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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. - -Title: The Ancient Volcanoes of Great Britain, Volume I (of 2) - -Author: Archibald Geikie - -Release Date: January 4, 2022 [eBook #66492] - -Language: English - -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) - -*** START OF THE PROJECT GUTENBERG EBOOK THE ANCIENT VOLCANOES OF -GREAT BRITAIN, VOLUME I (OF 2) *** - - - - - -Transcriber Note - -Text emphasis denoted by _Italics_ and =Bold=. - - - - -THE ANCIENT VOLCANOES OF GREAT BRITAIN - -[Illustration] - - - - - THE - - ANCIENT VOLCANOES - - OF - - GREAT BRITAIN - - BY - - SIR ARCHIBALD GEIKIE, F.R.S. - - D.C.L. Oxf., D. Sc. Camb., Dubl.; LL.D. St. And., Edinb. - - 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. - - - WITH SEVEN MAPS AND NUMEROUS ILLUSTRATIONS - - - - - IN TWO VOLUMES - - VOL. I - - - - - London - - MACMILLAN AND CO., Limited. - - NEW YORK: THE MACMILLAN COMPANY. - - 1897 - - _All rights reserved_ - - - - - - TO - - M. Ferdinand Fouqué - - MEMBER OF THE INSTITUTE - - PROFESSOR OF THE NATURAL HISTORY OF INORGANIC BODIES - IN THE COLLÈGE DE FRANCE - - AND - - M. Auguste Michel-Lévy - - MEMBER OF THE INSTITUTE - - DIRECTOR OF THE GEOLOGICAL SURVEY OF FRANCE - - DISTINGUISHED REPRESENTATIVES - OF THAT FRENCH SCHOOL OF GEOLOGY - WHICH BY THE HANDS OF DESMAREST FOUNDED THE - STUDY OF ANCIENT VOLCANOES - AND HAS SINCE DONE SO MUCH TO - PROMOTE ITS PROGRESS - THESE VOLUMES ARE INSCRIBED - WITH THE HIGHEST ADMIRATION AND - ESTEEM - - - - -PREFACE - - -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. - -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 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. - -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 _Theory of the Earth_, 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. - -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 -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. - -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. - -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. - -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. - -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. 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. - -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. - -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. - -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. - -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 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. - -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. - -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. - -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 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. - -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. - -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 -_Transactions_ and _Journal_. 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. - -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. - -Geological Survey Office, - 28 Jermyn Street, London, - _1st January 1897_. - - - - - CONTENTS - - - BOOK I - - GENERAL PRINCIPLES AND METHODS OF INVESTIGATION - - - CHAPTER I - - PAGE - - 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 1 - - - CHAPTER II - - The Nature and Causes of Volcanic Action--Modern Volcanoes 10 - - - CHAPTER III - - 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 14 - - - CHAPTER IV - - Materials erupted at the Surface--Extrusive Series--_continued_. - 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 39 - - - CHAPTER V - - 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 52 - - - CHAPTER VI - - Underground Phases of Volcanic Action--_continued_. 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 77 - - - CHAPTER VII - - Influence of Volcanic Rocks on the Scenery of the Land--Effects of - Denudation 100 - - - BOOK II - - VOLCANIC ACTION IN PRE-CAMBRIAN TIME - - - CHAPTER VIII - - Pre-Cambrian Volcanoes - - 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 109 - - - BOOK III - - THE CAMBRIAN VOLCANOES - - - CHAPTER IX - - Characteristics of the Cambrian System in Britain - - 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 139 - - - CHAPTER X - - The Cambrian Volcanoes of South Wales 145 - -CHAPTER XI - - The Cambrian Volcanoes of North Wales, the Malvern Hills and - Warwickshire 159 - - - BOOK IV - - THE SILURIAN VOLCANOES - - CHAPTER XII - - Characters of the Silurian System in Britain. The Arenig Volcanoes - - 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 173 - - - CHAPTER XIII - - The Eruptions of Llandeilo and Bala Age - - 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 202 - - - CHAPTER XIV - - The Silurian Volcanoes of Ireland 239 - - - BOOK V - - THE VOLCANOES OF DEVONIAN AND OLD RED SANDSTONE TIME - - - CHAPTER XV - - The Devonian Volcanoes 257 - - CHAPTER XVI - - The Volcanoes of the Old Red Sandstone - - 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 263 - - CHAPTER XVII - - Distribution of the Volcanic Centres in the Lower Old Red - Sandstone--Characters of the Materials Erupted by the Volcanoes 271 - - CHAPTER XVIII - - Structure and Arrangement of the Lower Old Red Sandstone Volcanic - Rocks in the Field 281 - - CHAPTER XIX - - Volcanoes of the Lower Old Red Sandstone of "Lake Caledonia" - - 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 294 - - CHAPTER XX - - Volcanoes of the Lower Old Red Sandstone of - "Lake Caledonia"--_continued_ - - The Southern Chain--The Pentland Volcano--The Biggar Centre--The - Duneaton Centre--The Ayrshire Volcanoes 317 - - CHAPTER XXI - - Volcanoes of the Lower Old Red Sandstone of the Cheviot Hills, - Lorne, "Lake Orcadie" and Killarney 336 - - CHAPTER XXII - - Volcanoes of the Upper Old Red Sandstone--The South-West of - Ireland, the North of Scotland 348 - - - BOOK VI - - THE CARBONIFEROUS VOLCANOES - - CHAPTER XXIII - - The Carboniferous System of Britain and its Volcanic Records - - 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 355 - - - CHAPTER XXIV - - Carboniferous Volcanic Plateaux of Scotland - - I. The Plateau-type restricted to Scotland--i. Distribution in the - Different Areas of Eruption--ii. Nature of the Materials Erupted 367 - - CHAPTER XXV - - Geological Structure of the Carboniferous Volcanic - Plateaux of Scotland - - 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 383 - - CHAPTER XXVI - - The Carboniferous Puys of Scotland - - 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 414 - - CHAPTER XXVII - - Geological Structure of the Carboniferous Puys of Scotland - - 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 424 - - CHAPTER XXVIII - - Illustrative Examples of the Carboniferous Puys of Scotland - - The Basin of the Firth of Forth--North Ayrshire--Liddesdale 462 - - - - - LIST OF ILLUSTRATIONS - - - FIG. PAGE - - 1. Vesicular structure, Lava from Ascension Island, slightly less - than natural size 15 - - 2. Elongation and branching of steam-vesicles in a lava, Kilninian, - Isle of Mull, a little less than natural size 17 - - 3. Microlites of the Pitchstone of Arran (magnified 70 diameters) 19 - - 4. Perlitic structure in Felsitic Glass, Isle of Mull (magnified) 19 - - 5. Spherulitic structure (magnified) 19 - - 6. Micropegmatitic or Granophyric structure in Granophyre, Mull - (magnified) 20 - - 7. Ophitic structure in Dolerite, Gortacloghan, Co. Derry - (magnified) 20 - - 8. Variolitic or orbicular structure, Napoleonite, Corsica - (nat. size) 22 - - 9. Flow-structure in Rhyolite, Antrim, slightly reduced 23 - - 10. Lumpy, irregular trachytic lava-streams (Carboniferous), East - Linton, Haddingtonshire 24 - - 11. View at the entrance of the Svinofjord, Faroe Islands, - illustrating the terraced forms assumed by basic lavas 25 - - 12. Sack-like or pillow-form structure of basic lavas (Lower - Silurian), Bennan Head, Ballantrae, Ayrshire 26 - - 13. Alternations of coarser and finer Tuff 34 - - 14. Alternations of Tuff with non-volcanic sediment 35 - - 15. Ejected block of basalt which has fallen among Carboniferous - shales and limestones, shore, Pettycur, Fife 37 - - 16. Effects of denudation on a Vesuvian cone 40 - - 17. Section to illustrate the structure of the Plateau type 43 - - 18. Diagram illustrating the structure and denudation of Puys 45 - - 19. Section illustrating submarine eruptions; alternations of lavas - and tuffs with limestones and shales full of marine organisms 48 - - 20. Diagram illustrating volcanic eruptions on a river-plain 49 - - 21. Diagram illustrating volcanic eruptions on a land-surface 50 - - 22. Ground-plans of some volcanic vents from the Carboniferous - districts of Scotland 55 - - 23. View of an old volcanic "Neck" (The Knock, Largs, Ayrshire, a - vent of Lower Carboniferous age) 56 - - 24. Section of neck of agglomerate, rising through sandstones and - shales 58 - - 25. Neck filled with stratified tuff 64 - - 26. Section of neck of agglomerate with plug of lava 65 - - 27. Section of agglomerate neck with dykes and veins 66 - - 28. Section of neck filled with massive rock 68 - - 29. Successive shiftings of vents giving rise to double or triple - cones 70 - - 30. Section to show the connection of a neck with a cone and - surrounding bedded tuffs 71 - - 31. Diagram illustrating the gradual emergence of buried volcanic - cones through the influence of prolonged denudation 75 - - 32. Dyke, Vein, and Sill 80 - - 33. Section of Sill or Intrusive Sheet 83 - - 34. Ideal section of three Laccolites. (After Mr. Gilbert) 86 - - 35. Diagram illustrating the stratigraphical relations of the - pre-Cambrian and Cambrian rocks of the North-west Highlands - of Scotland 112 - - 36. Map of a portion of the Lewisian gneiss of Ross-shire 118 - - 37. Section showing the position of sills in the mica-schist series - between Loch Tay and Amulree 124 - - 38. Sketch of crushed basic igneous rock among the schists, E. - side of Porth-tywyn-mawr, E. side of Holyhead Straits 128 - - 39. Section across the Uriconian series of Caer Caradoc 132 - - 40. Map of the volcanic district of St. David's 146 - - 41. Section showing the interstratification of tuff and - conglomerate above Lower Mill, St. David's 154 - - 42. Basic dyke traversing quartz-porphyry and converted into a - kind of slate by cleavage. West side of Llyn Padarn 162 - - 43. 