><a id="Footnote_137" href="#FNanchor_137" class="label">[137]</a> <i>Trans. Roy. Soc. Edin.</i> vol. xiv. (1840), p. 667.</p> - -</div> - -<p>Among the foreign geologists who have been drawn to the Scottish -mountains and islands by the interest of their Tertiary volcanic rocks, I have -already spoken of Faujas St. Fond. Much more important, however, were -the observations made some thirty years later by two German men of -science, Von Oyenhausen and Von Dechen. Their careful descriptions of -the geology of Skye, Eigg and Arran added new materials to the knowledge -already acquired by native geologists.<a id="FNanchor_138" href="#Footnote_138" class="fnanchor">[138]</a> To some of the more interesting -parts of their work reference will be made in later pages.</p> - -<div class="footnote"> - -<p><a id="Footnote_138" href="#FNanchor_138" class="label">[138]</a> Karsten's <i>Archiv</i> (1829), vol. i. p. 56.</p> - -</div> - -<p>The numerous trap-dykes of Northumberland, Durham and Northern -<span class="pagenum" id="Page_113">- 113 -</span> -Yorkshire at an early date attracted the attention of geologists. As far -back as 1817, they had been the subject of a memoir by N. J. Winch,<a id="FNanchor_139" href="#Footnote_139" class="fnanchor">[139]</a> who -gave an account of their effects on the adjacent rocks. More important -were the subsequent papers on the same subject by Sedgwick, who, discussing -the lithological characters, probable origin and geological age of the dykes, -pointed out that while the Cleveland dyke was undoubtedly younger than a -large part of the Jurassic rocks, there was no direct evidence to determine -whether dykes farther north were earlier or later than the time of the -Magnesian Limestone.<a id="FNanchor_140" href="#Footnote_140" class="fnanchor">[140]</a> Subsequent accounts of the dykes of the same -region were given by Buddle,<a id="FNanchor_141" href="#Footnote_141" class="fnanchor">[141]</a> M. Forster,<a id="FNanchor_142" href="#Footnote_142" class="fnanchor">[142]</a> N. Wood,<a id="FNanchor_143" href="#Footnote_143" class="fnanchor">[143]</a> H. T. M. Witham,<a id="FNanchor_144" href="#Footnote_144" class="fnanchor">[144]</a> -Tate <a id="FNanchor_145" href="#Footnote_145" class="fnanchor">[145]</a> and others, while in more recent years important additions to our -knowledge of these dykes and of their effects have been made by Sir J. -Lowthian Bell<a id="FNanchor_146" href="#Footnote_146" class="fnanchor">[146]</a> and Mr. J. J. H. Teall.<a id="FNanchor_147" href="#Footnote_147" class="fnanchor">[147]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_139" href="#FNanchor_139" class="label">[139]</a> <i>Trans. Geol. Soc.</i> vol. iv. (1817), p. 21. See also Tilloch's <i>Phil. Mag.</i> vols. xlix. and l.</p> - -<p><a id="Footnote_140" href="#FNanchor_140" class="label">[140]</a> <i>Cambridge Phil. Trans.</i> vol. ii. (1827), pp. 21, 139.</p> - -<p><a id="Footnote_141" href="#FNanchor_141" class="label">[141]</a> <i>Trans. Nat. Hist. Soc. Northumberland</i>, i. (1831), p. 9.</p> - -<p><a id="Footnote_142" href="#FNanchor_142" class="label">[142]</a> <i>Op. cit.</i> i. p. 44.</p> - -<p><a id="Footnote_143" href="#FNanchor_143" class="label">[143]</a> <i>Op. cit.</i> i. pp. 305, 306, 308, 309.</p> - -<p><a id="Footnote_144" href="#FNanchor_144" class="label">[144]</a> <i>Op. cit.</i> ii. (1838), p. 343.</p> - -<p><a id="Footnote_145" href="#FNanchor_145" class="label">[145]</a> <i>Trans. Northumberland and Durham</i>, ii. (1868), p. 30.</p> - -<p><a id="Footnote_146" href="#FNanchor_146" class="label">[146]</a> <i>Proc. Roy. Soc.</i> xxiii. (1875), p. 543.</p> - -<p><a id="Footnote_147" href="#FNanchor_147" class="label">[147]</a> <i>Quart. Journ. Geol. Soc.</i> xl. (1884), p. 209.</p> - -</div> - -<p>The geological age of the great series of Tertiary volcanic rocks has only -been determined district by district, and at wide intervals. That some -part of the Antrim basalts is younger than the Chalk of that region was -clearly shown by Berger, Conybeare and Buckland. Portlock, however, -referred to the occurrence of detached blocks of basalt which he supposed -to be immersed in the Chalk near Portrush, and which inclined him to -believe that "the basaltic flows commenced at a remote period of the -Cretaceous system."<a id="FNanchor_148" href="#Footnote_148" class="fnanchor">[148]</a> Macculloch showed that the corresponding basaltic -plateaux of the Inner Hebrides were certainly younger than the Oolitic rocks -of that region. But no nearer approximation to their date had yet been -made when in the year 1850 the Duke of Argyll announced the discovery -of strata containing fossiliferous chalk-flints and dicotyledonous leaves, lying -between the bedded basalts of Ardtun Head, in the Isle of Mull.<a id="FNanchor_149" href="#Footnote_149" class="fnanchor">[149]</a> In the -following year these fossil leaves were described by Edward Forbes, who -regarded them as decidedly Tertiary, and most probably Miocene. This was -the first palæontological evidence for the determination of the geological -age of any portion of the basalt-plateaux, and it indicated that the basalts -of the south-west of Mull were of older Tertiary date. Taken also in connection -with the occurrence of lignite-beds between the basalts of Antrim, -it suggested that these volcanic plateaux were not due to submarine eruptions, -<span class="pagenum" id="Page_114">- 114 -</span> -as the earlier geologists had supposed, but were rather the result of -the subærial outpouring of lava at successive intervals, during which terrestrial -vegetation sprang up upon the older outflows.</p> - -<div class="footnote"> - -<p><a id="Footnote_148" href="#FNanchor_148" class="label">[148]</a> <i>Report on the Geology of Londonderry</i>, p. 93. There can be no doubt that this was an error -of observation. The Antrim basalts are all certainly younger than the Chalk. The supposed -"lumps of basalt" were probably the ends of veins intruded into the Chalk, and perhaps partially -disconnected from the main parts of the veins. Such apparently detached masses of intrusive -rock are not infrequent occurrence in connection with the Tertiary intrusive sills. An example -will be found represented in <a href="#v2fig321">Fig. 321</a>.</p> - -<p><a id="Footnote_149" href="#FNanchor_149" class="label">[149]</a> <i>Brit. Assoc. Report</i>, 1850, Sections, p. 70; and <i>Quart. Jour. Geol. Soc.</i> vii. (1851), p. 87.</p> - -</div> - -<p>While Forbes brought forward palæontological proofs of the Tertiary -age of the volcanic rocks of the south-west of Mull, he at the same time -laid before the Geological Society a paper on the Estuary Beds and the -Oxford Clay of Loch Staffin, in Skye, wherein, while admitting the -existence of appearances which might be regarded as favourable to the -view that the intercalated basalts of that region were of much later date -than the Oolitic strata between which they might have been intrusively -injected, he stated his own belief that they were really contemporaneous -with the associated stratified rocks, and thus marked an outbreak of volcanic -energy at the close of the Middle Oolitic period.<a id="FNanchor_150" href="#Footnote_150" class="fnanchor">[150]</a> The Duke of Argyll, in -the paper which he on the same occasion communicated to the Geological -Society, adopted this view of the probable age of most of the basalts of the -Western Islands. He looked upon the Tertiary volcanic rocks of Mull as -occupying a restricted area, the great mass of the basalt of that island, like -that of Skye, being regarded by him as probably not later than some part -of the Secondary period.</p> - -<div class="footnote"> - -<p><a id="Footnote_150" href="#FNanchor_150" class="label">[150]</a> <i>Quart. Journ. Geol. Soc.</i> vol. vii. (1851), p. 104.</p> - -</div> - -<p>It must be granted that the appearances of contemporaneous intercalation -of the basalt among the Secondary strata are singularly deceptive. -When, several years after the announcement of the Tertiary age of the -basalts of Ardtun, I began my geological work in the Inner Hebrides, I was -led to the same conclusion as Edward Forbes, and expressed it in an early -paper.<a id="FNanchor_151" href="#Footnote_151" class="fnanchor">[151]</a> All over the north of Skye I traced what appeared to be evidence -of the contemporaneous interstratification of basalts with the Jurassic rocks -and I concluded (though with some reservation) that the whole of the vast -basaltic plateaux of that island were not younger than some late part of the -Jurassic period. In that same paper the attention of geologists was called -to the probable connection of the great system of east-and-west dykes -traversing Scotland and the North of England, with the basalt-plateaux of -the Inner Hebrides, and as I believed the latter to be probably of the age -of the Oolitic rocks, I assigned the dykes to the same period in geological -history. But subsequent explorations enabled me to correct the mistake into -which, with other geologists, I had fallen regarding the age of the volcanic -phenomena of the Western Islands. In 1867 I showed that instead of -being confined to a mere corner of Mull, the Tertiary basalts, with younger -associated trachytic or granitic rocks, covered nearly the whole of that -island, and that in all likelihood the long chain of basaltic masses, extending -from the North of Ireland along the west coast of Scotland to the Faroe -Islands, and beyond these to Iceland, was all erupted during the Tertiary -period. At the same time I drew special attention to the system of east-and-west -dykes as proofs of the vigour of volcanic action at that period, and -<span class="pagenum" id="Page_115">- 115 -</span> -I furnished evidence that this action was prolonged through a vast interval -of time, during which great subærial denudation of the older lavas took -place before the outflow of the younger.<a id="FNanchor_152" href="#Footnote_152" class="fnanchor">[152]</a> Later in the same year, in an -address to the Geological Section of the British Association, I reiterated -these views, and more particularly emphasized the importance of the system -of dykes, which in my opinion was possibly the most striking manifestation -of the vigour of Tertiary volcanic action.<a id="FNanchor_153" href="#Footnote_153" class="fnanchor">[153]</a> In 1871, after further explorations -in the field, I gave a detailed account of the structure which had led -to the mistake as to the age of the Tertiary volcanic rocks of the Western -Islands; and in a description of the island of Eigg, I brought forward data -to show the enormous duration of the Tertiary volcanic period in the west -of Britain.<a id="FNanchor_154" href="#Footnote_154" class="fnanchor">[154]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_151" href="#FNanchor_151" class="label">[151]</a> "On the Chronology of the Trap-rocks of Scotland," <i>Trans. Roy. Soc. Edin.</i> xxii. (1861), -p. 649.</p> - -<p><a id="Footnote_152" href="#FNanchor_152" class="label">[152]</a> <i>Proc. Roy. Soc. Edin.</i> vi. (1867), p. 71.</p> - -<p><a id="Footnote_153" href="#FNanchor_153" class="label">[153]</a> <i>Brit. Assoc. Report</i> (Dundee), 1867, Sections, p. 49.</p> - -<p><a id="Footnote_154" href="#FNanchor_154" class="label">[154]</a> <i>Quart. Journ. Geol. Soc.</i> xxvii. (1871), p. 279.</p> - -</div> - -<p>Three years later Mr. J. W. Judd read before the Geological Society a -paper "On the Ancient Volcanoes of the Highlands."<a id="FNanchor_155" href="#Footnote_155" class="fnanchor">[155]</a> The most novel feature -of this paper was the announcement that the author had recognized the basal -wrecks of five great central volcanoes in the Western Islands, among which -that of Mull was inferred by him to have been at least 14,500 feet high. -He was led to the conclusion that the volcanic period in these regions was -divisible into three sections—the first marked by the outburst of acid rocks -(felspathic lavas and ashes, connected with deeper and more central granitic -masses); the second by the extrusion of basic lavas and tuffs (the basaltic -plateaux); the third by the appearance of small sporadic volcanic cones -("felspathic, basaltic, or intermediate in composition") after the great central -cones had become extinct. It will be seen in the following pages that these -conclusions of Professor Judd are not supported by a more detailed study of -the region.</p> - -<div class="footnote"> - -<p><a id="Footnote_155" href="#FNanchor_155" class="label">[155]</a> <i>Quart. Journ. Geol. Soc.</i> xxx. (1874), p. 220.</p> - -</div> - -<p>In the year 1879, during a traverse of some portions of the volcanic -region of Wyoming, Montana and Utah, I was vividly impressed by the -identity of structure between the basaltic plateaux of these territories and -the youngest volcanic areas of Britain. It then appeared to me that some -of the puzzling features in the Tertiary volcanic series of the Inner Hebrides -might be explained by the structures so admirably displayed in these lava-fields -of the Far West.<a id="FNanchor_156" href="#Footnote_156" class="fnanchor">[156]</a> Riding over the great basalt-plains of the Snake -River and looking at the sections cut by the river through the thick series -of horizontal basalt-beds, I appreciated for the first time the significance of -Baron von Richthofen's views regarding "massive" or "fissure" eruptions, -as contradistinguished from those of great central cones of the type of Etna -or Vesuvius, and I gathered so many suggestions from my examination of -these American regions that I renewed with increased interest the investigation -of the Tertiary volcanic tracts of Britain. At last, after another -interval of nine years, during which my weeks of leisure were given to the -<span class="pagenum" id="Page_116">- 116 -</span> -task, I was able to complete a discussion of the whole history of Tertiary -volcanic action in this country, which was communicated to the Royal -Society of Edinburgh in the early summer of 1888.<a id="FNanchor_157" href="#Footnote_157" class="fnanchor">[157]</a> Since that time I -have continued the research, and have from time to time communicated -my results to the Geological Society. These various memoirs are combined -with hitherto unpublished details in the following account of the British -Tertiary Volcanic Rocks.</p> - -<div class="footnote"> - -<p><a id="Footnote_156" href="#FNanchor_156" class="label">[156]</a> <i>Geological Essays at Home and Abroad</i> (1882), pp. 271, 274; <i>Nature</i>, November 1880.</p> - -<p><a id="Footnote_157" href="#FNanchor_157" class="label">[157]</a> <i>Trans. Roy. Soc. Edin.</i> vol. xxxv. part ii. (1888), pp. 23-184.</p> - -</div> - -<p>Professor Judd has also prosecuted the investigation of the petrography -of the rocks, and has published his observations in the <i>Quarterly Journal of -the Geological Society</i>.<a id="FNanchor_158" href="#Footnote_158" class="fnanchor">[158]</a> To these papers by him more detailed reference -will be made in later Chapters.</p> - -<div class="footnote"> - -<p><a id="Footnote_158" href="#FNanchor_158" class="label">[158]</a> <i>Quart. Journ. Geol. Soc.</i> vols. xlv. (1889), xlvi. (1890), xlix. (1893). In the first of these -volumes Professor Judd offered a detailed criticism of my views as to the order of succession and -history of the volcanic rocks of the Inner Hebrides. Subsequent investigation having entirely -confirmed my main conclusions, it is not necessary to enter here upon matters of controversy. -Reference, however, will be made in subsequent Chapters to some of the points in dispute.</p> - -</div> - -<p>In describing the geological history of a great series of rocks, chronological -order is usually the most convenient method of treatment. Where, -however, the rocks are of volcanic origin, and do not always precisely -indicate their relative age, and where moreover the same kinds of rock may -appear on widely-separated geological horizons, it is not always possible or -desirable to adhere to the strict order of sequence. With this necessary -latitude, I propose to follow the chronological succession from the older to -the newer portions of the series. I shall treat first of the system of dykes, -by which so large a part of Scotland and of the north of England and Ireland -is traversed. Many of the dykes are undoubtedly among the youngest -members of the volcanic series, and in no case has their age been as yet -determined except relatively to the antiquity of the rocks which they -traverse. They must, of course, be posterior to these rocks, and hence it -would be quite logical to reserve them for discussion at the very end of the -whole volcanic phenomena. My reason for taking them at the beginning -will be apparent in the sequel. After the dykes, I shall describe the great -volcanic plateaux which, in spite of vast denudation, still survive in extensive -fragments in Antrim, the Inner Hebrides and the Faroe Islands. The -eruptive bosses of basic rocks that have broken through the plateaux will -next be discussed. An account will then be given of the protrusions of acid -rocks which have disrupted these basic bosses. The last chapters will contain -a sketch of the subsidences and dislocations which the basalt-plateaux -have suffered, and of the denudation to which they have been subjected.</p> - -<p>As has been explained in Chapter iii., the volcanic cycle of any district, -during a given geological period, embraces the whole range of erupted products -from the beginning to the end of a complete series of eruptions. -Reference was made in Book I. to the remarkable variation in the -character of the lavas successively poured out from the same volcanic reservoir -during the continuance of a single cycle, and it was pointed out that -Richthofen's law generally holds good that while the first eruptions may be -<span class="pagenum" id="Page_117">- 117 -</span> -of a basic or average and intermediate nature, those of succeeding intervals -become progressively more acid, but are often found to return again at the -close to thoroughly basic compounds.</p> - -<p>This law is well illustrated by the volcanic history of Tertiary time in -Britain. We shall find that the earliest eruptions of which the relative date -is known consisted generally of basic lavas (dolerites and basalts), but including -also more sparingly andesites, trachytes and rhyolites; that the oldest -intrusive masses consisted of bosses, sills and dykes of dolerite and gabbro; -that these intrusions were followed by others of a much more acid character—felsites, -pitchstones, quartz-porphyries or rhyolites, granophyres and -granites; that the latest lava is a somewhat acid rock, being a vitreous -form of dacite; and that the most recent volcanic products of all are dykes -of a thoroughly basic nature, like some of the earlier intruded masses.</p> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_118">- 118 -</span></p> - -<h2 class="nobreak" id="CHAPTER_XXXIV">CHAPTER XXXIV<br /> - -<span class="smaller">THE SYSTEM OF DYKES IN THE TERTIARY VOLCANIC SERIES</span></h2> -</div> - -<div class="blockquot"> - -<p>Geographical Distribution—Two Types of Protrusion—Nature of Component Rocks—Hade—Breadth—Interruptions -of Lateral Continuity—Length—Persistence of -Mineral Characters.</p> -</div> - - -<p>If a geologist were asked to select that feature in the volcanic geology of -the British Isles which, more than any other, marks this region off from the -rest of the European area, he would probably choose the remarkable system -of wall-like masses of erupted igneous rock, to which the old Saxon word -"dykes" has been affixed. From the moors of eastern Yorkshire to the -Perthshire Highlands, and from the basins of the Forth and Tay to the west -of Donegal and the far headlands of the Hebrides, the country is ribbed -across with these singular protrusions to such an extent that it may be -regarded as a typical region for the study of the phenomena of dykes. -That all the dykes in this wide tract of country are of Tertiary age cannot -be maintained. It has been shown in previous Chapters that each of the -great volcanic periods has had its system of dykes, even as far back as the -time of the Lewisian Gneiss.</p> - -<p>But when all the dykes which can reasonably be referred to older -geological periods are excluded, there remains a large series which cannot -be so referred, but which are connected together by various kinds of -evidence into one great system that must be of late geological date, and -can be assigned to no other than the Tertiary period in the volcanic history -of Britain. As far back as the year 1861, when I first drew attention to -this great system of dykes in connection with the progress of volcanic -action in the country, I pointed out the grounds on which it seemed to me -that these rocks belong to a comparatively recent geological period.<a id="FNanchor_159" href="#Footnote_159" class="fnanchor">[159]</a> My -own subsequent experience and the full details of structure collected by -my colleagues of the Geological Survey in all parts of the country, have -amply confirmed this view. The characters which link this great series of -dykes together as one connected system of late geological date are briefly -enumerated in the following list, and will be more fully discussed in later -pages.</p> - -<div class="footnote"> - -<p><a id="Footnote_159" href="#FNanchor_159" class="label">[159]</a> <i>Trans. Roy. Soc. Edin.</i> vol. xxii. (1861), p. 650.</p> - -</div> - -<p>1. The prevalent tendency of the dykes to take a north-westerly -course. There are exceptions to this normal trend, especially where the -<span class="pagenum" id="Page_119">- 119 -</span> -dykes are small and locally numerous; but it remains singularly characteristic -over the whole region.</p> - -<p>2. The increasing abundance of the dykes as they are traced to the -west coast and the line of the great Tertiary volcanic plateaux of Antrim -and the Inner Hebrides.</p> - -<p>3. The rectilinear direction so characteristic of them and so different -from the tortuous course of local groups of dykes. The exceptions to this -normal feature are as a rule confined to the same localities where departures -from the prevalent westerly trend occur.</p> - -<div class="figcenter" id="v2fig233" style="width: 530px;"> - <img src="images/v2fig233.png" width="530" height="345" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 233.</span>—Dyke on the south-east coast of the Island of Mull.</div> -</div> - -<p>4. The great breadth of the larger dykes of the system and their -persistence for long distances. This is one of their most remarkable and -distinctive characters.</p> - -<p>5. The posteriority of the dykes to the rest of the geological structure -of the regions which they traverse. They are not only younger than the -other rocks, but younger than nearly all the folds and faults by which -the rocks are affected.</p> - -<p>6. The manner in which they cut the Jurassic, Cretaceous and older -Tertiary rocks in the districts through which they run. At the south-eastern -end of the region they rise through the Lias and Oolite formations, -in the west they intersect the Chalk and also the Tertiary volcanic -plateaux together with their later eruptive bosses.</p> - -<p>7. Their petrographical characters, among which perhaps the most -distinctive is the frequent appearance of the original glass of the plagioclase-pyroxene-magnetite -(olivine) rock, of which they mostly consist. This -<span class="pagenum" id="Page_120">- 120 -</span> -glass, or its more or less completely devitrified representative, often still -recognizable with the microscope among the individualized microlites and -crystals throughout the body of a dyke, is also not infrequent as a black -vitreous varnish-like coating on the outer walls, and occasionally appears in -strings and veins even in the centre.</p> - -<p>It is the assemblage of dykes presenting these features which I propose -to describe. Obviously, the age of each particular dyke can only be fixed -relatively for itself. But when this remarkable community of characters is -considered, and when the post-Mesozoic age of at least a very large number -of the dykes can be demonstrated, the inference is reasonable that one great -system of dykes was extravasated during a time of marked volcanic disturbance, -which could not have been earlier than the beginning of the Tertiary -period. And this inference may be maintained even when we frankly admit -that every dyke within the region is by no means claimed as belonging to -the Tertiary series.</p> - -<div class="figleft" id="v2fig234" style="width: 215px;"> - <img src="images/v2fig234.png" width="215" height="225" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 234.</span>—Fissure left by the weathering out -of a dyke.</div> -</div> - -<p>In spite of their number and the extraordinary volcanic activity to -which they bear witness, the dykes form a much less prominent feature in -the landscape than might have been anticipated. In the lowlands of the -interior, they have for the most part been concealed under a cover of superficial -accumulations, though in the water-courses they not infrequently project -as hard rocky barriers across the channels, and occasionally form -picturesque waterfalls. On the barer uplands, they protrude in lines of -broken crag and scattered boulders, which by their decay give rise to a -better soil covered by a greener vegetation than that of the surrounding -brown moorland. Among the Highland hills, they are often traceable from -a distance as long black ribs that project from the naked faces of crag -and corry. Along the sea-coast, their -peculiarities of scenery are effectively -displayed. Where they consist of a -close-grained rock, they often rise from -the beach as straight walls which, -with a strangely artificial look, mount -into the face of the cliffs on the one -side, and project in long black reefs -into the sea on the other (<a href="#v2fig233">Fig. 233</a>). -Every visitor to the islands of the -Clyde will remember how conspicuous -such features are there. But it is -among the Inner Hebrides that this -kind of scenery is to be found in -greatest perfection. The soft dark -Lias shales of the island of Pabba, for -example, are ribbed across with scores of dykes which strike boldly out to -sea. Where, on the other hand, the material of the dykes is coarse in grain, -or is otherwise more susceptible to the disintegrating influences of the -weather, it has often rotted away and left yawning clefts behind, the vertical -<span class="pagenum" id="Page_121">- 121 -</span> -walls of which are those of the fissures up which the molten rock ascended -(<a href="#v2fig234">Fig. 234</a>). Some good instances of this kind are well known to summer -visitors on the eastern shores of Arran. Others, on a large scale, may be -seen in the interior of the same island along the crests of the granite ridges, -and still more conspicuously on the jagged summits of Blath Beinn and the -Cuillin Hills (<a href="#v2fig333">Fig. 333</a>), and intersecting the Jurassic strata along the -cliffs of Strathaird in Skye.</p> - - -<h3>1. GEOGRAPHICAL DISTRIBUTION</h3> - -<p>The limits of the region within which the dykes occur cannot be very -precisely fixed. There can be no doubt, however, that on their southern -side they reach to the Cleveland Hills of Yorkshire and the southern borders -of Lancashire, perhaps even as far as North Staffordshire (<a href="#Page_106">p. 106</a>), and on the -northern side to the farther shores of the island of Lewis—a direct distance -of 360 miles. They stretch across the basin of the Irish Sea, including the -Isle of Man, and appear in Ireland north of a line drawn from Dundalk Bay -to the Bays of Sligo and Donegal. Dykes are of frequent occurrence over -the north of England and south of Scotland, at least as far north as a line -drawn from the coast of Kincardineshire along the southern flank of the -Grampian Hills, by the head of Glen Shee and Loch Tay, to the north-western -coast of Argyleshire. They abound all along the line of the Inner -Hebrides and on parts of the adjacent coasts of the mainland, from the -remoter headlands of Skye to the shores of County Louth. They traverse -also the chain of the Long Island in the Outer Hebrides. So far as I am -aware, they are either absent or extremely rare in the Highlands north of the -line I have indicated. But a good many have been found by my colleagues -in the course of the Geological Survey of the northern lowlands of Aberdeenshire -and Banffshire. The longest of these has been traced by Mr. L. Hinxman -for rather more than two miles running in a nearly east and west direction -through the Old Red Sandstone of Strathbogie, with an average width of -about 35 feet. Another in the same district has a width of from 45 to 90 -feet, and has been followed for a third of a mile. But far beyond these -northern examples, I have found a number of narrow basalt-veins traversing -the Old Red flagstones of the Mainland of Orkney, which I have little doubt -are also a prolongation of the same late series. Taking, however, only those -western and southern districts in which the younger dykes form a notable -feature in the geology, we find that the dyke-region embraces an area of -upwards of 40,000 square miles—that is, a territory greater than either -Scotland or Ireland, and equal to more than a third of the total land-surface -of the British Isles (Map I.).</p> - -<p>Of this extensive region the greater portion has now been mapped in -detail by the Geological Survey. Every known dyke has been traced, and -the appearances it presents at the surface have been recorded. We are -accordingly now in possession of a larger body of evidence than has ever -before been available for the discussion of this remarkable feature in the -<span class="pagenum" id="Page_122">- 122 -</span> -geology of the British Isles. I have made use of this detailed information, -and besides the data accumulated in my own note-books, I have availed -myself of those of my colleagues in the Survey, for which due acknowledgment -is made where they are cited.</p> - -<p>The Tertiary basalt-plateaux of Britain have their counterpart in the -Faroe Islands and in Iceland, and whether or not the lava-fields stretched -throughout North-western Europe from Antrim to the farthest headlands of -<i>Ultima Thule</i>, there can hardly be any doubt that, if not continuous, these -volcanic areas were at least geologically contemporaneous in their activity. -Their characteristic scenery and structure are prolonged throughout the -whole region, reappearing with all their familiar aspects alike in Faroe and -in Iceland. I have not seen the latter island, but in the Faroe archipelago -I have found the dykes to be sufficiently common, and to cut the basalt-plateaux -there in the same way as they do those of the Inner Hebrides. -On the whole, however, dykes do not play, in these northern isles, the important -part which they take in the geology and scenery of the West of -Scotland. I have not had sufficient opportunity to ascertain whether there -is a general direction or system among the Faroe dykes. In the fjords -north of Thorshaven, and again along the west side of Stromö, many of them -show an E. and W. strike or one from E.N.E. to W.S.W.</p> - - -<h3>2. TWO TYPES OF PROTRUSION</h3> - -<p>The dykes are far from being equally distributed over the wide region -within which they occur. In certain limited areas they are crowded -together, sometimes touching each other to the almost entire exclusion of -the rocks through which they ascend, while elsewhere they appear only -at intervals of several miles. Viewed in a broad way, they may be -conveniently grouped in two types, which, though no hard line can be -drawn between them, nevertheless probably point to two more or less -distinct phases of volcanic action and to more than one period of intrusion. -In the first, which for the sake of distinction we may term the -Solitary type, there is either a single dyke separated from its nearest -neighbours by miles of intervening and entirely dykeless ground, or a group -of two or more running parallel to each other, but sometimes a mile or more -apart. The rock of which they consist is, on the whole, less basic than in -the second type; it includes the andesitic varieties. It is to this type that -the great dykes of the north of England and the south and centre of Scotland -belong. The Cleveland dyke, for example, at its eastern end has no -known dyke near it for many miles. The coal-field of Scotland is traversed -by five main dykes, which run in a general sense parallel to each other, with -intervals of from half a mile to nearly five miles between them. Dykes of -this type display most conspicuously the essential characters of the dyke-structure, -in particular the vertical marginal walls, the parallelism of their -sides, their great length, and their persistence in the same line.</p> - -<p><span class="pagenum" id="Page_123">- 123 -</span></p> - -<p>In the second, or what for brevity may be called the Gregarious type, -the dykes occur in great abundance within a particular district. They are -on the whole narrower, shorter, less strikingly rectilinear, more frequently -tortuous and vein-like, and generally more basic in composition than -those of the first type. They include the true basalts and dolerites. Illustrative -districts for dykes of this class are the islands of Arran, Mull, Eigg -and Skye.</p> - -<p>The great single or solitary dykes may be observed to increase in -number, though very irregularly, from south to north, and also in Central -Scotland from east to west. They are specially abundant in the tract -stretching from the Firth of Clyde along a belt of country some thirty miles -broad on either side of the Highland line, as far at least as the valley of the -Tay. They form also a prominent feature in the islands of Jura and -Islay.</p> - -<p>Dykes of the gregarious type are abundantly and characteristically displayed -in the basin of the Firth of Clyde. Their development in Arran -formed the subject of the interesting paper by Necker, already mentioned, -who catalogued and described 149 of them, and estimated their total number -in the whole island to be about 1500.<a id="FNanchor_160" href="#Footnote_160" class="fnanchor">[160]</a> As the area of Arran is 165 square -miles, there would be, according to this computation, about nine dykes to -every square mile. But they are far from being uniformly distributed. -While appearing only rarely in many inland tracts, they are crowded -together along the shore, particularly at the south end of the island, where -the number in each square mile must far exceed the average just given. -The portion of Argyleshire, between the hollow of Loch Long and the Firth -of Clyde on the east and Loch Fyne on the west, has been found by my -colleague, Mr. C. T. Clough, to contain an extraordinary number of dykes -(see <a href="#v2fig257">Fig. 257</a>). The coast line of Renfrewshire and Ayrshire shows that -the same feature is prolonged into the eastern side of the basin of the Clyde -estuary. But immediately to the westward of this area the crowded dykes -disappear from the basin of Loch Fyne. In Cantire their scarcity is as -remarkable as their abundance in Cowal.</p> - -<div class="footnote"> - -<p><a id="Footnote_160" href="#FNanchor_160" class="label">[160]</a> <i>Trans. Roy. Soc. Edin.</i> xiv. (1840), p. 677.</p> - -</div> - -<p>Both in the North of Ireland and through the Inner Hebrides, dykes -are singularly abundant in and around, but particularly beneath, the great -plateaux of basalt. Their profusion in Skye was described early in this -century by Macculloch, who called attention more especially to their extraordinary -development in the district of Strathaird. "They nearly equal in -some places," he says, "when collectively measured, the stratified rock through -which they pass. I have counted six or eight in the space of fifty yards, -of which the collective dimensions could not be less than sixty or seventy -feet." He supposed that it would not be an excessive estimate to regard -the igneous rock as amounting to one-tenth of the breadth of the strata -which it cuts.<a id="FNanchor_161" href="#Footnote_161" class="fnanchor">[161]</a> This estimate, however, falls much short of the truth in -some parts of Strathaird, where the dykes are almost or quite contiguous, -<span class="pagenum" id="Page_124">- 124 -</span> -and the Jurassic strata, through which they rise, are hardly to be seen -at all.</p> - -<div class="footnote"> - -<p><a id="Footnote_161" href="#FNanchor_161" class="label">[161]</a> <i>Trans. Geol. Soc.</i> iii. (1815), p. 79. This locality is further noticed on p. 164.</p> - -</div> - -<p>Among the districts where dykes of the gregarious type abound at a -distance from any of the basalt-plateaux, reference should be made to the -curious isolated tract of the central granite core of Western Donegal. In -that area a considerable number of dykes rises through the granite, to -which they are almost wholly confined. Again, far to the east another -limited district, where dykes are crowded together, lies among the Mourne -Mountains. These granite hills are probably to be classed with those of -Arran, as portions of a series of granite protrusions belonging to a late part -of the Tertiary volcanic period which will be treated of in Chapter -xlvii.</p> - -<p>Though the dykes may be conveniently grouped in two series or types, -which on the whole are tolerably well marked, it is not always practicable -to draw any line between them, or to say to which group a particular dyke -should be assigned. In some districts, however, in which they are both -developed, we can separate them without difficulty. In the Argyleshire -region above referred to, for example, which Mr. Clough has mapped, he -finds that the abundant dykes belonging to the gregarious type run in a -general N.W. or N.N.W. direction, and distinctly intersect the much -scarcer and less basic dykes of the solitary type, which here run nearly E. -and W. (<a href="#v2fig257">Fig. 257</a>). Hence, besides their composition, distinction in number, -breadth, rectilinearity and persistence, the two series in that region -demonstrably belong to distinct periods of eruption.<a id="FNanchor_162" href="#Footnote_162" class="fnanchor">[162]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_162" href="#FNanchor_162" class="label">[162]</a> Mr. Clough is inclined to suspect that the E. and W. dykes are older than the Tertiary -series and may be later Palæozoic.</p> - -</div> - -<p>The characteristic habit in gregarious dykes of occurring in crowded -groups which are separated from each other by intervals of variable dimensions, -marked by the presence of comparatively few dykes, is well illustrated -in the district of Strath in Skye, which indeed may be taken as a typical -area for this peculiarity of distribution. While the dykes are there -singularly abundant in the Cambrian Limestone and the Liassic strata, -they have been found by Mr. Clough and Mr. Harker to be comparatively -infrequent in the tracts of Torridon Sandstone. It is not easy to understand -this peculiar arrangement. As the Torridon Sandstone is the most -ancient rock of the district, it probably underlies all the Cambrian and -Jurassic formations, so that the dykes which penetrate these younger strata -must also rise through the Torridonian rocks. Some formations appear to -have been fissured more readily than others, and thus to have provided more -abundant openings for the uprise of the basaltic magma from below. To -the effect of such local differences in the structure of the terrestrial -crust we have to add the concentration of the volcanic foci in certain -areas, though there seems no means of ascertaining what part each of these -causes has played in the distribution of the dykes of any particular -district.</p> - -<p><span class="pagenum" id="Page_125">- 125 -</span></p> - - -<h3>3. NATURE OF COMPONENT ROCKS</h3> - -<p>The Tertiary dykes of Britain include representatives of four distinct -groups of igneous rocks. 1st, The vast majority of them consist of plagioclase-pyroxene-magnetite -rocks with or without olivine. These are the -normal basalts and dolerites. 2nd, A number of large dykes have a rather -more acid composition and are classed as andesites. 3rd, A few dykes of -trachyte have been observed in Cowal and in Skye cutting the dykes of -basalt (<a href="#Page_138">p. 138</a>). 4th, In some districts large numbers of still more acid -dykes occur. These are sometimes crystalline in structure (granophyre), -more frequently felsitic (felsite, spherulitic quartz-porphyry), and often glassy -(pitchstone). In some exceptional cases the basic and acid materials are -conjoined in the same dyke. Such compound varieties are described at p. -161. The acid dykes, connected as they so generally are with the large -bodies of granophyre or granite, are doubtless younger than the great -majority of the basic dykes. They will be treated in connection with the -acid intrusions in Chapter xlviii.</p> - -<p>By far the greater number of the dykes of the Tertiary volcanic -series belong to the first group, and it is these more especially which -will be discussed in the present and the following Chapter. As, however, -the andesitic group is intimately linked with the basaltic it will be here -included with them.</p> - -<p>1. Basalt, Dolerite and Andesite Dykes.—To the field-geologist, -who regards merely their external features, the Tertiary dykes present a -striking uniformity in general petrographical character. They vary indeed -in fineness or coarseness of texture, in the presence or absence of porphyritic -crystals, amygdales, glassy portions and other points of structure. But there -is seldom any difficulty in perceiving that they generally belong to one or other -of the types of the basalts, dolerites, diabases or andesites. This sameness of -composition, traceable from Yorkshire to Skye and from Donegal to Perthshire, -is one of the strongest arguments for referring this system of dykes to -one geological period. At the same time, there are enough of minor variations -and local peculiarities to afford abundant exercise for the observing faculties -alike in the field and in the study, and to offer materials for arriving at some -positive conclusions regarding the geological processes involved in the uprise -of the dykes.</p> - -<p>There appears to be reason to believe that, when the petrography of the -dykes is more minutely studied, marked differences of material will be -found to denote distinct periods of eruption. Already Mr. A. Harker of the -Geological Survey, who is engaged in mapping the interesting and complicated -district of Strath in Skye, has observed that the dykes which are -older than the great granophyre bosses of that tract may be distinguished -from those which are later than these protrusions. The older basic dykes -are not conspicuously porphyritic, are frequently marked by a close-grained -margin or even with a veneer of basalt-glass, sometimes have an inclination -of as much as 45°, are occasionally discontinuous, and not infrequently -<span class="pagenum" id="Page_126">- 126 -</span> -branch or send out veins. The younger dykes, on the other hand, as will be -more particularly noticed in the following chapter, are distinguished by the -frequent and remarkable character of their porphyritic inclusions, by the -presence of foreign fragments in them, by the greater perfection of their -jointing, and by their seldom departing much from the vertical.<a id="FNanchor_163" href="#Footnote_163" class="fnanchor">[163]</a> They are -likewise often markedly acid in composition, including such rocks as -granophyre, felsite and pitchstone.</p> - -<div class="footnote"> - -<p><a id="Footnote_163" href="#FNanchor_163" class="label">[163]</a> In the Blath Bheinn group of gabbro-hills, however, it is the youngest dykes which have -been found by Mr. Harker to possess the lowest hade.</p> - -</div> - -<p>(1) <i>External Characters.</i>—As regards the grain of the rock, every -gradation may be found, from a coarsely crystalline mass, in which the -component minerals are distinctly traceable with the naked eye, to a black -lustrous basalt-glass. Each dyke generally preserves the same character -throughout its extent. As a rule, broad and long dykes are coarser in grain -than narrow and short ones. For the most part, there runs along each -side of a dyke a selvage of finer grain than the rest of the mass. This -marginal strip varies in breadth from an inch or less up to a foot or more, -and obviously owes its origin to the more rapid chilling of the molten rock -along the walls of the fissure. It usually shades away inperceptibly into -the larger-grained inner portion. Even with the naked eye its component -materials can be seen to be more finely crystalline than the rest of the dyke, -though where dispersed porphyritic felspars occur they are as large in -the marginal strip as in any other part of a dyke, for they belong to an -earlier period of crystallization than the smaller felspars of the groundmass -and were already floating in the magma while it was still in a molten state.</p> - -<p>This finer-grained external band, so distinctive of an eruptive and -injected rock, is of great service in enabling us to trace dykes when they -traverse other dykes or masses of igneous rock of similar characters to their -own. When one dyke crosses another, that which has its marginal -band of finer grain unbroken must obviously be the younger of the two.</p> - -<div class="figleft" id="v2fig235" style="width: 161px;"> - <img src="images/v2fig235.png" width="161" height="196" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 235.</span>—Plan of basalt-veins with -selvages of black basalt-glass, east -side of Beinn Tighe, Isle of Eigg.</div> -</div> - - -<p>But in many examples in the south of -Scotland, Argyleshire and the Inner Hebrides, -the fineness of grain of the outer band culminates -in a perfect volcanic glass. Where this -occurs, the glass is usually jet black, more rarely -greenish or bluish black in tint, and varies in -thickness from about a couple of inches to a -mere varnish-like film on the outer face of the -dyke, the average width being probably less -than a quarter of an inch (<a href="#v2fig235">Fig. 235</a>). On -their weathered surface these external glassy -layers generally present a pattern of rounded or -polygonal prominences, varying up to four or -five lines or even more in diameter, and separated -by depressions or narrow ribs. The transition -from the glass to the crystalline part of the marginal fine-grained strip is -<span class="pagenum" id="Page_127">- 127 -</span> -usually somewhat abrupt, insomuch that on weathered faces it is often -difficult to get good specimens, owing to the tendency of the vitreous -portion to fly off when struck with the hammer. The glass doubtless -represents the original condition of the rock of the dyke. It was -suddenly chilled and solidified by contact with the cold walls of the -fissure. Inside this external glassy coating, the molten material could probably -still move, and had time to assume a more or less completely crystalline -condition before solidification. Not infrequently the glass shows spherulitic -forms, visible to the naked eye, and likewise a more or less distinctly -developed perlitic structure. These features, however, are best studied in -thin sections of the rock with the aid of the microscope, as will be subsequently -referred to.</p> - -<p>In some dykes, the glass is not confined to the edges, but runs in -strings or broader bands along the central portions, or has been squeezed into -little cavities like steam-holes or into minute fissures. One of the most -remarkable examples of this peculiarity occurs in the well-known dyke of -Eskdale, which runs for so many miles across the southern uplands of -Scotland.<a id="FNanchor_164" href="#Footnote_164" class="fnanchor">[164]</a> This dyke throughout most of its course is a crystalline rock of -the andesitic type. At Wat Carrick, in Eskdale, it presents an arrangement -into three parallel bands. On either side, a zone about eight feet broad -consists of the usual crystalline material. Between these two marginal -portions lies an intercalated mass 16 to 18 feet broad, of a very compact and -more or less vitreous rock. The demarcation between this central band and -the more crystalline zones of the outside is quite sharp, and the two kinds -of rock show a totally distinct system of jointing. There can, therefore, be -little doubt that the glassy centre belongs to a later uprise than the outer -portions, though possibly it may still have been included in the long process -of solidification of one original injected mass of molten material. If -the marginal parts adhered firmly to the walls, the centre, which with its -band of vesicles seems often to have been a line of weakness, might be -ruptured and subsequent intrusions would find their way along the rent. -Examples of this splitting of dykes with the intrusion of later eruptive -Material will be cited in later pages.</p> - -<div class="footnote"> - -<p><a id="Footnote_164" href="#FNanchor_164" class="label">[164]</a> See <i>Proc. Roy. Phys. Soc. Edin.</i> v. (1880), p. 241.</p> - -</div> - -<p>Mr. Clough, while mapping for the Geological Survey the extraordinarily -numerous dykes in the eastern part of Argyleshire between the -Firth of Clyde and Upper Loch Fyne, observed six or seven examples of -dykes showing glassy bands in their centres, with characters similar to -those of the Eskdale dyke. He found an absence of definite and regular -joints in the central glassy band, and on the other hand, an irregular set of -divisional planes by which the rock is traversed, and which he compared to -those seen in true perlitic structure.</p> - -<p>While, as a general rule, the external portions of a dyke are closer-grained -than the centre, rare cases occur where the middle is the most finely -crystalline part. I am disposed to regard these cases and the glassy centres -as forming in reality no true exceptions to the rule, that the outer portions -<span class="pagenum" id="Page_128">- 128 -</span> -of a dyke consolidated first, and are therefore finest in texture. For the -most part, each dyke appears to be due to a single uprise of molten matter, -though considerable movements may have taken place within its mass before -the whole stiffened into stone. Some particulars regarding these movements -will be given in section 12 of the next Chapter. It has already been mentioned -that in large dykes which have served as volcanic pipes, it is conceivable -that while the material next the outside consolidated and adhered -to the walls, the central portion may have remained liquid, and may even -have been propelled upward and have been succeeded by a different kind of -magma, as has been suggested by Mr. Iddings. In such cases, which, if -they occur, are probably excessively rare, we may expect that the earlier and -later material will not be sharply marked off from each other, unless we -suppose that the whole of the earlier liquid magma was so entirely ejected -that only its congealed marginal selvage was left as bounding walls for the -newer injection.</p> - -<p>Where, after more or less complete consolidation had taken place, the -fissure opened again, or from any other cause the dyke was split along its -centre, any lava which rose up the rent would tend to take a finer grain -than the material of the rest of the dyke, and might even solidify as glass.</p> - -<p>Large scattered crystals of felspar, of an earlier consolidation than that -of the minuter forms of the same mineral in the general groundmass of the -rock, give a porphyritic structure and andesitic character to many dykes. -Occasionally such crystals attain a considerable size. Mr. Clough has -observed them in some of the Argyleshire dykes reaching a length of -between three and four inches, with a thickness of two inches. Sometimes -they are distributed with tolerable uniformity through the substance of the -dyke. But not infrequently they may be observed in more or less definite -bands parallel with the boundary walls. Unlike the younger lath-shaped -and much smaller felspars of the groundmass, they show no diminution -either in size or abundance towards the edge of the dyke. On the contrary, -as already mentioned, they are often conspicuous in the close-grained -marginal strip, and may be found even in the glassy selvage, or touching -the very wall of the fissure. Indeed, they are sometimes more abundant in -the outer than in the inner portions of a dyke, having travelled outwards to -the surfaces of earliest cooling and crystallization.</p> - -<p>Mr. Clough has given me the details of an interesting case of this kind -observed by him in Glen Tarsan, Eastern Argyleshire:—"For an inch or so -from the edge of this dyke," he remarks, "porphyritic felspars giving -squarish sections, and ranging up to one-third of an inch in length, are so -abundant as nearly to equal in bulk the surrounding groundmass. For the -next inch and a half, they are decidedly fewer, occupying perhaps hardly an -eighth of the area exposed. Then for a breadth of three inches they come -in again nearly as abundantly as at the sides; after which they diminish -through a band 27 inches broad, where they may form from <sup>1</sup>/<sub>8</sub> to <sup>1</sup>/<sub>12</sub> of the -rock." He found another case where, in a dyke several yards wide, porphyritic -felspars, sometimes an inch long, are common along the eastern -<span class="pagenum" id="Page_129">- 129 -</span> -margin of the dyke in a band about two inches broad, but nearly absent -from the rest of the rock. Elsewhere the crystals are grouped rather in -patches than in bands. Among the dykes south of Oban some similar -instances of coarsely porphyritic felspars may be observed.</p> - -<p>Not only are these porphyritic felspars apt to occur in bands parallel -with the outer margins of the dykes, but they tend to range themselves -with their longer axis in the same direction, thus even on a large scale, -visible at some distance, showing the flow-structure, which is so often -erroneously regarded as essentially a microscopic arrangement, and as -specially characteristic of superficial lava-streams.</p> - -<p>Mr. Harker in his survey of Strath, Skye, has met with some remarkable -examples of the enclosure and incorporation of foreign materials in the -younger group of dykes which in that district traverse the granophyres -and gabbros. He remarks that the great majority of these dykes are basic, -and he has found them to be capable of convenient division into two groups. -1st, Non-porphyritic basic dykes with a specific gravity between 2·87 and -2·97, and an amygdaloidal structure affording clear indication of flowing -movement, either at the sides or along a central band. These dykes do not -greatly differ from those of pre-granophyre eruption. 2nd, Porphyritic basic -dykes which present features of peculiar interest. The porphyritic (or -pseudo-porphyritic) elements, according to Mr. Harker's observations, are -constantly felspar, frequently subordinate augite, and exceptionally quartz. -The felspars have for the most part rounded outlines with a bordering zone -of glass cavities apparently of secondary origin. The augite, in rounded -composite crystal-grains, differs from that of the groundmass and resembles -the augite of the gabbros. The quartz-grains are likewise rounded, and -show sometimes a distinct corroded border.</p> - -<p>These characters, Mr. Harker observes, are those of crystals derived from -some foreign source, and it can scarcely be doubted that this is the explanation -of their presence. He noticed that the dykes in question frequently -enclose fragments, varying up to several inches in diameter, of gabbro, -granite or granophyre, bedded lava, quartzite, etc., which show clear evidence -of having been rounded and corroded by an enveloping magma, and recognizable -crystals from some of the fragments may be observed in the -surrounding parts of the matrix of the dykes. Most of the felspar and -augite crystals disseminated through these porphyritic basic dykes may be -referred to the partial reabsorption of enclosed fragments of gabbro. The -same observer has found that many of the dykes which rise through the -basalt-plateau of Strathaird are crowded with gabbro fragments.</p> - -<p>Another megascopic character of the material composing the dykes is -the frequent presence of amygdales. It has sometimes been supposed that -amygdaloidal structure may be relied upon as a test to distinguish a mass of -molten rock which has reached the surface from one which has consolidated -under considerable pressure below ground. That this supposition, however, -is erroneous is demonstrated by hundreds of dykes in the great system -which I am now describing. But the amygdales of a dyke offer certain -<span class="pagenum" id="Page_130">- 130 -</span> -peculiarities which serve in a general way to mark them off from those of -an outflowing lava. They are usually smaller and more uniform in size -than in the latter rock. They are also more regularly spherical and less -frequently elongated in the direction of flow. Moreover, they are not -usually distributed through the -whole breadth of a dyke, but tend -to arrange themselves in lines -especially towards its centre (Fig. -236). In these central bands the -cavities are largest and depart -farthest from the regular spherical -form, so that for short spaces they -may equal in bulk the mass of -enclosing rock. In some rare instances, -a whole dyke is composed -of cellular basalt, like one of the -lava-sheets in the plateaux, as may -be seen on the north flank of Beinn -Suardal, Skye. Mr. Harker has -observed that an amygdaloidal structure is more common among the earlier -than among the later dykes of that district.</p> - -<div class="figleft" id="v2fig236" style="width: 240px;"> - <img src="images/v2fig236.png" width="240" height="190" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 236.</span>—Arrangement of lines of amygdales in -a dyke, Strathmore, Skye.</div> -</div> - -<p>Besides the common arrangement of fine-grained edges and a more -coarsely crystalline centre, instances are found where one of the contrasted -portions of a dyke traverses the other in the form of veins. Of these, I -think, there are two distinct kinds, probably originating in entirely different -conditions. In the first place, they may be of coarser grain than the rest -of the rock; but such a structure appears to be of extremely rare occurrence. -I have noticed some examples on the coast of Renfrewshire, where strings -of a more coarsely crystalline texture traverse the finer-grained body of the -rock. Veins of this kind are probably of the same nature as the so-called -"segregation-veins," to be afterwards referred to as of frequent occurrence -among the thicker Tertiary sills. They consist of the same minerals as the -rest of the rock, but in a different and more developed crystalline arrangement, -and they contain no glassy or devitrified material, except such portions -of that of the surrounding groundmass as may have been caught between -their crystalline constituents.</p> - -<p>The second kind of veins, which, though not common, is of much more -frequent occurrence than the first, is more particularly to be met with among -the broader dykes, and is distinguished by a remarkable fineness of grain, -sometimes approaching the texture of felsite or jasper, and occasionally -taking the form of actual glass. Such veins vary from half an inch or less, -up to four or five inches in breadth. They run sometimes parallel with the -walls of the dyke, but often irregularly in all directions, and for the most -part avoid the marginal portions, though now and then coming up to the -edge. They never extend beyond the body of the dyke itself into the -surrounding rock. Though they have obviously been injected after the -<span class="pagenum" id="Page_131">- 131 -</span> -solidification of the rock which they traverse, they may quite possibly be -extrusions of a deeper unconsolidated portion of the same rock into rents of -the already stiffened overlying parts. The field-geologist cannot fail to be -struck with the much greater hardness of these fine-grained veins and strings -that ramify through the coarsely crystalline dolerite, andesite or other -variety of the broader dykes. He can readily perceive in many cases their -more siliceous composition, and the inferences he deduces from the rough -observations he can make in the field are confirmed by the results of -chemical analysis (see <a href="#Page_137">p. 137</a>).</p> - -<p>In connection with veins of finer material, that may belong to a late -stage of the consolidation of the general body of a dyke, reference may be -made here to the occasional occurrence of patches of an exceedingly compact -or homogeneous texture immersed in the usual finely crystalline marginal -material. They look like angular and subangular portions of the more -rapidly cooled outer edge, which have been broken off and carried upward -by the still moving mass in the fissure.<a id="FNanchor_165" href="#Footnote_165" class="fnanchor">[165]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_165" href="#FNanchor_165" class="label">[165]</a> See Mr. J. J. H. Teall, <i>Quart. Journ. Geol. Soc.</i> xl. (1884), p. 214.</p> - -</div> - -<p>In general, each dyke is composed of one kind of rock, and retains its -chemical and mineralogical characters with singular persistence. The -difference of texture between the fine-grained chilled margin, with its -occasional glassy coating, and the more coarsely crystalline centre is due to -cooling and crystalline segregation in what was no doubt originally one -tolerably uniform molten mass. The glassy central bands, too, though they -indicate a rupture of the dyke up the middle, may at the same time quite -conceivably be, as I have said, extrusions from a lower portion of the dyke -before the final solidification of the whole. The ramifying veins of finer -grain that now and then traverse one of the large dykes are likewise -explicable as parts of a stage towards entire consolidation. All these -vitreous portions, whether still remaining as glass or having undergone -devitrification, are more acid than the surrounding crystalline parts of the -rock. They represent the siliceous "mother-liquor," so to speak, which was -left after the separation from it of the crystallized minerals, and which, -perhaps, entangled here and there in vesicles of the slowly cooling and -consolidating rock, was ready to be forced up into cracks of the overlying -mass during any renewal of terrestrial disturbance.</p> - -<p>But examples occur where a dyke, instead of consisting of one rock, is -made up of two or more bands of rock which, even if they resemble each -other closely, can be shown to be the results of separate eruptions. These, -which are obviously not exceptions to the general rule of the homogeneity -of dykes, I will consider in the next Chapter.</p> - -<p>Among the petrographical varieties observable in the field is the -occasional envelopment of portions of the surrounding rocks in the body of -a dyke. Angular fragments torn off from the fissure-walls have been carried -upwards in the ascending lava, and now appear more or less metamorphosed, -the amount of alteration seeming to depend chiefly upon the susceptibility -of the enclosed rock to change from the effects of heat. Cases of such -<span class="pagenum" id="Page_132">- 132 -</span> -entanglement, however, are of less common occurrence than those already -referred to, where pieces of some deep-seated rock, such as the gabbros of -Skye, have been carried up in the ascending magma. Occasionally, where -the enclosed fragments are oblong, they are arranged with their longer axes -parallel to the walls of the dyke, showing flow-structure on a large scale. -Mr. Clough has found some dykes near Dunoon which enclose fragments of -schist nearly three feet in length.</p> - -<p>One of the most interesting of the megascopic features of the dykes is -the joints by which they are traversed. These divisional planes are no -doubt to be regarded as consequences of the contraction of the original -molten rock during cooling and consolidation between its fissure-walls. -They are of considerable interest and importance, inasmuch as they furnish -a ready means of tracing a dyke when it runs through rock of the same -nature as itself, and also help to throw some light on the stages in the consolidation -of the material of the dyke.</p> - -<div class="figleft" id="v2fig237" style="width: 267px;"> - <img src="images/v2fig237.png" width="267" height="171" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 237.</span>—Systems of joints in the dykes.<br /><br /> -<i>a</i>, parallel; <i>b</i>, transverse.</div> -</div> - -<p>Two distinct systems of joints are recognizable (Fig 237). Though -sometimes combined in the same -dyke, they are most conspicuously -displayed when each occurs, -as it generally does, by itself. -The first and less frequent system -of joints (<i>a</i>) has been determined -by lines of retreat, which -are parallel to the walls of the -dyke. The joints are then -closest together at the margin, -and may be few or altogether -absent in the centre. They -are sometimes so numerous, -parallel and defined towards the -borders of the dyke, as to split the rock up into thin flags. Where transverse -joints are also present these flags are divided into irregular <i>tesseræ</i>.</p> - -<div class="figright" id="v2fig238" style="width: 226px;"> - <img src="images/v2fig238.png" width="226" height="191" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 238.</span>—Section of cylindrical vein or dyke, - cutting the bedded lavas, east side of Fuglö, - Faroe Islands.</div> -</div> - -<p>In the second or transverse system of joints (<i>b</i>), which is the more -usual, the divisional lines pass across the breadth of the dyke, either completely -from side to side, or from one wall for a longer or shorter distance -towards the other. Where this series of joints is most completely developed -the dyke appears to be built up of prisms piled horizontally, or -nearly so, one above another. These prisms, in rare instances, are as -regular as the columns of a basalt-sheet (see <a href="../../66492/66492-h/66492-h.htm#v1fig166">Fig. 166</a>). Usually, however, -they have irregularly defined faces, and merge into each other. Where the -prismatic structure is not displayed, the joints, starting sharply at the wall -of the dyke, strike inwards in irregular curving lines. It is such transverse -joints that enable the eye, even from a distance, to distinguish readily the -course of a dyke up the face of a cliff of basalt-beds, for they belong to -the dyke itself, are often at right angles to those of the adjacent basalt, and -by their alternate projecting and re-entering angles seam the dyke with -<span class="pagenum" id="Page_133">- 133 -</span> -parallel bars of light and shade (see the double dyke in <a href="#v2fig333">Fig. 333</a>). Where -they traverse not only the general mass of a dyke, but also the "contemporaneous -veins" which cross it, it may be inferred that these veins were -injected before the final solidification and contraction of the whole dyke.</p> - - -<p>An interesting modification of the transverse joints may sometimes be -observed, where, as in the case of the -Palæozoic "Rock and Spindle," at St. -Andrews (<a href="#v2fig222">Fig. 222</a>), the molten material -has solidified in a tubular or spherical -cavity. The joints then radiate inwards -from the outer curved surface. -The most remarkable instance of this -structure which I have found among -the Tertiary volcanic plateaux occurs -on the east side of the island Fuglö, -the most north-easterly of the group of -the Faroes. It is cut in section by -the face of the precipice, where it -appears as a round mass about 40 -or 50 feet in diameter piercing the -plateau-basalts. A selvage of finer material round its outer edge shows the -effect of rapid chilling, while the joints diverge from the periphery and -extend in fan-shape towards the centre (<a href="#v2fig238">Fig. 238</a>).</p> - -<div class="figcenter" id="v2fig239" style="width: 486px;"> - <img src="images/v2fig239.png" width="486" height="223" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 239.</span>—Joint-structures in the central vitreous portion of the Eskdale Dyke (B. N. Peach).<br /><br /> - A, View of a square yard of the outer wall of the vitreous central band, showing the polygonal arrangement of - the prisms and their investing sheath of ribs.<br /> - B, View of a smaller portion of the same wall to show the detailed structure of the ribs (<i>a</i> <i>a</i>) and their vitreous - cores (<i>b</i> <i>b</i>).<br /> - C, Profile of a part of the weathered face of the wall, showing the way in which the hard ribs or sheaths project - at the surface.</div> -</div> - - -<p>One of the most remarkable exhibitions of joint-structure hitherto -noticed among the Tertiary dykes is that which occurs in the central -vitreous band of the Eskdale dyke already referred to. The rock is divided -into nearly horizontal prisms, each of which consists of an inner more vitreous -<span class="pagenum" id="Page_134">- 134 -</span> -core and an outer more lithoid sheath. By the coherence of their polygonal -and irregular faces, and the greater durability of their material, these -sheaths project on the weathered wall of the vitreous centre of the dyke in -a curiously reticulated grouping of prominent ribs each about two inches -broad (<a href="#v2fig239">Fig. 239</a>, A), while the vitreous cores, being more readily acted on by -the weather, are hollowed out into little cup-shaped depressions. Each rib is -thus composed of the sheaths or outer lithoid portions of two prisms, the line -of separation being marked by a suture along the centre (B). Between this -median suture and the inner glassy core the rib is further cut into small -segments by a set of close joints, which are placed generally at right angles -to the course of the rib (C). Examined with a lens, the lithoid substance of -these sheaths has a dull finely granular aspect, like that of felsitic rocks, -with scattered felspars. It is obviously a more devitrified condition of the -material which forms the core of each prism. This material presents on a -fresh fracture a deep iron-black colour, dull resinous lustre and vitreous texture. -It at once recalls the aspect of many acid pitchstones, and in the early -days of petrography was naturally mistaken for one of these rocks. Through -its substance numerous kernels of more glassy lustre are dispersed, each of -which usually contains one or more amygdales of dull white chalcedony, but -sometimes only an empty black cavity. These black glistening kernels of -glass, of all sizes up to that of a small bean, scattered through the dull -resinous matrix, form with the white amygdales the most prominent feature -in the cores; but crystals of felspars may also be observed. Some details -of the microscopic characters of this remarkable structure will be given in a -subsequent page. The relation of the cores and sheaths to the prismatic -jointing of the rock seems to show that devitrification had not been -completed when these joints were established, and that it proceeded from -the faces of each prism inwards.</p> - -<p>(2) <i>Microscopic Characters.</i>—Much information has now been obtained -regarding the microscopic structure of the basaltic, doleritic and andesitic -dykes. The crystalline characters of those in the North of England have -been studied by Mr. Teall,<a id="FNanchor_166" href="#Footnote_166" class="fnanchor">[166]</a> and some of those from the West of Scotland -have been investigated by Professors Judd and Cole.<a id="FNanchor_167" href="#Footnote_167" class="fnanchor">[167]</a> Taken as a whole, -the rocks composing the dykes are found, when examined microscopically, -to consist essentially of mixtures of a plagioclase felspar, pyroxene and iron -oxide, with or without olivine, and usually with more or less interstitial matter.</p> - -<div class="footnote"> - -<p><a id="Footnote_166" href="#FNanchor_166" class="label">[166]</a> <i>Quart. Journ. Geol. Soc.</i> vol. xl. (1884).</p> - -<p><a id="Footnote_167" href="#FNanchor_167" class="label">[167]</a> <i>Op. cit.</i> vol. xxxix. (1883) p. 444 (basalt-glass); xlii. (1886) p. 49, where Professor Judd -discusses the gabbros, dolerites and basalts as a whole.</p> - -</div> - -<p>The felspar appears to be in some cases labradorite, in others anorthite, -but there may be a mingling of several species in many of the dykes, as in -the augite-andesite of the Santorin eruption in 1866, wherein Professor Fouqué -found that the larger porphyritic felspars were mainly labradorite, but partly -anorthite, while those of the groundmass were microlites of albite and -oligoclase.<a id="FNanchor_168" href="#Footnote_168" class="fnanchor">[168]</a> The large felspars scattered porphyritically through the groundmass -<span class="pagenum" id="Page_135">- 135 -</span> -are evidently the result of an early consolidation, unless where they -are survivals from fragments of older porphyritic rocks which have been -enveloped and partially dissolved in the dykes. They are often cracked, -penetrated by the groundmass, or even broken into fragments, and have -corroded borders. They sometimes include portions of the groundmass, and -present the zonal growth structure in great perfection. The small felspars -of the groundmass, on the other hand, are as obviously the result of a -later crystallization, for they vary in size and crystallographic development -according to their position in the dyke. Those from the centre are often -in well-formed crystals, which sometimes pass round their borders into -acicular microlites. Those in the marginal parts of the dyke occur chiefly -in the form of these microlites, forming the felted aggregate so characteristic -of the andesites. Curious skeleton forms, composed of aggregates of -microlites, connect the latter with the more completely developed crystals, -and illustrate the mode of crystallization of the felspathic constituents of -the dykes.<a id="FNanchor_169" href="#Footnote_169" class="fnanchor">[169]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_168" href="#FNanchor_168" class="label">[168]</a> <i>Santorin et ses Éruptions</i>, 1879, p. 203.</p> - -<p><a id="Footnote_169" href="#FNanchor_169" class="label">[169]</a> See Mr. Teall's excellent description of the Cleveland dyke, in the paper above cited.</p> - -</div> - -<p>The pyroxene is probably in most cases monoclinic (black or common -augite), but is sometimes rhombic (usually enstatite, less frequently perhaps -hypersthene). It occurs in (<i>a</i>) well-developed crystals, (<i>b</i>) crystalline -masses with some of the faces of the crystals developed, (<i>c</i>) granular -aggregates which polarise in one plane, (<i>d</i>) separate granules and -microscopic microlites, which may be spherical (globulites) or oblong -(longulites).</p> - -<p>The black iron-oxide is sometimes magnetite, sometimes ilmenite, or -other titaniferous ore. Apatite not infrequently occurs among the original -constituents. Olivine is entirely absent from most of the large solitary -dykes, especially at a distance from the great volcanic centres, and no serpentinous -matter remains to indicate that it was ever present in them. But it is -to be met with in numerous basalt-dykes in the volcanic areas, either in -sparsely scattered or in tolerably abundant crystals. Biotite occasionally -appears. Among the secondary products, calcite and pyrites are doubtless -the most common. To these must be added quartz, chalcedony and various -zeolitic substances, besides the aggregates which result from the decomposition -of the ferro-magnesian constituents and the oxidation of the -ferrous oxides.</p> - -<p>In many dykes there is little or no interstitial matter between the -crystalline constituents of the groundmass. In others this matter amounts -to a half or more of the whole composition, and from such cases a series of -gradations may be traced into a complete glass containing only the rudimentary -forms of crystals (globulites, longulites, etc.), with scattered -porphyritic crystals of an earlier consolidation. The process of the disappearance -of this original glass may be admirably studied in many dykes. -At the outer wall, the glass remains nearly as it was when contact with -the cold walls of the fissure solidified it. From that external vitreous layer -the successive devitrification products and crystalline growths may be -<span class="pagenum" id="Page_136">- 136 -</span> -followed inwards until in the central parts of a broad dyke little or no trace -of the interstitial matter may be left.</p> - - -<table id="v2fig240" summary="image"> -<tr> - <td style="width:200px;"><img src="images/v2fig240.png" width="186" height="183" alt="" /></td> - <td style="width:300px;"><div class="figcaption"> - <div class="tdc"><span class="smcap">Fig. 240.</span>—Microscopic structure of the - vitreous part of the Eskdale Dyke.<br /><br /></div> - <div class="justify">This section shows a crystal of augite, enclosing - magnetite and surrounded with - microlites, each of which consists of a central - pale yellow rod crusted with pale - yellow isotropic globulites. The glass - around this aggregation is clear, but at a - little distance globulites (many of them - elongated and dichotomous) abound, - with here and there scattered microlites, - some of which are curved and spiral. - (800 diameters.)<a id="FNanchor_170" href="#Footnote_170" class="fnanchor">[170]</a></div> - </div></td> -</tr> -</table> - -<div class="footnote"> - -<p><a id="Footnote_170" href="#FNanchor_170" class="label">[170]</a> <i>Proc. Roy. Phys. Soc. Edin.</i> v. (1880), p. 255.</p> - -</div> - -<p>The most instructive example of the process of devitrification which has -come under my observation occurs in the Eskdale dyke. The central -"cores" already referred to present a true glass, which in thin sections is -perfectly transparent and almost colourless, -but by streaks and curving lines of darker tint -shows beautiful flow-structure. The devitrification -of this glass has been accomplished by -the development of crystallites and crystals, -which increase in number until all the -vitreous part of the rock disappears. What -seems under a low power to be a structureless -or slightly dusty glass can be resolved with a -higher objective into an aggregate of minute -globules or granules (globulites), which average -perhaps <sup>1</sup>/<sub>20,000</sub> of an inch in diameter. -Some of these bodies are elongated and even -dichotomous at the ends. These granules are -especially crowded upon clear yellow dart-shaped -rods, which in turn are especially prominent -upon crystals and crystalline grains of augite -that bristle with them, while the immediately -surrounding glass has become clear. There -can be little doubt that these rudimentary -bodies are stages in the arrested development of augite crystals. There -occur also opaque grains, rods and trichites, which no doubt consist in -whole of magnetite (or other iron oxide), or are crusted over with that -mineral.</p> - -<p>At least two broad types of microscopic structure may be recognized -among the basic and intermediate dykes. (1) Holocrystalline, or with only -a trifling proportion of interstitial matter. This type includes the dolerites -and basalts, as well as rocks which German petrographers would class as -diabases or diabase-porphyrites. The rocks are very generally characterized -by ophitic structure, where the lath-shaped felspars penetrate the augite, -and are therefore of an earlier consolidation. In such cases there is a -general absence of any true interstitial matter. The rocks of this type are -often rich in olivine, and appear to be on the whole considerably more basic -than those of the second group. It is observable that they increase in -numbers from the centre of Scotland westwards, and throughout the region -of the basalt-plateaux they form the prevailing type. (2) In this type -there is a marked proportion of interstitial substance, which is inserted in -wedge-shaped portions among the crystallised constituents ("intersertal -structure" of Rosenbusch). The ophitic structure appears to be absent, -and olivine is either extremely rare or does not occur at all. The rocks of -<span class="pagenum" id="Page_137">- 137 -</span> -this group are obviously less basic than those of the other. They form the -large dykes that rise so conspicuously through the South of Scotland and -North of England, and their general characters are well described by Mr. -Teall in the paper already cited. In some instances they enclose abundant -porphyritic felspars of earlier consolidation, and then present most of the -characters of andesites. Professor Rosenbusch has extended the name of -"Tholeiites" to rocks of this group in the North of England.<a id="FNanchor_171" href="#Footnote_171" class="fnanchor">[171]</a> The -vitreous condition is found in both types, but is perhaps more frequent in -the second. The glass of the basalts, however, even in thin slices, is -characteristically opaque from its crowded inclusions; while that of the -andesitic forms, though black in hand specimens, appears perfectly transparent -and sometimes even colourless in thin slices.</p> - -<div class="footnote"> - -<p><a id="Footnote_171" href="#FNanchor_171" class="label">[171]</a> <i>Mikroskopische Physiographie</i>, 3rd edit. 1071 <i>et seq.</i></p> - -</div> - -<p>(3) <i>Chemical Characters.</i>—The only one of these to which reference will -be made here is the varying proportion of silica. While the dykes as a -whole are either intermediate or basic, some of them contain so high a percentage -of silica as to link them with the acid rocks. The average proportions -of this ingredient range from less than 50 to nearly 60 per cent. -The rocks with the lower percentage of acid are richer in the heavy bases, -and have a specific gravity which sometimes rises above 3·0. They include -the true dolerites and basalts. Those, on the other hand, with the higher -ratio of silica, are poorer in the heavy bases, and have a specific gravity -from 2·76 to 2·96. They comprise the tholeiites, andesites and other -more coarsely crystalline rocks of the great eastern and south-eastern dykes.<a id="FNanchor_172" href="#Footnote_172" class="fnanchor">[172]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_172" href="#FNanchor_172" class="label">[172]</a> For analyses of dykes, see Sir I. L. Bell, <i>Proc. Roy. Soc.</i> xxiii. p. 546; Mr. J. S. Grant Wilson, -<i>Proc. Roy. Phys. Soc. Edin.</i> v. p. 253; Mr. Teall, <i>Quart. Journ. Geol. Soc.</i> xl. p. 209; Professors -Judd and Cole, <i>Quart. Jour. Geol. Soc.</i> xxxix. p. 444.</p> - -</div> - -<p>Not only do the dykes differ considerably from each other in their -relative proportions of silica, but even the same dyke may sometimes be -found to present a similar diversity in different parts of its mass. It has -long been a familiar fact that the glassy parts of such rocks are more acid -than the surrounding crystalline portions. The original magma may be -regarded as a natural glass or fused silicate, in which all the elements of -the rock were dissolved, and which necessarily became more acid as the -various basic minerals crystallised out of it.<a id="FNanchor_173" href="#Footnote_173" class="fnanchor">[173]</a> In the Eskdale dyke the -silica percentage of this glassy portion is 58·67, that of the little kernels -of black glass dispersed through the rock as much as 65·49.<a id="FNanchor_174" href="#Footnote_174" class="fnanchor">[174]</a> In the -Dunoon dyke observed by Mr. Clough the siliceous finer-grained veins contain -no less than 68·05 per cent of silica, while the mass of the dyke itself -shows on analysis only 47·36 per cent.<a id="FNanchor_175" href="#Footnote_175" class="fnanchor">[175]</a> Similar red strings have been -noticed by the same careful observer in an east and west dyke near Lochgoilhead. -From Mr. Teall's examination a large part of the felspar in -<span class="pagenum" id="Page_138">- 138 -</span> -these veins is probably orthoclase. It forms a much larger percentage of -the entire rock than the felspar does in normal dolerites.</p> - -<div class="footnote"> - -<p><a id="Footnote_173" href="#FNanchor_173" class="label">[173]</a> On this subject see a paper by Dr. A. Lagorio, "Ãœber die Natur der Glasbasis sowie der -Krystallisationsvorgänge im eruptiven Magma," Tschermak's <i>Mineralog. Mittheil.</i> viii. (1887), -p. 421.</p> - -<p><a id="Footnote_174" href="#FNanchor_174" class="label">[174]</a> Mr. J. S. Grant Wilson, <i>Proc. Roy. Soc. Phys. Edin.</i> v. (1880) p. 253.</p> - -<p><a id="Footnote_175" href="#FNanchor_175" class="label">[175]</a> Unpublished analyses made by the late Professor Dittmar of Glasgow, and communicated -to me by Mr. Clough.</p> - -</div> - -<p>2. Trachyte Dykes.—In the Cowal District of Argyleshire, and -in the south of Skye, Mr. Clough has encountered a limited number -of dykes of trachyte. On a hasty inspection these are not always readily -distinguished from the basalt-dykes with which they agree in general -external aspect and in direction. Where their relation to these dykes, -however, can be determined they are found to traverse them, and thus to -be on the whole later, though one case has been observed where a trachytic -dyke is in turn traversed by one of the basic series. Mr. Clough has -supplied me with the following notes of his observations regarding the -trachytic dykes. They are all characterized by the possession of spherulitic -structures near their margins. These features, easily perceptible to the -naked eye, afford the readiest means of distinguishing the dykes of this -group. So abundant are the spherulites that they not infrequently impinge -on each other in long parallel rows forming rod-like aggregates. -Thus in a dyke near Craigendavie, at the head of Loch Striven, numerous -planes about a quarter of an inch apart, and composed of such close-set -rods, may be observed running parallel to the marginal wall for a distance -of several inches from the edge. Most of these planes show on their surfaces -that the rods are always parallel to each other, but may run in different -directions in the different layers, being sometimes horizontal, sometimes -vertical, or at any angle between. On examination, each rod is found to -be made up of polygonal bodies, the angles of which are quite sharp, but -with their sides often slightly curved, as if they had assumed their forms -from the mutual pressure of original spherical bulbs. Further scrutiny -shows that the polygonal bodies often exhibit an internal radiate structure.</p> - -<p>In the central parts of the dyke the spherulitic arrangement is not -traceable. About a foot from the margin it begins to be recognizable. At -a distance of three or four inches the spherulites are about the size of peas, -and gradually diminish towards the edge until they can no longer be seen.</p> - -<p>Another characteristic of the trachyte dykes has been found by Mr. -Clough to be a useful guide in discriminating them from the basalt-group. -While the amygdales in the latter are generally rudely spherical, those in -the trachytes are commonly elongated in the direction of the length of the -dyke, and are frequently three quarters of an inch, sometimes even an inch -and a half, in length, though less than a quarter of an inch in breadth.</p> - -<p>A good example of these trachytic dykes, which occurs at Dunans, -about the head of Glendaruel, has been examined microscopically and -chemically. The central better crystallised portion was found by Mr. -Teall to be composed mainly of small lath-shaped crystals of orthoclase, -together with scales of brown biotite, a few prismatic crystals of pale -somewhat altered pyroxene and scattered granules of magnetite. The -chemical analysis of this rock by Mr. J. H. Player gave the following -composition:—</p> - -<p><span class="pagenum" id="Page_139">- 139 -</span></p> - -<table summary="data"> -<tr> - <td class="tdl">Silica</td> - <td class="tdr">56·4</td> -</tr> -<tr> - <td class="tdl">Alumina</td> - <td class="tdr">19·0</td> -</tr> -<tr> - <td class="tdl">Ferric oxide</td> - <td class="tdr">3·5</td> -</tr> -<tr> - <td class="tdl">Ferrous oxide</td> - <td class="tdr">4·8</td> -</tr> -<tr> - <td class="tdl">Lime</td> - <td class="tdr">2·6</td> -</tr> -<tr> - <td class="tdl">Magnesia</td> - <td class="tdr">1·5</td> -</tr> -<tr> - <td class="tdl">Soda</td> - <td class="tdr">4·5</td> -</tr> -<tr> - <td class="tdl">Potash</td> - <td class="tdr">5·0</td> -</tr> -<tr> - <td class="tdl">Loss on ignition</td> - <td class="tdr">2·6</td> -</tr> -<tr> - <td></td> - <td class="bdt bdb2 tdr">99·9</td> -</tr> -</table> - - -<h3>4. HADE</h3> - -<p>In the majority of cases, especially among the great single dykes, the -intrusive rock has assumed a position nearly or quite vertical. But occasionally, -where one of these solitary examples crosses a deep valley, a slight -hade is perceptible by the deviation of the line of the dyke from its -normal course. Sedgwick long ago noticed that the Cleveland dyke has, -in places, an inclination of at least 80° to its N.E. side.<a id="FNanchor_176" href="#Footnote_176" class="fnanchor">[176]</a> In the coal-workings, -also, a trifling deviation from the vertical is sometimes perceptible, -especially where a dyke has found its way along a previously existing line of -fault, as in several examples in Stirlingshire. But in those districts where -the dykes are gregarious, departures from the vertical position are not infrequent, -more particularly near the great basalt-plateaux. It was noticed by -Necker, that even in such a dyke-filled region as Arran, almost all of the dykes -are vertical, though sometimes deviating from that position to the extent of -20°.<a id="FNanchor_177" href="#Footnote_177" class="fnanchor">[177]</a> Berger found that the angle of deviation among those of the north -of Ireland ranges from 9° to 20°, with a mean of 13°.<a id="FNanchor_178" href="#Footnote_178" class="fnanchor">[178]</a> The most oblique -examples are probably those which occur in the basalt-plateaux of the Inner -Hebrides, where the same dyke in some parts of its course runs horizontally -between two beds, across which it also descends vertically (see Figs. <a href="#v2fig251">251</a>, <a href="#v2fig252">252</a>, -<a href="#v2fig374">374</a>). But with these minor exceptions, the verticality of the great system of -dykes, pointing to the perpendicular fissure-walls between which the molten -rock ascended, is one of the most notable features in their geological -structure. In the Strath district of Skye Mr. Harker has noticed that -while the earlier dykes have sometimes a hade of 45°, those younger than -the granophyre are generally vertical or nearly so. In the Blath Bheinn -group of hills, however, as already alluded to, he has observed that it is the -youngest dykes which are inclined in a north-westerly direction, with a hade -of as much as 40° from the horizon.</p> - -<div class="footnote"> - -<p><a id="Footnote_176" href="#FNanchor_176" class="label">[176]</a> <i>Cambridge Phil. Trans.</i> ii. p. 28.</p> - -<p><a id="Footnote_177" href="#FNanchor_177" class="label">[177]</a> <i>Trans. Roy. Soc. Edin.</i> xiv. p. 677.</p> - -<p><a id="Footnote_178" href="#FNanchor_178" class="label">[178]</a> <i>Trans. Geol. Soc.</i> iii. p. 227.</p> - -</div> - - -<h3>5. BREADTH</h3> - -<p>An obvious characteristic of most dykes is the apparent uniformity of -their breadth. Many of them, as exposed along shore-sections, vary as -little in dimensions as well-built walls of masonry do. Departures from -such uniformity may often indeed be noted, whether a dyke is followed -<span class="pagenum" id="Page_140">- 140 -</span> -laterally or vertically. The largest amount of variation is, of course, to be -found among the dykes of the gregarious type, the thinner examples of -which may diminish to a width of only one inch or less, while their -average breadth is much smaller than in the case of the great solitary -dykes. In the district of Strathaird, in Skye, Macculloch estimated that -the remarkably abundant dykes there developed vary from 5 to 20 feet in -breadth, but with an average breadth of not more than 10 feet.<a id="FNanchor_179" href="#Footnote_179" class="fnanchor">[179]</a> In the -isle of Arran, according to Necker's careful measurements, most of the dykes -range from 2 or 3 to 10 or 15 feet, but some diminish to a few inches, -while others reach a width of 20, 30, or even 50 feet.<a id="FNanchor_180" href="#Footnote_180" class="fnanchor">[180]</a> In the North of -Ireland, Berger observed that the average breadth of thirty-eight dykes -traversing primitive rocks (schist, granites, etc.) was 9 feet; and of twenty-four -in Secondary rocks, 24 feet.<a id="FNanchor_181" href="#Footnote_181" class="fnanchor">[181]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_179" href="#FNanchor_179" class="label">[179]</a> <i>Trans. Geol. Soc.</i> iii. p. 80.</p> - -<p><a id="Footnote_180" href="#FNanchor_180" class="label">[180]</a> <i>Trans. Roy. Soc. Edin.</i> xiv. p. 690 et seq.</p> - -<p><a id="Footnote_181" href="#FNanchor_181" class="label">[181]</a> <i>Trans. Geol. Soc.</i> iii. p. 226. He believed that dykes in Secondary rocks reach a much -greater thickness than in other formations. My own observations do not confirm this -generalisation.</p> - -</div> - -<p>But when we pass to the great solitary dykes, that run so far and so -continuously across the country, we encounter much thicker masses of -igneous rock. Most of the measurements of these dykes have been made at -the surface, and the variations noted in their breadth occur along their -horizontal extension. The Cleveland dyke, which is the longest in Britain, -varies from 15 feet to more than 100 feet, with perhaps an average width -of between 70 and 90 feet.<a id="FNanchor_182" href="#Footnote_182" class="fnanchor">[182]</a> Some of the great dykes that cross Scotland -are of larger dimensions. Most of them, however, like that of Cleveland, -are liable to considerable variations in breadth when followed along their -length. The dyke which runs from the eastern coast across the Cheviot -Hills and Teviotdale to the head of the Ale Water, is in some places only -10 feet broad, but at its widest parts is probably about 100 feet. The -Eskdale and Moffat dyke is in parts of its course 180 feet wide, but elsewhere -it diminishes to not more than 40 feet. These variations are -repeated at irregular intervals, so that the dyke alternately widens and -contracts as its course is traced across the hills. Some of the dykes further -to the north and west attain yet more gigantic proportions. That which -crosses Cantyre opposite Ardlamont Point has been measured by Mr. J. B. -Hill, of the Geological Survey, who finds it to be from 150 to 180 feet -broad on the shore of Loch Fyne, and to swell out beyond the west side of -Loch Tarbert to a breadth of 240 to 270 feet. A dyke near Strathmiglo, -in Fife, is about 400 feet wide. The broadest dyke known to me is one -which I traced near Beith, in Ayrshire, traversing the Carboniferous Limestone. -Its maximum width is 640 feet.</p> - -<div class="footnote"> - -<p><a id="Footnote_182" href="#FNanchor_182" class="label">[182]</a> At Cockfield, where it has long been quarried, it varies from 15 to 66 feet; at Armathwatie, -in the vale of the Eden, it is about 54 feet (Mr. Teall, <i>Quart. Journ. Geol. Soc.</i> xl. p. 211).</p> - -</div> - -<p>Unfortunately, it is much less easy to get evidence of the width of dykes -at different levels in their vertical extension. Yet this is obviously an -important point in the theoretical discussion of their origin. Two means -<span class="pagenum" id="Page_141">- 141 -</span> -are available of obtaining information on the subject—(<i>a</i>) from mining -operations, and (<i>b</i>) from observations at precipices and between hill-crests -and valley-bottoms.</p> - -<p>(<i>a</i>) In the Central Scottish coal-field and in that of Ayrshire, some large -dykes have been cut through at depths of two or three hundred feet beneath -the surface. But there does not appear to be any well-ascertained variation -between their width so far below ground and at the surface. In not a few -cases, indeed, dykes are met with in the lower workings of the coal-pits which -do not reach the surface or even the workings in the higher coals. Such -upward terminations of dykes will be afterwards considered, and it will be -shown that towards its upper limit a dyke may rapidly diminish in width.</p> - -<p>(<i>b</i>) More definite information, and often from a wider vertical range, is -to be gathered on coast-cliffs and in hilly districts, where the same dyke can -be followed through a vertical range of many hundred feet. But so far as -my own observations go, no general rule can be established that dykes -sensibly vary in width as they are traced upward. Every one who has -visited the basalt-precipices of Antrim or the Inner Hebrides, where dykes -are so numerous, will remember how uniform is their breadth as they run -like ribbons up the faces of the escarpments.<a id="FNanchor_183" href="#Footnote_183" class="fnanchor">[183]</a> Now and then one of them -may be observed to die out, but in such cases (which are far from common) -the normal width is usually maintained up to within a few feet of the -termination.</p> - -<div class="footnote"> - -<p><a id="Footnote_183" href="#FNanchor_183" class="label">[183]</a> This point did not escape the attention of that excellent observer, Berger, in his examination -of the dykes in the North of Ireland. We find him expressing himself thus:—"The depth -to which the dykes descend is unknown; and after having observed the sections of a great many -along the coast in cliffs from 50 to 400 feet in height, I have not been able to ascertain (except -in one or two cases) that their sides converge or have a wedgeform tendency" (<i>Trans. Geol. Soc.</i> -iii. p. 227).</p> - -</div> - -<p>All over the southern half of Scotland, where the dykes run along the -crests of the hills and also cross the valleys, a difference of level amounting -to several hundred feet may often be obtained between adjacent parts of the -same dyke. But the breadth of igneous rock is not perceptibly greater in -the valleys than on the ridges. The depth of boulder clay and other superficial -deposits on the valley bottoms, however, too frequently conceals the -dykes at their lowest levels. Perhaps the best sections in the country for -the study of this interesting part of dyke-structure are to be found among -the higher hills of the Inner Hebrides, such as the quartzites of Jura and -the granophyres and gabbros of Skye. On these bare rocky declivities, -numerous dykes may be followed from almost the sea-level up to the rugged -and splintered crests, a vertical distance of between 2000 and 3000 feet. -The dykes are certainly not as a rule sensibly less in width on the hill-tops -than in the glens. So far, therefore, as I have been able to gather the -evidence, there does not appear to me to be, as a general rule, any appreciable -variation in the width of dykes for at least 2000 or 3000 feet of their -descent. The fissures which they filled must obviously have had nearly -parallel walls for a long way down.</p> - -<p><span class="pagenum" id="Page_142">- 142 -</span></p> - - -<h3>6. INTERRUPTIONS OF LATERAL CONTINUITY</h3> - -<p>In tracing the great solitary dykes across the country, the geologist is -often surprised to meet with gaps, varying in extent from a few hundred -feet to several miles, in which no trace whatever of the igneous rock can be -detected at the surface. This disappearance is not always explicable by the -depth of the cover of superficial accumulations; for it may be observed -over ground where the naked rocks come almost everywhere to the surface, -and where, therefore, if the conspicuous material of the dykes existed, it -could not fail to be found. No dyke supplies better illustrations of this -discontinuity than that of Cleveland. Traced north-westward across the -Carboniferous tracts that lie between the mouth of the Tees and the Yale -of the Eden, this dyke disappears sometimes for a distance of six or eight -miles. In the mining ground round the head of the South Tyne the rocks -are bare, so that the absence of the dyke among them can only be accounted -for by its not reaching the surface. Yet there can be no doubt that the -various separated exposures, which have the same distinctive lithological -characters and occur on the same persistent line, are all portions of one dyke -which is continuous at some depth below ground. We have thus an indication -of the exceedingly irregular upward limit of the dykes, as will be -more particularly discussed further on.</p> - -<p>But there are also instances where the continuity is interrupted and -then resumed on a different line. One of the best illustrations of this -character is supplied by the large dyke which rises through the hills about -a mile south of Linlithgow and runs westward across the coal-field. At -Blackbraes it ends off in a point, and is not found again to the westward in -any of the coal-workings. But little more than a quarter of a mile to the -south a precisely similar dyke begins, and strikes westward parallel to the -line of the first one. The two separated strips of igneous rock overlap -each other for about three-quarters of a mile. But that they are merely -interrupted portions of what is really a single dyke can hardly be questioned. -A second example is furnished by another of the great dykes of -the same district, which after running for about twelve miles in a nearly -east and west direction suddenly stops at Chryston, and begins again in the -same direction, but on a line about a third of a mile further north. Such -examples serve to mark out irregularities in the great fissures up which the -materials of the dykes rose.</p> - - -<h3>7. LENGTH</h3> - -<p>In those districts where the small and crowded dykes of the gregarious -type are developed, one cannot usually trace them for more than a short -distance. The longest examples known to me are those which have been -mapped with much patience and skill by Mr. Clough in Eastern Argyleshire. -Some of them he has been able to track over hill and valley for -four or five miles, though the great majority are much shorter. In Arran -and in the Inner Hebrides, it is seldom possible to follow what we can be -sure is the same dyke for more than a few hundred yards. This difficulty -<span class="pagenum" id="Page_143">- 143 -</span> -arises partly, no doubt, from the frequent spread of peat or other superficial -accumulation which conceals the rocks, and partly also from the great -number of dykes and the want of sufficiently distinct lithological characters -for the identification of any particular one. But making every allowance -for these obstacles, we are compelled, I think, to regard the gregarious dykes -as essentially short as well as relatively irregular.</p> - -<p>In striking contrast to these, come the great solitary dykes. In estimating -their length, as I have already remarked, we must bear in mind the -fact that they occasionally undergo interruptions of continuity owing to the -local failure of the igneous material to rise to the level of what is now the -surface of the ground. A narrow wall-like mass of andesite or dolerite, -which sinks beneath the surface for a few hundred yards, or for several -miles, and reappears on the same line with the same petrographical characters, -while there may be no similar rock for miles to right and left, can only be -one dyke prolonged underneath in the same great line of fissure. But even -if we restrict our measurements of length to those dykes or parts of dykes -where no serious interruption of continuity takes place, we cannot fail to be -astonished at the persistence of these strips of igneous rock through the -most diverse kinds of geological structure. A few illustrative examples of -this feature may be selected. It will be observed that the longest and -broadest dykes are found furthest from the basalt-plateaux, while the shortest -and narrowest are most abundant near these plateaux.</p> - -<p>Not far from what I have taken provisionally as the northern boundary -of the dyke region, two dykes occur which have been mapped from the head -of Loch Goil by Arrochar across Lochs Lomond and Katrine by Ben Ledi to -Glen Artney, whence they strike into the Old Red Sandstone of Strathmore, -and run on to the Tay near Perth—a total distance of about 60 miles. If -the dyke which continues in the same line on the other side of the estuary -of the Tay beyond Newburgh, is a prolongation of one of these, then its -entire length exceeds 70 miles. A few miles further south, one of a group -of dykes can be followed from the heart of Dumbartonshire by Callander -across the Braes of Doune to Auchterarder—a distance of 47 miles, with an -average breadth of more than 100 feet. In the district between the Forth -and Clyde a number of long parallel dykes can be traced for many miles -across hill and plain, and through the coal-fields. One of these is continuous -for 25 miles from the heart of Linlithgowshire into Lanarkshire. Still -longer is the dyke which runs from the Firth of Forth at Grangemouth -westward to the Clyde, opposite Greenock—a distance of about 36 miles. -Coming southward, we encounter a striking series of single dykes on the -uplands between the counties of Lanark and Ayr, whence they strike into -the Silurian hills of the southern counties. One of these runs across the -crest of the Haughshaw Hills, and can be followed for some 30 miles. But -if, as is probable, it is prolonged in one of the dykes that traverse the moorlands -of the north of Ayrshire and south of Renfrewshire to the Clyde, its -actual length must be at least twice that distance. The great Moffat and -Eskdale dyke strikes for more than 50 miles across the South of Scotland -<span class="pagenum" id="Page_144">- 144 -</span> -and North of England. The Hawick and Cheviot dyke runs for 26 miles in -Scotland and for 32 miles in Northumberland.</p> - -<p>But the most remarkable instance of persistence is furnished by the -Cleveland dyke. From where it is first seen near the coast-cliffs of -Yorkshire the strip of igneous rock can be followed, with frequent -interruptions, during which for sometimes several miles no trace of it -appears at the surface, across the North of England as far as Dalston Hall -south of Carlisle, beyond which the ground onwards to the Solway Firth is -deeply covered with superficial deposits. The total distance through which -this dyke can be recognized is thus about 110 miles. But it probably goes -further still. On the opposite side of the Solway, a dyke which runs in the -same line, rises through the Permian strata a little to the east of the mouth -of the Nith. Some miles further to the north-west, near Moniaive, Mr. J. -Horne, in the progress of the Geological Survey, traced a dark compact dyke -with kernels of basalt-glass near its margin, running in the same north-westerly -direction. Still further on in the same line, another similar rock -is found high on the flanks of the lofty hill known as Windy Standard. -And lastly, in the Ayrshire coal-field, a dyke still continuing the same -trend, runs for several miles, and strikes out to sea near Prestwick. It -cannot, of course, be proved that these detached Scottish protrusions belong -to one great dyke, or that if such a continuous dyke exists, it is a prolongation -of that from Cleveland. At the same time, I am on the whole inclined -to connect the various outcrops together as those of one prolonged subterranean -wall of igneous rock. The distance from the last visible portion -of the Cleveland dyke near Carlisle to the dyke that runs out into the Firth -of Clyde near Prestwick, is about 80 miles. If we consider this extension -as a part of the great North of England dyke, then the total length of this -remarkable geological feature will be about 190 miles.</p> - - -<h3>8. PERSISTENCE OF MINERAL CHARACTERS</h3> - -<p>Not less remarkable than their length is the preservation of their -normal petrographical characters by some dykes for long distances. In this -respect the Cleveland dyke may again be cited as a typical example. The -megascopic and microscopic structures of the rock of this dyke distinguish -it among the other eruptive rocks of the North of England. And these -peculiarities it maintains throughout its course.<a id="FNanchor_184" href="#Footnote_184" class="fnanchor">[184]</a> Similar though less prominent -uniformity may be traced among the long solitary dykes of the South -of Scotland, the chief variations in these arising from the greater or less -extent to which the original glassy magma has been retained. The same -dyke will at one part of its course show abundant glassy matter even to the -naked eye, while at a short distance the vitreous groundmass has been -devitrified, and its former presence can only be detected with the aid of the -microscope. Where a dyke has caught up and absorbed abundant foreign -materials its composition naturally varies considerably from point to point. -Mr. Harker has observed some good examples of this variation in Skye.</p> - -<div class="footnote"> - -<p><a id="Footnote_184" href="#FNanchor_184" class="label">[184]</a> See the careful examination of this dyke by Mr. Teall, <i>Quart. Journ. Geol. Soc.</i> xl. p. 209.</p> - -</div> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_145">- 145 -</span></p> - -<h2 class="nobreak" id="CHAPTER_XXXV">CHAPTER XXXV<br /> - -<span class="smaller">THE SYSTEM OF DYKES—<i>continued</i></span></h2> -</div> - -<div class="blockquot"> - -<p>Direction—Termination upward—Known vertical Extension—Evidence as to the movement -of the Molten Rock in the Fissures—Branches and Veins—Connection of Dykes -with Intrusive Sheets—Intersection of Dykes—Dykes of more than one infilling—Contact -Metamorphism of the Dykes—Relation of the Dykes to the Geological Structure -of the Districts which they traverse—Data for estimating the Geological Age of -the Dykes—Origin and History of the Dykes.</p> -</div> - - -<h3>9. DIRECTION</h3> - -<p>Another characteristic feature of the dykes is their generally rectilinear -course. So true are the solitary dykes to their normal trend that, in spite -of varying inequalities of surface and wide diversities of geological structure -in the districts which they traverse, they run over hill and dale almost -with the straightness of lines of Roman road. In the districts where the -gregarious type prevails, the dykes depart most widely from the character of -the great solitary series, but still tend to run in straight or approximately -straight lines, or, if wavy in their course, to preserve a general parallelism -of direction.</p> - -<p>Yet even among the great persistent dykes instances may be cited where -the rectilinear trend is exchanged for a succession of zig-zags, though the -normal direction is on the whole maintained. In such cases, it is evident -that the fissures were not long straight dislocations, like the larger lines of -fault in the earth's crust, but were rather notched rents or cracks which, -though keeping, on the whole, one dominant direction, were continually being -deflected for short distances to either side. As a good illustration of this -character, reference may be made to the Cheviot and Hawick dyke. In -Teviotdale, this dyke can be followed continuously among the rocky knolls, -so that its deviations can be seen and mapped. From the median line of -average trend the salient angles sometimes retire fully a quarter of a mile -on either side. Some examples of the same feature may be noticed in the -Eskdale dyke. The large dyke which runs westward from Dunoon has been -observed by Mr. Clough to change sharply in direction three times in four -<span class="pagenum" id="Page_146">- 146 -</span> -miles, running occasionally for a short distance at a right angle to its general -direction (see <a href="#v2fig257">Fig. 257</a>).</p> - -<p>Among these solitary dykes also, though the persistence of their trend -is so predominant, there occur instances where the general direction undergoes -great change. Some of the most remarkable cases of this kind -have been mapped by Mr. B. N. Peach and Mr. R. L. Jack, in the course -of the Geological Survey of Perthshire. Several important dykes strike -across the Old Red Sandstone plain for many miles in a direction slightly -south of west. But when they approach the rocks of the Highland border -in Glen Artney, they bend round to south-west, and continue their course -along that new line.</p> - -<p>Many years ago I called attention to the dominant trend of the dykes -from north-west to south-east.<a id="FNanchor_185" href="#Footnote_185" class="fnanchor">[185]</a> Subsequent research has shown this to be on -the whole the prevalent direction throughout the whole region of dykes. But -the detailed mapping, carried on by my colleagues and myself in the Geological -Survey, has brought to light some curious and interesting variations from -the normal trend. In the districts where dykes of the gregarious type -abound there is sometimes no one prevalent direction, but the dykes strike -to almost all points of the compass. Of the Arran dykes, so carefully -catalogued by Necker, only about a third have a general north-westerly -course. But in Eastern Argyleshire the abundant dykes mapped by Mr. -Clough trend almost without exception towards N.N.W. In the North of -Ireland, Berger found the direction of thirty-one dykes to vary from 17° to -71° W. of N., giving a mean of N. 36° W.<a id="FNanchor_186" href="#Footnote_186" class="fnanchor">[186]</a> In Islay, Jura, Eigg, Mull, -and Skye the mean of several hundred observations has given me similar -results. Among the Inner Hebrides, however, though the general north-westerly -trend is characteristic, many of the later dykes show marked -departures from it. Thus in Strath, Skye, some of the youngest follow a -nearly north and south direction (<a href="#v2fig253">Fig. 253</a>). In the Blath Bhein hill-range, -Mr. Harker has found that the latest dykes cut the gabbro at right -angles to the prevalent trend and are further distinguished by their low -hade.</p> - -<div class="footnote"> - -<p><a id="Footnote_185" href="#FNanchor_185" class="label">[185]</a> <i>Trans. Roy. Soc. Edin.</i> xxii. (1861), p. 650.</p> - -<p><a id="Footnote_186" href="#FNanchor_186" class="label">[186]</a> <i>Trans. Geol. Soc.</i> iii. p. 225.</p> - -</div> - -<p>It appears, therefore, that though there is sometimes extraordinary local -diversity in the direction of the dykes in those districts where they present -the gregarious type, the general north-westerly trend can usually still be -recognized. But when we turn to the long massive solitary dykes, we soon -perceive a remarkable change in their direction as we follow them northward -into Scotland. I formerly pointed out how the general north-westerly -trend becomes east and west in the Lothians, with a tendency to veer a little -to the south of west and north of east.<a id="FNanchor_187" href="#Footnote_187" class="fnanchor">[187]</a> This departure from the normal -direction is now seen to be part of a remarkable radial arrangement of the -dykes. Beginning at the southern margin of the dyke-region, we have the -notable example of the Cleveland dyke, which in its course from Cleveland -to Carlisle runs nearly W. 15° N. The Eskdale dyke has an average trend -<span class="pagenum" id="Page_147">- 147 -</span> -of W. 32° N., and the same general direction is maintained by the group of -dykes which run from the Southern Uplands across the south-west of -Lanarkshire and north-east of Ayrshire. But proceeding northwards we -observe the trend to turn gradually round towards the west. The dyke -that runs from near the mouth of the Coquet across the Cheviot Hills to -beyond Hawick has a general course of W. 8° N. In the great central -coal-field of Scotland the average direction may be taken to be nearly east -and west, the same dyke running sometimes to the north, and sometimes to -the south of that line. But immediately to the north a decided tendency -to veer round southwards makes its appearance. Thus the long dyke -which runs from the Carse of Stirling through the Campsie Fells to the -Clyde west of Leven, has a mean direction of W. 5° S. This continues to -be the prevalent trend of the remarkable series of dykes which crosses the -Old Red Sandstone plains, though some of these revert in whole or in part -to the more usual direction by keeping a little to the north of west. Even -as far as Loch Tay and the head of Strathardle, the course of the dykes -continues to be to the south of west. Tracing these lines upon a map of -the country we perceive that they radiate from an area lying along the -eastern part of Argyleshire and the head of the Firth of Clyde (see Map I.).</p> - -<div class="footnote"> - -<p><a id="Footnote_187" href="#FNanchor_187" class="label">[187]</a> <i>Trans. Roy. Soc. Edin.</i> xxii. p. 651.</p> - -</div> - -<div class="figcenter" id="v2fig241" style="width: 409px;"> - <img src="images/v2fig241.png" width="409" height="92" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 241.</span>—Section along the line of the Cleveland Dyke at Cliff Ridge, Guisbrough (G. Barrow).<br /><br /> -Scale, 12 inches to 1 mile.</div> -</div> - - -<h3>10. TERMINATION UPWARDS</h3> - -<p>It was pointed out many years ago by Winch that some of the dykes -which traverse the Northumberland coal-field do not cut the overlying -Magnesian Limestone. The Hett dyke, south of Durham, is said to end off -abruptly against the floor of the limestone.<a id="FNanchor_188" href="#Footnote_188" class="fnanchor">[188]</a> Here and there, among the -precipices of the Inner Hebrides, a dyke may be seen to die out before it -reaches the top of the cliff. But in the vast majority of cases, no evidence -remains as to how the dykes terminated upwards. I have referred to the -occasional interruptions of the continuity of a dyke, where, though the rock -does not reach the surface, it must be present in the fissure underneath. -Such interruptions show that, in some places at least, there was no rise of -the rock even up to the level of what is now the surface of the ground, and -that the upward limit of the dykes must have been exceedingly irregular.</p> - -<div class="footnote"> - -<p><a id="Footnote_188" href="#FNanchor_188" class="label">[188]</a> This is expressed in the Geological Survey Map, Sheet 93, N.E.</p> - -</div> - -<p>Excellent illustrations of this feature are supplied by sections on the -line of the Cleveland dyke. Towards its south-easterly extremity, this great -band of igneous rock ascends from the low Triassic plain of the Tees into -<span class="pagenum" id="Page_148">- 148 -</span> -the high uplands of Cleveland. Its course across -the ridges and valleys there has been carefully -traced for the Geological Survey by Mr. G. Barrow, -who has shown that over certain parts of its course -it does not reach the surface, but remains concealed -under the Jurassic rocks, which it never succeeded -in penetrating. But that in places it comes -within a few feet of the soil is shown by the -baked shale at the surface, for the alteration -which it has induced on the surrounding rocks -only extends a few feet from its margin. These -interruptions of continuity show how uneven is -the upper limit of the dyke. The characteristic -porphyritic rock may be observed running up -one side of a hill to the crest, but never reaching -the surface on the other side. At Cliff Ridge, -for example, about three miles south-west of -Guisbrough, Mr. Barrow has followed it up to the -summit on the west side; but has found that on -the east side it does not pierce the shales, which -there form the declivity. This structure is -represented in Fig 241. The vertical distance -between the summit to the left, where the dyke -(<i>b</i>) disappears, and the point to the right, where -the Lias shale (<i>a</i>) of the hillside is concealed by -drift (<i>c</i>), amounts to 250 feet, the horizontal -distance being a little more than 900 feet. But -as the shale when last seen at the foot of the -slope is quite unaltered, the dyke must there be -still some little distance beneath the surface, so -that the vertical extension of this upward -tongue of the dyke must be more than 250 feet. -Mr. Barrow, to whom I am indebted for these -particulars, has also drawn the accompanying -section (<a href="#v2fig242">Fig. 242</a>) along the course of the dyke -for a distance of nearly 11 miles eastward from -the locality represented in <a href="#v2fig241">Fig. 241</a>. From this -section it will be observed that in that space -there are at least three tongues or upward projections -of the upper limit of the dyke. Several -additional examples of the same structure are -to be seen further east towards the last visible -outcrop of the dyke.</p> - -<div class="figcenter" id="v2fig242" style="width: 811px;"> - <img src="images/v2fig242.png" width="811" height="135" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 242.</span>—Section along the course of the Cleveland Dyke, at the head of Lonsdale, Yorkshire (G. Barrow, in the <i>Memoirs of the Geol. Surrey</i>, Geology of Cleveland, p. 61).<br /><br /> - <i>a</i>, Liassic shales, sandstones and ironstones; <i>b</i>, the dyke.</div> -</div> - -<div class="figright" id="v2fig243" style="width: 280px;"> - <img src="images/v2fig243.png" width="280" height="257" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 243.</span>—Section across the extreme upper limit of Cleveland - Dyke, on the scale of 20 feet to one inch (Mr. G. Barrow).<br /><br /> - <i>a</i> <i>a</i>, Jurassic shales, etc.; <i>b</i>, Dyke.</div> -</div> - -<p>Another feature connected with the upward -termination of the dyke is well seen in some parts -of the ground through which the two foregoing -<span class="pagenum" id="Page_149">- 149 -</span> -sections are taken. Mr. Barrow informs me that at Ayton a level course -has been driven into the hill for mining operations, at a height of 400 feet -above sea-level, and the dyke has there been ascertained to be 80 feet broad. -Higher on the hill, close to the -750 feet contour—line, its -breadth is only 20 feet, so that -it narrows upward as much as -60 feet in a vertical height -of 350 feet. Its contraction -in width during the last -twenty feet is still more rapid, -and in the last few yards it -diminishes to two or three -feet, and has a rounded top -over which the strata are bent -upward. The accompanying -section (<a href="#v2fig243">Fig. 243</a>) across the -upper part of the dyke will -make these features clear.</p> - -<div class="figright" id="v2fig244" style="width: 212px;"> - <img src="images/v2fig244.png" width="212" height="120" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 244.</span>—Upper limit of Cleveland Dyke in - quarry near Cockfield (after Mr. Teall).<br /><br /> - <i>a</i> <i>a</i>, Carboniferous shales; <i>b</i>, dyke.</div> -</div> - -<p>Further to the west an -exposure of the upper limit -of the dyke has been described -and figured by Mr. Teall. In 1882, at one of the Cockfield quarries -(<a href="#v2fig244">Fig. 244</a>), the dyke was "seen to terminate upwards very abruptly in -the form of a low and somewhat irregular dome, over which the Coal-measure -shales passed without any fracture, and only with a slight upward -arching."<a id="FNanchor_189" href="#Footnote_189" class="fnanchor">[189]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_189" href="#FNanchor_189" class="label">[189]</a> <i>Quart. Jour. Geol. Soc.</i> xl. p. 210.</p> - -</div> - -<p>Near the other or north-western termination of this great dyke, similar -evidence is found of an uneven upper limit. After an interrupted course -through the Alston moors, the dyke reaches the ground that slopes eastward -from the edge of the Cross Fell escarpment. Its highest visible outcrop is -at a height of 1700 feet. But westwards from that point the dyke -disappears under the Carboniferous rocks, -and does not emerge along the front of -the great escarpment that descends upon -the valley of the Eden, where among -the naked scarps of rock it would unquestionably -be visible if it reached the -surface. Its upper edge must rapidly -descend somewhere behind the face of -the escarpment, for the igneous rock -crops out a little to the west of the -foot of the cliff, about 1000 feet -below the point where it is last seen on the hills above. Here the top -of the dyke has a vertical drop of not less than 1000 feet, in a horizontal -<span class="pagenum" id="Page_150">- 150 -</span> -distance of five miles, as shown in <a href="#v2fig245">Fig. 245</a>, which has been -drawn for me by Mr. J. G. Goodchild.</p> - -<p>It will be observed that in these sections (Figs. <a href="#v2fig241">241</a>, -<a href="#v2fig242">242</a> and <a href="#v2fig245">245</a>) there is a curiously approximate coincidence -between the inequalities in the upper surface of the dyke -and those in the form of the overlying ground. The -coincidence is too marked and too often repeated to be -merely accidental. Whether the ancient topographical -features had any influence in determining, by cooling or -otherwise, the limit of the upward rise of the lava, or -whether the dyke, even though concealed, has affected the -progress of the denudation of the ground overlying it, is a -question worthy of fuller investigation.</p> - -<div class="figcenter" id="v2fig245" style="width: 793px;"> - <img src="images/v2fig245.png" width="793" height="63" alt="" /> - <div class="figcaption"><span class="smcap">Fig 245.</span>—Section along the course of the Cleveland Dyke across the Cross Fell escarpment. The shaded part shows the position of the dyke, the unshaded part - overlying it marks where the dyke does not reach the surface. Scale of one inch to one mile.</div> -</div> - - -<h3>11. KNOWN VERTICAL EXTENSION</h3> - -<p>Closely connected with the determination of the -upper limit reached by the dykes, is the total vertical -distance to which they can be traced. Of course, the depth -of the original reservoir of molten rock which supplied -them remains unknown, and probably undiscoverable. -But it is possible, in many cases, to determine at least the -inferior limit of the thickness of rock through which the -molten material of the dykes has ascended. Along the -great basalt-escarpments of Mull and Skye, the ascent of -dykes from base to summit may often be observed. Thus, -on the cliffs of Dunvegan Head, on the west coast of Skye, -which rise out of the sea to a height of about 1000 feet, -several dykes may be observed rising through the whole -series of basalts up to the crest of the precipice. In the -dark gabbro hills of the same island, numerous dykes -may be seen climbing from the glens right up the steep -rugged acclivities and over the crests, through a vertical -thickness of more than 3000 feet of rock (<a href="#v2fig333">Fig. 333</a>). -The dykes which cross Loch Lomond, and ascend the hills -on either side of that deep depression, must rise through -at least as great a thickness. But where a knowledge of -the geological structure of the ground enables us to -estimate the bulk of the successive rock-formations which -underlie the surface, it can be shown that the lava ascended -through a much greater depth of rock. Measurements of -this kind can best be made towards the eastern end of the -Cleveland dyke, where the different sedimentary groups -have not been seriously disturbed, and where, from natural -sections and artificial borings, their thicknesses are capable -of satisfactory computation. The highest bed of the -<span class="pagenum" id="Page_151">- 151 -</span> -Jurassic series anywhere touched by the dyke is the Cornbrash. It is -certain, therefore, that the igneous rock rises through all the subjacent members -of the Jurassic series up to that horizon. There can be no doubt also -that the Trias and Magnesian Limestone continue in their normal thickness -underneath the Jurassic strata. To what extent the Coal-measures exist -under Cleveland has not been ascertained; possibly they have been entirely -denuded from that area, as from the ground to the west. But the Millstone -Grit and Carboniferous Limestone probably extend over the district in full -development; and below them there must lie a vast depth of Upper and -Lower Silurian strata, probably also of still older Palæozoic rocks and -beneath all the thick Archæan platform. Tabulating these successive -geological formations, and taking only the ascertained thickness of each in the -district, we find that they give the results shown in the subjoined table.<a id="FNanchor_190" href="#Footnote_190" class="fnanchor">[190]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_190" href="#FNanchor_190" class="label">[190]</a> Drawn up for me by Mr. G. Barrow.</p> - -</div> - - -<p class="caption4">STRATA CUT BY THE CLEVELAND DYKE</p> - -<table summary="data"> -<tr> - <td class="tdl"> Cornbrash—<br /></td> - <td class="vbot tdc">Feet.</td> -</tr> -<tr> - <td class="tdl">Lower Oolite and Upper Lias, as proved by bore-hole<br /> - on Gerrick Moor,</td> - <td class="vbot tdr">950</td> -</tr> -<tr> - <td class="tdl">Middle and Lower Lias, ascertained from measurement of<br /> - cliff-sections and from mining operations to be more than</td> - <td class="vbot tdr">850</td> -</tr> -<tr> - <td class="tdl">New Red Sandstone and Marl, found by boring close<br /> - to the Tees to exceed</td> - <td class="vbot tdr">1,600</td> -</tr> -<tr> - <td class="tdl">Magnesian Limestone, at least</td> - <td class="tdr">500</td> -</tr> -<tr> - <td class="tdl">Coal-measures, possibly absent</td> - <td class="tdr">0</td> -</tr> -<tr> - <td class="tdl">Millstone Grit, not less than</td> - <td class="tdr">500</td> -</tr> -<tr> - <td class="tdl">Carboniferous Limestone series at least</td> - <td class="tdr">3,000</td> -</tr> -<tr> - <td class="tdl">Silurian rocks, probably not less than</td> - <td class="tdr">10,000</td> -</tr> -<tr> - <td></td> - <td class="bdt">17,400</td> -</tr> -</table> - -<p>There is thus evidence that this dyke has risen through probably more -than three miles of stratified rocks. How much deeper still lay the original -reservoir of molten material that supplied the dyke, we have at present no -means of computing.</p> - - -<h3>12. EVIDENCE AS TO MOVEMENT OF THE MOLTEN ROCK IN THE FISSURES</h3> - -<p>It is usual to speak of the molten material of the dykes as having -risen vertically within the fissures. And doubtless, on the whole, the -expression is sufficiently accurate. In the case of such long dykes as those of -Central Scotland and the North of England, where the petrographical -character of the material remains so uniform throughout, it is obvious that -the andesite or dolerite cannot have come from a mere single pipe like -a volcanic orifice. Nor can we easily understand how it could have been -supplied even from a series of such pipes. The general aspect and structure -of the dykes suggest that the fissures were rent so profoundly in the crust -of the earth as to reach down to a reservoir of molten rock which straightway -rose in them. The roof of such a reservoir, however, may have been -irregular and uneven, so that a fissure need not have traversed it continuously, -<span class="pagenum" id="Page_152">- 152 -</span> -but may have only touched its upward projecting vaults. Hence -gaps would arise in the continuity of the dyke-material.</p> - -<p>The ascent of lava from a line of such separate openings along a fissure -would necessarily involve lateral as well as vertical movements in the -molten mass which would be forced along the open rent until the several -streams united and filled it up. We might therefore expect somewhere to -find instances of flow-structure in the dykes pointing to these movements. -I have already referred to the lines of amygdales frequently noticed in -dykes, especially towards the centre. Occasionally these steam-vesicles may -be observed to be drawn out in one general direction indicative of the -trend of motion of the molten rock.</p> - -<p>Some of the best examples of this feature which have come under my -observation occur among the trachytic dykes of the south-east coast of Skye -between Kyle Rhea and Loch na Daal, where they have been mapped and -carefully investigated by Mr. Clough, who has conducted me over the sections. -In some of these dykes, as already narrated, the marginal portions -display a finely spherulitic structure, the small pea-like spherulites being -grouped into fine ribs or rods. It is also observable that the steam-vesicles -which may retain their spherical forms in the centre are elongated in the -same direction as the rows of spherulites. Where this lineation is -developed vertically, it no doubt points to the vertical ascent of the lava -between the two walls of the fissure.</p> - -<p>But in other examples, the elongation is nearly horizontal, and between -the two positions Mr. Clough has registered many intermediate trends. It -would thus appear that in some places the lava has certainly flowed laterally -between the fissure-walls. Moreover, the trend of the spherulitic rods and -of the amygdales is found to vary in closely adjoining planes at different -distances from the margin, as if after the outer portions of the dyke had -consolidated into position, there was still movement enough to drag the rows -of spherulites and vesicles up or down along the trend of the fissure.</p> - -<p>Mr. Clough has observed that in some dykes, while the amygdaloidal -vesicles are large and undeformed in the centre, they become elongated and -inclined downward in the direction of the margin, as if the central portions -had not only remained fluid longer than the rest, but had a tendency to rise -upwards in the fissure, though there was obviously less motion after these -central vesicles appeared than in the marginal parts where the vesicles are -so much drawn out.</p> - - -<h3>13. BRANCHING DYKES AND VEINS</h3> - -<div class="figleft" id="v2fig246" style="width: 274px;"> - <img src="images/v2fig246.png" width="274" height="173" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 246.</span>—Branching portion of the great Dyke near - Hawick (length about one mile).</div> -</div> - -<p>It might have been anticipated that the uprise of such abundant masses -of molten rock, in so many long and wide fissures, would generally be attended -with the intrusion of the same material into lateral rents and irregular -openings, so that each dyke would have a kind of fringe of offshoots or -processes striking from it into the surrounding ground. It might have -been expected also that dykes would often branch, and that the arms would -<span class="pagenum" id="Page_153">- 153 -</span> -come together again and enclose portions of the rocks through which they -rise. But in reality such excrescences and bifurcations are of comparatively -rare occurrence. As a rule, each dyke is a mere wall of igneous rock, with -little more projection or ramification than may be seen in a stone field-fence. -Among the short, narrow and -irregular dykes of the gregarious -type branchings are -occasionally seen, and in some -districts are extraordinarily -abundant. But among the -great single dykes such irregularities -are far less common -than might have been looked -for. A few characteristic -examples from each type of -dyke may here be given.</p> - -<div class="figcenter" id="v2fig247" style="width: 362px;"> - <img src="images/v2fig247.png" width="362" height="106" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 247.</span>—Branching Dyke at foot of Glen Artney (length about four miles).</div> -</div> - -<p>The Cleveland dyke, which -in so many respects is typical of the great solitary dykes of the -country, has been traced for many miles without the appearance -of a single offshoot of any kind. Yet here and there along its course, -it departs from its usual regularity. As it crosses the Carboniferous -tracts of Durham and Cumberland, there appear near its course lateral -masses of eruptive rock, most of which doubtless belong to the much older -"Whin Sill." But there is at least one locality, at Bolam near Cockfield, in -the county of Durham, where the dyke, crossing the Millstone Grit, suddenly -expands into a boss, and immediately contracts to its usual dimensions. -Around this knot several short dykes or veins seem to radiate from it. The -dyke has been quarried here, and its relations to the surrounding strata -have been laid bare, as will be again referred to a little further on.<a id="FNanchor_191" href="#Footnote_191" class="fnanchor">[191]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_191" href="#FNanchor_191" class="label">[191]</a> This locality was well described by Sedgwick, in his early paper on Trap-Dykes in Yorkshire -and Durham, <i>Trans. Cambridge Phil. Soc.</i> ii. p. 27.</p> - -</div> - -<p>Among the great persistent dykes of Scotland the absence of bifurcation -and lateral offshoots offers a striking contrast to the behaviour of the dykes in -those districts where they are small in size and many in number. But -exceptions to the general rule may be gathered. Thus the Eskdale dyke is -flanked at Wat Carrick with a large lateral vein, which is almost certainly -connected with the main fissure. The Hawick and Cheviot dyke splits up -on the hill immediately to the east of the town of Hawick, sends off some -<span class="pagenum" id="Page_154">- 154 -</span> -branches, and then resumes its normal course (<a href="#v2fig246">Fig. 246</a>). Again, one of the -two nearly parallel dykes which run from Lochgoilhead across Ben Ledi -into Glen Artney bifurcates at the foot of that valley, its northern limb -(about two miles long) speedily dying out, and its southern branch throwing -off another lateral vein, and then continuing eastward as the main dyke -(<a href="#v2fig247">Fig. 247</a>).</p> - -<p>In the districts of gregarious dykes, however, abundant instances may -be found of dykes that branch, and of others that lose the parallelism of -their walls, become irregular in breadth, direction, and inclination, so as to -pass into those intrusive forms that are more properly classed as veins. -Excellent illustrations of bifurcating dykes may be observed along the shores -of the Firth of Clyde, particularly on the eastern coast-line of the isle of -Arran. The venous character has become familiar to geologists from the -sketches given by Macculloch from the lower parts of the cliffs of Trotternish -in Skye.<a id="FNanchor_192" href="#Footnote_192" class="fnanchor">[192]</a> Still more striking examples are to be seen in the breaker-beaten -cliffs of Ardnamurchan. The pale Secondary limestones and calcareous -sandstones of that locality are traversed by a series of dark basic veins, and -the contrast of tint between the two kinds of rock is so marked as even to -catch the eye of casual tourists in the passing steamboats. The veins vary -in width from less than an inch to several feet or yards. They run in all -directions and intersect each other, forming such a confused medley as -requires some patience on the part of the geologist who would follow out -each independent ribbon of injected material in its course up the cliffs, or -still more, would sketch their ramifications in his note-book. A good, -though perhaps somewhat exaggerated, illustration of their general character -was given by Macculloch.<a id="FNanchor_193" href="#Footnote_193" class="fnanchor">[193]</a> The accompanying figure (<a href="#v2fig248">Fig. 248</a>) is less -sensational, but represents with as much accuracy as I could reach, the network -of veins near the foot of the cliffs. One conspicuous group of veins, -which, seen from a distance, looks like a rude sketch of a lug-sail traced in -black outline upon a pale ground, is known to the boatmen as "M'Niven's -Sail." Another admirable locality for the study of dykes and tortuous veins -is the northern coast of the Sound of Soa, where an extraordinary number -of injections traverse the Torridon Sandstones on which the plateau-basalts -rest (<a href="#v2fig323">Fig. 323</a>).</p> - -<div class="footnote"> - -<p><a id="Footnote_192" href="#FNanchor_192" class="label">[192]</a> <i>Western Islands</i>, plate xvii.</p> - -<p><a id="Footnote_193" href="#FNanchor_193" class="label">[193]</a> <i>Op. cit.</i> plate xxxiii. Fig. 1.</p> - -</div> - -<p>As a general rule, the narrower the vein the finer in grain is the rock -of which it consists. This compact dark homogeneous material has -commonly passed by the name of "basalt." Its minuteness of texture probably -in most cases arises from local rapidity of cooling, and it is doubtless -the same substance which, where in larger mass in the immediate neighbourhood, -has solidified as one of the other pyroxene-plagioclase-magnetite -rocks.</p> - -<p>With regard to the places where such abundant tortuous veins are more -especially developed, I may remark that they are particularly prominent -under a thick overlying mass of erupted rock, such as a great intrusive -sheet, or the bedded basalts of the plateaux, or where there is good reason -<span class="pagenum" id="Page_155">- 155 -</span> -to believe that such a deep cover, though now removed by denudation, once -overspread the area in which they appear. It will be shown in the sequel -that such horizons have been peculiarly liable to intrusions of igneous -material of various kinds, and at many different intervals, during the -volcanic period. A thick cake of crystalline rock seems to have offered such -resistance to the uprise of molten material through it, that when the subterranean -energy was not sufficient to rend it open by great fissures, and -thus give rise to dykes, the lavas were either forced into such irregular -cracks as were made partly in the softer rocks underneath and partly in the -cake itself, or found escape along pre-existing divisional planes. In Ardnamurchan, -round the Cuillin Hills of Skye, and in Rum, the overlying -resisting cover now consists mainly of gabbro sheets. In the east of Skye, -in Eigg, and in Antrim, it is made up of the thick mass of the plateau-basalts.</p> - -<div class="figcenter" id="v2fig248" style="width: 509px;"> - <img src="images/v2fig248.png" width="509" height="307" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 248.</span>—Basic veins traversing secondary limestone and sandstone on the coast cliffs, - Ardnamurchan.</div> -</div> - - -<h3>14. CONNECTION OF DYKES WITH SILLS</h3> - -<p>Every field-geologist is aware how seldom he can actually find the vent -or pipe up which rose the igneous rock that supplied the material of sills -and laccolites. He might well be pardoned were he to anticipate that, in -a district much traversed by dykes, there should be many examples of -intrusive sheets and frequent opportunities of tracing the connection of such -sheets with the fissures from which their material might be supposed to -have been supplied. But such an expectation is singularly disappointed by -an actual examination of the Tertiary volcanic region of Britain. That -there are many intrusive sheets belonging to the great volcanic period with -<span class="pagenum" id="Page_156">- 156 -</span> -which I am now dealing, I shall endeavour to show in the sequel. But it -is quite certain that though these sheets have of course each had its subterranean -pipe or fissure of supply, they can only in rare instances be -directly traced to the system of dykes. On the other hand, the districts -where great single dykes are most conspicuous, are for the most part free -from intrusive sheets, except those of much older date, like the Carboniferous -Whin Sill of Durham and those of Linlithgowshire, Stirlingshire and Fife.</p> - -<p>Yet a few interesting examples of the relation of dykes to sheets have -been noticed among British Tertiary volcanic rocks. The earliest observed -instances were those figured and described by Macculloch. Among them -one has been familiar to geologists from having done duty in text-books of -the science for more than half a century. I allude to the diagram of "Trap -and Sandstone near Suishnish."<a id="FNanchor_194" href="#Footnote_194" class="fnanchor">[194]</a> In that drawing seven dykes are shown -as rising vertically through the horizontal sandstone, and merging into a -thick overlying mass of "trap." The author in his explanation leaves it an -open question "whether the intruding material has ascended from below -and overflowed the strata, or has descended from the mass," though from the -language he uses in his text we may infer that he was inclined to regard -the overlying body as the source of the veins below it.<a id="FNanchor_195" href="#Footnote_195" class="fnanchor">[195]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_194" href="#FNanchor_194" class="label">[194]</a> <i>Western Islands of Scotland</i>, pl. xiv. Fig. 4.</p> - -<p><a id="Footnote_195" href="#FNanchor_195" class="label">[195]</a> <i>Op. cit.</i> vol. i. pp. <a href="../../66492/66492-h/66492-h.htm#Page_384">334</a>, <a href="../../66492/66492-h/66492-h.htm#Page_385">385</a>.</p> - -</div> - -<div class="figcenter" id="v2fig249" style="width: 551px;"> - <img src="images/v2fig249.png" width="551" height="126" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 249.</span>—Section showing the connection of a Dyke with an Intrusive Sheet, Point of Suisnish, Skye.<br /><br /> - <i>g</i>, Granophyre of Carn Dearg; <i>f</i>, similar rock, which appears eastward under the "sill" (<i>d</i>); <i>e</i>, intrusive sheet of - fine-grained "basalt"; <i>d</i>, intrusive sheet or sill of coarse dolerite, 200 feet thick at its maximum, and rapidly - thinning out; <i>c</i>, dyke or pipe of finer grain than <i>d</i>; <i>b</i>, yellowish-brown shaly sandstones, and <i>a</i>, dark sandy - shales (Lias).</div> -</div> - - -<p>The section given by Macculloch, however, does not quite accurately -represent the facts. The narrow dykes there drawn have no connection -with the overlying sheet, but are part of the abundant series of basaltic -dykes found all over Skye. The feeder of the gabbro sill was presumably -the broad dyke which descends the steep bank immediately on the southern -front of Carn Dearg (636 feet high). The accompanying figure (Fig. -249) shows what seemed to me to be the structure of the locality, but the -actual junction of the dyke and sheet is concealed under the talus of the -slope.<a id="FNanchor_196" href="#Footnote_196" class="fnanchor">[196]</a> I shall have occasion in a later Chapter to refer again to this -<span class="pagenum" id="Page_157">- 157 -</span> -section in connection with the history of intrusive sheets, and also to cite -from the neighbouring island of Raasay another good example of the same -relation between dyke and sill.</p> - -<div class="footnote"> - -<p><a id="Footnote_196" href="#FNanchor_196" class="label">[196]</a> In more recently surveying this ground, Mr. Harker has been led to regard the coarse sill as -independent of the other intrusions, and as almost certainly later than the basalt-sheets of the -same locality. When it reaches the base of these sills it turns so as to pass beneath them as a -gabbro-sill, which is conspicuous near the summit of Carn Dearg. It runs westward for some -distance, almost immediately breaking across the bedding so as to leave the basalt, and rapidly -tapering until it dies out.</p> - -</div> - -<p>Sedgwick, in the paper above quoted, gave an account and figure of -the expansion of the Cleveland dyke at Bolam, to which allusion has -already been made. He showed that from a part of the dyke which is -unusually contracted a great lateral extension of the igneous rock takes place -on either side over beds of shale and coal. While in the dyke the prisms -are as usual directed horizontally inward from the two walls, those in the -connected sheet are vertical, and descend upon the surface of highly -indurated strata on which the sheet rests.</p> - -<p>The most important examples known to me are those which occur in -the coal-field of Stirlingshire. In that part of the country, the remarkable -group of dykes already referred to, lying nearly parallel to each other and -from half a mile to about three miles apart, runs in a general east and -west direction. From one of these dykes no fewer than four sills strike off -into the surrounding Coal-measures. The largest of them stretches southwards -for three miles, but the same rock is probably continued in a -succession of detached areas which spread westwards through the coal-field -and circle round to near the two western sheets that proceeded from the -same dyke. Another thick mass of similar rock extends on the north side -of the dyke for two and a half miles down the valley of the river Avon. -These various processes, attached to or diverging from the dyke, are -unquestionably intrusive sheets, which occupy different horizons in the -Carboniferous series. The one on the north side has inserted itself a little -above the top of the Carboniferous Limestone series. Those on the south -side lie on different levels in the Coal-measures, or, rather, they pass transgressively -from one platform to another in that group of strata.</p> - -<div class="figcenter" id="v2fig250" style="width: 453px;"> - <img src="images/v2fig250.png" width="453" height="102" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 250.</span>—Section to show the connection of a Dyke with an Intrusive Sheet, - Stirlingshire Coal-field.<br /><br /> - <i>a</i>, Dyke in line of fault; <i>b</i>, Sill traversing and altering the coal; <i>i</i>, Slaty-band Ironstone.</div> -</div> - -<p>No essential difference can be detected by the naked eye between the -material of the dyke and that of the sheets. If a series of specimens from -the different exposures were mixed up, it would be impossible to separate -those of the dyke from those of the sheets. A microscopical examination -of the specimens likewise shows that they are perfectly identical in composition -and structure, being chiefly referable to rocks of the dolerite, but -<span class="pagenum" id="Page_158">- 158 -</span> -partly of the tholeiite type. I have therefore little doubt that these remarkable -appendages to this dyke are truly offshoots from it, and are not -to be classed with the general mass of the sills of Central Scotland, which -are of Carboniferous, partly of Permian, age. The accompanying diagrammatic -section (<a href="#v2fig250">Fig. 250</a>) explains the geological structure of the ground.</p> - -<p>An interesting and important fact remains to be stated in connection -with these sheets. They are traversed by some of the other east and west -dykes. This is particularly observable in the case of the sheet which -extends northwards from the dyke through the parish of Torphichen. Two -well-marked dykes can be seen running westwards among the ridges of the -sheet. It is obvious, therefore that these particular dykes are younger than -the sheet. But, as will be shown in the sequel, there is abundant evidence that -all the dykes of a district are not of one eruption. The intersection of one -eruptive mass by another does not necessarily imply any long interval of time -between them. They mark successive, but it may be rapidly successive, -manifestations of volcanic action. Hence the cutting of the sheets by other -dykes does not invalidate the identification of these sheets as extravasations -from the great dyke by which they are bounded.</p> - - -<h3>15. INTERSECTION OF DYKES</h3> - -<div class="figleft" id="v2fig251" style="width: 217px;"> - <img src="images/v2fig251.png" width="217" height="316" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 251.</span>—Intersection of Dykes in bedded - basalt, Calliach Point, Mull.</div> -</div> - -<p>Innumerable instances may be cited, where one dyke, or one set of dykes, -cuts across another. To some of these I shall refer in discussing the data -for estimating the relative ages of -dykes. In considering the intersection -from the point of view of geological -structure, we are struck with the clean -sharp way in which it so generally -takes place. The rents into which the -younger dykes have been injected seem, -as a rule, not to have been sensibly -influenced in width and direction by -the older dykes, but go right across -them. Hence the younger dykes retain -their usual breadth and trend (Fig. -251). In trying to ascertain the -relative ages of such dykes we obtain a -valuable clue in studying the respective -"chilled edges" of the two intersecting -masses, as has already been pointed -out.</p> - -<p>Not only do dykes cross each -other, but still more is this the case -among the narrower tortuous intrusions -known as Veins (<a href="#v2fig252">Fig. 252</a>). Among the illustrations which the dykes -of the Inner Hebrides supply of these features one further characteristic -<span class="pagenum" id="Page_159">- 159 -</span> -example may be culled from the shore of Skye, near Broadford, where -the gently-inclined sheets of Lias limestone are traversed by three systems -of dykes (<a href="#v2fig253">Fig. 253</a>). One of these systems runs in a N.W. or N.N.W. -direction, a second follows a more nearly easterly trend, while the third -and youngest runs nearly north and south.</p> - -<table summary="rocks"> -<tr> - <td> -<div class="figcenter" id="v2fig252" style="width: 151px;"> - <img src="images/v2fig252.png" width="151" height="208" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 252.</span>—Basalt Veins traversing -bedded dolerites, Kildonan, Eigg.</div> -</div> - </td><td> </td><td> -<div class="figcenter" id="v2fig253" style="width: 239px;"> - <img src="images/v2fig253.png" width="239" height="215" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 253.</span>—Ground-plan of intersecting Dykes in -Lias limestone, Shore, Harrabol, East of -Broadford, Skye.</div> -</div> - </td> -</tr> -</table> - - -<h3>16. DYKES OF MORE THAN ONE IN-FILLING</h3> - -<p>The intersections of dykes prove that the process of fissuring in the -earth's crust took place at more than one period, and prepare us for the -reception of evidence that the same line of fissure might be again re-opened, -even after it had been filled with molten material. Numerous instances -have now been accumulated in which dykes are not single or simple intrusions, -but where the original dyke-fissure has been re-opened and has been -invaded by successive uprisings of lava.<a id="FNanchor_197" href="#Footnote_197" class="fnanchor">[197]</a> Compound dykes have thus -been formed, consisting of two or more parallel bands of similar or dissimilar -rock.</p> - -<div class="footnote"> - -<p><a id="Footnote_197" href="#FNanchor_197" class="label">[197]</a> See an example figured by Macculloch, <i>Western Isles</i>, plate xviii. Fig. 1.</p> - -</div> - -<p>While it is not difficult to conceive of the re-opening of a vertical -fissure during terrestrial strain, and the injection into it of later intrusions -of a volcanic magma, it is not so easy to understand the mechanism where -the line of weakness has been slightly inclined or horizontal, and where, consequently, -there has been the enormous superincumbent pressure of the -overlying part of the earth's crust to overcome. Yet gently inclined compound -dykes exhibit their parallel bands with hardly less regularity than do -those that are vertical. The difficulty of explanation is felt most strongly -in the attempt to realize the origin of the compound sills described in -Chapter xlviii.</p> - -<p>In the re-opening of dyke-fissures the later intrusions have generally -<span class="pagenum" id="Page_160">- 160 -</span> -taken place along the walls, or where the dykes were already compound, -between some of the component bands. Less frequently the first dyke has -been split open along the middle, and a second injection has forced its way -along the rent.</p> - -<p>Of the first of these two types, numerous instances have now been -observed in the West of Scotland. If the portion of a compound -dyke exposed at the surface be limited in extent, we may be unable to -determine which is the older of two parallel bands of igneous rock, -though the fact that they present to each other the usual fine-grained edge -due to more rapid cooling, shows that they are not one but two dykes, -belonging to distinct eruptions. So far as I have noticed, where one of the -dykes can be continuously traced for a considerable distance, the other is -comparatively short. I infer that the shorter one is the younger of -the two.</p> - -<p>In the Strath district of Skye, Mr. Harker has recently observed that -many of the basic dykes, both those older and those younger than the -granophyre protrusions, are double, triple or multiple. Thus in a conspicuous -dyke, more than 100 feet wide, to the south-east of Loch Kilchrist, -belonging to the older series, he has detected at least six contiguous dykes -which as they are traced south-eastward, in spite of their interruption by -the Beinn an Dubhaich granite, can be seen to separate and take different -courses, or successively die out. He remarks, further, that "many cases of -apparent bifurcation of dykes are really due to the separation of distinct -dykes which have run for some distance in one fissure. Sometimes apparent -variations in the width of a dyke are to be explained by this dying out of -one member of a double dyke. These multiple dykes are less easily detected -in the newer than the older set, owing to greater uniformity of -lithological type in the prevalent kinds and to the frequent absence of -chilled selvages."<a id="FNanchor_198" href="#Footnote_198" class="fnanchor">[198]</a> An example of a compound basic dyke cutting the crest -of the gabbro-mass of the Cuillin Hills is shown in <a href="#v2fig333">Fig. 333</a>.</p> - -<div class="footnote"> - -<p><a id="Footnote_198" href="#FNanchor_198" class="label">[198]</a> MS. notes supplied by Mr. Harker.</p> - -</div> - -<p>Instances of the second type of compound dykes are less common. Here, -instead of being re-opened along one of the walls, the fissure has been ruptured -along the centre of the dyke, and a second injection of molten material has -then taken place. This structure may be observed where the materials of the -compound dyke are on the whole similar, such as varieties of dolerite, basalt, -diabase or andesite. In these cases the rock of the central dyke is generally -rather fine-grained, sometimes decidedly porphyritic, and often a true basalt. -Where broad enough to show the difference of texture between margin and -centre, it exhibits the usual close grain along its edges, indicative of quicker -cooling. The older dyke presenting no such change at its junction with -the younger, was obviously already cooled and consolidated before its rupture.</p> - -<p>Whilst the centre of a dyke has occasionally proved to be a line of -weakness which has given way under intense strains in the terrestrial crust, -this rupture and the accompanying or subsequent ascent of molten material -in the re-opened fissure may sometimes have been included as phases of one -<span class="pagenum" id="Page_161">- 161 -</span> -connected volcanic episode. In those instances, for example, which have -been above described, where a central vitreous band has risen along the -heart of a dyke, the petrographical affinities of the rocks may be so close as to -suggest that although the main dyke had consolidated and had subsequently -been ruptured along its centre by powerful earth-movements, these changes -all belonged to the same period of dyke-making, and the subsequent uprise -of glassy material was merely a later phase in the movements of the same -subterranean magma.</p> - -<p>But where, as probably happens in the large majority of compound -dykes, there is a strongly marked difference between the respective bands of -rock, we must either infer that two essentially different magmas co-existed -in the volcanic reservoirs underneath, and were successively injected into -the same fissures, or that a sufficient lapse of time occurred to permit a -total renewal of the nature of the magma, and an uprise of this changed -material into fissures which sometimes coincided with older dykes. If any -interlocking of the crystals of the several bands of a compound dyke could -be detected, we might suppose that the first-injected material had not become -consolidated and cold before the uprise of the newer rock. But in general -it would seem that so sharp a line of demarcation can be drawn between the -two rocks as to indicate that their protrusion was due to two distinct and -perhaps widely-separated volcanic paroxysms.</p> - -<p>Compound dykes of basic material occur not only among the ordinary -straight north-westerly series, but also among the less regular gregarious dykes -and veins, such as abundantly intersect the gabbro bosses. Moreover they -are to be found among the youngest intrusions, for they traverse the masses -of granophyre. Conspicuous examples of such late compound dykes are -displayed along the cliffs of St. Kilda, as will be more particularly described -in a later Chapter. These St. Kilda dykes often occupy not vertical fissures -but parallel rents with a gentle inclination (see Figs. <a href="#v2fig367">367</a>, <a href="#v2fig368">368</a>).</p> - -<p>The Tertiary volcanic series of Scotland furnishes many examples of -compound dykes of a much more complex character where parallel bands of -some acid (granophyre, felsite, quartz-porphyry) or intermediate (andesite) -rock is associated with others of the more usual basic material (dolerite, -basalt, diabase). As the acid intrusions belong to a comparatively late part -of the volcanic history, their modes of occurrence will be discussed in -Chapters xlvi., xlvii. and xlviii. But no account of the general system of -dykes would be complete without some reference to these compound -examples, which will therefore be briefly described in the present section -of this work.</p> - -<p>Early in this century some striking illustrations of the association of -acid and more basic rocks within the same fissure were noticed by Jameson -in the island of Arran. He described and figured instances at Tormore, on -the west side of that island, where a group of pitchstones and "basalts" or -andesites have been successively protruded into the same fissures in the -(probably Permian) red sandstones of that district.<a id="FNanchor_199" href="#Footnote_199" class="fnanchor">[199]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_199" href="#FNanchor_199" class="label">[199]</a> <i>Mineralogy of the Scottish Isles</i>, 1800.</p> - -</div> - -<p><span class="pagenum" id="Page_162">- 162 -</span></p> - -<p>In some instances the more basic rock has been first injected, and has -subsequently been disrupted, by the more acid pitchstone. In other cases -the order has been the reverse. The successive ruptures have taken place -sometimes along the centre, sometimes at the margins, and sometimes -irregularly along the breadth of the dykes. Professor Judd has recently -studied these rocks, and has given descriptions of their chemical composition -and microscopic characters. He regards them as having been successively -injected into the fissures from the same subterranean reservoir, in which two -magmas of very different chemical constitution were simultaneously present.<a id="FNanchor_200" href="#Footnote_200" class="fnanchor">[200]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_200" href="#FNanchor_200" class="label">[200]</a> <i>Quart. Jour. Geol. Soc.</i> vol. xlix. (1893), p. 536. Full details of the compound dykes of -Tormore and Cir Mhor in Arran, and references to previous writers will be found in this paper. -The probable age of the youngest eruptive rocks of this island will be discussed in <a href="#Page_418">Chapter xlvii. -p. 418</a>.</p> - -</div> - -<p>Nowhere in the Tertiary volcanic regions of Britain do compound dykes -appear to be so abundant as in the centre and southern part of the island of -Skye. During the progress of the Geological Survey in that district, Mr. -Clough and Mr. Harker have mapped a large number in the ground between -the Sound of Sleat and the Red Hills. With regard to these dykes Mr. -Harker observes that the several members are generally petrographically -different, some being basic, others intermediate, and others acid. "There is -usually," he remarks, "a symmetrical disposition, two similar and more -basic dykes being divided by a more acid one; for example, two andesites -separated by a pitchstone. Thus at the mouth of the little stream which -runs from Torran into the bay east from Dùn Beag a dyke, apparently 18 -feet wide, is found on examination to consist of a central dyke (specific -gravity 2·86) flanked by two more basic dykes (specific gravity 3·02)."</p> - -<p>In the great majority of examples hitherto observed in Skye the two -lateral dykes consist of some basic rock (diabase or basalt), while the central -and thickest band is of some acid material (granophyre or quartz-felsite). -This triple arrangement occurs both in dykes and sills.</p> - -<div class="figleft" id="v2fig254" style="width: 209px;"> - <img src="images/v2fig254.png" width="209" height="119" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 254.</span>—Compound dyke, Market - Stance, Broadford, Skye.<br /><br /> - <i>a</i>, Granophyre; <i>b</i> <i>b</i>, Basalt; <i>c</i> <i>c</i>, Torridon - sandstone.</div> -</div> - -<p>As an illustration of the association of the two kinds of rock in dykes -I may cite an example which appears on the southern edge of the Market -Stance of Broadford (<a href="#v2fig254">Fig. 254</a>). Here the characteristic triple arrangement -is typically developed. A central light-coloured -band, about eight to ten feet -broad, consists of a spherulitic granophyre -in which the spherulites are -crowded together and project from the -weathered surface like peas, though they -do not here show the curious rod-like -aggregation so marked in some other -dykes. On either side of this acid centre -a narrow basalt dyke intervenes as a wall -next to the Torridon sandstone which -here forms the country-rock. Such compound dykes have sometimes a -total width of 100 feet or more.</p> - -<p><span class="pagenum" id="Page_163">- 163 -</span></p> - -<p>In this instance, and generally throughout the district, there is nothing -to indicate that the different bands of the dyke have any relation to each -other as connected uprises of material from the same original magma which -was either heterogeneous or was undergoing a process of differentiation -beneath the terrestrial crust. On the contrary, the several parts of each -dyke are as distinctly marked off from each other as they could have been -had they been injected at widely separated intervals of volcanic activity.</p> - -<p>Mr. Harker, in the course of his survey of this Skye ground, has observed -that "where evidence is available, the central acid dyke is found to be newer -than the basic ones. It has not split a single basic dyke, but has insinuated -itself between the two members of a double dyke. This is more clearly seen -when the acid magma has been forced into a triple or multiple basic dyke; -the perfect symmetry of arrangement may in this case be lost. For instance, -on the shore north-east of Corry, Broadford, a 13 feet dyke of granophyre -occurs in a multiple dyke of basalt, but it has taken its line so as to leave -only a one-foot dyke on one side, and a group with a total width of 12 feet -on the other. Also it has not accurately kept its course, but has cut -obliquely across one of the group of dykes alluded to. In some cases it is -certain that the acid magma has to some extent dissolved a portion of the -wall of a basic dyke with which it has come in contact. This may account -for the magma finding its easiest path along, and especially between, pre-existing -more basic dykes." This subject will be again referred to in Chapter -xlviii., when the phenomena of compound sills are discussed.</p> - -<p>Before closing this account of compound dykes, I may remark that no -examples have yet been observed among the ordinary Tertiary dykes of -Britain where, by a process of differentiation between the walls of a fissure, -successive zones have been developed in the dyke, differing from each other -in structure and composition, but becoming progressively and insensibly -more acid towards the centre, such as have been described from the older -rocks of Norway and Canada. Among the Tertiary gabbro bosses, indeed, -there occur sheets or dykes which present a remarkably banded structure, -to which full reference will be made in later pages. But I have never seen -anything at all resembling such a structure among the dykes of andesite, -dolerite, or basalt.</p> - - -<h3>17. CONTACT-METAMORPHISM OF THE DYKES</h3> - -<p>A geologist might naturally expect that such abundant intrusions of -igneous rock as those of the dykes should be accompanied with plentiful -proofs of contact-metamorphism. But in actual fact, evidence of any -serious amount of alteration is singularly scarce. A slight induration of the -rocks on either side of a dyke is generally all the change that can be -detected.</p> - -<div class="figleft" id="v2fig255" style="width: 265px;"> - <img src="images/v2fig255.png" width="265" height="166" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 255.</span>—Section of coal rendered columnar by intrusive - basalt, shore, Saltcoats, Ayrshire.<br /><br /> - <i>a</i>, Fireclay; <i>b</i>, Coal rendered prismatic near the basalt; - <i>c</i>, Dark shale; <i>d</i>, Basalt-rock.</div> -</div> - -<p>Some of the larger dykes, however, show more marked metamorphism, -the nature of which appears in many cases to be chiefly determined by the -chemical composition of the rock affected. Thus a considerable alteration -<span class="pagenum" id="Page_164">- 164 -</span> -has been superinduced on carbonaceous strata, particularly on seams -of coal. In the Ayrshire coal-field the alteration of the coal extends -sometimes 150 feet from the dyke, the extent of the change depending -not merely on the mass of the igneous rock, but on the nature of the -coal, and possibly on other causes. -Close to a dyke, coal passes into -a kind of soot or cinder, sometimes -assumes the form of a -finely columnar coke (<a href="#v2fig255">Fig. 255</a>), -and occasionally has become -vesicular after being fused.<a id="FNanchor_201" href="#Footnote_201" class="fnanchor">[201]</a> -Shales are converted into a -hard flinty substance that breaks -with a conchoidal fracture and -rings under the hammer. Fireclay -is baked into a porcelain-like -material. Limestone is -changed for a few inches into -marble. As an illustration of this alteration, I may cite a dyke ten feet -broad which cuts through the chalk in the Templepatrick Quarry, Antrim. -For about six inches from the igneous rock the chalk has passed into a finely -saccharoid condition, and its organisms are effaced. But beyond that distance -the crystalline structure rapidly dies away, the micro-organisms begin to -make their appearance, and within a space of one foot from the dyke the -chalk assumes its ordinary character.</p> - -<div class="footnote"> - -<p><a id="Footnote_201" href="#FNanchor_201" class="label">[201]</a> Explanation of Sheet 22, Geological Survey of Scotland, p. 26.</p> - -</div> - -<p>Sandstones are indurated by dykes into a kind of quartzite, sometimes -assume a columnar structure (the columns being directed away from the -dyke-walls), and for several feet or yards have their yellow or red colours -bleached out of them. The granite of Ben Cruachan where quarried on -Loch Awe, as I am informed by Mr. J. S. Grant Wilson, is traversed by -a basic dyke, and for a distance of about 20 feet is rendered darker in -colour, becomes granular, and cannot be polished and made saleable.</p> - -<p>Where many dykes have been crowded together, their collective effects -in the alteration of the strata traversed by them have sometimes been -strongly developed. One of the most remarkable illustrations of this -influence is presented by the district of Strathaird, which was cited by -Macculloch for the abundance of its dykes. In recently mapping this -ground for the Geological Survey, Mr. Harker has observed in some places a -score or more dykes in actual juxtaposition, while over considerable distances -he found it difficult to detect any trace of the Jurassic strata, through -which the igneous rocks have ascended. As might be expected under these -circumstances, such portions of the strata as can be seen display an altogether -exceptional amount of contact-metamorphism. Mr. Harker has -noticed some limestones at Camasunary which have been changed into very -remarkable lime-silicate rocks, with singular bunches of diopside crystals.</p> - -<p><span class="pagenum" id="Page_165">- 165 -</span></p> - -<p>These, however, are the extremes of contact-metamorphism by the -Tertiary basic dykes. A geologist visiting the Liassic shores of Strath in -Skye will not fail to be surprised at the very slight degree of alteration in -circumstances where he would have expected to find it strongly pronounced. -The dark shales, though ribbed across with dykes, are sometimes hardly -even hardened, and at the most are only indurated from an inch or two to -about two feet. These baked bands project above the rest of the more -easily denuded shales, and so adhere to the dykes as almost to seem part of -them. Again the limestones, where traversed by dykes some distance -apart, are not rendered in any appreciable degree more crystalline even -up to the very margin of the intrusive rock. Where the igneous material -has been thrust between the strata in sills, it has produced far more general -and serious metamorphism than when it occurs in the form of single -dykes. The famous rock of Portrush, already referred to as having -been once gravely cited as an example of fossiliferous basalt, is a -good illustration of the way in which Lias shale is porcellanized when -the intruded igneous material has been thrust between the planes of -bedding.</p> - -<p>In the West of Scotland, where dykes are so abundantly developed, considerable -differences can be observed between the amount of metamorphism -superinduced by adjacent dykes which may be of the same thickness, and -cut through the same kind of strata. Such variations have not probably -arisen from differences in the temperature of the original molten rock. -Perhaps they are rather to be assigned to the length of time occupied by -the ascent of the lava in the fissure. If, for instance, the fissure opened to -the surface and discharged lava there, the rocks of its walls would be exposed -to a continuous stream of molten rock as long as the outflow lasted. They -would thus have their temperature more highly raised, and maintained -at such an elevation for a longer time than where the magma, at once -arrested within the fissure, immediately proceeded to cool and consolidate -there. It would be an interesting and important conclusion if we could, -from the nature or amount of their contact-metamorphism, distinguish those -dykes which for some time served as channels for the discharge of lava -above ground.</p> - -<p>Some dykes which have caught up fragments of older rocks in their -ascent have exercised a considerable solvent action on these inclusions. -Examples of this feature have already been cited from Skye, where they -have been studied by Mr. Harker (pp. <a href="#Page_129">129</a>, <a href="#Page_163">163</a>).</p> - -<p>In connection with the metamorphism superinduced by dykes, reference -may again be made to the alteration which they themselves undergo where -they have invaded a carbonaceous shale or coal. The igneous rock, as we -have seen, loses its dark colour and obviously crystalline structure, and becomes -a pale yellow or white, dull, earthy substance, or "white trap." The -chemical changes involved in this alteration have been described by Sir J. -Lowthian Bell.<a id="FNanchor_202" href="#Footnote_202" class="fnanchor">[202]</a> Dr. Stecher has also discussed the alterations traceable by -<span class="pagenum" id="Page_166">- 166 -</span> -the aid of the microscope.<a id="FNanchor_203" href="#Footnote_203" class="fnanchor">[203]</a> Though most of the instances of such transformation -in Britain occur in the Carboniferous system, and have taken place -in intrusive rocks of probably, for the most part, Carboniferous or Permian -age, yet they are not unknown in the Tertiary volcanic series. Some of the -"white trap" of the Coal-measures may indeed belong to the Tertiary period, -but the coals and carbonaceous shales interstratified in the Tertiary basalt-plateaux -have reacted on both the superficial -lavas and the sills, and have given rise to the -same kind of alteration as in the Carboniferous -system, as will be shown in a later Chapter.</p> - -<div class="footnote"> - -<p><a id="Footnote_202" href="#FNanchor_202" class="label">[202]</a> <i>Proc. Roy. Soc.</i> xxiii. (1875), p. 543.</p> - -<p><a id="Footnote_203" href="#FNanchor_203" class="label">[203]</a> Tschermak's <i>Mineralogische Mittheilungen</i>, ix. (1887), p. 145, and <i>Proc. Roy. Soc. Edin.</i> -1888.</p> - -</div> - -<div class="figright" id="v2fig256" style="width: 171px;"> - <img src="images/v2fig256.png" width="171" height="238" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 256.</span>—Dolerite dyke with marginal - bands of "white trap," in black - shale, Lower Lias, Pabba.<br /><br /> - <i>a</i>, Black carbonaceous Lower Lias Shale; - <i>b</i> <i>b</i>, bands of indurated shale from 15 - inches to 2 feet broad; <i>c</i>, dolerite dyke - 3 feet 3 inches broad; <i>d</i> <i>d</i>, bands of - altered dolerite or "white trap," 3 to 5 - inches broad.</div> -</div> - - -<p>Some marked examples of this alteration -of intrusive igneous material are to be observed -among the basalt dykes which cut -the Lower Lias Shales of Skye. These -shales, where black and carbonaceous, as in -the island of Pabba, have exercised an unmistakable -influence on the abundant dykes -which intersect them. The chilled selvage -of each dyke has assumed the dull earthy -pale-grey or yellowish aspect, which extends -for a few inches from the wall into the -interior, where it rapidly passes into the -ordinary black crystalline basalt. These -features will be readily understood from the -accompanying diagram (<a href="#v2fig256">Fig. 256</a>). Where -the dykes give off narrow veins a few inches -broad, these consist entirely of the "white -trap." The shales are often traversed with strong joints parallel to the -walls of the dykes, and the transverse joints of the dykes are sometimes -prolonged into the bands of indurated shale.</p> - - -<h3>18. RELATION OF DYKES TO THE GEOLOGICAL STRUCTURE OF THE -DISTRICTS WHICH THEY TRAVERSE.</h3> - -<p>In no respect do the Tertiary dykes of Britain stand more distinguished -from all the other rocks of the country than in their extraordinary independence -of geological structure. The successive groups of Palæozoic and -Mesozoic strata have been so tilted as to follow each other in approximately -parallel bands, which run obliquely across the island from south-west to -north-east. The most important lines of fault take the same general line. -The contemporaneously included igneous rocks follow, of course, the trend -of the stratified deposits among which they lie, and even the intrusive sills -group themselves along the general strike of the whole country. But the -Tertiary dykes have their own independent direction, to which they adhere -amid the extremest diversities of geological arrangement.</p> - -<p><span class="pagenum" id="Page_167">- 167 -</span></p> - -<p>In the first place, the dykes intersect nearly the whole range of the -geological formations of the British Islands. In the Outer Hebrides and -north-west Highlands, they rise through the most ancient (Lewisian) -gneisses, through the red pre-Cambrian (Torridon) sandstones, and through -the oldest members of the Cambrian system. In the southern Highlands, -they pursue their course across the gnarled and twisted schists of the -younger crystalline (Dalradian) series. In the South of Scotland and North -of England, they traverse the various subdivisions of the Lower and Upper -Silurian rocks. In the basins of the Tay, Forth, and Clyde they cross the -plains and ridges of the Old Red Sandstone, with its deep pile of intercalated -volcanic material. In Central Scotland, and the northern English counties, -they occur abundantly in the Carboniferous system, and have destroyed the -seams of coal. In Cumberland and Durham, they traverse the Permian and -Trias groups. In Yorkshire, and along the West of Scotland, they are found -running through Jurassic strata. In Antrim, they intersect the Chalk. -Both in the North of Ireland, and all through the chain of the Inner -Hebrides, they abound in the great sheets and bosses of Tertiary volcanic -rocks. These are the youngest formations through which they rise. But -it is deserving of note, that they intersect every great group of these -Tertiary volcanic products, so that they include in their number the latest -known manifestations of eruptive action in the geological history of -Britain.<a id="FNanchor_204" href="#Footnote_204" class="fnanchor">[204]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_204" href="#FNanchor_204" class="label">[204]</a> They have not been found cutting the pitchstone-lava of the Scuir of Eigg.</p> - -</div> - -<p>In the second place, in ranging across groups of rock belonging to such -widely diverse periods, the dykes must necessarily often pass abruptly from -one kind of material and geological structure to another. But, as a rule, -they do so without any sensible deviation from their usual trend, or any -alteration of their average width. Here and there, indeed, we may observe -a dyke to follow a more wavy or more rapidly sinuous or zig-zag course in -one group of rocks than in another. Yet, so far as I have myself been able -to observe, such sinuosities may occur in almost any kind of material, and -are not satisfactorily explicable by any difference of texture or arrangement -in the rocks at the surface. No dyke traverses a greater variety of sedimentary -formations than that of Cleveland. In the eastern part of its -course, it rises through all the Mesozoic groups up to the Cornbrash. -Further west it cuts across each of the different subdivisions of the -Carboniferous system; and, of course, it must traverse all the older formations -which underlie these. But the occasional rapid changes noticeable in -its width and direction do not seem to be referable to any corresponding -structure in the surrounding rocks. The Cheviot dyke crosses from the -Carboniferous area of Northumberland into the Upper Silurian rocks and -Lower Old Red Sandstone volcanic tract of the Cheviot Hills. It then -strikes across the Upper Old Red Sandstone of Roxburghshire, and still -maintaining the same persistent trend, sweeps westward into the intensely -plicated Silurian rocks of the Southern Uplands. Its occasional deviations -have no obvious reference to any visible change of structure in the adjacent -<span class="pagenum" id="Page_168">- 168 -</span> -formations. Again, some of the great dykes at the head of Clydesdale -furnish striking illustrations of entire indifference to the nature of the rock -through which they run. Quitting the Silurian uplands, they keep their line -across Old Red Sandstone and Carboniferous rocks, and through large masses -of eruptive material.</p> - -<p>In the third place, not only are the dykes not deflected by great -diversities in the lithological character of the rocks which they traverse, -they even cross without deviation some of the most important geological -features in the general framework of the country. Some of the Scottish -examples are singularly impressive in this respect. Those which strike -north-westward from the uplands of Clydesdale cross without deflection the -great boundary-fault which, by a throw of several thousand feet, brings the -Lower Old Red Sandstone against Silurian rocks. They traverse some large -faults in the valley of the Douglas coal-field, pass completely across the axis -of the Haughshaw Hills, where the Upper Silurian rocks are once more -brought up to the surface, and also the long felsite ridge of Priesthill. The -dykes in the centre of the kingdom maintain their line across some of the -large masses of igneous rock that protrude through the Carboniferous -system. Further north, the dykes of Perthshire cut across the great sheets -of volcanic material that form the Ochil Hills, as well as through the piles -of sandstone and conglomerate of the Lower Old Red Sandstone, and then go -right across the boundary-fault of the Highlands, to pursue their way in the -same independent manner through grit, quartzite, or mica-schist, and across -glen and lake, moor and mountain.</p> - -<p>No one can contemplate these repeated examples of an entire want of -connection between the dykes and the nature and arrangement of the rocks -which they traverse without being convinced that the lines of rent up which -the material of the dykes rose were not, as a rule, old fractures in the -earth's crust, but were fresh fissures, opened across the course of the older -dislocations and strike of the country by the same series of subterranean -operations to which the uprise of the molten material of the dykes was also -due.</p> - -<p>In the fourth place, the dykes for the most part are not coincident with -visible lines of fault. After the examination of hundreds of dykes in all parts -of the country, and with all the help which bare hillsides and well-exposed -coast-sections can afford, the number of instances which have been met with -where dykes have availed themselves of lines of fault is surprisingly small. -Some of these cases will be immediately cited. To whatever cause we may -ascribe the rupture of the solid crust of the earth, which admitted the rise -of molten rock to form the dykes, there can be no doubt that it was not -generally attended with that displacement of level on one or both sides of -the dislocation, which we associate with the idea of a fault. Nowhere can -this important part of dyke-structure be more clearly illustrated than along -the Cleveland dyke, where the igneous rock rises through almost horizontal -Jurassic strata and gently inclined Coal-measures (Figs. <a href="#v2fig241">241</a>, <a href="#v2fig242">242</a>, <a href="#v2fig243">243</a>, <a href="#v2fig244">244</a>). -Besides the localities already cited, mining operations both for coal and for -<span class="pagenum" id="Page_169">- 169 -</span> -the Liassic ironstone have proved over a wide area that the dyke has not -risen along a line of fault. Again, in Skye, Raasay, Eigg, and other parts -of the west coast, where Jurassic strata and the horizontal basalts of the -plateaux are plentifully cut through by dykes, the same beds may generally -be seen at the same level on either side of them.</p> - -<p>In the fifth place, while complete indifference to geological structure is -the general rule among the dykes, instances do occur in which the molten -material has found its way upward along old lines of rupture. Most of such -instances are to be found in districts where previously existing faults -happened to run in the same general direction as that followed by the -dykes. These lines of fracture might naturally be re-opened by any great -earth-movements acting in their direction, and would afford ready channels -for the ascent of the lava, as we have seen to have not infrequently -happened in the case of dyke-fissures, which are shown by compound dykes -to have sometimes been re-opened several times in succession even after -having been filled up with basalt. Yet it is curious that, even when their -trend would have suited the line of the dykes, faults have not been more -largely made use of for the purpose of relief. Some of the best examples of -the coincidence of dykes with pre-existing faults in the same direction are -to be found in the Stirlingshire coal-field. The dyke that runs from -Torphichen for 23 miles to Cadder occupies a line of fault which at -Slamannan has a down-throw of more than 70 fathoms. The next dyke -further south has also risen along an east and west fault.</p> - -<p>But other examples may be observed where pre-existing fissures have -served to deflect dykes from their usual line of trend. Thus the Cleveland -dyke, after crossing several faults in the Coal-measures, at last encounters -one near Cockfield Fell, which lies obliquely across its path. Instead of -crossing this fault it bends sharply round a few points south of west, and -after keeping along the southern flank of the fault for about a mile, sinks -out of reach. Some of the Scottish examples are more remarkable. One of -the best of them occurs in the Sanquhar coal-field, where a dyke runs for -two miles and a half along the large fault that here brings down the Coal-measures -against the Lower Silurian rocks. At the north-western end of -the basin, this fault makes an abrupt bend of 60° to W.S.W., and the dyke -turns round with it, keeping this altered course for a mile and a half, when -it strikes away from the fault, crosses a narrow belt of Lower Silurian rocks, -and finds its way into the parallel boundary fault which defines the north-western -margin of the Southern Uplands.</p> - - -<p><span class="pagenum" id="Page_170">- 170 -</span></p> - -<div class="figcenter" id="v2fig257" style="width: 595px;"> - <img src="images/v2fig257.png" width="595" height="539" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 257.</span>—Map of the chief dykes between Lochs Riddon and Striven (C. T. Clough, Geological Survey, - Sheet 29). The large E. and W. dyke is a continuation of that which reaches the shore of the - Firth of Clyde at Dunoon.</div> -</div> - - -<p>Some of the Perthshire dykes, where they reach the great boundary-fault -of the Highlands, present specially interesting features. There can be -no doubt that this dislocation is one of the most important in the general -framework of the British Isles, though no definite estimate has yet been -formed of how much rock has been actually displaced by it. The fact -that in one place the beds of Old Red Sandstone are thrown on end for -some two miles back from it, shows that it must be a very powerful fracture. -Here, therefore, if anywhere, either an entire cessation of the dykes, or at -least a complete deflection of their course might be anticipated. It would -require, we might suppose, a singularly potent dislocation to open a way for -the ascent of the lava through such crushed and compressed rocks, and still -more to prolong the general line of a fracture across the old fault. Two -great dykes, about half a mile apart, run in a direction a little south of west -across the plain of Strathearn. Passing to the south of the village of -Crieff, they hold on their way until they reach the highly-inclined beds of -sandstone and conglomerate which here lean against the Highland fault in -<span class="pagenum" id="Page_171">- 171 -</span> -Glen Artney. They then turn round towards south-west, and run up the -glen along the strike of the beds, keeping approximately parallel to the fault -for about three miles, when they both strike across the fault, and pursue a -W.S.W. line through the contorted crystalline rocks of the Highlands. -About two miles further south, another dyke continues its normal course -across the belt of upturned Old Red Sandstone; but when it reaches the -fault it bends round and follows the line of dislocation, sometimes coinciding -with, sometimes crossing or running parallel with that line, at a short -distance (see <a href="#v2fig247">Fig. 247</a>).</p> - -<p>Some remarkable examples have been mapped by Mr. Clough in -Eastern Argyleshire, where broad bands of basalt or other allied rock run in -a N. and S. direction, and are formed by the confluence of N.W and S.E. or -N.N.W. and S.S.E. dykes, where these are drawn into a line of fault (Fig. -257). These broad bands, he has found to be not usually traceable for -more than a mile or so, for the dykes of which they are made up will not -be diverted from their regular paths for more than a certain distance, so -that one by one the dykes leave the compound band to pursue their normal -course. He has observed that the occasional great thickness of these compound -bands depends partly on the size and partly on the number of -separate dykes that are diverted into the line of transverse fissure; for, -where the fissure crosses an area with fewer north-west dykes, the band -becomes thinner or ceases altogether.