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-} - -.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; -} -/* 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; -} -/* 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;} */ - -.fl_left {float: left; clear: left;} -.fl_right {float: right; clear: right;} /*padding-right: 4em;*/ - - -/* Footnotes */ -.footnote {margin-left: 8%; margin-right: 8%; font-size: 0.9em;} - -.footnote .label {position: absolute; right: 88%; text-align: right; font-size: 0.75em;} - -.fnanchor { - vertical-align: super; - font-size: .75em; - text-decoration: - none; -} - - -/* Transcriber's notes */ -.transnote {background-color: #E6E6FA; - color: black; - font-size:smaller; - padding:0.5em; - margin-bottom:5em; - font-family:sans-serif, serif; } - -.antiqua {font-family: "Old English Text MT", "Lucida Calligraphy", "Perpetua", "Times New Roman", - Arial, Helvetica, sans-serif; font-size: 1.25em;} - -sub, sup {font-size: 0.6em;} - -table.plateaus td {line-height: 0.9em; padding: 0 0.25em;} - - </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 I (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 I (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 #66492]</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 I (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. I, by Sir Archibald Geikie" /> -</div> - - - - - -<p><span class="pagenum" id="Page_i">- i -</span></p> - - -<p class="caption2nb">THE ANCIENT VOLCANOES OF GREAT BRITAIN</p> - -<p><span class="pagenum" id="Page_ii">- ii -</span></p> - -<div class="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. I</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> - -<p> <span class="pagenum" id="Page_iv">- iv -</span></p> - - -<p><span class="pagenum" id="Page_v">- v -</span></p> - - - -<p class="pmt4 pmb4 tdc"> -TO<br /> -<br /> -<span class="antiqua">M. Ferdinand Fouqué</span><br /> -<br /> -MEMBER OF THE INSTITUTE<br /> -<br /> -PROFESSOR OF THE NATURAL HISTORY OF INORGANIC BODIES<br /> -IN THE COLLÈGE DE FRANCE<br /> -<br /> -AND<br /> -<br /> -<span class="antiqua">M. Auguste Michel-Lévy</span><br /> -<br /> -MEMBER OF THE INSTITUTE<br /> -<br /> -DIRECTOR OF THE GEOLOGICAL SURVEY OF FRANCE<br /> -<br /> -DISTINGUISHED REPRESENTATIVES<br /> -OF THAT FRENCH SCHOOL OF GEOLOGY<br /> -WHICH BY THE HANDS OF DESMAREST FOUNDED THE<br /> -STUDY OF ANCIENT VOLCANOES<br /> -AND HAS SINCE DONE SO MUCH TO<br /> -PROMOTE ITS PROGRESS<br /> -THESE VOLUMES ARE INSCRIBED<br /> -WITH THE HIGHEST ADMIRATION AND<br /> -ESTEEM<br /> -</p> - - -<p><span class="pagenum" id="Page_vi">- vi -</span></p> -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_vii">- vii -</span></p> - -<h2 class="nobreak" id="PREFACE">PREFACE</h2> -</div> - - -<p>In no department of science is the slow and chequered progress of investigation -more conspicuous than in that branch of Geology which treats of -volcanoes. Although from the earliest dawn of history, men had been -familiar with the stupendous events of volcanic eruptions, they were -singularly slow in recognizing these phenomena as definite and important -parts of the natural history of the earth. Even within the present century, -the dominant geological school in Europe taught that volcanoes were mere -accidents, due to the combustion of subterranean beds of coal casually set -on fire by lightning, or by the decomposition of pyrites. Burning mountains, -as they were called, were believed to be only local and fortuitous appearances, -depending on the position of the coal-fields, and having no essential connection -with the internal structure and past condition of our planet. So long -as such fantastic conceptions prevailed, it was impossible that any solid -progress could be made in this branch of science. A juster appreciation of -the nature of the earth's interior was needed before men could recognize -that volcanic action had once been vigorous and prolonged in many countries, -where no remains of volcanoes can now be seen.</p> - -<p>To France, which has led the way in so many departments of human -inquiry, belongs the merit of having laid the foundations of the systematic -study of ancient volcanoes. Her groups of Puys furnished the earliest -inspiration in this subject, and have ever since been classic ground to which -the geological pilgrim has made his way from all parts of the world. As -far back as the year 1752, Guettard recognised that these marvellous hills -were volcanic cones that had poured forth streams of lava. But it was -reserved for Desmarest twelve years later to examine the question in detail, -and to establish the investigation of former volcanic action upon a broad -and firm basis of careful observation and sagacious inference. His method -of research was as well conceived as the region of Auvergne was admirably -fitted to be the field of exploration. He soon discovered that the volcanoes -<span class="pagenum" id="Page_viii">- viii -</span> -of Central France were not all of one age, but had made their appearance in -a long series, whereof the individual members became less perfect and distinct -in proportion to their antiquity. Beginning with the cones, craters, and -lava-streams which stand out so fresh that they might almost be supposed -to have been erupted only a few generations ago, Desmarest traced the -volcanic series backward in time, through successive stages of the decay and -degradation wrought upon them by the influence of the atmosphere, rain -and running water. He was thus able, as it were, to watch the gradual -obliteration of the cones, the removal of the ashes and scoriæ, and the -erosion of the lava-streams, until he could point to mere isolated remnants -of lava, perched upon the hills, and overlooking the valleys which had been -excavated through them. He showed how every step in this process of -denudation could be illustrated by examples of its occurrence in Auvergne, -and how, in this way, the various eruptions could be grouped according to -their place in the chronological sequence. To this illustrious Frenchman -geology is thus indebted, not only for the foundation of the scientific study -of former volcanic action, but for the first carefully worked out example of -the potency of subærial erosion in the excavation of valleys and the transformation -of the scenery of the land.</p> - -<p>While these fruitful researches were in progress in France, others of -hardly less moment were advancing in Scotland. There likewise Nature had -provided ample material to arrest the attention of all who cared to make -themselves acquainted with the past history of our globe. Hutton, as a -part of his immortal <i>Theory of the Earth</i>, had conceived the idea that -much molten material had been injected from below into the terrestrial -crust, and he had found many proofs of such intrusion among the rocks -alike of the Lowlands and Highlands of his native country. His observations, -confirmed and extended by Playfair and Hall, and subsequently by -Macculloch, opened up the investigation of the subterranean phases of -ancient volcanic action.</p> - -<p>Under the influence of these great pioneers, volcanic geology would have -made steady and perhaps rapid progress in the later decades of last century, -and the earlier years of the present, but for the theoretical views unfortunately -adopted by Werner. That illustrious teacher, to whom volcanoes seemed to -be a blot on the system of nature which he had devised, did all in his -power to depreciate their importance. Adopting the old and absurd notion -that they were caused by the combustion of coal under ground, he laboured -to show that they were mere modern accidents, and had no connection with -his universal formations. He proclaimed, as an obvious axiom in science, -that the basalts, so widely spread over Central and Western Europe, and -which the observations of Desmarest had shown to mark the sites of old -<span class="pagenum" id="Page_ix">- ix -</span> -volcanoes, were really chemical precipitates from a primeval universal ocean. -Yet he had actually before him in Saxony examples of basalt hills which -entirely disprove his assertions.</p> - -<p>Fortunately for the progress of natural knowledge, Werner disliked the -manual labour of penmanship. Consequently he wrote little. But his wide -range of acquirement, not in mineralogy only, his precision of statement, his -absolute certainty about the truth of his own opinions, and his hardly -disguised contempt for opinions that differed from them, combined with his -enthusiasm, eloquence and personal charm, fired his pupils with emulation -of his zeal and turned them into veritable propagandists. Misled as to the -structure of the country in which their master taught, and undisciplined to -investigate nature with an impartial mind, they travelled into other lands -for the purpose of applying there the artificial system which they had learnt -at Freiberg. The methodical but cumbrous terminology in which Werner -had trained them was translated by them into their own languages, where it -looked still more uncouth than in its native German. Besides imbibing -their teacher's system, they acquired and even improved upon his somewhat -disdainful manner towards all conclusions different from those of the Saxon -Mining School.</p> - -<p>Such was the spirit in which the pupils of Werner proceeded to set the -"geognosy" of Europe to rights. The views, announced by Desmarest, that -various rocks, far removed from any active volcano, were yet of volcanic origin, -had been slowly gaining ground when the militant students from Saxony -spread themselves over the Continent. These views, however, being irreconcilable -with the tenets enunciated from the Freiberg Chair, were now either -ignored or contemptuously rejected. Werner's disciples loved to call themselves -by their teacher's term "geognosts," and claimed that they confined themselves -to the strict investigation of fact with regard to the structure of the -earth, in apparent unconsciousness that their terminology and methods were -founded on baseless assumptions and almost puerile hypotheses.</p> - -<p>With such elements ready for controversy, it was no wonder that before -long a battle arose over the origin of basalt and the part played by volcanoes -in the past history of the globe. The disciples of Werner, champions of a -universal ocean and the deposition of everything from water, were dubbed -Neptunists, while their opponents, equally stubborn in defence of the potency -of volcanic fire, were known as Vulcanists or Plutonists. For more than a -generation this futile warfare was waged with extraordinary bitterness—dogmatism -and authority doing their best to stop the progress of impartial -observation and honest opinion.</p> - -<p>One of the most notable incidents in the campaign is to be found in the -way in which the tide of battle was at last turned against the Wernerians. -<span class="pagenum" id="Page_x">- x -</span> -Cuvier tells us that when some of the ardent upholders of the Freiberg faith -came to consult Desmarest, the old man, who took no part in the fray, would -only answer, "Go and see." He felt that in his memoir and maps he had -demonstrated the truth of his conclusions, and that an unprejudiced observer -had only to visit Auvergne to be convinced.</p> - -<p>By a curious irony of fate it was from that very Auvergne that the -light broke which finally chased away the Wernerian darkness, and it was -by two of Werner's most distinguished disciples that the reaction was -begun.</p> - -<p>Daubuisson, a favourite pupil of the Freiberg professor, had written and -published at Paris in 1803 a volume on the Basalts of Saxony, conceived -in the true Wernerian spirit, and treating these rocks, as he had been -taught to regard then, as chemical precipitates from a former universal -ocean. In the following year the young and accomplished Frenchman -went to Auvergne and the Vivarais that he might see with his own eyes -the alleged proofs of the volcanic origin of basalt. Greatly no doubt to his -own surprise, he found these proofs to be irrefragable. With praiseworthy -frankness he lost no time in publicly announcing his recantation of the -Wernerian doctrine on the subject, and ever afterwards he did good service -in making the cause of truth and progress prevail.</p> - -<p>Still more sensational was the conversion of a yet more illustrious -prophet of the Freiberg school—the great Leopold von Buch. He too -had been educated in the strictest Wernerian faith. But eventually, after -a journey to Italy, he made his way to Auvergne in 1802, and there, in -presence of the astonishing volcanic records of that region, the scales seem -to have fallen from his eyes also. With evident reluctance he began to -doubt his master's teaching in regard to basalt and volcanoes. He went into -raptures over the clear presentation of volcanic phenomena to be found in -Central France, traced each detail among the puys, as in the examination of -a series of vast models, and remarked that while we may infer what takes -place at Vesuvius, we can actually see what has transpired at the Puy de -Pariou. With the enthusiasm of a convert he rushed into the discussion -of the phenomena, but somehow omitted to make any mention of Desmarest, -who had taught the truth so many years before.</p> - -<p>Impressed by the example of such men as Daubuisson and Von Buch, -the Wernerian disciples gradually slackened in zeal for their master's tenets. -They clung to their errors longer perhaps in Scotland than anywhere else -out of Germany—a singular paradox only explicable by another personal -influence. Jameson, trained at Freiberg, carried thence to the University of -Edinburgh the most implicit acceptance of the tenets of the Saxon school, -and continued to maintain the aqueous origin of basalt for many years -<span class="pagenum" id="Page_xi">- xi -</span> -after the notion had been abandoned by some of his most distinguished -contemporaries. But the error, though it died hard, was confessed at last -even by Jameson.</p> - -<p>After the close of this protracted and animated controversy the study -of former volcanic action resumed its place among the accepted subjects -of geological research. From the peculiarly favourable structure of the -country, Britain has been enabled to make many important contributions -to the investigation of the subject. De la Beche, Murchison and Sedgwick -led the way in recognizing, even among the most ancient stratified formations -of England and Wales, the records of contemporaneous volcanoes and -of their subterranean intrusions. Scrope threw himself with ardour into -the study of the volcanoes of Italy and of Central France. Maclaren -made known the structure of some of the volcanic groups of the lowlands of -Scotland. Ramsay, Selwyn, and Jukes, following these pioneers, were the -first to map out a Palæozoic volcanic region in ample detail. Sorby, applying -to the study of rocks the method of microscopic examination by -thin slices, devised by William Nicol of Edinburgh for the study of fossil plants, -opened up a new and vast field in the domain of observational geology, and -furnished the geologist with a key to solve many of the problems of volcanism. -Thus, alike from the stratigraphical and petrographical sides, the -igneous rocks of this country have received constantly increasing attention.</p> - -<p>The present work is intended to offer a summary of what has now been -ascertained regarding the former volcanoes of the British Isles. The -subject has occupied much of my time and thought all through life. -Born among the crags that mark the sites of some of these volcanoes, I was -led in my boyhood to interest myself in their structure and history. The -fascination which they then exercised has lasted till now, impelling me to -make myself acquainted with the volcanic records all over our islands, -and to travel into the volcanic regions of Europe and Western America for -the purpose of gaining clearer conceptions of the phenomena.</p> - -<p>From time to time during a period of almost forty years I have communicated -chiefly to the Geological Society of London and the Royal Society -of Edinburgh the results of my researches. As materials accumulated, the -desire arose to combine them into a general narrative of the whole progress -of volcanic action from the remotest geological periods down to the time -when the latest eruptions ceased. An opportunity of partially putting this -design into execution occurred when, as President of the Geological Society, -the duty devolved upon me of giving the Annual Addresses in 1891 and -1892. Within the limits permissible to such essays, it was not possible to -present more than a full summary of the subject. Since that time I have -continued my researches in the field, especially among the Tertiary volcanic -<span class="pagenum" id="Page_xii">- xii -</span> -areas, and have now expanded the two Addresses by the incorporation -of a large amount of new matter and of portions of my published -papers.</p> - -<p>In the onward march of science a book which is abreast of our knowledge -to-day begins to be left behind to-morrow. Nevertheless it may -serve a useful purpose if it does no more than make a definite presentation -of the condition of that knowledge at a particular time. Such a statement -becomes a kind of landmark by which subsequent progress may be measured. -It may also be of service in indicating the gaps that have to be filled up, -and the fields where fresh research may most hopefully be undertaken.</p> - -<p>I have to thank the Councils of the Royal Society of Edinburgh and -the Geological Society for their permission to use a number of the illustrations -which have accompanied my papers published in their <i>Transactions</i> -and <i>Journal</i>. To Colonel Evans and Miss Thom of Canna I am indebted for the photographs -which they have kindly taken for me. To those of my colleagues -in the Geological Survey who have furnished me with information -my best thanks are due. Their contributions are acknowledged -where they have been made use of in the text.</p> - -<p>The illustrations of these volumes are chiefly from my own note-books -and sketch-books. But besides the photographs just referred to, I have -availed myself of a series taken by Mr. Robert Lunn for the Geological -Survey among the volcanic districts of Central Scotland.</p> - -<p class="tdl"> -<span class="smcap">Geological Survey Office</span>,<br /> -<span class="smcap" style="padding-left: 2em;">28 Jermyn Street, London</span>,<br /> -<span style="padding-left: 4em;"><i>1st January 1897</i></span>.</p> - - -<p><span class="pagenum" id="Page_xiii">- xiii -</span></p> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<h2 class="nobreak" id="CONTENTS">CONTENTS</h2> -</div> - -<table summary="TOC"> -<tr> - <td colspan="2"> - <p class="caption2">BOOK I<br /> - GENERAL PRINCIPLES AND METHODS OF INVESTIGATION</p> - <p class="caption2">CHAPTER I</p> - </td> -</tr> -<tr> - <td></td> - <td class="fl_right smaller"> PAGE </td> -</tr> -<tr> - <td class="tdl">Earliest Knowledge of Volcanoes—Their Influence on Mythology and Superstition—Part - taken by Volcanic Rocks in Scenery—Progress of the Denudation of Volcanoes—Value - of the Records of former Volcanoes as illustrating Modern Volcanic Action—Favourable - Position of Britain for the Study of this Subject</td> - <td class="vbot tdr"><a href="#CHAPTER_I">1</a></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">CHAPTER II</p></td> -</tr> -<tr> - <td><p class="tdc smcap">The Nature and Causes of Volcanic Action—Modern Volcanoes</p></td> - <td class="vbot tdr"><a href="#CHAPTER_II">10</a></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">CHAPTER III</p></td> -</tr> -<tr> - <td class="tdl">Ancient Volcanoes: Proofs of their existence derived from the Nature of the Rocks erupted - from the Earth's Interior. A. Materials erupted at the Surface—Extrusive Series. - i. Lavas, their General Characters. Volcanic Cycles. ii. Agglomerates, Breccias - and Tuffs</td> - <td class="vbot tdr"><a href="#CHAPTER_III">14</a></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">CHAPTER IV</p></td> -</tr> -<tr> - <td class="tdl">Materials erupted at the Surface—Extrusive Series—<i>continued</i>. iii. Types of old Volcanoes—1. - The Vesuvian Type; 2. The Plateau or Fissure Type; 3. The Puy Type. iv. - Determination of the relative Geological Dates of Ancient Volcanoes. v. How the - Physical Geography associated with Ancient Volcanoes is ascertained</td> - <td class="vbot tdr"><a href="#CHAPTER_IV">39</a></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">CHAPTER V</p></td> -</tr> -<tr> - <td class="tdl">Underground Phases of Volcanic Action. B. Materials injected or consolidated beneath the - Surface—Intrusive Series: I. Vents of Eruption—i. Necks of Fragmentary - Materials; ii. Necks of Lava-form Materials; iii. Distribution of Vents in relation to - Geological Structure-Lines; iv. Metamorphism in and around Volcanic Cones, Solfataric - <span class="pagenum" id="Page_xiv">- xiv -</span> - Action; v. Inward Dip of Rocks towards Necks; vi. Influence of contemporaneous - Denudation upon Volcanic Cones; vii. Stages in the History of old Volcanic - Vents</td> - <td class="vbot tdr"><a href="#CHAPTER_V">52</a></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">CHAPTER VI</p></td> -</tr> -<tr> - <td class="tdl">Underground Phases of Volcanic Action—<i>continued</i>. II. Subterranean Movements of the - Magma: i. Dykes and Veins; ii. Sills and Laccolites; iii. Bosses (Stocks, Culots), - Conditions that govern the Intrusion of Molten Rock within the Terrestrial Crust</td> - <td class="vbot tdr"><a href="#CHAPTER_VI">77</a></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">CHAPTER VII</p></td> -</tr> -<tr> - <td class="tdl">Influence of Volcanic Rocks on the Scenery of the Land—Effects of Denudation</td> - <td class="vbot tdr"><a href="#CHAPTER_VII">100</a></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">BOOK II<br /> - VOLCANIC ACTION IN PRE-CAMBRIAN TIME</p></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">CHAPTER VIII</p> - <p class="tdc smcap"><span class="smcap">Pre-Cambrian Volcanoes</span></p></td> -</tr> -<tr> - <td class="tdl">The Beginnings of Geological History—Difficulties in fixing on a generally applicable - Terminology—i. The Lewisian (Archæan) Gneiss; ii. The Dalradian or Younger - Schists of Scotland; iii. The Gneisses and Schists of Anglesey; iv. The Uriconian - Volcanoes; v. The Malvern Volcano; vi. The Charnwood Forest Volcano</td> - <td class="vbot tdr"><a href="#CHAPTER_VIII">109</a></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">BOOK III<br /> - THE CAMBRIAN VOLCANOES</p> - - <p class="caption2">CHAPTER IX</p> - <p class="smcap tdc">Characteristics of the Cambrian System in Britain</p></td> -</tr> -<tr> - <td class="tdl">The Physical Geography of the Cambrian Period—The Pioneers of Palæozoic Geology in - Britain—Work of the Geological Survey in Wales—Subdivisions of the Cambrian - System in Britain</td> - <td class="vbot tdr"><a href="#CHAPTER_IX">139</a></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">CHAPTER X</p></td> -</tr> -<tr> - <td class="tdl"><span class="smcap">The Cambrian Volcanoes of South Wales</span></td> - <td class="vbot tdr"><a href="#CHAPTER_X">145</a></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">CHAPTER XI</p></td> -</tr> -<tr> - <td class="tdl"><span class="smcap">The Cambrian Volcanoes of North Wales, the Malvern Hills and - Warwickshire</span></td> - <td class="vbot tdr"><a href="#CHAPTER_XI">159</a></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">BOOK IV<br /> - THE SILURIAN VOLCANOES</p> - - <p><span class="pagenum" id="Page_xv">- xv -</span></p></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">CHAPTER XII</p> - <p class="tdc"><span class="smcap">Characters of the Silurian System - in Britain. The Arenig Volcanoes</span></p></td> -</tr> -<tr> - <td class="tdl">The Land and Sea of Silurian time—Classification of the Silurian System—General Petrography - of the Silurian Volcanic Rocks—I. The Eruptions of Arenig Age</td> - <td class="vbot tdr"><a href="#CHAPTER_XII">173</a></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">CHAPTER XIII</p> - <p class="tdc"><span class="smcap">The Eruptions of Llandeilo and Bala Age</span></p></td> -</tr> -<tr> - <td class="tdl">i. The Builth Volcano—ii. The Volcanoes of Pembrokeshire—iii. The Caernarvonshire - Volcanoes of the Bala Period—iv. The Volcanic District of the Berwyn Hills—v. - The Volcanoes of Anglesey—vi. The Volcanoes of the Lake District; Arenig to close - of Bala Period—vii. Upper Silurian (?) Volcanoes of Gloucestershire</td> - <td class="vbot tdr"><a href="#CHAPTER_XIII">202</a></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">CHAPTER XIV</p></td> -</tr> -<tr> - <td class="tdl"><span class="smcap">The Silurian Volcanoes of Ireland</span></td> - <td class="vbot tdr"><a href="#CHAPTER_XIV">239</a></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">BOOK V<br /> - THE VOLCANOES OF DEVONIAN AND OLD RED SANDSTONE TIME</p> - - <p class="caption2">CHAPTER XV</p></td> -</tr> -<tr> - <td class="tdl"><span class="smcap">The Devonian Volcanoes</span></td> - <td class="vbot tdr"><a href="#CHAPTER_XV">257</a></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">CHAPTER XVI</p> - <p class="tdc"><span class="smcap">The Volcanoes of the Old Red Sandstone</span></p></td> -</tr> -<tr> - <td class="tdl">Geological Revolutions at the close of the Silurian Period Physical Geography of the Old - Red Sandstone—Old Lake-basins, their Flora and Fauna—Abundance of Volcanoes—History - of Investigation in the Subject</td> - <td class="vbot tdr"><a href="#CHAPTER_XVI">263</a></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">CHAPTER XVII</p> - <p><span class="pagenum" id="Page_xvi">- xvi -</span></p></td> -</tr> -<tr> - <td class="tdl"><span class="smcap">Distribution of the Volcanic Centres in the Lower Old Red Sandstone—Characters - of the Materials Erupted by the Volcanoes</span></td> - <td class="vbot tdr"><a href="#CHAPTER_XVII">271</a></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">CHAPTER XVIII</p></td> -</tr> -<tr> - <td class="tdl"><span class="smcap">Structure and Arrangement of the Lower Old Red Sandstone Volcanic - Rocks in the Field</span></td> - <td class="vbot tdr"><a href="#CHAPTER_XVIII">281</a></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">CHAPTER XIX</p> - <p class="tdl"><span class="smcap">Volcanoes of the Lower Old Red Sandstone of "Lake Caledonia"</span></p></td> -</tr> -<tr> - <td class="tdl">Description of the several Volcanic Districts: "Lake Caledonia," its Chains of Volcanoes—The - Northern Chain: Montrose Group—Ochil and Sidlaw Hills—the Arran and - Cantyre Centre—the Ulster Centre</td> - <td class="vbot tdr"><a href="#CHAPTER_XIX">294</a></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">CHAPTER XX</p> - <p class="tdc"><span class="smcap">Volcanoes of the Lower Old Red Sandstone of "Lake Caledonia"</span>—<i>continued</i></p></td> -</tr> -<tr> - <td class="tdl">The Southern Chain—The Pentland Volcano—The Biggar Centre—The Duneaton Centre—The - Ayrshire Volcanoes</td> - <td class="vbot tdr"><a href="#CHAPTER_XX">317</a></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">CHAPTER XXI</p></td> -</tr> -<tr> - <td class="tdl"><span class="smcap">Volcanoes of the Lower Old Red Sandstone of the Cheviot Hills, Lorne, - "Lake Orcadie" and Killarney</span></td> - <td class="vbot tdr"><a href="#CHAPTER_XXI">336</a></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">CHAPTER XXII</p></td> -</tr> -<tr> - <td class="tdl"><span class="smcap">Volcanoes of the Upper Old Red Sandstone—The South-West of Ireland, - the North of Scotland</span></td> - <td class="vbot tdr"><a href="#CHAPTER_XXII">348</a></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">BOOK VI<br /> - THE CARBONIFEROUS VOLCANOES</p> - - <p class="caption2">CHAPTER XXIII</p> - <p class="tdc"><span class="smcap">The Carboniferous System of Britain and its Volcanic Records</span></p></td> -</tr> -<tr> - <td class="tdl">Geography and Scenery of the Carboniferous Period—Range of Volcanic Eruptions during - that time—I. The Carboniferous Volcanoes of Scotland—Distribution, Arrangement - and Local Characters of the Carboniferous System in Scotland—Sketch of the Work - of previous Observers in this Subject</td> - <td class="vbot tdr"><a href="#CHAPTER_XXIII">355</a></td> -</tr> -<tr> - <td colspan="2"> - <p><span class="pagenum" id="Page_xvii">- xvii -</span></p> - - <p class="caption2">CHAPTER XXIV</p> - <p class="tdc"><span class="smcap">Carboniferous Volcanic Plateaux of Scotland</span></p></td> -</tr> -<tr> - <td class="tdl">I. The Plateau-type restricted to Scotland—i. Distribution in the Different Areas of - Eruption—ii. Nature of the Materials Erupted</td> - <td class="vbot tdr"><a href="#CHAPTER_XXIV">367</a></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">CHAPTER XXV</p> - <p class="tdc"><span class="smcap">Geological Structure of the Carboniferous Volcanic - Plateaux of Scotland</span></p></td> -</tr> -<tr> - <td class="tdl">1. Bedded Lavas and Tuffs; Upper Limits and Original Areas and Slopes of the Plateaux; - 2. Vents; Necks of Agglomerate and Tuff; Necks of Massive Rock; Composite - Necks; 3. Dykes and Sills; 4. Close of the Plateau-eruptions</td> - <td class="vbot tdr"><a href="#CHAPTER_XXV">383</a></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">CHAPTER XXVI</p> - <p class="tdc"><span class="smcap">The Carboniferous Puys of Scotland</span></p></td> -</tr> -<tr> - <td class="tdl">i. General Character and Distribution of the Puys; ii. Nature of the Materials Erupted—Lavas - Ejected at the Surface—Intrusive Sheets—Necks and Dykes—Tuffs</td> - <td class="vbot tdr"><a href="#CHAPTER_XXVI">414</a></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">CHAPTER XXVII</p> - <p class="tdc"><span class="smcap">Geological Structure of the Carboniferous Puys of Scotland</span></p></td> -</tr> -<tr> - <td class="tdl">1. Vents: Relation of the Necks to the Rocks through which they rise—Evidence of the - probable Subærial Character of some of the Cones or Puys of Tuff—Entombment of - the Volcanic Cones and their Relation to the Superficial Ejections. 2. Bedded Tuffs - and Lavas—Effects of Subsequent Dislocations. 3. Sills, Bosses, and Dykes</td> - <td class="vbot tdr"><a href="#CHAPTER_XXVII">424</a></td> -</tr> -<tr> - <td colspan="2"> - <p class="caption2">CHAPTER XXVIII</p> - <p class="tdc"><span class="smcap">Illustrative Examples of the Carboniferous Puys of Scotland</span></p></td> -</tr> -<tr> - <td class="tdl">The Basin of the Firth of Forth—North Ayrshire—Liddesdale</td> - <td class="vbot tdr"><a href="#CHAPTER_XXVIII">462</a></td> -</tr> -</table> - - -<p><span class="pagenum" id="Page_xviii">- xviii -</span></p> - -<hr class="chap x-ebookmaker-drop" /> - - -<div class="chapter"> -<p><span class="pagenum" id="Page_xix">- xix -</span></p> - -<h2 class="nobreak" id="LIST_OF_ILLUSTRATIONS">LIST OF ILLUSTRATIONS</h2> -</div> - - -<table class="tblcont" summary="Illustrations"> -<tr> - <td class="smaller">FIG.</td> - <td></td> - <td class="tdr smaller">PAGE</td> -</tr> -<tr> - <td class="tdr vtop">1.</td> - <td><p class="hanging">Vesicular structure, Lava from Ascension Island, slightly less than - natural size</p></td> - <td class="tdr vbot"><a href="#v1fig1">15</a></td> -</tr> -<tr> - <td class="tdr vtop">2.</td> - <td><p class="hanging">Elongation and branching of steam-vesicles in a lava, Kilninian, Isle - of Mull, a little less than natural size</p></td> - <td class="tdr vbot"><a href="#v1fig2">17</a></td> -</tr> -<tr> - <td class="tdr vtop">3.</td> - <td><p class="hanging">Microlites of the Pitchstone of Arran (magnified 70 diameters)</p></td> - <td class="tdr vbot"><a href="#v1fig3">19</a></td> -</tr> -<tr> - <td class="tdr vtop">4.</td> - <td><p class="hanging">Perlitic structure in Felsitic Glass, Isle of Mull (magnified)</p></td> - <td class="tdr vbot"><a href="#v1fig4">19</a></td> -</tr> -<tr> - <td class="tdr vtop">5.</td> - <td><p class="hanging">Spherulitic structure (magnified)</p></td> - <td class="tdr vbot"><a href="#v1fig5">19</a></td> -</tr> -<tr> - <td class="tdr vtop">6.</td> - <td><p class="hanging">Micropegmatitic or Granophyric structure in Granophyre, Mull (magnified)</p></td> - <td class="tdr vbot"><a href="#v1fig6">20</a></td> -</tr> -<tr> - <td class="tdr vtop">7.</td> - <td><p class="hanging">Ophitic structure in Dolerite, Gortacloghan, Co. Derry (magnified)</p></td> - <td class="tdr vbot"><a href="#v1fig7">20</a></td> -</tr> -<tr> - <td class="tdr vtop">8.</td> - <td><p class="hanging">Variolitic or orbicular structure, Napoleonite, Corsica (nat. size)</p></td> - <td class="tdr vbot"><a href="#v1fig8">22</a></td> -</tr> -<tr> - <td class="tdr vtop">9.</td> - <td><p class="hanging">Flow-structure in Rhyolite, Antrim, slightly reduced</p></td> - <td class="tdr vbot"><a href="#v1fig9">23</a></td> -</tr> -<tr> - <td class="tdr vtop">10.</td> - <td><p class="hanging">Lumpy, irregular trachytic lava-streams (Carboniferous), East Linton, - Haddingtonshire</p></td> - <td class="tdr vbot"><a href="#v1fig10">24</a></td> -</tr> -<tr> - <td class="tdr vtop">11.</td> - <td><p class="hanging">View at the entrance of the Svinofjord, Faroe Islands, illustrating the - terraced forms assumed by basic lavas</p></td> - <td class="tdr vbot"><a href="#v1fig11">25</a></td> -</tr> -<tr> - <td class="tdr vtop">12.</td> - <td><p class="hanging">Sack-like or pillow-form structure of basic lavas (Lower Silurian), - Bennan Head, Ballantrae, Ayrshire</p></td> - <td class="tdr vbot"><a href="#v1fig12">26</a></td> -</tr> -<tr> - <td class="tdr vtop">13.</td> - <td><p class="hanging">Alternations of coarser and finer Tuff</p></td> - <td class="tdr vbot"><a href="#v1fig13">34</a></td> -</tr> -<tr> - <td class="tdr vtop">14.</td> - <td><p class="hanging">Alternations of Tuff with non-volcanic sediment</p></td> - <td class="tdr vbot"><a href="#v1fig14">35</a></td> -</tr> -<tr> - <td class="tdr vtop">15.</td> - <td><p class="hanging">Ejected block of basalt which has fallen among Carboniferous shales - and limestones, shore, Pettycur, Fife</p></td> - <td class="tdr vbot"><a href="#v1fig15">37</a></td> -</tr> -<tr> - <td class="tdr vtop">16.</td> - <td><p class="hanging">Effects of denudation on a Vesuvian cone</p></td> - <td class="tdr vbot"><a href="#v1fig16">40</a></td> -</tr> -<tr> - <td class="tdr vtop">17.</td> - <td><p class="hanging">Section to illustrate the structure of the Plateau type</p></td> - <td class="tdr vbot"><a href="#v1fig17">43</a></td> -</tr> -<tr> - <td class="tdr vtop">18.</td> - <td ><p class="hanging">Diagram illustrating the structure and denudation of Puys</p></td> - <td class="tdr vbot"><a href="#v1fig18">45</a></td> -</tr> -<tr> - <td class="tdr vtop">19.</td> - <td ><p class="hanging">Section illustrating submarine eruptions; alternations of lavas and - tuffs with limestones and shales full of marine organisms</p></td> - <td class="tdr vbot"><a href="#v1fig19">48</a></td> -</tr> -<tr> - <td class="tdr vtop">20.</td> - <td ><p class="hanging">Diagram illustrating volcanic eruptions on a river-plain</p></td> - <td class="tdr vbot"><a href="#v1fig20">49</a></td> -</tr> -<tr> - <td class="tdr vtop">21.</td> - <td ><p class="hanging">Diagram illustrating volcanic eruptions on a land-surface</p></td> - <td class="tdr vbot"><a href="#v1fig21">50</a></td> -</tr> -<tr> - <td class="tdr vtop">22.</td> - <td ><p class="hanging">Ground-plans of some volcanic vents from the Carboniferous districts - of Scotland</p></td> - <td class="tdr vbot"><a href="#v1fig22">55</a></td> -</tr> -<tr> - <td class="tdr vtop">23.</td> - <td ><p class="hanging">View of an old volcanic "Neck" (The Knock, Largs, Ayrshire, a vent - of Lower Carboniferous age)</p></td> - <td class="tdr vbot"><a href="#v1fig23">56</a></td> -</tr> -<tr> - <td class="tdr vtop">24.</td> - <td ><p class="hanging">Section of neck of agglomerate, rising through sandstones and shales</p></td> - <td class="tdr vbot"><a href="#v1fig24">58</a></td> -</tr> -<tr> - <td class="tdr vtop">25.</td> - <td ><p class="hanging">Neck filled with stratified tuff</p></td> - <td class="tdr vbot"><a href="#v1fig25">64</a></td> -</tr> -<tr> - <td class="tdr vtop">26.</td> - <td ><p class="hanging">Section of neck of agglomerate with plug of lava</p></td> - <td class="tdr vbot"><a href="#v1fig26">65</a></td> -</tr> -<tr> - <td class="tdr vtop">27.</td> - <td ><p class="hanging">Section of agglomerate neck with dykes and veins</p></td> - <td class="tdr vbot"><a href="#v1fig27">66</a></td> -</tr> -<tr> - <td class="tdr vtop">28.</td> - <td ><p class="hanging">Section of neck filled with massive rock</p></td> - <td class="tdr vbot"><a href="#v1fig28">68</a></td> -</tr> -<tr> - <td class="tdr vtop">29.</td> - <td ><p class="hanging">Successive shiftings of vents giving rise to double or triple cones</p></td> - <td class="tdr vbot"><a href="#v1fig29">70</a></td> -</tr> -<tr> - <td class="tdr vtop">30.</td> - <td ><p class="hanging">Section to show the connection of a neck with a cone and surrounding - bedded tuffs</p></td> - <td class="tdr vbot"><a href="#v1fig30">71</a></td> -</tr> -<tr> - <td class="tdr vtop">31.</td> - <td ><p class="hanging">Diagram illustrating the gradual emergence of buried volcanic cones - through the influence of prolonged denudation - <span class="pagenum" id="Page_xx">- xx -</span></p></td> - <td class="tdr vbot"><a href="#v1fig31">75</a></td> -</tr> -<tr> - <td class="tdr vtop">32.</td> - <td ><p class="hanging">Dyke, Vein, and Sill</p></td> - <td class="tdr vbot"><a href="#v1fig32">80</a></td> -</tr> -<tr> - <td class="tdr vtop">33.</td> - <td ><p class="hanging">Section of Sill or Intrusive Sheet</p></td> - <td class="tdr vbot"><a href="#v1fig33">83</a></td> -</tr> -<tr> - <td class="tdr vtop">34.</td> - <td ><p class="hanging">Ideal section of three Laccolites. (After Mr. Gilbert)</p></td> - <td class="tdr vbot"><a href="#v1fig34">86</a></td> -</tr> -<tr> - <td class="tdr vtop">35.</td> - <td ><p class="hanging">Diagram illustrating the stratigraphical relations of the pre-Cambrian - and Cambrian rocks of the North-west Highlands of Scotland</p></td> - <td class="tdr vbot"><a href="#v1fig35">112</a></td> -</tr> -<tr> - <td class="tdr vtop">36.</td> - <td ><p class="hanging">Map of a portion of the Lewisian gneiss of Ross-shire</p></td> - <td class="tdr vbot"><a href="#v1fig36">118</a></td> -</tr> -<tr> - <td class="tdr vtop">37.</td> - <td ><p class="hanging">Section showing the position of sills in the mica-schist series - between Loch Tay and Amulree</p></td> - <td class="tdr vbot"><a href="#v1fig37">124</a></td> -</tr> -<tr> - <td class="tdr vtop">38.</td> - <td ><p class="hanging">Sketch of crushed basic igneous rock among the schists, E. side of - Porth-tywyn-mawr, E. side of Holyhead Straits</p></td> - <td class="tdr vbot"><a href="#v1fig38">128</a></td> -</tr> -<tr> - <td class="tdr vtop">39.</td> - <td ><p class="hanging">Section across the Uriconian series of Caer Caradoc</p></td> - <td class="tdr vbot"><a href="#v1fig39">132</a></td> -</tr> -<tr> - <td class="tdr vtop">40.</td> - <td ><p class="hanging">Map of the volcanic district of St. David's</p></td> - <td class="tdr vbot"><a href="#v1fig40">146</a></td> -</tr> -<tr> - <td class="tdr vtop">41.</td> - <td ><p class="hanging">Section showing the interstratification of tuff and conglomerate - above Lower Mill, St. David's</p></td> - <td class="tdr vbot"><a href="#v1fig41">154</a></td> -</tr> -<tr> - <td class="tdr vtop">42.</td> - <td ><p class="hanging">Basic dyke traversing quartz-porphyry and converted into a kind of - slate by cleavage. West side of Llyn Padarn</p></td> - <td class="tdr vbot"><a href="#v1fig42">162</a></td> -</tr> -<tr> - <td class="tdr vtop">43.</td> - <td ><p class="hanging">Section of well-cleaved tuff, grit and breccia passing up into - rudely-cleaved conglomerate and well-bedded cleaved fine - conglomerate and grit. East side of Llyn Padarn</p></td> - <td class="tdr vbot"><a href="#v1fig43">163</a></td> -</tr> -<tr> - <td class="tdr vtop">44.</td> - <td ><p class="hanging">Section of Clegyr on the north-east side of Llyn Padarn, near the - lower end</p></td> - <td class="tdr vbot"><a href="#v1fig44">164</a></td> -</tr> -<tr> - <td class="tdr vtop">45.</td> - <td ><p class="hanging">Section across the Cambrian formations of the Malvern Hills, showing - the position of the intercalated igneous rocks. After Phillips</p></td> - <td class="tdr vbot"><a href="#v1fig45">170</a></td> -</tr> -<tr> - <td class="tdr vtop">46.</td> - <td ><p class="hanging">Section across Rhobell Fawr</p></td> - <td class="tdr vbot"><a href="#v1fig46">178</a></td> -</tr> -<tr> - <td class="tdr vtop">47.</td> - <td ><p class="hanging">Section at the Slate Quarry, Penrhyn Gwyn, north slopes of Cader Idris</p></td> - <td class="tdr vbot"><a href="#v1fig47">180</a></td> -</tr> -<tr> - <td class="tdr vtop">48.</td> - <td ><p class="hanging">Sketch-section across Cader Idris</p></td> - <td class="tdr vbot"><a href="#v1fig48">182</a></td> -</tr> -<tr> - <td class="tdr vtop">49.</td> - <td ><p class="hanging">Section across the Moelwyn Range</p></td> - <td class="tdr vbot"><a href="#v1fig49">185</a></td> -</tr> -<tr> - <td class="tdr vtop">50.</td> - <td ><p class="hanging">Section across the anticline of Corndon</p></td> - <td class="tdr vbot"><a href="#v1fig50">190</a></td> -</tr> -<tr> - <td class="tdr vtop">51.</td> - <td ><p class="hanging">Structure in finely-amygdaloidal diabase lava, south of mouth of - Stinchar River, Ayrshire</p></td> - <td class="tdr vbot"><a href="#v1fig51">193</a></td> -</tr> -<tr> - <td class="tdr vtop">52.</td> - <td ><p class="hanging">View of Knockdolian Hill from the east</p></td> - <td class="tdr vbot"><a href="#v1fig52">194</a></td> -</tr> -<tr> - <td class="tdr vtop">53.</td> - <td ><p class="hanging">Section across the Lower Silurian volcanic series in the south of - Ayrshire (B. N. Peach)</p></td> - <td class="tdr vbot"><a href="#v1fig53">197</a></td> -</tr> -<tr> - <td class="tdr vtop">54.</td> - <td ><p class="hanging">Section of part of the Arenig volcanic group, stream south of Bennane - Head, Ayrshire</p></td> - <td class="tdr vbot"><a href="#v1fig54">198</a></td> -</tr> -<tr> - <td class="tdr vtop">55.</td> - <td ><p class="hanging">Flow-structure in the lowest felsite on the track from Llanberis to - the top of Snowdon</p></td> - <td class="tdr vbot"><a href="#v1fig55">211</a></td> -</tr> -<tr> - <td class="tdr vtop">56.</td> - <td ><p class="hanging">Section of Snowdon</p></td> - <td class="tdr vbot"><a href="#v1fig56">212</a></td> -</tr> -<tr> - <td class="tdr vtop">57.</td> - <td ><p class="hanging">Section across the Berwyn Hills. (Reduced from Horizontal Section, - Geol. Surv. Sheet 35)</p></td> - <td class="tdr vbot"><a href="#v1fig57">219</a></td> -</tr> -<tr> - <td class="tdr vtop">58.</td> - <td ><p class="hanging">Section of the strata on the shore at Porth Wen, west of Amlwch</p></td> - <td class="tdr vbot"><a href="#v1fig58">223</a></td> -</tr> -<tr> - <td class="tdr vtop">59.</td> - <td ><p class="hanging">Section of intercalated black shale in the volcanic series at Porth - yr hwch, south of Carmel Point, Anglesey</p></td> - <td class="tdr vbot"><a href="#v1fig59">224</a></td> -</tr> -<tr> - <td class="tdr vtop">60.</td> - <td ><p class="hanging">Green slates overlain with volcanic breccia, Carmel Point</p></td> - <td class="tdr vbot"><a href="#v1fig60">224</a></td> -</tr> -<tr> - <td class="tdr vtop">61.</td> - <td ><p class="hanging">Blue shale or slate passing into volcanic breccia east of Porth - Padrig, near Carmel Point</p></td> - <td class="tdr vbot"><a href="#v1fig61">225</a></td> -</tr> -<tr> - <td class="tdr vtop">62.</td> - <td ><p class="hanging">Section of felsites in the Coniston Limestone group, west of Stockdale</p></td> - <td class="tdr vbot"><a href="#v1fig62">232</a></td> -</tr> -<tr> - <td class="tdr vtop">63.</td> - <td ><p class="hanging">Fine tuff with coarser bands near Quayfoot Quarries, Borrowdale</p></td> - <td class="tdr vbot"><a href="#v1fig63">234</a></td> -</tr> -<tr> - <td class="tdr vtop">64.</td> - <td ><p class="hanging">Diagram of the general relations of the different groups of rock in - the Lower Silurian volcanic district along the western shore of - Lough Mask</p></td> - <td class="tdr vbot"><a href="#v1fig64">253</a></td> -</tr> -<tr> - <td class="tdr vtop">65.</td> - <td ><p class="hanging">Veins and nests of sandstone due to the washing of sand into fissures - and cavities of an Old Red Sandstone lava. Turnberry Point, Ayrshire</p></td> - <td class="tdr vbot"><a href="#v1fig65">283</a></td> -</tr> -<tr> - <td class="tdr vtop">66.</td> - <td ><p class="hanging">Ground-plan of reticulated cracks in the upper surface of an Old Red - Sandstone lava filled in with sandstone. Red Head, Forfarshire</p></td> - <td class="tdr vbot"><a href="#v1fig66">284</a></td> -</tr> -<tr> - <td class="tdr vtop">67.</td> - <td ><p class="hanging">Section across the volcanic series of Forfarshire - <span class="pagenum" id="Page_xxi">- xxi -</span></p></td> - <td class="tdr vbot"><a href="#v1fig67">286</a></td> -</tr> -<tr> - <td class="tdr vtop">68.</td> - <td ><p class="hanging">Section across two necks above Tillicoultry, Ochil Hills</p></td> - <td class="tdr vbot"><a href="#v1fig68">288</a></td> -</tr> -<tr> - <td class="tdr vtop">69.</td> - <td ><p class="hanging">Section of the granite core between Merrick and Corscrine</p></td> - <td class="tdr vbot"><a href="#v1fig69">290</a></td> -</tr> -<tr> - <td class="tdr vtop">70.</td> - <td ><p class="hanging">Section across the three Dirrington Laws, Berwickshire</p></td> - <td class="tdr vbot"><a href="#v1fig70">291</a></td> -</tr> -<tr> - <td class="tdr vtop">71.</td> - <td ><p class="hanging">Section of Papa Stour, Shetlands, showing sill of spherulitic felsite - traversing Old Red Sandstone and bedded porphyrites (Messrs. Peach - and Horne)</p></td> - <td class="tdr vbot"><a href="#v1fig71">292</a></td> -</tr> -<tr> - <td class="tdr vtop">72.</td> - <td ><p class="hanging">Section across Northmavine, from Okrea Head to Skea Ness, Shetland, - showing dykes and connected sill of granite and felsite (Messrs. - Peach and Horne)</p></td> - <td class="tdr vbot"><a href="#v1fig72">292</a></td> -</tr> -<tr> - <td class="tdr vtop">73.</td> - <td ><p class="hanging">Section at the edge of one of the bays of Lower Old Red Sandstone along - the northern margin of Lake Caledonia, near Ochtertyre</p></td> - <td class="tdr vbot"><a href="#v1fig73">295</a></td> -</tr> -<tr> - <td class="tdr vtop">74.</td> - <td ><p class="hanging">Craig Beinn-nan-Eun (2067 feet), east of Uam Var, Braes of Doune. Old Red - Conglomerate, with the truncated ends of the strata looking across into - the Highlands; moraines of Corry Beach in the foreground</p></td> - <td class="tdr vbot"><a href="#v1fig74">296</a></td> -</tr> -<tr> - <td class="tdr vtop">75.</td> - <td ><p class="hanging">Section showing the top of the volcanic series at the foot of the - precipice of the Red Head, Forfarshire</p></td> - <td class="tdr vbot"><a href="#v1fig75">300</a></td> -</tr> -<tr> - <td class="tdr vtop">76.</td> - <td ><p class="hanging">Andesite with sandstone veinings and overlying conglomerate. Todhead, - south of Caterline, coast of Kincardineshire</p></td> - <td class="tdr vbot"><a href="#v1fig76">303</a></td> -</tr> -<tr> - <td class="tdr vtop">77.</td> - <td ><p class="hanging">Section across the Boundary-fault of the Highlands at Glen Turrit, - Perthshire</p></td> - <td class="tdr vbot"><a href="#v1fig77">305</a></td> -</tr> -<tr> - <td class="tdr vtop">78.</td> - <td ><p class="hanging">Section across the chain of the Sidlaw Hills near Kilspindie</p></td> - <td class="tdr vbot"><a href="#v1fig78">306</a></td> -</tr> -<tr> - <td class="tdr vtop">79.</td> - <td ><p class="hanging">Section across the Eastern Ochil Hills from near Newburgh to near - Auchtermuchty</p></td> - <td class="tdr vbot"><a href="#v1fig79">307</a></td> -</tr> -<tr> - <td class="tdr vtop">80.</td> - <td ><p class="hanging">Generalized section across the heart of the Ochil Hills from Dunning on - the north to the Fife coal-field near Saline on the south</p></td> - <td class="tdr vbot"><a href="#v1fig80">308</a></td> -</tr> -<tr> - <td class="tdr vtop">81.</td> - <td ><p class="hanging">Diagram of the volcanic series of the Western Ochil Hills</p></td> - <td class="tdr vbot"><a href="#v1fig81">309</a></td> -</tr> -<tr> - <td class="tdr vtop">82.</td> - <td ><p class="hanging">View of Cnoc Garbh, Southend, Campbeltown. A volcanic neck of Lower Old - Red Sandstone age, about 400 yards wide in its longer diameter</p></td> - <td class="tdr vbot"><a href="#v1fig82">312</a></td> -</tr> -<tr> - <td class="tdr vtop">83.</td> - <td ><p class="hanging">Section of volcanic series on beach, Southend, Campbeltown</p></td> - <td class="tdr vbot"><a href="#v1fig83">313</a></td> -</tr> -<tr> - <td class="tdr vtop">84.</td> - <td ><p class="hanging">Section of the base of the volcanic series, Reclain, five miles south of - Pomeroy</p></td> - <td class="tdr vbot"><a href="#v1fig84">316</a></td> -</tr> -<tr> - <td class="tdr vtop">85.</td> - <td ><p class="hanging">Section of shales and breccias at Crossna Chapel, north-east of Boyle</p></td> - <td class="tdr vbot"><a href="#v1fig85">316</a></td> -</tr> -<tr> - <td class="tdr vtop">86.</td> - <td ><p class="hanging">Section across the north end of the Pentland Hills, from Warklaw Hill to - Pentland Mains. Length about five miles</p></td> - <td class="tdr vbot"><a href="#v1fig86">318</a></td> -</tr> -<tr> - <td class="tdr vtop">87.</td> - <td ><p class="hanging">View of the lava-escarpments of Warklaw Hill, Pentland chain, from the - north-west</p></td> - <td class="tdr vbot"><a href="#v1fig87">319</a></td> -</tr> -<tr> - <td class="tdr vtop">88.</td> - <td ><p class="hanging">Section across the Pentland Hills through North Black Hill and Scald Law. - Length about three miles</p></td> - <td class="tdr vbot"><a href="#v1fig88">322</a></td> -</tr> -<tr> - <td class="tdr vtop">89.</td> - <td ><p class="hanging">Section from the valley of the Gutterford Burn through Green Law and Braid - Law to Eight-Mile Burn</p></td> - <td class="tdr vbot"><a href="#v1fig89">322</a></td> -</tr> -<tr> - <td class="tdr vtop">90.</td> - <td ><p class="hanging">Section across the north end of the Pentland Hills, and the southern edge - of the Braid Hill vent. Length about two miles</p></td> - <td class="tdr vbot"><a href="#v1fig90">324</a></td> -</tr> -<tr> - <td class="tdr vtop">91.</td> - <td ><p class="hanging">Section across the northern end of the Biggar volcanic group, from Fadden - Hill to beyond Mendick Hill</p></td> - <td class="tdr vbot"><a href="#v1fig91">326</a></td> -</tr> -<tr> - <td class="tdr vtop">92.</td> - <td ><p class="hanging">Section across the southern part of the Biggar volcanic group from - Covington to Culter</p></td> - <td class="tdr vbot"><a href="#v1fig92">328</a></td> -</tr> -<tr> - <td class="tdr vtop">93.</td> - <td ><p class="hanging">Section from Thankerton Moor across Tinto to Lamington</p></td> - <td class="tdr vbot"><a href="#v1fig93">328</a></td> -</tr> -<tr> - <td class="tdr vtop">94.</td> - <td ><p class="hanging">Section across the Duneaton volcanic district from the head of the - Duneaton Water to Kirklea Hill</p></td> - <td class="tdr vbot"><a href="#v1fig94">330</a></td> -</tr> -<tr> - <td class="tdr vtop">95.</td> - <td ><p class="hanging">Cavernous spaces in andesite, filled in with sandstone, John o' Groats - Port, Turnberry, Ayrshire</p></td> - <td class="tdr vbot"><a href="#v1fig95">333</a></td> -</tr> -<tr> - <td class="tdr vtop">96.</td> - <td ><p class="hanging">Section of andesites, Turnberry Castle, Ayrshire</p></td> - <td class="tdr vbot"><a href="#v1fig96">334</a></td> -</tr> -<tr> - <td class="tdr vtop">97.</td> - <td ><p class="hanging">Lenticular form of a brecciated andesite (shown in <a href="#v1fig96">Fig. 96</a>), Turnberry, - Ayrshire</p></td> - <td class="tdr vbot"><a href="#v1fig97">334</a></td> -</tr> -<tr> - <td class="tdr vtop">98.</td> - <td ><p class="hanging">Section across the volcanic area of St. Abb's Head (after Prof. J. Geikie)</p></td> - <td class="tdr vbot"><a href="#v1fig98">339</a></td> -</tr> -<tr> - <td class="tdr vtop">99.</td> - <td ><p class="hanging">View of terraced andesite hills resting on massive conglomerate, south of - Oban</p></td> - <td class="tdr vbot"><a href="#v1fig99">341</a></td> -</tr> -<tr> - <td class="tdr vtop">100.</td> - <td ><p class="hanging">Section of lava-escarpment at Beinn Lora, north side of mouth of Loch - Etive, Argyllshire</p></td> - <td class="tdr vbot"><a href="#v1fig100">342</a></td> -</tr> -<tr> - <td class="tdr vtop">101.</td> - <td ><p class="hanging">Section across Strathbogie, below Rhyme, showing the position of the - volcanic band - <span class="pagenum" id="Page_xxii">- xxii -</span></p></td> - <td class="tdr vbot"><a href="#v1fig101">344</a></td> -</tr> -<tr> - <td class="tdr vtop">102.</td> - <td ><p class="hanging">View of Knockfeerina, Limerick, from the north-east—a volcanic neck of - Upper Old Red Sandstone age</p></td> - <td class="tdr vbot"><a href="#v1fig102">349</a></td> -</tr> -<tr> - <td class="tdr vtop">103.</td> - <td ><p class="hanging">Section of the volcanic zone in the Upper Old Red Sandstone, Cam of Hoy, - Orkney</p></td> - <td class="tdr vbot"><a href="#v1fig103">351</a></td> -</tr> -<tr> - <td class="tdr vtop">104.</td> - <td ><p class="hanging">Section of the volcanic zone in the Upper Old Red Sandstone at Black - Ness, Rackwick, Hoy</p></td> - <td class="tdr vbot"><a href="#v1fig104">351</a></td> -</tr> -<tr> - <td class="tdr vtop">105.</td> - <td ><p class="hanging">Section across the volcanic band and its associated necks, Hoy, Orkney</p></td> - <td class="tdr vbot"><a href="#v1fig105">352</a></td> -</tr> -<tr> - <td class="tdr vtop">106.</td> - <td ><p class="hanging">Ground-plan of volcanic neck piercing the Caithness Flagstone series on - the beach near John o' Groat's House</p></td> - <td class="tdr vbot"><a href="#v1fig106">353</a></td> -</tr> -<tr> - <td class="tdr vtop">107.</td> - <td ><p class="hanging">View of the escarpment of the Clyde Plateau in the Little Cumbrae, from - the south-west</p></td> - <td class="tdr vbot"><a href="#v1fig107">368</a></td> -</tr> -<tr> - <td class="tdr vtop">108.</td> - <td ><p class="hanging">View of the edge of the Volcanic Plateau south of Campbeltown, - Argyllshire</p></td> - <td class="tdr vbot"><a href="#v1fig108">370</a></td> -</tr> -<tr> - <td class="tdr vtop">109.</td> - <td ><p class="hanging">View of North Berwick Law from the east, a phonolite neck marking one of - the chief vents of the Garleton Plateau. (From a photograph)</p></td> - <td class="tdr vbot"><a href="#v1fig109">371</a></td> -</tr> -<tr> - <td class="tdr vtop">110.</td> - <td ><p class="hanging">The Bass Rock, a trachytic neck belonging to the Garleton plateau, from - the shore at Canty Bay</p></td> - <td class="tdr vbot"><a href="#v1fig110">372</a></td> -</tr> -<tr> - <td class="tdr vtop">111.</td> - <td ><p class="hanging">Corston Hill—a fragment of the Midlothian Plateau, seen from the north</p></td> - <td class="tdr vbot"><a href="#v1fig111">373</a></td> -</tr> -<tr> - <td class="tdr vtop">112.</td> - <td ><p class="hanging">View of Arthur Seat from Calton Hill to the north</p></td> - <td class="tdr vbot"><a href="#v1fig112">374</a></td> -</tr> -<tr> - <td class="tdr vtop">113.</td> - <td ><p class="hanging">View of Arkleton Fell, part of the Solway Plateau, from the south-west</p></td> - <td class="tdr vbot"><a href="#v1fig113">376</a></td> -</tr> -<tr> - <td class="tdr vtop">114.</td> - <td ><p class="hanging">Vertical sections of the escarpment of the Clyde plateau from north-east - to south-west</p></td> - <td class="tdr vbot"><a href="#v1fig114">384</a></td> -</tr> -<tr> - <td class="tdr vtop">115.</td> - <td ><p class="hanging">Section of Craiglockhart Hill, Edinburgh</p></td> - <td class="tdr vbot"><a href="#v1fig115">387</a></td> -</tr> -<tr> - <td class="tdr vtop">116.</td> - <td ><p class="hanging">Section of the bottom of the Midlothian Plateau, Linnhouse Water above - Mid-Calder Oilworks</p></td> - <td class="tdr vbot"><a href="#v1fig116">387</a></td> -</tr> -<tr> - <td class="tdr vtop">117.</td> - <td ><p class="hanging">Section of the top of the Midlothian Plateau in the Murieston Water</p></td> - <td class="tdr vbot"><a href="#v1fig117">388</a></td> -</tr> -<tr> - <td class="tdr vtop">118.</td> - <td ><p class="hanging">Section of Calton Hill, Edinburgh</p></td> - <td class="tdr vbot"><a href="#v1fig118">389</a></td> -</tr> -<tr> - <td class="tdr vtop">119.</td> - <td ><p class="hanging">Cliff of tuff and agglomerate, east side of Oxroad Bay, a little east - from Tantallon Castle, East Lothian</p></td> - <td class="tdr vbot"><a href="#v1fig119">391</a></td> -</tr> -<tr> - <td class="tdr vtop">120.</td> - <td ><p class="hanging">Section across part of the Clyde Plateau to the west of Bowling (reduced - from Sheet 6 of the Horizontal Sections of the Geological Survey of - Scotland)</p></td> - <td class="tdr vbot"><a href="#v1fig120">392</a></td> -</tr> -<tr> - <td class="tdr vtop">121.</td> - <td ><p class="hanging">Diagram illustrating the thinning away southwards of the lavas of the - Clyde Plateau between Largs and Ardrossan. Length about 10 miles</p></td> - <td class="tdr vbot"><a href="#v1fig121">393</a></td> -</tr> -<tr> - <td class="tdr vtop">122.</td> - <td ><p class="hanging">Diagram illustrating the thinning away eastwards of the lavas of the - Clyde Plateau in the Fintry Hills. Length about 12 miles</p></td> - <td class="tdr vbot"><a href="#v1fig122">394</a></td> -</tr> -<tr> - <td class="tdr vtop">123.</td> - <td ><p class="hanging">View of the two necks Dumgoyn and Dumfoyn, Stirlingshire, taken from - the south</p></td> - <td class="tdr vbot"><a href="#v1fig123">395</a></td> -</tr> -<tr> - <td class="tdr vtop">124.</td> - <td ><p class="hanging">Ground-plan of Plateau-vents near Strathblane, Stirlingshire, on the - scale of 6 inches to a mile</p></td> - <td class="tdr vbot"><a href="#v1fig124">395</a></td> -</tr> -<tr> - <td class="tdr vtop">125.</td> - <td ><p class="hanging">Ground-plans of double and triple necks in the Plateau series, on the - scale of 6 inches to a mile</p></td> - <td class="tdr vbot"><a href="#v1fig125">396</a></td> -</tr> -<tr> - <td class="tdr vtop">126.</td> - <td ><p class="hanging">Ground-plan of tuff-neck, shore east of Dunbar</p></td> - <td class="tdr vbot"><a href="#v1fig126">398</a></td> -</tr> -<tr> - <td class="tdr vtop">127.</td> - <td ><p class="hanging">Section across the vents Dumgoyn and Dumfoyn, and the edge of the Clyde - plateau above Strathblane, Stirlingshire</p></td> - <td class="tdr vbot"><a href="#v1fig127">400</a></td> -</tr> -<tr> - <td class="tdr vtop">128.</td> - <td ><p class="hanging">Section through the large vent of the Campsie Hills</p></td> - <td class="tdr vbot"><a href="#v1fig128">400</a></td> -</tr> -<tr> - <td class="tdr vtop">129.</td> - <td ><p class="hanging">Diagrammatic section across the central vent of the Clyde plateau in - Renfrewshire</p></td> - <td class="tdr vbot"><a href="#v1fig129">400</a></td> -</tr> -<tr> - <td class="tdr vtop">130.</td> - <td ><p class="hanging">Section across Southern Berwickshire, to show the relation of the - volcanic plateau to the vents lying south from it</p></td> - <td class="tdr vbot"><a href="#v1fig130">401</a></td> -</tr> -<tr> - <td class="tdr vtop">131.</td> - <td ><p class="hanging">Section of south end of Dumbuck Hill. East of Dumbarton</p></td> - <td class="tdr vbot"><a href="#v1fig131">403</a></td> -</tr> -<tr> - <td class="tdr vtop">132.</td> - <td ><p class="hanging">Section across the East Lothian plateau, to show the relative position - of one of the necks</p></td> - <td class="tdr vbot"><a href="#v1fig132">403</a></td> -</tr> -<tr> - <td class="tdr vtop">133.</td> - <td ><p class="hanging">View of Traprain Law from the south, a phonolite neck of the Garleton - Plateau</p></td> - <td class="tdr vbot"><a href="#v1fig133">405</a></td> -</tr> -<tr> - <td class="tdr vtop">134.</td> - <td ><p class="hanging">Veins and dykes traversing the agglomerate and tuff of the great - Renfrewshire vent</p></td> - <td class="tdr vbot"><a href="#v1fig134">408</a></td> -</tr> -<tr> - <td class="tdr vtop">135.</td> - <td ><p class="hanging">"The Yellow Man," a dyke in volcanic tuff and conglomerate on the shore - a little east of North Berwick</p></td> - <td class="tdr vbot"><a href="#v1fig135">409</a></td> -</tr> -<tr> - <td class="tdr vtop">136.</td> - <td ><p class="hanging">Trachytic sills, Knockvadie, Kilpatrick Hills - <span class="pagenum" id="Page_xxiii">- xxiii -</span></p></td> - <td class="tdr vbot"><a href="#v1fig136">410</a></td> -</tr> -<tr> - <td class="tdr vtop">137.</td> - <td ><p class="hanging">Section across the edge of the Clyde plateau, south-east of Beith</p></td> - <td class="tdr vbot"><a href="#v1fig137">411</a></td> -</tr> -<tr> - <td class="tdr vtop">138.</td> - <td ><p class="hanging">Section across the upper part of the Clyde plateau at Kilbirnie, Ayrshire</p></td> - <td class="tdr vbot"><a href="#v1fig138">411</a></td> -</tr> -<tr> - <td class="tdr vtop">139.</td> - <td ><p class="hanging">Section across the upper surface of the Clyde volcanic plateau, Burnhead, - north-west of Kilsyth</p></td> - <td class="tdr vbot"><a href="#v1fig139">412</a></td> -</tr> -<tr> - <td class="tdr vtop">140.</td> - <td ><p class="hanging">Section across the upper surface of the Clyde volcanic plateau at Campsie</p></td> - <td class="tdr vbot"><a href="#v1fig140">412</a></td> -</tr> -<tr> - <td class="tdr vtop">141.</td> - <td ><p class="hanging">Section across western edge of the Garlton plateau</p></td> - <td class="tdr vbot"><a href="#v1fig141">412</a></td> -</tr> -<tr> - <td class="tdr vtop">142.</td> - <td ><p class="hanging">Section across the Solway plateau</p></td> - <td class="tdr vbot"><a href="#v1fig142">413</a></td> -</tr> -<tr> - <td class="tdr vtop">143.</td> - <td ><p class="hanging">Section of volcanic vent at East Grange, Perthshire coal-field, - constructed by Mr. B. N. Peach from the rocks exposed in a - railway-cutting, and from plans of ironstone- and coal-pits</p></td> - <td class="tdr vbot"><a href="#v1fig143">426</a></td> -</tr> -<tr> - <td class="tdr vtop">144.</td> - <td ><p class="hanging">View of the Binn of Burntisland—a volcanic neck of agglomerate</p></td> - <td class="tdr vbot"><a href="#v1fig144">428</a></td> -</tr> -<tr> - <td class="tdr vtop">145.</td> - <td ><p class="hanging">View of part of the cliffs of vertical agglomerate, Binn of Burntisland</p></td> - <td class="tdr vbot"><a href="#v1fig145">431</a></td> -</tr> -<tr> - <td class="tdr vtop">146.</td> - <td ><p class="hanging">Diagram of buried volcanic cone near Dalry, Ayrshire. Constructed from - information obtained in mining operations</p></td> - <td class="tdr vbot"><a href="#v1fig146">434</a></td> -</tr> -<tr> - <td class="tdr vtop">147.</td> - <td ><p class="hanging">Diagram to illustrate how Volcanic Necks may be concealed and exposed</p></td> - <td class="tdr vbot"><a href="#v1fig147">434</a></td> -</tr> -<tr> - <td class="tdr vtop">148.</td> - <td ><p class="hanging">Section across the Saline Hills, Fife</p></td> - <td class="tdr vbot"><a href="#v1fig148">435</a></td> -</tr> -<tr> - <td class="tdr vtop">149.</td> - <td ><p class="hanging">Section across the Binn of Burntisland, in an east and west direction</p></td> - <td class="tdr vbot"><a href="#v1fig149">436</a></td> -</tr> -<tr> - <td class="tdr vtop">150.</td> - <td ><p class="hanging">Section in old quarry, west of Wester Ochiltree, Linlithgowshire. - Calciferous Sandstone series</p></td> - <td class="tdr vbot"><a href="#v1fig150">437</a></td> -</tr> -<tr> - <td class="tdr vtop">151.</td> - <td ><p class="hanging">Ejected volcanic block in Carboniferous strata, Burntisland</p></td> - <td class="tdr vbot"><a href="#v1fig151">438</a></td> -</tr> -<tr> - <td class="tdr vtop">152.</td> - <td ><p class="hanging">View of volcanic agglomerate becoming finer above east end of Kingswood - Craig, two miles east from Burntisland</p></td> - <td class="tdr vbot"><a href="#v1fig152">439</a></td> -</tr> -<tr> - <td class="tdr vtop">153.</td> - <td ><p class="hanging">Alternations of basalt and tuff, with shale, etc., of Kingswood Craig, - Burntisland</p></td> - <td class="tdr vbot"><a href="#v1fig153">441</a></td> -</tr> -<tr> - <td class="tdr vtop">154.</td> - <td ><p class="hanging">Section of the upper surface of a diabase ("leckstone") sheet, Skolie - Burn, south-east of Bathgate</p></td> - <td class="tdr vbot"><a href="#v1fig154">443</a></td> -</tr> -<tr> - <td class="tdr vtop">155.</td> - <td ><p class="hanging">Section across the volcanic ridge of the Linlithgow and Bathgate Hills, - showing the intercalation of limestones that mark important - stratigraphical horizons</p></td> - <td class="tdr vbot"><a href="#v1fig155">444</a></td> -</tr> -<tr> - <td class="tdr vtop">156.</td> - <td ><p class="hanging">Section in Wardlaw Quarry, Linlithgowshire</p></td> - <td class="tdr vbot"><a href="#v1fig156">445</a></td> -</tr> -<tr> - <td class="tdr vtop">157.</td> - <td ><p class="hanging">Section from Linlithgow Loch to the Firth of Forth</p></td> - <td class="tdr vbot"><a href="#v1fig157">446</a></td> -</tr> -<tr> - <td class="tdr vtop">158.</td> - <td ><p class="hanging">Section across the Campsie Fells illustrating the contrast between the - sills below and above the plateau-lavas</p></td> - <td class="tdr vbot"><a href="#v1fig158">447</a></td> -</tr> -<tr> - <td class="tdr vtop">159.</td> - <td ><p class="hanging">Section showing the position of the basic sills in relation to the - volcanic series at Burntisland, Fife</p></td> - <td class="tdr vbot"><a href="#v1fig159">448</a></td> -</tr> -<tr> - <td class="tdr vtop">160.</td> - <td ><p class="hanging">Sills between shales and sandstones, Hound Point, Linlithgowshire</p></td> - <td class="tdr vbot"><a href="#v1fig160">449</a></td> -</tr> -<tr> - <td class="tdr vtop">161.</td> - <td ><p class="hanging">Section of Sill, Cramond Railway, Barnton, near Edinburgh</p></td> - <td class="tdr vbot"><a href="#v1fig161">450</a></td> -</tr> -<tr> - <td class="tdr vtop">162.</td> - <td ><p class="hanging">Intrusive dolerite sheet enclosing and sending threads into portions - of shale, Salisbury Crags, Edinburgh</p></td> - <td class="tdr vbot"><a href="#v1fig162">452</a></td> -</tr> -<tr> - <td class="tdr vtop">163.</td> - <td ><p class="hanging">Intrusive sheet invading limestone and shale, Dodhead Quarry, near - Burntisland</p></td> - <td class="tdr vbot"><a href="#v1fig163">452</a></td> -</tr> -<tr> - <td class="tdr vtop">164.</td> - <td ><p class="hanging">Spheroidal weathering of dolerite sill, quarry east of North Queensferry, - Fife.</p></td> - <td class="tdr vbot"><a href="#v1fig164">455</a></td> -</tr> -<tr> - <td class="tdr vtop">165.</td> - <td ><p class="hanging">Two thin sills of "white trap" injected into black carbonaceous shale - overlying the Hurlet Limestone, Hillhouse Quarry, Linlithgow</p></td> - <td class="tdr vbot"><a href="#v1fig165">456</a></td> -</tr> -<tr> - <td class="tdr vtop">166.</td> - <td ><p class="hanging">Dyke cutting the agglomerate of a neck. Binn of Burntisland</p></td> - <td class="tdr vbot"><a href="#v1fig166">457</a></td> -</tr> -<tr> - <td class="tdr vtop">167.</td> - <td ><p class="hanging">Boss of diabase cutting the Burdiehouse Limestone and sending sills and - veins into the overlying shales. Railway cutting, West Quarry, East - Calder, Midlothian</p></td> - <td class="tdr vbot"><a href="#v1fig167">458</a></td> -</tr> -<tr> - <td class="tdr vtop">168.</td> - <td ><p class="hanging">Side of columnar basalt-dyke in the same agglomerate as in <a href="#v1fig166">Fig. 166</a></p></td> - <td class="tdr vbot"><a href="#v1fig168">459</a></td> -</tr> -<tr> - <td class="tdr vtop">169.</td> - <td ><p class="hanging">Dyke rising through the Hurlet Limestone and its overlying shales. - Silvermine Quarry, Linlithgowshire</p></td> - <td class="tdr vbot"><a href="#v1fig169">460</a></td> -</tr> -<tr> - <td class="tdr vtop">170.</td> - <td ><p class="hanging">Junction of amygdaloidal basalt with shales and limestone, shore, half - a mile east from Kinghorn, Fife</p></td> - <td class="tdr vbot"><a href="#v1fig170">464</a></td> -</tr> -<tr> - <td class="tdr vtop">171.</td> - <td ><p class="hanging">Columnar basalt, Pettycur, Kinghorn, Fife - <span class="pagenum" id="Page_xxiv">- xxiv -</span></p></td> - <td class="tdr vbot"><a href="#v1fig171">469</a></td> -</tr> -<tr> - <td class="tdr vtop">172.</td> - <td ><p class="hanging">Section across the Fife band of Sills</p></td> - <td class="tdr vbot"><a href="#v1fig172">473</a></td> -</tr> -<tr> - <td class="tdr vtop">173.</td> - <td ><p class="hanging">Section across the upper volcanic band of north Ayrshire. Length about - four miles</p></td> - <td class="tdr vbot"><a href="#v1fig173">474</a></td> -</tr> -<tr> - <td class="tdr vtop">174.</td> - <td ><p class="hanging">Section showing the connection of the two volcanic bands in Liddesdale</p></td> - <td class="tdr vbot"><a href="#v1fig174">476</a></td> -</tr> -<tr> - <td class="tdr vtop">175.</td> - <td ><p class="hanging">Diagram to show the position of a mass of Upper Old Red Sandstone which - has fallen into the great vent near Tudhope Hill, east of Mosspaul</p></td> - <td class="tdr vbot"><a href="#v1fig175">476</a></td> -</tr> -</table> - -<h2>MAPS</h2> - -<table style="width: 42em;" summary="Maps"> -<tr> - <td class="tdr vtop">I.</td> - <td class="tdl">General map of the Volcanic districts of the British Isles—<a href="#Map_I">At the end of - the volume</a></td> -</tr> -<tr> - <td class="tdr vtop">II.</td> - <td class="tdl">Map of the Cambrian and Silurian volcanic region of North Wales - <span class="fl_right"><a href="#Map_II">To face p. 256</a></span></td> -</tr> -<tr> - <td class="tdr vtop">III.</td> - <td class="tdl">Map of the Old Red Sandstone volcanic region of "Lake Caledonia"<br /> - in Central Scotland and North Ireland - <span class="fl_right"><a href="#Map_III">To face p. 334</a></span></td> -</tr> -<tr> - <td class="tdr vtop">IV.</td> - <td class="tdl">Map of the Carboniferous volcanic districts of Scotland - <span class="fl_right"><a href="#Map_IV">To face p. 476</a></span></td> -</tr> -</table> - - -<hr class="chap x-ebookmaker-drop" /> - - -<div class="chapter"> -<p><span class="pagenum" id="Page_1">- 1 -</span></p> - -<h2 class="nobreak" id="BOOK_I">BOOK I<br /> - -<span class="smaller">GENERAL PRINCIPLES AND METHODS OF INVESTIGATION</span></h2> -</div> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<h2 class="nobreak" id="CHAPTER_I">CHAPTER I</h2> -</div> - -<div class="blockquot"> - -<p>Earliest Knowledge of Volcanoes—Their Influence on Mythology and Superstition—Part -taken by Volcanic Rocks in Scenery—Progress of the Denudation of Volcanoes—Value -of the Records of former Volcanoes as illustrating Modern Volcanic Action—Favourable -Position of Britain for the Study of this Subject.</p> -</div> - - -<p>Among the influences which affected the infancy of mankind, the most -potent were those of environment. Whatever in outer nature stimulated or -repressed courage, inventiveness, endurance, whatever tended to harden or -to weaken the bodily faculties, whatever appealed to the imagination or -excited the fancy, became a powerful factor in human development.</p> - -<p>Thus, in the dawn of civilization, the frequent recurrence of earthquakes -and volcanic eruptions throughout the basin of the Mediterranean could not -but have a marked effect on the peoples that dwelt by the borders of that -sea. While every part of the region was from time to time shaken by -underground commotion, there were certain places that became specially -noteworthy for the wonder and terror of their catastrophes. When, after -successive convulsions, vast clouds of black smoke rose from a mountain and -overspread the sky, when the brightness of noon was rapidly replaced by -the darkness of midnight, when the air grew thick with stifling dust and a -rain of stones and ashes fell from it on all the surrounding country, when -streams of what looked like liquid fire poured forth and desolated gardens, -vineyards, fields and villages—then did men feel sure that the gods were -angry. The contrast between the peacefulness and beauty of the ordinary -landscape and the hideous warfare of the elements at these times of volcanic -fury could not but powerfully impress the imagination and give a colour to -early human conceptions of nature and religion.</p> - -<p>It was not only in one limited district that these manifestations of -underground convulsion showed themselves. The islands of the Ægean -had their volcanoes, and the Greeks who dwelt among them watched their -<span class="pagenum" id="Page_2">- 2 -</span> -glowing fires by night and their clouds of steam by day, culminating now -and then in a stupendous explosion, like that which, in prehistoric time, -destroyed the island of Santorin. As the islanders voyaged eastward they -would see, on the coast of Asia Minor, the black bristling lavas of the -"Burnt Country," perhaps even then flowing from their rugged heaps of -cinders. Or when, more adventurously still, they sailed westward into the -Tyrrhenian waters, they beheld the snowy cone of Etna, with its dark -canopy of smoke and the lurid nocturnal gleam of its fires; while from time -to time they witnessed there on a still more stupendous scale the horrors of -a great volcanic eruption.</p> - -<p>From all sides, therefore, the early Greek voyagers would carry back -to the mother-country marvellous tales of convulsion and disaster. They -would tell how the sky rapidly darkened even in the blaze of mid-day, how -the land was smothered with dust and stones, how over the sea there spread -such a covering of ashes that the oarsmen could hardly drive their vessels -onward, how red-hot stones, whirling high overhead, rained down on sails and -deck, and crushed or burnt whatever they fell upon, and how, as the earth -shook and the sea rose in sudden waves and the mountain gave forth an -appalling din of constant explosion, it verily seemed that the end of the -world had come.</p> - -<p>To the actual horrors of such scenes there could hardly fail to be added -the usual embellishments of travellers' tales. Thus, in the end, the volcanoes -of the Mediterranean basin came to play a not unimportant part in Hellenic -mythology. They seemed to stand up as everlasting memorials of the -victory of Zeus over the giants and monsters of an earlier time. And as -the lively Greek beheld Mount Etna in eruption, his imagination readily -pictured the imprisoned Titan buried under the burning roots of the mountain, -breathing forth fire and smoke, and convulsing the country far and -near, as he turned himself on his uneasy pallet.</p> - -<p>When in later centuries the scientific spirit began to displace the -popular and mythological interpretation of natural phenomena, the existence -of volcanoes and their extraordinary phenomena offered a fruitful field for -speculation and conjecture. As men journeyed outward from the Mediterranean -cradle of civilization, they met with volcanic manifestations in -many other parts of the world. When they eventually penetrated into the -Far East, they encountered volcanoes on a colossal scale and in astonishing -abundance. When they had discovered the New World they learnt that, in -that hemisphere also, "burning mountains" were numerous and of gigantic -dimensions. Gradually it was ascertained that vast lines of volcanic activity -encircle the globe. By slow degrees the volcano was recognized to be as -normal a part of the mechanism of our planet as the rivers that flow on the -terrestrial surface. And now at last men devote themselves to the task of -critically watching the operations of volcanoes with as much enthusiasm as -they display in the investigation of any other department of nature. They -feel that their knowledge of the earth extends to little beyond its mere outer -skin, and that the mystery which still hangs over the vast interior of the -<span class="pagenum" id="Page_3">- 3 -</span> -planet can only, if ever, be dispelled by the patient study of these vents of -communication between the interior and the surface.</p> - -<p>If, however, we desire to form some adequate idea of the part which -volcanic action has played in the past history of the earth, we should be -misled were we to confine our attention to the phenomena of the eruptions -of the present day. An attentive examination of any modern volcano will -convince us that of some of the most startling features of an eruption no -enduring memorial remains. The convulsive earthquakes that accompany a -great volcanic paroxysm, unless where they actually fissure the ground, leave -little or no trace behind them. Lamentably destructive as they are to -human life and property, the havoc which they work is mostly superficial. -In a year or two the ruins have been cleared away, the earth-falls have been -healed over, the prostrated trees have been removed, and, save in the memories -and chronicles of the inhabitants, no record of the catastrophe may survive. -The clouds of dust and showers of ashes which destroyed the crops and -crushed in the roofs of houses soon disappear from the air, and the covering -which they leave over the surface of a district gradually mingles with -the soil. Vegetation eventually regains its place, and the landscape becomes -again as smiling as before.</p> - -<p>Even where the materials thrown out from the crater accumulate in -much greater mass, where thick deposits of ashes or solid sheets of lava bury -the old land-surface, the look of barren desolation, though in some cases it -may endure for long centuries, may in others vanish in a few years. The -surface-features of the district are altered indeed, but the new topography -soon ceases to look new. Another generation of inhabitants loses recollection -of the old landmarks, and can hardly realize that what has become so -familiar to itself differs so much from what was familiar to its fathers.</p> - -<p>But even when the volcanic covering, thus thrown athwart a wide tract -of country, has been concealed under a new growth of soil and vegetation, it -still remains a prey to the ceaseless processes of decay and degradation which -everywhere affect the surface of the land. No feature of a modern -volcano is more impressive than the lesson which it conveys of the reality -and potency of this continual waste. The northern slopes of Vesuvius, for -example, are trenched with deep ravines, which in the course of centuries -have been dug out of the lavas and tuffs of Monte Somma by rain and -melted snow. Year by year these chasms are growing deeper and wider, -while the ridges between them are becoming narrower. In some cases, -indeed, the intervening ridges have been reduced to sharp crests which are -split up and lowered by the unceasing influence of the weather. The slopes -of such a volcanic cone have been aptly compared to a half-opened umbrella. -It requires little effort of imagination to picture a time, by no means -remote in a geological sense, when, unless renovated by the effects of fresh -eruptions, the cone will have been so levelled with the surrounding country -that the peasants of the future will trail their vines and build their cots over -the site of the old volcano, in happy ignorance of what has been the history -of the ground beneath their feet.</p> - -<p><span class="pagenum" id="Page_4">- 4 -</span></p> - -<p>What is here predicted as probable or certain in the future has -undoubtedly happened again and again in the past. Over many districts of -Europe and Western America extinct volcanoes may be seen in every stage -of decay. The youngest may still show, perfect and bare of vegetation, their -cones and their craters, with the streams of lava that escaped from them. -Those of older date have been worn down into mere low rounded hills, or -the whole cone has been cleared away, and there is only left the hard core -of material that solidified in the funnel below the surface. The lava-sheets -have been cut through by streams, and now remain in mere scattered patches -capping detached hills, which only a trained eye can recognize as relics of -a once continuous level sheet of solid rock.</p> - -<p>By this resistless degradation, a volcanic district is step by step stripped -of every trace of its original surface. All that the eruptions did to change -the face of the landscape may be entirely obliterated. Cones and craters, -ashes and lavas, may be gradually effaced. And yet enough may be left to -enable a geologist to make sure that volcanic action was once rife there. -As the volcano marks a channel of direct communication between the -interior of the earth and the atmosphere outside, there are subterranean as -well as superficial manifestations of its activity, and while the latter are -removed by denudation, the former are one by one brought into light. The -progress of denudation is a process of dissection, whereby every detail in the -structure of a volcano is successively cut down and laid bare. But for this -process, our knowledge of the mechanism and history of volcanic action -would be much less full and definite than happily it is. In active volcanoes -the internal and subterranean structure can only be conjectured; in those -of ancient date, which have been deeply eroded, this underground structure -is open to the closest examination.</p> - -<p>By gathering together evidence of this nature over the surface of the -globe, we learn that abundantly as still active volcanoes are distributed on -that surface, they form but a small fraction of the total number of vents -which have at various times been in eruption. In Italy, for example, while -Vesuvius is active on the mainland, and Etna, Stromboli and Volcano -display their vigour among the islands, there are scores of old volcanoes that -have been silent and cold ever since the beginning of history, yet show by -their cones of cinders and streams of bristling lava that they were energetic -enough in their day. But the Italian volcanic region is only one of many -to be found on the European Continent. If we travel eastward into Hungary, -or northward into the Eifel, or into the heart of France, we encounter -abundant cones and craters, many of them so fresh that, though there is no -historical record of their activity, they look as if they had been in eruption -only a few generations ago.</p> - -<p>But when the geologist begins to search among rocks of still older date -than these comparatively recent volcanic memorials, he meets with abundant -relics of far earlier eruptions. And as he arranges the chronicles of the -earth's history, he discovers that each section of the long cycle of geological -ages has preserved its records of former volcanoes. In a research of this -<span class="pagenum" id="Page_5">- 5 -</span> -kind he can best realize how much he owes to the process of denudation. -The volcanic remains of former geological periods have in most cases been -buried under younger deposits, and have sunk sometimes thousands of feet -below the level of the sea. They have been dislocated and upheaved again -during successive commotions of the terrestrial crust, and have at last been -revealed by the gradual removal of the pile of material under which they -had lain.</p> - -<p>Hence we learn that the active volcanoes of the present time, which -really embrace but a small part of the volcanic history of our planet, are -the descendants of a long line of ancestors. Their distribution and activity -should be considered not merely from the evidence they themselves supply, -but in the light derived from a study of that ancestry. It is only when we -take this broad view of the subject that we can be in a position to form -some adequate conception of the nature and history of volcanoes in the -geological evolution of the globe.</p> - -<p>In this research it is obvious that the presently active volcano must be -the basis and starting-point of inquiry. At that channel of communication -between the unknown inside and the familiar outside of our globe, we can -watch what takes place in times of quiescence or of activity. We can -there study each successive phase of an eruption, measure temperatures, -photograph passing phenomena, collect gases and vapours, register the fall -of ashes or the flow of lavas, and gather a vast body of facts regarding the -materials that are ejected from the interior, and the manner of their -emission.</p> - -<p>Indispensable as this information is for the comprehension of volcanic -action, it obviously affords after all but a superficial glimpse of that action. -We cannot see beyond the bottom of the crater. We cannot tell anything -about the subterranean ducts, or how the molten and fragmental materials -behave in them. All the underground mechanism of volcanoes is necessarily -hidden from our eyes. But much of this concealed structure has -been revealed in the case of ancient volcanic masses, which have been buried -and afterwards upraised and laid bare by denudation.</p> - -<p>In yet another important aspect modern volcanoes do not permit us to -obtain full knowledge of the subject. The terrestrial vents, from which we -derive our information, by no means represent all the existing points of -direct connection between the interior and the exterior of the planet. We -know that some volcanic eruptions occur under the sea, and doubtless vast -numbers more take place there of which we know nothing. But the conditions -under which these submarine discharges are effected, the behaviour -of the outflowing lava under a body of oceanic water, and the part -played by fragmentary materials in the explosions, can only be surmised. -Now and then a submarine volcano pushes its summit above the sea-level, -and allows its operations to be seen, but in so doing it becomes practically -a terrestrial volcano, and the peculiar submarine phenomena are still effectually -concealed from observation.</p> - -<p>The volcanic records of former geological periods, however, are in large -<span class="pagenum" id="Page_6">- 6 -</span> -measure those of eruptions under the sea. In studying them we are permitted, -as it were, to explore the sea-bottom. We can trace how sheets of -coral and groves of crinoids were buried under showers of ashes and stones, -and how the ooze and silt of the sea-floor were overspread with streams of -lava. We are thus, in some degree, enabled to realize what must now -happen over many parts of the bed of the existing ocean.</p> - -<p>The geologist who undertakes an investigation into the history of -volcanic action within the area of the British Isles during past time, with a -view to the better comprehension of this department of terrestrial physics, -finds himself in a situation of peculiar advantage. Probably no region on -the face of the globe is better fitted than these islands to furnish a large -and varied body of evidence regarding the progress of volcanic energy in -former ages. This special fitness may be traced to four causes—1st, The remarkable -completeness of the geological record in Britain; 2nd, The geographical -position of the region on the oceanic border of a continent; 3rd, The singularly -ample development to be found there of volcanic rocks belonging to a -long succession of geological ages; and 4th, The extent to which this full -chronicle of volcanic activity has been laid bare by denudation.</p> - -<p>1. In the first place, the geological record of Britain is singularly complete. -It has often been remarked how largely all the great periods of -geological time are represented within the narrow confines of these islands. -The gaps in the chronicle are comparatively few, and for the most part are -not of great moment.</p> - -<p>Thanks to the restricted area of the country and to the large number of -observers, this remarkably full record of geological history has been studied -with a minute care which has hardly been equalled in any other country. -The detailed succession of all the formations has been so fully determined -in Britain that the very names first applied here to them and to their subdivisions -have in large measure passed into the familiar language of geology -all over the globe. Every definite platform in the stratigraphical series -has been more or less fully worked out. A basis has thus been laid for -referring each incident in the geological history of the region to its proper -relative date.</p> - -<p>2. In the second place, the geographical position of Britain gives it -a notable advantage in regard to the manifestations of volcanic energy. -Rising from the margin of a great ocean-basin and extending along the edge -of a continent, these islands have lain on that critical border-zone of the -terrestrial surface, where volcanic action is apt to be most vigorous and -continuous. It has long been remarked that volcanoes are generally placed -not far from the sea. From the earliest geological periods the site of -Britain, even when submerged below the sea, has never lain far from the -land which supplied the vast accumulations of sediment that went to form -the Palæozoic and later formations, while, on the other hand, it frequently -formed part of the land of former geological periods. It was thus most -favourably situated as a theatre for both terrestrial and submarine volcanic -activity.</p> - -<p><span class="pagenum" id="Page_7">- 7 -</span></p> - -<p>3. In the third place, this advantageous geographical position is found -to have been attended with an altogether remarkable abundance and persistence -of volcanic eruptions. No tract of equal size yet known on the -face of the globe furnishes so ample a record of volcanic activity from the -earliest geological periods down into Tertiary time. Every degree of -energy may be signalized in that record, from colossal eruptions which piled -up thousands of feet of rock down to the feeblest discharge of dust and -stones. Every known type of volcano is represented—great central cones -like Etna or Vesuvius, scattered groups of small cones like the <i>puys</i> of -France, and fissure- or dyke-eruptions like those of recent times in Iceland.</p> - -<p>Moreover, the accurate manner in which the stratigraphy of the country -has been established permits each successive era in the long volcanic history -to be precisely determined, and allows us to follow the whole progress of -that history stage by stage, from the beginning to the end.</p> - -<p>These characteristics may be instructively represented on a map, such -as that which accompanies the present volume (<a href="#Map_I">Map I.</a>). The reader will -there observe how repeatedly volcanic eruptions have taken place, not -merely within the general area of the British Isles, but even within the -same limited region of that area. The broad midland valley of Scotland -has been especially the theatre for their display. From the early part of -the Lower Silurian period, through the ages of the Old Red Sandstone, -Carboniferous and Permian systems, hundreds of volcanic vents were active -in that region, while in long subsequent time there came the fissure-eruptions -of the Tertiary series.</p> - -<p>4. In the fourth place, the geological revolutions of successive ages -have made this long volcanic chronicle fully accessible to observation. Had -the lavas and ashes of one period remained buried under the sedimentary -accumulations of the next, their story would have been lost to us. We -should only have been able to decipher the latest records which might -happen to lie on the surface. Fortunately for the progress of geology, -the endless vicissitudes of a continental border have brought up the very -oldest rocks once more to the surface. All the later formations of the -earth's crust have likewise been upraised and exposed to denudation during -long cycles of time. In this manner, the rocky framework of the country -has been laid bare, and each successive chapter of its geological history may -be satisfactorily deciphered. The singularly complete volcanic chronicle, -after being entombed under younger deposits, has been broken up and -raised once more into view. The active vents of former periods have been -dissected, submarine streams of lava have been uncovered, sheets of ashes -that fell over the sea-bottom have been laid bare. The progress of denudation -is specially favoured in such a variable and moist climate as that -of Britain, and thus by the co-operation of underground and meteoric causes -the marvellous volcanic records of this country have been laid open in -minutest detail.</p> - -<p>There is yet another respect in which the volcanic geology of Britain -possesses a special value. Popular imagination has long been prone to see -<span class="pagenum" id="Page_8">- 8 -</span> -signs of volcanic action in the more prominent rocky features of landscape. -A bold crag, a deep and precipitous ravine, a chasm in the side of a mountain, -have been unhesitatingly set down as proof of volcanic disturbance. -Many a cauldron-shaped recess, like the corries of Scotland or the cwms -of Wales, has been cited as an actual crater, with its encircling walls still -standing almost complete.</p> - -<p>The relics of former volcanoes in this country furnish ample proofs to -dispel these common misconceptions. They show that not a single crater -anywhere remains, save where it has been buried under lava; that no trace -of the original cones has survived, except in a few doubtful cases where -they may have been preserved under subsequent accumulations of material; -that in the rugged tracts, where volcanic action has been thought to have -been most rife, there may be not a vestige of it, while, on the other hand, -where the uneducated eye would never suspect the presence of any remnant -of volcanic energy, lavas and ashes may abound. We are thus presented -with some of the most impressive contrasts in geological history, while, at -the same time, this momentous lesson is borne in upon the mind, that the -existing inequalities in the configuration of a landscape are generally due -far less to the influence of subterranean force than to the action of the -superficial agents which are ceaselessly carving the face of the land. -Those rocks which from their hardness or structure are best able to withstand -that destruction rise into prominence, while the softer material around -them is worn away. Volcanic rocks are no exception to this rule, as the -geological structure of Britain amply proves.</p> - -<p>In the following chapters, forming Book I. of this work, I propose to -begin by offering some general remarks regarding the nature and causes of -volcanic action, so far as these are known to us. I shall then proceed to -consider the character of the evidence that may be expected to be met with -respecting the former prevalence of that action at any particular locality -where volcanic disturbances have long since ceased. The most telling evidence -of old volcanoes is naturally to be found in the materials which they -have left behind them, and the reader's attention will be asked to the special -characteristics of these materials, in so far as they give evidence of former -volcanic activity.</p> - -<p>As has been already remarked, many of the most prominent phenomena -of a modern volcano are only of transient importance. The earthquakes -and tremors, and the constant disengagement of steam and gases, that play so -conspicuous a part in an eruption, may leave no sensible record behind them. -But even the cones of ashes and lava, which are piled up into mountainous -masses, have no true permanence: they are liable to ceaseless erosion by -the meteoric agencies of waste, and every stage in their degradation may be -traced. In successive examples we can follow them as they are cut down -to the very core, until in the end they are entirely effaced.</p> - -<p>We may well, therefore, ask at the outset by what more enduring -records we may hope to detect the traces of former volcanic action. The -following introductory chapters will be devoted to an attempt to answer -<span class="pagenum" id="Page_9">- 9 -</span> -this question. I shall try to show the nature and relative importance -of the records of ancient volcanoes; how these records, generally so -fragmentary, may be pieced together so as to be made to furnish the -history which they contain; how their relative chronology may be established; -how their testimony may be supplemented in such wise that the -position of long vanished seas, lands, rivers, and lakes may be ascertained; -and how, after ages of geological revolution, volcanic rocks that have lain -long buried under the surface now influence the scenery of the regions -where they have once more been exposed to view.</p> - -<p>From this groundwork of ascertained fact and reasonable inference, we -shall enter in Book II. upon the story of the old volcanoes of the British Isles. -It is usual to treat geological history in chronological order, beginning with -the earliest ages. And this method, as on the whole the most convenient, -will be adopted in the present work. At the same time, the plan so persistently -followed by Lyell, of working backward from the present into -the past, has some distinct advantages. The volcanic records of the later -ages are much simpler and clearer than those of older times, and the -student may, in some respects, profitably study the history of the Tertiary -eruptions before he proceeds to make himself acquainted with the scantier -chronicles of the eruptions of the Palæozoic periods. But as I wish to -follow the gradual evolution of volcanic phenomena, and to show how volcanic -energy has varied, waxing and waning through successive vast intervals -of time, I will adhere to the chronological sequence.</p> - -<hr class="chap x-ebookmaker-drop" /> - - -<div class="chapter"> -<p><span class="pagenum" id="Page_10">- 10 -</span></p> - -<h2 class="nobreak" id="CHAPTER_II">CHAPTER II</h2> -</div> - -<p class="tdc">The Nature and Causes of Volcanic Action—Modern Volcanoes.</p> - - -<p>A volcano is a conical or dome-shaped hill or mountain, consisting of -materials which have been erupted from an orifice leading down from the -surface into the heated interior of the earth. Among modern and recent -volcanoes three types may be recognized. In the first and most familiar -of these, the lavas and ashes ejected from the central vent have gathered -around it by successive eruptions, until they have built up a central cone -like those of Etna and Vesuvius. As this cone grows in height and -diameter, lateral or parasitic cones are formed on its flanks, and may become -themselves the chief actively erupting vents. This type of volcano, which -has been so long well known from its Mediterranean examples, was until -recently believed by geologists to be the normal, or indeed the only, phase of -volcanic energy on the face of the earth.</p> - -<p>A modification of this type is to be found in a few regions where fragmentary -discharges are small in amount and where the eruptions are almost -wholly confined to the emission of tolerably liquid lava. A vast dome -with gently sloping declivities may in this way be formed, as in the -Sandwich Islands and in certain parts of Iceland.</p> - -<p>The second type of volcano is at the present day extensively developed -only in Iceland, but in Tertiary time it appears to have had a wide range -over the globe, for stupendous memorials of it are preserved in North-Western -Europe, in Western America, and in India. It is distinguished -by the formation of numerous parallel fissures from which the lava gushes -forth, either with or without the formation of small cinder-cones along the -lines of the chasms.</p> - -<p>The third type is distinguished by the formation of groups of cinder-cones -or lava-domes, which from their admirable development in Central -France have received the name of <i>Puys</i>. From these vents considerable -streams of lava have sometimes been discharged.</p> - -<p>Without entering here into a detailed inquiry regarding the nature and -causes of Volcanic Action, we may with advantage consider briefly the two -main factors on which this action appears to depend.</p> - -<p>1. Much uncertainty still exists as to the condition and composition of -the earth's interior. The wide distribution of volcanoes over the globe, -together with the general similarity of materials brought by them up to the -<span class="pagenum" id="Page_11">- 11 -</span> -surface, formerly led to the belief that our planet consists of a central mass -of molten rock enclosed within a comparatively thin solid crust. Physical -arguments, however, have since demonstrated that the earth, with such a -structure, would have undergone great tidal deformation, but that in actual -fact it has a greater rigidity than if it were made of solid glass or steel.</p> - -<p>From all the evidence obtainable it is certain that the temperature -of the earth's interior must be high. The rate of increase of this temperature -downward from the surface differs from place to place; but an -increase is always observed. At a depth of a few miles, every known substance -must be much hotter than its melting point at the surface. But at -the great pressures within the earth, actual liquefaction is no doubt prevented, -and the nucleus remains solid, though at a temperature at which, -but for the pressure, it would be like so much molten iron.</p> - -<p>Any cause which will diminish the pressure may allow the intensely -hot material within the globe to pass into the liquid state. There is one -known cause which will bring about this result. The downward increment -of temperature proves that our planet is continually losing heat. As the -outer crust is comparatively cool, and does not become sensibly hotter by -the uprise of heat from within, the hot nucleus must cool faster than the -crust is doing. Now cooling involves contraction. The hot interior is contracting -faster than the cooler shell which encloses it, and that shell is thus -forced to subside. In its descent it has to adjust itself to a constantly -diminishing diameter. It can do so only by plication or by rupture.</p> - -<p>When the terrestrial crust, under the strain of contraction, is compressed -into folds, the relief thus obtained is not distributed uniformly over the -whole surface of the planet. From an early geological period it appears to -have followed certain lines. How these came to be at first determined we -cannot tell. But it is certain that they have served again and again, during -successive periods of terrestrial readjustment. These lines of relief coincide, -on the whole, with the axes of our continents. The land-areas of the globe -may be regarded as owing their existence above sea-level to this result of -terrestrial contraction. The crust underneath them has been repeatedly -wrinkled, fractured and thrust upward by the vast oceanic subsidence around -them. The long mountain-chains are thus, so to speak, the crests of the -waves into which the crust has from time to time been thrown.</p> - -<p>Again, the great lines of fracture in the crust of the earth probably lie -in large measure within the land-areas, or at least parallel with their axes -and close to their borders. Where the disposition of the chief ruptures and -of the predominant plications can be examined, these leading structural -features are found to be, on the whole, coincident. In the British Islands, -for instance, the prevalent trend of the axes of folding from early Palæozoic -to Tertiary time has been from south-west to north-east. How profoundly -this direction of earth-movement has affected the structure of the region is -shown by any ordinary map, in the long hill-ranges of the land and in the -long inlets of the sea. A geological map makes the dependence of the -scenery upon the building of the rocks still more striking. Not only have -<span class="pagenum" id="Page_12">- 12 -</span> -these rocks been plicated into endless foldings, the axes of which traverse -the British Islands with a north-easterly trend: they have likewise been -dislocated by many gigantic ruptures, which tend on the whole to follow -the same direction. The line of the Great Glen, the southern front of the -Highlands, and the northern boundary of the Southern Uplands of Scotland, -are conspicuous examples of the position and effect of some of the greater -fractures in the structure of this country.</p> - -<p>The ridging up of any part of the terrestrial crust will afford some relief -from pressure to the parts of the interior immediately underneath. If, as -is probable, the material of the earth's interior is at the melting point -proper for the pressure at each depth, then any diminution of the pressure -may allow the intensely heated substance to pass into the liquid state. It -would be along the lines of terrestrial uplift that this relief would be given. -It is there that active volcanoes are found. The molten material is forced -upward under these upraised ridges by the subsidence of the surrounding -regions. And where by rupture of the crust this material can make its way -to the surface, we may conceive that it will be ejected as lava or as stones -and ashes.</p> - -<p>Viewed in a broad way, such appears to be the mechanism involved in -the formation and distribution of volcanoes over the surface of the earth. -But obviously this explanation only carries us so far in the elucidation of -volcanic action. If the molten magma flowed out merely in virtue of the -influence of terrestrial contraction, it might do so for the most part tranquilly, -though it would probably be affected by occasional sudden snaps, as -the crust yielded to accumulations of pressure. Human experience has -no record of the actual elevation of a mountain-chain. We may believe -that if such an event were to happen suddenly or rapidly, it would be -attended with gigantic catastrophes over the surface of the globe. We -can hardly conceive what would be the scale of a volcanic eruption -attending upon so colossal a disturbance of the terrestrial crust. But -the eruptions which have taken place within the memory of man have -been the accompaniments of no such disturbance. Although they have -been many in number and sometimes powerful in effect, they have -seldom been attended with any marked displacement of the surrounding -parts of the terrestrial crust. Contraction is, of course, continuously and -regularly in progress, and we may suppose that the consequent subsidence, -though it results in intermittent wrinkling and uplifting of the terrestrial -ridges, may also be more or less persistent in the regions lying outside these -ridges. There will thus be a constant pressure of the molten magma into -the roots of volcanoes, and a persistent tendency for the magma to issue at -the surface at every available rent or orifice. The energy and duration of -outflow, if they depended wholly upon the effects of contraction, would thus -vary with the rate of subsidence of the sinking areas, probably assuming -generally a feeble development, but sometimes bursting into fountains of -molten rock hundreds of feet in height, like those observed from time to -time in Hawaii.</p> - -<p><span class="pagenum" id="Page_13">- 13 -</span></p> - -<p>2. The actual phenomena of volcanic eruptions, however, show that a -source of explosive energy is almost always associated with them, and that -while the transference of the subterranean molten magma towards the -volcanic vents may be referred to the results of terrestrial contraction, the -violent discharge of materials from those vents must be assigned to some -kind of energy stored up in the substance of the earth's interior.</p> - -<p>The deep-seated magma from which lavas ascend contains various -vapours and gases which, under the enormous pressure within and beneath -the terrestrial crust, are absorbed or dissolved in it. So great is the tension -of these gaseous constituents, that when from any cause the pressure on the -magma is suddenly relieved, they are liberated with explosive violence.</p> - -<p>A volcanic paroxysm is thus immediately the effect of the rapid escape -of these imprisoned gases and vapours. With such energy does the -explosion sometimes take place, that the ascending column of molten lava is -blown into the finest impalpable dust, which may load the air around a -volcano for many days before it falls to the ground, or may be borne in the -upper regions of the atmosphere round the globe.</p> - -<p>The proportion of dissolved gases varies in different lavas, while the -lavas themselves differ in the degree of their liquidity. Some flow out -tranquilly like molten iron, others issue in a pasty condition and rapidly -congeal into scoriæ and clinkers. Thus within the magma itself the amount -of explosive energy is far from being always the same.</p> - -<p>It is to the co-operation of these two causes—terrestrial contraction and -its effects on the one hand, and the tension of absorbed gases and vapours -the other—that the phenomena of volcanoes appear to be mainly due. -There is no reason to believe that modern volcanoes differ in any essential -respect from those of past ages in the earth's history. It might, indeed, -have been anticipated that the general energy of the planet would manifest -itself in far more stupendous volcanic eruptions in early times than those of -the modern period. But there is certainly no geological evidence in favour -of such a difference. One of the objects of the present work is to trace the -continuity of volcanic phenomena back to the very earliest epochs, and to -show that, so far as the geological records go, the interior of the planet has -reacted on its exterior in the same way and with the same results.</p> - -<p>We may now proceed to inquire how far volcanoes leave behind them -evidence of their existence. I shall devote the next two or three chapters -to a consideration of the proofs of volcanic action furnished by the very -nature of the materials brought up from the interior of the earth, by the -arrangement of these materials at the surface, by the existence of the actual -funnels or ducts from which they were discharged above ground, and by the -disposition of the masses of rock which, at various depths below the surface, -have been injected into and have solidified within the terrestrial crust.</p> - -<hr class="chap x-ebookmaker-drop" /> - - -<div class="chapter"> -<p><span class="pagenum" id="Page_14">- 14 -</span></p> - -<h2 class="nobreak" id="CHAPTER_III">CHAPTER III</h2> -</div> - -<div class="blockquot"> - -<p>Ancient Volcanoes: Proofs of their existence derived from the Nature of the Rocks erupted -from the Earth's Interior. A. Materials erupted at the Surface—Extrusive Series. -i. Lavas, their General Characters. Volcanic Cycles. ii. Volcanic Agglomerates, -Breccias and Tuffs.</p> -</div> - - -<p>The materials brought by volcanic action from the earth's interior have -certain common characters which distinguish them from other constituents -of the terrestrial crust. Hence the occurrence of these materials on any -part of the earth's surface affords convincing proofs of former volcanic -eruptions, even where all outward trace of actual volcanoes may have been -effaced from the topographical features of the ground.</p> - -<p>Volcanic products may be classed in two divisions—1st, Those which -have been ejected at the surface of the earth, or the Extrusive series; and -2nd, Those which have been injected into the terrestrial crust at a greater -or less distance below the surface, and which are known as the Intrusive -series. Extrusive rocks may be further classified in two great groups—(i.) -The Lavas, or those which have been poured out in a molten condition -at the surface; and (ii.) The Fragmental Materials, including all kinds of -pyroclastic detritus discharged from volcanic vents.</p> - -<p>Taking first the Extrusive volcanic rocks, we may in the present chapter -consider those characters in them which are of most practical value in the -investigation of the volcanic phenomena of former geological periods.</p> - - -<h4>i. <span class="allsmcap">LAVAS</span></h4> - -<p>The term Lava is a convenient and comprehensive designation for all -those volcanic products which have flowed out in a molten condition. They -differ from each other in composition and structure, but their variations are -comprised within tolerably definite limits.</p> - -<p>As regards their composition they are commonly classed in three divisions—1st, -The Acid lavas, in which the proportion of silicic acid ranges from -a little below 70 per cent upwards; 2nd, The Intermediate lavas, wherein -the percentage of silica may vary from 55 to near 70; and 3rd, The Basic -lavas, where the acid constituent ranges from 55 per cent downwards. -Sometimes the most basic kinds are distinguished as a fourth group under -the name of Ultrabasic, in which the percentage of silica may fall below 40.</p> - -<p><span class="pagenum" id="Page_15">- 15 -</span></p> - -<p>The structures of lavas, however, furnish their most easily appreciated -characteristics. Four of these structures deserve more particular attention: -1st, Cellular, vesicular or pumiceous structure; 2nd, The presence of glass, or -some result of the devitrification of an original glass; 3rd, Flow-structure; -and 4th, The arrangement of the rocks in sheets or beds, with columnar and -other structures.</p> - -<div class="figcenter" id="v1fig1" style="width: 412px;"> - <img src="images/v1fig1.png" width="412" height="428" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 1.</span>—Vesicular structure, Lava from Ascension Island, slightly less than natural size.</div> -</div> - -<p>1. The <span class="allsmcap">CELLULAR</span>, <span class="allsmcap">VESICULAR</span>, <span class="allsmcap">SCORIACEOUS</span> or <span class="allsmcap">PUMICEOUS STRUCTURE</span> of -volcanic rocks (<a href="#v1fig1">Fig. 1</a>) could only have arisen in molten masses from the -expansion of imprisoned vapours or gases, and is thus of crucial importance -in deciding the once liquid condition of the rocks which display it. The -vesicles may be of microscopic minuteness, but are generally quite visible -to the naked eye, and are often large and conspicuous. Sometimes these -cavities have been subsequently filled up with calcite, quartz, agate, zeolites -or other mineral deposition. As the kernels thus produced are frequently -flattened or almond-shaped (<i>amygdales</i>), owing to elongation of the steam-holes -by movement of the lava before its consolidation, the rocks containing -them are said to be <i>amygdaloidal</i>.</p> - -<p>This structure, though eminently characteristic of superficial lavas, is -<span class="pagenum" id="Page_16">- 16 -</span> -not always by itself sufficient to distinguish them from the intrusive rocks. -Examples will be given in later chapters where dykes, sills and other -masses of injected igneous material are conspicuously cellular in some parts. -But, in such cases, the cavities are generally comparatively small, usually -spherical or approximately so, tolerably uniform in size and distribution, -and, especially when they occur in dykes, distributed more particularly -along certain lines or bands, sometimes with considerable regularity (see -Figs. <a href="#v1fig90">90</a>, <a href="#v1fig91">91</a>, and <a href="../../66493/66493-h/66493-h.htm#v2fig236">236</a>).</p> - -<p>Among the superficial lavas, however, such regularity is rarely to be -seen. Now and then, indeed, a lava, which is not on the whole cellular, -may be found to have rows of vesicles arranged parallel to its under or -upper surface, or it may have acquired a peculiar banded structure from the -arrangement of its vesicles in parallel layers along the direction of flow. The -last-named peculiarity is widely distributed among the Tertiary lavas of -North-Western Europe, and gives to their weathered surfaces a deceptive -resemblance to tuffs or other stratified rocks (see Figs. <a href="../../66493/66493-h/66493-h.htm#v2fig260">260</a>, <a href="../../66493/66493-h/66493-h.htm#v2fig310">310</a> and <a href="../../66493/66493-h/66493-h.htm#v2fig311">311</a>). -It will be more particularly referred to a few pages further on. In general, -however, we may say that the steam-cavities of lavas are quite irregular in -size, shape and distribution, sometimes increasing to such relative proportions -as to occupy most of the bulk of the rock, and in other places disappearing, -so as to leave the lava tolerably compact. When a lava presents -an irregularly vesicular character, like that of the slags of an iron-furnace, it -is said to be <i>slaggy</i>. When its upper surface is rugged and full of steam-vesicles -of all sizes up to large cavernous spaces, it is said to be <i>scoriaceous</i>, -and fragments of such a rock ejected from a volcanic vent are spoken of as -<i>scoriæ</i>.</p> - -<p>Attention to the flattening of the steam-vesicles in cellular lavas, which -has just been alluded to as the result of the onward movement of the still -molten mass, may show, by the trend and grouping of these elongated cavities, -the probable direction of the flow of the lava before it came to rest. Sometimes -the vesicles have been drawn out and flattened to such a degree -that the rock has acquired in consequence a fissile structure. In other -instances, the vesicles have been originally formed as long parallel and even -branching tubes, like the burrows of Annelids or the borings of <i>Teredo</i>. -Some remarkable examples of this exceptional structure have been obtained -from the Tertiary plateau-basalts of the Western Isles, of which an example -is represented in <a href="#v1fig2">Fig. 2</a>.</p> - -<p>In many cases the vesicles extend through the whole thickness of a -lava. Frequently they may be found most developed towards the top and -bottom; the central portion of the sheet being compact, while the top and -bottom are rugged, cavernous or scoriaceous.</p> - -<p>Though originally the vesicles and cavernous spaces, blown open by the -expansion of the vapours dissolved in molten lava, remained empty on the -consolidation of the rock, they have generally been subsequently filled up -by the deposit within them of mineral substances carried in aqueous solution. -The minerals thus introduced are such as might have been derived -<span class="pagenum" id="Page_17">- 17 -</span> -from the removal of their constituent ingredients by the solvent action of -water on the surrounding rock. And as amygdaloids are generally more -decayed than the non-vesicular lavas, it has been generally believed that the -abstraction of mineral material and its re-deposit within the steam-vesicles -have been due to the influence of meteoric water, which at atmospheric -temperatures and pressures has slowly percolated from the surface through -the cellular lava, long after the latter had consolidated and cooled, and even -after volcanic energy at the locality had entirely ceased.</p> - -<div class="figcenter" id="v1fig2" style="width: 341px;"> - <img src="images/v1fig2.png" width="341" height="302" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 2.</span>—Elongation and branching of steam-vesicles in a lava, Kilninian, Isle of Mull, a little less -than natural size.</div> -</div> - -<p>Examples, however, are now accumulating which certainly prove that, in -some cases, the vesicles were filled up during the volcanic period. Among -the Tertiary basalt-plateaux of the Inner Hebrides, for instance, it can be -shown that the lavas were already amygdaloidal before the protrusion of the -gabbros and granophyres which mark later stages of the same continuous -volcanic history, and even before the outpouring of much of the basalt of -these plateaux. Not improbably the mineral secretions were largely due to -the influence of hot volcanic vapours during the eruption of the basalts. -This subject will be again referred to in the description of the Tertiary -volcanic series.</p> - -<p>Vesicular structure is more commonly and perfectly developed among -the lavas which are basic and intermediate in composition than among -those which are acid.</p> - -<p>While the existence of a highly vesicular or scoriaceous structure may -generally be taken as proof that the rock displaying it flowed out at the -surface as a lava, other evidence pointing to the same conclusion may often -be gathered from the rocks with which the supposed lava is associated. -<span class="pagenum" id="Page_18">- 18 -</span> -Where, for example, a scoriaceous lava is covered with stratified deposits -which contain pieces of that lava, we may be confident that the rock is an -interstratified or contemporaneous sheet. It has been erupted after the -deposition of the strata on which it rests, and before that of the strata -which cover it and contain pieces of it. In such a case, the geological date -of the eruption could be precisely defined. Illustrations of this reasoning -will be given in <a href="#CHAPTER_IV">Chapter iv.</a>, and in the account of the volcanic series -of Carboniferous age in Central Scotland, where a basic lava can sometimes -be proved to be a true flow and not an intrusive sill by the fact that -portions of its upper slaggy surface are enclosed in overlying sandstone, -shale or limestone.</p> - -<p>2. The presence of <span class="allsmcap">GLASS</span>, or of some result of the devitrification of an -original glass, is an indication that the rock which exhibits it has once been -in a state of fusion. Even where no trace of the original vitreous condition -may remain, stages in its devitrification, that is, in its conversion into a -stony or lithoid condition, may be traceable. Thus what are called spherulitic -and perlitic structures (which will be immediately described), either -visible to the naked eye or only observable with the aid of the microscope, -afford evidence of the consolidation and conversion of a glassy into a lithoid -substance.</p> - -<p>Striking evidence of the former glassy, and therefore molten, condition -of many rocks now lithoid is to be gained by the examination of thin -slices of them under the microscope. Not only are vestiges of the original -glass recognizable, but the whole progress of devitrification may be followed -into a crystalline structure. The primitive crystallites or microlites of -different minerals may be seen to have grouped themselves together into -more or less perfect crystals, while scattered crystals of earlier consolidation -have been partially dissolved in and corroded by the molten glass. These -and other characteristics of once fused rocks have to a considerable extent -been imitated artificially by MM. Fouqué and Michel Lévy, who have fused -the constituent minerals in the proper proportions.</p> - -<p>Since traces of glass or of its representative devitrified structures are so -abundantly discoverable in lavas, we may infer the original condition of most -lavas to have been vitreous. Where, for instance, the outer selvages of a -basic dyke or sill are coated with a layer of black glass which rapidly passes -into a fine-grained crystalline basalt, and then again into a more largely -crystalline or doleritic texture in the centre, there can be no hesitation in -believing that glassy coating to be due to the sudden chilling and consolidation -of the lava injected between the cool rocks that enclose it. The part -that solidified first may be regarded as probably representing the condition -of the whole body of lava at the time of intrusion. The lithoid or crystalline -portion between the two vitreous outer layers shows the condition -which the molten rock finally assumed as it cooled more slowly.</p> - -<p>Some lavas, such as obsidians and pitchstones, have consolidated in the -glassy form. More usually, however, a lithoid structure has been developed, -the original glass being only discoverable by the microscope, and often not -<span class="pagenum" id="Page_19">- 19 -</span> -even by its aid. Two varieties of devitrification may be observed among -lavas, which, though not marked off from each other by any sharp lines, are -on the whole distinctive of the two great groups of acid and basic rocks.</p> - -<div class="figright" id="v1fig3" style="width: 186px;"> - <img src="images/v1fig3.png" width="186" height="184" alt="" /><br /> - <div class="hanging2"><span class="smcap">Fig. 3.</span>—Microlites of the Pitchstone of - Arran (magnified 70 diameters).</div> -</div> - -<p>(1) Among the acid rocks, what is called the Felsitic type of devitrification -is characteristic. Thus, obsidians pass by intermediate stages from a -clear transparent or translucent glass into a -dull flinty or horny mass. When thin slices -of these transitional forms are examined under -the microscope, minute hairs and fibres or -trichites, which may be observed even in the -most perfectly glassy rocks, are seen to increase -in number until they entirely take -the place of the glass. Microlites of definite -minerals may likewise be observed, together -with indefinite granules, and the rock finally -becomes a rhyolite, felsite or allied variety -(<a href="#v1fig3">Fig. 3</a>).</p> - -<p>At the same time it should be observed -that, even in the vitreous condition of a -lava, definite crystals of an early consolidation were generally already -present. Felspars and quartz, usually in large porphyritic forms, may -be seen in the glass, often so corroded as to indicate that they were in -course of being dissolved in the magma at the time of the cooling and solidification -of the mass. In obsidians and pitchstones such relics of an earlier -or derived series of crystallized minerals may often be recognized, while in -felsites and quartz-porphyries they are equally prominent. Where large -dispersed crystals form a prominent characteristic in a rock they give rise -to what is termed the <i>Porphyritic</i> structure.</p> - -<table summary="illo"> -<tr> - <td class="tdc" style="width: 185px;"><a id="v1fig4"></a><img src="images/v1fig4.png" width="184" height="183" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 4.</span>—Perlitic structure in Felsitic - Glass, Isle of Mull (magnified).</div></td> - <td class="tdc" style="width: 185px;"><a id="v1fig5"></a><img src="images/v1fig5.png" width="183" height="181" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 5.</span>—Spherulitic structure - (magnified).</div></td> -</tr> -</table> - -<p>Accompanying the passage of glass into stone, various structures make -their appearance, sometimes distinctly visible to the naked eye, at other -times only perceptible with the aid of the microscope. One of these structures, -known as <i>Perlitic</i> (<a href="#v1fig4">Fig. 4</a>), consists in the formation of minute curved -<span class="pagenum" id="Page_20">- 20 -</span> -or straight cracks between which the vitreous or felsitic substance, during its -contraction in cooling, assumed a finely globular form.</p> - -<p>Another structure, termed <i>Spherulitic</i> (<a href="#v1fig5">Fig. 5</a>), shows the development -of globules or spherules which may range from grains of microscopic minuteness -up to balls two inches or more in diameter. These not infrequently -present a well-formed internal fibrous radiation, which gives a black cross -between crossed Nicol prisms. Spherulites are more especially developed -along the margins of intrusive rocks, and may be found in dykes, sills and -bosses (see Figs. <a href="../../66493/66493-h/66493-h.htm#v2fig375">375</a> and <a href="../../66493/66493-h/66493-h.htm#v2fig377">377</a>). Where the injected mass is not thick it -may be spherulitic to the very centre, as can be seen among the felsitic -and granophyric dykes of Skye.</p> - -<p>Some felsitic lavas possess a peculiar nodular structure, which was -developed during the process of consolidation. So marked does this arrangement -sometimes become that the rocks which display it have actually been -mistaken for conglomerates. It is well exhibited among the Lower Silurian -lavas of Snowdon, the Upper Silurian lavas of Dingle, and the Lower Old -Red Sandstone lavas near Killarney.</p> - -<table summary="illo"> -<tr> - <td class="tdc" style="width: 185px;"><a id="v1fig6"></a><img src="images/v1fig6.png" width="185" height="180" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 6.</span>—Micropegmatitic or Granophyric - structure in Granophyre, Mull (magnified).</div></td> - <td class="tdc" style="width: 185px;"><a id="v1fig7"></a><img src="images/v1fig7.png" width="185" height="185" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 7.</span>—Ophitic structure in Dolerite, - Gortacloghan, Co. Derry (magnified).</div></td> -</tr> -</table> - -<p>A marked structure among some intrusive rocks, especially of an acid -composition, is that called <i>Micropegmatitic</i> or <i>Granophyric</i>. It consists in a -minute intergrowth of two component minerals, especially quartz and felspar, -and is more especially characteristic of certain granitic or granitoid rocks -which have consolidated at some distance from the surface and occur as -bosses, sills and dykes. It is also met with, however, in some basic sills. -Examples of all these and other structures will occur in the course of the -following description of British volcanic rocks.</p> - -<p>(2) The second type of devitrification, conspicuous in rocks of more -basic composition, is marked by a more complete development of crystallization. -Among basic, as among acid rocks, there are proofs of the consolidation -of definite minerals at more than one period. Where the molten -material has suddenly cooled into a black glass, porphyritic felspars or -other minerals are often to be seen which were already floating in the -<span class="pagenum" id="Page_21">- 21 -</span> -magma in its molten condition. During devitrification, however, other felspars -of a later period of generation made their appearance, but they are -generally distinguishable from their predecessors. Probably most basic and -intermediate rocks, when poured out at the surface as lavas, were no longer -mere vitreous material, but had already advanced to various stages of progress -towards a stony condition. These stages are still to some extent traceable -by the aid of the microscope.</p> - -<p>Microlites of the component minerals are first developed, which, if the -process of aggregation is not arrested, build up more or less perfect crystals -or crystalline grains of the minerals. Eventually the glass may be so completely -devitrified by the development of its constituent minerals as to be -wholly used up, the rock then becoming entirely crystalline, or to survive -only in scanty interstitial spaces. In the family of the basalts and dolerites -the gradual transition from a true glass into a holocrystalline compound -may be followed with admirable clearness. The component minerals have -sometimes crystallized in their own distinct crystallographic forms (idiomorphic); -in other cases, though thoroughly crystalline, they have assumed -externally different irregular shapes, fitting into each other without their -Proper geometric boundaries (allotriomorphic).</p> - -<p>A specially characteristic feature of many basic rocks is the presence of -what is termed an <i>Ophitic</i> structure (<a href="#v1fig7">Fig. 7</a>). Thus the component crystals -of pyroxene occur as large plates separated and penetrated by small needles -and crystals of felspar. The portions of pyroxene, divided by the enclosed -felspar, are seen under the microscope to be in optical continuity, and to have -crystallized round the already formed felspar. This structure is never found -in metamorphic crystalline rocks. It has been reproduced artificially from -fusion by Messrs. Fouqué and Michel Lévy.</p> - -<p>The name <i>Variolitic</i> is applied to another structure of basic rocks -(<a href="#v1fig8">Fig. 8</a>), in which, especially towards the margin of eruptive masses, abundant -spheroidal aggregates have been developed from the size of a millet-seed -to that of a walnut, imbedded in a fine-grained or compact greenish matrix -into which the kernels seem to shade off. These kernels consist of silicates -arranged either radially or in concentric zones.</p> - -<p>3. <span class="smcap">Flow-structure</span> is an arrangement of the crystals, vesicles, spherulites, -or devitrification-streaks in bands or lines, which sweep round any -enclosed object, such as a porphyritic crystal or detached spherulite, and -represent the curving flow of a mobile or viscous mass. Admirable examples -of this structure may often be observed in old lavas, as well as in dykes and -sills, the streaky lines of flow being marked as distinctly as the lines of foam -that curve round the boulders projecting from the surface of a mountain-brook.</p> - -<p>Flow-structure is most perfectly developed among the obsidians, rhyolites, -felsites and other acid rocks, of which it may be said to be a frequently -conspicuous character (<a href="#v1fig9">Fig. 9</a>). In these rocks it is revealed by the parallel -arrangement of the minute hair-like bodies and crystals, or by alternate -layers of glassy and lithoid material. The streaky lines thus developed are -sometimes almost as thin and parallel as the leaves of a book. But they -<span class="pagenum" id="Page_22">- 22 -</span> -generally show interruptions and curvatures, and may be seen to bend round -larger enclosed crystals, or to gather into eddy-like curves, in such a manner -as vividly to portray the flow of a viscous substance. These lines represent -on a minute scale the same flow-structure which may be traced in large -sheets among the lavas. The porphyritic crystals and the spherulites are -also drawn out in rows in the same general direction. Sometimes, indeed, -the spherulites have been so symmetrically grouped in parallel rows that -they appear as rod-like aggregates which extend along the margin of a dyke.</p> - -<div class="figcenter" id="v1fig8" style="width: 294px;"> - <img src="images/v1fig8.png" width="375" height="242" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 8.</span>—Variolitic or Orbicular structure, Napoleonite, Corsica (nat. size).</div> -</div> - -<p>Among lavas of more basic composition flow-structure is not so often -well displayed. It most frequently shows itself by the orientation of -porphyritic felspars or of lines of steam-vesicles. Occasionally, however, -sheets of basalt may be found in which a distinct streakiness has been -developed owing to variations in the differentiation of the original molten -magma. A remarkable and widespread occurrence of such a structure -is met with among the Tertiary basalt-plateaux of the Inner Hebrides and -the Faroe Islands. In the lower parts of these thick accumulations of successive -lava-sheets, a banded character is so marked as to give the rocks the -aspect of truly stratified deposits. The observer, indeed, can hardly undeceive -himself as to their real nature until he examines them closely. As a full -description of this structure will be given in a later chapter, it may suffice -to state here that the banding arises from two causes. In some cellular -lavas, the vesicles are arranged in layers which lie parallel with the upper -and under surfaces of the sheets. These layers either project as ribs or -recede into depressions along the outcrop, and thus impart a distinctly -stratified aspect to the rock. More frequently, however, the banded -structure is produced by the alternation of different varieties of texture, -and even of composition, in the same sheet of basalt. Lenticular seams of -olivine-basalt may be found intercalated in a more largely crystalline -dolerite. These differences appear to point to considerable variations in -<span class="pagenum" id="Page_23">- 23 -</span> -the constitution of the magma from which the lavas issued—variations -which already existed before the discharge of these lavas, and which showed -themselves in the successive outflow of basaltic and doleritic material during -the eruption of what was really, as regards its appearance at the surface, one -continuous stream of molten rock. It is impossible to account for such -variations in the same sheet of lava by any process of differentiation in -the melted material during its outflow and cooling. Analogous variations -occur among the basic sills and bosses of the Tertiary volcanic series of -Britain. These, as will be more fully discussed in later chapters, indicate -a considerable amount of heterogeneity in the deep-seated magma from which -the intrusive sheets and bosses were supplied (see vol. ii. pp. <a href="../../66493/66493-h/66493-h.htm#Page_329">329</a>, <a href="../../66493/66493-h/66493-h.htm#Page_342">342</a>).</p> - -<div class="figcenter" id="v1fig9" style="width: 410px;"> - <img src="images/v1fig9.png" width="518" height="384" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 9.</span>—Flow-structure in Rhyolite, Antrim, slightly reduced.</div> -</div> - -<p>It is a common error to assume that flow-structure is a distinctive -character of lavas that have flowed out at the surface. In reality some -of the most perfect examples of the structure occur in dykes and sills, both -among acid and basic rocks. Innumerable instances might be quoted from -the British Isles in support of this statement.</p> - -<p>Although, in the vast majority of cases, the presence of flow-structure -may be confidently assumed to indicate a former molten condition of the -rock in which it occurs, it is not an absolutely reliable test for an igneous -rock. Experiment has shown that under enormous pressure even solid -metals may be made to flow into cavities prepared for their reception. -<span class="pagenum" id="Page_24">- 24 -</span> -Under the vast compression to which the earth's crust is subjected during -terrestrial contraction, the most obdurate rocks are crushed into fragments -varying from large blocks to the finest powder. This comminuted -material is driven along in the direction of thrust, and when it comes to -rest presents a streakiness, with curving lines of flow round the larger -fragments, closely simulating the structure of many rhyolites and obsidians. -It is only by attention to the local surroundings that such deceptive resemblances -can be assigned to their true cause.</p> - -<div class="figcenter" id="v1fig10" style="width: 346px;"> - <img src="images/v1fig10.png" width="425" height="77" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 10.</span>—Lumpy, irregular trachytic Lava-streams (Carboniferous), East Linton, Haddingtonshire.</div> -</div> - -<p>4. The <span class="allsmcap">DISPOSITION OF LAVAS IN SHEETS OR BEDS</span> is the result of -successive outflows of molten rock. Such sheets may range from only a -yard or two to several hundred feet in thickness. As a rule, though with -many exceptions, the basic lavas, such as the basalts, appear in thinner beds -than the acid forms. This difference is well brought out if we compare, -for instance, the massive rhyolites or felsites of North Wales with the thin -sheets of basalt in Antrim and the Inner Hebrides. The regularity of the -bedded character is likewise more definite among the basic than among -the acid rocks, and this contrast also is strikingly illustrated by the two -series of rocks just referred to. The rhyolites and felsites, sometimes also -the trachytes and andesites, assume lumpy, irregular forms, and some little -care may be required to trace their upper and under surfaces, and to -ascertain that they really do form continuous sheets, though varying much -in thickness from place to place (<a href="#v1fig10">Fig. 10</a>). Like modern acid lavas, they -seem to have flowed out in a pasty condition, and to have been heaped up -round the vents in the form of domes, or with an irregular hummocky or -mounded surface. The basalts, and dolerites, and sometimes the andesites, -have issued in a more fluid condition, and have spread out in sheets of more -uniform thickness, as may be instructively seen in the sea-cliffs of Antrim, Mull, -Skye, and the Faroe Islands, where the horizontal or gently-inclined flows -of basalt lie upon each other in even parallel beds traceable for considerable -distances along the face of the precipices (Figs. <a href="#v1fig11">11</a>, <a href="../../66493/66493-h/66493-h.htm#v2fig265">265</a>, and <a href="../../66493/66493-h/66493-h.htm#v2fig286">286</a>). The andesites -of the Old Red Sandstone (Figs. <a href="#v1fig99">99</a>, <a href="#v1fig100">100</a>) and Carboniferous series (Figs. -<a href="#v1fig107">107</a>, <a href="#v1fig108">108</a>, <a href="#v1fig111">111</a>, <a href="#v1fig112">112</a>, <a href="#v1fig113">113</a>, <a href="#v1fig123">123</a>) in Scotland likewise form terraced hills.</p> - -<p>The length of a lava-stream may vary within wide limits. Sometimes an -outflow of lava has not reached the base of the cone from the side of which -it issued, like the obsidian stream on the flanks of the little cone of the -island of Volcano. In other cases, the molten rock has flowed for forty or -fifty miles, like the copious Icelandic lava-floods of 1783. In the basalt-plateaux -of the Inner Hebrides a single sheet may sometimes be traced for -several miles.</p> - -<p><span class="pagenum" id="Page_25">- 25 -</span></p> - -<div class="figcenter" id="v1fig11" style="width: 510px;"> - <img src="images/v1fig11.png" width="510" height="337" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 11.</span>—View at the entrance of the Svinofjord, Faroe Islands, illustrating the terraced forms - assumed by basic lavas. The island on the left is Borö, - that in the centre Viderö, and that on the right Svinö.</div> -</div> - -<p>Some lavas, more especially among the basic series, assume in cooling a -<i>Columnar structure</i>, of which two types may be noticed. In one of these the -columns pass with regularity and parallelism from the top to the bottom of -a bed (Figs. <a href="#v1fig171">171</a>, <a href="../../66493/66493-h/66493-h.htm#v2fig225">225</a>). The basalt in which Fingal's Cave, in the isle of -Staffa, has been hollowed out may be taken as a characteristic example -(Fig. 266a). Not infrequently the columns are curved, as at the well-known -Clam-shell Cave of Staffa. In the other type, the columns or prisms are -not persistent, but die out into each other and have a wavy, irregular shape, -somewhat like prisms of starch. These two types may occur in successive -sheets of basalt, or may even pass into each other. At Staffa the regularly -columnar bed is immediately overlain with one of the starch-like character. -The columnar structure in either case is a contraction phenomenon, produced -during the cooling and shrinking of the lava. But it is difficult to say what -special conditions in the lava were required for its production, or why it -should sometimes have assumed the regular, at others the irregular form. -It may be found not only in superficial lavas but in equal perfection in -some dykes and intrusive sills or injections, as among the Tertiary volcanic -rocks of the island of Canna (Figs. <a href="../../66493/66493-h/66493-h.htm#v2fig307">307</a> and <a href="../../66493/66493-h/66493-h.htm#v2fig308">308</a>).</p> - -<p>The precipitation of a lava-stream into a lake or the sea may cause the -outer crust of the rock to break up with violence, so that the still molten -material inside may rush into the water. Some basic lavas on flowing into -water or into a watery silt have assumed a remarkable spheroidal sack-like -<span class="pagenum" id="Page_26">- 26 -</span> -or pillow-like structure, the spheroids being sometimes pressed into shapes -like piles of sacks. A good instance of this structure occurs in a basalt at -Acicastello in Sicily.<a id="FNanchor_1" href="#Footnote_1" class="fnanchor">[1]</a> A similar appearance will be described in a later -chapter as peculiarly characteristic of certain Lower Silurian lavas associated -with radiolarian cherts in Britain and in other countries (<a href="#v1fig12">Fig. 12</a>).</p> - -<div class="footnote"> - -<p><a id="Footnote_1" href="#FNanchor_1" class="label">[1]</a> See Prof. G. Platania in Dr. Johnston-Lavis' <i>South Italian Volcanoes</i>, Naples (1891), p. 41 -and plate xii.</p> - -</div> - -<div class="figcenter" id="v1fig12" style="width: 332px;"> - <img src="images/v1fig12.png" width="332" height="353" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 12.</span>—Sack-like or pillow-form structure of basic lavas (Lower Silurian), Bennan Head, - Ballantrae, Ayrshire.</div> -</div> - -<p>It probably seldom happens that a solitary sheet of lava occurs among -non-volcanic sedimentary strata, with no other indication around it of former -volcanic activity. Such an isolated record does not seem to have been met -with in the remarkably ample volcanic register of the British Isles. The -outpouring of molten rock has generally been accompanied with the ejection -of fragmentary materials. Hence among the memorials of volcanic eruptions, -while intercalated lavas are generally associated with sheets of tuff, -bands of tuff may not infrequently be encountered in a sedimentary series -without any lava. Instances in illustration of these statements may be -culled from the British Palæozoic formations back even into the Cambrian -system.</p> - -<p>A characteristic feature of some interest in connection with the flow of -lava is the effect produced by it on the underlying rocks. If these are not -firmly compacted they may be ploughed up or even dislocated. Thus the -tuffs of the Velay have sometimes been plicated, inverted, and fractured by -<span class="pagenum" id="Page_27">- 27 -</span> -the movement of a flowing current of basalt.<a id="FNanchor_2" href="#Footnote_2" class="fnanchor">[2]</a> The great heat of the lava -has frequently induced considerable alteration upon the underlying rocks. -Induration is the most common result, often accompanied with a reddening -of the altered substance. Occasionally a beautifully prismatic structure has -been developed in the soft material immediately beneath a basalt, as in -ferruginous clay near the village of Esplot in the Velay, in which the close-set -columns are 50 centimetres long and 4 to 5 centimetres in diameter.<a id="FNanchor_3" href="#Footnote_3" class="fnanchor">[3]</a> -Changes of this nature, however, are more frequent among sills than among -superficial lavas. Many examples of them may be gathered from the -Scottish Carboniferous districts.</p> - -<div class="footnote"> - -<p><a id="Footnote_2" href="#FNanchor_2" class="label">[2]</a> M. Boule, <i>Bull. Cart. Géol. France</i>, No. 28, tom. iv. (1892), p. 235.</p> - -<p><a id="Footnote_3" href="#FNanchor_3" class="label">[3]</a> M. Boule. <i>Op. cit.</i> p. 234.</p> - -</div> - -<p><span class="smcap">Variations of structure in single lava-sheets.</span>—From what has -been said above in regard to certain kinds of flow-structure among basic -rocks, it will be evident that some considerable range of chemical, but more -particularly of mineralogical, composition may be sometimes observed even -within the same sheet of lava. Such differences, it is true, are more frequent -among intrusive rocks, especially thick sills and large bosses. But they -have been met with in so many instances among superficial lavas as to show -that they are the results of some general law, which probably has a wide -application among the surface-products of volcanic action. Scrope expressed -the opinion that in the focus of a volcano there may be a kind of filtration -of the constituents of a molten mass, the heavier minerals sinking through -the lighter, so that the upper portions of the mass will become more felspathic -and the lower parts more augitic and ferruginous.<a id="FNanchor_4" href="#Footnote_4" class="fnanchor">[4]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_4" href="#FNanchor_4" class="label">[4]</a> <i>Volcanoes</i>, p. 125.</p> - -</div> - -<p>Leopold von Buch found that in some of the highly glassy lavas of the -Canary Islands the felspar increases towards the bottom of the mass, becoming -so abundant as almost to exclude the matrix, and giving rise to a compound -that might be mistaken for a primitive rock.<a id="FNanchor_5" href="#Footnote_5" class="fnanchor">[5]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_5" href="#FNanchor_5" class="label">[5]</a> <i>Description Physique des Isles Canaries</i> (1836), p. 190.</p> - -</div> - -<p>Darwin observed that in a grey basalt filling up the hollow of an old -crater in James Island, one of the Galapagos group, the felspar crystals -became much more abundant in the lower scoriaceous part, and he discussed -the question of the descent of crystals by virtue of their specific gravity -through a still molten lava.<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> <i>Geological Observations on Volcanic Islands</i> (1844), p. 117.</p> - -</div> - -<p>Mr. Clarence King during a visit to Hawaii found that in every case -where he broke newly-congealed streamlets of lava, "the bottom of the flow -was thickly crowded with triclinic felspars and augites, while the whole -upper part of the stream was of nearly pure isotropic and acid glass."<a id="FNanchor_7" href="#Footnote_7" class="fnanchor">[7]</a> This -subject will be again referred to when we come to discuss the characters of -intrusive sills and bosses, for it is among them that the most marked petrographical -variations may be observed. Examples will be cited both from -the intrusive and extrusive volcanic groups of Britain.</p> - -<div class="footnote"> - -<p><a id="Footnote_7" href="#FNanchor_7" class="label">[7]</a> <i>U.S. Geol. Exploration of the Fortieth Parallel</i>, vol. i. (1878), p. 716.</p> - -</div> - -<p><span class="smcap">Volcanic cycles.</span>—Closely related to the problem of the range of structure -<span class="pagenum" id="Page_28">- 28 -</span> -and composition in a single mass of lava is another problem presented -by the remarkable sequence of different types of lava which are erupted -within a given district during a single volcanic period. Nearly thirty years -ago Baron von Richthofen drew attention to the sequence of volcanic -materials erupted within the same geographical area. He showed, more -especially from observations in Western America, that a definite order of -appearance in the successive species of lava could be established, the earliest -eruptions consisting of materials of an intermediate or average composition, -and those of subsequent outflows becoming on the whole progressively more -acid, but finishing by an abrupt transition to a basic type. His sequence was -as follows: 1. Propylite; 2. Andesite; 3. Trachyte; 4. Rhyolite; 5. Basalt.<a id="FNanchor_8" href="#Footnote_8" class="fnanchor">[8]</a> -This generalisation has been found to hold good over wide regions of -the Old World as well as the New. It is not, however, of universal application.<a id="FNanchor_9" href="#Footnote_9" class="fnanchor">[9]</a> -Examples are not uncommon of an actual alternation of acid and -basic lavas from the same, or at least from adjacent vents. Such an alternation -occurs among the Tertiary eruptions of Central France and among those -of Old Red Sandstone age in Scotland.</p> - -<div class="footnote"> - -<p><a id="Footnote_8" href="#FNanchor_8" class="label">[8]</a> <i>Trans. Acad. California</i>, 1868. Prof. Iddings' <i>Journ. Geol.</i>, vol. i. (1893), p. 606.</p> - -<p><a id="Footnote_9" href="#FNanchor_9" class="label">[9]</a> See Prof. Brögger, "Die Eruptivgesteine des Kristianiagebietes," part ii. (1895), p. 175; <i>Zeitsch. -Kryst. und Mineral</i>, vol. xvi. (1890) p. 83. This author would, from this point of view, draw -a distinction between rocks which have consolidated deep within the earth and those which have -flowed out at the surface, since he thinks that we are not justified in applying our experience of -the order of sequence in the one series to the other. Yet there can be no doubt that in many old -volcanic districts the masses that may be presumed to have consolidated at a great depth have -been in unbroken connection with masses that reached the surface. These latter, as Prof. Iddings -has urged, furnish a much larger body of evidence than the intrusive sheets and bosses.</p> - -</div> - -<p>The range of variation in the nature of the eruptive rocks during the -whole of a volcanic period in any district may be termed "a volcanic cycle." -In Britain, where the records of many volcanic periods have been preserved, -a number of such cycles may be studied. In this way the evolution of the -subterranean magma during one geological age may be compared with that -of another. It will be one of the objects of the following chapters to trace -out this evolution in each period where the requisite materials for the -purpose are available. We shall find that back to Archæan time a number -of distinct cycles may be observed, differing in many respects from each -other, but agreeing in the general order of development of the successive -eruptions. Leaving these British examples for future consideration, it may -be useful to cite here a few from the large series now collected from the -European continent and North America.<a id="FNanchor_10" href="#Footnote_10" class="fnanchor">[10]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_10" href="#FNanchor_10" class="label">[10]</a> Prof. M. Bertrand in a suggestive paper published in 1888 dealt with the general order of -appearance of eruptive rocks in different provinces of Europe. But the materials then at his -command probably did not warrant him in offering more than a sketch of the subject, <i>Bull. Soc. -Geol.</i>, France, xvi. p. 573. In the same volume there is a paper by M. Le Verrier, who announces -his opinion that the eruption of the basic rocks takes place in times of terrestrial calm, while -that of the acid rocks occurs in periods of great disturbance, <i>op. cit.</i> p. 498. Compare also -Prof. Brögger, <i>Die Eruptivgesteine des Kristianiagebietes</i>, ii. p. 169.</p> - -</div> - -<p>Among the older rocks of the European continent, Prof. Brögger has -shown that in the Christiania district the eruptive rocks which traverse the -Cambrian and Silurian formations began with the outburst of basic material -<span class="pagenum" id="Page_29">- 29 -</span> -such as melaphyre, augite-porphyrite, and gabbro-diabase, having from about -44 to about 52 per cent of silica. These were followed by rocks with a -silica-percentage ranging from about 50 to 61, including some characteristic -Norwegian rocks, like the rhomben-porphyry. The acidity continued to -increase, for in the next series of eruptions the silica-percentage rose to -between 60 and 67, the characteristic rock being a quartz-syenite. Then -came deep-seated protrusions of highly acid rocks, varieties of granite, -containing from 68 to 75 per cent of silica. The youngest eruptive -masses in the district show a complete change of character. They are -basic dykes (proterobase, diabase, etc.).<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> <i>Eruptivgest. Kristianiageb.</i>, 1895.</p> - -</div> - -<p>The same author institutes a comparison between the post-Silurian -eruptive series of Christiania and that of the Triassic system in the Tyrol, -and believes that the two cycles closely agree.<a id="FNanchor_12" href="#Footnote_12" class="fnanchor">[12]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_12" href="#FNanchor_12" class="label">[12]</a> <i>Op. cit.</i> He supposes in each case the pre-existence of a parent magma from which the -eruptive series started and which had a silica-percentage of about 64 or 65. In this difficult -subject it is of the utmost importance to accumulate fact before proceeding to speculation.</p> - -</div> - -<p>During Tertiary time in Central France more than one cycle may be -made out in distinct districts. Thus in the Velay, during the Miocene -Period, volcanic activity began with the outpouring of basalts, followed -successively by trachytes, labradorites and augitic andesites, phonolites -and basalts. The Pliocene eruptions showed a reversion to the intermediate -types of augitic andesites and trachytes, followed by abundant -basalts, which continued to be poured forth in Pleistocene time.<a id="FNanchor_13" href="#Footnote_13" class="fnanchor">[13]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_13" href="#FNanchor_13" class="label">[13]</a> M. Boule, "Description Géologique du Velay," <i>Bull. Carte. Géol. France</i>, 1892. This author -draws special attention to the evidence for the alternation of basic and more acid material in the -Tertiary eruptions of Central France.</p> - -</div> - -<p>Further north, in Auvergne, where the eruptions come down to a later -period, the volcanic sequence appears to have been first a somewhat acid -group of lavas (trachytes or domites), followed by a series of basalts, then -by andesites and labradorites, the latest outflows again consisting of basalts.<a id="FNanchor_14" href="#Footnote_14" class="fnanchor">[14]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_14" href="#FNanchor_14" class="label">[14]</a> M. Michel Lévy, <i>Bull. Soc. Géol. France</i>, 1890, p. 704.</p> - -</div> - -<p>Not less striking is the succession of lavas in the Yellowstone region, -as described by Mr. Iddings. The first eruptions consisted of andesites. -These were followed by abundant discharges of basalt, succeeded by later -outflows of andesite, and of basalt like that previously erupted. After a -period of extensive erosion, occupying a prolonged interval of time, volcanic -energy was renewed by the eruption of a vast flood of rhyolite, after which -came a feebler outflow of basalt that brought the cycle to a close, though -geysers and fumaroles show that the volcanic fires are not yet entirely -extinguished below.<a id="FNanchor_15" href="#Footnote_15" class="fnanchor">[15]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_15" href="#FNanchor_15" class="label">[15]</a> <i>Journal of Geology</i>, Chicago, i. (1893) p. 606. See also this author's excellent monograph -on "Electric Peak and Sepulchre Mountain," <i>12th Ann. Rep. U.S. Geol. Survey</i> (1890-91), and -Mr. H. W. Turner on "The Succession of Tertiary Volcanic Rocks in the Sierra Nevada of North -America," <i>14th Ann. Rep. U.S. Geol. Survey</i> (1892-93), p. 493.</p> - -</div> - -<p>But not only is there evidence of a remarkable evolution or succession -or erupted material within the volcanic cycle of a single geological period. -One of the objects of the present work is to bring forward proofs that such -<span class="pagenum" id="Page_30">- 30 -</span> -cycles have succeeded each other again and again, at widely separated -intervals, within the same region. After the completion of a cycle and the -relapse of volcanic energy into repose, there has been a renewal of the -previous condition of the subterranean magma, giving rise ultimately to a -similar succession of erupted materials.</p> - -<p>If we are at a loss to account for the changes in the sequence of lavas -during a single volcanic cycle, our difficulties are increased when we find -that in some way the magma is restored each time to somewhat the same -initial condition. Analogies may be traced between the differentiation -which has taken place within a plutonic intrusive boss or sill and the -sequence of lavas in volcanic cycles. It can be shown that though the -original magma that supplied the intrusive mass may be supposed to have -had a fairly uniform composition deep down in its reservoir, differentiation -set in long before the intrusive mass consolidated, the more basic constituents -travelling outwards to the margin and leaving the central parts -more acid. If some such process takes place within a lava-reservoir, it -may account for a sequence of variations in composition. But this would -not meet all the difficulties of the case, nor explain the determining cause -of the separation of the constituents within the reservoir of molten rock, -whether arising from temperature, specific gravity, or other influence. This -subject will be further considered in connection with intrusive Bosses.</p> - -<p>Another fact which may be regarded as now well established is the -persistence of composition and structure in the lavas of all ages. Notwithstanding -the oft-repeated cycles in the character of the magma, the materials -erupted to the surface, whether acid or basic, have retained with wonderful -uniformity the same fundamental characteristics. No part of the contribution -of British geology to the elucidation of the history of volcanic action is -of more importance than the evidence which it furnishes for this persistent -sameness of the subterranean magma. An artificial line has sometimes been -drawn between the volcanic products of Tertiary time and those of earlier -ages. But a careful study of the eruptive rocks of Britain shows that -no such line of division is based upon any fundamental differences.</p> - -<p>The lavas of Palæozoic time have of course been far longer exposed to -alterations of every kind than those of the Tertiary periods, and certain -superficial distinctions may be made between them. But when these accidental -differences are eliminated, we find that the oldest known lavas exhibit the -same types of structure and composition that are familiar in those of Tertiary -and recent volcanoes. Many illustrations of this statement will be furnished -in later chapters. As a particularly striking instance, I may cite here the -most ancient and most modern lavas of the Grand Cañon of the Colorado. -Mr. Walcott and Mr. Iddings have shown that in the Lower Cambrian, or -possibly pre-Cambrian, formations of that great gorge, certain basic lavas -were contemporaneously interstratified, which, in spite of their vast antiquity, -are only slightly different from the modern basalts that have been poured -over the surrounding plateau.<a id="FNanchor_16" href="#Footnote_16" class="fnanchor">[16]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_16" href="#FNanchor_16" class="label">[16]</a> <i>14th Annual Report U.S. Geol. Survey</i> (1892-93).</p> - -</div> - -<p><span class="pagenum" id="Page_31">- 31 -</span></p> - -<p><span class="smcap">The chief lavas found in Britain.</span>—Of the lavas which have been -poured out at the surface within the region of the British Isles, the following -varieties are of most frequent occurrence. In the acid series are Rhyolites -and Felsites, but the vitreous forms are probably all intrusive. The intermediate -series is represented by Trachytes and Andesites (Porphyrites), -which range from a glassy to a holocrystalline structure. The basic series -includes Dolerites, Diabases, Basalts, Limburgites (or Magma-basalts, containing -little or no felspar), and Picrites or other varieties of Peridotites. -The intrusive rocks display a greater variety of composition and structure.</p> - - -<h4>ii. <span class="allsmcap">VOLCANIC AGGLOMERATES, BRECCIAS AND TUFFS</span></h4> - -<p>The coarser fragmentary materials thrown from volcanic vents are -known as Agglomerates where they show no definite arrangement, and -especially where they actually fill up the old funnels of discharge. When -they have accumulated in sheets or strata of angular detritus outside an -active vent they are termed Breccias, or if their component stones have -been water-worn, Conglomerates. The finer ejected materials may be comprehended -under the general name of Tuffs.</p> - -<p>Although these various forms of pyroclastic detritus consist as a rule -of thoroughly volcanic material, they may include fragments of non-volcanic -rocks. This is especially the case among those which represent the earliest -explosions of a volcano. The first efforts to establish an eruptive vent -lead to the shattering of the terrestrial crust, and the consequent discharge -of abundant debris of that crust. The breccias or agglomerates thus produced -may contain, indeed, little or no truly volcanic material, but may be -made up of fragments of granite, gneiss, sandstone, limestone, shale, or -whatever may happen to be the rocks through which the eruptive orifice -has been drilled. If the first explosions exhausted the energy of the vent, -it may happen that the only discharges from it consisted merely of non-volcanic -debris. Examples of this kind have been cited from various old -volcanic districts. A striking case occurs at Sepulchre Mountain in the -Yellowstone Park, where the lower breccias, the product of the earliest -explosions of the Electric Peak volcano, and attaining a thickness of 500 -feet, are full of pieces of the Archæan rocks which underlie the younger -formations of that district.<a id="FNanchor_17" href="#Footnote_17" class="fnanchor">[17]</a> These non-volcanic stones do not occur among -the breccias higher up. Obviously, however, though most abundant at -first, pieces of the underlying rocks may reappear in subsequent discharges, -wherever by the energy of explosion, fragments are broken from the walls -of a volcanic chimney and hurled out of the crater. Illustrations of these -features will be given in the account of the British Carboniferous, Permian -and Tertiary volcanic rocks.</p> - -<div class="footnote"> - -<p><a id="Footnote_17" href="#FNanchor_17" class="label">[17]</a> Prof. J. P. Iddings, <i>12th Ann. Rep. U.S. Geol. Survey</i> (1890-91), p. 634.</p> - -</div> - -<p>It will be obvious that where the component materials of such fragmentary -accumulations consist entirely of non-volcanic rocks, great caution -must be exercised in attributing them to volcanic agency. Two sources of -<span class="pagenum" id="Page_32">- 32 -</span> -error in such cases may here be pointed out. In the first place, where -angular detritus has been precipitated into still water, as, for instance, -from a crag or rocky declivity into a lake, a very coarse and tumultuous -kind of breccia may be formed. It is conceivable that, in course of -time, such a breccia may be buried under ordinary sediments, and may -thereby be preserved, while all trace of its parent precipice may have -disappeared. The breccia might resemble some true volcanic agglomerates, -but the resemblance would be entirely deceptive.</p> - -<p>In the second place, notice must be taken of the frequent results of -movements within the terrestrial crust, whereby rocks have not only been -ruptured but, as already pointed out, have been crushed into fragments. -In this way, important masses of breccia or conglomerate have been formed, -sometimes running for a number of miles and attaining a breadth of several -hundred feet. The stones, often in huge blocks, have been derived from -the surrounding rocks, and while sometimes angular, are sometimes well-rounded. -They are imbedded in a finer matrix of the same material, and -may be scattered promiscuously through the mass, in such a way as to -present the closest resemblance to true volcanic breccia. Where the -crushed material has included ancient igneous rocks it might deceive even -an experienced geologist. Indeed, some rocks which have been mapped and -described as volcanic tuffs or agglomerates are now known to be only -examples of "crush-conglomerates."<a id="FNanchor_18" href="#Footnote_18" class="fnanchor">[18]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_18" href="#FNanchor_18" class="label">[18]</a> For an account of "crush-conglomerates," see Mr. Lamplugh's paper on those of the Isle of -Man, <i>Quart. Journ. Geol. Soc.</i>, li. (1895), p. 563. Mr. M'Henry has pointed to probable cases of -mistake of this kind in Ireland, <i>Nature</i>, 5th March 1896. A. Geikie, <i>Geol. Mag.</i> November 1896.</p> - -</div> - -<p>Not only have vast quantities of detritus of non-volcanic rocks been -shot forth from volcanic vents, but sometimes enormous solid masses of -rock have been brought up by ascending lavas or have been ejected by explosive -vapours. Every visitor to the puys of Auvergne will remember the -great cliff-like prominence of granite and mica-schist which, as described long -ago by Scrope, has been carried up by the trachyte of the Puy Chopine, -and forms one of the summits of the dome (Fig. 344). The same phenomenon -is observable at the Puy de Montchar, where large blocks of granite -have been transported from the underlying platform. Abich has described -some remarkable examples in the region of Erzeroum. The huge crater of -Palandokän, 9687 feet above the sea contains, in cliff-like projections from -its walls as well as scattered over its uneven bottom, great masses of -marmorised limestone and alabaster, associated with pieces of the green -chloritic schists, serpentines and gabbros of the underlying non-volcanic -platform. These rocks, which form an integral part of the structure of the -crater, have been carried up by masses of trachydoleritic, andesitic and -quartz-trachytic lavas.<a id="FNanchor_19" href="#Footnote_19" class="fnanchor">[19]</a> Examples will be given in a later chapter showing -how gigantic blocks of mica-schist and other rocks have been carried many -hundred feet upwards and buried among sheets of lava or masses of agglomerates -during the Tertiary volcanic period in Britain (<a href="../../66493/66493-h/66493-h.htm#v2fig262">Fig. 262</a>).</p> - -<div class="footnote"> - -<p><a id="Footnote_19" href="#FNanchor_19" class="label">[19]</a> Abich, <i>Geologie des Armenischen Hochlandes</i> (Part ii., western half), 1882, p. 76.</p> - -</div> - -<p><span class="pagenum" id="Page_33">- 33 -</span></p> - -<p>In the vast majority of cases, the fragmentary substances ejected by -ancient volcanic explosions, like those of the present day, have consisted -wholly or mainly of material which existed in a molten condition within -the earth, and which has been violently expelled to the surface. Such -ejected detritus varies from the finest impalpable dust or powder up to -huge masses of solid rock. These various materials may come from more -than one source. Where a volcanic orifice is blown out through already -solidified lavas belonging to previous eruptions, the fragments of these lavas -may accumulate within or around the vent, and be gradually consolidated -into agglomerate or breccia. Again, explosions within the funnel may -break up lava-crusts that have there formed over the cooling upper surface -of the column of molten rock. Or the uprising lava in the chimney may -be spurted out in lumps of slag and bombs, or may be violently blown out -in the form of minute lapilli, or of extremely fine dust and ashes.</p> - -<p>Although in theory these several varieties of origin may be discriminated, -it is hardly possible always to distinguish them among the products of -ancient volcanic action. In the great majority of cases old tuffs, having -been originally deposited in water, have undergone a good deal of decomposition, -and such early alteration has been aggravated by the subsequent -influence of percolating meteoric water.</p> - -<p>Where disintegration has not proceeded too far, the finer particles of -tuffs may be seen to consist of minute angular pieces of altered glass, or of -microlites or crystals, or of some vitreous or semi-vitreous substance, in -which such microlites and crystals are enclosed. It has already been stated -that the occurrence of glass, or of any substance which has resulted from -the devitrification of glass, may be taken as good evidence of former volcanic -activity.</p> - -<p>Most commonly, especially in the case of tuffs of high antiquity, like -those associated with the Palæozoic formations, the fresh glassy and microlitic -constituents, so conspicuous in modern volcanic ashes, are hardly to be -recognised. The finer dust which no doubt contained these characteristic -substances has generally passed into dull, earthy, granular, or structureless -material, though here and there, among basic tuffs, remaining as palagonite. -In the midst of this decayed matrix, the lapilli of disrupted lavas may -endure, but it may be difficult or impossible to decide whether they were -derived from the breaking up of older lavas by explosion, or from the -blowing out of the lava-crusts within the funnel.</p> - -<p>The cellular structure, which we have seen to be a markedly volcanic -peculiarity among the lavas, is not less so in their fragments among the -agglomerates, breccias and tuffs; indeed it may be said to be eminently -characteristic of them. The vesicles in the lapilli, bombs, and blocks are -sometimes of large size, as in masses of ejected slag, but they range down -to microscopic minuteness. The lapilli of many old tuffs are sometimes -so crowded with such minute pores, as to show that they were originally -true pumice.</p> - -<p>The composition of tuffs must obviously depend upon that of the lavas -<span class="pagenum" id="Page_34">- 34 -</span> -from which they were derived. But their frequently decayed condition -makes it less easy, in their case, to draw definite boundary-lines between -varieties. In a group of acid lavas, the tuffs may be expected to be also -acid, while among intermediate or basic lavas, the tuffs will generally be -found to correspond. There are, however, exceptions to this general rule. -As will be afterwards described in detail, abundant felsitic tuffs may be -seen among the andesitic lavas of Lower Old Red Sandstone age in Scotland, -and rhyolitic tuffs occur also among the Tertiary basalts of Antrim.</p> - -<p>As a rule, basic and intermediate tuffs, like the lavas from which they -have been derived, are rather more prone to decomposition than the acid -varieties. One of their most characteristic features is the presence in them -of lapilli of a minutely vesicular pumice, which will be more particularly -described in connection with volcanic necks, of which it is a characteristic -constituent. Occasional detached crystals of volcanic minerals, either entire -or broken, may be detected in them, though perhaps less frequently than -in the agglomerates. The earthy matrix is generally greenish in colour, -varying into shades of brick-red, purple and brown.</p> - -<p>The acid tuffs are, on the whole, paler in colour than the others, sometimes -indeed they are white or pale grey, but graduate into tones of -hæmatitic red or brown, the varying ferruginous tints being indicative of -stages in the oxidation of the iron-bearing constituent minerals. Small -rounded lapilli or angular fragments of felsite or rhyolite may be noticed -among them, sometimes exhibiting the most perfect flow-structure. As typical -examples of such tuffs, I may refer to those of the Pentland Hills, near -Edinburgh, and those that lie between the two groups of basalt in Antrim.</p> - -<div class="figleft" id="v1fig13" style="width: 186px;"> - <img src="images/v1fig13.png" width="186" height="181" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 13.</span>—Alternations of coarser and finer - Tuff.</div> -</div> - -<p>Thrown out promiscuously from active vents, the materials that form -tuffs arrange themselves, on the whole, -according to relative size over the surface -on which they come to rest, the largest -being generally grouped nearest to the -focus of discharge, and the finest extending -farthest from it. As the volcanoes of which -records have been preserved among the geological -formations were chiefly subaqueous, -the fragmentary substances, as they fell into -water, would naturally be to some extent -spread out by the action of currents or -waves. They would thus tend to take a -more or less distinctly stratified arrangement. -Moreover, as during an eruption there might be successive paroxysms of -violence in the discharges, coarser and finer detritus would successively -fall over the same spot. In this way, rapid alternations of texture -would arise (<a href="#v1fig13">Fig. 13</a>). A little experience will enable the observer to -distinguish between such truly volcanic variations and those of ordinary -sedimentation, where, for instance, layers of gravel and sand repeatedly -alternate. Besides the volcanic nature of the fragments and their non-water-worn -<span class="pagenum" id="Page_35">- 35 -</span> -forms, he will notice that here and there the larger blocks may -be placed on end—a position the reverse of that usual in the disposal of -aqueous sediments, but one which is not infrequently assumed by ejected -stones, even when they fall through some little depth of water. Further, -the occurrence of large pieces of lava, scattered at random through deposits -of fine tuff, would lead him to recognize the tumultuous discharges of a -volcanic focus, rather than the sorting and sifting action of moving water.</p> - -<p>Admirable illustrations of these various characteristics may be gathered -in endless number from the Palæozoic volcanic chronicles of Britain. I may -especially cite the basin of the Firth of Forth as a classical region for the -study of Carboniferous examples.</p> - -<div class="figright" id="v1fig14" style="width: 202px;"> - <img src="images/v1fig14.png" width="202" height="182" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 14.</span>—Alternations of Tuff (<i>t</i>, <i>t</i>,) with -non-volcanic sediment (<i>l</i>, <i>l</i>).</div> -</div> - -<p>When the conditions of modern volcanic eruptions are considered, it will -be seen that where ejected ashes and stones fall into water, they will there -mingle with any ordinary sediment that may be in course of deposition at -the time. There will thus be a blending of volcanic and non-volcanic -detritus, and the transition between the two may be so insensible that no -hard line of demarcation can be drawn. -Such intermingling has continually taken -place during past ages. One of the first -lessons learnt by the geologist, who begins -the study of ancient volcanic records, is -the necessity of recognizing this gradation -of material, and likewise the frequently -recurring alternations of true tuff with -shale, sandstone, limestone or other entirely -non-volcanic detritus (<a href="#v1fig14">Fig. 14</a>). He soon -perceives that such facts as these furnish -him with some of the most striking proofs -of the reality and progress of former -eruptions. The intermingling of much ordinary detritus with the volcanic -material may be regarded as indicative either of comparatively feeble activity, -or at least of considerable distance from the focus of discharge. It -is sometimes possible to trace such intermixtures through gradually augmenting -proportions of volcanic dust and stones, until the deposit becomes wholly -volcanic in composition, and so coarse in texture as to indicate the proximity -of the eruptive vent. On the other hand, the gradual decrease of the -volcanic ejections can be followed in the upward sequence of a series of -stratified deposits, until the whole material becomes entirely non-volcanic.</p> - -<p>The occurrence of thin partings of tuff between ordinary sedimentary -strata points to occasional intermittent eruptions of ashes or stones, the -vigour and duration of each eruptive interval being roughly indicated by the -thickness and coarseness of the volcanic detritus. The pauses in the volcanic -activity allowed the deposit of ordinary sediment to proceed unchecked. -The nature of such non-volcanic intercalations gives a clue to the physical -conditions of sedimentation at the time, while their thickness affords -some indication of the relative duration of the periods of volcanic repose.</p> - -<p><span class="pagenum" id="Page_36">- 36 -</span></p> - -<p>A little reflection will convince the observer that in such a section as -that represented in <a href="#v1fig14">Fig. 14</a> the volcanic intercalations must be regarded as -a mere local accident. Evidently the normal conditions of sedimentation at -the time these strata were accumulated are indicated by the limestone bands -(<i>l</i>, <i>l</i>). Had there been no volcanic eruptions, a continuous mass of limestone -would have been deposited, but this continuity was from time to time -interrupted by the explosions that gave rise to the intercalated bands of -tuff (<i>t</i>, <i>t</i>).</p> - -<p>The application of these rules of geological evidence will be best understood -from actual examples of their use. Many illustrations of them will -be subsequently given, more especially from the volcanic records of the -Carboniferous period.</p> - -<p>One of the most interesting peculiarities of interstratified tuffs is the -not infrequent occurrence of the remains of plants and animals imbedded -in them. Such remains would have been entombed in the ordinary sediment -had there been no volcanic eruptions, and their presence in the tuffs -is another convincing proof of contemporaneous volcanic action during the -deposition of a sedimentary series. But they may be made to furnish much -more information as to the chronology of the eruptions and the physical -geography of the localities where the volcanoes were active, as will be set -forth farther on.</p> - -<p>Tuffs, as already remarked, frequently occur without any accompaniment -of lava, although lavas seldom appear without some tuff. We thus -learn that in the past, as at present, discharges of fragmentary materials -alone were more common than the outflow of lava by itself. The relative -proportions of the lavas and tuffs in a volcanic series vary indefinitely. In -the Tertiary basalt-plateaux of Britain, the lavas succeed each other, sheet -above sheet, for hundreds of feet, with few and trifling fragmental intercalations. -Among the Carboniferous volcanic ejections, on the other hand, many -solitary or successive bands of tuff may be observed without any visible -sheets of lava. Viewed broadly, however, in their general distribution -during geological time, the two great groups of volcanic material may be -regarded as having generally appeared together. In all the great volcanic -series, from the base of the Palæozoic systems up to the Tertiary plateaux, -lavas and tuffs are found associated, much as they are among the ejections -of modern volcanoes. They often alternate, and thus furnish evidence as to -oscillations of energy at the eruptive vents.</p> - -<p>Now and then, by the explosions from a volcano at the present day, a single -stone may be ejected at such an angle and with such force as to fall to the -ground at a long distance from the vent. In like manner, among the volcanic -records of former periods, we may occasionally come upon a single block of -lava imbedded among tuffs or even in non-volcanic strata. Where such a -stone has fallen upon soft sediment, it can be seen to have sunk into it, -pressing down the layers beneath it, and having the subsequently deposited -layers heaped over it. An ejected block of this nature is represented among -the tuffs shown in <a href="#v1fig13">Fig. 13</a>. Another instance from a group of non-volcanic -<span class="pagenum" id="Page_37">- 37 -</span> -sediments is given in <a href="#v1fig15">Fig. 15</a>, and is selected from a number of illustrations -of this interesting feature which have been observed among the Lower -Carboniferous formations of the basin of the Firth of Forth. A solitary -block, imbedded in a series of strata, would not, of course, by itself afford a -demonstration of volcanic activity. There are various ways in which such -stones may be transported and dropped over a muddy water-bottom. They -may, for example, be floated off attached to sea-weeds, or wrapped round -by the roots of trees. But where a block of basalt or other volcanic rock -has obviously descended with such force as to crush down the deposits on -which it fell, and when lavas and tuffs are known to exist in the vicinity, -there can be little hesitation in regarding such a block as having been -ejected from a neighbouring vent, either during an explosion of exceptional -violence or with an unusually low angle of projection.</p> - -<div class="figcenter" id="v1fig15" style="width: 351px;"> - <img src="images/v1fig15.png" width="351" height="135" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 15.</span>—Ejected block of Basalt which has fallen among Carboniferous shales and limestones, - shore, Pettycur, Fife.</div> -</div> - -<p>In conclusion, reference may conveniently be made here to another -variety of fragmental volcanic materials which cannot always be satisfactorily -distinguished from true tuffs, although arising from a thoroughly -non-volcanic agency. Where a mass of lava has been exposed to denudation, -as, for instance, when a volcanic island has been formed in a lake or -in the sea, the detritus worn away from it may be spread out like any -other kind of sediment. Though derived from the degradation of lava, -such a mechanical deposit is not properly a tuff, nor can it even be included -among strictly volcanic formations. It may be called a volcanic conglomerate, -rhyolitic conglomerate, diabase sandstone, felsitic shale, or by any -other name that will adequately denote its composition and texture. But -it may not afford proof of strictly contemporaneous volcanic activity. All -that we are entitled to infer from such a deposit is that, at the time -when it was laid down, volcanic rocks of a certain kind were exposed -at the surface and were undergoing degradation. But the date of their -original eruption may have been long previous to that of the formation of -the detrital deposit from their waste.</p> - -<p>Nevertheless, it is sometimes possible to make sure that the conglomerate -or sandstone, though wholly due to the mechanical destruction of already -erupted lavas, was in a general sense contemporaneous with the volcanoes -that gave forth these lavas. The detrital formation may be traced perhaps -up to the lavas from which it was derived, and may be found to be intercalated -<span class="pagenum" id="Page_38">- 38 -</span> -in the same sedimentary series with which they are associated. -Or it may contain large bombs and slags, such as most probably came either -directly from explosions or from the washing down of cinder-cones or other -contemporaneously existing volcanic heaps. Examples of such intercalated -conglomerates will be given from the Lower Old Red Sandstone of Central -Scotland and from the Tertiary volcanic plateaux of the Inner Hebrides.</p> - -<hr class="chap x-ebookmaker-drop" /> - - -<div class="chapter"> -<p><span class="pagenum" id="Page_39">- 39 -</span></p> - -<h3 class="nobreak" id="CHAPTER_IV">CHAPTER IV</h3> -</div> - -<div class="blockquot"> - -<p>Materials erupted at the Surface—Extrusive Series—<i>continued</i>. iii. Types of old Volcanoes—1. -The Vesuvian Type; 2. The Plateau or Fissure Type; 3. The Puy Type. iv. -Determination of the relative Geological Dates of ancient Volcanoes. v. How the -Physical Geography associated with ancient Volcanoes is ascertained.</p> -</div> - - -<p>Having now taken note of the various materials ejected to the surface from -volcanic orifices, we may pass to the consideration of these orifices themselves, -with the view of ascertaining under what various conditions volcanic -action has taken place in the geological past. We have seen that modern -and not long extinct volcanoes may be grouped into three types, and a -study of the records of ancient volcanoes shows that the same types may -be recognized in the eruptions of former ages. The following chapters will -supply many illustrations of each type from the geological history of the -British Isles. In dealing with these illustrations, however, we must ever -bear in mind the all-powerful influence of denudation. We ought not to -expect to meet with the original forms of the volcanoes. Some little -reflection and experience may be required before we can realize under what -aspect we may hope to recognize ancient and much-denuded volcanoes. It -may therefore be of advantage to consider here, in a broad way, which of -the original characters are most permanent, and should be looked for as -mementoes of ancient volcanoes after long ages of denudation.</p> - - -<h4>iii. <span class="allsmcap">TYPES OF OLD VOLCANOES</span></h4> - -<p>The three forms of ancient volcanoes now to be discussed are—1st, the -Vesuvian type; 2nd, the Plateau or Fissure type; and 3rd, the Puy type.</p> - -<p>1. <i>The Vesuvian Type.</i>—In this kind of volcano, lavas and fragmental -ejections are discharged from a central vent, which is gradually built up -by successive eruptions of these materials. As the cone increases in size, -parasitic cones appear on its sides, and in the energy and completeness of -their phenomena become true volcanoes, almost rivalling their parent -mountain. Streams of lava descend upon the lower grounds, while showers -of dust and ashes are spread far and wide over the surrounding country.</p> - -<p>If a transverse section could be made of a modern Vesuvian cone, the -volcanic pile would be found to consist of alternations of lavas and tuffs, -thickest at the centre, and thinning away in all directions. At some -<span class="pagenum" id="Page_40">- 40 -</span> -distance from the crater, these volcanic materials might be seen to include -layers of soil and remains of land-vegetation, marking pauses between the -eruptions, during which soil accumulated and plants sprang up upon it. Where -the lavas and ashes had made their way into sheets of fresh water or into -the sea, they would probably be found interstratified with layers of ordinary -sediment containing remains of the animal or vegetable life of the time.</p> - -<div class="figcenter" id="v1fig16" style="width: 510px;"> - <img src="images/v1fig16.png" width="510" height="169" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 16.</span>—Effects of denudation on a Vesuvian cone.</div> -</div> - -<p>Conceive now the effects of prolonged denudation upon such a pile of -volcanic rocks. The cone will eventually be worn down, the crater will -disappear, and the only relics of the eruptive orifice may be the top of the -central lava-column and of any fragmental materials that solidified within -the vent (<a href="#v1fig16">Fig. 16</a>). The waste will, on the whole, be greater at the cone -than on the more level areas beyond. It might, in course of time, reach -the original surface of the ground on which the volcano built up its heap -of ejected material. The central lava-plug might thus be left as an isolated -eminence rising from a platform of older non-volcanic rocks, and the distance -between it and the dwindling sheets of lava and tuff which came out of it -would then be continually increased as their outcrop receded under constant -degradation.</p> - -<p>This piece of volcanic history is diagrammatically illustrated in <a href="#v1fig16">Fig. 16</a>. -The original forms of the central volcano and of its parasitic cones are -suggested by the dotted lines in the upper half of the Figure. All this -upper portion has been removed by denudation, and the present surface of -the ground is shown by the uppermost continuous line. The general structure -of the volcanic pile is indicated underneath that line—the lenticular sheets -of lava and tuff (<i>l</i>, <i>l</i>), the dykes (<i>d</i>, <i>d</i>), and the lavas (<i>p</i>, <i>p</i>) and agglomerates -(<i>a</i>, <i>a</i>) of the central vent and of the subordinate cones.</p> - -<p>The waste, though greatest on the higher ground of the great cone, -would not stop there. It would extend over the flatter area around the -volcano. Streams flowing over the plain would cut their way down through -the lavas and tuffs, eroding ravines in them, and leaving them in detached -and ever diminishing outliers on the crests of the intervening ridges. We -can easily picture a time when the last of these relics would have been worn -away, and when every vestige of the volcanic ejections would have been -removed, save the lava-column marking the site of the former vent.</p> - -<p><span class="pagenum" id="Page_41">- 41 -</span></p> - -<p>Every stage in this process of effacement may be recognized in actual -progress among the extinct volcanoes of the earth's surface. Probably -nowhere may the phenomena be more conveniently and impressively studied -than among the volcanic districts of Central France. On the one hand, we -meet there with cinder-cones so perfect that it is hard to believe them to -have been silent ever since the beginnings of history. On the other hand, -we see solitary cones of agglomerate or of lava, which have been left -isolated, while their once overlying and encircling sheets of ejected material -have been so extensively worn away as to remain merely in scattered patches -capping the neighbouring hills. Valleys many hundreds of feet in depth -have been cut by the rivers through the volcanic sheets and the underlying -Tertiary strata and granite since the older eruptions ceased. And yet these -eruptions belong to a period which, in a geological sense, is quite recent. -It is not difficult to contemplate a future time, geologically not very remote, -when in the valley of the Loire not a trace will remain of the wonderfully -varied and interesting volcanic chronicle of that district, save the plugs that -will mark the positions of the former active vents.</p> - -<p>In the British Islands, ancient volcanoes of the Vesuvian type are well -represented among the Palæozoic systems of strata. Their preservation has -been largely due to the fact that they made their appearance in areas that -were undergoing slow subsidence. Their piles of erupted lava and ashes -were chiefly heaped up over the sea-floor, and were buried under the sand, -silt and ooze that gathered there. Thus covered up, they were protected -from denudation. It is only in much later geological ages that, owing to -upheaval, gradual degradation of the surface, and removal of their overlying -cover of stratified formations, they have at last been exposed to waste. The -process of disinterment may be observed in many different stages of progress. -In some localities, only the tops of the sheets of lava and tuff have begun -to show themselves; in others, everything is gone except the indestructible -lava-plug.</p> - -<p>These inequalities of denudation arise not only from variations in the -durability of volcanic rocks, but still more from the relative position of -these rocks in the terrestrial crust, and the geological period at which, in -the course of the general lowering of the surface, they have been laid bare. -Not only are volcanic rocks of many different ages, and lie, therefore, on -many successive platforms within the crust of the earth: their places have -been still further dependent upon changes in the arrangement of that crust. -Fracture, upheaval, depression, curvature, unconformable deposition of strata, -have contributed to protect some portions, while leaving others exposed to -attack. Hence it happens that the volcanic record varies greatly in its -fulness of detail from one geological system or one district to another. -Some chapters have been recorded with the most surprising minuteness, so -that the events which they reveal can be realized as vividly as those of a -modern volcano. Others, again, are meagre and fragmentary, because the -chronicle is still for the most part buried underground, or because it has been -so long exposed at the surface that only fragments of it now remain there.</p> - -<p><span class="pagenum" id="Page_42">- 42 -</span></p> - -<p>In the descriptions which will subsequently be given of ancient British -volcanoes of the Vesuvian type, it will be shown that at many successive -periods during Palæozoic time, and at many distinct centres, lavas and tuffs -have been piled up to a depth of frequently more than 5000 feet—that is to -say, higher than the height of Vesuvius. Sometimes the vent from which -these materials were ejected can be recognized. In other places, it is still -buried under later formations, or has been so denuded as to be represented -now merely by the column of molten or fragmental rock that finally -solidified in it. Examples will be quoted of such ancient vents, measuring -not less than two miles in diameter, with subsidiary "necks" on their flanks, -like the parasitic cones on Etna.</p> - -<p>I shall show that while the ejected volcanic products have accumulated -in greatest depth close to the vent that discharged them, they die away as -they recede from it, sometimes so rapidly that a volcanic pile which is -7000 feet thick around its source may entirely thin out and disappear in a -distance of not more than ten or twelve miles. I shall point out how, as -the lavas and tuffs are followed outwards from their centre, they not only -get thinner, but are increasingly interstratified among the sedimentary -deposits with which they were coeval, and that in this way their limits, -their age, and the geographical conditions under which they were accumulated -can be satisfactorily fixed.</p> - -<p>As illustrations of the Vesuvian type in the volcanic history of Britain, -I may refer to the great Lower Silurian volcanoes of Cader Idris, Arenig, -Snowdon and the Lake District, and to the Old Red Sandstone volcanoes -of Central Scotland.</p> - -<p>2. <i>The Plateau</i> or <i>Fissure type</i> is, among modern volcanoes, best -developed in Iceland, as will be more fully detailed in <a href="../../66493/66493-h/66493-h.htm#CHAPTER_X">Chapter xl.</a> In that -island, during a volcanic eruption, the ground is rent open into long parallel -fissures, only a few feet or yards in width, but traceable sometimes for -many miles, and descending to an unknown depth into the interior. From -these fissures lava issues—in some cases flowing out tranquilly in broad -streams from either side, in other cases issuing with the discharge of slags -and blocks of lava which are piled up into small cones set closely together -along the line of the rent. It was from a fissure of this kind that the -great eruption of 1783 took place—the most stupendous outpouring of lava -within historic time.</p> - -<p>By successive discharges of lava from fissures, or from vents opening on -lines of fissure, wide plains may be covered with a floor of rock hundreds or -thousands of feet in thickness, made up of horizontal beds. The original -topography, which might have been undulating and varied, is completely -buried under a vast level lava-desert.</p> - -<p>The rivers which drained the country before the beginning of the -volcanic history will have their channels filled up, and will be driven to -seek new courses across the lava-fields. Again and again, as fresh -eruptions take place, these streams will be compelled to shift their line of -flow, each river-bed being in turn sealed up in lava, with all its gravels, -<span class="pagenum" id="Page_43">- 43 -</span> -silts and drift-wood. But the rain will continue to fall, and the drainage -to seek its way seaward. When the last eruption ceases, and the rivers are -at length left undisturbed at their task of erosion, they will carve that lava-floor -into deep gorges or open valleys. Where they flow between the -lavas and the slopes against which these ended, they will cut back the -volcanic pile, until in course of time the lavas will present a bold mural -escarpment to the land that once formed their limit. The volcanic plain -will become a plateau, ending off in this vertical wall and deeply trenched by -the streams that wind across it. And if the denudation is continued long -enough, the plateau will be reduced to detached hills, separated by deep and -wide valleys.</p> - -<div class="figcenter" id="v1fig17" style="width: 504px;"> - <img src="images/v1fig17.png" width="504" height="111" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 17.</span>—Section to illustrate the structure of the Plateau type.</div> -</div> - -<p>This geological history is illustrated by the diagram in <a href="#v1fig17">Fig. 17</a>. The -stippled ground underneath (<i>x</i>, <i>x</i>) represents the original undulating surface of -the country on which the plateau eruptions were poured out. The lavas of -these eruptions are shown by the horizontal lines to have entirely buried -the heights and hollows of the old land, and to have risen up to the upper -dotted line, which may be taken to mark the limit reached by the accumulation -of volcanic material. The dark lines (<i>d</i>, <i>d</i>) which come up -through the bedded lavas indicate the dykes with their connected vents. -Denudation has since stripped off the upper part of the volcanic series -down to the uppermost continuous black line which represents the existing -surface of the ground. The level sheets of lava have been deeply trenched, -and in one instance the valley has not only been cut through the volcanic -pile, but has been partly eroded out of the older rocks below. To the -right and left, the lavas end off abruptly in great escarpments.</p> - -<p>The succession of events here depicted has occurred more than once in -Britain. The Plateau type is chiefly developed in this country among the -great Tertiary basalt districts of Antrim and the Inner Hebrides, which -reappear in the Faroe Islands, and again still farther north in Iceland. -But it also occurs among the volcanic rocks of the Old Red Sandstone and -Carboniferous periods.</p> - -<p>As compared with the other volcanic types, that of the Plateaux is -distinguished by the wide extent of surface which its rocks cover, by the -great preponderance of lavas over tuffs, and by the regularity and persistence -of the individual sheets of rock. In Britain, the plateau-lavas are even still -often approximately horizontal, and lie piled on each other in tolerably regular -beds to a thickness of 1000, and in one place to more than 3000 feet. -<span class="pagenum" id="Page_44">- 44 -</span> -They form wide level or gently undulating tablelands, which rise in bold -escarpments from the surrounding country and have been deeply carved -into valleys. The sides of their cliffs and slopes are marked by parallel -lines of terrace, arising from the outcrop of successive sheets of lava (Figs. -<a href="#v1fig11">11</a>, <a href="../../66493/66493-h/66493-h.htm#v2fig265">265</a>).</p> - -<p>With the Tertiary basalt-plateaux are connected thousands of dykes, that -follow each other along nearly parallel lines in a general north-westerly direction, -and mark the position of fissures up which the molten lava ascended. -Occasional necks of agglomerate or basalt indicate the sites of some of the -eruptive vents.</p> - -<p>The Carboniferous volcanic plateaux have been more extensively denuded -than those of Tertiary age, so that a large number of their vents have been -laid bare. In general these vents are of comparatively small size, though -larger than those of the Carboniferous Puys. In some districts, abundant -dykes traverse the rocks on which the plateaux rest, though the fissures -seem to have been less numerous than in Tertiary time.