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 163 - - 44. Section of Clegyr on the north-east side of Llyn Padarn, - near the lower end 164 - - 45. Section across the Cambrian formations of the Malvern Hills, - showing the position of the intercalated igneous rocks. - After Phillips 170 - - 46. Section across Rhobell Fawr 178 - - 47. Section at the Slate Quarry, Penrhyn Gwyn, north slopes of - Cader Idris 180 - - 48. Sketch-section across Cader Idris 182 - - 49. Section across the Moelwyn Range 185 - - 50. Section across the anticline of Corndon 190 - - 51. Structure in finely-amygdaloidal diabase lava, south of mouth - of Stinchar River, Ayrshire 193 - - 52. View of Knockdolian Hill from the east 194 - - 53. Section across the Lower Silurian volcanic series in the south - of Ayrshire (B. N. Peach) 197 - - 54. Section of part of the Arenig volcanic group, stream south of - Bennane Head, Ayrshire 198 - - 55. Flow-structure in the lowest felsite on the track from - Llanberis to the top of Snowdon 211 - - 56. Section of Snowdon 212 - - 57. Section across the Berwyn Hills. (Reduced from Horizontal - Section, Geol. Surv. Sheet 35) 219 - - 58. Section of the strata on the shore at Porth Wen, west of Amlwch 223 - - 59. Section of intercalated black shale in the volcanic series at - Porth yr hwch, south of Carmel Point, Anglesey 224 - - 60. Green slates overlain with volcanic breccia, Carmel Point 224 - - 61. Blue shale or slate passing into volcanic breccia east of Porth - Padrig, near Carmel Point 225 - - 62. Section of felsites in the Coniston Limestone group, west of - Stockdale 232 - - 63. Fine tuff with coarser bands near Quayfoot Quarries, Borrowdale 234 - - 64. Diagram of the general relations of the different groups of - rock in the Lower Silurian volcanic district along the - western shore of Lough Mask 253 - - 65. Veins and nests of sandstone due to the washing of sand into - fissures and cavities of an Old Red Sandstone lava. Turnberry - Point, Ayrshire 283 - - 66. Ground-plan of reticulated cracks in the upper surface of an - Old Red Sandstone lava filled in with sandstone. Red Head, - Forfarshire 284 - - 67. Section across the volcanic series of Forfarshire 286 - - 68. Section across two necks above Tillicoultry, Ochil Hills 288 - - 69. Section of the granite core between Merrick and Corscrine 290 - - 70. Section across the three Dirrington Laws, Berwickshire 291 - - 71. Section of Papa Stour, Shetlands, showing sill of spherulitic - felsite traversing Old Red Sandstone and bedded porphyrites - (Messrs. Peach and Horne) 292 - - 72. Section across Northmavine, from Okrea Head to Skea Ness, - Shetland, showing dykes and connected sill of granite and - felsite (Messrs. Peach and Horne) 292 - - 73. Section at the edge of one of the bays of Lower Old Red - Sandstone along the northern margin of Lake Caledonia, - near Ochtertyre 295 - - 74. 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 296 - - 75. Section showing the top of the volcanic series at the foot of - the precipice of the Red Head, Forfarshire 300 - - 76. Andesite with sandstone veinings and overlying conglomerate. - Todhead, south of Caterline, coast of Kincardineshire 303 - - 77. Section across the Boundary-fault of the Highlands at Glen - Turrit, Perthshire 305 - - 78. Section across the chain of the Sidlaw Hills near Kilspindie 306 - - 79. Section across the Eastern Ochil Hills from near Newburgh to - near Auchtermuchty 307 - - 80. Generalized section across the heart of the Ochil Hills from - Dunning on the north to the Fife coal-field near Saline on - the south 308 - - 81. Diagram of the volcanic series of the Western Ochil Hills 309 - - 82. View of Cnoc Garbh, Southend, Campbeltown. A volcanic neck of - Lower Old Red Sandstone age, about 400 yards wide in its - longer diameter 312 - - 83. Section of volcanic series on beach, Southend, Campbeltown 313 - - 84. Section of the base of the volcanic series, Reclain, five miles - south of Pomeroy 316 - - 85. Section of shales and breccias at Crossna Chapel, north-east - of Boyle 316 - - 86. Section across the north end of the Pentland Hills, from - Warklaw Hill to Pentland Mains. Length about five miles 318 - - 87. View of the lava-escarpments of Warklaw Hill, Pentland chain, - from the north-west 319 - - 88. Section across the Pentland Hills through North Black Hill and - Scald Law. Length about three miles 322 - - 89. Section from the valley of the Gutterford Burn through Green - Law and Braid Law to Eight-Mile Burn 322 - - 90. Section across the north end of the Pentland Hills, and the - southern edge of the Braid Hill vent. Length about two miles 324 - - 91. Section across the northern end of the Biggar volcanic group, - from Fadden Hill to beyond Mendick Hill 326 - - 92. Section across the southern part of the Biggar volcanic group - from Covington to Culter 328 - - 93. Section from Thankerton Moor across Tinto to Lamington 328 - - 94. Section across the Duneaton volcanic district from the head - of the Duneaton Water to Kirklea Hill 330 - - 95. Cavernous spaces in andesite, filled in with sandstone, - John o' Groats Port, Turnberry, Ayrshire 333 - - 96. Section of andesites, Turnberry Castle, Ayrshire 334 - - 97. Lenticular form of a brecciated andesite (shown in Fig. 96), - Turnberry, Ayrshire 334 - - 98. Section across the volcanic area of St. Abb's Head (after Prof. - J. Geikie) 339 - - 99. View of terraced andesite hills resting on massive conglomerate, - south of Oban 341 - - 100. Section of lava-escarpment at Beinn Lora, north side of mouth - of Loch Etive, Argyllshire 342 - - 101. Section across Strathbogie, below Rhyme, showing the position - of the volcanic band 344 - - 102. View of Knockfeerina, Limerick, from the north-east--a volcanic - neck of Upper Old Red Sandstone age 349 - - 103. Section of the volcanic zone in the Upper Old Red Sandstone, - Cam of Hoy, Orkney 351 - - 104. Section of the volcanic zone in the Upper Old Red Sandstone at - Black Ness, Rackwick, Hoy 351 - - 105. Section across the volcanic band and its associated necks, - Hoy, Orkney 352 - - 106. Ground-plan of volcanic neck piercing the Caithness Flagstone - series on the beach near John o' Groat's House 353 - - 107. View of the escarpment of the Clyde Plateau in the Little - Cumbrae, from the south-west 368 - - 108. View of the edge of the Volcanic Plateau south of Campbeltown, - Argyllshire 370 - - 109. View of North Berwick Law from the east, a phonolite neck - marking one of the chief vents of the Garleton Plateau. - (From a photograph) 371 - - 110. The Bass Rock, a trachytic neck belonging to the Garleton - plateau, from the shore at Canty Bay 372 - - 111. Corston Hill--a fragment of the Midlothian Plateau, seen from - the north 373 - - 112. View of Arthur Seat from Calton Hill to the north 374 - - 113. View of Arkleton Fell, part of the Solway Plateau, from the - south-west 376 - - 114. Vertical sections of the escarpment of the Clyde plateau from - north-east to south-west 384 - - 115. Section of Craiglockhart Hill, Edinburgh 387 - - 116. Section of the bottom of the Midlothian Plateau, Linnhouse - Water above Mid-Calder Oilworks 387 - - 117. Section of the top of the Midlothian Plateau in the - Murieston Water 388 - - 118. Section of Calton Hill, Edinburgh 389 - - 119. Cliff of tuff and agglomerate, east side of Oxroad Bay, a - little east from Tantallon Castle, East Lothian 391 - - 120. 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) 392 - - 121. Diagram illustrating the thinning away southwards of the lavas - of the Clyde Plateau between Largs and Ardrossan. Length - about 10 miles 393 - - 122. Diagram illustrating the thinning away eastwards of the lavas of - the Clyde Plateau in the Fintry Hills. Length about 12 miles 394 - - 123. View of the two necks Dumgoyn and Dumfoyn, Stirlingshire, - taken from the south 395 - - 124. Ground-plan of Plateau-vents near Strathblane, Stirlingshire, - on the scale of 6 inches to a mile 395 - - 125. Ground-plans of double and triple necks in the Plateau series, - on the scale of 6 inches to a mile 396 - - 126. Ground-plan of tuff-neck, shore east of Dunbar 398 - - 127. Section across the vents Dumgoyn and Dumfoyn, and the edge of - the Clyde plateau above Strathblane, Stirlingshire 400 - - 128. Section through the large vent of the Campsie Hills 400 - - 129. Diagrammatic section across the central vent of the Clyde - plateau in Renfrewshire 400 - - 130. Section across Southern Berwickshire, to show the relation - of the volcanic plateau to the vents lying south from it 401 - - 131. Section of south end of Dumbuck Hill. East of Dumbarton 403 - - 132. Section across the East Lothian plateau, to show the relative - position of one of the necks 403 - - 133. View of Traprain Law from the south, a phonolite neck of the - Garleton Plateau 405 - - 134. Veins and dykes traversing the agglomerate and tuff of the - great Renfrewshire vent 408 - - 135. "The Yellow Man," a dyke in volcanic tuff and conglomerate on - the shore a little east of North Berwick 409 - - 136. Trachytic sills, Knockvadie, Kilpatrick Hills 410 - - 137. Section across the edge of the Clyde plateau, south-east - of Beith 411 - - 138. Section across the upper part of the Clyde plateau at - Kilbirnie, Ayrshire 411 - - 139. Section across the upper surface of the Clyde volcanic - plateau, Burnhead, north-west of Kilsyth 412 - - 140. Section across the upper surface of the Clyde volcanic - plateau at Campsie 412 - - 141. Section across western edge of the Garlton plateau 412 - - 142. Section across the Solway plateau 413 - - 143. 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 426 - - 144. View of the Binn of Burntisland--a volcanic neck of agglomerate 428 - - 145. View of part of the cliffs of vertical agglomerate, Binn of - Burntisland 431 - - 146. Diagram of buried volcanic cone near Dalry, Ayrshire. - Constructed from information obtained in mining operations 434 - - 147. Diagram to illustrate how Volcanic Necks may be concealed - and exposed 434 - - 148. Section across the Saline Hills, Fife 435 - - 149. Section across the Binn of Burntisland, in an east and - west direction 436 - - 150. Section in old quarry, west of Wester Ochiltree, - Linlithgowshire. Calciferous Sandstone series 437 - - 151. Ejected volcanic block in Carboniferous strata, Burntisland 438 - - 152. View of volcanic agglomerate becoming finer above east end - of Kingswood Craig, two miles east from Burntisland 439 - - 153. Alternations of basalt and tuff, with shale, etc., of - Kingswood Craig, Burntisland 441 - - 154. Section of the upper surface of a diabase ("leckstone") sheet, - Skolie Burn, south-east of Bathgate 443 - - 155. Section across the volcanic ridge of the Linlithgow and - Bathgate Hills, showing the intercalation of limestones that - mark important stratigraphical horizons 444 - - 156. Section in Wardlaw Quarry, Linlithgowshire 445 - - 157. Section from Linlithgow Loch to the Firth of Forth 446 - - 158. Section across the Campsie Fells illustrating the contrast - between the sills below and above the plateau-lavas 447 - - 159. Section showing the position of the basic sills in relation - to the volcanic series at Burntisland, Fife 448 - - 160. Sills between shales and sandstones, Hound Point, - Linlithgowshire 449 - - 161. Section of Sill, Cramond Railway, Barnton, near Edinburgh 450 - - 162. Intrusive dolerite sheet enclosing and sending threads into - portions of shale, Salisbury Crags, Edinburgh 452 - - 163. Intrusive sheet invading limestone and shale, Dodhead Quarry, - near Burntisland 452 - - 164. Spheroidal weathering of dolerite sill, quarry east of North - Queensferry, Fife. 455 - - 165. Two thin sills of "white trap" injected into black - carbonaceous shale overlying the Hurlet Limestone, Hillhouse - Quarry, Linlithgow 456 - - 166. Dyke cutting the agglomerate of a neck. Binn of Burntisland 457 - - 167. Boss of diabase cutting the Burdiehouse Limestone and sending - sills and veins into the overlying shales. Railway cutting, - West Quarry, East Calder, Midlothian 458 - - 168. Side of columnar basalt-dyke in the same agglomerate as in - Fig. 166 459 - - 169. Dyke rising through the Hurlet Limestone and its overlying - shales. Silvermine Quarry, Linlithgowshire 460 - - 170. Junction of amygdaloidal basalt with shales and limestone, - shore, half a mile east from Kinghorn, Fife 464 - - 171. Columnar basalt, Pettycur, Kinghorn, Fife 469 - - 172. Section across the Fife band of Sills 473 - - 173. Section across the upper volcanic band of north Ayrshire. - Length about four miles 474 - - 174. Section showing the connection of the two volcanic bands - in Liddesdale 476 - - 175. 