</p> - -<p>In some rare cases, the dykes have been shifted by more recent faults. -I shall have occasion to show that faults of more than 1000 feet have taken -place since the Tertiary basalt-plateaux were formed. There is therefore no -reason why here and there a fault with a low hade should not have shifted -the outcrop of a dyke. But the fact remains, that, as a general rule, the -dykes run independently of faults even where they approach close to them. -Mr. Clough has observed some interesting cases in South-eastern Argyleshire, -where the apparent shifting of a dyke by faults proves to be deceptive, and -where the dyke has for short distances merely availed itself of old lines of -fracture. One of the most remarkable of these is presented by the large -dyke which runs westward from Dunoon. No fewer than three times, in -the course of four miles between Lochs Striven and Riddon, does this dyke -make sharp changes of trend nearly at right angles to its usual direction, -where it encounters north and south faults (<a href="#v2fig257">Fig. 257</a>). It would be natural to -conclude that these changes are actual dislocations due to the faults. But -the careful observer just cited has been able to trace the dyke in a very -attenuated and uncrushed form along some of these cross faults, and thus to -prove that the faults are of older date, but that they have modified the line -of the long east and west fissure up which the material of the dyke ascended.</p> - - -<h3>19. DATA FOR ESTIMATING THE GEOLOGICAL AGE OF THE DYKES</h3> - -<p>I have already assigned reasons for regarding the system of north-west -and south-east or east and west dykes as belonging to the Tertiary volcanic -<span class="pagenum" id="Page_172">- 172 -</span> -period in the geographical history of the British Islands. But I have no -evidence that they were restricted to any part of that period. On the -contrary, there is every reason to consider the uprise of the earliest and -latest dykes to have been separated by a protracted interval. That they -do not all belong to one epoch has been already indicated, and may -now be more specially proved.</p> - -<p>The intersection of one dyke by another furnishes an obvious criterion -of relative age. Macculloch drew attention to this test, and stated that it -had enabled him to make out two distinct sets of dykes in Skye and Rum. -But he confessed that it failed to afford any information as to the length of -the interval of time between them.<a id="FNanchor_205" href="#Footnote_205" class="fnanchor">[205]</a> It is not always so easy as might be -thought to make sure which of two intersecting dykes is the older. As -was explained in <a href="../../66492/66492-h/66492-h.htm#CHAPTER_VI">Chapter vi</a>. (<a href="../../66492/66492-h/66492-h.htm#Page_81">vol. i. p. 81</a>), we have to look for the finer-grained -marginal strip at the edge of a dyke, which, where traceable across -another dyke, marks at once their relative age. The cross joints of the two -dykes also run in different directions. Reference may again be made to the -illustration given in <a href="#v2fig253">Fig. 253</a> where three distinct groups of dykes intersect -each other as they traverse the Lias limestones of Skye. The chilled edges -and the different arrangement of joints mark these dykes out from each -other, while the order in which they cross each other furnishes a clue to -their relative age. If from such sections, repeated in different parts of a -district, certain persistent petrographical characters can be ascertained to -distinguish each particular system of dykes, a guide may thereby be -obtained for the chronological grouping of the intrusions even where -evidence of actual intersection is not visible. In the case just cited from -Skye, the later north and south dykes are characterized by their lines of -vesicular cavities and by the large porphyritic felspars which they contain.</p> - -<div class="footnote"> - -<p><a id="Footnote_205" href="#FNanchor_205" class="label">[205]</a> <i>Trans. Geol. Soc.</i> iii. p. 75.</p> - -</div> - -<p>It is obvious, however, that although sections of this kind suffice to -prove the dykes to belong to distinct periods of intrusion, no longer -interval need have elapsed between their successive production than was -required for the solidification and assumption of a joint-structure by an older -dyke before a younger broke through it. They may both belong to one -brief period of volcanic activity. But when we pass to a series of dykes -traversing a considerable district of country, and find that those which run -in one direction are invariably cut by those which run in another, the -inference can hardly be resisted that they do not belong to the same period -of eruption, but mark successive epochs of volcanic energy. An excellent -example of this kind of evidence is furnished by Mr. Clough from Eastern -Argyleshire. The east and west dykes in that district are undoubtedly -older than those which run in a N.N.W. direction (<a href="#v2fig257">Fig. 257</a>).<a id="FNanchor_206" href="#Footnote_206" class="fnanchor">[206]</a> The latter -are by far the most abundant, and are on the whole much narrower, less -persistent, and finer in grain. On the opposite coast of the Clyde, a similar -double set of dykes may be traced through Renfrewshire, those in an east -<span class="pagenum" id="Page_173">- 173 -</span> -and west direction being comparatively few, while the younger N.N.W. -series is well developed. The great sheets or "sills" connected with one of -the Stirlingshire dykes, already described, appear to me to furnish similar -evidence in the younger dykes which run through them. And this -evidence is peculiarly valuable, for it shows a succession even among -adjacent dykes which all run in the same general direction.</p> - -<div class="footnote"> - -<p><a id="Footnote_206" href="#FNanchor_206" class="label">[206]</a> As already stated, Mr. Clough and also Mr. Gunn are inclined to separate these older east -and west dykes from the Tertiary series and to regard them as probably of late Palæozoic age.</p> - -</div> - -<p>But in all these cases it is obvious that we have little indication of the -length of time that intervened between the successive injections of the -dykes. In Skye, however, more definite evidence presents itself that the -interval must have been in some cases a protracted one. As far back as -the year 1857,<a id="FNanchor_207" href="#Footnote_207" class="fnanchor">[207]</a> I showed that the basic dykes of Strath in Skye are -of two ages; that one set was erupted before the appearance of the -"syenite" (granophyre) of that district, and was cut off by the latter rock; -and that the other arose after the "syenite" which it intersected. -Recent re-examination has enabled me to confirm and extend this observation. -The younger series which traverses the granophyre is much less numerous -than the older series in the same districts. In Chapter xlvi., where the -relations of the granophyres to other members of the volcanic series will -be discussed, further details will be given from that region of Skye to -demonstrate that there is a pre-granophyre and a post-granophyre series of -basic dykes. As a good illustration of the younger series I may refer to -the way in which these rocks make their appearance in the island group of St. -Kilda, where both the gabbros and granophyres of the Tertiary volcanic -series are characteristically developed. Numerous dykes traverse both -these rocks. Those in the gabbro are more abundant than those in the -granophyre—a circumstance which is exactly paralleled among the basic and -acid bosses of Skye. It is not improbable that in these remote islands a -similar difference in age and in petrographical character may be made out -between two series of dykes, one older and the other younger than the -granophyre. There is ample proof, at all events, of a post-granophyre series.</p> - -<div class="footnote"> - -<p><a id="Footnote_207" href="#FNanchor_207" class="label">[207]</a> <i>Quart. Jour. Geol. Soc.</i> vol. xiv. p. 16.</p> - -</div> - -<div class="figcenter" id="v2fig258" style="width: 382px;"> - <img src="images/v2fig258.png" width="382" height="178" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 258.</span>—Basalt-veins traversing granophyre, St. Kilda.</div> -</div> - -<p>The pale colour of the precipices in which the St. Kilda granophyre -plunges into the sea gives special prominence to the dark ribbon-like -<span class="pagenum" id="Page_174">- 174 -</span> -streaks which mark the course of basalt-dykes through that rock. Moreover -the greater liability of the material of the dykes to decay causes them to -weather into long lines of notch or recess. Four or five such dykes follow -each other in nearly parallel bands, which slant upward from the sea-level -on the eastern face of the hill Conacher to a height of several hundred -feet.<a id="FNanchor_208" href="#Footnote_208" class="fnanchor">[208]</a> (<a href="#v2fig258">Fig. 258</a>, see also <a href="#v2fig367">Fig. 367</a>.)</p> - -<div class="footnote"> - -<p><a id="Footnote_208" href="#FNanchor_208" class="label">[208]</a> This relation of the later dykes to the granophyre was observed here by Macculloch (<i>Western -Isles</i>, vol. ii. p. 55).</p> - -</div> - -<p>The acid eruptions of the Inner Hebrides are marked by so varied -a series of rocks, and so complex a geological structure, that they may, -with some confidence, be regarded as having occupied a considerable -interval of geological time. Yet we find that this prolonged episode in the -volcanic history was both preceded and followed by the extravasation of -basic dykes.</p> - -<p>Reference has already been made to recent observations by Mr. Harker, -who, in mapping the Strath district of Skye for the Geological Survey, has -not only confirmed the generalization as to the existence of a series of dykes -earlier, and another later, than the great granophyre protrusions of the Inner -Hebrides, but has made some progress towards the detection of a means of -distinguishing the two series even where no direct test of their relative age -may be available. He thinks that the general habit and petrographical -characters of the dykes may on further investigation be found to afford a -sufficiently reliable basis for discrimination. He finds that where the -relative ages of the dykes with reference to the granophyre can be fixed, -the earlier or pre-granophyre series is without exception basic. It consists -of fine-textured basalts or diabases, without any conspicuous porphyritic -crystals. Its dykes are less regular and persistent in their bearing than -those of the later series; have frequently a considerable hade, even as much as -45°, and often show chilled edges with tachylitic selvages. In Skye many of -these earlier dykes may be connected with the gabbro. They appear to be -more basic and to have a higher specific gravity than those of the later -series which most resemble them.</p> - -<p>The later or post-granophyre dykes include several types, the relative -ages of which are not yet definitely fixed. They run in straight parallel -lines, and thus seldom intersect each other. They are generally vertical or -highly inclined, and are much more frequently characterized by amygdaloiclal -structure than the earlier series. Mr. Harker distinguishes the -following varieties among them: (<i>a</i>) Quartz-felsites and other acid rocks; -these are not very common. (<i>b</i>) Pitchstones and various spherulitic and -variolitic rocks: the actual pitchstones observed are comparatively few -in number, but it is certain that some of spherulitic varieties are devitrified -pitchstones. (<i>c</i>) Basic rocks, not conspicuously porphyritic and less decidedly -basic than the dykes of the pre-granophyre series; most of the later -groups come into this or the next group, (<i>d</i>) Porphyritic basic dykes -not infrequently carrying inclusions of gabbro, granophyre or other rocks. -The porphyritic felspars seem to be in great part of foreign derivation, and -<span class="pagenum" id="Page_175">- 175 -</span> -the same is certainly true of the augite which occasionally accompanies them -and of the quartz that appears in some examples.<a id="FNanchor_209" href="#Footnote_209" class="fnanchor">[209]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_209" href="#FNanchor_209" class="label">[209]</a> Annual Report of the Director-General of the Geological Survey in Report of Science and Art -Department for 1895.</p> - -</div> - -<p>In the Carlingford district of the North-east of Ireland, similar evidence -has been obtained that one series of dykes preceded and another followed -the protrusion of the granites and granophyre which are in all probability -geologically coeval with the acid bosses of the Inner Hebrides. The -distinction was observed and mapped by Mr. Traill for the Geological -Survey. Professor Sollas in recently confirming these observations has not -noticed any striking difference between the pre-granite and post-granite -dykes, the whole appearing to consist of the same coarsely porphyritic -material.<a id="FNanchor_210" href="#Footnote_210" class="fnanchor">[210]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_210" href="#FNanchor_210" class="label">[210]</a> See Sheets 59, 60, and 71 of the Geological Survey Map of Ireland; Professor Sollas, <i>Trans. -Roy. Irish Acad.</i> vol. xxx. (1894), p. 477; and Annual Report of the Director-General of the -Geological Survey for 1895.</p> - -</div> - -<p>While the eruption of the granophyre bosses furnishes proof that the -dykes are not all of the same age, other evidence may be gathered to show -how much older some of the dykes are than the youngest lava-streams in -the volcanic history of Tertiary time in Britain. The Scuir of Eigg, to -which fuller reference will be made in Chapter xxxviii., is formed of a -mass of pitchstone, which has filled up an ancient valley eroded out of -the terraced basalts of the plateaux. At both ends of the ridge, these basalts -are seen to be traversed by dykes that are abruptly cut off by the shingle -of the old river-bed which the pitchstone has occupied (Figs. <a href="#v2fig279">279</a>, <a href="#v2fig282">282</a>). -It is thus evident that, though these dykes are younger than the plateau-basalts, -they are much older than the excavation of the valley out of these -basalts, and still older than the eruption of pitchstone. The latter rock -probably belongs to the close of the period of lava-eruptions. The enormous -denudation of the basalt-plateaux after the injection of the dykes and before -the outflow of the pitchstone affords a convincing proof of the vastness of -the interval between the eruption of the two kinds of rock.<a id="FNanchor_211" href="#Footnote_211" class="fnanchor">[211]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_211" href="#FNanchor_211" class="label">[211]</a> <i>Quart. Jour. Geol. Soc.</i> xiv. p. 1.</p> - -</div> - -<p>It is thus demonstrable that the dykes which in Britain form part of -the great Tertiary volcanic series, were not all produced at one epoch, but -belong to at least two (and probably to many more) episodes in one long -volcanic history. As they rise through every member of that series -of rocks (save the pitchstones), some of them must be among the latest -records of the prolonged volcanic activity. But, on the other hand, some -probably go back to the very beginning of the Tertiary volcanic period.</p> - - -<h3>20. ORIGIN AND HISTORY OF THE DYKES</h3> - -<p>Reference has already been made to the doubt expressed by Macculloch -whether the dykes in Skye had been filled in from above or from below. -That the dykes of the country as a whole were supplied from above, was -the view entertained and enforced by Boué. He introduces the subject with -<span class="pagenum" id="Page_176">- 176 -</span> -the following remarks:—"Scotland is renowned for the number of its basaltic -veins, which gave Hutton his ideas regarding the injection of lava from -below; but, as the greatest genius is not infallible, and as volcanic countries -present us with examples of such veins arising evidently from accidental -fissures that were filled up by currents of lava which moved over them, and -as the Scottish instances are of the same kind, we regard it as infinitely -probable that all these veins have been formed in the same way notwithstanding -the enormous denudation which this supposition involves; and -that only rarely do cases occur where they have been filled laterally or in -some other irregular manner."<a id="FNanchor_212" href="#Footnote_212" class="fnanchor">[212]</a> I need not say that this view, which, -except among Wernerians, had never many supporters, has long ago been -abandoned and forgotten. There is no further question that the molten -material came from below.</p> - -<div class="footnote"> - -<p><a id="Footnote_212" href="#FNanchor_212" class="label">[212]</a> <i>Essai Géologique sur l'Écosse</i>, p. 272.</p> - -</div> - -<p>1. In discussing the history of the dykes, we are first confronted with -the problem of the formation of the fissures up which the molten material -rose. From what has been said above regarding the usual want of relation -between dykes and the nature and arrangements of the rocks which they -traverse, it is, I think, manifest that the fissures could not have been caused -by any superficial action, such as that which produces cracks of the ground -during earthquake-shocks. The fact that they traverse rocks of the most -extreme diversities of elasticity, structure, and resistance, and yet maintain -the same persistent trend through them all, shows that they originated far -below the limits to which the known rocks of the surface descend. We -have seen that in the case of the Cleveland dyke, the fissure can be proved -to be at least some three miles deep. But the seat of the origin of the rents -no doubt lay much deeper down within the earth's crust.</p> - -<p>It is also evident that the cause which gave rise to these abundant -fissures must have been quite distinct from the movements that produced -the prevalent strike and the main faults of this country. From early -geological time, as is well known, the movements of the earth's crust -beneath the area of Britain, have been directed in such a manner as to give -the different stratified formations a general north-east and south-west -strike, and to dislocate them by great faults with the same average trend. -But the fissures of the Tertiary dykes run obliquely and even at a right -angle across this prevalent older series of lines and are distinct from any -other architectonic feature in the geology of the country. They did not -arise therefore by a mere renewal of some previous order of disturbances, -but were brought about by a new set of movements to which it is difficult -to find any parallel in the earlier records of the region.<a id="FNanchor_213" href="#Footnote_213" class="fnanchor">[213]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_213" href="#FNanchor_213" class="label">[213]</a> The only other known example of such a dyke-structure in Britain is that of the Pre-Cambrian -series of dykes in the Lewisian gneiss of Sutherland, described in Chapter viii.</p> - -</div> - -<p>We have further to remember that the fissures were not produced -merely by one great disturbance. The evidence of the dykes proves beyond -question that some of them are earlier than others, and hence that the cause -to which the fissures owed their origin came into operation repeatedly during -<span class="pagenum" id="Page_177">- 177 -</span> -the protracted Tertiary volcanic period. One of the most instructive lessons -in this respect is furnished by the huge eruptive masses of gabbro and granitoid -rocks in Skye. These materials have been erupted through the plateau-basalts. -The granitoid bosses are the younger protrusions, for they send -veins into the gabbros; but their appearance was later than that of some of -the dykes and older than that of others. Nevertheless, the youngest dykes -generally maintain the usual north-westerly trend across the thickest masses -of the granophyre. Thus we perceive that, even after the extrusion of -thousands of feet of such solid crystalline igneous rocks, covering areas of -many square miles, the fissuring of the ground was renewed, and rents -were opened through these new piles of material. From the evidence of -the dykes also we learn that some fissures were repeatedly re-opened and -admitted a new ascent of molten magma between their walls. The general -direction of the fissures remained from first to last tolerably uniform. Here -and there indeed, where one set of dykes traverses another, as in Skye and -the basin of the Clyde, we meet with proofs of a deviation from the normal -trend. But it is remarkable that dykes which pierce the latest eruptive -bosses of the Inner Hebrides rose in fissures that were opened in the -normal north-westerly line through these great protrusions of basic and acid -rock.</p> - -<p>Such a gigantic system of parallel fissures points to great horizontal -tension of the terrestrial crust over the area in which they are developed. -Hopkins, many years ago, discussed from the mathematical side the cause of -the production of such fissures.<a id="FNanchor_214" href="#Footnote_214" class="fnanchor">[214]</a> He assumed the existence of some elevatory -force acting under considerable areas of the earth's crust at any assignable -depth, either with uniform intensity at every point or with a somewhat -greater intensity at particular points. He did not assign to this force -any definite origin, but supposed it "to act upon the lower surface of the -uplifted mass through the medium of some fluid, which may be conceived to -be an elastic vapour, or, in other cases, a mass of matter in a state of -fusion from heat."<a id="FNanchor_215" href="#Footnote_215" class="fnanchor">[215]</a> He showed that such an upheaving force would produce -in the affected territory a system of parallel longitudinal fissures, -which, when not far distant from each other, could only have been formed -simultaneously, and not successively; that each fissure would begin not at -the surface but at some depth below it, and would be propagated with -great velocity; that there would be more fissures at greater than at lesser -depths, many of them never reaching the surface; that they would be of -approximately uniform width, the mean width tending to increase downwards; -that continued elevation might increase these fissures, but that new -fissures in the same direction would not arise in the separated blocks which -would now be more or less independent of each other; that subsequent subsidences -would give rise to transverse fissures, and by allowing the separated -blocks to settle down would cause irregularities in the width of the great -parallel fissures. He considered also the problem presented by those cases -where the ruptures of the terrestrial crust have been filled with igneous -<span class="pagenum" id="Page_178">- 178 -</span> -matter, and now appear as dykes. "The results above obtained," he says, -"will manifestly hold equally, whether we suppose the uplifted mass acted -upon immediately through the medium of an elastic vapour or by matter in -a state of fusion in immediate contact with its lower surface. In the latter -case, however, this fused matter will necessarily ascend into the fissures, and -if maintained there till it cools and solidifies, will present such phenomena -as we now recognize in dykes and veins of trap."</p> - -<div class="footnote"> - -<p><a id="Footnote_214" href="#FNanchor_214" class="label">[214]</a> <i>Cambridge Phil. Trans.</i> vi. (1835), p. 1.</p> - -<p><a id="Footnote_215" href="#FNanchor_215" class="label">[215]</a> <i>Ibid.</i> p. 10.</p> - -</div> - -<p>The existence of a vast lake or reservoir of molten rock under the -fissure-region of Britain is demonstrated by the dykes. But, if we inquire -further what terrestrial operation led to the uprise of so vast a body of lava -towards the surface in older Tertiary time, we find that as yet no satisfactory -answer can be given.</p> - -<p>2. In some districts the dykes can be connected with the gabbros which -occur as intrusive sills and irregular bosses in the basalt-plateaux and -among older rocks. The gabbros, however, are traversed by still later dykes, -which must then be independent of any visible mass of these rocks. The connection -of dykes with the gabbros is what we might naturally expect to find, -if the more coarsely crystalline rock represents portions of the basic magma -which consolidated at some depth below the surface. If we could penetrate -deep enough, it is not improbable that the dykes might be found in large -measure to shade downward into vast bodies of gabbro. Such a relation -has been observed in the Yellowstone district, where Mr. Iddings has -noticed that the centre toward which the dykes of the Old Crandale volcano -converge is a large mass of granular gabbro, passing into diorite, the dykes -becoming rapidly coarser in grain as they approach the gabbro-core.<a id="FNanchor_216" href="#Footnote_216" class="fnanchor">[216]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_216" href="#FNanchor_216" class="label">[216]</a> <i>Journ. Geol.</i> i. (1893), p. 608.</p> - -</div> - -<p>3. The rise of molten rock in thousands of fissures over so wide a region -is to my mind by far the most wonderful feature in the history of volcanic -action in Britain. The great plateaux of basalt, and the mountainous bosses -of rock by which they have been disrupted, are undoubtedly the most -obvious memorials of Tertiary volcanism. But, after all, they are merely -fragments restricted to limited districts. The dykes, however, reveal to us -the extraordinary fact that, at a period so recent as older Tertiary time, -there lay underneath the area of Britain a reservoir or series of reservoirs of -lava, the united extent of which must have exceeded 40,000 square miles.</p> - -<p>That the material of the dykes rose in general directly from below, and -was not, except locally, injected laterally along the open fissures, may be inferred, -although proof of such lateral injection on a small scale may here and -there be detected. The narrowness of the rents, and their enormous relative -length, make it physically impossible that molten rock could have moved -along them for more than short distances. The usual homogeneous character -of the dyke-rocks, the remarkable scarcity of any broken-up consolidated -fragments of them immersed in a matrix of different grain, the general -uniformity of composition and structure from one end of a long dyke to -another, the spherical form of the amygdales, the usual paucity of fragments -from the fissure walls—all point to a quiet welling of the lava upward. -<span class="pagenum" id="Page_179">- 179 -</span> -Over the whole of the region traversed by the dykes, from the hills of -Yorkshire and Lancashire to the remotest Hebrides, molten rock must have -lain at a depth, which, in one case, we know to have exceeded three miles, -and which was probably everywhere considerably greater than that limit.</p> - -<p>Forced upwards, partly perhaps by pressure due to terrestrial contraction -and partly by the enormous expansive force of the gases and vapours -absorbed within it, the lava rose in thousands of fissures that had been -opened for it in the solid overlying crust. That in most cases its ascent -terminated short of the surface of the ground may reasonably be inferred. -At least, we know, that many dykes do not reach the present surface, and -that those which do have shared in the enormous denudation of the -surrounding country. That even in the same dyke the lava rose hundreds -of feet higher at one place than at another is abundantly proved. When, -however, we consider the vast number of dykes that now come to the light -of day, and reflect that the visible portions of some of them differ more than -3000 feet from each other in altitude, we can hardly escape the conviction -that it would be incredible that nowhere should the lava have flowed out at -the surface. Subsequent denudation has undoubtedly removed a great -thickness of rock from what was the surface of the ground during older -Tertiary time, and hundreds of dykes are now exposed that doubtless -originally lay deeply buried beneath the overlying part of the earth's crust -through which they failed to rise. But some relics, at least, of the outflow -of lava might be expected to have survived. I believe that such relics -remain to us in the great basalt-plateaux of Antrim and the Inner Hebrides. -These deep piles of almost horizontal sheets of basalt, emanating from no -great central volcanoes, but with evidence of many local vents, appear to me -to have proceeded in large measure from dykes which, communicating with -the surface of the ground, allowed the molten material to flow out in -successive streams with occasional accompaniments of fragmentary ejections.<a id="FNanchor_217" href="#Footnote_217" class="fnanchor">[217]</a> -The structure of the basalt-plateaux, and their mode of origin, will form the -subject of the next division of this volume.</p> - -<div class="footnote"> - -<p><a id="Footnote_217" href="#FNanchor_217" class="label">[217]</a> It is interesting to note that in the great paper on Physical Geology already cited, Hopkins -considered the question of the outflow of lava from the fissures which he discussed. "If the -quantity of fluid matter forced into these fissures," he says, "be more than they can contain, it -will, of course, be ejected over the surface; and if this ejection take place from a considerable -number of fissures, and over a tolerably even surface, it is easy to conceive the formation of a bed of -the ejected matter of moderate and tolerably uniform thickness, and of any extent" (<i>op. cit.</i> <a href="#Page_71">p. 71</a>).</p> - -</div> - -<p>We can hardly suppose that the lava flowed out only in the western -region of the existing plateaux. Probably it was most frequently emitted -and accumulated to the greatest depth in that area. But over the centre of -Scotland and North of England there may well have been many places -where dykes actually communicated with the outer air, and allowed their -molten material to stream over the surrounding country, either from -open fissures or from vents that rose along these. The disappearance of -such outflows need cause no surprise, when we consider the extent of the -denudation which many dykes demonstrate. I have elsewhere shown that -all over Scotland there is abundant proof that hundreds and even thousands -<span class="pagenum" id="Page_180">- 180 -</span> -of feet of rock have been removed from parts of the surface of the land since -the time of the uprise of the dykes.<a id="FNanchor_218" href="#Footnote_218" class="fnanchor">[218]</a> The evidence of this denudation is -singularly striking in such districts as that of Loch Lomond, where the -difference of level between the outcrop of the dykes on the crest of the -ridges and in the bottom of the valleys exceeds 3000 feet. It is quite -obvious, for example, that had the deep hollow of Loch Lomond lain, as it -now does, in the pathway of these dykes, the molten rock, instead of -ascending to the summits of the hills, would have burst out on the floor of -the valley. We are, therefore, forced to admit that a deep glen and lake-basin -have been in great measure hollowed out since the time of the dykes. -If a depth of many hundreds of feet of hard crystalline schists could have -been removed in the interval, there need be no difficulty in understanding -that by the same process of waste, many sheets of solid basalt may have been -gradually stripped off the face of Central Scotland and Northern England.</p> - -<div class="footnote"> - -<p><a id="Footnote_218" href="#FNanchor_218" class="label">[218]</a> <i>Scenery of Scotland</i>, 2nd edit. (1887), p. 149. But see the remarks already made (p. 150) on -the curious coincidence sometimes observable between the upper limit of a dyke and the overlying -inequalities of surface.</p> - -</div> - -<p>The association of fissures and dykes with the accumulation of thick -and extensive volcanic plateaux, over so wide a region of North-western -Europe as from Antrim to the North of Iceland, finds its parallel in different -parts of the world. One of the closest analogies presents itself among the -Ghauts of the Bombay Presidency, where vast basaltic sheets, probably of -Cretaceous age, display topographical and structural features closely similar -to those of the Tertiary volcanic plateaux of the British Isles. The dykes -connected with these Indian basaltic outflows correspond almost exactly in -their general character and stratigraphical relations to those of this country. -They occur in great numbers, rising through every rock in the district up -to the crests of the Ghauts, 4000 feet above the sea. They vary from 1 or -2 to 10, 20, 40, and even occasionally 100 or 150 feet in width, and are -often many miles in length. They observe a general parallelism in one -average direction, and show no perceptible difference in character even when -traced up to elevations of 3000 and 4000 feet.<a id="FNanchor_219" href="#Footnote_219" class="fnanchor">[219]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_219" href="#FNanchor_219" class="label">[219]</a> Mr. G. T. Clark, <i>Quart. Journ. Geol. Soc.</i> xxv. (1869) p. 163. For remarks on the connection -of dykes with superficial lavas, see <i>postea</i>, <a href="#Page_268">p. 268</a>.</p> - -</div> - -<p>Thousands of square miles in the Western States and Territories of the -American Union have been similarly flooded with basic lavas. Denudation -has not yet advanced far enough to lay bare much of the platform on which -these lavas rest. But the dykes that traverse the rocks outside of the lava-deserts -afford an example of the structure which will ultimately be revealed -when the wide and continuous basalt-plains shall have been trenched by -innumerable valleys and reduced to fragmentary plateaux with lofty -escarpments (<a href="#Page_267">p. 267</a>).</p> - -<p>It is to the modern eruptions of Iceland, however, that we turn for the -completest illustration of the phenomena connected with dykes and fissures. -An account of these eruptions will therefore be given in Chapter xl. as -an explanation of the history of the Tertiary basalt-plateaux of Britain.</p> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_181">- 181 -</span></p> - -<h2 class="nobreak" id="CHAPTER_XXXVI">CHAPTER XXXVI<br /> - -<span class="smaller">THE PLATEAUX</span></h2> -</div> - -<div class="blockquot"> - -<p>Nature and Arrangement of the Rocks: 1. <span class="smcap">Lavas.</span>—Basalts, Dolerites, Andesites—Structure -of the Lavas in the Field—2. <span class="smcap">Fragmental Rocks.</span>—Agglomerates, -Conglomerates, and Breccias—Tuffs and their accompaniments.</p> -</div> - - -<p>We have now to consider the structure and history of those volcanic masses -which, during Tertiary time, were ejected to the surface within the area of -the British Islands, and now remain as extensive plateaux. Short though -the interval has been in a geological sense since these rocks were erupted, it -has been long enough to allow of very considerable movements of the ground -and of enormous denudation, as will be more fully discussed in Chapters -xlviii. and xlix. Hence the superficial records of Tertiary volcanic -action have been reduced to a series of broken and isolated fragments. -I have already stated that no evidence now remains to show to what -extent there were actual superficial outbursts of volcanic material over -much of the dyke-region of Britain. The subsequent waste of the surface -has been so enormous that various lava-fields may quite possibly have -stretched across parts of England and Scotland, whence they have since -been wholly stripped off, leaving behind them only that wonderful system -of dykes from which their molten materials were supplied.</p> - -<p>There can be little doubt, however, that whether or not other Phlegrean -fields extended over portions of the country whence they have since been -worn away, the chief volcanic tract lay in a broad and long hollow that -stretched from the south of Antrim to the Minch. From the southern to -the northern limit of the fragmentary lava-fields that remain in this -depression is a distance of some 250 miles, and the average breadth of -ground within which these lava-fields are preserved may be taken to range -from 20 to 50 miles. If, therefore, the sheets of basalt and layers of tuff -extended over the whole of this strip of country, they covered a space of -some 7000 or 8000 square miles. But they were not confined to the area -of the British Islands. Similar rocks rise into an extensive plateau in the -Faroe Islands, and it may reasonably be conjectured that the remarkable -submarine ridge which extends thence to the North-west of Scotland, and -separates the basin of the Atlantic from that of the Arctic Ocean, is partly -<span class="pagenum" id="Page_182">- 182 -</span> -at least of volcanic origin. Still further north come the extensive Tertiary -basaltic plateaux of Iceland, while others of like aspect and age cover a vast -area in Southern Greenland. Without contending that one continuous belt -of lava-streams stretched from Ireland to Iceland and Greenland, we can -have no doubt that in older Tertiary time the north-west of Europe was the -scene of more widely-extended volcanic activity than had shown itself at any -previous period in the geological history of the whole continent. The present -active vents of Iceland and Jan Mayen are not improbably the descendants -in uninterrupted succession of those that supplied the materials of the Tertiary -basaltic plateaux, the volcanic fires slowly dying out from south to north. -But so continuous and stupendous has been the work of denudation in these -northern regions, where winds and waves, rain and frost, floe-ice and glaciers -reach their highest level of energy, that the present extensive sheets of -igneous rock can be regarded only as magnificent relics, the grandeur of -which furnishes some measure of the magnitude of the last episode in the -extended volcanic history of Britain.</p> - -<p>The long and wide western valley in which the basalt-plateaux of this -country were accumulated seems, from a remote antiquity, to have been a -theatre of considerable geological activity. There are traces of some such -valley or depression even back in the period of the Torridon Sandstone of -the north-west. This formation, as we have seen, was laid down between -the great ridge of the Outer Hebrides and some other land to the east, of -which a few of the higher mountains, once buried under the sandstone, are now -being revealed by denudation between Loch Maree and Loch Broom, and also -in Assynt. The conglomerates and volcanic rocks of Lorne may represent -the site of one of the older water-basins of this ancient hollow. The -Carboniferous rocks, which run through the North of Ireland, cross into -Cantyre, and are found even as far north as the Sound of Mull, mark how, -in later Palæozoic time, the same strip of country was a region of subsidence -and sedimentation. During the Mesozoic ages, similar operations were -continued; the hollow sank several thousand feet, and Jurassic strata to -that depth filled it up. Before the Cretaceous period, underground movements -had disrupted and irregularly upheaved the Jurassic deposits, and -prolonged denudation had worn them away, so that when the Cretaceous -formations came to be laid down on the once more subsiding depression, -they were spread out with a strong unconformability on everything older -than themselves, resting on many successive horizons of the Jurassic system, -and passing from these over to the submerged hillsides of the crystalline -schists. Yet again, after the accumulation of the Chalk, the sea-floor along -the same line was ridged up into land, and the Chalk, exposed to denudation, -was deeply trenched by valleys, and entirely removed from wide tracts which -it once covered.</p> - -<p>It was in this long broad hollow, with its memorials of repeated subsidences -and upheavals, sedimentation and denudation, that the vigour of -subterranean energy at last showed itself in volcanic outbreaks, and in the -gradual piling up of the materials of the basalt-plateaux. So far as we -<span class="pagenum" id="Page_183">- 183 -</span> -know, these outbursts were subærial. At least no trace of any marine -deposit has yet been found even at the base of the pile of volcanic rocks. -Sheet after sheet of lava was poured out, until several thousand feet had -accumulated, so as perhaps to fill up the whole depression, and once more to -change entirely the aspect of the region. But the volcanic period, long and -important as it was in the geological history of the country, came to an end. -It, too, was merely an episode during which denudation still continued active, -and since which subterranean disturbance and superficial erosion have again -transformed the topography. In wandering over these ancient lava-fields, -we see on every hand the most stupendous evidence of change. They have -been dislocated by faults, sometimes with a displacement of hundreds of feet, -and have been hollowed out into deep and wide valleys and arms of the sea. -Their piles of solid rock, hundreds of feet thick, have been totally stripped -off from wide tracts of ground which were once undoubtedly buried under -them. Hence, late though the volcanic events are in the long history of the -land, they are already separated from us by so vast an interval that there -has been time for cutting down the wide plateaux of basalt into a series of -mere scattered fragments. But the process of land-sculpture has been of -the utmost service to geology, for, by laying bare the inner structure of -these plateaux, it has provided materials of almost unequalled value and -extent for the study of one type of volcanic action.</p> - - -<h3>I. NATURE AND ARRANGEMENT OF THE ROCKS OF THE PLATEAUX</h3> - -<p>The superficial outbursts of volcanic action during Tertiary time in Britain -are represented by a comparatively small variety of rocks. These consist -almost wholly of basalts, but include a number of less basic rocks which may -be classed as andesites. Many andesitic sheets, like the andesitic dykes, -have been intruded into the basalts, and are really sills.</p> - -<p>Besides the lavas of the basaltic-plateaux there are intercalated deposits -of tuffs and breccias and large masses of agglomerate. A brief notice of the -general petrography of the various constituents of the plateaux and their -mode of occurrence will here be given. The intrusive bosses which have -disrupted the superficial lavas will be discussed in subsequent chapters.</p> - - -<h3> i. <span class="allsmcap">LAVAS</span></h3> - - -<h3>1. <i>Petrographical Characters</i></h3> - -<p>(<i>a</i>) <i>Basalts and Dolerites.</i>—In external characters these rocks range from -coarsely crystalline varieties, in which the constituent minerals may be -more or less readily detected with the naked eye or a field-lens, to dense -black compounds in which only a few porphyritic crystals may be megascopically -visible. One of their characteristic features is the presence of the -ophitic structure, sometimes only feebly developed, sometimes showing itself -<span class="pagenum" id="Page_184">- 184 -</span> -in great perfection. Many of the rocks are holocrystalline, but usually -show more or less interstitial matter; in others the texture is finer, and the -interstitial matter more developed; in no case, as far as I have observed, -are there any glassy varieties, which are restricted to the dykes and sills, -though in some of the basalts the proportion of glassy or incompletely -devitrified substance is considerable. The felspars are generally of the -characteristic lath-shaped forms, and are usually quite clear and fresh. -The augite resembles that of the dykes, occurring sometimes in large plates -that enclose the felspars, at other times in a finely granular form. Olivine -is frequently not to be detected, even by green alteration products. -Magnetite is sometimes present in such quantity as to affect the compass of -the field-geologist. Porphyritic varieties occur with large felspar phenocrysts; -but such varieties are, I think, less frequent among the plateau-rocks -than among the dykes. They are well developed in the west part of the -island of Canna, and have been described from the Faroe islands. Occasionally -the plateau lavas are full of enclosed fragments of other rocks which -have been carried up in the ascending magma.</p> - -<p>(<i>b</i>) <i>Andesites and Trachytes.</i>—Probably the majority of these rocks where -they occur intercalated between the basalts of the plateaux are, as already -remarked, intrusive sheets rather than true lavas. But they have also been -poured out intermittently among the basalts and dolerites. The most extensive -development of lavas which are readily distinguishable from the -group of plateau-basalts, and must be placed in the present series, occurs in -the island of Mull. These rocks form part of a group of pale lavas which -overlie the main mass of the plateau-basalts, and cap the mountain Ben -More, together with several of its lofty neighbours. They are interstratified -with true ophitic dolerites, and basalts showing characteristic granular augite. -They are not so heavy as the ordinary plateau-lavas, their specific gravity -ranging from 2·55 to 2·74. Externally they are light grey in colour and -dull in texture, sometimes strongly amygdaloidal, sometimes with a remarkable -platy structure, which, in the process of weathering, causes them to -split up like stratified rocks. In some of their amygdaloidal varieties the -cells are filled with epidote, which also appears in the fissures, and sometimes -even as a constituent of the rock.</p> - -<p>Specimens from this "pale group" of Ben More, when examined in -thin slices under the microscope, were found by Dr. Hatch to consist almost -wholly of felspar in minute laths or microlites, but in no instance -sufficiently definite for satisfactory determination. In one of them he -observed that each lath of felspar passed imperceptibly into those adjacent -to it; the double refraction being very weak, and the twin-striation, if -present, not being traceable.<a id="FNanchor_220" href="#Footnote_220" class="fnanchor">[220]</a> More recently my colleague, Mr. W. W. -Watts, has looked at some of the same slides. He is disposed to class the -<span class="pagenum" id="Page_185">- 185 -</span> -rocks rather with the trachytes than the andesites. He remarks that "in -the apparent holocrystalline character, the size and shape of the felspars, -the sort of damascened appearance in polarized light, the finely scattered -iron-ores and the presence of a pale green hornblende, possibly augite, in -small, often complex, grains, these rocks much resemble the Carboniferous -trachytes of the Garlton Hills in Scotland."</p> - -<div class="footnote"> - -<p><a id="Footnote_220" href="#FNanchor_220" class="label">[220]</a> In the course of my investigations I have had many hundreds of thin slices cut from the Tertiary -volcanic rocks for microscopic determination. These I have myself studied in so far as their -microscopic structure appeared likely to aid in the investigation of those larger questions of geological -structure in which I was more especially interested. But for further and more detailed study I -placed them with Dr. Hatch, who submitted to me the results of his preliminary examination, and -where these offered points of geological import I availed myself of them in the memoir published -in 1888 in the <i>Transactions of the Royal Society of Edinburgh</i>. I have retained most of these -citations in their place in the present volume, and have supplemented them by notes supplied -to me from fresh observations by Mr. Watts and Mr. Harker. Professor Judd, in a series of -valuable papers, has discussed the general petrography of the Tertiary volcanic rocks (<i>Quart. -Jour. Geog. Soc.</i> vols. xxxix. xli. xlii. xlvi. xlix.)</p> - -</div> - -<p>One of the most interesting lavas of the Tertiary volcanic series is the -"pitchstone-porphyry" of the Scuir of Eigg. This rock, the latest known outflow -of lava in any of the volcanic areas of Britain, was formerly classed with -the acid series. Microscopical and chemical analyses prove it, however, to be -of intermediate composition, and to be referable to the andesites or dacites. -It is more particularly described in <a href="#CHAPTER_XXXVIII">Chapter xxxviii</a>.</p> - -<p>Professor Judd, collecting the andesitic rocks as a whole (both lavas and -sills), has grouped them into amphibole and mica-andesites, and pyroxene-andesites.<a id="FNanchor_221" href="#Footnote_221" class="fnanchor">[221]</a> -The thick lumpy and non-persistent sheets of these rocks -sometimes found near the centres of protrusion of the gabbros and granophyres -are probably sills.</p> - -<div class="footnote"> - -<p><a id="Footnote_221" href="#FNanchor_221" class="label">[221]</a> <i>Quart. Journ. Geol. Soc.</i> vol. xlvi. (1890), p. 356. Professor Judd has there described -under the name of "propylites" various members of the volcanic series which he believes to -have undergone alteration from solfataric action. I have not been able to discover any trace of -such action, but I have found that the lavas of the plateaux assume a peculiar condition where -they have been affected by large intrusive masses of granophyre or gabbro. (See <i>postea</i>, -<a href="#CHAPTER_XLVI">Chapter xlvi</a>.)</p> - -</div> - -<p>(<i>c</i>) <i>Rhyolites.</i>—In the Antrim plateau a group of rhyolite bosses occurs, -some of which have been claimed as superficial lavas. In some cases it can -be demonstrated that they are intrusive, and in no instance can they be -decisively shown to have escaped in streams at the surface. It is probable, -however, that some of these bosses did actually communicate with the outer -air, for between the lower and upper group of basalts in this plateau, -bands of rhyolitic conglomerate occur which may indicate the degradation -of exposed masses of rhyolite. The description of these Antrim bosses -will be given in <a href="#CHAPTER_XLVII">Chapter xlvii</a>., in connection with the acid eruptive rocks -of the Tertiary volcanic series.</p> - - -<h3>2. <i>Structure in the Field</i></h3> - -<p>Passing now to the consideration of the lavas as they are built up into -the plateaux, we have to note their distinctive characters as individual -sheets of rock, and their influence on the topography of the regions in -which they occur. Every tourist who has sailed along the cliffs of Antrim, -Mull, Skye, or the Faroe Islands is familiar with the singular terraced -structure of the great volcanic escarpments which stretch as mural precipices -<span class="pagenum" id="Page_186">- 186 -</span> -along these picturesque shores. Successive sheets of lava, either horizontal -or only gently inclined, rise above each other from base to summit of the -cliffs as parallel bars of brown rock with intervening strips of bright green -grassy slope.</p> - -<p>The geologist who for the first time visits these coast-lines is impressed -by the persistence of the same lithological characters giving rise to the same -topographical features. He soon realises that the plateaux, so imposingly -truncated by the great escarpments that spring from the edge of the sea, -are built up essentially of dark lavas—basalts and dolerites—and that -fragmental volcanic accompaniments, though here and there well developed, -play, on the whole, a quite insignificant part in the structure and composition -of these thick piles of volcanic material. Closer examination -in the field enables him to ascertain that, regarded as rock-masses, the lavas -include four distinct types:—</p> - -<p>1st. Thick, massive, prismatic or rudely-jointed sheets, rather more -coarsely crystalline and obviously more durable than the other types, inasmuch -as they project in tabular ledges and tend to retain perpendicular -faces owing to the falling away of slices of the rock along lines of vertical -joints. Many rocks of this type are undoubtedly intrusive sheets, and as -such will be further referred to in a later chapter. But the type includes -also true superficial lavas which show the characteristic slaggy or vesicular -bands at their upper and lower surfaces. The mere presence of such bands -may not be enough, indeed, absolutely to establish that the rock possessing -them flowed at the surface as a lava, for they are occasionally, though it -must be confessed rarely, exhibited by true sills. But the rough scoriaceous -top of a lava-stream, and the presence of fragments of this surface in the -overlying tuff, or wrapped round by the next succeeding lava, sufficiently -attest the true superficial outflow of the mass.</p> - -<p>2nd. Prismatic or columnar basalts, which, as at the Giant's Causeway -and Staffa, have long attracted notice as one of the most striking topographical -elements of the plateaux. Columnar structures are typical of the -more compact heavy basalts. A considerable variety is observable in the -degree of perfection of their development. Where they are least definite, -the rock is traversed by vertical joints, somewhat more regular and close-set -than those in the dolerites, by the intersection of which it is separated into -rude quadrangular or polygonal columns. The true columnar structure is -shown in two chief forms. (<i>a</i>) The rock is divided into close-fitting parallel, -usually six-sided columns; the number of sides varying, however, from three -up to nine. The columns run the whole thickness of the bed, and vary -from 8 or 10 to 40 or even 80 feet in length. They are segmented -by cross joints which sometimes, as at Giant's Causeway, take the ball-and-socket -form. Occasionally they are curved, as at the well-known Clam-shell -cave of Staffa. (<i>b</i>) The prisms are much smaller, and diverge in wavy -groups crowded confusedly over each other, but with a general tendency upwards. -This starch-like aggregation may be observed superposed directly -upon the more regular columnar form as at the Giant's Causeway and also -<span class="pagenum" id="Page_187">- 187 -</span> -at Staffa. Excellent illustrations of both these types may be seen at many -points along the sea-cliffs of the Inner Hebrides; the western coast of Skye, -the south-west side of Mull, and the cliffs of the island of Canna may be -specially cited.</p> - -<div class="figright" id="v2fig259" style="width: 326px;"> - <img src="images/v2fig259.png" width="326" height="86" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 259.</span>—Section of scoriaceous and prismatic Basalt, Camas - Tharbernish, north shore of Canna Island.</div> -</div> - -<p>Though generally rather compact, becoming indeed dense, almost -vitreous rocks in some -sheets, the columnar -basalts are often more -or less cellular throughout, -and highly slaggy -along their upper and -under surfaces. In some -cases, as in that of a -prismatic sheet which overlies the rough scoriaceous lava of Camas Tharbernish, -in the island of Canna, the rows of vesicles are disposed in lines parallel -to the under surface of the sheet (<a href="#v2fig259">Fig. 259</a>.)</p> - -<p>As already remarked with regard to the massive, rudely-jointed sheets, -many of the most perfectly columnar rocks of the plateaux are not superficial -lavas, but intrusive sills, bosses or dykes. Conspicuous examples of -such sills are displayed on the coast of Trotternish in Skye, and of the bosses -and dykes at the eastern end of Canna. To these further reference will be -made in the sequel. It is not always possible to be certain that columnar sheets -which appear to be regularly intercalated among the undoubted lavas of -the volcanic series may not be really intrusive. In some instances, indeed, -we can demonstrate that they are so, when after continuing perfectly parallel -with the lavas above and below them, they eventually break across them. -One of the most remarkable examples of this feature is supplied by the great -sill of the south-west of Stromö, in the Faroe Islands, of which I shall give -some account in <a href="#CHAPTER_XLII">Chapter xlii</a>. (Figs. <a href="#v2fig312">312</a>, <a href="#v2fig328">328</a>, <a href="#v2fig329">329</a>).</p> - -<p>3rd. Slaggy or amygdaloidal lavas without any regular jointed structure, -but often with roughly scoriform upper and under layers, and tending to decay -into brown earthy debris. Some of the upper surfaces of such sheets among -the Tertiary basalt-plateaux must have resembled the so-called "Aa" of the -Sandwich Islands. A striking example of the structure may be noticed at -Camas Tharbernish, on the north coast of the Island of Canna. There the -hummocks on the upper surface of a slaggy basalt measure about 15 feet -in breadth, and rise about three feet above the hollows between them, like a -succession of waves (see <a href="#v2fig259">Fig. 259</a>). The steam-holes are disposed in a general -direction parallel to the strike of the hummocks.</p> - -<p>Great variety obtains in the size and shape of the vesicles. Huge -cavities a foot or more in diameter may occasionally be found, and from such -extremes every gradation may be traced down to minute pore-like vacuoles -that can hardly be made out even with a strong lens. In regard to the -deformation of the vesicles, it is a familiar general rule that they have -been drawn out in the direction of the flow of the original lava. Occasionally -they have become straight, narrow, sometimes bifurcating pipes, several -<span class="pagenum" id="Page_188">- 188 -</span> -inches long, and only an eighth of an inch or so in diameter.<a id="FNanchor_222" href="#Footnote_222" class="fnanchor">[222]</a> A number of -such pipes, parallel to each other, resembles a row of worm-burrows (see -<a href="../../66492/66492-h/66492-h.htm#v2fig2">Fig. 2</a>).</p> - -<div class="footnote"> - -<p><a id="Footnote_222" href="#FNanchor_222" class="label">[222]</a> Some examples have been deposited by me in the Museum of Practical Geology, Jermyn -Street, in the case illustrating rock-structures. The elongation of the vesicles into annelide-like -tubes may also be observed among the stones in the volcanic agglomerates.</p> - -</div> - -<p>It may often be noticed that, even where the basalt is most perfectly -prismatic, it presents a cellular and even slaggy structure at the bottom. -The rock that forms the Giant's Causeway, for instance, is distinctly -vesicular, the vesicles being drawn out in a general east and west direction. -The beautiful columnar bed of Staffa is likewise slaggy and amygdaloidal -for a foot or so upwards from its base, and portions of this lower layer have -here and there been caught up and involved in the more compact material -above it. Even the bottom of the confusedly prismatic bed above the -columnar one on that island also presents a cellular texture. A similar -rock at Ardtun, in Mull, passes upward into a rugged slag and confused -mass of basalt blocks, over which the leaf-beds lie.</p> - -<p>Amygdaloidal structure is more or less developed throughout the -whole series of basalts. But it is especially marked in certain abundant -sheets, which, for the sake of distinction, are called amygdaloids. These -beds, which form a considerable proportion of the materials of every one of the -plateaux, are distinguished by the abundance and large size of their vesicles. -In some places, the cavities occupy at least as much of the rock as the solid -matrix in which they lie. They have generally been filled up with some -infiltrated mineral—calcite, chalcedony, zeolites, etc. The amygdales of the -west of Skye and of Antrim have long been noted for their zeolites. As a -consequence of their cellular texture and the action of infiltrating water -upon them, these amygdaloidal sheets are always more or less decomposed. -Their dull, lumpy, amorphous aspect contrasts well with the sharply-defined -columnar sheets above and below them, and as they crumble down they are -apt to be covered over with vegetation. Hence, on a sea-cliff or escarpment, -the green declivities between the prominent columnar basalts usually mark -the place of such less durable bands.</p> - -<p>Exceedingly slag-like lavas are to be seen among the amygdaloids, -immediately preceded and followed by beds of compact black basalt with -few or no vesicles. From the manner in which such rocks yield to the -weather, they often assume a singularly deceptive resemblance to agglomerates. -One of the best examples of this resemblance which have come under -my notice is that of the rock on which stands Dunluce Castle, on the north -coast of Antrim. Huge rounded blocks of a harder consistency than the rest -of the rock project from the surface of the cliffs, like the bombs of a true -volcanic agglomerate, while the matrix in which they are wrapped has -decayed from around them. But an examination of this matrix will soon -convince the observer that it is strongly amygdaloidal, and that the apparent -"bombs" are only harder and less cellular portions of it. The contrast -between the weathering of the two parts of the rocks seems to have arisen -<span class="pagenum" id="Page_189">- 189 -</span> -from an original variety in the relative abundance of steam-cavities. The -origin of such nodular or pillow-like blocks has been already referred to at -pp. <a href="#Page_26">26</a> and <a href="#Page_193">193</a>. Another singular instance occurs at the foot of the -outlier of Fionn Chro (<a href="#v2fig360">Fig. 360</a>), in the island of Rum. A conspicuous band -underlying the basalts there might readily be taken for a basalt-conglomerate. -But in this case, also, the apparent matrix is found to be amygdaloidal, -and the rounded blocks are really amygdales, sometimes a foot in -length, filled or lined with quartz, chalcedony, &c.</p> - -<p>A somewhat different structure, in which, however, the appearance of -volcanic breccia or agglomerate due to explosion from a vent is simulated, -may be alluded to here. The best instance which I have observed of it -occurs at the south end of Loch-na-Mna, in the island of Eigg, within a -basalt which is remarkable for a streaky flow-structure. On the weathered -faces the streaky layers may be observed to have been broken up, and their -disconnected fragments have been involved in ordinary basalt wherein this -flow-structure is not developed, while large blocks and irregular masses are -wrapped round in a more decomposing matrix. There can be no doubt -that in such cases we see the effects of the disruption of chilled crusts, and -the entanglement of the broken pieces in the still fluid lava.</p> - -<p>It is a common belief that the filling in of the steam-cavities has taken -place long subsequent to the volcanic period, by the slow percolation of -meteoric water through the rock. I believe, however, that at least in some -cases, if not in all, the conversion of the vesicular lavas into amygdaloids was -effected during the volcanic period. Thus it can be shown that the basalts -which have been disrupted by the gabbros and granophyres were already -amygdaloids before these basic intrusions disturbed them, for the kernels -of calcite, zeolite, etc., have shared in the general metamorphism induced in -the enclosing rock. Again, the blocks of amygdaloid contained in the -agglomerates of the volcanic series are in every respect like the amygdaloidal -lavas of the plateaux. It would thus seem that the infilling of the cavities -with mineral secretions was not merely a long secular process of infiltration -from the cool atmosphere, but was more rapidly completed by the operation of -warmer water, either supplied from volcanic sources or heated by the still -high temperature of the cellular lavas into which it descended from the -surface.<a id="FNanchor_223" href="#Footnote_223" class="fnanchor">[223]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_223" href="#FNanchor_223" class="label">[223]</a> Professor J. D. Dana, originally an advocate of infiltration from above, subsequently -supported the view that the kernels of amygdaloids were filled in by the action of moisture -within the rocks during the time of cooling.—<i>Amer. Journ. Sci.</i> ser. 3, vol. xx. (1880), p. 331. -Messrs. Harker and Marr have demonstrated that the Lower Silurian vesicular lavas of the Lake -district had already become amygdaloids before the uprise of the Shap granite.—<i>Quart. Journ. -Geol. Soc.</i> vol. xlix. (1893).</p> - -</div> - -<p>4th. Banded or stratiform lavas, consisting of successive parallel layers -or bands which weather into projecting ribs and flutings. The deceptive -resemblance to sedimentary rocks thus produced has no doubt frequently led -to these lavas being mistaken for tuffs. As I have recently found them to -be much more plentiful than I had supposed, a more detailed description of -them seems to be required.</p> - -<p><span class="pagenum" id="Page_190">- 190 -</span></p> - -<p>The banded character arises from marked distinctions in the texture of -different layers of a lava-sheet. In some cases (<i>a</i>) these distinctions arise -from differences in the size of the crystals or in the disposition of the component -minerals of the rock; in others (<i>b</i>) from the varying number and -size of the vesicles, which may be large or abundantly crowded together in -some layers, and small or only sparsely developed in others. The structure -thus points to original conditions of the lava at the time of its emission and -may be regarded as, to some extent, a kind of flow-structure on a large scale.</p> - -<p>(<i>a</i>) Where the banding is due to differences of crystalline texture, the -constituent felspars, augites, and iron-ores may be seen even with the naked -eye as well-defined minerals along the prominent surfaces of the harder ribs, -while the broader intervening flutings of finer material show the same -minerals in minuter forms. The alternating layers of coarser and finer -crystallization lie, on the whole, parallel with the upper and under surfaces -of the sheets in which they occur. But they likewise undulate like the -streaky lines in ordinary flow-structure.</p> - -<p>Banded structure of this type may be seen well developed in the lower -parts of the basalt-plateaux throughout the Inner Hebrides and the Faroe -Islands. A specimen taken from the west end of the island of Sanday, near -Canna, which showed the structure by a conspicuous parallel fluting on -weathered surfaces, was sliced for microscopical examination. Mr. Harker -has been kind enough to supply me with the following observations regarding -this slice:—</p> - -<p>"In the slice [6660]<a id="FNanchor_224" href="#Footnote_224" class="fnanchor">[224]</a> the banding becomes less conspicuous under the -microscope. The rock is of basaltic composition, and, with reference to its -micro-structure, might be styled a fine-grained olivine-diabase or olivine-dolerite -in some parts of the slice, an olivine-basalt in others. It consists -of abundant grains of olivine, imperfect octahedra and shapeless granules of -magnetite, little simple or twinned prisms of labradorite, and a pale brown -augite. The last-named mineral is always the latest product of consolidation, -but it varies in habit, being sometimes in ophitic patches moulded upon or -enclosing the other minerals, sometimes in small granules occupying the -interstices between the felspars and other crystals. The ophitic habit predominates -in the slice, while the granulitic comes in especially along certain -bands. If the former be taken as indicative of tranquil conditions, the -latter of a certain amount of movement in the rock during the latest stages -of its consolidation, the banding, though not strictly a flow-structure, may be -ascribed in some degree to a flowing movement of the nearly solidified rock. -There is, however, more than this merely structural difference between the -several bands. They differ to some extent in the relative proportions of the -minerals, especially of olivine and augite; which points to a considerable -flowing movement at an early stage in a magma which was initially not -homogeneous."</p> - -<div class="footnote"> - -<p><a id="Footnote_224" href="#FNanchor_224" class="label">[224]</a> The figures within square brackets throughout the following pages refer to the numbers of -the microscopic slides in the Geological Survey collection, where I have deposited all those prepared -from my specimens.</p> - -</div> - -<p><span class="pagenum" id="Page_191">- 191 -</span></p> - -<p>(<i>b</i>) Where the banding arises from the distribution of the vesicles, somewhat -similar weathered surfaces are produced. In some instances, while the basalt -is throughout finely cellular, interposed bands of harder, rather finer-grained -and less thoroughly vesicular character serve to give the stratified appearance. -Instances may be observed where the vesicles have been crowded together in -certain bands, which consequently weather -out differently from the layers above and -below them. An excellent illustration of -this arrangement occurs in the lowest -lava but one of the largest of the three -picturesque stacks known as Macleod's -Maidens on the west coast of Skye (Figs. -260, 283, 284 and 287). This lava is -thoroughly amygdaloidal, but the vesicles -are specially crowded together in certain -parallel bands from an inch to three or four -inches thick. Some of these layers lie close to each other, while elsewhere -there may be a band of more close-grained, less vesicular material between -them. But the most singular feature of the rock is to be seen in the shape -and position of the vesicles that are crowded together in the cellular bands. -Instead of being drawn out into flattened forms in the general direction of -banding, they are placed together at high angles. Each layer remains -parallel to the general bedding, but its vesicles are steeply inclined in one -direction, which was doubtless that of the flow of the still unconsolidated -lava.<a id="FNanchor_225" href="#Footnote_225" class="fnanchor">[225]</a> Weathering along these bands, the lava might easily be mistaken at -a little distance for a tuff or other stratified intercalation.</p> - -<div class="footnote"> - -<p><a id="Footnote_225" href="#FNanchor_225" class="label">[225]</a> This elongation of vesicles, more or less perpendicular to the general bedding, may be noticed -sometimes even in sills, as will be shown in a later Chapter.</p> - -</div> - -<div class="figright" id="v2fig260" style="width: 205px;"> - <img src="images/v2fig260.png" width="205" height="125" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 260.</span>—Banded amygdaloidal basalt - showing layers of elongated and steeply - inclined vesicles, Macleod's Maidens, - Skye.</div> -</div> - - -<p>Banded lavas possessing the characters now described are of frequent -occurrence among the Inner Hebrides. Many striking examples of them -may be seen along the west coast of Skye. Still more abundant in Faroe, -they form one of the most conspicuous features in the geology of that group -of islands. Along the whole of its western seaboard, on island after island, -they are particularly prominent in the lower parts of the precipices, while -the upper parts consist largely of amorphous or prismatic sheets. So much -do they resemble stratified rocks that it was not until I had landed at -various points that I could satisfy myself that they are really banded lavas.<a id="FNanchor_226" href="#Footnote_226" class="fnanchor">[226]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_226" href="#FNanchor_226" class="label">[226]</a> For recent contributions to the Geology of the Faroe Islands, see Prof. James Geikie, <i>Trans. -Roy. Soc. Edin.</i> vol. xxx. (1880), p. 217, where the banding of the basalts is noticed; Prof. A. -Helland, <i>Dansk. Geografisk. Tidskr.</i> (1881); R. Bréon, <i>Notes pour servir à l'étude de la Géologie -de l'Islande et des Isles Faeroe</i> (1884); Mr. J. Lomas, <i>Proc. Geol. Soc. Liverpool</i>, vol. vii. (1895), p. -292. Various writers have treated of the petrography of Faroe, particularly A. Osann, <i>Neues -Jahrb.</i> (1884), vol. i. p. 45, and M. Bréon in the volume here cited.</p> - -</div> - -<p>5th. Ordinary flow-structure, save in these banded lavas, is rather rare -among the plateaux. It may, however, be occasionally observed, where there -is no distinct banding. On a weathered surface it appears in fine, widely -parallel streaks, which are sometimes wavy, puckered and broken up, as in -<span class="pagenum" id="Page_192">- 192 -</span> -rhyolites and felsites, while the porphyritic felspars are arranged with their -long axes in the direction of flow. A good example of these characters -may be seen on the summit of the Dùn Can—the remarkable truncated cone -which forms the highest point on the Island of Raasay. The rock is a -black olivine-basalt, partly amygdaloidal, with zeolites filling up the cavities, -and its flow-lines are prominent on the weathered faces where they lie -parallel to the general bedding of the lavas. Another illustration may be -observed in the basalt already cited from Loch-na-Mna, in the island of -Eigg, where the rock presents in places a remarkable streaky structure -which, though hardly visible on a fresh fracture, reveals itself on a weathered -face in thin nearly parallel ribs coincident in direction with the upper and -under surfaces of the mass.</p> - -<p>Great variety is to be found in the thickness of different sheets of lava -in the plateaux. Some of them are not more than 6 or 8 feet; others reach -to 80 or 100 feet, and sometimes, though rarely, to even greater dimensions. -In Antrim, the average thickness of the flows is probably from 15 to 20 -feet.<a id="FNanchor_227" href="#Footnote_227" class="fnanchor">[227]</a> In the fine coast-sections at the Giant's Causeway, however, some -bands may be seen far in excess of that measurement. The bed that forms -the Causeway, for instance, is about 60 or 70 feet thick, and seems to -become even thicker further east. Along the great escarpment, 700 feet -high, which rises from the shores of Gribon, on the west coast of Mull, there -are twenty separate beds, which give an average of 35 feet for the thickness -of each flow. On the great range of sea-precipices along the west coast of -Skye, which present the most stupendous section of the basalts anywhere to -be seen within the limits of the British Islands, the average thickness of the -beds can be conveniently measured. At the Talisker cliffs some of the flows -are not more than 6 or 8 feet; others are 30 or 40 feet. The chief precipice, -957 feet high (<a href="#v2fig286">Fig. 286</a>), contains at least 18 or 20 separate lava-sheets, -which thus average of from 47 to 53 feet in thickness. In the cliffs that form -the seaward margin of the tableland of Macleod's Tables (<a href="#v2fig283">Fig. 283</a>) fourteen -successive beds of basalt can be counted in a vertical section of 400 feet, -which is equal to an average thickness of about 28 feet. But some of the -basalts are only about 6 feet thick, while others are 50 or 60. The Hoe of -Duirinish, 759 feet high, is composed of about sixteen distinct beds, which -thus have a mean thickness of 46 feet. The average thickness of the -successive flows on Dunvegan Head, which is 1000 feet high and contains -at least twenty-five separate sheets, is about 40 feet. Still further north, -the cliffs, 800 feet high, comprise sixteen successive flows, which have thus -an average of 50 feet each. Among the Faroe Islands the average thickness -of the basalt-sheets seems to be nearly the same as in Britain. Thus in the -magnificent ranges of precipices of Kalsö, Kunö and Borö, forty or more -sheets may be counted in the vast walls of rock some 2000 feet high, giving -a mean of about 50 feet.</p> - -<div class="footnote"> - -<p><a id="Footnote_227" href="#FNanchor_227" class="label">[227]</a> See Explanation of Sheet 20, Geol. Survey, Ireland, p. 11.</p> - -</div> - -<p>Each bed appears, on a cursory inspection, to retain its average thickness, -and to be continuous for a long distance. But I believe that this -<span class="pagenum" id="Page_193">- 193 -</span> -persistence is in great measure deceptive. We can seldom follow the same -bed with absolutely unbroken continuity for more than a mile or two. Even -in the most favourable conditions, such as are afforded by a bare sea-cliff on -which every sheet can be seen, there occur small faults, gullies where the -rocks are for the time concealed, slopes of debris, and other failures of continuity; -while the rocks are generally so like each other, that on the further -side of any such interruption, it is not always possible to make sure that we -are still tracing the same bed of basalt which we may have been previously -following. On the other hand, a careful examination of one of these great -natural sections will usually supply us with proofs that, while the bedded -character may continue well marked, the individual sheets die out, and are -replaced by others of similar character. Cases may not infrequently be -observed where the basalt of one sheet abruptly wedges out, and is replaced -by that of another. Where both are of the same variety of rock, it requires -close inspection to make out the difference between them; but where one is -a green, dull, earthy, amorphous amygdaloid, and the other is a compact, -black, prismatic basalt, the contrast between the two beds can be recognized -from a distance (<a href="#v2fig261">Fig. 261</a>). In the basaltic cliffs of the west coast of Skye, -the really lenticular character of the flows can be well seen. I may -especially cite the great headland south of Talisker Bay, already referred to, -where, in the pile of nearly horizontal sheets, two beds may be seen to die -out, one towards the north, the other towards the south. Further north, in -the cliff of the Hoe of Duirinish, a similar structure presents itself. Along -the coast-cliffs of Mull, Morven and Canna the same fact is clearly displayed. -Thus on the west side of the Sound of Mull the slopes above -Fishnish Bay show a group of basalts, which die out southward, and -are overlapped by a younger group that has been poured over their ends. -Such sections are best seen in the evening, when the grass-covered lavas -show their successive sheets by their respective shadows, their individuality -being lost in the full light of day. A more striking example occurs beyond -the west end of Glen More in Mull, where one series of basalts has been -tilted up, probably during some volcanic episode, and has had a younger -series banked up against its edges.</p> - -<div class="figcenter" id="v2fig261" style="width: 383px;"> - <img src="images/v2fig261.png" width="383" height="156" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 261.</span>—Termination of Basalt-beds, Carsaig, Mull.</div> -</div> - -<p>In Antrim also, remarkable evidence is presented of the rapid attenuation -<span class="pagenum" id="Page_194">- 194 -</span> -not of single beds only, but of a whole series of basalts. Thus, at -Ballycastle, the group of lavas known as the Lower Basalts, which underlie -the well-known horizon of iron-ore, are at least 350 feet thick. But, as we -trace them westwards, bed after bed thins out until, a little to the west of -Ballintoy, in a distance of only about 6 miles, the whole depth of the group -has diminished to somewhere about 40 feet. A decrease of more than 300 -feet in six miles or 50 feet per mile points to considerable inequalities in -the accumulation of the lavas. If the next series of flows came from -another vent and accumulated against such a gentle slope, it would be -marked by a slight unconformability. Structures of this kind are much -rarer than we should expect them to be, considering the great extent to -which the plateaux have been dissected and laid open in cliff-sections.</p> - -<p>The basalt-plateau of the Faroe Islands exhibits with remarkable clearness -the lenticular character of the basalt-sheets, and a number of examples -will be cited in the description of that region to be given in Chapter xxxix. -In these northern climes vegetation spreads less widely over rock and slope -than it does in the milder air of the Inner Hebrides. Hence the escarpments -sweep in precipices of almost bare rock from the level of the sea up -to the serrated crests of the islands, some 2000 feet in height. Each -individual bed of basalt can thus be followed continuously along the fjords, -and its variation or disappearance can be readily observed. Coasting along -these vast natural sections, we readily perceive that, as among the Western -Isles, the successive sheets of basalt have proceeded from no one common -centre of eruption. They die out now towards one quarter, now towards -another, yet everywhere retain the universal regularity and gentle inclinations -of the whole volcanic series.</p> - - -<h3> ii. <span class="allsmcap">FRAGMENTAL ROCKS</span></h3> - -<p>While the plateaux are built up mainly of successive flows of basaltic -lavas, they include various intercalations of fragmental materials, which, -though of trifling thickness, are of great interest and importance in regard -to the light which they cast on the history of the different regions during -the volcanic period. I shall enumerate the chief varieties of these rocks -here, and afterwards give fuller details regarding their stratigraphical -relations and mode of occurrence in connection with the succession of beds -in each of the plateaux.</p> - -<p>(<i>a</i>) <i>Volcanic Agglomerates.</i>—In the tumultuous unstratified masses -of fragmentary materials which fill eruptive vents in and around the -plateaux, the stones, which vary in size up to blocks several feet in -diameter, consist for the most part of basalts, often highly slaggy and -scoriaceous. They include also fragments of different acid eruptive rocks -(generally felsitic or rhyolitic in texture), with pieces of the non-volcanic rocks -through which the volcanic pipes have been drilled. The paste is granular, -dirty-green or brown in colour, and seems generally to consist chiefly of comminuted -basalt. As in the Carboniferous and Permian necks, the Tertiary -<span class="pagenum" id="Page_195">- 195 -</span> -agglomerates contain abundant detritus of a basic minutely cellular -pumice.</p> - -<p>(<i>b</i>) <i>Volcanic Conglomerates and Breccias in beds intercalated between the -flows of Basalt.</i>—These are of at least three kinds. (<i>a</i>) Basalt-conglomerates, -composed mainly of rounded and subangular blocks of basalt (or allied basic -lava), sometimes a yard or more in diameter, not unfrequently in the form -of pieces of rough slag or even of true bombs, imbedded in a granular -matrix of comminuted basalt-debris. In some cases, the stones form by far -the most abundant constituents of the rock, which then resembles some of -the coarse agglomerates just described. Perhaps the most remarkable accumulations -of this kind are those intercalated among the basalts in the islands -of Canna and Sanday, of which a detailed account will be given in Chapter -xxxviii. These conglomerates, besides their volcanic materials, contain rounded -blocks of Torridon sandstone and other rocks, which must have been carried -from the east by some tolerably powerful river that flowed across the basalt-plains -during the volcanic period. Again, on the east side of Mull, the -slaggy basalts of Beinn Chreagach Mhor are occasionally separated by volcanic -conglomerates. As a rule, however, such intercalations are seldom -more than a few feet or yards in thickness. Their coarseness and repetition -on successive horizons indicate that they probably accumulated in the near -neighbourhood of one or more small vents, from which discharges of fragmentary -materials took place at the beginning or at the close of an outflow of lava, -and that the stones were sometimes swept away from the cones and rolled -about by streams before being buried under the succeeding lava-sheets. -More commonly the dirty-green or dark-brown granular matrix exceeds in -bulk the stones embedded in it. It has obviously been derived mainly from -the trituration of already cooled basalt—masses, and probably also from -explosions of the still molten rock in the vents. A striking illustration of -this type of rock may be seen on the south side of Portree Harbour, where -a mass of dark-green basalt-conglomerate, with a coaly layer above it, lies -near the base of the bedded basalts, and attains at one part of its course a -thickness of about 200 feet. This rock will be again referred to in connection -with the vent from which its materials were probably derived. As -in the case of the agglomerates of the vents, pieces of older acid lavas, and -still more of the non-volcanic rocks that underlie the plateaux, are found in -the bedded conglomerates and breccias. In Antrim and Mull, for instance, -fragments of flint and chalk are of common occurrence. A characteristic -example of this kind of rock forms the platform of the columnar bed -out of which Fingal's Cave, Staffa, has been excavated (<a href="#v2fig266">Fig. 266</a><i>a</i>).</p> - -<p>(β) Felsitic Breccia.—This variety, though of rare occurrence, is to be -seen in a number of localities in the island of Mull. It is composed in -great measure of angular fragments of close-grained flinty felsitic or rhyolitic -rocks, sometimes showing beautiful flow-structure, together with pieces of -quartzite and amygdaloidal basalt, the dull dirty-green matrix appearing to -be made up chiefly of basalt-dust.</p> - -<p>(γ) Rhyolitic Conglomerate.—Between the upper and lower group of -<span class="pagenum" id="Page_196">- 196 -</span> -basalts in the Antrim plateau there occur bands of a pale fawn-coloured -conglomerate largely made up of more or less rounded fragments of rhyolite, -like some of the varieties of the rock which occur in place on the plateau. -The rhyolitic debris is often mixed with pebbles of basalt. Sometimes it -becomes so fine as to pass into pale clays.</p> - -<p>(δ) Breccias of non-volcanic materials.—These, the most exceptional of -all the fragmentary intercalations in the plateaux, consist almost wholly of -angular blocks of rocks which are known to underlie the basalts, but with -a variable admixture of basalt fragments. They are due to volcanic -explosions which shattered the subjacent older crust of rocks, and discharged -fragments of these from the vents or allowed them to be borne -upwards on an ascending column of lava. Pieces of the non-volcanic -platform are of common occurrence among the fragmentary accumulations, -especially in the lower parts of the plateaux basalts. But I have never -seen so remarkable an example of a breccia of this kind as that which occurs -near the summit of Sgurr Dearg, in the south-east of Mull. The bedded -basalt encloses a lenticular band of exceedingly coarse breccia, consisting -mainly of angular pieces of quartzite, with fragments of amygdaloidal -basalt. In the midst of the breccia lies a huge mass or cake of erupted -mica-schist, at least 100 yards long by 30 yards wide, as measured across -the strike up the slope of the hill. To the west, owing to the thinning out -of the breccia, this piece of schist comes to lie between two beds of basalt. -A little higher up, other smaller but still large blocks of similar schist are -involved in the basalt, as shown in <a href="#v2fig262">Fig. 262</a>. As the huge cake of mica-schist -plunges into the hill, its whole dimensions cannot be seen; but there -are visible, at least, 15,000 cubic yards, which must weigh more than -30,000 tons. Blocks of quartzite of less dimensions occur in the basalts -on Loch Spelve, in the same district. There can be no doubt, I think, that -these enormous fragments were torn off from the underlying crystalline -schists which form the framework of the Western Highlands, and were -floated upward in an ascending flow of molten basalt. Had the largest -mass occurred at or near the base of the volcanic series, its size and position -would have been less remarkable. But it lies more than 2000 feet up in -the basalts, and hence must have been borne upward for more than that -height. A similar but less striking breccia occurs on the south coast of -the same island, near Carsaig, made up chiefly of pieces of quartzite -and quartz.<a id="FNanchor_228" href="#Footnote_228" class="fnanchor">[228]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_228" href="#FNanchor_228" class="label">[228]</a> This is noticed by Mr. Starkie Gardner, <i>Quart. Journ. Geol. Soc.</i> xliii. (1887), p. 283, note.</p> - -</div> - -<p>Some remarkable agglomerates, near Forkhill, Armagh, probably belonging -to the Tertiary volcanic series, will be described in the account of the -Irish acid rocks (Chapter xlvii.). They consist entirely of non-volcanic -stones and dust and are traceable for some miles along the line of a fissure. -Where they have been discharged through granite they consist entirely of -the detritus of that rock, but where they have been erupted in the Silurian -area they consist of fragments of grits and shales. They seem to have been -produced by æriform discharges, without the uprise of any volcanic magma, -<span class="pagenum" id="Page_197">- 197 -</span> -though eventually andesite and rhyolite ascended the fissure and became -full of granitic and Silurian fragments.</p> - -<p>Some remarkable necks filled almost entirely with fragments of Torridon -Sandstone have been observed in the west of Applecross, Ross-shire, and -some curious plug-like masses of breccia, also made up of fragments of -Torridonian strata, occur in the island of Raasay. These examples will be -more particularly described on later pages (pp. <a href="#Page_292">292</a>, <a href="#Page_293">293</a>).</p> - -<p>(<i>c</i>) <i>Tuffs.</i>—The tuffs intercalated in the basalt-plateaux generally -consist essentially of basic materials, derived from the destruction of -different varieties of basalts, though also containing occasional fragments -of older felsitic rocks, as well as pieces of chalk, flint, quartz, and other non-volcanic -materials. They are generally dull, dirty-green in colour, but -become red, lilac, brown, and yellow, according to the amount and state of -combination and oxidation of their ferruginous constituents. They usually -contain abundant fragments of amygdaloidal and other basalts. As a rule, -they are distinctly stratified, and occur in bands from a few inches to 50 -feet or more in thickness. The matrix being soft and much decomposed, -these bands crumble away under the action of the weather, and contribute -to the abruptness of the basalt-escarpments that overlie them.</p> - -<div class="figcenter" id="v2fig262" style="width: 524px;"> - <img src="images/v2fig262.png" width="524" height="175" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 262.</span>—Breccia and Blocks of mica-schist, quartzite, etc., lying between - bedded Basalts, Isle of Mull.<br /><br /> - <i>a a</i>, Bedded basalts; <i>b</i>, Breccia; <i>d</i>, Basic dyke.</div> -</div> - -<p>In the group of strata between the two series of basalts in Antrim, some -of the tuffs consist chiefly of rhyolitic detritus, both glassy and lithoid.</p> - -<p>Where the tuffs become fine-grained and free from imbedded stones, -they pass into variously-coloured clays. Among these are the "bauxite" -and "lithomarge" of Antrim, probably derived from pale rhyolitic tuffs -and conglomerates (p. <a href="#Page_204">204</a>). Associated with these deposits in the -same district, is a pisolitic hæmatite, which has been proved to occur -over a considerable area on the same horizon. Many of the clays are highly -ferruginous. The red streaks that intervene between successive sheets of -basalt are of this nature (bole, plinthite, etc.). The source of the iron-oxide -is doubtless to be traced to the decomposition of the basic lavas during the -volcanic period.</p> - -<p><span class="pagenum" id="Page_198">- 198 -</span></p> - -<p>(<i>d</i>) There occur also grey and black clays and shales, of ordinary sedimentary -materials, containing leaves of terrestrial plants (leaf-beds), with -occasional wing-cases of beetles, sometimes associated with impure limestones, -but more frequently with sandstones and indurated gravels or -conglomerates containing pieces of fossil wood. These intercalated bands -undoubtedly indicate the action of running water, sometimes even of river-floods, -and the accumulation of sediment in hollows of the exposed flows of -basalt at intervals during the piling up of the successive lava-sheets that -form the plateaux. The alternation of fluviatile gravels with volcanic tuffs, -fluviatile conglomerates, and lava-streams, is admirably displayed in the -island of Canna, as will be narrated in detail in Chapter xxxviii.</p> - -<p>The vegetable matter has in some places gathered into lenticular seams -of lignite, and even occasionally of black glossy coal. Amber also has been -found in the lignite. Where the vegetation has been exposed to the action -of intrusive dykes or sheets, it has sometimes passed into the state of -graphite.</p> - -<p>The remarkable terrestrial flora found in the leaf-beds, and in association -with the lignites, was first made known by the descriptions of Edward -Forbes already referred to, and has subsequently been studied and -described by Heer, W. H. Baily, and Mr. Starkie Gardner.<a id="FNanchor_229" href="#Footnote_229" class="fnanchor">[229]</a> It was regarded -by Forbes as of Miocene age, and this view has generally been adopted by -geologists. Mr. Starkie Gardner, however, contends that it indicates a -much wider range of geological time. He believes that a succession of -floras may be recognised, the oldest belonging to an early part of the Eocene -period. Terrestrial plants, it must be admitted, are not always a reliable -test of geological age, and I am not yet satisfied that in this instance they -afford evidence of such a chronological sequence as Mr. Gardner claims, -though I am convinced that the Tertiary volcanic period was long enough -to have allowed of the development of considerable changes in the character -of the vegetation.</p> - -<div class="footnote"> - -<p><a id="Footnote_229" href="#FNanchor_229" class="label">[229]</a> On this subject consult Duke of Argyll, <i>Quart. Journ. Geol. Soc.</i> vol. vii. (1851), p. 89; E. -Forbes, <i>Ibid.</i> p. 103; W. H. Baily, <i>op. cit.</i> xxv. (1869), pp. 162, 357; <i>Brit. Assoc. Rep.</i> (1879) -p. 162; (1880) p. 107; (1881) p. 151; (1884) p. 209; Mr. J. Starkie Gardner, <i>Palæontographical -Society</i>, vols. xxxviii. xxxix. In the last of Mr. Baily's papers he notices that "the Rev. -Dr. Grainger found a portion of a fish (<i>Percidæ</i>, possibly <i>Lates</i>)." The discovery of the remains -of a fresh-water fish is an important additional testimony to the terrestrial conditions under -which the lavas were erupted. The genus <i>Lates</i> now inhabits the Nile and the Ganges.</p> - -</div> - -<p>For the purpose of the present volume, however, the precise stage in the -geological record, which this flora indicates, is of less consequence than the -broad fact that the plants prove beyond all question that the basalts among -which they lie were erupted on land during the older part of the long -succession of Tertiary periods. Their value in this respect cannot be overestimated. -Stratigraphical evidence shows that the eruptions must be later -than the Upper Chalk; but the imbedded plants definitely limit them to -the earlier half of Tertiary time.</p> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_199">- 199 -</span></p> - -<h2 class="nobreak" id="CHAPTER_XXXVII">CHAPTER XXXVII<br /> - -<span class="smaller">THE SEVERAL BASALT-PLATEAUX AND THEIR GEOLOGICAL HISTORY, -ANTRIM, MULL, MORVEN AND ARDNAMURCHAN</span></h2> -</div> - - -<p>There are five districts in North-western Europe where the original widespread -Tertiary lava-fields have been less extensively eroded than elsewhere, or -at least where they have survived in larger and thicker masses. Whether or -not each of them was an isolated area of volcanic activity cannot now be -determined. Their several outflows of lava within the area of the British -Isles may have united into one continuous volcanic tract, and their present -isolation there may be due entirely to subterranean movements and denudation. -There is a certain convenience, however, in treating the districts -separately. They are—1. Antrim; 2. Mull, Morven and Ardnamurchan; -3. Small Isles; 4. Skye; 5. The Faroe Islands.</p> - - -<h3>i. <span class="allsmcap">ANTRIM</span><span class="smaller"><a id="FNanchor_230" href="#Footnote_230" class="fnanchor">[230]</a></span></h3> - -<div class="footnote"> - -<p><a id="Footnote_230" href="#FNanchor_230" class="label">[230]</a> The basalts of Antrim are the subject of an abundant literature. I may refer particularly -to the papers of Berger and Conybeare (<i>Trans. Geol. Soc.</i> iii.), the Geological Report of Portlock, -and the Explanations of the Sheets of the Geological Survey of Ireland. Other papers will be -afterwards cited. The general features of the Antrim plateau are shown on Map VII.</p> - -</div> - -<p>The largest of the basalt-plateaux of Britain is that which forms so -prominent a feature in the scenery and geology of the North of Ireland, -stretching from Lough Foyle to Belfast Lough, and from Rathlin Island to -beyond the southern margin of Lough Neagh. Its area may be roughly -computed at about 2000 square miles. But, as its truncated strata rise -high along its borders, and look far over the surrounding low grounds, it -must be regarded as a mere fragment of the original volcanic plain. It -may be described as an undulating tableland, which almost everywhere -terminates in a range of bold cliffs, but which, towards the centre and -south, sinks gently into the basin of Lough Neagh. The marginal line of -escarpment, however, presents considerable irregularity both in height and -form, besides being liable to frequent local interruptions. It is highest on -the west side, one of its crests reaching at Mullaghmore, in County -Londonderry, a height of 1825 feet. It sinks down into the valley -of the Bann, east of which it gradually ascends, forming the well-known -range of cliffs from the Giant's Causeway and Bengore Head to Ballycastle. -<span class="pagenum" id="Page_200">- 200 -</span> -It then strikes inland, and making a wide curve in which it reaches a height -of more than 1300 feet, comes to the sea again at Garron Point. From that -headland the cliffs of basalt form a belt of picturesque ground southwards -beyond Belfast, interrupted only by valleys that convey the drainage of the -interior of the plateau to the North Channel. Above the valley of the -Lagan the crest of the plateau rises to a height of more than 1500 -feet.</p> - -<p>Throughout most of its extent the basalt-escarpment rests on the white -limestone or Chalk of Antrim, beneath which lie soft Lias shales and -Triassic marls. Here and there, where the substratum of Chalk is thin, -the action of underground water on the crumbling shales and marls below -it has given rise to landslips. The slopes beneath the base of the basalt -are strewn with slipped masses of that rock, almost all the way from -Cushendall to Larne, some of the detached portions being so large as to be -readily taken for parts of the unmoved rock. On the west side also, a -group of huge landslips cumbers the declivities beneath the mural front of -Benevenagh.</p> - -<p>I have found some difficulty in the attempt to ascertain what was the -probable form of surface over which the volcanic rocks of this plateau began -to be poured out. The Chalk sinks below the sea-level on the north coast, -but, in the outlier of Slieve Gallion, three miles beyond the western base of -the escarpment, it rises to a height of 1500 feet above the sea. On the -east side also, it shows remarkable differences of level. Thus, below the -White Head at the mouth of Belfast Lough, it passes under the sea-level, -but only 16 miles to the south, where it crops out from under the basalt, -its surface is about 1000 feet above that level. If these variations in height -existed at the time of the outpouring of the basalt, the surface of the -ground over which the eruptions took place was so irregular that some -hundreds of feet of lava must have accumulated before the higher chalk -hills were buried under the volcanic discharges. But it seems to me that -much of this inequality in the height of the upper surface of the Chalk is -to be attributed to unequal movements since the volcanic period, which -involved the basalt in their effects, as well as the platform of Chalk below -it. Had the present undulations of that platform been older than the -volcanic discharges, it is obvious that upper portions of the basalt-series -would have overlapped lower, and would have come to rest directly on the -Chalk. But this arrangement, so far as I am aware, never occurs, except -on a trifling scale. Wherever the Chalk appears, it is covered by sheets of -the lower and not of the upper of the two groups into which the Antrim -basalts are divisible. We have actual proof of considerable terrestrial disturbance, -subsequent to the date of the formation of the volcanic plateau. -Thus, near Ballycastle, a fault lets down the basalt and its Chalk platform -against the crystalline schists of that district. On the east side of the -fault, the Chalk is found far up the slope, circling round the base of the -beautiful cone of Knocklayd—an outlier of the basalt which reaches a -height of 1695 feet (<a href="#v2fig263">Fig. 263</a>). The amount of vertical displacement of -<span class="pagenum" id="Page_201">- 201 -</span> -the volcanic sheets is here 700 feet.<a id="FNanchor_231" href="#Footnote_231" class="fnanchor">[231]</a> Many other displacements, as shown -by the mapping of my colleagues in the Geological Survey, have shifted -the base of the escarpment from a few inches up to several hundred feet. -Besides actual dislocations, the Antrim plateau has undergone some marked -subsidences of which the most notable is that of Lough Neagh.<a id="FNanchor_232" href="#Footnote_232" class="fnanchor">[232]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_231" href="#FNanchor_231" class="label">[231]</a> Explanatory Memoir of Sheets 7 and 8, Geological Survey, Ireland, by Messrs. Symes, -Egan, and M'Henry (1888), p. 37.</p> - -<p><a id="Footnote_232" href="#FNanchor_232" class="label">[232]</a> These inequalities in the level of the base of the Antrim plateau will be more particularly -discussed in Chapter xlix., in connection with the subsidences and dislocations which have -affected the region since the close of the volcanic period.</p> - -</div> - -<p>It is evident, therefore, that the present position of the Chalk platform -is far from agreeing with that which it presented to the outflow of the sheets -of basalt. But, on the other hand, there can be no doubt that its surface -at the beginning of the volcanic outbursts was not a level plain. It was -probably a rolling country of low bare chalk-downs, like parts of the South-east -of England. The Irish Chalk attains its maximum thickness of -perhaps 250 feet at Ballintoy. But it is liable to rapid diminution. On -the shore at Ballycastle about 150 feet of it can be seen, its base being -concealed; but only two and a half miles to the south, on the outlier of -Knocklayd, the thickness is not quite half so much. On the west side of the -plateau also, there are rapid changes in the thickness of the Chalk. Such -variations appear to be mainly attributable to unequal erosion before the -overflow of the basalts. So great indeed had been the denudation of the -Cretaceous and underlying Secondary formations previous to the beginning of -the volcanic eruptions, that in some places the whole of these strata had been -stripped off the country, so that the older platform of Palæozoic or still more -ancient masses was laid bare. Thus, on the west side of the escarpment, -the basalt steals across the Chalk and comes to rest directly upon Lower -Carboniferous rocks.</p> - -<p>The authors who have described the junction of the Chalk and basalts -in Antrim have generally referred to the uneven surface of the former rock -as exposed in any given section. The floor on which the basalt lies is -remarkably irregular, rising into ridges and sinking into hollows or trenches, -but almost everywhere presenting a layer of earthy rubbish made of brown -ferruginous clays, mixed with pieces of flint, chalk, and even basalt.<a id="FNanchor_233" href="#Footnote_233" class="fnanchor">[233]</a> The -flints are generally reddened and shattery. The chalk itself has been -described as indurated, and its flints as partially burned by the influence of -the overlying basalt. But I have not noticed, at any locality, evidence of -alteration of the solid chalk, except where dykes or intrusive sheets have -penetrated it.<a id="FNanchor_234" href="#Footnote_234" class="fnanchor">[234]</a> There can be no doubt that the hardness of the rock is -an original peculiarity, due to the circumstances of its formation. The -irregular earthy rubble, that almost always intervenes between the chalk -and the base of the basalt, like the "clay with flints" so general over the -Chalk of Southern England, no doubt represents long-continued subærial -weathering previous to the outflow of the basalt. Even, therefore, if there -<span class="pagenum" id="Page_202">- 202 -</span> -were no other evidence, we might infer with some confidence from this -layer of rubble, that the surface over which the lavas were poured was a -terrestrial one. Here and there, too, we may detect traces of the subsidence -of the basalt into swallow-holes dissolved in the chalk subsequent to the -outflow of the basalt-sheets.</p> - -<div class="footnote"> - -<p><a id="Footnote_233" href="#FNanchor_233" class="label">[233]</a> Portlock, <i>Report on Geology of Londonderry</i>, etc. (Geological Survey), p. 117.</p> - -<p><a id="Footnote_234" href="#FNanchor_234" class="label">[234]</a> See Portlock, <i>op. cit.</i> <a href="#Page_116">p. 116</a>.</p> - -</div> - -<p>The Antrim plateau is not only the largest in the British Islands, it -is also the most continuous and regular. It may be regarded, indeed, as -one unbroken sheet of volcanic material, not disrupted by any such mountainous -masses of intrusive rock as in the other plateaux interrupt the -continuity of the horizontal or gently inclined sheets of basalt. Around its -margin, indeed, a few outliers tower above the plains, and serve as impressive -memorials of its losses by denudation. Of these, by much the most -picturesque and imposing, though not the loftiest, is Knocklayd already -referred to, which forms so striking a feature in the north-east of -Antrim (<a href="#v2fig263">Fig. 263</a>).</p> - -<div class="figcenter" id="v2fig263" style="width: 485px;"> - <img src="images/v2fig263.png" width="485" height="128" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 263.</span>—Section of Knocklayd, an outlier of the Antrim basalt-plateau lying on Chalk.<br /><br /> - 1. Crystalline schists; 2. Cretaceous strata; 3. Lower basalts; 4. Group of tuffs, clays and iron-ore; - 5. Upper basalts; <i>f</i>. Fault.</div> -</div> - -<p>The total thickness of volcanic rocks in the Antrim plateau exceeds -1000 feet; but, as the upper part of the series has been removed by denudation, -the whole depth of lava originally poured out cannot now be -told. A well-marked group of tuffs and clays, traceable throughout a large -part of Antrim, forms a good horizon in the midst of the basalts, which are -thus divisible into a lower and upper group (Fig 264).</p> - -<p>The Lower Basalts have a thickness of from 400 to 500 feet. But, -as already mentioned (<a href="#Page_194">p. 194</a>), they die out in about six miles to no -more than 40 feet at Ballintoy. They are distinguished by their generally -cellular and amygdaloidal character, and less frequently columnar structure. -The successive flows, each averaging perhaps above 15 feet in thickness, are -often separated by thin red ferruginous clayey partings, sometimes by -bands of green or brown fine gravelly tuff. The most extensive of these -tuff-bands occurs in the lower part of the group at Ballintoy, and can be -traced along the coast for about five miles. In the middle of its course, -near the picturesque Carrick-a-raide, it reaches a maximum thickness -of about 100 feet and gradually dies out to east and west. The neck of -coarse agglomerate at Carrick-a-raide, is doubtless the vent from which this -mass of tuff was discharged (see <a href="#v2fig301">Fig. 301</a>). Owing to the thinning out of -<span class="pagenum" id="Page_203">- 203 -</span> -the sheets of basalts, as they approach the vent, the tuff comes to rest -directly on the Chalk, and for some distance westwards forms the actual base -of the volcanic series.<a id="FNanchor_235" href="#Footnote_235" class="fnanchor">[235]</a> Occasional seams of carbonaceous clays, or of lignite, -appear in different horizons among the basalts. Beneath the whole mass of -basalt, indeed, remains of terrestrial vegetation here and there occur. Thus, -near Banbridge, County Down, a patch of lignite, four feet ten inches thick, -underlies the basalt, and rests directly on Silurian rocks. Such fragmentary -records are an interesting memorial of the wooded land-surface over which -the earliest outflows of basalt spread.</p> - -<div class="footnote"> - -<p><a id="Footnote_235" href="#FNanchor_235" class="label">[235]</a> See Explanation of Sheets 7 and 8 of the Geological Survey of Ireland (1888), p. 23.</p> - -</div> - -<div class="figcenter" id="v2fig264" style="width: 360px;"> - <img src="images/v2fig264.png" width="360" height="200" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 264.</span>—Diagram-Section of the Antrim Plateau.<br /><br /> - 1. Triassic series; 2, 3. Rhaetic strata and Lias; 4. Greensand; 5. Chalk; 6. Gravel and soil; 7. Lower group - of basalts; 8. Group of tuffs, clays and iron-ore; 9. Upper group of basalts.</div> -</div> - -<p>In looking at the great basalt-escarpments of Antrim, the Inner Hebrides -or the Faroe Islands, and in following with the eye the successive sheets of -lava in orderly sequence of level bands from the breaking waves at the base -to the beetling crest above, we are apt to take note only of the proofs of -regularity and repetition in the outflows of molten rock and to miss the -evidence that these outflows did not always rapidly follow each other, but -were separated by intervals of varying, sometimes even of long duration. -One of the most frequent and conspicuous proofs of such intervals is to be -found in the red layers or partings above referred to which, throughout all -the basalt-plateaux, so commonly intervene between successive sheets of -basalt. These red streaks cannot fail to arrest the eye on the coast-precipices -where by their brilliant contrast of colour, they help to -emphasize the bedded character of the whole volcanic series.</p> - -<p>Examined more closely, they are found to consist of clay or bole which -shades into the decomposed top of the bed whereon it lies, and is usually -somewhat sharply marked off from that which covers it. This layer has -long, and I think correctly, been regarded as due to the atmospheric disintegration -of the surface of the basalt on which it rests, before the eruption -of the overlying flow. It varies in thickness from a mere line up to a foot -or more, and it passes into the tuffs and clays which are sometimes interposed -<span class="pagenum" id="Page_204">- 204 -</span> -between the sheets of basalts. It may be looked upon as probably -furnishing evidence of the lapse of an interval sufficiently extended to -permit a considerable subserial decay of the surface of a lava-sheet before -the outflow of the next lava. But an attentive study of the plateaux discloses -other and even more remarkable indications that the pauses between -the consecutive basalt-beds were frequently so prolonged as to allow extensive -topographical changes to be made in a district. Nowhere is the long -duration of some of these intervals more impressively taught than in -the central zone of sedimentary strata in Antrim.</p> - -<p>This persistent group of tuffs, clays, and iron-ore is generally from 30 -to 40 and sometimes as much as 70 feet thick. From the occurrence of the -ore in it, it has been explored more diligently in recent years than any -other group of rocks in the district, and its outcrop is now known over -most of the plateau. The iron-ore bed varies from less than an inch -up to 18 inches in thickness, and consists of pisolitic concretions of -hæmatite, from the size of a pea to that of a hazel nut, wrapped up in a -soft ochreous clayey matrix.<a id="FNanchor_236" href="#Footnote_236" class="fnanchor">[236]</a> Where it is absent, its place is sometimes -taken by an aluminous clay, worked as "bauxite," which has yielded stumps -of trees and numerous leaves and cones. Beneath the iron-ore or its -representative, lies what is called the "pavement,"—a ferruginous tuff, 8 to -10 feet thick, resting on "lithomarge,"—a lilac or violet mottled aluminous -earth sometimes full of rounded blocks or bombs of basalt. The well-known -horizon for fossil plants at Ballypallidy is a red tuff in this zone. -The section of strata between the two basalt-groups at this locality may -serve as an illustration of the nature and arrangement of the deposits.<a id="FNanchor_237" href="#Footnote_237" class="fnanchor">[237]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_236" href="#FNanchor_236" class="label">[236]</a> Consult a good essay on the Iron-ore and Basalts of North-east Ireland by Messrs. Tate and -Holden, <i>Quart. Journ. Geol. Soc.</i> xxvi. (1870), p. 151. In this paper the nature, composition and -modes of origin of the iron-ore and its associated strata are fully discussed.</p> - -<p><a id="Footnote_237" href="#FNanchor_237" class="label">[237]</a> A. M'Henry, <i>Geol. Mag.</i> (1895), p. 263.</p> - -</div> - -<div style="margin-left:4em;"> - Upper Basalt, compact and often columnar sheets.<br /> - Brown laminated tuff and volcanic clays.<br /> - Laminated brown impure earthy lignite, 2 feet 3 inches.<br /> - Brown and red variegated clays, tuffs and sandy layers, with irregular<br /> - seams of coarse conglomerate composed of rounded and subangular<br /> - fragments of rhyolite and basalt, 3 feet 4 inches.<br /> - Brown, red and yellowish laminated tuffs, mudstones, and bole, with<br /> - occasional layers of fine conglomerate (rhyolitic and basaltic),<br /> - pisolitic iron-ore band and plant-beds, 8 feet 10 inches.<br /> - Lower basalt, amygdaloidal.<br /> -</div> - -<p>In some of the Ballypallidy tuffs the most frequent lapilli are pieces of -green and brown glass, which Mr. Watts compares with the pitchstone of -Sandy Braes, though rarely containing phenocrysts as that rock does. He -has found also in these strata a smaller proportion of lithoidal rhyolites -and occasionally fragments of basic rock.</p> - -<p>The pale and coloured clays that occur in this marked sedimentary -intercalation have doubtless been produced by the decomposition of the -<span class="pagenum" id="Page_205">- 205 -</span> -volcanic rocks and the washing of their fine detritus by water. Possibly -this decay may have been in part the result of solfataric action. From -true bauxite or aluminium-hydrate, the sediments vary in composition and -specific gravity and pass into aluminous silicates and iron-ores. They -seem to indicate a prolonged interval of volcanic quiescence when the lavas -and tuffs already erupted were denuded and decomposed.<a id="FNanchor_238" href="#Footnote_238" class="fnanchor">[238]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_238" href="#FNanchor_238" class="label">[238]</a> See a note on Bauxite by Professor G. A. Cole, <i>Scientif. Trans. Royal Dublin Soc.</i> vol. vi. -series ii. (1896), p. 105.</p> - -</div> - -<p>The area over which this interesting series of stratified deposits now -extends is obviously much less than it was originally. It has indeed been -so reduced by denudation into mere scattered patches that it probably does -not exceed 170 square miles. But the group can be traced from Divis -Hill, near Belfast, to Rathlin Island, a distance of 50 miles, and from the -valley of the Bann to the coast above Glenarm, more than 20 miles. There -can be little doubt that it was once continuous over all that area, and that it -probably extended some way further on each side. If the so-called Pliocene -clays of Lough Neagh be regarded as parts of this group of strata, its extent -will be still further increased. Hence the original area over which the iron-ore -and its accompanying tuffs and clays were laid down can hardly have -been less than 1000 square miles. This extensive tract was evidently the -site of a lake during the volcanic period, formed by a subsidence of the -floor of the lower basalts. The salts of iron contained in solution in the -water, whether derived from the decay of the surrounding lavas or from -the discharges of chalybeate springs, were precipitated as peroxide in -pisolitic form, as similar ores are now being formed on lake-bottoms in -Sweden. For a long interval, quiet sedimentation went on in this lake, -the only sign of volcanic energy during that time being the dust and stones -that were thrown out and fell over the water-basin, or were washed into -it by rains from the cones of the lava-slopes around.</p> - -<p>It may here be remarked that the tendency to subsidence in the Antrim -plateau seems to have characterized this region since an early part of -the volcanic period. The lake in which the deposits now described accumulated -was entirely effaced and overspread by the thick group of upper basalts. -But long after the eruptions had ceased, a renewed sinking of the ground -gave rise to the sheet of water which now forms Lough Neagh.<a id="FNanchor_239" href="#Footnote_239" class="fnanchor">[239]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_239" href="#FNanchor_239" class="label">[239]</a> This subject will be discussed in Chapter xlix.</p> - -</div> - -<p>Nowhere else among the Tertiary basalt-plateaux of Britain has any -trace been found of so marked and prolonged a pause in the volcanic -activity as is indicated by the Antrim zone of tuffs and clays. Throughout -the Inner Hebrides, indeed, numerous intercalations of sedimentary -material occur among the basalts, but these consist mainly of tuffs and -volcanic conglomerates with less frequent shales and coal-seams, and -they never suggest so distinct and lengthened an interval as is indicated by -the Antrim deposit.</p> - -<p>It is not improbable that this interval was marked by the outbreak of -rhyolitic eruptions somewhere in the region. The abundance of rhyolite -<span class="pagenum" id="Page_206">- 206 -</span> -fragments in some of the tuffs is striking evidence that acid rocks were in -one way or other brought to the surface at this time. At Glenarm one of -the members of the stratified series is a marked rhyolitic conglomerate, composed -of rounded pebbles of a rock not unlike the well-known rhyolite of -Tardree and Carnearny. These fragments, obviously of local origin, must -either have been derived from a surface of acid rock laid bare by denudation, -or from rhyolite ejected in lapilli or poured out in streams. I formerly -believed that all the Antrim rhyolites had been injected into the basalts -after the close of the plateau-period. But the proved abundance and wide -extent of the rhyolitic detritus among the sediments associated with the -iron-ore point to a possible outflow of acid lavas with accompanying tuffs -during the sedimentary interval between the two groups of basalt. The -characters of the Antrim rhyolites, however, will be more particularly discussed -in Chapter xlvii., in connection with the acid rocks of the Tertiary -volcanic series.</p> - -<p>Immediately above the iron-ore of Antrim, or separated from it in -places by only a few inches of tuff, comes the group of Upper Basalts, -which varies up to 600 feet in thickness, though as the upper portion has -been everywhere removed by denudation, no measure remains of what may -have been the original depth of the group. The general character of these -basalts is more frequently columnar, black and compact, and with fewer -examples of a strongly amygdaloidal structure than in the lower group. -But this distinction is less marked in the south than in the north -of Antrim, so that where the intervening zone of tuffs and iron-ore disappears, -no satisfactory line of division can be traced between the two -groups of basalt. The occurrence of that zone, however, by giving rise to a -hollow or slope, from which the upper basalts rise as a steep bank or cliff, -furnishes a convenient topographical feature for mapping the boundary of -these rocks. Among the upper basalts, also, there is perhaps a less frequent -occurrence of those thin red partings of bole between successive -flows, so conspicuous in the lower group. But the flows are not less distinctly -marked off from each other. Nowhere can their characteristic -features be better seen than along the magnificent range of cliffs from the -Giant's Causeway eastwards. The columnar bed that forms the Causeway -is the lowest sheet of the upper group, and may be seen resting directly on -the zone of grey and red tuffs. It is about 60 or 70 feet thick; and, -while perfectly regular in its columnar structure at the Causeway and the -"Organ," assumes further eastward the confusedly starch-like arrangement -of prisms already referred to. But in the great cliff section of the "Amphitheatre," -the more regular structure is resumed, the bed swells out to about -80 feet in thickness, and columns of that length run up the face of the -precipice, weathering out at the top into separate pillars, which, perched on -the crest of an outstanding ridge, are known as the "Chimneys." The -basalt-beds that succeed the lowest one are each only about 10 to 15 feet -thick (<a href="#v2fig265">Fig. 265</a>).</p> - -<p><span class="pagenum" id="Page_207">- 207 -</span></p> - -<div class="figcenter" id="v2fig265" style="width: 778px;"> - <img src="images/v2fig265.png" width="778" height="518" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 265.</span>—View of Basalt escarpment, Giant's Causeway, with the Amphitheatre and Chimneys. (From a photograph by Mr. R. Welch.)</div> -</div> - -<p><span class="pagenum" id="Page_208">- 208 -</span></p> - -<p>Between the successive sheets of the Upper Basalts thin seams of red -ferruginous clay though, as I have said, less frequent perhaps than in the -lower group, continue to show that the intervals between successive eruptions -were of sufficient duration to admit of some subærial decay of -the surface of a lava before the outflow of the next bed. Occasional thin -layers of tuff also, and even of pisolitic iron-ore, have been observed -among these higher basalts. But the most interesting and important intercalations -are inconstant seams of lignite. One of the most conspicuous of -these lies immediately above the basalt of the "Causeway," where it was -long worked for fuel, and was found to be more than six feet thick. But -it is quite local, as may be seen at the "Organ" over which it lies, having a -thickness of only 12 inches and rapidly dying out so as to allow the basalts -above and below it to come together. The removal of the upper portion -of the basalts by denudation has destroyed the records of the latest part of -the volcanic history of the Irish plateaux.</p> - -<p>It is obvious that nowhere in Antrim does any trace exist of a central -vent or cone from which the volcanic materials were discharged. There is -no perceptible thickening of the individual basalt-sheets, nor of the whole -series in one general direction, in such a manner as to point to the site of -some chief focus of eruption. Nor can we place reliance on the inclination -of the several parts of the plateau. I have pointed out that the varying -dip of the beds must be attributed mainly to post-volcanic movements, or at -least to movements which, if not later than all the phases of volcanic action, -must have succeeded the outpouring of the plateau-basalts. There has been -a general subsidence towards the central and southern tracts now occupied -by the valley of the Bann and Lough Neagh. But nowhere in the depression -is there any trace of the ruins of a central cone or focus of discharge.</p> - -<p>The Antrim plateau, in these respects, resembles the others. But as has -already been remarked, it differs from them in one important particular. -It has nowhere been disrupted by huge bosses of younger rocks, such as -have broken up the continuity of the old lava-fields further north. Yet -it also is not without its memorials of younger protrusions. It -contains not a few excellent examples of true volcanic vents, and, as above -stated, it includes some small acid bosses that may represent the great -protrusions of the Inner Hebrides, and may have been connected with -superficial outflows of rhyolitic lava and showers of rhyolitic tuff.</p> - - -<p>ii. <span class="allsmcap">MULL, MORVEN AND ARDNAMURCHAN</span></p> - -<p>This plateau covers nearly the whole of the island of Mull, embraces a -portion of Morven on the Argyleshire mainland, and, stretching across Loch -Sunart, includes the western part of the peninsula of Ardnamurchan (Map VI.). -That these now disconnected areas were once united into a continuous lava-field -which extended far beyond its present limits is impressively indicated -by their margin of cliffs and fringe of scattered islands and outliers. The -plateau went west, at least, as far as the Treshnish Isles, which are composed of -basalt. On its eastern border, a capping of basalt on the top of Beinn Iadain -<span class="pagenum" id="Page_209">- 209 -</span> -(1873 feet) in Morven, and others further north, prove that its volcanic -sheets once spread into the interior of Argyleshire (<a href="#v2fig266">Fig. 266</a>). On the -south, its fine range of lofty cliffs, with their horizontal bars of basalt, bear -witness to the diminution which it has undergone on that side; while, on -the north, similar sea-walls tell the same tale. Not only has it suffered -by waste along its margin, it has also been deeply trenched by the excavation -of glens and arms of the sea. The Sound of Mull cuts it in two, and -the mainland portion is further bisected by Loch Sunart, and again by Loch -Aline. The island of Mull is so penetrated by sea-lochs and divided by -deep valleys that a comparatively slight depression would turn it into a -group of islands. But, besides its enormous denudation, this plateau has -been subjected to disruption, and perhaps also to subsidence, from subterranean -movements. In the southern portion of the island of Mull it has -been broken up by the intrusion of large bosses and sheets of gabbro, and by -masses as well as innumerable veins of various granitoid and felsitic rocks. In -Ardnamurchan, it has suffered so much disturbance from the same cause that -its original structure has been almost obliterated over a considerable area. -Moreover, it has been dislocated by many faults, by which different portions -have been greatly shifted in level. The most important of these breaks is one -noticed by Professor Judd, and visible to every tourist who sails up the -Sound of Mull. It traverses the cliffs on the Morven side, opposite Craignure, -bringing the basalts against the crystalline schists, and strikes thence -inland, wheeling round into the long valley in which Lochs Arienas and -Teacus lie. On its western side, the base of the basalt-series is almost at -the sea-level; on its eastern side, that platform rises high into the outliers -of Beinn na h-Uamha (1521 feet) and Beinn Iadain. The amount of displacement -is probably more than 1000 feet. Many other minor faults -in the same district show how much the crust of the earth has been -fractured here since older Tertiary time.</p> - -<div class="figcenter" id="v2fig266" style="width: 355px;"> - <img src="images/v2fig266.png" width="355" height="226" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 266.</span>—Basalt-capping on top of Beinn Iadain, Morven.<br /><br /> - The hummocky ground to the right consists of the Highland schists against which the basalts are - brought by lines of dislocation.<a id="FNanchor_240" href="#Footnote_240" class="fnanchor">[240]</a></div> -</div> - -<div class="footnote"> - -<p><a id="Footnote_240" href="#FNanchor_240" class="label">[240]</a> There are no fewer than three faults in the basalt-capping on the summit of Beinn Iadain. -By bringing the basalts and schists into juxtaposition, they have given rise to topographical -features that can be seen even from a distance.</p> - -</div> - -<p><span class="pagenum" id="Page_210">- 210 -</span></p> - -<p>A little to the west of Mull, and belonging originally to the same -plateau, lies the isle of Staffa, the famous columnar basalts of which first -attracted the attention of travellers, and gave to the Tertiary volcanic rocks -of Scotland their celebrity (<a href="#v2fig266">Fig. 266</a><i>a</i>).</p> - -<div class="figcenter" id="v2fig266a" style="width: 521px;"> - <img src="images/v2fig266a.png" width="521" height="321" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 266</span><i>a</i>.—View of the south side of Staffa, showing the bedded and columnar structure of the basalt. - The rock in which the cave to the left hand has been eroded is a conglomeratic tuff underlying the - basalt; to the right is Fingal's Cave. These caverns bear witness to the enormous erosive power - of the Atlantic breakers.</div> -</div> - - -<p>In spite of the extent to which it has suffered from denudation -and subterranean disturbance, and indeed in consequence thereof, the -Mull plateau presents clear sections of many features in the history of the -basalt-outflows and of the subsequent phases of Tertiary volcanic action -which cannot be seen in the more regular and continuous tableland of -Antrim. Moreover, it still possesses in its highest mountain, Ben More -(3169 feet), a greater thickness, and probably a higher series, of lavas than -can now be seen in any other of the plateaux.</p> - -<p>The difficulties, already referred to in regard to Antrim, of tracing the -probable form of ground on which the volcanic eruptions began, are even -greater in the case of the Mull plateau. We can dimly perceive that the -depression in the crystalline rocks of the Highlands which had, from at -least the older part of the Jurassic period, stretched in a N.N.W. direction -along what is now the western margin of Argyleshire, lay beneath the sea -in Jurassic time, and was then more or less filled up with sedimentary -<span class="pagenum" id="Page_211">- 211 -</span> -deposits. The hollow appears thereafter to have become a land-valley, -whence the Jurassic strata were to a large extent cleared out by denudation -before its subsequent submergence under the sea in which the upper Cretaceous -deposits accumulated. Professor Judd has shown that relics of these -Cretaceous strata appear on both sides of the plateau from under the -protecting cover of basalt-sheets. But, before the volcanic eruptions began, -the area had once again been raised into land, and the youngest Secondary -formations had been extensively eroded.</p> - -<p>In their general aspect the basalts of Mull agree with those of Antrim, -and the circumstances under which they were erupted were no doubt -essentially the same. But considerable differences in detail are observable -between the succession of rocks in the two areas. When I first visited the -island in 1866, the only available maps, with any pretensions to accuracy, -were the Admiralty charts; but, as these do not give the interior except in -a generalized way, it was difficult to plot sections from them, and to arrive -at satisfactory conclusions as to the thickness of different groups of rock. -Accordingly, as the successive nearly flat flows of basalt can be traced from -the sea-level up to the top of Ben More, I contented myself with the fact -that the total depth of lava-beds in Mull was at least equal to the height of -that mountain, or 3169 feet. The publication of the Ordnance Survey -Maps now enables us to make a nearer approximation to the truth. From -the western base of the magnificent headland of Gribon, the basalts in -almost horizontal beds rise in one vast sweep of precipice and terraced slope -to a height of over 1600 feet, and then stretch eastwards to pass under the -higher part of Ben More, at a distance of some eight miles. They have a -slight easterly inclination, so that the basement sheets seen at the sea-level, -at the mouth of Loch Scridain, gradually sink below that level as they go -eastward. It is not easy to get a measurement of dip among these basalts, -except from a distance. If we take the inclination at only 1°, the beds -which are at the base of the cliff on the west, must be about 700 feet below -the sea on the line of Ben More, which would give a total thickness of -nearly 3900 feet of bedded lava below the top of that mountain. We shall -not probably overestimate the thickness of the Mull plateau if we put it at -3500 feet.</p> - -<p>The base of the volcanic series of Mull can best be seen on the south -coast at Carsaig, and at the foot of the precipices of Gribon. As already -stated, it is there found resting above Cretaceous and Jurassic rocks. The -lowest beds are basalt-tuffs, of the usual dull green colour. They are in -places much intermingled with sandy and gravelly sediment, as if the -volcanic debris had fallen into water where such sediment was in course of -deposition. One of the most interesting features, indeed, in this basement -part of the series, is the occurrence of bands of non-volcanic material which -accumulated after the tuffs and some of the lavas had been erupted, but -before the main mass of basalts. Those at Carsaig include a lenticular bed, -25 feet thick, of rolled flints, which, with some associated sandy bands, lies -between sheets of basalt. On the opposite side of the promontory is the -<span class="pagenum" id="Page_212">- 212 -</span> -well-known locality of Ardtun, from which the first land-plants in the -volcanic series were determined. The actual base of the basalts is not there -seen, being covered by the sea. The "leaf-beds," with their accompanying -sandstones, gravels, and limestone, lie upon a sheet of basalt, which in some -parts is exceedingly slaggy on the top, passing down into a black compact -structure, and assuming at the base of the cliff a columnar arrangement, with -the prisms curved and built up endways towards each other. Some of the -gravels exceed 30 feet in thickness, and consist of rolled flints, bits of chalk, -and pieces of basalt and other basic igneous rocks. But some of their -most interesting ingredients are pebbles of sanidine lavas, which have been -recognized in them by Prof. G. Cole.<a id="FNanchor_241" href="#Footnote_241" class="fnanchor">[241]</a> No known protrusions of such lavas -occur anywhere beneath or interstratified with the plateau-basalts of this -district. As will be afterwards shown, all the visible acid rocks, the -geological relations of which can be ascertained, are here of younger date -than these basalts. I am disposed to regard the fragments found in the -Ardtun conglomerates as probably derived from some of the basalt-conglomerates -of the plateau, in which fragments of siliceous igneous rocks do -occur. Though there is no evidence that any lavas of that nature were here -poured out at the surface before or during the emission of the basalts, the -contents of these fragmental volcanic accumulations suggest that such lavas, -already consolidated, lay at some depth beneath the surface, and that fragments -were torn off from them during the explosions that threw out the -materials of the basalt-conglomerates to the surface.</p> - -<div class="footnote"> - -<p><a id="Footnote_241" href="#FNanchor_241" class="label">[241]</a> <i>Quart. Jour. Geol. Soc.</i> xliii. (1887) p. 277.</p> - -</div> - -<p>The succession of strata at the Ardtun headland varies considerably in -a short distance, some of the sedimentary deposits rapidly increasing or -diminishing in thickness. The section as measured by Mr. Starkie Gardner -is as follows<a id="FNanchor_242" href="#Footnote_242" class="fnanchor">[242]</a>:—</p> - -<div style="margin-left:4em;"> - Columnar basalt, 40 feet.<br /> - Position of first leaf-bed, obscured by grass, about 2 feet.<br /> - Gravel varying from about 25 feet to a maximum of nearly 40 feet.<br /> - Black or second leaf-bed, 2<sup>1</sup>/<sub>2</sub> feet.<br /> - Gravel about 7 feet.<br /> - Grey clay, 2 feet.<br /> - Laminated sandstone, 6 inches, with 3 inches of fine limestone,<br /> - containing leaves at the base.<br /> - Clay, with leaves at base, 1 foot.<br /> - Clunch, with rootlets, 7 inches.<br /> - Amorphous basalt, becoming columnar at base, about 60 feet. -</div> - -<div class="footnote"> - -<p><a id="Footnote_242" href="#FNanchor_242" class="label">[242]</a> <i>Op. cit.</i> <a href="#Page_280">p. 280</a>.</p> - -</div> - -<p>Mr. Starkie Gardner has called attention to the extraordinarily fresh condition -of the vegetation in some of the layers of the Ardtun section. One -of the leaf-beds he has found to be made up for an inch or two of a pressed -mass of leaves, lying layer upon layer, and retaining almost the colours of -dead vegetation. Among the plants represented is a large purple <i>Ginkgo</i> -and a fine <i>Platanites</i>, one leaf measuring 15<sup>1</sup>/<sub>2</sub> inches long by 10<sup>1</sup>/<sub>2</sub> broad. -<span class="pagenum" id="Page_213">- 213 -</span> -The characteristic dicotyledonous leaves at this locality possessed relatively -large foliage.<a id="FNanchor_243" href="#Footnote_243" class="fnanchor">[243]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_243" href="#FNanchor_243" class="label">[243]</a> For fuller local details regarding the Ardtun leaf-beds, I may refer to the original paper by -the Duke of Argyll (<i>Quart. Jour. Geol. Soc.</i> vii. p. 89), and to the memoir by Mr. Starkie -Gardner (<i>op. cit.</i> xliii. (1887), p. 270).</p> - -</div> - -<p>To the early observations of Macculloch we are indebted for the record -of an interesting fact in connection with the vegetation of the land-surface -over which the first lava-flows spread. He figured a vertical tree trunk, -imbedded in prismatic basalt, and rightly referred it to some species of fir.<a id="FNanchor_244" href="#Footnote_244" class="fnanchor">[244]</a> -This relic may still be seen under the basalt precipices of Gribon. Mr. -Gardner found it to be "a large trunk of a coniferous tree, five feet in -diameter, perhaps <i>Podocarpus</i>, which has been enveloped, as it stood, in one -of the flows of trap to the height of 40 feet. Its solidity and girth -evidently enabled it to resist the fire, but it had decayed before the next flow -passed over it, for its trunk is a hollow cylinder filled with debris, and lined -with the charred wood. A limb of another, or perhaps the same tree, is in -a fissure not far off."<a id="FNanchor_245" href="#Footnote_245" class="fnanchor">[245]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_244" href="#FNanchor_244" class="label">[244]</a> <i>Western Islands</i>, vol. i. p. 568, and plate xxi. Fig. 1.</p> - -<p><a id="Footnote_245" href="#FNanchor_245" class="label">[245]</a> <i>Quart. Jour. Geol. Soc.</i> xliii. p. 283.</p> - -</div> - -<p>At different levels in the volcanic series of Mull, beds of lignite and -even true coal are observable. These seem to be always mere lenticular -patches, only a few square yards in extent. The best example I have met -with lies among the basalts near Carsaig. It is in part a black glossy coal, -and partly dull and shaly. Some years ago it was between two and three -feet thick, but now, owing to its having been dug away by the shepherds, -only some six or eight inches are to be seen. It lies between two basalt-flows, -and rapidly disappears on either side.</p> - -<p>More frequent than these inconstant layers of fossil vegetation are the -thin partings of tuff and layers of red clay, sometimes containing iron-ore, -which occur at intervals throughout the series between different flows of -basalt. But even such intercalations are of trifling thickness, and only of -limited extent. The magnificent precipices of M'Gorry's Head and Gribon -expose a succession of beds of columnar amorphous and amygdaloidal basalt, -which must attain a thickness of at least 2500 feet, before they are overlain -by the higher group of pale lavas in Ben More. On the east side of the -island, thin tuffs and bands of basalt-conglomerate occur on different horizons -among the bedded basalts, from near the sea-level up to the summit of the -ridge which culminates in Beinn Meadhon (2087 feet), Dùn-da-Ghaoithe -(2512 feet), and Mainnir-nam-Fiadh (2483 feet). Reference has already -been made to the remarkably coarse character of some of the breccias intercalated -among the basalts in this part of Mull, and to the enormous dimensions -of some of the masses of mica-schist and quartzite which have been -carried up from a depth of 2000 feet or more by volcanic agency (see <i>ante</i>, -<a href="#Page_196">p. 196</a>, and <a href="#v2fig262">Fig. 262</a>).</p> - -<p>Above the ordinary compact and amygdaloidal basalt comes the higher -group of pale lavas already referred to as forming the uppermost part of -Ben More, whence it stretches continuously along the pointed ridge of -<span class="pagenum" id="Page_214">- 214 -</span> -A'Chioch, and thence northwards into Beinn Fhada. The same lavas are -likewise found in two outliers, capping Beinn a' Chraig, a mile further north, -and I have found fragments of them on some of the loftier ridges to the -south-east. This highest and youngest group of lavas in the plateaux has -been reduced to mere isolated patches, and a little further denudation will -remove it altogether. Yet it is not less than about 800 feet thick, and -consists of bedded andesitic or trachytic lavas, which alternate with and follow -continuously and conformably upon the top of the ordinary plateau-basalts. -These dull, finely crystalline or compact, light-grey rocks weather with a -characteristic platy form, which has been mistaken for the bedding of tuffs. -The fissility, however, has none of the regularity or parallelism of true -bedding, and may be observed to run sometimes parallel with the bedding -of the sheets, sometimes obliquely or even at right angles to it. Even -where this structure is best developed, the truly crystalline nature of the -rocks can readily be detected. Some of them are porphyritic and -amygdaloidal, the very topmost bed of the mountain being a coarse -amygdaloid. Intercalated with these curious rocks there are others in -which the ordinary characters of the dolerites and basalts of the plateaux -can be recognised. The amygdaloids are often full of delicate prisms of -epidote.</p> - -<p>In Mull, as in the other areas of terraced basalts, we everywhere meet -with gently inclined sheets, which do not thicken out individually or -collectively in any given direction, except as the result of unequal denudations. -So far as I have been able to discover, they afford no evidence of any -great volcanic cone from which they proceeded. Their present inclinations -are unquestionably due, as in Ireland, to movements subsequent to the formation -of the plateau. In Loch-na-keal they dip gently to the E.N.E.; -in Ulva and the north-west coast to N.N.E.; near Salen to W.S.W. on -the one side, and N.W. on the other. Round the southern and eastern -margins of the mountainous tract of the island, they dip generally inwards -to the high grounds.</p> - -<p>The Mull plateau presents a striking contrast to that of Antrim, in the -extraordinary extent to which it has been disrupted by later protrusions of -massive basic and acid rocks over a rudely circular area, extending from the -head of Loch Scridain to the Sound of Mull, and from Loch-na-keal to Loch -Buy. The bedded basalts have been invaded by masses of dolerite, gabbro, -and granophyre, with various allied kinds of rock. They have not only -been disturbed in their continuity, but have undergone considerable metamorphism.</p> - -<p>Again, further to the north, in the promontory of Ardnamurchan, the -plateau has been disrupted in a similar way, and only a few recognisable -fragments of it have been left. These changes will be more appropriately -discussed in connection with similar phenomena in the other plateaux -further north.</p> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_215">- 215 -</span></p> - -<h2 class="nobreak" id="CHAPTER_XXXVIII">CHAPTER XXXVIII<br /> - -<span class="smaller">THE BASALT-PLATEAU OF THE PARISH OF SMALL ISLES—RIVERS OF -THE VOLCANIC PERIOD</span></h2> -</div> - - -<h3>iii. <span class="allsmcap">PARISH OF SMALL ISLES PLATEAU</span></h3> - -<p>The parish of Small Isles includes the islands of Eigg, Rum, Canna, Sanday -and Muck (Map VI.). The fragmentary basalt-plateau which it contains, -although the smallest of the whole series, is surpassed by none in the variety -and interest of its geology. It contains by far the most complete records of -the rivers which, during the volcanic period, flowed across the lava plains. -And it alone has preserved a relic of the latest lava which, after the -basalt-plateau had been built up and had been greatly eroded, flowed over -the denuded surface in streams of volcanic-glass that found their way into -a river-channel and sealed it up.</p> - -<p>That the fragments of the basaltic plateau preserved in each member of -the group of the Small Isles were once connected as a continuous volcanic -plain can hardly be doubted. Indeed, as already stated, they were not -improbably united with the plateau of Skye on the north, and with that -of Mull, Morven and Ardnamurchan oh the south. Taking the whole -space of land and sea within which the basalt of Small Isles is now -confined, we may compute it at not much less than 200 square miles. In -Eigg, Muck, Canna and Sanday the basalts retain their almost horizontal -position, and from underneath them the Jurassic strata emerge in the -first of these islands. The central part of the plateau in the island of Rum -has suffered greatly from denudation. It now consists of four small outliers -of basalt, which lie at levels of 1200 feet and upwards, on the western -slope. The basalt is underlain by a thick mass of red Torridon Sandstone, -which, with some gneisses and schists, forms the general underlying platform -of this island. These rocks are doubtless a continuation of the red sandstone -and schists of Sleat, in Skye, and like them have been subjected to -those post-Cambrian convolutions and metamorphism whereby the Lewisian -Gneiss and Torridon Sandstone have been brought above younger rocks, and -have been crushed and rolled out so as to assume a new schistose arrangement. -Before the time when volcanic action began, a mass of high ground, -consisting of these ancient rocks, stood where the island of Rum is now -<span class="pagenum" id="Page_216">- 216 -</span> -situated. The streams of basalt spread around it, not only covering the -surrounding low tracts of Jurassic rocks, but gradually accumulating against -the hills, and thus reducing them both in area and in height above the -plain.<a id="FNanchor_246" href="#Footnote_246" class="fnanchor">[246]</a> Viewed from Canna the western coast of Rum presents a striking -picture of the general relations of the volcanic masses of the Inner Hebrides -and of the enormous denudation which they have undergone (<a href="#v2fig267">Fig. 267</a>). The -Torridon Sandstones are there seen to mount into ranges of hills, capped with -outliers of the basalt-plateau, while behind rise the great eruptive bosses of -gabbro and granophyre. The edges of the sheets that form the outliers -would, if prolonged, cover the northern or lower half of the island, where -pre-Cambrian rocks form the surface. In the southern half, the continuity -of the basalt has been partly obscured and partly destroyed by the protrusion -of the great masses of gabbro that form the singularly picturesque -mountain group to which this island owes its prominence as a landmark far -and wide along the West Coast of Scotland.</p> - -<div class="footnote"> - -<p><a id="Footnote_246" href="#FNanchor_246" class="label">[246]</a> That the lava-fields did not completely bury this nucleus of older rocks has been supposed -to be shown by the fragments of red sandstone found in the ancient river-bed of Eigg, which was -scooped out of the basalt-plateau and sealed up under pitchstone. But I am disposed to think -that these fragments, together with those of Jurassic sandstone, came, not from Rum, but from -some district more to the north and east, as will be explained in a later page. At Canna, a few -miles to the west, fragments of red sandstone not improbably derived from Rum are abundant in -the conglomerates between the basalts.</p> - -</div> - -<div class="figcenter" id="v2fig267" style="width: 478px;"> - <img src="images/v2fig267.png" width="478" height="275" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 267.</span>—View of Rum from the harbour of Canna.<br /><br /> - The ground indicated by single birds is the area of Torridon Sandstone; two birds, the plateau basalts; three - birds, the gabbro just seen at one point above the granophyre hills; four birds, the granophyre.</div> -</div> - -<p>The most varied and interesting of the fragments of the basaltic plateau -in the area of the Small Isles is that which forms the island of Canna, with -its appendage Sanday. Canna measures five miles in length by from half a -mile to a mile in breadth, and consists entirely of the rocks of the plateau -and their accompaniments. The basalts are exposed along the north coast -<span class="pagenum" id="Page_217">- 217 -</span> -in a range of mural precipices rising to a height of about 600 feet above -the sea. From the top of that escarpment the ground falls by successive -rocky terraces and grassy slopes to the southern shore-line. Sanday, connected -with the large island by a shoal and foot-bridge, is two miles long and -220 to about 1200 yards broad. Its highest cliffs range along its southern -shore to a height of 193 feet, whence they slope gently northward into -the hollow between the two islands. This peculiar topography accounts for -the manner in which the geological sections of most interest are distributed.</p> - -<p>The first, and still the best, account of the geology of these islands is -that of Macculloch. He showed that the rocks all belong to the series of the -plateau-basalts, and he described the presence among them of a "trap-conglomerate." -He noticed the occurrence also of trap-tuff and the occasional -appearance of carbonized wood in these deposits. Reasoning upon these -observations in his characteristically vague and verbose manner, "bewildered -in the regions of conjecture," he concludes that the basalts instead of -belonging to "one general formation" have been successively deposited on -the same spot, "since lapse of time is evidently implied in the formation of -a conglomerate." He inclines to believe that they have been discharged by -ancient volcanoes from which in the course of time all traces of their -original outline have been more or less completely removed, the existing -basalts being merely fragments of once more extensive masses.<a id="FNanchor_247" href="#Footnote_247" class="fnanchor">[247]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_247" href="#FNanchor_247" class="label">[247]</a> <i>Western Isles</i>, vol. i. pp. 448-459, and pl. xix. Figs. 2, 3 and 4. See also Jameson's -<i>Mineralogy of the Scottish Isles</i>.</p> - -</div> - -<p>Macculloch regarded the intercalated-conglomerates as having been -arranged under water and as marking pauses in the deposition of the sheets -of "trap." He gave two diagrams in illustration of the relations of these -detrital deposits, but he expressed no definite opinion as to their origin, -though from one passage it would seem that he inclined towards the belief -that they were formed in the sea.<a id="FNanchor_248" href="#Footnote_248" class="fnanchor">[248]</a> Since his time, so far as I am aware, no -fresh light has been thrown upon the subject.</p> - -<div class="footnote"> - -<p><a id="Footnote_248" href="#FNanchor_248" class="label">[248]</a> <i>Op. cit.</i> pp. 449, 457, pl. xix. Figs. 2 and 3.</p> - -</div> - -<p>During a yachting cruise in the summer of 1894 I visited Canna for -the first time and found so much that was new to me in regard to the -history of Tertiary volcanic action, and which demanded a careful survey, -that I returned to the locality the following summer and remained in the -island until I had mapped it and its dependencies upon the Ordnance Survey -sheets on the scale of six inches to a mile. The following narrative is the -result of the observations then made.</p> - -<p>As far back as the year 1865 I published an account of an ancient -river-channel which, during the volcanic period, had been eroded on the -surface of the basalt-plateau, and of which a small portion had been preserved -under a stream of pitchstone-lava that had flowed into and buried it.<a id="FNanchor_249" href="#Footnote_249" class="fnanchor">[249]</a> -This water-course, now marked by the picturesque ridge of the Scuir of Eigg, -was shown to have been excavated by a stream which came from the north-east -or east, and to be younger, not only than the plateau-basalts of the -district, but than even the dykes which cut these basalts. Yet that it -<span class="pagenum" id="Page_218">- 218 -</span> -belonged to the volcanic period was proved by the manner in which it had -been sealed up and preserved under the black glassy lava of the Scuir. Its -history and the data from which this history is compiled will be narrated -in a later part of this chapter.</p> - -<div class="footnote"> - -<p><a id="Footnote_249" href="#FNanchor_249" class="label">[249]</a> <i>Scenery of Scotland</i> (1865); <i>Quart. Journ. Geo. Soc.</i> vol. xxvii. (1871), p. 303.</p> - -</div> - -<div class="figleft" id="v2fig268" style="width: 311px;"> - <img src="images/v2fig268.png" width="311" height="490" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 268.</span>—Section of the cliffs below Compass Hill, Isle of Canna.</div> -</div> - -<p>My examination of the islands of Canna and Sanday, however, brought to -light other and more abundant evidence of river-action in the same region -of the Inner Hebrides, but belonging to an earlier part of the volcanic period. -This evidence reveals that a powerful river, flowing westwards from the -Highland mountains, swept over the volcanic plain, while the sheets of -basalt were still being poured forth, and while volcanic eruptions were -taking place from cones of slag.</p> - -<p>The basalt-plateau of Canna resembles in all essential particulars those -of the other Western Isles. -Its base is everywhere concealed -under the sea, but -from the fragments of -Torridon Sandstone in its -agglomerates we may infer -that it probably rests on -that formation, like the -volcanic outliers in Rum. -It is formed of successive -sheets of different basalts -including the usual banded, -amygdaloidal and columnar -forms. Some of them towards -the west are specially -marked by the great -abundance and large size -of their porphyritic felspars. -The magnetic properties -of the basalts at -the east end of the island -have long been known, and -have given rise to various -modern myths regarding -their influence on the compasses -of passing vessels.</p> - -<p>But it is in its conglomerates, -tuffs and agglomerates -and the light -they cast on some aspects -of the volcanic period, -elsewhere hardly recorded, that the geology of Canna possesses a special -importance. To these, therefore, we may at once turn.</p> - -<p>The conglomerates are best developed at the eastern end of the island, -<span class="pagenum" id="Page_219">- 219 -</span> -where the cliffs present the structure represented in <a href="#v2fig268">Fig. 268</a>. At the -base, and passing under the level of the sea, lies the agglomerate (<i>a</i>) of -a vent which will be described in Chapter xli., together with other eruptive -orifices of the various plateaux (<a href="#Page_288">p. 288</a>). This rock has a somewhat uneven -upper surface which rises in places about 150 feet above high tide-mark. -Here and there it shades off upward into the conglomerate that overlies it; -water-worn pebbles appear among its contents, and rude traces of bedding -begin to show themselves, until, within the course of a few feet, we pass -upward into an undoubted conglomerate. Elsewhere, however, and particularly -along the precipices west of Compass Hill, the two deposits are -more distinctly marked off from each other. The agglomerate has there a -hummocky, irregular upper surface, as if it had been thrown down in heaps. -The hollows between these protuberances have been filled up with conglomerate -and sandstone, forming the base of the thick overlying deposit.</p> - -<p>It is thus clear that the loose materials of the vent were directly -exposed at the surface when the conglomerate was accumulated, and, -indeed, that these materials served to supply some of the detritus of which -the conglomerate consists. The absence of any trace of a cone and crater -at the vent may perhaps be explicable on the supposition that their incoherent -material was washed down by the currents that swept along and -deposited the conglomerate.</p> - -<p>The mass of sedimentary material (<i>b</i>) which overlies the agglomerate -of the vent forms a conspicuous feature along the lower half of the precipices -at the eastern end of Canna. It rises to a height of 250 to 300 feet above -sea-level, and must reach a maximum thickness of probably not less than -100 to 150 feet. It gradually descends in a westward direction, both along -the northern cliffs and in the lower ground round Canna Harbour, insomuch -that in about a mile, owing to the gentle westerly dip of the whole volcanic -series, combined with the effect of a number of small faults, it passes -under the level of the sea.</p> - -<p>Great variation in the character of the detritus composing this thick -group of strata may be observed as it is followed westward. On the cliffs -below Compass Hill, as represented in <a href="#v2fig268">Fig. 268</a>, the coarse conglomerate -with water-worn stones, hardly to be distinguished from the volcanic -agglomerate of the vent, shows more or less distinct bedding, or at least -a succession of coarser and finer bands. Towards its base it encloses -numerous pieces of Torridon Sandstone, sometimes subangular, but often so -well and smoothly rounded as to show that they must have been long subjected -to the action of moving water. It is further observable that, while -in the agglomerate the volcanic stones have rough surfaces, those in the -conglomerate begin to show increasing evidence of attrition, until, as the -deposit is traced upwards, they become almost as well rounded and water-worn -as the non-volcanic stones.</p> - -<p>Yet amidst and overlying these proofs of transport from some little -distance lie abundant huge slags and blocks of amygdaloidal lava, sometimes -closely aggregated, sometimes scattered through a volcanic tuff or ashy sandstone. -<span class="pagenum" id="Page_220">- 220 -</span> -The composition and structure of these stones, and the manner of -their dispersion through the deposit, leave little doubt that they were -ejected from the vent. We are thus confronted with the interesting fact -that, while the materials of the volcanic cone were being washed down by -running water, eruptions were still taking place. But by degrees these -indications of contemporaneous volcanic activity diminish. The detrital -materials become coarser and more distinctly water-rolled until they pass -into greenish sandstones and fine conglomerates. Yet the matrix even of -these higher sediments is largely composed of fine volcanic detritus, and -probably points to occasional discharges of dust and ashes.</p> - -<p>Various sills or intrusive sheets have been injected into this sedimentary -group along the precipices at the east end of Canna, and form there lenticular -bands. One of these (<i>c</i>) is shown in <a href="#v2fig268">Fig. 268</a>.</p> - -<p>Immediately above the massive greenish pebbly sandstone (<i>d</i>) which -caps the stratified series lies a group of basalts (<i>e</i>), composed of several -distinct beds, having a united thickness of from 80 to 100 feet. The lowest -of these has a regular columnar structure, while those overlying it exhibit -the confused starch-like grouping of curved and rather indistinctly-formed -prisms.</p> - -<p>The next band in upward succession is one of conglomerate (<i>f</i>), which -runs as a continuous and conspicuous feature along the upper part of the -cliff. This rock presents in many respects a strong contrast to the conglomerates -underneath. It is dull-green to yellow in colour, and is well -stratified, being marked by the interstratification of finer layers, and passing -down into a band of pebbly sandstone, which rests immediately on the -basalt (<i>e</i>). Its component stones are thoroughly water-worn, ranging up to -six inches or even more in length. But its most distinctive character lies in -the nature of its pebbles. Instead of consisting mainly of volcanic materials, -these stones have almost all been transported for some distance. They -include abundant fragments of Torridon Sandstone, gneiss, schists, grits, and -other rocks like those in Rum and Western Inverness-shire. No such rocks -exist <i>in situ</i> in Canna. The nearest tract of Torridon Sandstone is in Rum, -about four miles to the eastward. But the pieces of schist and epidotic -grit like the rocks of the Western Highlands, have probably travelled at least -30 miles.</p> - -<p>It is important to observe that all these transported stones indicate a -derivation from some source lying to the eastward of Canna. The evidence -in this respect agrees with that furnished by the ancient river-gravel under -the pitchstone of the Scuir of Eigg. It is clear that the waters which -found their way across the lava-fields of this part of the Inner Hebrides -took their rise somewhere to the eastward, probably among the mountains -of Inverness-shire.</p> - -<p>The conglomerate now described is from 40 to 50 feet thick. It can be -followed along the face of the cliffs for more than a mile on the north side -of Canna. Less persistent on the south side, its outcrop strikes from the -edge of the precipice inland, keeping to the south of the top of Compass -<span class="pagenum" id="Page_221">- 221 -</span> -Hill. It is well seen in the ravine above the Coroghon, but cannot be -followed further westward among the basalt-terraces. Yet, though this -stratified intercalation is not traceable far as a band of conglomerate, the -same stratigraphical horizon is probably indicated elsewhere by other kinds of -sedimentary deposits, to which further reference will be made in the sequel.</p> - -<p>The section now described establishes the existence of at least two -successive platforms of conglomerate in the volcanic series. Following -these platforms along their outcrop, we obtain additional light on their -origin, and on the topographical conditions under which they were deposited, -and we learn further that other prolonged intervals, which were -likewise marked by intercalations of sedimentary material, occurred in the -outpouring of the basalts.</p> - -<p>Taking first the lower conglomerate of Compass Hill and tracing it -westward, we find it to form the depression in which the sheltered inlet of -Canna Harbour lies. It is exposed along the shores and also in the islands -enclosed within the same bay. But it is not traceable further west, possibly -because it seems to sink beneath the level of the sea. To the south-east, -though it is there likewise for the most part concealed under the waves, it -rises above them in one or two parts of the coast-line of Sanday, particularly -at the Uamh Ruadh or Red Cave, and likewise on a surf-beaten skerry off -Ceann an Eilein, the highest part of the Sanday cliffs—a distance of about -a mile and a half from Compass Hill. Throughout this space it retains its -remarkably coarse character and is mainly made up of volcanic material.</p> - -<p>The numerous sections exposed in Canna Harbour enable us to study -the composition and local variations of this curious deposit. On the north -side of the basin, while the lower part of the sedimentary series continues -to be an exceedingly coarse volcanic conglomerate, it passes upward into -finer conglomerates, tuffs, and shales. In front of Canna House the -imbedded blocks are of large size, occasionally as much as three or four -feet in diameter. They are still more gigantic on the island of Eilean a' -Bhaird, where I found one to contain 150 cubic feet in the exposed part, the -rest being still concealed in the matrix. As they are generally somewhat -rounded, here and there markedly so, most of these stones have probably -undergone a certain amount of attrition in water. The great majority -of them, and certainly all those of larger size, are pieces of basalt, -dolerite, andesite, etc. Among them huge blocks of amygdaloid and -coarsely vesicular lava are specially abundant. Some of these look like -pieces of slag torn from the upper surface of lava-streams. Others, displaying -a highly vesicular centre and a close-grained outer crust, are suggestive -of bombs. It is interesting to note here again that the amygdaloidal blocks -present their zeolitic infiltrations so precisely like those of the amygdaloids -of the plateau that it seems reasonable to suppose the carbonate of lime, -zeolites, etc. to have been introduced before the blocks were imbedded in the -conglomerate.</p> - -<p>The whole aspect of this deposit is eminently volcanic. It looks like a -vast sheet of lava-fragments swept away from one or more cones of slags and -<span class="pagenum" id="Page_222">- 222 -</span> -cinders, or from the scoriaceous surface of a lava-stream. Where the -vesicles were still empty, the large boulders could be more easily swept -along by moving water. But a powerful current must have been needed -to transport and wear down into more or less rounded forms blocks of basic -lava, many of which must weigh several tons. The large block on Eilean -a' Bhaird probably exceeds 12 tons in weight.</p> - -<p>Besides the obviously volcanic contents of the conglomerate there occur -here also, as in the Compass Hill cliffs, abundant pieces of Torridon Sandstone. -These stones are notably smaller in size and more perfectly water-worn -and even polished than the blocks of lava. Obviously they have -travelled further and have undergone more prolonged attrition.</p> - -<p>The matrix of the rock consists essentially of the fine detritus of basic -lavas, probably mingled with true volcanic dust. The coarser parts display -only the feeblest indication of stratification; indeed, in a limited -exposure the rock might be regarded as a tumultuous agglomerate. But the -manner in which the deposit is intercalated with, and sometimes overlies, -green tuffs and shales, together with the water-worn condition of its stones, -shows that it has not been accumulated in a volcanic chimney, but has been -thrown down by some powerful body of water, with probably the co-operation -of volcanic discharges.</p> - -<p>While the composition of the conglomerate suffices to indicate that this -deposit was formed at a time when some volcano was active in the immediate -neighbourhood, singularly convincing proofs of the work of this vent -are to be seen in the form of intercalated sheets of lava. Thus on Eilean -a' Bhaird the boulders of the conglomerate are overlain and wrapped round -by a sheet of rudely prismatic basalt, with lines of vesicles arranged in the -direction of the bedding. A similar relation can be traced along the beach -between Canna House and the wooden pier, where successive sheets of basalt -have flowed over the conglomerate (<a href="#v2fig269">Fig. 269</a>).</p> - -<p>But, besides coarse volcanic detritus, the sedimentary platform represented -by the lower conglomerate of Compass Hill includes other deposits -of which good sections may be examined all round Canna Harbour. Beds -of fine well-stratified dull-green tuff pass by an admixture of pebbles into -fine ashy conglomerate or pebbly sandstone, and by an increase in the proportion -of their fine detritus into volcanic mudstone and fine shales. The -shales vary from a pale grey or white tone into blackish grey, brown, and -black. They are well stratified and are frequently interleaved with layers -of fine tuff. The darker bands are carbonaceous, and are not infrequently -full of ill-preserved vegetation. Indeed, leaves and stems in a rather -macerated condition are of common occurrence in all the shaly layers. -Here and there, especially in some ashy shales in front of Canna House, -I observed a recognisable <i>Sequoia</i>. The mudstones are dull green, close-grained -shattery rocks composed of fine volcanic detritus, and pass both -laterally and vertically into shales, tuffs, and conglomerates. They suggest -showers of fine dust or streams of volcanic mud. They, too, contain -fragmentary plants.</p> - -<p><span class="pagenum" id="Page_223">- 223 -</span></p> - -<p>It is a noteworthy fact that the sedimentary intercalations among the -Canna basalts generally end upward in carbonaceous shales or coaly layers. -The strong currents and overflows of water, which rolled and spread out the -coarse materials of the conglomerates, gave way to quieter conditions that -allowed silt and mud to gather over the water-bottom, while leaves and -other fragments of vegetation, blown or washed into these quiet reaches, -were the last of the suspended materials to sink to the bottom. Good -illustrations of this sequence in the case of the lower conglomerate zone of -Canna may be studied along the shores of Sanday, from the Catholic Chapel -eastwards. The fine pebbly sandstones, tuffs, and shales, which there overlie -the coarse conglomerate, are surmounted by dark brown or black carbonaceous -shale, with lenticles of matted vegetation that pass into impure coal. Immediately -overlying this coaly layer comes a sheet of prismatic vesicular basalt, -followed by another with an exceedingly slaggy texture.</p> - -<p>Lenticles of shale and mudstone likewise occur in the heart of the finer -parts of the conglomerate, especially towards the top, as may be seen in the -section exposed beneath the basalt behind the first cottage west from Canna -House. One of the most interesting layers in this section is a seam of -tuff, varying up to about two inches in thickness, which lies at the top of -the lenticular band of tuffs and shales, and immediately beneath the band -of basalt-conglomerate, on which a basalt, carrying a vesicular band near -its bottom, rests. Traced laterally, the dark brown tuff of this seam -gradually passes into a series of rounded bodies and flattened shells composed -of a colourless mineral which has evidently been developed <i>in situ</i> -after the deposition of the tuff. Mr. Harker's notes on thin slices made -from this band are as follows:—</p> - -<p>"This is a rusty-brown, dull-looking rock, rather soft and seemingly -light, but too absorbent to permit of its specific gravity being tested. The -dark brown mass is in great part studded with little spheroidal bodies, <sup>1</sup>/<sub>50</sub> -to <sup>1</sup>/<sub>10</sub> inch in diameter, of paler colour, but the larger ones having a dark -nucleus. In other parts larger flat bodies have been formed, as if by the -coalescence of the spheroids, extending as inconstant bands in the direction -of lamination for perhaps <sup>1</sup>/<sub>2</sub> inch, with a thickness of <sup>1</sup>/<sub>10</sub> inch or less. The -appearance is that of a spherulitic rather than an oolitic structure.</p> - -<p>"A slice [6658 <span class="allsmcap">A</span>] shows the general mass of the rock to be of an -extremely finely divided but coherent substance of brown colour, which can -scarcely be other than a fine volcanic dust, composed of minute particles of -basic glass or 'palagonite' compacted together. Scattered through this are -fragments of crystals recognizable as triclinic and perhaps monoclinic -felspars, green hornblende, augite, olivine (?), and magnetite, usually quite -fresh.</p> - -<p>"The curious spheroidal and elongated growths already mentioned are -better seen in another slide [6658 <span class="allsmcap">B</span>], where they occupy the larger part of -the field, leaving only an interstitial framework of the brown matrix. The -substance of the little spheroids is clear, colourless, and apparently structureless. -The centre is often occupied by an irregularly stellate patch of brown -<span class="pagenum" id="Page_224">- 224 -</span> -colour, and sometimes cracks tend to run in radiating fashion, but these are -the only indications of radial structure. The outer boundary is sharply -defined, and where the slice is shattered the spheroids have separated from -the matrix. The matrix is darker than in the normal rock, being obscured -by iron-oxide which we may conceive as having been expelled from the -spaces occupied by the spheroids. The little crystal-fragments are enclosed -in the spheroids as well as in the matrix, but there is no appearance of their -having served as starting-points for radiate growths. The flat elongated -bodies are like the spheroids, with merely the modifications implied in their -different shape.</p> - -<p>"The identity of the clear colourless substance seems to be rather -doubtful. It is sensibly isotropic and of refractive power distinctly lower -than that of felspar. These characters would agree with analcime, which is -not unknown as a contact-mineral; but it is difficult to understand how -analcime, even a lime-bearing variety like that of Plas Newydd,<a id="FNanchor_250" href="#Footnote_250" class="fnanchor">[250]</a> could be -formed in abundance from palagonitic material. An alternative supposition, -perhaps more probable, is that the clear substance is a glass, modified from -its former nature, especially by the expulsion of the iron-oxide into the -remaining matrix. A comparison is at once suggested with certain types of -'Knotenschiefer,' but respecting the thermal metamorphism of fine volcanic -tuffs there seems to be little or no direct information."</p> - -<div class="footnote"> - -<p><a id="Footnote_250" href="#FNanchor_250" class="label">[250]</a> Henslow, <i>Trans. Camb. Phil. Soc.</i> (1821), vol. i. p. 408; Mr. Harker, <i>Geol. Mag.</i> (1887), -p. 414. Mr. W. W. Watts suggests a comparison with the hexagonal bodies figured by Mr. -Monckton in an altered limestone from Stirlingshire: <i>Quart. Journ. Geol. Soc.</i>, vol. li. p. 487.</p> - -</div> - -<p>Lenticular interstratifications of shale and mudstone make their appearance -even in the coarser parts of the conglomerate, as may be observed on -the beach below Canna House where, as shown in <a href="#v2fig269">Fig. 269</a>, some shales and -tuffs (<i>a</i>) full of ill-defined leaves are surmounted by a conglomerate (<i>b</i>). The -deposition of this overlying bed of boulders has given rise to some scooping-out -of the finer strata underneath. Subsequently both the conglomerate -and shales have been overspread -by a stream of dolerite -(<i>c</i>), the slaggy bottom of which -has ploughed its way through -them.</p> - -<div class="figleft" id="v2fig269" style="width: 274px;"> - <img src="images/v2fig269.png" width="274" height="160" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 269.</span>—Lava cutting out conglomerate and shale. - Shore below Canna House.</div> -</div> - -<p>Before discussing the probable -conditions under which -the group of sedimentary deposits -now described was formed, -we may conveniently follow -the upper conglomerate band -of the Compass Hill, and note -the variations in structure and composition which its outcrop presents.</p> - -<p>This yellowish conglomerate can be traced along the cliffs for more than -a mile, when it descends below the sea-level at the solitary stack of Bod an -Stòl. A few hundred yards further west, what is probably the same band -<span class="pagenum" id="Page_225">- 225 -</span> -appears again at the base of the precipice overlain by prismatic basalts. -But the conglomerate, here only 12 feet thick, is made of much finer -detritus which, largely composed of volcanic material, includes small well-rounded -and polished pebbles of Torridon Sandstone. Beneath it lies a bed -of dark shale, with remains of plants, resting immediately on a zeolitic -amygdaloid which plunges into the sea. The chief interest of this locality -is to be found in the shale which, instead of being at the top of the sedimentary -group, lies at the bottom. I was informed by Mr. A. Thom that -leaves had been obtained from this shale; but I was not successful in my -search for them. The locality is only accessible by boat, and, as the coast is -fully exposed to the Atlantic swell, landing at the place is usually difficult -and often impossible.</p> - -<p>About a mile and a half still further west, where a foreshore fronts the -precipice of Earnagream at the Camas Tharbernish, a band of intercalated -sedimentary material underlies the great escarpment of basalts and rests -upon the slaggy sheet with the singular surface already referred to -(<a href="#Page_187">p. 187</a>). This band not improbably occupies the same platform as the -upper conglomerate of Compass Hill. It is only about seven feet thick, the -lower four feet consisting of a dull green pebbly tuff or ashy sandstone, with -small rounded pieces of Torridon Sandstone, while the upper three feet are -formed of dark shale with crowded but indistinct remains of plants. Here -the more usual order in the sequence of deposition is restored. The shale is -indurated and shattery, so that no slabs can be extracted without the use of -quarrying tools.</p> - -<div class="figright" id="v2fig270" style="width: 239px;"> - <img src="images/v2fig270.png" width="239" height="98" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 270.</span>—Section of shales and tuffs, with a coniferous - stump lying between two basalt-sheets, - Cùl nam Marbh, Canna.</div> -</div> - - -<p>Rather less than half a mile towards the south, on the roadside at the -gully of Cùl nam Marbh, the basalts enclose a sedimentary interstratification -which not improbably lies on the same horizon as those just described along -the northern shore. The relations of the rocks at this locality are shown in -<a href="#v2fig270">Fig. 270</a>. A remarkable slaggy basalt (<i>a</i>) rises into a hummock, against -which have been deposited some fine granular tuffs (<i>b</i>) whereof only a few -inches are visible, that pass up into a thin band of dark shale (<i>c</i>), including -a layer of pebbly ferruginous tuff, with small rounded pea-like pieces of -basalt, basic pumice, bole, limonite, -etc. At the top of this shale an -irregular parting of coaly material -(<i>d</i>) lies immediately under the slaggy -base of the succeeding basalt (<i>e</i>). It -will be observed that this upper lava -cuts out the shale and thus comes to -rest directly upon the lower sheet. At -the point where it begins to descend -it has caught up and enclosed a small tree-stump (<i>d′</i>) which stands -upright on the coaly parting and shale. This stump, at the time -of my visit, measured five inches in height by three inches in breadth; -it had been thoroughly charred and was crumbling away on exposure, -but among the pieces which I took from it sufficient trace of structure -<span class="pagenum" id="Page_226">- 226 -</span> -can be detected with the microscope to show the tree to have been a -conifer.</p> - -<p>We have here another instance of the deposition of volcanic dust and -fine mud in a pool that filled a hollow in the lava-field. Again we see -that the closing act of sedimentation was the subsidence of vegetable -matter in the pool, which was finally buried under another outflow of -basalt.</p> - -<div class="figcenter" id="v2fig271" style="width: 479px;"> - <img src="images/v2fig271.png" width="479" height="410" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 271.</span>—Dùn Mòr, Sanday. (From a photograph by Miss Thom.)</div> -</div> - -<p>It is on the southern coast of the isle of Sanday that the higher intercalations -of sedimentary material among the basalts are most instructively -displayed. At the eastern end of this island, as already stated, the lowest -and coarsest conglomerate is visible on a skerry immediately to the south of -the headland Ceann an Eilein. It doubtless underlies the Sanday cliffs, but -is not there visible, for the basalts descend below sea-level. These volcanic -sheets have a slight inclination westward; hence in that direction we -gradually pass into higher parts of the series. In the Creag nam Faoileann -(Seamews' Crag) and the gully that cuts its eastern end, likewise in the two -singularly picturesque stacks of Dùn Mòr and Dùn Beag (Big and Little Gull -Rocks), which here rise from the foreshore, two distinct platforms of detrital -<span class="pagenum" id="Page_227">- 227 -</span> -material may be noticed among the basalts. Both of these can be well seen -on Dùn Mòr, about 100 feet high, which is represented in <a href="#v2fig271">Fig. 271</a>. The lower -band, four or live feet thick, is here a rather coarse conglomerate which lies -upon a sheet of scoriaceous basalt that extends up to the base of the Creag nam -Faoileann. It is directly overlain by another basalt, about 30 feet thick, -which dips seawards and forms a broad shelving platform, whereon the tides -rise and fall. On this stack a second coarse conglomerate, about 10 feet -thick, forms a conspicuous band about a third of the height from the -bottom; it is composed mainly of well-rounded blocks of various lavas up to -18 inches or more in diameter, but it contains also pieces of Torridon Sandstone. -It is covered by about 60 feet of basalt, which towards the base is -somewhat regularly columnar, but passes upward into the wavy, starch-like, -prismatic structure.</p> - -<p>If now we trace these two intercalated zones of conglomerate along the -shore, we find them both rapidly to change their characters and to disappear. -The lower, though formed of coarse detritus under the Dùn Mòr, passes on -the opposite cliff in a space of not more than 60 yards, into fine tuff and -shale, about six feet thick, which become carbonaceous at the top where they -are overlain by the next basalt. A hundred yards to the east, the band -likewise consists of tuffs and ashy shales, which underlie the basalts on the -Dùn Beag, and again show the usual coaly layers at the top. On the east -side of the gully in the coast, about 160 yards to the north-east of Dùn Mòr, -the same band is reduced to not more than three feet in thickness, consisting -chiefly of fine conglomerate, wherein well water-worn pebbles of Torridon -Sandstone and epidotic grit appear among the predominant volcanic -detritus. This conglomerate is surmounted by a few inches of dark carbonaceous -mudstone or shale. Rough slaggy basalts lie above and below the -band.</p> - -<p>The upper conglomerate dies out, both towards the east and the west, in -the cliff opposite to Dùn Mòr, dwindling down at last to merely a few -pebbles between the basalts. It lies in a kind of channel or hollow among -these lavas. This depression, in an east and west direction, cannot be more -than about 65 yards broad.</p> - -<p>Probably still higher in the series of basalts is another intercalation of -sedimentary layers which may be seen in the little bay to the east of -Tallabric, rather more than a mile to the west of the Creag nam Faoileann. -It rests upon a coarsely slaggy amygdaloid, and is from six to ten feet in -thickness. The lower and larger part of the deposit consists of greenish -pebbly sandstone and fine conglomerate, largely composed of basaltic -detritus, but including abundant well-smoothed and polished pebbles of -Torridon Sandstone, green grit, quartzite, etc. The stones vary from mere -pea-like pebbles up to pieces two or three inches long, the largest being -generally fragments of slag and amygdaloid which are less water-worn than -the sandstones and other foreign ingredients. The uppermost two or three -feet of the intercalation consist of dark carbonaceous mudstone or shale, -made up in large measure of volcanic detritus, which may have been -<span class="pagenum" id="Page_228">- 228 -</span> -derived partly from eruptions of fine dust, partly from subærial disintegration -of the basalt-sheets. Some layers of these finer strata are full of -remains of much macerated plants.</p> - -<p>Other thin coaly intercalations have been observed among the basalts of -Canna, some of which may possibly mark still higher horizons than those -now described. But, confining our attention to the regular sequence of -intercalations exposed along the Sanday coast, we find at least four distinct -platforms of interstratified sediment among the plateau-basalts of this -district. Each of these marks a longer or shorter interval in the outflow -of lava, and points to the action of moving-water over the surface of the -lava-fields.</p> - -<p>We may now consider the probable conditions under which this intervention -of aqueous action took place. The idea that the sea had anything -to do with these conglomerates, sandstones, and shales may be summarily -dismissed from consideration. The evidence that the basalt-eruptions took -place on a terrestrial surface is entirely convincing, and geologists are now -agreed upon this question.</p> - -<p>Excluding marine action, we have to choose among forms of fresh water—between -lakes on the one hand and rivers on the other. That the -agency concerned in the transport and deposition of these strata was that -of a river may be confidently concluded on the following grounds:—</p> - -<p>1. The large size and rolled shape of the boulders in the conglomerates. -To move blocks several tons in weight, and not only to move them but to -wear them into more or less rounded forms, must have required the operation -of strong currents of water. The coarse detritus intercalated among the -basalts is quite comparable to the shingle of a modern river, which descends -with rapidity and in ample volume from a range of hills.</p> - -<p>2. The evidence that the materials of the conglomerates are not entirely -local, but include a marked proportion of foreign stones. The proofs of -transport are admirably exhibited by pieces of Torridon Sandstone, epidotic -grit, quartzite, and other hard rocks none of which occur <i>in situ</i> except at -some distance from Canna. These stones are often not merely rounded, but -so well smoothed and polished as to show that they must have been rolled -along for some considerable time in water.</p> - -<p>3. The lenticular character and rapid lithological variations of the -strata, both laterally and vertically. The coarse conglomerates die out as -they are followed along their outcrop and pass into finer sediment. They -seem to occur in irregular banks, which may not be more than 200 feet -broad, like the shingle-banks of a river. The coarser sediment generally lies -in the lower part of the sedimentary group. But cases may be observed, -such as that shown in <a href="#v2fig269">Fig. 269</a>, where fine sediment, laid down upon the -bottom conglomerate, has subsequently been overspread by another inroad of -coarse shingle. Such alternations are not difficult to understand if they are -looked upon as indicating the successive floods and quieter intervals of a -river.</p> - -<p>For these reasons I regard the platforms of sedimentary materials intercalated -<span class="pagenum" id="Page_229">- 229 -</span> -among the basalts of Canna and Sanday as the successive flood-plains -of a river which, like the rivers that traverse the lava-deserts of Iceland, -flowed perhaps in many separate channels across the basalt-fields of the -Inner Hebrides, and was liable to have its course shifted from time to time -by fresh volcanic eruptions. That this river came from the east or north-east -and had its source among the Western Highlands of Inverness-shire, may -be inferred from the nature of the stones which it has carried for 30 miles -or more along its bed. And that it crossed in its course the tract of -Torridon Sandstone, of which a portion still remains in Rum, is manifest -from the abundance of the fragments of that formation in the conglomerates.</p> - -<p>With the remarkable exception of the section on Dùn Beag, to be -immediately referred to, no trace of any eroded channel of this river through -the lavas of the great volcanic plain has been preserved. Possibly frequent -invasions of its bed by streams of basalt from different vents hindered it -from remaining long enough in one course to erode anything like a gorge or -canon. But, in any case, the main channel of the river probably lay rather -to the east of the present islands of Canna and Sanday, on ground -which is now covered by the sea. The banks or sheets of boulder-conglomerate -undoubtedly show where its current swept with great force over the -lava-plain, but the manner in which these coarser materials are so often -covered with fine silt suggests that the sedimentary materials now visible -were rather deposited on the low grounds over which the steam rushed in -times of flood. Pools of water would often be left after such inundations, -and in these depressions silt would gradually accumulate, partly carried in -suspension by the river, partly washed in by rain, while drift-wood that -found its way into these eddies, and leaves blown into them from the trees -and shrubs of the surrounding country, would remain for some time afloat -and would be the last of the detritus to sink to the bottom. Hence, no -doubt, the carbonaceous character of the hardened silt in the upper part of -each intercalation of sediment.</p> - -<p>If we were to look upon the volcanic materials in the conglomerates as -derived from the subærial disintegration of the fields of basalt, we should be -compelled to admit a very large amount of erosion of the surface of the volcanic -plain during the period when the river flowed over that tract. It would be -necessary to suppose not only that there was a considerable rainfall, but -that the differences of temperature, either from day to night, or from -summer to winter, were so great as to split up the lavas at the surface, in -order to provide the river with the blocks which it has rolled into rounded -boulders. I do not think, however, that such a deduction would be sound. -If we compare the materials that have filled up the large eruptive vent at -the east end of Canna (to be afterwards described) with the great majority -of the blocks in the coarse conglomerates, we cannot fail to note their strong -resemblance. The abundance of lumps of slaggy lava in the river-shingle -corresponds with their predominance in the agglomerate of the vent. The -boulders of basalt, dolerite, and andesite which crowd the conglomerates -<span class="pagenum" id="Page_230">- 230 -</span> -need not have been derived from the action of atmospheric waste on the -lava-fields, but might quite well have been mainly supplied by the demolition -of volcanic cones of fragmental materials.</p> - -<div class="figcenter" id="v2fig272" style="width: 466px;"> - <img src="images/v2fig272.png" width="466" height="411" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 272.</span>—View of the Dùn Beag, Sanday, seen from the south.<br /><br /> - (From a Photograph by Miss Thom.)