</p> - -<p>3. <i>The Puy type</i>, as before remarked, takes its name from the well-known -<i>puys</i>, or volcanic cones, of Central France. Volcanoes of this type -form conical hills, generally of small size, consisting usually of fragmental -materials, sometimes of lava. Where a cone is partially effaced by a second, -and even by a third, successive slight shiftings of the vent are to be inferred -(see Figs. <a href="#v1fig29">29</a> and <a href="../../66493/66493-h/66493-h.htm#v2fig214">214</a>). In many cases, no lava has issued from such cones, -nor were the ashes and cinders dispersed far from the vent. Hence, in the -progress of denudation, cones of this kind are easily effaced.</p> - -<p>From the uniformity of composition of their materials, the simplicity and -regularity of their forms, and their small size, it may be inferred that many -of these cones were the products of single eruptions. They may conceivably -have been thrown up in a few days, or even in a single day. The history of -Monte Nuovo, in the Bay of Naples, which was formed within twenty-four -hours in the year 1538, is a memorable example of the rapidity with which -a cone more than 400 feet high may be thrown up at some distance from a -central vent.</p> - -<p>The smallest independent volcanoes are included in the Puy type. In -many instances the diameter of the funnel has not exceeded a few yards; -the largest examples of the type seldom exceed 1000 feet in width.</p> - -<p>Where lavas have been discharged, as well as ashes and stones, a more -vigorous activity is indicated than where merely cones of tuff were formed. -The lavas may come from more than one side of a cone, and may flow in -opposite directions for a distance of several miles. It is observable that -considerable streams of lava have issued from the base of a cinder-cone -without disturbing it. The molten rock has found a passage between the -loose materials and the surface on which they rest,<a id="FNanchor_20" href="#Footnote_20" class="fnanchor">[20]</a> though, in some cases, -the cone may have been thrown up after the emission of the lava.</p> - -<div class="footnote"> - -<p><a id="Footnote_20" href="#FNanchor_20" class="label">[20]</a> M. Boule, <i>Bull. Carte Géol. France</i>, No. 28, tome iv. p. 232.</p> - -</div> - -<p>In the history of a puy there is commonly a first discharge of fragmentary -material; then lava may flow out, followed by a final discharge of loose stones -<span class="pagenum" id="Page_45">- 45 -</span> -and ashes. Hence the products of such a vent group themselves into three -layers—two of breccia separated by an intervening sheet of lava.<a id="FNanchor_21" href="#Footnote_21" class="fnanchor">[21]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_21" href="#FNanchor_21" class="label">[21]</a> M. Boule, <i>Bull. Carte Géol. France</i>, No. 28, tome iv.</p> - -</div> - -<p>Great changes are wrought on puys and their connected lavas and tuffs -during the progress of denudation. The cones are eventually destroyed, and -only a stump of agglomerate or lava is left to mark its place. The connection -of a lava-stream with its parent vent may likewise be effaced, and the lava -itself may be reduced to merely a few separate patches, perhaps capping a -ridge, while the surrounding ground has been hollowed into valleys. If the -waste continues long enough, even these outliers will disappear, and nothing -but the neck or stump of the little volcano will remain.</p> - -<div class="figcenter" id="v1fig18" style="width: 502px;"> - <img src="images/v1fig18.png" width="502" height="101" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 18.</span>—Diagram illustrating the structure and denudation of Puys.</div> -</div> - -<p>The accompanying diagram (<a href="#v1fig18">Fig. 18</a>) may help to make these changes -more intelligible. The upper dotted lines show the original forms of three -puys with the covering of loose materials discharged by them over the surrounding -ground. The lower shaded portion represents the surface as left by -denudation, and a section of the three vents beneath that surface. The whole -of the cones and craters has here been swept away, and only the volcanic -"neck" is in each case left. In the vent to the right, the material that -fills it up is a coarse agglomerate, which projects as a rounded dome above -the surrounding country. The central pipe is filled with fragmentary -materials, through which molten rock has risen, giving off dykes and -veins. In the vent to the left hand, only lava is seen to occupy the orifice, -representing the column of molten rock which solidified there and brought -the activity of this little volcano to an end. It will be observed that in -each of these volcanic hills the present outlines are very far from being those -of the original volcano, and that the eminence projects because of its greater -resistance to the forces of denudation that have not only removed the superficial -volcanic material, but have made some progress in lowering the level of -the ground on which that material was accumulated.</p> - -<p>The typical area for the study of Puys is the extraordinarily interesting -volcanic region of Central France. There the volcanic cones are clustered -in irregular groups, sometimes so close as to be touching each other; elsewhere -separated by intervals of several miles. They may be traced in all -stages of decay, from the most perfect cones and craters to the isolated -eminence that marks the site of a once active chimney. Their lavas, too, -may be seen as detached fragments of plateaux, many hundred feet above the -valleys that have been excavated since they flowed.<a id="FNanchor_22" href="#Footnote_22" class="fnanchor">[22]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_22" href="#FNanchor_22" class="label">[22]</a> See Desmarest's classic map and his papers in <i>Mem. Acad. Roy. Sciences</i>, Paris, 1774, 1779; -<i>Journ. de Physique</i>, 1779; Scrope's <i>Geology of Central France</i>, 1827, and <i>Extinct Volcanoes of -Central France</i>, 1858; Lecoq's <i>Époques Géologiques de l'Auvergne</i>, 1867; M. Michel Lévy, <i>Bull. -Soc. Géol. France</i>, 1890, p. 688; M. Boule, <i>Bull. Carte Géol. France</i>, No. 28, tome iv. 1892.</p> - -</div> - -<p><span class="pagenum" id="Page_46">- 46 -</span></p> - -<p>Another well-known region of modern Puys is that of the Eifel, where -the cones and craters are often so fresh that it is difficult to believe them -to be prehistoric.<a id="FNanchor_23" href="#Footnote_23" class="fnanchor">[23]</a> One of the most remarkable denuded puy-regions in -Europe covers a wide territory in the Swabian Alps of Würtemberg. No -fewer than 125 denuded necks filled with tuff, agglomerate and basalt have -there been mapped and described. They are of higher antiquity than the -Upper Miocene strata, and have thus probably been exposed to prolonged -denudation. In external aspect and internal structure they present the -closest parallel to the Carboniferous and Permian "necks" of Britain described -in Books VI. and VII. of the present work.<a id="FNanchor_24" href="#Footnote_24" class="fnanchor">[24]</a></p> - -<p>Among the Palæozoic volcanoes of Britain many admirable illustrations -of the Puy type are to be found. Their cones are almost always entirely -gone, though traces of them occasionally appear. The "necks" that show the -position of the vents are in some districts crowded together as thickly as -those of Auvergne. During the Carboniferous and Permian periods in -Central Scotland, clusters of such little volcanoes must have risen among -shallow lagoons and inland sheets of water, casting out their ashes and -pouring forth their little streams of lava over the water-bottom around them -and then dying out. As these eruptions took place in a region that -was gradually subsiding, the cones and their ejected ashes and lavas -were one by one submerged, the looser materials being washed down and -spread out among the silt, sand or mud, and enveloping the remains of any -plants or animals that might be strewn over the floor of the lake or sea. -Hence the Puys of Palæozoic time in Britain have been preserved with -extraordinary fulness of detail. They have been dissected by denudation, -both among the hills of the interior and along the margin of the sea. Their -structure can thus be in some respects made out even more satisfactorily -than that of the much younger and more perfect cones of Central France.</p> - -<div class="footnote"> - -<p><a id="Footnote_23" href="#FNanchor_23" class="label">[23]</a> The Eifel district has been fully described by Hibbert, Von Dechen, and other writers. Von -Dechen's little handbooks to the Eifel and Siebengebirge are useful guides.</p> - -<p><a id="Footnote_24" href="#FNanchor_24" class="label">[24]</a> These Würtemberg vents have been elaborately described and discussed by Professor W. -Branco of Tübingen in his <i>Schwabens 125 Vulkan-Embryonen und deren tufferfülte Ausbruchsröhren, -das grösste Gebiet chemaliger Maare auf der Erde</i>, Stuttgart, 1894.</p> - -</div> - - -<h4>iv. <span class="allsmcap">DETERMINATION OF THE RELATIVE GEOLOGICAL DATES OF ANCIENT -VOLCANOES</span></h4> - -<p>In themselves, accumulations of volcanic materials do not furnish any -exact or reliable evidence of the geological period in which they were -erupted. The lavas of the early Palæozoic ages may, indeed, on careful -examination, be distinguished from those of Tertiary date, but, as we have -seen, the difference is rather due to the effects of age and gradual alteration -than to any inherent fundamental distinction between them. In all essential -particulars of composition and internal structure, the lavas of the Cambrian -or Silurian period resemble those of Tertiary and modern volcanoes. The -<span class="pagenum" id="Page_47">- 47 -</span> -igneous magmas which supply volcanic vents thus appear to have been very -much what they are now from early geological epochs. At least no important -difference, according to relative age, has yet been satisfactorily -established among them.</p> - -<p>But although the rocks themselves afford no precise or trustworthy clue -to their date, yet where they have been intercalated contemporaneously -among fossiliferous stratified formations, of which the geological horizon can -be determined from included organic remains, it is easy to assign them -to their exact place in geological chronology. A determination of this kind -is only an application of the general principle on which the sequence of the -geological record is defined. A few illustrations will suffice to make this -point quite obvious.</p> - -<p>Among the volcanic tuffs in the upper part of Snowdon various fossils -occur, which are identical with those found in the well-known Bala Limestone. -As the accepted reading of such evidence, we conclude that these -tuffs must therefore be of the same geological age as that limestone. Now -the position of this seam of rock has been well established as a definite -horizon in the series of Lower Silurian formations. And we consequently -without hesitation place the eruptions of the Snowdon volcano on that same -platform, and speak of them as belonging to the Bala division of the Lower -Silurian period.</p> - -<p>Again, in West Lothian the tuffs and lavas ejected from many scattered -puys were interstratified among shales and limestones in which the characteristic -fossils of the Carboniferous Limestone are abundant. There cannot, -therefore, be any doubt that these eruptions were much younger than those -of Snowdon, and that they took place at the time when the Carboniferous -Limestone was being deposited. We thus speak of them as belonging to -volcanoes which were active in that early part of the Carboniferous period -to which the thick Mountain Limestone of Ireland and Derbyshire belongs.</p> - -<p>As yet another illustration of the determination of geological age, an -example from the plateau-type of eruption may be given. The great basalt-plateaux -of Antrim and the Inner Hebrides are built up of lavas that lie -unconformably on the Chalk. They are thus proved to be later than the -Cretaceous system, and this deduction would hold true even if no organic -remains were found associated with the volcanic rocks. But here and there, -intercalated between the basalts, lie layers of shale, limestone and tuff containing -well-preserved remains of plants which are recognizable as older -Tertiary forms of vegetation. This fossil evidence definitely places the date -of the eruptions in older Tertiary time.</p> - -<p>It is clear that, proceeding on this basis of reasoning, we may arrange -the successive volcanic eruptions of any given district, make out their order -of sequence in time, and thus obtain materials for a consecutive history of -them. Or, proceeding from that district into other regions, we may compare -its volcanic phenomena with theirs, determine the relative dates of their -respective eruptions, and in this way compile a wider history of volcanic -action in past time. It is on these principles that the general and detailed -<span class="pagenum" id="Page_48">- 48 -</span> -chronology of the volcanic rocks of the British Isles has been worked out, -and that the following chapters have been arranged.</p> - - -<h4>v. <span class="allsmcap">HOW THE PHYSICAL GEOGRAPHY ASSOCIATED WITH ANCIENT -VOLCANOES IS ASCERTAINED</span></h4> - -<p>While the materials erupted from old volcanic vents tell plainly enough -their subterranean origin, they may leave us quite in the dark as to the -conditions under which they were thrown out at the surface. Yet a careful -examination of the strata associated with them may throw much light -on the circumstances in which the eruptions took place. Many of the -results of such examination will be given in subsequent chapters. I will -here submit illustrations of how four different phases of physical geography -during former volcanic eruptions may be satisfactorily determined.</p> - -<div class="figright" id="v1fig19" style="width: 165px;"> - <img src="images/v1fig19.png" width="165" height="289" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 19.</span>—Section illustrating submarine - eruptions; alternations of lavas and tuffs with limestones - and shales full of marine organisms.</div> -</div> - -<p>1. <i>Submarine Eruptions.</i>—As by far the largest accessible part of the -crust of the earth consists of old marine sediments, it is natural that the -volcanic records preserved in that crust should -be mainly those of submarine eruptions. That -many lavas during the geological past were poured -out upon the sea-bottom is plainly shown by the -thick beds of marine organisms which they have -overspread and which lie above them (<a href="#v1fig19">Fig. 19</a>). -In Central Scotland, for example, sheets of basalt -have flowed over a sea-bottom on which thick -groves of crinoids, bunches of coral and crowds -of sea-shells were living. Not less striking is -the evidence supplied by bands of tuff. Around -Limerick, for instance, the thick Carboniferous -Limestone is interrupted by many thin layers -of tuff marking intervals when showers of volcanic -dust fell over the sea-bottom, killing off the -organisms that lived there. But the limestone -that overlies these volcanic intercalations is again -crowded with fossils, proving that the crinoids, -corals and shells once more spread over the place -and flourished as abundantly as ever above the -tuff.</p> - -<p>The accompanying diagram (<a href="#v1fig19">Fig. 19</a>) illustrates -these statements. At the bottom a thick mass of limestone (<i>l</i>) full of crinoids, -corals, brachiopods and other marine organisms bears witness to a long time -of repose, when the clear sea-water teemed with life. At last a volcanic -explosion took place, which threw out the first seam of tuff (<i>t</i>). But this -was only a transient interruption, for the accumulation of calcareous sediment -was immediately resumed, and the next band of limestone was laid down. -Thereafter a more prolonged or vigorous eruption ejected a larger mass of -dust and stones, which fell over the bottom and prevented the continuation -<span class="pagenum" id="Page_49">- 49 -</span> -of the limestone. But that the sea still abounded in life is shown by the -numerous organisms imbedded in the second stratified band of tuff. At -last an access of volcanic vigour gave vent to a stream of slaggy lava, which -rolled over the sea-bottom and solidified in the thick sheet of amydaloidal -basalt marked B. This outflow was followed by a further discharge of -ashes and stones, which, from their absence of stratification, may be supposed -to have been the result of a single explosion, or at least to have fallen too -rapidly for the marine currents to rearrange them in layers. When the -water cleared, the abundant sea-creatures returned, and from their crowded -remains limestone once more gathered over the bottom. Yet the volcanic -history had not then reached its close, for again there came a discharge of -ashes, followed by the outpouring of a second lava, which consolidated as a -sheet of columnar basalt (B').</p> - -<p>It is not necessary, in order to prove the eruptions to have been submarine, -that organic remains should be found in the tuffs or between them. -If the volcanic ejections are intercalated among strata which elsewhere can -be proved to be marine, their discharge must obviously have taken place -under the sea. The vent that discharged them may have raised its head -above the sea-level, but its lavas and tuffs were spread out over the adjoining -sea-floor.</p> - -<p>2. <i>Lacustrine Eruptions.</i>—The same line of evidence furnishes proof -that some volcanoes arose in inland sheets of water. If their products are -interstratified among sandstones, gravels and shell-marls, wherein the remains -of land-plants, insects and lacustrine shells, are preserved, we may be confident -that the eruptions took place in or near to some lake-basin. The -older lavas and tuffs of Central France supply an instructive example of such -an association. In Britain, the abundant and extensive outpouring of lavas -and tuffs during the time of the Lower Old Red Sandstone probably occurred -in large lakes. Among the sediments of these bodies of water, interstratified -between the volcanic sheets, remains of land-plants are abundant, together -with, here and there, those of myriapods washed down from the woodlands, -and of many forms of ganoid fishes.</p> - -<div class="figleft" id="v1fig20" style="width: 272px;"> - <img src="images/v1fig20.png" width="272" height="127" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 20.</span>—Diagram illustrating volcanic eruptions on a - river-plain.</div> -</div> - -<p>3. <i>Fluviatile Eruptions.</i>—Volcanoes have sometimes arisen on river-plains -or on the edges of valleys -and gorges, and have poured -out their lavas and discharged -their ashes over the channels -or alluvial lands of the streams. -Volcanic materials, usurping the -water-channels, bury or are -interstratified with fluviatile -sand or shingle, containing perhaps -remains of the vegetation -or animal life of the surrounding land. There may thus be a constant -shifting of the river-courses, and a consequent deposit of fluviatile sediment -at many successive levels among the lavas and tuffs. In <a href="#v1fig20">Fig. 20</a> some -<span class="pagenum" id="Page_50">- 50 -</span> -of these changes are indicated in a series of bedded lavas (<i>l</i>). The lower -part of the diagram shows the dying out of a bed of river gravel (<i>g</i>) -against the sloping end of a lava-stream, and the sealing up of this intercalation -by a fresh outpouring of lava. Higher up in the diagram a section -is shown of a gully or ravine which has been cut out of the lavas by a -stream, and has become choked up with water-worn detritus. Subsequent -outflows of lava have rolled over this channel and sealed it up. Examples -of such intercalations of lava with old river deposits, and of the burying -of water-courses, will be cited in the account of the Tertiary volcanic plateaux -of Britain in <a href="../../66493/66493-h/66493-h.htm#CHAPTER_XXXVIII">Chapter xxxviii</a>.</p> - -<p>4. <i>Terrestrial Eruptions.</i>—That volcanoes in former times broke out on -land as well as in water may readily be expected. But it is obvious that -the proofs of a terrestrial origin may not be always easy to obtain, for every -land-surface is exposed to denudation; and thus the relics of the eruptions -of one age may be effaced by the winds, rains, frosts and rivers of the next. -In assigning any volcanic group to a terrestrial origin, we may be guided -partly by negative evidence, such as the absence of all trace of marine -organisms in any of the sedimentary layers associated with the group. But -such evidence standing by itself would not be satisfactory or sufficient. If, -however, between the sheets of lava there occur occasional depressions, -filled with hardened sediment full of land-plants, with possibly traces -of insects and other terrestrial organisms, we may with some confidence -infer that these silted-up hollows represent pools or lakes that gathered -on the surface of the lava-sheets, and into which the vegetation of the -surrounding ground was blown or washed. Rain falling on the rugged -surface of a lava-field would naturally gather into pools and lakes, as the -bottoms of the hollows became "puddled" by the gradual decay of the rock -and the washing of fine silt into the crevices of the lava.</p> - -<div class="figleft" id="v1fig21" style="width: 270px;"> - <img src="images/v1fig21.png" width="270" height="112" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 21.</span>—Diagram illustrating volcanic eruptions on a -land-surface.</div> -</div> - -<p>Again, it may be expected that prolonged exposure to the air would give -rise to disintegration of the lava -and to the consequent formation -of soil. Terrestrial vegetation -would naturally spring up -on such soil; trees might take -root upon it. Hence, if another -lava-flood deluged the surface, -the soil and its vegetable mantle -would be entombed under the -molten rock.</p> - -<p>These geological changes are represented diagrammatically in <a href="#v1fig21">Fig. 21</a>. -Two hollows among the lavas are there shown to have been filled with silt, -including successive layers of vegetation now converted into coal. One of -the soils (<i>s</i>) is marked between the lavas, and the charred stump of a tree -with its roots still in another layer of soil higher up is seen to have been -engulphed in the overlying sheet of melted rock.</p> - -<p>Admirable illustrations of this succession of events are to be encountered -<span class="pagenum" id="Page_51">- 51 -</span> -among the great Tertiary basaltic plateaux which cover so large an area in -the north-west of Europe. Not only has no trace of any marine organism -been found among their interstratified sedimentary layers, but they have -yielded a terrestrial flora which is preserved in hollows between the -successive sheets of basalt. A full account of these rocks will be given in -Book VIII.</p> - -<hr class="chap x-ebookmaker-drop" /> - - -<div class="chapter"> -<p><span class="pagenum" id="Page_52">- 52 -</span></p> - -<h2 class="nobreak" id="CHAPTER_V">CHAPTER V</h2> -</div> - -<div class="blockquot"> - -<p>Underground Phases of Volcanic Action. B. Materials injected or consolidated beneath the -Surface—Intrusive Series: I. Vents of Eruption—i. Necks of Fragmentary Materials; -ii. Necks of Lava-form Materials; iii. Distribution of Vents in relation to Geological -Structure-Lines; iv. Metamorphism in and around Volcanic Cones, Solfataric Action; -v. Inward Dip of Rocks towards Necks; vi. Influence of contemporaneous Denudation -upon Volcanic Cones; vii. Stages in the History of old Volcanic Vents.</p> -</div> - - -<p>In our profound ignorance of the nature of the earth's interior, we know as -yet nothing certain regarding the condition and distribution there of those -molten materials which form the prime visible source of volcanic energy. -By the study of volcanoes and their products we learn that the fused substances -are not everywhere precisely the same and do not remain absolutely -uniform, even in the same volcanic region. But in what manner and from -what causes these variations arise is still unknown. We are further aware -that the molten magma, under a centre of volcanic disturbance, manifests -from time to time energetic movements which culminate in eruptions at the -surface. But what may be the exciting cause of these movements, to what -depth they descend, and over what extent of superficies they spread, are -matters regarding which nothing better than conjecture can as yet be -offered. It is true that, in some cases, a magma of fairly uniform composition -has been erupted over a vast tract of the earth's surface, and must -have had a correspondingly wide extent within the terrestrial crust. Thus -in the case of the older Tertiary volcanic eruptions of North-Western -Europe, basalt of practically the same composition was discharged from -thousands of fissures and vents distributed from the south of Antrim northward -beyond the Inner Hebrides, through the chain of the Faroe Islands -and over the whole breadth of Iceland. Under the British Isles alone, the -subterranean reservoirs of molten lavas must have been at least 40,000 square -miles in united area. If they stretched continuously northwards below the -Faroe Islands and Iceland, as is highly probable, that is, for 600 miles -further, their total extent may have been comparable to such a region as -Scandinavia.</p> - -<p>Was this vast underground body of lava part of a universal liquid mass -within the globe, or was it rather of the nature of one or more lakes or large -vesicles within the crust? We can only offer speculation for answer. On -the other hand, there seems to be good proof that in some districts, both now -<span class="pagenum" id="Page_53">- 53 -</span> -and in former geological periods, such differences exist between the materials -ejected from vents not far distant from each other as to show the existence -of more limited distinct reservoirs of liquid rock underneath.</p> - -<p>Some of the questions here asked will be further dealt with in later -pages in connection with such geological evidence as can be produced -regarding them. But it will be found that at every step in the endeavour -to ascertain the origin of volcanic phenomena difficulties present themselves -which are now and may long remain insoluble.</p> - - -<h3><span class="smcap">I. Vents of Eruption</span></h3> - -<p>It is a general belief that the first stage in the formation of a volcano -of the Vesuvian type by the efforts of subterranean energy is the rending of -the terrestrial crust in a line of fissure. Some of the most remarkable -groups of active volcanoes on the face of the globe are certainly placed in -rows, as if they had risen along some such great rents. The actual fissure, -however, is not there seen, and its existence is only a matter of probable -inference. Undoubtedly the effect of successive eruptions must be to conceal -the fissure, even if it ever revealed itself at the surface.</p> - -<p>What is supposed to have marked the initial step in the formation of a -great volcano is occasionally repeated in the subsequent history of the mountain. -During the convulsive shocks that precede and accompany an eruption, -the sides of the cone, and even sometimes part of the ground beyond, -are rent open, occasionally for a distance of several miles, and on the fissures -thus formed minor volcanoes are built up.</p> - -<p>It is in Iceland, as already stated, that the phenomena of fissures are -best displayed. There the great deserts of lava are from time to time -dislocated by new lines of rent, which ascend up to the surface and stretch -for horizontal distances of many miles. From these long narrow chasms -lava flows out to either side; while cones of slag and scoriæ usually form -upon them. This interesting eruptive phase will be more fully described -in the chapters dealing with the Tertiary volcanic rocks of Britain.</p> - -<p>There can be no doubt, however, that in a vast number of volcanic vents -of all geological periods no trace can be discovered of their connection with -any fissure in the earth's crust. Such fissures may indeed exist underneath, -and may have served as passages for the ascent of lava to within a greater -or less distance from the surface. But it is certain that volcanic energy has -the power of blowing out an opening for itself through the upper part of the -crust without the existence of any visible fissure there. What may be the -limits of depth at which this mode of communication with the outer air is -possible we do not yet know. They must obviously vary greatly according -to the structure of the terrestrial crust on the one hand, and the amount -and persistence of volcanic energy on the other. We may suppose that -where a fissure terminates upward under a great depth of overlying rock, -the internal magma may rise up to the end of the rent, and even be injected -laterally into the surrounding parts of the crust, but may be unable to complete -<span class="pagenum" id="Page_54">- 54 -</span> -the formation of a volcano by opening a passage to the surface. But -where the thickness of rock above the end of the fissure is not too great, -the expansive energy of the vapours absorbed in the magma may overcome -the resistance of that cover, and blow out an orifice by which the volcanic -materials can reach the surface. In the formation of new cones within the -historic period at a distance from any central volcano, the existence of an -open fissure at the surface has not been generally observed. When, for -example, Monte Nuovo was formed, it rose close to the shore among fields -and gardens, but without the appearance of any rent from which its -materials were discharged.</p> - -<p>That in innumerable instances during the geological past, similar vents -have been opened without the aid of fissures that reached the surface, will be -made clear from the evidence to be drawn from the volcanic history of the -British Isles. So abundant, indeed, are these instances that they may be -taken as proving that, at least in the Puy type of volcanoes, the actual vents -have generally been blown out by explosions rather than by the ascent of -fissures to the open air.</p> - -<p>In cases where, as in Iceland, fissures open at the surface and discharge -lava there, the channel of ascent is the open space between the severed walls -of the rent. Within this space the lava will eventually cool and solidify as -a <i>dyke</i>. It is obvious that a comparatively small amount of denudation -will suffice to remove all trace of the connection of such a dyke with the -stream of lava that issued from it. Among the thousands of dykes belonging -to the Tertiary period in the British Islands, it is probable that many -may have served as lines of escape for the basalt at the surface. But it is -now apparently impossible to distinguish between those which had such a -communication with the outer air and those that ended upward within the -crust of the earth. The structure of dykes will be subsequently discussed -among the subterranean intrusions of volcanic material.</p> - -<p>In an ordinary volcanic orifice the ground-plan is usually irregularly -circular or elliptical. If that portion of the crust of the earth through -which the vent is drilled should be of uniform structure, and would thus -yield equally to the effects of the volcanic energy, we might anticipate that -the ascent and explosion of successive globular masses of highly heated -vapours would give rise to a cylindrical pipe. But in truth the rocks of -the terrestrial crust vary greatly in structure; while the direction and force -of volcanic explosions are liable to change. Hence considerable irregularities -of ground-plan are to be looked for among vents.</p> - -<p>Some of these irregularities are depicted in <a href="#v1fig22">Fig. 22</a>, which represents -the ground plan of some vents from the Carboniferous volcanic districts of -Scotland. They are all drawn on the same scale. Other examples will be -cited in later chapters from the same and other parts of the British Isles.</p> - -<p>Some of the most marked departures from the normal and simple type -of vent occur where two orifices have been opened close to each other, or -where the same vent has shifted its position (Figs. <a href="#v1fig29">29</a>, <a href="#v1fig125">125</a>, <a href="../../66493/66493-h/66493-h.htm#v2fig205">205</a>, and <a href="../../66493/66493-h/66493-h.htm#v2fig214">214</a>). -Curiously irregular or elongated forms may thus arise in the resultant -<span class="pagenum" id="Page_55">- 55 -</span> -"necks" now visible at the surface. Many striking examples of these -features may be seen among the Carboniferous and Permian volcanoes to be -afterwards described. Occasionally where an open fissure has served as a -vent it has given rise to a long dyke-like mass (No. 1 in <a href="#v1fig22">Fig. 22</a>).</p> - -<div class="figright" id="v1fig22" style="width: 252px;"> - <img src="images/v1fig22.png" width="252" height="291" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 22.</span>—Ground-plans of some Volcanic vents from - the Carboniferous districts of Scotland.<br /> - 1. Linhope Burn, near Mosspaul, Roxburghshire; the - shaded parts are intrusions of trachytic material. 2. - Hazelside Hill, two miles W. from Newcastleton, - Roxburghshire. 3. St. Magdalen's, Linlithgow. 4. - South-west side of Coom's Fell (see <a href="#v1fig174">Fig. 174</a>). 5. Neck - on Greatmoor, Roxburghshire. 6. Pester Hill, Tarras - Water. 7. Head of Routing Burn, S.E. side of Hartsgarth - Fell, Liddesdale. 8. Hartsgarth Flow, Liddesdale.</div> -</div> - -<p>The size of a volcanic vent may vary indefinitely from a diameter of -not more than a yard or two up to -one or two or more miles. As a -rule, the smaller the vents the more -numerously are they crowded together. -In the case of large central -volcanoes like Etna, where many -subsidiary vents, some of them forming -not inconsiderable hills, may -spring up along the sides of the -parent cone, denudation will ultimately -remove all the material that -was heaped up on the surface, and -leave the stumps or necks of the -parasitic vents in groups around the -central funnel.</p> - -<p>Each volcanic chimney, by which -vapours, ashes or lava are discharged -at the surface, may be conceived to -descend in a more or less nearly -vertical direction until it reaches the -surface of the lava whence the eruptions -proceed. After the cessation -of volcanic activity, this pipe will be -left filled up with the last material -discharged, which will usually take -the form of a rudely cylindrical column reaching from the bottom of the crater -down to the lava-reservoir. It will be obvious that no matter how great may -be the denudation of the volcano, or how extensive may be the removal of -the various materials discharged over the surrounding ground, the pipe or -funnel with its column of solid rock must still remain. No amount of -waste of the surface of the land can efface that column. Successively lower -and yet lower levels may be laid bare in it, but the column itself goes still -further down. It will continue to make its appearance at the surface until -its roots are laid bare in the lava of the subterranean magma. Hence, of -all the relics of volcanic action, the filled-up chimney of the eruptive vent -is the most enduring. Save where it may have been of the less deep-seated -nature of a "hornito" upon a lava-stream, we may regard it as practically -permanent. The full meaning of these statements will be best understood -from a consideration of the numerous illustrations to be afterwards given.</p> - -<p>The stumps of volcanic columns of this nature, after prolonged denudation, -generally project above the surrounding ground as rounded or conical -<span class="pagenum" id="Page_56">- 56 -</span> -eminences known as "Necks" (<a href="#v1fig23">Fig. 23</a>. See also Figs. <a href="#v1fig52">52</a>, <a href="#v1fig82">82</a>, <a href="#v1fig102">102</a>, <a href="#v1fig109">109</a>, -<a href="#v1fig123">123</a>, <a href="#v1fig133">133</a>, <a href="#v1fig144">144</a>, <a href="../../66493/66493-h/66493-h.htm#v2fig178">178</a>, <a href="../../66493/66493-h/66493-h.htm#v2fig192">192</a>, <a href="../../66493/66493-h/66493-h.htm#v2fig195">195</a>, <a href="../../66493/66493-h/66493-h.htm#v2fig203">203</a>, <a href="../../66493/66493-h/66493-h.htm#v2fig204">204</a>, <a href="../../66493/66493-h/66493-h.htm#v2fig209">209</a>, <a href="../../66493/66493-h/66493-h.htm#v2fig294">294</a>, <a href="../../66493/66493-h/66493-h.htm#v2fig298">298</a>, <a href="../../66493/66493-h/66493-h.htm#v2fig306">306</a> and <a href="../../66493/66493-h/66493-h.htm#v2fig310">310</a>). -Their outlines, however, vary with the nature of their component materials. -The softer rocks, such as tuffs and agglomerates, are apt to assume the form -of smooth domes or cones, while the harder and especially the crystalline -rocks rise into irregular, craggy hills. Occasionally, indeed, it may happen -that a neck makes no prominence on the surface of the ground, and its -existence may only be discoverable by a careful examination of the geological -structure of the locality. Now and then an old vent will be found not to -form a hill, but to sink into a hollow. Such variations, however, have -little or no reference to original volcanic contours in the history of the -localities which display them. They arise mainly from the differing hardness -and structure of the materials that have filled the vents, and the consequent -diversity in the amount of resistance which they have offered -to the progress of denudation.</p> - -<div class="figcenter" id="v1fig23" style="width: 506px;"> - <img src="images/v1fig23.png" width="506" height="246" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 23.</span>—View of an old volcanic "Neck" (The Knock, Largs, Ayrshire, a vent of -Lower Carboniferous age).</div> -</div> - -<p>The materials now found in volcanic funnels are of two kinds: -1st, Fragmentary, derived from volcanic explosions; and 2nd, Lava-form, -arising from the ascent and consolidation of molten rock within the funnel.</p> - - -<h4>i. <i>Necks of Fragmentary Materials</i></h4> - -<p>By far the most satisfactory evidence of a former volcanic orifice is -furnished by a neck of fragmentary materials. Where "bosses" of crystalline -rock rise to the surface and assume the outward form of necks, we cannot -always be certain that they may not have been produced by subterranean -intrusions that never effected any connection with the surface. In other -words, such bosses may not mark volcanic orifices at all, though they may -have been part of the underground protrusions of volcanoes in their -<span class="pagenum" id="Page_57">- 57 -</span> -neighbourhood. But where the chimney has been filled with debris, there -can be no doubt that it truly marks the site of a once active volcano. The -fragmentary material is an eloquent memorial of the volcanic explosions -that drilled the vent, kept it open, and finally filled it up. These -explosions could not have taken place unless the elastic vapours which -caused them had found an escape from the pressure under which they lay -within the crust of the earth. Now and then, indeed, where the outpouring -of lava or some other cause has left cavernous spaces within the crust, there -may conceivably be some feeble explosion there, and some trifling accumulation -of fragmentary materials. But we may regard it as practically certain -that the mass of tumultuous detritus now found in volcanic necks could -not have been formed unless where a free passage had been opened from -the molten magma underneath to the outer surface of the planet.</p> - -<p>Considerable diversity may be observed in the nature and arrangement -of the fragmentary materials in volcanic necks. The chief varieties may be -arranged in four groups: (1) Necks of non-volcanic detritus; (2) Necks of -volcanic agglomerate or tuff; (3) Necks of agglomerate or tuff with a -central plug of lava; and (4) Necks of agglomerate or tuff with veins, -dykes or some lateral irregular mass of lava.</p> - -<p>(1) <i>Necks of non-volcanic Detritus.</i>—During the first convulsive efforts of -a volcanic focus to find a vent at the surface, the explosions that eventually -form the orifice do so by blowing out in fragments the solid rocks of -the exterior of the terrestrial crust. Of the detritus thus produced, shot -up the funnel and discharged into the air, part may gather round the mouth -of the opening and build up there a cone with an enclosed crater, while part -will fall back into the chimney, either to accumulate there, should the explosions -cease, or to be thrown out again, should they continue. In the -feeblest or most transient kinds of volcanic energy, the explosive vapours -may escape without any accompanying ascent of the molten magma to the -surface, and even without any sensible discharge of volcanic "ashes" from -that magma. In such cases, as I have already pointed out, the detritus of -the non-volcanic rocks, whatever they may be, through which volcanic energy -has made an opening, accumulate in the pipe and eventually consolidate -there. Examples of this nature will be adduced in later chapters from the -volcanic districts of Britain.</p> - -<p>Where only non-volcanic materials fill up a vent we may reasonably -infer that the eruptions were comparatively feeble, never advancing beyond -the initial stage when elastic vapours made their escape with explosive -violence, but did not lead to the outflow of lava or the discharge of ashes. -In the great majority of necks, however, traces of the earliest eruptions have -been destroyed by subsequent explosions, and the uprise of thoroughly -volcanic fragments. Yet even among these fragments, occasional blocks -may be detected which have been detached from the rocks forming the walls -of the funnel.</p> - -<p>The general name of Agglomerate, as already stated, is given to all -accumulations of coarse, usually unstratified, detritus in volcanic funnels, -<span class="pagenum" id="Page_58">- 58 -</span> -irrespective of the lithological nature of the materials. For further and -more precise designation, when an agglomerate is mainly made up of -fragments of one particular rock, the name of that rock may be prefixed as -sandstone-agglomerate, granite-agglomerate, basalt-agglomerate, trachyte-agglomerate. -Volcanic agglomerate is a useful general term that may -include all the coarser detritus ejected by volcanic action.</p> - -<p>Where volcanic explosions have been of sufficient violence or long -continuance, the upper part of the funnel may be left empty, and on the -cessation of volcanic activity, may be filled with water and become a lake. -The ejected detritus left round the edge of the orifice sometimes hardly -forms any wall, the crater-bottom being but little below the level of the -surrounding ground. Explosion-lakes are not infrequent in Central France -and the Eifel (Maare). A more gigantic illustration is afforded by the -perfectly circular crater of Coon Butte in Arizona, about 4000 feet in -diameter and 600 feet deep. It has been blown out in limestone, the -debris of which forms a rampart 200 feet high around it. Examples will -afterwards be cited from the Tertiary volcanic plateaux of North-Western -Europe. Vents may also be formed by an engulphment or subsidence of -the material, like that which has taken place at the great lava cauldron of -Hawaii, still an active volcano. The picturesque Crater Lake of Oregon is -an admirable instance of this structure.</p> - -<p>(2) <i>Necks of Agglomerate or Tuff.</i>—In the vast majority of cases, the explosions -that clear out a funnel through the rocks of the upper part of the -crust do not end by merely blowing out these rocks in fragments. The -elastic vapours that escape from the molten lava underneath are usually -followed by an uprise of the lava within the pipe. Relieved from the -enormous pressure under which it had before lain, the lava as it ascends is -kept in ebullition, or may be torn into bombs which are sent whirling up -into the air, or may even be blown into the finest dust by the sudden -expansion of the imprisoned steam. If its ascent is arrested within the -vent, and a crust is formed on the upper surface of the lava-column, -this congealed crust may be disrupted and thrown out in scattered pieces -by successive explosions, but may re-form again and again.</p> - -<div class="figleft" id="v1fig24" style="width: 198px;"> - <img src="images/v1fig24.png" width="198" height="90" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 24.</span>—Section of neck of agglomerate, -rising through sandstones and shales.</div> -</div> - -<p>In many vents, both in recent and in ancient times, volcanic progress -has never advanced beyond this early stage of the ejection of stones and -dust. The column of lava, though rising near enough to the surface to -supply by its ebullition abundant pyroclastic -detritus, coarse and fine, has not flowed out -above ground, nor even ascended to the top -of the funnel. It may have formed, at the -surface, cones of stones and cinders with -enclosed craters. But thereafter the eruptions -have ceased. The vents, filled up with -the fragmentary ejected material, have given -passage only to hot vapours and gases. As these gradually ceased, the -volcanoes have become finally extinct. Denudation has attacked their sides -<span class="pagenum" id="Page_59">- 59 -</span> -and crests. If submerged in the sea or a lake, the cones have been washed -down, and their materials have been strewn over the bottom of the water. -If standing on the land, they have been gradually levelled, until perhaps -only the projecting knob or neck of solidified rubbish in each funnel has -remained to mark its site. The buried column of compacted fragmentary -material will survive as the only memorial of the eruptions (<a href="#v1fig24">Fig. 24</a>. For -views of necks formed of agglomerate or tuff see Figs. <a href="#v1fig23">23</a>, <a href="#v1fig82">82</a>, <a href="#v1fig102">102</a>, <a href="#v1fig123">123</a>, -<a href="#v1fig144">144</a>, <a href="../../66493/66493-h/66493-h.htm#v2fig178">178</a>, <a href="../../66493/66493-h/66493-h.htm#v2fig192">192</a>, <a href="../../66493/66493-h/66493-h.htm#v2fig203">203</a>, <a href="../../66493/66493-h/66493-h.htm#v2fig204">204</a>, <a href="../../66493/66493-h/66493-h.htm#v2fig209">209</a>, <a href="../../66493/66493-h/66493-h.htm#v2fig210">210</a>, <a href="../../66493/66493-h/66493-h.htm#v2fig212">212</a>, <a href="../../66493/66493-h/66493-h.htm#v2fig216">216</a>).</p> - -<p>The volcanic agglomerates of such vents sometimes include, among -their non-volcanic materials, pieces of rock which bear evidence of having -been subjected to considerable heat (see <a href="../../66493/66493-h/66493-h.htm#Page_78">vol. ii. p. 78</a>). Carbonaceous shales, -for instance, have had their volatile constituents driven off, limestones have -been converted into marble, and a general induration or "baking" may be -perceptible. In other cases, however, the fragments exhibit no sensible -alteration. Fossiliferous limestones and shales often retain their organic -remains so unchanged that specimens taken out of the agglomerate cannot -be distinguished from those gathered from the strata lying <i>in situ</i> outside. -Some stones have evidently been derived from a deeper part of the chimney, -where they have been exposed to a higher temperature than others, or they -may have been lain longer within the influence of hot ascending vapours.</p> - -<p>The volcanic materials in agglomerate range in size from the finest dust -to blocks several yards in length, with occasionally even much larger -masses. The proportions of dust to stones vary indefinitely, the finer -material sometimes merely filling in the interstices between the stones, at -other times forming a considerable part of the whole mass.</p> - -<p>The stones of an agglomerate may be angular or subangular, but are -more usually somewhat rounded. Many of them are obviously pieces that -have been broken from already solid rock and have had their edges rounded -by attrition, probably by knocking against each other and the walls of the -chimney as they were hurled up and fell back again. Their frequently -angular shapes negative the supposition that they could have been produced -by the discharge of spurts of still liquid lava. As already stated, they -have probably been in large measure derived from the violent disruption of -the solidified cake or crust on the top of the column of lava in the pipe. -Many of them may have been broken off from the layer of congealed lava -that partially coated the rough walls of the funnel after successive uprises -of the molten material. Among them may be observed many large and -small blocks that appear to have been derived from the disruption of true -lava-streams, as if beds of lava had been pierced in the formation of the -vent, or as if those that congealed on the slopes of the cone had been broken -up by subsequent explosions. These fragments of lava are sometimes strongly -amygdaloidal. A characteristic feature, indeed, of the blocks of volcanic -material in the agglomerates is their frequent cellular structure. Many -of them may be described as rough slags or scoriæ. These have generally -come from the spongy crust or upper part of the lava where the imprisoned -steam, relieved from pressure, is able to expand and gather into vesicles.</p> - -<p><span class="pagenum" id="Page_60">- 60 -</span></p> - -<p>Less frequently evidence is obtainable that the blocks were partially or -wholly molten at the time of expulsion. Sometimes, for example, a mass -which presents on one side such a broken face as to indicate that it came -from already solidified material, will show on the other that its steam-vesicles -have been pulled out in such a way as to conform to the rounded -surface of the block. This elongation could only take place in lava that -was not yet wholly consolidated. It seems to indicate that such blocks -were derived from a thin hardened crust lying upon still molten material, -and that they carried up parts of that material with them. As each stone -went whirling up the funnel into the open air, its melted part would be -drawn round the gyrating mass, and would rapidly cool there.</p> - -<p>In other cases, we encounter true volcanic bombs, that is, rounded or -bomb-shaped blocks of lava, with their vesicles elongated all round them -and conforming to their spherical shape. Sometimes such blocks are -singularly vesicular in the centre, with a more close-grained crust on the -outside. Their rapid centrifugal motion during flight would allow of the -greater expansion of the dissolved steam in the central part of each mass, -while the outer parts would be quickly chilled, and would assume a more -compact texture. Bombs of this kind are met with among ancient volcanic -products, and, like those of modern volcanoes, have obviously been produced -by the ejection of spurts or gobbets of lava from the surface of a mass -in a state of violent ebullition. Occasionally they are hollow inside, the -rotation in these cases having probably been exceptionally rapid.</p> - -<p>Passing from the larger blocks to the smaller fragments, we notice the -great abundance of nut-like subangular or rounded pieces of lava in the -agglomerates. These include lumps of fine grain not specially vesicular, -and probably derived from the disruption of solidified rock. But in many -agglomerates, especially those associated with the outpouring of basalts or -other basic lavas (as those of Carboniferous and Tertiary age described in -later chapters), they comprise also vast numbers of very finely cellular -material or pumice. These pumiceous lapilli have been already alluded to -as ingredients of the stratified tuffs. But they are still more characteristic -of the necks, and reach there a larger size, ranging from the finest grains up -to lumps as large as a hen's egg, or even larger.</p> - -<p>The peculiar distinctions of this ejected pumice are the extreme minuteness -of its vesicles, their remarkable abundance, their prevalent spherical -forms, and the thinness of the walls which separate them. In these -respects they present a marked contrast to the large irregularly-shaped -steam-cavities of the outflowing lavas, or even of the scoriæ in the -agglomerates.</p> - -<p>This characteristic minutely vesicular pumice is basic in composition. -Where not too much decayed, it may be recognized as a basic glass. Thus -among the remarkable agglomerates which fill up the Pliocene or Pleistocene -vents of the Velay, the fragments consist of a dark very basic glass, which -encloses such a multitude of minute steam-cavities that, when seen under the -microscope, they are found to be separated from each other by walls so thin -<span class="pagenum" id="Page_61">- 61 -</span> -that the slice looks like a pattern of delicate lace.<a id="FNanchor_25" href="#Footnote_25" class="fnanchor">[25]</a> In necks of earlier -date, such as those of older Tertiary, and still more of Palæozoic, time, the -glass has generally been altered into some palagonitic material.</p> - -<div class="footnote"> - -<p><a id="Footnote_25" href="#FNanchor_25" class="label">[25]</a> M. Boule, <i>Bull. Cart. Géol. France</i>, No. 28, tome iv. (1892) p. 193.</p> - -</div> - -<p>This finely pumiceous substance appears to be peculiar to the vents and -to the deposits of tuff immediately derived from them. It is not found, so -far as I know, among any of the superficial lavas, and, of course, would not be -looked for among intrusive rocks. It was evidently a special product of the -volcanic chimney, as distinguished from the mass of the magma below. We -may perhaps regards it as in some way due to a process of quiet simmering -within the vent, when the continual passage of ascending vapours kept -the molten lava there in ebullition, and gave it its special frothy or finely -pumiceous character.</p> - -<p>The compacted dust, sand or gravelly detritus found in necks, and -comprised under the general name of Tuff, consists partly of the finer particles -produced during the violent disruption of already solidified rocks, partly of -the detritus arising from the friction and impact of stones ascending and -descending above an active vent during times of eruption, and partly of -the extremely light dust or ash into which molten lava may be blown by -violent volcanic explosions. In old volcanic necks, where the rocks have -long been subjected to the influence of percolating meteoric water, it is not -perhaps possible to discriminate, except in a rough way, the products from -these three sources. The more minutely comminuted material has generally -undergone considerable alteration, so that under the microscope it seldom -reveals any distinctive structures. Here and there in a slide, traces may -occasionally be detected of loose volcanic microlites, though more usually -these can only be found in lapilli of altered glass or finely pumiceous -lava.</p> - -<p>The composition of the detritus in a neck of agglomerate or tuff has -almost always a close relation to that of any lavas which may have been -emitted from that vent. If the lavas have been of an acid character, such -as rhyolites, felsites or obsidians, the pyroclastic materials will almost -always be found to be also acid. Where, on the other hand, the lavas -have been intermediate or basic, so also will be the tuffs and agglomerates. -Occasionally, however, as has already been pointed out, from the same or -closely adjoining vents lavas of very different chemical composition have been -successively erupted. Felsites or rhyolites have alternated with diabases, -basalts or andesites. In such cases, a commingling of acid and basic -detritus may be observed, as, for example, among the volcanoes of the Old -Red Sandstone. It has even happened sometimes that such a mixture of -material has taken place when only one class of lavas has been poured out -at the surface, as in the agglomerates that fill vents among the basalts of -the Inner Hebrides. But we may be sure that, though not discharged at -the surface, the lavas of which pieces are found in the tuffs must have -risen high enough in the vents to be actually blown out in a fragmentary -form. The occurrence of felsitic fragments among the otherwise basic -<span class="pagenum" id="Page_62">- 62 -</span> -agglomerates of Mull and Skye will be described in subsequent pages, -likewise the intercalation of rhyolitic detritus between the basalts of -Antrim. A similar association occurs among the modern vents of Iceland.</p> - -<p>Among the contents of the tuffs and agglomerates that occupy old -volcanic vents, some are occasionally to be observed of which the source is -not easily conjectured. Detached crystals of various minerals sometimes -occur abundantly which were certainly not formed <i>in situ</i>, but must have -been ejected as loose lapilli with the other volcanic detritus. Where these -crystals belong to minerals that enter into the composition of the lavas of -the district in which they are found, they may be regarded as having -probably been derived from the explosion of such lavas in the vents, the -molten magma being blown into dust, and its already formed crystals -being liberated and expelled as separate grains. But it seems to be -extremely rare to find any neighbouring lava in which the minerals in -question are so largely and so perfectly crystallized as they are in these -loose crystals of the neck. The beautifully complete crystals of augite -found in the old tuffs of Vesuvius and on the flanks of Stromboli may -be paralleled among Palæozoic tuffs and agglomerates in Britain. Thus the -necks belonging to the Arenig and Llandeilo volcanoes of southern Scotland -are sometimes crowded with augite, varying from minute seed-like grains -up to perfectly formed crystals as large as hazel nuts. The conditions -under which such well-shaped idiomorphic minerals were formed were -probably different from those that governed the cooling and consolidation -of the ordinary lavas.</p> - -<p>But besides the minerals that may be claimed as belonging to the -volcanic series of a district, others occur not infrequently in some tuff-necks, -the origin of which is extremely puzzling. Such are the large felspars, -micas, garnets and the various gems that have been obtained from necks. -The large size of some of these crystals and their frequently perfect crystallographic -forms negative the idea that they can, as a rule, be derived from -the destruction of any known rocks, though they may sometimes be conceivably -the residue left after the solution of the other constituents of a -rock by the underground magma, like the large residual felspars enclosed in -some dykes. The crystals in question, however, seem rather to point to -some chemical processes still unknown, which, in the depths of a volcanic -focus, under conditions of pressure and temperature which we may speculate -about but can perhaps hardly ever imitate in our laboratories, lead to the -elaboration of the diamond, garnet, sahlite, smaragdite, zircon and other -minerals.<a id="FNanchor_26" href="#Footnote_26" class="fnanchor">[26]</a> Examples of such foreign or deep-seated crystals will be -described from the probably Permian necks of Central Scotland.</p> - -<div class="footnote"> - -<p><a id="Footnote_26" href="#FNanchor_26" class="label">[26]</a> For lists of the minerals found in the diamond-bearing necks of Kimberley, see M. Boutan in -Frémy's <i>Encyclopédie Chimique</i> (1886), vol. ii. p. 168; Dr. M. Bauer's <i>Edelsteinkunde</i> (1895), -p. 223.</p> - -</div> - -<p>Whatsoever may be the source and nature of the fragmentary materials -that fill old volcanic vents, they present, as a general rule, no definite arrangement -in the necks. Blocks of all sizes are scattered promiscuously through -<span class="pagenum" id="Page_63">- 63 -</span> -the agglomerate, just as they fell back into the chimney and came to rest there. -The larger masses are placed at all angles, or stand on end, and are sometimes -especially conspicuous in the centre of a neck, though more usually dispersed -through the whole. Such a thoroughly tumultuous accumulation is precisely -what might be expected where explosions have taken place in still liquid -and in already consolidated lavas, and where the materials, violently discharged -to the surface, have fallen back and come finally to rest in the chimney of -the volcano.</p> - -<p>Nevertheless, this absence of arrangement sometimes gives place to a -stratification which becomes more distinct in proportion as the material of -the vent passes from coarse agglomerate into fine tuff. It is possible that -the existence and development of this structure depend on the depth at -which the materials accumulate in the funnel. We may conceive, for -instance, that in the lower parts of the chimney, the stones and dust, -tumultuously falling and rebounding from projections of the rugged walls, -will hardly be likely to show much trace of arrangement, though even there, -if the explosions continue to keep an open though diminishing passage in -the vent, alternations of coarser and finer layers, marking varying phases of -eruptivity, may be formed in the gradually heightening pile of agglomerate. -Rude indications of some such alternations may sometimes be detected in -what are otherwise quite unstratified necks.</p> - -<p>In the upper part of a volcanic funnel, however, close to and even -within the crater, the conditions are not so unfavourable to the production -of a stratified arrangement. As the pipe is filled up, and the activity of -eruption lessens, explosions may occur only from the very middle of the -orifice. The debris that falls back into the vent will gather most thickly -round the walls, whence it will slide down to the central, still eruptive hole. -It will thus assume a stratified arrangement, the successive layers lying at -the steepest angles of repose, or from 30° to 35°, and dipping down in an -inverted conical disposition towards the centre. If the process should -continue long enough, the crater itself may be partially or completely filled -up with detritus (<a href="#v1fig25">Fig. 25</a>).</p> - -<p>Of this gradual infilling of a volcanic chimney with stratified agglomerate -and tuff, examples belonging to different geological periods will be cited in -subsequent chapters. I may here especially allude to one of the most recently -observed and best marked illustrations, which occurs on the west side of -Stromö, in the Faroe Islands (see Figs. <a href="../../66493/66493-h/66493-h.htm#v2fig310">310</a>, <a href="../../66493/66493-h/66493-h.htm#v2fig311">311</a>, <a href="../../66493/66493-h/66493-h.htm#v2fig312">312</a>). A neck has there been -filled up with coarse agglomerate, which is rudely stratified, the layers dipping -steeply into the centre, where the tumultuous assemblage of large blocks -no doubt points to the final choking up of the diminished orifice of explosion. -The walls of the neck are nearly vertical, and consist of the bedded basaltic -lavas through which the vent has been opened. They terminate upward -in a conical expansion, evidently the old crater, which has subsequently -been filled up by the inroads of several lava-streams from adjacent vents. -It is here manifest that the bedded agglomerate belongs to the uppermost -part of the volcanic funnel.</p> - -<p><span class="pagenum" id="Page_64">- 64 -</span></p> - -<div class="figcenter" id="v1fig25" style="width: 511px;"> - <img src="images/v1fig25.png" width="511" height="251" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 25.</span>—Neck filled with stratified tuff. A. ground plan; B. transverse section.</div> -</div> - -<p>Where vents have been filled up with tuff rather than with agglomerate, -the stratified structure is best developed. Alternations of coarser and -finer detritus give rise to more or less definite layers, which, though inconstant -and irregular, serve to impart a distinctly stratified character -to the mass. Where there has been no subsequent disturbance within a -vent, these layers show the same inward dip towards the centre just referred -to, at the ordinary angles of repose. Now and then, where a neck with -this structure has been laid bare on a beach, its denuded cross-section presents -a series of concentric rings of strata from the walls towards the centre. -Good illustrations of these features are supplied by the probably Permian -necks of eastern Fife (Figs. <a href="#v1fig25">25 A</a> and <a href="../../66493/66493-h/66493-h.htm#v2fig217">217</a>).<a id="FNanchor_27" href="#Footnote_27" class="fnanchor">[27]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_27" href="#FNanchor_27" class="label">[27]</a> See also the sections of vents on the west coast of Stromö Faroes, above referred to.</p> - -</div> - -<p>It has frequently happened, however, that, owing to subsidence of the -materials filling up the vents or to later volcanic disturbances, the compacted -tuffs have been broken up and thrown into various positions, large masses -being even placed on end. Among the Carboniferous and Permian necks -of Central Scotland such dislocated and vertical tuffs are of common occurrence -(see Figs. <a href="#v1fig145">145</a>, <a href="../../66493/66493-h/66493-h.htm#v2fig218">218</a>). If, as is probable, we are justified in regarding -the stratified parts of necks as indicative of the uppermost parts of volcanic -funnels, not far from the surface, the importance of this inference -will be best understood when the Carboniferous and Permian volcanoes are -described.</p> - -<p>(3) <i>Necks with a central Lava-plug.</i>—Some vents of agglomerate or tuff -are pierced by a plug of lava, as may be instructively seen in many of the -Carboniferous and Permian necks of the centre and south of Scotland (<a href="#v1fig26">Fig. 26</a>; -compare also Figs. <a href="#v1fig148">148</a>, <a href="#v1fig174">174</a>, <a href="../../66493/66493-h/66493-h.htm#v2fig207">207</a>, and <a href="../../66493/66493-h/66493-h.htm#v2fig226">226</a>). Where this structure -shows itself, the contrast in hardness and durability between the more -destructible fragmentary material and the solid resisting lava leads to a -topographical distinction in the outer forms of necks. The smooth declivities -<span class="pagenum" id="Page_65">- 65 -</span> -of the friable tuffs are crowned or interrupted by more craggy features, -which mark the position of the harder intrusive rock.</p> - -<div class="figright" id="v1fig26" style="width: 244px;"> - <img src="images/v1fig26.png" width="244" height="123" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 26.</span>—Section of neck of agglomerate (<i>a</i> <i>a</i>) with - plug of lava (<i>b</i>).</div> -</div> - -<p>The plug, like the pipe up which it has risen, is in general irregularly -circular in ground-plan. It may be conceived to be a column of rock, -descending to an unknown depth into the interior, with a casing of -pyroclastic debris surrounding it. It may vary considerably in the proportion -which its cross-section bears to that of the surrounding fragmental -material. Sometimes it does not occupy more than a small part of the -whole, often appearing in the centre. -In other cases, it more than equals -all the rest of the material in the -vent, while instances may be noted -where only occasional patches of -tuff or agglomerate are visible between -the lava-plug and the wall -of the pipe. From these we naturally -pass to the second type of vent, -where no fragmentary material is to -be seen, but where the chimney is now entirely filled with some massive -once-molten rock.</p> - -<p>A neck with a lava-plug probably contains the records of two stages in -volcanic progress, the first of which, indicated by the tuff or agglomerate, -was confined to the discharge of fragmentary materials; while the second, -shown by the lava-plug, belonged to the time when, after the earlier -explosions, lava ascended in the vent and solidified there, thus bringing -the eruptions from that particular orifice to an end. Where a small central -column of lava rises through the tuff, we may suppose that the funnel had -been mainly choked up by the accumulation in it of ejected detritus, which -was compacted to a solid mass adhering to the wall of the funnel, but -leaving a central orifice to be kept open by the gradually waning energy of -the volcano. By a final effort that impelled molten rock up that duct and -allowed it to consolidate there, the operations of the vent were brought -to a close.</p> - -<p>Where, on the other hand, only occasional strips of tuff or agglomerate -are to be found between the lava-plug and the wall of the pipe, the last -uprise of lava may be supposed to have been preceded by more vigorous -explosions which cleared the throat of the volcano, driving out the accumulated -detritus and leaving only scattered patches adhering to the sides of -the funnel.</p> - -<p>There is, no doubt, some downward limit to the production of fragmentary -material, and if we could lay bare successive levels in the chimney of -a volcano we should find the agglomerate eventually replaced entirely -by lava.</p> - -<p>The materials of the lava-plugs vary widely in composition. Sometimes -they are remarkably basic, and present rocks of the picrite or limburgite -type; in other cases they are thoroughly acid rocks such as felsite and -<span class="pagenum" id="Page_66">- 66 -</span> -granophyre. Many intermediate varieties may be found between these -extremes. It is noteworthy that, in districts where the lavas erupted to -the surface have been andesitic or basaltic, the material which has finally -solidified in the vents is often more acid in composition, trachytic rocks -being specially frequent.</p> - -<div class="figleft" id="v1fig27" style="width: 251px;"> - <img src="images/v1fig27.png" width="251" height="128" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 27.</span>—Section of agglomerate neck (<i>a</i> <i>a</i>) with - dykes and veins (<i>b</i> <i>b</i>).</div> -</div> - -<p>(4) <i>Necks with Dykes, Veins, or irregular intrusions of Lava.</i>—While -the presence of a central plug of -lava in a neck of fragmental material -may indicate that the vent was -still to some extent open, there is -another structure which seems to -point to the ascent of lava after -the funnel has been choked up. -Numerous instances have been -observed where lava has been -forced upward through rents in a -mass of tuff or agglomerate, and has solidified there in the form of dykes -or veins (<a href="#v1fig27">Fig. 27</a>). Illustrations of this structure abound among the Carboniferous -and Permian necks of Britain. Here, again, though on a less -marked scale, the contrast in the amount and character of the weathering -of the two groups of rock gives rise to corresponding topographical features, -which are especially observable in cliffs and coast-sections, where the dykes -and veins project out of the tuffs as dark prominent walls (Figs. <a href="#v1fig135">135</a>, <a href="#v1fig149">149</a>, -<a href="#v1fig166">166</a>, <a href="#v1fig168">168</a>, <a href="../../66493/66493-h/66493-h.htm#v2fig219">219</a>, <a href="../../66493/66493-h/66493-h.htm#v2fig221">221</a>, <a href="../../66493/66493-h/66493-h.htm#v2fig222">222</a>).</p> - -<p>These intrusive injections are generally irregular in their forms, the -lava having evidently been driven through a mass of material which, not -having yet consolidated sufficiently to acquire a jointed structure, afforded -few dominant lines of division along which it could ascend. Now and -then, however, sharply defined dykes or veins, which at a distance look -like dark ribbons, may be seen running vertically or at a high angle, and -with a straight or wavy course, through the fine compacted tuff of a vent. -Frequently the injected material has found its readiest line of ascent along -the walls of the funnel, between the tuff and the surrounding rocks. -Occasionally it has made its way into rents in these rocks, as well as -into the body of the neck.</p> - -<p>It is worthy of remark in passing that complete consolidation of the fragmentary -material does not appear to be always requisite in order to allow -of the formation of such fissures as are needed for the production of -dykes. A singularly interesting illustration of this fact may be seen on -the northern crest of the outer crater of the Puy Pariou in Auvergne. A -dyke of andesite 8 or 10 feet broad may there be traced running for a -distance of about 300 yards through the loose material of the cone. The -rock is highly vesicular, and the vesicles have been elongated in the direction -of the course of the dyke so as to impart a somewhat fissile structure to -the mass.</p> - -<p>There can be little doubt that the dykes and veins which traverse necks -<span class="pagenum" id="Page_67">- 67 -</span> -of agglomerate belong to one of the closing phases in the history of the -vents in which they occur. They could only have been injected after the -pipes had been so choked up that explosions had almost or entirely ceased, -and eruptions had consequently become nearly or quite impossible. They -show, however, that volcanic energy still continued to manifest itself by -impelling the molten magma into these extinct funnels, while at the same -time it may have been actively discharging materials from other still open -vents in the same neighbourhood.</p> - -<p>With regard to the composition of these dykes and veins, it may be -remarked that in a district of acid lavas they may be expected to be felsitic -or rhyolitic, sometimes granophyric. Where, on the other hand, the lavas -poured out at the surface have been intermediate or basic, the veins in the -necks may be andesites, basalts or other still more basic compounds. But -it is observable, as in the case of the lava-plugs, that the injections into the -necks may be much more acid than any of the superficial lavas. The -advent of acid material in the later part of a volcano's history has been -already alluded to, and many examples of it will be given in this work.</p> - -<p>After all explosions and eruptions have ceased, heated vapours may -still for a long period continue to make their way upward through the -loose spongy detritus filling up the vent. The ascent of such vapours, and -more particularly of steam, may induce considerable metamorphism of the -agglomerate, as is more particularly noticed at <a href="#Page_71">p. 71</a>.</p> - - -<h4>ii. <i>Necks of Lava-form Material</i></h4> - -<p>The second type of neck is that in which the volcanic pipe has been -entirely filled up with some massive or crystalline rock. As already -remarked, it is not always possible to be certain that bosses of rock, having -the external form of necks of this kind, mark the sites of actual volcanic -orifices. Eruptive material that has never reached the surface, but has been -injected into the crust of the earth, has sometimes solidified there in forms -which, when subsequently exposed by denudation, present a deceptive -resemblance to true volcanic necks. Each example must be examined by -itself, and its probable origin must be determined by a consideration of all -the circumstances connected with it. Where other evidence exists of -volcanic activity, such, for instance, as the presence of bedded tuffs or -intercalated sheets of lava, the occurrence of neck-like eminences or bosses -of felsite, andesite, dolerite, basalt or other eruptive rock, would furnish a -presumption that these marked the sites of some of the active vents of the -period to which the tuffs and lavas belonged.</p> - -<p>If a neck-like eminence of this kind were found to possess a circular -or elliptical ground-plan, and to descend vertically like a huge pillar into -the crust of the earth; if the surrounding rocks were bent down towards -it and altered in the manner which I shall afterwards describe in detail; -if, moreover, the material composing the eminence were ascertained to be -closely related petrographically to some parts of the surrounding volcanic -<span class="pagenum" id="Page_68">- 68 -</span> -series, it might with some confidence be set down as marking the place of -one of the active vents from which that series was ejected.</p> - -<p>The chief contrast in external form between this type of neck and that -formed of fragmentary material arises from differences in the relative durability -of their component substance. The various kinds of lava-form rock -found in necks are, as a whole, much harder and more indestructible than -agglomerates and tuffs. Consequently bosses of them are apt to stand out -more prominently. They mount into higher points, present steeper declivities, -and are scarped into more rugged crags. But essentially they are -characterized by similar conical outlines, and by rising in the same solitary -and abrupt way from lower ground around them (see Figs. <a href="#v1fig109">109</a>, <a href="#v1fig133">133</a>, and -<a href="../../66493/66493-h/66493-h.htm#v2fig195">195</a>, <a href="../../66493/66493-h/66493-h.htm#v2fig294">294</a>).</p> - -<div class="figleft" id="v1fig28" style="width: 297px;"> - <img src="images/v1fig28.png" width="297" height="155" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 28.</span>—Section of neck filled with massive rock.</div> -</div> - -<p>Various joint-structures may be observed in these necks. In some -cases there is a tendency to separate into joints parallel to the bounding -walls, and occasionally this arrangement goes so far that the rock has -acquired a fissile structure as if it were composed of vertical strata. In -other instances, the rock -shows a columnar structure, -the columns diverging -from the outer margin, or -curving inwards, or displaying -various irregular -groupings. More usually, -however, this jointing is -so indefinite that no satisfactory -connection can be -traced between it and the -walls of the orifice in which the rock has solidified.</p> - -<p>Some of the most remarkable examples of necks ever figured and -described are those to which attention was called by Captain Dutton as -displayed in the Zuni plateau of New Mexico, where, amid wide denuded -sheets of basalt, numerous prominent crags mark the sites of eruptive vents. -The basalt of these eminences is columnar, the columns standing or lying -in all sorts of attitudes, and in most cases curved.<a id="FNanchor_28" href="#Footnote_28" class="fnanchor">[28]</a> In the Upper Velay, in -Central France, numerous conspicuous domes and cones of phonolite rise -amidst the much-worn basalt-plateau of that region (<a href="../../66493/66493-h/66493-h.htm#v2fig345">Fig. 345</a>). Many -instances will be cited in later chapters from the British Isles.</p> - -<div class="footnote"> - -<p><a id="Footnote_28" href="#FNanchor_28" class="label">[28]</a> <i>U.S. Geol. Survey, 6th Annual Report</i>, 1884-85, p. 172.</p> - -</div> - - -<h4>iii. <i>Distribution of Vents in Relation to Geological Structure-lines</i></h4> - -<p>Where the positions of true volcanic necks can be accurately determined, -it is interesting to study their distribution and their relation to the main -lines of geological structure around them. Sometimes a distinct linear -arrangement can be detected in their grouping. Those of the Lower Old -Red Sandstone of Central Scotland, for instance, can be followed in lines for -<span class="pagenum" id="Page_69">- 69 -</span> -distances of many miles (Map No. III). Yet when we try to trace the connection -of such an arrangement with any known great lines of dislocation in -the terrestrial crust, we can seldom establish it satisfactorily. In the case of -the Scottish Old Red Sandstone just cited, it is obvious that the vents were -opened along a broad belt of subsidence between the mountains of crystalline -schist on the north, and those of convoluted Silurian strata on the south, -either margin of that belt being subsequently, if not then, defined by lines -of powerful fault. No vents have risen along these faults, nor has any -relation been detected between the sites of the volcanic foci and dislocations -in the area of ancient depression.</p> - -<p>Indeed, it may be asserted of the vents of Britain that they are usually -entirely independent of any faults that traverse at least the upper visible -part of the earth's crust. They sometimes rise close to such lines of -fracture without touching them, but they are equally well developed where -no fractures are to be found. Now and then one of them may be observed -rising along a line of fault, but such a coincidence could hardly fail occasionally -to happen. From the evidence in the British Isles, it is quite -certain that if volcanic vents have, as is possible, risen preferably along -lines of fissure in the terrestrial crust, these lines are seldom those of the -visible superficial faults, but must lie much deeper, and are not generally -prolonged upward to the surface. The frequent recurrence of volcanic -outbursts at successive geological periods from the same or adjacent vents -seems to point to the existence of lines or points of weakness deep down -in the crust, within reach of the internal molten magma, but far beneath -the horizon of the stratified formations at the surface, with their more -superficial displacements.</p> - -<p>While sometimes running in lines, old volcanic vents of the Vesuvian -and Puy types often occur also in scattered groups. Two or three may be -found together within an area of a few hundred yards. Then may come -an interval where none, or possibly only a solitary individual, may appear. -And beyond that space may rise another sporadic group. These features -are well exhibited by the Carboniferous and Permian series of Scotland, to -the account of which the reader is referred.</p> - -<p>A large neck may have a number of smaller ones placed around it, just -as a modern Vesuvian cone has smaller parasitic cones upon its flanks. An -instructive example of this arrangement is to be seen at the great vent of -the Braid Hills belonging to the Lower Old Red Sandstone and described in -<a href="#CHAPTER_XX">Chapter xx.</a> Other instances may be cited from the Carboniferous and -Permian volcanic series (see Figs. <a href="#v1fig90">90</a>, <a href="#v1fig148">148</a>, <a href="../../66493/66493-h/66493-h.htm#v2fig213">213</a>).</p> - -<p>Not infrequently the irregularities in the ground-plan of a neck, as -already remarked, may be accounted for on the supposition that they -mark the site of more than one vent. Sometimes, indeed, it is possible to -demonstrate the existence of two or even more vents which have been -successively opened nearly on the same spot. The first orifice having -become choked up, another has broken out a little to one side, which in -turn ceasing to be effective from the same or some other cause, has been -<span class="pagenum" id="Page_70">- 70 -</span> -succeeded by a third (<a href="#v1fig29">Fig. 29</a>). The three cones and craters of the little -island of Volcanello supply a singularly perfect recent instance of this -structure (<a href="../../66493/66493-h/66493-h.htm#v2fig214">Fig. 214</a>). Here the funnel has twice shifted its position, each -cone becoming successively smaller and partially effacing that which preceded -it. In Auvergne, the Puy de Pariou has long been celebrated as -an example of a fresh cinder-cone partially effacing an earlier one. In the -much denuded Palæozoic volcanic tracts of Britain, where the cones have -long since disappeared and only the stumps of the volcanic cylinders are -left, many illustrations occur of a similar displacement of the funnel, especially -among the volcanoes of the Carboniferous system.</p> - -<p>Among the irregularities of necks that may indicate a connection with -lines of fissure, reference may be made here to dykes or dyke-like masses -of agglomerate which are sometimes to be seen among the volcanic districts -of Britain. In these cases the fragmentary materials, instead of lying in a -more or less cylindrical pipe, appear to fill up a long fissure. We may -suppose that the explosions which produced them did actually occur in -fissures instead of in ordinary vents. The remarkable Icelandic fissures -with their long rows of cinder cones are doubtless, at least in their upper -parts, largely filled up with slag and scoriæ. Some illustrations of this -structure will be given in the account of the Carboniferous volcanic rocks -of Scotland (see No. 1 in <a href="#v1fig22">Fig. 22</a>).</p> - -<div class="figcenter" id="v1fig29" style="width: 498px;"> - <img src="images/v1fig29.png" width="498" height="150" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 29.</span>—Successive shiftings of vents giving rise to double or triple cones. - A, ground-plan; B, vertical section.</div> -</div> - -<p>There is yet another consideration in regard to the form and size of -necks which deserves attention. Where the actual margin of a neck and -its line of vertical junction with the rocks through which it has been -drilled can be seen, there is no room for dispute as to the diameter of the -original funnel, which must have been that of the actual neck. But in -many cases it is impossible to observe the boundary; not merely because -of superficial soil or drift, but occasionally because the volcanic detritus -extends beyond the actual limits of the funnel. In such cases the necks -have retained some portion of the original volcanic cone which accumulated -on the surface around the eruptive vent. It may even chance that what -appears to be a large neck would be considerably reduced in diameter, and -might be shown to include more than one pipe if all this outer casing could -be removed from it. In <a href="#v1fig30">Fig. 30</a>, for example, a section is given of a neck (<i>n</i>) -<span class="pagenum" id="Page_71">- 71 -</span> -from which on the right-hand side all the cone and surrounding tuffs (<i>t</i>) -have been removed by denudation, the original form of the volcano being -suggested by the dotted lines. On the left side, however, the tuffs which -were interstratified with the contemporaneous sediments are still connected -with the neck, denudation not having yet severed them from it. The overlying -strata (<i>l</i>, <i>l</i>) which originally overspread the extinct volcano have been -bent into an anticline, and the neck of the vent has thus been laid bare by -the removal of the crest of the arch.</p> - -<div class="figcenter" id="v1fig30" style="width: 427px;"> - <img src="images/v1fig30.png" width="427" height="137" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 30.</span>—Section to show the connection of a neck with a cone and surrounding bedded tuffs.</div> -</div> - -<p>The instances where a structure of this kind is concealed are probably -fewer in number in proportion to their antiquity. But among Tertiary -cones they may perhaps not be so rare. The possibility of their occurrence -should be kept in view during the investigation of extinct volcanoes. The -term Neck ought not properly to be applied to such degraded volcanic -cones. The true neck still remains preserved in the inside of them. As -illustrative of the structure here referred to, I may cite the example of the -Saline Hill (<a href="#v1fig148">Fig. 148</a>) and of Largo Law (<a href="../../66493/66493-h/66493-h.htm#v2fig226">Fig. 226</a>), both in Fife.</p> - - -<h4>iv. <i>Metamorphism in and around Volcanic Vents—Solfataric Action</i></h4> - -<p>The prolonged ascent of hot vapours, stones, dust and lava, in the -funnel of a volcano must necessarily affect the rocks through which the -funnel has been driven. We may therefore expect some signs of alteration -in the material forming the walls of a volcanic neck. The nature of the -metamorphism will no doubt depend, in the first place, on the character and -duration of the agents producing it, and in the second, on the susceptibility -of the rocks to undergo change. Mere heat will indurate rocks, baking -sandstone, for instance, into quartzite, and shales into porcellanite. But -there will almost invariably be causes of alteration other than mere high -temperature. Water-vapour, for instance, has probably always been one of -the most abundant and most powerful of them. The copious evolution of -steam from volcanoes is one of their most characteristic features at the -present day, and that it was equally so in past time seems to be put beyond -question by the constantly recurring vesicular structure in ancient lavas -and in the lapilli and ejected blocks of old agglomerates and tuffs. Direct -experiment has demonstrated, in the hands of various skilful observers, -<span class="pagenum" id="Page_72">- 72 -</span> -from the time of Sir James Hall to that of Professor Daubrée, how powerfully -rocks are acted upon when exposed to superheated vapour of water -under great pressure. But the steam of volcanoes often contains other -vapours or mineralizing agents dissolved in it, which increase its metamorphic -influence. The mineral acids, for instance, must exert a powerful -effect in corroding most minerals and rocks. At the Solfatara of Naples -and at other volcanic orifices in different parts of Italy, considerable alteration -is seen to be due to this cause.</p> - -<p>Bearing these well-known facts in mind, we may be prepared to find -various proofs of metamorphism around and within old volcanic vents. -The surrounding rocks are generally much hardened immediately contiguous -to a neck, whether its materials be fragmental or massive. Sandstones, -for example, are often markedly bleached, acquire the vitreous lustre -and texture of quartzite, lose their usual fissility, break irregularly into angular -blocks, and on an exposed surface project above the level of the unaltered -parts beyond. Shales are baked into a kind of porcelain-like substance. -Coal-seams are entirely destroyed for economic purposes, having been burnt -into a kind of cinder or fused into a blistered slag-like mass. Limestones -likewise lose their usual bluish-grey tint, become white and hard, and -assume the saccaroid texture of marble.</p> - -<p>The distance to which this metamorphism extends from the wall is, -among the exposed necks in Britain, smaller than might be anticipated. -Thus I have seldom been able to trace it among those of Carboniferous or -Permian age for more than 15 or 20 yards in ordinary arenaceous and -argillaceous strata, even where every detail of a neck and its surroundings -has been laid bare in plan upon a beach. The alteration seems to reach -furthest in carbonaceous seams, such as coals.</p> - -<p>It is evident that the element of time must enter into the question of -the amount of metamorphism produced in the terrestrial crust immediately -surrounding a volcanic pipe. A volcano, of which the eruptions -begin and end within an interval of a few days or hours, cannot be -expected to have had much metamorphic influence on the rocks through -which its vent was opened. On the other hand, around a funnel which -served for many centuries as a channel for the escape of hot vapours, ashes -or lava to the surface, there could hardly fail to be a considerable amount of -alteration. The absence or comparatively slight development of metamorphism -at the Carboniferous and Permian necks of Scotland may perhaps -be regarded as some indication that these volcanoes were generally short-lived. -On the other hand, more extensive alteration may be taken as -pointing to a longer continuance of eruptive vigour.</p> - -<p>The same causes which have induced metamorphism in the rocks surrounding -a volcanic vent might obviously effect it also among the fragmentary -materials by which the vent may have been filled up. When the -eruptions ceased and the funnel was left choked with volcanic debris, hot -vapours and gases would no doubt still continue for a time to find their -way upward through the loose or partially compacted mass. In their ascent -<span class="pagenum" id="Page_73">- 73 -</span> -they would permeate this material, and in the end produce in it a series of -changes similar to, and possibly even more pronounced than, those traceable -in the walls of the vent. Instances of this kind of metamorphism will be -cited in the following chapters (see in particular <a href="#Page_404">p. 404</a>).</p> - - -<h4>v. <i>Inward Dip of Docks towards Necks</i></h4> - -<p>One concluding observation requires to be made regarding the relation -of old volcanic necks to the rocks which immediately surround them. -Where a vent has been opened through massive rocks, such as granite, -felsite, andesite or basalt, it is generally difficult or impossible to determine -whether there has been any displacement of these rocks, beyond the -disruption of them caused by the explosions that blew out the orifice. -But where the pipe has been drilled through stratified rocks, especially -when these still lie nearly flat, the planes of stratification usually -supply a ready test and measure of any such movement. Investigation of -the volcanic rocks of Britain has shown me that where any displacement -can be detected at a neck, it is almost invariably in a downward direction. -The strata immediately around the vent tend to dip towards it, whatever may -be their prevalent inclination in the ground beyond (<a href="#v1fig24">Fig. 24</a>). This is the -reverse of the position which might have been expected. It is so frequent, -however, that it appears to indicate a general tendency to subsidence at the -sites of volcanic vents. After copious eruptions, large cavernous spaces may -conceivably be left at the roots of volcanoes, and the materials that have filled -the vents, losing support underneath, will tend to gravitate downwards, and -if firmly welded to their surrounding walls may drag these irregularly down -with them. Examples of such sagging structures are abundantly to be seen -among the dissected vents of the Carboniferous and Permian volcanic series -of Scotland.</p> - - -<h4>vi. <i>Influence of Contemporaneous Denudation upon Volcanic Cones</i></h4> - -<p>It must be remembered that former vents, except those of the later geological -periods, are revealed at the surface now only after extensive denudation. -As a rule, the volcanoes that formed them appeared and continued in eruption -during periods of general subsidence, and were one by one submerged -and buried beneath subaqueous deposits. We can conceive that, while a -volcanic cone was sinking under water, it might be seriously altered in form -and height by waves and currents. If it consisted of loose ashes and stones, -it might be entirely levelled, and its material might be strewn over the floor -of the sea or lake in which it stood. But, as has been already pointed out, -the destruction of the cone would still leave the choked-up pipe or funnel -from which the materials of that cone had been ejected. Though, during -the subsidence, every outward vestige of the actual volcano might disappear, -yet the agglomerate or lava that solidified in the funnel underneath would -remain. And if these materials had risen some way within the cone or -<span class="pagenum" id="Page_74">- 74 -</span> -crater, or if they reached at least a higher level in the funnel than the surrounding -water-bottom or land-surface, the destruction of the cone might -leave a projecting knob or neck to be surrounded and covered by the accumulating -sediments of the time. It is thus evident that the levelling of a -cone of loose ashes during gradual subsidence, and the deposition of a contemporary -series of sedimentary deposits, might give rise to a true neck, -which would be coeval with the geological period of the volcano itself.</p> - -<p>In practice it is extremely difficult to decide how far any now visible -neck may have been reduced to the condition of a mere stump or core of a -volcano before being buried under the stratified accumulations of its time. -In every case the existence of the neck is a proof of denudation, and perhaps, -in most cases, the chief amount of that denudation is to be ascribed -not to the era of the original volcano, but to the comparatively recent -interval that has elapsed since, in the progress of degradation, the volcanic -rocks, after being long buried within the crust, were once more laid bare by -the continuous waste and lowering of the level of the land.</p> - - -<h4>vii. <i>Stages in the History of old Volcanic Vents</i></h4> - -<p>Let us now try to follow the successive stages in the history of a -volcano after its fires had quite burnt out, and when, slowly sinking in the -waters of the sea or lake wherein it had burst forth, it was buried under an -ever-growing accumulation of sedimentary material. The sand, mud, calcareous -ooze, shell-banks, or whatever may have been the sediment that was -gathering there, gradually crept over the submerged cone or neck, and -would no doubt be more or less mixed with any volcanic detritus which -waves or currents could stir up. If the cone escaped being levelled, or if -it left a projecting neck, this subaqueous feature would be entombed and -preserved beneath these detrital deposits. Hundreds or thousands of feet of -strata might be laid down over the site of the volcano, which would then -remain hidden and preserved for an indefinite period, until in the course of -geological revolutions it might once again be brought to the surface.</p> - -<p>These successive changes involve no theory or supposition. They must -obviously have taken place again and again in past time. That they -actually did occur is demonstrated by many examples in the British Isles. -I need only refer here to the interesting cases brought to light by mining -operations in the Dairy coal-fields of Ayrshire, which are more fully described -in <a href="#CHAPTER_XXVII">Chapter xxvii</a>. (<a href="#Page_433">p. 433</a>). In that district a number of cones of tuff, one -of which is 700 feet in height, have been met with in the course of boring -and mining for ironstone and coal. The well-known mineral seams of the -coal-field can be followed up to and over these hidden hills of volcanic tuff -which in the progress of denudation have not yet been laid bare (<a href="#v1fig146">Fig. 146</a>).</p> - -<p>The subsidence which carried down the water-bottom and allowed the -volcanic vents to be entombed in sedimentary deposits may have been in -most cases tolerably equable, so that at any given point these deposits -would be sensibly horizontal. But subsequent terrestrial disturbances -<span class="pagenum" id="Page_75">- 75 -</span> -might seriously affect this regularity. The sedimentary formations, piled -above each other to a great depth, and acquiring solidity by compression, -might be thrown into folds, dislocated, upheaved or depressed. The buried -volcanic funnels would, of course, share in the effects of these disturbances, -and eventually might be so squeezed and broken as to be with difficulty recognizable. -It is possible that some of the extreme stages of such subterranean -commotions are revealed among the "Dalradian" rocks of Scotland. -Certain green schists which were evidently originally sediments, and -probably tuffs, are associated with numerous sills and bosses of eruptive -material. The way in which these various rocks are grouped together -strikingly suggests a series of volcanic products, some of the crushed bosses -recalling the forms of true necks in younger formations. But they have -been so enormously compressed and sheared that the very lavas which -originally were massive amorphous crystalline rocks have passed into fissile -hornblende-schists.</p> - -<div class="figcenter" id="v1fig31" style="width: 518px;"> - <img src="images/v1fig31.png" width="518" height="205" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 31.</span>—Diagram illustrating the gradual emergence of buried volcanic cones through the influence - of prolonged denudation.</div> -</div> - -<p>Among the Palæozoic systems of Britain, however, where considerable -fracture and displacement have taken place, examples of successive stages in -the reappearance of buried volcanic cones and necks may be gathered in -abundance. As an illustrative diagram of the process of revelation by the -gradual denudation of an upheaved tract of country, <a href="#v1fig31">Fig. 31</a> may be -referred to (compare also <a href="#v1fig147">Fig. 147</a>).</p> - -<p>Here three volcanic vents are represented in different stages of re-emergence. -In the first (A) we see a cone and funnel which, after having been -buried under sedimentary deposits (<i>s</i>, <i>s</i>,) have been tilted up by subterranean -movements. The overlying strata have been brought within the influence -of denudation, and their exposed basset edges along the present surface of -the land (<i>g</i>, <i>g</i>) bear witness to the loss which they have suffered. Already, -in the progress of degradation, a portion of the volcanic materials which, -ejected from that vent, were interstratified with the contemporaneous -sediments of the surrounding sea-floor, has been exposed at <i>t</i>. A geologist -coming to that volcanic intercalation would be sure that it pointed to the -<span class="pagenum" id="Page_76">- 76 -</span> -existence of some volcanic vent in the neighbourhood, but without further -evidence he would be unable to tell whether it lay to right or left, whether -it was now at the surface or lay still buried under cover of the stratified -deposits which were laid down upon it.</p> - -<p>In the second or central example (B) we have a pipe and cone which -have been similarly disturbed. But in this case denudation has proceeded -so far as to reveal the cone and even to cut away a portion of it, as shown -by the dotted lines to the right hand. Owing, however, to the general -inclination of the rocks towards the left, that side of the cone, together with -the tuffs or lavas connected with it, still lies buried and protected under -cover of the sedimentary formations (<i>s</i>, <i>s</i>).</p> - -<p>The third example (C) shows a much more advanced stage of destruction. -Here the whole of the cone has been worn away. All the lavas and tuffs -which were ejected from it towards the right have likewise disappeared, -and strata older than the eruptions of this vent now come to the surface -there. To the left, however, a little portion of its lavas still remains at <i>l</i>, -though all the intervening volcanic material has been removed. That -solitary fragment of the outpourings of this volcano once extended further -to the left hand, but the occurrence of the large dislocation (<i>f</i>) has carried -this extension for down below the surface. The vent in this instance, -owing to its position, has suffered more from denudation than the other two. -Yet, judged by the size of its neck, it was probably larger than either -of them, and threw out a more extensive pile of volcanic material. Its -funnel has been filled with agglomerate (<i>a</i>), through which a central plug -of lava (<i>p</i>) has ascended, and into which dykes or veins (<i>d</i>, <i>d</i>), the last efforts -of eruption, have been injected.</p> - -<p>This diagram will serve to illustrate the fact already so often insisted on, -that although denudation may entirely remove a volcanic cone, and also all -the lavas and tuffs which issued from it, the actual filled-up pipe cannot be -so effaced, but is practically permanent.</p> - -<hr class="chap x-ebookmaker-drop" /> - - -<div class="chapter"> -<p><span class="pagenum" id="Page_77">- 77 -</span></p> - -<h2 class="nobreak" id="CHAPTER_VI">CHAPTER VI</h2> -</div> - -<div class="blockquot"> - -<p>Underground Phases of Volcanic Action—<i>continued</i>. II. Subterranean Movements of the -Magma: i. Dykes and Veins; ii. Sills and Laccolites; iii. Bosses (Stocks, Culots)—Conditions -that govern the Intrusion of Molten Rock within the Terrestrial Crust.</p> -</div> - - -<h3><span class="smcap">II. Subterranean Movements of the Magma</span></h3> - -<p>In the foregoing pages attention has been more specially directed to those -aspects of volcanic energy which reveal themselves above ground and in -eruptive vents. We have now to consider the various ways in which the -molten magma is injected into the crust of the earth.</p> - -<p>Such injection must obviously take place during the expulsion of volcanic -materials to the surface. If the explosive violence of an eruption, -or the concomitant movements of the earth's crust, should lead to ruptures -among the subterranean rocks, the molten magma will be forced into these -rents. It is evident that this may happen either with or without any -discharge of lava at the surface. It may be either entirely a plutonic, that -is, a deep-seated phenomenon, or it may be part of a truly volcanic series -of events.</p> - -<p>It is clear that, by the study of old volcanoes that have had their -structure laid bare by denudation, we may hope to obtain fresh light in -regard to some of the more deeply-seated features of volcanic energy, which -in a modern volcano are entirely concealed from view. A little reflection -will convince us that the conditions for consolidation within the crust are -so different from those at the surface that we may expect them to make -themselves visible in the internal characters of the rocks.</p> - -<p>An essential distinction between underground propulsions of molten -rock and superficial outflows of the same material lies in the fact that -while the latter are free to take any shape which the form and slope of -the ground may permit, the subterranean injections, like metal poured into -a mould, are always bounded by the walls of the aperture into which they -are thrust. According, therefore, to the shape of this aperture a convenient -classification of such intrusions may be made. Where the molten -material has risen up vertical fissures or irregular cracks, it has solidified -as Dykes and Veins. Where it has been thrust between the divisional -planes either of stratified or unstratified rocks, so as to form beds, these are -conveniently known as Sills, Laccolites or Intrusive Sheets. Where it has -taken the form of large cylindrical masses, which, ascending through the -<span class="pagenum" id="Page_78">- 78 -</span> -crust, appear at the surface in rounded, elliptical or irregularly-shaped -eminences, these are called Bosses (Stocks, Culots).</p> - -<p>Further contrasts between the superficial and subterranean consolidation -of molten material are to be found in the respective textures and minute -structures of the rocks. The deep-seated intrusions are commonly characterized -by a general and markedly greater coarseness of crystallization than -is possessed by lavas poured out at the surface. This difference of texture, -obviously in great measure the result of slower cooling, shows itself in acid, -intermediate, and basic magmas. A lava which at the surface has cooled as -a fine-grained, compact black basalt, in which neither with the naked eye nor -with the lens can the constituent minerals be distinctly determined, may -conceivably be represented at the roots of its parent volcano by a coarse-textured -gabbro, in which the felspars and pyroxenes may have grown into -crystals or crystalline aggregates an inch or more in length. Mr. Iddings -has pointed out that the various porphyrites which form the dykes and -sills of Electric Peak are connected with a central boss of coarsely crystalline -diorite.<a id="FNanchor_29" href="#Footnote_29" class="fnanchor">[29]</a> Examples of the same relation from different volcanic centres in -Britain will be cited in later chapters.</p> - -<div class="footnote"> - -<p><a id="Footnote_29" href="#FNanchor_29" class="label">[29]</a> <i>12th Ann. Rep. U.S. Geol. Survey</i> (1890-91), p. 595.</p> - -</div> - -<p>This greater coarseness of texture is shown by microscopic examination -to be accompanied by other notable differences. In particular, the glassy -residuum, or its devitrified representatives, which may be so frequently -detected among the crystals of outflowing lavas, is less often traceable in the -body of subterranean intrusive rocks, though it may sometimes be noticed -at their outer margins where they have been rapidly chilled by contact with -the cool upper part of the crust into which they have been impelled. -Various minerals, the constituents of which exist in the original magma, -but which may be hardly or not all recognisable in the superficial lavas, -have had leisure to crystallize out in the deep-seated intrusions and appear -sometimes among the components of the general body of the rock, or as well-terminated -crystals in its drusy cavities.</p> - -<p>Considerable though the variations may be between the petrographical -characters of the intrusive and extrusive rocks of a given district and of the -same eruptive period, they appear generally to lie within such limits as to -suggest a genetic relation between the whole series. Conditions of temperature -and pressure, and the retention or escape of the absorbed vapours which -play so large a part in volcanic activity, must exercise great influence on -the crystallization of constituent minerals, and on the consolidation and -ultimate texture of the rocks. Slow cooling under great pressure and with -the mineralizing vapours still largely retained seems to be pre-eminently -favourable for the production of a holocrystalline texture in deep-seated -portions of the magma, while rapid cooling under merely atmospheric -pressure and with a continuous disengagement of vapours, appears to be -required for the finer grain, more glassy structure, and more vesicular -character of lavas poured out at the surface.</p> - -<p>Besides these differences, however, there is evidence of a migration of -<span class="pagenum" id="Page_79">- 79 -</span> -the constituent minerals in the body of large intrusive masses before consolidation. -In particular, the heavier and more basic constituents travel -towards the cooling margin, leaving the central portions more acid. This -subject will be more fully considered in connection with the internal constitution -of Bosses, and some British examples will then be cited.</p> - -<p>Reference, however, may here be made to one of the most exhaustive and -instructive studies of the relations of the subterranean and superficial erupted -rocks of an old volcano, which will be found in the monograph by Mr. Iddings -on Electric Peak and Sepulchre Mountain in the Yellowstone Park of -Western America. From the data there obtainable he draws the deduction -that one parent magma, retaining the same chemical composition, may result -in the ultimate production of rocks strikingly different from each other in -structure and mineralogical constitution, yet chemically identical. Electric -Peak includes the central funnel filled up with coarsely crystalline diorite, -and having a connected series of sills and dykes of various porphyrites. -Sepulchre Mountain, separated from its neighbouring eminence by a fault of -4000 feet, displays some of the superficial discharges from the vent—coarse -breccias with andesite-lavas. These rocks are not chemically distinguishable -from the intrusive series, but the lavas are, on the whole, more glassy, while -the materials of the bosses, sills and dykes are more crystalline. The latter -display much more visible quartz and biotite.<a id="FNanchor_30" href="#Footnote_30" class="fnanchor">[30]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_30" href="#FNanchor_30" class="label">[30]</a> <i>12th Ann. Rep. U.S. Geol. Survey</i>, 1890-91. As already stated, the eruptions of this volcanic -centre became progressively more acid, and this change appears to be exhibited by the extrusive -lavas as well as by the intrusive rocks.</p> - -</div> - -<p>By practice in the field, supplemented by investigation with the aid of -the microscope, a geologist acquires a power of discriminating with fair -accuracy, even in hand specimens, the superficial from the subterranean -igneous rocks of an old volcanic district.</p> - -<p>Denudation, while laying bare the underground mechanism of an ancient -volcano, has not always revealed the evidence of the actual structural relations -of the rocks, or has first exposed and then destroyed it. Sometimes -a mass of eruptive rock has been worn down and left in such an isolated -condition that its connection with the rest of the volcanic network cannot -be determined. So far as its position goes, it might perhaps be either a -remnant of a lava-stream or the projecting part of some deeper-seated -protrusion. But its texture and internal structure will often enable a -confident opinion to be expressed regarding the true relations of such a -solitary mass.</p> - - -<h4>i. <i>Dykes and Veins</i></h4> - -<p>For the study of these manifestations of volcanic energy, the British -Isles may be regarded as a typical region. It was thence that the word -"dyke" passed into geological literature. Thousands of examples of both -dykes and veins may be seen from the Outer Hebrides southwards across -the length and breadth of the southern half of Scotland, far into the north -of England and towards the centre of Ireland. They may be found cutting -<span class="pagenum" id="Page_80">- 80 -</span> -the crests of the mountains and extending as reefs below the level of -the sea. They are thus exposed in every conceivable divergence of position -and in endless varieties of enclosing rock. Moreover, they can be shown -to represent a vast range of geological time. One system of them belongs -to some remote part of the Archæan periods, another is as young as the -older Tertiary ages.</p> - -<div class="figcenter" id="v1fig32" style="width: 330px;"> - <img src="images/v1fig32.png" width="330" height="195" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 32.</span>—Dyke, Vein and Sill. - The dyke (<i>d</i>) rises along a small fault among sandstones, shales, and ironstones (<i>sh</i>), and gives off a vein (<i>v</i>) and an - intrusive sheet or sill (<i>b</i>).</div> -</div> - -<p>Full details regarding these interesting relics of volcanic activity will -be given in later chapters, especially in Chapters <a href="../../66493/66493-h/66493-h.htm#CHAPTER_XXXIV">xxxiv.</a> and <a href="../../66493/66493-h/66493-h.htm#CHAPTER_XXXV">xxxv.</a> It -may suffice here to note that each of the three types of old volcanoes above -described has, in Britain, its accompaniment of dykes and veins. The plateaux, -however, present by far the most abundant and varied development of them. -The dykes of this series are characterized not only by their prodigious -numbers in and around some of the plateaux, but by the long distances to -which they may be traced beyond these limits. They are chiefly found in -connection with the Tertiary basalt-plateaux, though the Carboniferous -andesite-plateaux present a feebler display of them. The Tertiary dykes -are pre-eminently distinguished by their persistent rectilinear lines, sometimes -for distances of many miles, and their general north-westerly direction. -They form a vast system extending over an area of some 40,000 -square miles. Throughout that wide region their persistence of direction -and of petrographical characters point to the former existence of one or -more reservoirs of an andesitic and basaltic magma underneath the northern -half of Britain, and to the rupture of the crust overlying this subterranean -reservoir by thousands of parallel fissures. They thus constitute perhaps -the most astonishing feature in the volcanic history of Tertiary time.</p> - -<p>The dykes and veins connected with the puys are mainly to be found at -or close to the vents. Not infrequently they traverse the agglomerates of -the necks, and are sometimes to be traced to a central pipe or core of -basalt.</p> - -<p>The larger cones are likewise intersected with similar vertical, inclined -<span class="pagenum" id="Page_81">- 81 -</span> -or tortuously irregular walls of intruded lava. Occasionally a radiate -arrangement may be observed in such cases, like that noticeable at some -modern volcanoes, the dykes diverging from the eruptive centre.</p> - -<p>Many dykes exist regarding which there is no evidence to connect -them with any actual volcanic rocks. They have been injected into fissures, -but whether this took place during volcanic paroxysms, or owing to some -subterranean movements which never culminated in any eruption, cannot be -decided.</p> - -<p>The question of the age of dykes, like that of intrusive masses of all -kinds, is often difficult or impossible to decide. A dyke must of course be -younger than the rocks which it traverses, and a limit to its antiquity is thus -easily fixed. But we cannot always affirm that because a dyke stops short -of a particular rock, or series of rocks, it is older than these. The Hett -Dyke, in the north of England, rises through the Coal-measures, but stops -at the Magnesian Limestone; yet this cessation does not necessarily imply -that the dyke was in place before the deposition of that limestone. The -structure may have arisen from the dyke-fissure having ended at the bottom -of the limestone. Where dykes rise up to the base of an unconformable -formation without in any single case entering it, and where fragments of -them are enclosed in that formation, they must be of higher antiquity, and -must have been laid bare by extensive denudation before the unconformable -strata were deposited upon them. The great system of dykes in the Lewisian -Gneiss of the north-west of Scotland is in this way proved to be much more -ancient than the Torridon Sandstones under which it passes (Figs. <a href="#v1fig35">35</a>, <a href="#v1fig36">36</a>).</p> - -<p>Where two dykes cross each other, it is sometimes not difficult to decide -upon their relative antiquity. In intrusive rocks, the finest-grained parts -are those which lie nearest the outer margin, where the molten material was -rapidly chilled by coming in contact with cool surfaces of rock. Such -"chilled margins" of closer grain are common characteristics of dykes. -Wherever a dyke carries its chilled margin across another dyke, it must be -the younger of the two, and wherever such a margin is interrupted by -another dyke, it must belong to the older.</p> - -<p>As a rule, the uprise of molten material in a fissure has so effectually -sealed it up that in the subsequent disturbances of the terrestrial crust the -fissure has not been reopened, though others may have been produced near -it, or across it. Sometimes, however, the enormous tension to which the -crust was exposed opened the fissure once more, sometimes even splitting -a dyke along its centre, and a new ascent of molten rock took place within -the rent. Hence double or treble or compound dykes have been produced. -The second or later infillings are generally somewhat different from the -original dyke. Occasionally, indeed, they present a strong contrast to it. -Thus, among the dykes of Skye examples occur where the centre is occupied -by an acid granophyre, while the sides are occupied by dykes of basalt. -Instances of this compound type of dyke will be given in the account of -the Tertiary volcanic rocks of Britain.</p> - -<p>It is obvious that in a wide fissure the central portion may remain -<span class="pagenum" id="Page_82">- 82 -</span> -molten for some time after the sides have consolidated. If the fissure -served as a channel for the ascent of lava to the surface, it is conceivable -that the central still fluid part might be driven out and be replaced by -other material from below, and that this later material might differ considerably -in composition from that which first filled the opening. Such, -according to Mr. Iddings, has been the probable history of some of the -dykes at the old volcano of Electric Peak.<a id="FNanchor_31" href="#Footnote_31" class="fnanchor">[31]</a> But we can hardly suppose -that this explanation of compound dykes can have any wide application. It -could only hold good of broad fissures having an outlet, and is probably -inadmissible in the case of the numerous compound dykes not more than -10 or 15 feet in diameter, where the several bands of rock are sharply -marked off from each other. The abrupt demarcation of the materials in -these dykes, their closer texture along their mutual boundaries, the indications -of solution of the older parts of the group by the younger, and of -injection of the latter into the former, show that they belong to separate and -unconnected intrusions. These questions will be again referred to in the -account of the British Tertiary dykes (<a href="../../66493/66493-h/66493-h.htm#CHAPTER_XXXV">Chapter xxxv. vol. ii. p. 159</a>).</p> - -<div class="footnote"> - -<p><a id="Footnote_31" href="#FNanchor_31" class="label">[31]</a> <i>12th Ann. Rep. U.S. Geol. Survey</i> (1890-91), p. 587.</p> - -</div> - -<p>Another kind of compound dyke has arisen from the manner in which -the original fissure has been produced. While, in general, the dislocation -has taken the form of a single rectilinear rent, which on opening has left -two clean-cut walls, cases occur where the rupture has followed several -parallel lines, and the magma on rising into the rents appears as two or -more vertical sheets or dykes, separated by intervening partitions of the surrounding -rock. Examples of this structure are not infrequent among the -Tertiary dykes of Scotland. One of these may be noticed rising through -the cliffs of Lewisian gneiss on the east coast of the island of Lewis, -south of Stornoway. One of the most extraordinary instances of the same -structure yet observed is that described by Professor A. C. Lawson from the -Laurentian rocks at the mouth of White Gravel River, on the N.E. -coast of Lake Superior. In a breadth of only about 14 feet no less than -28 vertically intrusive sheets or dykes of diabase, from 1 inch to 6½ -inches broad, rise through the granite, which is thus split into 27 thin -sheets. The diabase undoubtedly cuts the granite, some of the sheets -actually anastomosing and sending veins into the older rock.<a id="FNanchor_32" href="#Footnote_32" class="fnanchor">[32]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_32" href="#FNanchor_32" class="label">[32]</a> <i>American Geologist</i> (1894), p. 293.</p> - -</div> - -<p>From the evidence supplied by the modern eruptions of Iceland, it is -evident that gaping fissures, which are filled by ascending lava and -thereby converted into dykes, in many instances serve as channels by -which molten rock escapes to the surface. It would be interesting if -any test could be discovered whereby those dykes could be distinguished -which had ever established a connection with the outer air. If the lava -continued to ascend in the fissures, and to pour out in superficial streams -for a long time, the rocks on either side would be likely to undergo -considerably more metamorphism than where there was only one rapid -injection of the magma, which would soon cool. Possibly in the much -<span class="pagenum" id="Page_83">- 83 -</span> -greater alteration of the same rocks by some dykes than by others, a sign of -such a connection with the surface may survive. This subject will be again -referred to in the account of the Tertiary dykes of Britain in Book VIII., -where the whole of the phenomena of this phase of volcanic action will be -fully discussed (see <a href="../../66493/66493-h/66493-h.htm#Page_163">vol. ii. p. 163</a>).</p> - - -<h4>ii. <i>Sills and Laccolites</i></h4> - -<p>The word "sill," derived from a remarkable sheet of eruptive rock in the -north of England, known as the Great Whin Sill (<a href="../../66493/66493-h/66493-h.htm#CHAPTER_XXIX">Chapter xxix.</a>), is now -applied as a convenient general term to masses of intrusive material, which -have been injected between such divisional planes as those of stratification, -and which now appear as sheets or beds (<a href="#v1fig33">Fig. 33</a>). These masses are likewise -called Intrusive Sheets, and where the injected material has accumulated -in large blister-like expansions, these are known as Laccolites (<a href="#v1fig34">Fig. 34</a>).</p> - -<div class="figcenter" id="v1fig33" style="width: 459px;"> - <img src="images/v1fig33.png" width="459" height="128" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 33.</span>—Section of Sill or Intrusive Sheet.</div> -</div> - -<p>Sills vary from only an inch or two up to 500 feet or more in thickness. -Lying, as they frequently do, parallel with strata above and below them, -they resemble in some respects true lava-sheets erupted contemporaneously -with the series of sediments among which they are intercalated. And, -indeed, cases occur in which it is hardly possible to decide whether to regard -a given mass as a sill or as a superficial lava. In general, however, sills -exhibit the coarser texture above referred to as specially characteristic of -subterranean eruptive masses. Moreover they are usually, though not -always, free from the vesicular and amygdaloidal structures of true surface-lavas. -Their under and upper surfaces, unlike the more scoriaceous parts of -lavas, are commonly much closer in grain than the general body of the -mass; in other words, they possess chilled borders, the result of more rapid -consolidation by contact with cooler rock. Again, instead of conforming to -the stratification of the formations among which they lie, as truly interstratified -lavas do, they may be seen to break across the bedding and pursue -their course on a higher or lower platform. The strata that overlie them, -instead of enclosing pieces of them and wrapping round irregularities on -their surface, as in the case of contemporaneously erupted lava-sheets, are -usually indurated, sometimes even considerably altered, while in many cases -they are invaded by veins from the eruptive sheet, or portions of them are -involved in it, and are then much hardened or metamorphosed.</p> - -<p>The petrographical character of the sills in a volcanic district depends -<span class="pagenum" id="Page_84">- 84 -</span> -primarily on the constitution of the parent magma, whence both they and the -outflowing lavas have issued. Where the lavas are rhyolites or felsites the -sills are acid, where basalts have been erupted the sills are basic, though -there has often been a tendency towards the appearance of more acid -material, such as trachyte. As we have seen, considerable differences in -petrographical characters may arise between the intrusive and extrusive -offshoots from the same parent magma during the course of a volcanic -cycle. This question will be more appropriately discussed together with -the leading characters of Bosses.</p> - -<p>Between the upper and under surface of a thick sill considerable petrographical -variation may sometimes be observed, especially where the rock is -of basic constitution. Differences both of texture and even to some extent -of composition can be detected. Sometimes what have been called "segregation -veins" traverse the mass, consisting of the same minerals as the -general body of the rock, but in larger crystals and in somewhat different -proportions. That these veins belong to the period of original consolidation -appears to be shown by the absence of fine-grained, chilled margins, and by -the way in which the component crystals of the veins are interlocked with -those of the body of the rock. Other veins of finer grain and more acid -composition probably belong to a later phase of consolidation, when, after -the separation and crystallization of the more basic minerals, the more acid -mother liquor that remained was, in consequence of terrestrial movements, -injected into cracks in the now solidified, though still highly heated, rock. -Examples of these features will be cited from various geological formations -in the following chapters.</p> - -<p>Reference has already been made to the difference occasionally perceptible -between the constitution of the upper and that of the under portions of superficial -lavas. A similar variation is sometimes strongly marked among sills, -especially those of a basic character, the felspars remaining most abundant -above, while the olivines and augites preponderate below. Mr. Iddings has -observed some excellent illustrations of this character in the great series of -sills connected with the volcanic pipe of Electric Peak in the Yellowstone -country.<a id="FNanchor_33" href="#Footnote_33" class="fnanchor">[33]</a> Some examples of the same structure will subsequently be cited -from the Carboniferous volcanic series of Central Scotland.</p> - -<div class="footnote"> - -<p><a id="Footnote_33" href="#FNanchor_33" class="label">[33]</a> "Electric Peak and Sepulchre Mountain," <i>12th Ann. Rep. U.S. Geol. Survey</i> (1890-91), -p. 584.</p> - -</div> - -<p>The greatest extreme of difference which I have observed in the petrographical -characters of any group of sills is that displayed by the Tertiary -gabbros of Skye. These rocks occur as sheets interposed among the bedded -basalts, and injected between each other in such a manner as to form thick -piles of rudely stratified sills. They possess a remarkable banded structure, -due to the aggregation of their component minerals in distinct layers, some -of which are dark in colour, from the abundance of their iron-ore, pyroxene -and olivine; while others are light-coloured, from the predominance of their -felspar. From the manner in which the component minerals of one band -interlace with those of the contiguous bands, it is quite certain that the -<span class="pagenum" id="Page_85">- 85 -</span> -structure is not due to successive injections of material among already -consolidated rocks, but belongs to the original conditions of expulsion of the -gabbro as a whole. It seems to indicate that the magma which supplied -the sills was at the time of its extrusion heterogeneous in composition, and -that the banding arises from the simultaneous or rapidly successive protrusion -of different portions of this variously-constituted magma. The details of -the structure will be described in the general account to be given of the -Tertiary volcanic rocks (Chapters <a href="../../66493/66493-h/66493-h.htm#CHAPTER_XLIII">xliii.</a> and <a href="../../66493/66493-h/66493-h.htm#CHAPTER_XLIV">xliv.</a>).</p> - -<p>Besides such visible differences in the composition of sills, others much -less obtrusive may occasionally be detected with the aid of microscopic or -chemical research. The outer parts of some sills are thus discovered to be -more basic or more acid than the inner portions. Or evidence may be -obtained pointing to the probable melting down of surrounding rocks by -the erupted magma, with a consequent local change in the chemical and -mineralogical constitution of the mass.</p> - -<p>In regard to their position in the geological structure of an old volcanic -district I may here remark that sills, seldom entirely absent, are more -especially developed either among the rocks through which the volcano has -driven its vent, or about the base of the erupted lavas and tuffs. Many -illustrations of this distribution will be described from the various volcanic -areas of Britain belonging to Palæozoic and Tertiary time. At the base of -the great Cambrian and Lower Silurian volcanic series of Merionethshire, -sills are admirably developed, while among the basaltic eruptions which closed -the long volcanic record in the north of Ireland and the Inner Hebrides, -they play a notable part.</p> - -<p>From the frequent place which sills take at the base of a volcanic series, -it may be inferred that they generally belong to a late phase in the history -of an eruptive episode or cycle, when the orifices of discharge had become -choked up, and when the volcanic energy found an easier passage laterally -between the strata underneath the volcanic pile or between the sheets of -that pile itself, than upward through the ever-increasing thickness of ejected -material.</p> - -<p>While there is an obvious relation between most sills and some eruptive -centre in their neighbourhood, cases occur in which no trace of any contemporaneous -volcano can be found, but where the intrusive sheet remains as the -sole evidence of the movements of the subterranean magma. The Great -Whin Sill, one of the most extensive intrusive sheets in the British Isles, is an -instance of this kind. Though this large mass of injected material can be -traced for a distance of about 80 miles, and though the strata beneath and -above it are well exposed in innumerable sections, no evidence has yet been -detected to show that it was connected with any vent that formed a volcano -at the surface (see <a href="../../66493/66493-h/66493-h.htm#Page_2">vol. ii. p. 2</a>). The absence of this evidence may, of -course, arise from the failure of denudation to uncover the site of the vent, -which may possibly still remain buried under the Carboniferous strata that -overlie the sill towards the south-east. But it may be due to the non-existence -of any such vent. We can quite conceive that volcanic energy should -<span class="pagenum" id="Page_86">- 86 -</span> -sometimes have failed to complete the formation of an actual volcano. -Aided by subterranean movements, it might have been potent enough -to disrupt the lower parts of the terrestrial crust, to propel the molten -magma into fissures, even to inject it for many miles between the planes -of stratification, which would be lines of least resistance, and yet in default -of available rents, might have been unable to force its way through the -upper layers and so reach the surface. Examples of such incompleted -volcanoes are perhaps to be recognized among solitary sills, which not infrequently -present themselves in the geological structure of Britain. But -the positive decision of this question is almost always frustrated by the -imperfection of the evidence, and the consequent possibility that a connected -vent may still lie concealed under overlying strata.</p> - -<p>Besides the more usual intrusions of molten material in the form of sheets -of which the vertical thickness bears but a small proportion to the horizontal -extent, there occur also large and thick cakes of intruded material in which -the vertical thickness may approach, or perhaps even surpass, the horizontal -diameter. These dome-shaped or irregular expansions form a connecting -link between ordinary sills and the bosses to be subsequently described. -They have received the name of <i>Laccolites</i> from Mr. G. K. Gilbert, who worked -out this peculiar type of structure in the case of the Henry Mountains in -southern Utah<a id="FNanchor_34" href="#Footnote_34" class="fnanchor">[34]</a> (<a href="#v1fig34">Fig. 34</a>). The same type has since been found distributed -over Arizona and Colorado, and it has been recognized as essentially that of -many eruptive masses or bosses in all parts of the world.</p> - -<div class="footnote"> - -<p><a id="Footnote_34" href="#FNanchor_34" class="label">[34]</a> "Geology of the Henry Mountains," <i>U.S. Geog. and Geol. Survey of the Rocky Mountain -Region</i>, 1877. For a review of the whole subject of laccolites in Western America see a -paper by Mr. Whitman Cross, in the <i>14th Annual Report of the Director of the U.S. Geological -Survey</i>, 1892-93 (pub. 1895), p. 157.</p> - -</div> - -<div class="figcenter" id="v1fig34" style="width: 338px;"> - <img src="images/v1fig34.png" width="338" height="173" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 34.</span>—Ideal section of three Laccolites. (After Mr. Gilbert.)</div> -</div> - -<p>In Western America, owing in large measure to the previously undisturbed -condition of the sedimentary formations, the relations of the injected -igneous material to these formations can be satisfactorily ascertained. The -geological structure of the various isolated laccolites thus clearly presented, -helps to explain the structure of other intrusive bodies which, having -been injected among plicated and dislocated rocks, do not so readily admit -of interpretation.</p> - -<p><span class="pagenum" id="Page_87">- 87 -</span></p> - -<p>In Colorado, Utah and Arizona the eruptive magma, usually a porphyrite, -diorite or quartz-porphyry, has risen in one or more pipes, and has then -intruded itself laterally between the planes of the sedimentary formations -which, over the centre of intrusion, have been pushed upward into a vast dome-shaped -or blister-like elevation. The horizon on which this lateral and -vertical expansion of the intruded material took place would seem to have -lain several thousand feet below the surface. It ranges from the Cambrian -to the Tertiary formations. Subsequent denudation has cut down the upraised -mantle of sedimentary layers, and has revealed more or less of the -igneous rock underneath, which is thus allowed to protrude and to be affected -by atmospheric erosion. In this way, wide plains of horizontal or gently -undulating Secondary and Tertiary strata have been diversified by the -appearance of cones, detached or in groups, which have become more peaked -and varied in outline in proportion as their original sedimentary covering -has been removed from them. The largest of the laccolitic masses in the -Henry Mountains is about 7000 feet deep and about 4 miles in diameter. -Less than one-half of the cover of overarching strata has been removed, and -denudation has cut deeply into the remaining part.</p> - -<p>That the type of structure, so well exhibited among the Henry -Mountains, has not been more abundantly recognized elsewhere probably -arises from the fact not that it is rare, but that the conditions for its -development are seldom so favourable as in Western America. Obviously -where stratified rocks have been much disturbed, they cease to furnish -definite or regular platforms for the reception of eruptive material, and to -afford convenient datum-lines for estimating what was probably the shape -of the intruded magma. We may believe that the effect of the propulsion -of eruptive material is usually to upheave the overlying crust, and thus to -give rise to a laccolitic form of intrusion. The upheaval relatively to the -surrounding country will be apt to be practically permanent, the intruded -body of rock being welded to the surrounding formations, and forming in this -way a solid and resisting core directly united by pipes or funnels with the -great magma-reservoir underneath. On the other hand, where the molten -rock, instead of consolidating underground, has been copiously discharged at -the surface, its emission must tend towards the production of cavernous -spaces within the crust. The falling in of the roofs of such caverns -will give rise to shocks of earthquakes. Subsequent uprisings of the -magma may fill these spaces up, and when the rock has solidified in the -form of laccolites or bosses, it may effectually put an end there to further -eruptions.</p> - -<p>Some contact metamorphism may be observed along the upper and under -surfaces of large sills. The rocks over the American laccolites have sometimes -been highly altered. But as the change is the same in kind as that -attendant upon Bosses, though generally less in degree, it will be considered -with these intrusive masses. The problems in terrestrial physics suggested -by the intrusion of such thick and persistent masses of eruptive material as -those which form sills and laccolites will likewise be discussed in connection -<span class="pagenum" id="Page_88">- 88 -</span> -with the mechanism of the remaining intrusive masses which have now to -be described.</p> - - -<h4>iii. <i>Bosses (Stocks, Culots)</i></h4> - -<p>The term Boss has been applied to masses of intrusive rock which form -at the surface rounded, craggy or variously-shaped eminences, having a -circular, elliptical or irregular ground-plan, and descending into the terrestrial -crust with vertical or steeply-inclined sides (<a href="#v1fig28">Fig. 28</a>). Sometimes they can -be seen to have pushed the surrounding rocks aside. In other places they -seem to occupy the place of these rocks through which, as it were, an -opening has been punched for the reception of the intrusive material.</p> - -<p>Occasionally, more especially in the case of large bosses, like those in -which granite so frequently appears, the eruptive mass may be observed to -rise here and there in detached knobs through the surrounding rocks, or to -enclose patches of these, in such a manner as to indicate that the large body -of eruptive material terminates upward in a very irregular surface, of which -only the more prominent parts project through the cake of overlying rocks. -In true bosses, unlike sills or laccolites, we do not get to any bottom on -which the eruptive material rests. Laccolites, indeed, may be regarded as -intermediate between the typical sill and the typical boss. The difference -between a laccolite and a boss lies in the fact that the body of the laccolite -does not descend into an unknown depth in the crust, but lies upon a -platform on which it has accumulated, the magma having ascended by one -or more ducts, which generally bear but a small proportion in area to the -mass of the laccolite. The boss, on the other hand, is not known to lie on -any horizon, nor to proceed from smaller ducts underneath, but plunges as a -great pillar or irregular mass, which may frequently be noticed to widen downwards -into the crust. There can be no doubt, however, that many masses of -eruptive rock, which, according to the definition here given, should be called -bosses, would be found to be truly laccolites if their structure below ground -could be ascertained. It is obvious that our failure to find any platform -on which the body of a boss lies, may arise merely from denudation having -been as yet insufficient to lay such a platform bare. It is hardly probable -that a boss several miles in diameter should descend as a column of that -magnitude to the magma-reservoir from which its material came. More -probably it has been supplied through one or more smaller ducts. The large -boss now visible at the surface may thus be really a laccolitic expansion on -one or more horizons. M. Michel Lévy lays stress on the general widening -of granitic bosses as they descend into the crust.<a id="FNanchor_35" href="#Footnote_35" class="fnanchor">[35]</a> While his observations -are supported by many illustrations from all parts of the globe, and -are probably true of the deeper-seated masses of granite, it is no less -true that numerous examples have been met with where a granite boss is -sharply marked off from the rocks which it has invaded and on which it may -<span class="pagenum" id="Page_89">- 89 -</span> -be seen to lie. Apart from the cases where granite seems to form part of a -vast internal, once molten mass, into which its encircling gneisses seem -to graduate, there are others in which this rock, as now visible, has been -injected into the crust as a boss or as a laccolite. Instances will be described -in later chapters where such bosses have risen through Cambrian, Silurian, -Devonian and Carboniferous formations. It may be said that between such -granitic intrusions and volcanic operations no connection can be traced. But -reasons will be brought forward in later chapters to regard some of the -granitic bosses as parts of the mechanism of Palæozoic volcanoes. It will -also be shown that among the intrusive rocks of the Tertiary volcanic series -of Britain there occur bosses of truly granophyric and granitic material. -Hence, though mainly what is called a "plutonic" rock, granite has made -its appearance among the subterranean protrusions of volcanoes.</p> - -<div class="footnote"> - -<p><a id="Footnote_35" href="#FNanchor_35" class="label">[35]</a> M. Michel Lévy, <i>Bull. Carte Géol. France</i>, No. 35, tome v. (1893), p. 32. The view stated -in the text is also that adopted by Prof. Brögger with reference to the granite of the Christiania district. -"Die Eruptivgesteine des Kristianiagebietes."</p> - -</div> - -<p>It is no doubt true that many intrusive masses, which must be included -under the general name of bosses, have probably had no connection whatever -with volcanic action properly so called. They are plutonic injections, -that is, portions of the subterranean magma which have been intruded into -the terrestrial crust during its periods of disturbance, and have not been -accompanied with any superficial discharges, which are essential in truly -volcanic energy. It has been proposed to draw a distinction between such -deep-seated intrusions and those which represent volcanic funnels.<a id="FNanchor_36" href="#Footnote_36" class="fnanchor">[36]</a> If this -were always practicable it would certainly be desirable. But the distinction is -not one that can in every case be satisfactorily drawn. Even in regard to -granitic bosses, which may generally be assumed to be plutonic in origin, -the British examples just referred to have in all likelihood been connected -with undoubted volcanic outbursts. Without, therefore, attempting here to -separate the obviously volcanic necks of eruptive material from the probably -plutonic bosses, I propose to describe briefly the general characters of bosses -considered as a group of intrusive rocks, together with the phenomena -which accompany them, and the conditions under which they may have -been injected.</p> - -<div class="footnote"> - -<p><a id="Footnote_36" href="#FNanchor_36" class="label">[36]</a> M. Michel Lévy, <i>Bull. Carte Géol. France</i>, No. 35, tome v. (1893).</p> - -</div> - -<p>Bosses, whether of plutonic or volcanic origin, are frequently not merely -single masses of eruptive rock, but are accompanied with a system of dykes -and veins, some of which can be traced directly into the parent-mass, while -others traverse it as well as the surrounding rocks. Hence the history of a -boss may be considerably more complex than the external form of the mass -might suggest.</p> - -<p>The petrographical characters of bosses link them with the other underground -injections of igneous material, more especially with sills and laccolites. -Indeed, on mere lithological grounds no satisfactory line could be drawn -between these various forms of intrusive rocks. The larger the mass the -more coarsely crystalline it may be expected to be. But the whole range -of structure, texture and composition, from those of the narrowest vein to -those of the widest boss, constitutes one connected series of gradations.</p> - -<p>Acid, intermediate and basic rocks are abundantly displayed among the -<span class="pagenum" id="Page_90">- 90 -</span> -bosses. Huge masses of granite, granophyre, quartz-porphyry, felsite or -rhyolite, represent the acid series. Intermediate varieties consist of trachyte, -phonolite, diorite, andesite or other rock. The basic bosses include varieties -of gabbro, dolerite, basalt, picrite, and other compounds.</p> - -<p>In a boss of large size, a considerable range of texture, composition and -structure may often be observed. The rock is generally much coarser in -grain than that of thin sills or dykes. Sometimes it exhibits a finer texture -along the margin than in the centre, though this variation is not usually so -marked as in sills and dykes. The rapidly-chilled and therefore more -close-textured selvage seems to have been developed much more fully in -small than in large masses of eruptive material. The latter, cooling more -slowly, allowed even their marginal parts to retain their heat, and sometimes -perhaps even their molten condition, longer than small injections. -Some influence must also have been exercised by the temperature of the -rocks into which the eruptive material was intruded. Where this temperature -was high, as in deep-seated parts of the crust, it would allow the -intrusive magma to cool more slowly, and thus to assume a more coarsely -crystalline condition. The absence of a close grain round the margins of -granitic bosses may be due to this cause.</p> - -<p>But a much more important distinction may be traced between the -central and marginal parts of some large bosses and thick sills. I have -already alluded to the fact that while the middle of a large intrusive mass -may be decidedly acid, taking even the form of granite, the outer borders -are sometimes found to be much more basic, passing into such a rock as -gabbro, or even into some ultra-basic compound. Between these extremes of -composition no sharp division is sometimes discoverable, such as might have -been expected had the one rock been intruded into the other. The differences -graduate so insensibly into each other as to suggest that originally -the whole mass of the rock formed one continuous body of eruptive material. -It is possible that in some cases the magma itself was heterogeneous at the -time of intrusion.<a id="FNanchor_37" href="#Footnote_37" class="fnanchor">[37]</a> But the frequency of the distribution of the basic -ingredients towards the outer margin, and the acid towards the centre, points -rather to a process of differentiation among the constituents of the boss -before consolidation. In some instances the differentiation would appear -to have taken place before crystallization to any great extent had set in, -because the minerals ultimately developed in the central parts differ from -those at the sides. In other cases, the transference of material would seem -to have been in progress after the component minerals had crystallized out -of the magma, for they are the same throughout the whole intrusive mass, -but differ in relative proportions from centre to circumference.<a id="FNanchor_38" href="#Footnote_38" class="fnanchor">[38]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_37" href="#FNanchor_37" class="label">[37]</a> The Tertiary gabbros of the Inner Hebrides have already been cited, and will be more fully -described in a later chapter as exhibiting the heterogeneousness of an eruptive magma.</p> - -<p><a id="Footnote_38" href="#FNanchor_38" class="label">[38]</a> See Messrs. Dakyns and Teall, <i>Quart. Journ. Geol. Soc.</i> xlviii. (1892), p. 104; Prof. Brögger, -<i>op. cit.</i> 1. (1894), p. 15; Mr. A. Harker, <i>op. cit.</i> p. 320; Prof. Iddings, <i>Journ. Geol. Chicago</i>, i. -(1893), p. 833; <i>Bull. Phil. Soc. Washington</i>, ii. (1890), p. 191; 1892, p. 89.</p> - -</div> - -<p>As illustrations of these features I may cite two good examples, one -from Scotland and one from England. The mass of Garabol Hill, in the -<span class="pagenum" id="Page_91">- 91 -</span> -Loch Lomond district, consists mainly of granite, occupying an area of about -12½ square miles. Messrs. Dakyns and Teall have shown that while the -central portions consist of granite, the south-eastern margin affords a remarkable -series of intermediate rocks, such as hornblende-biotite-granite, tonalite -(quartz-mica-diorite), diorite and augite-diorite, which lead us outwards into -highly basic compounds, including wehrlites (olivine-diallage rocks), picrites -(olivine-augite rocks), serpentine (possibly representing dunites, saxonites, -and lherzolites), and a peculiar rock consisting essentially of enstatite, -diallage, brown hornblende and biotite. The authors regard the whole of -these widely different rocks as the products of one original magma, the more -basic marginal area having consolidated first as peridotites, followed by -diorites, tonalites and granites in the order of increasing acidity. The most -acid rock in the whole series consists of felspar and quartz, is almost devoid -of ferro-magnesian minerals, and occurs in narrow veins in the granite and -tonalite. It indicates that after the segregation and consolidation of the -whole boss, ruptures occurred which were filled in by the ascent of the very -latest and most acid remaining portion of still fluid magma.<a id="FNanchor_39" href="#Footnote_39" class="fnanchor">[39]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_39" href="#FNanchor_39" class="label">[39]</a> Messrs. Dakyns and Teall, <i>Quart. Journ. Geol. Soc.</i> xlviii. (1892), p. 104.</p> - -</div> - -<p>The case of Carrock Fell in Cumberland has been described by Mr. A. -Harker, who has ascertained that the gabbro of this boss has in its central -portions a specific gravity of less than 2·85 and a silica-percentage sometimes -as high as 59·46, whilst its marginal zone gives a specific gravity -above 2·95 and a silica-percentage as low as 32·50. The migration of the -heavy iron ores towards the margin is readily apparent to the naked eye, -and is well established by chemical analysis, the oxides of iron amounting in -the centre to 6·24 (Fe<sub>2</sub>O<sub>3</sub> 3·60, FeO 2·64), and at the margin to 25·54 -(Fe<sub>2</sub>O<sub>3</sub> 8·44, FeO 17·10).<a id="FNanchor_40" href="#Footnote_40" class="fnanchor">[40]</a> Neither in this instance nor in that of Garabol -Hill has any evidence been noticed which would suggest that the basic and -acid rocks belong to different periods of intrusion. They pass so insensibly -into each other as to form in each case one graduated mass.</p> - -<div class="footnote"> - -<p><a id="Footnote_40" href="#FNanchor_40" class="label">[40]</a> Mr. A. Harker, <i>op. cit.</i> p. 320.</p> - -</div> - -<p>From these and other examples which have been observed, it is difficult -to escape the conclusion that the differences between the basic margin and -the acid centre are due to some process of segregation or differentiation while -the mass was still in a liquid condition, and its constituents could pass from -one part of the boss to another. According to Professor Brögger, it may be -stated as a general law that differentiation sets in during consolidation, -and is determined by, and dependent on, the laws of crystallization in a -magma, in so far as the compounds which, on given conditions, would first -crystallize out, diffuse themselves towards the cooling margin so as to produce -in the contact-stratum a peculiar chemical composition in the still -liquid material before crystallization takes place.<a id="FNanchor_41" href="#Footnote_41" class="fnanchor">[41]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_41" href="#FNanchor_41" class="label">[41]</a> This general conclusion is stated by Professor Brögger from his investigation of the rocks of -Gran, <i>Quart. Journ. Geol. Soc.</i> l. (1894), p. 36.</p> - -</div> - -<p>If during the process of differentiation, and before consolidation, injections -of the magma occur, they may be expected to differ in character according -<span class="pagenum" id="Page_92">- 92 -</span> -to the portion of the magma from which they are derived. Professor Brögger -believes that among the basic eruptive rocks of Gran in the Christiania district, -one and the same magma has in the bosses solidified as olivine-gabbro-diabases, -and in the dykes as camptonites, bostonites, pyroxenites, hornblendites, -and more acid augite-diorites.<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>Quart. Journ. Geol. Soc.</i> l. (1894), p. 35.</p> - -</div> - -<p>Various opinions have been propounded as to the cause or causes of this -so-called differentiation, but none of them are entirely satisfactory. We -must await the results of further exploration in the field and of continued -research in the laboratory.</p> - -<p>What appears to have taken place within a subterranean molten magma -which has been propelled into the earth's crust as a boss or laccolite, with -or without a connected system of dykes, may possibly be made to throw -some light on the remarkable changes in the characters of lavas successively -erupted from the same vent during the continuance of a volcanic cycle. -Whether or not any such process of differentiation can be proved to take -place within a subterranean volcanic reservoir, the sequence of erupted lavas -bears a curious resemblance to the order in which the constituents of some -large bosses succeed each other from margin to centre. The earliest lavas -may be of an intermediate or even basic character, but they generally tend -to become more acid. Nevertheless alternations of basic and acid lavas -which have been noted in various districts would seem to show that if there -be a process of differentiation in the magma-basins, it is not regular and -continuous, but liable to interruption and renewal. The return to basic -eruptions, which so often marks the close of a volcanic cycle, is likewise not -easily explicable on the supposition of continuous differentiation.</p> - -<p>Where no sensible evidence of differentiation is traceable in the general -body of a large intrusive mass, indications that some such process has there -been in progress are perhaps supplied by the more acid dykes or veins, -and the so-called "segregation veins," which have been already alluded to -as traversing large intrusive masses. Though these portions differ to a -greater or less extent in texture and composition from the main substance -of the boss, the differences are not such as to prevent us from regarding them -as really parts of the same parent magma. The veins, which are more -acid than the rock that they traverse, may be regarded as having emanated -from some central or deeper-seated part of a boss, which still remained fluid -after the marginal or upper portion had consolidated sufficiently far to be -capable of being rent open during subterranean disturbance. But that the -mass, though coherent enough to be fissured, still remained at a high temperature, -may be inferred from the general absence of chilled edges to these -veins. The evidence of differentiation supplied by "segregation veins" has -been referred to in the case of Sills.</p> - -<p>The study of the petrographical variations in the constitution of large -eruptive bosses has a twofold interest for the geologist. In the first place, -it affords him material for an investigation of the changes which a volcanic -magma undergoes during its eruption and consolidation, and thereby provides -<span class="pagenum" id="Page_93">- 93 -</span> -him with some data for an elucidation of the cause of the sequence of -erupted products during a volcanic cycle. In the second place, it yields to -him some interesting analogies with the structures of ancient gneisses, and -thus helps towards the comprehension of the origin and history of these -profoundly difficult but deeply fascinating rocks.</p> - -<p>Bosses, like sills, occur in the midst of volcanic sheets, and also as solitary -protrusions. Where they rise amidst interstratified lavas and tuffs they -may often be recognized as occupying the position of volcanic vents. They -are then necks, and their characters in this connection have already been -given. Where, however, as so frequently happens, they appear among rocks -in which no trace of any contemporaneous volcanic material is to be detected, -their relation to former volcanic activity remains uncertain.</p> - -<p>Of this doubtful nature some of the most notable examples are supplied -by the great granitic bosses which occur so frequently among the older -Palæozoic rocks of Britain. The age of these can sometimes be approximately -fixed, and is then found to correspond more or less closely with some volcanic -episode. Thus the granite-bosses of Galloway, in the south of Scotland, -disrupt Upper Silurian strata, but are older than the Upper Old Sandstone. -Hence they probably belong to the period of the Lower Old Red Sandstone, -which was eminently characterized by the vigour and long continuance of -its volcanoes. The granite of Arran and of the Mourne Mountains can be -shown by one line of reasoning to be younger than surrounding Carboniferous -formations, by other arguments to be probably later than the Permian period, -and by a review of the whole evidence to form almost certainly part of the -volcanic history of Tertiary time.</p> - -<p>But even where it can be shown that the uprise of a huge boss of -eruptive material was geologically contemporaneous with energetic volcanic -action, this coincidence may not warrant the conclusion that the boss therefore -marks one of the volcanic centres of activity. Each example must be -judged by itself. There have, doubtless, been many cases of the intrusion -of molten material in bosses, as well as in sills, without the establishment -of any connection with the surface. Such incompleted volcanoes have been -revealed by denudation after the removal of a great thickness of superincumbent -rock. The evidence which would have decided the question to -what extent any of them became true volcanic vents has thus been destroyed. -We can only reason tentatively from a careful collation of all the facts that -are now recoverable. Illustrations of this kind of reasoning will be fully -given in subsequent chapters.</p> - -<p>It has been supposed that a test for the discrimination of a subterranean -protrusion from a true volcanic chimney may be found in the condition of -the surrounding rocks, which in the case of the prolonged flow of molten -matter up a vent would be likely to undergo far more metamorphism than -would be the case in the injection of a single eruptive mass.<a id="FNanchor_43" href="#Footnote_43" class="fnanchor">[43]</a> But, as -has been already pointed out, no special or excessive metamorphism of the -encircling rocks is noticeable around many vents. There is certainly no -<span class="pagenum" id="Page_94">- 94 -</span> -more alteration contiguous to numerous true necks than around bosses, which -there is no reason to suppose ever communicated directly with the surface, -and which were probably the result of a single intrusion. We must always -remember that the denudation which has revealed these bosses has generally -removed the evidence of their upward termination and of their possible -connection with any volcanic ejections. Many of them may mark the sites -of true vents from which only single eruptions took place. The opening of -a volcanic vent does not necessarily imply a prolonged ascent of volcanic -material. In a vast number of cases the original eruption was the first and -last effort of the volcano, so that in such circumstances there seems no more -reason for much alteration of the walls of the chimney than for the metamorphism -of the rocks round a boss, laccolite, sill or dyke.</p> - -<div class="footnote"> - -<p><a id="Footnote_43" href="#FNanchor_43" class="label">[43]</a> See, for example, Mr. Harker, <i>Quart. Journ. Geol. Soc.</i> l. (1894), p. 329.</p> - -</div> - -<p>The metamorphism produced by intrusions of molten material upon the -rocks with which they have come in contact has long been studied. Its -amount varies so greatly in different cases that the conditions on which it has -specially depended are not easily determined. Three factors have obviously -been of great importance—first, the bulk of the intruded material; secondly, -the chemical composition and lithological texture and structure of the -rocks affected; and thirdly, the constitution and temperature of the invading -magma.</p> - -<p>1. It is clear that a huge boss of eruptive material will be likely to -effect much more alteration of the surrounding rocks than a small boss, sill -or dyke. Its initial temperature will probably be higher at the time of its -assuming its final place than that of the same material after it has found -its way into the narrower space of a thin sill or dyke. It will likewise -take much longer to cool. Hence the influence of its heat and its vapours -will continue to act long after those of the dyke or sill have ceased to -manifest themselves.</p> - -<p>2. It is equally evident that much of the resultant metamorphism will -depend on the susceptibility of the rocks to change. An obdurate material -such as pure quartz-sand, for example, will resist further alteration than -mere hardening into quartzite. Shales and mudstones may be indurated -into cherty substances of various textures. Limestones and dolomites, on -the other hand, may become entirely crystalline, and may even have new -minerals, such as garnet, tremolite, pyroxene, etc., developed in them. Hence -in comparing the amount of metamorphism attendant on two separate bosses -we must always take into account the nature of the rocks in which it -has been induced.</p> - -<p>3. But perhaps the most effective cause of variation in the nature and -amount of contact metamorphism has been the constitution of the eruptive -magma. A broad distinction may be drawn between the alteration produced -by basic and by acid rocks. The intrusion of basic material has -often produced singularly little change, even when the eruptive mass has -been of considerable size. The greatest amount of alteration is to be found -where the basic boss has caught up and enveloped portions of the surrounding -rocks. Thus where the gabbro of Carrock Fell has invaded the basic -<span class="pagenum" id="Page_95">- 95 -</span> -Lower Silurian lavas of the Lake District, the enveloped portions of the -latter show considerable modification. Their groundmass becomes darker -and more lustrous, the felspars assume a clearer appearance and lose some -of their conspicuous inclusions, the pyroxenic constituents are converted -into pale amphibole, and the glassy base disappears. At the actual line of -contact the felspars of the lavas have become disengaged from their original -matrix, which seems to have been dissolved and absorbed in the gabbro-magma. -Brown mica has been exceptionally developed in the altered lava. -At the same time, a change is noticeable in the character of the gabbro -itself near the contact. Brown mica is there to be seen, though not a -constituent of the rock elsewhere. The eruptive material has incorporated -the basic groundmass of the lavas, leaving the felspars undissolved.<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> Mr. Harker, <i>Quart. Journ. Geol. Soc.</i> vol. l. (1894), p. 331.</p> - -</div> - -<p>Much more serious are the changes produced by intrusions of acid -material, though here again the metamorphism varies within wide limits, -being sometimes hardly perceptible, and in other cases advancing so far as -to convert mere sedimentary material into thoroughly crystalline rocks. -Small sills and dykes of felsite and granophyre may produce very slight -change even upon shales and limestones, as may be seen among the eruptive -rocks of Skye and Raasay. Large bosses of granophyre, and still more -of granite, have been accompanied with the most extensive metamorphism. -Round these eruptive masses every gradation may be traced among sandy -and argillaceous sediments, until they pass into crystalline mica-schists, -which do not appear to be distinguishable from rocks of Archæan age. -Admirable examples of this extreme alteration may be observed around the -great granite bosses of Galloway.<a id="FNanchor_45" href="#Footnote_45" class="fnanchor">[45]</a> Again, among calcareous rocks a transition -may be traced from dull grey ordinary fossiliferous limestones and -dolomites into pure white crystalline marbles, full of crystals of tremolite, -zoisite, garnet and other minerals. The alteration of the fossiliferous Cambrian -limestones of Strath in Skye by the intrusive bosses of Tertiary granite -well illustrates this change.<a id="FNanchor_46" href="#Footnote_46" class="fnanchor">[46]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_45" href="#FNanchor_45" class="label">[45]</a> See Explanation to Sheet 9 of the <i>Geological Survey of Scotland</i>, p. 22; Prof. Bonney and -Mr. Allport, <i>Proc. Roy. Soc.</i> xvi. (1889); Miss Gardiner, <i>Quart. Journ. Geol. Soc.</i> vol. xlvi. -(1890), p. 569.</p> - -<p><a id="Footnote_46" href="#FNanchor_46" class="label">[46]</a> Macculloch, <i>Trans. Geol. Soc.</i> vol. iii. (1816), p. 1; <i>Description of the Western Isles</i>, vol. i. -p. 322. See also <i>Quart. Journ. Geol. Soc.</i> vol. xiv. (1857), p. 1; and vol. xliv. (1888), p. 62.</p> - -</div> - -<p>Without entering further here into the wide subject of contact metamorphism, -to which a large literature has now been devoted, we may note -the effects which have been produced in the eruptive material itself by its -contact with the surrounding rocks. Not only have these rocks been -altered, but very considerable modifications have likewise taken place in the -active agent of the change.</p> - -<p>Sometimes the alteration of the invading material has been effected -without any sensible absorption of the mineral constituents of the rocks -invaded. This appears to be the case in those instances where sheets of -basalt, intruded among coals or highly carbonaceous shales, have lost their -compact crystalline character and have become mere clays. In the coal-fields -<span class="pagenum" id="Page_96">- 96 -</span> -of Britain, where many examples of this change have been noted, the -igneous material is known as "white trap." The iron oxides have been in -great part removed, or, together with the lime of the component minerals, -have been converted into carbonates. Traces of the original felspar crystals -may still be detected, but the groundmass has been changed into a dull, -earthy, friable and decomposed substance.</p> - -<p>Nearly always, however, the alteration of the intrusive magma has -resulted from the incorporation of portions of the surrounding rocks. -Reference has been made above to the alteration of the Carrock Fell gabbro -by the absorption of some of the basic lavas around it. But still more -remarkable is the change produced in some acid rocks by the incorporation -of basic material into their substance. Professor Sollas has described in -great detail a remarkable instance of this effect in the probably Tertiary -eruptive rocks of the Carlingford district in the north-east of Ireland. -He has ascertained that the eruptive gabbro of that district is older -than the granite, for it is traversed by granophyre dykes which enclose -pieces of it. The granophyre dykes, on the other hand, often show a -lithoidal or chilled margin, which is not visible in the gabbro. He believes -that the gabbro is not only older than the acid protrusions, but was -already completely solid, traversed by contraction-joints, and probably fractured -by earth-movements, before the injection of the granophyric material, -which at the time of its intrusion was in a state of extreme fluidity, for it -has found its way into the minutest cracks and crevices. He has especially -studied the alteration produced by the granophyre upon the enclosed -pieces of basic rock. The diallage, isolated from the other constituents -of the gabbro, may commonly be seen to have broken up into numerous -granules, like the augite grains of basalt, while in some cases biotite and -hornblende have been developed with the concomitant excretion of magnetite. -The acid rock itself has undergone considerable modification owing -to the incorporation of basic material into its substance. Professor Sollas -distinguishes the following varieties of the rock:—Biotite-granophyre, biotite-amphibole-granophyre, -augite-granophyre, diallage-amphibole-augite-granophyre.<a id="FNanchor_47" href="#Footnote_47" class="fnanchor">[47]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_47" href="#FNanchor_47" class="label">[47]</a> <i>Trans. Roy. Irish Acad.</i> xxx. (1894), part xii. p. 477.</p> - -</div> - -<p>Similar phenomena have been described by Mr. Harker as occurring -where granophyre has invaded the gabbro of Carrock Fell.<a id="FNanchor_48" href="#Footnote_48" class="fnanchor">[48]</a> The same -observer has more recently detected some interesting examples furnished -by injections of Tertiary granophyre in the agglomerates of Skye. The -acid rock is roughly estimated by him to have taken up about one-fourth of -its bulk of gabbro fragments. He has investigated the minute structure of -the rock thus constituted, and has been able to recognize the augite of the -original gabbro, in various stages of alteration and completely isolated, the -other minerals having been dissolved in the acid magma.<a id="FNanchor_49" href="#Footnote_49" class="fnanchor">[49]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_48" href="#FNanchor_48" class="label">[48]</a> <i>Quart. Journ. Geol. Soc.</i> li. (1895), p. 183.</p> - -<p><a id="Footnote_49" href="#FNanchor_49" class="label">[49]</a> <i>Op. cit.</i> lii. The metamorphism produced upon fragments of different kinds of foreign -material enclosed within various igneous rocks has in recent years been studied in great detail by -Professor Lacroix—<i>Les Enclaves des Roches Volcaniques</i>, Macon, 1893.</p> - -</div> - -<p><span class="pagenum" id="Page_97">- 97 -</span></p> - -<p>It is not easy to comprehend the conditions under which large masses -of molten material have been injected into the crust of the earth. The two -main factors in volcanic action—terrestrial contraction and the energy of the -vapours in the magma—have no doubt played the chief part in the process. -But the relative share of each and the way in which the enormous load of -overlying rock has been overcome are not readily intelligible.</p> - -<p>Let us first consider for a moment the pressure of the superincumbent -crust under which the injection in many cases took place. The Whin Sill -of England may serve as a good illustration of the difficulties of the problem. -This notable mass of intrusive rock has been forced between the stratification -planes of the Carboniferous Limestone series in one, or sometimes more than -one, sheet. It stretches for a horizontal distance of not less than 80 miles -with an average thickness of between 80 and 100 feet. From the area -over which it can be traced its total extent underground must be at least -400 square miles (see <a href="../../66493/66493-h/66493-h.htm#CHAPTER_XXIX">Chapter xxix.</a>).</p> - -<p>In any single section the Whin Sill might be supposed to be a truly -interstratified sheet, so evenly does it seem to be intercalated between the -sedimentary strata. But here and there it diverges upward or downward -in such a way as to prove it to be really a vast injected sheet. The age of -the injection cannot be precisely fixed. It must be later than the Carboniferous -Limestone. There is no trace of any stratigraphical break in the -Carboniferous system of the region traversed by the sill. If the injection -took place during the Carboniferous period, it does not appear to have been -attended with any local disturbance, such as we might suppose would have -been likely to accompany the extravasation of so enormous a mass of igneous -material. If the date of injection be assigned to the next volcanic episode -in the geological history of Britain—that of the Permian period—it will -follow that the Whin Sill was intruded into its present position under the -superincumbent weight of the whole of the Carboniferous system higher -than the platform followed by the injected rock. The overlying body of -strata would thus exceed 5000 feet in thickness, or in round numbers would -amount at least to an English mile. The pressure of this mass of superincumbent -material, at the depth at which the injected magma was forced -between the strata, must have been so gigantic that it is difficult to believe -that the energy of the magma would have been able to achieve of itself so -stupendous a task as the formation of the Great Whin Sill.</p> - -<p>The volume of injected material is likewise deserving of special attention. -Many sills exceed 300 or 400 feet in thickness; and some laccolites must -enormously surpass these limits. The intrusion of so vast a body of new -material into the terrestrial crust will necessitate either a corresponding elevation -of that part of the crust overlying the injected magma or a subsidence -of that part underlying it, or some combination of both movements. It is -conceivable that, where the body of protruded magma was large and the -thickness of overlying crust was small, the expansive force of the vapours -under high tension in the molten rock may have sufficed for the uplift. -This result will be most likely to be effected around a volcanic chimney -<span class="pagenum" id="Page_98">- 98 -</span> -where the magma has the least amount of overlying load, and encounters -that relief from pressure which enables it to become a powerful agent in -terrestrial physics.</p> - -<p>But in the case of the larger bodies of injected rock, especially where -they do not seem to have been accompanied by the opening of any volcanic -vents, the propulsion of the igneous material into the crust has probably been -effected as a consequence of disturbance of the terrestrial crust. When the -strain of contraction leads to the pushing upward of the terrestrial areas -intervening between wide regions of subsidence, even though the differential -movement may be slight, the isogeotherms undergo deformation. The -intensely hot nucleus is squeezed upward, and if in the process of compression -ruptures take place in the crust, and cavities in it are consequently -opened, the magma will at once be forced into them. Such ruptures may -be expected to take place along lines of weakness. Rocks will split along -their stratification-planes, and the tendency to separation along these lines -may be aided by the readiness of the energetic magma to find its way into -and to enlarge every available opening. Hence we may expect that, besides -vertical fractures, leading to the production of dykes and bosses, there will -often be horizontal thrusts and ruptures, which will give rise to the formation -of sills.</p> - -<p>There is still another feature of terrestrial contraction which may help -us to follow the behaviour of the magma within the crust. Plication of the -crust is one of the most characteristic results of the contracting strain. -Where a great series of sedimentary formations has been violently compressed -so that its component strata have been thrown into rapid folds and squeezed -into a vertical position, the portion of the crust thus treated may possibly be -on the whole strengthened against the uprise of molten material through it. -But the folding is often accompanied with dislocation. Not only are the -rocks thrown into endless plications, but portions of them are ruptured and -even driven horizontally over other parts. Such greatly disturbed areas of -the crust are not infrequently found to have been plentifully injected with -igneous rocks in the form of dykes, veins, sills, laccolites and bosses.</p> - -<p>The elevation of a mountain-chain is known to be accompanied with a -diminution of density in the crust underneath. Mr. O. Fisher has suggested -that along such lines of terrestrial uplift there may be a double -bulge in the crust, one portion rising to form the upheaved land and the -other sinking down into the hot nucleus. If the lighter descending crust -were there melted it might form a magma ready to be poured out as lava -on the opening of any vent. The lava thus ejected would be of the lighter -kinds. It has been remarked as certainly a curious fact that the lavas -which issue from high mountain ranges are generally much more acid than -the heavy basic lavas which are so characteristic of volcanoes close to the -level of the sea.</p> - -<p>But even where no actual mountain-chain is formed, there are gentle -undulations of the crust which no doubt also affect the isogeotherms. If -any series of disturbances should give rise to a double system of such -<span class="pagenum" id="Page_99">- 99 -</span> -undulations, one crossing the other, there would be limited dome-shaped -elevations at the intersections of these waves, and if at the same time actual -rupture of the crust should take place, the magma might find its way -upward under such domes and give rise to the formation of laccolitic -intrusions. Cessation of the earth-movements might allow the intruded -material slowly to solidify without ever making an opening to the surface -and forming a volcano. Doubtless many sills, laccolites and bosses represent -such early or arrested stages in volcanic history.</p> - -<p>Propelled into the crust at a high temperature, and endowed with great -energy from the tension of its absorbed vapours and gases, the magma will -avail itself of every rent which may be opened in the surrounding crust, -and where it has succeeded in reaching the surface, its own explosive -violence may enable it to rupture the crust still further, and open for -itself many new passages. Thus an eruptive laccolite or boss is often fringed -with veins, dykes and sills which proceed from its mass into the rocks around.</p> - -<p>The question how far an ascending mass of magma can melt down -its walls is one to which no definite answer can yet be given. Recent -observations show that where the difference in the silica percentage between -the magma and the rock attacked is great, there may be considerable -dissolution of material from this cause. Allusion has already been made -to Mr. Harker's computation that some of the acid granophyres of Skye have -melted down about a fourth of their bulk of the basic gabbros. If such a -reaction should take place between the magma of a boss, sill or laccolite and -the rocks among which it has been intruded, great changes might result in -the composition of the intruded rock. We are not yet, however, in -possession of evidence to indicate that absorption of this kind really takes -place on an extensive scale within the earth's crust. If it did occur to a -large extent, we should expect much greater varieties in the composition of -eruptive rocks than usually occur, and also some observable relation between -the composition of the igneous material and that of the rocks into which it -has been injected. But enough is not yet known of this subject to warrant -any decided opinion regarding it.</p> - -<hr class="chap x-ebookmaker-drop" /> - - -<div class="chapter"> -<p><span class="pagenum" id="Page_100">- 100 -</span></p> - -<h3 class="nobreak" id="CHAPTER_VII">CHAPTER VII</h3> -</div> - -<p class="tdc">Influence of Volcanic Rocks on the Scenery of the Land—Effects of Denudation.</p> - - -<p>As considerable popular misapprehension exists respecting the part which -volcanism has played in the evolution of the existing topography of the -earth's surface, and as the British Isles, from their varied geological -structure, offer special facilities for the discussion of this subject, it may -not be out of place to devote a final section of the present Introduction to -a consideration of the real topographical influence of volcanic action.</p> - -<p>With modern, and especially with active, volcanoes we need not here -concern ourselves. Their topographical forms are well known, and give rise -to no difficulty. The lofty cones of the Vesuvian type, with their widespread -lavas and ashes, their vast craters and their abundant parasitic -volcanoes; the crowded, but generally diminutive, cones and domes of the -puy type, so well displayed in Auvergne, the Eifel and the Bay of Naples; -and the vast lava deserts of the plateaux, so characteristically developed in -Iceland and Western America, illustrate the various ways in which volcanic -energy directly changes the contours of a terrestrial surface.</p> - -<p>But the circumstances are altered when we deal with the topographical -influence of long extinct volcanoes. Other agencies then come into play, -and some caution may be needed in the effort to disentangle the elements -of the complicated problem, and to assign to each contributing cause its own -proper effect.</p> - -<p>Reference has already been made to the continuous denudation of -volcanic hills from the time that they are first erupted. But the comparative -rapidity of the waste and the remarkable topographical changes -which it involves can hardly be adequately realized without the inspection -of an actual example. A visit to the back of Monte Somma, already alluded -to, will teach the observer, far more vividly than books can do, how a -volcanic cone is affected by daily meteoric changes. The sides of such -a cone may remain tolerably uniform slopes so long as they are always -being renewed by deposits from fresh eruptions. But when the volcanic -activity ceases, and the declivities undergo no such reparation, they are -rapidly channelled by the descent of rain-water, until the furrows grow by -degrees wide and deep ravines, with only narrow and continually-diminishing -<span class="pagenum" id="Page_101">- 101 -</span> -crests between them. If unchecked by any fresh discharge of volcanic -material, the degradation will at last have removed the whole cone.</p> - -<p>It is thus obvious that purely volcanic topography, that is, the terrestrial -scenery due directly to the eruption of materials from within the earth, can -never become in a geological sense very old. It can only endure so long as -it is continually renewed by fresh eruptions, or where it is carried down by -subsidence under water and is there buried under a cover of protecting -sediments. When, therefore, we meet with volcanic rocks of ancient date -exposed at the surface, we may be quite certain that their present contours -are not those of the original volcano, but have been brought about by the -processes of denudation.</p> - -<p>It is true that, in the general erosion of the surface of the land, volcanic -rocks of ancient date sometimes rise into wonderfully craggy heights, including, -perhaps, cones and deep crater-like hollows, which to popular imagination -betoken contours left by now extinguished volcanic fires. Examples of -such scenery are familiar in various parts of Britain; but the resemblance -to recent volcanic topography is deceptive. There are, indeed, a few hills -wherein the progress of denudation seems not as yet to have entirely -removed the lavas and tuffs that gathered round the original vents. Some -of the tuff-cones of eastern Fife, for example, present cases of this kind. -Again, the great granophyre domes and cones of the Tertiary volcanic series -of the Inner Hebrides, though they have undoubtedly been extensively -denuded, may possibly retain contours that do not greatly differ from those -which these protruded bosses originally assumed under the mass of rock -which has been removed from them. Nevertheless, putting such doubtful -exceptions aside, we may confidently affirm that hills composed of ancient -volcanic material give no clue to the forms of the original volcanoes.</p> - -<p>It can hardly be too often repeated that the fundamental law in the -universal decay and sculpture of the land is that the waste is proportioned -to the resistance offered to it: the softer rocks are worn down with comparative -rapidity, while the harder varieties are left projecting above them. -As a general rule, volcanic rocks are more durable than those among which -they are interstratified, and hence project above them, but this is not always -the case. No universal rule can, indeed, be laid down with regard to the -relative durability of any rocks. While, therefore, topographic contours -afford a valuable indication of the nature and disposition of the rocks below -the surface, they cannot be relied upon as in all circumstances an infallible -guide in this respect. No better proof can be offered of the caution that -is needed in tracing such contours back to their origin than is furnished -by the old volcanic rocks of Britain. These eruptive masses, consisting -usually of durable materials and ranging through a vast cycle of geological -time, usually rise into prominent features and thus support the general law. -But they include also many easily eroded members, which, instead of -forming eminences, are worn into hollows. They include, in short, every -type of scenery, from featureless plains and rolling lowlands to craggy and -spiry mountains.</p> - -<p><span class="pagenum" id="Page_102">- 102 -</span></p> - -<p>The first point, then, which is established in an investigation of the -topographical influence of old volcanic rocks is that their prevailing -prominence arises from relative durability amidst universal degradation. -When we proceed further to inquire why they vary so much from each -other in different places, and how their complicated details of feature have -been elaborated, we soon learn that such local peculiarities have arisen -mainly from variations in the internal structure and grouping of the rocks -themselves.</p> - -<p>Here again the general law of sculpture comes into play. The local -features have depended upon the comparative resistance offered to the -sculpturing agents by the different portions of a volcanic series. Each -distinct variety of rock possesses its own characteristic internal structure. -The lines along which atmospheric disintegration will most effectually carry -on its carving work are thus already traced in the very substance and -architecture of the rock itself. Each rock consequently yields in its own -way to the processes of disintegration, and thus contributes its own distinctive -share to topographical feature.</p> - -<p>Among the massive rocks abundant examples of such special types of -weathering may be cited, from the acid and basic series, and from superficial -lavas as well as from intrusive bosses and sills. Acid bosses, such as those -of granite, granophyre and quartz-porphyry, tend to weather into blocks and -finally into sand, and as this tendency is somewhat uniformly distributed -through the rocks, they are apt to assume rounded, dome-shaped or conical -forms which, at a distance, may seem to have smooth declivities, but on -examination are generally found to be covered with a slowly-descending -sheet of disintegrated blocks and debris (<a href="../../66493/66493-h/66493-h.htm#v2fig346">Fig. 346</a>). When less prone to -decay, and especially where traversed by a strongly-defined system of vertical -joints, they may shoot up into tower-like heights, with prominent spires and -obelisks. Basic bosses, when their materials decay somewhat rapidly, give -rise to analogous topographical forms, though the more fertile soils which -they produce generally lead to their being clothed with vegetation. Where -they consist of an obdurate rock, much jointed and fissured, like the gabbro -of the Inner Hebrides, they form exceedingly rugged mountains, terminating -upward in serrated crests and groups of aiguilles (Figs. <a href="../../66493/66493-h/66493-h.htm#v2fig331">331</a>, <a href="../../66493/66493-h/66493-h.htm#v2fig333">333</a>).</p> - -<p>Acid lavas that have been superficially erupted weather into irregularly -craggy hills, like the flanks of Snowdon. Those of intermediate -composition, where they have accumulated in thick masses, are apt to -weather into conical forms, as may be seen among the Cheviot, Pentland -and Garleton Hills (Figs. <a href="#v1fig109">109</a>, <a href="#v1fig110">110</a>, <a href="#v1fig133">133</a>); but where they have been poured -out in successive thin sheets they have built up undulating plateaux with -terraced sides, as among the Ayrshire and Campsie Fells and the hills of -Lorne (Figs. <a href="#v1fig99">99</a>, <a href="#v1fig107">107</a>). Basic lavas have issued in comparatively thin sheets, -frequently columnar or slaggy, forming flat-topped hills and terraced escarpments, -such as are typically developed among the Tertiary basalt-plateaux -of the Inner Hebrides and the Faroe Islands (Figs. <a href="#v1fig11">11</a>, <a href="../../66493/66493-h/66493-h.htm#v2fig265">265</a>, <a href="../../66493/66493-h/66493-h.htm#v2fig283">283</a>, <a href="../../66493/66493-h/66493-h.htm#v2fig284">284</a>, <a href="../../66493/66493-h/66493-h.htm#v2fig286">286</a>).</p> - -<p>One of the most frequent causes of local peculiarities of topography -<span class="pagenum" id="Page_103">- 103 -</span> -among old volcanic rocks is the intercalation of very distinct varieties of -material in the same volcanic series. Where, for instance, lavas and tuffs -alternate, great inequalities of surface may be produced. The tuffs, being -generally more friable, decay faster and give rise to hollows, while the -lavas, being more durable, project in bold ridges or rise into mural escarpments -(<a href="../../66493/66493-h/66493-h.htm#v2fig265">Fig. 265</a>). Again, where dykes weather more readily than the rocks -which they traverse, they originate deep narrow clefts, while where they -weather more slowly than the rocks around them, they project as dark ribs. -Thus in Skye some dykes which rise through the obdurate gabbro are -marked by chasms which reach up even to the highest crests of the -mountains (<a href="../../66493/66493-h/66493-h.htm#v2fig333">Fig. 333</a>), while of those which run in the pale crumbling -granophyre, some stand up as black walls that can be followed with the -eye across the ridges even from a long distance.</p> - -<p>Many further illustrations of these principles might be cited here from -the old volcanic districts of Britain. But they will present themselves -successively in later chapters. For my present purpose it is enough to -show that the scenery of these districts is not directly due to volcanic action, -but is the immediate result of denudation acting upon volcanic rocks, -modified and directed by their geological structure.</p> - -<p>It may, however, be useful, in concluding the discussion of this subject, -to cite some typical volcanic regions in the British Isles as illustrations of -the relations between geology and topography, which, besides impressing the -main lesson here enforced, may serve also to show some of the striking contrasts -which geology reveals between the present and former conditions of -the surface of the globe. Among these contrasts none are more singular -than those offered by tracts where volcanic action has once been rife, and -where the picture of ancient geography presented in the rocks differs so -widely from the scenery of the same places to-day as to appeal vividly to the -imagination.</p> - -<p>The first district to which I may refer where ancient volcanic rocks are -well developed is that of Devonshire. The story of the Devonian volcanoes -will be told in some detail in later chapters, when it will be shown that the -eruptions were again and again renewed during a long course of ages. Yet, -abundant as the intercalated lavas and tuffs are, they can hardly be said to -have had any marked effect on the scenery, though here and there a harder -or larger mass of diabase rises into a prominent knoll or isolated hill. -When the amount of volcanic material in this region is considered, we may -feel some surprise at the trifling influence which it has exerted in the general -denudation of the surface.</p> - -<p>To one who wanders over the rich champaign of southern Devonshire, -and surveys from some higher prominence the undulating tree-crowned -ridges that slope down into orchard-filled hollows, and the green uplands -that sweep in successive waves of verdure to the distant blue tors of Dartmoor, -the scene appears as a type of all that is most peaceful, varied and -fertile in English landscape. In the trim luxuriance that meets the eye on -every side, the hand of man is apparent, though from many a point of -<span class="pagenum" id="Page_104">- 104 -</span> -vantage no sound may be heard for a time to show that he himself is anywhere -near us. Yet ever and anon from the deep lanes, hidden out of -sight under their canopy of foliage, there will come the creak of the groaning -waggon and the crack of the waggoner's whip, as evidence that there are -roads and human traffic through this bosky silent country.</p> - -<p>Amid so much quiet beauty, where every feature seems to be eloquent -of long generations of undisturbed repose, it must surely stir the imagination -to be told that underneath these orchards, meadows and woodlands lie -the mouldering remnants of once active and long-lived volcanoes. Yet we -have only to descend into one of the deep lanes to find the crumbling lavas -and ashes of the old eruptions. The landscape has, in truth, been carved out -of these volcanic rocks, and their decomposition has furnished the rich loam -that nourishes so luxuriant a vegetation.</p> - -<p>Not less impressive is the contrast presented between the present and -former condition of the broad pastoral uplands of the south of Scotland. -Nowhere in the British Islands can the feeling of mere loneliness be more -perfectly experienced than among these elevated tracts of bare moorland. -They have nothing of the grandeur of outline peculiar to mountain tracts. -Sometimes, for miles around one of their conspicuous summits, we may see -no projecting knob or pinnacle. The rocks have been gently rounded off -into broad featureless hills, which sink into winding valleys, each with its -thread of streamlet and its farms along the bottom, and its scattered remnants -of birch-wood or alder-copse along its slopes and dingles. Across miles of -heathy pasture and moorland, on the summits of this great tableland, we may -perchance see no sign of man or his handiwork, though the bleating of the -sheep and the far-off barking of the collie tell that we are here within the -quiet domain of the south-country shepherd.</p> - -<p>In this pastoral territory, also, though they hardly affect the scenery, -volcanic rocks come to the surface where the foldings of the earth's crust -have brought up the oldest formations. Their appearance extends over -so wide an area as to show that a large part of these uplands lies on -a deeply-buried volcanic floor. A whole series of submarine volcanoes, -extending over an area of many hundreds of square miles, and still in great -part overlain with the accumulated sands and silts of the sea-bottom, now -hardened into stone, underlies these quiet hills and lonely valleys.</p> - -<p>A contrast of another type meets us in the broad midland valley of -Scotland. Around the city of Edinburgh, for instance, the landscape is -diversified by many hills and crags which show where harder rocks project -from amidst the sediments of the Carboniferous system. On some of these -crags the forts of the early races, the towers of Celt and Saxon, and the -feudal castles of the middle ages were successively planted, and round their -base clustered for protection the cots of the peasants and the earliest homesteads -of the future city. Beneath these crags many of the most notable -events in the stormy annals of the country were transacted. Under their -shadow, and not without inspiration from their local form and colour, literature, -art and science have arisen and flourished. Nowhere, in short, within -<span class="pagenum" id="Page_105">- 105 -</span> -the compass of the British Isles has the political and intellectual progress of -the people been more plainly affected by the environment than in this -central district of Scotland.</p> - -<p>When now we inquire into the origin and history of the topography -which has so influenced the population around it, we find that its prominences -are relics of ancient volcanoes. The feudal towers are based on sills and -dykes and necks. The fields and gardens, monuments and roadways, overlie -sheets of lava or beds of volcanic ashes. Not only is every conspicuous -eminence immediately around of volcanic origin, but even the ranges of blue -hills that close in the distant view to south and north and east and west are -mainly built up of lavas and tuffs. The eruptions of which these heights -are memorials belong to a vast range of geological ages, the latest of them -having passed away long before the advent of man. But they have left -their traces deeply engraven in the rocky framework of the landscape. -While human history, stormy or peaceful, has been slowly evolving itself -during the progress of the centuries in these fertile lowlands, the crags and -heights have remained as memorials of an earlier history when Central -Scotland continued for many ages to be the theatre of vigorous volcanic -activity.</p> - -<p>As a final illustration of the influence of volcanic rocks in scenery, and -of the contrast between their origin and their present condition, I may cite -the more prominent groups of hills in the Inner Hebrides. In the singularly -varied landscapes of that region three distinct types of topography -attract the eye of the traveller. These are best combined and most fully -developed in the island of Skye. Throughout the northern half of that -picturesque island, the ground rises into a rolling tableland, deeply penetrated -by arms of the sea, into which it slopes in green declivities, while along -its outer borders it plunges in ranges of precipice into the Atlantic. Everywhere, -alike on the cliffs and the inland slopes, long parallel lines of rock-terrace -meet the eye. These mount one above another from the shores -up to the flat tops of the highest hills, presenting level or gently-inclined -bars of dark crag that rise above slopes of debris, green sward and bracken. -It is these parallel, sharply-defined bars of rock, with their intervening strips -of verdure, that give its distinctive character to the scenery of northern -Skye. On hillside after hillside and in valley after valley, they reappear -with the same almost artificial monotony. And far beyond the limits of -Skye they are repeated in one island after another, all down the chain of the -Inner Hebrides.</p> - -<p>In striking contrast to this scenery, and abruptly bounding it on the -south, rise the Red Hills of Skye—a singular group of connected cones. -Alike in form and in colour, these hills stand apart from everything around -them. The verdure of the northern terraced tableland here entirely disappears. -The slopes are sheets of angular debris,—huge blocks of naked -stone and trails of sand, amidst which hardly any vegetation finds a footing. -The decay of the rock gives it a pale yellowish-grey hue, which after rain -deepens into russet, so that in favourable lights these strange cones gleam -<span class="pagenum" id="Page_106">- 106 -</span> -with a warm glow as if they, in some special way, could catch and reflect -the radiance of the sky.</p> - -<p>Immediately to the west of these pale smooth-sloped cones, the dark -mass of the Cuillin Hills completes the interruption of the northern tableland. -In almost every topographical feature these hills present a contrast to the -other two kinds of scenery. Their forms are more rugged than those of -any other hill-group in Britain (<a href="../../66493/66493-h/66493-h.htm#v2fig331">Fig. 331</a>). Every declivity among them is -an irregular pile of crags, every crest is notched like a saw, every peak is -sharpened into a pinnacle. Instead of being buried under vast sheets of -their own debris, these hills show everywhere their naked rock, which seems -to brave the elements as few other rocks can do. Unlike the pale Red -Hills, they are dark, almost black in tone, though when canopied with cloud -they assume a hue of deepest violet.