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 476 - - - MAPS - - I. General map of the Volcanic districts of the British - Isles--_At the end of the volume_ - - II. Map of the Cambrian and Silurian volcanic region of - North Wales _To face p. 256_ - - III. Map of the Old Red Sandstone volcanic region of "Lake - Caledonia" in Central Scotland and North Ireland _To face p. 334_ - - IV. Map of the Carboniferous volcanic districts of - Scotland _To face p. 476_ - - - - -BOOK I - -GENERAL PRINCIPLES AND METHODS OF INVESTIGATION - - - - -CHAPTER I - - 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. - - -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. - -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. - -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 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. - -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. - -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. - -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 planet can only, if ever, be dispelled by the -patient study of these vents of communication between the interior and -the surface. - -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. - -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. - -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. - -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. - -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. - -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. - -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 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. - -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. - -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. - -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. - -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. - -The volcanic records of former geological periods, however, are in -large 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. - -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. - -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. - -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. - -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. - -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 _puys_ of France, and fissure- or -dyke-eruptions like those of recent times in Iceland. - -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. - -These characteristics may be instructively represented on a map, such -as that which accompanies the present volume (Map I.). 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. - -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. - -There is yet another respect in which the volcanic geology of Britain -possesses a special value. Popular imagination has long been prone to -see 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. - -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. - -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. - -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. - -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 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. - -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. - - - - -CHAPTER II - - The Nature and Causes of Volcanic Action--Modern Volcanoes. - - -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. - -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. - -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. - -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 _Puys_. From these vents -considerable streams of lava have sometimes been discharged. - -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. - -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 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. - -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. - -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. - -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. - -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 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. - -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. - -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. - -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. - -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. - -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. - -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. - -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. - -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. - - - - -CHAPTER III - - 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. - - -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. - -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. - -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. - - -i. LAVAS - -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. - -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. - -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. - -[Illustration: Fig. 1.--Vesicular structure, Lava from Ascension -Island, slightly less than natural size.] - -1. The CELLULAR, VESICULAR, SCORIACEOUS or PUMICEOUS STRUCTURE of -volcanic rocks (Fig. 1) 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 -(_amygdales_), owing to elongation of the steam-holes by movement of -the lava before its consolidation, the rocks containing them are said -to be _amygdaloidal_. - -This structure, though eminently characteristic of superficial lavas, -is 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. 90, 91, and 236). - -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. 260, 310 and 311). 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 _slaggy_. When its upper surface is -rugged and full of steam-vesicles of all sizes up to large cavernous -spaces, it is said to be _scoriaceous_, and fragments of such a rock -ejected from a volcanic vent are spoken of as _scoriæ_. - -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 _Teredo_. 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 Fig. 2. - -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. - -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 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. - -[Illustration: Fig. 2.--Elongation and branching of steam-vesicles in a -lava, Kilninian, Isle of Mull, a little less than natural size.] - -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. - -Vesicular structure is more commonly and perfectly developed among the -lavas which are basic and intermediate in composition than among those -which are acid. - -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. 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 -Chapter iv., 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. - -2. The presence of GLASS, 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. - -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. - -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. - -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 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. - -(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 (Fig. 3). - -[Illustration: Fig. 3.--Microlites of the Pitchstone of Arran -(magnified 70 diameters).] - -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 _Porphyritic_ structure. - -[Illustration: Fig. 4.--Perlitic structure in Felsitic Glass, Isle of -Mull (magnified).] - -[Illustration: Fig. 5.--Spherulitic structure (magnified).] - -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 _Perlitic_ (Fig. 4), consists in -the formation of minute curved or straight cracks between which the -vitreous or felsitic substance, during its contraction in cooling, -assumed a finely globular form. - -Another structure, termed _Spherulitic_ (Fig. 5), 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. 375 and 377). 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. - -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. - -[Illustration: Fig. 6.--Micropegmatitic or Granophyric structure in -Granophyre, Mull (magnified).] - -[Illustration: Fig. 7.--Ophitic structure in Dolerite, Gortacloghan, -Co. Derry (magnified).] - -A marked structure among some intrusive rocks, especially of an acid -composition, is that called _Micropegmatitic_ or _Granophyric_. 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. - -(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 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. - -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). - -A specially characteristic feature of many basic rocks is the presence -of what is termed an _Ophitic_ structure (Fig. 7). 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. - -The name _Variolitic_ is applied to another structure of basic rocks -(Fig. 8), 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. - -3. Flow-structure 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. - -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 (Fig. 9). 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 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. - -[Illustration: Fig. 8.--Variolitic or Orbicular structure, Napoleonite, -Corsica (nat. size).] - -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 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. 329, 342). - -[Illustration: Fig. 9.--Flow-structure in Rhyolite, Antrim, slightly -reduced.] - -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. - -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. 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. - -[Illustration: Fig. 10.--Lumpy, irregular trachytic Lava-streams -(Carboniferous), East Linton, Haddingtonshire.] - -4. The DISPOSITION OF LAVAS IN SHEETS OR BEDS 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 (Fig. 10). 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. 11, 265, and 286). The andesites of the Old -Red Sandstone (Figs. 99, 100) and Carboniferous series (Figs. 107, 108, -111, 112, 113, 123) in Scotland likewise form terraced hills. - -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. - -[Illustration: Fig. 11.--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.] - -Some lavas, more especially among the basic series, assume in cooling -a _Columnar structure_, 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. 171, 225). 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. 307 and 308). - -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 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.[1] 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 (Fig. 12). - -[Footnote 1: See Prof. G. Platania in Dr. Johnston-Lavis' _South -Italian Volcanoes_, Naples (1891), p. 41 and plate xii.] - -[Illustration: Fig. 12.--Sack-like or pillow-form structure of basic -lavas (Lower Silurian), Bennan Head, Ballantrae, Ayrshire.] - -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. - -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 the movement of a flowing current of basalt.[2] 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.[3] 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. - -[Footnote 2: M. Boule, _Bull. Cart. Géol. France_, No. 28, tom. iv. -(1892), p. 235.] - -[Footnote 3: M. Boule. _Op. cit._ p. 234.] - -Variations of structure in single lava-sheets.--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.[4] - -[Footnote 4: _Volcanoes_, p. 125.] - -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.[5] - -[Footnote 5: _Description Physique des Isles Canaries_ (1836), p. 190.] - -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.[6] - -[Footnote 6: _Geological Observations on Volcanic Islands_ (1844), p. -117.] - -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."[7] 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. - -[Footnote 7: _U.S. Geol. Exploration of the Fortieth Parallel_, vol. i. -(1878), p. 716.] - -Volcanic cycles.--Closely related to the problem of the range of -structure 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.[8] -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.