</div> -</div> - -<p>That such has really been the chief source of the blocks in the conglomerates -I cannot doubt. At the east end of Canna we actually detect a -volcanic cone, partly washed down and overlain by a pile of river-shingle. -There were probably many such mounds of slag and stones along lines of -fissure all over the lava-fields. The river in its winding course might come -upon one cone after another, and during times of flood, or when its waters -burst through any temporary barrier created by volcanic operations it would -attack the slopes of loose material and sweep their detritus onward. At the -same time, the current would carry forward its own natural burden of far-transported -sediment, and hence on its flood-plains, buried and preserved -under sheets of basalt, we find abundant pebbles of the old Highland rocks -which it had borne across the whole breadth of the basaltic lowland.</p> - -<p>But the destruction of volcanic cones was probably not the only source of -the basaltic detritus in the conglomerates of Canna and Sanday. I have -shown that these conglomerates pass laterally into tuffs, and are sometimes -<span class="pagenum" id="Page_231">- 231 -</span> -underlain, sometimes overlain, with similar material. It is quite obvious that -their deposition was contemporaneous with volcanic action in the immediate -neighbourhood, and that at least part of their finer sediment was obtained -directly from volcanic explosions. In wandering over the coast-sections of -these coarse deposits, I have been impressed with the enormous size -of many of the stones, their resemblance to the ejected blocks of the -agglomerate, and the distinction that may sometimes be made with more or -less clearness between their rather angular forms and the more rounded and -somewhat water-worn aspect of the other boulders. It seems to me not -improbable that some of the remarkably coarse masses of unstratified conglomerate -in Canna Harbour consist largely of ejected blocks from the -adjacent vent.</p> - -<div class="figcenter" id="v2fig273" style="width: 469px;"> - <img src="images/v2fig273.png" width="469" height="421" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 273.</span>—View of Dùn Beag, Sanday, from the north. The island of Rum in the distance.<br /><br /> - (From a Photograph by Miss Thom.)</div> -</div> - -<p>The only instance which I have observed of erosion of the basalt contemporaneous -with the operations of the river that spread out this conglomerate -is to be found in the striking stack of Dùn Beag already alluded to. -<span class="pagenum" id="Page_232">- 232 -</span><a id="FNanchor_251" href="#Footnote_251" class="fnanchor">[251]</a> -This extraordinary monument of geological history forms an outlying obelisk -which rises from the platform of the shore to a height of about 70 feet. -Seen from the south-west, it appears to consist entirely of bedded basalt -resting on some stratified tuff and shale which intervene between these lavas -and that of the broad platform of basalt on which the obelisk stands. On -that side it presents no essential difference from the structure of the Dùn -Mòr to the west, save that the lower conglomerate of that outlier is here -represented by fine sediment, and the upper conglomerate is wanting. The -general aspect of this south-western front of the stack is shown in <a href="#v2fig272">Fig. 272</a>. -If, however, we approach the rock from the coast-gully to the north, we -form a very different impression of its structure. It then appears to consist -chiefly of conglomerate with a capping of basalt on the top (Fig. -273). Not until a close scrutiny is made of the eastern and western -faces of the column do the true structure and history of this singular -piece of topography become apparent.</p> - -<div class="footnote"> - -<p><a id="Footnote_251" href="#FNanchor_251" class="label">[251]</a> This pinnacle of rock is referred to by Macculloch in his account of Canna, and is figured in -Plate xix. Fig. 3 of his work already cited. But neither his description nor his drawing conveys -any idea of the real structure of the rock.</p> - -</div> - -<div class="figcenter" id="v2fig274" style="width: 414px;"> - <img src="images/v2fig274.png" width="414" height="330" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 274.</span>—Section of eastern front of Dùn Beag.<br /><br /> - <i>a</i>, Very shaggy amygdaloidal basalt; <i>b</i>, shales and tuff; <i>c</i>, slaggy and jointed basalts; <i>d</i>, conglomerate; - <i>e</i>, prismatic basalt.<br /><br /> - The dotted lines indicate the supposed form of the ravine.</div> -</div> - -<p>On the eastern front, the section represented in <a href="#v2fig274">Fig. 274</a> is exposed. -At the bottom, forming the pediment of the column, lies a sheet of slaggy -and vesicular or amygdaloidal basalt (<i>a</i>), which shelves gently in a south-westerly -direction into the sea. The lowest band (<i>b</i>) in the structure of the -stack is a thin group of lilac, brown, and green shale and volcanic mudstone -or tuff, which encloses pieces of coniferous wood, and becomes markedly -carbonaceous in its uppermost layers. Above these strata on the south -<span class="pagenum" id="Page_233">- 233 -</span> -front comes the pile of bedded basalts (<i>c</i>) with their slaggy lower and upper -surfaces. But as we follow them round the east side, we find them to be -abruptly cut off by a mass of conglomerate (<i>d</i>). That the vertical junction-line -is not a fault is speedily ascertained. The lower platform of slaggy -basalt runs on unbroken under both shales and conglomerate. Moreover, -the line of meeting of this conglomerate -with the basalts that overlie the shales is -not a clean-cut straight wall, but displays -projections and recesses of the igneous rocks, -round and into which the materials of the -conglomerate have been deposited. The -pebbles may be seen filling up little crevices, -passing under overhanging ledges of the -basalts, and sharply truncating lines of -scoriaceous structure in these rocks. The -same relations may be observed on the west -front of the stack. There the ashy shales -and tuffs are sharply cut out by the conglomerate, -which wraps round and underlies -a projecting cornice of the slaggy bottom of the basalt that rests on the -stratified band (<a href="#v2fig275">Fig. 275</a>).</p> - -<div class="figright" id="v2fig275" style="width: 192px;"> - <img src="images/v2fig275.png" width="192" height="164" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 275.</span>—Enlarged Section on the western - side of Dùn Beag.<br /><br /> - <i>a</i>, amygdaloid; <i>b</i>, tuff; <i>c</i>, ashy shales; <i>d</i>, layer - of coaly shale; <i>e</i>, amygdaloidal basalts conglomerate.</div> -</div> - - -<p>The conglomerate is rudely stratified horizontally, its bedding being -best shown by occasional partings of greenish sandstone. It consists of -well-rounded, polished, and water-worn stones, chiefly of members of the -volcanic series—basalts, and dolerites, both compact and amygdaloidal or -slaggy—but with a conspicuous admixture of Torridon Sandstone, gneiss, -grey granite, grit and different schists. The coarsest part of the deposit -lies toward the bottom where the volcanic blocks, some of them being six -and eight feet in diameter, may have originally fallen from the basalts -against which the conglomerate now reposes. The far-transported stones -are also of considerable size, pieces of granite and gneiss frequently -exceeding a foot in length. The well-rounded pebbles of foreign materials -have been washed into the interstices between the large volcanic blocks.</p> - -<p>It is, I think, tolerably clear that the wall of basalt against which this -conglomerate has been laid down is one of erosion. The beds of basalt have -here been trenched by some agent which has likewise scooped out the soft -underlying shales, and even cut them away from under their protecting -cover of basalt. There can be little hesitation in regarding this agent as a -water-course, which for some considerable interval of time continued to dig -its channel through the hard basalts. There is not room enough between -the basalt-wall of Dùn Beag and the opposite cliffs of the shore (where no -trace of this conglomerate is to be seen) for any large stream to have found -its way. I do not therefore seek to identify this relic of an ancient waterway -with the channel of the main river which deposited the conglomerate -bands of Canna and Sanday. More probably it was either a mere torrential -chasm, or a tributary stream draining a certain part of the volcanic plateau -<span class="pagenum" id="Page_234">- 234 -</span> -and allowed to retain its channel long enough to be able to erode it to a -depth of nearly 50 feet. Erosion had reached down through the underlying -tuffs to the slaggy basalt below, but before it had made any progress in -that sheet its operations were brought to an end at this locality by the floods -that swept in the coarse shingle, and by the subsequent stream of basalt of -which a mere outlying fragment now forms the upper third of the stack -(<i>e</i>, <a href="#v2fig274">Fig. 274</a>).</p> - -<p>That the ravine or gully of Dùn Beag probably lay within the reach of -the floods of the main river, may be inferred from the number and size of -the far-transported rocks in its conglomerate. It was filled up gradually, -but the conditions of deposition remained little changed during the process, -except that the largest blocks of rock were swept into the chasm in the -earlier part of its history, while much smaller and more water-worn shingle -were introduced towards the close.</p> - -<p>Denudation, which has performed such marvels in the topography of -the West of Scotland since older Tertiary time, has here obliterated every -trace of this ancient gully, save the little fragment of one of the walls which -survives in the stack of Dùn Beag. When in the course of centuries this -picturesque obelisk shall have yielded to the action of the elements, the last -leaflet of one of the most interesting chapters in the geological history of the -Inner Hebrides will have been destroyed.</p> - -<p>The question naturally arises—What was the subsequent history of the -river which has left so many records of its floods entombed among the -basalts of Canna and Sanday? In particular, can any connection be traced -or plausibly conjectured between it and the river-bed preserved under the -Scuir of Eigg? To this question I shall return after the evidence for the -existence and date of the latter stream has been laid before the reader.</p> - -<p>In the chain of the Inner Hebrides, broken as it is in outline and varied -in its types of scenery, there is no object more striking than the island of -Eigg. Though only about five miles long and from a mile and a half to -three miles and a half broad, and nowhere reaching a height of so much as -1300 feet, this little island, from the singularity of one feature of its -surface, forms a conspicuous and familiar landmark. Viewed in the -simplest way, Eigg may be regarded as consisting of an isolated part of -the basaltic plateau which, instead of forming a rolling tableland or a -chain of hills with terraced sides, as in Antrim, Mull and Skye, has been so -tilted that, while it caps a lofty cliff about 1000 feet above the waves at the -north end, it slopes gently along the length of the island to the south end. -In its southern half, however, the ground rises, owing to the preservation -of an upper mass of lavas, which denudation has removed from the northern -half. On this thicker part of the plateau stands the distinguishing -feature of the island, the strange fantastic ridge of the Scuir, which, seen -from the north or south, looks like a long steep hill-crest, ending in a sharp -precipice on the east. Viewed from the east, this precipice is seen to be the end -of a huge mountain-wall, which rises vertically above the basalt-plateau to -a height of more than 350 feet. The accompanying map (<a href="#v2fig276">Fig. 276</a>) shows -<span class="pagenum" id="Page_235">- 235 -</span> -that the ridge of the Scuir corresponds with the area occupied by a mass of -pitchstone, and that while the basaltic rocks cover the whole of the rest of -the southern half of the island, they gradually rise towards the north, -successive members of the Jurassic series making their appearance until, at -the cliffs of Dunan Thalasgair, the latter cover the greater part of the -surface, and leave the volcanic rocks as a mere stripe capping the cliffs. -In the section (<a href="#v2fig277">Fig. 277</a>) the general structure of the island is represented.</p> - -<div class="figcenter" id="v2fig276" style="width: 431px;"> - <img src="images/v2fig276.png" width="431" height="582" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 276.</span>—Geological Map of the Island of Eigg.<br /><br /> - P, Pitchstone-lava of the Scuir; R, old river gravel under pitchstone; <i>p p</i>, small veins of Pitchstone; <i>b b</i>, dykes, - veins and sheets of intrusive basalt; the short black lines running north-west and south-east are basalt dykes; - <i>f f</i>, granophyre sills; D, bedded basalts with occasional tuffs; F, andesite; 1, 2, 3, 4, clays, shales, sandstones, - limestones, etc. (Jurassic); xx, Loch Beinn Tighe; x, Loch a Bhealaich. —> General dip of the rocks.</div> -</div> - - -<p><span class="pagenum" id="Page_236">- 236 -</span></p> - -<div class="figcenter" id="v2fig277" style="width: 691px;"> - <img src="images/v2fig277.png" width="691" height="78" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 277.</span>—Section of the geological structure of the Island of Eigg.<br /><br /> - P, Pitchstone-lava of Scuir; <i>c</i>, ancient river-gravel; <i>p p</i>, pitchstone veins; <i>f f</i>, intrusive granophyre, etc.; <i>b b</i>, dolerite and basalt dykes and veins; B, intrusive - dolerite and basalt-sheets; D, bedded dolerites and basalts; F, andesite bed; 1-4, Jurassic rocks.</div> -</div> - -<p>In Eigg the fragment of the basalt-plateau which has been preserved, -rests unconformably on successive platforms of the Jurassic formations. Its -component sheets of lava rise in cliffs around the -greater part of the island. As they dip gently southwards -their lower members are seen along the -northern and eastern shores, while on the south-west -side their higher portions are exposed in the lofty -precipices which there plunge vertically into the sea. -The total thickness of the volcanic series may here -be about 1100 feet. The rocks consist of the usual -types—black, fine-grained, columnar and amorphous -basalts, more coarsely crystalline dolerites, dull earthy -amygdaloids with red partings, and occasional thin -bands of basalt-conglomerate or tuff. The individual -beds range in thickness from 20 to 50 or 60 feet. -Though they seem quite continuous when looked at -from the sea, yet, on closer examination, they are -found not unfrequently to die out, the place of one -bed being taken by another, or even by more than one, -in continuation of the same horizon. The only -marked petrographical variety which occurs among -them is a light-coloured band which stands out conspicuously -among the darker ordinary sheets of the -escarpment on the east side of the island. The -microscopic characters of this rock show it to belong -to the same series of highly felspathic, andesitic, or -trachitic lavas as the "pale group" of Ben More, in -Mull. It is strongly vesicular, and the cells are in -some parts so flattened and elongated as to impart a -kind of fissile texture to the rock. There can be no -doubt that this band is a true lava, and that it was -poured out during the accumulation of the basalt-plateau. -It supplies an interesting example of the -intercalation of a lighter and less basic lava among the -ordinary heavy basic basalts and dolerites.</p> - -<p>That feature of the island of Eigg which renders -it so remarkable and conspicuous an object on the -west coast is the long ridge of the Scuir. Rising -gently from the valley which crosses the island from -Laig Bay to the Harbour, the basaltic plateau ascends -south-westwards in a succession of terraces, until -along its upper part it forms a long crest, from 900 -to 1000 feet above the sea, to which it descends on -the other or south-west side, first by a sharp -slope, and then by a range of precipices. Along the watershed of -this crest runs, in a graceful double curve, the abrupt ridge of the Scuir, -<span class="pagenum" id="Page_237">- 237 -</span> -terminating on the north-west at the edge of the great sea-cliff (975 -feet), and ending off on the south-east in that strange well-known mountain-wall -(1272 feet high) which rises in a sheer cliff nearly 300 feet above the -basalt-plateau on the one side and more than 400 feet on the other (<a href="#v2fig278">Fig. 278</a>). -The total length of the Scuir ridge is two miles and a quarter, its greatest -breadth 1520, its least breadth 350 feet. Its surface is very irregular, -rising into minor hills and sinking into rock-basins, of which nine are small -tarns, besides still smaller pools, while six others, also filled with water, lie -partly on the ridge and partly on the basaltic plateau. No one, indeed, -who looks on the Scuir from below, and notes how evenly it rests upon the -basalt-plateau, would be prepared for so rugged a landscape as that which -meets his eye everywhere along the top of the ridge. Two minor arms project -from the east side of the ridge; one of these forms the rounded hill -called Beinn Tighe (968 feet), the other the hill of A chor Bheinn.</p> - -<div class="figcenter" id="v2fig278" style="width: 379px;"> - <img src="images/v2fig278.png" width="379" height="228" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 278.</span>—View of the Scuir of Eigg from the east.</div> -</div> - -<p>Singular as the Scuir of Eigg is, regarded merely as one of the landmarks -of the Hebrides, its geological history is not less peculiar. The -natural impression which arises in the mind when this mountain comes into -view for the first time is, that the huge wall is part of a great dyke or intrusive -mass which has been thrust through the older rocks.<a id="FNanchor_252" href="#Footnote_252" class="fnanchor">[252]</a> It was not until after -some time that the influence of this first impression passed off my own mind, -and the true structure of the mass became apparent.</p> - -<div class="footnote"> - -<p><a id="Footnote_252" href="#FNanchor_252" class="label">[252]</a> Hay Cunningham remarks:—"In regard to the relations of the pitchstone-porphyry of the -Scuir and the trap-rocks with which it is connected, it can, after a most careful examination -around the whole mass, be confidently asserted that it exists as a great vein which has been -erupted through the other Plutonic rocks—thus agreeing in age with all the other pitchstones -of the island." Macculloch leaves us to infer that he regarded the rock of the Scuir to be -regularly interstratified with the highest beds of the dolerite series (<i>Western Isles</i>, i. p. 522). -Hugh Miller speaks of the Scuir of Eigg as "resting on the remains of a prostrate forest."—<i>Cruise -of the Betsy</i>, p. 32.</p> - -</div> - -<p>The ridge of the Scuir, presenting as it does so strong a topographical -<span class="pagenum" id="Page_238">- 238 -</span> -contrast to the green terraced slopes of the plateau-basalts on which it rests, -consists of some very distinct bands of black and grey lava, long known as -"pitchstone-porphyry." To the nature and history of these rocks I shall -return after we have considered a remarkable bed of conglomerate which lies -below them. On the lower or southern side of the ridge the bottom of the -pitchstone, dipping into the hill, is exposed on the roof of a small cave where -the ends of its columns form a polygonal reticulation. It is there seen to -repose upon a bed of breccia or conglomerate, having a pale-yellow or grey -felspathic matrix like the more decomposing parts of the grey devitrified -parts of the pitchstone. Through this deposit are dispersed great numbers -of angular and subangular pieces of pitchstone, some of which have a striped -texture. Fragments of basalt, red (Torridon) sandstone, and other rocks are -rare; and the bed suggests the idea that it is a kind of brecciated base or -floor of the main pitchstone mass. A similar rock is found along the bottom -of the pitchstone on both sides of the ridge (<i>c</i>, in <a href="#v2fig279">Fig. 279</a>). Here and there -where this breccia is only a yard or two in thickness, it consists of subangular -fragments of the various dolerites and basalts of the neighbourhood, -together with pieces of red sandstone, quartzite, clay-slate, etc. The matrix -is in some places a mass of hard basalt debris; in others it becomes more -calcareous, passing into a sandstone or grit in which chips and angular or -irregular-shaped pieces of coniferous wood are abundant.<a id="FNanchor_253" href="#Footnote_253" class="fnanchor">[253]</a> A little further -east, beyond the base of the Scuir, a patch of similar breccia is seen, but -with the stones much more rounded and smoothed. This outlier rests -against the denuded ends of the basalt-beds forming the side of the hill. -Its interest arises from the evidence it affords of the prolongation of the -deposit eastward, and consequently of the former extension of the precipice -of the Scuir considerably beyond its present front.</p> - -<div class="footnote"> - -<p><a id="Footnote_253" href="#FNanchor_253" class="label">[253]</a> The microscopic structure of this wood was briefly described by Witham (<i>Fossil Vegetables</i>, -p. 37), and two magnified representations were given to show its coniferous character. Lindley -and Hutton further described it in their "Fossil Flora," naming it <i>Pinites eiggensis</i>, and regarding -it as belonging to the Oolitic series of the Hebrides—an inference founded perhaps on the -erroneous statement of Witham to that effect. William Nicol corrected that statement by -showing that the wood-fragments occurred, not among the "lias rocks," but "among the debris -of the pitchstone" (<i>Edin. New Phil. Journal</i>, xviii. p. 154). Hay Cunningham, in the paper -already cited, states that the fossil wood really lies in the pitchstone itself! The actual position -of the wood, however, in the breccia and conglomerates underlying the pitchstone is beyond all -dispute. I have myself dug it out of the bed. The geological horizon assigned to this conifer, -on account of its supposed occurrence among Oolitic rocks, being founded on error, no greater -weight can be attached to the identification of the plant with an Oolitic species. Our knowledge -of the specific varieties of the microscopic structure of ancient vegetation is hardly precise enough -to warrant us in definitely fixing the horizon of a plant merely from the examination of the -minute texture of a fragment of its wood. From the internal organization of the Eigg pine, there -is no evidence that the fossil is of Jurassic age. From the position of the wood above the -dolerites and underneath the pitchstone of the Scuir it is absolutely certain that the plant is not -of Jurassic but of Tertiary date.</p> - -</div> - -<p>It is at the extreme north-western extremity of the pitchstone ridge, -however, that the most remarkable exposure of this intercalated detrital -band is now to be seen. Sweeping along the crest of the plateau the ridge -reaches the edge of the great precipice of Bideann Boidheach, by which its -<span class="pagenum" id="Page_239">- 239 -</span> -end is truncated, so as to lay open a section of the gravelly deposit along -which the pitchstone flowed.</p> - -<p>The accompanying diagram (<a href="#v2fig279">Fig. 279</a>) represents the natural section -there exposed. Rising over each other in successive beds, with a hardly -perceptible southerly dip of 2°, the sheets of basalt form a mural cliff about -700 feet high. The bedded character of these rocks and their alternations -of compact, columnar, amorphous and amygdaloidal beds are here strikingly -seen. They are traversed by veins and dykes of an exceedingly close-grained, -sometimes almost flinty, basalt. But the conspicuous feature of -the cliff is the hollow which has been worn out of these rocks, and which, -after being partially filled with coarse conglomerate, has been buried under -the huge pitchstone mass of the Scuir. The conglomerate consists of water-worn -fragments, chiefly of dolerite and basalt, but with some also of the -white Jurassic sandstones, imbedded in a compacted sand derived from the -waste of the older volcanic rocks. The grey devitrified bands in the pitchstone, -so conspicuous at the east end of the Scuir, here disappear and leave -the conglomerate covered by one huge overlying mass of glassy pitchstone.</p> - -<div class="figcenter" id="v2fig279" style="width: 371px;"> - <img src="images/v2fig279.png" width="371" height="263" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 279.</span>—Natural Section at the Cliff of Bideann Boidheach, north-west end of the Scuir of Eigg.<br /><br /> - <i>a a</i>, Bedded dolerites and basalts; <i>b</i>, basalt dykes and veins; <i>c</i>, ancient river-bed filled with - conglomerate; <i>p</i>, pitchstone of the Scuir.</div> -</div> - -<p>If any doubt could arise as to the origin of the mass of detritus exposed -under the pitchstone at the east end of the Scuir it would be dispelled by -the section at the west end, which shows with unmistakable clearness that -the conglomerate is a fluviatile deposit and lies in the actual channel of the -ancient river which was eroded out of the basalt plateau, and was subsequently -sealed up by streams of pitchstone-lava.</p> - -<p>An examination of the fragments of rock found in the conglomerate -affords here, as in Canna and Sanday, some indication of the direction in -which the river flowed. The occurrence of pieces of red sandstone, which -<span class="pagenum" id="Page_240">- 240 -</span> -no one who knows West-Highland geology can fail to recognize as of -Torridonian derivation, at once makes it clear that the higher grounds from -which they were borne probably lay to the north or north-east. The fragments -of white sandstone may also have been derived from the same quarter, -for the thick Jurassic series of Eigg once extended further in that -direction. The pieces of quartzite and clay-slate bear similar testimony to -an eastern or north-eastern source. In short, there seems every probability -that this old Tertiary river flowed through a forest-clad region, of which -the red Torridon mountains of Ross-shire, the white sandstone cliffs of -Raasay and Skye, and the quartzite and schist uplands of Western Inverness-shire -are but fragments, that it passed over a wide and long tract of the -volcanic plateau, and continued to flow long enough to be able to carve out -for itself a channel on the surface of the basalt. Its course across what is -now the island of Eigg took a somewhat north-westerly direction, probably -guided by inequalities on the surface of the lava-plain. It is there marked -by the winding ridge of the Scuir, the pitchstone of which flowed into the -river-bed and sealed it up. Several minor spurs, which project from the -eastern side of the main ridge, show the positions of small tributary rivulets -that entered the principal channel from the slopes of the basaltic tableland. -One of these, on the south-east side of the hill called Corven, must have -been a gully in the basalt with a rapid or waterfall. The pitchstone has -flowed into it, and some of the rounded pebbles that lay in the channel of -this vanished brook may still be gathered where the degradation of the -pitchstone has once more exposed them to the light. That the Eigg river -here flowed in a westerly direction may be inferred from the angle at which -the beds of the small tributaries meet the main stream, and also from the -fact that the old river-bed at the east end of the Scuir is considerably higher -than at the west end.</p> - -<p>Several features in the geological structure of this locality serve to -impress on the mind the great lapse of time represented by the erosion of -the river-channel of Eigg. Thus at the narrowest point of the pitchstone -ridge, near the little Loch a' Bhealaich, the bottom of the glassy -lava is about 200 feet above its base on the south side, so that the valley -cut out of the plateau-basalts must have been more than 200 feet deep. -Even the little tributaries had cut ravines or cañons in the basalts before -the ground was buried under the floods of pitchstone. In the most northerly -spur of the ridge, for example, the hill of Beinn Tighe, which represents -one of these tributaries, shows a considerable difference between the level -of the bottom of the pitchstone on the east and west sides.</p> - -<p>Again, all along the ridge of the Scuir, the basalt-dykes are abruptly -cut off at the denuded surface on which the pitchstone rests. This -feature is conspicuously displayed on the great sea-wall at the west end -(<a href="#v2fig279">Fig. 279</a>). The truncation of the dykes demonstrates that a considerable -mass of material must have been eroded before these lava-filled fissures -could be laid bare at the surface. And the removal of this material shows -that the denudation must have been continued for a long period of time.</p> - -<p><span class="pagenum" id="Page_241">- 241 -</span></p> - -<p>The river-channel of Eigg, since it was eroded long after the cessation -of the outflows of basalt in the plateau of Small Isles, must be much -later in origin than those of Canna and Sanday which, as we have seen, -were contemporaneous with the basalt-eruptions. But the river that excavated -the channels and deposited the gravels may have been the same -in both areas.</p> - -<p>In dealing with this subject, though the evidence is admittedly scanty, -we are not left wholly to conjecture. A consideration of the general -topographical features of the wide region of the Inner Hebrides, from the -beginning of the volcanic period onward, will convince us that, in spite of -the effects of prolonged basalt-eruptions, the persistent flow of the drainage -of the Western Highlands must have taken a westerly direction. It was -towards the west that the low grounds lay. Though the long and broad -valley which stretched northwards from Antrim, between the line of the -Outer Hebrides and the West of Scotland, was gradually buried under a -depth of two or three thousand feet of lava, the volcanic plain that overspread -it probably remained even to the end lower than the mountainous -Western Highlands. Hence the rivers, no matter how constantly they may -have had their beds filled up and may have been driven into new channels, -would nevertheless always seek their way westwards into the Atlantic.</p> - -<p>On Canna and Sanday the traces of a river are preserved which poured -its flood-waters across the lava-fields in that part of the volcanic region, -while streams of basalt were still from time to time issuing from vents and -fissures. Not more than fourteen miles to south-east stands the Scuir of -Eigg, with its buried river-channel and its striking evidence that there, -also, a river flowed westwards, but at a far later time, when the basalt-eruptions -had ceased and the volcanic plain had been already deeply -trenched by erosion, yet before the subterranean fires were finally quenched, -as the pitchstone of the Scuir abundantly proves.</p> - -<p>When one reflects upon the enormous denudation of this region, to -which more special reference will be made in the sequel, one is not surprised -that many connecting links should have been effaced. The astonishment -rather arises that so continuous a story can still be deciphered. Even, however, -had the original record been left complete, it would have been exceedingly -difficult to trace the successive mutations of a river-channel during -long ages of volcanic eruptions. Such a channel would have been concealed -from view by each lava-stream that poured into it, and would not have -been again exposed save by the very process of erosion that destroys while -it reveals.</p> - -<p>While, therefore, there is not and can never be any positive proof that -in the fluviatile records of Canna, Sanday and Eigg successive phases are -registered in the history of one single stream, I believe that this identity -is highly probable. It was a river which seems to have risen among the -mountains of Western Inverness-shire, and it had doubtless already taken -its course to the sea before any volcanic eruptions began. It continued to -flow westwards across the lava-floor that gradually spread over the plains. -<span class="pagenum" id="Page_242">- 242 -</span> -Its channel was constantly being filled up by fresh streams of basalt or -deflected by the uprise of new cinder-cones. But, fed by the Atlantic -rains, it maintained its seaward flow until the general subsidence which -carried so much of the volcanic plain below the sea. Yet the higher part -of this ancient water-course is no doubt unsubmerged, still traversing the -schists of the Western Highlands as it has done since older Tertiary time. -It may, perhaps, be recognized in one of the glens which carry seaward the -drainage of the districts of Morar, Arisaig, or Moidart.</p> - -<div class="figcenter" id="v2fig280" style="width: 407px;"> - <img src="images/v2fig280.png" width="407" height="199" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 280.</span>—View of the Scuir of Eigg from the South.</div> -</div> - -<p>Let us now turn to the remarkable lava which has sealed up the river-channel -of Eigg, and of which the remaining fragment stands up as the -great ridge of the Scuir. This rock presents characters that strongly distinguish -it from the surrounding basalts. It is not one single uniform -mass, but consists of a number of distinct varieties, some of which are a -volcanic glass, while others are a grey "porphyry," or devitrified pitchstone. -These are arranged in somewhat irregular, but well-marked, and, in a general -sense, horizontal sheets. On the great eastern terminal gable of the Scuir -this bedded structure is not clearly displayed, for the cliff seems there to be -built up of one homogeneous mass, save a markedly columnar band that -runs obliquely up the base of the precipice (<a href="#v2fig278">Fig. 278</a>). If, however, the -ridge is looked at from the south, the truly bedded character of its materials -becomes a conspicuous feature. Along the cliffs on that side the two -varieties of rock are strongly distinguished by their contrasting colour and -mode of weathering, the sombre-hued pitchstone standing up in a huge -precipice striped with columns, and barred horizontally with bands of the -pale-grey "porphyry," which, from its greater proneness to decay, seems sunk -into the face of the cliff. At the south-east end of the ridge the bedding -is especially distinct. West of the precipices, to the south of the Loch a' -Bhealaich, the dark pitchstone which forms the main mass is divided -by two long parallel intercalations of grey rock, and two other short -lenticular seams of the same material (see Figs. <a href="#v2fig280">280</a>, <a href="#v2fig281">281</a>). It is clear -from these features, which are not seen by most travellers who pass -<span class="pagenum" id="Page_243">- 243 -</span> -Eigg in the tourist-steamer that the Scuir is in no sense of the word a -dyke.</p> - -<p>But although the Scuir is thus a bedded mass, the bedding is far -different from the regularity and parallelism of that which obtains among -the bedded basalt-rocks below. Even where no intervening "porphyry" -occurs, the pitchstone can be recognized as made up of many beds, each -marked by the different angle at which its columns lie. And when the -"porphyry" does occur and forms so striking a division in the pitchstone, -its beds die out rapidly, appearing now on one horizon, now on another, -along the face of the cliffs, and thickening and thinning abruptly in short -distances along the line of the same bed. Perhaps the best place for -examining these features is at the Bhealaich, the only gully practicable for -ascent or descent, at the south-eastern face of the ridge.</p> - -<div class="figcenter" id="v2fig281" style="width: 426px;"> - <img src="images/v2fig281.png" width="426" height="250" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 281.</span>—View of the Scuir of Eigg from the South-west of the Loch a' Bhealaich, - showing the bedded character of the mass.</div> -</div> - -<p>By much the larger part of the mass of the Scuir consists of vitreous -material. As a rule this rock is columnar, the columns being much -slimmer and shorter than those of the basalt-rocks. They rise sometimes -vertically, and often obliquely, or project even horizontally from the -face of the cliff. They are seldom quite straight, but have a wavy outline; -and when grouped in knolls here and there along the top of the -ridge they remind one of gigantic bunches of some of the Palæozoic -corals, such as <i>Lithostrotion</i>. In other cases they slope out from a common -centre, and show an arrangement not very unlike that of a Highland -peat-stack.</p> - -<p>The pitchstone of the Scuir differs considerably in petrographical -character from other pitchstones of the island which occur in dykes and -veins. Its base is of a velvet-black colour, and is so much less vitreous -in aspect than ordinary pitchstone as to have been described by Jameson -<span class="pagenum" id="Page_244">- 244 -</span> -and later writers as intermediate between pitchstone and basalt.<a id="FNanchor_254" href="#Footnote_254" class="fnanchor">[254]</a> A -chemical analysis of the rock by Mr. Barker North,<a id="FNanchor_255" href="#Footnote_255" class="fnanchor">[255]</a> gave the following -composition:—</p> - -<table summary="data"> -<tr> - <td class="tdl">Silica</td> - <td class="tdr">65·81</td> -</tr> -<tr> - <td class="tdl">Alumina</td> - <td class="tdr">14·01</td> -</tr> -<tr> - <td class="tdl">Ferric oxide</td> - <td class="tdr">4·43</td> -</tr> -<tr> - <td class="tdl">Lime</td> - <td class="tdr">2·01</td> -</tr> -<tr> - <td class="tdl">Magnesia</td> - <td class="tdr">0·89</td> -</tr> -<tr> - <td class="tdl">Soda</td> - <td class="tdr">4·15</td> -</tr> -<tr> - <td class="tdl">Potash</td> - <td class="tdr">6·08</td> -</tr> -<tr> - <td class="tdl">Loss in ignition</td> - <td class="tdr">2·70</td> -</tr> -<tr> - <td></td> - <td class="bdt tdr">100·08</td> -</tr> -</table> - -<div class="footnote"> - -<p><a id="Footnote_254" href="#FNanchor_254" class="label">[254]</a> <i>Mineralogy of the Scottish Isles</i>, vol. ii. p. 47. See also Macculloch, <i>Western Isles</i>, vol. i. -p. 521, and Hay Cunningham, <i>Mem. Wern. Soc.</i> vol. viii. p. 155.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_255" href="#FNanchor_255" class="label">[255]</a> <i>Quart. Journ. Geol. Soc.</i> vol. xlvi. (1890), p. 379.</p> - -</div> - -<p>The grey devitrified bands, which occur as a subordinate part -of the mass of the Scuir ridge, are usually somewhat decomposed. Where -a fresh fracture is obtained, the material shows a fine-grained, sometimes -almost flinty, grey felsitic base, containing clear granules of quartz, -and facets of glassy felspar. In some places the rock is strongly porphyritic. -Examined under the microscope it presents a more thoroughly -devitrified groundmass, with the minutest depolarizing microlites, large -porphyritic crystals of plagioclase and sanidine, grains of augite, and sometimes -exceedingly abundant particles of magnetite.<a id="FNanchor_256" href="#Footnote_256" class="fnanchor">[256]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_256" href="#FNanchor_256" class="label">[256]</a> The microscopic structure of the identical pitchstone of Hysgeir is given on <a href="#Page_247">p. 247</a>.</p> - -</div> - -<div class="figleft" id="v2fig282" style="width: 265px;"> - <img src="images/v2fig282.png" width="265" height="257" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 282.</span>—Section at the base of the Scuir of - Eigg (east end).</div> -</div> - -<p>Although the line of separation between the grey dull felsitic sheets and -the more ordinary glassy pitchstone -is usually well defined, -the two rocks may be observed -to shade into each other in such -a manner as to show that the -lithoid material is only a devitrified -and somewhat decomposed -condition of the glassy -rock. This connection is -particularly to be observed -under the precipice at the east -end of the Scuir. At that -locality the pitchstone is underlain -by a very hard flinty -band, varying in colour from -white through various shades -of flesh-colour and brown into -black, containing a little free -quartz and crystals of glassy felspar. Where it becomes black it passes -into a rock like that of the main mass of the Scuir. Such vitreous parts -<span class="pagenum" id="Page_245">- 245 -</span> -of the bed lie as kernels in the midst of the more lithoid and decomposed -rock. The lower six feet of the "porphyry" are white and still more -decomposed. The relations of this mass are represented in <a href="#v2fig282">Fig. 282</a>, where -the basalt-rocks of the plateau (<i>a</i>) are shown to be cut through by basalt -dykes (<i>b b</i>), and overlain by the "porphyry" (<i>c</i>) and the pitchstone (<i>d</i>). In -the porphyry are shown several pitchstone kernels (<i>p</i>, <i>p</i>). It is deserving of -remark also that in different parts of the Scuir, particularly along the north -side, the bottom of the pitchstone beds passes into a dull grey earthy lithoid -substance, like that now under description.</p> - -<p>The bedded character of the rock of the Scuir and the well-marked -lithological distinction between its several component sheets show the lava -to have been the product of a number of separate outflows that found their -way one after another into the river-valley, which was the lowest ground in -the vicinity of the active vent. There can be little doubt, I think, that the -lava flowed down the valley. Its successive streams are still inclined from -east to west. The vent of eruption, therefore, ought to be looked for towards -the east. Nowhere within the Tertiary volcanic region is there any boss of -pitchstone or any mass the shape or size of which is suggestive of this -vent. In the island of Eigg no boss of any kind exists, save those of -granophyric porphyry to be afterwards referred to. But none of these -affords any satisfactory links of connection with the rock of the Scuir. -More probably the vent lay somewhere to the east on ground now overflowed -by the sea. The pitchstone veins of Eigg may represent some of the subterranean -extrusions from the same volcanic pipe, and if so, its site could -not be far off.</p> - -<p>The rock of the Scuir of Eigg has a special importance in the history -of the volcanic plateaux. It is, so far as we know, the latest of all the -superficial lavas of Britain.<a id="FNanchor_257" href="#Footnote_257" class="fnanchor">[257]</a> From the basalts on which it rests it was -separated by an enormous interval of time, during which these older lavas -were traversed by dykes and were worn down into valleys. Its presence -shows that long after the basalts of Small Isles had ceased to be erupted, a -new outbreak of volcanic activity took place in this district, when lavas of a -more acid composition flowed out at the surface. Whether this outburst -was synchronous with the appearance of the great granophyric protrusions -of the Inner Hebrides, or with the still later extravasation of pitchstone -dykes, can only be surmised.</p> - -<div class="footnote"> - -<p><a id="Footnote_257" href="#FNanchor_257" class="label">[257]</a> The rocks of Beinn Hiant in Ardnamurchan have been claimed by Professor Judd as superficial -lavas. For reasons to be afterwards given (<a href="#Page_318">p. 318</a>) I regard them as intrusive sheets. -Professor Cole believes the rhyolites and pitchstones of Tardree to be probably evidence of a -volcano later than the basalts of Antrim. As I have not been able to detect any actual proofs of -superficial outflow there, I relegate the description of the rocks to a future chapter, in which -the acid protrusions will be discussed (<a href="#Page_426">p. 426</a>).</p> - -</div> - -<p>When one scans the great precipice on the west side of Eigg, with its -transverse section of the pitchstone-lava, buried river-bed and basalt-plateau -underneath, there seems no chance of any further westward trace of -the pitchstone being ever found. The truncated end of the Scuir looks -from the top of the cliff out to sea, and the progress of denudation might -<span class="pagenum" id="Page_246">- 246 -</span> -have been supposed to have effectually destroyed all evidence of the continuation -of the rock in a westerly direction. Some years ago, however, my -friend Prof. Heddle, while cruising among the Inner Hebrides, landed upon -the little uninhabited islet of Hysgeir, which, some eighteen miles to the -westward of Eigg, rises out of the open sea. He at once recognized the -identity of the rock composing this islet with that of the Scuir, and in the -year 1892 published a brief account of this interesting discovery.<a id="FNanchor_258" href="#Footnote_258" class="fnanchor">[258]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_258" href="#FNanchor_258" class="label">[258]</a> Appendix C to <i>A Vertebrate Fauna of Argyle and the Inner Hebrides</i>, by Messrs. J. A. -Harvie-Brown and Thomas E. Buckley, p. 248.</p> - -</div> - -<p>I have myself been able to land on Hysgeir in two successive summers, -and can entirely confirm Prof. Heddle's identification. The islet stands on -the eastern edge of the submarine ridge which, running in a north-easterly -direction, culminates in the island of Canna. Hysgeir is a mere reef or -skerry, of which the top rises only 38 feet above the Ordnance datum-level. -Its surface is one of bare rock, save where a short but luxuriant growth of -grasses has found root on the higher parts of two or three of its ridges, and -on the old storm-beach of shingle which remains on the summit. The rock -undulates in long low swells, that run in a general direction 20° to 45° west -of north, and are separated by narrow channels or hollows. The place is a -favourite haunt of gulls, terns, eider-ducks and grey seals, and is used by the -proprietor of Canna for the occasional pasturage of sheep or cattle. So -numerous are the sea-fowl during the breeding-season that the geologist, -intent upon his own pursuits, may often tread on their nests unawares, while -he is the centre of a restless circle of white wings and anxious cries.</p> - -<p>The pitchstone of Hysgeir, like that of Eigg, is columnar, the columns -being irregularly polygonal and varying from three to ten inches in diameter. -They are packed so close together that the domes of rock on which their -ends appear look like rounded masses of honeycomb. They may here and -there be observed to be arranged radially with their ends at right angles to -the curved exterior of the ridges, as if this external surface represented the -original form of the cooled pitchstone, and were not due to mere denudation. -There can be no doubt, however, that the island has been well ice-worn.</p> - -<p>At the north-west promontory a beautiful example of fan-shaped -grouping of columns may be observed on a face of rock which descends -vertically into the sea. Here, too, is almost the only section on which the -sides of the columns may be examined, for, as a rule, it is merely their ends -on the rounded domes which are to be observed, and which everywhere slip -under the waves. The columns in a cliff from 15 to 20 feet high show the -slightly wavy, starch-like arrangement so often to be met with among the -plateau-basalts.</p> - -<p>The rock presents a tolerably uniform texture throughout, though in -some parts it is blacker, more resinous, and less charged with porphyritic -enclosures than in the general body of the rock. Large fresh felspars are -generally scattered through it. To the naked eye it reproduces every -feature of the pitchstone of the Scuir of Eigg.</p> - -<p>A microscopic examination completes our recognition of the identity of -<span class="pagenum" id="Page_247">- 247 -</span> -these two rocks. Mr. Harker has examined a thin slice prepared from the -Hysgeir pitchstone, and remarks regarding it that "the large felspars are -not the only porphyritic element. The microscope shows the presence also -of smaller imperfect crystals of augite, very faint green in the slice, and -small grains of magnetite. The felspars have been deeply corroded by the -enveloping magma, and irregular included patches of the groundmass occupy -nearly half the bulk of some of the crystals. This latter feature is seen -especially in some of the larger crystals, which seem to be sanidine. They -are, for the most part, apparently simple crystals, but in places there is a -scarcely defined lamellar twinning, or, again, small patches not extinguishing -with the rest; so that we are probably dealing with some perthitic intergrowth -on a minute scale.<a id="FNanchor_259" href="#Footnote_259" class="fnanchor">[259]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_259" href="#FNanchor_259" class="label">[259]</a> Comp. Prof. Judd's remarks on the Scuir of Eigg rock, <i>Quart. Journ. Geol. Soc.</i> vol. xlvi. -(1890), p. 380.</p> - -</div> - -<p>"Rather smaller felspar-crystals are rounded by corrosion, but lack the -inclusions of groundmass; these have albite-and sometimes pericline-lamellation, -and may be referred to oligoclase-andesine. The groundmass of -the rock is a brown glass with perlitic cracks, enclosing very numerous -microlites of felspar about ·001 inch in length [6619]. The rock is probably -to be regarded as a dacite rather than a rhyolite, and thus agrees with -Mr. Barker North's analysis of the Eigg pitchstone."<a id="FNanchor_260" href="#Footnote_260" class="fnanchor">[260]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_260" href="#FNanchor_260" class="label">[260]</a> <i>Op. cit.</i> <a href="#Page_379">p. 379</a>.</p> - -</div> - -<p>There is no trace of any conglomerate <i>in situ</i> like that under the Scuir -of Eigg, nor of any other rock, aqueous or igneous. As the pitchstone -everywhere slips under the sea, its geological relations are entirely concealed.</p> - -<p>The great variety of materials met with in the form of boulders on the -island is a testimony to the transport of erratics from the neighbouring -islands and the mainland during the Glacial Period. The most abundant -rock in these boulders is Torridon Sandstone, derived no doubt from the hills -of Rum, but there occur also various kinds of schist, gneisses, quartzites, -granites, porphyries, probably from the west of Inverness-shire, as well as -pieces of white sandstone, probably Jurassic, which may have come from -Eigg.</p> - -<p>That the pitchstone of Hysgeir is a continuation of that of the Scuir -may be regarded as highly probable. If not a continuation, it must be -another stream of the same kind, and doubtless of the same date. If it be -regarded as probably a westward prolongation of the Eigg rock, and if it be -about as thick as that mass at the west end of the Scuir, then its bottom -lies 200 or 300 feet under the waves. The river-channel occupied by the -Eigg pitchstone undoubtedly sloped from east to west. The position of -Hysgeir, 18 miles further west, may indicate a further fall in the same direction -at the rate of perhaps as much as 35 feet in the mile.<a id="FNanchor_261" href="#Footnote_261" class="fnanchor">[261]</a> Unfortunately, -however, as no trace of the river-bed can now be seen on this island, any -statement in regard to it must rest on mere conjecture.</p> - -<div class="footnote"> - -<p><a id="Footnote_261" href="#FNanchor_261" class="label">[261]</a> <i>Rep. Brit. Assoc.</i> 1894, p. 653.</p> - -</div> - -<p>Although the question of the denudation of the basalt-plateaux since -<span class="pagenum" id="Page_248">- 248 -</span> -the close of the volcanic period will be the subject of a special chapter in a -later part of this volume, I cannot here refrain from calling attention to the -pitchstone of Eigg and Hysgeir as one of the most impressive monuments of -denudation to be found within the British Isles. Though now so prominent -an object in the West Highlands, this rock once occupied the bottom of a -valley worn out of the basaltic tableland. Prolonged and stupendous denudation -has destroyed the connection with its source, has cut down its ends -into beetling precipices, has reduced the former surrounding hills into -gentle slopes and undulating lowland, and has turned the bottom of the -ancient valley into a long, narrow and high crest. Moreover, we see that -the erosion has not been uniform. The great wall of the Scuir does not -stand fairly on the crest of the basalt-plateau but on the south side of it, so -that the southern half of the old valley, with all its surrounding hills, has -been entirely cut away. That subsidence has also come into play in the -destruction of even the youngest parts of the volcanic plateaux will be more -fully discussed in a later chapter. I need only remark here that the submergence -of Hysgeir probably points to extensive depression of the land-surface -on which the lavas were poured out.</p> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_249">- 249 -</span></p> - -<h2 class="nobreak" id="CHAPTER_XXXIX">CHAPTER XXXIX<br /> - -<span class="smaller">THE BASALT-PLATEAUX OF SKYE AND OF THE FAROE ISLES</span></h2> -</div> - - -<h3>iv. <span class="allsmcap">THE SKYE PLATEAU</span></h3> - -<p>This largest and geologically most important of all the Scottish plateaux -comprises the island of Skye, at least as far south as Loch Eishort, and -the southern half of Raasay, but is shown by its sills to stretch as far as the -Shiant Isles on the north, and the Point of Sleat on the south (see Map VI.). -It may be reckoned to embrace an area of not less than 800 square miles. -The evidence that its limits, like those of the other plateaux, are now greatly -less than they originally were, is abundant and impressive. The truncated -edges of its basalts, rising here and there for a thousand feet as a great sea-wall -above the breakers at their base, and presenting everywhere their succession -of level or gently inclined bars, are among the most impressive monuments -of denudation in this country. But still more striking to the geologist is the -proof, furnished beyond the margins of the plateau, that the Jurassic and -other older rocks there visible were originally buried deep under the basalt-sheets, -which have thus been entirely stripped off that part of the country.</p> - -<p>Throughout most of the district, wherever the base of the basalts can -be seen, it is found to rest upon some member of the Jurassic series, but -with a complete unconformability. The underlying sedimentary strata had -been dislocated and extensively denuded before the volcanic period began. -On the southern margin, however, the red (Torridon) sandstones emerge -from under the basalts of Loch Scavaig, and extending into the island of -Soay are prolonged under the sea into Rum. This ridge probably -represents the range of the ancient high ground of the latter island already -referred to.</p> - -<p>Nowhere are the distinctive topographical features and geological -structure of the basalt-plateaux better displayed than in the northern half -of the island of Skye. The green terraced slopes, with their parallel bands -of brown rock formed by the outcrop of the nearly flat basalt-beds, rise from -the bottoms of the valleys into flat-topped ridges and truncated cones (Fig. -283). The hills everywhere present a curiously tabular form that bears -witness to the horizontal sheets of rock of which they are composed.<a id="FNanchor_262" href="#Footnote_262" class="fnanchor">[262]</a> And -<span class="pagenum" id="Page_250">- 250 -</span> -along the sea-precipices, each excessive sheet of basalt can be counted from -base to summit, and followed from promontory to promontory (Figs. <a href="#v2fig284">284</a>, <a href="#v2fig286">286</a>). -In the district of Trotternish, the basalt hills reach a height of 2360 feet. -Further west, the singular flat-topped eminences, called "Macleod's Tables" -(<a href="#v2fig283">Fig. 283</a>) ascend to 1600 feet.</p> - -<div class="footnote"> - -<p><a id="Footnote_262" href="#FNanchor_262" class="label">[262]</a> These features are more fully described in my <i>Scenery of Scotland</i>, 2nd edit (1887), pp. 74, -145, 216.</p> - -</div> - -<div class="figcenter" id="v2fig283" style="width: 355px;"> - <img src="images/v2fig283.png" width="355" height="155" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 283.</span>—Terraced Hills of Basalt Plateau (Macleod's Tables), Skye.</div> -</div> - -<p>Along the western side of Skye, the basalts descend beneath the level of -the Atlantic, save at Eist in Duirinish, where the Secondary strata, with their -belt of intrusive sills, rise from underneath them, and at the Sound of Soa, -where they rest on the Torridon Sandstone. Along the eastern side, their -base runs on the top of the great Jurassic escarpment, whose white and -yellow sandstones rise there, and on the east side of Raasay, into long lines -of pale cliffs. To the south-east, the regularity of the volcanic plateau is -effaced, as in Mull and Ardnamurchan, by the protrusion of extensive -masses of eruptive rocks constituting the Cuillin and Red Hills, east of -which the basalts have been almost entirely removed by denudation, so as to -expose the older rocks which they once covered, and through which the -younger eruptive bosses made their way. This is undoubtedly the most -instructive district for the study of that late phase in the volcanic history of -Britain comprised in the eruptive bosses of basic and acid rocks.</p> - -<p>The magnificent plateau of this island has been so profoundly cut -down into glens and arms of the sea, and its component layers are exposed -along so many leagues of precipice, that its structure is perhaps more completely -laid open than that of any of the other Tertiary volcanic areas in -Britain. It is built up of a succession of basalts and dolerites of the usual -types, which still reach a thickness of more than 2000 feet, though in -this instance, also, denudation has left only a portion of them, without any -evidence by which to reckon what their total original depth may have been. -In rambling over Skye, the geologist is more than ever struck with the -remarkable scarcity and insignificance of the interstratifications of tuff or of -any other kind of sedimentary deposit between the successive lava-sheets. -One of the thickest accumulations of volcanic tuff and conglomerate has -already been referred to as occurring on the south side of Portree Harbour, -where it attains a depth of about 200 feet. As it is in immediate connection -with its parent vent, it will be more fully alluded to in Chapter xli. -Here, as is so generally observable among the basalt-plateaux, traces of -<span class="pagenum" id="Page_251">- 251 -</span> -vegetation are plentiful among the stratified intercalations, even forming -thin seams of lignite and coal, one of which was formerly worked. That -volcanic eruptions, though possibly of a feebler kind, continued during the -interval between the basalt-outflows at this locality, is shown by the thick -accumulation of tuff and by the occurrence of abundant lapilli of fine basic -pumice among the shales, even to a distance of several miles from the -vent.</p> - -<div class="figcenter" id="v2fig284" style="width: 358px;"> - <img src="images/v2fig284.png" width="358" height="226" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 284.</span>—"Macleod's Maidens" and part of Basalt Cliffs of Skye.</div> -</div> - -<p>Another conspicuous intercalation of sedimentary materials in the Skye -plateau occurs on the Talisker cliffs at the mouth of Loch Bracadale, where, -on the face of the great precipice of Rudha nan Clach, some conspicuous -bands of lilac and red are interspersed among the basalts. These bands -were noticed by Macculloch, who described them as varieties of "iron-clay."<a id="FNanchor_263" href="#Footnote_263" class="fnanchor">[263]</a> -I have not had an opportunity of examining them except from the sea at a -little distance. But they suggest a similarity to some of the variegated -clays between the upper and lower basalt series of Antrim.</p> - -<div class="footnote"> - -<p><a id="Footnote_263" href="#FNanchor_263" class="label">[263]</a> <i>Western Islands</i>, vol. i. p. 376.</p> - -</div> - -<div class="figleft" id="v2fig285" style="width: 166px;"> - <img src="images/v2fig285.png" width="166" height="178" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 285.</span>—Intercalated group of - strata between Basalts, An Ceannaich, western side of Skye.</div> -</div> - -<p>Though good coal is not well developed in the Tertiary volcanic -plateaux of the British Isles, it has already been pointed out that coaly -layers are abundant, and that as the vegetable matter may confidently be -assumed always to indicate terrestrial vegetation, the presence of the carbonaceous -bands may be regarded as good evidence of some lapse of time -between the eruption of the basalts which they separate. I have also called -attention to the fact that the vegetable material is more especially observable -in the highest parts of a group of intercalated sediments between two -sheets of basalt. This relation, so strikingly exhibited in the isle of Canna, -as already observed, is also to be remarked in the Skye plateau. I may -here cite an interesting example which occurs at the base of the lofty sea-cliff -of An Ceannaich, to the south of Dunvegan Head, on the west coast of -Skye (<a href="#v2fig285">Fig. 285</a>). At the base of the precipice, ledges of a highly cellular -basalt (<i>a</i>) show a singularly scoriaceous and amygdaloidal structure, with -<span class="pagenum" id="Page_252">- 252 -</span> -abundant and beautiful zeolites, the hollows of the upper surface of the -sheet being filled in with dark brown carbonaceous shale, forming a layer -from one to fourteen inches thick, marked by coaly streaks and lenticles (<i>b</i>). -A band of green and yellow sandstone (<i>c</i>) next supervenes, which, from its -pale colour, attracts attention from a distance, and led me, while yachting -along the coast, to land at the locality in the -hope that it might prove to be a plant-bearing -limestone. This sandy stratum is only some -three or four inches thick at the north end of -the section, but increases rapidly southward -to a thickness of as many feet or more, when, -owing to the cessation of the underlying shale, -it comes to lie directly on the amygdaloid and -to enclose slaggy portions of that rock. Immediately -above the sandstone two or three feet -of fissile shale, black with plant-remains (<i>d</i>), -include brown layers that yield to the knife -like some oil-shales. The next stratum is a -seam of coal (<i>e</i>) about a foot thick, of remarkable -purity. It is glossy, hard, and cubical, including layers that break -like jet. It has been succeeded by a deposit of green sand (<i>f</i>), but while -this material was in course of deposition another outpouring of lava (<i>g</i>) took -place, whereby the terrestrial pool or hollow of the lava-field, in which the -group of sedimentary materials accumulated, was filled up and buried. -This lava is about 20 feet thick, and consists of a coarsely-crystalline, -jointed dolerite with highly amygdaloidal upper and under surface. Its -slaggy bottom has caught up or pushed aside the layer of green sand, so as -to lie directly on the coal, and has there been converted into the earthy -modification so familiar under the name of "white trap" among our coal-fields. -It is interesting to find that this kind of alteration, where molten -rock comes in contact with carbonaceous materials, is not confined to -subterranean sills, but may show itself in lavas that have flowed over a -terrestrial surface.</p> - -<p>From the frequent intercalation of such local deposits of sedimentary -material between the basalts, we may reasonably infer that during older -Tertiary time the rainfall in North-Western Europe was copious enough -to supply many little lakes and streams of water. As the surface of the -lava-fields decayed into soil, vegetation spread over it, so that, perhaps for -long intervals, some tracts remained green and forest-clad. But volcanic -action still continued to show itself, now from one vent, now from another. -These wooded tracts were buried under overflows of lava, and, the water-courses -being filled up, their streams were driven into new channels, and -other pools and lakes were formed.</p> - -<p><span class="pagenum" id="Page_253">- 253 -</span></p> - -<div class="figcenter" id="v2fig286" style="width: 696px;"> - <img src="images/v2fig286.png" width="696" height="400" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 286.</span>—Escarpment of Plateau-basalts, Cliffs of Talisker, Skye.</div> -</div> - -<p>In no part of the Tertiary volcanic area of Britain can the characters of -the lavas and the structure of the plateaux be better seen than along the -west side of Skye, north of Loch Bracadale. The precipices rise sheer out -of the sea, to heights of sometimes 1000 feet, and from base to summit -every individual bed may be counted. Some particulars have already been -given (<a href="#Page_192">p. 192</a>) regarding the average thickness of the basalt-sheets on this -coast-line. The general aspect of these cliffs and the arrangement of their -<span class="pagenum" id="Page_254">- 254 -</span> -component lavas is shown in <a href="#v2fig286">Fig. 286</a>. As a further detailed illustration -of the general succession of the basalts in the Skye plateau, I give a diagrammatic -view of the largest of Macleod's Maidens—the three weird sea-stalks -that rise so grandly in front of the storm-swept precipice at the -mouth of Loch Bracadale. The height of the stack must be at least 150 -feet (Figs. <a href="#v2fig284">284</a> and <a href="#v2fig287">287</a>). About ten distinct sheets of igneous rock can be -counted in it, which gives an average thickness of 15 feet for the individual -beds. It will be observed that there is a kind of alternation between the -compact, prismatic basalts and the more earthy amygdaloids, but that the -former are generally thickest.<a id="FNanchor_264" href="#Footnote_264" class="fnanchor">[264]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_264" href="#FNanchor_264" class="label">[264]</a> A striking and illustrative contrast between the relative thickness of the beds of the two kinds -of rock is supplied by the fine sections of this district. The amygdaloids range from perhaps 6 -or 8 to 25 or 30 feet; but the prismatic basalts, while never so thin as the others, sometimes -enormously exceed them in bulk. In the island of Wiay, for example, a bed of compact black -basalt, with the confused starch-like grouping of columns, reaches a thickness of no less than 170 -feet. Its bottom rests upon a red parting on the top of a dull greenish earthy amygdaloid. It is -possible, however, that some of these columnar sheets of basalt are really sills.</p> - -</div> - -<div class="figcenter" id="v2fig287" style="width: 346px;"> - <img src="images/v2fig287.png" width="346" height="390" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 287.</span>—Section of the largest of Macleod's Maidens.</div> -</div> - -<p>These features, which are repeated on cliff after cliff, may be considered -typical for all the plateaux. Another characteristic point, well -displayed here, is the intervening red parting between the successive -beds. If the occurrence and thickness of this layer could be assumed -<span class="pagenum" id="Page_255">- 255 -</span> -as an indication of the relative lapse of time between the different -flows of lava, it would furnish us with a rude kind of chronometer for -estimating the proportionate duration of the intervals between the eruptions. -It is to be noticed on the top both of the compact prismatic and of the -earthy amygdaloidal sheets; but it is more frequent and generally thicker -on the latter than on the former, which may only mean that the surfaces of -the cellular lavas were more prone to subærial decay than those of the -compact varieties. Nevertheless, I am disposed to attach some value to it, -as an index of time. In the present instance, for example, it seems to me -probable that the lavas in the lower half of Macleod's Maiden, where the red -layers are very prominent, were poured out at longer intervals than those -that form the upper half. The remarkable banded arrangement of the -vesicles in one of the cellular lavas of this sea-stack has been already -referred to (<a href="#Page_191">p. 191</a>).</p> - -<p>Another characteristic plateau-feature is admirably displayed in Skye—the -flatness of the basalts and the continuity of their level terraces (though -not of individual sheets) from cliff to cliff and hillside to hillside. This -feature may be followed with almost tiresome monotony over the whole of -the island, north of a line drawn from Loch Brittle to Loch Sligachan. -Throughout that wide region, the regularity of the basalt-plateau is unbroken, -except by minor protrusions of eruptive rock, which, as far as I have noticed, -do not seriously affect the topography. But south of the line just indicated, -the plateau undergoes the same remarkable change as in Rum, Ardnamurchan -and Mull. Portions of it which have survived indicate with sufficient -clearness that it once spread southwards and eastwards over the mountainous -district, and even farther south into the low parts of the island. Its removal -from that tract has been of the utmost value to geological research, for -some of the subterranean aspects of volcanism have thereby been revealed, -which would otherwise have remained buried under the thick cover of basalt. -Denudation has likewise cut deeply into the eruptive bosses, and has carved -out of them the groups of the Red Hills and the Cuillins, to whose picturesque -forms Skye owes so much of its charm.</p> - -<p>In this, as in each of the other plateaux, there is no trace of any -thickening of the basalts towards a supposed central vent of eruption. -The nearly level sheets may be followed up to the very edge of the great -mountainous tract of eruptive rocks, retaining all the way their usual characters; -they do not become thicker there either collectively or individually, nor -are they more abundantly interstratified with tuffs or volcanic conglomerates. -On the contrary, their very base is exposed around the mountain ground, and -the thickest interstratifications of fragmentary materials are found at a -distance from that area. So far as regards the structure of the remaining -part of the plateau, the eruption of the gabbros and granitoid rocks might -apparently have taken place as well anywhere further north.</p> - -<p><span class="pagenum" id="Page_256">- 256 -</span></p> - - -<h3>v. THE FAROE ISLANDS<span class="smaller"><a id="FNanchor_265" href="#Footnote_265" class="fnanchor">[265]</a></span></h3> - -<div class="footnote"> - -<p><a id="Footnote_265" href="#FNanchor_265" class="label">[265]</a> For references to the recent geological literature connected with these islands see the footnote -<i>ante</i>, <a href="#Page_191">p. 191</a>.</p> - -</div> - -<p>Though these islands lie beyond the limits of the region embraced by -the present work, I wish to cite them for the singular confirmation and -extension they afford to observations made among British Tertiary volcanic -rocks. Over a united extent of coast-cliffs which may be roughly -estimated at about 500 English miles, the nearly level sheets of basalts, -with their occasional tuffs, conglomerates, leaf-beds and coals, can be -followed with singular clearness. Although the Faroe Islands have been so -frequently visited and so often described that their general structure is -sufficiently well known, they present in their details such a mass of new -material for the illustration of volcanic action that they deserve a far more -minute and patient survey than they have yet received. They cannot be -adequately mapped and understood by the traveller who merely sails round -them. They must be laboriously explored, island by island and cliff -by cliff.</p> - -<p>While I cannot pretend to more than a mere general acquaintance with -their structure, I have learnt by experience that one may sail near their -precipices and yet miss some essential features of their volcanic structure. -In the summer of the year 1894 I passed close to the noble range of precipices -on the west side of Stromö, at the mouth of the Vaagöfjord, and -sketched the sill which forms so striking a part of the geology of that district -(Figs. <a href="#v2fig312">312</a>, <a href="#v2fig328">328</a> and <a href="#v2fig329">329</a>). But I failed to observe a much more remarkable -and interesting feature at the base of the same sea-cliffs. The following -summer, probably under better conditions of light, I was fortunate enough -to detect with my field-glass, from the deck of the yacht, what looked like -a mass of agglomerate, and found on closer examination the interesting -group of volcanic vents described in Chapter xli. The magnificent precipices -of Faroe, which in Myling Head reach a height of 2260 feet, present -a series of natural sections altogether without a rival in the rest of Europe. -They are less concealed with verdure than those of Mull and Skye, and -therefore display their geological details with even greater clearness than -can be found either in Scotland or in Ireland. I would especially refer to -the bare precipitous sides of the long narrow islands of Kalsö and Kunö, -as admirable sections wherein the characters of the plateau-basalts are -revealed as in a series of gigantic diagrams. The scarcity of vegetation, and -the steepness of the declivities which prevents the abundant accumulation of -screes of detritus, enable the observer to trace individual beds of basalt with -the eye for several miles. Thus on the west side of Kunö, one conspicuous -dark sheet in the lower part of the section can be followed from opposite -Mygledahl in Kalsö to the southern end of the island. There is one concealed -space at the mouth of the corrie behind Kunö village, but the same, -or at least a similar band of rock at the same level, emerges from the -detritus on the further side, and may possibly run into the opposite -<span class="pagenum" id="Page_257">- 257 -</span> -promontory of Bodö. It extends in Kunö for at least six geographical -miles.</p> - -<div class="figright" id="v2fig288" style="width: 252px;"> - <img src="images/v2fig288.png" width="252" height="105" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 288.</span>—Dying out of Lava-beds, east side of - Sandö, Faroe Isles.</div> -</div> - -<p>These vast escarpments of naked rock show, with even greater clearness -than the precipices of the Inner Hebrides, how frequently the basalts die -out, now in one direction now in -another. The two sides of the -Kalsöfjord exhibit many examples -of this structure, and some striking -instances of it are to be seen on the -west side of Haraldsfjord. In these -cliffs, which must be about 2000 -feet high, upwards of forty distinct -flows can sometimes be traced -from the sea-level to the crest. The average thickness of each bed -is thus somewhat less than 50 feet. Such vast escarpments, with wide -semicircular corries scooped out of their sides, such serrated crests and dark -rifts in the precipices, such deep fjords winding through nearly horizontal -basalts, of which the parallel sheets can be followed by the eye from island -to island, fill the mind with a vivid conception at once of the enormous -scale of the volcanic eruptions and of the stupendous denudation which -this portion of North-Western Europe has undergone since Tertiary time.</p> - -<p>As the lenticular character of the basalts, and the evidence they supply -of having been discharged from many small local vents are of great importance -in the comprehension of the volcanic history of the plateaux, some -further illustrations of these features may with advantage be given here. -Thus the traveller who skirts the western precipices of Suderö will notice -some good examples to the north of the highest part of the cliffs. On -Stromö he will detect other cases of the same structure. Similar features -will arrest his attention on the precipices of Sandö, where, though at first -sight the basalts seem to be regular and continuous, a nearer view of them -reveals such sections as that shown in <a href="#v2fig288">Fig. 288</a>, where a group of sheets -rapidly dies out towards the north against a thicker band that thins away -in the opposite direction. Further north he will come upon other examples -in the range of low cliffs between -Kirkebonaes and Thorshaven, and -more impressive still in the rugged -precipices that front the Atlantic -on the western front of Hestö (Fig. -289), where the disappearance is in -a northerly direction.</p> - -<div class="figright" id="v2fig289" style="width: 253px;"> - <img src="images/v2fig289.png" width="253" height="104" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 289.</span>—Lenticular lavas, western front of Hestö, - Faroe Isles.</div> -</div> - -<p>But it is in the northern part -of the Faroes, where the basalt-plateau -has been so deeply trenched by parallel fjords as to be broken -up into a group of long, narrow, lofty, and precipitous insular ridges, that -the really local and non-persistent character of the lavas can best be -seen. The eastern cliffs of Svinö present admirable examples, where in the -<span class="pagenum" id="Page_258">- 258 -</span> -same vertical wall of rock some of the basalts die out to the south, others -to the north, while occasionally a shorter sheet may be seen to disappear in -both directions as if it were the end of a stream that flowed at right angles -to the others (<a href="#v2fig290">Fig. 290</a>).</p> - -<div class="figleft" id="v2fig290" style="width: 319px;"> - <img src="images/v2fig290.png" width="319" height="120" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 290.</span>—Lenticular lavas east side of Svinö, Faroe Isles.</div> -</div> - -<p>The more the basalt-plateaux of Britain and the Faroe Islands are -studied, the more certain -does the conclusion become -that these widespread -sheets of lava -never flowed from a few -large central volcanoes -of the type of Etna -or Vesuvius, but were -emitted from innumerable -minor vents or from open fissures. In a later chapter an account -will be given of the vents, which may still be seen under the overlying -sheets of basalt, and, in particular, a remarkable group in the Faroe Islands -will be described.</p> - -<div class="figright" id="v2fig291" style="width: 152px;"> - <img src="images/v2fig291.png" width="152" height="268" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 291.</span>—Section at Frodbonyp, -Suderö, Faroe.</div> -</div> - -<p>The occurrence of tuffs, leaf-beds and thin coals between the plateau-basalts -of the Faroe Islands has long been known. These stratified deposits -are well seen in the island of Suderö, where they serve to divide two distinct -series of basalts, like the iron-ore and its accompaniments in Antrim. As -a characteristic illustration of the same diversity -of deposits observable between the lava-sheets of -the basalt-plateaux of the British Isles I give -here a section exposed on the east side of this -island—a locality often visited and described in -connexion with its coal-seams (<a href="#v2fig291">Fig. 291</a>). At the -base lies a sheet of basalt (<i>a</i>) with an irregularly -lumpy upper surface. It may be remarked that -the lower group of basalts is marked by the occurrence -of numerous columnar sheets, some of them -possibly sills, and also more massive, solid, and -durable basalts than the sheets above. The lowest -of the intercalated sediments are light-coloured -clays, passing down into dark nodular mudstone -and dark shale, the whole having a thickness of at -least 20 feet (<i>b</i>). These strata are succeeded by (<i>c</i>) -pale clays with black plant-remains, about three -feet thick. Immediately above this band comes -the coal or coaly layer (<i>d</i>), here about six inches -thick, which improves in thickness and quality further inland, where it has -been occasionally worked for economic purposes. A deposit of green and -brown volcanic mudstone (<i>e</i>), twelve feet in thickness, overlies the coal and -passes under a well-bedded granular green tuff and mudstone three feet thick -(<i>f</i>). The uppermost band is another volcanic mudstone (<i>g</i>) four feet in -<span class="pagenum" id="Page_259">- 259 -</span> -thickness, dark green in colour, and more or less distinctly stratified, with -irregular concretions, and also pieces of wood. Above this layer comes -another thick overlying group of basalts (<i>h</i>) distinguished by their -abundantly amygdaloidal character, and by their weathering into globular -forms which at a little distance give them a resemblance to agglomerates.</p> - -<p>We have here an intercalated group of strata upwards of 40 feet thick, -consisting partly of tuffs and partly of fine clays, which may either have -been derived from volcanic explosions or from the atmospheric disintegration -of basaltic lavas. Through some of these strata abundant carbonaceous -streaks and other traces of plants are distributed, while among them lies a -band almost wholly composed of compressed vegetation. Unfortunately -none of the strata at this locality seem to have preserved the plant-remains -with sufficient definiteness for identification. There can be no -doubt, however, that they were terrestrial forms like those of Mull and -Antrim.</p> - -<p>This coal, with its accompanying sedimentary deposits, has been traced -through Suderö, and another outcrop, possibly of the same horizon, occurs -on Myggenaes, the extreme western member of the group of islands, at a -distance of some 40 miles.<a id="FNanchor_266" href="#Footnote_266" class="fnanchor">[266]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_266" href="#FNanchor_266" class="label">[266]</a> See in particular Prof. J. Geikie, <i>Trans. Roy. Soc. Edin.</i> vol. xxx. (1880), p. 229.</p> - -</div> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_260">- 260 -</span></p> - -<h2 class="nobreak" id="CHAPTER_XL">CHAPTER XL<br /> - -<span class="smaller">THE MODERN VOLCANOES OF ICELAND AS ILLUSTRATIVE OF THE TERTIARY -VOLCANIC HISTORY OF NORTH-WESTERN EUROPE</span></h2> -</div> - - -<p>From the facts stated in the foregoing chapters concerning the structure -of the basalt-plateaux of North-Western Europe, it is evident that in none -of these areas have the eruptions come from one great central volcano like -Etna or Vesuvius. On the contrary, in every instance there is abundant -evidence that the basalt has flowed from many scattered points of eruption. -The uniformity of the lava-sheets in petrographical characters, their -continuity when viewed in mass, their general horizontality, and their -constant thinning away in different directions, show that the eruptive -vents must have been distributed over the whole plateau-areas.</p> - -<p>The conditions under which such eruptions took place can be most -readily understood by a comparison of the phenomena with those observable -in modern volcanic tracts where extensive outflows of lava have taken place -without the existence of any great central cones. Of these regions the -most instructive is undoubtedly to be found among the recent lava-deserts -of Iceland. There the parallels to the structures described from the British -and Faroe plateaux are so numerous and so close that an account of the -Icelandic region may appropriately be inserted here.</p> - -<p>The evidence furnished by Iceland is of special value in our present -enquiry, inasmuch as that island, besides its modern eruptions, includes vast -basaltic plateaux of Tertiary age. These areas of nearly level sheets of -basalt belong to the same geological period as those of the British and -Faroe Islands, and display the same internal structure and external features. -But they have this distinguishing peculiarity that the volcanic fires beneath -them are not yet extinguished. They have been broken through again and -again in recent times by volcanic eruptions which have repeated many of -the characteristics of their Tertiary predecessors. The old and the new -development of the same volcanic type are thus visible side by side.</p> - -<p>The Tertiary volcanic series of Iceland reaches a thickness of upwards of -3000 metres, or nearly 10,000 English feet, but as its base is nowhere seen, it -may be still thicker. Its successive sheets, piled over each other in parallel -layers, form terraced hills and bold escarpments along the coast, whence they -slope gently inland. The plateau, as in the Faroe Islands and in Scotland, -<span class="pagenum" id="Page_261">- 261 -</span> -has been extensively eroded, and has been trenched by many long valleys -and fjords The composition of the basalts remains remarkably uniform -over the island. The lava sheets are often decomposing, amygdaloidal, -and filled with zeolites; while higher in the series compact basalts abound, -the uppermost fine-grained sheets being especially constant in structure and -composition. Numerous dykes traverse the plateau, and some of them cut -even its highest members. The parallel with the geological structure of the -Inner Hebrides is continued in Iceland by the appearance of intrusive masses -of gabbro and granophyre, which represent the deeper parts of the Tertiary -volcanic series, while the basalts were poured out at the surface. Thus, at -Papafjord, the gabbro rises into mountainous peaks and, like the similar -rock in Mull and Skye, is intersected by dykes of a coarse-grained granitoid -liparite or granophyre. Large dykes and ramifying veins of the same acid -material, often with a thoroughly granitic aspect, extend into the basalts.<a id="FNanchor_267" href="#Footnote_267" class="fnanchor">[267]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_267" href="#FNanchor_267" class="label">[267]</a> Mr. Thoroddsen, <i>Dansk. Geografisk Tidsskrift</i>, vol. xiii.</p> - -</div> - -<p>A long series of eruptions has taken place in Iceland since the Glacial -Period. There were likewise pre-glacial eruptions. The glaciated lava-streams -are found underneath the modern lavas. So far indeed as is -known, no evidence exists of any important cessation of subterranean -activity there since Tertiary time.<a id="FNanchor_268" href="#Footnote_268" class="fnanchor">[268]</a> The existing volcanic phenomena may -with probability be regarded as the survival of those which were so widely -manifested over the Icelandic area and the north-west of Europe in the -older Tertiary ages. A careful study of them may therefore be expected to -throw light on the history of the Tertiary basaltic plateaux; while, on the -other hand, the thorough dissection of these plateaux by the denuding -agencies will not improbably be found to explain some parts of the subterranean -mechanism of the modern Icelandic volcanoes.</p> - -<div class="footnote"> - -<p><a id="Footnote_268" href="#FNanchor_268" class="label">[268]</a> See Dr. Johnston-Lavis, <i>Scottish Geographical Magazine</i>, 1895, p. 442.</p> - -</div> - -<p>In calling attention to some of the more obvious analogies which may -be traced between the modern and the ancient volcanoes, I am more -particularly indebted to the excellent memoirs of the resident Icelandic -geologist, Mr. Th. Thoroddsen, who has examined so large a part of the -island.<a id="FNanchor_269" href="#Footnote_269" class="fnanchor">[269]</a> The account given by Mr. A. Holland of the Laki craters has likewise -been of much service to me.<a id="FNanchor_270" href="#Footnote_270" class="fnanchor">[270]</a> Among other recent observers I may -cite Dr. Tempest Anderson,<a id="FNanchor_271" href="#Footnote_271" class="fnanchor">[271]</a> who has made himself familiar with extensive -tracts of Iceland. He was accompanied one year by Dr. Johnston-Lavis, -who has published a narrative of the journey.<a id="FNanchor_272" href="#Footnote_272" class="fnanchor">[272]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_269" href="#FNanchor_269" class="label">[269]</a> See In particular his paper on the volcanoes of north-east Iceland (<i>Bihang till. k. Svensk. -Vet. Akad. Handl.</i> xiv. ii. No. 5, 1888) and that on Snaefell and Faxebugt in the south-west of -the island (<i>op. cit.</i> xvii. ii. No. 2, 1891); also papers in <i>Dansk. Geografisk Tidsskrift</i>, vols. xii. -xiii. (1893-95); <i>Verhand. Gesellsch. Erdkunde zu Berlin</i>, 1894-95.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_270" href="#FNanchor_270" class="label">[270]</a> "Lakis Kratere og Lavaströmme, Universitætsprogram," Christiania, 1885. See Mr. -Thoroddsen's remarks on this paper, <i>Verhand. Gesell. Erdkunde</i>, 1894, p. 289.</p> - -<p><a id="Footnote_271" href="#FNanchor_271" class="label">[271]</a> <i>Brit. Assoc. Rep.</i> 1894, p. 650.</p> - -<p><a id="Footnote_272" href="#FNanchor_272" class="label">[272]</a> Dr. Johnston-Lavis, <i>Scottish Geographical Magazine</i>, September 1895.</p> - -</div> - -<p>It is a mistake to suppose that the Icelandic volcanoes are generally -built on the plan of such mountains as Vesuvius or Etna. Mr. Thoroddsen -can evidently hardly repress his impatience to find these two Italian cones -<span class="pagenum" id="Page_262">- 262 -</span> -cited in almost every handbook of geology as types of modern volcanoes and -their operations. The regular volcanic cone, composed of alternations of -lavas and tuffs, plays a very subordinate part in Iceland.</p> - -<div class="figcenter" id="v2fig292" style="width: 315px;"> - <img src="images/v2fig292.png" width="315" height="259" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 292.</span>—Fissure (gjá) in a lava-field, Iceland. (From a photograph by Dr. Tempest Anderson.)</div> -</div> - -<p>The fundamental feature in the Icelandic eruptions is the production of -fissures which reach the surface and discharge streams of lava from many -points. Two systems of such fissures appear to be specially marked, one -in southern Iceland running from south-west to north-east, the other, in -the north part of the island, stretching from south to north.<a id="FNanchor_273" href="#Footnote_273" class="fnanchor">[273]</a> Hekla -and Laki belong to the former. The dislocations have often followed the -boundaries of the "horsts," or solid blocks of country which have withstood -terrestrial displacement. The vast outbreaks of Odádahraun and Myvatn -have almost all issued from fissures of that nature.</p> - -<div class="footnote"> - -<p><a id="Footnote_273" href="#FNanchor_273" class="label">[273]</a> In the Snaefell promontory they run nearly east and west. Mr. Thoroddsen, <i>Bihang. Svensk. -Akad.</i> xvii. (ii.) No. 2, p. 91.</p> - -</div> - -<p>The violent eruption of 1875 in Askja found its exit at the intersection -of two lines of fissures. Many large fissures were opened on the surface in -a nearly north and south direction, which could be followed for 80 kilometres -or nearly 50 English miles. Some of them became the theatre -of intense volcanic activity.<a id="FNanchor_274" href="#Footnote_274" class="fnanchor">[274]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_274" href="#FNanchor_274" class="label">[274]</a> Mr. Thoroddsen, <i>op. cit.</i> xiv. ii. No. 5, p. 63.</p> - -</div> - -<p>Many lines of fissure are traceable at the surface as clefts or "gjás," that -run nearly straight for long distances, with a width of one to three yards, -and sometimes of unknown depth.<a id="FNanchor_275" href="#Footnote_275" class="fnanchor">[275]</a> The most stupendous example of the -structure yet discovered is probably the Eldgjá found by Dr. Thoroddsen in -the year 1893, below the Mýrdalsjökull. This gigantic chasm has a length -<span class="pagenum" id="Page_263">- 263 -</span> -of 30 kilometres (more than 18 English miles), and a depth of 130 to 200 -metres (426 to 656 feet). Over its vertical walls lofty waterfalls plunge -from the crest to the bottom.</p> - -<div class="footnote"> - -<p><a id="Footnote_275" href="#FNanchor_275" class="label">[275]</a> On the various modes of origin of these chasms, see Dr. Tempest Anderson, <i>Brit. Assoc. Rep.</i> -p. 650. The gjá shown in <a href="#v2fig292">Fig. 292</a> is not an eruptive fissure. For this and the following illustration -I am indebted to the kindness of Dr. Tempest Anderson, who himself photographed -the scenes.</p> - -</div> - -<p>Occasionally a fissure has not been continuously opened to the surface. -An interesting example of such intermittent chasms is supplied by the -great rent which gave forth the enormous volume of lava in 1783. The -mountain of Laki, composed of palagonite tuff, stands on the line of this -dislocation, but has not been entirely ruptured. The fissure has closed up -beneath the mountain, a short distance above the bottom of the slope, as -is shown by the position of a couple of small craters.<a id="FNanchor_276" href="#Footnote_276" class="fnanchor">[276]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_276" href="#FNanchor_276" class="label">[276]</a> Mr. A. Helland, <i>op. cit.</i> <a href="#Page_25">p. 25</a>.</p> - -</div> - -<p>Some fissures have remained mere open chasms without any discharge of -volcanic material; others have served as passages for the escape of lava and -the ejection of loose slags and cinders.<a id="FNanchor_277" href="#Footnote_277" class="fnanchor">[277]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_277" href="#FNanchor_277" class="label">[277]</a> Mr. Thoroddsen has observed that in the Reykjanes peninsula in the south-west of Iceland, -by the subsidence of one side of a fissure, a row of four craters has been cut through, leaving their -segments perched upon the upper side. <i>Globus.</i> vol. lxix. No. 5.</p> - -</div> - -<div class="figcenter" id="v2fig293" style="width: 321px;"> - <img src="images/v2fig293.png" width="321" height="249" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 293.</span>—Cones on the great Laki fissure, Iceland. (From a photograph by Dr. Tempest Anderson.)</div> -</div> - -<p>In some instances, according to Mr. Thoroddsen, lava wells out from the -whole length of a fissure without giving rise to the formation of cones, -the molten material issuing either from one or from both sides and -flowing out tranquilly. Thus from three points on the great Eldgjá chasm -lava spread out quietly without giving rise to any craters, though at the -southern prolongation of the fissure, where it becomes narrower, a row of -low slag-cones was formed. The three lava-streams flooded the low ground -over an area of 693 square kilometres, or 270 English square miles. In -the great majority of cases, however, the lava as it ascends in the fissure -gives rise to long ramparts of slags and blocks of lava piled up on either side, -or to a row of cones along the line of the open chasm. Thus, on the Laki -<span class="pagenum" id="Page_264">- 264 -</span> -fissure, which runs for about 20 miles in a north-east direction, -the cones amount to some hundreds in number.</p> - -<div class="figright" id="v2fig293a" style="width: 100px;"> - <img src="images/v2fig293a.png" width="45" height="774" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 293</span><i>a</i>.—Plan of small craters along the line of great Laki fissure, Iceland. (After Mr. Helland, reduced.)</div> -</div> - -<p>The cones consist generally of slags, cinders, and blocks -of lava. They are on the whole not quite circular but oblong, -their major axis coinciding with the line of the chasm on -which they have been piled up, as along the marvellous line -of the Laki fissure. In many places they are exceedingly -irregular in form, changes in the direction of outflow of lava or -of escape of steam having caused the cones partially to efface -each other.</p> - -<p>As regards their size, the cones present a wide range. -Some of them are only a few yards in diameter, others several -hundred yards. Generally they are comparatively low mounds. -On a fissure hardly 30 feet long, Mr. Thoroddsen found a -row of twelve small cones built exactly like those of largest -size, but with craters less than three feet in diameter. On -the Laki fissure some are only a couple of yards high; the -majority are much less than 50 yards in height, and hardly -one is as much as 100 yards.<a id="FNanchor_278" href="#Footnote_278" class="fnanchor">[278]</a> And yet these little monticules, -as Mr. Helland remarks, represent the pipes from -which milliards of cubic metres of lava have issued. While -other European volcanoes form conspicuous features in the -landscape, the Icelandic volcanoes of the Laki district, from -which the vastest floods of lava have issued in modern times, -are so low that they might escape notice unless they were -actually sought for.<a id="FNanchor_279" href="#Footnote_279" class="fnanchor">[279]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_278" href="#FNanchor_278" class="label">[278]</a> Mr. Thoroddsen, however, states that there are about 100 ranging between -20 and 100 metres in height.</p> - -<p><a id="Footnote_279" href="#FNanchor_279" class="label">[279]</a> <i>Op. cit.</i> <a href="#Page_27">p. 27</a>.</p> - -</div> - -<p>As they have generally arisen along lines of fissure, the -cones are, for the most part, grouped in rows. The hundreds -of cones that mark the line of the Laki fissure present an -extraordinary picture of volcanic energy of this type. In -other instances the cones occur in groups, though this -distribution may have arisen from the irregular uprise of -scattered vents along a series of parallel fissures. Thus to the -north-east of Laki a series of old cones entirely surrounded by -the lavas of 1783 lie in groups, the most northerly of which -consists of about 100 exceedingly small craters that have sent -out streams of lava towards the N.N.E.<a id="FNanchor_280" href="#Footnote_280" class="fnanchor">[280]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_280" href="#FNanchor_280" class="label">[280]</a> <i>Op. cit.</i> <a href="#Page_25">p. 25</a>. The great lava-fields of Iceland are likewise dotted over -with secondary craters or "hornitos" which have no direct connection with the -magma below, but arise from local causes affecting the outflowing lava. They -are grouped in hundreds over a small space.</p> - -</div> - -<p>It would appear from Mr. Helland's observations that the -same fissure has sometimes been made use of at more than one -<span class="pagenum" id="Page_265">- 265 -</span> -period of eruption. He describes some old craters on the line of the Laki -fissure, which had been active long before the outbreak of 1783.<a id="FNanchor_281" href="#Footnote_281" class="fnanchor">[281]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_281" href="#FNanchor_281" class="label">[281]</a> <i>Op. cit.</i> <a href="#Page_26">p. 26</a>.</p> - -</div> - -<p>When the lava issues from fissures it is in such a condition of plasticity -that it can be drawn out into threads and spun into ropes. When the slope -over which it flows is steep it often splits up into blocks on the surface. -Where the ground is flat the lava spreads out uniformly on all sides, forming -wide plains as level as a floor. Thus the vast lava-desert of Odádahraun -covers a plain 3640 square kilometres in area, or, if the small-lava-streams -north from Vatnajökull be included, 4390 square kilometres. This vast -flood of lava (about 1700 English square miles in extent) would, according -to Mr. Thoroddsen, cover Denmark to a depth of 16 feet. The whole of -this enormous discharge has been given forth from more than twenty vents -situated for the most part on parallel fissures.</p> - -<p>Not less striking is the picture of fissure-eruption to be met with at -Laki—the scene of the great lava-floods of 1783. "Conceive now," says -Mr. Helland, "these hundreds of craters, or, as they are called by the Icelanders, -'borge,' lying one behind another in a long row; every one of them -having sent out two or more streams of lava, now to the one side, now to -the other. Understand further that these streams merge into each other, so -as to flow wholly round the cones and form fields of lava miles in width, -which, like vast frozen floods, flow down to the country districts, and you -may form some idea of this remarkable region."<a id="FNanchor_282" href="#Footnote_282" class="fnanchor">[282]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_282" href="#FNanchor_282" class="label">[282]</a> <i>Op. cit.</i> <a href="#Page_24">p. 24</a>. Mr. Helland allows an average thickness of 30 metres for the mass of lava -which issued in two streams, one 80 kilometres (nearly 50 miles), the other 45 kilometres (about -28 miles) long. He estimates the total volume of lava discharged in the 1783 eruption at 27 -milliards of cubic metres, equal to a block 10 kilometres (6 miles 376 yards) long, 5 kilometres (3 -miles 188 yards) broad, and 540 metres (1771 feet) high; <i>op. cit.</i> <a href="#Page_31">p. 31</a>. Mr. Thoroddsen remarks -that the older estimates of the volume of lava discharged by this eruption have been greatly -exaggerated. He puts the area covered by lava at 565 square kilometres and the contents at 12<sup>1</sup>/<sub>3</sub> -cubic kilometres. Verhand. <i>Gesell. Erdkunde Berlin</i>, 1894, p. 296.</p> - -</div> - -<p>The basaltic lavas have issued in a comparatively liquid state, form -thin sheets and reach to great distances. The western stream from the -Laki eruption of 1783 flowed for upwards of 40 miles; a prehistoric lava -from Trölladyngjá in Odádahraun flowed for more than 60 miles.</p> - -<p>In the course of time the successive streams of lava poured out upon -one of these wide volcanic plains gradually increase the height of the ground, -while preserving its generally level aspect. The loose slag-cones of earlier -eruptions are effaced or swallowed up, as one lava-stream follows another. -Eventually, when, by the operation of running water or by fissure and subsidence, -transverse sections are cut through these lava-sheets, the observer -can generally notice only horizontal beds of lava piled one above another, -including the dykes connected with them and intercalated masses of loose -slag, that remain as relics of the old craters.</p> - -<p>In some places the lava has gradually built up enormous domes, like -those of Hawaii, having a gentle inclination in every direction, as may -be seen especially in the district between Floderne Skjalfanafljot and -Jökulsà Most of the large volcanic piles of North Iceland are of this -<span class="pagenum" id="Page_266">- 266 -</span> -nature. The highest of them are 1209 and 1491 metres high by from 6 -to 15 kilometres in diameter. The elliptical crater of the highest of these -eminences measures 1100 by 380 metres.<a id="FNanchor_283" href="#Footnote_283" class="fnanchor">[283]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_283" href="#FNanchor_283" class="label">[283]</a> Mr. Thoroddsen, <i>op. cit.</i> xiv. ii. No. 5, pp. 10, 23.</p> - -</div> - -<p>Large conical volcanoes of the Vesuvian type built up of alternating -lavas and tuffs are not common in Iceland, but some occur and rise -into lofty glacier-covered mountains, such as Öræfajökull (6241 feet), -Eyjafjallajökull (5432), and Snaefellsjökull (4577). Hekla (4961) also -is similarly composed of sheets of lava and tuffs, but has not been built -as a cone. It forms an oblong ridge which has been fissured in the direction -of its length and bears a row of craters along the fissure.<a id="FNanchor_284" href="#Footnote_284" class="fnanchor">[284]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_284" href="#FNanchor_284" class="label">[284]</a> Mr. Thoroddsen, <i>Dansk. Geograf. Tidsskrift</i>, vol. xiii.</p> - -</div> - -<p>Explosion-craters likewise occur among the modern volcanic phenomena -of Iceland. One of these was formed by a violent explosion at Askja on -29th March 1875. It has a diameter of only about 280 feet, yet so -great was the vigour of the outburst that pumiceous stones were spread -over an area of more than 100 Danish (468 English) square miles, and -the dust was carried as far as Norway and Sweden. Nine years later Mr. -Thoroddsen found the bottom of this crater filled with bluish-green boiling -mud, which will probably in the end become a sheet of still water. The -borders of these Icelandic explosion-craters seem to be very little higher -than the ground around them. Most of the ejected material is expelled -with such force and to such a distance that only a small fraction of it -falls down around the orifice of eruption.<a id="FNanchor_285" href="#Footnote_285" class="fnanchor">[285]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_285" href="#FNanchor_285" class="label">[285]</a> Mr. Thoroddsen, <i>op. cit.</i></p> - -</div> - -<p>There is still another feature of the Icelandic volcanic regions which -may be cited as an interesting parallel to the sequence of eruptive discharges -among the Inner Hebrides. While the lavas are as a rule more or less -basic—many of them being true basalts—they have been at different times -pierced by much more acid liparites and obsidians. Examples of these -rocks of post-Glacial age have recently been traced on the ground by -Mr. Thoroddsen,<a id="FNanchor_286" href="#Footnote_286" class="fnanchor">[286]</a> and their petrographical characters have been studied by -Mr. Bäckström.<a id="FNanchor_287" href="#Footnote_287" class="fnanchor">[287]</a> The wide distribution of such rocks all over the island, -their occurrence in isolated bosses among the more basic lavas, and their -remarkable internal structures have been noted by several observers.<a id="FNanchor_288" href="#Footnote_288" class="fnanchor">[288]</a> The -liparites and obsidians are contrasted with the basalt by the colours and -forms of their streams. Some of them are so black as to look like heaps -of coal, though their surfaces pass into grey pumice. They have flowed -out in a much less liquid condition than the basalts, and have consequently -formed short, thick and irregular sheets. The liparites and basalts appear -to have been nearly contemporaneous. They certainly belong to the same -volcanic cycle and their vents lie close to each other. Though none of the -<span class="pagenum" id="Page_267">- 267 -</span> -acid eruptions are known to have occurred in modern times, some of the -liparites are crusted with sulphur and from the connected fissures steam -still rises.</p> - -<div class="footnote"> - -<p><a id="Footnote_286" href="#FNanchor_286" class="label">[286]</a> <i>Geol. Fören. Stockholm Förhandl.</i> xiii. (1891), p. 609; <i>Bihang. Svensk. Vet. Akad. Handl.</i> -xvii. ii. p. 21 (1891); <i>Dansk. Geograf. Tidsskrift</i>, xiii. (1895).</p> - -<p><a id="Footnote_287" href="#FNanchor_287" class="label">[287]</a> <i>Geol. Fören. Stockholm Förhandl.</i> xiii. (1891), p. 637.</p> - -<p><a id="Footnote_288" href="#FNanchor_288" class="label">[288]</a> See in particular C. W. Schmidt, <i>Zeitsch. Deutsch. Geol. Gesellsch.</i> xxxvii. (1885), -p. 737.</p> - -</div> - -<p>It will thus be seen how entirely the modern volcanic eruptions of -Iceland agree with the phenomena presented by our Tertiary basalt-plateaux. -It is, therefore, to the Icelandic type of fissure-eruptions, and -not to great central composite cones like Vesuvius or Etna that we -must look for the modern analogies that will best serve as commentary -and explanation for the latest chapter in the long volcanic history of the -British Isles.<a id="FNanchor_289" href="#Footnote_289" class="fnanchor">[289]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_289" href="#FNanchor_289" class="label">[289]</a> In his memoir of 1874, Professor Judd announced his conclusion that there were formerly -five great volcanoes amongst the Western Isles, and that the lavas of the plateaux had issued -from these. He subsequently reiterated this view (<i>Quart. Journ. Geol. Soc.</i> xlv., 1890, p. 187), -and ridiculed the explanation of fissure-eruptions. The evidence adduced by me in a paper -published in 1896 (same journal, vol. lii. p. 331) and reprinted with additions in this chapter, -will, I trust, be regarded by geologists as having finally settled this question.</p> - -</div> - -<p>As a further but more ancient illustration of the type of volcanic action -which appears to have been prevalent during the formation of the Tertiary -volcanic plateaux of Britain, I may again refer to the vast basalt-fields of -Western America. The basalt of Idaho stretches out as an apparently -limitless plain. Along its northern boundary, this sea of black lava runs up -the valleys and round the promontories of the older trachytic hills with -almost the flatness of a sheet of water. It has been deeply trenched, however, -by the streams that wind across it, and especially by the Snake River, -which has cut out a gorge some 700 feet deep, on the walls of which the -successive beds of basalt lie horizontally one upon another, winding along -the curving face of the precipice exactly as those of Antrim and the Inner -Hebrides do along their sea-worn escarpments. Here and there, a low -cinder-cone on the surface of the plain marks the site of a late outflow. -One is struck, however, with the singular absence of tuffs and volcanic conglomerates. -The basalts appear to have flowed out stream after stream with -few fragmentary discharges.</p> - -<p>These characteristic features of one distinctive type of volcanic action -have been repeated over a vast region, or rather a whole series of regions, in -Western America, the united area of which must equal that of a considerable -part of Europe. From Idaho, the basalt-fields may be followed southwards -interruptedly into Utah and Nevada, and across the great plateau-country -of the cañons into Arizona and New Mexico, northwards into -Montana, and westwards into Oregon. The tract which has as yet been -most carefully traversed and described is probably that of the high plateaux -of Utah and Arizona. Thus on the Uinkaret plateau, which measures some -45 to 50 miles in length by 8 to 12 in breadth, a thick covering of basalt -has been spread composed of many successive flows. Between 160 and 170 -separate cones have been counted on this area, most of them quite small, -mere low mounds of scoriæ, though a few reach a height of 700 or 800 feet, -with a diameter of a mile. From three to seven or eight may be found in -a row, as if springing from a single line of fissure. But generally the -<span class="pagenum" id="Page_268">- 268 -</span> -grouping is quite irregular.<a id="FNanchor_290" href="#Footnote_290" class="fnanchor">[290]</a> My friend Captain C. E. Dutton, from whose -admirable memoir these details are quoted, remarks further that among the -Utah plateaux no trace of a cone is to be found at or near some of the most -recent basalt-fields, and that the most extensive outpours are most frequently -without cones. "The lavas," he adds, "appear to have reached the surface -and overflowed like water from a spring, spreading out immediately and -deluging a broad surface around the orifice."<a id="FNanchor_291" href="#Footnote_291" class="fnanchor">[291]</a> The deep gorges cut by the -rivers through these thick accumulations of horizontal or nearly horizontal -basalts, have here and there revealed parallel dykes that traverse the rocks, -and in at least one case have shown the dyke running for half a mile up a -cliff and actually communicating with a crater of scoriæ at the top.<a id="FNanchor_292" href="#Footnote_292" class="fnanchor">[292]</a> Again, -in New Mexico, Captain Dutton noticed vast tracts of younger basalt, about -which "a striking fact is the entire absence of all distinguishable traces of -the vents from which they came. Some of them, however, indicate unmistakably -their sources in small depressed cones of very flat profiles. No -fragmental ejecta (scoriæ, lapilli, etc.) have been found in connection with -these young eruptions."<a id="FNanchor_293" href="#Footnote_293" class="fnanchor">[293]</a> Such I believe to have been the general -conditions under which the basalts of the Tertiary plateaux of the British -Isles were also erupted.<a id="FNanchor_294" href="#Footnote_294" class="fnanchor">[294]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_290" href="#FNanchor_290" class="label">[290]</a> Captain C. E. Dutton, "Tertiary History of the Grand Cañon District," <i>U.S. Geol. Survey</i> -(1882), p. 104.</p> - -<p><a id="Footnote_291" href="#FNanchor_291" class="label">[291]</a> Captain C. E. Dutton, "Geology of the High Plateaux of Utah," <i>U.S. Geol. Survey of the -Rocky Mountain Region</i> (1880), pp. 198, 200. See also pp. 232, 234, 276 of the same Monograph -for additional examples.</p> - -<p><a id="Footnote_292" href="#FNanchor_292" class="label">[292]</a> <i>Tertiary History of the Grand Cañon</i>, etc., p. 95.</p> - -<p><a id="Footnote_293" href="#FNanchor_293" class="label">[293]</a> <i>Nature</i>, xxxi. (1884), p. 49.</p> - -<p><a id="Footnote_294" href="#FNanchor_294" class="label">[294]</a> I may again refer to Hopkins's <i>Researches in Physical Geology</i>, where the conditions of the -problem here discussed have been distinctly realized. Speaking of the ejection of lava from a -number of fissures, he remarks that the imperfect fluidity of the melted material "would seem to -require a number of points or lines of ejection as a necessary condition." "If there were only a -single centre of eruption, a bed of such matter approximating to uniformity of thickness, could -only be produced on a surface of a conical form." "Where no such tendency to this conical -structure can be traced, it would probably be in vain to look for any single centre of eruption. -On the supposition, too, of ejection through continued fissures, or from a number of points, that -minor unevenness of surface which must probably have existed under all circumstances during -the formation of the earth's crust, would not necessarily destroy the continuity of a comparatively -thin extensive bed of the ejected matter, in the same degree in which it would inevitably produce -that effect in the case of central ejection" (<i>Cambridge Phil. Trans.</i> vi. 1835, p. 71).</p> - -</div> - -<p>Although we may be convinced, from their general structure and -relations, that the stratified lavas of these plateaux have been poured -out from fissures and not from great central cones, it must obviously be -difficult to obtain demonstrative evidence of this origin from any single -section. Of the thousands of dykes which traverse the British plateaux -and the ground around them, I am not aware of a single one which can -be actually seen to have ever communicated with the surface. The very -process of denudation which has revealed these dykes has at the same -time removed all trace of any former connection they may have had with -the surface. The only places where we may hopefully search for the -missing evidence are the fronts of the escarpments. On these precipices -dykes may sometimes be seen to end off at some particular platform -<span class="pagenum" id="Page_269">- 269 -</span> -among the basalt-sheets, but I have never found a case which could be -confidently cited as an example of lava rising in a fissure and spreading -out as a superficial sheet. That this connection may eventually be found -when a more detailed survey is made of these great sea-walls I fully -anticipate.</p> - -<p>In recently mapping the basalt-plateau of Strathaird in Skye, Mr. -Harker has made some interesting observations regarding the probable connection -of the dykes with the plateau basalts. He has noticed that the -flanks of Slat Bheinn, a portion of the plateau, are abundantly traversed -by dykes containing numerous enclosed pieces of gabbro, while the basalt -on the summit of the plateau is full of similar fragments—an occurrence -not observed elsewhere. It is conceivable that the gabbro-bearing basalt-sheets -are sills, but Mr. Harker has found no proof that they are so, the -evidence so far as it has been collected being rather in favour of the view -that these sheets are superficial lavas, and that they have been supplied -from the dyke-fissures.</p> - -<p>Various considerations suffice to assure us that actual instances of the outflow -of the basalt from its parent fissures should be expected to be exceptional. -The absence or scarcity of beds of scoriæ among the basalt-plateaux -may be taken as an indication that the lava as a rule flowed out without -the formation of cinder-cones, and therefore that these conspicuous monuments -of the eruptive vents were probably always rare in Britain. If the -lava was poured out tranquilly from one or two points along a fissure which -were subsequently buried under floods of similar lava issuing from other -fissures, the chances that such points of emission should be laid open along -the front of any escarpment are small. And, even when so exposed, it -might be difficult to feel sure that the dyke below was really the feeder of -the basalt above, unless the cliff were accessible and the rocks could be -scrutinized foot by foot. These elements of uncertainty are happily -removed where the volcanic energy has drilled well-marked funnels of discharge -and left them filled with the erupted materials, as will be narrated -in the next chapter.</p> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_270">- 270 -</span></p> - -<h2 class="nobreak" id="CHAPTER_XLI">CHAPTER XLI<br /> - -<span class="smaller">THE ERUPTIVE VENTS OF THE BASALT-PLATEAUX</span></h2> -</div> - -<div class="blockquot"> - -<p>Vents filled with Basalt or other Lava-form Rock—Vents filled with Agglomerate</p> - -</div> - -<p>It is one of the most interesting points in the Tertiary volcanic history -that, in spite of the enormous geological revolutions that have passed since -they became extinct, the sites of many scattered vents can still be recognized. -A far greater number must lie buried under the basalts, and of -others the positions are concealed by the sea, which now covers so large -an area of the old lava-fields. Nevertheless, partly within the area of the -plateaux, but still more on the surrounding tracts from which the basalts -have been removed by denudation, the traces of unmistakable vents of discharge -may be recognized amid the general wreck.</p> - -<p>In Britain and the Faroe Isles, it is chiefly along the coast-line that the -process of denudation has revealed the volcanic vents of Tertiary time. The -interior of the country is often loaded with peat, covered with herbage, or -strewn with glacial detritus: and even where indications of the vents are -to be detected, it is not always possible to ascertain their true limits -and connections. But where the structure of the plateaux has been laid -bare along ranges of rocky precipice, the vents have sometimes been so -admirably dissected by the sea that every feature of their arrangements can -be satisfactorily determined.</p> - -<p>As the actual physical connexion of these volcanic orifices with the -plateaux has been in most cases removed by denudation, we can usually only -by inference place them in what was probably their true relation to the -plateau-eruptions. Those which project from the surface of the plateaux -must, of course, be younger than the basalts through which they rise; how -much younger we cannot tell. They may possibly be later than any of the -plateau-sheets; they may even belong to a subsequent and waning condition -of volcanic action. On the other hand, the vents which can now be traced -outside of the present limits of the edges of the plateaux may, like those -just mentioned, be younger than the basalt-sheets, or, on the contrary, they -may be records of a period of eruptivity anterior to the emission of any of -the rocks of the plateaux, and may have been deeply buried under a mass -of basalt-beds subsequently removed. Positive demonstration is, from the -<span class="pagenum" id="Page_271">- 271 -</span> -nature of the case, impossible in these instances. But examples will be cited -from the Western Isles and from Faroe, where the vents can be proved to -belong to the time of the plateau-eruptions, for they are seen to have broken -through some of the basalt-sheets and to have been buried under others. -With this clear evidence of relationship in some cases, there need be little -hesitation in believing that in other instances where no such positive connexion -can be found, but where the vents are obviously such as the general -structure of the plateaux would have led us to expect, they may be confidently -regarded as part of the phenomena of the plateau-eruptions.</p> - -<p>Sometimes the vents can be linked with lines of fissures or dykes. -This is especially the case where they are small in size. More usually, -however, no such relation can be demonstrated. It will be remembered that -among the modern Icelandic eruptions, some eruptive vents, like the later -cinder-cones of Laki, are ranged in a linear direction along the great fissure, -while others, of an older series in the same district, almost engulphed -amidst the more recent lavas, are clustered irregularly in groups. A similar -diversity of arrangement has been observed among the volcanic cones of the -Velay in Central France.</p> - -<p>Considering as a whole the volcanic necks or eruptive vents which rise -from the older rocks around the Tertiary basalt-plateaux, and sometimes -even from the surface of these plateaux themselves, we may conveniently -follow the same classification as was adopted in dealing with those of -Palæozoic age, and, according to the nature of the material that now fills -them, arrange them in two series: (1) Those occupied by some form of -crystalline eruptive rock, and (2) those filled with volcanic agglomerate.</p> - - -<h3>i. <span class="allsmcap">VENTS FILLED WITH DOLERITE, BASALT, ETC.</span></h3> - -<p>These, as the composition of the plateaux would lead us to anticipate, are -numerous. They perhaps attain their most conspicuous development in -Antrim, either on the tableland or among the underlying rocks round its -edges. The finest example in that district is undoubtedly furnished by the -lofty eminence called Slemish, which rises above the surrounding basalt-terrace, -to a height of 1437 feet above the sea (<a href="#v2fig294">Fig. 294</a>). It is elliptical in -ground-plan, measuring some 4000 feet in length by 1000 in breadth. -Seen from the north, it appears as a nearly perfect cone. The material of -which it consists is a coarsely crystalline olivine-dolerite, presenting under -the microscope a nearly holocrystalline aggregate, in which the lath-shaped -felspars penetrate the augite, with abundant fresh olivine, and wedge-shaped -patches of interstitial matter. The rock is massive and amorphous, -except that it is divided by parallel joints into large quadrangular blocks -like a granitic rock, and wholly different from the character of the surrounding -basalts. The latter, which possess the ordinary characters of the -rocks of the plateaux, can be followed to within 80 yards of this neck, which -rises steeply from them, but their actual junction with it is concealed under -the depth of talus.</p> - -<p><span class="pagenum" id="Page_272">- 272 -</span></p> - -<div class="figcenter" id="v2fig294" style="width: 478px;"> - <img src="images/v2fig294.png" width="478" height="213" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 294.</span>—Slemish, a Volcanic Neck or Vent on the Antrim Plateau, seen from the north.</div> -</div> - - -<div class="figcenter" id="v2fig295" style="width: 426px;"> - <img src="images/v2fig295.png" width="426" height="98" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 295.</span>—Section of Volcanic Vent at Carnmony Hill (E. Hull).<br /><br /> - T, Lower basalt; C, Cretaceous strata; L, Lower Lias; M, Triassic marls; V, Vent.</div> -</div> - -<p>At the nearest point to which the two rocks are traceable, -the basalts appear somewhat indurated, break with a peculiar splintery -fracture, and weather with a white crust. These characters are still better -shown on abundant fragments which may be picked up among the debris -further up the slope. There can be no doubt, I think, that a ring of flinty -basalt, differing considerably in texture from the usual aspect of that rock -in the district, surrounds the neck. The meaning of this ring will be more -clearly seen from the description of another example in Mull. About four -miles to the north-east of Slemish, a smaller and less conspicuous neck rises -out of the plateau-basalts. The rock of which it consists is less coarsely -crystalline than that of Slemish, but its relations to the surrounding volcanic -rocks are obviously the same. On the west side of Belfast Lough a -boss of similar rock, about 1200 feet in diameter, rises at the very edge -of the basalt escarpment into the eminence known as Carnmony Hill (Fig. -295). On its northern side it presents along its wall a mass of interposed -volcanic agglomerate.<a id="FNanchor_295" href="#Footnote_295" class="fnanchor">[295]</a> On visiting with Mr. M'Henry the quarry opened -on the eastern face of this vent, I was much struck with the remarkable -cellular structure of some parts of the dolerite. Many of the vesicles are -lined with a thin pellicle of black glass, and the same substance occurs in -minute patches in the body of the rock. A thin slice exhibiting this -structure was found by Mr. Watts to possess the following characters:—"The -<span class="pagenum" id="Page_273">- 273 -</span> -rock is an ophitic dolerite consisting of plagioclase, augite, and iron ores, -without olivine, enclosing one or two patches of finer basalt. The vesicles -in the latter, and certain angular spaces between the crystals of the former, -have been wholly or partially filled with brown glass, the outer part of which -has been converted into radiating crystals of a brown mineral." The -occurrence of patches of -glass which seem to have -been squeezed into vesicles -or cracks in the body of a -dolerite or andesite has been -noticed in some of the -Tertiary dykes. But in -the present case the glass -occurs as a mere coating on -the walls of the larger -spheroidal vesicles, the interior -of which generally -remains empty.</p> - -<div class="footnote"> - -<p><a id="Footnote_295" href="#FNanchor_295" class="label">[295]</a> This neck was recognised by Du Noyer in 1868 as "one of the great pipes or feeders of the -basaltic flows." See Prof. Hull, Explanation of Sheets 21, 28 and 29, <i>Geol. Survey of Ireland</i> -(1876), p. 30.</p> - -</div> - -<div class="figright" id="v2fig296" style="width: 298px;"> - <img src="images/v2fig296.png" width="298" height="197" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 296.</span>—Section of the east side of Scawt Hill, near Glenarm.<br /><br /> - <i>a</i>, bedded basalt; <i>b</i>, mass of chalk; <i>c</i>, basalt neck.</div> -</div> - -<p>Of the other doleritic -necks scattered over the -surface of the Antrim plateau, I will refer to only one which occurs on -the hillslopes between Glenarm and Larne. It forms a prominence known -as the Scawt Hill, and consists of a boss of basalt, which, in rising -through a vent in the plateau-sheets, has carried up with it and converted -into marble a large mass of chalk which is now exposed along its eastern -wall (<a href="#v2fig296">Fig. 296</a>).</p> - -<div class="figcenter" id="v2fig297" style="width: 380px;"> - <img src="images/v2fig297.png" width="380" height="129" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 297.</span>—Section of Neck of Basalt, Bendoo, Ballintoy.<br /><br /> - <i>a</i> <i>a</i>, Chalk; <i>b</i>, neck.</div> -</div> - -<p>As examples of similar necks which have been exposed by denudation -outside the present limits of the same plateau, I may allude to those which -rise through the Cretaceous and other Secondary strata on the northern -coast near Ballintoy. One of the most striking of these may be seen at -Bendoo, where a plug of basalt, measuring about 1400 feet in one diameter -and 800 feet in another, rises through the Chalk, and alters it around the -line of contact (<a href="#v2fig297">Fig. 297</a>). Another remarkably picturesque example is to -be seen near Cushendall, where a prominent doleritic cone rises out of the -<span class="pagenum" id="Page_274">- 274 -</span> -platform of Old Red Sandstone, some distance to the north of the present -edge of the volcanic escarpment (<a href="#v2fig298">Fig. 298</a>).</p> - -<div class="figcenter" id="v2fig298" style="width: 424px;"> - <img src="images/v2fig298.png" width="424" height="199" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 298.</span>—Volcanic Neck of Dolerite near Cushendall.</div> -</div> - -<p>The greater coarseness of grain of the material filling these pipes, -compared with that of the sheets in the terraces, is only what the very -different conditions of cooling and consolidation would lead us to expect. -There is no essential difference of composition between the two rocks. -Where the erupted material has been poured out at the surface, it has -assumed a finely crystalline texture, while, where it has slowly solidified -within a volcanic pipe at some depth beneath the surface, and where consequently -its component crystals have had more time for development, the -resulting structure is much more largely crystalline, with a more or less -complete development of the ophitic structure.</p> - -<div class="figcenter" id="v2fig299" style="width: 441px;"> - <img src="images/v2fig299.png" width="441" height="85" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 299.</span>—Section of Volcanic Neck at 'S Airde Beinne, near Tobermory, Mull.<br /><br /> - <i>a</i> <i>a</i>, bedded basalts; <i>b</i> <i>b</i>, bedded basalts altered along the side of vent; <i>c</i> <i>c</i>, dolerite.</div> -</div> - -<p>In the island of Mull, another instance of the same kind of vent has -been observed and described by Professor Judd.<a id="FNanchor_296" href="#Footnote_296" class="fnanchor">[296]</a> It rises in the conspicuous -hill, 'S Airde Beinne (Sarta Beinn), about two miles south-west from Tobermory, -and consists of a coarsely crystalline dolerite, which becomes finer in -grain towards the outer margin (<a href="#v2fig299">Fig. 299</a>). No bedding, or structure of -any kind beyond jointing, is perceptible in it. Examined in thin sections -under the microscope, this rock is found to be another typical ophitic -dolerite, consisting of lath-shaped felspars embedded in augite, with here and -there wedge-shaped portions of interstitial matter and grains of olivine. -Dr. Hatch found the felspars to contain spherical inclusions of devitrified -glass, filled with black granules and trichites, and he observed that, -<span class="pagenum" id="Page_275">- 275 -</span> -under a high power, the interstitial matter is seen to consist mainly of a -greenish-brown isotropic substance, in which are inclosed small crystals of -augite, skeleton-forms and microlites of felspar, sometimes in stellate aggregates, -as well as club-shaped, cruciform, arrow-headed and often crested -microlites of magnetite.</p> - -<div class="footnote"> - -<p><a id="Footnote_296" href="#FNanchor_296" class="label">[296]</a> <i>Quart. Jour. Geol. Soc.</i>, xxx. (1874), p. 264.</p> - -</div> - -<div class="figcenter" id="v2fig300" style="width: 481px;"> - <img src="images/v2fig300.png" width="481" height="303" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 300.</span>—Interior of the Volcanic Neck of 'S Airde Beinne, near Tobermory, Mull.</div> -</div> - -<p>Towering prominently above the flat basalt sheets, this neck has an oval -form, measuring about half a mile in length by a quarter of a mile in -breadth. Its central portion, however, instead of rising into a rugged hill-top, -as is usually the case, sinks into a deep hollow, which is filled with -water, and reminds one of a true crater-lake (Figs. <a href="#v2fig299">299</a>, <a href="#v2fig300">300</a>). The -middle of the neck is thus concealed from view, and we can only examine -the hard prominent ring of dolerite that surrounds the tarn. The -material occupying the hollow may be softer than that of the ring, and -may have been scooped out by denudation. What we now see may not be -the original surface, but may have been exposed after the removal of possibly -hundreds of feet of overlying material. On the other hand, it is conceivable -that the hollow is really a crater-lake which was filled up with detritus -and may have been overspread with basalt, since removed. It may be -suggestively compared with the crater-hollows revealed by denudation on -the cliffs of Stromö and Portree Harbour, which will be described in a later -part of this chapter. Possibly some more easily removable agglomerate, -representing an eruption later than that of the dolerite, may occupy the -centre of the volcanic pipe.</p> - -<p>One of the most interesting features of this vent is to be found in its -relation to the surrounding basalts. The marginal parts of the rock along -<span class="pagenum" id="Page_276">- 276 -</span> -the line of contact are much finer in grain than the rest, and have obviously -cooled more rapidly. The contrast between them and the ordinary dolerite -nearer the centre, however, cannot be properly understood, except in thin -sections under the microscope. Dr. Hatch, to whom I submitted my -specimens, observed that, in place of the structure above described, the -marginal parts show an absence of the ophitic grouping except in small -isolated patches. Instead of occurring in large grains or plates enveloping -the felspars, the augite is found in numerous small roundish grains, together -with grains of magnetite, in equal abundance and of similar size. The -felspars are speckled over with opaque particles; olivine has not been -detected.</p> - -<p>For miles around the vent, the plateau-rocks are of the usual type—black, -compact, sometimes amygdaloidal, alternating with more coarsely -crystalline decomposing bands, the separation between different sheets -being often marked by the ordinary red ferruginous partings. But around -the margin of the neck, they have undergone a remarkable metamorphism. -The portions of them which adhere to the outer wall of the neck have lost -their distinct bedding, and have been, as it were, welded together into an -indurated compact, black to dull-grey rock, so shattery and jointed that -fresh hand-specimens, three or four inches in length, are not easily obtainable. -Especially marked is one set of joints which, running approximately -parallel, cause the rock to split into plates or slabs. These joints are sometimes -curved. Yet, in spite of the alteration from its normal character, -the basalt retains in places some of its more usual external features, such, -for instance, as its amygdaloidal structure, the amygdales consisting of calcite, -finely acicular mesotype, and other minerals.</p> - -<p>Examined under the microscope, this altered basalt presents "a confused -aggregate of colourless microlites (felspar?) and innumerable minute -granules of magnetite, these two constituents being very unequally distributed. -Sometimes the colourless portions preponderate, in other places the -opaque granules are heaped together in black patches, which may possibly -mark the position of fused augites."<a id="FNanchor_297" href="#Footnote_297" class="fnanchor">[297]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_297" href="#FNanchor_297" class="label">[297]</a> Notes by Dr. Hatch.</p> - -</div> - -<p>In the zone of contact-metamorphism around some of the volcanic pipes -in the plateaux, we see changes analogous to, but less developed than, those -which have been superinduced on so large a scale round the great eruptive -bosses of gabbro, granophyre, etc., that have broken up the terraced basalts -along the west coast of Scotland. I shall accordingly return to this subject -in connection with phenomena presented by these younger rocks (<a href="#Page_386">p. 386</a>).</p> - - -<h3>ii. <span class="allsmcap">VENTS FILLED WITH AGGLOMERATE</span></h3> - -<p>While the necks of dolerite or basalt cannot always be satisfactorily -discriminated from bosses which may never have established a connection -with the surface, there is no room for any doubt in this respect in the case -of those filled with fragmentary materials. As has been already pointed out, -<span class="pagenum" id="Page_277">- 277 -</span> -the occurrence of true volcanic agglomerate may be accepted as evidence of the -existence of an eruptive vent communicating with the surface of the earth. -The agglomerate in the vents associated with the basalt-plateaux, like that of -the Palæozoic vents, is generally exceedingly coarse, and without any trace of -structure. Blocks of all sizes up to masses some -yards in length, and of the most diversified -materials, both volcanic and non-volcanic, are -dispersed confusedly through a granular paste -of similar miscellaneous composition.</p> - -<div class="figcenter" id="v2fig301" style="width: 573px;"> - <img src="images/v2fig301.png" width="573" height="125" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 301.</span>—Diagram to show the probable relation of the Neck at Carrick-a-raide, Antrim, to an adjacent group of tuffs.<br /><br /> - <i>a</i> <i>a</i>, Chalk; <i>b</i> <i>b</i>, lower group of bedded basalts; <i>c</i>, vent of Carrick-a-raide, filled with coarse volcanic agglomerate; <i>d</i> <i>d</i>, bedded tuffs; - <i>e</i> <i>e</i>, large veins of basalt traversing the agglomerate; <i>f</i> <i>f</i>, zone of tuffs and pisolitic iron ore; <i>g</i> <i>g</i>, upper group of bedded basalts.</div> -</div> - -<p>An instructive example of the general -characteristics of agglomerate-vents, and of the -relation of these vents to the surrounding tuffs -and basalts, is to be found at the island of -Carrick-a-raide, on the north coast of Antrim, -and on the opposite mainland. The visible -mass of this neck is about 1000 feet in -diameter, but the boundaries, except on the -land side, are concealed by the sea. The -material filling up the vent is a coarse agglomerate, -in which blocks and bombs of basalt, -with pieces of chalk and flint, are stuck at all -angles in a dull dirty-green granular tuff. Some -large and small intrusions of basalt rise through -it. Owing partly to these intrusions, and partly -to the grass-covered slope that separates it -from the line of cliff, the actual contact of this -neck with the volcanic beds of the escarpment -cannot be seen. I have no doubt, however, -that the tuff, which has already been referred -to as so conspicuous a member of the series -here, was discharged from this vent.<a id="FNanchor_298" href="#Footnote_298" class="fnanchor">[298]</a> The -materials are as usual coarser in the pipe than -beyond it, but the finer portion or matrix of -the agglomerate is similar to many bands of -the tuff. The structure of the locality may -be diagrammatically represented as in <a href="#v2fig301">Fig. 301</a>. -The bedded tuff is thickest in the neighbourhood -of the vent, and gradually dies away on -either side of it.</p> - -<div class="footnote"> - -<p><a id="Footnote_298" href="#FNanchor_298" class="label">[298]</a> See Explanation of Sheets 7 and 8, <i>Geol. Survey of Ireland</i> (1888), p. 31.</p> - -</div> - -<p>But another important inference may be drawn from this locality. I -have already pointed out that the lower basalts here reach their minimum -thickness. Their basement beds thin away towards the vent as markedly -as the tuff thickens. Obviously they cannot have proceeded from that -point of eruption. Yet, that they had begun to be poured out before the -discharge of the tuff is shown by their underlying as well as overlying -<span class="pagenum" id="Page_278">- 278 -</span> -that rock, though westward, owing to the thinning away of the undermost -basalts, the tuff comes to lie directly on the Chalk. Hence, we may legitimately -infer that in this neighbourhood one or more other vents supplied -the sheets of the lower basalts.</p> - -<p>In the island of Mull a number of detached bosses or patches of -agglomerate much obscured by invasions of granophyre probably mark the -sites of volcanic vents. They will be more particularly noticed in Chapter -xlvii. One of their most interesting features is the large number of -fragments of felsitic or rhyolitic rocks which they contain.</p> - -<p>In the promontory of Ardnamurchan, where the basalt-plateau has been -invaded and displaced by later intrusions of crystalline rocks, and has likewise -been reduced to such a fragmentary condition by denudation, some -interesting examples of agglomerate necks have been laid bare. One of the -largest of these occurs on the north shore at Faskadale. Cut open by the -sea for more than a quarter of a mile, this neck is seen to be filled with -a coarse agglomerate, composed mainly of basalt-blocks and debris, but -crowded also with angular and subangular pieces of different close-grained -andesitic, felsitic and porphyritic rocks belonging to the acid series to be afterwards -described.<a id="FNanchor_299" href="#Footnote_299" class="fnanchor">[299]</a> Some of these stones exhibit a very perfect flow-structure, -and closely resemble certain fine-grained, flinty, intrusive rocks in Mull, to -which allusion will subsequently be made. The matrix of the agglomerate -is of the usual dull dirty-green colour, but is so intensely indurated that on a -fresh fracture it can hardly be distinguished from some of the crystalline -rocks of the locality. The neck is pierced in all directions with dykes and -veins of basalt, dolerite, andesite, gabbro, and felsitic rocks. Similar intrusions -continue and increase in numbers farther west until the cliffs become a labyrinth -of dykes and veins running through a mass of rocks which appears to -consist mainly of dull dolerites and fine gabbros. Though the relations of this -vent to the plateau-basalts are not quite plain, the agglomerate seemed to -me to rise out of these rocks. At least the basalts extend from Achateny -to Faskadale, but, as they are followed westwards, they are more and more -invaded by eruptive sheets, and assume the indurated character to which I -have already referred.</p> - -<div class="footnote"> - -<p><a id="Footnote_299" href="#FNanchor_299" class="label">[299]</a> One of these felsites when viewed under a high magnifying power is seen to present an -abundant development of exceedingly minute micropegmatite arranged in patches and streaks -parallel with the lines of flow-structure in the general cryptocrystalline groundmass. The close -relationship between the felsites, quartz-porphyries, and granophyres will be afterwards pointed -out in the description of the acid rocks. It is remarkable that, though these rocks occur -abundantly in fragments in the volcanic necks and agglomerates of the plateaux, not a single -instance has been observed of their intercalation as contemporaneous sheets among the basic -lavas. The analogous case of the interstratification of felsitic tuffs among basic lavas in the -volcanic series of the Old Red Sandstone of Central Scotland has been described (<a href="../../66492/66492-h/66492-h.htm#Page_279">vol. i. p. 279</a>). It -is interesting to note that liparitic pumice and dykes have been erupted by some of the basaltic -craters of Iceland, for example at Askja, Öræfajökull and Snaefellsjökull. (Mr. Thoroddsen, -<i>Dansk. Geograf. Tidsskrift</i>, vol. xiii. 7th and 8th parts.)</p> - -</div> - -<p>On the south side of the peninsula of Ardnamurchan, another agglomerate, -noticed by Professor Judd,<a id="FNanchor_300" href="#Footnote_300" class="fnanchor">[300]</a> rises into the bold headland of -<span class="pagenum" id="Page_279">- 279 -</span> -Maclean's Nose, at the mouth of Loch Sunart, and affords better evidence -of its relation to the bedded basalts. It measures about 1000 yards in -length by 300 in breadth, and its summit rises more that 900 feet above -the sea, which washes the base of its southern front. It is filled with an -agglomerate even coarser than that on the northern coast. The blocks are -of all sizes, up to eight or ten feet in diameter. By far the largest proportion -of them consists of varieties of basalt and andesite, slaggy and -vesicular structures being especially conspicuous. There are also large -blocks of different andesitic porphyries and felsitic rocks like those just -referred to, a porphyry with felspar crystals two inches long being particularly -abundant. All the stones are more or less rounded, and are wrapped -up in a dull-green compact matrix of basalt-debris. There is no stratification -or structure of any kind in the mass. Numerous dykes or veins of -basalt, of andesite, and of a porphyry, resembling that of Craignure, in Mull, -traverse the agglomerate. Some of the narrow basalt-dykes cut through -the others.</p> - -<div class="footnote"> - -<p><a id="Footnote_300" href="#FNanchor_300" class="label">[300]</a> <i>Quart. Journ. Geol. Soc.</i> xxx. (1874), p. 261. Professor Judd has subsequently (<i>op. cit.</i> -xlvi. 1890, pp. 374 <i>et seq.</i>) given a map, section and description of what he believes to be the -structure of this ground, with numerous details as to the petrography of the rocks. The -geological structure of this area is more fully referred to on pp. 318 <i>et seq.</i></p> - -</div> - -<div class="figcenter" id="v2fig302" style="width: 497px;"> - <img src="images/v2fig302.png" width="497" height="188" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 302.</span>—Section of agglomerate Neck at Maclean's Nose, Ardnamurchan.<br /><br /> - <i>a</i> <i>a</i>, quartzites and schists; <i>b</i>, bedded basalts lying partly on the schists and partly on patches of Jurassic sandstones - that occupy hollows of the older crystalline rocks; <i>c</i>, agglomerate; <i>d</i> <i>d</i>, dykes and veins traversing the agglomerate; - <i>e</i>, dolerite sheets of Ben Hiant.</div> -</div> - - -<p>The position of the vent, with reference to the surrounding rocks, -will be understood from the accompanying section (<a href="#v2fig302">Fig. 302</a>). On the -eastern side, the agglomerate can be seen to abut against the truncated -ends of the flat beds of the plateau-basalts, which are of the usual bedded -compact and amygdaloidal character. There can be no doubt, therefore, -that the vent has been opened through these basalts. But it will be -observed that the latter belong to the lower part of the volcanic series. -These lowest sheets are exposed on the slope, resting upon yellowish and -spotted grey sandstone, with seams of jet and a reddish breccia, which, lying -in hollows of the quartzites, quartz-schists, and mica-schists, form no doubt the -local base of the Jurassic rocks of the district. Hence, the vent, though -younger than the older sheets of the plateau, may quite well be contemporaneous -with some of the later sheets.<a id="FNanchor_301" href="#Footnote_301" class="fnanchor">[301]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_301" href="#FNanchor_301" class="label">[301]</a> It may here be remarked that there is evidence of great differences in the level of the base -of the Jurassic series and the bottom of the volcanic plateau in this district. On the south and -west sides of Ben Hiant the Jurassic conglomerates may be seen lying on the edges of the -crystalline schists only a little above high-water mark, while on the north side, the schists, -with their overlying unconformable cake of limestones, rise several hundred feet above sea-level. -The surface on which the basalts were poured out was probably very uneven, but there may also -have been some considerable displacements of these basalts either before or during the injection -of the dolerite sills of Ben Hiant.</p> - -</div> - -<p><span class="pagenum" id="Page_280">- 280 -</span></p> - -<p>An interesting feature at this locality is the peculiar grouping of some -of the large dykes in the area around the agglomerate. They run in the -direction of the vent, and one or other of them may represent the fissure or -fissures on which the volcanic orifice was blown open to the surface. -Another notable element in the geological structure of the ground is the -vast amount of intrusive material, both in dykes and sheets, which has been -erupted. The intrusive sheets of Ben Hiant form the most prominent -eminence in this part of Ardnamurchan. Reserving them for description in -the following Chapter (<a href="#Page_318">p. 318</a>), I will only remark here that they partly overlie -the agglomerate, and are therefore, to some extent at least, younger than the -vent. They belong to that late stage in the history of the basalt-plateaux -when the molten material, no longer getting ready egress to the surface, -forced its way among the rocks about the base of the bedded basalts, and -more especially on the sites of older vents, which were doubtless weak places, -where it could more easily find relief.</p> - -<p>The large neck now described is only one of a group scattered around -it in the ground to the north. Two of these may be seen rising through a -detached area of Jurassic limestones and shales at the northern base of -Ben Hiant. A third, almost obliterated by the intrusive sheets, may be traced -at the western end of that mountain above Coiremhuilinn. Two others -rising through the schists on either side of Beinn na h-Urchrach, have been -much invaded by the sills of that eminence (<a href="#v2fig326">Fig. 326</a>). It is doubtless owing -to the extensive denudation of the basalt-plateau, and the consequent uncovering -of the rocks underneath it, that this series of vents has been laid bare.<a id="FNanchor_302" href="#Footnote_302" class="fnanchor">[302]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_302" href="#FNanchor_302" class="label">[302]</a> Professor Judd has united these scattered vents into a continuous platform of volcanic -agglomerates, which he represents as underlying the supposed lavas of Ben Hiant. Since the -publication of his map and description, I have re-examined the ground without being able to discover -any trace of this platform. All the visible agglomerates are separate necks, their actual walls -being sometimes exposed, as in the neck immediately north of the base of Ben Hiant, where the -limestone in contact is marmorised, though twelve yards of it is an ordinary dull blue rock.</p> - -</div> - -<p>By far the largest mass of agglomerate in any of the Tertiary volcanic -areas of Britain is that which occurs on the north side of the main valley -of Strath, in Skye.<a id="FNanchor_303" href="#Footnote_303" class="fnanchor">[303]</a> Unfortunately, it has been so seriously invaded -by the eruptive rocks of the Red Hills, that its original dimensions and -its relations to the surrounding rocks, especially to the bedded basalts, are -much obscured (see <a href="#v2fig348">Fig. 348</a>). It can be followed continuously from the lower -end of Loch Kilchrist along the southern slopes of Beinn Dearg Bheag round -<span class="pagenum" id="Page_281">- 281 -</span> -to the western roots of Beinn Dearg Mhor—a distance of more than two -miles in a straight line, and from Kilbride to the flank of Beinn na Caillich -above Coire-chat-achan—a direct distance of two miles and a quarter. A -similar rock, possibly a portion of the same mass, appears in Creagan Dubha, -on the north side of the Red Hills. If the whole of this agglomerate forms -part of one originally continuous mass, it must have been upwards of two -miles in diameter. There may, however, have been two or three closely -adjacent vents. The Beinn na Caillich patch, for example, appears to -belong to a different area, and that of Creagan Dubha is also probably -distinct. But there seems no reason to doubt that the mass which forms -Cnoc nam Fitheach, and all the long declivity on the southern flank of -Beinn Dearg Bheag, occupies part of the site of a single volcano. Owing -to the absence of sufficient sections, it is hardly possible to determine how -much of this fragmentary material should be assigned to the actual chimney. -The diameter of the whole mass is almost two miles. But possibly a considerable -proportion of this accumulation belongs to the external cone which -gathered round the vent, so that the eruptive pipe might thus be of much -smaller dimensions than the superficial area of the agglomerate. The subsequent -invasion of so much granophyre, not only that of the Red Hills, but -that of numerous smaller intrusions, has indurated the agglomerate and -made the investigation of its structure somewhat unsatisfactory.</p> - -<div class="footnote"> - -<p><a id="Footnote_303" href="#FNanchor_303" class="label">[303]</a> This extensive mass was not separated from the "syenite" of the Red Hills by Macculloch. -Von Oeynhausen and Von Dechen noticed it as a conglomerate with quartz pebbles, but did not -realise its volcanic nature (<i>Karsten's Archiv</i>, i. p. 90). In my map of Strath (<i>Quart. Jour. Geol. -Soc.</i> xiv. plate i.) I distinguished it from the rock of the Red Hills, but no name for it appears -in the legend of the map, nor is it referred to in the text. Its character as a true volcanic -agglomerate was recognised by Professor Judd, <i>op. cit.</i> p. 255. See <i>postea</i>, pp. 384 <i>et seq.</i></p> - -</div> - -<p>It might be supposed that the mere existence of intrusive bosses and -veins rather furnishes an argument in favour of considering the visible -agglomerate to belong to a deeper-seated part of the erupted material than -the external cone. But, as will be afterwards shown, there is some reason to -regard the present conical or dome-shaped outlines of the granophyre hills as -not far from their original forms, and to believe that, like the trachytic Puys -of Auvergne, they were much more superficial than plutonic eruptions. A -study of the cinder cones of Central France shows that even these superficial -accumulations have been invaded not only by bosses but by dykes.<a id="FNanchor_304" href="#Footnote_304" class="fnanchor">[304]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_304" href="#FNanchor_304" class="label">[304]</a> The existence of a small dyke of andesite on the northern rim of the well-known crater of -the Puy Parion has already been noticed.</p> - -</div> - -<p>The agglomerate of the great Strath vent is a coarse tumultuous -assemblage of blocks and bombs, imbedded in the usual dull, dirty-green -matrix. Among the stones, grit and sandstone, together with scoriaceous, -vesicular and amygdaloidal basalts are specially abundant; also pieces of -various quartz-porphyries and granophyres, among which a black felsite like -that of Mull may often be recognised. In some places, large masses of -altered limestone and quartzite (Cambrian) are included; in others, pieces of -yellow sandstone and dark shale (Jurassic), or of the bedded lavas. Some of -these masses may be 100 yards or more in length. Occasionally a breccia, -mainly made up of acid materials—granophyre or granite,—has been noticed -by Mr. Harker along the north side of the Red Hills, which he thinks may -rather be of the nature of a crush-breccia than a part of the true agglomerate.</p> - -<p>The agglomerate of this district is wholly without stratification or -structure of any kind. On the north-west side of Loch Kilchrist, indeed, it -<span class="pagenum" id="Page_282">- 282 -</span> -weathers into large tabular forms, the parallel surfaces of which dip to south-west; -but this is probably due only to jointing. Here and there, dykes of -basalt cut the rock in a general north-westerly direction, but their number -is remarkably small when compared with the prodigious quantity of them in -the limestone at the bottom and opposite side of the valley, some of which -may possibly mark the fissure on which the vent was placed. More abundant -and extensive are the masses of granophyre that rise particularly along -the outer margin of the agglomerate near Loch Kilchrist. These may be -connected with the great boss that forms the Red Hills, of which further -details will be given in Chapter xlvi.<a id="FNanchor_305" href="#Footnote_305" class="fnanchor">[305]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_305" href="#FNanchor_305" class="label">[305]</a> The granophyre intrusions in this agglomerate have been found by Mr. Harker to have taken -up and dissolved a considerable proportion of fragments of gabbro, Chapter xlvi. p. 392.</p> - -</div> - -<p>The important question of the relation of this agglomerate to the plateau-basalts -does not admit of satisfactory treatment, owing to destruction of the -evidence by the intrusion of the granophyre, and likewise to enormous -denudation. Nevertheless, some traces still remain to indicate that the -basalts once stretched over the site of the vent, which probably rose through -them. Looking westward from the Hanks of Beinn Dearg Bheag to the other -side of Loch Slapin, the geologist sees the bold basalt-escarpment of Strathaird -presenting its truncated beds to him at a distance of only two miles. That -these lavas were once prolonged eastwards beyond their present limits is -obvious, and that they stretched at least over these two intervening miles -can hardly be doubted. But we can still detect relics of them on the flanks -of Beinn Dearg. As we follow the agglomerate round the margin of the -granophyre that mounts steeply from it, we lose it here and there under -beds of amygdaloidal basalt. The rocks next the great eruptive mass of the -mountain are so indurated and shattered that it is difficult to separate them -from each other and determine their relative positions. But, so far as I -could ascertain, these basalts are fragments of beds that overlie the agglomerate -(<a href="#v2fig303">Fig. 303</a>). This is not the only place along the flanks of the Red -Hills where portions of the bedded basalts have -survived. Other localities will be subsequently -alluded to.</p> - -<div class="figleft" id="v2fig303" style="width: 163px;"> - <img src="images/v2fig303.png" width="163" height="125" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 303.</span>—Diagram to show the - probable relations of the rocks - on the southern flank of Beinn - Dearg Bheag.<br /><br /> - <i>a</i>, agglomerate; <i>b</i>, amygdaloidal and compact - basalt-rocks; <i>c</i>, granophyre.</div> -</div> - - -<p>The Strath vent has been drilled through -the Cambrian limestone, and as the result of -protracted denudation it now towers steeply 500 -or 600 feet above that formation on the floor -of the valley. Of the material discharged from -it over the surrounding country no certain -trace now remains. We may infer from the -nature of the rock which fills it that towards -the end, if not from the beginning of its activity, -its discharges consisted mainly of dust and -stones. A cone, of which the remains are two miles in diameter, -must surely have sent its fragmentary materials far and wide over -the surrounding region. But on the bare platform of older rocks to the -<span class="pagenum" id="Page_283">- 283 -</span> -south, beyond the bottom of the agglomerate declivities, not a vestige -of these erupted materials can now be found. Westward the escarpment of -Strathaird remains to assure us that no thick showers of ashes fell at even -so short a distance as two miles, either before or during the outpouring of -the successive basalt sheets still remaining there. We may therefore conclude -with some confidence that here, as at Ardnamurchan, the vent is -younger than at least the older parts of the basalt-plateau. Unfortunately -the uprise of the large bosses of granophyre that stretch from the Red -Hills to Loch Sligachan has entirely destroyed the vent and its connections -in that direction. There is no certain proof that any molten rock ever -issued from this orifice, unless we suppose the fragmentary patches of -amygdaloid on the southern flank of Beinn Dearg Bheag to be portions of -flows that proceeded from this centre of eruption. The basalt-plateau which -still remains in Strathaird no doubt formerly extended eastwards over -Strath and northwards across the site of the Red Hills and Cuillins, joining -on to the continuous tableland north of Lochs Brittle and Sligachan. How -much of the plateau had been built up here before the outburst of the vent -cannot be ascertained. The agglomerate may possibly, of course, belong to -the very latest period of the plateau-eruptions, or even to a still younger -phase of Tertiary volcanic history. The impression, however, made on my -mind by a study of the evidence from the Western and Faroe Isles is that -the necks of agglomerate, like those of dolerite and basalt, really belong to -different epochs of the plateau period itself; and mark some of the vents -from which the materials of the plateaux were successively emitted.</p> - -<p>The example of Carrick-a-raide (p. 277) is peculiarly suggestive when we -regard it in connexion with the great Strath vent. Already the progress of -denudation has removed at least half of the layer of dust and stones which, -thrown out from that little orifice, fell over the bare chalk-wolds and black -basalt-fields of Antrim. The neck that marks the position of the volcanic -funnel has been largely cut away by the waves, and is almost entirely -isolated among them. The vents at Canna, Portree and the Faroe Isles, to -be afterwards described, unquestionably belong to the eruptions of the -plateau-period, for their connection with the basalts can be clearly established. -At the Strath vent, however, the march of destruction has been -greater. The connexion between this vent and the materials ejected from -it has been entirely removed, and we can only guess from the size of the -remaining neck what may have been the area covered by the discharges -from this largest of all the volcanic cones of the Inner Hebrides.</p> - -<p>Other masses of similar agglomerate are observable in the same region -of Skye, where they not improbably mark the sites of other vents. Unfortunately -their original limits and relations to the rocks through which -the eruptive orifices were drilled have been much obscured by the uprise -of the great masses of gabbro and granophyre of the Cuillin Hills. Several -of these isolated intrusions occur in the midst of the gabbro, as in Harta -Corry and on the west side of the Blaven ridge. Another mass is interposed -between the gabbro and granophyre on Druim an Eidhne and at the base -<span class="pagenum" id="Page_284">- 284 -</span> -of the lavas between Druim an Eidhne and the Camasunary valley. Mr. -Harker has found a huge mass of agglomerate underlying the bedded basalts -to the north and west of Belig, one of the hills on the west side of the -large valley that runs from the head of Loch Slapin to Loch Aynort. This -mass has its bottom concealed by the granophyre which underlies it; but it -reaches a maximum thickness of perhaps 1000 feet, rapidly thinning out -and disappearing. It generally resembles the Strath agglomerate, but is -distinguished by including a large proportion of fragments of gabbro. Mr. -Harker remarks that "a study of these agglomerates points to the existence -of both gabbros and granophyres older than the volcanic series, and -therefore distinct from the gabbros and granophyres now exposed at the -surface."</p> - -<p>It is a suggestive fact that so many detached masses of agglomerate -should occur around and within the areas of the great eruptive bosses of -gabbro and granophyre. They seem to indicate the former existence of -groups of volcanic vents in these tracts, and may thus account for the uprise -of such large bodies of intrusive material through what must have been -a weakened part of the terrestrial crust.</p> - -<p>Further north in Skye a much smaller but more perfectly preserved -vent has been laid open by denudation on the south side of Portree Bay—a -deep inlet which has been cut out of the plateau-basalts and their underlying -platform of Jurassic sandstones and shales. The great escarpment of -the basalts has, at the recess of Camas Garbh, been trenched by a small -rivulet, aided by the presence of two dykes. The gully thus formed exposes -a section of a neck of agglomerate that underlies the basalts of the upper -half of the cliff. This neck is connected with a thick deposit of volcanic -conglomerate and tuff which, lying between the basalts, extends from the -neck to a considerable distance on either hand. The general relations of the -rocks at this locality are represented in <a href="#v2fig304">Fig. 304</a>.</p> - -<div class="figcenter" id="v2fig304" style="width: 399px;"> - <img src="images/v2fig304.png" width="399" height="170" alt="" /> - <div class="figcaption">Fig. 304.—Section of Volcanic Vent and connected lavas and tuffs, Scorr, Camas Garbh, - Portree Bay, Skye.<br /><br /> - <i>a</i>, Rudely-bedded dull green tuff; <i>b</i>, coarse agglomerate; <i>c</i>, prismatic basalt; <i>d</i>, massive jointed basalt; <i>e</i>, red - banded decomposing rock, probably of detrital origin; <i>f</i>, plateau-basalts, prismatic and rudely columnar; <i>g</i>, - dyke of dolerite, somewhat vesicular, five to six feet broad; <i>h</i>, basalt dyke two to three feet broad; <i>i</i>, dyke - or sill of similar basalt to <i>h</i>, and possibly connected with it.</div> -</div> - -<p>The agglomerate (<i>b</i>) is quite tumultuous, and here and there strikingly -coarse. Some of its included blocks measure five feet in length. These -<span class="pagenum" id="Page_285">- 285 -</span> -fragments represent most of the varieties of the lavas of the district. Large -slaggy masses are abundant among them, and sometimes exhibit the -annelide-like elongation of the vesicles which I have referred to as occasionally -displayed by the plateau-basalts. More than 60 feet of agglomerate -are visible in vertical height from where its base is concealed by debris and -vegetation to where its upper surface passes under a banded rock to be -afterwards described. That this unstratified mass of volcanic detritus marks -the site of a vent can hardly be doubted, although denudation has not -revealed the actual walls of the chimney. The steep grassy slopes do not -permit the relations of the rocks to be everywhere seen, but the agglomerate -appears to pass laterally into finer, rudely-stratified material of a similar -kind, which extends towards east and west as a thick deposit between the -bedded basalts. Possibly denudation has only advanced far enough to lay -bare the crater and its surrounding sheets of fragmentary material, while the -chimney lies still buried underneath.</p> - -<p>To the east or left of the agglomerate the detritus becomes less coarse, and -shows increasing indications of a bedded arrangement. Close to the agglomerate -the dip of the coarse tuff is towards that rock at about 10°. A few -yards further east a sheet of very slaggy basalt is seen to lie against the -tuff, which it does not pierce. The vesicles in this adhering cake of lava -have been pulled out in the direction of the slope till they have become -narrow tubes four or five inches long and parallel to each other. Some parts -of this rock have a curved ropy surface, like that of well-known Vesuvian -lavas, suggestive of the molten rock having flowed in successive thin viscous -sheets down the slope, which has a declivity of about 30°. This part of the -section may possibly preserve a fragment of the actual inner slope of the -crater formed of rudely-bedded tuffs.</p> - -<p>Continuing still eastward, we find the feebly stratified tuff (<i>a</i>) to be perhaps -200 feet thick. It forms a grassy declivity that descends from the basalt-escarpment -above to the grass-covered platform which overlies a lower group -of basalts. The visible portion of this tuff presents a thoroughly volcanic -character, being made up of the usual dull dirty-green granular paste, -through which are dispersed angular and rough lumps of slag and pieces of -more solid basalt, varying up to a foot or two feet in length. These stones -are generally disposed parallel to the indistinct bedding, but are sometimes -placed on end, as if they had assumed that position on falling from an -explosive shower. Among the smaller stones, pieces of a finely vesicular -basic pumice are frequent and are among the most strikingly volcanic -products of the deposit. From a characteristic sample of these stones, a -thin slice was prepared and placed in Mr. Harker's hands. The following -are his observations on it:—"A very compact dark grey rock, amygdaloidal -on a minute scale. The lighter grey crust is probably due merely to -weathering, and the specimen seems to be a distinct fragment, not a true -bomb. The slice shows it to be essentially a brown glass with only -occasional microscopic crystals of a basic plagioclase. It has been highly -vesicular, and the vesicles are now filled by various secondary products, -<span class="pagenum" id="Page_286">- 286 -</span> -including a chloritic mineral, nearly colourless and singly refracting in thin -section, and a zeolite."</p> - -<p>Tracing now the tuff from the west or right side of the vent, we can follow -it to a greater distance. No abrupt line can be detected here, any more than -on the other side, between the agglomerate and the tuff. The latter rock -extends under the overlying plateau of basalt, at least as far west as Portree -Loch, a distance of fully a mile, but rapidly diminishes in thickness in that -direction. Traces of what is probably the same tuff can be detected between -the basalts at Ach na Hannait, more than three miles to the south (<a href="#v2fig305">Fig. 305</a>). -It is thus probable that from the Portree vent fragmentary discharges took -place over an area of several square miles.</p> - -<p>Above the agglomerate of this vent two lavas may be seen to start -towards opposite directions. One of these (<i>c</i>), already referred to, is a dull -prismatic basalt with a slaggy bottom, its vesicles being pulled out in the -direction of the general bedding of the section. It descends by a twist -or step, and then lies on the inclined surface of the tuff which dips towards -the agglomerate and seems to pass into that rock. Further east this -basalt increases in thickness and forms the lowest of the basalt-sheets of -the cliff. The lava that commences on the west side of the agglomerate (<i>d</i>) -is a massive jointed basalt, which, though not seen at the vent, appears -immediately to the west of it and rapidly swells out so as to become one -of the thickest sheets of the locality. It lies upon the rudely-bedded tuff, -and is covered by the other basalts of the cliff.</p> - -<p>That these two basalts came out of this vent cannot be affirmed. If -they did so at different times, their emission must have been followed by -the explosion which cleared the funnel and left the central mass of agglomerate -there. But that some kind of saucer-shaped depression was still left -above the site of the vent is indicated by the curious elliptical mass of rock -(<i>e</i>) that lies immediately above the agglomerate, from which it is sharply -marked off. This is one of the most puzzling rocks in the district, probably -in large measure owing to its advanced state of decay. It is dull-red in -colour, and decomposes into roughly parallel layers, so that at a short distance -it looks like a bedded tuff, or like some of the crumbling varieties of -banded lavas. I could not obtain specimens fresh enough to put its nature -and origin beyond dispute. Whatever may have been its history, this -ferruginous rock rests in a flat basin-shaped hollow directly above the -agglomerate of the vent. The form of this depression corresponds fairly -well with what we may suppose to have been the final position and shape of -the crater of the little volcano. The rock that occupies the bowl dies out -towards the east on the face of the cliff, and the prismatic basalt (<i>c</i>) is then -immediately covered by the rest of the basalt-sheets of the plateau (<i>f</i>). On -the west side its precise termination is concealed by grass. But it must -rapidly dwindle in that direction also, for not many yards away it is found -to have disappeared, and the basalts (<i>d</i> and <i>f</i>) come together.</p> - -<p>Though the decayed state of this rock does not warrant any very confident -opinion regarding its history, I am inclined to look upon it as a -<span class="pagenum" id="Page_287">- 287 -</span> -deposit of much disintegrated volcanic detritus washed into the hollow of -the old crater when it had become filled with water, and had passed into -the condition of a <i>maar</i>. The peculiarly oxidized condition of its materials -points probably to long atmospheric exposure, and an examination of the -surrounding parts of the district furnishes more or less distinct evidence -that a considerable lapse of time did actually intervene between the cessation -of the eruptions of the Portree volcano and the next great basalt-floods of -this part of Skye.</p> - -<p>That volcanic eruptions from other vents continued after the Portree -vent had become extinct is proved by the great sheets of basalt (<i>f</i>) that -overspread it, and still bury a large tract of the fragmentary material which -it discharged. At a later time a fissure that was opened across the vent, -allowed the uprise of a basalt dyke (<i>g</i>), and subsequently another injection -of similar material took place along the same line of weakness (<i>h</i>).</p> - -<p>Before leaving this interesting locality we may briefly take note of the -distribution of the ashes and stones ejected by the volcano, and the evidence -for the relative length of the interval between the outflow of the lavas below -and that of those above the tuff and volcanic conglomerate. These -deposits may be traced in clear sections along the base of the cliffs for a -mile to the west of the vent. They thin away so rapidly in that direction -that at a distance of three-quarters of a mile they do not much exceed fifty -feet in thickness. At Camas Bà n they consist mainly of a fine, dull-green, -granular, rudely-stratified basalt-tuff, through which occasional angular -pieces of different lavas and rough slags are irregularly dispersed. These -stones occur here and there in rows, suggestive of more vigorous discharges, -the layers between the platforms of coarser detritus being occupied by fine -tuff. Some of the ejected blocks are imbedded on end—an indication of the -force with which they were projected so as to fall nearly a mile from the -crater.</p> - -<p>The upper parts of the tuff pass upward into fine yellow, brown, and -black clays a few feet in thickness, the darker layers being full of carbonaceous -streaks. On this horizon the coal of Portree was formerly mined. -The workings, however, have long been abandoned, and, owing to the fall of -large blocks from the basalt-cliff overhead, the entrance to the mine is almost -completely blocked up. One wooden prop may still be seen keeping up the -roof of the adit, which is here a slaggy basalt.</p> - -<p>To the east and south-east of the Portree vent, extensive landslips of the -volcanic series and of the underlying Jurassic formations make it hardly -possible to trace the continuation of the tuff-zone in that direction. To the -south, however, at a distance of rather more than three miles, what is probably -the same stratigraphical horizon may be conveniently examined from -Ach na Hannait for some way to the north of Tianavaig Bay. At the -former locality the calcareous sandstones of the Inferior Oolite are unconformably -covered by the group of rocks represented in <a href="#v2fig305">Fig. 305</a>. At the -bottom of the volcanic series lies a sheet of nodular dolerite with a slaggy -upper surface (<i>a</i>). Wrapping round the projections and filling up the -<span class="pagenum" id="Page_288">- 288 -</span> -depressions of this lava comes a thin group of sedimentary strata from an -inch or two to eighteen inches or more in thickness (<i>b</i>). These deposits -consist of hardened shale charged with macerated fragments of linear leaves -and other plant-remains, including and passing into streaks of coal, which -may be looked upon as probably occupying the -same horizon with the coal of Portree. But here, -instead of reposing on a mass of stratified tuff, -the carbonaceous layers lie on one of the bedded -lavas. The tuff has died out in the intervening -three miles, yet that some of the discharges of -volcanic detritus reached even to this distance, and -that they took place during the accumulation of -these layers of mud and vegetation, is shown -by the occurrence in the shales of pieces of -finely amygdaloidal basalt, from less than an -inch to six inches in length, likewise lapilli of -a fine minutely cellular basic pumice, like -some varieties of palagonite. The overlying -dolerite (<i>c</i>) becomes finely prismatic at its junction with the sedimentary -layers and has probably indurated them.</p> - -<div class="figleft" id="v2fig305" style="width: 161px;"> - <img src="images/v2fig305.png" width="161" height="187" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 305.</span>—Section of the Volcanic -Series at Ach na Hannait, -south of Portree, Skye.</div> -</div> - - -<p>This intercalation of a shaly and coaly band among the lavas can be -followed northward along the coast. In some places it has been invaded by -dykes, sills, and threads of basalt on the most remarkably minute scale, of -which I shall give some account in <a href="#CHAPTER_XLII">Chapter xlii</a>. (see <a href="#v2fig321">Fig. 321</a>). North -of Tianavaig Bay—that is, about three-quarters of a mile nearer to the -Portree vent—a perceptible increase in the amount of volcanic material is -observable among the shales and leaf-beds. Not only are lapilli of basic -pumice abundant, but the volcanic detritus has accumulated here and there -in sufficient amount to form a band of dull greenish-brown tuff.</p> - -<p>These coast-sections in the neighbourhood of Portree afford additional -illustrations of the characteristic fact, on which I have already insisted, -that the interstratifications of sedimentary material in the basalt-plateaux -frequently terminate upward in leaf-beds, thin coals, or layers of shale, full -of indistinctly preserved remains of plants. As I have endeavoured to -show, this vegetation, which was undoubtedly terrestrial, probably grew not -far from the sites where its remains have been preserved. Leaves and seeds -would naturally be blown or washed into pools on the lava-fields, and would -gather there among the mud and sand carried by rain from the surrounding -ground. Such a topography and such a sequence of events point to intervals -of longer or shorter duration between the successive outpourings of basalt. -It was probably during one of these intervals of quietude that the crater of -the Portree volcano became a <i>maar</i> and was finally silted up.</p> - -<p>Reference has already been made to a conspicuous mass of agglomerate -which occurs at the east end of the island of Canna, and marks the site of -an important volcanic vent belonging to the Small Isles plateau. A portion -of it projects from the grassy slopes, and rises vertically above the beach as -<span class="pagenum" id="Page_289">- 289 -</span> -a picturesque crag, in front of the precipice of Compass Hill (<a href="#v2fig306">Fig. 306</a>). -But the same rock may be traced southward to the Coroghon Mòr, and -north-westward in the lower part of the cliffs to a little beyond the sea-stack -of An Stòll. It has thus a diameter of at least 3000 feet. Westward it -passes under the conglomerate described in Chapter xxxviii. Its eastern -extension has been concealed by the sea.</p> - -<div class="figcenter" id="v2fig306" style="width: 568px;"> - <img src="images/v2fig306.png" width="568" height="395" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 306.</span>—View of part of a Volcanic Neck at the eastern end of the island of Canna. (From a photograph by Miss Thom.)</div> -</div> - -<p>The materials that fill this vent consist of a typical agglomerate composed -entirely, or almost entirely, of volcanic detritus. The embedded blocks -vary up to eight feet in diameter or even more. They are chiefly fragments -of various basalts and andesites, generally vesicular or amygdaloidal. Some -<span class="pagenum" id="Page_290">- 290 -</span> -of these, which have evidently been broken off from already consolidated -lavas, are angular or subangular in shape, and their steam-holes are cut -across by the outer surfaces of the stones. Where they consist of calcite, -zeolite, etc., the amygdales so exactly resemble those of the bedded basalts -of the plateaux that, as already remarked, we must believe them to have -been already filled by infiltration before the disruption of the rocks by -volcanic explosions. Other blocks are true bombs, with a fine-grained crust -outside and a more cellular texture inside, the vesicles of the outer crust -being sometimes dragged round the surface of the stone. The variety of -materials included among the ejected blocks and the abundance of pieces of -the red bole which so generally separates the plateau-basalts indicate that -a considerable thickness of bedded lavas has probably been broken through -by the vent.</p> - -<p>Beside the volcanic materials, occasional angular pieces of red (Torridon) -sandstone may be observed in the agglomerate. The paste is a comminuted -mass of the same material as the blocks, tolerably compact, and entirely -without any trace of stratification.</p> - -<p>The actual margin of this vent has nowhere been detected by me. We -never reach here the base of the volcanic series, for it is sunk under the -sea-level. On the other hand, the upper limits of the agglomerate have -been partially effaced or obscured by the conglomerates which overlie it. -From the breadth of ground across which the agglomerate can be followed -along the shore, the vent might be regarded as having been perhaps not less -than three-quarters of a mile in diameter. But there is the same difficulty -here as at the Strath vent in Skye in determining the actual limits of the -volcanic funnel. Possibly there may have been more than one vent in close -proximity. Even if there was only one, the existing agglomerate may -include not only what filled the chimney, but also a portion of what had -accumulated round the orifice and formed the external cone. That the -volcano continued for some time in vigorous eruption may be judged from -the amount of material ejected from it, the large size of its blocks, and the -distance to which they were sometimes thrown.</p> - -<p>The pieces of Torridon Sandstone were no doubt derived from the -extension of that formation underneath Canna. On the opposite island of -Rum, where these pre-Cambrian red sandstones are copiously developed, they -form the platform through which the Tertiary volcanic series has been erupted. -The several remaining outliers of the bedded basalts, referred to in a previous -chapter (p. 215 and <a href="#v2fig267">Fig. 267</a>) as visible on the west side of this island, -show that the basalt-plateau of Small Isles, which once covered that area, -rested immediately on the inclined edges of the Torridon Sandstones. -Probably the same structure stretches westward under Canna and Sanday. -No traces of any Jurassic strata have been detected beneath the volcanic -rocks of Rum, though they are so well developed a few miles to the east in -the island of Eigg. Either they were not deposited over the pre-Cambrian -rocks of Rum, or they had been removed from that ancient ridge before the -beginning of the Tertiary volcanic period. Certainly I have not detected -<span class="pagenum" id="Page_291">- 291 -</span> -a single recognizable fragment of any Jurassic sedimentary rock in the -agglomerate of Canna.</p> - -<p>This Canna vent exhibits, better than is usually shown, the occurrence -of dykes and irregular injections of lava through the agglomerate. A large -mass of a finely columnar basalt runs up from the beach at Garbh Asgarnish. -A similar rock forms several detached crags a little further south, -particularly in the headland of Coroghon Mòr and the island of Alman. -Here the basalt is beautifully columnar, its slender prisms curving from a -central line until their ends abut against the agglomerate. The truly -intrusive character of this basalt is well shown on the southern front -of Coroghon Mòr, and on the northern face of Alman, as represented in -the accompanying diagrams (Figs. <a href="#v2fig307">307</a> and <a href="#v2fig308">308</a>).</p> - -<table summary="images"> -<tr> - <td> -<div class="figcenter" id="v2fig307" style="width: 214px;"> - <img src="images/v2fig307.png" width="214" height="194" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 307.</span>—Columnar Basalt invading Agglomerate - of Volcanic Vent, Coroghon Mòr, - Isle of Canna. (Height above 20 feet.)</div> -</div> - </td><td> </td><td> -<div class="figcenter" id="v2fig308" style="width: 256px;"> - <img src="images/v2fig308.png" width="256" height="205" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 308.</span>—Columnar Basalt invading Volcanic - Conglomerate, north side of Alman Islet, - Canna.</div> -</div> - </td> -</tr> -</table> - -<p>Although there is no conclusive evidence that these intrusions belong to -the time of the activity of the vent, yet they differ so much from the -ordinary dykes (one of which also cuts the agglomerate and ascends through -the conglomerates and basalts above), are confined so markedly to the vent -and its immediate proximity, and resemble so closely the basalt-injections of -other vents, such as those of the Carboniferous and Permian necks of Scotland, -that they may with probability be regarded as part of the mechanism -of the Canna volcano.</p> - -<p>Though the form and size of the vent of this volcano cannot be precisely -defined, the upper part of its agglomerate, as we have seen (<i>ante</i>, p. -219), is dovetailed in the most interesting way with the series of coarse conglomerates, -which indicate strong river-action in this part of the volcanic -area during the time of the eruption of the plateau-basalts.</p> - -<p>The agglomerate vents described in the foregoing pages as occurring in -Antrim and among the Inner Hebrides all appear either in the midst of the -plateau-basalts or in close proximity to them. Before quitting the Scottish -examples, I may refer to some that rise through much more ancient formations -at a distance from any portion of the volcanic plateaux, and yet may -with probability be assigned to the Tertiary volcanic period.</p> - -<p><span class="pagenum" id="Page_292">- 292 -</span></p> - -<p>During the progress of the Geological Survey through the district of -Applecross, in the western part of the mainland of Ross-shire, and far away -from the basalt-plateau of Skye, Mr. John Horne<a id="FNanchor_306" href="#Footnote_306" class="fnanchor">[306]</a> has found two small -necks rising on each side of a line of fracture, through gently inclined -Torridon Sandstones. They are conspicuous from a distance by the verdure -of their slopes, in contrast with the brown tints of the surrounding moorland. -The larger of the two necks measures about 180 by 150 feet, and -abruptly truncates the beds of Torridon Sandstone, which as they approach -it assume a bleached aspect and become indurated. The material filling -this vent is an agglomerate made up mainly of pieces of Torridon Sandstone -and grit which, though generally small, occasionally measure a foot across, -and in one case were found to reach a length of four feet. They are not -as a rule markedly altered, but some of them have acquired a glazed or -vitreous texture. Besides these fragments of the general rock of the district, -there occur abundant lapilli of a basic volcanic rock, found by Mr. -Teall to consist of porphyritic felspar, extremely minute acicular microlites -of felspar, somewhat irregular transparent spaces now occupied by a -yellowish-green substance, and interstitial matter. At the south end of -the vent a small mass of decayed basalt appears to pierce the agglomerate.</p> - -<div class="footnote"> - -<p><a id="Footnote_306" href="#FNanchor_306" class="label">[306]</a> <i>Trans. Geol. Soc. Edin.</i> vii. (1894), p. 35.</p> - -</div> - -<div class="figcenter" id="v2fig309" style="width: 475px;"> - <img src="images/v2fig309.png" width="475" height="197" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 309.</span>—View of neck-like mass of breccia, Brochel, Raasay.</div> -</div> - -<p>Though there is no indication of the age of these necks, they agree so -closely in general character with known vents of the Tertiary volcanic -plateaux that there cannot be much hesitation in regarding them as dating -from the same great period of basalt-eruption. But no relic now exists -anywhere around of lavas or tuffs ejected from them. They rise on the bare -Applecross hills, 1000 feet above sea-level, two miles from the shore, and about -ten miles from the nearest outlier of the basalt-plateau in the Dùn Can of -Raasay. If they once discharged streams of lava that united with the rest of -the plateau, the total destruction of this lava affords another impressive picture -of the waste which the volcanic rocks of the Inner Hebrides have undergone.</p> - -<p>The large proportion of Torridon Sandstone blocks in these two Applecross -necks suggests, however, that the orifices never became active volcanic -<span class="pagenum" id="Page_293">- 293 -</span> -vents. They may have been mere spiracles, or blow-holes, where the funnels -drilled by explosive vapours were filled up with the debris of the rocks that -were blown out. But that lava did rise within them is shown by the basic -lapilli in the agglomerate, and by the basalt which in both vents has found -its way up the chimney.</p> - -<p>In the island of Raasay Mr. Teall, during the summer of 1894, observed -a group of curious neck-like masses of breccia which pierce the Torridon Sandstone -near Brochel (<a href="#v2fig309">Fig. 309</a>). The blocks in them are large angular unaltered -pieces of the surrounding sandstones and shales, sometimes ten feet or more -in length, and the matrix is sometimes pure crystalline calcite like Iceland -spar. The breccia is generally coarsest towards the outer margin. But -though the Lewisian gneiss exists immediately below the thin cake of -Torridonian strata, not a fragment of it could either Mr. Teall or I, when I -visited the locality with him, find among the components of the breccia. -Nor did we detect any trace of volcanic material. The general ground-plan -of these masses is elliptical, the most northerly measuring 30 yards in -diameter. Where the junction of the breccia with the Torridon strata can -be seen it is a nearly vertical one, the sandstones and shales being much -jumbled and broken, but not sensibly indurated. This little cluster of -patches of breccia can hardly be due to local crushing of the rocks. Their -definite outlines and composition seem rather to indicate spiracles of Tertiary -time, which never became vents erupting lava or ashes. The absence of -fragments of the underlying gneiss may be accounted for if we suppose that -the orifices were completely cleared out by the violence of the explosions -and were afterwards filled up by the falling in of the walls of the higher -parts now removed by denudation, which consisted of Torridon Sandstone -and shale.<a id="FNanchor_307" href="#Footnote_307" class="fnanchor">[307]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_307" href="#FNanchor_307" class="label">[307]</a> It is on one of these neck-like patches of breccia that Brochel Castle stands, of which Macculloch -gave so sensational a picture in one of the plates of his <i>Western Isles</i>.</p> - -</div> - -<p>Further research may detect at still greater distances from the -basalt-plateaux ancient volcanic necks that might, with more or less probability, -be referred to the Tertiary period. As an instance of this kind, -I refer to the neck at Bunowen, County Galway, recently described by -Mr. M'Henry and Professor Sollas. Though so remote from the Tertiary -basalt-plateaux, the rock of this boss is an olivine-basalt presenting a close -resemblance to some of the rocks of Antrim.<a id="FNanchor_308" href="#Footnote_308" class="fnanchor">[308]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_308" href="#FNanchor_308" class="label">[308]</a> <i>Trans. Roy. Irish Acad.