</p> - -<p>Each of these three distinct types of topography owes its existence to -the way in which a special kind of volcanic rock yields to the influences of -denudation. The terraced tableland of the north is built up of hundreds of -sheets of basaltic lava, each of the long level ledges of brown rock marking -the outcrop of one or more of these once molten streams. The black rugged -mass of the Cuillin Hills consists of a vast protruded body of eruptive -material, which, in the form of endless sills and bosses of gabbro and -dolerite, has invaded the basalt-plateau, and has now been revealed by the -gradual removal of the portion of that plateau which it upraised. The -pale cones and domes of the Red Hills mark the place of one of the last -protrusions in the volcanic history of Britain—that of large masses of an -acid magma, which broke through the basalt-plateau and also disrupted -the earlier gabbro.</p> - -<p>In no part of North-Western Europe has volcanic activity left more -varied and abundant records of its operations than in these three contiguous -tracts of Skye. It is interesting therefore to note the striking contrast -between the former and the present landscapes of the region. The lavas -of the basaltic tableland crumble into a rich loam, that in the mild moist -climate of the Hebrides supports a greener verdure than any of the other -rocks around will yield. The uplands have accordingly become pasture-grounds -for herds of sheep and cattle. The strips of lowland along the -valleys and in the recesses of the coast-line furnish the chief tracts of -arable land in the island, and are thus the main centres of the crofter -population. The bays and creeks of the much-indented shores form -natural harbours, which in former days attracted the Norse sea-rovers, and -supplied them with sites for their settlements. Norse names still linger -on headland and inlet, but the spirit of adventure has passed away, and a -few poor fishing-boats, here and there drawn up on the beach, are usually -the only token that the islanders make any attempt to gather the harvest -of the sea.</p> - -<p>The mountain groups which so abruptly bound the basalt-plateau on -the south, and present in their topographical features such distinctive -scenery, comprise a region too lofty, too rugged and too barren for human -<span class="pagenum" id="Page_107">- 107 -</span> -occupation. The black Cuillins and the pale Red Hills are solitudes left -to the few wild creatures that have not yet been exterminated. The -corries are the home of the red deer. The gabbro cliffs are haunts of the -eagle and the raven. Where patches of soil have gathered in the crannies -of the gabbro, alpine plants find their home. In the chasms left by the -decay of the dykes between the vertical walls of their fissures, the winter -snows linger into summer, and conceal with their thick drifts the -mouldering surface of the once molten rock beneath them. On every side -and at every turn a mute appeal is made to the imagination by the strange -contrasts between the quiet restfulness of to-day, when the sculpture-tools -of nature are each busily carving the features of the landscape, and the -tumult of the time when the rocks, now so silent, were erupted.</p> - -<hr class="tb" /> - -<p>The general discussion of the subject of Volcanism in this Introduction -will, I hope, have prepared the reader who has no special geological -training for entering upon the more detailed descriptions in the rest of this -treatise. As already stated, the chronological order of arrangement will be -followed. Beginning with the records of the earliest ages, we shall follow -the story of volcanic action down to the end of the latest eruptions.</p> - -<p>Each great geological system will be taken as a whole, representing a -long period of time, and its volcanic evolution will be traced from the -beginning of the period to the close. Some variety of treatment is -necessarily entailed by the wide range in the nature and amount of the -evidence for the volcanic history of different ages. But where practicable, -an outline will first be given of what can be gathered respecting the -physical geography of each geological period in Britain. In the description -which will then follow of the volcanic phenomena, an account of the -general characters of the erupted rocks will precede the more detailed -narrative of the history of the volcanic eruptions in the several regions -where they took place. References to the published literature of each -formation will be given in the first part of each section, or will be introduced -in subsequent pages, as may be found most convenient.</p> - -<hr class="chap x-ebookmaker-drop" /> - - -<div class="chapter"> -<p><span class="pagenum" id="Page_109">- 109 -</span></p> - -<h2 class="nobreak" id="BOOK_II">BOOK II<br /> - -<span class="smaller">VOLCANIC ACTION IN PRE-CAMBRIAN TIME</span></h2> -</div> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<h3 class="nobreak" id="CHAPTER_VIII">CHAPTER VIII<br /> - -<span class="smaller">PRE-CAMBRIAN VOLCANOES</span></h3> -</div> - -<div class="blockquot"> - -<p>The Beginnings of Geological History—Difficulties in fixing on a generally-applicable -Terminology—i. The Lewisian (Archæan) Gneiss; ii. The Dalradian or Younger -Schists of Scotland; iii. The Gneisses and Schists of Anglesey; iv. The Uriconian -Volcanoes; v. The Malvern Volcano; vi. The Charnwood Forest Volcano.</p> -</div> - - -<p>The early geological history of this globe, like the early history of mankind, -must be drawn from records at once scanty and hardly decipherable. Exposed -to the long series of revolutions which the surface of the planet has undergone, -these records, never perhaps complete at the first, have been in large -measure obliterated. Even where they still exist, their meaning is often -so doubtful that, in trying to interpret it, we find little solid footing, and -feel ourselves to be groping, as it were, in the dimness of mythological -legend, rather than working in the light of trustworthy and intelligible -chronicles. These primeval records have been more particularly the objects -of sedulous study during the last twenty years all over Europe and in North -America. A certain amount of progress in their decipherment has been -made. But the problems they still present for solution are numerous and -obscure. Fortunately, with many of these problems the subject of the -present treatise is not immediately connected. We need only concern -ourselves with those which are related to the history of primeval volcanic -activity.</p> - -<p>To the earliest and least definite division of the geological annals -various names have been applied. Some writers, believing that this period -preceded the first appearance of plants or animals upon the globe, have -named it Azoic—the lifeless age of geological history. But the absence of -any hitherto detected trace of organic existence among the oldest known -rocks cannot be held to prove that these rocks were formed before the -<span class="pagenum" id="Page_110">- 110 -</span> -advent of living things on the surface of the earth. The chance discovery -of a single fossil, which might at any moment be made, would show the -name "Azoic" to be a misnomer. Other geologists, believing that, as a -matter of fact, organic structures of low types do actually occur in them, -have called these old rocks "Eozoic," to denote that they were deposited -during the dawn of life upon our planet. But the supposed organisms -have not been everywhere accepted as evidence of former life. By many -able observers they are regarded as mere mineral aggregates. Another -term, "Archæan," has been proposed for the primeval ages of geological -history, which are recorded in rocks that carry us as far as may ever be -possible towards the beginnings of that history.</p> - -<p>In choosing some general term to include the oldest known parts of the -earth's crust, geologists are apt unconsciously to assume that the rocks thus -classed together represent a definite section of geological time, comparable, -for instance, to that denoted by one of the Palæozoic systems. Yet it is -obvious that, under one of these general terms of convenient classification, a -most multifarious series of rocks may be included, representing not one but -possibly many, and widely separated, periods of geological history.</p> - -<p>In many countries the oldest sedimentary accumulations, whether -fossiliferous or not, are underlain by a series of crystalline rocks, which -consist in great part of coarse massive gneisses and other schists. All over -the world these rocks present a singular sameness of structure and composition. -What might be found below them no man can say. They are in -each country the oldest rocks of which anything is yet known, and whatsoever -may be our theory of their origin, we must, at least for the present, -start from them as the fundamental platform of the terrestrial crust.</p> - -<p>But though crystalline rocks of this persistent character are widely -distributed, both in the Old World and in the New, they in themselves -furnish no means of determining their precise geological age. No method -has yet been devised whereby the oldest gneiss of one country can be shown -to be the true stratigraphical equivalent of the oldest gneiss of another. -Palæontology is here of no avail, and Petrology has not yet provided us -with such a genetic scheme as will enable us to make use of minerals and -rock-structures, as we do of fossils, in the determination of geological -horizons. All that can be positively affirmed regarding the stratigraphical -relations of the rocks in question is that they are vastly more ancient than -the oldest sedimentary and fossiliferous formations in each country where -they are found. The "Lewisian" gneiss of the north-west of Scotland, the -"Urgneiss" of Central Europe, and the "Laurentian" gneiss of Canada -occupy similar stratigraphical positions, and present a close resemblance in -lithological characters. We may conveniently class them under one common -name to denote this general relationship. But we have, as yet, no means of -determining how far they belong to one continuous period of geological -history. They may really be of vastly different degrees of antiquity.</p> - -<p>From the very nature of the case, any name by which we may choose to -designate such ancient rocks cannot possess the precise stratigraphical value -<span class="pagenum" id="Page_111">- 111 -</span> -of the terms applied to the fossiliferous formations. Yet the convenience of -possessing such a general descriptive epithet is obvious.</p> - -<p>Until much more knowledge of the subject has been gained, any -terminology which may be proposed must be regarded as more or less -provisional. The comprehensive term "pre-Cambrian" may be usefully -adopted as a general designation for all rocks older than the base of the -Cambrian system, irrespective of their nature and origin. Already it is well -known that under this term a vast series of rocks, igneous and sedimentary, -is included. In some regions several successive formations, or systems of -formations, may be recognized in this series. But until some method has -been devised for determining the stratigraphical relations of these formations -in different regions, it would seem safest not to attempt to introduce general -names for universal adoption, but to let the sequence of rocks in each distinct -geological province be expressed by a local terminology. This caution is -more especially desirable in the case of sedimentary deposits. We may -surmise as to the equivalence of the rocks called Huronian, Torridonian -and Longmyndian, but whilst so much is mere conjecture, it is certainly -injudicious to transfer the local names of one province to the rocks of another.</p> - -<p>The only relaxation of this general precaution which I think may at -present be made is the adoption of a common name for the oldest type of -gneisses. The term "Archæan" has been applied to these rocks, and if it -is used simply to express a common petrographical type, occupying the -lowest horizon in the stratigraphical series of a country, it has obvious -advantages. But I would still retain the local names as subordinate terms -to mark the local characteristics of the Archæan rocks of each province. -Thus the "Laurentian" rocks of Canada and the "Lewisian" rocks of -Scotland are widely-separated representatives of the peculiar stratigraphical -series which is known as Archæan.</p> - -<p>The pre-Cambrian rocks of Britain include several distinct systems or -groups. How far those of even one part of this comparatively limited -region are the proper equivalents of those of another and distant part is a -problem still unsolved. Hence each distinct area, with its own type of rocks, -will here be treated by itself. The following rock-types will be described: -<span class="allsmcap">I.</span> The Lewisian (Archæan) Gneiss; <span class="allsmcap">II.</span> The Younger (Dalradian) Schists -of Scotland; <span class="allsmcap">III.</span> The Gneisses and Schists of Anglesey; <span class="allsmcap">IV.</span> The Uriconian -Group; <span class="allsmcap">V.</span> The Malvern Group; <span class="allsmcap">VI.</span> The Charnwood Forest Group (see <a href="#Map_I">Map I.</a>).</p> - - -<h4>i. <span class="allsmcap">THE LEWISIAN (ARCHAÆN) GNEISS</span></h4> - -<p>The British Isles are singularly fortunate in possessing an admirable -development of pre-Cambrian rocks. These ancient masses rise up in -various parts of the islands, but the region where they are most extensively -displayed, and where their stratigraphical position and sequence are most -clearly shown, lies in the north-west of Scotland.<a id="FNanchor_50" href="#Footnote_50" class="fnanchor">[50]</a> In that territory they -<span class="pagenum" id="Page_112">- 112 -</span> -form the whole chain of the Outer Hebrides, and likewise extend as an -irregular selvage along the western margin of the counties of Sutherland -and Ross. The lowest known platform of the fossiliferous formations has -there been discovered and has been traced for a distance of more than 100 -miles. From this definite horizon, the high antiquity of all that lies -below it is impressively demonstrated. The accompanying diagram (<a href="#v1fig35">Fig. 35</a>) -will explain the general relations of the various geological formations -of the region.</p> - -<div class="footnote"> - -<p><a id="Footnote_50" href="#FNanchor_50" class="label">[50]</a> These rocks have been the subject of much discussion, but geologists are now agreed as to -their succession and structure. A full summary of the literature of the controversy regarding -them will be found in the <i>Quarterly Journal of the Geological Society</i>, vol. xliv. (1888), p. 378.</p> - -</div> - -<p>In certain dark shales (<i>b</i>) which occupy a well-defined and readily-traceable -position among the rocks of Sutherland and Ross, numerous specimens of -the trilobite genus <i>Olenellus</i>, together with other fossils, have been found. -By common consent among geologists, the zone of rock in which this genus -appears is taken as the lowest stage of the Cambrian system. In Britain -it marks the oldest known group of fossiliferous strata—the platform on -which the whole of the Palæozoic systems rest.</p> - -<div class="figcenter" id="v1fig35" style="width: 492px;"> - <img src="images/v1fig35.png" width="492" height="141" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 35.</span>—Diagram illustrating the stratigraphical relations of the pre-Cambrian and Cambrian rocks - of the North-west Highlands of Scotland.<br /> - - <i>c</i>, Durness Limestones, with Upper Cambrian and perhaps Lower Silurian fossils, 1500 feet, top nowhere seen. - <i>b</i>, Serpulite grit and "fucoid" shales, 70 to 80 feet, containing the <i>Olenellus</i>-zone. <i>a</i>, Quartzite, with - abundant annelid tubes, about 600 feet. II. Red Sandstones and Conglomerates, sometimes 8000 feet or more - (Torridonian). I. Gneiss with dykes, etc. (Lewisian).</div> -</div> - -<p>From the definite geological epoch indicated by this platform, we can go -backward into pre-Cambrian time, and realize in some measure how prodigious -must be the antiquity of the successive groups of rock which emerge -from beneath the base of the Palæozoic systems. Nowhere is this antiquity -more impressively proclaimed than in the north-west of Scotland. From -below the <i>Olenellus</i>-zone with its underlying sheets of quartzite (<i>a</i>), a thick -group of dull red sandstones and conglomerates (II.) rises into a series of -detached conical or pyramidal mountains, which form one of the most -characteristic features in the scenery of that region. As this detrital formation -is well developed around Loch Torridon, it has been termed Torridonian. -It attains a thickness of at least 8000 or 10,000 feet, and is traceable all -the way from the extreme northern headlands of Sutherland to the southern -cliffs of the island of Rum.</p> - -<p>In judging of the chronological significance of the geological structure -of the north-west of Scotland, we are first impressed by the stratigraphical -break between the base of the Cambrian system and the Torridonian -deposits below. This break is so complete that here and there the thick -<span class="pagenum" id="Page_113">- 113 -</span> -intervening mass of sandstones and conglomerates has been nearly or -wholly removed by denudation before the lowest Cambrian strata were -laid down. Such a discordance marks the passage of a protracted interval -of time.</p> - -<p>Again, when the composition of the Torridonian rocks is considered, -further striking evidence is obtained of the lapse of long periods. The -sandstones, conglomerates and shales of this pre-Cambrian system present -no evidence of cataclysmal action. On the contrary, they bear testimony -that they were accumulated much in the same way and at the same rate -as the subsequent Palæozoic systems. In that primeval period, as now, -sand and silt were spread out under lakes and seas, were ripple-marked by -the agitation of the water, and were gradually buried under other layers of -similar sediment. The accumulation of 10,000 feet of such gradually-assorted -detritus must have demanded a long series of ages. Here, then, in -the internal structure of the Torridonian rocks, there is proof that in -passing across them, from their summit to their base, we make another vast -stride backward into the early past of geological history.</p> - -<p>But when attention is directed to the relations of the Torridonian strata -to the rocks beneath them, a still more striking proof of an enormously -protracted period of time is obtained. Between the two series of formations -lies one of the most marked stratigraphical breaks in the geological structure -of the British Isles. There is absolutely nothing in common between them, -save that the conglomerates and sandstones have been largely made out of -the waste of the underlying gneiss. The denudation of the crystalline -rocks before the deposition of any of the Torridonian sediments must have -been prolonged and gigantic. The more, indeed, we study the gneiss, the -more do we feel impressed by the evidence for the lapse of a vast interval -of time, here unrecorded in rock, between the last terrestrial movements -indicated by the gneiss and the earliest of the Torridonian sediments.</p> - -<p>In this manner, reasoning backward from the horizon of the <i>Olenellus</i>-zone, -we are enabled to form some conception of the vastness of the antiquity -of the fundamental rocks of the North-west Highlands. The nature and -origin of these rocks acquire a special interest from a consideration of their -age. They contain the chronicles of the very beginnings of geological -history, in so far as this history is contained in the crust of the earth. -No part of the geological record is so obscure as this earliest chapter, but -we need not here enter further into its difficulties than may be necessary -for the purpose of understanding what light it can be made to throw on -the earliest manifestations of volcanic action.</p> - -<p>Under the term Lewisian Gneiss (<a href="#v1fig35">I. in Fig. 35</a>) a series of rocks is comprised -which differ from each other in composition, structure and age, -though most of them possess such crystalline and generally foliated characters -as may be conveniently included under the designation of gneiss. The complexity -of these ancient crystalline masses was not recognized at the time -when Murchison called them the "Fundamental" or "Lewisian" gneiss. -It is only since the Geological Survey began to study and map them -<span class="pagenum" id="Page_114">- 114 -</span> -in full detail that their true nature and history have begun to be understood.<a id="FNanchor_51" href="#Footnote_51" class="fnanchor">[51]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_51" href="#FNanchor_51" class="label">[51]</a> See the Report of this Survey work by Messrs. Peach, Horne, Gunn, Clough, Cadell and -Hinxman, <i>Quart. Journ. Geol. Soc.</i> vol. xliv. (1888), pp. 378-441; and Annual Reports of Director-General -of the Geological Survey in the <i>Report of The Science and Art Department for 1894</i>, p. -279, and 1895, p. 17 of reprint. The general area of the gneiss is shown in <a href="#Map_I">Map I.</a></p> - -</div> - -<p>The researches of the Survey have shown the so-called Lewisian -gneiss to comprise the following five groups of rock: 1. A group of -various more or less banded and foliated rocks which form together the -oldest and chief part of the gneiss (Fundamental complex); 2. Highly basic -dykes cutting the first group; 3. Dykes and sills of dolerite, epidiorite and -hornblende-schist; 4. A few dykes of peculiar composition; 5. Gneissose -granite and pegmatite.</p> - -<p>The first of these groups, forming the main body of the gneiss, has -been critically studied on the mainland from Cape Wrath to Skye. But -its development in the Outer Hebrides has not yet been worked out, -although the name "Lewisian" was actually taken from that chain of -islands. So far as at present known, however, the gneiss of the Hebrides -repeats the essential characters of that of the mainland.</p> - -<p>Mr. Teall, as the result of a careful investigation in the field and with -the microscope, has ascertained that on the mainland between Skye and -Cape Wrath the rocks of the "fundamental complex" are essentially composed -of olivine, hypersthene, augite (including diallage), hornblende, -biotite, plagioclase, orthoclase, microcline and quartz. He has further -observed that these minerals are associated together in the same manner as -in peridotites, gabbros, diorites and granites. Treating the rocks in -accordance with their composition and partly with their structure, but -excluding theoretical considerations, he has arranged them in the following -five subdivisions:—</p> - -<div class="blockquot"> -<p>1. Rocks composed of ferro-magnesian minerals, without felspar or quartz—Pyroxenites, -Hornblendites.</p> - -<p>2. Rocks in which pyroxenes are the dominating ferro-magnesian constituents, felspar -always being present, sometimes quartz: A, Without quartz, Hypersthene-augite-rocks -(pyroxene granulites; rocks of the Baltimore-gabbro type) and -augite-rocks (gabbros); B, With quartz, Augite-gneiss.</p> - -<p>3. Rocks in which hornblende is the prevalent ferro-magnesian constituent: A, Without -quartz, or containing it only in small quantity; rocks basic in composition: -(<i>a</i>) massive or only slightly foliated (Amphibolites, as epidote-amphibolite, -zoisite-amphibolite, garnet-amphibolite); (<i>b</i>) foliated (Hornblende-schist). B, -With quartz; rocks intermediate or acid in composition: (<i>a</i>) with compact -hornblende and a granular structure (Hornblende-gneiss proper); (<i>b</i>) with hornblende -occurring in fibrous or other aggregates; (<i>c</i>) with compact hornblende -and a more or less granulitic structure (Granulitic hornblende-gneiss).</p> - -<p>4. Rocks in which biotite is the predominant ferro-magnesian constituent; felspar and -quartz both present: (<i>a</i>) Biotite occurring as independent plates or in aggregates -of two or three large individuals (Biotite-gneiss); (<i>b</i>) Biotite occurring in -aggregates of numerous small individuals (rare type); (<i>c</i>) Biotite occurring as -independent plates in a granulitic structure.</p> - -<p>5. Rocks in which muscovite and biotite are present, together with felspar and quartz—Muscovite-biotite-gneiss. -These, though not forming a well-defined natural -<span class="pagenum" id="Page_115">- 115 -</span> -group, are placed together for purposes of description. They are all foliated, -some having the aspect of mica-schists, others being typical augen-gneisses, -or light grey gneisses with abundant oligoclase and inclusions of microlitic -epidote.</p> -</div> - -<p>The rocks of each of these types are usually restricted to relatively -small areas, and they succeed each other with much irregularity all the way -from Skye to Cape Wrath. Their chemical and mineralogical composition -proves them to have decided affinities with the plutonic igneous masses of -the earth's crust.</p> - -<p>The only exceptions to this prevalent igneous type occur in the -districts of Gairloch and Loch Carron, where the gneiss appears to be -associated with a group of mica-schists, graphitic-schists, quartzites and -siliceous granulites, limestones, dolomites, chlorite-schists and other schists. -That these are altered sedimentary formations can hardly be doubted. -What their precise relations to the fundamental complex of the gneiss may -be has not yet been satisfactorily determined. They are certainly far older -than the Torridon sandstone which covers them unconformably. Possibly -they may represent a sedimentary formation still more ancient than the -gneiss.</p> - -<p>Save these obscure relics of a pre-Torridonian system of strata, the -gneiss never presents any structure which suggests the alteration of clastic -constituents. Everywhere its mineral composition points to a connection -with the subterranean intrusions of different igneous magmas, while the -manner in which its different rock-groups are associated together, and the -internal structure of some of them, still further link it with phenomena -which will be described in succeeding chapters as parts of the records of -volcanic action.</p> - -<p>An interesting feature of the fundamental complex, as bearing on the -origin of the gneiss, is to be found in the occurrence of bosses and bands -which are either non-foliated or foliated only in a slight degree. These -comparatively structureless portions present much of the character of bosses -or sills of true eruptive rocks. They occur in various parts of Sutherland -and Ross. Their external margins are not well defined, and they pass -insensibly into the ordinary gneiss, the dark basic massive rocks shading -off into coarse basic gneisses, and the pegmatites of quartz and felspar -which traverse them merging into bands of grey quartzose gneiss.</p> - -<p>So far, therefore, as present knowledge goes, the main body or fundamental -complex of the Lewisian gneiss in the North-west Highlands of -Scotland consists of what may have been originally a mass of various -eruptive rocks. It has subsequently undergone a succession of deformations -from enormous stresses within the terrestrial crust, which have been -investigated with great care by the Geological Survey. But it presents -structures which, in spite of the abundant proofs of great mechanical -deformation, are yet, I venture to think, original, or at least belong to the -time of igneous protrusion before deformation took place. The alternation -of rocks of different petrographical constitution suggests a succession of -<span class="pagenum" id="Page_116">- 116 -</span> -extravasations of eruptive materials, though it may not be always possible -now to determine the order in which these followed each other. In the -feebly foliated or massive bands and bosses there is a parallel arrangement -of their constituent minerals or of fine and coarse crystalline layers which -recalls sometimes very strikingly the flow-structure of rhyolites and -other lavas. This resemblance was strongly insisted on by Poulett Scrope, -who believed that the laminar structure of such rocks as gneiss and mica-schist -was best explained by the supposition of the flow of a granitic magma -under great pressure within the earth's crust.<a id="FNanchor_52" href="#Footnote_52" class="fnanchor">[52]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_52" href="#FNanchor_52" class="label">[52]</a> <i>Volcanoes</i>, pp. 140, 283, 299.</p> - -</div> - -<p>The conviction that these parallel structures do, in some cases, really -represent traces of movements in the original unconsolidated igneous -masses, not yet wholly effaced by later mechanical stresses, has been greatly -strengthened in my mind by a recent study of the structures of various -eruptive bosses, especially those of gabbro in the Tertiary volcanic series of -the Inner Hebrides. The banded structure, the separation of the constituent -minerals into distinct layers or zones, the alternation of markedly -basic with more acid layers, and the puckering and plication of those bands, -can be seen as perfectly among the Tertiary gabbro bosses of Skye as in -the Lewisian gneiss (see Figs. <a href="../../66493/66493-h/66493-h.htm#v2fig336">336</a> and <a href="../../66493/66493-h/66493-h.htm#v2fig337">337</a>). It cannot be contended that -such structures in the gabbro are due to any subsequent terrestrial disturbance -and consequent deformation. They must be accepted as part of the -original structure of the molten magma.<a id="FNanchor_53" href="#Footnote_53" class="fnanchor">[53]</a> It seems to me, therefore, highly -probable that the parallel banding in the uncrushed cores of the Lewisian -gneiss reveals to us some of the movements of the original magma at the -time of its extrusion and before it underwent those great mechanical -stresses which have so largely contributed to the production of many of its -most characteristic structures.</p> - -<div class="footnote"> - -<p><a id="Footnote_53" href="#FNanchor_53" class="label">[53]</a> See A. Geikie and J. J. H. Teall, <i>Quart. Journ. Geol. Soc.</i> vol. 1. (1894), p. 645.</p> - -</div> - -<p>While the material of the oldest gneiss presents many affinities to plutonic -rocks of much younger date, a wide region of mere speculation opens out -when we try to picture the conditions under which this material was -accumulated. Some geologists have boldly advanced the doctrine that the -Archæan gneisses represent the earliest crust that consolidated upon the -surface of the globe. But these rocks offer no points of resemblance to the -ordinary aspect of superficial volcanic ejections. On the contrary, the -coarsely-crystalline condition even of those portions of the gneiss which -seem most nearly to represent original structure, the absence of anything -like scoriæ or fragmental bands of any kind, and the resemblances which -may be traced between parts of the gneiss and intrusive bosses of igneous -rock compel us to seek the nearest analogies to the original gneiss in deep-seated -masses of eruptive material. It is difficult to conceive that any -rocks approaching in character to the gabbros, picrites, granulites and -other coarsely-crystalline portions of the old gneiss could have consolidated -at or near the surface.</p> - -<p>When the larger area of gneiss forming the chain of the Outer -<span class="pagenum" id="Page_117">- 117 -</span> -Hebrides is studied, we may obtain additional information regarding the -probable origin and the earliest structures of the fundamental complex of -the Lewisian gneiss. In particular, we may look for some unfoliated -cores of a more acid character, and perhaps for evidence which will show -that both acid and basic materials were successively protruded. We may -even entertain a faint hope that some trace may be discovered of superficial -or truly volcanic products connected with the bosses which recall those of -later date and obviously eruptive nature. But up to the present time no -indication of any such superficial accompaniments has been detected. If -any portions of the old gneiss represent the deeper parts of columns of -molten rock that flowed out at the surface as lava, with discharges of fragmentary -materials, all this superincumbent material, at least in the regions -which have been studied in detail, had disappeared entirely before the -deposition of the very oldest part of the Torridonian rocks, unless some -trace of it may remain among the pebbles of the Torridonian conglomerates, -to which reference will be immediately made.</p> - -<p>So far, then, as the evidence now available allows a conclusion to be -drawn, the Lewisian gneiss reveals to us a primeval group of eruptive rocks -presenting a strong resemblance to some which in later formations are connected, -as underground continuations, with bedded lavas and tuffs that -were erupted at the surface; and although no proof has yet been obtained -of true volcanic ejections associated with the fundamental complex, the -rocks seem to be most readily understood if we regard them as having -consolidated from igneous fusion at some depth, and we may plausibly infer -that they may have been actually connected with the discharge of volcanic -materials at the surface. The graphite-schists, mica-schists, and limestones -of the Gairloch and Loch Carron may thus be surviving fragments -of the stratified crust into which these deep-seated masses were intruded, -and through which any volcanic eruptions that were connected with them -had to make their way.</p> - -<p>The limited areas occupied by the several varieties of rock in the -fundamental complex suggests the successive protrusion of different magmas, -or of different portions from one gradually changing magma. Mr. Teall -has ascertained that whenever in this series of rocks the relative ages of -two petrographical types can be clearly ascertained, the more basic is older -than the more acid.</p> - -<p>But besides all the complexity arising from original diversity of area, -structure and composition among the successive intrusions, a further -intricacy has been produced by the subsequent terrestrial disturbances, which -on a gigantic scale affected the north-west of Europe after the formation of -the fundamental complex of the old gneiss, but long before the Torridonian -period. By a series of terrestrial stresses that came as precursors of those -which in later geological times worked such great changes among the rocks -of the Scottish Highlands, the original bosses and sheets of the gneiss were -compressed, plicated, fractured and rolled out, acquiring in this process a -crumpled, foliated structure. Whether or not these disturbances were -<span class="pagenum" id="Page_118">- 118 -</span> -accompanied by any manifestations of superficial volcanic action has not -yet been determined. But we know that they were followed by a succession -of dyke-eruptions, to which, for extent and variety, there is no parallel -in the geological structure of Britain, save in the remarkable assemblage -of dykes belonging to the Tertiary volcanic period<a id="FNanchor_54" href="#Footnote_54" class="fnanchor">[54]</a> (<a href="#v1fig36">Fig. 36</a>).</p> - -<div class="footnote"> - -<p><a id="Footnote_54" href="#FNanchor_54" class="label">[54]</a> <i>Quart. Journ. Geol. Soc.</i> vol. xliv. (1888), p. 389 <i>et seq.</i></p> - -</div> - -<div class="figcenter" id="v1fig36" style="width: 427px;"> - <a href="images/v1fig36_lg.png"><img src="images/v1fig36.png" width="427" height="308" alt="" /></a> - - <div class="fl_left vsmall">Click on image to view larger.</div> - <div class="fl_right vsmall"><i>Walker & Boutall sc.</i></div> - - <div class="hanging2" style="clear: both;"><span class="smcap">Fig. 36.</span>—Map of a portion of the Lewisian gneiss of Ross-shire.<br /> - - Taken from Sheet 107 of the Geological Survey of Scotland on the scale of one inch to a mile. The white ground - (A) marks the general body of the Lewisian gneiss. This is traversed by dykes of dolerite (B), which are cut by - later dykes of highly basic material (peridotite, picrite, etc., P). The gneiss and its system of dykes is overlain - unconformably by the nearly horizontal Torridon Sandstone (<i>t</i>), which is injected by sheets of oligoclase-porphyry - (F).</div> -</div> - -<p>For the production of these dykes a series of fissures was first opened -through the fundamental complex of the gneiss, having a general trend -from E.S.E. to W.N.W., running in parallel lines for many miles, and so -close together in some places that fifteen or twenty of them occurred -within a horizontal space of one mile. The fissures were probably not all -formed at the same time; at all events, the molten materials that rose in -them exhibit distinct evidence of a succession of upwellings from the -igneous magma below.</p> - -<p>Considered simply from the petrographical point of view, the materials -that have filled the fissures have been arranged by Mr. Teall in the following -groups: 1. Ultrabasic dykes, sometimes massive (peridotites), sometimes -foliated (talcose schists containing carbonates and sometimes gedrite); -2. Basic dykes which where massive take the forms of dolerite and epidiorite, -and where foliated appear as hornblende-schist, the same dyke often -presenting the three conditions of dolerite, epidiorite and hornblende-schist; -3. Dykes of peculiar composition, comprising microcline-mica rocks and -<span class="pagenum" id="Page_119">- 119 -</span> -biotite-diorite with macro-poikilitic plagioclase; 4. Granites and gneissose -granites (biotite-granite with microcline); 5. Pegmatites (microcline-quartz -rocks with a variable amount of oligoclase or albite).<a id="FNanchor_55" href="#Footnote_55" class="fnanchor">[55]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_55" href="#FNanchor_55" class="label">[55]</a> <i>Annual Report of Geological Survey for 1895</i>, p. 18 of reprint.</p> - -</div> - -<p>Distinct evidence of a succession of eruptions can be made out among -these rocks. By far the largest proportion of the dykes consists of basic -materials. The oldest and most abundant of them are of plagioclase-augite -rocks, which, where uncrushed, differ in no essential feature of structure or -composition from the dolerites and basalts of more modern periods, though -they have been plentifully changed into epidiorite and hornblende-schist.<a id="FNanchor_56" href="#Footnote_56" class="fnanchor">[56]</a> -They present, too, most of the broad features that characterize the dykes -of later times—the central more coarsely-crystalline portion, the marginal -band of finer grain, passing occasionally into what was probably a basic -glass, and the transverse jointing. They belong to more than one period -of emission, for they cross each other. They vary in width up to nearly -200 feet, and sometimes run with singular persistence completely across -the whole breadth of the strip of gneiss in the west of Sutherland and -Ross. Dozens of dykes have been followed by the Geological Survey for -distances of ten or twelve miles.</p> - -<div class="footnote"> - -<p><a id="Footnote_56" href="#FNanchor_56" class="label">[56]</a> See Mr. Teall, <i>Quart. Journ. Geol. Soc.</i> vol. xli. (1885), p. 133.</p> - -</div> - -<p>Later in time, and much less abundant, are certain highly basic dykes—peridotites -with schistose modifications—which cut across the dolerites -in a more nearly east-and-west direction. There are likewise occasional -dykes of peculiar composition, which, as above stated, have been distinguished -by Mr. Teall as microcline-mica rocks and biotite-diorite.</p> - -<p>Last of all comes a group of thoroughly acid rocks—varieties of granite -and pegmatite—which form intrusive sheets and dykes. The granites -contain biotite with microcline, and are sometimes gneissose. The pegmatites -are microcline-quartz rocks with a variable amount of oligoclase or -albite. These dykes coincide in direction with the basalts and dolerites, -but they are apt to run together into belts of granite and pegmatite, sometimes -1500 feet broad.</p> - -<p>Up to the present time no evidence has been found of any superficial -outpouring of material in connection with this remarkable series of dykes -in the Lewisian gneiss. That they may have been concomitant with true -volcanic eruptions may be plausibly inferred from the close analogy which, -in spite of their antiquity and the metamorphism they have undergone, they -still present to the system of dykes that forms a part of the great Tertiary -volcanic series of Antrim and the Inner Hebrides. The close-set fissures -running in a W.N.W direction, the abundant uprise into these fissures of -basic igneous rocks, followed by a later and more feeble extravasation of -acid material, are features which in a singular manner anticipate the -volcanic phenomena of Tertiary time.</p> - -<p>There can be no question as to the high antiquity of these dykes. -They were already in place before the advent of those extraordinary vertical -lines of shearing which have so greatly affected both the gneiss and the -<span class="pagenum" id="Page_120">- 120 -</span> -dykes; and these movements, in turn, had long been accomplished before -the Torridon Sandstone was laid down, for the dykes, with their abundant -deformation, run up to and pass beneath the sandstone which buries them -and all the rocks with which they are associated. Though later than the -original fundamental complex, the dykes have become so integral and -essential a part of the gneiss as it now exists that they must be unhesitatingly -grouped with it.</p> - -<p>With so wide an extension of the subterranean relics of volcanic -energy, it is surely not too much to hope that somewhere there may have -been preserved, and may still be discovered, proofs that these eruptive rocks -opened a connection with the surface, and that we may thus recognize -vestiges of the superficial products of actual Archæan volcanoes. Among -the pebbles in the conglomerates of the Torridon Sandstone there occur, -indeed, fragments of felsites which possess great interest from the perfection -with which they retain some of the characteristic features of younger -lavas. Mr. Teall has described their minute structure. They are dark, -purplish, compact rocks, consisting of a spherulitic micropegmatitic, micropoikilitic -or microcrystalline groundmass, in which are imbedded porphyritic -crystals or crystal-groups of felspar, often oligoclase. These spherulitic -rocks occasionally show traces of perlitic structure. They bear a striking -resemblance to some of the Uriconian felsites of Shropshire, pebbles from -which occur in the Longmynd rocks.<a id="FNanchor_57" href="#Footnote_57" class="fnanchor">[57]</a> These fragments suggest the -existence of volcanic materials at the surface when the Torridon Sandstone -was deposited. Possibly they may represent some vanished Lewisian lavas. -But the time between the uprise of the dykes and the formation of the -Torridonian series was vast enough for the advent of many successive -volcanic episodes. The pebbles may therefore be the relics of eruptions -that took place long after the period of the dykes.</p> - -<div class="footnote"> - -<p><a id="Footnote_57" href="#FNanchor_57" class="label">[57]</a> <i>Annual Report of Geological Survey for 1895</i>, p. 21 of reprint.</p> - -</div> - -<p>Among the Torridonian strata no undoubted trace of any contemporaneous -volcanic eruptions has been met with.<a id="FNanchor_58" href="#Footnote_58" class="fnanchor">[58]</a> The only relics of volcanic -rocks in this enormous accumulation of sediments are the pebbles just -referred to, which may be referable to a time long anterior to the very -oldest parts of the Torridonian series.</p> - -<div class="footnote"> - -<p><a id="Footnote_58" href="#FNanchor_58" class="label">[58]</a> The supposed tuff referred to in <i>Quart. Journ. Geol. Soc.</i> vol. xlviii. (1892), p. 168, is -probably not of truly volcanic origin.</p> - -</div> - -<p>That Archæan time witnessed volcanic eruptions on a considerable scale, -and with great variety of petrographical material, has recently been shown -in detail by Mr. Otto Nordenskjöld from a study of the rocks of Småland -in Sweden. He has described a series of acid outbursts, including masses -of rhyolite and dacite, together with agglomerates and tuffs, likewise basic -eruptions, with dioritic rocks, augite-porphyrite and breccia. He refers -these rocks to the same age as most of the Scandinavian gneisses, and remarks -that though they have undergone much mechanical deformation and -metamorphism, they have yet here and there retained some of their distinctive -volcanic structures, such as the spherulitic.<a id="FNanchor_59" href="#Footnote_59" class="fnanchor">[59]</a> When the large -area of Lewisian gneiss forming the chain of the Outer Hebrides is investigated -it may possibly supply examples of a similar series of ancient volcanic -masses.</p> - -<div class="footnote"> - -<p><a id="Footnote_59" href="#FNanchor_59" class="label">[59]</a> "Über Archæische Ergussgesteine aus Småland," <i>Sveriges Geol. Undersökn</i>, No. 135 (1894).</p> - -</div> - -<p><span class="pagenum" id="Page_121">- 121 -</span></p> - - -<h4>ii. <span class="allsmcap">THE DALRADIAN OR YOUNGER SCHISTS OF SCOTLAND</span></h4> - -<p>We now come to one of the great gaps in the geological record. The -Lewisian gneiss affords us glimpses of probable volcanic activity at the very -beginning of geological history. An enormous lapse of time, apparently -unrepresented in Britain by any geological record, must be marked by the -unconformability between the gneiss and the Torridon Sandstone. Another -prodigious interval is undoubtedly shown by the Torridonian series. Neither -this thick accumulation of sediment nor the Cambrian formations, which to -a depth of some 2000 feet overlie the Torridon Sandstone, have yielded -any evidence of true superficial eruptions, though they are traversed by -numerous dykes, sills and bosses. The age of these intrusive masses -cannot be precisely fixed; a large proportion of them is certainly older -than the great terrestrial displacements and concurrent metamorphism of the -North-West Highlands.</p> - -<p>While from the Lewisian gneiss upward to the highest visible Cambrian -platform in Sutherland, no vestige of contemporaneous volcanic rocks is to -be seen, the continuity of the geological record is abruptly broken at the -top of the Durness Limestone. By a series of the most stupendous dislocations, -the rocks of the terrestrial crust have there been displaced to -such a degree that portions have been thrust westward for a horizontal -distance of sometimes as much as ten miles, while they have been so -crushed and sheared as to have often lost entirely their original structures, -and to have passed into the crystalline and foliated condition of -schists. Portions of the floor of Lewisian gneiss, and large masses of the -Torridon Sandstone, which had been buried under the Cambrian sediments, -have been torn up and driven over the Durness Limestone and quartzite.</p> - -<p>Though much care has been bestowed by the officers of the Geological -Survey on the investigation of the complicated mass of material which, -pushed over the Cambrian strata, forms the mountainous ground that lies -to the east of a line drawn from Loch Eribol, in the north of Sutherland, to -the south-east of Skye, some uncertainty still exists as to the age and -history of the rocks of that region. For the purposes of this work, -therefore, the rest of the country eastwards to the line of the Great Glen—that -remarkable valley which cuts Scotland in two—may be left out of -account.</p> - -<p>To the east of the Great Glen the Scottish Highlands display a vast -succession of crystalline schists, the true stratigraphical relations of which -to the Lewisian gneiss have still to be determined, but which, taken as a -whole, no one now seriously doubts must be greatly younger than that -ancient rock. Murchison first suggested that the quartzites and limestones -found in this newer series are the equivalents of those of the North-West. -<span class="pagenum" id="Page_122">- 122 -</span> -This identification may yet be shown to be correct, but must be regarded as -still unproved. Traces of fossils (annelid-pipes) have been found in some of -the quartzites, but they afford little or no help in determining the horizons -of the rocks. In Donegal, where similar quartzites, limestones and schists -are well developed, obscure indications of organic remains (corals and -graptolites) have likewise been detected, but they also fail to supply any -satisfactory basis for stratigraphical comparison.</p> - -<p>Essentially the schists of the Scottish Highlands east of the Great -Glen consist of altered sedimentary rocks. Besides quartzites and limestones, -there occur thick masses of clay-slate and other slates and schists, -with bands of graphitic schist, greywacke, pebbly grit, quartzite, boulder-beds -and conglomerates. Among rocks that have been so disturbed and -foliated it is necessarily difficult to determine the true order of succession. -In the Central Highlands, however, a certain definite sequence has been -found to continue as far as the ground has yet been mapped. Were the -rocks always severely contorted, broken and placed at high angles, this -sequence might be deceptive, and leave still uncertain the original order of -deposition of the whole series. But over many square miles the angles of -inclination are low, and the successive bands may be traced from hill to -hill, across strath and glen, forming escarpments along the slopes and outliers -on the summits, precisely as gently-undulating beds of sandstone and -limestone may be seen to do in the dales of Yorkshire. It is difficult to -resist the belief, though it may, perhaps, be premature to conclude, that this -obvious and persistent order of succession really marks the original sequence -of deposition. In Donegal also a definite arrangement of the rock-groups -has been ascertained which, when followed across the country, gives the -key to its geological structure.<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> <i>Geol. Survey Memoirs: Geology of N.W. Donegal</i>, 1891.</p> - -</div> - -<p>In the order of succession which has been recognized during the -progress of the Geological Survey through the Central and Southern Highlands, -it is hard in many places to determine whether the sequence that -can be recognized is in an upward or downward direction. Two bands of -limestone, which appear to retain their relative positions across Scotland for -a distance of some 230 miles, may afford a solution of this difficulty, and if, -as is probable, they are to be identified with the similar limestones of -Donegal, Mayo and Galway, their assistance will thus be available across a -tract of more than 400 miles. What is regarded as the lower zone of -limestone is particularly well seen about Loch Tay; what is believed to be -the upper is typically displayed in the heart of Perthshire, about Blair-Athol.</p> - -<p>From under the Loch Tay Limestone a great thickness of mica-schists, -"green schists," schistose grits and conglomerates, slates and greywackes, -emerges up to the border of the Highlands. Above that calcareous band -thick masses of mica-schist and sericite-schist are succeeded by a well-marked -zone of quartzite, which forms the mountains of Ben-y-Glo and Schihallion, -and stretches south-westward across Argyllshire into Islay and Jura. The -<span class="pagenum" id="Page_123">- 123 -</span> -second or Blair-Athol Limestone lies next to this quartzite. If the limestones -are identical with those of Donegal, Mayo and Galway, the quartzites -may doubtless be also regarded as continued in those of the same Irish -counties, where they form some of the most conspicuous features in the -scenery, since they rise into such conspicuous mountains as Erigal, Slieve -League, Nephin, and the twelve Bins of Connemara.</p> - -<p>The age of this vast system of altered rocks has still to be determined. -It is possible that they may include some parts of the Torridonian series, or -even here and there a wedge of the Lewisian gneiss driven into position by -gigantic disruptions, like those of the North-West Highlands. But there -can be no doubt that the schists, quartzites and limestones form an assemblage -of metamorphosed sedimentary strata which differs much in variety -of petrographical character, as well as in thickness, from the Torridonian -sandstone, and which has not been identified as the equivalent of any known -Palæozoic system or group of formations in Britain. It may conceivably -embrace the Cambrian series of the North-West Highlands, and also the -sedimentary deposits that succeeded the Durness Limestone, of which no -recognizable vestige remains in Sutherland or Ross.</p> - -<p>That the metamorphic rocks east of the line of the Great Glen are at -least older than the Arenig formation of the Lower Silurian system may be -inferred from an interesting discovery recently made by the officers of the -Geological Survey. A narrow strip of rocks has been found which, from -their remarkable petrographical characters, their order of sequence and their -scanty fossil contents (<i>Radiolaria</i>), are with some confidence identified with -a peculiar assemblage of rocks on the Arenig horizon of the Silurian system -in the Southern Uplands of Scotland, to which fuller reference will be made -in <a href="#CHAPTER_XII">Chapter xii.</a> This strip or wedge of probably Lower Silurian strata -intervenes between the Highland schists and the Old Red Sandstone in -Kincardineshire, Forfarshire and Dumbartonshire. It has been recognized -also, occupying a similar position, in Tyrone in Ireland. The schists in some -places retain their foliated character up to the abrupt line of junction with -the presumably Lower Silurian strata, while in other districts, as at Aberfoyle, -they have been so little affected that it is hardly possible to draw a line -between the Highland rocks and those of this border-zone, which indeed are -there perhaps more metamorphosed than the Highland grits to the north -of them. The metamorphism of the schists may have been mainly effected -before the final disturbances that wedged in this strip of Silurian strata -along the Highland border, though some amount of crushing and schist-making -seems to have accompanied these disturbances. No trace of any -similar strip of Palæozoic rocks has ever been detected among the folds of -the schists further into the Highlands. But some of the Highland rocks -in the region of Loch Awe lose their metamorphosed character, and pass -into sedimentary strata which, so far as petrographical characters are concerned, -might well be Palæozoic.</p> - -<p>Until some clue is found to the age of the Younger or Eastern schists, -quartzites and limestones of the Highlands, it is desirable to have some short -<span class="pagenum" id="Page_124">- 124 -</span> -convenient adjective to distinguish them. As a provisional term for them -I have proposed the term "Dalradian," from Dalriada, the name of the old -Celtic kingdom of the north of Ireland and south-west of Scotland.<a id="FNanchor_61" href="#Footnote_61" class="fnanchor">[61]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_61" href="#FNanchor_61" class="label">[61]</a> <i>Presidential Address to Geological Society</i>, 1891, p. 39.</p> - -</div> - -<p>The special feature for which this Dalradian series is cited in the present -volume is the evidence it furnishes of powerful and extensive volcanic -action. In a series of rocks so greatly dislocated, crumpled and metamorphosed, -we cannot look for the usual clear proofs of contemporaneous -eruptions. Nevertheless all over the Scottish Highlands, from the far coast -of Aberdeenshire to the Mull of Cantyre, and across the west of Ireland -from the headlands of Donegal into Galway, there occurs abundant -evidence of the existence of rocks which, though now forming an integral -part of the schists, can be paralleled with masses of undoubtedly volcanic -origin.</p> - -<div class="figcenter" id="v1fig37" style="width: 475px;"> - <img src="images/v1fig37.png" width="475" height="76" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 37.</span>—Section showing the position of Sills in - the mica-schist series between Loch Tay and Amulree.<br /> - <i>a</i>, Mica-schist; <i>b</i>, <i>b</i>, Sills.</div> -</div> - -<p>Intercalated in the vast pile of altered sediments lie numerous sheets -of epidiorite and hornblende-schist, which were erupted as molten materials, -not improbably as varieties of diabase-lava. Most of these sheets are -doubtless intrusive "sills," for they can be observed to break across from one -horizon to another. But some of them may possibly be contemporaneous -lava-streams. A sheet may sometimes be followed for many miles, occupying -the same stratigraphical platform. Thus a band of sills may be traced from -the coast of Banffshire to near Ben Ledi, a distance of more than 100 -miles. Among the hornblendic sills of this band some occur on a number -of horizons between the group of Ben Voirlich grits and the Ben-y-Glo -quartzite. One of the most marked of these is a sheet, sometimes 200 feet -thick, which underlies the Loch Tay Limestone. Another interesting group -in the same great band has been mapped by the Geological Survey on the -hills between Loch Tay and Amulree, some of them being traceable for several -miles among the mica-schists with which they alternate (<a href="#v1fig37">Fig. 37</a>).</p> - -<p>In Argyllshire also, between Loch Tarbert and Loch Awe, and along the -eastern coasts of the islands of Islay and Jura, an abundant series of sheets -of epidiorite, amphibolite and hornblende-schist runs with the prevalent -strike of the schists, grits and limestones of that region. Similar rocks -reappear in a like position in Donegal, where, as in Scotland, the frequency -of the occurrence of these eruptive rocks on the horizons of the limestones -is worthy of remark. The persistence, number and aggregate thickness of -the sills in this great band mark it out as the most extensive series of intrusive -sheets in the British Isles.</p> - -<p><span class="pagenum" id="Page_125">- 125 -</span></p> - -<p>In addition to the sills there occur also bosses of similar material, which -in their form and their obvious relation to the sheets recall the structure -of volcanic necks. They consist of hornblendic rocks, like the sills, but are -usually tolerably massive, and show much less trace of superinduced foliation.</p> - -<p>Besides the obviously eruptive masses there is another abundant group -of rocks which, I believe, furnishes important evidence as to contemporaneous -volcanic action during the accumulation of the Dalradian series. Throughout -the Central and South-Western Highlands certain zones of "green schist" -have long occupied the attention of the officers of the Geological Survey. -They occur more especially on two horizons between the Loch Tay Limestone -and a much lower series of grits and fine conglomerates, which run through -the Trossachs and form the craggy ridges of Ben Ledi, Ben Voirlich and -other mountains near the Highland border. In the lower group of "green -schists," thick hornblendic sills begin to make their appearance, increasing -in number upwards. The upper group of "green schists" lies between two -bands of garnetiferous mica-schist, above the higher of which comes the -Loch Tay Limestone. The peculiar greenish tint and corresponding mineral -constituents of these schists, however, are likewise found diffused through -higher parts of the series.</p> - -<p>So much do the "green schists" vary in structure and composition that -no single definition of them is always applicable. At one extreme are -dull green chlorite-schists, passing into a "potstone," which, like that of -Trondhjem, can be cut into blocks for architectural purposes.<a id="FNanchor_62" href="#Footnote_62" class="fnanchor">[62]</a> At the -other extreme lie grits and quartzites, with a slight admixture of the same -greenish-coloured constituent. Between these limits almost every stage may -be met with, the proportion of chlorite or hornblende and of granular or -pebbly quartz varying continually, not only vertically, but even in the -extension of the same bed. The quartz-pebbles are sometimes opalescent, -and occasionally larger than peas. An average specimen from one of the -zones of "green schists" is found, on closer examination, to be a thoroughly -schistose rock, composed of a matrix of granular quartz, through which -acicular hornblende and biotite crystals, or actinolite and chlorite, are -ranged along the planes of foliation.</p> - -<div class="footnote"> - -<p><a id="Footnote_62" href="#FNanchor_62" class="label">[62]</a> From such a rock, which crosses the upper part of Loch Fyne, the Duke of Argyll's residence -at Inveraray has been built.</p> - -</div> - -<p>That these rocks are essentially of detrital origin admits of no doubt. -They differ, however, from the other sedimentary members of the Dalradian -series in the persistence and abundance of the magnesian silicates diffused -through them. The idea which they suggested to my mind some years ago -was that the green colouring-matter represents fine basic volcanic dust, -which was showered out during the accumulation of ordinary quartzose, -argillaceous and calcareous sediments, and that, under the influence -of the metamorphism which has so greatly affected all the rocks of the -region, the original pyroxenes and felspars suffered the usual conversion -into hornblendes, chlorites and micas. This view has occurred also to my -colleagues on the Survey, and is now generally adopted by them.</p> - -<p><span class="pagenum" id="Page_126">- 126 -</span></p> - -<p>Not only are these "green schists" traceable all through the Central -and South-Western Highlands, rocks of similar character, and not improbably -on the same horizons, reappear in the north-west of Ireland, and -run thence south-westward as far as the Dalradian rocks extend. If we -are justified in regarding them as metamorphosed tuffs and ashy sediments, -they mark a widespread and long-continued volcanic period during the time -when the later half of the Dalradian series was deposited.</p> - -<p>Besides the extensive development of basic sills which, though probably -in great part later than the "green schists," may belong to the same prolonged -period of subterranean activity, numerous acid protrusions are to -be observed in the Dalradian series of Scotland and Ireland. That these -masses were erupted at several widely-separated intervals is well shown by -their relation to the schists among which they occur. Some of the great -bosses and sills of granite were undoubtedly injected before the metamorphism -of the schists was completed, for they have shared in the foliation of the -region. Others have certainly appeared after the metamorphism was complete, -for they show no trace of having suffered from its effects. Thus some -of the vast tracts of newer granite in the Grampian chain, which cover -many square miles of ground, must be among the newest rocks of that area. -They have recently been found by Mr. G. Barrow, of the Geological Survey, -to send veins into the belt of probably Lower Silurian strata which flanks -the Highland schists. They are thus later than the Arenig period. Not -impossibly they may be referable to the great granite intrusions which formed -so striking a feature in the history of the Lower Old Red Sandstone.</p> - - -<h4>iii. <span class="allsmcap">THE GNEISSES AND SCHISTS OF ANGLESEY</span></h4> - -<p>In the island of Anglesey an interesting series of schists and quartzites -presents many points of resemblance to the Dalradian or younger schists of -the Highlands. At present the geologist possesses no means of determining -whether these Welsh rocks are the equivalents of the Scottish in stratigraphical -position, but their remarkable similarity justifies a brief allusion -to them in this place. Much controversy has arisen regarding the geology -of Anglesey, but into this dispute it is not necessary for my present purpose -to enter.<a id="FNanchor_63" href="#Footnote_63" class="fnanchor">[63]</a> I will content myself with expressing what seems to me, -after several traverses, to be the geological structure of the ground.</p> - -<div class="footnote"> - -<p><a id="Footnote_63" href="#FNanchor_63" class="label">[63]</a> The literature of Anglesey geology is now somewhat voluminous, but I may refer to the -following as the chief authorities. The island is mapped in Sheet 78 of the Geological Survey -of England and Wales, and its structure is illustrated in Horizontal Sections, Sheet 40. A full -account of its various formations and of their relations to each other is given in vol. iii. of the -<i>Memoirs of the Geological Survey</i>, "The Geology of North Wales," by Sir A. C. Ramsay, 2nd -edit. 1881. The subject has been discussed by Professor Hughes, <i>Quart. Journ. Geol. Soc.</i> vols. -xxxiv. (1878) p. 137, xxxv. (1879) p. 682, xxxvi. (1880) p. 237, xxxviii. (1882) p. 16; <i>Brit. -Assoc. Rep.</i> (1881) pp. 643, 644; <i>Proc. Camb. Phil. Soc.</i> vol. iii. pp. 67, 89, 341; by Professor -Bonney, <i>Quart. Journ. Geol. Soc.</i> vol. xxxv. (1879) pp. 300, 321; <i>Geol. Mag.</i> (1880) p. 125; -by Dr. H. Hicks, <i>Quart. Journ. Geol. Soc.</i> vols. xxxiv. (1878) p. 147, xxxv. (1879) p. 295; <i>Geol. -Mag.</i> (1879) pp. 433, 528 (1893) p. 548; by Dr. C. Callaway, <i>Quart. Journ. Geol. Soc.</i> vols. -xxxvii. (1881) p. 210, xl. (1884) p. 567; and by the Rev. J. F. Blake, <i>Quart. Journ. Geol. Soc.</i> -vol. xliv. (1888) p. 463. Further references to the work of these observers in Anglesey are -given in <a href="#CHAPTER_XIII">Chapter xiii. p. 220</a> <i>et seq.</i> The Pre-Cambrian areas of Anglesey are shown in <a href="#Map_II">Map II.</a></p> - -</div> - -<p><span class="pagenum" id="Page_127">- 127 -</span></p> - -<p>There are two groups of rocks in Anglesey to which a pre-Cambrian -age may with probability be assigned. In the heart of the island lies a -core of gneiss which, if petrographical characters may be taken as a guide, -must certainly be looked upon as Archæan. In visiting that district with -my colleague Mr. Teall I was much astonished to find there so striking a -counterpart to portions of the Lewisian gneiss of the north-west of Sutherland -and Ross. The very external features of the ground recall the -peculiar hummocky surface which so persistently characterizes the areas of -this rock throughout the north-west of Scotland. If the geologist could be -suddenly transported from the rounded rocky knolls of Sutherland, Ross-shire -or the Hebrides to those in the middle of Anglesey, south of Llanerchymedd, -he would hardly be aware of the change, save in the greater verdure -of the hollows, which has resulted from a more advanced state of decomposition -of the rocks at the surface, as well as from a better climate and -agriculture.</p> - -<p>When we examine these rocky hummocks in detail we find them to -consist of coarse gneisses, the foliation of which has a prevalent dip to -N.N.W. Some portions abound in dark hornblende and garnets, others -are rich in brown mica, the folia being coarsely crystalline and rudely -banded, as in the more massive gneisses of Sutherland. Abundant veins -of coarse pegmatite may here and there be seen, with pinkish and white -felspars and milky quartz. Occasionally the gneiss is traversed by bands -of a dark greenish-grey rock, which remind one of the dykes of the north-west -of Scotland. There are other rocks, some of them probably intrusive -and of later date, to be seen in the same area; but they require more -detailed study than they have yet received.</p> - -<p>The relation of this core of gneiss and its associated rocks to the second -group of pre-Cambrian rocks has not hitherto been satisfactorily ascertained. -The core may conceivably be an eruptive boss in that group, and may have -acquired its foliation during the movements that produced the foliation of -the surrounding schists. But it seems more probable that the gneiss is -much older than these schists, though it would undoubtedly participate in -the effects of the mechanical movements which gave rise to their deformation, -cleavage and foliation.</p> - -<p>The second group of rocks occupies a large area in the west and in the -centre and south of Anglesey. The schists of which it consists are -obviously in the main a clastic series. One of their most conspicuous -members is quartzite, which, besides occurring sporadically all over the -island, forms the prominent mass of Holyhead Mountain. There are likewise -flaggy chloritic schists, green and purple phyllites or slates, and bands -of grit, while parts of the so-called "grey gneiss" consist of pebbly sandstones -that have acquired a crystalline structure. That some order of -sequence among these various strata may yet be worked out is not impossible, -but the task will be one of no ordinary difficulty, for the plications -<span class="pagenum" id="Page_128">- 128 -</span> -and fractures are numerous, and much of the surface of the ground is -obscured by the spread of Palæozoic formations and superficial deposits.</p> - -<p>These Anglesey schists are so obviously an altered sedimentary series -that it is not surprising that they should have been regarded as metamorphosed -Cambrian strata. All that can be positively affirmed regarding -their age is that they are not only older than the lowest fossiliferous rocks -around them—that is, than Arenig or even Tremadoc strata—but that -they had already acquired their present metamorphic character before these -strata were laid down unconformably upon them. There is no actual proof -that they include no altered Cambrian rocks. But when we consider their -distinctly crystalline structure, and the absence of such a structure from any -portion of the Cambrian areas of the mainland; when, moreover, we reflect -that the metamorphism which has affected them is of the regional type, -and can hardly have been restricted to merely the limited area of Anglesey; -we must agree with those observers who, in spite of the absence of positive -proof of their true geological horizon, have regarded these rocks as of much -higher antiquity than the Cambrian strata of the neighbourhood. No one -familiar with the Dalradian rocks of Scotland and Ireland can fail to be -struck with the close resemblance which these younger Anglesey schists bear -to them, down even into the minutest details. Petrographically they are -precisely the counterparts of the quartzites and schists of Perthshire and -Donegal, and a further connection may be established of a palæontological -kind. The upper part of the Holyhead quartzite was found by Mr. B. N. -Peach and myself in the autumn of the year 1890 to be at one place -crowded with annelid-pipes, and I subsequently found the same to be the -case with some of the flaggy quartzites near the South Stack.</p> - -<div class="figcenter" id="v1fig38" style="width: 357px;"> - <img src="images/v1fig38.png" width="357" height="229" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 38.</span>—Sketch of crushed basic igneous rock among the schists, E. side of Porth-tywyn-mawr, - E. side of Holyhead Straits.</div> -</div> - -<p>For the purpose of the inquiry which forms the theme of this work, -the feature of greatest interest about these younger schists of Anglesey is -the association of igneous rocks with them. They include bands of dark -basic material, the less crushed parts of which resemble the diabases of -<span class="pagenum" id="Page_129">- 129 -</span> -later formations, while the sheared portions pass into epidiorites and true -hornblende-schists. As in other regions where eruptive rocks have been -crushed down and changed into the schistose modification, it is frequently -possible to see groups of uncrushed cores round which, under severe -mechanical stresses, the rock has undergone this conversion. Lines of -movement through the body of the rock may be detected by bands of schist, -the gradation from the solid core to the hornblende-schist being quite -gradual. The accompanying figure (<a href="#v1fig38">Fig. 38</a>) represents a portion of one of -these crushed basic igneous rocks on the east side of Holyhead Straits.</p> - -<p>As in the Dalradian series of the Highlands, many, perhaps most, of -these igneous bands are probably intrusive sills, but others may be -intercalated contemporaneous sheets. They occur across the whole breadth -of the island from the Menai Strait to the shores of Holyhead.</p> - -<p>Besides these undoubtedly igneous rocks, the green chloritic slates of -Anglesey deserve notice. They are well-bedded strata, consisting of -alternations of foliated fine grit or sandstone, with layers more largely -made up of schistose chlorite. The gritty bands sometimes contain pebbles -of blue quartz, and evidently represent original layers of sandy sediment, -but with an admixture of chloritic material. The manner in which this -green chloritic constituent is diffused through the whole succession of -strata, and likewise aggregated into bands with comparatively little quartzose -sediment, reminds one of the "green schists" of the Central Highlands -and Donegal, and suggests a similar explanation. Taken in connection -with the associated basic igneous rocks, these chloritic schists seem to me -to represent a thick group of volcanic tuffs and interstratified sandy and -clayey layers. If this inference is well founded, and if we are justified -in grouping these Anglesey rocks with the Dalradian schists of Scotland -and Ireland, a striking picture is presented to the mind of the wide -extent and persistent activity of the volcanoes of that primeval period -in Britain.<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> Mr. E. Greenly, late of the Geological Survey of Scotland, has recently established himself on -the Menai Strait for the purpose of working out in detail the geological structure of this interesting -and complicated region. We may therefore hope that some of the still unsolved problems -presented by the rocks of Anglesey will before long be satisfactorily explained.</p> - -</div> - - -<h4>iv. <span class="allsmcap">THE URICONIAN VOLCANOES</span></h4> - -<p>Along the eastern borders of Wales a ridge of ancient rocks, much broken -by faults and presenting several striking unconformabilities, has long -been classic ground in geology from the descriptions and illustrations of -Murchison's <i>Silurian System</i>.<a id="FNanchor_65" href="#Footnote_65" class="fnanchor">[65]</a> The main outlines of the structure of -that district, first admirably worked out by this great pioneer, were -delineated on the maps and sections of the Geological Survey, wherein it -was shown that in the Longmynd an enormously thick group of stratified -rocks, which, though unfossiliferous, were referred to the Cambrian system, -rose in the very heart of the country; that to the east of these rocks lay -<span class="pagenum" id="Page_130">- 130 -</span> -strata of Caradoc or Bala age; that by a great hiatus in the stratigraphy -the Upper Silurian series transgressively wrapped round everything below it; -that yet again the Coal-measures crept over all these various Palæozoic formations, -followed once more unconformably by Permian and Triassic deposits.<a id="FNanchor_66" href="#Footnote_66" class="fnanchor">[66]</a> -Besides all this evidence of extraordinary and repeated terrestrial movement, -it was found that the region was traversed by some of the most -powerful dislocations in this country, while to complete the picture of -disturbance, many protrusions of igneous rocks were recognized.</p> - -<div class="footnote"> - -<p><a id="Footnote_65" href="#FNanchor_65" class="label">[65]</a> See especially chap. xix. vol. i. p. 225.</p> - -<p><a id="Footnote_66" href="#FNanchor_66" class="label">[66]</a> The area is embraced in Sheet 61 of the Geological Survey, and is illustrated by Nos. 33 and -36 of the sheets of Horizontal Sections. In the early editions of the Survey maps the "felspathic -traps" and the "greenstones" of the Wrekin district were distinguished by separate colours, -but unfortunately this useful and so far correct discrimination was given up in subsequent -editions, where all the acid and basic rocks are merged into one.</p> - -</div> - -<p>In a territory so complicated, though it had been sedulously and -skilfully explored, there could hardly fail to remain features of structure -which had escaped the notice of the first observers. In particular, the -igneous rocks had been dealt with only in a general way, and they -consequently offered a favourable field for more detailed study; while by a -more searching examination of some of the rocks for fossils, important -corrections of the earlier work might yet be made.</p> - -<p>A notable step towards a revision of the received opinions regarding the -igneous rocks of this region was taken by Mr. Allport, who showed that the -so-called "greenstone" included masses of devitrified spherulitic pitchstones -and perlites, together with indurated volcanic breccias, agglomerates and -ashes.<a id="FNanchor_67" href="#Footnote_67" class="fnanchor">[67]</a> Subsequently Professor Bonney described more fully the petrographical -characters of the Wrekin igneous rocks, confirming and extending -the observations of Mr. Allport.<a id="FNanchor_68" href="#Footnote_68" class="fnanchor">[68]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_67" href="#FNanchor_67" class="label">[67]</a> <i>Quart. Journ. Geol. Soc.</i> vol. xxxiii. (1877) p. 449.</p> - -<p><a id="Footnote_68" href="#FNanchor_68" class="label">[68]</a> <i>Op. cit.</i> vol. xxxv. (1879) p. 662; vol. xxxviii. (1882) p. 124.</p> - -</div> - -<p>But the correction of the prevalent error as to the geological age of -these rocks was due to Dr. Callaway, who, after spending much time and -labour in ascertaining, by a careful search for fossils, the position of the -superincumbent rocks (wherein he discovered Cambrian organisms), and in -a detailed investigation of the structure and relationships of the igneous -masses themselves, was led to regard them as part of an ancient pre-Cambrian -ridge; and he proposed for the volcanic group the name of -Uriconian, from the name of the former Roman town which stood not far -to the west of them.<a id="FNanchor_69" href="#Footnote_69" class="fnanchor">[69]</a> He has shown how essentially volcanic this ancient -series of rocks is, how seldom they present any clearly-marked evidence of -stratification, and how small is the proportion of sedimentary material -associated with them.<a id="FNanchor_70" href="#Footnote_70" class="fnanchor">[70]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_69" href="#FNanchor_69" class="label">[69]</a> <i>Quart. Journ. Geol. Soc.</i> vols. xxx. (1874) p. 196, xxxiv. (1878) p. 754, xxxv. (1879) p. 643, -xlii. (1886) p. 481. For a criticism of Dr. Callaway's views as to the order of succession among -the rocks of this district, see Prof. Blake, <i>op. cit.</i> vol. xlvi. (1890) p. 386, and Dr. Callaway's reply, -vol. xlvii. (1891) p. 109.</p> - -<p><a id="Footnote_70" href="#FNanchor_70" class="label">[70]</a> <i>Op. cit.</i> vol. xlvii. (1891) p. 123.</p> - -</div> - -<p>Subsequently Professor Lapworth, by his discovery of the <i>Olenellus</i>-fauna, -marking the lowest known fossiliferous Cambrian zone in the Wrekin -<span class="pagenum" id="Page_131">- 131 -</span> -district, and his recognition of Cambrian fossils under the Coal-measures of -Warwickshire, supplied valuable evidence for the discussion of the geological -position of the older rocks of the Midlands. He has mapped in minute -detail the rocks of the Wrekin, and has exhausted all the evidence that is -at present obtainable on the subject. But unfortunately the publication of -his researches is still delayed.<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>Geol. Mag.</i> (1882) p. 563, (1886) p. 319, (1887) p. 78, (1888) p. 484; and a joint paper -with Mr. W. W. Watts on the Geology of South Shropshire, <i>Proc. Geol. Assoc.</i> vol. xiii. (1894) -pp. 302, 335.</p> - -</div> - -<p>It is now recognized that the core of the ancient ridge, extending from -near Wellington through the Wrekin, Caer Caradoc and other hills, until it -sinks beneath the Upper Silurian formations, is formed of igneous rocks -that consist partly of lavas, partly of volcanic breccias and fine tuffs. The -lavas are thoroughly acid rocks of the felsitic or rhyolitic type. One of -them, about 100 feet thick, which forms a prominent feature on the flanks -and crest of Caer Caradoc, shows abundant finely-banded flow-structure, -often curved or on end, while its bottom and upper parts are strongly -amygdaloidal, the cavities being occasionally pulled out in the direction of -flow and lined with quartz or chalcedony. Some of the detached areas of -eruptive rocks show the beautiful spherulitic and perlitic structures first -noticed in this region by Mr. Allport. More recently the structures of -these acid rocks have been described by Mr. F. Rutley.<a id="FNanchor_72" href="#Footnote_72" class="fnanchor">[72]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_72" href="#FNanchor_72" class="label">[72]</a> <i>Quart. Journ. Geol. Soc.</i> vol. xlvii. (1891) p. 540. Mr. Rutley more particularly describes -those of Caradoc Hill.</p> - -</div> - -<p>The breccias and tuffs appear to consist mainly of felsitic material. In -the coarser varieties, fragments of finely-banded felsite may be noticed, while -the finer kinds pass into a kind of hornstone (hälleflinta), which in hand-specimens -could hardly be distinguished from close-grained felsite. In -some places, these pyroclastic rocks are well stratified, but elsewhere no -satisfactory bedding can be recognized in them. Various other rocks, which -are probably intrusive, occur in the ridge. At either end of the Wrekin -there is a mass of pink microgranite, while at Caer Caradoc numerous sheets -of "greenstone," intercalated in the fine tuffs, sweep across the hill. Mr. -Rutley has published an account of these basic rocks, which he classes as -"melaphyres," or altered forms of basalt or andesite.<a id="FNanchor_73" href="#Footnote_73" class="fnanchor">[73]</a> That at least some -of them are intrusive is manifest by the way in which they ramify through -the surrounding strata. But others are so strongly amygdaloidal and slaggy -that they may possibly be true interbedded lavas, though there may be -some hesitation in admitting that such basic outflows could be erupted in -the midst of thoroughly acid ejections.<a id="FNanchor_74" href="#Footnote_74" class="fnanchor">[74]</a> Leaving these doubtful flows out -of account, we have here a group of undoubted volcanic rocks represented -by acid lavas and pyroclastic materials, by intrusive bosses of acid rocks, -and by younger basic sills. The general lithological characters of these -masses and the sequence of their appearance thus strongly resemble those of -subsequent Palæozoic volcanic episodes.</p> - -<div class="footnote"> - -<p><a id="Footnote_73" href="#FNanchor_73" class="label">[73]</a> <i>Op. cit.</i> p. 534.</p> - -<p><a id="Footnote_74" href="#FNanchor_74" class="label">[74]</a> This difficulty, however, need not be in itself insuperable, as is evident from the remarkable -alternation of basic and acid lavas and tuffs in the Cambrian volcanic group of St. David's -and in the Old Red Sandstone series of the Pentland Hills.</p> - -</div> - -<p><span class="pagenum" id="Page_132">- 132 -</span></p> - -<p>The geological age of this volcanic group is a question of much interest -and importance in regard to the history of volcanism in this country. An -inferior limit to the antiquity of the group can at once be fixed by the fact -that, as originally pointed out by Dr. Callaway, the quartzite which overlies -the volcanic rocks passes under a limestone containing Cambrian fossils in -which Professor Lapworth has since recognized <i>Olenellus</i>, <i>Paradoxides</i> and -other Lower Cambrian forms. The eruptions, therefore, must be at least as -old as the earlier part of the Cambrian period. But it is affirmed that the -quartzite rests with a complete unconformability on the volcanic rocks. If -this be so, then the epoch of eruption must be shifted much farther back.</p> - -<div class="figcenter" id="v1fig39" style="width: 496px;"> - <img src="images/v1fig39.png" width="496" height="78" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 39.</span>—Section across the Uriconian series of Caer Caradoc.<br /> - S3, Upper Silurian; S2, Bala group; S1, Arenig group; C, Cambrian; L, Longmyndian; <i>u</i>, Uriconian; - <i>f</i> <i>f</i>, faults.</div> -</div> - -<p>The evidence adduced in favour of this great break appears to me to be -threefold. In the first place, the quartzite contains fragments of the -volcanic rocks. I do not think much stress can be laid on this fact. When -I visited the ground, what struck me most in the composition of the -quartzite was its singularly pure quartzose character, and the comparative -scarcity of felsite-pebbles in it. Any deposit laid down conformably upon -the top of the breccias and tuffs might obviously contain some of these -materials, while, if laid down unconformably, it might reasonably be -expected to be full of them. In the second place, this quartzite is alleged -to pass transgressively across the edges of successive sheets of the volcanic -group, and thus to have a quite discordant dip and strike. I failed to find -satisfactory evidence of this unconformability in the northern part of the -district. But in the Caer Caradoc area the quartzite does appear to steal -across the outcrops of the older rocks, which plunge at nearly right angles -in an opposite direction. In the third place, the felsitic volcanic group is -believed by Professor Lapworth to pass upwards into the Longmynd rocks. -Obviously, if this group lies at the very bottom of the vast Longmynd -series, the discordance between it and the quartzite must be enormous, and -the date of the volcanic eruptions must be placed vastly farther back in -geological antiquity. Though the evidence does not seem to me to amount -to clear proof, I am disposed, in the meantime, to accept it as affording the -most probable solution of the difficulties presented by the structure of the -ground.</p> - -<p>The sequence of the rocks around Caer Caradoc is partly concealed by -surface accumulations, but if these could be cleared away the structure of -the ground would be, according to Messrs. Lapworth and Watts, as shown -in <a href="#v1fig39">Fig. 39</a>.<a id="FNanchor_75" href="#Footnote_75" class="fnanchor">[75]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_75" href="#FNanchor_75" class="label">[75]</a> <i>Proc. Geol. Assoc.</i> vol. xiii. (1894), pp. 314, 315.</p> - -</div> - -<p><span class="pagenum" id="Page_133">- 133 -</span></p> - -<p>If, then, this volcanic group underlies the whole of the Longmynd -series, and if, as it now appears, that series is older than the <i>Olenellus</i>-zone -of the Lower Cambrian rocks, we can hardly include the volcanic rocks of -the Wrekin and Caer Caradoc in the Cambrian system. They must belong -to a still older geological formation, and I think we cannot do better than -adopt for them Dr. Callaway's name, Uriconian.</p> - -<p>There are still, however, many problems to be solved before the -geological history of that region is completely understood. The rocks of the -Longmynd must be more fully worked out. It is improbable that strata -which look so likely to yield fossils should for ever prove barren. The -lower half at least may be hopefully searched, although the upper massive -reddish sandstones and conglomerates offer less prospect of success. On the -west side of the Longmynd, above Pontesbury, there occurs a small area of -volcanic rocks like those of the Wrekin district, including a well-marked -nodular felsite and fine tuffs. These rocks have been regarded by Dr. -Callaway as another axis of the Uriconian series. It is very difficult, however, -by any combination of geological structures, to bring up a portion of -the very bottom of the Longmynd series and place it apparently at the top. -This is a feat which a detailed study of the region, and the detection of -unconformabilities in the Longmynd, may possibly accomplish. In the -meantime, however, I would venture to suggest whether it is not more -probable that we have here a detached area of much younger volcanic rocks, -like those which, in various districts, may be included in the Cambrian -system, and which will be referred to in some detail in subsequent pages.</p> - - -<h4>v. <span class="allsmcap">THE MALVERN VOLCANO</span></h4> - -<p>Regarding the age and origin of the oldest rocks of the Malvern Hills -some controversy has arisen, and no general agreement has yet been reached.<a id="FNanchor_76" href="#Footnote_76" class="fnanchor">[76]</a> -It is clear that the core of crystalline rocks which is overlain unconformably -by the Hollybush Sandstone must be older than the Upper Cambrian rocks. -There is no good evidence of any stratigraphical break in the Cambrian -system of England or Wales, and it may be reasonably inferred that the -break seen at the base of the Hollybush Sandstones indicates that the rocks -underneath that horizon are pre-Cambrian. Some portions of these -certainly very ancient rocks are gneisses or schists; others have been -described as "felsites," and have been regarded as passing into schists, and -as the original material from which portions of the foliated series of the -range have been produced by mechanical deformation. Not improbably the -whole series of rocks is of igneous origin, but has been subsequently -rendered more or less schistose.</p> - -<div class="footnote"> - -<p><a id="Footnote_76" href="#FNanchor_76" class="label">[76]</a> There is no room here for a full bibliography of the geological literature devoted to this -locality. In the monograph by J. Phillips in vol. ii. part i. of the <i>Memoirs of the Geological -Survey</i>, a list of writings is given up to the time of its publication in 1848. Since that year -many additional papers have appeared. I may especially refer to H. B. Holl, <i>Quart. Journ. -Geol. Soc.</i> xxi. (1865) p. 72; J. H. Timins, <i>op. cit.</i> xxii. (1867); Mr. F. Rutley, <i>op. cit.</i> xliii. (1887) -p. 481; Dr. Callaway, <i>op. cit.</i> xliii. (1887) p. 525, xlv. (1889) p. 475, xlix. (1893) p. 398; Prof. -Green, <i>op. cit.</i> li. (1895) p. 1; Mr. H. D. Acland, <i>Geol. Mag.</i> 1894, p. 48.</p> - -</div> - -<p><span class="pagenum" id="Page_134">- 134 -</span></p> - -<p>There is one area where the rocks have escaped metamorphism, and -where they present some of the well-known features of ancient volcanic -materials. This tract was first indicated by Dr. H. B. Holl as one occupied -by "altered primordial rocks and post-primordial trap." Its evidently -igneous materials have been examined and described by different observers, -among whom Dr. Callaway has contributed some detailed papers on the -subject. More recently Professor Green, who had the advantage of sections -exposed in the excavations for the construction of a reservoir for supplying -water to Great Malvern, came to the conclusion that the rocks -consist mainly of felsites, having many of the characters of rhyolites. -With these are associated felsitic tuffs, while bands of dolerite, probably -intrusive, form likewise part of the series. So far as the somewhat meagre -evidence allows an opinion to be formed, there appears to be an alternation -of felsites, lavas and tuffs placed in a more or less vertical position, striking -in a northerly direction, and traversed by several sheets of intrusive dolerite.</p> - -<p>No junction has been found between these unfoliated volcanic rocks -and the schists that form the core of the range. Judging merely from -their present relative condition, one would naturally infer that the volcanic -rocks must be the younger of the two groups. But, as Professor Green has -pointed out, it is conceivable that the latter may have locally escaped -crushing, and yet be of the same age as the felsites and epidiorites of the -neighbouring Raggedstone Hill, which have been in part considerably -affected by mechanical movements.<a id="FNanchor_77" href="#Footnote_77" class="fnanchor">[77]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_77" href="#FNanchor_77" class="label">[77]</a> <i>Op. cit.</i> p. 7. The metamorphism of the igneous rocks of the Malvern Hills into schists has -been especially investigated by Dr. Callaway.</p> - -</div> - -<p>For our present inquiry it is perhaps sufficient to take note that in the -heart of the Malvern Hills there lies a remnant of a volcanic district, probably -of pre-Cambrian age, the rocks of which had been raised up into a -vertical position so as to form islets or reefs in the sea in which the Upper -Cambrian strata (Hollybush Sandstone and Upper Lingula shales) were -deposited. Until some more precise evidence is obtained as to the geological -age of these rocks it may be convenient to place them provisionally with -the volcanic Uriconian series.</p> - - -<h4>vi. <span class="allsmcap">THE CHARNWOOD FOREST VOLCANO</span></h4> - -<p>In the heart of England the great Triassic plain is diversified by the -uprise through it of the peaks and crests of an old Triassic land-surface, -which are embraced in the district known as Charnwood Forest. These -scattered eminences consist of materials not only immensely older than the -Trias, but once doubtless buried under thousands of feet of Palæozoic strata. -They had been laid bare by denudation and carved into picturesque crags -and pinnacles before the New Red Sandstone was deposited around and -above them.</p> - -<p><span class="pagenum" id="Page_135">- 135 -</span></p> - -<p>To these vestiges of an early Mesozoic land, still half buried among -Triassic strata, a peculiar interest attaches from the obviously high antiquity -of their rocks and their uprise in the very centre of the island. Various -opinions have been expressed as to the age of their component rocks. -When they were mapped by the Geological Survey they were recognized to -be as old as any group of rocks then known, and they were accordingly -placed in the Cambrian system. More recent research has suggested that -they may be still more ancient, and may be regarded as pre-Cambrian.</p> - -<p>The rocks of Charnwood Forest have been the subject of an exhaustive -research by the Rev. E. Hill and Professor Bonney, to whom most of our -knowledge regarding them is due. These observers first pointed out the -truly volcanic nature of the coarse clastic rocks of the district. They have -traced their relations in the field, and have likewise described their structure -and composition as shown by the microscope. Subsequently the district -has been re-mapped on the scale of six inches to a mile by Mr. Fox -Strangways for the Geological Survey, while Mr. W. W. Watts, another -member of the Survey, has studied the petrography of the ground, and has -traced the boundaries of the several rock-groups so far as these can be -determined. Confirming generally the stratigraphical arrangement sketched -by Messrs. Hill and Bonney, Mr. Watts has proposed the following classification -of the rocks:—<a id="FNanchor_78" href="#Footnote_78" class="fnanchor">[78]</a></p> - -<table summary="rocks"> -<tr> - <td class="tdr">7.</td> - <td class="tdl">Groby and Swithland slates.</td> - <td class="tdl" rowspan="2"><img src="images/bracer_60.png" width="11" height="60" alt="" /></td> - <td class="tdl" rowspan="2">The Brand series.</td> -</tr> -<tr> - <td class="tdr">6.<br /> </td> - <td class="tdl">Hanging Rocks conglomerate<br /> and Bradgate quartzite.</td> -</tr> -<tr> - <td class="tdr">5.</td> - <td class="tdl">Woodhouse beds (ashy grits).</td> - <td class="tdl" rowspan="4"><img src="images/bracer_160.png" width="11" height="160" alt="" /></td> - <td class="tdl" rowspan="4">The Maplewell series<br /> (volcanic tuffs<br /> and agglomerates).</td> -</tr> -<tr> - <td class="tdr">4.</td> - <td class="tdl">Slate-agglomerate of Roecliffe.</td> -</tr> -<tr> - <td class="tdr">3.</td> - <td class="tdl">Hornstone beds of Beacon Hill.</td> -</tr> -<tr> - <td class="tdr">2.</td> - <td class="tdl">Felsitic agglomerate of Benscliffe.</td> -</tr> -<tr> - <td class="tdr vtop">1.</td> - <td class="tdl">Quartzose, felspathic and felsitic grits.</td> - <td class="tdl" colspan="2">The Blackbrook series.</td> -</tr> -</table> - -<div class="footnote"> - -<p><a id="Footnote_78" href="#FNanchor_78" class="label">[78]</a> Annual Report of Director-General of the Geological Survey, in the <i>Report of Science and -Art Department for 1895</i>.</p> - -</div> - -<p>Under any computation or measurement, the total thickness of detrital -material in this series of formations must amount to several thousand feet. -The chief interest centres in the middle series, which consists largely of -fragmental volcanic rocks, with intercalations of slate and grit. As was -first shown by Mr. Hill and Professor Bonney, these volcanic materials -vary from exceedingly coarse agglomerates to fine, ashy or felspathic -slates. In most cases distinct bedding can be recognized in them, but -more particularly in the fine-grained material. Yet even among the -massive agglomerates a tendency may be seen towards an orientation of -the blocks with their long axes parallel. That this arrangement is not -entirely due to the effects of cleavage may be inferred from the many -exceptions to it, which would hardly have occurred had such powerful -cleavage affected the whole district, as would be needed to rearrange the -large blocks in the agglomerates. Besides, the coarser parts often intercalate -<span class="pagenum" id="Page_136">- 136 -</span> -with fine felspathic grits, which distinctly mark the stratification of -the whole.</p> - -<p>The remarkably coarse breccia of Benscliffe is mainly made up of blocks -of quartz-porphyry, felsite or rhyolite, with slate fragments. The Roecliffe -agglomerate, another extraordinarily coarse rock, consists of slate fragments -imbedded in an andesitic matrix, some of the blocks of slate being six -feet long. The finer tuffs have been ascertained to consist of felsitic or -andesitic detritus, sometimes forming exceedingly compact flinty rocks or -hornstones.</p> - -<p>In this thick accumulation of detrital rocks we are presented with a -series of alternations of coarser and finer pyroclastic material, interstratified -among green, grey and purple slates and grits, which probably represent -the non-volcanic sediments of the time of eruption. The succession of -strata bears witness to a long series of eruptions of varying intensity, but -culminating at two distinct periods in the discharge of huge blocks of rock -(Benscliffe and Roecliffe agglomerates).</p> - -<p>After some search I have been unable to detect a single vesicular fragment -among the stones in the breccias and tuffs, and Messrs. Hill and -Bonney were not more successful. Not a trace of anything in the least -degree scoriaceous is anywhere to be found. The paste in which the blocks -lie consists of such fine material as would result from the trituration of -felsite and slate. It contains many broken crystals of felspar, with grains -of clear quartz. A gradation can be traced from the coarser into the finer -bands of volcanic and non-volcanic material, fine slates being also interleaved -with highly-felspathic partings of grit.</p> - -<p>Having looked with some care for a trace of a true volcanic neck in -the district, I have not seen anything that could be unhesitatingly so -designated. Even in the north-western part of the district, where the -breccias are coarsest, and there is least trace of ordinary sediments, some -signs of bedding can usually be detected in the position of the imbedded -stones and the partings of finer tuff. Both the coarser and finer detritus -suggest the kind of material discharged from vents before the uprise of any -lava. The entire absence of scoriaceous fragments is noteworthy, and the -abundance of slate blocks rather points to the early eruptions of a volcanic -focus. Possibly, while the chief centre of eruption lay towards the north-west, -numerous vents may have been opened all over the district, discharging -abundant showers of dust and stones, but seldom or never culminating in -the actual outpouring of lava.</p> - -<p>No indubitable lava-sheet has, in my judgment, been yet recognized in -Charnwood Forest. Various opinions have been expressed as to some of -the more compact close-grained rocks, and even the verdicts of the same -observers have varied from time to time, the rocks once considered as felsites -being afterwards regarded as tuffs, and subsequently placed with the felsites -or andesites after all. It is not necessary for my present purpose to enter -into these questions, which are rather of local interest. I will only say -that, in my opinion, the rocks of Sharpley, Peldar, and Bardon Hill are -<span class="pagenum" id="Page_137">- 137 -</span> -massive rocks, as they have finally been classed by Messrs. Hill and Bonney. -But I cannot look upon them as lavas, at least I have seen no evidence to -lead me to believe that they were ever erupted at the surface. I have fully -considered the arguments of Mr. Hill and Professor Bonney on this point.<a id="FNanchor_79" href="#Footnote_79" class="fnanchor">[79]</a> -There can, I think, be no doubt of the close association of these felsitic -rocks and the breccias, but the structure of the rocks in the field seems to -me to be decidedly in favour of the view expressed above. The microscope -affords no assistance in the question.<a id="FNanchor_80" href="#Footnote_80" class="fnanchor">[80]</a> The doubtful rocks seem to me rather -to be intrusive masses which have been protruded into the volcanic sedimentary -series among which they rise. They are acid, fine-grained, porphyritic -rocks, which would formerly have been included under the general name of -felsites or quartz-porphyries. Their coarse porphyritic parts rapidly pass -into close-grained felsitic material. Many of the blocks in the breccias -are precisely like parts of these rocks. It might hence be asserted -that these fragmental deposits are later than the eruptive bosses. At -least it is obvious that rocks of the same type as those of Sharpley, -Peldar, and Bardon Hill must have been disrupted to produce the coarse -breccias.</p> - -<div class="footnote"> - -<p><a id="Footnote_79" href="#FNanchor_79" class="label">[79]</a> <i>Quart. Journ. Geol. Soc.</i> xlvii. (1891), pp. 80-88.</p> - -<p><a id="Footnote_80" href="#FNanchor_80" class="label">[80]</a> See Messrs. Hill and Bonney, <i>op. cit.</i> xxxiv. (1878), p. 211.</p> - -</div> - -<p>Later eruptive rocks, consisting of masses of syenite and granite, with -still younger dykes of dolerite, andesite, diorite and felsite, have successively -made their appearance, and add to the diversity of the igneous phenomena -of this district.</p> - -<p>The question of the age of this isolated volcanic series is one of much -interest, but of great perplexity. Though a resemblance may be admitted -to exist between some of the slates and parts of the Cambrian system of -North Wales, the difference between the Charnwood rocks and the undoubted -Cambrian series of Warwickshire, only thirteen miles to the south-west, -is such as to indicate that the former are probably older than the -latter. While the Charnwood rocks have been intensely cleaved and -crushed, those of Warwickshire have undergone no such change. The -argillaceous strata in the one region have been converted into slates, in the -other they remain mere shales. Though cleavage is sometimes irregularly -developed, its rapid disappearance in so short a distance as the interval -between Charnwood Forest and Nuneaton seems most explicable if we -suppose that the rocks at the more easterly locality were cleaved before -those towards the west were deposited. If this inference be well grounded -the pre-Cambrian age of the Charnwood volcanoes would be established. -But the argument is not conclusive. No fossils of any kind have yet been -found in any of the old rocks of Charnwood.<a id="FNanchor_81" href="#Footnote_81" class="fnanchor">[81]</a> Merely lithological resemblances -or differences are all that can be used as a guide to the geological -age of these masses. Mr. Watts has suggested that possibly the quartzite -<span class="pagenum" id="Page_138">- 138 -</span> -of Bradgate (No. 6 of the Charnwood groups) may be the equivalent of the -quartzite which in Shropshire and Warwickshire forms the base of the -sedimentary Cambrian formations. If that correlation could be established, -the volcanic series below the quartzite in Charnwood might be regarded as -representing the Uriconian volcanic series of Shropshire.</p> - -<div class="footnote"> - -<p><a id="Footnote_81" href="#FNanchor_81" class="label">[81]</a> Since this page was in type, Professor Lapworth has found a worm-burrow low down in the -Brand Series, and one or two additional examples have since been obtained by Mr. J. Rhodes of -the Geological Survey. These are the first undoubted organisms from the Charnwood Forest -rocks. Mr. Watts, <i>Geol. Mag.</i> 1896, p. 487.</p> -</div> -<hr class="chap x-ebookmaker-drop" /> - - -<div class="chapter"> -<p><span class="pagenum" id="Page_139">- 139 -</span></p> - -<h2 class="nobreak" id="BOOK_III">BOOK III<br /> - -<span class="smaller">THE CAMBRIAN VOLCANOES</span></h2> -</div> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<h3 class="nobreak" id="CHAPTER_IX">CHAPTER IX<br /> - -<span class="smaller">CHARACTERISTICS OF THE CAMBRIAN SYSTEM IN BRITAIN</span></h3> -</div> - -<div class="blockquot"> - -<p>The Physical Geography of the Cambrian Period—The Pioneers of Palæozoic Geology in -Britain—Work of the Geological Survey in Wales—Subdivisions of the Cambrian -System in Britain.</p> -</div> - - -<p>In leaving the investigation of the pre-Cambrian rocks and entering upon -that of the Palæozoic systems, that is, the great series of sedimentary formations -which include the earliest records of organized life upon the surface of -the globe, the geologist feels much as the historian when, quitting the domain -of legend and tradition, he can tread firmly in the region of documentary -evidence. From the bottom of the Cambrian system upward through the -long series of geological formations, the chronicle, though often sadly incomplete, -is usually clear and legible. As we follow the lowest fossiliferous -strata across a territory, we recognize that they bear witness to the -same processes of denudation and deposition which have been going on -uninterruptedly on the face of the globe ever since. The beds of conglomerate -represent the gravels and shingles of old coast-lines and river-beds. -The sandstones recall the familiar features of sandy sea-bottoms not far -from land. The shales bear witness to the fall of fine sediment in stiller -water, such as now takes place in the deeper parts of seas and lakes. Notwithstanding -their vast antiquity, the strata themselves exhibit no exceptional -peculiarities of origin. They seem to be just such familiar deposits as are -gathering under fitting conditions at the present time.</p> - -<p>Some writers have speculated on the far greater intensity of all geological -activities in the early times of the planet's history. But if we may -interpret the record of the stratified formations by the phenomena of to-day, -there is for these speculations no confirmation in the sedimentation of -the oldest stratified deposits. It is of course quite intelligible, if not probable, -<span class="pagenum" id="Page_140">- 140 -</span> -that many geological forces may have been more vigorous in primeval -times than they afterwards became. But of the gigantic tides, prodigious -denudation and violent huddling together of the waste of the earth's -surface, which have been postulated for the early Palæozoic ages, there is -assuredly nowhere any indication among the stratified formations. In -those vast orderly repositories, layer succeeds layer among thinly-laminated -shales, as gently and equably as the fine silt of each tide sinks to-day -over the floor of a sheltered estuary. At the primeval period of -which these sediments are the memorial, the waters receded from flat -shores and left tracts of mud bare to the sky, precisely as they do still. -Then as now, the sun shone and dried such mud-flats, covering their surfaces -with a network of cracks; the rain fell in heavy drops, that left their -imprints on the drying mud; and the next tide rose so gently as to overflow -these records of sunshine and shower without effacing them, but -spreading over them a fresh film of sediment, to be succeeded by other -slowly-accumulating layers, under which they have lain preserved during -the long cycles of geological history.</p> - -<p>That organized creatures had already appeared upon the earth's surface -before the beginning of the Cambrian period cannot be doubted. The -animal remains in the lowest Cambrian strata are far from being the simple -forms which might be expected to indicate the first start of animal life -upon the surface of the earth. On the contrary, though they are comparatively -scanty in types, and often rare or absent throughout a thick -mass of sedimentary deposits, they show beyond dispute that, when they -flourished, invertebrate life had already reached such a stage of advancement -and differentiation that various leading types had appeared which have -descended, in some cases with generic identity, down to our own day. There -must have been a long pedigree to these organisms of the oldest known -fossiliferous rocks. And somewhere on the earth's surface we may yet hope -to find the remains of their progenitors in pre-Cambrian deposits.</p> - -<p>The researches of many explorers in Europe and North America have -brought to light an interesting series of organic remains from the Cambrian -system. Of the plants of the time hardly any traces have survived, -save some markings which have been referred to sea-weeds. The earliest -known sponges and corals occur in this system, likewise the ancestors of -the graptolites, which played so prominent a part in the life of the next or -Silurian period. There were already representatives of crinoids and star-fishes, -besides examples of the extinct group of cystideans. Sea-worms -crawled over the muddy and sandy sea-bottom, for they have left their -trails and burrows in the hardened sediments. Molluscs had by this time -appeared in their four great divisions of Brachiopods, Lamellibranchs, Gasteropods -and Cephalopods, though the forms yet discovered among Cambrian -rocks are comparatively few. The most abundant and characteristic inhabitants -of the Cambrian seas were the trilobites, of which many genera have -been disinterred from the strata. In the lowest fossiliferous Cambrian -group the trilobitic genus <i>Olenellus</i>, already referred to, is the characteristic -<span class="pagenum" id="Page_141">- 141 -</span> -form. Higher up <i>Paradoxides</i> is predominant, while towards the top of the -system the most characteristic genus is <i>Olenus</i>.</p> - -<p>From the organic remains which have been preserved, we may legitimately -infer the existence of others which have entirely disappeared. There -seems no reason to doubt that the leading grades of invertebrate life which are -wanting in the known Cambrian fauna were really represented in the Cambrian -seas. The chance discovery of a band of limestone may any day -entirely alter our knowledge as to the relative proportions of the several -divisions of the animal kingdom in the earliest Palæozoic rocks. Sand -is rather adverse to the preservation of a varied representation of the -organisms of the overlying sea-water. Mud is generally favourable, but calcareous -accumulations are greatly more so, and they usually consist almost -entirely of organic remains. Thus in the Cambrian series of the north-west -of Scotland the quartzites that form the lower group, though sometimes -crowded with worm-burrows, contain hardly any other sign of organisms. -The overlying shales, besides their abundant worm-castings, have yielded -perfect specimens of <i>Olenellus</i> and other fossils. But in the uppermost group, -consisting of limestones, every particle of the sediment appears to have passed -through the intestines of worms, and as it gathered on the sea-bottom it -enclosed and has preserved a varied and abundant assemblage of organisms, -including trilobites, gasteropods and a number of cephalopods. While in -the Cambrian rocks of Europe calcareous bands are comparatively rare, in -those of North America they are not infrequent. Hence it is largely from -American deposits that our knowledge of the Cambrian fauna has been -derived.</p> - -<p>Not a vestige of any vertebrate organism has yet been detected among -the earlier Palæozoic sediments. So far as we know, there were no fishes -in the Cambrian seas. The highest organisms then existing were chambered -shells, a once abundant and singularly varied class, of which the living -Nautilus is now the sole representative.</p> - -<p>In trying to realize the general geographical conditions of Cambrian -time, the geologist finds himself entirely without any evidence as to the -character of the terrestrial vegetation. We can hardly doubt that the land -was clothed with plants, probably including lycopods and ferns, possibly -even cycads and conifers. But no remains of this flora have yet been -recovered. Nor have any traces of land-animals been detected. All that -we yet know of the life of the period has been gleaned from marine sediments, -which show that the invertebrate population by which the sea was -then tenanted embraced some of the leading types of structure that have -survived through all the long vista of geological time down to our own -day.</p> - -<p>Some of the shore-lines of the Cambrian waters may still be traced, and -it is possible to say where the land of the time stood and where lay the sea. -In the British area the largest relic of Cambrian land is found in the far -north-west of Scotland. Formed partly of the Lewisian Gneiss and partly -of the Torridon Sandstone, it takes in the whole chain of the Outer -<span class="pagenum" id="Page_142">- 142 -</span> -Hebrides and likewise part of the present western seaboard of Sutherland -and Ross. Along the margin of that northern land the white sand was laid -down which now gleams in sheets of snow-like quartzite on most of the higher -mountains from Cape Wrath to Skye. The sea lay to the east and, so far -as we know, may have stretched across the rest of Scotland, and the north -and centre of England. Another vestige of the land of this ancient era occurs -in Anglesey. There, and likewise over scattered tracts in the Midlands, -and in the south-west of England, the geologist seems to descry the last -relics of islets that rose out of the Cambrian sea, and are now surrounded -with its hardened sediments.</p> - -<p>While such was the general aspect of the region of the British Isles -during Cambrian time, volcanic action manifested itself at various localities -over the area, breaking out on the sea-bottom, and pouring forth sheets of -lava and showers of ashes, which mingled with the sand and silt that were -settling there at the time. In the northern or Scottish tract no trace of -this subterranean activity has been found; but in the English Midlands -and over much of Wales abundant evidence has been obtained to show that -in those districts the Cambrian period was marked by frequent and prolonged -eruptions.</p> - -<p>As its name denotes, the Cambrian system is typically developed in -Wales. It was there that Sedgwick first worked out the stratigraphical -relations of its ancient sediments, and that Murchison demonstrated the -succession of organic remains contained in them, applying to them the -principles of classification laid down by William Smith in regard to the -Secondary formations. It was there too that some of the earliest and -most memorable achievements were made in the investigation of ancient -volcanic rocks. Sedgwick and Murchison, besides the admirable work -which they accomplished in establishing the stratigraphy of the older -Palæozoic formations, clearly recognized that among these formations there -were preserved the records of contemporaneous submarine eruptions. Sedgwick -showed that the mountainous masses of eruptive rock in North Wales -were really lavas and ashes, which had been discharged over the sea-floor at -the time when the ancient sediments of that region were deposited, while -Murchison established the same fact by numerous observations in the east -and south of Wales, and in the bordering English counties. De la Beche -had found similar evidence among the "grauwacke" rocks of Devonshire.<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> For early researches on the older Palæozoic volcanic rocks of Britain, see Sedgwick, <i>Proc. -Geol. Soc.</i> vols. ii. (1838) pp. 678, 679, iii. (1841) p. 548, iv. (1843) p. 215; <i>Quart. Journ. Geol. -Soc.</i> vols. i. (1845) pp. 8-17, iii. (1847) p. 134. Murchison, <i>Proc. Geol. Soc.</i> vol. ii. (1833-34) p. -85; <i>Silurian System</i> (1839) pp. 225, 258, 268, 287, 317, 324, 401; <i>Siluria</i>, 4th edit. -(1867) p. 76 <i>et seq.</i> De la Beche, <i>Mem. Geol. Survey</i>, vol. i. (1846) pp. 29-36. A. C. Ramsay -in the Maps and Horizontal Sections of Wales published by the Geological Survey; also -Descriptive Catalogue of the Rock-Specimens in the Museum of Practical Geology, 1st edit. -(1858), 2nd edit. (1859), 3rd edit. (1862); "The Geology of North Wales," forming vol. iii. of -<i>Memoirs of the Geological Survey</i>, 1st edit. (1866), 2nd edit. (1881).</p> - -</div> - -<p>Following in the track thus opened up by these great masters, the officers -of the Geological Survey were enabled to unravel, as had never before been -attempted, the complicated structure of the old volcanic regions of Wales. -<span class="pagenum" id="Page_143">- 143 -</span> -At the outset of the following discussion I wish to express my admiration -of the labours of the early pioneers who thus laid for us the foundation of -our knowledge of volcanic action in the Palæozoic periods. To De la Beche -and his associates in the Survey a special measure of gratitude is due from -all who have followed in their steps and profited by their work. When we -consider the condition of geological science, and especially of the department -of petrography, in this country at the time when these early and detailed -investigations were carried on, when we remember the imperfection of much -of the topography on the old one-inch Ordnance maps (which were the only -maps then available), when we call to mind the rugged and lofty nature of -the ground where some of the most complicated geological structures are -displayed, we must admit that at the period when these maps and sections -were produced they could not have been better done; nay, that as in some -important respects they were distinctly in advance of their time, their -publication marked an era in the progress of structural, and especially of -volcanic, geology. The separation of lavas and tuffs over hundreds of -square miles in a mountainous region, the discrimination of intrusive sheets -and eruptive bosses, the determination of successive stratigraphical zones of -volcanic activity among some of the oldest fossiliferous formations, were -achievements which will ever place the names of Ramsay, Selwyn, Jukes -and their associates high in the bede-roll of geological science. No one -ever thinks now of making a geological excursion into Wales without -carrying with him the sheets of the Geological Survey map. These form -his guide and handbook, and furnish him with the basis of information from -which he starts in his own researches.</p> - -<p>But science does not stand still. The most perfect geological map that -can be made to-day will be capable of improvement thirty or forty years -hence. The maps of the Geological Survey are no exception to this rule. -In criticizing and correcting them, however, let us judge them not by the -standard of knowledge which we have now reached, but by that of the time -when they were prepared. It is easy to criticize; it is not so easy to -recognize how much we owe to the very work which we pronounce to be -imperfect.</p> - -<p>The ancient volcanoes of Wales, thanks mainly to the admirable labours -of my former friend and chief, Sir Andrew C. Ramsay, have taken a familiar -place in geological literature. But a good deal has been learnt regarding -them since he mapped and wrote. The volcanic history, as he viewed it, -began in the Arenig period. The progress of subsequent inquiry, however, -has shown that there are volcanic rocks in Wales of much older date. I -shall show that the Cambrian period, both in South and North Wales, was -eminently volcanic.</p> - -<p>Much controversy having arisen as to the respective limits and nomenclature -of the older Palæozoic rocks, let me state, at the outset of the -inquiry into the volcanic eruptions of Cambrian time, that under the term -"Cambrian" I class all the known Palæozoic rocks which lie below the -bottom of what is termed the Arenig group. It was maintained by Sir -<span class="pagenum" id="Page_144">- 144 -</span> -Andrew Ramsay and his colleagues on the Geological Survey that on the -mainland of Wales no base is ever found to the Cambrian system. More -recently certain conglomerates have been fixed upon as the true Cambrian -base, both in South and North Wales, and endeavours have been made to -trace an unconformability at that line, all rocks below it being treated as -pre-Cambrian. But conglomerates do not necessarily mark a stratigraphical -discordance, and in South Wales there is no trace of any unconformability -between the strata above and below the supposed line of break.<a id="FNanchor_83" href="#Footnote_83" class="fnanchor">[83]</a> Professor -Bonney has shown that in North Wales several zones of conglomerate have -been erroneously identified as the supposed basal platform of the Cambrian -series, and more recently Mr. Blake has pointed out that some of these -conglomerates are unquestionably Lower Silurian.</p> - -<div class="footnote"> - -<p><a id="Footnote_83" href="#FNanchor_83" class="label">[83]</a> See a discussion of this subject in <i>Quart. Journ. Geol. Soc.</i> vol. xxxix. (1883), p. 305.</p> - -</div> - -<p>My own examination so far confirms the conclusions arrived at by these -observers. Like my predecessors in the Geological Survey, however, I have -been unable to detect anywhere in Caernarvonshire or Merionethshire a base -to the Cambrian system, and I am compelled to agree with them in -regarding as Cambrian (partly even as Lower Silurian) all the rocks from -Bangor to Llanllyfni, which have more recently been classed as pre-Cambrian. -But though thus supporting their general stratigraphy, I am bound to -acknowledge that they failed to recognize the existence of a great volcanic -series below the Arenig horizon. The existence of this series, noticed by -Sedgwick, was first definitely stated by Professor Hughes,<a id="FNanchor_84" href="#Footnote_84" class="fnanchor">[84]</a> and his statements -have been confirmed and extended by subsequent observers, notably by -Professor Bonney and Mr. Blake. The Cambrian period is thus proved to -have been perhaps even more continuously volcanic than the Lower Silurian -period was in Wales.</p> - -<div class="footnote"> - -<p><a id="Footnote_84" href="#FNanchor_84" class="label">[84]</a> <i>Proc. Camb. Phil. Soc.</i> vol. iii. (1877), p. 89. The Cambrian volcanic areas of North Wales -are represented in <a href="#Map_II">Map II.</a></p> - -</div> - -<p>The following table shows the subdivisions of the Cambrian system now -recognized in Britain:—</p> - -<table style="width: 40em;" summary="data"> -<tr> - <td class="tdc bdt bdl bdb" colspan="3"><span class="smcap">Wales.</span><br />(Ranging up to 12,000 feet or more.)</td> - <td class="tdc bdt bdl bdb"><span class="smcap">Western England.</span><br />(About 3000 feet.)</td> - <td class="tdc bdt bdl bdb bdr"><span class="smcap">N.W. Scotland.</span><br />(About 2000 feet.)</td> -</tr> -<tr> - <td class="bdl vtop"><div class="hanging3">Upper or <i>Olenus</i> Zones.</div></td> - <td class="vtop"><img src="images/bracel_60.png" width="11" height="60" alt="" /></td> - <td class="vtop"><div class="hanging3">Tremadoc Slates Lingula Flags (<i>Lingulella</i>, <i>Olenus</i>, etc.</div></td> - <td class="bdl vtop"><div class="hanging3">Shineton Shales (<i>Dictyograptus</i> or <i>Dictyonema</i>, <i>Olenus</i>, etc.).</div></td> - <td class="bdl bdr" rowspan="2"><div class="hanging3">Limestones, about 1500 feet thick, divisible into seven groups - (<i>Archæocyathus</i>, <i>Maclurea</i>, <i>Ophileta</i>, <i>Murchisonia</i>, - <i>Orthoceras</i>, and vast quantities of annelid castings).</div></td> -</tr> -<tr> - <td class="bdl"><div class="hanging3">Middle or <i>Paradoxides</i> Zones.</div></td> - <td><img src="images/bracel_36.png" width="11" height="36" alt="" /></td> - <td><div class="hanging3">Menevian group (<i>Paradoxides</i>).</div></td> - <td class="bdl vtop"><div class="hanging3">Conglomerates and limestones (Comley), with <i>Paradoxides</i>, etc.</div></td> -</tr> -<tr> - <td class="bdl bdb"><div class="hanging3">Lower or <i>Olenellus</i> Zones.</div></td> - <td class="bdb"><img src="images/bracel_116.png" width="11" height="116" alt="" /></td> - <td class="bdb"><div class="hanging3">Harlech and Llanberis group with basement volcanic rocks; bottom not seen.</div></td> - <td class="bdl bdb"><div class="hanging3">Thin quartzite passing up into green flags, grits, shales - and sandstone (Comley Sandstone), containing <i>Olenellus</i>.</div></td> - <td class="bdl bdb bdr"><div class="hanging3">Shales ("fucoid beds"), with <i>Olenellus</i>, <i>Salterella</i>, etc.</div> - <div class="hanging3">Quartzites with annelid burrows. The base of the series lies unconformably on pre-Cambrian rocks.</div></td> -</tr> -</table> - - -<hr class="chap x-ebookmaker-drop" /> - - -<div class="chapter"> -<p><span class="pagenum" id="Page_145">- 145 -</span></p> - -<h3 class="nobreak" id="CHAPTER_X">CHAPTER X<br /> - -<span class="smaller">THE CAMBRIAN VOLCANOES OF SOUTH WALES</span></h3> -</div> - - -<p>In the southern part of the Principality of Wales a remarkably varied -display of British Cambrian volcanic rocks has been preserved. The -district around St. David's has the distinction of being the first in which -volcanic rocks of such high antiquity were recognized. As far back as the -year 1842, Ramsay found that "felspathic volcanic ash" was associated with -other proofs of igneous action, and this fact was recorded by him on the -published Horizontal Sections of the Geological Survey. Unfortunately he -afterwards came to regard the rocks as "altered Cambrian," thus following -certain hypothetical views which, as will be further alluded to in the sequel, -he had adopted in explanation of the phenomena in Caernarvonshire and in -Anglesey. The volcanic nature of these ancient materials was subsequently -rediscovered by Dr. Hicks, who has devoted much time and labour to their -study. Distinguishing the volcanic series of St. David's by the name -"Pebidian," he has contended that it forms a pre-Cambrian system separated -by an unconformability from the base of the Cambrian formations. He -likewise endeavoured to show that an older system of rhyolitic lavas, felsitic -breccias and hälleflintas could be distinguished, which he termed "Arvonian"; -and more ancient still, a core of granitoid or gneissic rocks, which he -separated under the name of "Dimetian." My own investigation of the -ground thoroughly convinced me that there are no pre-Cambrian rocks at -St. David's; that the "Arvonian" and "Dimetian" series are merely -intrusive rocks (quartz-porphyry, granite, etc.) which have invaded the -volcanic series; and that the "Pebidian," instead of being a pre-Cambrian -formation on which the Cambrian base rests unconformably, is a group of -volcanic rocks into which the Cambrian strata pass down conformably, and -which in the St. David's district constitutes the lowest group of the Cambrian -system.<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> For Dr. Hicks' views, see especially his papers in the <i>Quart. Journ. Geol. Soc.</i> vols. xxxi. -xxxiii. xxxiv. xl. My criticism of them will be found in <i>op. cit.</i> vol. xxxix. (1883), subsequently -in the main confirmed by Prof. Lloyd Morgan, <i>op. cit.</i> xlvi. p. 241. See also Prof. Blake, -<i>op. cit.</i> xl. (1884). Dr. Hicks in his more recent papers has merely reiterated his previously -published opinions.</p> - -</div> - -<p><span class="pagenum" id="Page_146">- 146 -</span></p> - -<div class="figcenter" id="v1fig40" style="width: 625px;"> - <a href="images/v1fig40_lg.png"><img src="images/v1fig40.png" width="625" height="401" alt="" /></a> - -<div class="fl_left smaller">Click on image to view larger</div> -<div class="fl_right smaller"><i>Walker & Boutall sc.</i></div> - -<div class="tdc smaller" style="clear: both;"><span class="smcap">Fig. 40.</span>—Map of the volcanic district of St. David's.</div> -</div> - - -<p>The volcanic geology of St. David's possesses a special interest inasmuch -as it embraces a tolerably full development of various features which -characterize the volcanic groups of later Palæozoic systems. Though the -rocks are chiefly tuffs, they include also sheets of lava, as well as sills, dykes and -bosses. They show a remarkable range in chemical composition from quite -basic to highly acid materials. They present the amplest proofs of having -been erupted and spread out over the sea-bottom, and they likewise afford -<span class="pagenum" id="Page_147">- 147 -</span> -clear evidence of alternation with the ordinary non-volcanic sediment of the -time to which they belong. In these respects they are particularly noteworthy, -for they prove that in the earliest Palæozoic ages the essential features of -volcanic action were already as well developed as in any subsequent epoch -of geological history.</p> - -<p>The volcanic group of St. David's attains a visible thickness of about -1800 feet. Its upper part graduates upward into purple and green Lower -Cambrian sandstones. The base of the group is not seen owing to the -plicated structure of the district. Hence the total thickness of volcanic -material cannot be determined, neither can we tell on what it rests, whether -on a still lower sedimentary series or on some platform of pre-Cambrian -rocks.</p> - -<p>The structure of the group, notwithstanding all that has been written -about it, has never yet been adequately worked out. The unfortunate and -barren controversy about supposed pre-Cambrian rocks at St. David's has -tended to obscure the real importance of these rocks as the oldest well-preserved -record of volcanic action in Britain. They deserve to be carefully -surveyed on maps of a large scale, in the same detailed manner as has been -so successfully applied to the elucidation of younger volcanic tracts. Until -such detailed investigation is made, any account of them which is given can -be little more than a general outline of the subject. The following description -is the result of my examination of the ground in company with my colleague -Mr. B. N. Peach, and afterwards with the late Mr. W. Topley.<a id="FNanchor_86" href="#Footnote_86" class="fnanchor">[86]</a> A few -additional observations, from the subsequent exploration of Professor Lloyd -Morgan,<a id="FNanchor_87" href="#Footnote_87" class="fnanchor">[87]</a> are incorporated in the narrative.</p> - -<div class="footnote"> - -<p><a id="Footnote_86" href="#FNanchor_86" class="label">[86]</a> <i>Quart. Journ. Geol. Soc.</i> vol. xxxix. (1883), p. 294 <i>et seq.</i> While the essential parts of the -investigation are given in the following pages, I would refer the reader to this paper for details -not transferred to the present volume.</p> - -<p><a id="Footnote_87" href="#FNanchor_87" class="label">[87]</a> <i>Op. cit.</i> vol. xlvi. (1890), p. 241.</p> - -</div> - -<p>The geologist who traces these St. David's rocks in the field cannot -fail to be struck with their general resemblance to volcanic masses of later -Palæozoic date. Many of the lavas and tuffs are in outward characters -quite indistinguishable from those of the Lower Old Red Sandstone and -Carboniferous systems of Britain. So many points of detail may be observed -to be common to the Palæozoic eruptive rocks all over the country from -the Cambrian to the Permian periods as to indicate that volcanic -phenomena must have recurred under much the same conditions throughout -Palæozoic time.</p> - -<p>By far the larger part of the Cambrian volcanic group of St. David's -consists of bedded tuffs, though a few lavas are interstratified in it, -particularly towards the top. The whole has subsequently been invaded by -acid protrusions, and lastly by basic dykes.</p> - -<p>1. <i>Bedded Tuffs and Lavas.</i>—The tuffs, which are the predominant -members of the volcanic group, present many varieties of colour, from dark -purple, through tints of brick-red and lilac, to pale pink, yellow and creamy -white, but not unfrequently assume various shades of dull green. They -<span class="pagenum" id="Page_148">- 148 -</span> -vary likewise in texture from somewhat coarse breccias or agglomerates, -through many gradations, into fine silky schists in which the tuffaceous -character is almost lost. Generally they are distinctly granular, presenting -to the naked eye abundant angular and subangular lapilli, among which -broken crystals of a white, somewhat kaolinized, felspar and fragments of -fine-grained felsite are often conspicuous. The greater part of the tuffs, -particularly the purple, red and dark-green varieties, which constitute so -large a proportion of the whole, has been derived from the explosion of -basic rocks similar in character to the diabases now found associated with -them. On the other hand, the paler varieties, both in the form of fine -tuffs and of breccias, have probably resulted mainly from the destruction of -more siliceous lavas, probably felsites (rhyolites) or other acid rocks.</p> - -<p>That many of the tuffs are due to the destruction of diabase-lavas -may be surmised from their close general external resemblance to these -rocks, and from the way in which they are associated with the contemporaneous -sheets of diabase. Some of the dull dark-purple tuffs -might almost at first sight be mistaken for truly eruptive rocks. The -analyses of two typical examples of these basic tuffs (Nos. I. and II.), and -one (No. III.) of an intermediate variety containing an admixture of acid -fragments, are given in the subjoined table.</p> - -<table summary="data"> -<tr> - <td class="bdl bdt bdb"></td> - <td class="bdl bdt bdb tdc">SiO<sub>2</sub></td> - <td class="bdl bdt bdb tdc">Al<sub>2</sub>O<sub>3</sub></td> - <td class="bdl bdt bdb tdc">Fe<sub>2</sub>O<sub>3</sub></td> - <td class="bdl bdt bdb tdc">FeO</td> - <td class="bdl bdt bdb tdc">MnO</td> - <td class="bdl bdt bdb tdc">CaO</td> - <td class="bdl bdt bdb tdc">MgO</td> - <td class="bdl bdt bdb tdc">K<sub>2</sub>O</td> - <td class="bdl bdt bdb tdc">Na<sub>2</sub>O</td> - <td class="bdl bdt bdb tdc">H<sub>2</sub>O and<br />Loss on<br />Ignition.</td> - <td class="bdl bdt bdb tdc">Total.</td> - <td class="bdl bdt bdb bdr tdc">Specific<br />Gravity.</td> -</tr> -<tr> - <td class="bdl tdr">I.</td> - <td class="bdl tdc">51·25</td> - <td class="bdl tdc">20·41</td> - <td class="bdl tdc">3·02</td> - <td class="bdl tdc">3·91</td> - <td class="bdl tdc">0·21</td> - <td class="bdl tdc">4·53</td> - <td class="bdl tdc">7·22</td> - <td class="bdl tdc">2·93</td> - <td class="bdl tdc">1·82</td> - <td class="bdl tdc">5·02</td> - <td class="bdl tdc">100·32</td> - <td class="bdl bdr tdc">2·84</td> -</tr> -<tr> - <td class="bdl tdr">II.</td> - <td class="bdl tdc">48·11</td> - <td class="bdl tdc">13·30</td> - <td class="bdl tdc">3·70</td> - <td class="bdl tdc">8·10</td> - <td class="bdl tdc">1·43</td> - <td class="bdl tdc">8·48</td> - <td class="bdl tdc">9·51</td> - <td class="bdl tdc">1·57</td> - <td class="bdl tdc">1·96</td> - <td class="bdl tdc">4·21</td> - <td class="bdl tdc">100·37</td> - <td class="bdl bdr tdc">2·92</td> -</tr> -<tr> - <td class="bdl bdb tdr">III.</td> - <td class="bdl bdb tdc">61·54</td> - <td class="bdl bdb tdc">16·30</td> - <td class="bdl bdb tdc">4·40</td> - <td class="bdl bdb tdc">3·66</td> - <td class="bdl bdb tdc">0·32</td> - <td class="bdl bdb tdc">3·08</td> - <td class="bdl bdb tdc">2·99</td> - <td class="bdl bdb tdc">1·62</td> - <td class="bdl bdb tdc">2·81</td> - <td class="bdl bdb tdc">2·99</td> - <td class="bdl bdb tdc">99·71</td> - <td class="bdl bdb bdr tdc">...</td> -</tr> -</table> - -<table summary="rocks"> -<tr> - <td class="tdr vtop">I.</td> - <td class="tdl">Purplish-red shaly tuff from below olivine-diabase, Crag Rhosson. Analysis by Mr. J. S. - Grant Wilson.</td> -</tr> -<tr> - <td class="tdr vtop">II.</td> - <td class="tdl">Dull purple and green tuff from the lowest group of tuffs between Pen-maen-melyn and - Pen-y-foel. Analysis by Mr. Wilson.</td> -</tr> -<tr> - <td class="tdr vtop">III.</td> - <td class="tdl">Greenish shaly finely granular tuff, from road-side, north of Board Schools, St. David's. - Analysis by Prof. A. Renard of Ghent.</td> -</tr> -</table> - -<p>Although the majority of the tuffs are more or less basic, they frequently -contain evidence in the form of small felsitic lapilli that acid lavas were -present in the eruptive vents, while the pale tuffs show that at the time of -their discharge it was these acid lavas and not the diabases that were -blown out by the explosions. Appended are three analyses of the acid -tuffs (Nos. IV. V. and VI.).</p> - -<p><span class="pagenum" id="Page_149">- 149 -</span></p> - -<table summary="data"> -<tr> - <td class="bdl bdt bdb"></td> - <td class="bdl bdt bdb tdc">SiO<sub>2</sub></td> - <td class="bdl bdt bdb tdc">Al<sub>2</sub>O<sub>3</sub></td> - <td class="bdl bdt bdb tdc">Fe<sub>2</sub>O<sub>3</sub></td> - <td class="bdl bdt bdb tdc">FeO</td> - <td class="bdl bdt bdb tdc">MnO</td> - <td class="bdl bdt bdb tdc">CaO</td> - <td class="bdl bdt bdb tdc">MgO</td> - <td class="bdl bdt bdb tdc">K<sub>2</sub>O</td> - <td class="bdl bdt bdb tdc">Na<sub>2</sub>O</td> - <td class="bdl bdt bdb tdc">H<sub>2</sub>O and<br />Loss on<br />Ignition.</td> - <td class="bdl bdt bdb tdc">Total.</td> - <td class="bdl bdt bdb bdr tdc">Specific<br />Gravity.</td> -</tr> -<tr> - <td class="bdl tdr">IV.</td> - <td class="bdl tdc">80·59</td> - <td class="bdl tdc">11·29</td> - <td class="bdl tdc">0·28</td> - <td class="bdl tdc">1·41</td> - <td class="bdl tdc">trace</td> - <td class="bdl tdc">0·52</td> - <td class="bdl tdc">0·95</td> - <td class="bdl tdc">2·98</td> - <td class="bdl tdc">0·72</td> - <td class="bdl tdc">1·96</td> - <td class="bdl tdc">100·70</td> - <td class="bdl bdr tdc">2·55</td> -</tr> -<tr> - <td class="bdl tdr">V.</td> - <td class="bdl tdc">73·42</td> - <td class="bdl tdc">12·09</td> - <td class="bdl tdc">0·91</td> - <td class="bdl tdc">3·13</td> - <td class="bdl tdc">0·25</td> - <td class="bdl tdc">2·94</td> - <td class="bdl tdc">1·12</td> - <td class="bdl tdc">1·67</td> - <td class="bdl tdc">3·88</td> - <td class="bdl tdc">1·28</td> - <td class="bdl tdc">100·69</td> - <td class="bdl bdr tdc">2·74</td> -</tr> -<tr> - <td class="bdl bdb tdr">VI.</td> - <td class="bdl bdb tdc">72·63</td> - <td class="bdl bdb tdc">16·23</td> - <td class="bdl bdb tdc">2·70</td> - <td class="bdl bdb tdc">0·48</td> - <td class="bdl bdb tdc">...</td> - <td class="bdl bdb tdc">0·18</td> - <td class="bdl bdb tdc">1·36</td> - <td class="bdl bdb tdc">3·35</td> - <td class="bdl bdb tdc">0·15</td> - <td class="bdl bdb tdc">3·00</td> - <td class="bdl bdb tdc">100·12</td> - <td class="bdl bdb bdr tdc">...</td> -</tr> -</table> - -<table summary="rocks"> -<tr> - <td class="tdr vtop">IV.</td> - <td class="tdl">Greenish felsitic breccia, Clegyr Hill; angular fragments of various felsites in a greenish - base. Analysis by Mr. J. S. Grant Wilson.</td> -</tr> -<tr> - <td class="tdr vtop">V.</td> - <td class="tdl">Grey granular felsitic tuff, Bridge over Allan River north from St. David's Board Schools. - Analysis by Mr. Wilson.</td> -</tr> -<tr> - <td class="tdr vtop">VI.</td> - <td class="tdl">Pale pinkish-white, finely schistose tuff—a characteristic sample of the "Porth-lisky - schists." Analysis by Prof. Renard.</td> -</tr> -</table> - -<p>Many varieties of texture can be traced among the tuffs, from coarse -breccias or agglomerates, with blocks a yard or more in length, to fine -schistose mudstones or sericitic schists. One of the most remarkable of the -finer kinds, found near Pen-y-foel, is externally dirty-green, compact and -tolerably homogeneous, but with distinct evidence of its clastic character. -Under the microscope it is found to be composed mainly of lapilli of a -peculiar rock, which is characterized by the abundance and freshness of its -plagioclase (an unusual feature in the volcanic group of St. David's); by the -large, well-defined crystals (one of which measured 0·022 inch by 0·0125 -inch) of augite; by large crystals replaced by green decomposition-products, -but having the external form of olivine; by the absence or scantiness of -any base or groundmass; and, in many of the lapilli, by the abundance of -spherical cells, either empty or filled up as amygdales with decomposition-products. -These spherical vapour-vesicles, so characteristic of the basic or -palagonitic lapilli in many Palæozoic volcanic vents, were found in one -fragment, where they were particularly abundant, to range from a -minimum of 0·0008 inch to a maximum of 0·0033 inch, with a mean of -about 0·0018. The rock from which these lapilli have been derived comes -nearest to one of the diabases from the same part of the district (which -will afterwards be referred to), but shows a closer approach to basalt rocks.</p> - -<p>Another interesting tuff is that of which the analysis (No. II.) has -been given. It occurs not far from the horizon of the rock just described. -Under a low power, it is seen to be composed mainly of fragments of -diabase like the rocks of Rhosson and Clegyr Foig. These fragments are -subangular, or irregular in shape, and vary considerably in size. They are -sometimes finely cellular—the cavities, as in the case just referred to, being -spherical. The plagioclase crystals in the diabase-lapilli are everywhere -conspicuous; so also is the augite, which occurs in larger forms than in the -rock of Rhosson or Clegyr Foig. Next in abundance to these basic fragments -are rounded or subangular pieces of felsite. These weather out in -conspicuous grey rough projections on the exposed face of the rock; under -the microscope they are seen to consist of fine granular felsite, which shows -<span class="pagenum" id="Page_150">- 150 -</span> -a groundmass remaining dark between crossed nicols, but with luminous -points and filaments, and an occasional spherulite giving the usual cross in -polarized light. Lapilli of an older tuff may here and there be detected. -A few angular and subangular grains of quartz are scattered through -the rock. The lapilli are bound together by a finely-granular dirty-green -substance.</p> - -<p>As a typical illustration of the minute structure of the felsitic tuffs, -I may refer to the rock No. V. of the foregoing analyses. It is composed -mainly of fragments of various felsites, many of which show good flow-structure. -Large, and usually broken, crystals of orthoclase are dispersed -among the other ingredients. Here and there a fragment of diabase may -be detected; but I could find no trace of pieces of the peculiar microcrystalline -spherulitic quartz-porphyries of St. David's. There is but little -that could be called matrix cementing the lapilli together. The presence -of fragments of diabase may possibly reduce the proportion of silica and -increase that of magnesia, as compared with what would otherwise have -been present in the rock.</p> - -<p>Some of the tuffs appear to have been a kind of volcanic mud. A -specimen of this nature collected from the road-side section, north of the -Board School, presents a finely-granular paste enclosing abundant angular -and subangular lapilli of diabase, a smaller proportion of felsite (sometimes -displaying perfect flow-structure), broken plagioclase crystals, and a greenish -micaceous mineral which has been subsequently developed out of the matrix -between the lapilli.</p> - -<p>Though they lie in the sedimentary series above the main volcanic -group, I may refer here to certain thin bands of tuff at Castell, on account -of their interest in relation to the true Cambrian age of the volcanic group. -They are not quite so fresh as the tuff that occurs in thicker masses, but -their volcanic origin is readily observable. One band appears to be made -up of the debris of some basic rock, like the diabase of the district, through -which detached plagioclase crystals are scattered. The lapilli are subangular; -and around their border a granular deposit of hæmatite has taken -place, giving a red colour to the rock. Another band presents small angular -lapilli, almost entirely composed of a substance which to the naked eye, or -with a lens, is dull, white and clay-like, easily scratched, and slightly unctuous -to the touch. Under the microscope, with a low power, it becomes pale -greyish-green and transparent, and is seen to consist in large part of altered -felspar crystals, partially kaolinized and partially changed into white mica -and calcite. These scattered crystals are true volcanic lapilli, and have not -been derived from the mechanical waste of any pre-existing volcanic rock. -In the tuffs interstratified with the conglomerate, at the quarry above -Porth-clais, though much decomposed, crystals of plagioclase can likewise -still be traced. These strata are also true tuffs, and not mere detritus due -to mechanical degradation (see <a href="#v1fig41">Fig. 41</a>).</p> - -<p>The general result of the study of the microscopic structure of the -Cambrian tuffs of St. David's may be briefly summed up as follows:—</p> - -<p><span class="pagenum" id="Page_151">- 151 -</span></p> - -<p>1. These pyroclastic deposits are almost wholly composed of fragments -of eruptive rocks, sometimes rounded, but usually angular or subangular. -In the more granular varieties very little matrix is present; it consists of -fine debris of the same materials. No detached microlites have been noted, -such as are common among modern volcanic ashes; but there are abundant -ejected crystals. In these respects the Cambrian tuffs resemble those of -the other Palæozoic systems. A mingling of grains of quartz-sand may -indicate the intermixture of ordinary with volcanic sediment.</p> - -<p>2. They may be divided into two groups—one composed mainly of -fragments of diabase or other similar basic rocks, the other of felsite. The -former group has doubtless been derived from the explosion of such rocks -as the diabase-sheets of the district. The felsitic tuffs have not been -observed to contain any fragments of the microcrystalline quartz-porphyries -of St. David's. They have been derived from true fine-grained felsites or -rhyolites. There are various intermediate varieties of tuff, due to the -mingling in various proportions of the two kinds of debris.</p> - -<p>3. They are marked by the presence of some characteristic features -of the volcanic vents of later Palæozoic time, and in particular by presenting -the following peculiarities: (<i>a</i>) lapilli of a minutely-cellular pumice with -spherical cells; (<i>b</i>) lapilli with well-developed flow-structure; (<i>c</i>) lapilli -consisting of a pale green serpentinous substance resembling altered -palagonite and probably originally glass; (<i>d</i>) lapilli derived from the destruction -of older tuffs; and (<i>e</i>) lapilli consisting of ejected crystals, especially -of felspars, sometimes entire, often broken.</p> - -<p>4. They frequently show that they have undergone metamorphism, by -the development of a pale greenish micaceous mineral between the lapilli, -the change advancing until the fine tuffs occasionally pass into fine silky -schists.</p> - -<p>In my study of the St. David's district, I was unable to observe -any evidence that the basic and siliceous tuffs characterize two distinct -periods of volcanicity. From the foregoing analyses it appears that some -of the oldest visible tuffs which are seen between Pen-maen-melyn and -Pen-y-foel contain only 48·11 per cent of silica; while a specimen from -Porth-lisky yielded 72·63 per cent of that ingredient. Specimens taken -even from adjacent beds show great differences in the percentage of silica, -as may be seen in the analyses Nos. III. and V.</p> - -<p>This alternation of basic and siliceous fragmental materials has its -parallel in the neighbouring eruptive rocks, some of which are olivine-diabases -containing only 45 per cent of silica, while others are highly -siliceous quartz-porphyries. But all the siliceous eruptive rocks, so far as I -have been able to discover, are intrusive, and belong, I believe, to a later -period than that of the volcanic group; in no single instance do they -appear to me to be true superficial lava-flows. On the other hand, the -basic eruptive rocks occur both as contemporaneous sheets and as intrusive -masses. The presence of both siliceous and basic lavas in the Cambrian -volcanic reservoirs, however, is proved by the character of the tuffs. It -<span class="pagenum" id="Page_152">- 152 -</span> -would appear from the evidence at present known, that while the basic -lavas were most abundant in the vents during the volcanic period -recorded by the rocks of St. David's, furnishing the material for most of -the fragmental eruptions, and issuing in occasional superficial streams of -molten rock, the siliceous lavas did not flow forth at the surface, though -their debris was copiously discharged in the form of dust and lapilli.</p> - -<p>The rise of both basic and acid lavas at different periods in the same or -adjoining vents, so familiar in recent volcanic phenomena, thus appears to -have also characterized some of the oldest examples of volcanic action. An -interesting parallel may be traced between the succession of events at St. -David's and that which occurred in the volcanic group of the Lower Old -Red Sandstone of the Pentland Hills, near Edinburgh, of which a detailed -account will be given in <a href="#CHAPTER_XX">Chapter xx.</a> of this volume. It is also worthy -of remark that in the latest of the volcanic episodes in British geology a -remarkable similarity to the St. David's volcanic group may be observed. -Some of the older Tertiary agglomerates are full of pieces of acid rocks -(felsites, rhyolites or granophyres), while the lavas poured out at the surface -were mainly basalts.</p> - -<p>In the volcanic group of St. David's the tuffs contain evidence that -ordinary sedimentation was not entirely interrupted by the volcanic discharges. -Thus, in the Allan valley, west from the Cathedral, one of the -schistose tuffs is full of well-rounded pebbles of white quartz. Occasional -shaly bands indicate the deposit of mud with the tuffs.</p> - -<p>Excluding the granites and porphyries (which are described at <a href="#Page_155">p. 155</a>), -two kinds of eruptive rocks are associated with the volcanic group. One -of these is certainly intrusive and of late date, viz. dykes and veins of -diabase, to be afterwards referred to. The other kind occurs in long parallel -sheets, some of which, if not all, are true contemporaneous lava-streams, -erupted at intervals during the accumulation of the volcanic group. They -form prominent crags to the west of St. David's, such as Clegyr Foig, Rhosson, -and the rocky ground rising from the eastern shores of Ramsey Sound. -Their dip and strike coincide with those of the tuffs above and below them. -It is possible that some of these sheets may be intrusive sills intercalated -along the bedding of the tuffs; and in one or two cases I have observed -indications of what, on further and more careful exploration, may prove to -be disruption across the bedding.</p> - -<p>But it is the interbedded sheets that possess the chief interest as -superficial lava-streams of such venerable antiquity. They present many -of the ordinary features of true lava-flows. In particular a slaggy structure -may be detected at the bottom of a sheet, the vesicles being here and there -lengthened in the direction of flow. Some of the sheets are in part -amygdaloidal. The alternation of these sheets with tuffs, evidently derived -from lavas of similar character, is another argument in favour of their -contemporaneous date. One of the best localities for studying these features -lies between Clegyr Foig and the coast, west of Rhosson.</p> - -<p>The eruptive rocks thicken towards the south-west, as if the main vents -<span class="pagenum" id="Page_153">- 153 -</span> -had lain in that direction. There are doubtless intrusive as well as -contemporaneously interbedded masses in the rough ground between Pen-maen-melyn -and Treginnis. To separate these out would be a most -interesting and beautiful piece of mapping for any competent geologist in -possession of a good map on a sufficiently large scale.</p> - -<p>The interbedded lavas, so far as I have had an opportunity of studying -them, appear to present remarkable uniformity of petrographical characters. -Megascopically they are dull, fine-grained to compact, sparingly porphyritic, -ranging in colour from an epidote-green to dull blackish-green and dark -chocolate-brown. Some of them are finely porphyritic from the presence -of small glistening surfaces which present the colour and metallic lustre of -hæmatite and yield its characteristic streak. Obviously basic rocks, they -present, as I have said, a close external resemblance to many of the lavas of -the Lower Old Red Sandstone and Carboniferous districts of Scotland. -From their chemical composition and microscopic structure they may be -most appropriately ranged among the diabases. The analyses of two of -the most conspicuous diabases of this class in the district, those of Rhosson -(VII.) and Clegyr Foig (VIII.), by Mr. J. S. Grant Wilson, are shown in -the following table:—</p> - -<table summary="data"> -<tr> - <td class="bdl bdt bdb"></td> - <td class="bdl bdt bdb tdc">SiO<sub>2</sub></td> - <td class="bdl bdt bdb tdc">Al<sub>2</sub>O<sub>3</sub></td> - <td class="bdl bdt bdb tdc">Fe<sub>2</sub>O<sub>3</sub></td> - <td class="bdl bdt bdb tdc">FeO</td> - <td class="bdl bdt bdb tdc">MnO</td> - <td class="bdl bdt bdb tdc">CaO</td> - <td class="bdl bdt bdb tdc">MgO</td> - <td class="bdl bdt bdb tdc">K<sub>2</sub>O</td> - <td class="bdl bdt bdb tdc">Na<sub>2</sub>O</td> - <td class="bdl bdt bdb tdc">H<sub>2</sub>O and<br />Loss on<br />Ignition.</td> - <td class="bdl bdt bdb tdc">Insoluble<br />Residue.</td> - <td class="bdl bdt bdb tdc">Total.</td> - <td class="bdl bdt bdb bdr tdc">Specific<br />Gravity.</td> -</tr> -<tr> - <td class="bdl tdr">VII.</td> - <td class="bdl tdc">45·92</td> - <td class="bdl tdc">18·16</td> - <td class="bdl tdc">1·18</td> - <td class="bdl tdc">9·27</td> - <td class="bdl tdc">0·19</td> - <td class="bdl tdc">7·19</td> - <td class="bdl tdc">10·07</td> - <td class="bdl tdc">1·78</td> - <td class="bdl tdc">2·12</td> - <td class="bdl tdc">4·22</td> - <td class="bdl tdc">0·04</td> - <td class="bdl tdc">100·14</td> - <td class="bdl bdr tdc">2·96</td> -</tr> -<tr> - <td class="bdl bdb tdr">VIII.</td> - <td class="bdl bdb tdc">45·38</td> - <td class="bdl bdb tdc">16·62</td> - <td class="bdl bdb tdc">4·06</td> - <td class="bdl bdb tdc">8·63</td> - <td class="bdl bdb tdc">0·14</td> - <td class="bdl bdb tdc">8·19</td> - <td class="bdl bdb tdc">9·41</td> - <td class="bdl bdb tdc">0·71</td> - <td class="bdl bdb tdc">2·20</td> - <td class="bdl bdb tdc">4·34</td> - <td class="bdl bdb tdc">0·08</td> - <td class="bdl bdb tdc">99·76</td> - <td class="bdl bdb bdr tdc">2·99</td> -</tr> -</table> - -<p>The two rocks here analyzed, likewise that from the crag south of Castell -and that from the cliffs at the southern end of the promontory between -Ramsey Sound and Pen-y-foel, show under the microscope a general similarity -of composition and structure. They present a variable quantity of a base, -which under a ⅕ objective is resolved into ill-defined coalescent globulites -and fibre-like bodies, that remain dark when rotated between crossed -nicols. In some varieties, as in part of Rhosson Crag, the base is nearly -lost in the crowd of crystalline constituents; in others, as in the crag south -of Castell, it forms a large part of the whole mass, and may be seen in -distinct spaces free from any crystalline particles. Through this base are -diffused, in vast numbers, irregularly-shaped grains of augite, seldom showing -idiomorphic forms. These grains, or granules, may perhaps average -about 0·003 inch in diameter. Plagioclase is generally hardly to be recognized, -though here and there a crystal with characteristic twinning may be -detected in the base. Magnetite occurs abundantly—its minute octahedra, -with their peculiar colour and lustre, being apparent with reflected light on -the fresher specimens, though apt to be lost as diffused ferruginous blotches -in the more decomposed varieties. But perhaps the most remarkable ingredient -<span class="pagenum" id="Page_154">- 154 -</span> -is olivine. Red hæmatitic crystals are visible, even to the naked -eye, dispersed through the groundmass of the rocks. With a lens these -may be observed to be orthorhombic in form, and to be evidently pseudomorphs -after some mineral which has been converted chiefly into hæmatite. -Such red pseudomorphs are common in Carboniferous and Old Red Sandstone -lavas, where in some cases they appear to be after hornblende, and in -others after augite, but occasionally are suggestive of olivine, though with -no trace of the original substance of that mineral. In the lava associated -with the tuffs at the south end of the promontory between Ramsey Sound -and Pen-y-foel, however, certain large, well-developed pseudomorphs are -undoubtedly after olivine. They have the characteristic contour of that -mineral and its peculiar transverse curved and irregular fractures. The -average length of these pseudomorphs was found, from the measurement of -six examples, to be 0·023 inch, the largest being 0·034, and the smallest -0·014. Seen by transmitted light they present a structureless pale-green -material nearly inert in polarized light, round the borders and across -fissures in which an opaque substance has been developed, as serpentine -and magnetite have been grouped in the familiar alteration of olivine. The -opaque material is bright brick-red in reflected light, and is evidently -now chiefly oxidized into hæmatite. Every stage may be traced, from -orthorhombic forms with the incipient development of transverse fissures -filled with iron-oxide, to others of distorted shapes in which the ferruginous -matter occupies the whole, or nearly the whole, of the mould of the -original crystal.</p> - -<p>The rocks now described differ from the Palæozoic andesites or "porphyrites," -with which I am acquainted, in their more basic composition, in the -less abundance of their microscopic base, in the comparatively inconspicuous -development of their felspars of later consolidation, in the absence of large -porphyritic felspars of earlier consolidation, in the extraordinary prominence -of the granular augite, and in the presence of olivine. In composition and -structure they are essentially forms of olivine-diabase.</p> - -<div class="figleft" id="v1fig41" style="width: 161px;"> - <img src="images/v1fig41.png" width="161" height="156" alt="" /> - <div class="hanging2"><p><span class="smcap">Fig. 41.</span>—Section showing the interstratification - of tuff and conglomerate above Lower Mill, St. David's.</p></div> -</div> - -<p>Above the volcanic group of St. David's lies a band of quartz-conglomerate -which has been taken by Dr. Hicks -to mark the base of the Cambrian system. This -rock, though mainly composed of quartz and -quartzite, contains fragments of the underlying -volcanic rocks. But that it does not mark any -decided break in the sedimentation, much less -the violent unconformability and vast interval -of time which it has been erroneously supposed -to do, is well illustrated by the occurrence of -bands of tuff, as well as diffused volcanic dust, -in the conglomerate and also in the green and -red shales and sandstones which conformably -overlie it. An example of this intercalation of -volcanic material is represented in <a href="#v1fig41">Fig. 41</a>. On the left side vertical layers -<span class="pagenum" id="Page_155">- 155 -</span> -of fine reddish tuff (<i>a</i>) are succeeded by a band of quartz conglomerate -(<i>b</i>) of the usual character. Parallel to this conglomerate comes a band, -about six inches thick, of fine tuff (<i>c</i>), followed by ashy sandstone (<i>d</i>), which -graduates into conglomerate (<i>e</i>). No more complete evidence could be -desired of the perfect inosculation of the conglomerate with the volcanic -group. On the coast at Nun's Chapel similar evidence presents itself. The -conglomerate there contains some thin seams of tuff, and is intercalated -between a series of dull green agglomerates and tuffs and some greenish -shales and sandstones with layers of tuff.</p> - -<p>Not less striking is the evidence of the contemporaneous eruption of fine -volcanic dust in the overlying shales and sandstones.<a id="FNanchor_88" href="#Footnote_88" class="fnanchor">[88]</a> Some of the red -shales are full of this material, which here and there is gathered into the -thin seams or ribs of which the microscopic characters have already been -described. This diffused volcanic detritus marks, no doubt, the enfeebled -discharges of fine dust towards the close of the volcanic episode in the -Lower Cambrian period at St. David's. It would be difficult to find an -instance of a more perfect transition from a series of thoroughly volcanic -masses into a series of ordinary mechanical sediments.</p> - -<div class="footnote"> - -<p><a id="Footnote_88" href="#FNanchor_88" class="label">[88]</a> These are a portion of Dr. Hicks' "Caerfai group" in the Lower Cambrian series. They -have yielded Lower Cambrian fossils.</p> - -</div> - -<p>2. <i>Intrusive Granite and Quartz-Porphyry.</i>—The heart of the volcanic -group is pierced by a mass of granite which also cuts the conglomerate and -overlying shales and sandstones on the east side. The age of this intrusive -boss cannot be more definitely fixed than by saying that it must be later -than the volcanic group. This rock has been the subject of a remarkable -amount of description, and has been dignified by being actually elevated into -a distinct Archæan "formation" composed of "highly crystalline gneissic -rocks," with "bands of limestone, hornblende, chlorite, and micaceous -schists."<a id="FNanchor_89" href="#Footnote_89" class="fnanchor">[89]</a> Into this somewhat dreary chapter of English geological literature -it is fortunately not necessary to enter here. I will only say that the -rock is unquestionably a granite, with no essential differences from many -other bosses regarding which there has been no controversy. It is a holocrystalline -rock with a thoroughly granitic texture, and composed of the -ordinary minerals of granite—quartz, orthoclase and plagioclase, among -which a green chloritic mineral, more or less plentiful, probably represents -original hornblende, biotite or augite. Sometimes the quartz and felspar in -the body of the rock show a micropegmatitic arrangement, and the same -structure occurs in veins that traverse it. This structure gives the rock -some resemblance to the Tertiary granites and granophyres of Ireland and -Scotland.</p> - -<div class="footnote"> - -<p><a id="Footnote_89" href="#FNanchor_89" class="label">[89]</a> See the papers cited on <a href="#Page_145">p. 145</a> and my discussion of the relations of this granite in <i>Quart. -Journ. Geol. Soc.</i> vol. xxxix.; also Prof. Lloyd Morgan, <i>op. cit.</i> vol. xlvi. (1890).</p> - -</div> - -<p>This granite has undergone a good deal of decomposition, for its felspars -are turbid, and its original ferro-magnesian constituents are always -replaced by green chloritic aggregates, while epidote is also present. The -rock tends to become finer in grain towards the margin, and then sometimes -<span class="pagenum" id="Page_156">- 156 -</span> -assumes a more decidedly pegmatitic structure, like graphic granite. -At the northern end of the granite ridge, a gradation can be traced from the -ordinary texture through increasingly fine-grained varieties until we pass -into a microcrystalline spherulitic porphyry. After a careful examination -of the ground I satisfied myself that the spherulitic quartz-porphyries, which -form a conspicuous feature in the geology of St. David's, are really offshoots -from this granitic core.<a id="FNanchor_90" href="#Footnote_90" class="fnanchor">[90]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_90" href="#FNanchor_90" class="label">[90]</a> These apophyses from the granite constitute the "Arvonian" formation of Dr. Hicks' -pre-Cambrian series of St. David's.</p> - -</div> - -<p>These spherulitic rocks have been fully described.<a id="FNanchor_91" href="#Footnote_91" class="fnanchor">[91]</a> They consist of a -base composed of a microcrystalline aggregate of quartz and orthoclase, -which is distributed between the spherulites. These have been developed in -remarkable beauty and perfection. While the microcrystalline structure is -everywhere recognizable, the spherulites occasionally disappear. But their -absence is merely local, and they may be found both in large dykes and -narrow veins. A further porphyritic structure is given to the rocks by the -presence in them of abundant quartz, which takes the form of conspicuous -rounded blebs or worn crystals sometimes distinctly dihexihedral, but with -somewhat blunted angles. Porphyritic plagioclase is also common. Flow-structure -is occasionally traceable. Some parts of these rocks where the -porphyritic elements are locally absent might be cursorily classed as felsites; -but they all possess a microcrystalline and not a felsitic base. They -cannot be confounded with the true felsites of which fragments occur in -the tuffs.</p> - -<div class="footnote"> - -<p><a id="Footnote_91" href="#FNanchor_91" class="label">[91]</a> See, for example, J. Davies, <i>Quart. Journ. Geol. Soc.</i> vol. xxiv. p. 164, xxxv. p. 203; also -the paper already referred to, <i>op. cit.</i> xxxix. p. 315; and Mr. Teall's <i>British Petrography</i>, p. 334.</p> - -</div> - -<p>In addition to the parallelism that may be traced between the earliest -Palæozoic agglomerates and those of the youngest volcanic series of -Britain, a close analogy may also be noticed between the acid intrusive rocks -of the two widely-separated periods. In both cases we have a granitic -core sending out apophyses which assume a spherulitic structure and traverse -earlier volcanic products of the district.</p> - -<p>These spherulitic quartz-porphyries of St. David's occur as bosses, dykes -(elvans) or veins, cutting through all horizons of the volcanic group, and in -one case apparently, if not actually, reaching the quartz conglomerate. One -of the best exposures of this intrusive character may be seen in the cliff -below Nun's Chapel, where the elvan runs along the face of the cliff through -the uppermost zone of the volcanic group, cutting the strata somewhat -irregularly. Apparently in connection with this dyke, a network of -intrusions of decomposed quartz-porphyry may be observed in the shales -along the face of the cliff immediately below Nun's Chapel. On the -whole, the intruded material has forced its way along the bedding-planes -of the shales, but has also broken across them, sending out finger-like -branches.</p> - -<p>3. <i>Diabase Dykes and Sills.</i>—The latest rocks of the St. David's district -are dykes and intrusive sheets of diabase, which traverse all the other -<span class="pagenum" id="Page_157">- 157 -</span> -formations. The dykes are specially abundant in the granite. One or two -may be detected in almost every artificial opening which has been made in -that rock; while on the coast-section they are here and there profusely -abundant. They are likewise frequent in the quartz-porphyries, and occur -also in the volcanic group as well as in the sandstones and shales above the -conglomerate, but become fewer in number as they recede from the granite -centre.<a id="FNanchor_92" href="#Footnote_92" class="fnanchor">[92]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_92" href="#FNanchor_92" class="label">[92]</a> The occurrence of these dykes is paralleled by that of the similar intrusions in the quartz-felsite -of Llyn Padarn to be afterwards described.</p> - -</div> - -<p>In external characters, the rock composing these dykes and sheets may -be described as usually a dull dirty-green or yellowish-brown mass, to -which the old name of "wacke" might appropriately be given. It exhibits -the texture and mode of weathering of the more distinctly crystalline -members of the basalt family. It is occasionally amygdaloidal or cellular, -the kernels or cavities being arranged parallel with the sides of the -dyke. Here and there a rudely prismatic structure extends between the -walls.</p> - -<p>The microscopic structure of this rock has been described by Professor -Judd, Mr. Davies and Mr. Tawney. It is a diabase, but more allied in -structure to true basalt than the olivine-diabase of the volcanic group. It -especially differs from the older rock in the abundance and freshness of its -felspars, in the comparative scarcity of its augite, and in the absence of -olivine. The magnesian silicates are very generally replaced by green -decomposition-products diffused through the mass. An occasional crystal -of hornblende, recognizable by its cleavages and dichroism, may be detected. -Some of the diabase dykes present excellent examples of flow-structure. -A beautiful instance occurs in a dyke that cuts the shales, in a small -cove to the east of Nun's Chapel. The shale and eruptive rock are in -contact; and the small acicular prisms of felspar, besides ranging themselves -in line parallel to the side of the dyke, stream round the larger -felspar crystals.</p> - -<p>Some of the dykes or veins are only three inches broad. They send -out fingers, and sometimes break abruptly across from one line to another. -They appear generally to have followed the lines of joint in the granite, as -Mr. Tawney has observed;<a id="FNanchor_93" href="#Footnote_93" class="fnanchor">[93]</a> consequently they must be posterior to the -development of the system of jointing in that rock.</p> - -<div class="footnote"> - -<p><a id="Footnote_93" href="#FNanchor_93" class="label">[93]</a> <i>Proc. Nat. Hist. Soc. Bristol</i>, vol. ii. part ii. (1879), p. 115.</p> - -</div> - -<p>Besides the abundant dykes, there has been a more limited extrusion of -the same material in sheets parallel (or approximately so) to the bedding of -the sandstones and shales. These sheets are well displayed at St. John's -Point, where evidence of their being intrusive, and not truly bedded, may -be seen along the fine cliffs which have been cut by the waves on this part -of the coast-line.</p> - -<p>The sedimentary series which overlies the volcanic group of St. David's, -and contains the fossils of the lower part of the Cambrian system, gradually -loses all trace of volcanic material, as its members are followed upward in -<span class="pagenum" id="Page_158">- 158 -</span> -stratigraphical order.<a id="FNanchor_94" href="#Footnote_94" class="fnanchor">[94]</a> We thus learn that the eruptions of this district -came to an end in an early part of the Cambrian period. But as we shall -see in the following pages, volcanic activity was subsequently renewed at no -great distance in the next or Silurian period.</p> - -<div class="footnote"> - -<p><a id="Footnote_94" href="#FNanchor_94" class="label">[94]</a> Dr. Hicks has noted the occurrence of "volcanic tuff" in the Lower Lingula Flags of Porth-y-Rhaw, -a little to the east of St. David's (<i>Quart. Journ. Geol. Soc.</i> vol. xx. 1864, p. 240). This -intercalation is marked as a "dyke" in the MS. notes of Sir A. C. Ramsay on a copy of the -Geological Survey map of the district.</p> - -</div> - -<hr class="chap x-ebookmaker-drop" /> - - -<div class="chapter"> -<p><span class="pagenum" id="Page_159">- 159 -</span></p> - -<h2 class="nobreak" id="CHAPTER_XI">CHAPTER XI<br /> - -<span class="smaller">THE CAMBRIAN VOLCANOES OF NORTH WALES, THE MALVERN HILLS -AND WARWICKSHIRE</span></h2> -</div> - - -<h3>NORTH WALES</h3> - -<p>The Cambrian volcanic rocks in the northern part of the Welsh Principality -have their main development in Caernarvonshire. Southwards from that tract, -though the Lower Cambrian strata form a vast pile of sedimentary material -in the Harlech anticline, which is estimated by the Geological Survey to be -from 6000 to 7000 feet thick, they have yielded no trace of any contemporaneous -volcanic rocks.<a id="FNanchor_95" href="#Footnote_95" class="fnanchor">[95]</a> The purple slates that rise along the centre of -the anticline dip below the grits and conglomerates on either side without -disclosing a glimpse of the base of the system. This enormous accumulation -of sedimentary deposits seems to diminish in thickness as it is traced -northwards, for towards the Menai Strait it does not reach more than a -fourth part of the depth which it is said to display in the Harlech anticline.<a id="FNanchor_96" href="#Footnote_96" class="fnanchor">[96]</a> -In the Pass of Llanberis the series of grits that overlies the purple slates is -estimated to be about 1300 feet thick.<a id="FNanchor_97" href="#Footnote_97" class="fnanchor">[97]</a> This gradual thinning away of -the Cambrian series towards the north was, in the opinion of Sir Andrew -Ramsay, accompanied by an increasing metamorphism of the lower portions -of the system. In his view, the long ridge of quartz-porphyry which -crosses the lower end of Llyn Padarn represents the result of the extreme -alteration of the stratified rocks. He believed that he could trace an -insensible passage from the slates, grits and conglomerates into the -porphyry, and he was led to the "conviction that the solid porphyry itself -is nothing but the result of the alteration of the stratified masses carried a -stage further than the stage of porcellanite, into the condition of that -kind of absolute fusion that in many other regions seems to have resulted -<span class="pagenum" id="Page_160">- 160 -</span> -in the formation of granites, syenites and other rocks, commonly called -intrusive."<a id="FNanchor_98" href="#Footnote_98" class="fnanchor">[98]</a> Certain structural lines in the quartz-porphyry he looked -upon as indicating "traces of stratification in a rock, the original felspathic -and quartzose material of which has been metamorphosed into true -porphyry."<a id="FNanchor_99" href="#Footnote_99" class="fnanchor">[99]</a> In conformity with these ideas, the remarkable felspathic -strata which lie nearest the porphyry were regarded as metamorphosed -Cambrian rocks, and where similar rocks reappear over a large area near -Bangor they were coloured on the map with the same tint and lettering as -were used for the so-called "altered Cambrian" of Anglesey.</p> - -<div class="footnote"> - -<p><a id="Footnote_95" href="#FNanchor_95" class="label">[95]</a> <i>Mem. Geol. Surv.</i> vol. iii. 2nd edit. "Geology of North Wales," p. 21. It is possible that -this thickness has been somewhat overestimated. Dr. Hicks (<i>Geol. Mag.</i> 1880, p. 519) has referred -to certain "highly felsitic rocks, for the most part a metamorphic series of schists, -alternating with harder felsitic bands, probably originally felsitic ashes," lying at the bottom -of the whole pile, and he has claimed them as pre-Cambrian. But I have not found any -evidence of such rocks, nor any trace of igneous materials save dykes and sills, acid and basic, -such as are indicated on the Survey map.</p> - -<p><a id="Footnote_96" href="#FNanchor_96" class="label">[96]</a> <i>Ibid.</i> p. 24.</p> - -<p><a id="Footnote_97" href="#FNanchor_97" class="label">[97]</a> <i>Ibid.</i> p. 173.</p> - -<p><a id="Footnote_98" href="#FNanchor_98" class="label">[98]</a> <i>Mem. Geol. Surv.</i> vol. iii. 2nd edit. p. 173.</p> - -<p><a id="Footnote_99" href="#FNanchor_99" class="label">[99]</a> <i>Ibid.</i> p. 174.</p> - -</div> - -<p>No one who has examined this Caernarvonshire ground can have failed -to find the sections which doubtless led my predecessor to form the convictions -to which he gave expression in the passages I have just quoted. -It is easy to see how these sections, wherein it is certainly difficult to draw -a sharp line between the igneous rock and the clastic materials derived -from it, would be welcomed as appearing to offer confirmation of the ideas -concerning metamorphism which were then in vogue. There cannot, however, -be any doubt that my friend was mistaken in his interpretation of the -structure of that part of the country. It is to me a subject of keen regret -that in his later years, when the subject was revived, he was no longer able -to re-examine this ground himself, for no one would have confessed more -frankly his error, and done more ample justice to those who, coming after -him, have been able in some parts to correct his work.</p> - -<p>The quartz-porphyry, felsite or rhyolite of Llyn Padarn, as well as that -of Llandeiniolen, is not a metamorphic but an eruptive rock, as has been -demonstrated by Professors Hughes and Bonney. There is no true passage -of the sedimentary rocks into it; on the contrary, the conglomerates which -abut against it are in great part made out of its fragments, so that it was -already in existence before these Cambrian strata were deposited upon it. -These conclusions must be regarded as wholly indisputable. But most of -the critics of the work of the Geological Survey have proceeded to certain -further deductions. They have maintained that the presence of fragments -of the porphyry in the overlying conglomerate marks an unconformability -between the two rocks, that the conglomerate shows the base of the Cambrian -system, and that the porphyry is therefore pre-Cambrian.</p> - -<p>These assertions and inferences do not seem to me to be warranted. -They have, in my judgment, been disproved by Mr. Blake,<a id="FNanchor_100" href="#Footnote_100" class="fnanchor">[100]</a> who shows that -there is no break in the Cambrian series, that the various porphyries and -their accompaniments are parts of that series, and that there is no certain -proof of the existence of any pre-Cambrian rocks in the whole district.<a id="FNanchor_101" href="#Footnote_101" class="fnanchor">[101]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_100" href="#FNanchor_100" class="label">[100]</a> In an excellent memoir read before the Geological Society in 1888, with the main conclusions -of which I agree.</p> - -<p><a id="Footnote_101" href="#FNanchor_101" class="label">[101]</a> <i>Quart. Journ. Geol. Soc.</i> vol. xliv. p. 271. For subsequent papers by Mr. Blake, see <i>op. cit.</i> -vols. xlviii. (1892) p. 243, xlix. (1893) p. 441.</p> - -</div> - -<p>That the igneous rocks of the Llyn Padarn area mark a volcanic period -has been recognized by most writers since Professor Bonney pointed out the -flow-structure of the quartz-porphyry, and other proofs of active volcanic -<span class="pagenum" id="Page_161">- 161 -</span> -eruptions have been traced by him, as well as by Professor Hughes and Mr. -Blake, in the stratified rocks which stretch north-eastwards to Bangor. -The extent and persistence of these ancient volcanic phenomena, and their -probable connection with the remarkable northward attenuation of the Cambrian -sedimentary rocks, deserve special attention.</p> - -<p>It is generally agreed that the rocks variously termed quartz-porphyries, -felsites or rhyolites form the oldest members of this volcanic series.<a id="FNanchor_102" href="#Footnote_102" class="fnanchor">[102]</a> -They come to the surface in two long ridges, one running from Caernarvon to -near Bangor, the other from Llanllyfni to Ann's Chapel, at the mouth of Nant -Francon (Map. II.). Whether the materials of these two ridges are parts of -one originally continuous sheet or group of sheets, or, if different protrusions, -whether they belong to the same geological horizon, or whether, as Mr. -Blake believes, they are distinct masses, separated by a considerable thickness -of detrital material, cannot in the present state of our knowledge be -positively decided. It seems to me probable that they are connected -underground, as a continuous platform beneath the overlying pyroclastic -materials.</p> - -<div class="footnote"> - -<p><a id="Footnote_102" href="#FNanchor_102" class="label">[102]</a> Whether the granitic rock of Twt Hill, Caernarvon, is connected with the porphyry or -belongs to an older eruption is immaterial for my present purpose.</p> - -</div> - -<p>These acid rocks have been regarded by some observers as intrusive -sheets, by others as lava-streams that were poured out at the surface. If -account be taken simply of their petrographical characters, they find their -nearest analogies among the intrusive quartz-porphyries of older geological -periods. The presence of flow-structure in them has been thought to -indicate that they were superficial streams. But this structure may be -found in dykes and intrusive sheets as perfectly as in lava-flows, so that it -cannot by itself be taken as proof of a surface-discharge of lava. It must -be confessed that, both in the main mass of quartz-porphyry and in the -abundant fragments of it in the overlying conglomerates and breccias, there -is an absence of such scoriform portions as one would naturally look for in -a superficial lava-stream;<a id="FNanchor_103" href="#Footnote_103" class="fnanchor">[103]</a> while, on the other hand, the rock generally -presents the tolerably uniform flinty texture so familiar in intrusive sheets -of similar material.<a id="FNanchor_104" href="#Footnote_104" class="fnanchor">[104]</a> My own impression is that these igneous masses were -probably erupted to the surface as long banks which rose above the waves; -that they were thus exposed to prolonged subærial and marine denudation; -that by this means any upper more cellular portions of the lava which may -have existed were broken up and pounded down into detritus, and thus that -what is now visible is a part of the eruptive rock which originally lay -at some depth within its body. This view is confirmed by a study of other -lavas which are found on different platforms in the detrital deposits that -overlie the Llyn Padarn quartz-porphyry.</p> - -<div class="footnote"> - -<p><a id="Footnote_103" href="#FNanchor_103" class="label">[103]</a> But the Llyn Padarn rock, like many acid lavas, may never have possessed a vesicular -structure in any portion of its mass. The sheets of felsite which occur among the overlying tuffs -are not cellular, but present the closest resemblance to the main mass below.</p> - -<p><a id="Footnote_104" href="#FNanchor_104" class="label">[104]</a> Mr. Blake brought forward the evidence of a section on the north or under side of the -Llyn Padarn ridge to show that the rock has there been intruded into the Cambrian strata -(<i>Quart. Journ. Geol. Soc.</i> vol. xliv. (1888), p. 283). But the rock supposed by him to be altered -slate has been shown to be intrusive "greenstone" (Miss Raisin, <i>op. cit.</i> vol. xlvii. (1891), p. 336).</p> - -</div> - -<p><span class="pagenum" id="Page_162">- 162 -</span></p> - -<p>That the material of each of the two main ridges is the result of more -than one eruption has been inferred from the supposed intercalation of -bands of slate and of breccia in the rock.<a id="FNanchor_105" href="#Footnote_105" class="fnanchor">[105]</a> Considerable lithological -differences may be detected in each mass, but they are not greater than may -be observed in single sills and bosses. In some parts of the Llyn Padarn -porphyry a distinct nodular structure appears which shades off into bands -and lenticular streaks, reminding one of the characters of some of the Bala -rhyolites. Other portions are markedly brecciated, the separated fragments -being surrounded in a matrix of the rock, which shows flow-structure -sweeping past them. On Moel Gronw angular fragments of a dark pinkish -tint are scattered through the general mass. Again, some parts are -crowded with quartz-grains, while others are comparatively free of these, -and occasionally a spherulitic structure has been observed.<a id="FNanchor_106" href="#Footnote_106" class="fnanchor">[106]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_105" href="#FNanchor_105" class="label">[105]</a> See for example, Prof. Bonney, <i>Quart. Journ. Geol. Soc.</i> vol. xxxv. (1879), p. 312; Mr. Blake, -<i>op. cit.</i> vol. xliv. (1888), pp. 277, 287. But some at least of the supposed "slates," as stated -in a previous footnote, have been since shown to be dykes.</p> - -<p><a id="Footnote_106" href="#FNanchor_106" class="label">[106]</a> Mr. Blake, <i>ibid.</i> p. 277.</p> - -</div> - -<p>The microscopic structure of this ancient eruptive rock has been studied -by Professor Bonney, who found that the general type was a compact dull -grey felsite, with porphyritic crystals of felspar and grains of quartz, closely -resembling some modern rhyolites. Though unable to detect any actual glass -in the base, he had no doubt that the rock was originally vitreous, and he -found abundant and fresh examples of the most perfect flow-structure.<a id="FNanchor_107" href="#Footnote_107" class="fnanchor">[107]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_107" href="#FNanchor_107" class="label">[107]</a> <i>Op. cit.</i> vol. xxxv. p. 312.</p> - -</div> - -<div class="figright" id="v1fig42" style="width: 211px;"> - <img src="images/v1fig42.png" width="211" height="214" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 42.</span>—Basic dyke traversing quartz-porphyry - and converted into a kind of slate by cleavage. West side of Llyn - Padarn.<br /> - - <i>p</i> <i>p</i>, quartz-porphyry; <i>d</i> <i>d</i>, dyke and connected - veins.</div> -</div> - -<p>Reference may be made here to the remarkable influence of the intense -cleavage of the district upon this rock.<a id="FNanchor_108" href="#Footnote_108" class="fnanchor">[108]</a> Along its southern margin, where -it has been exposed to pressure from the south-east, the quartz-porphyry -has been so crushed that it passes here -and there into a fine unctuous slate or -almost a schist. Nowhere can this change -be more clearly seen than on the slopes -of Mynydd y Cilgwyn. The cleavage -planes strike about N. 40° E., with an -inclination to dip towards the N.W. -Within a space of a few yards a series -of specimens may be collected showing at -one end an ordinary or only slightly-sheared -quartz-porphyry with abundant -quartz-blebs, and at the other a fine -greenish sericitic slate or phyllite, wherein -the quartz has been almost entirely -crushed down. Lines of shearing may be -detected across the breadth of the porphyry -ridge, each of them coinciding with -the prevalent trend of the cleavage. Sometimes -also certain basic dykes, which traverse the porphyry in some numbers, -<span class="pagenum" id="Page_163">- 163 -</span> -have undergone considerable deformation from the same cause. Their thinner -portions are so well cleaved that they have been mistaken for included bands -of green slate (<a href="#v1fig42">Fig. 42</a>). But these cleaved branches may sometimes be -traced into a thicker and more solid dyke, whose uncrushed cores still -preserve the original character of the rock and prove it to be eruptive.</p> - -<div class="footnote"> - -<p><a id="Footnote_108" href="#FNanchor_108" class="label">[108]</a> The secondary planes due to cleavage must not be confounded with the original flow-structure.</p> - -</div> - -<div class="figright" id="v1fig43" style="width: 403px;"> - <img src="images/v1fig43.png" width="403" height="246" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 43.</span>—Section of well-cleaved tuff, grit and breccia passing up into rudely-cleaved conglomerate and - well-bedded cleaved fine conglomerate and grit. East side of Llyn Padarn.</div> -</div> - -<p>The rocks which succeed the porphyry in the Valley of Llanberis are of -great interest, for they contain abundant proof of contemporaneous volcanic -activity, and they show that, so far from there being any marked hiatus -here, there is evidence of the persistence of eruptions even into the time of -the Llanberis Slates.<a id="FNanchor_109" href="#Footnote_109" class="fnanchor">[109]</a> Considerable misapprehension has arisen from the -attempt to make one of the conglomerates the base of the Cambrian series, -and the real significance of the volcanic detrital strata in association with -it was consequently missed. The conglomerate does not lie on one definite -horizon. In truth, there are several zones of conglomerate, each with some -difference of composition, thickness or extent.<a id="FNanchor_110" href="#Footnote_110" class="fnanchor">[110]</a> These may be well studied -both on the south and the north side of the porphyry ridge at the lower -<span class="pagenum" id="Page_164">- 164 -</span> -end of Llyn Padarn. They are intercalated among fine tuffs, grits, -volcanic breccias and purple slates, sometimes full of fine ashy material. -On the south-east side of the ridge, where the rocks have suffered intense -cleavage, they assume a fissile unctuous character, and then resemble parts -of the cleaved Cambrian tuffs at St. David's. But on the north-west side, -where they have in large measure escaped the effects of the cleavage-movements, -their original structures are well preserved.</p> - -<div class="footnote"> - -<p><a id="Footnote_109" href="#FNanchor_109" class="label">[109]</a> The sections in the Vale of Llanberis on either side of Llyn Padarn have been again and -again described and fought over. Some of the papers are cited in the following pages, but it -would be impossible in this volume to find room for a full discussion of the differences of opinion. -What is stated in the text is the result of my own study of the rocks on the ground, coupled -with a careful consideration of the work of other observers.</p> - -<p><a id="Footnote_110" href="#FNanchor_110" class="label">[110]</a> I can find no evidence of unconformability beneath any of the conglomerates. The section -described by Professor Green, <i>Quart. Journ. Geol. Soc.</i> vol. xli. (1885), p. 74, merely shows the -difference between the effects of cleavage on the fine tuffs and the more massive resisting conglomerate -which overlies them. This section is represented in <a href="#v1fig43">Fig. 43</a>. At first sight the -conglomerate appears to be lying on the vertical edges of an older group of slates, but any one -acquainted with cleavage can trace this structure from the tuffs into the conglomerate and -resuming its course again in the finer sediments above. The whole series of deposits in the -section is continuous and conformable. The section on the slate railway has deceived Mr. -Blake as well as Professor Green (<i>Quart. Journ. Geol. Soc.</i> vol. xlix. (1893), p. 445). The correct -interpretation is given by Professor Bonney and Miss Raisin (<i>op. cit.</i> vol. l. p. 592).</p> - -</div> - -<p>One of the first features of these detrital deposits to arrest attention is -the amount and variety of the fragments of igneous rocks in them. Some -of the conglomerates, though enclosing pebbles of quartz, quartzite, granite -and other rocks not found <i>in situ</i> in the immediate district, are mainly composed -of the debris of the quartz-porphyry of the ridge. Indeed, this -latter material appears to have contributed a large proportion of the -detritus of which the general body of strata here is made up. But there -are to be noticed among the contents of the conglomerates and breccias -pieces of many volcanic rocks not to be found on the porphyry ridge. -Among these, besides felsites showing sometimes beautiful flow-structure -(rhyolites) and various quartz-porphyries, there occur abundant fragments -of less acid lavas (andesites) and pieces of older tuffs. Some of the fragmental -rocks are green in colour, probably from the abundance of fine -basic volcanic dust in them. Certain bands are full of large angular pieces -of shale, similar in character to the Cambrian slates, and doubtless due to -the disruption of pre-existing Cambrian strata by volcanic explosions. It -is clear that from vents in this neighbourhood there continued to be an -abundant discharge of dust and various andesitic and other lapilli, which, -falling on the sea-floor, mingled there with the ordinary mechanical sediment -that was being deposited at the time.<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 conglomerates or breccias, see Professor Bonney and Miss Raisin, -<i>Quart. Jour. Geol. Soc.</i> vol. l. (1894), p. 598.</p> - -</div> - -<div class="figcenter" id="v1fig44" style="width: 372px;"> - <img src="images/v1fig44.png" width="372" height="116" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 44.</span>—Section of Clegyr on the north-east side of Llyn Padarn, near the lower end.</div> -</div> - -<p>But we have evidence that, during the period when these showers of -volcanic detritus were thrown out, streams of lava, though on a greatly -diminished scale, continued to be poured forth. The hill of Clegyr (<a href="#v1fig44">Fig. 44</a>), -near the lower end of Llyn Padarn, on the north-east side, consists -mainly of cleaved tuffs (<i>t</i>) and slates with conglomerates (<i>c</i>), overlying the -quartz-porphyry (<i>p</i>). Near the summit a band of felsite is intercalated in -these rocks.</p> - -<p><span class="pagenum" id="Page_165">- 165 -</span></p> - -<p>Still more striking are the sections on the south-west side of the lake.<a id="FNanchor_112" href="#Footnote_112" class="fnanchor">[112]</a> -Starting from the porphyry of the ridge, we cross a zone of conglomerate -and grit largely composed of the debris of the porphyry, until we reach a -band of felsite or quartz-porphyry, which at its eastern end is about ten -feet thick, while it seems to increase in dimensions westwards.<a id="FNanchor_113" href="#Footnote_113" class="fnanchor">[113]</a> In the -centre the rock is dark purplish-red, exceedingly compact or flinty, -sprinkled with a variable proportion of quartz-blebs and felspar crystals. -Towards its southern or upper edge (for the rocks, though nearly vertical, -dip southwards) it has been cleaved into a variety of purple slate, and -would there at once be classed among the ordinary slates of the neighbourhood. -But the fissile character is merely a marginal structure which -the rock shares with the highly-cleaved tuffs that follow it. Traced westwards, -this bed is found to enclose a core of quartziferous porphyry, which, -though it has escaped from the most complete results of crushing, is nevertheless -cleaved along its margin as well as partially in its interior. It -would not be possible to distinguish parts of this intercalated less crushed -core from portions of the porphyry of the main ridge. The difference of -colour does not count for much, for even in this core the purple tint gives -place to greenish grey, and what in the centre at the east end is a solid -dark purplish-red felsite passes westward into a greenish slate, like that -already noticed on Mynydd y Cilgwyn.</p> - -<div class="footnote"> - -<p><a id="Footnote_112" href="#FNanchor_112" class="label">[112]</a> For various readings of these sections, compare Mr. Blake (<i>Quart. Jour. Geol. Soc.</i> vol. xlix. -(1893), p. 450) with Professor Bonney and Miss Raisin (<i>op. cit.</i> vol. l. (1894), p. 581).</p> - -<p><a id="Footnote_113" href="#FNanchor_113" class="label">[113]</a> See Professor Bonney and Miss Raisin, <i>op. cit.</i> p. 593 <i>et seq.</i></p> - -</div> - -<p>The microscopical examination of this rock shows it to be a true felsite -of the rhyolitic type, which in the central uncleaved part exhibits a wavy -flow-structure like that found in the quartz-porphyry of the ridge. So -intense has been the cleavage in its upper part that the original structure -of the rock is there effaced. The immediately overlying tuffs, which are -likewise so thoroughly cleaved that it is not easy to draw a sharp and -continuous line between them and the intercalated lava, precisely resemble -those found below the conglomerate on the opposite side of the lake. -They include bands of coarse volcanic breccia as well as fine compact -material, showing the varying intensity of the volcanic discharges. Their -included stones consist of various felsites, andesites and slates.<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> I see no reason to doubt that the less acid igneous fragments were ejected during the -closing phases of volcanic action, even though no such rocks have been found at the surface <i>in -situ</i>. We must remember how frequently mixtures of acid and basic materials are to be found -in the same continuous series of volcanic ejections and even in the same vent, of which illustration -will be given in subsequent pages. Nor should we forget what a mere fragment of a -volcanic group is exposed at the surface in the Llanberis district. See Professor Bonney and -Miss Raisin, <i>op. cit.</i> p. 596, <i>footnote</i>.</p> - -</div> - -<p>The thin sheet of interstratified quartz-porphyry here described is not -the only one to be found in the section. Others thinner and more intensely -cleaved lie among the tuffs higher up. They have been sheared into mere -pale unctous slates, but the remains of their quartz-blebs may still be -detected in them.</p> - -<p>The tuffs, with their interstratified bands of porphyry, become more -<span class="pagenum" id="Page_166">- 166 -</span> -and more mingled with ordinary argillaceous and sandy sediment as they -are followed in upward succession. Among them occur bands of grit and -fine conglomerate containing pebbles of porphyry and pieces of slate. Some -of these grits are mainly composed of white felspar, felsite and clear grains -of quartz, evidently derived from the disintegration of a rock like the -porphyry of the main ridge. As the ordinary sediment of the Llanberis -group sets in, the tuffs are restricted to thinner and more widely-separated -bands. Some thin layers of felspathic breccia, seen among the slates close -to the Glyn Peris Hotel, probably mark the last discharges of the slowly-expiring -vents of this region. Here, as at St. David's, from the most -ancient of our volcanic records, striking evidence is furnished of the gradual -extinction of volcanic action. Through many hundreds of feet of strata -which now supervene, representing the closing ages of the Cambrian and -the earlier ages of the Silurian period, no trace of volcanic material has -been found in this district until we reach the Bala lavas and agglomerates -of Snowdon and the Pass of Llanberis.</p> - -<p>In the neighbourhood of Bangor another area of similar rocks wraps -round the northern end of the western porphyry ridge. The Geological -Survey map, in conformity with the ideas that governed its representation -of the older rocks of Anglesey and Caernarvon, colours -these as altered Cambrian. That this error should have been made, -or, when made, should not have been speedily corrected, is all the more -surprising when we consider the thorough mastery which the surveyors had -acquired of the aspects and the interpretation of ancient volcanic rocks in -Wales, and when, moreover, we remember that as far back as 1843, long -before the Survey of Caernarvonshire was published, Sedgwick had pointed -out the true volcanic nature of the rocks. That great pioneer recognized -the presence of "trappean conglomerates" and "trappean shales (Schaalstein)" -among these deposits at Bangor; but he could not separate them -from the Cambrian series of the rest of Wales.<a id="FNanchor_115" href="#Footnote_115" class="fnanchor">[115]</a> And in his section he -represents them as undulating towards the east and passing under the -great mass of the Caernarvonshire slates and porphyries.</p> - -<div class="footnote"> - -<p><a id="Footnote_115" href="#FNanchor_115" class="label">[115]</a> <i>Proc. Geol. Soc.</i> vol. iv. p. 212; <i>Quart. Journ. Geol. Soc.</i> vol. iii. (1847), p. 136.</p> - -</div> - -<p>This interpretation, which I believe to be essentially accurate, was -modified by Professor Hughes, who, fixing on a conglomerate as the base of -the Cambrian system, regarded all the rocks below it, or what he termed -his "Bangor group," as pre-Cambrian.<a id="FNanchor_116" href="#Footnote_116" class="fnanchor">[116]</a> He has been followed in this view -by subsequent writers;<a id="FNanchor_117" href="#Footnote_117" class="fnanchor">[117]</a> but Mr. Blake has argued that here, as in the -Llanberis district, there is no evidence to separate the volcanic detrital -deposits above the porphyry from the Cambrian system.<a id="FNanchor_118" href="#Footnote_118" class="fnanchor">[118]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_116" href="#FNanchor_116" class="label">[116]</a> <i>Quart. Journ. Geol. Soc.</i> vol. xxxiv. (1878), p. 137.</p> - -<p><a id="Footnote_117" href="#FNanchor_117" class="label">[117]</a> Prof. Bonney, <i>op. cit.</i> vol. xxxv. (1879), p. 316; Dr. Hicks, <i>ibid.</i> p. 296.</p> - -<p><a id="Footnote_118" href="#FNanchor_118" class="label">[118]</a> <i>Op. cit.