[9] 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. - -[Footnote 8: _Trans. Acad. California_, 1868. Prof. Iddings' _Journ. -Geol._, vol. i. (1893), p. 606.] - -[Footnote 9: See Prof. Brögger, "Die Eruptivgesteine des -Kristianiagebietes," part ii. (1895), p. 175; _Zeitsch. Kryst. und -Mineral_, 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.] - -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.[10] - -[Footnote 10: 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, _Bull. Soc. Geol._, 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, -_op. cit._ p. 498. Compare also Prof. Brögger, _Die Eruptivgesteine des -Kristianiagebietes_, ii. p. 169.] - -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 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.).[11] - -[Footnote 11: _Eruptivgest. Kristianiageb._, 1895.] - -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.[12] - -[Footnote 12: _Op. cit._ 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.] - -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.[13] - -[Footnote 13: M. Boule, "Description Géologique du Velay," _Bull. -Carte. Géol. France_, 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.] - -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.[14] - -[Footnote 14: M. Michel Lévy, _Bull. Soc. Géol. France_, 1890, p. 704.] - -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.[15] - -[Footnote 15: _Journal of Geology_, Chicago, i. (1893) p. 606. See -also this author's excellent monograph on "Electric Peak and Sepulchre -Mountain," _12th Ann. Rep. U.S. Geol. Survey_ (1890-91), and Mr. H. -W. Turner on "The Succession of Tertiary Volcanic Rocks in the Sierra -Nevada of North America," _14th Ann. Rep. U.S. Geol. Survey_ (1892-93), -p. 493.] - -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 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. - -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. - -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. - -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.[16] - -[Footnote 16: _14th Annual Report U.S. Geol. Survey_ (1892-93).] - -The chief lavas found in Britain.--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. - - -ii. VOLCANIC AGGLOMERATES, BRECCIAS AND TUFFS - -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. - -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.[17] 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. - -[Footnote 17: Prof. J. P. Iddings, _12th Ann. Rep. U.S. Geol. Survey_ -(1890-91), p. 634.] - -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 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. - -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."[18] - -[Footnote 18: For an account of "crush-conglomerates," see Mr. -Lamplugh's paper on those of the Isle of Man, _Quart. Journ. Geol. -Soc._, li. (1895), p. 563. Mr. M'Henry has pointed to probable cases of -mistake of this kind in Ireland, _Nature_, 5th March 1896. A. Geikie, -_Geol. Mag._ November 1896.] - -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.[19] 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 (Fig. 262). - -[Footnote 19: Abich, _Geologie des Armenischen Hochlandes_ (Part ii., -western half), 1882, p. 76.] - -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. - -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. - -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. - -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. - -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. - -The composition of tuffs must obviously depend upon that of the lavas -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. - -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. - -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. - -[Illustration: Fig. 13.--Alternations of coarser and finer Tuff.] - -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 (Fig. 13). 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 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. - -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. - -[Illustration: Fig. 14.--Alternations of Tuff (_t_, _t_,) with -non-volcanic sediment (_l_, _l_).] - -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 (Fig. 14). 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. - -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. - -A little reflection will convince the observer that in such a section -as that represented in Fig. 14 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 (_l_, _l_). 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 (_t_, _t_). - -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. - -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. - -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. - -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 -Fig. 13. Another instance from a group of non-volcanic sediments is -given in Fig. 15, 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. - -[Illustration: Fig. 15.--Ejected block of Basalt which has fallen among -Carboniferous shales and limestones, shore, Pettycur, Fife.] - -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. - -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 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. - - - - -CHAPTER IV - - Materials erupted at the Surface--Extrusive Series--_continued_. - 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. - - -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. - - -iii. TYPES OF OLD VOLCANOES - -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. - -1. _The Vesuvian Type._--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. - -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 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. - -[Illustration: Fig. 16.--Effects of denudation on a Vesuvian cone.] - -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 (Fig. 16). 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. - -This piece of volcanic history is diagrammatically illustrated in Fig. -16. 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 (_l_, -_l_), the dykes (_d_, _d_), and the lavas (_p_, _p_) and agglomerates -(_a_, _a_) of the central vent and of the subordinate cones. - -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. - -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. - -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. - -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. - -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. - -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. - -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. - -2. _The Plateau_ or _Fissure type_ is, among modern volcanoes, best -developed in Iceland, as will be more fully detailed in Chapter xl. -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. - -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. - -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, 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. - -[Illustration: Fig. 17.--Section to illustrate the structure of the -Plateau type.] - -This geological history is illustrated by the diagram in Fig. 17. -The stippled ground underneath (_x_, _x_) 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 (_d_, _d_) 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. - -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. - -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. 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. 11, 265). - -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. - -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. - -3. _The Puy type_, as before remarked, takes its name from the -well-known _puys_, 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. 29 and 214). 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. - -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. - -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. - -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,[20] though, in some cases, the cone may have been thrown up after -the emission of the lava. - -[Footnote 20: M. Boule, _Bull. Carte Géol. France_, No. 28, tome iv. p. -232.] - -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 and ashes. Hence the products of such a vent -group themselves into three layers--two of breccia separated by an -intervening sheet of lava.[21] - -[Footnote 21: M. Boule, _Bull. Carte Géol. France_, No. 28, tome iv.] - -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. - -[Illustration: Fig. 18.--Diagram illustrating the structure and -denudation of Puys.] - -The accompanying diagram (Fig. 18) 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. - -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.[22] - -[Footnote 22: See Desmarest's classic map and his papers in _Mem. Acad. -Roy. Sciences_, Paris, 1774, 1779; _Journ. de Physique_, 1779; Scrope's -_Geology of Central France_, 1827, and _Extinct Volcanoes of Central -France_, 1858; Lecoq's _Époques Géologiques de l'Auvergne_, 1867; M. -Michel Lévy, _Bull. Soc. Géol. France_, 1890, p. 688; M. Boule, _Bull. -Carte Géol. France_, No. 28, tome iv. 1892.] - -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.[23] 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.[24] - -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. - -[Footnote 23: 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.] - -[Footnote 24: These Würtemberg vents have been elaborately described -and discussed by Professor W. Branco of Tübingen in his _Schwabens 125 -Vulkan-Embryonen und deren tufferfülte Ausbruchsröhren, das grösste -Gebiet chemaliger Maare auf der Erde_, Stuttgart, 1894.] - - -iv. DETERMINATION OF THE RELATIVE GEOLOGICAL DATES OF ANCIENT VOLCANOES - -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 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. - -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. - -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. - -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. - -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. - -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 chronology of the volcanic -rocks of the British Isles has been worked out, and that the following -chapters have been arranged. - - -v. HOW THE PHYSICAL GEOGRAPHY ASSOCIATED WITH ANCIENT VOLCANOES IS -ASCERTAINED - -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. - -[Illustration: - - Fig. 19.--Section illustrating submarine eruptions; alternations - of lavas and tuffs with limestones and shales full of marine - organisms. -] - -1. _Submarine Eruptions._--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 -(Fig. 19). 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. - -The accompanying diagram (Fig. 19) illustrates these statements. At -the bottom a thick mass of limestone (_l_) 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 (_t_). -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 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'). - -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. - -2. _Lacustrine Eruptions._--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. - -[Illustration: Fig. 20.--Diagram illustrating volcanic eruptions on a -river-plain.] - -3. _Fluviatile Eruptions._--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 Fig. 20 some of these changes -are indicated in a series of bedded lavas (_l_). The lower part of the -diagram shows the dying out of a bed of river gravel (_g_) 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 Chapter xxxviii. - -4. _Terrestrial Eruptions._--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. - -[Illustration: Fig. 21.