</i>, 1896</p> - -</div> - -<p>As a final illustration of Tertiary volcanic vents I will now describe the -Faroe group already alluded to (<a href="../../66492/66492-h/66492-h.htm#Page_63">vol. i. p. 63</a>, <a href="../../66492/66492-h/66492-h.htm#Page_256">vol. ii. p. 256</a>). It was almost by -a kind of happy accident that these vents were discovered. Noticing at a -distance of a mile or more from the deck of a steam-yacht that the base of the -great basalt cliffs on the west side of Stromö were varied by what looked -like agglomerate, I steamed inshore, and was delighted to find, as the vessel -drew near to the cliff, that the agglomerate assumed definite boundaries and -occurred in several distinct patches, until at last it presented the unmistakable -outlines of a group of vents underlying and overspread by the bedded -<span class="pagenum" id="Page_294">- 294 -</span> -basalts of the plateau. Favoured by an unusually calm sea, I was enabled -to boat into every nook and round every buttress and islet of this part of -the coast-line.</p> - -<div class="figcenter" id="v2fig310" style="width: 600px;"> - <img src="images/v2fig310.png" width="600" height="426" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 310.</span>—View of Volcanic Neck piercing and overlain by the Plateau-Basalts, Stromö, entrance of Vaagöfjord, Faroe Islands.<br /><br /> - (From a photograph by Colonel Evans.)</div> -</div> - -<p>The basalt-plateau here presents to the western ocean a nearly vertical -escarpment which must reach a height of at least 1000 feet (see <a href="#v2fig328">Fig. 328</a>), -and displays a magnificent section of the bedded lavas. The lower part of -this section shows chiefly the banded structure already described, the layers -of different consistency being etched out by the weather in such a way as to -give them the look of stratified rocks. In the upper part of the precipice -<span class="pagenum" id="Page_295">- 295 -</span> -columnar and jointed or prismatic sheets are more common, but the most -prominent band is the great sill, to which further reference will be made in -the next Chapter.</p> - -<p>In the course of the gradual retreat of the cliff, as the waves tunnel its -base, and slice after slice is detached from its vertical front, a group of at -least five small vents has been uncovered lying along a nearly north and -south line. Of two of these a segment remains still on the cliff-wall and -passes under the basalts; the others have been dissected and half cut away -from the cliff, while groups of stacks and rocky islets of agglomerate may -mark the position of others almost effaced. The horizontal distance within -which the vents are crowded is probably less than half a mile, but the lofty -proportions of the precipice tend to lead the eye to underestimate both -heights and distances.</p> - -<div class="figcenter" id="v2fig311" style="width: 405px;"> - <img src="images/v2fig311.png" width="405" height="232" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 311.</span>—Section of the same Neck as that shown in <a href="#v2fig310">Fig. 310</a>.</div> -</div> - -<p>The agglomerate is a thoroughly volcanic rock, consisting of large and small -blocks of various basalts, among which large slags are specially conspicuous, -the whole being wrapped in a granular matrix of comminuted volcanic -detritus. The arrangement of this material is best seen in the fourth vent -(Figs. <a href="#v2fig310">310</a> and <a href="#v2fig311">311</a>). In this characteristic volcanic neck (<i>b</i> in <a href="#v2fig311">Fig. 311</a>) -the boundary walls, as laid bare on the face of the precipice, are vertical, and -are formed of the truncated ends of the banded lavas (<i>a</i> <i>a</i>) which have been -blown out at the time of the formation of the orifice. The visible diameter -of the vent was roughly estimated by me to be about 100 yards. No -appreciable alteration was observed in the ends of the lavas next the -vent.</p> - -<p>The agglomerate is coarsest in the centre, where huge blocks of slaggy -lava lie imbedded in the amorphous mass of compacted debris. On either -side of this structureless central portion the agglomerate is distinctly stratified -from the walls towards the middle, at angles of 30° to 35°. Even from -a distance it can be observed that the upper limit of the agglomerate is -saucer-shaped, the sloping sides of the depression dipping towards the centre -<span class="pagenum" id="Page_296">- 296 -</span> -of the neck at about the same angle as the rudely-stratified agglomerate -underneath. From the bottom of this basin to the sea-level may be a -vertical distance of some 30 yards. The basin itself has been filled up by -three successive flows of basalt, of which the first (<i>c</i>) has merely overflowed -the bottom, the second (<i>d</i>), entering from the northern rim of the basin, -extends across to the southern slope, while -the third (<i>e</i>), also flowing from the north, -has filled up the remainder of the hollow -and extended completely across it. The -next succeeding lava (<i>f</i>) stretched over the -site in such a way as to bury it entirely, -and to provide a level floor for the piling -up of the succeeding sheets of basalt.</p> - -<table summary="images"> -<tr> - <td> -<div class="figleft" id="v2fig312" style="width: 189px;"> - <img src="images/v2fig312.png" width="189" height="219" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 312.</span>—Volcanic Neck close to that - shown in Figs. <a href="#v2fig310">310</a> and <a href="#v2fig311">311</a>.</div> -</div> - </td><td> </td><td> -<div class="figcenter" id="v2fig313" style="width: 144px;"> - <img src="images/v2fig313.png" width="144" height="203" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 313.</span>—Section of wall of - another Neck of agglomerate - in the same group with those - represented in Figs. <a href="#v2fig310">310</a>, - <a href="#v2fig311">311</a>, and <a href="#v2fig312">312</a>.</div> -</div> - </td> -</tr> -</table> - - -<p>The second vent, which is represented -in Fig 312, exhibits the same features, but -with some additional points of interest. It -measures roughly about 20 yards in diameter -at the sea-level, rises through the same group -of banded basalt (<i>a</i> <i>a</i>), and is filled with a -similar agglomerate (<i>b</i>). Its more northerly -wall is now coincident with a line of fault (<i>h</i>) which ascends the cliff, and -probably marks some subsidence after the eruptions had ceased. The -southern wall shows that a dyke of basalt (<i>g</i>) has risen between the agglomerate -and the banded basalts, and that a second dyke (<i>g′</i>) traverses the latter -at a distance of a few feet. In this instance, also, the upper surface of the -agglomerate forms a cup-shaped depression which has been filled in by -two successive streams of lava (<i>c</i>, <i>d</i>). Among the -succeeding lavas (<i>e</i>) the prominent sill (<i>f</i>) has been -intruded, to which further allusion is made on p. 323.</p> - -<p>These necks are obviously volcanic vents belonging -to the time of the basaltic eruptions. They -have been drilled through the basalts of the lower -part of the cliff, but have been buried under those of -the central and higher parts. The arrangement of -their component materials in rude beds dipping -towards the middle of each vent shows that the -ejected dust and stones must have fallen back into -the orifice so as to be rudely stratified towards the -centre of the chimney, which was finally closed by -its own last discharges of coarse detritus. The -saucer-shaped upper limit of the agglomerate seems -to indicate, as has been suggested above in the -case of the Portree volcano, that after the eruptions ceased each vent -remained as a hollow or <i>maar</i> on the surface of the lava-fields. And the -manner in which they are filled with successive sheets of basalt shows -that in course of time other eruptions from neighbouring orifices gave forth -streams of lava which, in flowing over the volcanic fields, eventually buried -and obliterated each of the vents.</p> - - -<table style="border: #000 1px;" summary="Map VI"> -<tr> - <td class="tdl vsmall" colspan="2"><a id="v2map6"></a>TO ACCOMPANY SIR ARCHIBALD GEIKIE'S "ANCIENT VOLCANOES OF BRITAIN"</td> - <td class="tdr vsmall">Map VI.</td> -</tr> -<tr> - <td colspan="3"><a href="images/v2map6lg.png"><img src="images/v2map6.png" width="469" height="648" alt="" /></a></td> -</tr> -<tr> - <td style="width: 33%;" class="tdl vsmall">The Edinburgh Geographical Institute</td> - <td style="width: 33%;" class="tdc vsmall">Copyright</td> - <td style="width: 33%;" class="tdr vsmall">J. G. Bartholomew</td> -</tr> -<tr> - <td colspan="3" class="tdc smaller" style="padding-top: 0.5em;">MAP OF THE TERTIARY VOLCANIC REGION OF THE INNER HEBRIDES<br /> - Click on map to view larger sized.</td> -</tr> -</table> - - -<p><span class="pagenum" id="Page_297">- 297 -</span></p> - -<p>In the destruction of the precipice some of the vents have been so -much cut away that only a small part of the wall is left, with a portion of -the agglomerate adhering to it. The third neck, for instance, affords the -section represented in <a href="#v2fig313">Fig. 313</a>, where the horizontal sheets of basalt (<i>a</i>) have -still a kind of thick pellicle of the volcanic detritus (<i>b</i>) adhering to what -must have been part of the side of the orifice of eruption. The waves -have cut out a cave at the base, so that we can, by boat, get behind -the agglomerate and see the margin of the volcanic funnel in the roof -overhead.</p> - -<p>The fragment of geological history so picturesquely laid bare on the -Stromö cliffs presents a significant illustration of what seems to have been -a frequent, if not the normal type of volcanic vent in the Tertiary basalt-plateaux. -By the fortunate accident that denudation has not proceeded -too far, we are able to observe the original tops of at least two of the vents, -and to see how such volcanic orifices, which were doubtless abundant all -over these plateaux, came to be entombed under the ever-increasing pile of -accumulating basalt.</p> - -<p>There is still one feature of interest in these cliff-sections which deserves -notice here. Every geologist who has studied the composition of the basalt-plateaux -has remarked the comparatively insignificant part played by tuffs -in these volcanic accumulations. Hundreds of feet of successive basalt-sheets -may often be examined without the discovery of any intercalation of -fragmental materials, and even where such intercalations do occur they are -for the most part quite thin and extremely local. I found it impossible to -scale the precipice for the purpose of ascertaining whether around the -Stromö vents, and connected with them, there might not be some beds of tuff -interstratified between the basalts. If such beds exist, they can only be of -trifling thickness and extent. Here, then, are examples of once active vents, -the funnels of which are still choked up with coarse fragmentary ejections, -yet from which little or no discharge of ashes and stones took place over the -surrounding ground. They seem to have been left as crater-like hollows on -the bare surface of the lava-fields.</p> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_298">- 298 -</span></p> - -<h2 class="nobreak" id="CHAPTER_XLII">CHAPTER XLII<br /> - -<span class="smaller">THE BASIC SILLS OF THE BASALT-PLATEAUX</span></h2> -</div> - - -<p>We have now followed the distribution of the basalt-plateaux, the arrangement -of their component materials which were erupted at the surface, and -the character of the dyke-fissures and vents from which these materials were -ejected. But there remains to be considered an extensive series of rocks -which display some of the underground phenomena of the Tertiary volcanoes. -The injection of many basaltic sheets had been clearly enforced by Macculloch. -In 1871 I pointed out that at different horizons in the plateau-basalts, -but especially at their base and among the stratified rocks underneath -them, sheets of basalt and dolerite occur which, though lying parallel with -the stratification of the volcanic series, are not truly bedded, but intrusive, -and therefore younger than the rocks between which they lie.<a id="FNanchor_309" href="#Footnote_309" class="fnanchor">[309]</a> The non-recognition -of their true nature had led to their being regarded as proofs of -volcanic intercalations in the Jurassic series of Scotland. There is, however, -no trace of the true interstratification of a volcanic band in that series, -every apparent example being due to the way in which intrusive sheets -simulate the characters of contemporaneous flows.</p> - -<div class="footnote"> - -<p><a id="Footnote_309" href="#FNanchor_309" class="label">[309]</a> <i>Quart. Jour. Geol. Soc.</i> xxvii. (1871), p. 296.</p> - -</div> - -<p>If such sheets had been met with only at one or two localities, we might -regard them as due to some mere local accident of structure in the overlying -crust through which the erupted material had to make its way. But when -we find them everywhere from the cliffs of Antrim to the far headlands of -Skye and the Shiant Isles, and see them reappear among the Faroe Islands, it -is obvious that, like those of Palæozoic time, they must be due to some -general cause, and that they contain the record of a special period or phase -in the building up of the Tertiary volcanic tablelands. I will first describe -some typical examples of them from different districts, and then discuss -their probable relations with the other portions of the plateaux.</p> - - -<h3> i. <span class="allsmcap">ANTRIM</span></h3> - -<p>First to be examined, and now most familiar to geologists, are the -remarkable sheets that underlie the plateau of Antrim, and project at various -parts of the picturesque line of coast between Portrush and Fair Head. -<span class="pagenum" id="Page_299">- 299 -</span> -From the shore at Portrush, as I have already remarked, came the evidence -that was supposed to prove basalt to be a rock of aqueous origin, inasmuch -as shells were obtained there from what was believed to be basalt. The -long controversy to which this supposed discovery gave rise is one of the -most curious in the history of geology.<a id="FNanchor_310" href="#Footnote_310" class="fnanchor">[310]</a> It continued even after the -illustrious Playfair had shown that the pretended basalt was in reality -highly indurated shale, and hence that, instead of furnishing proof of the -aqueous formation of basalt, the Portrush sections only contributed another -strong confirmation of the Huttonian theory, which claimed basalt to be a -rock of igneous origin.</p> - -<div class="footnote"> - -<p><a id="Footnote_310" href="#FNanchor_310" class="label">[310]</a> For an excellent summary of this controversy and an epitome of the descriptions of the -Portrush section, see the <i>Report on the Geology of Londonderry</i>, etc. (<i>Mem. Geol. Survey</i>), by -J. E. Portlock (1843), p. 37.</p> - -</div> - -<p>It is now well known that the rock which yielded the fossils is a -Liassic shale, that it is traversed by several sheets of eruptive rock, and -that by contact-metamorphism it has been changed into a highly indurated -substance, breaking with a splintery, conchoidal fracture, but still retaining -its ammonites and other fossils. The eruptive material is a coarse, -distinctly crystalline dolerite, in some parts of which the augite, penetrated -by lath-shaped crystals of plagioclase, is remarkably fresh, while the olivine -has begun to show the serpentinous change along its cracks.<a id="FNanchor_311" href="#Footnote_311" class="fnanchor">[311]</a> This rock -has been thrust between the bedding planes of the shales, but also breaks -across them, and occurs in several sheets, though these may all be portions -of one subterranean mass. Some of the sheets are only a few inches thick, -and might at first be mistaken for sedimentary alternations in the shale. -But their mode of weathering soon enables the observer readily to distinguish -them. It is to be noticed that these thin layers of eruptive material assume -a fine grain, and resemble the ordinary dykes of the district. This closeness -of texture, as Griffith long ago pointed out,<a id="FNanchor_312" href="#Footnote_312" class="fnanchor">[312]</a> is also to be noticed along the -marginal portions of the thicker sheets where they lie upon or are covered -by the shales. But away from the surfaces of contact, the rock assumes a -coarser grain, insomuch that in its thickest mass it presents crystals -measuring sometimes an inch in length, and then externally resembles a -gabbro. A more curious structure is shown in one of these coarsely -crystalline portions by the occurrence of a band a few inches broad which is -strongly amygdaloidal, the cells, sometimes three inches or more in diameter, -being filled with zeolites.<a id="FNanchor_313" href="#Footnote_313" class="fnanchor">[313]</a> The general dip of the shales and of the intrusive -sheets which have been injected between them is towards the east. From -underneath them a thick mass of dolerite rises up to form the long promontory -that here projects northwards from the coast-line, and is prolonged -seawards in the chain of the Skerries.</p> - -<div class="footnote"> - -<p><a id="Footnote_311" href="#FNanchor_311" class="label">[311]</a> Dr. F. Hatch, Explanation of Sheets 7 and 8, <i>Geol. Survey of Ireland</i>, p. 40.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_312" href="#FNanchor_312" class="label">[312]</a> "Address to Geological Society of Dublin, 1835," p. 13, <i>Jour. Geol. Soc. Dublin</i>, vol. i. -The varieties of the Portrush rock were described by the late Dr. Oldham, in Portlock's <i>Report on -the Geology of Londonderry</i>, p. 150; see also the same work for Portlock's own remarks, p. 97.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_313" href="#FNanchor_313" class="label">[313]</a> For a list of the minerals in this rock, see Oldham, <i>op. cit.</i> p. 151.</p> - -</div> - -<p>An interesting feature of the Portrush sections is the clear way in -<span class="pagenum" id="Page_300">- 300 -</span> -which they exhibit the phenomena of "segregation-veins"—so characteristic -of the thicker and more coarsely crystalline sills. These veins or -seams here differ from the rest of the rock mainly in the much larger size -and more definitely crystalline form of their component minerals. Though -sharply defined, when looked at from a little distance, they are found on -closer inspection to shade into the surrounding rock by a complete interlacing -of crystals. On the shore, they can be seen to lie, on the whole, -parallel with the bedding of the sheets in which they occur, but without -rigidly following it, since they undulate and even ramify. A good section -across their dip has been exposed in a quarry near the end of the promontory, -and shows that they are considerably less regular than the plan -of their outcrop on the shore would have led us to anticipate. The accompanying -drawing (<a href="#v2fig314">Fig. 314</a>) represents the veins laid bare on a face of rock -nine feet in length by five feet in height. It will be seen that while there is -a general tendency to conform to the dip-slope, which is here from right to -left, the seams or layers unite into a large rudely-bedded mass, which sends -out processes at different angles. The peculiar aggregation of minerals -which distinguishes such veins is perhaps best seen at Fair Head, and I -reserve for the description of that locality what I have to say on the subject, -only remarking with regard to the Portrush rock that the felspar -shows a disposition to collect in the centre of the veins with the augite and -the other dark minerals at the outer margins.</p> - -<div class="figcenter" id="v2fig314" style="width: 426px;"> - <img src="images/v2fig314.png" width="426" height="256" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 314.</span>—View of "Segregation-Veins" in a dolerite sill, Portrush, Antrim.</div> -</div> - -<p>The contact-metamorphism at this locality is of more historical interest -in connection with the progress of geological theory than of scientific -importance. It consists mainly in an intense induration of the argillaceous -strata. These pass here from their usual condition of fissile, laminar, dull, -dark shales into an exceedingly compact, black, flinty substance, which in -its fracture, colour and hardness reminds one of Lydian stone. Yet the -<span class="pagenum" id="Page_301">- 301 -</span> -ammonites and other organic remains have not been destroyed. They are -preserved in pyrites.</p> - -<div class="figcenter" id="v2fig315" style="width: 535px;"> - <img src="images/v2fig315.png" width="535" height="373" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 315.</span>—View of Fair Head, from the east, showing the main upper sill and a thinner sheet -cropping out along the talus slope.</div> -</div> - -<p>Of all the examples of Tertiary sills in Britain few are more imposing -than that of the noble range of precipices which form the promontory of -Fair Head. Leaving out of account the minor masses of eruptive rock which -occur underneath it, we find the main sheet to extend along the coast for -nearly four miles, to rise to a height of 636 feet above the sea, and to attain -a maximum thickness of 250 feet. This enormous bed dies out rapidly -both to the east and west, and seems also to thin away inland. Seen from -the north, it stands upon a talus of blocks as a sheer vertical wall, 250 -feet high, and the rude prisms into which it is divided are continuous from -top to bottom (<a href="#v2fig315">Fig. 315</a>). So regular is this prismatic structure, and so -much does it recall the more minute columnar grouping of the bedded -basalts, that at a little distance we can hardly realize the true scale of the -structure. It is only when we stand at the base of the cliff or scramble -down its one accessible gully, the "Grey Man's Path," that we appreciate -how long and thick each of the prisms actually is (<a href="#v2fig316">Fig. 316</a>). It -may here be remarked that this regular prismatic jointing is one of the -distinguishing features of the large sills, and serves to mark them off -from the bedded basalts, even when these have assumed a columnar -structure. The prisms are much larger than the basalt-columns, and never -display the irregular starch-like arrangement so common among the -plateau-basalts.</p> - -<p><span class="pagenum" id="Page_302">- 302 -</span></p> - -<div class="figcenter" id="v2fig316" style="width: 780px;"> - <img src="images/v2fig316.png" width="780" height="501" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 316.</span>—View of Fair Head from the shore. (From a Photograph by Mr. R. Welch.)</div> -</div> - -<p><span class="pagenum" id="Page_303">- 303 -</span></p> - -<p>The rock composing this magnificent sheet is a coarsely crystalline, -ophitic, olivine-dolerite.<a id="FNanchor_314" href="#Footnote_314" class="fnanchor">[314]</a> The same diminution of the component crystals, -which is so marked along the margins of the eruptive masses at Portrush, is -strikingly exhibited at the borders of the Fair Head sill. For about 18 or -20 inches upward from the bottom, where the bed rests on the black, Carboniferous -shales, the dolerite is dark and finely crystalline, weathering -spheroidally in the usual manner. But immediately above that bottom -layer of closer grain, the normal coarsely crystalline texture rapidly supervenes. -A similar closeness of grain is observable at the surfaces of contact -where the sheet splits up on its western border.</p> - -<div class="footnote"> - -<p><a id="Footnote_314" href="#FNanchor_314" class="label">[314]</a> Professor Judd has described what he calls a "glomero-porphyritic structure" in this rock -(<i>Quart. Journ. Geol. Soc.</i> xlii. (1886), p. 71).</p> - -</div> - -<p>Nowhere, so far as I know, can the phenomena of "segregation-veins" -be so instructively studied as along the abundant exposures of this great -sheet. The veins are most conspicuous where the rock occurs in thickest -mass. They vary up to three or four feet in thickness, and, as at Portrush -and elsewhere, lie on the whole parallel to the upper and under surfaces of -the sheet. An erroneous impression may be conveyed by the term "veins" -applied to them. They are quite as much layers, parallel on the whole with -the bedding of the sheet, yet not adhering rigidly to one plane, but passing -across here and there from one horizon to another. That they are not due -to any long subsequent protrusion of younger material through the main -sheet is made manifest by the thorough interlocking of their component -crystals with those of the body of the rock in which they lie. The material -that fills these veins has obviously been introduced into them while there -was still some freedom of movement among the crystals of the surrounding -rock, which must thus have been still not quite consolidated and therefore -intensely hot. Both crystallized slowly, and in so doing their component -minerals dovetailed with each other. The constituents of the veins consist -of an exceedingly coarse aggregate of crystals, or rather of crystalline lumps -of the same minerals that constitute the general mass of the rock, the -felspar and augite showing the ophitic intergrowth of the main rock, but -on a far larger scale. Some of the pieces of augite measure two inches or -more in diameter. The conditions under which these veins were produced -must have differed in some essential respects from those that prevailed -during the formation of the fine-grained, highly siliceous veins already -described as occurring in some dykes and sills.</p> - -<p>This great Fair Head sill lies upon Carboniferous strata, but that it is -to be classed with the Tertiary volcanic series is, I think, demonstrated by -its relations to the Chalk at its eastern end. It has there broken through -that rock, and converted it for a short distance into a white, granular -marble. But it is at the western side that the most interesting sections -occur to show the truly intrusive nature of the mass. The rock there splits -up into about a dozen sheets, which, keeping generally parallel with each -<span class="pagenum" id="Page_304">- 304 -</span> -other, have forced their way between and partly across the bedding planes -of the Carboniferous shales (<a href="#v2fig317">Fig. 317</a>). In this way the huge, unbroken -mass, 250 feet thick, subdivides itself and disappears in a few hundred -yards, though it continues a little further inland, and approaches the shore -again half a mile to the south-west. Further evidence of the intrusive -nature of this rock may be observed along the base of the precipice, where -at least one sheet 70 feet thick diverges from the main mass and runs eastwards -between the Carboniferous shales (<a href="#v2fig315">Fig. 315</a>). At the contact with the -eruptive rock the shales are everywhere much indurated.</p> - -<div class="figcenter" id="v2fig317" style="width: 527px;"> - <img src="images/v2fig317.png" width="527" height="260" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 317.</span>—Section at Farragandoo Cliff, west end of Fair Head, showing the rapid splitting - up and dying out of an Intrusive Sheet.<br /><br /> - <i>a</i>, Carboniferous sandstone; <i>b</i>, Carboniferous shale; <i>c</i>, intrusive sheet.</div> -</div> - - -<h3> ii. <span class="allsmcap">SKYE</span></h3> - -<p>All through the Inner Hebrides the base of the basalt-plateaux presents -abundant examples of sills. The general parallelism of these intrusive sheets -to the bedding of the Jurassic strata among which they lie has been above -referred to as having given rise to the erroneous conclusion that in Skye and -elsewhere the basalts are interstratified with Jurassic rocks, and are consequently -of Jurassic age. It was Macculloch who first described and figured -in detail the proofs of their intrusive nature. His well-known sections in -plate xvii. of the illustrations to his work on the <i>Western Islands</i> have been -repeatedly copied, and have served as typical figures of intrusive igneous -rocks.</p> - -<p>Nowhere in North-Western Europe can the phenomena of sills be studied -so fully and with such exuberance and variety of detail as in the island of -Skye and its surrounding islets. On the western coast the greater subsidence -of the basaltic plateau has for the most part submerged the platform -of intrusive sheets, though wherever the base of the bedded lavas is brought up -<span class="pagenum" id="Page_305">- 305 -</span> -to the surface the accompanying -sills are exposed to view. The -east coast of the island has -been classic ground for this part -of volcanic geology since it -supplied the materials for -Macculloch's descriptions and -diagrams. From the mouth -of Loch Sligachan to Rudha -Hunish, at the north end of -Skye, a series of sills may be -traced, sometimes crowning the -cliffs as a columnar mural escarpment, -sometimes burrowing in -endless veins and threads through -the Jurassic rocks. The horizontal -distance to which this -continuous band of sills extends -in Skye is not far short of 30 -miles. But it stretches beyond -the limits of the island. It -forms the group of islets which -prolongs the geological structure -and topographical features of -Trotternish for 4 miles further -to the north-west. It reappears -10 miles still further on in the -Shiant Isles. Thus its total -visible length is fully 40 miles, -or if we include some outlying -sills near the Point of Sleat, to be -afterwards described, it extends -over a distance of not less than -60 miles. From the last outlier -in Skye to the sills of the Isle of -Eigg is a distance of only 8 -miles, thence to those of Ardnamurchan -17 miles, and to those -of the south coast of Mull 25 -miles. Thus this platform of -intrusive sheets of the Inner -Hebrides can be interruptedly -followed for a space of not less -than 110 miles.</p> - -<div class="figcenter" id="v2fig318" style="width: 835px;"> - <img src="images/v2fig318.png" width="835" height="223" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 318.</span>—View of the Trotternish Coast, showing the position of the band of Sills.<br /><br /> - The dark band crowning the first slope above sea-level marks a conspicuous band of sills which towards the right descends to the beach and is prolonged seaward in the group of islands. - The Storr Rock appears as a slanting obelisk of rock on the hill to the left.</div> -</div> - -<p>Though none of the sills in -Skye itself attain the dimensions -<span class="pagenum" id="Page_306">- 306 -</span> -of the Fair Head sheet, they present a greater variety of rock and of -geological structure than is to be found in Antrim. They are specially -developed at the base of the -thick, overlying, basalt-plateau—a -platform on which such -a prodigious quantity of eruptive -material has been injected. -Part of this material -consists of basic rocks in the -form of dykes, veins, or sills; -part of it is included in the -intermediate and acid groups, -and comprises veins, sheets, -and bosses of granitoid, felsitic, -rhyolitic, trachytic, and -pitchstone rocks. One of the -peculiarities of the Skye sills -is the occurrence among them -of compound examples, where -sheets of basic and acid -material have been injected -along the same general platform. -These will be more -specially referred to in Chapter xlviii. With regard to the basic -sills (dolerites, basalts, etc.), I -would remark that while in -Western Scotland the Antrim -type of short, thick intrusions, -or laccolites, is also found, -the vast majority of the sheets -are much thinner, more persistent, -and less easily distinguishable -from the bedded -basalts.</p> - -<div class="figcenter" id="v2fig319" style="width: 713px;"> - <img src="images/v2fig319.png" width="713" height="253" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 319.</span>—Columnar Sill intrusive in Jurassic Strata east of Kilmartin, Trotternish, Skye.<br /> - [The high ground to the left is a portion of the basalt-plateau to the north of the well-known Quiraing.]</div> -</div> - - -<p>In describing the sills of -Skye I shall take first those -of the eastern and then those -of the western side of the -island. Along the east coast, -from Loch Sligachan to the -most northerly headlands and -islets the sills play a notable -part in the scenery, inasmuch -as they cap the great sea-cliff -of Trotternish and run as a line of ridges parallel to the trend of the coast, -<span class="pagenum" id="Page_307">- 307 -</span> -while the plateau-basalts rise above them further inland as a lofty escarpment, -which includes the picturesque landslips of the Storr Rock and Quiraing (Figs. -318, 319). Beneath the thick sills, the Jurassic sandstones form a range of -pale yellow precipices, along which many thinner sheets of eruptive material -have been intruded. As Macculloch well showed, many of these sheets, if -seen only at one point, might readily be taken for regularly interstratified -beds, but perhaps only a few yards distant they may be found to break -across the strata and to resume their course on a different level.</p> - -<p>The sills of this Trotternish coast may be distinguished even at some -distance from the bedded basalts by the regular prismatic jointing, already -referred to, and by their frequently greater thickness, while on closer -inspection they are characterized by their much coarser texture. They are -generally somewhat largely crystalline ophitic dolerites, gabbros or diabases, -and exhibit the persistent uniformity of composition and structure so characteristic -of intrusive sheets and dykes. These characters are well exhibited -in the Kilt Rock, a columnar sill capping the cliffs to the south of Loch -Staffin (<a href="#v2fig319">Fig. 319</a>).</p> - -<p>These massive sills are prolonged in a series of picturesque flat tabular -islets beyond the most northerly headlands of Skye. They probably continue -northwards under the sea at least 12 miles further, for sills of the same -type rise there in the singularly striking group of the Shiant Isles (<a href="#v2fig320">Fig. 320</a>). -These lonely islets, extending in an east and west direction for about three -miles, display in great perfection most of the chief characters of the Skye sills. -They are especially noteworthy for including the thickest intrusive sheet and -the noblest columnar cliff in the whole of the Tertiary volcanic series of -Britain. The larger of the two chief islands consists of two masses of rock -connected by a strip of shingle-beach, and having a united length from north -to south of about two miles. The northern half, or Garbh Eilean, presents -towards the north a sheer precipice 500 feet high. This magnificent face of -rock consists of one single sill, but as its original upper limit has been -removed by denudation and its base, where it is thickest, is concealed under -the sea, the sill may exceed 500 feet in thickness. The rock has the usual -prismatic structure, which imparts to it an impressive appearance of -regularity. The columns retain their individuality to a great height, and -though none of them perhaps can be followed from base to crest of the cliff, -many of them are evidently at least 300 or 400 feet long.</p> - -<p>Macculloch, who gave the first geological description of the Shiant Isles, -showed the intrusive nature of the igneous rocks, and described the remarkable -globular or botryoidal structure of the Jurassic shales between which -they have been injected.<a id="FNanchor_315" href="#Footnote_315" class="fnanchor">[315]</a> Professor Heddle has published a brief account of -the geology of the islands.<a id="FNanchor_316" href="#Footnote_316" class="fnanchor">[316]</a> Professor Judd visited the group and brought -away a series of specimens of their eruptive rocks, which he found to include -basic and ultra-basic varieties.<a id="FNanchor_317" href="#Footnote_317" class="fnanchor">[317]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_315" href="#FNanchor_315" class="label">[315]</a> <i>Western Islands</i>, vol. i. p. 441.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_316" href="#FNanchor_316" class="label">[316]</a> <i>Trans. Norfolk Nat. Hist. Soc.</i> vol. iii. (1880) p. 61.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_317" href="#FNanchor_317" class="label">[317]</a> <i>Quart. Journ. Geol. Soc.</i> vol. xxxiv. (1878) p. 677, and xli. (1885) p. 393. My description -in the text is the result of three successive visits to the islands.</p> - -</div> - -<p><span class="pagenum" id="Page_308">- 308 -</span></p> - -<div class="figcenter" id="v2fig320" style="width: 672px;"> - <img src="images/v2fig320.png" width="672" height="382" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 320.</span>—View of the northern precipice (500 feet high) of the largest of the Shiant Isles.<br /> - (From a Photograph by Colonel Evans.)</div> -</div> - -<p>In Garbh Eilean, where the thickest mass of erupted material presents -itself, at least three sills may be observed. Some low reefs that run parallel -with the northern coast of the island consist of coarse ophitic gabbro in -two or more sheets which have been intruded between the Jurassic shales. -<span class="pagenum" id="Page_309">- 309 -</span> -Above these strata comes the great columnar sill, its base gradually sinking -towards the west until it passes under the sea, and the vertical columns -then plunge abruptly into the water. The rock of which this massive sill -consists is another large-grained gabbro or dolerite, with an ophitic structure. -Owing to the form of the ground it cannot be so satisfactorily examined -as the neighbouring island of Eilean Mhuire, which, though less lofty and -rather smaller than Garbh Eilean, affords a succession of admirable and -easily examined sections along its precipitous shores.</p> - -<p>Professor Judd found that while the rocks are mainly ophitic gabbros and -dolerites, they include such highly basic compounds as dunite. An examination -of the Eilean Mhuire cliffs enables the observer to ascertain that the -sills display considerable variety in texture and in the character and -arrangement of their component minerals. They are marked by a persistent, -more or less distinct disposition in rude beds, and these again often display -a banding of their constituents in lines parallel with the general -bedding. Some of these bands are largely felspathic, and are thus paler in -colour. Others, where the ferro-magnesian minerals and ores are more -specially aggregated, are dark in colour. In some layers the long -black prisms of augite are ranged in a general parallelism with the -banding.</p> - -<p>A specimen selected as typical of the ordinary coarse-grained amorphous -rock was sliced and placed in Mr. Harker's for microscopic examination, -and he has supplied the following observations regarding it: "The gabbro -from Eilean Mhuire [7110] is a crystalline rock showing to the eye lustrous -black augites, half an inch long, and (predominating) felspar. The microscope -reveals, in addition, irregular grains of black iron-ore and little -hexagonal prisms of apatite. No olivine is to be detected. As regards -structure, the augite has tended to crystallise out in advance of the felspar, -but this relation is not constant.</p> - -<p>"The augite is of a light-brown tint in slices, and has an unusual kind -of pleochroism. The colour for vibrations parallel to the β-axis is of the -purplish-brown tone seen in some soda-bearing augites; parallel to γ and α -it has a yellow or citron tint. The colour and pleochroism are more marked -in the interior of a crystal than towards the margin, but some crystals -pass at the margin into a slightly pleochroic, pale-green, recalling ægerine-augite. -The felspar tends to build elongated crystals. It is a rather finely -lamellated labradorite, sometimes showing pericline- as well as albite-lamellae."</p> - -<p>Another specimen from one of the black bands in the same island, with -a linear arrangement of its component minerals, is thus described by the -same petrographer: "This rock [7111] is of darker appearance than the -preceding, and contains abundant black iron-ore, besides some pyrites. It -also differs in having a marked parallel disposition of its crystals.</p> - -<p>"Except for the greater prominence of large irregular grains of iron-ore, -this rock under the microscope closely resembles the last described, the -parallel structure not being conspicuous in the slice. The augite has the -<span class="pagenum" id="Page_310">- 310 -</span> -peculiar colour and pleochroism already noted, and the felspar is of the same -kind as before."</p> - -<p>I did not succeed in finding in place any bands of dunite, but this basic -material probably occurs at the base of some of the sills where it has segregated -from the rest of the mass, like the picrite at the bottom of the -Bathgate diabase.</p> - -<p>The amount of contact-metamorphism effected even by such thick sills -as those of Trotternish and Shiant is much less than might be expected. It -seldom goes beyond a mere induration of the strata for a few yards, often -only for a few inches from the surface of junction. In the Shiant Isles, -however, the shales between the sills have undergone a more remarkable -alteration. They have not only been greatly indurated, but have acquired -the globular or botryoidal structure so fully described by Macculloch. The -spheroidal aggregates vary from not more than a line to more than half -an inch in diameter, and appear on the surface as dark, irregularly grouped, -pea-like aggregates. This structure is perhaps best developed immediately -under the thick sill on the west side of Eilean Mhuire.</p> - -<p>The massive sills are not the only evidence of the injection of -igneous material on the Shiant Isles. The sill, or more probably group of -sills, forming Eilean Mhuire is traversed by a number of sheets of basalt -varying from only two or three inches to 20 feet in thickness. These black -fine-grained rocks invariably present chilled selvages next the coarse -gabbro, and though they have been on the whole injected parallel to the -general bedding or banding, they here and there break across it as veins. -The most important of these later intrusions forms a columnar sill on the -eastern side of the island, and can be followed for several hundred yards. -It consists of a dark finely crystalline olivine-basalt, which towards the -margin assumes a dense black texture. Under the microscope Mr. Harker -found a thin slice of this rock to be "an olivine-basalt of semi-ophitic, -semi-granulitic structure [7112]. The olivine is mostly fresh, but part of -it is converted into a yellowish-brown pseudomorph like iddingsite. Magnetite -occurs chiefly in imperfect octohedra. The felspar is in little -lath-shaped sections, many of which are finely striated, and give extinction-angles -indicating a labradorite. The augite, light brown in the slice, never -has crystal-boundaries, and often enwraps the felspars."</p> - -<p>The narrow veins are composed of a much closer-grained basalt in which -a few scattered felspars are visible. Mr. Harker remarks, with regard to -a thin slice of one of these rocks [7113], that "the microscope shows this, -too, to be an olivine-basalt. The porphyritic felspars are twinned on the -Carlsbad and albite laws. Olivine and pseudomorphs after it are well -represented. Magnetite is only sparingly present. The general mass of -the rock consists of very small striated prisms of labradorite, granules of -augite, and interstitial matter which must be partly glassy."</p> - -<p>This is perhaps the most striking of all the examples known to me -where an older sill has been split open to receive a subsequent injection of -molten material. The Eilean Mhuire gabbro must be at least 200 feet -<span class="pagenum" id="Page_311">- 311 -</span> -thick, and it not impossibly passes under the still thicker pile of Garbh -Eilean. Yet it has been horizontally ruptured near its base, and into the -rent thus produced another mass of molten matter has been thrust. This -subject will be again referred to in connection with another remarkable -example on the west coast of Skye.</p> - -<div class="figcenter" id="v2fig321" style="width: 359px;"> - <img src="images/v2fig321.png" width="359" height="265" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 321.</span>—Section of thin Intrusive Sheets and Veins in carbonaceous shales lying among the Plateau-basalts, - cliffs north of Ach na Hannait, between Portree Bay and Lock Sligachan.</div> -</div> - -<p>In contrast to such enormous thicknesses of intrusive material as those -of Trotternish and the Shiant Isles, instances may be culled from the same -belt of sills where the molten rock has been injected in thin leaves and mere -threads into the Jurassic sandstones and shales, or into the shales and coals -intercalated among the plateau-basalts. Thus, on the cliff immediately to -the north of Ach na Hannait, between Loch Sligachan and Portree Bay, the -section may be seen which is represented in <a href="#v2fig321">Fig. 321</a>. At the base lies a -vesicular dolerite with a slaggy upper surface (<i>a</i>). Next comes a zone of sedimentary -material about five or six feet thick, the lower portion consisting of an -impure coal, which passes towards the right hand into brown and grey carbonaceous -shale with plant-remains (<i>b</i>). This coaly layer has been already -alluded to as probably lying on the same horizon with the coal of Portree -(p. 288). Traced northward, it is found to have a bed of fine tuff beneath it, -and sometimes a volcanic breccia or conglomerate. It fills up rents in the -underlying slaggy lava, and was undoubtedly deposited upon the cooled surface -of that rock. Immediately above this lower band the black carbonaceous -shale which follows has been invaded by an extraordinary number of -thin cakes or sills and also by veins or threads of basalt. For a thickness -of two or three feet the band (<i>d</i>) consists mainly of these intrusions, which, -in the form of a fine grey basalt, vary from less than an inch to three or -four inches in thickness. They are separated by thin partings of coaly -shale, and as they tend to break up into detached nodule-like portions, -<span class="pagenum" id="Page_312">- 312 -</span> -especially towards the right hand of the section, they might, on casual -inspection, be easily mistaken for nodules in the dark shales. Somewhat -later in the time of intrusion are veins of basalt which, as at <i>c</i>, break across -the nodular sills, and sometimes expand into thicker beds (<i>c′</i>).</p> - -<p>I have never seen such a congeries of minute sills among the Tertiary -basalt-plateaux as that here exhibited. In a space of about three feet of -vertical height there must be more than a dozen of roughly parallel leaves -of intrusive rock. Veins (<i>e</i>) run up from the chief band of eruptive material -into the overlying finely vesicular basalt (<i>f</i>). The dyke (<i>g</i>) is probably the -youngest rock in the section.</p> - -<p>The more general and extensive submergence of the base of the basalt-plateau -on the west side of Skye has for the most part carried the platform -of sills below sea-level, so that it is only exceptionally where, owing to local -irregularities, that base has been brought up to the air, that the intrusive -sheets show themselves. Yet the persistence of the platform on that side -is indicated by its extension even as far as the southern promontory of the -island.</p> - -<p>The Trotternish type of sill extends down the west coast under the -headlands of Duirinish. Thus at the mouth of Dunvegan Loch, where the -underlying Jurassic platform has been ridged up above the surface of the -sea, it has carried with it the marked sill which forms the islets of Mingay -and Clett that lie as a protecting breakwater across the entrance of the inlet. -The intrusive rock rests on shell-limestones full of oysters (<i>Ostrea hebridica</i>), -and referable to the Loch Staffin group of the Great Oolite Series. This -sill, when observed from a little distance, presents the usual regularly prismatic -or columnar structure so well developed among the Trotternish -examples, but on a closer view shows this structure less distinctly. It is an -olivine-dolerite of medium and fine texture, which in thin slices displays -under the microscope a distinctly ophitic structure, the abundant light-brown -augite enclosing the striated felspars. Its lowest portion, from three to seven -or eight inches upward from the bottom, is much closer-textured than the -rest of the rock and is finely amygdaloidal. Its vesicles are in many cases -drawn out to a length of three or four inches, and the zeolites which now -fill them look like parallel annelid tubes or stems of <i>Lithostrotion</i>. It is -noteworthy also that the elongation of the vesicles has sometimes taken -place at a right angle to the surface of contact with the underlying strata. -But the most remarkable feature in this sill is the surface which it presents -to the oyster-beds on which it rests. The fine-grained dark dolerite has -there assumed the aspect of a sheet of iron-slag, with a smooth or wrinkled, -twisted, ropy surface, which displays fine curving flow-lines. No one looking -at a detached specimen of this surface would be ready to admit that it could -possibly have come from anything but a true lava-stream that flowed out at -the surface. The contours of a viscous lava are here precisely reproduced -on the under surface of a massive sill.</p> - -<p>A little further south, the promontory of Eist, forming the western -breakwater of Moonen Bay, consists of an important sill or group of sills -<span class="pagenum" id="Page_313">- 313 -</span> -which has insinuated itself among shales, shell-limestones, and shaly sandstones, -full of <i>Ostrea hebridica</i>, <i>Cyrena aurata</i>, etc., and belonging to the -Loch Staffin group of the Great Oolite Series. The shore-cliff below the -waterfall affords the section given in <a href="#v2fig322">Fig. 322</a>, illustrating the manner in -which a thick intrusive sheet may sometimes give off thin veins from its -mass. The rock attains on the Eist promontory a thickness of probably at -least 100 feet, where it is thickest and undivided. But the two main -sheets, or branches of one great sheet, on this peninsula have probably a -united depth of more than 300 feet. Landwards the rock splits up and -encloses cakes of the Jurassic strata. It possesses the usual prismatic structure -and doleritic composition. In Moonen Bay, as shown in <a href="#v2fig322">Fig. 322</a>, it -presents a banded structure, marked especially by an alternation of lines of -amygdales and layers of more compact and solid dolerite, with occasional -enclosed cakes of baked shale or sandstone. Its upper surface is somewhat -uneven, and from it are given off narrow, wavy, ribbon-like veins (<i>d</i>), from -less than an inch to three inches or more in width, which keep in a general -sense parallel to the top of the sill, but at a distance of a few inches or feet -from it. The sill becomes as usual fine-grained towards the contact, the -shales and sandstones being indurated and the limestone marmorized.</p> - -<div class="figcenter" id="v2fig322" style="width: 404px;"> - <img src="images/v2fig322.png" width="404" height="171" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 322.</span>—Upper part of Sill, Moonen Bay, Waternish, Skye, showing the divergence of veins.<br /><br /> - <i>a</i>, false-bedded shaly sandstone; <i>b</i>, shell-limestone; <i>c</i>, dolerite sill; <i>d</i>, veins proceeding from the sill.<br /> - Length of section about five yards.</div> -</div> - -<p>The next uprise of the base of the basalt-plateau on the west side of -Skye lies about 25 miles to the south-east, where it emerges from the sea in -the Sound of Soa (<a href="#v2fig323">Fig. 323</a>). A vast volcanic pile has there been heaped -up on the Torridon sandstone, the whole of the thick Jurassic series, which -is found in force only three miles distant in Strathaird, having been removed -by denudation from this area before the beginning of the Tertiary volcanic -period. The plateau-basalts rests on the upturned edges of the Torridonian -sandstones and shales, and are accompanied as usual by their underlying network -of intrusive rocks. It is hardly possible to exaggerate the wild confusion -of sills, dykes and veins which have been injected among the rocks, at and on -both sides of the unconformability. Endless sheets of basalt and dolerite -have forced their way between the bedded basalts and the sandstones, while -across the whole rise vast numbers of dykes and veins. Narrow, black, -<span class="pagenum" id="Page_314">- 314 -</span> -wavy ribbons of basic material cross many of these veins, while the later -north-west dykes cut sharply through everything older than themselves. -As a natural section for the study of the phenomena of intrusion in many -of their most characteristic phases, I know no locality equal to the northern -coast-line of the Sound of Soa, unless it be the cliffs of Ardnamurchan. -But the Skye cliffs, though less imposing than those of the great Argyllshire -headland, have this advantage, that instead of being exposed to the full roll -of the open Atlantic, they form the margin of a comparatively sheltered -strait, and can thus be conveniently examined.</p> - -<div class="figcenter" id="v2fig323" style="width: 375px;"> - <img src="images/v2fig323.png" width="341" height="176" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 323.</span>—Section of the base of the Basalt-plateau with sill and dykes, Sound of Soa, Skye.<br /><br /> - <i>a</i> <i>a</i>, Torridon Sandstone; <i>b</i>, Bedded basalts; <i>c</i>, Sill; <i>d</i> <i>d</i>, Dykes.</div> -</div> - -<p>Following still the western seaboard of Skye, we meet with other striking -examples of sills at a distance of some eight miles in a straight line -eastward where, between Lochs Slapin and Eishort, the prominent headland -of Suisnish juts out into the sea. This promontory has long been known -to geologists from the section of it given by Macculloch as an instance of -the connection between overlying rocks and dykes. I have already alluded -to it in that relation, and refer to it again as an example of one of the -thicker intrusive sheets of the Inner Hebrides. Denudation has here also -proceeded so far that the whole of the volcanic plateau has been stripped -off, only some of the underlying sills being left, together with the platform -of older rocks between which and the vanished basalts they were -injected. Most of these sills consist of granophyres belonging to the acid -group of rocks to be afterwards described. But basic sheets occur not infrequently -interposed between the granophyres and the subjacent Lias, and -sometimes even intercalated in the former rock. Though at first sight it -might be thought that these sills had insinuated themselves after the eruption -of the granophyre, and there are instances where this cannot be shown -not to be the case, I have obtained so many proofs of the invasion of the -basic by the acid rock that I have no doubt the former is, as a general -rule, the older of the two.</p> - -<p>The Suisnish headland exhibits the structure represented in <a href="#v2fig249">Fig. 249</a>. -For about 300 feet above the sea-level the steep grassy slope shows -outcrops of the dark, sandy shales and yellowish brown, shaly sandstones -of the Lias which form the range of cliffs to the eastward. These gently -<span class="pagenum" id="Page_315">- 315 -</span> -inclined strata are cut through by many vertical basalt-dykes, some of which -intersect each other, but among which by far the largest is the mass shown -in the figure. This broad dyke consists of a dolerite or gabbro the largely -crystalline texture of which marks it off at once from the others, which are -of the usual dark, heavy, fine-grained type, with an occasional less basic and -porphyritic variety. Traced up from the sea-margin, the dyke loses itself in -a talus of blocks from the cliff above, so that its actual junction with the -mural front of the sill cannot be seen. But that it joins that mass, with -which it agrees in petrographical characters, hardly admits of question. -The cliff consists of a thick sheet of coarsely crystalline dolerite or gabbro -(<i>d</i> in <a href="#v2fig249">Fig. 249</a>), which in its general aspect at once recalls the rock of Fair -Head. It varies considerably in texture, some parts of the mass are exceedingly -coarse, like the Skye gabbros, and present a fibrous structure in their -augite resembling that of the diallage in these rocks; other portions assume -the compactness of basalt. A specimen of medium grain under the microscope -shows the typical ophitic structure so generally found among the -dolerites both of the plateaux and of the intrusive sheets. This sill must -be about 200 feet thick, and like the rock at Fair Head is traversed from -top to bottom by joints that divide it into prisms. It appears to bifurcate -eastward, one portion running with a tolerably uniform thickness of a few -feet as a prominent band at the top of the shales and sandstones, the other -slanting upwards and gradually thinning away in the granophyre.</p> - -<p>Towards its base, near the contact with the underlying shales, the rock -as usual becomes finer grained, and the thin band just referred to resembles -in texture one of the wider basalt-dykes. Westwards the rock can be -followed round the top of the grassy slopes formed by the decay of the shales. -Though concealed by intervals of moorland and peat, it is visible in the -stream sections, and I think must be continuous, as a band only a few -yards thick, round the northern side of the hills as far as Beinn Bhuidhe, -where a similar sill makes a prominent crag. Its total area measures a mile -and a quarter in length by half a mile in breadth. The granophyre which -overlies it forms part of an interesting series of sheets which I have traced -all the way from Suisnish to the braes above Skulamus.</p> - -<p>Whether or not the whole sheet of basic rock is continuous, and -whether it all proceeded from the great Suisnish dyke, cannot be confidently -decided until the ground is mapped in detail, though from the great thickness -of the sill at the dyke, its attenuation outwards from that centre and -its uniformity of petrographical character, I am disposed to answer affirmatively. -There is no other probable vent to be seen in the neighbourhood, -unless a massive dyke that runs from Loch Fada north-westwards into Glen -Boreraig can be so regarded.</p> - -<p>Not far from the extreme southern point of Skye a singularly interesting -example of a sill remains as a detached survival of the basaltic plateau and its -accompaniments. In his map of Skye, Macculloch showed the position of -this outlier, which he classed with the general "trap" formation of the -island. The locality was visited by Professor Judd, who regarded the intrusive -<span class="pagenum" id="Page_316">- 316 -</span> -rock as a "phonolite"<a id="FNanchor_318" href="#Footnote_318" class="fnanchor">[318]</a> In 1894, during an excursion with my colleague -Mr. C. T. Clough, I had an opportunity of examining the rocks and collecting -notes for the following account of them.</p> - -<div class="footnote"> - -<p><a id="Footnote_318" href="#FNanchor_318" class="label">[318]</a> <i>Quart. Jour. Geol. Soc.</i> vol. xxxiv. (1878) p. 692.</p> - -</div> - -<p>At Rudh' an Iasgaich, about two miles from the Point of Sleat, a small -outlier of conglomerate lies on the edges of the Torridon Sandstone. This -deposit has been correctly identified by Professor Judd with the similar -strata which, in Skye and elsewhere on the west coast of Scotland, underlie -the Liassic series. It is here about 10 or 12 feet thick, reddish and -yellowish in colour, and distinctly calcareous. Its component pebbles consist -largely of Cambrian (Durness) limestone, quartzite, and Torridon Sandstone—rocks -which all occur <i>in situ</i> in Sleat. It may be compared with -the limestone conglomerates of Strath and those which underlie the Lias at -Heast on Loch Eishort.<a id="FNanchor_319" href="#Footnote_319" class="fnanchor">[319]</a> That here, as elsewhere in this region, the basement -conglomerate was followed by the rest of the Lias and Oolites may -be inferred with some confidence from the copious development of the -Jurassic series a few miles off, both to north and south. But the whole -of this overlying succession of formations has here been swept away, and, but -for the protection afforded by the eruptive rocks of Rudh' an Iasgaich, the -conglomerate would likewise have disappeared.</p> - -<div class="footnote"> - -<p><a id="Footnote_319" href="#FNanchor_319" class="label">[319]</a> <i>Op. cit.</i> vol. xiv. (1857), p. 9; vol. xliv. (1888), p. 71.</p> - -</div> - -<div class="figcenter" id="v2fig324" style="width: 345px;"> - <img src="images/v2fig324.png" width="345" height="142" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 324.</span>—Section of Dolerite Sill cut by another sill, both being traversed by dykes, Rudh' an - Iasgaich, western side of Sleat, Skye.</div> -</div> - -<p>Above the conglomeratic band lies a sheet of intrusive rock, which in -one place has apparently cut it out, so as to rest directly upon the -Torridon Sandstone (<i>a</i>, <a href="#v2fig324">Fig. 324</a>). The decay of the softer detrital rock -underneath has caused the sill to break off in slices, which have left behind -them a bold mural escarpment (<i>b</i> <i>b</i>).</p> - -<p>The rock of this sill is a rather coarsely crystalline porphyritic olivine-dolerite, -which towards the north attains a thickness of about 70 feet. It -exhibits the usual prismatic jointing, but less perfectly than some of the -Trotternish sills already referred to. Besides these vertical joints, it is also -traversed by a system of horizontal divisional planes which, though somewhat -irregular in their course, run, in a general sense, parallel to the upper -and under surfaces of the sill.</p> - -<p>It seems to have been along this transverse series of joints that a -second sill (<i>c</i>), five or six feet thick, has been injected. The material of this -<span class="pagenum" id="Page_317">- 317 -</span> -younger intrusion is a black, finely crystalline dolerite or basalt, with rudely -prismatic jointing. Its most striking feature, besides its regularity of -position and persistency for several hundred yards as a platform along the -shore, is the basalt-glass which marks both its under and upper surfaces of -contact, and which is here developed upon a scale to which I have not met -with an equal among the Tertiary sills of this country.</p> - -<p>The selvage of glass appears as a black tar-like layer, varying from a -mere film to two or three inches in thickness. It is found not only on the -upper and under surfaces, but descends along abrupt step-like interruptions -of the upper surface, a foot or more in height, as if the sill had been broken -by a series of subsidences. The apparent fracture, however, is probably due -to the irregularities of the passage forced for itself by the molten rock as it -passed from one line of horizontal joint to another through the heart of the -older sheet.</p> - -<p>The exposed surface of black glass on the top of the younger sill exhibits -long parallel lines, probably marking flow-structure, which are made -conspicuous by a pale yellow ferruginous weathered crust. Portions of the -larger intrusive sheet have been broken off and involved in the later rock. -The younger sill disappears to the north, and is not found in the cliff of Rudha -Chà rn nan Cearc, where the thick sill, lying once more on the band of conglomerate, -forms a fine escarpment above the shore. Dykes of fine-grained -basalt (<i>d</i> <i>d</i>) with compact chilled margins rise through both sills, together -with veins which pursue a wavy upward path like strips of black ribbon.</p> - -<p>This example, and that of the Shiant Isles already described, cannot but -impress the observer with the prodigious force with which the material of -the sills was injected. In these instances solid sheets of intrusive rock have -subsequently been rent open, doubtless under a superincumbent pressure of -many hundreds of feet of the terrestrial crust, and a new injection of molten -magma has made its way into the rents thus caused. In each case, the -position of the rents was obviously determined by structural lines in the -older sills, but we are lost in astonishment at the energy required to split -open, even along these lines, such solid crystalline masses as the thick -sills, and to overcome the superincumbent pressure of so deep a pile of rock.</p> - -<p>The isolation of a relic of the Tertiary sills on the west side of the promontory -of Sleat presents some interesting problems to the mind of the -geologist. The locality lies about midway between the basalt-plateau of -Strathaird and that of Eigg, and some eight or nine miles in a direct line from -either. The basalts cannot be proved to have once stretched continuously -between Eigg and Strathaird, and to have covered this part of Sleat; but -the position of the Sleat sills makes it probable that this continuation did -formerly exist. The denudation of the West of Scotland since early Tertiary -time has been so stupendous that I am prepared for almost any seemingly -incredible evidence of its effects. There can hardly be any doubt, however, -that the sills here described belong to the great platform of intrusive sheets, -and that they were injected under a pile of Secondary strata, if not also of -Tertiary basalts, which has here been entirely removed.</p> - -<p><span class="pagenum" id="Page_318">- 318 -</span></p> - -<p>Reference may be made, in conclusion, to a not infrequent feature of the -Skye sills. Like the dykes, they are often double or multiple, molten -material having been successively injected along the same plane. The -example just cited from the west side of Sleat illustrates one type of such -compound sills. More frequently, however, the subsequent injections have -been made along the floor or roof of the first sheet. Mr. Harker has found -numerous cases of this structure in the Strath district. They are recognizable -even from a distance by their terraced contours when seen in profile. -They often vary considerably in thickness owing to the dying out or coming-in -of their separate bands; while, on the other hand, single sills tend to -maintain a uniform thickness for long distances, or taper away gradually. -The compound arrangement of the basic sills is well brought out where acid -material has been injected between the sheets, as will be more fully described -in Chapter xlviii.</p> - - -<h3>iii. <span class="allsmcap">EIGG, ARDNAMURCHAN</span></h3> - -<p>The phenomena of the coasts of Skye are repeated on the east side of -Raasay, in Eigg, and still more magnificently along the south coast of -Mull. A single example is here given (<a href="#v2fig325">Fig. 325</a>) from the east side of -Eigg. Over the Jurassic sandstones (<i>a</i> <i>a</i>) a sill of basalt (1) four to six feet -thick has been injected between the -stratification, and another (2) two -to four feet thick has forced its way -across the middle of one of the bedded -basalts (<i>b</i> <i>b</i>) in which it bifurcates, -and above which comes the thick -series of lavas of the plateau (<i>c</i>, <i>d</i>). -In one of the streamlets, which -exposes a section of the Jurassic -strata below the volcanic escarpment, -more than twenty intrusive -sheets may be counted among the -shales and limestones. They are -sometimes not six inches thick, and -seldom exceed six or eight feet.</p> - -<div class="figleft" id="v2fig325" style="width: 247px;"> - <img src="images/v2fig325.png" width="247" height="283" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 325.</span>—Section to show Bedded and Intrusive - Sheets, Eigg.</div> -</div> - -<p>I will conclude this account of -the Tertiary basic sills of Britain -by referring to one further district -in the West of Scotland, where -they are well displayed on bare -hillslopes and also along a picturesque sea-coast. In the promontory -of Ardnamurchan in the west of Argyleshire, one of the most conspicuous -eminences, known as Ben Hiant, affords a striking mass of intrusive -material, which, extending along a rugged shore for three-quarters of a mile, -mounts thence inland in a series of rocky knolls, and in rather less than a -<span class="pagenum" id="Page_319">- 319 -</span> -mile culminates in a summit, 1729 feet above sea-level.<a id="FNanchor_320" href="#Footnote_320" class="fnanchor">[320]</a> The rocks which -cover this large space are disposed in numerous rude beds, which have a -seaward dip of perhaps 15° to 20°, and are sometimes distinctly prismatic, -the prisms being not infrequently grouped in fan-shape. They are evidently -due to successive intrusions. Although generally coarsely crystalline in -texture, they include also intermediate and fine-grained sheets. They -are never, so far as I have been able to discover, amygdaloidal,<a id="FNanchor_321" href="#Footnote_321" class="fnanchor">[321]</a> nor do -they present the ordinary external characters of the beds of the plateaux, -though here and there they appear to have caught up portions of the -plateau-series. They distinctly overlie the bedded basalts on their eastern -and southern margins; but westwards they appear to lie transgressively -across the edges of these rocks, while to the north-west and north they rest -on quartzites and schists and on Jurassic limestones. An outlier from the -main mass forms the prominent hill of Sròn Mhor, and can be seen distinctly -overlying the bedded basalts as well as the neck of agglomerate already -described (<a href="#v2fig302">Fig. 302</a>).</p> - -<div class="footnote"> - -<p><a id="Footnote_320" href="#FNanchor_320" class="label">[320]</a> This locality has been described by Professor Judd (<i>Quart. Jour. Geol. Soc.</i> xxx. (1874), p. -261; and xlvi. (1890), p. 373).</p> - -<p><a id="Footnote_321" href="#FNanchor_321" class="label">[321]</a> As amygdaloidal structure is occasionally to be found among both dykes and sills its -presence in the Ben Hiant rocks would not be inconsistent with their intrusive origin.</p> - -</div> - -<p>The prevalent rocks of Ben Hiant are well crystallized, ophitic olivine-dolerites -and gabbros. A specimen taken from the shore on the west side -of the mass was found by Dr. Hatch to present under the microscope its -augite in large plates, which enclose narrow laths and needles of plagioclase -felspar as well as grains of olivine. All the felspars are in lath-shapes, -sometimes extremely long and narrow. The iron-ore likewise assumes an -ophitic character, enclosing rectangular portions of felspar. Dr. Hatch -observed in another specimen, taken from the south-east side of the hill, "a -curious intermixture of two different structures. Scattered portions which -show the usual ophitic structure, their felspar and augite occurring in large -crystals, are, so to speak, imbedded in a groundmass which presents rather -a basaltic type, its felspar, augite, and magnetite, in long thin needles, -microlites, and other skeleton forms, being enclosed in a dark devitrified -base." A third specimen, selected from one of the columnar sheets near the -top of Ben Hiant, is "a fine-grained dolerite (or gabbro) showing little -ophitic structure, the augite occurring in roundish grains, and only slightly -intergrown with the felspars, which are more or less lath-shaped. The rock -contains a considerable quantity of black iron-ore in irregular grains and -some dirty-green viridite." Still another variety of structure occurs in a -specimen which I broke from one of the shore crags on the south-west side -of the hill. Under the microscope, Dr. Hatch found in it a beautiful -aggregate of "skeleton crystals and microlites of plagioclase, with here and -there a rectangular crystal, long slender microlites of augite, and short -serrated microlites of magnetite, the whole being confusedly imbedded in -a dark glassy base powdered over with a fine magnetite dust."<a id="FNanchor_322" href="#Footnote_322" class="fnanchor">[322]</a> A sill -<span class="pagenum" id="Page_320">- 320 -</span> -of pitchstone lies among the bedded basalts on the east side of the -hill.</p> - -<div class="footnote"> - -<p><a id="Footnote_322" href="#FNanchor_322" class="label">[322]</a> Professor Judd has called the rocks of Beinn Hiant augite-andesites, and has given descriptions -and figures of their structure, and analyses of their chemical composition (<i>op. cit.</i>).</p> - -</div> - -<p>From a number of specimens collected by me during a second visit to -this district in the summer of 1896, I selected some for microscopic examination -and submitted them to Mr. Harker, who has furnished me with -the following descriptions of them: "The sill at the north end of Camas -na Cloiche, Ben Hiant [7114] is an olivine-gabbro of medium grain and -fresh appearance. Olivine, fresh or partly serpentinized, is plentiful. The -felspar is a labradorite with Carlsbad- and albite- (rarely pericline-) twinning, -and some of it has zonary banding. It is for the most part in crystals -giving rectangular sections, but there are some of allotriomorphic form. -Magnetite occurs chiefly in shapeless grains of later crystallization than the -felspar, but sometimes presenting crystal-faces to the augite. The augite -is light-brown in the slice, without any true diallage-structure, and tends -to enwrap the earlier minerals in ophitic patches.</p> - -<p>"The sill south of Uamh na Creadha, on the west side of Ben Hiant -[7115], is a rock of different type, having porphyritic crystals of felspar, up -to an inch or more in length, in a rather finely-crystalline groundmass. -The microscope shows it to be a dolerite of granulitic structure, the main -mass of the rock consisting of little striated labradorite-crystals, grains of -pyroxene, and rather abundant crystal-grains of magnetite. The pyroxene -seems to be chiefly augite, but hypersthene is also present, and builds rather -larger and more idiomorphic crystals with characteristic pleochroism."</p> - -<p>In rambling over this Ardnamurchan district I have often been reminded -of the great intrusive sheets of Fair Head. One of the features in -which the rocks of the two localities resemble each other is their tendency -to assume a coarsely crystalline texture. In some parts of Ben Hiant the -individual crystals reach an inch or more in length. These more largely -crystalline portions, however, do not form distinct bands so much as patches -in the midst of the general mass; at least I have not noticed any examples -of such veins of segregation as are so prominent in Antrim.</p> - -<p>No one familiar with the well-marked distinctions between the lavas of -the plateaux and the sills which traverse them can hesitate in which series -to place the rocks of Ben Hiant. Since, however, these rocks have been -claimed by Professor Judd as the superficial lava-currents of a volcano -which broke out after the time of the plateau-basalts, like the Scuir of -Eigg, some further details in regard to the geological structure of the -district, which would otherwise be superfluous, may here be given.</p> - -<p>The number of sills and dykes in Ardnamurchan is astonishingly great. -There must be hundreds of them visible, and perhaps as many more -concealed under superficial coverings. They are well exposed on the shore -traversing the Jurassic strata and the schists. The sills become especially -large and abundant in the direction of Ben Hiant, which has evidently -been the principal centre from which their materials were injected. -The rocks composing these sills are quite similar to those of Ben -Hiant, save that, as they occur in thinner sheets than in that mountain, -<span class="pagenum" id="Page_321">- 321 -</span> -they do not attain the same coarseness of texture which the -more massive beds there display. They generally possess fine-grained -chilled selvages along their upper and under surfaces.</p> - -<div class="figleft" id="v2fig326" style="width: 238px;"> - <img src="images/v2fig326.png" width="238" height="257" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 326.</span>—Ground-plan of Sills at Ben Hiant, - Ardnamurchan.<br /><br /> - <i>a</i> <i>a</i>, crystalline schists; <i>b</i> <i>b</i>, necks of volcanic agglomerate; - <i>c</i> <i>c</i>, numerous thin sills; D, massive sill of Beinn na - h-Urchrach; E, north side of Ben Hiant; F, sill proceeding - from the series forming Ben Hiant and joining that - of Beinn na h-Urchrach. The arrows mark the dip.</div> -</div> - -<p>These abundant sills may be traced up into the mass of Ben -Hiant from which they have issued, -and of the individual sheets of -which they are a continuation. -One of the most striking and -easily-followed examples of this -connection is to be seen on the -north side of the mountain. A -thick sheet in the middle of Ben -Hiant descends from among its contiguous -sheets and, as a prominent -rib, runs down the scree-slope -into the valley below, where it -forms a prominent feature. Crossing -the streamlet in the middle -of the valley, where a section has -been cut through its upper surface, -it gradually bends round towards -the north-east, mounts the side of -Beinn na h-Urchrach until it -reaches the crest of the ridge and -joins the other sills of which this -eminence is built up. The route of this band of rock will be understood -from the annexed ground-plan (<a href="#v2fig326">Fig. 326</a>).</p> - -<p>That this prolongation of one of the thick beds of Ben Hiant is in no -respect a superficial lava-stream but a true sill, is proved not only by its -escarpment and dip-slope, but by its actually passing under and indurating -the schistose grits, as may be seen in the stream-section. Again Beinn na -h-Urchrach, which is mapped by Professor Judd as a northern expansion -of Ben Hiant, is likewise not a lava but a true sill. Not only does it dip -northwards at an angle of about 20°, having the schists immediately below -its crest on the one side and descending with a long dip-slope on the other, -but dwindling down rapidly from a thickness of 100 or 200 feet in the -centre to no more than a few feet in a south-westerly direction, it there -passes under schistose grits like those on which it lies. The strata that -adhere to its upper surface are as usual indurated.</p> - -<p>A section drawn across this attenuated development of the Beinn na -h-Urchrach sill and that from Ben Hiant shows the structure represented -in the accompanying diagram (<a href="#v2fig327">Fig. 327</a>), which simply gives the facts as -exposed on the ground. The lower sill is that which issues from the main -body of Ben Hiant, massive at first but diminishing in thickness as it -recedes from its source.</p> - -<p>Again, among the sheets which descend from the northern face of the -<span class="pagenum" id="Page_322">- 322 -</span> -summit of Ben Hiant and strike into the Jurassic outlier below, intensely -indurated shale may be seen lying between two of the dolerites, which are -unquestionably sills that have been injected into the Jurassic series.</p> - -<p>The ridge of Ben Hiant is thus found to consist of a thick and complex -series of sills, some of which are not traceable beyond the side of the -mountain, while others can be followed outwards among the surrounding -rocks. The specially marked dyke-like sills diverge from the main mass -and run for some distance north-eastward, one of them, fully a mile long, -descending among the schists into the valley and ascending into the basalt-plateau -on the opposite side.<a id="FNanchor_323" href="#Footnote_323" class="fnanchor">[323]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_323" href="#FNanchor_323" class="label">[323]</a> The sills of Ben Hiant descend on the south-west side into the sea, and can be examined -along the slopes and the beach, where Professor Judd has mapped a continuous platform of -agglomerate. The broad hollow between that mountain and Beinn na h-Urchrach, over which -he has spread his "augite-andesite lavas," appears to be underlain mainly by the crystalline -schists through which sills from Ben Hiant have been injected. The northern eminence, which -he has united with Ben Hiant, is entirely separate and, as above shown, is an obvious sill.</p> - -</div> - -<div class="figcenter" id="v2fig327" style="width: 409px;"> - <img src="images/v2fig327.png" width="409" height="73" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 327.</span>—Section of two Sills in schistose grits, west end of Beinn na h-Urchrach, Ardnamurchan.<br /><br /> - <i>a</i> <i>a</i>, crystalline schists; <i>b</i>, neck of volcanic agglomerate; <i>c</i>, small sill; D, massive sill of Beinn na h-Urchrach; - F, sill proceeding from the series forming Ben Hiant and joining that of Beinn na h-Urchrach.</div> -</div> - -<p>On the south-east side of the mountain where the bedded basalts can -be traced close up to the intrusive dolerites, they are found to present the -usual dull indurated aspect so characteristic of contact alteration among -these rocks. There cannot therefore be any doubt that Ben Hiant never -was itself a volcano. Its rocks are characteristically those of subterranean -intrusions. They seem to have been injected from a line of fissure or from -several such lines, running in a general north-easterly direction, at some -late part of the volcanic period. The group of agglomerate necks of older -date shows that already the ground underneath had been drilled by a -number of distinct volcanic funnels, and discloses a weak part in the -terrestrial crust.</p> - - -<h3> iv. <span class="allsmcap">FAROE ISLES</span></h3> - -<p>In the Faroe Islands the actual base of the volcanic series is nowhere -visible. Hence, the great lower platform of intrusive sheets being there -concealed, this feature of the basalt-plateaux is less conspicuous than it is -in the Inner Hebrides. A number of sills, however, have been noticed by -previous observers,<a id="FNanchor_324" href="#Footnote_324" class="fnanchor">[324]</a> and I have observed others on the sides of Stromö, -Kalsö, Kunö and other islands. In the lofty precipices of the Haraldsfjord, -many of the massive light-coloured prismatic sheets are intrusive, for -though they preserve their parallelism with the bedded sheets for considerable -<span class="pagenum" id="Page_323">- 323 -</span> -distances, they may be seen sometimes to break across these, as is -strikingly shown in one of the great corries on the east side of Kunö.</p> - -<div class="footnote"> - -<p><a id="Footnote_324" href="#FNanchor_324" class="label">[324]</a> See in particular Prof. James Geikie and Mr. Lomas, in the papers already cited on p. 191.</p> - -</div> - -<div class="figright" id="v2fig328" style="width: 320px;"> - <img src="images/v2fig328.png" width="320" height="142" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 328.</span>—Sill traversing bedded Basalts, cliffs of Stromö, at - entrance of Vaagöfjord.<br /><br /> - The caves and notches shown at the bottom of the precipice mark the position - of the vents represented in Figs. <a href="#v2fig311">311</a>, <a href="#v2fig312">312</a>, <a href="#v2fig313">313</a>, <a href="#v2fig314">314</a>.</div> -</div> - -<p>One of the most remarkable sills in the Faroe Islands is probably that -which forms so prominent an object on the western cliffs of Stromö, at the -entrance into the Vaagöfjord -(Figs. <a href="#v2fig328">328</a>, <a href="#v2fig329">329</a>). -It is prismatic in structure, -and where it runs -along the face of the -cliffs, parallel to the -bedded basalts among -which it has been intruded, -presents the -familiar characters of -such sheets. The precipice -of which it forms a -part is that which rises -above the row of volcanic vents already described. But it there begins to -ascend the cliffs obliquely across the basalts until it reaches the crest of the -great wall of volcanic rock at a height of probably about 1000 feet above -the waves. From the crest of the precipice the upward course of the sill -is continued into the interior of the island. It pursues its way as a line -of bold crag along the ridges of the plateau, gradually ascending till it forms -the summit of one of the most prominent hills in the district (<a href="#v2fig329">Fig. 329</a>).</p> - -<p>Some further idea of the enormous energy with which the sills were -injected may be formed from this example, where the eruptive materials -followed neither the line of bedding nor a vertical fissure, but took an -oblique course through the plateau-basalts for a vertical distance of probably -more than 1500 feet.</p> - - -<h3> v. <span class="allsmcap">GENERAL DEDUCTIONS REGARDING THE TERTIARY BASIC SILLS</span></h3> - -<p>If we consider the facts which have now been adduced regarding the -position and structure of the sills, we are led, I think, to regard these masses -as certainly belonging to the history of the basalt-plateaux, but, on the -whole, to a comparatively late part of it. They consist of essentially the -same materials as the lavas that form these plateaux, though with the -differences of structure which the conditions of their production would lead -us naturally to expect. Where they occur in thick masses, which must -obviously have cooled much more slowly at some depth beneath the surface -than the comparatively thin sheets could do that were poured out above -ground, they have assumed a far more largely crystalline texture than that -of the superficial lavas. As a rule, we may say that the thicker the sill the -coarser is its texture, while the thinnest sheets are the most close-grained. -Sills are especially abundant about the base of the basaltic-plateaux. We -may examine miles of the central and higher parts of the escarpments without -<span class="pagenum" id="Page_324">- 324 -</span> -detecting a single example of them, but if the escarpment is cut down to the -base we seldom need to search far to find them.</p> - -<div class="figcenter" id="v2fig329" style="width: 632px;"> - <img src="images/v2fig329.png" width="632" height="144" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 329.</span>—View of the same Sill seen from the channel opposite the island of Kolter.</div> -</div> - -<p>That the efforts of the internal magma to establish an outlet towards -the surface were accompanied by powerful disturbances of the terrestrial -crust is shown by the abundant dykes which -traverse all the volcanic districts from Antrim -to Iceland, and some of which ascend even to -the very highest remaining lavas of the basalt-plateaux. -The parallel fissures filled by these -dykes prove that even after the accumulation of -more than 3000 feet of basalt-sheets, the movements -continued to be so powerful as to disrupt -these vast piles of volcanic material. But undoubtedly -the highest parts of the plateau-basalts -are less cut by dykes than the lower parts. -There would no doubt come a time when the dislocations -would more seldom reach the surface, -when dykes would not be formed so abundantly -or up to such a high level, and when the volcanic -energies would more and more sparingly result in -the opening of new vents or in the discharge of -fresh eruptions from old ones.</p> - -<p>It appears to me most probable that the injection -of the sills was connected with the same -terrestrial disturbances that produced the dykes -which traverse the plateaux. Besides being -dislocated by parallel fissures, the earth's crust -in North-Western Europe seems to have been -ruptured internally along lines more or less at -right angles to the vertical fissures. The deep -accumulation of bedded basalts presented an -increasing obstacle to the ascent of the magma to -the surface. Unable to gain ample enough egress -through such vertical fissures as might be formed -in the volcanic pile, the molten rock would find -its lines of least resistance along the planes of -the strata and the lower basalt-beds, either by -the aid of terrestrial ruptures there, or in virtue -of its own energy. On these horizons, accordingly, -the sills occur in extraordinary profusion throughout -the volcanic regions. They are no doubt -of all ages in the progress of the building up of -the volcanic plateaux, but I am disposed to believe that a large number of -them may belong to the very latest period of the uprise of basalt within the -area of Britain.</p> - -<p>One of the most suggestive features of the abundant Tertiary sills lies in -<span class="pagenum" id="Page_325">- 325 -</span> -the evidence they furnish of the enormous energy concerned in the ascent -and intrusion of volcanic material. The infilling of dykes or the outpouring -of successive streams of lava at the surface hardly appeals to our imagination -so strikingly as the proof that the sills have been impelled into their places -with a vigour which, even when guided and aided by gigantic terrestrial -ruptures, was capable of overcoming the vertical pressure of hundreds, or even -thousands of feet of overlying rock. Had these intrusive sheets been mere -thin layers, their horizontal extent and persistence would still have excited -our astonishment, but when we find them sometimes several hundred feet -thick, and to extend in a continuous series for horizontal distances of 50 -miles or more, we are lost in wonder at the prodigious expansive strength of -the volcanic forces. Again, the intrusions have not always taken place -between the bedding-planes of the stratified or igneous rocks, but, as we have -seen, solid sheets of already deeply buried lavas have sometimes been split -open and the intrusive material has forced itself between the disrupted -portions. Such subterranean proofs of the vigour of volcanic energy teach -some of the most impressive lessons in the chronicles of volcanic action in -the British Isles.</p> - -<hr class="tb" /> - -<p>In closing this history of the accumulation of the great Tertiary volcanic -plateaux of North-Western Europe, I would remark that as the result of -prolonged eruptions from innumerable vents, the depression that extended -from the south of Antrim to the Minch was gradually filled up with -successive sheets of basalt to a depth of more than 3000 feet. A succession -of lava-fields stretched from the North of Ireland across the West of Scotland, -and perhaps even to the Faroe Islands, Iceland and Greenland. That -the lava spread round the base of the Highland mountains and ran up -the Highland glens, much as the sea now does, is made clear from -the position of the outliers of it which have been left perched on the -ridges of Morven and Ardnamurchan. So far as can now be surmised, -these wide Phlegræan fields were only varied by occasional volcanic cones -scattered over their surface, marking some of the last vents from which -streams of basalt had flowed. But the volcanic energy was still far -from exhaustion. After the accumulation of such a deep and far-extended -sheet of lava, those underground movements which produced the fissures -that served as channels for the uprise of the earliest dykes continued -to show their vigour. The pile of bedded lavas was rent open by -innumerable long parallel fissures in the prevalent north-westerly direction, -up which basic lavas rose to form dykes, while vast numbers of sills were -injected underneath. Whether the outflow of basalt at the surface had wholly -ceased when the last of these dykes were injected into the plateaux cannot be -told. Nor is there any evidence whether it had ended before the next great -episode of the volcanic history—the extravasation of the gabbro bosses. All -that we can affirm with certainty is, that the formation of north-west fissures -and the uprise of basalt in them were again repeated, for we find north-west -dykes traversing even the crests of the later eruptive masses of basic and acid -<span class="pagenum" id="Page_326">- 326 -</span> -rocks. It is difficult to suppose that none of these latest dykes communicated -with the surface, and gave rise to cones with the outpouring of -basalt and the ejection of dust and stones. But of such later outflows -of basic material over the surface of the plateaux no undoubted trace -has yet been recognised.</p> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_327">- 327 -</span></p> - -<h2 class="nobreak" id="CHAPTER_XLIII">CHAPTER XLIII<br /> - -<span class="smaller">THE BOSSES AND SHEETS OF GABBRO</span></h2> -</div> - -<div class="blockquot"> - -<p>Petrography of the Rocks—Relations of the Gabbros to the other members of the Volcanic -series—Description of the Gabbro districts—Skye</p> -</div> - - -<p>In singular contrast to the nearly flat basalts of the plateaux, another series -of rocks rises high and abruptly above these tablelands into groups of dome-shaped, -conical, spiry, and rugged hills. It is these heights which, more -than any other feature, relieve the monotony of the wide areas of almost -horizontal stratification so characteristic of the volcanic region of the north-west. -Their geological structure and history are much less obvious than -those of the bedded basalts. Their mountainous forms at once suggest a -wholly different origin. Some portions of them have even been compared -with the oldest or Archæan rocks.<a id="FNanchor_325" href="#Footnote_325" class="fnanchor">[325]</a> That they are really portions of the -Tertiary volcanic series, and that they reveal a wholly distinct phase in the -history of volcanic action, is now frankly admitted. Whether we regard -them from the petrographical or structural point of view, they naturally -arrange themselves into two well-defined groups. Of these one consists of -highly basic compounds, of which olivine-gabbro is the most prominent. -The other comprises numerous varieties—granite, granophyre, felsite, quartz-porphyry, -pitchstone and others—all of them being more or less decidedly -acid, and some of them markedly so. For reasons which will appear in the -sequel, the former group must be considered as the older of the two, and it -will therefore be described first.</p> - -<div class="footnote"> - -<p><a id="Footnote_325" href="#FNanchor_325" class="label">[325]</a> This was my own first impression, when I began, as a boy, to ramble among them. The -remarkable resemblance of some parts of them to ancient gneisses will be afterwards dwelt upon. -Macculloch had correctly grouped them with the other overlying rocks, and this conclusion was -afterwards confirmed by Prof. Zirkel.</p> - -</div> - - -<h3> i. <span class="allsmcap">PETROGRAPHY OF THE GABBRO AREAS</span></h3> - -<p>Since the publications of Macculloch, the occurrence of beautiful -varieties of highly basic rocks among the igneous masses of the Western -Isles has been familiar to geologists. They were named by him "hypersthene -rock" and "augite rock,"<a id="FNanchor_326" href="#Footnote_326" class="fnanchor">[326]</a> names which continued in use until 1871, -<span class="pagenum" id="Page_328">- 328 -</span> -when my friend Professor Zirkel published the results of his tour through -the West of Scotland, and showed that the rocks in question were mostly -true gabbros.<a id="FNanchor_327" href="#Footnote_327" class="fnanchor">[327]</a> Since his observations were published some of these rocks -have formed the subject of important papers by Professor Judd.<a id="FNanchor_328" href="#Footnote_328" class="fnanchor">[328]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_326" href="#FNanchor_326" class="label">[326]</a> <i>Western Islands</i>, vol. i. pp. 385, 484.</p> - -<p><a id="Footnote_327" href="#FNanchor_327" class="label">[327]</a> <i>Zeitschrift. Deutsch. Geol. Gesellsch.</i> xxiii. (1871), p. 1.</p> - -<p><a id="Footnote_328" href="#FNanchor_328" class="label">[328]</a> <i>Quart. Jour. Geol. Soc.</i> xli. (1885), p. 354; xlii. (1886), p. 49.</p> - -</div> - -<p>The general petrographical characters of the gabbro areas of Western -Scotland may be summarized as follows:—A very considerable variety of -petrological structure and chemical composition is observable among the -rocks. At the one end of the series are compounds of plagioclase and -augite, which, though wanting in olivine, have the general structure and -habit of dolerites. At the other end are mixtures wherein felspar is scarce -or absent, and where olivine becomes the chief constituent. Between these -two extremes are many intermediate grades, of which the most important -are those containing the variety of augite known as diallage and also olivine. -These are the olivine-gabbros, which form so marked a feature in the central -parts of the great basic bosses. That some of these varieties of rock pass -into each other cannot be doubted. Their distinctive composition and -structure appear to have been largely determined by their position in the -eruptive mass. The outer and thinner sheets are in great measure dolerites, -with little or no olivine. Coarse gabbros are abundant in the inner portions. -Rocks rich in olivine, however, occur at the outer and especially the lower -part of the gabbro masses of Rum and in some parts of Skye. The following -leading varieties may be enumerated:—</p> - -<p>Dolerite.—This rock varies from an exceedingly close grain (when it -approaches and graduates into basalt) up to a coarse granular crystalline -texture, in which the component minerals are distinctly visible to the naked -eye. An average sample is found to consist of plagioclase, usually lath-shaped, -and crystals or grains of augite with or without olivine. Under the -microscope, the different varieties are distinguished by the presence of more -or less distinct ophitic structure, the felspar being enveloped in the augite. -For the most part they are holocrystalline, but occasionally show traces of -a glassy base. Ilmenite is not infrequent, with its characteristic turbid -decomposition product (leucoxene). In other cases, the iron-ore is probably -magnetite. Between the dolerites and gabbros no line of demarcation can -be drawn in the field, nor can a much more satisfactory limitation be made -even with the aid of the microscope. As a rule, the thickest and largest -intrusive masses or bosses are gabbro, those of less size are dolerite, while -the smallest (and sometimes the edges of the others) assume externally the -aspect of basalts.</p> - -<p>Gabbro.—Under this term I arrange, as proposed by Professor Judd, -all the coarse-grained granitoid basic rocks of the region without reference -to the variety of augite present in them. Under the microscope, they are -found to be holocrystalline, but with a granitic or granulitic rather than an -ophitic structure, though traces of the latter are by no means rare. To the -naked eye their component minerals are usually recognizable. Professor -<span class="pagenum" id="Page_329">- 329 -</span> -Zirkel, from his examination of the Mull gabbros, believed them to consist of -three parts of plagioclase, two parts of olivine, and one part of diallage.<a id="FNanchor_329" href="#Footnote_329" class="fnanchor">[329]</a> -Olivine, however, is not invariably present.<a id="FNanchor_330" href="#Footnote_330" class="fnanchor">[330]</a> The pyroxene also does not -always show the peculiar fibrous structure of diallage. Professor Judd, -indeed, maintains that the diallagic form is due to a deep-seated process of -alteration (schillerization), and that the same crystal may consist partly of -ordinary augite and partly of diallage.<a id="FNanchor_331" href="#Footnote_331" class="fnanchor">[331]</a> Ilmenite (with leucoxene), -magnetite, apatite, biotite, and epidote are not infrequent constituents.</p> - -<div class="footnote"> - -<p><a id="Footnote_329" href="#FNanchor_329" class="label">[329]</a> <i>Zeitschr. Deutsch. Geol. Gesellsch.</i> xxiii. (1871), p. 59.</p> - -<p><a id="Footnote_330" href="#FNanchor_330" class="label">[330]</a> Professor Judd (<i>Quart. Jour. Geol. Soc.</i> xlii. p. 62) believes that originally all the gabbros -contained olivine, and that where it is now absent, it has been altered into magnetite or serpentine. -But in some coarse massive gabbros this mineral does not appear to have been an essential constituent. -See <i>op. cit.</i> vol. l. p. 654.</p> - -<p><a id="Footnote_331" href="#FNanchor_331" class="label">[331]</a> <i>Op. cit.</i> xli. In a later paper he insists on the gradation of the coarse granitoid varieties -(gabbros) into holocrystalline compounds, where the felspar appears in lath-shapes with crystals -or rounded grains of augite and olivine (dolerites), and thence into true basalts, magma-basalts, -and tachylytes (<i>op. cit.</i> xlii. p. 62).</p> - -</div> - -<p>In a recent study of the gabbros of the Cuillin Hills of Skye by Mr. -J. J. H. Teall and myself, four characteristic types have been recognized.<a id="FNanchor_332" href="#Footnote_332" class="fnanchor">[332]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_332" href="#FNanchor_332" class="label">[332]</a> <i>Quart. Journ. Geol. Soc.</i> vol. 1. (1894), pp. 645-659, and Plates xiii. xxvi.-xxviii. See also -Prof. Judd's paper, <i>op. cit.</i> (1886), p. 49.</p> - -</div> - -<p>(1) <i>Granulitic Gabbros.</i>—These are dark, fine-grained rocks which -externally resemble some of the altered basalts of the plateau-series. They -occur in bands or sheets which, so far as can be made out, are the oldest -portions of the whole gabbro mass. Under the microscope they are found -to possess a finely granulitic structure, and to consist of grains of pyroxene -(augite, but more usually with the inclusions characteristic of diallage and -pseudo-hypersthene), and of felspar allied to labradorite, with green pseudomorphs -agreeing in form and size with the pyroxene-grains, but made of -minute prisms and fibres of green hornblende and a little chlorite.</p> - -<p>(2) <i>Banded Gabbros.</i>—These are characterized by a remarkable arrangement -in parallel bands of different mineral composition like the banding of -ancient gneisses. This structure will be more particularly described in later -pages. They are coarse-grained rocks composed of pyroxene, plagioclase, -olivine and magnetite. But these minerals are not distributed equally -through the mass. The pale bands contain much felspar; the dark bands -are largely composed of the ferro-magnesian minerals and magnetite. The -pyroxene, occurring as ordinary augite, not uncommonly shows a tendency -to ophitic structure. The felspar, a variety closely allied to labradorite, -occurs as grains, as irregular ophitic patches, and also in forms that give broad -rectangular sections. Olivine in an unaltered condition has been detected -by Mr. Teall in only one specimen, and he thinks that this mineral probably -never played an important part in the original constitution of these rocks. -Its rounded grains may be observed to have the other minerals moulded round -them, whence it may be inferred to be of older consolidation. Magnetite is -generally present, either in rounded grains or in large irregular masses. -Though it occurs also in strings traversing the other minerals as a secondary -product, it must undoubtedly have entered largely into the original composition -<span class="pagenum" id="Page_330">- 330 -</span> -of these rocks. It is found enclosing the augite grains and -behaving like a groundmass between the felspars. Among the dark bands -there occur narrow lenticular black layers ('schlieren') composed entirely of -augite and iron-ore.</p> - -<p>The extraordinary differences between the composition of the pale -felspathic and the dark ultra-basic bands are well brought out in the -following analyses by Mr. J. Hort Player, No. 1 being from a light-coloured -band consisting mainly of labradorite with some augite, uralitic hornblende -and magnetite; No. 2 from a dark band composed of augite, magnetite and -labradorite; and No. 3 from a thin ultra-basic layer mainly formed of augite -and magnetite. All these specimens were taken from the ridge of Druim -an Eidhne, on the eastern side of the Cuillin Hills, Skye.<a id="FNanchor_333" href="#Footnote_333" class="fnanchor">[333]</a></p> - -<table summary="data"> -<tr> - <td></td> - <td class="tdc">I.</td> - <td rowspan="15"> </td> - <td class="tdc">II.</td> - <td rowspan="15"> </td> - <td class="tdc">III.</td> -</tr> -<tr> - <td class="tdl">Silica</td> - <td class="tdr">52·8</td> - <td class="tdr">40·2</td> - <td class="tdr">29·5</td> -</tr> -<tr> - <td class="tdl">Titanic acid</td> - <td class="tdr">·5</td> - <td class="tdr">4·7</td> - <td class="tdr">9·2</td> -</tr> -<tr> - <td class="tdl">Alumina</td> - <td class="tdr">17·8</td> - <td class="tdr">9·5</td> - <td class="tdr">3·8</td> -</tr> -<tr> - <td class="tdl">Ferric oxide</td> - <td class="tdr">1·2</td> - <td class="tdr">9·7</td> - <td class="tdr">17·8</td> -</tr> -<tr> - <td class="tdl">Ferrous oxide</td> - <td class="tdr">4·8</td> - <td class="tdr">12·2</td> - <td class="tdr">18·2</td> -</tr> -<tr> - <td class="tdl">Ferric sulphide</td> - <td class="tdr">···</td> - <td class="tdr">·4</td> - <td class="tdr">·4</td> -</tr> -<tr> - <td class="tdl">Oxide of manganese</td> - <td class="tdr">···</td> - <td class="tdr">·4</td> - <td class="tdr">·3</td> -</tr> -<tr> - <td class="tdl">Lime</td> - <td class="tdr">12·9</td> - <td class="tdr">13·1</td> - <td class="tdr">10·0</td> -</tr> -<tr> - <td class="tdl">Magnesia</td> - <td class="tdr">4·8</td> - <td class="tdr">8·0</td> - <td class="tdr">8·7</td> -</tr> -<tr> - <td class="tdl">Soda</td> - <td class="tdr">3·0</td> - <td class="tdr">·8</td> - <td class="tdr">·2</td> -</tr> -<tr> - <td class="tdl">Potash</td> - <td class="tdr">·5</td> - <td class="tdr">·2</td> - <td class="tdr">·1</td> -</tr> -<tr> - <td class="tdl">Loss by ignition</td> - <td class="tdr">1·2</td> - <td class="tdr">·5</td> - <td class="tdr">1·0</td> -</tr> -<tr> - <td></td> - <td class="bdt tdr">99·5</td> - <td class="bdt tdr">99·7</td> - <td class="bdt tdr">99·2</td> -</tr> -<tr> - <td class="tdl">Spec. grav.</td> - <td class="bdt bdb2 tdr">2·91</td> - <td class="bdt bdb2 tdr">3·36</td> - <td class="bdt bdb2 tdr">3·87</td> -</tr> -</table> - -<div class="footnote"> - -<p><a id="Footnote_333" href="#FNanchor_333" class="label">[333]</a> <i>Quart. Journ. Geol. Soc.</i> vol. 1. (1894), p. 653. Banded structures have been recognized in -many gabbros of different ages. See the references in this paper; also Mr. W. S. Bayley, -<i>Journ. Geol.</i> Chicago, ii. (1895), p. 814, and vol. iii. p. 1.</p> - -</div> - -<p>(3) <i>Coarse-grained massive Gabbros.</i>—These rocks, so abundant among -the great basic bosses of the Inner Hebrides, are characterized by their -coarse granitic structure, their component crystals being sometimes more -than an inch long. They occur as sheets, veins and irregular masses -traversing the varieties of gabbro already mentioned. They consist of the -same minerals as the banded forms, and indeed are themselves sometimes -banded. They are more uniform in composition than the typical banded -gabbros, though showing also some variation in the relative proportions of -their constituents. The specific gravity of three specimens was found to be -2·82, 2·97, and 3·06.</p> - -<p><span class="pagenum" id="Page_331">- 331 -</span></p> - -<div class="figcenter" id="v2fig330" style="width: 520px;"> - <img src="images/v2fig330.png" width="520" height="695" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 330.</span>—Granulitic and coarsely foliated gabbro traversed by later veins of felspathic gabbro, - Druim an Eidhne, Cuillin Hills, Skye.</div> -</div> - -<p><span class="pagenum" id="Page_332">- 332 -</span></p> - -<p>(4) <i>Pale Gabbros of the Veins.</i>—These occur abundantly as irregular -branching veins, from less than an inch to several yards in width, and cross -all the other varieties (<a href="#v2fig330">Fig. 330</a><a id="FNanchor_334" href="#Footnote_334" class="fnanchor">[334]</a>). Their whiteness on weathered surfaces -makes them conspicuous by contrast with the dark brown or black hue of -the rocks which they traverse, and shows at once that they must be poorer -in bases than these. They are found on microscopic examination to consist -of the same minerals as the more coarsely crystalline gabbros, but with a -much greater abundance of the felspar. They contain also apatite, and -hornblende appears to predominate in them over augite. They are to be -distinguished from the pale veins that form apophyses from the intrusive -granophyres.</p> - -<div class="footnote"> - -<p><a id="Footnote_334" href="#FNanchor_334" class="label">[334]</a> Figs. <a href="#v2fig330">330</a>, <a href="#v2fig336">336</a> and <a href="#v2fig337">337</a> are from photographs taken for the Geological Survey by Mr. R. -Lunn.</p> - -</div> - -<p>Troctolite (Forellenstein).—This beautiful variety of plagioclase-olivine -rock occurs as a conspicuous feature on the east side of the gabbro-area -of the island of Rum. It forms a sill on the side of the mountain Allival, in -which the component minerals are drawn out parallel with the upper and -under surfaces of the bed (<a href="#v2fig341">Fig. 341</a>). So marked is this flow-structure that -hand-specimens might readily be taken at the first glance for ancient schistose -limestone. "The felspathic ingredient (probably labradorite or anorthite) is -white, and its lath-shaped crystals have ranged themselves with their long -axes parallel to the line of flow. The olivine occurs in perfectly fresh grains, -which in hand-specimens have a delicate green tint. Under the microscope -they appear colourless, and are penetrated by the felspar prisms in ophitic -intergrowth. There is a small quantity of a pale brownish augite, which -not only occurs in wedge-shaped portions between the felspars, but also as -a narrow zone round the olivines."<a id="FNanchor_335" href="#Footnote_335" class="fnanchor">[335]</a> Considerable differences are visible in the -development of the flow-structure, and with these there appear to be accompanying -variations in the microscopic structure. Dr. Hatch, to whom I -submitted my specimens, informed me that in one of them, where the flow-structure -is so marked as to give a finely schistose aspect to the rock, "there -is a larger proportion of augite, some of which exhibits a distinct diallagic -striping; the olivine grains show no ophitic structure, but are sometimes -completely embedded in the augite." To this remarkable flow-structure I -shall again refer in connection with the light it throws on the bedded -character of much of the gabbro bosses.</p> - -<div class="footnote"> - -<p><a id="Footnote_335" href="#FNanchor_335" class="label">[335]</a> MS. of Dr. Hatch.</p> - -</div> - -<p>Between the different basic intrusive igneous rocks of the Inner Hebrides, -as Professor Judd has shown, there are many gradations according to the varying -proportions of the chief component minerals. Thus from the olivine-gabbros, -by the diminution or disappearance of the augite we get such rocks -as troctolite; where the plagioclase diminishes or vanishes, we have different -forms of picrite; where the olivine is left out, we come to compounds, like -eucrite; while by the lessening or disappearance of the felspar and augite, -we are led to ultra basic compounds, consisting in greatest part of olivine, -like lherzolite, dunite and serpentine. To some of the features and probable -origin of these chemical and mineralogical diversities in the same great -eruptive mass further reference will be made in later pages.</p> - -<p><span class="pagenum" id="Page_333">- 333 -</span></p> - - -<h3>ii. <span class="allsmcap">RELATIONS OF THE GABBROS TO THE OTHER MEMBERS OF THE -VOLCANIC SERIES</span></h3> - -<p>Various opinions have been expressed regarding the connection between -the amorphous eruptive rocks of the hill-groups and the level basalt-sheets -of the plateaux. Jameson, though he landed at Rudh' an Dunain, in Skye, -where this connection can readily be found, does not seem to have made any -attempt to ascertain it. He noticed that the lower grounds were formed of -basalt, and that the mountains "appeared to be wholly composed of syenite -and hornblende rock, traversed by basalt veins."<a id="FNanchor_336" href="#Footnote_336" class="fnanchor">[336]</a> Macculloch, in many -passages of his <i>Western Islands</i>, alludes to the subject as one which he -knew would interest geologists, but about which he felt that he could give -no satisfactory information, and with characteristic verbiage he refers to the -impossibility of determining boundaries, to the transition from one rock into -another, to the inaccessible nature of the ground, to the almost insuperable -obstacles that impede examination, to the distance from human habitation, -and to the stormy climate,—a formidable list of barriers, in presence of -which he leaves the relative position and age of the rocks unsettled.<a id="FNanchor_337" href="#Footnote_337" class="fnanchor">[337]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_336" href="#FNanchor_336" class="label">[336]</a> <i>Mineralogical Travels</i> (1813), vol. ii. p. 72.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_337" href="#FNanchor_337" class="label">[337]</a> See his <i>Western Islands</i>, vol. i. pp. 368, 374, 385, 386. With much admiration for the -insight and zeal, amounting almost to genius, which Macculloch displayed in his work among -the Western Islands, at a time when, with poor maps and inadequate means of locomotion, -geological surveying was a more difficult task than it is now, I have found it impossible to follow -in his footsteps with his descriptions in hand, and not to wish that for his own fame he had been -content to claim credit only for what he had seen. His actual achievements were enough to -make the reputation of half a dozen good geologists. It was unfortunate that he did not realize -how inexhaustible nature is, how impossible it is for one man to see and understand every fact -even in the little corner of nature which he may claim to have explored. He seems to have had -a morbid fear lest any one should afterwards discover something he had missed; he writes -as if with the object of dissuading men from travelling over his ground, and he indeed tacitly -lays claim to anything they may ascertain by averring that those who may follow him "will -find a great deal that is not here described, although little that has not been examined" (p. -373). Principal Forbes long ago exposed this weak side of Macculloch and his work (<i>Edin. New -Phil. Journ.</i> xl. 1846, p. 82).</p> - -</div> - -<p>Von Oyenhausen and Von Dechen, who wrote so excellent an account of -their visit to Skye, and who traced much of the boundary-line between the -gabbros and the other mountainous eruptive masses ("syenite"), seem to -have made no attempt to work out the connection between the former and -the rest of the volcanic rocks.<a id="FNanchor_338" href="#Footnote_338" class="fnanchor">[338]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_338" href="#FNanchor_338" class="label">[338]</a> Karsten's <i>Archiv</i>, i. p. 99. They frankly admit that "the relation of the hypersthene -rock to the other trap rocks was not ascertained."</p> - -</div> - -<p>J. D. Forbes, in his able sketch of the <i>Topography and Geology of the -Cuchullin Hills</i>, was the first to recognize the superposition of the "hypersthene -rock" upon the "common trap rocks"—that is, the plateau-basalts. -He was disposed to consider the "hypersthene mass as a vast bed, thinning -out both ways, and inclined at a moderate angle towards the S.E."<a id="FNanchor_339" href="#Footnote_339" class="fnanchor">[339]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_339" href="#FNanchor_339" class="label">[339]</a> <i>Edin. New Phil. Journ.</i> xl. (1846), pp. 85, 86.</p> - -</div> - -<p>Professor Judd regarded the bosses of basic and acid rocks that rise -out of the bedded basalts as the basal cores of enormously denuded volcanic -cones. He believed the granitoid rocks to have been first erupted, and -<span class="pagenum" id="Page_334">- 334 -</span> -that after a long interval the basic masses were forced through them, partly -consolidating underneath and partly appearing at the surface as the plateau-basalts.<a id="FNanchor_340" href="#Footnote_340" class="fnanchor">[340]</a> -That the order of appearance of the several rocks has been exactly -the reverse of this supposed sequence was fully established by me in the -year 1888, and has since been amply confirmed.<a id="FNanchor_341" href="#Footnote_341" class="fnanchor">[341]</a> Professor Zirkel recognized -that the gabbros are a dependence of the basalts, that they overlie -them, and that on the naked flanks of the mountains they are regularly -bedded with them.<a id="FNanchor_342" href="#Footnote_342" class="fnanchor">[342]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_340" href="#FNanchor_340" class="label">[340]</a> <i>Quart. Journ. Geol. Soc.</i> xxx. (1874), p. 249.</p> - -<p><a id="Footnote_341" href="#FNanchor_341" class="label">[341]</a> <i>Trans. Roy. Soc. Edin.</i> xxxv. (1888), pp. 122 <i>et seq.</i>; <i>Quart. Journ. Geol. Soc.</i> vol. 1. (1894), -pp. 216, 645; vol. lii. (1896), p. 384, and Mr. Harker, <i>ibid.</i> p. 320.</p> - -<p><a id="Footnote_342" href="#FNanchor_342" class="label">[342]</a> <i>Zeitschrift. Deutsch. Geol. Gesellsch.</i> xxiii. (1871), pp. 58, 92.</p> - -</div> - -<p>Up to the time of the publication of my memoir in 1888 no one had -traced out in more detail the actual boundaries of the several rocks on the -ground, so as to obtain evidence of their true relations to each other as -regards structure and age. Some of the numerous impediments recorded by -Macculloch no doubt retarded the investigation. But, as Forbes so well -pointed out, there is really no serious difficulty in determining the true -structural connection of the amorphous rocks with each other and with the -bedded basalts of the plateaux. I have ascertained them in each of the -districts,<a id="FNanchor_343" href="#Footnote_343" class="fnanchor">[343]</a> and have found that there cannot be the least doubt that the -amorphous bosses, both basic and acid, are younger than the surrounding -bedded basalts, and that the acid protrusions are on the whole younger than -the basic, I shall now proceed to show how these conclusions are established -by the evidence of each of the areas where the several kinds of rock -occur.</p> - -<div class="footnote"> - -<p><a id="Footnote_343" href="#FNanchor_343" class="label">[343]</a> In two of my excursions in Mull, and once in Skye, I was accompanied by my former -colleague Mr. H. M. Cadell, who rendered me great assistance in mapping those regions.</p> - -</div> - - -<h3>iii. <span class="allsmcap">DESCRIPTIONS OF THE SEVERAL GABBRO-DISTRICTS</span></h3> - - -<h3>1. <i>The Gabbro of Skye</i></h3> - -<p>The largest, most picturesque, and to the geologist most important area of -Tertiary gabbro in Britain, is that of Skye (Map. VI.). Though, like every -other portion of the Tertiary volcanic districts, it has suffered enormous denudation, -and has thereby been trenched to the very core, it reveals, more conspicuously -and clearly than can be seen anywhere else, the relation of the gabbro -to the bedded basalts on the one hand, and to the acid protrusions on the other. -Its chief portion is that which rises into the group of the Cuillin Hills, which -for blackness of hue, ruggedness of surface, jaggedness of crest, and general -grimness of aspect, have certainly no rivals within the limits of the British -Isles (<a href="#v2fig331">Fig. 331</a>). It has long been known to extend eastwards into Blath -Bheinn (Blaven) and its immediate northern neighbours. There is, indeed, -no break whatever between the rock of the Cuillins and that of the hills on -the east side of Strath na Creitheach. In Strath More the gabbro is interrupted -by the granitoid mass of the Red Hills. Patches of it, however, -occur further to the east, even as far as the Sound of Scalpa.</p> - -<p><span class="pagenum" id="Page_335">- 335 -</span></p> - -<div class="figcenter" id="v2fig331" style="width: 803px;"> - <img src="images/v2fig331.png" width="803" height="505" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 331.</span>—Scuir na Gillean, Cuillin Hills, shewing the characteristic craggy forms of the Gabbro. (From a photograph by Mr. Abraham, Keswick.)</div> -</div> - -<p><span class="pagenum" id="Page_336">- 336 -</span></p> - -<p>If we throw out of account the invading granitoid rocks, and look upon the whole tract -within which the gabbro occurs as originally one connected area, we find -that it covered an elliptical space measuring about nine miles from south-west -to north-east and six miles from north-west to south-east, and embracing -at least 40 square miles.<a id="FNanchor_344" href="#Footnote_344" class="fnanchor">[344]</a> But that its original size was greater is -strikingly shown more particularly on the western margin, which like -that of the basalt-escarpments, has obviously been determined by denudation, -for its separate beds present their truncated ends to the horizon all -along the flanks of the Cuillins, from the head of Glen Brittle round to -Loch Scavaig (<a href="#v2fig332">Fig. 332</a>), and from Strath na Creitheach round the -southern flanks of Blath Bheinn to Loch Slapin and Strath More.</p> - -<div class="footnote"> - -<p><a id="Footnote_344" href="#FNanchor_344" class="label">[344]</a> Though this and the other bosses are here spoken of as consisting of gabbro, it will be understood -that this rock only constitutes the larger portion of their mass, which includes also -dolerites and other more basic compounds, together with involved portions of the plateau-basalts -and masses of agglomerate which probably mark the position of older vents.</p> - -</div> - -<div class="figcenter" id="v2fig332" style="width: 490px;"> - <img src="images/v2fig332.png" width="490" height="120" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 332.</span>—Section across Glen Brittle, to show the general relations of the Bedded Basalts (<i>a</i>) and the -Gabbros (<i>b</i>).</div> -</div> - - -<p>The first point to be ascertained in regard to the gabbro and associated -basic rocks of the mountainous tract is their connection in geological structure -and age with the bedded basalts of the plateau. This initial and -fundamental relation, as Forbes long ago said, can be examined along the -whole western and southern flank of the Cuillin Hills, from the foot of Glen -Sligachan round to the mouth of Loch Scavaig. Even from a distance, the -observer, who is favoured with clear weather, can readily trace the almost -level sheets of basalt till they dip gently under the darker, more massive -rock of the hills. Tourists, who approach Skye by way of Loch Coruisk, -have an opportunity, as the steamer nears the island of Soay, of following -with the eye the basalt-terraces of the promontory of Rudh' an Dunain -until they disappear under the gabbro of the last spur of the Cuillins that -guards the western entrance to Loch Scavaig.</p> - -<p>What is so evident at a distance becomes still more striking when viewed -from nearer ground. Nowhere can it be more impressively seen than at the -head of Glen Brittle. Looking westwards, the traveller sees in front of him -only the familiar level terraces and green slopes of the basalt-plateau, rising -platform above platform to a height of nearly 1500 feet above the sea. But -turning to the east, he beholds the dark, gloomy, cauldron-like Corry na -Creiche, from which rise some of the ruggedest and loftiest crests of the -Cuillins. On the hills that project from either side of this recess and half -<span class="pagenum" id="Page_337">- 337 -</span> -enclose it, the bedded basalts mount from the bottom of the valley, with -their lines of parallel terrace dipping gently inward below the black rugged -gabbro that crowns them and sweeps round to form the back or head of the -corry. Down the whole length of Glen Brittle the same structure conspicuously -governs the topographical features. On the right hand, the -ordinary terraced basalts form the slopes; and they rise for some 500 or -600 feet up the eastern side, until they pass under the darker, more rugged, -and less distinctly bedded rocks of the mountains (<a href="#v2fig332">Fig. 332</a>). The dip of -the whole series is here at a gentle angle towards south-east, that is, into or -under the main mass of the Cuillin group.</p> - -<p>When, however, we proceed to examine the junction between the two -rocks we find it to be less simple than it appears. It is not an instance of -mere superposition. The gabbro unquestionably overlies the basalts, and is -therefore of younger date. But it overlies them, not as they rest on each -other, in regular conformable sequence of eruption, but intrusively, as a sill -does upon the rocks on which it appears to follow in the unbroken order of -accumulation. This important structure may be ascertained in almost any -of the many sections cut by the torrents which have so deeply trenched -with gullies the flanks of the hills. Starting from the ordinary bedded -basalts, we observe, in mounting the slopes and approaching the gabbro, that -the rocks insensibly assume that indurated shattery character, which has -been referred to as characteristic of them round the margins of vents, and -which will be shown to be not less so in contact with large eruptive masses -of basic or acid rock.<a id="FNanchor_345" href="#Footnote_345" class="fnanchor">[345]</a> Beds of dolerite make their appearance among the -basalts, so distinctly crystalline, and so similar in character to the rocks of -the sills, that there can be little hesitation in regarding them as intrusive. -These sills increase in size and number as we ascend, though hardened -amygdaloidal basalts may still be observed. True gabbros then supervene -in massive beds, and at last we find ourselves entirely within the gabbro -area, where, however, thin bands of highly altered basalt may still for some -distance appear. One further fact will generally be noticed, viz. that before -reaching the main mass of gabbro, veins and sills of basalt, as well as of -various felsitic and porphyritic members of the acid group, come in abundantly, -crossing and recrossing each other in the most intricate network. -The base of the thick gabbro-sheets is thus another horizon on which, as on -that below the plateau-basalts, intrusive masses have been especially developed. -Through all these rocks numerous parallel basalt-dykes, running -in a general persistent N.N.W. direction, with a later N.E. series, rise from -below the sea-level up even to the very crests of the Cuillins (<a href="#v2fig333">Fig. 333</a>).</p> - -<div class="footnote"> - -<p><a id="Footnote_345" href="#FNanchor_345" class="label">[345]</a> This indurated, altered character of the bedded basalts near the intrusive bosses and sills -will be more particularly described in a later chapter in connection with the granophyre intrusions -(see <a href="#Page_386">p. 386</a>). The metamorphism induced by the basic rocks has generally been less -pronounced than that effected by the acid masses.</p> - -</div> - -<p>The sections on the western side of the gabbro area of Skye thus prove -that this rock inosculates with the bedded basalts by sending into them, -between their bedding planes, sheets which vary in texture from fine dolerites -at the outside into coarse gabbros further towards the central mass, and -that this intrusion has been accompanied by a certain amount of induration -of the older rocks.</p> - -<p><span class="pagenum" id="Page_338">- 338 -</span></p> - -<div class="figcenter" id="v2fig333" style="width: 482px;"> - <img src="images/v2fig333.png" width="482" height="798" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 333.</span>—View of the crest of the Cuillin Hills, showing the weathering of the gabbro along its joints, - and of a compound basic dyke which rises through it. (From a photograph by Mr. Abraham, - Keswick.)</div> -</div> - -<p><span class="pagenum" id="Page_339">- 339 -</span></p> - -<p>On the eastern side, the same structure can be even more distinctly -seen, for it is not only exposed in gullies and steep declivities, but can be -traced outward into the basalt-plateau. In the promontory of Strathaird, -Jurassic sandstones and shales, which form the coast-line and lower -grounds, are surmounted by the bedded basalts. Denudation has cut -the plateau into two parts. The smaller of these makes the outlier that -rises into Ben Meabost (1128 feet). The larger stretches continuously -from Glen Scaladal and Strathaird House northward into Blath Bheinn. -Hence from the ordinary terraced basalts, with their amygdaloids, thin tuffs, -red partings, and seams of lignite, every step can be followed into the huge -gabbro mountain. Starting from the black Jurassic shales on which the -lowest basalt lies, we walk over the successive terraces up into the projecting -ridge of An da Bheinn. But as we ascend, sheets of dolerite and gabbro -make their appearance between the basalts, which gradually assume the -altered aspect already noticed. The dip of the whole series is at a low -angle northwards, and the beds can be followed round the head of the Glen -nan Leac into the southern slopes of Blath Bheinn. Seen from the eastern -side of this valley, the bedded character of that mountain is remarkably -distinct, but it becomes less marked towards the upper part of the ridge -where the gabbros preponderate. One of the most striking features of the -locality is the number and persistence of the dykes, which strike across -from the ordinary unaltered basalts of the plateau up into the highest -gabbros of the range. Where less durable than the intractable gabbro, they -have weathered out on the face of the precipices, thereby causing the vertical -rifts and gashes and the deep notches on the crest that form so marked a -feature in the scenery. On the other hand, they are often less destructible -than the plateau-basalts, and hence in the Glen nan Leac they may be seen -projecting as low dams across the stream which throws itself over them in -picturesque waterfalls. The youngest dykes in the Blath Bheinn group of -hills, have been found by Mr. Harker to have a north-easterly trend, and -a north-westerly hade of about 40°, and to give a stratified appearance to -the gabbro when viewed from a distance.</p> - -<p>The deep dark hollow of the Coire Uaigneich has been cut out of the -very core of Blath Bheinn, and lays bare the structure of the east part of -the mountain in the most impressive as well as instructive way (<a href="#v2fig334">Fig. 334</a>). -By ascending into this recess from Loch Slapin, we pass over the whole -series of rocks, and can examine them in an almost continuous section in -the bed of the stream and on the bare rocky slopes on either side. Sandstones -and shales of the Jurassic series extend up the Allt na Dunaiche for -nearly a mile, much veined with basalt and quartz-porphyry, by which the -sandstones are locally indurated into quartzite. At last these strata are -overlapped by the basalts of the Strathaird plateau, which with a marked -inclination to N.N.W., here dip towards the mountains. But by the time -<span class="pagenum" id="Page_340">- 340 -</span> -these rocks have reached the valley, they have already lost their usual -brown colour and crumbling surfaces, and have assumed the indurated -splintery character, though still showing their amygdaloidal structure. They -are much traversed by felsitic veins and strings which proceed from a -broad band of fine-grained hornblende-granite that runs up the bottom of the -Coire Uaigneich and, ascending the col, crosses it south-westwards into the -Glen nan Leac. On the left or south-eastern side of this intrusive mass, a -portion of Lias shales and limestone (here and there altered into white -marble) is traceable for several hundred yards up the stream.<a id="FNanchor_346" href="#Footnote_346" class="fnanchor">[346]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_346" href="#FNanchor_346" class="label">[346]</a> This limestone was formerly identified by me with the Cambrian strata of the district. It -was noticed by Von Oyenhausen and Von Dechen, who, as Mr. Harker has recently ascertained, -correctly believed it to be a portion of the Lias torn off and carried upward by the eruptive -rocks (Karsten's <i>Archiv</i>, i. p. 79).</p> - -</div> - -<p>The bedded basalts of Strathaird, after dipping down towards the N.N.W., -bend up where they are interbanded with dolerites and gabbros, and form -the prominence called An Stac, which rises as the eastern boundary of the -Coire Uaigneich. Their steep dip away from the mountain is well seen -from the east side, and their outward inclination is continued into the ridge -to the southward. Similar rocks appear on the other flank of the band of -granite, and form the base of Blath Bheinn. They are likewise continued -in the mountains further north called Sgurr nan Each and Belig, where -they dip in a northerly direction away from Blath Bheinn, which seems -to be the centre of uprise, with the gabbro-sheets dipping away from -it. The bedded basalts have been traced by Mr. Harker up to a height of -well over 2000 feet on the Blath Bheinn range. They are of the usual -altered, indurated, and splintery character. The intrusive sheets interposed -between them become thicker and more abundant higher up, until they -constitute the main mass of the mountain. But that they are in separate -sheets, and not in one amorphous mass, can be recognized by the parallel -lines that mark their boundaries. The junction of the gabbro sills and -the lavas is a very irregular one, portions of the latter rocks being enveloped -in the intrusive sills.</p> - -<p>The granite which sends out veins into the surrounding rocks is -obviously the youngest protrusion of the locality, except of course the -basalt-dykes which cross it, and which are nowhere seen in a more imposing -display than round the flanks of Blath Bheinn. A section across the corry -shows the structure represented in <a href="#v2fig334">Fig. 334</a>.</p> - -<p>It is thus demonstrable that when its line of junction with the surrounding -plateau-basalts is traced in some detail, the gabbro is found to -overlie them as a whole, but also to be intercalated with them in innumerable -beds, bands, or veins which rapidly die out as they recede outwards -from the main central mass; that these interposed beds are intrusive sheets -or sills from that mass which have cut off and enveloped portions of the -basalts, and that the contiguous bedded basalts show more or less marked -metamorphism.</p> - -<p>We have now to consider the structure of the interior of the gabbro -<span class="pagenum" id="Page_341">- 341 -</span> -area of the Cuillin Hills. The first impression of the geologist who visits -that wild district is that the main mass of rock is as thoroughly amorphous -as a core of granite. Yet a little further examination will reveal to him -many varieties of texture, sometimes graduating into, sometimes sharply -marked off from, each other, and suggesting that the rock is not the -product of one single protrusion. He will notice further indications of -successive discharges or extravasations of crystalline material during probably -a protracted period of time, and in the intricate network of veins -crossing each other and the general body of the rock in every direction, as -well as in the system of basalt-dykes that traverse all the other rocks, he -will recognize the completion of the evidence of repeated renewals of subterranean -energy.</p> - -<div class="figcenter" id="v2fig334" style="width: 494px;"> - <img src="images/v2fig334.png" width="494" height="215" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 334.</span>—Section across the Coire Uaigneich, Skye.<br /><br /> - <i>a</i>, <i>b</i>, Jurassic sandstones and shales; <i>c</i> <i>c</i>, bedded basalts and dolerites; <i>d</i> <i>d</i>, gabbros and dolerites with indurated - basalts; <i>e</i>, fine-grained hornblende-granite sending veins into surrounding rocks; <i>f</i> <i>f</i>, basalt-dykes running - through all the other rocks.</div> -</div> - - -<p>But the observer will be struck with the absence of the more usual -proofs of volcanic activity in such forms as vesicular lavas and abundant -masses of slag, bombs and tuffs, which are commonly associated with the -idea of the centre of a volcanic orifice, though he will meet with isolated -masses of coarse volcanic agglomerate within the gabbro area and along -some parts of its junction with the granophyre. The general characters of -the rocks around him suggest that he stands, as it were, far beneath that -upper part of the earth's crust which is familiar to us in the phenomena of -modern volcanoes; that he has been admitted into the heart of one of the -deeper layers, where he can study the operations that go on at the very -roots of an active vent.</p> - -<p>When the geologist begins a more leisurely and systematic examination -of the interior of the gabbro area of Skye he soon sees reason to modify the -impression he may at first have received that this rugged region presents -the characters of one single eruptive mass. The more he climbs among -the hills the more will he meet with evidence of long-continued and oft-repeated -extravasation, one portion having solidified before another broke -<span class="pagenum" id="Page_342">- 342 -</span> -through it, and both having been subsequently disrupted by still later -protrusions.</p> - -<p>But if by chance he should begin his examination of the ground upon -some of the more typically banded varieties of rock, he may for a time -almost refuse to admit that these can be either of volcanic origin or of -Tertiary age.<a id="FNanchor_347" href="#Footnote_347" class="fnanchor">[347]</a> He will find among them such startling counterparts of the -structure of the ancient Lewisian gneiss of the North-West of Scotland that -he may well be pardoned if for a time he seeks for evidence that they really -do belong to that primeval formation, and have only been accidentally -involved among the Tertiary volcanic rocks. If, for instance, he should land -in Loch Scavaig, and first set foot upon the gabbros as they appear around -Loch Coruisk, he would find himself upon masses of grey coarsely crystalline, -rudely banded rock, like much of the old gneiss of Sutherland and Ross. -Ascending over the ice-worn domes, he would notice that the banding -becomes here and there more definitely marked by strong differences in -texture and colour, while elsewhere it disappears and is replaced by a granitoid -arrangement of the crystals, which are often as large as walnuts.</p> - -<div class="footnote"> - -<p><a id="Footnote_347" href="#FNanchor_347" class="label">[347]</a> See <i>Quart. Journ. Geol. Soc.</i> vol. l. pp. 217, 657, and a paper by the author, "Sur la -Structure rubannée des plus anciens Gneiss et des Gabbros Tertiaires," <i>Compt. rend. Cong. Géol. -Internat.</i> 1894, p. 139.</p> - -</div> - -<div class="figcenter" id="v2fig335" style="width: 426px;"> - <img src="images/v2fig335.png" width="426" height="137" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 335.</span>—Banded and puckered gabbro, Druim an Eidhne, Glen Sligachan, Skye.</div> -</div> - -<p>Nowhere is the gneissoid banding more beautifully developed than on -the east side of the Cuillin group near the head of Glen Sligachan along the -ridge of Druim an Eidhne. It was at this locality that the four typical -structures were observed which have already been referred to (p. 329). The -varieties of colour and composition depend upon the exceedingly irregular -distribution of the component minerals. The paler bands, rich in felspar, -lie parallel with dark brown bands full of pyroxene, olivine and magnetite, -in which, moreover, thin ribs of glistening black consist in large part of the -iron ore. These layers vary in thickness from mere pasteboard-like laminæ -to beds a yard or more in thickness. Within a space of a few square yards -their parallelism reminds one of stratified deposits (<a href="#v2fig336">Fig. 336</a>), but traced -over a wider space they are found to be more or less irregular in thickness -and lenticular in form.</p> - -<p><span class="pagenum" id="Page_343">- 343 -</span></p> - -<div class="figcenter" id="v2fig336" style="width: 672px;"> - <img src="images/v2fig336.png" width="672" height="514" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 336.</span>—Banded structure in the Gabbro, from the ridge of Druim an Eidhne between Loch Coruisk and Glen Sligachan.</div> -</div> - -<p><span class="pagenum" id="Page_344">- 344 -</span></p> - -<p>The resemblance to gneisses, and sometimes to the flow-structure of -coarse rhyolites, is still further sustained by occasional undulations or minute -puckerings (<a href="#v2fig335">Fig. 335</a>). Still more extraordinary are the examples of the -actual plication of a group of successive bands, as shown in <a href="#v2fig337">Fig. 337</a>, -wherein such a group about ten feet thick is shown to have been -doubly folded between parallel bands above and below. This structure -is not due to any deformation of the gabbro long subsequent to the -consolidation of the mass. It belongs to the phenomena of protrusion -and solidification. An examination of thin slices of these rocks under the -microscope reveals no evidence of crushing. On the contrary, the minerals -of one band interlock with those of the band adjoining, in such a manner as -to prove that the differences of composition cannot be due to crushing and -shearing or to successive intrusion, but must have been present before the -final consolidation of the whole rock.<a id="FNanchor_348" href="#Footnote_348" class="fnanchor">[348]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_348" href="#FNanchor_348" class="label">[348]</a> Mr. J. J. H. Teall and A. G., <i>Quart. Journ. Geol. Soc.</i> vol. 1. (1894), p. 652.</p> - -</div> - -<p>The conclusion which seems most consonant with the facts is that the -magma which supplied the visible masses of gabbro in Skye existed below -in a heterogeneous condition, that portions of it, differing considerably from -each other in composition, were simultaneously intruded, and that by the -deformation of these portions during their intrusion their present plicated -structures were produced. A careful study of these banded gabbros offers -many suggestive points of comparison with the gneisses and anorthosite -(Norian) rocks of pre-Cambrian age. It seems in the highest degree probable -that the banded structures and peculiar mineral aggregation in these -ancient rocks arose under conditions closely analogous to, if not identical -with, those in which the Tertiary gabbros of Skye originated.<a id="FNanchor_349" href="#Footnote_349" class="fnanchor">[349]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_349" href="#FNanchor_349" class="label">[349]</a> Consult the Memoirs cited in the footnote on p. 342.</p> - -</div> - -<p>Similar structures are found to be widely developed through the -gabbros of the Cuillin Hills. Not only are these rocks disposed in distinct -beds, but many of the beds display the most perfect banding. Thus the -mountains that surround the head of Loch Scavaig and sweep round Loch -Coruisk up to the great splintered crests of Sgurr na Banachdich display -on their bare black crags a distinct bedded structure. On the east side of -Loch Scavaig the rock presents a rudely-banded character, the bands or -beds being piled over each other from the sea-level up to the summits of the -rugged precipices, and dipping into the hill at angles of 25° to 35°. Abundant -dykes and veins of various basic, intermediate and acid rocks cut this -structure. The individual layers here show sometimes the wavy and -puckered condition already referred to.</p> - -<p>Even from a distance the alternating lighter and darker bands can -readily be seen, so that this structure, with the variations in its inclination, can -be followed from hill to hill (<a href="#v2fig338">Fig. 338</a>). The regularity of the arrangement, -however, is often less pronounced on closer inspection. While the gabbro is -rudely disposed in thick beds, indicative of different intrusive sheets or sills, -with which the banding is generally parallel, considerable irregularities may -be observed in the arrangement of the structure of individual sheets. These -sheets may be parallel to each other, and yet, while in some the banding is -tolerably regular in the direction of the planes of the sheets, in others it is -much twisted or inclined at various angles.</p> - -<p><span class="pagenum" id="Page_345">- 345 -</span></p> - -<div class="figcenter" id="v2fig337" style="width: 683px;"> - <img src="images/v2fig337.png" width="683" height="487" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 337.</span>—Banded and doubly-folded Gabbro, Druim an Eidhne, 10 feet broad.</div> -</div> - -<p><span class="pagenum" id="Page_346">- 346 -</span></p> - -<p>On the west side of the Coruisk river the banding is vertical; southward -from that stream it inclines slightly towards the south, but soon again -becomes vertical, and continues conspicuously so at the junction of the -gabbro with the Torridon sandstones and plateau-basalts on the west side of -Loch Scavaig.</p> - -<p>Thus, instead of being one great eruptive boss, the gabbro of this -district is in reality an exceedingly complicated network of sills, veins and -dykes. While the general inclination of the bedding sometimes continues -uniform in direction and amount from one ridge to another, it is apt to -change rapidly, as if the complex assemblage of intruded masses had been -disrupted and had subsided in different directions. For example, after overlying -the bedded basalts of the plateau all the way from Glen Brittle to the -west side of Loch Scavaig, the gabbro descends abruptly across these basalts -and also across the Torridon sandstones, on which they unconformably rest. -These two groups of rocks are not only truncated by the gabbro, but are -traversed by the intricate system of sills, dykes and veins already referred to. -Where it abuts against the sandstones and basalts in Loch Scavaig, the gabbro -is arranged in vertical bands of different mineral composition and texture. -Much of it is remarkably coarse, some bands displaying pyroxene crystals -more than an inch in length. There is no fine-grained selvage here, indicative -of more rapid cooling. So coarse, indeed, is the rock close up against -the sandstone, that the junction-line can hardly be supposed to be the normal -contact of the intrusive rock. This inference is confirmed by the existence -of a singular kind of breccia between the gabbro and the sandstones. It is -a tumultuous mass of fragments of coarse and fine gabbro, Torridon sandstone -and shale, and plateau-basalts, embedded in a pale crystalline matrix -of fine granular granophyre; veins from this acid intrusion run off into the -gabbro on the one side as well as into the Torridon sandstones on the other. -It would seem that this junction-line has been one of great movement, that -the gabbro-sheets have subsided against a fault-wall of plateau-basalt and -Torridon sandstone, and that subsequently an intrusion of finely granular -granophyre has come up the fissure, involving in its ascent fragments of all -the materials around.</p> - -<p>The rocks for a considerable distance to the south of the gabbro are -intensely altered. The Torridon sandstone has been so indurated as to pass into -a bleached white quartzite, while the shales interstratified with it have been -converted into a kind of porcellanite. But the most interesting alterations -are those to be observed in the plateau-basalts, which at a height of about -300 feet above the sea, are to be seen in nearly horizontal sheets that lie -immediately on the upturned edges of the Torridon sandstones. These lavas -have suffered great metamorphism, to which more particular reference will -be made in Chapter xlvi. in connection with the action of the granophyre. -Whether this alteration has been produced by the intrusion of the gabbro or -of some concealed mass of granophyre underneath, of which only projecting -dykes and veins reach the surface, must remain a matter of doubt. On the -whole, as the gabbro is here undoubtedly thrown against the basalts and -<span class="pagenum" id="Page_347">- 347 -</span> -Torridon sandstone by a fault, it seems most probable that the change has -been mainly due to the influence of the acid rock.</p> - -<p>In the Blath Bheinn group of hills the relations of the gabbro to the -bedded basalts have recently been mapped in detail by Mr. Harker during -the progress of the Geological Survey of Skye. He has observed that, allowing -for irregularities of form, the mass of gabbro obliquely overlies the basalts -as a great sheet, not necessarily due to a single intrusion, which dips towards -the west. He has found the rock to vary from a coarse gabbro to a diabasic -type, and to vary also in mineralogical constitution, becoming in places very -rich in olivine, though the banded structure is here only exceptionally -developed. North of Garbh Bheinn the gabbro is much crushed and the -outlying patch to the north of Belig is in part a crush-breccia. Mr. -Harker remarks that similar brecciated structures are common among the -granophyres of the Red Hills, and that it is sometimes difficult to distinguish -their structure from that of the true volcanic agglomerates.</p> - -<div class="figcenter" id="v2fig338" style="width: 469px;"> - <img src="images/v2fig338.png" width="469" height="311" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 338.</span>—Sketch of Banded Structure in the Gabbros of the hills at the head of Loch Scavaig.</div> -</div> - -<p>Besides the main area of gabbro in Skye, a great many small detached -bosses, sills and dykes lie further east on the flanks of the Red Hills. One -of the best marked of these detached areas forms a conspicuous crag on the -east side of Strath More, immediately to the north of Beinn na Cro. It -consists of beds of coarse gabbro, with others of dolerite intercalated in an -outlier of the plateau-basalts, and is traversed by veins from the granophyre of -the glen, as well as by the usual north-west basalt dykes (<a href="#v2fig349">Fig. 349</a>). It appears -to be a marginal portion of the main gabbro area separated by the intrusion -of the great granitoid boss of the Red Hills. On the north-eastern side of -<span class="pagenum" id="Page_348">- 348 -</span> -Beinn na Caillich numerous intrusive sheets of gabbro and dolerite traverse -the quartzite and limestone, and extend down to the sea-margin in the Sound -of Scalpa.</p> - -<p>There is an important feature in the main gabbro area of Skye not yet -clearly understood, and which only a minute and patient survey can elucidate. -Though I have found among the Cuillin Hills no distinct proof that -the mass of gabbro ever gave rise to discharges of material, either lava-form -or fragmentary, which reached the surface, the gabbro area, as already -remarked, contains unquestionable evidence of explosions and the production -of pyroclastic masses. Among the moraine-mounds of Harta Corry, -blocks of basalt-agglomerate are strewn about, full of angular fragments of -altered basalt, sometimes highly amygdaloidal, and also boulders in which -lumps of coarse gabbro are enveloped in a matrix of finer material. I -did not find the parent rocks from which these glacier-borne masses had -been derived, but there can be no doubt that they exist among the gabbro -crags that surround that deep glen. Reference has already been made -to the similar rock found <i>in situ</i> on the opposite side of the Cuillin ridge -at the head of the great cauldron of Corry na Creich; likewise to the mass -of coarse agglomerate which forms a group of knolls and crags on the east -side of Druim an Eidhne above the head of Glen Sligachan. This rock -contains abundant blocks of various slaggy lavas like those of the basalt-plateau, -and runs for some distance along the eastern limit of the gabbro, -between that rock and the granophyre. It is intersected by numerous -basalt-veins. Mr. Harker, as above mentioned, has recently found some -considerable strips of agglomerate which, like that which I traced round -the west side of Beinn Dearg, are interposed between the gabbro and the -bosses of granophyre, or lie at the base of the volcanic series (p. 284).</p> - -<p>There does not, however, appear to be any evidence to connect these -isolated masses of agglomerate with the phenomena attending the uprise of -the gabbro. They seem to be more probably related to the plateau eruptions, -and may be compared with those of Strath, Ardnamurchan and Mull -(pp. 278, 280, 384). That the huge gabbro mass of Skye, besides invading -and altering the bedded basalts, may have communicated eventually with the -surface, and have given rise to superficial discharges, is not at all improbable, -but of any such outflows not a vestige appears now to remain. We must -remember, however, that the gabbro no doubt in many places found its -readiest upward ascent in vents belonging to the plateau-period, and that -portions of the agglomerates of these earlier vents may be expected to be -found involved in it, as the agglomerate of the great vent of Strath has -been invaded by the granophyre.</p> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_349">- 349 -</span></p> - -<h2 class="nobreak" id="CHAPTER_XLIV">CHAPTER XLIV</h2> -</div> - -<div class="blockquot"> - -<p>THE BOSSES AND SHEETS OF GABBRO IN THE DISTRICTS OF RUM, ARDNAMURCHAN, -MULL, ST. KILDA AND NORTH-EAST IRELAND. HISTORY OF THE -GABBRO INTRUSIONS</p> -</div> - - -<h3>2. <i>The Island of Rum</i></h3> - -<p>The mountains of the island of Rum, rising as they do from a wide expanse -of open sea, present one of the most prominent and picturesque outlines in -the West Highlands (Map VI.). More inaccessible than most of the other -parts of the volcanic region, they have been less visited by geologists. They -were described by Macculloch as composed of varieties of "augite rock." He -noticed in this rock "a tendency to the same obscurely bedded disposition -as is observed in other rocks of the trap family," and found at one place that -it assumed "a regularly bedded form, being disposed in thin horizontal -strata, among which are interposed equally thin beds of a rock resembling -basalt in its general characters."<a id="FNanchor_350" href="#Footnote_350" class="fnanchor">[350]</a> Professor Judd repeats Macculloch's -observation, that "the great masses of gabbro in Rum often exhibit that -pseudo-stratification so often observed in igneous rocks." He regards these -masses, like those of Skye and Mull, as representing the core of a volcano -from which the superficial discharges have been entirely removed, and he -gives a section of the island in which the gabbro is represented as an amorphous -boss sending veins into a surrounding mass of granite.<a id="FNanchor_351" href="#Footnote_351" class="fnanchor">[351]</a> In a subsequent -paper he gave an excellent detailed account of the mineralogical -composition of some of the remarkably varied and beautiful basic rocks -constituting the hills of Rum, but added no further information regarding -the geological structure of the island.<a id="FNanchor_352" href="#Footnote_352" class="fnanchor">[352]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_350" href="#FNanchor_350" class="label">[350]</a> <i>Western Islands</i>, i. p. 486.</p> - -<p><a id="Footnote_351" href="#FNanchor_351" class="label">[351]</a> <i>Quart. Journ. Geol. Soc.</i> xxx. p. 253.</p> - -<p><a id="Footnote_352" href="#FNanchor_352" class="label">[352]</a> <i>Op. cit.</i> xli. (1885) p. 354. See also his paper in vol. xlii. of the same Journal.</p> - -</div> - -<p>Even from a distance of eight or ten miles, the hills of Rum are seen to -be obviously built up of successive nearly horizontal tiers of rock. As the -summer tourist is carried past the island, in that wonderful moving panorama -revealed to him by the "swift steamer" of modern days, these great -dark cones remind him of colossal pyramids, and as the ever-varying lights -and shadows reveal more prominently the alternate nearly level bars of crag -and stripes of slope, the resemblance to architectural forms stamps these hills -with an individuality which strikes his imagination and fixes itself in his -memory. If choice or chance should give him a nearer view of the scene, he -<span class="pagenum" id="Page_350">- 350 -</span> -would not fail to notice that it is among the northern hills of the island -that the bedded character is so conspicuous, and that it ceases to be prominent -in the southern heights, though here and -there, as in the upper part of Scuir na Gillean, it -may in certain lights be detected even from a -distance. Crossing over from Eigg, he would recognize -each of the features represented in the -sketch reproduced in <a href="#v2fig339">Fig. 339</a>. Along the shore, -red sandstones rise in naked cliffs, from the top -of which the ground slopes upward in brown -moors to the bare rocky declivities. A deep -valley (Glen Dibidil) is seen to run into the heart -of the hills, between the bedded group to the north -and the structureless group to the south. If the -weather is favourable, some eight or more prominent -parallel bars of rock may be counted on the -two higher cones to the right. These bars are -not quite level, but slope gently from right to -left. They remind one of the terraced basalts of -the plateaux, but present a massiveness and a -breadth of intervening bare talus-slope such as are -not usual among those rocks.</p> - -<div class="figcenter" id="v2fig339" style="width: 696px;"> - <img src="images/v2fig339.png" width="696" height="132" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 339.</span>—Outline of the Hills of the Island of Rum, sketched from near the Isle of Eigg.</div> -</div> - -<p>Nor is this impression of regularity and -bedded arrangement lessened when we actually -climb the slopes of the hills. I had for years been -familiar with the outlines of Rum as seen from -a distance, and had sketched them from every side, -but I shall never forget the surprise and pleasure -when my first ascent of the cones revealed to me -the meaning of these parallel tiers of rock. I -found it to be the structure of the Cuillin Hills -repeated, but with some minor differences which -are of interest, inasmuch as they enlarge our -conceptions of the process by which the gabbro-bosses -were formed.</p> - -<p>The northern half of the island of Rum -consists almost entirely of red sandstone, which, -as already stated, is a continuation of the same -formation (Torridonian) so well developed in the -south-east of Skye, Applecross and Loch Torridon, -and traceable between the Archæan gneiss and the -Cambrian strata up as far as Cape Wrath. The -sandstones, though full of false bedding, show quite -distinctly their true stratification, which is inclined -with singular persistence towards W.N.W., at angles averaging from 15° to -20°. If they are not repeated by folds or faults, they must reach in this -<span class="pagenum" id="Page_351">- 351 -</span> -island a thickness of some 10,000 feet. Their red or rather pinkish tint -seems mainly to arise from the pink felspar so abundant in them, for in -many places they really consist of a kind of arkose. Pebbly bands with -rounded pieces of quartz are of common occurrence throughout the whole -formation. Dykes and veins of basalt are profusely abundant. Sometimes -these run with the bedding, and might at a distance be taken for dark layers -among the pink sandstones. They often also strike obliquely up the face of -the cliffs like ribbons.</p> - -<p>But, notwithstanding their apparent continuity, there can be no doubt -that these sandstones have suffered from those powerful terrestrial disturbances -which have affected all the older rocks of the North-West Highlands. -On the west side, where they plunge steeply into the sea, they have undergone -a change into fine laminated rocks, which might at first be mistaken -for shales, but which owe their fissility to shearing movements. Along their -southern border, from a point on the east coast near Bagh-na-h-Uamha, south -of Loch Scresort, to the head of Kilmory Glen, they are abruptly truncated -against a group of dark, flaggy and fissile schists and fine quartzites or grits, -which in some places are black and massive like basalt, and in others are -associated with coarse grey gneiss. That some of these rocks are portions of -the Lewisian series can hardly be doubted, and their structure and relations -are probably repetitions of those between the Lewisian gneiss and Torridon -sandstone of Sleat in Skye. I found also on the northern slopes of -Glen Dibidil a patch of much altered grey and white limestone or marble, -which reminded me of the Cambrian limestone of Skye. The red sandstones -in a more or less altered condition are prolonged to the south-east promontory -of the island.</p> - -<p>In passing over the zone of these more ancient rocks, we find them to -present increasing signs of alteration as they are traced up the slopes towards -the great central mass of erupted material. The pink sandstones gradually -lose their characteristic tint, and grow much harder and more compact, while -the veins and dykes of basalt and sheets of dolerite intersecting them increase -in number. The zone of black compact quartzite, which lies to the south of -the sandstones, and which at one point reminds us of basalt, at another of -the flinty slate of the schistose series, likewise displays increasing induration. -Its bedding, not always to be detected, is often vertical and crumpled. But -the most remarkable point in its structure is the intercalation in it of bands -of breccia. These vary from less than an inch to several yards in diameter; -they run mostly with the bedding, but occasionally across it. The stones -in them are fragments of the surrounding rock embedded in a matrix of the -same material, but also with pieces of a somewhat coarser grit or quartzite. -A band of coarse breccia forms the southern limit of this zone along the -northern base of Barkeval and Allival. In general character it resembles -the thinner seams of the same material just referred to. The matrix so -closely agrees with the black flinty quartzite, that but for the included stones -it could hardly be distinguished; so greatly has the mass been indurated -that the stones seem to shade off into the rest of the rock. But here and -<span class="pagenum" id="Page_352">- 352 -</span> -there its true brecciated nature is conspicuously revealed by prominent -blocks of hardened sandstone. This band of breccia must in some places be -150 or 200 feet broad. It has no distinct bedding, but seems to lie as a -highly inclined bed dipping into the hill. It may possibly be a crush-breccia -belonging to a period earlier than the volcanic eruptions. It is at once -succeeded by a black flinty felsite like that of Mull. The groundmass of -this rock, so thickly powdered with magnetite grains as to be almost opaque -under the microscope, displays good flow-structure round the turbid crystals -of orthoclase and the clear granules of quartz. Further up the hill, the -rock becomes lighter in colour and less flinty in texture—a change which is -found to arise from more complete devitrification, the groundmass having -become a crystalline granular aggregate of quartz and felspar with scattered -porphyritic crystals of these minerals (microgranite). In some places, the -felsite incloses fragments of other rocks. A specimen of this kind, taken -from the head of Coire Dubh, shows under the microscope a brown micro-felsitic -groundmass, with crystals of felspar and augite, inclosing a piece -of basalt, composed of fine laths of plagioclase, abundant magnetite and a -smaller proportion of granules of augite.</p> - -<div class="figcenter" id="v2fig340" style="width: 496px;"> - <img src="images/v2fig340.png" width="496" height="369" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 340.</span>—View of Allival, Rum, sketched from the base of the north-east side of the cone.</div> -</div> - -<p>This band of felsite and microgranite may be traced continuously from -Loch Gainmich along the base of Barkeval and Allival, and similar rocks -appear at intervals on the same line round the eastern base of the hills. -<span class="pagenum" id="Page_353">- 353 -</span> -Immediately above this belt of felsitic protrusions comes the great body of -gabbro. It will be observed that here, as in Skye, the base of the gabbro -mass presents a horizon on which injections of acid rocks have been particularly -abundant. Whether the breccias be regarded as the result of earlier -rock-crushings, or as due to volcanic explosions during the Tertiary period, -they are evidently older than the eruption of the gabbros. In that respect -they may be compared with the agglomerates through which the youngest -eruptive bosses of Skye have made their way; but their component materials -have been derived from the surrounding platform of ancient rocks, and not -from subterranean lavas.</p> - -<div class="figcenter" id="v2fig341" style="width: 425px;"> - <img src="images/v2fig341.png" width="425" height="138" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 341.</span>—Section of foliated gabbros in the Tertiary volcanic series of Allival, Rum.<br /><br /> - <i>a</i>, massive gabbro with rude lamination parallel to bedding, only seen in some weathered surfaces; <i>b</i>, laminated - troctolite; <i>c</i>, massive coarsely crystalline gabbro rudely laminated.</div> -</div> - -<p>For my present purpose, however, the chief point of importance is the -structure of the gabbro mass that springs from that platform into the great -conical hills of Rum. The accompanying sketch (<a href="#v2fig340">Fig. 340</a>) will convey a -better idea of this structure than a mere description. At the base, -immediately above the felsite just referred to, bedded dolerites make their -appearance, much intersected with veins from the siliceous rock. Veins and -dykes of basalt also cut all the rocks here, the newest being those which run -in a north-west direction. The lowest sheets of dolerite are succeeded by -overlying sills of coarser dolerites, gabbros, troctolites, etc., which are as regular -in their thickness and continuity as the ordinary basalts of the plateaux. The -band of light-coloured troctolite, in particular (<a href="#v2fig341">Fig. 341</a>), about 20 to 30 feet -thick, which has been already referred to for its remarkable laminar structure, -can be followed for some distance along the base of the hill as a -marked projecting escarpment. This rock at once arrests attention by its -platy or fissile structure, parallel to the bedding-surfaces of the sheet. -Indeed hand-specimens of it, as I have said, might readily pass for pieces of -schistose limestone, especially if taken from the upper part. It consists of -successive layers, which on the weathered surface divide it into beds almost -as regular as those of a flagstone, each bed being further separated into -laminæ marked off by the darker and lighter tints of their mineral constituents. -The darker layers consist of olivine, and the lighter of plagioclase. -This segregation here and there takes the form of rounded masses, where the -minerals are more indefinitely gathered together. The affinity of the -rock with intrusive sheets is further displayed by the occurrence of abundant -<span class="pagenum" id="Page_354">- 354 -</span> -nut-like aggregates of pale green olivine. Examined under the microscope, -flow-structure is admirably seen, the lath-shaped felspars being drawn out -parallel to the planes of movement, and giving thereby the peculiarly -schistose structure which is so deceptive.</p> - -<p>The massive and coarsely crystalline gabbros below and above this -troctolite are all more or less affected by the same laminar structure. Some -of those in higher parts of the mountain are quite massive in part, but also -include bands of lamination. Banding like that of the Skye gabbros is -generally developed among them, the individual bands varying from less than -an inch to a foot or more in thickness. This structure, like the lamination, -is parallel to the general bedding of the sheets. As in the Cuillin Hills, the -bands differ from each other in the relative proportions of the constituent -minerals, especially the predominant pyroxene and olivine. The crystals or -crystalline aggregates are often from a quarter of an inch to an inch in -diameter, and in these large forms are crowded together in certain bands. -Magnetite, on the whole, is rather less conspicuous than in the Cuillin gabbro: -at least, it is not so prominently aggregated in special layers. In one or -two instances I have observed curvature of the banding, but no example so -striking as that cited from the Cuillin area (<a href="#v2fig337">Fig. 337</a>).</p> - -<p>On weathered surfaces, where the felspars decay into a creamy white and -the ferro-magnesian minerals assume tints of green, brown and red, the -resemblance of the rocks to schists is striking. This external likeness is -combined with a tendency to split into thin plates parallel to the lamination, -which still further increases their schistose appearance. Though less -developed than in Skye, the banding appears to be of the same kind and -origin; but in Rum it is combined with the remarkable lamination above -mentioned, produced by the arrangement of the component minerals with -their longer axes parallel to the planes of bedding, as in flow-structure—a -combination which I have not yet observed in Skye.</p> - -<p>The bedded arrangement of the gabbros of Rum, so conspicuous in the -great eastern cones (Figs. <a href="#v2fig339">339</a> and <a href="#v2fig340">340</a>), is emphasized by the fact that some -sheets, of a more durable kind, stand out boldly as prominent ribs, while -the softer crumble into a kind of sand, which forms talus-slopes between the -others. Alternations of this nature are continued up to the very top of the -mountains. The beds are nearly flat, but dip slightly into the interior or -towards the south-west. On the west side of the island also, beyond Loch -Sgathaig, a distinct bedding may be traced, the inclination being here once -more inwards or to the east. But from Glen Harris and the base of Askival -this structure becomes less marked, and gradually disappears. There is thus -a central or southern more amorphous region, while round the margin towards -the north and east the rock appears in frequent alternating beds.</p> - -<p>It is clear that in the broad features of their architecture the hills of -Rum follow closely the plan shown in the Cuillin Hills of Skye. But, unfortunately, -in the former island denudation has gone so far that no connection -can be traced on the ground between the gabbros and the plateau-basalts. -As already stated, the latter rocks have been almost entirely -<span class="pagenum" id="Page_355">- 355 -</span> -stripped off from the platform of sandstones and schists which they undoubtedly -at one time covered, and the few outliers of them that remain lie -at some little distance from the margin of the gabbro area (<i>ante</i>, p. 216). -Nevertheless, we are not without some indications of them underneath the -gabbros. I have alluded to the basalts that lie at the base of the eastern -cones. As we follow the bottom of the gabbro southward round the flanks -of the hills, dull compact black shattery basalts, with a white crust, appear -from under the more crystalline sheets. These at once remind one of the -altered basalts of Skye and Mull. On the west side also, beds of basalt -emerge from under the gabbro, but they have been so veined and indurated -by the granophyre of that district, that their relations to the gabbro are -somewhat obscured. If we could restore the lost portions of the plateau, I -believe we should find the gabbros of Rum resting on part of the volcanic -plateau, and some of the gabbro-beds prolonged as sills between the sheets -of basalt.</p> - - -<h3>3. <i>The Gabbro of Ardnamurchan</i></h3> - -<p>The promontory of Ardnamurchan reveals as clearly as the flanks of -the Cuillin Hills, though in a less imposing way, the relations of the gabbros -to the plateau-basalts (Map VI.). From the southern shore at Kilchoan to the -northern shore at Kilmory, bedded basalts, of the usual type, amygdaloidal -and compact, weathering into brown soil, may be followed along the eastern -slopes of the hills, resting upon the schists and Jurassic series of western -Argyleshire. These rocks are a continuation of those that cap the ridges -further to the south-east and cross Loch Sunart into Morven. They dip -westwards, and followed upwards in that direction, they soon present -the usual marks of alteration. They weather with a white crust and -become indurated and splintery. Sheets of dolerite with many veins and -dykes of basalt run between and across them. Bands of gabbro make their -appearance, and these, as we advance westwards, increase in number and in -coarseness of grain until this rock, in its rudely bedded form, constitutes -practically the whole of the promontory from Meall nan Con to the light-house. -Many admirable sections may be seen on the coast-cliffs and in the -rugged interior, showing the irregular bedding of the gabbro, and how prone -this rock is to develop its component minerals in bands or ribbons, sometimes -made up of large crystals, as in Skye, Rum and Mull.</p> - - -<h3>4. <i>The Gabbro of Mull</i></h3> - -<p>In the island of Mull, the conclusions to which the geology of the other -volcanic districts leads us as to the position of the gabbros in the series of -volcanic phenomena, are further confirmed. The first geologist who appears -to have observed the relation of these rocks in that island was Jameson, who -classed them under the old name of "greenstone," including in the same designation -rocks now termed dolerites and gabbros. He ascended one of the -<span class="pagenum" id="Page_356">- 356 -</span> -hills above Loch Don, probably Mainnir nam Fiadh (2483 feet), which he -found to consist of "strata of basalt and greenstone," with some basalt-breccia -or tuff and a capping of basalt. He speaks of the "singular scorified-like -aspect" of the weathered greenstone—a description which applies to some -of the coarser gabbro bands of that locality. But he appears to have -recognized the general bedded arrangement of the rocks up even to the -summit of the hill.<a id="FNanchor_353" href="#Footnote_353" class="fnanchor">[353]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_353" href="#FNanchor_353" class="label">[353]</a> <i>Mineralogy of the Scottish Isles</i> i. p. 205.</p> - -</div> - -<p>It was not, however, until the visit of Professor Zirkel in 1868, that the -true petrographical characters of the gabbro of Mull were recognized. This -observer remarked that the rock is regularly interstratified with the -basalt.<a id="FNanchor_354" href="#Footnote_354" class="fnanchor">[354]</a> Professor Judd, as already stated, has supposed the gabbros to -be the deep-seated portion of the masses which when poured out at the surface -became the plateau-basalts, and he represents them in his map and sections -of Mull as ramifying through the granitic rocks.<a id="FNanchor_355" href="#Footnote_355" class="fnanchor">[355]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_354" href="#FNanchor_354" class="label">[354]</a> <i>Zeitsch. Deutsch. Geol. Gesellsch.</i> xxiii. (1871) p. 58.</p> - -<p><a id="Footnote_355" href="#FNanchor_355" class="label">[355]</a> <i>Quart. Jour. Geol. Soc.</i> xxx. (1874).</p> - -</div> - -<p>In Mull the disposition of the gabbro in beds, sheets or sills is well -displayed, for there is here no great central complicated mass of interlacing -banded and amorphous sheets. We have seen that a higher group of -plateau-basalts has survived in this island better than in the other plateaux, -and it would seem that denudation has not yet succeeded here in cutting -down so deeply into the gabbro core as in Skye, Rum and Ardnamurchan. -Only the upper or outer fringe of intrusive sheets among the bedded basalts -has been laid bare. The district within which this fringe may be observed -is tolerably well-defined by the difference of contour between the long terraced -uplands of the ordinary basalts and the more conical forms of the southern -group of gabbro hills between Loch na Keal and Loch Spelve. The number -and thickness of the gabbro-sheets increase as we proceed inwards from the -basalt-plateau. These sheets are specially prominent along the higher parts -of the ridge that runs northwards from the northern end of Loch Spelve, and -along the west side of Glen Forsa. But they swell out into the thickest -mass in the south-western part of the hilly ground, where, from above Craig, -in Glenmore, they cross that valley, and form the rugged ridge that rises -into Ben Buy (2354 feet), and stretches eastward to near Ardara (Map VI.). -It is in this southern mass that the Mull gabbro approaches nearest in general -characters to that of Skye. But even here its true intercalation above a -great mass of bedded basalt may readily be ascertained in any of the numerous -ravines and rocky declivities.</p> - -<p>One of the best lines of section for exhibiting the relations of the rocks is -the declivity to the west of Ben Buy and Loch Fhuaran. Ascending from the -west side, we walk over successive low escarpments and terraces of the plateau-basalts -with a gentle inclination towards north-east or east. These rocks -weather in the usual way, some into a brown loam, others into spheroidal exfoliating -masses. But as we advance uphill they gradually assume the peculiar -indurated shattery character already referred to. The soft earthy amygdaloids -become dull splintery rocks, in which the amygdales are no longer sharply -<span class="pagenum" id="Page_357">- 357 -</span> -defined from the matrix, but rather seem to shade off into it, sometimes with -a border of interlacing fibres of epidote. The compact basalts have undergone -less change, but they too have become indurated, and generally assume -a white or grey crust, and none of them weather out into columnar forms. -Strings and threads full of epidote run through much of these altered rocks. -Abundant granophyric and felsitic veins traverse them. Sheets of dolerite -likewise make their appearance between the basalts, followed further up the -slope by sheets of gabbro until the latter form the main body of the hill.</p> - -<p>On the north side of the same ridge similar evidence is obtainable, -though somewhat complicated by the injections of granophyric and felsitic -veins and bosses, to which more detailed reference will afterwards be made. -But the altered basalts with their amygdaloidal bands and their intercalated -basalt-tuffs and breccias, can be followed from the bottom of the glen up to -a height of some 1700 feet, above which the main gabbro mass of Ben Buy -sets in. Many minor sheets of dolerite and gabbro make their appearance -along the side of the hill before the chief overlying body of the rock is -reached. Some of these can be distinctly seen breaking across or ending off -between the bedded basalts which here dip gently into the hill (<a href="#v2fig342">Fig. 342</a>). -A conspicuous band of coarse basalt-agglomerate, containing blocks of compact -and amygdaloidal basalt a yard or more in diameter, shows by the -excessive induration of its dull-green matrix the general alteration which -the rocks of the basalt-plateau have here undergone. An almost incredible -number of veins of fine basalt, porphyry and felsite has been injected into -these rocks—a structure which is precisely a counterpart of what occurs -under the main body of gabbro in Skye, Ardnamurchan and Rum.</p> - -<div class="figright" id="v2fig342" style="width: 308px;"> - <img src="images/v2fig342.png" width="308" height="178" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 342.</span>—Altered Plateau-Basalts invaded by Gabbro, and with - a Dyke of prismatic Basalt cutting both rocks, north slope of - Ben Buy, Mull.<br /><br /> - <i>a</i> <i>a</i>, amygdaloidal basalt, much altered; <i>b</i>, gabbro; <i>c</i>, finely prismatic - basalt.</div> -</div> - - -<p>The gabbro mass of the Ben Buy ridge is thus undoubtedly a huge overlying -sheet, which probably -reaches a thickness -of at least 800 feet. It -seems to descend rather -across the bedding into -the hollow of Glen More, -and possibly its main pipe -of supply lay in that -direction. Being enormously -thicker than any -other sheet in the island, -it exhibits the crystalline -peculiarities which are so -well developed in the -central portions of the -larger bosses of gabbro. It presents more coarsely crystalline varieties than -appear in the thinner sheets, some portions showing crystals of diallage and -felspar upwards of an inch in length. It likewise contains admirable examples -of banded structure, which, as in Skye and elsewhere, is best developed where -the texture becomes especially coarse. Veins or bands, in which the constituent -<span class="pagenum" id="Page_358">- 358 -</span> -minerals have crystallized out in more definite and conspicuous -forms, here and there succeed each other so quickly as to impart a bedded or -foliated look to the body of rock, recalling, as in Skye, the aspect of some -coarsely crystalline granitoid gneiss. In these respects the Mull gabbro -closely resembles that of the Cuillin Hills. Occasionally, on the exposed -faces of crags, portions of such bands or veins are seen to be detached and -enveloped in a finer surrounding matrix. The thick belts or bands of -coarser and finer texture alternate, and give an appearance of bedding to the -mass. Nevertheless they are really intrusive sills, which run generally -parallel with beds of finer gabbro or with sheets of highly indurated basalt, -that may be detached portions of the ordinary rocks of the plateau. The -thick sheet of Ben Buy, like the mass of the Cuillin Hills, is thus the result -not of one but of many uprises of gabbro.</p> - -<p>Of the thinner sheets of dolerite and gabbro in Mull little need here be -said. I have referred to their great abundance in the range of eastern hills -that rise from the Sound of Mull between Loch Spelve and Fishnish Bay. -Though obviously intrusive, they lie on the whole parallel to the bedding of -the basalts. The latter rocks exhibit the usual dull indurated shattery -character which they assume where large bosses of gabbro have invaded -them, and which gradually disappears as we follow them down hill away -from the intrusive sheets to the shores of the Sound. They dip towards -the centre of the hill group, that is, to south-west in the ridge of Mainnir nam -Fiadh, Dùn da Ghaoithe, and Beinn Meadhon, the angle increasing southwards -to 15°-20°, and at the south end reaching as much as 35°-40°. Some -fine crags of gabbro and dolerite form a prominent spur on the east side of -the ridge of Ben Talaidh, in the upper part of Glen Forsa. These consist -of successive sheets bedded with the basalts, and dipping south-west. A large -sheet stands out conspicuously on the north front of Ben More, lying at the -base of the "pale lavas," and immediately above the ordinary basalts. It -circles round the fine corry between Ben More and A'Chioch, some of its -domes being there beautifully ice-worn. This is the highest platform to -which I have satisfactorily traced any of the intrusive sheets of Mull. -Another dyke-like mass emerges from beneath the talus slopes of A'Chioch, -on the southern side, and runs eastward across the col between the Clachaig -Glen and Loch Scridain.</p> - - -<h3>5. <i>The Gabbros of St. Kilda and North-east Ireland</i></h3> - -<p>Sixty miles to the westward of the Outer Hebrides lies the lonely group -of islets of which St. Kilda is the chief. As the main feature of geological -interest in this group is the relation of the acid protrusions to the other -rocks, the account of the geology will be more appropriately given as a whole -in Chapter xlvii. I need only remark here that the predominant rocks of -these islands are dark basic masses, chiefly varieties of gabbro, but including -also dolerites and basalts. Reasons will be afterwards brought forward for -regarding these rocks as parts of the Tertiary volcanic series. They present -<span class="pagenum" id="Page_359">- 359 -</span> -a close parallel to the gabbros and associated rocks of Skye. But in one -important respect they stand alone. No certain trace remains of any -basalt-plateau at St. Kilda such as those through which the gabbros of Skye, -Mull and Ardnamurchan have been injected. In regard to their mode of production -they have doubtless been intruded at some considerable depth beneath -the surface. But no relic appears to have survived of the overlying cover of -rock under which they consolidated, and into which they were injected.</p> - -<p>In the remarkable volcanic district of the north-east of Ireland a series -of basic rocks appears, which in its mode of occurrence and its relation to -the other members of the series presents many points of resemblance to the -gabbros of the Inner Hebrides. The Irish gabbros are well developed in the -Carlingford district, where they form intrusive bosses and sheets which have -been erupted through the Palæozoic rocks (Map VII.). They are themselves -pierced by later masses of granophyre and other acid rocks. Further reference -will be made to these gabbros in later pages, where an account will be -given of the granite masses of Mourne, Barnavave and Slieve Gullion.</p> - -<hr class="tb" /> - -<p>It is interesting to observe that, while in St. Kilda no relic of any -basaltic plateau has been preserved, in the Faroe Islands, on the other hand, -no sign has been revealed by denudation that the volcanic plateau of that -region is pierced by any eruptive core of gabbro or of granophyre. During -my cruises round these islands and through their channels, I was ever on -the outlook for any difference in topography that might indicate the presence -of some eruptive boss like the gabbro and granophyre masses of the Inner -Hebrides. But nothing of that nature could be discerned. Everywhere the -long level lines of the bedded basalts were seen mounting up to the crests of -the ridges and the tops of the highest peaks. Though I cannot assert that -no intrusions of gabbro or of granophyre exist among the Faroe Islands, I -feel confident that any such masses which may appear at the surface must -be of quite insignificant dimensions, and do not make the important feature -in geology and topography which they do among the Inner Hebrides. It -is, of course, possible that, vast as the denudation of these islands has -undoubtedly been, it has not yet trenched the plateau deeply enough to -expose any great intrusive bosses and sills which may underlie and invade -the basalts.</p> - - -<h3>iv. <span class="allsmcap">HISTORY OF THE GABBRO INTRUSIONS</span></h3> - -<p>We are now in a position to draw, from the observations which have -been given in this and the preceding chapter regarding the different areas -of gabbro in the Tertiary volcanic region of Britain, some general conclusions -with respect to the type of geological structure and the phases of volcanic -energy which they illustrate.</p> - -<p>1. No evidence exists to show that the masses of gabbro ever communicated -directly with the surface. They never exhibit the cellular, slaggy -and other structures so characteristic of surface-flows. They are, on the -whole, free from included pyroclastic material, though masses of agglomerate -<span class="pagenum" id="Page_360">- 360 -</span> -are enclosed in, and have probably been invaded by, the gabbro of the Cuillin -Hills. If the gabbro-bosses ever were continuous with sheets of rock -emitted above ground, all such upward continuations have been entirely -removed. In any case, we may be quite certain that in an outburst at the -surface, the rock would not have appeared in the form of a coarsely crystalline -or granitoid gabbro.</p> - -<p>2. The crystalline structures of the gabbros point unmistakably to -slow cooling and consolidation at some depth beneath the surface. -The most coarsely-crystalline varieties, and those with the best developed -banded structure, occur in the largest bodies of rock, where the cooling and -consolidation would be most prolonged.<a id="FNanchor_356" href="#Footnote_356" class="fnanchor">[356]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_356" href="#FNanchor_356" class="label">[356]</a> On this subject, see the papers by Professor Judd already cited.</p> - -</div> - -<p>3. The remarkable differences in composition between the dark and pale -layers in the banded gabbros cannot be accounted for by segregation or -successive intrusion, but seem to point to the existence of a heterogeneous -magma from which these distinct varieties of material were simultaneously -intruded.</p> - -<p>4. From the prevalence of a bedded structure and the occurrence of -bands and more irregular portions of considerably different texture and even -mineralogical composition which intersect each other, it may be confidently -inferred that even what appears now as one continuous mass was produced -by more than one intrusion.</p> - -<p>5. In every case there would necessarily be one or more pipes up which -the igneous material rose. These channels might sometimes be wider -parts of fissures, such as those filled by the dykes. In other places, -they may have been determined by older vents, which had served -for the emission of the plateau-basalts and their pyroclastic accompaniments. -There can be no doubt that some of these vents afforded -egress for the subsequent eruption of granitoid rocks, as will be pointed -out in the following chapters. In the case of the gabbros, however, -the position of the vents seems to have been generally concealed by the -tendency of these rocks to spread out laterally. Denudation has cut deeply -into the gabbro-masses, but apparently not deep enough to isolate any of the -pipes from the larger bodies of material which issued from them, and thus -to leave solitary necks like those in and around the basalt-plateaux. In -Skye, where the central core of gabbro is largest and most completely -encircled, we cannot tell how much of it lies above the true pipe or pipes, -and has spread out on all sides from the centre of eruption. The prevalence -of rude bedding and a banded structure indicate that most of the visible -rock occurs in the form of sills, successively injected not only into the -plateau-basalts, but between and across each other. Round the margin of -the gabbro we undoubtedly reach horizons below that rock, and see that it -lies as a cake or series of cakes upon the plateau-basalts. The actual pipe -or fissure of supply must in each case lie further inward, away from the -margin, and may be of comparatively small diameter.</p> - -<p>6. From the central pipe or group of pipes or fissures which rose from the -<span class="pagenum" id="Page_361">- 361 -</span> -platform of older rocks into the thick mass of the basalt-plateaux, successive -sheets of dolerite and gabbro were forced outward between the layers of basalt. -This took place all round the orifices of supply, on many different horizons, and -doubtless at many different times. In some cases, the intrusive sheets were -injected into the very bottom of the basalts, and even between these rocks -and the older surface on which they rested. This is particularly the case -in Rum, where the gabbro-cones spring almost directly from the ancient -grits, schists and sandstones on which they rest. The intrusive sheets have -likewise found egress at every higher platform in the basalt-series, up at -least to the base of the "pale group" in Mull—that is, through a continuous -pile of more than 2000 feet of bedded basalt. But the intrusion did not -proceed equally all round an orifice. At all events, the progress of denudation -has revealed that on one side of a gabbro area the injected portions -may occur on a lower stratigraphical level than they do on the opposite side. -At the Cuillin Hills, for example, the visible sheets of dolerite and gabbro -to the north of Coire na Creiche begin about 1600 feet above the sea, which -must be much more than that distance above the bottom of the basalts. On -the south-east side, however, they come down to near the base of the basalts -at Loch Scavaig; that is to say, their lowest members lie at least 1600 -feet below those on the opposite margin.