</i> vol. xliv. (1888), p. 278.</p> - -</div> - -<p>A little southward from Bangor the quartz-porphyry is overlain by a -most interesting group of fragmental rocks, the "Bangor group" of Professor -Hughes. Largely of volcanic origin, they must be some hundreds of -<span class="pagenum" id="Page_167">- 167 -</span> -feet thick, and pass under the dark shales and grits of the Lower Silurian -(Arenig) series. Some of the most persistent bands among them are conglomerates, -which differ from each other in composition, but most of which -consist largely of fragments of various igneous rocks. Some of the coarser -masses might be termed agglomerates, for they show little or no trace of -bedding, and are essentially made up of blocks of volcanic material. There -are abundant beds of grit, sometimes pebbly or finely conglomeratic, alternating -with tuffs and with bands of more ordinary sediment. Courses of -purple shale and sandstone, green shale and dark grey sandy shale occasionally -occur to mark pauses in the volcanic explosions. Perhaps the most -striking feature in the pyroclastic materials is the great abundance of very -fine compact pale tuffs (hälleflintas of some writers), sometimes thinly -laminated, sometimes occurring in ribbon-like bands, each of which presents -internally a close-grained, almost felsitic or flinty texture.<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> The occurrence of flinty or cherty deposits, in association with volcanic rocks of Lower -Silurian age, is well established in Britain, and will be more particularly referred to in the -sequel.</p> - -</div> - -<p>A cursory examination of the contents of the conglomerates, breccias -and grits shows them to consist largely of different felsites, with fragments -of more basic lavas. Some of these might obviously have been derived -from the rock of the porphyry ridge, but, as at Llyn Padarn, there is a far -greater variety of material than can be found in that ridge. Some of the -fragments show perfect flow-structure. Professor Bonney has described the -microscopic characters of some of these fragments, and has especially -remarked upon their glassy character. Among the slides prepared from -specimens collected by myself, besides the abundant fragments of felsite -(rhyolite), there are also numerous pieces of different andesitic lavas and fine -tuffs, as well as grains of quartz and felspar, and sometimes a good deal of -granular iron-ore.</p> - -<p>That a large proportion of the material of the so-called "Bangor beds" -was directly derived from volcanic explosions can hardly be doubted. -There appears to have been a prolonged succession of eruptions, varying in -intensity, and somewhat also in the nature as well as in the relative fineness -of the material discharged. On the one hand, coarse massive agglomerates -were probably accumulated not far from the active vents, as the result -of more violent or transient explosions; on the other hand, exceedingly fine -and well-stratified tuffs, which attain a great thickness, serve to indicate -a phase of eruptivity marked by the long-continued discharge of fine -volcanic dust. Ordinary sediment was doubtless drifted over the sea-bottom -in this district during the volcanic episode, but the comparative -infrequence of distinct interstratifications of shale or sandstone may be taken -to imply that as a rule the pauses between the eruptions were not long -enough to allow any considerable accumulation of sand or mud to take -place.</p> - -<p>No satisfactory proof has yet been obtained of any interstratified lavas -among the tuffs of the Bangor district. Some rocks, indeed, can be seen -<span class="pagenum" id="Page_168">- 168 -</span> -on the road between the George Hotel and Hendrewen, which, if there -were better exposures, might possibly furnish the required proof; but at -present little can be made of them, for their relations to the surrounding -rocks are everywhere concealed.</p> - -<p>From what I have now adduced, it is obvious that while both felsitic -and andesitic lavas existed within the volcanic foci, and were ejected in -fragments to form the tuffs and breccias, the lavas poured out at the -surface during the Cambrian period in Caernarvonshire were mainly, if not -entirely, felsites (rhyolites) in which the chief porphyritic constituent was -quartz. These lavas thus stand entirely by themselves in the volcanic -history of Wales. Though felsites of various types were afterwards poured -out, nothing of the same quartziferous kind, so far as we yet know, ever -again appeared. Further south, in Merionethshire, as will be shown in -<a href="#CHAPTER_XII">Chapter xii.</a>, the Cambrian volcanic eruptions appear to have been on -the whole less acid, and to have begun with the outpouring of andesitic -lavas.</p> - -<p>I have now to consider the relation of the volcanic group of Bangor -to the strata which overlie it. The geological horizon of these strata is -not, perhaps, very definitely fixed. It may be Arenig, possibly even older. -But for my present purpose it will be sufficient to consider the strata in -question as lying at the bottom of the Lower Silurian series. Professors -Hughes and Bonney have taken as their base a marked but impersistent -band of conglomerate. Mr. Blake, however, has more recently shown -that, as this band is succeeded by tuffs like those below it, it cannot be -claimed as marking the upper limit of the volcanic group. He therefore -classes it in that group and traces what he thinks is an overlap or -unconformability at the bottom of the Lower Silurian strata to the east. -Mr. B. N. Peach, who accompanied me in an examination of this ground, -agreed with me in confirming Mr. Blake's observation as to the position -of the conglomerate, which is undoubtedly overlain by the same flinty -felsitic tuffs as are found below it. But we were unable to trace any -unconformability. According to the numerous observations which we -made, there does not seem to be any discordance in strike or dip between -the flinty tuffs and the overlying shales and grits. The two groups of -rock appeared to us to be conformable and to pass into each other, as at -Llyn Padarn.<a id="FNanchor_120" href="#Footnote_120" class="fnanchor">[120]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_120" href="#FNanchor_120" class="label">[120]</a> See Mr. Blake on this point, <i>Quart. Journ. Geol. Soc.</i> vol. xlviii. (1892), p. 252, <i>note</i>. I -retain the opinion expressed above.</p> - -</div> - -<p>An unconformable junction here would, in some respects, have been -welcome, for it would at once have accounted for the superposition of -Lower Silurian strata directly upon the Cambrian volcanic series, and for -the disappearance of the Llanberis slates and grits which form so conspicuous -a feature above the tuffs and conglomerates at Llyn Padarn. In the -absence of such a structure we must accept the order of succession as -apparently unbroken, and rely on some such explanation as was proposed by -Sir Andrew Ramsay to account for the overlap of the Arenig rocks on -<span class="pagenum" id="Page_169">- 169 -</span> -everything older than themselves as they are traced northwards.<a id="FNanchor_121" href="#Footnote_121" class="fnanchor">[121]</a> But -this explanation will not entirely remove the difficulties of the case. The -inosculation of the volcanic group of Bangor with the base of the Lower -Silurian series cannot be accounted for by any such overlap; it seems only -explicable on the supposition that the volcanic activity, which ceased in the -Llyn Padarn district about the time that the Llanberis Slates were deposited, -was continued in the Bangor area until Arenig time, or was then renewed. -The thick volcanic group of Bangor would thus be the stratigraphical -equivalent not only of the thin volcanic group of Llyn Padarn, but of the -overlying mass of strata up to the Arenig rocks. In confirmation of this -view, I shall show in a later chapter that volcanic action seems to have -been prolonged in Anglesey to a still later geological period, that it -appeared during the deposition of the Arenig strata, and that it attained a -great development throughout the time of the Bala group. That a series -of volcanic rocks, with associated cherty strata, may be the stratigraphical -equivalent of a great thickness of ordinary sediments in other districts -will be dwelt upon in the description of the Lower Silurian volcanic geology -of the Southern Uplands of Scotland.<a id="FNanchor_122" href="#Footnote_122" class="fnanchor">[122]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_121" href="#FNanchor_121" class="label">[121]</a> <i>Mem. Geol. Surv.</i> vol. iii. 2nd edit. p. 252.</p> - -<p><a id="Footnote_122" href="#FNanchor_122" class="label">[122]</a> A group of cherts and mudstones not more than 60 or 70 feet thick appear in that region -to be stratigraphically equivalent to the great depth of sedimentary material which elsewhere -constitutes the Upper Arenig and Lower and Middle Llandeilo formations. See <i>Annual Report of -the Geological Survey for 1895</i>, p. 27 of reprint.</p> - -</div> - -<p>In the areas of North Wales which have now been described, volcanic -action appears to have begun and ended within the limits of the Cambrian -period. Southwards, in the district of Dolgelly, another distinct and, in -some respects, very different development of Cambrian volcanic activity may -be recognized. In that district there is evidence that the volcanoes which -distinguished the earlier part of the Silurian period had already begun -their eruptions during Cambrian time. As their records, however, are -intimately linked with those of Silurian age, an account of them is deferred -to the next chapter.</p> - - -<h3>THE MALVERN HILLS</h3> - -<p>Although the chief surviving records of Cambrian volcanic action in -Britain are found in Wales, there is no evidence that the volcanoes of the -period lay chiefly in that region. It is certainly a suggestive fact that, in -the few districts where Cambrian strata appear from under younger formations -in England, they are generally accompanied with igneous rocks, though -the age of the latter may be older or later than the Cambrian period. If -the oldest Palæozoic rocks could be uncovered over the English counties, a -more abundant development of volcanic materials might be laid bare than -is now to be seen in Wales.</p> - -<p>Taking, however, the extremely limited exposures of Cambrian strata, -we find two tracts that specially deserve attention. Reference has already -been made to the ancient eruptive rocks of the Malvern Hills, the -<span class="pagenum" id="Page_170">- 170 -</span> -antiquity of which is proved by the position of the Cambrian fossiliferous -strata that overlie them. But these strata themselves include certain -igneous rocks which point to a recrudescence of eruptive energy in a far -later geological period.</p> - -<p>Nearly half a century has passed away since John Phillips mentioned -the intercalation of igneous rocks in the series of strata which is -now classed as Upper Cambrian in the Malvern Hills. Since that date -hardly anything has been added to the information which he collected. -The existence of a group of rocks of such high antiquity, asserted to be of -truly volcanic origin, and the precise horizon of which could be fixed by the -stratigraphical aid of organic remains, seems to have almost dropped out of -sight. Phillips noted the occurrence of what he regarded as truly volcanic -materials in the Hollybush Sandstone and the overlying dark (Lingula) -shales, and he clearly recognized that a wide difference of age separated -them from the far more ancient igneous rocks of the central core of the -chain. The Hollybush Sandstones were observed by him to have "often a -trappean aspect and to be traversed with felspathic dykes." He found -the overlying black shales to include "layers of trappean ashy sandstone." -But it was at the top of these shales that he obtained what he regarded as -the most conspicuous evidence of contemporaneous volcanic action. He -there encountered a zone of "interposed trap rocks" varying up to 50 feet -in thickness, consisting of "porphyritic and greenstone masses, which, -erupted from below, have flowed in limited streams over the surface of the -black shales." He recognized amygdaloidal and prismatic structures among -them.<a id="FNanchor_123" href="#Footnote_123" class="fnanchor">[123]</a> The position of these eruptive rocks is shown in <a href="#v1fig45">Fig. 45</a>.</p> - -<div class="footnote"> - -<p><a id="Footnote_123" href="#FNanchor_123" class="label">[123]</a> <i>Mem. Geol. Survey</i>, vol. ii. part i. pp. 52, 55; also Horizontal Sections of the Geol. Survey, -Sheet 13, No. 8, and Sheet 15. Reference to the igneous rocks of this area will be found in the -remarkable essay by De la Beche in vol. i. of the <i>Mem. Geol. Surv.</i> pp. 34, 38.</p> - -</div> - -<div class="figcenter" id="v1fig45" style="width: 505px;"> - <img src="images/v1fig45.png" width="505" height="55" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 45.</span>—Section across the Cambrian formations of the Malvern Hills, showing the position of the - intercalated igneous rocks (<i>p</i> <i>p</i>). After Phillips.</div> -</div> - -<p>These rocks were afterwards observed and described by Dr. Holl, who -found what he considered to be four true lava sheets interstratified in the -Hollybush Sandstones. He noted the intercalation of "numerous beds of -volcanic ash, grit and lava" in the black shales.<a id="FNanchor_124" href="#Footnote_124" class="fnanchor">[124]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_124" href="#FNanchor_124" class="label">[124]</a> <i>Quart. Journ. Geol. Soc.</i> vol. xxi. (1865), pp. 87-91.</p> - -</div> - -<p>So far as I am aware, no more recent account of these rocks has been -published. Their true stratigraphical and petrographical relations require -to be more precisely determined. If they are really contemporaneous lavas, -they point to volcanic eruptions at the time when the middle division of -the Cambrian system was being deposited. If, on the other hand, they -should prove to be intrusive, they would indicate probable volcanic activity -in this part of England at some time later than the middle of the Cambrian -period.</p> - -<p><span class="pagenum" id="Page_171">- 171 -</span></p> - - -<h3>WARWICKSHIRE</h3> - -<p>Some fifty miles to the north-east of the Malvern Hills, in the heart of -the rich Midlands, and among the coal-fields and the New Red Sandstone to -which these Midlands owe so much of their manufacturing industry and -their agricultural fertility, another little tract of Cambrian rocks rises to -the surface on the east side of the Warwickshire coal-field between -Nuneaton and Atherstone. So unobtrusively do these ancient strata take -their place among their younger peers, that their venerable antiquity was -for a long time undetected.<a id="FNanchor_125" href="#Footnote_125" class="fnanchor">[125]</a> They were actually regarded as parts of the -Carboniferous series, which at first sight they seem to underlie conformably. -It was not until 1882 that the mistake was corrected by Professor -Lapworth, who proved the rocks to be Cambrian by finding undoubted -Upper Cambrian fossils in them.<a id="FNanchor_126" href="#Footnote_126" class="fnanchor">[126]</a> Subsequent investigation enabled him -to work out the detailed sequence of these strata. He found that the -supposed "Millstone Grit" is a thick-bedded quartzite perhaps 1000 feet -in thickness, and resembling the well-known quartzites of the Lickey and -Caer Caradoc. The "Coal-shales" proved to be a series (possibly 2000 feet -thick) of purple, green, grey and black shales, which from their fossils -could be paralleled with the dark shales of the Upper Cambrian series of -the Malvern Hills.<a id="FNanchor_127" href="#Footnote_127" class="fnanchor">[127]</a> These shales are immediately overlain by the Coal-measures.</p> - -<div class="footnote"> - -<p><a id="Footnote_125" href="#FNanchor_125" class="label">[125]</a> Their antiquity was recognized by Yates as far back as 1825 (<i>Trans. Geol. Soc.</i> 2nd series, -vol. ii. p. 261). They had been confounded with "Millstone Grit" and "Coal-shale" by -Conybeare and Phillips, and this mistake was adopted on the maps and memoirs of the Geological -Survey.</p> - -<p><a id="Footnote_126" href="#FNanchor_126" class="label">[126]</a> <i>Geol. Mag.</i> (1882), p. 563.</p> - -<p><a id="Footnote_127" href="#FNanchor_127" class="label">[127]</a> <i>Op. cit.</i> (1886), p. 319.</p> - -</div> - -<p>For our present inquiry, however, the chief feature of interest in these -discoveries is the recognition of a group of volcanic rocks underneath the -quartzite. This group was named the "Caldecote Volcanic Rocks" by -Professor Lapworth, who first recognized its nature and relations. Its -rocks have been studied by Mr. T. H. Waller<a id="FNanchor_128" href="#Footnote_128" class="fnanchor">[128]</a> and Mr. F. Rutley,<a id="FNanchor_129" href="#Footnote_129" class="fnanchor">[129]</a> and have -been traced upon a revised edition of the Geological Survey map by Mr. -A. Strahan.<a id="FNanchor_130" href="#Footnote_130" class="fnanchor">[130]</a> They consist of a thin series of well-stratified tuffs -apparently derived from andesitic lavas. Their base is not seen owing to -the fault which brings down the New Red Sandstone against them. They -are surmounted by the quartzite, which at its base is conglomeratic and -contains blocks of the tuff. A mass of quartz-felsite is possibly intrusive -in these strata, and is associated with a diabase-porphyrite. In these -rocks, but still more in the shales which overlie them, numerous sills of -diorite and diabase occur. The total thickness of rocks from the lowest -visible part of the Caldecote volcanic series to the base of the Coal-measures -is probably between 2000 and 3000 feet.</p> - -<div class="footnote"> - -<p><a id="Footnote_128" href="#FNanchor_128" class="label">[128]</a> <i>Op. cit.</i> p. 323.</p> - -<p><a id="Footnote_129" href="#FNanchor_129" class="label">[129]</a> <i>Op. cit.</i> p. 557.</p> - -<p><a id="Footnote_130" href="#FNanchor_130" class="label">[130]</a> <i>Geol. Mag.</i> (1886), p. 540. In this paper full references will be found to the previous papers -on the geology of the district. Jukes had recognized that the rocks below the coal-bearing -strata were "older than the Upper Silurian, perhaps older than any Silurian," <i>Mem. Geol. -Survey</i>, "South Staffordshire Coal-field" (1859), p. 134.</p> - -</div> - -<p><span class="pagenum" id="Page_172">- 172 -</span></p> - -<p>There can be no doubt as to the geological position of the dark fossiliferous -shales and their underlying quartzite. The fact that the basement -conglomerate of the quartzite is partly made up of the underlying volcanic -series may possibly mark a wide difference of age between them, and may -indicate that the eruption of the tuffs took place long before Upper -Cambrian time. On the other hand, the tuffs have the same strike and -angle of dip with the quartzite, and as Professor Lapworth admits, the -break between them may not be of great moment. It is at least certain -that the intrusive sills of the district are later than the tuffs, and later also -than the sedimentary Cambrian groups.</p> - -<hr class="chap x-ebookmaker-drop" /> - - -<div class="chapter"> -<p><span class="pagenum" id="Page_173">- 173 -</span></p> - -<h2 class="nobreak" id="BOOK_IV">BOOK IV<br /> - -<span class="smaller">THE SILURIAN VOLCANOES</span></h2> -</div> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<h3 class="nobreak" id="CHAPTER_XII">CHAPTER XII<br /> - -<span class="smaller">CHARACTERS OF THE SILURIAN SYSTEM IN BRITAIN. THE ARENIG VOLCANOES</span></h3> -</div> - -<div class="blockquot"> - -<p>The Land and Sea of Silurian time—Classification of the Silurian System—General -Petrography of the Silurian Volcanic Rocks—I. The Eruptions of Arenig Age.</p> -</div> - - -<p>The next great geological period, to which Murchison gave the name of -Silurian, has in Britain a fuller record than the period which preceded it. -The rocks that tell its history are more varied in origin and structure. -They are displayed at the surface over a far wider area, and, what gives -them special interest and value, they contain a much larger assemblage of -organic remains. For the immediate subject of the present volume, they -have likewise the additional attraction that they include a singularly complete -and widespread volcanic chronicle. They display in many admirable -sections the piled-up lavas and tuffs of scores of volcanoes, scattered all -over the three kingdoms, from the headlands of Kerry to the hills of -Lammermuir. They thus enable us to form a truer conception of what the -early Palæozoic volcanoes were than is possible from the more limited -evidence furnished by the Cambrian system.</p> - -<p>At the beginning of the Silurian period most of the area of the British -Isles lay under the sea. But if we may judge from the sedimentary strata -which represent the floor of that sea, the water, during most of the time, -was of no great depth. There is evidence, indeed, that during a part of -the period the sea was deep enough to admit of the accumulation of wide -tracts of radiolarian ooze, with but little admixture of mechanical sediment. -But, for the most part, sand and mud were drifted from neighbouring -lands, the more important of which probably lay to the north, over what -are now the Highlands of Scotland and the north and north-west districts -of Ireland. No general change in topography or in physical conditions -took place at the close of Cambrian time. The older era glided insensibly -<span class="pagenum" id="Page_174">- 174 -</span> -into the newer, unmarked by any such catastrophe as was once supposed -to have intervened at the end of each great geological period. There -are traces, indeed, of slight local disturbances, but these only make the -general gradual transition more marked.</p> - -<p>Of the vegetation which covered the Silurian lands hardly anything is -known. Traces of lycopods and ferns have been detected, and these probably -formed the chief constituents in what must have been rather a sombre and -monotonous flora. The character of the terrestrial fauna is still hidden -from us, though we do know that insects winged their way through those -green flowerless forests, and that scorpions likewise harboured there. -That these primeval arachnoids were air-breathers is shown by their -breathing stigmata; and from the fact that they possessed a well-developed -poison-gland and sting, we may believe that there were already living at -the same time other land-animals, possibly of higher grade, on which they -preyed. But of these ancestral types no actual relics have yet been -discovered.</p> - -<p>It is the life of the sea-floor that has mainly been chronicled among the -sedimentary formations. Taking the Silurian system as a whole, we find it -to be the repository of a remarkably varied assemblage of organisms. -Among the simpler forms, Radiolaria deserve especial notice, from their wide -range in space and time, and the comparative indestructibility of the -highly-siliceous, fine-grained, flinty strata, which have preserved them in -abundance and have a wide distribution over the British Isles. The -Graptolites, so specially characteristic of the system, range entirely through -it, and by their successive differences of specific and generic forms, -furnish a basis for the division of the whole series of rocks into more -or less definite stratigraphical zones. Hardly less important for purposes -of correlation are the Trilobites which in the Silurian period reached the -culmination of their development in regard to number of species and genera. -These interesting extinct types of crustacean life must have swarmed over -some parts of the sea-bottom, for their remains abound in its hardened silts. -The Brachiopods are likewise numerously represented among Silurian strata; -and since the vertical range of the species is generally not great, they serve -as useful guides in fixing stratigraphical horizons. Lamellibranchs, -Gasteropods, and Cephalopods become increasingly numerous and varied as -we follow the succession of strata from the base to the summit of the -Silurian system. That there were fishes also in the Silurian seas is -proved by the occurrence of their remains, more particularly in the higher -formations.</p> - -<p>From the organic remains which have been preserved in the rocks, it -may be inferred that the animal life of the globe became more varied in -Silurian time; higher types made their appearance, until vertebrates took -the place of pre-eminence which they have ever since maintained.</p> - -<p>The volcanic activity that had marked the passage of Cambrian time -in Britain was prolonged into the Silurian period. In North Wales, -indeed, it is clear that though the eruptions began in the earlier era of -<span class="pagenum" id="Page_175">- 175 -</span> -geological history they continued to be comparatively feeble until they -broke out into full activity in the succeeding epoch. There is no hiatus -or essential difference between the volcanic phenomena, any more than -there is between the sedimentary deposits, of the two periods.</p> - -<p>Although it may be only owing to the fact that the Silurian formations -come much more extensively to the surface of the land than the underlying -Cambrian are permitted to do, yet it is at least noteworthy that the relics -of Silurian volcanoes are spread over a far wider area of the British Isles -than those of the earlier period. Throughout a large part of Wales they -form some of the most prominent mountains, such as Cader Idris, the Arans, -Arenig Fawr, Moel Wyn, Moel Siabod, and Snowdon. They rise into the -picturesque hill-groups of the Lake District, they appear at many detached -places throughout the south of Scotland, and form conspicuous eminences in -Carrick. In Ireland they abound all down the east side of the island, and -even reappear on the far western headlands of the Dingle coast-line.</p> - -<p>To the same pioneers, by whom the foundations of our knowledge of the -Cambrian volcanoes were laid, we are indebted for the first broad outlines -of the history of volcanic action in Silurian time. The writings of -Sedgwick and Murchison, but still more the detailed mapping of De la -Beche, Ramsay, Selwyn, Jukes, and the other members of the Geological -Survey, have given to the Silurian volcanic rocks of Wales a classic interest -in the history of geology. To these labours further reference will be made -in subsequent pages.<a id="FNanchor_131" href="#Footnote_131" class="fnanchor">[131]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_131" href="#FNanchor_131" class="label">[131]</a> For references to the older literature see <i>ante</i>, <a href="#Page_142">p. 142</a>.</p> - -</div> - -<p>The amount of material being so ample for the compilation of a record -of volcanic action in Britain during Silurian time, it will be desirable to -arrange it in stratigraphical order. For this purpose invaluable assistance -is afforded by the evidence of organic remains, whereby the whole Silurian -system has been subdivided into sections, each characterized throughout the -whole region by certain distinctive fossils. The following tabular statement -exhibits the chief stratigraphical divisions of the system, and the -short black lines in it mark the positions of separate volcanic platforms in -each of the three kingdoms:—</p> - -<table summary="data"> -<tr> - <td class="bdl bdt bdb" colspan="3"></td> - <td class="bdl bdt bdb tdc">England</td> - <td class="bdl bdt bdb tdc">Wales</td> - <td class="bdl bdt bdb tdc">Scotland</td> - <td class="bdl bdt bdb bdr tdc">Ireland</td> -</tr> -<tr> - <td class="bdl tdc" rowspan="3">Upper Silurian</td> - <td rowspan="3"><img src="images/bracel_60.png" width="11" height="60" alt="" /></td> - <td class="tdl">Ludlow Group</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">Wenlock Group</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl">...</td> - <td class="tdc bdl bdr"><em>---</em></td> -</tr> -<tr> - <td class="tdl">Llandovery Group</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" colspan="3"> </td> - <td class="bdl tdc"> </td> - <td class="bdl tdc"> </td> - <td class="bdl tdc"> </td> - <td class="tdc bdl bdr"> </td> -</tr> -<tr> - <td class="bdl bdb tdl" rowspan="3">Lower Silurian</td> - <td class="bdb" rowspan="3"><img src="images/bracel_60.png" width="11" height="60" alt="" /></td> - <td class="tdl">Bala and Caradoc Group</td> - <td class="tdc bdl"><em>---</em></td> - <td class="tdc bdl"><em>---</em></td> - <td class="tdc bdl"><em>---</em></td> - <td class="tdc bdl bdr"><em>---</em></td> -</tr> -<tr> - <td class="tdl">Llandeilo Group</td> - <td class="tdc bdl"><em>---</em></td> - <td class="tdc bdl"><em>---</em></td> - <td class="tdc bdl"><em>---</em></td> - <td class="tdc bdl bdr"><em>---</em></td> -</tr> -<tr> - <td class="tdl bdb">Arenig Group</td> - <td class="tdc bdl bdb"><em>---</em></td> - <td class="tdc bdl bdb"><em>---</em></td> - <td class="tdc bdl bdb"><em>---</em></td> - <td class="tdc bdl bdb bdr"><em>---</em></td> -</tr> -</table> - -<p>It will be most convenient, following the combined stratigraphical and -geographical arrangement of this table, to discuss first the volcanic history -of the Lower Silurian period as recorded in each of the three kingdoms, and -then that of the Upper Silurian.</p> - -<p><span class="pagenum" id="Page_176">- 176 -</span></p> - - -<h3>I. THE ERUPTIONS OF ARENIG AGE</h3> - - -<h4>i. <span class="allsmcap">MERIONETHSHIRE</span></h4> - -<p>Placing the upper limit of the Cambrian system at the top of the -Tremadoc group, we pass into the records of another series of volcanic -eruptions which marked various epochs during the Silurian period over the -area of the British Isles. The earliest of these volcanic episodes has left its -memorials in some of the most impressive scenery of North Wales. To the -picturesque forms sculptured out of the lavas and ashes of that early time, -we owe the noble range of cliffs and peaks that sweeps in a vast semicircle -through the heights of Cader Idris, Aran Mawddwy, Arenig, and Moel -Wyn. To the east other volcanic masses, perhaps in part coeval with -these, rise from amidst younger formations in the groups of the Berwyn and -Breidden Hills, and the long ridges of the Shelve and Corndon country. -Far to the south, traces of Silurian volcanoes are met with near Builth, -while still more remote are the sheets of lava and tuff interstratified -among the Lower Silurian rocks of Pembrokeshire, and those which extend -into Skomer Island.</p> - -<p>The most important of these districts is unquestionably that of -Merionethshire. In this area, as was pointed out in the last chapter, the -eruptions certainly began before the close of the Cambrian period, for traces -of them occur in the Tremadoc and Lingula Flag groups. But below these -strata, in the vast pile of grits and conglomerates of the Harlech anticline, -there does not appear to be any trace of contemporaneous volcanic action.</p> - -<p>At the time when the Geological Survey maps of this region were -prepared, the Cambrian and Lower Silurian rocks had not been subdivided -into the various palæontological groups which are now recognized. Nor -had any attempt been made to separate the various kinds of contemporaneous -igneous masses from each other and from the tuffs in so extensive -and complicated a mountain-region. The task undertaken by the Survey -was beset with difficulties, some of which geologists, furnished with the -advantages of a later time, can hardly perhaps realize. The imperfections -of the mapping were long ago recognized by the original surveyors, and -various corrections of them were made from time to time. First of all, the -volcanic rocks, which originally had been all massed under one colour, were -traced out separately on the ground, according to their structure and mode -of origin, and were distinguished from each other on the maps.<a id="FNanchor_132" href="#Footnote_132" class="fnanchor">[132]</a> Subsequently -divisional lines were followed out between some of the larger -stratigraphical groups, the maps and sections were still further modified, and -the results were summed up in the volume on the <i>Geology of North Wales</i>. -<span class="pagenum" id="Page_177">- 177 -</span><a id="FNanchor_133" href="#Footnote_133" class="fnanchor">[133]</a> -But short of actually resurveying the whole of that rugged tract, it was -impossible to bring the maps abreast of the onward march of science. -They consequently remain, as a whole, very much as they were some thirty -or forty years ago.</p> - -<div class="footnote"> - -<p><a id="Footnote_132" href="#FNanchor_132" class="label">[132]</a> <i>Mem. Geol. Surv.</i> vol. iii. 2nd edit. p. 95, note.</p> - -<p><a id="Footnote_133" href="#FNanchor_133" class="label">[133]</a> Some of the modifications introduced are, I think, to be regretted, for the earlier editions of -the maps and sections are in certain respects more accurate than the later. On this point I -concur with the criticism made by Messrs. Cole and Jennings, <i>Quart. Journ. Geol. Soc.</i> vol. xlv. -(1889), p. 436.</p> - -</div> - -<p>Sir Andrew Ramsay, in his great Monograph on the geology of North -Wales, has described the Merionethshire volcanic district in considerable -detail. He seems finally to have come to the conclusion that the eruptions -of that area were included within the Arenig period.<a id="FNanchor_134" href="#Footnote_134" class="fnanchor">[134]</a> He shows, indeed, -that on Rhobell Fawr the ejected materials lie directly on disturbed Lingula -Flags without the intervention of the Tremadoc group, which is nevertheless -present in full development in the near neighbourhood.<a id="FNanchor_135" href="#Footnote_135" class="fnanchor">[135]</a> And in trying to -account for this remarkable fact he evidently had in his mind the possibility -that volcanic eruptions had taken place long before as well as after -the beginning of the deposition of the Arenig grit and slates.<a id="FNanchor_136" href="#Footnote_136" class="fnanchor">[136]</a> He seems -eventually, however, to have looked on the Rhobell Fawr sections as exceptional -and possibly to be accounted for by some local disturbance and -intrusion of eruptive rock.<a id="FNanchor_137" href="#Footnote_137" class="fnanchor">[137]</a> He clearly recognized that there were two -great epochs of volcanic activity during the Silurian period in Wales, one -belonging to the time of the Arenig, the other to that of the Bala rocks, -and he pointed out that the records of these two periods are separated by -a thick accumulation of sedimentary strata which, being free from interstratifications -of contemporaneous igneous rocks, may be taken to indicate -a long interval of quiescence among the subterranean forces.<a id="FNanchor_138" href="#Footnote_138" class="fnanchor">[138]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_134" href="#FNanchor_134" class="label">[134]</a> <i>Mem. Geol. Survey</i>, vol. iii. 2nd ed., p. 96.</p> - -<p><a id="Footnote_135" href="#FNanchor_135" class="label">[135]</a> The ashes and agglomerates of Rhobell Fawr can be seen in various places to rest on the -highest members of the Lingula Flags. See Messrs. Cole and Holland, <i>Geol. Mag.</i> (1890), p. 451.</p> - -<p><a id="Footnote_136" href="#FNanchor_136" class="label">[136]</a> <i>Op. cit.</i> <a href="#Page_72">p. 72</a>.</p> - -<p><a id="Footnote_137" href="#FNanchor_137" class="label">[137]</a> He was disposed to regard Rhobell Fawr as one of the great centres of eruption of the district. -See <i>Memoir of A. C. Ramsay</i>, p. 81, and <i>Geology of North Wales</i>, 2nd edit. p. 98.</p> - -<p><a id="Footnote_138" href="#FNanchor_138" class="label">[138]</a> <i>Op. cit.</i> pp. <a href="#Page_71">71</a>, <a href="#Page_96">96</a>, <a href="#Page_105">105</a>.</p> - -</div> - -<p>The lower limit of the Arenig rocks has been fixed at a band or bands -of grit or conglomerate (Garth grit) which can be followed with some slight -interruptions all round the great dome of Cambrian strata from Llanegrin -on the south to the shore at Criccieth on the north. The volcanic group -doubtless lies, generally speaking, above that basement platform. But, -besides the sections at Rhobell Fawr just referred to, where the volcanic -materials lie on the Lingula Flags, the same relation may, I think, be -observed on the north flank of Cader Idris. Messrs. Cole, Jennings, and -Holland have come to the conclusion that the eruptions began at a rather -earlier date than that assigned to them in the <i>Survey Memoirs</i>, and my -own examination of the ground led me to accept their conclusion.<a id="FNanchor_139" href="#Footnote_139" class="fnanchor">[139]</a> I -inferred that the earliest discharges in the southern part of the region took -place in Cambrian time, at or possibly before the close of the deposition -of the Lingula Flags, and that intermittent outbursts occurred at many -intervals during the time when the Tremadoc and Arenig rocks were -deposited.</p> - -<div class="footnote"> - -<p><a id="Footnote_139" href="#FNanchor_139" class="label">[139]</a> <i>Quart. Journ. Geol. Soc.</i> vol. xlv. (1889), p. 436; <i>Geol. Mag.</i> (1890), p. 447. <i>Pres. Address -Geol. Soc.</i> 1890, p. 107.</p> - -</div> - -<p><span class="pagenum" id="Page_178">- 178 -</span></p> - -<p>Important confirmation of this view of the Cambrian age of the earlier -volcanic eruptions of the Cader Idris region has recently been obtained by -Messrs. P. Lake and S. H. Reynolds who, in the ground intervening between -the lower slopes of Cader Idris and Dolgelly, have ascertained the existence -of a marked band of andesitic lava traceable for some distance in the upper -Lingula Flags. They have also observed a higher volcanic group reposing -upon the Tremadoc strata at the top of the Cambrian system, and consisting -of rhyolite with rhyolite-tuffs.<a id="FNanchor_140" href="#Footnote_140" class="fnanchor">[140]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_140" href="#FNanchor_140" class="label">[140]</a> <i>Quart. Journ. Geol. Soc.</i> vol. lii. (1896), p. 511.</p> - -</div> - -<p>Some of the most stupendous memorials of the earlier eruptions are -to be seen in the huge mountain mass of Rhobell Fawr (2403 feet). They -consist mainly of agglomerates and tuffs, one of the most remarkable varieties -of which is distinguished by its abundant scattered crystals of hornblende -and of augite. The fragments of rock included in these rocks are scoriæ -and lumps of various lavas, especially basaltic and trachytic andesites. The -tuffs become finer towards the top of the mountain where they are interleaved -with grits. Among the pyroclastic materials occasional lavas -(basaltic andesites) occur which may be contemporaneous streams, but -most of the lava-form rocks appear to be intrusive. They include dolerites -(augite-aphanites), basaltic andesites, and trachytic andesites.<a id="FNanchor_141" href="#Footnote_141" class="fnanchor">[141]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_141" href="#FNanchor_141" class="label">[141]</a> Prof. Cole, <i>Geol. Mag.</i> (1893), p. 337.</p> - -</div> - -<div class="figcenter" id="v1fig46" style="width: 499px;"> - <img src="images/v1fig46.png" width="499" height="77" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 46.</span>—Section across Rhobell Fawr.<a id="FNanchor_142" href="#Footnote_142" class="fnanchor">[142]</a><br /> - - L L, Lingula flags; <i>t</i>, tuffs and ashy slates; <i>s</i>, slates and grits; F F, Arenig volcanic series; D, dolerite.</div> -</div> - - -<div class="footnote"> - -<p><a id="Footnote_142" href="#FNanchor_142" class="label">[142]</a> After Messrs. Cole and Holland, <i>Geol. Mag.</i> (1890), p. 450.</p> - -</div> - -<p>The materials from the Rhobell Fawr volcano are clearly distinguishable -from those of the Arenig volcanoes in the neighbourhood. The latter begin -to make their appearance among the black slates at the base of the northern -declivities of Cader Idris, and extend upward through that mountain into -the country beyond.</p> - -<p>An upper limit to this volcanic group is not easily traceable; partly, -no doubt, from the gradual cessation of the eruptions and partly from the -want of any marked and persistent stratigraphical horizon near the top of -the group. Sir Andrew Ramsay, indeed, refers to the well-known band -of pisolitic iron-ore as lying at or near to the top of the Arenig rocks.<a id="FNanchor_143" href="#Footnote_143" class="fnanchor">[143]</a> -There can be no doubt, however, that the volcanic intercalations continue -far above that horizon in the southern part of the district.</p> - -<div class="footnote"> - -<p><a id="Footnote_143" href="#FNanchor_143" class="label">[143]</a> <i>Mem. Geol. Survey</i>, vol. iii. 2nd edit. pp. 249, 250.</p> - -</div> - -<p>In spite of the extent to which the volcanic masses of the Arenig period -have been covered by later Palæozoic formations, it is still possible to fix -approximately the northern, western, and southern limits of the district -over which the ashes and lavas were distributed. These materials die out -<span class="pagenum" id="Page_179">- 179 -</span> -as they are traced southwards from Cader Idris and north-westwards from -Tremadoc.<a id="FNanchor_144" href="#Footnote_144" class="fnanchor">[144]</a> The greatest diameter of ground across which they are now continuously -traceable is about twenty-eight miles. They attain their greatest -thickness, upwards of 5000 feet, in Aran Mawddwy, which rises from -their most easterly escarpment. We may therefore infer that the main -vent or vents lay somewhere in that direction. The noble range of precipices -facing westwards shows how greatly the limits of the volcanic rocks -have been reduced by denudation. There can be little doubt that at least -the finer tuffs extended westwards as far as a line drawn from Tremadoc -to Llanegrin—that is, some fifteen miles or more beyond the cliffs of Aran -Mawddwy, thus stretching across much of the site of what is now the -great Harlech anticline.</p> - -<div class="footnote"> - -<p><a id="Footnote_144" href="#FNanchor_144" class="label">[144]</a> <i>Op. cit.</i> <a href="#Page_96">p. 96</a>.</p> - -</div> - -<p>This compact, well-defined volcanic area, in spite of the faults which -traverse it and the disturbed positions into which its rocks have been -thrown, is, in many respects, one of the simplest and most easily studied -among the Palæozoic formations of this country. Its main features have -been delineated on the maps of the Geological Survey and have been -described in Sir Andrew Ramsay's monograph. But these publications cannot -be regarded as more than a first broad, though masterly, outline of the -whole subject. There is an ample field for further and more minute -research wherein, with the larger and better Ordnance maps now available, -and with the advantage of the numerous modern petrographical aids, a -more exhaustive account may be given of the district. The whole volcanic -succession from base to summit is laid bare in innumerable magnificent -natural sections along ranges of hills for a distance of some forty miles, and -a careful study and re-mapping of it could not fail to add greatly to our -knowledge of the early history of volcanic action.<a id="FNanchor_145" href="#Footnote_145" class="fnanchor">[145]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_145" href="#FNanchor_145" class="label">[145]</a> The excellent papers of Professor Cole, Mr. Jennings, Mr. Holland, Mr. G. J. Williams, Mr. -P. Lake and Mr. S. H. Reynolds are illustrations of how the published work of the Geological -Survey may be modified and elaborated.</p> - -</div> - -<p>According to the observations of the Geological Survey, the Arenig -volcanic rocks of Merionethshire naturally arrange themselves in three -great bands, each of which is described as tolerably persistent throughout -the whole district:—1st, a lower series of ashes and conglomerates, sometimes -3300 feet thick (Aran Mawddwy); 2nd, a middle group of "felstones" -and "porphyries," consisting partly of true contemporaneous lava-streams -and partly of intrusive sheets, and reaching a thickness of 1500 -feet; 3rd, an upper series of fragmental deposits like that beneath, the -extreme thickness of which is 800 feet (Arenig mountain). A re-mapping -of the ground on the six-inch maps would, no doubt, show many local -departures from this general scheme.</p> - -<p>The pyroclastic members of this volcanic series present many features -of interest both to the field-geologist and the petrographer; but they have -as yet been only partially studied. At the southern end of the district -it is remarkable to what a large extent the earliest eruptions must have -<span class="pagenum" id="Page_180">- 180 -</span> -been mere gaseous explosions, with the discharge of comparatively little -volcanic material. Many of the tuffs that are interstratified with black -slates (? Lingula Flags) at the foot of the long northern slope of Cader -Idris, consist mainly of black-slate fragments like the slate underneath, -with a variable proportion of grey volcanic dust.</p> - -<div class="figleft" id="v1fig47" style="width: 138px;"> - <img src="images/v1fig47.png" width="138" height="193" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 47.</span>—Section at the Slate - Quarry, Penrhyn Gwyn, north slopes of Cader Idris.</div> -</div> - -<p>The accompanying section (<a href="#v1fig47">Fig. 47</a>) represents the arrangement of the -rocks exposed at the Slate Quarry of Penrhyn Gwyn. About 50 feet of -black slate (<i>a</i>) are there seen, the bedding in which -dips S. at 20°, while the cleavage is inclined towards -S.W. at a slightly higher angle. The next 20 feet -of slate (<i>b</i>) are distinguished by many intercalations -of slate-tuff or breccia, varying from less than an -inch to three feet in thickness. An intrusive sheet -of andesite (<i>c</i>), which varies from two or three to -ten feet in thickness, and is strongly cellular in the -centre, interrupts the slates and hardens them. Above -this sill the indurated slate and tuff (<i>d</i>), containing -abundant felspar crystals, pass under a flinty porphyritic -felsite (<i>e</i>) or exceedingly fine tuff, enclosing -a band of granular tuff. Beyond this band the black -slates with their seams of tuff continue up the hill and -include a sheet of slaggy felsitic lava 8 or 10 feet thick.</p> - -<p>This section, affording as it does the first glimpse of the volcanic history -of Cader Idris, indicates a continued series of feeble gaseous discharges, probably -from one or more small vents, whereby the black clay on the sea-floor was -blown out, the fragments falling back again to be covered up under a gradual -accumulation of similar dark mud. By degrees, as the vigour of eruption -increased, lava-dust and detached felspar crystals were ejected, and eventually -lava rose to the surface and flowed over the sea-bottom in thin sheets.</p> - -<p>But elsewhere, and likewise at a later period in this same southern part -of the district, the fragmental discharges consisted mainly of volcanic material. -Sir Andrew Ramsay has described the coarse conglomerates composed of -subangular and rounded blocks of different "porphyries," sometimes 20 -inches in diameter, embedded in a fine matrix of similar materials. The -true nature of the component fragments in these rocks has still to be -worked out.</p> - -<p>Messrs. Cole and Jennings have noticed that the grey volcanic dust of -the older slate-tuff of Cader Idris is seen under the microscope "to abound -in particles of scoriaceous andesite-glass, now converted into a green -palagonite."<a id="FNanchor_146" href="#Footnote_146" class="fnanchor">[146]</a> Their investigations show that while the same kinds of -volcanic rocks continue to be met with from the bottom to the top, -nevertheless there is an increase in the acid character of the lapilli as the -section is traced upwards. Some of the fragments consist of colourless -devitrified glass, with pieces of pumice, as if derived from the breaking up -of previously-formed tuffs. Others resemble quartz-andesites, rhyolites, or -<span class="pagenum" id="Page_181">- 181 -</span> -trachytes, while in at least one instance, somewhat low down in the section, -quartz-grains with intruded material point to the existence of some fairly -acid and vitreous lava.<a id="FNanchor_147" href="#Footnote_147" class="fnanchor">[147]</a> On the south side of Llyn Cau, that is towards the -top of the volcanic group, I found a coarse agglomerate with blocks of felsitic -lavas, sometimes three feet across (see <a href="#v1fig48">Fig. 48</a>). This gradual increase of -acidity in the lapilli of the tuffs finds an interesting confirmation in the -contemporaneous lava-sheets to which I shall afterwards allude.</p> - -<div class="footnote"> - -<p><a id="Footnote_146" href="#FNanchor_146" class="label">[146]</a> <i>Quart. Journ. Geol. Soc.</i>, vol. xlv. (1889), p. 424; <i>Geol. Mag.</i> (1890), p. 447.</p> - -<p><a id="Footnote_147" href="#FNanchor_147" class="label">[147]</a> <i>Op. cit.</i> <a href="#Page_429">p. 429</a>. A tuff lying below the ironstone near Cross Foxes, east of Dolgelly, likewise -contains fragments of trachytic lavas.</p> - -</div> - -<p>One of the most noticeable features in the tuffs of this volcanic group -is the great abundance of entire and broken crystals dispersed through them. -These crystals have certainly not been formed <i>in situ</i>, but were discharged -from the vents as part of the volcanic dust. They usually consist of felspar -which, at least in the southern portion of the district, appears generally to -be plagioclase. Frequent reference to these crystals as evidence of volcanic -explosions may be found in the publications of the Survey. Nowhere can -they be better seen than in the black slate-tuffs of Cader Idris. They are -there white, more or less kaolinized, and as they lie dispersed through the -black base, they give the rock a deceptive resemblance to some dark -porphyry. The large crystals of hornblende and augite abundantly scattered -through much of the tuff of Rhobell Fawr have been already referred to.</p> - -<p>In the central parts of the district thick bands of ashes were mapped -by the Survey, and described as consisting almost wholly of volcanic materials, -but containing occasional thin bands of slate which suffice to mark pauses -in the eruptions, when ordinary sediment was strewn over the sea-bottom. -In the Cader Idris ground, on the other hand, interstratifications of non-volcanic -material are of such frequent recurrence as to show that there, -instead of constant and vigorous discharges accumulating a vast pile of -ashes, the eruptions followed each other after intervals of sufficient duration -to allow of the usual dark sediment spreading for a depth of many feet -over the sea-bottom.</p> - -<p>One of the most interesting deposits of these interludes of quiescence is -that of the pisolitic ironstone and its accompanying strata on the north -front of Cader Idris (<i>i</i> in <a href="#v1fig48">Fig. 48</a>). A coarse pumiceous conglomerate with -large slag-like blocks of andesite and other rocks, seen near Llyn-y-Gadr, -passes upward into a fine bluish grit and shale, among which lies the bed -of pisolitic (or rather oolitic) ironstone which is so widely diffused over -North Wales. The finely-oolitic structure of this band is obviously original, -but the substance was probably deposited as carbonate of lime under quiet -conditions of precipitation. The presence of numerous small <i>Lingulæ</i> in -the rock shows that molluscan life flourished on the spot at the time. The -iron exists in the ore mainly as magnetite, the original calcite or aragonite -having been first replaced by carbonate of iron, which was subsequently -broken up so as to leave a residue of minute cubes of magnetite.<a id="FNanchor_148" href="#Footnote_148" class="fnanchor">[148]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_148" href="#FNanchor_148" class="label">[148]</a> Messrs. Cole and Jennings, <i>op. cit.</i> p. 426.</p> - -</div> - -<p>Above the ironstone some more blue and black shale and grit pass under -<span class="pagenum" id="Page_182">- 182 -</span> -a coarse volcanic conglomerate like that below, lying at the base of the high -precipice of Cader Idris. Hence this intercalated -group of sedimentary strata marks a -pause in the discharge of ashes and lavas, -during which the peculiar conditions of sedimentation -indicated by the ironstone spread -over at least the southern part of the volcanic -area. Some few miles to the east, where the -ironstone has been excavated near Cross Foxes, -the band is again found lying among tuffs and -grits full of volcanic lapilli.</p> - -<div class="figcenter" id="v1fig48" style="width: 716px;"> - <img src="images/v1fig48.png" width="716" height="148" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 48.</span>—Sketch-section across Cader Idris.<br /> - <i>st</i>, slates and tuffs and ashy slates; <i>s</i>, slates and grits; <i>i</i>, ironstone; <i>b</i>, volcanic breccias; <i>a</i>, slaggy andesitic and more basic lavas; <i>e</i>, microgranite or eurite; <i>f</i>, felsites; - <i>d</i>, "greenstone" (dolerites, diabases, etc.).</div> -</div> - -<p>Between a lower and an upper band of -tuff in the Arenig volcanic group the Maps -and Memoirs of the Geological Survey distinguish -a central zone of "felspathic porphyry," -which attains a maximum thickness -of 1500 feet (see <a href="#v1fig48">Fig. 48</a>). From Sir Andrew -Ramsay's descriptions, it is clear that he -recognized in this zone both intrusive and -extrusive sheets, and that the latter, where -thickest, were not to be regarded as one -mighty lava-flow, but rather as the result of -successive outpourings, with occasional intervals -marked by the intercalation of bands of slate -or of tuff. To a certain extent the intruded -sheets are separated on the map from the contemporaneous -lavas; but this has been done -only in a broad and sketchy way. One of the -most important, and at the same time most -difficult, tasks yet to be accomplished in this -district is the separation of the rocks which -were probably poured out at the surface from -those that were injected underneath it. My -own traverses of the ground have convinced -me that good evidence of superficial outflows -may be found in tracts which have been mapped -as entirely intrusive; while, on the other hand, -some of the so-called "lavas" may more probably -be of the nature of sills.</p> - -<p>The petrography of the rocks, moreover, -still requires much study. Among the so-called -"felspathic porphyries" of the Survey -maps a considerable variety of texture, structure -and composition will doubtless be detected. -In the <i>Descriptive Catalogue of Rock-Specimens -in the Museum of Practical Geology</i> (3rd edit., 1862) the rocks that form -<span class="pagenum" id="Page_183">- 183 -</span> -the "lava-streams of Llandeilo age," in Merionethshire, are named "felstone," -"felspar-porphyry," "felstone-porphyry," "felspathic-porphyry," and "calcareous -amygdaloid."</p> - -<p>The most interesting feature which my own slight personal acquaintance -with the region has brought before me is the clear evidence of a succession -from comparatively basic lavas in the lower part of the group to much more -acid masses in the higher part. In the Survey map numerous sheets of -intrusive "greenstone" are shown traversing the Lingula Flags, Tremadoc -slates, and lower part of the volcanic group along the northern slopes of -Cader Idris. The true intrusive nature of much of this material is clearly -established by transgressive lines of junction and by contact-metamorphism, -as well as by the distinctive crystalline texture of the rocks themselves. -But the surveyors were evidently puzzled by some parts of the ground. -Sir Andrew Ramsay speaks of "the great mass of problematical vesicular -and sometimes calcareous rock which is in places almost ashy-looking." -After several oscillations of opinion, he seems to have come finally to the -conclusion that this vesicular material, which occurs also in the upper part -of the mountain, passes into, and cannot be separated from, the undoubted -intrusive "greenstones."<a id="FNanchor_149" href="#Footnote_149" class="fnanchor">[149]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_149" href="#FNanchor_149" class="label">[149]</a> <i>Mem. Geol. Surv.</i> vol. iii. 2nd edit. p. 36; see also pp. 31, 32.</p> - -</div> - -<p>The true solution of the difficulty will be found, I believe, in the -recognition of a group of scoriaceous lavas among these greenstones. The -presence of a cellular structure might not be sufficient to demonstrate that -the rocks in which it appears are true lava-beds, for such a structure is far -from unknown both among dykes and sills. But in the present case there -is other corroborative testimony that some of these Cader Idris amygdaloids -were really poured out at the surface. Below Llyn-y-Gadr—the dark tarn -at the foot of the vast wall of Cader Idris—the beds of coarse volcanic -conglomerate (<i>b</i> in <a href="#v1fig48">Fig. 48</a>), to which I have already alluded, are largely -composed of blocks of the vesicular "greenstones" on which they lie. -These "greenstones," moreover, have many of the most striking characteristics -of true lavas (<i>a</i> in <a href="#v1fig48">Fig. 48</a>). They are extraordinarily cellular; their upper -surfaces sometimes present a mass of bomb-like slags with flow-structure, -and the vesicles are not infrequently arranged in rows and bands along -the dip-planes.</p> - -<p>A microscopic examination of two slides cut from these rocks shows them -to be of a trachytic or andesitic type, with porphyritic crystals of a kaolinized -felspar embedded in a microlitic groundmass. The rocks are much impregnated -with calcite, which fills their vesicles and ramifies through their mass.</p> - -<p>A few miles to the east some remarkable felsitic rocks take the place -of these vesicular lavas immediately below the pisolitic iron ore. I have -not determined satisfactorily their relations to the surrounding rocks, and -in particular am uncertain whether they are interbedded lavas or intrusive -sheets. Dr. F. H. Hatch found that their microscopic characters show a close -resemblance to the soda-felsites described by him from the Bala series of -the south-east of Ireland.</p> - -<p><span class="pagenum" id="Page_184">- 184 -</span></p> - -<p>The slopes of Cader Idris are partly obscured with debris, from above -which rises the great precipitous face formed by the escarpment of -"porphyry," here intrusively interposed among the Arenig volcanic rocks. -This enormous sill will be referred to a little further on in connection -with the other intrusive sheets of the region.</p> - -<p>The remarkably cellular rock which forms the peak of Cader Idris is -coloured on the Survey map as an intrusive sill of "greenstone," which in -the Memoir is said to alter the contiguous slates and to appear to cut -across them diagonally. I am disposed, however, to think that these -appearances of intrusion are deceptive. On the southern declivity of the -mountain this rock presents one of the most curious structures to be seen -in the whole district. Its surface displays a mass of spheroidal or pillow-shaped -blocks aggregated together, each having a tendency to divide -internally into prisms which diverge from the outside towards the centre.<a id="FNanchor_150" href="#Footnote_150" class="fnanchor">[150]</a> -Some portions are extremely slaggy, and round these more solid portions finely -crystalline parts are drawn, suggestive rather of free motion at the surface -than of the conditions under which a subterranean sill must be formed. -The idea occurred to me on the ground that while the band of rock marked -as "greenstone" on the map is probably, in the main, an interstratified -lava, there may nevertheless be basic intrusions along its course, as in the -lower part of the mountain. The minute structure of this amygdaloid, -as revealed by the microscope, shows it to be an epidiorite wherein the -hornblende, paramorphic after augite, has been again partially altered along -the margins into chlorite.</p> - -<div class="footnote"> - -<p><a id="Footnote_150" href="#FNanchor_150" class="label">[150]</a> This peculiar structure of the more basic Arenig lavas, where the rock looks as if built up -of irregularly-spheroidal, sack-like or pillow-shaped blocks, will be again referred to in connection -with the Arenig (and Llandeilo) lavas of Scotland and Ireland. It appears to be widely distributed, -and especially in connection with the occurrence of radiolarian cherts. The black slate above the -Cader Idris amygdaloid would, in a similar position in Scotland, be associated with such cherts, -but these have not yet been noticed at this locality. With the spheroidal internally-radiating -prismatic structure of the Cader Idris rock, compare that of the lava at Acicastello already -noticed on <a href="#Page_26">p. 26</a>.</p> - -</div> - -<p>The highest lavas of Cader Idris, forming the ridge to the south of -Llyn Cau, are separated from the amygdaloid just described by a thick -zone of black slate with thin ashy intercalations, beyond which comes the -coarse volcanic agglomerate already referred to as containing blocks of -felsite a yard or more in diameter. These lavas are true felsites, sometimes -beautifully spherulitic and exhibiting abundant flow-structure, like some -of the felsites of the next or Bala volcanic period.<a id="FNanchor_151" href="#Footnote_151" class="fnanchor">[151]</a> The petrography of -these rocks still remains to be worked out.</p> - -<div class="footnote"> - -<p><a id="Footnote_151" href="#FNanchor_151" class="label">[151]</a> Messrs. Cole and Jennings, <i>Quart. Journ. Geol. Soc.</i> vol. xlv. (1889), p. 430. From the examination -of slices prepared from a few of the felsites of the Dolgelly district, Dr. Hatch observed a -"striking difference between their characters and those of the Cambrian felsites of Caernarvonshire. -The porphyritic constituent is now no longer quartz, but felspar (plagioclase), and the rocks -belong, not to the rhyolitic, but rather to the less acid trachytes, perhaps even to the andesites."</p> - -</div> - -<p>The volcanic series of Cader Idris sweeps northward through the chain -of Aran and Arenig, and then curves westward through the group of -Manod and Moelwyn, beyond which it rapidly dies out. In its course of -<span class="pagenum" id="Page_185">- 185 -</span> -about 45 miles it undergoes considerable variation, as may be seen by -comparing a section through Moelwyn with that through Cader Idris already -given. According to the researches of Mr. Jennings and Mr. Williams,<a id="FNanchor_152" href="#Footnote_152" class="fnanchor">[152]</a> the -main mass of volcanic material in the northern part of the region consists of -fragmentary rocks varying in texture from agglomerates into fine tuffs, but -showing some differences in the succession of beds in different localities.</p> - -<div class="footnote"> - -<p><a id="Footnote_152" href="#FNanchor_152" class="label">[152]</a> <i>Quart. Journ. Geol. Soc.</i> xlvii. (1891), p. 368.</p> - -</div> - -<p>The Tremadoc group of strata clearly underlies the volcanic series of -these more northerly tracts. But it contains, so far as appears, no intercalation -of volcanic material. The inference may thus be drawn that the -eruptions began in the Cader Idris district, and did not extend into that of -Manod and Moelwyn until after the beginning of the Arenig period. -Above the Tremadoc group lies the well-marked and persistent band, about -13 feet thick, known as the Garth grit, which has been already referred to -as a convenient base-line to the Arenig group.</p> - -<div class="figcenter" id="v1fig49" style="width: 392px;"> - <img src="images/v1fig49.png" width="392" height="110" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 49.</span>—Section across the Moelwyn Range.<a id="FNanchor_153" href="#Footnote_153" class="fnanchor">[153]</a><br /> - 1, Tremadoc Group; 2, Garth or Arenig grit (base of Arenig group); 3, Arenig slates, etc.; 3<sup>1</sup>, Lower slate band; - 3<sup>2</sup>, Middle slate band; 3<sup>3</sup>, Upper slate band; 4<sup>1</sup>, Lower agglomerate; 4<sup>2</sup>, Middle agglomerate; 4<sup>3</sup>, Upper - agglomerate; 5, Llandeilo group; G, Granite boss of Moel tan y Grisiau.</div> -</div> - -<div class="footnote"> - -<p><a id="Footnote_153" href="#FNanchor_153" class="label">[153]</a> After Messrs. Jennings and Williams, <i>Quart. Journ. Geol. Soc.</i> vol. xlvii. (1891), p. 371, -and Horizont. Sect. Geol. Surv. Sheet 28.</p> - -</div> - -<p>In this northern district, among the sediments which overlie the Garth -grit, layers of fine tuff begin to make their appearance, which north of -Cwm Orthin thicken out into a considerable mass between the grit and the -lowest of the great agglomerates. These tuffs, which mark the beginning of -the volcanic eruptions of the district, are followed by a band of slate which -in some places has yielded a <i>Lingula</i>, <i>Orthis Carausii</i>, and a <i>Tetragraptus</i>, -and points to an interval of quiescence in the volcanic history. We now enter -upon an enormous thickness of agglomerates and tuffs separated by several -bands of slate. Taking advantage of the slaty intercalations, Messrs. -Jennings and Williams have divided this great accumulation of fragmentary -volcanic material into three beds (<a href="#v1fig49">Fig. 49</a>). The matrix of the agglomerates -is compact and pale, so as to resemble and to have been called "felstone," -but showing its fragmentary nature on weathered surfaces. The blocks -imbedded in this paste range up to sometimes as much as 11 feet in length -by 4 feet in width. Their minute petrographical characters have not been -studied, but the blocks are stated to consist for the most part of "slaty -and schistose fragments mixed with rounded pebbles of fine-grained -'felstone.'" They are heaped together as in true agglomerates. In the -<span class="pagenum" id="Page_186">- 186 -</span> -upper agglomerate, fragments of cleaved slate containing <i>Lingula</i> have -been observed.</p> - -<p>The name of "felstone" is restricted by Messrs. Jennings and Williams -to certain fine-grained varieties of rock, of which a thin band lies at the -base of the lower agglomerate, while another of considerably greater importance -occurs in the middle of the upper agglomerate. These bands -consist of a fine compact greenish base, and weather with a dull white crust; -sometimes, as in the thicker sheet, a columnar structure shows itself. -Whether these rocks are to be regarded as lavas or sills, or even as finer -varieties of tuff, is a question that awaits further inquiry. But it is clear, -from the investigation of the two observers just cited, that the pyroclastic -constituents must vastly preponderate in the volcanic series over the -northern part of the region. All these rocks, whether coarse or fine-grained, -appear to be rather acid in composition, and no evidence has yet been -obtained of a sequence among them from a more basic to a more acid series, -as in Cader Idris.</p> - -<p>The highest agglomerate bed of the Manod and Moelwyn area is covered -by slates which contain Llandeilo graptolites. In this way, by means of -palæontological evidence, the upward and downward limits of the Arenig -volcanic series in this part of Wales are definitely fixed.</p> - -<p>Hardly any information has yet been obtained as to the situation and -character of the vents from which the lavas and ashes of Merionethshire -were discharged. In the course of the mapping of the ground, the Geological -Survey recognized that, as the greatest bulk of erupted material lies in the -eastern and south-eastern parts of the region, the chief centres of emission -were to be looked for in that quarter, and that possibly some of the intrusive -masses which break through the rocks west of the great escarpment may -mark the site of vents, such as Tyddyn-rhiw, Gelli-llwyd-fawr, Y-Foel-ddu, -Rhobell Fawr, and certain bosses near Arenig.<a id="FNanchor_154" href="#Footnote_154" class="fnanchor">[154]</a> The distribution of the -volcanic materials indicates that there were certainly more than one active -crater. While the southward thickening of the whole volcanic group points -to some specially vigorous volcano in that quarter, the notable thinning -away of the upper tuffs southward and their great depth about Arenig -suggest their having come from some vent in this neighbourhood. On the -other hand, the lower tuffs are absent at Arenig, while on Aran Mawddwy, -only nine miles to the south, they reach a depth of 3000 feet. Still farther -to the south these volcanic ejections become more and more divided by -intercalated bands of ordinary sediment. One of the most important -volcanoes of the region evidently rose somewhere in the neighbourhood of -what is now Aran Mawddwy. There seems reason to surmise that the -sites of the chief vents now lie to the east and south of the great escarpment, -buried under the thick sedimentary formations which cover all that -region.</p> - -<div class="footnote"> - -<p><a id="Footnote_154" href="#FNanchor_154" class="label">[154]</a> <i>Mem. Geol. Surv.</i> vol. iii. 2nd edit. p. 98; see also pp. 44, 54, 58, 71.</p> - -</div> - -<p>If we are justified, on stratigraphical and petrographical grounds, in -connecting the lowest volcanic rocks of the Berwyn range with those of -<span class="pagenum" id="Page_187">- 187 -</span> -Merionethshire, we may speculate on the existence of a group of submarine -vents, coming into eruption at successive intervals, from some epoch during -the period of the Lingula Flags up to that of the Bala rocks, and covering with -lavas and ashes a space of sea-bottom at least forty miles from east to west -by more than twenty miles from north to south, or roughly, an area of some -800 square miles.<a id="FNanchor_155" href="#Footnote_155" class="fnanchor">[155]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_155" href="#FNanchor_155" class="label">[155]</a> The Berwyn Hills, however, will be described in later pages as a distinct volcanic district.</p> - -</div> - -<p>Besides the materials ejected to the surface, the ancient volcanic region -of Merionethshire was marked by the intrusion of a vast amount of igneous -rock between and across the bedding-planes of the strata deep underground. -One of the most prominent features of the Geological Survey map is the -great number of sills represented as running with the general strike of the -strata, especially between the top of the Harlech grits and the base of the -volcanic series. On the north side of the valley of the Mawddach, between -Barmouth and Rhaiadr Mawddach, in a distance of twelve miles the Survey -mapped "more than 150 intrusions varying from a few yards to nearly a -mile in length."<a id="FNanchor_156" href="#Footnote_156" class="fnanchor">[156]</a> This zone of sills is equally marked on the south side of -the valley. It may be traced all round the Harlech anticline until it dies -out, as the bedded masses also do, towards Towyn on the south and about -Tremadoc on the north.</p> - -<div class="footnote"> - -<p><a id="Footnote_156" href="#FNanchor_156" class="label">[156]</a> <i>Mem. Geol. Surv.</i> vol. iii. p. 26.</p> - -</div> - -<p>The presence of such a zone of intrusive sheets at the base of an ancient -volcanic series is a characteristic feature in the geology of Britain. It is -met with again and again among the Palæozoic systems, and appears on a -striking scale in association with the Tertiary basaltic plateaux of Antrim -and the Inner Hebrides. But nowhere, perhaps, is it more strongly developed -than beneath the Arenig group of lavas and tuffs in North Wales. -Abundant as are the protrusions marked on the Geological Survey map, they -fall short of the actual number to be met with on the ground. Indeed, to -represent them as they really are would require laborious surveying and the -use of maps on a far larger scale than one inch to a mile.</p> - -<p>The vast majority of these sills are basic rocks, or, in the old and convenient -terminology, "greenstones." Those of the Cader Idris district have -been examined by Messrs. Cole and Jennings, who found that, notwithstanding -the considerable alteration everywhere shown by the abundant -epidote and calcite, the coarser varieties may be recognized as having originally -been dolerites approaching gabbro, with a well-developed ophitic character, -the general range of structure being from dolerites without olivine and -aphanites to andesitic rocks with an originally glassy matrix.<a id="FNanchor_157" href="#Footnote_157" class="fnanchor">[157]</a> Dr. Hatch -confirmed this diagnosis from slides prepared from my specimens. The -ophitic structure is usually characteristic and well preserved, in spite of the -alteration indicated by epidote, chlorite, uralite, and leucoxene.</p> - -<div class="footnote"> - -<p><a id="Footnote_157" href="#FNanchor_157" class="label">[157]</a> <i>Quart. Journ. Geol. Soc.</i> vol. xlv. (1889), p. 432.</p> - -</div> - -<p>That this zone of "greenstone" sills belongs to the period of the -Merionethshire volcanoes may be reasonably concluded. The way in which -they follow the line of the great escarpment, their almost entire absence -from the Cambrian dome to the west, their cessation as the overlying lavas -<span class="pagenum" id="Page_188">- 188 -</span> -and tuffs die out laterally, and their scarcity above the lower part of the -volcanic group, seem to indicate their close relationship to that group. Moreover, -that they must have been as a whole later than the main part of the lavas -and tuffs may be inferred from their position. The molten material of which -they were formed could hardly have forced its way between and across the -strata unless egress to the surface had been impeded by some thick overlying -mass. The "greenstones" may therefore be regarded as lateral emanations from -funnels of more basic lava towards the close of the volcanic period. Possibly -some at least of the highly slaggy and vesicular bands to which I have -referred may represent portions of this material, which actually flowed out -as streams of lava at the surface.</p> - -<p>But there is likewise evidence of extensive intrusion of more siliceous -rocks. On the Geological Survey map, besides the numerous "greenstones," -various sheets of "felspathic porphyry" are represented as running with the -general strike of the region, but here and there breaking across it. One of -the most remarkable of these acid sills is that which, in the noble precipice -of Cader Idris, has a thickness of about 1500 feet and a length of three or -four miles. It is shown on the map to be transgressive across other rocks, -and, as seen on the ground, it maintains the uniformity of texture which is -characteristic rather of sheets that have solidified underneath than of those -which have congealed with comparative rapidity at the surface. On a fresh -fracture the rock presents a pale bluish-grey tint, becoming yellowish or -brownish as the result of weathering. Its texture is finely granular, with -occasional disseminated felspars. Under the microscope a section of it was -found by Dr. Hatch to exhibit the characteristic structure of a microgranite, -a confused holocrystalline aggregate of quartz and felspar, with a few porphyritic -felspars. Messrs. Cole and Jennings have proposed to revive for -this rock Daubuisson's name "Eurite."<a id="FNanchor_158" href="#Footnote_158" class="fnanchor">[158]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_158" href="#FNanchor_158" class="label">[158]</a> Mr. Harker speaks of the rock as a granophyre.</p> - -</div> - -<p>A similar rock occurs at a lower horizon among the Lingula Flags at -Gelli-llwyd-fawr, two miles south-west of Dolgelly,<a id="FNanchor_159" href="#Footnote_159" class="fnanchor">[159]</a> and much microgranite -has been injected along the slopes above Tyddyn-mawr.</p> - -<div class="footnote"> - -<p><a id="Footnote_159" href="#FNanchor_159" class="label">[159]</a> Messrs. Cole and Jennings, <i>op. cit.</i> p. 435.</p> - -</div> - -<p>The chronological relation of these acid sheets and bosses to the more -basic intrusions has not yet been definitely determined. That some of them -may have solidified in vents and may have been directly connected with the -protrusion of the later or more highly siliceous lavas is not at all improbable. -Others again would seem to belong to a much later geological period -than the Arenig volcanoes. In this late series the well-known boss of -Tan-y-grisiau near Festiniog should probably be included. This mass -of eruptive material was mapped by the Geological Survey as "intrusive -syenite." It has been more recently examined and described by Messrs. -Jennings and Williams as a granitite.<a id="FNanchor_160" href="#Footnote_160" class="fnanchor">[160]</a> These observers have noticed not -only that it intrusively traverses and alters the Tremadoc group, but that -its intrusion appears to have taken place subsequent to the cleavage which -<span class="pagenum" id="Page_189">- 189 -</span> -affects the Llandeilo as well as older formations. This granitic boss has -thus probably no connection with the Arenig volcanoes, but belongs to a -later period in the volcanic history of the Principality.</p> - -<div class="footnote"> - -<p><a id="Footnote_160" href="#FNanchor_160" class="label">[160]</a> <i>Quart. Journ. Geol. Soc.</i> vol. xlvii. (1891), p. 379.</p> - -</div> - -<p>The remarkable scarcity of dykes in the volcanic districts of Wales has -been noticed by more than one observer. Among the intrusive "greenstones" -of Merionethshire some occasionally assume the dyke form, and -through the agglomerates and tuffs of Rhobell Fawr dykes of olivine-diabase -have worked their way. In the Festiniog district various altered andesitic -dykes have been noted. But there has been no widespread fissuring of the -ground and uprise of lava in the rents, such as may be seen in the Archæan -gneiss, and in the later Palæozoic, but still more in the Tertiary volcanic -regions. This feature becomes all the more notable when it is viewed in -connection with the great development of sills, and the evidence thereby -afforded of widespread and extremely vigorous subterranean volcanic action.</p> - -<p>In the Merionethshire region there certainly was a long period of -quiescence between the close of the Arenig and the beginning of the Bala -eruptions. Moreover, no evidence has yet been found that active vents -ever again appeared in that district, the subterranean energy at its next -outburst having broken out farther to the east and north. In Anglesey, -however, where, as I shall point out, there is proof of contemporaneous -tuffs among the Arenig rocks, it is possible that a continuous record of -volcanic action may yet be traced from Arenig well onward into Bala time.</p> - - -<h4>ii. <span class="allsmcap">SHROPSHIRE</span></h4> - -<p>About 35 miles to the south-east of the great volcanic range of -Merionethshire a small tract of Arenig rocks rises from amidst younger -formations, and forms the picturesque country between Church Stoke and -Pontesbury. Murchison in his excellent account of this district clearly -recognized the presence of both intrusive and interstratified igneous rocks.<a id="FNanchor_161" href="#Footnote_161" class="fnanchor">[161]</a> -The ground has in recent years been more carefully worked over by Mr. -G. H. Morton<a id="FNanchor_162" href="#Footnote_162" class="fnanchor">[162]</a> and Professor Lapworth.<a id="FNanchor_163" href="#Footnote_163" class="fnanchor">[163]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_161" href="#FNanchor_161" class="label">[161]</a> <i>Silurian System</i> (1839), chap. xix.; <i>Siluria</i>, 4th edit. (1867), pp. 26, 49.</p> - -<p><a id="Footnote_162" href="#FNanchor_162" class="label">[162]</a> <i>Proc. Liverpool Geol. Soc.</i> x. (1854), p. 62.</p> - -<p><a id="Footnote_163" href="#FNanchor_163" class="label">[163]</a> <i>Geol. Mag.</i> (1887), p. 78.</p> - -</div> - -<p>At the top of the Arenig group of this district lies a zone of well-stratified -andesitic tuff and breccia (Stapeley Ash), with frequent intercalations -of shales, and occasionally fossiliferous.<a id="FNanchor_164" href="#Footnote_164" class="fnanchor">[164]</a> There is thus satisfactory -proof of contemporaneous eruptions at intervals during the accumulation of -the later Arenig sediments. That there were also outflows of lava is -shown by the presence of sheets of augite- and hypersthene-andesite. -These volcanic intercalations form marked ridges, having a general northerly -trend. They are folded over the broad laccolitic ridge of Corndon, on the -east side of which they are thrown into a synclinal trough, so that successive -parallel outcrops of them are exposed. According to the mapping of -<span class="pagenum" id="Page_190">- 190 -</span> -the Geological Survey they are thickest towards the west, and become -more split up with intercalated sediments as they range eastward.</p> - -<div class="footnote"> - -<p><a id="Footnote_164" href="#FNanchor_164" class="label">[164]</a> Prof. Lapworth and Mr. W. W. Watts, <i>Proc. Geol. Assoc.</i> xiii. (1894), pp. 317, 337.</p> - -</div> - -<p>Volcanic eruptions in this Shropshire region continued from the Arenig -into the Bala period. They are marked among the Llandeilo strata by -occasional tuffs and by two massive beds of "volcanic grit," described by -Murchison,<a id="FNanchor_165" href="#Footnote_165" class="fnanchor">[165]</a> but they appear to have been rather less vigorous in the -interval represented by this subdivision of the Silurian system. Those of -Bala time gave forth abundant discharges of ash, of which the lowest -accumulation, locally known as the Hagley Ash, consists of andesitic -detritus. Occasional layers of tuff are intercalated in the overlying Hagley -Shales, above which comes an important band called the Whittery Ash, -"consisting of andesitic and rhyolitic breccias and conglomerates, fine ashes -with curious spherulitic or pisolitic structures, and bands of shale often -fossiliferous."<a id="FNanchor_166" href="#Footnote_166" class="fnanchor">[166]</a> It is evident that the eruptions of the Shelve district came -from independent vents in that neighbourhood, and never reached the -importance of the great volcanoes of Arenig age in Montgomeryshire or of -Bala age in Caernarvonshire.</p> - -<div class="footnote"> - -<p><a id="Footnote_165" href="#FNanchor_165" class="label">[165]</a> <i>Silurian System</i>, p. 229.</p> - -<p><a id="Footnote_166" href="#FNanchor_166" class="label">[166]</a> Messrs. Lapworth and Watts, <i>op. cit.</i> p. 318.</p> - -</div> - -<div class="figcenter" id="v1fig50" style="width: 486px;"> - <img src="images/v1fig50.png" width="486" height="92" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 50.</span>—Section across the anticline of Corndon.<a id="FNanchor_167" href="#Footnote_167" class="fnanchor">[167]</a><br/> - A, Arenig flags and shales; B, andesites and tuffs; C, intrusive dolerite.</div> -</div> - -<div class="footnote"> - -<p><a id="Footnote_167" href="#FNanchor_167" class="label">[167]</a> After Prof. Lapworth and Mr. Watts, <i>op. cit.</i> p. 342.</p> - -</div> - -<p>Numerous dykes and sills traverse the rocks of this district. They -consist chiefly of hypersthene-dolerite. They appear to belong to a much -later period than the interstratified volcanic series; at least some of them -are found altering the Pentamerus limestones, and these must be later than -the Llandovery rocks.<a id="FNanchor_168" href="#Footnote_168" class="fnanchor">[168]</a> The most important sill is that which forms -Corndon, the central igneous mass of the district. This body of dolerite -was ascertained by Mr. Watts not to be a boss but a laccolite, which -wedges out both towards the north-west and south-east, as shown in -<a href="#v1fig50">Fig. 50</a>.</p> - -<div class="footnote"> - -<p><a id="Footnote_168" href="#FNanchor_168" class="label">[168]</a> <i>Op. cit.</i> p. 339.</p> - -</div> - -<p>Six miles to the north of the Shelve and Corndon district the Breidden -Hills rise on the border of Shropshire and Montgomeryshire, and include a -mass of volcanic material belonging to a distinct area of eruption. In the -ridge that extends for about three and a half miles through Moel-y-golfa -and Middletown Hill, a synclinal trough of volcanic rocks lies upon shales, -which from their fossils have been placed in the Bala group. The volcanic -series appears to exceed 1000 feet in thickness. The lowest part of it on -Moel-y-golfa consists of andesitic lavas about 400 feet thick, followed by -tuffs and volcanic conglomerates. The lavas resemble some of the "porphyrites" -<span class="pagenum" id="Page_191">- 191 -</span> -of the Old Red Sandstone, and contain two forms of pyroxene—one -rhombic, probably enstatite, and the other monoclinic augite. There are -likewise considerable masses of intrusive rock, which are varieties of diabase -or dolerite.<a id="FNanchor_169" href="#Footnote_169" class="fnanchor">[169]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_169" href="#FNanchor_169" class="label">[169]</a> See Mr. W. W. Watts on the Igneous and Associated Rocks of the Breidden Hills, <i>Quart. -Journ. Geol. Soc.</i> vol. xli. (1885), p. 532.</p> - -</div> - - -<h4>iii. <span class="allsmcap">SCOTLAND</span></h4> - -<p>From the centre of England we must in imagination transport ourselves -into the Southern Uplands of Scotland, where a widely distributed -series of Silurian volcanic rocks has been preserved. It was, until -recently, supposed that the Silurian system north of the Tweed contains no -contemporaneously erupted volcanic rocks. Yet, as far back as the year -1860, I pointed to the abundant existence of volcanic detritus in these -strata throughout the southern counties as a probable indication of volcanic -activity at the time and in the area within which the strata were deposited.<a id="FNanchor_170" href="#Footnote_170" class="fnanchor">[170]</a> -Some years later, when the microscope had been introduced as an aid to -field-geology, I sliced some of the Silurian sediments of that region and -found them, particularly certain shales and grits of Moffatdale, to contain a -large admixture of perfectly fresh unworn felspar crystals, which I felt -tolerably certain had been supplied by volcanic explosions. As no trace, -however, had then been detected of an intercalated volcanic group in any -part of the Silurian series of the south of Scotland, I used at that time to -speculate on the possibility of the volcanic detritus having been wind-borne -from the volcanoes of the Lake District. I had at that time no suspicion -that its source was rather to be sought under my feet. The presence of -volcanic rocks underneath the uplands of the south of Scotland would have -been a welcome explanation of the frequent felspathic composition of many -of the Silurian greywackes and shales of that region, and particularly the -abundance of andesitic and felsitic fragments in them.</p> - -<div class="footnote"> - -<p><a id="Footnote_170" href="#FNanchor_170" class="label">[170]</a> <i>Trans. Roy. Soc. Edin.</i> xxii. (1860), p. 636.</p> - -</div> - -<p>It had been long known that the Scottish Silurian formations, besides -having undergone extensive plication, have also been injected by protrusions -of igneous material of various kinds. The intrusive character of many of -these is so obvious that a similar origin was attributed even to those bosses -which could not be proved to be intrusive. Recent work of the Geological -Survey, however, and more especially the numerous and careful traverses of -my friend and colleague Mr. Peach, have revealed the unlooked-for and -important fact that a large number of these supposed intrusions are really -portions of a volcanic group brought up on the crests of anticlinal folds, and -laid bare by denudation. This group can be traced for at least 100 miles -from north-east to south-west over a belt of country sometimes 30 miles -broad. Its original limits cannot be ascertained, but they obviously exceeded -those within which the rocks can now be seen. Nevertheless the present -boundaries embrace an area of nearly 2000 square miles. This Palæozoic -volcanic region is thus one of the most extensive in the British Isles. -<span class="pagenum" id="Page_192">- 192 -</span> -Owing, however, to the constant plication of the strata, and the wide space -which the overlying sedimentary deposits are thus made to cover, the -volcanic group only comes occasionally into view, and thus occupies but a -mere fraction of the superficial extent of the region over which its scattered -outcrops appear. These exposures, sometimes only a few square yards in -extent, may always be looked for where the anticlinal folds bring up a -sufficiently low portion of the Silurian system; they prove that a vast -volcanic floor underlies the visible Lower Silurian grits and shales over the -length and breadth of the Southern Uplands of Scotland.</p> - -<p>Without anticipating details which will properly appear in the official -<i>Memoirs</i> of the Geological Survey, I may briefly indicate the visible boundaries -of the volcanic group, and refer to some of the localities where it may best be -seen. The most easterly points where it has been recognized by Mr. Peach -stand on the crests of some sharp anticlinal folds near St. Mary's Loch and -near Leadburn and Winkstone in Peeblesshire. Farther westwards it appears -at many places along the northern border of the Silurian territory, as at -Romanno Bridge, Wrae, Kilbucho, Culter Water and Abington, the length and -breadth of each exposure depending partly on the breadth of the anticline -and partly on the depth to which it has been cut down by denudation. -Near Sanquhar the volcanic series opens out for a breadth of more than a -mile, and is seen at intervals across the wild moorlands of Carrick, until -from the Stinchar valley it widens out seaward and occupies much of the -coast-line of Ayrshire between Girvan and the mouth of Loch Ryan. It -probably rises again along a fold near Portpatrick, and it is seen at -various points along the southern borders of the Silurian uplands, as near -Castle-Douglas, at Glenkiln, Bell Craig near Moffat, and the head of Ettrickdale.</p> - -<p>The best sections are those exposed along the coast to the north and -south of Ballantrae. When that ground was first examined by the Geological -Survey, the hypothetical views in regard to metamorphism already -referred to were in full ascendant, and the rocks were mapped on the same -general principles as those which had been followed in Wales. Professor -Bonney, however, a few years later recognized the true igneous nature of -many of the rocks. He found among them porphyrite lavas and agglomerates -which he regarded as of Old Red Sandstone age, likewise intrusive -serpentines and gabbros.<a id="FNanchor_171" href="#Footnote_171" class="fnanchor">[171]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_171" href="#FNanchor_171" class="label">[171]</a> <i>Quart. Journ. Geol. Soc.</i> vol. xxxiv. (1878), p. 769.</p> - -</div> - -<p>The volcanic rocks of this wide district include both lavas and their -pyroclastic accompaniments, as well as intrusive sills and bosses of various -materials. They have recently been studied by Mr. J. J. H. Teall, and full -descriptions of them by him will appear in a forthcoming volume of the -<i>Memoirs</i> of the Geological Survey. He has ascertained that though -generally more or less decomposed, the lavas would be classed by German -petrographers as diabases and diabase-porphyrites. The former are compact -dark-green non-porphyritic rocks, often containing numerous small -spherical amygdales; while the latter are markedly porphyritic, enclosing -<span class="pagenum" id="Page_193">- 193 -</span> -large phenocrysts of more or less altered plagioclase, often measuring half -an inch across. These two groups of rock are connected by transitional -varieties. They were probably, in the first instance, composed of plagioclase, -augite, iron-ores, and a variable quantity of imperfectly crystallized interstitial -matter.</p> - -<p>Some of these rocks closely resemble in outward appearance the -andesites ("porphyrites") of the Old Red Sandstone of the district not -many miles to the north, that is, fine purplish-red rocks with a compact -base through which porphyritic felspars are abundantly scattered. Occasionally -they are markedly slaggy, and show even a ropy surface, while the -breccias associated with them contain blocks of similar slag.</p> - -<div class="figcenter" id="v1fig51" style="width: 418px;"> - <img src="images/v1fig51.png" width="418" height="396" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 51.</span>—Structure in finely-amygdaloidal diabase lava, south of mouth of Stinchar River, Ayrshire. - The fine dots and circles mark the lines of amygdales.</div> -</div> - -<p>But the most characteristic external feature of these lavas is their -tendency to assume irregularly-elliptical, sack-like or pillow-shaped forms. -On a weathered face they sometimes look like a pile of partially-filled sacks -heaped on each other, the prominences of one projecting into corresponding -hollows in the next. The general aspect of this structure is shown in <a href="#v1fig12">Fig. 12</a>, -which represents a face of rock about eight feet high and six feet broad. -The rocks exhibiting this peculiarity are usually finely amygdaloidal, and it -may be observed that the vesicles are grouped in lines parallel to the -outer surface of the pillow-like block in which they occur. The diagram in -<span class="pagenum" id="Page_194">- 194 -</span> -<a href="#v1fig51">Fig. 51</a> represents in ground-plan a surface about twelve feet square on -the shore immediately to the south of the mouth of the River Stinchar. -In the heart of the spheroids enclosed fragments of other lavas are sometimes -observable.</p> - -<p>This singular structure has already (<a href="#Page_184">p. 184</a>) been referred to as -strikingly displayed in a rock at the top of Cader Idris. It is found in -dark basic lavas probably of Arenig age, which will be afterwards referred -to as occurring along the southern flanks of the Scottish Highlands and -also in the north of Ireland. It has been observed by Mr. Teall among the -rocks of the Lizard, and has been described as occurring in Saxony and -California.<a id="FNanchor_172" href="#Footnote_172" class="fnanchor">[172]</a> In these different localities it is associated with jaspers and -cherts, some of which contain abundant Radiolaria. The same structure has -been found among the variolitic diabases of Mont Genèvre,<a id="FNanchor_173" href="#Footnote_173" class="fnanchor">[173]</a> and likewise in -some modern lavas, as in that of Acicastello already referred to (<i>ante</i>, <a href="#Page_26">p. 26</a>).</p> - -<div class="footnote"> - -<p><a id="Footnote_172" href="#FNanchor_172" class="label">[172]</a> Mr. J. J. H. Teall, <i>Roy. Geol. Soc. Cornwall</i>, 1894, p. 3. Mr. L. Ransome, <i>Bull. Depart. -Geol. University of California</i>, vol. i. p. 106.</p> - -<p><a id="Footnote_173" href="#FNanchor_173" class="label">[173]</a> Messrs. Cole and Gregory, <i>Quart. Journ. Geol. Soc.</i> vol. xlvi. (1890), p. 311.</p> - -</div> - -<div class="figcenter" id="v1fig52" style="width: 496px;"> - <img src="images/v1fig52.png" width="496" height="197" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 52.</span>—View of Knockdolian Hill from the east.</div> -</div> - -<p>The volcanic agglomerates and breccias, in the south-west of Ayrshire, -attain a great development in several centres probably at or near the -original volcanic vents. They present several distinct petrographical types. -The remarkable neck-like hill of Knockdolian in the Stinchar Valley is made -of a coarse breccia composed mainly of angular pieces of dull greyish-green -fine-grained diabase. The breccias and agglomerates of Bennane Head in -some parts consist largely of broken-up shales, flinty mudstone, black -radiolarian flint or chert, and abundant fragments of andesites and felsites. -In other parts the volcanic material predominates, including angular and -subangular fragments of various somewhat basic lavas, lumps of vesicular slag -and pieces of pumice. Here and there much calcite is diffused through -the matrix in strings, veins and patches, which enclose the lapilli. The -agglomerate north of Lendalfoot possesses a greenish, somewhat serpentinous -matrix, through which immense numbers of tabular felspar crystals are -scattered. Similar crystals also occur abundantly in embedded blocks of -<span class="pagenum" id="Page_195">- 195 -</span> -one of the purplish diabase-porphyrites, which occurs in mass on the shore -and inland, and closely resembles the rock of Carnethy in the Old Red -Sandstone volcanic series of the Pentland Hills.</p> - -<p>Yet another and very distinct type of agglomerate is to be seen on the -Mains Hill south-east of Ballantrae. It is a coarse rock, enclosing blocks -up to a yard or more in diameter, of a fine compact purplish porphyrite, -with large crystals of plagioclase and smaller ones of augite. In some -places immense numbers of the small lapilli in the matrix consist of an -extremely fine vesicular pumice. Small perfect and larger broken crystals -of augite are likewise abundant in some of the greenish, more basic parts of -the mass. These greenish serpentinous parts and the numerous augite -crystals point to the explosion of some tolerably basic pyroxenic lava. A -similar dark green, almost black, rock, with augite crystals, which sometimes -measure a quarter of an inch in diameter, occurs near Sanquhar in -Nithsdale. It presents a close resemblance to the agglomerate of Rhobell -Fawr, already alluded to. So far as these Scottish agglomerates have yet -been microscopically examined, they have been found to be composed of -crystals, crystal-fragments, and lapilli derived partly from lavas similar to -those above described, and partly from felsitic and other rocks which have -not yet been observed here in the form of lavas.</p> - -<p>The finer tuffs show likewise a considerable range of composition. -According to Mr. Peach's observations along the south-eastern parts of the -volcanic area, the ejected materials have consisted largely of fine dust -(probably in great measure felsitic), which towards the north-east is gradually -interleaved with ordinary sediment till the ashy character disappears. -As I have already remarked, there is reason to believe that the overlying -greywackes and shales derived part of their material either directly from -volcanic explosions or from the attrition of banks of lavas and tuffs exposed -to denudation.</p> - -<p>But besides the interstratified lavas and fragmental rocks there occur -numerous intrusive masses which are so intimately associated with the -volcanic series that they may with little hesitation be regarded as forming -part of it. They consist of various gabbros and serpentines, which are -especially developed where the volcanic series comes out in greatest force in -the south-west of Ayrshire. They also include more acid intrusions which, -as in the case of the rock of Byne Hill, near Girvan, even assume the -characters of granite.</p> - -<p>The dying out of the volcanic material towards the north-east probably -indicates that the vents of the period lay rather in the central or south-western -parts of the district. Unfortunately, the limited extent of the -exposures of the rocks makes it a hopeless task to search for traces of these -vents over by far the largest part of the area. There are two localities, -however, where the search may be made with better prospect of success. -One of these is a tract to the north of Sanquhar in Nithsdale, which -still requires to be studied in detail with reference to the sequence -and structure of its volcanic rocks. The other area is that south-western -<span class="pagenum" id="Page_196">- 196 -</span> -part of Ayrshire which has been already cited as displaying so large a -development of the volcanic series. Here the coast-sections reveal the -intercalation of fossiliferous bands which show the true stratigraphical -horizons of the lavas and tuffs. Under Bennane Head, Professor Lapworth -some years ago found, in certain hardened black shales, a group of graptolites -which mark an undoubted Arenig platform.<a id="FNanchor_174" href="#Footnote_174" class="fnanchor">[174]</a> Recently the ground has -been carefully re-examined by Messrs Peach and Horne, who have detected a -number of other fossiliferous zones which confirm and extend previous -observations. They have also been able to unravel the complicated -structure of the volcanic series, and to represent it on the 6-inch maps of -the Geological Survey, of which a reduction on the scale of 1 inch to a -mile is now in course of preparation. The following tabular summary, -taken partly from notes made by myself during a series of traverses of the -ground with Mr. Peach when the revision was begun, and partly from -memoranda supplied by that geologist himself, may suffice as a general -outline of the volcanic history of this exceedingly interesting and important -region.</p> - -<div class="footnote"> - -<p><a id="Footnote_174" href="#FNanchor_174" class="label">[174]</a> <i>Geol. Mag.</i> 1889, p. 22.</p> - -</div> - -<table summary="data"> -<tr> - <td class="tdl">Llandovery.</td> - <td><img src="images/bracer_36.png" width="11" height="36" alt="" /></td> - <td class="tdl">Pentamerus grit.<br /> Conglomerate (Mulloch Hill).</td> -</tr> -<tr> - <td class="tdl" rowspan="4">Caradoc.</td> - <td rowspan="4"><img src="images/bracel_116.png" width="11" height="116" alt="" /></td> - <td class="tdl">Shales, sandstones, grits, etc. (Ardmillan, Balcletchie).</td> -</tr> -<tr> - <td class="tdl">Thick conglomerate (Byne Hill, Bennane, etc.).</td> -</tr> -<tr> - <td class="tdl">Thick fossiliferous limestone (Stinchar, Girvan). (On this horizon come - the perlitic felsites and soda-felsites of Winkstone and Wrae.)</td> -</tr> -<tr> - <td class="tdl">Sandstone (<i>Orthis confinis</i>) passing down into thick conglomerate.</td> -</tr> -<tr> - <td class="tdc" colspan="3">[Unconformability.]</td> -</tr> -<tr> - <td class="tdl" rowspan="2">Upper Llandeilo.</td> - <td rowspan="2"><img src="images/bracer_36.png" width="11" height="36" alt="" /></td> - <td class="tdl">Green mudstones, grits and greywackes.</td> -</tr> -<tr> - <td class="tdl">Thin band of dark mudstone with Upper Llandeilo graptolites.</td> -</tr> -<tr> - <td class="tdl" rowspan="9">Arenig and Lower<br /> and Middle Llandeilo.</td> - <td rowspan="9"><img src="images/bracel_180.png" width="15" height="180" alt="" /></td> - <td class="tdl">Group of Radiolarian cherts (about 70 feet) with alternating tuffs.</td> -</tr> -<tr> - <td class="tdl">Tuff or volcanic conglomerate, with occasional lava-flows.</td> -</tr> -<tr> - <td class="tdl">Black shale (10 feet) with Arenig graptolites.</td> -</tr> -<tr> - <td class="tdl">Volcanic breccias around local centres (Knockdolian, etc.).</td> -</tr> -<tr> - <td class="tdl">Thick group of porphyrite and diabase lavas.</td> -</tr> -<tr> - <td class="tdl">Red flinty mudstones with Arenig graptolites.</td> -</tr> -<tr> - <td class="tdl">Porphyrites, etc.</td> -</tr> -<tr> - <td class="tdl">Fine tuffs, etc., with Lower Arenig fossils.</td> -</tr> -<tr> - <td class="tdl">Diabase lavas, etc. (base not seen).</td> -</tr> -</table> - -<p>It will be noticed from this table that the bottom of the volcanic series is -not reached, so that no estimate can be formed of its full thickness, nor -on what geological platform it begins. Possibly its visible portions represent -merely the closing scenes of a long volcanic history, which, over the -area of the south of Scotland, extended into Cambrian time, like the contemporary -series of Cader Idris.</p> - -<p>Among the lowest lavas there are interstratified courses of fine tuffs, -<span class="pagenum" id="Page_197">- 197 -</span> -flinty shales and thin limestones, which sometimes fill in the hollows -between the pillow-like blocks above referred to. Among the characteristic -Lower Arenig graptolites of these intercalated layers are <i>Tetragraptus bryonoides</i>, -<i>T. fruticosus</i>, <i>T. quadribrachiatus</i>, and <i>T. Headi</i> together with <i>Caryocaris -Wrightii</i>. Considerable variation is to be seen in the development of the upper -part of the volcanic series. In some places the lavas ascend almost to the -top; in others, thick masses of breccia or agglomerate take their place. These -fragmentary materials are locally developed round particular centres, which -probably lie near the sites of active vents whence large quantities of pyroclastic -material were discharged. One of the volcanic centres must have -been situated close to the position of Knockdolian Hill already referred to. -The exceedingly coarse breccia of that eminence is rudely stratified in -alternations of coarser and finer material, which was probably to some extent -assorted under water around the cinder-cone that discharged it. The -date of the explosions of this hill has been ascertained by Mr. Peach from -the intercalation of black shales containing Arenig graptolites among the -breccias. Another vent lay somewhere in the immediate neighbourhood of -the Mains Hill agglomerate, if not actually on part of the site of that rock. -Though probably not more than a mile from the Knockdolian volcano, and -belonging to the same epoch of eruption, this vent, to judge from the -peculiarities of its ejected material, must have been quite distinct in its -source. A third vent lay somewhere in the immediate vicinity of Bennane -Head, and threw out the extraordinary masses of agglomerate and the sheets -of lava seen on the coast at that locality. A fourth may be traced by its -separate group of fine tuffs on the coast three miles south of Ballantrae.</p> - -<div class="figcenter" id="v1fig53" style="width: 518px;"> - <img src="images/v1fig53.png" width="518" height="110" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 53.</span>—Section across the Lower Silurian volcanic series in the south of Ayrshire (B. N. Peach).<br /> - B, Interstratified lavas in Arenig group; <i>t</i>, tuffs; <i>r</i>, radolarian cherts; <i>b</i><sup>2</sup>, Llandeilo group; <i>b</i><sup>3</sup> Caradoc group. - Σ, Serpentine. G, Gabbro.</div> -</div> - -<p>A feature of singular interest in the material erupted from these various -centres of activity consists in the evidence that the explosions occurred at -intervals during the deposition of the Lower Silurian formations, and that -these formations were successively disrupted by submarine explosions. Mr. -Peach has found, for example, abundant pieces of the peculiar and easily -recognized radiolarian cherts imbedded in the volcanic series. That these -cherts were deposited contemporaneously with the volcanic eruptions is -proved by their intercalation among the breccias. Yet among these very -breccias lie abundant fragments of chert which must have already solidified -before disruption. It is thus evident that this siliceous ooze not only -accumulated but set into solid stone on the sea-floor, between periods of -<span class="pagenum" id="Page_198">- 198 -</span> -volcanic outburst, and that such an occurrence took place several times in -succession over the same area.</p> - -<p>These facts derive further interest from the organic origin of the chert. -It is now some years since Mr. Peach and his colleagues observed that between -the Glenkiln Shale with its Upper Llandeilo graptolites and the top of the -volcanic group in the central part of the Silurian uplands, alternations of -green, grey or red shaly mudstones and flinty greywackes are interleaved -with fine tuffs, and are specially marked by the occurrence in them of -nodules and bands of black, grey and reddish chert. This latter substance, on -being submitted to Dr. Hinde, was found by him to yield twenty-three new -species of Radiolaria belonging to twelve genera, of which half are new. It -thus appears that during the volcanic activity there must have been intervals -of such quiescence, and such slow, tranquil sedimentation in clear, perhaps -moderately deep water, that a true radiolarian ooze gathered over the -sea-bottom.<a id="FNanchor_175" href="#Footnote_175" class="fnanchor">[175]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_175" href="#FNanchor_175" class="label">[175]</a> <i>Ann. Mag. Nat. Hist.</i> (1890), 6th ser. vi. p. 40.</p> - -</div> - -<p>That the deposition of this ooze probably occupied a prolonged lapse of -time seems clearly indicated by the evidence of the fossils that occur below -and above the cherts. The graptolites underneath indicate a horizon in the -Middle Arenig group, those overlying the cherts are unmistakably Upper -Llandeilo. Thus the great depth of strata which elsewhere constitute the -Upper Arenig and Lower and Middle Llandeilo subdivisions is here represented -by only some 60 or 70 feet of radiolarian cherts. These fine -siliceous, organic sediments probably accumulated with extreme slowness in -a sea of some depth and over a part of the sea-floor which lay outside the -area of the transport and deposit of the land-derived sediment of the time.<a id="FNanchor_176" href="#Footnote_176" class="fnanchor">[176]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_176" href="#FNanchor_176" class="label">[176]</a> <i>Annual Report of the Geol. Surv. for 1895</i>, p. 27 of reprint.</p> - -</div> - -<div class="figleft" id="v1fig54" style="width: 235px;"> - <img src="images/v1fig54.png" width="235" height="125" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 54.</span>—Section of part of the Arenig volcanic - group, stream south of Bennane Head, Ayrshire.</div> -</div> - -<p>As an illustration of some of the characteristic features in the succession -of deposits in the volcanic series of the south-west of Ayrshire, the accompanying -section (<a href="#v1fig54">Fig. 54</a>) is inserted. In descending order we come first upon -a group of greywackes and grey -shattery mudstones (<i>a</i>), followed by -grey-green and dark banded cherts, -containing Radiolaria and much plicated. -Next comes a group of dark-grey, -black and red cherts, with -numerous partings and thin bands -of tuff and volcanic conglomerate (<i>c</i>). -The siliceous bands were certainly -deposited during the volcanic eruptions, -and they are moulded round the rugose, slaggy upper surface of the -band of lavas (<i>d</i>) on which they directly lie. These lavas have the sack-like -or pillow structure already described, and they enclose lumps of chert containing -Radiolaria. A few yards to the west of the line of section bands of -nodular tuff are interposed between the top of the lavas and the overlying -cherts, with which also they are interstratified. These tuffs contain blocks -<span class="pagenum" id="Page_199">- 199 -</span> -of lava six inches or more in diameter. Below the belt of lavas come -black cherts and shales (<i>e</i>) succeeded below by volcanic breccias and tuffs -(<i>f</i>) alternating with shales in thin inconstant courses. These coarse detrital -rocks are thoroughly volcanic in origin, and they contain fragments of the -black cherts which lie still lower in the series. The whole depth of strata -represented in this section does not amount to much more than 100 feet.</p> - -<p>While in some parts of the Ayrshire district the coarse breccias that -accumulated around their parent vents form most of the upper part of -the volcanic series, in others the lavas are succeeded by fine tuffs -which are intercalated among the ordinary sediments, and show a -gradual decline and cessation of volcanic energy. South of Ballantrae, -for example, the lavas occupy more than two miles of coast, in which -space they display hardly any intercalations of sedimentary material, though -they show more or less distinctly that they consist of many separate flows. -Where they at last end, bands of nodular and fine tuff make their appearance, -together with bands of ashy shale and the characteristic zone of the -red radiolarian cherts or flints. Above these, in conformable sequence, -come bands of black shale, containing abundant Upper Llandeilo graptolites, -overlain by greenish or olive-coloured shaly mudstones, which pass upward -into a thick overlying group of greywackes.</p> - -<p>In this section the alternation of fine pyroclastic with ordinary sediment -shows that the volcanic eruptions in the southern part of the Ballantrae -district came to an end by a slowly-lessening series of explosions. The -ashy material gradually dies out, and does not reappear all through the -thick group of sandy and muddy sediments which here overlies the volcanic -series.</p> - -<p>We thus learn from the evidence of the Ayrshire sections that volcanic -action was in full vigour in the south-west of Scotland during the Arenig -period, but gradually died out before the end of the Llandeilo period. The -rocks in which this volcanic history is chronicled have been very greatly -disturbed and plicated, so that though from their frequently vertical position -they might be thought to attain a vast depth, they very possibly do not -exceed 500 feet in thickness.</p> - -<p>As the volcanic series is followed north-eastwards it exhibits a gradual -diminution in extent and variety, but this may be at least partly due to the -much less depth of it exposed on the crests of the narrow anticlines that -bring it to the surface. There is evidence in that region that the eruptions -did not everywhere terminate in the Llandeilo period, but were in some -districts prolonged into the age of the Bala rocks. Thus in the neighbourhood -of Sanquhar volcanic breccias, tuffs and lavas have been found by -Messrs. Peach and Horne intercalated in strata apparently belonging to -the Bala group. Again, in the district of Hartfell, a moderately coarse -volcanic agglomerate occurs in the heart of the so-called "barren mudstones" -of the Hartfell black-shale group, which, from its graptolites, is -placed on the horizon of the Bala rocks. At Winkstone, Hamilton Hill, -and Wrae in Peeblesshire, perlitic felsites and soda-felsites have been -<span class="pagenum" id="Page_200">- 200 -</span> -detected by Messrs. Peach and Horne and determined by Mr. Teall. They -are associated with the Bala limestone, which in some of its conglomeratic -bands contains pebbles of felsite.</p> - -<p>The intrusive rocks which accompany the Lower Silurian volcanic series -of the south of Scotland are best displayed in the south-west of Ayrshire, -between Girvan and Ballantrae, where they appear to be on the whole -later than at least the great mass of the interstratified lavas and -tuffs. The most abundant rocks and the earliest to be injected are -complex basic masses which include serpentine, olivine-enstatite rock, -troctolite, gabbro and other compounds, all which may be different modifications -of the same original basic magma. They do not show a finer texture -where they respectively meet, nor any other symptom of having been subsequently -intruded into each other, though they do exhibit such structures -along their lines of contact with the surrounding rocks, into which they -are intrusive. These more basic masses have subsequently been invaded by -irregular bosses and dyke-like protrusions, which, when small, are fine-grained -dolerites, but when in larger bodies take the form of gabbro, -sometimes exhibiting a mineral banding and foliated structure. These -banded varieties much resemble the banded Tertiary gabbros of Skye and -some parts of the Lewisian gneiss.</p> - -<p>At the Byne Hill, near Girvan, a large intrusive boss or ridge displays -on its outer margin a fine-grained texture, where it comes in contact with -the serpentine. Further inwards it becomes a fine dolerite, passing into gabbro -and increasing in coarseness of grain as well as in acidity of composition, -through stages of what in the field would be called diorite and quartz-diorite, -into a central granitic rock, whereof milky or blue quartz forms the prominent -constituent. The intrusive rocks of this district have generally been injected -parallel to the stratification-planes, and take on the whole the form of sills.</p> - -<p>Some time after the close of the volcanic episode in the Silurian period -of the south of Scotland, the rocks were locally subjected to considerable -disturbance and elevation, whereby parts of the volcanic series were exposed -to extensive denudation. Hence the overlying unconformable Caradoc -conglomerates are in some places largely made up of the detritus of the -volcanic rocks. It is interesting to find this evidence of waste during -the very next stage of the Silurian period, for it affords good evidence -that the extensive sheets of intrusive material could not have had any -large amount of overlying strata resting upon them at the time of their -injection. Pieces of these intrusive rocks, such as the serpentine, occur -abundantly in the Caradoc conglomerates, some of which indeed are -almost wholly composed of their detritus. Probably the total thickness -of the overlying cover of rock under which the sills were injected did -not amount to as much as 200 or 300 feet. Yet we see that among -the sills were coarse gabbros and granitoid rocks. We may therefore infer -that for the injection of such intrusive masses, great depth and enormous -superincumbent pressure are possibly not always necessary.</p> - -<p><span class="pagenum" id="Page_201">- 201 -</span></p> - -<p>During the progress of the Geological Survey along the southern borders -of the Highlands, a remarkable group of rocks has been observed, intervening -as a narrow interrupted strip between the schistose masses to the north and -the great boundary-fault which brings the Old Red Sandstone in vertical -strata against them. Between Cortachy in Forfarshire and Stonehaven on -the east coast, these rocks have been mapped by Mr. G. Barrow, who has -carefully worked out their relations. They appear again between Callander -and Loch Lomond, where their extent and structure have been mapped by -Mr. C. T. Clough. For the purpose of our present inquiry two chief -features of interest are presented by these rocks. They include a group -of sedimentary strata among which occur bands of jasper or chert containing -radiolaria, and one of their most conspicuous members is a series of -volcanic rocks consisting chiefly of dolerites and basalts, some of which have -been much crushed and cleaved, but in which vesicular structures can still -occasionally be recognized.</p> - -<p>The striking resemblance of both the aqueous and igneous members of -this marginal strip of rocks along the Highland border to the Arenig cherts -and their accompanying lavas in the south of Scotland, the remarkable -association of the same kinds of material in the same order of sequence, the -occurrence of radiolaria in the siliceous bands in both regions, furnish -strong presumptive evidence that a strip of Arenig rocks has been wedged -in against the Highland schists.</p> - -<p>In many respects, these dull green diabasic lavas of the Highland -border resemble those of the Ayrshire coast. In particular, the same -peculiar sack-like or pillow-shaped masses are conspicuous in the Forfarshire -ravines. As in Ayrshire, igneous materials underlie the cherts -which are doubled over and repeated by many successive folds. Unfortunately, -it is only a narrow strip of these probably Arenig lavas that has -been preserved, and no trace has been detected of tuffs, agglomerates or -necks. If, however, we may regard the rocks as truly of Arenig age, they -furnish interesting additional proof of the wide extent of the earliest Silurian -volcanoes. The distance between the last Arenig volcanic outcrop in the -Southern uplands and the band of similar lavas along the margin of the -Highlands is about 50 miles. If the volcanic ejections were continuous -across the intervening tract, the total area over which the lavas and tuffs of -the Arenig volcanoes were distributed must be increased by at least 6000 -square miles in Scotland.</p> - -<p>But it is in the north of Ireland that this northern extension of what -may probably be regarded as an Arenig series of volcanic rocks attains its -greatest development. Of this Irish prolongation a brief account is given -in <a href="#CHAPTER_XIV">Chapter xiv.</a>, where the whole of the Silurian volcanic rocks of the -island are discussed.</p> - -<hr class="chap x-ebookmaker-drop" /> - - -<div class="chapter"> -<p><span class="pagenum" id="Page_202">- 202 -</span></p> - -<h3 class="nobreak" id="CHAPTER_XIII">CHAPTER XIII<br /> - -<span class="smaller">THE ERUPTIONS OF LLANDEILO AND BALA AGE</span></h3> -</div> - -<div class="blockquot"> - -<p>i. The Builth Volcano—ii. The Volcanoes of Pembrokeshire—iii. The Caernarvonshire -Volcanoes of the Bala Period—iv. The Volcanic District of the Berwyn Hills—v. -The Volcanoes of Anglesey—vi. The Volcanoes of the Lake District; Arenig -to close of Bala Period—vii. Upper Silurian (?) volcanoes of Gloucestershire.</p> -</div> - - -<p>The stratigraphical subdivisions of geology are necessarily more or less -arbitrary. The sequence in the sedimentary deposits of one region always -differs in some degree from that of adjoining regions. In drawing up a -table of stratigraphical equivalents for separate countries, we must be content -to accept a general parallelism, without insisting on too close an identity -in either the character of the strata or the grouping of their organic remains. -We need especially to guard against the assumption that the limit assigned -to a geological formation in any country marks a chronological epoch which -will practically agree with that denoted by the limit fixed for the same -formation in another country. The desirability of caution in this respect is -well shown by the vagueness of the horizons between the several subdivisions -of the Lower Silurian system. So long as the areas of comparison are near -each other, no great error may perhaps be committed if their stratigraphical -equivalents are taken to have been in a broad geological sense contemporary. -But in proportion as the element of distance comes in, there enters with it -the element of uncertainty.</p> - -<p>Even within so limited a region as the British Isles, this difficulty makes -itself strongly felt. Thus, in the typical regions of Wales, the several subdivisions -of the Lower Silurian strata are tolerably well marked, both by -lithological nature and by fossils. But as they are followed into other parts -of the country, they assume new features, sometimes increasing sometimes -diminishing in thickness, changing their sedimentary character, and altering -the association or range of their organisms. The subdivisions into which the -geologist groups them may thus be vaguely defined by limits which, in -different parts of the region, may be far from representing the same periods -of time.</p> - -<p>Hence, in trying to ascertain how far the volcanic eruptions of one area -during the Silurian period may have been contemporary with those of another -<span class="pagenum" id="Page_203">- 203 -</span> -area, we must be content to allow a wide margin for error. It is hardly -possible to adhere strictly to the stratigraphical arrangement, for the geological -record shows that in the volcanic districts the sedimentary formations -by which the chronology might have been worked out are not infrequently -absent or obscure. It will be more convenient to treat the rest of the Lower -Silurian formations as the records of one long and tolerable definite section -of geological time, without attempting in each case to distinguish between -the eruptions of the successive included periods, so long as the actual -volcanic sequence is distinctly kept in view. I will therefore take the -history of each district in turn and follow its changes from the close of -the Arenig period to the end of Upper Silurian time. The stages in the -volcanic evolution of each tract will thus be clearly seen.</p> - -<p>Above the Arenig group with its voluminous volcanic records comes the -great group of sediments known as the Llandeilo formation, in which also -there are proofs of contemporaneous volcanic activity over various parts of the -sea-floor within the site of Britain. We have seen that in the south of -Scotland the eruptions of Arenig time were probably continued into the -period of the Llandeilo rocks, or even still later into that of the Bala group. -But it is in Wales that the history of the Llandeilo volcanoes is most fully -preserved. A series of detached areas of volcanic rocks, intercalated among -the Llandeilo sediments, may be followed for nearly 100 miles, from the -northern end of the Breidden Hills in Montgomeryshire, by Shelve, Builth, -Llanwrtyd and Llangadock, to the mouth of the Taf river. But some 35 miles -further west another group of lavas and tuffs appears on the coast of Pembrokeshire, -from Abereiddy Bay to beyond Fishguard. The want of continuity in -these scattered outcrops is no doubt partly due to concealment by geological -structure. But from the comparative thinness of the volcanic accumulations -and their apparent thinning out along the strike it may be inferred that no -large Llandeilo volcano existed in Wales. There would rather seem to -have been a long line of minor vents which in the south-east part of the -area appear to have only discharged ashes. Certainly, if we may judge -from their visible relics, these eruptions never rivalled the magnitude of the -discharges from the Arenig volcanoes that preceded, or the Bala volcanoes -that followed them.</p> - - -<h4>i. <span class="allsmcap">THE VOLCANO OF BUILTH AND ITS NEIGHBOURS</span></h4> - -<p>So far as the available evidence goes, the most important volcanic centre -down the eastern side of Wales during the Llandeilo period was one which -lay not far from the centre of the long line of vents just referred to. Its -visible remains form an isolated tract of hilly ground, some seven miles long, -and four or five miles broad, immediately north from the town of Builth. -This area is almost entirely surrounded by unconformable Upper Silurian -strata, so that its total extent is not seen, and may be much more considerable -than the area now laid bare by denudation.</p> - -<p><span class="pagenum" id="Page_204">- 204 -</span></p> - -<p>The volcanic rocks of Builth were first described in the "Silurian -System." Murchison clearly recognized that they included some which -were "evolved from volcanic apertures during the submarine accumulation -of the Lower Silurian rocks," and also "unbedded volcanic masses which had -been intruded subsequently, dismembering and altering all the strata with -which they came in contact."<a id="FNanchor_177" href="#Footnote_177" class="fnanchor">[177]</a> These igneous rocks were mapped in some -detail by the Geological Survey, and their general relations were expressed -in lines of horizontal section.<a id="FNanchor_178" href="#Footnote_178" class="fnanchor">[178]</a> They were likewise described by Ramsay in -the <i>Catalogue of the Rock-specimens in the Jermyn Street Museum</i>, specimens -of them being displayed in that collection.<a id="FNanchor_179" href="#Footnote_179" class="fnanchor">[179]</a> The tuffs and lavas were -distinguished, and likewise the intrusive "greenstones." But no attempt -was made towards petrographical detail.</p> - -<div class="footnote"> - -<p><a id="Footnote_177" href="#FNanchor_177" class="label">[177]</a> <i>Silurian System</i>, 1839, p. 330. The occurrence of "trappean ash" with fossils in the Builth -district was noticed by De la Beche, <i>Mem. Geol. Surv.</i> vol. i. (1846), p. 31.</p> - -<p><a id="Footnote_178" href="#FNanchor_178" class="label">[178]</a> See Sheet 56 of the one-inch map and Sheets 5 and 6 of the Horizontal Sections.</p> - -<p><a id="Footnote_179" href="#FNanchor_179" class="label">[179]</a> <i>Catalogue of Rock Specimens</i>, 3rd edit. 1862, p. 36 <i>et seq.</i></p> - -</div> - -<p>This interesting district has recently been studied by Mr. Henry Woods,<a id="FNanchor_180" href="#Footnote_180" class="fnanchor">[180]</a> -who has grouped the igneous rocks in probable order of appearance, as -follows:—1st, Andesites; 2nd, Andesitic ash; 3rd, Rhyolites; 4th, Diabase-porphyrite; -and 5th, Diabase.</p> - -<div class="footnote"> - -<p><a id="Footnote_180" href="#FNanchor_180" class="label">[180]</a> <i>Quart. Journ. Geol. Soc.</i> vol. l. (1894), p. 566.</p> - -</div> - -<p>Some of the andesites are described as intrusive in the Llandeilo strata. -The ash in its lower part contains numerous well-rounded pebbles of -andesite, usually five or six inches in diameter, but sometimes having a -length of two feet. It contains fossils (<i>Orthis calligramma</i>, <i>Leptæna sericea</i>, -<i>Serpulites dispar</i>, etc.), and as it is overlain with shales containing <i>Ogygia -Buchii</i>, it may be regarded as probably of Lower Llandeilo age. The -rhyolites are feebly represented, and some of them may possibly be intrusive. -Among them a nodular variety has been noticed, the nodules being solid -throughout, varying up to two inches in diameter, and formed of microcrystalline -quartz and felspar, with no trace of any radial or concentric -internal arrangement. The diabase-porphyrite, the most conspicuous rock -of the district, is intrusive in the andesites and ashes, and occurs in four -separate masses or sills. The diabases are all intrusive and of later date -than any of the other igneous rocks, and as they traverse also the Llandeilo -shales, they are probably considerably later than the previous eruptions. -But as they do not enter the surrounding Llandovery and Wenlock strata, -they are regarded by Mr. Woods as of intermediate age between the time -of the Llandeilo and that of the Upper Silurian formations.</p> - -<p>About nine miles in a west-south-westerly direction from the southern -extremity of the Builth volcanic area, another much smaller exposure of -igneous rocks has been mapped by the Geological Survey at the village of -Llanwrtyd. This tract is only about three miles long and half a mile -broad. The volcanic rocks are represented as consisting of three or more -bands of "felspathic trap" interstratified in the Lower Silurian strata, and -folded into an anticline along the ridge of Caer Cwm. No published line of -<span class="pagenum" id="Page_205">- 205 -</span> -section runs across this ground, and the band of rock does not appear to -have been described.<a id="FNanchor_181" href="#Footnote_181" class="fnanchor">[181]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_181" href="#FNanchor_181" class="label">[181]</a> The locality is referred to by De la Beche, <i>Mem. Geol. Surv.</i> vol. i. p. 31, and by Ramsay -in the <i>Descriptive Catalogue of Rock-specimens in the Museum of Practical Geology</i>, 3rd edit. -p. 38, but no specimens from it are in the collection.</p> - -</div> - -<p>Seventeen miles to the south-west a still feebler display of intercalated -volcanic material occurs in the Llandeilo formation near the village of -Llangadock. The Geological Survey map represents one or more bands of -ash associated with limestone, and thrown into a succession of folds. In -the <i>Horizontal Section</i> (Sheet III. Section 3) a band, 100 to 200 feet thick, -of "trappean ash" with fossils is shown among the shales, limestones and grits, -and in the <i>Catalogue of Rock-specimens</i> the same rock is referred to as -brecciated ash in connection with specimens of it in the Museum, which are -described as not purely ashy, but containing many slate-fragments and -broken felspar-crystals together with organic remains.<a id="FNanchor_182" href="#Footnote_182" class="fnanchor">[182]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_182" href="#FNanchor_182" class="label">[182]</a> <i>Op. cit.</i> p. 38.</p> - -</div> - -<p>About twenty-four miles still further in the same south-westerly -direction, two patches of "ash" are shown upon the Survey map, near the -mouth of the river Taf. No description of these rocks is given.<a id="FNanchor_183" href="#Footnote_183" class="fnanchor">[183]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_183" href="#FNanchor_183" class="label">[183]</a> One of the patches was shown by J. Phillips in <i>Horizontal Section</i>, Sheet III. Section 6, -as a "felspathic trap," near which the shales are bleached. The map, however, was subsequently -altered, so as to make the igneous rocks pyroclastic.</p> - -</div> - - -<h4>ii. <span class="allsmcap">THE VOLCANOES OF PEMBROKESHIRE</span></h4> - -<p>In north-western Pembrokeshire, the observations of Murchison, De la -Beche and Ramsay showed the existence of an important volcanic district, -where numerous igneous bands are interstratified among the Lower Silurian -rocks, over an area extending from St. David's Head for thirty miles to -the eastward.<a id="FNanchor_184" href="#Footnote_184" class="fnanchor">[184]</a> On the maps of the Geological Survey, lavas, tuffs, sills -and bosses were discriminated, but no description of these rocks was -published. Since the publication of the Survey map very little has yet -been added to our information on the subject.</p> - -<div class="footnote"> - -<p><a id="Footnote_184" href="#FNanchor_184" class="label">[184]</a> See <i>Silurian System</i>, p. 401; Sheet 40 of the Geological Survey Map; <i>Memoir of A. C. -Ramsay</i>, p. 232 <i>et seq.</i>; De la Beche, <i>Trans. Geol. Soc.</i> 2nd series, vol. ii. part i. (1826), p. 3.</p> - -</div> - -<p>There appear to have been at least three principal groups of vents. -One may be indicated by the bands of "felspathic trap" which have been -mapped as extending from near St. Lawrence for fourteen miles to the east. -Another must have existed in the neighbourhood of Fishguard. A third is -shown to have lain between Abereiddy Bay and Mathry, by the abundant -bands of lava and tuff and intrusive sills there to be seen.</p> - -<p>Of these areas the only one which has yet been examined and described -in some detail is that of Fishguard, of which an account has recently been -published by Mr. Cowper Reed.<a id="FNanchor_185" href="#Footnote_185" class="fnanchor">[185]</a> This observer has shown that the -eruptions began there during the deposition of the Lower Llandeilo rocks, -and continued intermittently into the Bala period. The earliest consisted -of felsites and tuffs intercalated between Lower Llandeilo black slates containing -<span class="pagenum" id="Page_206">- 206 -</span> -<i>Didymograptus Murchisoni</i>, the tuffs themselves being sometimes -fossiliferous. A second great volcanic belt, composed of felsitic lavas, -breccias and tuffs, lies at the base of the Upper Llandeilo strata and shows -the maximum of volcanic energy. The breccias are partly coarse agglomerates, -which probably represent, or lie not far from, some of the eruptive -vents of the time. A higher band of lavas and breccias appears to -be referable to the Bala formation. The whole volcanic series is stated to -thin out towards the south-west, so that the chief focus of eruption probably -lay somewhere in the neighbourhood of Fishguard.</p> - -<div class="footnote"> - -<p><a id="Footnote_185" href="#FNanchor_185" class="label">[185]</a> <i>Quart. Journ. Geol. Soc.</i> vol. li. (1895), p. 149.</p> - -</div> - -<p>The lavas may all be included under the general term felsite. Their -specific gravity ranges from 2·60 to 2·76, and their silica percentage from -68 to 72. Mr. Cowper Reed observed among them three conspicuous types -of structure. Some are characterized by a distinct arrangement in fine -light and dark bands which rapidly alternate, and are sometimes thrown -into folds and convolutions. A second structure, observed only at one -locality, consists in the development of pale grey or whitish ovate nodules, -about half an inch in length, with a clear quartz-grain in their centre, or -else hollow. The third type is shown by the appearance of perlitic structure -on the weathered surface.<a id="FNanchor_186" href="#Footnote_186" class="fnanchor">[186]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_186" href="#FNanchor_186" class="label">[186]</a> Mr. Cowper Reed enters into a detailed account of the microscopic structures and chemical -composition of these rocks. They have rather a high percentage of alumina, potash and soda, -and are obviously akin to the keratophyres of other districts.</p> - -</div> - -<p>The tuffs and breccias are chiefly developed at the base and top of each -volcanic group. Some of them contain highly vesicular fragments, as well -as pieces of slate and broken crystals of quartz and felspar.</p> - -<p>A characteristic feature of this volcanic district is the occurrence in it -of sills and irregularly-intruded masses of "greenstone." Under that name -are comprised basalts, dolerites, andesitic dolerites with tachylitic modifications, -as well as diabases and gabbros.<a id="FNanchor_187" href="#Footnote_187" class="fnanchor">[187]</a> Some of these rocks exhibit a -variolitic structure. As regards age, some of the intrusions appear to -have taken place before the tilting, cleavage and faulting of the strata. They -have not been noticed in the surrounding Upper Silurian strata, and we may -perhaps infer that here, as at Builth, they are of Lower Silurian date. Mr. -Cowper Reed, however, is inclined to regard the large Strumble Head masses -as later than the tilting and folding of the rocks.<a id="FNanchor_188" href="#Footnote_188" class="fnanchor">[188]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_187" href="#FNanchor_187" class="label">[187]</a> Mr. Cowper Reed, <i>op. cit.</i> p. 180.</p> - -<p><a id="Footnote_188" href="#FNanchor_188" class="label">[188]</a> <i>Op. cit.</i> p. 193.</p> - -</div> - -<p>A few miles to the south-west of the Fishguard district, on the coast of -Abereiddy Bay, good sections have been laid bare of the volcanic rocks of -this region. Dr. Hicks has shown that the bands of tuff there displayed -are intercalated among the black slates of the Lower Llandeilo group, and -that there was probably a renewal of volcanic activity during the deposition -of the upper group.<a id="FNanchor_189" href="#Footnote_189" class="fnanchor">[189]</a> But the volcanic history of this area still remains to -be properly investigated.</p> - -<div class="footnote"> - -<p><a id="Footnote_189" href="#FNanchor_189" class="label">[189]</a> <i>Quart. Journ. Geol. Soc.</i> xxxi. (1875), p. 177.</p> - -</div> - -<p>In southern Pembrokeshire two conspicuous bands of eruptive rocks -have long been known and described. Their general characters and distribution -<span class="pagenum" id="Page_207">- 207 -</span> -were sketched by De la Beche,<a id="FNanchor_190" href="#Footnote_190" class="fnanchor">[190]</a> and further details were afterwards -added by Murchison.<a id="FNanchor_191" href="#Footnote_191" class="fnanchor">[191]</a> As traced by the officers of the Geological Survey, -they were represented as consisting of "greenstone," "syenite" and "granite." -The more northerly band was shown to run in a nearly east and west line -from Lawrenny to the Stack Rock, west of Talbenny, a distance of about -fourteen miles. The second band, placed a short way farther south, stretches -in the same general line, from Milford Haven at Dall Road into Skomer -Island, a distance of about seven miles.</p> - -<div class="footnote"> - -<p><a id="Footnote_190" href="#FNanchor_190" class="label">[190]</a> <i>Trans. Geol. Soc.</i>, 2nd ser. vol. ii. (1823), p. 6 <i>et seq.</i></p> - -<p><a id="Footnote_191" href="#FNanchor_191" class="label">[191]</a> <i>Silurian System</i>, p. 401 <i>et seq.</i></p> - -</div> - -<p>The relations of these rocks to the surrounding formations and their -geological age have been variously interpreted. De la Beche regarded the -different masses as intrusive, and probably later than even the adjoining Coal-measures.<a id="FNanchor_192" href="#Footnote_192" class="fnanchor">[192]</a> -Murchison, on the other hand, considered the bedded eruptive -rocks of Skomer Island to be undoubtedly lavas contemporaneous with the -strata among which they are intercalated.<a id="FNanchor_193" href="#Footnote_193" class="fnanchor">[193]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_192" href="#FNanchor_192" class="label">[192]</a> <i>Mem. Geol. Survey</i>, vol. i. p. 231.</p> - -<p><a id="Footnote_193" href="#FNanchor_193" class="label">[193]</a> <i>Silurian System</i>, p. 404.</p> - -</div> - -<p>The rocks have been studied petrographically by various observers. Mr. -Rutley gave a full description of the remarkable nodular and banded felsites -of Skomer Island.<a id="FNanchor_194" href="#Footnote_194" class="fnanchor">[194]</a> Mr. Teall has also noticed these rocks, likewise "a -magnificent series of basic lava-flows" in the same island, and a number of -"porphyrites." The basic lavas seemed to him to contain too much felspar -and too little olivine to be regarded as perfectly typical olivine-basalts, and -he found them to lie sometimes in very thin and highly vesicular sheets. -The "porphyrites" he placed "on the border-line between basic and intermediate -rocks."<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> "The Felsitic Lavas of England and Wales," <i>Mem. Geol. Survey</i> (1885), pp. 16, 18.</p> - -<p><a id="Footnote_195" href="#FNanchor_195" class="label">[195]</a> <i>British Petrography</i>, pp. 224, 284, 336.</p> - -</div> - -<p>More recently this southern district of Pembrokeshire has been examined -by Messrs. F. T. Howard and E. W. Small, who have obtained further -evidence of the interbedded character of the igneous series. Below an -upper basalt they have noted the occurrence of bands of felsitic conglomerate, -sandstone, shale and breccia lying upon and obviously derived -from a banded spherulitic felsite, below which comes a lower group of -basalts. The age of this interesting alteration of basic and acid eruptions -has not been precisely determined, but is conjectured to be that of the -Bala or Llandovery rocks.<a id="FNanchor_196" href="#Footnote_196" class="fnanchor">[196]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_196" href="#FNanchor_196" class="label">[196]</a> <i>Rep. Brit. Assoc.</i> 1893, p. 766; <i>Geol. Mag.</i> 1896, p. 481.</p> - -</div> - - -<h4>iii. <span class="allsmcap">THE CAERNARVONSHIRE VOLCANOES OF THE BALA PERIOD</span></h4> - -<p>Owing to the effects partly of plication and partly of denudation, the -rocks of the next volcanic episode in Wales, that of the Bala period, occupy -a less compact and defined area than those of the Arenig group in Merionethshire. -From the latter they are separated, as we have seen, by a considerable -depth of strata,<a id="FNanchor_197" href="#Footnote_197" class="fnanchor">[197]</a> whence we may infer, with the Geological Survey, -that the eruptions of Arenig, the Arans and Cader Idris were succeeded by -a long period of repose, the Llandeilo outbreaks described in the foregoing -<span class="pagenum" id="Page_208">- 208 -</span> -pages not having extended apparently into North Wales. When the next -outbreaks took place, the vents are found to have shifted northwards into -Caernarvonshire, where they fixed themselves along a line not much to the -east of where the Cambrian porphyries and tuffs now appear at the surface. -The lavas and ashes that were thrown out from these vents form the highest -and most picturesque mountains of North Wales, culminating in the noble -cone of Snowdon. They stretch northwards to Diganwy, beyond Conway, -and southwards, at least as far as the neighbourhood of Criccieth. They -die out north-eastwards beyond Bala Lake, and there can be but little doubt -that they thin out also eastwards under the Upper Bala rocks. The lavas -and tuffs that rise up on a similar horizon among the Bala rocks of the -Berwyn Hills evidently came not from the Snowdonian vents, but from -another minor volcanic centre some miles to the east, while still more -remote lay the vents of the Breidden Hills and the sheets of andesitic tuff -that probably spread from them over the ground east of Chirbury (<a href="#Map_II">Map II.</a>).</p> - -<div class="footnote"> - -<p><a id="Footnote_197" href="#FNanchor_197" class="label">[197]</a> Estimated at from 6000 to 7000 feet, <i>Mem. Geol. Surv.</i> vol. iii. 2nd edit. p. 131.</p> - -</div> - -<p>The Caernarvonshire volcanic group extends from north to south for -fully thirty miles, with an extreme breadth of about fifteen miles; while, if -we include the rocks of the Lleyn peninsula, the area will be prolonged some -twenty miles farther to the south-west.</p> - -<p>The general stratigraphical horizon of this volcanic group has been -well determined by the careful mapping of Ramsay, Selwyn and Jukes on -the maps of the Geological Survey. These observers brought forward -ample evidence to show that the lavas and tuffs were erupted during the -deposition of the Bala strata of the Lower Silurian series, that the Bala -Limestone is in places full of ashy material, and that this well-marked fossiliferous -band passes laterally into stratified volcanic tuffs containing the -same species of fossils.<a id="FNanchor_198" href="#Footnote_198" class="fnanchor">[198]</a> But the progress of stratigraphical geology, and -the increasing value found to attach to organic remains as marking even -minor stratigraphical horizons, give us reason to believe that a renewed and -still more detailed study of the Bala rocks of North Wales would probably -furnish data for more precisely defining the platforms of successive eruptions, -and would thus fill in the details of the broad sketch which Sir Andrew -Ramsay and his associates so admirably traced. Besides the Bala Limestone -there may be other lithological horizons which, like the Garth grit and the -pisolitic iron-ore of the Arenig group, might be capable of being followed -among the cwms and crests as well as the opener valleys of Caernarvonshire. -Until some such detailed mapping is accomplished, we cannot safely -advance much beyond the point where the stratigraphy was left by the -Survey.</p> - -<div class="footnote"> - -<p><a id="Footnote_198" href="#FNanchor_198" class="label">[198]</a> <i>Mem. Geol. Surv.</i> vol. iii. 2nd edit. pp. 126, 128, 131, 139, etc.</p> - -</div> - -<p>From the Survey maps and sections it is not difficult to follow the general -volcanic succession, and to perceive that the erupted materials must altogether -be several thousand feet in thickness from the lowest lavas in the north to -the highest on the crest of Snowdon. In that mountain the total mass of -volcanic material is set down as 3100 feet. But this includes only the -higher part of the whole volcanic group. Below it come the lavas of Y -<span class="pagenum" id="Page_209">- 209 -</span> -Glyder-Fach, which, according to the Survey measurements, are about 1500 -feet thick, while still lower lie the ancient <i>coulées</i> of Carnedd Dafydd and -those that run north from the vent of Y-foel-frâs, which must reach a united -thickness of many hundred feet. We can thus hardly put the total depth -of volcanic material at a maximum of less than 6000 to 8000 feet. The -pile is, of course, thickest round the vents of discharge, so that no measurement, -however carefully made at one locality, would be found to hold good -for more than a short distance.</p> - -<p>Though little is said in the Survey Memoir of the vents from which this -vast amount of volcanic material was erupted, the probable positions of a -number of these orifices may be inferred from the maps. From the shore -west of Conway a series of remarkable eminences may be traced south-westwards -for a distance of nearly forty miles into the peninsula of Lleyn. At -the northern extremity of this line stands the prominent boss of Penmaen-mawr, -while southward beyond the large mass of Y-foel-frâs, with the smaller -knobs west of Nant Francon, and the great dome of Mynydd-mawr, the -eye ranges as far as the striking group of <i>puy</i>-like cones that rise from the -sea around Yr Eifl and Nevin. Some of these hills, particularly Y-foel-frâs, -were recognized by the Survey as vents.<a id="FNanchor_199" href="#Footnote_199" class="fnanchor">[199]</a> But the first connected account -of them and of their probable relation to the volcanic district in which they -occur has been given by Mr. Harker in his exceedingly able essay on "The -Bala Volcanic Series of Caernarvonshire,"<a id="FNanchor_200" href="#Footnote_200" class="fnanchor">[200]</a>—the most important contribution -to the volcanic history of Wales which has been made since the publications -of the Geological Survey appeared. I shall refer to these vents more specially -in the sequel. I allude to them here for the purpose of showing at the -outset the marvellous completeness of the volcanic records of Caernarvonshire. -So great has been the denudation of the region that the pile of lavas -and tuffs which accumulated immediately around and above these orifices has -been swept away. No trace of any portion of that pile has survived to the -west of the line of bosses; while to the east, owing to curvature and subsequent -denudation, the rocks have been dissected from top to bottom, until almost -every phase of the volcanic activity is revealed.</p> - -<div class="footnote"> - -<p><a id="Footnote_199" href="#FNanchor_199" class="label">[199]</a> <i>Op. cit.</i> pp. 137, 220.</p> - -<p><a id="Footnote_200" href="#FNanchor_200" class="label">[200]</a> This was the Sedgwick Prize Essay for 1888, and was published in 1889.</p> - -</div> - -<p>The volcanic products discharged from these vents consist of a succession -of lava-streams separated by bands of slate, tuff, conglomerate and breccia. -These fragmental intercalations, which vary from a few yards to many -hundred feet in thickness, are important not only as marking pauses in the -emission of lava or in the activity of the volcanoes, but as affording a means -of tracing the several lavas to their respective vents. Essentially, however, -the volcanic materials consist of lava-flows, the intercalations of fragmentary -materials, though numerous, being comparatively thin. The thickest accumulation -of tuffs is that forming much of the upper part of Snowdon. It is -set down by my predecessor at 1200 feet in thickness, but I should be -inclined to reduce this estimate. I shall have occasion to show that the -summit and upper shoulders of Snowdon are capped with andesites interstratified -<span class="pagenum" id="Page_210">- 210 -</span> -among the tuffs. Sir Andrew Ramsay has referred with justice to -the difficulty of always discriminating in the field between the fine tuffs and -some of the lavas.<a id="FNanchor_201" href="#Footnote_201" class="fnanchor">[201]</a> Yet I am compelled to admit that, if the ground were -to be re-mapped now, the area represented as covered by fragmental rocks -would be considerably restricted. Mr. Harker is undoubtedly correct when -he remarks that, taken "as a whole, the Bala volcanic series of Caernarvonshire -is rather remarkable for the paucity of genuine ashes and -agglomerates."<a id="FNanchor_202" href="#Footnote_202" class="fnanchor">[202]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_201" href="#FNanchor_201" class="label">[201]</a> <i>Op. cit.</i> p. 148.</p> - -<p><a id="Footnote_202" href="#FNanchor_202" class="label">[202]</a> <i>Bala Volcanic Rocks</i>, p. 25.</p> - -</div> - -<p>The lavas of the Bala volcanic group, like those of the Arenig series, -were mapped by the Survey as "porphyries," "felstones," or "felspathic -traps." They were shown to be acid-lavas, having often a well-developed -flow-structure comparable with that of obsidian and pitchstone, and to -consist of successive sheets that were poured out over the sea-floor. Their -petrography has subsequently been studied more in detail by many observers, -among whom I need only cite Professor Bonney, Professor Cole, Mr. Rutley, -Mr. Teall, and Miss Raisin; the most important recent additions to our -knowledge of this subject having been made by Mr. Harker in the Essay -to which I have just referred.</p> - -<p>The great majority of these lavas are thoroughly acid rocks, and present -close analogies of composition and structure to modern rhyolites, though I -prefer to retain for them the old name of "felsites." Their silica-percentage -ranges from 75 to more than 80. To the naked eye they are externally -pale greyish, or even white, but when broken into below the thick decomposed -and decoloured crust, they are bluish-grey to dark iron-grey, or even -black. They break with a splintery or almost conchoidal fracture, and -show on a fresh surface an exceedingly fine-grained, tolerably uniform -texture, with minute scattered felspars.</p> - -<p>One of their most striking features is the frequency and remarkable -development of their flow-structure. Not merely as a microscopic character, -but on such a scale as to be visible at a little distance on the face of a cliff -or crag, this structure may be followed for some way along the crops of -particular flows. The darker and lighter bands of devitrification, with their -lenticular forms, rude parallelism and twisted curvature, have been compared -to the structure of mica-schist and gneiss. One aspect of this -structure, however, appears to have escaped observation, or, at least, has -attracted less notice than it seems to me to deserve. In many cases it is -not difficult to detect, from the manner in which the lenticles and strips of -the flow-structure have been curled over and pushed onward, what was the -direction in which the lava was moving while still a viscous mass. By -making a sufficient number of observations of this direction, it might in -some places be possible to ascertain the quarter from which the several -flows proceeded. As an illustration, I would refer to one of the basement-felsites -of Snowdon, which forms a line of picturesque crags on the slope -facing Llanberis. The layers of variously-devitrified matter curl and fold -over each other, and have been rolled into balls, or have been broken up -<span class="pagenum" id="Page_211">- 211 -</span> -and enclosed one within the other (<a href="#v1fig55">Fig. 55</a>). The general push indicated -by them points to a movement from the westward. Turning round from -the crags, and looking towards the west, we see before us on the other side -of the deep vale of Llyn Cwellyn, at a distance of little more than three -miles, the great dome-shaped Mynydd-mawr, which, there is every reason to -believe, marks one of the orifices of eruption. It might in this way be -practicable to obtain information regarding even some of the vents that still -lie deeply buried under volcanic or sedimentary rocks.</p> - -<p>That these felsites were poured forth in a glassy condition may be -inferred from the occurrence of the minute perlitic and spherulitic forms so -characteristic of the devitrification of once vitreous rocks. Mr. Rutley was -the first who called attention to this interesting proof of the close resemblance -between Palæozoic felsites and modern obsidians, and other observers have -since confirmed and extended his observations.<a id="FNanchor_203" href="#Footnote_203" class="fnanchor">[203]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_203" href="#FNanchor_203" class="label">[203]</a> <i>Quart. Journ. Geol. Soc.</i> vol. xxxv. (1879), p. 508.</p> - -</div> - -<div class="figcenter" id="v1fig55" style="width: 374px;"> - <img src="images/v1fig55.png" width="374" height="213" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 55.</span>—Flow-structure in the lowest felsite on the track from Llanberis to the top of Snowdon. - Length about 4 feet, height 2½ feet.</div> -</div> - -<p>Another remarkable aspect of the felsites is that nodular structure so -often to be seen among them, and regarding the origin of which so much -has already been written. I agree with Professor Cole and Mr. Harker in -looking upon the "nodules" as derived from original spherulites by a process -of alteration, of which almost every successive stage may be traced until the -original substance of the rock has been converted into a flinty or agate-like -material. If this be the true explanation of the structure, some of the -original lavas must have exhibited perlitic and spherulitic forms on a -gigantic scale. There can, I think, be little doubt that this peculiar structure -was very generally misunderstood by the earlier observers, who naturally -looked upon it as of clastic origin, and who therefore believed that large -beds of rock consisted of volcanic conglomerate, which we should now map -as nodular felsite (pyromeride).<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> Another source of error may probably be traced in the occasional brecciated structure of the -felsites, which has been mistaken for true volcanic breccia, but which can be traced disappearing -into the solid rock. Sometimes this structure has resulted from the breaking up of the lenticles -of flow, sometimes from later crushing.</p> - -</div> - -<p><span class="pagenum" id="Page_212">- 212 -</span></p> - -<p>While by far the larger proportion of the Caernarvonshire lavas consists -of thoroughly acid rocks, the oldest outflows are much less acid than -those erupted at the height of the volcanic activity, when the rocks of Snowdon -were poured forth.<a id="FNanchor_205" href="#Footnote_205" class="fnanchor">[205]</a> But towards the close of the period there was apparently -a falling off in the acidity of the magma, for at the top of the group the -andesitic lavas to which I have already alluded are encountered. Sir -Andrew Ramsay has shown the existence of an upper "felstone" or "felspathic -porphyry," almost entirely removed by denudation, but of which -outliers occur on Crib-goch, Lliwedd, and other crests around Snowdon, and -likewise on Moel Hebog.<a id="FNanchor_206" href="#Footnote_206" class="fnanchor">[206]</a> Mr. Harker alludes to these remnants, and -speaks of them as less acid than the older lavas, but he gives no details as -to their structure and composition.<a id="FNanchor_207" href="#Footnote_207" class="fnanchor">[207]</a> In an examination of Snowdon I was -surprised to find that the summit of the mountain, instead of consisting of -bedded ashes as hitherto represented, is formed of a group of lava-sheets -having a total thickness of perhaps from 100 to 150 feet (6 in <a href="#v1fig56">Fig. 56</a>). -The apex of Yr Wyddfa, the peak of Snowdon, consists of fossiliferous shale -lying on a dull grey rock that weathers with elongated vesicles, somewhat -like a cleaved amygdaloid, but a good deal decomposed. A thin slice of this -latter rock shows under the microscope irregular grains and microlites of -felspar, with a few grams of quartz, the whole much sheared and calcified. -Below this bed comes a felsite, or devitrified obsidian, showing in places -good spherulitic structure, and followed by a grey amygdaloid. The latter -is a markedly cellular rock, and, though rather decayed, shows under the -microscope a microlitic felspathic groundmass, through which granules of -magnetite are dispersed.</p> - -<div class="footnote"> - -<p><a id="Footnote_205" href="#FNanchor_205" class="label">[205]</a> Mr. Harker, <i>op. cit.</i> p. 127.</p> - -<p><a id="Footnote_206" href="#FNanchor_206" class="label">[206]</a> <i>Mem. Geol. Surv.</i> vol. iii. 2nd edit. pp. 141, 144, 145, 147, 161.</p> - -<p><a id="Footnote_207" href="#FNanchor_207" class="label">[207]</a> <i>Bala Volcanic Series</i>, pp. 10, 23, 125. He refers also to lavas occupying a similar position -at Nant Gwynant and Moel Hebog; but he adds that he had not had an opportunity of studying -them.</p> - -</div> - -<div class="figcenter" id="v1fig56" style="width: 489px;"> - <img src="images/v1fig56.png" width="489" height="127" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 56.</span>—Section of Snowdon.<a id="FNanchor_208" href="#Footnote_208" class="fnanchor">[208]</a><br /> - 1. Grits and slates; 2. Felsite with good flow-structure; 3. Volcanic tuffs; 4. Felsite; 5. Tuffs with sheets of - felsite and andesite; 6. Group of andesitic lavas on summit of Snowdon; 7. Intrusive "greenstones."</div> -</div> - - -<div class="footnote"> - -<p><a id="Footnote_208" href="#FNanchor_208" class="label">[208]</a> After the Geological Survey Section (Horizont. Sect. Sheet 28), slightly modified.</p> - -</div> - -<p>Underneath this upper group of lavas lie the tuffs for which Snowdon -has been so long celebrated. But, as I have already stated, there does -not appear to me to be such a continuous thickness of fragmental material -as has been supposed. There cannot, I think, be any doubt that not -only at the top, but at many horizons throughout this supposed thick -<span class="pagenum" id="Page_213">- 213 -</span> -accumulation of tuff, some of the beds of rock are really lava-flows. -Some of these lavas have suffered considerably from the cleavage which -has affected the whole of the rocks of the mountain, while the results -of centuries of atmospheric disintegration, so active in that high exposed -locality, have still further contributed to alter them. They consequently -present on their weathered faces a resemblance to the pyroclastic rocks -among which they lie. Where, however, the lavas are thicker and more -massive, and have resisted cleavage better, some of them appear as cellular -dull grey andesites or trachytes, while a few are felsites. Many instructive -sections of such bands among the true tuffs may be seen on the eastern -precipices of Snowdon above Glas-lyn.</p> - -<p>It thus appears that the latest lavas which flowed from the Snowdonian -vent were, on the whole, decidedly more basic than the main body of felsites -that immediately preceded them. They occur also in thinner sheets, and -are far more abundantly accompanied with ashes. At the same time it is -deserving of special notice that among these less acid outflows there are -intercalated sheets of felsite, and that some of these still retain the -spherulitic structure formed by the devitrification of an original volcanic -glass.</p> - -<p>Far to the south-west, in the promontory of Lleyn, another group of -volcanic rocks exists which may have been in a general sense contemporaneous -with those of the Snowdon region, but which were certainly -erupted from independent vents. Mr. Harker has described them as quartzless -pyroxene-andesites, sometimes markedly cellular, and though their -geological relations are rather obscure, he regards them as lava-flows interbedded -among strata of Bala age and occurring below the chief rhyolites of -the district. If this be their true position, they indicate the outflow of -much less highly siliceous lavas before the eruption of the acid felsites. In -the Snowdon area any such intermediate rocks which may have been -poured out before the time of the felsitic outflows have been buried under -these.</p> - -<p>The tuffs of the Bala series in Caernarvonshire have not received the -same attention as the lavas. One of the first results of a more careful -study of them will probably be a modification of the published maps by a -reduction of the area over which these rocks have been represented. They -range from coarse volcanic breccias to exceedingly fine compacted volcanic -dust, which cannot easily be distinguished, either in the field or under the -microscope, from the finer crushed forms of felsite. Among the oldest tuffs -pieces of dark blue shale as well as of felsite may be recognized, pointing to -the explosions by which the vents were drilled through the older Silurian -sediments already deposited and consolidated. Sometimes, indeed, they -recall the dark slate-tuffs of Cader Idris, like which they are plentifully -sprinkled with kaolinized felspar crystals. The beds of volcanic breccia -intercalated between the lower felsites of Snowdon include magnificent -examples of the accumulation of coarse volcanic detritus. The blocks of -various felsites in them are often a yard or more in diameter. Among the -<span class="pagenum" id="Page_214">- 214 -</span> -felsite fragments smaller scattered pieces of andesitic rocks may be found. -This mixture of more basic materials appears to increase upwards, the -highest ashes containing detritus of andesitic lavas like those which occur -among them as flows.</p> - -<p>The tuffs in the upper part of Snowdon are well-bedded deposits made -up partly of volcanic detritus and partly of ordinary muddy sediment.<a id="FNanchor_209" href="#Footnote_209" class="fnanchor">[209]</a> -Layers of blue shale or slate interstratified among them indicate that the -enfeebled volcanic activity marked by the fine tuffs passed occasionally into -a state of quiescence. As is well known, numerous fossils characteristic of -the Bala rocks occur in these tuffs. The volcanic discharges are thus -proved to have been submarine and to have occurred during Bala time.</p> - -<div class="footnote"> - -<p><a id="Footnote_209" href="#FNanchor_209" class="label">[209]</a> See the interesting account of these tuffs given by Sir A. Ramsay, <i>Mem. Geol. Survey</i>, vol. -iii. 2nd edit. p. 142.</p> - -</div> - -<p>I have already alluded to some of the probable vents from which the -lavas and tuffs were discharged, and to their position along a line drawn -from Penmaen-mawr into the peninsula of Lleyn. It will be observed that -they lie outside the area of the bedded volcanic rocks and rise through -parts of the Silurian system older than these rocks. The largest and most -important of them is unquestionably that formed by Y-foel-frâs and its -neighbouring heights. As mapped by the Geological Survey, this mass of -igneous rock is irregularly elliptical, measures about six square miles in -area, and consists mainly of intrusive "felstone-porphyry" passing into -"hornblendic greenstone."<a id="FNanchor_210" href="#Footnote_210" class="fnanchor">[210]</a> Mr. Harker, however, has made an important -correction of this petrography, by showing that a large part of the area -consists of augitic granophyre, while the so-called "greenstone" is partly -diabase and partly andesitic ashes and agglomerates. He suggests that an -older vent has here been destroyed by a later and larger protrusion of -igneous matter.<a id="FNanchor_211" href="#Footnote_211" class="fnanchor">[211]</a> This high and somewhat inaccessible tract of ground is -still in need of detailed mapping and closer study, for undoubtedly it is the -most important volcanic vent now visible in North Wales. My former -colleague in the Geological Survey, Mr. E. Greenly, spent a week upon it -some years ago, and kindly supplied me with the following notes of his -observations:—"The central and largest area of the neck is mainly occupied -with diabases and andesites, while the ashes and agglomerates, which are -intimately connected with them, seem to run as a belt or ring round them, -and to occur in one or more patches in the midst of them. Portions of -green amygdaloid run through the pyroclastic masses. Outside the ring of -agglomerate and ashes an interrupted border of felsite can be traced, which -may be presumed to be older than they, for a block of it was observed in -them. The granophyre, on the other hand, which is interposed between -the fragmental masses and the surrounding rocks on the western wall of the -vent, seems to be of later date. Dykes or small bosses of diabase, like the -material of the sills, pierce both the agglomerates and the rocks of the centre."<a id="FNanchor_212" href="#Footnote_212" class="fnanchor">[212]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_210" href="#FNanchor_210" class="label">[210]</a> <i>Mem. Geol. Survey</i>, vol. iii. 2nd edit. pp. 137, 139.</p> - -<p><a id="Footnote_211" href="#FNanchor_211" class="label">[211]</a> <i>Bala Volcanic Series</i>, pp. 41, 71, 72, 123.</p> - -<p><a id="Footnote_212" href="#FNanchor_212" class="label">[212]</a> Mr. Greenly has made a sketch map of this interesting locality. As he has now established -his home in North Wales, I trust he may find an opportunity of returning to Y-foel-frâs and -completing his investigations.</p> - -</div> - -<p><span class="pagenum" id="Page_215">- 215 -</span></p> - -<p>No agglomerate appears to have been noticed by any observer among -the other supposed vents along the line that runs south-westwards from -Penmaen-mawr, to the promontory of Lleyn. These bosses are rudely -circular in ground-plan and rise vertically out of the Lower Silurian or -Cambrian strata, or partake more of the nature of lenticular sheets or -laccolites which have been thrust between the planes of bedding. There is -usually an observable alteration of the surrounding rocks along the line of -contact.</p> - -<p>The material of these bosses is sometimes thoroughly acid, as is the -granophyre of Y-foel-frâs, the microgranite of Mynydd-mawr with its -riebeckite crystals, the augite-granite-porphyry of Clynog-fawr, and the -granophyric and rhyolitic quartz-porphyries of the Rivals. In other cases -the rock is of an intermediate grade, as in the enstatite-diorite of Penmaen-mawr, -the pyroxene-andesite of Carn Boduan, and the quartz-augite-syenite -of Llanfoglen.<a id="FNanchor_213" href="#Footnote_213" class="fnanchor">[213]</a> A few bosses of still more basic material occur in the Sarn -district, including hornblende-diabase and hornblende-picrite. Sometimes -both the acid and the more basic rocks are found in the same boss, as in the -large mass of Y-foel-frâs.</p> - -<div class="footnote"> - -<p><a id="Footnote_213" href="#FNanchor_213" class="label">[213]</a> The geological relations and petrographical characters of these various rocks are given by -Mr. Harker in the fourth and fifth sections of his Essay.</p> - -</div> - -<p>It must be confessed that there is no absolute proof that any of these -masses mark the actual sites of eruptive vents, except probably the boss of -Y-foel-frâs. Some of them may have been intruded without establishing -any outlet to the surface.<a id="FNanchor_214" href="#Footnote_214" class="fnanchor">[214]</a> But that a few of them really represent orifices -from which the Bala volcanic group was erupted may be plausibly inferred -from their neck-like form, from their positions with reference to the volcanic -district, from the obvious thickening of the lavas and tuffs in the direction -of these bosses, and from the petrographical relation that exists between -their component materials and rocks that were discharged at the surface. -This last-named feature has been well pointed out by Mr. Harker, who has -established, by a study of microscopic slides, a gradation from the granophyric -material of the bosses into structures greatly resembling those of the -bedded felsites, and likewise a close similarity between the intermediate -rocks of the other bosses and the andesites which have elsewhere been -poured out at the surface.<a id="FNanchor_215" href="#Footnote_215" class="fnanchor">[215]</a> But perhaps the most impressive evidence -as to the sites of the chief centres of eruption is supplied by the lavas and -tuffs themselves as they thicken in certain directions and thin away in -others. This feature of their distribution has been well expressed in the -maps and sections of the Survey, and has been clearly summarized by Mr. -Harker.<a id="FNanchor_216" href="#Footnote_216" class="fnanchor">[216]</a> The oldest lavas now visible lie at the northern end of the -district, and the vents from which they proceeded may, with considerable -probability, be placed somewhere in the tract which includes the chain -of bosses of Penmaen-mawr, Y-foel-frâs, and Y Drosgl. The chief centre -of eruption no doubt lay somewhere in the Snowdon tract, where the -<span class="pagenum" id="Page_216">- 216 -</span> -lavas and tuffs attain their greatest thickness, and whence they thin away -in all directions. The Mynydd-mawr boss may be presumed to have been -one of the main vents. But there were not improbably others, now -concealed under the deep cover of their own ejections.</p> - -<div class="footnote"> - -<p><a id="Footnote_214" href="#FNanchor_214" class="label">[214]</a> Mr. Harker speaks of some of them as laccolites.</p> - -<p><a id="Footnote_215" href="#FNanchor_215" class="label">[215]</a> <i>Op. cit.</i> pp. 57, 72.</p> - -<p><a id="Footnote_216" href="#FNanchor_216" class="label">[216]</a> See especially pp. 9, 120 <i>et seq.</i>, and fig. 6 of his Essay.</p> - -</div> - -<p>More diligent search, with a special eye to the discovery of such vents, -might indeed be rewarded, even in the midst of the volcanic district itself. -To the north-east of Capel Curig, for example, there is a prominent knob of -agglomerate,<a id="FNanchor_217" href="#Footnote_217" class="fnanchor">[217]</a> which I visited with Mr. B. N. Peach, and which we regarded -as probably marking one of the minor vents. The material of this eminence -has a base which by itself would probably be regarded by the field-geologist -as a felsite. But through this compact matrix are dispersed abundant -stones of all sizes up to six inches or more in diameter. They are mostly -subangular or somewhat rounded-off at the edges, and generally markedly -cellular. Among them may be observed pieces of trachyte, felsite, and a -rock that is probably a devitrified pitchstone or obsidian. The vesicles in -these stones are sometimes lined with an acicular zeolite. Traces of rude -bedding can be detected, dipping at high angles. On the north-east side of -the hill finer agglomerate is seen to alternate with ashy grits and grey -shales, which, dipping E.N.E. at 20°-30°, pass under a group of felsites, one -at least of which retains a very fine perlitic structure and evidently flowed -as a true glass. Some of these lavas are full of enclosed pieces of various -flinty cellular and porphyritic felsites and andesites or trachytes, like the -stones which occur abundantly in the agglomerate. The connection of these -bedded lavas and tuffs with the agglomerate-neck seems obvious.</p> - -<div class="footnote"> - -<p><a id="Footnote_217" href="#FNanchor_217" class="label">[217]</a> This rock is referred to in the <i>Geological Survey Memoir</i> as "a short thick band of conglomeratic -ash, which strikes northwards about half a mile and then disappears" (<a href="#Page_134">p. 134</a>).</p> - -</div> - -<p>The Caernarvonshire volcanic area furnishes another admirable example -of the intrusion of basic sills as a final phase of eruptivity. These masses -have been carefully separated out on the maps of the Geological Survey, -which present a striking picture of their distribution and their relation to -the other igneous rocks. An examination of the maps shows at once that -the basic sheets tend to lie parallel with the bedding along certain horizons. -In the southern and western portions of the area they have forced themselves -among the Lower Silurian sedimentary strata that underlie the Bala volcanic -group—a position analogous to that taken by the corresponding sills of the -Arenig series. But they likewise invade the volcanic group itself. Along -the eastern borders of the district they abound, especially in the higher parts -of the volcanic pile, where they have been injected between the flows, and -have subsequently participated in the abundant plication of the rocks between -the mountains and the line of the River Conway.</p> - -<p>The curvatures into which the rocks of the region have been thrown, -and the consequent breadth of country over which the volcanic sheets can -now be examined, furnish a much better field than Merionethshire for the -attempt to trace the probable centre or centres from which the basic magma -of the sills was protruded. A study of the Survey maps soon leads to a conviction -that the intrusions were not connected, except perhaps to a trifling -<span class="pagenum" id="Page_217">- 217 -</span> -extent, with the great line of western vents. It is remarkable that the older -strata which emerge from under the volcanic group on its western outcrop are, -on the whole, singularly free from sills, though some conspicuous examples -are shown opposite to Mynydd-mawr, while a few more occur further north -along the same line. Their lenticular forms, their short outcrops, and their -appearance on different horizons at widely separated points seem to indicate -that the sills probably proceeded from many distinct subterranean pipes. -Their greater abundance along the eastern part of the district may be taken -to indicate that the ducts lay for the most part considerably to the eastward -of the line of western vents. They may have risen in minor funnels, like -that of Capel Curig.</p> - -<p>It is noteworthy that so abundant an extravasation of basic material -should have taken place without the formation of numerous dykes. We -have here a repetition of the phenomena that distinguished the preceding -Arenig volcanic period in Merionethshire, and it will be remembered that -the Llandeilo eruptions of Builth were likewise followed by the injection of -large bodies of basic rock. As an enormous amount of igneous magma may -thus be impelled into the Earth's crust without the formation of dykes, it is -evident that the conditions for the production of sills must be in some -important respects different from those required for dykes.</p> - -<p>No evidence has yet been obtained that any one of these sills established -a connection with the surface. Not a trace can be found of the outpouring -of any such basic lava-streams, nor have fragments of such materials been -met with in any of the tuffs. On the other hand, there is abundant proof -of the usual contact-metamorphism. Though the sills conform on the whole -to the bedding, they frequently break across it. They swell into thick -irregular masses, and thin out rapidly. In short, they behave as true -intrusive sheets, and not as bedded lavas.</p> - -<p>In regard to their internal character, they show the customary uniformity -of texture throughout each mass. They are mapped under the general -name of "greenstones" by the Geological Survey, and are described in the -<i>Memoir</i> as hornblendic.<a id="FNanchor_218" href="#Footnote_218" class="fnanchor">[218]</a> The more precise modern methods of examination, -however, prove them to be true diabases, in which the felspar has, as a rule, -consolidated before the augite, giving as a result the various types of diabasic -structure.<a id="FNanchor_219" href="#Footnote_219" class="fnanchor">[219]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_218" href="#FNanchor_218" class="label">[218]</a> <i>Op. cit.</i> p. 156.</p> - -<p><a id="Footnote_219" href="#FNanchor_219" class="label">[219]</a> Mr. Harker, <i>Bala Volcanic Series</i>, p. 83.</p> - -</div> - -<p>The date of the intrusion of these basic sills can be fixed by the same -process of reasoning as was applied to those of the Arenig volcanic group. -Their connection with the other igneous rocks of Caernarvonshire is so -obvious that they must be included as part of the volcanic history of the Bala -period. But they clearly belong to a late stage, perhaps the very latest -stage, of that history. They probably could not have been injected into their -present positions, unless a considerable mass of rocky material had overlain -them. Some of them are certainly younger than the tuffs of Snowdon and -Moel Hebog, which belong to a late part of the volcanic period. On the other -hand, they had been intruded before the curvature and compression of the -<span class="pagenum" id="Page_218">- 218 -</span> -region, for they share in the foldings and cleavage of the rocks among which -they lie. The terrestrial movements that produced this disturbance have -been proved to have occurred after the time when the uppermost Bala rocks -were deposited, and before that of the accumulation of the Upper Silurian -formations.<a id="FNanchor_220" href="#Footnote_220" class="fnanchor">[220]</a> The epoch of intrusion is thus narrowed down to some part of -the Upper Bala period. With this subterranean manifestation, volcanic -action in this part of the country finally died out.</p> - -<div class="footnote"> - -<p><a id="Footnote_220" href="#FNanchor_220" class="label">[220]</a> <i>Mem. Geol. Sur.</i> vol. iii. 2nd edit. p. 326. See also Mr. Harker's <i>Bala Volcanic Series</i>, p. 76.</p> - -</div> - - -<h4>iv. <span class="allsmcap">THE VOLCANIC CENTRE OF THE BERWYN HILLS</span></h4> - -<p>Among the thick group of sedimentary formations which overlies the -great volcanic ridge of the Arans and Arenig, and undulates eastwards across -the Bala Valley, occasional thin intercalations of tuff point to the existence -of another centre of volcanic activity which lay somewhere in the region of -the Berwyn Hills. The structure of this ground, first indicated by Sedgwick, -was investigated in detail by J. B. Jukes and his colleagues, whose work -was embodied in the Maps, Sections and Memoirs of the Geological Survey.<a id="FNanchor_221" href="#Footnote_221" class="fnanchor">[221]</a> -The distinguishing characteristics of the volcanic rocks of this district are -the occurrence of both lavas and tuffs as comparatively thin solitary bands -in the midst of the ordinary sediments, and the persistence of these bands -for a distance of sometimes more than 24 miles. The position of the vent -or vents from which this extensive outpouring of volcanic material took -place has not been revealed. As the bands tend to thin away eastwards, it -may be surmised that the chief focus of eruption lay rather towards the -west, perhaps under the trough of Upper Silurian strata somewhere in the -neighbourhood of Llandderfel. There was probably another in the Hirnant -district.</p> - -<div class="footnote"> - -<p><a id="Footnote_221" href="#FNanchor_221" class="label">[221]</a> See Sheet 74 of the one-inch map; Sheets 32, 35, 37 and 38 of the Horizontal Sections; and -chapter xxxi. of the <i>Memoir</i> on the Geology of North Wales.</p> - -</div> - -<p>The mapping of the officers of the Survey showed that in the Berwyn -Hills there are representatives of both the great volcanic periods of North -Wales. A lower series of "felstones and greenstones" probably belongs to -the older period, which began towards the end of Cambrian time and lasted -in some districts even into the time of the Llandeilo formation. An upper -group of tuffs, lying among the Bala rocks, is evidently equivalent, on the -whole, to the much thicker volcanic series of the Snowdon region.</p> - -<p>The lowest visible volcanic rocks occur among the hills to the north-west -of Llanrhaiadr yn Mochnant. They are described as consisting of -felstone of a pale greenish-grey colour and compact texture, like those of -Arenig, and ashes distinctly interstratified with the slates. No exact -petrographical examination of these rocks has yet been made. From the -account given in the Survey <i>Memoir</i> there appears to be here a group of -lavas and tuffs intercalated in Llandeilo perhaps partly in Upper Arenig, -strata which form the broken dome of the Berwyn anticline. The lavas -are represented as lying on four or five platforms, a single band reaching a -<span class="pagenum" id="Page_219">- 219 -</span> -thickness of 300 feet and separated from the next band by sometimes 1000 -or 1500 feet of non-volcanic sediment.</p> - -<p>These lower lavas, according to the measurements -of Jukes, are overlain by more than 4000 feet of -sedimentary strata before the upper or Bala volcanic -series is reached. Three successive "ash-beds" -constitute this upper series. Of these the lowest -band, about 50 or 60 feet thick, was named a -"greenstone ash" in contradistinction to a felstone -ash, and was not traceable for more than a short -distance. Above it, after an intervening thickness -of several hundred feet of sedimentary strata, comes -a second and much more continuous band of tuff, -known as the "Lower ash-bed," about 100 feet thick -on the west front of the Berwyn range. Still higher, -after an interval of about 1500 feet of slates, lies -the "Upper ash-bed," which on the same line of -section has a thickness of about 200 feet. This -is the most persistent of all the volcanic horizons, -for it can be followed continuously round the whole -range of the Berwyns until it is overlain by the -Carboniferous Limestone near Selattyn, a distance -of not less than twenty-four miles. The same band, -but much more feebly developed, has been traced -through the faulted country on both sides of Bala -Lake, where it formed a useful platform in the investigation -of the complicated geological structure -of that area. Along the north side of the Berwyn -Hills another thin band of tuff lies from 150 to -200 feet still higher up in the series, and has been -traced for a distance of about twelve miles. The -Bala limestone comes in about 800 or 1000 feet -above the "Upper ash-bed."</p> - -<div class="figcenter" id="v1fig57" style="width: 718px;"> - <img src="images/v1fig57.png" width="718" height="112" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 57.</span>—Section across the Berwyn Hills. (Reduced from Horizontal Section, Geol. Surv., Sheet 35).<br /> - <i>L</i>, Llandeilo Flags; <i>B</i>, Bala group; <i>B L</i>, Bala Limestone; <i>t</i> <i>t</i>, volcanic tuffs; <i>D</i>, intrusive "greenstones."</div> -</div> - -<p>Besides the rocks now enumerated, the Survey -maps show the intercalation of four or five sheets -of "greenstone," which are represented as following -with marked regularity the strike of the strata. -Until these sheets have been more precisely examined -it is impossible to decide regarding their true petrographical -character, or to determine whether they -are sills, or interstratified lavas, or include rocks of -both these types.</p> - - -<h4>V. <span class="allsmcap">THE VOLCANOES OF ANGLESEY</span></h4> - -<p>We now turn to another part of the country, about which much has -<span class="pagenum" id="Page_220">- 220 -</span> -been written and keen controversy has arisen. In the centre of Anglesey, -among the rocks grouped together by the Geological Survey as "altered -Cambrian," there occur masses of breccia, the probable volcanic origin -of which was, so far as I know, first suggested by Professor Hughes.<a id="FNanchor_222" href="#Footnote_222" class="fnanchor">[222]</a> Dr. -Callaway regards them as pre-Cambrian,<a id="FNanchor_223" href="#Footnote_223" class="fnanchor">[223]</a> while Professor Blake places -them in his "Monian system."<a id="FNanchor_224" href="#Footnote_224" class="fnanchor">[224]</a> When I went over them some years ago, -I accepted the view that they are volcanic agglomerates.<a id="FNanchor_225" href="#Footnote_225" class="fnanchor">[225]</a> Subsequent -examination, however, has convinced me that notwithstanding their remarkable -resemblance to true agglomerates they are not really of volcanic origin, -but are essentially "crush-conglomerates," like those in the Isle of Man, so -well described by Mr. Lamplugh.<a id="FNanchor_226" href="#Footnote_226" class="fnanchor">[226]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_222" href="#FNanchor_222" class="label">[222]</a> <i>Proc. Camb. Phil. Soc.</i> vol. iii. (1880), p. 347.</p> - -<p><a id="Footnote_223" href="#FNanchor_223" class="label">[223]</a> <i>Quart. Journ. Geol. Soc.</i></p> - -<p><a id="Footnote_224" href="#FNanchor_224" class="label">[224]</a> <i>Op. cit.</i></p> - -<p><a id="Footnote_225" href="#FNanchor_225" class="label">[225]</a> <i>Presidential Address Geol. Soc.</i> vol. xlvii. (1891), p. 130.</p> - -<p><a id="Footnote_226" href="#FNanchor_226" class="label">[226]</a> <i>Quart. Journ. Geol. Soc.</i> vol. li. (1895), p. 563. See <i>Geol. Mag.</i> 1896, p. 481.</p> - -</div> - -<p>But though their present coarse, agglomerate-like structure is, I think, -entirely due to the mechanical crushing of the rocks <i>in situ</i> and not to -volcanic explosions, it does not follow that the rocks which have been -broken up do not contain evidence of volcanic action contemporaneous with -their original formation. Obviously, pyroclastic materials may be subjected -to deformation and disruption as well as any other components of the earth's -crust, and may be equally converted into crush-conglomerates. And in -Anglesey it can, I think, be shown that some of the rocks which have been -broken up were originally tuffs and volcanic breccias.</p> - -<p>Throughout Anglesey the stratified rocks present evidence of having -undergone very great compression, deformation and rupture. Thus at -Llanerchymedd thick-bedded Lower Silurian grits, with their intercalations -of shale, have been broken up by numerous small faults, and have been -pushed over each other in large irregular blocks, the shales being now -pinched out, and now pressed up into the interstices between the dislocated -harder and more resisting grits. This condition of rupture may be regarded -as one of the stages towards the formation of a conglomerate by the crushing -together of rocks <i>in situ</i>. A few miles further south at the beginning -of the railway cuttings of Llangefni, green, red and purple slates and grits -appear in a rather more crushed state, and immediately beyond these strata -come the coarse breccias. Neither in their composition nor in their -structural condition do these Llangefni strata appear to be marked off from -the undoubted Lower Silurian rocks as parts of a different system.</p> - -<p>The railway cuttings at Llangefni reveal a series of rocks which appear -to have been originally shales, with thin bands of siliceous grit. The -argillaceous portions of this series are now green and phyllitic, and remind -one of the finer parts of some basic tuffs among the older Palæozoic systems. -They include, however, pale flinty bands, such as might have been derived -from fine felsitic dust. The grits are for the most part fine-grained and -highly siliceous, but they include also coarser varieties with clear quartz-grains. -The enormous deformation which these strata have undergone is -<span class="pagenum" id="Page_221">- 221 -</span> -at once apparent. They seem to have been plicated, ruptured and thrust -over each other, the harder parts surviving longest, but being eventually -broken into small fragments. Every stage may be traced from a recognizable -band of grit down to the rounded or elliptical pebbles of the same -material entirely isolated in this phyllitic matrix of crushed shale.</p> - -<p>But while the volcanic origin of these coarsely-fragmental masses -cannot be maintained, there is elsewhere evidence that the older Palæozoic -rocks of Anglesey include relics of contemporaneous volcanic eruptions. -Seven miles to the south-east of Holyhead, in the basal Lower Silurian -conglomerates which, as before referred to, Mr. Selwyn found lying unconformably -on the green schists, there occur abundant fragments of -volcanic rocks, besides the prevalent detritus of the schists of the neighbourhood. -Some of the bands have somewhat the character of volcanic -breccias or tuffs, and they show an evident resemblance to portions of the -Bangor group and the rocks of Llyn Padarn, though they are doubtless of -much later age. That these volcanic fragments were not derived from the -waste of rocks of a much earlier period is made tolerably certain by the -intercalation of true tuffs among the black shales higher up in the order -of succession. Here, then, we have evidence of contemporaneous volcanic -action in the very basement Lower Silurian strata of Anglesey, which by -their fossil contents are shown to be on the horizon of the lowest Arenig -or even Tremadoc group.</p> - -<p>But still further and fuller evidence of Silurian volcanism is to -be obtained by an examination of the northern coast-line. I have already -referred to the elliptical fault which is marked on the Geological Survey -map as running from the north-western headland to the eastern coast beyond -Amlwch. The necessity for inserting this fault, apart from any actual -visible trace of its occurrence, arose when the conclusion was arrived at -that the rocks of the extreme north of Anglesey were essentially altered -Cambrian strata.<a id="FNanchor_227" href="#Footnote_227" class="fnanchor">[227]</a> For immediately to the south of these rocks black -shales, obviously Silurian, were seen to dip to the north—a structure which -could only be accounted for by a dislocation letting them down into that -position. The same necessity for a fault has of course been felt by all -writers who have subsequently treated the northern area as pre-Cambrian. -But it is deserving of notice that in the original mapping of the Survey no -continuous abrupt hiatus is shown by the line which was afterwards marked -as a continuous line of fault. On the contrary, on one of the field-maps -in, I believe, Mr. Selwyn's handwriting the remark occurs:—"The gradual -passage from the black shale to the upper green gritty slates of Llanfechell -is best seen at Bothedd, on road from Llanfaethlu to Llyn-llygeirian."<a id="FNanchor_228" href="#Footnote_228" class="fnanchor">[228]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_227" href="#FNanchor_227" class="label">[227]</a> I have fully considered the evidence adduced by Dr. Callaway and Professor Blake, and -have examined the ground, and can come to no other conclusion than that stated in the text. -But see Mr. Blake's remarks, <i>Geol. Mag.</i> 1891, p. 483.</p> - -<p><a id="Footnote_228" href="#FNanchor_228" class="label">[228]</a> There is no continuous section now visible at this place, but the two groups of rock can be -traced to within a few feet of each other, both inclined as usual in the same direction, and the -black shales appearing to pass under the others.</p> - -</div> - -<p>It is no part of my aim to disprove the existence of faults along the line -<span class="pagenum" id="Page_222">- 222 -</span> -referred to. These may quite well exist; but there is assuredly no one -gigantic displacement, such as the theory I am combating would require; -while any faults which do occur cannot be greatly different from the others -of the district, and do not prevent the true relations of the rocks from being -discoverable.</p> - -<p>Where the supposed elliptical fault reaches the shore at Carmel Point, -the two groups of rock seem to me to follow each other in unbroken -sequence.<a id="FNanchor_229" href="#Footnote_229" class="fnanchor">[229]</a> The black slates, which are admittedly Lower Silurian, dip -underneath a breccia and greenish (Amlwch) slates. Not only so, but -bands of similar black slates occur higher up, interstratified with and -shading-off into tuffs and greenish slates. Further, bands of coarse volcanic -breccia occur among the black slates south of the supposed break. -These, in accordance with the exigencies of theory, are represented as -separated by a network of faults from the black slates amid which they lie. -But good evidence may be found that they are truly interbedded in these -slates. In short, the whole of the rocks in that part of Anglesey form one -great series, consisting partly of black slates, partly of greenish slates, with -abundant intercalations of volcanic detritus. The age of the base of this -series is moreover determined by the occurrence of Bala fossils in a band of -limestone near Carmel Point.</p> - -<div class="footnote"> - -<p><a id="Footnote_229" href="#FNanchor_229" class="label">[229]</a> I cannot admit that there is any evidence of a thrust-plane here. To insert one is merely to -modify field-evidence to suit theory. See <i>Geol. Mag.</i> 1891, p. 483.</p> - -</div> - -<p>The rocks which extend eastward along the coast from the north-western -headland of Anglesey are marked on the Survey map as "green, grey and -purple slates with conglomeratic and siliceous beds." The truly volcanic -nature of a considerable proportion of these strata has been clearly stated -by Mr. Blake.<a id="FNanchor_230" href="#Footnote_230" class="fnanchor">[230]</a> As they dip in a general northerly direction, higher portions -of the series present themselves as far as the most northern projection of the -island near Porth Wen (<a href="#v1fig58">Fig. 58</a>). They have been greatly crumpled and -crushed, so that the slates pass into phyllites. They include some thick -seams of blue limestone and white quartzite, also courses of black shale containing -Lower Silurian graptolites. Among their uppermost strata several -(probably Bala) fossils, including <i>Orthis Bailyana</i>, have been obtained by -Professor Hughes. It has been supposed that the higher bands of black -shale may also have been brought into their present positions by faults, and -that they do not really belong to the series of strata among which they lie. -But this suggestion is completely disproved by the coast-sections, which -exhibit many thin interstratified leaves of black shale, sometimes less than -an inch thick. These and the ashy layers containing the <i>Orthis</i> and other -fossils form an integral part of the so-called "Amlwch slates."<a id="FNanchor_231" href="#Footnote_231" class="fnanchor">[231]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_230" href="#FNanchor_230" class="label">[230]</a> <i>Quart. Journ. Geol. Soc.</i> vol. xliv. (1888), p. 517. See his further remarks in <i>Geol. Mag.</i> -1891, p. 483.</p> - -<p><a id="Footnote_231" href="#FNanchor_231" class="label">[231]</a> The Amlwch slates exhibit on a great scale the puckering that points to intense compression. -This "gnarled" structure, as Prof. Hughes called it, has been illustrated by Mr. Harker, <i>British -Assoc. Report</i> (1885), pp. 839, 840.</p> - -</div> - -<p>As evidence of the regular intercalation of the black shales and tuffs in -this sedimentary series, a portion of the coast section at Porth Wen is here -<span class="pagenum" id="Page_223">- 223 -</span> -given (<a href="#v1fig58">Fig. 58</a>). The lowest member (1) of the series is a white quartzite -much jumbled in its bedding, but yet distinctly interstratified with the -other sediments, and containing intercalated courses of green tuff and highly -carbonaceous shale. Markings like worm-pipes are here and there to be -seen. The next group of strata (2) consists of black shale followed by yellow -conglomeratic sandstone and pebbly tuffs. The shales enclose rounded and -angular fragments of quartzite. The sandstone passes upward into pinkish -and yellowish conglomerate (3), with an abundant lustrous phyllitic matrix, -which when free from pebbles closely resembles some of the tuffs of Llyn -Padarn. The next band (4) is one of yellow, sandy, felspathic grit, -quartz-conglomerate and fine tuffs, with leaves of dark shale towards the -base. It was in the lower part of this band that the <i>Orthis</i> above mentioned -was found. The black shales contain markings which are probably -graptolites. Reddish quartzite and quartz-conglomerate (5) next succeed. -These strata have the same phyllitic base just noticed. The highest group -here shown is one of black, yellow and green shales mixed with patches -and bands of volcanic breccia and tuff, the whole being greatly disturbed, -cleavage and bedding seeming as it were to be struggling for the mastery. -These last strata look as if they were about to pass up vertically into the -ordinary dark Lower Silurian shales or slates.</p> - -<div class="figcenter" id="v1fig58" style="width: 417px;"> - <img src="images/v1fig58.png" width="417" height="86" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 58.</span>—Section of the strata on the shore at Porth Wen, west of Amlwch.</div> -</div> - -<p>There can be no doubt regarding the serious amount of crushing which -the rocks of this coast-line have undergone. Some of the bands might even -be described as "crush-conglomerates." Yet the intercalation of seams of -black shale and limestone, and the occurrence of the exactly similar but -thicker group of black shales at Porth Prydd, which are admitted to be -Lower Silurian, unite the whole series of strata as parts of one formation.</p> - -<p>It thus appears that the area coloured "altered Cambrian" on the -Survey map, and regarded as pre-Cambrian by some later observers, is -proved by the evidence of fossils at its base, towards its centre and at its -top, to belong to the Lower Silurian series, probably to the Bala division. -That this was the geological horizon of part at least of the area was -recognized by Sir A. Ramsay, though he confessed himself unable "precisely -to determine on the north coast of Anglesey how much of the strata are of -Silurian and how much of Cambrian age."<a id="FNanchor_232" href="#Footnote_232" class="fnanchor">[232]</a> Professor Hughes was the first -to suggest that the whole of these rocks should be referred to the Bala -group.<a id="FNanchor_233" href="#Footnote_233" class="fnanchor">[233]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_232" href="#FNanchor_232" class="label">[232]</a> <i>Mem. Geol. Surv.</i> vol. iii. 2nd edit. p. 242.</p> - -<p><a id="Footnote_233" href="#FNanchor_233" class="label">[233]</a> <i>Proc. Camb. Phil. Soc.</i> vol. iii. (1880), pp. 341-348.</p> - -</div> - -<p><span class="pagenum" id="Page_224">- 224 -</span></p> - -<div class="figright" id="v1fig59" style="width: 206px;"> - <img src="images/v1fig59.png" width="206" height="207" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 59.</span>—Section of intercalated black shale - in the volcanic series at Porth yr hwch, south of Carmel Point.</div> -</div> - -<div class="figright" id="v1fig60" style="width: 211px;"> - <img src="images/v1fig60.png" width="211" height="263" alt="" /> - <div class="hanging2"><p><span class="smcap">Fig. 60.</span>—Green slates overlain with volcanic -breccia, Carmel Point, Anglesey.