--Diagram illustrating volcanic eruptions on a -land-surface.] - -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. - -These geological changes are represented diagrammatically in Fig. 21. -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 (_s_) 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. - -Admirable illustrations of this succession of events are to be -encountered 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. - - - - -CHAPTER V - - 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. - - -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. - -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 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. - -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. - - -I. Vents of Eruption - -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. - -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. - -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. - -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 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. - -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. - -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 _dyke_. 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. - -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. - -Some of these irregularities are depicted in Fig. 22, 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. - -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. 29, 125, 205, -and 214). Curiously irregular or elongated forms may thus arise in the -resultant "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 Fig. 22). - -[Illustration: Fig. 22.--Ground-plans of some Volcanic vents from the -Carboniferous districts of Scotland. - - 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 Fig. 174). - 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. -] - -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. - -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. - -The stumps of volcanic columns of this nature, after prolonged -denudation, generally project above the surrounding ground as rounded -or conical eminences known as "Necks" (Fig. 23. See also Figs. 52, -82, 102, 109, 123, 133, 144, 178, 192, 195, 203, 204, 209, 294, 298, -306 and 310). 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. - -[Illustration: Fig. 23.--View of an old volcanic "Neck" (The Knock, -Largs, Ayrshire, a vent of Lower Carboniferous age).] - -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. - - -i. _Necks of Fragmentary Materials_ - -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 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. - -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. - -(1) _Necks of non-volcanic Detritus._--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. - -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. - -The general name of Agglomerate, as already stated, is given to all -accumulations of coarse, usually unstratified, detritus in volcanic -funnels, 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. - -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. - -(2) _Necks of Agglomerate or Tuff._--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. - -[Illustration: Fig. 24.--Section of neck of agglomerate, rising through -sandstones and shales.] - -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 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 (Fig. 24. For views of necks formed of -agglomerate or tuff see Figs. 23, 82, 102, 123, 144, 178, 192, 203, -204, 209, 210, 212, 216). - -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 vol. ii. p. 78). 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 _in situ_ 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. - -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. - -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. - -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. - -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. - -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. - -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. - -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 that the slice looks like a pattern -of delicate lace.[25] 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. - -[Footnote 25: M. Boule, _Bull. Cart. Géol. France_, No. 28, tome iv. -(1892) p. 193.] - -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. - -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. - -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 -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. - -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 -_in situ_, 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. - -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.[26] Examples of such foreign or deep-seated crystals -will be described from the probably Permian necks of Central Scotland. - -[Footnote 26: For lists of the minerals found in the diamond-bearing -necks of Kimberley, see M. Boutan in Frémy's _Encyclopédie Chimique_ -(1886), vol. ii. p. 168; Dr. M. Bauer's _Edelsteinkunde_ (1895), p. -223.] - -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 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. - -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. - -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 (Fig. 25). - -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. 310, 311, 312). 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. - -[Illustration: Fig. 25.--Neck filled with stratified tuff. A. ground -plan; B. transverse section.] - -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. 25 A and -217).[27] - -[Footnote 27: See also the sections of vents on the west coast of -Stromö Faroes, above referred to.] - -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. 145, 218). 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. - -(3) _Necks with a central Lava-plug._--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 (Fig. 26; compare also Figs. 148, 174, 207, and 226). 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 of the friable tuffs are crowned or -interrupted by more craggy features, which mark the position of the -harder intrusive rock. - -[Illustration: Fig. 26.--Section of neck of agglomerate (_a_ _a_) with -plug of lava (_b_).] - -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. - -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. - -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. - -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. - -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 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. - -[Illustration: Fig. 27.--Section of agglomerate neck (_a_ _a_) with -dykes and veins (_b_ _b_).] - -(4) _Necks with Dykes, Veins, or irregular intrusions of Lava._--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 (Fig. 27). 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. 135, 149, 166, 168, 219, 221, 222). - -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. - -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. - -There can be little doubt that the dykes and veins which traverse -necks 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. - -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. - -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 p. 71. - - -ii. _Necks of Lava-form Material_ - -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. - -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 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. - -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. 109, 133, and 195, 294). - -[Illustration: Fig. 28.--Section of neck filled with massive rock.] - -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. - -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.[28] In -the Upper Velay, in Central France, numerous conspicuous domes and -cones of phonolite rise amidst the much-worn basalt-plateau of that -region (Fig. 345). Many instances will be cited in later chapters from -the British Isles. - -[Footnote 28: _U.S. Geol. Survey, 6th Annual Report_, 1884-85, p. 172.] - - -iii. _Distribution of Vents in Relation to Geological Structure-lines_ - -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 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. - -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. - -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. - -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 Chapter xx. Other instances may be cited from the -Carboniferous and Permian volcanic series (see Figs. 90, 148, 213). - -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 succeeded by a third (Fig. 29). The three cones and -craters of the little island of Volcanello supply a singularly perfect -recent instance of this structure (Fig. 214). 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. - -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 Fig. 22). - -[Illustration: Fig. 29.--Successive shiftings of vents giving rise to -double or triple cones. A, ground-plan; B, vertical section.] - -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 -Fig. 30, for example, a section is given of a neck (_n_) from which on -the right-hand side all the cone and surrounding tuffs (_t_) 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 (_l_, _l_) 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. - -[Illustration: Fig. 30.--Section to show the connection of a neck with -a cone and surrounding bedded tuffs.] - -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 (Fig. 148) and of Largo Law -(Fig. 226), both in Fife. - - -iv. _Metamorphism in and around Volcanic Vents--Solfataric Action_ - -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, 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. - -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. - -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. - -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. - -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 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 p. 404). - - -v. _Inward Dip of Docks towards Necks_ - -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 (Fig. 24). 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. - - -vi. _Influence of Contemporaneous Denudation upon Volcanic Cones_ - -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 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. - -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. - - -vii. _Stages in the History of old Volcanic Vents_ - -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. - -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 Chapter xxvii. (p. 433). 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 (Fig. 146). - -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 -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. - -[Illustration: Fig. 31.--Diagram illustrating the gradual emergence of -buried volcanic cones through the influence of prolonged denudation.] - -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, -Fig. 31 may be referred to (compare also Fig. 147). - -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 (_s_, _s_,) 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 (_g_, _g_) 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 _t_. A geologist coming -to that volcanic intercalation would be sure that it pointed to the -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. - -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 (_s_, -_s_). - -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 _l_, 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 (_f_) 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 (_a_), through which a central plug of lava (_p_) has -ascended, and into which dykes or veins (_d_, _d_), the last efforts of -eruption, have been injected. - -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. - - - - -CHAPTER VI - - Underground Phases of Volcanic Action--_continued_. 