</p> - -<p>7. The uprise of so much igneous material in one or more funnels, -and its injection between the beds of plateau-basalt, would necessarily -elevate the surface of the ground immediately above, even if we believe that -surface to have been eventually disrupted and superficial discharges to have -been established. If no disruption took place, then the ground would probably -be upraised into a smooth dome, the older lavas being bent up over -the cone of injected gabbro until the portion of the plateau so pushed upward -had risen some hundreds of feet above the surrounding country. The amount -of elevation, which would of course be greatest at the centre of the dome, -might be far from equable all round, one side being pushed up further or -with a steeper slope than another side. But even in the case of the Cuillin -Hill area, it is conceivable that the total uplift produced at the surface a -gentle inclination of no more than 8° or 10°.</p> - -<p>It is along the periphery of a gabbro area that we may most hopefully -search for traces of this uplift. But unfortunately it is just there that the -work of denudation has been most destructive. There appears also to have -been a general tendency to sagging subsequent to the gabbro protrusions, -and the inward dip thereby produced has probably been instrumental in -effacing at least the more gentle outward inclinations caused by the uprise -of the eruptive rock. In one striking locality, however, to which I have -already referred, the effects of both movements are, I think, preserved. The -basalt-plateau of Strathaird, which in its southern portion exhibits the -ordinary nearly level bedding, dips in its northern part at an unusually -steep angle to the north-west, towards the gabbro mass of Blath Bheinn. But -before reaching that mountain the basalts, much interbanded with sheets of -dolerite and gabbro, suddenly bend up to form the prominent eminence of An -<span class="pagenum" id="Page_362">- 362 -</span> -Stac, where they dip rapidly towards south-east and south (<a href="#v2fig334">Fig. 334</a>). This -steep dip away from the central mass of gabbro, is repeated in the hills to the -north, where the beds are inclined to north-east, the angle gradually lessening -northwards till they are truncated by the granophyre of Strathmore. The -mass of Blath Bheinn thus occupies the centre of the dome or anticline. The -theoretical structure of one of the gabbro bosses is represented in <a href="#v2fig343">Fig. 343</a>. -It will be understood, however, that what for the sake of clearness is here -represented in one uniform tint of black in reality consists of an exceedingly -complex network of sheets and dykes differing from each other in texture -and structure, as well as in the relative dates of their intrusion.</p> - -<p>8. The injection of so much igneous material among the bedded basalts -has induced in these rocks a certain amount of contact metamorphism. I -have referred to it as showing itself in the field as a marked induration, the -rocks becoming closer grained, dull, splintery, and weathering, with a grey -or white crust, while their amygdales lose their definite outlines, and epidote -and calcite run in strings, veins and patches through many parts of the -rocks. As already remarked, it is difficult to determine how much of this -change should be referred to the influence of the gabbro, and how much to -that of the numerous intrusions of granophyre which may be apophyses -of much larger bodies of that rock lying not far underneath. On account of -this difficulty, the more detailed description of the metamorphism of the -plateau-basalts is reserved for Chapter xlvi., where it will find a place in connection -with the effects produced by the intruded granophyres, which have -undoubtedly been more extensive than those effected by the gabbros.</p> - -<div class="figcenter" id="v2fig343" style="width: 495px;"> - <img src="images/v2fig343.png" width="495" height="210" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 343.</span>—Theoretical representation of the structure of one of the Gabbro Bosses of the Inner - Hebrides.<br /><br /> - <i>a</i> <i>a</i>, platform of older rock on which the bedded basalts (<i>b</i> <i>b</i>) have been poured out; <i>c</i>, gabbro.</div> -</div> - -<p>The structure and history of the gabbro bosses of the Inner Hebrides -find a close parallel in those of the Henry Mountains of Southern Utah, so -well described by Mr. G. K. Gilbert of the United States Geological Survey.<a id="FNanchor_357" href="#Footnote_357" class="fnanchor">[357]</a> -In that fine group of mountains, rising to an extreme height of 5000 feet -above the surrounding plateau, and 11,000 feet above the level of the sea, -<span class="pagenum" id="Page_363">- 363 -</span> -masses of trachyte have been injected between sedimentary strata belonging -to the Jura-Triassic and Cretaceous systems. These masses, thirty-six in -number, have consolidated in dome-shaped bodies, termed by Mr. Gilbert -"laccolites," which have arched up the overlying strata, sending sheets, veins -and dykes into them, and producing in them the phenomena of contact -metamorphism. There is no proof that any of these protrusions communicated -with the surface, and there is positive evidence that most if not all of -them did not. The progress of denudation has laid bare the inner structure -of this remarkable type of hill, and yet has left records of every stage -in its sculpture. In one place are seen only arching strata, the process -of erosion not having yet cut down through the dome of stratified rocks -into the trachyte that was the cause of their uprise. In another place, a -few dykes pierce the arch; in a third, where a greater depth has been bared -away, a network of dykes and sheets is revealed; in a fourth, the surface -of the underlying "laccolite" is exposed; in a fifth, the laccolite, long -uncovered, has been carved into picturesque contours by the weather, and -its original form is more or less destroyed.<a id="FNanchor_358" href="#Footnote_358" class="fnanchor">[358]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_357" href="#FNanchor_357" class="label">[357]</a> See the remarks and diagram, <i>ante</i>, p. 86.</p> - -<p><a id="Footnote_358" href="#FNanchor_358" class="label">[358]</a> "Geology of the Henry Mountains," by Mr. G. K. Gilbert, <i>U.S. Geographical and Geological -Survey of the Rocky Mountain Region</i>, 1877.</p> - -</div> - -<p>The gabbro "laccolites" of the West of Scotland belong to an older -geological period than those of Utah, and have, therefore, been longer subject -to the processes of denudation. They have been enormously eroded. The -overlying cover of basalt has been stripped off from them, though from the -escarpments beyond them it is not difficult in imagination to restore it. In -Rum it has been so completely removed, that only a few fragments remain -at some distance from the core of gabbro, which now stands isolated. In -Ardnamurchan, and still more in Skye, the surrounding plateau of basalt -remains in contact with the gabbro bosses. But in Mull, where the plateau-basalts -reach now, and perhaps attained originally a greater thickness than -anywhere else, they have protected the intrusive sheets, which are therefore -less deeply cut away than in any of the other districts, and no great central -core of gabbro has yet been uncovered.</p> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_364">- 364 -</span></p> - -<h2 class="nobreak" id="CHAPTER_XLV">CHAPTER XLV<br /> - -<span class="smaller">THE ACID ROCKS</span></h2> -</div> - -<div class="blockquot"> - -<p>Their Petrography—Their Stratigraphical Position and its Analogies in Central France</p> - -</div> - -<p>We now come to the examination of another distinct phase of volcanic -action during Tertiary time in Britain. The igneous rocks that have been -under consideration in the foregoing chapters, whether poured out at the -surface or injected below ground, have been chiefly of basic, partly indeed, -like the peridotites, of ultra-basic character. Some, however, have shown an -andesitic or intermediate composition. Reference has also been made to the -probable eruption of acid rhyolites in the long interval between the outflow -of the lower and the upper basalts in Antrim. But we now encounter a -great series, decidedly acid in composition, in the more largely crystalline -members of which the excess of silica is visible to the eye in the form of -free quartz. While there is a strong contrast in chemical composition -between this series and the rocks hitherto under discussion, there are also -marked differences in structure and mode of occurrence. Like the gabbros, -all the masses of acid rock now visible appear to be intrusive. They have -been injected beneath the surface, and therefore record for us subterranean -rather than superficial manifestations of volcanic action.</p> - -<p>The existence of rocks of this class in the midst of the basic masses has -long been recognized. They were noticed by Jameson, who described the -hills between Loch Sligachan and Broadford as composed of "a compound of -felspar and quartz, or what may be called a granitel, with occasional veins -of pitchstone."<a id="FNanchor_359" href="#Footnote_359" class="fnanchor">[359]</a> Macculloch gave a fuller account of the same region, and -classed the rocks as chiefly "syenite" and "porphyry."<a id="FNanchor_360" href="#Footnote_360" class="fnanchor">[360]</a> In Antrim, also, -even in the midst of the basalt-tableland, masses of "pitchstone-porphyry -"pearlstone-porphyry," "clay-porphyry," and "greystone" were observed and -described.<a id="FNanchor_361" href="#Footnote_361" class="fnanchor">[361]</a> In more recent years Professor Zirkel has given a brief account -of the so-called "syenite and porphyry" of Mull and Skye,<a id="FNanchor_362" href="#Footnote_362" class="fnanchor">[362]</a> and the late -<span class="pagenum" id="Page_365">- 365 -</span> -Professor Von Lasaulx fully described the "trachyte" or rhyolite of -Antrim.<a id="FNanchor_363" href="#Footnote_363" class="fnanchor">[363]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_359" href="#FNanchor_359" class="label">[359]</a> <i>Mineralogical Travels</i>, ii. 90.</p> - -<p><a id="Footnote_360" href="#FNanchor_360" class="label">[360]</a> <i>Western Isles</i>, see the descriptions of Skye, Mull and Rum.</p> - -<p><a id="Footnote_361" href="#FNanchor_361" class="label">[361]</a> Berger, <i>Trans. Geol Soc.</i> iii. (1816), p. 190; Portlock, <i>Journ. Geol. Soc. Ireland</i>, vol. i. -(1834), p. 9.</p> - -<p><a id="Footnote_362" href="#FNanchor_362" class="label">[362]</a> <i>Zeitsch. Deutsch. Geol. Gesellsch.</i> xxiii. (1871), pp. 54, 77, 84, 88.</p> - -<p><a id="Footnote_363" href="#FNanchor_363" class="label">[363]</a> Tschermak's <i>Min. und Petrog. Mittheilungen</i>, 1878, p. 412. The chemical composition of -this rock and its place among the rhyolites had already been determined by E. T. Hardman -from analysis, <i>Journ. Geol. Soc. Ireland</i>, vol. iii. (1871), p. 32.</p> - -</div> - -<p>This interesting series of rocks embraces a greater variety of petrographical -characters than any other portion of the British Tertiary volcanic -rocks. On the one hand, it presents thoroughly vitreous masses, some of -which in their colour, lustre and microscopic structure remind us of recent -obsidians. On the other hand, it affords coarsely crystalline compounds, to -which no other name than granite can be assigned, and which, did we not -know their geological position, might almost be classed with some of the -most ancient eruptive rocks. Between these two extremes abundant gradations -may be found, including beautiful spherulitic rocks, felsites and -rhyolites.</p> - -<p>In dealing with such a series of intrusive rocks, we again encounter the -difficulty of reaching certainty as to their relative dates of eruption, since in -each case all that can usually be affirmed is that the intrusive mass is -younger than that into which it is injected. It is quite possible that protrusions -of acid rocks occurred at intervals during the accumulation of the -basic masses, as may perhaps be inferred from the rhyolite-tuffs and conglomerates -of Antrim and from the occurrence of fragments of siliceous lavas -in the gravels near the base of the basalt-plateau of Mull, and in the -agglomerates of that island as well as of other districts.<a id="FNanchor_364" href="#Footnote_364" class="fnanchor">[364]</a> It is probable, -therefore, that at the time when the basalts of the plateaux were emitted, -there existed, within reach of volcanic explosions, masses of granophyric, felsitic -or rhyolitic rocks, fragments from which were shot up the funnels of discharge. -That portions of these rocks were actually intruded into the basalt-sheets -before the building up of the plateaux was completed appears to be proved -in Antrim. Elsewhere, however, no evidence has yet been obtained of any -such intrusion until after the close of the plateau-period. On the contrary, -in every case where the relative ages of the rocks can be fixed, the acid are -younger than the basic protrusions.</p> - -<div class="footnote"> - -<p><a id="Footnote_364" href="#FNanchor_364" class="label">[364]</a> Reference may also again be made to the agglomerates of Strath, Skye, which contain in -some parts abundant fragments of acid rocks that closely resemble some of the masses of granophyre -which disrupt these agglomerates.</p> - -</div> - -<p>The only known exceptions to this rule are the latest basalt-dykes. -Hence, while amid the large and varied series of acid rocks, which no doubt -represents a wide interval of time, some may belong to comparatively early -epochs in the protracted volcanic period, the actual available evidence places -the emission of these rocks, as a whole, towards the end of the volcanic -history. This evidence I shall bring, forward in full detail, since it -necessitates an abandonment of what has been the general belief in regard -to the relative ages of the rocks.</p> - -<p><span class="pagenum" id="Page_366">- 366 -</span></p> - - -<h3>i. <span class="allsmcap">PETROGRAPHY OF THE ACID ROCKS</span></h3> - -<p>The classification of the rocks which best harmonizes the field-evidence -and the detailed study of their mineralogical composition, is one that arranges -these volcanic protrusions into two series. In the one, the orthoclase is -sanidine, and the rocks range from the most vitreous pitchstone through -perlitic and spherulitic varieties to rhyolite ("quartz-trachyte"). In the -other series, which embraces by far the largest proportion of the whole, the -orthoclase is always turbid, and in this respect as well as in many others the -rocks remind us rather of ancient eruptive masses than of those which have -appeared in Tertiary time. They range from flinty felsitic varieties, which -are obviously devitrified glasses, through different textures of quartz-porphyry -into granophyre, and finally into granite. As I have been unable to recognize -any essential difference of structure and composition between these acid -Tertiary rocks and those of far earlier geological time, I give them the names -which no petrographer would hesitate to apply to them if they were of -Palæozoic age. It has long appeared to me that these rocks furnish conclusive -evidence of the misleading artificiality of any petrographical nomenclature -in which relative antiquity is made an essential element of discrimination.</p> - -<p><i>Granite.</i>—That true granites form part of the Tertiary volcanic series -of the British Isles has now been completely established. They occur as -bosses and sills which have been intruded into the gabbros and all older -rocks. They are thus proved not only to belong to the Tertiary period, -but to one of the latest phases of its volcanic history. But besides these -granites, the relative age of which can be definitely fixed, there occur others -which, standing alone and at some distance from the basaltic plateaux, can -only be inferentially classed in the Tertiary series. To this group belong -the granite masses of the Isle of Arran and the Mourne Mountains in north-eastern -Ireland.</p> - -<p>Taking first the unquestionably Tertiary granites which occur as bosses -and intrusive sheets, we have to note that the more coarsely crystalline -granophyres are hardly to be distinguished externally from granite. As the -dark ferro-magnesian constituent of these rocks was generally believed to be -hornblende, they were called by the older petrographers "syenite"; that is, -granite with hornblende instead of mica. The peculiar micropegmatitic -groundmass, which constitutes the distinguishing feature of the granophyres, -may occasionally be observed so reduced in amount as only to appear here -and there between the other minerals, which are grouped in a granitic -structure. From this condition, one step further carries us into a true -granite, from which all trace of the granophyric character has disappeared. -Such gradations may be traced even within short distances in the same -boss of rock. Thus, in the hornblende-biotite-granite boss of Beinn-an-Dubhaich, -Skye, a thoroughly granitic arrangement of the component -minerals is observable in the centre, while a specimen taken from near the -edge on the shore of Camas Malag shows the development of a granophyric -<span class="pagenum" id="Page_367">- 367 -</span> -groundmass. But, though the large bosses are usually somewhat -coarsely crystalline in the centre, and tend to assume finer felsitic textures -around their borders, as was observed long ago by Oeynhausen and Von -Dechen,<a id="FNanchor_365" href="#Footnote_365" class="fnanchor">[365]</a> the granitic structure is sometimes exhibited even at the very -edge, and not only so, but in the dykes that protrude from the bosses into -the surrounding rocks. Thus the Beinn-an-Dubhaich mass, at its margin -in Camas Malag, sends a vein into the surrounding limestone, but though -more close-grained than the main body of the rock, this vein is neither -felsitic nor granophyric, but truly granitic in structure.</p> - -<div class="footnote"> - -<p><a id="Footnote_365" href="#FNanchor_365" class="label">[365]</a> Karsten's <i>Archiv</i>, i. p. 89.</p> - -</div> - -<p>So far as I have observed, the true granites contain a brown mica and -also a little hornblende, both visible to the naked eye, but generally somewhat -decomposed. These rocks are thus hornblende-biotite-granites (amphibole-granitites -of Rosenbusch). They may be defined as medium-grained -aggregates of quartz, orthoclase (also plagioclase), biotite and hornblende, -with sometimes magnetite, apatite, epidote and zircon. Dr. Hatch found -that in some instances (Beinn-an-Dubhaich) the quartz contains minute -inclusions (glass?), bearing immovable bubbles with strongly-marked contours; -while in others (Beinn-na-Chro, Skye) this mineral is full of liquid -inclusions with bubbles, sometimes vibratile, sometimes fixed. He remarked -that the quartz and felspar have consolidated almost simultaneously, but -that in some instances (Marsco, Glen Sligachan) there are isolated roughly -idiomorphic crystals, of a white, less turbid orthoclase, which belong to a -slightly earlier consolidation than that of the more kaolinized felspar of the -rest of the rock.</p> - -<p>The granite of the island of Arran, in the Birth of Clyde, which -is here included in the Tertiary volcanic series, has long been -recognized as consisting of two distinct portions, an eastern or coarse-grained, -and a western or fine-grained variety. The latter sends -veins into the former. These granites contain orthoclase, plagioclase, -quartz and dark mica, the quartz being often idiomorphic with respect -to the felspar, and a tendency towards a micropegmatitic structure -being sometimes observable. A distinguishing characteristic of the Arran -granite is the cavernous or drusy structure which it presents, the cavities being -often lined with well-crystallized orthoclase and smoky quartz.<a id="FNanchor_366" href="#Footnote_366" class="fnanchor">[366]</a> The granite -of the Mourne Mountains in Ireland closely resembles that of Arran. Its -druses, with their beautifully terminated minerals, have long been well -known.</p> - -<div class="footnote"> - -<p><a id="Footnote_366" href="#FNanchor_366" class="label">[366]</a> See Mr. Teall's <i>British Petrography</i>, p. 328.</p> - -</div> - -<p><i>Microgranite.</i>—This term is applied to certain intrusive masses, which -megascopically may be classed with the quartz-porphyries and felsites, but -which microscopically are found to possess a holocrystalline granitic groundmass -of quartz and orthoclase, through which are scattered porphyritic -crystals of the same two minerals, sometimes also with plagioclase, augite, -magnetite or apatite. Rocks of this type do not appear to be abundant. -They occur as dykes and bosses, but occasionally also as sheets. I have -collected them from Skye, Rum and Ardnamurchan.</p> - -<p><span class="pagenum" id="Page_368">- 368 -</span></p> - -<p><i>Granophyre.</i>—Under this name may be grouped the large majority of the -acid rocks which play an important part in the geology of the West of Scotland. -They are typically developed in the islands of Mull and Skye. Generally -pale grey or buff in colour, they range in texture from the true granites, into -which, as above stated, they graduate, to exceedingly close-grained varieties -like the felsites of Palæozoic formations. In the great majority of them the -micrographic intergrowth of quartz and felspar, known as micropegmatite, -is their conspicuous structure, and even constitutes most of their substance. -They may thus be classed generally as granophyres, in the sense in which -this term is employed by Rosenbusch, but without his limitation of it to -pre-Tertiary rocks.</p> - -<p>The specific gravity of these rocks has been determined from a series of -specimens by Mr. A. Harker to range from about 2·3 among the felsites to -2·7 among the granites. No chemical analyses of these rocks have yet been -made, but they have been subjected to microscopical examination, and their -general structure and composition are now known.</p> - -<p>The typical granophyre of the Inner Hebrides outwardly closely -resembles an ordinary granite of medium grain, in which the component -dull felspar and clear quartz can be readily distinguished by the naked eye. -Throughout all the varieties of texture there is a strong tendency to the -development of minute irregularly-shaped drusy (miarolitic) cavities, which -here and there give a carious aspect to the rock. That these cavities, however, -are part of the original structure of the rock, and are not due to mere -weathering, is shown by the well-terminated crystals of quartz and felspar -which project into them. On a small scale, it is the same structure so -characteristic of the granite of the Mourne Mountains and of parts of that -of Arran.</p> - -<p>Examined under the microscope, a normal specimen of the granophyre of -the Western Isles presents a holocrystalline groundmass, which fills all the -interspaces between the crystals of earlier consolidation. This groundmass -consists of an aggregate of clear quartz and turbid orthoclase, arranged as -micropegmatite, but also in more or less idiomorphic crystals. In some -parts, the two dominant minerals are grouped in alternate parallel fibres, -diverging from the surface of the enclosed crystals, which are thus more -or less completely surrounded by a radially fibrous mass. The felspathic -portion of the micropegmatite which usually surrounds the orthoclase -crystals, when viewed between crossed Nicols, is found to extinguish -simultaneously with the central crystal.<a id="FNanchor_367" href="#Footnote_367" class="fnanchor">[367]</a> In other parts, the felspar forms -a kind of network, the meshes of which are filled up with quartz. Through -the groundmass, besides the clear quartz and dull orthoclase, some ferro-magnesian -or other additional constituent is generally distributed, but -usually somewhat decomposed. In certain varieties Dr. Hatch found an -abundant brown mica, as in the rock at Camas Malag, Skye. In others, a -pyroxene occurs, which he observed in minute greenish grains, sometimes -completely enclosed in the quartz. In a third variety, the dark constituent -<span class="pagenum" id="Page_369">- 369 -</span> -is hornblende, the most remarkable example of which is one to be seen at -Ishriff, in the Glen More of Mull, where the ferro-magnesian mineral takes -the form of long dirty-green needles, conspicuous on a weathered surface of -the rock. A fourth variety is distinguished by containing plagioclase in -addition to or instead of orthoclase. In the rock of the sheet forming Cnoc -Carnach, near Heast, in Skye, Dr. Hatch observed both orthoclase and -plagioclase scattered through a fine micropegmatitic groundmass, and in a -part of the boss at Ishriff he found the rock to be composed mainly of -plagioclase, in a micropegmatitic groundmass of quartz and felspar, with a -few scattered grains of a pale brown augite and grains of magnetite. A fifth -variety is marked by the prominence of the crystals of quartz and felspar -of earlier consolidation, and by the fineness of grain in the surrounding -micropegmatitic groundmass, whereby a distinct porphyritic structure is -developed. Rocks of this kind are megascopically like ordinary quartz-porphyries. -Still another variety has been detected by Mr. Teall in the rock -of Meall Dearg, at the head of Glen Sligachan, Skye, in which, besides -irregular patches which may represent decayed biotite, and others which are -possibly ilmenite, the rare mineral riebeckite is present.<a id="FNanchor_368" href="#Footnote_368" class="fnanchor">[368]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_367" href="#FNanchor_367" class="label">[367]</a> Mr. Teall, <i>Quart. Journ. Geol. Soc.</i> vol. 1. (1894) p. 219. See also his <i>British Petrography</i>, p. 327.</p> - -<p><a id="Footnote_368" href="#FNanchor_368" class="label">[368]</a> <i>Quart. Journ. Geol. Soc.</i> vol. 1. (1894), p. 219.</p> - -</div> - -<p><i>Felsite.</i>—The close-grained rocks into which the ordinary granophyres -frequently graduate may be conveniently grouped under the general name -of Felsite. They differ in no essential feature from the felsites of the -Palæozoic formations. They are more particularly developed, as might be -expected, in those places where the conditions have been most favourable -for rapid cooling, while the more coarsely crystalline granophyres occur -where the material may be supposed to have consolidated most slowly. -Where the acid magma has been injected into chinks and fissures so as to -take the form of veins or dykes, it is sometimes felsitic, sometimes granophyric, -in texture. Along the margin of large bosses, like those of Mull -and Skye, it frequently though not invariably has assumed a fine texture, -with even spherulitic and flow-structures. But in the centre of large -bosses it usually appears as coarse granophyre or as granite.</p> - -<p>The felsites vary in texture from flinty or horny to dull finely-granular, -and in colour from white through shades of grey, buff and lilac, -to black, generally with porphyritic felspars and blebs of quartz. Where -these porphyritic enclosures increase in size and number, the rocks cannot -be distinguished externally from ancient quartz-porphyries. In general the -groundmass of these rocks has been completely devitrified. But in some -dykes enough of the glassy base remains to show their original vitreous -condition. A gradation can thus be traced from thoroughly glassy pitchstone -into completely lithoid felsites and crystalline granophyres.</p> - -<p>A characteristic feature of the felsitic varieties of acid rock is their -flow-structure, which they often display in great perfection. Sometimes, -indeed, this structure has been so strongly developed as to cause the rock to -weather along the planes of flow and to break up into thin slabs.</p> - -<p>Many of these rocks also present admirably developed spherulitic structures, -<span class="pagenum" id="Page_370">- 370 -</span> -varying from microscopic minuteness up to large round or egg-shaped -balls nearly two inches in diameter, and often distributed in lines along -those of flow-structure. They likewise exhibit a frequent development of -micropegmatite. No line indeed can be drawn between these felsites and -the granitoid varieties, for the same characteristic granophyric intergrowth -of felspar and quartz runs through them all.</p> - -<p><i>Pitchstone.</i>—This name is applied to the glassy varieties apart from -their chemical composition, and specially denotes the possession of a vitreous -structure. Some of the rocks to which it has been applied are probably -glassy varieties of andesite, others are dacites, while some may be as -acid as the most acid felsites and granophyres. The pitchstones are found -in veins or dykes which traverse different geological formations up to and -including the great granophyre bosses of the Inner Hebrides. They vary in -colour from a deep jet-black or raven-black to a pale bottle-green, and in -lustre from an almost glassy obsidian-like to a dull resinous aspect. -Occasionally they assume a felsitic texture, owing to devitrification, and -also a finely spherulitic structure. Some varieties appear to the naked eye -to be perfectly homogeneous, others become porphyritic by the appearance -of abundant sanidine crystals.</p> - -<p>The microscopic structure of the British pitchstones has not yet been -fully worked out. The beautiful feathery microlites of the Arran dykes, -first made known by David Forbes, and subsequently described by Zirkel, -Allport and others, are well known objects to geological collectors. Dr. Hatch, -in whose hands I placed my tolerably large collection of specimens and their -thin slides, furnished me with some preliminary notes on the slides, from -which the following generalized summary is compiled.</p> - -<p>At the one end of the pitchstone group we have a nearly pure glass, -with no microlites, and only a few scattered crystals of sanidine, quartz, -augite or magetite. The glass in thin slices is almost colourless, but -generally inclines to yellow, sometimes to dark-grey. Some varieties of the -rock are crowded with microlites, in others these bodies are gathered into -groups, the glass between which is nearly free from them. Among the -minerals that have been observed in this microlitic form are sanidine, -augite, hornblende (forming the beautiful green feathery or fern-like aggregates -in the Arran pitchstones, <a href="../../66492/66492-h/66492-h.htm#v1fig3">Fig. 3</a>) and magnetite. Sometimes the rudimentary -forms appear as globulites, or as belonites, but more commonly as -dark trichites. Among the more definite mineral forms are grains of sanidine, -quartz and augite. The porphyritic crystals are chiefly sanidine, augite and -magnetite, but plagioclase occasionally occurs. The development of spherulites -is well seen in a few of the slides, and occasionally perlitic structure makes -its appearance.</p> - -<p>The interesting rhyolitic areas of Antrim include several varieties of -pitchstone. One of these is described by Professor Cole as "a glassy -pyroxene-rhyolite, on the verge of the rhyolitic andesites." Another is a -blue-black porphyritic obsidian.<a id="FNanchor_369" href="#Footnote_369" class="fnanchor">[369]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_369" href="#FNanchor_369" class="label">[369]</a> <i>Scientif. Trans. Roy. Dublin Soc.</i> vol. vi. (ser. ii.) 1896, p. 77.</p> - -</div> - -<p><span class="pagenum" id="Page_371">- 371 -</span></p> - -<p><i>Rhyolite (Quartz-Trachyte).</i>—This rock has been abundantly erupted -in north-east Ireland, where it rises in occasional bosses among the plateau-basalts.<a id="FNanchor_370" href="#Footnote_370" class="fnanchor">[370]</a> -It is best exposed at the Tardree and Carnearny Hills, where it -has long been quarried. Its petrographical characters at that locality were -described by Von Lasaulx as those of a typical quartz-trachyte rich in -tridymite, and containing large crystals of glassy sanidine, isolated narrow -laths of plagioclase (probably andesine), grains of smoky-grey quartz, partly -bounded by dihexahedral faces, and a few scattered flakes of a dark-coloured -mica. The groundmass is microgranitic, and under a high power is resolvable -into a confused aggregate of minute microlites of felspar, with interstitial -quartz-granules.<a id="FNanchor_371" href="#Footnote_371" class="fnanchor">[371]</a> More recently a detailed investigation of the petrography -of the Antrim rhyolites has been conducted by Professor Cole, who -has called attention to their remarkable varieties of structure, ranging from -perfect volcanic glass to a thoroughly lithoidal texture, and exhibiting flow, -perlitic and spherulitic structures.<a id="FNanchor_372" href="#Footnote_372" class="fnanchor">[372]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_370" href="#FNanchor_370" class="label">[370]</a> Fragments of acid rock were detected by Prof. Cole in the gravel among the Ardtun basalt -of Mull, as already noticed on p. 212.</p> - -<p><a id="Footnote_371" href="#FNanchor_371" class="label">[371]</a> Tschermak's <i>Min. und Pet. Mittheil.</i> 1878, p. 412.</p> - -<p><a id="Footnote_372" href="#FNanchor_372" class="label">[372]</a> <i>Scientif. Trans. Roy. Dublin Soc.</i> vol. vi. (ser. ii.) 1896, p. 77. This paper gives an excellent -account of the microscopical character and mineralogical and chemical compositions of these -rocks.</p> - -</div> - -<p>Intrusive masses of rhyolite are also found in the Carlingford region. -One of these, seen at Forkhill, is a velvet-black almost resinous rock with -abundant quartz and felspar, and sometimes displaying beautiful flow-structure. -It will be more particularly described in Chapter xlvii. Some -of the acid dykes and sills of the Inner Hebrides are varieties of rhyolite. -No undoubted example has yet been observed of a superficial rhyolite-lava, -though such not improbably appeared in the interval between the lower and -upper basalts of Antrim.</p> - - -<h3>ii. <span class="smcap">STRATIGRAPHICAL POSITION.—ANALOGIES FROM CENTRAL FRANCE</span></h3> - -<p>In the history of opinion regarding the relative position of the Tertiary -eruptive rocks, no feature is so remarkable as the universal acceptance of the -misconception regarding the place of the acid protrusions. In tracing this -mistake to its source, we find that it probably arose from the fact that along -their line of junction the granitoid masses generally underlie the basic. -This order of superposition, which would usually suffice to fix the age of -two groups of stratified rocks, is obviously not of itself enough to settle the -relative epochs of two groups of intrusive rocks. Yet it has been assumed -as adequate for this purpose, and hence what can be proved to be really the -youngest has been placed as the oldest part of the Tertiary volcanic series.</p> - -<p>Macculloch, who showed that his "syenites" and "porphyries" had -invaded the Secondary strata of the Inner Hebrides, and must therefore be -of younger date than these, left their relations to the other igneous rocks of -the region in a curiously indefinite position. He was disposed to regard -them all as merely parts of one great series; and seems to have thought that -<span class="pagenum" id="Page_372">- 372 -</span> -they graduate into each other, and that any attempt to discriminate between -them as to relative age is superfluous. Yet he evidently felt that the -contrasts of topography which he described could hardly fail to raise the -question of whether rocks so distinct in outward form did not differ also in -relative antiquity. But he dismissed the question without answering it, -remarking that if there is any difference of age between the two kinds of rock, -"there appears no great prospect of discovering it."<a id="FNanchor_373" href="#Footnote_373" class="fnanchor">[373]</a> He records an instance -of a vein of "syenite" traversing the "hypersthene rock" in the valley of -Coruisk. "If this vein," he says, "could be traced to the mass of syenite, -it might be held a sufficient ground of judgment, but under the present circumstances -it is incapable of affording any assistance in solving the difficulty."<a id="FNanchor_374" href="#Footnote_374" class="fnanchor">[374]</a> -Instead, however, of being a solitary instance, it is only one of hundreds of -similar intrusions which can be connected with the general body of granitic -and granophyric masses, and which put the relative ages of the several -groups of rock beyond any further doubt.</p> - -<div class="footnote"> - -<p><a id="Footnote_373" href="#FNanchor_373" class="label">[373]</a> <i>Western Islands</i>, i. p. 368; see also pp. 488, 575, 578.</p> - -<p><a id="Footnote_374" href="#FNanchor_374" class="label">[374]</a> <i>Op. cit.</i> p. 370.</p> - -</div> - -<p>Boué, who knew the geology of some of the extinct volcanic regions of -Europe, recognized the similarity of the Scottish masses to those of the -Continent, and classed the acid rocks as "trachytes." He saw in each of -the volcanic areas of the West of Scotland a trachytic centre, and supposed -that the more granitoid parts might represent the centres in the European -trachytic masses. He traced in imagination the flow of the lava-streams -from these foci of volcanic activity, distinguishing them as products of -different epochs of eruption, among the last of which he thought that the -trachytic porphyries might have been discharged. He admitted, however, -that his restoration could not be based on the few available data without -recourse to theoretical notions drawn from the analogy of other regions.<a id="FNanchor_375" href="#Footnote_375" class="fnanchor">[375]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_375" href="#FNanchor_375" class="label">[375]</a> <i>Essai Géologique sur l'Écosse</i>, pp. 291, 322, 327.</p> - -</div> - -<p>In the careful exploration of the central region of Skye made by Von -Oeynhausen and Von Dechen, these able observers traced the boundary -between the "syenite" and the "hypersthene rock"; and as they found the -former lying underneath the latter, they seem naturally to have considered -it to be the older protrusion of the two.<a id="FNanchor_376" href="#Footnote_376" class="fnanchor">[376]</a> Principal Forbes came to a -similar conclusion from the fact that he found the dark gabbro always overlying -the light-coloured felspathic masses.<a id="FNanchor_377" href="#Footnote_377" class="fnanchor">[377]</a> Professor Zirkel also observed -the same relative position, and adopted the same inference as to the relative -age of the rocks.<a id="FNanchor_378" href="#Footnote_378" class="fnanchor">[378]</a> Professor Judd followed these writers in placing the acid -rocks before the basic. He supposed the granitoid masses to form the -cores of volcanic piles probably of Eocene age, through and over which -the protrusions of gabbro and the eruptions of the plateau-basalts took -place.<a id="FNanchor_379" href="#Footnote_379" class="fnanchor">[379]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_376" href="#FNanchor_376" class="label">[376]</a> Karsten's <i>Archiv</i>, i. p. 82. It will be shown in later pages that the apparent infraposition -of the granophyre is often deceptive, the real junction being vertical.</p> - -<p><a id="Footnote_377" href="#FNanchor_377" class="label">[377]</a> <i>Edin. New Phil. Jour.</i> xl. (1846) p. 84.</p> - -<p><a id="Footnote_378" href="#FNanchor_378" class="label">[378]</a> <i>Zeitsch. Deutsch. Geol. Gesellsch.</i> xxiii. (1871) pp. 90, 95. He says that the gabbro seems -to be the younger rock, so far as their relations to each other can be seen.</p> - -<p><a id="Footnote_379" href="#FNanchor_379" class="label">[379]</a> <i>Quart. Jour. Geol. Soc.</i> xxx. (1874) p. 255.</p> - -</div> - -<p><span class="pagenum" id="Page_373">- 373 -</span></p> - -<p>The evidence for the posteriority of the acid rocks will be fully detailed -in later pages. Before entering upon its consideration, however, I would -remark that the uprise of the British granophyres presents so many points -of resemblance to that of the trachytes and phonolites among the basalt-plateaux -of Auvergne and the Velay in Central France, that a brief account of -the acid protrusions of these regions may be suitably given here as an introduction -to the account of those of the Inner Hebrides. A succession of stages -in the progress of denudation allows us to follow the gradual isolation and -dissection of the French volcanic groups. The youngest examples occur in the -chain of cones and craters, in the region of the Puy de Dôme. These may -be of Pleistocene, or even of more recent date. Older and more deeply -eroded than these are the numerous domes and cones in the territory of -Haute Loire. Yet more ancient and still more stupendously denuded come -the bosses, sills and dykes of Britain. Nevertheless, the geologist, by -the methods so admirably devised by Desmarest, may follow the chain of -relationship through these different regions and trace a remarkable continuity -of structure. The younger rocks serve to illustrate the original -condition of the more ancient, while the latter, by their extensive denudation, -permit points of structure to be seen which in the former are still concealed.</p> - -<p>No feature in the interesting volcanic district of Auvergne has attracted -more attention than the trachytic protrusions.<a id="FNanchor_380" href="#Footnote_380" class="fnanchor">[380]</a> Rising conspicuously along -the chain of puys, they claim notice even from a distance owing to the topographical -contrast which their pale rounded domes offer to the truncated, -crater-bearing cones of dark cinders around them. They consist of masses of -a pale variety of trachyte (domite), which in ground-plan present a circular -or somewhat elliptical outline. They vary in size from the nearly circular -dome of the Grand Sarcoui, which measures about 400 yards in diameter, to -the largest mass of all—that of the Puy de Dôme, which extends for some -1500 yards from north to south with a breadth varying from 500 to 800 -yards. They are likewise prominent from their height; in the Puy de -Dôme they form the highest elevation of the whole region (1465 metres), and -even in the less conspicuous hills they rise from 500 to 600 feet above the -surrounding plateau.</p> - -<div class="footnote"> - -<p><a id="Footnote_380" href="#FNanchor_380" class="label">[380]</a> The admirable Map and Memoirs of Desmarest on Auvergne are classics in geology. Scrope's -work, vol. i. p. 45, gives still the best published account of this district. See also the work -of Lecoq (<i>ibid.</i>). The results of more detailed petrographical research regarding the rocks -will be found in the essays of M. Michel Lévy (<i>Bull. Soc. Géol. France</i>, 1890, p. 688) and in -the Clermont sheet of the Geological Survey Map of France (Feuille, 166). A bibliography of the -district up to the year 1890 is given in the volume of the <i>Bull. Soc. Géol. France</i> just cited, p. 674.</p> - -</div> - -<p>Five such dome-shaped protrusions of trachyte have made their appearance -among the cinder-cones in a space of about five English miles in length -by about two miles in extreme breadth. Though opinions have varied as -to the mode of formation of these domes, there has been a general agreement -that their present topographic contours cannot be far from the -original outlines assumed by the masses at the time of their production. -The position of the trachyte bosses among the puys serves to show that they -were not deep-seated masses which have been entirely uncovered by denudation, -<span class="pagenum" id="Page_374">- 374 -</span> -but were essentially superficial, and were protruded to the surface at -various points along the plateau in the midst of already existing cinder-cones. -In some cases, they have risen on or near the position of the vents -of these cones. Thus the Puy de Chopine is half encircled by the crater of -the Puy de la Goutte, and the Grand Sarcoui stands in a similar relation to -the fragmentary crater-wall of the Petit Sarcoui.</p> - -<p>M. Michel Lévy, in pointing out the superficial character of the -domitic protrusions, has forcibly dwelt on the evidence that these rocks have -undergone a comparatively trifling denudation, and that they could never have -extended much beyond their present limits.<a id="FNanchor_381" href="#Footnote_381" class="fnanchor">[381]</a> As Scrope pointed out, they -were obviously protruded in a pasty condition, not flowing out in streams -like the other lavas of the district, but consolidating within their chimneys -and rising from these in rounded domes.</p> - -<div class="footnote"> - -<p><a id="Footnote_381" href="#FNanchor_381" class="label">[381]</a> <i>Op. cit.</i> p. 711.</p> - -</div> - -<div class="figcenter" id="v2fig344" style="width: 440px;"> - <img src="images/v2fig344.png" width="440" height="138" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 344.</span>—Section through the Puy de la Goutte and Puy de Chopine.<br /><br /> - 1, Mica-schist; 2 2, Granite; 3 3, Tuffs; 4, Trachyte; 5, Basalt dyke.</div> -</div> - -<p>Undoubtedly denudation, cannot have left them altogether unaffected, -but must have removed some amount of material from their surface. There -is reason to believe that the material so removed may have been in large -part of a fragmental character, and that it was under a covering of loose -pyroclastic debris that the upward termination of the trachyte column -assumed its typical dome-form. Thus in the crater-wall of the Puy de la -Goutte, layers of buff-coloured trachytic tuff dip gently away from the central -domite mass of the Puy de Chopine. That this material was thrown out from -the vent previous to the uprise of the domite may be inferred from the way in -which the latter rock has obliterated the northern half of the crater. The -relations of the rocks are somewhat obscured by talus and herbage, but when -I last visited the locality in the spring of 1895 the structure seemed to me -to be as expressed in the accompanying diagram (<a href="#v2fig344">Fig. 344</a>).<a id="FNanchor_382" href="#Footnote_382" class="fnanchor">[382]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_382" href="#FNanchor_382" class="label">[382]</a> Compare M. Michel Lévy, <i>ibid.</i></p> - -</div> - -<p>The relative date of the protrusion of the trachytic domes cannot be -very precisely defined. There can, indeed, be no doubt that it belongs to a -late phase of the volcanic history. It came long after the outpouring of the -older basaltic plateaux, of which large fragments emerge from beyond the -limits of the younger lavas on both sides of the great ridge of the puys, and -not only long after that outpouring, but even after the widespread sheets of -basalt had been deeply trenched by valleys and isolated into outliers capping -the hill-tops. Yet there is good evidence also that the uprise of the comparatively -<span class="pagenum" id="Page_375">- 375 -</span> -acid trachytes was not the last volcanic episode of the district. -The abundance of dark slags and fragments of basalt lying on the domite hills -shows that discharges of more basic detritus occurred after these hills had -taken their place in the landscape.</p> - -<p>Since the latest eruptions, a gradual alteration of the topographical features -by denudation has been slowly but continuously going on. The Grand -Sarcoui, possibly from having originally had a considerable covering of fragmentary -material, shows least the effects of this waste. Its remarkably -regular form, like that of an inverted cauldron (the "Chaudron," as it is -called in the district), presents, in a distant view, a smooth grassy surface -which slopes steeply down into the great volcanic plain. But on a nearer -examination these declivities are found to be seamed with trenches which -the rain-storms of centuries have dug out. The covering of loose debris has -been largely washed away, though many fragments of dark slag are still -strewn over the slopes, and the scars are now being cut into the domite -below. A more advanced stage of decay may be seen on the Puy de Dôme, -where, from greater elevation and exposure, the domite is already deeply -gashed by gullies and ravines, while the slopes below are strewn with its -detritus.</p> - -<p>The region of the Velay displays on a far more extensive scale the protrusion -of trachytic and phonolitic bosses, but as its volcanic history goes -back beyond the time of the Puys of Auvergne, its volcanic monuments -have consequently been more extensively affected by denudation.<a id="FNanchor_383" href="#Footnote_383" class="fnanchor">[383]</a> A series -of basaltic eruptions forming extensive sheets can there be traced, the oldest -dating from Miocene time, the youngest coming down to the age of the -mammoth, cave-bear and early man. During this prolonged outpouring -of basic lavas there were several intervals during which materials of a more -acid nature—trachytes and phonolites—were erupted. These rocks occur -partly as extensive tracts, covering five or six square miles, like those of the -Mezenc, the Megal, the Pic de Lizieux, and the Rand, and partly in isolated -conical or dome-shaped prominences, sometimes only a few hundred feet in -diameter. Upwards of one hundred distinct eruptions of phonolite have -been observed in the Velay. Even in the tracts where they cover the -largest space, several prominent eminences may usually be observed, not -unlike in general shape the isolated cones and domes of Auvergne. In -these wider areas there appears to be evidence of the outcome of the lava -from one or more vents, either as superficial streams or as underground -intrusive sheets. M. Boule has expressed his opinion that most of the -masses of trachyte and phonolite have been the result of local and limited -eruptions, the pasty rock having risen in and accumulated around its pipe, -without flowing far in any direction. A section across one of these masses -would present a somewhat mushroom-shaped form.<a id="FNanchor_384" href="#Footnote_384" class="fnanchor">[384]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_383" href="#FNanchor_383" class="label">[383]</a> In addition to the work of Scrope, the student of this important volcanic district will find -an invaluable guide in the Le Puy Sheet (No. 186) of the Geological Survey Map of France, and -in the <i>Bulletins</i> of the Survey, particularly those by MM. Termier and Boule, No. 13 (1890) and -No. 28 (1892).</p> - -<p><a id="Footnote_384" href="#FNanchor_384" class="label">[384]</a> <i>Bull. Carte. Géol. France</i>, No. 28 (tome iv.) p. 125.</p> - -</div> - -<p><span class="pagenum" id="Page_376">- 376 -</span></p> - -<p>That fragmentary ejections accompanied the protrusion of these rocks, -though probably on a very limited scale, is shown by the occasional survival -of portions of trachyte tuff around them. One of the most notable of these -deposits occurs in the hollow between the Suc du Pertuis and the next dome -to the south. It consists of fine and coarse, trachytic detritus, which in -one place is rudely bedded and appears to dip away from the phonolite -dome behind it at an angle of 30°. This material and its inclination are -what might be expected to occur round an eruptive vent, and may be compared -with those of the crater-wall of the Puy de la Goutte in relation to -the domite boss of the Puy de Chopine.</p> - -<p>The denudation of Velay has undoubtedly advanced considerably further -than that of the Puys of Auvergne. The pyroclastic material which may -have originally covered the domes of trachyte and phonolite has been in -great part swept away. The surrounding rocks, too, both aqueous and -igneous, have been extensively removed from around the necks of more -enduring material. Hence the trachyte and phonolite bosses stand out with so -striking a prominence as to arrest the eye even for a distance of many miles.</p> - -<div class="figcenter" id="v2fig345" style="width: 452px;"> - <img src="images/v2fig345.png" width="452" height="142" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 345.</span>—View of the Huche Pointue and Huche Platte west of Le Pertuis.<br /><br /> - The cone is one of the trachytic domes, while the flat plateau to the left is a denuded outlier of the basalt sheets.</div> -</div> - -<p>There cannot be any doubt that these necks have pierced the older -basalts, and therefore belong to a later epoch in the volcanic history. The -approximately horizontal sheets of basalt have been deeply eroded and reduced -to mere fragments, and in some instances their existing portions owe -their survival to the protection afforded to them by the immense protrusions -of more acid material. But there is here, as well as in Auvergne, evidence -of the uprise of a later more basic magma, for sheets of basalt are found -overlying some parts of the trachytes and phonolites.</p> - -<p>While the external forms of these Velay necks recall with singular -vividness the features of many more ancient necks in Britain, an examination -of the internal structure of some of them affords some further interesting -points of resemblance. The slabs into which, by means of weathering -along the joints, the rock is apt to split up are sometimes arranged with a -general dip outwards from the centre of the hill, so that their flat surfaces -roughly coincide with the hillslopes. In other cases the peculiar platy structure, -so characteristic of phonolite, is disposed vertically or dips at a steep -angle into the hill, so that the edges of the slabs are presented to the -declivities, which consequently become more abrupt and rugged.</p> - -<p><span class="pagenum" id="Page_377">- 377 -</span></p> - -<p>Though none of the volcanic series in Auvergne or the Velay is so acid -in composition as the more acid members of the Tertiary volcanic series of -Britain, the manner in which the trachytes and phonolites of the French -region make their appearance presents some suggestive analogies to that of -the corresponding rocks in this country. We see that they were erupted long -after the outpouring of extensive basaltic plateaux, that they belonged to -successive epochs of volcanic activity, that they were protruded in a pasty -condition to the surface, where, more or less covered with fragmentary -ejections, they terminated in dome-shaped hills or spread out to a limited -distance around the vents, and lastly, that they were succeeded by a still -later series of more basic eruptions, which completed the long volcanic -history. We shall see in the following pages how closely the various stages -in this complex record of volcanic activity may be paralleled in the geological -records of Tertiary time in Britain.<a id="FNanchor_385" href="#Footnote_385" class="fnanchor">[385]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_385" href="#FNanchor_385" class="label">[385]</a> The phonolite necks of Bohemia, which form so prominent a feature in the Tertiary geology -of that country, might likewise be cited here in illustration of the acid domes and bosses of the -British Isles.</p> - -</div> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_378">- 378 -</span></p> - -<h2 class="nobreak" id="CHAPTER_XLVI">CHAPTER XLVI<br /> - -<span class="smaller">TYPES OF STRUCTURE IN THE ACID ROCKS—BOSSES</span></h2> -</div> - - -<p>Returning now to the consideration of the acid rocks as these manifest -themselves in the volcanic areas of Britain, I would remark that three -distinct types of structure may be noted among them, viz. (1) bosses, (2) -sills or intrusive sheets, (3) veins and dykes. These types, as above remarked, -belong entirely to the underground operations of volcanism, for -though the rhyolitic fragments in the tuffs and agglomerates of the plateaux -prove that acid lavas existed near the surface, no undoubted case of superficial -lava belonging to the acid series has yet been observed.<a id="FNanchor_386" href="#Footnote_386" class="fnanchor">[386]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_386" href="#FNanchor_386" class="label">[386]</a> The rhyolites of Tardree in Antrim have recently been claimed by Professor Cole as true -lavas grouped round an eruptive vent. For reasons to be given in the next chapter I regard -them as intrusive masses, though they may not improbably have been connected with streams of -lava now entirely removed.</p> - -</div> - -<p>The bosses of acid material in the British Tertiary volcanic series -are irregular protrusions, varying in size from knobs only a few -square yards in area up to huge masses many square miles in extent, and -comprising groups of lofty hills. As a rule, their outlines are markedly -irregular. Beneath the surface they plunge down almost vertically through -the rocks which they traverse, but in not a few instances their boundaries -are inclined to the horizon, so that the contiguous rocks seem to rest against -them, and sometimes lie in outliers on their sides and summits. From the -margins of these bosses apophyses are given off into the surrounding rocks, -sometimes only rarely and at wide intervals, in other places in prodigious -numbers. Sometimes the acid material has been injected in thousands of -veins and minute threads, which completely enclose fragments of the surrounding -rock.</p> - -<p>The rock of which the bosses consist is generally granophyric in texture, -passing on the one hand, particularly in the central parts, into granite, and -on the other, and especially towards the margin, into various more compact -felsitic varieties, and sometimes exhibiting along the outer edge more or less -developed spherulitic and flow-structures.</p> - -<p>Decided contact metamorphism is traceable round the bosses, but is by -no means uniform even in the same rock, some parts being highly altered, -while others, exposed apparently to the same influences, have undergone -<span class="pagenum" id="Page_379">- 379 -</span> -little change. The most marked examples of this metamorphism are those -in which the Cambrian limestone of Skye has been converted into a pure -white saccharoid marble. But the most interesting to the student of volcanic -action are those where the altered rocks are older parts of the volcanic series. -As the bosses of each volcanic area offer distinctive peculiarities they will -here be described geographically.</p> - - -<h3>i. <span class="allsmcap">THE ACID BOSSES OF SKYE</span></h3> - -<p>It is in the island of Skye that the granophyre and granite bosses attain -their largest dimensions and afford, on the whole, the most complete evidence -of their structures and their relations to the other parts of the volcanic series -(Map VI.). They cover there a total area of about 25 square miles, and form -characteristic groups of hills from 2000 to 2500 feet in height. On the -south-east side, three conspicuous cones (the Red Hills) rise from the valley of -Strath (Beinn Dearg Mhor, Beinn Dearg Bheag and Beinn na Caillich). -A solitary graceful pointed cone (Beinn na Cro) stands between Strathmore -and Strathbeg, while to the north-west a continuous chain of connected -cones runs from Loch Sligachan up into the heart of the Cuillin Hills. -Their conical outlines, their smooth declivities, marked with long diverging -lines of screes, and their pale reddish or reddish-yellow hue, that deepens -after a shower into glowing orange, mark off these hills from all the surrounding -eminences, and form in especial a singular contrast to the black, -spiry, and rugged contours of the gabbro heights to the west of them.</p> - -<p>Besides this large continuous mass, a number of minor bosses are scattered -over the district. Of these the largest forms the ridge of Beinn an Dubhaich, -south of Loch Kilchrist. Several minor protrusions lie between that -ridge and the flank of Beinn Dearg. Others protrude through the moory -ground above Corry; several occur on the side of the Sound of Scalpa, about -Strollamus; and one, already referred to, lies at the eastern base of Blath -Bheinn. In the neighbouring island of Raasay, a large area of granophyre -likewise occurs, which will be described with the Sills in later pages.</p> - -<p>In so extensive a district there is room for considerable diversity of -composition and texture among the rocks. As already stated, in some places, -more particularly in the central parts of the hills, the acid material -assumes the character of a granite, being made up of a holocrystalline aggregate -of quartz, orthoclase, plagioclase, hornblende and biotite, without granophyric -structure, and thus becomes a hornblende-biotite-granite (quartz-syenite, -granite-syenite of Zirkel, or amphibole-granitite of Rosenbusch). By -the development of the micropegmatitic structure and radiated spherical concretions, -it passes into granophyre. By the appearance of a felsitic groundmass, -it shades off into different varieties of quartz-porphyry or rhyolite, -sometimes with distinct bi-pyramidal crystals of quartz.<a id="FNanchor_387" href="#Footnote_387" class="fnanchor">[387]</a> This change, -which here and there is observable along the edge of a boss, is sometimes -accompanied with an ample development of spherulitic and flow-structures. -<span class="pagenum" id="Page_380">- 380 -</span> -As it is convenient to adopt some general term to express the whole series -of varieties, I have used the word granophyre for this purpose.</p> - -<div class="footnote"> - -<p><a id="Footnote_387" href="#FNanchor_387" class="label">[387]</a> The best account yet published of these varieties in Skye is that by Prof. Zirkel, <i>Zeitsch. -Deutsch. Geol. Gesellsch.</i> xxiii. (1871) p. 88.</p> - -</div> - -<div class="figcenter" id="v2fig346" style="width: 555px;"> - <img src="images/v2fig346.png" width="555" height="373" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 346.</span>—View of Glamich, 2537 feet, Glen Sligachan. (From a photograph by R. J. A. Berry, M.D., lent by the - Scottish Mountaineering Club).</div> -</div> - -<p>That the large area of these rocks in Skye was the result of many -separate protrusions from distinct centres of emission may be inferred, I -think, not only from the varieties of petrographical character in the material, -but also from the peculiar topography of the ground, and perhaps from the -curious relation which seems, in some instances at least, to be traceable -between the external features and apparent internal structure of the hills. -It will be seen from the Map (No. VI.) that in the area lying to the east -of Strath More the granophyre is broken up into nearly detached portions -by intervening patches of older rocks. There can be little doubt that the -<span class="pagenum" id="Page_381">- 381 -</span> -mass of Beinn na Caillich and the two Beinn Deargs is the product of a -distinct orifice, if not of more than one. Beinn na Cro, lying between its -two deep bounding glens, is another protrusion. The western cones stand -so closely together that their screes meet at the bottoms of the intervening -valleys. Yet each group is not improbably the result of emission from an -independent funnel, like the separate domite puys of Auvergne.</p> - -<p>But, though I believe this large area of granitoid rock to have proceeded -not from one but from many orifices, I have only here and there obtained, -from the individual hills themselves, indications of an internal structure -suggestive of distinct and successive protrusions of material from the same -vent of discharge. On the outer declivities of some of the cones we may -detect a rudely bedded structure, which will be subsequently referred to as -well displayed in Rum (p. 403). This structure is specially observable along -the east side of Glen Sligachan. Down the northern slopes of Marsco the -granophyre (here in part a hornblende-biotite-granite) is disposed in massive -sheets or beds that plunge outwards from the centre of the hill at angles of -30° to 40°. On the southern front of the same graceful cone, as well as -on the flanks of its neighbour, Ruadh Stac, still plainer indications of a -definite arrangement of the mass of the rock in irregular lenticular beds -may be noticed. These beds, folding over the axis of the hill, dip steeply -down as concentric coats of rock. The external resemblance of the red -conical mountains of Skye to the trachyte puys of Auvergne was long ago -remarked by J. D. Forbes,<a id="FNanchor_388" href="#Footnote_388" class="fnanchor">[388]</a> and in this internal arrangement of their -materials, indefinite though it may be, there is a further resemblance to the -onion-like coatings which Von Buch and Scrope remarked in the structure -of the interior of the Grand Sarcoui.<a id="FNanchor_389" href="#Footnote_389" class="fnanchor">[389]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_388" href="#FNanchor_388" class="label">[388]</a> <i>Edin. New Phil. Jour.</i> xl. p. 78.</p> - -<p><a id="Footnote_389" href="#FNanchor_389" class="label">[389]</a> Von Buch, <i>Geognostische Beobachtungen auf Reisen durch Deutschland und Italien</i>, vol. ii. -(1809) p. 245; Scrope, <i>Geology and Extinct Volcanoes of Central France</i>, 2nd edit. p. 68. Von -Buch regarded the external form of this Puy as having been determined by its internal structure.</p> - -</div> - -<p>Where the contour of the cones is regular, and the declivities are not -marked by prominent scars and ribs of rock, this monotony of feature -betokens a corresponding uniformity of petrographical character. But -where, on the other hand, the slopes are diversified by projecting crags and -other varieties of outline, a greater range of texture and composition in the -material of the hills is indicated. This relation is well brought out on the -western front of Marsco, where numerous alternations of granitoid and felsitic -textures occur. On many declivities also, which at a distance look quite -smooth, but which are really rough with angular blocks detached from the -parent mass underneath, an occasional basalt-dyke will be observed to rise -as a prominent dark rib. A good example of this structure is to be seen on -the south front of Beinn na Caillich. Where a group of dark parallel -dykes runs along the sides of one of these pale cones, it sometimes produces a -curiously deceptive appearance of bedding. A conspicuous illustration may -be noticed on the southern front of Beinn Dearg Meadhonach, north from -Marsco. When I first saw that hillside I could not realize that the parallel -<span class="pagenum" id="Page_382">- 382 -</span> -bars were actually dykes until I had crossed the valley and climbed the -slopes of the hill.<a id="FNanchor_390" href="#Footnote_390" class="fnanchor">[390]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_390" href="#FNanchor_390" class="label">[390]</a> The difference of contour and colour between the ordinary reddish smooth-sloped "syenite" -and the black craggy "hypersthene rock" and "greenstone" in the Glamaig group of hills caught -the eyes of Von Oeynhausen and Von Dechen (Karsten's <i>Archiv</i>, i. p. 83).</p> - -</div> - -<p>Good evidence of successive protrusions of the acid rock within -the great area of the Red Hills may be found on the south side of -Meall Dearg at the head of Glen Sligachan, where the granophyre is traversed -by a younger band or dyke of fine-grained spherulitic material about -ten feet broad. The rock exhibits there the same beautiful flow-structure -with rows of spherulites as is to be seen along the contact of the main granophyre -mass with the gabbro on the same hill, which will be afterwards described. -This dyke, vein or band, though possibly belonging to the same -epoch of protrusion as the surrounding granophyre, must obviously be later -than the consolidation of the rock which it traverses.</p> - -<p>Occasionally round the margin of the granophyre a singular brecciated -structure is to be seen. I have found it well marked on weathered -faces, along the flanks of Glamaig and of Marsco, and Mr. Harker has -observed many examples of it on the north side of the granophyre mass of -the Red Hills. When the rock is broken open, it is less easy to detect the -angular and subangular fragments from the surrounding matrix, which is -finely crystalline or felsitic.</p> - -<p>The actual junction of the eruptive mass with the surrounding rocks -through which it has ascended is generally a nearly vertical boundary, but -the granophyre sometimes plunges at a greater or less angle under the rocks -that lie against or upon it. On the north side of Glamaig, for instance, -the prophyritic and felsitic margin of the great body of eruptive rock descends -as a steeply inclined wall, against which the red sandstones and marls -at the base of the Secondary formations are sharply tilted. On the south -side of the area a similar steep face of fine-grained rock forms the edge of -the granophyre of the great southern cones, and plunges down behind Lias -limestone and shale, Cambrian limestone and quartzite, or portions of the -Tertiary volcanic series. Where the granophyre cuts vertically through the -gabbro, the latter rock being more durable is apt to rise above the more decomposable -granophyre as a crag or wall, and thus the deceptive appearance -arises of the basic overlying the acid rock. As above mentioned, there seems -every reason to believe that this peculiarity of weathering has given rise to or confirmed -the mistaken impression that the granophyre is older than the gabbro.</p> - -<p>There can be no doubt, however, that along many parts of the boundary-line -the acid eruptive mass extends underneath the surface far beyond the -actual base of the cones, for projecting knobs as well as veins and dykes -of it rise up among the surrounding rocks. This is well seen along the -northern foot of Beinn na Caillich. But of all the Skye bosses none exhibits -its line of junction with the surrounding rocks so well and continuously -as Beinn an Dubhaich. This isolated tract of eruptive material lies -entirely within the area of the Cambrian limestone, and its actual contact -<span class="pagenum" id="Page_383">- 383 -</span> -with that rock, and with the basalt-dykes that traverse it, can be examined -almost everywhere. The junction is usually vertical or nearly so, sometimes -inclining outwards, sometimes inwards. It is notched and wavy, the -granite sending out projecting spurs or veins, and retiring into little bays, -which are occupied by the limestone. The subdivisions of the latter rock -have recently been traced by Mr. Harker up to one side of the granite -and recognized again on the other side, with no apparent displacement, as if -so much limestone had been punched out to make way for the uprise of the -acid boss. The older dykes, too, are continuous on either side of the ridge. -The granite is massive and jointed, splitting up into great quadrangular -blocks like an ancient granite, and weathering into rounded boulders. Its -granitic composition and texture are best seen where the mass is broadest, -south of Kilbride. Towards its margin, on the shore of Camas Malag, the -granophyric structure appears, especially in narrow ribbons or veins that -run through the more granitic parts of the rock. These may be compared -with the much larger dyke of spherulitic rock above noticed as traversing -the granophyre of Meall Dearg.</p> - -<div class="figcenter" id="v2fig347" style="width: 372px;"> - <img src="images/v2fig347.png" width="372" height="193" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 347.</span>—Section across the north slope of Beinn an Dubhaich, Skye.<br /><br /> - <i>a</i> <i>a</i>, Cambrian limestone; <i>b</i> <i>b</i>, basalt dykes; <i>c</i>, granite.</div> -</div> - -<p>Immediately to the south of Camas Malag the junction with the limestone -is well displayed, and the eruptive rock, which is there granitic in -character, sends out into the limestone a vein or dyke about two feet broad, -of closer grain than the main body of the boss, but still distinctly granitic -in structure. The junction on the north side is equally well seen below the -crofts of Torran. Here the rock of the boss, for a few yards from its margin, -assumes a fine-grained felsitic aspect, and under the microscope presents a -curious brecciated appearance, suggestive of its having broken up at the -margin before final consolidation. Portions of the already crystallized -granite seem to be involved in a microgranitic base. The rock has here -truncated a number of basalt-dykes which intersect the Cambrian limestone. -To one of these further reference will be made in the sequel.</p> - -<p>On the surface of the mass of Beinn an Dubhaich, a few little patches of -limestone occur to the south of Kilchrist Loch. Considering the nearly -vertical wall which the granophyre presents to the adjacent rock all round -<span class="pagenum" id="Page_384">- 384 -</span> -its margin, we may perhaps reasonably infer that these outliers of limestone -are remnants of a once continuous limestone sheet that overlay the eruptive -rock, and hence that, with due allowance for considerable denudation, the -present surface of the boss represents approximately the upper limit to -which the granophyre ascended through the limestone. The actual facts are -shown in <a href="#v2fig347">Fig. 347</a>.</p> - -<p>All round the margin of this boss, the limestone has been converted for -a variable distance of a few feet or many yards into a granular crystalline -marble. The lighter portions of the limestone have become snowy white; -but some of the darker carbonaceous beds retain their dark tint. The -nodules of chert, abundant in many of the limestones, project from the -weathered faces of the marble. The dolomitic portions of the series have -likewise undergone alteration into a thoroughly crystalline-granular or -saccharoid rock. The most thorough metamorphism is exhibited by portions -of the limestone which are completely surrounded by and rest upon the -granite. The largest of these overlying patches was many years ago -quarried for white marble above the old Manse of Kilchrist. I have shown -by lithological, stratigraphical and palæontological evidence that this limestone, -instead of belonging to the Lias, as was formerly believed, forms a -part of the Cambrian or possibly the very lowest Silurian series, being a -continuation of the fossiliferous limestone of western Sutherland and Ross-shire.<a id="FNanchor_391" href="#Footnote_391" class="fnanchor">[391]</a> -Mr. Clough and Mr. Harker, in the progress of the Geological -Survey in Skye, have ascertained that the distinctive characters of the three -groups of strata into which the limestone can be divided may be recognized -even through the midst of the metamorphism.<a id="FNanchor_392" href="#Footnote_392" class="fnanchor">[392]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_391" href="#FNanchor_391" class="label">[391]</a> <i>Quart. Journ. Geol. Soc.</i> vol. xliv. (1888) p. 62.</p> - -<p><a id="Footnote_392" href="#FNanchor_392" class="label">[392]</a> <i>Annual Report of Director-General of the Geological Survey for 1895.</i></p> - -</div> - -<p>The generally vertical line of separation between the rock of Beinn an -Dubhaich and the contiguous limestone has been taken advantage of for the -segregation of mineral veins. On the southern boundary at Camas Malag, a -greenish flinty layer, from less than an inch to two or three inches in width, -consisting of a finely-granular aggregate of some nearly colourless mineral, -which polarizes brilliantly, coats the wall of the granophyre, and also both -sides of the vein which proceeds from that rock into the limestone. But the -most abundant and interesting deposits are metalliferous. Fragments of a -kind of "gossan" may be noticed all along the boundary-line of the boss, -and among these are pieces of magnetic iron-ore and sulphides of iron and -copper. The magnetite may be seen in place immediately to the south of -Kilbride. A mass of this ore several feet in diameter sends strings and disseminated -particles through the surrounding granophyre, and is partially -coated along its joints with green carbonate of copper.</p> - -<p>From the Skye area important evidence is obtainable in regard to the -relation of the acid eruptions to (1) earlier eruptive vents filled with agglomerate; -(2) the bedded basalts of the plateaux; (3) the bosses, sills and -dykes of gabbro and dolerite; and (4) the great system of basic dykes.</p> - -<p>(1) <i>Relation of the Granophyre to older Eruptive Vents.</i>—The granophyre -<span class="pagenum" id="Page_385">- 385 -</span> -of Beinn na Caillich and the two Beinn Deargs has invaded on its -north-eastern side the Cambrian limestone and quartzite, and has truncated the -sheets of intrusive dolerite and gabbro that have there been injected into -them. But to the south-west it rises through the great Strath agglomerate -already described, and continues in that rock round to the entrance into -Strath Beg. The eruptive mass is in great part surrounded with a ring of -agglomerate, as if it had risen up a huge volcanic chimney and solidified -there, though probably there were more than one vent in this agglomerate -area. Again the thick mass of agglomerate north of Belig is interposed -between the bedded lavas and the great granophyre mass which extends -northwards to Loch Sligachan. On the west side of the Blaven ridge, a -number of masses of agglomerate are found on both sides of Glen Sligachan, -along the border of the same great tract of acid rock.</p> - -<div class="figcenter" id="v2fig348" style="width: 432px;"> - <img src="images/v2fig348.png" width="432" height="179" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 348.</span>—Section from Beinn Dearg to Beinn an Dubhaich, Skye.<br /><br /> - <i>a</i> <i>a</i>, Cambrian limestone; <i>b</i> <i>b</i>, volcanic agglomerate; <i>c</i> <i>c</i> <i>c</i>, basalt-dykes older than granophyre; <i>d<sup>1</sup></i>, granophyre of - Beinn Dearg; <i>d<sup>2</sup></i>, granophyre in the agglomerate neck; <i>d<sup>3</sup></i>, granite of Beinn an Dubhaich; <i>e</i>, basalt-dyke - younger than granite.</div> -</div> - - -<p>With regard to the relation of the granophyre of the Red Hills to the -great agglomerate of Strath, we may infer that the granophyre has not risen -exactly in the centre of the old funnel, but rather to the north of it, -unless we suppose, as already suggested, that some of the agglomerate -belongs to the cone that gathered round the eruptive orifice. It is -interesting to observe, however, that granophyre, from the same or from -another centre of protrusion, has likewise risen along the outer or southern -margin of the agglomerate, generally between that rock and the limestone, -but sometimes entirely within the agglomerate. The distance between the -nearest part of this ring of eruptive rock and the edge of the boss of Beinn -an Dubhaich is under 400 yards, the intervening space being occupied by -limestone (or marble), much traversed by north-west basalt-dykes. Most of -these dykes do not enter the rocks of the vent, and are abruptly truncated -by the mass of Beinn an Dubhaich. The probable structure of this locality -is shown in Fig 348.</p> - -<p>The masses of agglomerate which further westward so curiously follow -the margin of the great granophyre bosses, and those which are entangled -in that rock and in the gabbro, probably indicate, as already suggested, the -<span class="pagenum" id="Page_386">- 386 -</span> -position of a group of older volcanic funnels which provided facilities for -the uprise of the basic and acid magmas. The group of vents which, as -we have seen, probably rose out of the plateau-basalts, and first served for -the rise of the masses of gabbro, has by the subsequent protrusion of the -granophyres been still further destroyed and concealed.</p> - -<p>The granophyre intrusions in the great Strath agglomerate have lately -been mapped and described by Mr. Harker. As regards their internal -structure and composition, this observer remarks that compared with the -normal granophyres of the Red Hills and other bosses of the district, these -smaller intrusive masses are darker and manifestly richer in the iron-bearing -minerals, and have a slightly higher specific gravity. But in their general -characters they agree with the other granophyres. The most interesting -feature in them is the evidence they afford that they have enclosed and -partially dissolved fragments of basic rocks. To this evidence further -reference will be made on a later page (see <a href="#Page_392">p. 392</a>).</p> - -<p>(2) <i>Relation of the Granophyre to the Bedded Basalts of the Plateaux. -Metamorphism of the Basalts.</i>—On the north-west side, the granophyre of -Glamaig and Glen Sligachan mounts directly out of the bedded basalts. -These latter rocks, which rise into characteristic terraced slopes on the north -side of Loch Sligachan, appear on the south side immediately to the west of -Sconser, and stretch westwards round the roots of Glamaig into the Coire -na Sgairde. As they approach that hill they assume the usual dull, -indurated, splintery, veined character of their contact metamorphism, -and weather with a pale crust. Some of them are highly amygdaloidal, -and between their successive beds thin bands of basalt-breccia, also much -hardened, occasionally appear. Veins of granophyre become more numerous -nearer the main mass of that rock. The actual line of junction runs into -the Coire na Sgairde and slants up the Druim na Ruaige, ascending to -within a few feet of the top of that ridge. A dark basic rock lies on the -granophyre, the latter being here finer grained and greenish in colour, and -projecting up into the former.<a id="FNanchor_393" href="#Footnote_393" class="fnanchor">[393]</a> There is so much detritus along the sides -and floor of Glen Sligachan that the relations of the two groups of rock -cannot be well examined there. But the basalts, which present their -ordinary characters to the north of the Inn, are observed to become more -and more indurated, close-grained, dull and splintery, as they draw nearer to -the granophyre of Marsco. This part of the district furnishes the clearest -evidence of the posteriority of the great cones of Glamaig and its neighbours -to the plateau-basalts which come up to the very base of these hills.<a id="FNanchor_394" href="#Footnote_394" class="fnanchor">[394]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_393" href="#FNanchor_393" class="label">[393]</a> I think it probable that some of the greenish portions of the granophyre along this part of -the junction-line will be found to have had their structure and composition altered by having -incorporated into their substance a proportion of the bedded basalts through which they have -been disrupted.</p> - -<p><a id="Footnote_394" href="#FNanchor_394" class="label">[394]</a> The dykes of granophyre in these basalts are referred to at p. 444.</p> - -</div> - -<p>Round the eastern group of cones some interesting fragments of the once -continuous sheet of plateau-basalts remain, and show the same relation of the -acid protrusions on that side. One of these lies on the granophyre of the -flanks of Beinn na Caillich, a little to the west of the loch at the northern -<span class="pagenum" id="Page_387">- 387 -</span> -base of that hill. Another of larger size forms a prominent knob about -three-quarters of a mile further west, and is prolonged into the huge dark -excrescence of Creagan Dubha, which rises in such striking contrast to the -smooth red declivities of the granophyre cones around it. This prominence -at its eastern and northern parts consists of highly indurated splintery -basalt in distinct beds, some of which are strongly amygdaloidal. The -bedding is nearly vertical, but with an inclination inwards to the hill. -Towards the south-west end a thin band of basalt-breccia makes its -appearance between two beds of basalt. Its thickness rapidly increases -southward until it is the only rock adhering to the granophyre. Beyond -the foot of the hill, limestone and quartzite occupy for some distance the -bottom of Strath Beg, much invaded by masses of quartz-porphyry. At the -summit of Creagan Dubha abundant veins run into the basic rocks from the -granophyre, which is here finer grained towards the margin; and there -are likewise veins of quartz-porphyry which, though their actual connection -with the main mass of granophyre cannot be seen, are no doubt apophyses -from it.</p> - -<p>This outlier of altered basalt and breccia appears to me to be a fragment -of the plateau-basalts which once overlay the Cambrian and Jurassic rocks -of Strath Beg, and were disrupted by the uprise of the granophyre. It -continues to adhere to the wall of the eruptive mass that broke up and -baked its rocks. Its breccia, passing southward into a coarse agglomerate, -may be a product of the same vent or group of vents that discharged the -great agglomerate mass above Kilbride and Kilchrist. I have already -(p. 282) referred to what appears to be another outlier of the basalts on the -south side of Beinn Dearg.</p> - -<p>On the northern and southern flanks of Beinn na Cro, similar evidence -may be observed of the posteriority of the granophyre to the basic rocks. -Round the northern base of the hill a continuous tract of plateau-basalts, -dolerites and gabbros forms the ridge between Strathmore and Strathbeg. -There is an admirable section of the relation of the two groups of rock on -the eastern side of the western glen. Along the lower part of the declivity, -coarsely-crystalline gabbros, like some of those in the Cuillin Hills, are -succeeded by sheets of dolerite and basalt, the whole forming an ascending -succession of beds to the summit of the ridge. The edges of these beds are -obliquely truncated by the body of granophyre, which slants up the hill -across them and sends veins into them. They are further traversed by -basalt dykes, which here, as almost everywhere, abound (<a href="#v2fig349">Fig. 349</a>). On the -south side of Beinn na Cro, highly indurated black and grey Lias shales and -sandstones have been tilted up steeply and indurated by the eruptive rock -of the hill; and at one place some 800 feet above the sea, a little patch of -altered basalt, lying on the shale, but close up against the steep declivity of -granophyre, forms a conspicuous prominence on the otherwise featureless -slope.</p> - -<p>Reference has already been made to the mass of fine-grained hornblende-granite -which runs for several miles at the base of the volcanic series on -<span class="pagenum" id="Page_388">- 388 -</span> -the eastern side of the Blaven group of hills. Mr. Harker has traced a great -development of granophyre on the west side of these hills, where the acid -rock sends apophyses both into the bedded basalts and into the gabbros.</p> - -<div class="figcenter" id="v2fig349" style="width: 345px;"> - <img src="images/v2fig349.png" width="345" height="168" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 349.</span>—Section at north end of Beinn na Cro, Skye.<br /><br /> - <i>a</i>, basalt, dolerite and gabbro; <i>b</i>, granophyre of Beinn na Cro; <i>b′</i>, dyke of granophyre; - <i>c</i> <i>c</i>, basalt dykes.</div> -</div> - -<p>Combining the results of observations made not only in Skye but in Mull, -Rum and Ardnamurchan, I shall here give a fuller account of the metamorphism -of the basalts, to which frequent allusion has been made as one of -the evidences of the posteriority of the eruptive bosses of rock round which -it occurs.<a id="FNanchor_395" href="#Footnote_395" class="fnanchor">[395]</a> The field-geologist observes that the basalts, as they are traced -towards these bosses, lose their usual external characters. They no longer -weather into spheroidal blocks with a rich brown loam, but project in much -jointed crags, and their hard rugged surface shows when broken a thin white -crust, beneath which the rock appears black or dark bluish-grey, dull and -splintery. They are generally veined with minute threads or strings of -calcite, epidote and quartz, which form a yellowish-brown network that -projects above the rest of the weathered surface. Where they are amygdaloidal, -the kernels no longer decay away or drop out, leaving the empty -smooth-surfaced cells, but remain as if they graduated into the surrounding -rock by an interlacing of their crystalline constituents. They then look at -a distance more like spots of decoloration, and even when seen close at hand -would hardly at first betray their real nature.</p> - -<div class="footnote"> - -<p><a id="Footnote_395" href="#FNanchor_395" class="label">[395]</a> Many years ago I was much struck with the evidence of alteration in the igneous rocks of -Mull, and referred to it in several papers, <i>Proc. Roy. Soc. Edin.</i> (1866-67) vol. vi. p. 73; <i>Quart. -Journ. Geol. Soc.</i> xxvii. (1871) p. 282, note. The subject was more fully discussed in my memoir -in the <i>Trans. Roy. Soc. Edin.</i> vol. xxxv. (1888) p. 167, from which the account in the text is -taken. Prof. Judd has more recently referred the alteration to solfataric action (<i>Quart. Journ. -Geol. Soc.</i> xlvi. 1890, p. 341). As already mentioned, I have been unable to detect evidence of such -action. The alteration is always intimately connected with the presence of intrusive masses, and -it affects indifferently any part of the basalt-plateaux which may chance to lie next to these -masses. The bedded lavas can be traced step by step from their usual unaltered condition in the -plateaux to their metamorphosed state next to the eruptive rocks. The nature or degree of the -metamorphism has doubtless somewhat varied with the composition and structure of the rocks -affected, and with the character and mass of the eruptive material; but it is certainly not confined -to the older parts of the plateaux, nor to any supposed pre-basaltic group of andesites. I -have found no evidence that such a group anywhere preceded the plateau-basalts. The andesites, -so far at least as my observations go, were erupted at intervals during the plateau period, -and alternate with the true basalts. The greatest accumulation of them lies not below but above -the general body of the basalts, in the "pale group" of Mull. Nor even if the term "propylite" -be adopted for these altered rocks, can it be applied to any special horizon in the volcanic series. -The alteration of the basic rocks by the granophyre of St. Kilda will be described in the account -of that island in Chapter xlvii.</p> - -</div> - -<p><span class="pagenum" id="Page_389">- 389 -</span></p> - -<p>From the specimens collected by me among the Inner Hebrides up to -the year 1888, I selected two dozen which seemed to be fairly typical of these -altered rocks, and placed thin slices of them for microscopic examination -in Dr. Hatch's hands. His notes may be condensed into the following -summary. One of the most frequent features in the slides is the tendency -in the component minerals to assume granular forms. In one specimen from -Loch Spelve, Mull, the rock, probably originally a dolerite, shows only a few -isolated recognizable crystals of plagioclase and augite, the whole of the rest -of the rock consisting of roundish granules embedded in a felspathic matrix. -The felspar crystals are sometimes broken up into a mosaic, though retaining -their external contours. Besides the granules, which are no doubt augite, a -few grains of magnetite are scattered through the rock, aggregated here and -there into little groups. In another specimen, taken from the junction with -the granophyre in Glenmore in the same island, parts of the augite crystals -are converted into granular aggregates associated with large grains and -patches of magnetite. The latter mineral also assumes in some of the rocks -granular and even globular shapes suggestive of fusion.</p> - -<p>The felspars, which in most of the basic rocks are usually remarkably -clear and fresh, show marked kaolinization in some of these altered masses. -Minute dusky scales of kaolin are developed, sometimes also with the separation -of minute grains of quartz. The augite shows frequent alteration to -hornblende, proceeding as usual from the exterior inward. In some cases -only an envelope of uralite appears round the augite, while in others only a -kernel of the original mineral is left, or the whole crystal has been changed. -In many cases the altered substance appears as minute needles, blades and -fibres of actinolite. Occasionally, besides the green hornblende, shred-like -pieces of a strongly pleochroic brown hornblende make their appearance. -Serpentinous and chloritic substances are not infrequent. Epidote is sometimes -abundant. The titaniferous iron has commonly passed more or less -completely into leucoxene. Here and there a dark mica may be detected.</p> - -<p>Since the year 1888 I have continued the investigation of this subject, -and have especially studied the metamorphism of the bedded basalts on the -western shores of Loch Scavaig, where, as already described, they are truncated -by vertical beds of gabbro, and are traversed by basalt-dykes and by -abundant veins of fine-grained granophyre. The alteration here effected -affords excellent materials for study, as the very same sheets of basalt can be -followed from the normal conditions outside to the altered state within the -influence of the metamorphic agent. The alternations of amygdaloidal and -more compact sheets can still be recognized, although their enclosed amygdales -have in places been almost effaced. They show the dull, indurated, -splintery character, with the white weathered crust, so distinctive of this -type of contact-metamorphism. They are traversed by numerous sills and -veins of gabbro. As has been already suggested, although no large mass of -<span class="pagenum" id="Page_390">- 390 -</span> -granophyre appears here at the surface, the alteration of the basalts is -probably to be attributed not so much to the influence of the gabbro, as to -the abundant acid sills, dykes and veins, for there may be a considerable -body of granophyre underneath the locality, the dykes and veins being -indications of its vicinity.</p> - -<p>In the summer of 1895 I examined the locality with much care, and -collected some typical specimens illustrative of the conditions of metamorphism -presented by different varieties of the bedded basalts. Thin -slices cut from these specimens were placed in Mr. Harker's hands for -microscopical examination, and he furnished me with the following notes -regarding them.</p> - -<p>"In hand-specimens the bedded basalts from the neighbourhood of the -gabbro of Loch Scavaig [6613-6618] do not appear very different from the -normal basalts of this region. The most conspicuous secondary mineral is -yellowish-green epidote in patches, and especially in the amygdales.</p> - -<p>"The texture of the rocks varies, and the slices show that the micro-structure -also varies, the augite occurring sometimes in small ophitic plates, -sometimes in small rounded granules. The chief secondary change in the -body of the rock is shown by the augite, which is seen in various stages of -conversion to greenish fibrous hornblende. Some round patches seem also -to consist mainly of the latter mineral, and are probably pseudomorphs after -olivine. Here the little fibres are confusedly matted together, without the -parallelism proper to uralite derived from augite. No fresh olivine has been -observed. The felspar and magnetite of the basalts show little or no sign -of metamorphic processes, unless a rather unusual degree of clearness in the -felspar crystals is to be regarded in that light.</p> - -<p>"The contents of the metamorphosed amygdales are not always the -same. Epidote is usually present in some abundance, and in well-shaped -crystals. It has a pale citron tint in the slices, with marked pleochroism; -but a given crystal is not always uniform in its optical characters. Frequently -the interior is pale, and has a quite low birefringence. This is -probably to be regarded as an intergrowth of zoisite in the epidote, and -there are a few distinct crystals of zoisite seen in some places.</p> - -<p>"In the slide which best exhibits these features [6613] the crystals of -epidote are in part enwrapped and enclosed by what are doubtless zeolitic -minerals. At least two of these are to be distinguished. One, very nearly -isotropic, and with a pale-brownish tint, is probably analcime. Associated -with this is a colourless mineral with partial radiate arrangement and with -twin lamellation; the birefringence is somewhat higher than that of quartz, -and the γ-axis of optic elasticity makes a small angle with the twin-line. -These characters agree with those of epistilbite. In other parts of the same -large amygdale, the epidote crystals are embedded in what seems to be a -felspar. This latter mineral is rather obscure, and twin-lamellation is rarely -to be detected; but it seems highly probable that felspar has here been -developed by metamorphic agency at the expense of zeolites which once -occupied the amygdale. I have observed undoubted examples of this in -<span class="pagenum" id="Page_391">- 391 -</span> -metamorphosed basalts from other parts of Skye, <i>e.g.</i> from Creagan Dubha, -near the granophyre mass of Beinn Dearg.<a id="FNanchor_396" href="#Footnote_396" class="fnanchor">[396]</a> The felspar occurs there in the -same fashion, and in the same relation to epidote [2700, 2701]. In the -specimens now described the chief minerals in the metamorphosed amygdales -are those already named: others occur more sparingly, associated with them. -In some cases there is a grass-green, strongly pleochroic, actinolitic hornblende, -accompanied by a little iron pyrites [6615].</p> - -<div class="footnote"> - -<p><a id="Footnote_396" href="#FNanchor_396" class="label">[396]</a> Compare <i>Trans. Roy. Soc. Edin.</i> vol. xxxv. p. 166.</p> - -</div> - -<p>"Epidote and various hornblendic and augitic minerals are characteristic -products in the metamorphism of amygdaloidal basalts in other regions: -felspar with this mode of occurrence I have not seen except in Skye, where -it seems to connect itself naturally with the abundance of zeolites in the -amygdales of the non-metamorphosed lavas. It is to be observed that in -these basalts from Loch Scavaig the alteration is shown especially in the -amygdales, the body of the rock not being greatly affected: this indicates a -not very advanced stage of metamorphism. The production of uralitic -hornblende, rather than brown mica, from the augite and its decomposition-products, -seems to be characteristic of the metamorphism of basaltic as distinguished -from andesitic rocks, and is well illustrated by a comparison of -the two sets of lavas near the Shap granite."<a id="FNanchor_397" href="#Footnote_397" class="fnanchor">[397]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_397" href="#FNanchor_397" class="label">[397]</a> <i>Quart. Journ. Geol. Soc.</i> vol. xlix. (1893) p. 361.</p> - -</div> - -<p>Mr. Harker, who is at present engaged in mapping the central region of -Skye, has had occasion to go over a number of the localities (Creagan -Dubha, etc.) originally cited by me, and, while corroborating my general -conclusions regarding them, has been able to obtain much fresh evidence -regarding the nature and extent of the metamorphism which the bedded -basalts have undergone. The results of his investigations will be published -when the Geological Survey of Skye is further advanced.</p> - -<p>(3) <i>Relation of the Granophyre to the Gabbros.</i>—That the granophyres -invade the gabbros has been incidentally illustrated in the foregoing pages. -But as the mutual relations of the two rocks in the island of Skye have been -the subject of frequent reference in previous writings of geologists, it is -desirable to adduce some detailed evidence from a region which has been -regarded as the typical one for this feature in the geological structure of -the Inner Hebrides. No geological boundary is more easily traced than -that between the pale reddish granophyre and the dark gabbro. It can be -followed with the eye up a whole mountain side, and can be examined so -closely that again and again the observer can walk or climb for some distance -with one foot on each rock. That there should ever have been any -doubt about the relations of the two eruptive masses is possibly explicable -by the very facility with which their junction can be observed. Their -contrasts of form and colour make their boundary over crag and ridge so -clear that geologists do not seem to have taken the trouble to follow it out -in detail. And as the pale rock undoubtedly often underlies the dark, they -have assumed this infraposition to mark its earlier appearance.</p> - -<p>I will only cite one part of the junction line, which is easily accessible, -<span class="pagenum" id="Page_392">- 392 -</span> -for it lies in Glen Sligachan immediately to the south of the mouth of -Harta Corry. The rounded eminence of Meall Dearg, which rises to the -south of the two Black Lochs, belongs to the granophyre, while the rugged -ground to the west of it lies in the gabbro. The actual contact between the -two rocks can be followed from the side of Harta Corry over the ridge and -down into Strath na Creitheach, whence it sweeps northward between the -red cone of Ruadh Stac and the black rugged declivities of Garbh Beinn. -There is no more singular scene in Skye than the lonely tract on the south -side of Meall Dearg. The ground for some way is nearly level, and strewn with -red shingle from the decomposing granophyre underneath. It reminds -one of some parts of the desert "Bad lands" of Western America. Grim -dark crags of gabbro, with veins from the granophyre, rise along its -western border, beyond which tower the black precipices of the Cuillins, -while the flaming reddish-yellow cones of Glen Sligachan stand out against -the northern sky.</p> - -<p>Having recently described in some detail the relations of the boss of -granophyre at this interesting locality, I will only here offer a brief summary -of the chief features.<a id="FNanchor_398" href="#Footnote_398" class="fnanchor">[398]</a> The granophyre of Meall Dearg forms a marginal -portion of the great mass of the Red Hills. It has broken across the -banded gabbros, and also cuts an isolated boss of agglomerate in the ridge of -Druim an Eidhne. Its line of junction is nearly vertical, but along part of -its course the wall of gabbro rises higher than that of the more decomposable -granophyre. Hence the origin of the black crags that crown the red -slopes of granophyre debris. Seen from a distance the basic rock seems to -rest as a great bed upon the acid mass.</p> - -<div class="footnote"> - -<p><a id="Footnote_398" href="#FNanchor_398" class="label">[398]</a> See <i>Quart. Journ. Geol. Soc.</i> vol. 1. (1894) p. 212.</p> - -</div> - -<p>The younger date and intrusive nature of the granophyre are well -shown by the change in the texture of the mass as it approaches the rocks -against which it has cooled. The ordinary granophyric characters rapidly pass -into a fine-grained felsitic texture, and this change is accompanied with the -development of a remarkably well-defined flow-structure and of rows of -spherulites which run parallel to the boundary wall. In a ravine on the -west side of Meall Dearg, the lines of flow-structure and rows of large -spherulites are seen to be arranged vertically against the face of gabbro.</p> - -<p>Further proof of the later date of the protrusion of the granophyre is -supplied by abundant felsitic dykes and veins which traverse the gabbro, and -some of which can be seen to proceed from the main body of granophyre. -These intrusions will be described in the next chapter, in connection with -the dykes and veins of the acid rocks.</p> - -<p>Additional evidence as to the posteriority of the granophyre to -the gabbro has recently been obtained by Mr. Harker from a study -of the internal structure and composition of the masses of these rocks -which have been intruded into the agglomerate above Loch Kilchrist -in Strath. He has found that the granophyre has there caught up -from some subterranean depth portions of gabbro, and has partially -dissolved them, thereby undergoing a modification of its own composition. -<span class="pagenum" id="Page_393">- 393 -</span> -"The gabbro-debris," he remarks, "has been for the most part completely disintegrated -by the caustic or solvent action of the acid magma on some of its -minerals. Those constituents which resisted such action have been set free -and now figure as xenocrysts [foreign crystals], either intact or more or less -perfectly transformed into other substances. At the same time the material -absorbed has modified the composition of the magma, in the general sense -of rendering it less acid." Mr. Harker has traced the fate of each of the -minerals of the gabbro in the process of solution and isolation in the acid -magma, which, where this process has been most developed, is believed by -him to have taken up foreign material amounting to fully one-fourth of its -own bulk, derived not from the rocks immediately around, but from a gabbro -probably at a considerable depth beneath.<a id="FNanchor_399" href="#Footnote_399" class="fnanchor">[399]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_399" href="#FNanchor_399" class="label">[399]</a> <i>Quart. Journ. Geol. Soc.</i> vol. lii. (1896) p. 320.</p> - -</div> - -<div class="figright" id="v2fig350" style="width: 171px;"> - <img src="images/v2fig350.png" width="171" height="166" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 350.</span>—Ground-plan of basic - dyke in Cambrian Limestones - truncated by granophyre which - encloses large blocks of the dyke, - Torrin, Skye.</div> -</div> - - -<p>(4) <i>Relation of the Granophyre to the Basic Dykes and Veins.</i>—Reference -has already been made to the fact that the "syenite" bosses of Skye cut off -most of the basalt-dykes, but are themselves traversed by a few others.<a id="FNanchor_400" href="#Footnote_400" class="fnanchor">[400]</a> -The locality that furnished me with the evidence on which this statement -was originally made nearly forty years ago affords in small compass a clearer -presentation of the facts than I have elsewhere met with. The sections -described by me are visible at the eastern end of the boss of Beinn an -Dubhaich, Strath; but similar and even better examples may be cited from -the whole northern and southern margins of that eruptive mass. On the -north side an extraordinary number of dykes may be traced in the Cambrian -limestone from the shores of Loch Slapin eastwards. They have a general -north-westerly trend, but one after another, as I have already remarked, is -abruptly cut off by the granophyre. As an example of the way in which -this truncation takes place, I may site a single illustration from the northern -margin of the eruptive mass, near Torrin. It might perhaps be contended -that the numerous dykes which traverse the -limestone and stop short at the edge of the acid -rock, are not necessarily older than the granophyre, -but may actually be younger, their sudden -termination at the edge of the acid boss being due -to their inability to traverse that rock. That this -explanation is untenable is readily proved by -such sections as that given in <a href="#v2fig350">Fig. 350</a>, where -a basic dyke (<i>b</i>) 9 or 10 feet broad running -through the Cambrian Limestone (<i>a</i> <i>a</i>) is abruptly -cut off by the edge of the great granophyre boss. -Not only is the dyke sharply truncated, but -numerous pieces of it, from an inch to more than -a foot in length, are enclosed in the granophyre. -The latter is well exposed along the shore of -Loch Slapin in an almost continuous section of nearly a mile in length. -The contrast therefore between the development of dykes within and beyond -<span class="pagenum" id="Page_394">- 394 -</span> -its area cannot but arrest the attention of the observer. Though I was on -the outlook for dykes in the granophyre, I found only one. Yet immediately -beyond the eruptive boss they at once appear on either side up to its very -edge, where they suddenly cease. The conclusion cannot be resisted that -the protrusion of the acid rock took place after most of the dykes of the -district had been formed, but before the emission of the very latest dykes, -which pursue a north-west course across the boss (<a href="#v2fig348">Fig. 348</a>).</p> - -<div class="footnote"> - -<p><a id="Footnote_400" href="#FNanchor_400" class="label">[400]</a> <i>Ante</i>, p. 173, and <i>Quart. Journ. Geol. Soc.</i> vol. xiv. (1857) p. 16.</p> - -</div> - -<p>Some sections on the southern margin of Beinn an Dubhaich complete -the demonstration that such has been the order of appearance of the rocks. -Near the head of the Allt Lèth Slighe (or Half-way Burn), where the -granite has pushed a long tongue into the limestone, a north-west basalt-dyke -is abruptly cut off by the main body of the boss and by the protruded -vein (<a href="#v2fig351">Fig. 351</a>). Besides this truncation, the acid rock sends out strings -and threads of its own substance into and across the dyke, these injected -portions being as usual of an exceedingly fine felsitic texture.</p> - -<div class="figcenter" id="v2fig351" style="width: 372px;"> - <img src="images/v2fig351.png" width="372" height="160" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 351.</span>—Section on south side of Beinn an Dubhaich, Skye, showing the truncation of a basalt-dyke - (<i>b</i>), in Cambrian Limestone (<i>a</i>), by the granite (<i>c</i>) of Beinn an Dubhaich, Skye.</div> -</div> - -<p>Similar evidence may be gathered from the area of the great granophyre -cones further north. The profusion of basalt-dykes in the surrounding -rocks stops short at the margin of that area. The comparatively few -dykes which cross the boundary pursue a general north-west course through -the granophyre, and, as already remarked, from their dark colour, greater -durability and straightness of direction, stand out as prominent ribs on the -flanks of the pale cones which they traverse.</p> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_395">- 395 -</span></p> - -<h2 class="nobreak" id="CHAPTER_XLVII">CHAPTER XLVII<br /> - -<span class="smaller">THE ACID BOSSES OF MULL, SMALL ISLES, ST. KILDA, ARRAN AND THE -NORTH-EAST OF IRELAND</span></h2> -</div> - - -<h3>ii. <span class="allsmcap">THE ACID BOSSES OF MULL</span></h3> - -<p>Though of comparatively small extent, the granophyre bosses of the island -of Mull afford to the geologist a large amount of instruction in regard to -the relations of the different members of the volcanic series to each other. -Especially important is the evidence which they contain of the connection -between the acid and basic groups of rocks. They have been laid bare in -many natural sections, some of which, forming entire hillsides, are among -the most astonishing in the whole wonderful series which, dissected by denudation, -reveal to us the structure of these volcanic regions. They lie in two -chief areas. One of these extends along the northern flanks of the mountainous -tract from the western side of Beinn Fhada across Loch Ba' to the -west side of Glen Forsa. The other occupies for over three miles the bottom -of Glen More, the deep valley which, skirting the southern side of the chief -group of hills, connects the east side of the island by road with the head of -the great western inlet of Loch Scridain. There are other minor areas. One -of these extends for about a mile along the declivities to the south of Salen, -across the valley of the Allt na Searmoin; another occurs at Salen; a -third runs along the shore at Craignure. In the interior also, many -isolated areas of similar rocks, besides thousands of veins, occur in the -central group of hills and valleys which form the basins of the Glencannel -and Forsa rivers (Map VI.).</p> - -<p>The chief northern boss, which for the sake of convenience of reference -may be called that of Loch Ba', has a length of nearly six miles, with a -breadth varying from a quarter of a mile to about a mile and a quarter. It -descends to within 50 feet of the sea-level, and is exposed along the crest of -Beinn Fhada at a height of more than 1800 feet. It chiefly consists of -a grey crystalline rock which might readily be identified as a granite, but -which when examined microscopically is found to possess the granophyric -structure. With this distinctly granular-crystalline rock are associated -various porphyritic and felsitic masses, which pass into it, and are more -specially observable along its border. An exceedingly compact black -<span class="pagenum" id="Page_396">- 396 -</span> -quartz-felsite or rhyolite forms its southern boundary, runs as a broad -dyke-like ridge from the head of the Scarrisdale Water north-eastward across -Loch Ba' (<a href="#v2fig352">Fig. 352</a>), and spreads out eastward into a mass more than a -mile broad on the heights above Kilbeg in Glen Forsa. The sharp line -of demarcation of this felsite, and its mass and extent, point to a different -period of extravasation.</p> - -<div class="figcenter" id="v2fig352" style="width: 554px;"> - <img src="images/v2fig352.png" width="554" height="239" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 352.</span>—View of the hills on the south side of the head of Loch na Keal, showing the junction of the granophyre - and the bedded basalts.<br /><br /> - One bird, the bedded basalts of the Gribon plateau; two birds, the bedded dolerites and basalts of Beinn a' Chraig adhering to the - northern slope and capping the hill; three birds, summit of Ben More, with A'Chioch to the left and the top of Beinn Fhada - appearing in the middle distance between them; four birds, the granophyre slopes of Beinn a' Chraig with the great dyke-like - mass of felsite on the left.</div> -</div> - - -<p>The geologist, who -approaches this district -from the north-east, has his -attention arrested, even at -a distance of several miles, -by the contrast between the -outer and inner parts of -the hills that lie to the -south-west of Loch Ba'. -He can readily trace from -afar the dark bedded basic -rocks rising terrace above -terrace, from the shores of -Loch na Keal, to form the -seaward faces of the hills -along the southern side of -that fjord. But he observes -that immediately behind -these terraces the mass of -the rising ground obviously -consists of some amorphous -rock, which weathers into -white debris. Nothing -can be sharper than the -contrast of colour and form -between the two parts of -the hills. The bedded -plateau-rocks lie as a kind -of wall or veneer against a -steep face of the structureless -interior (<a href="#v2fig352">Fig. 352</a>). -Seen from the other or -hilly side, the contrast is -perhaps even more striking. But the astonishment with which it -is beheld at a distance becomes intensified when one climbs the slopes, -and finds that the sheets of dolerite and basalt (which from some -points of view look quite level, yet dip towards the north-east at a -gentle angle) are immediately behind the declivity abruptly truncated by -a mass of granophyre. Of all the junction-lines between the acid bosses -and the lavas of the plateaux, those exposed on these Mull hillsides are -<span class="pagenum" id="Page_397">- 397 -</span> -certainly the most extraordinary. So little disturbed are the lavas, that -one's first impulse is to search for pebbles of the granophyre between the -basalts, for it seems incredible that the inner rock should be anything but a -central core of older eruptive material, against and round which the younger -basic rocks have flowed. But, though the granophyre is so decomposing and -covers its slopes with such "screes" of debris, that had the basalts been -poured round it, they must infallibly have had some of its fragments washed -down between their successive flows, not a single pebble of it is there to be -found. This might not be considered decisive evidence, but it is extended -and confirmed by the fact that the acid rock gives off veins which ramify -through the basalts.</p> - -<p>Before examining the actual contact of the two rocks, however, the -geologist will not fail to observe here an admirable example of the gradual -change which was described in the foregoing chapter as coming over the -bedded basalts near the acid bosses. As he approaches the nucleus of white -rock, the basalts assume the usual hard indurated character, not decaying -into brown sand as on the plateaux, but often standing out as massive crags -with vertical clean-cut joint-faces. This metamorphosed condition extends -in some cases to a considerable distance from the main body of acid rock, -especially where knobs of that material, protruding through the more basic -lavas, show that it must extend in some mass underneath. Thus along the -shore at Saline the bedded basalts succeed each other in well-defined sheets, -some being solid, massive and non-amygdaloidal, others quite vesicular, and -recalling the black scoriform surfaces of recent Vesuvian lavas; yet they -are all more indurated than in the normal plateau-country, and they break -with a hard splintery fracture. Immense numbers of dykes cut these rocks, -and they are likewise pierced by occasional felsitic intrusions.</p> - -<p>If we cross to the other side of the island and trace the bedded basalts -away from the central masses of acid rock we meet with so gradual a diminution -of the induration that no definite boundary-line for the metamorphism -can be drawn. As we recede from the centre of alteration, the rocks -insensibly begin to show brown weathered crusts, with spheroidal exfoliation, -the reticulations of epidote and calcite become much less abundant, -the amygdaloids gradually assume their normal earthy character, and -eventually we find ourselves on the familiar types of the plateau. This -transition is well seen along the shores of Loch na Keal.<a id="FNanchor_401" href="#Footnote_401" class="fnanchor">[401]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_401" href="#FNanchor_401" class="label">[401]</a> Some of the thick massive sheets of basic rock along the south side of this inlet may -possibly be altered sills.</p> - -</div> - -<p>These proofs of the alteration of the plateau-basalts are accompanied in -Mull as in Skye by further abundant evidence that the acid rocks are of -younger date than the basic. In particular, dykes and veins may be traced -proceeding from the former and intersecting the latter. Thus, in the bed -of the south fork of the Scarrisdale stream, a separate mass of granophyre -(which under the microscope exhibits in perfection the characteristic structure -of this rock) protrudes through the basalts in advance of the main -mass, and a little higher up on the outskirts of that mass narrow ribbons -<span class="pagenum" id="Page_398">- 398 -</span> -of the granophyre run through the basic rocks. The contrast of colour -between the pale veins of the intrusive rock and the dark tint of the basalts -is well shown in the channel of the water. Similar sections may be seen on -the flanks of Beinn Fhada, especially in the great corry north of Ben More, -where the granophyre sends a tongue of finer grain between the beds of -basalt. On the east side of Loch Ba' numerous proofs of similar intrusion -may be observed. Thus at the east end of Loch na Dà iridh, where the -granophyre has been intruded into the basalts, hand-specimens may be -obtained showing the two rocks welded together. On the slopes of Cruach -Tòrr an Lochain, where the granophyre has a felsitic selvage, the bedded -basalts are traversed by veins of the latter material (<a href="#v2fig353">Fig. 353</a>). A little -further east, at the head of the Allt na Searmoin, the bedded basalts, -some of which are separated by slaggy scoriaceous surfaces, are intersected -by another protrusion from the compact felsitic porphyry (<a href="#v2fig354">Fig. 354</a>).<a id="FNanchor_402" href="#Footnote_402" class="fnanchor">[402]</a> A -mile lower down the same valley a separate mass of granophyre sends out -veins into the basalt, which as usual is dark bluish-grey in colour, indurated -and splintery.</p> - -<div class="footnote"> - -<p><a id="Footnote_402" href="#FNanchor_402" class="label">[402]</a> This rock appears to the eye as a black finely crystalline-granular felsite. Under the -microscope, it was found by Dr. Hatch to "present a markedly granulitic structure, consisting -mainly of small rounded grains of dirty brown turbid felspar, with isolated granules of colourless -quartz. Scattered through the rock, or accumulated in patches, are small spherical or drop-like -granules of a bright green augite (coccolite)."</p> - -</div> - -<div class="figcenter" id="v2fig353" style="width: 365px;"> - <img src="images/v2fig353.png" width="365" height="124" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 353.</span>—Section on south side of Cruach Tòrr an Lochain, Mull.<br /><br /> - <i>a</i>, bedded basalts and dolerites; <i>b</i>, granophyre; <i>c</i>, marginal finer-grained band; <i>d</i> <i>d</i>, veins from the granophyre - traversing the basic rocks.</div> -</div> - -<div class="figleft" id="v2fig354" style="width: 261px;"> - <img src="images/v2fig354.png" width="261" height="159" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 354.</span>—Section at head of Allt na Searmoin, Mull.<br /><br /> - <i>a</i>, basalts and dolerites, with slaggy upper surfaces; <i>b</i>, felsite.</div> -</div> - -<p>As the posteriority of the Mull granophyre and felsites to the basalts is -thus proved, the further question -remains as to their manner of -intrusion. Here and there, -especially on the south-eastern -side, between the head of the -Scarrisdale river and Loch Ba', -the line of junction between the -two rocks is nearly vertical, but -a body of black felsite intervenes -as a huge wall between the -ordinary granophyre and the -basalt. On Beinn Fhada and -Beinn a' Chraig the line of separation, as I have above remarked, is inclined -<span class="pagenum" id="Page_399">- 399 -</span> -outwards, and plunges under the basalts at an angle of 30° to 40°. The -terraced basalts and dolerites are not sensibly disturbed, but end off abruptly -against the steep face of intrusive rock. We might suppose that in this -case the younger rock had merely carried upward the continuation of the -beds that are truncated by it, as if an orifice had been punched out for its -ascent. But on the top of the ridge of Beinn a' Chraig we find that the -outliers which there remain are not portions of the lower basalts, but of the -upper "pale group" of Ben More. The same rocks are prolonged on the -other side of the Scarrisdale Glen, sweep over the summit of Beinn Fhada, -and run on continuously into the crest of A'Chioch and the upper part of -Ben More. The granophyre has usurped the place of the lower dolerites -and basalts, but has left the more felspathic lavas of the "pale group" in -their proper position. And to make this remarkable structure still more -clear, sections may be seen on the southern flanks of Beinn Fhada, where the -upper surface of the granophyre comes down obliquely across the edges of the -lavas, and allows the junction of the basalts and the "pale group" to be seen -above it (<a href="#v2fig355">Fig. 355</a>). As in the case of Beinn an Dubhaich, it is as if the granophyre -had eaten its way upward and dissolved the rocks which it has replaced.</p> - -<div class="figcenter" id="v2fig355" style="width: 386px;"> - <img src="images/v2fig355.png" width="386" height="117" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 355.</span>—Section on south side of Beinn Fhada, Mull.<br /><br /> - <i>a</i>, bedded basalts and dolerites; <i>b</i>, "pale group" of Ben More; <i>c</i>, granophyre.</div> -</div> - -<p>The usual kind of contact-metamorphism has been produced around this -intrusive boss. It is most marked in the outliers that cap Beinn a' Chraig -and on the two ridges to the south-west, where it is seen to consist in a -high degree of induration, the production of a shattery, irregularly-jointed -structure, and the effacement of the obvious bedding which characterizes the -unaltered rocks.</p> - -<p>The position of this eruptive mass, quite a mile broad, breaking through, -without violently tilting, more than 1800 feet of the bedded basalts, -and then stopping short about the base of the "pale group," presents a -curious problem to the student of geological physics. It at once reminds -him of many sections among Palæozoic granites where an eruptive boss has -ascended and taken the place of an equivalent volume of the surrounding -rocks, which, though more or less metamorphosed, are not made to dip away -from it as from a solid wedge driven upwards through them. In this Mull -case, however, there are some peculiar features that deserve consideration, for -they seem to show that here, as elsewhere, passages for the uprise of the -intrusive rock were already provided by the presence of volcanic pipes, which, -<span class="pagenum" id="Page_400">- 400 -</span> -even if filled up with fragmentary materials, would no doubt continue to be -points of weakness. Round the flanks of the Loch Ba' boss, and here and -there on its surface, patches of intensely indurated volcanic agglomerate may -be detected. A little to the south of the tarn called Loch na Dà iridh, the -granophyre is succeeded by the black, flinty felsite or rhyolite already referred -to. This rock in some places exhibits a beautiful flow-structure, with large -porphyritic felspars, and encloses a great many fragments of dolerite and -gabbro, varying from the size of a pea up to blocks several inches in -diameter. Lying on its surface are detached knolls of much altered -dolerite, basalt, and coarse breccia or agglomerate. On its southern margin -one of these patches of agglomerate contains abundant fragments of various -felsitic rocks, among which are pieces of a compact rock with flow-structure -like that found in place immediately to the north; also rounded pieces of -quartzite, and of compact and amygdaloidal basalt wrapped up in a very -hard matrix which seems to consist largely of basalt-dust. No bedding can -be made out in this rock, and the mass looks like part of a true neck. -Further down the slope the bedded basalts appear. The actual junctions -of the different rocks cannot be satisfactorily traced, but the structure of -the ground appears to me to be as shown in <a href="#v2fig356">Fig. 356</a>. A patch of similar -agglomerate appears a little to the south-west of the last section in front of -a cliff of the felsite, and seems to be enclosed in the latter rock, and other -exposures of agglomerate, underlain and intensely indurated by the felsite, -may be noticed on the ground that slopes towards Loch Ba'.</p> - -<div class="figcenter" id="v2fig356" style="width: 414px;"> - <img src="images/v2fig356.png" width="414" height="91" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 356.</span>—Section to south of Loch na Dà iridh, Mull.<br /><br /> - <i>a</i>, basalts; <i>b</i>, dolerite; <i>c</i>, volcanic agglomerate; <i>d</i>, black felsite; <i>e</i>, granophyre.</div> -</div> - -<p>That these agglomerates do not belong to the period of the eruption of -the granophyre and felsite, but to that of the bedded basalts, may be inferred -from their intense induration next the acid rocks, and also from the fact that -similar breccias are actually found here interposed between the bedded -basalts. This is well shown on the hill above the Coille na Sròine, where -the accompanying section can be seen (<a href="#v2fig357">Fig. 357</a>). The broad dyke-like -mass of black flinty felsite already referred to runs as a prominent rib -over the southern end of Beinn a' Chraig into the head of the Scarrisdale -glen (see <a href="#v2fig352">Fig. 352</a>). It cuts across the bedded basalts, and immediately to -the south of where these appear, a thin intercalated bed of breccia crops out, -of the usual dull-green colour, with abundant fragments of basalt and many -of yellow and grey felsite.</p> - -<p>From these various facts we may, I think, conclude that along the strip -of ground now occupied by the Loch Ba' boss of granophyre and felsite, -there once stood a line or group of vents, from which, besides the usual -<span class="pagenum" id="Page_401">- 401 -</span> -basalt-debris, there were ejected many pieces of different felsitic or rhyolitic -rocks, and that these eruptions of fragmentary material took place during -the accumulation of the plateau-basalts. These volcanic funnels occasioned -a series of points or a line of weakness of which, in a long subsequent episode -of the protracted volcanic period, the acid rocks took advantage, forcing -themselves upwards therein, and leaving only slight traces of the vents -which assisted their ascent. The mingling of acid and basic fragments in -the material ejected from these vents is another proof of the existence of -acid rocks in the volcanic reservoirs before the advent of the great granophyre -intrusions. The evidence thus entirely confirms the conclusions -deduced from the Skye area.</p> - -<div class="figcenter" id="v2fig357" style="width: 324px;"> - <img src="images/v2fig357.png" width="324" height="202" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 357.</span>—Section of junction of south side of Loch Ba' granophyre boss, with - the bedded basalts, Mull.<br /><br /> - <i>a</i>, bedded basalts; <i>b</i> <i>b</i>, basalt-tuff and breccia; <i>c</i>, granophyre; <i>d</i>, black felsite; <i>e</i>, coarse dolerite dyke, - 30 or 40 feet wide.</div> -</div> - -<p>The second or Glen More boss, instead of rising into hilly ground, is -confined to the bottom of the main and tributary valleys, and has only been -revealed by the extensive denudation to which these hollows owe their -origin. It begins nearly a mile below Torness and extends up to Loch -Airdeglais—a distance of almost four miles. Though singularly devoid of -topographical feature, it exhibits with admirable clearness the relation of -the granophyres to the gabbros, and thus deserves an important place among -the tracts of acid rocks in the Western Islands. Its petrographical characters -change considerably from one part of its body to another. For the -most part, it is a true granophyre, sometimes with orthoclase, sometimes -with plagioclase as its predominant felspar. At Ishriff, as already stated, it -is sprinkled with long acicular decayed crystals of hornblende; but at the -watershed the ferro-magnesian mineral is augite. The surrounding rocks -are mainly the plateau-basalts, with their sills of dolerite and gabbro.</p> - -<p><span class="pagenum" id="Page_402">- 402 -</span></p> - -<div class="figcenter" id="v2fig358" style="width: 339px;"> - <img src="images/v2fig358.png" width="339" height="217" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 358.</span>—Mass of dark gabbro about two feet in diameter traversed by pale veins of granophyre, - lying on north slope of Creag na h-Iolaire, Mull.</div> -</div> - -<div class="figcenter" id="v2fig359" style="width: 428px;"> - <img src="images/v2fig359.png" width="428" height="206" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 359.</span>—Section at Creag na h-Iolaire, Glen More, Mull, showing basalts and gabbros - resting on and pierced by granophyre.<br /><br /> - <i>a</i>, much indurated and altered basalts and dolerites; <i>b</i> <i>b</i>, gabbro; <i>c</i>, granophyre; <i>d</i> <i>d</i>, basalt dykes.</div> -</div> - -<p>This strip of granophyre sends abundant apophyses from its mass into -the dark basic rocks around it. Some of the best sections to show the -nature of these offshoots are to be found on the steep hillslope which -mounts from the watershed in Glen More southward into the Creag na -h-Iolaire (Eagle's Crag), and thence up into the great gabbro ridge of Ben -Buy. From the main body of granophyre a multitude of veins ascends -through the basalts and gabbros from two feet or more in breadth down to -mere filaments (<a href="#v2fig358">Fig. 358</a>). Even at a height of 300 feet up the hill some -of these veins are still three inches broad, and present the usual granophyric -structure, though rather finer in grain than the general mass of the boss, -and sometimes assuming a compact felsitic and spherulitic texture at the -immediate contact with the surrounding rock. One of the most striking -proofs of the posteriority of these veins is furnished by the perfect flow-structure -they not infrequently exhibit along their margins, their long -felspar crystals being arranged parallel to the walls in lines that follow the -sinuosities of the boundary between the two rocks. Patches of gabbro -and of the indurated basalts may be seen lying on the granophyre, from -which veins and strings ramify through them (<a href="#v2fig359">Fig. 359</a>). Similar veins -<span class="pagenum" id="Page_403">- 403 -</span> -can be traced upward into the main body of coarse gabbro, forming the -ridge of Ben Buy. Some of them are of the usual granular granophyric -texture, others are dull and fine-grained (claystones of the older authors).</p> - -<p>Hence it is evident that the granophyres of Mull have been protruded -not only after the accumulation of the plateau-basalts, but after these were -traversed by the sheets and veins of gabbro. The amount of acid rock -injected into these older rocks over the mountainous part of the island is -enormous; but I reserve further reference to it for the section on acid -Dykes and Veins, for these are the forms in which it chiefly occurs in that -region. It should be added, that in the localities here referred to basalt-veins -and dykes are generally abundant, cutting through all the other -rocks (<a href="#v2fig359">Fig. 359</a>). So numerous are they that the geologist ceases to take -note of them when his thoughts are engaged upon the problems presented -by the masses through which they rise.</p> - - -<h3>iii. <span class="allsmcap">THE ACID BOSSES OF SMALL ISLES</span></h3> - -<p>In the island of Eigg three small bosses or sheets of acid rock occur. -That at the northern end rises through the Jurassic sedimentary rocks, and -forms a bold cliff from 150 to 200 feet high. It is a light grey granophyric -porphyry, with rounded blebs of quartz in a micropegmatic base -of quartz and felspar. The other two masses, of smaller size, cut through -the bedded basalts<a id="FNanchor_403" href="#Footnote_403" class="fnanchor">[403]</a> (Map VI.).</p> - -<div class="footnote"> - -<p><a id="Footnote_403" href="#FNanchor_403" class="label">[403]</a> <i>Quart. Journ. Geol. Soc.</i> xxvii. (1871) p. 294.</p> - -</div> - -<p>In the opposite island of Rum, the acid protrusions play a much more -important part. On the east side of the hills, they occur in sheets at the -base of the gabbros; on the west side, they form a large tract of hilly -ground, which, stretching along the coast-line for about three and a -half miles from the headland of A' Bhrideanach to Harris, forms there a -range of shattered sea-cliffs, that tower for 1000 feet above the Atlantic -breakers that beat about their base. The area extends inland to the slopes -on the west side of Loch Sgathaig, a distance of about three and a half -miles, descending in a range of precipices along its northern front, and -reaching in its culminating summit, Orval, a height of 1868 feet above the -sea. The rocks of which this triangular area consists resemble those of the -Mull bosses. They are chiefly quartz-porphyries, becoming felsitic in -texture towards their contact with adjacent rocks. In some places, as was -noticed by Macculloch on the sea-cliffs,<a id="FNanchor_404" href="#Footnote_404" class="fnanchor">[404]</a> they have a rudely bedded structure. -Thus on the north-west front of Orval, this structure is shown by parallel -planes that dip outwards or north-west at 30° to 40°, and which are made -still more distinct by an occasional intrusive dyke or sheet of basalt between -their surfaces. I have already alluded to indications of an internal arrangement -in the granitoid bosses of Skye (p. 381).</p> - -<div class="footnote"> - -<p><a id="Footnote_404" href="#FNanchor_404" class="label">[404]</a> <i>Western Islands</i>, vol. i. p. 487.</p> - -</div> - -<p><span class="pagenum" id="Page_404">- 404 -</span></p> - -<div class="figcenter" id="v2fig360" style="width: 463px;"> - <img src="images/v2fig360.png" width="463" height="109" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 360.</span>—Section on north side of Orval, Rum.<br /><br /> - <i>a</i>, Torridon sandstones; <i>b</i>, bedded basalts of Fionn Chro; <i>c</i>, dolerite; <i>d</i>, quartz-porphyry.</div> -</div> - - -<div class="figcenter" id="v2fig361" style="width: 337px;"> - <img src="images/v2fig361.png" width="337" height="175" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 361.</span>—Junction of Quartz-Porphyry (Microgranite) and Basic Rocks, south-east side of Orval, Rum.<br /><br /> - <i>a</i>, basalts and dolerites; <i>b</i>, dolerite and gabbro veins; <i>c</i>, quartz-porphyry cutting <i>a</i> and <i>b</i>.</div> -</div> - -<p>As in the other islands, the granophyres, porphyries and felsites of Rum -have been intruded at the base of the volcanic series. Over much, if not -all, of their area they lie directly on the red Torridon sandstone. That the -bedded basalts once covered them is indicated by the position of the three -outliers of the basalt-plateau already noticed. But a fourth outlier still lies -upon the porphyry of Orval as a cake that dips gently northward. It consists -of a bedded, dark, finely-crystalline, ophitic dolerite, porphyritic in -places, with a rudely prismatic or columnar structure (<a href="#v2fig360">Fig. 360</a>). It has -undergone contact-metamorphism, and tongues from the underlying rock -project up into it. On the south-eastern side of the same hill, still more -striking evidence is presented of the posteriority of the acid to the basic -rocks. The porphyry shows here the same tendency to assume a bedded structure, -the parallel "beds" again dipping outward or south-east at 40°. They -plunge under the body of gabbro, dolerite and other intrusive masses which -from this point stretch eastward into the great cones of Allival and its -neighbours. The rock at the junction is a fine microgranite with traces of -micropegmatite. It is composed of a holocrystalline base of quartz and -orthoclase, with porphyritic crystals of microcline, blebs of quartz and -scattered granules of augite. The rocks that rest immediately next it are -basalt and dolerite, into which it has sent an intricate network of veins -(<a href="#v2fig361">Fig. 361</a>).<a id="FNanchor_405" href="#Footnote_405" class="fnanchor">[405]</a> It has also pushed long tongues down the slope into them, -which may be seen traversing the dolerite and gabbro veins that -cut the basalts. The basic rocks next the porphyry have been intensely -<span class="pagenum" id="Page_405">- 405 -</span> -altered. They seem in places as if they have been shattered by some explosive -force, and had then been invaded by the mass that rushed into all the -rents thus caused. This remarkable structure is still better displayed on St. -Kilda, and is more fully described in the following account of the geology -of that island.</p> - -<div class="footnote"> - -<p><a id="Footnote_405" href="#FNanchor_405" class="label">[405]</a> In a thin slice cut from a specimen showing the junction, there is a minute vein of the -porphyry penetrating the basalt which is much altered, while the porphyry becomes much finer -in grain than at a distance from the contact.</p> - -</div> - - -<h3>iv. <span class="allsmcap">THE ROCKS OF ST. KILDA</span></h3> - -<p>Brief allusions to St. Kilda and its rocks have already been made (pp. -173, 358). We may now enter more fully upon the consideration of its -geological structure and history.</p> - -<p>When the weather is clear there may be seen from the western headlands -of the Outer Hebrides a small blue cone rising above the Atlantic -horizon at a distance of about 60 miles. As the voyager approaches this -distant land it gradually shapes itself into a group of islets of which St. -Kilda, the largest and only inhabited, has an extreme length of about four -miles, a breadth of less than two miles, and a height of 1262 feet above the -sea. Four miles to the north-east Borrera, about one square mile in extent, -rises with precipitous sides to a height of 1000 feet. Off the north-western -promontory of St. Kilda the huge rock of Soay, half a square mile in area, -towers from 600 to 800 feet above the waves. Borrera has two attendant -rocks—Stack Li and Stack an Armin—huge pyramidal masses several -hundred feet high, and the home of thousands of gannets. St. Kilda -possesses two less imposing islets between its north-western headland and -Soay, and a third to the south-east known as Levenish.</p> - -<p>The scenery of this picturesque group affords a good indication of its -geological structure. It displays two distinct types of topographical form. -In Borrera the marvellous combination of spiry ridges, deep gullies and -clefts, notched crests and splintered pinnacles, at once reminds the visitor of -the outlines of the Cuillin Hills of Skye. The same features are repeated on -a less magnificent scale in Soay and along the whole of the south-western -precipitous coast-line of St. Kilda.</p> - -<p>In marked contrast to these varied outlines, the eastern half of St. Kilda -rises with a smooth green surface, varied with sheets of grey screes, up to the -rounded summit of Conagher, the highest point in the island. If the dark -crags of the rest of the island group remind one of the Cuillins, this eastern -tract recalls at once the form and colour of the Red Hills of Skye. A closer -examination shows that in each case the topography arises from the influence -of the very same rocks and geological structure as in that island.</p> - -<p>There is, however, one aspect in which St. Kilda has no rival throughout -the Western Isles. Its russet-coloured cone, though rising on the west side -with gentle green slopes from the central valley, plunges on the eastern side -in one vast precipice from a height of 1000 feet or more into the surge at -its base. Nowhere among the Inner Hebrides, not even on the south-western -side of Rum, is there any such display of the capacity of the -youngest granite to assume the most rugged and picturesque forms. It is -hardly possible to exaggerate the variety of outline assumed by the rock as -<span class="pagenum" id="Page_406">- 406 -</span> -it yields along its system of joints to the influence of a tempestuous climate. -It has been carved into huge projecting buttresses and deep alcoves, the -naked stone glowing with tints of orange and fawn colour, veiled here and -there with patches of bright green slope, or edged with fringes of sea-pink -and camomile. Every outstanding bastion is rent with chasms and split into -blocks, which accumulate on the ledges like piles of ruined walls. To one -who boats underneath these cliffs the scene of ceaseless destruction which -they present is vividly impressive.</p> - -<p>The geology of St. Kilda was sketched by Macculloch, who recognized -the close resemblance of its two groups of rock to the "augite-rock" (gabbro) -and "syenite" (granophyre) of Skye and other islands of the Inner Hebrides. -But he left the relations of the two groups to each other undetermined.<a id="FNanchor_406" href="#Footnote_406" class="fnanchor">[406]</a> -Professor Heddle has published a brief reference to the rocks of St. Kilda, -without, however, offering any definite opinion as to the geological structure -of the islands.<a id="FNanchor_407" href="#Footnote_407" class="fnanchor">[407]</a> The best account of the geology has been given by Mr. -Alexander Ross, who obtained evidence that the acid sends veins into the -basic rock. He brought away specimens clearly showing this relation, but -in his description left the question open for further inquiry.<a id="FNanchor_408" href="#Footnote_408" class="fnanchor">[408]</a> To some of -the observations in these papers reference will be made in the sequel. The -following account is based on the results of two visits paid by me to St. -Kilda in the summers of 1895 and 1896, during which I was enabled to -examine the rocks on land, and to sail several times round the islands, boating -along those parts of the cliffs which presented features of special geological -importance.</p> - -<div class="footnote"> - -<p><a id="Footnote_406" href="#FNanchor_406" class="label">[406]</a> <i>Description of the Western Isles</i>, vol. ii. p. 54.</p> - -<p><a id="Footnote_407" href="#FNanchor_407" class="label">[407]</a> In an article on the general geological features of the Outer Hebrides contributed to <i>A -Vertebrate Fauna of the Outer Hebrides</i>, by J. A. Harvie-Brown and T. E. Buckley, 1888.</p> - -<p><a id="Footnote_408" href="#FNanchor_408" class="label">[408]</a> <i>British Association Report</i>, 1885, p. 1040, and a much fuller paper in the <i>Proceedings of the -Inverness Field Club</i>, vol. iii. (1884), p. 72.</p> - -</div> - -<p>In the St. Kilda islets three groups of rock differing from each other in -age may be recognized. 1st, A series of gabbros, dolerites and basalts which -have been intruded through and between each other as sills; 2nd, a mass of -granophyre which invades these sills; and 3rd, abundant dykes and veins of -basalt which occur both in the basic and acid masses.</p> - -<p>From the extension of the basalt-dykes across the Outer Hebrides it is -clear that the Tertiary volcanic region reached at least to within 60 miles -of St. Kilda. Whether or not it stretched over the intervening space -now overflowed by the Atlantic must be matter for conjecture. There -can be no doubt that the intrusive rocks of St. Kilda are in age and -origin the equivalents of those of the Inner Hebrides. The remnants -left of them were assuredly not superficial extrusions, but are characteristic -examples of the more deep-seated intrusions of the Tertiary -volcanic period. Down to the most minute details of structure they -reproduce the features so well displayed by the gabbros and granophyres -of Skye, Rum and Mull. If it is demonstrable in the case of -these islands that the intrusions have taken place under a deep cover of -basalt-sheets, now in large part removed, the inference may legitimately be -<span class="pagenum" id="Page_407">- 407 -</span> -drawn that at St. Kilda a basalt-plateau once existed which has been more -completely destroyed than in the other regions. Not a fragment of such a -plateau has survived, unless we may perhaps be allowed to recognize it in -some of the basalts enclosed among the gabbro-sills. Placed far amid the -melancholy main and exposed to the full fury of the Atlantic gales, these -islets must be regarded as the mere fragmentary cores of a once much more -extensive volcanic area. The geologist who visits them is deeply impressed -at every turn by the evidence of the active and unceasing destruction which -their cliffs are undergoing. Nothing now remains save the deep-seated -nucleus of intrusive sills, bosses and dykes.</p> - -<p>1. <i>The Gabbro Sills.</i>—The rudely-bedded arrangement of these rocks is -conspicuous along the west side of St. Kilda, in Soay and in Borrera. They -consist of coarse and fine varieties disposed in successive sheets which dip at -angles varying from as little as 15° up to as much as 60° or even more. In -St. Kilda they form the picturesque promontory of the Dune, and extend -thence along the western side of the island to its extreme northern end. -Their escarpments face the ocean, and their dip-slopes descend towards the -north-east in grassy declivities to the south bay and the long verdant glen -which runs thence across to the north bay. The same strike is prolonged into -Soay, but further east in Borrera the direction curves so as to present vast -escarpments towards the west and shelving sheets of rock towards the east.</p> - -<p>None of the gabbros seen by me are as coarse as the large-grained -varieties of Skye, nor does there appear ever to be such a marked banded -structure among them as that displayed by the Cuillin rocks. Faint -banding, however, may be noticed. A series of specimens which I collected -from the west side of the island has been sliced for microscopic examination, -and Mr. Harker has furnished me with the following notes regarding them.</p> - -<p>"An olivine-gabbro from the west side of St. Kilda [7107] is a dark, -heavy, medium-grained rock, in which augite and felspar are conspicuous. -The microscope shows, in addition, plentiful grains of olivine, with but -little original iron-ore, and some apatite-needles. The structure is ophitic, -the plates of pale-brown augite enveloping both olivine and felspar. A -little brown hornblende and red-brown mica are probably original, the rock -showing little sign of alteration. The felspar is labradorite, with albite- -and Carlsbad-twinning, and forms elongated rectangular crystals.</p> - -<p>"Another specimen [7108] is a rock of similar appearance but somewhat -coarser texture, and structurally is a more typical gabbro than the -preceding, the felspar having little of the 'lath' shape, while the augite, -though still moulded on the felspar, scarcely assumes an ophitic habit. A -striking feature in this rock is the way in which the augite is crowded -with 'schiller'-inclusions, in places so closely as to be almost opaque. A -high magnification shows that these inclusions are dark, linear in form, and -disposed along two directions intersecting at a high angle. The labradorite -has unusually close twin-lamellation on both albite and pericline laws, and -it is possible that this is a strain-effect.</p> - -<p>"A third specimen [7109] is from a rock in every respect identical -<span class="pagenum" id="Page_408">- 408 -</span> -with the preceding, except that the olivine is rather more plentiful, and -in some grains is partially serpentinized."</p> - -<p>While the gabbros of St. Kilda are not a mere uniform boss, but a series -of sills and irregular masses which have been successively injected into each -other, they have subsequently been cut through by many basalt-dykes and -veins. These, which are sometimes as abundant as in the gabbro of the -Cuillin Hills, traverse the rocks both in the line of bedding and also at -many different angles across it. As they generally weather faster than the -gabbros, they give rise to deep narrow clefts which may be traced up the -whole height of the precipices, occasioning sea-caves below and sharp notches -on the crests above.</p> - -<p>These scenic features, so indicative of the geological structure that causes -them, are specially well seen on the western face of the Dune or south-western -promontory of the island, and likewise in the strangely rifted precipices -further north and in Soay. They are, however, most impressively -displayed around the naked walls of Borrera, which in their marvellous -combination of spiry ridges, deep straight gullies, and splintered crests, -remind one at every turn of the scenery of Blaven and the Cuillin Hills.</p> - -<p>2. <i>The Granophyre Boss and its Apophyses.</i>—The eastern half of the -island of St. Kilda consists of a pale rock which Macculloch long ago -identified with the granophyre of Skye, and which, as he pointed out, has -much resemblance to parts of the granite of Arran.<a id="FNanchor_409" href="#Footnote_409" class="fnanchor">[409]</a> Not only does it give -rise to topographical forms like those of the Red Hills, but it weathers, like -the Skye granophyre and the Arran granite, into thick bed-like sheets -divided by transverse joints into large quadrangular blocks. On closer -inspection it is found to resemble still more precisely the acid rocks of the -Inner Hebrides. It possesses the same drusy micropegmatitic structure as -the granophyres of Skye, Rum and Mull. The ferro-magnesian constituents -are present in small quantity, hence the pale hue of the stone. The quartz -and felspar project in well-terminated crystals into the drusy cavities, which -are sometimes further adorned with delicate tufts of clear crystallized -epidote. In these and other respects the rock displays the familiar external -forms of the younger or Tertiary granites of Britain.</p> - -<div class="footnote"> - -<p><a id="Footnote_409" href="#FNanchor_409" class="label">[409]</a> <i>Description</i>, vol. ii. p. 54.</p> - -</div> - -<p>Mr. Harker's notes on the microscopic structure of this granophyre are -as follows:—"The prevailing felspar is orthoclase, often very turbid from -secondary products. Even what appear to be distinct crystals are sometimes -seen in the slices to be invaded on the margin by quartz in rough micrographic -intergrowths, and much of the finer intergrowth occurs as a fringe -to the crystals. In this case the felspar of the micropegmatite can often -be verified to be in crystalline continuity with the crystal which has served -as a nucleus [6624]. Quartz occurs in distinct crystals and grains as well -as in the micropegmatite. There is a more granitoid variety of the rock, in -which only a very rude approach to micrographic intergrowths is seen -[6623]. In both varieties there is but little trace of any ferro-magnesian -mineral; the more typical granophyre has what seems to be destroyed -<span class="pagenum" id="Page_409">- 409 -</span> -augite, while the granitoid rock contains a little deep-brown biotite. -Scattered crystal-grains of magnetite occur in both."</p> - -<p>Narrow ribbon-like veins of a finer material, sometimes only an inch in -breadth, traverse the ordinary granophyre. Similar veins run through the -rock of the Red Hills in Skye; they are sharply defined from the enclosing -rock, as if the latter had already solidified before their intrusion. With -regard to the microscopic structure of some thin slices prepared from these -veins, Mr. Harker remarks that "the material of the veins is of a type intermediate -between granophyre and microgranite [6622, 6623]. The chief -bulk is a finely-granular aggregate of quartz and felspar, the latter very -turbid; but in this aggregate are imbedded numerous patches of micropegmatite, -often of perfect and delicate structure. These areas of micropegmatite -show some approach to a radiate or rudely spherulitic structure, and, -in some cases, are clustered round a crystal of felspar or quartz. Some -granules of magnetite and rare flakes of brown biotite are the only other constituents -of the rock. Although they must be of somewhat later date, there -is evidently nothing in the petrographical characters of these fine-textured -veins to separate them widely from the ordinary granophyres of the region."</p> - -<p>These veins may be compared with the spherulitic dyke that traverses -the granophyre of Meall Dearg at the head of Glen Sligachan (described at -p. 381), which, though undoubtedly somewhat younger than the rock that -contains it, yet presents the very same structures as are visible at the margin -of that rock.<a id="FNanchor_410" href="#Footnote_410" class="fnanchor">[410]</a> The material of this dyke and of the finer veins of St. Kilda -and the Red Hills probably belongs to a later period of protrusion from a -deeper unconsolidated portion of the same acid magma as at first supplied -the general body of granophyre.</p> - -<div class="footnote"> - -<p><a id="Footnote_410" href="#FNanchor_410" class="label">[410]</a> <i>Quart. Journ. Geol. Soc.</i> vol. 1. (1894), p. 220.</p> - -</div> - -<p>Undoubtedly the most interesting feature in the granophyre of St. Kilda -is its junction with the mass of basic rock to the west of it. This junction-line -runs from about the middle of the chief or south bay (where, however, -its precise position is concealed under detritus) across the island to the north -shore, where it descends the face of the precipice and plunges under the sea. -Important as the actual contact of the two rocks obviously is in regard to -their relative date, it has not hitherto been observed or described. Macculloch -noticed "numerous fragments of trap penetrated by veins of -syenite," but he did not see these rocks in place, and, in spite of their -apparent testimony to the posteriority of the acid intrusions, he was inclined -to believe that the veins were not real veins, but that the "trap" and -"syenite" had a common origin and would be found to pass into each other, -as he thought also occurred in Mull and Rum. In recent years Mr. Alexander -Ross, during his visit to St. Kilda, collected specimens illustrating -the varieties of gabbro, dolerite and basalt, and showing the intrusion of the -acid into the basic rocks. As already stated, he was disposed to regard the -"granite" as of younger date than the gabbros, but left the question -undecided.<a id="FNanchor_411" href="#Footnote_411" class="fnanchor">[411]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_411" href="#FNanchor_411" class="label">[411]</a> In his paper, <i>Proceed. Inverness Field Club</i>, vol. iii. (1884), p. 78, Mr. Ross quotes a letter -<span class="pagenum" id="Page_410">- 410 -</span> -from Prof. Judd, who there states that the rock supposed to be granite "is seen under the microscope -to be a quite different rock—a quartz-diorite." Some of the specimens from St. Kilda collected -by Mr. Ross were exhibited at a meeting of the Geological Society on 25th January 1893. -With regard to these Prof. Judd, in the course of the discussion on his paper on "Inclusions of -Tertiary Granite in the Gabbro of the Cuillin Hills," remarked:—"They show a dark rock traversed -by veins of a light one, but the dark rock is not a gabbro and the light one is not a -granite" (<i>Quart. Journ. Geol. Soc.</i> vol. xlix. (1893), p. 198).</p> - -</div> - -<p>The best locality for the examination of the junction of the main granophyre -mass with the gabbros is inaccessible save by boat, and only in the -calmest weather. It occurs in the great cliff on the northern side of the -island between the north bay and the sea-stack known as the Bragstack. -The line of contact emerges from below the sea-level, and ascends the cliff -with a westward inclination of from 60° to 80°. Here, as in Skye, the acid -rock underlies the basic masses, which are rudely bedded and much jointed. -About 150 feet above the sea-level, the nearly vertical cliff breaks up into -an exceedingly rocky and rugged acclivity, across which the junction seems -to slope at a lower angle. But the place is hardly reachable, save perhaps -by the intrepid, barefooted cragsmen of St. Kilda.</p> - -<div class="figcenter" id="v2fig362" style="width: 359px;"> - <img src="images/v2fig362.png" width="359" height="241" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 362.</span>—Junction of granophyre and gabbro, north side of St. Kilda.</div> -</div> - -<p>Along the sharply defined line of contact the granophyre is close-grained, -and sends a network of veins into the dark sheets of gabbro. The -general features of the junction are represented in <a href="#v2fig362">Fig. 362</a>. The veins are -narrow, those nearest the main body of granophyre diverging from it at a -still more acute angle than those from the mass of Meall Dearg (<a href="#v2fig376">Fig. 376</a>), -and then branching so as to enclose masses of the gabbro and to run across -them in long parallel veins. A characteristic feature of many of these veins, -besides their narrowness, is their tendency to split up at the ends into mere -fingers and threads as represented in <a href="#v2fig363">Fig. 363</a>.</p> - -<p>Owing to the depth of soil on the cultivated land, and of boulders and -sand on the beach, the actual junction of the main body of granophyre with -the gabbro is not seen on the southern shore. But a few yards to the -<span class="pagenum" id="Page_411">- 411 -</span>westward of where it must lie, the beach is cumbered with large blocks of -rock broken up from the mass, which can be seen <i>in situ</i> a little further -south forming a line of low cliff with a rugged foreshore. These rocks -consist of various gabbros and basalts of rather fine grain, profusely traversed -with veins of white granophyre. Some of these veins are two feet or more -in breadth, and, when of that size, show the distinctive granular texture and -drusy structure of the main part of the acid rock. But from these dimensions -they can be traced through every stage of diminution until they -become mere threads. When they are only an inch or two broad, they -assume a finely granular texture like that of the veins that run through the -body of the granophyre.</p> - -<div class="figcenter" id="v2fig363" style="width: 448px;"> - <img src="images/v2fig363.png" width="448" height="116" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 363.</span>—Veins of granophyre traversing gabbro and splitting up into thin threads, - north side of St. Kilda.</div> -</div> - -<p>The amount of injected material in the dark basic rocks is here and -there so great as to form a kind of breccia (<a href="#v2fig364">Fig. 364</a>), which, from the contrast -of tone between its two constituents, makes a conspicuous object on the -shore. Here, as in the example already cited from Rum, the basic rocks seem -to have been shattered into fragments, and the acid material to have been -injected into the minutest interstices between them. The enclosed fragments -are of all sizes from mere grains up to blocks a foot or more in length. -They are generally angular, like rock-chips from a quarry. Moreover, they -are not all of the same kind of material. While at this locality most of -them consist of basalt, they include also a few large and small pieces of -rather coarse gabbro. There has evidently been a certain amount of transport -of material, as well as an extensive disruption of the rocks <i>in situ</i>. -The granophyre here and there assumes a darker or greener tint, as if it -had dissolved and absorbed some portion of the older rock.</p> - -<p>Still more astonishing are the sections to be seen on the western cliffs -and rocky declivities of the ridge to the north of the Dune, at a distance of -perhaps 500 or 600 yards westwards from those of the South Bay. Here -the gabbro-sheets are traversed by a number of conspicuous white bands, -which on examination prove to be veins or dykes of granophyre. As viewed -from the sea, the general disposition of the two groups of rocks is represented -in <a href="#v2fig366">Fig. 366</a>. The broadest mass of granophyre breaks out -towards the bottom of the precipice, and slants upward as a sheet -intercalated between the gabbro sills, with a breadth of about 40 or -50 feet, but rapidly thinning away in its ascent. One of the bands below -it has a breadth of about 15 feet. The material of these intrusions is a -<span class="pagenum" id="Page_412">- 412 -</span> -pale fine-grained granophyre like that of the South Bay, I did not detect, -either here or anywhere else in St. Kilda, a definite spherulitic structure such -as is so common in the granophyre dykes of Skye.</p> - -<div class="figcenter" id="v2fig364" style="width: 645px;"> - <img src="images/v2fig364.png" width="645" height="436" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 364.</span>—Pale granophyre injected into dark basalt, South Bay, St. Kilda.<br /><br /> - The crags on the further side of the bay are the gabbro sheets of the Dune. (From a photograph by Colonel Evans.)</div> -</div> - -<p>Though the acid intrusions are somewhat irregular both in thickness -and direction, they lie generally parallel to each other in the line of strike of -the bedding of the gabbros. They are no doubt apophyses from the main -<span class="pagenum" id="Page_413">- 413 -</span> -body of granophyre, which emerges to the surface about a third of a mile to -the eastward, but may of course be at no great depth underneath.</p> - -<div class="figright" id="v2fig365" style="width: 173px;"> - <img src="images/v2fig365.png" width="173" height="160" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 365.</span>—Veins of granophyre traversing - a line-grained gabbro and - scarcely entering a coarse-grained - sheet, west side of Rueval, St. Kilda.</div> -</div> - - -<p>Besides the broader bands of acid rock, and diverging from them, a complicated -network of veins ramifies in all directions through the gabbros, as -at the South Bay. The extraordinary degree to which the basic rocks have -been shattered into fragments is strikingly -displayed here, likewise the extreme liquidity -of the acid magma, whereby it was able to -insinuate itself into every chink and cranny. -But the observer notices that this condition of -excessive disruption is not shared by all the -basic sills, and is not attendant upon all the -acid dykes. As an example of this irregular -distribution of the structure, I give the accompanying -sketch (<a href="#v2fig365">Fig. 365</a>), where a fine-grained -gabbro has been completely broken -up and intersected with granophyre veins, -while the coarser sheet overlying it has almost -entirely escaped. The dark basalt-like sheets -appear generally to have been much more disrupted than the more largely-crystalline -varieties. It is noticeable here, also, that the fragments entangled -in the network of granophyre veinings do not entirely belong to -the rock that has been shattered, but sometimes include large and small -lumps of different gabbros, showing some transference of material with the -inrush of the acid magma.</p> - -<p>Though closer in grain where it comes in contact with the gabbro, the -granophyre never assumes any vitreous texture along its margin. A series of -thin slices, prepared from specimens collected by me in the South Bay in the -summer of 1895, was examined by Mr. Harker, who furnished the following -notes regarding them:—"The basalt traversed by the granophyre is a fine-textured -variety with small porphyritic felspars. These latter seem to be -usually unaltered, retaining the glass cavities which in some of the crystals -are abundant. The groundmass, however, shows minerals of metamorphic -origin which must be derived mainly from the original augite. A brown mica -is the most conspicuous; but with it are associated some brownish-green -hornblende and certain chloritic and perhaps serpentinous substances. It is -chiefly near the margin of a fragment of basalt that the mica gives place to -these minerals. The basalt still retains plenty of unaltered granules of -augite in the central parts of a fragment. It is not certain that the -secondary minerals named come exclusively from the augite of the basalt; -from their form and mode of occurrence they may in part have replaced -olivine or even rhombic pyroxene.</p> - -<p><span class="pagenum" id="Page_414">- 414 -</span></p> - -<div class="figcenter" id="v2fig366" style="width: 706px;"> - <img src="images/v2fig366.png" width="706" height="408" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 366.</span>—View of sills and veins of pale granophyre traversing dark sheets of gabbro, west side of St. Kilda.<br /><br /> - (From a photograph by Colonel Evans.)</div> -</div> - -<p><span class="pagenum" id="Page_415">- 415 -</span></p> - -<p>"The acid rock, though styled granophyre above, belongs to a granitoid -variety of that group of rocks, and has but little indication of micrographic -structures. Compared with the other granophyres from St. Kilda, sliced and -examined, these examples show a less acid composition. This is expressed -mineralogically in the presence of a somewhat larger proportion of ferro-magnesian -minerals and of soda-lime felspar. These features might indeed be -matched in many normal granophyres among the Western Isles, but in the -present case it can hardly be doubted that they are to be explained, at least -in some degree, by the acid magma having taken up a certain amount of -material from the basalt. Many of these Tertiary granophyres have undoubtedly -been modified by the incorporation of pieces of basalt and gabbro, -and a collection made in the Strath district of Skye will furnish examples for -future study. Professor Sollas's description of similar phenomena in the -Carlingford district has already proved the importance of this kind of action.<a id="FNanchor_412" href="#Footnote_412" class="fnanchor">[412]</a> -In the present instance, both brown mica and hornblende occur plentifully -in the granophyre, and especially round the basalt fragments. This latter -point is conclusive as to the derivation of the basic material, and further -proves a certain degree of viscosity in the acid magma at the time of its -intrusion."</p> - -<div class="footnote"> - -<p><a id="Footnote_412" href="#FNanchor_412" class="label">[412]</a> <i>Trans. Roy. Irish Acad.</i> vol. xxx. (1894), pp. 477-572.</p> - -</div> - -<p>Another series of specimens which I collected in the following year was -submitted to Mr. Harker for petrographical determination, and his observations -on two of the microscopic slices are as follow: "A breccia from the -South Bay, St. Kilda [7105], consists of angular fragments up to two inches -in diameter set in a matrix of grey granophyre of medium texture. The -fragments belong to two types—one of very close texture (basalt), the other -more evidently crystalline (diabase). Both are cut by the slice.</p> - -<p>"The basalt shows very evident metamorphism, its augite being wholly -transformed into greenish-brown hornblende. The little felspar-laths and -granules of iron-ore seem to be unaltered, though the latter may perhaps -have contributed to the formation of the hornblende. Another fragment of -basalt has some larger crystal-grains of augite, and these are not converted -into hornblende.</p> - -<p>"The diabase shows a less marked boundary under the microscope, but -otherwise has similar characters to the preceding. The striated felspar-crystals -and grains of iron-ore have not been re-crystallized. A considerable -amount of pale augite remains, but there is also plenty of deeply-coloured -hornblende, both fibrous and compact. This diabase is certainly an intrusive -rock, but the basalt, from its petrographic character, might be from a lava-flow -or from a dyke.</p> - -<p>"The granophyre is of somewhat coarse texture, the micrographic -structure being only of a rude type. It is notably richer in the darker -constituents than is usual in such rocks. Further, the hornblende and -magnetite tend to cluster in little patches which suggest destroyed fragments -of basic rocks. A grain or two of sphene occur, a mineral foreign to the -normal granophyres.</p> - -<p>"Another similar specimen [7106] from the same locality shows a basic -rock of coarser texture, approaching some of the gabbros in appearance and -with boundaries in places not very sharply defined. The grey matrix is -again relatively rich in the dark elements, and the manner in which they -<span class="pagenum" id="Page_416">- 416 -</span> -occur in little patches, like nearly obliterated 'xenoliths,' points unmistakably -to a certain amount of absorption of basic material by the acid magma, with -consequent enrichment in the ferro-magnesian minerals.</p> - -<p>"The slice cuts only the acid rock, which is seen to be of granitoid -rather than granophyric structure, though the tendency of the felspar to -enclose quartz-grains is unlike a typical granite. Oligoclase, with combined -albite- and Carlsbad-twinning, is well represented in addition to orthoclase, -and some zoned crystals seem to be of albite with a border of oligoclase. -Brown hornblende and a little brown mica are the coloured constituents. -Magnetite and apatite are also observed."</p> - -<p>The testimony of the rocks of St. Kilda to the posteriority of the granophyre -to the gabbros and basalts is thus clear and emphatic. It entirely -confirms my previous observations regarding the order of sequence of these -rocks in Mull, Rum and Skye. But the St. Kilda sections display, even -more strikingly than can be usually seen in these islands, the intricate network -of veins which proceed from the granophyre, the shattered condition of -the basic rocks which these veins penetrate, the remarkable liquidity of the -acid magma at the time of its intrusion, and the solvent action of this -magma on the basic fragments which it enveloped.</p> - -<p>3. <i>The Basic Dykes.</i>—Reference has already been made to the numerous -dykes by which the gabbros of the St. Kilda group of islets is traversed. -Similar dykes occur also, though less plentifully, in the granophyre. It -remains for future observation to determine whether there is one series older -and another later than the intrusion of the acid rock. In any case, it is -quite certain that the dykes in the gabbro do not all belong to one period of -injection, for frequent examples of intersection may be noticed, especially on -the cliffs of Borrera, and also cases of double and even treble dykes which -have been formed by successive infillings within the same fissure. The -remarkably varied precipices of that island are marked by the long narrow -rifts left by the weathering of vertical dykes, which, as above remarked, may be -followed with the eye from the sea-level to the sky-line, ascending obliquely -across the bedding of the gabbro sheets. Another group of dykes may be -traced sloping upward at low angles along the face of the cliffs and affording -admirable ledges with overarching roofs for innumerable gannets, kittywakes -and guillemots. Other dykes and ribbon-like veins may be seen traversing -the gabbro in many different directions, precisely as among the Cuillin Hills. -As no similar network of dykes and veins is to be observed in the granophyre, -I am disposed to regard a large number of these intrusions as older than -that rock. But I did not observe any actual example of a basic dyke truncated -by the granophyre.</p> - -<p>There can be no doubt, however, that an injection of similar dykes and -veins took place after the invasion of the granophyre. These later intrusions -are conspicuously displayed along the cliffs that extend from the gabbro -junction on the north side of St. Kilda round the eastern coast into the -South Bay. They maintain a general parallelism and ascend from the sea-level -at varying angles of inclination, running up the pale sea-wall as dark -<span class="pagenum" id="Page_417">- 417 -</span> -bands. They consist of basalt-rocks, and may often be seen to branch and to -die out. Like those in the gabbro, they are not infrequently compound, -being made up of two or three or even more distinct dykes. This is well -seen on the great precipice below Conacher, where the section given in -<a href="#v2fig367">Fig. 367</a> is displayed. Here in a vertical height of about 800 or 900 feet, -there must be at least seven dykes, simple and compound. A little further -south a triple dyke may be seen to be composed of a thick central zone -and two thinner marginal bands, of which the lower strikes off from -the others and maintains an independent course through the granophyre -(<a href="#v2fig368">Fig. 368</a>).</p> - -<div class="figcenter" id="v2fig367" style="width: 461px;"> - <img src="images/v2fig367.png" width="461" height="423" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 367.</span>—Section of the sea-cliff below Conacher, St. Kilda, showing basic dykes in granophyre.</div> -</div> - -<div class="figcenter" id="v2fig368" style="width: 393px;"> - <img src="images/v2fig368.png" width="393" height="100" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 368.</span>—Triple basic dyke, sea-cliff, east side of St. Kilda.</div> -</div> - -<p><span class="pagenum" id="Page_418">- 418 -</span></p> - - -<h3>V. <span class="allsmcap">THE GRANITE OF ARRAN</span></h3> - -<p>The northern half of the island of Arran is mainly occupied by one of -the most compact and picturesque groups of granite mountains in Scotland.<a id="FNanchor_413" href="#Footnote_413" class="fnanchor">[413]</a> -These heights, rising out of the Firth of Clyde to a height of 2866 feet, -present, in their spiry and serrated crests, a contrast to the smoother contours -of the older granitic elevations of this country. The granite is surrounded -by a ring of schistose rocks, belonging to the metamorphic series of the -Southern Highlands, save for a short distance on the eastern margin, where -it comes in contact with and indurates the Lower Old Red Sandstone. -Macculloch long ago pointed out that no pebbles of the granite are to be -found in the surrounding conglomerates and red sandstones of Carboniferous -and younger age.<a id="FNanchor_414" href="#Footnote_414" class="fnanchor">[414]</a> Geologists accordingly came to the conclusion that the -protrusion of the granite took place after Carboniferous time, and hence that -it had no connection with the appearance of the far older granites of the -Highlands. In the year 1873 I gave reasons for believing the granite to -be not only younger than the Carboniferous formations, but to be referable -with most probability to the Tertiary volcanic series.<a id="FNanchor_415" href="#Footnote_415" class="fnanchor">[415]</a> The progress of -inquiry has tended to confirm this inference, though no direct proof of -its correctness has been obtained. Two lines of investigation may be -pursued, and each leads to the conclusion of the probability of the Tertiary -age of the granite. One of these proceeds on a comparison of the petrographical -characters of the Arran rocks with those of undoubted members of -the Tertiary series among the Western Isles. The other inquiry deals with -the relation of the rocks to each other in the general geological structure of -Arran itself.</p> - -<div class="footnote"> - -<p><a id="Footnote_413" href="#FNanchor_413" class="label">[413]</a> The rocks of Arran have often been described. Besides the work of Macculloch above -quoted, reference may be made to the paper by Sedgwick and Murchison, <i>Trans. Geol. Soc.</i> 2nd -Ser. vol. iii. p. 21; A. C. Ramsay's <i>Geology of the Island of Arran</i>, 1841, the paper of Necker -de Saussure quoted on p. 412; J. Bryce's <i>Geology of Clydesdale and Arran</i>, 3rd edit. 1865. The -island is at present being surveyed for the Geological Survey by Mr. W. Gunn.</p> - -<p><a id="Footnote_414" href="#FNanchor_414" class="label">[414]</a> <i>Description of the Western Islands of Scotland</i>, vol. ii. p. 388.</p> - -<p><a id="Footnote_415" href="#FNanchor_415" class="label">[415]</a> <i>Trans. Edin. Geol. Soc.</i> vol. ii. part iii.</p> - -</div> - -<p>Macculloch first remarked the strong lithological resemblance of the -Arran granite to the "syenite," or granophyre, of Skye and St. Kilda.<a id="FNanchor_416" href="#Footnote_416" class="fnanchor">[416]</a> -More recent petrographical investigation, as already stated, has furnished -additional proofs of the connection between the acid rocks of these islands. So -closely indeed are these rocks linked by megascopic and microscopic characters, -that the petrologist has no hesitation in placing them together as probably -products of the same period of igneous activity.</p> - -<div class="footnote"> - -<p><a id="Footnote_416" href="#FNanchor_416" class="label">[416]</a> <i>Description</i>, vol. ii. p. 352.</p> - -</div> - -<p>From the general geological structure of Arran, a further strong argument -may be deduced in favour of the late date of the eruptions of granite. -Good reasons have been given for classing as Permian the bright red sandstones -which occupy much of the central and southern parts of this island, -and include the little volcanic group already referred to. These sandstones -have been invaded by a complex series of eruptive rocks which would -thus be later than the Permian period. No igneous masses posterior to this -<span class="pagenum" id="Page_419">- 419 -</span> -period are certainly known in Britain save those of Tertiary age. The larger -body of granite in the northern half of the island nowhere comes into direct -contact with the newer red sandstones, but these strata are pierced by smaller -bodies of granite. Hence, both by the evidence of their internal structure and -by the stratigraphy of the ground, the later igneous rocks of Arran may be -reasonably grouped together as one important and consecutive series, comparable -in age and general characters with those of Tertiary date in the -Inner Hebrides.</p> - -<div class="figcenter" id="v2fig369" style="width: 519px;"> - <img src="images/v2fig369.png" width="519" height="376" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 369.</span>—Jointed structure of the granite near the top of Goatfell Arran.<br /><br /> - (From a photograph by Mr. W. Douglas, lent by the Scottish Mountaineering Club.)</div> -</div> - -<p>The igneous rocks of Arran, later than the probably Permian sandstones, -range from acid to basic in composition. Besides the northern -granite, there are in the southern part of the island acid rocks that -include granite, coarse-grained quartz-porphyry and fine-grained felsite. -Where the relations of these rocks to each other can be seen, the felsite -<span class="pagenum" id="Page_420">- 420 -</span> -is found by Mr. Gunn to be newer than the porphyry, into which it sends -sills and dykes.</p> - -<p>A feature observed by the same geologist in Arran offers a further point -of resemblance to the acid sills and dykes of Skye. He has noticed that -accompanying the quartz-porphyry of Drumadoon and Bennan, a mass of -basic rock forms a kind of fringe or selvage round it, sometimes with what -appears to be a rock of intermediate character between them. Basic sills -are abundant south of Glen Ashdale, though to the west of Whiting Bay -most of the intrusive sheets are of acid material.</p> - -<p>Some of the quartz-porphyry sheets are markedly columnar. One of -them, near Corriegills, displays a divergent grouping of the prisms, not unlike -parts of the pitchstone sheets of Eigg and Hysgeir, and suggestive of the -rock having flowed along a hollow like that of a valley. No certain trace, -however, has been found of any Tertiary lava-stream in Arran, nor has -evidence of tuffs been detected in any part of the younger igneous series. All -the rocks appear to be intrusive, though so abundant and varied are they -as to indicate that they belong to a vigorous eruptive centre, which may -have poured out at the surface lavas and ashes, since entirely removed by -denudation.</p> - -<p>The numerous basic dykes for which the south end of Arran has long -been celebrated have a general northerly trend, and appear to be all of the -same or nearly the same age. They undoubtedly cut through the quartz-porphyries -and the coarse-grained basic sills, but are less numerously visible -in the finer-grained basic sills, while in the felsitic sheets they are seldom to -be seen. In several places dykes running in an E.N.E. direction cut the -others, and are therefore of later date.<a id="FNanchor_417" href="#Footnote_417" class="fnanchor">[417]</a> The compound dykes of Tormore -on the west side of the island have been already noticed (p. 161).</p> - -<div class="footnote"> - -<p><a id="Footnote_417" href="#FNanchor_417" class="label">[417]</a> <i>Ann. Rep. of Geol. Surv.</i> for 1894, p. 286.</p> - -</div> - - -<h3>VI. <span class="allsmcap">THE NORTH-EAST OF IRELAND</span></h3> - -<p>In the north-eastern counties of Ireland there are two regions which -afford ample material for discussion in connection with the protrusion of acid -rocks during the Tertiary volcanic period. One of these, which for distinction -may be called the Carlingford region, embraces the tract of country -which includes the Mourne Mountains on the north-east side of Carlingford -Lough and the ranges of Slieve Foye and Slieve Gullion on the south-west -side. The other lies mainly within the basaltic plateau, the largest of its -scattered portions forming parts of the hills of Carnearny and Tardree in the -county of Antrim (Map VII.).</p> - - -<h3>1. The Carlingford Region</h3> - -<p>a. <i>The Mourne Mountains.</i>—This compact and picturesque group of -hills, about twelve miles long and six miles broad, and reaching a height of -2798 feet in Slieve Donard, presents a comparatively simple geological structure, -since it consists almost entirely of granitic rocks which pierce, overlie and -<span class="pagenum" id="Page_421">- 421 -</span> -underlie Upper Silurian grits and shales. So far as regards the contact of -these rocks with the disrupted sedimentary formations, all that can be -asserted is that the granite must be later than at least the older part of the -Upper Silurian period. But for at least two reasons, the eruptive rocks may be -regarded with some confidence as part of the Tertiary series. In the first -place, there is a strong petrographical resemblance between the Mourne -Mountain granite and that of the Island of Arran and the granitic parts of -the granophyre of the Western Isles. And this resemblance is so close as to -furnish a cogent argument in favour of grouping all these rocks together as -parts of one geologically contemporaneous series. In the second place, the -Mourne Mountain granite abruptly cuts off a large number of basic dykes -which, running in a general N.N.W. direction, may be looked upon as almost -certainly members of the Tertiary system of protrusions.</p> - -<p>The manner in which the granite of the district behaves towards -certain detached areas of Silurian strata with their accompanying dykes is -one of the most astonishing features in the whole assemblage of intrusive -rocks in Britain. As has been excellently shown in the Geological Survey -Map and sections by Mr. W. A. Traill, the granite has carried up on its -surface broad cakes of vertical Silurian strata, together with all their network -of dykes.<a id="FNanchor_418" href="#Footnote_418" class="fnanchor">[418]</a> A cake of this kind, from 50 to about 200 feet thick and nearly -two miles broad, has been bodily uplifted from the rest of the mass and -carried upward by the granite, so that the truncated ends of the beds of -grit and shale with their system of dykes stand upon a platform of granite, -from which also numerous veins penetrate them. There can be little doubt -that the basic dykes thus broken through are parts of the great Tertiary -system, and if so, the granite which disrupts them cannot be older than -Tertiary time.</p> - -<div class="footnote"> - -<p><a id="Footnote_418" href="#FNanchor_418" class="label">[418]</a> See Sheets 60, 61 and 71 of the one-inch map of the Geological Survey of Ireland, and -Sheets 22, 23 and 24 of the Horizontal Sections. The Explanation to these Sheets of the map -was written by Professor Hull, Mr. Traill having previously retired from the service. The -Mourne Mountain area is now undergoing critical revision by Prof. Sollas for the Geological -Survey, and important additional material for the elucidation of this district may be expected -from him.</p> - -</div> - -<p>Besides the older basic dykes disrupted by the granite, a younger but -much less abundant series traverses that rock, and also follows a general -north-westerly direction. These later dykes in some cases cross more acid -dykes which have risen through the granite. There is no trace of any -superficial discharge from the Mourne Mountain area. But from the analogy -of other districts we may easily conceive that the granite represents the -underground parts of volcanic material which has now been entirely -removed.</p> - -<p>b. <i>Slieve Foye and Barnavave District.</i>—This area embraces the mountainous -ground lying between Carlingford Lough and Dundalk Bay, and -culminating in Slieve Foye (1935 feet). It measures roughly about six -miles in extreme length and four miles in breadth.</p> - -<p>The remarkable assemblage of basic and acid materials in this area has -received considerable attention from geologists. The relative order of the -<span class="pagenum" id="Page_422">- 422 -</span> -two groups of rocks was first clearly recognized by Griffith, who showed that -the granite (granophyre) is intruded into the gabbro.<a id="FNanchor_419" href="#Footnote_419" class="fnanchor">[419]</a> Professor Haughton -subsequently confirmed this observation, and proved the post-Carboniferous -date of the intrusive materials, which he compared with those of Skye.<a id="FNanchor_420" href="#Footnote_420" class="fnanchor">[420]</a> The -general distribution of the rocks was traced out in some detail by the -Geological Survey, and described in the official <i>Memoirs</i>.<a id="FNanchor_421" href="#Footnote_421" class="fnanchor">[421]</a> More recently -the district has been examined by Professor Sollas, who, bringing the -photographic camera and the microscope to the aid of field-geology, has -elucidated the structure and relations of the rocks, and has obtained abundant -evidence that the acid and basic rocks maintain there the same relative -order as among the Inner Hebrides.<a id="FNanchor_422" href="#Footnote_422" class="fnanchor">[422]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_419" href="#FNanchor_419" class="label">[419]</a> <i>Journ. Geol. Soc. Ireland</i> (1843), p. 113.</p> - -<p><a id="Footnote_420" href="#FNanchor_420" class="label">[420]</a> <i>Quart. Journ. Geol. Soc.</i> vol. xii. (1856), p. 171; xiv. p. 300; and <i>Journ. Geol. Soc. Ireland</i> -(1876), p. 91.</p> - -<p><a id="Footnote_421" href="#FNanchor_421" class="label">[421]</a> Sheet 71 of the Geol. Surv. Ireland, and accompanying Explanation. These were the -work of Mr. W. A. Traill.</p> - -<p><a id="Footnote_422" href="#FNanchor_422" class="label">[422]</a> <i>Trans. Roy. Irish Acad.</i> vol. xxx. (1894), p. 477. This is part i. of what is intended to be a -series of papers.</p> - -</div> - -<p>One of the first features in this tract of country to arrest the eye of the -geologist is the situation of this centre of protrusion and that of Slieve -Gullion along a north-west line, coincident with the general direction of the -numerous basic dykes of the region. Whether or not the successive intrusions -took place contemporaneously in the two areas, they have followed each -other in the same order. In the Barnavave district the igneous rocks occupy -an area of about 20 square miles. They consist of a central and chief mass -composed of acid materials, which have risen through the basic rocks now -found as an interrupted ring round them.</p> - -<p>In his more recent examination, Prof. Sollas has devoted special attention -to the influence of the solvent action of the acid magma upon the basic rocks -and upon its own composition and structure. Besides confirming the work of -previous observers as to the order of appearance of the two kinds of material, -he has obtained evidence that the gabbro had not only completely solidified, -but was traversed by contraction-joints, possibly even fractured by earth-movements, -before the injection of the granophyric material. He found that this -material, like that of the Inner Hebrides and St. Kilda, must have been in -a state of great fluidity at the time of its intrusion, and made its way into -the minutest cracks and crevices. In observing the solvent action of the -granophyre, he ascertained that this action took place even in comparatively -narrow dykes, which probably consolidated at no great depth beneath the -surface.<a id="FNanchor_423" href="#Footnote_423" class="fnanchor">[423]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_423" href="#FNanchor_423" class="label">[423]</a> <i>Op. cit.</i></p> - -</div> - -<p>c. <i>The Slieve Gullion District.</i>—This area is separated from that just -described by a narrow strip of Silurian strata, so that its isolation as a -separate igneous district is complete. It will be observed from the map to -continue the same north-westerly line as the Slieve Foye tract, the two -together running in that direction for a distance of some 16 miles. It -is interesting to note the adoption of this predominant north-westerly trend -even by eruptive masses which were mainly of acid material.</p> - -<p><span class="pagenum" id="Page_423">- 423 -</span></p> - -<p>This district measures about ten miles in length by from one to five -miles in breadth. The rocks are, on the whole, similar to those in the area -south of Carlingford Lough, and bear the same relation to each other, the -acid being intrusive in the basic series. It is worthy of remark that the -Tertiary eruptive rocks have made their appearance in the midst of the older -granite of Newry. This granite has been already alluded to as disrupting -Upper Silurian strata, and being probably of the age of the Lower Old Red -Sandstone (<a href="../../66492/66492-h/66492-h.htm#Page_290">vol. i. p. 290</a>). In long subsequent ages, after protracted -denudation, during which its cover of Silurian and Carboniferous formations -was stripped off and it was laid bare, it was broken through by the whole -series of basic and acid protrusions of Slieve Gullion.</p> - -<p>This district is portrayed on Sheets 59, 60, 70 and 71 of the Geological -Survey of Ireland, which show a central core of basic and acid material -piercing the Newry granite.