</p></div> -</div> - -<p>I have dwelt on the determination of the true geological age of the rocks -of the north of Anglesey because of the diversity of opinion respecting -them, and because of their great interest in regard to the history of -volcanic action in Wales. These rocks contain a record of volcanic -eruptions, probably contemporaneous on the whole with those of the Bala -period in Caernarvonshire, yet independent -of them and belonging to a different type -of volcanic energy. Some of the vents -probably lay in the north-western part of -Anglesey. The materials ejected from -them were, so far as we know, entirely of -a fragmentary kind. Vast quantities of -detritus, largely in the form of fine dust, -were thrown out; but no trace has yet -been found of the outflow of any lava. -The lower part of this volcanic series consists -of bedded breccias which are sometimes -remarkably coarse. Their included -stones, ranging up to six inches or more -in diameter, are usually more or less -angular, and consist mainly of various felsites. -Layers of more rounded pebbles occasionally occur, while the bedding -is still further indicated by finer and coarser bands, and even by intercalations -of fine tuffs and ashy shales. Towards their upper limits some of -these volcanic bands shade off into pale -grey or greenish ashy shale, followed by -black sandy shale of the usual kind. The -relation of the peculiar greenish shale of -the Amlwch type to these tuffs and -breccias is well shown east of Carmel -Point. This shale is interleaved with -tuff and contains frequent repetitions of -finer or coarser volcanic breccia, as well -as occasional seams of black shale. An -illustration of this structure is given in -<a href="#v1fig59">Fig. 59</a>, where some yellow decomposing -breccias (1), cut by a fault (<i>f</i>), are overlain -by about 40 or 50 feet of black shale -(2), above which lies a flinty felsitic rock -(3) that appears to run in bands or dykes -through the agglomerate. At Carmel -Point (<a href="#v1fig60">Fig. 60</a>) a similar structure -may be observed to that at Llyn Padarn -already referred to (<a href="#Page_163">p. 163</a>). The cleavage, which is well developed in the -green slates (<i>a</i>), is much more faintly marked in the overlying breccia (<i>b</i>), -but the bedding can still be detected in both rocks running parallel to -their mutual boundary-line. Beyond Porth Padrig, which lies east from -<span class="pagenum" id="Page_225">- 225 -</span> -Carmel Point, the section may be seen which is shown in <a href="#v1fig61">Fig. 61</a>. Here -the blue or lead-coloured shale or slate (<i>a</i>) marked as Silurian on the -Geological Survey map passes up into a mass of fine yellowish felsitic tuff -and breccia (<i>b</i>). The shale at the junction intercalates in thin leaves with -the tuff.</p> - -<div class="figright" id="v1fig61" style="width: 371px;"> - <img src="images/v1fig61.png" width="371" height="234" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 61.</span>—Blue shale or slate passing into volcanic breccia east of Porth Padrig, near Carmel - Point, Anglesey.</div> -</div> - -<p>The breccias south of Carmel Point, though they are chiefly made up -of felsitic detritus, sometimes show a preponderance of fragments of shale. -They vary also rapidly in texture and composition. These variations may -indicate that the vent or vents from which their materials were derived -stood somewhere in the near neighbourhood, if indeed they are not to be -recognized in some of the boss-like eminences that rise above the shore. -At the same time, the enormous amount of crushing and shearing which -the rocks of this region have undergone has doubtless introduced crush-conglomerates -into the structure of the ground. And some patient labour -may be required before the nature and origin of the different fragmental -masses are determined.</p> - -<p>Certain remarkably coarse, tumultuous breccias, exposed on the coast at -Mynyddwylfa and Cemmaes, were formerly regarded by me as volcanic -agglomerates. But more recent examination has satisfied me that these, -like the breccias at Llangefni, are not of volcanic origin but are crush-conglomerates.<a id="FNanchor_234" href="#Footnote_234" class="fnanchor">[234]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_234" href="#FNanchor_234" class="label">[234]</a> Presidential Address, <i>Quart. Journ. Geol. Soc.</i> vol. xlvii. p. 134; <i>Rep. Brit. Assoc.</i> 1896, -Section C; <i>Geol. Mag.</i> 1896, p. 481.</p> - -</div> - -<p>While the lower breccias are sometimes tolerably coarse, the volcanic -detritus becomes much finer in the higher parts of the Amlwch slates. Above -the limestones and black shales of Cemmaes volcanic breccias and ashes, -with limestone, quartzite, conglomerate and thin seams of black shale, continue -to the extreme northern headlands. The amount of fine volcanic -detritus distributed through these strata is very great. We can clearly make -<span class="pagenum" id="Page_226">- 226 -</span> -out that while ordinary sedimentation was in progress, an almost constant -but variable discharge of fragmental materials took place from the vents in -the neighbourhood. Sometimes a special paroxysm of explosion would give -rise to a distinct band of breccia or of tuff, but even where, during a time -of comparative quiescence, the ordinary sand or mud predominated, it was -generally mingled with more or less volcanic dust.</p> - -<p>Some bands of conglomerate in this group of strata deserve particular -notice. The most conspicuous of these, already referred to as seen at Porth -Wen, is made up of quartz and quartzite blocks, embedded in a reddish matrix -largely composed of ashy material, and recalling the red spotted tuffs of Llyn -Padarn. The occurrence of strong conglomerates near the top of a volcanic -series has been noted at St. David's, Llyn Padarn and Bangor. In none of -these localities, as I have tried to show, do the conglomerates mark an -unconformability or serious break between two widely-separated groups of -rock. The Anglesey section entirely supports this view, for the conglomerates -are there merely intercalations in a continuous sequence of deposits; they -are succeeded by tuffs and shales like those which underlie them. The -interposition of such coarse materials, however, may undoubtedly indicate -local disturbance, connected, perhaps, in this and the other localities, -with terrestrial readjustments consequent upon the waning of volcanic -energy.</p> - -<p>The detailed geological structure of Anglesey is still far from being completely -understood. Besides the serious crushing here referred to, there is -reason to suspect that considerable plication, perhaps even inversion, of the -strata has taken place, and that, by denudation, detached portions of some -of the higher groups have been left in different parts of the island. The -occurrence of Upper Silurian fossils in several localities adds to the perplexity -of the problem by indicating that, among the folds and hardly distinguishable -from the older slates, portions of Upper Silurian formations -may have been caught and preserved. These difficulties, moreover, involve -in some obscurity the closing phases of volcanic activity in Wales; for until -they are, to some extent at least, removed, we shall be left in doubt whether -the vents in the north of Anglesey, which were in eruption probably during -Bala time, were the last of the long succession of Welsh volcanoes. If the -black shales of Parys Mountain are really referable to the horizon of the -Mayhill Sandstone, the two great igneous bands between which they lie would -seem to mark an outbreak of volcanic energy during Upper Silurian time. -No other indications, however, of eruptions of that age having been met -with in Great Britain (though they occur in the south-west of Ireland and -possibly in Gloucestershire), more careful investigation is required before -such a position can be safely assigned to any rocks in Anglesey.</p> - -<p>Putting these doubtful rocks aside for the present, we may, in conclusion, -contrast the type of eruption in Anglesey with that of the great Snowdonian -region. While the Caernarvonshire volcanoes were pouring forth their -volumes of felsitic lava, and piling them up for thousands of feet on the -sea-floor, the northern Anglesey vents, not more than some five-and-twenty -<span class="pagenum" id="Page_227">- 227 -</span> -miles away, threw out only stones and dust, but continued their intermittent -explosions until they had strewn the sea-bottom with detritus to a depth of -many hundred feet.</p> - -<p>There is yet another feature of interest in this independent group of -submarine vents in Anglesey. Their operations appear to have begun -before the earliest eruptions of the Bala volcanoes in Caernarvonshire. Their -first beginnings may, indeed, have been coeval with the explosions that -produced the older Arenig tuffs of Merionethshire; their latest discharges -were possibly the last manifestations of volcanic energy in Wales. They -seem thus to bridge over the vast interval from Tremadoc to Upper Bala, -possibly even to Upper Silurian time. But we may, perhaps, connect them -with the still earlier period of Cambrian volcanism. I have referred to the -evidence which appears to show that the vents whence the lavas and tuffs of -Moel Trefan and Llyn Padarn were erupted gradually moved northwards, and -continued in eruption until after the beginning of the deposition of the black -slates that are generally regarded as Arenig. The Anglesey tuffs and breccias -may thus be looked upon as evidence of a still further shifting of the active -orifices northward. In this view, while the Aran and Cader Idris volcanoes -broke out in Upper Cambrian and continued through Arenig time, and the -Snowdonian group was confined to Bala time, a line of vents opened to the -north-west in the Cambrian period before the epoch of the Llanberis slates, -and, dying out in the south, continued to manifest a minor degree of energy, -frequently discharging fragmental materials, but no lava, over the sea-bottom, -until, towards the close of the Bala period, possibly even in Upper -Silurian time, they finally became extinct.</p> - - -<h4>vi. <span class="allsmcap">THE VOLCANOES OF THE LAKE DISTRICT (ARENIG TO CLOSE -OF BALA PERIOD)</span></h4> - -<p>From the time of the appearance of Sedgwick's classic letters to Wordsworth, -no volcanic area of Britain has probably been so well known in a -general sense to the ordinary travelling public as the district of the English -Lakes. Many geologists have since then visited the ground, and not a few -of them have published additions to our knowledge respecting what is now -known as the Borrowdale Volcanic Series. The most elaborate and detailed -account of any part of it is that given by the late Mr. J. C. Ward in the -<i>Geological Survey Memoirs</i>, wherein he embodied the results of his minute -investigation and mapping of the northern portion of the district.<a id="FNanchor_235" href="#Footnote_235" class="fnanchor">[235]</a> Notices -of the petrography of some of the more interesting rocks have subsequently -been published by Mr. Rutley, Professor Bonney, Mr. Harker, Mr. Marr, -Mr. Hutchings and others. But up to the present time no complete -memoir on the volcanic geology of the Lake District as a whole has -<span class="pagenum" id="Page_228">- 228 -</span> -appeared. The sheets of the Geological Survey map present a graphic -view of the general distribution of the rocks, but so rapid has the progress -of certain branches of geology been since these sheets were published, that -the map is even now susceptible of considerable improvement.</p> - -<div class="footnote"> - -<p><a id="Footnote_235" href="#FNanchor_235" class="label">[235]</a> Sheet 101 S. E. of the Geological Survey of England and Wales and Explanation illustrating -the same; and papers by him in <i>Quart. Journ. Geol. Soc.</i> vols. xxxi. xxxii. (1875-76). See also -Messrs. Aveline and Hughes, <i>Mem. Geol. Survey</i>, Sheet 98 N.E. (Kendal, Sedbergh, etc.).</p> - -</div> - -<p>In estimating the area over which the volcanic rocks of the Lake -District are spread, geologists are apt to consider only the tract which lies -to the south of Keswick and stretches southward to a line drawn from the -Duddon Sands to Shap. But it can easily be shown that this area falls -far short of the extent of that wherein the rocks can still be traced, and -yet further short of that over which the lavas and ashes originally spread. -For, in the first place, the volcanic group can be followed round the eastern -end of the mountain-group which culminates in Skiddaw, and along the -northern base of these heights to Cockermouth, though only a narrow fringe -of it emerges from underneath the Carboniferous series. It is thus manifest -that the volcanic rocks once stretched completely across Skiddaw and its -neighbours, and that they extend northwards below the Whitehaven Coal-field. -But, in the next place, far beyond these limits, volcanic rocks, which -there can be little doubt were originally continuous with those of the Lakes, -emerge from beneath the base of the Cross Fell escarpment,<a id="FNanchor_236" href="#Footnote_236" class="fnanchor">[236]</a> and still further -to the east a prolongation of the same group rises for a brief space to the -surface from under the great limestone sheets of Upper Teesdale. Between -the north-western and south-eastern limits within which the rocks can now -be seen there intervenes a distance of some 11 miles, while the extreme -length of the tract from south-west to north-east is about 50 miles. Even -if we take these figures as marking the approximate boundaries of the -region covered by the volcanic ejections, it cannot be less than 550 square -miles. But this is probably much less than the original area.</p> - -<div class="footnote"> - -<p><a id="Footnote_236" href="#FNanchor_236" class="label">[236]</a> For an account of the Cross Fell inlier of Silurian rocks see the paper by Professor Nicholson -and Mr. Marr, with the petrographical appendix by Mr. Harker. <i>Quart. Journ. Geol. Soc.</i> vol. -xlvii. (1891), pp. 500, 512.</p> - -</div> - -<p>The thickness of the accumulated volcanic materials is proportionate to -the large tract of country over which they have been spread. From -various causes, it is difficult to arrive satisfactorily at any precise statement -on this question. In a volcanic series bedding is apt to be obscure where, -as in the present case, there are no interstratified bands of ordinary sedimentary -strata to mark it off. It tends, moreover, to vary considerably and -rapidly within short distances, not only from subsequent unequal movements -of subsidence or elevation, but from the very conditions of original -accumulation. Mr. Ward considered that the maximum thickness of the -volcanic group of the Lake District might be taken to range from 12,000 -to 15,000 feet.<a id="FNanchor_237" href="#Footnote_237" class="fnanchor">[237]</a> Professors Harkness and Nicholson, on the other hand, -gave the average thickness as not more than 5000 feet.<a id="FNanchor_238" href="#Footnote_238" class="fnanchor">[238]</a> My own impression -is that the truth is to be found somewhere between these two estimates, -and that the maximum thickness probably does not exceed 8000 or 9000 feet. -In any case there cannot, I think, be much doubt that we have here the -<span class="pagenum" id="Page_229">- 229 -</span> -thickest accumulation of volcanic material, belonging to a single geological -period, anywhere known to exist in Britain.</p> - -<div class="footnote"> - -<p><a id="Footnote_237" href="#FNanchor_237" class="label">[237]</a> Ward, <i>op. cit.</i> p. 46.</p> - -<p><a id="Footnote_238" href="#FNanchor_238" class="label">[238]</a> <i>Brit. Assoc. (1870) Sectional Reports</i>, p. 74.</p> - -</div> - -<p>The geological age of this remarkable volcanic episode is fortunately -fixed by definite palæontological horizons both below and above. The base -of the volcanic group rests upon and is interstratified with the upper part -of the Skiddaw Slate,<a id="FNanchor_239" href="#Footnote_239" class="fnanchor">[239]</a> which from the evidence of its fossils is paralleled with -the Arenig rocks of Wales. The highest members of the group are interstratified -with the Coniston Limestone, which, from its abundant fauna, can -without hesitation be placed on the same platform as the Bala Limestone of -Wales, and is immediately followed by the Upper Silurian series. Thus -the volcanic history comprises the geological interval that elapsed between -the later part of the Arenig period and the close of the Bala period. It -begins probably not so far back as that of the Arenig group of Merionethshire, -and its termination was perhaps coincident with the dying out of the -Snowdonian volcanoes. But it contains no record of a great break or -interval of quiescence like that which separated the Arenig from the Bala -eruptions in Wales.</p> - -<div class="footnote"> - -<p><a id="Footnote_239" href="#FNanchor_239" class="label">[239]</a> Mr. Dakyns has expressed his belief that the volcanic group lies unconformably on the -Skiddaw Slate (<i>Geol. Mag.</i> 1869, pp. 56, 116), and Professor Nicholson has formed the same -opinion (<i>op. cit.</i> pp. 105, 167; <i>Proc. Geol. Assoc.</i> vol. iii. p. 106). Mr. Goodchild, however, has -shown that in the Cross Fell inlier the oldest tuffs are interstratified with the Skiddaw Slates -(<i>Proc. Geol. Assoc.</i> vol. xi. (1889), p. 261). Mr. Ward in mapping the district inserted a complex -series of faults along the junction-line between the volcanic series and the Skiddaw Slates. When -I went over the ground with him some years before his death I discussed this boundary-line with -him and could not adopt his view that it was so dislocated. More recent re-examination has confirmed -me in my dissent. A large number of the faults inserted on the Geological Survey map to -separate the Skiddaw Slates from the Borrowdale volcanic series cannot be proved, and probably -do not exist. Others may be of the nature of "thrust-planes." But see Mr. Ward's explanation -of his views, <i>op. cit.</i> p. 48.</p> - -</div> - -<p>The materials that form this enormous volcanic pile consist entirely of -lavas and ashes. No intercalations of ordinary sedimentary material have -been met with in it, save at the bottom and at the top. The lower lavas, -well seen among the hills to the south of Keswick, were shown by Mr. -Ward to be intermediate between felsites and dolerites in regard to their -silica percentage, and he proposed for them the name of felsi-dolerites. They -are comprised in the group of the andesites or "porphyrites." From the -analyses published by Mr. Ward, the amount of silica appears to range up -to about 60 per cent.<a id="FNanchor_240" href="#Footnote_240" class="fnanchor">[240]</a> They are usually close-grained, dull dark-grey to -black rocks, breaking, where fresh, with a splintery or conchoidal fracture, -showing a few minute striated felspars, apt to weather with a pale-brown or -yellowish-grey crust, and sometimes strongly vesicular or amygdaloidal. -They present many external resemblances to some of the "porphyrites" or -altered andesites of the Lower Old Red Sandstone of Scotland. A microscopic -examination of specimens collected by Dr. Hatch and myself from the -hills to the south of Keswick showed the rocks to be true andesites, composed -of a multitude of slender laths (sometimes large porphyritic crystals) of -<span class="pagenum" id="Page_230">- 230 -</span> -felspar with a brownish glassy groundmass, and with some chloritic material -probably representing augite, but with no trace of quartz.<a id="FNanchor_241" href="#Footnote_241" class="fnanchor">[241]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_240" href="#FNanchor_240" class="label">[240]</a> <i>Quart. Journ. Geol. Soc.</i> vol. xxxi. (1875) p. 408, vol. xxxii. (1876) p. 24. Geology of -Northern Part of Lake District (<i>Mem. Geol. Survey</i>), p. 22. In a subsequent paper the more basic -lavas of Eycott Hill are compared with dolerites (<i>Monthly Microscopical Journ.</i> 1877, p. 246).</p> - -<p><a id="Footnote_241" href="#FNanchor_241" class="label">[241]</a> These rocks were mapped as tuffs by Mr. Ward. Their microscopic characters have been -described by Messrs. Harker and Marr, <i>Quart. Journ. Geol. Soc.</i> xlvii. (1891), p. 292; by Mr. -Harker, <i>op. cit.</i> p. 517; and by Mr. W. M. Hutchings, <i>Geol. Mag.</i> 1891, p. 537; 1892, pp. 227, 540.</p> - -</div> - -<p>Another type of andesite has been found by Mr. Hutchings to occur -abundantly at Harter Fell, Mardale, between the Nan Bield Pass and High -Street, and in the cliffs on the right side of the Kentmere Valley. It consists -of rocks mostly of a grey-green or grey-blue colour with resinous lustre -and extremely splintery fracture. They are augite-andesites of a much more -vitreous nature than the dominant type of lavas of the Lake District. Their -groundmass under the microscope is seen to have originally varied from a -wholly glassy base to an intimate mixture of glass and exceedingly minute -felspar-microlites. This groundmass is permeated with chlorite in minute -flakelets, and encloses numerous porphyritic sharply-defined felspar-crystals, -together with chlorite-pseudomorphs after augite.<a id="FNanchor_242" href="#Footnote_242" class="fnanchor">[242]</a> Gradations from these -rocks to the ordinary more coarse-grained andesites may be observed.</p> - -<div class="footnote"> - -<p><a id="Footnote_242" href="#FNanchor_242" class="label">[242]</a> Mr. Hutchings, <i>Geol. Mag.</i> 1891, p. 539. This observer describes a quartz-andesite or dacite -from near Dunmail Raise.</p> - -</div> - -<p>Some of the andesites appear to have a trachytic facies, where the -felspars of the groundmass consist largely of untwinned laths and appear to -be mainly orthoclase.<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> <i>Op. cit.</i> p. 543.</p> - -</div> - -<p>Among the lavas of the Lake District there occur many which are -decidedly more basic than the andesites, and which should rather be classed -among the dolerites and basalts, though they do not appear to contain olivine. -These rocks occur at Eycott Hill, above Easedale Tarn, Scarf Gap Pass, Dale -Head, High Scawdell, Seatoller Fell and other places. Analyses of those -from Eycott Hill were published by Mr. Ward, and their silica percentage -was shown to range from 51 to 53·3.<a id="FNanchor_244" href="#Footnote_244" class="fnanchor">[244]</a> The microscopic characters of the -group have been more recently determined by Mr. Hutchings<a id="FNanchor_245" href="#Footnote_245" class="fnanchor">[245]</a> and Messrs. -Harker and Marr.<a id="FNanchor_246" href="#Footnote_246" class="fnanchor">[246]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_244" href="#FNanchor_244" class="label">[244]</a> <i>Monthly Microscopical Journal</i>, 1877, p. 246.</p> - -<p><a id="Footnote_245" href="#FNanchor_245" class="label">[245]</a> <i>Geol. Mag.</i> 1891, p. 538.</p> - -<p><a id="Footnote_246" href="#FNanchor_246" class="label">[246]</a> <i>Quart. Journ. Geol. Soc.</i> vol. xlix. (1893), p. 389. Mr. Harker, <i>op. cit.</i> vol. xlvii. (1891).</p> - -</div> - -<p>The andesitic and more basic lavas are particularly developed in the -lower and central part of the volcanic group. They rise into ranges of -craggy hills above the Skiddaw Slates, and form, with their accompanying -tuffs, the most rugged and lofty ground in the Lake District. They extend -even to the southern margin of the volcanic area at one locality to the south-west -of Coniston, where they may be seen with their characteristic vesicular -structure forming a succession of distinct flows or beds, striking at the -Coniston Limestone which lies upon them with a decided, though probably -very local, unconformability.<a id="FNanchor_247" href="#Footnote_247" class="fnanchor">[247]</a> One of the flows from this locality was found -by Dr. Hatch, under the microscope, to belong to the more basic series. It -<span class="pagenum" id="Page_231">- 231 -</span> -approaches a basalt, containing porphyritic crystals of fresh augite instead of -the usual felspars, and showing a groundmass of felspar microlites with some -granules of augite and dispersed magnetite. This local increase of basic -composition is interesting as occurring towards the top of the volcanic group. -A porphyritic and somewhat vesicular andesite, with large crystals of striated -felspar in a dark almost isotropic groundmass, occurs under the Coniston -Limestone near Stockdale.</p> - -<div class="footnote"> - -<p><a id="Footnote_247" href="#FNanchor_247" class="label">[247]</a> This unconformability has been described and discussed by various observers. The general -impression has been, I think, that the break is only of local importance. Mr. Aveline, however, -believed it to be much more serious, and he regarded the volcanic rocks which were ejected during -the deposition of the Coniston Limestone series as much later in date than those of the Borrowdale -group. See <i>Mem. Geol. Survey</i>, Explanation to Sheet 98 N.E. 2nd edit. p. 8 (1888).</p> - -</div> - -<p>Mr. Ward was much impressed with the widespread metamorphism which -he believed all the volcanic rocks of this region had undergone, and as a -consequence of which arose the difficulty he found in discriminating between -close-grained lavas and fine tuffs. There is, of course, a general induration -of the rocks, while cleavage has widely, and sometimes very seriously, -affected them. There is also local metamorphism round such bosses as the -Shap granite, but the evidence of any general and serious metamorphism -of the whole area does not seem to me to be convincing.<a id="FNanchor_248" href="#Footnote_248" class="fnanchor">[248]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_248" href="#FNanchor_248" class="label">[248]</a> The metamorphism of all the rocks, aqueous and igneous, around the Shap granite has been -well worked out by Messrs. Harker and Marr, <i>Quart. Journ. Geol. Soc.</i> vol. xlvii. (1891) p. 266, -xlix. (1893) p. 359.</p> - -</div> - -<p>With regard to the original structure and subsequent alteration of some -of the andesitic lavas, an interesting section has recently been cut along the -road up Borrowdale a little south of the Bowder Stone. Several bands of -coarse amygdaloidal lava may there be seen interstratified among tuffs. The -calcite amygdales in these rocks are arranged parallel to the bedding and -therefore in the planes of flow, while those lined with chlorite are more -usually deformed parallel to the direction of the cleavage. This difference -suggests that before the cleavage took place, not improbably during the -volcanic period, the rocks had been traversed by heated water producing -internal alteration and rearrangements, in virtue of which the vesicles along -certain paths of permeation were filled up with calcite, so as then to offer -some resistance to the cleavage, while those which remained empty, or which -had been merely lined with infiltrated substance, were flattened and pulled -out of shape. Messrs. Harker and Marr have shown that the amygdaloidal -kernels had already been introduced into the cellular lavas before the intrusion -of the Shap granite. In the account to be given of the Tertiary -plateau-basalts (<a href="../../66493/66493-h/66493-h.htm#CHAPTER_XXXVI">Chapter xxxvi.</a>) evidence will be adduced that this filling -up of the steam-cavities of lava may take place during a volcanic period, -and that it is probably connected with the passage of heated vapours or -water through the rocks.</p> - -<p>Though acid lavas are not wholly absent from the central and lower -parts of the volcanic group, it is at the top that their chief development -appears to occur. These rocks may be grouped together as felsites or -rhyolites. They probably play a much larger part in the structure of the -southern part of the volcanic area than the published maps would suggest, -and a detailed survey and petrographical study of them would well reward -the needful labour.<a id="FNanchor_249" href="#Footnote_249" class="fnanchor">[249]</a> A fine series of felsites is interbedded in the lower -<span class="pagenum" id="Page_232">- 232 -</span> -part of the Coniston Limestone, and spreads out underneath it along the -southern margin of the volcanic district from the Shap granite south-westward -for some miles<a id="FNanchor_250" href="#Footnote_250" class="fnanchor">[250]</a> (<a href="#v1fig62">Fig. 62</a>). Between the valleys of the Sprint and Kent -these felsites (which farther east are said to be 700 feet thick) may be seen -interposed between the limestone and the fossiliferous calcareous shales -below it, while from underneath the latter other sheets rise up into the range -of hills behind.</p> - -<div class="footnote"> - -<p><a id="Footnote_249" href="#FNanchor_249" class="label">[249]</a> See Mr. F. Rutley, "The Felsitic Lavas of England and Wales," <i>Mem. Geol. Surv.</i> 1885, pp. -12-15; also the description of Messrs. Harker and Marr, <i>Quart. Journ. Geol. Soc.</i> xlvii. (1891), p. 301.</p> - -<p><a id="Footnote_250" href="#FNanchor_250" class="label">[250]</a> Unfortunately these acid lavas are not distinguished from the others in the Geological -Survey maps.</p> - -</div> - -<div class="figcenter" id="v1fig62" style="width: 381px;"> - <img src="images/v1fig62.png" width="381" height="104" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 62.</span>—Section of felsites on the Coniston Limestone group, west of Stockdale.<br /> - <i>a</i>, Felsites more or less cleaved; <i>b</i>, Calcareous shales with fossils, much cleaved; <i>c</i>, Cleaved felsite; - <i>d</i>, Coniston Limestone; <i>e</i>, Stockdale Shales (with graptolites).</div> -</div> - -<p>These acid lavas are generally grey, cream-coloured, or pink, with a -white weathered crust. Their texture when fresh is flinty or horny, or at -least extremely fine-grained and compact. They are seldom markedly -porphyritic. They frequently display good flow-structure, and sometimes -split up readily along the planes of flow. Occasionally the flow-lines on the -outer crust have broken up in the movement of the rock, giving rise to -irregular fragments which have been carried forward. Short, extremely -irregular, branching veins of a fine cherty felsitic substance, which occasionally -shows a well-marked flow-structure parallel to the walls, traverse -certain parts of a dark-grey felsite, near Brockstones, between the valleys of -the Kent and Sprint.<a id="FNanchor_251" href="#Footnote_251" class="fnanchor">[251]</a> Occasionally a distinct nodular structure may be -observed in these acid lavas, sometimes minute, like an oolite, in other parts, -as on Great Yarlside, presenting large rounded balls. This nodular structure -is not confined to the lava-flows, but has been detected by Messrs. Harker -and Marr in what appears to be an intrusive rock near Shap Wells. The -microscopic characters of some of the Lake District rhyolites were described -by Mr. Rutley, who found them to exhibit beautiful perlitic and spherulitic -structures.<a id="FNanchor_252" href="#Footnote_252" class="fnanchor">[252]</a> That such rocks as these were poured out in a vitreous condition, -like obsidian or pitchstone, cannot be doubted. Chemical analysis -shows that the Lake District rhyolites agree exactly with those of North -Wales in their composition. They contain about 76 per cent of silica.<a id="FNanchor_253" href="#Footnote_253" class="fnanchor">[253]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_251" href="#FNanchor_251" class="label">[251]</a> Compare the structure described by Mr. Harker from the Cross Fell inlier, <i>Quart. Journ. Geol. -Soc.</i> xlvii. (1891), p. 518.</p> - -<p><a id="Footnote_252" href="#FNanchor_252" class="label">[252]</a> "Geology of Kendal," etc., <i>Mem. Geol. Survey</i>, Sheet 98 N.E. 2nd edit. p. 9.</p> - -<p><a id="Footnote_253" href="#FNanchor_253" class="label">[253]</a> Messrs. Harker and Marr, <i>op. cit.</i> p. 302.</p> - -</div> - -<p>The rhyolitic lavas have been seriously affected by the general cleavage -of the region. In some places they have been so intensely cleaved as to -become a kind of fissile slate, and there seems good reason to believe that in -<span class="pagenum" id="Page_233">- 233 -</span> -this altered condition they have often been mistaken for tuffs. Where they -assume a nodular structure, the nodules have sometimes been flattened and -elongated in the direction of the prevalent cleavage.</p> - -<p>The abundance and persistence of thoroughly acid lavas along the -southern edge of the volcanic area where the youngest outflows are found, -is a fact of much interest and importance in the history of the eruptions of -this region. It harmonizes with the observations made in Wales, where in -the Arenig, and less distinctly in the Bala group, a marked increase in acidity -is noticeable in the later volcanic products. At the same time, as above -mentioned, there is evidence also of the discharge of more basic materials -towards the close of the eruptions, and even of the outflow of a lava -approaching in character to basalt.</p> - -<p>According to the Geological Survey maps, by far the largest part of the -volcanic district consists of pyroclastic materials. When my lamented friend, -the late Mr. Ward, was engaged in mapping the northern part of the district, -which he did with so much enthusiasm, I had an opportunity of going over -some of the ground with him, and of learning from him his ideas as to the -nature and distribution of the rocks and the general structure of the region. -I remember the difficulty I had in recognizing as tuff much of what he had -mapped as such, and I felt that had I been myself required, without his -experience of the ground, to map the rocks, I should probably have greatly -enlarged the area coloured as lava, with a corresponding reduction of that -coloured as tuff. A recent visit to the district has revived these doubts. It -is quite true, as Mr. Ward maintains, that where the finer-grained tuffs have -undergone some degree of induration or metamorphism, they can hardly, by -any test in the field, be distinguished from compact lavas. He was himself -quite aware of the objections that might be made to his mapping,<a id="FNanchor_254" href="#Footnote_254" class="fnanchor">[254]</a> but the -conclusions he reached had been deduced only after years of unremitting -study in the field and with the microscope, and in the light of experience -gained in other volcanic regions. Nevertheless I think that he has somewhat -exaggerated the amount of fragmental material in the northern part of -the Lake District, and that the mapping, so consistently and ably carried out -by him, and followed by those members of the Survey who mapped the rest -of the ground, led to similar over-representation there. Some portions of the -so-called tuffs of the Keswick region are undoubtedly andesites; other parts -in the southern tracts include intercalated bands of felsite as well as andesite.</p> - -<div class="footnote"> - -<p><a id="Footnote_254" href="#FNanchor_254" class="label">[254]</a> He says: "I shall be very much surprised if my mapping of many parts of the district be -not severely criticized and found fault with by those who examine only one small area and do not -take into consideration all the facts gathered together, during the course of several years, from -every mountain flank and summit" (<i>op. cit.</i> p. 25). Mr. Hutchings has expressed his agreement -with the opinions stated in the text. He likewise coincides in the belief that there are many of -these Lake District volcanic rocks, regarding which it is impossible to decide whether they are -lavas or ashes (<i>Geol. Mag.</i> 1891, p. 544).</p> - -</div> - -<p>But even with this limitation, the pyroclastic material in the Lake -District is undoubtedly very great in amount. It varies in texture from -coarse breccia or agglomerate, with blocks measuring several yards across, -to the most impalpable compacted volcanic dust. In the lower parts of the -<span class="pagenum" id="Page_234">- 234 -</span> -group some of the tuffs abound in blocks and chips of Skiddaw Slate. -Some good examples of this kind may be seen in Borrowdale, below Falcon -Crag and at the Quayfoot quarries. Where the tuff is largely made up of -fragments of dark blue -slate, it much resembles -the slate-tuffs of Cader -Idris. Some of the pieces -of slate are six or eight -inches long and are now -placed parallel to the -cleavage of the rock. -Among the slate debris, -however, felspar crystals -and felsitic fragments -may be observed. Bands -of coarser and finer green tuff show very clearly the bedding in spite of -the marked cleavage (<a href="#v1fig63">Fig. 63</a>).</p> - -<div class="figleft" id="v1fig63" style="width: 321px;"> - <img src="images/v1fig63.png" width="321" height="130" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 63.</span>—Fine tuff with coarser bands near Quayfoot quarries, - Borrowdale.<br /> - The highly-inclined fine lines show the cleavage. The more gently dipping - bands and lines mark the bedding.</div> -</div> - -<p>But throughout the whole volcanic group the material of the tuff is -chiefly of thoroughly volcanic origin, and its distribution appears to agree on -the whole with that of the bedded lavas. In the older portions of the group -it is probably mainly derived from andesitic rocks, though with an occasional -intermingling of felsitic or rhyolitic detritus, while in the higher parts many -of the tuffs are markedly rhyolitic. Among the lapilli minute crystals of -felspar, broken or entire, may be detected with the microscope. Some -of the ejected ash must have been an exceedingly fine dust. Compacted -layers of such material form bands of green slates, which may occasionally -be seen to consist of alternations of coarser and finer detritus, now and then -false-bedded. Such tuffs bring vividly before the mind the intermittent -explosions, varying a little in intensity, by which so much of the fabric of -the Lake mountains was built up.</p> - -<p>Breccias of varying coarseness are likewise abundant, composed of fragments -of andesite and older tuffs in the central and lower parts of the volcanic -group, and mainly of felsitic or rhyolitic detritus in the upper parts. Some -of these rocks, wherein the blocks measure several yards across, are probably -not far from the eruptive vents, as at Sourmilk Gill and below Honister -Pass. Generally the stones are angular, but occasionally more or less -rounded. Stratification can generally be detected among these fragmental -rocks, but it is apt to be concealed or effaced by the cleavage, while it is -further obscured by that widespread induration on which Mr. Ward has -laid so much stress. The extreme state of comminution of the volcanic -dust that went to form the tuffs has probably caused them to be more -liable to metamorphism than the lavas.<a id="FNanchor_255" href="#Footnote_255" class="fnanchor">[255]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_255" href="#FNanchor_255" class="label">[255]</a> The microscopic and chemical characters of the Ash-Slates of the Lake District have been -investigated by Mr. Hutchings, <i>Geol. Mag.</i> 1892, pp. 155, 218.</p> - -</div> - -<p>Little has yet been done in identifying any of the vents from which -the vast mass of volcanic material in the Lake District was ejected. Mr.<span class="pagenum" id="Page_235">- 235 -</span> -Ward believed that the diabase boss forming the Castle Head of Keswick -marks the site of "one of the main volcanic centres of this particular district,"<a id="FNanchor_256" href="#Footnote_256" class="fnanchor">[256]</a> -whence the great lava sheets to the southward flowed out. There -are obviously two groups of bosses on the northern side of the district, some -of which may possibly mark the position of vents. A few of them are -occupied by more basic, others by more acid rocks. It is not necessary -to suppose that the andesitic lavas ascended only from the former and the -felsites from the latter. While the felsites on the whole are younger than -the more basic lavas, they may have been erupted from vents which had -previously emitted andesites, so that the present plug may represent only -the later and more acid protrusions.</p> - -<div class="footnote"> - -<p><a id="Footnote_256" href="#FNanchor_256" class="label">[256]</a> <i>Op. cit.</i> p. 70.</p> - -</div> - -<p>Besides the boss of Castle Head there are numerous smaller basic -intrusions farther down the Derwent Valley on either side of Bassenthwaite -Lake. Among these are the highly basic rocks forming the picrite on the -east side of the Dash Beck and the dykes on Bassenthwaite Common. All -these bosses, sills, and dykes rise through the Skiddaw Slates, but there is -no positive proof that they belong to the Lower Silurian volcanic series; -they may possibly be much later.</p> - -<p>The most important and most interesting of all the intrusive masses -of basic material is that which constitutes a large part of the eminence -that culminates in Carrock Fell. The remarkable variations in the -composition of this mass have been already referred to. Mr. Harker has -shown that while the centre of the mass is a quartz-gabbro, it becomes -progressively more basic towards the margin. Through the gabbro a mass -of granophyre has subsequently made its way, and along the line of junction -has incorporated into its own substance so much of the basic rock as to -undergo a marked modification in its structure and composition. Whether -these intruded bodies of basic and acid material have ascended in one of -the old volcanic funnels and have been injected laterally in laccolitic fashion -has not been ascertained. Mr. Harker, indeed, is rather inclined to refer -the intrusions to a time not only later than the Borrowdale volcanoes, but -later even than the terrestrial movements that subsequently affected the -district and gave the rocks their present cleaved and faulted structures. -Besides the gabbro and granophyre of this locality, igneous activity has -manifested itself in the uprise of numerous later dykes and veins, intermediate -to basic in composition. Some of these are glassy (tachylyte) and -spherulitic or variolitic.<a id="FNanchor_257" href="#Footnote_257" class="fnanchor">[257]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_257" href="#FNanchor_257" class="label">[257]</a> Mr. Harker, <i>Quart. Journ. Geol. Soc.</i> vol. l. (1894) p. 312, li. (1895) p. 125. <i>Geol. Mag.</i> -1894, p. 551.</p> - -</div> - -<p>Throughout the Lake District a considerable number of bosses of more -acid rocks rise through the Skiddaw Slates, and likewise through the volcanic -group even up to its highest members. Some of these bosses may possibly -indicate the site of volcanic vents. Two of them, which form conspicuous -features on either side of the Vale of St. John, consist of microgranite, and -rise like great plugs through the Skiddaw Slates, as well as through the -<span class="pagenum" id="Page_236">- 236 -</span> -base of the volcanic group. The view of the more eastern hill, as seen -from the west, is at once suggestive of a "neck." These masses measure -roughly about a square mile each.</p> - -<p>With the acid intrusions may possibly be associated some of the other -masses of granophyre, microgranite and granite (felsite, felstone, quartz-felsite, -syenitic granite, quartz-syenite, elvanite), which have long attracted -attention in this region. The largest of these intrusions is the tract of -granite which stretches from Eskdale down to near the sea-coast as a -belt about eleven miles long and from one to three miles broad. Another -large mass is the granophyre or "syenite" of Ennerdale. Numerous other -intrusions of smaller dimensions have been mapped.</p> - -<p>To what extent any of these eruptive masses were associated with the -volcanic phenomena remains still to be worked out. There seems to be -little doubt that a number of them must belong to a much later period. -Mr. Harker has expressed his belief that the intrusion of some of these -igneous rocks was intimately associated with the post-Silurian terrestrial -movements of which cleavage is one of the memorials.<a id="FNanchor_258" href="#Footnote_258" class="fnanchor">[258]</a> The Skiddaw -granite, though it does not touch any part of the volcanic group, but is confined -to the underlying Skiddaw Slates, was erupted after the cleavage of the -district, which affects the volcanic as well as the sedimentary series. In -other instances also, as in that of Carrock Fell, the intrusion seems to have -been later than the disturbances of the crust.<a id="FNanchor_259" href="#Footnote_259" class="fnanchor">[259]</a> The amount of metamorphism -around some of the bosses of granite is considerable. That of -the Skiddaw region has been well described by J. C. Ward,<a id="FNanchor_260" href="#Footnote_260" class="fnanchor">[260]</a> while that of -the volcanic group by the Shap granite has been carefully worked out by -Mr. Harker and Mr. Marr.<a id="FNanchor_261" href="#Footnote_261" class="fnanchor">[261]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_258" href="#FNanchor_258" class="label">[258]</a> <i>Quart. Journ. Geol. Soc.</i> vol. li. (1895), p. 144.</p> - -<p><a id="Footnote_259" href="#FNanchor_259" class="label">[259]</a> <i>Op. cit.</i> p. 126.</p> - -<p><a id="Footnote_260" href="#FNanchor_260" class="label">[260]</a> "Geology of Northern Part of the English Lake District," <i>Mem. Geol. Surv.</i> 1876, chap. iii. -The metamorphism around the diorites and dolerites, and the granophyres and felsites, is described -in the same chapter.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_261" href="#FNanchor_261" class="label">[261]</a> <i>Quart. Journ. Geol. Soc.</i> xlvii. (1891) p. 266, xlix. (1893) p. 359.</p> - -</div> - -<p>The Shap granite comes through the very highest member of the -volcanic series, and even alters the Upper Silurian strata. It must thus be -of much younger date than the volcanic history of the Lake District. It -presents some features in common with the granite bosses of the south -of Scotland. Like these, it is later than Upper Silurian and older than -Lower Carboniferous or Upper Old Red Sandstone time. Its protrusion -may thus have been coeval with the great volcanic eruptions of the period -of the Lower Old Red Sandstone. It will accordingly be again referred to -in a later chapter.</p> - -<p>It must be confessed that none of the large bosses of massive rocks, -whether diabases, gabbros, felsites, granophyres, or granites, appear to afford -any satisfactory proof of the position of the vents which supplied the lavas -and tuffs of the Lake District. Nor can such a decided accumulation of -the volcanic materials in certain directions be established as to indicate the -quarters where the centres of eruption should be sought. On the contrary, -the confused commingling of materials, and the comparative shortness of the -<span class="pagenum" id="Page_237">- 237 -</span> -outcrop of the several sheets which have been traced, rather suggest that -if any one great central volcano existed, its site must lie outside of the -present volcanic district, or more probably, that many scattered vents threw -out their lavas and ashes over no very wide area, but near enough to each -other to allow their ejected materials to meet and mingle. The scene may -have been rather of the type of the Phlegræan fields than of Etna and -Vesuvius. If this surmise be true, we may expect yet to recognize little -necks scattered over the volcanic district and marking the positions of some -of these vanished cones.</p> - -<p>What appears to have been one of these small vents stands near Grange -at the mouth of Borrowdale, where I came upon it in 1890. In the little -Comb Beck, the Skiddaw Slates are pierced by a mass of extremely coarse -agglomerate, forming a rudely-circular boss. The slates are greatly disturbed -along the edges of the boss, so much so, indeed, that it is in some places -difficult to draw a line between them and the material of the agglomerate. -That material is made up of angular blocks, varying in size up to three feet -long, stuck in every position and angle in an intensely-indurated matrix -formed apparently of comminuted debris like the stones. The blocks consist -of a finely-stratified shale, which is now hardened into a kind of hornstone, -with some felsitic fragments. I could see no slags or bombs of any -kind. There is no trace of cleavage among the blocks, nor is the matrix -itself sensibly cleaved. I believe this to be a small volcanic neck and not -a "crush-conglomerate." It has been blown through the Skiddaw Slates, and -is now filled up with the debris of these slates. Its formation seems to -have taken place before the cleavage of the strata, and its firm position and -great induration enabled it to resist the cleavage which has so powerfully -affected the slates and many members of the volcanic group.</p> - -<p>It was the opinion of my predecessor, Sir Andrew Ramsay, and likewise -of Mr. Ward, that the Cumbrian volcanic action was mainly subærial. -This opinion was founded chiefly on the fact that, save at the bottom and -top of the series, there is no evidence of any interstratified sediment of non-volcanic -kind. The absence of such interstratification may undoubtedly -furnish a presumption in favour of this view, but, of course, it is by no -means a proof. Better evidence is furnished by the unconformability already -mentioned between the Coniston Limestone and the lavas on which it lies. -Besides angular pieces of lava, probably derived from direct volcanic explosion, -this limestone contains fragments of amygdaloidal andesite, and also -rolled crystals of striated felspar.<a id="FNanchor_262" href="#Footnote_262" class="fnanchor">[262]</a> These ingredients seem to indicate that -some part of the volcanic group was above water when the Coniston Limestone -was deposited.</p> - -<div class="footnote"> - -<p><a id="Footnote_262" href="#FNanchor_262" class="label">[262]</a> Messrs. Harker and Marr, <i>Quart. Journ. Geol. Soc.</i> vol. xlvii. (1891), p. 310.</p> - -</div> - -<p>The absence of interstratifications of ordinary non-volcanic sediment in -the Borrowdale group might conceivably arise from the eruptions following -each other so continuously on the sea-floor, and at so great a distance from -land that no deposition of sand or mud from the outside could sensibly -affect the accumulation of volcanic material. Certainly some miles to the -<span class="pagenum" id="Page_238">- 238 -</span> -east at the Cross Fell inlier, as already mentioned, there is evidence of the -alternation of tuffs with the sandy and muddy sediment of the sea-bottom. -Here, at the outer confines of the volcanic district, the ejected materials -evidently fell on the sea-floor, mingled there with ordinary sediment, and -enclosed the same organic remains. The well-defined stratification of many -of the fine tuffs is rather suggestive to my mind of subaqueous than of -subærial accumulation. At the same time, there seems no reason why, -here and there at least, the volcanic cones should not have risen above -the water, though their materials would be washed down and spread out by -the waves.</p> - -<p>One of the most marked points of contrast between the Cumbrian and -the Welsh volcanic districts is to be found in the great paucity of sills in -the former region. A few sheets of diorite and diabase have been mapped, -especially in the lower parts of the volcanic group and in the underlying -Skiddaw Slates. On the other hand, dykes are in some parts of the district -not unfrequent, and certainly play a much more prominent part here than -they do in the Welsh volcanic districts. The majority of them consist of -felsites, quartz-porphyries, diorites, and mica-traps. But there is reason to -suspect that where they are crowded together near the granite, as around -Shap Fells, they ought to be connected with the uprise of the post-Silurian -granitic magma rather than with the history of the volcanic group.<a id="FNanchor_263" href="#Footnote_263" class="fnanchor">[263]</a> If this -series of dykes be eliminated, there remain comparatively few that can with -any confidence be associated with the eruption of the Borrowdale rocks.</p> - -<div class="footnote"> - -<p><a id="Footnote_263" href="#FNanchor_263" class="label">[263]</a> For a description of the dykes around the Shap granite see the paper by Messrs. Harker -and Marr, <i>Quart. Journ. Geol. Soc.</i> vol. xlvii. (1891), p. 285.</p> - -</div> - - -<p>vii. <span class="allsmcap">UPPER SILURIAN (?) VOLCANOES OF GLOUCESTERSHIRE</span></p> - -<p>A remarkable group of igneous materials has long been known to rise -among the Silurian rocks of the Tortworth district at the north end of the -Bristol coal-field. They were believed to be aqueous deposits in the Wernerian -sense by Weaver.<a id="FNanchor_264" href="#Footnote_264" class="fnanchor">[264]</a> Murchison regarded them as intrusive sheets;<a id="FNanchor_265" href="#Footnote_265" class="fnanchor">[265]</a> Phillips -looked on them as partly intrusive and partly interstratified.<a id="FNanchor_266" href="#Footnote_266" class="fnanchor">[266]</a> They consist -largely of coarsely-amygdaloidal basalts, some of which have been microscopically -examined.<a id="FNanchor_267" href="#Footnote_267" class="fnanchor">[267]</a> But their field-relations as well as their petrography -have not yet been adequately determined. They are represented on the -Geological Survey Map as forming a number of parallel bands in strata -classed as Upper Llandovery. If, as seems probable, some of them are -really interstratified, they form the youngest group of Silurian volcanic -rocks in England, Scotland, or Wales.</p> - -<div class="footnote"> - -<p><a id="Footnote_264" href="#FNanchor_264" class="label">[264]</a> <i>Trans. Geol. Soc.</i> 2nd ser. vol. i. (1819), pp. 324-334.</p> - -<p><a id="Footnote_265" href="#FNanchor_265" class="label">[265]</a> <i>Silurian System</i> (1839), p. 457.</p> - -<p><a id="Footnote_266" href="#FNanchor_266" class="label">[266]</a> <i>Mem. Geol. Surv.</i> vol. ii. part i. (1848), p. 194.</p> - -<p><a id="Footnote_267" href="#FNanchor_267" class="label">[267]</a> "Geology of East Somerset," etc., in <i>Mem. Geol. Surv.</i> (1876), p. 210; descriptions by Mr. -F. Rutley.</p> - -</div> -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_239">- 239 -</span></p> - -<h3 class="nobreak" id="CHAPTER_XIV">CHAPTER XIV<br /> - -<span class="smaller">THE SILURIAN VOLCANOES OF IRELAND</span></h3> -</div> - - -<p>Abundant as are the volcanic records of the Silurian period in England, -Wales and Scotland, the description of them would be incomplete without -an account of those of Ireland. The eruptions of Arenig, Llandeilo and -Bala time, which we have followed from the south of Caermarthenshire to -the borders of the Scottish Highlands, had their counterparts all down the -east of Ireland. The Irish register of them, however, supplies some details -which are less clearly preserved in the sister island. But the most distinctive -feature of the Silurian volcanic history in Ireland is the preservation -of memorials of eruptions during the Upper Silurian period. In no part -of Great Britain has any unquestionable trace been found of volcanic activity -during that part of the geological record, the last eruptions of which the age -is known being those of the Bala rocks. But in the south-west of Ireland -there is evidence that for a time active vents appeared over the sea-floor on -which the earlier deposits of Upper Silurian time were laid down.</p> - - -<h3>I. The Lower Silurian Series</h3> - - -<h4>i. <i>Eruptions probably of Arenig Age</i></h4> - -<p>It is in that part of Ireland which lies east of a line drawn from Strabane to -Dungarvan Harbour that the records of Lower Silurian volcanic activity are to -be found. In the north the development of volcanic rocks resembles that in -Scotland, in the south it corresponds rather with the volcanic districts of Wales.</p> - -<p>The Irish Silurian volcanic rocks have been traced with more or less -detail on the maps of the Geological Survey. Since these maps were -published, however, great advances have been made in the study of the -petrography of volcanic rocks, as well as in the art of tracing their structure -upon maps. Much, therefore, now remains to be done to bring our knowledge -of the older volcanic history of Ireland abreast of that of the rest of -the British Isles. In the following summary I have had to rely mainly on -my own traverses of the ground, guided by the maps and memoirs of the -Survey, and with the personal assistance of some of my colleagues.</p> - -<p>The remarkable zone of crushed cherts, igneous rocks and sandstones, -<span class="pagenum" id="Page_240">- 240 -</span> -probably of Lower Silurian age, which I have referred to (<a href="#Page_201">p. 201</a>) as wedged -in between the schists and the Old Red Sandstone along the southern margin -of the Highlands of Scotland, reappears in Ireland. It occupies an area in -the County Tyrone, about 24 miles long and about 9 miles broad at the -broadest part, but disappearing towards the north-east and south-west.<a id="FNanchor_268" href="#Footnote_268" class="fnanchor">[268]</a> -Lying between the Palæozoic formations on the south and the schists on -the north, it occupies a similar position to the Scottish belt, but presents -a much broader area, and thus affords greater facilities for examining -the rocks. It presents the same indefinite or faulted boundaries as in -Scotland, so that its relations to the rocks along its flanks have not been -satisfactorily determined. That the rocks of this area are older than the -Silurian strata to the south of them seems to be established by the occurrence -of fragments of them in these strata, and that they are younger than -the schists may be inferred from their non-foliated character. But they -have undoubtedly undergone considerable crushing by powerful terrestrial -movements which have placed them in their present position.</p> - -<div class="footnote"> - -<p><a id="Footnote_268" href="#FNanchor_268" class="label">[268]</a> This area was mapped by Mr. J. Nolan for the Geological Survey, and was described by him -in the <i>Geol. Mag.</i> for 1879. I visited it in company with my colleagues, Mr. B. N. Peach and Mr. -A. M'Henry, in 1890 and again in 1894. My first conclusion was that the volcanic rocks should -be regarded as part of the schistose series lying to the north of them (<i>Pres. Address Geol. Soc.</i> -1891, p. 77). But on the second visit, after having studied the rocks of the border of the Scottish -Highlands, I formed the opinion stated in the text.</p> - -</div> - -<p>The special feature of interest in this Irish area is the remarkable -development of volcanic materials which is there to be seen, spreading over -a far wider area than in Scotland. The rocks include lavas associated with -tuffs and agglomerates, likewise a varied series of intrusive masses.</p> - -<p>The lavas are chiefly dull greenish, fine-grained rocks, having the general -character of diabases and "porphyrites." They are sometimes quite slaggy, -and where the amygdaloidal kernels remain, these are usually of calcite. -Under the microscope, the diabases show in some parts that their lath-shaped -felspars, and the augite which these penetrate, are tolerably fresh, -while in other parts fibrous chlorite, granular epidote and veins of calcite -bear witness to the metamorphism which they have undergone.</p> - -<p>One of the most conspicuous features in some of these lavas is the -occurrence of the same sack-like or pillow-shaped structure which has been -already referred to as so marked among the Arenig lavas of Scotland. -Though the vesicles of these rocks are often quite uncrushed, showing that -there has been no general subsequent deformation of the whole mass, there -occur local tracts where evidence of considerable movement may be noticed. -Thus close to a mass of gneiss, and elsewhere along their margin, the lavas -are apt to be much jointed and broken with numerous lines of shear, along -which the crushed material assumes more or less of a schistose structure. -Yet in the solid cores between these bands of crushing the original forms of -the vesicles are retained.</p> - -<p>These greenish lavas are occasionally interleaved with grey flinty mudstones, -cherts and red jaspers, which are more particularly developed immediately -above. In lithological character, and in their relation to the -<span class="pagenum" id="Page_241">- 241 -</span> -diabases, these siliceous bands bear the closest resemblance to those of -Arenig age in Scotland. But no recognizable Radiolaria have yet been -detected in them.</p> - -<p>Besides the more basic lavas, there occur also, but less abundantly, platy -felsitic rocks which have suffered much from shearing, and consequently -have acquired a fissile slaty structure.</p> - -<p>The agglomerates are made up of angular, subangular and rounded -fragments imbedded in a matrix of similar composition. This matrix has -in places become quite schistose, and then closely resembles some parts of -the "green schists" of the Scottish Highlands. Of the inclosed stones -the great majority consist of various felsites, which, weathering with a thick -white opaque crust, are internally close-grained, dull-grey or even black, sometimes -showing flow-structure, and of all sizes up to eight inches in diameter -or more. There are also fragments of the basic lavas, and likewise pieces -of chert and jasper. On many of the rocky hummocks no distinct bedding -can be made out in the agglomerate, but in others the rock is tolerably well -stratified.</p> - -<p>The tuffs are fine silky schistose rocks, and seem to have been largely -derived from basic lavas. They have suffered more than any of the other -rocks from mechanical deformation, for they pass into green chloritic schists. -Some portions of them are not unlike the slaty tuffs of Llyn Padarn in -Caernarvonshire.</p> - -<p>Accompanying the fragmental volcanic rocks, some ordinary sedimentary -intercalations are to be found—red shales and pebbly quartzites, that -seem to have escaped much crushing. The true order of succession -in the volcanic series has not yet been determined. But apparently above -this series come some dark shales, such as might yield graptolites, pale grits -and occasional limestones.</p> - -<p>Later than the lavas and the pyroclastic material are various intrusive -masses, which in bands and bosses form numerous craggy hills throughout -the area. So far as I have been able to observe, these rocks include two -groups. Of these the older consists of basic injections, such as gabbros -and allied rocks, some of which remind me of the so-called "hypersthene-rock" -of Lendalfoot, in Ayrshire. The coarser varieties, as at Carrickmore -or Termon rock, are sometimes traversed by fine-grained veins from an inch -to several feet in breadth. Portions of the slaggy diabases may be observed -inclosed in these intrusive masses. The younger group is of more acid -composition (granite, quartz-porphyry, etc.), and sends veins into the older.</p> - - -<h4>ii. <i>Eruptions of Llandeilo and Bala Age</i></h4> - -<p>Into the east of Ireland the Lower Silurian rocks are prolonged from -Scotland, from the Lake District and from Wales. Though greatly concealed -under younger formations across the breadth of the island, and occasionally -interrupted by what are regarded as older strata of Cambrian age, they -nevertheless occupy by much the larger part of the maritime counties -<span class="pagenum" id="Page_242">- 242 -</span> -from Belfast Lough to the southern coast-line of Waterford, even as far as -Dungarvan Harbour. With the same lithological types of sedimentary -deposits as in other parts of the United Kingdom, they carry with them -here also their characteristic records of contemporaneous volcanic action. -Though nowhere piled into such magnificent mountain-masses as in Westmoreland -and North Wales, these records become increasingly abundant and -interesting as they are traced southwards, until they are abruptly terminated -by the coast-line along the south of the counties of Wexford and Waterford.</p> - -<p>While much remains to be done, both in the field and in the laboratory -and microscope-room, before our acquaintance with the Irish Silurian volcanic -rocks is as complete as our knowledge of their equivalents in other portions of -the United Kingdom, a serious preliminary difficulty must be recognized in the -fact that the several geological horizons of these rocks have only been approximately -fixed. Great difficulty was experienced by the Geological Survey -in drawing any satisfactory line between the Llandeilo and Bala formations. -This arose not so much from deficiency of fossil evidence as from the way -in which the fossils of each group seemed to occur in alternating bands -in what were regarded as a continuous series of strata. Indeed, in some -localities it almost appeared as if the occurrence of one or other <i>facies</i> of -fossils depended mainly on lithological characters indicative of original -conditions of deposit, for the Llandeilo forms recurred where black shales -set in, while Bala forms made their reappearance where calcareous and -gritty strata predominated.<a id="FNanchor_269" href="#Footnote_269" class="fnanchor">[269]</a> More recent work among the Silurian formations -in England and Scotland, however, indicates that the parallel repetition -of the two types of fossils is due to rapid and constant plication of the -rocks, whereby the two formations, neither of them, perhaps, of great thickness, -have been folded with each other in such a way that without the -evidence of an established sequence of fossils, or the aid of continuous -sections, it becomes extremely difficult to make out the stratigraphical order -in any district. When the ground is attacked anew in detail, with the -assistance of such palæontological and lithological horizons as have permitted -the complicated structure of the southern uplands of Scotland to be -unravelled, we may be enabled to tabulate the successive phases of the -volcanic history of the region in a way which is for the present impossible. -We have as yet no palæontological evidence that in the Silurian region of -the east of Ireland, which extends from Belfast Lough to the south coast of -County Waterford, any of the anticlinal folds bring up to the surface a -portion of the Lower Arenig formation, though possibly some of the lowest -visible strata may be of Upper Arenig age. A considerable part of the -region must be referred to the Llandovery and other Upper Silurian -formations, but the precise limits of the two divisions of the Silurian -system have not yet been determined, except for the region north of Dublin, -which has recently been re-examined for the Geological Survey by Mr. -F. W. Egan and Mr. A. M'Henry.</p> - -<div class="footnote"> - -<p><a id="Footnote_269" href="#FNanchor_269" class="label">[269]</a> Jukes was disposed to regard the two faunas as essentially coeval, but inhabiting different -kinds of sea-bottom. See his note, Explanation of Sheets 167, 168, 178, 179, p. 30.</p> - -</div> - -<p><span class="pagenum" id="Page_243">- 243 -</span></p> - -<p>These observers have ascertained that, as in Southern Scotland, by far -the larger part of the Silurian region of the north-east of Ireland is occupied -by strata belonging to the upper division of the system. The Lower Silurian -formations, including the Llandeilo and Bala groups, form a belt varying up -to six miles in breadth, which stretches from the coast of Down, between the -mouth of Belfast Lough and Copeland Island, in a south-westerly direction -to near the valley of the Shannon in County Longford. South of this belt -the Lower Silurian rocks rise to the surface only here and there on the -crests of anticlinal folds, and it is in these scattered "inliers" that the volcanic -and intrusive rocks are found. So far as the available evidence goes, -the volcanic history of this part of Ireland is entirely to be assigned to -Lower Silurian time, and more especially to the interval between the beginning -of the Llandeilo and the close of the Bala period. I must for the -present content myself with this general limit of geological chronology, and -make no attempt to trace the relative antiquity of the igneous rocks in the -several districts in which they are distributed.<a id="FNanchor_270" href="#Footnote_270" class="fnanchor">[270]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_270" href="#FNanchor_270" class="label">[270]</a> The task of revising the Irish maps and tracing out the respective areas of Upper and Lower -Silurian rocks over the whole island is now in progress by the Geological Survey, Mr. Egan and Mr. -M'Henry being entirely engaged on it.</p> - -</div> - -<p>Viewing the volcanic region of Eastern Ireland as a whole, we are first -struck by the feebleness of the manifestations of eruptivity in the north, and -their increasing development as we advance southwards. At the northern -end of the Silurian area in County Down, thin bands of "felstone" and -"ash" have been mapped by the Geological Survey as interstratified with -the highly inclined and plicated Silurian rocks.<a id="FNanchor_271" href="#Footnote_271" class="fnanchor">[271]</a> As the latter are plainly -a continuation of the strata which have been mapped out zone by zone in -the south of Scotland, their igneous intercalations may be looked upon as -probably equivalents of some of those in the Silurian districts of Wigtonshire -and Kirkcudbrightshire. But in County Down no representative has -yet been detected of the Arenig and Llandeilo volcanic series of the southern -uplands of Scotland. Nor has more precise petrographical examination confirmed -the reference of any of the igneous rocks in the Silurian area of that -district to truly contemporaneously intercalated volcanic rocks. All the -eruptive material appears to be of an intrusive character. It occurs in the -form of dykes of lamprophyre or mica-trap belonging to the groups of minettes -and kersantites. Nothing definite is known of the age of these intrusions: -they are possibly referable to the time of the Lower Old Red Sandstone.<a id="FNanchor_272" href="#Footnote_272" class="fnanchor">[272]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_271" href="#FNanchor_271" class="label">[271]</a> See Sheet 49 Geol. Survey, Ireland, and Explanation thereto (1871), pp. 16, 37, 39. The so-called -"ashes" of the Explanation are probably parts of dykes which have been more or less -crushed.</p> - -<p><a id="Footnote_272" href="#FNanchor_272" class="label">[272]</a> <i>Guide to the Collection of Rocks and Fossils belonging to the Geological Survey of Ireland</i>, -by Messrs. M'Henry and Watts, Dublin, 1895, p. 74.</p> - -</div> - -<p>Far in the interior several bands of "felspathic ash" and "massive -agglomerate" are shown on the Survey map as running through the counties -of Monaghan and Cavan.<a id="FNanchor_273" href="#Footnote_273" class="fnanchor">[273]</a> In one locality south of the Drumcalpin Loughs -a large exposure of this ash is visible: "brown crumbly beds, with small -<span class="pagenum" id="Page_244">- 244 -</span> -rounded pebbles, give place to a massive bed of agglomerate, the enclosed -blocks of which are always of one species of felstone, sometimes measuring -10 × 12 × 18 inches, and not always rounded." South of Carrickatee Lough, -and a few miles farther to the south-west, near Lackan Bridge, considerable -exposures of these rocks occur. One crag in particular displays a thickness -of more than 70 feet of "tough flaky breccias," "thick agglomerates with -small and large blocks of felstone," and "thin beds of fine pale green compact -grit without pebbles, and a few flags." "One of the flaky beds contains -numerous white worn crystals of felspar"; "the imbedded blocks of felstone -are of the usual kind—pale compact matrix showing dark oblong patches, -vesicular and amygdaloidal, the cavities being filled with chlorite."</p> - -<div class="footnote"> - -<p><a id="Footnote_273" href="#FNanchor_273" class="label">[273]</a> Sheet 69 Geol. Survey, Ireland, and Explanation of Sheets 68 and 69, pp. 9, 13, 15.</p> - -</div> - -<p>Further south a more extensive area of igneous rocks has been mapped -on the borders of Louth and Meath, where, according to the Geological -Survey map, a group of lavas and tuffs extends for about twelve miles near -Slane.<a id="FNanchor_274" href="#Footnote_274" class="fnanchor">[274]</a> Other bands of "ash" and "felstone" have been mapped in the -Silurian area south of Drogheda. Thus at Hilltown, west from the racecourse, -a "bluish crystalline felstone, showing in places lines of viscous -flow," is stated to be overlain by "indurated felspathic ash and tuff, felstone, -and indurated shale" in alternating beds.<a id="FNanchor_275" href="#Footnote_275" class="fnanchor">[275]</a> On a recent visit to this locality -I found that the "porcellanite or indurated shale" is a greenish-grey chert, -full of Radiolaria and finely-diffused volcanic dust. This association of -radiolarian chert with contemporaneous volcanic activity is of much interest, -as showing the extension of the same physical conditions of the Lower -Silurian sea from Scotland into Ireland. The Lower Llandeilo age of the -volcanic intercalations in County Meath is further indicated by the occurrence -of <i>Didymograptus Murchisoni</i> in grey shales in the same neighbourhood -with the radiolarian cherts. In the Lower Silurian district of -Balbriggan numerous intrusive bosses and sills have been mapped by the -Geological Survey. I have found, however, that among these rocks there -occur bands of volcanic breccia, containing abundant angular fragments of a -minutely-vesicular pumice, and also that some of the diabase-masses display -the pillow-structure and amygdaloidal texture. Hence, though most of the -igneous rocks are no doubt intrusive, they appear to include lavas and tuffs -of Bala age.</p> - -<div class="footnote"> - -<p><a id="Footnote_274" href="#FNanchor_274" class="label">[274]</a> <i>Ibid.</i> Sheets 81 and 91. These rocks are chiefly augitic andesites, a few are basalts, and -some seem related to felstones. Probably many of them are intrusive sills of uncertain age. The -"ashes" contain fragments of felsite and porphyrite often of considerable size (<i>Guide to Irish -Rock-Collection</i>, p. 36).</p> - -<p><a id="Footnote_275" href="#FNanchor_275" class="label">[275]</a> <i>Ibid.</i> Sheets 91 and 92 and Explanation to these Sheets (1871), p. 10; <i>Guide to Irish Rock-Collection</i>, -p. 36. Some of these lavas are andesites, others are felsites. Mr. M'Henry has contended -that certain "ashes" and "agglomerates," particularly those exposed on the coast at Portraine, -opposite Lambay Island, are "crush-conglomerates" due to terrestrial disturbances, which have -affected both intrusive igneous rocks and the sedimentary series into which these have been injected.</p> - -</div> - -<p>When the numerous Silurian cores of the mountain-groups in the -interior of Ireland shall have been searched for traces of contemporaneous -volcanic action, it is not improbable that these will be found. One of the -smaller Silurian inliers which diversify the great Carboniferous plain, that -<span class="pagenum" id="Page_245">- 245 -</span> -of the Chair of Kildare, has long been known to have igneous rocks associated -with its abundantly fossiliferous Bala limestone.<a id="FNanchor_276" href="#Footnote_276" class="fnanchor">[276]</a> On recently visiting -this locality I found that, besides the amygdaloidal and porphyritic -andesites and basalts described by Jukes and Du Noyer, the fossiliferous -conglomerates contain pebbles of rocks like those of the Chair, together -with worn crystals of felspar, while intercalated with them are thin courses -of volcanic tuff. There is thus evidence here of contemporaneous volcanic -activity during the accumulation of the Bala group of strata. The limited -area over which the rocks are exposed, however, affords merely a glimpse -of this volcanic centre.</p> - -<div class="footnote"> - -<p><a id="Footnote_276" href="#FNanchor_276" class="label">[276]</a> See Explanation to Quarter Sheet 35 N.E. (Sheet 119 of newer numeration) of Geol. Survey -Ireland (1858), p. 16. (See note, p. 256.)</p> - -</div> - -<p>Crossing over the broad belt of Carboniferous Limestone through which -the Liffey flows into Dublin Bay, we come to the great continuous tract of -older Palæozoic rocks which stretches southward to the cliffs of Waterford. -Through this tract runs the huge ridge of the Wicklow and Carlow granite. -On the west side of this intrusive mass, bands of "greenstone-ash," as well -as "felspathic ashes," have been traced among the Silurian rocks by the -Geological Survey. But it is on the south-east side of the granite that the -volcanic intercalations are best displayed. Indeed, from Wicklow Head to -Dungarvan Harbour there is an almost continuous development of igneous -rocks, rising into rocky eminences, trenched into ravines by the numerous -streams, and laid bare by the waves in fine coast-cliffs. It is in this south-eastern -region, comprising the counties of Wicklow, Wexford and Waterford, -that the Irish Lower Silurian igneous rocks can best be studied.</p> - -<p>There are obviously various distinct centres of eruption in this long belt -of country. The Rathdrum and Castletimon tract forms one of these. -Another of less size culminates in Kilpatrick Hill, a few miles to the -southward. Arklow Head marks the position of a third. The lavas and -tuffs which set in a few miles to the south of that promontory, and may -be said to extend without interruption to the south coast, were probably -thrown out by a series of vents which, placed along a north-east and south-west -line, united their ejections into one long submarine volcanic bank. -There can be no doubt that the most active vents lay at the southern end -of the belt, for there the volcanic materials are piled up in thickest mass, -and succeed each other with comparatively trifling intercalations of ordinary -sedimentary material. Some of these vents, as I shall relate in the sequel, -have been cut open by the sea along a range of precipitous cliffs.</p> - -<p>The comparatively feeble character of the volcanic energy during Lower -Silurian time over the greater part of the south-east of Ireland is shown by -the great contrast between the thickness of the volcanic intercalations there -and in Wales and the Lake country, but still more strikingly by innumerable -sections where thin interstratifications of fine tuff or volcanic breccia occur -among the ordinary sedimentary strata, and are sometimes crowded with -Bala fossils. Some interesting illustrations of this feature are to be seen in -the Enniscorthy district, where layers of fine felsitic tuff, sometimes less than -<span class="pagenum" id="Page_246">- 246 -</span> -an inch in thickness, lie among the shales. In some of the tuffs the lapilli -are fragments of trachytic or andesitic rocks.</p> - -<p>A striking example of rapid alternations of pyroclastic material with -ordinary sediment lies far to the south in County Waterford, close to Dunhill -Bridge, where a group of fine volcanic breccias and grits has been -laid bare by quarrying.<a id="FNanchor_277" href="#Footnote_277" class="fnanchor">[277]</a> These strata consist of coarser and finer -detritus, enclosing angular fragments of felsites and grey and black shale. -The felsite-lapilli vary in texture, some of them presenting beautiful flow-structure. -The stones are stuck at random through each bed, the largest -being often at the bottom. The beds of breccia vary from a few inches to -a foot or more in thickness. There can, I think, be little doubt that each -of these breccia-bands points to a single volcanic explosion, whereby felsitic -fragments were thrown out, mingled with pieces of the Silurian strata through -which the vents were drilled. In a vertical thickness of some fifty feet of -rock there must thus be a record of ten or twelve such explosions.</p> - -<div class="footnote"> - -<p><a id="Footnote_277" href="#FNanchor_277" class="label">[277]</a> See Explanation of Sheets 167, 168, 178 and 179, Geol. Surv. Ireland, p. 56.</p> - -</div> - -<p>Nearer the active vents the fragmental deposits become, as usual, coarser -and thicker. But I have not observed any thick masses of tuff like those -of North Wales. So far as my examination has gone, the tuffs are mainly -felsitic. The so-called "greenstone-ash" of the Survey maps is certainly in -many cases not a true tuff. This term was proposed by Jukes for certain -apple-green to olive-brown flaky fissile rocks only found "in association with -masses of greenstone."<a id="FNanchor_278" href="#Footnote_278" class="fnanchor">[278]</a> Some years ago I had occasion to make a series of -traverses in Wicklow and Wexford, and then convinced myself that in that -part of the country the "greenstone-ashes" were probably crushed bands of -basic sills. Dr. Hatch has proved this to be their origin from a series -of microscopic slides prepared from specimens collected by himself on the -ground.<a id="FNanchor_279" href="#Footnote_279" class="fnanchor">[279]</a> In other cases the "greenstone-ashes" seem to be excessively-cleaved -or sheared felsites, which have acquired a soapy feel and a dull green -colour; but they also do include true tuffs. Thus, in one instance, at Ballyvoyle -cross-roads, in the south of County Waterford, a "greenstone-ash" -is a dull green tuff full of fragments of felspar (chiefly plagioclase) and -pieces of dark andesitic lavas. Another example may be found to the west -of the Metal Man, near Tramore, where the tuff is full of fragments of -felspar and shale cemented in a greenish-yellow material which may be -palagonite.</p> - -<div class="footnote"> - -<p><a id="Footnote_278" href="#FNanchor_278" class="label">[278]</a> Explanation of Sheets 129, 130, p. 13 (1869).</p> - -<p><a id="Footnote_279" href="#FNanchor_279" class="label">[279]</a> Explanation of Sheets 138, 139.</p> - -</div> - -<p>The felsites of the south-east of Ireland form by much the largest -proportion of the whole volcanic series. They occur as lenticular sheets -from a few feet to several hundred feet in thickness, and occasionally -traceable for some miles. On the whole, they are compact dull grey rocks, -weathering with a white crust. A geologist familiar with the contemporary -lavas of North Wales cannot fail to be struck with the absence of the coarse -flow-structure so often characteristic of the felsites in that region. This -structure, indeed, is not entirely absent from the Irish rocks, but it occurs, -so far, at least, as I have seen, rather as a fine streakiness than in the bold -<span class="pagenum" id="Page_247">- 247 -</span> -lenticular bands so common in Caernarvonshire. In like manner the nodular -structure, though not entirely absent, is rare.<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> In Waterford nodular felsites occur with concretions varying from the size of a pea to several -inches in diameter. Explanation to Sheets 167, 168, 178 and 179, p. 11.</p> - -</div> - -<p>Until these felsites have been subjected to more detailed investigation, -little can be said as to their petrography, and as to the points of resemblance -or difference between them and those of other Lower Silurian districts in -the United Kingdom. An important step, however, in this direction was -taken by Dr. Hatch, who studied them on the ground, in the laboratory, -and with the microscope. He found that some of them were soda-felsites -or keratophyres (with albite as their felspar), that others were potash-felsites -(with orthoclase as their felspar), while a third group contained both soda -and potash, the last-named greatly preponderating.<a id="FNanchor_281" href="#Footnote_281" class="fnanchor">[281]</a> The existence of soda-felsites -had not been previously detected among British volcanic rocks, and -it remains to be seen how far they may occur in the large and somewhat -varied group of rocks combined under the general term "felsites." Dr. -Hatch believed that these rocks probably graduate into the normal or -orthoclase felsites; but it has not yet been possible to test this view on -the ground, nor to ascertain whether there is any essential difference between -the mode of occurrence of the two types.</p> - -<div class="footnote"> - -<p><a id="Footnote_281" href="#FNanchor_281" class="label">[281]</a> Explanation of Sheets 138, 139, p. 49; and <i>Geol. Mag.</i> 1889, p. 545.</p> - -</div> - -<p>Besides the more abundant felsites, occasional bands of andesite have -been detected. Various other eruptive rocks occur, probably in most or -all cases intrusive. Such are quartz-mica-diorites, quartz-diorites, augite-diorites -or proterobases, dolerites, gabbros, diabases and epidiorites.<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>Guide to Irish Rock-Collections</i>, pp. 34, 35.</p> - -</div> - -<p>I have said that the chief theatre of eruption lay towards the south-west -end of the volcanic belt of the south-east of Ireland. The coast-line of -County Waterford, from Tramore westward to Ballyvoyle Head—a distance -of nearly fifteen miles—presents, perhaps, the most wonderful series of sections -of volcanic vents within the British Islands. No one coming from the -inland is prepared for either the striking character of the cliff scenery or -the extraordinary geological structure there presented, for the country is, -on the whole, rather featureless, and much of it is smoothed over and -obscured by a covering of drift, through which occasional knobs of the -harder felsites protrude. The cliffs for mile after mile range from 100 -to 150 or 200 feet in height, and present naked vertical walls of rock, -trenched by occasional gullies, through which a descent may be made to -the beach. Throughout the whole distance agglomerates and felsites succeed -each other in bewildering confusion, varied here and there by the intercalation -of Lower Silurian shales and limestones involved and pierced by the -igneous rocks. Hardly any bedded volcanic material is to be recognized from -one end to the other. The sea has laid bare a succession of volcanic vents -placed so close to each other that it will be difficult or impossible to separate -them out. A careful study and detailed mapping of this marvellous coast-section, -however, is a task well worthy of the labour of any one desirous -<span class="pagenum" id="Page_248">- 248 -</span> -of making himself acquainted with some of the conditions of volcanism -during older Palæozoic time.</p> - -<p>At the east end of the section, black shales containing Llandeilo -graptolites, and calcareous bands full of Bala fossils, dip westward below a -group of soda-felsites and felsitic tuffs, which seem to lie quite conformably -on these strata. Here, then, we start with proof that the volcanic eruptions -of this locality began during some part of the Bala period. But immediately -to the west, these bedded igneous rocks are broken through by a neck -of coarse agglomerate stuck full of chips and blocks of shale, some of them -a foot long, with abundant fragments of scoriform and flinty felsites. Some -columnar dykes of dolerite cut through the neck, and a larger intrusion -seems to have risen up the same funnel. The bedded tuffs appear again for -a short distance, but they are soon replaced by a tumultuous mass of -agglomerate. And from this part of the coast onwards for some distance -all is disorder.</p> - -<p>The agglomerates are crowded with blocks of various felsites and micro-granites -sometimes 18 inches in diameter, many of them presenting the -most exquisite streaky flow-structure. The angularity of these stones and -the abrupt truncation of their lines of flow prove that they were derived -from the shattering of already consolidated rocks. In other places the -ejected materials consist almost wholly of black shale fragments, but with -an intermixture of felsite-lapilli.</p> - -<p>It is difficult to convey an adequate idea of the way in which the -agglomerates are traversed by dykes, veins and bosses of various felsites, -and of how these break in endless confusion through each other. Some -of the intrusive rocks are compact and amorphous, others are vesicular, -others close-grained and columnar. Again and again they present the -most perfect flow-structure, and it is noticeable that the lines of flow follow -the inequalities of the walls of the fissure up which the rock has ascended, -and not only so, but even of the surfaces of detached blocks of shale or -felsite which have been caught up and enclosed in the still moving -mass.</p> - -<p>A few of these intrusive rocks were examined in thin slices by Dr. -Hatch. Most of them appear to be soda-felsites, but they include also -rather decomposed rocks, some of which are probably diorites and quartz-diorites. -Occasionally, thoroughly basic dykes (dolerite) may be observed.</p> - -<p>In the midst of this tumultuous assemblage of volcanic masses, representing -the roots of a group of ancient vents, there occur occasional -interspaces occupied by ordinary stratified rocks. In the eastern part of -the section these consist mainly of black shale, sometimes with calcareous -bands, from which a series of Bala fossils has been obtained.<a id="FNanchor_283" href="#Footnote_283" class="fnanchor">[283]</a> A very -cursory examination suffices to show that these intercalations do not mark -pauses in the volcanic eruptions. They are, in fact, portions of the marine -accumulations under the sea-floor through which the vents were blown;<span class="pagenum" id="Page_249">- 249 -</span> -they have been tossed about, crushed and invaded by dykes and veins of -felsite.</p> - -<div class="footnote"> - -<p><a id="Footnote_283" href="#FNanchor_283" class="label">[283]</a> But see the <i>Geol. Survey Memoir</i> on Sheets 167, 168, 178 and 179, Ireland (1865), p. 28, for -a description of the association of Bala and Llandeilo fossils on that coast-line.</p> - -</div> - -<p>But certain other intercalated strips of stratified rocks present a special -interest, for they bring before us examples of volcanic ashes that gathered -on the sea-floor, but which were disrupted by later explosions. Thus, at the -Knockmahon headland, well-bedded felspathic grits and ashy shales occur, -thrown in among the general mass of eruptive material. As I have already -remarked, it is difficult or impossible to fix the horizons of the stratified -patches that are involved among the igneous ejections of this coast-section, -save where they contain recognizable fossils, but the intercalation of true -bedded tuffs among them is a proof that volcanic action had been in -operation there long before the outbreak of the vents which are now laid -bare along the cliffs.</p> - -<p>In the south-east of Ireland there is the usual association of acid and -basic sills with the evidence of a superficial outpouring of lavas and ashes. -But these intrusive masses play a much less imposing part than in Wales. -They may be regarded, indeed, as bearing somewhat the same proportion to -the comparatively feeble display of extrusive rocks in this region that the -abundant and massive sheets of Merionethshire and Caernarvonshire do to -the enormous piles of lavas and tuffs which overlie them.</p> - -<p>Among the acid intrusive sheets the most conspicuous are those mapped -by the Survey as "elvans." These rocks, as they occur in Wicklow and -Wexford, have been examined by Dr. Hatch, who finds them to be micro-granitic -in structure, occasionally exhibiting micropegmatitic or granophyric -modifications.<a id="FNanchor_284" href="#Footnote_284" class="fnanchor">[284]</a> The true stratigraphical relations of these rocks have not -yet been adequately investigated. Those of them which occur on the flanks -of the great granite ridge are not improbably connected with that mass, and -if so are much younger than the Lower Silurian volcanoes.<a id="FNanchor_285" href="#Footnote_285" class="fnanchor">[285]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_284" href="#FNanchor_284" class="label">[284]</a> Explanation of Sheets 138 and 139, p. 53.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_285" href="#FNanchor_285" class="label">[285]</a> The Leinster granite is certainly later than the Lower Silurian rocks and older than the -Carboniferous rocks of the south-east of Ireland. It may belong to the great epoch of granite -protrusion during the Old Red Sandstone period.</p> - -</div> - -<p>The basic sills, or "greenstones," consist largely of diabase, frequently -altered into epidiorite; they include also varieties of diorite.<a id="FNanchor_286" href="#Footnote_286" class="fnanchor">[286]</a> That they -were intruded before the plication and cleavage of the rocks among which -they lie is well shown by their crushed and sheared margins where they are -in thick mass, and by their cleaved and almost schistose condition where -they are thinner. The intense compression and crushing to which they -have been subjected are well shown by the state of their component -minerals, and notably by the paramorphism of the original augite into -hornblende.</p> - -<div class="footnote"> - -<p><a id="Footnote_286" href="#FNanchor_286" class="label">[286]</a> Dr. Hatch, <i>op. cit.</i> p. 49.</p> - -</div> - -<p>The scarcity of dykes associated with Silurian volcanic action is as -noticeable in the south-east of Ireland as it is in Wales. I have observed -a considerable number, indeed, but they are confined to the line of old vents -on the Waterford coast, and, but for the clear cliff-sections cut by the -sea, they would certainly have escaped observation, for they make no -<span class="pagenum" id="Page_250">- 250 -</span> -feature on the ground in the interior. They are sometimes distinctly -columnar, and vary from less than a foot to many yards in width. They -traverse both the agglomerates and the intrusive felsites. Most of them are -of felsite, sometimes cellular; but in some cases they are dolerites. There -is obviously no clue to the dates of these dykes.</p> - -<p>That some at least of the vents along the south coast of County -Waterford may be vastly younger than the Lower Silurian rocks through -which they have forced their way is suggested, if not proved, by a section -which is in some respects the most extraordinary of the whole of this -remarkable series. The occurrence of a group of red strata was carefully -noted by the late Mr. Du Noyer at Ballydouane Bay, when he was engaged -in carrying on the Geological Survey of that part of the country. At first -he regarded them as belonging to the Old Red Sandstone, which comes on -in great force only a few miles to the west; but he subsequently arrived at -the belief that they are really an integral part of the Lower Silurian rocks -of the district. Professor Jukes had previously expressed himself in favour -of this latter idea, which was thought to receive support from the occurrence -of some reddish strata in the Lower Silurian rocks of Tagoat, County -Wexford.<a id="FNanchor_287" href="#Footnote_287" class="fnanchor">[287]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_287" href="#FNanchor_287" class="label">[287]</a> Explanation of Sheets 167, 168, 178 and 179 of the Geological Survey of Ireland (1865), pp. -10, 59.</p> - -</div> - -<p>The occurrence of red rocks among Silurian strata, which are not -usually red, might quite reasonably be looked for in the neighbourhood of -Old Red Sandstone, Permian or Triassic deposits. If these deposits once -spread over the Silurian formations, a more or less decided "raddling" of -the latter may have taken place. But in the present instance, though the -Old Red Sandstone begins not many miles to the west, no such explanation -of the colour of the strata is possible. The cliffs of Ballydouane Bay -consist of red sandstone, red sandy shale and conglomerate. The red tint -is of that dull chocolate tone so characteristic of the Lower Old Red Sandstone. -The conglomerates are immense accumulations of ancient shingle, -consisting largely of pieces of white vein-quartz and quartzite, sometimes a -foot long and often well water-worn. Some of the sandy beds are full of -large scales of white mica, as if derived from some granitic or schistose -region at no great distance. Taken as a whole, the strata are much less -indurated and broken than the Silurian grits and shales of the district; -some of them, indeed, weather into mere incoherent sand that crumbles -under the fingers. There does not appear to be any positive proof that the -red rocks are truly bedded with the ordinary Silurian strata, the junctions -being faulted or obscured by intrusive igneous masses.</p> - -<p>Nowhere in the British Islands, so far as I am aware, is there a similar -group of strata among the Lower Silurian rocks. If they belong to so -ancient a series, they show that in the south of Ireland, during Lower -Silurian time, there arose a set of peculiar physical conditions precisely like -those that determined the accumulation of the Old Red Sandstone in the -same region at a later geological period. And in that case it is hardly -<span class="pagenum" id="Page_251">- 251 -</span> -possible to conceive that these conditions could have been confined to the -extreme south of Ireland. We should certainly expect to meet with -evidence of them elsewhere, at least in the same Silurian region.<a id="FNanchor_288" href="#Footnote_288" class="fnanchor">[288]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_288" href="#FNanchor_288" class="label">[288]</a> The nearest approach of any Silurian group of strata to the character of these conglomerates -is furnished by the remarkably coarse conglomerates, boulder-beds and pebbly grits of the Bala -and Llandovery series in the region between Killary Harbour and Lough Mask, to which further -reference is made in a later part of this chapter.</p> - -</div> - -<p>While I hesitate to express a decided opinion in opposition to the -conclusions of such experienced observers as Jukes and Du Noyer, I incline -to believe that the rocks in question really belong to the Old Red Sandstone. -If such shall finally be determined to be their geological position, they will -supply evidence that some at least of the volcanic vents of the coast-line -cannot be older than the Old Red Sandstone. They are pierced by masses -of soda-felsite and by a coarse red agglomerate containing abundant pieces -of felsite. These volcanic rocks belong to the same type as those which -break through the undoubted Silurian rocks on either side. They may -thus come to prove a recrudescence of volcanic energy in this same district -at a much later geological period; and a new problem will arise to task the -skill of the most accomplished field-geologist and petrographer—to unravel -the structure and history of this chain of volcanic vents, and, in so doing, -to detect and separate the eruptions of Lower Silurian time from those of -the Lower Old Red Sandstone.</p> - -<p>In the far west of Ireland, another group of Lower Silurian volcanoes has -left its remains in the mountainous tract of country between the western -shores of Lough Mask and Killary Harbour.<a id="FNanchor_289" href="#Footnote_289" class="fnanchor">[289]</a> There appear to have been at -least three separate centres of eruption along a line stretching in a north-easterly -direction for about 16 miles from the western end of Lough Nafooey -to the hamlet of Derrindaffdery beyond Tourmakeady, where the older rocks -are unconformably overlain by the lower Carboniferous strata. As shown -by the mapping of the Geological Survey, the most northerly area, which -may be called the Tourmakeady centre, has a breadth of about a mile, and -dies out southward after a course of nearly six miles. About a mile to the -south-west of the last visible prolongation of its rocks, we encounter a second -volcanic centre which occupies an area of about a square mile in the valley -of Glensaul. The third centre stretches from the western shores of Lough -Mask across Lough Nafooey, where it forms a mass of high rugged ground, -and reaches a length of some six or seven miles before it finally dies out.<a id="FNanchor_290" href="#Footnote_290" class="fnanchor">[290]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_289" href="#FNanchor_289" class="label">[289]</a> This group was placed in the Upper Silurian series by the officers of the Geological Survey who -mapped the region (see Sheets 84, 85, 94 and 95 of the Geological Map of Ireland and accompanying -Explanation), and on their testimony I formerly referred to the volcanic rocks as of Upper -Silurian age. Mr. Baily, however, had pointed out that the limestone associated with the lavas -and agglomerates contains Bala fossils. Yet, in spite of this palæontological testimony, the fossils -were considered to be "derivative," and the rocks were removed from the series of formations to -which they would naturally be assigned. A recent examination of the ground, in company with -Mr. J. R. Kilroe of the Geological Survey, has satisfied me that the volcanic rocks are interstratified -with sedimentary deposits of Bala age, and must consequently be grouped with the -rest of the Lower Silurian series of Ireland. The results of this examination are given in the text.</p> - -<p><a id="Footnote_290" href="#FNanchor_290" class="label">[290]</a> These areas were carefully mapped for the Survey by Mr. Nolan, and the lines of division -marked by him fairly represent the general distribution of the rocks.</p> - -</div> - -<p><span class="pagenum" id="Page_252">- 252 -</span></p> - -<p>The rocks in each of these three areas are similar. One of their distinguishing -features is the intercalation among them of a fossiliferous limestone -and calcareous fossiliferous tuffs, which contain well-preserved species -of organisms characteristic of the Bala division of the Lower Silurian rocks.<a id="FNanchor_291" href="#Footnote_291" class="fnanchor">[291]</a> -There cannot be any question that these organisms were living at the time -the strata in which their remains occur are found. The most delicate -parts of the sculpture on <i>Illænus Bowmanni</i> and <i>Orthis elegantula</i> are well -preserved. Nor have the limestones been pushed into their present places -by volcanic agency, or by faults in the terrestrial crust. They are not only -regularly intercalated among the volcanic rocks, but the limestone in some -places abounds in volcanic dust, while above it come calcareous tuffs, also containing -the same fossils. It is thus clearly established that the volcanic -series now to be described has its geological age definitely fixed as that of -the Bala period.</p> - -<div class="footnote"> - -<p><a id="Footnote_291" href="#FNanchor_291" class="label">[291]</a> See the list of fossils as determined by Mr. Baily in <i>Explanatory Memoir</i> to accompany Sheets -73, 74, 83 and 84 of the Geological Survey of Ireland, p. 68 (1876).</p> - -</div> - -<p>The lavas of the Lough Mask region consist of felsites and andesites -with rocks of probably more basic composition. The felsites are generally -quartziferous porphyries, which occupy a considerable space in each of the -three districts. To what extent they are intrusive rather than interstratified -remains for investigation. Some of them have undoubtedly invaded -other members of the volcanic series. But, on the other hand, fragments -of similar quartz-porphyries and felsites abound in the intercalated bands of -volcanic breccia.</p> - -<p>The andesites and more basic lavas are finely-crystalline or compact, dull-green -to chocolate-purple rocks, often resembling the "porphyrites" of the -Old Red Sandstone. Some of them are strongly vesicular, the cavities being -filled with calcite on fresh fracture, though empty on weathered surfaces. -The sack-like or pillow structure, already referred to as characteristic of many -Lower Silurian lavas, appears conspicuously among some of these rocks. At -Bohaun, nine miles south from Westport, where a prolongation of the volcanic -series rises to the surface from under the overlying coarse conglomerates, I -observed that, owing to the compression which the rocks have there -undergone, the pillow-shaped blocks have been squeezed together into rudely -polygonal forms, while their vesicles have been greatly drawn out in the -direction of tension. Where the rocks have been still more sheared, the -distinct pillow-shaped blocks with their vesicular structure disappear, while -the more fine-grained crusts that surround them have been broken up and -appear as fragments involved in a matrix of green schist.</p> - -<p>Intercalated with the lavas are numerous bands of volcanic breccia and -fine tuff. The stones in these breccias consist chiefly of various felsites with -andesites and more basic lavas. But pieces of jasper, chert, shale and grit -are not infrequent. In some places abundant blocks of black shale are to be -noticed, probably derived from the Llandeilo group which exists below, and -which has here and there been ridged up to the surface in the midst of the -<span class="pagenum" id="Page_253">- 253 -</span> -volcanic rocks.<a id="FNanchor_292" href="#Footnote_292" class="fnanchor">[292]</a> Near Shangort I noticed in one of these breccias one block -measuring 12 feet, another 20 feet in length and 3 or 4 feet thick, composed -of alternating bands of grit and slate. It is interesting to note that these -strata had already undergone cleavage before disruption, the bands of slate -being strongly cleaved obliquely to the bedding. None of the Llandeilo or -other rocks in the neighbourhood display this structure. The blocks seem -to have been derived from some deeper group of strata. They are laid -down parallel with the rude bedding of the breccia in which they lie.</p> - -<div class="footnote"> - -<p><a id="Footnote_292" href="#FNanchor_292" class="label">[292]</a> In re-examining this region, Mr. Kilroe has found in the stream west of the monastery, -Tourmakeady, an uprise of graptolitic black shale containing forms belonging to the very lowest -Llandeilo or Upper Arenig strata, and a similar band above Leenane, Killary Harbour.</p> - -</div> - -<p>The fine tuffs and thin ashy limestones associated with the thicker -band of limestone show the renewal of volcanic explosions after the interval -marked by the calcareous deposit which is sometimes 20 or 40 feet thick. -In many places this limestone is brecciated and much mingled with volcanic -dust and lapilli. At Shangort, for example, the thick tolerably pure -limestone is truncated on the west and north sides by a coarse agglomerate -probably filling a volcanic vent. A few hundred yards further north, -beyond the interrupting agglomerate, the limestone reappears on the same -line of strike, but is then found to be nodular and brecciated and much -mingled with volcanic detritus. It lies among ashy grits and tuffs.</p> - -<div class="figcenter" id="v1fig64" style="width: 492px;"> - <img src="images/v1fig64.png" width="492" height="92" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 64.</span>—Diagram of the general relations of the different groups of rock in the Lower Silurian volcanic - district along the western shore of Lough Mask.<br /> - <i>a</i>, Llandeilo shales, cherts and grits; <i>b</i>, Volcanic breccias; <i>c</i>, Felsites and andesites; <i>d</i>, Tuffs and ashy grits and - shales; <i>e</i>, Limestone with Bala fossils; <i>f</i>, Calcareous tuffs and thin bands of ashy limestone with fossils; <i>g</i>, - Coarse conglomerate and grits; <i>h</i>, Wenlock strata resting unconformably on the Bala rocks and passing southwards - from these to overlie an older series of schists; *, Fault.</div> -</div> - -<p>The general structure of the ground occupied by the Lough Mask volcanic -rocks is diagrammatically represented in <a href="#v1fig64">Fig. 64</a>. The thickness of the -volcanic series must amount to many hundred feet, but it has not been -precisely determined. The uppermost parts of the series pass under a great -thickness of coarse conglomerates and pebbly grits which form the ridge -of Formnamore, and stretch thence westwards along Killary Harbour and -through the Mweelrea mountains. These strata are classed as the Upper -Silurian on the Geological Survey map. Since, however, they conformably -overlie rocks containing Bala fossils, and in the Killary district include -green shales which have yielded fossils of the same age, they doubtless -belong in large part to the Lower Silurian division. The remarkable -coarseness of these conglomerates towards the south, and their rapid -passage into much finer grits and shales towards the north, probably -<span class="pagenum" id="Page_254">- 254 -</span> -indicate that they were formed close to the shores of a land composed of -schistose rocks, quartzite and granite, of which the mountainous tracts of -Connemara are the last relics.</p> - -<p>A base to the volcanic series is found in the occasional uprise of a short -axis of Llandeilo, or perhaps even upper Arenig strata, containing bands of -dark chert and black graptolitic shales. Unfortunately the relations of these -underlying rocks to the volcanic masses are not very clear, being obscured -by superficial accumulations and also by faulting. It is thus hardly -possible to be certain whether they pass up conformably into the base of the -volcanic series, or are covered by it unconformably.</p> - -<p>The position of this isolated volcanic district in the far west of Ireland, -the abundance, variety and thickness of the erupted materials, and the definite -intercalation of these materials in the Bala or highest division of the Lower -Silurian series, acquire a special interest from the history of the nearest -Silurian volcanic area which has now to be described—that of the western -shores of the Dingle promontory.</p> - - -<h3>II. <span class="smcap">The Upper Silurian Series</span></h3> - -<p>The latest volcanic eruptions of Silurian time yet definitely known took -place during the accumulation of the Wenlock and Ludlow rocks in the far -west of Ireland. No satisfactory record of any contemporaneous phenomena -of a like kind has yet been met with in any other Upper Silurian district -in the British Isles, unless at Tortworth in Gloucestershire, as above -described. So far as at present known, only one centre of activity has -been preserved. It lies among the headlands of Kerry, where the land -projects furthest west into the stormy Atlantic. The occurrence of volcanic -rocks in this remote area and their geological horizon have been clearly -indicated on the maps of the Geological Survey. More than thirty years, -however, have elapsed since some of the mapping was done, and we must -therefore be prepared to find it, more especially in its petrography, capable -of modification and improvement now.</p> - -<p>In the country known as the Dingle promontory, these traces of contemporaneous -volcanic rocks are to be observed at various localities and -on several horizons. To the east, near Anascaul, on the northern shore -of Dingle Bay, some tuffs occur in what are believed to be Llandovery -strata. But it is on the western coast, among the headlands and coves -that lie to the north and south of Clogher Head, that the best sections -are to be seen. The succession of the rocks in this locality was well -worked out by Du Noyer, and the Memoir prepared by him, with the -general introduction by Jukes, is an invaluable guide to the geologist who -would explore this somewhat inaccessible region.<a id="FNanchor_293" href="#Footnote_293" class="fnanchor">[293]</a> The most important -correction that will require to be made in the work arises from a mistake as -to the true nature of certain rocks which were described as pisolitic tuffs, -but which are nodular felsites.</p> - -<div class="footnote"> - -<p><a id="Footnote_293" href="#FNanchor_293" class="label">[293]</a> Sheets 160 and 171 of the one-inch map, and Memoir on Sheets 160, 161, 171 and 172.</p> - -</div> - -<p><span class="pagenum" id="Page_255">- 255 -</span></p> - -<p>By far the most striking geological feature of this singularly interesting -and impressive coast-line is to be found in the interstratification of lavas -with bands of tuff among abundantly fossiliferous strata which, from their -organic contents, are unmistakably of the age of the Wenlock group. These -lavas occur in a number of sheets, separated from each other by tuffs and -other fragmental deposits. They thus point to a series of eruptions over -a sea-bottom that teemed with Upper Silurian life. They consist for the -most part of remarkably fine typical nodular felsites. The nodules vary in -dimensions from less than a pea to the size of a hen's egg. They are sometimes -hollow and lined with quartz-crystals. They vary greatly in number, -some parts being almost free from them and others entirely made up of -them. The matrix, where a fresh fracture can be obtained, is horny in -texture, and often exhibits an exceedingly beautiful and fine flow-structure. -On weathered faces there may be seen thick parallel strips and lenticles of -flow-structure like those of the Snowdon lavas. The upper portions of some -of the sheets enclose fragments of foreign rocks. The microscopic examination -of a few slices cut from these lavas shows them to be true felsites -(rhyolites) composed of a microcrystalline aggregate of quartz and felspar, -with layers and patches of cryptocrystalline matter, and only occasional -porphyritic crystals of orthoclase and plagioclase.</p> - -<p>The pyroclastic rocks associated with these lavas vary from exceedingly -fine tuff to coarse agglomerate. Some of the finer tuffs contain pumiceous -fragments and pieces of grey and red shale; they pass into fine ashy sandstones -and shales, crowded with fossils, and into gravelly breccias made up -of fragments of different volcanic rocks.</p> - -<p>But the most extraordinary of these intercalated fragmental strata is -a breccia or agglomerate, about 15 feet thick, which lies in a thick group of -fossiliferous dull-yellow, ashy and ochreous sandstones. The stones of this -bed consist chiefly of blocks of different felsites, varying up to three feet in -length. Some of them show most perfect flow-structure; others are spongy -and cellular, like lumps of pumice. The calcareous sandstone on the top of -the breccia is crowded with fossils chiefly in the form of empty casts, and -the same material, still full of brachiopods, crinoids, corals, etc., fills up the -interstices among the blocks down to the bottom of the breccia, where -similar fossiliferous strata underlie it.</p> - -<p>Nowhere has the volcanic history of a portion of Palæozoic time been -more clearly and eloquently recorded than in this remote line of cliffs swept -by the gales of the Atlantic. We see that the ordinary sedimentation -of Upper Silurian time was quietly proceeding, fine mud and sand being -deposited, and enclosing the remains of the marine organisms that swarmed -over the sea-bottom when volcanic eruptions began. First came discharges -of fine dust and small stones, which sometimes fell so lightly as not seriously -to disturb the fauna on the sea-floor, but at other times followed so rapidly -and continuously as to mask the usual sediment and form sheets of tuff and -volcanic gravel. Occasionally there would come more paroxysmal explosions, -whereby large blocks of lava were hurled forth until they gathered in a -<span class="pagenum" id="Page_256">- 256 -</span> -thick layer over the bottom. But the life that teemed in the sea, though -temporarily destroyed or driven out, soon returned. Corals, crinoids and -shells found their way back again, and fine sediment carried their remains -with it and filled up the crevices. The ejected volcanic blocks are thus -enclosed in a highly fossiliferous matrix.</p> - -<p>A succession of lava-streams, of which the strongly-nodular sheet of -Clogher Head is the thickest and most conspicuous, mark the culmination -of the volcanic energy, and show how at this late part of the Silurian period -felsites that reproduce some of the most striking peculiarities of earlier time -were once more poured out at the surface. A few more discharges of tuff -and the outflow of a greenish flinty felsite brought this series of eruptions -to an end, and closed in Britain the long and varied record of older Palæozoic -volcanic activity.<a id="FNanchor_294" href="#Footnote_294" class="fnanchor">[294]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_294" href="#FNanchor_294" class="label">[294]</a> As this sheet is passing through the press, the interesting paper by Messrs. S. H. Reynolds -and C. J. Gardiner, "On the Kildare Inlier" has appeared (<i>Quart. Journ. Geol. Soc.</i> vol. lii. p. -587). These authors give petrographical details regarding the lavas, which they show to be -andesites and basalts of Bala age, associated with highly fossiliferous tuffs.</p> - -</div> - -<p><span class="pagenum" id="Map_II"></span></p> - - -<p class="pmt2 pmb1 tdc"><span class="smaller">MAP OF THE SILURIAN VOLCANIC DISTRICTS OF NORTH WALES</span><br /> -<span class="vsmall">Reduced from the Maps of the Geological Survey.</span></p> - -<div class="figcenter" id="v1map2" style="width: 523px;"> - <span class="fl_left vsmall">TO ACCOMPANY SIR ARCHIBALD GEIKIE'S "ANCIENT VOLCANOES OF BRITAIN"</span> - <span class="fl_right vsmall">Map II</span> - <div style="clear: both;"> - <a href="images/v1map2lg.png"><img src="images/v1map2.png" width="523" height="577" alt="" /></a> - <span class="fl_left vsmall">The Edinburgh Geographical Institute</span> - <span class="vsmall">Copyright</span> - <span class="fl_right vsmall">J. G. Bartholomew.</span> - </div> - <div class="tdc smaller">Click on map to view larger sized.</div> -</div> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_257">- 257 -</span></p> - -<h2 class="nobreak" id="BOOK_V">BOOK V<br /> - -<span class="smaller">THE VOLCANOES OF DEVONIAN AND OLD RED -SANDSTONE TIME</span></h2> -</div> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<h3 class="nobreak" id="CHAPTER_XV">CHAPTER XV<br /> - -<span class="smaller">THE DEVONIAN VOLCANOES</span></h3> -</div> - - -<p>Throughout the whole region of the British Isles, wherever the uppermost -strata of the Silurian system can be seen to graduate into any later series of -sedimentary deposits, they are found to pass up conformably into an -enormous accumulation of red sandstones, marls, cornstones, and conglomerates, -which have long been grouped together under the name of -"Old Red Sandstone." In England and Wales, in Scotland and in Ireland, -this upward succession is so well shown that at first British geologists -were naturally disposed to believe it to represent the normal order of the -geological record. When, however, Sedgwick and Murchison demonstrated -that in the counties of Devon and Cornwall a very different group of strata -contained an abundant assemblage of organic remains, including types which -Lonsdale showed to be intermediate between those of the Silurian and the -Carboniferous systems; when, moreover, this palæontological facies of the -south-west of England, termed by its discoverers "Devonian," was found to -be abundantly developed on the Continent, and to be there indeed the -prevalent stratigraphical type of the formations intervening between Silurian -and Carboniferous, geologists began to perceive that the Old Red Sandstone -must be regarded as the record of peculiar local conditions of sedimentation, -while the Devonian type was evidently the more usual development of the -same geological period.</p> - -<p>From the remote Shetland Isles, across the whole of Scotland and -England, down to the northern shores of the Bristol Channel, the Old Red -Sandstone maintains its general characters. Nowhere, indeed, are these -characters more typically developed than in South Wales, where many -thousands of feet of red sediments, almost entirely devoid of organic -<span class="pagenum" id="Page_258">- 258 -</span> -remains, emerge from under the escarpments of Carboniferous Limestone, -and stretch into broad uplands until they are lost at the top of the Silurian -system.</p> - -<p>But when the geologist crosses the Bristol Channel to the opposite -shores of North Devon, he encounters a remarkably different assemblage of -rocks. It is true that he has not yet been able to detect there any equivalents -of the uppermost Silurian strata of Glamorganshire, nor does he find any -conspicuous band of Carboniferous Limestone, such as that which encircles -the Welsh Coal-field. He is thus unable to start from a known definite -horizon in the attempt to work out the order of succession, either in an -upward or downward direction. Lithological characters likewise afford him -no means of establishing any satisfactory parallelism. As he follows the -Devonian strata, however, he finds them to disappear conformably under the -Culm-measures, which, though strangely unlike the Carboniferous strata on -the opposite coast, are yet proved by their fossils to belong to the Carboniferous -system. Hence the Devonian type, like the Old Red Sandstone, is -proved to be immediately anterior to, and to graduate into, the Carboniferous -rocks.</p> - -<p>There is no stratigraphical change in Britain so rapid and complete as -that from the Old Red Sandstone on the one side of the Bristol Channel to -the Devonian series on the other. No satisfactory explanation has yet been -found for this sudden transformation, which still remains one of the -unsolved problems in British geology.</p> - -<p>As the observer follows the Devonian assemblage across the land to the -southern coast-line, he is conscious that its general characters, both lithological -and palæontological, depart more and more from the type of the Old -Red Sandstone, and approach more closely to the common Devonian facies of -the Continent. He is forced to admit that the Old Red Sandstone, notwithstanding -its extensive development in Britain, must be regarded as an -exceptional type of sedimentation, while the Devonian facies represents -that which is most widely prevalent, not only in Europe, but generally -over the world.</p> - -<p>The broad estuary of the Bristol Channel unfortunately conceals from -view the tract which lies between the typical Old Red Sandstone of -Glamorganshire and the typical Devonian formations of Devonshire. -Whether this intervening space of some fifteen miles was occupied by -a physical barrier, which separated the respective areas of deposit of -these two types, or the circumstances of sedimentation in the one region -merged insensibly into those of the other, must remain matter for -speculation.</p> - -<p>The geographical conditions betokened by the Old Red Sandstone will -be considered in the next chapter. There can be no doubt that those -indicated by the Devonian system were marine. The organic remains so -plentifully distributed through the argillaceous and arenaceous sediments of -that system, and so crowded together in its limestones, were obviously -denizens of the open sea. Yet the only tract of Britain over which this -<span class="pagenum" id="Page_259">- 259 -</span> -sea can be shown to have spread was the south of England. To the north -of that belt, the site of Britain during Devonian time appears to have been -partly land and partly wide water-basins in which the Old Red Sandstone -was deposited.</p> - -<p>In that half terrestrial half lacustrine territory that stretched northwards -to beyond the Shetland Isles, many volcanoes were active, of which -the chronicles will be described in later pages. The most southerly of -these centres of eruption yet known was the district of the Cheviot Hills. -Throughout the rest of England and Wales no trace of any contemporary -volcanic action has been detected in the Old Red Sandstone. It is true -that over most of that region rocks of this age have been concealed under -younger formations. Yet throughout Wales, where the Old Red Sandstone -attains so vast a thickness, and covers so wide an area, it has not yet -yielded a vestige of any contemporaneous volcanic eruptions.</p> - -<p>But over the sea-floor that covered the south of England, and stretched -thence into the heart of Europe, abundant volcanoes have left behind them -proofs of their activity. The first geologist who recognized these proofs -and traced their extent on the ground appears to have been De la Beche, -who, by his detailed maps and careful description of the igneous rocks of -Devonshire, did so much to advance the study of ancient volcanic action. -This great pioneer not only determined the former existence of Devonian -volcanoes, but he was likewise the first to detect and map the volcanic rocks -associated with the Carboniferous and "New Red Sandstone" formations of -the same region. The broad outlines traced by him among the volcanic -products of these three geological periods in the south-west of England -still remain but little changed. Nor are they likely to be much improved -until the ground is resurveyed on a larger and more accurate map, and -with better petrographical equipment than were available in his day.</p> - -<p>Not long after the observations of De la Beche came those of A. C. -Godwin-Austen, who devoted much time to a sedulous exploration of the -rocks of South Devon, and satisfied himself that contemporaneous volcanic -sheets were intercalated among the limestones of that district. "The coral -limestones," he says, "are in many places superincumbent on great sheets of -volcanic materials, with which, in some instances, as at North Whilborough, -they alternate." He pointed out that the interstratified volcanic rocks are -of two periods, one Devonian and the other Carboniferous.<a id="FNanchor_295" href="#Footnote_295" class="fnanchor">[295]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_295" href="#FNanchor_295" class="label">[295]</a> <i>Trans. Geol. Soc.</i> 2nd ser. vol. vi. (1842), pp. 465, 470, 473.</p> - -</div> - -<p>In his Geological Maps of Devon and Cornwall, which are to the present -time those issued by the Geological Survey, De la Beche made no attempt -to discriminate between the varieties of igneous rocks, save that the basic -"greenstones" were distinguished from the acid bosses of granite and the -elvans. But in his classic "Report" much more detail was inserted, showing -that he clearly recognized the existence both of volcanic ashes and of lavas, -as well as of intrusive sheets. At the outset of his account of the "Grauwacke," -he remarks that the sedimentary deposits are accompanied with -igneous products, "a portion of which may also be termed sedimentary, -<span class="pagenum" id="Page_260">- 260 -</span> -inasmuch as it would seem to have been deposited in beds among contemporaneous -rocks of the former description by the agency of water, after -having been ejected from fissures or craters in the shape of ashes and -cinders, precisely as we may now expect would happen with the ashes and -cinders ejected from volcanoes, particularly insular and littoral volcanoes, -into the sea."<a id="FNanchor_296" href="#Footnote_296" class="fnanchor">[296]</a> Again he speaks of "two kinds of trappean rocks having -probably been erupted, one in the state of igneous fusion, and the other in -that of ash, during the time that the mud, now forming slates, was deposited, -the mixtures of volcanic and sedimentary materials being irregular from the -irregular action of the respective causes which produced them; so that though -the one may have been derived from igneous action, and the other from -the ordinary abrasion of pre-existing solid rocks, they were geologically contemporaneous."<a id="FNanchor_297" href="#Footnote_297" class="fnanchor">[297]</a> -He recognized the origin of the amygdaloidal varieties of -rock, and by dissolving out the calcite from their cells showed how close -was their resemblance to modern pumice.<a id="FNanchor_298" href="#Footnote_298" class="fnanchor">[298]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_296" href="#FNanchor_296" class="label">[296]</a> "Report on the Geology of Cornwall, Devon and West Somerset," <i>Mem. Geol. Survey</i>, -1839, p. 37.</p> - -<p><a id="Footnote_297" href="#FNanchor_297" class="label">[297]</a> <i>Op. cit.</i> p. 57.</p> - -<p><a id="Footnote_298" href="#FNanchor_298" class="label">[298]</a> <i>Op. cit.</i> pp. 57, 61.</p> - -</div> - -<p>Since these early researches many geologists have studied the igneous -rocks of Devonshire. I would especially refer to the labours of Mr. -Allport,<a id="FNanchor_299" href="#Footnote_299" class="fnanchor">[299]</a> the late J. A. Phillips,<a id="FNanchor_300" href="#Footnote_300" class="fnanchor">[300]</a> Mr. Rutley,<a id="FNanchor_301" href="#Footnote_301" class="fnanchor">[301]</a> the late Mr. Champernowne,<a id="FNanchor_302" href="#Footnote_302" class="fnanchor">[302]</a> -Mr. W. A. E. Ussher,<a id="FNanchor_303" href="#Footnote_303" class="fnanchor">[303]</a> Mr. Hobson,<a id="FNanchor_304" href="#Footnote_304" class="fnanchor">[304]</a> and General M'Mahon.<a id="FNanchor_305" href="#Footnote_305" class="fnanchor">[305]</a> Mr. -Champernowne in particular has shown the abundance of volcanic material -among the rocks of Devonshire, and the resemblance which in this respect -they offer to the Devonian system of North Germany.</p> - -<div class="footnote"> - -<p><a id="Footnote_299" href="#FNanchor_299" class="label">[299]</a> <i>Quart. Journ. Geol. Soc.</i> xxxii. (1876), p. 418.</p> - -<p><a id="Footnote_300" href="#FNanchor_300" class="label">[300]</a> <i>Op. cit.</i> xxxi. (1875) p. 325, xxxii. (1876) p. 155, xxxiv. (1878) p. 471.</p> - -<p><a id="Footnote_301" href="#FNanchor_301" class="label">[301]</a> "Brent Tor," <i>Mem. Geol. Surv.</i> p. 18; <i>Quart. Journ. Geol. Soc.</i> lii. (1896), p. 66.</p> - -<p><a id="Footnote_302" href="#FNanchor_302" class="label">[302]</a> See in particular his last paper "On the Ashprington Volcanic Series of South Devon," -<i>Quart. Journ. Geol. Soc.</i> vol. xlv. (1889), p. 369.</p> - -<p><a id="Footnote_303" href="#FNanchor_303" class="label">[303]</a> This geologist has spent many laborious years in the investigation of the geology of Devonshire, -and has published numerous papers on the subject, in the <i>Transactions of the Devonshire -Association</i> and of the <i>Royal Cornwall Geological Society</i>, in the <i>Proceedings of the Somersetshire -Archæological and Natural History Society</i>, and of the <i>Geologists' Association</i>, in the <i>Geological -Magazine</i>, and the <i>Quarterly Journal of the Geological Society</i>. Reference may especially be made -to his Memoir in the last named journal, vol. xlvi. (1890), p. 487.</p> - -<p><a id="Footnote_304" href="#FNanchor_304" class="label">[304]</a> <i>Quart. Journ. Geol. Soc.</i> xlviii. (1892), p. 496.</p> - -<p><a id="Footnote_305" href="#FNanchor_305" class="label">[305]</a> <i>Op. cit.</i> xlix. (1893), p. 385.</p> - -</div> - -<p>Unfortunately the geological structure of the Palæozoic rocks of the -South-west of England has been complicated to an amazing extent by -plication and fracture, with concomitant cleavage and metamorphism. -Hence it is a task of extreme difficulty to trace out with any certainty -definite stratigraphical horizons, and to determine the range of contemporaneous -volcanic action. Mr. Ussher has shown with what success this -task may be accomplished when it is pursued on a basis of minute mapping, -combined with a sedulous collection and determination of fossils.<a id="FNanchor_306" href="#Footnote_306" class="fnanchor">[306]</a> But -years must necessarily elapse before such detailed work is carried over the -whole Devonian region, and probably not till then will the story of the -volcanic history of the rocks be adequately made out.</p> - -<div class="footnote"> - -<p><a id="Footnote_306" href="#FNanchor_306" class="label">[306]</a> See Memoir cited in a previous note.</p> - -</div> - -<p>In the meantime, it has been established that while there is a singular -<span class="pagenum" id="Page_261">- 261 -</span> -absence of igneous rocks in North Devon, a strip of country extending from -Newton Abbot and Torquay westwards by Plymouth across Cornwall to -Penzance contains abundant records of volcanic action. It has not yet -been possible to map out, among what were formerly all grouped together -as "greenstones," the respective limits of the bedded lavas and the tuffs, to -distinguish the true sills, and to fix on the position of the chief vents of -eruption. So intense have been the compression and shearing of the rocks -that solid sheets of diabase have been crushed into fissile schists, which can -hardly be distinguished from tuffs. Moreover, owing perhaps in large -measure to the mantle of red Permian (or Triassic) strata, which has been -stripped off by denudation from large tracts of this region once overspread -by it, the Devonian rocks have been deeply "raddled," or stained red. But -probably one of the main sources of difficulty in studying the petrography -of the area is to be found in the results of atmospheric weathering. Devonshire -lies in that southern non-glaciated strip of England, where the rocks -have been undergoing continuous decay since long before the Ice Age. -No ploughshare of ice has there swept off the deep weathered crust, so as to -leave hard surfaces of rock, fresh and bare, under a protecting sheet of -boulder-clay. It is seldom that a really fresh piece of any igneous rock -can be procured among the lanes and shallow pits of Devon, where alone, -for the most part, the materials are exposed.</p> - -<p>Much, therefore, remains to be done, both in the stratigraphy and -petrography of the Devonian volcanic rocks of this country. To the late -J. A. Phillips geology is indebted for the first detailed chemical and -microscopical investigation of these rocks. He clearly showed the truly -volcanic origin of many of the so-called "greenstones." He believed that -certain "slaty blue elvans," which he found to have a composition -identical with that of altered dolerites, might be highly metamorphosed -tuffs, and that others might have been originally sheets of volcanic mud. -After studying the chemical composition and minute structure of a large -collection of "greenstones," he demonstrated that in all essential particulars -they were dolerites, though somewhat altered from their original character.<a id="FNanchor_307" href="#Footnote_307" class="fnanchor">[307]</a> -Subsequently they were studied by Dr. Hatch, who found the fresher -specimens generally to possess an ophitic structure, while some are granular, -others porphyritic.<a id="FNanchor_308" href="#Footnote_308" class="fnanchor">[308]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_307" href="#FNanchor_307" class="label">[307]</a> See especially <i>Quart. Journ. Geol. Soc.</i> vols. xxxii. and xxxiv.</p> - -<p><a id="Footnote_308" href="#FNanchor_308" class="label">[308]</a> A few of the eruptive rocks of Devonshire have recently been studied by K. Busz. He -finds most of his specimens (chiefly from the Torquay district) to be varieties of diabase, but -describes a palæopicrite from Highweek near Newton Bushel, and a kersantite from South -Brent on the south-east edge of Dartmoor (<i>Neues Jahrb.</i> 1896, p. 57).</p> - -</div> - -<p>Although the rocks have undergone so much crushing, solid cores of -them, showing the original structure, may be obtained, also examples of the -amygdaloidal, vesicular or slaggy character. They occur in sheets either -singly or in groups, and appear generally to be regularly interstratified in -the slates and grits. While some of these intercalations, especially the -amygdaloidal sheets, may be true superficial lavas, it can hardly be doubted -that others are sills, especially those which assume the crystalline structure -<span class="pagenum" id="Page_262">- 262 -</span> -and composition of gabbros, and show an entire absence of the vesicular -structure. But no one has yet attempted to separate the two types from -each other.</p> - -<p>With these rocks are associated abundant diabase-tuffs (schalstein), -frequently mingled with ordinary non-volcanic detrital matter, and shading -off into the surrounding grits and slates. There is thus clear evidence of -the outpouring of basic lavas and showers of ashes during the Devonian -period in the south-west of England, under conditions analogous to those -which characterized the deposition of the Devonian system in Nassau and -the Harz.</p> - -<p>The exact range of these eruptions in geological time has still to be -ascertained. So far as at present determined, volcanic activity was not -awakened during the accumulation of the Lower Devonian formations. It -was not until the sporadic coral-reefs and shell-banks had grown up, which -form the basement limestones of the Middle Devonian group, that the first -eruptions took place. As Godwin-Austen, Champernowne and Mr. Ussher -have shown, some of these reefs were overwhelmed with streams of lava or -buried under showers of ashes. The volcanic discharges, however, were -peculiarly local, probably from many scattered vents, and never on any -great scale. Some districts remained little or not at all affected by them, -so that the growth of limestone went on without interruption, or at -least with no serious break. In other areas again the place of the limestone -is taken by volcanic materials.</p> - -<p>The chief epoch of this volcanic action, marked by the "Ashprington -Volcanic Series," appears to have occurred about midway in the Middle -Devonian period. But in certain districts it extended into Upper -Devonian time. Intrusive sills of diabase may mark the later phases of the -volcanic history. But the occurrence of such sills even in the Upper -Devonian rocks, and the alteration of the strata in contact with them -(spilosite), point to the continuance or renewal of subterranean disturbance -even in the later Devonian ages, if not in subsequent geological time. -That volcanic activity accompanied the deposition of the Carboniferous rocks -of Devonshire has long been well known (see <a href="../../66493/66493-h/66493-h.htm#CHAPTER_XXIX">Chapter xxix.</a>).</p> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_263">- 263 -</span></p> - -<h2 class="nobreak" id="CHAPTER_XVI">CHAPTER XVI<br /> - -<span class="smaller">THE VOLCANOES OF THE OLD RED SANDSTONE</span></h2> -</div> - -<div class="blockquot"> - -<p>Geological Revolutions at the close of the Silurian Period—Physical Geography of the -Old Red Sandstone—Old Lake-basins, their Flora and Fauna—Abundance of -Volcanoes—History of Investigation in the Subject.</p> -</div> - - -<p>We now enter upon the consideration of the records of a notable era in the -geological evolution of north-western Europe. Up to the close of the -Silurian period the long history embodied in the rocks presents a constant -succession of slowly sinking sea-floors. Wide tracts of ocean stretched over -most of Europe, and across the shifting bottom, sand and mud, washed from -lands that have long vanished, spread in an ever-accumulating pile. Now -and then, some terrestrial movement of more than usual potency upraised -this monotonous sea-bed, but the old conditions of ceaseless waste continued, -and fresh sheets of detritus were thrown down upon the broken-up -heaps of older sediment. All through the vast cycles of time denoted by -these accumulations of strata, generations of sea-creatures came and went in -long procession, leaving their relics amidst the ooze of the bottom. Genera -and families, once abundant, gradually died out, and gave place to others, the -onward march of life being slow but uninterrupted. Of the land of the -time or of the plants and animals that lived on its surface, hardly anything -is known. The chronicles that have come down to us are almost wholly -records of the vicissitudes of the ocean-bed.</p> - -<p>Over the centre and south of Europe, the marine conditions of Silurian -time were prolonged, as we have seen, into the next period, when the -Devonian formations were deposited. In that wide region, no marked break -has been traced between either the sedimentation or the animal life of the -Silurian and Devonian periods. But in the north-west of Europe a striking -departure took place from the protracted monotony of marine conditions. -By a series of terrestrial movements that affected the area lying to the -north of the line of the Bristol Channel, and extended not only to the -furthest limit of the British Isles, but probably as far as Norway, and -perhaps even into northern Russia, the previous widespread conditions of -marine sedimentation were entirely altered. Instead of the fine oceanic -silts and sands with their abundant organic remains, and the thick limestones -<span class="pagenum" id="Page_264">- 264 -</span> -with their masses of coral and crowds of crinoids, there were now -laid down, over these northern regions, vast piles of deep red sediment, from -which traces of animal life are almost wholly absent. The shelving land -against which these ferruginous sands and gravels gathered can still in part -be recognized. As the observer follows its margin, notes the varying local -peculiarities of its sediment, and detects, sometimes in great abundance, -remains of the vegetation which clothed it, the conviction grows in his -mind that the remarkable contrast between these deposits, known as the -Old Red Sandstone, and those of the Silurian and Devonian systems is not -to be accounted for by any mere rearrangement of the sea-bottom, or redistribution -of the land that supplied that sea-bottom with sediment. It -has long been the general belief among geologists that the subterranean -movements which, over the greater part of Britain, brought the deposition -of the Upper Silurian formations to a close, led to a total alteration of the -geography of the region affected, that the sea-floor was elevated, and -that, over the upraised tract, large lakes or inland seas were eventually -formed, in which the peculiar sediments of the Old Red Sandstone were -accumulated.</p> - -<p>The records of this series of geographical changes are too fragmentary -to enable us to follow, except in a very general way, the sequence of events -in the transformation of the Silurian sea into the peculiar topographical -conditions in which the Old Red Sandstone was laid down. While there -was a widespread elevation of the sea-floor, and of such ridges of insular -land as may have risen above sea-level, the upheaval appears to have been -of a somewhat complicated kind, and to have been combined with many -local subsidences. The area of disturbance was probably thrown into a -series of parallel ridges and troughs, the former continuing to be pushed -upward, while the latter tended to subside. The ridges thus became land -surfaces, and their prolonged elevation may have more or less compensated -for the denudation to which, on their emergence, they were necessarily -exposed. The troughs, on the other hand, which sank down, may in many -cases have subsided below the sea. But where the general upheaval of the -crust was most pronounced, some of the depressions would be isolated above -sea-level and become lake-basins in the terrestrial areas.</p> - -<p>Of some of these water-basins the outlines can still in some measure be -defined. The rocks that rose into hills around them, and from which their -enormous accumulations of detritus were derived, still partially survive. We -can explore these piles of sediment, and from them can form some idea of the -condition of the water in the lakes, and the nature of the vegetation on -the surrounding land. The frequent occurrence and exceeding coarseness of -the conglomerates, which appear on many successive horizons throughout -the deposits of these basins, probably indicate contemporaneous terrestrial -disturbances. The same causes that led to the wrinkling of the crust into -parallel ridges and troughs no doubt still continued in operation. From -time to time the ridges, much worn down by prolonged denudation, were -pushed upward, either by gradual uprise or by more rapid jerks. The -<span class="pagenum" id="Page_265">- 265 -</span> -troughs may in like manner have been still affected by their old tendency -to subsidence. Hence, in spite of the effects of degradation and deposition, -it is possible that the ridges may not, on the whole, have varied much in -height, nor the basins in depth, during the time when thousands of feet were -stripped off the land and strewn in detritus over the bottoms of the lakes.</p> - -<p>Let us try to realize a little more definitely the general aspect of the -region in which the Old Red Sandstone water-basins lay. As the axes of the -folds into which the crust of the earth was thrown ran in a north-east and -south-west direction, they gave this trend to the lakes and to the tracts of -land that separated them. These intervening ridges must in some instances -have been hilly or even mountainous. Thus, the Scottish Highlands rose -between two of the lakes, and poured into them an abundant tribute of -gravel, sand and silt. The terrestrial vegetation of the time has been -partially preserved. The hills seem to have been clothed with conifers, -while the lower slopes and swamps were green with sigillariæ, lepidodendra -and calamites. One of the most characteristic plants was <i>Psilophyton</i>, of -which large matted sheets were drifted across the lakes and entombed in -the silt of the bottom. A grass-like vegetation, with long linear leaves, -seems to have grown thickly in some of the shallows of the lakes.</p> - -<p>Of the land animals we have still less knowledge than of the vegetation. -Doubtless various forms of insect life flitted through the woodlands, though -no relics of their forms have yet been recovered. But the remains of -myriapods have been found in Forfarshire.<a id="FNanchor_309" href="#Footnote_309" class="fnanchor">[309]</a> These early relics of the -animal life of the land inhabited the woodlands, like our modern gally-worms, -and were swept down into the lakes, together with large quantities -of vegetation.</p> - -<div class="footnote"> - -<p><a id="Footnote_309" href="#FNanchor_309" class="label">[309]</a> Mr. B. N. Peach, <i>Proceedings of Royal Physical Society of Edinburgh</i>, vol. vii. (1882).</p> - -</div> - -<p>Some of the lakes, especially in the earlier part of their history, -abounded in eurypterid crustacea. These animals inhabited the seas in -Upper Silurian time, and appear to have been isolated in the water-basins -of the Old Red Sandstone. Certain species of Pterygotus, a Silurian genus -found also in the Lower Old Red Sandstone, reached a length of six feet. -But the most abundant forms of animal life were fishes. These furnish -additional evidence in favour of the lacustrine nature of the waters in which -they lived. Such characteristically marine forms as the sharks and rays of -the Silurian seas were replaced by genera of Acanthodians, Ostracoderms, -Dipnoids, Teleostomes, Placoderms, and Palæoniscids, which abounded in -the more northerly waters. The distinctive outward characters of many -of these early vertebrates were their bony scales and plates. Some of them -had their heads encased in an armature of bone, of large size and massive -thickness. In several genera the bone was coated with a layer of glittering -enamel. Even now, after the vast lapse of time since their day, the -cuirasses and scale-armour of these fishes keep their bright sheen in the -hardened sand and mud from which they are disinterred.</p> - -<p>A difference is observable between the faunas of the different water-basins. -Even where the same genus occurs in two adjacent areas, the -<span class="pagenum" id="Page_266">- 266 -</span> -species are often distinct. Two large lakes, separated by the tract of the -Scottish Highlands, had each its own assemblage of fishes, not a single -genus being common to the two basins. Such contrasts, whether the two -lakes were geologically contemporaneous, or the northern arose later than -the southern, undoubtedly indicate long-continued isolation and the gradual -evolution of new forms under different conditions of environment.<a id="FNanchor_310" href="#Footnote_310" class="fnanchor">[310]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_310" href="#FNanchor_310" class="label">[310]</a> In my memoir "On the Old Red Sandstone of Western Europe" (<i>Trans. Roy. Soc. Edin.</i> -vol. xxviii. 1878), I argued for the probable geological contemporaneity of the conglomerates, -sandstones and flagstones on either side of the Grampian chain, even although their organic -contents were so unlike. The stratigraphical evidence favours this view. In each case a thick -series of strata is covered unconformably by Upper Old Red Sandstone, containing <i>Holoptychius -nobilissimus</i> and other fishes. The question cannot perhaps be definitely settled by the data -available in Scotland. It is quite possible that the basin on the northern side of the Grampians, -which I have termed "Lake Orcadie," came into existence after that on the southern side. But -I do not think the differences in their respective faunas are to be accounted for simply by lapse -of time and the gradual organic evolution in progress over one continuous region. The more -the Old Red Sandstone is studied, the more local do its various fish-faunas appear to have been. -These strongly-marked diversities appear to me rather to point to prolonged isolation of the -basins from each other, as stated above. Dr. Traquair has drawn attention to the remarkable -fact that, even in what appears to be one continuous series of strata of no great thickness forming -the Upper Old Red Sandstone of the Moray Firth basin, the fishes found about Nairn are -entirely different from those met with in the rest of the region.</p> - -</div> - -<p>Such, in brief, were the aspects of the physical geography of the time on -the further consideration of which we are now to enter. The subterranean -disturbances, so characteristic of the period, were accompanied by a display -of volcanic activity more widespread, perhaps, than any which had yet taken -place in the geological history of Britain. Nevertheless, it is worthy of remark -that this manifestation of underground energy did not begin with the commencement -of these displacements of the crust. The earliest eruptions only -took place after the geography of the region had been completely changed; -at least no trace of them is to be found in the earliest portions of the Old -Red Sandstone. After the last lingering Silurian volcanoes in the west of -Ireland had died out, a protracted quiescence of the subterranean fires -ensued. In the latest ages of Silurian time there was not in Britain, so far -as at present known, a single volcanic eruption. Not until after the inauguration -of the Old Red Sandstone topography, when the lakes had -taken shape and had begun to be filled with sediment from the surrounding -hills, did a series of new volcanoes burst into activity over the northern -half of Britain. Rising in the midst of the lakes in groups of separate -cones, these vents poured out floods of lava, together with clouds of ashes -and stones. Their sites, the history of their eruptions, and the piles of -material ejected by them, can still be ascertained, and I shall now proceed -to give some account of them.</p> - -<p>The thick mass of sedimentary material known as the Old Red Sandstone, -lying between the top of the Silurian and the base of the Carboniferous -system, has been divided into two sections, which, however, are of -unequal dimensions, and doubtless represent very unequal periods of time. -The older series, or Lower Old Red Sandstone, is by far the more important -and interesting in its extent, thickness, palæontological riches, and, what -<span class="pagenum" id="Page_267">- 267 -</span> -specially concerns us in the present inquiry, in its volcanic records. -Wherever its true base can be seen, this series passes down conformably -into Upper Silurian strata. It sometimes reaches a thickness of 15,000 -and even 20,000 feet. There is generally a marked break between its -highest visible strata and all younger formations. Even the upper -division of the Old Red Sandstone rests unconformably upon the -lower.<a id="FNanchor_311" href="#Footnote_311" class="fnanchor">[311]</a> Such a hiatus undoubtedly points to a considerable lapse of -geological time, and to the advent of important geographical changes -that considerably modified the remarkable topography of the older part -of the period.</p> - -<div class="footnote"> - -<p><a id="Footnote_311" href="#FNanchor_311" class="label">[311]</a> <i>Quart. Journ. Geol. Soc.</i> vol. xvi. (1860), p. 312. In Wales no break has actually been discovered -between the two divisions of the Old Red Sandstone, though it is suspected to exist -there also.</p> - -</div> - -<p>The younger division or Upper Old Red Sandstone passes upward conformably -into the base of the Carboniferous system. Its red and yellow -sandstones, conglomerates and breccias, covering much more restricted areas, -and attaining a much less thickness than those of the lower division, -indicate the diminution and gradual effacement of the lakes of the older -time, and the eventual return of the sea to the tracts from which it had -been so long excluded. So vast an interval elapsed between the time -recorded in the deposits respectively of the two sections of the Old Red -Sandstone that the characteristic forms of animal life in the earlier ages -had entirely passed away, and their places had been taken by other types -when the diminished lake-basins of the second period began to be filled up. -Volcanic action also dwindled to such a degree that in contrast to the -abundant vents of the older period, only one or two widely scattered groups -of vents are known to have existed in the area of the British Isles during -the later period, and these, after a feeble activity, gave way to a prolonged -volcanic quiescence, which lasted until the earlier ages of the succeeding or -Carboniferous period.</p> - -<p>Although geologists are in the habit of grouping the Old Red Sandstone -and the Devonian rocks as equivalent or homotaxial formations, deposited in -distinct areas under considerably different conditions of sedimentation, the -attempt to follow out the sequence of strata in Devonshire, and to trace some -analogy between the Devonian succession and that of the Old Red Sandstone, -presents many difficulties for which no obvious solution suggests itself. -Into these problems it is not needful to enter further than was done in the -last chapter. We may assume that not improbably some of the eruptions -now to be described were coeval with those of Devonian time in the south-west -of England, though we may hesitate to decide which of them should be -brought into parallelism.</p> - -<p>As we trace the shore-lines of the ancient basins of the Lower Old Red -Sandstone, and walk over the shingle of their beaches, or as we examine the -silt of their deeper gulfs, and exhume the remains of the plants that shaded -their borders, and of the fishes that swarmed in their waters, we gradually -learn that although the sediments which accumulated in some of these basins -<span class="pagenum" id="Page_268">- 268 -</span> -amount to many thousand feet in thickness; yet from bottom to top they -abound in evidence of shallow-water conditions of deposit. The terrestrial -disturbances above referred to continued for a vast interval, and while, as -already suggested, the floors of the basins sank, and the intervening tracts -were ridged up, as the results of one great movement of the earth's crust, -the denudation of the surface of the land contributed to the basins such a -constant influx of sediment as, on the whole, compensated for the gradual -depression of their bottoms.</p> - -<p>We need not suppose that these movements of subsidence and upheaval -were uninterrupted and uniform. Indeed, the abundant coarse conglomerates, -which play so prominent a part in the materials thrown into the basins, -suggest that at various intervals during the prolonged sedimentation -subterranean disturbances were specially vigorous. But the occurrence of -strong unconformabilities among the deposits of the basins sets this question -at rest, by proving that the terrestrial movements were so great as sometimes -to break up the floor of a lake, and to place its older sediments on -end, in which position they were covered up and deeply buried by the -succeeding deposits.<a id="FNanchor_312" href="#Footnote_312" class="fnanchor">[312]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_312" href="#FNanchor_312" class="label">[312]</a> An unconformability of this kind occurs between the south end of the Pentland Hills and -Tinto in Lanarkshire, and another in Ayrshire.</p> - -</div> - -<p>It is not surprising to discover, among these evidences of great terrestrial -disturbance, that eventually groups of volcanoes rose in long lines from the -waters of most of the lakes, and threw out enormous quantities of lava and -ashes over tracts hundreds of square miles in extent. So vast, indeed, were -these discharges, across what is now the Midland Valley of Scotland, that the -portions of sheets of lava and tuff visible at the surface form some of the -most conspicuous ranges of hills in that district, stretching continuously for -40 or 50 miles and reaching heights of more than 2000 feet above the -sea. Exposed in hundreds of ravines and escarpments, and dissected by the -waves along both the eastern and western coasts of the country, these -volcanic records may be studied with a fulness of detail which cannot be -found among earlier Palæozoic formations.</p> - -<p>It might have been supposed that a series of rocks so well displayed -and so full of interest, would long ere this have been fully examined and -described. But they can hardly be said to have yet received, as a whole, -the attention they deserve. Without enumerating all the writers who, each -in his own measure, have added to the sum of our knowledge of the subject, -I may refer to the labours of Jameson,<a id="FNanchor_313" href="#Footnote_313" class="fnanchor">[313]</a> Macknight<a id="FNanchor_314" href="#Footnote_314" class="fnanchor">[314]</a> and Fleming,<a id="FNanchor_315" href="#Footnote_315" class="fnanchor">[315]</a> among -the observers who began the investigation. But of the early pioneers, by far -the most important in regard to the igneous rocks of the Old Red Sandstone -was Ami Boué. While attending the University of Edinburgh, where -he took the degree of M.D. in the year 1816, he imbibed from Jameson a -love of mineralogy and geognosy, and for several years spent his leisure time -in personally visiting many parts of Scotland, in order to study the geological -<span class="pagenum" id="Page_269">- 269 -</span> -structure of the country. Probably in 1820 he published in French his -now classic <i>Essai</i>.<a id="FNanchor_316" href="#Footnote_316" class="fnanchor">[316]</a> The value of this work as an original contribution -to the geology of the British Isles has probably never been adequately -acknowledged. For this want of due recognition the author himself was no -doubt in some measure to blame. He refers distinctly enough to various -previous writers, notably to Jameson and Macculloch, but he mingles the -results of his own personal examinations with theirs in such a way that -it is hardly possible to ascertain what portions are the outcome of his own -original observations. Less credit has accordingly been given to him than -he could fairly have claimed for solid additions to the subjects of which -he treated. In the later years of his life I had opportunities of learning -personally from him how extensive had been his early peregrinations in -Scotland, and how vivid were the recollections which, after the lapse of half -a century, he still retained of them. Judged simply as a well-ordered -summary of all the known facts regarding the geology of Scotland, his -<i>Essai</i> must be regarded as a work of very great value. Especially important -is his arrangement of the volcanic phenomena of the country, which stands -far in advance of anything of the kind previously attempted. Under the -head of the "Terrain Volcanique," he treats of the basaltic formations, distinguishing -them as sheets (<i>nappes</i>, <i>coulées</i>) and dykes; and of the felspathic -or trachytic formations, which he subdivides into phonolites, trachytes, -porphyries (forming mountains and also sheets) and felspathic or trachytic -dykes. In the details supplied under each of these sections he gives facts -and deductions which were obviously the result of his own independent -examination of the ground, and he likewise marshals the data accumulated -by Jameson, Macculloch and others, in such a way as to present a more -comprehensive and definite picture of the volcanic phenomena of Scotland -than any previous writer had ventured to give.</p> - -<div class="footnote"> - -<p><a id="Footnote_313" href="#FNanchor_313" class="label">[313]</a> <i>Memoirs of the Wernerian Society</i>, vol. ii. (1811), pp. 178, 217, 618; vol. iii. (1820), p. 220, -225.</p> - -<p><a id="Footnote_314" href="#FNanchor_314" class="label">[314]</a> <i>Op. cit.</i> vol. ii. pp. 123, 461.</p> - -<p><a id="Footnote_315" href="#FNanchor_315" class="label">[315]</a> <i>Op. cit.</i> vol. i. (1808), p. 162; vol. ii. (1811), pp. 138, 339.</p> - -<p><a id="Footnote_316" href="#FNanchor_316" class="label">[316]</a> <i>Essai géologique sur l'Écosse</i> (Paris; no date, but probably about 1820). He acknowledges -his indebtedness to Jameson, whose demonstrations of the geology of the Edinburgh district he -partly reproduced in his book. Jameson's early writings in the <i>Wernerian Memoirs</i> and in separate -works were mere mineralogical or "geognostical" descriptions. His later lectures became -more valuable but were never published, save indirectly in so far as they influenced the opinions -of his pupils who published writings on the same subjects. See, for instance, Hay Cunningham's -<i>Geology of the Lothians</i>, p. 59, footnote. Compare an article on Boué, <i>Edinburgh Review</i> for May -1823 (vol. xxxviii. p. 413).</p> - -</div> - -<p>The account which Boué wrote of the Old Red Sandstone and its -associated igneous rocks marked the first great forward step in the investigation -of this section of the geological record. He was the earliest observer to -divide what he calls the "roches feldspathiques et trappéennes" into groups -according to their geological position and mineralogical character, and to -regard them as of igneous origin and of the age, or nearly of the age, of the -red sandstone of Central Scotland.</p> - -<p>Of later writers who have treated of the volcanic rocks of the Old Red -Sandstone, my old friend Charles Maclaren deserves special recognition. -His survey and description of the Pentland Hills embodied the first detailed -and accurate investigation of any portion of these rocks, and his <i>Geology -<span class="pagenum" id="Page_270">- 270 -</span> -of Fife and the Lothians</i> may still be read with pleasure and instruction.<a id="FNanchor_317" href="#Footnote_317" class="fnanchor">[317]</a> -Boué had indicated roughly on the little sketch-map accompanying his -<i>Essai</i> the chief bands of his felspathic and trappean rocks of the Old -Red Sandstone, but their position and limits were more precisely defined in -Macculloch's "Geological Map of Scotland," which was published in 1840, -five years after the sudden and tragic death of its author. The observers -who have more recently studied these rocks have been chiefly members of -the Geological Survey, and to some of the more important results obtained -by them I shall refer in the sequel.</p> - -<div class="footnote"> - -<p><a id="Footnote_317" href="#FNanchor_317" class="label">[317]</a> <i>Geology of Fife and the Lothians</i>, 1839. More detailed reference will be made in later -pages to this classic.</p> - -</div> - -<p>For many years I have devoted much time to the investigation of the -Old Red Sandstone and its volcanic rocks. In the year 1859 I ascertained -the existence of the great hiatus between the Lower and Upper divisions -of the system.<a id="FNanchor_318" href="#Footnote_318" class="fnanchor">[318]</a> A first sketch of the volcanic history of the Old Red -Sandstone was given by me in 1861,<a id="FNanchor_319" href="#Footnote_319" class="fnanchor">[319]</a> which was subsequently enlarged and -filled in with more detail in 1879.<a id="FNanchor_320" href="#Footnote_320" class="fnanchor">[320]</a> But it was not until 1892 that I -published a somewhat detailed outline of the whole subject, tracing the -history of volcanic action during the period of the Old Red Sandstone, the -distribution of the volcanoes, and the character of the materials erupted by -them.<a id="FNanchor_321" href="#Footnote_321" class="fnanchor">[321]</a> This outline I now proceed to amplify, filling in details that were -necessarily omitted before, though there are still several districts regarding -which information is scanty.</p> - -<div class="footnote"> - -<p><a id="Footnote_318" href="#FNanchor_318" class="label">[318]</a> "On the Old Red Sandstone of the South of Scotland," <i>Quart. Journ. Geol. Soc.</i> xvi. -(1860), p. 312.</p> - -<p><a id="Footnote_319" href="#FNanchor_319" class="label">[319]</a> "On the Chronology of the Trap-Rocks of Scotland," <i>Trans. Roy. Soc. Edin.</i> vol. xxii. -(1861), p. 63.</p> - -<p><a id="Footnote_320" href="#FNanchor_320" class="label">[320]</a> Article "Geology," in Ninth Edition of the <i>Encyclopædia Britannica</i>, vol. x. (1879), p. 343. -Reprinted in my <i>Text-Book of Geology</i>, of which the first edition appeared in 1882.</p> - -<p><a id="Footnote_321" href="#FNanchor_321" class="label">[321]</a> "Presidential Address to the Geological Society," <i>Quart. Journ. Geol. Soc.</i> vol. xlviii. -(1892).</p> - -</div> - -<p>In arranging the treatment of the subject I shall divide the record into -two main sections, the first and much the more important being devoted to -the Lower and the second to the Upper Old Red Sandstone. In the first -of these divisions it will be convenient to begin by taking note of the -distribution of the various districts over which the geological evidence is -spread. We may then proceed to consider the general character of the -volcanic rocks and the manner in which they are arranged in the stratigraphy -of the country, taking in consecutive order (1) the superficial lavas -and tuffs; (2) the vents; (3) the dykes and sills. From these general -considerations we may pass to the detailed history of events in each of the -separate volcanic areas, and thus obtain, as far as the evidence at present -permits, a broad view of the progress of volcanic action during the time of -the Lower Old Red Sandstone in Britain.</p> -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_271">- 271 -</span></p> - - -<h2 class="nobreak" id="CHAPTER_XVII">CHAPTER XVII</h2> -</div> - -<p>DISTRIBUTION OF THE VOLCANIC CENTRES IN THE LOWER OLD RED SANDSTONE—CHARACTERS -OF THE MATERIALS ERUPTED BY THE VOLCANOES</p> - - -<p>i. <span class="allsmcap">DISTRIBUTION OF VOLCANIC CENTRES</span></p> - -<p>The area within which volcanic rocks belonging to the Lower Old Red Sandstone -appear is one of the most extensive regions over which the volcanic -eruptions of any geological period can be traced in the British Isles (<a href="#Map_I">Map I.</a>). -Its northern limit reaches as far as the islet of Uya in Shetland, and its -southern appears in England in the Cheviot Hills—a distance of about -250 miles. But volcanic rocks of probably corresponding age occur even -as far to the south as the hills near Killarney. The most easterly -margin of this area is defined by the North Sea on the coast of Berwickshire, -and its extreme western boundary extends to near Lough Erne in -the north of Ireland—a distance of some 230 miles. If we include the post-Silurian -bosses and dykes, like those of Shap, and likewise the Devonian -volcanic rocks of Devon and Cornwall, as contemporaneous with those of -the Old Red Sandstone, the area of eruption will be greatly enlarged. -But leaving these out of account for the present, and confining our attention -to the Lower Old Red Sandstone series, we find that, within the wide -limits over which the volcanic rocks are distributed, a number of distinct -and often widely separated centres of eruption may be traced. Taking -these as they lie from north to south, we may specially enumerate the -following:—</p> - -<p>1. The Shetland and Orkney Islands, together with the basin of the -Moray Firth. This region includes several distinct volcanic groups, of -which the most northerly extends through the centre to the north-western -headlands of the mainland of Shetland, another lies in the island of Shapinshay, -one of the Orkneys, while at least two can be recognized on the south -side of the Moray Firth. To this wide region of Old Red Sandstone I -have given the general designation of "Lake Orcadie."<a id="FNanchor_322" href="#Footnote_322" class="fnanchor">[322]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_322" href="#FNanchor_322" class="label">[322]</a> <i>Trans. Roy. Soc. Edin.</i> vol. xxviii. (1878), p. 354.</p> - -</div> - -<p>2. The basin of Lorne, on the west of the mainland of Argyllshire, -extending from Loch Creran to Loch Melfort and the hills on the west side -of Loch Awe.</p> - -<p><span class="pagenum" id="Page_272">- 272 -</span></p> - -<p>3. The great central basin of Scotland, which, for the sake of distinctness, -I have called "Lake Caledonia,"<a id="FNanchor_323" href="#Footnote_323" class="fnanchor">[323]</a> stretching between the Highlands -and the Southern Uplands, from the east coast south-westwards across Arran -and the south end of Cantire into Ireland as far as Lough Erne. Numerous -distinct volcanic groups occur in this great basin, and their volcanic -history will be discussed in detail in later chapters (see <a href="#Map_III">Map III</a>.).</p> - -<div class="footnote"> - -<p><a id="Footnote_323" href="#FNanchor_323" class="label">[323]</a> <i>Op. cit.</i></p> - -</div> - -<p>4. The basin of the Cheviot Hills and Berwickshire, with these hills as -the chief area, but including also other tracts, probably independent, which -are cut off by the sea along the eastern coast of Berwickshire between St. -Abb's Head and Eyemouth.</p> - -<p>5. The Killarney tract, including the hills lying around Lough Guitane -in the east of County Kerry.</p> - -<p>At the outset we may take note of a feature in the volcanic history -of Britain, first prominently noticeable in the records of the Old Red -Sandstone, and becoming increasingly distinct during the rest of the long -sequence of Palæozoic eruptions, namely, the persistence with which -the vents have been opened in the valleys and have avoided the high -grounds. I formerly dwelt on this relation, with reference to the Carboniferous -volcanic phenomena,<a id="FNanchor_324" href="#Footnote_324" class="fnanchor">[324]</a> but the observation may be greatly extended. -With regard to the Old Red Sandstone of Central Scotland, though the -lavas and tuffs that were discharged over the floor of the sheet of water -which occupied that region gradually rose along the flanks of the northern -and southern hills, yet it was on the lake-bottom and not among the hills -that the orifices of eruption broke forth.</p> - -<div class="footnote"> - -<p><a id="Footnote_324" href="#FNanchor_324" class="label">[324]</a> <i>Trans. Roy. Soc. Edin.</i> vol. xxix. (1879), p. 454.</p> - -</div> - -<p>So far as I am aware, no undoubted vents of the age of the Lower Old -Red Sandstone have been detected among the high grounds of the Highlands -on the one hand, or among the Silurian uplands on the other, -although a fringe of the lavas may be traced here and there along the base of -the hills.<a id="FNanchor_325" href="#Footnote_325" class="fnanchor">[325]</a> In some cases, doubtless, the position of the valleys may have -been determined by lines of fault that might well serve as lines of relief along -which volcanic vents would be opened. But in many instances it can be -proved that, though the vents have risen in valleys and low grounds, they -have not selected lines of fault visible at the surface, even when these -existed in their neighbourhood. Any fissures up which the volcanic -ejections made their way must have lain at great depths beneath the formations -that now form the surface rocks.</p> - -<div class="footnote"> - -<p><a id="Footnote_325" href="#FNanchor_325" class="label">[325]</a> Certain remarkable necks of breccia have been detected by Mr. J. R. Dakyns rising -through the schists at the upper end of Loch Lomond; but there is not sufficient evidence to -connect them with the volcanic series of the Lower Old Red Sandstone. Some of the younger -granite bosses are not improbably to be referred to this volcanic series. The latest granites of the -eastern Grampians, as already stated, have lately been found by Mr. Barrow cutting the band of -probably Lower Silurian strata along the southern border of the Highlands. Those of Galloway -are younger than the Upper Silurian formations, which they invade, and older than the -conglomerates of the Upper Old Red Sandstone, which contain pebbles of them. These -eruptive bosses will be further discussed in the sequel.</p> - -</div> - -<p><span class="pagenum" id="Page_273">- 273 -</span></p> - - -<p>ii. <span class="allsmcap">CHARACTERS OF THE MATERIALS ERUPTED BY THE VOLCANOES</span></p> - -<p>A general summary of the petrographical characters of the igneous rocks -of the Lower Old Red Sandstone may here find a place. Further details -will be given in the account of "Lake Caledonia," which is the typical area -for them; but, on the whole, the prevailing types in one region are found -to be repeated in the others.</p> - -<p>1. <i>Bedded Lavas.</i>—Beginning with the lavas which were poured out at -the surface, we have to notice a considerable range of chemical composition -among them, although, as a rule, they are characterized by general similarity -of external appearance. At the one end, come diabases and other ancient -forms of basalt or dolerite, wherein the silica percentage is below or little -above 50. By far the largest proportion of the lavas, however, are porphyrites -or altered andesites, having about 60 per cent of silica. With these -are associated lavas containing more or less unstriped felspar and a somewhat -higher proportion of silica, which may be grouped as trachytes, -though no very sharp line can be drawn between them and the andesites. In -the Pentland Hills, and some other areas, orthophyres flowed out alternately -with the more basic lavas, and were associated with felsitic tuffs and -breccias.</p> - -<p>It is noteworthy that the lava-sheets of the Lower Old Red Sandstone, -if we consider the character of the prevalent type, hold an intermediate -grade between the average chemical composition of those of Silurian and of -those of later Carboniferous time. On the one hand, they rarely assume -the character of thoroughly acid rocks, like the nodular rhyolites of the Bala -and Upper Silurian series;<a id="FNanchor_326" href="#Footnote_326" class="fnanchor">[326]</a> on the other hand, they seldom include such -basic lavas as the basalts, so common among the puy-eruptions of the -Carboniferous system, and never, so far as I know, contain varieties comparable -to the "ultra-basic" compounds which I shall have occasion to -allude to as characteristic of a particular volcanic zone in that system.</p> - -<div class="footnote"> - -<p><a id="Footnote_326" href="#FNanchor_326" class="label">[326]</a> The only examples known to me are those of Benaun More and other hills in County Kerry.</p> - -</div> - -<p>(<i>a</i>) The <span class="smcap">Diabase-lavas</span> are typically developed in the chain of the -Pentland Hills, where they form long bands intercalated between felsitic -tuffs—a remarkable association, to which I shall make more detailed -reference in a later chapter. They range in texture from a compact dark -greenish base to a dull earthy amygdaloid. One of their most remarkable -varieties is a fine-grained green porphyry, with large flat tabular crystals -of plagioclase arranged parallel to the direction of flow (Carnethy Hill). -Most of them, however, are more or less amygdaloidal, and some of them -(Warklaw Hill) strongly so. The following analyses, made in the laboratory -of the Royal School of Mines under the direction of Prof. E. Frankland, -show the chemical composition of some of the diabases of the Pentland -Hills:<a id="FNanchor_327" href="#Footnote_327" class="fnanchor">[327]</a>—</p> - -<div class="footnote"> - -<p><a id="Footnote_327" href="#FNanchor_327" class="label">[327]</a> For analyses of some Shetland diabases of Old Red Sandstone age, see Mr. R. R. Tatlock, -<i>Trans. Roy. Soc. Edin.</i> vol. xxxii. (1887), p. 387.</p> - -</div> - -<p><span class="pagenum" id="Page_274">- 274 -</span></p> - -<table summary="data"> -<tr> - <td class="bdl bdt bdb" colspan="3"></td> - <td class="bdl bdt bdb tdc">SiO<sub>2</sub></td> - <td class="bdl bdt bdb tdc">Al<sub>2</sub>O<sub>3</sub></td> - <td class="bdl bdt bdb tdc">Fe<sub>2</sub>O<sub>3</sub></td> - <td class="bdl bdt bdb tdc">FeO</td> - <td class="bdl bdt bdb tdc">CaO</td> - <td class="bdl bdt bdb tdc">MgO</td> - <td class="bdl bdt bdb tdc">K<sub>2</sub>O</td> - <td class="bdl bdt bdb tdc">Na<sub>2</sub>O</td> - <td class="bdl bdt bdb tdc">H<sub>2</sub>O</td> - <td class="bdl bdt bdb tdc">P<sub>2</sub>O<sub>5</sub></td> - <td class="bdl bdt bdb bdr tdc">CO<sub>2</sub></td> -</tr> -<tr> - <td class="bdl tdl" colspan="3">Carnethy Hill<a id="FNanchor_328" href="#Footnote_328" class="fnanchor">[328]</a></td> - <td class="bdl tdc">51·16</td> - <td class="bdl tdc">22·27</td> - <td class="bdl tdc">2·94</td> - <td class="bdl tdc">4·02</td> - <td class="bdl tdc">5·61</td> - <td class="bdl tdc">3·46</td> - <td class="bdl tdc">2·42</td> - <td class="bdl tdc">2·58</td> - <td class="bdl tdc">3·42</td> - <td class="bdl tdc">0·48</td> - <td class="bdl bdr tdc">1·28</td> -</tr> -<tr> - <td class="bdl tdl vtop" rowspan="2">Buiselaw.<br />Sp. grav.<br /> 2·80.</td> - <td rowspan="2"><img src="images/bracel_60.png" width="11" height="60" alt="" /></td> - <td class="tdl">Soluble<br /> in HCl</td> - <td class="bdl tdc">...</td> - <td class="bdl tdc">1·30</td> - <td class="bdl tdc">1·53</td> - <td class="bdl tdc">1·14</td> - <td class="bdl tdc">2·43</td> - <td class="bdl tdc">0·98</td> - <td class="bdl tdc">...</td> - <td class="bdl tdc">...</td> - <td class="bdl tdc">...</td> - <td class="bdl tdc">0·32</td> - <td class="bdl bdr tdc">...</td> -</tr> -<tr> - <td class="tdl">Insoluble<br /> in HCl</td> - <td class="bdl tdc">52·00</td> - <td class="bdl tdc">17·46</td> - <td class="bdl tdc">7·85</td> - <td class="bdl tdc">...</td> - <td class="bdl tdc">6·80</td> - <td class="bdl tdc">1·06</td> - <td class="bdl tdc">1·66</td> - <td class="bdl tdc">4·17</td> - <td class="bdl tdc">2·68</td> - <td class="bdl tdc">...</td> - <td class="bdl bdr tdc"> ...</td> -</tr> -<tr> - <td class="bdl bdb tdl vtop" rowspan="2">Warklaw<br /> Hill.<br />Sp. grav.<br /> 2·77.</td> - <td class="bdb" rowspan="2"><img src="images/bracel_60.png" width="11" height="60" alt="" /></td> - <td class="tdl">Soluble<br /> in HCl</td> - <td class="bdl tdc">...</td> - <td class="bdl tdc">5·23</td> - <td class="bdl tdc">7·32</td> - <td class="bdl tdc">...</td> - <td class="bdl tdc">7·88</td> - <td class="bdl tdc">3·65</td> - <td class="bdl tdc">...</td> - <td class="bdl tdc">...</td> - <td class="bdl tdc">...</td> - <td class="bdl tdc">0·12</td> - <td class="bdl bdr tdc">5·01</td> -</tr> -<tr> - <td class="bdb tdl">Insoluble<br /> in HCl</td> - <td class="bdl bdb tdc">47·77</td> - <td class="bdl bdb bdb tdc">13·08</td> - <td class="bdl bdb tdc">0·84</td> - <td class="bdl bdb tdc">...</td> - <td class="bdl bdb tdc">4·07</td> - <td class="bdl bdb tdc">0·30</td> - <td class="bdl bdb tdc">1·17</td> - <td class="bdl bdb tdc">2·30</td> - <td class="bdl bdb tdc">2·48</td> - <td class="bdl bdb tdc">...</td> - <td class="bdl bdb bdr tdc">...</td> -</tr> -</table> - -<div class="footnote"> - -<p><a id="Footnote_328" href="#FNanchor_328" class="label">[328]</a> There was a trace of manganous oxide in this specimen.</p> - -</div> - -<p>(<i>b</i>) The <span class="smcap">Andesites</span>, or, as they were formerly called, <span class="smcap">Porphyrites</span>, -which constitute by far the largest proportion of the lavas, have a -characteristic but limited range of lithological varieties. The prevailing -type presents a close-grained, rather dull texture, and a colour varying from -pinkish grey, through many shades of green and brown, to purplish red, -which last is, on the whole, the predominant hue. Minute lath-shaped -felspars may frequently be detected with the naked eye on fresh surfaces, -while scattered crystals, which are generally hæmatitic pseudomorphs after -some pyroxene, occasionally after hornblende or mica, may often be -observed. The usual porphyritic constituents are plagioclase felspars, -occasionally in abundant tabular crystals measuring half an inch or more -across, also one or more pyroxenes (augite, enstatite), and sometimes brown -or black mica. Where large felspar-crystals occur in a compact green -matrix, the rock assumes a resemblance to the <i>verde antique</i> of the ancients.<a id="FNanchor_329" href="#Footnote_329" class="fnanchor">[329]</a> -One of the Cheviot andesites lying at the bottom of the series is distinguished -by its large and abundant plates of black mica.<a id="FNanchor_330" href="#Footnote_330" class="fnanchor">[330]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_329" href="#FNanchor_329" class="label">[329]</a> An instance of this rock occurs in Kincardineshire, from which the large flat twins of -labradorite have been analyzed by Dr. Heddle (<i>Trans. Roy. Soc. Edin.</i> vol. xxviii. (1879), -p. 257).</p> - -<p><a id="Footnote_330" href="#FNanchor_330" class="label">[330]</a> C. T. Clough, "The Cheviot Hills," <i>Mem. Geol. Survey</i> (1888), p. 12.</p> - -</div> - -<p>The texture of the andesites occasionally becomes faintly resinous, where -a considerable proportion of glass still remains undevitrified, as in the well-known -varieties from the Cheviot Hills, and in another pitchstone-like rock -from above Airthrey Castle in the Ochil Hills, near Bridge of Allan. It -sometimes presents a nodular or coarsely perlitic character, weathering out -in nut-like balls, like the rock of Buckham's Wall Burn in the Cheviot -Hills.<a id="FNanchor_331" href="#Footnote_331" class="fnanchor">[331]</a> Much more frequent is a well-developed amygdaloidal structure, -which indeed may be said to be the most obvious characteristic of these -rocks as a whole. The steam-vesicles, now filled with agate, quartz, calcite -or zeolite, vary in size from mere granules up to large irregular cavities a -foot or more in diameter. Where the kernels are coated with pale-green earth -and lie in a dark brown matrix, they give rise to some of the most beautiful -varieties of rock in any volcanic series in this country, as may be seen on -<span class="pagenum" id="Page_275">- 275 -</span> -the Ayrshire coast at Culzean and Turnberry. Some rocks contain the -vesicles only as rare individuals, others have them so crowded together as -to form the greater part of the cubic contents of the mass. When the -infiltration-products have weathered out, some of the amygdaloids present a -striking resemblance to recent slaggy brown lavas; lumps of them must -have been originally light enough to float in water.</p> - -<div class="footnote"> - -<p><a id="Footnote_331" href="#FNanchor_331" class="label">[331]</a> <i>Ibid.</i> p. 11.</p> - -</div> - -<p>My colleague in the Geological Survey, Mr. J. S. Grant Wilson, some -years ago made for me a large series of determinations of the specific gravity -of the volcanic rocks of the Lower Old Red Sandstone of Scotland. He -found that the andesites collected from various districts to illustrate the -more typical varieties of rock averaged about 2·66. He also made a series -of chemical analyses of a number of the same rocks from the Cheviot Hills, -where they are well preserved. The results are shown in the following -table:—</p> - -<table summary="data"> -<tr> - <td class="bdl bdt bdb"></td> - <td class="bdl bdt bdb tdc">SiO<sub>2</sub></td> - <td class="bdl bdt bdb tdc">Al<sub>2</sub>O<sub>3</sub></td> - <td class="bdl bdt bdb tdc">Fe<sub>2</sub>O<sub>3</sub></td> - <td class="bdl bdt bdb tdc">FeO</td> - <td class="bdl bdt bdb tdc">MnO</td> - <td class="bdl bdt bdb tdc">CaO</td> - <td class="bdl bdt bdb tdc">MgO</td> - <td class="bdl bdt bdb tdc">K<sub>2</sub>O</td> - <td class="bdl bdt bdb tdc">Na<sub>2</sub>O</td> - <td class="bdl bdt bdb tdc">H<sub>2</sub>O</td> - <td class="bdl bdt bdb tdc">H<sub>2</sub>SO<sub>4</sub></td> - <td class="bdl bdt bdb bdr tdc">Loss.</td> -</tr> -<tr> - <td class="bdl tdl">Scawd Law</td> - <td class="bdl tdc">59·29</td> - <td class="bdl tdc">16·30</td> - <td class="bdl tdc">1·77</td> - <td class="bdl tdc">3·70</td> - <td class="bdl tdc">·41</td> - <td class="bdl tdc">4·81</td> - <td class="bdl tdc">3·15</td> - <td class="bdl tdc">4·19</td> - <td class="bdl tdc">3·44</td> - <td class="bdl tdc">3·84</td> - <td class="bdl tdc"> ...</td> - <td class="bdl bdr tdc">...</td> -</tr> -<tr> - <td class="bdl tdl">Rennieston</td> - <td class="bdl tdc">62·81</td> - <td class="bdl tdc">16·40</td> - <td class="bdl tdc">·55</td> - <td class="bdl tdc">3·27</td> - <td class="bdl tdc">·81</td> - <td class="bdl tdc">4·46</td> - <td class="bdl tdc">1·64</td> - <td class="bdl tdc">3·60</td> - <td class="bdl tdc">3·02</td> - <td class="bdl tdc">4·04</td> - <td class="bdl tdc">...</td> - <td class="bdl bdr tdc">...</td> -</tr> -<tr> - <td class="bdl tdl">Cunrieston</td> - <td class="bdl tdc">63·38</td> - <td class="bdl tdc">15·77</td> - <td class="bdl tdc">·73</td> - <td class="bdl tdc">2·65</td> - <td class="bdl tdc">·08</td> - <td class="bdl tdc">4·44</td> - <td class="bdl tdc">1·88</td> - <td class="bdl tdc">1·88</td> - <td class="bdl tdc">4·54</td> - <td class="bdl tdc">4·69</td> - <td class="bdl tdc">...</td> - <td class="bdl bdr tdc">...</td> -</tr> -<tr> - <td class="bdl tdl">Duncan's Dubs</td> - <td class="bdl tdc">59·44</td> - <td class="bdl tdc">16·15</td> - <td class="bdl tdc">1·05</td> - <td class="bdl tdc">2·83</td> - <td class="bdl tdc">·37</td> - <td class="bdl tdc">6·70</td> - <td class="bdl tdc">2·46</td> - <td class="bdl tdc">3·18</td> - <td class="bdl tdc">3·70</td> - <td class="bdl tdc">3·35</td> - <td class="bdl tdc">...</td> - <td class="bdl bdr tdc">...</td> -</tr> -<tr> - <td class="bdl tdl">Whitton Hill</td> - <td class="bdl tdc">60·70</td> - <td class="bdl tdc">17·98</td> - <td class="bdl tdc">·66</td> - <td class="bdl tdc">2·58</td> - <td class="bdl tdc">·20</td> - <td class="bdl tdc">7·07</td> - <td class="bdl tdc">2·20</td> - <td class="bdl tdc">3·57</td> - <td class="bdl tdc">2·95</td> - <td class="bdl tdc">3·45</td> - <td class="bdl tdc">...</td> - <td class="bdl bdr tdc">...</td> -</tr> -<tr> - <td class="bdl tdl">Cuddies' Tops</td> - <td class="bdl tdc">60·58</td> - <td class="bdl tdc">12·25</td> - <td class="bdl tdc">1·01</td> - <td class="bdl tdc">4·13</td> - <td class="bdl tdc">·15</td> - <td class="bdl tdc">4·40</td> - <td class="bdl tdc">2·86</td> - <td class="bdl tdc">2·19</td> - <td class="bdl tdc">3·61</td> - <td class="bdl tdc">...</td> - <td class="bdl tdc">·55</td> - <td class="bdl bdr tdc">2·15</td> -</tr> -<tr> - <td class="bdl tdl">Cocklawfoot</td> - <td class="bdl tdc">62·29</td> - <td class="bdl tdc">17·03</td> - <td class="bdl tdc">·93</td> - <td class="bdl tdc">2·44</td> - <td class="bdl tdc">·21</td> - <td class="bdl tdc">3·92</td> - <td class="bdl tdc">2·71</td> - <td class="bdl tdc">1·14</td> - <td class="bdl tdc">3·20</td> - <td class="bdl tdc">·29<a id="FNanchor_332" href="#Footnote_332" class="fnanchor">[332]</a></td> - <td class="bdl tdc">·37</td> - <td class="bdl bdr tdc">4·81</td> -</tr> -<tr> - <td class="bdl bdb tdl">Morebattle</td> - <td class="bdl bdb tdc">59·82</td> - <td class="bdl bdb tdc">16·96</td> - <td class="bdl bdb tdc">·20</td> - <td class="bdl bdb tdc">6·57</td> - <td class="bdl bdb tdc">·15</td> - <td class="bdl bdb tdc">4·73</td> - <td class="bdl bdb tdc">2·84</td> - <td class="bdl bdb tdc">2·63</td> - <td class="bdl bdb tdc">3·04</td> - <td class="bdl bdb tdc">...</td> - <td class="bdl bdb tdc">trace</td> - <td class="bdl bdb bdr tdc">1·98</td> -</tr> -</table> - -<div class="footnote"> - -<p><a id="Footnote_332" href="#FNanchor_332" class="label">[332]</a> This is CO<sub>2</sub>.</p> - -</div> - -<p>The microscopic structure of the andesites of the Lower Old Red -Sandstone has been partially investigated, especially those of the Cheviot -Hills, by Mr. Teall<a id="FNanchor_333" href="#Footnote_333" class="fnanchor">[333]</a> and by Dr. Petersen,<a id="FNanchor_334" href="#Footnote_334" class="fnanchor">[334]</a> who both give chemical analyses -of the rocks. Much, however, still remains to be done before our knowledge -of this branch of British petrography can be regarded as adequate. The -groundmass in some of the rocks consists mainly of a brown glass with a -streaky structure (as in the well-known variety of Kirk Yetholm, and in -the rock, still more like pitchstone, from near Airthrey Castle in the Ochil -chain); more usually it has been devitrified more or less completely by -the appearance of felspathic microlites, until it presents a confused felspar -aggregate. The porphyritic felspars are often large, generally striped, but -sometimes including crystals that show no striping. They are frequently -found to be full of inclusions of the base, and these sometimes consist of -glass. The ferro-magnesian constituents are usually rather decomposed, -being now represented by chloritic pseudomorphs; but augite, and perhaps -still more frequently enstatite, may be recognized, or its presence may be -<span class="pagenum" id="Page_276">- 276 -</span> -inferred among them. The beautiful resinous or pitchstone-like rock from -near Airthrey Castle has been found by Mr. Watts to be a glassy hypersthene-augite-andesite, -since among its phenocrysts of plagioclase, augite and -hypersthene both occur. Magnetite is commonly traceable, and apatite may -be occasionally detected. As the result of decomposition, calcite, chlorite and -limonite are very generally diffused through the rocks.<a id="FNanchor_335" href="#Footnote_335" class="fnanchor">[335]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_333" href="#FNanchor_333" class="label">[333]</a> <i>Geol. Mag. for 1883</i>, pp. 100, 145, 252.</p> - -<p><a id="Footnote_334" href="#FNanchor_334" class="label">[334]</a> <i>Mikroskopische und chemische Untersuchungen am Enstatit-porphyrit aus den Cheviot Hills</i>, -Inaug. Dissert. Kiel, 1884. Descriptions have also been published of detached rocks from -other districts, such as those by Prof. Judd and Mr. Durham of specimens from the Eastern -Ochils, <i>Quart. Journ. Geol. Soc.</i> vol. xlii. (1886), p. 418.</p> - -<p><a id="Footnote_335" href="#FNanchor_335" class="label">[335]</a> Dr. F. H. Hatch supplied notes on microscopic structure which are incorporated in the text, -together with particulars afterwards furnished by Mr. Watts.</p> - -</div> - -<p>(<i>c</i>) The lavas which may be separated as <span class="smcap">Trachytes</span> offer no distinctive -features externally by which they may be distinguished from the andesites. -Indeed, both groups of rocks appear to be connected by intermediate -varieties. In the Cheviot Hills some of the lavas are found, on microscopic -examination, to contain a large admixture of unstriped porphyritic felspars, -which can occasionally be recognized as sanidine in Carlsbad twins. The -groundmass is sometimes a brown glass, but is usually more or less completely -devitrified, portions of it being inclosed in the large felspars. Chlorite, -pseudomorphic after augite or enstatite, may be detected, and sometimes a -brown mica. A specimen of one of these rocks, from a locality to the north-west -of Whitton, near Jedburgh, was found by Mr. J. S. Grant Wilson to -have the following composition:—</p> - -<table summary="data"> -<tr> - <td class="bdl bdt bdb"></td> - <td class="bdl bdt bdb tdc">SiO<sub>2</sub></td> - <td class="bdl bdt bdb tdc">Al<sub>2</sub>O<sub>3</sub></td> - <td class="bdl bdt bdb tdc">Fe<sub>2</sub>O<sub>3</sub></td> - <td class="bdl bdt bdb tdc">FeO</td> - <td class="bdl bdt bdb tdc">MnO</td> - <td class="bdl bdt bdb tdc">CaO</td> - <td class="bdl bdt bdb tdc">MgO</td> - <td class="bdl bdt bdb tdc">K<sub>2</sub>O</td> - <td class="bdl bdt bdb tdc">Na<sub>2</sub>O</td> - <td class="bdl bdt bdb tdc">H<sub>2</sub>O</td> - <td class="bdl bdt bdb bdr tdc">Total.</td> -</tr> -<tr> - <td class="bdl bdb tdc">N.W. of Whitton Hill,|<br /> - Jedburgh (No. 1938)<br /> - Sp. gr. 2·55.</td> - <td class="bdl bdb tdc">62·44</td> - <td class="bdl bdb tdc">18·99</td> - <td class="bdl bdb tdc">3·35</td> - <td class="bdl bdb tdc">1·8</td> - <td class="bdl bdb tdc">·25</td> - <td class="bdl bdb tdc">1·84</td> - <td class="bdl bdb tdc">1·37</td> - <td class="bdl bdb tdc">5·02</td> - <td class="bdl bdb tdc">2·65</td> - <td class="bdl bdb tdc">2·48</td> - <td class="bdl bdb bdr tdc">100·19</td> -</tr> -</table> - -<p>(<i>d</i>) Acid rocks such as <span class="smcap">Felsites</span> and <span class="smcap">Rhyolites</span> are rare among the -lavas poured out at the surface during the time of the Lower Old Red -Sandstone. They occur in the Pentland Hills, also near Dolphinton in the -Biggar district, and in the Ochil Hills near Auchterarder, associated with -extensive accumulations of felsitic tuffs and breccias. They are usually -so much decomposed that it is hardly possible to procure fresh specimens -of them. Some of them display beautiful flow-structure. They appear to -be generally orthoclase-felsites or orthophyres. Dull, fine-grained to flinty -in texture, they hardly ever display free quartz, so that they can seldom be -placed among the typical rhyolites, though in their banded flow-structure -they often strongly resemble some lithoid varieties of these rocks, especially -such varieties as that represented in <a href="#v1fig9">Fig. 9</a>.</p> - -<p>Mr. Watts, to whom I submitted, for microscopic examination, a -number of specimens from the Pentland and Ochil Hills, has found them to -"consist of a brown felsitic groundmass in which are embedded a generation -of small stumpy prisms of orthoclase and a set of larger phenocrysts, -generally consisting of orthoclase and plagioclase in equal proportions. -Brown mica is usually present and zircons are not uncommon." The rocks, -<span class="pagenum" id="Page_277">- 277 -</span> -when they undergo weathering, pass into the varieties formerly comprised -under the name claystone.</p> - -<p>The only nodular felsite of this age which I have met with is that -of Lough Guitane among the "Dingle Beds," near Killarney, to which -reference will be made in later pages.</p> - -<p>2. <i>Intrusive Bosses, Sills and Dykes.</i>—While the interbedded lava-sheets -are mainly andesites, the intrusive rocks are generally more acid, and most -of them may be grouped under the convenient head of felsites. Some -intrusive andesites, and even more basic rocks, do indeed occur in dykes and -sills, as well as also filling vents. But the rule remains of general application -over the whole country that the materials which have consolidated in -the volcanic orifices of the Old Red Sandstone, or have been thrust among -the rocks in dykes, bosses or sills, are decidedly acid. In this series of -rocks a greater range of types may be traced than among the extrusive -lavas. At the one end we find true granites or granitites, as in the intrusive -bosses of Spango Water and of Galloway, which, for reasons which -I will afterwards adduce, may with some probability be assigned to the -volcanic history of the Lower Old Red Sandstone period. Among the -bosses, many of which probably mark the positions of eruptive vents, orthophyres -are especially prominent. These rocks frequently contain no -mica, but, on the other hand, they sometimes show abundant quartz in -their groundmass. The augite-granitite of the Cheviot Hills, so well -described by Mr. Teall, has invaded the bedded andesites of that region.<a id="FNanchor_336" href="#Footnote_336" class="fnanchor">[336]</a> -A similar rock has been noticed by my brother, Prof. James Geikie, -associated with the Lower Old Red Sandstone volcanic rocks of the east of -Ayrshire. A remarkable petrographical variety has been mapped by Mr. -B. N. Peach, rising as a small boss through the lower part of the great -lava-sheets of the Ochil Hills, above Tillicoultry. It is a granophyric -quartz-diorite, which, under the microscope, is seen to be composed of short, -thick-set prisms of plagioclase, with abundant granophyric quartz, a -pleochroic hypersthene, and needles of apatite. Sometimes the pyroxene is -replaced by green chloritic pseudomorphs.<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>Geol. Mag.</i> for 1883, pp. 100, 145, 252; and <i>British Petrography</i>, pp. 272, 278.</p> - -<p><a id="Footnote_337" href="#FNanchor_337" class="label">[337]</a> Notes by Dr. Hatch.</p> - -</div> - -<p>At the other end of the series come the felsites, quartz-porphyries, mica-porphyrites, -minettes, vogesites, "hornstones" and "claystones" (or decayed -felsites), which have a close-grained texture, often with porphyritic felspars, -quartz or black mica, generally a whitish, pale buff, orange, pink or purplish-grey -colour, and a specific gravity of about 2·55.<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> The intrusive "porphyry" of Lintrathen in Forfarshire (which may be younger than -the Old Red Sandstone) is a bright red rock with porphyritic felspar, quartz, white mica and a -very singular black mica (Mr. Teall's <i>British Petrography</i>, p. 286).</p> - -</div> - -<p>Though I class these rocks as intrusive, I am not prepared to assert -that in none of the instances where they occur as sheets may they possibly -have been erupted at the surface as lavas. In one or two cases the evidence -either way is doubtful, but as the great majority of the acid rocks can be -<span class="pagenum" id="Page_278">- 278 -</span> -shown to be intrusive in their behaviour, I have preferred to keep them all -in the same category. I am prepared to find, however, that, as so vast an -amount of felsitic debris was ejected to form the tuffs, more of this material -may have flowed out in streams of lava than is at present recognized.</p> - -<p>The following table shows the chemical composition of some acid sills -and dykes from the Lower Old Red Sandstone, as determined in the -laboratory of Prof. E. Frankland: -<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> Two analyses of rhyolites from Shetland by Mr. Tatlock will be found in <i>Trans. Roy. Soc. -Edin.</i> vol. xxxii. (1887), p. 387. Their silica percentage is 72·32 and 73·70. An analysis of a -quartz-felsite from the Cheviot Hills by Mr. T. Waller is given in the Geological Survey -Memoir on the Cheviot Hills, p. 25. The proportion of silica in this rock is 67·9.</p> - -</div> - -<table summary="data"> -<tr> - <td class="bdl bdt bdb"></td> - <td class="bdl bdt bdb tdc">SiO<sub>2</sub></td> - <td class="bdl bdt bdb tdc">Al<sub>2</sub>O<sub>3</sub></td> - <td class="bdl bdt bdb tdc">Fe<sub>2</sub>O<sub>3</sub></td> - <td class="bdl bdt bdb tdc">MnO</td> - <td class="bdl bdt bdb tdc">CaO</td> - <td class="bdl bdt bdb tdc">MgO</td> - <td class="bdl bdt bdb tdc">K<sub>2</sub>O</td> - <td class="bdl bdt bdb tdc">Na<sub>2</sub>O</td> - <td class="bdl bdt bdb tdc">P<sub>2</sub>O<sub>3</sub></td> - <td class="bdl bdr bdt bdb tdc">H<sub>2</sub>O</td> -</tr> -<tr> - <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"> </td> - <td class="bdl"> </td> - <td class="bdl bdr"> </td> -</tr> -<tr> - <td class="bdl bdl tdl">"Hornstone."<br /> Torgeith Knowe,<br /> Pentlands</td> - <td class="bdl tdr">73·91</td> - <td class="bdl tdr">14·41</td> - <td class="bdl tdr">·76</td> - <td class="bdl tdr">·07</td> - <td class="bdl tdr">1·21</td> - <td class="bdl tdr">4·90</td> - <td class="bdl tdr">3·36</td> - <td class="bdl tdr">1·57</td> - <td class="bdl tdr">...</td> - <td class="bdl bdr tdr">·90</td> -</tr> -<tr> - <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"> </td> - <td class="bdl"> </td> - <td class="bdl bdr"> </td> -</tr> -<tr> - <td class="bdl tdl">"Hornstone."<br /> Braid Hills<a id="FNanchor_340" href="#Footnote_340" class="fnanchor">[340]</a></td> - <td class="bdl tdr">64·73</td> - <td class="bdl tdr">17·01</td> - <td class="bdl tdr">2·35</td> - <td class="bdl tdr">·24</td> - <td class="bdl tdr">4·19</td> - <td class="bdl tdr">·66</td> - <td class="bdl tdr">3·27</td> - <td class="bdl tdr">3·75</td> - <td class="bdl tdr">·26</td> - <td class="bdl bdr tdr">2·78</td> -</tr> -<tr> - <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"> </td> - <td class="bdl"> </td> - <td class="bdl bdr"> </td> -</tr> -<tr> - <td class="bdl tdl">Tinto, Lanarkshire:</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"> </td> - <td class="bdl bdr"> </td> -</tr> -<tr> - <td class="bdl tdl"> Soluble in hydro-<br /> chloric acid</td> - <td class="bdl tdr">·04</td> - <td class="bdl tdr">1·01</td> - <td class="bdl tdr">1·24</td> - <td class="bdl tdc">...</td> - <td class="bdl tdr">·92</td> - <td class="bdl tdr">·52</td> - <td class="bdl tdc">...</td> - <td class="bdl tdc">...</td> - <td class="bdl tdr">·16</td> - <td class="bdl bdr tdc">...</td> -</tr> -<tr> - <td class="bdl bdb tdl"> Insoluble in ditto.</td> - <td class="bdl bdb tdr">70·28</td> - <td class="bdl bdb tdr">12·54</td> - <td class="bdl bdb tdr">·43</td> - <td class="bdl bdb tdc">...</td> - <td class="bdl bdb tdr">·91</td> - <td class="bdl bdb tdc">...</td> - <td class="bdl bdb tdr">3·92</td> - <td class="bdl bdb tdr">5·84</td> - <td class="bdl bdb tdc">...</td> - <td class="bdl bdb bdr tdr">1·99</td> -</tr> -</table> - -<div class="footnote"> -<p><a id="Footnote_340" href="#FNanchor_340" class="label">[340]</a> This specimen also yielded 0·13 of ferrous oxide, and 2·42 of carbon dioxide.