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. - - -II. Subterranean Movements of the Magma - -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. - -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. - -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. - -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 crust, appear at the -surface in rounded, elliptical or irregularly-shaped eminences, these -are called Bosses (Stocks, Culots). - -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.[29] Examples of the same relation -from different volcanic centres in Britain will be cited in later -chapters. - -[Footnote 29: _12th Ann. Rep. U.S. Geol. Survey_ (1890-91), p. 595.] - -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. - -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. - -Besides these differences, however, there is evidence of a migration -of 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. - -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.[30] - -[Footnote 30: _12th Ann. Rep. U.S. Geol. Survey_, 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.] - -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. - -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. - - -i. _Dykes and Veins_ - -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 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. - -[Illustration: Fig. 32.--Dyke, Vein and Sill. - -The dyke (_d_) rises along a small fault among sandstones, shales, and -ironstones (_sh_), and gives off a vein (_v_) and an intrusive sheet or -sill (_b_).] - -Full details regarding these interesting relics of volcanic activity -will be given in later chapters, especially in Chapters xxxiv. and -xxxv. 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. - -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. - -The larger cones are likewise intersected with similar vertical, -inclined 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. - -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. - -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. 35, 36). - -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. - -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. - -It is obvious that in a wide fissure the central portion may remain -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.[31] -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 (Chapter xxxv. vol. ii. p. -159). - -[Footnote 31: _12th Ann. Rep. U.S. Geol. Survey_ (1890-91), p. 587.] - -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.[32] - -[Footnote 32: _American Geologist_ (1894), p. 293.] - -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 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 vol. ii. p. 163). - - -ii. _Sills and Laccolites_ - -The word "sill," derived from a remarkable sheet of eruptive rock in -the north of England, known as the Great Whin Sill (Chapter xxix.), -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 (Fig. -33). These masses are likewise called Intrusive Sheets, and where the -injected material has accumulated in large blister-like expansions, -these are known as Laccolites (Fig. 34). - -[Illustration: Fig. 33.--Section of Sill or Intrusive Sheet.] - -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. - -The petrographical character of the sills in a volcanic district -depends 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. - -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. - -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.[33] -Some examples of the same structure will subsequently be cited from the -Carboniferous volcanic series of Central Scotland. - -[Footnote 33: "Electric Peak and Sepulchre Mountain," _12th Ann. Rep. -U.S. Geol. Survey_ (1890-91), p. 584.] - -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 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 xliii. and xliv.). - -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. - -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. - -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. - -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 vol. ii. p. 2). 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 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. - -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 -_Laccolites_ from Mr. G. K. Gilbert, who worked out this peculiar type -of structure in the case of the Henry Mountains in southern Utah[34] -(Fig. 34). 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. - -[Footnote 34: "Geology of the Henry Mountains," _U.S. Geog. and Geol. -Survey of the Rocky Mountain Region_, 1877. For a review of the -whole subject of laccolites in Western America see a paper by Mr. -Whitman Cross, in the _14th Annual Report of the Director of the U.S. -Geological Survey_, 1892-93 (pub. 1895), p. 157.] - -[Illustration: Fig. 34.--Ideal section of three Laccolites. (After Mr. -Gilbert.)] - -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. - -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. - -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. - -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 with the mechanism of the -remaining intrusive masses which have now to be described. - - -iii. _Bosses (Stocks, Culots)_ - -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 (Fig. 28). -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. - -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.[35] -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 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. - -[Footnote 35: M. Michel Lévy, _Bull. Carte Géol. France_, 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."] - -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.[36] 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. - -[Footnote 36: M. Michel Lévy, _Bull. Carte Géol. France_, No. 35, tome -v. (1893).] - -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. - -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. - -Acid, intermediate and basic rocks are abundantly displayed among the -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. - -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. - -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.[37] 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.[38] - -[Footnote 37: 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.] - -[Footnote 38: See Messrs. Dakyns and Teall, _Quart. Journ. Geol. Soc._ -xlviii. (1892), p. 104; Prof. Brögger, _op. cit._ 1. (1894), p. 15; Mr. -A. Harker, _op. cit._ p. 320; Prof. Iddings, _Journ. Geol. Chicago_, -i. (1893), p. 833; _Bull. Phil. Soc. Washington_, ii. (1890), p. 191; -1892, p. 89.] - -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 -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.[39] - -[Footnote 39: Messrs. Dakyns and Teall, _Quart. Journ. Geol. Soc._ -xlviii. (1892), p. 104.] - -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_{2}O_{3} 3·60, FeO 2·64), and at the margin to 25·54 (Fe_{2}O_{3} -8·44, FeO 17·10).[40] 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. - -[Footnote 40: Mr. A. Harker, _op. cit._ p. 320.] - -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.[41] - -[Footnote 41: This general conclusion is stated by Professor Brögger -from his investigation of the rocks of Gran, _Quart. Journ. Geol. Soc._ -l. (1894), p. 36.] - -If during the process of differentiation, and before consolidation, -injections of the magma occur, they may be expected to differ in -character according 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.[42] - -[Footnote 42: _Quart. Journ. Geol. Soc._ l. (1894), p. 35.] - -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. - -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. - -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. - -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 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. - -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. - -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. - -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. - -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.[43] 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 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. - -[Footnote 43: See, for example, Mr. Harker, _Quart. Journ. Geol. Soc._ -l. (1894), p. 329.] - -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. - -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. - -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. - -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 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.[44] - -[Footnote 44: Mr. Harker, _Quart. Journ. Geol. Soc._ vol. l. (1894), p. -331.] - -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.[45] 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.[46] - -[Footnote 45: See Explanation to Sheet 9 of the _Geological Survey of -Scotland_, p. 22; Prof. Bonney and Mr. Allport, _Proc. Roy. Soc._ xvi. -(1889); Miss Gardiner, _Quart. Journ. Geol. Soc._ vol. xlvi. (1890), p. -569.] - -[Footnote 46: Macculloch, _Trans. Geol. Soc._ vol. iii. (1816), p. 1; -_Description of the Western Isles_, vol. i. p. 322. See also _Quart. -Journ. Geol. Soc._ vol. xiv. (1857), p. 1; and vol. xliv. (1888), p. -62.] - -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. - -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 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. - -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.[47] - -[Footnote 47: _Trans. Roy. Irish Acad._ xxx. (1894), part xii. p. 477.] - -Similar phenomena have been described by Mr. Harker as occurring -where granophyre has invaded the gabbro of Carrock Fell.[48] 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.[49] - -[Footnote 48: _Quart. Journ. Geol. Soc._ li. (1895), p. 183.] - -[Footnote 49: _Op. cit._ 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--_Les Enclaves des Roches Volcaniques_, Macon, 1893.] - -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. - -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 Chapter -xxix.). - -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. - -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 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. - -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. - -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. - -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. - -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 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. - -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. - -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. - - - - -CHAPTER VII - - Influence of Volcanic Rocks on the Scenery of the Land--Effects of - Denudation. - - -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. - -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. - -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. - -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 crests between them. If -unchecked by any fresh discharge of volcanic material, the degradation -will at last have removed the whole cone. - -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. - -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. - -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. - -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. - -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. - -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 (Fig. 346). 