<a id="FNanchor_424" href="#Footnote_424" class="fnanchor">[424]</a> Round this core and touching it at its north-western -and south-eastern end, but elsewhere separated from it by a space of -several miles, runs a curiously continuous band of igneous material which is -marked as "quartziferous porphyry" and "felstone-porphyry" on the Survey -maps.</p> - -<div class="footnote"> - -<p><a id="Footnote_424" href="#FNanchor_424" class="label">[424]</a> The ground was chiefly mapped and described by Mr. Joseph Nolan and Mr. F. W. Egan.</p> - -</div> - -<p>The south-western portion of this elliptical ring possesses a peculiar -interest from its including certain remarkable masses of breccia or agglomerate. -These rocks have been mapped by Mr. Nolan, and are described by him in -the official <i>Explanation</i>, but in more detail in two separate papers.<a id="FNanchor_425" href="#Footnote_425" class="fnanchor">[425]</a> Having -had an opportunity of paying a brief visit to the ground, I can confirm the -general accuracy of his mapping and description, and am able to add a few -further particulars to the facts enumerated by him.</p> - -<div class="footnote"> - -<p><a id="Footnote_425" href="#FNanchor_425" class="label">[425]</a> Sheet 70 of the Geol. Surv. Map of Ireland and Explanation thereto; also <i>Journ. Roy. Geol. -Soc. Ireland</i>, vol. iv. (1877), p. 233; <i>Geol. Mag.</i> 1878.</p> - -</div> - -<p>The tract of ground where these agglomerates appear forms a prominent -ridge which rises several hundred feet above the lower country on either -side, and extends in a W.N.W. direction for about seven miles, nearly -along the line of junction between the Newry granite and the Silurian -strata. The ridge has a breadth varying from a few hundred yards to -upwards of a mile. It is separated from the main igneous mass of the -Slieve Gullion area by an intervening strip of lower ground from three-quarters -of a mile to about a mile and a half in width, which is occupied by the -Newry granite. At the north-west end of the ridge the newer eruptive -rocks lie within the area of that granite, while at the south-east end they -rise entirely amongst the Silurian strata.</p> - -<p>Beginning at the south-eastern extremity, we find the agglomerate -occupying several detached eminences and surrounded by altered Silurian -grits and shales. Further west the rock occurs in larger and more continuous -masses, appearing at intervals, especially along the southern borders of the -quartz-porphyry which forms by much the greater part of the ridge. Actual -junctions of the agglomerate with the older rocks around seem to be seldom -visible. I found one, however, above the gamekeeper's house on the southern -<span class="pagenum" id="Page_424">- 424 -</span> -flanks of the hill called Tievecrom. The Upper Silurian grits and shales, -in a much indurated and shattered condition, are there traceable for several -hundred feet up the slope, until they are abruptly cut off by the agglomerate. -The line of separation appears to be nearly vertical, the truncated ends of -the strata being wrapped round by the mass of fragmental material.</p> - -<p>The most remarkable features of this agglomerate, which has been well -described by Mr. Nolan, are the notable absence of truly volcanic stones in -it, and the derivation of its materials from the rocks around it. I found -only one piece of amygdaloid, but not a single lump of slag, no bombs, no -broken fragments of lava-crusts, and no fine volcanic dust or enclosed lapilli. -The rock may be said to consist entirely of fragments of Silurian grits and -shales where it lies among these strata, and of granite where it comes -through that rock. Blocks of these materials, of all sizes up to two feet in -breadth, are confusedly piled together in a matrix made of comminuted -debris of the same ingredients.</p> - -<p>The agglomerate on the ridge of Carrickbroad has no definite boundary, -but seems to graduate into an andesitic rock, and then into a quartz-felsite -or rhyolite. This apparent gradation is one of the most singular features -of the ridge. The andesite resembles some of the "porphyrites" of the -Old Red Sandstone. It is close-grained, with abundant minute felspar-laths, -and numerous large porphyritic felspars, which latter are sometimes -aggregated in patches, as in the old porphyries of Portraine, Lambay Island -and the Chair of Kildare. This rock has undoubtedly been erupted at the -time of the formation of the agglomerate, or at least before the loose materials -were compacted together; for it is full of separate stones of the same materials, -and becomes so charged with them as to become itself a kind of agglomerate, -with a small proportion of andesitic matrix cementing the blocks.</p> - -<p>A thin slice prepared from one of the specimens obtained by me from -this hill has been studied by Mr. Watts, who reports that the fine-grained -andesitic matrix in which the stones are imbedded has often been injected -into their minute fissures, and that the minute fragments enclosed in this -matrix consist here of a trachyte-like porphyry, felsite, andesites, basalts of -various degrees of fineness and olivine-basalt, together with isolated grains -of felspar, such as might have been derived from the breaking up of some -of these fragments.</p> - -<p>Westward from Carrickbroad, the chief eruptive rock is a dark, sometimes -nearly velvet-black, flinty, occasionally almost resinous, quartz-porphyry -or rhyolite, with abundant quartz and large felspars and occasional well-marked -flow-structure. This material, near the much smaller protrusion -of andesite, is curiously mixed up with that rock, as if the two -had come up together. Sometimes they seem to pass into each other, -at least the separation between them cannot be sharply drawn. -There can be little doubt, however, that the acid magma continued to -ascend after the other, for it sends veins and strings into the more basic -material, and encloses blocks of it. This thoroughly acid porphyry plays -the same part as the andesite in regard to the stones of the agglomerate. -<span class="pagenum" id="Page_425">- 425 -</span> -Throughout its whole extent, it is found to enclose these stones, which here -and there become so numerous as to form the main bulk of the mass, leaving -only a limited amount of quartz-porphyry (rhyolite) matrix to bind the -whole into an exceedingly compact variety of breccia. Occasionally the acid -rock cuts through the ordinary clastic agglomerate, as may be well seen -on the southern face of Tievecrom.</p> - -<p>A specimen of this porphyry with its enclosed fragments, which was -collected by me from above the old tower at Glendovey, Carrickbroad, -has been sliced and examined by Mr. Watts under the microscope, and is -thus described by him: "The large fragment in this slide consists of -ophitic olivine-dolerite full of large phenocrysts of olivine. It is broken up -and penetrated by veins of quartz-porphyry, rich in quartz, which exhibits -a beautiful flow-structure. The felspars and augite of the dolerite do not -appear to have suffered much alteration at the margin of the fragment, but -the olivines are much serpentinized, the serpentine passing into a border of -actinolite which runs in veins into the neighbouring rock and even passes -out into the quartz-porphyry at the junction, impregnating it with actinolite -and chlorite for some distance. A few particles of basalt also occur -and a portion of a granite-fragment comes into the slide, from the edge of -which a piece of biotite has floated off into the quartz-porphyry."</p> - -<p>The essentially non-volcanic material of the agglomerate shows, as Mr. -Nolan pointed out, that it was produced by æriform explosions, which blew -out the Silurian strata and granite in fragments and dust. These discharges -probably took place either from a series of vents placed along a line of -fissure running in a north-westerly line, or directly from the open fissure -itself. Possibly both of these channels of escape were in use; detached -vents appearing at the east end and a more continuous discharge from the -fissure further west.</p> - -<p>After the earliest explosions had thrown out a large amount of granitic -and Silurian detritus, andesitic lava rose in the fissure, and solidifying there -enclosed a great deal of the loose fragmentary material that fell back into -the chasm. Subsequently, and on a more extensive scale, a much more acid -magma ascended from below, likewise involving and carrying up a vast -quantity of loose stones, among which are pieces of basalt and dolerite.</p> - -<p>No evidence remains as to the extent of the material discharged over -the surface from this fissure. Denudation has removed all the surrounding -fragmental sheets as well as any lava that may have flowed out upon or -become intercalated among them. There remains now only the cores of the -little necks at the east end, and the indurated agglomerate and lava that -consolidated along the mouth of the fissure or vents.</p> - -<p>This is the only example of such a line of fissure-eruption which has -yet been met with in the British Isles. Its connection with the eruptive -masses of Slieve Gullion and Carlingford links it with the Tertiary volcanic -series. But no evidence appears to remain regarding the epoch in the long -volcanic period when the eruptions from it took place. They may possibly -date back to the time of the plateau-basalts; but the abundant acid magma, -<span class="pagenum" id="Page_426">- 426 -</span> -which constitutes one of their distinguishing characteristics, suggests that -they more probably belong to the later time when the main protrusions of -acid material took place. They suggest that coeval with the uprise of the -great domes of Slieve Gullion, Carlingford and the Mourne Mountains -there may have been many superficial eruptions of which, after prolonged -denudation, all trace has now been effaced.</p> - - -<h3>2. The Antrim Region</h3> - -<p>Reference was made in <a href="#CHAPTER_XXXVII">Chapter xxxvii</a>. to the occurrence of rhyolitic -conglomerate and tuff between the lower and upper series of basalts in -the Antrim plateau, and to the evidence furnished by these detrital -deposits either that masses of rhyolite appeared at the surface, or that -rhyolitic ashes were discharged from volcanic vents in the long interval that -elapsed between the two groups of basalt. The further consideration of this -question, and an account of the rhyolite bosses, were reserved for the -present chapter, that they might be taken in connection with the other acid -eruptions of Tertiary time in Britain.<a id="FNanchor_426" href="#Footnote_426" class="fnanchor">[426]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_426" href="#FNanchor_426" class="label">[426]</a> For an early account of the Antrim trachytic rocks, see Berger, <i>Trans. Geol. Soc.</i> iii. (1816), -p. 190. Professor Hull has described the Tardree rock in the Explanation to Sheets 21, 28 and -29, <i>Geol. Survey of Ireland</i> (1876), p. 17, and has supposed it to be older than the basalts, referring -it to the Eocene period (<i>Physical Geology and Geography of Ireland</i>, 2nd edit. (1891), pp. 87, 95). -Duffin (quoted by Mr. Kinahan) believed that "the trachytes occur at the centre of eruption, -and were probably poured out at the end of the outburst." Du Noyer also (quoted by the same -writer) thought them to be newer than the plateau-basalts, and to have lifted up masses of these -rocks. Mr. Kinahan himself (<i>Geology of Ireland</i>, p. 172) has pointed to the absence of any -rhyolitic fragments between the basalts as an argument against the supposed antiquity of the -acid protrusions. A petrographical account of the Tardree rock is given by Von Lasaulx in the -paper already cited, Tschermak's <i>Min. Pet. Mittheil.</i> (1878), p. 412. A more elaborate discussion -of the petrography by Prof. Cole will be found in the Memoir above referred to (<i>Scientif. Trans. -Roy. Dublin Soc.</i> vol. vi. 1896), and the geological relations of the rocks are discussed by him in -another shorter paper, <i>Geol. Mag.</i> (1895), p. 303. See also Mr. M'Henry on the trachytic rocks -of Antrim, <i>Geol. Mag.</i> (1895), p. 260, and <i>Proc. Geol. Assoc.</i> vol. xiv. (1895), p. 140.</p> - -</div> - -<p>With one exception, all the known protrusions of acid material in the -Antrim area lie within the limits of the basalt-plateau (see Map. No. VII.). -They occur along a line at intervals for a distance of about 17 miles, from -Templepatrick to a point four miles north of Ballymena. It is worthy of -remark that here again the line of protrusion has a north-west trend. It -not improbably indicates the position of a fissure up which the acid material -rose at various points.</p> - -<p>The petrography of the rocks has been frequently discussed. They -include several varieties of rhyolite, generally rather coarsely crystalline, but -sometimes becoming compact, and even passing into dark obsidian. No -undoubted tuff occurs associated with them in any of the exposures, nor do -the rhyolites anywhere display structures that point to their having flowed -out at the surface.<a id="FNanchor_427" href="#Footnote_427" class="fnanchor">[427]</a> That the masses now visible may have communicated -<span class="pagenum" id="Page_427">- 427 -</span> -with the surface is quite conceivable, but what we now see appears -in every case to be a subterranean and not a superficial part of the protrusion.</p> - -<div class="footnote"> - -<p><a id="Footnote_427" href="#FNanchor_427" class="label">[427]</a> At Sandy Braes an exposure is visible of what at first might be thought to be a volcanic -conglomerate, but closer examination shows the rock to consist of obsidian, which decomposes -into a clay, leaving round sharply-defined glassy cores enclosed in the decayed material. The -"banded rhyolites" do not exhibit any kind of flow-structure that may not be met with in -dykes and bosses. Nor have any satisfactory traces been found of vesicular or pumiceous bands -such as might mark the upper surfaces of true lava-streams. Professor Cole has described what -he calls "The Volcanoe of Tardree" (<i>Geol. Mag.</i> July 1895). If the Tardree mass ever was a -volcano, which is far from improbable, its superficial ejections have long ago disappeared. At -least, after the most diligent search, I have been unable to discover any trace of them, all that -now remains appearing to me to be the neck or core of protruded material.</p> - -</div> - -<div class="figcenter" id="v2fig370" style="width: 493px;"> - <img src="images/v2fig370.png" width="493" height="98" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 370.</span>—Intrusive rhyolite in the Lower Basalt group of Antrim, Templepatrick.<br /><br /> - 1 1, Chalk; 2 2, Gravel; 3 3, Bedded basalt; 4 4, Rhyolite, intrusive.</div> -</div> - -<p>Most of the rhyolitic exposures are extremely limited in area—mere -little knobs, sometimes rising in the middle of a bog, and never forming -conspicuous features in the landscape. The relation of these rocks to the -basalts are generally concealed, but the isolation of the small rhyolitic -patches leaves no doubt that they are intrusive as regards the surrounding -basalts. This relation is well seen at Templepatrick, where it was first -observed by Mr. M'Henry of the Geological Survey (<a href="#v2fig370">Fig. 370</a>). The rhyolite -there forms a sill which has been thrust between the basalts and the gravel -that underlies them, the basalts being bent back and underlain by the -acid rock.<a id="FNanchor_428" href="#Footnote_428" class="fnanchor">[428]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_428" href="#FNanchor_428" class="label">[428]</a> The progress of quarrying operations during the last eight years has somewhat destroyed -the section as exposed in 1888. But we now see that the basalt has not only been bent back -but is underlain by the acid rock.</p> - -</div> - -<p>The largest and most interesting of the Antrim rhyolite tracts covers a -space of about ten square miles in the heart of the basalt-plateau to the -north-east of the town of Antrim. It rises to about 1000 feet above the sea, -and forms a few featureless hills, some of which are capped with basalt. -The best known localities in this tract are Tardree and Carnearny. The -rock is chiefly a somewhat coarse-textured lithoidal rhyolite, but includes -also vitreous portions.</p> - -<div class="figcenter" id="v2fig371" style="width: 518px;"> - <img src="images/v2fig371.png" width="518" height="88" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 371.</span>—Section across the southern slope of Carnearny Hill, Antrim.<br /><br /> - <i>a</i> <i>a</i> <i>a</i>, bedded basalts; <i>b</i>, rhyolite.</div> -</div> - -<p>Owing to the cover of soil and turf, the junction of this mass with the -surrounding basalts cannot be so clearly seen as in the sections of the Inner -<span class="pagenum" id="Page_428">- 428 -</span> -Hebrides, and hence the stratigraphical relations of the two groups are apt -to be misunderstood. What is actually seen is represented in <a href="#v2fig371">Fig. 371</a>. -The lithoidal rhyolite emerges from underneath the basalts which abut against -its sloping surface, forming on the north side of Carnearny Hill a steep bank -about 150 feet above the more gently inclined slope below. The basalts -consist of successive nearly level sheets of compact and amygdaloidal rock.</p> - -<p>It is obvious that only two explanations of this section are possible. -Either the rhyolite was in existence before the basalts which flowed round -it and gradually covered it, or it has been erupted through these rocks, and -is therefore of later date.</p> - -<p>The former supposition has been the more usually received. The -rhyolite has been supposed to form the summit of an ancient volcanic dome, -perhaps of Eocene age, which had been worn down before the outflow -of the plateau-basalts under which it was eventually entombed. Had this -been the true history of the locality, it is inconceivable that of a rock which -decays so rapidly as this rhyolite, and strews its slopes with such abundance -of detritus, not a single fragment should occur between the successive beds -of basalt which are supposed to have surrounded and buried it. Though the -several beds of basalt are well exposed all round, I could not, on my first -visit, find a trace of any rhyolitic fragments between them, nor had Mr. -Symes, who mapped the ground in detail for the Geological Survey, been -more successful. I have since made a second search with Mr. M'Henry, but -without detecting a single pebble of the acid rock among the basalts. Yet -it is clear from the upper surfaces of some of these lavas that a considerable -interval of time separated their successive outflows, so that there was opportunity -enough for the scattering of rhyolite-debris had any hill of that rock -existed in the vicinity.</p> - -<p>Again, little more than a mile to the east of Carnearny Hill, an outlier -of the basalts forming the prominent height of the Brown Dod lies upon and -is completely surrounded by the rhyolite, which along the east side of the -hill can be traced as it passes under the level sheets of basalt. The line of -junction ascends and descends on that flank of the outlier, so that successive -flows of basalt are truncated by the acid rock. But I could find no rhyolitic -debris between them.</p> - -<p>It appears to me, therefore, that the relations between the two groups of -rock in this area are similar to those between the granophyres and bedded -basalts on the south side of Loch na Keal in Mull (p. 396). In other -words, the rhyolites have risen through the basalts, and are therefore younger -than these lavas. This conclusion is corroborated by the actual proofs of -the intrusion of rhyolite into the basalts at Templepatrick.</p> - -<p>All the known rhyolitic masses in Antrim are confined to the Lower -group of basalts.<a id="FNanchor_429" href="#Footnote_429" class="fnanchor">[429]</a> And as they traverse some of its highest members, -<span class="pagenum" id="Page_429">- 429 -</span> -they may be regarded as certainly younger than that group. Mr. -M'Henry, who first indicated this relation, suggested that the rhyolites -were erupted in the interval between the two basaltic series, and he -connected with their eruption the rhyolitic detritus found in association with -the iron-ore at so many places in Antrim. It appears to me that this -suggestion carries with it much probability. The rhyolitic conglomerate -of Glenarm proves that, in the long period represented by the iron-ore and -its associated group of sedimentary deposits, there were masses of rhyolite -at the surface, the waste of which could supply such detritus. The -resemblance between the material of that conglomerate and the rhyolites -now visible at Tardree and elsewhere is so close that we cannot doubt that, -if not derived from some of the known rhyolitic protrusions, this material -certainly came from exposed masses that had the same general petrographic -characters.</p> - -<div class="footnote"> - -<p><a id="Footnote_429" href="#FNanchor_429" class="label">[429]</a> The only exception to this rule was believed to be that of the mass at Eslerstown, four -miles east of Ballymena, which, as originally mapped, was shown as crossing from the Lower -into the Upper basalts. Mr. M'Henry, however, has recently ascertained that the acid rock is -entirely restricted to the area of the older group.</p> - -</div> - -<p>While the rhyolite pebbles in the Glenarm conglomerate are distinctly -rounded and water-worn, showing that some prominences of acid rock were -undergoing active denudation at the time when this conglomerate was laid -down, the finer rhyolitic detritus in the tuffs of Ballypallidy rather suggests -the actual discharge of rhyolitic ashes during the same period. But it would -appear that the superficial outbursts of rhyolitic material, whether in the -form of lava or of tuff, were only of trifling extent, or else that the interval -between the eruption of the two basalt-groups was so prolonged that any -such superficial material was then removed by denudation. The varieties of -lithological character to be met with among the acid protrusions of Antrim -suggests a succession of uprises of rhyolites differing from each other more -or less in composition and structure. Unfortunately the ground is generally -so covered with superficial accumulations, and the exposures of rock are so poor -and limited, that no sequence has yet been determined among the several -kinds of acid rock. The only locality where I have observed clear evidence -of such a sequence is on the old quarries half a mile west of Shankerburn -Bridge, and three miles north-west of Dromore, County Down. A small -boss of rhyolite there rises through the Silurian strata. It consists partly -of a coarse-grained lithoidal rhyolite, with large smoky quartzes and felspars, -and partly of a much finer textured variety. The latter, on the south side -of the small brook which separates the quarries, can be seen to ascend -vertically through the coarse-grained rock into which it sends a projecting -vein. Its margin shows a streaky flow-structure parallel with its vertical -wall and is in places spherulitic. Here the closer-grained rock is certainly -later than the rest of the mass.</p> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_430">- 430 -</span></p> - -<h2 class="nobreak" id="CHAPTER_XLVIII">CHAPTER XLVIII<br /> - -<span class="smaller">THE ACID SILLS, DYKES AND VEINS</span></h2> -</div> - - -<h3>i. <span class="allsmcap">THE SILLS</span></h3> - -<p>Not only have the acid rocks been protruded in small and large bosses, they -have also been injected as sills between the bedding-planes of stratified rocks, -between the surfaces of the basalt-beds, and between the bottom of the -plateau-basalts or of the gabbros and the platform of older rock on which -the volcanic series has been piled up. Every gradation of size may be -observed, from mere partings not more than an inch or two in thickness, up -to massive sheets, which now, owing to the removal of their original covering -of rock by denudation, form minor groups and ranges of hills. Where the -sheets are numerous, they are usually small in size; where, on the other -hand, they are few in number, they reach their greatest dimensions.</p> - -<p>It is not always possible to discriminate between bosses and large irregular -sills. A good illustration of the connection between these two forms of -intrusion will be cited from the island of Raasay, where a widespread intrusive -sheet is in part connected with a true boss.</p> - -<p>In Mull, sills of acid eruptive rocks are profusely abundant throughout -the central mountainous tract between Loch na Keal and Loch Spelve. If we -ascend the slopes from the Sound of Mull, for instance, we have not gone far -before some of these sheets make their appearance. They are usually dull -granular quartz-porphyries, or granophyres, often only two or three feet in -thickness, and interposed between the beds of basalt that form the mass of -the hills. Along the crest of the ridge that stretches through Beinn -Chreagach Mhor to Mainnir nam Fiadh they take a prominent place among -the ledges of basalt, basalt-conglomerate and dolerite. The largest sheet in -Mull is probably that which has thrust itself between the base of the basalts -and the underlying Jurassic strata and crystalline-schists on the shore of the -Sound of Mull at Craignure. The porphyry of this sheet is referred to by -Professor Zirkel as only a finer-grained variety of the same quartziferous -rock, with hornblende and orthoclase crystals, which in Skye breaks through -the Lias.<a id="FNanchor_430" href="#Footnote_430" class="fnanchor">[430]</a> On the south coast also, at the base of the thick basalt series, -similar porphyries have been injected into the underlying strata; and under -<span class="pagenum" id="Page_431">- 431 -</span> -the great gabbro mass of Ben Buy similar protrusions occur. But as we -retire from the mountainous tract into the undisturbed basalts of the plateau, -these acid intercalations gradually disappear.</p> - -<div class="footnote"> - -<p><a id="Footnote_430" href="#FNanchor_430" class="label">[430]</a> <i>Zeitsch. Deutsch. Geol. Gesellsch.</i> xxiii. p. 54.</p> - -</div> - -<p>In the islands of Eigg and Rum, excellent examples occur of the -tendency which the sheets of porphyry or granophyre manifest to appear at -or about the base of the bedded basalts. I have already alluded to -the boss or sheet at the north end of the former island. A still -more striking illustration occurs in Rum. All along the base of the great -mass of gabbro, protrusions of various kinds of acid rock have taken place. -The great mass of Orval, already described, is one of these. Below Barkeval -and round the foot of the hills to the south-east of that eminence an interrupted -band of quartz-porphyry may be traced, from which veins proceed -into the gabbros and dolerites.</p> - -<p>But it is in Skye and Raasay that the intrusive sheets of the acid group -of rocks reach their chief development. They have been most abundantly -injected underneath the bedded basalts, particularly among the Jurassic -strata. A band or belt of them, though not continuous, can be traced round -the east side of the main body of granophyre, at a distance of from a mile -and a half to about three miles. Beginning near the point of Suisnish, -this belt curves through the hilly ground for some five miles, until it dies -out on the slopes above Skulamus. It may be found again on the west -side of the ridge of Beinn Suardal, and on the moors above Corry, till it -reaches the shore at the Rudh' an Eireannich (Irishman's Point). It skirts -the west side of Scalpa Island, and runs for some miles through Raasay. -Another series of sills occurs below the basalts and gabbros in the Blaven -group of hills.</p> - -<p>Over a large part of their course, the rocks of the eastern belt rest in great -overlying sheets upon the Jurassic strata, which may almost everywhere be -seen dipping under them. From the analogy of other districts, we may, I -think, infer that the position of these sills here points to their having been intruded -at the base of the plateau-basalts which have since been removed -from almost the whole tract. Fortunately, a portion of the basalts remains in -Raasay, and enables us to connect that island with the great plateau of Skye -of which it once formed a part. There can be no doubt that the basalts -of the Dùn Caan ridge once extended westwards across the tract of granophyre -which now forms most of the surface between that ridge and the Sound of -Raasay. A thin sheet of quartz-porphyry, interposed among the Oolitic -strata, may be seen a little inland from the top of the great eastern cliff and -below the position of the bedded basalts.</p> - -<p>The great sheet, or rather series of sheets, which stretches north-eastwards -from Suisnish at the mouth of Loch Eishort in Skye, consists of a -rock which for the most part may readily be distinguished in the field from -the granitoid material of the bosses. It appears to the naked eye to be a -rather close-grained or finely crystalline-granular quartz-porphyry, with -scattered blebs or bi-pyramidal crystals of quartz and crystals of orthoclase. -At the contact with adjacent rocks, the texture becomes more felsitic, sometimes -<span class="pagenum" id="Page_432">- 432 -</span> -distinctly spherulitic (west side of Carn Nathragh, next Lias shale). -Under the microscope the rock is seen to be a fine-grained granophyric -porphyry or porphyritic granophyre. It caps Carn Dearg (636 feet) above -Suisnish, where it covers a space of nearly a square mile, and reaches at its -eastern extremity (Beinn Bhuidhe), a height of 908 feet above the sea (Fig. -249). This rock rests upon a sill of dolerite, and is apparently split up by -it. But, as I have already stated, the basic rock is probably the older of -the two, and the granophyre seems to have wedged itself between two earlier -doleritic sheets. To the north-west of Carn Dearg, above the northern end -of the crofts of Suisnish, the same sill, or one occupying a similar position, -crops out between masses of granophyre, and is intersected by narrow veins -from that rock.</p> - -<p>Though severed by denudation, the large sheets of granophyre to the -east of Beinn Bhuidhe are no doubt continuations of the Carn Dearg mass, -or at least occupy a similar position. That they are completely unconformable -to the Jurassic strata is shown by the fact, that while at Suisnish they -lie on sandstones which must be fully 1000 feet above the bottom of the -Lias, only two miles to the east they are found resting on the very basement -limestones, within a few yards from the underlying quartzite and Torridon -sandstone. I do not think that this transgression can be accounted for by -intrusion obliquely across the stratification. I regard it as arising from the -eruptive rock having forced its way between the bottom of the now -vanished basalt-plateau and the denuded surface of Jurassic rocks, over which -the basalts were poured. The platform underneath these granophyre sills -thus represents, in my opinion, the terrestrial surface before the beginning -of the volcanic period.</p> - -<p>But there is abundant proof that though the intruded granophyre sills -followed generally this plane of separation, they did not rigidly adhere to it, -but burrowed, as it were, along lower horizons. Thus on the south-east -front of Beinn a' Chà irn, which forms so fine an escarpment above the valley -of Heast, the base of the granophyre, after creeping upward across successive -beds of limestone, sends out a narrow tongue into these strata, and continues -its course a little higher up in the Lias. The same rock, after spreading out -into the broad flat tableland of Beinn a' Chà irn (983 feet), rapidly contracts -north-eastwards into a narrow strip which forms the crest of the -ridge, and at once suggests a much-weathered lava-stream. The resemblance -to a <i>coulée</i> is heightened by the curious thinning off of the rocks where the two -streams emerge from the Heast lochs; it looks as if the igneous mass were -a mere superficial ridge which had been cut down by erosion, so as to -expose the shales beneath it. But that the granophyre is really a sill becomes -abundantly clear at its eastern end, where we find that it consists of two -separate sheets with intervening Liassic shales. The structure of this -interesting locality is shown in <a href="#v2fig372">Fig. 372</a>. In this instance also, there is -evidence that the acid sills are younger than the basic, for the upper sheet -of granophyre sends up into the overlying dark basaltic rock narrow vertical -felsitic veins, a quarter of an inch to an inch in width, which being more -<span class="pagenum" id="Page_433">- 433 -</span> -durable, stand out above the decomposable surface of the containing rock, -and show their quartz-blebs and felspar crystals on the weathered surface.</p> - -<p>Perhaps the most striking feature of the granophyre sills of Skye is -their general association with thinner basic intrusive sheets between which -they have insinuated themselves. This characteristic structure, pointed -out by me in 1888, has recently been more minutely mapped in the progress -of the Geological Survey. Mr. Harker has found the typical arrangement to -be the occurrence of a thick sill of granophyre interposed between two sills -of basalt, each of which is usually not more than six or eight feet thick. -Where the granophyre has been intruded independently among the Lias -formations, it does not assume the regularity and persistence which mark it -where it has followed the course of basic sills.</p> - -<div class="figcenter" id="v2fig372" style="width: 457px;"> - <img src="images/v2fig372.png" width="457" height="96" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 372.</span>—Section across the Granophyre Sills at Loch a' Mhullaich, above Skulamus, Skye.<br /><br /> - <i>a</i>, Jurassic sandstones and shales; <i>b</i>, Jurassic dark brown sandy shales; <i>c</i>, sills of basalt, some bands highly - cellular; <i>c′</i>, basalt-sill with veins of felsite rising into it from the granophyre below; <i>d</i> <i>d</i>, intrusive sheets or - sills of granophyre.</div> -</div> - - -<p>"The acid rock," Mr. Harker observes, "is invariably the later intrusion, -for it sends narrow veins into the basalts, metamorphosing them -to some extent and frequently enclosing fragments of them. These fragments -are always rounded by corrosion, and show various stages of dissolution -down to mere darker patches as seen by the naked eye. Such -inclusions and patches are found in the marginal part of a granophyre, -where no continuous basalt occurs, but where the acid magma has evidently -in places completely destroyed the earlier basic sheets between which it was -forced. It seems probable that in all cases a certain amount of solution of -the basalt by the granophyre magma took place at their contact, facilitating -the injection of the later intrusion and accounting for its persistent choice -of the contact-plane of two basalt-sills as the surface offering least resistance -to its injection."</p> - -<p>These observations throw fresh light on the remarkable original regularity -and persistence of the basic sills. Where one of these sills disappears -above or below a granophyre sheet its probable former presence is often -indicated by corroded fragments of the basic in the acid rock. Mr. Harker -remarks that the acid magma seems to have been "in itself less adapted -than the basic to follow accurately a definite horizon and to maintain a -uniform thickness in its intruded sheets, but could do both when guided by -a pre-existing basalt-sill, or especially when insinuated between contiguous -basalt-sills." The corrosive action of the acid magma on the surface of -the basalt, which enabled it to force its way more readily between the basic -sills, might proceed so far as partially or wholly to destroy these sills.</p> - -<p><span class="pagenum" id="Page_434">- 434 -</span></p> - -<p>This solvent action may serve to explain some of the irregularities of the -granophyre intrusions. According to the same observer, such irregularities -are found "where the granophyre sheet and its encasing basalt-sills are not co-extensive, -or again where the two basalt-sills separate, owing to one of them -cutting obliquely across the bedding. In the latter case, which is not -common, the granophyre follows one of the basalt-sills, necessarily parting -from the other. When one of the two guiding basalt-sills dies out, the -granophyre may still continue, following the sill which persists. If the -latter also dies out, while the granophyre is still in some force, the acid -magma seems to have been reluctant to travel beyond the limit of the basalt, -but has drawn towards it, and the granophyre presents a blunt laccolitic -form, which contrasts with the acutely tapering edge of a granophyre which -dies out before reaching the limit of its basalt-sills. If, on the other hand, -on reaching the limit of the basalt, the acid magma has been in such force as -to be driven further, it is usually found to lose something of its regularity -and to depart from the exact horizon which it has hitherto followed. This -seems to happen, for instance, in the Beinn a' Chà irn sheet, which, when -traced westward, is found to behave as a 'boss' and is obviously transgressive, -having cut across the bedding of the strata so as to enter the -limestones, where it no longer behaves in any degree as a sill. The -district affords many examples of the tendency of intrusive masses in -general to cut sharply across the beds when they enter a group of limestones."</p> - -<p>More complex examples of acid sills are to be found where there have -been three or more basic sheets together. The great granophyre sheet -already referred to at Suisnish affords the best illustration of this structure. -Mr. Harker has noticed that "round most of its circumference there is seen -merely a single basalt-sill passing under the granophyre. Probably there -has been another similar sheet over the acid rock, but if so, it has been -removed by erosion, the granophyre itself forming everywhere the surface -of the plateau. On the southern side, however, we see that the original -basalt must have been at least triple, or counting the uppermost member, -now removed, quadruple. The granophyre has forced its way in between -the several members of the multiple basalt-sill, the intermediate ones being -thus completely enveloped. They are evidently metamorphosed as well as -veined by the granophyre, and when traced onward they give place to -detached portions which, floating as it were in the acid rock, are soon lost."</p> - -<p>It is seldom easy to determine where lay the vent or vents from which -the granophyre sills proceeded. Those of the Skye platform just described -may be chiefly concealed under some of the larger areas of the rock, such as -the sheets of Carn Dearg or Beinn a' Chà irn. But in several places, in close -association with the compound sills of granophyre and basalt, Mr. Harker -has found large dyke-like bodies of the acid rock, which may with considerable -probability be regarded as marking the position of the channels by -which the material of the sills ascended. "These bodies," he remarks, -"either occur isolated by erosion, the sills or the parts of the sills presumed -<span class="pagenum" id="Page_435">- 435 -</span> -to have been in connection with the dykes having been removed, or are only -very partially exhibited in direct connection with sills still remaining. -Where they can be examined in detail they are seen to be dykes varying up -to about 100 feet in width, but of no great longitudinal extent. Between -Suisnish and Cnoc Carnach they bear E.N.E., that is, at right angles to the -ordinary basic dykes of the district and parallel to the general direction of -the axes of folding, though further north they change this trend, but still -remain parallel to the strike of the Lias.</p> - -<p>"These dykes are composed essentially of granophyre, identical with that -of the sills. In some cases, they are flanked with basalt-dykes on one or -both sides, or the former existence of such lateral dykes is indicated by -partly-destroyed inclusions of the basic rock in the granophyre. The basalt -found in these cases is identical with that of the basic sills, and shows the -same relation to the granophyre. Discontinuity and failure of the basalt -are commoner, however, in the dykes than in the sills—a difference presumably -attributable to more energetic destructive action of the acid magma -when it was hotter and fresher. These supposed feeders of the granophyre -sills are certainly in some cases, and have possibly been in all, double or -triple dykes. The acid magma thus appears not only to have spread laterally -along the same platforms as the earlier basalts, but to have reached these -levels by rising through the same fissures which had already given passage -to the basic magma."<a id="FNanchor_431" href="#Footnote_431" class="fnanchor">[431]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_431" href="#FNanchor_431" class="label">[431]</a> MS. notes supplied by Mr. Harker.</p> - -</div> - -<p>The granophyre sills which, as already stated, can be followed as an -interrupted band from Suisnish Point to the Sound of Scalpa, emerge again -beyond Loch Sligachan and also in the island of Raasay, where a great sheet -of the acid rock covers an area of about five square miles. This tract has -recently been mapped for the Geological Survey by Mr. H. B. Woodward, -who has found it to have been intruded across the Jurassic series, a large -part of its mass coming in irregularly about the top of the thick white sandstones -of the Inferior Oolite. But it descends beneath the Secondary rocks -altogether, and in some places intervenes between the base of the Infra-liassic -conglomerates and the Torridon sandstone. Its irregular course transgressively -across the Mesozoic formations is probably to be regarded as another -example of the intrusion of the acid material preferentially along the line of -unconformability between the older rocks and the Tertiary basalts, now -nearly all removed from Raasay by denudation, though the intrusion does -not rigidly follow that line of division, but sometimes descends below it.</p> - -<p>The central portions of this Raasay granophyre possess the ordinary -structures of the corresponding rocks in Skye. They show a finely crystalline-granular, -micropegmatitic base, through which large felspars and quartzes -are dispersed. But at the upper and under junction with the sedimentary -rocks, beautiful spherulitic structures are developed. This is well seen on -the shore near the Point of Suisnish (Raasay), where, below the Lias Limestones, -the top of the granophyre appears, and where its bottom is seen to lie -on the Torridon sandstone.</p> - -<p><span class="pagenum" id="Page_436">- 436 -</span></p> - -<p>This granophyre sheet presents a further point of interest inasmuch as -it appears to have preserved one of the dyke-like masses which may mark -channels of escape from the general body of the acid magma below. Near -the Manse the section represented in <a href="#v2fig373">Fig. 373</a> may be observed. Owing to -great denudation, the massive sheet of granophyre has been cut into isolated -outliers which cap the low hills, and the rock may be seen descending -through the Jurassic sandstones, which in places are much indurated. It is -observable that the amount of contact-metamorphism induced by the granophyre -sills upon the rocks between which they have been injected is, in -general, comparatively trifling. It is for the most part a mere induration, -sometimes accompanied with distortion and fracture.</p> - -<div class="figcenter" id="v2fig373" style="width: 504px;"> - <img src="images/v2fig373.png" width="504" height="150" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 373.</span>—Section to show the connection of a sill of Granophyre with its probable funnel of supply, - Raasay.<br /><br /> - <i>a</i> <i>a</i>, Jurassic sandstones; <i>b</i>, granophyre.</div> -</div> - -<div class="figleft" id="v2fig374" style="width: 332px;"> - <img src="images/v2fig374.png" width="332" height="193" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 374.</span>—Granophyre sill resting on Lower Lias shales with a dyke of - basalt passing laterally into a sill, Suisnish Point, Isle of Raasay.</div> -</div> - -<p>Although the intrusion of the granophyre sills has been subsequent to -that of the basalt-sheets with which they are so generally associated, we may -expect that as there is a series of post-granophyre basic dykes, so there may -be some basic sills later than the injections of the acid sheets. The Raasay -granophyre appears to -furnish an example of -such a later basic intrusion. -At the Point -of Suisnish on that -island I have observed -the relations shown in -<a href="#v2fig374">Fig. 374</a>. There the -dark shales of the -Lower Lias (<i>a</i> <i>a</i>) are -immediately overlain -by the granophyre sill -(<i>b</i>), and are cut by a -basalt-dyke which, -when it rises to the base of the granophyre, turns abruptly to one side, and -then pursues its course as a sill (<i>c</i>) between the granophyre and the shales. -There can be little doubt that this intrusion is later than the granophyre. -Here a basic sill is interposed at the bottom of the acid sheet; and is -<span class="pagenum" id="Page_437">- 437 -</span> -visibly connected with the actual fissure up which its molten material was -impelled.</p> - - -<h3>ii. <span class="allsmcap">THE ACID DYKES AND VEINS</span></h3> - -<p>Besides bosses and sills, the acid rocks of the Inner Hebrides take the -form of Dykes and Veins which have invaded the other members of the -volcanic series. Some of these have already been referred to; but a more -particular description of the venous development of the acid rocks as a whole -is now required.</p> - -<p>As regards their occurrence and distribution, they present two phases, -which, however, cannot always be distinguished from each other. On the -one hand, they are found abundantly either directly proceeding from the -bosses (more rarely from the sills), or in such immediate proximity and close -relationship to these as to indicate that they must be regarded as apophyses -from the larger bodies of eruptive material. On the other hand, they present -themselves as solitary individuals, or in groups at a distance of sometimes -several miles from any visible boss of granophyre. In such cases, it is of -course obvious that though not exposed at the surface, there may be a large -mass of the acid magma at no great distance beneath, and that these isolated -dykes and veins do not essentially differ in origin from those of which the -relations to eruptive bosses can be satisfactorily observed or inferred.</p> - -<p>Considered as a petrographical group, these Dykes and Veins are marked -by the following characters. At the one extreme, we have thoroughly -vitreous rocks in the pitchstones. From these, through various degrees of -devitrification, we are led to completely lithoid felsites, quartz-porphyries -or rhyolites. Micropegmatitic structure is commonly present, and as it -increases in development, the rocks assume the ordinary characters of granophyre. -Occasionally the structure becomes microgranitic in the immediate -periphery of a boss wherein a granitic character has been assumed. Viewed -as a whole, however, it may be said that the dull lithoid rocks of the dykes -and veins can generally be resolved under the microscope into some variety -of granophyric porphyry or granophyre.</p> - -<p>A characteristic feature in the granophyric, felsitic or rhyolitic dykes -and veins is the presence of spherulitic structure (Figs. <a href="#v2fig375">375</a>, <a href="#v2fig377">377</a>). In some -cases this structure is hardly traceable save with the aid of the microscope, -but from these minute proportions it may be followed up to such a strong -development that the individual spherulites may be an inch or two in -diameter, and lie crowded together, like the round pebbles of a conglomerate. -The structure is a contact phenomenon, being specially marked along the -margin of the dykes, as it is on the edge of sills and bosses. In the Strath -district of Skye, Mr. Clough and Mr. Harker have observed that the -spherulites are apt to be grouped in parallel lines so as to form rod-like -aggregates along the walls, and that where the rock is fairly fresh the centre -of the dyke sometimes consists of glassy pitchstone, so that the spherulitic -felsite or granophyre is probably devitrified pitchstone. Frequently flow-structure -is admirably developed in these dykes, the streaky layers of -devitrification flowing round the spherulites and any enclosed fragments as -perfectly as in any rhyolitic lava (<a href="#v2fig378">Fig. 378</a>).</p> - -<p><span class="pagenum" id="Page_438">- 438 -</span></p> - -<div class="figcenter" id="v2fig375" style="width: 746px;"> - <img src="images/v2fig375.png" width="746" height="509" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 375.</span>—Weathered surface of spherulitic granophyre from dyke in banded gabbros, Druim an Eidhne, Meall Dearg, Glen Sligachan, Skye. Natural size.</div> -</div> - -<p><span class="pagenum" id="Page_439">- 439 -</span></p> - -<p>In regard to their modes of occurrence, the dykes of acid material differ -in some important respects from those of basic composition. More especially -they are apt to assume the irregular venous form, rather than the vertical -wall-like character of ordinary dykes. They take the form of dykes, -particularly where their material has been guided in its uprise by one or -more already existent basic or intermediate dykes, as in the compound dykes, -already described. The conditions for their production must thus have been -essentially different from those of the great body of the basic dykes. Their -intrusion was not marked by any general and widespread fissuring of the -earth's crust, such as prepared rents for the reception of the basalt and andesite -dykes. They were rather -accompaniments of the protrusion -of large masses of acid -magma into the terrestrial crust. -This magma, as we have seen, -was often markedly liquid, and -was impelled, sometimes with -what might be called explosive -violence, into the irregular cracks -of the shattered surrounding -rocks or into pre-existing dyke-fissures. -Hence long straight -dykes of the acid rocks are much -less common than short irregular -tortuous veins and strings.</p> - -<div class="figright" id="v2fig376" style="width: 255px;"> - <img src="images/v2fig376.png" width="255" height="409" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 376.</span>—Plan of portion of the ridge north of Druim - an Eidhne, Glen Sligachan, Skye, showing three dykes - issuing from a mass of granophyre.<br /><br /> - <i>a</i>, gabbros; <i>b</i>, granophyre; I. II. III., three dykes proceeding - from the granophyre. The arrows show the direction of dip - of the bands of gabbro.</div> -</div> - - -<p>Much difference may be -noticed among the granophyre -bosses in regard to their giving -off a fringe of apophyses. Thus, -along the well-exposed boundary -of Beinn-an-Dubhaich in Skye, -though the edge of the boss is -remarkably notched, hardly any -veins deserving the name diverge -from it. On the other hand, the -ridge of Meall Dearg at the head -of Glen Sligachan, already referred -to, is distinguished by the -number and variety of the dykes -and veins which proceed from the granophyre and traverse the banded -gabbros. As this locality has been elsewhere fully described, I will give here -only the leading structural features which it presents.<a id="FNanchor_432" href="#Footnote_432" class="fnanchor">[432]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_432" href="#FNanchor_432" class="label">[432]</a> Professor Judd (<i>Quart. Journ. Geol. Soc.</i> vol. xlix. (1893), p. 175) described the granophyre -dykes of this locality as inclusions of Tertiary granite in the gabbro, and cited them in proof of -his contention that the acid eruptions of the Western Isles are older than the basic. Their true -character was shown by me in a paper published in the <i>Quart. Journ. Geol. Soc.</i> vol. 1. (1894), -p. 212.</p> - -</div> - -<p><span class="pagenum" id="Page_440">- 440 -</span></p> - -<div class="figcenter" id="v2fig377" style="width: 745px;"> - <img src="images/v2fig377.png" width="745" height="478" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 377.</span>—Weathered surface of spherulitic granophyre from dyke in banded gabbros, Druim an Eidhne, Meall Dearg, Glen Sligachan, Skye. Natural size.</div> -</div> - -<p><span class="pagenum" id="Page_441">- 441 -</span></p> - -<p>Within a horizontal distance of less than 100 yards three well-marked -dykes issue from the spherulitic edge of the Meall Dearg granophyre, and -run in a south-easterly direction in the handed gabbros (<a href="#v2fig376">Fig. 376</a>). The -most northerly of these is traceable in a nearly straight line for 800 feet. -The central dyke, which can be followed for 200 feet or more, rises as a -band six to ten feet broad between the dark walls of gabbro as represented in -<a href="#v2fig379">Fig. 379</a>.</p> - -<p>These dykes are marked by the most perfectly developed spherulitic and -flow-structures (Figs. <a href="#v2fig375">375</a>, <a href="#v2fig377">377</a>). Numerous detached portions of other -dykes and also irregular veins are to be observed cutting the banded -gabbros all over the ridge of Druim an Eidhne for a distance of a mile -or more. Many of these exhibit the same exquisitely beautiful spherulitic -and flow-structure displayed by the dykes which can actually be traced into -the main body of granophyre. The lines of flow conform to every sinuosity -in the boundary-walls of gabbro, and sometimes sweep round and enclose -blocks of that rock. The example of this structure, given in <a href="#v2fig378">Fig. 378</a>, -shows how these lines, curving round projections and bending into eddy-like -swirls, exhibit the motion of a viscous lava flowing in a cleft between two -walls of solid rock. Sometimes the laminæ of flow have been disrupted, -and broken portions of them have been carried onward and enveloped in the -yet unconsolidated material. Certain portions of this dyke are richly -spherulitic, the spherulites varying from the size of small peas up to that of -tennis-balls. Occasionally two large spherulites have coalesced into an -8-shaped concretion, and it may be observed in some cases that the -spherulites are hollow shells.</p> - -<div class="figcenter" id="v2fig378" style="width: 389px;"> - <img src="images/v2fig378.png" width="389" height="206" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 378.</span>—Plan of pale granophyric dyke, with spherulitic and flow-structure, cutting and enclosing - dark gabbro, Druim an Eidhne.</div> -</div> - -<p><span class="pagenum" id="Page_442">- 442 -</span></p> - -<div class="figcenter" id="v2fig379" style="width: 661px;"> - <img src="images/v2fig379.png" width="661" height="474" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 379.</span>—Dyke (six to ten feet broad) proceeding from a large body of granophyre and traversing gabbro, from the same locality as Figs. <a href="#v2fig375">375</a> and <a href="#v2fig377">377</a>.</div> -</div> - -<p><span class="pagenum" id="Page_443">- 443 -</span></p> - -<p>A remarkable feature has been recently observed by Mr. Harker among -the abundant granophyre dykes and veins which intersect the gabbros and -older rocks, along the eastern flanks of the Red Hills of Skye between -Broadford and the Sound of Scalpa. Broad dykes of granophyre which -traverse the Cambrian limestone of that district might be supposed at first -sight to be cut off by the intrusions of gabbro. But closer examination -proves that their apparent truncation arises from their suddenly breaking -up into a network of small veins where they abut against the basic rock. -This structure evidently belongs to the same type as that of the St. Kilda -granophyre.</p> - -<div class="figcenter" id="v2fig380" style="width: 343px;"> - <img src="images/v2fig380.png" width="343" height="278" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 380.</span>—Section of intruded veins of various acid rocks above River Clachaig, Mull.<br /><br /> - <i>a</i> <i>a</i>, basalt, dolerite, etc.; <i>b</i> <i>b</i>, granophyre.</div> -</div> - -<p>Compound dykes and sills, where one or more of the injections has consisted -of acid material, have been already noticed as intimately associated -together in Skye (p. 162). Dykes of this nature are more particularly -abundant in Strath, especially along its eastern side. In addition to the -examples cited already from that district, I may refer to other two which -intersect the Middle Lias shales and limestones in the island of Scalpa. -They are both compound dykes, but the more basic marginal bands are not -always continuous, having possibly been here and there dissolved by the -acid invasion. Though they do not show any distinct spherulitic forms, -the presence of flow-structure is indicated by the thin slabs into which the -rocks weather parallel to the dyke-walls. The rock in each case is a fine-grained -felsitic mass, with bi-pyramidal crystals of quartz. It is observable -that where these dykes come directly against the Liassic strata, the latter -are more seriously indurated than where they are traversed by the ordinary -basic dykes.</p> - -<p>In the central mountainous tract of the island of Mull veins of acid -material are extraordinarily abundant. They probably proceed from a much -larger subterranean body of granophyre than any of the comparatively small -bosses of this rock which appear at the present surface of the ground. They -<span class="pagenum" id="Page_444">- 444 -</span> -show themselves partly at the margins of the visible bosses, but much more -profusely in that tract of altered basalt, with intrusive sheets and dykes of -basalt, dolerite and gabbro, which lies within the great ring of heights -between Loch na Keal and Loch Spelve. In some areas, the amount of -injected material appears to equal the mass of more basic rock into which it -has been thrust. Pale grey and yellowish porphyries and granophyres, -varying from thick dykes down to the merest threads, ramify in an intricate -network through the dark rocks of the hills, as shown in the accompanying -illustration (<a href="#v2fig380">Fig. 380</a>), which represents a portion of the hillside between -Beinn Fhada and the Clachaig River. Such a profusion of veins probably -indicates the existence here of some large mass of granophyre or granite, -at no great depth beneath the surface.</p> - -<p>In Mull, as in the other islands of the Inner Hebrides, two horizons on -which protrusions of acid materials have been specially abundant, are the -base of the bedded basalts of the plateau and the bottom of the thick sheets -of gabbro. Dykes and veins of granophyre, quartz-porphyry, felsite and -other allied rocks are sometimes crowded together along these two horizons, -though they may be infrequent above or below them.</p> - -<p>Illustrations of solitary veins in the midst of unaltered plateau-basalts -or in older rocks may be gathered from many parts of the Western Isles. -Some remarkable instances are to be seen among the basalts that form the -terraced slopes on the north side of Loch Sligachan. Several thick dykes -of granophyre run up the declivity, cutting across hundreds of feet of the -nearly level basalt-beds. Some of them can be seen on the shore passing -under the sea. They trend in a S.S.E. direction towards Glamaig, and they -are not improbably apophyses from that huge boss, the nearest edge of -which is three-quarters of a mile distant. Another example may be cited -from the basalt-outlier of Strathaird, where two veins of felsite, one of them -a pale flinty rock showing flow-structure parallel to the walls, may be -seen on the west front of Ben Meabost. In this case, the veins are -three miles and a half from the granophyre mass of Strath na Creitheach -to the north, four miles from that of Beinn an Dubhaich to the north-east, -and nearly three miles from that of Coire Uaigneich at the foot of -Blath Bheinn.</p> - -<p>A special place must be reserved for the pitchstone-veins. Ever since -the early explorations of Jameson and Macculloch, the West of Scotland has -been noted as one of the chief European districts for these vitreous rocks. -From Skye to Arran, and thence to Antrim, many localities have furnished -examples of them, but always within the limits of the Tertiary volcanic -region. That all of the pitchstones are of Tertiary age cannot, of course, be -proved, for some of them are found traversing only Palæozoic rocks, and of -these all that can be absolutely affirmed is that they must be younger than -the Carboniferous or even the Permian system. But, as most of them are -unquestionably parts of the Tertiary volcanic series, they are probably all -referable to that series. Not only so, but there is, I think, good reason to -place them among its very youngest members. It is a significant fact that -<span class="pagenum" id="Page_445">- 445 -</span> -they almost always occur either in or close to granophyre or granite bosses, -the comparatively late origin of which has now been proved.</p> - -<div class="figright" id="v2fig381" style="width: 195px;"> - <img src="images/v2fig381.png" width="195" height="251" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 381.</span>—Pitchstone vein traversing the -bedded basalts, Rudh an Tangairt, Eigg.</div> -</div> - - -<p>The first pitchstone observed in Skye was found by Jameson on the -flanks of the great granophyre cone of Glamaig. Another rises on the side -of the porphyry mass of Glas Bheinn Bheag, in Strath Beg. Several occur -at the foot of Beinn na Callich. In Rum, -I found a pitchstone vein traversing the -western slopes of the wide granophyre -boss of Orval. In Eigg, the well-known -veins of this rock intersect the plateau-basalts -(<a href="#v2fig381">Fig. 381</a>), but they are accompanied, -even within the same fissure, with -granophyre, and in their near neighbourhood -lie the masses of this rock already -alluded to.<a id="FNanchor_433" href="#Footnote_433" class="fnanchor">[433]</a> In Antrim, pitchstone and -obsidian occur in the midst of the rhyolite. -The only marked exceptions to the general -rule, with which I am acquainted, are -those of the island of Arran. Most of -the pitchstone-veins in that district traverse -the red sandstones which may be -Permian. But none of them are far -removed from the great granite boss of the -northern half of the island, while large masses of quartz-porphyry, which -strikingly resemble some of those of Skye and Mull, lie still nearer to -them. It is also worthy of notice that pitchstone-veins rise through the -Arran granite boss itself, the probably Tertiary date of which has been -already discussed.</p> - -<div class="footnote"> - -<p><a id="Footnote_433" href="#FNanchor_433" class="label">[433]</a> For an account of the pitchstone veins of Eigg, see <i>Quart. Journ. Geol. Soc.</i> xxvii. p. 299.</p> - -</div> - -<p>This common association of pitchstone-veins with the Tertiary eruptive -bosses of acid rocks can hardly be a mere accidental coincidence. It seems -to prove a renewed extravasation of acid material, now in vitreous form, -from the same vents that had supplied the granitoid, granophyric, porphyritic -and felsitic varieties of earlier protrusions. We must remember that -the pitchstone-veins are not mere local glassy parts of the larger bodies of -granophyre or granite in which they lie. Their margins are sharply -defined; they are indeed in all respects as manifestly intruded, and therefore -later masses, as are the basalt-dykes. Their occurrence, therefore, -within the acid bosses proves them to be younger than these members of -the Tertiary volcanic series. Whether they are also later than the latest -basalt-dykes cannot yet be decided, for I have never succeeded in finding -an example of the intersection of these two groups of veins and dykes. -But, with this possible exception, the pitchstones are the most recent of all -the eruptive rocks of Britain.</p> - -<p>As a rule, the intrusive pitchstones occur as veins which cannot be -traced far, and which vary from a few yards to less than an inch in width. -<span class="pagenum" id="Page_446">- 446 -</span> -They generally show considerable irregularity in breadth and direction, -sometimes sending out strings into the surrounding rock (<a href="#v2fig381">Fig. 381</a>). The -outer portions are not infrequently more glassy and obsidian-like than the -interior. Occasionally the vitreous character disappears by devitrification, -and the rock assumes the texture of a compact felsite or of a spherulitic -rock.</p> - -<p>Among the later movements of the acid magma account must be taken -here of the pale fine-grained veins which have already been referred to as -traversing the granophyre bosses. These intrusions, so well seen in the -bosses of Skye and St. Kilda, are often so close in texture that they may be -called quartz-felsites. Their sharply-defined edges and felsitic character -suffice to separate them from what are termed "veins of segregation." In -at least one instance, that of Meall Dearg, already cited, a mass of typical -granophyre which has developed spherulitic and flow-structures along its -margin, and which sends out dykes having the very same structures for a -distance of several hundred feet across the banded gabbros, is itself traversed -by a dyke of precisely similar character. Here we see that after the intrusion -of its apophyses, and after its own consolidation in the upper parts, the -granophyric magma that rose into rents in the solidified portion retained -the same tendency to produce large spherulites as it had shown at first.</p> - -<p>The fine felsitic veins that traverse the granophyre of the Red Hills are -now being mapped by Mr. Harker during the progress of the Geological -Survey. He has not yet obtained evidence of the age of these veins in -relation to the latest basic dykes. He has observed that they appear to be -on the whole rather less acid than the material of the surrounding bosses, -though they were probably all connected with the same underlying acid -magma from which the bosses were protruded. A somewhat similar relation -has been noticed between older granites and their surrounding dykes, as in -Cornwall and Galloway.</p> - - -<hr class="chap x-ebookmaker-drop" /> - -<table style="border: #000 1px;" summary="Map VII"> -<tr> - <td class="tdl vsmall" colspan="2"><a id="v2map7"></a>TO ACCOMPANY SIR ARCHIBALD GEIKIE'S "ANCIENT VOLCANOES OF BRITAIN"</td> - <td class="tdr vsmall">Map VII.</td> -</tr> -<tr> - <td colspan="3"><a href="images/v2map7lg.png"><img src="images/v2map7.png" width="482" height="659" alt="" /></a></td> -</tr> -<tr> - <td style="width: 33%;" class="tdl vsmall">The Edinburgh Geographical Institute</td> - <td style="width: 33%;" class="tdc vsmall">Copyright</td> - <td style="width: 33%;" class="tdr vsmall">J. G. Bartholomew</td> -</tr> -<tr> - <td colspan="3" class="tdc smaller" style="padding-top: 0.5em;">MAP OF THE TERTIARY VOLCANIC DISTRICT OF NORTH EAST IRELAND<br /> - Click on map to view larger sized.</td> -</tr> -</table> - - -<hr class="chap x-ebookmaker-drop" /> - -<p><span class="pagenum" id="Page_447">- 447 -</span></p> - -<div class="chapter"> -<h2 class="nobreak" id="CHAPTER_XLIX">CHAPTER XLIX<br /> - -<span class="smaller">THE SUBSIDENCES AND DISLOCATIONS OF THE PLATEAUX</span></h2> -</div> - - -<p>There can be no doubt that considerable alterations of level have taken -place over the volcanic areas of North-Western Europe since the eruptions -that produced the basalt-plateaux, These alterations embrace general and -local subsidences, and also dislocations by which considerable displacements -of the crust either in a downward or upward direction have been effected.</p> - - -<h3>i. <span class="allsmcap">SUBSIDENCES</span></h3> - -<p>The mere fact that in many places the lower members of the series of -terrestrial lavas have been submerged under the sea may be taken to prove -a subsidence since older Tertiary time. Along the west coast of Skye -this depression is well shown by the almost entire concealment of the bottom -of the plateau under the Atlantic. In the Faroe Isles the subsidence has -advanced still further, for not a trace of the underlying platform on which -the basalts rest remains above water. In Iceland, too, the complete submergence -of the base of the Tertiary volcanic sheets points to a widespread -subsidence of that region.</p> - -<p>Another strong argument in favour of considerable depression may be -derived from a comparison of the submarine topography with that of the -tracts above sea-level. It is obvious that the same forms of contour which -are conspicuous on the land are prolonged under the Atlantic. If we are -correct in regarding the valleys as great lines of subærial erosion, their -prolongations as fjords and submarine troughs must be considered as having -had a similar origin. We can thus carry down the surface of erosion -several hundred feet lower than the line along which it disappears under -the waves.</p> - -<p>I know no locality where this kind of reasoning is so impressively -enforced upon the mind as the west end of the Scuir of Eigg. The old -river-bed and its pitchstone terminate abruptly at the top of a great precipice. -Assuredly they must once have continued much further westward, -as well as the sheets of basalt that form the main part of the cliff. Yet the -sea in front of this truncated face of rock rapidly deepens to fully 500 feet -in some places. Had any such hollow existed in the volcanic period it -<span class="pagenum" id="Page_448">- 448 -</span> -would have been filled up by the long-continued outflowings of basalt. -Making every allowance for concealed faults and local subsidences, we -can only account for this submarine topography by regarding it as having -been carved out, together with the topography of the land, at a time when -the level of the latter was at least 500 feet higher than it is now.</p> - -<p>The subsidence which is thus indicated along the whole of the North-West -of Europe probably varied in amount from one region to another. We -seem to have traces of such inequalities in the varying inclinations of -different segments of the basalt-plateaux. The angles of inclination are -almost always gentle, but they differ so much in direction from island to -island, and even among the several districts of the same island, as to indicate -that certain portions of the volcanic plain have sunk rather more than -other portions.</p> - -<p>Thus in the Faroe Islands, where the bare cliffs allow the varying angles -of inclination to be easily determined, a general gentle dip of the basalts in -a south-easterly direction has been noted among the central and northern -islands by previous observers. This inclination, however, is replaced among -the southern islands by an equally gentle dip towards the north-east. The -centre of depression would thus seem to lie somewhere about Sandö and -Skuö. The highest angle of inclination which I noticed anywhere was at -Myggenaes, where the basalts dip E.S.E. at about 15°.</p> - -<p>Among the Western Isles, also, where similar variations in the inclination -of the basalt-sheets are observable, it might be possible by careful -survey to ascertain the probable position of the areas of maximum depression, -and to show to what extent differential movements have affected the -originally nearly level volcanic floor. It would doubtless be found that -everywhere the dominant movement has been one of subsidence. The vast -outpourings of lava would tend to leave the overlying crust unsupported, -and to cause it to sink into the cavities thus produced.</p> - -<p>Perhaps the most extensive subsidence of this kind, at least that which -admits of most satisfactory investigation, because it still remains above -sea-level, is displayed by the vast hollow in the Antrim plateau, which -embraces the basin of Lough Neagh and the valley of the Lower Bann. -This depression measures about 60 miles in length by about 20 in breadth. -Its axis follows the N.N.W. trend so characteristic of the volcanic features -of Tertiary time. The depression may be said to involve the entire -basaltic plateau of Antrim, for with the exception of a few insignificant -areas along the borders, especially on the east side between Larne and -Cushendall, the whole region slopes inward from its marginal line of escarpments, -which reach heights of 1800 feet and upwards, towards the great -hollow in its centre (see Map VII.).</p> - -<p>Lough Neagh, which occupies the deepest part of this hollow, and covers -about one-eighth of the whole area of subsidence, is the largest sheet of fresh -water in the British Isles, for it exceeds 150 square miles in extent of -surface. Yet, for its size, it is one of the shallowest of our lakes, its average -depth being less than 40 feet. Its shallowness, compared with its wide -<span class="pagenum" id="Page_449">- 449 -</span> -area, marks it out in strong contrast to most of the larger British lakes. -Its surface is only 48 feet above the level of the sea.</p> - -<p>The origin of Lough Neagh, the theme of various legends, has been -seriously discussed by different writers, but most exhaustively by the late E. -T. Hardman of the Geological Survey.<a id="FNanchor_434" href="#Footnote_434" class="fnanchor">[434]</a> This author connected the formation -of the lake-basin with a series of large faults which are found intersecting -the rocks around the basin, and passing under the water in a general -north-easterly direction. He showed that these faults have produced serious -displacements of the strata, amounting sometimes to as much as 2000 feet, -and he believed that it was by the concurrent effect of such dislocations that -the depression of Lough Neagh had been caused.</p> - -<div class="footnote"> - -<p><a id="Footnote_434" href="#FNanchor_434" class="label">[434]</a> "On the Age and Formation of Lough Neagh," <i>Journ. Roy. Geol. Soc. Ireland</i>, vol. iv. -(1875-76), p. 170; also Explanation of Sheet 35 of the <i>Geol. Surv. Ireland</i> (1877), p. 72.</p> - -</div> - -<p>It is possible that these displacements may have contributed to at least -the earlier stages in the history of the Antrim subsidence. They have -undoubtedly taken place after the outpouring of the basalts, for these rocks -are involved in their effects. But in the hollow of the Bann valley north -of Lough Neagh the faults which have been detected in the basaltic plateau -are few and trifling. The bold and bare escarpments, that so clearly display -the relations of the rocks, reveal few traces of any important transverse -dislocations. Nor has any proof of large longitudinal faults parallel with -the axis of depression been obtained within the area of the Bann valley.</p> - -<p>The earliest evidence for the existence of a lake on the site of the -present Lough Neagh has been supposed to be furnished by certain fine -clays, sands, seams of lignite and clay-ironstone, which have been referred -to the Pliocene period. These deposits have been regarded as indicating -the accumulation of fine sediment with drift vegetation brought down into -a quiet lake by streams entering from the south. Their fresh-water origin -was believed to be further corroborated by the occurrence of shells belonging -to the lacustrine or fluviatile genus, <i>Unio</i>.<a id="FNanchor_435" href="#Footnote_435" class="fnanchor">[435]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_435" href="#FNanchor_435" class="label">[435]</a> These shells were regarded as forms of <i>Unio</i> by the late W. H. Baily; but Dr. Henry -Woodward assigned them to <i>Mytilus</i>. See Prof. Hull's <i>Physical Geology and Geography of Ireland</i>, -2nd edit. p. 101. The shells have been more recently dug out by Mr. Clement Reid, who has -found them to be the common <i>Mytilus edulis</i>.</p> - -</div> - -<p>The thickness of this series of strata, their position above sea-level, and -their distribution are important parts of the evidence for the geological -history of the locality. At one place the deposits are said to have been bored -through to a depth of 294 feet, and Mr. Hardman believed them to be not -less than 500 feet deep. The same observer found that they certainly reach -a height of 120 feet above the sea, and he was of opinion that in some -places their height was not less than 140 feet. The deposition of strata to -the depth of 300 feet below a level of 120 feet above the sea would, of -course, entirely fill up Lough Neagh, and spread over a large tract of low -ground around it. The pottery-clays and lignites, however, appear to be -confined to the southern half of the lake, from which they rise gently -into the low country around.</p> - -<p>The distribution of these deposits and their extraordinary variations in -<span class="pagenum" id="Page_450">- 450 -</span> -altitude, as described by Mr. Hardman, present great difficulties in the -attempt to regard them as the sediments of a Pliocene lake. A more -recent examination of the ground by Mr. Clement Reid of the Geological -Survey has led that able observer to believe that two totally different -groups of strata at Lough Neagh have been confounded. He noticed the -<i>Mytilus</i>-clay to be a dark blue mass full of derived boulder-clay stones, and -yielding <i>Mytilus edulis</i> and seeds of a sedge. This deposit cannot be -Pliocene, but must be of Glacial or post-Glacial age, possibly contemporary -with the Clyde beds. The junction of this clay with the pipe-clays is not -at present seen, but the lithological contrast between the two groups of -strata is so strong as to indicate their independence of each other. Mr. -Reid found the white, red and mottled pipe-clays with their masses of -lignite to present a strong resemblance to the Bagshot group in the -Tertiary series. It is possible, as already suggested, that the pipe-clays and -lignites may belong to the sedimentary zone that separates the lower and -upper basalts of Antrim. At all events they furnish no proof of any Pliocene -lake, and may not indicate more than a deeper part of the depression -in which the tuffs, lignites and iron-ore were laid down.</p> - -<p>The existence of the <i>Mytilus</i>-clay shows that in Glacial or post-Glacial -times the valley of the Bann was a strait or fjord into which the sea -entered. Thick masses of drift have been laid down all round and over -the depression now occupied by Lough Neagh, insomuch that had any -older lake existed here in Glacial times, it could hardly have escaped being -filled up.</p> - -<p>The observer, who from one of the basalt-heights looks down upon the -expanse of Lough Neagh and the broad peat-covered plain that continues the -level platform of the lake-surface down the valley of the Bann, cannot but -be impressed with the size of this wide hollow in the heart of the Antrim -plateau, and with the evident continuity of the whole depression from the -lake to the sea. If he be a geologist, he will be further struck by the fact -that while the Chalk and other older rocks appear from under the basalt-escarpments -all round the plateau, at heights of many hundred feet above the -sea, the floor of this wide hollow is entirely covered with basalt. Had the -depression been merely due to denudation, the rocks that underlie the -volcanic series would have been exposed to view. The base of the basalts -which, on either side of the depression, is often more than 1000 feet above -the sea-level, sinks below that level in the hollow of the Bann and Lough -Neagh.</p> - -<p>This inequality of position may have been partially brought about by -faults like those around Lough Neagh, and may thus have been begun long -before the Glacial period. But it appears to me to be mainly due to a -wide subsidence, of which the axis ran in a N.N.W. and S.S.E. direction -from the present coast up the valley of the Bann and the basin of Lough -Neagh to beyond Portadown.</p> - -<p>We may conceive that after the cessation of the outflows of basalt, the -territory overlying the lava-reservoir that had been emptied would tend to -<span class="pagenum" id="Page_451">- 451 -</span> -subside, partly by ruptures of the crust producing faults and partly by a -downward movement of a more general kind. In course of time, these -disturbances turned the drainage into the hollow now traversed by the -Bann. Denudation would necessarily accompany them, and the surface of -the country would be continually eroded and lowered.</p> - -<p>Lough Neagh has been carefully sounded by the Admiralty, and its chart -affords much suggestive material for the consideration of the geologist.<a id="FNanchor_436" href="#Footnote_436" class="fnanchor">[436]</a> -From the soundings there given it has long been known that the lake -deepens towards its northern end, and attains a maximum depth of 102 feet. -But it is not until we trace on the chart a series of contour-lines for -successive depths, as shown by the soundings, that we realize the remarkable -form of the lake bottom. We then discover that below a depth of 50 feet -a well-defined channel extends for rather more than half the length of the -lake. This channel begins to be distinctly perceptible between Kiltagh -Point and Langford Lodge. It first runs in a northerly course on the west -side of the centre of the Lough, but when it comes into a line with Saltera -Castle on the western shore, it wheels round so as to conform to the curve -of the Antrim coast-line, which it follows northward until, about two miles -from the exit of the lake, its outline ceases to be traceable on the gently -shelving bottom. Its total length is thus about 12 miles.</p> - -<div class="footnote"> - -<p><a id="Footnote_436" href="#FNanchor_436" class="label">[436]</a> Lough Neagh surveyed and sounded by Lieut. Thomas Graves, R.N.</p> - -</div> - -<p>There can hardly be any doubt that this channel is a former bed of the -River Bann. It occupies exactly the position which that stream would take -if the lake were drained, and its depth and breadth correspond to those of -the valley-bottom of the present river. If this conclusion be accepted, some -important conclusions may be further deduced from it.</p> - -<p>1. The presence of a former course of the Bann on the bottom of Lough -Neagh proves the lake to be much younger than the Ice Age. The thick -boulder-clays and Glacial gravels which so encumber the country around and -descend under the lake, would assuredly have filled up the river-channel had -it existed at the time of their deposition. The channel has obviously been -cut out of these drifts since the Glacial period. When the erosion took -place, the present Lough Neagh could not have existed, but the Bann -followed a continuous course across the plain which the lake now covers. -The river probably maintained its place for a long period, so as to be able to -excavate so wide and deep a bed in the drifts, if, indeed, it did not to some -extent slowly carve its bed out of the underlying basalts. It must be -remembered that sediment is being continually poured into Lough Neagh, -and that some of the silt must have accumulated in the submerged river-course, -thus lessening its depth and width. That the channel should still -be so marked may be used as an argument for the comparatively late date -of the subsidence.</p> - -<p>2. The submerged river-course is a clear proof of subsidence. The -present Lough Neagh cannot be looked upon as a glacial lake formed by -rock-erosion or by irregular deposition of drift. Its floor must have been a -land surface when the Bann cut out its bed upon it. The whole area has -<span class="pagenum" id="Page_452">- 452 -</span> -sunk down, the drainage has been arrested, and some 20 miles of the -course of the Bann are now under a sheet of shallow water. This subsidence -was not brought about by faults. It seems rather to have resulted from a -general sinking of the ground. The movement was probably comparatively -rapid, otherwise the river-course would hardly have survived so well.</p> - -<p>3. These inferences, based upon purely geological considerations, have an -interesting bearing upon the allusions to the origin of Lough Neagh contained -in some ancient historical documents. Various legends have from -an early period been handed down as to the first appearance of this sheet of -water. These myths, though differing in details, agree in describing such a -sudden or rapid accumulation of water as destroyed human life, in a district -which had previously been inhabited by man. The earliest records indicate -that the alleged catastrophe took place in the first century of the Christian -era.<a id="FNanchor_437" href="#Footnote_437" class="fnanchor">[437]</a> It appears to me not improbable that the tradition,thus preserved in these -legends, may have had its basis in the actual disturbance which, on geological -grounds, can be shown to have determined the existence of Lough Neagh. -Though the event may go back far beyond the first century, there can be no -doubt that, in a geological sense, it was one of the most recent topographical -changes which the British Isles have undergone.</p> - -<div class="footnote"> - -<p><a id="Footnote_437" href="#FNanchor_437" class="label">[437]</a> For versions of the legends, see Dr. Todd's "Irish Version of the Historia Britonum of -Nennius," <i>Roy. Hist, and Archæol. Assoc. Ireland</i>; Dr. Reeves' "Ecclesiastical Antiquities of -Down," etc., p. 370; Mr. J. O'Beirne Crowe's "Ancient Lake Legends of Ireland," No. 1 in -<i>Journ. Roy. Hist. and Archæol. Assoc. Ireland</i>, vol. i. (1870-71), p. 94; <i>Giraldus Cambrensis</i>, -vol. v. cap. ix. p. 91—"de lacu magno miram originem habente." Moore's well-known lines -embody the popular belief that round towers and other buildings were submerged by the inundation.</p> - -</div> - -<p>Thus the Antrim basalt-plateau, in addition to the high interest of its -volcanic history, has the additional claim to our attention that it has -preserved, more fully and clearly than any other of the plateaux, the -evidence for the latest subterranean movements that followed the long series -of volcanic eruptions during Tertiary time. It contains the record of a -post-Glacial subsidence that gave birth to the largest lake in Britain.</p> - - -<h3>ii. <span class="allsmcap">DISLOCATIONS</span></h3> - -<p>Though I have not observed any features among the Tertiary basalt-plateaux -of the British Isles that can be compared to the remarkable rifts -and subsidences of Iceland, it can be shown that these piles of volcanic -material have undoubtedly been fractured, and that portions of them have -subsided along the lines of dislocation.</p> - -<p>Careful examination of the basalt-escarpments of the Inner Hebrides -discloses the existence of numerous faults which, though generally of small -displacement, nevertheless completely break the continuity of all the rocks -in a precipice of 700 or 1000 feet in height. Not infrequently such -dislocations give rise to clefts in the cliffs. Some good illustrations of this -feature may be noticed on the north side of the island of Canna, where the -precipice has been fissured by a series of dislocations, having a hade towards -<span class="pagenum" id="Page_453">- 453 -</span> -the west and a throw which may in some cases amount to about 20 or 25 -feet. The cumulative effect of this system of faulting, combined with a -gentle westerly dip, is to bring down to the sea-level the upper band of conglomerate -which further to the east lies at the top of the cliff. Again, the -basalt-escarpment on the west side of Skye, from Dunvegan Head to Loch -Eynort, is traversed by a series of small faults. On the east side of Skye -and in Raasay, a number of faults, some of them having perhaps a throw of -several hundred feet, has been mapped by Mr. H. B. Woodward.</p> - -<p>The largest dislocation observed by me among the basalt-plateaux of the -Inner Hebrides is that already referred to (p. 209), which runs at the back -of the Morven outlier, in the west of Argyllshire, from the Sound of Mull -by the head of Loch Aline to the mouth of Loch Sunart, along the -line of valley that contains the salt-water fjord Loch Teacus and the fresh-water -lakes Loch Durinemast and Loch Arienas. While the Cretaceous -deposits and the bottom of their overlying basalts rise but little above the -sea-level on the south-west side of this line, they are perched as outliers on -hill-tops on the north-east side, where they rise to 1300 feet above the sea. -The amount of vertical displacement here probably exceeds 1000 feet. The -fault runs in a north-westerly direction, and has obviously been the guiding -influence in the erosion of the broad and deep valley which marks its course -at the surface.</p> - -<p>This dislocation is only the largest of a number by which the basalt-plateau -has been broken in the district of Morven. Their effects are well -shown in the outlier of basalt which caps Ben Iadain, where two parallel -faults bring down the lavas against the platform of schists on which they -lie (see <a href="#v2fig266">Fig. 266</a>).</p> - -<p>Many faults have been traced in the Antrim plateau, and are represented -on the Geological Survey Maps. In general they are of comparatively trifling -displacement. Occasionally, however, they amount to several hundred feet, -as in those already referred to as occurring near Ballycastle and around the -southern part of the basin of Lough Neagh.</p> - -<p>To what extent the dislocations that traverse the British Tertiary -basalts are to be regarded as comparable to those which in Iceland have -been referred to subsidence caused by the tapping and outflow of the lower -still liquid parts of lava-sheets must be matter for further inquiry. So far -as my own observations have yet gone, the faults do not seem explicable by -any mere superficial action of the kind supposed. Where they descend -through many hundreds of feet of successive sheets of basalt, and dislocate -the Secondary formations underneath, they must obviously have been produced -by much more general and deep-seated causes.</p> - -<p>It is conceivable that, if these dislocations took place during the -volcanic period, they broke up the lava-plains into sections, some of which -sank down so as to leave a vertical wall at the surface on one side of the -rent, or even to form open "gjás," like those of Iceland. But it is noteworthy -that the fissures, which have been filled with basalt and now appear -as dykes, comparatively seldom show any displacement in the relative levels -<span class="pagenum" id="Page_454">- 454 -</span> -of their two sides. In Iceland, also, the great lava-emitting fissures seem to -be in general free from marked displacements of that kind.</p> - -<div class="figleft" id="v2fig382" style="width: 192px;"> - <img src="images/v2fig382.png" width="192" height="157" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 382.</span>—Reversed fault on the eastern - side of Svinö, Faroe Isles.</div> -</div> - -<p>The faults in the Inner Hebrides, so far as I have observed, are all -normal, and indicate nothing more than gentle subsidence. But among -the Faroe Islands I have come upon -several instances of reversed faults, which, -in spite of the usually gentle inclinations -of the basalts, probably point to more -vigorous displacement within the terrestrial -crust.</p> - -<p>On the east side of Svinö a fault with -a low hade runs from sea-level up to the top -of the cliff, a height of several hundred feet. -It has a down-throw of a few yards, but is -a reversed fault, as will be seen from Fig. -382. Another similar instance may be -noticed on the north-east headland of Sandö, where, however, on the upcast -side, the basalts appear as if they had been driven upward, a portion -of them having been pushed up into a low arch (<a href="#v2fig383">Fig. 383</a>).</p> - -<div class="figcenter" id="v2fig383" style="width: 198px;"> - <img src="images/v2fig383.png" width="198" height="153" alt="" /> - <div class="figcaption"><span class="smcap">Fig. 383.</span>—Reversed fault on the north-east - headland of Sandö, Faroe Isle.</div> -</div> - -<p>When the Tertiary basalt-plateaux of the Hebrides and the Faroe -Isles come to be worked out in detail, -many examples of dislocation will doubtless -be discovered. We shall then learn -more of the amount and effects of the -terrestrial disturbances which have affected -North-Western Europe since older Tertiary -time. In the meantime evidence enough -has been adduced to prepare us for -proofs of very considerable recent displacements -even among regions of crystalline -schists, like that which has been disrupted -by the Morven faults above alluded to. -While the study of the Tertiary volcanic rocks demonstrates the vast -general denudation of the country since older Tertiary time, the proofs that -these rocks have been faulted acquire a special interest in relation to the -origin and evolution of the topography of the region.</p> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_455">- 455 -</span></p> - -<h2 class="nobreak" id="CHAPTER_L">CHAPTER L<br /> - -<span class="smaller">EFFECTS OF DENUDATION</span></h2> -</div> - - -<p>Among the more impressive lessons which the basalt-plateaux of North-Western -Europe teach the geologist, the enormous erosion of the surface of this part -of the continental area since older Tertiary time takes a foremost place. He -may be ready almost without question to accept the evidence adduced in -favour of a vast amount of denudation among such soft and incoherent -strata as those of the older Tertiary formations of the south-east of England -or the north-west of France. But he is hardly prepared for the proofs -which meet him among the north-western isles that such thick masses of -solid volcanic rocks have been removed during the same geological interval.</p> - -<p>To gain some idea of the amount of this waste we must, in the first -place, picture to our minds the extent of ground over which the lavas were -poured, and the depth to which they were piled upon it. Though we may -never be able to ascertain whether the now isolated basalt-plateaux of Britain -were once united into a continuous plain of lava, we can be quite certain -that every one of these plateaux was formerly more extensive than it is -now, for each of them presents, as its terminal edge, a line of wall formed by -the truncated ends of horizontal basalt-sheets. And there seems no -improbability in the assumption that the whole of the great hollow from the -centre of Antrim up to the Minch was flooded with lavas which flowed from -many vents between the hills of ancient crystalline rocks forming the line -of the Outer Hebrides on the west, and those of the mainland of Scotland -on the east.</p> - -<p>It is certain that the depth to which some parts of this long hollow -were overflowed with lava exceeded 3000 feet, for more than that depth of -rock can be shown to have been in some places removed. The original -inequalities of surface were buried under the volcanic materials which were -spread out in a vast plain or series of plains, like those that have been -deluged by modern eruptions in Iceland. Owing, however, to a general but -unequal movement of subsidence, the lava-fields sank down here and there -to, perhaps, an extent of several hundred feet, so that the old land-surface -on which they began to be poured out now lies in those places below the -level of the sea.</p> - -<p>I have shown that even during the volcanic period, while the lavas were -<span class="pagenum" id="Page_456">- 456 -</span> -still flowing from time to time, erosion was in active progress over the surface -of the volcanic plain. The records of river-action in Canna and Sanday, -and the buried river-channel of the Scuir of Eigg, prove that, while eruptions -still continued, rivers descending from the mountains of the Western -Highlands carried the detritus of these uplands for many miles across the -lava-fields, swept away the loose material of volcanic cones, and cut channels -for themselves out of the black rugged floor of basalt.</p> - -<p>The erosion thus early begun has probably been carried on continuously -ever since. The present streams may be looked upon as practically the -same as those which were flowing in the Tertiary period. There may have -been slight changes of level, oscillations both upward and downward in the -relative positions of land and sea, and shiftings of the water-courses to one -side or other; but there seems no reason to doubt that the existing basalt-plateaux, -which were built up as terrestrial areas, have remained land-surfaces -with little intermission ever since, although their lower portions may -have been in large measure submerged.</p> - -<p>In the existing valleys, fjords and sea-straits by which these plateaux -have been so deeply and abundantly trenched, we may recognize some of the -drainage-lines traced out by the rivers which flowed across the volcanic plains. -The results achieved by this prolonged denudation are of the most stupendous -kind. The original lava-floor has been cut down into a fragmentary tableland. -Hundreds of feet of solid rock have been removed from its general -surface. Outliers of it may be seen scattered over the mountains of Morven, -whence they look into the heart of the Highlands. Others cap the hills -of Rum, where they face the open Atlantic. Several miles from the main -body of the plateau in Skye, a solitary remnant, perched on the highest -summit of Raasay, bears eloquent witness that the basaltic tableland once -stretched far to the east of its present limits.</p> - -<p>Two lines of observation and of argument may be followed in the effort -to demonstrate how great the denudation has been since older Tertiary time. -In the first place, there is the evidence of the level or nearly level sheets of -basalt that form the plateaux, and, in the second place, there is the testimony -of the dykes, sills and bosses by which these lavas have been disrupted.</p> - -<p>1. The study of the denudation of the Tertiary volcanic rocks of North-Western -Europe is most satisfactorily begun by an attempt to measure the -minimum amount of waste which in certain places the basalt-plateaux can -be proved to have undergone. For the purposes of this study, the stratification -of the lavas and their nearly horizontal, or at least very slightly disturbed, -position afford exceptional facilities. Amorphous rocks, such as -granites and gabbros, or even foliated masses like the old gneisses and schists, -may have been enormously denuded. Their mere presence at the existing -surface may be taken as proof of such waste, yet they furnish in themselves -no criterion by which the amount of removed material may be estimated.</p> - -<p>But in the case of the basalt-plateaux, as in that of horizontal sedimentary -formations, the successive lines of superposition of the component -beds of the whole stratigraphical series supply admirable datum-lines which, -<span class="pagenum" id="Page_457">- 457 -</span> -on the one hand, vividly impress the imagination by the demonstration which -they afford of the reality and magnitude of the denudation, and, on the other -hand, furnish a measure by which the minimum amount of this denudation -may be actually computed.</p> - -<p>Availing ourselves of this kind of evidence it is easy to show that -valleys many miles long, several miles broad, and from crest to bottom -several thousand feet deep, have been excavated out of the basalt-plateaux -since the close of the volcanic period. And if this conclusion can be -demonstrated for these plateaux, it must obviously apply equally to the rest -of the country. We thus obtain a most important contribution to the -investigation of the origin and relative age of the present topographical -features of the surface of the land.</p> - -<p>Let me give a few illustrations of the nature of the investigation and of -the results to which it leads. Throughout the Western and Faroe Islands -the level bars of basalt present their truncated ends in the great escarpment-cliffs -which wind mile after mile along their picturesque coasts. Where -they front the open sea, it is obviously impossible to say how much further -seaward they once extended. But where they retire in fjords or sea-lochs, -and sweep inland into glens, it is easy to measure the distance from the -bottom of the eroded hollow to its bounding watersheds, and to estimate the -amount of material that has been worn out of it. The only uncertainty in -this computation arises from our inability to determine to what extent -movements of subsidence may have come into play to aid in the disappearance -of the basalts. Where the bottom of the lavas can be seen at the same -level on either side of an inlet, with no evidence of faulting, or where a -definite horizon in the volcanic series can be traced round the head of a glen -or sea-loch, the influence of underground movements may be eliminated. -The evidence of vast denudation is always visible, the proofs of subsidence -are much less frequently observable.</p> - -<p>The island of Mull supplies many striking examples of the enormous -waste of the basalt-plateau. The Sound of Mull, for instance, has been -eroded out of the volcanic series for a distance of 20 miles, with a mean -breadth of about two miles. From the deepest part of this fjord to the -summit of the Mull plateau is a vertical height of 3600 feet. The whole -of this vast excavation has taken place since older Tertiary time. On the -opposite side of Mull the hollow of Loch Scridain has been eroded to a mean -depth of at least 1200 feet below the average level of the surrounding -plateau, with a breadth of rather more than a mile.</p> - -<p>The scattered islands which lie to the west of Mull tell the same tale. -They are all outliers of the same basalt-plateau, and have not only been -greatly lowered by the removal of their upper lavas, but have been separated -by the erosion of long and deep hollows between them. Thus from the -summit of the Gribon cliffs in Mull to the deepest part of the sea-floor -between that precipice and the Treshnish Isles a vertical depth of at -least 2000 feet of rock has been removed since the basalts ceased to be -erupted.</p> - -<p><span class="pagenum" id="Page_458">- 458 -</span></p> - -<p>I have referred to the impressive evidence of denudation displayed -on the west side of the island of Eigg. The vertical distance from the -summit of the Eigg plateau to the bottom of the submarine valley between -this island and Rum is about 1500 feet, but as that summit lies below the -original surface of the lava-field, the depth of rock which has been removed -must exceed 1500 feet. We thus learn that since the close of the volcanic -period the hollow between the islands of Eigg and Rum has been eroded to -this great depth.</p> - -<p>Still more striking is the evidence of enormous waste presented by the -Faroe Islands. The cliffs there are loftier and barer, and the fjords have -been cut more deeply and precipitously out of the basalt-plateau. I shall -never forget the first impression made on my mind when the dense curtain -of mist within which I had approached the southern end of the archipelago -rapidly cleared away, and the sunlit slopes and precipices of Suderö, the -two Dimons, Skuö and Sandö, rose out of a deep blue sea. Each island -showed its prolongation of the same long level lines of rock-terrace. The -eye at once seized on these features as the dominant element in the geology -and the topography, for they revealed at a glance the true structure of the -islands, and gave a measure of the amount and irregularity of the erosion -of the original basalt-plateau. And this first impression of stupendous -degradation only deepened as one advanced further north into the more -mountainous group of islands. Probably nowhere else in Europe is the -potency of denudation as a factor in the evolution of topographical features -so marvellously and instructively displayed as among the north-eastern -members of the Faroe group.</p> - -<p>Availing ourselves of the datum-lines supplied by the nearly level bars -of basalt, we easily perceive that in many parts of the Faroe Isles the -amount of volcanic material left behind, stupendous though it be, is less -than the amount which has been removed. Thus the island of Kalsö is -merely a long narrow ridge separating two broad valleys which are now -occupied by fjords. The material carved out of these valleys would make -several islands as large as Kalsö. Again, the lofty precipice of Myling -Head, 2260 feet high, built up of bedded basalts from the summit to below -sea-level, faces the north-western Atlantic, and the sea rapidly deepens in -front of it to the surface of the submarine ridge 200 to 300 feet below. -The truncated ends of the vast pile of basalt-sheets which form that loftiest -sea-wall of Europe bears testimony to the colossal denudation which has -swept away all of the volcanic plateau that once extended further towards -the west.</p> - -<p>Nevertheless, enormous as has been the waste of this plateau of the -Faroe Islands, we may still trace some of its terrestrial features that date -back probably to the volcanic period. Even more distinctly, perhaps, than -among the Western Isles of Scotland, we may recognize the position of the -original valleys, and trace some of the main drainage lines of the area when -it formed a wide and continuous tract of land.</p> - -<p>A line of watershed can be followed in a south-westerly direction from -<span class="pagenum" id="Page_459">- 459 -</span> -the east side of Viderö, across Borö to the centre of Osterö, and thence by -the Sund across Stromö and Vaagö. From this line the fjords and valleys -diverge towards the north-west and south-east. There can hardly be any -doubt that on the whole this line corresponds with the general trend of the -water-parting at the time when the Tertiary streams were flowing over the -still continuous volcanic plain. Considerable depression of the whole region -has since then sent the sea up the lower and wider valleys, converting them -into fjords, and isolating their intervening ridges into islands.</p> - -<p>The topography of the Faroe Islands seems to me eminently deserving -of careful study in the light of its geological origin. There is assuredly no -other region in Europe where the interesting problems presented by this -subject could be studied so easily, where the geological structure is throughout -so simple, where the combined influences of the atmosphere and of the -sea could be so admirably worked out and distinguished, and where the -imagination, kindled to enthusiasm by the contemplation of noble scenery, -could be so constantly and imperiously controlled by the accurate observation -of ascertainable fact.</p> - -<p>2. Impressive and easily comprehended as are the proofs of denudation -supplied by the basalts of the plateaux, they are perhaps to a geological eye -less overwhelming than those furnished by the eruptive rocks which have -been injected into these plateaux. In the case of at least the basic intrusions, -we may reasonably infer that they assumed their present position -under a greater or less depth of overlying rock which has since been -removed. When, therefore, they are found at or above the summits of the -plateaux, they demonstrate that a vast amount of material has been removed -from these summits.</p> - -<p>The argument from the position of the dykes has already been enforced. -It is absolutely certain that valleys several thousand feet deep must have -been excavated since these dykes were erupted, for had such valleys existed -at the time when the dykes were injected across their site, the molten rock, -instead of ascending to the tops of the surrounding mountains, would -obviously have rushed forth over the valley-bottoms. I have shown that -this reasoning applies not merely to the volcanic districts, but to the whole -surface of the country within the region of dykes. Thus the uplands of -Southern Scotland, and wide areas in the Southern and Western Highlands, -can be proved to have had glens cut out of their mass to a depth of -hundreds of feet since the Tertiary volcanic period.</p> - -<p>Not less convincing is the evidence afforded by the great eruptive masses -of gabbro. We have seen that these complex accumulations of sills, dykes, -and bosses include rocks so coarse in grain as to show that they must have -consolidated at some considerable depth, but that they now appear in hill-groups -2000 to 3000 feet in height, the whole of the original basaltic cover -having been stripped off from them. But these gabbro hills have been in -turn traversed up to the very crests by later basalt-dykes, which thus -supply additional proof that the erosion here has been stupendous.</p> - -<p>The granophyre bosses tell the same tale. Though, like the domite -<span class="pagenum" id="Page_460">- 460 -</span> -Puys of Auvergne, they may still retain, in their conical forms, indications -of the original shapes which their component material assumed at the -time of its protrusion, we may be confident that their existing surfaces -have been reached after the removal of much rock which once lay above -them. This inference is confirmed by the fact that these eruptive bosses -have been invaded by a younger system of dykes. The black ribs of basalt -which may be traced along their pale declivities, which cross the glens that -have been eroded in them and which mount up to their very crests, prove that -since the latest manifestations of volcanic energy in the West of Scotland, -extensive changes in the topography of the land have been effected by the -operation of the subærial agents of degradation.</p> - -<p>So much for what can be demonstrated. But how much more may, -with the highest probability, be inferred! The original limits of the -plateaux are unknown. The waves of the wide Atlantic now roll over -many a square league of the old lava-plains, and wide tracts of the islands -and the mainland from which the basalt has been entirely stripped, or where -it remains only in scattered outliers, were once deeply buried under piles of -lava-sheets. It would probably be no exaggeration to affirm that over the -British area, as well as over the Faroe Isles, the amount of Tertiary volcanic -rock that now remains, large as it is, falls short in amount of what has -been removed. The geologist who has made himself familiar with the -effects of denudation in other Tertiary volcanic districts, such as Central -France, Saxony and Bohemia, will be prepared for almost any conceivable -amount of erosion among the far older volcanic series of the north-west of -Europe.</p> - -<p>To the student of the origin of the existing topography of the land -there is a profound interest in the demonstration which these volcanic rocks -supply of the vast changes which the terrestrial surface has undergone -within a period geologically so recent as older Tertiary time. When, -on the one hand, he finds himself more and more restricted in his demands -for time by the confident assertions of the physicist that all the -phenomena of geological history must have been comprised within a few -millions of years, and when, on the other hand, he watches the seemingly -feeble and tardy operations of the forces of denudation and sedimentation -which have played the chief parts in that history, he may well be excused -if sometimes he is apt to despair of ever reconciling the facts which he -observes with the physical deductions that are somewhat dogmatically -brought forward in opposition to his interpretation of them. He may feel -sure that his facts cannot be gainsaid, and he may be unable to find any -other way of comprehending them save by the admission that they necessitate -a liberal allowance of time. Yet he may not feel himself to be in a -position to offer any valid objections to the arguments from physical considerations -that would so seriously abridge the length of time which geology -requires.</p> - -<p>In these circumstances it is some satisfaction to be provided with -definite measurements of the amount of geological change which has been -<span class="pagenum" id="Page_461">- 461 -</span> -effected within a limited and relatively recent period of time. This change -has resulted from the operation of the same agents by which it is still being -carried on. No break in the history can be detected. There is not the -least reason to suppose that the agents of denudation and sedimentation -have, during the period in question, differed in their rate of working. -Their activity at the present time is probably neither greater nor less than -it was then. If, therefore, during so recent an interval such a stupendous -amount of material has been worn away from the surface of the land and -deposited on the sea-floor as the Tertiary volcanic rocks demonstrate, the -geologist may surely contemplate without misgiving the lapse of time -required for the completion of older geological revolutions. He may oppose -to the arguments of the physicist the measurements and computations -which he himself makes from data which are at least as reliable as the -postulates whereon these arguments are based. The rate at which denudation -and sedimentation are now taking place has been measured with tolerable -accuracy, and a fair average for it has been obtained. Whatever may be -maintained as to this rate in early geological ages, there can be no serious -opposition to its being taken as fairly constant since older Tertiary time. -We are thus provided with data for estimating the minimum amount of -time that can have elapsed since the volcanic plateaux began to be denuded. -But as no relic remains of the original upper surface of those plateaux, -and as we are consequently ignorant of how much rock has been removed -from their highest surviving outliers, it is obvious that such estimates are -more likely to err in understating than overstating the amount of time -required.</p> - -<p>It would be beyond the scope of the present volume to enter fully into -the measurements and calculations required for the adequate treatment of -this subject. I will merely illustrate my argument by again taking a few -data from the plateau of Mull. The original height of this plateau is -shown by the outlier of Ben More to have been at least 3200 feet. If to -this figure we add the portion of the basalt-group submerged under the sea -the height will probably be increased by several hundred feet. But let us -take 3000 feet as a moderate computation for the average thickness of the -volcanic series here at the close of the plateau-period. Until a number of -sections have been carefully plotted from the Ordnance Maps, in order to -ascertain with approximate accuracy the average height of the present surface -of the Mull basaltic plateau, making due allowance for the vast erosion of the -Sound of Mull and the numerous glens and sea-lochs that traverse the -island, any estimate which may be offered as to this average must be merely -provisional. If, in the meantime, we suppose the present mean level of the -plateau to be 1000 feet above the sea, the difference between this amount -and the assumed original height will be 2000 feet. If, further, we take -the present average rate of degradation of the Mull plateau to be <sup>1</sup>/<sub>6000</sub> of -a foot in a year, which has been shown to be probably a fair estimate, then -the time required for the lowering of the Mull plateau from its original to -its present average level amounts to twelve millions of years. Yet this -<span class="pagenum" id="Page_462">- 462 -</span> -period, vast though it be, does not carry us back even as far as the beginning -of Tertiary time.</p> - -<hr class="tb" /> - -<p>In concluding this lengthened discussion of the Tertiary volcanic history -of Britain, I may, perhaps, usefully add a brief summary of the leading features -of the long record.</p> - -<p>The region within which volcanic activity displayed itself during older -Tertiary time in the British Isles, if our estimate of its area is restricted to -those parts of the country where igneous rocks, probably of that age, now -appear at the surface, embraces the North of England and of Ireland, the -southern half and the west coast of Scotland—a total area of more than -40,000 square miles. Over that extensive region volcanic phenomena -were displayed during an enormously protracted interval of geological time. -The earliest beginnings of disturbance may possibly have started in the -Eocene, and the final manifestations may not have ceased until the Miocene -period. So prolonged was the duration of the eruptions, that enormous -topographical changes from denudation, and probably also considerable -variation in the fauna and flora, alike of land and sea, may have been -effected.</p> - -<p>Owing to some cause which has not yet in this relation been investigated, -but which is probably referable to secular terrestrial contraction, the volcanic -region underwent elevation, while, at the same time, a vast subterranean -lake or sea of molten rock existed underneath it. Enormous horizontal -tension thus arose, and at last the stretched terrestrial crust gave way. A -system of approximately parallel fissures opened in it, having a general direction -towards north-west. The rapid and simultaneous production of such a -gigantic series of rents must have given rise to earthquakes of enormous -magnitude and destructive force. The great majority of the fractures, -doubtless, did not reach to the surface of the ground, though probably not a -few did so. Such was the potency of this development of terrestrial energy, -that the fissures ran through the most varied kinds of rocks and the most -complicated geological structures, crossing even earlier lines of powerful -dislocation, and yet retaining their direction and parallelism for sometimes -50 or 100 miles.</p> - -<p>Into the fissures thus formed the molten magma from underneath was -forced for many hundreds or even thousands of feet above the surface of the -subterranean lava-reservoir. Solidifying between the fissure walls, it formed -the crowd of basic dykes that stand out as the most widespread and -distinctive feature of the volcanic region.</p> - -<p>Where the fissures reached the surface or near to it, the molten rock -would seek relief by egress in streams of lava. This probably occurred in -many places from which subsequent denudation has removed all vestige of -superficial volcanic manifestations. But, in the great range of basalt-plateaux, -from Antrim northwards through the chain of the Inner Hebrides, there are -still left abundant remains of the surface-outflows. Like the modern lavas -of Iceland, the molten material probably flowed out sometimes from the -<span class="pagenum" id="Page_463">- 463 -</span> -open fissures, sometimes from vents formed along the chasms. After the -convulsions ceased which produced the earliest dykes, the communication -that had been established between the magma-reservoir underneath and the -air above would be maintained, and repeated eruptions might take place, -either from the original fissures and vents or from others afterwards opened -by the volcanic energy.</p> - -<p>As in the modern eruptions of Iceland, new fissures are successively -opened through the older lava-sheets, so in the Tertiary volcanic areas, -renewed ruptures of the earth's crust allowed later dykes to be formed. The -basalt-plateaux are traversed by such dykes, even up to their highest sheets. -It is impossible to say how often the process of dyke-making may have been -repeated. Not improbably it recurred again and again during the building -of the basalt-plateaux, and we know that it was renewed even after the -protrusion of the granophyre bosses which mark one of the latest phases of -volcanism in the region.</p> - -<p>For a protracted geological period, with long intervals of quiescence, -various basic lavas (basalts, dolerites, etc.), with occasionally some of intermediate -composition (andesites, trachytes), and perhaps in Antrim acid -rhyolites, flowed out from fissures and vents until they had filled up the -hollows of the great valley, which then stretched from the south of Antrim -northwards between the west coast of Scotland and the chain of the Outer -Hebrides. In some places the accumulated pile of these ejections even -now exceeds 3000 feet in thickness, but we cannot tell how much -material has been bared away from its top by denudation. The volcanic -discharges consisted mostly of lava, fragmentary materials being comparatively -insignificant in amount and local in origin, though layers of fine tuff and -basalt-breccias occur in all the plateaux. None of the erupted materials -thicken towards any centres that might be taken to mark volcanoes of the -type of Vesuvius or Etna. On the contrary, the persistent flatness and -uniformity of the volcanic series, and the thinning out of the separate beds -in different directions, show that the lavas issued from many points all over -the region. The positions of some of the actual vents can still be ascertained. -They are now filled sometimes with dolerite, sometimes with coarse -agglomerate.</p> - -<p>The surface over which the lava flowed seems to have been mainly -terrestrial. Here and there, between the successive sheets of basalt, -the leaves, stems, and fruit of land-plants, sometimes in most perfect -preservation, may be observed, together with the remains of insects and -fresh-water fish. Distinct relics of old river-channels can be recognized -which have been buried under streams of lava. Among the deposits left -by these streams the uppermost layers are commonly dark with decayed -vegetation, while layers of coal are found here and there between the -basalts.</p> - -<p>As the pile of erupted materials gradually thickened, and the subterranean -energy possibly grew feebler, the ascending magma was forced -between the layers of sedimentary strata underneath the basalts, or between -<span class="pagenum" id="Page_464">- 464 -</span> -these strata and the overlying volcanic series, or along any other plane of -weakness in the terrestrial crust. In this way arose the multitudinous -sills or intrusive sheets.</p> - -<p>When the great volcanic plateaux had been built up to a thickness of -several thousand feet, another remarkable episode in the history occurred. -At certain points large bodies of coarsely crystalline basic rocks were pushed -into and through the plateaux-basalts, upraising them in dome-shaped -elevations, and ultimately solidifying as dolerites, gabbros, troctolites, -picrites, etc. There is reason to believe that the points of extravasation -of these materials were mainly determined by the positions of -the larger or more closely clustered vents of the plateau-period, where -points of weakness consequently existed in the terrestrial crust. Rising as -huge bosses through such weak places, the gabbros and associated rocks -raised up the overlying bedded basalts, and forced themselves between them, -forming thus a fringe of finer-grained intrusive sills and veins around the -central banded and amorphous masses of more coarsely crystalline material. -Whether, in any of these vast domes of upheaval, the summit was disrupted, -so as to allow the basic intrusion to flow out as lava at the surface, cannot -now be told, owing to enormous subsequent denudation.</p> - -<p>The next chapter in the chronicle shows us that probably long after the -eruption of the gabbros, when possibly all outward symptom of volcanic -action had ceased, a renewed outbreak of subterranean activity gave rise to -the protrusion of another and wholly different class of materials. This -time the rocks were of a markedly acid type. They included varieties that -range from obsidians, pitchstones, flinty felsites and rhyolites, through -porphyries and granophyres, into compounds which cannot be classed under -any other name than granite. These masses likewise availed themselves of -older vents in the plateaux, and broke through them. They now form huge -conical hills, which, in their outer aspect, and even to some extent in their -inner structure, recall the trachytic puys of Auvergne. But the granophyres -not only ascended through the basalt-plateaux and the gabbro-bosses; they -sent into these rocks a network of veins, pushed their way in huge sheets -or sills between the strata below, and actually incorporated a considerable -proportion of the basic materials into their own substance. Around the -bosses of gabbro and granophyre, the bedded basalts have undergone considerable -contact-metamorphism.</p> - -<p>The gabbro and granophyre bosses of the Inner Hebrides demonstrate -with singular force how unreliable petrographical characters are as a test -of the relative age of rocks. No one, looking at hand-specimens of these -rocks, or even studying them in the field, would at first suspect them to -be of Tertiary date. They closely resemble rocks of similar kinds in -Palæozoic and even Archæan formations. Yet, of their late appearance in -geological time, there cannot be any possibility of doubt.</p> - -<p>After the uprise of the granophyre, and the injection of the network of -felsitic veins, there came once more a period of terrestrial convulsion, like -that of the earliest basic dykes, but of less intensity. Again, the crust of -<span class="pagenum" id="Page_465">- 465 -</span> -the earth over the volcanic region was pushed upward and rent open by -another system of parallel fissures. Again, from a reservoir or basin of -basic lava underneath, molten rock was forced upwards into the rents, and -thus another system of basic dykes was formed. These dykes are found -crossing those of earlier date, and rising through the other volcanic rocks. -They traverse the plateau-basalts from bottom to top; they climb to the -summits of the gabbro mountains, and they even pursue their undeviating -course over the huge domes of granophyre. No proof has yet been found -that from any of these dykes there was a superficial outflow of lava. But -so great has been the subsequent denudation of the areas, that such outflows -might quite well have taken place, and have subsequently been destroyed.</p> - -<p>Whether these basic dykes were the last manifestation of volcanic energy -in our region cannot yet be decidedly affirmed. So far as the evidence -at present goes, they are possibly older than another series of acid veins and -dykes (pitchstone, felsite, and granophyre), which are found at many points -from Antrim to the far end of the Inner Hebrides. These protrusions -traverse every other member of the volcanic series, except some of the -youngest basic dykes, and do not appear to be themselves cut by any.</p> - -<p>Since the close of the volcanic period considerable disturbance of the -basalt-plateaux has taken place. The whole volcanic region has subsided, -some districts having sunk more than others. In Britain the most striking -evidence of such depression is supplied by the basin of Lough Neagh. But -throughout the Inner Hebrides much of the lower portion of the terrestrial -lava-plateaux is now below sea-level. In the Faroe Islands and in Iceland -the subsidence has been still more marked. Dislocations, also, sometimes -amounting to more than a thousand feet of displacement, have occurred -among the volcanic masses. The bedded basalts, originally on the whole -nearly flat, have thus been broken up into large blocks of country wherein -the sheets are now inclined in various directions.</p> - -<p>One of the most important lessons taught by the Tertiary volcanic series -of the north-west of Europe is the extent of the denudation of the land since -the close of the volcanic period. The horizontal or gently inclined layers of -bedding among the basalts afford datum-lines from which the minimum -amount of material removed may be measured. As a reasonable estimate -it may be inferred that in the case of the Mull plateau, for example, the -average amount by which its surface has been lowered since the close of the -volcanic period cannot be less than 2000 feet. If the rate of lowering of -the land-surface in western Europe by subærial denudation be taken as -1/6000 of a foot in a year, then the lapse of time required for the degradation -of the Mull plateau must amount to about twelve millions of years. Some -such interval has therefore elapsed since the last Tertiary volcanoes became -extinct.</p> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_466">- 466 -</span></p> - -<h2 class="nobreak" id="CHAPTER_LI">CHAPTER LI<br /> - -<span class="smaller">SUMMARY AND GENERAL DEDUCTIONS</span></h2> -</div> - - -<p>The foregoing chapters comprise a connected narrative of the history of -volcanic action in the area of the British Isles during the vast succession of -ages from the early Archæan dawn down to the latest eruptions of Tertiary -time. In this final chapter I propose to present a brief summary of the -facts of largest import and widest interest which this protracted history has -placed before us, together with a statement of deductions which may be -drawn from them regarding the nature and progress of volcanism in the -evolution of the globe.</p> - -<p>1. Among the broad features which soonest arrest attention in such a -survey is the geographical position of the theatre of this volcanic activity. -In the distribution of volcanoes at the present time we are familiar with -their tendency to range themselves along continental borders or in oceanic -islands. The volcanic energy so conspicuous in the geological history of -Britain has shown itself along the western or Atlantic margin of the -European continent. When the eruptions have not been actually on the -land itself, they have taken place within the shallow tracts near the land, -where the lavas and tuffs have been interstratified with sediments derived -from the adjacent coasts.</p> - -<p>Moreover the volcanic rocks in Britain are ranged along the greatest -length of the group of islands, in a general north and south line, from the -south of Devonshire to the far Shetlands. It is on the western side of the -country that they occur. East of a line drawn from Berwick by Leicester -to Exeter, although the geological formations, ranging from the Carboniferous -Limestone to the latest Pleistocene deposits, are there abundantly exposed -to view, they include no contemporaneous volcanic rocks.</p> - -<p>2. A second and still more remarkable feature in the geological history -of Western Europe is the persistence of volcanic activity along the site of -the British Isles. Evidence has been brought forward in these volumes -that from the primeval time vaguely termed Archæan, onward to that of the -older Tertiary clays and sands of the south-east of England—that is to say, -through by far the largest part of geological history, as chronicled in the -stratified crust of the globe—this long strip of territory continued to be -intermittently a theatre of volcanic action. Every great division of Palæozoic -<span class="pagenum" id="Page_467">- 467 -</span> -time was marked by volcanic eruptions, sometimes over tracts hundreds of -square miles in area and on a colossal scale. After a long period of -quiescence during the Mesozoic ages, the renewed outbreak of volcanic energy -in older Tertiary time, so marked over the western half of Europe, reached -its maximum of development along the Atlantic border, from the north of -England and Ireland through the chain of the Inner Hebrides to the Faroe -Islands, Iceland and Greenland.</p> - -<p>3. Not only has there been a remarkable persistence of volcanic activity -over the comparatively limited area of the British Isles, viewed as a whole, -but if we examine the different parts of this area we perceive that many of -them, of relatively restricted extent, have been the sites of a recrudescence -of volcanic action, again and again, through a vast succession of geological -periods. While the whole region has been in different quarters and at -different times affected, there have been districts where the volcanic fires -have been rekindled after long intervals of quiescence, the new vents being -opened among or near to the sites of earlier volcanoes. In the south-west -of England, for example, the Middle Devonian tuffs and diabases were -succeeded in the Carboniferous period by the eruptions of the Culm-measures, -and in the very same tracts came last of all the lavas and tuffs of the -Permian conglomerates. Still more astonishing is the record of volcanic -energy in the south of Scotland, where, within a space of not many hundred -square miles, there are the chronicles of the Arenig, Llandeilo and Bala -eruptions of the Southern Uplands, the huge piles of lavas and tuffs of -the Lower Old Red Sandstone, the long succession of the plateaux and -then of the puys of the Carboniferous period, the groups of tuff-cones of the -Permian period, and, lastly, the numerous dykes connected with the Tertiary -volcanoes.</p> - -<p>While some portions of the region have been specially liable to exhibitions -of volcanic action, others have continuously escaped. Some of these "horsts," -or stationary and unaffected blocks of country, have been surrounded by or -have risen close to the borders of this volcanic district, yet have maintained -their immunity through a long series of ages. Thus the Central Highlands -of Scotland, though they were flanked on the south and south-west by the -active volcanoes of the Old Red Sandstone, and again on the south by those of -Carboniferous time, had no vents opened on their surface after the metamorphism -of their schists. Still more striking perhaps is the immunity of -the Southern Uplands. Though they were in large measure surrounded by -the volcanoes of the Lower Old Red Sandstone, then by those of the -Calciferous Sandstones and Carboniferous Limestone, and though they looked -down on the Permian eruptions of Ayrshire and Nithsdale, which spread -streams of lava and showers of ash along their flanks, these hills formed a -solid block that seems to have resisted perforation by the volcanic funnels. -Again, the tracts covered with Carboniferous Limestone in England and -Ireland almost entirely escaped from invasion by volcanic eruptions.</p> - -<p>We thus learn that even within comparatively restricted regions some -portions of the terrestrial crust have been areas of weakness, liable to serve -<span class="pagenum" id="Page_468">- 468 -</span> -again and again as lines of escape for volcanic energy, while close to them -other portions of greater solidity have been persistently left intact.</p> - -<p>4. The sites of volcanic vents in all the geological systems wherein they -occur in Britain have not usually been determined by any obvious structure -in the rocks now visible. They comparatively seldom depend on ascertainable -lines of fault, even when faults, probably already existent, occur in their -near neighbourhood. This independence, to which, however, there are -occasional marked exceptions, comes out more particularly in the coal-fields -pierced by vents, for mining operations have there revealed the positions of -many more faults than can be traced at the surface. If the sites of the -vents have been fixed by dislocations or lines of weakness in the terrestrial -crust, these must generally lie below the formations now visible at the surface.</p> - -<p>There is one striking connection between the sites of the vents and -ancient topographical features to which frequent reference has been made in -the foregoing chapters. All through the long volcanic history, as far back -as such features can be traced, we see that orifices of discharge for the -erupted materials have been opened along low grounds and valleys rather -than on ridges and hills. The great central hollow of the Scottish midlands -was a depression even as long ago as the time of the Lower Old Red Sandstone, -and though it has probably been several times since then filled up, -and more or less completely effaced, its ancient features have been partially -revealed by extensive denudation. This vast depression, 40 miles broad, -between the Highland mountains on the one side and the Southern Uplands -on the other, was the chief centre of volcanic activity in western Europe -during the latter half of Palæozoic time. The vents of the Old Red Sandstone, -Carboniferous and Permian series are scattered all over it, but few or -none of them are to be found on the high grounds that bound it. Again, -in Tertiary time, the great outpouring of lava took place in the hollow that -lay between the ridge of the Outer Hebrides and the mainland of Scotland. -This wide and long tract of low ground was buried under upwards of 3000 -feet of lava and tuff, but these materials were erupted from fissures and vents -within its own border and not from the mountains on either side.</p> - -<p>But perhaps the most conspicuous example of any in which the vents keep -to the valleys is that supplied by the Permian necks of Nithsdale and the -neighbouring glens. These depressions are as old as Permian, and even as -Carboniferous time, but they appear to be entirely hollows of erosion; at -least they have yielded no evidence that their direction has been determined -by lines of fault. The chain of vents can be followed from the lowlands of -Ayrshire up to the base of the Southern Uplands, down the wide valley cut -by the Nith in these hills and up some of the tributary valleys, and though -the volcanoes continued for some time in vigorous eruption, not a trace of -any contemporary vent has yet been met with on the surrounding hills.</p> - -<p>While the position of volcanic vents in lines of valley may be generally -due to guiding lines of fissure in the crust underneath, either within or below -the rocks visible at the surface, there may sometimes be conditions in which -other dominant causes come into play. The curious coincidence between -<span class="pagenum" id="Page_469">- 469 -</span> -variations in the upper limit of dykes and inequalities in the configuration -of the overlying ground, suggest that where the subterranean magma has -ascended to within a comparatively short distance from the surface, a -difference of a few hundreds or thousands of feet in the depth of overlying -rock, such as the difference of height between the bottom of a valley and the -tops of the adjacent hills, may determine the path of escape for the magma -through the least thickness of overarching roof.</p> - -<p>5. Volcanic phenomena cannot be regarded as a mere isolated and -incidental feature in the physics of the globe. During the short time within -which man has been observing the operations of existing volcanoes, he has -hardly yet had sufficient opportunity of watching how far they can be -correlated with other terrestrial movements. Nor, when he endeavours to -trace some such connection among the records of the geological past, has he -yet collected materials enough to furnish a sufficiently broad and firm basis -of comparison. One formidable obstacle is presented by the difficulty in -determining chronological equivalents in separated groups of rock. Geologists -have tried to discover whether the volcanoes of some particular period or -region were in any way connected with such geological changes as extensive -plication, dislocations of the crust, or elevation of mountain-chains. In -regard to the volcanic history of Britain, various possible relations of this -kind obviously suggest themselves. Thus the division of geological time -comprised within the Lower Silurian period was undoubtedly an interval of -considerable terrestrial disturbance in western Europe. The unconformabilities -and overlaps in the series of formations belonging to that period, the -frequent conglomerates, the great and often rapid changes in the thickness -and lithological characters of the strata, all point to instability of land-surface -and sea-floor. During these oscillations a prolonged and widespread series -of volcanic eruptions took place. The volcanic manifestations began in -Cambrian time and continued in intermittent activity till towards the close -of the deposition of the Lower Silurian formations. It is certainly a -significant fact that the Upper Silurian deposits, in their lithological -characters, present a strong contrast to those that preceded them. They -point, on the whole, to quiet sedimentation, during an interval of comparative -calm in the terrestrial crust. With this evidence of tranquillity there is, -over almost the whole of the British Isles, an entire absence of any trace of -renewed volcanic activity. With the exception of the Dingle lavas and tuffs, -in the extreme west of Ireland, not a single undoubted instance is yet known -of an Upper Silurian volcano.</p> - -<p>After the deposition of the Upper Silurian rocks an interval of great -terrestrial disturbance ensued, and these rocks over a large part of Britain -were intensely plicated and crushed. The movements, continued into the -period of the Lower Old Red Sandstone, were, in their later stages, accompanied -or, at least, followed by the vast outpourings of lava which now cover -so much of the tracts of Old Red Sandstone in Scotland and Ireland.<a id="FNanchor_438" href="#Footnote_438" class="fnanchor">[438]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_438" href="#FNanchor_438" class="label">[438]</a> <i>Trans. Geol. Soc. Edin.</i> vol. ii. part iii. (1874).</p> - -</div> - -<p>In proportion as the volcanic energy was vigorous, widespread and long-continued, -<span class="pagenum" id="Page_470">- 470 -</span> -we may expect it to have been connected with important -terrestrial movements affecting extensive regions of the earth. The Tertiary -volcanic history seems to afford a remarkable instance of this connection. A -wide area of the European continent is dotted over with old centres of volcanic -activity which were in eruption at successive epochs throughout the Tertiary -period. Of all these centres the most important was that of the north-western -basalt-plateaux, where floods of lava were discharged over many -thousand square miles from Ireland to Greenland. The geological date of -these outpourings probably coincides with the last great orographic movements -that gave to the mountain-chains of Europe their latest elevation and -dimensions.</p> - -<p>But without entering into what must be for the present a field of -speculation, we can be assured of one important fact in the connection of -ancient volcanoes with movements of the terrestrial crust. A study of the -records of volcanic action in Britain proves beyond dispute that the volcanoes -of past time have been active on areas of the earth's surface that were -sinking and not rising. We usually associate volcanic action with elevation -rather than subsidence, and there are certainly abundant proofs of such -elevation around active or recently extinct volcanoes. Many of the active -vents of the present time, like Vesuvius and Etna, began with submarine -eruptions and have been gradually upraised into land. It may be, however, -that such uprise is merely a temporary incident, and that if we could survey -the whole geological period of which human history chronicles so small a -part, we might find that subsidence, and not upheaval, is ultimately the rule -over volcanic areas.</p> - -<p>Be this as it may, there can be no question that with the one solitary -exception of the Tertiary volcanoes, which were terrestrial and not submarine, -all the British vents were carried down and eventually buried under aqueous -sediments. Even the Tertiary lava-fields have in many places sunk down -below sea-level since their eruptions ceased.</p> - -<p>That there are any Palæozoic volcanic rocks now visible at the surface is -obviously due to subsequent movements not immediately connected with -their original conditions of eruption, and to gigantic denudation. The -amount of subsidence which followed on a volcanic episode was sometimes -enormous, even within the same geological period, as one may see by observing -the prodigious piles of sedimentary material heaped over the lavas and tuffs -of Arenig time, or over those of the Lower Old Red Sandstone. I do not wish -to maintain that the downward movement was necessarily a consequence of -volcanic ejections, for we know that it took place over tracts remote from -centres of eruption. But I have sometimes asked myself whether it was not -possibly increased as a sequel to vigorous volcanic action; whether, for -instance, the great depth of the Palæozoic sedimentary rocks in some regions, -as compared with their feeble development in others, may not have been due -to an acceleration of subsidence consequent upon volcanic action.</p> - -<p>6. A review of the geological history of Britain cannot but impress the -geologist with a conviction of the essential uniformity of volcanism in its -<span class="pagenum" id="Page_471">- 471 -</span> -manifestations since the early beginnings of geological time. The composition -and structure of the materials erupted from the interior have remained with -but little change. The manner in which these materials have been discharged -has likewise persisted from the remotest periods. The three modern -types of Vesuvian cones, puys and fissure-eruptions can be seen to have -played their parts in the past as they do to-day.</p> - -<p>Among the earliest igneous masses of which the relative geological date -can be fixed are the dykes which form so striking a system among the -Archæan rocks of the north-west, and show how far back the modern type of -volcanic fissures and dykes can be traced. No relic, indeed, has survived of -any lavas that may have flowed out from these ancient fissures, but so far as -regards underground structure, the type is essentially the same as that of the -Tertiary and modern Icelandic lava-fields.</p> - -<p>The early Palæozoic volcanoes formed cones of lava and tuff comparable -to those of such vents as Vesuvius and Etna. In the Lake District the -pile of material ejected during Lower Silurian time was at least 8000 or -9000 feet thick. In the Old Red Sandstone basins of Central Scotland -there were more than one mass of lavas and tuffs thicker than those of -Vesuvius.</p> - -<p>The puys of the later half of Palæozoic time closely resembled their -Tertiary successors in Central France, the Eifel, and the Phlegræan Fields.</p> - -<p>Nor, as regards extent and vigour, did the eruptions of the geological -past differ in any important respect from those of the present time. There -is assuredly no evidence that volcanic energy has gradually waned since the -dawn of geological history. The latest eruptions of North-Western Europe, -forming the Tertiary basalt-plateaux, far exceeded in area, and possibly also -in bulk of material discharged, all the eruptions that had preceded them in -the geological record.</p> - -<p>7. Nevertheless, while the Tertiary eruptions showed no diminution of -vigour, it is undoubtedly true that the volcanic energy has not manifested -itself in a uniform way since the beginning of geological time. There have -been periods of maximum activity followed by others of lessened force. -Thus if we take a broad view of the general features of volcanic action -during the Palæozoic ages in Britain, we see clear evidence of a gradual -diminution in its vigour. The widespread outpourings of lava and tuff in -the Silurian period in England, Wales, Scotland and Ireland were succeeded -by the somewhat diminished, though still important, eruptions of the Lower -Old Red Sandstone basins. The latter were followed by the still lessened -outflows of the Carboniferous plateaux, which in turn were succeeded by -the yet feebler and more localized eruptions of the Carboniferous puys, the -whole prolonged volcanic succession ending in the small scattered vents of -the Permian period. There were of course oscillations of relative energy -during this history, some of the maxima and minima being of considerable -moment. But though progress towards extinction was not regular and -uniform, it was a dominant feature of the phenomena.</p> - -<p>8. The Permian volcanoes were the last of the long Palæozoic series, -<span class="pagenum" id="Page_472">- 472 -</span> -and, so far as we yet know, the whole of the Mesozoic periods within the area -of Britain were absolutely unbroken by a single volcanic eruption. The -chronological value of this enormous interval of quiescence may, perhaps, -never be ascertainable, but the interval must assuredly cover a large part -of geological time. It was an era of geological calm, during which the -Triassic, Jurassic and Cretaceous formations were slowly accumulated over -the larger part of Europe. The stratigraphical quietude was not indeed -unbroken. The widespread subsidence of the sea-bottom was interrupted -here and there by important upheavals, and considerable geographical -changes were in process of time accomplished. But, save in one or two -widely separated areas of Europe, there were no active volcanoes over the -whole continent.<a id="FNanchor_439" href="#Footnote_439" class="fnanchor">[439]</a> Here again the scarcity or absence of intercalated -volcanic rocks is in harmony with the general stratigraphy of the -formations.</p> - -<div class="footnote"> - -<p><a id="Footnote_439" href="#FNanchor_439" class="label">[439]</a> The Triassic eruptions of Predazzo and Monzoni were important, and traces of others are -said to occur in the Cretaceous system in Portugal and Silesia.</p> - -</div> - -<p>9. After the prodigious interval represented by the whole of the -Mesozoic and the earlier part of the Tertiary formations, a time of disturbance -arose once more, and the great basalt-floods of the north-west -were poured forth. Evidence has been adduced in the foregoing chapters -that this latest volcanic period was one of vast duration; that it was -marked by long intervals of quiescence, and by repeated renewals of -volcanic energy. Yet over the area of Britain the whole of its manifestations -were probably comprised within the earlier (Oligocene and perhaps -early Miocene) part of older Tertiary time. Since its eruptions ceased, -another interval of profound quiescence has succeeded, which still continues. -But this interval is almost certainly of less duration than that -which elapsed between the Palæozoic and Tertiary outbursts. In other -words, remote as the date of these Tertiary volcanoes appears to be from -our own day, it comes much nearer to us than did the era of the last -Permian eruptions to the earliest of the Tertiary series.</p> - -<p>10. By the dissection which prolonged denudation has effected among -the old volcanic centres of Britain, materials are supplied for studying the -sequence of events from the beginning to the end of a volcanic period. -These events have generally followed the same tolerably well-defined -order.</p> - -<p>In the case of fissure-eruptions, rents formed in the crust of the earth -and communicating with the surface have allowed lava to rise and flow out -above ground, either from the lips of the fissures or from vents opened along -the lines of chasm. The thousands of parallel dykes in Britain remain as -evidence of this mode of the ascent of the molten magma. Lines of large -cones of the Vesuvian type may be presumed to have risen along guiding -fissures in the terrestrial crust.</p> - -<p>But it is evident from a study of the British examples that the existence -of a fissure in the visible part of the crust is not always necessary for -the production of a volcanic vent. In hundreds of instances, communication -<span class="pagenum" id="Page_473">- 473 -</span> -from the internal magma to the surface was effected by successive explosions, -which finally blew out an orifice at the surface with no visible relation -to any fissures or dykes. Of course, beneath the formations that now form -the surface, and through which the necks rise, there may be lines of fault -or weakness in older rocks which we cannot see. But, in what can be -actually examined, vents have commonly been drilled through rocks independently -of faults.</p> - -<p>The discharge of explosive vapours was sometimes the first and only -effort of volcanic energy. Generally, however, fragmentary volcanic -materials were ejected, or, if the eruption was more vigorous, lava was -poured out. In a vast number of cases, especially in the later ages of -Palæozoic time, only ashes were projected, and cones of tuff were formed. -In the earlier ages, on the other hand, there was a much larger proportion -of lava expelled. Towards the close of a volcanic period, the vents were -gradually choked up with the fragmentary materials that were ejected from -and fell back into them. Occasionally, during the process of extinction, -an explosion might still occur and clear the chimney, so as to allow of the -uprise of a column of molten rock which solidified there; or the sides of -the crater, as well as of the cavernous funnel underneath, fell in and filled -up the passage. Heated vapours sometimes continued to ascend through -the debris in the vent, and to produce on it a marked metamorphism.</p> - -<p>There seems to have been commonly a contraction and subsidence of -the materials in the vents, with a consequent dragging down or sagging of -the rocks immediately outside, which are thus made to plunge steeply -towards the necks.</p> - -<p>When the vents were plugged up by the consolidation of fragmentary -matter or the uprise of lava in them, the final efforts of the volcanoes led -to the intrusion of sills and dykes, not only into the rocks beneath the -volcanic sheets, but also, in many instances, into at least the older parts -of the sheets themselves. These subterranean manifestations of volcanic -action may be recognized in almost every district. They vary greatly in -the degree to which they are developed. Sometimes, as in the Cader -Idris, Arenig and Snowdon regions, they attain considerable importance, -alike as regards the number and thickness of the sheets. In other cases, -they are exhibited on so small a scale that they might be overlooked, as in -the tract of Carboniferous puy-eruptions in the north of Ayrshire. But -they are so generally present as to form a remarkably characteristic feature -of the volcanic activity of each geological period from the earliest time to -the latest. The basic sheets in the Dalradian series of Scotland display -early and colossal examples. All through the successive eruptive periods -of Palæozoic time, sills are found as accompaniments of superficial ejections.</p> - -<p>The Tertiary basalt-plateaux supply numerous and gigantic examples of -intruded sheets. Tertiary cones of Vesuvian type are not found in Britain, but -where on the continent they have been sufficiently laid open by denudation, -they present sometimes an astonishing series of sills. As a striking illustration -of this structure reference may be made to the sheets of trachyte that -<span class="pagenum" id="Page_474">- 474 -</span> -have been injected between and have marmorized the Cretaceous strata on -which Monte Venda stands, among the Euganean Hills.<a id="FNanchor_440" href="#Footnote_440" class="fnanchor">[440]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_440" href="#FNanchor_440" class="label">[440]</a> G. vom Rath, <i>Zeitsch. Deutsch. Geol. Gesellsch.</i>, xvi. (1864), p. 461. E. Suess, <i>Sitzungsber. -k. Akad. Wien.</i>, lxxi. (1875), p. 7; <i>Antlitz der Erde</i>, vol. i. p. 193. E. Reyer, <i>Die Euganeen</i>, -1877. This volcano is further referred to, <i>postea</i>, p. 477.</p> - -</div> - -<p>It is obvious that the time of intrusion of the sills cannot be precisely -determined. They were not likely to be injected at an epoch when the volcanic -magma could find ready egress to the surface. That they did not arise -before such egress was obtained may be inferred from their petrographical -characters, which are usually those of the later and not of the earlier outflows -of the magma; and from the fact that they not only lie among the -rocks below the volcanic series, but intersect the lower parts of that -series, sometimes even the higher parts. We may therefore, with every -probability, regard the sills as among the closing phases of a volcanic -period.</p> - -<p>As the lavas and tuffs of each volcanic period are intercalated among -the successive geological formations, a definite beginning and end to the -period are stratigraphically fixed. We see exactly where in the sedimentary -series the first showers of ashes fell, and where the last mingled -with the ordinary sand and mud of the sea-door. The same record shows -that the volcanic accumulations were finally washed down, that they subsided -with the rest of the ground around them, and that usually they were -buried under overlying conformable sedimentary deposits. Thus cones of -ashes and lava which may have been several thousand feet high completely -disappeared.</p> - -<p>10. A consideration of the distribution of the volcanic rocks in time -shows not only how singularly uniform the course of volcanic activity has -been, but that there is no evidence of the cessation of any of the broader -petrographical types during geological history. Quite as much variety may -be observed among the erupted materials of Tertiary time in Britain as -among those of the early ages, when the earth was younger and its volcanic -vigour might be supposed to have been greater and more varied than it is -now. The table on the following page will make these features at once -apparent. From this table it will be seen that while some of the acid -rocks have not always been extruded, the basic masses have played their -part in every volcanic period.</p> - -<p>11. A study of the volcanic products of a long series of eruptions -within the same geographical region may be expected to throw light on the -changes that take place during the course of ages in the character of the -internal molten magma. In a former chapter (<a href="../../66492/66492-h/66492-h.htm#Page_27">vol. i. p. 27</a>) reference was -made to the subject of volcanic cycles and to the sequence, observed in -various widely separated parts of the world, among the materials erupted -from below. Allusion was likewise made in a later chapter (<a href="../../66492/66492-h/66492-h.htm#Page_90">vol. i. p. 90</a>) to the -remarkable differences in texture and composition noticeable within some -large bodies of eruptive material, and to the evidence which these differences -furnish of a segregation or differentiation among the constituents of -<span class="pagenum" id="Page_475">- 475 -</span> -an eruptive mass after it has been injected into its position within the -crust of the earth.</p> - -<p class="caption3nb"><span class="smcap">Table of the Periods of Volcanic Action in the British Isles and<br /> -of the Chronological Distribution of the Volcanic Products.</span></p> - -<table summary="data"> -<tr> - <td class="bdl bdt bdb bdr" colspan="2"></td> - <td class="tdc bdl bdt bdb">Granites,<br />Granophyres,<br />etc.</td> - <td class="tdc bdl bdt bdb">Felsites,<br />Rhyolites,<br />etc.</td> - <td class="tdc bdl bdt bdb">Dacite,<br />"Pitchstone"<br />of Eigg.</td> - <td class="tdc bdl bdt bdb">Trachytes.</td> - <td class="tdc bdl bdt bdb">Andesites<br />(Porphyrites).</td> - <td class="tdc bdl bdt bdb">Gabbros.</td> - <td class="tdc bdl bdt bdb">Dolerites,<br />Basalts<br />(Diabases).</td> - <td class="tdc bdl bdt bdb">Picrites and<br />highly basic<br />lavas.</td> - <td class="tdc bdl bdt bdb bdr">Tuffs,<br />acid or<br />basic.</td> -</tr> -<tr> - <td class="bdl"> </td> - <td></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl bdr"></td> -</tr> -<tr> - <td class="bdl"></td> - <td class="tdl"><span class="smcap">Older Tertiary</span></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl bdr"></td> -</tr> -<tr> - <td class="bdl"></td> - <td class="tdl"> (Plateaux, dykes,<br /> - necks, bosses,<br /> sills)</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl bdr">*</td> -</tr> -<tr> - <td class="bdl"> </td> - <td></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl bdr"></td> -</tr> -<tr> - <td class="bdl"></td> - <td class="tdl"><span class="smcap">Mesozoic</span></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl bdr"></td> -</tr> -<tr> - <td class="bdl"></td> - <td class="tdl"> No volcanic rocks.</td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl bdr"></td> -</tr> -<tr> - <td class="bdl"> </td> - <td></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl bdr"></td> -</tr> -<tr> - <td class="bdl"></td> - <td class="tdl"><span class="smcap">Permian</span></td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl bdr">*</td> -</tr> -<tr> - <td class="bdl"> </td> - <td></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl bdr"></td> -</tr> -<tr> - <td class="bdl"></td> - <td class="tdl"><span class="smcap">Carboniferous</span></td> - <td class="tdc bdl">?</td> - <td class="tdc bdl"></td> - <td class="tdc bdl"></td> - <td class="tdc bdl"></td> - <td class="tdc bdl"></td> - <td class="tdc bdl"></td> - <td class="tdc bdl"></td> - <td class="tdc bdl"></td> - <td class="tdc bdl bdr"></td> -</tr> -<tr> - <td class="bdl"></td> - <td class="tdl"> Puy type</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl bdr">*</td> -</tr> -<tr> - <td class="bdl"></td> - <td class="tdl">Plateau type</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl bdr">*</td> -</tr> -<tr> - <td class="bdl"> </td> - <td></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl bdr"></td> -</tr> -<tr> - <td class="bdl" rowspan="5">{</td> - <td class="tdl"><span class="smcap">Devonian</span></td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl bdr">*</td> -</tr> -<tr> - <td></td> - <td class="bdl"> </td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl bdr"></td> -</tr> -<tr> - <td class="tdl"><span class="smcap">Old Red<br /> Sandstone</span></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl bdr"></td> -</tr> -<tr> - <td class="tdl"> Upper</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl bdr">*</td> -</tr> -<tr> - <td class="tdl"> Lower</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl bdr">*</td> -</tr> -<tr> - <td class="bdl"> </td> - <td></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl bdr"></td> -</tr> -<tr> - <td class="bdl"></td> - <td class="tdl"><span class="smcap">Silurian</span></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl bdr"></td> -</tr> -<tr> - <td class="bdl"></td> - <td class="tdl"> Upper</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl bdr">*</td> -</tr> -<tr> - <td class="bdl"></td> - <td class="tdl"> Lower, Bala</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl bdr">*</td> -</tr> -<tr> - <td class="bdl"></td> - <td class="tdl"> " Arenig</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl bdr">*</td> -</tr> -<tr> - <td class="bdl"> </td> - <td></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl bdr"></td> -</tr> -<tr> - <td class="bdl"></td> - <td class="tdl"><span class="smcap">Cambrian</span></td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl bdr">*</td> -</tr> -<tr> - <td class="bdl"> </td> - <td></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl bdr"></td> -</tr> -<tr> - <td class="bdl"></td> - <td class="tdl"><span class="smcap">Uriconian</span></td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl bdr">*</td> -</tr> -<tr> - <td class="bdl"> </td> - <td></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl bdr"></td> -</tr> -<tr> - <td class="bdl"></td> - <td class="tdl"><span class="smcap">Dalradian</span></td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl bdr">?</td> -</tr> -<tr> - <td class="bdl"> </td> - <td></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl bdr"></td> -</tr> -<tr> - <td class="bdl"></td> - <td class="tdl"><span class="smcap">Torridonian</span></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl bdr"></td> -</tr> -<tr> - <td class="bdl"> </td> - <td></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl bdr"></td> -</tr> -<tr> - <td class="bdl"></td> - <td class="tdl"><span class="smcap">Lewisian</span></td> - <td class="tdc bdl">*</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl">*</td> - <td class="tdc bdl bdr">...</td> -</tr> -<tr> - <td class="bdb bdl"> </td> - <td class="bdb"></td> - <td class="bdb bdl"></td> - <td class="bdb bdl"></td> - <td class="bdb bdl"></td> - <td class="bdb bdl"></td> - <td class="bdb bdl"></td> - <td class="bdb bdl"></td> - <td class="bdb bdl"></td> - <td class="bdb bdl"></td> - <td class="bdb bdl bdr"></td> -</tr> -</table> - -<p>From the history of volcanic action in the British Isles it is clear that -differentiation is effected under three distinct conditions.</p> - -<p>In the first place, a notable difference may be occasionally observed -between two adjacent parts of the same mass of lava which has flowed out -at the surface. Thus, in the Carboniferous picrite of Blackburn, there has -been a separation of the heavy basic constituents, which have in great part -settled down into the lower part of the sheet, while the lighter felspar has -mainly come to the top. In this case the gradual transition from top to -bottom suggests that the separation occurred after the lava had reached -the surface and taken the form of a stream or sheet.</p> - -<p>In the second place, segregation has taken place in the magma within the -terrestrial crust after intrusion, for it is frequently observable in large bosses -<span class="pagenum" id="Page_476">- 476 -</span> -and sometimes in sills, the basic elements having tended to mass themselves -towards the margins of the rock, leaving more acid material in the centre. -The cases of Garabol Hill among the Dalradian schists of Scotland, of -Carrock Fell among the Silurian strata of the Lake District, and of the -Cramond picrite among the Carboniferous formations of Midlothian, with -others that might be cited from various other regions and geological formations -in Britain, prove to what a considerable extent a separation of ingredients -may take place in a boss, and even sometimes in a comparatively -thin sill before the molten mass consolidates.</p> - -<p>In the third place, there is good evidence that already before the -magma is either intruded or extruded, and while it still lies within the -internal reservoir, it may not possess a general uniformity of composition, -but may have become more or less heterogeneous. In regard to intrusive -rocks, the extraordinarily banded gabbros of the Tertiary series of Skye -obviously proceeded from a magma in which the molten material consisted -in some parts mainly of felspar, and in others mainly of the ferro-magnesian -minerals and iron-ores. Streams from these differently constituted parts -of the magma were simultaneously or successively injected as sills into the -older portions of the volcanic series, while, as the process of differentiation -within the magma proceeded, still more felspathic liquid was left behind, -to be thrust into cracks in the sills previously consolidated.</p> - -<p>Moreover, the banded basalts of the Tertiary plateaux show that this -heterogeneity was not confined to internal intrusions, but maintained its -place even when the molten material was ejected to the surface. The -differentiation indeed is not so striking there as among the sills of gabbro; -but its presence, even in a less degree, proves that the separation of constituent -minerals was not due to any general cooling of an erupted body of igneous -rock, but was already developed in the reservoir from which the molten -material was propelled to the surface.</p> - -<p>Attention has been called to the remarkable similarity of structure -between these banded intrusive rocks and some of the ancient gneisses. The -resemblance is so close that we may with every probability infer that the -gneisses acquired their characteristic banding as intrusive masses of igneous -rocks, discharged from heterogeneous magmas, like that which supplied the -gabbros of the Cuillin Hills. And as these gneisses belong to pre-Cambrian -formations, we are thus led to the interesting result that the tendency to -develop heterogeneity was already as characteristic of the magma-basins of -the earliest geological time as it has been of those of later periods.</p> - -<p>The evidence of differentiation presented by superficial lavas, and by -intrusive sills and bosses, acquires great interest when considered in connection -with the changes which are seen to have occurred in the character -of the materials erupted during the course of a definite volcanic period. An -attentive examination of the volcanic products of the various ages, so fully -recorded in the geological structure of the British Isles, shows that a recognizable -sequence in the nature of the materials erupted during a single -volcanic period can be traced from the earliest to the latest times, and -<span class="pagenum" id="Page_477">- 477 -</span> -that, in spite of occasional departures, the normal order remains broadly -uniform.</p> - -<p>With the important exception of the Snowdonian region and possibly -others, we find that the earlier eruptions of each period were generally most -basic, and that the later intrusions were most acid. Thus the diabase-lavas -and tuffs at the base of the Cambrian series of St. David's are pierced by -quartz-porphyry veins. The andesites of the Lower Old Red Sandstone -were succeeded by bosses, sills, and dykes of granite, felsite, and lamprophyre. -The eruptions of the Carboniferous plateaux began with extremely basic -lavas, and ended with trachytes, felsites, and quartz-porphyries. The basalts -of the great lava-fields of the Tertiary period are pierced by masses of -granophyre and even granite.</p> - -<p>There has evidently been, on the whole, a progressive diminution in the -quantity of bases and a corresponding increase in the proportion of acid in the -lavas erupted during the lapse of one volcanic period. This sequence is so -well marked and so common that it cannot be merely accidental. The acid -and basic rocks, occurring as they do at each volcanic centre in the same -relation to each other, are obviously parts of one connected series of -eruptions. We seem to see in this sequence an indication of what was -taking place within the subterranean magma. There was first an extensive -separation of the more basic constituents, such as the ferro-magnesian -minerals and ores, and the lavas which came off at that time were heavy -and basic basalts, and even picrites. The removal of these elements left the -magma more acid, and such rocks as andesites were poured out, until at last -the deeper intrusive sills, dykes and bosses became thoroughly acid rocks, -such as felsite, quartz-porphyry and granite, while if any superficial outflow -took place it was such a rock as dacite.</p> - -<p>In the case of the Tertiary volcanic series there is evidence that after -the acid protrusions a final uprise of basic material occurred. No satisfactory -proof of any similar return to basic eruptions has been detected among the -Palæozoic formations. But it is possible that some of the basic sills and -dykes, the precise age of which cannot be fixed, may really mark such a -reversion, even in the earlier volcanic periods.</p> - -<p>Some illustrative examples of volcanic cycles from other countries -were cited in Chapter iii. To these I may add another instance which -presents a close analogy to some of the phenomena characteristic of the -British examples of Palæozoic as well as of Tertiary age. Monte Venda in -the Euganean Hills, already alluded to (p. 474), may be cited as an interesting -specimen of an older Tertiary volcano, which has been so dissected by -denudation as to show not only the succession of its superficial discharges, -but the position and order of its subterranean intrusions. The volcanic -eruptions of this neighbourhood, judging from the area which they still cover -and the height they reach, may have piled up a mountain rivalling or -surpassing Etna in dimensions. In Monte Venda the lowest visible igneous -rocks are sills of oligoclase-trachyte that have been thrust between and have -highly altered Cretaceous (Tithonian) limestones. Other intrusive sheets -<span class="pagenum" id="Page_478">- 478 -</span> -of trachyte follow in the overlying Cretaceous strata (Neocomian and -<i>Scaglia</i>). It is not until the older Tertiary formations are reached that -undoubted tuffs and lavas occur, indicative of truly interstratified volcanic -materials. These formations, consisting of nummulitic limestones and other -strata together with fossiliferous tuffs, show that the volcano began as a -submarine vent. It discharged dark basic dolerites and tuffs. The highest -lava, however, crowning the summit of the mountain is a trachyte. There -appears to have been a rapid decrease of the bases in the magma, for the -later lavas were rhyolites, accompanied with rhyolitic tuffs of Oligocene age, -and followed in the end by the black vitreous trachyte of Monte Sieva.</p> - -<p>12. From the evidence detailed in these volumes, it appears that the -sequence from basic to acid discharges was on the whole characteristic of -each eruptive period. It is obvious, however, that as the protrusions of -successive periods took place within the same limited geographical area, the -internal magma during the interval between two such periods must in some -way have been renewed as regards its constitution, for when, after long -quiescence, eruptions began once more, basic lavas appeared first and were -eventually followed by acid kinds. This cycle of transformation is admirably -exhibited in Central Scotland, where the andesites of the Old Red Sandstone -with their felsite sills are followed by the limburgites, picrites and other highly -basic lavas at the bottom of the Carboniferous plateaux, succeeded in turn -by the andesites, trachytes and acid sills of that series. When the puy -eruptions ensued, the magma had once more become decidedly basic.</p> - -<p>That the true explanation of these alterations is of a complex order may -be inferred from the exceptions which occur to the general rule. I have -alluded to the Snowdon region, where the acid rhyolites are followed by more -basic andesites, and where the sills are also more basic than the superficial -lavas. In the Arenig and Cader Idris country the sills are likewise more -basic than the bedded lavas. Among the Carboniferous puys of the basin of -the Firth of Forth, the sills are not sensibly more acid than many of the -superficial basalts, and they even include such rocks as picrite. Possibly in -this last-named region we see an arrested sequence, the volcanic protrusions -having from some cause ceased before the general uprise of the more acid -magma.</p> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_479">- 479 -</span></p> - -<h2 class="nobreak" id="INDEX">INDEX</h2> -</div> - - -<div class="tdc"> -[<a href="#A">A</a>] [<a href="#B">B</a>] [<a href="#C">C</a>] [<a href="#D">D</a>] [<a href="#E">E</a>] [<a href="#F">F</a>] [<a href="#G">G</a>] -[<a href="#H">H</a>] [<a href="#I">I</a>] [<a href="#J">J</a>] [<a href="#K">K</a>] [<a href="#L">L</a>] [<a href="#M">M</a>] [<a href="#N">N</a>]<br /> -[<a href="#O">O</a>] [<a href="#P">P</a>] [<a href="#R">R</a>] [<a href="#S">S</a>] [<a href="#T">T</a>] [<a href="#U">U</a>] -[<a href="#V">V</a>] [<a href="#W">W</a>] [<a href="#Y">Y</a>] -</div> - -<p> -<a id="A"></a>AA form of lava in the Sandwich Islands, ii. <a href="#Page_187">187</a><br /> -Abereiddy Bay, i. <a href="../../66492/66492-h/66492-h.htm#Page_i">206</a><br /> -Abich, H., i. <a href="../../66492/66492-h/66492-h.htm#Page_32">32</a><br /> -Acid igneous rocks, silica percentage of, i. <a href="../../66492/66492-h/66492-h.htm#Page_14">14</a>;<br /> -<span style="margin-left: 1em;">devitrification of, <a href="../../66492/66492-h/66492-h.htm#Page_19">19</a>;</span><br /> -<span style="margin-left: 1em;">flow-structure of, <a href="../../66492/66492-h/66492-h.htm#Page_21">21</a>;</span><br /> -<span style="margin-left: 1em;">occur in thicker sheets than basic, <a href="../../66492/66492-h/66492-h.htm#Page_24">24</a>;</span><br /> -<span style="margin-left: 1em;">alternations of, with basic, <a href="../../66492/66492-h/66492-h.htm#Page_28">28</a>, <a href="../../66492/66492-h/66492-h.htm#Page_61">61</a>, <a href="../../66492/66492-h/66492-h.htm#Page_152">152</a>, <a href="../../66492/66492-h/66492-h.htm#Page_157">157</a>, <a href="../../66492/66492-h/66492-h.htm#Page_165">165</a>, <a href="../../66492/66492-h/66492-h.htm#Page_207">207</a>, <a href="../../66492/66492-h/66492-h.htm#Page_213">213</a>, <a href="../../66492/66492-h/66492-h.htm#Page_233">233</a>, <a href="../../66492/66492-h/66492-h.htm#Page_284">284</a>, <a href="../../66492/66492-h/66492-h.htm#Page_318">318</a>; -ii. <a href="#Page_236">236</a>, <a href="#Page_266">266</a>, <a href="#Page_278">278</a>;</span><br /> -<span style="margin-left: 1em;">metamorphic action of, i. <a href="../../66492/66492-h/66492-h.htm#Page_95">95</a>, <a href="../../66492/66492-h/66492-h.htm#Page_96">96</a>;</span><br /> -<span style="margin-left: 1em;">connection with mountains, ii. <a href="#Page_98">98</a>;</span><br /> -<span style="margin-left: 1em;">scenery of, <a href="#Page_102">102</a>.