</p> - -</div> - -<p>The rock of Tinto, which may be considered typical of the prevailing -acid intrusive rocks of the series, presents several slightly different -varieties. Dr. Hatch, as the result of his examination of a number of -microscopic slides prepared from specimens taken by me from various parts -of the hill, found some to be minettes, showing small isolated crystals of -orthoclase and rare flakes of biotite, sometimes granules of quartz, imbedded -in a brown, finely microlitic groundmass of felspar powdered over with -calcite; while other specimens had a granular instead of a microlitic -groundmass, and contained a considerable amount of quartz in addition to -the constituents just mentioned. A conspicuous knob on the south side of -Tinto, called the Pap Craig, is a mass of augite-diorite, which has risen -through the other rocks<a id="FNanchor_341" href="#Footnote_341" class="fnanchor">[341]</a> (see <a href="#v1fig93">Fig. 93</a>). The sills in the same region show -still further differences. Some are true "felspar-porphyries," and "quartz-porphyries" -varying in the relative abundance and size of their porphyritic -orthoclase and quartz, while others, by the introduction of hornblende or -pseudomorphs after that mineral, pass into vogesites.</p> - -<div class="footnote"> - -<p><a id="Footnote_341" href="#FNanchor_341" class="label">[341]</a> This rock differs considerably from the other intrusive masses in its neighbourhood. Dr. -Hatch found it to be composed chiefly of lath-shaped striped felspar, with some granular augite, -magnetite and interstitial quartz.</p> - -</div> - -<p>Basic sills and bosses are chiefly developed among the Ochil and Sidlaw -Hills. They may generally be classed as diabases. But sometimes their -pyroxenic constituent is partly hypersthene, as in a coarsely crystalline boss -about a mile south of Dunning, which has been determined by Mr. Watts -<span class="pagenum" id="Page_279">- 279 -</span> -to "consist of augite and hypersthene imbedded in and occurring amongst -large plagioclase prisms. Some iron-ore is also present; the rock is a -hyperite."</p> - -<p>3. <i>Tuffs and Agglomerates.</i>—The fragmental materials, ejected from or -filling up the vents, vary from the finest compacted dust up to some of the -coarsest agglomerates in this country. In general they consist mainly of -detritus of andesite, and have been derived from the blowing up of already -consolidated masses of that rock. The fragments are usually angular, and -range from minute grains up to blocks as large as a cottage. The tuffs are -often more or less mixed with ordinary non-volcanic sediment, and as they -are traced away from the centres of eruption they pass insensibly into sandstones -and conglomerates.</p> - -<p>But while, as might be expected, the tuffs are most commonly made up -of debris of the same kind of lavas as those that usually form the sheets -which were poured out at the surface, they include also bands of material -derived from the destruction of much more acid rocks. Throughout the -chain of the Ochil Hills, for example, in the midst of the bedded andesite-lavas, -many of the thin courses of fine tuff consist largely of felsitic -fragments, with scattered felspar crystals. The most remarkable examples -of this nature, however, are to be met with at the great vent of the Braid -Hills, in the chain of the Pentland Hills which runs south-westward from -it, and in the Biggar volcanic district still further south. These acid tuffs -are generally pale flesh-coloured or lilac in tint, and compact in texture, but, -like the felsitic lavas from which they were derived, they are apt to weather -into yellow or buff "claystones." The finer varieties are so compact as to -present to the naked eye no distinguishable grains; they might be mistaken -for felsites, and indeed, except where they contain recognizable fragments of -rock or broken crystals of felspar, can hardly be discriminated from them. -They consist of an exceedingly fine compacted felsitic dust. Here and -there, however, the scattered crystals of felspar and small angular fragments -of felsite, which may be detected in them, increase in number until -they form the whole of the rock, which is then a brecciated tuff or fine -volcanic breccia, made up of different felsites, among which, even with the -naked eye, delicate flow-structures may be detected. In these pale acid -tuffs, fragments of different andesites may often be observed, which increase -in number as the rocks are traced away from the main vents of eruption.</p> - -<p>At my request my colleague, Mr. George Barrow, determined the silica -percentages in a few specimens which I selected as showing some of the -more characteristic varieties of these tuffs from the Braid and Pentland -Hills. His results are exhibited in the following table:—</p> - -<table summary="data"> -<tr> - <td></td> - <td class="smaller">Silica percentage.</td> -</tr> -<tr> - <td class="tdl">1. Quarry above Woodhouselee</td> - <td class="tdc">63·3 </td> -</tr> -<tr> - <td class="tdl">2. South-west side of Castlelaw Hill</td> - <td class="tdc">73·15</td> -</tr> -<tr> - <td class="tdl">3. Quarry on road, ½ mile N.E. of Swanston (Braid Hill vent)</td> - <td class="tdc">74·1 </td> -</tr> -<tr> - <td class="tdl">4. South-west side of Castlelaw Hill</td> - <td class="tdc">75·0 </td> -</tr> -<tr> - <td class="tdl">5. Castlelaw Hill</td> - <td class="tdc">76·00</td> -</tr> -<tr> - <td class="tdl">6. South side of White Hill Plantation</td> - <td class="tdc">90·00</td> -</tr> -</table> - -<p><span class="pagenum" id="Page_280">- 280 -</span></p> - -<p>From these analyses it may be inferred that the average amount of -silica in the more typical varieties is between 70 and 75 per cent. The -last specimen in the table, with its abnormally high percentage of acid, -must be regarded as an exceptional variety, where there has either been an -excessive removal of some of the bases, or where silica has been added by -infiltration.</p> - -<p>The microscopic examination of these rocks has not added much to the -information derivable from a study of them in the field. In their most -close-grained varieties, as above remarked, they are hardly to be distinguished -from felsites. But they generally show traces of the minute -detrital particles of felsite of which they are essentially composed. The -brecciated varieties exhibit finely-streaked flow-structure in some of the -fragments. Pieces of andesite, grains of quartz, and other extraneous -ingredients appear in these rocks towards the southern limits of the -volcanic area of the Pentland Hills, where the acid tuffs are associated -with and pass laterally and vertically into ordinary non-volcanic sedimentary -strata. Further details as to the part which these tuffs play in -the volcanic history of the regions wherein they occur will be given in -later pages.</p> - -<hr class="chap x-ebookmaker-drop" /> - - -<div class="chapter"> -<p><span class="pagenum" id="Page_281">- 281 -</span></p> - -<h2 class="nobreak" id="CHAPTER_XVIII">CHAPTER XVIII<br /> - -<span class="smaller">STRUCTURE AND ARRANGEMENT OF THE LOWER OLD RED SANDSTONE -VOLCANIC ROCKS IN THE FIELD</span></h2> -</div> - - -<p>We have now to consider the manner in which the various volcanic -products have been grouped around and within the orifices of discharge. -The first feature to arrest the eye of a trained geologist who approaches -them as they are displayed in one of the ranges of hills in Central Scotland -is the bedded aspect of the rocks. If, for example, he looks eastward from -the head of the Firth of Tay, he marks on the right hand, running for -many miles through the county of Fife, a succession of parallel escarpments, -of which the steep fronts face northwards, while their long dip-slopes -descend towards the south. On his left hand a similar but higher series -of escarpments, stretching far eastwards into Forfarshire, through the chain -of the Sidlaw Hills, repeats the same features, but in opposite directions. -If he stands on the alluvial plain of the Forth, near Stirling, and looks -towards the north, he can trace bar after bar of brown rock and grassy -slope rising from base to summit of the western end of the Ochil Hills. -If, again, from any height on the southern outskirts of the city of Edinburgh, -he lets his eye range along the north-western front of the chain of the -Pentland Hills, especially towards evening, he can follow the same parallel -banding as a conspicuous feature on each successive hill that mounts above -the plain. Or if, as he traverses the west of Argyllshire, he comes in -sight of the uplands of Lorne, he at once recognizes the terraced contours of -the hills between Loch Awe and the western sea, presenting so strange a -contrast to the rugged and irregular outlines of the more ancient schist and -granite mountains all around (see <a href="#v1fig99">Fig. 99</a>).</p> - - -<h3>i. <span class="allsmcap">BEDDED LAVAS AND TUFFS</span></h3> - -<p>On a nearer inspection, the dominant topographical features are found -to correspond with a well-marked stratification of the whole volcanic series. -Where two sheets of andesite are separated by layers of tuff, sandstone or -conglomerate, a well-marked hollow will often be found to indicate the -junction-line; but even where the lavas follow each other without such -<span class="pagenum" id="Page_282">- 282 -</span> -interstratifications, their differences of texture and consequent variations in -mode and amount of weathering usually suffice to mark them off from each -other, and to indicate their trend along the surface in successive terraces. -Even where the angles of inclination are high, the bedded arrangement can -generally be detected.</p> - -<p>It is in the picturesque and instructive coast-sections, however, that the -details of this bedded structure are most clearly displayed. On both sides -of the country, along the shores of Ayrshire on the west, and those -of Kincardineshire and Forfarshire on the east, the volcanic group has -been admirably dissected by the waves. The lava-beds have been cut in -vertical section, so that their structure and their mode of superposition, one -over another, can be conveniently studied, while at the same time, the upper -surfaces of many of the flows have been once more laid bare as they existed -before they were buried under the sedimentary accumulations of the waters -in which they were erupted.</p> - -<p>Though distinctly bedded, the Lavas show little of the regularity and -persistence so characteristic of those of Carboniferous and of Tertiary time. -Some of them are not more than from four to ten feet thick, and generally, -on the coast-cliffs, they appear to be less than fifty feet. A continuous -group of sheets can sometimes be traced for ten miles or more from the -probable vent of discharge.</p> - -<p>That many of these lavas were erupted in a markedly pasty condition -may be inferred from certain of their more prominent characteristics. -Sometimes, indeed, they appear as tolerably dense homogeneous masses, -breaking with a kind of prismatic jointing; but more frequently they are -strongly amygdaloidal, and sometimes so much so that, as already stated, -the amygdales form the larger proportion of their bulk. Where the secondary -infiltration-products have weathered out, the rough scoriform rock looks as -if it might only recently have been erupted. In a few instances I have -observed an undulating rope-like surface, which reminded me of well-known -Vesuvian lavas. Usually the top and bottom of each sheet assume a -strikingly slaggy aspect, which here and there is exaggerated to such an -extent that between the more solid and homogeneous parts of two consecutive -flows an intermediate band occurs, ten or twelve feet thick, made up of -clinker-like lumps of slag, the interspaces being filled in with hardened -sand. In some cases these agglomeratic layers may actually consist in part -of ejected blocks; but the way in which many of the lavas have cooled in -rugged scoriaceous surfaces is as conspicuous as on any modern <i>coulée</i>. The -loosened slags, or the broken-up cakes and blocks of lava, have sometimes -been caught up in the still moving, pasty current, which has congealed -with its vesicles drawn out round the enclosed fragments, giving rise to a -mass that might be taken for a breccia or agglomerate. Now and then we -may observe that the upper slaggy portion of a sheet has assumed a bright -red colour from the oxidation of its ferruginous minerals; and from the -contrast it thus presents to the rest of the rock we may perhaps legitimately -infer that the disintegration took place before the outflow of the next -<span class="pagenum" id="Page_283">- 283 -</span> -succeeding lava. If this inference be well founded, and it is confirmed by -other evidence which will be subsequently adduced, it points to the probable -lapse of considerable intervals -of time between some of the -outflows of lava.</p> - -<div class="figright" id="v1fig65" style="width: 290px;"> - <img src="images/v1fig65.png" width="290" height="262" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 65.</span>—Veins and nests of sandstone due to the washing - of sand into fissures and cavities of an Old Red Sandstone - lava. Turnberry Point, Ayrshire.</div> -</div> - -<p>But perhaps the most -singular structure displayed -by these lavas is to be seen -in the manner in which they -are traversed by and enclose -portions of sandstone. Since I -originally observed this feature -on the Ayrshire coast, near -Turnberry Point, many years -ago,<a id="FNanchor_342" href="#Footnote_342" class="fnanchor">[342]</a> I have repeatedly met -with it in the various volcanic -districts of the Lower -Old Red Sandstone across the -whole of the Midland Valley -of Scotland. The first and -natural inference which a -cursory examination of it suggests is that the molten rock has caught -up and carried along pieces of already consolidated sandstone. But a -little further observation will show that the lines of stratification in the -sandstone, even in what appear to be detached fragments, are marked by -a general parallelism, and lie in the same general plane with the surface -of the bed of lava in which the sandy material is enclosed. In a vertical -section the sandstone is seen to occur sometimes in narrow dykes with -even, parallel walls, but more usually in irregular twisting and branching -veins, or even in lumps which, though probably once connected with -some of these veins, now appear as if entirely detached from them -(<a href="#v1fig65">Fig. 65</a>). Frequently, indeed, the nodular slaggy andesite and the -sandstone are so mixed up that the observer may hesitate whether -to describe the mass as a sandstone enclosing balls and blocks of -lava, or as a scoriaceous lava permeated with hardened sand. A -close connection may be traced between these sandstone-inclosures and -the beds of sandstone interstratified between the successive lavas. We -can follow the sandy material downwards from these intercalated beds into -the andesites below them. On exposed upper surfaces of the lava, an -intricate reticulation of sandstone veins may be noticed, in each of which -the stratification of the material runs across the veins, showing sometimes -distinct current-bedding, but maintaining a general parallelism with the -bedding of the volcanic sheets and their fragmentary accompaniments (<a href="#v1fig66">Fig. 66</a>). -If we could remove the sandstone-veinings and aggregates, we should -find the upper surfaces of these igneous masses to present a singularly fissured -<span class="pagenum" id="Page_284">- 284 -</span> -and slaggy appearance, reminding us of the rugged, rent and clinker-loaded -slopes of a modern viscous lava, like some of those in the Atrio del -Cavallo on Vesuvius. -There cannot, therefore, -be any doubt -that the sandstone, -so irregularly dispersed -through these -lavas, was introduced -originally as loose -sand washed in from -above so as to fill -the numerous rents -and cavernous interspaces -of the volcanic -rock. A more striking -proof of the subaqueous -character of -the eruptions could -hardly be conceived. -This interesting feature -in lavas erupted -under water is not confined to the volcanic series of the Old Red Sandstone. -We shall find that it is hardly less distinct among the basic lavas of the -Permian series both in Scotland and in Devonshire.</p> - -<div class="footnote"> - -<p><a id="Footnote_342" href="#FNanchor_342" class="label">[342]</a> See Jukes' <i>Manual of Geology</i>, 3rd edit. (1872), Fig. 111, p. 276.</p> - -</div> - -<div class="figleft" id="v1fig66" style="width: 333px;"> - <img src="images/v1fig66.png" width="333" height="278" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 66.</span>—Ground-plan of reticulated cracks in the upper surface of an Old - Red Sandstone lava filled in with sandstone. Red Head, Forfarshire.</div> -</div> - -<p>A remarkable exception to the general type of dark basic and intermediate -lavas is furnished by the pale, decomposing felsites of the Pentland -and Dolphinton Hills. Those which issue from the great eruptive -centre of the Braid Hills, alternate with the andesites and the diabases, -gradually diminishing like these in a southward direction and dying out -in some six or seven miles. Beyond the limits of these lavas, another -similar thick group was erupted from a separate vent at the northern end -of the Biggar district near Dolphinton. The same occurrence has been -ascertained also in the area of the Ochil chain. Fuller reference will be -made to these interesting rocks in the descriptions to be afterwards given -of the structure and history of the volcanic areas of the Pentland Hills, the -Biggar centre and the Ochil Hills.</p> - -<p>It is certainly a notable feature in the volcanism of Old Red Sandstone -time that from the same, or from closely adjoining vents, lavas should be -alternately poured forth, differing so much from each other, alike in chemical -composition and petrographical characters, as andesites and diabases on the -one hand, and felsites on the other. Additional examples, from widely -different geological systems, will be cited in subsequent pages. It will be -shown that even in the very latest volcanic period in Britain, that of -older Tertiary time, highly basic and markedly acid materials were ejected -from the same centres of eruption.</p> - -<p><span class="pagenum" id="Page_285">- 285 -</span></p> - -<p>The part taken by the Tuffs in the structure of the ground agrees with -what might have been expected in the accompaniments of extremely slaggy -and viscid lavas. These pyroclastic intercalations are, in most of the -volcanic districts, comparatively insignificant in amount, by far the largest -proportion of solid material ejected from the various vents having consisted -of streams of lava. Round or within some of the vents the fragmentary -materials attain a remarkable coarseness, as may be seen in the great -agglomerates of Dumyat, near Stirling, the largest of which is more than -700 feet thick. These massive accumulations doubtless represent a long -series of explosive discharges from the summit of the lava column in one or -more adjacent vents. Traced away from the orifices of emission, the tuffs -rapidly grow finer in grain, less in thickness, and more mixed with ordinary -detritus, until they pass into ordinary non-volcanic sediment or die out -between the lava-sheets.</p> - -<p>Good sections, showing the nature and arrangement of the thin intercalations -of andesite-tuff between the successive outpourings of lava, may be -examined on the coast. Thus, near Turnberry Point, in Ayrshire, upwards -of a dozen successive flows of lava, with their sandy and ashy intervening -layers, are exposed in plan upon the beach, and partly also in section along -the cliffs on which the ruins of the historic castle of Turnberry stand. -(Figs. <a href="#v1fig95">95</a>, <a href="#v1fig96">96</a>, <a href="#v1fig97">97</a>). Again, along the coast of Forfarshire, from the Red -Head to Montrose, the numerous sheets of andesite are separated by layers -of dull purplish tuff passing into conglomerate, with blocks of porphyrite a -yard or more in diameter.</p> - -<p>The most remarkable interstratified tuffs in the Lower Old Red Sandstone -are the felsitic varieties. Those which proceed from the great vent of -the Braid Hills, extend south-westwards for eight or nine miles, and their -peculiar materials, mixed with ordinary sediment, may be traced several miles -further. They occur in successive sheets, which, from a maximum thickness -and number at the north end, gradually thin away southwards, like -the felsitic lavas which they accompany, and from the explosion of which -they no doubt were derived. They consist to a large extent of extremely fine -volcanic dust, and since they are generally much decomposed, it is often, as -already remarked, hardly possible to distinguish between them and the -equally decayed felsites. In some parts of the hills they present a distinct -fissile bedding; but still more satisfactory is the occasional fine brecciated -structure which they assume, when they are seen to consist of angular -lapilli of different felsites.</p> - -<p>The amount of volcanic material ejected from the more important vents -was much greater than the height of the present hills would lead us to -suppose. The rocks have generally been tilted into positions much more -inclined than those which they originally occupied, so that to measure their -actual thickness we must take a line approximately perpendicular to the -dip. In this way we ascertain that the accumulated mass of lavas and -tuffs immediately outside the vent at the north end of the Pentland Hills -must be at least 7000 feet thick, for the base of the series is concealed -<span class="pagenum" id="Page_286">- 286 -</span> -under the unconformable overlap of the Lower Carboniferous Sandstones, while -the top is cut off by a fault which brings down the -Carboniferous formations against the eastern flank of -the hills. Probably not less voluminous is the pile of -ejected material in the Ochil Hills, where, though the -base of the whole is concealed by the fault which throws -down the coal-field, some 6500 feet of lavas, tuffs and -conglomerates can be seen. There were thus, during -the time of the Lower Old Red Sandstone, more than -one volcano in Central Scotland which might be compared -in bulk of ejected material to Vesuvius.</p> - -<div class="figcenter" id="v1fig67" style="width: 715px;"> - <img src="images/v1fig67.png" width="715" height="93" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 67.</span>—Section across the volcanic series of Forfarshire. - <i>a</i>, conglomerates, sandstones and flagstones; <i>b</i>, sheets of andesitic lava.</div> -</div> - -<p>That the eruptions were mainly subaqueous is indicated, -as I have shown, by the intercalated bands of -sandstone and conglomerate between the successive lavas, -as these are traced away from the centres of discharge, -and likewise, even more impressively, by the hardened -sand which has been washed into former fissures and -crevices in the lava. But that, in some cases, the volcanic -cones were built up above the surface of the lake -may be legitimately inferred from the remarkable -volcanic conglomerates which occur, more particularly -in the great chain of the Ochil and Sidlaw Hills. -These thick accumulations of well-rounded and water-worn -blocks are interspersed between sheets of andesite, -and are mainly made up of andesite fragments. Impressive -sections of them may be seen along the Kincardineshire -coast. The conglomerates are sometimes -so remarkably coarse, many of their blocks exceeding -two feet in diameter, and so rudely bedded, that it is -only by noting the position of oblong boulders that one -can make out the general direction of the stratification. -In their smooth rounded forms, these blocks resemble -the materials of storm-beaches on an exposed coast. -The trituration of the andesite fragments has given rise -to a certain amount of green paste, which firmly wraps -round the stones, and retains casts of them after they -have dropped out. It is further deserving of remark -that while in some districts, as in the central Ochils, the -materials were entirely derived from the destruction -of volcanic rocks, in others a large proportion of non-volcanic -materials is mingled with the debris of the -lavas. South of Stonehaven, for example, large boulders -of quartzite form a conspicuous feature in the conglomerates, -of which in places they make up quite half -of the total constituents. There can be little doubt, I -think, that the materials of these coarse detrital accumulations were gathered -<span class="pagenum" id="Page_287">- 287 -</span> -together as shingle-beaches, and were derived in part from volcanic cones -which had risen above the level of the lake. They seem to suggest considerable -degradation of these cones by breaker-action, whereby blocks of -rock a yard or more in diameter could be rounded and smoothed.</p> - -<p>Another inference deducible from such conglomerates, and to which I -have already alluded, is that considerable intervals of time took place -between some of the eruptions. Round the vents, indeed, where the -successive sheets of volcanic material follow each other continuously, it is -perhaps impossible to form any definite opinion as to the relative chronological -value of the lines of separation between different ejections. But -where some hundreds of feet of coarse conglomerate, chiefly composed of -well-rounded andesite blocks, intervene between two streams of lava, we -may conclude that the interval between the outpouring of these streams -must have been of considerable duration. Other evidence of a similar -tendency may be recognized in the intercalation of groups of varied sedimentary -accumulations, such as those which were deposited over the site of -Eastern Forfarshire and Kincardineshire during the time that elapsed -between two successive floods of lava. In the Den of Canterland, for -example, in the midst of the volcanic sheets we find interesting evidence of -one of these intervals of quiescence, during which layers of fine olive shales -were laid quietly down, while macerated vegetation, drifting over the lake-bottom, -was buried with remains of fishes, and abundant gally-worms -(<i>Kampecaris</i>, <i>Archidesmus</i>), washed from the neighbouring land.<a id="FNanchor_343" href="#Footnote_343" class="fnanchor">[343]</a> So undisturbed -were the conditions of deposition that calcareous sediment gathered -round some of the organisms and encased them in limestone nodules.</p> - -<div class="footnote"> - -<p><a id="Footnote_343" href="#FNanchor_343" class="label">[343]</a> An abundant organism in some of these deposits, named <i>Parka</i>, was first regarded as a plant, -was afterwards believed to be the egg-packets of crustacea, and is now pronounced by competent -authorities to belong to an aquatic plant with creeping stems, linear leaves and sessile sporocarps.</p> - -</div> - -<p>In some of the districts the discharges of volcanic material were so -abundant or so continuous that no recognizable deposition of ordinary -sediment has taken place between them. Thus, at the north end of the -Pentland Hills the rocks are entirely of volcanic origin, and though, as we -trace them southwards away from the centre of eruption, they diminish in -thickness, they include hardly any interstratified sandstones and conglomerates -until they finally begin to die out.</p> - -<p>The distances to which the lavas and tuffs have been erupted from the -chief vents of a district vary up to 15 or 20 miles. Those of the Pentland -Hills extend from the Braid Hill vent for 10 miles to the south-west. -Those of the Biggar centre stretch for about 16 miles to the north-east. -Those of the Ochil Hills, which probably came from a number of distinct -vents, can be traced for nearly 50 miles.</p> - - -<h3>ii. <span class="allsmcap">VENTS</span></h3> - -<p>On the whole the actual vents of the volcanoes of Lower Old Red Sandstone -time are less clearly distinguishable than those of subsequent volcanic -<span class="pagenum" id="Page_288">- 288 -</span> -periods. This deficiency doubtless arises from the geological structure of -the districts in which the formation is chiefly developed. Thus, in the -great Midland Valley of Scotland, where the Old Red Sandstone covers a -large part of the surface, the vents seem to have been placed along the -central parts of the long trough rather than among the older rocks on either -margin. Hence they are in large measure buried either under the volcanic -and sedimentary accumulations of their own period or under Carboniferous -strata.</p> - -<div class="figcenter" id="v1fig68" style="width: 467px;"> - <img src="images/v1fig68.png" width="467" height="145" alt="" /> - <div class="tdc"><span class="smcap">Fig. 68.</span>—Section across two necks above Tillicoultry, Ochil Hills.<br /> - 1 1, Andesite lavas; 2 2, Tuffs and volcanic conglomerates; 3 3, The two necks; 4 4, Dykes of felsite, etc.; - 5, Coal-measures; <i>f</i>, Fault.</div> -</div> - -<p>Certain bosses of massive rocks lying well within the volcanic area -may with some confidence be regarded as the sites of eruptive centres. -They occur either singly or in groups, and may be specially noticed along -the chain of the Ochil and Sidlaw Hills. Yet it seems to me probable -that these visible bosses, even if we are correct in regarding them as -marking the positions of true vents, do not indicate the chief orifices of -discharge. If we consider their size and their distribution with reference -to the areas of lava and tuff discharged at the surface, we are rather led to -look upon them as subsidiary vents, the more important orifices, from which -the main bulk of the eruptions took place, being still concealed under the -Carboniferous rocks of the Midland Valley. The bosses which rise through -different portions of the volcanic series are obviously not the oldest or original -vents. The great felsitic mass of Tinto in Lanarkshire (<a href="#v1fig93">Fig. 93</a>), indeed, -pierces strata which lie near the base of the Lower Old Red Sandstone, but -the smaller cone of Quothquan in its neighbourhood appears in the midst of -the lavas (<a href="#v1fig92">Fig. 92</a>). In the south-western part of the Ochil chain the bosses -or necks are chiefly small in size, seldom exceeding half a mile in diameter. -They have been filled sometimes with crystalline, sometimes with fragmental -materials. Two of them, containing the remarkable granophyric quartz-diorite -already referred to, emerge from among the tuffs in a low part of -the series, immediately above the village of Tillicoultry in Clackmannan -(<a href="#v1fig68">Fig. 68</a>). Two or three more, which are occupied by orthophyres and -quartz-felsites, pierce the volcanic group a few miles to the west of Loch -Leven. The whole of the visible bosses of the Ochil Hills may be regarded -as one connected group, subsidiary to the main orifices which lay rather -<span class="pagenum" id="Page_289">- 289 -</span> -further to the south and west. More particular reference to this district -will be made in the following chapter (p. 303).</p> - -<p>Vents which have been filled up with agglomerate, and which thus -furnish the most obvious proofs of their connection with the eruptions of -the volcanic series, though not frequent, may be observed in a number of -the volcanic districts. Their fragmentary materials generally consist mainly -of the detritus of andesites or diabases like those which form the bedded -lavas. But where more acid lavas have risen to the surface, fragments of -felsite may occur more or less abundantly. In the great vent of the Braid -Hills the tuffs and breccias are almost wholly acid. Non-volcanic materials -may often be found in the agglomerates, and occasionally even to the exclusion -of volcanic detritus. Thus, in the far north of Scotland several -examples occur among the Shetland Isles of necks filled entirely with -blocks of the surrounding flagstones and sandstones. Such cases, as has -been already pointed out, probably represent incompleted volcanoes, when -the explosive vapours were powerful enough to drill orifices in the crust of -the earth and eject the shattered debris from them, but were not sufficiently -vigorous or lasting to bring up any solid or liquid volcanic material to the -surface. These Shetland examples are further noticed on p. 345.</p> - -<p>Necks of agglomerate in the Lower Old Red Sandstone vary in size from -a great orifice measuring two miles across to little plugs only a few yards in -diameter. They may be found in limited numbers in most of the volcanic -districts. No examples have been observed rising through older rocks than -the Old Red Sandstone, all the known instances being eruptive through -some part of the volcanic series or of the sandstones, and therefore not -belonging to the earliest eruptions.</p> - -<p>The largest, and in some respects the most interesting, vent in the -Lower Old Red Sandstone, that of the Braid Hills near Edinburgh, described -in <a href="#CHAPTER_XX">Chapter xx.</a>, covers an area of more than two square miles, and is filled -with felsitic breccias and tuffs, through which bosses and veins of acid and -basic rocks have been injected. It completely truncates the bedded lavas -and tuffs of the Pentland Hills, and not improbably marks the chief centre -from which these rocks were erupted. Several smaller necks rise a little -beyond its southern margin, marking, perhaps, lateral cones on the main -volcano.</p> - -<p>In the small area of Lower Old Red Sandstone lying between Campbeltown -and the Mull of Cantyre, several necks of agglomerate occur, which -have been partly dissected by the waves along the shore, thus revealing their -internal structure and their relation to the surrounding conglomerates. An -account of them will be found at p. 311. One of the series, which lies back -from the coast-line, forms a prominent rounded hill measuring about 400 -yards in its longest diameter. Its general contour is represented in -<a href="#v1fig82">Fig. 82</a>.</p> - -<p>Of the eruptive bosses of massive rock outside the limits of the Old Red -Sandstone which may be plausibly referred to the volcanic phenomena of the -period, though they cannot be proved to be actually part of them, the most -<span class="pagenum" id="Page_290">- 290 -</span> -notable are the bosses of granite and other acid material which rise through -the Silurian strata of the Southern Uplands of Scotland.<a id="FNanchor_344" href="#Footnote_344" class="fnanchor">[344]</a> The largest are the -well-known masses of Galloway (<a href="#v1fig69">Fig. 69</a>), with which must be grouped the -bosses near New Cumnock, that of the Spango Water (<a href="#v1fig94">Fig. 94</a>), and those of -Cockburn Law and Priestlaw in Lammermuir, together with a number of -masses of felsitic material scattered over the same region, such as the Dirrington -Laws of Berwickshire (<a href="#v1fig70">Fig. 70</a>). These bosses present some points of -structure in common with true vents. They come like great vertical columns -through highly-folded and puckered strata, and, as they truncate the Llandovery -and Wenlock formations, they are certainly younger than the greater -part of the Upper Silurian series. They must be later, too, than the chief -plication and cleavage of these strata; but they are older than the Upper -Old Red Sandstone or basement Carboniferous rocks which contain pebbles -of them. Their date of eruption is thus narrowed to the interval between -the later part of the Upper Silurian period and the beginning of the Upper -Old Red Sandstone. I have myself little doubt that they are to be associated -with the volcanic epoch we are now considering, as it was the only -known great episode of igneous activity in this region during the interval -within which the protrusion of these granites must have taken place. In -the Cheviot Hills, indeed, we have evidence of the eruption of a large -mass of augite-granitite through the porphyrite-lavas of the Lower Old Red -Sandstone, with abundant veins projecting from it into them, as will be -narrated in later pages.<a id="FNanchor_345" href="#Footnote_345" class="fnanchor">[345]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_344" href="#FNanchor_344" class="label">[344]</a> I suggested this possible connection many years ago in <i>Trans. Geol. Soc. Edin.</i> vol. ii. (1874) -p. 21.</p> - -<p><a id="Footnote_345" href="#FNanchor_345" class="label">[345]</a> The volcanic geology of the Cheviot Hills is described by Mr. Teall, <i>Geol. Mag. for 1883</i>, p. -106; and by Mr. Clough, <i>Mem. Geol. Survey</i>, "Geology of the Cheviot Hills," Sheet 108 N.E., -1888, p. 24.</p> - -</div> - -<div class="figcenter" id="v1fig69" style="width: 450px;"> - <img src="images/v1fig69.png" width="450" height="112" alt="" /> - <div class="tdc"><span class="smcap">Fig. 69.</span>—Section of the granite core between Merrick and Corscrine.<br /> - <i>a</i>, Silurian greywackes, grits and shales; <i>b</i>, granite.</div> -</div> - -<p>Not improbably many other granite protrusions throughout the British -Isles are to be referred to the volcanic operations of the Lower Old Red -Sandstone. Such are those of the Lake District, notably that of Shap,<a id="FNanchor_346" href="#Footnote_346" class="fnanchor">[346]</a> the -granites of Newry and Leinster in the east of Ireland, which are later than -the Silurian rocks and older than the Carboniferous Limestone, and the -younger Grampian granites, which pierce the presumably Arenig belt along -the Highland border. Whether or not these granitic protrusions were -<span class="pagenum" id="Page_291">- 291 -</span> -connected with superficial volcanic discharges of which no remains have -survived, they seem to indicate the wide extent and remarkable vigour of -the subterranean igneous action of this geological period.</p> - -<div class="footnote"> - -<p><a id="Footnote_346" href="#FNanchor_346" class="label">[346]</a> See the descriptions of the Shap granite by Messrs. Marr and Harker, <i>Quart. Journ. Geol. -Soc.</i> xlvii. (1891) p. 266, and xlix. (1893) p. 359.</p> - -</div> - -<div class="figcenter" id="v1fig70" style="width: 383px;"> - <img src="images/v1fig70.png" width="383" height="101" alt="" /> - <div class="tdc"><span class="smcap">Fig. 70.</span>—Section across the three Dirrington Laws, Berwickshire.<br /> - <i>a</i>, Upper Silurian strata; <i>b</i>, Necks probably of Lower Old Red Sandstone age; <i>c</i>, Upper Old Red Sandstone lying - unconformably both on <i>a</i> and <i>b</i>.</div> -</div> - -<p>Viewed as a whole, the materials which now occupy the vents of the -volcanic chains in the Lower Old Red Sandstone of the British Isles are -more acid than the lavas erupted at the surface. In the Pentland district, -indeed, and in some other areas this acid material was ejected at intervals in -abundant discharges of dust and lapilli and in outflows of felsitic lavas, -while between these successive discharges copious streams of diabasic and -andesitic lavas, either from the same or from some closely-adjoining vent, -were poured out. Throughout the whole region, however, as a closing phase -of the volcanic history, the acid magma rose after the outpouring of the more -basic lavas and filled such chimneys of the volcanoes as were not already -blocked with agglomerate. It was probably after these pipes were plugged -that the final efforts of volcanic energy were expended in the protrusion of -the acid material as sills between the bedding-planes of the surrounding -rocks, and as dykes and veins in and around the vents.</p> - - -<h3>iii. <span class="allsmcap">SILLS AND DYKES</span></h3> - -<p>Nowhere throughout the volcanic tracts of the Lower Old Red Sandstone -is there any such development of sills as may be seen beneath the Silurian -volcanic sheets of North Wales. Those which occur are most abundant in -the Lanarkshire district, to the north-west and south-west of Tinto, and in -the south of Ayrshire. From the village of Muirkirk to the gorge of the -Clyde, below the Falls, the Upper Silurian and Lower Old Red Sandstone -strata are traversed by numerous intrusive sheets of pink and yellow felsite, -quartz-porphyry, minette, lamprophyre and allied rocks, which are no doubt -to be regarded as part of the volcanic phenomena with which we are here -concerned. In the south of Ayrshire, between the villages of Dalmellington -and Barr, there is a copious development of similar sills, especially along one -or more horizons near the base of the Old Red Sandstone. Garleffin Fell, -Glenalla Fell, Turgeny and other heights are conspicuous prominences -formed of these rocks; above the sills lie thick conglomerates and sandstones -on which the great andesite-sheets rest.</p> - -<p>In the Pentland Hills, as will be described in <a href="#CHAPTER_XX">Chapter xx.</a>, a massive -<span class="pagenum" id="Page_292">- 292 -</span> -felsitic sill forms a conspicuous feature along the north side of the chain, -and there are probably others which have not yet been separated from -the felsitic tuffs and orthophyres which they so much resemble.</p> - -<p>Perhaps the most remarkable acid -sills in the Old Red Sandstone of -Britain are those which occur at the -extreme northern end of the region -among the volcanic phenomena of the -Shetland Isles (Figs. <a href="#v1fig71">71</a>, <a href="#v1fig72">72</a>). The -largest of them, consisting mainly of -granite and felsite, is believed to reach -a length of 20 and a breadth of from -three to four miles.<a id="FNanchor_347" href="#Footnote_347" class="fnanchor">[347]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_347" href="#FNanchor_347" class="label">[347]</a> Messrs. B. N. Peach and J. Horne, <i>Trans. Roy. Soc. Edin.</i> xxxii. (1884), p. 359.</p> - -</div> - -<div class="figcenter" id="v1fig71" style="width: 660px;"> - <img src="images/v1fig71.png" width="660" height="64" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 71.</span>—Section of Papa Stour, Shetlands, showing sill of spherulitic felsite traversing Old Red Sandstone and bedded porphyrites - (Messrs. Peach and Horne).<br /> - 1. Red sandstones and flagstones; 2. Purple diabase-porphyrites; 3. Great sheet of pink spherulitic felsite.</div> -</div> - -<div class="figcenter" id="v1fig72" style="width: 659px;"> - <img src="images/v1fig72.png" width="659" height="60" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 72.</span>—Section across Northmavine, from Okrea Head to Skea Ness, Shetland, showing dykes and connected sill of granite and felsite - (Messrs. Peach and Horne).<br /> - 1. Schists, etc.; 2. Serpentine; 3. Granite and quartz-felsite; 4. Breccia of serpentine fragments; 5. Bedded andesites and tuffs. <i>f</i>, Fault.</div> -</div> - -<p>A group of sills composed of a -bright red quartz-porphyry has been -traced along the southern flanks of -the Highlands for upwards of 18 -miles.<a id="FNanchor_348" href="#Footnote_348" class="fnanchor">[348]</a> This rock, already referred to -as the "Lintrathen porphyry," lies -chiefly among the conglomerates and -sandstones, but also intersects the lavas, -and may be later than the Old Red -Sandstone (p. 277). An extension of -it is found even on the north side of -the boundary fault, cutting the andesites -which there lie unconformably -on the schists.</p> - -<div class="footnote"> - -<p><a id="Footnote_348" href="#FNanchor_348" class="label">[348]</a> See Sheet 56 of the Geological Survey of Scotland.</p> - -</div> - -<p>Examples, however, occur of sills -much less acid in composition. In -the Dundee district, for instance, the -intrusive sheets are andesites and -diabases. They send veins into and -bake the sandstones among which they -have been intruded, and are sometimes -full of fragments of such indurated -sandstone, as may be well seen on -the northern shore of the Firth of -Tay, west of Dundee.</p> - -<p>A conspicuous characteristic of -most of the volcanic tracts of the -Lower Old Red Sandstone is the comparative -scarcity of contemporaneous -dykes. In the band of acid sills -between Muirkirk and the Clyde, a considerable number of dykes -<span class="pagenum" id="Page_293">- 293 -</span> -have been mapped, which must be regarded as due to the same series -of movements and protrusions of the magma that produced the adjacent -sills. Throughout the length of the Southern Uplands dykes of felsite, -minette, lamprophyre, vogesite and other varieties, which may also be -connected with the volcanic phenomena of the Lower Old Red Sandstone, -not infrequently occur among the Silurian rocks. On the -Kincardineshire coast, south of Bervie, a number of dykes of pink -quartz-porphyry traverse the conglomerates and sandstones. The coast -south of Montrose displays some singularly picturesque sections, where a -porphyry dyke running through andesitic lavas and agglomerates stands up -in wall-like and tower-like projections. On the shore at Gourdon, as well -as inland, intrusive dykes of serpentine occur. A line of these, possibly -along the same fissure, has been traced for more than a dozen of miles from -above Cortachy Castle to near Bamff. But there is no evidence to connect -them with the volcanic phenomena of the Old Red Sandstone. Not improbably -they belong to a later geological period.</p> - -<p>One would expect to meet with a network of dykes in and around the -volcanic vents; but even there they are usually not conspicuous either for -number or size. In the great vent of the Braid Hills only a few have been -noticed. In the Ochil Hills groups of dykes of felsite and andesite may -be observed, especially near the necks. They are fairly numerous in the -neighbourhood of Dollar (see <a href="#v1fig68">Fig. 68</a>). One of the most abundant series -yet observed traverses the tract around the granite boss of the Cheviot -Hills, from which many dykes of granite, felsite, quartz-porphyry and andesite -radiate. This district will be more fully referred to in <a href="#CHAPTER_XXI">Chapter xxi</a>. -Another remarkable development of dykes occurs in Shetland (<a href="#v1fig72">Fig. 72</a>), -where they consist of granite, felsite and rhyolite, and are associated with -the acid sills above referred to.</p> - -<hr class="chap x-ebookmaker-drop" /> - - -<div class="chapter"> -<p><span class="pagenum" id="Page_294">- 294 -</span></p> - -<h2 class="nobreak" id="CHAPTER_XIX">CHAPTER XIX<br /> - -<span class="smaller">VOLCANOES OF THE LOWER OLD RED SANDSTONE OF "LAKE CALEDONIA"</span></h2> -</div> - -<div class="blockquot"> - -<p>Description of the several Volcanic Districts: "Lake Caledonia," its Chains of Volcanoes—The -Northern Chain: Montrose Group, Ochil and Sidlaw Hills, the Arran and -Cantyre Centre, the Ulster Centre.</p> -</div> - - -<p>I now propose to give some account of each of the districts which have been -separate areas of volcanic action during the time of the Lower Old Red -Sandstone, tracing its general structure, the arrangement and sequence of its -volcanic rocks and the history of its eruptions. As by far the most varied -development of the Old Red Sandstone is to be found in the great Midland -Valley of Scotland, and as it is there that the remarkable volcanic phenomena -of the system have been most abundantly displayed and are most clearly -recorded, I shall begin my description of the volcanic eruptions of the Lower -Old Red Sandstone with a detailed account of the different centres of -volcanic activity in that region. The phenomena are so fully displayed there -that a more summary treatment of the subject will suffice for the other -regions.</p> - -<p>Under the designation of "Lake Caledonia," as already remarked, I -include the whole of the Midland Valley of Scotland between the Highlands -and the Southern Uplands, likewise the continuation of the same ancient -hollow by Arran and the south of Cantyre across the north of Ireland to -Lough Erne.<a id="FNanchor_349" href="#Footnote_349" class="fnanchor">[349]</a> Throughout most of the area thus defined, the present limits -of the Lower Old Red Sandstone are sharply marked off by large parallel -faults. On the north-west side one, or rather a parallel series, of such dislocations -runs from Stonehaven along the flank of the Highland mountains -to the Clyde, thus traversing the whole breadth of the island. On the -<span class="pagenum" id="Page_295">- 295 -</span> -south-east side another similar series of faults, which there skirts the edge -of the Silurian tableland, has nearly the same effect in precisely defining -the margin of the Old Red Sandstone. As thus limited, the tract has a -breadth of about 50 miles in Scotland, while the portion of it now visible -in the British Isles has an extreme length of about 280 miles (<a href="#Map_III">Map III.</a>).</p> - -<div class="footnote"> - -<p><a id="Footnote_349" href="#FNanchor_349" class="label">[349]</a> My own investigations of this region have been continued over an interval of forty years. -Besides personally traversing every portion of it, I have mapped in detail, for the Geological Survey, -many hundreds of square miles of its area from the outskirts of Edinburgh south-westwards into -Lanarkshire, in Ayrshire, and in the counties of Fife, Perth and Kinross. The Geological Survey -maps of the volcanic tracts of the Sidlaw Hills have been prepared by my brother, Prof. James -Geikie, and Messrs. H. M. Skae and D. R. Irvine. The Western Ochils were mapped chiefly by -Mr. B. N. Peach, partly by Prof. J. Young, Mr. R. L. Jack and myself; the Eastern Ochils -were surveyed mainly by Mr. H. H. Howell; while the volcanic belt between the tracts mapped -by me in Lanarkshire and in Ayrshire was chiefly traced out by Mr. Peach. As a rule, each of -these geologists has described in the Survey Memoirs the portions of country surveyed by him.</p> - -</div> - -<p>But though the boundary-faults determine, on the whole, the present -limits of the tract of Old Red Sandstone, they do not necessarily indicate -the shore-lines of the sheet of water in which that great series of deposits -was laid down. They point to an enormous subsidence of the tract between -them—a prolonged and extensive sagging of the strip of country that stretches -across the Midland Valley of Scotland into the north of Ireland.<a id="FNanchor_350" href="#Footnote_350" class="fnanchor">[350]</a> This -downward movement began as far back as the close of the Silurian period, -but the marginal fractures and the disruption and plication of the thick -masses of sandstone and conglomerate which were accumulated in the lake -chiefly took place after the close of the period of the Lower Old Red Sandstone. -I think we may reasonably connect these movements with the general -sinking of the area consequent upon the enormous outpouring of volcanic -materials during that period.</p> - -<div class="footnote"> - -<p><a id="Footnote_350" href="#FNanchor_350" class="label">[350]</a> In some of the dislocations along the Highland border, the Old Red Sandstone is bent back -upon itself, and the older schists are thus made to recline upon it, as if there had been a push over -from the Highland area.</p> - -</div> - -<p>Along both the northern and southern margins of the basin there -occur, on the farther side of the boundary faults, outlying patches of Lower -Old Red Sandstone that rest unconformably on the rocks forming the -flanks of the hills. These areas possess a peculiar interest, inasmuch as -they reveal some parts of the shore-line of the lake, and show the relation -between the earlier rocks and the sediments of the Old Red Sandstone. -We learn from them that the shore-line was indented with wide bays, but -nevertheless ran in a general north-easterly direction. It thus corresponded -in trend with the present Midland Valley, with the axes of plication among -the schists of the Highlands as well as among the Silurian rocks of the -Southern Uplands, and with the subsequent faulting and folding of the -Old Red Sandstone.</p> - -<div class="figcenter" id="v1fig73" style="width: 431px;"> - <img src="images/v1fig73.png" width="431" height="132" alt="" /> - <div class="hanging2"><p><span class="smcap">Fig. 73.</span>—Section at the edge of one of the bays of Lower Old Red Sandstone along the northern - margin of Lake Caledonia, near Ochtertyre.<br /> - <i>a</i>, slates and phyllites; <i>b</i>, volcanic conglomerates; <i>c</i>, andesite-lava.</p></div> -</div> - -<p>I may remark in passing that the conglomerates and other associated -materials which have been preserved in these bays and hollows beyond the -<span class="pagenum" id="Page_296">- 296 -</span> -lines of the great faults, though they lie unconformably on the rocks beneath, -are not the basement portions of the Old Red Sandstone. On the contrary, -where their probable stratigraphical horizons can be recognized or inferred, -they are found to belong to parts of the series considerably above the base -of the whole. They point to the gradual sinking of the basin and the -creeping of the waters with their littoral shingles further and further up -the slopes of the hills on either side (<a href="#v1fig73">Fig. 73</a>).</p> - -<p>But this is not all the evidence that can be adduced to show that the -limits of the lake extended considerably beyond the lines of dislocation -between which the present area of Old Red Sandstone mainly lies. No -one can look at the noble escarpments of the Braes of Doune on the one -side (<a href="#v1fig74">Fig. 74</a>), or walk over the upturned conglomerates and andesites which -flank the Lanarkshire uplands on the other, without being convinced that if -the effects of the boundary faults could be undone, so as to restore the original -structure of the ground, the prolongations of the rocks, now removed by -denudation, would be found sweeping far into the Highlands on the north -and into the Silurian Uplands on the south.</p> - -<div class="figcenter" id="v1fig74" style="width: 542px;"> - <img src="images/v1fig74.png" width="542" height="306" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 74.</span>—Craig Beinn nan-Eun (2067 feet), east of Uam Var, Braes of Doune. Old Red Conglomerate, - with the truncated ends of the strata looking across into the Highlands; moraines of Corry Beach - in the foreground.</div> -</div> - -<p>If the area of "Lake Caledonia" were taken to be defined by the boundary -faults, it covered a space of about 10,000 square miles. But, as we know -that it certainly stretched beyond the limits marked by these faults, it -must have been of still greater extent. We shall probably not exaggerate -if we regard it as somewhat larger than the present Lake Erie, the superficies -of which is about 9900 square miles. In this long narrow basin the -<span class="pagenum" id="Page_297">- 297 -</span> -remarkable volcanic history was enacted of which I now proceed to give -some account.</p> - -<p>The Lower Old Red Sandstone of Central Scotland may be conveniently -divided into three great groups, each of which marks a distinct epoch in -the history of the basin wherein they were successively accumulated. The -lowest of these groups indicates a time of quiet sedimentation during which -the basin was defined by plication of the terrestrial crust, and when, by the -same subterranean movements, some parts of the floor of the lake were -pushed upward above water, and were then denuded and buried. The -middle group consists largely of volcanic rocks. It points to the existence -of lines of active volcanic cones situated along the length of the lake. The -uppermost group records the extinction of volcanic action and the gradual -obliteration of the lake, partly by the pouring of sediment into it, and -partly no doubt by the continued terrestrial movements which had originally -produced the basin.</p> - -<p>It is evident from these records that though volcanic activity continued -vigorous for a vast period of time, it had entirely ceased in "Lake Caledonia" -long before the last sediments of the Lower Old Red Sandstone were -laid down. The great cones of the Ochil Hills, for example, sank below -the waters of the lake in which they had long been a conspicuous -feature, and so protracted was the subsidence of the lake-bottom that the -site of these volcanoes was buried under 8000 or 9000 feet of sandstones -and conglomerates, among which no trace of any volcanic eruptions has -yet been found. The sagging of the terrestrial crust over an area from -which such an enormous amount of volcanic products had been discharged -would doubtless be a protracted process. Long after the subsidence of the -lake-bottom and the accumulation of its thick mass of sediments, after even -the entire effacement of the topography and the deposition of the thick -Carboniferous formations over its site, the downward movement showed -itself in the production of gigantic north-east faults, and the sinking -of the Carboniferous rocks for several thousand feet. These dislocations, -as was natural, have run through the heart of some of the volcanic groups, -carrying much of the evidence of the ancient volcanoes out of sight, and -leaving us only fragments from which to piece together the records of a -volcanic period which is by no means the least interesting in the geological -history of this country.</p> - -<p>Confining our attention for the present to the records of the middle or -volcanic group, we find evidence of a number of distinct clusters of volcanoes -ranged along the whole length of the basin. The independence of these -volcanic districts may be inferred from the following facts:—1st, The actual -vents of discharge may in some cases be recognized; 2nd, Even where these -vents have been buried, we may often observe, as we approach their probable -sites, a marked increase in the thickness of the volcanic accumulations, as -well as a great development of agglomerates and tuffs; 3rd, Traced in -opposite directions, the volcanic materials are found to thin away or even -to disappear. Those from one centre of discharge may be observed -<span class="pagenum" id="Page_298">- 298 -</span> -now and then to overlap those from another, but the two series remain -distinct.</p> - -<p>Reasoning from these data and studying the distribution of the various -volcanic areas, we are led to recognize the former existence of two parallel -chains of vents, running along the length of the lake at a distance from -each other of somewhere about twenty miles. They may be conveniently -distinguished as the northern and the southern chain.</p> - -<p>The northern band runs from the coast-line near Stonehaven south-westward -through the Sidlaw and Ochil Hills. It is then abruptly truncated by a -large fault and by the unconformable superposition of the Carboniferous formations. -But 60 miles further to the south-west, where the Old Red Sandstone -comes out on the west side of the Firth of Clyde, a continuation of the volcanic -band has recently been detected by Mr. W. Gunn of the Geological Survey in -the Island of Arran. Twenty-five miles still further in the same direction a -much ampler development of the volcanic rocks occurs to the south of -Campbeltown in Cantyre. If we cross the 22 miles of sea that separate -the Argyllshire coast-sections from those of Red Bay in Ireland, we find -near Cushendall a repetition of the Scottish volcanic conglomerates, -while still further along the same persistent line, some 50 miles into the -interior, the hills of Tyrone include sheets of lava precisely like those -of Central Scotland. The total length of this northern chain of volcanoes -is thus not much less than 250 miles, and as its north-eastern end -is now cut off by the North Sea it must have been still longer. It ran -parallel to the north-western coast-line of the lake, at a distance which, -over the site of the Midland Valley of Scotland, seems to have varied from -10 to 20 miles, but which greatly lessened further to the south-west.</p> - -<p>At a distance of some twenty miles to the south of the northern belt, the -second parallel chain of volcanoes ran in a nearly straight line, which is now -traceable from the southern suburbs of Edinburgh to the coast of Ayrshire, -a distance of about 75 miles, but as its north-eastern end is concealed -by Carboniferous formations, and its south-western passes under the sea, its -true length is probably considerably more.</p> - -<p>If the areas which present evidence of distinct and independent vents -are grouped according to their positions on these two lines, they naturally -arrange themselves as in the following list:—</p> - -<table class="pmt1 pmb2" summary="data"> -<tr> - <td class="vtop tdr" style="padding: 1em 0;" rowspan="5">I. </td> - <td class="tdl" style="padding: 1em 0;"><span class="smcap">Northern Chain of Volcanoes</span></td> -</tr> -<tr> - <td class="tdl">1. The Montrose Centre.</td> -</tr> -<tr> - <td class="tdl">2. The Sidlaw and Ochil Group.</td> -</tr> -<tr> - <td class="tdl">3. The Arran and Cantyre Centre.</td> -</tr> -<tr> - <td class="tdl">4. The Ulster Centres.</td> -</tr> -<tr> - <td class="vtop tdr" style="padding: 1em 0;" rowspan="5">II. </td> - <td class="tdl" style="padding: 1em 0;"><span class="smcap">Southern Chain of Volcanoes</span></td> -</tr> -<tr> - <td class="tdl">5. The Pentland Volcano.</td> -</tr> -<tr> - <td class="tdl">6. The Biggar Centre.</td> -</tr> -<tr> - <td class="tdl">7. The Duneaton Centre.</td> -</tr> -<tr> - <td class="tdl">8. The Ayrshire Group.</td> -</tr> -</table> - -<p><span class="pagenum" id="Page_299">- 299 -</span></p> - -<p>The distribution of these various volcanic areas will be most easily -understood from an examination of <a href="#Map_III">Map III.</a> accompanying this volume.</p> - - -<h3>I. <span class="allsmcap">THE NORTHERN CHAIN OF VOLCANOES IN "LAKE CALEDONIA"</span></h3> - - -<h4>1. <i>The Montrose Centre</i></h4> - -<p>Beginning at the north-eastern end of the area, we first encounter a -series of volcanic rocks which attain their maximum thickness in Forfarshire -around the town of Montrose. The main vents probably lay somewhere to the -east of the present coast, under the floor of the North Sea; at least no clear -indication of their existence either on the coast or inland has been detected. -From Montrose, both to the north-east and south-west, the lavas thin away, -becoming intercalated among the sandstones, flagstones and conglomerates, -and gradually dying out. The total length of the volcanic belt is about 18 -miles, that is nine miles from the central thick mass in a north-easterly and -the same distance in a south-westerly direction.<a id="FNanchor_351" href="#Footnote_351" class="fnanchor">[351]</a> The volcanic pile must be -several thousand feet thick, but owing to the prolongation of the great Ochil -anticline, the lavas roll over and do not allow their base to be seen. The -axis of the fold must pass out to sea, through the hollow on which the town -of Montrose stands. The volcanic series consists of andesite-sheets with -volcanic conglomerates. It contains little ordinary tuff, but the conglomerates -no doubt partly represent ejected fragmental material, as well as the -waste of exposed lavas. A section across the anticlinal fold from Forfar to -Panbride, a little to the south-west of Montrose, would reveal the structure -shown in <a href="#v1fig67">Fig. 67</a>.</p> - -<div class="footnote"> - -<p><a id="Footnote_351" href="#FNanchor_351" class="label">[351]</a> The south-western part of this area from Arbroath to Johnshaven was mapped for the -Geological Survey by the late Mr. H. M. Skae, the north-eastern part by Mr. D. R. Irvine. My -account of it is mainly taken from notes made by myself on the ground preliminary to the -commencement of the mapping of the Survey.</p> - -</div> - -<p>In the north-eastern prolongation of the volcanic series from the -Montrose centre, successively lower members are exposed along the coast-line. -But the lavas are dying out in that direction, and sometimes many -hundreds of feet of ordinary sediment intervene between two successive -flows. It was in one of these long pauses near the top of the whole -pile of lavas that the strata of Canterland were deposited, to which reference -has already been made. South-west from Montrose the thick volcanic -mass rapidly diminishes, and is prolonged to the end only by three or four -bands separated by sandstones and flagstones. It is in these intercalated -groups of sedimentary material that the "Forfarshire flags" occur.</p> - -<p>Nowhere can the details of the Old Red Sandstone volcanic rocks be -more conveniently studied than along the coast-section in this district from -the Red Head to Stonehaven. The rocks have not only been cut into -vertical cliffs, but along many parts of the shore they have been also laid bare -in ground-plan, so that a complete dissection of them is presented to the -geologist. At the south end, the top of the volcanic series appears at the -<span class="pagenum" id="Page_300">- 300 -</span> -bold promontory of the Red Head. There, at the base of the cliffs of red -sandstone, the accompanying section may be seen. Beneath the red false-bedded -and sometimes pebbly sandstones (<i>e</i>), which form nearly the whole -precipice, lies a band of dull purplish ashy conglomerate (<i>d</i>), composed -almost wholly of fragments -of different andesites, imbedded -in a paste of the -same comminuted material. -Towards the south, this -rock rapidly becomes -coarser, until it passes into -a kind of agglomerate, in -which the andesite blocks -are sometimes a yard or -more in diameter. It includes -bands of sandstone, -which increase in number -and thickness towards the -north, and sometimes intervene -underneath the conglomerate. -The lowest rocks here visible are sheets of andesite or "porphyrite" -(<i>a</i>), separated from each other by irregular bright red layers of -tufaceous sand and agglomerate. These lavas are dull purplish-grey to -green, some of them being tolerably compact, others highly amygdaloidal, -with large steam-cavities often drawn out in the direction of flow.</p> - -<div class="figleft" id="v1fig75" style="width: 301px;"> - <img src="images/v1fig75.png" width="301" height="201" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 75.</span>—Section showing the top of the volcanic series at the - foot of the precipice of the Red Head, Forfarshire.<br /> - <i>a</i>, Top of slaggy andesite; <i>b</i>, coarse volcanic conglomerate; <i>c</i>, Red - sandstone; <i>d</i>, Tuff and volcanic conglomerate; <i>e</i>, Red sandstones.</div> -</div> - -<p>One of the most striking features in the andesites of this coast is the -remarkable manner in which they include the veinings of pale green and red -sandstone already described (see Figs. <a href="#v1fig65">65</a>, <a href="#v1fig66">66</a>). Some of the sheets have in -cooling cracked into rude polygons. They are likewise traversed by large -cavernous spaces and intricate fissures or steam-cavities. Into all these -openings the sand has been washed, filling them up and solidifying into well-stratified -sandstone, the bedding of which is generally parallel with that -of the rocks that enclose it, the dip of the whole series of strata being -gently seawards. But a still more intimate mixture of the sand with the -lava-sheets is to be remarked where these rocks assume their most slaggy -character. In some of them the upper part, to a depth of ten or twelve feet, -consists of mere rugged lumps of slag which, while the mass was in motion, -were probably in large measure loose, and rolled over each other as they -were borne onward. The sand has found its way into all the interstices of -these clinker-beds, and now binds the whole mass firmly together. At first -sight, these bands might be taken for agglomerates of ejected blocks, and -as already suggested, some of the slags may have been thrown out as loose -pieces, but a little examination will show that in the main the rough -scoriaceous lumps are pieces of the lava underneath. In these instances, -also, it is clear that the blocks were in position before the fine sand was -sifted into their interspaces, for the pale green sandstone is horizontally -<span class="pagenum" id="Page_301">- 301 -</span> -stratified through its intricate ramifications among the pile of dark -clinkers.</p> - -<p>The seaward inclination of the rocks allows the succession of lavas to -be seen as the coast is followed westward into Lunan Bay. On the further -side of that inlet, after passing over a group of sandstones that underlie -the volcanic series of the Red Head, the observer meets with a second and -lower succession of lavas which in the five miles northward to Montrose -Harbour are admirably exposed both along coast-cliffs and on the beach. -They resemble those of the Red Head, being made up of alternations of highly -vesicular andesite with more compact varieties, and showing similar sandstone -veinings. Here and there, as at Fishtown of Usar, the sea has cut them -down into a platform from which the harder parts rise as fantastic half-tide -stacks. In some cases, the more durable rock consists of the slaggy upper -portions of the flows, and in one case this material stands up as a rude -pillar twelve feet high, composed of clinkers firmly cemented with veinings -of sandstone. The geologist who wanders over this coast-line is arrested at -every turn by the marvellously fresh volcanic aspect of many of the lavas. -Their upper parts are so cellular that if the calcite, chalcedony and other -infiltrated minerals were removed from their vesicles, they would be transformed -into surfaces of mere slag. In one respect would their antiquity -still be evident. These slaggy bands are generally a good deal reddened, as -if they had been long exposed to oxidation before being covered by the -overlying sheets of lava—a feature already cited, as probably indicating the -lapse of some considerable interval of time between successive outflows.</p> - -<p>Along this coast-section the absence of intercalated tuffs is soon remarked. -The volcanic ejections seem to have consisted almost entirely -of andesitic lavas, though it is possible that here and there the very -slaggy bands between the more solid parts of the sheets may include a -little pyroclastic material. The lowest portion of the volcanic group here -visible is reached at Montrose Harbour, where, in the flagstones and shales -of Ferryden, the late Rev. Hugh Mitchell obtained some of the fossil-fishes -of the formation.</p> - -<p>A space of more than three miles now intervenes where the rocks are -concealed by blown sand and other superficial accumulations. It is through -this hollow, as already stated, that the great Ochil anticline runs out to sea. -On the north side of the North Esk River, we again come upon the same -band of lavas as to the south of Montrose, but with a dip to the north-west. -This inclination, however, soon bends round more westerly, and the result -of the change is to expose a slowly descending section all the way to the -Highland fault at Stonehaven.</p> - -<p>A picturesque line of high inland cliff, running northwards beyond St. -Cyrus, reveals with great clearness the bedded structure of the andesites. -But as one moves northward, owing to the change in the direction of dip, -one finally passes out of this volcanic belt and begins gradually to descend -into the thick Kincardineshire Old Red Sandstone. The amount of conglomerate -exposed along this part of the coast-line probably considerably -<span class="pagenum" id="Page_302">- 302 -</span> -surpasses in thickness any other conglomerate series in the Lower Old Red -Sandstone of Britain. Throughout the enormous depth of sedimentary -material, the conglomerates are well-bedded, consisting of a dull green paste, -composed in large degree of comminuted andesitic debris, and interstratified -with green felspathic sandstones. They are often remarkably -coarse, the pebbles sometimes measuring three feet in length. Interposed -among them are some ten or twelve bands, probably often single outflows -of andesite, sometimes compact and porphyritic, at other times highly -amygdaloidal. Such is the succession of rocks for many miles along the -shore; and as the inclination varies from a little north of west to west, or even -west by south, the observer gradually passes over a thickness of rather more -than 2000 feet from the base of the St. Cyrus andesites to Gourdon. In this -accumulation of coarse, well water-worn material, with abundant intercalations -of finer sandstone and occasional sheets of lava, there is the record of prolonged -and powerful denudation with intermittent volcanic activity. Dykes -of a quartziferous porphyry cut the conglomerates, and at Gourdon they are -pierced by the intrusion of serpentine above referred to.</p> - -<p>The proportion of andesite fragments in the conglomerates of this part -of the coast varies, but is generally much lower than that of the rocks from -the Highlands. Thus at Johnshaven, out of 100 blocks, broken promiscuously -from the conglomerate, I found that only 8 per cent were of andesite, -while 44 per cent were of quartzite, and the remainder consisted of various -quartz-porphyries, granites and schists. It is evident, therefore, that some -area of crystalline rocks was subjected to enormous waste, and that its detritus -was strewn over the floor of Lake Caledonia, at the same time that from the -Montrose volcanic vents many streams of andesitic lava were poured forth.</p> - -<p>A vast mass of coarse conglomerate intervenes between Gourdon and -Dunnottar, and forms a nearly continuous line of precipices which in some -places rise 200 feet above the waves. The bedding is everywhere distinctly -marked, so that there is no difficulty in following the succession of the strata, -and estimating their thickness. From the last of the lavas at Gourdon -to the base of the conglomerates near Stonehaven, there lies an accumulation -of conglomerate at least 8000 feet thick. The boulders and pebbles in -these deposits are generally well-rounded, and vary up to four feet or more -in length. I observed one of quartz-porphyry at Kinneff which measured -seven feet long and six feet broad. The proportion of andesite fragments -in these conglomerates continues to be small. I ascertained that in the -coarsest mass at Kinneff they numbered only 14 per cent; at Todhead Point, -a mile and a half to the north, 20 per cent, and at Caterline, three quarters -of a mile further in the same direction, 21 per cent.</p> - -<div class="figright" id="v1fig76" style="width: 323px;"> - <img src="images/v1fig76.png" width="323" height="234" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 76.</span>—Andesite with sandstone veinings and overlying conglomerate. - Todhead, south of Caterline, coast of Kincardineshire.</div> -</div> - -<p>In the midst of this gigantic accumulation of the very coarsest water-worn -detritus, there are still records of contemporaneous volcanic action. -Near Kinneff the beautiful andesite, with large tabular crystals of plagioclase, -alluded to on <a href="#Page_274">p. 274</a>, occurs in the conglomerate.<a id="FNanchor_352" href="#Footnote_352" class="fnanchor">[352]</a> South of Caterline -<span class="pagenum" id="Page_303">- 303 -</span> -two flows, lying still lower in the system, project into the sea. One of -these presents a section of much interest. It shows a central solid portion, -jointed into rudely prismatic blocks, with an indefinite platy structure, which -gives it a roughly-bedded aspect. Its upper ten or twelve feet are sharply -marked off by their slaggy structure, ending upwards in a wavy surface like -that of the Vesuvian lava of 1858. Into its fissures, steam-cavities and -irregular hollows, fine sand has been washed from above, as at Red Head, -while immediately above it comes a coarse conglomerate of the usual character -(<a href="#v1fig76">Fig. 76</a>). Still lower -down, beneath some 900 -feet of remarkably coarse -conglomerate, another -group of sheets of andesite -abuts at Crawton -upon the coast, with -which, at a short distance -inland, it runs -parallel for more than -two miles, coming back -to the sea at Thornyhive -Bay and at Maidenkaim. -We have then to pass -over about 5000 feet of -similar conglomerates, -until, after having crossed -several intercalated sheets of andesite, we meet with the last and lowest of -the whole volcanic series of this region in the form of some bands of porphyrite -at the Bellman's Head, Stonehaven. The peculiar geographical conditions that -led to the formation of the coarse conglomerates appear to have been established -at the same time that the volcanic eruptions began, for as we descend in -the long coast section, we find that the coarse sediment and the intercalated -lavas cease on the same general horizon. Below that platform lie some -5000 feet of red sandstones and red shales, yet the base of the series is -not seen, for the lowest visible strata have been faulted against the schists -of the Highlands. It is thus obvious that more than 5000 feet of sediment -had been laid down over this part of the floor of Lake Caledonia before the -first lavas were here erupted.</p> - -<div class="footnote"> - -<p><a id="Footnote_352" href="#FNanchor_352" class="label">[352]</a> For an analysis of the felspar in this rock, see Prof. Heddle's paper, <i>Trans. Roy. Soc. Edin.</i> -xxviii. (1879), p. 257.</p> - -</div> - - -<h4>2. <i>The Sidlaw and Ochil Group</i></h4> - -<p>The volcanoes which poured out the masses of material that now form -the chain of the Ochil and Sidlaw Hills appear to have been among the most -vigorous in the whole region of Lake Caledonia. Their chief vents -probably lay towards the south-west in the neighbourhood of Stirling, where -the lavas, agglomerates and tuffs discharged from them reach a thickness of -not less than 6500 feet, without revealing their bottom. From that centre -<span class="pagenum" id="Page_304">- 304 -</span> -the lavas range continuously for nearly fifty miles to the north-east, until -they reach the sea at Tayport; but they are prolonged on the north side of -the Firth of Tay from Broughty Ferry to near Arbroath, so as to overlap those -of the Montrose group. They thus attain a total length of nearly sixty -miles in a north-easterly line. How far they stretched south-west cannot -now be ascertained, for they have been dislocated and buried in that direction -under the Carboniferous formations of the Midland Valley.</p> - -<p>It will be observed from the map (No. III.) that the great volcanic -ridge of the Ochil Hills continues unbroken for twenty-two miles, from -Stirling to Bridge of Earn. Thereafter it branches into two divergent portions, -one of which runs on through the north of Fife to the southern promontory -of the estuary of the Tay, while the other, after sinking below the alluvial -plains of the Earn and the Tay, mounts once more into a high ridge near -Perth, and thence stretches eastward into Forfarshire as the chain of the -Sidlaw Hills. This bifurcation is due to the opening out and denudation of -the great anticlinal fold above mentioned. The rocks in the northern limb -dip north-westward, those in the southern limb dip south-eastward. The -lower members of the Old Red Sandstone, underlying the volcanic series, -ought to be seen beneath them along the crest of the anticline. Unfortunately, -however, partly by the action of faults along the boundaries of the -volcanic bands, but chiefly from the unconformable overspread of Upper Old -Red Sandstone and Lower Carboniferous rocks across the plains of the Carse -of Gowrie and of the Earn, the lower parts of the system are there concealed -(see <a href="#v1fig78">Fig. 78</a>). As already remarked, this important anticlinal fold runs to -the north-east across Forfarshire, and passes out to sea north of Montrose.</p> - -<p>Through the Ochil chain the fold runs obliquely in a south-westerly -direction, until it is truncated by the great fault which lets down the -Clackmannan coal-field. The total traceable length of this anticline is thus -about sixty miles. It flattens down towards the south-west; consequently -the rocks in the western part of the Ochil Hills are so gently inclined that -the same bands may be followed winding round the sides of the valleys, and -giving to the steep declivities the terraced contours to which allusion has -already been made (see <a href="#v1fig68">Fig. 68</a>). Another result of this structure is that -the base of the volcanic series is entirely concealed by its higher portions.</p> - -<p>From an examination of the map it will be further obvious that the -whole wide plain of Strathmore—that is the great hollow, more than 80 -miles long and about ten or twelve miles broad, which stretches between the -base of the Highland mountains and the north-western slopes of the Ochil -and Sidlaw chain—is underlain with volcanic rocks of Lower Old Red Sandstone -age. This plain lies on a broad synclinal fold, along the south-east -side of which the lavas, tuffs and conglomerates of the Ochil and Sidlaw -Hills dip under a thick accumulation of red sandstone and flagstone. On -the north-west side similar lavas and tuffs rise again to the surface, both on -the southern side of the great boundary faults, and also in the little bays -which here and there survive on the northern side of the dislocations -(<a href="#v1fig77">Fig. 77</a>). I have already alluded to these interesting relics of the shore-line -<span class="pagenum" id="Page_305">- 305 -</span> -of Lake Caledonia, and to the fact that though they lie unconformably -on the Highland schists, they do not belong to the actual basement members -of the Old Red Sandstone (<i>ante</i>, <a href="#Page_295">p. 295</a>, and <a href="#v1fig73">Fig. 73</a>). We have seen that -below the bottom of the volcanic series a thickness of 5000 feet of sandstones -and shales emerges on the Stonehaven coast, and yet that even there -the base of the whole system is not visible, owing to the effect of the Highland -boundary fault.</p> - -<p>It is thus evident that over the bottom of Lake Caledonia a very thick -deposit of tolerably fine sedimentary material was spread before the commencement -of the Ochil and Sidlaw eruptions,—that when the lavas were poured -out and the coarse conglomerates began to be formed, these materials overlapped -the older deposits and gradually encroached upon the subsiding area -of the Highlands. The lavas rolled across the floor of the lake and entered -the successive bays of the northern coast-line, where their outlying patches -may still be seen.</p> - -<div class="figcenter" id="v1fig77" style="width: 358px;"> - <img src="images/v1fig77.png" width="358" height="93" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 77.</span>—Section across the Boundary-fault of the Highlands at Glen Turrit, Perthshire.<br /> - <i>s</i>, Crystalline schists of the Highlands; <i>c</i> <i>c</i>, conglomerates and sandstones (Lower Old Red Sandstone) with interstratified - volcanic rocks (<i>v</i> <i>v</i>); <i>f</i>, fault.</div> -</div> - -<p>From these facts it is clear that to the actually visible area of volcanic -material in the Ochil and Sidlaw region, and to the anticlinal tract whence -the andesites have been removed by denudation, we have to add the area -that lies under the plain of Strathmore, which may be computed to be at -least 800 square miles, making a total of probably not less than 1300 -square miles. But it will be remembered that practically only one side of -the anticlinal fold is accessible to observation. We cannot tell how far in a -southerly direction the lavas of the Ochil Hills may extend. It is quite -possible that not a half of the total area covered by the eruptions of this -volcanic group is now within reach, either of observation or of well-founded -inference.</p> - -<p>One further general characteristic of this volcanic district will be obvious -from an inspection of the map. While the thickest mass of lavas and tuffs, -lying towards the south-west, points to the existence of the most active vents -in that part of the area, the actual positions of these vents have not been -detected. Probably they lie somewhere to the south of the edge of the -Ochil chain, under the tract which is overspread with the coal-field. But -other and possibly minor orifices of eruption appear to have risen at -irregular intervals towards the north-east along the length of the lake. Thus -there are numerous bosses of felsitic and andesitic rocks among the central -Ochils, some of which may mark the positions of active vents. For some -miles to the east of that area an interval occurs, marked by the presence of -<span class="pagenum" id="Page_306">- 306 -</span> -only a few small intrusive masses. But as the broad anticline of the Firth -of Tay opens out and allows the lower or pre-volcanic -members of the Old Red Sandstone to approach the -surface, another group of bosses emerges from the -lower sandstones and flagstones. Some of these -cover a considerable space at the surface, though -a portion of their visible area may be due to lateral -extravasation from adjacent pipes, the true dimensions -of which are thereby obscured. Some of the masses -are undoubtedly sills. In the case of Dundee Law -we probably see both the pipe and the sill which -proceeded from it; the prominent, well-defined hill -marking the former, while the band of rock which -stretches from it south-westwards to the shore belongs -to the latter. The material that forms the bosses and -sills in this neighbourhood is generally a dark compact -andesite. The rock of Dundee Law was found -by Dr. Hatch to show under the microscope "striped -lath-shaped felspars abundantly imbedded in a finely -granular groundmass, speckled with granules of magnetite, -but showing no unaltered ferro-magnesian -constituents." Here and there in the same district -a solitary neck may be observed filled with agglomerate -(<a href="#v1fig78">Fig. 78</a>).</p> - -<div class="figcenter" id="v1fig78" style="width: 679px;"> - <img src="images/v1fig78.png" width="679" height="106" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 78.</span>—Section across the chain of the Sidlaw Hills, near Kilspindie.<br /> - 1. Lower Old Red Flagstones and Sandstones; 2. Andesite lavas; 3. Volcanic tuff; 4. Volcanic conglomerates and sandstones; N, Volcanic neck; 5. Upper - Old Red Sandstone under Carse of Gowrie, lying unconformably on the lower division; <i>f</i>, Fault; <i>d</i>, Basic dyke.</div> -</div> - -<p>The variations in the structure of the Ochil and -Sidlaw volcanic group will be most easily understood -from a series of parallel sections. Beginning on the -north-eastern or Sidlaw branch of the volcanic band, -we find the arrangement of the rocks to be as is -shown in the accompanying figure<a id="FNanchor_353" href="#Footnote_353" class="fnanchor">[353]</a> (<a href="#v1fig78">Fig. 78</a>). As -is usually the case in this region, the base of the -volcanic series is here concealed by the fault which -brings down the Upper Old Red Sandstone under -the alluvial deposits of the Carse of Gowrie. The -total thickness of the series in this section is about -2500 feet. The rocks consist of successive sheets -of andesite of the familiar types, varying in colour -through shades of blue, purple and red, and in texture -from a dull compact almost felsitic character to -more coarsely crystalline varieties. They are often -amygdaloidal, especially in the upper and lower -portions of the individual flows. They are not infrequently -separated from each other by courses of conglomerate or ashy -<span class="pagenum" id="Page_307">- 307 -</span> -sandstone and grit. Of these intercalations four are of sufficient thickness -and persistence to be mapped, and are shown on the Geological Survey -Sheet 48. The stones in the conglomerates vary up to blocks two feet in -diameter, and consist chiefly of andesites, but include also some pink felsites -and pieces of greenish hardened sandstone. Generally they are more or -less well-rounded; but occasionally they become angular like those of volcanic -agglomerates.</p> - -<div class="footnote"> - -<p><a id="Footnote_353" href="#FNanchor_353" class="label">[353]</a> This section and the notes accompanying it have been supplied by Prof. James Geikie, who -mapped the western half of the Sidlaw range for the Geological Survey. The eastern half was -mapped by the late Mr. H. M. Skae.</p> - -</div> - -<p>One of the most interesting features in this section is the neck which at -Over Durdie rises through the volcanic series. Oval in form, it measures -630 yards in one diameter and 350 in another, and is filled with pinkish -granular tuff, full of andesitic lapilli and blocks. A much smaller neck of -similar material lies about 100 yards further to the south-west. There -seems no reason to doubt that these necks mark two of the volcanic vents -belonging to a late part of the volcanic history of the district.</p> - -<p>The structure of the Sidlaw range is repeated among the hills of east -Fife on the southern side of the great anticlinal fold.<a id="FNanchor_354" href="#Footnote_354" class="fnanchor">[354]</a> Thus a section from -near Newburgh on the Firth of Tay southward to near Auchtermuchty in -Stratheden gives the arrangement of rocks shown in <a href="#v1fig79">Fig. 79</a>. In this -traverse a thick mass of fragmental material occurs in the higher part of the -series of volcanic rocks. Though on the whole stratified and forming a -group of conglomerate-beds between the lavas, the material is in places an -amorphous agglomerate of volcanic blocks varying in size up to two feet in -diameter. These portions show abundant angular and subangular blocks, -many of which, after having undergone some attrition, have been finally -broken across before reaching their present resting-places. Sharply fractured -surfaces can be picked out of the felspathic ashy matrix. The stones are -chiefly varieties of andesite, but they include also pink felsites and pieces of -some older fine-grained tuff.</p> - -<div class="footnote"> - -<p><a id="Footnote_354" href="#FNanchor_354" class="label">[354]</a> The eastern part of the Ochils was mapped for the Geological Survey by Mr. H. H. Howell -and Mr. B. N. Peach.</p> - -</div> - -<div class="figcenter" id="v1fig79" style="width: 434px;"> - <img src="images/v1fig79.png" width="434" height="62" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 79.</span>—Section across the Eastern Ochil Hills from near Newburgh to near Auchtermuchty.<br /> - 1. Lower Old Red Sandstones and conglomerates; 2. Andesite lavas; 3. Volcanic conglomerates; 4. Upper Old - Red Sandstone.</div> -</div> - -<p>These fragmental materials form a local deposit about nine miles long, -and probably not less than 1700 feet thick. They are partly interstratified -with flows of andesite. Though, from the rounded forms of some of the -pebbles, wave-action may be inferred to have been concerned in their -accumulation, they seem to be mainly due to volcanic explosions. No trace, -however, has been found of the vent from which the eruptions took place. -Not improbably its site lies somewhere to the south in the area now concealed -under the Upper Old Red Sandstone and Carboniferous formations. The -large size of many of the blocks suggests that they do not lie far from their -<span class="pagenum" id="Page_308">- 308 -</span> -parent focus of discharge. It is impossible to tell -how much of the volcanic series is here concealed by -the unconformable overlap of the younger formations.</p> - -<div class="figcenter" id="v1fig80" style="width: 716px;"> - <img src="images/v1fig80.png" width="716" height="88" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 80.</span>—Generalized section across the heart of the Ochil Hills, from Dunning on the north to the Fife Coal-field near Saline on the south.<br /> - 1. Volcanic tuffs and agglomerates; 2. Andesite lavas; 3. Lower Old Red Sandstone and conglomerate; 4. Necks of felsitic rocks; 5. Upper Old Red Sandstone and - Calciferous Sandstones; 6. Representative of the Plateau lavas and tuffs of the Lower Carboniferous series; 7. Hurlet (Carboniferous) Limestone; 8. Dolerite sill; - 9. Sandstones, shales and coals of the Carboniferous Limestone series; 10. Neck of the Puy series (Carboniferous); <i>f</i>, Fault.</div> -</div> - -<p>A section across the centre of the Ochil chain,<a id="FNanchor_355" href="#Footnote_355" class="fnanchor">[355]</a> -from Dunning in Strathearn to the Crook of Devon -and the Fife Coal-field, gives the structure which is -generalized in <a href="#v1fig80">Fig. 80</a>. At the north end the volcanic -series is found to be gradually split up into -separate lava-sheets until it dips under the red sandstones -of Strathearn. Traced southwards the rocks -become entirely volcanic. Some of their most conspicuous -and interesting members are pale felsitic -tuffs, which occupy a considerable tract of ground -about Craig Rossie, south-east of Auchterarder. As -the dip gradually lessens the harder lavas are able to -spread over wider tracts of ground, capping the hills -and ridges, while underneath them thick masses of -tuff and conglomerate are laid bare in the valleys. -A number of bosses of orthophyre rise through these -rocks and are accompanied by many veins and dykes -of similar material. It is not improbable that some -of these bosses, as already suggested, may represent -vents. They are especially prominent among the hills -due south of Auchterarder. One of these eminences, -known as the Black Maller, is composed of a typical -orthoclase-felsite without mica. Another, about four -and a half miles further south, forms the conspicuous -summit of Ben Shee overlooking Glen Devon, and -consists of a similar rock with a characteristic platy -structure.</p> - -<div class="footnote"> - -<p><a id="Footnote_355" href="#FNanchor_355" class="label">[355]</a> The central portion of the Ochils was mapped for the Geological Survey by Mr. B. N. Peach, -Prof. James Geikie, Prof. J. Young, Mr. R. L. Jack and myself.</p> - -</div> - -<p>No necks of agglomerate have been observed in -this part of the chain. It will be seen from the -section that the lowest visible parts of the Ochil -volcanic series are here truncated by a fault which -brings in the lower part of the Carboniferous system. -By a curious conjuncture, immediately on the south -side of this fault, a band of tuff appears, lying on -the platform of the Carboniferous "plateau-lavas," -to be hereafter considered, and passing below the -well-known Hurlet seam of the Carboniferous Limestone, -while through these strata rises one of the puys -belonging to the second phase of volcanic activity in -Carboniferous time in Scotland.</p> - -<p>The best sections to show the nature and sequence -<span class="pagenum" id="Page_309">- 309 -</span> -of the volcanic series of the Ochil Hills are to be -observed at the west end of the chain. But as -the whole succession of rocks cannot conveniently -be obtained along one line, it is better to make -several traverses, starting in each case from a known -horizon. In this way, by means of three parallel -sections, we may obtain the whole series of lavas -and tuffs in continuous order. The first line of -section starts in the lowest part of the tuffs -represented at the bottom of the group in <a href="#v1fig80">Fig. 80</a>, -and runs up to the first thick ashy intercalation -among the lavas. Following this bed south-westward -to the Burn of Sorrow, we make from -that horizon a second traverse across the strike -to the summit of King's Seat Hill (2111 feet -above the sea), where we meet with a well-marked -lava which can be traced south-westwards, gradually -descending the southern escarpment of the -hills until it reaches the boundary fault near the -village of Menstrie. Starting again from this -definite horizon, we take a third line across the -top of Dumyat (1373 feet) to the plain of Sheriffmuir, -and there pass beyond the volcanic series -into the overlying red sandstones. Arranged thus -in continuous vertical sequence the succession is -found to be as represented in <a href="#v1fig81">Fig. 81</a>. The total -thickness of volcanic material amounts to more -than 6500 feet.</p> - -<div style="float:right;width: 250px; padding: 6px;"> - <div class="figcenter" id="v1fig81" style="width: 123px;"> - <img src="images/v1fig81.png" width="123" height="772" alt="" /> - </div> - <div class="hanging2"><span class="smcap">Fig. 81.</span>—Diagram of the volcanic series of the Western Ochil Hills.<br /> - The bands with vertical lines are various lavas (<i>a</i>); the tuffs and volcanic breccias are shown by the dotted bands (<i>b</i>); the uppermost portion of the section above the last thick - group of lavas consists of conglomerates and sandstones (<i>c</i>) with a sheet of lava. - </div> -</div> - -<p>In this vast pile of volcanic ejections the -lavas are almost entirely andesites of the usual -characters. They include many slaggy and -amygdaloidal varieties, some beautiful porphyries -with large tabular felspars, likewise the resinous -or glassy variety already referred to as -occurring above Airthrey Castle. Their upper -and under surfaces show the same structure as -already described in those of the coast-sections in -the Montrose tract. They include also more acid -lavas, like the pale pink decomposing felsites of -the Pentland Hills.</p> - -<p>The tuffs and conglomerates occur on many -platforms throughout the succession of lava-sheets. -They form the lowest visible part of the whole -volcanic series, but they are most abundant towards -the top, and are best displayed at the western -end of the hills. In Dumyat they form a conspicuous -<span class="pagenum" id="Page_310">- 310 -</span> -feature. The whole of that hill consists of a constant alternation -of lavas (chiefly slaggy andesites, but including also one felsitic flow) with -bands of coarse and finer tuff and volcanic conglomerate. The greatest -continuous mass of this fragmental material is 600 or 700 feet thick. -From the extraordinary size of its included blocks it obviously must have -been formed of ashes, stones and huge pieces of lava ejected from some -vent in the near neighbourhood. Some of the individual blocks in this -mass are as large as a Highland crofter's cottage.</p> - -<p>The uppermost lavas of Dumyat dip under a still higher series of coarse -volcanic conglomerates entirely made up of andesitic debris and reaching a -thickness of about 1000 feet. This enormous accumulation was probably -due partly to the abrasion of exposed cones and lava-ridges, and partly to -volcanic discharges of fragmentary materials. Yet it is worthy of note that -even amidst these evidences of the most vigorous volcanic activity we have -also proofs of quiet sedimentation and traces of the fishes that lived in the -waters of the lake. This particular zone of coarse conglomerate as it -extends in a south-westerly direction becomes finer, and its upper part -passes into a chocolate-coloured sandstone which has been quarried at -Wolfe's Hole, Westerton, Bridge of Allan, at a distance of about three -miles from where the line of section runs, which is embodied in the diagram, -<a href="#v1fig81">Fig. 81</a>. It was from this locality that the specimens of <i>Eucephalaspis</i>, -<i>Pteraspis</i> and <i>Scaphaspis</i> were obtained which were described by Professor -Ray Lankester.<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> <i>Palæontographical Society</i>, vols. xxi. (1867) and xxiii. (1869).</p> - -</div> - -<p>Above the last-named thick group of coarse volcanic conglomerates a -solitary sheet of dark slaggy andesite may be observed. This lava is then -overlain by the great depth of chocolate-coloured and red sandstones and -marls of the plain of Strathmore (<i>c</i> in <a href="#v1fig81">Fig. 81</a>). Nevertheless a few -hundred feet up in these sedimentary deposits we meet with yet one -further thin sheet of lava—the last known eruption of the long volcanic -history of this district.</p> - -<p>Before quitting the Ochil range I may refer to the evidence there -obtainable as to the horizontal extent of separate sheets of lava. The -western end of this range affords great facilities for following out individual -beds of andesite along the bare terraced front of the great escarpment. -Thus, the easily recognizable porphyrite which caps King's Seat Hill, above -Tillicoultry (see <a href="#v1fig68">Fig. 68</a>), can be traced winding along the hill-slopes until -it descends to the plain, and is then lost under the great fault, at the foot -of Dumyat—a distance of more than six miles. There is, therefore, no -difficulty in supposing that from the Ochil line of vents streams of lava -should have rolled along the floor of the lake across to the base of the -Highland slopes, 10 or 12 miles distant. We cannot tell, of course, -whether any buried vents lie below the plain of Strathmore, but certainly -no unquestionable trace of vents has yet been found among the crystalline -rocks along the borders of the Highlands.<a id="FNanchor_357" href="#Footnote_357" class="fnanchor">[357]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_357" href="#FNanchor_357" class="label">[357]</a> Allusion has already been made to the possible connection of the younger Highland granites -with the volcanic series of the north-eastern part of Lake Caledonia; also to the occurrence of -isolated masses of breccia piercing the crystalline schists near Loch Lomond (<i>ante</i>, <a href="#Page_272">p. 272</a>).</p> - -</div> - -<p><span class="pagenum" id="Page_311">- 311 -</span></p> - -<p>Reference has already been made to the comparative scarcity of sills in -this region, and to the occurrence of the acid group of Lintrathen porphyry -and the more basic sheets between the Firth of Tay and Forfar. This -scarcity no doubt arises in part from the extent to which the rocks that -underlie the volcanic series are concealed. Yet it is noteworthy that along -the coast-section of these rocks near Stonehaven hardly any intrusive sheets -are to be seen.</p> - - -<h4>3. <i>The Arran and Cantyre Centre</i></h4> - -<p>It is unfortunate that the Ochil chain should be broken across and -buried under younger formations at the very place where some of the most -interesting vents in the whole area of the Old Red Sandstone might have -been looked for.<a id="FNanchor_358" href="#Footnote_358" class="fnanchor">[358]</a> We have to pass westwards across the Firth of Clyde to -the Isle of Arran before we again meet with rocks of the same age and -character.</p> - -<div class="footnote"> - -<p><a id="Footnote_358" href="#FNanchor_358" class="label">[358]</a> The Ochil area is not the only example of the abrupt termination of a volcanic band near -its centre owing to faults or overlaps. The sudden disappearance of the Pentland lavas and tuffs -on the northern side of the Braid Hills is another striking illustration.</p> - -</div> - -<p>In the course of the recent work of the Geological Survey in that -island, Mr. W. Gunn has discovered that the Lower Old Red Sandstone -includes some interstratified volcanic rocks on the north side of North Glen -Sannox, and he has supplied me with the following notes regarding them. -"The area in which the volcanic intercalations occur is much faulted and -only a part of it has been mapped in detail, but the position of the interbedded -igneous rocks is quite clear. The Old Red Sandstone here consists -of three distinct members, the lowest of which is made up of coarse, well-rounded -conglomerates, alternating with sandstones and purple mudstones. -Above this, and apparently unconformable to it, is a middle series of light -coloured conglomerates and sandstones, the pebbles in which are mainly of -quartz. Finally comes an upper series of red sandstones and conglomerates, -which occupy nearly the whole of the coast section, and it is this series which -has generally been taken as the typical Old Red Sandstone of the island. -The volcanic series is intercalated between the middle and upper divisions -given above, and may be seen in several places on the hillside between the -shepherd's house at North Sannox and Laggan. It consists mainly of old -lava-beds of a dull reddish or purplish colour, often soft, and in places much -decomposed. It seems basic in character. A specimen from near the -Fallen Rocks, examined by Mr. Teall, was found to be too much altered for -precise determination, but was probably a basalt originally. These rocks do -not occur on the coast."</p> - -<p>In the southern extremity of Cantyre some important relics of the -volcanic rocks of the Lower Old Red Sandstone have been recently -detected and mapped for the Geological Survey by Mr. R. G. Symes.<a id="FNanchor_359" href="#Footnote_359" class="fnanchor">[359]</a> -<span class="pagenum" id="Page_312">- 312 -</span> -This division of the system has been ascertained by him to be extensively -developed to the south of Campbeltown, and to include some small but -interesting remains of the volcanic action which was so marked a feature in -the areas of Lake Caledonia, lying further to the east. To the student of -volcanic geology, indeed, this small tract at the extreme southern end of -Argyllshire has a peculiar interest, for in no other part of the British Isles -have the phenomena of the eruptive vents of the Lower Old Red Sandstone -been more admirably laid bare. Not only are there necks in the interior -like that represented in <a href="#v1fig82">Fig. 82</a>; but others have been dissected by the -waves along the southern shore, and their relations to the deposits of -fragmentary material showered over the bottom of the lake have been -more or less clearly exposed.</p> - -<div class="footnote"> - -<p><a id="Footnote_359" href="#FNanchor_359" class="label">[359]</a> The late Prof. James Nicol published in 1852 an account of the geology of the southern -portion of Cantyre. He grouped all the igneous rocks of the district as one series, which he -regarded as later than the Coal-formation and possibly of the same age as those of the north-east -of Ireland. He made no distinction between the Lower Old Red Sandstone and the younger -unconformable conglomerates (<i>Quart. Journ. Geol. Soc.</i> vol. viii. (1852), p. 406).</p> - -</div> - -<div class="figcenter" id="v1fig82" style="width: 517px;"> - <img src="images/v1fig82.png" width="517" height="329" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 82.</span>—View of Cnoc Garbh, Southend, Campbeltown. A volcanic neck of Lower Old Red Sandstone - age, about 400 yards wide in its longer diameter.</div> -</div> - -<p>At Keil Point, a little to the east of the most southerly headland of -the Mull of Cantyre, some reddish and purplish highly felspathic sandstones -(<i>a</i> in <a href="#v1fig83">Fig. 83</a>) dipping towards the east are found to pass upward into -coarse volcanic breccias (<i>b</i>), which, followed eastwards, lose almost all trace -of stratification, and are then abruptly succeeded by a neck of coarse -agglomerate (<i>c</i>) measuring 25 yards from north to south, where its limits -can be seen, and at least 12 yards from west to east. It is hardly possible -to distinguish between the breccias to the west and the agglomerate of the -<span class="pagenum" id="Page_313">- 313 -</span> -neck, except by the rude bedding of the former which pass down into the -well-bedded sandstones.</p> - -<p>The agglomerate is a thoroughly volcanic rock. The materials consist -chiefly of angular blocks of a pale purplish or lilac highly porphyritic mica-porphyrite, -with large white felspars and hexagonal tables of black mica. -These blocks might sometimes be mistaken for slags from their cavernous, -weathered surfaces, but this rough aspect is found on examination to be -due to the decay of their felspars.</p> - -<div class="figcenter" id="v1fig83" style="width: 550px;"> - <img src="images/v1fig83.png" width="355" height="83" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 83.</span>—Section of volcanic series on beach, Southend, Campbeltown.<br /> - <i>a</i>, Fine reddish and purplish highly felspathic sandstones, largely composed of porphyry-debris and passing up into - coarse breccias; <i>b</i>, volcanic breccias, coarse and only rudely stratified, formed of blocks of porphyry, sandstone - fine tuff and andesite, together with water-worn quartzite pebbles derived from some conglomerate; - <i>c</i>, coarse unstratified agglomerate forming a neck.</div> -</div> - -<p>Perhaps the most singular feature among the contents of this neck is -the number of well-rounded and smoothed pebbles and boulders of quartzite. -These are dispersed at random through the mass, and are often placed -on end. There can be no doubt that they are water-worn stones, but the -contrast of their smooth surfaces and rounded forms with the rough angular -blocks of igneous material is so striking as to lead at once to the conclusion -that they cannot have acquired their water-worn character in the deposit -where they now lie. Their positions and their occurrence with ejected -volcanic blocks suggest that they too were discharged by volcanic explosions. -They so exactly resemble the quartzite boulders and pebbles in -the neighbouring Old Red Conglomerates that there can be little hesitation -in regarding them as derived from these conglomerates. They seem to me -to have come from a lower part of the Old Red Sandstone, which was -shattered by volcanic energy either before the conglomerates were firmly -consolidated or afterwards by such violent explosions as served to separate -the pebbles from the matrix of the rock.</p> - -<p>There occur also in the agglomerate blocks of fine tuff and ashy sandstone -sometimes four feet long, and often stuck on end, showing that the deposits -of earlier eruptions were broken up during the drilling of this little vent.</p> - -<p>A few hundred yards further east a larger neck rises on the beach, -immediately to the south of the old Celtic chapel of St. Columba. It consists -also of exceedingly coarse agglomerate, with andesite blocks three and -four yards in diameter. It is about 125 yards broad from east to west, -on which sides it is seen to be flanked by coarse volcanic breccias and -conglomerates, resembling in composition the materials of the neck, but -showing an increasingly definite stratification as they are traced eastward -in the ascending succession of deposits. Following the section in still -the same easterly direction along the coast, we find that bands of fine -felspathic sandstone, marking probably intervals of quiescence, are again -<span class="pagenum" id="Page_314">- 314 -</span> -and again succeeded by coarse brecciated conglomerates of igneous materials, -which may be inferred to have been due to a renewal of violent eruptions. -By degrees the evidence of stratification and of attrition among the -volcanic materials becomes more pronounced as the ascending section is -followed; blocks of andesite, even 18 inches or two feet in diameter, assume -well-rolled, rounded, water-worn forms, like the pebbles of quartzite associated -with them, and eventually the strata return to the usual aspect of the -conglomerates of the district.</p> - -<p>I have never seen anywhere better proofs of volcanic explosions, -contemporaneous with a group of strata, and of the distribution of volcanic -fragmentary material round the vents. A further point of much interest is -the additional evidence furnished by this shore-section of considerable wave-action -during the accumulation of the coarse conglomerates. To give to -blocks of porphyrite two feet in diameter a smoothed and rounded form -must have required the action of water in considerable agitation.</p> - - -<h4>4. <i>The Ulster Centres</i></h4> - -<p>From the volcanic breccias and conglomerates of the Mull of Cantyre to -the coast of Antrim in a straight line is a distance of little more than -twenty miles. On a clear day the Old Red Sandstone of Cross Slieve, and -the range of cliffs in which it abruptly descends to the sea between Cushendall -and Cushendun, can be distinctly seen from the Argyllshire shore. The -geologist who passes from the Scottish to the Irish sections cannot fail to -be impressed with the resemblance of the rocks in the two countries, and -with the persistence of the types of conglomerate in Lake Caledonia.</p> - -<p>A picturesque section has been laid bare between the Coastguard -Station south of Cushendall and Cushendun Bay.<a id="FNanchor_360" href="#Footnote_360" class="fnanchor">[360]</a> At the south side of the -little inlet of Cushendall, a compact dull quartz-porphyry is exposed in crags -along the shore. This rock ranges in colour from dark brown and purple to -pale-green and buff. Its texture also varies, as well as the proportion of its -felspar-crystals and quartz-blebs. Some parts have a cavernous structure, -like that of an amygdaloid, the small globular cavities being filled with -green decomposition products.</p> - -<div class="footnote"> - -<p><a id="Footnote_360" href="#FNanchor_360" class="label">[360]</a> For descriptions of this district see J. Bryce, <i>Proc. Geol. Soc.</i> i. (1834) p. 396, v. (1837) p. -69; J. Kelly, <i>Proc. Roy. Irish Acad.</i> x. (1868), p. 239. The area is contained in Sheet 14 of the -Geological Survey of Ireland, and was mapped by Mr. A. M'Henry and described by him in the -accompanying Explanatory Memoir (1886), pp. 12, 25.</p> - -</div> - -<p>The stratigraphical relations of this rock are not quite clear, but it is -certainly older than the Old Red conglomerates which lie to the north of it, -for these are largely made up of its fragments. The matrix of these detrital -masses consists mainly of the comminuted debris of the porphyry. The -pebbles include all the varieties of that rock, and are tolerably well-rounded. -There is no distinct evidence of volcanic action among these -conglomerates. They resemble, however, many of the conglomerates in the -Midland Valley of Scotland, which, as in the case of those on the Forfarshire -and Kincardineshire coast, are in great part made of the detritus -<span class="pagenum" id="Page_315">- 315 -</span> -of andesitic lavas. The Cushendall rocks become coarser as they are traced -northwards into lower members of the series, while at the same time the -proportion of porphyry-debris in their constitution diminishes, and materials -from the metamorphic series take its place. Thus at Cushendun the percentage -of quartz-pebbles rises to 70 or 80. These blocks, of all sizes up -to two feet or more in diameter, are admirably rounded and smoothed, like -those in the Stonehaven section and those among the conglomerates at the -south end of Cantyre. Fragments of the porphyry, however, still continue to -appear, and the matrix shows an admixture of the finer detritus of that rock. -I may remark in passing that no conglomerates of the Old Red Sandstone -show more strikingly than these at Cushendun the effects of mechanical crushing -subsequent to deposition and consolidation. In many parts of the rock it -is hardly possible to find a rounded block that has not been fractured. Some -of them, indeed, may be seen cut into half a dozen slices, which have been -pushed over each other under the strain of strong lateral or vertical pressure.</p> - -<p>In the interior of the country, after passing over the broad Tertiary -basaltic plateau of Antrim, we come upon a large area of Lower Old Red -Sandstone in Tyrone. It stretches from Pomeroy to Loch Erne, a distance -of about 30 miles, and is about 12 miles broad. In lithological character -the strata of this tract exactly resemble parts of the deposits of Lake -Caledonia in Central Scotland. They include also a volcanic series which, -down to the smallest points of detail, may be paralleled in the sister island.<a id="FNanchor_361" href="#Footnote_361" class="fnanchor">[361]</a> -This interesting westward prolongation of the volcanic record consists of -a number of outlying patches confined to the eastern part of the district.</p> - -<div class="footnote"> - -<p><a id="Footnote_361" href="#FNanchor_361" class="label">[361]</a> This area of Old Red Sandstone is represented on Sheets 33, 34, 45 and 46 of the Geological -Survey of Ireland, and the igneous rocks are described in the Memoirs on Sheets 33 (1886, p. 17) -by Mr. J. R. Kilroe, and 34 (1878, p. 16) by Mr. J. Nolan.</p> - -</div> - -<p>The largest of these patches lies to the south of Pomeroy, where it forms -a line of hills about four miles long, and covers an area of some five square -miles. The rocks consist of successive sheets of andesite-lavas. These, as -a rule, are not markedly cellular, though they include some characteristic -amygdaloids. A distinguishing feature of some of the sheets is their remarkably -well-developed flow-structure. Thus on Sentry Box, at the north-western -end of the ridge, the fissility resulting from this structure so perfectly divides -the rock into parallel flags that the material might easily be mistaken for a -bedded rock. Where this structure has been produced in a cellular lava, -the cavities have been drawn out and flattened in the direction of flow.</p> - -<p>I have not observed true tuffs in any of the sections traversed by me -in this district. But the conglomerates furnish abundant evidence of the -contemporaneous outpouring of the lavas. Thus, in a brook a little west of -Reclain, five miles south of Pomeroy, the section shown in <a href="#v1fig84">Fig. 84</a> may be -seen. At the base lies a coarse conglomerate (<i>a</i>) largely composed of -andesite-debris, the stones being here, as elsewhere in the district, well -rounded. Then comes a series of green and reddish highly-felspathic sandstones -(<i>b</i>), followed by an exceedingly coarse conglomerate (<i>c</i>), formed mainly -of the debris of andesites, especially lumps of slag. Some of the stones -<span class="pagenum" id="Page_316">- 316 -</span> -measure 18 inches in diameter, and all are well water-worn. Immediately -over this mass of detritus lies the lowest sheet of andesite-lava (<i>d</i>).</p> - -<div class="figcenter" id="v1fig84" style="width: 423px;"> - <img src="images/v1fig84.png" width="423" height="66" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 84.</span>—Section of the base of the volcanic series, Reclain, five miles south of Pomeroy.</div> -</div> - -<p>Some sections visible in the neighbourhood of Omagh afford further -evidence of volcanic action at the time of the deposition of the Old Red -Sandstone of this region. At Farm Hill, a little to the east of the town, -felspathic sandstones and breccias enclose angular and subangular pieces of -various andesites, and occasionally even pieces of tuff. Near these strata a -decayed andesite occurs in the bed of a stream, and a fresher variety is -quarried at Farm Hill. A little further south another variety of andesite -is exposed in two quarries at Recarson Meeting-House—a fine granular -purplish-grey rock, with abundantly-diffused hæmatite -pseudomorphs, probably after a pyroxene, and sometimes -strongly amygdaloidal.</p> - -<div id="v1fig85" style="float:right;width: 350px; padding: 6px;"> - <div class="tdc"> - <img src="images/v1fig85.png" width="110" height="240" alt="" /> - </div> - <div class="hanging2"><span class="smcap">Fig. 85.</span> Section of - shales and breccias at Crossna Chapel, north-east of Boyle.<br /> - <i>a</i> <i>a</i>, Green and grey shales; <i>b</i> <i>b</i>, green - and grey hard sandstones and grits, some bands strongly felspathic; - <i>c</i>, fine compact felspathic breccia, with angular chips of - different felsites and andesites, etc.</div> -</div> - -<p>There can thus be no doubt that this region of Ulster -included several centres of volcanic activity during the -deposition of the red sandstones and conglomerates, and -that the lavas and volcanic conglomerates belonged to -precisely the same types as those of the same geological -age which occur so abundantly in Scotland.</p> - -<p>Further south-west, near Boyle, in the county of -Roscommon, certain curious felspathic breccias in the Old -Red Sandstone have been mapped as "felstone."<a id="FNanchor_362" href="#Footnote_362" class="fnanchor">[362]</a> So far -as I have been able to examine them, however, they are -entirely of fragmental origin. They contain pieces of -andesitic and felsitic rocks, with fragments of devitrified -glass, which undoubtedly point to the occurrence of -volcanic eruptions during their deposition, though no -tuffs and lavas appear to crop out in the narrow strip of -the formation there exposed.</p> - -<div class="footnote"> - -<p><a id="Footnote_362" href="#FNanchor_362" class="label">[362]</a> See Sheet 66 Geological Survey of Ireland, and Explanation to that sheet (1878), p. 15. The -rocks were previously described by Jukes and Foot, <i>Journ. Roy. Geol. Soc. Ireland</i>, vol. i. (1866), -p. 249.</p> - -</div> - -<p>The accompanying section (<a href="#v1fig85">Fig. 85</a>) may be seen on -the hills to the north-east of Boyle. Where quarried -on the road-side to the north of Boyle, the series -of deposits here represented contains a bed of coarse -and exceedingly compact breccia, similar to that just -referred to, but containing angular and subangular fragments six or eight -inches long. The joints of these compact strata are remarkably sharp and -clean cut, so that where the fragmentary character is not very distinct -the rocks might easily be mistaken on casual inspection for felsites.</p> - -<hr class="chap x-ebookmaker-drop" /> - - -<div class="chapter"> -<p><span class="pagenum" id="Page_317">- 317 -</span></p> - -<h2 class="nobreak" id="CHAPTER_XX">CHAPTER XX<br /> - -<span class="smaller">VOLCANOES OF THE LOWER OLD RED SANDSTONE OF "LAKE -CALEDONIA"—<i>continued</i></span></h2> -</div> - -<div class="blockquot"> - -<p>The Southern Chain—The Pentland Volcano—The Biggar Centre—The Duneaton -Centre—The Ayrshire Volcanoes.</p> -</div> - - -<p>We have now to note the leading features of the groups of volcanic rocks -distributed along the southern line of vents already described. At least -four different centres of eruption may be observed on that line. Their -mutual limits are, on the whole, better seen than those of the northern line, -for from the north-eastern to the south-western end of the volcanic belt the -Old Red Sandstone and rocks of older date are almost continuously exposed -at the surface. The encroaching areas of Carboniferous formations in -Lanarkshire and Ayrshire interrupt but do not entirely conceal the volcanic -tracts.</p> - - -<h3>II. THE SOUTHERN CHAIN OF VOLCANOES IN "LAKE CALEDONIA"</h3> - - -<h4>5. <i>The Pentland Volcano</i></h4> - -<p>Beginning at the north-east end of the line we first come upon the -classic area of the Pentland Hills, for the study of which the geologist is -prepared by the admirable description of Charles Maclaren,<a id="FNanchor_363" href="#Footnote_363" class="fnanchor">[363]</a> and the earlier -geognostical papers of Jameson.<a id="FNanchor_364" href="#Footnote_364" class="fnanchor">[364]</a> The area mapped in detail is represented -in Sheet 32 of the Geological Survey of Scotland, published in 1859, and -described in the Memoir accompanying that sheet.</p> - -<div class="footnote"> - -<p><a id="Footnote_363" href="#FNanchor_363" class="label">[363]</a> <i>A Sketch of the Geology of Fife and the Lothians</i>, 1839. The detailed descriptions in this -work are accompanied with a map and two plates of sections. In the map all the volcanic rocks -are represented by one colour. In the sections the bedding of the rocks is shown, and an -indication is given of the succession of their chief varieties.</p> - -<p><a id="Footnote_364" href="#FNanchor_364" class="label">[364]</a> See specially <i>Mem. Wernerian Soc.</i> vol. ii.; also MacKnight in vol. i. The account of the -Pentland Hills by Hay Cunningham in vol. vii. (1838) is clear but brief.</p> - -</div> - -<p>When in these early days I surveyed this ground I found it extremely -difficult to understand. Being then myself but a beginner in geology, and -the study of old volcanic rocks not having yet advanced much beyond its -elementary stage, I failed to disentangle the puzzle. Not until after more -than twenty years, largely spent in the investigation of volcanic rocks elsewhere, -had I an opportunity of resurveying the ground and bringing to its -<span class="pagenum" id="Page_318">- 318 -</span> -renewed study a wider knowledge of the subject. A new edition of the -map was issued in 1892, and I shall here embody in my summary the -chief results obtained in the course of this revision.</p> - -<p>The most obvious features in the Pentland area are the marked development -of the volcanic rocks at the north end of the chain, their rapid -diminution and disappearance towards the south-west, the abrupt truncation -of the bedded masses by the line of craggy declivity which forms the -northern termination of the hills, and lastly, the continuation of the volcanic -series northward in a totally different form in the lower eminences of the -Braid Hills.</p> - -<p>The length of the whole volcanic tract is about eleven miles; its breadth -at the widest northern part is four miles, but from that maximum it dwindles -southwards and dies out in seven miles. Its western side is in large measure -flanked by the unconformable overlap of the Upper Old Red Sandstone and -Lower Carboniferous formations, though in some places the base of the -volcanic series is seen. The eastern boundary is chiefly formed by a large -fault which brings down the Carboniferous rocks against the volcanic ridge. -At the northern end, this ridge plunges unconformably under the Upper Old -Red Sandstone of the southern outskirts of Edinburgh.</p> - -<p>The bedded aspect of the truncated end of the Pentland chain, as seen -from the north, has been already alluded to (<a href="#Page_281">p. 281</a>). The rocks dip to the -south-east, hence the lower members of the series are to be found along the -north-west side of the hills.</p> - -<div class="figcenter" id="v1fig86" style="width: 519px;"> - <img src="images/v1fig86.png" width="519" height="94" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 86.</span>—Section across the north end of the Pentland Hills, from Warklaw Hill to Pentland Mains. - Length about five miles.<br /> - 1. Upper Silurian grits and shales, not seen where the line of section crosses; 2 2. Andesites and diabases in - numerous interstratified sheets; 2 <i>s</i>. Intercalated sandstones and conglomerates; 3. Felsitic tuffs and - breccias and orthophyre sheets; <i>n</i>, Volcanic neck; 4. Lower Carboniferous strata lying unconformably on and - overlapping the volcanic series; 5. Calciferous Sandstones and Carboniferous Limestone series brought down - against the volcanic series by a fault (<i>f</i>).</div> -</div> - -<p>It will be noticed from the Geological Survey map that the volcanic -rocks of the main body of the Pentland Hills are arranged in alternations of -somewhat basic and more acid bands. The most basic sheets are some -amygdaloidal diabases at the bottom of the whole series which make their -appearance in Warklaw Hill (<a href="#v1fig86">Fig. 86</a>). The greater number of the dark -lavas are varieties of andesite, sometimes tolerably compact, sometimes -highly cellular and amygdaloidal. But interstratified with these are thick -sheets of what used to be called "claystone," a term which here comprised -decayed felsites (orthophyres), and also felsitic tuffs and breccias. The remarkably -acid nature of some of these rocks has been already pointed out.</p> - -<p>The total thickness of the volcanic series at the north end of the hills is -about 7000 feet, but as neither the top nor the bottom is there visible, it -<span class="pagenum" id="Page_319">- 319 -</span> -may be considerably greater. At these maximum dimensions the rocks form -the high scarped front of the Pentland Hills, which rises into so prominent -a feature in the southern landscape of Edinburgh. A series of transverse -sections across the chain from north to south will illustrate its structure -and history. These I shall here describe, reserving for subsequent consideration -the great vent of the Braid Hills.</p> - -<p>A section taken through the north end of the chain, where the maximum -depth of volcanic material is exposed, presents the arrangement -represented in <a href="#v1fig86">Fig. 86</a>. It will be seen that the base of the series is here -concealed by the unconformable overlap of the Lower Carboniferous rocks -on the west side, while the top is cut off by the great fault which on the -east side brings down the Midlothian Coal-field.</p> - -<div class="figcenter" id="v1fig87" style="width: 507px;"> - <img src="images/v1fig87.png" width="507" height="244" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 87.</span>—View of the lava-escarpments of Warklaw Hill, Pentland chain, from the north-west.</div> -</div> - -<p>The Lower Carboniferous conglomerates (4) creep over the edge and up the -slopes of the volcanic series of the Pentland Hills. They contain abundant -pebbles of the lavas, and were evidently laid down along a shore from which -the Pentland rocks rose steeply into land. Though the actual base of the -lavas is not seen here, two miles further to the south highly-inclined -Upper Silurian shales and mudstones are found emerging unconformably -from under the volcanic pile, and similar strata probably underlie Warklaw -Hill as indicated in the figure. The Upper Silurian strata pass up into a -lower group of the Lower Old Red Sandstone, which has also been covered -unconformably by the volcanic series. In these underlying deposits we -have evidence of the pre-volcanic accumulations of the lake, which were -broken up and tilted at the beginning of the volcanic eruptions.</p> - -<p>The lowest lavas, consisting of well-marked beds of diabase (2), present -their escarpments to the north-west and dip into the rising ground, as -sketched in <a href="#v1fig87">Fig. 87</a>. Their characters have been already noticed in the -general petrography of the Old Red Sandstone volcanic rocks. Dark solid -compact portions of them pass rapidly into coarsely cellular slag, especially -<span class="pagenum" id="Page_320">- 320 -</span> -along the upper and under parts of the several sheets. No tuff has been -noticed between these basic flows, but here and there thin lenticular layers -of sandstone, lying in hollows of the lava-sheets, are connected with vertical -or highly-inclined ramifying veins of similar material, with the plains of -stratification passing across the breadth of the veins. These features are an -exact reproduction of those above described in Forfarshire and Kincardineshire. -The amygdales consist of chalcedony, crystallized quartz and calcite.</p> - -<p>Torduff Hill, which rises to the east of Warklaw, consists of a mass of -coarse volcanic breccia or agglomerate (<i>n</i>), markedly felsitic in its materials. -It probably forms a neck marking a small volcanic vent, like some others -at the north end of the chain to be afterwards referred to.</p> - -<p>In the lower part of Capelaw Hill, the next eminence in an easterly -direction, bedded andesites, with an intercalated band of sandstone and -conglomerate (2<i>s</i>), appear and pass under rocks of so decomposing a kind -that no good sections of them are to be found. The hill is covered with -grass, but among the rubbish of the screes pieces of felsite-like rocks and -breccias may be observed. Some of these blocks show an alternation of -layers of felsitic breccia with a fine felsite-like material which may be a -tuff. These rocks, conspicuous by the light colours of their screes, alternate -further up with other dark andesitic lavas, and run south-westward for -about five miles.</p> - -<p>Beyond Capelaw Hill, upon a band of these pale rocks, comes a thick -group of sheets of dark andesite, which form the main mass of Allermuir -Hill. They are well seen from the south side and likewise from the north, -dipping towards the south-east at angles of from 35° to 40°, and weathering -along the crest of the hills into a succession of scars and slopes which show -the bedded character of the lavas.</p> - -<p>At Caerketton Hill another band of pale material forms the conspicuous -craggy face so familiar in the aspect of the Pentland Hills as seen from -Edinburgh. This band consists of pale felsitic breccia, and amorphous, -compact, much-decayed rock, regarding which it is difficult to decide -whether it should be considered as a fine felsitic tuff, or as a decomposed -felsite. The band is better seen when traced southwards. The light colour -of its screes makes it easily followed by the eye even from a distance along -the hill-tops and declivities.</p> - -<p>On the next hill to the south-west, known as Castlelaw Hill, this pale -band of rock is exposed in a few crags and quarries, and its debris, protruding -through the scanty herbage, slips down the slopes. On its north -side the screes display the same felsitic breccias and compact, decayed felsitic -rocks, occasionally showing a structure like the flow-structure of rhyolite. -The breccia which projects in blocks from the summit of the hill has been -quarried immediately below the crest on the south side, where it overlies -a thin intercalated band of a dull, much-decomposed porphyry.</p> - -<p>The breccias are composed almost entirely of thoroughly acid rock-fragments, -as may be judged from the percentage of silica shown to occur -in them. These fragments vary from the finest lapilli up to angular pieces -<span class="pagenum" id="Page_321">- 321 -</span> -several inches long. They not infrequently display a fine and extremely -beautiful flow-structure. It is thus quite certain that there are acid -breccias intercalated among the more basic lavas of the northern Pentlands, -and that among the constituents of these breccias are fragments of felsite -or perhaps even lithoid rhyolite.</p> - -<p>We may therefore be prepared to find that actual outflows of felsitic -lava accompanied the discharge of these highly-siliceous tuffs. Unfortunately -the manner in which the rocks decay and conceal themselves under -their own debris makes it difficult to separate the undoubtedly fragmental -bands from those which may be true lavas. But an occasional opening, -and here and there a scattered loose block, serve to indicate that the two -groups of rock certainly do coexist in this pale band, which can be followed -through the chain for upwards of six miles until it is cut off by the eastern -boundary fault.</p> - -<p>At the south-west end of Castlelaw Hill, where a quarry has been -opened above the Kirk Burn, blocks of felsite may be observed showing -flow-structure on a large scale. The bands of varied devitrification are sometimes -a quarter of an inch broad, and weather out in lighter and darker -tints. Some of them have retained their felsitic texture better than others, -which have become more thoroughly kaolinized. That these are not deceptive -layers of different texture in fine tuffs is made quite clear by some -characteristic rhyolitic structures. The bands are not quite parallel, but, -on the contrary, are developed lenticularly, and may be observed to be -occasionally puckered, and to be even bent back and folded over as in -ordinary rhyolites. There is no contortion to be observed among the -stratified tuffs of the hills. This irregularity in the layers is obviously -original, and can only be due to the flow of a moving lava.</p> - -<p>On the east side of Castlelaw Hill, as shown in <a href="#v1fig86">Fig. 86</a>, dull reddish -andesites overlie the pale belt of felsitic rocks. Their lower bands are -marked by the presence of well-formed crystals of a dark green mica. Their -central and higher portions consist of porphyrites of the prevalent type, -both compact and vesicular. These lavas continue as far as any rock can -be seen. Beyond the boundary fault, the Burdiehouse Limestone and oil-shales -of the Lower Carboniferous series are met with, inclined at high -angles against the hills. It is impossible to say how much of the volcanic -series has here been removed from sight by the dislocation.</p> - -<p>If now we move three miles further to the south-west and take a -second section across the Pentland Hills, it will be found to expose the -arrangement of rocks represented in <a href="#v1fig88">Fig. 88</a>. At the western end the -Upper Old Red Sandstones (4) and Lower Carboniferous series (5) are seen -lying unconformably on the upturned edges of the Upper Silurian shales (1). -North Black Hill consists of a large intrusive sheet of pale felsite (F) that -has broken through the Silurian strata and has in places thrust itself between -them and the conglomerates of the Lower Old Red Sandstone which lie -unconformably upon them. In the neighbouring Logan Burn, at the bottom -of the Habbie's Howe Waterfall, the felsite can be seen injected into the conglomerate. -<span class="pagenum" id="Page_322">- 322 -</span> -The felsitic sill of North Black Hill runs for a mile and a half -along the western base of the volcanic series, and has a breadth of about -half a mile. It is the only important intrusive mass in the Pentland Hills.</p> - -<div class="figcenter" id="v1fig88" style="width: 500px;"> - <img src="images/v1fig88.png" width="500" height="116" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 88.</span>—Section across the Pentland Hills through North Black Hill and Scald Law (length about - three miles).</div> -</div> - -<p>To the south of the Silurian shales that lie against the southern flank -of North Black Hill, pale felsitic tuffs (3) occur, which are a continuation of -those already referred to as running southwards from Capelaw Hill. Above -them a series of andesites (2), with intercalated bands of tuff, sandstone -and conglomerate (2<i>s</i>), occupy the bottom of the Logan valley and part -of the slopes on both sides. In the thickest band of tuffs, which is well-exposed -along the road by the side of the Loganlee Reservoir, a group of -well-bedded strata occurs from less than an inch to a foot or more in thickness. -Generally they are pale in colour, and are made up of white felsitic -detritus, but with a sprinkling of dull purplish-red fragments, and occasional -larger rounded pieces of different andesites. Some of the rocks might be -called felspathic sandstones. Other bands in the group are dark purplish-red -in tint, and consist mainly of andesitic debris, with a dusting of white -felsitic grains and fragments. There would thus seem to have been showers -both of felsitic and of andesitic ashes and lapilli.</p> - -<p>The dark lavas that overlie the tuffs are likewise well displayed along -the same road-section. They vary rapidly from extremely compact homogeneous -dark blue rocks, that weather with a greenish crust, to coarse, -slaggy masses and amygdaloids.</p> - -<div class="figcenter" id="v1fig89" style="width: 471px;"> - <img src="images/v1fig89.png" width="471" height="92" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 89.</span>—Section from the valley of the Gutterford Burn through Green Law and Braid Law to - Eight-Mile Burn.</div> -</div> - -<p>These more basic lavas are a continuation of those of Allermuir Hill, -and, as at that locality, they plunge here also under the same band of white -tuffs, breccias and felsites (3), which has been referred to as stretching -southward from Caerketton Crags. This band must here be at least 500 feet -thick. It forms Scald Law (1898 feet) and the surrounding summits, and -thus occupies the highest elevations in the Pentland chain. It dips beneath -<span class="pagenum" id="Page_323">- 323 -</span> -the uppermost group of andesites, which, as before, are here truncated by -the eastern fault (<i>f</i>), the Calciferous Sandstones and Carboniferous Limestone -series (6) being thrown against them.</p> - -<p>A third section (<a href="#v1fig89">Fig. 89</a>), taken two miles still further south, shows a -remarkable attenuation of the volcanic series, and the appearance of a thick -group of conglomerates (2) lying conformably below that series, but resting -on the upturned edges of the upper Silurian shales (1). The thick Allermuir -porphyrites are here reduced to a few thin beds (3) intercalated among -the conglomerates and sandstones, amidst which the whole volcanic series -dies out southward. A detailed section of the rocks exposed on the western -front of Braid Law shows the following succession:—</p> - -<div class="blockquot"> - -<p>White felsitic rocks of Braid Law (4 in <a href="#v1fig89">Fig. 89</a>).</p> - -<p>Coarse conglomerate passing down into sandstone. About 20 feet visible.</p> - -<p>Dark andesite, 4 feet.</p> - -<p>Parting of yellow felspathic grit, 8 or 10 inches.</p> - -<p>Andesite, 10 feet.</p> - -<p>Hard felspathic grit, 6 feet.</p> - -<p>Dark green amygdaloidal andesite, 2 feet.</p> - -<p>Yellow felspathic sandstone and grit, 2 feet.</p> - -<p>Dark green amygdaloidal andesite, 6 feet.</p> - -<p>Felspathic grit and red and brown sandstone, 4 feet.</p> - -<p>Dark andesite, perhaps 6 or 8 feet.</p> - -<p>Great conglomerate with alternating courses of sandstone, rapidly increasing in -thickness southwards.</p> -</div> - -<p>Above these dwindling representatives of the northern andesitic lavas -comes the continuation of the white band of tuffs and breccias of Caerketton -and Scald Law (4), which in turn dips under the highest group of -andesites. The Carboniferous strata (5) are brought in by the fault (<i>f</i>). -In little more than two miles beyond this line of section the volcanic -series disappears, and the Old Red Sandstone for a brief space consists only -of sedimentary deposits.</p> - -<p>Besides the remarkable alternation of basic and acid ejections, there is a -further notable feature in the geology of the Pentland Hills. This volcanic -centre presents us with one of the most remarkable vents anywhere to be -seen among the volcanic rocks of Britain. The full significance of this -feature may best be perceived if we advance along the hills from their -south-western end. As has now been made clear, the volcanic materials -which begin about the line of the North Esk near Carlops rapidly augment -in thickness until, in a distance of not more than seven miles, they attain a -thickness of about 7000 feet, and then form the great scarped front of the -hills that look over Edinburgh. But at the base of that wall their continuity -abruptly ceases. The lower ground, which extends thence to the -southern suburbs of Edinburgh, and includes the group of the Braid Hills, is -occupied by another and more complex group of rocks in which the parallelism -and persistence so marked in the Pentland chain entirely disappear.</p> - -<p>This abrupt truncation of the bedded lavas and tuffs marks approximately -the southern margin of a large vent from which at least some, if not most, -<span class="pagenum" id="Page_324">- 324 -</span> -of these rocks were probably ejected. The size of this vent cannot be precisely -ascertained on account of the unconformable overspread of Lower -Carboniferous strata. But that it must have been a large and important -volcanic orifice may be inferred from the fact that the visible area of the -materials that fill it up measures two miles from north-east to south-west, -and a mile and a half from south-east to north-west, thus including a space -of rather more than two square miles. Its original limits towards the north -and south can be traced by help of the bedded lavas that partially surround -it, but on the two other sides they are concealed by the younger formations. -We shall probably not over-estimate the original area of the vent if we state -it at about four square miles.</p> - -<div class="figcenter" id="v1fig90" style="width: 452px;"> - <img src="images/v1fig90.png" width="452" height="114" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 90.</span>—Section across the north end of the Pentland Hills, and the southern edge of the Braid Hill - vent. Length about two miles.<br /> - 1 1. Andesites; 2. Fine tuffs, etc., of the Braid Hill vent; 3 3 3. Agglomerate in lateral necks with - felsitic intrusions (4).</div> -</div> - -<p>The materials that now fill this important orifice consist mainly of "claystones," -like those of the Pentland series—dull rocks, meagre to the touch, -varying in texture from the rough porous aspect of a sinter through stages -of increasing firmness till they become almost felsitic, and ranging in colour -from a dark purple-red, through shades of lilac and yellow, to nearly white, -but often strikingly mottled. A more or less laminar structure is often to -be observed among them, indicating a dip in various directions (but especially -towards the north) and at considerable angles. Throughout this exceedingly -fine-grained material, lines of small lapilli may occasionally be detected, also -bands of breccia, consisting of broken-up tuff of the same character, and -of fine "hornstone" and felsite, with delicate flow-structure. Exhibiting -on the whole so little structure, this tract may be regarded as consisting -largely of fine volcanic dust derived from the explosion of felsitic or orthophyric -lavas. Some portions indeed are not improbably composed of -decayed felsites, like those which present so many difficulties to the geologist -who would try to trace their course among the other lavas and tuffs of the -Pentland chain. Various veins, dykes and small bosses of felsite, andesite -and even more basic material, such as fine dolerite, have been intruded into -the general body of the mass.</p> - -<p>On the outskirts of the main vent some subordinate necks may be -observed (3, 3 in <a href="#v1fig90">Fig. 90</a>), perhaps, like Torduff Hill, already noticed (<a href="#v1fig86">Fig. 86</a>), -marking lateral eruptions from the flanks of the great cone. Three -of these occur in a line more than half a mile long, possibly indicating a -fissure on the side of the old volcano, running in a south-westerly direction -<span class="pagenum" id="Page_325">- 325 -</span> -from the southern edge of the vent. The smallest of them measures about -500 feet in diameter; the largest is oblong in shape, its shorter diameter -being about 500 feet, and its longer about 1000 feet. The materials that -fill these lateral vents are coarse agglomerates, traversed by veins and -irregular intrusions of a fine horny or flinty felsite.</p> - -<p>From the acid character of most of the rocks that now fill the wide vent -of the Braid Hills it may be inferred that at least the last eruptions from it -consisted chiefly of acid tuffs and lavas. The upper portion of the volcanic -series being everywhere concealed, there are no means left to verify this -inference from an examination of the ejected material. It may be remarked, -however, that the pale yellow sandstones which lie on the east side of the -fault and are exposed in the Lyne Water above West Linton are in great -measure composed of fine felsitic material.<a id="FNanchor_365" href="#Footnote_365" class="fnanchor">[365]</a> They certainly belong to a -higher horizon than the most southerly lavas of the Pentland Hills, and -if they have not derived their volcanic detritus from the Biggar volcanic -area, it may be assumed that they obtained it from the vent of the Braid -Hills. In any case they show that after the lavas of the southern end of -the Pentland Hills were buried, acid volcanic detritus continued to be -abundantly distributed over this part of the floor of Lake Caledonia.</p> - -<div class="footnote"> - -<p><a id="Footnote_365" href="#FNanchor_365" class="label">[365]</a> Explanation to Sheet 24 of the Geological Survey of Scotland, pp. 10, 12.</p> - -</div> - - -<h4>6. <i>The Biggar Centre</i><a id="FNanchor_366" href="#Footnote_366" class="fnanchor">[366]</a></h4> - -<div class="footnote"> - -<p><a id="Footnote_366" href="#FNanchor_366" class="label">[366]</a> This area is included in Sheets 23 and 24 of the Geological Survey of Scotland. It was mapped -and described by myself. (Explanations of Sheets 23 and 24.) Various parts of it have been -referred to by earlier writers, particularly Maclaren, <i>Geology of Fife</i>, etc., p. 176.</p> - -</div> - -<p>Another distinct group of volcanoes had its centre about 25 miles -south-westward from the Braid vent, and on the same line as those of the -Pentland Hills. In no part of the basin can the isolation of the different -volcanic clusters be so impressively observed as in the area to the south-west -of these hills. On the one hand, the lavas and tuffs from the Braid -vent die out, and on the other, as we follow the conglomerates south-westwards, -a new volcanic series immediately makes its appearance.</p> - -<p>The space between the last extremity of the Pentland lavas and the -beginning of the Biggar series does not exceed some 500 yards. It will be -remembered that the lower half of the Pentland volcanic series dies out long -before it reaches the southern end of the hills, and that it is by lavas on the -horizon of some of the dark andesites of Allermuir Hill that the volcanic -band is finally prolonged to its extreme southern limit. The most northerly -extension of the Biggar lavas lies somewhere on the same general platform. -But whereas, at the north end of the Pentland chain, the volcanic sheets -rest on the edges of the Upper Silurian shales, at the south end, several -hundred feet of coarse conglomerate and sandstone intervene between the -Silurian shales and the porphyrites. So rapidly does the bulk of these -sedimentary formations increase that in the course of two miles they must -be 3000 feet in thickness below the most northerly of the Biggar lavas just -<span class="pagenum" id="Page_326">- 326 -</span> -referred to. But after that point, when they cross the Lyne Water, they -begin to be more and more interstratified with thin sheets of andesite. These -lavas, the beginning of the Biggar series, soon number nine or ten distinct -bands, and so quickly do they usurp the place of the sedimentary materials -that in a distance of not more than twelve miles they form, where traversed -by the river Clyde, the whole breadth of the visible tract of Old Red Sandstone, -to the exclusion of the conglomerates.</p> - -<p>Unfortunately, soon after the lavas make their appearance at the north -end they are in great measure overlapped unconformably by the red sandstones -at the base of the Carboniferous system, but where the Medwin Water -has cut through this covering, they can be seen here and there underneath -on their southerly course.</p> - -<p>A section through the northern end of the Biggar series, where the -successive lavas are dying out northwards among the conglomerates, shows -the structure given in <a href="#v1fig91">Fig. 91</a>. The sedimentary strata consist largely of -debris of andesite, and the lavas include dark red or purple andesites and -also pale felsites, both having the same characters as those of the Pentland -Hills.</p> - -<div class="figcenter" id="v1fig91" style="width: 454px;"> - <img src="images/v1fig91.png" width="454" height="103" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 91.</span>—Section across the northern end of the Biggar volcanic group, from Fadden Hill to - beyond Mendick Hill.<br /> - 1. Conglomerates and sandstones; 2. Lavas, the lowest being an olivine-diabase or basalt, the main mass - being andesites; 3. Felsites and tuffs; 4. Upper Old Red Sandstone. <i>f</i>, Fault.</div> -</div> - -<p>In one important respect the volcanic series in the northern part of the -Biggar area differs from that of the Pentland Hills, for whereas the uppermost -parts of the latter are concealed by faults which bring down the Carboniferous -strata against the base of the hills, the lavas at the north end of -the Biggar district pass conformably under a thick group of Lower Old Red -conglomerates and sandstones. We thus learn that here the volcanic -eruptions ceased long before the close of the deposition of the Lower Old -Red Sandstone. The overlying sedimentary series is disposed in a long -synclinal trough, corresponding in direction with the general north-easterly -strike of the volcanic rocks which reappear from under the sandstones and -conglomerates along its south-eastern border, where they are abruptly truncated -by the fault (<i>f</i>, <a href="#v1fig92">Fig. 92</a>), which brings them against the flanks of the -Silurian Uplands. It is interesting to note that by this dislocation the lavas -of the Lower Old Red Sandstone are placed almost in immediate contact -with those of the Lower Silurian series, which appear here on the crests of -numerous anticlinal folds that are obliquely cut off by the fault.</p> - -<p>There is yet another feature of interest in the northern part of the -Biggar volcanic centre. While the lowest visible lava is an olivine-diabase -<span class="pagenum" id="Page_327">- 327 -</span> -not unlike parts of the Warklaw group of the Pentland Hills, those which -occur above it are partly andesites and partly orthoclase-felsites. The latter -form, among the hills near Dolphinton, an important group which reaches -its greatest development in the Black Mount (1689 feet). These rocks -cover a breadth of more than a mile of ground, and probably attain a thickness -of not less than 2000 feet. They so closely resemble in their general -characters the corresponding rocks of the Pentland Hills that a brief description -of them may suffice. As in that chain of hills, they are so prone -to decomposition that they are in large part concealed under a covering of -their own debris and of herbage, though their fragments form abundant -screes, and numerous projecting knobs of rock suffice to show the main -features of the lavas and their accompaniments.</p> - -<p>The felsites weather into pale yellow and greyish "claystones," but -where fresher sections can be procured they often show darker tints of lilac -and purple. They are close-grained, sometimes flinty, generally porphyritic -with scattered highly-kaolinized white felspars, but without quartz, often -presenting beautiful flow-structure, and not infrequently showing a brecciated -appearance, which in the usual weathered blocks is hardly to be distinguished -from the breccia of interstratified tuffs.</p> - -<p>A locality where some of these features may be satisfactorily examined -is a dry ravine in the farm of Bank, on the south-east side of the Black Mount. -Here the felsite possesses such a perfectly developed flow-structure as to -split into slabs which, dipping S.E. at about 25°, might deceive the observer -into the belief that it is a sedimentary rock. A fresh fracture shows the -laminæ of flow, many of which are as thin as sheets of paper, to be lilac in -colour, some of the more decomposed layers assuming tints of grey. The -felspars and micas are arranged with their long axes parallel to the lines -of flow. The rock is not vesicular, but it breaks up here and there into the -brecciated condition just referred to. Below the sheet which displays the -most perfect flow-structure, what is probably a true volcanic breccia makes -its appearance. It consists of angular fragments of a similar lilac felsite, of -all sizes up to pieces two or three inches in length, cemented in a matrix of -the same material stained reddish-brown. In this breccia the stones show -little or no flow-structure.</p> - -<p>Above the group of felsites and felsitic breccias, grey andesites make -their appearance, like some of those in the Pentland Hills. They are sometimes -extraordinarily vesicular, the vesicles in the body of the rock being -filled with calcite, agate, etc. Such lavas must have been originally sheets -of rough slag. The elongated steam-vesicles have been partly filled up with -micaceous sand and fine red mud that were washed into crannies of the lava -in direct communication with the overlying water. It is evident that in the -northern part of the Biggar centre the succession of volcanic events followed -closely the order observable in the Pentland Hills, but on a feebler scale. -We may suppose that the lower diabases and andesites are the equivalents of -those of Warklaw and Allermuir, that the felsites and breccias were contemporaneous -with those of Capelaw, Caerketton and Castlelaw, and that the -<span class="pagenum" id="Page_328">- 328 -</span> -last andesites made their appearance together with those which form the -highest lavas of the Pentland chain.</p> - -<div class="figcenter" id="v1fig92" style="width: 438px;"> - <img src="images/v1fig92.png" width="438" height="100" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 92.</span>—Section across the southern part of the Biggar volcanic group from Covington to Culter.<br /> - 1. Lower Silurian strata; 2. Lower Old Red Sandstone (pre-volcanic group); 3. Andesite lavas with intercalated - sandstones and conglomerates; 4. Felsite neck. <i>f</i>, The boundary-fault on northern edge of Southern Uplands.</div> -</div> - -<p>A section across the southern end of the Biggar volcanic belt shows less -diversity of structure (<a href="#v1fig92">Fig. 92</a>). The lavas (3) are there found to flatten -out and to spread unconformably over the older part of the Lower Old -Red Sandstone (2), which, as already stated, passes down into the Upper -Silurian shales. A few intercalations of conglomerate, mainly made up of -volcanic detritus, are here and there to be detected among these lavas. But -the chocolate sandstones and conglomerates that lie unconformably below -them contain no such detritus, for they belong to the pre-volcanic part of -the history of Lake Caledonia, and were here locally upraised, perhaps as -an accompaniment of the terrestrial disturbances that preceded or attended -the first outburst of volcanic energy. Followed south-westwards, the stratigraphical -break in the Lower Old Red Sandstone disappears, and, as will be -shown in the account of the Duneaton centre, a continuous succession can -there be traced from the Upper Silurian shales up into the volcanic series.</p> - -<p>An interesting feature in this district is the felsitic boss of Quothquan -already alluded to (<a href="#Page_288">p. 288</a>) as rising up through the andesites, and possibly -marking one of the vents of the district. It is one of a number of felsitic -intrusions in this neighbourhood, of which the most important is Tinto.</p> - -<div class="figcenter" id="v1fig93" style="width: 450px;"> - <img src="images/v1fig93.png" width="450" height="105" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 93.</span>—Section from Thankerton Moor across Tinto to Lamington.<br /> - 1<i>a</i>. Lower Silurian; 1. Upper Silurian strata; 2. Lower Old Red Sandstone with two marked bands of conglomerate; - 3. Lower Old Sandstone (pre-volcanic chocolate sandstones); 4. Andesite lavas with sandstones, - conglomerates and tuffs lying unconformably on No. 3; 5. Felsite sill of Tinto with the smaller sill of the - Pap Craig (6). <i>f</i>, Fault bounding the Silurian uplands on the north. A small patch of the unconformable - Lower Old Red conglomerate is seen on the south side of the fault.</div> -</div> - -<p>A third section taken across Tinto, from Thankerton Moor on the north -side to Lamington on the south, will serve further to illustrate the great -unconformability in the Lower Old Red Sandstone of this district, and to show -the relation of the largest felsitic intrusion to the surrounding rocks (<a href="#v1fig93">Fig. 93</a>). -The conglomerates and sandstones that appear on the south slopes of Tinto -<span class="pagenum" id="Page_329">- 329 -</span> -lie near the base of the Old Red Sandstone, and if we could bore among the -overlying andesites we should probably meet with the Upper Silurian shales -among or conformably beneath the red passage-beds, as in the Lesmahagow -district.</p> - -<p>The andesitic lavas creep over the upturned denuded edges of these -strata and sweep round the flanks of Tinto. This conspicuous hill reaches -a height of 2335 feet above the sea, and consists of the felsitic rocks already -described (<a href="#Page_278">p. 278</a>). Seen from many points of view it rises as a graceful cone, -distinguished from all the other eminences around it by the pinkish colour of -its screes. In reality it forms a continuous ridge which runs in an east and -west direction for about five and a half miles, with a breadth of about a -mile. Some part at least, and possibly the whole of this oblong mass, is in -the form of a sill or laccolite which dips towards the north. Conglomerates -and sandstones plunge under it on the southern side, and similar sandstones -overlie it on the north. If there be a neck in this mass, as one might infer -from the shape of the hill, its precise limits are concealed. The rock does -not break through the andesites, and may belong to an earlier period of -eruptivity than the lavas immediately around it. There were other, though -smaller, vents in the immediate neighbourhood. Besides the cone of -Quothquan just referred to, another may be marked by the felsite boss -which overlooks the village of Douglas, four miles to the south-west of the -Tinto ridge, while a third rises into a low rounded hill close to the village -of Symington.</p> - -<p>The lavas spread out again to the south-west of Tinto in a group of -hills, until they are interrupted by a fault which brings in the Douglas -coal-field.<a id="FNanchor_367" href="#Footnote_367" class="fnanchor">[367]</a> This dislocation abruptly terminates the Biggar volcanic band -in a south-westerly direction, after extending for a length of 26 miles, with -a breadth of sometimes as much as five miles.</p> - -<div class="footnote"> - -<p><a id="Footnote_367" href="#FNanchor_367" class="label">[367]</a> See Explanation to Sheet 23 of the Geological Survey of Scotland (1873), p. 15. This -ground was mapped and described by Mr. B. N. Peach.</p> - -</div> - - -<h4>7. <i>The Duneaton Centre</i></h4> - -<p>Among the high bleak muirlands on the confines of the three counties of -Lanark, Ayr and Dumfries, traversed by the Duneaton Water, a distinct -volcanic area may be traced.<a id="FNanchor_368" href="#Footnote_368" class="fnanchor">[368]</a> Its boundaries, however, cannot be satisfactorily -fixed. It is overspread with Carboniferous rocks both to the -north-east and south-west, so that its rocks are only visible along a strip -about seven miles long and two miles broad. On the north-western side its -lower members are seen lying interstratified among the sandstones and conglomerates -which thence pass down conformably into the Upper Silurian -series (<a href="#v1fig94">Fig. 94</a>). But although we thus get below the volcanic series we meet -with no vents or sills among the lower rocks. On the south-east side the -highest lavas and tuffs are overlain by some 5000 feet of red sandstones and -conglomerates (2, 3), which completely bury all traces of the volcanic history.</p> - -<div class="footnote"> - -<p><a id="Footnote_368" href="#FNanchor_368" class="label">[368]</a> This area was mapped by Mr. B. N. Peach in Sheet 15 of the Geological Survey of Scotland, -and is described by him in the accompanying Memoir.</p> - -</div> - -<p><span class="pagenum" id="Page_330">- 330 -</span></p> - -<p>The volcanic series in this limited district reaches an estimated thickness -of 4000 feet, built up of purple and green -slaggy andesites, dark heavy diabases (melaphyres) -and tuffs, with abundant interstratification of sandstone, -especially towards the base. One of its chief -features of interest is the manner in which it -exhibits, better, perhaps, than can be found in any -of the other volcanic areas, the frequent and rapid -alternations of lavas and tuffs with sandstone and -conglomerate. In this part of the region the -volcanic discharges were obviously frequent and -intermittent, while at the same time the transport -and deposition of sediment were continuous. This -sediment consisted largely, indeed, of volcanic detritus -mixed with ordinary sand and silt. That -these conditions of sedimentation were not wholly -inimical to animal life is shown by the occasional -occurrence of worm-burrows in the ashy sandstones.<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> Memoir on Sheet 15 Geol. Surv. Scotland (1871), p. 22.</p> - -</div> - -<div class="figcenter" id="v1fig94" style="width: 715px;"> - <img src="images/v1fig94.png" width="715" height="105" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 94.</span>—Section across the Duneaton volcanic district from the head of the Duneaton Water to Kirklea Hill.<br /> - 1. Silurian strata; 2. Lower Old Red Sandstone and conglomerate; 3. Coarse conglomerate; 4. Andesite-lavas; 5. Stratified tuffs; 6. Spango granite; 7. Upper Old - Red Sandstone.</div> -</div> - -<p>The thick accumulation of sandstones and conglomerates -above the main mass of lavas has been -derived almost wholly from the waste of the volcanic -rocks (3). Blocks of andesite, well rounded -and often from six to twelve inches in diameter, may -be seen in the remarkable band of coarse conglomerate -which runs as a nearly continuous ridge -from the Nith to the Clyde—a distance of more -than twenty miles. Nothing impresses the geologist -more, as he wanders over this district, than the -evidence of the prodigious waste which the volcanic -series underwent before it was finally buried. Some -part of the detritus may have been supplied, indeed, -by occasional discharges of fragmental matter, -as has already been suggested in the case of the -Ochil and Montrose conglomerates. But the nature -of the pebbles in these masses of ancient shingle -shows them to be not bombs, but pieces worn away -from sheets of lava.</p> - -<p>That the lavas underlie these piles of detritus -and extend southwards even up to the very edge -of the Silurian Uplands is shown by the rise of a -number of successive beds from under the trough -into which the conglomerate has been thrown. -These lavas, however, are almost immediately cut -off by the great boundary fault (<i>f</i>) which flanks the -Silurian territory. That they are not met with now to the south-east of -<span class="pagenum" id="Page_331">- 331 -</span> -the dislocation, where they must once have lain, is an evidence of the great -denudation which the district has undergone. <a href="#v1fig94">Fig. 94</a>, which gives a section -across the broadest part of the area, from the edge of the Muirkirk coal-field -to the Silurian uplands, shows the general structure of the ground.</p> - -<p>No satisfactory evidence regarding the position of any of the vents -of the period has been met with in this district. The rocks to the -south of the boundary-fault are older than the Old Red Sandstone, and -as they must have been for some distance overspread by the conglomerates, -sandstones and volcanic series, we might hope to find somewhere among -them traces of necks or bosses. The only mass of eruptive rock in -that part of the district is the tract of Spango granite which has been -already referred to in connection with the subject of the vents and granite -protrusions of Old Red Sandstone time. This mass, about four miles long -and two miles broad, rises through Silurian strata, and by means of the -boundary fault is brought against the higher group of conglomerates and -sandstones. The Silurian shales and sandstones around the granite have -undergone contact-metamorphism, becoming highly micaceous and schistose. -The ascent of this granite must have taken place between the upheaval and -contortion of the Upper Silurian strata, and the deposition of the higher -parts of the Lower Old Red Sandstone of this region. Its date might thus -come within the limits of the volcanic period. But one must frankly own -that there is no positive evidence to connect its production with the volcanic -history.</p> - - -<h4>8. <i>The Ayrshire Group of Vents</i></h4> - -<p>The original limits of the volcanic districts in the remaining portion of the -Old Red Sandstone area on the mainland of Scotland, from the valley of the -Nith to the Firth of Clyde, can only be vaguely indicated.<a id="FNanchor_370" href="#Footnote_370" class="fnanchor">[370]</a> There is a difficulty -in ascertaining the south-western termination of the Duneaton area, -and in deciding whether the lavas and tuffs of Corsincone in Nithsdale -should be assigned to that district or be placed with those further to the -south-west. Between Corsincone and the next visible volcanic rocks of the -Lower Old Red Sandstone there intervenes a space of six miles, along which, -owing to the effect of the great fault that flanks the north-western margin -of the Southern Uplands, the Carboniferous Limestone and even the Coal-measures -are brought against the Silurian formations, every intermediate -series of rocks being there cut out. It may therefore be, on the whole, -better to include all the volcanic rocks on the left side of the Nith as part -of the Duneaton series. There will still remain a tract of five miles of -blank intermediate ground before we enter upon the volcanic rocks of -Ayrshire.</p> - -<div class="footnote"> - -<p><a id="Footnote_370" href="#FNanchor_370" class="label">[370]</a> The mapping of the Old Red Sandstone volcanic areas of Ayrshire for the Geological Survey -was thus distributed:—The district east of Dalmellington was surveyed by Mr. B. N. Peach, that -between Dalmellington and Straiton by Prof. James Geikie, and all from the line of the Girvan -Valley south of Straiton westward to the sea by myself. The ground is embraced in Sheets 8, 13 -and 14 of the Map of Scotland, and is described in accompanying Explanations.</p> - -</div> - -<p><span class="pagenum" id="Page_332">- 332 -</span></p> - -<p>Owing to complicated faults, extensive unconformable overlaps of the -Carboniferous formations, and enormous denudation, the volcanic tracts of -Old Red Sandstone age in Ayrshire have been reduced to mere scattered -patches, the true relations of which are not always easily discoverable. One -of these isolated areas flanks the Silurian Uplands as a belt from a mile to -a mile and a half in breadth and about six miles long, but with its limits -everywhere defined by faults. A second much more diversified district -extends for about ten miles to the south-west of Dalmellington. It too -forms a belt, averaging about four miles in breadth, but presenting a singularly -complicated geological structure. Owing to faults, curvatures and denudation, -the volcanic rocks have there been isolated into a number of detached -portions, between some of which the older parts of the Old Red Sandstone, -and even the Silurian rocks, have been laid bare, while between others the -ground is overspread with Carboniferous strata. A third unbroken area -forms the Brown Carrick Hills, south of the town of Ayr, and is of special -interest from the fact that its rocks have been exposed along a range of sea-cliffs -and of beach-sections for a distance of nearly four miles. Other detached -tracts of volcanic rocks are displayed on the shore at Turnberry and Port -Garrick, on the hills between Mochrum and the vale of the Girvan, and on -the low ground between Dalrymple and Kirkmichael.</p> - -<p>The isolation of these various outliers and separated districts is probably -not entirely due to the effects of subsequent geological revolutions. More -probably some of the areas were always independent of each other, and their -igneous rocks were discharged from distinct volcanic centres. We may -conjecture that one of these centres lay somewhere in the neighbourhood of -New Cumnock, for the lavas between that town and Dalmellington appear -to diminish in thickness and number as they are traced south-westward. -Another vent, or more probably a group of vents, may have stood on the -site of the present hills to the right and left of the Girvan Valley, south of -the village of Straiton. A third probably rose somewhere between Dailly -and Crosshill, and poured out the lavas of the ridges between Maybole and -the Dailly coal-field. The important centre of eruption that produced the -thick and extensive lavas of the Brown Carrick Hills may be concealed -under these hills, or may have stood somewhere to the west of Maybole. -Still another vent, perhaps now under the sea, appears to be indicated by -the porphyrites of the coast-section between Turnberry and Culzean Bay.</p> - -<p>Owing to the complicated structure of the ground, several important -points in the history of the Old Red Sandstone of this region have not been -established beyond dispute. In particular, the unconformability which undoubtedly -exists in that system in the south-west of Ayrshire has not -been traced far enough eastwards to determine whether it affects the volcanic -belt east of Dalmellington, or whether the break took place before or after -the eruption of that belt. West of Dalmellington it clearly separates a -higher group of sandstones, conglomerates and volcanic rocks from everything -older than themselves. The structure is similar to that in the Pentland -Hills, a marked disturbance having taken place here as well as -<span class="pagenum" id="Page_333">- 333 -</span> -there after a considerable portion of the Lower Old Red Sandstone had been -deposited. These earlier strata were upraised, and on their denuded ends -another group of sandstones and conglomerates was laid down, followed by -an extensive eruption of volcanic materials.</p> - -<p>It is the upper unconformable series that requires to be considered here, -as it includes all the volcanic rocks of the Old Red Sandstone lying to the -west of the meridian of Dalmellington. The position of these rocks on -their underlying conglomerates is admirably exposed among the hills between -the valleys of the Doon and the Girvan, as well as on Bennan Hill to the -south of Straiton. The andesites rise in a craggy escarpment crowning long -green slopes that more or less conceal the conglomerates and sandstones below.</p> - -<p>Along the coast-sections the structure of the volcanic rocks may be -most advantageously studied. The shore from the Heads of Ayr to -Culzean Castle affords a fine series of exposures, where every feature in the -succession of the lavas may be observed. Still more instructive, perhaps, -is the mile and a half of beach between Turnberry Bay and Douglaston, of -which I shall here give a condensed account, for comparison with the coast-sections -of Kincardineshire and Forfarshire already described.</p> - -<p>The special feature of this part of the Ayrshire coast-line is the number -of distinct andesite sheets which can be discriminated by means of the thin -layers of sandstone and sandy tuff that intervene between them. In the -short space of a mile and a half somewhere about thirty sheets can be -recognized, each marking a separate outflow of lava. It was in this section -that I first observed the sandstone-veinings which have been described in -previous pages, and nowhere are they more clearly developed. Almost every -successive stream of andesite has been more or less fissured in cooling, and its -rents and irregular cavernous hollows have been filled with fine sand silted -in from above. The connection may often be observed between these sandstone -partitions or patches and the bed of the same material, which -overspread the surface of the lava at the time that the fissures were being -filled up.</p> - -<div class="figright" id="v1fig95" style="width: 276px;"> - <img src="images/v1fig95.png" width="276" height="155" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 95.</span>—Cavernous spaces in andesite, filled in with sandstone, - John o' Groat's Port, Turnberry, Ayrshire.</div> -</div> - -<p>The andesites of the Turnberry shore are of the usual dark purplish-red -to green colours, more or less -compact in the centre and -vesicular towards the top and -bottom. They display with -great clearness the large empty -spaces that were apt to be -formed in such viscous slaggy -lavas as they moved along the -lake-bottom. These spaces, -afterwards filled with fine sand, -now appear as irregular enclosures -of hard green sandstone -embedded in the andesite. The example shown in <a href="#v1fig95">Fig. 95</a> may be -seen in one of the lavas at John o' Groat's Port.</p> - -<p><span class="pagenum" id="Page_334">- 334 -</span></p> - -<div class="figleft" id="v1fig96" style="width: 211px;"> - <img src="images/v1fig96.png" width="211" height="321" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 96.</span>—Section of andesites, Turnberry - Castle, Ayrshire.</div> -</div> - -<p>From the arrangement of the veins of sandstone it is evident that -irregularly divergent, but often more or less stellate, fissures opened in the -lavas as they cooled. Sometimes, indeed, the molten rock appears to have -broken up into a shattered mass of fragments, as must often have happened -when lavas were poured over the lake-floor. What may be an instance of this -effect is to be seen on the cliff under the -walls of Turnberry Castle, whence the -annexed sketch (<a href="#v1fig96">Fig. 96</a>) was taken. -The lower andesite (<i>a</i>) is highly amygdaloidal -towards the top, and is traversed -in all directions with irregular veins and -nests of sandstone which can be traced -upward to the bed (<i>b</i>), consisting of sandstone, -but so full of lumps or slags of -amygdaloidal andesite that one is here -and there puzzled whether to regard it -as a sedimentary deposit, or as the upper -layer of clinkers of a lava-stream strewn -with sand. Above this fragmentary layer -lies another bed of andesite (<i>c</i>) of a -coarsely amygdaloidal structure, which -encloses patches of the underlying sandstone. -It passes upward, in a space of -from four to six feet, into a mass of -angular scoriaceous fragments (<i>d</i>) of all -sizes up to blocks 18 inches in length -cemented in a vein-stuff of calcite, chalcedony -and quartz. This brecciated structure ascends for about 13 or 14 -feet, and is then succeeded by a greenish compact andesite (<i>e</i>), which further -north becomes amygdaloidal and much veined with sandstone, passing into -a breccia of lava fragments and sandstone.</p> - -<div class="figcenter" id="v1fig97" style="width: 394px;"> - <img src="images/v1fig97.png" width="394" height="136" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 97.</span>—Lenticular form of a brecciated andesite (shown in <a href="#v1fig96">Fig. 96</a>), Turnberry, Ayrshire.</div> -</div> - -<p><span class="pagenum" id="Map_III"></span></p> - -<div class="pmt4 pmb4 smaller tdc">MAP OF THE VOLCANIC DISTRICTS OF THE LOWER OLD RED SANDSTONE<br /> - OF "LAKE CALEDONIA" IN CENTRAL SCOTLAND & NORTH EAST IRELAND<br /> - - <img src="images/v1map3.png" alt="" usemap="#zoomarea" width="693" height="214" /> - <map name="zoomarea" id="zoomarea"> - <area shape="rect" coords="0,0,214,260" alt="West" href="images/v1map3lgpt1.png" /> - <area shape="rect" coords="280,0,518,214" alt="Middle" href="images/v1map3lgpt2.png" /> - <area shape="rect" coords="519,0,693,214" alt="Right" href="images/v1map3lgpt3.png" /> - </map> - <div class="tdc"><span class="fl_left vsmall">The Edinburgh Geographical Institute</span> - <span class="vsmall">Copyright</span> - <span class="fl_right vsmall">J. G. Bartholomew.</span></div> - - -<div class="tdc smaller">Click on map's left, middle, or right to view larger sized version.</div> -</div> - -<p><span class="pagenum" id="Page_335">- 335 -</span></p> - -<p>The remarkable brecciated band (<i>d</i>) in this cliff, though 13 or 14 feet -in the centre, immediately thins out on either side, until in the course of a -few yards it completely disappears and allows the lavas <i>c</i> and <i>e</i> to come -together, as shown in <a href="#v1fig97">Fig. 97</a>. We may suppose that this section reveals -the structure of the terminal portion of a highly viscous lava which was -shattered into fragments as it moved along under water.</p> - -<p>No clear evidence of the sites of any of the volcanic vents has yet been -detected in the Old Red Sandstone of Ayrshire. Possibly some of the -numerous felsitic bosses to the south-west of Dalmellington may partly mark -their positions. But the sills connected with the volcanic series are well -exposed in the 12 miles of hilly ground between Dalmellington and Barr. -Two groups of intrusive sheets may there be seen. The most numerous -consist of pale or dark-pink felsite, often full of crystals of mica. They -form prominent hills, such as Turgeny, Knockskae and Garleffin Fell. The -second group comprises various diabase-sheets which have been intruded -near the base of the red sandstones and conglomerates, over a distance of seven -miles on the north side of the Stinchar Valley above Barr. They attain -their greatest development on Jedburgh Hill, where they form a series of -successive sills, the largest of which unite northwards into one thick mass -and die out southward among the sandstones and conglomerates.</p> - -<hr class="chap x-ebookmaker-drop" /> - - -<div class="chapter"> -<p><span class="pagenum" id="Page_336">- 336 -</span></p> - -<h2 class="nobreak" id="CHAPTER_XXI">CHAPTER XXI<br /> - -<span class="smaller">VOLCANOES OF THE LOWER OLD RED SANDSTONE OF THE CHEVIOT HILLS, -LORNE, "LAKE ORCADIE" AND KILLARNEY</span></h2> -</div> - - -<h3>THE CHEVIOT AND BERWICKSHIRE DISTRICT</h3> - -<p>In the south-east of Scotland, and extending thence into the north of -England, the remains of several distinct volcanic centres of the Lower Old -Red Sandstone may still be recognized. Of these the largest and most -interesting forms the mass of the Cheviot Hills; a second has been partially -dissected by the sea along the coast south from St. Abb's Head; while -possibly relics of others may survive in detached bosses of eruptive rock -which rise through the Silurian formations of Berwickshire. The water-basin -in which these volcanic groups were active was named by me "Lake -Cheviot,"<a id="FNanchor_371" href="#Footnote_371" class="fnanchor">[371]</a> to distinguish it from the other basins of the same geological -period (<a href="#Map_I">Map I.</a>).</p> - -<div class="footnote"> - -<p><a id="Footnote_371" href="#FNanchor_371" class="label">[371]</a> <i>Trans. Roy. Soc.</i> Edin. xxviii. (1878), p. 354.</p> - -</div> - -<p>The volcanic rocks of the Cheviot Hills, though their limits have been -reduced by faults, unconformable overlap of younger formations and severe -denudation, still cover about 230 square miles of ground, and rise to -a height of 2676 feet above the sea. As they have been mapped in detail -by the Geological Survey, both on the English and the Scottish sides of the -Border, their structure is now known.<a id="FNanchor_372" href="#Footnote_372" class="fnanchor">[372]</a> No good horizontal section, however, -has yet been constructed to show this structure—a deficiency which, it is -hoped, may before long be supplied.</p> - -<div class="footnote"> - -<p><a id="Footnote_372" href="#FNanchor_372" class="label">[372]</a> The Geology of the Cheviot Hills is comprised in Sheets 108 N.E., 109 N.W., and 110 S.W. -of the Geological Survey of England and Wales, and in Sheets 17, 18 and 26 of the Geological -Survey of Scotland. For descriptive accounts the Memoirs to some of these Sheets may be consulted, -particularly "Geology of the Cheviot Hills" (English side), by C. T. Clough (<i>Mem. Geol. -Surv.</i> 1888); "Geology of Otterburn and Elsdon," by H. Miller and C. T. Clough (<i>Mem. Geol. -Surv.</i> 1887); "Geology of Part of Northumberland between Wooler and Coldstream," by W. -Gunn and C. T. Clough, with Petrographical Notes by W. W. Watts (<i>Mem. Geol. Surv.</i> 1895). -Other descriptions have been published by Professor James Geikie, <i>Good Words</i>, vol. xvii. (1876), -reprinted in <i>Fragments of Earth-lore</i> (1893), and by Prof. Lebour, <i>Outlines of the Geology of -Northumberland</i>, 2nd edit. 1886. For the petrography of the rocks consult Mr. J. J. H. Teall, -<i>Geol. Mag.</i> 1883, pp. 100, 145, 252, 344; 1884, p. 226; 1885, p. 106; <i>Proc. Geol. Assoc.</i> ix. (1886) -p. 575; and his <i>British Petrography</i>, 1888; Dr. J. Petersen, <i>Mikroskopische und chemische Untersuchungen -am Enstatit-porphyrit aus den Cheviot Hills</i>, Inaugural Dissertation, Kiel, 1884.</p> - -</div> - -<p><span class="pagenum" id="Page_337">- 337 -</span></p> - -<p>This volcanic pile, consisting mainly of bedded andesites which rest -unconformably on the upturned edges of Wenlock shales and grits, presents -a most typical display of the lavas of the Lower Old Red Sandstone. These -rocks range from vitreous or resinous pitchstone-like varieties to coarsely -porphyritic forms, on the one hand, and to highly vesicular and amygdaloidal -kinds, on the other. Analyses of some of these rocks, and an account -of their petrography, have already been given.</p> - -<p>The lavas are often separated by thin partings of tuff, and their upper -surfaces show the fissured character with sandstone infillings, so characteristic -among the lavas of "Lake Caledonia."<a id="FNanchor_373" href="#Footnote_373" class="fnanchor">[373]</a> Tuffs form a very subordinate -part of the whole volcanic series. One of the most important bands is -a thick mass at the base of the series, lying immediately on the highly -inclined Silurian shales. The fragments are generally of a fine-grained -purple mica-andesite, often two or three feet and sometimes at least five -feet long. For a few feet near the bottom of this mass of tuff, pieces of -Silurian shale an inch in length may be noticed. Mr. Clough remarks that -distinct bedding is not usual among the tuffs. Though no doubt most -of the fragmental materials really lie intercalated between successive lava-streams, -yet some of the isolated patches of coarse volcanic breccia may -mark the sites of eruptive vents. One such probable neck has been mapped -on the Scottish side between Cocklawfoot, at the head of the Bowmont -Water, and King's Seat, while others may perhaps occur among the detached -patches that have been observed on the Northumbrian side. No thick conglomerates -or sandstones have been noticed in the Cheviot District. The -volcanic eruptions appear to have usually succeeded each other without the -spread of any notable amount of ordinary detritus over the floor of the -water-basin. It is difficult to estimate the total thickness of volcanic -material here piled up, but it probably amounts to several thousand feet. -The top of the series is not visible, having been partly removed by denudation -and partly buried under the Carboniferous formations.</p> - -<div class="footnote"> - -<p><a id="Footnote_373" href="#FNanchor_373" class="label">[373]</a> Clough, <i>Geology of the Cheviot Hills</i>, p. 15.</p> - -</div> - -<p>It will thus be seen that the Cheviot area stands apart from the other -volcanic districts of the Lower Old Red Sandstone in the great relative -thickness of its accumulated lavas, the comparative thinness of its tuffs, and -the absence of the thick intercalations of coarse conglomerate so abundantly -developed among the volcanic series all over Central Scotland. But there -is yet another characteristic in which this area is pre-eminently conspicuous. -In the heart of the andesites lies a core of augite-granitite, around -which these rocks are traversed with dykes.</p> - -<p>This interesting granitic boss rises into the highest summit of the whole -Cheviot range, and covers an area of rather more than 20 square miles. -While its petrographical characters have been described by Mr. Teall, -its boundary has been mapped by Mr. Clough, who found the line difficult -to trace, owing partly to the prevalent covering of peat, and partly to the -jagged and irregular junction caused by the protrusion of dykes from and -into the boss. He obtained evidence that the granite has broken through -<span class="pagenum" id="Page_338">- 338 -</span> -the bedded andesites, and that it is in turn traversed by dykes composed of -a material indistinguishable from that of some of the flows. He therefore -considered that it is essentially of the same age as the rest of the volcanic -series, and "not improbably the deep-seated source of it."<a id="FNanchor_374" href="#Footnote_374" class="fnanchor">[374]</a> Mr. Teall also, -from a chemical and microscopical examination of the rocks, drew a similar -conclusion.<a id="FNanchor_375" href="#Footnote_375" class="fnanchor">[375]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_374" href="#FNanchor_374" class="label">[374]</a> <i>Op. cit.</i> p. 24.</p> - -<p><a id="Footnote_375" href="#FNanchor_375" class="label">[375]</a> <i>Geol. Mag.</i> 1885, p. 106.</p> - -</div> - -<p>The andesites around the granite have undergone contact-metamorphism, -but the nature and extent of the change have not yet been studied. There -occur around the granite many dykes of felsite and quartz-felsite, to the -petrographical character of which reference has already been made. But -the most abundant and remarkable dykes of the district are those of a -reddish mica-porphyrite, of which Mr. Clough has mapped no fewer than -forty, besides those in the granitic area. He has called attention to the -significant manner in which all the dykes of the district tend to point in a -general way to the great core of granite, as if that were the nucleus from -which they had radiated.<a id="FNanchor_376" href="#Footnote_376" class="fnanchor">[376]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_376" href="#FNanchor_376" class="label">[376]</a> <i>Op. cit.</i> pp. 26-28.</p> - -</div> - -<p>The central granite of the Cheviot Hills, with its peripheral dykes, has -no accompanying agglomerates nor any decided proof that it ever communicated -with the surface. When, however, we consider its petrographical -and chemical constitution, its position as a core among the bedded lavas, -and the intimate way in which it is linked with these rocks by the network -of dykes, we are, I think, justified in accepting the inference that it -belongs to the volcanic series. It possesses some curious and interesting -features in common with the great granophyre bosses of Tertiary age in -the Inner Hebrides. Like these it has no visible accompaniment of -superficial discharges. Yet it may have ascended by means of some central -vent or group of vents which, offering to it a weak part of the crust, -allowed it to communicate with the surface and give rise to the outflow of -lavas and fragmental ejections. In any case, it affords us a most interesting -and instructive insight into one of the deeper-seated ducts of a volcanic -region, and the relation of a volcanic focus to the ascent of the granitic -magma.</p> - -<hr class="tb" /> - -<p>About twenty miles to the north of the Cheviot Hills, and separated -from them by the Carboniferous and Upper Old Red Sandstones which -spread across the broad plain of the Merse, a group of volcanic rocks has -been laid open in a singularly instructive manner along the coast of -Berwickshire, between the village of Eyemouth and the promontory of St. -Abb's Head. Not only the actual vents, but the lavas and tuffs connected -with them, have there been admirably dissected by the forces of denudation.</p> - -<p>That this volcanic area was quite distinct from that of the Cheviot -Hills may be inferred from its coarse agglomerates, and from the fact that -when the rocks are followed inland in a south-westerly direction, that is, -towards the Cheviot area, they are found to diminish in thickness and to -disappear among the ordinary sediments. For the same reason we may -<span class="pagenum" id="Page_339">- 339 -</span> -regard the area as independent of any vents which may have risen further -west about Cockburn Law and the Dirrington Laws. Unfortunately, however, -only a small part of the area comes into view, the rest of it lying -beneath the waters of the North Sea.<a id="FNanchor_377" href="#Footnote_377" class="fnanchor">[377]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_377" href="#FNanchor_377" class="label">[377]</a> This area lies in Sheet 34 Geological Survey of Scotland, and was described by myself in the -Memoir to accompany that Sheet ("Geology of Eastern Berwickshire," 1864, p. 20). More -recently the shore between St. Abb's Head and Coldingham has been re-mapped by Professor -James Geikie who has also studied the microscopic character of the rocks, <i>Proc. Roy. Soc. Edin.</i> -xiv. (1887).</p> - -</div> - -<p>Of the several vents dissected along this coast-line, one may be seen at -Eyemouth, filled with a very coarse tumultuous agglomerate of andesite -fragments embedded in a compact felspathic matrix, through which are -scattered broken crystals of felspar, and imperfect tabular crystals of black -mica. Another of similar character is exposed for more than a mile and a -half along the shore at Coldingham. It contains blocks, sometimes more -than a yard in diameter, of different varieties of andesite, and, as at Eyemouth, -is much invaded by veins and bosses of intrusive andesite.</p> - -<div class="figcenter" id="v1fig98" style="width: 436px;"> - <img src="images/v1fig98.png" width="436" height="103" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 98.</span>—Section across the volcanic area of St. Abb's Head (after Prof. J. Geikie).<br /> - 11. Silurian formations; 2. Lower Old Red Conglomerate and Sandstone; 3 3. Sheets of andesitic lava; 4. Volcanic - tuffs, largely composed of scoriæ in the higher parts; 5. Volcanic agglomerate of neck on shore; 6. Intrusive - andesites. <i>f</i>, Fault.</div> -</div> - -<p>To the north of Coldingham, a series of bedded volcanic rocks which -form the picturesque headland of St. Abb's Head, are, according to the -estimate of Professor James Geikie, about 1000 to 1200 feet thick, but -neither their bottom nor their top is seen. The same observer found them -to consist of three groups of andesite sheets separated and overlain by -bedded tuffs. The lowest lavas have their base concealed under the sea, -and are covered by a thick band of coarse agglomeratic tuff, above which -lies the second group of andesites, about 250 feet thick. An intercalation -of various tuffs from 40 to 50 feet thick then succeeds, followed by the -third lava-group, 250 or 300 feet in depth. The highest member of the -series is a mass of bedded tuffs some 400 feet thick.</p> - -<p>The andesites lie in beds varying from about 15 to about 50 feet or -more in thickness. They are fine-grained, purplish-blue, or greyish-blue, -often reddish rocks, of the usual type. Generally rather close-grained, they -are not as a rule very porphyritic, but often highly scoriaceous and amygdaloidal, -especially towards the top and bottom of each bed. The more -slaggy portions are sometimes so filled in with fine tuff that the rock might -be mistaken for one of fragmental origin.</p> - -<p>The bedded tuffs are usually well stratified deposits. The most important -<span class="pagenum" id="Page_340">- 340 -</span> -band of them is that which forms the highest member of the volcanic -series. It consists of successive beds that vary from fine red mudstones up -to volcanic breccias with blocks one foot or more in diameter. The -materials have been derived from the explosion of andesitic lavas. Most of -the lapilli are vesicular or amygdaloidal, and many of them have evidently -come from vitreous scoriaceous lavas. Professor Geikie remarks that "from -their highly vesicular character, they might well have floated in water at -the time of their ejection—they are in short mere cinders." He could -detect no trace of ordinary sediment in the matrix, the whole material -being thoroughly volcanic in origin.</p> - -<p>The lavas, tuffs and agglomerates have been abundantly invaded by -intrusive rocks, chiefly andesites.<a id="FNanchor_378" href="#Footnote_378" class="fnanchor">[378]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_378" href="#FNanchor_378" class="label">[378]</a> See Prof. J. Geikie, <i>op. cit.</i></p> - -</div> - -<p>The agglomerates of this Berwickshire coast extend for a short way inland -from the Coldingham and Eyemouth vents, but the fragmental material -soon becomes finer and more water-rolled, and assumes a distinctly stratified -structure, as it is gradually and increasingly interleaved with layers of -ordinary sediment. Hence in passing towards the south-west, away from -the coast-line, we are obviously receding from the vents of eruption and -entering into the usual non-volcanic deposits of the time. That these -deposits belong to the Lower Old Red Sandstone was first ascertained -during the progress of the Geological Survey in this district by the discovery -of abundant plant-remains in the form of linear grass-like strips, and also -pieces of <i>Pterygotus</i> in some of the green shales interstratified among fine -tuffs and ashy sandstones.<a id="FNanchor_379" href="#Footnote_379" class="fnanchor">[379]</a> Before the volcanic detritus disappears from the -strata as they are followed in a south-westerly direction, the whole series -is unconformably overlain by the Upper Old Red Sandstone. The lower -division of the formation is not again seen until it rises from under the -southern margin of the plain of the Merse into the Cheviot Hills.</p> - -<div class="footnote"> - -<p><a id="Footnote_379" href="#FNanchor_379" class="label">[379]</a> "Geology of Eastern Berwickshire," <i>Mem. Geol. Surv. Scotland</i> (1864), pp. 26, 27, 57.</p> - -</div> - -<p>About ten miles to the south-west of the large Coldingham neck the great -boss of Cockburn Law and Stoneshiel Hill rises out of the Silurian rocks.<a id="FNanchor_380" href="#Footnote_380" class="fnanchor">[380]</a> -Five miles still further in the same direction the group of the beautiful -cones of Dirrington (<a href="#v1fig70">Fig. 70</a>) overlooks the wide Merse of Berwickshire,<a id="FNanchor_381" href="#Footnote_381" class="fnanchor">[381]</a> -and six miles to the north of these hills, in the very heart of Lammermuir, -lies the solitary boss of the Priestlaw granite.<a id="FNanchor_382" href="#Footnote_382" class="fnanchor">[382]</a> To these protrusions of -igneous material reference has already been made as possible volcanic vents -connected with the eruptions of the Lower Old Red Sandstone. As regards -their age they must certainly be younger than the Llandovery rocks which -they disrupt, and older than the Upper Old Red Sandstone, of which the -conglomerates, largely made from their debris, lie on them unconformably. -It seems therefore probable that these great bosses may form a part of the -<span class="pagenum" id="Page_341">- 341 -</span> -volcanic history of the Lower Old Red Sandstone period. But no positive -proof has yet been obtained that any one of them was the site of an eruptive -vent, and no trace has been detected around them of any lavas or tuffs -which might have proceeded from them.</p> - -<div class="footnote"> - -<p><a id="Footnote_380" href="#FNanchor_380" class="label">[380]</a> See "The Geology of Eastern Berwickshire" (Sheet 34), <i>Mem. Geol. Surv. Scotland</i> (1864), -p. 29.</p> - -<p><a id="Footnote_381" href="#FNanchor_381" class="label">[381]</a> These hills are chiefly represented in Sheet 25. But see "The Geology of East Lothian," -<i>Mem. Geol. Surv. Scotland</i> (1866), p. 26.</p> - -<p><a id="Footnote_382" href="#FNanchor_382" class="label">[382]</a> "Geology of East Lothian," <i>Mem. Geol. Surv. Scotland</i>, p. 15, and authorities there cited.</p> - -</div> - - -<h3>"THE LAKE OF LORNE"</h3> - -<div class="figcenter" id="v1fig99" style="width: 458px;"> - <img src="images/v1fig99.png" width="458" height="190" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 99.</span>—View of terraced andesite hills resting on massive conglomerate, south of Oban.</div> -</div> - -<p>The basin of Lorne has not yet been carefully examined and described, -though various writers have referred to different parts of it (<a href="#Map_I">Map I.</a>). My -own observations have been too few to enable me to give an adequate account -of it. The volcanic sheets of this area form a notable feature in the scenery -of the West Highlands, for they are the materials out of which the remarkable -terraced hills have been carved, which stretch from Loch Melfort to -Loch Creran (<a href="#v1fig99">Fig. 99</a>), and which reappear in picturesque outliers among -the mountains traversed by Glen Coe. Between the ancient schists that -form the foundation-rocks of this district and the base of the volcanic series, -lies a group of sedimentary strata which in the western part of the district -must be 500 feet thick. This group consists of exceedingly coarse breccias -at the bottom, above which come massive conglomerates, volcanic grits or -tuffs, fine sandstones and courses of shale. While the basement-breccias -are composed mainly of detritus of the underlying schists, including blocks -six feet long, they pass up into coarse conglomerates made up almost -entirely of fragments of different lavas (andesites, diabases, etc.), and more -than 100 feet thick, which often show little or no trace of stratification, -but break up into large quadrangular blocks by means of joints which cut -across the imbedded boulders. These volcanic conglomerates form some of -the more conspicuous features of the coast to the south and north of Oban, -and are well exposed in the Isle of Kerrera. They offer many points of -resemblance to those of Lake Caledonia, but no certain proof has been -noted that they belong to the Lower Old Red Sandstone. They have -obviously been derived from lava-sheets that were exposed to strong breaker-action, -which at the same time transported and rounded blocks of granite, -schist and other crystalline rocks derived from the adjacent hills. During -<span class="pagenum" id="Page_342">- 342 -</span> -the intervals of quieter sedimentation indicated by the fine sandstones and -shales, volcanic explosions continued, as may be seen by the occurrence of -occasional large bombs which have fallen upon and pressed down the fine -ashy silt that was gathering on the bottom.</p> - -<p>It would seem from the characters of some of the strata in this sedimentary -series that over the area of deposit portions of the shallower waters -were occasionally laid bare to the sun and air. Among the conglomerates -there lie certain bands of reddish sandy, ripple-marked, sun-cracked and -rain-pitted shales and fine sandstones. Such accumulations, indicative of -the ultimate exposure of fine sediment that silted up hollows in the great -banks of coarse shingle, may be noticed at the south end of the Island of -Kerrera, on at least two horizons which are separated from each other by -thick masses of conglomerate and fine felspathic grit. We may infer, -therefore, that the fine littoral mud, which gathered during pauses in the -heaping up of the coarse gravel and shingle, was occasionally laid dry. -Similar strata may be observed behind Oban, where the alternation of -exceedingly fine sediment and granular ashy bands is well exhibited.</p> - -<div class="figleft" id="v1fig100" style="width: 284px;"> - <img src="images/v1fig100.png" width="284" height="163" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 100.