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. -331, 333). - -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. 109, 110, 133); 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. 99, 107). 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. 11, 265, 283, 284, 286). - -One of the most frequent causes of local peculiarities of topography -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 (Fig. 265). 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 (Fig. 333), 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. - -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. - -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. - -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. - -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 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. - -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. - -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. - -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. - -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 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. - -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. - -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. - -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 with a warm glow as if -they, in some special way, could catch and reflect the radiance of the -sky. - -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 (Fig. 331). 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. - -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. - -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. - -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 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. - - * * * * * - -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. - -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. - - - - -BOOK II - -VOLCANIC ACTION IN PRE-CAMBRIAN TIME - - - - -CHAPTER VIII - -PRE-CAMBRIAN VOLCANOES - - 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. - - -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. - -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 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. - -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. - -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. - -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. - -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 of the terms applied to the fossiliferous formations. Yet the -convenience of possessing such a general descriptive epithet is obvious. - -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. - -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. - -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: I. The Lewisian (Archæan) Gneiss; II. The Younger -(Dalradian) Schists of Scotland; III. The Gneisses and Schists of -Anglesey; IV. The Uriconian Group; V. The Malvern Group; VI. The -Charnwood Forest Group (see Map I.). - - -i. THE LEWISIAN (ARCHAÆN) GNEISS - -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.[50] In that territory they 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 (Fig. 35) will -explain the general relations of the various geological formations of -the region. - -[Footnote 50: 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 _Quarterly Journal of the Geological Society_, vol. xliv. -(1888), p. 378.] - -In certain dark shales (_b_) which occupy a well-defined and -readily-traceable position among the rocks of Sutherland and Ross, -numerous specimens of the trilobite genus _Olenellus_, 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. - -[Illustration: Fig. 35.--Diagram illustrating the stratigraphical -relations of the pre-Cambrian and Cambrian rocks of the North-west -Highlands of Scotland. - -_c_, Durness Limestones, with Upper Cambrian and perhaps Lower Silurian -fossils, 1500 feet, top nowhere seen. _b_, Serpulite grit and "fucoid" -shales, 70 to 80 feet, containing the _Olenellus_-zone. _a_, 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).] - -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 _Olenellus_-zone with its underlying sheets of -quartzite (_a_), 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. - -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 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. - -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. - -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. - -In this manner, reasoning backward from the horizon of the -_Olenellus_-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. - -Under the term Lewisian Gneiss (I. in Fig. 35) 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 in full detail that their true -nature and history have begun to be understood.[51] - -[Footnote 51: See the Report of this Survey work by Messrs. Peach, -Horne, Gunn, Clough, Cadell and Hinxman, _Quart. Journ. Geol. Soc._ -vol. xliv. (1888), pp. 378-441; and Annual Reports of Director-General -of the Geological Survey in the _Report of The Science and Art -Department for 1894_, p. 279, and 1895, p. 17 of reprint. The general -area of the gneiss is shown in Map I.] - -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. - -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. - -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:-- - - 1. Rocks composed of ferro-magnesian minerals, without felspar or - quartz--Pyroxenites, Hornblendites. - - 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. - - 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: (_a_) massive or - only slightly foliated (Amphibolites, as epidote-amphibolite, - zoisite-amphibolite, garnet-amphibolite); (_b_) foliated - (Hornblende-schist). B, With quartz; rocks intermediate or acid - in composition: (_a_) with compact hornblende and a granular - structure (Hornblende-gneiss proper); (_b_) with hornblende - occurring in fibrous or other aggregates; (_c_) with compact - hornblende and a more or less granulitic structure (Granulitic - hornblende-gneiss). - - 4. Rocks in which biotite is the predominant ferro-magnesian - constituent; felspar and quartz both present: (_a_) Biotite - occurring as independent plates or in aggregates of two or three - large individuals (Biotite-gneiss); (_b_) Biotite occurring - in aggregates of numerous small individuals (rare type); - (_c_) Biotite occurring as independent plates in a granulitic - structure. - - 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 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. - -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. - -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. - -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. - -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. - -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 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.[52] - -[Footnote 52: _Volcanoes_, pp. 140, 283, 299.] - -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. 336 and 337). 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.[53] 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. - -[Footnote 53: See A. Geikie and J. J. H. Teall, _Quart. Journ. Geol. -Soc._ vol. 1. (1894), p. 645.] - -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. - -When the larger area of gneiss forming the chain of the Outer 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. - -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. - -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. - -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 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[54] (Fig. -36). - -[Footnote 54: _Quart. Journ. Geol. Soc._ vol. xliv. (1888), p. 389 _et -seq._] - -[Illustration: _Walker & Boutall sc._ - -Fig. 36.--Map of a portion of the Lewisian gneiss of Ross-shire. - -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 (_t_), which is injected by -sheets of oligoclase-porphyry (F).] - -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. - -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 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).[55] - -[Footnote 55: _Annual Report of Geological Survey for 1895_, p. 18 of -reprint.] - -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.[56] 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. - -[Footnote 56: See Mr. Teall, _Quart. Journ. Geol. Soc._ vol. xli. -(1885), p. 133.] - -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. - -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. - -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. - -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 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. - -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.[57] 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. - -[Footnote 57: _Annual Report of Geological Survey for 1895_, p. 21 of -reprint.] - -Among the Torridonian strata no undoubted trace of any contemporaneous -volcanic eruptions has been met with.[58] 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. - -[Footnote 58: The supposed tuff referred to in _Quart. Journ. Geol. -Soc._ vol. xlviii. (1892), p. 168, is probably not of truly volcanic -origin.] - -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.[59] 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. - -[Footnote 59: "Über Archæische Ergussgesteine aus Småland," _Sveriges -Geol. Undersökn_, No. 135 (1894).] - - -ii. THE DALRADIAN OR YOUNGER SCHISTS OF SCOTLAND - -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. - -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. - -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. - -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. 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. - -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.[60] - -[Footnote 60: _Geol. Survey Memoirs: Geology of N.W. Donegal_, 1891.] - -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. - -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 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. - -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. - -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 (_Radiolaria_), 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 Chapter xii. 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. - -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 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.[61] - -[Footnote 61: _Presidential Address to Geological Society_, 1891, p. -39.] - -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. - -[Illustration: Fig. 37.--Section showing the position of Sills in the -mica-schist series between Loch Tay and Amulree. - -_a_, Mica-schist; _b_, _b_, Sills.] - -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 (Fig. 37). - -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. - -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. - -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. - -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.[62] 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. - -[Footnote 62: From such a rock, which crosses the upper part of Loch -Fyne, the Duke of Argyll's residence at Inveraray has been built.] - -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. - -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. - -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. - - -iii. THE GNEISSES AND SCHISTS OF ANGLESEY - -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.