</span><br /> -Acids, mineral, at volcanoes, i. <a href="../../66492/66492-h/66492-h.htm#Page_72">72</a><br /> -Acland, Mr. H. D., i. <a href="../../66492/66492-h/66492-h.htm#Page_133">133</a><br /> -Aegean Sea, volcanoes of, i. <a href="../../66492/66492-h/66492-h.htm#Page_1">1</a><br /> -<a id="Agglomerates"></a>Agglomerates, i. <a href="../../66492/66492-h/66492-h.htm#Page_31">31</a>, <a href="../../66492/66492-h/66492-h.htm#Page_57">57</a>, <a href="../../66492/66492-h/66492-h.htm#Page_58">58</a>;<br /> -<span style="margin-left: 1em;">in dykes, <a href="../../66492/66492-h/66492-h.htm#Page_70">70</a>;</span><br /> -<span style="margin-left: 1em;">Archæan, <a href="../../66492/66492-h/66492-h.htm#Page_120">120</a>, <a href="../../66492/66492-h/66492-h.htm#Page_130">130</a>, <a href="../../66492/66492-h/66492-h.htm#Page_135">135</a>;</span><br /> -<span style="margin-left: 1em;">Cambrian, <a href="../../66492/66492-h/66492-h.htm#Page_148">148</a>, <a href="../../66492/66492-h/66492-h.htm#Page_149">149</a>, <a href="../../66492/66492-h/66492-h.htm#Page_167">167</a>;</span><br /> -<span style="margin-left: 1em;">Silurian, <a href="../../66492/66492-h/66492-h.htm#Page_178">178</a>, <a href="../../66492/66492-h/66492-h.htm#Page_180">180</a>, <a href="../../66492/66492-h/66492-h.htm#Page_181">181</a>, <a href="../../66492/66492-h/66492-h.htm#Page_184">184</a>, <a href="../../66492/66492-h/66492-h.htm#Page_185">185</a>, <a href="../../66492/66492-h/66492-h.htm#Page_194">194</a>, <a href="../../66492/66492-h/66492-h.htm#Page_199">199</a>, <a href="../../66492/66492-h/66492-h.htm#Page_206">206</a>, <a href="../../66492/66492-h/66492-h.htm#Page_214">214</a>, <a href="../../66492/66492-h/66492-h.htm#Page_237">237</a>, <a href="../../66492/66492-h/66492-h.htm#Page_241">241</a>, <a href="../../66492/66492-h/66492-h.htm#Page_244">244</a>, <a href="../../66492/66492-h/66492-h.htm#Page_247">247</a>, <a href="../../66492/66492-h/66492-h.htm#Page_253">253</a>, <a href="../../66492/66492-h/66492-h.htm#Page_255">255</a>;</span><br /> -<span style="margin-left: 1em;">Old Red Sandstone, <a href="../../66492/66492-h/66492-h.htm#Page_279">279</a>, <a href="../../66492/66492-h/66492-h.htm#Page_285">285</a>, <a href="../../66492/66492-h/66492-h.htm#Page_289">289</a>, <a href="../../66492/66492-h/66492-h.htm#Page_300">300</a>, <a href="../../66492/66492-h/66492-h.htm#Page_313">313</a>, <a href="../../66492/66492-h/66492-h.htm#Page_325">325</a>, <a href="../../66492/66492-h/66492-h.htm#Page_338">338</a>, <a href="../../66492/66492-h/66492-h.htm#Page_349">349</a>, <a href="../../66492/66492-h/66492-h.htm#Page_352">352</a>;</span><br /> -<span style="margin-left: 1em;">Carboniferous, <a href="../../66492/66492-h/66492-h.htm#Page_381">381</a>, <a href="../../66492/66492-h/66492-h.htm#Page_399">399</a>, <a href="../../66492/66492-h/66492-h.htm#Page_402">402</a>, <a href="../../66492/66492-h/66492-h.htm#Page_404">404</a>, <a href="../../66492/66492-h/66492-h.htm#Page_427">427</a>, <a href="../../66492/66492-h/66492-h.htm#Page_429">429</a>, <a href="../../66492/66492-h/66492-h.htm#Page_439">439</a>, <a href="../../66492/66492-h/66492-h.htm#Page_440">440</a>; -ii. <a href="#Page_13">13</a>, <a href="#Page_24">24</a>, <a href="#Page_28">28</a>, <a href="#Page_29">29</a>;</span><br /> -<span style="margin-left: 1em;">Permian, <a href="#Page_62">62</a>, <a href="#Page_64">64</a>, <a href="#Page_99">99</a>;</span><br /> -<span style="margin-left: 1em;">Tertiary, <a href="#Page_194">194</a>, <a href="#Page_277">277</a>, <a href="#Page_278">278</a>, <a href="#Page_281">281</a>, <a href="#Page_289">289</a>, <a href="#Page_292">292</a>, <a href="#Page_293">293</a>, <a href="#Page_384">384</a>, <a href="#Page_400">400</a>, <a href="#Page_423">423</a></span><br /> -Allan, T., i. <a href="../../66492/66492-h/66492-h.htm#Page_363">363</a><br /> -Allotriomorphic minerals, i. <a href="../../66492/66492-h/66492-h.htm#Page_21">21</a><br /> -Allport, Mr., i. <a href="../../66492/66492-h/66492-h.htm#Page_95">95</a>, <a href="../../66492/66492-h/66492-h.htm#Page_130">130</a>, <a href="../../66492/66492-h/66492-h.htm#Page_131">131</a>, <a href="../../66492/66492-h/66492-h.htm#Page_260">260</a>, <a href="../../66492/66492-h/66492-h.htm#Page_451">451</a>; <a href="../../66492/66492-h/66492-h.htm#Page_ii">ii</a>. <a href="../../66492/66492-h/66492-h.htm#Page_11">11</a>, <a href="../../66492/66492-h/66492-h.htm#Page_42">42</a>, <a href="../../66492/66492-h/66492-h.htm#Page_102">102</a>, <a href="../../66492/66492-h/66492-h.htm#Page_103">103</a>, <a href="../../66492/66492-h/66492-h.htm#Page_104">104</a>, <a href="../../66492/66492-h/66492-h.htm#Page_106">106</a>, <a href="../../66492/66492-h/66492-h.htm#Page_370">370</a><br /> -Amber in Tertiary volcanic series, ii. <a href="#Page_198">198</a><br /> -America, Western North, volcanic rocks of, i. <a href="../../66492/66492-h/66492-h.htm#Page_10">10</a>, <a href="../../66492/66492-h/66492-h.htm#Page_100">100</a>; - ii, <a href="../../66492/66492-h/66492-h.htm#Page_267">267</a><br /> -Amygdales, origin of, i. <a href="../../66492/66492-h/66492-h.htm#Page_15">15</a>; - ii. <a href="../../66492/66492-h/66492-h.htm#Page_189">189</a>, <a href="../../66492/66492-h/66492-h.htm#Page_221">221</a>, <a href="../../66492/66492-h/66492-h.htm#Page_285">285</a>, <a href="../../66492/66492-h/66492-h.htm#Page_290">290</a><br /> -Amygdaloidal structure, i. <a href="../../66492/66492-h/66492-h.htm#Page_15">15</a>, <a href="../../66492/66492-h/66492-h.htm#Page_16">16</a>, <a href="../../66492/66492-h/66492-h.htm#Page_17">17</a>, <a href="../../66492/66492-h/66492-h.htm#Page_59">59</a>, <a href="../../66492/66492-h/66492-h.htm#Page_274">274</a>, <a href="../../66492/66492-h/66492-h.htm#Page_385">385</a>; ii. <a href="#Page_3">3</a>, <a href="#Page_31">31</a>, <a href="#Page_57">57</a>, <a href="#Page_129">129</a>, <a href="#Page_188">188</a><br /> -Analyses of Cambrian tuffs, i. <a href="../../66492/66492-h/66492-h.htm#Page_148">148</a>, <a href="../../66492/66492-h/66492-h.htm#Page_149">149</a>;<br /> -<span style="margin-left: 1em;">of Cambrian diabases, <a href="../../66492/66492-h/66492-h.htm#Page_153">153</a>;</span><br /> -<span style="margin-left: 1em;">of Old Red Sandstone diabases, <a href="../../66492/66492-h/66492-h.htm#Page_274">274</a>;</span><br /> -<span style="margin-left: 1em;">of Old Red Sandstone andesites, <a href="../../66492/66492-h/66492-h.htm#Page_275">275</a>;</span><br /> -<span style="margin-left: 1em;">of Old Red Sandstone trachytes, <a href="../../66492/66492-h/66492-h.htm#Page_276">276</a>;</span><br /> -<span style="margin-left: 1em;">of Old Red Sandstone felsites, <a href="../../66492/66492-h/66492-h.htm#Page_278">278</a>;</span><br /> -<span style="margin-left: 1em;">of Carboniferous limburgite, <a href="../../66492/66492-h/66492-h.htm#Page_377">377</a>;</span><br /> -<span style="margin-left: 1em;">of Carboniferous basalts, <a href="../../66492/66492-h/66492-h.htm#Page_379">379</a>;</span><br /> -<span style="margin-left: 1em;">of Carboniferous trachytes, <a href="../../66492/66492-h/66492-h.htm#Page_380">380</a>;</span><br /> -<span style="margin-left: 1em;">of Carboniferous phonolite, <a href="../../66492/66492-h/66492-h.htm#Page_381">381</a>;</span><br /> -<span style="margin-left: 1em;">of Tertiary trachyte, ii. <a href="#Page_139">139</a>;</span><br /> -<span style="margin-left: 1em;">of Tertiary dacite, <a href="#Page_244">244</a></span><br /> -Anderson, Dr. Tempest, ii <a href="#Page_261">261</a>, <a href="#Page_262">262</a>, <a href="#Page_263">263</a><br /> -Andesite, i. <a href="../../66492/66492-h/66492-h.htm#Page_24">24</a>, <a href="../../66492/66492-h/66492-h.htm#Page_131">131</a>, <a href="../../66492/66492-h/66492-h.htm#Page_136">136</a>, <a href="../../66492/66492-h/66492-h.htm#Page_164">164</a>, <a href="../../66492/66492-h/66492-h.htm#Page_165">165</a>, <a href="../../66492/66492-h/66492-h.htm#Page_167">167</a>, <a href="../../66492/66492-h/66492-h.htm#Page_178">178</a>, <a href="../../66492/66492-h/66492-h.htm#Page_180">180</a>, <a href="../../66492/66492-h/66492-h.htm#Page_184">184</a>, <a href="../../66492/66492-h/66492-h.htm#Page_189">189</a>, <a href="../../66492/66492-h/66492-h.htm#Page_190">190</a>, <a href="../../66492/66492-h/66492-h.htm#Page_204">204</a>, <a href="../../66492/66492-h/66492-h.htm#Page_212">212</a>, <a href="../../66492/66492-h/66492-h.htm#Page_213">213</a>, <a href="../../66492/66492-h/66492-h.htm#Page_214">214</a>, <a href="../../66492/66492-h/66492-h.htm#Page_215">215</a>, <a href="../../66492/66492-h/66492-h.htm#Page_229">229</a>, <a href="../../66492/66492-h/66492-h.htm#Page_230">230</a>, <a href="../../66492/66492-h/66492-h.htm#Page_245">245</a>, <a href="../../66492/66492-h/66492-h.htm#Page_246">246</a>, <a href="../../66492/66492-h/66492-h.htm#Page_247">247</a>, <a href="../../66492/66492-h/66492-h.htm#Page_252">252</a>, <a href="../../66492/66492-h/66492-h.htm#Page_274">274</a>, <a href="../../66492/66492-h/66492-h.htm#Page_275">275</a>, (analyses), <a href="../../66492/66492-h/66492-h.htm#Page_277">277</a>, <a href="../../66492/66492-h/66492-h.htm#Page_292">292</a>, <a href="../../66492/66492-h/66492-h.htm#Page_300">300</a>, <a href="../../66492/66492-h/66492-h.htm#Page_306">306</a>, <a href="../../66492/66492-h/66492-h.htm#Page_309">309</a>, <a href="../../66492/66492-h/66492-h.htm#Page_315">315</a>, <a href="../../66492/66492-h/66492-h.htm#Page_318">318</a>, <a href="../../66492/66492-h/66492-h.htm#Page_325">325</a>, <a href="../../66492/66492-h/66492-h.htm#Page_330">330</a>, <a href="../../66492/66492-h/66492-h.htm#Page_333">333</a>, <a href="../../66492/66492-h/66492-h.htm#Page_345">345</a>, <a href="../../66492/66492-h/66492-h.htm#Page_377">377</a>, <a href="../../66492/66492-h/66492-h.htm#Page_379">379</a>, <a href="../../66492/66492-h/66492-h.htm#Page_386">386</a>, <a href="../../66492/66492-h/66492-h.htm#Page_403">403</a>, <a href="../../66492/66492-h/66492-h.htm#Page_421">421</a>; ii. <a href="#Page_45">45</a>, <a href="#Page_57">57</a>, <a href="#Page_96">96</a>, <a href="#Page_125">125</a>, <a href="#Page_137">137</a>, <a href="#Page_184">184</a>, <a href="#Page_236">236</a>, <a href="#Page_424">424</a><br /> -Anglesey, gneisses and schists of, i. <a href="../../66492/66492-h/66492-h.htm#Page_126">126</a>;<br /> -<span style="margin-left: 1em;">volcanic rocks of, <a href="../../66492/66492-h/66492-h.htm#Page_189">189</a>, <a href="../../66492/66492-h/66492-h.htm#Page_219">219</a></span><br /> -Anhydrite deposits, ii. <a href="../../66492/66492-h/66492-h.htm#Page_54">54</a><br /> -Annandale, Permian volcanic rocks of, ii. <a href="#Page_56">56</a>, <a href="#Page_58">58</a>, <a href="#Page_60">60</a>, <a href="#Page_61">61</a>, <a href="#Page_66">66</a><br /> -Antrim, Old Red Sandstone volcanic rocks of, i. <a href="../../66492/66492-h/66492-h.htm#Page_314">314</a>;<br /> -<span style="margin-left: 1em;">Tertiary volcanic rocks of, <a href="../../66492/66492-h/66492-h.htm#Page_47">47</a>, <a href="../../66492/66492-h/66492-h.htm#Page_52">52</a>; ii. <a href="#Page_109">109</a>, <a href="#Page_110">110</a>, <a href="#Page_113">113</a>, <a href="#Page_139">139</a>, <a href="#Page_140">140</a>, <a href="#Page_199">199</a>;</span><br /> -<span style="margin-left: 1em;">basalts of, <a href="#Page_192">192</a>, <a href="#Page_193">193</a>, <a href="#Page_199">199</a>, <a href="#Page_202">202</a>, <a href="#Page_206">206</a>;</span><br /> -<span style="margin-left: 1em;">clays and iron-ore of, <a href="#Page_204">204</a>;</span><br /> -<span style="margin-left: 1em;">rhyolites of, <a href="#Page_185">185</a>, <a href="#Page_364">364</a>, <a href="#Page_370">370</a>, <a href="#Page_371">371</a>, <a href="#Page_426">426</a>, <a href="#Page_445">445</a>;</span><br /> -<span style="margin-left: 1em;">deceptive agglomerate of, <a href="#Page_188">188</a>;</span><br /> -<span style="margin-left: 1em;">rhyolitic conglomerate of, <a href="#Page_195">195</a>, <a href="#Page_206">206</a>;</span><br /> -<span style="margin-left: 1em;">plateau of, <a href="#Page_199">199</a>;</span><br /> -<span style="margin-left: 1em;">tuffs of, <a href="#Page_202">202</a>, <a href="#Page_204">204</a>;</span><br /> -<span style="margin-left: 1em;">vents of, <a href="#Page_271">271</a>, <a href="#Page_277">277</a>;</span><br /> -<span style="margin-left: 1em;">sills of, <a href="#Page_298">298</a>;</span><br /> -<span style="margin-left: 1em;">central subsidence of basalt-plateau of, <a href="#Page_448">448</a></span><br /> -Apatite, ii. <a href="#Page_135">135</a><br /> -Apjohn, J., <a href="../../66492/66492-h/66492-h.htm#Page_ii">ii</a>. <a href="#Page_42">42</a><br /> -Applecross, volcanic vents in, ii. <a href="#Page_292">292</a><br /> -Arans, the, i. <a href="../../66492/66492-h/66492-h.htm#Page_175">175</a>, <a href="../../66492/66492-h/66492-h.htm#Page_176">176</a>, <a href="../../66492/66492-h/66492-h.htm#Page_179">179</a>, <a href="../../66492/66492-h/66492-h.htm#Page_184">184</a>, <a href="../../66492/66492-h/66492-h.htm#Page_186">186</a>, <a href="../../66492/66492-h/66492-h.htm#Page_207">207</a><br /> -Archæan period, i. <a href="../../66492/66492-h/66492-h.htm#Page_110">110</a>, <a href="../../66492/66492-h/66492-h.htm#Page_111">111</a>;<br /> -<span style="margin-left: 1em;">volcanic rocks of, <a href="../../66492/66492-h/66492-h.htm#Page_120">120</a></span><br /> -Ardnamurchan, dykes and veins of, ii. <a href="#Page_154">154</a>, <a href="#Page_320">320</a>;<br /> -<span style="margin-left: 1em;">basalt-plateau of, <a href="#Page_208">208</a>;</span><br /> -<span style="margin-left: 1em;">vents of, <a href="#Page_287">287</a>;</span><br /> -<span style="margin-left: 1em;">sills of, <a href="#Page_318">318</a>;</span><br /> -<span style="margin-left: 1em;">gabbro of, <a href="#Page_355">355</a></span><br /> -Arenig group, i. <a href="../../66492/66492-h/66492-h.htm#Page_175">175</a>;<br /> -<span style="margin-left: 1em;">lower limit of, <a href="../../66492/66492-h/66492-h.htm#Page_177">177</a>, <a href="../../66492/66492-h/66492-h.htm#Page_185">185</a>;</span><br /> -<span style="margin-left: 1em;">top of, <a href="../../66492/66492-h/66492-h.htm#Page_178">178</a>, <a href="../../66492/66492-h/66492-h.htm#Page_228">228</a>, <a href="../../66492/66492-h/66492-h.htm#Page_246">246</a></span><br /> -—— volcano of, i. <a href="../../66492/66492-h/66492-h.htm#Page_42">42</a>, <a href="../../66492/66492-h/66492-h.htm#Page_175">175</a>, <a href="../../66492/66492-h/66492-h.htm#Page_176">176</a>, <a href="../../66492/66492-h/66492-h.htm#Page_179">179</a>, <a href="../../66492/66492-h/66492-h.htm#Page_186">186</a>, <a href="../../66492/66492-h/66492-h.htm#Page_207">207</a><br /> -—— rocks in Scottish Highlands, i. <a href="../../66492/66492-h/66492-h.htm#Page_123">123</a>, <a href="../../66492/66492-h/66492-h.htm#Page_126">126</a>;<br /> -<span style="margin-left: 1em;">in Merionethshire, <a href="../../66492/66492-h/66492-h.htm#Page_176">176</a>, <a href="../../66492/66492-h/66492-h.htm#Page_179">179</a>;</span><br /> -<span style="margin-left: 1em;">of Shropshire, <a href="../../66492/66492-h/66492-h.htm#Page_189">189</a>;</span><br /> -<span style="margin-left: 1em;">of Ayrshire, <a href="../../66492/66492-h/66492-h.htm#Page_196">196</a>;</span><br /> -<span style="margin-left: 1em;">of Scottish Highlands, <a href="../../66492/66492-h/66492-h.htm#Page_201">201</a>;</span><br /> -<span style="margin-left: 1em;">of Anglesey, <a href="../../66492/66492-h/66492-h.htm#Page_221">221</a>;</span><br /> -<span style="margin-left: 1em;">of Lake district, <a href="../../66492/66492-h/66492-h.htm#Page_229">229</a>;</span><br /> -<span style="margin-left: 1em;">of Ireland, <a href="../../66492/66492-h/66492-h.htm#Page_239">239</a></span><br /> -Argyll, Duke of, ii. <a href="#Page_113">113</a>, <a href="#Page_114">114</a>, <a href="#Page_198">198</a><br /> -Argyllshire, dykes of, ii. <a href="#Page_127">127</a>, <a href="#Page_128">128</a>, <a href="#Page_138">138</a>, <a href="#Page_142">142</a>, <a href="#Page_146">146</a>, <a href="#Page_171">171</a>, <a href="#Page_172">172</a>;<br /> -<span style="margin-left: 1em;">vents of, <a href="#Page_278">278</a></span><br /> -Arizona, explosion crater in, i. <a href="../../66492/66492-h/66492-h.htm#Page_58">58</a>;<br /> -<span style="margin-left: 1em;">laccolites in, <a href="../../66492/66492-h/66492-h.htm#Page_86">86</a></span><br /> -Arran, Old Red Sandstone volcanic rocks of, i. <a href="../../66492/66492-h/66492-h.htm#Page_298">298</a>, <a href="../../66492/66492-h/66492-h.htm#Page_311">311</a>;<br /> -<span style="margin-left: 1em;">Carboniferous volcanic rocks of, <a href="../../66492/66492-h/66492-h.htm#Page_386">386</a>, <a href="../../66492/66492-h/66492-h.htm#Page_392">392</a>;</span><br /> -<span style="margin-left: 1em;">possible Permian volcanic rocks of, ii. <a href="#Page_58">58</a>;</span><br /> -<span style="margin-left: 1em;">granite of, i. <a href="../../66492/66492-h/66492-h.htm#Page_93">93</a>; ii. <a href="#Page_366">366</a>, <a href="#Page_367">367</a>, <a href="#Page_418">418</a>;</span><br /> -<span style="margin-left: 1em;">pitchstone of, i. <a href="../../66492/66492-h/66492-h.htm#Page_19">19</a>; ii. <a href="#Page_445">445</a>;</span><br /> -<span style="margin-left: 1em;">dykes of, <a href="#Page_123">123</a>, <a href="#Page_139">139</a>, <a href="#Page_140">140</a>, <a href="#Page_142">142</a>, <a href="#Page_146">146</a>, <a href="#Page_154">154</a>, <a href="#Page_161">161</a></span><br /> -Arthur Seat, i. <a href="../../66492/66492-h/66492-h.htm#Page_364">364</a>, <a href="../../66492/66492-h/66492-h.htm#Page_373">373</a>, <a href="../../66492/66492-h/66492-h.htm#Page_378">378</a>, <a href="../../66492/66492-h/66492-h.htm#Page_385">385</a>, <a href="../../66492/66492-h/66492-h.htm#Page_386">386</a>; ii. <a href="#Page_67">67</a><br /> -"Arvonian," i. <a href="../../66492/66492-h/66492-h.htm#Page_145">145</a>, <a href="../../66492/66492-h/66492-h.htm#Page_156">156</a><br /> -Asbestos in volcanic breccia, ii. <a href="#Page_51">51</a><br /> -Ascension Island, cellular lava of, i. <a href="../../66492/66492-h/66492-h.htm#Page_15">15</a><br /> -Ashes, volcanic (<i>see</i> <a href="#Tuffs">Tuffs</a>)<br /> -Ashprington volcanic series, i. <a href="../../66492/66492-h/66492-h.htm#Page_262">262</a><br /> -Asphalt, ii. <a href="#Page_79">79</a><br /> -Atherstone, i. <a href="../../66492/66492-h/66492-h.htm#Page_170">170</a><br /> -Augite, loose crystals of, in volcanic vents, i. <a href="../../66492/66492-h/66492-h.htm#Page_62">62</a>, <a href="../../66492/66492-h/66492-h.htm#Page_178">178</a>, <a href="../../66492/66492-h/66492-h.htm#Page_181">181</a>; ii. <a href="#Page_58">58</a>, <a href="#Page_79">79</a>;<br /> -<span style="margin-left: 1em;">lumps of, in volcanic vents, i. <a href="../../66492/66492-h/66492-h.htm#Page_352">352</a></span><br /> -Augite-aphanites, i. <a href="../../66492/66492-h/66492-h.htm#Page_178">178</a><br /> -Auvergne, old volcanoes of, i. <a href="../../66492/66492-h/66492-h.htm#Page_29">29</a>, <a href="../../66492/66492-h/66492-h.htm#Page_32">32</a>, <a href="../../66492/66492-h/66492-h.htm#Page_66">66</a>, <a href="../../66492/66492-h/66492-h.htm#Page_70">70</a>, <a href="../../66492/66492-h/66492-h.htm#Page_100">100</a>; ii. <a href="#Page_373">373</a><br /> -</p> - -<p><span class="pagenum" id="Page_480">- 480 -</span></p> - -<p> -Aveline, Mr. W. T., i. <a href="../../66492/66492-h/66492-h.htm#Page_227">227</a>, <a href="../../66492/66492-h/66492-h.htm#Page_230">230</a>; ii. <a href="#Page_32">32</a><br /> -Ayrshire, example of volcanic neck in, i. <a href="../../66492/66492-h/66492-h.htm#Page_56">56</a>;<br /> -<span style="margin-left: 1em;">Silurian volcanic rocks of, <a href="../../66492/66492-h/66492-h.htm#Page_192">192</a>;</span><br /> -<span style="margin-left: 1em;">Old Red Sandstone volcanic rocks of, <a href="../../66492/66492-h/66492-h.htm#Page_275">275</a>, <a href="../../66492/66492-h/66492-h.htm#Page_282">282</a>, <a href="../../66492/66492-h/66492-h.htm#Page_283">283</a>, <a href="../../66492/66492-h/66492-h.htm#Page_285">285</a>, <a href="../../66492/66492-h/66492-h.htm#Page_291">291</a>, <a href="../../66492/66492-h/66492-h.htm#Page_331">331</a>;</span><br /> -<span style="margin-left: 1em;">Carboniferous volcanic plateau of, <a href="../../66492/66492-h/66492-h.htm#Page_102">102</a>, <a href="../../66492/66492-h/66492-h.htm#Page_368">368</a>, <a href="../../66492/66492-h/66492-h.htm#Page_388">388</a>, <a href="../../66492/66492-h/66492-h.htm#Page_393">393</a>, <a href="../../66492/66492-h/66492-h.htm#Page_398">398</a>, <a href="../../66492/66492-h/66492-h.htm#Page_410">410</a>;</span><br /> -<span style="margin-left: 1em;">Carboniferous Puys of, <a href="../../66492/66492-h/66492-h.htm#Page_415">415</a>, <a href="../../66492/66492-h/66492-h.htm#Page_416">416</a>, <a href="../../66492/66492-h/66492-h.htm#Page_434">434</a>, <a href="../../66492/66492-h/66492-h.htm#Page_440">440</a>, <a href="../../66492/66492-h/66492-h.htm#Page_474">474</a>;</span><br /> -<span style="margin-left: 1em;">Permian volcanic rocks of, ii. <a href="#Page_55">55</a>, <a href="#Page_58">58</a>, <a href="#Page_62">62</a></span><br /> -Azoic period, i. <a href="../../66492/66492-h/66492-h.htm#Page_109">109</a><br /> -<br /> -<a id="B"></a>Bäckström, Mr., ii. <a href="#Page_266">266</a><br /> -Baily, W. H., i. <a href="../../66492/66492-h/66492-h.htm#Page_251">251</a>, <a href="../../66492/66492-h/66492-h.htm#Page_252">252</a>; ii. <a href="#Page_198">198</a>, <a href="#Page_449">449</a><br /> -Bala group, i. <a href="../../66492/66492-h/66492-h.htm#Page_175">175</a>, <a href="../../66492/66492-h/66492-h.htm#Page_190">190</a>, <a href="../../66492/66492-h/66492-h.htm#Page_196">196</a>, <a href="../../66492/66492-h/66492-h.htm#Page_201">201</a>, <a href="../../66492/66492-h/66492-h.htm#Page_206">206</a>, <a href="../../66492/66492-h/66492-h.htm#Page_207">207</a>, <a href="../../66492/66492-h/66492-h.htm#Page_223">223</a>, <a href="../../66492/66492-h/66492-h.htm#Page_242">242</a>;<br /> -<span style="margin-left: 1em;">limestone of, <a href="../../66492/66492-h/66492-h.htm#Page_47">47</a>, <a href="../../66492/66492-h/66492-h.htm#Page_175">175</a>, <a href="../../66492/66492-h/66492-h.htm#Page_229">229</a>, <a href="../../66492/66492-h/66492-h.htm#Page_245">245</a>, <a href="../../66492/66492-h/66492-h.htm#Page_251">251</a>;</span><br /> -<span style="margin-left: 1em;">volcanic rocks of, <a href="../../66492/66492-h/66492-h.htm#Page_186">186</a>, <a href="../../66492/66492-h/66492-h.htm#Page_190">190</a>, <a href="../../66492/66492-h/66492-h.htm#Page_207">207</a>, <a href="../../66492/66492-h/66492-h.htm#Page_213">213</a>, <a href="../../66492/66492-h/66492-h.htm#Page_221">221</a>, <a href="../../66492/66492-h/66492-h.htm#Page_241">241</a>, <a href="../../66492/66492-h/66492-h.htm#Page_248">248</a></span><br /> -Balbriggan, igneous rocks of, i. <a href="../../66492/66492-h/66492-h.htm#Page_244">244</a><br /> -Ballagan beds (Lower Carboniferous), i. <a href="../../66492/66492-h/66492-h.htm#Page_384">384</a>, <a href="../../66492/66492-h/66492-h.htm#Page_387">387</a>, <a href="../../66492/66492-h/66492-h.htm#Page_392">392</a>, <a href="../../66492/66492-h/66492-h.htm#Page_393">393</a>, <a href="../../66492/66492-h/66492-h.htm#Page_412">412</a>, <a href="../../66492/66492-h/66492-h.htm#Page_447">447</a><br /> -Ballantrae, volcanic rocks at, i. <a href="../../66492/66492-h/66492-h.htm#Page_192">192</a>, <a href="../../66492/66492-h/66492-h.htm#Page_199">199</a><br /> -Ballypallidy, tuffs and leaf-beds of, ii. <a href="#Page_204">204</a>, <a href="#Page_429">429</a><br /> -Bamborough, Whin Sill at, ii. <a href="#Page_2">2</a>, <a href="#Page_3">3</a>, <a href="#Page_5">5</a><br /> -Banding of igneous rocks, i. <a href="../../66492/66492-h/66492-h.htm#Page_84">84</a>, <a href="../../66492/66492-h/66492-h.htm#Page_207">207</a>; ii. <a href="#Page_189">189</a>, <a href="#Page_294">294</a>, <a href="#Page_329">329</a>, <a href="#Page_354">354</a>, <a href="#Page_357">357</a>, <a href="#Page_476">476</a><br /> -—— of gneiss, i. <a href="../../66492/66492-h/66492-h.htm#Page_116">116</a><br /> -Bangor group, i. <a href="../../66492/66492-h/66492-h.htm#Page_166">166</a><br /> -Banks, Sir Joseph, ii. <a href="#Page_109">109</a><br /> -Barnavave, eruptive rocks of, ii. <a href="#Page_421">421</a><br /> -Barrow, Mr. G., i. <a href="../../66492/66492-h/66492-h.htm#Page_201">201</a>, <a href="../../66492/66492-h/66492-h.htm#Page_226">226</a>, <a href="../../66492/66492-h/66492-h.htm#Page_272">272</a>, <a href="../../66492/66492-h/66492-h.htm#Page_279">279</a>, <a href="../../66492/66492-h/66492-h.htm#Page_380">380</a>; ii. <a href="#Page_147">147</a>, <a href="#Page_148">148</a><br /> -Basalt, columnar structure of, i. <a href="../../66492/66492-h/66492-h.htm#Page_24">24</a>, <a href="../../66492/66492-h/66492-h.htm#Page_25">25</a>;<br /> -<span style="margin-left: 1em;">relation to gabbro, <a href="../../66492/66492-h/66492-h.htm#Page_78">78</a>;</span><br /> -<span style="margin-left: 1em;">altered by carbonaceous strata, <a href="../../66492/66492-h/66492-h.htm#Page_95">95</a>;</span><br /> -<span style="margin-left: 1em;">shells supposed to occur in, ii. <a href="#Page_110">110</a>;</span><br /> -<span style="margin-left: 1em;">banded, <a href="#Page_189">189</a>;</span><br /> -<span style="margin-left: 1em;">thickness of sheets of, <a href="#Page_192">192</a>;</span><br /> -<span style="margin-left: 1em;">meaning of red layer between sheets of, <a href="#Page_197">197</a>, <a href="#Page_203">203</a>, <a href="#Page_206">206</a>, <a href="#Page_254">254</a>;</span><br /> -<span style="margin-left: 1em;">metamorphism of, <a href="#Page_272">272</a>, <a href="#Page_276">276</a>, <a href="#Page_337">337</a>, <a href="#Page_339">339</a>, <a href="#Page_340">340</a>, <a href="#Page_347">347</a>, <a href="#Page_355">355</a>, <a href="#Page_356">356</a>, <a href="#Page_357">357</a>, <a href="#Page_358">358</a>, <a href="#Page_362">362</a>, <a href="#Page_378">378</a>, <a href="#Page_383">383</a>, <a href="#Page_386">386</a>, <a href="#Page_397">397</a>, <a href="#Page_399">399</a>, <a href="#Page_400">400</a>, <a href="#Page_404">404</a>, <a href="#Page_413">413</a></span><br /> -—— pre-Cambrian, i. <a href="../../66492/66492-h/66492-h.htm#Page_119">119</a>, <a href="../../66492/66492-h/66492-h.htm#Page_131">131</a>;<br /> -<span style="margin-left: 1em;">Silurian, <a href="../../66492/66492-h/66492-h.htm#Page_206">206</a>, <a href="../../66492/66492-h/66492-h.htm#Page_207">207</a>, <a href="../../66492/66492-h/66492-h.htm#Page_230">230</a>, <a href="../../66492/66492-h/66492-h.htm#Page_245">245</a>;</span><br /> -<span style="margin-left: 1em;">Carboniferous, <a href="../../66492/66492-h/66492-h.htm#Page_378">378</a>, <a href="../../66492/66492-h/66492-h.htm#Page_403">403</a>, <a href="../../66492/66492-h/66492-h.htm#Page_407">407</a>, <a href="../../66492/66492-h/66492-h.htm#Page_417">417</a>; ii. <a href="#Page_11">11</a>, <a href="#Page_45">45</a>, <a href="#Page_46">46</a>;</span><br /> -<span style="margin-left: 1em;">Permian, <a href="#Page_57">57</a>, <a href="#Page_96">96</a>;</span><br /> -<span style="margin-left: 1em;">Tertiary, <a href="#Page_125">125</a>, <a href="#Page_136">136</a>, <a href="#Page_183">183</a>, <a href="#Page_199">199</a>, <a href="#Page_208">208</a>, <a href="#Page_291">291</a></span><br /> -Basalt-conglomerate, ii. <a href="#Page_195">195</a><br /> -Basic volcanic rocks, silica-percentage of, i. <a href="../../66492/66492-h/66492-h.htm#Page_14">14</a>;<br /> -<span style="margin-left: 1em;">devitrification of, <a href="../../66492/66492-h/66492-h.htm#Page_20">20</a>;</span><br /> -<span style="margin-left: 1em;">flow-structure of, <a href="../../66492/66492-h/66492-h.htm#Page_21">21</a>;</span><br /> -<span style="margin-left: 1em;">occur in thinner sheets than the acid, <a href="../../66492/66492-h/66492-h.htm#Page_24">24</a>;</span><br /> -<span style="margin-left: 1em;">metamorphic action of, <a href="../../66492/66492-h/66492-h.htm#Page_94">94</a>;</span><br /> -<span style="margin-left: 1em;">erupted at low levels, <a href="../../66492/66492-h/66492-h.htm#Page_98">98</a>;</span><br /> -<span style="margin-left: 1em;">scenery of, <a href="../../66492/66492-h/66492-h.htm#Page_102">102</a>;</span><br /> -<span style="margin-left: 1em;">converted into schists by deformation, <a href="../../66492/66492-h/66492-h.htm#Page_75">75</a>, <a href="../../66492/66492-h/66492-h.htm#Page_114">114</a>, <a href="../../66492/66492-h/66492-h.htm#Page_118">118</a>, <a href="../../66492/66492-h/66492-h.htm#Page_119">119</a>, <a href="../../66492/66492-h/66492-h.htm#Page_124">124</a>, <a href="../../66492/66492-h/66492-h.htm#Page_129">129</a>;</span><br /> -<span style="margin-left: 1em;">alternation with acid, <a href="../../66492/66492-h/66492-h.htm#Page_28">28</a>, <a href="../../66492/66492-h/66492-h.htm#Page_61">61</a>, <a href="../../66492/66492-h/66492-h.htm#Page_131">131</a>, <a href="../../66492/66492-h/66492-h.htm#Page_157">157</a>, <a href="../../66492/66492-h/66492-h.htm#Page_165">165</a>, <a href="../../66492/66492-h/66492-h.htm#Page_207">207</a>, <a href="../../66492/66492-h/66492-h.htm#Page_213">213</a>, <a href="../../66492/66492-h/66492-h.htm#Page_233">233</a>, <a href="../../66492/66492-h/66492-h.htm#Page_284">284</a>, <a href="../../66492/66492-h/66492-h.htm#Page_318">318</a>; ii. <a href="#Page_236">236</a>, <a href="#Page_266">266</a>, <a href="#Page_278">278</a></span><br /> -Bass Rock, i. <a href="../../66492/66492-h/66492-h.htm#Page_372">372</a>, <a href="../../66492/66492-h/66492-h.htm#Page_373">373</a>, <a href="../../66492/66492-h/66492-h.htm#Page_403">403</a><br /> -Bassenthwaite Lake, i. <a href="../../66492/66492-h/66492-h.htm#Page_335">335</a><br /> -Bathgate, puy eruptions of, i. <a href="../../66492/66492-h/66492-h.htm#Page_440">440</a>, <a href="../../66492/66492-h/66492-h.htm#Page_442">442</a>, <a href="../../66492/66492-h/66492-h.htm#Page_445">445</a>, <a href="../../66492/66492-h/66492-h.htm#Page_456">456</a>, <a href="../../66492/66492-h/66492-h.htm#Page_461">461</a><br /> -Bauer, Dr. M., i. <a href="../../66492/66492-h/66492-h.htm#Page_62">62</a><br /> -Bauxite, ii. <a href="#Page_197">197</a>, <a href="#Page_204">204</a><br /> -Bayley, Mr. W. S., ii. <a href="#Page_330">330</a><br /> -Bedding in lavas, i. <a href="../../66492/66492-h/66492-h.htm#Page_24">24</a><br /> -Bell, Sir I. Lowthian, ii. <a href="#Page_1">1</a>, <a href="#Page_113">113</a>, <a href="#Page_137">137</a>, <a href="#Page_165">165</a><br /> -Bemrose, Mr. H. A., ii. <a href="#Page_10">10</a>, <a href="#Page_11">11</a>, <a href="#Page_13">13</a>, <a href="#Page_16">16</a>, <a href="#Page_17">17</a>, <a href="#Page_18">18</a>, <a href="#Page_20">20</a>, <a href="#Page_21">21</a><br /> -Ben Cruachan, alteration of granite at, i. <a href="../../66492/66492-h/66492-h.htm#Page_343">343</a><br /> -—— Hiant, basic sills of, ii. <a href="#Page_318">318</a><br /> -Benaun More, felsite of, i. <a href="../../66492/66492-h/66492-h.htm#Page_347">347</a><br /> -Berger, J. F., ii. <a href="#Page_22">22</a>, <a href="#Page_95">95</a>, <a href="#Page_110">110</a>, <a href="#Page_113">113</a>, <a href="#Page_139">139</a>, <a href="#Page_140">140</a>, <a href="#Page_141">141</a>, <a href="#Page_145">145</a>, <a href="#Page_199">199</a>, <a href="#Page_364">364</a>, <a href="#Page_426">426</a><br /> -Bertrand, Prof. M., i. <a href="../../66492/66492-h/66492-h.htm#Page_28">28</a><br /> -Berwickshire, i. <a href="#Page_272">272</a>, <a href="#Page_290">290</a>, <a href="#Page_338">338</a>, <a href="#Page_375">375</a>, <a href="#Page_385">385</a>, <a href="#Page_401">401</a>, <a href="#Page_413">413</a><br /> -Berwyn Hills, i. <a href="../../66492/66492-h/66492-h.htm#Page_176">176</a>, <a href="../../66492/66492-h/66492-h.htm#Page_186">186</a>, <a href="../../66492/66492-h/66492-h.htm#Page_208">208</a>, <a href="../../66492/66492-h/66492-h.htm#Page_218">218</a><br /> -Biggar, volcanic area, i. <a href="../../66492/66492-h/66492-h.htm#Page_287">287</a>, <a href="../../66492/66492-h/66492-h.htm#Page_325">325</a><br /> -Binney, E., ii. <a href="#Page_56">56</a><br /> -Binny Craig type of basalt, i. <a href="../../66492/66492-h/66492-h.htm#Page_419">419</a>, <a href="../../66492/66492-h/66492-h.htm#Page_421">421</a> (444)<br /> -Biotite (<i>see</i> <a href="#Mica">Mica</a>)<br /> -Bitumen in intrusive rocks, i. <a href="../../66492/66492-h/66492-h.htm#Page_421">421</a><br /> -Blackstone (Derbyshire), ii. <a href="#Page_18">18</a>, <a href="#Page_21">21</a><br /> -Blair-Atholl Limestone, i. <a href="../../66492/66492-h/66492-h.htm#Page_122">122</a><br /> -Blake, Rev. J. F., i. <a href="../../66492/66492-h/66492-h.htm#Page_126">126</a>, <a href="../../66492/66492-h/66492-h.htm#Page_130">130</a>, <a href="../../66492/66492-h/66492-h.htm#Page_144">144</a>, <a href="../../66492/66492-h/66492-h.htm#Page_160">160</a>, <a href="../../66492/66492-h/66492-h.htm#Page_161">161</a>, <a href="../../66492/66492-h/66492-h.htm#Page_162">162</a>, <a href="../../66492/66492-h/66492-h.htm#Page_163">163</a>, <a href="../../66492/66492-h/66492-h.htm#Page_165">165</a>, <a href="../../66492/66492-h/66492-h.htm#Page_166">166</a>, <a href="../../66492/66492-h/66492-h.htm#Page_168">168</a>, <a href="../../66492/66492-h/66492-h.htm#Page_220">220</a>, <a href="../../66492/66492-h/66492-h.htm#Page_221">221</a>, <a href="../../66492/66492-h/66492-h.htm#Page_222">222</a><br /> -Blocks, ejected, i. <a href="../../66492/66492-h/66492-h.htm#Page_36">36</a>, <a href="../../66492/66492-h/66492-h.htm#Page_423">423</a>, <a href="../../66492/66492-h/66492-h.htm#Page_438">438</a>; ii. <a href="#Page_197">197</a>, <a href="#Page_221">221</a><br /> -Bole between lavas, i. 442; ii. <a href="#Page_197">197</a>, <a href="#Page_203">203</a>, <a href="#Page_206">206</a>, <a href="#Page_254">254</a><br /> -Bombay, volcanic plateau of, ii. <a href="#Page_180">180</a><br /> -Bombs, volcanic, i. <a href="../../66492/66492-h/66492-h.htm#Page_60">60</a>; ii. <a href="#Page_39">39</a><br /> -Bonney, Prof., i. <a href="../../66492/66492-h/66492-h.htm#Page_95">95</a>, <a href="../../66492/66492-h/66492-h.htm#Page_126">126</a>, <a href="../../66492/66492-h/66492-h.htm#Page_130">130</a>, <a href="../../66492/66492-h/66492-h.htm#Page_136">136</a>, <a href="../../66492/66492-h/66492-h.htm#Page_144">144</a>, <a href="../../66492/66492-h/66492-h.htm#Page_160">160</a>, <a href="../../66492/66492-h/66492-h.htm#Page_162">162</a>, <a href="../../66492/66492-h/66492-h.htm#Page_163">163</a>, <a href="../../66492/66492-h/66492-h.htm#Page_164">164</a>, <a href="../../66492/66492-h/66492-h.htm#Page_165">165</a>, <a href="../../66492/66492-h/66492-h.htm#Page_166">166</a>, <a href="../../66492/66492-h/66492-h.htm#Page_167">167</a>, <a href="../../66492/66492-h/66492-h.htm#Page_168">168</a>, <a href="../../66492/66492-h/66492-h.htm#Page_192">192</a>, <a href="../../66492/66492-h/66492-h.htm#Page_210">210</a>, <a href="../../66492/66492-h/66492-h.htm#Page_227">227</a><br /> -Borrowdale Volcanic Series, i. <a href="../../66492/66492-h/66492-h.htm#Page_227">227</a><br /> -<a id="Bosses"></a>Bosses, volcanic, i. <a href="../../66492/66492-h/66492-h.htm#Page_56">56</a>, <a href="../../66492/66492-h/66492-h.htm#Page_78">78</a>, <a href="../../66492/66492-h/66492-h.htm#Page_88">88</a>;<br /> -<span style="margin-left: 1em;">petrography of, <a href="../../66492/66492-h/66492-h.htm#Page_89">89</a>;</span><br /> -<span style="margin-left: 1em;">differentiation in, <a href="../../66492/66492-h/66492-h.htm#Page_90">90</a>; ii. <a href="#Page_476">476</a>;</span><br /> -<span style="margin-left: 1em;">granitic, i. <a href="../../66492/66492-h/66492-h.htm#Page_93">93</a>;</span><br /> -<span style="margin-left: 1em;">metamorphism around, <a href="../../66492/66492-h/66492-h.htm#Page_94">94</a>, <a href="../../66492/66492-h/66492-h.htm#Page_95">95</a>;</span><br /> -<span style="margin-left: 1em;">conditions of their intrusion, <a href="../../66492/66492-h/66492-h.htm#Page_97">97</a>, <a href="../../66492/66492-h/66492-h.htm#Page_98">98</a>;</span><br /> -<span style="margin-left: 1em;">weathering of, <a href="../../66492/66492-h/66492-h.htm#Page_102">102</a></span><br /> -—— Silurian, i. <a href="../../66492/66492-h/66492-h.htm#Page_215">215</a>, <a href="../../66492/66492-h/66492-h.htm#Page_235">235</a>;<br /> -<span style="margin-left: 1em;">Old Red Sandstone, <a href="../../66492/66492-h/66492-h.htm#Page_277">277</a>, <a href="../../66492/66492-h/66492-h.htm#Page_288">288</a>;</span><br /> -<span style="margin-left: 1em;">Carboniferous, <a href="../../66492/66492-h/66492-h.htm#Page_403">403</a>, <a href="../../66492/66492-h/66492-h.htm#Page_458">458</a></span><br /> -—— Tertiary, ii. <a href="#Page_271">271</a>, <a href="#Page_284">284</a>, <a href="#Page_327">327</a>, <a href="#Page_366">366</a>, <a href="#Page_378">378</a>, <a href="#Page_395">395</a>, <a href="#Page_403">403</a>;<br /> -<span style="margin-left: 1em;">boundaries of, <a href="#Page_382">382</a>;</span><br /> -<span style="margin-left: 1em;">relation to older eruptive vents, <a href="#Page_280">280</a>, <a href="#Page_384">384</a>, <a href="#Page_399">399</a>;</span><br /> -<span style="margin-left: 1em;">relation to plateau basalts, <a href="#Page_386">386</a>, <a href="#Page_396">396</a>, <a href="#Page_402">402</a>, <a href="#Page_404">404</a>;</span><br /> -<span style="margin-left: 1em;">relation to gabbro intrusions, <a href="#Page_391">391</a>, <a href="#Page_402">402</a>, <a href="#Page_404">404</a>;</span><br /> -<span style="margin-left: 1em;">relation to the basic dykes, <a href="#Page_395">395</a></span><br /> -Bostonite, ii. <a href="#Page_47">47</a><br /> -Boué, Ami, i. <a href="../../66492/66492-h/66492-h.htm#Page_268">268</a>, <a href="../../66492/66492-h/66492-h.htm#Page_363">363</a>; ii. <a href="#Page_112">112</a>, <a href="#Page_372">372</a><br /> -Boule, M., i. <a href="../../66492/66492-h/66492-h.htm#Page_27">27</a>, <a href="../../66492/66492-h/66492-h.htm#Page_29">29</a>, <a href="../../66492/66492-h/66492-h.htm#Page_44">44</a>, <a href="../../66492/66492-h/66492-h.htm#Page_45">45</a>, <a href="../../66492/66492-h/66492-h.htm#Page_46">46</a>, <a href="../../66492/66492-h/66492-h.htm#Page_61">61</a>; ii. <a href="#Page_375">375</a><br /> -Boutan, M., i. <a href="../../66492/66492-h/66492-h.htm#Page_62">62</a><br /> -Bowden Hill, type of doleritic basalt, i. <a href="../../66492/66492-h/66492-h.htm#Page_418">418</a>, <a href="../../66492/66492-h/66492-h.htm#Page_421">421</a><br /> -Braid Hills, great vent of, i. <a href="../../66492/66492-h/66492-h.htm#Page_289">289</a>, <a href="../../66492/66492-h/66492-h.htm#Page_293">293</a>, <a href="../../66492/66492-h/66492-h.htm#Page_311">311</a>, <a href="../../66492/66492-h/66492-h.htm#Page_318">318</a>, <a href="../../66492/66492-h/66492-h.htm#Page_323">323</a><br /> -Branco, Prof. W., i. <a href="../../66492/66492-h/66492-h.htm#Page_46">46</a>, <a href="../../66492/66492-h/66492-h.htm#Page_417">417</a><br /> -Breccias, volcanic, i. <a href="../../66492/66492-h/66492-h.htm#Page_31">31</a>, <a href="../../66492/66492-h/66492-h.htm#Page_32">32</a>, <a href="../../66492/66492-h/66492-h.htm#Page_120">120</a>, <a href="../../66492/66492-h/66492-h.htm#Page_131">131</a>, <a href="../../66492/66492-h/66492-h.htm#Page_135">135</a>, <a href="../../66492/66492-h/66492-h.htm#Page_147">147</a>, <a href="../../66492/66492-h/66492-h.htm#Page_165">165</a>, <a href="../../66492/66492-h/66492-h.htm#Page_189">189</a>, <a href="../../66492/66492-h/66492-h.htm#Page_190">190</a>, <a href="../../66492/66492-h/66492-h.htm#Page_197">197</a>, <a href="../../66492/66492-h/66492-h.htm#Page_213">213</a>, <a href="../../66492/66492-h/66492-h.htm#Page_224">224</a>, <a href="../../66492/66492-h/66492-h.htm#Page_225">225</a>, <a href="../../66492/66492-h/66492-h.htm#Page_233">233</a>, <a href="../../66492/66492-h/66492-h.htm#Page_234">234</a>, <a href="../../66492/66492-h/66492-h.htm#Page_246">246</a>, <a href="../../66492/66492-h/66492-h.htm#Page_252">252</a>, <a href="../../66492/66492-h/66492-h.htm#Page_255">255</a>, <a href="../../66492/66492-h/66492-h.htm#Page_289">289</a>, <a href="../../66492/66492-h/66492-h.htm#Page_347">347</a>; ii. <a href="#Page_39">39</a>, <a href="#Page_41">41</a>, <a href="#Page_49">49</a>, <a href="#Page_195">195</a><br /> -—— of non-volcanic materials, ii. <a href="#Page_196">196</a>, <a href="#Page_423">423</a><br /> -Brecciated structure, i. <a href="../../66492/66492-h/66492-h.htm#Page_162">162</a>, <a href="../../66492/66492-h/66492-h.htm#Page_211">211</a><br /> -Breidden Hills, i. <a href="../../66492/66492-h/66492-h.htm#Page_176">176</a>, <a href="../../66492/66492-h/66492-h.htm#Page_190">190</a>, <a href="../../66492/66492-h/66492-h.htm#Page_208">208</a><br /> -Brent Tor, ii. <a href="#Page_33">33</a>, <a href="#Page_35">35</a>, <a href="#Page_36">36</a><br /> -Bréon, M. R., ii. <a href="#Page_191">191</a><br /> -Britain, advantageous position of, for the study of ancient volcanic action, i. <a href="../../66492/66492-h/66492-h.htm#Page_6">6</a>;<br /> -<span style="margin-left: 1em;">completeness of the Geological Record in, <a href="../../66492/66492-h/66492-h.htm#Page_6">6</a>;</span><br /> -<span style="margin-left: 1em;">direction of folds and fractures in, <a href="../../66492/66492-h/66492-h.htm#Page_11">11</a>;</span><br /> -<span style="margin-left: 1em;">chief lavas found in, <a href="../../66492/66492-h/66492-h.htm#Page_31">31</a>;</span><br /> -<span style="margin-left: 1em;">Vesuvian cones of, <a href="../../66492/66492-h/66492-h.htm#Page_42">42</a>;</span><br /> -<span style="margin-left: 1em;">volcanic plateaux of, <a href="../../66492/66492-h/66492-h.htm#Page_43">43</a>;</span><br /> -<span style="margin-left: 1em;">puys of, <a href="../../66492/66492-h/66492-h.htm#Page_46">46</a>;</span><br /> -<span style="margin-left: 1em;">lacustrine volcanoes of, <a href="../../66492/66492-h/66492-h.htm#Page_49">49</a>;</span><br /> -<span style="margin-left: 1em;">fissure eruptions of, <a href="../../66492/66492-h/66492-h.htm#Page_52">52</a>;</span><br /> -<span style="margin-left: 1em;">scenery of volcanic rocks of, <a href="../../66492/66492-h/66492-h.htm#Page_100">100</a>, <a href="../../66492/66492-h/66492-h.htm#Page_101">101</a>;</span><br /> -<span style="margin-left: 1em;">pre-Cambrian rocks of, <a href="../../66492/66492-h/66492-h.htm#Page_111">111</a>;</span><br /> -<span style="margin-left: 1em;">in Cambrian time, <a href="../../66492/66492-h/66492-h.htm#Page_141">141</a>;</span><br /> -<span style="margin-left: 1em;">in Silurian time, <a href="../../66492/66492-h/66492-h.htm#Page_173">173</a>;</span><br /> -<span style="margin-left: 1em;">in Devonian time, <a href="../../66492/66492-h/66492-h.htm#Page_258">258</a>;</span><br /> -<span style="margin-left: 1em;">in Old Red Sandstone time, <a href="../../66492/66492-h/66492-h.htm#Page_263">263</a>;</span><br /> -<span style="margin-left: 1em;">in Carboniferous time, <a href="../../66492/66492-h/66492-h.htm#Page_355">355</a>;</span><br /> -<span style="margin-left: 1em;">in Permian time, ii. <a href="#Page_53">53</a>;</span><br /> -<span style="margin-left: 1em;">in older Tertiary time, <a href="#Page_108">108</a></span><br /> -Brögger, Prof., i. <a href="../../66492/66492-h/66492-h.htm#Page_28">28</a>, <a href="../../66492/66492-h/66492-h.htm#Page_88">88</a>, <a href="../../66492/66492-h/66492-h.htm#Page_90">90</a>, <a href="../../66492/66492-h/66492-h.htm#Page_91">91</a>, <a href="../../66492/66492-h/66492-h.htm#Page_92">92</a><br /> -Bryce, J., i. <a href="../../66492/66492-h/66492-h.htm#Page_314">314</a>, <a href="../../66492/66492-h/66492-h.htm#Page_369">369</a><br /> -Buch, L. von, i. <a href="../../66492/66492-h/66492-h.htm#Page_27">27</a>; ii. <a href="#Page_381">381</a><br /> -Buckland, W., ii. <a href="#Page_95">95</a>, <a href="#Page_110">110</a>, <a href="#Page_113">113</a><br /> -Buddle, J., ii. <a href="#Page_113">113</a><br /> -Builth, i. <a href="../../66492/66492-h/66492-h.htm#Page_176">176</a>, <a href="../../66492/66492-h/66492-h.htm#Page_203">203</a><br /> -Burdiehouse Limestone, i. <a href="../../66492/66492-h/66492-h.htm#Page_361">361</a>, <a href="../../66492/66492-h/66492-h.htm#Page_374">374</a>, <a href="../../66492/66492-h/66492-h.htm#Page_388">388</a>, <a href="../../66492/66492-h/66492-h.htm#Page_415">415</a>, <a href="../../66492/66492-h/66492-h.htm#Page_463">463</a><br /> -Burnt Country of Asia Minor, i. <a href="../../66492/66492-h/66492-h.htm#Page_2">2</a><br /> -</p> - -<p><span class="pagenum" id="Page_481">- 481 -</span></p> - -<p> -Burntisland, Binn of, i. <a href="../../66492/66492-h/66492-h.htm#Page_428">428</a>, <a href="../../66492/66492-h/66492-h.htm#Page_429">429</a>, <a href="../../66492/66492-h/66492-h.htm#Page_433">433</a>, <a href="../../66492/66492-h/66492-h.htm#Page_435">435</a>, <a href="../../66492/66492-h/66492-h.htm#Page_457">457</a>, <a href="../../66492/66492-h/66492-h.htm#Page_459">459</a><br /> -Burntisland Sill type of dolerite, i. <a href="../../66492/66492-h/66492-h.htm#Page_418">418</a>, <a href="../../66492/66492-h/66492-h.htm#Page_421">421</a><br /> -Busz, Mr. K., i. <a href="../../66492/66492-h/66492-h.htm#Page_261">261</a><br /> -Bute, Isle of, i. <a href="../../66492/66492-h/66492-h.htm#Page_369">369</a>, <a href="../../66492/66492-h/66492-h.htm#Page_378">378</a>, <a href="../../66492/66492-h/66492-h.htm#Page_407">407</a><br /> -<br /> -<a id="C"></a>Cadell, Mr. H. M., i. <a href="../../66492/66492-h/66492-h.htm#Page_114">114</a>, <a href="../../66492/66492-h/66492-h.htm#Page_423">423</a>; ii. <a href="#Page_334">334</a><br /> -Cader Idris, volcanic rocks of, i. <a href="../../66492/66492-h/66492-h.htm#Page_42">42</a>, <a href="../../66492/66492-h/66492-h.htm#Page_175">175</a>, <a href="../../66492/66492-h/66492-h.htm#Page_176">176</a>, <a href="../../66492/66492-h/66492-h.htm#Page_177">177</a>, <a href="../../66492/66492-h/66492-h.htm#Page_178">178</a>, <a href="../../66492/66492-h/66492-h.htm#Page_179">179</a>, <a href="../../66492/66492-h/66492-h.htm#Page_180">180</a>, <a href="../../66492/66492-h/66492-h.htm#Page_181">181</a>, <a href="../../66492/66492-h/66492-h.htm#Page_182">182</a>, <a href="../../66492/66492-h/66492-h.htm#Page_188">188</a>, <a href="../../66492/66492-h/66492-h.htm#Page_207">207</a><br /> -Caer Caradoc, i. <a href="../../66492/66492-h/66492-h.htm#Page_131">131</a>, <a href="../../66492/66492-h/66492-h.htm#Page_132">132</a>, <a href="../../66492/66492-h/66492-h.htm#Page_170">170</a><br /> -Caerfai group (Cambrian), i. <a href="../../66492/66492-h/66492-h.htm#Page_155">155</a><br /> -Caernarvonshire, volcanic rocks of, i. <a href="../../66492/66492-h/66492-h.htm#Page_159">159</a>, <a href="../../66492/66492-h/66492-h.htm#Page_207">207</a><br /> -Caithness Flags, i. <a href="../../66492/66492-h/66492-h.htm#Page_343">343</a>, <a href="../../66492/66492-h/66492-h.htm#Page_352">352</a><br /> -—— volcanic vents in, i. <a href="../../66492/66492-h/66492-h.htm#Page_352">352</a><br /> -Calciferous Sandstones, i. <a href="../../66492/66492-h/66492-h.htm#Page_361">361</a>, <a href="../../66492/66492-h/66492-h.htm#Page_366">366</a>, <a href="../../66492/66492-h/66492-h.htm#Page_415">415</a><br /> -Calcite as a matrix of tuffs, ii. <a href="#Page_27">27</a>, <a href="#Page_39">39</a>, <a href="#Page_41">41</a><br /> -Caldecote volcanic rocks, i. <a href="../../66492/66492-h/66492-h.htm#Page_170">170</a><br /> -Callaway, Dr. C., i. <a href="../../66492/66492-h/66492-h.htm#Page_126">126</a>, <a href="../../66492/66492-h/66492-h.htm#Page_130">130</a>, <a href="../../66492/66492-h/66492-h.htm#Page_132">132</a>, <a href="../../66492/66492-h/66492-h.htm#Page_134">134</a>, <a href="../../66492/66492-h/66492-h.htm#Page_220">220</a>, <a href="../../66492/66492-h/66492-h.htm#Page_221">221</a><br /> -Calton Hill, lavas and tuffs of, i. <a href="../../66492/66492-h/66492-h.htm#Page_373">373</a>, <a href="../../66492/66492-h/66492-h.htm#Page_378">378</a>, <a href="../../66492/66492-h/66492-h.htm#Page_385">385</a>, <a href="../../66492/66492-h/66492-h.htm#Page_386">386</a>, <a href="../../66492/66492-h/66492-h.htm#Page_389">389</a><br /> -Cambrian system, i. <a href="../../66492/66492-h/66492-h.htm#Page_112">112</a>, <a href="../../66492/66492-h/66492-h.htm#Page_123">123</a>, <a href="../../66492/66492-h/66492-h.htm#Page_133">133</a>, <a href="../../66492/66492-h/66492-h.htm#Page_139">139</a>, <a href="../../66492/66492-h/66492-h.htm#Page_143">143</a>, <a href="../../66492/66492-h/66492-h.htm#Page_144">144</a>;<br /> -<span style="margin-left: 1em;">volcanoes of, <a href="../../66492/66492-h/66492-h.htm#Page_145">145</a>, <a href="../../66492/66492-h/66492-h.htm#Page_159">159</a></span><br /> -Campbeltown, volcanic rocks of, i. <a href="../../66492/66492-h/66492-h.htm#Page_312">312</a>, <a href="../../66492/66492-h/66492-h.htm#Page_386">386</a><br /> -Campsie Fells, i. <a href="../../66492/66492-h/66492-h.htm#Page_102">102</a>, <a href="../../66492/66492-h/66492-h.htm#Page_368">368</a>, <a href="../../66492/66492-h/66492-h.htm#Page_369">369</a>, <a href="../../66492/66492-h/66492-h.htm#Page_384">384</a>, <a href="../../66492/66492-h/66492-h.htm#Page_386">386</a>, <a href="../../66492/66492-h/66492-h.htm#Page_389">389</a>, <a href="../../66492/66492-h/66492-h.htm#Page_393">393</a>, <a href="../../66492/66492-h/66492-h.htm#Page_397">397</a>, <a href="../../66492/66492-h/66492-h.htm#Page_398">398</a>, <a href="../../66492/66492-h/66492-h.htm#Page_400">400</a>, <a href="../../66492/66492-h/66492-h.htm#Page_403">403</a>, <a href="../../66492/66492-h/66492-h.htm#Page_410">410</a>, <a href="../../66492/66492-h/66492-h.htm#Page_412">412</a>, <a href="../../66492/66492-h/66492-h.htm#Page_447">447</a><br /> -Canary Islands, i. <a href="../../66492/66492-h/66492-h.htm#Page_27">27</a><br /> -Canna, basalts of, ii. <a href="#Page_184">184</a>, <a href="#Page_187">187</a>, <a href="#Page_190">190</a>, <a href="#Page_215">215</a>, <a href="#Page_216">216</a>;<br /> -<span style="margin-left: 1em;">vent in, <a href="#Page_288">288</a></span><br /> -Cantyre, volcanic rocks of, i. <a href="../../66492/66492-h/66492-h.htm#Page_311">311</a>, <a href="../../66492/66492-h/66492-h.htm#Page_369">369</a>, <a href="../../66492/66492-h/66492-h.htm#Page_370">370</a>, <a href="../../66492/66492-h/66492-h.htm#Page_386">386</a><br /> -Caradoc group, i. <a href="../../66492/66492-h/66492-h.htm#Page_175">175</a>, <a href="../../66492/66492-h/66492-h.htm#Page_196">196</a><br /> -Carbonaceous rocks, influence of, on igneous masses, i. <a href="../../66492/66492-h/66492-h.htm#Page_95">95</a>, <a href="../../66492/66492-h/66492-h.htm#Page_426">426</a>, <a href="../../66492/66492-h/66492-h.htm#Page_449">449</a>, <a href="../../66492/66492-h/66492-h.htm#Page_456">456</a>; ii. <a href="#Page_65">65</a>, <a href="#Page_87">87</a>, <a href="#Page_104">104</a>, <a href="#Page_165">165</a><br /> -Carboniferous Limestone, origin of, i. <a href="../../66492/66492-h/66492-h.htm#Page_357">357</a><br /> -—— system, subdivisions of, in Britain, i. <a href="../../66492/66492-h/66492-h.htm#Page_358">358</a>, <a href="../../66492/66492-h/66492-h.htm#Page_360">360</a>, <a href="../../66492/66492-h/66492-h.htm#Page_366">366</a>;<br /> -<span style="margin-left: 1em;">ancient geography of, <a href="../../66492/66492-h/66492-h.htm#Page_355">355</a>, <a href="../../66492/66492-h/66492-h.htm#Page_361">361</a>, <a href="../../66492/66492-h/66492-h.htm#Page_362">362</a>, <a href="../../66492/66492-h/66492-h.htm#Page_432">432</a>, <a href="../../66492/66492-h/66492-h.htm#Page_462">462</a>;</span><br /> -<span style="margin-left: 1em;">flora and fauna of, <a href="../../66492/66492-h/66492-h.htm#Page_356">356</a></span><br /> -—— volcanic plateaux, distribution of, i. <a href="../../66492/66492-h/66492-h.htm#Page_364">364</a>, <a href="../../66492/66492-h/66492-h.htm#Page_367">367</a>;<br /> -<span style="margin-left: 1em;">nature of materials constituting, <a href="../../66492/66492-h/66492-h.htm#Page_377">377</a>;</span><br /> -<span style="margin-left: 1em;">structure of, <a href="../../66492/66492-h/66492-h.htm#Page_383">383</a>;</span><br /> -<span style="margin-left: 1em;">bedded lavas and tuffs of, <a href="../../66492/66492-h/66492-h.htm#Page_383">383</a>;</span><br /> -<span style="margin-left: 1em;">vents of, <a href="../../66492/66492-h/66492-h.htm#Page_54">54</a>, <a href="../../66492/66492-h/66492-h.htm#Page_394">394</a>, <a href="../../66492/66492-h/66492-h.htm#Page_399">399</a></span><br /> -—— Puys, i. <a href="../../66492/66492-h/66492-h.htm#Page_46">46</a>, <a href="../../66492/66492-h/66492-h.htm#Page_47">47</a>, <a href="../../66492/66492-h/66492-h.htm#Page_308">308</a>, <a href="../../66492/66492-h/66492-h.htm#Page_364">364</a>;<br /> -<span style="margin-left: 1em;">of Scotland, <a href="../../66492/66492-h/66492-h.htm#Page_414">414</a>;</span><br /> -<span style="margin-left: 1em;">nature of the materials erupted by, <a href="../../66492/66492-h/66492-h.htm#Page_416">416</a>;</span><br /> -<span style="margin-left: 1em;">necks of, <a href="../../66492/66492-h/66492-h.htm#Page_424">424</a>;</span><br /> -<span style="margin-left: 1em;">bedded lavas and tuffs of, <a href="../../66492/66492-h/66492-h.htm#Page_417">417</a>, <a href="../../66492/66492-h/66492-h.htm#Page_436">436</a>, <a href="../../66492/66492-h/66492-h.htm#Page_440">440</a>;</span><br /> -<span style="margin-left: 1em;">sills of, <a href="../../66492/66492-h/66492-h.htm#Page_446">446</a>, <a href="../../66492/66492-h/66492-h.htm#Page_472">472</a>;</span><br /> -<span style="margin-left: 1em;">bosses of, <a href="../../66492/66492-h/66492-h.htm#Page_458">458</a>, <a href="../../66492/66492-h/66492-h.htm#Page_465">465</a>;</span><br /> -<span style="margin-left: 1em;">dykes of, <a href="../../66492/66492-h/66492-h.htm#Page_460">460</a>;</span><br /> -<span style="margin-left: 1em;">of Derbyshire, ii. <a href="#Page_8">8</a>;</span><br /> -<span style="margin-left: 1em;">Isle of Man, <a href="#Page_22">22</a>;</span><br /> -<span style="margin-left: 1em;">of Somerset, <a href="#Page_32">32</a>;</span><br /> -<span style="margin-left: 1em;">of Devonshire, <a href="#Page_32">32</a>;</span><br /> -<span style="margin-left: 1em;">of King's County, <a href="#Page_37">37</a>;</span><br /> -<span style="margin-left: 1em;">of Limerick, <a href="#Page_41">41</a></span><br /> -Carlingford, igneous rocks of, i. <a href="../../66492/66492-h/66492-h.htm#Page_96">96</a>; ii. <a href="#Page_175">175</a>, <a href="#Page_371">371</a>, <a href="#Page_420">420</a><br /> -Carnedd Dafydd, i. <a href="../../66492/66492-h/66492-h.htm#Page_209">209</a><br /> -Carnmony Hill, ii. <a href="#Page_272">272</a><br /> -Carrock Fell, differentiation in rocks of, i. <a href="../../66492/66492-h/66492-h.htm#Page_91">91</a>;<br /> -<span style="margin-left: 1em;">metamorphism at, <a href="../../66492/66492-h/66492-h.htm#Page_94">94</a>, <a href="../../66492/66492-h/66492-h.htm#Page_96">96</a>;</span><br /> -<span style="margin-left: 1em;">as a volcanic boss, <a href="../../66492/66492-h/66492-h.htm#Page_235">235</a>, <a href="../../66492/66492-h/66492-h.htm#Page_236">236</a></span><br /> -Cement-stone group, i. <a href="../../66492/66492-h/66492-h.htm#Page_362">362</a>, <a href="../../66492/66492-h/66492-h.htm#Page_366">366</a>, <a href="../../66492/66492-h/66492-h.htm#Page_387">387</a>, <a href="../../66492/66492-h/66492-h.htm#Page_418">418</a>, <a href="../../66492/66492-h/66492-h.htm#Page_462">462</a><br /> -Cellular structure of volcanic rocks, i. <a href="../../66492/66492-h/66492-h.htm#Page_15">15</a>, <a href="../../66492/66492-h/66492-h.htm#Page_33">33</a><br /> -Chalk, metamorphism of, by a dyke, ii. <a href="#Page_164">164</a><br /> -Champernowne, A., i. <a href="../../66492/66492-h/66492-h.htm#Page_260">260</a>, <a href="../../66492/66492-h/66492-h.htm#Page_262">262</a><br /> -Charnwood Forest, i. <a href="../../66492/66492-h/66492-h.htm#Page_134">134</a>; ii. <a href="#Page_53">53</a><br /> -Cherts associated with volcanic rocks, i. <a href="../../66492/66492-h/66492-h.htm#Page_123">123</a>, <a href="../../66492/66492-h/66492-h.htm#Page_167">167</a>, <a href="../../66492/66492-h/66492-h.htm#Page_169">169</a>, <a href="../../66492/66492-h/66492-h.htm#Page_173">173</a>, <a href="../../66492/66492-h/66492-h.htm#Page_174">174</a>, <a href="../../66492/66492-h/66492-h.htm#Page_184">184</a>, <a href="../../66492/66492-h/66492-h.htm#Page_196">196</a>, <a href="../../66492/66492-h/66492-h.htm#Page_197">197</a>, <a href="../../66492/66492-h/66492-h.htm#Page_240">240</a>, <a href="../../66492/66492-h/66492-h.htm#Page_254">254</a>; ii. <a href="#Page_25">25</a>, <a href="#Page_36">36</a><br /> -Cheviot Hills, i. <a href="../../66492/66492-h/66492-h.htm#Page_102">102</a>, <a href="../../66492/66492-h/66492-h.htm#Page_271">271</a>, <a href="../../66492/66492-h/66492-h.htm#Page_272">272</a>, <a href="../../66492/66492-h/66492-h.htm#Page_274">274</a>, <a href="../../66492/66492-h/66492-h.htm#Page_275">275</a>, <a href="../../66492/66492-h/66492-h.htm#Page_277">277</a>, <a href="../../66492/66492-h/66492-h.htm#Page_278">278</a>, <a href="../../66492/66492-h/66492-h.htm#Page_290">290</a>, <a href="../../66492/66492-h/66492-h.htm#Page_293">293</a>, <a href="../../66492/66492-h/66492-h.htm#Page_336">336</a><br /> -Chilled margin in intrusive rocks, i. <a href="../../66492/66492-h/66492-h.htm#Page_81">81</a>, <a href="../../66492/66492-h/66492-h.htm#Page_83">83</a>; ii. <a href="../../66492/66492-h/66492-h.htm#Page_126">126</a>, <a href="../../66492/66492-h/66492-h.htm#Page_158">158</a>, <a href="../../66492/66492-h/66492-h.htm#Page_160">160</a>, <a href="../../66492/66492-h/66492-h.htm#Page_172">172</a>, <a href="../../66492/66492-h/66492-h.htm#Page_299">299</a>, <a href="../../66492/66492-h/66492-h.htm#Page_303">303</a>, <a href="../../66492/66492-h/66492-h.htm#Page_310">310</a>, <a href="../../66492/66492-h/66492-h.htm#Page_317">317</a>, <a href="../../66492/66492-h/66492-h.htm#Page_321">321</a>, <a href="../../66492/66492-h/66492-h.htm#Page_392">392</a>, <a href="../../66492/66492-h/66492-h.htm#Page_402">402</a><br /> -Christiania, eruptive rocks of, i. <a href="../../66492/66492-h/66492-h.htm#Page_28">28</a><br /> -Chronology, volcanic, how determined, i. <a href="../../66492/66492-h/66492-h.htm#Page_46">46</a><br /> -Clark, Mr. G. T., ii. <a href="#Page_180">180</a><br /> -Claystone, i. <a href="../../66492/66492-h/66492-h.htm#Page_277">277</a>, <a href="../../66492/66492-h/66492-h.htm#Page_279">279</a>, <a href="../../66492/66492-h/66492-h.htm#Page_318">318</a>, <a href="../../66492/66492-h/66492-h.htm#Page_324">324</a>, <a href="../../66492/66492-h/66492-h.htm#Page_327">327</a>; ii. <a href="#Page_403">403</a><br /> -Cleavage, effects of, on igneous rocks, i. <a href="../../66492/66492-h/66492-h.htm#Page_162">162</a>, <a href="../../66492/66492-h/66492-h.htm#Page_165">165</a>, <a href="../../66492/66492-h/66492-h.htm#Page_224">224</a>, <a href="../../66492/66492-h/66492-h.htm#Page_231">231</a>, <a href="../../66492/66492-h/66492-h.htm#Page_232">232</a>, <a href="../../66492/66492-h/66492-h.htm#Page_234">234</a>, <a href="../../66492/66492-h/66492-h.htm#Page_237">237</a>, <a href="../../66492/66492-h/66492-h.htm#Page_260">260</a>, <a href="../../66492/66492-h/66492-h.htm#Page_261">261</a>; ii. <a href="#Page_36">36</a><br /> -Clee Hills, ii. <a href="#Page_101">101</a>, <a href="#Page_102">102</a><br /> -Cleveland Dyke, ii. <a href="#Page_1">1</a>, <a href="#Page_122">122</a>, <a href="../../66492/66492-h/66492-h.htm#Page_139">139</a>, <a href="../../66492/66492-h/66492-h.htm#Page_140">140</a>, <a href="../../66492/66492-h/66492-h.htm#Page_142">142</a>, <a href="../../66492/66492-h/66492-h.htm#Page_144">144</a>, <a href="../../66492/66492-h/66492-h.htm#Page_146">146</a>, <a href="../../66492/66492-h/66492-h.htm#Page_147">147</a>, <a href="../../66492/66492-h/66492-h.htm#Page_150">150</a>, <a href="../../66492/66492-h/66492-h.htm#Page_153">153</a>, <a href="../../66492/66492-h/66492-h.htm#Page_167">167</a>, <a href="../../66492/66492-h/66492-h.htm#Page_168">168</a>, <a href="../../66492/66492-h/66492-h.htm#Page_169">169</a><br /> -Clough, Mr. C. T., i. <a href="../../66492/66492-h/66492-h.htm#Page_114">114</a>, <a href="../../66492/66492-h/66492-h.htm#Page_201">201</a>, <a href="../../66492/66492-h/66492-h.htm#Page_236">236</a>, <a href="../../66492/66492-h/66492-h.htm#Page_274">274</a>, <a href="../../66492/66492-h/66492-h.htm#Page_290">290</a>, <a href="../../66492/66492-h/66492-h.htm#Page_337">337</a>; ii. <a href="#Page_123">123</a>, <a href="#Page_124">124</a>, <a href="#Page_127">127</a>, <a href="#Page_128">128</a>, <a href="#Page_132">132</a>, <a href="#Page_137">137</a>, <a href="#Page_138">138</a>, <a href="#Page_142">142</a>, <a href="#Page_145">145</a>, <a href="#Page_146">146</a>, <a href="#Page_152">152</a>, <a href="#Page_162">162</a>, <a href="#Page_171">171</a>, <a href="#Page_172">172</a>, <a href="#Page_316">316</a>, <a href="#Page_384">384</a>, <a href="#Page_437">437</a><br /> -Clyde, Carboniferous volcanic plateau of, i. <a href="../../66492/66492-h/66492-h.htm#Page_368">368</a>, <a href="../../66492/66492-h/66492-h.htm#Page_384">384</a>, <a href="../../66492/66492-h/66492-h.htm#Page_385">385</a>, <a href="../../66492/66492-h/66492-h.htm#Page_393">393</a>, <a href="../../66492/66492-h/66492-h.htm#Page_400">400</a>, <a href="../../66492/66492-h/66492-h.htm#Page_407">407</a>, <a href="../../66492/66492-h/66492-h.htm#Page_411">411</a><br /> -Coal interbedded among volcanic rocks, i. <a href="../../66492/66492-h/66492-h.htm#Page_392">392</a>, <a href="../../66492/66492-h/66492-h.htm#Page_423">423</a>; ii. <a href="#Page_198">198</a>, <a href="#Page_213">213</a>, <a href="#Page_251">251</a>, <a href="#Page_287">287</a><br /> -—— alteration of, at volcanic vents, i. <a href="../../66492/66492-h/66492-h.htm#Page_72">72</a>; ii. <a href="#Page_64">64</a><br /> -—— alteration of intrusive rocks by, i. <a href="../../66492/66492-h/66492-h.htm#Page_95">95</a>, <a href="../../66492/66492-h/66492-h.htm#Page_451">451</a><br /> -—— alteration of by sills, dykes, etc., ii. <a href="#Page_67">67</a>, <a href="#Page_164">164</a>, <a href="#Page_166">166</a><br /> -Coal-measures, i. <a href="../../66492/66492-h/66492-h.htm#Page_358">358</a>, <a href="../../66492/66492-h/66492-h.htm#Page_360">360</a>, <a href="../../66492/66492-h/66492-h.htm#Page_366">366</a><br /> -Coalbrookdale Coal-field, ii. <a href="#Page_103">103</a><br /> -Cole, Prof. G. A., i. <a href="../../66492/66492-h/66492-h.htm#Page_176">176</a>, <a href="../../66492/66492-h/66492-h.htm#Page_177">177</a>, <a href="../../66492/66492-h/66492-h.htm#Page_178">178</a>, <a href="../../66492/66492-h/66492-h.htm#Page_179">179</a>, <a href="../../66492/66492-h/66492-h.htm#Page_180">180</a>, <a href="../../66492/66492-h/66492-h.htm#Page_181">181</a>, <a href="../../66492/66492-h/66492-h.htm#Page_184">184</a>, <a href="../../66492/66492-h/66492-h.htm#Page_187">187</a>, <a href="../../66492/66492-h/66492-h.htm#Page_188">188</a>, <a href="../../66492/66492-h/66492-h.htm#Page_210">210</a>, <a href="../../66492/66492-h/66492-h.htm#Page_211">211</a>; ii. <a href="#Page_134">134</a>, <a href="#Page_205">205</a>, <a href="#Page_212">212</a>, <a href="#Page_245">245</a>, <a href="#Page_370">370</a>, <a href="#Page_371">371</a>, <a href="#Page_378">378</a>, <a href="#Page_426">426</a><br /> -Colorado, Grand Cañon of, i. <a href="../../66492/66492-h/66492-h.htm#Page_30">30</a>;<br /> -<span style="margin-left: 1em;">laccolites of, <a href="../../66492/66492-h/66492-h.htm#Page_86">86</a></span><br /> -Columnar structure, i. <a href="../../66492/66492-h/66492-h.htm#Page_25">25</a>, <a href="../../66492/66492-h/66492-h.htm#Page_27">27</a>, <a href="../../66492/66492-h/66492-h.htm#Page_343">343</a>, <a href="../../66492/66492-h/66492-h.htm#Page_378">378</a>, <a href="../../66492/66492-h/66492-h.htm#Page_385">385</a>, <a href="../../66492/66492-h/66492-h.htm#Page_459">459</a>; ii. <a href="#Page_164">164</a>, <a href="#Page_186">186</a>, <a href="#Page_206">206</a>, <a href="#Page_301">301</a><br /> -Comley Sandstone, i. <a href="../../66492/66492-h/66492-h.htm#Page_144">144</a><br /> -Cones, volcanic, connection of, with necks, i. <a href="../../66492/66492-h/66492-h.htm#Page_70">70</a>;<br /> -<span style="margin-left: 1em;">contemporaneous denudation of, <a href="../../66492/66492-h/66492-h.htm#Page_73">73</a>; ii. <a href="#Page_202">202</a>, <a href="#Page_218">218</a>, <a href="#Page_230">230</a>;</span><br /> -<span style="margin-left: 1em;">entombment of, i. <a href="../../66492/66492-h/66492-h.htm#Page_433">433</a>, <a href="../../66492/66492-h/66492-h.htm#Page_463">463</a>; ii. <a href="#Page_66">66</a></span><br /> -<a id="Conglomerates"></a>Conglomerates, volcanic, i. <a href="../../66492/66492-h/66492-h.htm#Page_31">31</a>, <a href="../../66492/66492-h/66492-h.htm#Page_37">37</a>, <a href="../../66492/66492-h/66492-h.htm#Page_183">183</a>, <a href="../../66492/66492-h/66492-h.htm#Page_190">190</a>, <a href="../../66492/66492-h/66492-h.htm#Page_286">286</a>, <a href="../../66492/66492-h/66492-h.htm#Page_300">300</a>, <a href="../../66492/66492-h/66492-h.htm#Page_307">307</a>, <a href="../../66492/66492-h/66492-h.htm#Page_309">309</a>, <a href="../../66492/66492-h/66492-h.htm#Page_310">310</a>, <a href="../../66492/66492-h/66492-h.htm#Page_314">314</a>, <a href="../../66492/66492-h/66492-h.htm#Page_315">315</a>, <a href="../../66492/66492-h/66492-h.htm#Page_330">330</a>, <a href="../../66492/66492-h/66492-h.htm#Page_341">341</a>; ii. <a href="#Page_195">195</a>, <a href="#Page_198">198</a>, <a href="#Page_218">218</a>, <a href="#Page_284">284</a><br /> -Coniston Limestone, i. <a href="../../66492/66492-h/66492-h.htm#Page_229">229</a>, <a href="../../66492/66492-h/66492-h.htm#Page_231">231</a><br /> -Contemporaneity in Geology, <a href="../../66492/66492-h/66492-h.htm#Page_201">201</a><br /> -Continents, origin of, i. <a href="../../66492/66492-h/66492-h.htm#Page_11">11</a><br /> -Contraction, effects of terrestrial, i. <a href="../../66492/66492-h/66492-h.htm#Page_12">12</a>, <a href="../../66492/66492-h/66492-h.htm#Page_97">97</a>, <a href="../../66492/66492-h/66492-h.htm#Page_98">98</a><br /> -Conybeare, J. J., ii. <a href="#Page_95">95</a><br /> -—— W., i. <a href="../../66492/66492-h/66492-h.htm#Page_171">171</a>; ii. <a href="#Page_9">9</a>, <a href="#Page_95">95</a>, <a href="#Page_110">110</a>, <a href="#Page_113">113</a>, <a href="#Page_199">199</a><br /> -Cooling, effects of, in inducing varieties of texture in igneous rocks, i. <a href="../../66492/66492-h/66492-h.htm#Page_78">78</a>, <a href="../../66492/66492-h/66492-h.htm#Page_79">79</a>, <a href="../../66492/66492-h/66492-h.htm#Page_81">81</a>; ii. <a href="#Page_274">274</a>, <a href="#Page_275">275</a>, <a href="#Page_299">299</a>, <a href="#Page_303">303</a>, <a href="#Page_310">310</a>, <a href="#Page_317">317</a>, <a href="#Page_392">392</a>, <a href="#Page_402">402</a><br /> -Coon Butte, Arizona, i. <a href="../../66492/66492-h/66492-h.htm#Page_58">58</a><br /> -Cork, County, volcanic breccias of, ii. <a href="#Page_49">49</a><br /> -Corndon, sill of, i. <a href="../../66492/66492-h/66492-h.htm#Page_176">176</a>, <a href="../../66492/66492-h/66492-h.htm#Page_189">189</a>, <a href="../../66492/66492-h/66492-h.htm#Page_190">190</a><br /> -Corston Hill, i. <a href="../../66492/66492-h/66492-h.htm#Page_373">373</a>, <a href="../../66492/66492-h/66492-h.htm#Page_386">386</a>, <a href="../../66492/66492-h/66492-h.htm#Page_387">387</a><br /> -Craiglockhart type of dolerite and basalt, i. <a href="../../66492/66492-h/66492-h.htm#Page_418">418</a><br /> -Crater, consolidation of tuff within a, i. <a href="../../66492/66492-h/66492-h.htm#Page_429">429</a><br /> -Crater-lakes, i. <a href="../../66492/66492-h/66492-h.htm#Page_58">58</a>; ii. <a href="#Page_266">266</a>, <a href="#Page_275">275</a><br /> -Cretaceous period, geography of the, ii. <a href="#Page_108">108</a>, <a href="#Page_182">182</a><br /> -Cross Fell, i. <a href="../../66492/66492-h/66492-h.htm#Page_228">228</a>, <a href="../../66492/66492-h/66492-h.htm#Page_229">229</a>, <a href="../../66492/66492-h/66492-h.htm#Page_238">238</a><br /> -Cross, Mr. Whitman, i. <a href="../../66492/66492-h/66492-h.htm#Page_86">86</a><br /> -Crush-conglomerates or breccias, i. <a href="../../66492/66492-h/66492-h.htm#Page_32">32</a>, <a href="../../66492/66492-h/66492-h.htm#Page_220">220</a>, <a href="../../66492/66492-h/66492-h.htm#Page_223">223</a>, <a href="../../66492/66492-h/66492-h.htm#Page_225">225</a>, <a href="../../66492/66492-h/66492-h.htm#Page_244">244</a>; ii. <a href="#Page_281">281</a>, <a href="#Page_347">347</a>, <a href="#Page_352">352</a><br /> -Crushing, mechanical effects of, i. <a href="../../66492/66492-h/66492-h.htm#Page_315">315</a> (<i>see</i> <a href="#Schists">Schist</a>)<br /> -Crust, contraction of the terrestrial, i. <a href="../../66492/66492-h/66492-h.htm#Page_12">12</a>, <a href="../../66492/66492-h/66492-h.htm#Page_97">97</a>, <a href="../../66492/66492-h/66492-h.htm#Page_98">98</a>;<br /> -<span style="margin-left: 1em;">oldest rocks of, <a href="../../66492/66492-h/66492-h.htm#Page_110">110</a>;</span><br /> -<span style="margin-left: 1em;">deformation of, <a href="../../66492/66492-h/66492-h.htm#Page_117">117</a>, <a href="../../66492/66492-h/66492-h.htm#Page_121">121</a>, <a href="../../66492/66492-h/66492-h.htm#Page_264">264</a>, <a href="../../66492/66492-h/66492-h.htm#Page_295">295</a>, <a href="../../66492/66492-h/66492-h.htm#Page_297">297</a></span><br /> -Cryptocrystalline type of basalt, i. <a href="../../66492/66492-h/66492-h.htm#Page_419">419</a><br /> -Crystallites of volcanic rocks, i. <a href="../../66492/66492-h/66492-h.htm#Page_18">18</a><br /> -Crystals, different periods of formation of, in volcanic rocks, i. <a href="../../66492/66492-h/66492-h.htm#Page_19">19</a>, <a href="../../66492/66492-h/66492-h.htm#Page_20">20</a>, <a href="../../66492/66492-h/66492-h.htm#Page_21">21</a>, <a href="../../66492/66492-h/66492-h.htm#Page_421">421</a>; ii. <a href="#Page_128">128</a>, <a href="#Page_131">131</a>, <a href="#Page_134">134</a>, <a href="#Page_135">135</a>;<br /> -<span style="margin-left: 1em;">ejected by volcanic vents, i. <a href="../../66492/66492-h/66492-h.htm#Page_62">62</a>, <a href="../../66492/66492-h/66492-h.htm#Page_178">178</a>, <a href="../../66492/66492-h/66492-h.htm#Page_180">180</a>, <a href="../../66492/66492-h/66492-h.htm#Page_181">181</a>, <a href="../../66492/66492-h/66492-h.htm#Page_195">195</a>, <a href="../../66492/66492-h/66492-h.htm#Page_213">213</a>, <a href="../../66492/66492-h/66492-h.htm#Page_234">234</a>, <a href="../../66492/66492-h/66492-h.htm#Page_245">245</a>; ii. <a href="#Page_27">27</a>, <a href="#Page_49">49</a>, <a href="#Page_58">58</a>, <a href="#Page_79">79</a></span><br /> -</p> - -<p><span class="pagenum" id="Page_482">- 482 -</span></p> - -<p> -Cuillin Hills, scenery of, i. <a href="../../66492/66492-h/66492-h.htm#Page_106">106</a>;<br /> -<span style="margin-left: 1em;">gabbro of, ii. <a href="#Page_329">329</a>, <a href="#Page_361">361</a>;</span><br /> -<span style="margin-left: 1em;">acid rocks of, <a href="#Page_391">391</a></span><br /> -Culots, i. <a href="../../66492/66492-h/66492-h.htm#Page_78">78</a>, <a href="../../66492/66492-h/66492-h.htm#Page_88">88</a><br /> -Culm-measures, ii. <a href="#Page_33">33</a><br /> -Cumbrae Islands, i. <a href="../../66492/66492-h/66492-h.htm#Page_368">368</a>, <a href="../../66492/66492-h/66492-h.htm#Page_369">369</a>, <a href="../../66492/66492-h/66492-h.htm#Page_378">378</a>, <a href="../../66492/66492-h/66492-h.htm#Page_407">407</a><br /> -Cumming, J. G., ii. <a href="#Page_22">22</a><br /> -Cycles, volcanic, i. <a href="../../66492/66492-h/66492-h.htm#Page_27">27</a>, <a href="../../66492/66492-h/66492-h.htm#Page_92">92</a>; ii. <a href="#Page_116">116</a><br /> -<br /> -<a id="D"></a>Dacite, i. <a href="../../66492/66492-h/66492-h.htm#Page_230">230</a>; ii. <a href="#Page_185">185</a><br /> -Dakyns, Mr. J. R., i. <a href="../../66492/66492-h/66492-h.htm#Page_90">90</a>, <a href="../../66492/66492-h/66492-h.htm#Page_229">229</a>, <a href="../../66492/66492-h/66492-h.htm#Page_272">272</a>; ii. <a href="#Page_10">10</a><br /> -Dalmellington, volcanic rocks at, i. <a href="../../66492/66492-h/66492-h.htm#Page_333">333</a>; ii. <a href="#Page_62">62</a><br /> -Dalmeny type of dolerite and basalt, i. <a href="../../66492/66492-h/66492-h.htm#Page_418">418</a>, <a href="../../66492/66492-h/66492-h.htm#Page_420">420</a><br /> -Dalradian rocks, probable crushed necks of, i. <a href="../../66492/66492-h/66492-h.htm#Page_75">75</a>, <a href="../../66492/66492-h/66492-h.htm#Page_125">125</a>;<br /> -<span style="margin-left: 1em;">lavas and sills of, <a href="../../66492/66492-h/66492-h.htm#Page_121">121</a>;</span><br /> -<span style="margin-left: 1em;">green schists of, <a href="../../66492/66492-h/66492-h.htm#Page_124">124</a></span><br /> -Dalry, Ayrshire, buried volcanoes of, i. <a href="../../66492/66492-h/66492-h.htm#Page_434">434</a><br /> -Dana, J. D., ii. <a href="#Page_189">189</a><br /> -Darwin, C., i. <a href="../../66492/66492-h/66492-h.htm#Page_27">27</a><br /> -Daubrée, A., i. <a href="../../66492/66492-h/66492-h.htm#Page_72">72</a>, <a href="../../66492/66492-h/66492-h.htm#Page_404">404</a><br /> -Davies, J., i. <a href="../../66492/66492-h/66492-h.htm#Page_156">156</a>, <a href="../../66492/66492-h/66492-h.htm#Page_157">157</a><br /> -Dechen, H. von, i. <a href="../../66492/66492-h/66492-h.htm#Page_46">46</a>; ii. <a href="#Page_112">112</a>, <a href="#Page_280">280</a>, <a href="#Page_333">333</a>, <a href="#Page_340">340</a>, <a href="#Page_367">367</a>, <a href="#Page_372">372</a>, <a href="#Page_381">381</a><br /> -Deformation, effects of, on volcanic rocks, i. <a href="../../66492/66492-h/66492-h.htm#Page_75">75</a>, <a href="../../66492/66492-h/66492-h.htm#Page_115">115</a>, <a href="../../66492/66492-h/66492-h.htm#Page_117">117</a>, <a href="../../66492/66492-h/66492-h.htm#Page_119">119</a>, <a href="../../66492/66492-h/66492-h.htm#Page_121">121</a>, <a href="../../66492/66492-h/66492-h.htm#Page_127">127</a>, <a href="../../66492/66492-h/66492-h.htm#Page_129">129</a>, <a href="../../66492/66492-h/66492-h.htm#Page_162">162</a><br /> -De la Beche, H., i. <a href="../../66492/66492-h/66492-h.htm#Page_142">142</a>, <a href="../../66492/66492-h/66492-h.htm#Page_143">143</a>, <a href="../../66492/66492-h/66492-h.htm#Page_170">170</a>, <a href="../../66492/66492-h/66492-h.htm#Page_175">175</a>, <a href="../../66492/66492-h/66492-h.htm#Page_204">204</a>, <a href="../../66492/66492-h/66492-h.htm#Page_205">205</a>, <a href="../../66492/66492-h/66492-h.htm#Page_207">207</a>, <a href="../../66492/66492-h/66492-h.htm#Page_259">259</a>; ii. <a href="#Page_9">9</a>, <a href="#Page_10">10</a>, <a href="#Page_19">19</a>, <a href="#Page_33">33</a>, <a href="#Page_95">95</a>, <a href="#Page_96">96</a>, <a href="#Page_97">97</a><br /> -Delessite, ii. <a href="#Page_79">79</a><br /> -Denudation, influence of, on volcanoes, i. <a href="../../66492/66492-h/66492-h.htm#Page_3">3</a>, <a href="../../66492/66492-h/66492-h.htm#Page_4">4</a>, <a href="../../66492/66492-h/66492-h.htm#Page_8">8</a>, <a href="../../66492/66492-h/66492-h.htm#Page_40">40</a>, <a href="../../66492/66492-h/66492-h.htm#Page_43">43</a>, <a href="../../66492/66492-h/66492-h.htm#Page_45">45</a>, <a href="../../66492/66492-h/66492-h.htm#Page_46">46</a>, <a href="../../66492/66492-h/66492-h.htm#Page_54">54</a>, <a href="../../66492/66492-h/66492-h.htm#Page_58">58</a>, <a href="../../66492/66492-h/66492-h.htm#Page_71">71</a>, <a href="../../66492/66492-h/66492-h.htm#Page_73">73</a>, <a href="../../66492/66492-h/66492-h.htm#Page_75">75</a>, <a href="../../66492/66492-h/66492-h.htm#Page_79">79</a>, <a href="../../66492/66492-h/66492-h.htm#Page_87">87</a>, <a href="../../66492/66492-h/66492-h.htm#Page_100">100-107</a>, <a href="../../66492/66492-h/66492-h.htm#Page_370">370</a>, <a href="../../66492/66492-h/66492-h.htm#Page_433">433</a>, <a href="../../66492/66492-h/66492-h.htm#Page_434">434</a>, <a href="../../66492/66492-h/66492-h.htm#Page_436">436</a>, <a href="../../66492/66492-h/66492-h.htm#Page_476">476</a>; ii. <a href="#Page_55">55</a>, <a href="#Page_61">61</a>, <a href="#Page_62">62</a>, <a href="#Page_179">179</a>, <a href="#Page_181">181</a>, <a href="#Page_182">182</a>, <a href="#Page_241">241</a>, <a href="#Page_245">245</a>, <a href="#Page_248">248</a>, <a href="#Page_249">249</a>, <a href="#Page_255">255</a>, <a href="#Page_257">257</a>, <a href="#Page_282">282</a>, <a href="#Page_283">283</a>, <a href="#Page_292">292</a>, <a href="#Page_316">316</a>, <a href="#Page_317">317</a>, <a href="#Page_363">363</a>, <a href="#Page_373">373</a>, <a href="#Page_407">407</a>, <a href="#Page_455">455</a><br /> -Derbyshire, toadstones of, i. <a href="../../66492/66492-h/66492-h.htm#Page_359">359</a>; ii. <a href="#Page_8">8</a><br /> -Desmarest, ii. <a href="#Page_373">373</a><br /> -Devitrification of volcanic rocks, i. <a href="../../66492/66492-h/66492-h.htm#Page_18">18</a>, <a href="../../66492/66492-h/66492-h.htm#Page_19">19</a>, <a href="../../66492/66492-h/66492-h.htm#Page_78">78</a>; ii. <a href="#Page_437">437</a>, <a href="#Page_446">446</a><br /> -Devonian system, i. <a href="../../66492/66492-h/66492-h.htm#Page_257">257</a>;<br /> -<span style="margin-left: 1em;">volcanoes of, <a href="../../66492/66492-h/66492-h.htm#Page_259">259</a></span><br /> -Devonshire, volcanic scenery of, i. <a href="../../66492/66492-h/66492-h.htm#Page_103">103</a>;<br /> -<span style="margin-left: 1em;">Devonian volcanic rocks of, <a href="../../66492/66492-h/66492-h.htm#Page_259">259</a>;</span><br /> -<span style="margin-left: 1em;">Carboniferous volcanic rocks of, ii. <a href="#Page_32">32</a>;</span><br /> -<span style="margin-left: 1em;">Permian volcanic rocks of, <a href="#Page_94">94</a></span><br /> -Diabase, i. <a href="../../66492/66492-h/66492-h.htm#Page_151">151</a>, <a href="../../66492/66492-h/66492-h.htm#Page_153">153</a> (analyses), <a href="../../66492/66492-h/66492-h.htm#Page_156">156</a>, <a href="../../66492/66492-h/66492-h.htm#Page_192">192</a>, <a href="../../66492/66492-h/66492-h.htm#Page_194">194</a>, <a href="../../66492/66492-h/66492-h.htm#Page_204">204</a>, <a href="../../66492/66492-h/66492-h.htm#Page_206">206</a>, <a href="../../66492/66492-h/66492-h.htm#Page_214">214</a>, <a href="../../66492/66492-h/66492-h.htm#Page_217">217</a>, <a href="../../66492/66492-h/66492-h.htm#Page_235">235</a>, <a href="../../66492/66492-h/66492-h.htm#Page_240">240</a>, <a href="../../66492/66492-h/66492-h.htm#Page_247">247</a>, <a href="../../66492/66492-h/66492-h.htm#Page_249">249</a>, <a href="../../66492/66492-h/66492-h.htm#Page_273">273</a> (analyses), <a href="../../66492/66492-h/66492-h.htm#Page_278">278</a>, <a href="../../66492/66492-h/66492-h.htm#Page_292">292</a>, <a href="../../66492/66492-h/66492-h.htm#Page_318">318</a>, <a href="../../66492/66492-h/66492-h.htm#Page_320">320</a>, <a href="../../66492/66492-h/66492-h.htm#Page_330">330</a>, <a href="../../66492/66492-h/66492-h.htm#Page_335">335</a>, <a href="../../66492/66492-h/66492-h.htm#Page_344">344</a>, <a href="../../66492/66492-h/66492-h.htm#Page_345">345</a>, <a href="../../66492/66492-h/66492-h.htm#Page_351">351</a>, <a href="../../66492/66492-h/66492-h.htm#Page_403">403</a>; ii. <a href="#Page_5">5</a>, <a href="#Page_136">136</a>, <a href="#Page_415">415</a><br /> -Diabase-porphyrite, i. <a href="../../66492/66492-h/66492-h.htm#Page_192">192</a>, <a href="../../66492/66492-h/66492-h.htm#Page_204">204</a><br /> -Diamond found in volcanic vents, i. <a href="../../66492/66492-h/66492-h.htm#Page_62">62</a><br /> -Dick, Mr. A., jun., i. <a href="../../66492/66492-h/66492-h.htm#Page_380">380</a><br /> -Dickson, Mr. E., ii. <a href="#Page_23">23</a><br /> -<a id="Differentiation"></a>Differentiation in igneous rocks, i. <a href="../../66492/66492-h/66492-h.htm#Page_22">22</a>, <a href="../../66492/66492-h/66492-h.htm#Page_27">27</a>, <a href="../../66492/66492-h/66492-h.htm#Page_84">84</a>, <a href="../../66492/66492-h/66492-h.htm#Page_85">85</a>, <a href="../../66492/66492-h/66492-h.htm#Page_90">90</a>, <a href="../../66492/66492-h/66492-h.htm#Page_91">91</a>, <a href="../../66492/66492-h/66492-h.htm#Page_449">449</a>; ii. <a href="#Page_300">300</a>, <a href="#Page_475">475</a><br /> -"Dimetian," i. <a href="../../66492/66492-h/66492-h.htm#Page_145">145</a><br /> -Dingle-beds, i. <a href="../../66492/66492-h/66492-h.htm#Page_346">346</a><br /> -Dingle, Upper Silurian nodular lavas of, i. <a href="../../66492/66492-h/66492-h.htm#Page_20">20</a>, <a href="../../66492/66492-h/66492-h.htm#Page_254">254</a><br /> -Diorite, i. <a href="../../66492/66492-h/66492-h.htm#Page_78">78</a>, <a href="../../66492/66492-h/66492-h.htm#Page_247">247</a>, <a href="../../66492/66492-h/66492-h.htm#Page_249">249</a>, <a href="../../66492/66492-h/66492-h.htm#Page_277">277</a>, <a href="../../66492/66492-h/66492-h.htm#Page_278">278</a>, <a href="../../66492/66492-h/66492-h.htm#Page_288">288</a>; ii. <a href="#Page_36">36</a><br /> -Dirrington Law, i. <a href="../../66492/66492-h/66492-h.htm#Page_290">290</a><br /> -Dittmar, Prof., ii. <a href="#Page_137">137</a><br /> -Dolerite, i. <a href="../../66492/66492-h/66492-h.htm#Page_119">119</a>, <a href="../../66492/66492-h/66492-h.htm#Page_134">134</a>, <a href="../../66492/66492-h/66492-h.htm#Page_178">178</a>, <a href="../../66492/66492-h/66492-h.htm#Page_190">190</a>, <a href="../../66492/66492-h/66492-h.htm#Page_206">206</a>, <a href="../../66492/66492-h/66492-h.htm#Page_230">230</a>, <a href="../../66492/66492-h/66492-h.htm#Page_247">247</a>, <a href="../../66492/66492-h/66492-h.htm#Page_261">261</a>, <a href="../../66492/66492-h/66492-h.htm#Page_378">378</a>, <a href="../../66492/66492-h/66492-h.htm#Page_403">403</a>, <a href="../../66492/66492-h/66492-h.htm#Page_407">407</a>, <a href="../../66492/66492-h/66492-h.htm#Page_417">417</a>, <a href="../../66492/66492-h/66492-h.htm#Page_448">448</a>; ii. <a href="#Page_5">5</a>, <a href="#Page_11">11</a>, <a href="#Page_35">35</a>, <a href="#Page_49">49</a>, <a href="#Page_102">102</a>, <a href="#Page_103">103</a>, <a href="#Page_104">104</a>, <a href="#Page_125">125</a>, <a href="#Page_136">136</a>, <a href="#Page_157">157</a>, <a href="#Page_183">183</a>, <a href="#Page_271">271</a>, <a href="#Page_299">299</a>, <a href="#Page_303">303</a>, <a href="#Page_307">307</a>, <a href="#Page_319">319</a>, <a href="#Page_328">328</a><br /> -Dolgelli, i. <a href="../../66492/66492-h/66492-h.htm#Page_169">169</a>, <a href="../../66492/66492-h/66492-h.htm#Page_178">178</a>, <a href="../../66492/66492-h/66492-h.htm#Page_188">188</a><br /> -Donegal, Dalradian rocks of, i. <a href="../../66492/66492-h/66492-h.htm#Page_122">122</a>;<br /> -<span style="margin-left: 1em;">dykes in, ii. <a href="#Page_124">124</a></span><br /> -Drogheda, volcanic rocks near, i. <a href="../../66492/66492-h/66492-h.htm#Page_244">244</a><br /> -Duffin, W. le S., ii. <a href="#Page_426">426</a><br /> -Dumbarton, rocks near, i. <a href="../../66492/66492-h/66492-h.htm#Page_402">402</a>, <a href="../../66492/66492-h/66492-h.htm#Page_404">404</a><br /> -Dumfoyn, a volcanic neck, i. <a href="../../66492/66492-h/66492-h.htm#Page_395">395</a>, <a href="../../66492/66492-h/66492-h.htm#Page_398">398</a>, <a href="../../66492/66492-h/66492-h.htm#Page_400">400</a><br /> -Dumgoyn, a volcanic neck, i. <a href="../../66492/66492-h/66492-h.htm#Page_395">395</a>, <a href="../../66492/66492-h/66492-h.htm#Page_397">397</a>, <a href="../../66492/66492-h/66492-h.htm#Page_398">398</a>, <a href="../../66492/66492-h/66492-h.htm#Page_400">400</a><br /> -Dundee, sills and bosses near, i. <a href="../../66492/66492-h/66492-h.htm#Page_292">292</a>, <a href="../../66492/66492-h/66492-h.htm#Page_306">306</a><br /> -Duneaton Water, volcanic rocks of, i. <a href="../../66492/66492-h/66492-h.htm#Page_329">329</a><br /> -Dunite, ii. <a href="#Page_309">309</a><br /> -Du Noyer, G. V., i. <a href="../../66492/66492-h/66492-h.htm#Page_245">245</a>, <a href="../../66492/66492-h/66492-h.htm#Page_250">250</a>, <a href="../../66492/66492-h/66492-h.htm#Page_254">254</a>; ii. <a href="#Page_272">272</a>, <a href="#Page_426">426</a><br /> -Durham, Mr., i. <a href="../../66492/66492-h/66492-h.htm#Page_275">275</a><br /> -Durness Limestone, i. <a href="../../66492/66492-h/66492-h.htm#Page_112">112</a>, <a href="../../66492/66492-h/66492-h.htm#Page_121">121</a>, <a href="../../66492/66492-h/66492-h.htm#Page_123">123</a>, <a href="../../66492/66492-h/66492-h.htm#Page_141">141</a><br /> -Dust, volcanic, i. <a href="../../66492/66492-h/66492-h.htm#Page_13">13</a><br /> -Dutton, Capt. C. E., i. <a href="../../66492/66492-h/66492-h.htm#Page_68">68</a>; ii. <a href="#Page_267">267</a><br /> -<a id="Dykes"></a>Dykes, vitreous margins of, i. <a href="../../66492/66492-h/66492-h.htm#Page_18">18</a>;<br /> -<span style="margin-left: 1em;">formation of, <a href="../../66492/66492-h/66492-h.htm#Page_54">54</a>, <a href="../../66492/66492-h/66492-h.htm#Page_98">98</a>;</span><br /> -<span style="margin-left: 1em;">in necks, <a href="../../66492/66492-h/66492-h.htm#Page_66">66</a>;</span><br /> -<span style="margin-left: 1em;">filled with agglomerate, <a href="../../66492/66492-h/66492-h.htm#Page_70">70</a>;</span><br /> -<span style="margin-left: 1em;">grouping of, among intrusive rocks, <a href="../../66492/66492-h/66492-h.htm#Page_77">77</a>;</span><br /> -<span style="margin-left: 1em;">character of, <a href="../../66492/66492-h/66492-h.htm#Page_79">79</a>;</span><br /> -<span style="margin-left: 1em;">extent of, in Britain, <a href="../../66492/66492-h/66492-h.htm#Page_80">80</a>;</span><br /> -<span style="margin-left: 1em;">age of, <a href="../../66492/66492-h/66492-h.htm#Page_81">81</a>;</span><br /> -<span style="margin-left: 1em;">compound, <a href="../../66492/66492-h/66492-h.htm#Page_81">81</a>; ii. <a href="#Page_59">59</a>;</span><br /> -<span style="margin-left: 1em;">expulsion of lava from, i. <a href="../../66492/66492-h/66492-h.htm#Page_82">82</a>; ii. <a href="#Page_128">128</a>;</span><br /> -<span style="margin-left: 1em;">connected with the surface, i. <a href="../../66492/66492-h/66492-h.htm#Page_82">82</a>;</span><br /> -<span style="margin-left: 1em;">pre-Cambrian, <a href="../../66492/66492-h/66492-h.htm#Page_118">118</a>;</span><br /> -<span style="margin-left: 1em;">flow-structure in, <a href="../../66492/66492-h/66492-h.htm#Page_161">161</a>;</span><br /> -<span style="margin-left: 1em;">Cambrian, <a href="../../66492/66492-h/66492-h.htm#Page_156">156</a>;</span><br /> -<span style="margin-left: 1em;">Silurian, <a href="../../66492/66492-h/66492-h.htm#Page_187">187</a>, <a href="../../66492/66492-h/66492-h.htm#Page_216">216</a>, <a href="../../66492/66492-h/66492-h.htm#Page_235">235</a>, <a href="../../66492/66492-h/66492-h.htm#Page_237">237</a>, <a href="../../66492/66492-h/66492-h.htm#Page_248">248</a>, <a href="../../66492/66492-h/66492-h.htm#Page_249">249</a>;</span><br /> -<span style="margin-left: 1em;">Old Red Sandstone, <a href="../../66492/66492-h/66492-h.htm#Page_277">277</a>, <a href="../../66492/66492-h/66492-h.htm#Page_291">291</a>, <a href="../../66492/66492-h/66492-h.htm#Page_338">338</a>, <a href="../../66492/66492-h/66492-h.htm#Page_345">345</a>;</span><br /> -<span style="margin-left: 1em;">Carboniferous, <a href="../../66492/66492-h/66492-h.htm#Page_406">406</a>, <a href="../../66492/66492-h/66492-h.htm#Page_429">429</a>, <a href="../../66492/66492-h/66492-h.htm#Page_460">460</a>; ii. <a href="#Page_1">1</a>, <a href="#Page_30">30</a>;</span><br /> -<span style="margin-left: 1em;">Permian, <a href="#Page_83">83</a>;</span><br /> -<span style="margin-left: 1em;">amygdaloidal structure of, <a href="../../66492/66492-h/66492-h.htm#Page_85">85</a></span><br /> -—— Tertiary, ii. <a href="#Page_114">114</a>;<br /> -<span style="margin-left: 1em;">arguments for their geological age, <a href="#Page_118">118</a>, <a href="#Page_125">125</a>, <a href="#Page_171">171</a>;</span><br /> -<span style="margin-left: 1em;">geographical distribution, <a href="#Page_121">121</a>;</span><br /> -<span style="margin-left: 1em;">two types of protrusion of, <a href="#Page_122">122</a>;</span><br /> -<span style="margin-left: 1em;">nature of component rocks of, <a href="#Page_125">125</a>;</span><br /> -<span style="margin-left: 1em;">external character of, <a href="#Page_126">126</a>;</span><br /> -<span style="margin-left: 1em;">classification of basic, <a href="#Page_129">129</a>;</span><br /> -<span style="margin-left: 1em;">enclosed fragments in, <a href="#Page_129">129</a>, <a href="#Page_131">131</a>, <a href="#Page_144">144</a>;</span><br /> -<span style="margin-left: 1em;">porphyritic and amygdaloidal structures of, <a href="#Page_128">128</a>, <a href="#Page_129">129</a>, <a href="#Page_130">130</a>;</span><br /> -<span style="margin-left: 1em;">veins in, <a href="#Page_130">130</a>;</span><br /> -<span style="margin-left: 1em;">joints in, <a href="#Page_132">132</a>, <a href="#Page_166">166</a>;</span><br /> -<span style="margin-left: 1em;">microscopic characters of, <a href="#Page_134">134</a>;</span><br /> -<span style="margin-left: 1em;">chemical characters of, <a href="#Page_137">137</a>, <a href="#Page_139">139</a>;</span><br /> -<span style="margin-left: 1em;">hade of, <a href="#Page_139">139</a>;</span><br /> -<span style="margin-left: 1em;">breadth of, <a href="#Page_139">139</a>;</span><br /> -<span style="margin-left: 1em;">interruptions of, <a href="#Page_142">142</a>;</span><br /> -<span style="margin-left: 1em;">length of, <a href="#Page_142">142</a>;</span><br /> -<span style="margin-left: 1em;">persistence of mineral characters of, <a href="#Page_144">144</a>;</span><br /> -<span style="margin-left: 1em;">direction of, <a href="#Page_145">145</a>, <a href="#Page_159">159</a>;</span><br /> -<span style="margin-left: 1em;">upward termination of, <a href="#Page_147">147</a>;</span><br /> -<span style="margin-left: 1em;">known vertical extension of, <a href="#Page_150">150</a>;</span><br /> -<span style="margin-left: 1em;">evidence of movement of molten rock of, <a href="#Page_151">151</a>;</span><br /> -<span style="margin-left: 1em;">branches and veins from, <a href="#Page_152">152</a>;</span><br /> -<span style="margin-left: 1em;">connection with sills, <a href="#Page_155">155</a>;</span><br /> -<span style="margin-left: 1em;">intersecting, <a href="#Page_158">158</a>;</span><br /> -<span style="margin-left: 1em;">compound or of more than one infilling, <a href="#Page_159">159</a>;</span><br /> -<span style="margin-left: 1em;">double, treble, and multiple, <a href="#Page_160">160</a>, <a href="#Page_318">318</a>, <a href="#Page_417">417</a>, <a href="#Page_439">439</a>;</span><br /> -<span style="margin-left: 1em;">compound, with basic and acid bands, <a href="#Page_161">161</a>, <a href="#Page_435">435</a>;</span><br /> -<span style="margin-left: 1em;">contact metamorphism of, <a href="#Page_163">163</a>;</span><br /> -<span style="margin-left: 1em;">relation of, to geological structure, <a href="#Page_166">166</a>;</span><br /> -<span style="margin-left: 1em;">origin and history of, <a href="#Page_175">175</a>;</span><br /> -<span style="margin-left: 1em;">Icelandic example of, <a href="#Page_261">261</a>;</span><br /> -<span style="margin-left: 1em;">example communicating with cinder cone in Utah, <a href="#Page_268">268</a>;</span><br /> -<span style="margin-left: 1em;">connection of, with surface, <a href="#Page_179">179</a>, <a href="#Page_269">269</a>, <a href="#Page_280">280</a>;</span><br /> -<span style="margin-left: 1em;">latest protrusions of, <a href="#Page_381">381</a>, <a href="#Page_408">408</a>, <a href="#Page_416">416</a>;</span><br /> -<span style="margin-left: 1em;">of granophyre, <a href="#Page_435">435</a>, <a href="#Page_436">436</a>, <a href="#Page_437">437</a>, <a href="#Page_439">439</a></span><br /> -<br /> -<a id="E"></a>Earth, condition of the interior of the, i. <a href="../../66492/66492-h/66492-h.htm#Page_10">10</a>;<br /> -<span style="margin-left: 1em;">fractures in crust of the, <a href="../../66492/66492-h/66492-h.htm#Page_11">11</a></span><br /> -Earthquakes, influence of, on early man, i. <a href="../../66492/66492-h/66492-h.htm#Page_1">1</a>;<br /> -<span style="margin-left: 1em;">transient effects of, <a href="../../66492/66492-h/66492-h.htm#Page_3">3</a>, <a href="../../66492/66492-h/66492-h.htm#Page_8">8</a></span><br /> -East Lothian, trachyte lavas of, i. <a href="../../66492/66492-h/66492-h.htm#Page_24">24</a>;<br /> -<span style="margin-left: 1em;">Carboniferous volcanic plateau of, <a href="../../66492/66492-h/66492-h.htm#Page_370">370</a>, <a href="../../66492/66492-h/66492-h.htm#Page_389">389</a>, <a href="../../66492/66492-h/66492-h.htm#Page_403">403</a>, <a href="../../66492/66492-h/66492-h.htm#Page_409">409</a></span><br /> -Edinburgh, volcanic rocks near, i. <a href="../../66492/66492-h/66492-h.htm#Page_24">24</a>, <a href="../../66492/66492-h/66492-h.htm#Page_102">102</a>, <a href="../../66492/66492-h/66492-h.htm#Page_104">104</a>, <a href="../../66492/66492-h/66492-h.htm#Page_269">269</a>, <a href="../../66492/66492-h/66492-h.htm#Page_273">273</a>, <a href="../../66492/66492-h/66492-h.htm#Page_276">276</a>, <a href="../../66492/66492-h/66492-h.htm#Page_279">279</a>, <a href="../../66492/66492-h/66492-h.htm#Page_281">281</a>, <a href="../../66492/66492-h/66492-h.htm#Page_285">285</a>, <a href="../../66492/66492-h/66492-h.htm#Page_287">287</a>, <a href="../../66492/66492-h/66492-h.htm#Page_289">289</a>, <a href="../../66492/66492-h/66492-h.htm#Page_291">291</a>, <a href="../../66492/66492-h/66492-h.htm#Page_293">293</a>, <a href="../../66492/66492-h/66492-h.htm#Page_311">311</a>, <a href="../../66492/66492-h/66492-h.htm#Page_317">317</a>, <a href="../../66492/66492-h/66492-h.htm#Page_318">318</a>, <a href="../../66492/66492-h/66492-h.htm#Page_323">323</a>, <a href="../../66492/66492-h/66492-h.htm#Page_364">364</a>, <a href="../../66492/66492-h/66492-h.htm#Page_370">370</a>, <a href="../../66492/66492-h/66492-h.htm#Page_373">373</a>, <a href="../../66492/66492-h/66492-h.htm#Page_385">385</a>, <a href="../../66492/66492-h/66492-h.htm#Page_386">386</a>, <a href="../../66492/66492-h/66492-h.htm#Page_387">387</a>, <a href="../../66492/66492-h/66492-h.htm#Page_389">389</a>, <a href="../../66492/66492-h/66492-h.htm#Page_410">410</a>, <a href="../../66492/66492-h/66492-h.htm#Page_420">420</a>, <a href="../../66492/66492-h/66492-h.htm#Page_436">436</a>, <a href="../../66492/66492-h/66492-h.htm#Page_449">449</a><br /> -Egan, Mr. F. W., i. <a href="../../66492/66492-h/66492-h.htm#Page_242">242</a>; ii. <a href="#Page_201">201</a>, <a href="#Page_423">423</a><br /> -Eifel, i. <a href="../../66492/66492-h/66492-h.htm#Page_4">4</a>, <a href="../../66492/66492-h/66492-h.htm#Page_46">46</a>, <a href="../../66492/66492-h/66492-h.htm#Page_58">58</a>, <a href="../../66492/66492-h/66492-h.htm#Page_100">100</a><br /> -Eigg, Isle of, ii. <a href="#Page_115">115</a>;<br /> -<span style="margin-left: 1em;">pitchstone of, <a href="#Page_185">185</a>, <a href="#Page_217">217</a>, <a href="#Page_242">242</a>, <a href="#Page_445">445</a>;</span><br /> -<span style="margin-left: 1em;">brecciated basalt in, <a href="#Page_189">189</a>, <a href="#Page_192">192</a>;</span><br /> -<span style="margin-left: 1em;">basalt plateau of, <a href="#Page_215">215</a>, <a href="#Page_234">234</a>;</span><br /> -<span style="margin-left: 1em;">Scuir of, <a href="#Page_217">217</a>, <a href="#Page_234">234</a>, <a href="#Page_447">447</a>;</span><br /> -<span style="margin-left: 1em;">sills of, <a href="#Page_318">318</a>;</span><br /> -<span style="margin-left: 1em;">acid bosses of, <a href="#Page_403">403</a>;</span><br /> -<span style="margin-left: 1em;">acid sills of, <a href="#Page_431">431</a>;</span><br /> -<span style="margin-left: 1em;">proofs of subsidence at, <a href="#Page_447">447</a>;</span><br /> -<span style="margin-left: 1em;">enormous denudation of, <a href="#Page_239">239</a>, <a href="#Page_447">447</a>, <a href="#Page_458">458</a></span><br /> -</p> - -<p><span class="pagenum" id="Page_483">- 483 -</span></p> - -<p> -Eildon Hills, i. <a href="../../66492/66492-h/66492-h.htm#Page_375">375</a><br /> -Electric Peak, i. <a href="../../66492/66492-h/66492-h.htm#Page_79">79</a>, <a href="../../66492/66492-h/66492-h.htm#Page_82">82</a>, <a href="../../66492/66492-h/66492-h.htm#Page_84">84</a><br /> -Elvans, i. <a href="../../66492/66492-h/66492-h.htm#Page_249">249</a>, <a href="../../66492/66492-h/66492-h.htm#Page_281">281</a><br /> -Engulphment craters, i. <a href="../../66492/66492-h/66492-h.htm#Page_58">58</a><br /> -Ennerdale, granite of, i. <a href="../../66492/66492-h/66492-h.htm#Page_236">236</a><br /> -Enniscorthy, volcanic rocks near, i. <a href="../../66492/66492-h/66492-h.htm#Page_245">245</a><br /> -Environment, influence of, on early man, i. <a href="../../66492/66492-h/66492-h.htm#Page_1">1</a><br /> -Eozoic period, i. <a href="../../66492/66492-h/66492-h.htm#Page_110">110</a><br /> -Epidiorite, i. <a href="../../66492/66492-h/66492-h.htm#Page_118">118</a>, <a href="../../66492/66492-h/66492-h.htm#Page_124">124</a>, <a href="../../66492/66492-h/66492-h.htm#Page_129">129</a>, <a href="../../66492/66492-h/66492-h.htm#Page_184">184</a>, <a href="../../66492/66492-h/66492-h.htm#Page_247">247</a>, <a href="../../66492/66492-h/66492-h.htm#Page_249">249</a><br /> -Erosion, laws of, i. <a href="../../66492/66492-h/66492-h.htm#Page_101">101</a><br /> -Eruptions, transient effects of, i. <a href="../../66492/66492-h/66492-h.htm#Page_3">3</a>;<br /> -<span style="margin-left: 1em;">old submarine, how ascertained, <a href="../../66492/66492-h/66492-h.htm#Page_48">48</a>;</span><br /> -<span style="margin-left: 1em;">lacustrine, <a href="../../66492/66492-h/66492-h.htm#Page_49">49</a>;</span><br /> -<span style="margin-left: 1em;">fluviatile, <a href="../../66492/66492-h/66492-h.htm#Page_49">49</a>;</span><br /> -<span style="margin-left: 1em;">terrestrial, <a href="../../66492/66492-h/66492-h.htm#Page_50">50</a>;</span><br /> -<span style="margin-left: 1em;">evidence of intervals between, <a href="../../66492/66492-h/66492-h.htm#Page_283">283</a>, <a href="../../66492/66492-h/66492-h.htm#Page_287">287</a>, <a href="../../66492/66492-h/66492-h.htm#Page_300">300</a>, <a href="../../66492/66492-h/66492-h.htm#Page_442">442</a>; ii. <a href="#Page_42">42</a>, <a href="#Page_59">59</a>, <a href="#Page_203">203</a>, <a href="#Page_205">205</a>, <a href="#Page_221">221</a>, <a href="#Page_251">251</a>, <a href="#Page_254">254</a>, <a href="#Page_287">287</a></span><br /> -Erzeroum, old volcanoes near, i. <a href="../../66492/66492-h/66492-h.htm#Page_32">32</a><br /> -Eskdale Dyke, ii. <a href="#Page_127">127</a>, <a href="#Page_133">133</a>, <a href="#Page_136">136</a>, <a href="#Page_137">137</a>, <a href="#Page_140">140</a>, <a href="#Page_143">143</a>, <a href="#Page_145">145</a>, <a href="#Page_146">146</a>, <a href="#Page_153">153</a><br /> -—— (Lake District), granite of, i. <a href="../../66492/66492-h/66492-h.htm#Page_236">236</a><br /> -Etheridge, Mr. R, jun., ii. <a href="#Page_24">24</a><br /> -Etna, i. <a href="../../66492/66492-h/66492-h.htm#Page_2">2</a>, <a href="../../66492/66492-h/66492-h.htm#Page_4">4</a>, <a href="../../66492/66492-h/66492-h.htm#Page_10">10</a>, <a href="../../66492/66492-h/66492-h.htm#Page_55">55</a>; ii. <a href="#Page_261">261</a><br /> -Eurite, i. <a href="../../66492/66492-h/66492-h.htm#Page_188">188</a><br /> -Europe, basalt plateaux of north-western, i. <a href="../../66492/66492-h/66492-h.htm#Page_51">51</a>, <a href="../../66492/66492-h/66492-h.htm#Page_52">52</a>; ii. <a href="#Page_181">181</a>;<br /> -<span style="margin-left: 1em;">pre-Cambrian disturbances of north-western, i. <a href="../../66492/66492-h/66492-h.htm#Page_117">117</a></span><br /> -Explosion-craters, i. <a href="../../66492/66492-h/66492-h.htm#Page_58">58</a>; ii. <a href="#Page_266">266</a><br /> -Explosions, volcanic, i. <a href="../../66492/66492-h/66492-h.htm#Page_246">246</a>; ii. <a href="#Page_196">196</a>, <a href="#Page_266">266</a>, <a href="#Page_425">425</a>, <a href="#Page_472">472</a><br /> -Extrusive rocks, defined, i. <a href="../../66492/66492-h/66492-h.htm#Page_14">14</a>;<br /> -<span style="margin-left: 1em;">textures of, <a href="../../66492/66492-h/66492-h.htm#Page_78">78</a></span><br /> -<br /> -<a id="F"></a>Fair Head, sills of, ii. <a href="#Page_301">301</a><br /> -Farey, J., ii. <a href="#Page_9">9</a><br /> -Farne Islands, ii. <a href="#Page_2">2</a><br /> -Faroe Isles, basalt plateaux of, i. <a href="../../66492/66492-h/66492-h.htm#Page_52">52</a>, <a href="../../66492/66492-h/66492-h.htm#Page_102">102</a>; ii. <a href="#Page_191">191</a>, <a href="#Page_192">192</a>, <a href="#Page_194">194</a>, <a href="#Page_256">256</a>;<br /> -<span style="margin-left: 1em;">vents in, i. <a href="../../66492/66492-h/66492-h.htm#Page_63">63</a>; ii. <a href="#Page_293">293</a>;</span><br /> -<span style="margin-left: 1em;">dykes of, <a href="#Page_122">122</a>, <a href="#Page_133">133</a>;</span><br /> -<span style="margin-left: 1em;">tuffs and lignites of, <a href="#Page_258">258</a>;</span><br /> -<span style="margin-left: 1em;">sills of, <a href="#Page_322">322</a>;</span><br /> -<span style="margin-left: 1em;">absence of gabbro bosses in, <a href="#Page_355">355</a>;</span><br /> -<span style="margin-left: 1em;">subsidence of, <a href="#Page_447">447</a>;</span><br /> -<span style="margin-left: 1em;">dip of basalts in, <a href="#Page_448">448</a>;</span><br /> -<span style="margin-left: 1em;">proofs of denudation in, <a href="#Page_458">458</a></span><br /> -Faujas St. Fond, ii. <a href="#Page_109">109</a>, <a href="#Page_112">112</a><br /> -Faults, connexion of volcanic vents with, i. <a href="../../66492/66492-h/66492-h.htm#Page_69">69</a>; ii. <a href="#Page_65">65</a>;<br /> -<span style="margin-left: 1em;">boundary, i. <a href="../../66492/66492-h/66492-h.htm#Page_294">294</a>, <a href="../../66492/66492-h/66492-h.htm#Page_303">303</a>, <a href="../../66492/66492-h/66492-h.htm#Page_305">305</a>, <a href="../../66492/66492-h/66492-h.htm#Page_369">369</a>; ii. <a href="#Page_169">169</a>;</span><br /> -<span style="margin-left: 1em;">effects of, i. <a href="../../66492/66492-h/66492-h.htm#Page_446">446</a>; ii. <a href="#Page_200">200</a>;</span><br /> -<span style="margin-left: 1em;">connection with, dykes, <a href="#Page_168">168</a></span><br /> -—— of Tertiary basalt-plateau, ii. <a href="#Page_452">452</a><br /> -Felsite (Felstone), Torridonian, i. <a href="../../66492/66492-h/66492-h.htm#Page_120">120</a>;<br /> -<span style="margin-left: 1em;">Uriconian, <a href="../../66492/66492-h/66492-h.htm#Page_130">130</a>, <a href="../../66492/66492-h/66492-h.htm#Page_133">133</a>;</span><br /> -<span style="margin-left: 1em;">of Malverns, <a href="../../66492/66492-h/66492-h.htm#Page_134">134</a>;</span><br /> -<span style="margin-left: 1em;">Cambrian, <a href="../../66492/66492-h/66492-h.htm#Page_151">151</a>, <a href="../../66492/66492-h/66492-h.htm#Page_160">160</a>, <a href="../../66492/66492-h/66492-h.htm#Page_161">161</a>, <a href="../../66492/66492-h/66492-h.htm#Page_164">164</a>, <a href="../../66492/66492-h/66492-h.htm#Page_165">165</a>, <a href="../../66492/66492-h/66492-h.htm#Page_167">167</a>, <a href="../../66492/66492-h/66492-h.htm#Page_168">168</a>;</span><br /> -<span style="margin-left: 1em;">Silurian, <a href="../../66492/66492-h/66492-h.htm#Page_184">184</a>, <a href="../../66492/66492-h/66492-h.htm#Page_199">199</a>, <a href="../../66492/66492-h/66492-h.htm#Page_205">205</a>, <a href="../../66492/66492-h/66492-h.htm#Page_206">206</a>, <a href="../../66492/66492-h/66492-h.htm#Page_207">207</a>, <a href="../../66492/66492-h/66492-h.htm#Page_210">210</a>, <a href="../../66492/66492-h/66492-h.htm#Page_212">212</a>, <a href="../../66492/66492-h/66492-h.htm#Page_218">218</a>, <a href="../../66492/66492-h/66492-h.htm#Page_231">231</a>, <a href="../../66492/66492-h/66492-h.htm#Page_232">232</a>, <a href="../../66492/66492-h/66492-h.htm#Page_246">246</a>, <a href="../../66492/66492-h/66492-h.htm#Page_247">247</a>, <a href="../../66492/66492-h/66492-h.htm#Page_252">252</a>, <a href="../../66492/66492-h/66492-h.htm#Page_255">255</a>;</span><br /> -<span style="margin-left: 1em;">Old Red Sandstone, <a href="../../66492/66492-h/66492-h.htm#Page_276">276</a>, <a href="../../66492/66492-h/66492-h.htm#Page_277">277</a>, <a href="../../66492/66492-h/66492-h.htm#Page_291">291</a>, <a href="../../66492/66492-h/66492-h.htm#Page_293">293</a>, <a href="../../66492/66492-h/66492-h.htm#Page_321">321</a>, <a href="../../66492/66492-h/66492-h.htm#Page_327">327</a>, <a href="../../66492/66492-h/66492-h.htm#Page_335">335</a>, <a href="../../66492/66492-h/66492-h.htm#Page_346">346</a>;</span><br /> -<span style="margin-left: 1em;">Carboniferous, ii. <a href="#Page_36">36</a>, <a href="#Page_49">49</a>;</span><br /> -<span style="margin-left: 1em;">Permian, <a href="#Page_85">85</a>;</span><br /> -<span style="margin-left: 1em;">Tertiary, <a href="#Page_174">174</a>, <a href="#Page_369">369</a>, <a href="#Page_424">424</a>, <a href="#Page_446">446</a></span><br /> -Felsitic breccia, ii. <a href="#Page_195">195</a><br /> -—— type of devitrification, i. <a href="../../66492/66492-h/66492-h.htm#Page_19">19</a><br /> -Felspar, ejected crystals of, i. <a href="../../66492/66492-h/66492-h.htm#Page_181">181</a>; ii. <a href="#Page_58">58</a>, <a href="#Page_79">79</a>;<br /> -<span style="margin-left: 1em;">large porphyritic crystals of, in dykes, <a href="#Page_129">129</a>, <a href="#Page_135">135</a></span><br /> -Fife, Old Red Sandstone volcanic rocks of, i. <a href="../../66492/66492-h/66492-h.htm#Page_307">307</a>;<br /> -<span style="margin-left: 1em;">Carboniferous volcanic rocks of, <a href="../../66492/66492-h/66492-h.htm#Page_428">428</a>, <a href="../../66492/66492-h/66492-h.htm#Page_429">429</a>, <a href="../../66492/66492-h/66492-h.htm#Page_430">430</a>, <a href="../../66492/66492-h/66492-h.htm#Page_433">433</a>, <a href="../../66492/66492-h/66492-h.htm#Page_437">437</a>, <a href="../../66492/66492-h/66492-h.htm#Page_448">448</a>;</span><br /> -<span style="margin-left: 1em;">Permian volcanic rocks of, ii. <a href="#Page_56">56</a>, <a href="#Page_69">69</a></span><br /> -Fingal's Cave, i. <a href="../../66492/66492-h/66492-h.htm#Page_25">25</a>; ii. <a href="#Page_210">210</a><br /> -Fisher, Rev. O., i. <a href="../../66492/66492-h/66492-h.htm#Page_98">98</a><br /> -Fishguard, volcanic rocks at, i. <a href="../../66492/66492-h/66492-h.htm#Page_205">205</a><br /> -Fissure type of volcanoes, i. <a href="../../66492/66492-h/66492-h.htm#Page_42">42</a>, <a href="../../66492/66492-h/66492-h.htm#Page_52">52</a>; ii. <a href="#Page_108">108</a>, <a href="#Page_115">115</a>, <a href="#Page_267">267</a><br /> -Fissures, volcanic, i. <a href="../../66492/66492-h/66492-h.htm#Page_42">42</a>, <a href="../../66492/66492-h/66492-h.htm#Page_52">52</a>, <a href="../../66492/66492-h/66492-h.htm#Page_53">53</a>, <a href="../../66492/66492-h/66492-h.htm#Page_54">54</a>, <a href="../../66492/66492-h/66492-h.htm#Page_425">425</a>; ii. <a href="#Page_141">141</a>, <a href="#Page_145">145</a>, <a href="#Page_159">159</a>, <a href="#Page_176">176</a>;<br /> -<span style="margin-left: 1em;">filled with agglomerate, i. <a href="../../66492/66492-h/66492-h.htm#Page_70">70</a>;</span><br /> -<span style="margin-left: 1em;">filled with dykes, <a href="../../66492/66492-h/66492-h.htm#Page_81">81</a>, <a href="../../66492/66492-h/66492-h.htm#Page_118">118</a>;</span><br /> -<span style="margin-left: 1em;">compound, <a href="../../66492/66492-h/66492-h.htm#Page_82">82</a>, ii. <a href="#Page_159">159</a>;</span><br /> -<span style="margin-left: 1em;">pre-Cambrian, i. <a href="../../66492/66492-h/66492-h.htm#Page_118">118</a>, <a href="../../66492/66492-h/66492-h.htm#Page_119">119</a>;</span><br /> -<span style="margin-left: 1em;">Carboniferous, <a href="../../66492/66492-h/66492-h.htm#Page_425">425</a>;</span><br /> -<span style="margin-left: 1em;">Tertiary, ii. <a href="#Page_141">141</a>, <a href="#Page_159">159</a>, <a href="#Page_176">176</a>, <a href="#Page_425">425</a>;</span><br /> -<span style="margin-left: 1em;">modern of Iceland, <a href="#Page_262">262</a>;</span><br /> -<span style="margin-left: 1em;">cause of, <a href="#Page_177">177</a></span><br /> -Fleming, John, i. <a href="../../66492/66492-h/66492-h.htm#Page_268">268</a><br /> -Flow-structure, i. <a href="../../66492/66492-h/66492-h.htm#Page_21">21</a>, <a href="../../66492/66492-h/66492-h.htm#Page_157">157</a>, <a href="../../66492/66492-h/66492-h.htm#Page_160">160</a>, <a href="../../66492/66492-h/66492-h.htm#Page_161">161</a>, <a href="../../66492/66492-h/66492-h.htm#Page_162">162</a>, <a href="../../66492/66492-h/66492-h.htm#Page_184">184</a>, <a href="../../66492/66492-h/66492-h.htm#Page_210">210</a>, <a href="../../66492/66492-h/66492-h.htm#Page_232">232</a>, <a href="../../66492/66492-h/66492-h.htm#Page_246">246</a>, <a href="../../66492/66492-h/66492-h.htm#Page_248">248</a>, <a href="../../66492/66492-h/66492-h.htm#Page_255">255</a>, <a href="../../66492/66492-h/66492-h.htm#Page_315">315</a>, <a href="../../66492/66492-h/66492-h.htm#Page_321">321</a>, <a href="../../66492/66492-h/66492-h.htm#Page_327">327</a>, <a href="../../66492/66492-h/66492-h.htm#Page_346">346</a>; ii. <a href="#Page_129">129</a>, <a href="#Page_152">152</a>, <a href="#Page_190">190</a>, <a href="#Page_191">191</a>, <a href="#Page_332">332</a>, <a href="#Page_369">369</a>, <a href="#Page_392">392</a>, <a href="#Page_402">402</a>, <a href="#Page_424">424</a>, <a href="#Page_437">437</a>, <a href="#Page_441">441</a><br /> -Foot, F. J., i. <a href="../../66492/66492-h/66492-h.htm#Page_316">316</a><br /> -Forbes, D., ii. <a href="#Page_370">370</a><br /> -—— Edward, ii. <a href="#Page_66">66</a>, <a href="#Page_113">113</a>, <a href="#Page_114">114</a>, <a href="#Page_198">198</a><br /> -—— J. D., ii. <a href="#Page_112">112</a>, <a href="#Page_333">333</a>, <a href="#Page_372">372</a>, <a href="#Page_381">381</a><br /> -Forellenstein, ii. <a href="#Page_332">332</a><br /> -Forest of Wyre coal-field, ii. <a href="#Page_102">102</a><br /> -Forfarshire, volcanic rocks of, i. <a href="../../66492/66492-h/66492-h.htm#Page_285">285</a>, <a href="../../66492/66492-h/66492-h.htm#Page_299">299</a>;<br /> -<span style="margin-left: 1em;">flagstones of, <a href="../../66492/66492-h/66492-h.htm#Page_299">299</a></span><br /> -Forster, M., ii. <a href="#Page_113">113</a><br /> -Forth-basin, Carboniferous system of, i. <a href="../../66492/66492-h/66492-h.htm#Page_361">361</a>;<br /> -<span style="margin-left: 1em;">Carboniferous plateaux of, <a href="../../66492/66492-h/66492-h.htm#Page_370">370</a>;</span><br /> -<span style="margin-left: 1em;">Carboniferous puys of, <a href="../../66492/66492-h/66492-h.htm#Page_416">416</a>, <a href="../../66492/66492-h/66492-h.htm#Page_427">427</a>, <a href="../../66492/66492-h/66492-h.htm#Page_429">429</a>, <a href="../../66492/66492-h/66492-h.htm#Page_430">430</a>, <a href="../../66492/66492-h/66492-h.htm#Page_432">432</a>, <a href="../../66492/66492-h/66492-h.htm#Page_434">434</a>, <a href="../../66492/66492-h/66492-h.htm#Page_437">437</a>, <a href="../../66492/66492-h/66492-h.htm#Page_440">440</a>, <a href="../../66492/66492-h/66492-h.htm#Page_446">446</a>, <a href="../../66492/66492-h/66492-h.htm#Page_462">462</a>;</span><br /> -<span style="margin-left: 1em;">Permian volcanoes of, ii. <a href="#Page_55">55</a>, <a href="#Page_67">67</a></span><br /> -Foster, Mr. C. le Neve, ii. <a href="#Page_10">10</a><br /> -Fouqué, Prof., i. <a href="../../66492/66492-h/66492-h.htm#Page_18">18</a>, <a href="../../66492/66492-h/66492-h.htm#Page_21">21</a>; ii. <a href="#Page_134">134</a><br /> -Fox, Mr. Howard, ii. <a href="#Page_36">36</a><br /> -Fox Strangways, Mr. C., i, <a href="../../66492/66492-h/66492-h.htm#Page_135">135</a><br /> -Fragmental volcanic rocks, i. <a href="../../66492/66492-h/66492-h.htm#Page_14">14</a>;<br /> -<span style="margin-left: 1em;">only arise from explosions which reach the surface, <a href="../../66492/66492-h/66492-h.htm#Page_57">57</a> (<i>see</i> <a href="#Agglomerates">Agglomerates</a>, <a href="#Conglomerates">Conglomerates</a>, <a href="#Tuffs">Tuffs</a>)</span><br /> -France, Tertiary volcanoes of Central, i. <a href="../../66492/66492-h/66492-h.htm#Page_4">4</a>, <a href="../../66492/66492-h/66492-h.htm#Page_10">10</a>, <a href="../../66492/66492-h/66492-h.htm#Page_29">29</a>, <a href="../../66492/66492-h/66492-h.htm#Page_41">41</a>, <a href="../../66492/66492-h/66492-h.htm#Page_45">45</a>, <a href="../../66492/66492-h/66492-h.htm#Page_49">49</a>, <a href="../../66492/66492-h/66492-h.htm#Page_58">58</a>, <a href="../../66492/66492-h/66492-h.htm#Page_60">60</a>, <a href="../../66492/66492-h/66492-h.htm#Page_70">70</a>; ii. <a href="#Page_31">31</a>, <a href="#Page_271">271</a>, <a href="#Page_281">281</a>, <a href="#Page_373">373</a>;<br /> -<span style="margin-left: 1em;">Carboniferous volcanic action in, i. <a href="../../66492/66492-h/66492-h.htm#Page_357">357</a></span><br /> -Frankland, Prof. E., i. <a href="../../66492/66492-h/66492-h.htm#Page_273">273</a>, <a href="../../66492/66492-h/66492-h.htm#Page_278">278</a><br /> -Fundamental complex of oldest gneiss, i. <a href="../../66492/66492-h/66492-h.htm#Page_114">114</a>, <a href="../../66492/66492-h/66492-h.htm#Page_115">115</a><br /> -—— gneiss, i. <a href="../../66492/66492-h/66492-h.htm#Page_115">115</a><br /> -<br /> -<a id="G"></a>Gabbro, granulitic, ii. <a href="#Page_329">329</a>;<br /> -<span style="margin-left: 1em;">banded structure of, i. <a href="../../66492/66492-h/66492-h.htm#Page_116">116</a>; ii. <a href="#Page_329">329</a>, <a href="#Page_354">354</a>, <a href="#Page_357">357</a>, <a href="#Page_476">476</a>;</span><br /> -<span style="margin-left: 1em;">coarse-grained massive, <a href="#Page_330">330</a>;</span><br /> -<span style="margin-left: 1em;">pale varieties in veins, <a href="#Page_330">330</a>;</span><br /> -<span style="margin-left: 1em;">gneiss-like aspect of, <a href="#Page_342">342</a>, <a href="#Page_254">254</a>, <a href="#Page_358">358</a></span><br /> -—— of Carrock Fell, i. <a href="../../66492/66492-h/66492-h.htm#Page_91">91</a>;<br /> -<span style="margin-left: 1em;">Silurian, <a href="../../66492/66492-h/66492-h.htm#Page_195">195</a>, <a href="../../66492/66492-h/66492-h.htm#Page_206">206</a>, <a href="../../66492/66492-h/66492-h.htm#Page_247">247</a>;</span><br /> -<span style="margin-left: 1em;">Devonian, <a href="../../66492/66492-h/66492-h.htm#Page_262">262</a>;</span><br /> -<span style="margin-left: 1em;">Tertiary, <a href="../../66492/66492-h/66492-h.htm#Page_84">84</a>, <a href="../../66492/66492-h/66492-h.htm#Page_90">90</a>, <a href="../../66492/66492-h/66492-h.htm#Page_116">116</a>; ii. <a href="#Page_307">307</a>, <a href="#Page_308">308</a>, <a href="#Page_309">309</a>, <a href="#Page_319">319</a>, <a href="#Page_327">327</a>, <a href="#Page_334">334</a>, <a href="#Page_349">349</a>, <a href="#Page_355">355</a>, <a href="#Page_358">358</a>, <a href="#Page_391">391</a>, <a href="#Page_406">406</a>, <a href="#Page_407">407</a></span><br /> -Gairloch, peculiar pre-Cambrian rocks of, i. <a href="../../66492/66492-h/66492-h.htm#Page_115">115</a>, <a href="../../66492/66492-h/66492-h.htm#Page_117">117</a><br /> -Galapagos Islands, i. <a href="../../66492/66492-h/66492-h.htm#Page_27">27</a><br /> -Gallaston type of dolerite and basalt, i. <a href="../../66492/66492-h/66492-h.htm#Page_418">418</a><br /> -Galloway, granites of, i. <a href="../../66492/66492-h/66492-h.htm#Page_93">93</a>, <a href="../../66492/66492-h/66492-h.htm#Page_95">95</a>, <a href="../../66492/66492-h/66492-h.htm#Page_272">272</a>, <a href="../../66492/66492-h/66492-h.htm#Page_277">277</a>, <a href="../../66492/66492-h/66492-h.htm#Page_290">290</a>, <a href="../../66492/66492-h/66492-h.htm#Page_331">331</a><br /> -Garabol Hill, differentiation at, i. <a href="../../66492/66492-h/66492-h.htm#Page_90">90</a><br /> -Gardiner, Miss, i. <a href="../../66492/66492-h/66492-h.htm#Page_95">95</a><br /> -Gardiner, Mr. C. J., i. <a href="../../66492/66492-h/66492-h.htm#Page_256">256</a><br /> -Gardner, Mr. Starkie, ii. <a href="#Page_196">196</a>, <a href="#Page_198">198</a>, <a href="#Page_212">212</a><br /> -Garlton Hills, i. <a href="../../66492/66492-h/66492-h.htm#Page_102">102</a>, <a href="../../66492/66492-h/66492-h.htm#Page_370">370</a>, <a href="../../66492/66492-h/66492-h.htm#Page_377">377</a>, <a href="../../66492/66492-h/66492-h.htm#Page_378">378</a>, <a href="../../66492/66492-h/66492-h.htm#Page_379">379</a>, <a href="../../66492/66492-h/66492-h.htm#Page_380">380</a>, <a href="../../66492/66492-h/66492-h.htm#Page_386">386</a>, <a href="../../66492/66492-h/66492-h.htm#Page_390">390</a>, <a href="../../66492/66492-h/66492-h.htm#Page_405">405</a>, <a href="../../66492/66492-h/66492-h.htm#Page_412">412</a><br /> -Garnet found in volcanic vents, i. <a href="../../66492/66492-h/66492-h.htm#Page_62">62</a><br /> -Garth Grit, i. <a href="../../66492/66492-h/66492-h.htm#Page_177">177</a>, <a href="../../66492/66492-h/66492-h.htm#Page_185">185</a>, <a href="../../66492/66492-h/66492-h.htm#Page_208">208</a><br /> -Gases dissolved in the volcanic magma, i. <a href="../../66492/66492-h/66492-h.htm#Page_13">13</a>, <a href="../../66492/66492-h/66492-h.htm#Page_15">15</a>, <a href="../../66492/66492-h/66492-h.htm#Page_72">72</a>, <a href="../../66492/66492-h/66492-h.htm#Page_97">97</a>, <a href="../../66492/66492-h/66492-h.htm#Page_99">99</a><br /> -Geikie, Prof. J., i. <a href="../../66492/66492-h/66492-h.htm#Page_277">277</a>, <a href="../../66492/66492-h/66492-h.htm#Page_306">306</a>, <a href="../../66492/66492-h/66492-h.htm#Page_308">308</a>, <a href="../../66492/66492-h/66492-h.htm#Page_331">331</a>, <a href="../../66492/66492-h/66492-h.htm#Page_336">336</a>, <a href="../../66492/66492-h/66492-h.htm#Page_339">339</a>, <a href="../../66492/66492-h/66492-h.htm#Page_340">340</a>, <a href="../../66492/66492-h/66492-h.htm#Page_369">369</a>, <a href="../../66492/66492-h/66492-h.htm#Page_375">375</a>, <a href="../../66492/66492-h/66492-h.htm#Page_426">426</a>; ii. <a href="#Page_57">57</a>, <a href="#Page_191">191</a>, <a href="#Page_259">259</a>, <a href="#Page_322">322</a><br /> -Genèvre, Mont, i. <a href="../../66492/66492-h/66492-h.htm#Page_194">194</a><br /> -Geological action, supposed former greater intensity of, i, <a href="../../66492/66492-h/66492-h.htm#Page_139">139</a><br /> -—— contrasts, i. <a href="../../66492/66492-h/66492-h.htm#Page_103">103</a><br /> -—— history, i. <a href="../../66492/66492-h/66492-h.htm#Page_109">109</a>, <a href="../../66492/66492-h/66492-h.htm#Page_113">113</a><br /> -—— Survey of Great Britain, i. <a href="../../66492/66492-h/66492-h.htm#Page_113">113</a>, <a href="../../66492/66492-h/66492-h.htm#Page_115">115</a>, <a href="../../66492/66492-h/66492-h.htm#Page_118">118</a>, <a href="../../66492/66492-h/66492-h.htm#Page_119">119</a>, <a href="../../66492/66492-h/66492-h.htm#Page_121">121</a>, <a href="../../66492/66492-h/66492-h.htm#Page_122">122</a>, <a href="../../66492/66492-h/66492-h.htm#Page_123">123</a>, <a href="../../66492/66492-h/66492-h.htm#Page_124">124</a>, <a href="../../66492/66492-h/66492-h.htm#Page_125">125</a>, <a href="../../66492/66492-h/66492-h.htm#Page_126">126</a>, <a href="../../66492/66492-h/66492-h.htm#Page_129">129</a>, <a href="../../66492/66492-h/66492-h.htm#Page_130">130</a>, <a href="../../66492/66492-h/66492-h.htm#Page_133">133</a>, <a href="../../66492/66492-h/66492-h.htm#Page_135">135</a>, <a href="../../66492/66492-h/66492-h.htm#Page_142">142</a>, <a href="../../66492/66492-h/66492-h.htm#Page_143">143</a>, <a href="../../66492/66492-h/66492-h.htm#Page_144">144</a>, <a href="../../66492/66492-h/66492-h.htm#Page_145">145</a>, <a href="../../66492/66492-h/66492-h.htm#Page_159">159</a>, <a href="../../66492/66492-h/66492-h.htm#Page_160">160</a>, <a href="../../66492/66492-h/66492-h.htm#Page_166">166</a>, <a href="../../66492/66492-h/66492-h.htm#Page_170">170</a>, <a href="../../66492/66492-h/66492-h.htm#Page_171">171</a>, <a href="../../66492/66492-h/66492-h.htm#Page_175">175</a>, <a href="../../66492/66492-h/66492-h.htm#Page_176">176</a>, <a href="../../66492/66492-h/66492-h.htm#Page_179">179</a>, <a href="../../66492/66492-h/66492-h.htm#Page_181">181</a>, <a href="../../66492/66492-h/66492-h.htm#Page_182">182</a>, <a href="../../66492/66492-h/66492-h.htm#Page_183">183</a>, <a href="../../66492/66492-h/66492-h.htm#Page_186">186</a>, <a href="../../66492/66492-h/66492-h.htm#Page_187">187</a>, <a href="../../66492/66492-h/66492-h.htm#Page_188">188</a>, <a href="../../66492/66492-h/66492-h.htm#Page_190">190</a>, <a href="../../66492/66492-h/66492-h.htm#Page_196">196</a>, <a href="../../66492/66492-h/66492-h.htm#Page_198">198</a>, <a href="../../66492/66492-h/66492-h.htm#Page_201">201</a>, <a href="../../66492/66492-h/66492-h.htm#Page_204">204</a>, <a href="../../66492/66492-h/66492-h.htm#Page_205">205</a>, <a href="../../66492/66492-h/66492-h.htm#Page_207">207</a>, <a href="../../66492/66492-h/66492-h.htm#Page_208">208</a>, <a href="../../66492/66492-h/66492-h.htm#Page_212">212</a>, <a href="../../66492/66492-h/66492-h.htm#Page_214">214</a>, <a href="../../66492/66492-h/66492-h.htm#Page_215">215</a>, <a href="../../66492/66492-h/66492-h.htm#Page_216">216</a>, <a href="../../66492/66492-h/66492-h.htm#Page_217">217</a>, <a href="../../66492/66492-h/66492-h.htm#Page_218">218</a>, <a href="../../66492/66492-h/66492-h.htm#Page_219">219</a>, <a href="../../66492/66492-h/66492-h.htm#Page_220">220</a>, <a href="../../66492/66492-h/66492-h.htm#Page_221">221</a>, <a href="../../66492/66492-h/66492-h.htm#Page_225">225</a>, <a href="../../66492/66492-h/66492-h.htm#Page_227">227</a>, <a href="../../66492/66492-h/66492-h.htm#Page_228">228</a>, <a href="../../66492/66492-h/66492-h.htm#Page_232">232</a>, <a href="../../66492/66492-h/66492-h.htm#Page_233">233</a>, <a href="../../66492/66492-h/66492-h.htm#Page_238">238</a>, <a href="../../66492/66492-h/66492-h.htm#Page_239">239</a>, <a href="../../66492/66492-h/66492-h.htm#Page_240">240</a>, <a href="../../66492/66492-h/66492-h.htm#Page_242">242</a>, <a href="../../66492/66492-h/66492-h.htm#Page_243">243</a>, <a href="../../66492/66492-h/66492-h.htm#Page_244">244</a>, <a href="../../66492/66492-h/66492-h.htm#Page_245">245</a>, <a href="../../66492/66492-h/66492-h.htm#Page_250">250</a>, <a href="../../66492/66492-h/66492-h.htm#Page_251">251</a>, <a href="../../66492/66492-h/66492-h.htm#Page_254">254</a>, <a href="../../66492/66492-h/66492-h.htm#Page_259">259</a>, <a href="../../66492/66492-h/66492-h.htm#Page_270">270</a>, <a href="../../66492/66492-h/66492-h.htm#Page_275">275</a>, <a href="../../66492/66492-h/66492-h.htm#Page_278">278</a>, <a href="../../66492/66492-h/66492-h.htm#Page_294">294</a>, <a href="../../66492/66492-h/66492-h.htm#Page_299">299</a>, <a href="../../66492/66492-h/66492-h.htm#Page_306">306</a>, <a href="../../66492/66492-h/66492-h.htm#Page_307">307</a>, <a href="../../66492/66492-h/66492-h.htm#Page_308">308</a>, <a href="../../66492/66492-h/66492-h.htm#Page_314">314</a>, <a href="../../66492/66492-h/66492-h.htm#Page_315">315</a>, <a href="../../66492/66492-h/66492-h.htm#Page_317">317</a>, <a href="../../66492/66492-h/66492-h.htm#Page_318">318</a>, <a href="../../66492/66492-h/66492-h.htm#Page_325">325</a>, <a href="../../66492/66492-h/66492-h.htm#Page_329">329</a>, <a href="../../66492/66492-h/66492-h.htm#Page_331">331</a>, <a href="../../66492/66492-h/66492-h.htm#Page_336">336</a>, <a href="../../66492/66492-h/66492-h.htm#Page_339">339</a>, <a href="../../66492/66492-h/66492-h.htm#Page_340">340</a>, <a href="../../66492/66492-h/66492-h.htm#Page_344">344</a>, <a href="../../66492/66492-h/66492-h.htm#Page_346">346</a>, <a href="../../66492/66492-h/66492-h.htm#Page_349">349</a>, <a href="../../66492/66492-h/66492-h.htm#Page_350">350</a>, <a href="../../66492/66492-h/66492-h.htm#Page_352">352</a>, <a href="../../66492/66492-h/66492-h.htm#Page_364">364</a>, <a href="../../66492/66492-h/66492-h.htm#Page_369">369</a>, <a href="../../66492/66492-h/66492-h.htm#Page_372">372</a>, <a href="../../66492/66492-h/66492-h.htm#Page_373">373</a>, <a href="../../66492/66492-h/66492-h.htm#Page_375">375</a>, <a href="../../66492/66492-h/66492-h.htm#Page_397">397</a>, <a href="../../66492/66492-h/66492-h.htm#Page_403">403</a>, <a href="../../66492/66492-h/66492-h.htm#Page_404">404</a>, <a href="../../66492/66492-h/66492-h.htm#Page_406">406</a>, <a href="../../66492/66492-h/66492-h.htm#Page_407">407</a>, <a href="../../66492/66492-h/66492-h.htm#Page_411">411</a>, <a href="../../66492/66492-h/66492-h.htm#Page_423">423</a>, <a href="../../66492/66492-h/66492-h.htm#Page_425">425</a>, <a href="../../66492/66492-h/66492-h.htm#Page_434">434</a>, <a href="../../66492/66492-h/66492-h.htm#Page_449">449</a>, <a href="../../66492/66492-h/66492-h.htm#Page_462">462</a>, <a href="../../66492/66492-h/66492-h.htm#Page_475">475</a>, <a href="../../66492/66492-h/66492-h.htm#Page_476">476</a>; ii. <a href="#Page_3">3</a>, <a href="#Page_4">4</a>, <a href="#Page_9">9</a>, <a href="#Page_10">10</a>, <a href="#Page_12">12</a>, <a href="#Page_13">13</a>, <a href="#Page_16">16</a>, <a href="#Page_17">17</a>, <a href="#Page_20">20</a>, <a href="#Page_23">23</a>, <a href="#Page_33">33</a>, <a href="#Page_36">36</a>, <a href="#Page_37">37</a>, <a href="#Page_42">42</a>, <a href="#Page_43">43</a>, <a href="#Page_46">46</a>, <a href="#Page_48">48</a>, <a href="#Page_49">49</a>, <a href="#Page_56">56</a>, <a href="#Page_58">58</a>, <a href="#Page_65">65</a>, <a href="#Page_66">66</a>, <a href="#Page_68">68</a>, <a href="#Page_94">94</a>, <a href="#Page_95">95</a>, <a href="#Page_102">102</a>, <a href="#Page_103">103</a>, <a href="#Page_118">118</a>, <a href="#Page_121">121</a>, <a href="#Page_125">125</a>, <a href="#Page_127">127</a>, <a href="#Page_144">144</a>, <a href="#Page_145">145</a>, <a href="#Page_148">148</a>, <a href="#Page_162">162</a>, <a href="#Page_164">164</a>, <a href="#Page_170">170</a>, <a href="#Page_174">174</a>, <a href="#Page_175">175</a>, <a href="#Page_190">190</a>, <a href="#Page_192">192</a>, <a href="#Page_199">199</a>, <a href="#Page_201">201</a>, <a href="#Page_203">203</a>, <a href="#Page_253">253</a>, <a href="#Page_272">272</a>, <a href="#Page_277">277</a>, <a href="#Page_292">292</a>, <a href="#Page_347">347</a>, <a href="#Page_384">384</a>, <a href="#Page_391">391</a>, <a href="#Page_420">420</a>, <a href="#Page_422">422</a>, <a href="#Page_423">423</a>, <a href="#Page_426">426</a>, <a href="#Page_428">428</a>, <a href="#Page_433">433</a>, <a href="#Page_435">435</a>, <a href="#Page_446">446</a>, <a href="#Page_449">449</a><br /> -</p> - -<p><span class="pagenum" id="Page_484">- 484 -</span></p> - -<p> -Giant's Causeway, ii. <a href="#Page_80">80</a>, <a href="#Page_109">109</a>, <a href="#Page_186">186</a>, <a href="#Page_188">188</a>, <a href="#Page_192">192</a>, <a href="#Page_206">206</a><br /> -Gilbert, Mr. G. K., i. <a href="../../66492/66492-h/66492-h.htm#Page_87">87</a>; ii. <a href="#Page_362">362</a>, <a href="#Page_363">363</a><br /> -Girvan, i. <a href="../../66492/66492-h/66492-h.htm#Page_192">192</a>, <a href="../../66492/66492-h/66492-h.htm#Page_200">200</a><br /> -Glaciation, absence of, in Devonshire, i. <a href="../../66492/66492-h/66492-h.htm#Page_261">261</a><br /> -Glass in volcanic rocks, i. <a href="../../66492/66492-h/66492-h.htm#Page_18">18</a>, <a href="../../66492/66492-h/66492-h.htm#Page_33">33</a>, <a href="../../66492/66492-h/66492-h.htm#Page_60">60</a>, <a href="../../66492/66492-h/66492-h.htm#Page_78">78</a>, <a href="../../66492/66492-h/66492-h.htm#Page_180">180</a>, <a href="../../66492/66492-h/66492-h.htm#Page_211">211</a>, <a href="../../66492/66492-h/66492-h.htm#Page_216">216</a>, <a href="../../66492/66492-h/66492-h.htm#Page_230">230</a>, <a href="../../66492/66492-h/66492-h.htm#Page_232">232</a>, <a href="../../66492/66492-h/66492-h.htm#Page_235">235</a>, <a href="../../66492/66492-h/66492-h.htm#Page_316">316</a>; ii. <a href="#Page_85">85</a>, <a href="#Page_120">120</a>, <a href="#Page_126">126</a>, <a href="#Page_133">133</a>, <a href="#Page_135">135</a>, <a href="#Page_137">137</a>, <a href="#Page_184">184</a>, <a href="#Page_204">204</a>, <a href="#Page_247">247</a>, <a href="#Page_272">272</a>, <a href="#Page_285">285</a>, <a href="#Page_316">316</a>, <a href="#Page_317">317</a><br /> -Globulites, ii. <a href="#Page_135">135</a><br /> -Gloucestershire, Silurian volcanoes of, i. <a href="../../66492/66492-h/66492-h.htm#Page_238">238</a><br /> -Gneiss, analogies of, with igneous rocks, i. <a href="../../66492/66492-h/66492-h.htm#Page_93">93</a>; ii. <a href="#Page_476">476</a>;<br /> -<span style="margin-left: 1em;">oldest, i. <a href="../../66492/66492-h/66492-h.htm#Page_110">110</a>, <a href="../../66492/66492-h/66492-h.htm#Page_115">115</a></span><br /> -Godwin-Austen, A. C., i. <a href="../../66492/66492-h/66492-h.htm#Page_259">259</a>, <a href="../../66492/66492-h/66492-h.htm#Page_262">262</a><br /> -Goodchild, Mr. J. G., i. <a href="../../66492/66492-h/66492-h.htm#Page_229">229</a>, <a href="../../66492/66492-h/66492-h.htm#Page_449">449</a>; ii. <a href="#Page_150">150</a><br /> -Grainger, Rev. Dr., ii. <a href="#Page_198">198</a><br /> -Grand Sarcoui, ii. <a href="#Page_373">373</a>, <a href="#Page_374">374</a>, <a href="#Page_381">381</a><br /> -Granite, bosses of, i. <a href="../../66492/66492-h/66492-h.htm#Page_88">88</a>, <a href="../../66492/66492-h/66492-h.htm#Page_90">90</a>, <a href="../../66492/66492-h/66492-h.htm#Page_93">93</a>;<br /> -<span style="margin-left: 1em;">plutonic and volcanic, <a href="../../66492/66492-h/66492-h.htm#Page_89">89</a>;</span><br /> -<span style="margin-left: 1em;">metamorphism by, <a href="../../66492/66492-h/66492-h.htm#Page_95">95</a>;</span><br /> -<span style="margin-left: 1em;">altered by dykes, ii. <a href="#Page_164">164</a>;</span><br /> -<span style="margin-left: 1em;">pre-Cambrian, i. <a href="../../66492/66492-h/66492-h.htm#Page_119">119</a>;</span><br /> -<span style="margin-left: 1em;">post-Arenig in Highlands, <a href="../../66492/66492-h/66492-h.htm#Page_126">126</a>, <a href="../../66492/66492-h/66492-h.htm#Page_310">310</a>;</span><br /> -<span style="margin-left: 1em;">in Cambrian rocks, <a href="../../66492/66492-h/66492-h.htm#Page_155">155</a>;</span><br /> -<span style="margin-left: 1em;">in Silurian rocks, <a href="../../66492/66492-h/66492-h.htm#Page_200">200</a>, <a href="../../66492/66492-h/66492-h.htm#Page_229">229</a>, <a href="../../66492/66492-h/66492-h.htm#Page_236">236</a>, <a href="../../66492/66492-h/66492-h.htm#Page_238">238</a>, <a href="../../66492/66492-h/66492-h.htm#Page_249">249</a>;</span><br /> -<span style="margin-left: 1em;">of probably Old Red Sandstone age, <a href="../../66492/66492-h/66492-h.htm#Page_272">272</a>, <a href="../../66492/66492-h/66492-h.htm#Page_277">277</a>, <a href="../../66492/66492-h/66492-h.htm#Page_290">290</a>, <a href="../../66492/66492-h/66492-h.htm#Page_331">331</a>, <a href="../../66492/66492-h/66492-h.htm#Page_337">337</a>;</span><br /> -<span style="margin-left: 1em;">Tertiary, ii. <a href="#Page_366">366</a>, <a href="#Page_418">418</a>, <a href="#Page_420">420</a></span><br /> -Granitite, i. <a href="../../66492/66492-h/66492-h.htm#Page_188">188</a>, <a href="../../66492/66492-h/66492-h.htm#Page_277">277</a>, <a href="../../66492/66492-h/66492-h.htm#Page_290">290</a>, <a href="../../66492/66492-h/66492-h.htm#Page_337">337</a>; ii. <a href="#Page_367">367</a><br /> -Granophyre, alteration of rocks by, i. <a href="../../66492/66492-h/66492-h.htm#Page_95">95</a>, <a href="../../66492/66492-h/66492-h.htm#Page_96">96</a>;<br /> -<span style="margin-left: 1em;">scenery of, <a href="../../66492/66492-h/66492-h.htm#Page_105">105</a>;</span><br /> -<span style="margin-left: 1em;">solvent action of, <a href="../../66492/66492-h/66492-h.htm#Page_82">82</a>, <a href="../../66492/66492-h/66492-h.htm#Page_84">84</a>, <a href="../../66492/66492-h/66492-h.htm#Page_85">85</a>, <a href="../../66492/66492-h/66492-h.htm#Page_96">96</a>, <a href="../../66492/66492-h/66492-h.htm#Page_99">99</a>; ii. <a href="#Page_163">163</a>, <a href="#Page_392">392</a>, <a href="#Page_415">415</a>, <a href="#Page_422">422</a>, <a href="#Page_433">433</a>;</span><br /> -<span style="margin-left: 1em;">brecciated, <a href="#Page_382">382</a>;</span><br /> -<span style="margin-left: 1em;">spherulitic, <a href="#Page_381">381</a>;</span><br /> -<span style="margin-left: 1em;">bedded structure of, <a href="#Page_381">381</a>, <a href="#Page_403">403</a>, <a href="#Page_404">404</a>;</span><br /> -<span style="margin-left: 1em;">apt to be intruded at the base of a volcanic series, <a href="#Page_403">403</a>;</span><br /> -<span style="margin-left: 1em;">shattering of rocks invaded by, <a href="#Page_405">405</a>, <a href="#Page_411">411</a>, <a href="#Page_413">413</a>, <a href="#Page_416">416</a>, <a href="#Page_439">439</a>;</span><br /> -<span style="margin-left: 1em;">veins of, <a href="#Page_409">409</a>, <a href="#Page_410">410</a>, <a href="#Page_432">432</a>, <a href="#Page_437">437</a>;</span><br /> -<span style="margin-left: 1em;">Silurian, i. <a href="../../66492/66492-h/66492-h.htm#Page_214">214</a>, <a href="../../66492/66492-h/66492-h.htm#Page_215">215</a></span><br /> -—— Tertiary, i. <a href="../../66492/66492-h/66492-h.htm#Page_339">339</a>; ii. <a href="#Page_368">368</a>, <a href="#Page_395">395</a>, <a href="#Page_408">408</a>, <a href="#Page_430">430</a>;<br /> -<span style="margin-left: 1em;">boundaries of, <a href="#Page_382">382</a>, <a href="#Page_409">409</a>;</span><br /> -<span style="margin-left: 1em;">relation to older vents, <a href="#Page_280">280</a>, <a href="#Page_384">384</a>, <a href="#Page_399">399</a>;</span><br /> -<span style="margin-left: 1em;">relation to plateau-basalts, <a href="#Page_386">386</a>, <a href="#Page_396">396</a>, <a href="#Page_402">402</a>, <a href="#Page_404">404</a>;</span><br /> -<span style="margin-left: 1em;">relation to gabbro, <a href="#Page_391">391</a>, <a href="#Page_402">402</a>, <a href="#Page_404">404</a>, <a href="#Page_410">410</a>;</span><br /> -<span style="margin-left: 1em;">relation to basic dykes, <a href="#Page_395">395</a>;</span><br /> -<span style="margin-left: 1em;">proof of liquidity of, <a href="#Page_413">413</a>;</span><br /> -<span style="margin-left: 1em;">sills of, <a href="#Page_430">430</a>, <a href="#Page_436">436</a>, <a href="#Page_437">437</a>;</span><br /> -<span style="margin-left: 1em;">dykes of, <a href="#Page_435">435</a></span><br /> -Granophyric structure, i. <a href="../../66492/66492-h/66492-h.htm#Page_20">20</a>; ii. <a href="#Page_366">366</a><br /> -Graphite in Tertiary volcanic series, ii. <a href="#Page_198">198</a><br /> -Graptolites, i. <a href="../../66492/66492-h/66492-h.htm#Page_174">174</a>, <a href="../../66492/66492-h/66492-h.htm#Page_196">196</a>, <a href="../../66492/66492-h/66492-h.htm#Page_197">197</a><br /> -Granwacke or Devonian rocks, De la Beche on, i. <a href="../../66492/66492-h/66492-h.htm#Page_259">259</a>, ii. <a href="#Page_33">33</a><br /> -Graves, Lieut. T., ii. <a href="#Page_451">451</a><br /> -Great Glen of Scotland, i. <a href="../../66492/66492-h/66492-h.htm#Page_121">121</a><br /> -Greeks, influence of volcanoes on, i. <a href="../../66492/66492-h/66492-h.htm#Page_1">1</a><br /> -Green, A. H., i. <a href="../../66492/66492-h/66492-h.htm#Page_133">133</a>, <a href="../../66492/66492-h/66492-h.htm#Page_134">134</a>, <a href="../../66492/66492-h/66492-h.htm#Page_163">163</a>; ii. <a href="#Page_10">10</a>, <a href="#Page_12">12</a><br /> -Greenland, Tertiary basalts of, ii. <a href="#Page_182">182</a><br /> -Greenly, Mr. E., i. <a href="../../66492/66492-h/66492-h.htm#Page_129">129</a>, <a href="../../66492/66492-h/66492-h.htm#Page_214">214</a><br /> -Greenock, Lord, i. <a href="../../66492/66492-h/66492-h.htm#Page_363">363</a><br /> -Green schists of the Scottish Highlands, i. <a href="../../66492/66492-h/66492-h.htm#Page_122">122</a>;<br /> -<span style="margin-left: 1em;">of Anglesey, <a href="../../66492/66492-h/66492-h.htm#Page_129">129</a></span><br /> -Greenstone, i. <a href="../../66492/66492-h/66492-h.htm#Page_183">183</a>, <a href="../../66492/66492-h/66492-h.htm#Page_187">187</a>, <a href="../../66492/66492-h/66492-h.htm#Page_206">206</a>, <a href="../../66492/66492-h/66492-h.htm#Page_217">217</a>, <a href="../../66492/66492-h/66492-h.htm#Page_219">219</a>, <a href="../../66492/66492-h/66492-h.htm#Page_249">249</a>, <a href="../../66492/66492-h/66492-h.htm#Page_259">259</a>, <a href="../../66492/66492-h/66492-h.htm#Page_261">261</a>; ii. <a href="#Page_34">34</a>, <a href="#Page_35">35</a>, <a href="#Page_37">37</a>, <a href="#Page_103">103</a>, <a href="#Page_104">104</a>, <a href="#Page_355">355</a><br /> -Greenstone-ash, i. <a href="../../66492/66492-h/66492-h.htm#Page_219">219</a><br /> -Griffith, Sir R., ii. <a href="#Page_299">299</a>, <a href="#Page_422">422</a><br /> -Gunn, Mr. W., i. <a href="../../66492/66492-h/66492-h.htm#Page_114">114</a>, <a href="../../66492/66492-h/66492-h.htm#Page_298">298</a>, <a href="../../66492/66492-h/66492-h.htm#Page_311">311</a>, <a href="../../66492/66492-h/66492-h.htm#Page_336">336</a>, <a href="../../66492/66492-h/66492-h.htm#Page_369">369</a>, <a href="../../66492/66492-h/66492-h.htm#Page_407">407</a>, <a href="../../66492/66492-h/66492-h.htm#Page_410">410</a>; ii. <a href="#Page_58">58</a>, <a href="#Page_172">172</a>, <a href="#Page_420">420</a><br /> -Gypsum deposits, ii. <a href="#Page_54">54</a><br /> -<br /> -<a id="H"></a>Hade of dykes, ii. <a href="#Page_139">139</a><br /> -Hæmatitic iron-ore, ii. <a href="#Page_197">197</a><br /> -Hall, Sir James, i. <a href="../../66492/66492-h/66492-h.htm#Page_72">72</a>, <a href="../../66492/66492-h/66492-h.htm#Page_363">363</a><br /> -Hälleflinta, i. <a href="../../66492/66492-h/66492-h.htm#Page_131">131</a>, <a href="../../66492/66492-h/66492-h.htm#Page_167">167</a><br /> -Hardman, E. T., ii. <a href="#Page_365">365</a>, <a href="#Page_449">449</a><br /> -Harker, Mr. A., i. <a href="../../66492/66492-h/66492-h.htm#Page_90">90</a>, <a href="../../66492/66492-h/66492-h.htm#Page_91">91</a>, <a href="../../66492/66492-h/66492-h.htm#Page_93">93</a>, <a href="../../66492/66492-h/66492-h.htm#Page_95">95</a>, <a href="../../66492/66492-h/66492-h.htm#Page_96">96</a>, <a href="../../66492/66492-h/66492-h.htm#Page_99">99</a>, <a href="../../66492/66492-h/66492-h.htm#Page_188">188</a>, <a href="../../66492/66492-h/66492-h.htm#Page_209">209</a>, <a href="../../66492/66492-h/66492-h.htm#Page_210">210</a>, <a href="../../66492/66492-h/66492-h.htm#Page_211">211</a>, <a href="../../66492/66492-h/66492-h.htm#Page_212">212</a>, <a href="../../66492/66492-h/66492-h.htm#Page_213">213</a>, <a href="../../66492/66492-h/66492-h.htm#Page_214">214</a>, <a href="../../66492/66492-h/66492-h.htm#Page_217">217</a>, <a href="../../66492/66492-h/66492-h.htm#Page_218">218</a>, <a href="../../66492/66492-h/66492-h.htm#Page_222">222</a>, <a href="../../66492/66492-h/66492-h.htm#Page_227">227</a>, <a href="../../66492/66492-h/66492-h.htm#Page_228">228</a>, <a href="../../66492/66492-h/66492-h.htm#Page_230">230</a>, <a href="../../66492/66492-h/66492-h.htm#Page_231">231</a>, <a href="../../66492/66492-h/66492-h.htm#Page_232">232</a>, <a href="../../66492/66492-h/66492-h.htm#Page_235">235</a>, <a href="../../66492/66492-h/66492-h.htm#Page_236">236</a>, <a href="../../66492/66492-h/66492-h.htm#Page_237">237</a>, <a href="../../66492/66492-h/66492-h.htm#Page_238">238</a>, <a href="../../66492/66492-h/66492-h.htm#Page_290">290</a>; ii. <a href="#Page_124">124</a>, <a href="#Page_125">125</a>, <a href="#Page_126">126</a>, <a href="#Page_129">129</a>, <a href="#Page_130">130</a>, <a href="#Page_139">139</a>, <a href="#Page_144">144</a>, <a href="#Page_146">146</a>, <a href="#Page_160">160</a>, <a href="#Page_162">162</a>, <a href="#Page_163">163</a>, <a href="#Page_164">164</a>, <a href="#Page_174">174</a>, <a href="#Page_185">185</a>, <a href="#Page_189">189</a>, <a href="#Page_190">190</a>, <a href="#Page_223">223</a>, <a href="#Page_224">224</a>, <a href="#Page_247">247</a>, <a href="#Page_269">269</a>, <a href="#Page_281">281</a>, <a href="#Page_284">284</a>, <a href="#Page_285">285</a>, <a href="#Page_309">309</a>, <a href="#Page_310">310</a>, <a href="#Page_318">318</a>, <a href="#Page_320">320</a>, <a href="#Page_334">334</a>, <a href="#Page_339">339</a>, <a href="#Page_347">347</a>, <a href="#Page_348">348</a>, <a href="#Page_368">368</a>, <a href="#Page_382">382</a>, <a href="#Page_384">384</a>, <a href="#Page_385">385</a>, <a href="#Page_387">387</a>, <a href="#Page_389">389</a>, <a href="#Page_392">392</a>, <a href="#Page_407">407</a>, <a href="#Page_408">408</a>, <a href="#Page_409">409</a>, <a href="#Page_413">413</a>, <a href="#Page_415">415</a>, <a href="#Page_433">433</a>, <a href="#Page_434">434</a>, <a href="#Page_437">437</a>, <a href="#Page_441">441</a>, <a href="#Page_446">446</a><br /> -Harkness, R., i. <a href="../../66492/66492-h/66492-h.htm#Page_228">228</a>; ii. <a href="#Page_56">56</a><br /> -Harlech anticline, i. <a href="../../66492/66492-h/66492-h.htm#Page_159">159</a>, <a href="../../66492/66492-h/66492-h.htm#Page_179">179</a>, <a href="../../66492/66492-h/66492-h.htm#Page_187">187</a>;<br /> -<span style="margin-left: 1em;">group, <a href="../../66492/66492-h/66492-h.htm#Page_176">176</a></span><br /> -Hatch, Dr. F. H., i. <a href="../../66492/66492-h/66492-h.htm#Page_183">183</a>, <a href="../../66492/66492-h/66492-h.htm#Page_184">184</a>, <a href="../../66492/66492-h/66492-h.htm#Page_187">187</a>, <a href="../../66492/66492-h/66492-h.htm#Page_188">188</a>, <a href="../../66492/66492-h/66492-h.htm#Page_229">229</a>, <a href="../../66492/66492-h/66492-h.htm#Page_230">230</a>, <a href="../../66492/66492-h/66492-h.htm#Page_246">246</a>, <a href="../../66492/66492-h/66492-h.htm#Page_247">247</a>, <a href="../../66492/66492-h/66492-h.htm#Page_248">248</a>, <a href="../../66492/66492-h/66492-h.htm#Page_249">249</a>, <a href="../../66492/66492-h/66492-h.htm#Page_261">261</a>, <a href="../../66492/66492-h/66492-h.htm#Page_277">277</a>, <a href="../../66492/66492-h/66492-h.htm#Page_278">278</a>, <a href="../../66492/66492-h/66492-h.htm#Page_306">306</a>, <a href="../../66492/66492-h/66492-h.htm#Page_377">377</a>, <a href="../../66492/66492-h/66492-h.htm#Page_380">380</a>, <a href="../../66492/66492-h/66492-h.htm#Page_381">381</a>, <a href="../../66492/66492-h/66492-h.htm#Page_417">417</a>, <a href="../../66492/66492-h/66492-h.htm#Page_419">419</a>, <a href="../../66492/66492-h/66492-h.htm#Page_420">420</a>; ii. <a href="#Page_57">57</a>, <a href="#Page_96">96</a>, <a href="#Page_184">184</a>, <a href="#Page_274">274</a>, <a href="#Page_276">276</a>, <a href="#Page_299">299</a>, <a href="#Page_319">319</a>, <a href="#Page_332">332</a>, <a href="#Page_367">367</a>, <a href="#Page_368">368</a>, <a href="#Page_369">369</a>, <a href="#Page_370">370</a>, <a href="#Page_388">388</a>, <a href="#Page_398">398</a><br /> -Haughton, Prof. S., i. <a href="../../66492/66492-h/66492-h.htm#Page_346">346</a>; ii. <a href="#Page_422">422</a><br /> -Hawaii, lava-fountains of, i. <a href="../../66492/66492-h/66492-h.htm#Page_12">12</a>;<br /> -<span style="margin-left: 1em;">differentiation in lavas of, <a href="../../66492/66492-h/66492-h.htm#Page_27">27</a>;</span><br /> -<span style="margin-left: 1em;">lava-cauldron of, <a href="../../66492/66492-h/66492-h.htm#Page_58">58</a></span><br /> -Haworth, Mr. E., ii. <a href="#Page_96">96</a><br /> -Hay Cunningham, R. I., i. <a href="../../66492/66492-h/66492-h.htm#Page_269">269</a>, <a href="../../66492/66492-h/66492-h.htm#Page_317">317</a>, <a href="../../66492/66492-h/66492-h.htm#Page_363">363</a>, <a href="../../66492/66492-h/66492-h.htm#Page_372">372</a>, <a href="../../66492/66492-h/66492-h.htm#Page_373">373</a>, <a href="../../66492/66492-h/66492-h.htm#Page_449">449</a>, <a href="../../66492/66492-h/66492-h.htm#Page_451">451</a>; ii. <a href="#Page_237">237</a>, <a href="#Page_238">238</a>, <a href="#Page_244">244</a><br /> -Heaphy, Mr. C., i. <a href="../../66492/66492-h/66492-h.htm#Page_432">432</a><br /> -Hebrides, basalt-sheets of, i. <a href="../../66492/66492-h/66492-h.htm#Page_24">24</a>, <a href="../../66492/66492-h/66492-h.htm#Page_47">47</a>, <a href="../../66492/66492-h/66492-h.htm#Page_52">52</a>, <a href="../../66492/66492-h/66492-h.htm#Page_102">102</a>;<br /> -<span style="margin-left: 1em;">acid rocks of, <a href="../../66492/66492-h/66492-h.htm#Page_95">95</a>, <a href="../../66492/66492-h/66492-h.htm#Page_102">102</a>;</span><br /> -<span style="margin-left: 1em;">gabbros of, <a href="../../66492/66492-h/66492-h.htm#Page_84">84</a>, <a href="../../66492/66492-h/66492-h.htm#Page_90">90</a>, <a href="../../66492/66492-h/66492-h.htm#Page_102">102</a>;</span><br /> -<span style="margin-left: 1em;">scenery of, <a href="../../66492/66492-h/66492-h.htm#Page_105">105</a>;</span><br /> -<span style="margin-left: 1em;">pre-Cambrian rocks of, <a href="../../66492/66492-h/66492-h.htm#Page_112">112</a>, <a href="../../66492/66492-h/66492-h.htm#Page_114">114</a>, <a href="../../66492/66492-h/66492-h.htm#Page_117">117</a>, <a href="../../66492/66492-h/66492-h.htm#Page_121">121</a>;</span><br /> -<span style="margin-left: 1em;">Cambrian land of, <a href="../../66492/66492-h/66492-h.htm#Page_141">141</a>;</span><br /> -<span style="margin-left: 1em;">early observations on the Tertiary volcanic rocks of, ii. <a href="#Page_109">109</a>, <a href="#Page_110">110</a>, <a href="#Page_111">111</a>;</span><br /> -<span style="margin-left: 1em;">dykes of, <a href="#Page_118">118</a>, <a href="#Page_146">146</a>, <a href="#Page_158">158</a>, <a href="#Page_174">174</a>;</span><br /> -<span style="margin-left: 1em;">basalts of, <a href="#Page_181">181</a>, <a href="#Page_186">186</a>, <a href="#Page_215">215</a>;</span><br /> -<span style="margin-left: 1em;">pitchstone lava of, <a href="#Page_238">238</a>, <a href="#Page_246">246</a>;</span><br /> -<span style="margin-left: 1em;">plateau-scenery of, <a href="#Page_249">249</a>;</span><br /> -<span style="margin-left: 1em;">Tertiary rivers and lakes of, <a href="#Page_217">217</a>, <a href="#Page_228">228</a>, <a href="#Page_231">231</a>, <a href="#Page_234">234</a>, <a href="#Page_252">252</a>;</span><br /> -<span style="margin-left: 1em;">vents of, <a href="#Page_274">274</a>;</span><br /> -<span style="margin-left: 1em;">basic sills of, <a href="#Page_304">304</a>;</span><br /> -<span style="margin-left: 1em;">gabbro intrusions of, <a href="#Page_327">327</a>;</span><br /> -<span style="margin-left: 1em;">acid intrusions of, <a href="#Page_364">364</a>, <a href="#Page_379">379</a>, <a href="#Page_430">430</a>, <a href="#Page_437">437</a>;</span><br /> -<span style="margin-left: 1em;">dislocations of, <a href="#Page_452">452</a>;</span><br /> -<span style="margin-left: 1em;">denudation of, <a href="#Page_455">455</a></span><br /> -Heddle, Dr., i. <a href="../../66492/66492-h/66492-h.htm#Page_274">274</a>, <a href="../../66492/66492-h/66492-h.htm#Page_302">302</a>; ii. <a href="#Page_78">78</a>, <a href="#Page_79">79</a>, <a href="#Page_246">246</a>, <a href="#Page_307">307</a>, <a href="#Page_406">406</a><br /> -Helland, Prof. A., ii. <a href="#Page_191">191</a>, <a href="#Page_261">261</a>, <a href="#Page_263">263</a>, <a href="#Page_264">264</a><br /> -Henderson, Mr. J., i. <a href="../../66492/66492-h/66492-h.htm#Page_449">449</a><br /> -Henry Mountains, laccolites of, i. <a href="../../66492/66492-h/66492-h.htm#Page_86">86</a>; ii. <a href="#Page_362">362</a><br /> -Henslow, J. S., ii. <a href="#Page_22">22</a>, <a href="#Page_224">224</a><br /> -Heterogeneity in volcanic magmas, i. <a href="../../66492/66492-h/66492-h.htm#Page_85">85</a>, <a href="../../66492/66492-h/66492-h.htm#Page_90">90</a>; ii, <a href="#Page_190">190</a>, <a href="#Page_334">334</a><br /> -Hett Dyke, ii. <a href="#Page_1">1</a>, <a href="#Page_7">7</a>, <a href="#Page_147">147</a><br /> -Hibbert, S., i. <a href="../../66492/66492-h/66492-h.htm#Page_46">46</a><br /> -Hicks, Dr. H., i. <a href="../../66492/66492-h/66492-h.htm#Page_126">126</a>, <a href="../../66492/66492-h/66492-h.htm#Page_145">145</a>, <a href="../../66492/66492-h/66492-h.htm#Page_154">154</a>, <a href="../../66492/66492-h/66492-h.htm#Page_158">158</a>, <a href="../../66492/66492-h/66492-h.htm#Page_159">159</a>, <a href="../../66492/66492-h/66492-h.htm#Page_166">166</a>, <a href="../../66492/66492-h/66492-h.htm#Page_206">206</a><br /> -Hill, Mr. J. B., ii. <a href="#Page_140">140</a><br /> -Hill, Rev. E., i. <a href="#Page_135">135</a><br /> -Hinde, Dr. G. J., i. <a href="../../66492/66492-h/66492-h.htm#Page_198">198</a>; ii. <a href="#Page_35">35</a><br /> -Hinxman, Mr. L., i. <a href="../../66492/66492-h/66492-h.htm#Page_114">114</a>, <a href="../../66492/66492-h/66492-h.htm#Page_344">344</a>; ii. <a href="#Page_121">121</a><br /> -Hobson, Mr. B., i. <a href="../../66492/66492-h/66492-h.htm#Page_260">260</a>; ii. <a href="#Page_23">23</a>, <a href="#Page_27">27</a>, <a href="#Page_96">96</a>, <a href="#Page_99">99</a><br /> -Holden, Mr. J. S., ii. <a href="#Page_204">204</a><br /> -Holl, H. B., i. <a href="../../66492/66492-h/66492-h.htm#Page_133">133</a>, <a href="../../66492/66492-h/66492-h.htm#Page_134">134</a>, <a href="../../66492/66492-h/66492-h.htm#Page_170">170</a><br /> -Holland, Mr. P., i. <a href="../../66492/66492-h/66492-h.htm#Page_177">177</a>, <a href="../../66492/66492-h/66492-h.htm#Page_178">178</a>, <a href="../../66492/66492-h/66492-h.htm#Page_179">179</a>; ii. <a href="#Page_23">23</a><br /> -Hollybush Sandstone, i. <a href="../../66492/66492-h/66492-h.htm#Page_133">133</a>, <a href="../../66492/66492-h/66492-h.htm#Page_170">170</a><br /> -Holocrystalline structure, i. <a href="../../66492/66492-h/66492-h.htm#Page_78">78</a>; ii. <a href="#Page_136">136</a>, <a href="#Page_184">184</a><br /> -Hopkins, W., ii. <a href="#Page_177">177</a>, <a href="#Page_179">179</a>, <a href="#Page_268">268</a><br /> -</p> - -<p><span class="pagenum" id="Page_485">- 485 -</span></p> - -<p> -Hornblende, ejected crystals of, i. <a href="../../66492/66492-h/66492-h.htm#Page_178">178</a>, <a href="../../66492/66492-h/66492-h.htm#Page_181">181</a>; ii. <a href="#Page_49">49</a>, <a href="#Page_51">51</a>, <a href="#Page_58">58</a>, <a href="#Page_79">79</a><br /> -Hornblende-schists formed from basic igneous rocks, i. <a href="../../66492/66492-h/66492-h.htm#Page_75">75</a>, <a href="../../66492/66492-h/66492-h.htm#Page_114">114</a>, <a href="../../66492/66492-h/66492-h.htm#Page_118">118</a>, <a href="../../66492/66492-h/66492-h.htm#Page_119">119</a>, <a href="../../66492/66492-h/66492-h.htm#Page_124">124</a>, <a href="../../66492/66492-h/66492-h.htm#Page_129">129</a><br /> -Horne, Mr. John, i. <a href="../../66492/66492-h/66492-h.htm#Page_114">114</a>, <a href="../../66492/66492-h/66492-h.htm#Page_196">196</a>, <a href="../../66492/66492-h/66492-h.htm#Page_199">199</a>, <a href="../../66492/66492-h/66492-h.htm#Page_200">200</a>, <a href="../../66492/66492-h/66492-h.htm#Page_344">344</a>, <a href="../../66492/66492-h/66492-h.htm#Page_345">345</a>, <a href="../../66492/66492-h/66492-h.htm#Page_375">375</a>; ii. <a href="#Page_23">23</a>, <a href="#Page_144">144</a>, <a href="#Page_292">292</a><br /> -Hornito of a lava-stream, i. <a href="../../66492/66492-h/66492-h.htm#Page_55">55</a>; ii. <a href="#Page_264">264</a><br /> -Hornstone, i. <a href="../../66492/66492-h/66492-h.htm#Page_131">131</a>, <a href="../../66492/66492-h/66492-h.htm#Page_136">136</a>, <a href="../../66492/66492-h/66492-h.htm#Page_277">277</a>, <a href="../../66492/66492-h/66492-h.htm#Page_278">278</a> (analyses), <a href="../../66492/66492-h/66492-h.htm#Page_324">324</a><br /> -Houston Marls, i. <a href="../../66492/66492-h/66492-h.htm#Page_423">423</a>, <a href="../../66492/66492-h/66492-h.htm#Page_436">436</a>, <a href="../../66492/66492-h/66492-h.htm#Page_440">440</a>, <a href="../../66492/66492-h/66492-h.htm#Page_444">444</a>, <a href="../../66492/66492-h/66492-h.htm#Page_466">466</a><br /> -Howard, Mr. H. T., i. <a href="../../66492/66492-h/66492-h.htm#Page_207">207</a><br /> -Howell, Mr. H. H., i. <a href="../../66492/66492-h/66492-h.htm#Page_294">294</a>, <a href="../../66492/66492-h/66492-h.htm#Page_307">307</a>, <a href="../../66492/66492-h/66492-h.htm#Page_364">364</a><br /> -Hoy, Island of, i. <a href="../../66492/66492-h/66492-h.htm#Page_350">350</a><br /> -Hughes, Prof. T. M'K., i. <a href="../../66492/66492-h/66492-h.htm#Page_126">126</a>, <a href="../../66492/66492-h/66492-h.htm#Page_144">144</a>, <a href="../../66492/66492-h/66492-h.htm#Page_160">160</a>, <a href="../../66492/66492-h/66492-h.htm#Page_161">161</a>, <a href="../../66492/66492-h/66492-h.htm#Page_166">166</a>, <a href="../../66492/66492-h/66492-h.htm#Page_168">168</a>, <a href="../../66492/66492-h/66492-h.htm#Page_220">220</a>, <a href="../../66492/66492-h/66492-h.htm#Page_222">222</a>, <a href="../../66492/66492-h/66492-h.htm#Page_223">223</a>, <a href="../../66492/66492-h/66492-h.htm#Page_227">227</a><br /> -Hull, Mr. E., ii. <a href="#Page_42">42</a>, <a href="#Page_95">95</a>, <a href="#Page_103">103</a>, <a href="#Page_272">272</a>, <a href="#Page_421">421</a>, <a href="#Page_426">426</a>, <a href="#Page_449">449</a><br /> -Hurlet Limestone, i. <a href="../../66492/66492-h/66492-h.htm#Page_360">360</a>, <a href="../../66492/66492-h/66492-h.htm#Page_366">366</a>, <a href="../../66492/66492-h/66492-h.htm#Page_394">394</a>, <a href="../../66492/66492-h/66492-h.htm#Page_410">410</a>, <a href="../../66492/66492-h/66492-h.htm#Page_415">415</a>, <a href="../../66492/66492-h/66492-h.htm#Page_444">444</a>, <a href="../../66492/66492-h/66492-h.htm#Page_452">452</a>, <a href="../../66492/66492-h/66492-h.htm#Page_456">456</a>, <a href="../../66492/66492-h/66492-h.htm#Page_467">467</a>, <a href="../../66492/66492-h/66492-h.htm#Page_470">470</a>, <a href="../../66492/66492-h/66492-h.htm#Page_474">474</a><br /> -Huronian rocks, i. <a href="../../66492/66492-h/66492-h.htm#Page_111">111</a><br /> -Hutchings, Mr. W. M., i. <a href="../../66492/66492-h/66492-h.htm#Page_227">227</a>, <a href="../../66492/66492-h/66492-h.htm#Page_230">230</a>, <a href="../../66492/66492-h/66492-h.htm#Page_233">233</a><br /> -Hutton, James, i. <a href="../../66492/66492-h/66492-h.htm#Page_363">363</a>; ii. <a href="#Page_9">9</a>, <a href="#Page_110">110</a><br /> -Hutton, W., ii. <a href="#Page_3">3</a><br /> -Hyperite, i. <a href="../../66492/66492-h/66492-h.htm#Page_279">279</a><br /> -Hysgeir, pitchstone of, ii. <a href="#Page_246">246</a><br /> -<br /> -<a id="I"></a>Iceland, Tertiary basalts of, ii. <a href="#Page_182">182</a>, <a href="#Page_260">260</a>;<br /> -<span style="margin-left: 1em;">Tertiary gabbros and liparites or granophyres of, <a href="#Page_261">261</a>;</span><br /> -<span style="margin-left: 1em;">continuity of volcanic phenomenon of, <a href="#Page_261">261</a>;</span><br /> -<span style="margin-left: 1em;">lava-fields of, i. <a href="../../66492/66492-h/66492-h.htm#Page_24">24</a>, <a href="../../66492/66492-h/66492-h.htm#Page_42">42</a>, <a href="../../66492/66492-h/66492-h.htm#Page_53">53</a>, <a href="../../66492/66492-h/66492-h.htm#Page_100">100</a>; ii. <a href="#Page_260">260</a>;</span><br /> -<span style="margin-left: 1em;">lava-domes of, i. <a href="../../66492/66492-h/66492-h.htm#Page_10">10</a>; ii. <a href="#Page_265">265</a>;</span><br /> -<span style="margin-left: 1em;">fissures of, i. <a href="../../66492/66492-h/66492-h.htm#Page_70">70</a>; ii. <a href="#Page_262">262</a>, <a href="#Page_271">271</a>, <a href="#Page_454">454</a>;</span><br /> -<span style="margin-left: 1em;">dykes of, <a href="#Page_122">122</a>, <a href="#Page_261">261</a>;</span><br /> -<span style="margin-left: 1em;">cinder cones of, <a href="#Page_264">264</a>, <a href="#Page_271">271</a>;</span><br /> -<span style="margin-left: 1em;">subsidence of, <a href="#Page_447">447</a></span><br /> -Idahoe, lava-fields of, ii. <a href="#Page_267">267</a><br /> -Iddings, Prof., i. <a href="../../66492/66492-h/66492-h.htm#Page_28">28</a>, <a href="../../66492/66492-h/66492-h.htm#Page_29">29</a>, <a href="../../66492/66492-h/66492-h.htm#Page_30">30</a>, <a href="../../66492/66492-h/66492-h.htm#Page_78">78</a>, <a href="../../66492/66492-h/66492-h.htm#Page_79">79</a>, <a href="../../66492/66492-h/66492-h.htm#Page_82">82</a>, <a href="../../66492/66492-h/66492-h.htm#Page_84">84</a>, <a href="../../66492/66492-h/66492-h.htm#Page_90">90</a>; ii. <a href="../../66492/66492-h/66492-h.htm#Page_128">128</a>, <a href="../../66492/66492-h/66492-h.htm#Page_178">178</a><br /> -Idiomorphic crystals, i. <a href="../../66492/66492-h/66492-h.htm#Page_21">21</a>, <a href="../../66492/66492-h/66492-h.htm#Page_417">417</a>, <a href="../../66492/66492-h/66492-h.htm#Page_420">420</a>; ii. <a href="#Page_40">40</a><br /> -Index Limestone of the Scottish coal-fields, i. <a href="../../66492/66492-h/66492-h.htm#Page_360">360</a>, <a href="../../66492/66492-h/66492-h.htm#Page_444">444</a>, <a href="../../66492/66492-h/66492-h.htm#Page_452">452</a><br /> -India, fissure-eruptions of, i. <a href="../../66492/66492-h/66492-h.htm#Page_10">10</a>;<br /> -<span style="margin-left: 1em;">volcanic plateau of, ii. <a href="#Page_180">180</a></span><br /> -Intermediate volcanic rocks, silica-percentage of, i. <a href="../../66492/66492-h/66492-h.htm#Page_14">14</a><br /> -Intersertal structure, i. <a href="../../66492/66492-h/66492-h.htm#Page_417">417</a>; ii. <a href="#Page_136">136</a><br /> -Intrusive rooks, defined, i. <a href="../../66492/66492-h/66492-h.htm#Page_14">14</a>;<br /> -<span style="margin-left: 1em;">occasional cellular character of, <a href="../../66492/66492-h/66492-h.htm#Page_16">16</a>;</span><br /> -<span style="margin-left: 1em;">flow-structure in, <a href="../../66492/66492-h/66492-h.htm#Page_22">22</a>, <a href="../../66492/66492-h/66492-h.htm#Page_161">161</a>;</span><br /> -<span style="margin-left: 1em;">varieties of, <a href="../../66492/66492-h/66492-h.htm#Page_77">77</a>;</span><br /> -<span style="margin-left: 1em;">textures of, <a href="../../66492/66492-h/66492-h.htm#Page_78">78</a>, <a href="../../66492/66492-h/66492-h.htm#Page_449">449</a>; ii. <a href="#Page_274">274</a>, <a href="#Page_360">360</a>, <a href="#Page_392">392</a>;</span><br /> -<span style="margin-left: 1em;">in sheets, sills, and laccolites, i. <a href="../../66492/66492-h/66492-h.htm#Page_83">83</a>;</span><br /> -<span style="margin-left: 1em;">melting of rocks by, <a href="../../66492/66492-h/66492-h.htm#Page_82">82</a>, <a href="../../66492/66492-h/66492-h.htm#Page_84">84</a>; ii. <a href="#Page_129">129</a>, <a href="#Page_163">163</a>, <a href="#Page_392">392</a>;</span><br /> -<span style="margin-left: 1em;">consolidation of, i. <a href="../../66492/66492-h/66492-h.htm#Page_84">84</a>;</span><br /> -<span style="margin-left: 1em;">banding of, <a href="../../66492/66492-h/66492-h.htm#Page_84">84</a>, <a href="../../66492/66492-h/66492-h.htm#Page_450">450</a>; ii. <a href="#Page_329">329</a>, <a href="#Page_342">342</a>;</span><br /> -<span style="margin-left: 1em;">heterogeneity of, i. <a href="../../66492/66492-h/66492-h.htm#Page_85">85</a>; ii. <a href="#Page_344">344</a>;</span><br /> -<span style="margin-left: 1em;">metamorphism by, i. <a href="../../66492/66492-h/66492-h.htm#Page_94">94</a>, <a href="../../66492/66492-h/66492-h.htm#Page_451">451</a>;</span><br /> -<span style="margin-left: 1em;">influence of surrounding rocks on, <a href="../../66492/66492-h/66492-h.htm#Page_95">95</a>;</span><br /> -<span style="margin-left: 1em;">conditions of their intrusion, <a href="../../66492/66492-h/66492-h.htm#Page_97">97</a>;</span><br /> -<span style="margin-left: 1em;">columnar structure in, ii. <a href="#Page_187">187</a>, <a href="#Page_291">291</a>, <a href="#Page_301">301</a></span><br /> -—— Pre-Cambrian, i. <a href="../../66492/66492-h/66492-h.htm#Page_116">116</a>;<br /> -<span style="margin-left: 1em;">Cambrian, <a href="../../66492/66492-h/66492-h.htm#Page_156">156</a>;</span><br /> -<span style="margin-left: 1em;">Silurian, <a href="../../66492/66492-h/66492-h.htm#Page_187">187</a>, <a href="../../66492/66492-h/66492-h.htm#Page_195">195</a>, <a href="../../66492/66492-h/66492-h.htm#Page_206">206</a>, <a href="../../66492/66492-h/66492-h.htm#Page_216">216</a>, <a href="../../66492/66492-h/66492-h.htm#Page_235">235</a>, <a href="../../66492/66492-h/66492-h.htm#Page_237">237</a>, <a href="../../66492/66492-h/66492-h.htm#Page_248">248</a>, <a href="../../66492/66492-h/66492-h.htm#Page_249">249</a>;</span><br /> -<span style="margin-left: 1em;">Devonian, <a href="../../66492/66492-h/66492-h.htm#Page_261">261</a>;</span><br /> -<span style="margin-left: 1em;">Old Red Sandstone, <a href="../../66492/66492-h/66492-h.htm#Page_277">277</a>, <a href="../../66492/66492-h/66492-h.htm#Page_291">291</a>, <a href="../../66492/66492-h/66492-h.htm#Page_321">321</a>, <a href="../../66492/66492-h/66492-h.htm#Page_335">335</a>, <a href="../../66492/66492-h/66492-h.htm#Page_338">338</a>, <a href="../../66492/66492-h/66492-h.htm#Page_343">343</a>, <a href="../../66492/66492-h/66492-h.htm#Page_345">345</a>;</span><br /> -<span style="margin-left: 1em;">Carboniferous, <a href="../../66492/66492-h/66492-h.htm#Page_406">406</a>, <a href="../../66492/66492-h/66492-h.htm#Page_408">408</a>, <a href="../../66492/66492-h/66492-h.htm#Page_420">420</a>, <a href="../../66492/66492-h/66492-h.htm#Page_446">446</a>; ii. <a href="#Page_1">1</a>, <a href="#Page_21">21</a>, <a href="#Page_30">30</a>, <a href="#Page_33">33</a>, <a href="#Page_40">40</a>, <a href="#Page_48">48</a>;</span><br /> -<span style="margin-left: 1em;">Permian, <a href="#Page_58">58</a>, <a href="#Page_64">64</a>;</span><br /> -<span style="margin-left: 1em;">Tertiary, <a href="#Page_270">270</a>, <a href="#Page_298">298</a></span><br /> -Ireland, submarine eruptions of, i. <a href="../../66492/66492-h/66492-h.htm#Page_48">48</a>;<br /> -<span style="margin-left: 1em;">Dalradian rocks of, <a href="../../66492/66492-h/66492-h.htm#Page_122">122</a>, <a href="../../66492/66492-h/66492-h.htm#Page_123">123</a>, <a href="../../66492/66492-h/66492-h.htm#Page_126">126</a>;</span><br /> -<span style="margin-left: 1em;">Arenig rocks in, <a href="../../66492/66492-h/66492-h.htm#Page_123">123</a>;</span><br /> -<span style="margin-left: 1em;">Silurian volcanic rocks of, <a href="../../66492/66492-h/66492-h.htm#Page_239">239</a>, <a href="../../66492/66492-h/66492-h.htm#Page_251">251</a>;</span><br /> -<span style="margin-left: 1em;">granites of, <a href="../../66492/66492-h/66492-h.htm#Page_290">290</a>;</span><br /> -<span style="margin-left: 1em;">Old Red Sandstone volcanic rocks of, <a href="../../66492/66492-h/66492-h.htm#Page_346">346</a>, <a href="../../66492/66492-h/66492-h.htm#Page_348">348</a>;</span><br /> -<span style="margin-left: 1em;">Carboniferous volcanic rocks of, <a href="../../66492/66492-h/66492-h.htm#Page_359">359</a>; ii. <a href="#Page_37">37</a>;</span><br /> -<span style="margin-left: 1em;">early observers among the Tertiary volcanic rooks of, <a href="#Page_109">109</a>;</span><br /> -<span style="margin-left: 1em;">Tertiary basalt plateau of, <a href="#Page_364">364</a>, <a href="#Page_370">370</a>, <a href="#Page_371">371</a>;</span><br /> -<span style="margin-left: 1em;">gabbros of, <a href="#Page_359">359</a>;</span><br /> -<span style="margin-left: 1em;">acid rocks of, <a href="#Page_420">420</a></span><br /> -Iron-ore, pisolitic (Arenig), i. <a href="../../66492/66492-h/66492-h.htm#Page_181">181</a>, <a href="../../66492/66492-h/66492-h.htm#Page_208">208</a>;<br /> -<span style="margin-left: 1em;">Tertiary, of Antrim, ii. <a href="#Page_204">204</a></span><br /> -Irvine, Mr. D. R., i. <a href="../../66492/66492-h/66492-h.htm#Page_294">294</a>, <a href="../../66492/66492-h/66492-h.htm#Page_299">299</a><br /> -Irving, Rev. A., ii. <a href="#Page_95">95</a><br /> -Isogeotherms, shifting of, i. <a href="../../66492/66492-h/66492-h.htm#Page_98">98</a><br /> -Italy, old volcanoes of, i. <a href="../../66492/66492-h/66492-h.htm#Page_4">4</a>; ii. <a href="#Page_474">474</a>, <a href="#Page_477">477</a><br /> -<br /> -<a id="J"></a>Jack, Mr. R. L., i. <a href="../../66492/66492-h/66492-h.htm#Page_294">294</a>, <a href="../../66492/66492-h/66492-h.htm#Page_308">308</a>, <a href="../../66492/66492-h/66492-h.htm#Page_369">369</a>, <a href="../../66492/66492-h/66492-h.htm#Page_375">375</a>, <a href="../../66492/66492-h/66492-h.htm#Page_396">396</a>, <a href="../../66492/66492-h/66492-h.htm#Page_404">404</a>; ii. <a href="#Page_57">57</a>, <a href="#Page_145">145</a><br /> -Jameson, Robert, i. <a href="../../66492/66492-h/66492-h.htm#Page_268">268</a>, <a href="../../66492/66492-h/66492-h.htm#Page_269">269</a>, <a href="../../66492/66492-h/66492-h.htm#Page_317">317</a>, <a href="../../66492/66492-h/66492-h.htm#Page_363">363</a>; ii. <a href="#Page_109">109</a>, <a href="#Page_161">161</a>, <a href="#Page_244">244</a>, <a href="#Page_333">333</a>, <a href="#Page_355">355</a>, <a href="#Page_364">364</a><br /> -Jan Mayen, ii. <a href="#Page_182">182</a><br /> -Jedburgh type of dolerite and basalt, i. <a href="../../66492/66492-h/66492-h.htm#Page_418">418</a><br /> -Jennings, Mr. C. V., i. <a href="../../66492/66492-h/66492-h.htm#Page_176">176</a>, <a href="../../66492/66492-h/66492-h.htm#Page_177">177</a>, <a href="../../66492/66492-h/66492-h.htm#Page_179">179</a>, <a href="../../66492/66492-h/66492-h.htm#Page_180">180</a>, <a href="../../66492/66492-h/66492-h.htm#Page_181">181</a>, <a href="../../66492/66492-h/66492-h.htm#Page_184">184</a>, <a href="../../66492/66492-h/66492-h.htm#Page_185">185</a>, <a href="../../66492/66492-h/66492-h.htm#Page_186">186</a>, <a href="../../66492/66492-h/66492-h.htm#Page_187">187</a>, <a href="../../66492/66492-h/66492-h.htm#Page_188">188</a><br /> -Johnston-Lavis, Dr., ii. <a href="#Page_261">261</a><br /> -Joints in dykes, ii. <a href="#Page_132">132</a><br /> -Judd, Prof. J. W., i. <a href="../../66492/66492-h/66492-h.htm#Page_157">157</a>, <a href="../../66492/66492-h/66492-h.htm#Page_275">275</a>; ii. <a href="#Page_115">115</a>, <a href="#Page_116">116</a>, <a href="#Page_134">134</a>, <a href="#Page_137">137</a>, <a href="#Page_162">162</a>, <a href="#Page_185">185</a>, <a href="#Page_209">209</a>, <a href="#Page_211">211</a>, <a href="#Page_245">245</a>, <a href="#Page_247">247</a>, <a href="#Page_267">267</a>, <a href="#Page_274">274</a>, <a href="#Page_278">278</a>, <a href="#Page_280">280</a>, <a href="#Page_303">303</a>, <a href="#Page_307">307</a>, <a href="#Page_309">309</a>, <a href="#Page_315">315</a>, <a href="#Page_316">316</a>, <a href="#Page_319">319</a>, <a href="#Page_322">322</a>, <a href="#Page_328">328</a>, <a href="#Page_329">329</a>, <a href="#Page_332">332</a>, <a href="#Page_333">333</a>, <a href="#Page_349">349</a>, <a href="#Page_356">356</a>, <a href="#Page_360">360</a>, <a href="#Page_372">372</a>, <a href="#Page_388">388</a>, <a href="#Page_410">410</a>, <a href="#Page_439">439</a><br /> -Jukes, J. B., i. <a href="../../66492/66492-h/66492-h.htm#Page_143">143</a>, <a href="../../66492/66492-h/66492-h.htm#Page_171">171</a>, <a href="../../66492/66492-h/66492-h.htm#Page_175">175</a>, <a href="../../66492/66492-h/66492-h.htm#Page_208">208</a>, <a href="../../66492/66492-h/66492-h.htm#Page_218">218</a>, <a href="../../66492/66492-h/66492-h.htm#Page_219">219</a>, <a href="../../66492/66492-h/66492-h.htm#Page_245">245</a>, <a href="../../66492/66492-h/66492-h.htm#Page_246">246</a>, <a href="../../66492/66492-h/66492-h.htm#Page_250">250</a>, <a href="../../66492/66492-h/66492-h.htm#Page_254">254</a>, <a href="../../66492/66492-h/66492-h.htm#Page_316">316</a>; ii. <a href="#Page_10">10</a>, <a href="#Page_20">20</a>, <a href="#Page_42">42</a>, <a href="#Page_47">47</a>, <a href="#Page_49">49</a>, <a href="#Page_101">101</a>, <a href="#Page_103">103</a>, <a href="#Page_105">105</a><br /> -Jurassic period, physical conditions of the, ii. <a href="#Page_108">108</a>, <a href="#Page_182">182</a><br /> -<br /> -<a id="K"></a>Kelly, J., i. <a href="../../66492/66492-h/66492-h.htm#Page_314">314</a><br /> -Kenmare, Old Red Sandstone volcanic rocks of, i. <a href="../../66492/66492-h/66492-h.htm#Page_350">350</a><br /> -Keratophyre, i. <a href="../../66492/66492-h/66492-h.htm#Page_247">247</a><br /> -Kerrera, Isle of, i. <a href="../../66492/66492-h/66492-h.htm#Page_342">342</a><br /> -Kersantite, i. <a href="../../66492/66492-h/66492-h.htm#Page_261">261</a><br /> -Keswick, i. <a href="../../66492/66492-h/66492-h.htm#Page_229">229</a><br /> -Kildare, Chair of, Bala volcanic rocks at, i. <a href="../../66492/66492-h/66492-h.htm#Page_245">245</a>, <a href="../../66492/66492-h/66492-h.htm#Page_256">256</a><br /> -Killarney, nodular lavas of, i. <a href="../../66492/66492-h/66492-h.htm#Page_20">20</a>, <a href="../../66492/66492-h/66492-h.htm#Page_272">272</a>, <a href="../../66492/66492-h/66492-h.htm#Page_346">346</a><br /> -Kilpatrick Hills, i. <a href="../../66492/66492-h/66492-h.htm#Page_385">385</a>, <a href="../../66492/66492-h/66492-h.htm#Page_388">388</a>, <a href="../../66492/66492-h/66492-h.htm#Page_403">403</a>, <a href="../../66492/66492-h/66492-h.htm#Page_410">410</a><br /> -Kilroe, Mr. J. R., i. <a href="../../66492/66492-h/66492-h.htm#Page_251">251</a>, <a href="../../66492/66492-h/66492-h.htm#Page_253">253</a>, <a href="../../66492/66492-h/66492-h.htm#Page_315">315</a><br /> -Kilsyth type of dolerite and basalt, i. <a href="../../66492/66492-h/66492-h.htm#Page_418">418</a><br /> -Kinahan, Mr. G. H., i. <a href="../../66492/66492-h/66492-h.htm#Page_349">349</a>; ii. <a href="#Page_45">45</a>, <a href="#Page_49">49</a>, <a href="#Page_426">426</a><br /> -Kincardineshire, volcanic necks of, i. <a href="../../66492/66492-h/66492-h.htm#Page_281">281</a>, <a href="../../66492/66492-h/66492-h.htm#Page_286">286</a>, <a href="../../66492/66492-h/66492-h.htm#Page_293">293</a>, <a href="../../66492/66492-h/66492-h.htm#Page_299">299</a>;<br /> -<span style="margin-left: 1em;">Old Red Sandstone of, <a href="../../66492/66492-h/66492-h.htm#Page_301">301</a></span><br /> -King, Mr. Clarence, i. <a href="../../66492/66492-h/66492-h.htm#Page_27">27</a><br /> -King's County, volcanic necks of, ii. <a href="#Page_37">37</a><br /> -Kippie Law type of basalt, i. <a href="../../66492/66492-h/66492-h.htm#Page_418">418</a><br /> -Kirkby, Mr. J., ii. <a href="#Page_106">106</a><br /> -Kirwan, R., ii. <a href="#Page_110">110</a><br /> -Knockfeerina, Old Red Sandstone volcanic rocks of, i. <a href="../../66492/66492-h/66492-h.htm#Page_349">349</a><br /> -Knocklayd, ii. <a href="#Page_200">200</a><br /> -Kynaston, Mr. H., i. <a href="../../66492/66492-h/66492-h.htm#Page_343">343</a><br /> -<br /> -<a id="L"></a>Labyrinthodonts, i. <a href="../../66492/66492-h/66492-h.htm#Page_356">356</a><br /> -Laccolites, i. <a href="../../66492/66492-h/66492-h.htm#Page_77">77</a>, <a href="../../66492/66492-h/66492-h.htm#Page_83">83</a>, <a href="../../66492/66492-h/66492-h.htm#Page_86">86</a>, <a href="../../66492/66492-h/66492-h.htm#Page_88">88</a>, <a href="../../66492/66492-h/66492-h.htm#Page_98">98</a>, <a href="../../66492/66492-h/66492-h.htm#Page_99">99</a>, <a href="../../66492/66492-h/66492-h.htm#Page_190">190</a>; ii. <a href="#Page_363">363</a><br /> -Lacroix, Prof., i. <a href="../../66492/66492-h/66492-h.htm#Page_96">96</a><br /> -Lacustrine volcanic eruptions, i. <a href="../../66492/66492-h/66492-h.htm#Page_49">49</a><br /> -Lagorio, Dr. A., ii. <a href="#Page_137">137</a><br /> -Lake, Mr. P., i. <a href="../../66492/66492-h/66492-h.htm#Page_177">177</a>, <a href="../../66492/66492-h/66492-h.htm#Page_179">179</a><br /> -Lake-District, i. <a href="../../66492/66492-h/66492-h.htm#Page_227">227</a>, <a href="../../66492/66492-h/66492-h.htm#Page_290">290</a>;<br /> -<span style="margin-left: 1em;">Vesuvian cone of, i. <a href="../../66492/66492-h/66492-h.htm#Page_42">42</a>, <a href="../../66492/66492-h/66492-h.htm#Page_45">45</a></span><br /> -"Lake Caledonia," i. <a href="../../66492/66492-h/66492-h.htm#Page_272">272</a>, <a href="../../66492/66492-h/66492-h.htm#Page_294">294</a>, <a href="../../66492/66492-h/66492-h.htm#Page_296">296</a><br /> -"Lake of Lorne," i. <a href="../../66492/66492-h/66492-h.htm#Page_341">341</a><br /> -"Lake Orcadie," i. <a href="../../66492/66492-h/66492-h.htm#Page_266">266</a>, <a href="../../66492/66492-h/66492-h.htm#Page_271">271</a>, <a href="../../66492/66492-h/66492-h.htm#Page_343">343</a>, <a href="../../66492/66492-h/66492-h.htm#Page_350">350</a><br /> -Lakes, eruptions in, i. <a href="../../66492/66492-h/66492-h.htm#Page_49">49</a>;<br /> -<span style="margin-left: 1em;">crater, <a href="../../66492/66492-h/66492-h.htm#Page_58">58</a>;</span><br /> -<span style="margin-left: 1em;">of Old Red Sandstone, <a href="../../66492/66492-h/66492-h.htm#Page_264">264</a></span><br /> -Lambay Island, conglomerates of, i. <a href="../../66492/66492-h/66492-h.htm#Page_244">244</a><br /> -Lammermuir, granites of, i. <a href="../../66492/66492-h/66492-h.htm#Page_290">290</a>, <a href="../../66492/66492-h/66492-h.htm#Page_340">340</a><br /> -Lamplugh, Mr. G. W., i. <a href="../../66492/66492-h/66492-h.htm#Page_32">32</a>, <a href="../../66492/66492-h/66492-h.htm#Page_220">220</a>; ii. <a href="#Page_23">23</a>, <a href="#Page_28">28</a><br /> -</p> - -<p><span class="pagenum" id="Page_486">- 486 -</span></p> - -<p> -Lamprophyre, i. <a href="../../66492/66492-h/66492-h.htm#Page_291">291</a>, <a href="../../66492/66492-h/66492-h.htm#Page_293">293</a><br /> -Lanarkshire, i. <a href="../../66492/66492-h/66492-h.htm#Page_291">291</a>, <a href="../../66492/66492-h/66492-h.htm#Page_368">368</a>, <a href="../../66492/66492-h/66492-h.htm#Page_416">416</a><br /> -Land, sculpture of the, i. <a href="../../66492/66492-h/66492-h.htm#Page_101">101</a>, <a href="../../66492/66492-h/66492-h.htm#Page_102">102</a><br /> -Landslips, ii. <a href="#Page_200">200</a>, <a href="#Page_287">287</a><br /> -Lankester, Prof. E. Ray, i. <a href="../../66492/66492-h/66492-h.htm#Page_310">310</a><br /> -Lapilli, volcanic, i. <a href="../../66492/66492-h/66492-h.htm#Page_33">33</a>, <a href="../../66492/66492-h/66492-h.htm#Page_34">34</a>, <a href="../../66492/66492-h/66492-h.htm#Page_61">61</a>, <a href="../../66492/66492-h/66492-h.htm#Page_151">151</a><br /> -Lapworth, Prof. C., i. <a href="../../66492/66492-h/66492-h.htm#Page_130">130</a>, <a href="../../66492/66492-h/66492-h.htm#Page_132">132</a>, <a href="../../66492/66492-h/66492-h.htm#Page_137">137</a>, <a href="../../66492/66492-h/66492-h.htm#Page_171">171</a>, <a href="../../66492/66492-h/66492-h.htm#Page_172">172</a>, <a href="../../66492/66492-h/66492-h.htm#Page_189">189</a>, <a href="../../66492/66492-h/66492-h.htm#Page_190">190</a>, <a href="../../66492/66492-h/66492-h.htm#Page_196">196</a><br /> -Largs, volcanic vent near, i. <a href="../../66492/66492-h/66492-h.htm#Page_56">56</a>, <a href="../../66492/66492-h/66492-h.htm#Page_396">396</a>, <a href="../../66492/66492-h/66492-h.htm#Page_397">397</a>, <a href="../../66492/66492-h/66492-h.htm#Page_401">401</a><br /> -Lasaulx, Prof. von, ii. <a href="#Page_365">365</a>, <a href="#Page_371">371</a>, <a href="#Page_426">426</a><br /> -Laurentian gneiss, i. <a href="../../66492/66492-h/66492-h.htm#Page_110">110</a><br /> -<a id="Lavas"></a>Lavas, classification of, i. <a href="../../66492/66492-h/66492-h.htm#Page_14">14</a>;<br /> -<span style="margin-left: 1em;">flow-structure of, <a href="../../66492/66492-h/66492-h.htm#Page_16">16</a>, <a href="../../66492/66492-h/66492-h.htm#Page_21">21</a>;</span><br /> -<span style="margin-left: 1em;">vesicular structure of, <a href="../../66492/66492-h/66492-h.htm#Page_17">17</a>;</span><br /> -<span style="margin-left: 1em;">glass in, <a href="../../66492/66492-h/66492-h.htm#Page_18">18</a>;</span><br /> -<span style="margin-left: 1em;">devitrification of, <a href="../../66492/66492-h/66492-h.htm#Page_19">19</a>;</span><br /> -<span style="margin-left: 1em;">bedding of, <a href="../../66492/66492-h/66492-h.htm#Page_24">24</a>;</span><br /> -<span style="margin-left: 1em;">effect of water on molten, <a href="../../66492/66492-h/66492-h.htm#Page_25">25</a>, <a href="../../66492/66492-h/66492-h.htm#Page_334">334</a>;</span><br /> -<span style="margin-left: 1em;">sack-like or pillow-structure of, <a href="../../66492/66492-h/66492-h.htm#Page_26">26</a>, <a href="../../66492/66492-h/66492-h.htm#Page_184">184</a>, <a href="../../66492/66492-h/66492-h.htm#Page_193">193</a>, <a href="../../66492/66492-h/66492-h.htm#Page_201">201</a>, <a href="../../66492/66492-h/66492-h.htm#Page_240">240</a>, <a href="../../66492/66492-h/66492-h.htm#Page_244">244</a>, <a href="../../66492/66492-h/66492-h.htm#Page_252">252</a>;</span><br /> -<span style="margin-left: 1em;">seldom occur in solitary sheets, <a href="../../66492/66492-h/66492-h.htm#Page_26">26</a>;</span><br /> -<span style="margin-left: 1em;">variations in structure in, <a href="../../66492/66492-h/66492-h.htm#Page_27">27</a>;</span><br /> -<span style="margin-left: 1em;">sequence of, in eruptions, <a href="../../66492/66492-h/66492-h.htm#Page_28">28</a>, <a href="../../66492/66492-h/66492-h.htm#Page_92">92</a>, <a href="../../66492/66492-h/66492-h.htm#Page_377">377</a>, <a href="../../66492/66492-h/66492-h.htm#Page_386">386</a>;</span><br /> -<span style="margin-left: 1em;">crusts of, disrupted in volcanic explosions, <a href="../../66492/66492-h/66492-h.htm#Page_58">58</a>, <a href="../../66492/66492-h/66492-h.htm#Page_59">59</a>, <a href="../../66492/66492-h/66492-h.htm#Page_60">60</a>; ii. <a href="#Page_189">189</a>;</span><br /> -<span style="margin-left: 1em;">alternations of acid and basic, i. <a href="../../66492/66492-h/66492-h.htm#Page_28">28</a>, <a href="../../66492/66492-h/66492-h.htm#Page_61">61</a>, <a href="../../66492/66492-h/66492-h.htm#Page_152">152</a>, <a href="../../66492/66492-h/66492-h.htm#Page_157">157</a>, <a href="../../66492/66492-h/66492-h.htm#Page_165">165</a>, <a href="../../66492/66492-h/66492-h.htm#Page_207">207</a>, <a href="../../66492/66492-h/66492-h.htm#Page_213">213</a>, <a href="../../66492/66492-h/66492-h.htm#Page_284">284</a>, <a href="../../66492/66492-h/66492-h.htm#Page_318">318</a>; ii. <a href="#Page_236">236</a>, <a href="#Page_266">266</a>;</span><br /> -<span style="margin-left: 1em;">contrasted with intrusive rocks, i. <a href="../../66492/66492-h/66492-h.htm#Page_78">78</a>;</span><br /> -<span style="margin-left: 1em;">sandstone veins in, <a href="../../66492/66492-h/66492-h.htm#Page_283">283</a>, <a href="../../66492/66492-h/66492-h.htm#Page_300">300</a>, <a href="../../66492/66492-h/66492-h.htm#Page_303">303</a>, <a href="../../66492/66492-h/66492-h.htm#Page_320">320</a>, <a href="../../66492/66492-h/66492-h.htm#Page_327">327</a>, <a href="../../66492/66492-h/66492-h.htm#Page_333">333</a>, <a href="../../66492/66492-h/66492-h.htm#Page_337">337</a>; ii. <a href="#Page_59">59</a>, <a href="#Page_98">98</a>;</span><br /> -<span style="margin-left: 1em;">shattered or agglomerate structure of, <a href="#Page_99">99</a>;</span><br /> -<span style="margin-left: 1em;">metamorphism of, i. <a href="../../66492/66492-h/66492-h.htm#Page_231">231</a>, <a href="../../66492/66492-h/66492-h.htm#Page_240">240</a>, <a href="../../66492/66492-h/66492-h.htm#Page_338">338</a>; ii. <a href="#Page_272">272</a>, <a href="#Page_276">276</a>, <a href="#Page_337">337</a>, <a href="#Page_339">339</a>, <a href="#Page_340">340</a>, <a href="#Page_347">347</a>, <a href="#Page_355">355</a>, <a href="#Page_379">379</a>, <a href="#Page_386">386</a>, <a href="#Page_397">397</a>, <a href="#Page_399">399</a>, <a href="#Page_400">400</a>, <a href="#Page_404">404</a>, <a href="#Page_413">413</a></span><br /> -—— Cambrian, i. <a href="../../66492/66492-h/66492-h.htm#Page_152">152</a>, <a href="../../66492/66492-h/66492-h.htm#Page_168">168</a><br /> -—— Silurian of Merionethshire, i. <a href="../../66492/66492-h/66492-h.htm#Page_183">183</a>;<br /> -<span style="margin-left: 1em;">Scotland, <a href="../../66492/66492-h/66492-h.htm#Page_191">191</a>;</span><br /> -<span style="margin-left: 1em;">Builth, <a href="../../66492/66492-h/66492-h.htm#Page_203">203</a>;</span><br /> -<span style="margin-left: 1em;">Pembrokeshire, <a href="../../66492/66492-h/66492-h.htm#Page_205">205</a>;</span><br /> -<span style="margin-left: 1em;">Caernarvonshire, <a href="../../66492/66492-h/66492-h.htm#Page_207">207</a>;</span><br /> -<span style="margin-left: 1em;">Berwyn Hills, <a href="../../66492/66492-h/66492-h.htm#Page_218">218</a>;</span><br /> -<span style="margin-left: 1em;">Anglesey, <a href="../../66492/66492-h/66492-h.htm#Page_219">219</a>;</span><br /> -<span style="margin-left: 1em;">Lake District, <a href="../../66492/66492-h/66492-h.htm#Page_227">227</a>;</span><br /> -<span style="margin-left: 1em;">Gloucestershire, <a href="../../66492/66492-h/66492-h.htm#Page_238">238</a>;</span><br /> -<span style="margin-left: 1em;">Ireland, <a href="../../66492/66492-h/66492-h.htm#Page_239">239</a>, <a href="../../66492/66492-h/66492-h.htm#Page_254">254</a></span><br /> -—— Lower Old Red Sandstone, i. <a href="../../66492/66492-h/66492-h.htm#Page_273">273</a>, <a href="../../66492/66492-h/66492-h.htm#Page_281">281</a>, <a href="../../66492/66492-h/66492-h.htm#Page_294">294</a>, <a href="../../66492/66492-h/66492-h.htm#Page_317">317</a><br /> -—— Carboniferous, i. <a href="../../66492/66492-h/66492-h.htm#Page_377">377</a>, <a href="../../66492/66492-h/66492-h.htm#Page_384">384</a>, <a href="../../66492/66492-h/66492-h.htm#Page_417">417</a>, <a href="../../66492/66492-h/66492-h.htm#Page_436">436</a>, <a href="../../66492/66492-h/66492-h.htm#Page_440">440</a>, <a href="../../66492/66492-h/66492-h.htm#Page_443">443</a>; ii. <a href="#Page_8">8</a>, <a href="#Page_18">18</a>, <a href="#Page_34">34</a>, <a href="#Page_45">45</a><br /> -—— Permian, ii. <a href="#Page_68">68</a>, <a href="#Page_96">96</a><br /> -—— Tertiary, ii. <a href="#Page_183">183</a>;<br /> -<span style="margin-left: 1em;">types of, <a href="#Page_186">186</a>;</span><br /> -<span style="margin-left: 1em;">banding of, <a href="#Page_189">189</a>;</span><br /> -<span style="margin-left: 1em;">thickness of, <a href="#Page_192">192</a>;</span><br /> -<span style="margin-left: 1em;">lenticular character of, <a href="#Page_193">193</a>;</span><br /> -<span style="margin-left: 1em;">of Antrim, <a href="#Page_199">199</a>;</span><br /> -<span style="margin-left: 1em;">irregular bedding of the vitreous, <a href="#Page_243">243</a></span><br /> -—— modern Icelandic eruptions of, ii. <a href="#Page_261">261</a><br /> -Lava-domes, ii. <a href="#Page_265">265</a><br /> -Lava-plug of volcanic funnels, permanence of, i. <a href="../../66492/66492-h/66492-h.htm#Page_40">40</a>, <a href="../../66492/66492-h/66492-h.htm#Page_41">41</a>, <a href="../../66492/66492-h/66492-h.htm#Page_55">55</a>, <a href="../../66492/66492-h/66492-h.htm#Page_73">73</a>, <a href="../../66492/66492-h/66492-h.htm#Page_76">76</a>, <a href="../../66492/66492-h/66492-h.htm#Page_430">430</a><br /> -Lawson, Prof. A. C., i. <a href="../../66492/66492-h/66492-h.htm#Page_82">82</a><br /> -Leaf-beds, ii. <a href="#Page_198">198</a><br /> -Lebour, Prof., i. <a href="../../66492/66492-h/66492-h.htm#Page_336">336</a>; ii. <a href="#Page_2">2</a>, <a href="#Page_3">3</a>, <a href="#Page_5">5</a>, <a href="#Page_7">7</a><br /> -Leckstone, i. <a href="../../66492/66492-h/66492-h.htm#Page_419">419</a>, <a href="../../66492/66492-h/66492-h.htm#Page_442">442</a>, <a href="../../66492/66492-h/66492-h.htm#Page_443">443</a><br /> -Lecoq, H., i. <a href="../../66492/66492-h/66492-h.htm#Page_45">45</a>; ii. <a href="#Page_373">373</a><br /> -Leinster granite, ii. <a href="#Page_245">245</a>, <a href="#Page_249">249</a>, <a href="#Page_290">290</a><br /> -Lewisian Gneiss, i. <a href="../../66492/66492-h/66492-h.htm#Page_81">81</a>, <a href="../../66492/66492-h/66492-h.htm#Page_110">110</a>, <a href="../../66492/66492-h/66492-h.htm#Page_111">111</a>, <a href="../../66492/66492-h/66492-h.htm#Page_113">113</a>, <a href="../../66492/66492-h/66492-h.htm#Page_118">118</a><br /> -Liddesdale, Carboniferous volcanic vents of, i. <a href="../../66492/66492-h/66492-h.htm#Page_55">55</a>, <a href="../../66492/66492-h/66492-h.htm#Page_416">416</a>, <a href="../../66492/66492-h/66492-h.htm#Page_425">425</a>, <a href="../../66492/66492-h/66492-h.htm#Page_440">440</a>, <a href="../../66492/66492-h/66492-h.htm#Page_475">475</a><br /> -Life, earliest traces of, i. <a href="../../66492/66492-h/66492-h.htm#Page_140">140</a><br /> -Lignite in Tertiary volcanic series, ii. <a href="#Page_198">198</a><br /> -Limburgite, i. <a href="../../66492/66492-h/66492-h.htm#Page_377">377</a>, <a href="../../66492/66492-h/66492-h.htm#Page_408">408</a>, <a href="../../66492/66492-h/66492-h.htm#Page_417">417</a>, <a href="../../66492/66492-h/66492-h.htm#Page_420">420</a>, <a href="../../66492/66492-h/66492-h.htm#Page_448">448</a>; ii. <a href="#Page_40">40</a>, <a href="#Page_46">46</a><br /> -—— type, i. <a href="../../66492/66492-h/66492-h.htm#Page_418">418</a><br /> -Limerick, Carboniferous volcanic rocks of, i. <a href="../../66492/66492-h/66492-h.htm#Page_421">421</a>, <a href="../../66492/66492-h/66492-h.htm#Page_430">430</a>; ii. <a href="#Page_41">41</a>;<br /> -<span style="margin-left: 1em;">Old Red Sandstone volcanic rocks of, i. <a href="../../66492/66492-h/66492-h.htm#Page_348">348</a></span><br /> -Limestone, metamorphism of, i. <a href="../../66492/66492-h/66492-h.htm#Page_72">72</a>, <a href="../../66492/66492-h/66492-h.htm#Page_451">451</a>; ii. <a href="#Page_14">14</a>, <a href="#Page_22">22</a>, <a href="#Page_164">164</a>, <a href="#Page_280">280</a>, <a href="#Page_383">383</a><br /> -Lindley and Hutton on Eigg conifer, ii. <a href="#Page_238">238</a><br /> -Lingula Flags, i. <a href="../../66492/66492-h/66492-h.htm#Page_144">144</a>, <a href="../../66492/66492-h/66492-h.htm#Page_177">177</a><br /> -Linlithgowshire (<i>see</i> <a href="#West_Lothian">West Lothian</a>)<br /> -Lintrathen, porphyry of, i. <a href="../../66492/66492-h/66492-h.htm#Page_277">277</a>, <a href="../../66492/66492-h/66492-h.htm#Page_292">292</a>, <a href="../../66492/66492-h/66492-h.htm#Page_311">311</a><br /> -Lion's haunch type of dolerite and basalt, i. <a href="../../66492/66492-h/66492-h.htm#Page_418">418</a><br /> -Lithomarge, ii. <a href="#Page_197">197</a>, <a href="#Page_204">204</a><br /> -Lizard, rocks at the, i. <a href="../../66492/66492-h/66492-h.htm#Page_194">194</a><br /> -Llanberis, Pass of, i. <a href="../../66492/66492-h/66492-h.htm#Page_159">159</a>, <a href="../../66492/66492-h/66492-h.htm#Page_163">163</a><br /> -—— group, i. <a href="../../66492/66492-h/66492-h.htm#Page_144">144</a><br /> -Llandeilo group, i. <a href="../../66492/66492-h/66492-h.htm#Page_175">175</a>, <a href="../../66492/66492-h/66492-h.htm#Page_196">196</a>, <a href="../../66492/66492-h/66492-h.htm#Page_242">242</a>;<br /> -<span style="margin-left: 1em;">volcanic rocks of, <a href="../../66492/66492-h/66492-h.htm#Page_186">186</a>, <a href="../../66492/66492-h/66492-h.htm#Page_202">202</a>, <a href="../../66492/66492-h/66492-h.htm#Page_221">221</a>, <a href="../../66492/66492-h/66492-h.htm#Page_227">227</a>, <a href="../../66492/66492-h/66492-h.htm#Page_241">241</a></span><br /> -Llandeiniolen, i. <a href="../../66492/66492-h/66492-h.htm#Page_160">160</a><br /> -Llandovery group, i. <a href="../../66492/66492-h/66492-h.htm#Page_175">175</a>, <a href="../../66492/66492-h/66492-h.htm#Page_196">196</a>;<br /> -<span style="margin-left: 1em;">possible volcanic rocks of, <a href="../../66492/66492-h/66492-h.htm#Page_238">238</a></span><br /> -Llangadock, i. <a href="../../66492/66492-h/66492-h.htm#Page_205">205</a><br /> -Llangefni, i. <a href="../../66492/66492-h/66492-h.htm#Page_220">220</a><br /> -Llanllyfni, i. <a href="../../66492/66492-h/66492-h.htm#Page_161">161</a><br /> -Llanwrtyd, i. <a href="../../66492/66492-h/66492-h.htm#Page_204">204</a><br /> -Lleyn Peninsula, i. <a href="../../66492/66492-h/66492-h.htm#Page_208">208</a>, <a href="../../66492/66492-h/66492-h.htm#Page_209">209</a>, <a href="../../66492/66492-h/66492-h.htm#Page_213">213</a>, <a href="../../66492/66492-h/66492-h.htm#Page_215">215</a><br /> -Lloyd Morgan, Prof., i. <a href="../../66492/66492-h/66492-h.htm#Page_145">145</a>, <a href="../../66492/66492-h/66492-h.htm#Page_147">147</a>, <a href="../../66492/66492-h/66492-h.htm#Page_154">154</a><br /> -Llyn Padarn, i. <a href="../../66492/66492-h/66492-h.htm#Page_157">157</a>, <a href="../../66492/66492-h/66492-h.htm#Page_159">159</a>, <a href="../../66492/66492-h/66492-h.htm#Page_160">160</a><br /> -Loch Carron, pre-Cambrian rocks of, i. <a href="../../66492/66492-h/66492-h.htm#Page_115">115</a>, <a href="../../66492/66492-h/66492-h.htm#Page_117">117</a><br /> -Loch Lomond, dykes at, ii. <a href="#Page_180">180</a><br /> -Loch Tay Limestone, i. <a href="../../66492/66492-h/66492-h.htm#Page_122">122</a>, <a href="../../66492/66492-h/66492-h.htm#Page_124">124</a>, <a href="../../66492/66492-h/66492-h.htm#Page_125">125</a><br /> -Lomas, Mr. J., ii. <a href="#Page_191">191</a>, <a href="#Page_322">322</a><br /> -Longulites, ii. <a href="#Page_135">135</a><br /> -Lonsdale, W., i. <a href="../../66492/66492-h/66492-h.htm#Page_257">257</a><br /> -Longmyndian rocks, i. <a href="../../66492/66492-h/66492-h.htm#Page_111">111</a>, <a href="../../66492/66492-h/66492-h.htm#Page_129">129</a>, <a href="../../66492/66492-h/66492-h.htm#Page_132">132</a><br /> -Lorne, volcanic rocks of, i. <a href="../../66492/66492-h/66492-h.htm#Page_102">102</a>, <a href="../../66492/66492-h/66492-h.htm#Page_271">271</a>, <a href="../../66492/66492-h/66492-h.htm#Page_281">281</a>, <a href="../../66492/66492-h/66492-h.htm#Page_341">341</a><br /> -Lough Mask, Silurian volcanic rocks of, i. <a href="../../66492/66492-h/66492-h.htm#Page_251">251</a><br /> -—— Nafooey, Silurian volcanic rocks of, i. <a href="../../66492/66492-h/66492-h.htm#Page_251">251</a><br /> -—— Neagh, subsidence of site of, ii. <a href="#Page_201">201</a>, <a href="#Page_205">205</a>;<br /> -<span style="margin-left: 1em;">history of, <a href="#Page_448">448</a></span><br /> -Ludlow group, i. <a href="../../66492/66492-h/66492-h.htm#Page_175">175</a><br /> -Lycopods, fossil, i. <a href="../../66492/66492-h/66492-h.htm#Page_174">174</a><br /> -<br /> -<a id="M"></a>Maare, volcanic, i. <a href="../../66492/66492-h/66492-h.htm#Page_58">58</a>; ii. <a href="#Page_287">287</a>, <a href="#Page_288">288</a>, <a href="#Page_296">296</a><br /> -Macconochie, Mr. A., ii. <a href="#Page_58">58</a><br /> -Macculloch, John, i. <a href="../../66492/66492-h/66492-h.htm#Page_95">95</a>, <a href="../../66492/66492-h/66492-h.htm#Page_269">269</a>, <a href="../../66492/66492-h/66492-h.htm#Page_270">270</a>; ii. <a href="#Page_22">22</a>, <a href="#Page_111">111</a>, <a href="#Page_113">113</a>, <a href="#Page_123">123</a>, <a href="#Page_140">140</a>, <a href="#Page_154">154</a>, <a href="#Page_156">156</a>, <a href="#Page_159">159</a>, <a href="#Page_172">172</a>, <a href="#Page_174">174</a>, <a href="#Page_175">175</a>, <a href="#Page_213">213</a>, <a href="#Page_217">217</a>, <a href="#Page_231">231</a>, <a href="#Page_237">237</a>, <a href="#Page_244">244</a>, <a href="#Page_251">251</a>, <a href="#Page_280">280</a>, <a href="#Page_293">293</a>, <a href="#Page_304">304-307</a>, <a href="#Page_310">310</a>, <a href="#Page_314">314</a>, <a href="#Page_315">315</a>, <a href="#Page_327">327</a>, <a href="#Page_349">349</a>, <a href="#Page_364">364</a>, <a href="#Page_371">371</a>, <a href="#Page_403">403</a>, <a href="#Page_406">406</a>, <a href="#Page_408">408</a>, <a href="#Page_409">409</a>, <a href="#Page_418">418</a><br /> -Macknight, Dr., i. <a href="../../66492/66492-h/66492-h.htm#Page_268">268</a>, <a href="../../66492/66492-h/66492-h.htm#Page_317">317</a><br /> -Maclaren, Charles, i. <a href="../../66492/66492-h/66492-h.htm#Page_269">269</a>, <a href="../../66492/66492-h/66492-h.htm#Page_317">317</a>, <a href="../../66492/66492-h/66492-h.htm#Page_325">325</a>, <a href="../../66492/66492-h/66492-h.htm#Page_363">363</a>, <a href="../../66492/66492-h/66492-h.htm#Page_372">372</a>, <a href="../../66492/66492-h/66492-h.htm#Page_373">373</a>, <a href="../../66492/66492-h/66492-h.htm#Page_451">451</a>, <a href="../../66492/66492-h/66492-h.htm#Page_462">462</a>; ii. <a href="#Page_67">67</a><br /> -M'Henry, Mr. A., i. <a href="../../66492/66492-h/66492-h.htm#Page_32">32</a>, <a href="../../66492/66492-h/66492-h.htm#Page_240">240</a>, <a href="../../66492/66492-h/66492-h.htm#Page_242">242</a>, <a href="../../66492/66492-h/66492-h.htm#Page_244">244</a>, <a href="../../66492/66492-h/66492-h.htm#Page_314">314</a>, <a href="../../66492/66492-h/66492-h.htm#Page_347">347</a>; ii. <a href="#Page_201">201</a>, <a href="#Page_204">204</a>, <a href="#Page_272">272</a>, <a href="#Page_293">293</a>, <a href="#Page_426">426</a>, <a href="#Page_427">427</a>, <a href="#Page_428">428</a>, <a href="#Page_429">429</a><br /> -M'Mahon, General, i. <a href="../../66492/66492-h/66492-h.htm#Page_260">260</a>; ii. <a href="#Page_35">35</a>, <a href="#Page_36">36</a><br /> -Magma, volcanic, explosive energy of, i. <a href="../../66492/66492-h/66492-h.htm#Page_13">13</a>;<br /> -<span style="margin-left: 1em;">gases and vapours dissolved in, <a href="../../66492/66492-h/66492-h.htm#Page_13">13</a>, <a href="../../66492/66492-h/66492-h.htm#Page_72">72</a>, <a href="../../66492/66492-h/66492-h.htm#Page_97">97</a>, <a href="../../66492/66492-h/66492-h.htm#Page_99">99</a>;</span><br /> -<span style="margin-left: 1em;">differentiation of, <a href="../../66492/66492-h/66492-h.htm#Page_22">22</a>, <a href="../../66492/66492-h/66492-h.htm#Page_84">84</a>, <a href="../../66492/66492-h/66492-h.htm#Page_91">91</a>;</span><br /> -<span style="margin-left: 1em;">solvent action of, <a href="../../66492/66492-h/66492-h.htm#Page_82">82</a>, <a href="../../66492/66492-h/66492-h.htm#Page_84">84</a>, <a href="../../66492/66492-h/66492-h.htm#Page_85">85</a>, <a href="../../66492/66492-h/66492-h.htm#Page_99">99</a>; ii. <a href="#Page_392">392</a>, <a href="#Page_415">415</a>, <a href="#Page_422">422</a>, <a href="#Page_433">433</a>;</span><br /> -<span style="margin-left: 1em;">heterogeneity of, i. <a href="../../66492/66492-h/66492-h.htm#Page_85">85</a>, <a href="../../66492/66492-h/66492-h.htm#Page_90">90</a>, <a href="../../66492/66492-h/66492-h.htm#Page_91">91</a>; ii. <a href="#Page_344">344</a>, <a href="#Page_360">360</a>, <a href="#Page_476">476</a>;</span><br /> -<span style="margin-left: 1em;">metamorphic action of, i. <a href="../../66492/66492-h/66492-h.htm#Page_94">94</a>;</span><br /> -<span style="margin-left: 1em;">alteration of, by incorporation of foreign material, <a href="../../66492/66492-h/66492-h.htm#Page_96">96</a>; ii. <a href="#Page_386">386</a>, <a href="#Page_390">390</a>, <a href="#Page_392">392</a>;</span><br /> -<span style="margin-left: 1em;">conditions for the injection of, i. <a href="../../66492/66492-h/66492-h.htm#Page_97">97</a>, <a href="../../66492/66492-h/66492-h.htm#Page_99">99</a></span><br /> -Magnesian Limestone, ii. <a href="#Page_54">54</a><br /> -Malvern, pre-Cambrian volcanic rocks of, i. <a href="../../66492/66492-h/66492-h.htm#Page_133">133</a>;<br /> -<span style="margin-left: 1em;">Cambrian volcanic rocks of, <a href="../../66492/66492-h/66492-h.htm#Page_169">169</a></span><br /> -Man, Isle of, Carboniferous volcanic rocks of, ii. <a href="#Page_22">22</a><br /> -Manod, i. <a href="../../66492/66492-h/66492-h.htm#Page_184">184</a><br /> -Marl Slate, ii. <a href="#Page_54">54</a><br /> -Marl, volcanic, i. <a href="../../66492/66492-h/66492-h.htm#Page_423">423</a>, <a href="../../66492/66492-h/66492-h.htm#Page_436">436</a>, <a href="../../66492/66492-h/66492-h.htm#Page_440">440</a>, <a href="../../66492/66492-h/66492-h.htm#Page_444">444</a>, <a href="../../66492/66492-h/66492-h.htm#Page_466">466</a><br /> -Marr, Mr. J. E., i. <a href="../../66492/66492-h/66492-h.htm#Page_227">227</a>, <a href="../../66492/66492-h/66492-h.htm#Page_228">228</a>, <a href="../../66492/66492-h/66492-h.htm#Page_230">230</a>, <a href="../../66492/66492-h/66492-h.htm#Page_231">231</a>, <a href="../../66492/66492-h/66492-h.htm#Page_232">232</a>, <a href="../../66492/66492-h/66492-h.htm#Page_236">236</a>, <a href="../../66492/66492-h/66492-h.htm#Page_237">237</a>, <a href="../../66492/66492-h/66492-h.htm#Page_238">238</a>, <a href="../../66492/66492-h/66492-h.htm#Page_290">290</a>; ii. <a href="#Page_189">189</a><br /> -Matlock Bath, ii. <a href="#Page_13">13</a>, <a href="#Page_22">22</a><br /> -Mediterranean, earthquakes and volcanoes of, i. <a href="../../66492/66492-h/66492-h.htm#Page_1">1</a><br /> -Melaphyre, i. <a href="../../66492/66492-h/66492-h.htm#Page_131">131</a><br /> -Mello, Mr. J. M., ii. <a href="#Page_22">22</a><br /> -Melrose, rocks near, i. <a href="../../66492/66492-h/66492-h.htm#Page_397">397</a>, <a href="../../66492/66492-h/66492-h.htm#Page_398">398</a>, <a href="../../66492/66492-h/66492-h.htm#Page_400">400</a>, <a href="../../66492/66492-h/66492-h.htm#Page_425">425</a><br /> -</p> - -<p><span class="pagenum" id="Page_487">- 487 -</span></p> - -<p> -Melting of rocks by igneous intrusions, i. <a href="../../66492/66492-h/66492-h.htm#Page_82">82</a>, <a href="../../66492/66492-h/66492-h.htm#Page_84">84</a>, <a href="../../66492/66492-h/66492-h.htm#Page_85">85</a>, <a href="../../66492/66492-h/66492-h.htm#Page_96">96</a>, <a href="../../66492/66492-h/66492-h.htm#Page_99">99</a>; ii. <a href="#Page_163">163</a>, <a href="#Page_392">392</a>, <a href="#Page_415">415</a>, <a href="#Page_422">422</a>, <a href="#Page_433">433</a><br /> -Menai Strait, i. <a href="../../66492/66492-h/66492-h.htm#Page_159">159</a><br /> -Menevian group, i. <a href="../../66492/66492-h/66492-h.htm#Page_114">114</a><br /> -Merse, volcanic plateau of the, i. <a href="../../66492/66492-h/66492-h.htm#Page_375">375</a><br /> -Metamorphism of tuffs, i. <a href="../../66492/66492-h/66492-h.htm#Page_157">157</a>;<br /> -<span style="margin-left: 1em;">of lavas (<i>see</i> under <a href="#Lavas">Lavas</a>)</span><br /> -—— by lavas, i. <a href="../../66492/66492-h/66492-h.htm#Page_27">27</a>;<br /> -<span style="margin-left: 1em;">by sills and bosses, <a href="../../66492/66492-h/66492-h.htm#Page_87">87</a>, <a href="../../66492/66492-h/66492-h.htm#Page_94">94</a>, <a href="../../66492/66492-h/66492-h.htm#Page_95">95</a>, <a href="../../66492/66492-h/66492-h.htm#Page_216">216</a>, <a href="../../66492/66492-h/66492-h.htm#Page_236">236</a>, <a href="../../66492/66492-h/66492-h.htm#Page_331">331</a>, <a href="../../66492/66492-h/66492-h.htm#Page_338">338</a>, <a href="../../66492/66492-h/66492-h.htm#Page_431">431</a>; ii. <a href="#Page_7">7</a>, <a href="#Page_22">22</a>, <a href="#Page_36">36</a>, <a href="#Page_131">131</a>, <a href="#Page_148">148</a>, <a href="#Page_163">163</a>, <a href="#Page_299">299</a>, <a href="#Page_300">300</a>, <a href="#Page_310">310</a>, <a href="#Page_337">337</a>, <a href="#Page_339">339</a>, <a href="#Page_340">340</a>, <a href="#Page_347">347</a>, <a href="#Page_355">355</a>, <a href="#Page_356">356</a>, <a href="#Page_357">357</a>, <a href="#Page_358">358</a>, <a href="#Page_362">362</a>, <a href="#Page_378">378</a>, <a href="#Page_383">383</a>, <a href="#Page_386">386</a>, <a href="#Page_397">397</a>, <a href="#Page_399">399</a>, <a href="#Page_400">400</a>, <a href="#Page_404">404</a>, <a href="#Page_413">413</a></span><br /> -—— in vents, i. <a href="../../66492/66492-h/66492-h.htm#Page_67">67</a>, <a href="../../66492/66492-h/66492-h.htm#Page_71">71</a>, <a href="../../66492/66492-h/66492-h.htm#Page_82">82</a>, <a href="../../66492/66492-h/66492-h.htm#Page_93">93</a>, <a href="../../66492/66492-h/66492-h.htm#Page_399">399</a>, <a href="../../66492/66492-h/66492-h.htm#Page_404">404</a>; ii. <a href="#Page_39">39</a>, <a href="#Page_78">78</a>, <a href="#Page_292">292</a><br /> -—— around vents, i. <a href="../../66492/66492-h/66492-h.htm#Page_72">72</a>, <a href="../../66492/66492-h/66492-h.htm#Page_349">349</a>, <a href="../../66492/66492-h/66492-h.htm#Page_350">350</a>, <a href="../../66492/66492-h/66492-h.htm#Page_352">352</a>, <a href="../../66492/66492-h/66492-h.htm#Page_399">399</a>, <a href="../../66492/66492-h/66492-h.htm#Page_404">404</a>, <a href="../../66492/66492-h/66492-h.htm#Page_432">432</a>; ii. <a href="#Page_76">76</a>, <a href="#Page_272">272</a>, <a href="#Page_273">273</a>, <a href="#Page_276">276</a>, <a href="#Page_280">280</a>, <a href="#Page_292">292</a><br /> -—— regional, i. <a href="../../66492/66492-h/66492-h.htm#Page_121">121</a>, <a href="../../66492/66492-h/66492-h.htm#Page_123">123</a>; ii. <a href="#Page_35">35</a><br /> -<a id="Mica"></a>Mica, ejected crystals of, in volcanic breccias, ii. <a href="#Page_49">49</a>, <a href="#Page_58">58</a>, <a href="#Page_79">79</a>, <a href="#Page_80">80</a><br /> -Mica-porphyrite, i. <a href="../../66492/66492-h/66492-h.htm#Page_277">277</a>, <a href="../../66492/66492-h/66492-h.htm#Page_338">338</a><br /> -Michel Lévy, M., i. <a href="../../66492/66492-h/66492-h.htm#Page_18">18</a>, <a href="../../66492/66492-h/66492-h.htm#Page_21">21</a>, <a href="../../66492/66492-h/66492-h.htm#Page_46">46</a>, <a href="../../66492/66492-h/66492-h.htm#Page_88">88</a>; ii. <a href="#Page_373">373</a>, <a href="#Page_374">374</a><br /> -Microgranite, i. <a href="../../66492/66492-h/66492-h.htm#Page_131">131</a>, <a href="../../66492/66492-h/66492-h.htm#Page_215">215</a>, <a href="../../66492/66492-h/66492-h.htm#Page_235">235</a>, <a href="../../66492/66492-h/66492-h.htm#Page_249">249</a>; ii. <a href="#Page_367">367</a>, <a href="#Page_437">437</a><br /> -Microlites of igneous rocks, i. <a href="../../66492/66492-h/66492-h.htm#Page_18">18</a>, <a href="../../66492/66492-h/66492-h.htm#Page_21">21</a>, <a href="../../66492/66492-h/66492-h.htm#Page_33">33</a>; ii. <a href="#Page_135">135</a>, <a href="#Page_275">275</a><br /> -Micropegmatitic structure, i. <a href="../../66492/66492-h/66492-h.htm#Page_20">20</a>, <a href="../../66492/66492-h/66492-h.htm#Page_418">418</a>, <a href="../../66492/66492-h/66492-h.htm#Page_449">449</a>; ii. <a href="#Page_5">5</a>, <a href="#Page_368">368</a>, <a href="#Page_437">437</a><br /> -Microscopic examination of rocks, i. <a href="../../66492/66492-h/66492-h.htm#Page_18">18</a>, <a href="../../66492/66492-h/66492-h.htm#Page_21">21</a><br /> -Midlands, eruptive rocks of English, ii. <a href="#Page_100">100</a><br /> -Midlothian, Carboniferous volcanic plateau of, i. <a href="../../66492/66492-h/66492-h.htm#Page_373">373</a>, <a href="../../66492/66492-h/66492-h.htm#Page_385">385</a>, <a href="../../66492/66492-h/66492-h.htm#Page_387">387</a>;<br /> -<span style="margin-left: 1em;">sills of, <a href="../../66492/66492-h/66492-h.htm#Page_446">446</a></span><br /> -Miller, Hugh, ii. <a href="#Page_237">237</a><br /> -—— Mr. H., i. <a href="../../66492/66492-h/66492-h.htm#Page_336">336</a><br /> -Mills, Abraham, ii. <a href="#Page_109">109</a><br /> -Millstone grit, i. <a href="../../66492/66492-h/66492-h.htm#Page_358">358</a>, <a href="../../66492/66492-h/66492-h.htm#Page_360">360</a>, <a href="../../66492/66492-h/66492-h.htm#Page_366">366</a><br /> -Minette, i. <a href="../../66492/66492-h/66492-h.htm#Page_277">277</a>, <a href="../../66492/66492-h/66492-h.htm#Page_278">278</a>, <a href="../../66492/66492-h/66492-h.htm#Page_291">291</a>, <a href="../../66492/66492-h/66492-h.htm#Page_293">293</a><br /> -Minto Crags, i. <a href="../../66492/66492-h/66492-h.htm#Page_375">375</a>, <a href="../../66492/66492-h/66492-h.htm#Page_397">397</a><br /> -Mitchell, Rev. Hugh, i. <a href="../../66492/66492-h/66492-h.htm#Page_301">301</a><br /> -Moel Siabod, i. <a href="../../66492/66492-h/66492-h.htm#Page_175">175</a><br /> -—— Wyn, i. <a href="../../66492/66492-h/66492-h.htm#Page_175">175</a>, <a href="../../66492/66492-h/66492-h.htm#Page_176">176</a>, <a href="../../66492/66492-h/66492-h.htm#Page_184">184</a>, <a href="../../66492/66492-h/66492-h.htm#Page_185">185</a><br /> -Monckton, Mr. H. W., i. <a href="../../66492/66492-h/66492-h.htm#Page_449">449</a>; ii. <a href="#Page_224">224</a><br /> -Montana, lava-fields of, ii. <a href="#Page_115">115</a>, <a href="#Page_267">267</a><br /> -Montrose, volcanoes of, i. <a href="../../66492/66492-h/66492-h.htm#Page_299">299</a><br /> -Moray Firth, basin of, i. <a href="../../66492/66492-h/66492-h.htm#Page_271">271</a>, <a href="../../66492/66492-h/66492-h.htm#Page_343">343</a><br /> -Morton, Mr. G. H., i. <a href="../../66492/66492-h/66492-h.htm#Page_189">189</a><br /> -Morven, basalt-plateau of, ii. <a href="#Page_208">208</a><br /> -Mountain-chains, origin of, i. <a href="../../66492/66492-h/66492-h.htm#Page_11">11</a>, <a href="../../66492/66492-h/66492-h.htm#Page_12">12</a>, <a href="../../66492/66492-h/66492-h.htm#Page_98">98</a><br /> -Mourne mountains, granite of, i. <a href="../../66492/66492-h/66492-h.htm#Page_93">93</a>; ii. <a href="#Page_124">124</a>, <a href="#Page_366">366</a>, <a href="#Page_367">367</a>, <a href="#Page_420">420</a><br /> -Muck, Isle of, ii. <a href="#Page_215">215</a><br /> -Mud-lava, ii. <a href="#Page_85">85</a><br /> -Mudstone, volcanic, i. <a href="../../66492/66492-h/66492-h.htm#Page_423">423</a>, <a href="../../66492/66492-h/66492-h.htm#Page_436">436</a>, <a href="../../66492/66492-h/66492-h.htm#Page_440">440</a>, <a href="../../66492/66492-h/66492-h.htm#Page_444">444</a>, <a href="../../66492/66492-h/66492-h.htm#Page_466">466</a>; ii. <a href="#Page_86">86</a>, <a href="#Page_222">222</a>, <a href="#Page_258">258</a><br /> -Mull, branching amygdales of, i. <a href="../../66492/66492-h/66492-h.htm#Page_17">17</a>;<br /> -<span style="margin-left: 1em;">perlitic glass from, <a href="../../66492/66492-h/66492-h.htm#Page_19">19</a>;</span><br /> -<span style="margin-left: 1em;">pale lavas of, ii. <a href="#Page_184">184</a>, <a href="#Page_213">213</a>;</span><br /> -<span style="margin-left: 1em;">basalt of, <a href="#Page_188">188</a>, <a href="#Page_192">192</a>, <a href="#Page_193">193</a>;</span><br /> -<span style="margin-left: 1em;">breccias of, <a href="#Page_196">196</a>;</span><br /> -<span style="margin-left: 1em;">plateau of, i. <a href="../../66492/66492-h/66492-h.htm#Page_24">24</a>, ii. <a href="#Page_208">208</a>;</span><br /> -<span style="margin-left: 1em;">non-volcanic breccias in, <a href="#Page_196">196</a>, <a href="#Page_211">211</a>;</span><br /> -<span style="margin-left: 1em;">flint gravel in, <a href="#Page_211">211</a>;</span><br /> -<span style="margin-left: 1em;">leaf-beds of, <a href="#Page_212">212</a>;</span><br /> -<span style="margin-left: 1em;">vents in, <a href="#Page_274">274</a>, <a href="#Page_278">278</a>;</span><br /> -<span style="margin-left: 1em;">gabbro of, <a href="#Page_355">355</a>;</span><br /> -<span style="margin-left: 1em;">acid bosses of, i. <a href="../../66492/66492-h/66492-h.htm#Page_20">20</a>; ii. <a href="#Page_395">395</a>;</span><br /> -<span style="margin-left: 1em;">acid sills of, <a href="#Page_430">430</a>;</span><br /> -<span style="margin-left: 1em;">acid dykes and veins of, <a href="#Page_443">443</a>;</span><br /> -<span style="margin-left: 1em;">enormous denudation of, <a href="#Page_457">457</a>, <a href="#Page_461">461</a></span><br /> -Murchison, R. I., i. <a href="../../66492/66492-h/66492-h.htm#Page_113">113</a>, <a href="../../66492/66492-h/66492-h.htm#Page_121">121</a>, <a href="../../66492/66492-h/66492-h.htm#Page_129">129</a>, <a href="../../66492/66492-h/66492-h.htm#Page_142">142</a>, <a href="../../66492/66492-h/66492-h.htm#Page_173">173</a>, <a href="../../66492/66492-h/66492-h.htm#Page_175">175</a>, <a href="../../66492/66492-h/66492-h.htm#Page_189">189</a>, <a href="../../66492/66492-h/66492-h.htm#Page_204">204</a>, <a href="../../66492/66492-h/66492-h.htm#Page_205">205</a>, <a href="../../66492/66492-h/66492-h.htm#Page_207">207</a>, <a href="../../66492/66492-h/66492-h.htm#Page_238">238</a>, <a