</span>—Section of lava-escarpment at Beinn Lora, north - side of mouth of Loch Etive, Argyllshire.<br /> - 1. Phyllites; 2. Thick conglomerate; 3. Successive sheets of - andesite.</div> -</div> - -<p>But the explosions that gave rise to the volcanic materials so largely -represented in these lower conglomerates, were merely preliminary to those -which led to the outflow of -the great sheets of lava that -now constitute so large a part -of the hills of Lorne. In the -few traverses which I have -made across different parts of -this district I have noted the -general resemblance of the -lavas to those of the Lower -Old Red Sandstone of the -Midland Valley of Scotland, -their bedded character, and -the occurrence of occasional -layers of stratified material -between them. The prominent features of these rocks, and their relation to -the volcanic conglomerates below them, and to the underlying slates and -schists are well displayed on Beinn Lora at the mouth of Loch Etive (<a href="#v1fig100">Fig. 100</a>). -There the black slates of the district are unconformably covered by -the coarse volcanic conglomerate, formed chiefly of blocks of andesite, -cemented in a hard matrix of similar composition. About 150 or 200 feet -of this material underlie the great escarpment of the lavas, which here rise -in successive beds to the top of the hill, 1000 feet above its base.</p> - -<p>On the south side of Loch Etive the base of the volcanic series, with its -underlying conglomerate, may be followed westward to Oban and thence -southward to Loch Feochan. The lavas cover most of the ground from the -western shore eastwards to near Loch Awe. But this area is still very -imperfectly known. The Geological Survey, however, has now advanced -<span class="pagenum" id="Page_343">- 343 -</span> -to this part of the country, so that we shall before long be in possession -of more detailed information regarding the character and sequence of its -volcanic history and the geological age of the eruptions.</p> - -<p>Mr. H. Kynaston, who has begun the mapping of the eastern portion -of the district, finds that there, as further west, the bottom of the volcanic -series is generally a breccia or conglomerate. He has met with two leading -types among the lavas, the more abundant being strongly vesicular, the -other more compact. He has observed also numerous dykes and sills of -intrusive porphyrite, trending in a general N.N.E. and S.S.W. direction, and -pointing towards the great granite mass of Ben Cruachan.<a id="FNanchor_383" href="#Footnote_383" class="fnanchor">[383]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_383" href="#FNanchor_383" class="label">[383]</a> <i>Ann. Report Geol. Surv.</i> (1895), p. 29 of reprint.</p> - -</div> - -<p>Mr. R. G. Symes has traced the volcanic series to the north and south -of Oban. While visiting with him part of his ground, I was much struck -with the evidence of an intrusive mass at the base of the volcanic series in -the Sound of Kerrera. A prominent feature on the east side of the channel, -known as Dun Uabairtich and 270 feet high, consists of andesite which -appears to combine both a central boss and a sill. The rock breaks through -the black slates and the overlying conglomerates and sandstones, and has -wedged itself into the unconformable junction between the two formations. -It is beautifully columnar on its sea-covered face, some of the columns -being 120 feet or more in length, and gently curved.</p> - - -<h3>"LAKE ORCADIE"</h3> - -<p>We now cross the whole breadth of the Scottish Highlands in order to -peruse the records of another of the great detached water-basins of the Old -Red Sandstone, which for the sake of brevity of reference I have named and -described as "Lake Orcadie" (<a href="#Map_I">Map I.</a>). This area has its southern limits -along the base of the hills that enclose the wide Moray Firth. It spreads -northward over the Orkney and much of the Shetland Islands, but its boundaries -in that direction are lost under the sea. In the extensive sheet of -water which spread over all that northern region the peculiar Caithness Flags, -with their associated sandstones and conglomerates, were deposited to a total -depth of 16,000 feet. A sigillaroid and lycopodiaceous vegetation flourished -on the surrounding land, together with ferns, <i>Psilophyton</i> and conifers. The -waters teemed with fishes of which many genera and species have now -been described. The remains of these creatures lie crowded upon each -other in the flagstones in such a manner as to indicate that from time to -time vast quantities of fish were suddenly killed. Not impossibly, these -destructions may have been connected with the volcanic activity which has -now to be described.</p> - -<p>In the year 1878 I called attention to the evidence for the existence of -contemporaneous volcanic rocks in the Old Red Sandstone north of the range -of the Grampians, and specially noted three localities where this evidence -could be seen—Strathbogie, Buckie and Shetland.<a id="FNanchor_384" href="#Footnote_384" class="fnanchor">[384]</a> Since that time Messrs. -<span class="pagenum" id="Page_344">- 344 -</span> -B. N. Peach and J. Horne have added a fourth centre in the Orkney Islands. -At present, therefore, we are acquainted with the records of four separate -groups of volcanic vents in Lake Orcadie.</p> - -<div class="footnote"> - -<p><a id="Footnote_384" href="#FNanchor_384" class="label">[384]</a> <i>Trans. Roy. Soc. Edin.</i> xxviii. (1878).</p> - -</div> - -<p>The southern margin of this water-basin appears to have indented the -land with long fjord-like inlets. One of these now forms the vale of Strathbogie, -which runs into the hills for a distance of fully 20 miles beyond -what seems to have been the general trend of the coast-line. In this valley I -found a bed of dark vesicular diabase intercalated among the red sandstones -and high above the base of the formation, as exposed on the west side of the -valley near Burn of Craig. On the east side a similar band has since been -mapped for the Geological Survey by Mr. L. Hinxman who has traced it -for some miles down the Strath.<a id="FNanchor_385" href="#Footnote_385" class="fnanchor">[385]</a> This latter band, as shown in <a href="#v1fig101">Fig. 101</a>, -lies not far above the bottom of the Old Red Sandstone of this district, and is -thus probably distinct from the Craig outcrop. There would thus appear to -be evidence of two separate outflows of basic lava in this fjord of the Old -Red Sandstone period.</p> - -<div class="footnote"> - -<p><a id="Footnote_385" href="#FNanchor_385" class="label">[385]</a> See Sheet 76 of the Geological Survey of Scotland.</p> - -</div> - -<div class="figcenter" id="v1fig101" style="width: 434px;"> - <img src="images/v1fig101.png" width="434" height="98" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 101.</span>—Section across Strathbogie, below Rhynie, showing the position of the volcanic band.<br /> - 1. Knotted schists; 2. Diorite intrusive in schists; 3. Old Red Conglomerate; 4. Volcanic band; 5. Shales with - calcareous nodules; 6. Sandstones of Rhynie; 7. Shales and sandstones. <i>f</i>, Fault.</div> -</div> - -<p>No vestige has here been found of any vent, nor is the lava accompanied -with tuff. The eruptions took place some time after the earlier sediments -of the basin were accumulated, but ceased before the thick mass of upper -sandstones and shales was deposited. A section across the valley gives the -structure represented in the accompanying diagram (<a href="#v1fig101">Fig. 101</a>).</p> - -<p>Twenty-five miles further north a still smaller andesite band has been -detected by Mr. J. Grant Wilson among the sandstones and conglomerates -near Buckie.<a id="FNanchor_386" href="#Footnote_386" class="fnanchor">[386]</a> It is a truly contemporaneous flow, for pebbles of it occur in -the overlying strata. But again it forms only a solitary bed, and no trace -of any accompanying tuff has been met with, nor of the vent from which it -came. Both this vent and that of Strathbogie must have been situated near -the southern coast-line of the lake.</p> - -<div class="footnote"> - -<p><a id="Footnote_386" href="#FNanchor_386" class="label">[386]</a> See Sheet 95 of the Geological Survey of Scotland and <i>Trans. Roy. Soc. Edin.</i> vol. xxviii. -(1878), p. 435.</p> - -</div> - -<p>At a distance of some 90 miles northward from these Moray Firth -vents another volcanic district lies in the very heart of the Orkney Islands.<a id="FNanchor_387" href="#Footnote_387" class="fnanchor">[387]</a> -The lavas which were there ejected occur at the south-eastern corner -of the island of Shapinshay, where they are regularly bedded with -the flagstones. They consist of dark green olivine-diabases with highly -<span class="pagenum" id="Page_345">- 345 -</span> -amygdaloidal and vesicular upper surfaces. Their thickness cannot be -ascertained, as their base is not seen, but they have been cut by the -sea into trenches which show them to exceed 30 feet in depth. The -position of the vent from which they came has not been ascertained. -Neither here nor in the Moray Firth area do any sills accompany the -interbedded sheets, and in both cases the volcanic action would seem to have -been of a feeble and short-lived character.</p> - -<div class="footnote"> - -<p><a id="Footnote_387" href="#FNanchor_387" class="label">[387]</a> Messrs. B. N. Peach and J. Horne, <i>Proc. Roy. Phys. Soc. Edin.</i> vol. v. (1879), p. 80.</p> - -</div> - -<p>Much more important were the volcanoes that broke out nearly -100 miles still further north, where the Mainland of Shetland now -lies. I shall never forget the pleasure with which I first recognized the -traces of these eruptions, and found near the most northerly limits of the -British Isles proofs of volcanic activity in the Lower Old Red Sandstone. -Since my observations were published,<a id="FNanchor_388" href="#Footnote_388" class="fnanchor">[388]</a> Mr. Peach, who accompanied me in -Shetland, has returned to the district, and, in concert with his colleague -Mr. Horne, has extended our knowledge of the subject.<a id="FNanchor_389" href="#Footnote_389" class="fnanchor">[389]</a> The chief vent or -vents lay towards the west and north-west of the Mainland and North -Mavine; others of a less active and persistent type were blown out some -25 miles to the east, where the islands of Bressay and Noss now -stand. In the western district streams of slaggy andesite and diabase with -showers of fine tuff and coarse agglomerate were ejected, until the total -accumulation reached a thickness of not less than 500 feet. The volcanic -eruptions took place contemporaneously with the deposition of the red sandstones, -for the lavas and tuffs are intercalated in these strata. The lavas -and volcanic conglomerates are traceable from the southern coast of Papa -Stour across St. Magnus' Bay to the western headlands of Esha Ness, a -distance of more than 14 miles. They have been cut by the Atlantic -into a picturesque range of cliffs, which exhibit in some places, as at the -singular sea-stalk of Doreholm, rough banks of andesitic lava with the -conglomerate deposited against and over them, and in other places, as -along the cliffs of Esha Ness, sheets of lava overlying the conglomerates.</p> - -<div class="footnote"> - -<p><a id="Footnote_388" href="#FNanchor_388" class="label">[388]</a> <i>Trans. Roy. Soc. Edin.</i> vol. xxviii. (1878), p. 418.</p> - -<p><a id="Footnote_389" href="#FNanchor_389" class="label">[389]</a> <i>Ibid.</i> vol. xxxii. (1884), p. 359.</p> - -</div> - -<p>No trace of any vents has been found in the western and chief volcanic -district, but in Noss Sound a group of small necks occurs, filled with a coarse -agglomerate composed of pieces of sandstone, flagstone and shale. Messrs. -Peach and Horne infer that these little orifices never discharged any streams -of lava. More probably they were opened by explosions which only gave -forth vapours and fragmentary discharges, such as a band of tuff which is -intercalated among the flagstones in their neighbourhood.</p> - -<p>But one of the most striking features of the volcanic phenomena of this -remote region is the relative size and number of the sills and dykes which -here as elsewhere mark the latest phases of subterranean activity. Messrs. -Peach and Horne have shown us that three great sheets of acid rocks -(granites and spherulitic felsites, to which reference has already been made, -<a href="#Page_292">p. 292</a>) have been injected among the sandstones and basic lavas, that -abundant veins of granite, quartz-felsite and rhyolite radiate from these -acid sills, and that the latest phase of igneous action in this region was the -<span class="pagenum" id="Page_346">- 346 -</span> -intrusion of a series of bosses and dykes of basic rocks (diabases) which -traverse the sills.</p> - - -<h3><span class="smcap">The Killarney District</span></h3> - -<p>In the south of Ireland the Upper Silurian strata are followed upwards -conformably by the great series of red sandstones and conglomerates known -as the "Dingle Beds." Lithologically these rocks present the closest -resemblance to the Lower Old Red Sandstone of Central Scotland. They -occupy a similar stratigraphical position, and though they have not yielded -any palæontological data for comparison, there can, I think, be no hesitation -in classing them with the Scottish Lower Old Red Sandstone, and regarding -them as having been deposited under similar geographical conditions. They -offer one feature of special interest for the purpose of the present inquiry, -since they contain a well-marked group of contemporaneous volcanic rocks, -including nodular felsites, like those so characteristic of the Silurian period.</p> - -<p>The area where this remote and isolated volcanic group is best developed -forms a range of high rugged ground along the northern front of the hills -that stretch eastward from the Lakes of Killarney. Their general distribution -is shown on Sheets 184 and 185 of the Geological Survey of Ireland;<a id="FNanchor_390" href="#Footnote_390" class="fnanchor">[390]</a> -though I may again remark that petrography has made great strides during -the thirty years and more that have passed since these maps and their -accompanying Memoirs were published, and that, were the district to be -surveyed now, probably a considerable tract of ground coloured as ash would -be marked as felsite. At the same time the existence of both these rocks -here cannot be gainsaid.</p> - -<div class="footnote"> - -<p><a id="Footnote_390" href="#FNanchor_390" class="label">[390]</a> See the Memoir (by J. B. Jukes and G. V. Du Noyer) on Sheet 184, p. 15. Other volcanic -rocks have been mapped at Valentia Harbour in the Dingle Beds, but these I have not had an -opportunity of personally examining.</p> - -</div> - -<p>The felsite was long ago brought into notice by Dr. Haughton, who -published an analysis of it.<a id="FNanchor_391" href="#Footnote_391" class="fnanchor">[391]</a> It is also referred to by Mr. Teall for its -spherulitic structure.<a id="FNanchor_392" href="#Footnote_392" class="fnanchor">[392]</a> Seen on the ground it appears as a pale greenish-grey -close-grained rock, sometimes exhibiting flow-structure in a remarkably -clear manner, the laminæ of devitrification following each other in wavy -lines, sometimes twisted and delicately puckered or frilled, as in some -schists. Portions of the rock are strongly nodular, the nodules varying in -size from less than a pea to that of a hen's egg.</p> - -<div class="footnote"> - -<p><a id="Footnote_391" href="#FNanchor_391" class="label">[391]</a> <i>Trans. Roy. Irish Acad.</i> vol. xxiii. (1859), p. 615.</p> - -<p><a id="Footnote_392" href="#FNanchor_392" class="label">[392]</a> <i>British Petrography</i>, p. 349.</p> - -</div> - -<p>The close resemblance of this rock to many of the Lower Silurian -nodular felsites of Wales cannot but strike the geologist. It presents -analogies also to the Upper Silurian felsites of Dingle. But its chief -interest arises from the geological horizon on which it occurs. Lying in -the so-called "Dingle-Beds," which may be regarded as the equivalents -of the Lower Old Red Sandstone of England and Scotland, it is, so far -as my observations go, the only example of such a nodular felsite of -later date than the Silurian period. We recognize in it a survival, as it -were, of the peculiar Silurian type of acid lava, the last preceding eruption -<span class="pagenum" id="Page_347">- 347 -</span> -of which took place not many miles to the west, in the Dingle promontory. -But elsewhere this type does not appear to have survived the end of the -Silurian period.</p> - -<p>The detrital rocks accompanying the felsite, in the district east of -Killarney, vary from such closed-grained felsitic material as cannot readily -be distinguished from the felsite itself to unmistakable felsitic breccias. -Even in the finest parts of them, occasional rounded quartz-pebbles may -be detected, while here and there a reddish shaly band, or a layer of fine -pebbly conglomerate with quartz-pebbles an inch in length, shows at once -the bedding and the dip. Mr. W. W. Watts, who, with Mr. A. M'Henry of -the Irish Staff of the Geological Survey, accompanied me over this ground, -found that a microscopic examination of the slides which were prepared -from the specimens we collected completely confirmed the conclusions -reached from inspection of the rocks in the field.<a id="FNanchor_393" href="#Footnote_393" class="fnanchor">[393]</a> He detected among the -angular grains slightly damaged crystals of felspar, chiefly orthoclase. -Many portions of these felspathic grits much resemble the detrital Cambrian -rocks which in the Vale of Llanberis have been made out of the pale felsite -of that locality.</p> - -<div class="footnote"> - -<p><a id="Footnote_393" href="#FNanchor_393" class="label">[393]</a> Mr. Watts also examined the microscopic structure of the felsite of Benaun More. He -found that the spherulites appear to have a micropegmatitic structure, owing to the intergrowth -of quartz and felspar. In some parts of the rock the spherulites, from ·02 to ·01 inch in diameter, -are surrounded by exceedingly minute green needles, possibly of hornblende, while inside some -of them are small quartz-grains. Larger porphyritic felspars occur outside the spherulites, some -being of plagioclase, but most of orthoclase. The spherulitic structure is not so well developed -near the felspars. A few of the large nodules are hollow and lined with crystals, while some of -them show a finely concentric lamination like the successive layers of an agate.</p> - -</div> -<hr class="chap x-ebookmaker-drop" /> - - -<div class="chapter"> -<p><span class="pagenum" id="Page_348">- 348 -</span></p> - -<h2 class="nobreak" id="CHAPTER_XXII">CHAPTER XXII<br /> - -<span class="smaller">VOLCANOES OF THE UPPER OLD RED SANDSTONE—THE SOUTH-WEST OF -IRELAND, THE NORTH OF SCOTLAND</span></h2> -</div> - - -<p>In the northern half of Britain, where the Old Red Sandstone is so well -displayed, the two great divisions into which this series of sedimentary -deposits is there divisible are separated from each other by a strongly marked -unconformability. The interval of time represented by this break must -have been of long duration, for it witnessed the effacement of the old -water-basins, the folding, fracture, and elevation of their thick sedimentary -and volcanic accumulations, and the removal by denudation of, in some -places, several thousand feet of these rocks. The Upper Old Red Sandstone, -consisting so largely as it does of red sandstones and conglomerates, -indicates the return or persistence of geographical conditions not unlike -those that marked the deposition of the lower subdivision. But in one -important respect its history differs greatly from that which I have -sketched for the older part of the system. Though the Upper Old Red -Sandstone is well developed across the southern districts of Scotland from -the Ochil to the Cheviot Hills, and appears in scattered areas over so much -of England and Wales, no trace has ever been there detected in it of any -contemporaneously erupted volcanic rocks. The topographical changes -which preceded its deposition must have involved no inconsiderable -amount of subterranean disturbance, yet the volcanic energy, which had -died out so completely long before the close of the time of the Lower Old -Red Sandstone, does not appear to have been rekindled until the beginning -of the Carboniferous period.</p> - -<p>Two widely separated tracts in the British Isles have yielded traces of -contemporaneous volcanic rocks in the Upper Old Red Sandstone. One -of these lies in the south-west of Ireland, the other in the far north of -Scotland.</p> - - -<h3>THE SOUTH-WEST OF IRELAND</h3> - -<p>The Irish locality is situated a few miles to the south of the town of -Limerick, where the Carboniferous Limestone has been thrown into long -folds, and where, along the anticlines, strips of the underlying red sandstones -<span class="pagenum" id="Page_349">- 349 -</span> -have been brought up to the surface. Two such ridges of Upper Old Red -Sandstone bear, each on its crest, a small but interesting relic of volcanic -activity<a id="FNanchor_394" href="#Footnote_394" class="fnanchor">[394]</a> (<a href="#Map_I">Map I.</a>).</p> - -<div class="footnote"> - -<p><a id="Footnote_394" href="#FNanchor_394" class="label">[394]</a> See Sheet 153 of the Geological Survey of Ireland, and Explanation to that Sheet (1861), by -Messrs. G. H. Kinahan and J. O'Kelly. The account of the ground above given is from notes -which I made during a personal visit.</p> - -</div> - -<p>The more northerly ridge rises in the conical eminence of Knockfeerina -to a height of 949 feet above the sea. Even from a distance the -resemblance of this hill (<a href="#v1fig102">Fig. 102</a>) to many of the Carboniferous necks of -Scotland at once attracts the eye of the geologist. The resemblance is -found to hold still more closely when the internal structure of the ground is -examined. The cone consists mainly of a coarse agglomerate, with blocks -generally somewhat rounded and varying in size up to two feet in length. The -most prominent of these, on the lower eastern slopes, are pieces of a fine flinty -felsite weathering white, but there also occur fragments of grit and baked -shale. The matrix is dull-green in colour, and among its ingredients are -abundant small lapilli of a finely vesicular andesite or diabase. These -more basic ingredients increase in number towards the top of the -eminence, where much of the agglomerate is almost wholly made up of -them. No marked dip is observable over most of the hill, the rock appearing -as a tumultuous agglomerate, though here and there, particularly near -the top and on the south side, a rude bedding may be detected dipping -outwards. On the west side the agglomerate is flanked with yellow sandstone -baked into quartzite, so that the line of junction there between the -two rocks not improbably gives us the actual wall of the vent. The -induration of the surrounding sandstones is a familiar feature among the -Carboniferous vents. Some intrusive dark flinty rock traverses the agglomerate -near the top on the north side.</p> - -<div class="figcenter" id="v1fig102" style="width: 370px;"> - <img src="images/v1fig102.png" width="370" height="170" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 102.</span>—View of Knockfeerina, Limerick, from the north-east—a volcanic neck of - Upper Old Red Sandstone age.</div> -</div> - -<p>Retiring eastwards from the cone, the observer finds evidence of the -intercalation of tuff among the surrounding Upper Old Red Sandstone. At -the east end of the village of Knockfeerina a red nodular tuff, with rounded -pieces of andesite, grit and sandstone, sometimes 12 inches long, is seen -to dip under yellow, grey and red sandstones and shales, while other shales -<span class="pagenum" id="Page_350">- 350 -</span> -and sandstones underlie this tuff and crop out between it and the agglomerate. -There is thus evidence of the intercalation of volcanic tuff in the -Upper Old Red Sandstone of this district. And there seems no reason to -doubt that the tuff was ejected from the adjoining vent of Knockfeerina.</p> - -<p>On the next ridge of Old Red Sandstone, which runs parallel to that of -Knockfeerina at a distance of little more than a mile to the south, another -mass of volcanic material rises into a prominence at Ballinleeny. On the -north side it consists of agglomerate like that just described, and is -flanked by sandstone baked into quartzite. Here again we probably see -the edge of a volcanic funnel. There may possibly be more than one vent -in this area. But well-bedded tuffs can be observed to dip away from -the centre and to pass under sandstones and shales which are full of -fine ashy material. Gradations can be traced from the tuff into ordinary -sediment. In this instance, therefore, there is additional proof of contemporaneous -volcanic action in the Upper Old Red Sandstone. There can be -no uncertainty as to the horizon of the strata in which these records have -been preserved, for they dip conformably under the shales and limestones -at the base of the Carboniferous Limestone series. They are said to have -yielded the characteristic fern <i>Palæopteris</i> of Kiltorcan.<a id="FNanchor_395" href="#Footnote_395" class="fnanchor">[395]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_395" href="#FNanchor_395" class="label">[395]</a> There may be some other examples of Upper Old Red Sandstone volcanic rocks in Ireland -which I have not yet been able personally to examine. On the maps of the Geological Survey -(Sheet 198, and Explanation, pp. 8, 17) contemporaneous rocks of this age are marked as -occurring at Cod's Head and Dursey Island, on the south side of the mouth of the Kenmare -estuary.</p> - -</div> - - -<h3>THE NORTH OF SCOTLAND</h3> - -<p>The only district in England or Scotland wherein traces of volcanic -action during the time of the Upper Old Red Sandstone have been observed -lies far to the north among the Orkney Islands, near the centre of -the scattered outliers which I have united as parts of the deposits of "Lake -Orcadie"<a id="FNanchor_396" href="#Footnote_396" class="fnanchor">[396]</a> (<a href="#Map_I">Map I.</a>). The thick group of yellow and red sandstones which -form most of the high island of Hoy, and which, there can be little doubt, -are correctly referred to the Upper Old Red Sandstone, rest with a marked -unconformability on the edges of the Caithness flagstones (<a href="#v1fig103">Fig. 103</a>). At -the base of these pale sandstones, and regularly interstratified with them, -lies a band of lavas and tuffs which can be traced from the base of the -rounded hills to the edge of the cliffs at the Cam, along the face of which -it runs as a conspicuous feature, gradually sloping to a lower level, till it -reaches the sea. At the Cam of Hoy it is about 200 feet thick, and consists -of three or more sheets of andesite. The upper 50 or 60 feet show a -strongly slaggy structure, the central portion is rudely columnar, and the -lower part presents a kind of horizontal jointing or bedding. There can be -no question that this rock is not a sill but a group of contemporaneous lava-flows. -Beneath it, and lying across the edges of the flagstones below, there -<span class="pagenum" id="Page_351">- 351 -</span> -is a zone of dull-red, fine-grained tuff, banded with seams of hard red and -yellow sandstone. This tuff zone dies out to the eastward of the Cam.</p> - -<div class="footnote"> - -<p><a id="Footnote_396" href="#FNanchor_396" class="label">[396]</a> First noticed in <i>Geol. Mag.</i> February 1878; and <i>Trans. Roy. Soc. Edin.</i> xxviii. (1878), p. 411.</p> - -</div> - -<table summary="images"> -<tr> - <td style="width: 45%;" class="tdc"><a id="v1fig103"></a><img src="images/v1fig103.png" width="220" height="277" alt="" /></td> - <td rowspan="2"> </td> - <td style="width: 45%;" class="tdc"><a id="v1fig104"></a><img src="images/v1fig104.png" width="258" height="284" alt="" /></td> -</tr> -<tr> - <td><div class="hanging2"><span class="smcap">Fig. 103.</span>—Section of the volcanic zone in the - Upper Old Red Sandstone, Cam of Hoy, Orkney.<br /> - 1. Caithness flagstones; 2. Dull-red tuff and bands - of sandstone; 3. Lava zone in three bands; 4. - Yellow and red sandstone.</div></td> - <td class="vtop"><div class="hanging2"><span class="smcap">Fig. 104.</span>—Section of the volcanic zone in the Upper Old - Red Sandstone at Black Ness, Rackwick, Hoy.</div></td> -</tr> -</table> - -<p>A few miles south of the Cam the -volcanic zone appears again as the platform -on which the picturesque natural -obelisk of the Old Man of Hoy stands. -Here the lava runs out as a promontory -from the base of the cliff, and on this -projection the Old Man has been left -isolated from the main precipice. The -cliffs of Hoy are traversed by numerous -small faults which have shifted the -volcanic zone. But on the great face -of rock behind the Old Man there appears -to be a second volcanic zone lying -several hundred feet above that just -described. It is probably this upper -zone which emerges from under the -hills a mile and a half to the south at -Black Ness in the bay of Rackwick. -A good section is there visible, which -is represented in <a href="#v1fig104">Fig. 104</a>. The ordinary -red and yellow sandstones (<i>a</i>) appear -from under the volcanic rocks at this -locality, and stretch southwards to the most southerly headland of Hoy. -The lowest volcanic band in the -section is one of red sandy well-bedded -tuffs (<i>b</i>). Some of the -layers are coarse and almost -agglomeratic, while others are -fine marly and sandy, with dispersed -bombs, blocks and lapilli -of diabase and andesite. Hard -ribs of a sandy tufaceous material -also occur. These fragmental -deposits are immediately overlain -by a dark-blue rudely prismatic -diabase with slaggy top -(<i>c</i>). It is about 150 feet thick -at its thickest part, but rapidly -thins away in a westerly direction. -It passes under a zone -of red tuff (<i>d</i>) like that below, -and above this highest member -of the volcanic group comes the -great overlying pile of yellow and reddish sandstone of Hoy. Followed -<span class="pagenum" id="Page_352">- 352 -</span> -westward for a short distance, the whole volcanic zone is found to die out -and the sandstones below and above it then come together.</p> - -<p>The interest of this little volcanic centre in Hoy is heightened by -the fact that the progress of denudation has revealed some of the vents -belonging to it. On the low ground to the east of the Cam, and immediately -to the north of the volcanic escarpment, the flagstones which there -emerge from under the base of the unconformable upper sandstones are -pierced by three volcanic necks which we may with little hesitation -recognize as marking the sites of vents from which this series of lavas and -tuffs was discharged (<a href="#v1fig105">Fig. 105</a>). The largest of them forms a conspicuous -hill about 450 feet high, the smallest is only a few yards in diameter, and -rises from the surface of a flagstone ridge. They are filled with a coarse, -dull-green, volcanic agglomerate, made up of fragments of the lavas with -pieces of hardened yellow sandstone and flagstone. Around the chief vent -the flagstones through which it has been opened have been greatly hardened -and blistered. The most easterly vent, which has been laid bare on the -beach at Bring, due east of Hoy Hill, can be seen to pierce the flagstones, -which, although their general dip is westerly at from 10° to 15°, yet at -their junction with the agglomerate are bent in towards the neck, and are -otherwise much jumbled and disturbed.</p> - -<div class="figcenter" id="v1fig105" style="width: 391px;"> - <img src="images/v1fig105.png" width="391" height="149" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 105.</span>—Section across the volcanic band and its associated necks, Hoy, Orkney.<br /> - 1. Caithness flagstones; 2. Volcanic band lying on red sandstones and conglomerates and dying out eastwards; 3 3. - Two vents between the base of the hills and the sea; their connection with the volcanic band is shown by dotted - lines; 4. Overlying mass of Upper Old Red Sandstone forming the hills of Hoy.</div> -</div> - -<p>On the northern coast of Caithness I have described a remarkable -volcanic vent about 300 feet in diameter, which rises through the uppermost -group of the Caithness flagstones. It is filled with a coarse -agglomerate consisting of a dull-greenish diabase paste crowded with -blocks of diabase, sometimes three feet in diameter, and others of red -sandstone, flagstone, limestone, gneiss and lumps of black cleavable -augite (<a href="#v1fig106">Fig. 106</a>).<a id="FNanchor_397" href="#Footnote_397" class="fnanchor">[397]</a> The sandstones around it present the usual disrupted, -indurated and jointed character, and are traversed by a small diabase dyke -close to the western margin of the neck. Another similar neck has since -been found by the officers of the Geological Survey on the same coast. -<span class="pagenum" id="Page_353">- 353 -</span> -That these volcanic orifices were active about the same time with those in -the opposite island of Hoy may be legitimately inferred.</p> - -<div class="footnote"> - -<p><a id="Footnote_397" href="#FNanchor_397" class="label">[397]</a> See <i>Trans. Roy. Soc. Edin.</i> xxviii. (1878), p. 405; also p. 482 of the same volume for an -account of the cleavable augite.</p> - -</div> - -<div class="figcenter" id="v1fig106" style="width: 319px;"> - <img src="images/v1fig106.png" width="319" height="162" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 106.</span>—Ground-plan of volcanic neck piercing the Caithness Flagstone series on the beach near - John o' Groat's House.</div> -</div> - -<p>These northern volcanoes made their appearance in a district where -during the preceding Lower Old Red Sandstone period there had been -several widely separated groups of active volcanic vents. So far as the -fragmentary nature of the geological evidence permits an opinion to be -formed, they seem to have broken out at the beginning, or at least at an -early stage, of the deposition of the Upper Old Red Sandstone, and to have -become entirely extinct after the lavas of Hoy were poured forth. No -higher platform of volcanic materials has been met with in that region. -With these brief and limited Orcadian explosions the long record of Old -Red Sandstone volcanic activity in the area of the British Isles comes to -an end.<a id="FNanchor_398" href="#Footnote_398" class="fnanchor">[398]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_398" href="#FNanchor_398" class="label">[398]</a> There appear to be traces of volcanic eruptions contemporaneous with the Upper Old Red -Sandstone of Berwickshire, but as they merely formed a prelude to the great volcanic activity of -Carboniferous time, they are included in the account of the Carboniferous plateau of Berwickshire -in Chapters <a href="#CHAPTER_XXIV">xxiv.</a> and <a href="#CHAPTER_XXV">xxv.</a></p> - -</div> -<hr class="chap x-ebookmaker-drop" /> - - -<div class="chapter"> -<p><span class="pagenum" id="Page_355">- 355 -</span></p> - -<h2 class="nobreak" id="BOOK_VI">BOOK VI<br /> - -<span class="smaller">THE CARBONIFEROUS VOLCANOES</span></h2> -</div> - - -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<h2 class="nobreak" id="CHAPTER_XXIII">CHAPTER XXIII<br /> - -<span class="smaller">THE CARBONIFEROUS SYSTEM OF BRITAIN AND ITS VOLCANIC RECORDS</span></h2> -</div> - -<div class="blockquot"> - -<p>Geography and Scenery of the Carboniferous Period—Range of Volcanic Eruptions during -that time—I. The Carboniferous Volcanoes of Scotland—Distribution, Arrangement -and Local Characters of the Carboniferous System in Scotland—Sketch of the Work -of previous Observers in this Subject.</p> -</div> - - -<p>Within the area of the British Isles, the geological record is comparatively -full and continuous from the base of the Upper Old Red Sandstone -to the top of the Coal-measures. We learn from it that the local basins of -deposit in which the later portion of the Old Red Sandstone was accumulated -sank steadily in a wide general subsidence, that allowed the clear sea of the -Carboniferous Limestone ultimately to spread for some 700 miles from the -west coast of Ireland into Westphalia. Over the centre of England this -Carboniferous Mediterranean had a breadth of at least 150 miles, gradually -shallowing northwards in the direction of land in Scotland and Northern -Ireland. The gentle sinking of the floor of the basin continued until more -than 6000 feet of sediment, chiefly composed of the remains of crinoids, -corals and other marine organisms, had been piled up in the deeper parts. -Traces of the southern margin of this sea, or at least of a long insular -ridge that rose out of its waters, are to be seen in the protuberances -of older rocks which appear at intervals from under the Coal-measures -and later formations between the borders of Wales and the heart of -Leicestershire, and of which the crags of Charnwood Forest are among the -few peaks that still remain visible. To the south of this ridge, open sea -extended far southward and westward over the site of the Mendip Hills and -the uplands of South Wales.</p> - -<p>The Carboniferous period, as chronicled by its sedimentary deposits, was -a time of slow submergence and quiet sedimentation, terrestrial and marine -<span class="pagenum" id="Page_356">- 356 -</span> -conditions alternating along the margins of the sinking land, according as -the rate of depression surpassed or fell short of that of the deposition of -sediment. There is no trace of any general disturbance among the strata, -such as would be marked by an important and widely extended unconformability. -But many indications may be observed that the rate of -subsidence did not continue uniform, if, indeed, the downward movement -was not locally arrested, and even exchanged for a movement in the opposite -direction. It is difficult, for instance, to believe the ancient ridge of the -Midlands to have been so lofty that even the prolonged subsidence required -for the accumulation of the whole Carboniferous system was insufficient to -carry its highest crests below the level of the coal-jungles. More probably -the depression reached its maximum along certain lines or bands running in -a general north-easterly direction, the intervals between these lines sinking -less, or possibly even undergoing some measure of uplift. One of the subsiding -tracts, that of the wide lowlands of Central Scotland, was flanked on -the south by a ridge which, while its north-eastern portion was buried under -the Upper Old Red Sandstone and Lower Carboniferous rocks, remained -above water towards the south-west, and does not appear to have been -wholly submerged there even at the close of the Carboniferous period.</p> - -<p>So abundant and varied are the sedimentary formations of Carboniferous -time, and so fully have they preserved remains of the contemporary plants -and animals, that it is not difficult to realise in some measure the general -aspect of the scenery of the time, and the succession of changes which it -underwent from the beginning to the end of the period. The land was -green with a luxuriant if somewhat monotonous vegetation. Large pine -trees flourished on the drier uplands. The lower grounds nourished dense -groves of cycads or plants allied to them, which rose as slim trees twenty or -thirty feet high, with long hard green leaves and catkins that grew into -berries. The swamps and wetter lands bore a rank growth of various gigantic -kinds of club-moss, equisetaceous reeds and ferns.</p> - -<p>Nor was the hum of insect-life absent from these forests. Ancestral -types of cockroaches, mayflies and beetles lived there. Scorpions swarmed -along the margins of the shallow waters, for their remains, washed away -with the decayed vegetation among which they harboured, are now found -in abundance throughout many of the dark shales.</p> - -<p>The waters were haunted by numerous kinds of fish quite distinct from -those of the Old Red Sandstone. In the lagoons, shoals of small ganoids -lived on the cyprids that peopled the bottom, and they were in turn preyed -on by larger ganoids with massive armature of bone. Now and then a -shark from the opener sea would find its way into these more inland waters. -The highest types of animal life yet known to have existed at this time were -various amphibians of the extinct order of Labyrinthodonts.</p> - -<p>The open sea, too, teemed with life. Wide tracts of its floor supported -a thick growth of crinoids whose jointed stems, piled over each other -generation after generation, grew into masses of limestone many hundreds -of feet in thickness. Corals of various kinds lived singly and in colonies, -<span class="pagenum" id="Page_357">- 357 -</span> -here and there even growing into reefs. Foraminifera, sponges, sea-urchins, -brachiopods, gasteropods, lamellibranchs and cephalopods, in many genera -and species, mingled their remains with the dead crinoids and corals to -furnish materials for the wide and thick accumulation of Carboniferous -Limestone.</p> - -<p>Looking broadly at the history of the Carboniferous period, and bearing -in mind the evidence of prolonged depression already referred to, we can -recognize in it three great eras. During the first, the wide clear sea of the -Carboniferous Limestone spread over the centre and south of Britain, interrupted -here and there by islands that rose from long ridges whereby the sea-floor -was divided into separate basins. Next came a time of lessened -depression, when the sea-bottom was overspread with sand, mud and gravel, -and was even in part silted up, as has been chronicled in the Millstone -Grit. The third stage brings before us the jungles of the Coal-measures, -when the former sea-floor became a series of shallow lagoons where, as in -the mangrove-swamps of our own time, a terrestrial vegetation sprang up -and mingled its remains with those of marine shells and fishes.</p> - -<p>Such a state of balance among the geological forces as is indicated by -the stratigraphy of the Carboniferous system would not prepare us for the -discovery of the relics of any serious display of contemporary volcanic -activity. And, indeed, throughout the Carboniferous rocks of Western -Europe there is for the most part little trace of contemporaneous volcanic -eruptions. Yet striking evidence exists that, along the western borders of -the continental area, in France as well as over much of Britain, which had -for so many previous geological ages been the theatre of subterranean activity, -the older half of Carboniferous time witnessed an abundant, though less -stupendous and prolonged, renewal of volcanic energy.</p> - -<p>From the very commencement of the Carboniferous period to the epoch -when the Coal-measures began to be accumulated, the area of the British -Isles continued to be a scene of active volcanism. In the course of that -prolonged interval of geological time the vents shifted their positions, -and gradually grew less energetic, but there does not appear to have been -any protracted section of the interval when the subterranean activity became -everywhere entirely quiescent.</p> - -<p>The geologist who traces, from older to younger formations, the progress -of some persistent operation of nature, observes the evidence gradually to -increase in amount and clearness as it is furnished by successively later -parts of the record. He finds that the older rocks have generally been so -dislocated and folded, and are often so widely covered by younger formations, -that the evidence which they no doubt actually contain may be difficult to -decipher, or may be altogether concealed from view. In following, for -instance, the progress of volcanic action, he is impressed, as he passes from -the older to the younger Palæozoic chronicles, by the striking contrast -between the fulness and legibility of the Carboniferous records and the -comparative meagreness and obscurity of those of the earlier periods. The -Carboniferous rocks have undergone far less disturbance than the Cambrian -<span class="pagenum" id="Page_358">- 358 -</span> -and Silurian formations; while over wide tracts, where their volcanic -chapters are fullest and most interesting, they lie at the surface, and can -thus be subjected to the closest scrutiny. Hence the remains of the -volcanic phenomena of the later Palæozoic periods present a curiously -modern aspect, when contrasted with the fragmentary and antique look of -those of older date.</p> - -<p>The history of volcanic action during the Carboniferous period in Britain -is almost wholly comprised in the records of the earlier half of that period, -that is, during the long interval represented by the Carboniferous Limestone -series and the Millstone Grit. It was chiefly in the northern part of the -region that volcanic activity manifested itself. In Scotland there is the -chronicle of a long succession of eruptions across the district of the central -and southern counties, from the very beginning of Carboniferous time down -to the epoch when the Coal-measures began to be accumulated. In England, -on the other hand, the traces of Carboniferous volcanoes are confined within -a limited range in the Carboniferous Limestone, while in Ireland they appear -to be likewise restricted to the same lower division of the system. During -the whole of the vast interval represented by the Coal-measures volcanic -energy, so far as at present known, was entirely dormant over the region of -the British Isles.</p> - -<p>These general statements will be more clearly grasped from the accompanying -table, which shows the various sections into which the Carboniferous -system of Britain has been divided, and also, by black vertical lines, the -range of volcanic intercalations in each of the three kingdoms.</p> - -<table summary="data"> -<tr> - <td class="bdl bdt bdb" colspan="3"></td> - <td class="bdl bdt bdb tdc">England.</td> - <td class="bdl bdt bdb tdc">Scotland.</td> - <td class="bdl bdt bdb bdr tdc">Ireland.</td> -</tr> -<tr> - <td class="bdl tdl" colspan="3">Coal-measures.</td> - <td class="bdl tdc"> </td> - <td class="bdl tdc"> </td> - <td class="bdl bdr tdc"> </td> -</tr> -<tr> - <td class="bdl" rowspan="3"> </td> - <td rowspan="3"><img src="images/bracel_86.png" width="11" height="86" alt="" /></td> - <td class="tdl">Upper Red Sandstones with<br /> <i>Spirorbis</i>-limestone.</td> - <td class="bdl tdc"> </td> - <td class="bdl tdc"> </td> - <td class="bdl bdr tdc"> </td> -</tr> -<tr> - <td class="tdl">Middle or chief coal-bearing<br />measures.</td> - <td class="bdl tdc"> </td> - <td class="bdl tdc"> </td> - <td class="bdl bdr tdc"> </td> -</tr> -<tr> - <td class="tdl">Gannister group.</td> - <td class="bdl tdc"> </td> - <td class="bdl tdc"> </td> - <td class="bdl bdr tdc"> </td> -</tr> -<tr> - <td class="bdl tdl" colspan="3">Millstone Grit.</td> - <td class="bdl tdc"> </td> - <td class="bdl bdb tdc" rowspan="6"><img src="images/vertline.png" width="11" height="250" alt="" /></td> - <td class="bdl bdr tdc"> </td> -</tr> -<tr> - <td class="bdl"> </td> - <td><img src="images/bracel_36.png" width="11" height="36" alt="" /></td> - <td class="tdl">Grits, flagstones and shales with<br />thin coals.</td> - <td class="bdl tdc"> </td> - <td class="bdl bdr tdc"> </td> -</tr> -<tr> - <td class="bdl tdl" colspan="3">Carboniferous Limestone.</td> - <td class="bdl tdc"> </td> - <td class="bdl bdr tdc"> </td> -</tr> -<tr> - <td class="bdl bdb" rowspan="3"> </td> - <td class="bdb" rowspan="3"><img src="images/bracel_132.png" width="11" height="132" alt="" /></td> - <td class="tdl">Yoredale group of shales and grits<br />with limestones.</td> - <td class="bdl tdc"> </td> - <td class="bdl bdr tdc"> </td> -</tr> -<tr> - <td class="tdl">Thick (Scaur or Main) Limestone<br />of England, with sandstones<br />and coals in Scotland.</td> - <td class="bdl tdc"><img src="images/vertline.png" width="11" height="50" alt="" /></td> - <td class="bdl bdr tdc vbot"><img src="images/vertline.png" width="11" height="25" alt="" /></td> -</tr> -<tr> - <td class="bdb tdl">Lower Limestone Shale (Calciferous<br />Sandstones of Scotland).</td> - <td class="bdl bdb tdc"> </td> - <td class="bdl bdb bdr tdc"> </td> -</tr> -</table> - -<p>Such being the general range in time of the Carboniferous volcanic -phenomena, it may be convenient, in this preliminary survey, to take note of -the general distribution of the volcanic districts over the British Isles, as in -this way we may best realise the extent and grouping of the eruptions, -which will then be considered in further detail (see <a href="#Map_I">Map I.</a>).</p> - -<p><span class="pagenum" id="Page_359">- 359 -</span></p> - -<p>Not only were the Carboniferous volcanoes most abundant and persistent -in Scotland, but they attained there a variety and development which give -their remains an altogether exceptional interest in the study of volcanic -geology. They were distributed over the wide central valley, from the -south of Cantyre to beyond the mouth of the estuary of the Forth. On the -southern side of the Silurian Uplands, they were likewise numerous and -active. There is thus no considerable tract of Lower Carboniferous rocks in -Scotland which does not furnish its evidence of contemporaneous volcanic -action.</p> - -<p>Although some portions of the Scottish Carboniferous igneous rocks -run for a short distance into England, it is remarkable that, when these -at last die out southwards, no other relics of contemporaneous volcanic -energy take their place. Along the Pennine chain, from the Border into -the heart of England, though natural sections are abundant, no trace of -included volcanic rocks appears until we reach Derbyshire. The whole -of that wide interval of 150 miles, so far as the present evidence goes, -remained during Carboniferous time entirely free from any volcanic eruption. -But from the picturesque country of the Peak southwards, the sea-floor of -the Carboniferous Limestone, in what is now the heart of England, was -dotted with vents whence the sheets of "toadstone" were ejected, which -have so long been a familiar feature in English geology. Beyond this -limited volcanic district the Carboniferous formations of the south-west -of England remain, on the whole, devoid of contemporaneous volcanic -intercalations, traces of Carboniferous volcanic action having been recognized -only in West Somerset and Devonshire. In the Mendip district and in the -ridges of limestone near Weston-super-Mare bands of cellular lava and tuff -have been observed. To the west of Dartmoor, Brent Tor and some of the -surrounding igneous masses may mark the positions of eruptive vents during -an early part of the Carboniferous period.</p> - -<p>At the south end of the Isle of Man relics remain of a group of -vents among the Carboniferous limestones. Passing across to Ireland, -where these limestones attain so great a thickness and cover so large -a proportion of the surface of the island, we search in vain for any -continuation of the abundant and varied volcanic phenomena of Central -Scotland. So far as observation has yet gone, only two widely separated -areas of Carboniferous volcanic rocks are known to occur in Ireland.<a id="FNanchor_399" href="#Footnote_399" class="fnanchor">[399]</a> One -of these shows that a little group of vents probably rose from the floor of -the Carboniferous Limestone sea, near Philipstown, in King's County. The -other lies far to the west in the Golden Vale of Limerick, where a more -important series of vents poured out successive streams of lava with showers -of ashes, from an early part of the Carboniferous period up to about the -beginning of the time of the Coal-measures.</p> - -<div class="footnote"> - -<p><a id="Footnote_399" href="#FNanchor_399" class="label">[399]</a> The supposed Carboniferous volcanic rocks of Bearhaven on the coast of Cork are noticed on -<a href="../../66493/66493-h/66493-h.htm#Page_49">p. 49, vol. ii.</a></p> - -</div> - -<p>The total area within which the volcanic eruptions of Carboniferous -time took place was thus less than that over which the volcanoes of the -<span class="pagenum" id="Page_360">- 360 -</span> -Lower Old Red Sandstone were distributed, yet they were scattered -across the larger part of the site of the British Isles. From the vents -of Fife to those of Limerick is a distance of above 300 miles; from the -latter eastward to those of Devonshire is an interval of 250 miles; while -the space between the Devonshire volcanoes and those of Fife is about -400 miles. In this triangular space volcanic action manifested itself at -each of the apices, to a slight extent along the centre of the eastern side, -but with much the greatest vigour throughout the northern part of the -area.</p> - -<p>Since the volcanic phenomena of Carboniferous time are exhibited on a -much more extensive scale in Scotland than in any other region of the -world yet studied, it will be desirable to describe that area in considerable -detail. The other tracts in Britain where volcanic rocks of the same age -occur need not be so fully treated, except where they help to a better -comprehension of the general volcanic history.</p> - -<hr class="tb" /> - -<p>It is in the southern half of Scotland that the Carboniferous system is -developed (<a href="#Map_IV">Map IV.</a>). A line drawn from Machrihanish Bay, near the Mull -of Cantyre, north-eastward across Arran and Bute to the south end of Loch -Lomond, and thence eastward by Bridge of Allan, Kinross and Cupar to St. -Andrews Bay, forms the northern limit of this system. South of that line -Carboniferous volcanic intercalations are to be met with in nearly every -county across into the borders of Northumberland.</p> - -<p>That we may follow intelligently the remarkably varied volcanic history -of this region, it is desirable to begin by taking note of the nature and -sequence of the sedimentary formations among which the volcanic rocks are -intercalated, for these serve to bring before us the general conditions of -the geography of the period. The subjoined table exhibits the subdivisions -into which the Carboniferous system in Scotland has been grouped:—</p> - -<table style="width: 40em;" summary="data"> -<tr> - <td class="tdl" colspan="3">Coal-measures.</td> -</tr> -<tr> - <td rowspan="2"> </td> - <td class="tdc" rowspan="2"><img src="images/bracel_36.png" width="11" height="36" alt="" /></td> - <td><div class="hanging2">Upper Red Sandstone group, nearly devoid of coal-seams.</div></td> -</tr> -<tr> - <td><div class="hanging2">Coal-bearing, white, yellow and grey sandstones, dark shales and - ironstones (Upper Coal series).</div></td> -</tr> -<tr> - <td class="tdl" colspan="3">Millstone Grit.</td> -</tr> -<tr> - <td> </td> - <td class="tdc"><img src="images/bracel_32.png" width="11" height="32" alt="" /></td> - <td><div class="hanging2">Thick white and reddish sandstones and grits.</div></td> -</tr> -<tr> - <td class="tdl" colspan="3">Carboniferous Limestone series.</td> -</tr> -<tr> - <td rowspan="2"> </td> - <td class="tdc" rowspan="2"><img src="images/bracel_86.png" width="11" height="86" alt="" /></td> - <td><div class="hanging2">Sandstones, shales, fireclays, coal-seams, ironstones and three seams - of marine limestone, of which the uppermost is known as the Castlecary - seam, the second as the Calmy or Arden, and the lowest as the Index - (Lower Coal series).</div></td> -</tr> -<tr> - <td><div class="hanging2">Bands of marine limestone intercalated among sandstones, shales and some - coal-seams. A thick band of limestone lying at or near the bottom of - the group, traceable all over Central Scotland, is known as the Hurlet - or Main Limestone. Some higher and thinner seams are called Hosie's - (see <a href="#v1fig155">Fig. 155</a>).</div></td> -</tr> -<tr> - <td class="tdl" colspan="3">Calciferous Sandstones.<a id="FNanchor_400" href="#Footnote_400" class="fnanchor">[400]</a><span class="pagenum" id="Page_361">- 361 -</span></td> -</tr> -<tr> - <td rowspan="3"> </td> - <td class="tdc" rowspan="3"><img src="images/bracel_150.png" width="11" height="150" alt="" /></td> - <td><div class="hanging2">In the basin of the Firth of Forth, below the Hurlet Limestone, comes - a varied series of white and yellow sandstones, black shales - (oil-shales), cyprid shales and limestones (Burdiehouse), and occasional - coal-seams (Houston), having a total depth of about 3000 feet. This local - group abounds in fossil plants, entomostraca and ganoid fishes. It passes - down into the Cement-stone group, which, however, is feebly developed in - this district, unless it is partly represented by the sandstones, shales, - limestones and coals just mentioned.</div></td> -</tr> -<tr> - <td><div class="hanging2">Cement-stone group consisting of red, blue and green marls and shales, red - and grey sandstones, and thin bands of cement-stone: fossils scarce.</div></td> -</tr> -<tr> - <td><div class="hanging2">Reddish and grey sandstones and shales, with occasional plant-remains, - passing down into the deep red (sometimes yellow) sandstones, red marls - and cornstones of the Upper Old Red Sandstone.</div></td> -</tr> -</table> - -<div class="footnote"> - -<p><a id="Footnote_400" href="#FNanchor_400" class="label">[400]</a> The Calciferous Sandstones are the stratigraphical equivalents of the Limestone Shale and -lower portion of the Carboniferous Limestone of England.</p> - -</div> - -<p>From this table the gradual geographical evolution of the Carboniferous -period in Scotland may be gleaned. We observe that at the beginning, -the conditions under which the Old Red Sandstone had been accumulated -still in part continued. The great lacustrine basins of the Lower -Old Red Sandstone had indeed been effaced, and their sites were occupied -by comparatively shallow areas of fresh or brackish water in which -the Upper Old Red Sandstone was laid down. Their conglomerates and -sandstones had been uplifted and fractured. Their vast ranges of volcanic -material, after being deeply buried under sediment, had been once more laid -bare, and extended as ridges of land, separating the pools and lagoons -which they supplied with sand and silt. This singular topography had not -been entirely effaced at the beginning of the Carboniferous period, for we -find that many of the ridges which bounded the basins of the Upper Old -Red Sandstone remained as land until they sank beneath the waters in -which the earliest Carboniferous strata accumulated. Thus, while no trace -of an unconformability has yet been detected at the top of the Upper Old -Red Sandstone, there is often a strong overlap of the succeeding deposits. -At the south end of the Pentland Hills, for example, the Upper Old Red -Sandstone attains a thickness of 1000 feet, but only three miles further south -it entirely disappears, together with all the overlying mass of Calciferous -Sandstones, and the Carboniferous Limestone then rests directly on the -Lower Old Red Sandstone. Again, at the north end of the same chain the -upper division of the Old Red Sandstone dies out against the lower, which is -eventually overlapped by the Calciferous Sandstones.</p> - -<p>The change from the physical conditions of the Scottish Old Red Sandstone -to those of the Carboniferous system was no doubt gradual and slow. -The peculiar red sandy sediment continued to be laid down in basins that -were apparently being gradually widened by access of water from the open -sea. Yet it would seem that in Scotland these basins still for a long time -continued saline or, from some other cause, unfavourable to life; for the red, -blue and green shales or marls, and occasional impure limestones or cement-stones -and gypseous layers, which were deposited in them, are in general -unfossiliferous, though drifted plants from the neighbouring land are here -<span class="pagenum" id="Page_362">- 362 -</span> -and there common enough. The sediments of these early Carboniferous -waters are met with all over the southern half of Scotland, but in very unequal -development, and constitute what is known as the "Cement-stone -Group."</p> - -<p>It was while these strata were in course of deposition that the earliest -Carboniferous volcanoes broke into eruption. In some localities a thickness -of several hundred feet of the Cement-stone group underlies the lowest lavas. -In other places the lavas occur in and rest on the Upper Old Red Sandstone -and have the Cement-stone group wholly above them; while in yet other -districts the volcanic rocks seem entirely to take the place of that group. -So vigorous was the earliest display of volcanic action in Carboniferous -times that from the borders of Northumberland to the uplands of Galloway, -and from the slopes of the Lammermuirs to Stirlingshire and thence across -the estuary of the Clyde to Cantyre, innumerable vents were opened and -large bodies of lava and ashes were ejected.</p> - -<p>The Cement-stone group, save where succeeded by volcanic intercalations, -passes up conformably into the lowest crinoidal limestones of the -Carboniferous Limestone series. In the basin of the Firth of Forth, however, -the cement-stones, feebly represented there, are overlain by a remarkable -assemblage of white sandstones, black carbonaceous shales, or "oil-shales," -cyprid limestones, occasional marine limestones and thin seams of coal, the -whole having a thickness of more than 3000 feet. These strata, unlike the -typical Cement-stone group, abound in fossils both vegetable and animal. -They prove that, over the area of the Forth, the insalubrious basins wherein -the red and green sediments of the Cement-stone group were laid down, -gave place to opener and clearer water with occasional access of the sea. -The peculiar lagoon-conditions which favoured the formation of coal were -thus developed in Central Scotland earlier than elsewhere in Britain. We -shall see in later pages that these conditions were accompanied by a fresh -outbreak of volcanic activity, in a phase less vigorous but more enduring -and extensive than that of the first Carboniferous eruptions.</p> - -<p>The Carboniferous Limestone sea over the site of the southern half of -Scotland appears never to have reached the depth which it attained in -England and Ireland. To the north of it lay the land from which large -quantities of sand and mud were carried into it, as shown by the deep -accumulations of sandstone and shale, which far surpass in thickness the -few comparatively thin marine limestones intercalated in them. There is -thus a striking contrast between the thick masses of limestone in central -and south-western England and their dwindled representatives in the north. -Another marked difference between the Scottish and English developments -of this formation is to be noticed in the abundant proof that the comparatively -shallow waters of the northern basin were plentifully dotted over with -active volcanoes. The eruptions were especially vigorous and prolonged in -the basin of the Firth of Forth. They continued at intervals, even after the -peculiar geographical conditions of the Carboniferous Limestone had ceased. -But they had died out by the time of the beginning of the Coal-measures.</p> - -<p><span class="pagenum" id="Page_363">- 363 -</span></p> - -<p>Owing to the number and variety of the natural sections, the Carboniferous -volcanic rocks of Scotland have been the subject of numerous observations -and descriptions, from the early days of geology down to the present -time. The mere enumeration of the titles of the various publications -regarding them would make a long list. These rocks formed the subject of -some of Hutton's early observations, and furnished him with facts from -which he established the igneous origin of "whinstone."<a id="FNanchor_401" href="#Footnote_401" class="fnanchor">[401]</a> They supplied -Playfair with numerous apt illustrations in support of Hutton's views, and -he seems to have made himself thoroughly familiar with them.<a id="FNanchor_402" href="#Footnote_402" class="fnanchor">[402]</a> In the -hands of Sir James Hall they became the groundwork of those remarkable -experiments on the fusion of whinstone which may be said to have laid the -foundation of experimental geology.<a id="FNanchor_403" href="#Footnote_403" class="fnanchor">[403]</a> In the controversies of the Neptunian -and Plutonian schools these rocks were frequently appealed to by each side -in confirmation of its dogmas. The appointment in 1804 of Jameson to the -Chair of Natural History in the Edinburgh University gave increased -impetus to the study of the igneous rocks of Scotland. Though he did not -himself publish much regarding them, we know that he was constantly in -the habit of conducting his class to the hills, ravines and quarries around -Edinburgh, and that the views which he taught were imbibed and extended -by his pupils.<a id="FNanchor_404" href="#Footnote_404" class="fnanchor">[404]</a> Among the early writers the names of Allan,<a id="FNanchor_405" href="#Footnote_405" class="fnanchor">[405]</a> Townson,<a id="FNanchor_406" href="#Footnote_406" class="fnanchor">[406]</a> -Lord Greenock,<a id="FNanchor_407" href="#Footnote_407" class="fnanchor">[407]</a> and Ami Boué,<a id="FNanchor_408" href="#Footnote_408" class="fnanchor">[408]</a> deserve especial mention.</p> - -<div class="footnote"> - -<p><a id="Footnote_401" href="#FNanchor_401" class="label">[401]</a> Hutton's <i>Theory of the Earth</i>, vol. i. p. 155 <i>et seq.</i></p> - -<p><a id="Footnote_402" href="#FNanchor_402" class="label">[402]</a> Playfair's <i>Illustrations of the Huttonian Theory</i>, § 255 <i>et seq.</i></p> - -<p><a id="Footnote_403" href="#FNanchor_403" class="label">[403]</a> <i>Trans. Roy. Soc. Edin.</i> (1805), vol. v. p. 43.</p> - -<p><a id="Footnote_404" href="#FNanchor_404" class="label">[404]</a> <i>Mem. Wern. Soc.</i> ii. 178, 618; iii. 25; <i>Edin. Phil. Journ.</i> i. 138, 352; xv. 386.</p> - -<p><a id="Footnote_405" href="#FNanchor_405" class="label">[405]</a> <span class="smcap">Trans. Roy. Soc. Edin.</span> (1811), vi. p. 405.</p> - -<p><a id="Footnote_406" href="#FNanchor_406" class="label">[406]</a> <i>Tracts and Observations in Natural History and Physiology</i>, 8vo, Lond. 1799.</p> - -<p><a id="Footnote_407" href="#FNanchor_407" class="label">[407]</a> <i>Trans. Roy. Soc. Edin.</i> (1833), xiii. pp. 39, 107.</p> - -<p><a id="Footnote_408" href="#FNanchor_408" class="label">[408]</a> <i>Essai géologique sur l'Écosse.</i> Paris; no date, probably 1820.</p> - -</div> - -<p>The first broad general sketch of the Carboniferous igneous rocks of a large -district of the country was that given by Hay Cunningham in his valuable -essay on the geology of the Lothians.<a id="FNanchor_409" href="#Footnote_409" class="fnanchor">[409]</a> He separated them into two series, -the Felspathic, including "porphyry" and "clinkstone," and the Augitic or Trap -rocks. To these he added "Trap-tufa," which he considered to be identical -in origin with modern volcanic tuff. It was the eruptive character of the -igneous rocks on which he specially dwelt, showing by numerous sections -the effects which the protrusion of the molten masses have had upon the -surrounding rocks. He did not attempt to separate the intrusive from the -interstratified sheets, nor to form a chronological arrangement of the whole.</p> - -<div class="footnote"> - -<p><a id="Footnote_409" href="#FNanchor_409" class="label">[409]</a> <i>Mem. Wern. Soc.</i> vii. p. 1. Published separately, 1838.</p> - -</div> - -<p>Still more important was the sketch given by Maclaren, in his classic -<i>Geology of Fife and the Lothians</i>.<a id="FNanchor_410" href="#Footnote_410" class="fnanchor">[410]</a> This author clearly recognized that -many of the igneous rocks were thrown out contemporaneously with the -strata among which they now lie. He constantly sought for analogies -among modern volcanic phenomena, and presented the Carboniferous igneous -rocks of the Lothians not as so many petrographical varieties, but as monuments -<span class="pagenum" id="Page_364">- 364 -</span> -of different phases of volcanic action previous to the formation of -the Coal-measures. His detailed descriptions of Arthur Seat and the rocks -immediately around Edinburgh, which alone the work was originally intended -to embrace, may be cited as models of exact and luminous research. -The portions referring to the rest of the basin of the Forth did not profess -to be more than a mere sketch of the subject.</p> - -<div class="footnote"> - -<p><a id="Footnote_410" href="#FNanchor_410" class="label">[410]</a> Small 8vo, Edin. 1838, first partly published as articles in the <i>Scotsman</i> newspaper. A -second edition, which was little more than a reprint of the first, appeared in 1866.</p> - -</div> - -<p>Various papers of more local interest, to some of which allusion will be -made in the sequel, appeared during the next quarter of a century. But -no systematic study of the volcanic phenomena of any part of Scotland -was resumed until the extension in 1854 of the Geological Survey to the -north of the Tweed by A. C. Ramsay. The volcanic rocks of the Lothians -and Fife were mapped by Mr. H. H. Howell and myself. The maps of that -district began to be published in the year 1859, and the Memoirs two years -later. In 1861, in a chronological grouping of the whole of the volcanic phenomena -of Scotland, I gave an outline of the Carboniferous eruptions.<a id="FNanchor_411" href="#Footnote_411" class="fnanchor">[411]</a> By -degrees the detailed mapping of the Geological Survey was pushed across the -whole of the rest of the south of Scotland, and the Carboniferous volcanic -rocks of each area were then for the first time carefully traced and assigned -to their various stratigraphical horizons. In the following pages reference -will be given to the more important features of the Survey maps and -Memoirs. In the year 1879, availing myself of the large amount of -information which my own traverses and the work of the Survey had -enabled me to acquire, I published a Memoir on the geology and petrography -of the volcanic rocks of the basin of the Firth of Forth;<a id="FNanchor_412" href="#Footnote_412" class="fnanchor">[412]</a> and lastly, -in my Presidential Address to the Geological Society in 1892, I gave a -summary of all that had then been ascertained on the subject of the volcanic -rocks of Carboniferous time in the British Isles.<a id="FNanchor_413" href="#Footnote_413" class="fnanchor">[413]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_411" href="#FNanchor_411" class="label">[411]</a> <i>Trans. Roy. Soc. Edin.</i> vol. xxii.</p> - -<p><a id="Footnote_412" href="#FNanchor_412" class="label">[412]</a> <i>Ibid.</i> vol. xxix. (1879), p. 437.</p> - -<p><a id="Footnote_413" href="#FNanchor_413" class="label">[413]</a> <i>Quart. Journ. Geol. Soc.</i> xlviii. (1892), p. 104. This summary, with additional details and -illustrations, is embodied in the text.</p> - -</div> - -<p>Two well-marked types of volcanic accumulations are recognizable in -the British Isles, which may be conveniently termed Plateaux and Puys.</p> - -<p>1. <span class="smcap">Plateaux.</span>—In this type, the volcanic materials were discharged -over wide tracts of country, so that they now form broad tablelands or -ranges of hills, reaching sometimes an extent of many hundreds of square -miles and a thickness of more than 1000 feet. Plateaux of this character -occur within the British area only in Scotland, where they are the predominant -phase of volcanic intercalations in the Carboniferous system.</p> - -<p>It is noteworthy that the Carboniferous plateaux appeared during a -well-marked interval of geological time. The earliest examples of them date -from the close of the Upper Old Red Sandstone. They were all in vigorous -eruption during the time of the Calciferous Sandstones, but in no case did -they survive into that of the Hurlet and later limestones. They are thus -eminently characteristic of the earliest portion of the Carboniferous period.</p> - -<p>2. <span class="smcap">Puys.</span>—In this type, the ejections were often confined to the discharge -of a small amount of fragmentary materials from a single solitary -<span class="pagenum" id="Page_365">- 365 -</span> -vent, and even where the vents were more numerous and the outpourings -of lava and showers of ash more copious, the ejected material usually -covered only a small area round the centres of eruption. Occasionally -streams of basic lava and accumulations of tuff were piled up into long -ridges. Volcanoes of this character were specially abundant in the basin -of the Firth of Forth, and more sparingly in Ayrshire and Roxburghshire. -They form the persistent type throughout the rest of the British Isles.</p> - -<p>The Puys also occupy a well-defined stratigraphical position. They did -not begin until some of the volcanic plateaux had become extinct. From -the top of the Cement-stone group up into the Carboniferous Limestone -series, their lavas and tuffs are met with on many platforms, but none occur -above that series save in Ayrshire, where some of the eruptions appear to -have been as late as about the beginning of the Coal-measures.</p> - -<p>Arranged in tabular form the stratigraphical and geographical distribution -of the two great volcanic types of the Carboniferous system in -Scotland will be more easily followed. I have therefore drawn up the -accompanying scheme:—</p> - -<p><span class="pagenum" id="Page_366">- 366 -</span></p> - -<table summary="data"> -<tr> - <td class="tdl smaller" colspan="14"><span style="margin-left: 1em;">Location Key:</span><br /> - <span style="margin-left: 3em;">A. Ayrshire and Renfrewshire.</span><br /> - <span style="margin-left: 3em;">B. Stirlingshire.</span><br /> - <span style="margin-left: 3em;">C. West Lothian.</span><br /> - <span style="margin-left: 3em;">D. Midlothian.</span><br /> - <span style="margin-left: 3em;">E. East Lothian.</span><br /> - <span style="margin-left: 3em;">F. Fife.</span><br /> - <span style="margin-left: 3em;">G. Berwick & Roxburghshire.</span></td> -</tr> -</table> - -<table class="plateaus" summary="data"> -<tr> - <td class="bdl bdt bdb" colspan="3"></td> - <td class="tdc bdt bdb" colspan="5"><span class="smcap">Plateau-type.</span></td> - <td class="tdc bdl2 bdt bdb bdr" colspan="7"><span class="smcap">Puy-type.</span></td> -</tr> -<tr> - <td class="bdl bdt bdb" colspan="3"></td> - <td class="tdc bdl bdb">A.</td> - <td class="tdc bdl bdb">B.</td> - <td class="tdc bdl bdb">D.</td> - <td class="tdc bdl bdb">E.</td> - <td class="tdc bdl bdb">G.</td> - <td class="tdc bdl2 bdb">A.</td> - <td class="tdc bdl bdb">B.</td> - <td class="tdc bdl bdb">C.</td> - <td class="tdc bdl bdb">D.</td> - <td class="tdc bdl bdb">E.</td> - <td class="tdc bdl bdb">F.</td> - <td class="tdc bdl bdb bdr">G.</td> -</tr> -<tr> - <td class="tdl bdl" colspan="3">Coal Measures</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 bdl2">..</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 bdl" colspan="3">Millstone Grit</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 bdl2"><img src="images/vertline.png" height="100%" alt="" /></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 bdl" colspan="3">Carboniferous Limestone Series.</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 bdl2"><img src="images/vertline.png" height="100%" alt="" /></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" rowspan="4"> </td> - <td rowspan="4"><img src="images/bracel_86.png" width="11" height="86" alt="" /></td> - <td class="tdl">Castlecary Limestone</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 bdl2"><img src="images/vertline.png" height="100%" alt="" /></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">Calmy "</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 bdl2"><img src="images/vertline.png" height="100%" alt="" /></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">Index "</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 bdl2"><img src="images/vertline.png" height="100%" alt="" /></td> - <td class="tdc bdl">..</td> - <td class="tdc bdl"><img src="images/vertline.png" height="100%" alt="" /></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">Hurlet "</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 bdl2">..</td> - <td class="tdc bdl">..</td> - <td class="tdc bdl"><img src="images/vertline.png" height="100%" alt="" /></td> - <td class="tdc bdl">..</td> - <td class="tdc bdl">..</td> - <td class="tdc bdl">..</td> - <td class="tdc bdl bdr"><img src="images/vertline.png" height="100%" alt="" /></td> -</tr> -<tr> - <td class="bdl" colspan="3">Calciferous Sandstone Series.</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 bdl2">..</td> - <td class="tdc bdl">..</td> - <td class="tdc bdl"><img src="images/vertline.png" height="100%" alt="" /></td> - <td class="tdc bdl">..</td> - <td class="tdc bdl">..</td> - <td class="tdc bdl">..</td> - <td class="tdc bdl bdr"><img src="images/vertline.png" height="100%" alt="" /></td> -</tr> -<tr> - <td class="bdl bdb" rowspan="3"> </td> - <td class="bdb" rowspan="3"><img src="images/bracel_86.png" width="11" height="86" alt="" /></td> - <td>Burdiehouse Limestone<br /> and Oil-shale Group</td> - <td class="tdc bdl vtop"><img src="images/vertline.png" width="11" height="36" alt="" /></td> - <td class="tdc bdl vtop"><img src="images/vertline.png" width="11" height="24" alt="" /></td> - <td class="tdc bdl">..</td> - <td class="tdc bdl vbot">..<br /><img src="images/vertline.png" width="11" height="18" alt="" /></td> - <td class="tdc bdl">..</td> - <td class="tdc bdl2">..</td> - <td class="tdc bdl">..</td> - <td class="tdc bdl vtop"><img src="images/vertline.png" width="11" height="30" alt="" /></td> - <td class="tdc bdl vbot"><img src="images/vertline.png" width="11" height="24" alt="" /></td> - <td class="tdc bdl">..</td> - <td class="tdc bdl">..</td> - <td class="tdc bdl bdr vtop"><img src="images/vertline.png" width="11" height="24" alt="" /></td> -</tr> -<tr> - <td class="tdl">Cement-stone Group</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 vbot"><img src="images/vertline.png" height="100%" alt="" /></td> - <td class="tdc bdl2">..</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 bdb">Red Sandstones passing down<br /> into Upper Old Red Sandstone</td> - <td class="tdc bdl bdb">..</td> - <td class="tdc bdl bdb">..</td> - <td class="tdc bdl bdb">..</td> - <td class="tdc bdl bdb">..</td> - <td class="tdc bdl bdb vtop"><img src="images/vertline.png" width="11" height="12" alt="" /></td> - <td class="tdc bdl2 bdb">..</td> - <td class="tdc bdl bdb">..</td> - <td class="tdc bdl bdb">..</td> - <td class="tdc bdl bdb">..</td> - <td class="tdc bdl bdb">..</td> - <td class="tdc bdl bdb">..</td> - <td class="tdc bdl bdb bdr">..</td> -</tr> -</table> - -<hr class="chap x-ebookmaker-drop" /> - - -<div class="chapter"> -<p><span class="pagenum" id="Page_367">- 367 -</span></p> - -<h2 class="nobreak" id="CHAPTER_XXIV">CHAPTER XXIV<br /> - -<span class="smaller">CARBONIFEROUS VOLCANIC PLATEAUX OF SCOTLAND</span></h2> -</div> - -<div class="blockquot"> - -<p>I. The Plateau-type restricted to Scotland—i. Distribution in the Different Areas of -Eruption—ii. Nature of the Materials erupted.</p> -</div> - - -<p>In the division of the Plateaux I group all the more copious eruptions -during the Carboniferous period, when the fragmentary materials generally -formed but a small part of the discharges, but when the lavas were poured -out so abundantly and frequently as to form lava-fields sometimes more than -2000 square miles in area, and to build up piles of volcanic material sometimes -upwards of 3000 feet in thickness. As already remarked, this phase -of volcanic action, especially characteristic of the earlier part of the Carboniferous -period across the south of Scotland, but not found elsewhere in the -same system in Britain, preceded the type of the Puys. Its eruptions extended -from about the close of the Old Red Sandstone period through that -section of Carboniferous time which was marked by the deposition of the -Calciferous Sandstones, but they entirely ceased before the accumulation of the -Main or Hurlet Limestone, at the base of the Carboniferous Limestone Series -of Scotland. Its stratigraphical limits, however, are not everywhere the same. -In the eastern part of the region, the lavas appear to be intercalated with, and -certainly lie directly upon, the Upper Old Red Sandstone containing scales -of <i>Bothriolepis</i> and other characteristic fishes, and they are covered by the -Cement-stone group of the Calciferous Sandstones. In the western district -a considerable thickness of Carboniferous strata sometimes underlies the -volcanic sheets. On the other hand, the type of the Puys, although it -appeared in Fife, Linlithgowshire and Midlothian during the time of the -Calciferous Sandstones, attained its chief development during that of the -Carboniferous Limestone, and did not finally die out in Ayrshire until the -beginning of the deposition of the Coal-measures.</p> - - -<p>i. <span class="allsmcap">DISTRIBUTION OF THE PLATEAUX</span></p> - -<p>Notwithstanding the effects of many powerful faults and extensive denudation, -the general position of the Plateaux and their independence of each -<span class="pagenum" id="Page_368">- 368 -</span> -other can still be traced. They are entirely confined, as I have said, to the -southern half of Scotland (see <a href="#Map_IV">Map IV.</a>). In noting their situations we are once -more brought face to face with the remarkable fact, so strikingly manifested -in the geological history of Britain, that volcanic action has been apt to recur -again and again in or near to the same areas. The Carboniferous volcanic -plateaux were poured out from vents, some of which not impossibly rose -among the extinct vents of the Old Red Sandstone. Another fact, to which -also I have already alluded as partially recognizable in the records of Old Red -Sandstone volcanism, now becomes increasingly evident—the tendency of -volcanic vents to be opened along lines of valley rather than over tracts of hill. -The vents that supplied the materials of the largest of the Carboniferous -volcanic plateaux broke forth, like the Old Red Sandstone volcanoes, along -the broad Midland Valley of Scotland, between the ridge of the Highlands -on the north and that of the Southern Uplands on the south. Others -appeared in the long hollow between the southern side of these uplands, -and the Cheviot Hills and hills of the Lake District. It is not a question -of the rise of volcanic vents merely along lines of fault, but over broad -tracts of low ground rather than on the surrounding or neighbouring -heights. It can easily be shown that this distribution is not the result of -better preservation in the valleys and greater denudation from the higher -grounds, for, as has been already remarked in regard to the volcanoes of the -Old Red Sandstone, these higher grounds are singularly free from traces of -necks which, had any vents ever existed there, would certainly have remained -as memorials of them. The following summary of the position and -extent of the Plateaux will afford some idea of their general characters:—</p> - -<div class="figcenter" id="v1fig107" style="width: 510px;"> - <img src="images/v1fig107.png" width="510" height="163" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 107.</span>—View of the escarpment of the Clyde Plateau in the Little Cumbrae, from the south-west.</div> -</div> - -<p>1. <span class="smcap">The Clyde Plateau.</span>—The chief plateau rises into one of the most -conspicuous features in the scenery of Central Scotland. Beginning at Stirling, -it forms the tableland of the Fintry, Kilsyth, Campsie and Kilpatrick -Hills, stretching westwards to the Clyde near Dumbarton. It rises again on -the south side of that river, sweeping southwards into the hilly moorlands -which range from Greenock to Ardrossan, and spreading eastwards along the -high watershed between Renfrewshire, Ayrshire, and Lanarkshire to Galston -and Strathavon. But it is not confined to the mainland, for its prolongation -can be traced down the broad expanse of the Firth of Clyde by the islands -<span class="pagenum" id="Page_369">- 369 -</span> -of Cumbrae to the southern end of Bute, and thence by the east of Arran to -Campbeltown in Cantyre. Its visible remnants thus extend for more than -100 miles from north-east to south-west, with a width of some thirty-five -miles in the broadest part. We shall probably not exaggerate if we estimate -the original extent of this great volcanic area as not less than between 2000 -and 3000 square miles.</p> - -<p>It is in this tract that the phenomena of the plateaux are most admirably -displayed. Ranges of lofty escarpments reveal the succession of the several -eruptions, and the lower ground in front of these escarpments presents to us, -as the result of stupendous denudation, many of the vents from which the -materials of the plateau were ejected, while in the western portion of the area -admirable coast-sections lay bare to view the minutest details of structure.<a id="FNanchor_414" href="#Footnote_414" class="fnanchor">[414]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_414" href="#FNanchor_414" class="label">[414]</a> This plateau is represented in Sheets 12, 21, 29, 30, 31 and 39 of the Geological Survey, and is -described in the accompanying Memoirs as far as published. The eastern part of the Campsie -Hills was surveyed by Mr. B. N. Peach, the western part by Mr. R. L. Jack, who also mapped -the rest of the plateau to the Clyde, and a portion of the high ground of Renfrewshire and Ayrshire; -the rest of the area, south to Ardrossan, was surveyed by myself. The tract from Stewarton -to Strathavon was surveyed by Mr. James Geikie, the Cumbraes and Bute by Mr. W. Gunn, and -southern Cantyre by Mr. R. G. Symes. The Campsie Hills have been partly described by Mr. John -Young in the first volume of the <i>Transactions of the Glasgow Geological Society</i>. The occurrence -of plants in the tuffs of the east coast of Arran was discovered by Mr. E. Wunsch. The Campbeltown -igneous rocks were described by J. Nicol, <i>Quart. Journ. Geol. Soc.</i> viii. (1852), p. 406. See -also J. Bryce's <i>Arran and Clydesdale</i>.</p> - -</div> - -<p>It will be seen from the map (No. IV.), that the Clyde plateau extends in -a general north-east and south-west direction. It is inclined on the whole -towards the east, where, when not interrupted by faults, its highest lavas -and tuffs may be seen to pass under the Carboniferous Limestone series. -Its greatest elevations are thus towards its escarpment, which, commencing -above the plains of the Forth a little to the west of Stirling, extends as a -striking feature to the Clyde above Dumbarton. On the south side of the -great estuary the escarpment again stretches in a noble range of terraced -slopes for many miles into Ayrshire. It is well developed in the Little -Cumbrae Island (<a href="#v1fig107">Fig. 107</a>), and in the south of Bute, where its successive -platforms of lava mount in terraces and green slopes above the Firth. Even -as far as the southern coast of Cantyre the characteristic plateau scenery -reappears in the outliers which there cap the hills and descend the slopes -(<a href="#v1fig108">Fig. 108</a>).</p> - -<p>While the escarpment side of this plateau is comparatively unfaulted, so -that the order of succession of the lavas and their superposition in the sedimentary -rocks can be distinctly seen, the eastern or dip side is almost -everywhere dislocated. Innumerable local ruptures have taken place, -allowing the limestone series to subside, and giving to the margin of the -volcanic area a remarkably notched appearance. To the effects of this -faulting may be attributed the way in which the plateau has been separated -into detached blocks with intervening younger strata. Thus a complex -series of dislocations brings in a long strip of Carboniferous Limestone which -extends from Johnston to Ardrossan, while another series lets in the limestone -that runs from Barrhead to near Dalry. In each of these instances, the -<span class="pagenum" id="Page_370">- 370 -</span> -continuity of the volcanic plateau is interrupted. To the same cause we owe -the occasional reappearance of a portion of the plateau beyond the limits of -the main mass, as for instance in the detached area which occurs in the -valley of the Garnock above Kilwinning.</p> - -<div class="figcenter" id="v1fig108" style="width: 490px;"> - <img src="images/v1fig108.png" width="490" height="325" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 108.</span>—View of the edge of the Volcanic Plateau south of Campbeltown, Argyllshire.<br /> - The uppermost of the three zones is the volcanic series with its lava-ridges. The central band is the Upper Old - Red Sandstone, lying conformably beneath the lavas, with its cornstone which has been quarried. The lowest - band, tinted dark, is the Lower Old Red Sandstone, on which the other rocks rest unconformably.</div> -</div> - -<p>Denudation has likewise come into play, not only in reducing the area of -the plateau, but in isolating portions of it into outliers, with or without the -assistance of faults. The site of the Cumbraes and Bute was no doubt at -one time covered with a continuous sheet of volcanic material, and there -appears to be no reason for refusing to believe that this sheet formed part of -that which caps the opposite uplands of Ayrshire. From the southern end -of Bute it is only about seven miles across to the shore of Arran near Corrie, -where the lavas and tuffs reappear. They are so poorly represented there, -however, that we are evidently not far from the limit of the plateau in that -direction. So vast has been the denudation of the region that it is now -impossible to determine whether the volcanic ejections of Campbeltown, which -occupy the same geological platform as those of Arran, Bute and Ayrshire, -were also actually continuous with them. But as the distance between the -denuded fragments of the volcanic series in Arran and in Cantyre is only -about 20 miles it is not improbable that this continuity existed, and thus -that the volcanic accumulations reached at least as far as the southern end -of Argyllshire, where they now slip under the sea.</p> - -<p><span class="pagenum" id="Page_371">- 371 -</span></p> - -<div class="figcenter" id="v1fig109" style="width: 700px;"> - <img src="images/v1fig109.png" width="700" height="461" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 109.</span>—View of North Berwick Law from the east, a trachyte neck marking one of the chief vents of the Garleton Plateau. (From a photograph.)<br /> - This illustration and Figs. <a href="#v1fig119">119</a>, <a href="#v1fig133">133</a> and <a href="#v1fig135">135</a> are from photographs taken by Mr. Robert Lunn for the Geological Survey.</div> -</div> - - -<p><span class="pagenum" id="Page_372">- 372 -</span></p> - -<p>2. <span class="smcap">The East Lothian or Garleton Plateau.</span>—Some 50 miles to the -east of the Clyde volcanic district, and entirely independent of it, lies the -plateau of the Garleton Hills in East Lothian, which, as its limits towards -the east and north have been reduced by denudation, and towards the west -are hidden under the Carboniferous Limestone series of Haddington, covers -now an area of not more than about 60 square miles.<a id="FNanchor_415" href="#Footnote_415" class="fnanchor">[415]</a> That the eruptions -from this area did not extend far to the north is shown by the absence of -all trace of them among the Lower Carboniferous rocks of Fife. A relic of -them occurs above Borthwick, in Midlothian, about twelve miles to the -south-west of the nearest margin of the plateau. The area over which the -lavas and tuffs were discharged may not have exceeded 150 square miles. -Small though this plateau is, it possesses much interest from the remarkable -variety of petrographical character in its lavas, from the size and composition -of its necks, and from the picturesque coast-line where its details have -been admirably dissected by the waves. In many respects it stands by -itself as an exception to the general type of the other plateaux.</p> - -<div class="footnote"> - -<p><a id="Footnote_415" href="#FNanchor_415" class="label">[415]</a> This plateau is represented in Sheets 33 and 41 of the Geological Survey of Scotland, and is -described in the Explanation to accompany Sheet 33.</p> - -</div> - -<div class="figcenter" id="v1fig110" style="width: 319px;"> - <img src="images/v1fig110.png" width="319" height="360" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 110.</span>—The Bass Rock, a trachytic neck belonging to the Garleton plateau, from the shore - at Canty Bay.</div> -</div> - -<p>From its proximity to Edinburgh this volcanic area has been often -studied and described. The memoirs of Hay Cunningham and Maclaren -gave the fullest account of it until its structure was mapped by the Geological -Survey. Its scenery differs from that of the other plateaux chiefly in -the absence of the terraced contour which in them is so characteristic. The -<span class="pagenum" id="Page_373">- 373 -</span> -peculiar lavas of the Garleton Hills form irregularly-uneven ground, rising -to not more than 600 feet above the sea. They slope gradually down to -the coast, where a succession of fine sections of the volcanic series has been -laid bare for a distance of altogether about ten miles. Nowhere, indeed, -can the phenomena of the plateau-tuffs and their association with the -Carboniferous strata be so well studied as along the coast-line from North -Berwick to Dunbar. Among the necks of this plateau distinguished for -their size, conspicuous prominence and component materials, the most important -are those that form the conical eminences of North Berwick Law -(<a href="#v1fig109">Fig. 109</a>), Traprain Law (<a href="#v1fig133">Fig. 133</a>), and the Bass Rock (<a href="#v1fig110">Fig. 110</a>).</p> - -<p>3. <span class="smcap">The Midlothian Plateau.</span>—On the same general stratigraphical -horizon as the other volcanic plateaux, a narrow band of lavas and tuffs can -be followed from the eastern outskirts of the city of Edinburgh into Lanarkshire, -a distance of about 23 miles. It is not continuously visible, often -disappearing altogether, and varying much in thickness and composition. -This volcanic tract, which may be conveniently termed the Midlothian -Plateau, is the smallest and most fragmentary of all the series. Its most -easterly outliers form Arthur Seat and Calton Hill at Edinburgh.<a id="FNanchor_416" href="#Footnote_416" class="fnanchor">[416]</a> Three -miles to the south-west a third detached portion is known as Craiglockhart -Hill. After another interval of ten miles, the largest remaining fragment -forms the prominent ridge of Corston Hill (<a href="#v1fig111">Fig. 111</a>), whence a discontinuous -narrow strip may be traced nearly as far as the River Clyde.</p> - -<div class="footnote"> - -<p><a id="Footnote_416" href="#FNanchor_416" class="label">[416]</a> I formerly classed these eminences with the Puys, but I am now of opinion that they ought -rather to be regarded as fragments of a long and somewhat narrow plateau. Their basic lavas -and overlying sheets of porphyrite repeat the usual sequence of the plateaux, which is not met -with among the Puys. But, as will be pointed out in the sequel, Arthur Seat in long subsequent -time became again the site of a volcanic vent.</p> - -</div> - -<div class="figcenter" id="v1fig111" style="width: 428px;"> - <img src="images/v1fig111.png" width="428" height="159" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 111.</span>—Corston Hill—a fragment of the Midlothian Plateau, seen from the north.<br /> - The volcanic rocks form a cake on the top, the slopes lying across the edges of the Calciferous Sandstones.</div> -</div> - -<p>The well-known Arthur Seat and Calton Hill have been fully described -by Maclaren, and have been the subject of numerous observations by other -geologists.<a id="FNanchor_417" href="#Footnote_417" class="fnanchor">[417]</a> They have been likewise mapped in detail on a large scale by -<span class="pagenum" id="Page_374">- 374 -</span> -the Geological Survey, and have been described in the Survey Memoirs. -The rest of the plateau to the south-west is much less familiar.</p> - -<div class="footnote"> - -<p><a id="Footnote_417" href="#FNanchor_417" class="label">[417]</a> Maclaren's <i>Geology of Fife and the Lothians</i>, 1839, pp. 1-67; and Hay Cunningham, <i>Mem. -Wer. Soc.</i> vii. pp. 51-62. The plateau is represented in Sheets 24 and 32 of the Geological Survey, -and Arthur Seat and Calton Hill will be found on Sheet 2 of the Geological Survey map of -Edinburghshire on the scale of 6 inches to a mile.</p> - -</div> - -<p>In <a href="#v1fig112">Fig. 112</a> the great escarpment which descends from the right -towards the centre is the sill of Salisbury Crags. The long dark crag -(Long Row) rising between the two valleys is the lowest of the interstratified -lavas. The slope that rises above it has been cut out of well-bedded -tuffs, on which lie the basalts and andesites in successive sheets that form all -the eastern or left side of the hill. The rocks around the summit belong to -a much later period of volcanic eruption, and are referred to in <a href="../../66493/66493-h/66493-h.htm#CHAPTER_XXXI">Chapter xxxi.</a></p> - -<div class="figcenter" id="v1fig112" style="width: 508px;"> - <img src="images/v1fig112.png" width="508" height="168" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 112.</span>—View of Arthur Seat from Calton Hill to the north.</div> -</div> - -<p>The rocks of this plateau are comparatively limited in thickness, and -have a much more restricted vertical range than those of other districts. -At Arthur Seat and Corston Hill they begin above the cement-stones and -cease in a low part of the great group of white sandstones and dark shales -which form the upper half of the Calciferous Sandstones of Midlothian. -They do not ascend as high as the Burdiehouse Limestone, which to the -west of Corston Hill is seen to come on above them. One of their most -remarkable features is the manner in which they diminish to a single thin -bed and then die out altogether, reappearing again in a similar attenuated -form on the same horizon. This impersistence is well seen in the south-western -part of the area, between Buteland, in the parish of Currie, and -Crosswood, in the parish of Mid-Calder. The lowest more basic band may -there be traced at intervals for many miles without the overlying andesitic -group. Yet that andesites followed the basalts, as in other plateaux, is well -shown by large remnants of these less basic lavas left in Arthur Seat and -Calton Hill. On the extreme southern margin of the area also a thin band -of porphyrite with a group of overlying tuffs is seen above the red sandstones -near Dunsyre.<a id="FNanchor_418" href="#Footnote_418" class="fnanchor">[418]</a> The eruptions over the site of this plateau seem to -have been much more local and limited than in the other plateaux. They -appear to have gathered chiefly around two centres of activity, one of which -lay about the position of Edinburgh, the other in the neighbourhood of -Corston Hill. It is worthy of remark that this tract of volcanic material -flanks the much older range of lavas and tuffs of the Pentland Hills and -<span class="pagenum" id="Page_375">- 375 -</span> -wraps round the south-western end of this range, thus furnishing another -illustration of the renewal of volcanic activity in the same region during -successive geological periods.</p> - -<div class="footnote"> - -<p><a id="Footnote_418" href="#FNanchor_418" class="label">[418]</a> <i>Explanation, Geol. Surv. Scotland</i>, Sheet 24, p. 13 (1869).</p> - -</div> - -<p>4. <span class="smcap">The Berwickshire Plateau.</span>—Another and entirely disconnected -area occurs in the broad plain or Merse of the lower portion of the valley -of the Tweed.<a id="FNanchor_419" href="#Footnote_419" class="fnanchor">[419]</a> The northern limit of its volcanic tuff occurs in the River -Whitadder above Duns, whence the erupted materials rapidly widen -and thicken towards the south-west by Stitchell and Kelso, until they -die out against the flanks of the Cheviot Hills. The eastern extension of -the area is lost beneath the Cement-stone group which covers the Merse -down to the sea. Its western boundary must once have reached far beyond -its present limits, for the low Silurian ground in that direction is dotted -over with scattered vents to a distance of ten miles or more from the present -outcrop of the bedded lavas, extensive denudation having cleared away the -erupted materials and exposed the volcanic pipes over many square miles of -country. Among the more prominent of these old vents are the Eildon -Hills, Minto Crags and Rubers Law, as well as many other eminences -familiar in Border story.</p> - -<div class="footnote"> - -<p><a id="Footnote_419" href="#FNanchor_419" class="label">[419]</a> This plateau is shown on Sheets 17, 25, 26 and 33 of the Geological Survey Map of Scotland. -It was chiefly mapped by Prof. James Geikie and Mr. B. N. Peach.</p> - -</div> - -<p>The bedded volcanic rocks of this area form a marked feature in the -topography and geology of the district. They rise above the plain of the -Merse as a band of undulating hills, of which the eminence crowned by -Hume Castle, about 600 feet above the sea, is the most conspicuous height. -In the geological structure of this part of Scotland they are mainly interposed -between the Upper Old Red Sandstone and the base of the Carboniferous -system, which they thus serve to divide from each other. But their lowest -sheets appear to be in some places intercalated in the Old Red Sandstone, -so that their eruption probably began before the beginning of the Carboniferous -period. They form a band that curves round the end of the great -Carboniferous trough at Kelso and skirts the northern edge of the andesites -of the Lower Old Red Sandstone in the Cheviot Hills.</p> - -<p>5. <span class="smcap">The Solway Plateau.</span>—The last plateau, that of the Solway basin, -though its present visible eastern limits approach those reached by the lavas -from the Berwickshire area, was quite distinct, and had its chief vents -at some distance towards the south-west.<a id="FNanchor_420" href="#Footnote_420" class="fnanchor">[420]</a> On the north-western flanks -of the Cheviot Hills, the Upper Old Red Sandstone is overlain by the -lowest Carboniferous strata, without the intercalation of any volcanic zone, -so that there must have been some intermediate ground that escaped being -flooded with lava from the vents of the Merse on the one hand, and of the -Solway on the other. The Solway lavas form a much thinner group than -those of Berwickshire. From the wild moorland between the sources of the -<span class="pagenum" id="Page_376">- 376 -</span> -Liddell and the Rule Water, they run in a narrow and much-faulted band -south-westward across Eskdale and the foot of Annandale, and are traceable -in occasional patches on the farther side of the Nith along the southern -flanks of Criffel, even as far as Torrorie on the coast of Kirkcudbright—a -total distance of about 45 miles. It is probable that this long outcrop -presents merely the northern edge of a volcanic platform which is mainly -buried under the Carboniferous rocks of the Solway basin. Yet it exhibits -many of the chief characters of the other plateaux, and even occasionally -rivals them in the dignity of the escarpments which mark its progress -through the lonely uplands between the head of Liddesdale and the Ewes -Water (Figs. <a href="#v1fig113">113</a>, <a href="#v1fig142">142</a>).</p> - -<div class="footnote"> - -<p><a id="Footnote_420" href="#FNanchor_420" class="label">[420]</a> For a delineation of the distribution and structure of this plateau see Sheets 5, 6, 10, 11 and -17 of the Geological Survey of Scotland. In the upper part of Liddesdale, Ewesdale and Tarras it -was mapped by Mr. B. N. Peach; in lower Liddesdale and Eskdale by Mr. R. L. Jack and Mr. -J. S. Grant Wilson; from Langholm to the Annan by Mr. H. Skae; and in Kirkcudbright by -Mr. John Horne.</p> - -</div> - -<div class="figcenter" id="v1fig113" style="width: 476px;"> - <img src="images/v1fig113.png" width="476" height="245" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 113.</span>—View of Arkleton Fell, part of the Solway Plateau, from the south-west.<br /> - The lower slopes below the single bird, round to the left side of the sketch, are on the Upper Old Red Sandstone; - the line of crag below the two birds marks the volcanic group above which lies an outlier of the Calciferous Sandstone - series, forming the upper part of the hill (three birds). The knobs under the four birds are bosses of - andesite.</div> -</div> - -<p>The plateaux of the Merse and the Solway illustrate in a striking -manner the distribution of the volcanic eruptions along valleys and low -plains. The vents from which the lavas and tuffs proceeded are chiefly to -be found on the lower grounds, though these bedded volcanic rocks rise to a -height of 1712 feet (the Pikes) to the west of the Cheviot Hills. Between -the Silurian uplands of Selkirkshire and Berwickshire on the north and the -ridge of the Cheviot Hills on the south, the broad plain was dotted with -volcanic vents and flooded with lava, while to the south-west the corresponding -hollow between the uplands of Dumfries and Galloway on the one -side, and those of Cumberland on the other, was similarly overspread. The -significance of these facts will be more apparent when the grouping of the -vents has been described. We shall then also be better able to realize the -validity of the inference that the present plateaux are mere fragments -of what they originally were, wide areas having been removed from the one -side of them by denudation, and having been concealed on the other under -later portions of the Carboniferous system.</p> - -<p><span class="pagenum" id="Page_377">- 377 -</span></p> - -<p>The same two plateaux likewise supply further illustrations of the outflow -of similar volcanic materials in the same locality at widely separated -intervals of time. They may be traced up to and round the margin of -the great pile of andesites of Lower Old Red Sandstone age forming the -Cheviot Hills.</p> - - -<p>ii. <span class="allsmcap">NATURE OF THE MATERIALS ERUPTED</span></p> - -<p>The volcanic materials characteristic of the plateau-type of eruptions -consist mainly of lavas in successive sheets, but include also various tuffs -in frequent thin courses, and less commonly in thick local accumulations. -The lavas are chiefly andesites in the altered condition of porphyrites. -They vary a good deal in the relative proportions of silica. Some of them -are decidedly basic and take the form of dolerites and olivine-basalts. With -these rocks are occasionally associated "ultra-basic" varieties, where the -felspar almost disappears and the material consists mainly of ferro-magnesian -minerals. The more basic rocks are generally found towards the bottom of -the volcanic series, where they appear as the oldest flows. In the Garleton -Hills lavas of a much more acid nature are met with—true sanidine-trachytes, -which overlie the porphyrites and basalts of the earlier eruptions.</p> - -<p>No adequate investigation has yet been made of the chemical and microscopic -characters of these various rocks, regarded as a great volcanic series -belonging to a definite geological age, though many of the individual rocks -and the petrography of different districts have been more or less fully -described. I cannot here enter into much detail on the subject, but must -content myself with such a summary as will convey some idea of the general -composition and structure of this very interesting volcanic series.</p> - -<p>(<i>a</i>) <span class="smcap">Augite-olivine Rocks</span> (<span class="smcap">Picrites</span> and <span class="smcap">Limburgites</span>).—Towards the -bottom of the plateaux there are found here and there sheets of "ultra-basic" -material, some of which appear to be bedded with the other rocks and to -have flowed out as surface-lavas, though it may be impossible to prove that -they are not sills. Thus at Whitelaw Hill, on the south side of the Garleton -Hills, a dark heavy rock is found to contain hardly any felspar, but to be -made up mainly of olivine and augite. Dr. Hatch has published a description -and drawing of this rock, together with the following analysis by Mr. -Player:<a id="FNanchor_421" href="#Footnote_421" class="fnanchor">[421]</a>—</p> - -<table summary="data"> -<tr> - <td class="tdl">Silica</td> - <td class="tdr">40·2</td> -</tr> -<tr> - <td class="tdl">Titanic oxide</td> - <td class="tdr">2·9</td> -</tr> -<tr> - <td class="tdl">Alumina</td> - <td class="tdr">12·8</td> -</tr> -<tr> - <td class="tdl">Ferric oxide</td> - <td class="tdr">4·0</td> -</tr> -<tr> - <td class="tdl">Ferrous oxide</td> - <td class="tdr">10·4</td> -</tr> -<tr> - <td class="tdl">Lime</td> - <td class="tdr">10·4</td> -</tr> -<tr> - <td class="tdl">Magnesia</td> - <td class="tdr">11·9</td> -</tr> -<tr> - <td class="tdl">Potash</td> - <td class="tdr">0·8</td> -</tr> -<tr> - <td class="tdl">Soda</td> - <td class="tdr">2·7</td> -</tr> -<tr> - <td class="tdl">Loss by ignition</td> - <td class="tdr">3·4</td> -</tr> -<tr> - <td class="tdl">Spec. grav. 3·03.</td> - <td class="tdr bdt">99·5</td> -</tr> -</table> - -<div class="footnote"> - -<p><a id="Footnote_421" href="#FNanchor_421" class="label">[421]</a> <i>Trans. Roy. Soc. Edin.</i> vol. xxxvii. (1893), p. 116.</p> - -</div> - -<p><span class="pagenum" id="Page_378">- 378 -</span></p> - -<p>(<i>b</i>) <span class="smcap">Dolerites</span> and <span class="smcap">Basalts</span>.<a id="FNanchor_422" href="#Footnote_422" class="fnanchor">[422]</a>—These rocks are found both as interstratified -lavas and as intrusive masses. In the former condition they take a -conspicuous place among the sheets of the plateaux, but especially in the -lower parts of the series. They are dark, often black, usually more or less -porphyritic, with large felspars, frequently also large crystals of augite or -olivine, and may be described as porphyritic olivine-dolerites and olivine-basalts, -more rarely as olivine-free dolerites and basalts. Their groundmass -consists of short laths or microlites of felspar (probably labradorite) and -granules or small crystals of augite and magnetite, with sometimes a little -fibrous brown mica. The large porphyritic felspars are striped (probably -labradorite), the augites are frequently chloritized, and the olivines are -generally more or less serpentinized. But in some cases all these minerals -are as fresh as in a recent basalt. The rocks are sometimes beautifully -columnar, as at Arthur Seat.</p> - -<div class="footnote"> - -<p><a id="Footnote_422" href="#FNanchor_422" class="label">[422]</a> A general classification of the whole series of Scottish Carboniferous dolerites and basalts, -including both the plateau and puy examples, will be given in the account of the rocks of the puys -in <a href="#CHAPTER_XXVI">Chapter XXVI.</a> (<a href="#Page_418">p. 418</a>).</p> - -</div> - -<p>Of these basic lavas conspicuous examples may be seen at Arthur Seat, -Calton Hill and Craiglockhart Hill. The eastern part of Arthur Seat, -known as Whinny Hill, furnishes examples of olivine-dolerites of the Jedburgh -type (<a href="#Page_418">p. 418</a>). The beautiful basalt of Craiglockhart with its large -porphyritic olivines and augites has afforded a distinct type of Carboniferous -basalt (<a href="#Page_418">p. 418</a>). The same type occurs on the Calton Hill in the cliff -below the gaol. Similar basic lavas are especially abundant and remarkable -in the Clyde plateau near Campbeltown in Argyllshire, and at the south end -of Bute and in the Cumbraes, where they are associated with an interesting -series of dykes and sills. But even where, as in the Garleton Hills, the lavas -are for the most part somewhat acid in composition, those first poured out, -which form the lowest band, include some typical olivine-basalts, of which a -characteristic example occurs at Kippie Law at the base of the Garleton -plateau (<a href="#Page_418">p. 418</a>). It has been described by Dr. Hatch as exhibiting under -the microscope porphyritic crystals of felspar and olivine lying in a groundmass -composed of lath-shaped felspars, granular olivine and magnetite, and -microlitic augite. The olivine, originally the most abundant constituent, has -been converted into a fibrous aggregate of serpentine. All the minerals -are more or less idiomorphic, but especially the augite, which crowds the -groundmass in delicately-shaped prisms, most of which are terminated at -both ends by faces of the hemi-pyramid. The analysis of this rock is given -in the accompanying table of analyses of Garleton basalts. The Kippie Law -type of basalt was recognized by Dr. Hatch among the Geological Survey -collections from other districts, as in the intrusive bosses of Neides Law -and Bonchester near Jedburgh, and from the Campsie plateau a mile and a -half north of Lennoxtown.<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>Trans. Roy. Soc. Edin.</i> vol. xxxvii. (1893), pp. 117-119.</p> - -</div> - -<p>At Hailes Castle, in the Garleton plateau, the lower basic lavas include -another olivine-basalt somewhat more felspathic than that just described, and -<span class="pagenum" id="Page_379">- 379 -</span> -at Markle quarry the rock is still more felspathic and contains the olivine -only in small sporadic grains. The composition of these basic rocks of the -Garleton plateau is shown in the subjoined table of analyses by Mr. J. S. -Grant Wilson:—</p> - -<table summary="data"> -<tr> - <td class="bdl bdt bdb"></td> - <td class="bdl bdt bdb tdc">SiO<sub>2</sub></td> - <td class="bdl bdt bdb tdc">Al<sub>2</sub>O<sub>3</sub></td> - <td class="bdl bdt bdb tdc">Fe<sub>2</sub>O<sub>3</sub></td> - <td class="bdl bdt bdb tdc">FeO</td> - <td class="bdl bdt bdb tdc">MnO</td> - <td class="bdl bdt bdb tdc">CaO</td> - <td class="bdl bdt bdb tdc">MgO</td> - <td class="bdl bdt bdb tdc">K<sub>2</sub>O</td> - <td class="bdl bdt bdb tdc">Na<sub>2</sub>O</td> - <td class="bdl bdt bdb tdc">H<sub>2</sub>O</td> - <td class="bdl bdt bdb bdr tdc">Total</td> -</tr> -<tr> - <td class="bdl tdl">Kippie Law, specific<br /> gravity 2·8</td> - <td class="bdl tdc">46·01</td> - <td class="bdl tdc">19·19</td> - <td class="bdl tdc">5·91</td> - <td class="bdl tdc">6·75</td> - <td class="bdl tdc">0·19</td> - <td class="bdl tdc">8·68</td> - <td class="bdl tdc">6·81</td> - <td class="bdl tdc">1·20</td> - <td class="bdl tdc">3·27</td> - <td class="bdl tdc">3·07</td> - <td class="bdl bdr tdc">101·08</td> -</tr> -<tr> - <td class="bdl tdl">Hailes Castle, specific<br /> gravity 2·76</td> - <td class="bdl tdc">49·07</td> - <td class="bdl tdc">19·43</td> - <td class="bdl tdc">10·58</td> - <td class="bdl tdc">2·35</td> - <td class="bdl tdc">0·32</td> - <td class="bdl tdc">7·87</td> - <td class="bdl tdc">4·36</td> - <td class="bdl tdc">0·98</td> - <td class="bdl tdc">3·31</td> - <td class="bdl tdc">2·26</td> - <td class="bdl bdr tdc">100·53</td> -</tr> -<tr> - <td class="bdl bdb tdl">Markle Quarry, specific<br /> gravity 2·7</td> - <td class="bdl bdb tdc">49·54</td> - <td class="bdl bdb tdc">22·23</td> - <td class="bdl bdb tdc">9·55</td> - <td class="bdl bdb tdc">1·12</td> - <td class="bdl bdb tdc">0·08</td> - <td class="bdl bdb tdc">7·19</td> - <td class="bdl bdb tdc">2·80</td> - <td class="bdl bdb tdc">1·81</td> - <td class="bdl bdb tdc">4·56</td> - <td class="bdl bdb tdc">2·42</td> - <td class="bdl bdb bdr tdc">101·30</td> -</tr> -</table> - -<p>Olivine-dolerites are more especially developed in the district around -Jedburgh, where they form some of the most prominent bosses, such as -Dunian and Black Law. They show a sub-ophitic groundmass, with inconspicuous -porphyritic crystals, among which those of olivine are more prominent -than the felspars (<a href="#Page_418">p. 418</a>).</p> - -<p>(<i>c</i>) <span class="smcap">Andesites</span> (<span class="smcap">Porphyrites</span>).—These are the most abundant lavas of -the plateaux. They occur in every district, and usually form the main -constituents of the pile of volcanic material. They vary in colour from a -pale pinkish grey, through many shades of red, purple, brown and yellow, -to sometimes a dark green or nearly black rock. Their texture ranges -from almost semi-vitreous, through different degrees of compactness, to open, -cellular, slaggy masses. Generally through their base porphyritic felspars -are abundantly disseminated, sometimes in large, flat, tabular forms, like -those of the Lower Old Red Sandstone already referred to. The amygdaloidal -kernels consist of calcite, zeolites, chalcedony or quartz. It is -from the amygdaloids on either side of the Clyde that the fine examples of -zeolites have been chiefly obtained for which the south of Scotland has long -been famed. Occasionally, as at the south end of Bute, the andesitic lavas -display a marked columnar structure.</p> - -<p>Under the microscope these rocks present the usual fine felted aggregate -of felspar microlites, with granules or crystals of magnetite and sometimes -pyroxene. The porphyritic felspars, often large and well defined, generally -contain inclusions of the groundmass. Occasionally some of the large porphyritic -constituents are augite, or pseudomorphs after that mineral. The -alteration of the rocks has oxidized some of the iron-ore and given rise to -the prevalent purplish and reddish tints.</p> - -<p>(<i>d</i>) <span class="smcap">Trachytes.</span>—Some of the most remarkable lavas to be found in -any of the plateaux are those which constitute a large part of the Garleton -Hills. They overlie the lower andesite and basalt platform, which surrounds -them as a narrow belt, while they occupy the central and much the largest -part of the area. They have been included among the porphyrites, but are -pale rocks, generally with a yellowish crust, presenting when quite fresh a -grey, compact, felsitic base with large porphyritic crystals of unstriped felspar.</p> - -<p><span class="pagenum" id="Page_380">- 380 -</span></p> - -<p>A number of specimens selected as illustrative of the different varieties -have been analyzed and the results are stated in the subjoined table.<a id="FNanchor_424" href="#Footnote_424" class="fnanchor">[424]</a> The -specific gravity of the rocks is about 2·6.</p> - -<div class="footnote"> - -<p><a id="Footnote_424" href="#FNanchor_424" class="label">[424]</a> The first two analyses are by Mr. J. S. Grant Wilson, the last two by Mr. A. Dick jun., -and that from Hopetoun Monument by Mr. G. Barrow. <i>Trans. Roy. Soc. Edin.</i> vol. xxxvii. p. 122.</p> - -</div> - -<table summary="data"> -<tr> - <td class="bdl bdt bdb"></td> - <td class="bdl bdt bdb tdc">SiO<sub>2</sub></td> - <td class="bdl bdt bdb tdc">Al<sub>2</sub>O<sub>3</sub></td> - <td class="bdl bdt bdb tdc">Fe<sub>2</sub>O<sub>3</sub></td> - <td class="bdl bdt bdb tdc">FeO</td> - <td class="bdl bdt bdb tdc">MnO</td> - <td class="bdl bdt bdb tdc">CaO</td> - <td class="bdl bdt bdb tdc">MgO</td> - <td class="bdl bdt bdb tdc">K<sub>2</sub>O</td> - <td class="bdl bdt bdb tdc">Na<sub>2</sub>O</td> - <td class="bdl bdt bdb tdc">H<sub>2</sub>O</td> - <td class="bdl bdt bdb bdr tdc">Total</td> -</tr> -<tr> - <td class="bdl tdl">Pepper Craig</td> - <td class="bdl tdc">62·61</td> - <td class="bdl tdc">18·17</td> - <td class="bdl tdc">0·32</td> - <td class="bdl tdc">4·25</td> - <td class="bdl tdc">0·21</td> - <td class="bdl tdc">2·58</td> - <td class="bdl tdc">0·74</td> - <td class="bdl tdc">4·02</td> - <td class="bdl tdc">6·49</td> - <td class="bdl tdc">0·80</td> - <td class="bdl bdr tdc">100·19</td> -</tr> -<tr> - <td class="bdl tdl">Kae Heughs</td> - <td class="bdl tdc">61·35</td> - <td class="bdl tdc">16·88</td> - <td class="bdl tdc">0·41</td> - <td class="bdl tdc">5·01</td> - <td class="bdl tdc">0·26</td> - <td class="bdl tdc">2·39</td> - <td class="bdl tdc">0·44</td> - <td class="bdl tdc">6·12</td> - <td class="bdl tdc">5·26</td> - <td class="bdl tdc">1·70</td> - <td class="bdl bdr tdr">99·82</td> -</tr> -<tr> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl"></td> - <td class="bdl tdl" colspan="2"><img src="images/braceh_70.png" width="70" height="12" alt="" /></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 tdl">Hopetoun Monument</td> - <td class="bdl tdc">62·50</td> - <td class="bdl tdc">18·51</td> - <td class="bdl tdc" colspan="2">4·39</td> - <td class="bdl tdc">...</td> - <td class="bdl tdc">2·00</td> - <td class="bdl tdc">0·61</td> - <td class="bdl tdc">6·31</td> - <td class="bdl tdc">3·44</td> - <td class="bdl tdc">2·10</td> - <td class="bdl bdr tdr">99·86</td> -</tr> -<tr> - <td class="bdl tdl">Phantassie</td> - <td class="bdl tdc">59·50</td> - <td class="bdl tdc">18·25</td> - <td class="bdl tdc">4·81</td> - <td class="bdl tdc">2·34</td> - <td class="bdl tdc">...</td> - <td class="bdl tdc">2·10</td> - <td class="bdl tdc">0·70</td> - <td class="bdl tdc">6·30</td> - <td class="bdl tdc">5·03</td> - <td class="bdl tdc">1·60</td> - <td class="bdl bdr tdc">100·63</td> -</tr> -<tr> - <td class="bdl bdb tdl">Bangley Quarry</td> - <td class="bdl bdb tdc">58·50</td> - <td class="bdl bdb tdc">21·12</td> - <td class="bdl bdb tdc">4·68</td> - <td class="bdl bdb tdc">...</td> - <td class="bdl bdb tdc">...</td> - <td class="bdl bdb tdc">3·70</td> - <td class="bdl bdb tdc">0·93</td> - <td class="bdl bdb tdc">5·84</td> - <td class="bdl bdb tdc">3·90</td> - <td class="bdl bdb tdc">2·00</td> - <td class="bdl bdr bdb tdc">100·67</td> -</tr> -</table> - - -<p>The microscopic characters of these rocks, as worked out by Dr. Hatch, -show them to be well-marked and wonderfully fresh sanidine-trachytes. -Some of them are porphyritic, with large crystals of perfectly unaltered -sanidine, sometimes also oligoclase. Small but well-formed crystals of -yellowish-green augite, in addition to the porphyritic felspars, are imbedded -in a fine groundmass composed chiefly of microlites of sanidine, but -with granules of augite and magnetite plentifully interspersed, and occasionally -prisms of apatite. There is a group in which the porphyritic felspars -are scarce or absent. In these there is little or no ferro-magnesian constituent. -Other trachytes, rather less basic than the augite-bearing varieties -here referred to, occur as bosses in the Garleton Hills district, and are -referred to in the following section (<i>e</i>).<a id="FNanchor_425" href="#Footnote_425" class="fnanchor">[425]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_425" href="#FNanchor_425" class="label">[425]</a> For fuller petrographical details consult Dr. Hatch's paper above cited.</p> - -</div> - -<p>(<i>e</i>) <span class="smcap">Rocks of the Necks.</span>—In the necks connected with the plateaux -other types of massive rock are to be found. Among these perhaps the most -frequent are trachytes, grey to pink in colour, but apt to weather yellow, -exceedingly compact, sparingly porphyritic, and with a peculiar platy -structure and waxy lustre. Rocks of this character also appear as sills and -dykes. Other varieties that occur in similar positions are more basic in -composition, including dark, coarse, granular diabases. In the Jedburgh -district the most frequent rocks are beautiful varieties of olivine-dolerite -and olivine-basalt, which form most of the prominent hills of the neighbourhood. -These bosses are sometimes associated with agglomerates as at -Rubers Law.</p> - -<p>In the Garleton Hills district, some of the necks present another petrographical -type which directly connects them with the remarkable lavas of -the higher part of that plateau. Thus the rock of Traprain Law was ascertained -by Dr. Hatch to be a true phonolite. In its general platy structure -and sonorous ring under the hammer it reminds one of typical phonolites. -Under the microscope the rock is found to consist mainly of small lath-shaped -crystals of sanidine arranged in a marked minute flow-structure, but -with few porphyritic crystals. It contains small crystals and ophitic patches -<span class="pagenum" id="Page_381">- 381 -</span> -of a light green soda-augite, with practically no magnesia in it. A small -quantity of iron-ore and isolated granules of apatite are also present, together -with patches of nepheline which, though generally decomposed and -replaced with zeolitic products, occasionally display six- and four-sided -crystal-contours. An analysis of the Traprain phonolite by Mr. Player is -subjoined:—<a id="FNanchor_426" href="#Footnote_426" class="fnanchor">[426]</a></p> - -<table summary="data"> -<tr> - <td class="tdl">Silica</td> - <td class="tdr" style="width: 3em;">56·8</td> -</tr> -<tr> - <td class="tdl">Titanic acid</td> - <td class="tdr">0·5</td> -</tr> -<tr> - <td class="tdl">Alumina</td> - <td class="tdr">19·7</td> -</tr> -<tr> - <td class="tdl">Ferric oxide</td> - <td class="tdr">2·2</td> -</tr> -<tr> - <td class="tdl">Ferrous oxide</td> - <td class="tdr">3·5</td> -</tr> -<tr> - <td class="tdl">Manganous oxide</td> - <td class="tdr">0·2</td> -</tr> -<tr> - <td class="tdl">Lime</td> - <td class="tdr">2·2</td> -</tr> -<tr> - <td class="tdl">Magnesia</td> - <td class="tdr">0·4</td> -</tr> -<tr> - <td class="tdl">Soda</td> - <td class="tdr">4·3</td> -</tr> -<tr> - <td class="tdl">Potash</td> - <td class="tdr">7·1</td> -</tr> -<tr> - <td class="tdl">Loss by ignition</td> - <td class="tdr">2·5</td> -</tr> -<tr> - <td class="tdl">Spec. grav. 2·588</td> - <td class="tdr bdt">99·4</td> -</tr> -</table> - -<div class="footnote"> - -<p><a id="Footnote_426" href="#FNanchor_426" class="label">[426]</a> <i>Trans. Roy. Soc. Edin.</i> vol. xxxvii. p. 125.</p> - -</div> - -<p>The neck of North Berwick Law was found by Dr. Hatch to be a -trachyte, showing a plexus of lath-shaped sanidines that diminish in size -to minute microlites, but with no porphyritic or ferro-magnesian constituent. -The Bass Rock, though its geological relations are concealed by -the sea, is in all probability another neck of this district. It is likewise a -mass of trachyte, composed almost entirely of lath-shaped crystals of sanidine, -with no ferro-magnesian constituent, but a good deal of iron ore. It shows -none of the large porphyritic felspars so characteristic of the Garleton Hills -lavas, but it closely resembles the non-porphyritic varieties, particularly the -lavas of Score Hill, Pencraig, Lock Pit Hill, and Craigie Hill.<a id="FNanchor_427" href="#Footnote_427" class="fnanchor">[427]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_427" href="#FNanchor_427" class="label">[427]</a> The composition of the rocks of North Berwick Law and the Bass closely resembles that of -the trachytic lavas of the plateau. For analyses, see Dr. Hatch's Paper, <i>ibid.</i> pp. 123, 124.</p> - -</div> - -<p>(<i>f</i>) <span class="smcap">Tuffs.</span>—The fragmentary ejections of the plateaux vary in texture -from the finest-grained tuffs to coarse agglomerates.<a id="FNanchor_428" href="#Footnote_428" class="fnanchor">[428]</a> As they have been -derived from the explosion of andesite-lavas, they consist mainly of the debris -of these rocks. They are often deep red in colour, as for example those of -Dunbar, but are most frequently greenish. They have a granular texture, -due to the small lapilli of various porphyrites imbedded in a fine dust of -the same material. Grains of quartz, frequently to be detected even in -the finer tuffs, may either have been ejected from the volcanic vents, or -may have been grains of sand in the ordinary sediment of the sea-bottom. -Both at the base and at the top of the plateau-series, the tuffs are -interstratified with and blend into sandstones and shales, so that specimens -may be collected showing a gradual passage from volcanic into non-volcanic -detritus. In many of the tuffs of the necks fragments of sandstone and -<span class="pagenum" id="Page_382">- 382 -</span> -other stratified rocks occur, representing the strata through which the vents -were drilled. In the tuffs of the Eaglesham district pieces of grey and pink -granite have been met with which, if they are portions of an old granite -mass below, must have come from a great depth.<a id="FNanchor_429" href="#Footnote_429" class="fnanchor">[429]</a> In the coarser tuffs -and agglomerates a larger variety of lava-form rocks is to be found than -can be seen among the bedded lavas of the Plateaux. They include felsites -and quartz-porphyries, and more rarely basic lavas (diabases, etc.).</p> - -<div class="footnote"> - -<p><a id="Footnote_428" href="#FNanchor_428" class="label">[428]</a> For accounts of these rocks, see Explanation of Sheet 33 <i>Geol. Surv. Scot.</i> p. 32; Sheet 22, -pp. 11-14; Sheet 31, pp. 14-17.</p> - -</div> - -<div class="footnote"> - -<p><a id="Footnote_429" href="#FNanchor_429" class="label">[429]</a> Explanation of Sheet 22 <i>Geol. Surv. Scot.</i> p. 12.</p> - -</div> -<hr class="chap x-ebookmaker-drop" /> - -<div class="chapter"> -<p><span class="pagenum" id="Page_383">- 383 -</span></p> - -<h2 class="nobreak" id="CHAPTER_XXV">CHAPTER XXV<br /> - -<span class="smaller">GEOLOGICAL STRUCTURE OF THE CARBONIFEROUS<br /> -VOLCANIC PLATEAUX OF SCOTLAND</span></h2> -</div> - -<div class="blockquot"> - -<p>1. Bedded Lavas and Tuffs; Upper Limits and Original Areas and Slopes of the -Plateaux; 2. Vents; Necks of Agglomerate and Tuff; Necks of Massive Rock; -Composite Necks; 3. Dykes and Sills; 4. Close of the Plateau-eruptions.</p> -</div> - - -<p>The structure of the various plateaux presents a general similarity, with many -local variations. Each plateau is built up entirely, or almost entirely, of sheets -of volcanic material, the intercalations of ordinary sedimentary layers being, -for the most part, few and unimportant, and usually occurring either towards -the base or the top of the volcanic series, though at a few localities interstratifications -of shale and sandstone, marking pauses in the eruptions, -occur throughout that series. The vents of eruption are in some instances -still to be recognized on the plateaux themselves. More usually they occur -on the lower ground flanking the volcanic escarpments, where they have -been laid bare by denudation. Dykes, though seldom abundant, are associated -with the plateaux, while the sills which may mark the latest manifestations -of volcanic energy, though not developed on so large a scale as -among the Cambrian and Silurian volcanoes, can nevertheless be distinctly -recognized.</p> - -<p>It is a question of some interest to determine the geological date of the -commencement of the plateau-eruptions by fixing the precise stratigraphical -horizon on which the base of the volcanic series rests. I have already -referred to the fact that this base does not always lie on the same platform -among the Lower Carboniferous formations. In Berwickshire, as above -mentioned, the earliest eruptions appear to have taken place before the -close of the Upper Old Red Sandstone period. These are the earliest of -the whole series. In Cantyre, the lowest lavas and tuffs come directly upon -the sandstones, marls and cornstones of the Upper Old Red Sandstone. In -Stirlingshire, Renfrewshire and Ayrshire several hundred feet of the Cement-stone -group are sometimes interposed between the bottom of the volcanic -rocks and the top of the Old Red Sandstone. This divergence doubtless -indicates that the eruptions began earlier in some districts than in others. -But there were also probably unequal terrestrial movements preceding, and -<span class="pagenum" id="Page_384">- 384 -</span> -perhaps accompanying, the volcanic outbursts. In the case of the Clyde -plateau, for example, if we examine its base in the neighbourhood of Fintry, -we find that it lies upon some 500 feet of Carboniferous white sandstone, -red and green marls and cement-stones, which rest on the Upper Old -Red Sandstone. Yet only eight miles to the eastward, this considerable -mass of strata disappears, and the bottom of the lavas comes down upon -the red sandstones. Five miles still further in the same direction the -volcanic masses likewise die out, and then the Carboniferous Limestone -series is found at Abbey Craig to lie, with scarcely any representative of the -Cement-stone group, on the Upper Old Red Sandstone (<a href="#v1fig114">Fig. 114</a>). Again, to -the south-west of Fintry, the zone of cement-stones below the volcanic series -continues to vary considerably in thickness and sometimes almost to disappear, -while in Ayrshire the lavas lie immediately on the red sandstones.</p> - -<div class="figcenter" id="v1fig114" style="width: 510px;"> - <img src="images/v1fig114.png" width="510" height="245" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 114.</span>—Vertical sections of the escarpment of the Clyde plateau from north-east to south-west.</div> - -<div class="hanging2">I. Section at the east end of the Campsie Hills, four miles west from Stirling. II. Section above Glins, six miles -west from No. I. III. Section at Strathblane Hill, eight miles further south-west. IV. Section at Lang -Craig, east from Dumbarton, eight miles south-west from No. III. V. Section above Fort Matilda, Greenock, -eleven miles from the previous section and on the south side of the Clyde.</div> - -<div class="hanging2">1. Lower Old Red Sandstone; 2. Upper Old Red Sandstone; 3. Carboniferous shales, sandstones and cement-stones -(the "Ballagan beds"); 4. Thick white sandstone which comes in above the Ballagan beds; 5. -Andesite lava-sheets; 6. Interstratified tuffs. The dotted lines connect the base of the volcanic series.</div> -</div> - -<p>These irregularities, not improbably indicative of inequalities of subsidence -and of deposition, may have been connected with the subterranean -disturbances which culminated in the abundant outbreak of volcanic action. -But though the volcanic rocks of the plateaux may be traced overlapping the -underlying strata, no evidence has anywhere been detected of an unconformability -between them and the Lower Carboniferous or Upper Old Red -Sandstone series.</p> - - -<h3>1. BEDDED LAVAS AND TUFFS</h3> - -<p>The successive sheets of lava in a plateau usually form thin and widespread -beds which are only occasionally separated by intercalations of tuff -<span class="pagenum" id="Page_385">- 385 -</span> -or of red marl. In this, as well as in other respects, they present much -resemblance to the lavas of the Tertiary plateaux of Antrim and the Inner -Hebrides. They are generally marked off from each other by the slaggy -upper and under portions of the successive flows, and this structure gives a -distinctly bedded aspect to the escarpments, as in the Campsie and Largs -Hills, or still more conspicuously in Little Cumbrae (<a href="#v1fig107">Fig. 107</a>) and the -southern end of Bute. Considerable diversity of structure may be noticed -among these sheets. Some present a compact jointed centre passing up and -down into the slaggy material just referred to; others have assumed a vesicular -character throughout, the vesicles being often elongated in the direction of flow. -Where, as usually occurs, the vesicular is replaced by the amygdaloidal -structure, some of the rocks have long been famous for the minerals found -in their cavities. The beautiful zeolites of the Kilpatrick and Renfrewshire -Hills, for example, may be found in every large mineralogical collection in -the country. Well-developed columnar structure occasionally appears among -the lavas of the plateaux, but chiefly, so far as I have observed, in the lower -or more basic group, as in the basalts along the east side of the Dry Dam -at Arthur Seat.</p> - -<p>In each plateau the lavas may be observed to thicken in one direction, -or more usually towards more than one, and this increase no doubt indicates -in which quarters the chief centres of discharge lay. Thus in the Clyde -plateau, several areas of maximum development may be detected. In -the Kilpatrick Hills the total thickness of lavas and tuffs exceeds 3000 -feet (<a href="#v1fig120">Fig. 120</a>). Above Largs it is more than 1500 feet, rapidly thinning -away towards the south. The continuation of the plateau far to the north-east -in the Campsie Fells reveals a thickness of about 1000 feet of lavas -at Kilsyth, which become thicker further west, but eastward rapidly diminish -in collective bulk, until in about twelve or thirteen miles they disappear -altogether, and then, as already remarked, the Calciferous Sandstone series -closes up without any volcanic intercalation.</p> - -<p>In the Solway plateau, the lavas attain a maximum development about -Birrenswark, whence they diminish in bulk towards the north-east and -south-west. The Berwickshire plateau reaches its thickest mass about -Stitchill, whence it rapidly thins away towards the north-east, until at a -distance of some twelve miles it disappears altogether, the last trace of -it in that direction being a band of tuff which dies out in the Calciferous -Sandstones to the north of Duns.</p> - -<p>In the Midlothian Plateau, the development of the volcanic series is -more irregular than in any of the others. As already remarked, there -appear to have been at least two chief centres of discharge in this region, one -at Edinburgh and one some fourteen miles to the south-west. At the former, -the volcanic materials attain in Arthur Seat and Calton Hill a thickness of -about 1100 feet. In Craiglockhart Hill, three miles distant, they are still -about 600 feet thick. But beyond that eminence they cease to be traceable -for about eight miles, either because they entirely die out, or because their -dwindling outcrops are concealed under superficial deposits. As we approach -<span class="pagenum" id="Page_386">- 386 -</span> -the south-western centre of eruption around Corston Hill a new volcanic -group begins and soon increases in bulk.</p> - -<p>A distinguishing feature of the plateaux is found in the difference -between the lavas that were first erupted and those which followed them. -The earlier eruptions, as above remarked, were generally basic, sometimes -highly so. Thus at Arthur Seat the thick series of lavas which form -the eastern part of the hill have at their base several sheets of columnar -basalt, over which come the andesites that make up the main mass of the -erupted material. In the Calton Hill the same sequence may be observed. -Underneath the andesites of Campbeltown comes a well-marked and persistent -band of olivine-dolerite. Still more basic are some portions of the earliest -lavas of the Garleton plateau where, as already stated, rocks present themselves -composed mainly of olivine and augite.</p> - -<p>It is worthy of notice that where the lavas of a plateau diminish greatly -in thickness or become impersistent, the lowest basic group may continue -while the overlying andesites disappear. This feature has been already -mentioned as well seen in the Midlothian plateau. The thick group of -andesites in Arthur Seat and Calton Hill is not to be found in the next -volcanic eminence, Craiglockhart Hill; but the basalts with their underlying -tuffs continue. In the south-western tract from Harper Rig to Hare -Law in Lanarkshire, the thin lava-band, which can be found only at -intervals along the line of outcrop of the volcanic series for about nine -miles, is a dolerite often highly slaggy in structure. Again, at Corrie in -Arran, the lavas which appear upon the shore, apparently at the extreme -western limits of the Clyde plateau, are basic rocks.</p> - -<p>But whether or not the lowest and more basic lavas appear in -any plateau, the main mass of the molten material erupted has usually -consisted of varieties of andesite. The successive discharges of these intermediate -lavas have flowed out in sheets, some of which must have been -little more than heaps of clinkers and scoriæ, while others were more fluid -and rolled along with a ropy or slaggy surface. Occasionally the upper part -of an andesite shows the reddened and decomposed character that suggests -some degree of disintegration or weathering before the next lava-stream buried -it. The intervals between successive outflows of these lavas are not, as a rule, -defined by any marked breaks or by the intercalation of other material. In -general, the plateaux are mainly built up of successive sheets of lava which -have followed each other at intervals sufficiently short to prevent the accumulation -of much detritus between them. Thus the Campsie Hills have -the upper 600 feet of their mass formed of admirably-well-defined sheets of -andesite, separated sometimes by thin partings of tuff, but more usually only -by the slaggy vesicular surfaces between successive flows.</p> - -<p>Where the lavas consisted of trachytes they were apt to assume more -irregular forms. Of this tendency the rocks of the Garleton Hills supply -an excellent example. As already stated, their lumpy character gives to -these hills an outline which offers strong contrast to the ordinary symmetrical -terraced contours of the andesitic plateaux.</p> - -<p><span class="pagenum" id="Page_387">- 387 -</span></p> - -<div class="figcenter" id="v1fig115" style="width: 496px;"> - <img src="images/v1fig115.png" width="496" height="113" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 115.</span>—Section of Craiglockhart Hill, Edinburgh.<br /> - 1. Red sandstones and clays; 2. Green stratified tuffs; 3. Columnar basalt; 4. Dark shales, ironstones - and sandstones, with plants.</div> -</div> - -<div class="figcenter" id="v1fig116" style="width: 438px;"> - <img src="images/v1fig116.png" width="438" height="83" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 116.</span>—Section of the bottom of the Midlothian Plateau, Linnhouse Water above Mid-Calder Oilworks.<br /> - 1. Shales and cement-stones; 2. Sandstones; 3. Highly vesicular lava; 4. Tuffs and sandstone bands. <i>f</i>, Fault.</div> -</div> - -<p>Although tuffs play, on the whole, a comparatively unimportant part -among the constituents of the plateaux, they attain in a few localities an -exceptionally great development, and even where they occur only as thin -partings between the successive lava-flows, they are always interesting -memorials of the volcanic activity of a district. In many portions of the -plateaux, the lowest members of the volcanic series are tuffs and agglomerates, -showing that the eruptions often began with the discharge of fragmentary -materials. Thus in the Midlothian plateau at Arthur Seat, though -the lowest interbedded volcanic sheet is a dolerite, it is immediately followed -by a series of bedded tuffs, before the main mass of the lavas of that hill -make their appearance. At Craiglockhart Hill, three miles distant (<a href="#v1fig115">Fig. 115</a>), -this lowest lava is absent, and a group of tuffs about 300 feet thick rests -immediately on the red Carboniferous sandstones and shales, and is overlain -by sheets of columnar basalt. The scoriaceous bottom of the latter rock may -here and there be seen to have cut out parts of the tuff as it rolled over the -still unconsolidated material. In the same district, a few miles further to the -south-west, some interesting sections of the Midlothian plateau are laid bare in -the streams which descend from the western slopes of the Pentland Hills. I -may cite, in particular, those exposed in the course of the Linnhouse Water. -At the railway viaduct near the foot of Corston Hill, a good section is -displayed of the Cement-stone group—thick reddish, purplish, and greenish-blue -marly shales or clays, with thin ribs and bands of cement-stone and -grey compact cyprid-limestone, as well as lenticular seams and thicker beds -of grey shaly sandstone, sometimes full of ripple-marks and sun-cracks. -These strata, which exactly reproduce the typical lithological characters of -the Cement-stone group of Stirlingshire (Ballagan Beds), Ayrshire and -Berwickshire, are surmounted by a group of reddish, yellow and brown -sandstones, sometimes pebbly and containing a band of conglomerate. -<span class="pagenum" id="Page_388">- 388 -</span> -Among the stones in this band, pieces of the radiolarian cherts of the Lower -Silurian series of the Southern Uplands are conspicuous, likewise pieces of -andesite which may have come from the neighbouring Pentland Hills.</p> - -<p>Above these strata lie the lavas of Corston Hill. These are highly -vesicular in some parts, and include bands of tuff which are well exposed -further down the same stream, immediately above the railway bridge near -the Mid-Calder oilworks (<a href="#v1fig116">Fig. 116</a>). There the lavas, though much decomposed, -show a highly vesicular structure with a rugged upper surface, in the hollows -and over the prominences of which fine flaky and sandy tuffs have been -deposited, while thin seams of vesicular lava are intercalated among -these strata.</p> - -<div class="figright" id="v1fig117" style="width: 438px;"> - <img src="images/v1fig117.png" width="438" height="54" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 117.</span>—Section of the top of the Midlothian Plateau in the Murieston Water.</div> -</div> - -<p>The upper part of the same plateau, as exposed in the course of the -Murieston Water, contains evidence that the last eruptions consisted of -tuff. The highly slaggy lava (1 in <a href="#v1fig117">Fig. 117</a>) is there surmounted by a -thick mass of grey and greenish-white well-bedded granular tuff (2) including -occasional lumps of the basic lava, and passing up into black shale -(3). But that the volcanic eruptions continued during the accumulation of -the shale is proved by the intercalation of thin partings and thicker layers -of tuff in the black sediment. A short way higher up the Burdiehouse -Limestone comes in.</p> - -<p>The great lava-escarpment of the Kilpatrick Hills rests on a continuous -band of tuff which is thickest towards the west, near the group of -vents above Dumbarton, while it thins away eastward and disappears in -Strathblane, the lavas then forming the base of the volcanic series. But -perhaps the most remarkable group of basal tuffs is that which underlies -the lavas of the Garleton plateau, to which further reference will be immediately -made.</p> - -<p>Extensive accumulations of tuff form in one or two localities a large -proportion of the thickness of the whole volcanic series of a plateau. Thus -in the north-eastern part of Ayrshire, between Eaglesham and the valley of -the Irvine, the lavas die out for a space and give place to tuffs. During -the discharge of the fragmentary materials over that ground no lava seems -to have flowed out for a long period. Ordinary sediment, however, mingled -with the volcanic detritus, and there were even pauses in the eruptions when -layers of ironstone were deposited, together with thin impure limestone that -inclosed shells of <i>Productus giganteus</i>.<a id="FNanchor_430" href="#Footnote_430" class="fnanchor">[430]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_430" href="#FNanchor_430" class="label">[430]</a> Explanation of Sheet 22 <i>Geol. Surv. Scotland</i>, p. 12.</p> - -</div> - -<p>In some of the plateaux, particularly within the older part of the volcanic -series, intercalations of ordinary sediment among the tuffs and lavas show -that eruptions occurred only occasionally, and that during the long intervals -<span class="pagenum" id="Page_389">- 389 -</span> -between them the deposition of sand and mud went on as before. Thus -the lower 400 feet of the Campsie Fells are built up of slaggy andesites and -thick beds of fine-grained stratified tuff, with bands of red, green and grey -clays and cement-stone and a zone of white sandstone. The Calton Hill at -Edinburgh (<a href="#v1fig118">Fig. 118</a>) affords an excellent illustration of the interstratification -both of tuffs and ordinary sediments among the successive outflows of -lava. In the total thickness of about 1100 feet of volcanic material in -this hill, at least eight intervals in the discharge of the lavas are marked -by the intercalation of as many bands of nodular tuff, together with seams -of shale and sandstone more or less charged with volcanic detritus. The -highest lava is immediately covered by the white sandstones and black -shales of the Calciferous Sandstone series.</p> - - -<div class="figcenter" id="v1fig118" style="width: 461px;"> - <img src="images/v1fig118.png" width="461" height="106" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 118.</span>—Section of Calton Hill, Edinburgh.<br /> - 1. Lower Carboniferous sandstones; 2. Basic lava at the bottom of the volcanic series; 3. Tuff often interstratified - with sandstones and shales; 4. Sheets of andesite-lava frequently separated by layers of tuff; 5. Shale passing - into tuff; 6. White sandstone and black carbonaceous shales overlying the volcanic series.</div> -</div> - -<p>The tuffs, as might be expected, are coarsest in texture and thickest in -mass where they approach most nearly to some of the vents of eruption, -and, on the other hand, become finer as they recede from these. As a rule, -they are distinctly stratified, and consist of layers varying in the size of -their component lapilli. Here and there, near the centres of discharge, the -bedding becomes hardly traceable or disappears, and the fragmentary -materials take the form of agglomerate.</p> - -<p>In the admirable range of coast-cliffs which extend from North Berwick -to Dunbar, we learn that above the red sandstones at the base of the -Carboniferous system, a thick pile of volcanic ashes was accumulated by -numerous discharges from vents in the immediate neighbourhood. Some of -the explosions were so vigorous that blocks of different lavas, sometimes a -yard or more in length, were thrown out and heaped up in irregular mounds -and hollows. Others discharged exceedingly fine dust, and between these -two extremes every degree of coarseness of material may be recognized.</p> - -<p>As an illustration of the remarkable alternation of coarse and fine -materials, according to the varying intensity of the volcanic paroxysm, <a href="#v1fig119">Fig. 119</a> -is here introduced. It represents a portion of the tuff-cliffs east of -Tantallon Castle, and shows at the bottom fine well-stratified tuff, over which -a shower of large blocks of lava has fallen. Fine detritus is seen to cover -the deposits of this shower, and successive discharges of large stones may be -noticed higher up on more or less well-defined horizons.</p> - -<p>The space over which this pyroclastic material can now be traced, large -<span class="pagenum" id="Page_390">- 390 -</span> -though it is, does not represent the whole of the original area included -within the range of the discharges of ash and stones, for much has been -removed by denudation. During pauses of various length between the -eruptions, waves and currents washed down the heaps of volcanic material -and distributed ordinary sediment over the bottom of the water. Hence, -abundantly interstratified in some parts of the tuff, seams of sandstone, blue -and green shale, cement-stone and limestone occur. One thick band of -limestone may be traced from near Tynningham House to Whittinghame, -a distance of about four miles; another patch appears near Rockville House; -and a third at Rhodes, near North Berwick. No fossils have been noticed -in these limestones. The calcareous matter, together sometimes with silica, -appears to have been supplied, at least in part, by springs, which may have -been connected with the volcanic phenomena of the district. The North -Berwick limestone, in particular, has the peculiar carious wavy structure -with minute mamillated interstices so common among sinters. It contains -grains of pyrites, flakes of white kaolin, which probably represent decayed -prisms or tufts of natrolite, and cavities lined with dog-tooth spar. Some -portions give out a strongly fœtid odour when freshly broken.</p> - -<p>After the tuffs of the Garleton plateau had accumulated to a depth of -perhaps 200 feet or more, lavas began to be poured out. First came -basic outflows (olivine-basalts with picrites) and andesites (porphyrites), -which form a thin but continuous sheet all over the area. These were -succeeded by the series of trachytes which distinguish this area. Although -the observer remarks the absence there of the usual terraced arrangement, -yet from some points of view, particularly from the westward, a succession -of low escarpments and longer dip-slopes can be detected among -the trachytes of the Garleton Hills, while there can be no doubt that, in -spite of their irregular lumpy contours, these lavas lie as a great cake above -the lower platform of more basic flows (<a href="#v1fig10">Fig. 10</a>). There is evidence that -during the emission of the trachytes occasional eruptions of andesite took place. -Not the least striking and interesting feature of this plateau is the size and -distribution of its necks, to which reference will be made in the sequel.</p> - -<p>The latest eruption in the Garleton area had ceased and the cones and -lava sheets had probably been buried under sediment before the commencement -of the deposition of the Hurlet or thick Main Limestone of the -Carboniferous Limestone series which lies immediately to the west of the -plateau.</p> - -<p><span class="pagenum" id="Page_391">- 391 -</span></p> - -<div class="figcenter" id="v1fig119" style="width: 491px;"> - <img src="images/v1fig119.png" width="491" height="757" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 119.</span>—Cliff of tuff and agglomerate, east side of Oxroad Bay, a little east from Tantallon Castle, - East Lothian.</div> -</div> - -<p><span class="pagenum" id="Page_392">- 392 -</span></p> - -<p>The tuffs of the plateaux are seldom fossiliferous, probably for the same -reason that fossils are scarce in the Cement-stone group which the plateau -volcanic rocks overspread and with which they are interstratified. Occasional -stems and other fragments of vegetation occur in the plateau-tuffs, as -in those of North Berwick, where I have found a decayed coniferous trunk -three feet in length. The green tuff at the base of the volcanic group of -Arthur Seat contains abundant macerated plant-remains, together with -scales of <i>Rhizodus</i> and other fishes. In some places the plants are represented -by trunks or roots, which appear to remain in their positions of -growth. A remarkable instance of this nature occurs in some bands of tuff -in the volcanic group of the east coast of the Isle of Arran, first brought to -notice by Mr. E. Wunsch,<a id="FNanchor_431" href="#Footnote_431" class="fnanchor">[431]</a> and of which the plants have been so fully -investigated by Professor Williamson.<a id="FNanchor_432" href="#Footnote_432" class="fnanchor">[432]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_431" href="#FNanchor_431" class="label">[431]</a> <i>Trans. Geol. Soc. Glasgow</i>, vol. ii. (1867) p. 97.</p> - -<p><a id="Footnote_432" href="#FNanchor_432" class="label">[432]</a> <i>Phil. Trans.</i> 1871-1883.</p> - -</div> - -<p>Plant-remains also occasionally occur in the stratified layers intercalated -among the lavas and tuffs of the plateaux. Some of the best examples of -their occurrence are to be found in the shales and tuffs interstratified among -the enormous pile of volcanic material near Bowling. Not only does -abundant vegetable debris occur distributed through the detrital strata in the -volcanic series at that locality, but it is even aggregated into thin seams of -coal which have been examined and described by various observers.<a id="FNanchor_433" href="#Footnote_433" class="fnanchor">[433]</a> It may -be remarked that the plant remains thus found intercalated in the volcanic -series, especially when they have been entombed in tuff, have often had -their internal structure admirably preserved, the organic tissues having -been delicately replaced by calcite or other petrifying medium. The -remarkably perfect structure of some of these plants has been demonstrated -by Professor Williamson, especially in the case of the Arran deposit just -referred to. Mr. John Young has also found the structure well preserved -among the <i>Sigillariæ</i> and <i>Stigmariæ</i> that occur in the stratified intercalations -between the lavas near Bowling.</p> - -<div class="footnote"> - -<p><a id="Footnote_433" href="#FNanchor_433" class="label">[433]</a> See in particular J. Young, <i>Trans. Geol. Soc. Glasgow</i>, vol. iv. (1874) p. 123.</p> - -</div> - -<div class="figcenter" id="v1fig120" style="width: 487px;"> - <img src="images/v1fig120.png" width="487" height="100" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 120.</span>—Section across part of the Clyde Plateau to the west of Bowling (reduced from Sheet 6 - of the Horizontal Sections of the Geological Survey of Scotland).<br /> - 1. "Ballagan Beds"; 2. White sandstone; 3. Tuffs, 600 feet thick, with a thin sheet of andesite; 4. Andesite - sheets, 500 feet; 5. Stratified tuffs with thin coals, shales, fireclays and plant-remains, 500 or 600 feet; 6 6. - A series of andesite-lavas, about 1500 feet thick, enclosing a thin coal-seam at *; 7. Stratified tuffs, 200 feet; - 8. Shales with plants and coaly seams, 150 feet; 9. Base of another andesite series, which must be some - hundreds of feet thick; 10 and 11. Necks of agglomerate.</div> -</div> - -<p><i>Upper Limits and Original Areas and Slopes of the Plateaux.</i>—Where -the highest members of the volcanic series can be seen passing conformably -under the overlying Carboniferous strata they are frequently found to -be mainly composed of fine tuffs, the last feeble efforts of the plateau-volcanoes -having consisted in the discharge of showers of ashes. These -materials were mingled with a gradually increasing proportion of ordinary -mechanical sediment, which finally overspread and buried the volcanic tracts -of ground, as these slowly sank in the general subsidence of the region. -The characteristic corals, crinoids and shells of the Carboniferous Limestone -begin to appear in these ashy sediments. There is thus an insensible -passage from volcanic detritus into fossiliferous shales and limestones. -<span class="pagenum" id="Page_393">- 393 -</span> -Examples of this gradation may be seen in many natural sections along -the flanks of the Ayrshire plateau from above Kilbirnie to Strathavon.</p> - -<p>It is still possible to fix in some quarters the limits beyond which -neither the lavas nor the tuffs extended, and thus partially to map out -the original areas of the plateaux. For example, in certain directions the -Carboniferous formations can be followed continuously downward below the -Main Limestone, without the intervention of any volcanic material, or with -only a slight intermixture of fine volcanic lapilli, such as might have been -carried by a strong wind from some neighbouring active vents. By this -kind of evidence and by the proved thinning-out of the materials of the -plateau, we can demonstrate that in the north of Ayrshire the southern -limits of the great volcanic bank did not pass beyond a line drawn from near -Ardrossan to Galston. We can show, too, that the lavas of the Campsie -Fells ended off about a mile beyond Stirling before they reached the line of -the Ochil heights, and that the <i>coulées</i> which flowed from the Solway vents -did not quite join with those from the Berwickshire volcanoes.</p> - -<div class="figcenter" id="v1fig121" style="width: 517px;"> - <img src="images/v1fig121.png" width="517" height="96" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 121.</span>—Diagram illustrating the thinning away southwards of the lavas of the Clyde - Plateau between Largs and Ardrossan. Length about 10 miles.<br /> - 1. Upper Old Red Sandstone; 2. Sandstones, shales, etc., with "Ballagan Beds"; 3. Tuffs; 4. Andesite lavas; - 5. Carboniferous Limestone series.</div> -</div> - -<p>Moreover, evidence enough remains to enable us to form a tolerably -clear conception of the original average slopes of the surface of some of -the plateaux. Thus in the great escarpment above Largs and the high -ground eastward to Kilbirnie the volcanic series, as already stated, must -be at least 1500 feet thick. This thick mass of lavas and tuffs thins away -southwards and probably disappears a short distance south from Ardrossan -in a space of about ten miles (<a href="#v1fig121">Fig. 121</a>). The original southward slope of -the plateau would thus appear to have been about 1 in 35. Again, the -northward slope of the same plateau may be estimated from observations in -the Campsie Fells. We have seen that above Kilsyth the total depth of -the volcanic sheets is about 1000 feet, while to the westward it is much -thicker. From the top of the Meikle Bin (1870 feet) above Kilsyth north-eastwards -to Causewayhead, where the whole volcanic series has died out, -is a distance of 12 miles, so that the slope of the surface of erupted materials -on this side was about 1 in 63 (<a href="#v1fig122">Fig. 122</a>).</p> - -<p>Judging from the sections exposed along the faces of the escarpments, -we may infer that the volcanic sheets had a tolerably uniform surface -which sloped gently away from the chief vents, but with local inequalities -according to the irregularities of the lava-streams that were heaped up round -the vents and flowed outward in different directions and to various distances -<span class="pagenum" id="Page_394">- 394 -</span> -from them. At the beginning, these flat volcanic domes were certainly -subaqueous. While they were being formed, continuous subsidence appears -to have been in progress. But the great thickness of the volcanic accumulations, -as in the Kilpatrick and Renfrewshire areas, and the paucity of -ordinary sedimentary strata among them, make it not improbable that at least -their higher parts rose above the water. Where this was the case there -may have been considerable degradation of the lava-banks before these were -reduced or were by subsidence submerged beneath the water-level. Evidence -of this waste is probably to be recognized in the bands of conglomerate, -occasionally of considerable thickness, which, particularly in some parts of -Ayrshire, intervene between the top of the volcanic group and the Hurlet -Limestone. As I shall have occasion to point out further on, there seems -to be some amount of evidence in favour of the view that a considerable -interval of time elapsed between the close of the plateau-eruptions and the -date of that widespread depression which led to the deposition of the Hurlet -Limestone over the whole of Central Scotland. If such an interval did -occur it would include a prolonged abrasion of any projecting parts of the -plateaux, and the production and deposition of volcanic conglomerate.</p> - -<div class="figcenter" id="v1fig122" style="width: 516px;"> - <img src="images/v1fig122.png" width="516" height="114" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 122.</span>—Diagram illustrating the thinning away eastwards of the lavas of the Clyde - Plateau in the Fintry Hills. Length about 12 miles.<br /> - 1. Upper Old Red Sandstone; 2. White sandstone, blue shales and cement-stones ("Ballagan Beds"); 3. Andesite - sheet, about 100 feet thick; 4. Tuffs (250 feet), with an included band of ashy sandstone containing plant-remains; - 5. Andesite lavas; 6. Carboniferous Limestone series, which to the east lies immediately on the - Upper Old Red Sandstone.</div> -</div> - - -<h3>2. VENTS</h3> - -<p>We have now to consider the external forms, internal contents and -distribution of the vents from which the material of the plateaux was discharged. -In the Carboniferous system these interesting relics of former -volcanoes are far more distinctly defined and better preserved than in older -geological formations. Moreover, in Scotland, they are laid bare to greater -advantage, both inland and along the sea-coast, and may indeed be studied -there as typical illustrations of this kind of geological structure.</p> - -<p><span class="pagenum" id="Page_395">- 395 -</span></p> - -<div class="figcenter" id="v1fig123" style="width: 462px;"> - <img src="images/v1fig123.png" width="462" height="250" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 123.</span>—View of the two necks Dumgoyn and Dumfoyn, Stirlingshire, taken from the south.<br /> - These two necks form a conspicuous feature in front of and below the lava plateau, a portion of which is shown on - the right hand. The ground-plan of the same necks is shown in <a href="#v1fig124">Fig. 124</a>.</div> -</div> - -<div class="figcenter" id="v1fig124" style="width: 480px;"> - <img src="images/v1fig124.png" width="480" height="363" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 124.</span>—Ground-plan of Plateau-vents near Strathblane, Stirlingshire, on the scale of - 6 inches to a mile.</div> -</div> - -<p>In external form the necks connected both with the plateaux and the -puys generally rise from the surrounding ground as isolated, rounded, conical -or dome-shaped prominences, their details of contour depending mainly upon -the materials of which they consist. When these materials are of agglomerate, -tuff or other readily disintegrated rock, the surface of the domes is -generally smooth and grass-covered. Where, on the other hand, they consist -wholly or in part of dolerite, basalt, diabase, andesite, trachyte or other -crystalline rock, they present more irregular rocky outlines. Illustrations -of some of those varying forms are given in Figs. <a href="#v1fig23">23</a> and <a href="#v1fig123">123</a>. In rare -<span class="pagenum" id="Page_396">- 396 -</span> -instances the vent is marked at the surface not by a hill but by a hollow, -as in the great neck in the heart of the Campsie Fells (<a href="#v1fig128">Fig. 128</a>).</p> - -<div class="figcenter" id="v1fig125" style="width: 458px;"> - <img src="images/v1fig125.png" width="458" height="540" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 125.</span>—Ground-plans of double and triple necks in the Plateau series, on the scale of - 6 inches to a mile.<br /> - A. Barwood Hill and Ravenscraig, east of Dumbarton, double vent. B. The Knock Hill, Largs, Ayrshire, double - vent (see <a href="#v1fig23">Fig. 23</a>). C. Dumbowie and Dumbuck, east of Dumbarton, triple vent.</div> -</div> - -<p>As regards their ground-plan, which affords a cross-section of the original -volcanic funnel, the plateau-vents present considerable variety. The simplest -cases are those in which the form is approximately circular or somewhat -elliptical. Here the outline corresponds to the cross-section of a single and -normal orifice. Some examples of this simple type are given in <a href="#v1fig124">Fig. 124</a>, -which represents a group of vents on the edge of the Clyde plateau near Strathblane. -The two larger necks here shown are the same which appear in the -<span class="pagenum" id="Page_397">- 397 -</span> -view in <a href="#v1fig123">Fig. 123</a>.<a id="FNanchor_434" href="#Footnote_434" class="fnanchor">[434]</a> Where two vents have been successively opened close to -each other, or where the same vent has shifted its position, the ground-plan -may be greatly modified. In some instances the double funnel can be distinctly -traced. Thus in the conspicuous Knock Hill above Largs in Ayrshire (<a href="#v1fig125">Fig. -125, B</a>) there are two conjoined necks, and such appears to be also the -structure shown by the ground-plan of the neck of Barwood Hill and -Raven's Craig, east of Dumbarton (<a href="#v1fig125">Fig. 125, A</a>).<a id="FNanchor_435" href="#Footnote_435" class="fnanchor">[435]</a> But more complex -forms occur which point to a still larger number of coalescing necks. -A group of hills to the east of Dumbarton gives the ground-plan shown -in <a href="#v1fig125">C, Fig. 125</a>, where traces may be detected of three separate vents. -Still more irregular are long narrow dyke-like masses of tuff or agglomerate -which have probably risen along lines of fissure (<a href="#v1fig22">Fig. 22, No. 1</a>). The most -striking example of these, however, occur in association with the puys and -will be described in later pages.</p> - -<div class="footnote"> - -<p><a id="Footnote_434" href="#FNanchor_434" class="label">[434]</a> The illustrations in Figs. <a href="#v1fig124">124</a> and <a href="#v1fig125">125</a> are taken from the field-maps of the Geological Survey on -the scale of 6 inches to a mile. The ground represented in <a href="#v1fig124">Fig. 124</a> was mapped by Mr. R. L. Jack.</p> - -<p><a id="Footnote_435" href="#FNanchor_435" class="label">[435]</a> These ground-plans are likewise taken from the field-maps of the Geological Survey. A and -C were mapped by Mr. Jack, B by myself. The shaded parts are intrusive andesites and dolerites; -the dark bars in A and C being dolerite dykes of much later date than the necks. The dotted -portions mark tuff and agglomerate.</p> - -</div> - -<p>Connected with their ground-plan is the relative size of the plateau-vents. -On the whole they are larger than those of the puy series. The -simple circular or elliptical type presents the smallest necks, some of them -not exceeding 100 feet in diameter. The more complex forms are generally -also of larger dimensions. By much the largest vent or connected group -of vents is that which lies among the uplands of Misty Law in the heart -of the Renfrewshire part of the Clyde plateau, where a connected mass of -tuff and agglomerate now occupies a space of about 4 miles in length -by 2½ miles in breadth (<a href="#v1fig129">Fig. 129</a>). It has not been found possible, however, -to trace the boundaries of the separate vents of this tract, nor to distinguish -the material of the necks from that which surrounds them. -Another large mass which from its shape may be conjectured to represent -more than one vent is the great tract north of Melrose, which measures -8800 by 4200 feet.<a id="FNanchor_436" href="#Footnote_436" class="fnanchor">[436]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_436" href="#FNanchor_436" class="label">[436]</a> The following measurements are, like those in the text, taken from the field-maps of the -Geological Survey. Carewood Rig, on the borders of Roxburghshire and Dumfriesshire, -7000 × 2400 feet; the great vent in the middle of the Campsie Fells, 5200 × 2600; Black Law, -between Bedrule and Jedburgh, 3400 × 1600; Dumgoyn, Strathblane, 2300 × 1300; Rubers Law, -1500 × 1000; Minto Hill (south), 2300 × 1650; Minto Hill (north), 1500 × 1100; Doughnot Hill, -Kilpatrick range, 1000 × 700; four of the smallest agglomerate vents along the northern escarpment -of the Clyde plateau between Strathblane and Fintry, 500 × 450, 450 × 400, 250 × 100, -200 × 200; Pike Law, Arkleton, Tarras Water, 500 × 500; Harwood, Stonedge, 5 miles S.E. from -Hawick, 500 × 300; Arkleton Burn, Dumfriesshire, 400 × 100; Dalbate Burn, 250 × 120.</p> - -</div> - -<p>The distribution of the necks can best be understood from the maps of -the Geological Survey, where they have been carefully indicated. As might -have been expected, they are not found outside the original limits within -which it may be reasonably inferred that the lavas and tuffs were erupted. -They occur most abundantly and attain their largest size in and around the -districts where the plateaux are most extensively developed. No doubt a -<span class="pagenum" id="Page_398">- 398 -</span> -large number of them are concealed under these plateaux. A few appear at -the surface among the lavas and tuffs, but by far the largest number now -visible have been revealed by denudation, the escarpments having been cut -back so as to lay bare the underlying rocks through which the necks rise. -Thus, along the flanks of the great escarpment that extends from near Stirling -by Fintry and Strathblane to Dumbarton, more than two dozen of agglomerate -necks may be counted in a distance of about sixteen miles, while if the necks -of lava-form material are included, the number of vents must be about fifty. -Nowhere in Scotland do such necks form a more conspicuous feature in the -scenery as well as the geology than they do between Fintry and Strathblane, -where, standing out as bold isolated hills in front of the escarpments, their -conical and rounded outlines present a striking contrast to the terraced -escarpments behind them. I would especially refer again to the two -remarkable cones of Dumfoyn and Dumgoyn above Strathblane (Figs. -<a href="#v1fig123">123</a>, <a href="#v1fig124">124</a>, <a href="#v1fig127">127</a>). Along the west front of the hills between Gourock and -Ardrossan seventeen agglomerate-vents occur in a distance of sixteen miles. -In Roxburghshire a group of large agglomerate-necks is dotted over the -Silurian country around Melrose and Selkirk<a id="FNanchor_437" href="#Footnote_437" class="fnanchor">[437]</a> (see <a href="#v1fig130">Fig. 130</a>).</p> - -<div class="footnote"> - -<p><a id="Footnote_437" href="#FNanchor_437" class="label">[437]</a> In this region and farther southward, besides the plateau-eruptions, a later group of puys is -to be seen, and it is difficult to discriminate between the necks belonging to the two groups. Those -which lie to the east are probably connected with the plateaux, those to the west with the puys. -The latter are referred to on <a href="#Page_475">p. 475</a>.</p> - -</div> - -<div class="figcenter" id="v1fig126" style="width: 373px;"> - <img src="images/v1fig126.png" width="373" height="277" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 126.</span>—Ground-plan of tuff-neck, shore east of Dunbar.<br /> - The surrounding rocks are sandstones, which are much hardened round the vent in the zone marked by the short - divergent lines. The arrows mark the direction of dip. See "Geology of East Lothian," <i>Mem. Geol. Survey</i>, p. 44.</div> -</div> - -<p>From the evidence of these necks it is plain that the volcanic materials -of the plateaux must in each case have been supplied not from great -central orifices, but from abundant vents standing sometimes singly, with -intervening spaces of several miles, often in groups of four or five within a -single square mile.</p> - -<p><span class="pagenum" id="Page_399">- 399 -</span></p> - -<p>In the interior of the country, it is seldom possible to examine the actual -junction of necks with the rocks through which they rise, the boundary-line -being usually obscured by debris or herbage. On the coast, the vents of the -plateaux have not been bared by the sea so fully as in the case of the much -younger series of the east of Fife to be described in later pages. But -where the East Lothian plateau touches the shore, the waves have laid bare -a number of its minor vents, which have thus been dissected in ground-plan -on the beach. As an illustration of these vents an example is given in -<a href="#v1fig126">Fig. 126</a>, from the shore east of Dunbar. Here the sandstones, which are -inclined in an easterly direction at 20° to 25°, are pierced by an irregular -mass of tuff. It is observable that in this instance long tongue-like projections -of the sandstones protrude into the neck; more frequently the -material of a neck sends veins or dykes into the surrounding walls. A -volcanic chimney would seem to have been often much shattered and fissured -in the course of the volcanic explosions, and the fragmentary material has -fallen or been injected into the rents thus caused. As a rule, the rocks -immediately around the Carboniferous necks are more or less indurated, as -in this instance from the Dunbar shore.</p> - -<p>The materials which have filled up the vents connected with the plateau-eruptions -generally consist of (<i>a</i>) agglomerates or tuffs, but occasionally of -(<i>b</i>) some kind of lava, and frequently (<i>c</i>) of both these kinds of rock -combined.</p> - -<p>(<i>a</i>) <i>Necks of Agglomerate or Tuff.</i>—These materials vary greatly in the -nature and relative proportions of their constituents. Usually the included -blocks and lapilli are pieces of andesite, diabase, basalt or other lava, like -the rocks of the plateaux. But with these occur also fragments probably -detached from the sides of the funnels through which the explosions took -place, such as pieces of greywacke, sandstone, limestone and shale. Considerable -induration may be observed among these non-volcanic ingredients. -In some cases, as in that of the occurrence of pieces of granite referred -to on <a href="#Page_382">p. 382</a>, the stones have probably been brought up from some considerable -depth. In others it is easy to see that the blocks have slipped -down from some higher group of strata now removed from the surrounding -surface by denudation. Some striking illustrations of this feature will be -cited from necks of the puy-series in the south of Roxburghshire (<a href="#Page_476">p. 476</a>).</p> - -<p>The lava blocks in the tuffs and agglomerates are usually rounded or -subangular. Pear-shaped blocks, or flattened discs, or hollow spherical balls -are hardly ever to be observed, though I have noticed a few examples in -the tuffs of Dunbar. A frequent character of the blocks is that of roughly -rounded, highly amygdaloidal pieces of lava, the cellular structure being -specially developed in the interior, and the cells on the outside being often -much drawn out round the circumference of the mass. Such blocks were -probably torn from the cavernous, partially consolidated, or at least rather -viscous, top of a lava column. Most of the stones, however, suggest that -they were produced by the explosion of already solidified lava, and were -somewhat rounded by attrition in their ascent and descent. The vents filled -<span class="pagenum" id="Page_400">- 400 -</span> -with such materials must have been the scene of prolonged and intermittent -activity; successive paroxysms resulting in the clearing out of the hardened -lava column in the throat of the volcano, and in the rise of fresh lava, with -abundant ejection of dust and lapilli.</p> - -<div class="figcenter" id="v1fig127" style="width: 475px;"> - <img src="images/v1fig127.png" width="475" height="149" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 127.</span>—Section across the vents Dumgoyn and Dumfoyn, and the edge of the Clyde plateau above - Strathblane, Stirlingshire.<br /> - 1. Upper Old Red Sandstone; 2. Shales, cement-stones and sandstones ("Ballagan beds"); 3. White sandstone; - 4. Andesite lavas; 5. Agglomerate (shown by the dotted portions), traversed by intrusive diabase. <i>f</i>, Fault. <i>D.</i> - Late dolerite dyke.</div> -</div> - -<p>Necks formed entirely of agglomerate are abundant among the vents -connected with the plateaux. As examples of them I may refer to the series -already mentioned as fronting the escarpment of the Clyde plateau from -Fintry to Largs. Another interesting group rises through the Silurian and -Old Red Sandstone rocks to the west of the escarpment of the Berwickshire -plateau, that near Melrose forming one of the largest in Scotland.</p> - -<div class="figcenter" id="v1fig128" style="width: 475px;"> - <img src="images/v1fig128.png" width="475" height="100" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 128.</span>—Section through the large vent of the Campsie Hills.<br /> - 1. Andesite lavas; 2. Agglomerate and tuff; 3. Trachytic and andesitic intrusive rocks.</div> -</div> - -<div class="figcenter" id="v1fig129" style="width: 466px;"> - <img src="images/v1fig129.png" width="466" height="87" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 129.</span>—Diagrammatic section across the central vent of the Clyde plateau in Renfrewshire.<br /> - 1. Andesite lavas; 2. Agglomerates and fine tuffs often much altered; 3. Dykes of trachytic and andesitic rocks; - 4. Later dykes of dolerite and basalt.</div> -</div> - -<p>Illustrations of the varying structure of these vents are given in the -accompanying figures. In <a href="#v1fig127">Fig. 127</a>, a section is drawn through the two necks -Dumgoyn and Dumfoyn, which have already been shown in outline and in -ground-plan. The relation of these two vents to the neighbouring plateau to -the right can here be seen. <a href="#v1fig128">Fig. 128</a> gives a section taken through the -<span class="pagenum" id="Page_401">- 401 -</span> -great vent of the Campsie Hills, with the minor adjacent necks of Dungoil, -Bin Bairn, and the Meikle Bin.</p> - -<p>The diagram in <a href="#v1fig129">Fig. 129</a> is meant to convey in a -general way what appears to be the structure of the -central vent of the Renfrewshire plateaux, to be afterwards -referred to. But, as already mentioned, the limits -of the various rocks are too much obscured to allow an -accurate delineation to be given of their areas and relations -to each other. The Berwickshire plateau supplies -abundant interesting examples of tuff necks which rise -through the Old Red Sandstone many miles distant from -the edge of the lavas. This structure is shown in -<a href="#v1fig130">Fig. 130</a>.</p> - -<div class="figcenter" id="v1fig130" style="width: 735px;"> - <img src="images/v1fig130.png" width="735" height="82" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 130.</span>—Section across Southern Berwickshire to show the relation of the volcanic plateau to the vents lying south from it.<br /> - 1. Upper Silurian strata; 2. Upper Old Red Sandstone; 3. The volcanic plateau; 4. Agglomerate and tuff of the vents; 5. Basalt and dolerite; 6. Lower Carboniferous strata.</div> -</div> - -<p>Indications may occasionally be observed of an -agglomerate vent having been first occupied by one kind -of material and then, after being in great measure cleared -out by explosions, having been subsequently filled up -with another. As an example of this structure I may -cite again the double neck of the Knock Hill a little -to the north of Largs, of which the outline is shown -in <a href="#v1fig23">Fig. 23</a>, and the ground-plan in <a href="#v1fig125">Fig. 125, B</a>. This -hill rises from the red sandstone slopes that front the -great Ayrshire plateau and forms a conspicuous cone -the top of which is rather more than 700 feet above the -sea. Its summit commands a remarkably extensive and -interesting panorama of the scenery of the Clyde, but -to the geologist perhaps the most striking feature in the -landscape is the range of terraced hills behind, mounting -up into the great vents of the Renfrewshire uplands. -On these declivities the successive lava-streams that have -built up the plateau can be seen piled over each other -for a thickness of more than 1000 feet, and presenting -their escarpments as parallel lines of brown crag with -green slopes between.</p> - -<p>The Knock has had its upper part artificially dressed, -for lines of trench have been cut out of its rocks by some -early race that converted the summit of the hill into a -strongly intrenched camp. From the apex of the cone -the ground falls rapidly westward into a hollow, beyond -which rises a lower rounded ridge of similar materials. -It is possible that this western ridge may really form -part of the main hill, but the grass-covered ground does -not afford sufficient exposures of the rocks to settle this -point. From the contours of the surface, it may be -inferred that there are two closely adjacent vents, and -that the western and lower eminence is the older of the two. This -<span class="pagenum" id="Page_402">- 402 -</span> -hill or ridge consists partly of a coarse agglomerate, and partly of veins and -irregular protrusions of a dark, compact, slightly cellular lava. The stones -in the fragmental rock are different olivine-basalts, or other basic lavas, and -sandstones. The paste is rough, loose and granular. The sandstone fragments -are much indurated and sometimes bleached.</p> - -<p>The Knock itself is formed mainly of a remarkably coarse and strikingly -volcanic agglomerate. Round the outside, and particularly on the south-east, -the rock is finer in texture, compact, and gravelly, or like a mudstone, with -few or no imbedded blocks, dull-green to red in colour, and breaking with -a clean fracture which shows angular lapilli of various basalts or diabases. -At the southern end of the neck, where the surrounding red sandstone can -be seen within a few feet of the tuff, the latter is bright red in colour, and -contains much debris of red sandstone and marl. Possibly this finer tuff, -which is traceable as an irregular band round the outside of the neck, may -mark an older infilling of the vent than the agglomerate of the centre; but -there is no sharp line to be drawn between the two, though a hollow can -sometimes be traced on the surface where they join.</p> - -<p>The agglomerate of this locality is one of the most characteristic among -the plateau-necks of the Clyde region. Its blocks sometimes measure from two -to three feet in diameter. They consist almost wholly of a dark crystalline -porphyritic olivine-basalt. These blocks are subangular in form, often with -clean-fractured surfaces. Though occasionally slightly cellular, they are -never slaggy so far as I could see, nor are any true scoriæ to be noticed -among them. The blocks suggest that they were derived from the disruption -of an already solidified mass of lava. The agglomerate is entirely -without any trace of stratification.</p> - -<p>Through this tumultuous accumulation of volcanic debris some -irregular veins of olivine-basalt, sometimes glassy in structure, have been -injected, and reach nearly to the summit of the hill. This intrusive -material resembles generally some of the dark intrusive masses in the -Dumbartonshire necks. Like these, it exhibits a tendency to assume a -more or less distinctly columnar structure, its columns having the same -characteristic wavy sides and irregular curvature. The intrusive rocks in -the two eminences of the Knock may be paralleled among the stones in the -agglomerate. The neck on its north-eastern side rises steeply from the red -sandstones which it pierces, but which, although they are much jointed and -broken, are not sensibly indurated. Unfortunately the actual junction of -the igneous and sedimentary rocks is concealed under herbage.</p> - -<div class="figright" id="v1fig131" style="width: 295px;"> - <img src="images/v1fig131.png" width="295" height="99" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 131.</span>—Section of south end of Dumbuck Hill. East of - Dumbarton.</div> -</div> - -<p>As a rule, the fragmental materials of the plateau-necks are quite -unstratified. Their included blocks, distributed irregularly through the -mass, have evidently undergone little or no assortment after they fell -back into the vents. Occasionally, however, a more or less distinct -bedding of the agglomerate or tuff may be observed, the layers having -a tendency to dip inward into the centre. One of the most conspicuous -examples of this structure is to be found in the hill of Dumbuck, to -the east of Dumbarton. This neck, which forms so prominent a feature -<span class="pagenum" id="Page_403">- 403 -</span> -in the landscape, presents a precipitous face towards the south, and allows -the disposition of its component materials to be there seen. The agglomerate -consists of a succession of rudely stratified beds of coarser and finer detritus, -which on both sides are -inclined towards the centre, -where a plug of fine-grained -olivine-basalt has risen and -spread out into a columnar -sheet above (<a href="#v1fig131">Fig. 131</a>). In -general form this basalt -resembles such intrusions -as that of Largo Law, to be afterwards described (<a href="../../66493/66493-h/66493-h.htm#v2fig226">Fig. 226</a>), where -what may have been the hollow or bottom of the crater is filled with basalt.</p> - -<div class="figcenter" id="v1fig132" style="width: 509px;"> - <img src="images/v1fig132.png" width="509" height="100" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 132.</span>—Section across the East Lothian plateau to show the relative position of one of the necks.<br /> - 1. Lower Carboniferous sandstones and shales; 2. Red and green tuffs with a seam of limestone (<i>l</i>); 3. Band of - basic sheets at the base of the lavas; 4. Trachytes; 5. Phonolite neck.</div> -</div> - -<p>(<i>b</i>) <i>Necks of Andesite, Trachyte, Dolerite, Diabase, or other massive Rock.</i>—When -the vents have been filled by the uprise of some molten rock, it is -generally, as we have seen, of a more acid character than the ordinary lavas -of the plateaux. Frequently it consists of some variety of trachyte or -andesite, commonly of a dull yellow or grey tint and waxy lustre. Good -examples may be seen among the remarkable group of necks on either side -of the valley north of the village of Strathblane and in those above Bowling. -The three great necks in East Lothian, already alluded to,—Traprain Law -(Figs. <a href="#v1fig132">132</a>, <a href="#v1fig133">133</a>), North Berwick Law (<a href="#v1fig109">Fig. 109</a>), and the Bass Rock -(<a href="#v1fig110">Fig. 110</a>)—are masses of phonolite and trachyte, obviously related to the trachytes -of the adjacent plateau. A smaller but very perfect instance of a vent -similarly filled is to be seen in the same neighbourhood on the shore to the -east of North Berwick Law.<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> See "Geology of East Lothian," <i>Geological Survey Memoir</i>, p. 40.</p> - -</div> - -<p>Examples occur where the funnels of eruption have been finally sealed -up by the rise of more basic material, and this has happened even in a -district where most of the lava-form necks consist of trachyte or some other -intermediate lava. Thus, in the Campsie Fells, several such bosses appear, -of which the most conspicuous forms the hill of Dungoil (1396 feet,<a href="#v1fig128"> Fig. -128</a>). Further west, among the Kilpatrick Hills, bosses of this kind are -still more numerous. The group of bosses near Ancrum and Jedburgh is -mainly made up of olivine-dolerites and olivine-basalts (<a href="#v1fig130">Fig. 130</a>). This more -basic composition of itself suggests that these bosses may be connected rather -with the puy- than with the plateau-eruptions.</p> - -<p><span class="pagenum" id="Page_404">- 404 -</span></p> - -<p>(<i>c</i>) <i>Necks of Composite Character.</i>—In not a few examples, the vents -have been filled with agglomerate which has been pierced by a plug or veins -of lava-form material. Many illustrations of this composite structure may -be observed along the west front of the great escarpments from Fintry to -Ardrossan (see Figs. <a href="#v1fig124">124</a>, <a href="#v1fig125">125</a>, <a href="#v1fig127">127</a> and <a href="#v1fig128">128</a>). In that region the intruded -rock is often a dull yellowish or grey trachytic or andesitic material. -Olivine-basalt is the chief rock intruded in the vents in the Dumbarton -district. Among the Roxburghshire vents, where the injected material is -commonly olivine-basalt or dolerite, it occasionally happens, as in Rubers Law, -that the uprise of the lava has almost entirely cleared out or concealed the -agglomerate, and in some of the bosses, where no agglomerate is now to be -seen, the basalt may have taken its place (<a href="#v1fig130">Fig. 130</a>).</p> - -<p>The largest and most interesting vents connected with this type of -Carboniferous volcano, are those which occur within the limits of the -plateaux, where they are still surrounded with lavas and tuffs that probably -came out of them. Of these by far the most extensive and remarkable -lies among the high moorlands of Renfrewshire between Largs and -Lochwinnoch, where the ground rises to more than 1700 feet above the -sea (see <a href="#v1fig129">Fig. 129</a>). This area, as already remarked, is unfortunately much -obscured with drift and peat, so that the limits of its rocks cannot be so -satisfactorily traced as might be desired. I think it probable that several -successive vents have here been opened close to each other, but their erupted -ashes probably cannot be distinguished. Over a space measuring about four -miles in length by two and a half in breadth, the rocks exposed at the -surface are fine tuffs, breccias and coarse agglomerates, largely made up of -trachytic, andesitic or felsitic material, and pierced by innumerable protrusions -of various andesitic, trachytic or felsitic rocks in bosses and veins, as well -as also by dykes of a more basic kind, such as dolerites and basalts. Some -of the tuffs present a curiously indurated condition; and they are frequently -much decayed at the surface.<a id="FNanchor_439" href="#Footnote_439" class="fnanchor">[439]</a> Another large mass of tuff and agglomerate -lies a little to the south-west of the main area.</p> - -<div class="footnote"> - -<p><a id="Footnote_439" href="#FNanchor_439" class="label">[439]</a> This tract of ground was mapped for the Geological Survey by Mr. R. L. Jack, now in charge -of the Geological Survey of Queensland. See Sheet 31, <i>Geological Survey of Scotland</i>.</p> - -</div> - -<p>After the explosions ceased, by which the vents were opened and the -cones of debris were heaped up, heated vapours would in many cases, as in -modern volcanoes, continue for a long while to ascend in the funnels. The -experiments of Daubrée on the effects of water and vapour upon silicates -under great pressure and at a low red heat, have shown how great may -be the lithological changes thereby superinduced. It is improbable that -where a mass of tuff and lava, lying deep within a volcanic vent, was -thoroughly permeated with constantly ascending heated vapours, it should -escape some kind of change. I am inclined to attribute to this cause the -frequent conversion of the sandstones round the walls of the vents into -quartzite. The most remarkable example of metamorphism within a vent -which I have observed among the plateaux, occurs in the heart of the -Campsie Fells, where, instead of forming a prominence, the neck is marked -by a great hollow, measuring about a mile in length and half a mile in -breadth (<a href="#v1fig128">Fig. 128</a>).<a id="FNanchor_440" href="#Footnote_440" class="fnanchor">[440]</a> It is occupied mainly by a coarse tumultuous agglomerate, -like that of other necks in the same district, but with a matrix -rather more indurated, and assuming in certain parts a crystalline texture, -so as to be at first sight hardly distinguishable from some of the surrounding -andesites. Even in this altered condition, however, its included fragments -may be recognized, particularly blocks of sandstone which have been -hardened into quartzite. Numerous small veins of pink and yellow trachyte -traverse the agglomerate, and are found also cutting the bedded andesites -that encircle it.</p> - -<div class="footnote"> - -<p><a id="Footnote_440" href="#FNanchor_440" class="label">[440]</a> See Explanation to Sheet 31, <i>Geological Survey of Scotland</i>, par. 21 (1878).</p> - -</div> - -<p><span class="pagenum" id="Page_405">- 405 -</span></p> - -<div class="figcenter" id="v1fig133" style="width: 755px;"> - <img src="images/v1fig133.png" width="755" height="475" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 133.</span>—View of Traprain Law from the south, a phonolite neck of the Garleton Plateau.</div> -</div> - -<p><span class="pagenum" id="Page_406">- 406 -</span></p> - - -<h3>3. DYKES AND SILLS</h3> - -<p>Intrusive masses both in the form of dykes and of sills are of frequent -occurrence in connection with the Carboniferous volcanic plateaux. From -the variety of their component materials it may be inferred that these rocks -belong to different ages of intrusion.</p> - -<p><span class="smcap">Dykes.</span>—The great majority of the Dykes consist of trachyte or of -andesite, resembling in lithological characters the material of the necks and -doubtless connected with its uprise. There occur also dykes of diabase, -basalt or dolerite. Some of the latter, especially those which run for many -miles, cutting every rock in the districts in which they occur, and crossing -large faults without deviation, are certainly long posterior to the plateau -volcanic period. Whether the small inconstant dykes of more basic composition, -found in the same districts with the trachytes, are to be looked -upon as part of the volcanic phenomena of the plateaux, is a question to -which at present no definite answer can be given. I shall have occasion to -show that in the next volcanic period the lavas that flowed from the puys -are more basic than most of those of the plateaux, and that they are -associated with more basic dykes and sills. In Roxburghshire, where it is -so difficult to distinguish between the denuded vents of the two periods, -the dark heavy olivine-basalts and dolerites of the bosses may possibly -belong rather to the later than to the earlier volcanic episode. And if -that be their true age, the dykes of similar material may be connected -with them. At the same time it must be remembered that the earliest -eruptions of the plateaux were markedly basic, that many vents in the -plateaux are pierced by basic intrusions, and that basic dykes may have been -associated with the uprise of the same magma.</p> - -<p>The dykes occur in considerable numbers and in two distinct positions, -though these may be closely related to each other: 1st, among the rocks -outside and beneath the plateau-lavas, or cutting these lavas; and 2nd, in -and around the vents.</p> - -<p>1. Among the rocks which emerge from under the Carboniferous -volcanic plateaux, dykes are sometimes to be observed in considerable -numbers. They may be compared to the far more extensive series connected -<span class="pagenum" id="Page_407">- 407 -</span> -with the Tertiary basalt-plateaux, like which they may have had a -close relation to the actual building up of the successive sheets of andesite, -trachyte and basalt that were erupted at the surface. They are particularly -well developed in the Clyde plateau, where by extensive denudation they have -been admirably exposed. I would especially refer to those that traverse the -tract of red sandstones which underlie the volcanic series along the flanks of the -great escarpments from Fintry to Strathblane and Dumbarton, and between -Gourock and Ardrossan. These dykes have been dissected by the sea along -both sides of the estuary of the Clyde and in the islands of Cumbrae. In these -islands and in Bute they have recently been mapped in great detail for the -Geological Survey by my colleague, Mr. W. Gunn, who has supplied me with -notes of his observations on the subject, from which the following summary -is compiled.</p> - -<p>"There are at least four distinct groups of intrusive rocks in the Greater -Cumbrae. The oldest of these is trachytic in character, and occurs -both as dykes and sheets, which run generally in the same E.N.E. direction. -The rock is usually pinkish in colour, sometimes grey or purplish. A -specimen from the dyke of the Hawk's Nest, north of Farland Point, -analyzed by Mr. Teall, was found to contain 11 per cent of alkalies, -principally potash, while the percentages of lime and iron were very low. -Sometimes these rocks are fine in grain with only a few porphyritic orthoclase -crystals, though numerous small crystals of this mineral are found -with the aid of the microscope. These red trachyte dykes are almost confined -to the Upper Old Red Sandstone, rarely entering the overlying white -Calciferous Sandstones, and never invading the plateau-lavas. They are -therefore probably of early Carboniferous age.</p> - -<p>"The next group follows the same general direction, but clearly traverses -the trachytes, and must therefore be of later date. The dykes of this group -are the most numerous of the whole, the greater part of the island being -intersected by them. In the north-east corner about 40 of them may be -counted in half a mile of coast-line, some being of large size. All of them -which can be clearly made out are porphyritic olivine-basalts of the -type of the Lion's Haunch at Arthur's Seat. They are generally grey in -colour and finer at the edges than in the centre, which is often coarsely -porphyritic and amygdaloidal. Olivine seems always characteristic, but has -often been replaced by hæmatite or calcite. In Bute a good many dykes -have been mapped to the north of Kilchattan Bay resembling this basalt -series of Cumbrae, and running in the same direction. But they appear to -be all porphyritic andesites. The second group of dykes, though it cuts -the first and is thus proved to be later in date, is nevertheless confined -within the same stratigraphical limits. It may thus belong nearly to the -same period of intrusion.</p> - -<p>"The dykes of the third group are dolerites without olivine, and follow -on the whole an east and west direction. They cut both of the two foregoing -sets of dykes, and likewise the lavas of the plateau. They must thus -belong to a far later period of intrusion. They may be connected with -<span class="pagenum" id="Page_408">- 408 -</span> -other dykes and sills on the mainland, which traverse the Coal-measures, -and would thus be not older than late Carboniferous or Permian time.</p> - -<p>"The fourth group of dykes intersects all the others, and is probably of -Tertiary age. The prevalent direction of these dykes in the Cumbraes is -N.N.W." The Tertiary dykes are more fully described in Chapters <a href="../../66493/66493-h/66493-h.htm#CHAPTER_XXXIV">xxxiv.</a> and -<a href="../../66493/66493-h/66493-h.htm#CHAPTER_XXXV">xxxv.</a></p> - -<p>The great group of tuffs which underlies the lavas of the East Lothian -plateau is traversed by numerous dykes and sills, of which many good -examples may be seen in the coast-cliffs of North Berwick. Among these -rocks are beautiful olivine-basalts with singularly fresh olivine, as on the -shore at North Berwick. Some of them are still more basic, as in the case -of a limburgite intrusion at the Gin Head, Tantallon Castle.</p> - -<div class="figcenter" id="v1fig134" style="width: 417px;"> - <img src="images/v1fig134.png" width="417" height="301" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 134.</span>—Veins and dykes traversing the agglomerate and tuff of the great Renfrewshire vent.</div> -</div> - -<p>2. In the necks, dykes are sometimes abundant, and they may be -observed occasionally to traverse the surrounding lavas. They consist of -similar materials to those found outside the plateaux. Some of the larger -necks are intersected by a network of dykes and veins. The great vent or -group of vents among the uplands of Renfrewshire, already described (<a href="#v1fig129">Fig. -129</a>), furnishes some admirable examples of this characteristic volcanic -feature. An illustration from that locality forms the subject of <a href="#v1fig134">Fig. 134</a>. -The agglomerate which fills the large hollow among the Campsie Hills may -be quoted as another illustration (<a href="#v1fig128">Fig. 128</a>). Further instances will be -found in some of the sections given in preceding pages (see Figs. <a href="#v1fig124">124</a>, -<a href="#v1fig125">125</a>, <a href="#v1fig127">127</a>). The general aspect of a dyke in the volcanic series is shown -in <a href="#v1fig135">Fig. 135</a>.</p> - -<p><span class="pagenum" id="Page_409">- 409 -</span></p> - -<div class="figcenter" id="v1fig135" style="width: 488px;"> - <img src="images/v1fig135.png" width="488" height="753" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 135.</span>—"The Yellow Man," a dyke in volcanic tuff and conglomerate on the shore a little east of - North Berwick.</div> -</div> - -<p><span class="pagenum" id="Page_410">- 410 -</span></p> - -<p>The <span class="smcap">Sills</span> associated with the plateau-type of Carboniferous volcanic -action form a less prominent feature than they do among the earlier -Palæozoic formations or in the puy-type which succeeded them. They -consist in general of short lenticular sheets of andesite or trachyte, like the -necks and dykes in proximity to which they commonly appear. The best -area for the study of them is the ground which stretches out from the -base of the great escarpments of the Campsie, Kilpatrick and Ayrshire -Hills (<a href="#v1fig136">Fig. 136</a>), where, among the agglomerate-vents and abundant dykes, -intrusive sheets have likewise been injected between the bedding-planes of -the red sandstones. But these sheets are of comparatively trifling dimensions. -Very few of them reach a mile in length, the great majority falling far -short of that size. In the Cumbraes and in Bute, Mr. Gunn has observed -that the trachytic, olivine-basalt and dolerite dykes are apt to pass into -intrusive sheets. That the sills, as well as the dykes and bosses of the -same material, are not of older date than the lavas of the plateaux is proved -by the manner in which they pierce these lavas, especially towards the -bottom of the series. The general absence of basic sills, when we consider -how thick a mass of these rocks has sometimes been poured out in the -plateaux, is not a little remarkable. Only in the basin of the Firth of -Forth do we encounter thick basic sills near the plateaux, such, for instance, -as Salisbury Crags at Edinburgh. But it is doubtful whether they ought -not rather to be classed with the sills of the puys, to be afterwards -described.</p> - -<div class="figcenter" id="v1fig136" style="width: 434px;"> - <img src="images/v1fig136.png" width="434" height="125" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 136.</span>—Trachytic sills, Knockvadie, Kilpatrick Hills.<br /> - 1. Upper Old Red Sandstone; 2. "Ballagan Beds"; 3. Tuffs; 4. Lavas of the Plateau; 5. Agglomerate of necks; - 6. Trachyte sills; 7. Dolerite dyke (? Tertiary).</div> -</div> - - -<h3>4. <span class="smcap">Close of the Plateau-eruptions</span></h3> - -<p>The relative geological date when the eruptions of each plateau ceased -can fortunately be determined with much more precision than the time of -their beginning. The Hurlet Limestone, so well known as the lowest thick -calcareous seam in the Carboniferous Limestone series, of which it is -generally taken as the base, can be identified over the whole of Central -Scotland, and thus forms an excellent stratigraphical horizon, from which the -upward termination of the volcanic sheets underneath it can be measured.</p> - -<p>When the volcanic episode of the plateau-eruptions came to an end, -such banks or cones as rose above the level of the shallow sea which then -overspread Central Scotland were brought beneath the water, as I have -already remarked, either by prolonged denudation or more probably in large -<span class="pagenum" id="Page_411">- 411 -</span> -part by the continued subsidence of the region. The downward movement -may possibly for a time have been accelerated, especially in some districts. -Thus the Hurlet Limestone, though usually not more than five or six feet -thick, increases locally to a much greater thickness. At Petersfield, near -Bathgate, for example, it is between 70 and 80 feet in depth, while at -Beith, in North Ayrshire, it increases to 100 feet (<a href="#v1fig137">Fig. 137</a>), which is the -thickest mass of Carboniferous Limestone known to exist in Scotland. At -both of these localities the limestone lies upon a series of volcanic rocks, -and we may perhaps infer that the subsidence advanced there somewhat -more rapidly or to a greater extent, so as to form hollows in which the -limestone could gather to an abnormal depth. The water would appear to -have become for a time tolerably free from mechanical sediment. The -limestone is hence comparatively pure, and is extensively quarried all over -the country for industrial purposes. It is a crinoidal rock, abounding in -many species of corals, brachiopods, lamellibranchs, and gasteropods, with -trilobites, cephalopods, and fishes.</p> - -<div class="figcenter" id="v1fig137" style="width: 441px;"> - <img src="images/v1fig137.png" width="441" height="88" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 137.</span>—Section across the edge of the Clyde plateau, south-east of Beith.<br /> - 1. Plateau-lavas; 2. Tuffs and volcanic conglomerates; 3. Hurlet Limestone; 4. Coal-bearing strata above the - limestone; 5. Dolerite dyke.</div> -</div> - -<p>A variable thickness of strata intervenes between the top of the volcanic -series and the Main Limestone. Sometimes these deposits consist in large -measure of a mixture of ordinary sandy and muddy material with the -washed-down tuff of the cones, and probably with volcanic dust and lapilli -thrown out by the latest eruptions. Thus along the flank of the hills from -Barrhead to Strathavon, yellow and green ashy sandstones, grits and conglomerates -are succeeded by ordinary sandstones, black shales and ironstones, -while here and there true volcanic tuff and conglomerate make their appearance.<a id="FNanchor_441" href="#Footnote_441" class="fnanchor">[441]</a> -Further west, in the Kilbirnie district, the limestone lies directly on -the tuffs that rest upon the andesites (Figs. <a href="#v1fig137">137</a>, <a href="#v1fig138">138</a>).</p> - -<div class="footnote"> - -<p><a id="Footnote_441" href="#FNanchor_441" class="label">[441]</a> Explanation of Sheet 22, <i>Geol. Surv. Scotland</i>, p. 12.</p> - -</div> - -<div class="figcenter" id="v1fig138" style="width: 423px;"> - <img src="images/v1fig138.png" width="423" height="97" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 138.</span>—Section across the upper part of the Clyde plateau at Kilbirnie, Ayrshire.<br /> - 1 1. Plateau-lavas; 2 2. Tuffs; 3 3. Hurlet Limestone; 4. Black-band Ironstone. <i>f</i> <i>f</i>. Faults.</div> -</div> - -<p><span class="pagenum" id="Page_412">- 412 -</span></p> - -<p>But perhaps the most striking contrast between adjacent localities in -regard to the distance between the limestone and the top of the volcanic -series is to be observed along the southern front of the Campsie Fells. In -spite of the abundant faults which have there so broken up the regular -sequence of the rocks, we can see that at Banton and Burnhead the limestone -lies almost immediately on the volcanic series (<a href="#v1fig139">Fig. 139</a>). But a -little to the westward, sandstones, conglomerates, shales and thin limestones -begin to intervene between the volcanic series and the Hurlet Limestone -and swell out so rapidly that on Craigmaddie Muir and South Hill of -Campsie, only some five miles off, they must form a total thickness of not -less than from 600 to 800 feet of ordinary non-volcanic deposits, chiefly -thick pebbly sandstones (<a href="#v1fig140">Fig. 140</a>). Such local variations not improbably -serve to indicate hollows on the flanks of the plateaux that were filled up -with detritus before the depression and clearing of the water that led to the -deposition of the Hurlet Limestone.</p> - -<div class="figcenter" id="v1fig139" style="width: 312px;"> - <img src="images/v1fig139.png" width="312" height="77" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 139.</span>—Section across the upper surface of the Clyde volcanic plateau, Burnhead, north-west of - Kilsyth.<br /> - 1. Lavas of the plateau; 2. Tuffs; 3. Hurlet Limestone; 4. Hosie's Limestone; <i>f</i>, Fault.</div> -</div> - -<div class="figcenter" id="v1fig140" style="width: 372px;"> - <img src="images/v1fig140.png" width="372" height="110" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 140.</span>—Section across the upper surface of the Clyde volcanic plateau at Campsie.<br /> - 1. Shales, sandstones, cement-stones, etc. ("Ballagan Beds"); 2. Lavas of the plateau; 3. Thick white - sandstone and conglomerate; 4. Hurlet Limestone; 5. Hosie's Limestone; <i>f</i>. Fault.</div> -</div> - -<div class="figcenter" id="v1fig141" style="width: 399px;"> - <img src="images/v1fig141.png" width="399" height="79" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 141.</span>—Section across western edge of the Garleton plateau.<br /> - 1. Trachyte lavas of the plateau; 2. Calciferous Sandstones; 3. Hurlet Limestone.</div> -</div> - -<p>I have already remarked that the eruptions of the plateau period lasted -longer in the western than in the eastern parts of the region. In the -Garleton district, where the peculiar viscous trachytic lavas probably gave -rise to a more uneven surface or more prominent cones than was usual -among the andesitic plateaux, the eruptions ceased some time before the -<span class="pagenum" id="Page_413">- 413 -</span> -deposition of the Hurlet Limestone. As the area sank, the successive zones -of the Calciferous Sandstones crept over the flanks of -the trachytes, until at last they had completely buried -these rocks before the limestone spread over the area -(<a href="#v1fig141">Fig. 141</a>). In consequence, probably, of the uneven -surface of this plateau, there is here a strong overlap -of the higher part of the Calciferous Sandstones. On -the west side of the volcanic area there can hardly -be more than some 200 feet of strata between the -top of the trachytic series and the limestone, while -on the south side there must be greatly more than -that thickness. This structure probably indicates that -the Garleton volcanoes became extinct after having -piled up a mass of tuffs and lavas to such a height -that its summits were not submerged until the area -had subsided 800 or 1000 feet in the waters, over the -floor of which the Calciferous Sandstones were laid -down. Hence, in spite of the proximity of the lavas -to the limestone, there may have been a vast interval -of time between their respective epochs, as has been -already suggested with regard to other plateaux. -This subject will be again referred to in discussing -the relative chronology of the plateaux and puys.</p> - -<p>In the Berwickshire and Solway districts, the -extinction of the plateau-vents appears to have taken -place at a still earlier part of the Carboniferous period, -for there the andesites, while they rest on the Upper -Old Red Sandstone, are covered with at least the -higher group of the Calciferous Sandstones (<a href="#v1fig142">Fig. 142</a>). -The equivalent of the Hurlet Limestone of Central -Scotland must lie many hundred feet above them.</p> - -<p>The submergence of the plateaux, and their entombment -under the thick Carboniferous Limestone -series, did not mark the close of volcanic activity -in Central Scotland during Carboniferous time. The -plateau-type of eruption ceased and was not repeated, -but a new type arose, to which I would now call the -reader's attention.</p> - -<div class="figcenter" id="v1fig142" style="width: 739px;"> - <img src="images/v1fig142.png" width="739" height="100" alt="" /> - <div class="hanging2"><p><span class="smcap">Fig. 142.</span>—Section across the Solway plateau from Birrenswark to Kirtlebridge.<br /> - 1. Upper Silurian strata; 2. Upper Old Red Sandstone; 3. Plateau-lavas; 4. Calciferous Sandstones and Carboniferous Limestone series; 5. Trias.</p></div> -</div> - -<hr class="chap x-ebookmaker-drop" /> - - -<div class="chapter"> -<p><span class="pagenum" id="Page_414">- 414 -</span></p> - -<h2 class="nobreak" id="CHAPTER_XXVI">CHAPTER XXVI<br /> - -<span class="smaller">THE CARBONIFEROUS PUYS OF SCOTLAND</span></h2> -</div> - -<div class="blockquot"> - -<p>i. General Character and Distribution of the Puys; ii. Nature of the Materials -Erupted—Lavas Ejected at the Surface—Intrusive Sheets—Necks and Dykes—Tuffs.</p> -</div> - - -<h3>i. <span class="allsmcap">GENERAL CHARACTER AND DISTRIBUTION</span></h3> - -<p>After the beginning of the Carboniferous Limestone period, when eruptions -of the plateau-type had generally ceased, volcanic activity showed itself over -the area of the British Isles in a different guise both as regards the nature -of its products and the manner and scale of their discharge. Instead of -widely extended lava-sheets and tuffs, piled above each other sometimes to a -thickness of many hundred feet, and stretching over hundreds of square -miles, we have now to study the records of another phase of volcanism, -where scattered groups and rows of <i>Puys</i>, or small volcanic cones, threw out -in most instances merely tuffs, and these often only in trifling quantity, -though here and there their vents also poured forth lavas and gradually -piled up volcanic ridges which, in a few cases, almost rivalled some of the -plateaux. The evidence for these less vigorous manifestations of volcanic -activity is furnished (1) by layers of tuff and sheets of basaltic-lavas intercalated -among the strata that were being deposited at the time of the -eruptions, (2) by necks of tuff, agglomerate, or different lava-form rocks that -mark the positions of the orifices of discharge, and (3) by sills, bosses, and -dykes that indicate the subterranean efforts of the volcanoes. The comparatively -small thickness of the accumulations usually formed by these -vents, their extremely local character, the numerous distinct horizons on -which they appear, and the intimate way in which they mingle and alternate -with the ordinary Carboniferous strata are features which at once arrest the -attention of the geologist, presenting, as they do, so striking a contrast to -those of the plateaux.</p> - -<p>From the clear intercalation of these volcanic materials on successive -platforms of the Carboniferous system, the limits of geological time within -which they were erupted can be fixed with considerable precision. It may -be said that, in a broad sense, they coincided with the period of the -Carboniferous Limestone, and certainly it was during the deposition of that -<span class="pagenum" id="Page_415">- 415 -</span> -formation that the eruptions which produced them reached their greatest -vigour and widest extent. Here and there in Scotland evidence may be -found that the phase of the Puys began during that earlier section of -Carboniferous time recorded by the Calciferous Sandstones. This is -markedly the case in Liddesdale and the neighbouring territory. Over -the western part of Midlothian also, the eastern portion of West Lothian, -and the southern margin of Fife, abundant traces occur of puy-eruptions -during the deposition of the Calciferous Sandstones. Elsewhere in Central -Scotland there is no evidence of the vents having been opened until after -the deposition of the Hurlet Limestone, which, as we have seen, may conveniently -be taken as the base of the Scottish Carboniferous Limestone -series. The volcanoes remained active in West Lothian until near the close -of the time represented by that series; but in Ayrshire they continued in -eruption until the beginning of the accumulation of the Coal-measures. -These western examples of the puy-type are, so far as I am aware, the -latest known in Britain.</p> - -<p>Whether or not the earliest puy-eruptions began before the latest -plateau-lavas and tuffs were accumulated is a question that cannot be readily -answered. It will be remembered that in the basin of the Firth of Forth -a thickness of more than 3000 feet of sedimentary strata, including the -Burdiehouse Limestone and numerous oil-shales as well as thin coal-seams, -lies above the red and green marls, shales, sandstones and cement-stones of -the Calciferous Sandstone series. This remarkable assemblage of strata is -absent in the western parts of the country, where the top of the Clyde -volcanic plateau is almost immediately overlain by the Hurlet Limestone. -If we were to judge of the sequence of events merely from the stratigraphy, -as expressed in such sections as Figs. <a href="#v1fig137">137</a>, <a href="#v1fig138">138</a>, <a href="#v1fig139">139</a> and <a href="#v1fig140">140</a>, we might -naturally infer that as no trace of any break occurs at the top of the Clyde -plateau, the tuffs shading upward there into the limestone series, no -important pause in sedimentation took place, but that the last volcanic -eruptions were soon succeeded by the conditions that led to the deposition -of the widespread encrinite-limestones. If this inference were well founded -it would follow that while the plateau-eruptions in the west lasted till the -time of the Hurlet Limestone, those in the east ceased long before that time -and were succeeded by the puys of Fife and the Lothians. There would -thus be an overlap of the two phases of volcanic action.</p> - -<p>I am inclined to believe, however, that in spite of the superposition -of the Hurlet Limestone almost immediately upon the volcanic rocks of -the Clyde plateau, and the absence of any trace of a break in the process of -sedimentation, a long interval nevertheless elapsed between the last eruptions -and the deposit of that limestone. The Campsie section (<a href="#v1fig140">Fig. 140</a>) -shows us how rapidly a thick mass of strata can come in along that horizon. -The volcanic ridges may have remained partly unsubmerged for such time -as was required for the subsidence of the Forth basin and the deposit of the -thick Calciferous Sandstone series there, and their summits may only have -finally sunk under the sea not long before the Hurlet Limestone grew as a -<span class="pagenum" id="Page_416">- 416 -</span> -continuous floor of calcareous material over the whole area of central Scotland. -In these circumstances, the puy-eruptions of that basin would be -long subsequent to the eruptions of the Clyde plateau, as they certainly -were to those of the plateaux of Midlothian and the Garleton Hills.</p> - -<p>In tracing the geographical distribution of the puy-eruptions we are first -impressed with the force of the evidence for their extremely local and -restricted character (<a href="#Map_IV">Map IV.</a>). Thus in the area of the basin of the Firth -of Forth, which may be regarded as the typical region in Britain for the study -of this form of Carboniferous volcano, traces of them are abundant to the -west of the line of the Pentland Hills. To the east of that line, however, not -a vestige of puy-eruptions, save a few sills of uncertain relationship, can be -detected, though the same series of stratigraphical horizons is well developed -on both sides of the Lothian coal-field. Again, to the westward of the -Forth basin over the area of Stirlingshire, Lanarkshire and Renfrewshire -lying to the north of the great volcanic plateau, no record of puy-eruptions -has been noticed. Immediately to the south of that plateau, however, these -eruptions were numerous in the north of Ayrshire. Yet the rest of the -Carboniferous area in that large county has supplied no relics of these -eruptions save at one locality—the Heads of Ayr. Lastly, while no trace -of any younger display of volcanic activity occurs in the Merse of Berwickshire, -east of the plateau series of that district, the ground immediately to -the west abounds in puys, and contains likewise extensive sheets of tuff and -beds of basic lavas connected with these vents.</p> - -<p>Another fact which at once attracts notice in Scotland is the way in -which the puy-vents have generally avoided the areas of the plateaux, -though they sometimes approach them closely. As a rule, it is possible -to distinguish the tuffs and agglomerates which have filled up these vents -from those that mark the sites of the eruptive orifices of the plateaux. -There are, no doubt, some instances, as in Liddesdale, where puys have -appeared on the sites of the older lavas, but these are exceptional collocations.<a id="FNanchor_442" href="#Footnote_442" class="fnanchor">[442]</a> -On the other hand, many examples may be found where puys have -risen in the interspace between the limits of the eruptions of two plateau-areas. -Thus the tract between the Clyde plateau-eruptions on the west -and those of the Garleton Hills on the east was dotted over with puys. -Again, the southern margin of the Clyde plateau in Ayrshire, from Dalry to -Galston is flanked with puys and long sheets of their lavas and tuffs.<a id="FNanchor_443" href="#Footnote_443" class="fnanchor">[443]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_442" href="#FNanchor_442" class="label">[442]</a> A means of definitely placing some of these vents in the series of puy-eruptions is stated -further on, at <a href="#Page_476">p. 476</a>.</p> - -<p><a id="Footnote_443" href="#FNanchor_443" class="label">[443]</a> Reference may again be made here to the remarkable similarity between the Scottish Carboniferous -puy-vents and those of older Tertiary time in the Swabian Alps so fully described by -Professor Branco in the work already cited <a href="#Page_46">p. 46</a>. Denudation in that region has bared the -cones and exposed the structure of the necks which, down to even minute details, repeat the -phenomena of Carboniferous and Permian time in Scotland.</p> - -</div> - - -<h3>ii. <span class="allsmcap">NATURE OF THE MATERIALS ERUPTED</span></h3> - - -<h3>A. The Lava-form Rocks</h3> - -<p>We have now to consider the nature of the materials erupted by the -volcanic activity of the puys. The geologist who passes from the study -<span class="pagenum" id="Page_417">- 417 -</span> -of the plateau lavas to those of the puys at once remarks the prevalent -more basic character of the latter. The great majority of them are basalts, -generally olivine-bearing, in the various types embraced in the table on the -following page. The olivine-free dolerites are generally found as intrusive -bosses, sills and dykes. Such more acid rocks as andesites occur only -rarely, and still more seldom are quartziferous masses met with in some of -the bosses.</p> - -<p><span class="smcap">Dolerites</span> and <span class="smcap">Basalts</span>.—The great majority of the lava-form rocks -connected with the puys are basic in composition, and belong to the family -of the Dolerites and Basalts. They graduate, on the one hand, into ultra-basic -rocks such as limburgite and picrite, and on the other, into compounds -that approach andesites or trachytes in composition. A large series of -specimens from Central Scotland was studied a few years ago by Dr. Hatch, -who proposed a petrographical classification of the rocks, and arranged them -in a number of types which he named after localities where they are well -developed.<a id="FNanchor_444" href="#Footnote_444" class="fnanchor">[444]</a> More recently the rocks have again been subjected to microscopic -investigation by my colleague Mr. Watts, who, confirming generally -Dr. Hatch's discriminations, has made some modifications of them. He has -furnished me with a revised classification (<a href="#Page_418">p. 418</a>), based on purely petrographical -considerations. The doleritic and basaltic series may be grouped -into two divisions, one with, and the other without, olivine, and each division -may be further separated into a dolerite group, which presents an ophitic or -sub-ophitic structure, and a basalt-group in which the groundmass is made up -of felspar and granular augite, and possesses the "intersertal structure" of -Rosenbusch, or consists of idiomorphic augite embedded in felspar substance. -The term "sub-ophitic" is employed by Mr. Watts "to imply that the -augite grains are neither very large nor very continuous, optically, and that -they rarely contain entire felspar-crystals imbedded in them, merely the -ends of a group of these crystals as a rule penetrating into them."</p> - -<div class="footnote"> - -<p><a id="Footnote_444" href="#FNanchor_444" class="label">[444]</a> This classification was given in my Presidential Address to the Geological Society, 1892, -<i>Quart. Journ. Geol. Soc.</i> vol. xlviii. p. 129. See Report of Geological Survey for 1896.</p> - -</div> - -<p>Transitional forms occur between many of the following types by the -increase or diminution in the relative proportions of the constituents. Thus -it is not easy to draw a line between 2<i>b</i> and 2<i>c</i>; the latter again shades -into 2<i>d</i> and 2<i>e</i> by the decrease of the felspar.</p> - -<p>Mr. Watts has further observed that the rocks containing no olivine -offer greater difficulties in classification than those in which that mineral is -present. "The very distinction," he remarks, "between dolerites and -basalts is less marked, the types are much less sharply distinguished, and -decomposition and masking of the structure are more common. While -using the term Dolerite for such rocks as have a sub-ophitic structure, I -have extended it to those rocks in which evidence exists that a great part -of the crystallization took place under intratelluric conditions. Although -<span class="pagenum" id="Page_418">- 418 -</span> -not quite holocrystalline, the crystals of felspar, augite and magnetite are -large and the structure coarse-grained, while the groundmass is confined to -comparatively small interstitial patches. In these rocks there is usually no -one dominant porphyritic ingredient."</p> - -<table summary="data"> -<tr> - <td class="caption3nb" colspan="4">I. <span class="smcap">The Olivine-bearing Series</span></td> -</tr> -<tr> - <td class="tdc" colspan="4">1. <i>Olivine-Dolerites</i></td> -</tr> -<tr> - <td class="tdl vtop">1<i>a</i>.</td> - <td class="tdl">Porphyritic elements inconspicuous, olivine being - the principal, and felspar of secondary importance; groundmass - sub-ophitic.</td> - <td><span style="font-size: 2em;">}</span></td> - <td class="tdl">Jedburgh Type.</td> -</tr> -<tr> - <td class="tdl vtop">1<i>b</i>.</td> - <td class="tdl">Strongly porphyritic; felspar-phenocrysts large; - olivine smaller; groundmass sub-ophitic.</td> - <td><span style="font-size: 2em;">}</span></td> - <td class="tdl">Kilsyth Type.</td> -</tr> -<tr> - <td class="tdl">1<i>c</i>.</td> - <td class="tdl">Porphyritic olivine, but not felspar; groundmass - sub-ophitic.</td> - <td><span style="font-size: 2em;">}</span></td> - <td class="tdl">Gallaston Type.</td> -</tr> -<tr> - <td class="tdc" colspan="4">2. <i>Olivine-Basalts</i></td> -</tr> -<tr> - <td class="tdl vtop">2<i>a</i>.</td> - <td class="tdl">Porphyritic olivine, augite and felspar; groundmass - of felspar-laths imbedded in granules of augite.</td> - <td><span style="font-size: 2em;">}</span></td> - <td class="tdl">Lion's Haunch Type. (See <a href="../../66493/66493-h/66493-h.htm#v2fig207">Fig. 207</a>.)</td> -</tr> -<tr> - <td class="tdl vtop">2<i>b</i>.</td> - <td class="tdl">Porphyritic olivine and augite; groundmass of - felspar-laths imbedded in granules of augite. More rarely the - groundmass is made of idiomorphic augite imbedded in - felspar-substance.</td> - <td><span style="font-size: 3em;">}</span></td> - <td class="tdl">Craiglockhart Type.</td> -</tr> -<tr> - <td class="tdl vtop">2<i>c</i>.</td> - <td class="tdl">Porphyritic olivine abundant, augite much less common, - and felspar very rare or absent; groundmass with granular or - idiomorphic augite (one of the most common types).</td> - <td><span style="font-size: 3em;">}</span></td> - <td class="tdl">Dalmeny Type.</td> -</tr> -<tr> - <td class="tdl vtop">2<i>d</i>.</td> - <td class="tdl">Porphyritic olivine more common than augite; groundmass - of granules of augite set amongst lath-like felspars which are - much fewer in number than in 2<i>c</i>.</td> - <td><span style="font-size: 3em;">}</span></td> - <td class="tdl">Picrite Type.</td> -</tr> -<tr> - <td class="tdl vtop">2<i>e</i>.</td> - <td class="tdl">Porphyritic olivine more common than augite; groundmass - of idiomorphic augite imbedded in felspathic material which is - not abundant.</td> - <td><span style="font-size: 3em;">}</span></td> - <td class="tdl">Limburgite Type.</td> -</tr> -<tr> - <td class="tdl vtop">2<i>f</i>.</td> - <td class="tdl">Porphyritic olivine and felspar, without augite; - groundmass of granular or idiomorphic augite, with lath-shaped - felspars.</td> - <td><span style="font-size: 3em;">}</span></td> - <td class="tdl">Kippie Law Type. (For analysis see <a href="#Page_379">p. 379</a>).</td> -</tr> -<tr> - <td class="caption3nb" colspan="4">II. <span class="smcap">The Non-olivine-bearing Series</span></td> -</tr> -<tr> - <td class="tdc" colspan="4">3. <i>Olivine-free Dolerites</i></td> -</tr> -<tr> - <td></td> - <td class="tdl" colspan="3">Felspar, augite, magnetite in coarse-grained aggregate usually ophitic or sub-ophitic; - groundmass not plentiful.</td> -</tr> -<tr> - <td class="tdl">3<i>a</i>.</td> - <td class="tdl">Groundmass absent</td> - <td></td> - <td class="tdl">Ophitic Type.</td> -</tr> -<tr> - <td class="tdl">3<i>b</i>.</td> - <td class="tdl">Groundmass micropegmatitic</td> - <td></td> - <td class="tdl">Ratho Type.</td> -</tr> -<tr> - <td class="tdl vtop">3<i>c</i>.</td> - <td class="tdl">Groundmass an unstriated felspar (not orthoclase) and - occasionally some interstitial altered glass or a little quartz.</td> - <td><span style="font-size: 2em;">}</span></td> - <td class="tdl">Burntisland Sill Type.</td> -</tr> -<tr> - <td class="tdc" colspan="4">4. <i>Doleritic Basalts</i></td> -</tr> -<tr> - <td></td> - <td class="tdl" colspan="3">Felspar, augite and magnetite in coarse-grained aggregate; groundmass rather - more plentiful and often in large patches.</td> -</tr> -<tr> - <td class="tdl vtop">4<i>a</i>.</td> - <td class="tdl">Felspar and augite, related sub-ophitically where - together, but augite showing crystalline contours in contact - with the groundmass; some interstitial quartz and unstriated - felspar.</td> - <td><span style="font-size: 3em;">}</span></td> - <td class="tdl">Bowden Hill Type.</td> -</tr> -<tr> - <td class="tdc" colspan="4">5. <i>Basalts</i><span class="pagenum" id="Page_419">- 419 -</span></td> -</tr> -<tr> - <td></td> - <td class="tdl" colspan="3">Finer-grained rocks, generally with a porphyritic ingredient and much scattered - interstitial matter in the groundmass.</td> -</tr> -<tr> - <td class="tdl vtop">5<i>a</i>.</td> - <td class="tdl">Porphyritic felspar, and occasionally a little augite; - groundmass of granular augite, felspar needles and magnetite with - some interstitial matter.</td> - <td><span style="font-size: 3em;">}</span></td> - <td class="tdl">Binny Craig Type.</td> -</tr> -<tr> - <td class="tdl vtop">5<i>b</i>.</td> - <td class="tdl">Porphyritic felspars not conspicuous and small; the rock - mainly made up of a mesh of fine felspar-laths set amongst - granular augite, magnetite and base.</td> - <td><span style="font-size: 3em;">}</span></td> - <td class="tdl">Tholeiite Type.</td> -</tr> -<tr> - <td class="tdl vtop">5<i>c</i>.</td> - <td class="tdl">Similar to the last but even finer-grained, and with the } - - base in a cryptocrystalline condition.</td> - <td><span style="font-size: 2em;">}</span></td> - <td class="tdl">Cryptocrystalline Type.</td> -</tr> -</table> - -<p>Taking first the superficial lavas, I know of only one locality where -picrite occurs in such a position that it may be included among the -surface outflows. This is the quarry near Blackburn, to the east of Bathgate, -where I originally observed it.<a id="FNanchor_445" href="#Footnote_445" class="fnanchor">[445]</a> The rock occurs there on the line -of the basalt-flows from the Bathgate Hills, and I mapped it as one of them -before the microscope revealed the remarkable composition of the mass. I -still believe it to be a lava like the "leckstone" described on <a href="#Page_443">p. 443</a>, though -the other known examples of this rock in the basin of the Firth of Forth -are intrusive sheets. The rock locally known as "leckstone" or "lakestone" -has long been quarried for the purpose of constructing the soles of bakers' -ovens, as it stands a considerable temperature without cracking. Its microscopic -structure is now well known. As exposed in Blackburn quarry, an -interesting difference is observable between the lower and upper parts of the -sheet. The lower portion is a picrite, with abundant serpentinized olivine, -large crystals of augite, and a considerable amount of ores. The upper -portion, on the other hand, has plagioclase as its most abundant definite -mineral, with a minor quantity of minute prisms of augite and of iron-ores, -and scattered crystals of olivine. Here, within the compass of a few yards -and in one continuous mass of rock, we have a transition from a variety of -olivine-basalt into a picrite.</p> - -<div class="footnote"> - -<p><a id="Footnote_445" href="#FNanchor_445" class="label">[445]</a> <i>Trans. Roy. Soc. Edin.</i> vol. xxix. (1879) p. 506.</p> - -</div> - -<p>The great majority of the puy lavas belong to the olivine-bearing series. -A few of them are dolerites, but most are true basalts of the Dalmeny type, -of which typical examples may be seen at the Kirkton quarries, Bathgate, -and in the coast section between Pettycur and Kinghorn. Occasionally -they present transitions towards picrite, as in the sheet overlying the lowest -limestone at Kirkton, and in the lowest lava of King Alexander's Crag, -Burntisland. These puy lavas exhibit considerable variety of structure as -seen in the field. Some are solid, compact, black rocks, not infrequently -columnar and weathering into spheroidal exfoliating forms. Others are -somewhat granular in texture, acquiring green and brown tints by weathering, -often showing amygdaloidal kernels, and even passing into well-marked -amygdaloids. Many of them exhibit a slaggy structure at their upper and -under surfaces (Figs. <a href="#v1fig153">153</a>, <a href="#v1fig170">170</a>, <a href="#v1fig171">171</a>). These external differences are an -<span class="pagenum" id="Page_420">- 420 -</span> -index to the corresponding variations in composition and microscopic -structure enumerated in the foregoing tabular arrangement.</p> - -<p>As a rule, the basic rocks which occur intrusively in connection with -the puys, especially where they form a considerable mass, have assumed a -much more coarsely crystalline texture than those of similar composition -which have been poured out at the surface. They are generally dolerites -rather than basalts. But with this obvious distinction, the two groups have -so much in common, that the geologist who passes from the study of the -subterranean phenomena of the Plateaux to that of the corresponding phenomena -of the Puys is at once impressed with the close relationship between -the material which, in the case of the puys, has consolidated above ground, -and that which has been injected below. There is no such contrast between -them, for example, as that between the basic and intermediate lavas of the -plateaux and the more acid intrusions associated with them.</p> - -<p>By far the largest number of the basic sills, bosses and dykes associated -with the puys are somewhat coarsely crystalline dolerites without olivine. -They include, however, olivine-dolerites and basalts, and even some extremely -basic compounds. Of these last, a typical example is supplied by the now -well-known picrite of Inchcolm, in the Firth of Forth, which occurs as an -intrusive sheet among the Lower Carboniferous Sandstones.<a id="FNanchor_446" href="#Footnote_446" class="fnanchor">[446]</a> In recent -years one or two other picrite-sills have been observed in the same district. -An interesting example has been described from a railway cutting between -Edinburgh and Cramond where the rock invades and alters shales. More -detailed reference to it will be made in the account of the sills connected -with the puys. Another instance of the occurrence of this rock is in a -railway cutting immediately to the west of Burntisland where it has been -intruded among the Calciferous Sandstones below the Burdiehouse Limestone.</p> - -<div class="footnote"> - -<p><a id="Footnote_446" href="#FNanchor_446" class="label">[446]</a> <i>Trans. Roy. Soc. Edin.</i> vol. xxix. (1879) p. 506. Teall, <i>British Petrography</i>, p. 94.</p> - -</div> - -<p>Rocks approaching limburgite occur among the sills and bosses which -pierce the Carboniferous Limestone series of Fife between Cowdenbeath and -Inverkeithing. One of these is found at Pitandrew, near Fordel Castle. -Dr. Hatch observed that it consists of "numerous porphyritic crystals of -olivine, with a few grains of augite and an occasional small lath-shaped -crystal of felspar imbedded in a groundmass which is composed principally -of idiomorphic augite microlites, small crystals of a brown mica, granules of -magnetite and prisms of apatite. In addition, there is a considerable -amount of interstitial matter, which is partly colourless glass, and partly -shows a slight reaction between crossed nicols." Another example of the -same type of rock occurs as a plug or boss in the tuff-vent of the Hill -of Beath, and a further display of the limburgite type is to be seen in -Dunearn Hill near Burntisland.</p> - -<p>Although olivine-basalts of the Dalmeny type are most frequently met -with as interstratified lavas, they also occur as bosses and sills. The typical -example from Dalmeny is itself intrusive. Other illustrations are to be -found in the Castle Rock of Edinburgh and in the sheets near Crossgates -and Blairadam in Fife. The presence or absence of olivine, however, may -<span class="pagenum" id="Page_421">- 421 -</span> -sometimes be a mere accident of cooling or otherwise. I have shown that -in the same mass of rock at Blackburn a gradation can be traced from a -rock largely composed of altered olivine into one consisting mainly of felspar -with but little olivine, and another example occurs in the picrite-sill between -Edinburgh and Cramond. Dr. Stecher has ascertained that the marginal -portions of the sills in the basin of the Firth of Forth, which cooled first and -rapidly, and may be taken, therefore, to indicate the mineral composition of -the rock at the time of extrusion, are often rich in olivine, while that -mineral may be hardly or not at all discernible in the main body of the rock.<a id="FNanchor_447" href="#Footnote_447" class="fnanchor">[447]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_447" href="#FNanchor_447" class="label">[447]</a> Dr. Stecher, <i>Tschermak's Mineralog. Mittheil.</i> vol. ix. (1887) p. 193. <i>Proc. Roy. Soc. Edin.</i> -vol. xv. (1888) p. 162.</p> - -</div> - -<p>Of the ordinary and characteristic dolerites without olivine which constitute -most of the intrusive masses, the various types enumerated in the -tabular arrangement are abundantly developed in Central Scotland. Thus -the normal ophitic type is displayed by the uppermost sill of the Burntisland -series, and by the rock which forms the plug of the Binns Hill neck -in Linlithgowshire. The Ratho type is well seen in the large sill at Ratho, -likewise in the extensive intrusive sheets in the west of Linlithgowshire as -at Muckraw and Carribber. The Burntisland sill type is shown by the -lower sills of Burntisland and by some others in the same region, especially -by that of Colinswell, and by another on the shore east from the Poorhouse, -near Kinghorn. The great boss among the Bathgate Hills likewise displays -it. The Bowden Hill type occurs in well-marked development at Bowden -Hill, three miles south-west of Linlithgow, and in the massive sill at St. -Margaret's, west from North Queensferry.</p> - -<p>The non-olivine-bearing basalts are found in various bosses and sheets -in the basin of the Firth of Forth. Thus the Binny Craig type occurs in -the prominent and picturesque sill from which it is named, likewise among -the intrusive sheets near Kirkcaldy, in Fife. Sometimes the same mass of -rock displays more than one type of structure, as in the case of the great -Galabraes neck among the Bathgate Hills wherein both the Tholeiite and -Burntisland sill types may be recognized.</p> - -<p>Some of the sills in West Lothian, as I pointed out many years ago, -contain bitumen and give off a bituminous odour when freshly broken. -They have been injected into bituminous shales or coal-seams.<a id="FNanchor_448" href="#Footnote_448" class="fnanchor">[448]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_448" href="#FNanchor_448" class="label">[448]</a> <i>Geol. Survey Memoir on Geology of Edinburgh</i> (Sheet 32, Scotland), p. 46.</p> - -</div> - -<p>2. <span class="smcap">Andesites.</span>—Rocks referable to this series appear to have been of -rare occurrence among the puy-eruptions. Examples of them containing as -much as 60 per cent of silica occur among the lavas of the Limerick basin. -Some of the necks and what may be sills in the same district likewise -consist of them.</p> - -<p>3. <span class="smcap">Trachytes and Quartz-bearing Rocks.</span>—Acid rocks, as I have -already said, are extremely rare among the puy-eruptions. The only important -examples known to me are those around the Limerick basin, where -they rise apparently in old vents and form conspicuous rounded or conical -hills. These rocks have been examined microscopically by Mr. W. W. -<span class="pagenum" id="Page_422">- 422 -</span> -Watts. One of the most interesting varieties, which occurs at the Standing -Stone near Oola, was found by him to show quartz enclosing ophitically the -felspars which, with well-terminated prisms, project into it. Further west, -near Knockaunavoher, another boss occurs with conspicuous quartz. These -rocks have much in common with trachytes but have a wholly crystalline -structure. They will be described in the account of the Limerick basin.</p> - - -<h3>B. Tuffs</h3> - -<p>The fragmental rocks connected with the puy-eruptions form a well-marked -group, easily distinguishable, for the most part, from the tuffs of -the plateaux. They vary from exceedingly fine compacted dust or volcanic -mud, through various stages of increasing coarseness of texture, to basalt-conglomerates -and tumultuous agglomerates.</p> - -<p>The fragmentary material found in the necks of the puys is generally -an agglomerate of a dull dirty-green colour. The matrix ranges from a -fine compact volcanic mud to a thoroughly granular detritus, and sometimes -shows a spheroidal concentric structure in weathering. In this matrix the -lapilli are distributed with great irregularity and in constantly varying -proportions. They consist in large measure of a pale yellowish-green, sometimes -pale grey, very basic, finely vesicular, devitrified glass, which is generally -much decomposed and cuts easily with the knife. This highly basic -substance is a kind of palagonite. So minute are its vesicles that under -the microscope a thin slice may present a delicate lace-like network of -connected walls, the palagonite occupying much less space than the vesicles. -The material has been a finely frothed-up pumice.</p> - -<p>Besides this generally distributed basic pumice, the stones in the -agglomerate of the necks likewise include fragments of older volcanic grits -or tuffs, blocks of basalt or diabase, as well as pieces of the Carboniferous -strata of the district, especially shale, sandstone and limestone. Not -infrequently also, they comprise angular blocks of fossil wood.</p> - -<p>The materials which fill the necks are generally much coarser than -those that form intercalated beds. But while in numerous cases huge -blocks of basalt and large masses of sandstone, shale, limestone, ironstone or -other strata may be seen wrapped up in a matrix of coarse basalt-tuff, in -not a few instances the material in the necks may be observed to consist -of a tuff quite as fine as that of the interstratified bands. Such necks appear -to mark the sites of tuff-cones where only fine ashes and lapilli were ejected, -and where, after sometimes a brief and feeble period of activity, the orifice -became extinct.</p> - -<p>The bedded tuffs interstratified with the ordinary Carboniferous strata -do not essentially differ in composition from the material of the necks. -They are basalt- (diabase-) tuffs and basalt- (diabase-) conglomerates, usually -dull green in colour and granular in texture, the lapilli consisting in great -measure of various more or less decayed basalts, but containing the same -highly vesicular basic glass or pumice above referred to. They are mainly -<span class="pagenum" id="Page_423">- 423 -</span> -to be distinguished by their conspicuous stratification, and especially by their -rapid alternations of coarser and finer material, by the intercalation of shales, -limestones, sandstones or ironstones in them, and by the insensible gradations -by which they pass both vertically and laterally into ordinary sediments. -Occasional large blocks or bombs, indicating some paroxysm of -explosion, may be observed even among the finer tuffs, shales and other -strata, which round the sides of these masses have had their layers bent -down by the fall of heavy blocks.<a id="FNanchor_449" href="#Footnote_449" class="fnanchor">[449]</a> Many of the bedded tuffs contain -fossils, such as crinoids, corals, brachiopods, fish-teeth or macerated fragments -of land-plants. Coal-seams also are occasionally interstratified among them.</p> - -<div class="footnote"> - -<p><a id="Footnote_449" href="#FNanchor_449" class="label">[449]</a> <i>Ante</i>, <a href="#Page_36">p. 36</a>, and Figs. <a href="#v1fig15">15</a> and <a href="#v1fig151">151</a>. See also <i>Geol. Mag.</i> i. (1864), p. 22; <i>Trans. Roy. Soc. -Edin.</i> vol. xxix. (1879) p. 515.</p> - -</div> - -<p>Of the finer kinds, the best example is furnished by a remarkable group -of "green and red marls" which lie above a seam of coal (Houston Coal) -in the Calciferous Sandstones of West Lothian.<a id="FNanchor_450" href="#Footnote_450" class="fnanchor">[450]</a> These strata, which differ -much from any of the rocks with which they are associated, are exceedingly -fine in grain, dull sage-green and brownish or chocolate-red in colour, not -well laminated like the shales, but breaking under the influence of weathering -into angular fragments, sometimes with a conchoidal fracture. They look -like indurated mud. Mr. H. M. Cadell, who has recently re-examined -them in connection with a revision of the Geological Survey Map (Sheet 32) -has found them passing into ordinary granular tuff.</p> - -<div class="footnote"> - -<p><a id="Footnote_450" href="#FNanchor_450" class="label">[450]</a> Memoir on Sheet 32 <i>Geol. Surv. Scotland</i> (1861), p. 42. The stratigraphical position of these -"Houston Marls," as they are locally called, is indicated in <a href="#v1fig155">Fig. 155</a>.</p> - -</div> - -<p>Palagonitic-tuff is of frequent occurrence. It is met with in the Firth -of Forth district,<a id="FNanchor_451" href="#Footnote_451" class="fnanchor">[451]</a> and Mr. Watts has detected fragments of palagonite among -the tuffs of the Limerick basin.</p> - -<div class="footnote"> - -<p><a id="Footnote_451" href="#FNanchor_451" class="label">[451]</a> <i>Trans. Roy. Soc. Edin.</i> vol. xxix. (1819) p. 515.</p> - -</div> -<hr class="chap x-ebookmaker-drop" /> - - -<div class="chapter"> -<p><span class="pagenum" id="Page_424">- 424 -</span></p> - -<h2 class="nobreak" id="CHAPTER_XXVII">CHAPTER XXVII<br /> - -<span class="smaller">GEOLOGICAL STRUCTURE OF THE CARBONIFEROUS PUYS OF SCOTLAND</span></h2> -</div> - -<div class="blockquot"> - -<p>1. Vents: Relation of the Necks to the Rocks through which they rise—Evidence of the -probable Subærial Character of some of the Cones or Puys of Tuff—Entombment of -the Volcanic Cones and their Relation to the Superficial Ejections. 2. Bedded Tuffs -and Lavas—Effects of Subsequent Dislocations. 3. Sills, Bosses and Dykes.</p> -</div> - - -<p>The puy-type of volcanic hill differs widely in one respect from those -which we have hitherto been considering. In the earlier epochs of volcanism -within the British area, it is the masses of material discharged from the -vent, rather than the vents themselves which arrest attention. Indeed, so -copiously have these masses been erupted that the vents are often buried, or -their positions have been rendered doubtful, by the uprise in and around -them of sills and bosses of molten rock. But among the Carboniferous puys -the vent is often the only record that remains of the volcanic activity. In -some cases we know that it never ejected any igneous material to the surface. -In others, though it may be filled with volcanic agglomerate or tuff, -there is no record of any shower of such detritus having been discharged -from it. In yet a third class of examples, we see that lava rose in the -vent, but no evidence remains as to whether or not it ever flowed out above -ground. Other cases occur where beds of lava or of tuff, or of both together, -have been intercalated in a group of strata, but with no trace now visible of -the vent from which they came. The most complete chronicle, preserving -at once a record of the outflow of lava, of the showering forth of ashes and -bombs, and of the necks that mark the vents of eruption, is only to be found -in some of the districts.</p> - -<p>I shall therefore, in the present instance, reverse the order of arrangement -followed in the previous chapters, and treat first of the vents, then of -the materials emitted from them, and lastly of the sills and dykes.</p> - - -<h3>i. <span class="allsmcap">VENTS</span></h3> - -<p>A large number of vents rise through the Carboniferous rocks of Scotland. -Some of these are not associated with any interbedded volcanic material, -so that their geological age cannot be more precisely defined than by saying -<span class="pagenum" id="Page_425">- 425 -</span> -that they must be later than the particular formations which they pierce. -Some of them, as I shall endeavour to show, are in all probability of Permian -age. But many, from their position with reference to the nearest intercalated -lavas and tuffs, are to be regarded as almost certainly belonging -to the Carboniferous period. Those which are immediately surrounded by -sheets of lava and tuff, similar in character to the materials in the vents -themselves, may without hesitation be connected with these sheets as -marking the orifices of discharge.</p> - -<p>The vents of the puys are in general much less than those of the -plateaux. Their smallest examples measure only a few yards in diameter, -their largest seldom much exceed half a mile.<a id="FNanchor_452" href="#Footnote_452" class="fnanchor">[452]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_452" href="#FNanchor_452" class="label">[452]</a> The following measurements of necks belonging to the puy-eruptions in different parts of -Scotland are taken from the 6-inch field-maps of the Geological Survey:—Saline Hill, Fife, 6000 × -4000 feet; Binn of Burntisland, 3500 × 1500; Hill of Beath, Fife, 2900 × 1550; Binns Hill, Linlithgowshire, -4800 × 2200; Tor Hill, Ecclesmachan, Linlithgowshire, 1900 × 1000 (<a href="#v1fig155">Fig. 155</a>); Great -Moor, near Maiden Pap, Roxburghshire, 2600 × 2400; Tinnis Hill, Liddesdale, 1500 × 1000; Roan -Fell, Liddesdale, 300 × 200; Hadsgarth Burn, Liddesdale, 250 × 200; Dalbate Burn, 250 × 120. -In some cases, especially in those of the larger necks, it is probable that the tuff belongs to more -than one funnel. Thus the Binn of Burntisland almost certainly includes two necks, a smaller -one to the west and a much larger one to the east. Saline Hill may also conceal more than one -vent. But in the continuous mass of tuff at the surface it is at present impossible to determine -precisely the number and boundaries of the several orifices.</p> - -</div> - -<p>The dislocations of the Carboniferous system are probably on the whole -later than its volcanic phenomena. It is at least certain that the lavas and -tuffs of the puys have been extensively faulted, like the surrounding sedimentary -strata, and the vents seldom show any apparent relation to faults. -It may sometimes be observed, however, that the vents are arranged in lines -suggestive of fissures underneath. A remarkable instance of the linear distribution -is furnished by the chain of necks which extends from the Vale of -the Tweed at Melrose south-westwards across the watershed and down -Liddesdale. The most notable part of this line lies among the uplands to -the east of the Old Mosspaul Inn at the head of the Ewes Water. A string -of masses of agglomerate has there solidified in a fissure among the Silurian -greywackes and shales, running in a north-easterly direction for several -miles. The largest connected mass of agglomerate is 4700 feet long, and -from 350 to 600 feet broad (see No. 1 in <a href="#v1fig22">Fig. 22</a>). That this curious vent, -or connected line of vents along a great fissure, belongs to the puy-eruptions -of Liddesdale is shown by the abundant fragments of yellow sandstone and -cement-stone which occur in the agglomerate.<a id="FNanchor_453" href="#Footnote_453" class="fnanchor">[453]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_453" href="#FNanchor_453" class="label">[453]</a> These facts were ascertained by Mr. Peach in mapping the ground for the Geological Survey. -See Sheet 17, Scotland.</p> - -</div> - -<p>Most frequently the vents are distributed irregularly in groups. As -examples of this arrangement I may cite those of the west of Fife, of West -Lothian and of the north of Ayrshire.</p> - -<p>A convenient classification of the vents may be made by dividing them -into four groups according to the nature of the material that now fills them: -1st, Necks of non-volcanic debris; 2nd, Necks of tuff and agglomerate; 3rd, -Necks of similar materials, but with a central plug of basalt; 4th, Bosses of -basalt or other lava, without agglomerate or tuff.</p> - -<p><span class="pagenum" id="Page_426">- 426 -</span></p> - -<p>1. <i>Necks of Non-volcanic Debris.</i>—In a few instances the orifices of -eruption have been filled up entirely with non-volcanic debris. They have -served as funnels for the discharge of explosive vapours only, without the -expulsion of any solid volcanic materials. At least no trace of fragmentary -lavas is met with in them, nor are any beds of tuff or lava intercalated -among the surrounding strata. Some interesting examples of this kind -were laid bare in the open ironstone-workings near Carluke in Lanarkshire. -They were circular in ground-plan, descended vertically into the strata, and -were somewhat wider at the top of the quarry than at the bottom. They -were filled with angular pieces of Carboniferous sandstone, shale, limestone, -ironstone and other rocks, these materials being rudely arranged with a -dip towards the centre of the neck, where the blocks were largest in size. -Though no fragments of igneous rocks were observed among the debris, a -few string-like veins of "white trap," or altered basalt, were seen to traverse -the agglomerate here and there. The necks and the strata surrounding -them were highly impregnated with pyrites and sulphate of lime.<a id="FNanchor_454" href="#Footnote_454" class="fnanchor">[454]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_454" href="#FNanchor_454" class="label">[454]</a> Prof. Jas. Geikie, <i>Mem. Geol. Surv. Scotland</i>, Explanation of Sheet 23, p. 39.</p> - -</div> - -<div class="figcenter" id="v1fig143" style="width: 449px;"> - <img src="images/v1fig143.png" width="449" height="155" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 143.</span>—Section of volcanic vent at East Grange, Perthshire coal-field, constructed by Mr. B. N. - Peach from the rocks exposed in a railway-cutting, and from plans of ironstone- and coal-pits.<br /> - 1. Three feet coal; 2. Ontake coal; 3. Upper and Lower Black-band Ironstones; 4. Index Limestone; 5. Gas - Coal and Janet Peat Coal; 6. Calmy Limestone; 7. Neck.</div> -</div> - -<p>A vent of the same nature, but on a much larger scale, has been -mapped by Mr. Peach in the south of Perthshire, near East Grange, where -it rises through the higher coal-bearing part of the Carboniferous Limestone -series (<a href="#v1fig143">Fig. 143</a>). It has been encountered in the mining of coal and ironstone, -and its cross-section has been ascertained in the underground workings -which have been carried up to its margin. It measures 1500 feet in -diameter from east to west and 2000 feet from north to south. It does -not appear ever to have emitted any ashes or lava. Mr. Peach found it -filled with dark sandy crumbling clays, full of fragments of sandstone, shale -and coal. These sediments are arranged in layers that dip in the same -general direction as the strata surrounding the vent. They contain abundant -calcareous nodules of all sizes from that of a hazel-nut up to concretions -18 feet in diameter. The clays likewise include many of the common shells -and crinoids of the Carboniferous Limestone sea, and the same fossils are -enclosed in the nodules. A remarkable feature in this vent is the occurrence -<span class="pagenum" id="Page_427">- 427 -</span> -of abundant vertical rents, which have been filled partly with the -same material that forms the nodules, and partly with sandstone.</p> - -<p>The formation of the neck took place after the deposition of the Index -Limestone, and probably about the time of the accumulation of the next -limestone, which lies immediately to the west somewhat higher in the -series. It would appear that the eruption which produced this funnel gave -forth only gaseous explosions, and occurred on the sea-floor; that the low -crater-walls were washed down to such an extent that the sea entered and -carried some of its characteristic organisms into the lagoon or <i>maar</i> within; -further, that as the silt gathered inside, successive subsidences occurred, -whereby the sediment was rent by cracks into which sand and calcareous -mud were washed from above.<a id="FNanchor_455" href="#Footnote_455" class="fnanchor">[455]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_455" href="#FNanchor_455" class="label">[455]</a> The vent is shown in Sheet 39, <i>Geol. Surv. Scotland</i>.</p> - -</div> - -<p>Many necks occur wherein non-volcanic materials, though not forming -the whole of the agglomerate, make up by far the larger part, with only a -slight admixture of volcanic tuff between them. Among the Burntisland -necks of Fife, for instance, abundant fragments of the well-marked cyprid -limestone and shale may be observed, while at Niddry in Linlithgowshire -blocks several yards in length, and consisting of different layers of shale and -cement-stone still adhering to each other, may be seen imbedded at all angles -in the tuff.</p> - -<p>Where only the debris of non-volcanic rocks occupies a vent, we may -infer that the volcanic action was limited to the explosion of steam, whereby -the rocks were dislocated, and an orifice communicating with the surface -was drilled through them, and that while no true volcanic rock in such a -case appeared, the pipe was filled up to perhaps not far from the surface by -the falling back of the shattered detritus. A little greater intensity or -farther prolongation of the volcanic action would bring the column of lava -up the funnel, and allow its upper part to be blown out as dust and lapilli; -while still more vigorous activity would be marked by the rise of the lava -into rents of the cone or its actual outflow at the surface. Every gradation -in this scale of progress may be detected among the Carboniferous volcanoes -of the basin of the Firth of Forth.</p> - -<p>2. <i>Necks of Tuff and Agglomerate.</i>—The majority of the necks connected -with the puys consist of tuff or agglomerate. Externally they -generally appear as smooth rounded grassy hills that rise disconnected from -other eminences. In some districts their materials consist of a greenish -granular often stratified tuff, enclosing rounded balls of various basic lavas and -pieces of sandstone, shale, limestone or other strata through which they have -been drilled. This is their usual character in the Forth region. But in some -cases, the tuff becomes a coarse agglomerate, made up partly of large blocks -of basalt and other volcanic rocks and partly of the sedimentary strata -around them, of which large masses, many cubic yards in bulk, may be seen. -Among the enclosed fragments it is not unusual to find pieces of older -stratified tuff. These resemble in general petrographical character parts of -the tuff among which they are imbedded. Sometimes they have been -derived from previous tuffs which, interstratified among the sedimentary -strata, had been broken up by the opening of a new vent. But probably -in most cases they should be regarded as portions of the volcanic debris -which, having solidified inside the crater, was blown out in fragments by -subsequent explosions. In a modern volcano a considerable amount of -stratified tuff may be formed inside the crater. The ashes and stones -thrown out during a period of activity fall not only on the outer slopes of -the cone, but on the steep inner declivities of the crater, where they arrange -themselves in beds that dip at high angles towards the crater bottom. This -feature is well seen in some of the extinct cones in the Neapolitan -district. In some of the Scottish puys the tuff is stratified and has tumbled -down into a highly inclined or vertical position (<a href="#v1fig145">Fig. 145</a>).</p> - -<p><span class="pagenum" id="Page_428">- 428 -</span></p> - -<div class="figcenter" id="v1fig144" style="width: 762px;"> - <img src="images/v1fig144.png" width="762" height="506" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 144.</span>—View of the Binn of Burntisland—a volcanic neck of agglomerate. - (This illustration and Figs. <a href="#v1fig145">145</a>, <a href="#v1fig152">152</a>, <a href="#v1fig153">153</a>, <a href="#v1fig164">164</a>, <a href="#v1fig166">166</a> and <a href="#v1fig168">168</a> are from photographs taken by Mr. Robert Lunn for the Geological Survey.)</div> -</div> - -<p><span class="pagenum" id="Page_429">- 429 -</span></p> - -<p>As a good illustration of the variety and relative proportions of the -ejected blocks in the green tuff of the puy-vents, I may cite the following -table of percentages which I took many years ago in the tuff which rises -through the Cement-stone group on the beach at the Heads of Ayr.</p> - -<table summary="data"> -<tr> - <td class="tdl">Diabase and basalt</td> - <td class="tdr">57</td> - <td class="tdc">per cent.</td> -</tr> -<tr> - <td class="tdl">Older tuff</td> - <td class="tdr">3</td> - <td class="tdc">"</td> -</tr> -<tr> - <td class="tdl">Andesite (probably from Old Red Sandstone<br /> - volcanic series of the neighbourhood)</td> - <td class="tdr vbot">14</td> - <td class="tdc vbot">"</td> -</tr> -<tr> - <td class="tdl">Limestone (cement-stone, etc.)</td> - <td class="tdr">20</td> - <td class="tdc">"</td> -</tr> -<tr> - <td class="tdl">Shale</td> - <td class="tdr">3</td> - <td class="tdc">"</td> -</tr> -<tr> - <td class="tdl">Sandstone</td> - <td class="tdr">2</td> - <td class="tdc">"</td> -</tr> -<tr> - <td class="tdl">Fossil wood</td> - <td class="tdr bdb">1</td> - <td class="tdc">"</td> -</tr> -<tr> - <td></td> - <td class="tdr">100</td> - <td></td> -</tr> -</table> - -<p>While many examples might be cited where no molten rock of any -kind has risen in the vents, or where at least all the visible materials are of -a fragmentary character, yet small veins and dykes of basalt have not -infrequently been injected into the tuff or agglomerate. These seldom run -far, and usually present a more or less tortuous course. Thus, on the south -front of the Binn of Burntisland (Figs. <a href="#v1fig166">166</a>, <a href="#v1fig168">168</a>) a number of basalt-dykes, -which vary in breadth from five or six feet to scarcely so many inches, -bifurcate and rapidly disappear in the tuff, one of them ascending tortuously -to near the top of the cliff. They at once recall the appearance of the well-known -dykes in the great crater wall of Somma.</p> - -<p>Though not by any means the largest or most perfect of the vents -in the basin of the Firth of Forth, the Binn of Burntisland, of which a -view is given in <a href="#v1fig144">Fig. 144</a>, may be cited in illustration of their general -characters. It presents in detail some of the most strikingly volcanic -aspects of scenery anywhere to be seen in that region. Consisting -of a dull green granular volcanic tuff, it rises abruptly out of the Lower -Carboniferous formations to a height of 631 feet above the sea. The -southern edge of this neck has been so extensively denuded, that it -presents steep craggy slopes and rugged precipices, which descend from -the very summit of the cone to the plain below—a vertical distance of -<span class="pagenum" id="Page_430">- 430 -</span> -nearly 500 feet. Here and there the action of atmospheric waste has -hollowed out huge crater-like chasms in the crumbling tuff. Standing in -one of these, the geologist can realize what must have been the aspect of -the interior of these ancient Carboniferous volcanic cones. The scene at -once reminds him of the crater-walls of a modern or not long extinct -volcano. The dull-green rudely stratified tuff rises around in verdureless -crumbling sheets of naked rock, roughened by the innumerable blocks of lava, -which form so conspicuous an element in the composition of the mass. The -ribs or veins of columnar basalt run up the declivities as black shattered -walls. The frosts and rains of many centuries have restored to the tuff its -original loose gravelly character. It disintegrates rapidly, and rolls down -the slopes in long grey lines of volcanic sand, precisely as it no doubt did -at the time of its ejection, when it fell on the outer and inner declivities -of the original cone. Some of these features may be partly realized from -<a href="#v1fig145">Fig. 145</a>, which represents a portion of the south front of the hill. Sections -of this neck are given in Figs. <a href="#v1fig149">149</a> and <a href="#v1fig159">159</a>.</p> - -<p>(3) <i>Necks of Tuff or Agglomerate with a Central Plug of Basalt or other -Lava.</i>—It has often happened that, after the explosions in a vent have -begun to decrease in vigour, or have at last ceased, lava has risen in the -chimney and finally sealed it up. In such cases the main mass of the -rock may consist of tuff or agglomerate, which the enfeebled volcanic -activity has been unable to expel from the orifice, while a plug of basalt, -dolerite, or even more basic material, of much smaller dimensions, may have -risen up the pipe in the centre or towards one side. Binns Hill, West -Lothian, the Beath and Saline Hills of Fife, and Tinnis Hill in Liddesdale -are good examples of this structure. (See Figs. <a href="#v1fig26">26</a>, <a href="#v1fig148">148</a>, <a href="#v1fig149">149</a> and <a href="#v1fig174">174</a>).</p> - -<p>(4) <i>Necks of Basalt, Dolerite, etc.</i>—In other cases no fragmental material -is present in the vent, or possibly traces of it may be seen here and there -adhering to the walls of the funnel, the prevailing rock being some form of -lava. Necks of this kind are much less frequent in the puy- than in the -plateau-type. But examples may be found in several districts. The most -striking with which I am acquainted are those which form so picturesque a -group of isolated cones around the volcanic basin of Limerick, to be afterwards -described (Figs. <a href="../../66493/66493-h/66493-h.htm#v2fig195">195</a>, <a href="../../66493/66493-h/66493-h.htm#v2fig196">196</a>). The vents there have been filled by the -uprise of much more acid rocks than the lavas of the basin, for, as I have -already stated, they include even quartziferous trachytes. In the basin of -the Firth of Forth some prominent bosses of basalt probably mark the sites -of former vents, such as Dunearn Hill in Fife, the Castle Rock of Edinburgh, -and Galabraes Hill near Bathgate. Some striking vents which occur in -the Jedburgh district, in the debateable land between the plateau series on -the east and the puy-series on the west, show the nearly complete usurpation -of the funnel by basalt, but with portions of the tuff still remaining visible.</p> - -<p><span class="pagenum" id="Page_431">- 431 -</span></p> - -<div class="figcenter" id="v1fig145" style="width: 775px;"> - <img src="images/v1fig145.png" width="775" height="508" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 145.</span>—View of part of the - cliffs of vertical agglomerate, Binn of Burntisland.</div> -</div> - -<p><span class="pagenum" id="Page_432">- 432 -</span></p> - -<p><i>Relation of the Necks to the Rocks through which they rise.</i>—A remarkable -feature among the Carboniferous and Permian vents of central Scotland is -presented by the effect which has been produced on the strata immediately -surrounding them. In the interior of the country this effect is often -concealed by herbage, but where the rocks have been laid bare by the sea it -may be most instructively studied. In such shore-sections, a singular -change of dip is often observable among the strata round the edge of a vent. -No matter what may be the normal inclination at the locality, the beds are -bent sharply down towards the wall of the neck, and are frequently placed on -end. This structure (shown in Figs. <a href="#v1fig24">24</a>, <a href="#v1fig143">143</a>, <a href="#v1fig147">147</a>, <a href="#v1fig148">148</a> and <a href="#v1fig149">149</a>) is precisely -the reverse of what might have been anticipated, and can hardly be due to -upward volcanic explosions. It is frequently associated with considerable -metamorphism in the disturbed strata. Shales are converted into porcellanite -or various jaspery rocks, according to their composition. Sandstones -pass into quartzite, with its characteristic lustrous fracture. It is common -to find vents surrounded with a ring of this contact-metamorphism, which, -from the hardness and frequently vertical or highly inclined bedding of its -strata, stands up prominently on the beach (as in Figs. <a href="#v1fig126">126</a> and <a href="../../66493/66493-h/66493-h.htm#v2fig210">210</a>), and -serves to mark the position of the necks from a distance.</p> - -<p>I have not been able to find an altogether satisfactory explanation of -this inward dip of the strata around vents. Taking it in connection with -the metamorphism, I am inclined to believe that it arose after the close of -the long-continued volcanic action which had hardened the rocks around the -volcanic pipe, and as the result of some kind of subsidence within the vent. -The outpouring of so much tuff and lava as escaped from many of the volcanoes -would doubtless often be apt to produce cavities underneath them, and -on the decay of volcanic energy there might be a tendency in the solid or -cavernous column filling up the funnel, to settle down by mere gravitation. -So firmly, however, did much of it cohere to the sides of the pipe, that if it -sank at all, it could hardly fail to drag down a portion of these sides. So -general is this evidence of downward movement in all the volcanic districts -of Scotland where the necks have been adequately exposed, that the -structure may be regarded as normal to these volcanic vents. It has been -observed among the shore-sections of the volcanoes of the Auckland -district, New Zealand. Mr. C. Heaphy, in an interesting paper upon that -district, gives a drawing of a crater and lava-stream abutting on the edge of -a cliff where the strata bend down towards the point of eruption, as in the -numerous cases in Scotland.<a id="FNanchor_456" href="#Footnote_456" class="fnanchor">[456]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_456" href="#FNanchor_456" class="label">[456]</a> <i>Quart. Journ. Geol. Soc.</i> 1860, vol. xvi. p. 245.</p> - -</div> - -<p><i>Evidence for the probable subærial Character of some of the Cones or Puys -of Tuff.</i>—From the stratigraphical data furnished by the basin of the Firth -of Forth, it is certain that this region, during a great part of the Carboniferous -period, existed as a wide shallow lagoon, sometimes overspread with -sea-water deep enough to allow of the growth of corals, crinoids, and -brachiopods; at other times, shoaled to such an extent with sand and mud -as to be covered with wide jungles of a lepidodendroid and calamitoid -vegetation. As volcanic action went on interruptedly during a vast section -of that period, the vents, though generally submarine, may occasionally have -been subærial. Indeed, we may suppose that the same vent might begin as -a subaqueous orifice and continue to eject volcanic materials, until, as these -<span class="pagenum" id="Page_433">- 433 -</span> -rose above the level of the water, the vent became subærial. An instance -of a submarine vent has been cited from the Perthshire coal-field (<a href="#Page_426">p. 426</a>).</p> - -<p>Among the evidence which may be collected to show that some -Carboniferous volcanoes probably rose as insular cones of tuff above the -surrounding waters, the structure of the tuff in many necks may be -cited, for it suggests subærial rather than subaqueous stratification. The -way in which the stones, large and small, are grouped together in -lenticular seams may be paralleled on the slopes of many a modern -volcano. Another indication of this mode of origin is supplied by the -traces of wood to be met with in some of the tuff-necks. The -vents of Fife and Linlithgowshire contain these traces sometimes in -great abundance. The specimens are always angular fragments, and are -frequently encrusted with calcite.<a id="FNanchor_457" href="#Footnote_457" class="fnanchor">[457]</a> Sometimes they present the glossy -fracture and clear ligneous structure shown by sticks of well-made -wood charcoal. In a neck at St. Magdalen's, near Linlithgow, the wood -fragments occur as numerous black chips. So far as can be ascertained -from the slices already prepared for the microscope, the wood is always -coniferous. These woody fragments seldom occur in the interstratified tuffs -or in the associated strata where <i>Stigmaria</i>, <i>Lepidodendron</i>, etc., are common. -They are specially characteristic of the necks and adjacent tuffs. The -parent trees may have grown on the volcanic cones, which as dry insular -spots would support a different vegetation from the club-mosses and reeds -of the surrounding swamps. As the fragments occur in the tuffs which, on -the grounds already stated, may be held to have been deposited within the -crater, they seem to point to intervals of volcanic quiescence, when the -dormant or extinct craters were filled with a terrestrial flora, as Vesuvius -was between the years 1500 and 1631, when no eruptions took place. -Some of the cones, such as Saline Hill and the Binn of Burntisland, may -have risen several hundred feet above the water. Clothed with dark pine -woods, they would form a notable feature in the otherwise monotonous -scenery of central Scotland during the Carboniferous period.</p> - -<div class="footnote"> - -<p><a id="Footnote_457" href="#FNanchor_457" class="label">[457]</a> The largest I have observed is a portion of a stem about two feet long and six inches broad, -in the (Permian?) neck below St. Monan's church.</p> - -</div> - -<div class="figleft" id="v1fig146" style="width: 215px;"> - <img src="images/v1fig146.png" width="213" height="170" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 146.</span>—Diagram of buried volcanic cone - near Dalry, Ayrshire. Constructed from - information obtained in mining operations.<br /> - 1. Hurlet Limestone. 2. Clayband Ironstone. 3. - Black-band Ironstone. 4. Borestone Coal. - 5. Wee Coal. 6. Highfield Limestone. 7 and - 8. Thin Limestones. 9. Linn Limestone. 10. - Volcanic neck and cone of tuff.</div> -</div> - -<p><i>Entombment of the Volcanic Cones and their relation to the bedded Lavas -and Tuffs.</i>—From the facts above detailed, it is evident that in most cases -the necks represent, as it were, the mere denuded stumps of the volcanoes. -As the puys took their rise in areas which, on the whole, were undergoing -a movement of subsidence, they were eventually submerged and buried -under sedimentary accumulations. Their loose ashes would be apt to be -washed down and strewn over the sea-bottom, so that only the lower and -inner part of a cone might remain. We can hardly hope to discover any -of the actual craters among these volcanic relics. The cones having been -submerged and buried under many hundred feet of sediment, their present -position at the surface is due to subsequent elevation and prolonged denudation. -It is obvious that there must still be many buried cones which the -progress of denudation has not yet reached. Some of these have been -<span class="pagenum" id="Page_434">- 434 -</span> -revealed in the course of mining operations. Valuable seams of coal, ironstone -and oil-shale in the Scottish Carboniferous Limestone and Calciferous -Sandstone series are extensively worked, and in the underground operations -many illustrations of former volcanic action have been met with. -The most remarkable instances of the discovery of buried volcanoes have -occurred in the Dalry coal-field in the -north of Ayrshire. In one pit-shaft -about a mile and a half to the south-west -of the village of Dalry, a thickness -of 115 fathoms of tuff was passed -through, and in another pit 90 fathoms -of similar tuff were sunk into before the -position of the black-band ironstone of -that mineral field was reached by driving -levels through the tuff into the sedimentary -strata outside of it. Only a -short distance from these thick piles of -tuff, their place is entirely taken up by -the ordinary sedimentary strata of the -district. The working-plans of the -mines show the tuff to occur in irregular -patches and strips, between which the ironstone is workable. From these -data we perceive that the shafts have in some cases been sunk directly upon -the tops of puys of tuff, which were, in one case, nearly 700 feet, and in -another instance, 540 high<a id="FNanchor_458" href="#Footnote_458" class="fnanchor">[458]</a> (<a href="#v1fig146">Fig. 146</a>).</p> - -<div class="footnote"> - -<p><a id="Footnote_458" href="#FNanchor_458" class="label">[458]</a> Explanation of Sheet 22, <i>Geol. Surv. of Scotland</i>, p. 16.</p> - -</div> - -<p>It is obvious that from the condition of a completely buried and concealed -cone every stage may be expected to occur up to the deeply worn-down -neck representing merely the stump of the volcanic column. The -subjoined diagram (<a href="#v1fig147">Fig. 147</a>) may serve to illustrate this process of gradual -re-emergence.</p> - -<div class="figcenter" id="v1fig147" style="width: 502px;"> - <img src="images/v1fig147.png" width="502" height="132" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 147.</span>—Diagram to illustrate how Volcanic Necks may be concealed and exposed.<br /> - 1, Neck, still buried under the succeeding sedimentary accumulations; 2, Neck uncovered and denuded.</div> -</div> - -<p>When, in the progress of denudation, a volcanic cone began to show -itself from under the cover of removed strata, it would still for a time maintain -its connection with the sheets of tuff or of lava which, when active, it -had erupted. A number of examples of this structure may be observed in -the basin of the Firth of Forth, where the degradation of the surface has -<span class="pagenum" id="Page_435">- 435 -</span> -not yet proceeded so far as to isolate the column of agglomerate or tuff from -the sheets of tuff that were strewn around the old volcano. In such cases, the -actual limits of the vent are still more or less concealed, or at least no sharp -line can be drawn between the vent and its ejections. As an illustration -of this connection of a volcanic pipe with the materials ejected from it over -the surrounding country I would cite Saline Hill -in the west of Fife. That eminence rises to a -height of 1178 feet above the sea, out of a band -of tuff which can be traced across the country -for fully three miles. Numerous sections in the -water-courses show that this tuff is regularly -interbedded in the Carboniferous Limestone series, -so that the relative geological date of its eruption -can be precisely fixed. On the south of Saline -Hill, coal and ironstone, worked under the tuff, -prove that this portion of the mass belongs to -the general sheet of loose ashes and dust, extending -outwards from the original cone over the floor -of the sheet of water in which the Carboniferous -Limestone series of strata was being deposited. -But the central portion of the hill is occupied -by one or more volcanic pipes. A section across -the eminence from north-west to south-east would -probably show the structure represented in <a href="#v1fig148">Fig. -148</a>. Immediately to the east of the Saline Hill -lies another eminence, known as the Knock Hill, -which marks the site of another eruptive vent. -A coal-seam (the Little Parrot or Gas Coal) is -worked along its southern base, and is found to -plunge down steeply towards the volcanic rocks. -This seam, however, is not the same as that worked -under the Saline Hill, but lies some 600 feet below -it. Probably the whole of the Knock Hill occupies -the place of a former vent.</p> - -<div class="figcenter" id="v1fig148" style="width: 586px;"> - <img src="images/v1fig148.png" width="586" height="103" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 148.</span>—Section across the Saline Hills, Fife.<br /> - The thick parallel black lines mark the position of seams of coal and ironstone, some of which are worked under Saline Hill. T, Tuff of the - necks; <i>t</i>, Tuff at a little distance from the cone, interstratified with the ordinary sedimentary beds; B, Basalt. The larger eminence - is Saline Hill, the lower is Knock Hill.</div> -</div> - -<p>A further stage of decay and denudation brings -before us the entire severance of the volcanic -column from the materials that were ejected from -it. An excellent example of this isolation of the -neck in the midst of surrounding masses of tuff -and lava which proceeded from it is presented by -the Binn of Burntisland, to which I have already -alluded. A section across that eminence gives the geological structure -represented in <a href="#v1fig149">Fig. 149</a>. The dip of the rocks away from the volcanic -pipe at this locality has been produced long after the volcanic phenomena -had ceased. The arch here shown is really the prolongation and final disappearance -of the great anticlinal fold of which the Pentland Hills form the -<span class="pagenum" id="Page_436">- 436 -</span> -axis on the opposite side of the Firth. But if we restore the rocks to a -horizontal, or approximately horizontal position, we find the Binn of Burntisland -rising among them in one or more necks, which doubtless mark -centres of volcanic activity in that district. A series of smaller neck-like -eminences runs for two miles westward.</p> - -<p>Striking as the forms of many of the necks -are, and much as their present conical forms resemble -those of active and extinct volcanoes, the evidence -of extensive denudation proves that these contours -are not the original outlines of the Carboniferous -vents, but are in every case the result of prolonged -waste. What we now see is a section of the volcanic -chimney, and the conical form is due to the -way in which the materials filling the chimney have -yielded to the forces of denudation.</p> - -<div class="figcenter" id="v1fig149" style="width: 580px;"> - <img src="images/v1fig149.png" width="580" height="102" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 149.</span>—Section across the Binn of Burntisland, in an East and West direction.<br /> - 1, Sandstones; 2, Limestone (Burdiehouse); 3, Shales, etc.; <i>b</i>, <i>b</i>, Interstratified basalts; <i>t</i> <i>t</i>, Bedded tuff, etc.; T, Tuff of the - great neck of Burntisland; B, Basalt veins.</div> -</div> - - -<h3>ii. <span class="allsmcap">BEDDED TUFFS AND LAVAS</span></h3> - -<p>During at least the earlier part of the period of -the puys, in some districts or from certain vents, -such as those of East Fife, Western Midlothian, -Eastern Linlithgowshire, Northern Ayrshire, Heads -of Ayr and Lower Eskdale, only fine tuff seems to -have been thrown out, which we now find intercalated -among the surrounding strata. These eruptions, -neither so vigorous nor so long-continued as -those of the plateaux, never gave forth such thick -and widespread sheets of fragmentary materials as -those associated with the plateaux in East Lothian -and the north-east of Ayrshire. A single discharge -of ashes seems in many cases to have been the sole -achievement of one of those little volcanoes; at least -only one thin band of tuff may be discoverable to -mark its activity.</p> - -<p>The tuff of these solitary bands is seldom coarse -in texture. It usually consists of the ordinary dull -green paste, with dust and lapilli of basic pumice. -The local variations in the tuffs of the puys generally -arise mainly from differences in the composition, size -and numbers of the included ejected blocks. Generally -the most abundant stones are pieces of different diabases, or basalts; -then come fragments from the surrounding Carboniferous strata, from older -tuffs and rarely from rocks of much deeper-seated origin.</p> - -<p>Now and then the eruptions of tuff have consisted of extremely fine -volcanic dust, which, mingling with water, took the form of a compact mudstone, -as in the case of the Houston Marls (<a href="#Page_423">p. 423</a>), which remind one -<span class="pagenum" id="Page_437">- 437 -</span> -of a volcanic mud. But in most localities the discharge of tuff, though for -a time it may have completely obscured the ordinary contemporaneous -sedimentation, was intermittent, so that in the intervals between successive -showers of detritus, the deposition of non-volcanic sediment went on as -usual. Hence it is that bands of tuff, whether they lie among lavas or -among sedimentary formations, are apt to contain interstratifications of sandstone, -shale, limestone or other detrital deposit, and to pass insensibly into -these. The extremely gentle gradation from volcanic into non-volcanic -sediment, and the occasional reappearance of thin partings of tuff bring -vividly before the mind the slow dying out of volcanic energy among the -Carboniferous lagoons.</p> - -<div class="figright" id="v1fig150" style="width: 168px;"> - <img src="images/v1fig150.png" width="168" height="165" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 150.</span>—Section in old quarry, west of Wester - Ochiltree, Linlithgowshire. Calciferous Sandstone series.</div> -</div> - -<p>The comparatively quiet character of the volcanic explosions, and the -contemporaneous undisturbed deposition of sediment during the earlier part -of the puy period, are exemplified in many sections throughout the areas -above enumerated, as will be more fully illustrated in subsequent pages. -Two typical examples may suffice for this general statement of the -characters of the discharges of tuff in the puy-eruptions. In the Linlithgowshire -quarry represented in <a href="#v1fig150">Fig. 150</a>, where about ten feet of strata have -been exposed, a black shale (1) of the usual carbonaceous character, so -common in the Oil-shale series of this region, may be seen at the bottom of -the section. It is covered by a bed of nodular -bluish-grey tuff (2) containing black shale fragments. -A second black shale (3) is succeeded -by a second thin band of fine pale yellowish -tuff (4). Black shale (5) again supervenes, -containing rounded fragments of tuff, perhaps -ejected lapilli, and passing up into a layer of -tuff (6). It is evident that we have here a -continuous deposit of black shale which was -three times interrupted by showers of volcanic -dust and stones. At the close of the third -interruption, the deposition of the shale was -renewed and continued, with sufficient slowness -to permit of the segregation of thin seams and -nodules of clay ironstone round the decomposing organic remains of the -muddy bottom (7). A fourth volcanic interlude now took place, and the -floor of the water was once more covered with tuff (8). But the old conditions -of deposit were immediately afterwards resumed (9); the muddy -bottom was abundantly peopled with ostracod crustaceans, while many -fishes, whose coprolites have been left in the mud, haunted the locality. -At last, however, a much more serious volcanic explosion took place. A -coarse agglomeratic tuff (10), with blocks sometimes nearly three feet in -diameter, was then thrown out, and overspread the lagoon.<a id="FNanchor_459" href="#Footnote_459" class="fnanchor">[459]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_459" href="#FNanchor_459" class="label">[459]</a> See <i>Geol. Surv. Memoir of Edinburgh</i>, p. 45. These tuffs are further described on pp. 465 <i>et seq.</i></p> - -</div> - -<p>The second illustration may be taken from the admirable coast-section -between Burntisland and Kinghorn, where the number of intercalations of -<span class="pagenum" id="Page_438">- 438 -</span> -tuff is very great. Besides thicker well-marked bands, successive innumerable -thin layers occur there among the associated zones of sedimentary -strata which separate the sheets of basalt. The character of these -tuff-seams may be inferred from the following details of less than two feet -of rock at Pettycur Point:—</p> - -<table summary="data"> -<tr> - <td class="tdl">Tuff</td> - <td class="tdl"> 1·5</td> - <td class="tdc">inch.</td> -</tr> -<tr> - <td class="tdl">Limestone</td> - <td class="tdl"> 0·2</td> - <td class="tdc">"</td> -</tr> -<tr> - <td class="tdl">Tuff</td> - <td class="tdl"> 0·5</td> - <td class="tdc">"</td> -</tr> -<tr> - <td class="tdl">Shale</td> - <td class="tdl"> 0·2</td> - <td class="tdc">"</td> -</tr> -<tr> - <td class="tdl">Tuff</td> - <td class="tdl"> 0·1</td> - <td class="tdc">"</td> -</tr> -<tr> - <td class="tdl">Shale and tuff</td> - <td class="tdl"> 1·0</td> - <td class="tdc">"</td> -</tr> -<tr> - <td class="tdl">Shale</td> - <td class="tdl"> 0·2</td> - <td class="tdc">"</td> -</tr> -<tr> - <td class="tdl">Limestone</td> - <td class="tdl"> 0·5</td> - <td class="tdc">"</td> -</tr> -<tr> - <td class="tdl">Shale full of volcanic dust</td> - <td class="tdl"> 3·5</td> - <td class="tdc">"</td> -</tr> -<tr> - <td class="tdl">Shaly limestone</td> - <td class="tdl"> 1·5</td> - <td class="tdc">"</td> -</tr> -<tr> - <td class="tdl">Laminated tufaceous limestone</td> - <td class="tdl"> 2·0</td> - <td class="tdc">"</td> -</tr> -<tr> - <td class="tdl">Limestone in thin bands, with thin laminæ of tuff</td> - <td class="tdl"> 0·8</td> - <td class="tdc">"</td> -</tr> -<tr> - <td class="tdl">Granular tuff</td> - <td class="tdl"> 0·6</td> - <td class="tdc">"</td> -</tr> -<tr> - <td class="tdl">Argillaceous limestone, with diffused tuff</td> - <td class="tdl"> 0·9</td> - <td class="tdc">"</td> -</tr> -<tr> - <td class="tdl">Fine granular tuff</td> - <td class="tdl"> 0·7</td> - <td class="tdc">"</td> -</tr> -<tr> - <td class="tdl">Argillaceous limestone, with diffused tuff</td> - <td class="tdl"> 1·5</td> - <td class="tdc">"</td> -</tr> -<tr> - <td class="tdl">Laminated limestone</td> - <td class="tdl"> 0·1</td> - <td class="tdc">"</td> -</tr> -<tr> - <td class="tdl">Limestone, with parting of granular tuff in middle</td> - <td class="tdl"> 0·9</td> - <td class="tdc">"</td> -</tr> -<tr> - <td class="tdl">Tufaceous shale</td> - <td class="tdl"> 2·0</td> - <td class="tdc">"</td> -</tr> -<tr> - <td class="tdl">Limestone</td> - <td class="tdl"> 0·4</td> - <td class="tdc">"</td> -</tr> -<tr> - <td class="tdl">Shaly tuff</td> - <td class="tdl"> 1·25</td> - <td class="tdc">"</td> -</tr> -<tr> - <td class="tdl">Laminated limestone</td> - <td class="tdl"> 0·1</td> - <td class="tdc">"</td> -</tr> -<tr> - <td class="tdl">Tuff</td> - <td class="tdl bdb"> 1·2</td> - <td class="tdc">"</td> -</tr> -<tr> - <td></td> - <td class="tdl">21·65</td> - <td class="tdc">inches.</td> -</tr> -</table> - -<div class="figright" id="v1fig151" style="width: 221px;"> - <img src="images/v1fig151.png" width="221" height="110" alt="" /> - <div class="hanging2"><p><span class="smcap">Fig. 151.</span>—Ejected volcanic block in Carboniferous - strata, Burntisland.<br /> - 1. Brown shaly fire-clay with rootlets, about five - inches; 2. Impure coal, five or six inches, pressed - down in its upper layers by the impact and - weight of the stone; 3. Green crumbling ashy - fire-clay, one foot, with its lower layers pressed - down by the stone while the upper layers rise - over it, showing that the stone fell at the time - when half this seam was deposited. The fire-clay - passes up into dark greenish and black ashy - shale (4) about six inches thick and containing - plant-remains. The stone is a pale diabase - weighing about six or eight pounds.</p></div> -</div> - -<p>Such a section as this brings vividly before the mind a long-continued -intermittent feeble volcanic action during pauses between successive outbursts -of lava. In such intervals of -quiescence, the ordinary sediment of the -lagoons accumulated, and was mixed up -with the debris supplied by occasional -showers of volcanic dust. In this Fife -volcanic series, thin layers of sandstone, -streaked with remains of the Carboniferous -vegetation; beds of shale full of -cyprid-cases, ganoid scales, and fragmentary -ferns; thin beds of limestone, and -bands of fire-clay supporting seams of -coal, are interleaved with strata of tuff -and sheets of basalt. Now and then -a sharp discharge of larger stones is -seen to have taken place, as in the case -of the block many years ago described by me as having fallen and crushed -down a still soft bed of coal (<a href="#v1fig151">Fig. 151</a>).<a id="FNanchor_460" href="#Footnote_460" class="fnanchor">[460]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_460" href="#FNanchor_460" class="label">[460]</a> <i>Geol. Mag.</i> vol. i. p. 22. This Fife coast-section is given in full at <a href="#Page_470">p. 470</a>.</p> - -</div> - -<p><span class="pagenum" id="Page_439">- 439 -</span></p> - -<div class="figcenter" id="v1fig152" style="width: 761px;"> - <img src="images/v1fig152.png" width="761" height="507" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 152.</span>—View of volcanic agglomerate becoming finer above. East end of Kingswood Craig, two miles east from Burntisland.</div> -</div> - -<p><span class="pagenum" id="Page_440">- 440 -</span></p> - -<p>The Fife coast-section from which these details are taken supplies almost -endless instances of the varying characters of the pyroclastic materials of -the puy-eruptions. The very same cliff, bank or reef will show at one point -an accumulation of excessively coarse volcanic debris and at another thin -laminæ of the finest dust and lapilli. These rapid gradations are illustrated -in <a href="#v1fig152">Fig. 152</a>, which is taken from the east end of the Kingswood -Craig. The lower part of the declivity is a coarse agglomerate which passes -upward into finer tuff.</p> - -<p>Besides the thin partings and thicker layers of tuff which, intercalated -among the sedimentary strata of the Carboniferous system, mark a comparatively -feeble and intermittent volcanic activity, we meet in some localities -with examples where the puys have piled up much thicker accumulations of -fragmentary material without any intercalated streams of lava, or interstratified -sandstone, shale or limestone. Thus the widespread Houston -marls above described reach a thickness of some 200 feet. The vents of -the Saline Hills in Fife covered the sea-floor with volcanic ashes to a depth -of several hundred feet. In the north of Ayrshire the first eruptions of the -puys have formed a continuous band of fine tuff traceable for some 15 miles, -and in places at least 200 feet thick.</p> - -<p>Where volcanic energy reached its highest intensity during the time of -the puys, not only tuffs but sheets of lava were emitted, which, gathering -round the vents, formed cones or long, connected banks and ridges. Of -these there are four conspicuous examples in Scotland—the hills of -the Burntisland district, the Bathgate Hills, the ground between Dalry and -Galston in north Ayrshire, and a broken tract in Liddesdale. Nowhere -in the volcanic history of this country have even the minutest details of -that history been more admirably preserved than among the materials -erupted from puys in these respective districts.</p> - -<p>Lava-cones, answering to solitary tuff-cones among the fragmental -eruptions, do not appear to have existed, or, like some of those in the great -lava-fields of Northern Iceland and Western America, must have been mere -small heaps of slag and cinders at the top of the lava-column, which were -washed down and effaced during the subsidence and entombment of the -volcanic materials. The lavas never occur without traces of fragmentary -discharges. Two successive streams of basalt may indeed be found at a -given locality without any visible intercalation of tuff, but proofs of the -eruption of fragmental material will generally be observed to occur somewhere -in the neighbourhood, associated with one or both of them, or with -other lavas above or below them.</p> - -<p><span class="pagenum" id="Page_441">- 441 -</span></p> - -<div class="figcenter" id="v1fig153" style="width: 768px;"> - <img src="images/v1fig153.png" width="768" height="491" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 153.</span>—Alternations of - basalt and tuff with shale, etc., Kingswood Craig, Burntisland.</div> -</div> - -<p><span class="pagenum" id="Page_442">- 442 -</span></p> - -<p>Where the phenomena of the puys have been most typically developed, -lavas and tuffs succeed each other in rapid succession, with numerous or occasional -interstratifications of ordinary sediment. Perhaps the most complete -and interesting example of this association is to be found on the coast between -Burntisland and Kirkcaldy, where, out of a total thickness of rock which -may be computed to be between 1500 and 2000 feet, it will probably be a -fair estimate to say that the igneous materials constitute four-fifths, or from -1200 to 1600 feet. The lavas are varieties of basalt ranging in character -from a black compact columnar to a dirty green earthy cellular or slaggy -rock. Each separate flow may be on the average about 20 or 30 feet in -thickness. Columnar and amorphous sheets succeed each other without any -interposition of fragmentary material (<a href="#v1fig171">Fig. 171</a>). But along the junctions of -the separate flows layers of red clay, like the bole between the basalts of the -Giant's Causeway, may frequently be noticed. The characteristic slaggy -aspect of the upper parts of these ancient <i>coulées</i> is sometimes remarkably -striking. The full details of this most interesting section will be given in -later pages (<a href="#Page_470">p. 470</a>). But some of its more characteristic external features -may be understood from the views which are presented in Figs. <a href="#v1fig152">152</a>, <a href="#v1fig153">153</a>, -<a href="#v1fig170">170</a>, <a href="#v1fig171">171</a>.</p> - -<p>The general bedded character of the volcanic series is well shown in -<a href="#v1fig153">Fig. 153</a>, which represents the alternations of lavas and tuffs in the -Kingswood Craig two miles to the east of Burntisland. The harder -basalts will be seen to project as bold crags while the tuffs and other -stratified deposits between them give rise to grassy slopes and hollows. A -nearer view of the alternation of lavas and tuffs with non-volcanic sedimentary -deposits is supplied in <a href="#v1fig170">Fig. 170</a>, which is taken from a part of the Fife coast a -little further to the east than the last illustration. Here one of the limestones -of the Carboniferous Limestone series is overlain with shale and tuff, which, -being easily disintegrated, have been cut away by the waves, leaving the -lava above to overhang and fall off in blocks. The columnar structure of -some of the basalts of this coast is well brought out in <a href="#v1fig171">Fig. 171</a>, which -shows further how the columns sometimes merge into an amorphous part of -the same sheet.</p> - -<p>These Fife basalts illustrate admirably the peculiarities of the sheets -of lava which are intercalated among the Carboniferous strata. They show -how easy it generally is to discriminate between such sheets and intrusive -sills. The true lavas are never so largely crystalline, nor spread out in -such thick sheets as the sills; they are frequently slaggy and amygdaloidal, -especially towards the top and bottom, the central portion being generally -more fine-grained and sometimes porphyritic. Where most highly cellular -they often decompose into a dull, earthy, dirty-green rock. Where they form -a thick mass they are usually composed of different beds of varying texture. -Except the differences between the more compact centre and the slaggy -layer above and below, the bedded lavas do not present any marked variation -in composition or structure within the same sheet. A striking exception -to this rule, however, is furnished by the Bathgate "leckstone" already -described.<a id="FNanchor_461" href="#Footnote_461" class="fnanchor">[461]</a> This mass forms a continuation of the great basaltic ridge of -the Bathgate Hills, and though its exact relations to the surrounding strata -are concealed, it appears to be an interbedded and not an intrusive sheet. -The remarkable separation of its constituent minerals into an upper, lighter -felspathic layer, and a lower, heavier layer, rich in olivine, augite and iron-ores, -is a structure which might be more naturally expected to occur in -<span class="pagenum" id="Page_443">- 443 -</span> -a sill. An instance of its development in an undoubted sill will be -described further on. Nevertheless, if we follow the trend of the volcanic -band of the Bathgate Hills southward for only two miles beyond the picrite -quarry, we find in the Skolie Burn a rock in many respects similar, and -quarried for the same purpose of building oven-soles. This "leckstone" is -there seen to be surmounted by a group of calcareous shales and thin -limestones. The section laid bare in the stream is represented in <a href="#v1fig154">Fig. 154</a>. -Immediately above the diabase, which is highly cellular, lies a green felspathic -sandstone or shale containing detached fragments of the amygdaloid together -with <i>Lingulæ</i> and other shells. There seems no reason to doubt that this -is a true interstratified lava.<a id="FNanchor_462" href="#Footnote_462" class="fnanchor">[462]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_461" href="#FNanchor_461" class="label">[461]</a> <i>Trans. Roy. Soc. Edin.</i> xxix. (1879) p. 504.</p> - -<p><a id="Footnote_462" href="#FNanchor_462" class="label">[462]</a> <i>Trans. Roy. Soc. Edin.</i> xxix. (1879), pp. 505-507.</p> - -</div> - -<div class="figcenter" id="v1fig154" style="width: 362px;"> - <img src="images/v1fig154.png" width="362" height="128" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 154.</span>—Section of the upper surface of a diabase ("leckstone") sheet, Skolie Burn, south-east - of Bathgate.<br /> - 1. Slaggy diabase; 2. Green sandy shale and shaly sandstone containing <i>Lingulæ</i>, also pieces of slag from the underlying - lava, which are completely wrapped round in the sediment; 3. Yellow calcareous shelly sandstone; 4. - Dark shale with <i>Spiriferæ</i>, etc.; 5. Bed of blue crinoidal limestone; 6. Clays and thin coal; 7. Black and - blue calcareous shales and thin limestones.</div> -</div> - -<p>Where the puys attained their greatest development in Scotland, they -rose in the shallow lagoons, and here and there from deeper parts of the -sea-bottom, until by their successive discharges of lavas and tuffs they -gradually built up piles of material, which, in the Linlithgow and Bathgate -district, may have been nearly 2000 feet in thickness. It must be remembered, -however, that the eruptions took place in a subsiding area, and that even the -thickest volcanic ejections, if the downward movement kept pace with the -volcanic activity, need not have grown into a lofty volcanic hill. Indeed, -largely as the lavas and tuffs bulk in the geology of some parts of Central -Scotland, their eruption does not seem to have seriously interfered with the -broader physical changes that were in progress over the whole region. Thus -the subsidence which led to the spread of a marine and limestone-making -fauna over much of Central Scotland included also the volcanic districts. -The limestones, formed of crinoids, corals and other marine organisms, -extended over the submerged lavas and tuffs, and were even interstratified -with them.</p> - -<p>While the volcanic materials are found to replace locally the ordinary -Carboniferous sedimentary strata, it is interesting in this regard to note -that, during pauses in the volcanic activity, while the subsidence doubtless -was still going on, some groups of sandstones, shales or limestones extended -themselves across the volcanic ridges so as to interpose, on more than one -<span class="pagenum" id="Page_444">- 444 -</span> -platform, a mass of ordinary sediment between the -lavas or tuffs already erupted and those of succeeding -discharges, and thus to furnish valuable -geological chronometers by which to define the -stratigraphical horizons of the successive phases of -volcanic energy.</p> - -<div class="figcenter" id="v1fig155" style="width: 778px;"> - <img src="images/v1fig155.png" width="778" height="92" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 155.</span>—Section across the volcanic ridge of the Linlithgow and Bathgate Hills, showing the intercalation of limestones that mark important - stratigraphical horizons.<br /> - 1. Houston Coal; 2. Houston Marls and tuffs; 3. Interstratified sheets of basic - lavas with occasional tuffs and intercalations of shale, sandstone, etc.; - 4. Tartraven Limestone; 5. Hurlet Limestone with tuffs, shales and sandstones - above and below; 6. Wardlaw Limestone; 7. Index Limestone; 8. Highest band of - tuff—upward limit of the volcanic series; 9 9. Volcanic necks; 10. Sill of - basalt; 11. Levenseat or Castlecary Limestone; 12. Millstone Grit; 13. Base - of Coal-measures; 14. Thick doleritic sill; 15. Dolerite dyke (? Tertiary).</div> -</div> - -<p>The volcanic banks or ridges not improbably -emerged as islets out of the water, and were sometimes -ten miles or more in length. Their materials -were supplied from many separate vents along their -surface, but probably never attained to anything -approaching the elevation which they would have -reached had they been poured out upon a stable -platform. This feature in the history of the volcanic -ridges is admirably shown by the fact just -referred to, that recognizable stratigraphical horizons -can sometimes be traced right through the heart -of the thickest volcanic accumulations. One of -the largest areas of basalts and tuffs connected -with the puys is that of the Linlithgow and Bathgate -Hills, where, as already remarked, a depth of -some 2000 feet of igneous rocks has been piled -up. Yet several well-known seams of stone can -be traced through it, such as the Hurlet Limestone -and the Index Limestone (<a href="#v1fig155">Fig. 155</a>). Only -at the north end, where the volcanic mass is thickest -and the surface-exposures of rock are not continuous, -has it been impossible to subdivide the -mass by mapping intercalations of sedimentary -strata across it. It would thus seem that, even -where the amplest accumulations gathered round -the puys, they formed low flat domes, rather than -prominent hills, which, as subsidence went on and -the tuff-cones were washed down, gradually sank -under water, and were buried under the accumulating -silt of the sea-floor.</p> - -<p>As a detailed illustration of the manner in -which the growth of organically-formed limestones -and the deposit of ordinary sediment took place -concurrently with the occasional outflow of lava-streams -over the sea-bottom, I may cite the section -presented in another Linlithgowshire quarry (<a href="#v1fig156">Fig. -156</a>). At the bottom of the group of strata there -exposed, a pale amygdaloidal, somewhat altered -basalt (A) marks the upper surface of one of the -submarine lavas of the period. Directly over it -<span class="pagenum" id="Page_445">- 445 -</span> -comes a bed of limestone (B) 15 feet thick, the lower layers of which are -made up of a dense growth of the thin-stemmed coral <i>Lithostrotion irregulare</i>. -The next stratum is a band of dark shale (C) -about two feet thick, followed by about the same -thickness of an impure limestone with shale seams -(D). The conditions for coral and crinoid growth -were evidently not favourable, for this argillaceous -limestone was eventually arrested first by -the deposit of a dark mud, now to be seen in -the form of three or four inches of a black pyritous -shale (E), and next by the inroad of a large -quantity of dark sandy mud and drift vegetation, -which has been preserved as a sandy shale (F), -containing <i>Calamites</i>, <i>Producti</i>, ganoid scales and -other traces of the life of the time. Finally, -a great sheet of lava, represented by the uppermost -amygdaloid (G), overspread the area, and -sealed up these records of Palæozoic history.<a id="FNanchor_463" href="#Footnote_463" class="fnanchor">[463]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_463" href="#FNanchor_463" class="label">[463]</a> <i>Geol. Surv. Mem.</i> "Geology of Edinburgh," p. 58.</p> - -</div> - -<div class="figright" id="v1fig156" style="width: 171px;"> - <img src="images/v1fig156.png" width="171" height="272" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 156.</span>—Section in Wardlaw - Quarry, Linlithgowshire.</div> -</div> - -<p>Among the phenomena associated with the -Carboniferous volcanoes mention may, in conclusion, -be made of the evidence for the former -existence of thermal springs and saline sublimations or incrustations. -Among the plateau-tuffs of North Berwick, as has been already pointed out (<a href="#Page_390">p. -390</a>), a fœtid limestone has been quarried, which bears indications of having -been deposited by springs, probably in connection with the volcanic action -of the district. The lower limestones of Bathgate furnish abundant laminæ -of silica interleaved with calcareous matter, the whole probably due to the -action of siliceous and calcareous springs connected with the active puys of -that district. Some portions of the limestone are full of cellular spaces, -lined with chalcedony.<a id="FNanchor_464" href="#Footnote_464" class="fnanchor">[464]</a> A saline water has been met with among the -volcanic rocks to the west of Linlithgow, in a bore which was sunk to a -depth of 348 feet in these rocks without reaching their bottom. The -water that rose from the bore-hole was found to contain as much as 135 -grains of chloride of sodium in the gallon. It is not improbable that this -salt was originally produced by incrustations on the Carboniferous lavas -immediately after their eruption, as has happened so often in recent times -at Vesuvius, and that it was then buried under succeeding showers of tuff -and streams of lava.<a id="FNanchor_465" href="#Footnote_465" class="fnanchor">[465]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_464" href="#FNanchor_464" class="label">[464]</a> <i>Ibid.</i> p. 49, <i>et seq.</i></p> - -<p><a id="Footnote_465" href="#FNanchor_465" class="label">[465]</a> <i>Proc. Roy. Soc. Edin.</i> vol. ix. p. 367. Besides chloride of sodium the water contained also -chlorides of calcium, magnesium and potassium, carbonates of lime and magnesia, sulphate of -lime, and other ingredients in minute proportions.</p> - -</div> - -<p><i>Subsequent Dislocation of Bedded Lavas and Tuffs.</i>—As the interstratified -volcanic materials were laid down in sheets at the surface, they necessarily -behave like the ordinary sedimentary strata, and have undergone with them -the various curvatures and fractures which have occurred since Carboniferous -<span class="pagenum" id="Page_446">- 446 -</span> -times. Notwithstanding their volcanic nature, they can be traced -and mapped precisely as if they had been limestones or sandstones. This -perfect conformability with the associated stratified rocks is strikingly seen -in the case of the sheets of lava which lie imbedded in the heart of the -great volcanic ridge of Linlithgowshire. The overlying strata having been -removed from their surface for some distance, and the ground having been -broken by faults, these volcanic rocks might at first be taken for irregular -intrusive bosses, but their true character is that shown in <a href="#v1fig157">Fig. 157</a>, where -by a succession of faults, with a throw in the same direction, the upper -basalts of Bonnytoun Hill are gradually brought down to the level of the -Firth of Forth.</p> - -<div class="figcenter" id="v1fig157" style="width: 470px;"> - <img src="images/v1fig157.png" width="470" height="101" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 157.</span>—Section from Linlithgow Loch to the Firth of Forth.</div> -</div> - - -<h3>iii. <span class="allsmcap">SILLS, BOSSES AND DYKES</span></h3> - -<p>One of the characteristic features of Central Scotland is the great -number, and often the large size and extraordinary persistence, of the masses -of eruptive, more or less basic material, which have been injected among the -Carboniferous strata. The precise geological age of these intrusions cannot, -of course, be more exactly defined than by stating that they are younger -than the rocks which they traverse, though in many cases their association -with the necks, lavas and tuffs is such as to show that they must be -regarded as part of the Carboniferous volcanic phenomena.</p> - -<p><span class="smcap">Sills.</span>—With regard to the sills I have been led, for the following -reasons, to connect the great majority of them with the puys, though some -are certainly of far later date, while others should possibly be assigned -to the plateaux.</p> - -<p>In the first place, the sills obviously connected with the plateaux are in -great measure intermediate, or even somewhat acid rocks, while those of the -puy series are much more basic. It is hardly possible, however, in all cases -to decide to which series a particular sill should be assigned. This difficulty -is particularly manifest in the western part of Midlothian, where the plateau -of that district exhibits such frequent interruption, and where it often consists -only of a single basaltic sheet. To the west of it lie the abundant -puys with their lavas and tuffs, and between the two volcanic areas -numerous sills of dolerite and diabase make their appearance. In the -difficulty of deciding to which series these sills should be referred, it will be -convenient to consider them with those of the puys.</p> - -<p><span class="pagenum" id="Page_447">- 447 -</span></p> - -<div class="figcenter" id="v1fig158" style="width: 744px;"> - <img src="images/v1fig158.png" width="744" height="105" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 158.</span>—Section across the Campsie Fells illustrating the contrast between the sills below and above the plateau-lavas.<br /> - 1. Upper Old Red Sandstone; 2. "Ballagan Beds"; 3. Tuffs; 4. Lavas of the Campsie district of the Clyde plateau; 5 5. Necks belonging to the plateau volcanic series; 6. Trachytic - sills belonging to the plateau; 7. Carboniferous Limestone series; 8. Dolerite sills cutting the Carboniferous Limestone series. <i>f</i>, Fault.</div> -</div> - -<p>A remarkable illustration of the contrast in petrographical character -between the typical sills of the plateaux and those -of the puys is furnished by the chain of the Campsie -Fells, where, on the north side, among the Calciferous -Sandstones which emerge from under the andesitic -lavas of the Clyde plateau, many intrusive sheets and -bosses of trachytic material may be seen, while on -the southern side come the great basic sills which, -from Milngavie by Kilsyth to Stirling, run in the -Carboniferous Limestone series (<a href="#v1fig158">Fig. 158</a>). A similar -contrast may be observed in Renfrewshire between -the trachytic sills below the plateau-lavas south of -Greenock and the basic sills above these lavas in the -Carboniferous Limestone series around Johnstone and -Paisley.</p> - -<p>In the second place, the more basic sills, as a -rule, appear on platforms higher in stratigraphical -position than the plateaux, and wherever this is -their position there cannot be any hesitation in deciding -against their association with the older phase -of volcanic activity.</p> - -<p>In the third place, the basic sills often occur in -obvious connection with the vents or bedded lavas -and tuffs of the puy series. A conspicuous example -of this dependence is supplied by the intrusive sheets -of Burntisland, underlying the basalts and tuffs of -that district in the immediate neighbourhood of some -of the vents from which these bedded rocks were -erupted (<a href="#v1fig159">Fig. 159</a>).</p> - -<p>In the fourth place, even where no visible vents -appear now at the surface near the sills, the latter -generally occupy horizons within the stratigraphical -range indicated by the interbedded volcanic rocks. -It must be remembered that all the Carboniferous -vents were deeply buried under sedimentary deposits, -and that large as is the number of them which has -been exposed by denudation, it is probably much -smaller than the number still concealed from our -view. The sills are to be regarded as deep-seated -parts of the volcanic protrusions, and they more -especially appear at the surface where the strata -between which they were injected crop out from -under some of the higher members of the Carboniferous -system. Thus the remarkable group of sills -between Kilsyth and Stirling (<a href="#v1fig158">Fig. 158</a>) may quite -possibly be connected with a group of vents lying -<span class="pagenum" id="Page_448">- 448 -</span> -not far to the eastward, but now buried under the higher parts of the -Carboniferous Limestone, Millstone Grit and Coal-measures. Again, the -great series of sills that gives rise to such a conspicuous range of hills in -the north and middle of Fife may have depended for its origin upon the -efforts of a line of vents running east and west through the centre of the -county, but now buried under the Coal-measures. Some vents, indeed, have -been laid bare in that district, such as the conspicuous groups of the Saline -Hills and the Hill of Beath, but many more may be concealed under higher -Carboniferous strata further east.</p> - -<div class="figcenter" id="v1fig159" style="width: 382px;"> - <img src="images/v1fig159.png" width="382" height="78" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 159.</span>—Section showing the position of the basic sills in relation to the volcanic series at - Burntisland, Fife.<br /> - 1. Calciferous Sandstone series; 2. Burdiehouse Limestone; 3. Sandstones, shales and tuffs; 4. Basalts and tuffs, - with intercalations of sandstone, shale and limestone; 5. Agglomerate of the Binn of Burntisland neck; 6. - Basalt dyke; 7. Dyke and sill; 8 8 8. Three sills.</div> -</div> - -<p>In the fifth place, the materials of which the sills consist link them in -petrographical character with those that proceeded from the puys. The -rocks of the intrusive sheets in West Lothian, Midlothian and Fife are very -much what an examination of the bedded lavas of the puys in the same -region would lead us to expect. There is, of course, the marked textural -difference between masses of molten rock which have cooled very slowly -within the crust of the earth and those which have solidified with rapidity -at the surface, the sills being for the most part much more coarsely crystalline -than the lavas, and more uniform in texture throughout, though -generally finer at the margins than at the centre. There is likewise the -further contrast arising from differences in the composition of the volcanic -magma at widely-separated periods of its extravasation. At the time when -the streams of basalt flowed out from the puys its constitution was comparatively -basic, in some localities even extremely basic. Any sills dating from -that time may be expected to show an equal proportion of bases. But -those which were injected at a long subsequent stage in the volcanic period -may well have been considerably more acid.</p> - -<p>In actual fact the petrographical range of the sills reasonably referable -to the puy-eruptions varies from picrite or limburgite to dolerite without -olivine. The great majority of these sheets in the basin of the Firth of Forth, -where they are chiefly displayed, are dolerites (diabases), sometimes with, but -more frequently without, olivine. They include all the more coarsely crystalline -rocks of the region, though occasionally they are ordinary close-grained -basalts. Their texture may be observed to bear some relation to their mass, -so far at least as that, where they occur in beds only two or three feet or -yards in thickness, they are almost invariably closer-grained. A cellular or -amygdaloidal texture is seldom to be observed among them, and never where -<span class="pagenum" id="Page_449">- 449 -</span> -they are largely crystalline. This texture is most often to be found in thin -sills which have been injected among carbonaceous shales or coals. These -intrusive sheets are generally finely cellular, and more or less decayed -("white trap").</p> - -<div class="figcenter" id="v1fig160" style="width: 373px;"> - <img src="images/v1fig160.png" width="373" height="113" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 160.</span>—Sills between shales and sandstones, Hound Point, Linlithgowshire.</div> -</div> - -<p>Differences of texture may often be observed within short distances in -the same sill, and likewise considerable varieties in colour and composition. -The most finely crystalline portions are, as usual, those along the junction -with the stratified rocks, the most crystalline occurring in the central parts -of the mass. A diminution in the size of the crystalline constituents may -be traced not only at the base, but also at the top of a sheet, or at any -intermediate portion which has come in contact with a large mass of the -surrounding rock. A good illustration is supplied by the intrusive sheet at -Hound Point (<a href="#v1fig160">Fig. 160</a>), to the east of South Queensferry, where some -layers of shale have been involved in the igneous rock, which becomes -remarkably close-grained along the junction.<a id="FNanchor_466" href="#Footnote_466" class="fnanchor">[466]</a> This change in texture and -absence of cellular structure form a well-marked distinction between these -sheets and those which have flowed out at the surface as true lava-streams.</p> - -<div class="footnote"> - -<p><a id="Footnote_466" href="#FNanchor_466" class="label">[466]</a> See Hay Cunningham's "Essay," p. 66, and plate ix.; and <i>Geol. Survey Memoir</i> on "Geology -of Edinburgh," p. 114.</p> - -</div> - -<p>Some of the larger doleritic sills display a somewhat coarsely crystalline -texture in their central portions, and occasionally present a notable -micropegmatitic aggregate, which plays the part of interstitial substance -enclosing the other minerals. Mr. Teall has referred to the frequent occurrence -of this structure in the coarser parts of the Whin Sill of the north of -England.<a id="FNanchor_467" href="#Footnote_467" class="fnanchor">[467]</a> It occurs also in a marked degree in the Ratho sill and in some -portions of the great doleritic sill of which the crags of Stirling form a part.<a id="FNanchor_468" href="#Footnote_468" class="fnanchor">[468]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_467" href="#FNanchor_467" class="label">[467]</a> <i>British Petrography</i>, p. 208.</p> - -<p><a id="Footnote_468" href="#FNanchor_468" class="label">[468]</a> Mr. H. W. Monckton. <i>Quart. Journal Geol. Soc.</i> vol. li. (1895), p. 482.</p> - -</div> - -<p>But beside the differences in texture, mainly due to varying rates of -cooling, the sills sometimes exhibit striking varieties of composition in the -same mass of rock. These variations are more especially noticeable among -the larger sills, and particularly where the material is most markedly basic. -The special type of differentiation, so noticeable in the Bathgate diabase and -picrite mass already alluded to, is likewise well exhibited in an intrusive -sheet or group of sheets, recently exposed at Barnton, in the cutting of a -railway from Edinburgh to Cramond<a id="FNanchor_469" href="#Footnote_469" class="fnanchor">[469]</a> (<a href="#v1fig161">Fig. 161</a>). The intrusive nature of the -<span class="pagenum" id="Page_450">- 450 -</span> -several bands of igneous rock which occur here is made quite evident by the -alteration they have produced upon the shales with which they have come -in contact. It is the uppermost and most extensive of these sills which -specially deserves notice, for the differentiation of its constituents. It -stretches along the cutting for several hundred yards at an angle of dip of -about 15°. At the western or upper part of the mass its actual contact -with the superincumbent sedimentary strata is not visible, but as the -igneous rock is there a good deal finer in grain than elsewhere, its upper -surface cannot be many feet distant. The upper visible portion is a light -well-crystallized dolerite with a rudely bedded structure, the planes dipping -westwards at 15°. About 20 or 30 feet below the upper visible termination -of the mass, the dark ferro-magnesian minerals begin rapidly to increase in -relative proportion to the pale felspar, and the rock consequently becomes -dark-greenish brown. The change is particularly noticeable in certain -bands which run parallel with the general dip. There is no definite line -between the pale and dark body of the rock, the two graduating into each -other and the darker part becoming deeper in colour, heavier and more -decomposing, until it becomes a true typical picrite. Even in this ultra-basic -portion the same rude bedding or banding may be observed.</p> - -<div class="footnote"> - -<p><a id="Footnote_469" href="#FNanchor_469" class="label">[469]</a> This rock has been described by Mr. J. Henderson and Mr. Goodchild, <i>Trans. Geol. Soc. -Edin.</i> vi. (1893) pp. 297, 301, and by Mr. H. W. Monckton, <i>Quart. Journ. Geol. Soc.</i> l. (1894) -p. 39. Mr. Goodchild recognized the occurrence of picrite, and Mr. Monckton has described the -succession of rocks, and given a diagram of them.</p> - -</div> - -<div class="figcenter" id="v1fig161" style="width: 449px;"> - <img src="images/v1fig161.png" width="449" height="58" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 161.</span>—Section of Sill, Cramomd Railway, Barnton, near Edinburgh.<br /> - 1. Baked shale; 2. Sill of very felspathic dolerite about, nine feet thick; - 3. Baked shale, eight inches; 4. Dolerite showing chilled fine-grained edge - and adhering firmly to the shale below; it rapidly passes up into (5) Picrite - with white felspathic veins (6); 7. Junction of picrite and dolerite with a - similar vein along the line of contact; 8. Large globular body of dolerite - enclosing a mass of picrite.</div> -</div> - -<p>Veins in which felspar predominates over the darker minerals traverse -the rock, sometimes parallel with the bedding, sometimes across it. They -vary from less than an inch to a foot in width, sometimes dividing and -enclosing parts of the surrounding mass. But that they are on the whole -contemporaneous with the sill itself, and not long subsequent injections, is -shown by the way in which the dark ferro-magnesian minerals project from -the picrite into the veins and lock the two together.</p> - -<p>But besides these injections, which doubtless represent the last and -more acid portions of the magma injected into the basic parts before the -final consolidation of the whole, there are to be observed irregular concretionary -patches, of similar character to the veins, distributed through the -picrite. On the other hand, towards its base the sill becomes a coarse -dolerite round which the picrite is wrapped, and which encloses a detached -portion of that rock.</p> - -<p>It is deserving of note that while the ultra-basic portion descends -almost to the very bottom of the sill, the lowest five feet show the same -<span class="pagenum" id="Page_451">- 451 -</span> -change as occurs at the top of the mass. There the felspar rapidly begins -to predominate over the darker minerals, and the dolerite into which the -rock passes shows a fine-grained margin adhering firmly to the shales on -which it rests. This lower doleritic band, showing as it does the effect of -chilling upon its under surface, may be due to more rapid cooling and -crystallization, while in the overlying parts the mass remained sufficiently -mobile to allow of a separation of the heavier minerals from the felspars, -which appear in predominant quantity towards the top. It must be frankly -admitted, however, that we are still very ignorant of the causes which -led to this separation of ingredients in a few sills, while they were entirely -absent or non-efficient in most of them.</p> - -<p>The intrusive character of the Carboniferous sills of Central Scotland -and their contact-metamorphism have been fully described, and some of -them have become, as it were, "household words" in geology.<a id="FNanchor_470" href="#Footnote_470" class="fnanchor">[470]</a> Exposed in -so many fine natural sections in the vicinity of Edinburgh, they early -attracted the notice of geologists, and furnished a battle-ground on which -many a conflict took place between the Plutonist and Neptunist champions -at the beginning of the present century.</p> - -<div class="footnote"> - -<p><a id="Footnote_470" href="#FNanchor_470" class="label">[470]</a> See, for instance, Maclaren's <i>Geology of Fife and the Lothians</i>, 1839; Hay Cunningham's -<i>Essay</i>, previously cited; <i>Geological Survey Memoir on the Geology of Edinburgh</i> (Sheet 32), 1861; -Mr. Allport, <i>Quart. Journ. Geol. Soc.</i> vol. xxx. (1874) p. 553; Teall, <i>British Petrography</i>, p. 187; E. -Stecher, <i>Contacterscheinungen an schottischen Olivindiabasen</i>, Tschermak's <i>Mineralog. Mittheil.</i> vol. -ix. (1887) p. 145; <i>Proc. Roy. Soc. Edin.</i> vol. xv. (1888) p. 160.</p> - -</div> - -<p>As the sills frequently lie in even sheets perfectly parallel with the -bedding of the strata between which they have been injected, care is required -in some cases to establish that they are of intrusive origin. One of the -most obvious tests for this purpose is furnished by the alteration they -produce among the strata through which they have made their way, -whether these lie above or below them. The strata are sometimes crumpled -up in such a manner as to indicate considerable pressure. They are occasionally -broken into fragments, though this may have been due rather to -the effects of gaseous explosions than to the actual protrusion of melted -rock. But the most frequent change superinduced upon them is an -induration which varies greatly in amount even along the edge of the same -intrusive sheet. Sandstones are hardened into quartzite, breaking with a -smooth clear glistening fracture. Coals are converted into a soft sooty -substance, sometimes into anthracite. Limestones acquire a crystalline -saccharoid structure. Shales pass generally into a kind of porcellanite, but -occasionally exhibit other types of contact-metamorphism. Thus below the -thick picrite sill at Barnton, near Edinburgh, the shales have assumed a -finely concretionary structure by the appearance in them of spherical pea-like -aggregates.</p> - -<p>Another proof of intrusion is to be found in the manner in which sills -catch up and completely enclose portions of the overlying strata. The -well-known examples on Salisbury Crags (<a href="#v1fig162">Fig. 162</a>) are paralleled by scores -of other instances in different parts of the same region.</p> - -<p>Moreover, sills do not always remain on the same horizon; that is, -<span class="pagenum" id="Page_452">- 452 -</span> -between the same strata. They may be observed to steal across or break -through the beds, so as to lie successively between different layers. No -more instructive example of this relation on a small scale could be cited -than that of the intrusive sheet which has -been laid open in the Dodhead Limestone -Quarry, near Burntisland. As shown in the -accompanying figure (<a href="#v1fig163">Fig. 163</a>), this rock -breaks through the limestone and then spreads -out among the overlying shales, across which -it passes obliquely.</p> - -<div class="figleft" id="v1fig162" style="width: 174px;"> - <img src="images/v1fig162.png" width="174" height="202" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 162.</span>—Intrusive dolerite sheet enclosing - and sending threads into portions of shale, Salisbury Crags, - Edinburgh.</div> -</div> - -<p>Among the larger sills this transgressive -character is seen to be sometimes manifested -on a great scale. Thus, along the important -belt of intrusive rocks that runs from Kilsyth -to Stirling, the Hurlet Limestone lies in one -place below, in another above, the invading -mass, but in the intervening ground has been -engulphed in it. Similar evidence of the -widely separate horizons occupied by different -parts of the same sill is supplied at Kilsyth, where the intrusive sheet -lies about 70 or 80 fathoms below the Index Limestone, while at Croy, in -the same neighbourhood, it actually passes above that seam.<a id="FNanchor_471" href="#Footnote_471" class="fnanchor">[471]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_471" href="#FNanchor_471" class="label">[471]</a> Explanation of Sheet 31, <i>Geological Survey of Scotland</i>, §§ 43 and 83.</p> - -</div> - -<div class="figcenter" id="v1fig163" style="width: 438px;"> - <img src="images/v1fig163.png" width="438" height="198" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 163.</span>—Intrusive sheet invading limestone and shale, Dodhead Quarry, near Burntisland.</div> -</div> - -<p>Other interesting evidence of the intrusive nature of the Carboniferous -dolerite sills of Central Scotland is supplied by the internal modifications -which the eruptive rock has undergone by contact with the strata between -which it has been thrust. These alterations, though partly visible to the -naked eye, are best studied in thin slices with the aid of the microscope. -Tracing the variations of an intrusive dolerite outwards in the direction of -the rocks which it has invaded, we perceive change first in the augite. The -large crystals and kernels of that mineral grow smaller until they pass into -a granulated form like that characteristic of basalts. The large plates and -<span class="pagenum" id="Page_453">- 453 -</span> -amorphous patches of titaniferous iron or magnetite give place to minute -particles, which tend to group themselves into long club-shaped bodies. The -labradorite continues but little affected, except that its prisms, though as -defined, may not be quite so large. The interstitial glassy groundmass remains -in much the same condition and relative amount as in the centre of the rock.</p> - -<p>Along the line of contact, while the dolerite becomes exceedingly close-grained, -its felspar crystals are still quite distinct even up to the very edge. -But they become fewer in relative number, and still smaller in size, though -an occasional prism two or three millimetres in length may occur. They -retain also their sharpness of outline, and their comparative freedom from -enclosures of any kind. They tend to range themselves parallel with the -surface of the contact-rock. The augite exists as a finely granular pale green -substance, which might at first be taken for a glass, but it gives the characteristic -action of augite with polarized light. It is intimately mixed through -the clear glass of the groundmass, which it far exceeds in quantity. The -iron oxides now appear as a fine granular dust, which is frequently aggregated -into elongated club-shaped objects, as if round some inner pellucid or -translucent microlite. In patches throughout the field, however, the oxides -take the form of a geometrically perfect network of interlacing rods. This -beautiful structure, described and figured by Zirkel and others,<a id="FNanchor_472" href="#Footnote_472" class="fnanchor">[472]</a> is never to -be seen in any of the dolerites, except close to the line of contact with the -surrounding rocks. It occurs also in some of the dykes. I have not -succeeded in detecting any microlites in the sandstones at the edge of a -dolerite sheet, though I have had many slices prepared for the purpose.</p> - -<div class="footnote"> - -<p><a id="Footnote_472" href="#FNanchor_472" class="label">[472]</a> <i>Mikroskopische Beschaffenheit der Mineralien und Gesteine</i>, p. 273; Vogelsang's <i>Krystalliten</i>.</p> - -</div> - -<p>Where one of the dolerite sills has invaded sandstone, there is usually -a tolerably sharp line of demarcation between the two rocks, though it is -seldom easy to procure a hand-specimen showing the actual contact, for the -stone is apt to break along the junction-line. Where, however, the rock -traversed by the igneous mass is argillaceous shale, we may find a thorough -welding of the two substances into each other. In such cases the dolerite at -the actual contact becomes a dark opaque rock, which in thin slices under -the microscope is found to be formed of a mottled or curdled segregation -of exceedingly minute black grains and hairs in a clear glassy matrix, in -which the augite and felspar are not individualized. But even in this -tachylyte-like rock perfectly formed and very sharply defined crystals of -triclinic felspar may be observed ranging themselves as usual parallel to the -bounding surfaces of the rock. These characters are well seen in the -contact of the intrusive sheet of dolerite with shale and sandstone at Hound -Point (<a href="#v1fig160">Fig. 160</a>).</p> - -<p>Another instructive example is furnished by the small threads which proceed -from the dolerite of Salisbury Crags, and traverse enclosed fragments of -shale (<a href="#v1fig162">Fig. 162</a>). Some of these miniature dykes are not more than one-eighth -of an inch in diameter, and may therefore easily be included, together -with part of the surrounding rock, in the field of the microscope. The dolerite -in these ramifications assumes an exceedingly fine texture. The felspar is the -<span class="pagenum" id="Page_454">- 454 -</span> -only mineral distinctly formed into definite crystals. It occurs in prisms of an -early consolidation, sometimes one-fifth of an inch long, and therefore readily -recognizable by the naked eye. These prisms are perfectly shaped, contain -abundant twin lamellæ, and show enclosures of the iron of the base. They -had been already completely formed at the time of injection; for occasionally -they may be observed projecting beyond the wall of the vein into the -adjacent shale or sandstone, and they have ranged themselves parallel to the -sides of the vein.<a id="FNanchor_473" href="#Footnote_473" class="fnanchor">[473]</a> The black ground, from which these large well-defined -crystals stand out prominently, consists of a devitrified glass, rendered dark -by the multitude of its enclosed black opaque microlites. These are very -minute grains and rudely feathered rods, with a tendency to group themselves -here and there into forms like portions of the rhombohedral skeletons -of titaniferous iron. So thoroughly fused and liquid has the dolerite been at -the time of its injection, that little threads of it, less than 1/100 of an inch in -diameter, consisting of the same dark base, with well-defined felspars, may -be seen isolated within the surrounding sedimentary rock. Minute grains -and rounded portions of the latter may also be noticed in the marginal parts -of the dolerite.</p> - -<div class="footnote"> - -<p><a id="Footnote_473" href="#FNanchor_473" class="label">[473]</a> The infusibility of the felspar was well shown in some experiments on the rocks of the -neighbourhood of Edinburgh, made at my request by Dr. R. S. Marsden, who subjected some of -these rocks to fusion at the laboratory of the University of Edinburgh. Microscopic sections were -prepared of the products obtained. The basalt of Lion's Haunch is peculiarly instructive. Its -large labradorite crystals have resisted the intense white heat which, continued for four hours, has -reduced the rest of the minerals to a perfect glass. We can thus well understand how large -definite crystals of felspar should have survived or appeared in dykes and veins while the rock -was still thoroughly liquid. The glass obtained from the Lion's Haunch rock is of a honey-yellow, -and contains translucent tufted microlites. The iron forms beautiful dendritic films in -the cracks. Altogether, the glass presents a strong resemblance to the palagonitic substance so -abundant among the lapilli in the tuffs of the vents.</p> - -</div> - -<p>It is thus evident that specimens taken from the edge of an intrusive -sheet, where the rock has rapidly chilled and solidified, represent to us an -earlier stage in the history of the whole mass than specimens taken from -its central portions. In fact, a series of samples collected at short intervals -from the outer contact to the inner mass shows, as it were, the successive -stages in the consolidation of the molten rock.</p> - -<p>From the observations just described, it appears that the triclinic -felspars began to assume the shape of large definite crystals before any of -the other minerals. These felspars already existed when the molten mass -forced its way among the shales, for they can be seen lying with their long -axes parallel to the surface of shale, precisely as, in the well-known flow-structures, -they behave round a large crystal embedded in the heart of a -rock. A few feet from where the consolidation was not so rapid, the iron -oxides have grouped themselves into incipient crystalline forms and skeleton -crystals; the felspar crystals abundantly occur, and the augite has been left -in the finely granular condition. Still further towards the interior of the -mass, the normal character of the dolerite is gradually assumed.<a id="FNanchor_474" href="#Footnote_474" class="fnanchor">[474]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_474" href="#FNanchor_474" class="label">[474]</a> For a further and more detailed investigation of the contact phenomena of the Carboniferous -doleritic sills of the basin of the Firth of Forth, see the papers by Dr. Stecher, quoted -on <a href="#Page_451">p. 451</a>.</p> - -</div> - -<p><span class="pagenum" id="Page_455">- 455 -</span></p> - -<div class="figcenter" id="v1fig164" style="width: 747px;"> - <img src="images/v1fig164.png" width="747" height="499" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 164.</span>—Spheroidal weathering of dolerite sill, quarry east of North Queensferry, Fife.</div> -</div> - -<p><span class="pagenum" id="Page_456">- 456 -</span></p> - -<p>Where a sill has been injected among carbonaceous shales and coals it -has undergone great alteration along the contact, and if the sheet is only a -few inches or feet thick, the change extends throughout its whole mass. -Black basalts and dolerites, in such circumstances, pass into a substance -like a white or pale yellow clay, which at first might be mistaken for some -band of fire-clay intercalated among the other sediments. But evidence of -actual intrusion may usually be found, as where the igneous rock has caught -up or broken through the adjacent strata, besides altering them. Such -"white traps," as they have been called, have long been familiar in the coal-fields -of Scotland and Central England.</p> - -<div class="figleft" id="v1fig165" style="width: 215px;"> - <img src="images/v1fig165.png" width="215" height="179" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 165.</span>—Two thin sills of "White Trap" - injected into black carbonaceous shale overlying the Hurlet Limestone, - Hillhouse Quarry, Linlithgow.<br /> - 1. Hurlet Limestone; 2. Black shales; 3 3. Two sills - of "White Trap"; 4. Columnar Basalts.</div> -</div> - -<p>As a good illustration of the behaviour of such thin sills among -carbonaceous shales I give here a section -(<a href="#v1fig165">Fig. 165</a>) exposed in the old limestone -quarry of Hillhouse, south of Linlithgow. -At the bottom lies the Hurlet Limestone -which has once been extensively mined -at this locality. Above it comes a -group of black shales which in turn are -surmounted by a sheet of beautifully -columnar basalt. The shales seem at -first sight to include two layers of pale -fire-clay, each only a few inches in thickness. -Closer inspection, however, will -show that these two thin intercalations -are really sills which, though on the -whole parallel with the bedding of the -shale, may be seen to cut across it, and -even at one place to send a finger into it. The upper example may also -be observed to diminish rapidly in thickness in one direction.</p> - -<p>The dimensions of the sills vary within tolerably wide limits. Although -here and there the injected material dwindles down to an inch or less in -thickness, running away even into threads, it more usually forms sheets of -considerable depth. The rock of Salisbury Crags, for example, is fully 150 -feet thick at its maximum. That of Corstorphine Hill is probably about -350 feet. The great sheet which runs among the lower limestones from -Kilsyth by Denny to Stirling has been bored through to a depth of 276 -feet, but as the bore started on the rock, and not in overlying strata, some -addition may need to be made to that thickness.</p> - -<p>The spheroidal weathering so characteristic of basic eruptive rocks is -nowhere more characteristically displayed than among the great doleritic -sills of the basin of the Firth of Forth. As an illustration of this structure -an example is taken here from the large sheet at North Queensferry -(<a href="#v1fig164">Fig. 164</a>).</p> - -<p><span class="pagenum" id="Page_457">- 457 -</span></p> - -<div class="figcenter" id="v1fig166" style="width: 739px;"> - <img src="images/v1fig166.png" width="739" height="479" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 166.</span>—Dyke cutting the agglomerate - of a neck. Binn of Burntisland.</div> -</div> - -<p><span class="pagenum" id="Page_458">- 458 -</span></p> - -<p>While one is struck with the great size and extent of some of the sills -connected with the puys, as compared with the small and local sheets -underneath the plateaux, there is a further fact regarding them that -deserves remark—their capricious distribution. Their occurrence seems to -have little or no relation to the measure of volcanic energy as manifested -in superficial eruptions. Thus in the north of Ayrshire, where a long -band of lavas and tuffs, pointing to vigorous activity, lies at the top of the -Carboniferous Limestone series, and where the strata underneath it are -abundantly exposed at the surface, the sills occur as thin and inconstant -bands in the central and eastern parts of the district only. The bedded -lavas and tuffs at the head of the Slitrig Water have no visible accompaniment -of sills. On the other hand, in the Edinburgh and Burntisland -districts, the sills bear a large proportion to the amount of bedded lavas and -tuffs, while in the Bathgate and Linlithgow district, where the superficial -eruptions were especially vigorous and prolonged, the sills are of trifling -extent.</p> - -<p>It would seem from these facts that the extent to which the crust of -the earth round a volcanic orifice is injected with molten rock, in the form -of intrusive sheets between the strata, does not depend upon the energy of -the volcano as gauged by its superficial outpourings, but on other considerations -not quite apparent. Possibly, the more effectively volcanic energy -succeeded in expelling materials from the vent, the less opportunity was -afforded for subterranean injections. And if the protrusion of the sills took -place after the vents were solidly sealed up with agglomerate or lava, it -would doubtless often be easier for the impelled magma to open a way for -itself laterally between the bedding-planes of the strata than vertically -through the thick solid crust. The size and extent of the sills may thus -be a record of the intensity of this latest phase of the volcanic eruptions.</p> - -<div class="figcenter" id="v1fig167" style="width: 516px;"> - <img src="images/v1fig167.png" width="516" height="102" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 167.</span>—Boss of diabase cutting the Burdiehouse Limestone and sending sills and veins into the - overlying shales. Railway cutting, West Quarry, East Calder, Midlothian.<br /> - 1. Burdiehouse Limestone; 2. Shales; 3. Diabase.</div> -</div> - -<hr class="tb" /> - -<p><span class="smcap">Bosses.</span>—The rounded, oval or irregularly shaped masses of igneous rock -included under this head are found in some cases to be only denuded domes -of sills, as, for example, in the apparently isolated patches in the oil-shale -district of Linlithgowshire, which have been found to unite under ground. -(Compare <a href="#v1fig157">Fig. 157</a>). In other instances, bosses possibly, or almost certainly, -mark the position of volcanic funnels, as at the Castle Rock of Edinburgh, -Dunearn Hill, Burntisland, and Galabraes, near Bathgate. But many -examples occur which can only be regarded as the exposed ends of irregular -bodies of molten material which has been protruded upwards into the -Carboniferous formations. The area between Edinburgh and Linlithgow -and the hills of the north of Fife furnish many examples.</p> - - -<p><span class="pagenum" id="Page_459">- 459 -</span></p> - -<div class="figcenter" id="v1fig168" style="width: 761px;"> - <img src="images/v1fig168.png" width="761" height="485" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 168.</span>—Side of columnar basalt-dyke - in the same agglomerate as in <a href="#v1fig166">Fig. 166</a>.</div> -</div> - -<p><span class="pagenum" id="Page_460">- 460 -</span></p> - -<p>The connection between bosses and intrusive sheets is instructively -exhibited in a railway cutting to the west of Edinburgh, where the section -shown in <a href="#v1fig167">Fig. 167</a> may be seen. In the space of a few yards no fewer -than four distinct bands of diabase traverse the shale, thickening rapidly -in one direction and uniting with a large boss of more coarsely crystalline -material. Such connections must exist in all sills, for the material injected -as a sheet between stratified formations cannot but be united to some column, -dyke or irregular protrusion which descends to the parent magma in the -interior. But it is very rarely that the geologist is permitted to see them.</p> - -<hr class="tb" /> - -<div class="figcenter" id="v1fig169" style="width: 328px;"> - <img src="images/v1fig169.png" width="328" height="406" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 169.</span>—Dyke rising through the Hurlet Limestone and its overlying shales. Silvermine - Quarry, Linlithgowshire.</div> -</div> - -<p><span class="smcap">Dykes</span> take a comparatively unimportant place in the eruptive -phenomena of the puys. They occur in some numbers, but on a small -scale, among the tuff vents, and there they can without much hesitation be -set down as part of the phenomena of eruption through these pipes. The -Binn of Burntisland, which has been so often referred to in this Chapter, -may again be cited as a typical vent for the display of this series of dykes -(Figs. <a href="#v1fig149">149</a> and <a href="#v1fig159">159</a>). Two additional illustrations from this locality are -<span class="pagenum" id="Page_461">- 461 -</span> -here given. In <a href="#v1fig166">Fig. 166</a> a dyke of compact black basalt is seen on the -right hand running up the steep slopes of the agglomerate. Some of these -dykes are distinctly columnar, the columns diverging from the walls on each -side. Where the encasing agglomerate has been removed by the weather, -the side of the dyke presents a reticulated network of prism-ends. A -narrow basalt-dyke of this character near the top of the Binn vent is -represented in <a href="#v1fig168">Fig. 168</a>.</p> - -<p>But dykes also occur apart from vents and without any apparent -relation to these. They are sometimes associated with sills and bosses in -such a manner as to suggest that the whole of these forms of injected -material belong to one connected series of intrusions. Among the Bathgate -Hills, for example, from which I have already cited instances of sills and a -boss, the section represented in <a href="#v1fig169">Fig. 169</a> occurs. Yet in this same district -there is a group of large east and west dykes which cut all the other rocks -including the bedded lavas and tuffs, and must be of later date than the -highest part of the Coal-measures (<a href="#v1fig155">Fig. 155</a>).</p> - -<p>It is difficult to ascertain the age of the dykes which rise through the -Carboniferous formations at a distance from any interbedded sheets of lava -and tuff, or from any recognizable vent. The south-east and north-west -dykes, increasing in number as they go westward, and attaining a prodigious -development in the great volcanic area of Antrim and the Inner Hebrides, -are probably of Tertiary date.<a id="FNanchor_475" href="#Footnote_475" class="fnanchor">[475]</a> Others may possibly be Permian, while a -certain number may reasonably be looked upon as Carboniferous. In petrographical -characters the latter resemble the dolerites and basalts (diabases) -of the finer-grained sills.</p> - -<div class="footnote"> - -<p><a id="Footnote_475" href="#FNanchor_475" class="label">[475]</a> These are fully described in Chapters <a href="../../66493/66493-h/66493-h.htm#CHAPTER_XXXIV">xxxiv.</a> and <a href="../../66493/66493-h/66493-h.htm#CHAPTER_XXXV">xxxv.</a></p> - -</div> -<hr class="chap x-ebookmaker-drop" /> - - -<div class="chapter"> -<p><span class="pagenum" id="Page_462">- 462 -</span></p> - -<h2 class="nobreak" id="CHAPTER_XXVIII">CHAPTER XXVIII<br /> - -<span class="smaller">ILLUSTRATIVE EXAMPLES OF THE CARBONIFEROUS PUYS OF SCOTLAND</span></h2> -</div> - -<p class="tdc">The Basin of the Firth of Forth—North Ayrshire—Liddesdale.</p> - - -<p>Though many of the geological details of each of the Scottish districts of -Puys have been given in the foregoing pages, it will be of advantage to -describe in connected sequence the structure and geological history of a -few typical areas. By far the fullest and most varied record of this phase -of volcanic activity has been preserved in the basin of the Firth of Forth; -but the north of Ayrshire and the district of Liddesdale furnish also many -interesting characteristics.</p> - - -<h3>1. BASIN OF THE FIRTH OF FORTH</h3> - -<p>Reference has already been made to the remarkable peculiarity in the -development of the lower part of the Carboniferous system in this district.<a id="FNanchor_476" href="#Footnote_476" class="fnanchor">[476]</a> -Elsewhere throughout Scotland the Cement-stone group and the plateau -lavas are immediately overlain by the Carboniferous Limestone series. But -in the basin of the Firth of Forth a varied succession of strata, more than -3000 feet in thickness, intervenes between the Cement-stones and the -Hurlet Limestone. The lower portion of this thick mass of sediment may -represent a part of the Cement-stone group of other districts, but even if -some deduction is made on this account there remain many hundred feet of -stratified deposits, for which there does not appear to be any stratigraphical -equivalent elsewhere in Scotland. The distinguishing features of this series -of strata are the thick zones of white sandstone, with occasional bands of -fine conglomerate, the abundant seams of dark shale, often highly carbonaceous -(oil-shales), the cyprid limestones and the seams of coal. Such an association -of deposits may indicate a more humid climate and more varied conditions -of denudation and deposition than are presented by the typical Cement-stones. -The muddy floor of the shallow water must, in many places, have supported a -luxuriant growth of vegetation, which is preserved in occasional seams and -<span class="pagenum" id="Page_463">- 463 -</span> -streaks of coal. Numerous epiphytic ferns grew on the subærial stems and -branches of the lycopodiaceous trees. Large coniferae clothed the higher -grounds, from which the streams brought down copious supplies of sediment, -and whence a flood now and then transported huge prostrate trunks of pine. -In the lagoons animal life abounded. Cyprids swarmed to such a degree as -to form by their accumulated remains bands of limestone, which in the well-known -Burdiehouse seam sometimes attain a thickness of 70 feet. Fishes -of many genera haunted the waters, for their scales, bones and coprolites are -found in profusion among the shales and limestones.</p> - -<div class="footnote"> - -<p><a id="Footnote_476" href="#FNanchor_476" class="label">[476]</a> See Maclaren's "Geology of Fife and the Lothians," the <i>Memoirs of the Geological Survey of -Scotland</i>, on Sheets 31 and 32, and my Memoir, already cited, <i>Trans. Roy. Soc. Edin.</i> vol. xxix. -(1879) p. 437.</p> - -</div> - -<p>When the puys began their activity, this district was gradually dotted -over with little volcanic cones. At the same time it was affected by the -general movement of slow subsidence which embraced all Central Scotland. -Cone after cone, more or less effaced by the waters which closed over it, was -carried down and buried under the growing accumulation of sediment. New -vents, however, continued to be opened elsewhere, throwing out for a time -their showers of dust and stones, and then lapsing into quiescence as they -sank into the lagoon. Two groups of volcanoes emitted streams of lava and -built up two long volcanic ridges—those of Fife and West Lothian.</p> - -<p>The occasional presence of the sea over the area is well shown by the -occurrence of thin bands of limestone or shale, containing such fossils as -species of <i>Orthoceras</i>, <i>Bellerophon</i> and <i>Discina</i>, which suffice to prove the -strata to be stratigraphical equivalents of the Lower Limestone shale, and -part of the Carboniferous Limestone of England (<a href="#v1fig170">Fig. 170</a>). Yet the -general estuarine or freshwater character of the accumulations seems satisfactorily -established, not only by the absence of undoubtedly marine forms -from most of the strata, but by the abundance of cyprids and small ganoids, -the profusion of vegetable remains, and the occasional seams of coal.</p> - -<p>The portion of the Forth basin within which the puys are displayed -extends from near Leven in Fife, on the north, to Crosswood Burn near the -borders of Lanarkshire, on the south, a distance of about 36 miles, and from -near Culross in Fife and the line of the Almond River between Stirlingshire -and Linlithgowshire, on the west, to the island of Inchkeith on the east, a -distance of about 16 miles (<a href="#Map_IV">Map IV.</a>). But these limits do not precisely mark -the original boundaries of the eruptions. To the north and south, indeed, -we can trace the gradual dying out of the volcanic intercalations, until we -reach ground over which no trace of either lavas or tuffs can be detected. -To the east, the waters of the Firth conceal the geology of a considerable -area, the island of Inchkeith with its bedded lavas and tuffs showing that -these rocks extend some way farther eastwards than the position of that -island. But in Midlothian there is no evidence that any of the puy-eruptions -took place to the east of the line of the Pentland Hills. On the west side, -the volcanic rocks dip under the Millstone Grit and Coal-measures, so that -we do not know how far they extend in that direction. But as the Carboniferous -Limestone series, when it rises again to the surface on the west side -of the Stirlingshire coal-field, is destitute of included lavas and tuffs, the -westward limit of the eruptions cannot lie much beyond the line of the River -Almond. We shall probably be within the mark if we set down the original -area over which puys broke out and spread abroad their lavas and tuffs as -covering about 300 square miles of the lagoons and jungles of Central -Scotland.</p> - -<p><span class="pagenum" id="Page_464">- 464 -</span></p> - -<div class="figcenter" id="v1fig170" style="width: 747px;"> - <img src="images/v1fig170.png" width="747" height="509" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 170.</span>—Junction of amygdaloidal - basalt with shales and limestone, Shore, half a mile east from Kinghorn, Fife. - (From a photograph by Mr. R. Lunn.)</div> -</div> - -<p><span class="pagenum" id="Page_465">- 465 -</span></p> - -<p>I have already shown that the range in geological time of the puy-eruptions -in this district extends from a low horizon among the Calciferous -Sandstones through the Carboniferous Limestone series, up to nearly the -level of the Calmy Limestone, which lies not far from the top of that series. -The vertical thickness of strata between these two stratigraphical limits, when -there are no intercalated volcanic rocks, is probably more than 4000 feet.</p> - -<p>The vents from which the volcanic materials were ejected, so far as they -are now to be observed at the surface, may be divided into two groups, one -lying to the north, the other to the south of the Firth of Forth. The -northern or Fife group may be followed over an area 15 miles long, and -about three miles broad. Some fifteen separate vents may be recognized in -it, distributed chiefly at the two ends of the belt, a cluster of about six rising -around Burntisland, while another of nearly as many appears at Saline. The -characters of some of these necks have been already given in the foregoing -pages.</p> - -<p>The southern or West Lothian group includes about a dozen vents which -are scattered over an area of some 60 square miles, extending from the -coast-line between Borrowstounness and Queensferry southwards to Bathgate -and Uphall. In this group Binns Hill, a mile long by almost half a mile -broad, and rising to a height of nearly 300 feet above the sea, forms the -most prominent individual. But the vents are generally smaller in the -southern than in the northern group.</p> - -<p>The manner in which the vents have been left filled with volcanic -material has been described in previous pages. Most of them are occupied -by tuff or agglomerate. In many cases the neck consists entirely of pyroclastic -detritus, as in most of the vents of eastern Linlithgowshire and many -of those in Fife. Not infrequently a column of basalt has risen in the -funnel and solidified there, as exemplified by Binns Hill and Saline Hill, -or the basalt has filled rents in the tuff and now appears in dykes like those -on the Binn of Burntisland (Figs. <a href="#v1fig148">148</a>, <a href="#v1fig149">149</a>, <a href="#v1fig159">159</a>, <a href="#v1fig166">166</a>, <a href="#v1fig168">168</a>).</p> - -<p>But it is possible that in some cases vents may be represented by bosses -of basalt or dolerite, unaccompanied by any agglomerate or tuff. Perhaps -the best example of this suggested origin is supplied by Galabraes Hill, which -rises through the Hurlet limestone and the volcanic series of the Bathgate -Hills, about a mile north-east from the town of Bathgate. This eminence -rises to a height of 940 feet above the sea, and consists of a rudely elliptical -boss of basalt, measuring 3500 feet in its greater and 3000 feet in its minor -axis. It disrupts the sedimentary and volcanic series, which can be traced -up to it on all sides. Some of the smaller circular or elliptical bosses in -eastern Linlithgowshire and western Fife may perhaps belong to the same -category. But undoubtedly most of the intrusive basalts and dolerites of -this volcanic region are sills.</p> - -<p><span class="pagenum" id="Page_466">- 466 -</span></p> - -<p>Over the greater part of the district, only fine tuffs were ejected. -These occur as interstratifications among the ordinary sediments, and vary -from mere thin partings, marking the feeblest and briefest explosions, up to -continuous accumulations several hundred feet thick. As an example of the -least vigorous emission of tuff I may refer to the sections already given on -pp. <a href="#Page_437">437</a>, <a href="#Page_438">438</a>. The thicker bands are well illustrated by that which lies some -way above the Houston Coal, between Drumcross and West Broadlaw in Linlithgowshire, -and by the great mass of tuff which occurs immediately below the -Calmy Limestone on the River Avon near Linlithgow Bridge, and which -may be 300 feet thick.</p> - -<p>It is a striking characteristic of the tuffs that they may be met with in -their solitary beds intercalated in the midst of ordinary sediments at a -distance from any other trace of volcanic activity, their parent vents not -being visible. I may cite in illustration an interesting case in the Swear -Burn, near the southern end of the volcanic district. A band of tuff about -ten feet thick lies there intercalated in a group of dark shales and thin coals. -Below it there is a seam of coal four inches thick, and among the blue shales -overlying it is another coal ten inches thick. The tuff is pale green, almost -white in colour, fine in texture, like a volcanic mud, while some of its component -beds, one foot in thickness, are made up of fine laminæ and are even -false-bedded. We might infer that the volcanic vent lay at some -distance, so that only the finest dust fell over the swamps in which the coal-vegetation -was accumulating, but for the presence of occasional blocks of -basalt one foot in diameter scattered through the tuff. When the eruptions -ceased, the deposition of ordinary mud and the accumulation of plant-remains -went on as before, and animal life crowded in again over the spot, for between -the partings of the coal above the tuff, abundant fragments of eurypterids -and scorpions may be found.</p> - -<p>One of the most extensive volcanic discharges of fragmentary material -was that which produced the "Houston marls" already referred to. These -strata appear to mark a peculiar phase in the volcanic history of the Lower -Carboniferous rocks of the Firth of Forth, when exceedingly fine ash, or -perhaps even volcanic mud, was erupted in considerable quantity. The -"marls" attain in some places a thickness of nearly 200 feet, and can be -traced through most of the eastern part of Linlithgowshire, over an area of -perhaps more than 50 square miles. This volcanic platform, which has been -followed in mining for oil-shale, is one of the most extensive among the puy-eruptions. -The material, which probably came from one or more vents -among the Bathgate Hills, is not always of equal fineness, but passes into -and even alternates with ordinary granular tuff. Thus in the Niddry Burn, -above Ecclesmachan, the dull sage-green and brownish red Houston marls -contain a few inconstant layers of green tuff, in which may be noticed pieces -of black shale and lapilli of the usual basic pumice. Not far to the west, -beyond Wester Ochiltree, and thus probably nearer to the active vents that -ejected the dust and ashes, the Houston marls are replaced by or include a -bedded granular tuff or basalt-agglomerate, which lies above the 2-feet coal -<span class="pagenum" id="Page_467">- 467 -</span> -of the district. The matrix of this rock is in part a dull green granular -mudstone, wrapping round the lapilli and ejected stones, which, when they -fall out under the action of the weather, leave casts of their forms behind -them. The enclosed fragments vary in size up to blocks three feet in -diameter, and consist in great measure of a compact volcanic grit, composed of -a fine mud mixed with minute fragments of black shale, grains of sand and -flakes of mica. There are likewise blocks of cement-stone and shale. Thin -courses of black shale interlaminated with the tuff show its bedding.</p> - -<p>The thickest and most continuous accumulations of tuff occur round some -of the larger tuff cones, particularly round the Saline Hills, and in the -Burntisland district. In the first-named area the copious eruptions of -fragmentary material brought the volcanic history there to an end; but -around Burntisland they were only the prelude to a prolonged and varied -series of discharges.</p> - -<p>I have already remarked that in the area of the puys of the Forth-basin, -while the majority of the vents were tuff-cones, and emitted only fragmentary -discharges, there were two well-marked tracts where lavas were poured out -extensively and during a long geological interval. One of those lies in the -southern, the other in the northern area.</p> - -<p>The southern or Linlithgowshire lava-ridge forms now what are known -as the Bathgate and Linlithgow Hills. The lavas extend for about 14 miles -from north to south, dying out in both directions, while their present visible -breadth is about three miles at its widest part. The highest summit reaches -a height of about 1000 feet above the sea. The structure of this long ridge -reveals an interesting record of volcanic eruptions. It consists mainly of -sheets of basalt, sometimes separated by layers of tuff (<a href="#v1fig155">Fig. 155</a>). But -on one or two horizons the volcanic rocks cease, and ordinary sedimentary -deposits take their place. As has been already stated, the Main or Hurlet -Limestone can be traced through the heart of the volcanic masses. This -seam attains there an exceptional thickness of as much as 70 to 80 feet, -and is nowhere more abundantly fossiliferous. During its deposition there -seems to have been a subsidence of the area, together with a cessation of -volcanic activity for a time. The crinoids, corals, brachiopods, bryozoa, -lamellibranchs, gasteropods, cephalopods and fishes, which swarmed in the -clear water, built up a thick calcareous layer above the lavas and tuffs of -the sea-bottom.</p> - -<p>Among the sandstones and shales that cover the limestone, bands of tuff -make their appearance, indicating the renewal of volcanic activity. These -are immediately surmounted by another thick pile of basalt-sheets. Subsequently, -during pauses in the eruptions, while the general subsidence continued, -renewed deposits of sediment spread over the submerged volcanic -bank. One of the longest periods of quiescence was that during which the -coals and even the Index Limestone of Bathgate crept northwards over the -sunken lavas and tuffs. But the whole of the ridge does not seem to have -disappeared at that time under water, at least these intercalated strata have -not been traced across the thick pile of volcanic material near Linlithgow. -<span class="pagenum" id="Page_468">- 468 -</span> -During the final period of eruption, the outpouring of lava and discharge of -ashes, neither in united thickness nor in horizontal extent, equalled those -which had preceded them. When the volcanoes ceased their activity, the area -continued to sink, and over the submerged lavas marine organisms crowded -the sea-floor, so as to build up the Calmy Limestone. After that time -volcanic action seems to have become extinct in this region, for no trace of -any intercalated lava or tuff has been detected either in the overlying -Millstone Grit or in the Coal-measures. The total thickness of rock in the -Linlithgowshire volcanic ridge is about 2200 feet. It will probably not be -an exaggeration to place the proportion of lava and tuff in that depth of -material at nearly 2000 feet.</p> - -<p>The northern or Fifeshire district over which lavas were abundantly -erupted stretches along the coast from Aberdour to Kirkcaldy and inland to -near Lochgelly, as well as seawards to Inchkeith. It may comprise an area -of about 30 square miles. In many respects this is the most important -locality in Britain for the study of Carboniferous volcanic history. The sea -has cut an admirable coast-section in which the structures of the rocks are -laid bare. The bottom and top of the whole volcanic series can be seen. -The vents and their relation to the lavas and tuffs that were emitted from -them may easily be made out; while the interstratification of well-known -seams of rock in the Scottish Carboniferous system permits the sequence -and chronology of the whole volcanic series to be traced with great -clearness.</p> - -<p>Most of these features have already been described in foregoing pages, -for the district is a typical one for the study of Carboniferous volcanic -phenomena. Thus the group of vents about Burntisland has been illustrated -by the Binn of Burntisland rising among the bedded lavas and tuffs. The -characters of the Carboniferous basalt-sheets have been enumerated, together -with their intercalated layers of tuff and bole, and their fine partings of -ashy material that was thrown out over the lagoons during the intervals -between two outbursts of lava. But it may be of service if I insert here -a detailed section of the whole volcanic series as it is displayed along the -coast-section between Burntisland and Kinghorn. The lowest intercalated -lavas of that section lie a little above the horizon of the Burdiehouse -Limestone, and are thus probably rather earlier than those of Linlithgowshire. -The highest reach up to the base of the Hurlet Limestone. The -volcanic energy manifestly died out here long before it ceased on the south -side of the Firth. Yet so vigorous was its activity while it continued, -that it piled up one of the thickest masses of volcanic material anywhere -to be seen among the puy-eruptions of the British Isles. The following -tabular statement of the alternations of material in this great mass in -descending order, was drawn up by me on the ground many years ago, -before the construction of fortifications and other changes partly concealed -the rocks.</p> - -<p><span class="pagenum" id="Page_469">- 469 -</span></p> - -<div class="figcenter" id="v1fig171" style="width: 759px;"> - <img src="images/v1fig171.png" width="759" height="494" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 171.</span>—Columnar basalt, Pettycur, Kinghorn, Fife. (From a photograph taken for the Geological Survey by Mr. R. Lunn.)</div> -</div> - -<p><span class="pagenum" id="Page_470">- 470 -</span></p> - - -<p><span class="smcap">Section of the Volcanic Series below the Hurlet or Main Limestone on the -Coast of Fife, west of Kinghorn, in descending order</span><a id="FNanchor_477" href="#Footnote_477" class="fnanchor">[477]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_477" href="#FNanchor_477" class="label">[477]</a> The succession of rocks in this interesting coast-section was briefly given by Dr. P. Neill in -his translation of Daubuisson's <i>Basalts of Saxony</i>, Edinburgh, 1814, note <i>f</i>, p. 215. He was secretary -of the Wernerian Society, and in his enumeration the Wernerian terminology is used without -a hint that any single band in the whole series is of volcanic origin.</p> - -</div> - -<div class="blockquot"> -<p>75. Reddish and white sandstones.</p> - -<p>74. Shale with hard ribs of limestone and ironstone nodules. Fossils abundant.</p> - -<p>73. Limestone, crinoidal, 8 or 9 feet.</p> - -<p>72. Blue shale, becoming calcareous towards the top, where shells are plentiful.</p> - -<p>71. Reddish false-bedded sandstones, with bands of reddish and blue shale.</p> - -<p>70. Basalt in two sills separated by 2 or 3 feet of sandstone and shale.</p> - -<p>69. Dark fissile sandy shale, passing up into white shaly sandstone, and including a thin -parting of impure coal.</p> - -<p>68. Limestone (<span class="smcap">Hurlet</span> or <span class="smcap">Main Seam</span>) in a number of bands having a united thickness -of 25 feet. Abundant fossils.</p> - -<p>67. Black shale becoming calcareous at top, and then enclosing abundant <i>Productus</i>, etc., -8 or 10 feet.</p> - -<p>66. Red and green tufaceous marl and tuff. About 30 feet.</p> - -<p>65. Basalt, the lower part strongly amygdaloidal.</p> - -<p>64. Tufaceous red marl and tuff; comparatively coarse below, becoming finer above, 3 -or 4 feet.</p> - -<p>63. Basalt, earthy and amygdaloidal, with an irregular bottom involving masses of the -shales below.</p> - -<p>62. Dark calcareous shale and dull green tufaceous marly shale, 2 or 3 feet.</p> - -<p>61. Crinoidal limestone in several bands with a united thickness of 10 feet.</p> - -<p>60. Shale, 1 foot.</p> - -<p>59. Fine green sandy tuffs in a number of bands of varying coarseness, about 6 feet.</p> - -<p>58. Dark shale with abundance of <i>Aviculopecten</i> immediately under the tuffs above, -1½ feet.</p> - -<p>57. Soft, light, marly shale with fragmentary plants, 1½ feet.</p> - -<p>56. Dark fissile shale, full of fish-scales, plants, etc., 3 feet.</p> - -<p>55. Basalt, rudely columnar, dark fine-grained in centre, becoming highly amygdaloidal -and scoriaceous at bottom and top.</p> - -<p>54. Basalt, like the sheet above, vesicular at top and bottom, with a parting of red clay -on top.</p> - -<p>53. Fissile rippled sandy shale, with plants, having a red and green marly parting at the -top, 12 or 14 feet.</p> - -<p>52. Basalts; a group of beds, probably in part sills, involving three bands of sandstone -or quartzite.</p> - -<p>51. Quartzite—a hard white altered sandstone, 2 to 3 feet.</p> - -<p>50. Basalt, light green, earthy, amygdaloidal.</p> - -<p>49. Sandstones and shales with plants, 25 feet.</p> - -<p>48. Basalt, with a highly amygdaloidal central band. There may be several sheets here.</p> - -<p>47. Green tufaceous shale and marl, 1 foot.</p> - -<p>46. Basalt, dark, firm and amygdaloidal.</p> - -<p>45. Sandstones and shales with plants.</p> - -<p>44. Basalt forming west side of Kinghorn Bay, and including more than one sheet. -The rock is very black, compact, irregularly columnar, with the usual amygdaloidal -earthy band at the base, and forms the crag called the Carlinehead Rocks. An -irregular and inconstant band of dull green tufaceous shale, sometimes 2 feet thick, -serves to separate two of the basalt-sheets. Below it lies a remarkable scoriaceous -almost brecciated basalt, which has been broken up on cooling in such a manner -that at first it might be mistaken for a volcanic conglomerate.</p> -</div> - -<p><span class="pagenum" id="Page_471">- 471 -</span></p> - -<div class="blockquot"> -<p>43. Basalt, a compact black solid rock, with a vesicular and amygdaloidal bottom, about -40 feet. This sheet runs out into the promontory of Kinghorn Ness.</p> - -<p>42. Basalt, firm, compact and highly amygdaloidal throughout, 15 feet.</p> - -<p>41. Basalt, earthy, amygdaloidal and scoriaceous in the upper part, compact below.</p> - -<p>40. Red tufaceous marl, clay or bole, a few inches thick.</p> - -<p>39. Basalt: one of the most compact sheets of the whole series, about 40 feet. The -top is formed of a thick zone of scoriaceous and brecciated material, the bottom is -singularly uneven owing to the very irregular surface of the underlying bed.</p> - -<p>38. Basalt more or less scoriaceous throughout, especially at the bottom, the vesicles being -drawn out round the slag-like blocks.</p> - -<p>37. Green tufaceous shales with bands of fine green tuff, 7 to 8 feet. The lower bands -consist of a gravelly tuff passing up into a fine volcanic mudstone, with scattered -lapilli of basalt an inch or more in diameter.</p> - -<p>36. Basalt, with an upper, earthy and highly amygdaloidal portion, 30 feet.</p> - -<p>35. Tufaceous sandstone and shale, 6 to 8 feet.</p> - -<p>34. Basalt, in a thick bed, having an earthy, slaggy top and a scoriaceous bottom.</p> - -<p>33. Basalt, very slaggy below with a compact centre.</p> - -<p>32. Basalt, like that below it.</p> - -<p>31. Basalt, firm, compact, black rock, with a rough, green earthy band, from 6 inches to a -foot, at the bottom, and becoming again very slaggy at the top.</p> - -<p>30. Green shale like that below the underlying limestone, a few inches in thickness.</p> - -<p>29. Coarse, green, sandy tufaceous limestone, averaging 1 foot in thickness.</p> - -<p>28. Black shale with plants, 12 or 14 feet, becoming green and tufaceous at the top.</p> - -<p>27. Basalt—the most striking of the whole section—a fine compact black olivine-bearing -rock, beautifully columnar, 30 feet. The columns reach to within a foot of the -bottom of the bed and cease about 10 feet from the top, the upper portion of the -bed being massive, with vesicles which are drawn out parallel to the bedding of the -series. The lowest part of the bed is a broken brecciated band, 3 or 4 inches thick. -(See <a href="#v1fig171">Fig. 171</a>.)</p> - -<p>26. Black shale with fragmentary plants, 3 feet.</p> - -<p>25. Basalt, with plentiful olivine, 12 to 16 feet. The base is not highly scoriaceous, but -finely vesicular. Towards the top it becomes green, earthy and roughly brecciated. -It partly cuts out the tuff underneath.</p> - -<p>24. Tuff, green, fine-grained and well-stratified, consisting chiefly of fine volcanic dust, -but becoming coarser towards the top, where it contains lapilli and occasional -bombs of highly vesicular lavas.</p> - -<p>23. Black carbonaceous shale, 3 feet; approaching to the character of an impure coal in -the lower part, and becoming more argillaceous above with some thin nodular -calcareous bands.</p> - -<p>22. Green tuff, 12 feet, well stratified and fine-grained, with minute lapilli of highly -vesicular basic lavas; becomes shaly at the bottom.</p> - -<p>21. Basalt, compact, amygdaloidal, with highly vesicular upper surface, 20 feet.</p> - -<p>20. Basalt, hard, black and full of olivine; an irregular bed 3 to 6 feet thick.</p> - -<p>19. Basalt, dull brownish-green to black, full of kernels and strings of calcite, and showing -harder and softer bands parallel with upper and under surfaces, which give it -a stratified appearance.</p> - -<p>18. Basalt, some parts irregularly compact, others earthy and scoriaceous. The distinguishing -feature of this bed is the abundance of its enclosed fragments of shale, -ironstone and limestone, which here and there form half of its bulk. The -roughly scoriaceous upper portion is especially full of these fragments. In the -ironstone balls coprolites may be detected, and occasional pieces of plant-stems are -embedded in the basalt. This lava has evidently broken up and involved some -of the underlying strata over which it flowed. This rock overhangs Pettycur -Harbour.</p> - -<p>17. Shales and limestone bands more or less tufaceous, 8 to 10 feet, with plants, cyprids, -etc. The intercalation of fine partings of tuff in this band has been already -cited on <a href="#Page_438">p. 438</a>, as an illustration of the feeble intermittent character of many -of the volcanic explosions between successive outflowings of lava.</p> -</div> - -<p><span class="pagenum" id="Page_472">- 472 -</span></p> - -<div class="blockquot"> -<p><span style="padding-left: 2em;">Owing</span> to a change in the direction of strike the rocks now wheel round and for -a time run nearly parallel with the coast-line, while they are partly concealed by -blown sand and herbage. The shales and limestones just mentioned are not constant, -and are soon lost, but about a quarter of a mile westward a band of tuff begins -on the same horizon or near it, and increases in thickness towards the west, where -it is associated with other sediments. The shore ceases to furnish a continuous -section, so that recourse must be had to the craggy slopes immediately to the north, -where the rocks can be examined on a cliff face (<a href="#v1fig153">Fig. 153</a>). There the tuff just referred -to, together with some overlying bands of sandstone, is seen to pass under the -group of basalts last enumerated. It is a green, stratified rock, perhaps 60 feet thick -at its maximum, but dying out rapidly to north-west and south-east. It encloses balls -of basalt and subangular and rounded fragments of sandstone, limestone and shale. -A mass of coarse volcanic agglomerate which is connected with it and cuts across -the ends of some of the basalts below, probably marks the position of the vent from -which the tuff was ejected (<a href="#v1fig152">Fig. 152</a>).</p> - -<p>16. Black and grey shales forming a thin band at the summit of King Alexander's Crag.</p> - -<p>15. Basalt, dark compact rock, with an upper and lower highly scoriaceous and amygdaloidal -band, 15 feet.</p> - -<p>14. Black shales, tufaceous green shales, sandstone, and 6 inches of coal, forming a -group of strata about 12 feet thick between two basalts; plants and cyprids -abundant. (The coal seam is shown in <a href="#v1fig151">Fig. 151</a>.)</p> - -<p>13. Basalt, dull, earthy and highly amygdaloidal, with abundant calcite in kernels and -veins; about 15 feet, but varying in thickness.</p> - -<p>12. Basalt, forming a well-marked bed from 12 to 25 feet thick. It is a compact black -olivine-bearing rock, sparingly amygdaloidal, but showing the usual dull green, -earthy scoriform base. The upper surface is singularly irregular, having, in flowing, -broken up into large clinker-like blocks, which are involved in the immediately -overlying basalt. The bottom also is very uneven, for the basalt has in some places -cut out the underlying shales, so as to rest directly upon the basalt below.</p> - -<p>11. Black shale, varying up to 6 inches, but sometimes entirely removed by the overlying -lava-stream.</p> - -<p>10. Basalt, containing large irregularly spheroidal masses of hard black finely vesicular -material enclosed in more earthy and coarsely vesicular rock. The vesicles are -sometimes elongated parallel to the bedding, but have often been drawn out round -a spheroid; some of them measure nearly a foot in length by 2 or 3 inches in -breadth. The upper surface is uneven and coarsely amygdaloidal.</p> - -<p>9. Basalt, hard black, with abundant olivine, and a columnar structure.</p> - -<p>8. Green shale, 6 inches to 1 foot, much baked and involved in the overlying basalt.</p> - -<p>7. Basalt, dull-green, earthy, amygdaloidal, varying from 10 to 40 feet in thickness.</p> - -<p>6. Blue shale, disappearing where the basalt above it unites with that below.</p> - -<p>5. Basalt with olivine, forming a thick irregular bed, which in some places is black and -compact, in others green, earthy and amygdaloidal. The upper part is particularly -cellular.</p> - -<p>4. Sandstones forming a thick group of beds which have long been quarried for building-stone -at the Grange and elsewhere.</p> - -<p>3. Black shales.</p> - -<p>2. Limestone (<span class="smcap">Burdiehouse</span>).</p> - -<p>1. Sandstones, shales and thin limestones forming the strata at Burntisland through -which the sills of that district have been injected (<a href="#v1fig159">Fig. 159</a>).</p> -</div> - -<p>The phenomena of sills are abundantly developed among the Carboniferous -rocks of the basin of the Firth of Forth, and some of the more -remarkable examples in this district have been already cited. Taking now -a general survey of this part of the volcanic history, I may observe that if -the sills are for a moment considered simply as they appear at the surface, -<span class="pagenum" id="Page_473">- 473 -</span> -and apart from the geological horizons on which they lie, they form a wide -ring surrounding the Falkirk and Stirlingshire coal-field.</p> - -<p>Beginning at the Abbey Craig, near Stirling, we may trace this ring as -a continuous belt of high ground from Stirling to the River Carron. Thence -it splits up into minor masses in different portions of the Carboniferous -system, and doubtless belonging to different periods of volcanic disturbance, -but yet sweeping as a whole across the north-eastern part of the Clyde -coal-field, and then circling round into Stirlingshire and Linlithgowshire. -There are no visible masses to fill up the portion of the ring back to Abbey -Craig. But through Linlithgowshire and the west of Edinburghshire a -number of intrusive sheets form an eastward prolongation of the ring. -Large as some of these sheets are at the surface, for they sometimes exceed two -or three square miles in area, a much larger portion of their mass is generally -concealed below ground. Mining operations, for example, have proved that -in the south-east of Linlithgowshire areas of intrusive rock which appear as -detached bosses or bands at the surface are connected underneath as portions -of one continuous sill, which must be several square miles in extent.</p> - -<div class="figcenter" id="v1fig172" style="width: 457px;"> - <img src="images/v1fig172.png" width="457" height="91" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 172.</span>—Section across the Fife band of Sills.<br /> - 1. Upper Old Red Sandstone; 2. Calciferous Sandstones; 3. Carboniferous Limestone series; 4. Millstone Grit; - 5. Coal-measures; 6. Dolerite Sills. <i>f</i>, Fault.</div> -</div> - -<p>But it is in Fife that the sills reach their greatest development among -the Carboniferous rocks of Scotland (<a href="#v1fig172">Fig. 172</a>). A nearly continuous belt of -them runs from the Cult Hill near Saline on the west, to near St. Andrews -on the east, a distance of about 35 miles. This remarkable band -is connected with a less extensive one, which extends from Torryburn on -the west, to near Kirkcaldy on the east. In two districts of the Fife region -of sills, a connection seems to be traceable between the intrusive sheets and -volcanic vents, at least groups of necks are found in the midst of the sills. -One of these districts is that of the Saline Hills already described, the other -is that of Burntisland. In the latter case the evidence is especially striking, -for the vents are connected above with bedded lavas and tuffs, while below -lie three well-marked sills (<a href="#v1fig159">Fig. 159</a>).</p> - -<p>It is certainly worthy of remark that sills are generally absent from -those areas where no traces of contemporaneous volcanic activity are to be -found. No contrast in this respect can be stronger than that between the -ground to the east and west of the old axis of the Pentland Hills. In the -western district, where the puys are so well displayed, sills abound, but in -the eastern tract both disappear.</p> - -<p>Another question of importance in dealing with the history of these sills -is their stratigraphical position. By far the larger proportion of them lies -<span class="pagenum" id="Page_474">- 474 -</span> -in the Carboniferous Limestone series. Thus the great sill between Stirling -and Kilsyth keeps among the lower parts of that series. On the same -general horizon are the vast sheets of dolerite which stretch through Fife in -the chain of the Cult, Cleish, and Lomond Hills on the one side, and in the -eminences from Torryburn to Kinghorn on the other, though the intrusive -material sometimes descends almost to the Old Red Sandstone. In Linlithgowshire -and Edinburghshire, as well as in the south of Fife, the sills -traverse the Calciferous Sandstone groups.</p> - -<p>If the horizons of the sills furnished any reliable clue to their age, it -might be inferred that the rocks were all intruded during the Carboniferous -period, and as most of them lie beneath the upper stratigraphical limit of -the puy-eruptions, the further deduction might be drawn that they are -connected with these eruptions. I have little doubt that in a general sense -both conclusions are well-founded. But that there are exceptions to the -generalization must be frankly conceded. On close examination it will be -observed that the same intrusive mass sometimes extends from the lower -into the upper parts of the Carboniferous groups. Thus, in the west of -Linlithgowshire, a large protrusion which lies upon the Upper Limestones, -crosses most of the Millstone Grit, and reaches up almost as high as the -Coal-measures. Again, in Fife, to the east of Loch Leven, a spur of the -great Lomond sill, crossing the Carboniferous limestone, advances southward -into the coal-field of Kinglassie, In Stirlingshire and Lanarkshire numerous -large dolerite sheets have invaded the Millstone Grit and Coal-measures, -including even the upper red sandstones, which form the top of the Carboniferous -system in this region. It is thus obvious that if the puy-eruptions -in the basin of the Forth ceased towards the close of the deposition of the -Carboniferous Limestone series, there must have been a subsequent injection -of basic lava on a gigantic scale in central Scotland. I shall recur to this -subject in <a href="../../66493/66493-h/66493-h.htm#CHAPTER_XXXI">Chapter xxxi.</a></p> - -<div class="figcenter" id="v1fig173" style="width: 505px;"> - <img src="images/v1fig173.png" width="505" height="98" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 173.</span>—Section across the Upper Volcanic Band of north Ayrshire. Length about four miles.<br /> - 1. Andesite lavas of the Clyde Plateau; 2. Tuffs closing the Plateau volcanic series; 3. Hurlet Limestone; - 4. Carboniferous Limestone series with coal-seams; 5. Lower tuff zone of the Upper volcanic band; 6. - Basic lavas; 7. Upper tuff zone; 8. Basic sill; 9. Coal-measures.</div> -</div> - - -<h3>2. NORTH OF AYRSHIRE</h3> - -<p>In this part of the country another group of puys and their associated -tuffs and lavas may be traced from near Dairy on the west, to near Galston -on the east (<a href="#Map_IV">Map IV.</a>). The length of the tract is about sixteen miles, while -its breadth varies from about a furlong to nearly a mile and a half. I have -had occasion to allude to this marked band of volcanic materials which here -<span class="pagenum" id="Page_475">- 475 -</span> -intervenes between the Carboniferous Limestone and the Coal-measures, and -from its position appears to mark the latest Carboniferous volcanoes. Its -component rocks reach a thickness of sometimes 600 feet, and as they dip -southwards under the Coal-measures, they may extend for some distance in -that direction. They have been met with in borings sunk through the -northern part of the Irvine coal-field. Even what of them can be seen at -the surface, in spite of the effects of faults and denudation, shows that they -form one of the most persistent platforms of volcanic rock among the puy-eruptions -of Scotland.</p> - -<p>Where best developed this volcanic band has a zone of tuff at the -bottom, a central and much thicker zone of bedded basalts, and an upper -group of tuffs, on which the Coal-measures rest conformably. A few vents, -probably connected with it, are to be seen at the surface between Fenwick -and Ardrossan. But others have been buried under the Carboniferous sedimentary -rocks, and, as already described, have been discovered in the underground -workings for coal and ironstone (<a href="#Page_434">p. 434</a>). These mining operations -have, indeed, revealed the presence of far more volcanic material below -ground than would be surmized from what can be seen at the surface. -Here and there, thin layers of tuff appear in brook-sections, indicating what -might be conjectured to have been trifling discharges of volcanic material. -But the prosecution of the ironstone-mining has proved that, at the time -when the seam of Black-band Ironstone of that district was accumulated, the -floor of the shallow sea or lagoon where this deposition took place was dotted -over with cones of tuff, in the hollows between which the ferruginous and -other sediments gathered into layers. That seam is in one place thick and -of good quality; yet only a short distance off it is found to be so mixed -with fine tuff as to be worthless, and even to die out altogether.<a id="FNanchor_478" href="#Footnote_478" class="fnanchor">[478]</a></p> - -<div class="footnote"> - -<p><a id="Footnote_478" href="#FNanchor_478" class="label">[478]</a> See Explanation of Sheet 22, <i>Geol. Surv. of Scotland</i>, pars. 29, 33, 45.</p> - -</div> - - -<h3>3. LIDDESDALE</h3> - -<p>A remarkable development of puys lies in that little-visited tract of -country which stretches from the valleys of the Teviot and Rule Water -south-westwards across the high moorland watershed, and down Liddesdale. -Through this district a zone of bedded olivine-basalts and associated tuffs -runs in a broken band which, owing to numerous faults and extensive -denudation, covers now only a few scattered patches of the site over which -it once spread. The geological horizon of this zone lies in the Calciferous -Sandstones, many hundred feet above the position of the top of the plateau-lavas -(<a href="#Map_IV">Map IV.</a>).</p> - -<p>So great an amount of material has been here removed by denudation -that not only has the volcanic zone been bared away, but the vents which -supplied its materials have been revealed in the most remarkable manner -over an area some twenty miles long and eight miles broad. Upwards of -forty necks of agglomerate may be seen in this district, rising through the -Silurian, Old Red Sandstone, and lowest Carboniferous rocks. It fills the -<span class="pagenum" id="Page_476">- 476 -</span> -geologist with wonder to meet with those stumps of old volcanoes far to the -west among the Silurian lowlands, sometimes fully ten miles away from the -nearest relic of the bedded lavas connected with them.<a id="FNanchor_479" href="#Footnote_479" class="fnanchor">[479]</a> That these vents, -though they rose through ground which at the time of their activity was -covered with the volcanic series of the plateaux, do not belong to that series, -but are of younger date, has been proved in several cases by Mr. Peach. -He has found that among the blocks composing their agglomerates, pieces -of the sandstones, fossiliferous limestones and shales of the Cement-stone -group, overlying the plateau-lavas, are to be recognized. These vents were -therefore drilled through some part at least of the Calciferous Sandstones, -which are thus shown to have extended across the tract dotted with vents. -After the volcanic activity ceased, fragments of these strata tumbled down -from the sides into the funnels. Denudation has since stripped off the -Calciferous Sandstones, but the pieces detached from them, and sealed up at -a lower level in the agglomerates, still remain. Mr. Peach's observations -indicate to how considerable an extent sagging of the walls of these orifices -took place, with the precipitation not merely of blocks, but of enormous -masses of rock, into the volcanic chimneys. In one instance, between -Tudhope Hill and Anton Heights, a long neck, or perhaps group of necks, -which crosses the watershed, shows a mass of the red sandstone many acres -in extent, and large enough to be inserted on the one-inch map, which has -fallen into the vent (<a href="#v1fig175">Fig. 175</a>).</p> - -<div class="footnote"> - -<p><a id="Footnote_479" href="#FNanchor_479" class="label">[479]</a> They have been recognized and mapped by Mr. B. N. Peach for the Geological Survey. See -Sheets 11 and 17, <i>Geol. Surv. Scotland</i>.</p> - -</div> - -<div class="figcenter" id="v1fig174" style="width: 520px;"> - <img src="images/v1fig174.png" width="520" height="92" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 174.</span>—Section showing the connection of the two volcanic bands in Liddesdale.<br /> - 1. Upper Silurian strata; 2. Upper Old Red Sandstone; 3. The lavas of the Solway plateau; 4. Agglomerate - neck with lava plug, belonging to the plateau system; 5. Calciferous Sandstone series; 6. Thick Carboniferous - Limestones; 7. Tuff, and 8. Lavas, of the upper volcanic band, connected with the puys; 9. Agglomerate - neck with lava plug belonging to the puy-system; 10. Basic sill.</div> -</div> - -<div class="figcenter" id="v1fig175" style="width: 373px;"> - <img src="images/v1fig175.png" width="373" height="93" alt="" /> - <div class="hanging2"><span class="smcap">Fig. 175.</span>—Diagram to show the position of a mass of Upper Old Red Sandstone which has fallen into - the great vent near Tudhope Hill, east of Mosspaul.<br /> - 1. Upper Silurian strata; 2. Outlier of Upper Old Red Sandstone; 2´. Large mass of this formation in the vent; - 3. Agglomerate of the neck with andesite intrusion (4).</div> -</div> - -<p>The materials ejected from the Liddesdale vents include both basaltic -lavas and tuffs. The former are sometimes highly vesicular, especially -<span class="pagenum" id="Page_477">- 477 -</span> -along the upper parts of the flows. They are thickest towards the north, -and in Windburgh Hill attain a depth of at least 300 or 400 feet. In -that part of the district they form the lower and main part of the volcanic -series, being there covered by a group of tuffs. But a few miles southwards, -not far to the west of Kershopefoot, they die out. The tuffs then form the -whole of the volcanic band which, intercalated in a well-marked group of -limestones, can be followed across the moors for some six miles into the -valley of the Esk, where an interesting section of them and of the associated -limestone and shales is exposed (<a href="#v1fig174">Fig. 174</a>). At Kershopefoot, a higher -band of basic lava overlies the Kershopefoot limestone, and can be traced in -scattered patches both on the Scottish and English side of the Border.</p> - - - - -<p class="tdc caption3nb">END OF VOL. I.</p> - - - - -<p class="pmt2 pmb4 tdc smaller"><i>Printed by</i> -<span class="smcap">R. & R. 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