href="../../66492/66492-h/66492-h.htm#Page_257">257</a>; ii. <a href="#Page_56">56</a>, <a href="#Page_95">95</a><br /> -Mynydd-mawr, i. <a href="../../66492/66492-h/66492-h.htm#Page_209">209</a>, <a href="../../66492/66492-h/66492-h.htm#Page_211">211</a>, <a href="../../66492/66492-h/66492-h.htm#Page_216">216</a><br /> -Mythology, influence of earthquakes and volcanoes on, i. <a href="../../66492/66492-h/66492-h.htm#Page_2">2</a><br /> -<br /> -<a id="N"></a>Nant Francon, i. <a href="../../66492/66492-h/66492-h.htm#Page_161">161</a><br /> -Naples, puys of, i. <a href="../../66492/66492-h/66492-h.htm#Page_100">100</a>, <a href="../../66492/66492-h/66492-h.htm#Page_429">429</a><br /> -Napoleonite, i. <a href="../../66492/66492-h/66492-h.htm#Page_22">22</a><br /> -Necker, L. A., ii. <a href="#Page_112">112</a>, <a href="#Page_123">123</a>, <a href="#Page_139">139</a>, <a href="#Page_140">140</a>, <a href="#Page_146">146</a><br /> -Necks, volcanic, i. <a href="../../66492/66492-h/66492-h.htm#Page_56">56</a>;<br /> -<span style="margin-left: 1em;">of fragmentary materials, <a href="../../66492/66492-h/66492-h.htm#Page_56">56</a>;</span><br /> -<span style="margin-left: 1em;">of non-volcanic detritus, <a href="../../66492/66492-h/66492-h.htm#Page_57">57</a>, <a href="../../66492/66492-h/66492-h.htm#Page_289">289</a>, <a href="../../66492/66492-h/66492-h.htm#Page_343">343</a>, <a href="../../66492/66492-h/66492-h.htm#Page_426">426</a>;</span><br /> -<span style="margin-left: 1em;">of agglomerate, <a href="../../66492/66492-h/66492-h.htm#Page_58">58</a>; ii. <a href="#Page_276">276</a>;</span><br /> -<span style="margin-left: 1em;">internal stratification in, i. <a href="../../66492/66492-h/66492-h.htm#Page_63">63</a>; ii. <a href="#Page_80">80</a>, <a href="#Page_294">294</a>;</span><br /> -<span style="margin-left: 1em;">with central lava-plug, i. <a href="../../66492/66492-h/66492-h.htm#Page_64">64</a>, <a href="../../66492/66492-h/66492-h.htm#Page_430">430</a>;</span><br /> -<span style="margin-left: 1em;">with dykes and veins, <a href="../../66492/66492-h/66492-h.htm#Page_66">66</a>, <a href="../../66492/66492-h/66492-h.htm#Page_430">430</a>; ii, <a href="#Page_291">291</a>;</span><br /> -<span style="margin-left: 1em;">of lava-form material, i. <a href="../../66492/66492-h/66492-h.htm#Page_67">67</a>, <a href="../../66492/66492-h/66492-h.htm#Page_430">430</a>; ii. <a href="#Page_271">271</a>;</span><br /> -<span style="margin-left: 1em;">parasitic, i. <a href="../../66492/66492-h/66492-h.htm#Page_69">69</a>;</span><br /> -<span style="margin-left: 1em;">connection of, with cones, <a href="../../66492/66492-h/66492-h.htm#Page_70">70</a>, <a href="../../66492/66492-h/66492-h.htm#Page_435">435</a>; ii, <a href="#Page_70">70</a>, <a href="#Page_89">89</a>, <a href="#Page_277">277</a>, <a href="#Page_281">281</a>, <a href="#Page_290">290</a>;</span><br /> -<span style="margin-left: 1em;">metamorphism of, i. <a href="../../66492/66492-h/66492-h.htm#Page_67">67</a>, <a href="../../66492/66492-h/66492-h.htm#Page_71">71</a>, <a href="../../66492/66492-h/66492-h.htm#Page_82">82</a>, <a href="../../66492/66492-h/66492-h.htm#Page_93">93</a>, <a href="../../66492/66492-h/66492-h.htm#Page_399">399</a>; ii. <a href="#Page_39">39</a>, <a href="#Page_78">78</a>;</span><br /> -<span style="margin-left: 1em;">metamorphism of rocks around, i. <a href="../../66492/66492-h/66492-h.htm#Page_72">72</a>, <a href="../../66492/66492-h/66492-h.htm#Page_349">349</a>, <a href="../../66492/66492-h/66492-h.htm#Page_350">350</a>, <a href="../../66492/66492-h/66492-h.htm#Page_352">352</a>, <a href="../../66492/66492-h/66492-h.htm#Page_399">399</a>, <a href="../../66492/66492-h/66492-h.htm#Page_404">404</a>, <a href="../../66492/66492-h/66492-h.htm#Page_432">432</a>; ii. <a href="#Page_76">76</a>, <a href="#Page_272">272</a>, <a href="#Page_273">273</a>, <a href="#Page_274">274</a>, <a href="#Page_276">276</a>, <a href="#Page_280">280</a>, <a href="#Page_292">292</a>;</span><br /> -<span style="margin-left: 1em;">inward dip of rocks towards, i. <a href="../../66492/66492-h/66492-h.htm#Page_73">73</a>, <a href="../../66492/66492-h/66492-h.htm#Page_352">352</a>; ii. <a href="#Page_80">80</a>;</span><br /> -<span style="margin-left: 1em;">connection of, with bosses, i. <a href="../../66492/66492-h/66492-h.htm#Page_93">93</a>; ii. <a href="#Page_276">276</a>, <a href="#Page_284">284</a>;</span><br /> -<span style="margin-left: 1em;">entombment and exposure of, i. <a href="../../66492/66492-h/66492-h.htm#Page_434">434</a>;</span><br /> -<span style="margin-left: 1em;">connection of with valleys, <a href="../../66492/66492-h/66492-h.htm#Page_272">272</a>, <a href="../../66492/66492-h/66492-h.htm#Page_366">366</a>, <a href="../../66492/66492-h/66492-h.htm#Page_375">375</a>; ii. <a href="#Page_61">61</a>, <a href="#Page_65">65</a>;</span><br /> -<span style="margin-left: 1em;">relation between their size and the character of the agglomerate, <a href="#Page_76">76</a>;</span><br /> -<span style="margin-left: 1em;">connection of, with sheets of tuff or lava, <a href="#Page_70">70</a>, <a href="#Page_89">89</a>, <a href="#Page_277">277</a>, <a href="#Page_284">284</a></span><br /> -—— Silurian, i. <a href="../../66492/66492-h/66492-h.htm#Page_215">215</a>, <a href="../../66492/66492-h/66492-h.htm#Page_235">235</a>;<br /> -<span style="margin-left: 1em;">Old Red Sandstone, <a href="../../66492/66492-h/66492-h.htm#Page_277">277</a>, <a href="../../66492/66492-h/66492-h.htm#Page_288">288</a>, <a href="../../66492/66492-h/66492-h.htm#Page_293">293</a>, <a href="../../66492/66492-h/66492-h.htm#Page_311">311</a>, <a href="../../66492/66492-h/66492-h.htm#Page_318">318</a>, <a href="../../66492/66492-h/66492-h.htm#Page_323">323</a>, <a href="../../66492/66492-h/66492-h.htm#Page_328">328</a>;</span><br /> -<span style="margin-left: 1em;">Carboniferous, <a href="../../66492/66492-h/66492-h.htm#Page_394">394</a>, <a href="../../66492/66492-h/66492-h.htm#Page_399">399</a>, <a href="../../66492/66492-h/66492-h.htm#Page_400">400</a>, <a href="../../66492/66492-h/66492-h.htm#Page_404">404</a>, <a href="../../66492/66492-h/66492-h.htm#Page_406">406</a>, <a href="../../66492/66492-h/66492-h.htm#Page_424">424</a>, <a href="../../66492/66492-h/66492-h.htm#Page_465">465</a>; ii. <a href="#Page_13">13</a>, <a href="#Page_28">28</a>, <a href="#Page_47">47</a>;</span><br /> -<span style="margin-left: 1em;">Permian, ii. <a href="#Page_62">62</a>, <a href="#Page_67">67</a>;</span><br /> -<span style="margin-left: 1em;">Tertiary, ii. <a href="#Page_202">202</a>, <a href="#Page_270">270</a>, <a href="#Page_276">276</a></span><br /> -Neptunist and Plutonist controversy, i. <a href="../../66492/66492-h/66492-h.htm#Page_363">363</a>; ii. <a href="#Page_67">67</a>, <a href="#Page_95">95</a>, <a href="#Page_110">110</a>, <a href="#Page_112">112</a><br /> -New Mexico, necks in, i. <a href="../../66492/66492-h/66492-h.htm#Page_68">68</a><br /> -Newry granite, i. <a href="../../66492/66492-h/66492-h.htm#Page_290">290</a><br /> -Nicholson, Prof. Alleyne, i. <a href="../../66492/66492-h/66492-h.htm#Page_228">228</a>, <a href="../../66492/66492-h/66492-h.htm#Page_229">229</a><br /> -Nicol, James, i. <a href="../../66492/66492-h/66492-h.htm#Page_311">311</a>, <a href="../../66492/66492-h/66492-h.htm#Page_369">369</a><br /> -—— W., ii. <a href="#Page_238">238</a><br /> -Nigrine, ii. <a href="#Page_79">79</a><br /> -Nithsdale, Permian volcanic rocks of, ii. <a href="#Page_58">58</a>, <a href="#Page_60">60</a>, <a href="#Page_62">62</a>, <a href="#Page_65">65</a><br /> -Nodular structure of lavas, i. <a href="../../66492/66492-h/66492-h.htm#Page_20">20</a>, <a href="../../66492/66492-h/66492-h.htm#Page_162">162</a>, <a href="../../66492/66492-h/66492-h.htm#Page_204">204</a>, <a href="../../66492/66492-h/66492-h.htm#Page_206">206</a>, <a href="../../66492/66492-h/66492-h.htm#Page_207">207</a>, <a href="../../66492/66492-h/66492-h.htm#Page_211">211</a>, <a href="../../66492/66492-h/66492-h.htm#Page_232">232</a>, <a href="../../66492/66492-h/66492-h.htm#Page_247">247</a>, <a href="../../66492/66492-h/66492-h.htm#Page_255">255</a>, <a href="../../66492/66492-h/66492-h.htm#Page_274">274</a>, <a href="../../66492/66492-h/66492-h.htm#Page_346">346</a><br /> -Nolan, Mr. J., i. <a href="../../66492/66492-h/66492-h.htm#Page_240">240</a>, <a href="../../66492/66492-h/66492-h.htm#Page_251">251</a>, <a href="../../66492/66492-h/66492-h.htm#Page_315">315</a>; ii. <a href="#Page_423">423</a>, <a href="#Page_424">424</a>, <a href="#Page_425">425</a><br /> -Non-volcanic debris, among volcanic rocks, i. <a href="../../66492/66492-h/66492-h.htm#Page_31">31</a>, <a href="../../66492/66492-h/66492-h.htm#Page_57">57</a>, <a href="../../66492/66492-h/66492-h.htm#Page_289">289</a>, <a href="../../66492/66492-h/66492-h.htm#Page_313">313</a>, <a href="../../66492/66492-h/66492-h.htm#Page_345">345</a>, <a href="../../66492/66492-h/66492-h.htm#Page_381">381</a>, <a href="../../66492/66492-h/66492-h.htm#Page_399">399</a>, <a href="../../66492/66492-h/66492-h.htm#Page_402">402</a>, <a href="../../66492/66492-h/66492-h.htm#Page_422">422</a>, <a href="../../66492/66492-h/66492-h.htm#Page_426">426</a>, <a href="../../66492/66492-h/66492-h.htm#Page_437">437</a>; ii. <a href="#Page_18">18</a>, <a href="#Page_27">27</a>, <a href="#Page_28">28</a>, <a href="#Page_58">58</a>, <a href="#Page_64">64</a>, <a href="#Page_76">76</a>, <a href="#Page_78">78</a>, <a href="#Page_99">99</a>, <a href="#Page_195">195</a>, <a href="#Page_196">196</a>, <a href="#Page_281">281</a>, <a href="#Page_423">423</a>;<br /> -<span style="margin-left: 1em;">indicates comparatively feeble eruptions, i. <a href="../../66492/66492-h/66492-h.htm#Page_57">57</a>, <a href="../../66492/66492-h/66492-h.htm#Page_289">289</a>, <a href="../../66492/66492-h/66492-h.htm#Page_345">345</a>, <a href="../../66492/66492-h/66492-h.htm#Page_426">426</a>, <a href="../../66492/66492-h/66492-h.htm#Page_438">438</a>; ii. <a href="#Page_293">293</a>;</span><br /> -<span style="margin-left: 1em;">points to earliest eruptions of a vent, ii. <a href="#Page_76">76</a></span><br /> -Nordenskjöld, Mr. O., i. <a href="../../66492/66492-h/66492-h.htm#Page_120">120</a><br /> -North Berwick Law, i. <a href="../../66492/66492-h/66492-h.htm#Page_371">371</a>, <a href="../../66492/66492-h/66492-h.htm#Page_373">373</a>, <a href="../../66492/66492-h/66492-h.htm#Page_403">403</a><br /> -North, Mr. Barker, ii. <a href="#Page_244">244</a><br /> -Norway, eruptive rocks of, i. <a href="../../66492/66492-h/66492-h.htm#Page_28">28</a><br /> -Nuneaton, i. <a href="../../66492/66492-h/66492-h.htm#Page_171">171</a><br /> -<br /> -<a id="O"></a>Obsidian, i. <a href="../../66492/66492-h/66492-h.htm#Page_18">18</a>, <a href="../../66492/66492-h/66492-h.htm#Page_19">19</a>; ii. <a href="#Page_370">370</a><br /> -Ochil Hills, i. <a href="../../66492/66492-h/66492-h.htm#Page_274">274</a>, <a href="../../66492/66492-h/66492-h.htm#Page_276">276</a>, <a href="../../66492/66492-h/66492-h.htm#Page_277">277</a>, <a href="../../66492/66492-h/66492-h.htm#Page_279">279</a>, <a href="../../66492/66492-h/66492-h.htm#Page_281">281</a>, <a href="../../66492/66492-h/66492-h.htm#Page_286">286</a>, <a href="../../66492/66492-h/66492-h.htm#Page_287">287</a>, <a href="../../66492/66492-h/66492-h.htm#Page_288">288</a>, <a href="../../66492/66492-h/66492-h.htm#Page_293">293</a>, <a href="../../66492/66492-h/66492-h.htm#Page_303">303</a>, <a href="../../66492/66492-h/66492-h.htm#Page_308">308</a>, <a href="../../66492/66492-h/66492-h.htm#Page_311">311</a><br /> -Oil-shales of the Lothians, i. <a href="../../66492/66492-h/66492-h.htm#Page_361">361</a>, <a href="../../66492/66492-h/66492-h.htm#Page_362">362</a>, <a href="../../66492/66492-h/66492-h.htm#Page_462">462</a><br /> -O'Kelly, J., i. <a href="../../66492/66492-h/66492-h.htm#Page_349">349</a>; ii. <a href="#Page_49">49</a><br /> -Oldham, T., ii. <a href="#Page_299">299</a><br /> -Old Red Sandstone, lines of vents in, i. <a href="../../66492/66492-h/66492-h.htm#Page_69">69</a>;<br /> -<span style="margin-left: 1em;">granite protrusions of, <a href="../../66492/66492-h/66492-h.htm#Page_236">236</a>, <a href="../../66492/66492-h/66492-h.htm#Page_272">272</a>, <a href="../../66492/66492-h/66492-h.htm#Page_277">277</a>, <a href="../../66492/66492-h/66492-h.htm#Page_290">290</a>, <a href="../../66492/66492-h/66492-h.htm#Page_337">337</a>;</span><br /> -<span style="margin-left: 1em;">of County Waterford, <a href="../../66492/66492-h/66492-h.htm#Page_251">251</a>;</span><br /> -<span style="margin-left: 1em;">distribution in Britain, <a href="../../66492/66492-h/66492-h.htm#Page_257">257</a>;</span><br /> -<span style="margin-left: 1em;">an exceptional stratigraphical type, <a href="../../66492/66492-h/66492-h.htm#Page_258">258</a>;</span><br /> -<span style="margin-left: 1em;">conditions of its deposit, <a href="../../66492/66492-h/66492-h.htm#Page_259">259</a>, <a href="../../66492/66492-h/66492-h.htm#Page_263">263</a>, <a href="../../66492/66492-h/66492-h.htm#Page_297">297</a>;</span><br /> -<span style="margin-left: 1em;">original scenery of, <a href="../../66492/66492-h/66492-h.htm#Page_265">265</a>;</span><br /> -<span style="margin-left: 1em;">vegetation of, <a href="../../66492/66492-h/66492-h.htm#Page_265">265</a>;</span><br /> -<span style="margin-left: 1em;">isolation of the water-basins of, shown by fossil evidence, <a href="../../66492/66492-h/66492-h.htm#Page_265">265</a>;</span><br /> -<span style="margin-left: 1em;">classification of, <a href="../../66492/66492-h/66492-h.htm#Page_260">260</a>;</span><br /> -<span style="margin-left: 1em;">history of the investigation of, <a href="../../66492/66492-h/66492-h.htm#Page_268">268</a>;</span><br /> -<span style="margin-left: 1em;">volcanic centres in, <a href="../../66492/66492-h/66492-h.htm#Page_271">271</a>;</span><br /> -<span style="margin-left: 1em;">nature of volcanic products in, <a href="../../66492/66492-h/66492-h.htm#Page_273">273</a>;</span><br /> -<span style="margin-left: 1em;">structure of lavas and tuffs of, <a href="../../66492/66492-h/66492-h.htm#Page_281">281</a>;</span><br /> -<span style="margin-left: 1em;">volcanoes of, <a href="../../66492/66492-h/66492-h.htm#Page_259">259</a>, <a href="../../66492/66492-h/66492-h.htm#Page_263">263</a>, <a href="../../66492/66492-h/66492-h.htm#Page_294">294</a>, <a href="../../66492/66492-h/66492-h.htm#Page_303">303</a>, <a href="../../66492/66492-h/66492-h.htm#Page_323">323</a>, <a href="../../66492/66492-h/66492-h.htm#Page_325">325</a>, <a href="../../66492/66492-h/66492-h.htm#Page_337">337</a>, <a href="../../66492/66492-h/66492-h.htm#Page_341">341</a>, <a href="../../66492/66492-h/66492-h.htm#Page_343">343</a>, <a href="../../66492/66492-h/66492-h.htm#Page_346">346</a>, <a href="../../66492/66492-h/66492-h.htm#Page_348">348</a>, <a href="../../66492/66492-h/66492-h.htm#Page_352">352</a>;</span><br /> -<span class="pagenum" id="Page_488">- 488 -</span> -<span style="margin-left: 1em;">subdivisions of, <a href="../../66492/66492-h/66492-h.htm#Page_297">297</a>;</span><br /> -<span style="margin-left: 1em;">thickest conglomerates of, <a href="../../66492/66492-h/66492-h.htm#Page_301">301</a>;</span><br /> -<span style="margin-left: 1em;">composition of conglomerates of, <a href="../../66492/66492-h/66492-h.htm#Page_302">302</a>, <a href="../../66492/66492-h/66492-h.htm#Page_315">315</a>, <a href="../../66492/66492-h/66492-h.htm#Page_341">341</a>;</span><br /> -<span style="margin-left: 1em;">unconformabilities in, <a href="../../66492/66492-h/66492-h.htm#Page_267">267</a>, <a href="../../66492/66492-h/66492-h.htm#Page_328">328</a>, <a href="../../66492/66492-h/66492-h.htm#Page_333">333</a>;</span><br /> -<span style="margin-left: 1em;">Upper division of, <a href="../../66492/66492-h/66492-h.htm#Page_348">348</a>, <a href="../../66492/66492-h/66492-h.htm#Page_375">375</a>, <a href="../../66492/66492-h/66492-h.htm#Page_383">383</a>; ii. <a href="#Page_42">42</a></span><br /> -Olenellus-zone, i. <a href="../../66492/66492-h/66492-h.htm#Page_112">112</a>, <a href="../../66492/66492-h/66492-h.htm#Page_130">130</a>, <a href="../../66492/66492-h/66492-h.htm#Page_132">132</a>, <a href="../../66492/66492-h/66492-h.htm#Page_140">140</a>, <a href="../../66492/66492-h/66492-h.htm#Page_144">144</a><br /> -Olenus-zone, i. <a href="../../66492/66492-h/66492-h.htm#Page_144">144</a><br /> -Olivine, i. <a href="../../66492/66492-h/66492-h.htm#Page_154">154</a>, <a href="../../66492/66492-h/66492-h.htm#Page_418">418</a>, <a href="../../66492/66492-h/66492-h.htm#Page_420">420</a>; ii. <a href="#Page_58">58</a>, <a href="#Page_135">135</a><br /> -Omagh, i. <a href="../../66492/66492-h/66492-h.htm#Page_315">315</a><br /> -Ophitic structure, i. <a href="../../66492/66492-h/66492-h.htm#Page_21">21</a>, <a href="../../66492/66492-h/66492-h.htm#Page_417">417</a>; ii. <a href="#Page_136">136</a>, <a href="#Page_184">184</a>, <a href="#Page_274">274</a><br /> -—— type of dolerite, i. <a href="../../66492/66492-h/66492-h.htm#Page_418">418</a>, <a href="../../66492/66492-h/66492-h.htm#Page_421">421</a><br /> -Oregon, crater lake in, i. <a href="../../66492/66492-h/66492-h.htm#Page_58">58</a><br /> -Orkney Isles, i. <a href="../../66492/66492-h/66492-h.htm#Page_271">271</a>, <a href="../../66492/66492-h/66492-h.htm#Page_344">344</a>, <a href="../../66492/66492-h/66492-h.htm#Page_350">350</a>; ii. <a href="#Page_121">121</a><br /> -Orthoclase, ejected crystals of, ii. <a href="#Page_79">79</a><br /> -Orthophyre, i. <a href="../../66492/66492-h/66492-h.htm#Page_273">273</a>, <a href="../../66492/66492-h/66492-h.htm#Page_276">276</a>, <a href="../../66492/66492-h/66492-h.htm#Page_277">277</a>, <a href="../../66492/66492-h/66492-h.htm#Page_308">308</a><br /> -Osann, A., ii. <a href="#Page_191">191</a><br /> -Oyenhausen, C. von, ii. <a href="#Page_112">112</a>, <a href="#Page_280">280</a>, <a href="#Page_333">333</a>, <a href="#Page_340">340</a>, <a href="#Page_367">367</a>, <a href="#Page_372">372</a>, <a href="#Page_381">381</a><br /> -<br /> -<a id="P"></a>Palæopicrite, i. <a href="../../66492/66492-h/66492-h.htm#Page_261">261</a><br /> -Palæozoic systems, i. <a href="../../66492/66492-h/66492-h.htm#Page_139">139</a>;<br /> -<span style="margin-left: 1em;">volcanic rocks resemble modern, i. <a href="../../66492/66492-h/66492-h.htm#Page_30">30</a></span><br /> -Palagonite, i. <a href="../../66492/66492-h/66492-h.htm#Page_33">33</a>, <a href="../../66492/66492-h/66492-h.htm#Page_61">61</a>, <a href="../../66492/66492-h/66492-h.htm#Page_151">151</a>, <a href="../../66492/66492-h/66492-h.htm#Page_180">180</a>, <a href="../../66492/66492-h/66492-h.htm#Page_246">246</a>, <a href="../../66492/66492-h/66492-h.htm#Page_422">422</a>, <a href="../../66492/66492-h/66492-h.htm#Page_423">423</a>; ii. <a href="#Page_44">44</a>, <a href="#Page_46">46</a>, <a href="#Page_57">57</a>, <a href="#Page_223">223</a><br /> -Paradoxides-zone, i. <a href="../../66492/66492-h/66492-h.htm#Page_144">144</a><br /> -Paramorphism, i. <a href="../../66492/66492-h/66492-h.htm#Page_249">249</a><br /> -Peach, Mr. B. N., i. <a href="../../66492/66492-h/66492-h.htm#Page_114">114</a>, <a href="../../66492/66492-h/66492-h.htm#Page_128">128</a>, <a href="../../66492/66492-h/66492-h.htm#Page_147">147</a>, <a href="../../66492/66492-h/66492-h.htm#Page_168">168</a>, <a href="../../66492/66492-h/66492-h.htm#Page_191">191</a>, <a href="../../66492/66492-h/66492-h.htm#Page_192">192</a>, <a href="../../66492/66492-h/66492-h.htm#Page_195">195</a>, <a href="../../66492/66492-h/66492-h.htm#Page_196">196</a>, <a href="../../66492/66492-h/66492-h.htm#Page_197">197</a>, <a href="../../66492/66492-h/66492-h.htm#Page_198">198</a>, <a href="../../66492/66492-h/66492-h.htm#Page_199">199</a>, <a href="../../66492/66492-h/66492-h.htm#Page_200">200</a>, <a href="../../66492/66492-h/66492-h.htm#Page_216">216</a>, <a href="../../66492/66492-h/66492-h.htm#Page_240">240</a>, <a href="../../66492/66492-h/66492-h.htm#Page_277">277-294</a>, <a href="../../66492/66492-h/66492-h.htm#Page_307">307</a>, <a href="../../66492/66492-h/66492-h.htm#Page_308">308</a>, <a href="../../66492/66492-h/66492-h.htm#Page_329">329</a>, <a href="../../66492/66492-h/66492-h.htm#Page_331">331</a>, <a href="../../66492/66492-h/66492-h.htm#Page_344">344</a>, <a href="../../66492/66492-h/66492-h.htm#Page_345">345</a>, <a href="../../66492/66492-h/66492-h.htm#Page_369">369</a>, <a href="../../66492/66492-h/66492-h.htm#Page_375">375</a>, <a href="../../66492/66492-h/66492-h.htm#Page_425">425</a>, <a href="../../66492/66492-h/66492-h.htm#Page_426">426</a>, <a href="../../66492/66492-h/66492-h.htm#Page_476">476</a>; ii. <a href="#Page_133">133</a>, <a href="#Page_145">145</a><br /> -Pebidian, i. <a href="../../66492/66492-h/66492-h.htm#Page_145">145</a><br /> -Pegmatite, i. <a href="../../66492/66492-h/66492-h.htm#Page_20">20</a>, <a href="../../66492/66492-h/66492-h.htm#Page_119">119</a>, <a href="../../66492/66492-h/66492-h.htm#Page_127">127</a>, <a href="../../66492/66492-h/66492-h.htm#Page_418">418</a>, <a href="../../66492/66492-h/66492-h.htm#Page_449">449</a>; ii. <a href="#Page_5">5</a>, <a href="#Page_368">368</a>, <a href="#Page_437">437</a><br /> -Pembrokeshire, volcanic rocks of, i. <a href="../../66492/66492-h/66492-h.htm#Page_145">145</a>, <a href="../../66492/66492-h/66492-h.htm#Page_159">159</a>, <a href="../../66492/66492-h/66492-h.htm#Page_205">205</a><br /> -Penmaen-mawr, i. <a href="../../66492/66492-h/66492-h.htm#Page_209">209</a>, <a href="../../66492/66492-h/66492-h.htm#Page_215">215</a><br /> -Pennant, T., ii. <a href="#Page_109">109</a><br /> -Pennine chain, ii. <a href="#Page_8">8</a><br /> -Pentland Hills, volcanic series of the, i. <a href="../../66492/66492-h/66492-h.htm#Page_102">102</a>, <a href="../../66492/66492-h/66492-h.htm#Page_269">269</a>, <a href="../../66492/66492-h/66492-h.htm#Page_273">273</a>, <a href="../../66492/66492-h/66492-h.htm#Page_276">276</a>, <a href="../../66492/66492-h/66492-h.htm#Page_279">279</a>, <a href="../../66492/66492-h/66492-h.htm#Page_281">281</a>, <a href="../../66492/66492-h/66492-h.htm#Page_285">285</a>, <a href="../../66492/66492-h/66492-h.htm#Page_287">287</a>, <a href="../../66492/66492-h/66492-h.htm#Page_289">289</a>, <a href="../../66492/66492-h/66492-h.htm#Page_291">291</a>, <a href="../../66492/66492-h/66492-h.htm#Page_311">311</a>, <a href="../../66492/66492-h/66492-h.htm#Page_317">317</a><br /> -Perlite, i. <a href="../../66492/66492-h/66492-h.htm#Page_130">130</a><br /> -Perlitic structure, i. <a href="../../66492/66492-h/66492-h.htm#Page_19">19</a>, <a href="../../66492/66492-h/66492-h.htm#Page_196">196</a>, <a href="../../66492/66492-h/66492-h.htm#Page_199">199</a>, <a href="../../66492/66492-h/66492-h.htm#Page_206">206</a>, <a href="../../66492/66492-h/66492-h.htm#Page_211">211</a>, <a href="../../66492/66492-h/66492-h.htm#Page_216">216</a>, <a href="../../66492/66492-h/66492-h.htm#Page_232">232</a>, <a href="../../66492/66492-h/66492-h.htm#Page_274">274</a><br /> -Permian system, geographical conditions accompanying the deposition of, ii. <a href="#Page_53">53</a>, <a href="#Page_97">97</a>;<br /> -<span style="margin-left: 1em;">subdivisions of, in S. W. England, <a href="#Page_94">94</a>;</span><br /> -<span style="margin-left: 1em;">volcanic phenomena of, i. <a href="../../66492/66492-h/66492-h.htm#Page_46">46</a>; ii. <a href="#Page_55">55</a>;</span><br /> -<span style="margin-left: 1em;">lavas and tuffs of, <a href="#Page_57">57</a>, <a href="#Page_58">58</a>;</span><br /> -<span style="margin-left: 1em;">vents of, <a href="#Page_62">62</a>, <a href="#Page_67">67</a>, <a href="#Page_70">70</a>, <a href="#Page_96">96</a>;</span><br /> -<span style="margin-left: 1em;">sills of, <a href="#Page_91">91</a>, <a href="#Page_100">100</a></span><br /> -Permo-carboniferous strata, ii. <a href="#Page_54">54</a><br /> -Petersen, Dr., i. <a href="../../66492/66492-h/66492-h.htm#Page_275">275</a>, <a href="../../66492/66492-h/66492-h.htm#Page_336">336</a><br /> -Phillips, J., i. <a href="../../66492/66492-h/66492-h.htm#Page_133">133</a>, <a href="../../66492/66492-h/66492-h.htm#Page_170">170</a>, <a href="../../66492/66492-h/66492-h.htm#Page_205">205</a>, <a href="../../66492/66492-h/66492-h.htm#Page_238">238</a>; ii. <a href="#Page_3">3</a><br /> -—— J. A., i. <a href="../../66492/66492-h/66492-h.htm#Page_260">260</a>, <a href="../../66492/66492-h/66492-h.htm#Page_261">261</a><br /> -—— W., i. <a href="../../66492/66492-h/66492-h.htm#Page_171">171</a>; ii. <a href="#Page_95">95</a><br /> -Phonolite, i. <a href="../../66492/66492-h/66492-h.htm#Page_380">380</a> (analysis); ii. <a href="#Page_375">375</a><br /> -Phyllite, i. <a href="../../66492/66492-h/66492-h.htm#Page_162">162</a>, <a href="../../66492/66492-h/66492-h.htm#Page_222">222</a><br /> -Picrite, i. <a href="../../66492/66492-h/66492-h.htm#Page_377">377</a>, <a href="../../66492/66492-h/66492-h.htm#Page_417">417</a>, <a href="../../66492/66492-h/66492-h.htm#Page_420">420</a>, <a href="../../66492/66492-h/66492-h.htm#Page_448">448</a>, <a href="../../66492/66492-h/66492-h.htm#Page_450">450</a>; ii. <a href="#Page_57">57</a><br /> -—— type, i. <a href="../../66492/66492-h/66492-h.htm#Page_418">418</a><br /> -Pillow-structure in lavas, i. <a href="../../66492/66492-h/66492-h.htm#Page_26">26</a>, <a href="../../66492/66492-h/66492-h.htm#Page_184">184</a>, <a href="../../66492/66492-h/66492-h.htm#Page_193">193</a>, <a href="../../66492/66492-h/66492-h.htm#Page_201">201</a>, <a href="../../66492/66492-h/66492-h.htm#Page_240">240</a>, <a href="../../66492/66492-h/66492-h.htm#Page_244">244</a>, <a href="../../66492/66492-h/66492-h.htm#Page_252">252</a>; ii. <a href="#Page_189">189</a>, <a href="#Page_259">259</a><br /> -Pitchstone, i. <a href="../../66492/66492-h/66492-h.htm#Page_18">18</a>, <a href="../../66492/66492-h/66492-h.htm#Page_19">19</a>, <a href="../../66492/66492-h/66492-h.htm#Page_130">130</a>; ii. <a href="#Page_134">134</a>, <a href="#Page_174">174</a>, <a href="#Page_204">204</a>, <a href="#Page_238">238</a>, <a href="#Page_242">242</a>, <a href="#Page_246">246</a>, <a href="#Page_370">370</a>, <a href="#Page_437">437</a>, <a href="#Page_444">444</a><br /> -Plagioclase, ejected crystals of, ii. <a href="#Page_79">79</a><br /> -Plants fossil, in tuffs, i. <a href="../../66492/66492-h/66492-h.htm#Page_392">392</a>; ii. <a href="#Page_113">113</a>, <a href="#Page_198">198</a>, <a href="#Page_212">212</a>, <a href="#Page_222">222</a><br /> -Platania, G., i. <a href="../../66492/66492-h/66492-h.htm#Page_26">26</a><br /> -Plateau-type of volcanoes, i. <a href="../../66492/66492-h/66492-h.htm#Page_42">42</a>, <a href="../../66492/66492-h/66492-h.htm#Page_100">100</a>, <a href="../../66492/66492-h/66492-h.htm#Page_308">308</a>, <a href="../../66492/66492-h/66492-h.htm#Page_341">341</a>, <a href="../../66492/66492-h/66492-h.htm#Page_364">364</a><br /> -Plateaux, Carboniferous, of Scotland, i. <a href="../../66492/66492-h/66492-h.htm#Page_364">364</a>;<br /> -<span style="margin-left: 1em;">distribution of, <a href="../../66492/66492-h/66492-h.htm#Page_367">367</a>;</span><br /> -<span style="margin-left: 1em;">composition of, <a href="../../66492/66492-h/66492-h.htm#Page_377">377</a>;</span><br /> -<span style="margin-left: 1em;">structure of, <a href="../../66492/66492-h/66492-h.htm#Page_383">383</a>;</span><br /> -<span style="margin-left: 1em;">lavas and tuffs of, <a href="../../66492/66492-h/66492-h.htm#Page_383">383</a>;</span><br /> -<span style="margin-left: 1em;">vents of, <a href="../../66492/66492-h/66492-h.htm#Page_394">394</a>;</span><br /> -<span style="margin-left: 1em;">dykes and sills of, <a href="../../66492/66492-h/66492-h.htm#Page_406">406</a>;</span><br /> -<span style="margin-left: 1em;">close of eruption of, <a href="../../66492/66492-h/66492-h.htm#Page_410">410</a>;</span><br /> -<span style="margin-left: 1em;">Tertiary, ii. <a href="#Page_181">181</a>;</span><br /> -<span style="margin-left: 1em;">formation of modern Icelandic, <a href="#Page_265">265</a></span><br /> -Player, Mr. J. H., analyses by, i. <a href="../../66492/66492-h/66492-h.htm#Page_377">377</a>, <a href="../../66492/66492-h/66492-h.htm#Page_381">381</a>; ii. <a href="#Page_138">138</a>, <a href="#Page_330">330</a><br /> -Playfair, John, i. <a href="../../66492/66492-h/66492-h.htm#Page_363">363</a>; ii. <a href="#Page_110">110</a><br /> -Plinthite, ii. <a href="#Page_197">197</a><br /> -Pliocene (supposed) of Lough Neagh, ii. <a href="#Page_449">449</a><br /> -Plutonic operations of volcanoes, i. <a href="../../66492/66492-h/66492-h.htm#Page_77">77</a>;<br /> -<span style="margin-left: 1em;">granite, <a href="../../66492/66492-h/66492-h.htm#Page_88">88</a></span><br /> -Plutonists and Neptunists, i. <a href="../../66492/66492-h/66492-h.htm#Page_363">363</a>; ii. <a href="#Page_67">67</a>, <a href="#Page_95">95</a>, <a href="#Page_110">110</a>, <a href="#Page_112">112</a><br /> -Pomeroy, volcanic series near, i. <a href="../../66492/66492-h/66492-h.htm#Page_315">315</a><br /> -Porphyrite, i. <a href="../../66492/66492-h/66492-h.htm#Page_190">190</a>, <a href="../../66492/66492-h/66492-h.htm#Page_193">193</a>, <a href="../../66492/66492-h/66492-h.htm#Page_207">207</a>, <a href="../../66492/66492-h/66492-h.htm#Page_229">229</a>, <a href="../../66492/66492-h/66492-h.htm#Page_240">240</a>, <a href="../../66492/66492-h/66492-h.htm#Page_252">252</a>, <a href="../../66492/66492-h/66492-h.htm#Page_273">273</a>, <a href="../../66492/66492-h/66492-h.htm#Page_274">274</a>, <a href="../../66492/66492-h/66492-h.htm#Page_377">377</a>, <a href="../../66492/66492-h/66492-h.htm#Page_379">379</a><br /> -Porphyritic structure, i. <a href="../../66492/66492-h/66492-h.htm#Page_19">19</a>, <a href="#Page_274">274</a>; ii. <a href="#Page_128">128</a><br /> -Portlock, J. E., ii. <a href="#Page_110">110</a>, <a href="#Page_111">111</a>, <a href="#Page_113">113</a>, <a href="#Page_199">199</a>, <a href="#Page_201">201</a>, <a href="#Page_299">299</a>, <a href="#Page_364">364</a><br /> -Portraine, conglomerates at, i. <a href="../../66492/66492-h/66492-h.htm#Page_244">244</a><br /> -Portrush, shells in supposed basalt at, ii. <a href="#Page_110">110</a>, <a href="#Page_299">299</a><br /> -Potstone, i. <a href="../../66492/66492-h/66492-h.htm#Page_125">125</a><br /> -Pre-Cambrian rocks, i. <a href="../../66492/66492-h/66492-h.htm#Page_111">111</a>, <a href="../../66492/66492-h/66492-h.htm#Page_121">121</a>, <a href="../../66492/66492-h/66492-h.htm#Page_126">126</a><br /> -Pressure, experimental proof of effects of, i. <a href="../../66492/66492-h/66492-h.htm#Page_24">24</a><br /> -Prestwich, Sir J., ii. <a href="#Page_103">103</a><br /> -Propylites, ii. <a href="#Page_185">185</a>, <a href="#Page_388">388</a><br /> -Proterobase, i. <a href="../../66492/66492-h/66492-h.htm#Page_247">247</a><br /> -<i>Pterygotus</i>, i. <a href="../../66492/66492-h/66492-h.htm#Page_265">265</a><br /> -Pumice in tuffs, i. <a href="../../66492/66492-h/66492-h.htm#Page_244">244</a>, <a href="../../66492/66492-h/66492-h.htm#Page_422">422</a>; ii. <a href="#Page_17">17</a>, <a href="#Page_27">27</a>, <a href="#Page_28">28</a>, <a href="#Page_32">32</a>, <a href="#Page_44">44</a>, <a href="#Page_46">46</a>, <a href="#Page_286">286</a>, <a href="#Page_288">288</a>;<br /> -<span style="margin-left: 1em;">in volcanic necks, i. <a href="../../66492/66492-h/66492-h.htm#Page_60">60</a>, <a href="../../66492/66492-h/66492-h.htm#Page_180">180</a>; ii. <a href="#Page_17">17</a>, <a href="#Page_39">39</a>, <a href="#Page_195">195</a></span><br /> -Pumiceous structure, i. <a href="../../66492/66492-h/66492-h.htm#Page_15">15</a>, <a href="../../66492/66492-h/66492-h.htm#Page_33">33</a>, <a href="../../66492/66492-h/66492-h.htm#Page_34">34</a>, <a href="../../66492/66492-h/66492-h.htm#Page_60">60</a><br /> -Puys, as a type of volcano, i. <a href="../../66492/66492-h/66492-h.htm#Page_10">10</a>, <a href="../../66492/66492-h/66492-h.htm#Page_44">44</a>, <a href="../../66492/66492-h/66492-h.htm#Page_100">100</a>, <a href="../../66492/66492-h/66492-h.htm#Page_414">414</a>;<br /> -<span style="margin-left: 1em;">probable subærial nature of some, <a href="../../66492/66492-h/66492-h.htm#Page_432">432</a>;</span><br /> -<span style="margin-left: 1em;">Carboniferous, <a href="../../66492/66492-h/66492-h.htm#Page_308">308</a>, <a href="../../66492/66492-h/66492-h.htm#Page_364">364</a>, <a href="../../66492/66492-h/66492-h.htm#Page_414">414</a>, <a href="../../66492/66492-h/66492-h.htm#Page_424">424</a>, <a href="../../66492/66492-h/66492-h.htm#Page_463">463</a>; ii. <a href="#Page_13">13</a>, <a href="#Page_28">28</a>, <a href="#Page_34">34</a>, <a href="#Page_47">47</a>;</span><br /> -<span style="margin-left: 1em;">Permian, ii. <a href="#Page_62">62</a>;</span><br /> -<span style="margin-left: 1em;">Tertiary, <a href="#Page_271">271</a>, <a href="#Page_276">276</a></span><br /> -Puy de Chopine, i. <a href="../../66492/66492-h/66492-h.htm#Page_32">32</a>; ii. <a href="#Page_374">374</a><br /> -—— Dôme, ii. <a href="#Page_373">373</a><br /> -—— Montchar, i. <a href="../../66492/66492-h/66492-h.htm#Page_32">32</a><br /> -—— Pariou, i. <a href="../../66492/66492-h/66492-h.htm#Page_66">66</a>, <a href="../../66492/66492-h/66492-h.htm#Page_70">70</a>; ii. <a href="#Page_31">31</a>, <a href="#Page_281">281</a><br /> -Pyroclastic detritus, i. <a href="../../66492/66492-h/66492-h.htm#Page_31">31</a>, <a href="../../66492/66492-h/66492-h.htm#Page_58">58</a>, <a href="../../66492/66492-h/66492-h.htm#Page_61">61</a><br /> -Pyromeride, i. <a href="../../66492/66492-h/66492-h.htm#Page_211">211</a><br /> -Pyrope, ii. <a href="#Page_58">58</a>, <a href="#Page_79">79</a><br /> -Pyroxene, ii. <a href="#Page_135">135</a><br /> -<br /> -<a id="Q"></a>Quartz-porphyry, i. <a href="../../66492/66492-h/66492-h.htm#Page_19">19</a>, <a href="../../66492/66492-h/66492-h.htm#Page_156">156</a>, <a href="../../66492/66492-h/66492-h.htm#Page_159">159</a>, <a href="../../66492/66492-h/66492-h.htm#Page_160">160</a>, <a href="../../66492/66492-h/66492-h.htm#Page_161">161</a>, <a href="../../66492/66492-h/66492-h.htm#Page_165">165</a>, <a href="../../66492/66492-h/66492-h.htm#Page_277">277</a>, <a href="../../66492/66492-h/66492-h.htm#Page_291">291</a>, <a href="../../66492/66492-h/66492-h.htm#Page_314">314</a>; ii. <a href="#Page_96">96</a>, <a href="#Page_369">369</a>, <a href="#Page_420">420</a>, <a href="#Page_423">423</a>, <a href="#Page_430">430</a>, <a href="#Page_431">431</a><br /> -Quartz-trachyte, ii. <a href="#Page_371">371</a><br /> -Quartzite, i. <a href="../../66492/66492-h/66492-h.htm#Page_112">112</a>, <a href="../../66492/66492-h/66492-h.htm#Page_170">170</a><br /> -<br /> -<a id="R"></a>Raasay, basalt of, ii. <a href="#Page_192">192</a>;<br /> -<span style="margin-left: 1em;">neck-like breccias in, <a href="#Page_293">293</a>;</span><br /> -<span style="margin-left: 1em;">acid sill of, <a href="#Page_430">430</a></span><br /> -Raddling or red-staining of rocks, i. <a href="../../66492/66492-h/66492-h.htm#Page_250">250</a>, <a href="../../66492/66492-h/66492-h.htm#Page_261">261</a><br /> -Radiolarian cherts, i. <a href="../../66492/66492-h/66492-h.htm#Page_123">123</a>, <a href="../../66492/66492-h/66492-h.htm#Page_167">167</a>, <a href="../../66492/66492-h/66492-h.htm#Page_169">169</a>, <a href="../../66492/66492-h/66492-h.htm#Page_173">173</a>, <a href="../../66492/66492-h/66492-h.htm#Page_174">174</a>, <a href="../../66492/66492-h/66492-h.htm#Page_184">184</a>, <a href="../../66492/66492-h/66492-h.htm#Page_196">196</a>, <a href="../../66492/66492-h/66492-h.htm#Page_201">201</a>, <a href="../../66492/66492-h/66492-h.htm#Page_244">244</a><br /> -Rain-pittings in strata, i. <a href="../../66492/66492-h/66492-h.htm#Page_342">342</a><br /> -Raisin, Miss, i. <a href="../../66492/66492-h/66492-h.htm#Page_161">161</a>, <a href="../../66492/66492-h/66492-h.htm#Page_163">163</a>, <a href="../../66492/66492-h/66492-h.htm#Page_164">164</a>, <a href="../../66492/66492-h/66492-h.htm#Page_165">165</a>, <a href="../../66492/66492-h/66492-h.htm#Page_210">210</a><br /> -Ramsay, A. C., i. <a href="../../66492/66492-h/66492-h.htm#Page_126">126</a>, <a href="../../66492/66492-h/66492-h.htm#Page_142">142</a>, <a href="../../66492/66492-h/66492-h.htm#Page_143">143</a>, <a href="../../66492/66492-h/66492-h.htm#Page_144">144</a>, <a href="../../66492/66492-h/66492-h.htm#Page_145">145</a>, <a href="../../66492/66492-h/66492-h.htm#Page_158">158</a>, <a href="../../66492/66492-h/66492-h.htm#Page_159">159</a>, <a href="../../66492/66492-h/66492-h.htm#Page_168">168</a>, <a href="../../66492/66492-h/66492-h.htm#Page_175">175</a>, <a href="../../66492/66492-h/66492-h.htm#Page_176">176</a>, <a href="../../66492/66492-h/66492-h.htm#Page_177">177</a>, <a href="../../66492/66492-h/66492-h.htm#Page_178">178</a>, <a href="../../66492/66492-h/66492-h.htm#Page_179">179</a>, <a href="../../66492/66492-h/66492-h.htm#Page_180">180</a>, <a href="../../66492/66492-h/66492-h.htm#Page_182">182</a>, <a href="../../66492/66492-h/66492-h.htm#Page_183">183</a>, <a href="../../66492/66492-h/66492-h.htm#Page_204">204</a>, <a href="../../66492/66492-h/66492-h.htm#Page_205">205</a>, <a href="../../66492/66492-h/66492-h.htm#Page_208">208</a>, <a href="../../66492/66492-h/66492-h.htm#Page_210">210</a>, <a href="../../66492/66492-h/66492-h.htm#Page_212">212</a>, <a href="../../66492/66492-h/66492-h.htm#Page_214">214</a>, <a href="../../66492/66492-h/66492-h.htm#Page_223">223</a>, <a href="../../66492/66492-h/66492-h.htm#Page_237">237</a>, <a href="../../66492/66492-h/66492-h.htm#Page_364">364</a><br /> -Ratho type of dolerite, i. <a href="../../66492/66492-h/66492-h.htm#Page_418">418</a>, <a href="../../66492/66492-h/66492-h.htm#Page_421">421</a><br /> -Red Head, section at, i. <a href="../../66492/66492-h/66492-h.htm#Page_300">300</a><br /> -Red Hills, Skye, scenery of, i. <a href="../../66492/66492-h/66492-h.htm#Page_105">105</a>; ii. <a href="#Page_379">379</a><br /> -Reed, Mr. Cowper, i. <a href="../../66492/66492-h/66492-h.htm#Page_205">205</a><br /> -Reid, Mr. Clement, ii. <a href="#Page_449">449</a>, <a href="#Page_450">450</a><br /> -Renard, Prof. A., i. <a href="../../66492/66492-h/66492-h.htm#Page_148">148</a>, <a href="../../66492/66492-h/66492-h.htm#Page_149">149</a><br /> -Renfrewshire, Carboniferous volcanic rocks of, i. <a href="../../66492/66492-h/66492-h.htm#Page_368">368</a>, <a href="../../66492/66492-h/66492-h.htm#Page_385">385</a>, <a href="../../66492/66492-h/66492-h.htm#Page_397">397</a>, <a href="../../66492/66492-h/66492-h.htm#Page_400">400</a>, <a href="../../66492/66492-h/66492-h.htm#Page_404">404</a>, <a href="../../66492/66492-h/66492-h.htm#Page_408">408</a>, <a href="../../66492/66492-h/66492-h.htm#Page_416">416</a>, <a href="../../66492/66492-h/66492-h.htm#Page_430">430</a>, <a href="../../66492/66492-h/66492-h.htm#Page_447">447</a><br /> -</p> - -<p><span class="pagenum" id="Page_489">- 489 -</span></p> - -<p> -Reyer, Prof. E., ii. <a href="#Page_474">474</a><br /> -Reynolds, Mr. S. H., i. <a href="../../66492/66492-h/66492-h.htm#Page_177">177</a>, <a href="../../66492/66492-h/66492-h.htm#Page_179">179</a>, <a href="../../66492/66492-h/66492-h.htm#Page_256">256</a><br /> -Rhobel Fawr, i. <a href="../../66492/66492-h/66492-h.htm#Page_177">177</a>, <a href="../../66492/66492-h/66492-h.htm#Page_178">178</a>, <a href="../../66492/66492-h/66492-h.htm#Page_186">186</a><br /> -Rhyolite, i. <a href="../../66492/66492-h/66492-h.htm#Page_19">19</a>, <a href="../../66492/66492-h/66492-h.htm#Page_22">22</a>, <a href="../../66492/66492-h/66492-h.htm#Page_24">24</a>, <a href="../../66492/66492-h/66492-h.htm#Page_131">131</a>, <a href="../../66492/66492-h/66492-h.htm#Page_161">161</a>, <a href="../../66492/66492-h/66492-h.htm#Page_165">165</a>, <a href="../../66492/66492-h/66492-h.htm#Page_167">167</a>, <a href="../../66492/66492-h/66492-h.htm#Page_168">168</a>, <a href="../../66492/66492-h/66492-h.htm#Page_178">178</a>, <a href="../../66492/66492-h/66492-h.htm#Page_204">204</a>, <a href="../../66492/66492-h/66492-h.htm#Page_210">210</a>, <a href="../../66492/66492-h/66492-h.htm#Page_231">231</a>, <a href="../../66492/66492-h/66492-h.htm#Page_232">232</a>, <a href="../../66492/66492-h/66492-h.htm#Page_255">255</a>, <a href="../../66492/66492-h/66492-h.htm#Page_276">276</a>, <a href="../../66492/66492-h/66492-h.htm#Page_278">278</a>; ii. <a href="#Page_185">185</a>, <a href="#Page_205">205</a>, <a href="#Page_371">371</a>, <a href="#Page_424">424</a>, <a href="#Page_437">437</a><br /> -Rhyolitic conglomerate, ii. <a href="#Page_195">195</a>, <a href="#Page_206">206</a>, <a href="#Page_429">429</a><br /> -Richardson, Rev. W., ii. <a href="#Page_110">110</a><br /> -Richthofen, F. von, i. <a href="../../66492/66492-h/66492-h.htm#Page_28">28</a>; ii. <a href="#Page_115">115</a>, <a href="#Page_116">116</a><br /> -Rivers made to shift their channels by volcanic eruptions, i. <a href="../../66492/66492-h/66492-h.htm#Page_42">42</a>, <a href="../../66492/66492-h/66492-h.htm#Page_49">49</a>;<br /> -<span style="margin-left: 1em;">of the Tertiary volcanic period, ii. <a href="#Page_217">217</a>, <a href="#Page_228">228</a>, <a href="#Page_231">231</a>, <a href="#Page_234">234</a>, <a href="#Page_456">456</a></span><br /> -Rocks, oldest known, i. <a href="../../66492/66492-h/66492-h.htm#Page_110">110</a><br /> -Roscommon, volcanic rocks of, i. <a href="../../66492/66492-h/66492-h.htm#Page_316">316</a><br /> -Rosenbusch, Prof. H., ii. <a href="#Page_136">136</a>, <a href="#Page_137">137</a><br /> -Ross, Mr. Alexander, ii. <a href="#Page_406">406</a>, <a href="#Page_409">409</a><br /> -Rothliegende, ii. <a href="#Page_95">95</a><br /> -Roxburghshire, Carboniferous vents of, i. <a href="../../66492/66492-h/66492-h.htm#Page_55">55</a>, <a href="../../66492/66492-h/66492-h.htm#Page_403">403</a>, <a href="../../66492/66492-h/66492-h.htm#Page_404">404</a><br /> -Rubers Law, i. <a href="../../66492/66492-h/66492-h.htm#Page_375">375</a>, <a href="../../66492/66492-h/66492-h.htm#Page_380">380</a>, <a href="../../66492/66492-h/66492-h.htm#Page_404">404</a><br /> -Rum, i. <a href="../../66492/66492-h/66492-h.htm#Page_112">112</a>;<br /> -<span style="margin-left: 1em;">basalt-plateau of, ii. <a href="#Page_215">215</a>;</span><br /> -<span style="margin-left: 1em;">gabbros of, <a href="#Page_332">332</a>, <a href="#Page_349">349</a>;</span><br /> -<span style="margin-left: 1em;">acid bosses of, <a href="#Page_403">403</a>;</span><br /> -<span style="margin-left: 1em;">acid sills of, <a href="#Page_431">431</a>;</span><br /> -<span style="margin-left: 1em;">pitchstone of, <a href="#Page_445">445</a></span><br /> -Rutley, Mr. F., i. <a href="../../66492/66492-h/66492-h.htm#Page_131">131</a>, <a href="../../66492/66492-h/66492-h.htm#Page_133">133</a>, <a href="../../66492/66492-h/66492-h.htm#Page_170">170</a>, <a href="../../66492/66492-h/66492-h.htm#Page_207">207</a>, <a href="../../66492/66492-h/66492-h.htm#Page_210">210</a>, <a href="../../66492/66492-h/66492-h.htm#Page_227">227</a>, <a href="../../66492/66492-h/66492-h.htm#Page_231">231</a>, <a href="../../66492/66492-h/66492-h.htm#Page_232">232</a>, <a href="../../66492/66492-h/66492-h.htm#Page_238">238</a>, <a href="../../66492/66492-h/66492-h.htm#Page_260">260</a>; ii. <a href="#Page_23">23</a>, <a href="#Page_35">35</a><br /> -<br /> -<a id="S"></a>Sahlite found in volcanic vents, i. <a href="../../66492/66492-h/66492-h.htm#Page_62">62</a><br /> -Saline Hill, volcanic vents of, i. <a href="../../66492/66492-h/66492-h.htm#Page_433">433</a>, <a href="../../66492/66492-h/66492-h.htm#Page_435">435</a>, <a href="../../66492/66492-h/66492-h.htm#Page_440">440</a><br /> -Sanday, basalts of, ii. <a href="#Page_215">215</a>;<br /> -<span style="margin-left: 1em;">conglomerates of, <a href="#Page_226">226</a></span><br /> -Sandstone altered into quartzite, i. <a href="../../66492/66492-h/66492-h.htm#Page_72">72</a>, <a href="../../66492/66492-h/66492-h.htm#Page_349">349</a>, <a href="../../66492/66492-h/66492-h.htm#Page_350">350</a>, <a href="../../66492/66492-h/66492-h.htm#Page_404">404</a>, <a href="../../66492/66492-h/66492-h.htm#Page_432">432</a>, <a href="../../66492/66492-h/66492-h.htm#Page_451">451</a>; ii. <a href="#Page_76">76</a>, <a href="#Page_164">164</a>;<br /> -<span style="margin-left: 1em;">veinings of, in lava, i. <a href="../../66492/66492-h/66492-h.htm#Page_283">283</a>, <a href="../../66492/66492-h/66492-h.htm#Page_300">300</a>, <a href="../../66492/66492-h/66492-h.htm#Page_303">303</a>, <a href="../../66492/66492-h/66492-h.htm#Page_320">320</a>, <a href="../../66492/66492-h/66492-h.htm#Page_327">327</a>, <a href="../../66492/66492-h/66492-h.htm#Page_333">333</a>, <a href="../../66492/66492-h/66492-h.htm#Page_337">337</a>; ii. <a href="#Page_59">59</a>, <a href="#Page_98">98</a></span><br /> -Sandwich Islands, lava-cones of, i. <a href="../../66492/66492-h/66492-h.htm#Page_10">10</a><br /> -Sanidine ejected from volcanic vents, ii. <a href="#Page_58">58</a>, <a href="#Page_79">79</a><br /> -Sanquhar, Silurian volcanic rocks at, i. <a href="../../66492/66492-h/66492-h.htm#Page_192">192</a>, <a href="../../66492/66492-h/66492-h.htm#Page_195">195</a>, <a href="../../66492/66492-h/66492-h.htm#Page_199">199</a>;<br /> -<span style="margin-left: 1em;">Permian volcanic rocks at, ii. <a href="#Page_62">62</a></span><br /> -Santorin, ii. <a href="#Page_134">134</a><br /> -Saponite, ii. <a href="#Page_79">79</a><br /> -Scenery, origin of, i. <a href="../../66492/66492-h/66492-h.htm#Page_8">8</a>, <a href="../../66492/66492-h/66492-h.htm#Page_100">100</a><br /> -Schalstein, i. <a href="../../66492/66492-h/66492-h.htm#Page_262">262</a>; ii. <a href="#Page_36">36</a><br /> -<a id="Schists"></a>Schists, primeval, i. <a href="../../66492/66492-h/66492-h.htm#Page_110">110</a>, <a href="../../66492/66492-h/66492-h.htm#Page_114">114</a>, <a href="../../66492/66492-h/66492-h.htm#Page_118">118</a>, <a href="../../66492/66492-h/66492-h.htm#Page_119">119</a>;<br /> -<span style="margin-left: 1em;">produced by deformation of igneous rocks, <a href="../../66492/66492-h/66492-h.htm#Page_75">75</a>, <a href="../../66492/66492-h/66492-h.htm#Page_114">114</a>, <a href="../../66492/66492-h/66492-h.htm#Page_118">118</a>, <a href="../../66492/66492-h/66492-h.htm#Page_119">119</a>, <a href="../../66492/66492-h/66492-h.htm#Page_121">121</a>, <a href="../../66492/66492-h/66492-h.htm#Page_162">162</a>, <a href="../../66492/66492-h/66492-h.htm#Page_240">240</a>, <a href="../../66492/66492-h/66492-h.htm#Page_249">249</a>, <a href="../../66492/66492-h/66492-h.htm#Page_252">252</a>, <a href="../../66492/66492-h/66492-h.htm#Page_261">261</a></span><br /> -Schmidt, Dr. C. W., ii. <a href="#Page_266">266</a><br /> -Scoriaceous structure, i. <a href="../../66492/66492-h/66492-h.htm#Page_15">15</a>, <a href="../../66492/66492-h/66492-h.htm#Page_16">16</a>, <a href="../../66492/66492-h/66492-h.htm#Page_282">282</a>, <a href="../../66492/66492-h/66492-h.htm#Page_327">327</a>, <a href="../../66492/66492-h/66492-h.htm#Page_339">339</a><br /> -Scorpions, fossil, i. <a href="../../66492/66492-h/66492-h.htm#Page_174">174</a>, <a href="../../66492/66492-h/66492-h.htm#Page_356">356</a>, <a href="../../66492/66492-h/66492-h.htm#Page_466">466</a><br /> -Scotland, lines of fault in, i. <a href="../../66492/66492-h/66492-h.htm#Page_11">11</a>;<br /> -<span style="margin-left: 1em;">Vesuvian cones of, <a href="../../66492/66492-h/66492-h.htm#Page_42">42</a>;</span><br /> -<span style="margin-left: 1em;">plateaux of, <a href="../../66492/66492-h/66492-h.htm#Page_43">43</a>;</span><br /> -<span style="margin-left: 1em;">puys of, <a href="../../66492/66492-h/66492-h.htm#Page_46">46</a>;</span><br /> -<span style="margin-left: 1em;">submarine lavas of, <a href="../../66492/66492-h/66492-h.htm#Page_48">48</a>;</span><br /> -<span style="margin-left: 1em;">Carboniferous vents of, <a href="../../66492/66492-h/66492-h.htm#Page_55">55</a>;</span><br /> -<span style="margin-left: 1em;">volcanic scenery of, <a href="../../66492/66492-h/66492-h.htm#Page_104">104</a>;</span><br /> -<span style="margin-left: 1em;">pre-Cambrian rocks of, <a href="../../66492/66492-h/66492-h.htm#Page_111">111</a>;</span><br /> -<span style="margin-left: 1em;">Cambrian rocks of, <a href="../../66492/66492-h/66492-h.htm#Page_112">112</a>;</span><br /> -<span style="margin-left: 1em;">Lewisian gneiss of, <a href="../../66492/66492-h/66492-h.htm#Page_114">114</a>;</span><br /> -<span style="margin-left: 1em;">Dalradian rocks of, <a href="../../66492/66492-h/66492-h.htm#Page_121">121</a>;</span><br /> -<span style="margin-left: 1em;">Arenig rocks of, <a href="../../66492/66492-h/66492-h.htm#Page_123">123</a>, <a href="../../66492/66492-h/66492-h.htm#Page_191">191</a>;</span><br /> -<span style="margin-left: 1em;">Old Red Sandstone of, <a href="../../66492/66492-h/66492-h.htm#Page_266">266</a>, <a href="../../66492/66492-h/66492-h.htm#Page_273">273</a>, <a href="../../66492/66492-h/66492-h.htm#Page_281">281</a>, <a href="../../66492/66492-h/66492-h.htm#Page_291">291</a>;</span><br /> -<span style="margin-left: 1em;">Carboniferous geography of, <a href="../../66492/66492-h/66492-h.htm#Page_356">356</a>;</span><br /> -<span style="margin-left: 1em;">Carboniferous volcanoes of, <a href="../../66492/66492-h/66492-h.htm#Page_359">359</a>;</span><br /> -<span style="margin-left: 1em;">Carboniferous plateaux of, <a href="../../66492/66492-h/66492-h.htm#Page_367">367</a>;</span><br /> -<span style="margin-left: 1em;">Carboniferous puys of, <a href="../../66492/66492-h/66492-h.htm#Page_414">414</a>;</span><br /> -<span style="margin-left: 1em;">Permian volcanoes of, ii. <a href="#Page_55">55</a>;</span><br /> -<span style="margin-left: 1em;">Tertiary dykes of, <a href="#Page_122">122</a>;</span><br /> -<span style="margin-left: 1em;">Tertiary basalt-plateaux of, <a href="#Page_208">208</a>, <a href="#Page_274">274</a>, <a href="#Page_304">304</a>;</span><br /> -<span style="margin-left: 1em;">Tertiary gabbros of, <a href="#Page_327">327</a>;</span><br /> -<span style="margin-left: 1em;">Tertiary acid rocks of, <a href="#Page_379">379</a></span><br /> -Scrope, G. P., i. <a href="../../66492/66492-h/66492-h.htm#Page_27">27</a>, <a href="../../66492/66492-h/66492-h.htm#Page_82">82</a>, <a href="../../66492/66492-h/66492-h.htm#Page_45">45</a>, <a href="../../66492/66492-h/66492-h.htm#Page_116">116</a>; ii. <a href="#Page_373">373</a>, <a href="#Page_374">374</a>, <a href="#Page_381">381</a><br /> -Sedgwick, A., i. <a href="../../66492/66492-h/66492-h.htm#Page_142">142</a>, <a href="../../66492/66492-h/66492-h.htm#Page_166">166</a>, <a href="../../66492/66492-h/66492-h.htm#Page_175">175</a>, <a href="../../66492/66492-h/66492-h.htm#Page_218">218</a>, <a href="../../66492/66492-h/66492-h.htm#Page_227">227</a>, <a href="../../66492/66492-h/66492-h.htm#Page_257">257</a>; ii. <a href="#Page_1">1</a>, <a href="#Page_2">2</a>, <a href="#Page_3">3</a>, <a href="#Page_5">5</a>, <a href="#Page_113">113</a>, <a href="#Page_139">139</a>, <a href="#Page_153">153</a>, <a href="#Page_157">157</a><br /> -Segregation (<i>see</i> <a href="#Differentiation">Differentiation</a>)<br /> -Segregation-veins, i. <a href="../../66492/66492-h/66492-h.htm#Page_84">84</a>, <a href="../../66492/66492-h/66492-h.htm#Page_92">92</a>; ii. <a href="../../66492/66492-h/66492-h.htm#Page_66">66</a>, <a href="../../66492/66492-h/66492-h.htm#Page_130">130</a>, <a href="../../66492/66492-h/66492-h.htm#Page_300">300</a>, <a href="../../66492/66492-h/66492-h.htm#Page_303">303</a><br /> -Selwyn, Mr. A. C. R., i. <a href="../../66492/66492-h/66492-h.htm#Page_143">143</a>, <a href="../../66492/66492-h/66492-h.htm#Page_175">175</a>, <a href="../../66492/66492-h/66492-h.htm#Page_208">208</a>, <a href="../../66492/66492-h/66492-h.htm#Page_221">221</a><br /> -Semi-opal, ii. <a href="#Page_79">79</a><br /> -Sepulchre Mountain, i. <a href="../../66492/66492-h/66492-h.htm#Page_79">79</a><br /> -Serpentine, i. <a href="../../66492/66492-h/66492-h.htm#Page_195">195</a>, <a href="../../66492/66492-h/66492-h.htm#Page_293">293</a><br /> -Shale, alteration of, i. <a href="../../66492/66492-h/66492-h.htm#Page_72">72</a>, <a href="../../66492/66492-h/66492-h.htm#Page_451">451</a>; ii. <a href="#Page_164">164</a><br /> -Shap, granite of, i. <a href="../../66492/66492-h/66492-h.htm#Page_236">236</a>, <a href="../../66492/66492-h/66492-h.htm#Page_238">238</a>, <a href="../../66492/66492-h/66492-h.htm#Page_271">271</a>, <a href="../../66492/66492-h/66492-h.htm#Page_290">290</a><br /> -Sheets, intrusive (<i>see</i> <a href="#Sills">Sills</a>)<br /> -Shelve, i. <a href="../../66492/66492-h/66492-h.htm#Page_176">176</a>, <a href="../../66492/66492-h/66492-h.htm#Page_190">190</a><br /> -Shetland, i. <a href="../../66492/66492-h/66492-h.htm#Page_271">271</a>, <a href="../../66492/66492-h/66492-h.htm#Page_289">289</a>, <a href="../../66492/66492-h/66492-h.htm#Page_292">292</a>, <a href="../../66492/66492-h/66492-h.htm#Page_293">293</a>, <a href="../../66492/66492-h/66492-h.htm#Page_345">345</a><br /> -Shiant Isles, ii. <a href="#Page_307">307</a><br /> -Shineton Shales, i. <a href="../../66492/66492-h/66492-h.htm#Page_144">144</a><br /> -Shore-lines, traces of ancient, i. <a href="../../66492/66492-h/66492-h.htm#Page_295">295</a>, <a href="../../66492/66492-h/66492-h.htm#Page_305">305</a><br /> -Shropshire, ancient volcanic rocks of, i. <a href="../../66492/66492-h/66492-h.htm#Page_129">129</a>, <a href="../../66492/66492-h/66492-h.htm#Page_189">189</a>;<br /> -<span style="margin-left: 1em;">latest eruptive rocks of, ii. <a href="#Page_101">101</a></span><br /> -Sicily, i. <a href="../../66492/66492-h/66492-h.htm#Page_26">26</a><br /> -Sidlaw Hills, i. <a href="../../66492/66492-h/66492-h.htm#Page_286">286</a>, <a href="../../66492/66492-h/66492-h.htm#Page_294">294</a>, <a href="../../66492/66492-h/66492-h.htm#Page_303">303</a><br /> -<a id="Sills"></a>Sills, vitreous margins of, i. <a href="../../66492/66492-h/66492-h.htm#Page_18">18</a>;<br /> -<span style="margin-left: 1em;">tectonic relations of, <a href="../../66492/66492-h/66492-h.htm#Page_77">77</a>, <a href="../../66492/66492-h/66492-h.htm#Page_83">83</a>, <a href="../../66492/66492-h/66492-h.htm#Page_451">451</a>;</span><br /> -<span style="margin-left: 1em;">origin of name, <a href="../../66492/66492-h/66492-h.htm#Page_83">83</a>;</span><br /> -<span style="margin-left: 1em;">differentiation (segregation) in, <a href="../../66492/66492-h/66492-h.htm#Page_81">81</a>, <a href="../../66492/66492-h/66492-h.htm#Page_450">450</a>; ii. <a href="#Page_476">476</a>;</span><br /> -<span style="margin-left: 1em;">ordinary stratigraphical position of, i. <a href="../../66492/66492-h/66492-h.htm#Page_85">85</a>;</span><br /> -<span style="margin-left: 1em;">considered as parts of incompleted volcanoes, <a href="../../66492/66492-h/66492-h.htm#Page_86">86</a>;</span><br /> -<span style="margin-left: 1em;">metamorphism by, <a href="../../66492/66492-h/66492-h.htm#Page_87">87</a>, <a href="../../66492/66492-h/66492-h.htm#Page_94">94</a>, <a href="../../66492/66492-h/66492-h.htm#Page_451">451</a>; ii. <a href="#Page_299">299</a>, <a href="#Page_303">303</a>, <a href="#Page_310">310</a>;</span><br /> -<span style="margin-left: 1em;">conditions for injection of, i. <a href="../../66492/66492-h/66492-h.htm#Page_97">97</a>, <a href="../../66492/66492-h/66492-h.htm#Page_98">98</a>, <a href="../../66492/66492-h/66492-h.htm#Page_99">99</a>, <a href="../../66492/66492-h/66492-h.htm#Page_458">458</a>;</span><br /> -<span style="margin-left: 1em;">columnar structure of, ii. <a href="#Page_187">187</a>, <a href="#Page_291">291</a>, <a href="#Page_301">301</a>, <a href="#Page_306">306</a>, <a href="#Page_308">308</a>, <a href="#Page_319">319</a>;</span><br /> -<span style="margin-left: 1em;">amygdaloidal structure in, <a href="#Page_299">299</a>, <a href="#Page_312">312</a>;</span><br /> -<span style="margin-left: 1em;">banding of, <a href="#Page_309">309</a>;</span><br /> -<span style="margin-left: 1em;">split by later sills, <a href="#Page_310">310</a>, <a href="#Page_316">316</a>;</span><br /> -<span style="margin-left: 1em;">extreme subdivision of, <a href="#Page_311">311</a>;</span><br /> -<span style="margin-left: 1em;">slaggy surface in some, <a href="#Page_312">312</a>;</span><br /> -<span style="margin-left: 1em;">give off veins, <a href="#Page_313">313</a>;</span><br /> -<span style="margin-left: 1em;">double and multiple, <a href="#Page_318">318</a>, <a href="#Page_434">434</a>;</span><br /> -<span style="margin-left: 1em;">connection with vents, <a href="#Page_322">322</a></span><br /> -—— pre-Cambrian, i. <a href="../../66492/66492-h/66492-h.htm#Page_118">118</a>, <a href="../../66492/66492-h/66492-h.htm#Page_124">124</a>;<br /> -<span style="margin-left: 1em;">Cambrian, <a href="../../66492/66492-h/66492-h.htm#Page_155">155</a>, <a href="../../66492/66492-h/66492-h.htm#Page_170">170</a>, <a href="../../66492/66492-h/66492-h.htm#Page_171">171</a>;</span><br /> -<span style="margin-left: 1em;">Silurian, <a href="../../66492/66492-h/66492-h.htm#Page_187">187</a>, <a href="../../66492/66492-h/66492-h.htm#Page_195">195</a>, <a href="../../66492/66492-h/66492-h.htm#Page_206">206</a>, <a href="../../66492/66492-h/66492-h.htm#Page_216">216</a>, <a href="../../66492/66492-h/66492-h.htm#Page_237">237</a>, <a href="../../66492/66492-h/66492-h.htm#Page_249">249</a>;</span><br /> -<span style="margin-left: 1em;">Devonian, <a href="../../66492/66492-h/66492-h.htm#Page_261">261</a>;</span><br /> -<span style="margin-left: 1em;">Old Red Sandstone, <a href="../../66492/66492-h/66492-h.htm#Page_277">277</a>, <a href="../../66492/66492-h/66492-h.htm#Page_291">291</a>, <a href="../../66492/66492-h/66492-h.htm#Page_321">321</a>, <a href="../../66492/66492-h/66492-h.htm#Page_335">335</a>, <a href="../../66492/66492-h/66492-h.htm#Page_343">343</a>, <a href="../../66492/66492-h/66492-h.htm#Page_345">345</a>;</span><br /> -<span style="margin-left: 1em;">Carboniferous, <a href="../../66492/66492-h/66492-h.htm#Page_408">408</a>, <a href="../../66492/66492-h/66492-h.htm#Page_446">446</a>, <a href="../../66492/66492-h/66492-h.htm#Page_472">472</a>; ii. <a href="#Page_2">2</a>, <a href="#Page_21">21</a>, <a href="#Page_30">30</a>, <a href="#Page_48">48</a>;</span><br /> -<span style="margin-left: 1em;">Permian, <a href="#Page_64">64</a>, <a href="#Page_66">66</a>;</span><br /> -<span style="margin-left: 1em;">of Midlands, <a href="#Page_102">102</a>, <a href="#Page_103">103</a>;</span><br /> -<span style="margin-left: 1em;">Tertiary, (1) Basic, <a href="#Page_298">298</a>;</span><br /> -<span style="margin-left: 2em;">(2) Acid, <a href="#Page_366">366</a>, <a href="#Page_430">430</a></span><br /> -Silurian system, i. <a href="../../66492/66492-h/66492-h.htm#Page_173">173</a>;<br /> -<span style="margin-left: 1em;">vegetation of, <a href="../../66492/66492-h/66492-h.htm#Page_174">174</a>;</span><br /> -<span style="margin-left: 1em;">geography of, <a href="../../66492/66492-h/66492-h.htm#Page_263">263</a>;</span><br /> -<span style="margin-left: 1em;">volcanoes of, <a href="../../66492/66492-h/66492-h.htm#Page_175">175</a>;</span><br /> -<span style="margin-left: 1em;">classification of, <a href="../../66492/66492-h/66492-h.htm#Page_175">175</a>;</span><br /> -<span style="margin-left: 1em;">two volcanic series of, <a href="../../66492/66492-h/66492-h.htm#Page_177">177</a></span><br /> -—— volcanoes in Shropshire, i. <a href="../../66492/66492-h/66492-h.htm#Page_189">189</a>;<br /> -<span style="margin-left: 1em;">in Scotland, <a href="../../66492/66492-h/66492-h.htm#Page_191">191</a>;</span><br /> -<span style="margin-left: 1em;">at Builth, <a href="../../66492/66492-h/66492-h.htm#Page_203">203</a>;</span><br /> -<span style="margin-left: 1em;">in Pembrokeshire, <a href="../../66492/66492-h/66492-h.htm#Page_205">205</a>;</span><br /> -<span style="margin-left: 1em;">in Caernarvonshire, <a href="../../66492/66492-h/66492-h.htm#Page_207">207</a>;</span><br /> -<span style="margin-left: 1em;">in the Berwyn Hills, <a href="../../66492/66492-h/66492-h.htm#Page_218">218</a>;</span><br /> -<span style="margin-left: 1em;">in Anglesey, <a href="../../66492/66492-h/66492-h.htm#Page_219">219</a>;</span><br /> -<span style="margin-left: 1em;">in the Lake District, <a href="../../66492/66492-h/66492-h.htm#Page_227">227</a>;</span><br /> -<span style="margin-left: 1em;">in Gloucestershire, <a href="../../66492/66492-h/66492-h.htm#Page_238">238</a>;</span><br /> -<span style="margin-left: 1em;">in Ireland, <a href="../../66492/66492-h/66492-h.htm#Page_239">239</a>, <a href="../../66492/66492-h/66492-h.htm#Page_254">254</a></span><br /> -Skae, H. M., i. <a href="../../66492/66492-h/66492-h.htm#Page_294">294</a>, <a href="../../66492/66492-h/66492-h.htm#Page_299">299</a>, <a href="../../66492/66492-h/66492-h.htm#Page_306">306</a>, <a href="../../66492/66492-h/66492-h.htm#Page_375">375</a>; ii. <a href="#Page_57">57</a><br /> -Skiddaw, i. <a href="../../66492/66492-h/66492-h.htm#Page_228">228</a>;<br /> -<span style="margin-left: 1em;">granite of, <a href="../../66492/66492-h/66492-h.htm#Page_236">236</a></span><br /> -—— Slate, i. <a href="../../66492/66492-h/66492-h.htm#Page_229">229</a><br /> -Skomer Island, i. <a href="../../66492/66492-h/66492-h.htm#Page_207">207</a><br /> -Skye, spherulitic dykes and sills of, i. <a href="../../66492/66492-h/66492-h.htm#Page_20">20</a>;<br /> -<span style="margin-left: 1em;">ophitic structure from, <a href="../../66492/66492-h/66492-h.htm#Page_20">20</a>;</span><br /> -<span style="margin-left: 1em;">basalt-terraces of, <a href="../../66492/66492-h/66492-h.htm#Page_24">24</a>;</span><br /> -<span style="margin-left: 1em;">metamorphism by granophyre of, <a href="../../66492/66492-h/66492-h.htm#Page_95">95</a>, <a href="../../66492/66492-h/66492-h.htm#Page_96">96</a>;</span><br /> -<span style="margin-left: 1em;">volcanic scenery of, <a href="../../66492/66492-h/66492-h.htm#Page_103">103</a>, <a href="../../66492/66492-h/66492-h.htm#Page_105">105</a>;</span><br /> -<span style="margin-left: 1em;">dykes of, ii. <a href="#Page_123">123</a>, <a href="#Page_124">124</a>, <a href="#Page_129">129</a>, <a href="#Page_139">139</a>, <a href="#Page_140">140</a>, <a href="#Page_146">146</a>, <a href="#Page_150">150</a>, <a href="#Page_152">152</a>, <a href="#Page_154">154</a>, <a href="#Page_160">160</a>, <a href="#Page_162">162</a>, <a href="#Page_164">164</a>, <a href="#Page_165">165</a>, <a href="#Page_173">173</a>, <a href="#Page_269">269</a>;</span><br /> -<span style="margin-left: 1em;">bedded basalts of, <a href="#Page_192">192</a>, <a href="#Page_249">249</a>, <a href="#Page_269">269</a>;</span><br /> -<span style="margin-left: 1em;">tuffs of, <a href="#Page_251">251</a>;</span><br /> -<span style="margin-left: 1em;">connection of dykes and superficial lavas in, <a href="#Page_269">269</a>;</span><br /> -<span style="margin-left: 1em;">vents in, <a href="#Page_280">280</a>;</span><br /> -<span style="margin-left: 1em;">sills of, <a href="#Page_304">304</a>;</span><br /> -<span style="margin-left: 1em;">gabbro bosses of, i. <a href="../../66492/66492-h/66492-h.htm#Page_116">116</a>; ii. <a href="#Page_334">334</a>;</span><br /> -<span style="margin-left: 1em;">acid bosses of, <a href="#Page_379">379</a>;</span><br /> -<span style="margin-left: 1em;">acid sills of, <a href="#Page_431">431</a>;</span><br /> -<span style="margin-left: 1em;">acid dykes of, <a href="#Page_437">437</a>;</span><br /> -<span style="margin-left: 1em;">pitchstone veins of, <a href="#Page_445">445</a>;</span><br /> -<span style="margin-left: 1em;">subsidence of, <a href="#Page_447">447</a></span><br /> -Slaggy structure, i. <a href="../../66492/66492-h/66492-h.htm#Page_16">16</a>, <a href="../../66492/66492-h/66492-h.htm#Page_33">33</a>, <a href="../../66492/66492-h/66492-h.htm#Page_59">59</a>, <a href="../../66492/66492-h/66492-h.htm#Page_282">282</a>, <a href="../../66492/66492-h/66492-h.htm#Page_327">327</a>, <a href="../../66492/66492-h/66492-h.htm#Page_339">339</a>; ii. <a href="#Page_98">98</a>, <a href="#Page_187">187</a><br /> -Slane, volcanic rocks near, i. <a href="../../66492/66492-h/66492-h.htm#Page_244">244</a><br /> -Slate-tuffs, i. <a href="../../66492/66492-h/66492-h.htm#Page_180">180</a>, <a href="../../66492/66492-h/66492-h.htm#Page_213">213</a>, <a href="../../66492/66492-h/66492-h.htm#Page_234">234</a><br /> -Slemish a volcanic neck, ii. <a href="#Page_271">271</a><br /> -Slieve Foye, ii. <a href="#Page_421">421</a><br /> -—— Gallion, ii. <a href="#Page_200">200</a><br /> -—— Gullion, ii. <a href="#Page_422">422</a><br /> -</p> - -<p><span class="pagenum" id="Page_490">- 490 -</span></p> - -<p> -Small Isles, basalt-plateau of, ii. <a href="#Page_215">215</a>;<br /> -<span style="margin-left: 1em;">vents of, <a href="#Page_288">288</a>;</span><br /> -<span style="margin-left: 1em;">sills of, <a href="#Page_318">318</a>;</span><br /> -<span style="margin-left: 1em;">acid bosses of, <a href="#Page_403">403</a>;</span><br /> -<span style="margin-left: 1em;">acid sills of, <a href="#Page_431">431</a></span><br /> -Small, Mr. E. W., i. <a href="../../66492/66492-h/66492-h.htm#Page_207">207</a><br /> -Smaragdite found in volcanic vents, i. <a href="../../66492/66492-h/66492-h.htm#Page_62">62</a><br /> -Snowdon, volcanic rocks of, i. <a href="../../66492/66492-h/66492-h.htm#Page_20">20</a>, <a href="../../66492/66492-h/66492-h.htm#Page_42">42</a>, <a href="../../66492/66492-h/66492-h.htm#Page_47">47</a>, <a href="../../66492/66492-h/66492-h.htm#Page_102">102</a>, <a href="../../66492/66492-h/66492-h.htm#Page_175">175</a>, <a href="../../66492/66492-h/66492-h.htm#Page_208">208</a>, <a href="../../66492/66492-h/66492-h.htm#Page_209">209</a>, <a href="../../66492/66492-h/66492-h.htm#Page_210">210</a>, <a href="../../66492/66492-h/66492-h.htm#Page_211">211</a>, <a href="../../66492/66492-h/66492-h.htm#Page_212">212</a>, <a href="../../66492/66492-h/66492-h.htm#Page_213">213</a>, <a href="../../66492/66492-h/66492-h.htm#Page_218">218</a>, <a href="../../66492/66492-h/66492-h.htm#Page_226">226</a><br /> -Soda-felsites, i. <a href="../../66492/66492-h/66492-h.htm#Page_183">183</a>, <a href="../../66492/66492-h/66492-h.htm#Page_196">196</a>, <a href="../../66492/66492-h/66492-h.htm#Page_247">247</a><br /> -Solfataric action, i. <a href="../../66492/66492-h/66492-h.htm#Page_71">71</a>; ii. <a href="#Page_185">185</a>, <a href="#Page_205">205</a>, <a href="#Page_388">388</a><br /> -Sollas, Prof., i. <a href="../../66492/66492-h/66492-h.htm#Page_96">96</a>; ii. <a href="#Page_175">175</a>, <a href="#Page_293">293</a>, <a href="#Page_415">415</a>, <a href="#Page_421">421</a>, <a href="#Page_422">422</a><br /> -Solway, Carboniferous volcanic plateau of, i. <a href="../../66492/66492-h/66492-h.htm#Page_375">375</a>, <a href="../../66492/66492-h/66492-h.htm#Page_385">385</a>, <a href="../../66492/66492-h/66492-h.htm#Page_413">413</a><br /> -Somerset, volcanic rocks of, ii. <a href="#Page_32">32</a><br /> -Somma, denudation of, i. <a href="../../66492/66492-h/66492-h.htm#Page_3">3</a>, <a href="../../66492/66492-h/66492-h.htm#Page_100">100</a><br /> -Spheroidal structure of dolerite, i. <a href="../../66492/66492-h/66492-h.htm#Page_456">456</a><br /> -Spherulitic structure, i. <a href="../../66492/66492-h/66492-h.htm#Page_19">19</a>, <a href="../../66492/66492-h/66492-h.htm#Page_20">20</a>, <a href="../../66492/66492-h/66492-h.htm#Page_95">95</a>, <a href="../../66492/66492-h/66492-h.htm#Page_120">120</a>, <a href="../../66492/66492-h/66492-h.htm#Page_130">130</a>, <a href="../../66492/66492-h/66492-h.htm#Page_155">155</a>, <a href="../../66492/66492-h/66492-h.htm#Page_162">162</a>, <a href="../../66492/66492-h/66492-h.htm#Page_184">184</a>, <a href="../../66492/66492-h/66492-h.htm#Page_211">211</a>, <a href="../../66492/66492-h/66492-h.htm#Page_232">232</a>, <a href="../../66492/66492-h/66492-h.htm#Page_235">235</a>, <a href="../../66492/66492-h/66492-h.htm#Page_346">346</a>; ii. <a href="#Page_369">369</a>, <a href="#Page_381">381</a>, <a href="#Page_392">392</a>, <a href="#Page_432">432</a>, <a href="#Page_435">435</a>, <a href="#Page_437">437</a>, <a href="#Page_441">441</a>, <a href="#Page_446">446</a><br /> -Spilosite, i. <a href="../../66492/66492-h/66492-h.htm#Page_262">262</a><br /> -Springs, mineral, connected with volcanic action, i. <a href="../../66492/66492-h/66492-h.htm#Page_390">390</a>, <a href="../../66492/66492-h/66492-h.htm#Page_445">445</a><br /> -St. Abb's Head, i. <a href="../../66492/66492-h/66492-h.htm#Page_338">338</a><br /> -Staffa, i. <a href="../../66492/66492-h/66492-h.htm#Page_25">25</a>;<br /> -<span style="margin-left: 1em;">first notice of, ii. <a href="#Page_109">109</a>;</span><br /> -<span style="margin-left: 1em;">columnar basalts of, <a href="#Page_186">186</a>, <a href="#Page_188">188</a>, <a href="#Page_210">210</a>;</span><br /> -<span style="margin-left: 1em;">basalt conglomerate of, <a href="#Page_195">195</a></span><br /> -Staffordshire, latest eruptive rocks of, ii. <a href="#Page_101">101</a>, <a href="#Page_103">103</a><br /> -St. Andrews, old volcanoes near, ii. <a href="#Page_71">71</a>, <a href="#Page_73">73</a>, <a href="#Page_87">87</a><br /> -St. David's, Cambrian volcanic rocks of, i. <a href="../../66492/66492-h/66492-h.htm#Page_145">145</a><br /> -—— Head, i. <a href="../../66492/66492-h/66492-h.htm#Page_205">205</a><br /> -Steam in volcanic action, i. <a href="../../66492/66492-h/66492-h.htm#Page_13">13</a>, <a href="../../66492/66492-h/66492-h.htm#Page_15">15</a>, <a href="../../66492/66492-h/66492-h.htm#Page_16">16</a>, <a href="../../66492/66492-h/66492-h.htm#Page_71">71</a><br /> -Stecher, Dr., i. <a href="../../66492/66492-h/66492-h.htm#Page_421">421</a>, <a href="../../66492/66492-h/66492-h.htm#Page_451">451</a>; ii. <a href="#Page_165">165</a><br /> -St. Kilda, dykes of, ii. <a href="#Page_173">173</a>, <a href="#Page_416">416</a>;<br /> -<span style="margin-left: 1em;">gabbro of, <a href="#Page_358">358</a>;</span><br /> -<span style="margin-left: 1em;">general account of geology of, <a href="#Page_405">405</a>;</span><br /> -<span style="margin-left: 1em;">granophyre of, <a href="#Page_408">408</a></span><br /> -Stocks, or bosses, i. <a href="../../66492/66492-h/66492-h.htm#Page_78">78</a>, <a href="../../66492/66492-h/66492-h.htm#Page_88">88</a><br /> -Strahan, Mr. A., i. <a href="../../66492/66492-h/66492-h.htm#Page_171">171</a>; ii. <a href="#Page_10">10</a>, <a href="#Page_12">12</a>, <a href="#Page_23">23</a>, <a href="#Page_28">28</a>, <a href="#Page_32">32</a><br /> -Strathaird, ii. <a href="#Page_123">123</a>, <a href="#Page_140">140</a>, <a href="#Page_164">164</a>, <a href="#Page_269">269</a><br /> -Strathbogie, i. <a href="../../66492/66492-h/66492-h.htm#Page_344">344</a>; ii. <a href="#Page_121">121</a><br /> -Strathmore, i. <a href="../../66492/66492-h/66492-h.htm#Page_304">304</a><br /> -Stromboli, i. <a href="../../66492/66492-h/66492-h.htm#Page_4">4</a><br /> -Sublimations, traces of ancient, i. <a href="../../66492/66492-h/66492-h.htm#Page_445">445</a><br /> -Submarine eruptions, i. <a href="../../66492/66492-h/66492-h.htm#Page_48">48</a><br /> -Sub-ophitic structure, i. <a href="../../66492/66492-h/66492-h.htm#Page_417">417</a><br /> -Subsidence and volcanic action, i. <a href="../../66492/66492-h/66492-h.htm#Page_295">295</a>, <a href="../../66492/66492-h/66492-h.htm#Page_297">297</a>, <a href="../../66492/66492-h/66492-h.htm#Page_444">444</a>, <a href="../../66492/66492-h/66492-h.htm#Page_463">463</a>; ii. <a href="#Page_42">42</a>, <a href="#Page_205">205</a>, <a href="#Page_447">447</a>, <a href="#Page_470">470</a><br /> -Subterranean igneous injections, i. <a href="../../66492/66492-h/66492-h.htm#Page_77">77</a> (<i>see</i> <a href="#Bosses">Bosses</a>, <a href="#Dykes">Dykes</a>, <a href="#Sills">Sills</a>)<br /> -Suess, Prof. E., ii. <a href="#Page_474">474</a><br /> -Sun-cracks, i. <a href="../../66492/66492-h/66492-h.htm#Page_342">342</a><br /> -Sweden, Archæan volcanic rocks of, i. <a href="../../66492/66492-h/66492-h.htm#Page_120">120</a><br /> -Syenite, ii. <a href="#Page_366">366</a><br /> -Symes, Mr. R. G., i. <a href="../../66492/66492-h/66492-h.htm#Page_311">311</a>, <a href="../../66492/66492-h/66492-h.htm#Page_343">343</a>, <a href="../../66492/66492-h/66492-h.htm#Page_369">369</a>; ii. <a href="#Page_201">201</a>, <a href="#Page_428">428</a><br /> -<br /> -<a id="T"></a>Tate, G., ii. <a href="#Page_3">3</a>, <a href="#Page_113">113</a><br /> -—— R., ii. <a href="#Page_204">204</a><br /> -Tatlock, Mr. R. R., i. <a href="../../66492/66492-h/66492-h.htm#Page_273">273</a>, <a href="../../66492/66492-h/66492-h.htm#Page_278">278</a><br /> -Tawney, E. B., i. <a href="../../66492/66492-h/66492-h.htm#Page_157">157</a><br /> -Teall, Mr. J. J. H., i. <a href="../../66492/66492-h/66492-h.htm#Page_90">90</a>, <a href="../../66492/66492-h/66492-h.htm#Page_114">114</a>, <a href="../../66492/66492-h/66492-h.htm#Page_116">116</a>, <a href="../../66492/66492-h/66492-h.htm#Page_117">117</a>, <a href="../../66492/66492-h/66492-h.htm#Page_118">118</a>, <a href="../../66492/66492-h/66492-h.htm#Page_119">119</a>, <a href="../../66492/66492-h/66492-h.htm#Page_120">120</a>, <a href="../../66492/66492-h/66492-h.htm#Page_156">156</a>, <a href="../../66492/66492-h/66492-h.htm#Page_192">192</a>, <a href="../../66492/66492-h/66492-h.htm#Page_194">194</a>, <a href="../../66492/66492-h/66492-h.htm#Page_200">200</a>, <a href="../../66492/66492-h/66492-h.htm#Page_207">207</a>, <a href="../../66492/66492-h/66492-h.htm#Page_210">210</a>, <a href="../../66492/66492-h/66492-h.htm#Page_275">275</a>, <a href="../../66492/66492-h/66492-h.htm#Page_277">277</a>, <a href="../../66492/66492-h/66492-h.htm#Page_290">290</a>, <a href="../../66492/66492-h/66492-h.htm#Page_311">311</a>, <a href="../../66492/66492-h/66492-h.htm#Page_336">336</a>, <a href="../../66492/66492-h/66492-h.htm#Page_338">338</a>, <a href="../../66492/66492-h/66492-h.htm#Page_346">346</a>, <a href="../../66492/66492-h/66492-h.htm#Page_407">407</a>, <a href="../../66492/66492-h/66492-h.htm#Page_449">449</a>; ii. <a href="#Page_2">2</a>, <a href="#Page_3">3</a>, <a href="#Page_5">5</a>, <a href="#Page_7">7</a>, <a href="#Page_11">11</a>, <a href="#Page_32">32</a>, <a href="#Page_44">44</a>, <a href="#Page_113">113</a>, <a href="#Page_131">131</a>, <a href="#Page_134">134</a>, <a href="#Page_135">135</a>, <a href="#Page_137">137</a>, <a href="#Page_138">138</a>, <a href="#Page_140">140</a>, <a href="#Page_144">144</a>, <a href="#Page_149">149</a>, <a href="#Page_292">292</a>, <a href="#Page_293">293</a>, <a href="#Page_329">329</a>, <a href="#Page_367">367</a>, <a href="#Page_368">368</a>, <a href="#Page_369">369</a><br /> -Teesdale, i. <a href="../../66492/66492-h/66492-h.htm#Page_228">228</a><br /> -Termier, M. P., ii. <a href="#Page_375">375</a><br /> -Terrestrial volcanic eruptions, i. <a href="../../66492/66492-h/66492-h.htm#Page_50">50</a><br /> -Tertiary Volcanic Series, ii. <a href="#Page_181">181</a><br /> -<span style="margin-left: 1em;">Subaerial character of eruptions, ii. <a href="#Page_103">103</a>, <a href="#Page_198">198</a></span><br /> -<span style="margin-left: 1em;">Scenery of, ii. <a href="#Page_255">255</a>, <a href="#Page_256">256</a>, <a href="#Page_349">349</a>, <a href="#Page_391">391</a>, <a href="#Page_405">405</a>, <a href="#Page_408">408</a></span><br /> -<span style="margin-left: 1em;">The Plateaux, ii. <a href="#Page_183">183</a>, <a href="#Page_249">249</a>;</span><br /> -<span style="margin-left: 2em;">lavas of, <a href="#Page_183">183</a>, <a href="#Page_218">218</a>, <a href="#Page_236">236</a>, <a href="#Page_256">256</a>;</span><br /> -<span style="margin-left: 2em;">thickness of individual sheets, <a href="#Page_192">192</a>, <a href="#Page_206">206</a>, <a href="#Page_254">254</a>, <a href="#Page_257">257</a>;</span><br /> -<span style="margin-left: 2em;">lenticular character of lavas of, <a href="#Page_193">193</a>, <a href="#Page_257">257</a>;</span><br /> -<span style="margin-left: 2em;">greatest depth of, <a href="#Page_210">210</a>, <a href="#Page_211">211</a>, <a href="#Page_213">213</a>, <a href="#Page_260">260</a>;</span><br /> -<span style="margin-left: 2em;">tuffs and clays of, <a href="#Page_194">194</a>, <a href="#Page_202">202</a>, <a href="#Page_204">204</a>, <a href="#Page_211">211</a>, <a href="#Page_222">222</a>, <a href="#Page_225">225</a>, <a href="#Page_251">251</a>, <a href="#Page_258">258</a>, <a href="#Page_277">277</a>, <a href="#Page_284">284</a>, <a href="#Page_287">287</a>;</span><br /> -<span style="margin-left: 2em;">non-volcanic fragments in, <a href="#Page_196">196</a>, <a href="#Page_211">211</a>, <a href="#Page_213">213</a>, <a href="#Page_219">219</a>;</span><br /> -<span style="margin-left: 2em;">lignites of, <a href="#Page_198">198</a>, <a href="#Page_203">203</a>, <a href="#Page_208">208</a>, <a href="#Page_213">213</a>, <a href="#Page_251">251</a>;</span><br /> -<span style="margin-left: 2em;">gravels and conglomerates of, <a href="#Page_198">198</a>, <a href="#Page_212">212</a>, <a href="#Page_238">238</a>, <a href="#Page_256">256</a>;</span><br /> -<span style="margin-left: 2em;">coal of, <a href="#Page_213">213</a>, <a href="#Page_251">251</a>, <a href="#Page_256">256</a>, <a href="#Page_287">287</a>;</span><br /> -<span style="margin-left: 2em;">leaf-beds of, <a href="#Page_204">204</a>, <a href="#Page_212">212</a>, <a href="#Page_222">222</a>, <a href="#Page_225">225</a>, <a href="#Page_288">288</a>;</span><br /> -<span style="margin-left: 2em;">carbonaceous nature of the upper parts of intercalated sediments in, <a href="#Page_223">223</a>, <a href="#Page_226">226</a>, <a href="#Page_227">227</a>, <a href="#Page_229">229</a>, <a href="#Page_232">232</a>, <a href="#Page_251">251</a>, <a href="#Page_288">288</a>;</span><br /> -<span style="margin-left: 2em;">evidence for intervals between the eruptions in, <a href="#Page_203">203</a>, <a href="#Page_205">205</a>, <a href="#Page_208">208</a>, <a href="#Page_221">221</a>, <a href="#Page_228">228</a>, <a href="#Page_240">240</a>, <a href="#Page_245">245</a>, <a href="#Page_251">251</a>, <a href="#Page_254">254</a>, <a href="#Page_288">288</a>;</span><br /> -<span style="margin-left: 2em;">no evidence of great central vents in, <a href="#Page_208">208</a>, <a href="#Page_214">214</a>, <a href="#Page_255">255</a>, <a href="#Page_258">258</a>, <a href="#Page_260">260</a>, <a href="#Page_267">267</a>;</span><br /> -<span style="margin-left: 2em;">faulted condition of, <a href="#Page_200">200</a>, <a href="#Page_208">208</a>, <a href="#Page_209">209</a>, <a href="#Page_452">452</a>;</span><br /> -<span style="margin-left: 2em;">subsidences of, <a href="#Page_205">205</a>, <a href="#Page_208">208</a>, <a href="#Page_209">209</a>, <a href="#Page_214">214</a>, <a href="#Page_447">447</a>;</span><br /> -<span style="margin-left: 2em;">ancient river channels of, <a href="#Page_217">217</a>, <a href="#Page_228">228</a>, <a href="#Page_231">231</a>, <a href="#Page_234">234</a>, <a href="#Page_456">456</a>;</span><br /> -<span style="margin-left: 2em;">volcanic cones of, <a href="#Page_202">202</a>, <a href="#Page_218">218</a>, <a href="#Page_230">230</a>, <a href="#Page_277">277</a>, <a href="#Page_281">281</a>, <a href="#Page_285">285</a>;</span><br /> -<span style="margin-left: 2em;">paralleled by the modern Icelandic eruptions, <a href="#Page_260">260</a>;</span><br /> -<span style="margin-left: 2em;">vents of, <a href="#Page_202">202</a>, <a href="#Page_218">218</a>, <a href="#Page_230">230</a>, <a href="#Page_270">270</a>, <a href="#Page_276">276</a></span><br /> -<span style="margin-left: 1em;">The Basic sills, ii. <a href="#Page_298">298</a>, <a href="#Page_304">304</a></span><br /> -<span style="margin-left: 1em;">The Gabbro bosses, ii. <a href="#Page_327">327</a>, <a href="#Page_349">349</a>, <a href="#Page_355">355</a>, <a href="#Page_358">358</a>;</span><br /> -<span style="margin-left: 2em;">history of the gabbro intrusions, <a href="#Page_359">359</a></span><br /> -<span style="margin-left: 1em;">The Acid rocks, ii. <a href="#Page_364">364</a>;</span><br /> -<span style="margin-left: 2em;">petrography of, <a href="#Page_366">366</a>;</span><br /> -<span style="margin-left: 2em;">history of their investigation, <a href="#Page_371">371</a>;</span><br /> -<span style="margin-left: 2em;">analogies with trachytes of Central France, <a href="#Page_373">373</a>;</span><br /> -<span style="margin-left: 2em;">intruded at base of the gabbros or of the bedded basalts, <a href="#Page_337">337</a>, <a href="#Page_353">353</a>, <a href="#Page_357">357</a>, <a href="#Page_431">431</a>, <a href="#Page_432">432</a>, <a href="#Page_444">444</a>;</span><br /> -<span style="margin-left: 2em;">bosses of Skye, <a href="#Page_378">378</a>;</span><br /> -<span style="margin-left: 2em;">of Mull, <a href="#Page_395">395</a>;</span><br /> -<span style="margin-left: 2em;">of Small Isles, <a href="#Page_405">405</a>;</span><br /> -<span style="margin-left: 2em;">of St. Kilda, <a href="#Page_405">405</a>;</span><br /> -<span style="margin-left: 2em;">of Arran, <a href="#Page_418">418</a>;</span><br /> -<span style="margin-left: 2em;">of Carlingford, <a href="#Page_420">420</a>;</span><br /> -<span style="margin-left: 2em;">of Slieve Foye, and Barnavave, <a href="#Page_421">421</a>;</span><br /> -<span style="margin-left: 2em;">of Slieve Gullion, <a href="#Page_422">422</a>;</span><br /> -<span style="margin-left: 2em;">of Antrim, <a href="#Page_426">426</a>;</span><br /> -<span style="margin-left: 2em;">acid sills, <a href="#Page_430">430</a>;</span><br /> -<span style="margin-left: 2em;">acid dykes and veins, <a href="#Page_437">437</a></span><br /> -<span style="margin-left: 1em;">Metamorphism of the basalts, ii. <a href="#Page_272">272</a>, <a href="#Page_276">276</a>, <a href="#Page_337">337</a>, <a href="#Page_339">339</a>, <a href="#Page_340">340</a>, <a href="#Page_347">347</a>, <a href="#Page_355">355</a>, <a href="#Page_356">356</a>, <a href="#Page_357">357</a>, <a href="#Page_358">358</a>, <a href="#Page_362">362</a>, <a href="#Page_378">378</a>, <a href="#Page_383">383</a>, <a href="#Page_386">386</a>, <a href="#Page_397">397</a>, <a href="#Page_399">399</a>, <a href="#Page_400">400</a>, <a href="#Page_404">404</a>, <a href="#Page_413">413</a></span><br /> -Texture, varieties of, in igneous rocks, i. <a href="../../66492/66492-h/66492-h.htm#Page_78">78</a>, <a href="../../66492/66492-h/66492-h.htm#Page_449">449</a>; ii. <a href="#Page_5">5</a>, <a href="#Page_299">299</a>, <a href="#Page_360">360</a><br /> -Tholeiites, ii. <a href="#Page_137">137</a>, <a href="#Page_158">158</a><br /> -Tholeiite type of basalt, i. <a href="../../66492/66492-h/66492-h.htm#Page_419">419</a>, <a href="../../66492/66492-h/66492-h.htm#Page_421">421</a><br /> -Thornhill, volcanic rocks of, ii. <a href="#Page_60">60</a><br /> -Thoroddsen, Th., ii. <a href="#Page_261">261</a>, <a href="#Page_262">262</a>, <a href="#Page_263">263</a>, <a href="#Page_264">264</a>, <a href="#Page_265">265</a>, <a href="#Page_266">266</a>, <a href="#Page_278">278</a><br /> -Thrust-planes, i. <a href="../../66492/66492-h/66492-h.htm#Page_229">229</a><br /> -Time in geological history, ii. <a href="#Page_107">107</a>, <a href="#Page_461">461</a>, <a href="#Page_465">465</a><br /> -Timmins, J. H., i. <a href="../../66492/66492-h/66492-h.htm#Page_133">133</a><br /> -Tinto, i. <a href="../../66492/66492-h/66492-h.htm#Page_278">278</a>, <a href="../../66492/66492-h/66492-h.htm#Page_288">288</a>, <a href="../../66492/66492-h/66492-h.htm#Page_329">329</a><br /> -Titterstone Clee Hill, ii. <a href="#Page_101">101</a><br /> -Toadstones of Derbyshire, i. <a href="../../66492/66492-h/66492-h.htm#Page_359">359</a>; ii. <a href="#Page_8">8</a><br /> -Topley, W., i. <a href="../../66492/66492-h/66492-h.htm#Page_147">147</a>; ii. <a href="#Page_3">3</a>, <a href="#Page_5">5</a>, <a href="#Page_7">7</a><br /> -Torridonian rocks, i. <a href="../../66492/66492-h/66492-h.htm#Page_111">111</a>, <a href="../../66492/66492-h/66492-h.htm#Page_112">112</a>, <a href="../../66492/66492-h/66492-h.htm#Page_113">113</a>, <a href="../../66492/66492-h/66492-h.htm#Page_120">120</a>; ii. <a href="#Page_350">350</a><br /> -Tortworth, volcanic rocks at, i. <a href="../../66492/66492-h/66492-h.htm#Page_238">238</a><br /> -Tourmakeady, volcanic rocks of Bala age at, i. <a href="../../66492/66492-h/66492-h.htm#Page_251">251</a><br /> -Townson, R., i. <a href="../../66492/66492-h/66492-h.htm#Page_363">363</a><br /> -Trachyte, i. <a href="../../66492/66492-h/66492-h.htm#Page_183">183</a>, <a href="../../66492/66492-h/66492-h.htm#Page_230">230</a>, <a href="../../66492/66492-h/66492-h.htm#Page_246">246</a>, <a href="../../66492/66492-h/66492-h.htm#Page_273">273</a>, <a href="#Page_276">276</a> (analysis), <a href="#Page_379">379</a> (analysis), <a href="../../66492/66492-h/66492-h.htm#Page_386">386</a>, <a href="../../66492/66492-h/66492-h.htm#Page_403">403</a>, <a href="../../66492/66492-h/66492-h.htm#Page_407">407</a>, <a href="../../66492/66492-h/66492-h.htm#Page_421">421</a>; ii. <a href="#Page_36">36</a>, <a href="#Page_47">47</a>, <a href="#Page_96">96</a>, <a href="#Page_138">138</a>, <a href="#Page_152">152</a>, <a href="#Page_184">184</a>, <a href="#Page_236">236</a><br /> -Traill, Mr. W., ii. <a href="#Page_175">175</a>, <a href="#Page_421">421</a>, <a href="#Page_422">422</a><br /> -Traprain Law, i. <a href="../../66492/66492-h/66492-h.htm#Page_372">372</a>, <a href="../../66492/66492-h/66492-h.htm#Page_380">380</a>, <a href="../../66492/66492-h/66492-h.htm#Page_403">403</a>, <a href="../../66492/66492-h/66492-h.htm#Page_405">405</a><br /> -Traquair, Dr. R. H., i. <a href="../../66492/66492-h/66492-h.htm#Page_266">266</a><br /> -Tremadoc group, i. <a href="../../66492/66492-h/66492-h.htm#Page_144">144</a>, <a href="../../66492/66492-h/66492-h.htm#Page_177">177</a><br /> -Trevelyan, W. C., ii. <a href="#Page_3">3</a><br /> -Triassic eruptive rocks, i. <a href="../../66492/66492-h/66492-h.htm#Page_29">29</a>;<br /> -<span style="margin-left: 1em;">geography, ii. <a href="#Page_108">108</a></span><br /> -Trichites, i. <a href="../../66492/66492-h/66492-h.htm#Page_19">19</a>; ii. <a href="#Page_136">136</a><br /> -</p> - -<p><span class="pagenum" id="Page_491">- 491 -</span></p> - -<p> -Troctolite, ii. <a href="#Page_332">332</a><br /> -<a id="Tuffs"></a>Tuffs, i. <a href="../../66492/66492-h/66492-h.htm#Page_31">31</a>;<br /> -<span style="margin-left: 1em;">association of, <a href="../../66492/66492-h/66492-h.htm#Page_33">33</a>;</span><br /> -<span style="margin-left: 1em;">composition of, <a href="../../66492/66492-h/66492-h.htm#Page_34">34</a>;</span><br /> -<span style="margin-left: 1em;">alternations of, <a href="../../66492/66492-h/66492-h.htm#Page_34">34</a>, <a href="../../66492/66492-h/66492-h.htm#Page_61">61</a>;</span><br /> -<span style="margin-left: 1em;">blending of, with non-volcanic sediment, <a href="../../66492/66492-h/66492-h.htm#Page_35">35</a>, <a href="../../66492/66492-h/66492-h.htm#Page_437">437</a>;</span><br /> -<span style="margin-left: 1em;">fossiliferous, <a href="../../66492/66492-h/66492-h.htm#Page_36">36</a>;</span><br /> -<span style="margin-left: 1em;">without lava, <a href="../../66492/66492-h/66492-h.htm#Page_36">36</a>;</span><br /> -<span style="margin-left: 1em;">necks of, <a href="../../66492/66492-h/66492-h.htm#Page_58">58</a>;</span><br /> -<span style="margin-left: 1em;">relation of, to lavas, <a href="../../66492/66492-h/66492-h.htm#Page_61">61</a>;</span><br /> -<span style="margin-left: 1em;">metamorphism of, ii. <a href="#Page_224">224</a></span><br /> -—— pre-Cambrian, i, <a href="../../66492/66492-h/66492-h.htm#Page_125">125</a>, <a href="../../66492/66492-h/66492-h.htm#Page_135">135</a>;<br /> -<span style="margin-left: 1em;">Cambrian, <a href="../../66492/66492-h/66492-h.htm#Page_147">147</a>, <a href="../../66492/66492-h/66492-h.htm#Page_151">151</a>, <a href="../../66492/66492-h/66492-h.htm#Page_155">155</a>, <a href="../../66492/66492-h/66492-h.htm#Page_163">163</a>, <a href="../../66492/66492-h/66492-h.htm#Page_165">165</a>, <a href="../../66492/66492-h/66492-h.htm#Page_167">167</a>;</span><br /> -<span style="margin-left: 1em;">Silurian, <a href="../../66492/66492-h/66492-h.htm#Page_178">178</a>, <a href="../../66492/66492-h/66492-h.htm#Page_189">189</a>, <a href="../../66492/66492-h/66492-h.htm#Page_190">190</a>, <a href="../../66492/66492-h/66492-h.htm#Page_195">195</a>, <a href="../../66492/66492-h/66492-h.htm#Page_205">205</a>, <a href="../../66492/66492-h/66492-h.htm#Page_209">209</a>, <a href="../../66492/66492-h/66492-h.htm#Page_212">212</a>, <a href="../../66492/66492-h/66492-h.htm#Page_213">213</a>, <a href="../../66492/66492-h/66492-h.htm#Page_222">222</a>, <a href="../../66492/66492-h/66492-h.htm#Page_224">224</a>, <a href="../../66492/66492-h/66492-h.htm#Page_229">229</a>, <a href="../../66492/66492-h/66492-h.htm#Page_232">232</a>, <a href="../../66492/66492-h/66492-h.htm#Page_241">241</a>, <a href="../../66492/66492-h/66492-h.htm#Page_245">245</a>, <a href="../../66492/66492-h/66492-h.htm#Page_246">246</a>, <a href="../../66492/66492-h/66492-h.htm#Page_254">254</a>, <a href="../../66492/66492-h/66492-h.htm#Page_255">255</a>;</span><br /> -<span style="margin-left: 1em;">Devonian, <a href="../../66492/66492-h/66492-h.htm#Page_262">262</a>;</span><br /> -<span style="margin-left: 1em;">Old Red Sandstone, <a href="../../66492/66492-h/66492-h.htm#Page_279">279</a>, <a href="../../66492/66492-h/66492-h.htm#Page_281">281</a>, <a href="../../66492/66492-h/66492-h.htm#Page_289">289</a>, <a href="../../66492/66492-h/66492-h.htm#Page_337">337</a>, <a href="../../66492/66492-h/66492-h.htm#Page_339">339</a>, <a href="../../66492/66492-h/66492-h.htm#Page_351">351</a>;</span><br /> -<span style="margin-left: 1em;">Carboniferous, <a href="../../66492/66492-h/66492-h.htm#Page_381">381</a>, <a href="../../66492/66492-h/66492-h.htm#Page_384">384</a>, <a href="../../66492/66492-h/66492-h.htm#Page_387">387</a>, <a href="../../66492/66492-h/66492-h.htm#Page_399">399</a>, <a href="../../66492/66492-h/66492-h.htm#Page_422">422</a>, <a href="../../66492/66492-h/66492-h.htm#Page_427">427</a>, <a href="../../66492/66492-h/66492-h.htm#Page_429">429</a>, <a href="../../66492/66492-h/66492-h.htm#Page_432">432</a>, <a href="../../66492/66492-h/66492-h.htm#Page_436">436</a>, <a href="../../66492/66492-h/66492-h.htm#Page_466">466</a>; ii. <a href="#Page_11">11</a>, <a href="#Page_18">18</a>, <a href="#Page_24">24</a>, <a href="#Page_36">36</a>;</span><br /> -<span style="margin-left: 1em;">Permian, <a href="#Page_57">57</a>, <a href="#Page_58">58</a>;</span><br /> -<span style="margin-left: 1em;">Tertiary, <a href="#Page_194">194</a>, <a href="#Page_197">197</a>, <a href="#Page_202">202</a>, <a href="#Page_204">204</a>, <a href="#Page_211">211</a>, <a href="#Page_222">222</a></span><br /> -Tyrol, Triassic eruptive rocks of, i. <a href="../../66492/66492-h/66492-h.htm#Page_29">29</a><br /> -Tyrone, Old Red Sandstone of, i. <a href="../../66492/66492-h/66492-h.htm#Page_314">314</a><br /> -<br /> -<a id="U"></a>Ulster, Old Red Sandstone volcanic rocks of, i. <a href="../../66492/66492-h/66492-h.htm#Page_314">314</a><br /> -Ultra-basic rocks, i. <a href="../../66492/66492-h/66492-h.htm#Page_14">14</a>, <a href="../../66492/66492-h/66492-h.htm#Page_118">118</a>, <a href="../../66492/66492-h/66492-h.htm#Page_377">377</a>, <a href="../../66492/66492-h/66492-h.htm#Page_417">417</a><br /> -Unconformability, deceptive case of, i. <a href="../../66492/66492-h/66492-h.htm#Page_163">163</a><br /> -Urgneiss, i. <a href="../../66492/66492-h/66492-h.htm#Page_110">110</a><br /> -Uriconian volcanic rocks, i. <a href="../../66492/66492-h/66492-h.htm#Page_129">129</a><br /> -Ussher, Mr. W. A. E., i. <a href="../../66492/66492-h/66492-h.htm#Page_260">260</a>, <a href="../../66492/66492-h/66492-h.htm#Page_262">262</a>; ii. <a href="#Page_35">35</a>, <a href="#Page_95">95</a><br /> -Utah, laccolites of, i. <a href="../../66492/66492-h/66492-h.htm#Page_86">86</a>;<br /> -<span style="margin-left: 1em;">volcanic regions of, ii. <a href="#Page_115">115</a>, <a href="#Page_267">267</a></span><br /> -<br /> -<a id="V"></a>Valleys, tendency of vents to appear in, i. <a href="../../66492/66492-h/66492-h.htm#Page_272">272</a>, <a href="../../66492/66492-h/66492-h.htm#Page_368">368</a>, <a href="../../66492/66492-h/66492-h.htm#Page_376">376</a>; ii. <a href="#Page_61">61</a>, <a href="#Page_65">65</a>, <a href="#Page_96">96</a><br /> -Vapours, action of volcanic, i. <a href="../../66492/66492-h/66492-h.htm#Page_13">13</a>, <a href="../../66492/66492-h/66492-h.htm#Page_15">15</a>, <a href="../../66492/66492-h/66492-h.htm#Page_16">16</a>, <a href="../../66492/66492-h/66492-h.htm#Page_17">17</a>, <a href="../../66492/66492-h/66492-h.htm#Page_31">31</a>, <a href="../../66492/66492-h/66492-h.htm#Page_57">57</a>, <a href="../../66492/66492-h/66492-h.htm#Page_72">72</a>, <a href="../../66492/66492-h/66492-h.htm#Page_78">78</a>, <a href="../../66492/66492-h/66492-h.htm#Page_97">97</a>, <a href="../../66492/66492-h/66492-h.htm#Page_99">99</a>, <a href="../../66492/66492-h/66492-h.htm#Page_180">180</a>, <a href="../../66492/66492-h/66492-h.htm#Page_289">289</a>, <a href="../../66492/66492-h/66492-h.htm#Page_426">426</a><br /> -Variolitic structure, i. <a href="../../66492/66492-h/66492-h.htm#Page_21">21</a>, <a href="../../66492/66492-h/66492-h.htm#Page_206">206</a>, <a href="../../66492/66492-h/66492-h.htm#Page_235">235</a><br /> -Veins, intrusive, i. <a href="../../66492/66492-h/66492-h.htm#Page_66">66</a>, <a href="../../66492/66492-h/66492-h.htm#Page_77">77</a>, <a href="../../66492/66492-h/66492-h.htm#Page_79">79</a>, <a href="../../66492/66492-h/66492-h.htm#Page_98">98</a>, <a href="../../66492/66492-h/66492-h.htm#Page_426">426</a>, <a href="../../66492/66492-h/66492-h.htm#Page_429">429</a>; ii. <a href="#Page_311">311</a>, <a href="#Page_313">313</a>, <a href="#Page_400">400</a>, <a href="#Page_410">410</a>, <a href="#Page_432">432</a>, <a href="#Page_437">437</a><br /> -Velay, volcanic rocks of, i. <a href="../../66492/66492-h/66492-h.htm#Page_26">26</a>, <a href="../../66492/66492-h/66492-h.htm#Page_27">27</a>, <a href="../../66492/66492-h/66492-h.htm#Page_29">29</a>, <a href="../../66492/66492-h/66492-h.htm#Page_60">60</a>; ii. <a href="#Page_271">271</a>, <a href="#Page_373">373</a>, <a href="#Page_375">375</a><br /> -Vents, volcanic, i. <a href="../../66492/66492-h/66492-h.htm#Page_53">53</a>;<br /> -<span style="margin-left: 1em;">ground-plans of, <a href="../../66492/66492-h/66492-h.htm#Page_54">54</a>;</span><br /> -<span style="margin-left: 1em;">size of, <a href="../../66492/66492-h/66492-h.htm#Page_55">55</a>;</span><br /> -<span style="margin-left: 1em;">filled with non-volcanic detritus, <a href="../../66492/66492-h/66492-h.htm#Page_57">57</a>;</span><br /> -<span style="margin-left: 1em;">ejected crystals found in, <a href="../../66492/66492-h/66492-h.htm#Page_62">62</a>;</span><br /> -<span style="margin-left: 1em;">agglomerates of, <a href="../../66492/66492-h/66492-h.htm#Page_62">62</a>; ii. <a href="#Page_13">13</a>, <a href="#Page_28">28</a>, <a href="#Page_47">47</a>, <a href="#Page_61">61</a>, <a href="#Page_69">69</a>, <a href="#Page_276">276</a>, <a href="#Page_280">280</a>, <a href="#Page_284">284</a>, <a href="#Page_288">288</a>, <a href="#Page_289">289</a>;</span><br /> -<span style="margin-left: 1em;">stratification in, i. <a href="../../66492/66492-h/66492-h.htm#Page_63">63</a>; ii. <a href="#Page_80">80</a>;</span><br /> -<span style="margin-left: 1em;">metamorphism in, i. <a href="../../66492/66492-h/66492-h.htm#Page_67">67</a>, <a href="../../66492/66492-h/66492-h.htm#Page_71">71</a>;</span><br /> -<span style="margin-left: 1em;">metamorphism of rocks around, <a href="../../66492/66492-h/66492-h.htm#Page_72">72</a>, <a href="../../66492/66492-h/66492-h.htm#Page_349">349</a>, <a href="../../66492/66492-h/66492-h.htm#Page_350">350</a>, <a href="../../66492/66492-h/66492-h.htm#Page_352">352</a>, <a href="../../66492/66492-h/66492-h.htm#Page_399">399</a>, <a href="../../66492/66492-h/66492-h.htm#Page_404">404</a>, <a href="../../66492/66492-h/66492-h.htm#Page_432">432</a>; ii. <a href="#Page_76">76</a>, <a href="#Page_272">272</a>, <a href="#Page_280">280</a>;</span><br /> -<span style="margin-left: 1em;">connection of, with geological structure-lines, i. <a href="../../66492/66492-h/66492-h.htm#Page_68">68</a>;</span><br /> -<span style="margin-left: 1em;">occurrence of, in lines and in groups, <a href="../../66492/66492-h/66492-h.htm#Page_69">69</a>;</span><br /> -<span style="margin-left: 1em;">double and multiple, <a href="../../66492/66492-h/66492-h.htm#Page_69">69</a>;</span><br /> -<span style="margin-left: 1em;">possible indications of length of activity of, <a href="../../66492/66492-h/66492-h.htm#Page_72">72</a>;</span><br /> -<span style="margin-left: 1em;">inward dip of strata around, <a href="../../66492/66492-h/66492-h.htm#Page_73">73</a>, <a href="../../66492/66492-h/66492-h.htm#Page_352">352</a>; ii. <a href="#Page_76">76</a>, <a href="#Page_295">295</a>;</span><br /> -<span style="margin-left: 1em;">stages in history of, i. <a href="../../66492/66492-h/66492-h.htm#Page_74">74</a>;</span><br /> -<span style="margin-left: 1em;">tendency of, to rise in lines of valley, <a href="../../66492/66492-h/66492-h.htm#Page_272">272</a>, <a href="../../66492/66492-h/66492-h.htm#Page_368">368</a>, <a href="../../66492/66492-h/66492-h.htm#Page_376">376</a>; ii. <a href="#Page_61">61</a>, <a href="#Page_65">65</a>, <a href="#Page_96">96</a>, <a href="#Page_468">468</a>;</span><br /> -<span style="margin-left: 1em;">criteria for the relative ages of, <a href="#Page_270">270</a>;</span><br /> -<span style="margin-left: 1em;">connection of, with later eruptive bosses, <a href="#Page_280">280</a>, <a href="#Page_384">384</a>, <a href="#Page_399">399</a>, <a href="#Page_400">400</a></span><br /> -—— Silurian, i. <a href="../../66492/66492-h/66492-h.htm#Page_209">209</a>, <a href="../../66492/66492-h/66492-h.htm#Page_214">214</a>, <a href="../../66492/66492-h/66492-h.htm#Page_234">234</a>;<br /> -<span style="margin-left: 1em;">Old Red Sandstone, <a href="../../66492/66492-h/66492-h.htm#Page_272">272</a>, <a href="../../66492/66492-h/66492-h.htm#Page_287">287</a>, <a href="../../66492/66492-h/66492-h.htm#Page_298">298</a>, <a href="../../66492/66492-h/66492-h.htm#Page_305">305</a>, <a href="../../66492/66492-h/66492-h.htm#Page_323">323</a>, <a href="../../66492/66492-h/66492-h.htm#Page_328">328</a>, <a href="../../66492/66492-h/66492-h.htm#Page_337">337</a>;</span><br /> -<span style="margin-left: 1em;">Carboniferous, <a href="../../66492/66492-h/66492-h.htm#Page_394">394</a>, <a href="../../66492/66492-h/66492-h.htm#Page_399">399</a>, <a href="../../66492/66492-h/66492-h.htm#Page_400">400</a>, <a href="../../66492/66492-h/66492-h.htm#Page_404">404</a>, <a href="../../66492/66492-h/66492-h.htm#Page_406">406</a>, <a href="../../66492/66492-h/66492-h.htm#Page_424">424</a>, <a href="../../66492/66492-h/66492-h.htm#Page_465">465</a>; ii. <a href="#Page_13">13</a>, <a href="#Page_28">28</a>, <a href="#Page_47">47</a>;</span><br /> -<span style="margin-left: 1em;">Permian, <a href="#Page_61">61</a>, <a href="#Page_69">69</a>;</span><br /> -<span style="margin-left: 1em;">Tertiary, <a href="#Page_202">202</a>, <a href="#Page_270">270</a>, <a href="#Page_294">294</a>, <a href="#Page_400">400</a></span><br /> -Vesicular structure of lavas, i. <a href="../../66492/66492-h/66492-h.htm#Page_15">15</a>; ii. <a href="#Page_187">187</a><br /> -Vesuvius, denudation of, i. <a href="../../66492/66492-h/66492-h.htm#Page_3">3</a>;<br /> -<span style="margin-left: 1em;">as an active volcano, <a href="../../66492/66492-h/66492-h.htm#Page_4">4</a>;</span><br /> -<span style="margin-left: 1em;">as a type of volcano, <a href="../../66492/66492-h/66492-h.htm#Page_10">10</a>, <a href="../../66492/66492-h/66492-h.htm#Page_39">39</a>, <a href="../../66492/66492-h/66492-h.htm#Page_53">53</a>, <a href="../../66492/66492-h/66492-h.htm#Page_100">100</a>; ii. <a href="#Page_108">108</a>, <a href="#Page_115">115</a>, <a href="#Page_261">261</a>, <a href="#Page_266">266</a></span><br /> -Vicary, Mr. W., ii. <a href="#Page_95">95</a><br /> -Vogesite, i. <a href="../../66492/66492-h/66492-h.htm#Page_277">277</a>, <a href="../../66492/66492-h/66492-h.htm#Page_293">293</a><br /> -Volcanello Island, i. <a href="../../66492/66492-h/66492-h.htm#Page_70">70</a><br /> -Volcanic action, permanent traces of, i. <a href="../../66492/66492-h/66492-h.htm#Page_4">4</a>;<br /> -<span style="margin-left: 1em;">of present time elucidates that of the past, <a href="../../66492/66492-h/66492-h.htm#Page_5">5</a>;</span><br /> -<span style="margin-left: 1em;">submarine, <a href="../../66492/66492-h/66492-h.htm#Page_5">5</a>;</span><br /> -<span style="margin-left: 1em;">transient effects of, <a href="../../66492/66492-h/66492-h.htm#Page_8">8</a>;</span><br /> -<span style="margin-left: 1em;">chief factors in, <a href="../../66492/66492-h/66492-h.htm#Page_10">10</a>;</span><br /> -<span style="margin-left: 1em;">explosive energy of, <a href="../../66492/66492-h/66492-h.htm#Page_13">13</a>, <a href="../../66492/66492-h/66492-h.htm#Page_99">99</a>;</span><br /> -<span style="margin-left: 1em;">uniformity of, in geological time, <a href="../../66492/66492-h/66492-h.htm#Page_13">13</a>, ii. <a href="#Page_470">470</a>;</span><br /> -<span style="margin-left: 1em;">metamorphism by, i. <a href="../../66492/66492-h/66492-h.htm#Page_67">67</a>, <a href="../../66492/66492-h/66492-h.htm#Page_71">71</a>;</span><br /> -<span style="margin-left: 1em;">underground phases of, <a href="../../66492/66492-h/66492-h.htm#Page_77">77</a>;</span><br /> -<span style="margin-left: 1em;">proofs of gradual quiescence of, <a href="../../66492/66492-h/66492-h.htm#Page_155">155</a>, <a href="../../66492/66492-h/66492-h.htm#Page_157">157</a>, <a href="../../66492/66492-h/66492-h.htm#Page_166">166</a>;</span><br /> -<span style="margin-left: 1em;">connected with subsidence, <a href="../../66492/66492-h/66492-h.htm#Page_295">295</a>, <a href="../../66492/66492-h/66492-h.htm#Page_297">297</a>; ii. <a href="#Page_205">205</a>, <a href="#Page_444">444</a>, <a href="#Page_463">463</a>, <a href="#Page_470">470</a>;</span><br /> -<span style="margin-left: 1em;">repetition of, in the same region, i. <a href="../../66492/66492-h/66492-h.htm#Page_368">368</a>, <a href="../../66492/66492-h/66492-h.htm#Page_375">375</a>, <a href="../../66492/66492-h/66492-h.htm#Page_377">377</a>; ii. <a href="#Page_42">42</a>, <a href="#Page_69">69</a>, <a href="#Page_94">94</a>, <a href="#Page_467">467</a>;</span><br /> -<span style="margin-left: 1em;">developed along continental borders, <a href="#Page_466">466</a>;</span><br /> -<span style="margin-left: 1em;">persistence of, in Britain, i. <a href="../../66492/66492-h/66492-h.htm#Page_7">7</a>; ii. <a href="#Page_466">466</a>;</span><br /> -<span style="margin-left: 1em;">connection of, with lines of geological structure, <a href="#Page_468">468</a>;</span><br /> -<span style="margin-left: 1em;">connection of, with terrestrial disturbance, <a href="#Page_469">469</a>;</span><br /> -<span style="margin-left: 1em;">gradual decline of, during Palæozoic time, <a href="#Page_471">471</a>;</span><br /> -<span style="margin-left: 1em;">quiescence of, during Mesozoic time, <a href="#Page_472">472</a></span><br /> -Volcanic cycles, i. <a href="../../66492/66492-h/66492-h.htm#Page_27">27</a>, <a href="../../66492/66492-h/66492-h.htm#Page_92">92</a><br /> -—— products, general characters of, i. <a href="../../66492/66492-h/66492-h.htm#Page_14">14</a>;<br /> -<span style="margin-left: 1em;">persistent uniformity of, <a href="../../66492/66492-h/66492-h.htm#Page_30">30</a>, <a href="../../66492/66492-h/66492-h.htm#Page_46">46</a>;</span><br /> -<span style="margin-left: 1em;">thickest mass of, in Britain, <a href="#Page_229">229</a> (<i>see</i> <a href="#Agglomerates">Agglomerate</a>, <a href="#Lavas">Lava</a>, <a href="#Tuffs">Tuff</a>)</span><br /> -Volcano, Island of, i. <a href="../../66492/66492-h/66492-h.htm#Page_4">4</a>, <a href="../../66492/66492-h/66492-h.htm#Page_24">24</a><br /> -Volcanoes, their influence on mythology, i. <a href="../../66492/66492-h/66492-h.htm#Page_1">1</a>, <a href="../../66492/66492-h/66492-h.htm#Page_2">2</a>;<br /> -<span style="margin-left: 1em;">denudation of, <a href="../../66492/66492-h/66492-h.htm#Page_3">3</a>;</span><br /> -<span style="margin-left: 1em;">number of extinct, <a href="../../66492/66492-h/66492-h.htm#Page_4">4</a>;</span><br /> -<span style="margin-left: 1em;">ancient, of Britain, <a href="../../66492/66492-h/66492-h.htm#Page_6">6</a>;</span><br /> -<span style="margin-left: 1em;">influence on scenery, <a href="../../66492/66492-h/66492-h.htm#Page_8">8</a>, <a href="../../66492/66492-h/66492-h.htm#Page_100">100</a>, <a href="../../66492/66492-h/66492-h.htm#Page_102">102</a>;</span><br /> -<span style="margin-left: 1em;">defined, <a href="../../66492/66492-h/66492-h.htm#Page_10">10</a>;</span><br /> -<span style="margin-left: 1em;">types of, <a href="../../66492/66492-h/66492-h.htm#Page_10">10</a>, <a href="../../66492/66492-h/66492-h.htm#Page_39">39</a>; ii. <a href="#Page_471">471</a>;</span><br /> -<span style="margin-left: 1em;">determination of relative dates of, i. <a href="../../66492/66492-h/66492-h.htm#Page_46">46</a>;</span><br /> -<span style="margin-left: 1em;">their geographical condition in old times, how ascertained, <a href="../../66492/66492-h/66492-h.htm#Page_48">48</a>;</span><br /> -<span style="margin-left: 1em;">parasitic, <a href="../../66492/66492-h/66492-h.htm#Page_69">69</a>;</span><br /> -<span style="margin-left: 1em;">contemporaneous denudation of, <a href="../../66492/66492-h/66492-h.htm#Page_73">73</a>, <a href="../../66492/66492-h/66492-h.htm#Page_100">100</a>;</span><br /> -<span style="margin-left: 1em;">connected with granite, <a href="../../66492/66492-h/66492-h.htm#Page_89">89</a>;</span><br /> -<span style="margin-left: 1em;">incompleted, <a href="../../66492/66492-h/66492-h.htm#Page_86">86</a>, <a href="../../66492/66492-h/66492-h.htm#Page_93">93</a>, <a href="../../66492/66492-h/66492-h.htm#Page_99">99</a></span><br /> -Vom Rath, G., ii. <a href="#Page_474">474</a><br /> -<br /> -<a id="W"></a>Wacke, i. <a href="../../66492/66492-h/66492-h.htm#Page_157">157</a><br /> -Walcott, Mr., i. <a href="../../66492/66492-h/66492-h.htm#Page_30">30</a><br /> -Wales, pre-Cambrian rocks of, i. <a href="../../66492/66492-h/66492-h.htm#Page_126">126</a>, <a href="../../66492/66492-h/66492-h.htm#Page_142">142</a>;<br /> -<span style="margin-left: 1em;">early geological work in, <a href="../../66492/66492-h/66492-h.htm#Page_142">142</a>;</span><br /> -<span style="margin-left: 1em;">Cambrian volcanoes of, <a href="../../66492/66492-h/66492-h.htm#Page_145">145</a>, <a href="../../66492/66492-h/66492-h.htm#Page_159">159</a>;</span><br /> -<span style="margin-left: 1em;">volcanic scenery of, <a href="../../66492/66492-h/66492-h.htm#Page_176">176</a>;</span><br /> -<span style="margin-left: 1em;">Silurian volcanoes of, <a href="../../66492/66492-h/66492-h.htm#Page_176">176</a>, <a href="../../66492/66492-h/66492-h.htm#Page_202">202</a>, <a href="../../66492/66492-h/66492-h.htm#Page_205">205</a>, <a href="../../66492/66492-h/66492-h.htm#Page_207">207</a>, <a href="../../66492/66492-h/66492-h.htm#Page_218">218</a>, <a href="../../66492/66492-h/66492-h.htm#Page_219">219</a>;</span><br /> -<span style="margin-left: 1em;">Old Red Sandstone of, <a href="../../66492/66492-h/66492-h.htm#Page_257">257</a>, <a href="../../66492/66492-h/66492-h.htm#Page_259">259</a></span><br /> -Waller, Mr. T. H., i. <a href="../../66492/66492-h/66492-h.htm#Page_171">171</a>, <a href="../../66492/66492-h/66492-h.htm#Page_278">278</a><br /> -Ward, J. C., i. <a href="../../66492/66492-h/66492-h.htm#Page_227">227</a>, <a href="../../66492/66492-h/66492-h.htm#Page_228">228</a>, <a href="../../66492/66492-h/66492-h.htm#Page_229">229</a>, <a href="../../66492/66492-h/66492-h.htm#Page_230">230</a>, <a href="../../66492/66492-h/66492-h.htm#Page_231">231</a>, <a href="../../66492/66492-h/66492-h.htm#Page_233">233</a>, <a href="../../66492/66492-h/66492-h.htm#Page_234">234</a>, <a href="../../66492/66492-h/66492-h.htm#Page_235">235</a>, <a href="../../66492/66492-h/66492-h.htm#Page_236">236</a>, <a href="../../66492/66492-h/66492-h.htm#Page_237">237</a>; ii. <a href="#Page_23">23</a><br /> -Warwickshire, Cambrian rocks of, i. <a href="../../66492/66492-h/66492-h.htm#Page_137">137</a>, <a href="../../66492/66492-h/66492-h.htm#Page_171">171</a><br /> -Waterford, volcanic region of, i. <a href="../../66492/66492-h/66492-h.htm#Page_247">247</a><br /> -Watts, Mr. W. W., i. <a href="../../66492/66492-h/66492-h.htm#Page_131">131</a>, <a href="../../66492/66492-h/66492-h.htm#Page_132">132</a>, <a href="../../66492/66492-h/66492-h.htm#Page_135">135</a>, <a href="../../66492/66492-h/66492-h.htm#Page_137">137</a>, <a href="../../66492/66492-h/66492-h.htm#Page_189">189</a>, <a href="../../66492/66492-h/66492-h.htm#Page_190">190</a>, <a href="../../66492/66492-h/66492-h.htm#Page_191">191</a>, <a href="../../66492/66492-h/66492-h.htm#Page_243">243</a>, <a href="../../66492/66492-h/66492-h.htm#Page_276">276</a>, <a href="../../66492/66492-h/66492-h.htm#Page_278">278</a>, <a href="../../66492/66492-h/66492-h.htm#Page_336">336</a>, <a href="../../66492/66492-h/66492-h.htm#Page_347">347</a>, <a href="../../66492/66492-h/66492-h.htm#Page_417">417</a>, <a href="../../66492/66492-h/66492-h.htm#Page_421">421</a>, <a href="../../66492/66492-h/66492-h.htm#Page_423">423</a>; ii. <a href="#Page_40">40</a>, <a href="#Page_42">42</a>, <a href="#Page_43">43</a>, <a href="#Page_45">45</a>, <a href="#Page_57">57</a>, <a href="#Page_96">96</a>, <a href="#Page_184">184</a>, <a href="#Page_204">204</a>, <a href="#Page_224">224</a>, <a href="#Page_272">272</a>, <a href="#Page_424">424</a>, <a href="#Page_425">425</a><br /> -Weaver, T., i. <a href="../../66492/66492-h/66492-h.htm#Page_238">238</a><br /> -Wenlock group, i. <a href="../../66492/66492-h/66492-h.htm#Page_175">175</a>;<br /> -<span style="margin-left: 1em;">volcanic rocks of, <a href="../../66492/66492-h/66492-h.htm#Page_552">552</a></span><br /> -Wernerian School, ii. <a href="#Page_109">109</a><br /> -<a id="West_Lothian"></a>West Lothian, volcanic rocks of, i. <a href="../../66492/66492-h/66492-h.htm#Page_47">47</a>, <a href="../../66492/66492-h/66492-h.htm#Page_55">55</a>, <a href="../../66492/66492-h/66492-h.htm#Page_415">415</a>, <a href="../../66492/66492-h/66492-h.htm#Page_433">433</a>, <a href="../../66492/66492-h/66492-h.htm#Page_437">437</a><br /> -Whin Sill of England, i. <a href="../../66492/66492-h/66492-h.htm#Page_83">83</a>, <a href="../../66492/66492-h/66492-h.htm#Page_85">85</a>, <a href="../../66492/66492-h/66492-h.htm#Page_97">97</a>, <a href="../../66492/66492-h/66492-h.htm#Page_449">449</a>; ii. <a href="#Page_2">2</a><br /> -Whitehurst, J., ii. <a href="#Page_9">9</a>, <a href="#Page_109">109</a><br /> -White trap, i. <a href="../../66492/66492-h/66492-h.htm#Page_96">96</a>, <a href="../../66492/66492-h/66492-h.htm#Page_426">426</a>, <a href="../../66492/66492-h/66492-h.htm#Page_449">449</a>, <a href="../../66492/66492-h/66492-h.htm#Page_456">456</a>; ii. <a href="#Page_65">65</a>, <a href="#Page_87">87</a>, <a href="#Page_103">103</a>, <a href="#Page_165">165</a>, <a href="#Page_252">252</a><br /> -Williams, Mr. G. J., i. <a href="../../66492/66492-h/66492-h.htm#Page_179">179</a>, <a href="../../66492/66492-h/66492-h.htm#Page_185">185</a>, <a href="../../66492/66492-h/66492-h.htm#Page_186">186</a>, <a href="../../66492/66492-h/66492-h.htm#Page_188">188</a><br /> -Williamson, W. C., i. <a href="../../66492/66492-h/66492-h.htm#Page_392">392</a><br /> -Wilson, Mr. J. S. Grant, i. <a href="../../66492/66492-h/66492-h.htm#Page_148">148</a>, <a href="../../66492/66492-h/66492-h.htm#Page_149">149</a>, <a href="../../66492/66492-h/66492-h.htm#Page_153">153</a>, <a href="../../66492/66492-h/66492-h.htm#Page_276">276</a>, <a href="../../66492/66492-h/66492-h.htm#Page_344">344</a>, <a href="../../66492/66492-h/66492-h.htm#Page_375">375</a>, <a href="../../66492/66492-h/66492-h.htm#Page_379">379</a>, <a href="../../66492/66492-h/66492-h.htm#Page_380">380</a>; ii. <a href="#Page_137">137</a>, <a href="#Page_164">164</a><br /> -Wilson, Mr. A., ii. <a href="#Page_49">49</a><br /> -Winch, N. T., ii. <a href="#Page_113">113</a>, <a href="#Page_147">147</a><br /> -Witham, H. T. M., ii. <a href="#Page_113">113</a>, <a href="#Page_238">238</a><br /> -Wood, N., ii. <a href="#Page_113">113</a><br /> -Woods, Mr. H., i. <a href="../../66492/66492-h/66492-h.htm#Page_204">204</a><br /> -Woodward, Dr. Henry, ii. <a href="#Page_449">449</a><br /> -Woodward, Mr. H. B., ii. <a href="#Page_32">32</a>, <a href="#Page_435">435</a>, <a href="#Page_453">453</a><br /> -Worcestershire, latest eruptive rocks of, ii. <a href="#Page_101">101</a><br /> -Worth, Mr. R. N., ii. <a href="#Page_99">99</a><br /> -Wrekin, i. <a href="../../66492/66492-h/66492-h.htm#Page_130">130</a><br /> -Wright, J. R., ii. <a href="#Page_102">102</a><br /> -</p> - -<p><span class="pagenum" id="Page_492">- 492 -</span></p> - -<p> -Wunsch, E., i. <a href="../../66492/66492-h/66492-h.htm#Page_369">369</a>, <a href="../../66492/66492-h/66492-h.htm#Page_392">392</a><br /> -Würtemberg, puys of, i. <a href="../../66492/66492-h/66492-h.htm#Page_46">46</a><br /> -Wyoming, lava-fields of, ii. <a href="#Page_115">115</a><br /> -<br /> -<a id="Y"></a>Yates, J., i. <a href="../../66492/66492-h/66492-h.htm#Page_171">171</a><br /> -Yellowstone Park, volcanic phenomena of, i. <a href="../../66492/66492-h/66492-h.htm#Page_29">29</a>, <a href="../../66492/66492-h/66492-h.htm#Page_31">31</a><br /> -Y-foel-frâs, i. <a href="../../66492/66492-h/66492-h.htm#Page_209">209</a>, <a href="../../66492/66492-h/66492-h.htm#Page_214">214</a><br /> -Y Glyder-Fach, i. <a href="../../66492/66492-h/66492-h.htm#Page_209">209</a><br /> -Yoredale group, ii. <a href="../../66492/66492-h/66492-h.htm#Page_9">9</a>, <a href="../../66492/66492-h/66492-h.htm#Page_13">13</a>, <a href="../../66492/66492-h/66492-h.htm#Page_17">17</a><br /> -Young, Mr. John, i. <a href="../../66492/66492-h/66492-h.htm#Page_369">369</a>, <a href="../../66492/66492-h/66492-h.htm#Page_392">392</a><br /> -Young, Prof. John, i. <a href="../../66492/66492-h/66492-h.htm#Page_294">294</a>, <a href="../../66492/66492-h/66492-h.htm#Page_308">308</a><br /> -<br /> -<a id="Z"></a>Zircon, found in volcanic vents, i. <a href="../../66492/66492-h/66492-h.htm#Page_62">62</a><br /> -Zirkel, Prof., ii. <a href="#Page_327">327</a>, <a href="#Page_329">329</a>, <a href="#Page_334">334</a>, <a href="#Page_356">356</a>, <a href="#Page_364">364</a>, <a href="#Page_370">370</a>, <a href="#Page_372">372</a>, <a href="#Page_379">379</a>, <a href="#Page_430">430</a><br /> -</p> - -<p class="pmt2 pmb2 caption3nb">THE END</p> - -<p class="tdc"><i>Printed by</i> <span class="smcap">R. & R. Clark, Limited</span>, <i>Edinburgh</i>.</p> - - -<hr class="chap" /> - -<p class="pmt2 caption2nb">THE CAMBRIDGE NATURAL HISTORY.</p> - - -<p class="tdc">Edited by <span class="smcap">S. F. Harmer</span>, M.A, Fellow of King's College, Cambridge, Superintendent of the -University Museum of Zoology; and <span class="smcap">A. E. Shipley</span>, M.A., Fellow of Christ's College, -Cambridge, University Lecturer on the Morphology of Invertebrates.</p> - -<p class="tdc"><i>To be completed in Ten Volumes. 8vo. price 17s. net each.</i></p> - -<p>Intended in all respects to be a Standard Natural History accurate enough to be of use to the -Student, and at the same time popular enough for the general reader who desires trustworthy -information as to the structure and habits of all members of the Animal Kingdom, from the -Protozoa to the Mammals. The Volumes are fully illustrated by original figures drawn where -possible from nature. When complete, the Series is one which should be indispensable in all -libraries, whether public or private.</p> - - -<p class="tdc"><i>NOW READY.</i></p> - -<p class="caption3">WORMS, LEECHES, etc.,</p> - -<p class="tdc">VOLUME II.</p> - -<div class="blockquot"> - -<p>FLATWORMS. By <span class="smcap">F. W. Gamble</span>, M.Sc., Vict., Owens College.</p> - -<p>NEMERTINES. By Miss <span class="smcap">L. Sheldon</span>, Newnham College, Cambridge.</p> - -<p>THREAD-WORMS, <span class="smcap">Etc.</span> By <span class="smcap">A. E. Shipley</span>, M.A., Fellow of Christ's College, Cambridge.</p> - -<p>ROTIFERS. By <span class="smcap">Marcus Hartog</span>, M.A., Trinity College, Cambridge, D.Sc. Lond., Professor -of Natural History in the Queen's College, Cork.</p> - -<p>POLYCHAET WORMS. By <span class="smcap">W. Blaxland Benham</span>, D.Sc. Lond., Hon. M.A. Oxon., -Aldrichian Demonstrator of Comparative Anatomy in the University of Oxford.</p> - -<p>EARTHWORMS AND LEECHES. By <span class="smcap">F. E. Beddard</span>, M.A. Oxon., F.R.S., Prosector to the -Zoological Society, London.</p> - -<p>GEPHYREA, <span class="smcap">Etc.</span> By <span class="smcap">A. E. Shipley</span>, M.A., Fellow of Christ's College, Cambridge.</p> - -<p>POLYZOA. By <span class="smcap">S. F. Harmer</span>, M.A., Fellow of King's College, Cambridge.</p> -</div> - -<div class="blockquot"> - -<p><i>NATURAL SCIENCE.</i>—"This second volume of the Cambridge Natural History is certain to prove a most -welcome addition to English Zoological literature. It deals with a series of animal groups, all deeply interesting to -the specialist in Morphology; some important from their economic relations to other living things, others in their -life-histories rivalling the marvels of fairy-tales. And the style in which they are here treated is also interesting: -history and the early observations of the older writers lend their charm; accounts of habits, and mode of occurrence, -of life, in a word, from the cradle to the grave, are given in ample detail, accompanied by full references to modern -and current literature. The whole is admirably illustrated."</p> -</div> - - -<p class="caption3">SHELLS.</p> - -<p class="tdc">VOLUME III.</p> - -<div class="blockquot"> - -<p>MOLLUSCS AND BRACHIOPODS. By the Rev. <span class="smcap">A. H. Cooke</span>, M.A., <span class="smcap">A. E. Shipley</span>, M.A., -and <span class="smcap">F. R. C. Reed</span>, M.A.</p> -</div> - -<div class="blockquot"> - -<p><i>FIELD.</i>—"We know of no book available to the general reader which affords such a vast fund of information on -the structure and habits of molluscs."</p> - -<p><i>DAILY NEWS.</i>—"Promises to be, in its own department of science, the most important work of the day.... -This large and exhaustive work is written especially for general readers. No special training in science is needed for -the easy comprehension of this first volume at any rate. But while the writers treat their respective subjects in -what is called a popular way, they are careful to give due importance to the latest results of scientific research.... -The volume is liberally and admirably illustrated."</p> - -<p><i>KNOWLEDGE.</i>—"If succeeding volumes are like this one, the Cambridge Natural History will rank as one of the -finest works on natural history ever published."</p> - -<p><i>WESTMINSTER REVIEW.</i>—"Should find a place in the library of every naturalist."</p> -</div> - - -<p class="caption3">INSECTS and CENTIPEDES.</p> - -<p class="tdc">VOLUME V.</p> - -<div class="blockquot"> - -<p>PERIPATUS. By <span class="smcap">Adam Sedgwick</span>, M.A., F.R.S. MYRIAPODS. By <span class="smcap">F. G. Sinclair</span>, M.A. -INSECTS. Part I. By <span class="smcap">David Sharp</span>, M.A. Cantab., M.B. Edin., F.R.S.</p> -</div> - -<div class="blockquot"> - -<p><i>FIELD.</i>—"Although written for the student and the specialist, the book is not the less adapted to all intelligent -readers who wish to make themselves thoroughly acquainted with the habits, structure, and the modern classification -of the animals of which it treats. To such it cannot be recommended too strongly."</p> - -<p><i>ATHENÆUM.</i>—"This book will find a place in the library of most entomologists, prove a welcome boon to weak -brethren who are compelled to lecture—<i>docenda discimus</i>—and occupy by the side of Westwood's 'Modern -Classification' and Burmester's 'Manual' a niche that has long-been empty."</p> - -<p><i>NATURE NOTES.</i>—"When the following volume is completed, Mr. Sharp's work will, we think, last for many -years as the standard text-book on Entomology in England."</p> - -<p><i>SCIENCE PROGRESS.</i>—"The work is excellently illustrated, and besides being full of interest for the general -reader will prove extremely useful to the student."</p> -</div> - -<p>⁂ The next Volumes to appear will be Vol. IX. on BIRDS, by Mr. <span class="smcap">A. H. Evans</span>, and Vol. -VII. on FISHES and Kindred Forms, by Professor <span class="smcap">Bridge</span>, and other contributors. These will -be published in the course of 1897, and the remaining Volumes are all in active progress.</p> - -<p class="pmb4 tdc">MACMILLAN AND CO., <span class="smcap">Ltd.</span>, LONDON.</p> - - - -<p class="caption3nb">NATURE SERIES.</p> - -<p class="tdc"><i>Crown 8vo.</i></p> - -<div class="blockquot"> - -<p>POPULAR LECTURES AND ADDRESSES. By Lord <span class="smcap">Kelvin</span>, F.R.S. In -Three Vols.</p> -</div> - -<table summary="books"> -<tr> - <td>Vol.</td> - <td class="tdr">I.</td> - <td class="tdl"><span class="smcap">The Constitution of Matter.</span> Second Edition. 7s. 6d.</td> -</tr> -<tr> - <td>Vol.</td> - <td class="tdr">II.</td> - <td class="tdl"><span class="smcap">Geology and General Physics.</span> 7s. 6d.</td> -</tr> -<tr> - <td>Vol.</td> - <td class="tdr">III.</td> - <td class="tdl"><span class="smcap">Papers on Navigation.</span> 7s. 6d.</td> -</tr> -</table> - -<div class="blockquot"> - -<p>HOW TO DRAW A STRAIGHT LINE: a Lecture on Linkages. By <span class="smcap">A. B. -Kempe</span>, F.R.S. Illustrated. 1s. 6d.</p> - -<p>ON LIGHT. The Burnett Lectures. By Sir <span class="smcap">George Gabriel Stokes</span>, F.R.S. -Three Courses. I. <span class="smcap">On the Nature of Light.</span> II. <span class="smcap">On Light as a means -of Investigation.</span> III. <span class="smcap">On Beneficial Effects of Light.</span> 7s. 6d.</p> - -<p>LIGHT: a Series of Simple, Entertaining, and Useful Experiments. By <span class="smcap">A. M. -Mayer</span> and <span class="smcap">C. Barnard</span>. Illustrated. 2s. 6d.</p> - -<p>POLARISATION OF LIGHT. By <span class="smcap">W. Spottiswoode</span>, F.R.S. Illustrated. 3s. 6d.</p> - -<p>SOUND: a Series of Simple, Entertaining, and Inexpensive Experiments. By <span class="smcap">A. -M. Mayer</span>. 3s. 6d.</p> - -<p>MODERN VIEWS OF ELECTRICITY. By Prof. <span class="smcap">O. J. Lodge</span>, F.R.S. -Illustrated. 6s. 6d.</p> - -<p>THE CHEMISTRY OF THE SECONDARY BATTERIES OF PLANTE AND -FAURE. By <span class="smcap">J. H. Gladstone</span>, F.R.S., and <span class="smcap">A. Tribe</span>. 2s. 6d.</p> - -<p>THE CHEMISTRY OF PHOTOGRAPHY. By Prof. <span class="smcap">R. Meldola</span>, F.R.S. -Illustrated. 6s.</p> - -<p>CHARLES DARWIN. Memorial Notices reprinted from <i>Nature</i>. By <span class="smcap">Thomas -H. Huxley</span>, F.R.S.; <span class="smcap">G. J. Romanes</span>, F.R.S.; Sir <span class="smcap">Archibald Geikie</span>, -F.R.S.; and <span class="smcap">W. T. Dyer</span>, F.R.S. 2s. 6d.</p> - -<p>THE SCIENTIFIC EVIDENCES OF ORGANIC EVOLUTION. By <span class="smcap">George -J. Romanes</span>, F.R.S. 2s. 6d.</p> - -<p>TIMBER AND SOME OF ITS DISEASES. By Prof. <span class="smcap">H. M. Ward</span>, F.R.S. -Illustrated. 6s.</p> - -<p>ON THE COLOURS OF FLOWERS. By <span class="smcap">Grant Allen</span>. Illustrated. 3s. 6d.</p> - -<p>FLOWERS, FRUITS, AND LEAVES. By Sir <span class="smcap">John Lubbock</span>, M.P., F.R.S. -Illustrated. 4s. 6d.</p> - -<p>ON BRITISH WILD FLOWERS CONSIDERED IN RELATION TO INSECTS. -By Sir <span class="smcap">John Lubbock</span>, M.P., F.R.S. Illustrated. 4s. 6d.</p> - -<p>THE ORIGIN AND METAMORPHOSES OF INSECTS. By Sir <span class="smcap">John -Lubbock</span>, M.P., F.R.S. With Illustrations. 3s. 6d.</p> - -<p>SEEING AND THINKING. By Prof. <span class="smcap">W. K. Clifford</span>, F.R.S. Diagrams. -3s. 6d.</p> -</div> - -<p class="tdc">MACMILLAN AND CO., <span class="smcap">Ltd.</span>, LONDON.</p> - - - -<p class="caption2nb">BY THE SAME AUTHOR.</p> - -<div class="blockquot"> - -<p>TEXT-BOOK OF GEOLOGY. With Illustrations. Third Edition. -Revised and Enlarged. Medium 8vo. 28s.</p> - -<p>CLASS-BOOK OF GEOLOGY. Illustrated with woodcuts. Second -Edition. Crown 8vo. 4s. 6d.</p> - -<p>ELEMENTARY LESSONS IN PHYSICAL GEOGRAPHY. Illustrated -with woodcuts and ten plates. Fcap. 8vo. 4s. 6d. <span class="smcap">Questions for -the use of Schools.</span> Fcap. 8vo. 1s. 6d.</p> - -<p>PHYSICAL GEOGRAPHY. Illustrated. Pott 8vo. 1s.</p> - -<div class="tdr">[<i>Science Primers.</i></div> - -<p>THE SCENERY OF SCOTLAND VIEWED IN CONNEXION WITH -ITS PHYSICAL GEOLOGY. With a Geological Map and Illustrations. -Second Edition. Crown 8vo. 12s. 6d.</p> - -<p>GEOLOGY. With Illustrations. Pott 8vo. 1s.</p> - -<div class="tdr">[<i>Science Primers.</i></div> - -<p>OUTLINES OF FIELD GEOLOGY. New and revised Edition. Extra -Fcap. 8vo. 3s. 6d.</p> - -<p>GEOLOGICAL SKETCHES AT HOME AND ABROAD. With Illustrations. -8vo. 10s. 6d.</p> - -<p>THE TEACHING OF GEOGRAPHY. Suggestions regarding principles -and methods for the use of teachers. Globe 8vo. 2s.</p> - -<div class="tdr">[<i>Geographical Series.</i></div> - -<p>GEOGRAPHY OF THE BRITISH ISLES. Pott 8vo. 1s.</p> - -<div class="tdr">[<i>Geographical Series.</i></div> -</div> - -<hr class="tb" /> - -<p class="pmb2 tdc">Box of Geological Specimens to illustrate Geikie's Primer of Geology. -10s. 6d.</p> - -<p class="pmb4 tdc">MACMILLAN AND CO., <span class="smcap">Ltd.</span>, LONDON.</p> - - - -<p class="caption3nb">MACMILLAN AND CO.'S BOOKS FOR STUDENTS</p> - -<p class="tdc">OF</p> - -<p class="caption3nb"><b>PHYSICAL GEOGRAPHY, GEOLOGY, AND -MINERALOGY</b>.</p> - -<div class="blockquot"> - -<p>PHYSICS OF THE EARTH'S CRUST. By Rev. <span class="smcap">Osmond Fisher</span>, M.A., -F.G.S., Hon. Fellow of King's College, London. 2nd Ed., enlarged. -8vo. 12s.</p> - -<p>TABLES FOR THE DETERMINATION OF THE ROCK-FORMING -MINERALS. Compiled by <span class="smcap">F. L. Loewinson-Lessing</span>. Trans. by -<span class="smcap">J. W. Gregory</span>, B.Sc., F.G.S. Glossary by Prof. <span class="smcap">G. A. J. Cole</span>, F.G.S. -8vo. 4s. 6d. net.</p> - -<p>MICROSCOPICAL PHYSIOGRAPHY OF THE ROCK-MAKING -MINERALS. By <span class="smcap">H. Rosenbusch</span>. Trans. by <span class="smcap">J. P. Iddings</span>. 8vo. -24s.</p> - -<p>METEOROLOGY. By <span class="smcap">T. Russell</span>. 8vo. 16s. net.</p> - -<p>PHYSIOGRAPHY FOR BEGINNERS. By <span class="smcap">A. T. Simmons</span>, B.Sc. Globe -8vo. 2s. 6d.</p> - -<p>A POPULAR TREATISE ON THE WINDS. By <span class="smcap">W. Ferrel</span>, M.A., -Member of the American National Academy of Sciences. 8vo. 17s. -net.</p> - -<p>ECONOMIC GEOLOGY OF THE U.S. By <span class="smcap">R. S. Tarr</span>, B.S. 8vo. -16s. net.</p> - -<p>ELEMENTARY PHYSICAL GEOGRAPHY. By <span class="smcap">R. S. Tarr</span>, B.S. -Crown 8vo. 7s. 6d. net.</p> - -<p>ELEMENTS OF CRYSTALLOGRAPHY, for Students of Chemistry, -Physics, and Mineralogy. By <span class="smcap">G. H. Williams</span>, Ph.D. Crown -8vo. 6s.</p> - -<p>POPULAR LECTURES AND ADDRESSES. By Lord <span class="smcap">Kelvin</span>, P.R.S. -Vol. II. <span class="smcap">Geology and General Physics.</span> Crown 8vo. 7s. 6d.</p> - -<p>THE PLANET EARTH. By <span class="smcap">R. A. Gregory</span>, F.R.A.S. Crown 8vo. 2s.</p> - -<p>PHYSIOGRAPHY. An Introduction to the Study of Nature. By the -Eight Hon. <span class="smcap">T. H. Huxley</span>, F.R.S. Illustrated. Crown 8vo. 6s.</p> - -<p>SHORT STUDIES IN EARTH KNOWLEDGE. Introduction to Physiography. -By <span class="smcap">William Gee</span>. Illustrated. Globe 8vo. 3s. 6d.</p> - -<p>OUTLINES OF PHYSIOGRAPHY—THE MOVEMENTS OF THE -EARTH. By <span class="smcap">J. Norman Lockyer</span>, F.R.S. Illustrated. Crown 8vo. -Sewed, 1s. 6d.</p> -</div> - - -<p class="tdc">MACMILLAN AND CO., <span class="smcap">Ltd.</span>, LONDON.</p> - -<hr class="chap" /> - -<hr class="tb" /> - -<div class="transnote"> - -<p class="caption3">Transcriber Note</p> - -<p>Minor typos were corrected. A paragraph break was -inserted in the last paragraph on pages <a href="#Page_271">271</a> and <a href="#Page_334">334</a> to accomodate placement of -illustrations. To prevent images from splitting paragraphs, some text was moved.</p> - -</div> - - - - - -<div style='display:block; margin-top:4em'>*** END OF THE PROJECT GUTENBERG EBOOK THE ANCIENT VOLCANOES OF GREAT BRITAIN, VOLUME II (OF 2) ***</div> -<div style='text-align:left'> - -<div style='display:block; margin:1em 0'> -Updated editions will replace the previous one—the old editions will -be renamed. -</div> - -<div style='display:block; margin:1em 0'> -Creating the works from print editions not protected by U.S. copyright -law means that no one owns a United States copyright in these works, -so the Foundation (and you!) can copy and distribute it in the United -States without permission and without paying copyright -royalties. Special rules, set forth in the General Terms of Use part -of this license, apply to copying and distributing Project -Gutenberg™ electronic works to protect the PROJECT GUTENBERG™ -concept and trademark. Project Gutenberg is a registered trademark, -and may not be used if you charge for an eBook, except by following -the terms of the trademark license, including paying royalties for use -of the Project Gutenberg trademark. If you do not charge anything for -copies of this eBook, complying with the trademark license is very -easy. You may use this eBook for nearly any purpose such as creation -of derivative works, reports, performances and research. Project -Gutenberg eBooks may be modified and printed and given away--you may -do practically ANYTHING in the United States with eBooks not protected -by U.S. copyright law. Redistribution is subject to the trademark -license, especially commercial redistribution. -</div> - -<div style='margin:0.83em 0; font-size:1.1em; text-align:center'>START: FULL LICENSE<br /> -<span style='font-size:smaller'>THE FULL PROJECT GUTENBERG LICENSE<br /> -PLEASE READ THIS BEFORE YOU DISTRIBUTE OR USE THIS WORK</span> -</div> - -<div style='display:block; margin:1em 0'> -To protect the Project Gutenberg™ mission of promoting the free -distribution of electronic works, by using or distributing this work -(or any other work associated in any way with the phrase “Project -Gutenberg”), you agree to comply with all the terms of the Full -Project Gutenberg™ License available with this file or online at -www.gutenberg.org/license. -</div> - -<div style='display:block; font-size:1.1em; margin:1em 0; font-weight:bold'> -Section 1. General Terms of Use and Redistributing Project Gutenberg™ electronic works -</div> - -<div style='display:block; margin:1em 0'> -1.A. By reading or using any part of this Project Gutenberg™ -electronic work, you indicate that you have read, understand, agree to -and accept all the terms of this license and intellectual property -(trademark/copyright) agreement. If you do not agree to abide by all -the terms of this agreement, you must cease using and return or -destroy all copies of Project Gutenberg™ electronic works in your -possession. If you paid a fee for obtaining a copy of or access to a -Project Gutenberg™ electronic work and you do not agree to be bound -by the terms of this agreement, you may obtain a refund from the person -or entity to whom you paid the fee as set forth in paragraph 1.E.8. -</div> - -<div style='display:block; margin:1em 0'> -1.B. “Project Gutenberg” is a registered trademark. It may only be -used on or associated in any way with an electronic work by people who -agree to be bound by the terms of this agreement. There are a few -things that you can do with most Project Gutenberg™ electronic works -even without complying with the full terms of this agreement. See -paragraph 1.C below. There are a lot of things you can do with Project -Gutenberg™ electronic works if you follow the terms of this -agreement and help preserve free future access to Project Gutenberg™ -electronic works. See paragraph 1.E below. -</div> - -<div style='display:block; margin:1em 0'> -1.C. The Project Gutenberg Literary Archive Foundation (“the -Foundation” or PGLAF), owns a compilation copyright in the collection -of Project Gutenberg™ electronic works. Nearly all the individual -works in the collection are in the public domain in the United -States. If an individual work is unprotected by copyright law in the -United States and you are located in the United States, we do not -claim a right to prevent you from copying, distributing, performing, -displaying or creating derivative works based on the work as long as -all references to Project Gutenberg are removed. Of course, we hope -that you will support the Project Gutenberg™ mission of promoting -free access to electronic works by freely sharing Project Gutenberg™ -works in compliance with the terms of this agreement for keeping the -Project Gutenberg™ name associated with the work. You can easily -comply with the terms of this agreement by keeping this work in the -same format with its attached full Project Gutenberg™ License when -you share it without charge with others. -</div> - -<div style='display:block; margin:1em 0'> -1.D. The copyright laws of the place where you are located also govern -what you can do with this work. Copyright laws in most countries are -in a constant state of change. If you are outside the United States, -check the laws of your country in addition to the terms of this -agreement before downloading, copying, displaying, performing, -distributing or creating derivative works based on this work or any -other Project Gutenberg™ work. The Foundation makes no -representations concerning the copyright status of any work in any -country other than the United States. -</div> - -<div style='display:block; margin:1em 0'> -1.E. Unless you have removed all references to Project Gutenberg: -</div> - -<div style='display:block; margin:1em 0'> -1.E.1. The following sentence, with active links to, or other -immediate access to, the full Project Gutenberg™ License must appear -prominently whenever any copy of a Project Gutenberg™ work (any work -on which the phrase “Project Gutenberg” appears, or with which the -phrase “Project Gutenberg” is associated) is accessed, displayed, -performed, viewed, copied or distributed: -</div> - -<blockquote> - <div style='display:block; margin:1em 0'> - This eBook is for the use of anyone anywhere in the United States and most - other parts of the world at no cost and with almost no restrictions - whatsoever. You may copy it, give it away or re-use it under the terms - of the Project Gutenberg License included with this eBook or online - at <a href="https://www.gutenberg.org">www.gutenberg.org</a>. If you - are not located in the United States, you will have to check the laws - of the country where you are located before using this eBook. - </div> -</blockquote> - -<div style='display:block; margin:1em 0'> -1.E.2. If an individual Project Gutenberg™ electronic work is -derived from texts not protected by U.S. copyright law (does not -contain a notice indicating that it is posted with permission of the -copyright holder), the work can be copied and distributed to anyone in -the United States without paying any fees or charges. If you are -redistributing or providing access to a work with the phrase “Project -Gutenberg” associated with or appearing on the work, you must comply -either with the requirements of paragraphs 1.E.1 through 1.E.7 or -obtain permission for the use of the work and the Project Gutenberg™ -trademark as set forth in paragraphs 1.E.8 or 1.E.9. -</div> - -<div style='display:block; margin:1em 0'> -1.E.3. If an individual Project Gutenberg™ electronic work is posted -with the permission of the copyright holder, your use and distribution -must comply with both paragraphs 1.E.1 through 1.E.7 and any -additional terms imposed by the copyright holder. Additional terms -will be linked to the Project Gutenberg™ License for all works -posted with the permission of the copyright holder found at the -beginning of this work. -</div> - -<div style='display:block; margin:1em 0'> -1.E.4. Do not unlink or detach or remove the full Project Gutenberg™ -License terms from this work, or any files containing a part of this -work or any other work associated with Project Gutenberg™. -</div> - -<div style='display:block; margin:1em 0'> -1.E.5. Do not copy, display, perform, distribute or redistribute this -electronic work, or any part of this electronic work, without -prominently displaying the sentence set forth in paragraph 1.E.1 with -active links or immediate access to the full terms of the Project -Gutenberg™ License. -</div> - -<div style='display:block; margin:1em 0'> -1.E.6. You may convert to and distribute this work in any binary, -compressed, marked up, nonproprietary or proprietary form, including -any word processing or hypertext form. However, if you provide access -to or distribute copies of a Project Gutenberg™ work in a format -other than “Plain Vanilla ASCII” or other format used in the official -version posted on the official Project Gutenberg™ website -(www.gutenberg.org), you must, at no additional cost, fee or expense -to the user, provide a copy, a means of exporting a copy, or a means -of obtaining a copy upon request, of the work in its original “Plain -Vanilla ASCII” or other form. Any alternate format must include the -full Project Gutenberg™ License as specified in paragraph 1.E.1. -</div> - -<div style='display:block; margin:1em 0'> -1.E.7. Do not charge a fee for access to, viewing, displaying, -performing, copying or distributing any Project Gutenberg™ works -unless you comply with paragraph 1.E.8 or 1.E.9. -</div> - -<div style='display:block; margin:1em 0'> -1.E.8. You may charge a reasonable fee for copies of or providing -access to or distributing Project Gutenberg™ electronic works -provided that: -</div> - -<div style='margin-left:0.7em;'> - <div style='text-indent:-0.7em'> - • You pay a royalty fee of 20% of the gross profits you derive from - the use of Project Gutenberg™ works calculated using the method - you already use to calculate your applicable taxes. The fee is owed - to the owner of the Project Gutenberg™ trademark, but he has - agreed to donate royalties under this paragraph to the Project - Gutenberg Literary Archive Foundation. Royalty payments must be paid - within 60 days following each date on which you prepare (or are - legally required to prepare) your periodic tax returns. Royalty - payments should be clearly marked as such and sent to the Project - Gutenberg Literary Archive Foundation at the address specified in - Section 4, “Information about donations to the Project Gutenberg - Literary Archive Foundation.” - </div> - - <div style='text-indent:-0.7em'> - • You provide a full refund of any money paid by a user who notifies - you in writing (or by e-mail) within 30 days of receipt that s/he - does not agree to the terms of the full Project Gutenberg™ - License. You must require such a user to return or destroy all - copies of the works possessed in a physical medium and discontinue - all use of and all access to other copies of Project Gutenberg™ - works. - </div> - - <div style='text-indent:-0.7em'> - • You provide, in accordance with paragraph 1.F.3, a full refund of - any money paid for a work or a replacement copy, if a defect in the - electronic work is discovered and reported to you within 90 days of - receipt of the work. - </div> - - <div style='text-indent:-0.7em'> - • You comply with all other terms of this agreement for free - distribution of Project Gutenberg™ works. - </div> -</div> - -<div style='display:block; margin:1em 0'> -1.E.9. If you wish to charge a fee or distribute a Project -Gutenberg™ electronic work or group of works on different terms than -are set forth in this agreement, you must obtain permission in writing -from the Project Gutenberg Literary Archive Foundation, the manager of -the Project Gutenberg™ trademark. Contact the Foundation as set -forth in Section 3 below. -</div> - -<div style='display:block; margin:1em 0'> -1.F. -</div> - -<div style='display:block; margin:1em 0'> -1.F.1. Project Gutenberg volunteers and employees expend considerable -effort to identify, do copyright research on, transcribe and proofread -works not protected by U.S. copyright law in creating the Project -Gutenberg™ collection. Despite these efforts, Project Gutenberg™ -electronic works, and the medium on which they may be stored, may -contain “Defects,” such as, but not limited to, incomplete, inaccurate -or corrupt data, transcription errors, a copyright or other -intellectual property infringement, a defective or damaged disk or -other medium, a computer virus, or computer codes that damage or -cannot be read by your equipment. -</div> - -<div style='display:block; margin:1em 0'> -1.F.2. LIMITED WARRANTY, DISCLAIMER OF DAMAGES - Except for the “Right -of Replacement or Refund” described in paragraph 1.F.3, the Project -Gutenberg Literary Archive Foundation, the owner of the Project -Gutenberg™ trademark, and any other party distributing a Project -Gutenberg™ electronic work under this agreement, disclaim all -liability to you for damages, costs and expenses, including legal -fees. YOU AGREE THAT YOU HAVE NO REMEDIES FOR NEGLIGENCE, STRICT -LIABILITY, BREACH OF WARRANTY OR BREACH OF CONTRACT EXCEPT THOSE -PROVIDED IN PARAGRAPH 1.F.3. YOU AGREE THAT THE FOUNDATION, THE -TRADEMARK OWNER, AND ANY DISTRIBUTOR UNDER THIS AGREEMENT WILL NOT BE -LIABLE TO YOU FOR ACTUAL, DIRECT, INDIRECT, CONSEQUENTIAL, PUNITIVE OR -INCIDENTAL DAMAGES EVEN IF YOU GIVE NOTICE OF THE POSSIBILITY OF SUCH -DAMAGE. -</div> - -<div style='display:block; margin:1em 0'> -1.F.3. LIMITED RIGHT OF REPLACEMENT OR REFUND - If you discover a -defect in this electronic work within 90 days of receiving it, you can -receive a refund of the money (if any) you paid for it by sending a -written explanation to the person you received the work from. If you -received the work on a physical medium, you must return the medium -with your written explanation. The person or entity that provided you -with the defective work may elect to provide a replacement copy in -lieu of a refund. If you received the work electronically, the person -or entity providing it to you may choose to give you a second -opportunity to receive the work electronically in lieu of a refund. If -the second copy is also defective, you may demand a refund in writing -without further opportunities to fix the problem. -</div> - -<div style='display:block; margin:1em 0'> -1.F.4. Except for the limited right of replacement or refund set forth -in paragraph 1.F.3, this work is provided to you ‘AS-IS’, WITH NO -OTHER WARRANTIES OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT -LIMITED TO WARRANTIES OF MERCHANTABILITY OR FITNESS FOR ANY PURPOSE. -</div> - -<div style='display:block; margin:1em 0'> -1.F.5. Some states do not allow disclaimers of certain implied -warranties or the exclusion or limitation of certain types of -damages. If any disclaimer or limitation set forth in this agreement -violates the law of the state applicable to this agreement, the -agreement shall be interpreted to make the maximum disclaimer or -limitation permitted by the applicable state law. The invalidity or -unenforceability of any provision of this agreement shall not void the -remaining provisions. -</div> - -<div style='display:block; margin:1em 0'> -1.F.6. INDEMNITY - You agree to indemnify and hold the Foundation, the -trademark owner, any agent or employee of the Foundation, anyone -providing copies of Project Gutenberg™ electronic works in -accordance with this agreement, and any volunteers associated with the -production, promotion and distribution of Project Gutenberg™ -electronic works, harmless from all liability, costs and expenses, -including legal fees, that arise directly or indirectly from any of -the following which you do or cause to occur: (a) distribution of this -or any Project Gutenberg™ work, (b) alteration, modification, or -additions or deletions to any Project Gutenberg™ work, and (c) any -Defect you cause. -</div> - -<div style='display:block; font-size:1.1em; margin:1em 0; font-weight:bold'> -Section 2. Information about the Mission of Project Gutenberg™ -</div> - -<div style='display:block; margin:1em 0'> -Project Gutenberg™ is synonymous with the free distribution of -electronic works in formats readable by the widest variety of -computers including obsolete, old, middle-aged and new computers. It -exists because of the efforts of hundreds of volunteers and donations -from people in all walks of life. -</div> - -<div style='display:block; margin:1em 0'> -Volunteers and financial support to provide volunteers with the -assistance they need are critical to reaching Project Gutenberg™’s -goals and ensuring that the Project Gutenberg™ collection will -remain freely available for generations to come. In 2001, the Project -Gutenberg Literary Archive Foundation was created to provide a secure -and permanent future for Project Gutenberg™ and future -generations. To learn more about the Project Gutenberg Literary -Archive Foundation and how your efforts and donations can help, see -Sections 3 and 4 and the Foundation information page at www.gutenberg.org. -</div> - -<div style='display:block; font-size:1.1em; margin:1em 0; font-weight:bold'> -Section 3. Information about the Project Gutenberg Literary Archive Foundation -</div> - -<div style='display:block; margin:1em 0'> -The Project Gutenberg Literary Archive Foundation is a non-profit -501(c)(3) educational corporation organized under the laws of the -state of Mississippi and granted tax exempt status by the Internal -Revenue Service. The Foundation’s EIN or federal tax identification -number is 64-6221541. Contributions to the Project Gutenberg Literary -Archive Foundation are tax deductible to the full extent permitted by -U.S. federal laws and your state’s laws. -</div> - -<div style='display:block; margin:1em 0'> -The Foundation’s business office is located at 809 North 1500 West, -Salt Lake City, UT 84116, (801) 596-1887. Email contact links and up -to date contact information can be found at the Foundation’s website -and official page at www.gutenberg.org/contact -</div> - -<div style='display:block; font-size:1.1em; margin:1em 0; font-weight:bold'> -Section 4. Information about Donations to the Project Gutenberg Literary Archive Foundation -</div> - -<div style='display:block; margin:1em 0'> -Project Gutenberg™ depends upon and cannot survive without widespread -public support and donations to carry out its mission of -increasing the number of public domain and licensed works that can be -freely distributed in machine-readable form accessible by the widest -array of equipment including outdated equipment. Many small donations -($1 to $5,000) are particularly important to maintaining tax exempt -status with the IRS. -</div> - -<div style='display:block; margin:1em 0'> -The Foundation is committed to complying with the laws regulating -charities and charitable donations in all 50 states of the United -States. Compliance requirements are not uniform and it takes a -considerable effort, much paperwork and many fees to meet and keep up -with these requirements. We do not solicit donations in locations -where we have not received written confirmation of compliance. To SEND -DONATIONS or determine the status of compliance for any particular state -visit <a href="https://www.gutenberg.org/donate/">www.gutenberg.org/donate</a>. -</div> - -<div style='display:block; margin:1em 0'> -While we cannot and do not solicit contributions from states where we -have not met the solicitation requirements, we know of no prohibition -against accepting unsolicited donations from donors in such states who -approach us with offers to donate. -</div> - -<div style='display:block; margin:1em 0'> -International donations are gratefully accepted, but we cannot make -any statements concerning tax treatment of donations received from -outside the United States. U.S. laws alone swamp our small staff. -</div> - -<div style='display:block; margin:1em 0'> -Please check the Project Gutenberg web pages for current donation -methods and addresses. Donations are accepted in a number of other -ways including checks, online payments and credit card donations. To -donate, please visit: www.gutenberg.org/donate -</div> - -<div style='display:block; font-size:1.1em; margin:1em 0; font-weight:bold'> -Section 5. General Information About Project Gutenberg™ electronic works -</div> - -<div style='display:block; margin:1em 0'> -Professor Michael S. Hart was the originator of the Project -Gutenberg™ concept of a library of electronic works that could be -freely shared with anyone. For forty years, he produced and -distributed Project Gutenberg™ eBooks with only a loose network of -volunteer support. -</div> - -<div style='display:block; margin:1em 0'> -Project Gutenberg™ eBooks are often created from several printed -editions, all of which are confirmed as not protected by copyright in -the U.S. unless a copyright notice is included. Thus, we do not -necessarily keep eBooks in compliance with any particular paper -edition. -</div> - -<div style='display:block; margin:1em 0'> -Most people start at our website which has the main PG search -facility: <a href="https://www.gutenberg.org">www.gutenberg.org</a>. -</div> - -<div style='display:block; margin:1em 0'> -This website includes information about Project Gutenberg™, -including how to make donations to the Project Gutenberg Literary -Archive Foundation, how to help produce our new eBooks, and how to -subscribe to our email newsletter to hear about new eBooks. -</div> - -</div> -</body> -</html> diff --git a/old/66493-h/images/cover.png b/old/66493-h/images/cover.png Binary files differdeleted file mode 100644 index 212bbaa..0000000 --- a/old/66493-h/images/cover.png +++ /dev/null diff --git a/old/66493-h/images/cover_epub.jpg b/old/66493-h/images/cover_epub.jpg Binary files differdeleted file mode 100644 index 28c7c35..0000000 --- a/old/66493-h/images/cover_epub.jpg +++ /dev/null diff --git a/old/66493-h/images/logo.png b/old/66493-h/images/logo.png Binary files differdeleted file mode 100644 index dda8e0a..0000000 --- a/old/66493-h/images/logo.png +++ /dev/null diff --git a/old/66493-h/images/v2fig176.png b/old/66493-h/images/v2fig176.png Binary files differdeleted file mode 100644 index 3e4af12..0000000 --- a/old/66493-h/images/v2fig176.png +++ /dev/null diff --git a/old/66493-h/images/v2fig177.png b/old/66493-h/images/v2fig177.png Binary files differdeleted file mode 100644 index 1b91f04..0000000 --- a/old/66493-h/images/v2fig177.png +++ /dev/null diff --git a/old/66493-h/images/v2fig178.png b/old/66493-h/images/v2fig178.png Binary files differdeleted file mode 100644 index bff71b7..0000000 --- a/old/66493-h/images/v2fig178.png +++ /dev/null diff --git a/old/66493-h/images/v2fig179.png b/old/66493-h/images/v2fig179.png Binary files differdeleted file mode 100644 index 79ef8be..0000000 --- a/old/66493-h/images/v2fig179.png +++ /dev/null diff --git a/old/66493-h/images/v2fig180.png b/old/66493-h/images/v2fig180.png Binary files differdeleted file mode 100644 index c32e5a4..0000000 --- a/old/66493-h/images/v2fig180.png +++ /dev/null diff --git a/old/66493-h/images/v2fig181.png b/old/66493-h/images/v2fig181.png Binary files differdeleted file mode 100644 index f06ab0f..0000000 --- a/old/66493-h/images/v2fig181.png +++ /dev/null diff --git a/old/66493-h/images/v2fig182.png b/old/66493-h/images/v2fig182.png Binary files differdeleted file mode 100644 index 22f1d4a..0000000 --- a/old/66493-h/images/v2fig182.png +++ /dev/null diff --git a/old/66493-h/images/v2fig183.png b/old/66493-h/images/v2fig183.png Binary files differdeleted file mode 100644 index 958471c..0000000 --- a/old/66493-h/images/v2fig183.png +++ /dev/null diff --git a/old/66493-h/images/v2fig184.png b/old/66493-h/images/v2fig184.png Binary files differdeleted file mode 100644 index c40b543..0000000 --- a/old/66493-h/images/v2fig184.png +++ /dev/null diff --git a/old/66493-h/images/v2fig185.png b/old/66493-h/images/v2fig185.png Binary files differdeleted file mode 100644 index 70b41ca..0000000 --- a/old/66493-h/images/v2fig185.png +++ /dev/null diff --git a/old/66493-h/images/v2fig186.png b/old/66493-h/images/v2fig186.png Binary files differdeleted file mode 100644 index b7f38bb..0000000 --- a/old/66493-h/images/v2fig186.png +++ /dev/null diff --git a/old/66493-h/images/v2fig187.png b/old/66493-h/images/v2fig187.png Binary files differdeleted file mode 100644 index f6f3227..0000000 --- a/old/66493-h/images/v2fig187.png +++ /dev/null diff --git a/old/66493-h/images/v2fig188.png b/old/66493-h/images/v2fig188.png Binary files differdeleted file mode 100644 index f848df5..0000000 --- a/old/66493-h/images/v2fig188.png +++ /dev/null diff --git a/old/66493-h/images/v2fig189.png b/old/66493-h/images/v2fig189.png Binary files differdeleted file mode 100644 index c2008e2..0000000 --- a/old/66493-h/images/v2fig189.png +++ /dev/null diff --git a/old/66493-h/images/v2fig190.png b/old/66493-h/images/v2fig190.png Binary files differdeleted file mode 100644 index ff2c977..0000000 --- a/old/66493-h/images/v2fig190.png +++ /dev/null diff --git a/old/66493-h/images/v2fig191.png b/old/66493-h/images/v2fig191.png Binary files differdeleted file mode 100644 index a6cd7ed..0000000 --- a/old/66493-h/images/v2fig191.png +++ /dev/null diff --git a/old/66493-h/images/v2fig192.png b/old/66493-h/images/v2fig192.png Binary files differdeleted file mode 100644 index fac551c..0000000 --- a/old/66493-h/images/v2fig192.png +++ /dev/null diff --git a/old/66493-h/images/v2fig193.png b/old/66493-h/images/v2fig193.png Binary files differdeleted file mode 100644 index 7ad13e4..0000000 --- a/old/66493-h/images/v2fig193.png +++ /dev/null diff --git a/old/66493-h/images/v2fig194.png b/old/66493-h/images/v2fig194.png Binary files differdeleted file mode 100644 index ef68261..0000000 --- a/old/66493-h/images/v2fig194.png +++ /dev/null diff --git a/old/66493-h/images/v2fig195.png b/old/66493-h/images/v2fig195.png Binary files differdeleted file mode 100644 index 8b4c2e9..0000000 --- a/old/66493-h/images/v2fig195.png +++ /dev/null diff --git a/old/66493-h/images/v2fig196.png b/old/66493-h/images/v2fig196.png Binary files differdeleted file mode 100644 index 89522f8..0000000 --- a/old/66493-h/images/v2fig196.png +++ /dev/null diff --git a/old/66493-h/images/v2fig197.png b/old/66493-h/images/v2fig197.png Binary files differdeleted file mode 100644 index 44552c9..0000000 --- a/old/66493-h/images/v2fig197.png +++ /dev/null diff --git a/old/66493-h/images/v2fig198.png b/old/66493-h/images/v2fig198.png Binary files differdeleted file mode 100644 index 745ec7c..0000000 --- a/old/66493-h/images/v2fig198.png +++ /dev/null diff --git a/old/66493-h/images/v2fig199.png b/old/66493-h/images/v2fig199.png Binary files differdeleted file mode 100644 index 4df1162..0000000 --- a/old/66493-h/images/v2fig199.png +++ /dev/null diff --git a/old/66493-h/images/v2fig200.png b/old/66493-h/images/v2fig200.png Binary files differdeleted file mode 100644 index 11750f5..0000000 --- a/old/66493-h/images/v2fig200.png +++ /dev/null diff --git a/old/66493-h/images/v2fig201.png b/old/66493-h/images/v2fig201.png Binary files differdeleted file mode 100644 index f3dbb66..0000000 --- a/old/66493-h/images/v2fig201.png +++ /dev/null diff --git a/old/66493-h/images/v2fig202.png b/old/66493-h/images/v2fig202.png Binary files differdeleted file mode 100644 index b7e0824..0000000 --- a/old/66493-h/images/v2fig202.png +++ /dev/null diff --git a/old/66493-h/images/v2fig203.png b/old/66493-h/images/v2fig203.png Binary files differdeleted file mode 100644 index dc9dd5c..0000000 --- a/old/66493-h/images/v2fig203.png +++ /dev/null diff --git a/old/66493-h/images/v2fig204.png b/old/66493-h/images/v2fig204.png Binary files differdeleted file mode 100644 index 1921e1d..0000000 --- a/old/66493-h/images/v2fig204.png +++ /dev/null diff --git a/old/66493-h/images/v2fig205.png b/old/66493-h/images/v2fig205.png Binary files differdeleted file mode 100644 index 22dfbca..0000000 --- a/old/66493-h/images/v2fig205.png +++ /dev/null diff --git a/old/66493-h/images/v2fig206.png b/old/66493-h/images/v2fig206.png Binary files differdeleted file mode 100644 index 6c24308..0000000 --- a/old/66493-h/images/v2fig206.png +++ /dev/null diff --git a/old/66493-h/images/v2fig207.png b/old/66493-h/images/v2fig207.png Binary files differdeleted file mode 100644 index e1168ca..0000000 --- a/old/66493-h/images/v2fig207.png +++ /dev/null diff --git a/old/66493-h/images/v2fig208.png b/old/66493-h/images/v2fig208.png Binary files differdeleted file mode 100644 index 315254f..0000000 --- a/old/66493-h/images/v2fig208.png +++ /dev/null diff --git a/old/66493-h/images/v2fig209.png b/old/66493-h/images/v2fig209.png Binary files differdeleted file mode 100644 index 9edd68f..0000000 --- a/old/66493-h/images/v2fig209.png +++ /dev/null diff --git a/old/66493-h/images/v2fig210.png b/old/66493-h/images/v2fig210.png Binary files differdeleted file mode 100644 index 080438f..0000000 --- a/old/66493-h/images/v2fig210.png +++ /dev/null diff --git a/old/66493-h/images/v2fig211.png b/old/66493-h/images/v2fig211.png Binary files differdeleted file mode 100644 index c08d97c..0000000 --- a/old/66493-h/images/v2fig211.png +++ /dev/null diff --git a/old/66493-h/images/v2fig212.png b/old/66493-h/images/v2fig212.png Binary files differdeleted file mode 100644 index ee9516e..0000000 --- a/old/66493-h/images/v2fig212.png +++ /dev/null diff --git a/old/66493-h/images/v2fig213.png b/old/66493-h/images/v2fig213.png Binary files differdeleted file mode 100644 index d660bb2..0000000 --- a/old/66493-h/images/v2fig213.png +++ /dev/null diff --git a/old/66493-h/images/v2fig214.png b/old/66493-h/images/v2fig214.png Binary files differdeleted file mode 100644 index f370c64..0000000 --- a/old/66493-h/images/v2fig214.png +++ /dev/null diff --git a/old/66493-h/images/v2fig215.png b/old/66493-h/images/v2fig215.png Binary files differdeleted file mode 100644 index 6c94393..0000000 --- a/old/66493-h/images/v2fig215.png +++ /dev/null diff --git a/old/66493-h/images/v2fig216.png b/old/66493-h/images/v2fig216.png Binary files differdeleted file mode 100644 index df660e4..0000000 --- a/old/66493-h/images/v2fig216.png +++ /dev/null diff --git a/old/66493-h/images/v2fig217.png b/old/66493-h/images/v2fig217.png Binary files differdeleted file mode 100644 index 4f6520c..0000000 --- a/old/66493-h/images/v2fig217.png +++ /dev/null diff --git a/old/66493-h/images/v2fig218.png b/old/66493-h/images/v2fig218.png Binary files differdeleted file mode 100644 index 067b7d5..0000000 --- a/old/66493-h/images/v2fig218.png +++ /dev/null diff --git a/old/66493-h/images/v2fig219.png b/old/66493-h/images/v2fig219.png Binary files differdeleted file mode 100644 index d491bf5..0000000 --- a/old/66493-h/images/v2fig219.png +++ /dev/null diff --git a/old/66493-h/images/v2fig220.png b/old/66493-h/images/v2fig220.png Binary files differdeleted file mode 100644 index bb8cf51..0000000 --- a/old/66493-h/images/v2fig220.png +++ /dev/null diff --git a/old/66493-h/images/v2fig221.png b/old/66493-h/images/v2fig221.png Binary files differdeleted file mode 100644 index 8f74c50..0000000 --- a/old/66493-h/images/v2fig221.png +++ /dev/null diff --git a/old/66493-h/images/v2fig222.png b/old/66493-h/images/v2fig222.png Binary files differdeleted file mode 100644 index 501c276..0000000 --- a/old/66493-h/images/v2fig222.png +++ /dev/null diff --git a/old/66493-h/images/v2fig223.png b/old/66493-h/images/v2fig223.png Binary files differdeleted file mode 100644 index f21c361..0000000 --- a/old/66493-h/images/v2fig223.png +++ /dev/null diff --git a/old/66493-h/images/v2fig224.png b/old/66493-h/images/v2fig224.png Binary files differdeleted file mode 100644 index 9b836c0..0000000 --- a/old/66493-h/images/v2fig224.png +++ /dev/null diff --git a/old/66493-h/images/v2fig225.png b/old/66493-h/images/v2fig225.png Binary files differdeleted file mode 100644 index 8ab60a9..0000000 --- a/old/66493-h/images/v2fig225.png +++ /dev/null diff --git a/old/66493-h/images/v2fig226.png b/old/66493-h/images/v2fig226.png Binary files differdeleted file mode 100644 index d875369..0000000 --- a/old/66493-h/images/v2fig226.png +++ /dev/null diff --git a/old/66493-h/images/v2fig227.png b/old/66493-h/images/v2fig227.png Binary files differdeleted file mode 100644 index 3da6194..0000000 --- a/old/66493-h/images/v2fig227.png +++ /dev/null diff --git a/old/66493-h/images/v2fig228.png b/old/66493-h/images/v2fig228.png Binary files differdeleted file mode 100644 index 428fcaf..0000000 --- a/old/66493-h/images/v2fig228.png +++ /dev/null diff --git a/old/66493-h/images/v2fig229.png b/old/66493-h/images/v2fig229.png Binary files differdeleted file mode 100644 index 77c2ac5..0000000 --- a/old/66493-h/images/v2fig229.png +++ /dev/null diff --git a/old/66493-h/images/v2fig230.png b/old/66493-h/images/v2fig230.png Binary files differdeleted file mode 100644 index decd451..0000000 --- a/old/66493-h/images/v2fig230.png +++ /dev/null diff --git a/old/66493-h/images/v2fig231.png b/old/66493-h/images/v2fig231.png Binary files differdeleted file mode 100644 index 53f36a7..0000000 --- a/old/66493-h/images/v2fig231.png +++ /dev/null diff --git a/old/66493-h/images/v2fig232.png b/old/66493-h/images/v2fig232.png Binary files differdeleted file mode 100644 index d634567..0000000 --- a/old/66493-h/images/v2fig232.png +++ /dev/null diff --git a/old/66493-h/images/v2fig233.png b/old/66493-h/images/v2fig233.png Binary files differdeleted file mode 100644 index fba810f..0000000 --- a/old/66493-h/images/v2fig233.png +++ /dev/null diff --git a/old/66493-h/images/v2fig234.png b/old/66493-h/images/v2fig234.png Binary files differdeleted file mode 100644 index 748a87c..0000000 --- a/old/66493-h/images/v2fig234.png +++ /dev/null diff --git a/old/66493-h/images/v2fig235.png b/old/66493-h/images/v2fig235.png Binary files differdeleted file mode 100644 index 9a2c16f..0000000 --- a/old/66493-h/images/v2fig235.png +++ /dev/null diff --git a/old/66493-h/images/v2fig236.png b/old/66493-h/images/v2fig236.png Binary files differdeleted file mode 100644 index 7887fcb..0000000 --- a/old/66493-h/images/v2fig236.png +++ /dev/null diff --git a/old/66493-h/images/v2fig237.png b/old/66493-h/images/v2fig237.png Binary files differdeleted file mode 100644 index 39a0e34..0000000 --- a/old/66493-h/images/v2fig237.png +++ /dev/null diff --git a/old/66493-h/images/v2fig238.png b/old/66493-h/images/v2fig238.png Binary files differdeleted file mode 100644 index 05d204c..0000000 --- a/old/66493-h/images/v2fig238.png +++ /dev/null diff --git a/old/66493-h/images/v2fig239.png b/old/66493-h/images/v2fig239.png Binary files differdeleted file mode 100644 index d280374..0000000 --- a/old/66493-h/images/v2fig239.png +++ /dev/null diff --git a/old/66493-h/images/v2fig240.png b/old/66493-h/images/v2fig240.png Binary files differdeleted file mode 100644 index 1a77fb3..0000000 --- a/old/66493-h/images/v2fig240.png +++ /dev/null diff --git a/old/66493-h/images/v2fig241.png b/old/66493-h/images/v2fig241.png Binary files differdeleted file mode 100644 index 5cf5289..0000000 --- a/old/66493-h/images/v2fig241.png +++ /dev/null diff --git a/old/66493-h/images/v2fig242.png b/old/66493-h/images/v2fig242.png Binary files differdeleted file mode 100644 index 6877ebb..0000000 --- a/old/66493-h/images/v2fig242.png +++ /dev/null diff --git a/old/66493-h/images/v2fig243.png b/old/66493-h/images/v2fig243.png Binary files differdeleted file mode 100644 index 9204494..0000000 --- a/old/66493-h/images/v2fig243.png +++ /dev/null diff --git a/old/66493-h/images/v2fig244.png b/old/66493-h/images/v2fig244.png Binary files differdeleted file mode 100644 index ab121e7..0000000 --- a/old/66493-h/images/v2fig244.png +++ /dev/null diff --git a/old/66493-h/images/v2fig245.png b/old/66493-h/images/v2fig245.png Binary files differdeleted file mode 100644 index 7332e1c..0000000 --- a/old/66493-h/images/v2fig245.png +++ /dev/null diff --git a/old/66493-h/images/v2fig246.png b/old/66493-h/images/v2fig246.png Binary files differdeleted file mode 100644 index 6f7b630..0000000 --- a/old/66493-h/images/v2fig246.png +++ /dev/null diff --git a/old/66493-h/images/v2fig247.png b/old/66493-h/images/v2fig247.png Binary files differdeleted file mode 100644 index 538b3aa..0000000 --- a/old/66493-h/images/v2fig247.png +++ /dev/null diff --git a/old/66493-h/images/v2fig248.png b/old/66493-h/images/v2fig248.png Binary files differdeleted file mode 100644 index 81d23fc..0000000 --- a/old/66493-h/images/v2fig248.png +++ /dev/null diff --git a/old/66493-h/images/v2fig249.png b/old/66493-h/images/v2fig249.png Binary files differdeleted file mode 100644 index 1434166..0000000 --- a/old/66493-h/images/v2fig249.png +++ /dev/null diff --git a/old/66493-h/images/v2fig250.png b/old/66493-h/images/v2fig250.png Binary files differdeleted file mode 100644 index 7cd93e0..0000000 --- a/old/66493-h/images/v2fig250.png +++ /dev/null diff --git a/old/66493-h/images/v2fig251.png b/old/66493-h/images/v2fig251.png Binary files differdeleted file mode 100644 index 02dae82..0000000 --- a/old/66493-h/images/v2fig251.png +++ /dev/null diff --git a/old/66493-h/images/v2fig252.png b/old/66493-h/images/v2fig252.png Binary files differdeleted file mode 100644 index 7503878..0000000 --- a/old/66493-h/images/v2fig252.png +++ /dev/null diff --git a/old/66493-h/images/v2fig253.png b/old/66493-h/images/v2fig253.png Binary files differdeleted file mode 100644 index c7a6053..0000000 --- a/old/66493-h/images/v2fig253.png +++ /dev/null diff --git a/old/66493-h/images/v2fig254.png b/old/66493-h/images/v2fig254.png Binary files differdeleted file mode 100644 index 74dc089..0000000 --- a/old/66493-h/images/v2fig254.png +++ /dev/null diff --git a/old/66493-h/images/v2fig255.png b/old/66493-h/images/v2fig255.png Binary files differdeleted file mode 100644 index 46fb39e..0000000 --- a/old/66493-h/images/v2fig255.png +++ /dev/null diff --git a/old/66493-h/images/v2fig256.png b/old/66493-h/images/v2fig256.png Binary files differdeleted file mode 100644 index 494b0f6..0000000 --- a/old/66493-h/images/v2fig256.png +++ /dev/null diff --git a/old/66493-h/images/v2fig257.png b/old/66493-h/images/v2fig257.png Binary files differdeleted file mode 100644 index 41e3d7a..0000000 --- a/old/66493-h/images/v2fig257.png +++ /dev/null diff --git a/old/66493-h/images/v2fig258.png b/old/66493-h/images/v2fig258.png Binary files differdeleted file mode 100644 index 0a4ea7b..0000000 --- a/old/66493-h/images/v2fig258.png +++ /dev/null diff --git a/old/66493-h/images/v2fig259.png b/old/66493-h/images/v2fig259.png Binary files differdeleted file mode 100644 index 0f77e39..0000000 --- a/old/66493-h/images/v2fig259.png +++ /dev/null diff --git a/old/66493-h/images/v2fig260.png b/old/66493-h/images/v2fig260.png Binary files differdeleted file mode 100644 index 68ccdd2..0000000 --- a/old/66493-h/images/v2fig260.png +++ /dev/null diff --git a/old/66493-h/images/v2fig261.png b/old/66493-h/images/v2fig261.png Binary files differdeleted file mode 100644 index 918c8eb..0000000 --- a/old/66493-h/images/v2fig261.png +++ /dev/null diff --git a/old/66493-h/images/v2fig262.png b/old/66493-h/images/v2fig262.png Binary files differdeleted file mode 100644 index 9878a84..0000000 --- a/old/66493-h/images/v2fig262.png +++ /dev/null diff --git a/old/66493-h/images/v2fig263.png b/old/66493-h/images/v2fig263.png Binary files differdeleted file mode 100644 index 7c6ea86..0000000 --- a/old/66493-h/images/v2fig263.png +++ /dev/null diff --git a/old/66493-h/images/v2fig264.png b/old/66493-h/images/v2fig264.png Binary files differdeleted file mode 100644 index 70bc338..0000000 --- a/old/66493-h/images/v2fig264.png +++ /dev/null diff --git a/old/66493-h/images/v2fig265.png b/old/66493-h/images/v2fig265.png Binary files differdeleted file mode 100644 index 554df71..0000000 --- a/old/66493-h/images/v2fig265.png +++ /dev/null diff --git a/old/66493-h/images/v2fig266.png b/old/66493-h/images/v2fig266.png Binary files differdeleted file mode 100644 index 734ced0..0000000 --- a/old/66493-h/images/v2fig266.png +++ /dev/null diff --git a/old/66493-h/images/v2fig266a.png b/old/66493-h/images/v2fig266a.png Binary files differdeleted file mode 100644 index 4f5d0da..0000000 --- a/old/66493-h/images/v2fig266a.png +++ /dev/null diff --git a/old/66493-h/images/v2fig267.png b/old/66493-h/images/v2fig267.png Binary files differdeleted file mode 100644 index 7469526..0000000 --- a/old/66493-h/images/v2fig267.png +++ /dev/null diff --git a/old/66493-h/images/v2fig268.png b/old/66493-h/images/v2fig268.png Binary files differdeleted file mode 100644 index 8c000a3..0000000 --- a/old/66493-h/images/v2fig268.png +++ /dev/null diff --git a/old/66493-h/images/v2fig269.png b/old/66493-h/images/v2fig269.png Binary files differdeleted file mode 100644 index bcd32f4..0000000 --- a/old/66493-h/images/v2fig269.png +++ /dev/null diff --git a/old/66493-h/images/v2fig270.png b/old/66493-h/images/v2fig270.png Binary files differdeleted file mode 100644 index 2a19149..0000000 --- a/old/66493-h/images/v2fig270.png +++ /dev/null diff --git a/old/66493-h/images/v2fig271.png b/old/66493-h/images/v2fig271.png Binary files differdeleted file mode 100644 index 7a877e4..0000000 --- a/old/66493-h/images/v2fig271.png +++ /dev/null diff --git a/old/66493-h/images/v2fig272.png b/old/66493-h/images/v2fig272.png Binary files differdeleted file mode 100644 index 07e7967..0000000 --- a/old/66493-h/images/v2fig272.png +++ /dev/null diff --git a/old/66493-h/images/v2fig273.png b/old/66493-h/images/v2fig273.png Binary files differdeleted file mode 100644 index eaf55a7..0000000 --- a/old/66493-h/images/v2fig273.png +++ /dev/null diff --git a/old/66493-h/images/v2fig274.png b/old/66493-h/images/v2fig274.png Binary files differdeleted file mode 100644 index d621d61..0000000 --- a/old/66493-h/images/v2fig274.png +++ /dev/null diff --git a/old/66493-h/images/v2fig275.png b/old/66493-h/images/v2fig275.png Binary files differdeleted file mode 100644 index 3ccd1e3..0000000 --- a/old/66493-h/images/v2fig275.png +++ /dev/null diff --git a/old/66493-h/images/v2fig276.png b/old/66493-h/images/v2fig276.png Binary files differdeleted file mode 100644 index ebe1c11..0000000 --- a/old/66493-h/images/v2fig276.png +++ /dev/null diff --git a/old/66493-h/images/v2fig277.png b/old/66493-h/images/v2fig277.png Binary files differdeleted file mode 100644 index d0c58e9..0000000 --- a/old/66493-h/images/v2fig277.png +++ /dev/null diff --git a/old/66493-h/images/v2fig278.png b/old/66493-h/images/v2fig278.png Binary files differdeleted file mode 100644 index bdd69dd..0000000 --- a/old/66493-h/images/v2fig278.png +++ /dev/null diff --git a/old/66493-h/images/v2fig279.png b/old/66493-h/images/v2fig279.png Binary files differdeleted file mode 100644 index 4a82141..0000000 --- a/old/66493-h/images/v2fig279.png +++ /dev/null diff --git a/old/66493-h/images/v2fig280.png b/old/66493-h/images/v2fig280.png Binary files differdeleted file mode 100644 index 4402fe7..0000000 --- a/old/66493-h/images/v2fig280.png +++ /dev/null diff --git a/old/66493-h/images/v2fig281.png b/old/66493-h/images/v2fig281.png Binary files differdeleted file mode 100644 index d30bb90..0000000 --- a/old/66493-h/images/v2fig281.png +++ /dev/null diff --git a/old/66493-h/images/v2fig282.png b/old/66493-h/images/v2fig282.png Binary files differdeleted file mode 100644 index 1d26486..0000000 --- a/old/66493-h/images/v2fig282.png +++ /dev/null diff --git a/old/66493-h/images/v2fig283.png b/old/66493-h/images/v2fig283.png Binary files differdeleted file mode 100644 index fc7b52b..0000000 --- a/old/66493-h/images/v2fig283.png +++ /dev/null diff --git a/old/66493-h/images/v2fig284.png b/old/66493-h/images/v2fig284.png Binary files differdeleted file mode 100644 index fef5812..0000000 --- a/old/66493-h/images/v2fig284.png +++ /dev/null diff --git a/old/66493-h/images/v2fig285.png b/old/66493-h/images/v2fig285.png Binary files differdeleted file mode 100644 index 60d1853..0000000 --- a/old/66493-h/images/v2fig285.png +++ /dev/null diff --git a/old/66493-h/images/v2fig286.png b/old/66493-h/images/v2fig286.png Binary files differdeleted file mode 100644 index f29c31a..0000000 --- a/old/66493-h/images/v2fig286.png +++ /dev/null diff --git a/old/66493-h/images/v2fig287.png b/old/66493-h/images/v2fig287.png Binary files differdeleted file mode 100644 index 6bccd80..0000000 --- a/old/66493-h/images/v2fig287.png +++ /dev/null diff --git a/old/66493-h/images/v2fig288.png b/old/66493-h/images/v2fig288.png Binary files differdeleted file mode 100644 index ac72def..0000000 --- a/old/66493-h/images/v2fig288.png +++ /dev/null diff --git a/old/66493-h/images/v2fig289.png b/old/66493-h/images/v2fig289.png Binary files differdeleted file mode 100644 index d62638f..0000000 --- a/old/66493-h/images/v2fig289.png +++ /dev/null diff --git a/old/66493-h/images/v2fig290.png b/old/66493-h/images/v2fig290.png Binary files differdeleted file mode 100644 index 2ce237c..0000000 --- a/old/66493-h/images/v2fig290.png +++ /dev/null diff --git a/old/66493-h/images/v2fig291.png b/old/66493-h/images/v2fig291.png Binary files differdeleted file mode 100644 index 3a40e22..0000000 --- a/old/66493-h/images/v2fig291.png +++ /dev/null diff --git a/old/66493-h/images/v2fig292.png b/old/66493-h/images/v2fig292.png Binary files differdeleted file mode 100644 index d506881..0000000 --- a/old/66493-h/images/v2fig292.png +++ /dev/null diff --git a/old/66493-h/images/v2fig293.png b/old/66493-h/images/v2fig293.png Binary files differdeleted file mode 100644 index 5b82c98..0000000 --- a/old/66493-h/images/v2fig293.png +++ /dev/null diff --git a/old/66493-h/images/v2fig293a.png b/old/66493-h/images/v2fig293a.png Binary files differdeleted file mode 100644 index 46d0641..0000000 --- a/old/66493-h/images/v2fig293a.png +++ /dev/null diff --git a/old/66493-h/images/v2fig294.png b/old/66493-h/images/v2fig294.png Binary files differdeleted file mode 100644 index d785c51..0000000 --- a/old/66493-h/images/v2fig294.png +++ /dev/null diff --git a/old/66493-h/images/v2fig295.png b/old/66493-h/images/v2fig295.png Binary files differdeleted file mode 100644 index 207b66e..0000000 --- a/old/66493-h/images/v2fig295.png +++ /dev/null diff --git a/old/66493-h/images/v2fig296.png b/old/66493-h/images/v2fig296.png Binary files differdeleted file mode 100644 index f26adc5..0000000 --- a/old/66493-h/images/v2fig296.png +++ /dev/null diff --git a/old/66493-h/images/v2fig297.png b/old/66493-h/images/v2fig297.png Binary files differdeleted file mode 100644 index 5de7ead..0000000 --- a/old/66493-h/images/v2fig297.png +++ /dev/null diff --git a/old/66493-h/images/v2fig298.png b/old/66493-h/images/v2fig298.png Binary files differdeleted file mode 100644 index 6c7db61..0000000 --- a/old/66493-h/images/v2fig298.png +++ /dev/null diff --git a/old/66493-h/images/v2fig299.png b/old/66493-h/images/v2fig299.png Binary files differdeleted file mode 100644 index d5e65eb..0000000 --- a/old/66493-h/images/v2fig299.png +++ /dev/null diff --git a/old/66493-h/images/v2fig300.png b/old/66493-h/images/v2fig300.png Binary files differdeleted file mode 100644 index bed29a1..0000000 --- a/old/66493-h/images/v2fig300.png +++ /dev/null diff --git a/old/66493-h/images/v2fig301.png b/old/66493-h/images/v2fig301.png Binary files differdeleted file mode 100644 index 2343559..0000000 --- a/old/66493-h/images/v2fig301.png +++ /dev/null diff --git a/old/66493-h/images/v2fig302.png b/old/66493-h/images/v2fig302.png Binary files differdeleted file mode 100644 index 41bc81e..0000000 --- a/old/66493-h/images/v2fig302.png +++ /dev/null diff --git a/old/66493-h/images/v2fig303.png b/old/66493-h/images/v2fig303.png Binary files differdeleted file mode 100644 index 5bf5c70..0000000 --- a/old/66493-h/images/v2fig303.png +++ /dev/null diff --git a/old/66493-h/images/v2fig304.png b/old/66493-h/images/v2fig304.png Binary files differdeleted file mode 100644 index 3247ed8..0000000 --- a/old/66493-h/images/v2fig304.png +++ /dev/null diff --git a/old/66493-h/images/v2fig305.png b/old/66493-h/images/v2fig305.png Binary files differdeleted file mode 100644 index 130eb15..0000000 --- a/old/66493-h/images/v2fig305.png +++ /dev/null diff --git a/old/66493-h/images/v2fig306.png b/old/66493-h/images/v2fig306.png Binary files differdeleted file mode 100644 index 60bd2ac..0000000 --- a/old/66493-h/images/v2fig306.png +++ /dev/null diff --git a/old/66493-h/images/v2fig307.png b/old/66493-h/images/v2fig307.png Binary files differdeleted file mode 100644 index 6a5358c..0000000 --- a/old/66493-h/images/v2fig307.png +++ /dev/null diff --git a/old/66493-h/images/v2fig308.png b/old/66493-h/images/v2fig308.png Binary files differdeleted file mode 100644 index 40dfa79..0000000 --- a/old/66493-h/images/v2fig308.png +++ /dev/null diff --git a/old/66493-h/images/v2fig309.png b/old/66493-h/images/v2fig309.png Binary files differdeleted file mode 100644 index 1df0573..0000000 --- a/old/66493-h/images/v2fig309.png +++ /dev/null diff --git a/old/66493-h/images/v2fig310.png b/old/66493-h/images/v2fig310.png Binary files differdeleted file mode 100644 index dee893b..0000000 --- a/old/66493-h/images/v2fig310.png +++ /dev/null diff --git a/old/66493-h/images/v2fig311.png b/old/66493-h/images/v2fig311.png Binary files differdeleted file mode 100644 index 0bf5339..0000000 --- a/old/66493-h/images/v2fig311.png +++ /dev/null diff --git a/old/66493-h/images/v2fig312.png b/old/66493-h/images/v2fig312.png Binary files differdeleted file mode 100644 index 2ae0168..0000000 --- a/old/66493-h/images/v2fig312.png +++ /dev/null diff --git a/old/66493-h/images/v2fig313.png b/old/66493-h/images/v2fig313.png Binary files differdeleted file mode 100644 index 6802761..0000000 --- a/old/66493-h/images/v2fig313.png +++ /dev/null diff --git a/old/66493-h/images/v2fig314.png b/old/66493-h/images/v2fig314.png Binary files differdeleted file mode 100644 index b0a322b..0000000 --- a/old/66493-h/images/v2fig314.png +++ /dev/null diff --git a/old/66493-h/images/v2fig315.png b/old/66493-h/images/v2fig315.png Binary files differdeleted file mode 100644 index c41ab67..0000000 --- a/old/66493-h/images/v2fig315.png +++ /dev/null diff --git a/old/66493-h/images/v2fig316.png b/old/66493-h/images/v2fig316.png Binary files differdeleted file mode 100644 index 56de00b..0000000 --- a/old/66493-h/images/v2fig316.png +++ /dev/null diff --git a/old/66493-h/images/v2fig317.png b/old/66493-h/images/v2fig317.png Binary files differdeleted file mode 100644 index 59bc6bb..0000000 --- a/old/66493-h/images/v2fig317.png +++ /dev/null diff --git a/old/66493-h/images/v2fig318.png b/old/66493-h/images/v2fig318.png Binary files differdeleted file mode 100644 index 4afcfdc..0000000 --- a/old/66493-h/images/v2fig318.png +++ /dev/null diff --git a/old/66493-h/images/v2fig319.png b/old/66493-h/images/v2fig319.png Binary files differdeleted file mode 100644 index 13f4ece..0000000 --- a/old/66493-h/images/v2fig319.png +++ /dev/null diff --git a/old/66493-h/images/v2fig320.png b/old/66493-h/images/v2fig320.png Binary files differdeleted file mode 100644 index e95e274..0000000 --- a/old/66493-h/images/v2fig320.png +++ /dev/null diff --git a/old/66493-h/images/v2fig321.png b/old/66493-h/images/v2fig321.png Binary files differdeleted file mode 100644 index 755bbd8..0000000 --- a/old/66493-h/images/v2fig321.png +++ /dev/null diff --git a/old/66493-h/images/v2fig322.png b/old/66493-h/images/v2fig322.png Binary files differdeleted file mode 100644 index c508d8b..0000000 --- a/old/66493-h/images/v2fig322.png +++ /dev/null diff --git a/old/66493-h/images/v2fig323.png b/old/66493-h/images/v2fig323.png Binary files differdeleted file mode 100644 index 0e729aa..0000000 --- a/old/66493-h/images/v2fig323.png +++ /dev/null diff --git a/old/66493-h/images/v2fig324.png b/old/66493-h/images/v2fig324.png Binary files differdeleted file mode 100644 index 9b75774..0000000 --- a/old/66493-h/images/v2fig324.png +++ /dev/null diff --git a/old/66493-h/images/v2fig325.png b/old/66493-h/images/v2fig325.png Binary files differdeleted file mode 100644 index 685ef66..0000000 --- a/old/66493-h/images/v2fig325.png +++ /dev/null diff --git a/old/66493-h/images/v2fig326.png b/old/66493-h/images/v2fig326.png Binary files differdeleted file mode 100644 index 45d83ac..0000000 --- a/old/66493-h/images/v2fig326.png +++ /dev/null diff --git a/old/66493-h/images/v2fig327.png b/old/66493-h/images/v2fig327.png Binary files differdeleted file mode 100644 index b1fab3a..0000000 --- a/old/66493-h/images/v2fig327.png +++ /dev/null diff --git a/old/66493-h/images/v2fig328.png b/old/66493-h/images/v2fig328.png Binary files differdeleted file mode 100644 index f319cc4..0000000 --- a/old/66493-h/images/v2fig328.png +++ /dev/null diff --git a/old/66493-h/images/v2fig329.png b/old/66493-h/images/v2fig329.png Binary files differdeleted file mode 100644 index bafcb47..0000000 --- a/old/66493-h/images/v2fig329.png +++ /dev/null diff --git a/old/66493-h/images/v2fig330.png b/old/66493-h/images/v2fig330.png Binary files differdeleted file mode 100644 index 6ffc2bf..0000000 --- a/old/66493-h/images/v2fig330.png +++ /dev/null diff --git a/old/66493-h/images/v2fig331.png b/old/66493-h/images/v2fig331.png Binary files differdeleted file mode 100644 index 72c61ad..0000000 --- a/old/66493-h/images/v2fig331.png +++ /dev/null diff --git a/old/66493-h/images/v2fig332.png b/old/66493-h/images/v2fig332.png Binary files differdeleted file mode 100644 index 2127ce0..0000000 --- a/old/66493-h/images/v2fig332.png +++ /dev/null diff --git a/old/66493-h/images/v2fig333.png b/old/66493-h/images/v2fig333.png Binary files differdeleted file mode 100644 index 163571d..0000000 --- a/old/66493-h/images/v2fig333.png +++ /dev/null diff --git a/old/66493-h/images/v2fig334.png b/old/66493-h/images/v2fig334.png Binary files differdeleted file mode 100644 index ff5a507..0000000 --- a/old/66493-h/images/v2fig334.png +++ /dev/null diff --git a/old/66493-h/images/v2fig335.png b/old/66493-h/images/v2fig335.png Binary files differdeleted file mode 100644 index db9cd71..0000000 --- a/old/66493-h/images/v2fig335.png +++ /dev/null diff --git a/old/66493-h/images/v2fig336.png b/old/66493-h/images/v2fig336.png Binary files differdeleted file mode 100644 index 17f443b..0000000 --- a/old/66493-h/images/v2fig336.png +++ /dev/null diff --git a/old/66493-h/images/v2fig337.png b/old/66493-h/images/v2fig337.png Binary files differdeleted file mode 100644 index de7e1d4..0000000 --- a/old/66493-h/images/v2fig337.png +++ /dev/null diff --git a/old/66493-h/images/v2fig338.png b/old/66493-h/images/v2fig338.png Binary files differdeleted file mode 100644 index 9ed1459..0000000 --- a/old/66493-h/images/v2fig338.png +++ /dev/null diff --git a/old/66493-h/images/v2fig339.png b/old/66493-h/images/v2fig339.png Binary files differdeleted file mode 100644 index 4b9184c..0000000 --- a/old/66493-h/images/v2fig339.png +++ /dev/null diff --git a/old/66493-h/images/v2fig340.png b/old/66493-h/images/v2fig340.png Binary files differdeleted file mode 100644 index 9b2a80a..0000000 --- a/old/66493-h/images/v2fig340.png +++ /dev/null diff --git a/old/66493-h/images/v2fig341.png b/old/66493-h/images/v2fig341.png Binary files differdeleted file mode 100644 index 27922ea..0000000 --- a/old/66493-h/images/v2fig341.png +++ /dev/null diff --git a/old/66493-h/images/v2fig342.png b/old/66493-h/images/v2fig342.png Binary files differdeleted file mode 100644 index 8dbbaff..0000000 --- a/old/66493-h/images/v2fig342.png +++ /dev/null diff --git a/old/66493-h/images/v2fig343.png b/old/66493-h/images/v2fig343.png Binary files differdeleted file mode 100644 index dc66525..0000000 --- a/old/66493-h/images/v2fig343.png +++ /dev/null diff --git a/old/66493-h/images/v2fig344.png b/old/66493-h/images/v2fig344.png Binary files differdeleted file mode 100644 index 2046244..0000000 --- a/old/66493-h/images/v2fig344.png +++ /dev/null diff --git a/old/66493-h/images/v2fig345.png b/old/66493-h/images/v2fig345.png Binary files differdeleted file mode 100644 index 9ce9982..0000000 --- a/old/66493-h/images/v2fig345.png +++ /dev/null diff --git a/old/66493-h/images/v2fig346.png b/old/66493-h/images/v2fig346.png Binary files differdeleted file mode 100644 index fb4f73b..0000000 --- a/old/66493-h/images/v2fig346.png +++ /dev/null diff --git a/old/66493-h/images/v2fig347.png b/old/66493-h/images/v2fig347.png Binary files differdeleted file mode 100644 index bb65baf..0000000 --- a/old/66493-h/images/v2fig347.png +++ /dev/null diff --git a/old/66493-h/images/v2fig348.png b/old/66493-h/images/v2fig348.png Binary files differdeleted file mode 100644 index 83a63f2..0000000 --- a/old/66493-h/images/v2fig348.png +++ /dev/null diff --git a/old/66493-h/images/v2fig349.png b/old/66493-h/images/v2fig349.png Binary files differdeleted file mode 100644 index 1affe6e..0000000 --- a/old/66493-h/images/v2fig349.png +++ /dev/null diff --git a/old/66493-h/images/v2fig350.png b/old/66493-h/images/v2fig350.png Binary files differdeleted file mode 100644 index 9ecc1da..0000000 --- a/old/66493-h/images/v2fig350.png +++ /dev/null diff --git a/old/66493-h/images/v2fig351.png b/old/66493-h/images/v2fig351.png Binary files differdeleted file mode 100644 index 1ab2db5..0000000 --- a/old/66493-h/images/v2fig351.png +++ /dev/null diff --git a/old/66493-h/images/v2fig352.png b/old/66493-h/images/v2fig352.png Binary files differdeleted file mode 100644 index b4bd188..0000000 --- a/old/66493-h/images/v2fig352.png +++ /dev/null diff --git a/old/66493-h/images/v2fig353.png b/old/66493-h/images/v2fig353.png Binary files differdeleted file mode 100644 index de718be..0000000 --- a/old/66493-h/images/v2fig353.png +++ /dev/null diff --git a/old/66493-h/images/v2fig354.png b/old/66493-h/images/v2fig354.png Binary files differdeleted file mode 100644 index 8269829..0000000 --- a/old/66493-h/images/v2fig354.png +++ /dev/null diff --git a/old/66493-h/images/v2fig355.png b/old/66493-h/images/v2fig355.png Binary files differdeleted file mode 100644 index 219b4cc..0000000 --- a/old/66493-h/images/v2fig355.png +++ /dev/null diff --git a/old/66493-h/images/v2fig356.png b/old/66493-h/images/v2fig356.png Binary files differdeleted file mode 100644 index efb5ac7..0000000 --- a/old/66493-h/images/v2fig356.png +++ /dev/null diff --git a/old/66493-h/images/v2fig357.png b/old/66493-h/images/v2fig357.png Binary files differdeleted file mode 100644 index 36ec1d4..0000000 --- a/old/66493-h/images/v2fig357.png +++ /dev/null diff --git a/old/66493-h/images/v2fig358.png b/old/66493-h/images/v2fig358.png Binary files differdeleted file mode 100644 index 52a194f..0000000 --- a/old/66493-h/images/v2fig358.png +++ /dev/null diff --git a/old/66493-h/images/v2fig359.png b/old/66493-h/images/v2fig359.png Binary files differdeleted file mode 100644 index 40efdb6..0000000 --- a/old/66493-h/images/v2fig359.png +++ /dev/null diff --git a/old/66493-h/images/v2fig360.png b/old/66493-h/images/v2fig360.png Binary files differdeleted file mode 100644 index 831a050..0000000 --- a/old/66493-h/images/v2fig360.png +++ /dev/null diff --git a/old/66493-h/images/v2fig361.png b/old/66493-h/images/v2fig361.png Binary files differdeleted file mode 100644 index 0fe4417..0000000 --- a/old/66493-h/images/v2fig361.png +++ /dev/null diff --git a/old/66493-h/images/v2fig362.png b/old/66493-h/images/v2fig362.png Binary files differdeleted file mode 100644 index 0f2267d..0000000 --- a/old/66493-h/images/v2fig362.png +++ /dev/null diff --git a/old/66493-h/images/v2fig363.png b/old/66493-h/images/v2fig363.png Binary files differdeleted file mode 100644 index e0595b7..0000000 --- a/old/66493-h/images/v2fig363.png +++ /dev/null diff --git a/old/66493-h/images/v2fig364.png b/old/66493-h/images/v2fig364.png Binary files differdeleted file mode 100644 index 1333d21..0000000 --- a/old/66493-h/images/v2fig364.png +++ /dev/null diff --git a/old/66493-h/images/v2fig365.png b/old/66493-h/images/v2fig365.png Binary files differdeleted file mode 100644 index da0f906..0000000 --- a/old/66493-h/images/v2fig365.png +++ /dev/null diff --git a/old/66493-h/images/v2fig366.png b/old/66493-h/images/v2fig366.png Binary files differdeleted file mode 100644 index 27542fc..0000000 --- a/old/66493-h/images/v2fig366.png +++ /dev/null diff --git a/old/66493-h/images/v2fig367.png b/old/66493-h/images/v2fig367.png Binary files differdeleted file mode 100644 index b01646d..0000000 --- a/old/66493-h/images/v2fig367.png +++ /dev/null diff --git a/old/66493-h/images/v2fig368.png b/old/66493-h/images/v2fig368.png Binary files differdeleted file mode 100644 index 32d8cbc..0000000 --- a/old/66493-h/images/v2fig368.png +++ /dev/null diff --git a/old/66493-h/images/v2fig369.png b/old/66493-h/images/v2fig369.png Binary files differdeleted file mode 100644 index d5f15b5..0000000 --- a/old/66493-h/images/v2fig369.png +++ /dev/null diff --git a/old/66493-h/images/v2fig370.png b/old/66493-h/images/v2fig370.png Binary files differdeleted file mode 100644 index 53008e3..0000000 --- a/old/66493-h/images/v2fig370.png +++ /dev/null diff --git a/old/66493-h/images/v2fig371.png b/old/66493-h/images/v2fig371.png Binary files differdeleted file mode 100644 index c952a62..0000000 --- a/old/66493-h/images/v2fig371.png +++ /dev/null diff --git a/old/66493-h/images/v2fig372.png b/old/66493-h/images/v2fig372.png Binary files differdeleted file mode 100644 index 7592e4e..0000000 --- a/old/66493-h/images/v2fig372.png +++ /dev/null diff --git a/old/66493-h/images/v2fig373.png b/old/66493-h/images/v2fig373.png Binary files differdeleted file mode 100644 index 5c20503..0000000 --- a/old/66493-h/images/v2fig373.png +++ /dev/null diff --git a/old/66493-h/images/v2fig374.png b/old/66493-h/images/v2fig374.png Binary files differdeleted file mode 100644 index ae0a702..0000000 --- a/old/66493-h/images/v2fig374.png +++ /dev/null diff --git a/old/66493-h/images/v2fig375.png b/old/66493-h/images/v2fig375.png Binary files differdeleted file mode 100644 index fe89563..0000000 --- a/old/66493-h/images/v2fig375.png +++ /dev/null diff --git a/old/66493-h/images/v2fig376.png b/old/66493-h/images/v2fig376.png Binary files differdeleted file mode 100644 index 33a55b8..0000000 --- a/old/66493-h/images/v2fig376.png +++ /dev/null diff --git a/old/66493-h/images/v2fig377.png b/old/66493-h/images/v2fig377.png Binary files differdeleted file mode 100644 index c4fadcb..0000000 --- a/old/66493-h/images/v2fig377.png +++ /dev/null diff --git a/old/66493-h/images/v2fig378.png b/old/66493-h/images/v2fig378.png Binary files differdeleted file mode 100644 index 0031099..0000000 --- a/old/66493-h/images/v2fig378.png +++ /dev/null diff --git a/old/66493-h/images/v2fig379.png b/old/66493-h/images/v2fig379.png Binary files differdeleted file mode 100644 index 8c7963f..0000000 --- a/old/66493-h/images/v2fig379.png +++ /dev/null diff --git a/old/66493-h/images/v2fig380.png b/old/66493-h/images/v2fig380.png Binary files differdeleted file mode 100644 index 62b6489..0000000 --- a/old/66493-h/images/v2fig380.png +++ /dev/null diff --git a/old/66493-h/images/v2fig381.png b/old/66493-h/images/v2fig381.png Binary files differdeleted file mode 100644 index afc71ea..0000000 --- a/old/66493-h/images/v2fig381.png +++ /dev/null diff --git a/old/66493-h/images/v2fig382.png b/old/66493-h/images/v2fig382.png Binary files differdeleted file mode 100644 index 1c4faf9..0000000 --- a/old/66493-h/images/v2fig382.png +++ /dev/null diff --git a/old/66493-h/images/v2fig383.png b/old/66493-h/images/v2fig383.png Binary files differdeleted file mode 100644 index d4db555..0000000 --- a/old/66493-h/images/v2fig383.png +++ /dev/null diff --git a/old/66493-h/images/v2map5.png b/old/66493-h/images/v2map5.png Binary files differdeleted file mode 100644 index 1cfb7ad..0000000 --- a/old/66493-h/images/v2map5.png +++ /dev/null diff --git a/old/66493-h/images/v2map5lg.png b/old/66493-h/images/v2map5lg.png Binary files differdeleted file mode 100644 index 5bd98f2..0000000 --- a/old/66493-h/images/v2map5lg.png +++ /dev/null diff --git a/old/66493-h/images/v2map6.png b/old/66493-h/images/v2map6.png Binary files differdeleted file mode 100644 index ea55682..0000000 --- a/old/66493-h/images/v2map6.png +++ /dev/null diff --git a/old/66493-h/images/v2map6lg.png b/old/66493-h/images/v2map6lg.png Binary files differdeleted file mode 100644 index 96c6a7d..0000000 --- a/old/66493-h/images/v2map6lg.png +++ /dev/null diff --git a/old/66493-h/images/v2map7.png b/old/66493-h/images/v2map7.png Binary files differdeleted file mode 100644 index b130eaf..0000000 --- a/old/66493-h/images/v2map7.png +++ /dev/null diff --git a/old/66493-h/images/v2map7lg.png b/old/66493-h/images/v2map7lg.png Binary files differdeleted file mode 100644 index 262f0e3..0000000 --- a/old/66493-h/images/v2map7lg.png +++ /dev/null |