[63] -I will content myself with expressing what seems to me, after several -traverses, to be the geological structure of the ground. - -[Footnote 63: 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 _Memoirs of the -Geological Survey_, "The Geology of North Wales," by Sir A. C. Ramsay, -2nd edit. 1881. The subject has been discussed by Professor Hughes, -_Quart. Journ. Geol. Soc._ vols. xxxiv. (1878) p. 137, xxxv. (1879) p. -682, xxxvi. (1880) p. 237, xxxviii. (1882) p. 16; _Brit. Assoc. Rep._ -(1881) pp. 643, 644; _Proc. Camb. Phil. Soc._ vol. iii. pp. 67, 89, -341; by Professor Bonney, _Quart. Journ. Geol. Soc._ vol. xxxv. (1879) -pp. 300, 321; _Geol. Mag._ (1880) p. 125; by Dr. H. Hicks, _Quart. -Journ. Geol. Soc._ vols. xxxiv. (1878) p. 147, xxxv. (1879) p. 295; -_Geol. Mag._ (1879) pp. 433, 528 (1893) p. 548; by Dr. C. Callaway, -_Quart. Journ. Geol. Soc._ vols. xxxvii. (1881) p. 210, xl. (1884) p. -567; and by the Rev. J. F. Blake, _Quart. Journ. Geol. Soc._ vol. xliv. -(1888) p. 463. Further references to the work of these observers in -Anglesey are given in Chapter xiii. p. 220 _et seq._ The Pre-Cambrian -areas of Anglesey are shown in Map II.] - -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. - -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. - -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. - -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 and fractures are numerous, -and much of the surface of the ground is obscured by the spread of -Palæozoic formations and superficial deposits. - -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. - -[Illustration: Fig. 38.--Sketch of crushed basic igneous rock among the -schists, E. side of Porth-tywyn-mawr, E. side of Holyhead Straits.] - -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 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 (Fig. -38) represents a portion of one of these crushed basic igneous rocks on -the east side of Holyhead Straits. - -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. - -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.[64] - -[Footnote 64: 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.] - - -iv. THE URICONIAN VOLCANOES - -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 _Silurian System_.[65] 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 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.[66] 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. - -[Footnote 65: See especially chap. xix. vol. i. p. 225.] - -[Footnote 66: 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.] - -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. - -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.[67] Subsequently Professor Bonney -described more fully the petrographical characters of the Wrekin -igneous rocks, confirming and extending the observations of Mr. -Allport.[68] - -[Footnote 67: _Quart. Journ. Geol. Soc._ vol. xxxiii. (1877) p. 449.] - -[Footnote 68: _Op. cit._ vol. xxxv. (1879) p. 662; vol. xxxviii. (1882) -p. 124.] - -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.[69] 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.[70] - -[Footnote 69: _Quart. Journ. Geol. Soc._ 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, _op. cit._ vol. xlvi. -(1890) p. 386, and Dr. Callaway's reply, vol. xlvii. (1891) p. 109.] - -[Footnote 70: _Op. cit._ vol. xlvii. (1891) p. 123.] - -Subsequently Professor Lapworth, by his discovery of the -_Olenellus_-fauna, marking the lowest known fossiliferous Cambrian -zone in the Wrekin 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.[71] - -[Footnote 71: _Geol. Mag._ (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, _Proc. Geol. Assoc._ vol. xiii. (1894) pp. 302, 335.] - -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.[72] - -[Footnote 72: _Quart. Journ. Geol. Soc._ vol. xlvii. (1891) p. 540. Mr. -Rutley more particularly describes those of Caradoc Hill.] - -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.[73] 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.[74] 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. - -[Footnote 73: _Op. cit._ p. 534.] - -[Footnote 74: 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.] - -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 _Olenellus_, _Paradoxides_ 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. - -[Illustration: Fig. 39.--Section across the Uriconian series of Caer -Caradoc. - -S3, Upper Silurian; S2, Bala group; S1, Arenig group; C, Cambrian; L, -Longmyndian; _u_, Uriconian; _f_ _f_, faults.] - -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. - -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 Fig. 39.[75] - -[Footnote 75: _Proc. Geol. Assoc._ vol. xiii. (1894), pp. 314, 315.] - -If, then, this volcanic group underlies the whole of the Longmynd -series, and if, as it now appears, that series is older than the -_Olenellus_-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. - -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. - - -V. THE MALVERN VOLCANO - -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.[76] 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. - -[Footnote 76: 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 _Memoirs of the Geological Survey_, -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, _Quart. Journ. Geol. Soc._ xxi. (1865) p. 72; J. -H. Timins, _op. cit._ xxii. (1867); Mr. F. Rutley, _op. cit._ xliii. -(1887) p. 481; Dr. Callaway, _op. cit._ xliii. (1887) p. 525, xlv. -(1889) p. 475, xlix. (1893) p. 398; Prof. Green, _op. cit._ li. (1895) -p. 1; Mr. H. D. Acland, _Geol. Mag._ 1894, p. 48.] - -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. - -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.[77] - -[Footnote 77: _Op. cit._ p. 7. The metamorphism of the igneous rocks of -the Malvern Hills into schists has been especially investigated by Dr. -Callaway.] - -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. - - -vi. THE CHARNWOOD FOREST VOLCANO - -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. - -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. - -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:--[78] - - 7. Groby and Swithland slates. } - } - 6. Hanging Rocks conglomerate and } The Brand series. - Bradgate quartzite. } - - 5. Woodhouse beds (ashy grits). } - } - 4. Slate-agglomerate of Roecliffe. } The Maplewell series (volcanic - } tuffs and agglomerates). - 3. Hornstone beds of Beacon Hill. } - } - 2. Felsitic agglomerate of Benscliffe. } - - 1. Quartzose, felspathic and felsitic grits. The Blackbrook series. - -[Footnote 78: Annual Report of Director-General of the Geological -Survey, in the _Report of Science and Art Department for 1895_.] - -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 with fine felspathic -grits, which distinctly mark the stratification of the whole. - -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. - -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). - -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. - -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. - -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 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.[79] -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.[80] 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. - -[Footnote 79: _Quart. Journ. Geol. Soc._ xlvii. (1891), pp. 80-88.] - -[Footnote 80: See Messrs. Hill and Bonney, _op. cit._ xxxiv. (1878), p. -211.] - -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. - -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.[81] 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 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. - -[Footnote 81: 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, _Geol. Mag._ 1896, p. 487.] - - - - -BOOK III - -THE CAMBRIAN VOLCANOES - - - - -CHAPTER IX - -CHARACTERISTICS OF THE CAMBRIAN SYSTEM IN BRITAIN - - 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. - - -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. - -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, 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. - -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. - -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 -_Olenellus_, already referred to, is the characteristic form. Higher -up _Paradoxides_ is predominant, while towards the top of the system -the most characteristic genus is _Olenus_. - -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 -_Olenellus_ 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. - -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. - -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. - -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 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. - -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. - -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.[82] - -[Footnote 82: For early researches on the older Palæozoic volcanic -rocks of Britain, see Sedgwick, _Proc. Geol. Soc._ vols. ii. (1838) pp. -678, 679, iii. (1841) p. 548, iv. (1843) p. 215; _Quart. Journ. Geol. -Soc._ vols. i. (1845) pp. 8-17, iii. (1847) p. 134. Murchison, _Proc. -Geol. Soc._ vol. ii. (1833-34) p. 85; _Silurian System_ (1839) pp. 225, -258, 268, 287, 317, 324, 401; _Siluria_, 4th edit. (1867) p. 76 _et -seq._ De la Beche, _Mem. Geol. Survey_, 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 -_Memoirs of the Geological Survey_, 1st edit. (1866), 2nd edit. (1881).] - -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. 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. - -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. - -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. - -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 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.[83] 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. - -[Footnote 83: See a discussion of this subject in _Quart. Journ. Geol. -Soc._ vol. xxxix. (1883), p. 305.] - -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,[84] 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. - -[Footnote 84: _Proc. Camb. Phil. Soc._ vol. iii. (1877), p. 89. The -Cambrian volcanic areas of North Wales are represented in Map II.] - -The following table shows the subdivisions of the Cambrian system now -recognized in Britain:-- - - +----------------------+----------------------+-------------------------+ - | Wales. | | | - | (Ranging up to | Western England. | N.W. Scotland. | - | 12,000 feet | (About 3000 feet.) | (About 2000 feet.) | - | or more.) | | | - +----------------------+----------------------+-------------------------+ - |Upper or _Olenus_ Zones. | | - | Tremadoc Slates | Shineton Shales | Limestones, about 1500 | - | Lingula Flags | (_Dictyograptus_ | feet thick, divisible | - | (_Lingulella_, | or _Dictyonema_, | into seven groups | - | _Olenus_, etc.). | _Olenus_, etc.). | (_Archæocyathus_, | - | | | _Maclurea_, _Ophileta_,| - |Middle or _Paradoxides_ Zones. | _Murchisonia_, | - | Menevian group | Conglomerates and | _Orthoceras_, | - | (_Paradoxides_). | limestones (Comley),| and vast quantities of | - | | (Comley), with | annelid castings). | - | | _Paradoxides_, etc. | | - | | | | - |Lower or _Olenellus_ Zones. | | - | Harlech and | Thin quartzite | Shales ("fucoid beds"), | - | Llanberis | passing up into | with _Olenellus_, | - | group with | green flags, grits, | _Salterella_, etc. | - | basement volcanic | shales | Quartzites with annelid | - | rocks; bottom not | and sandstone | burrows. The base of | - | seen. | (Comley Sandstone), | the series lies | - | | containing | unconformably on | - | | _Olenellus_. | pre-Cambrian rocks. | - +----------------------+----------------------+-------------------------+ - - - - -CHAPTER X - -THE CAMBRIAN VOLCANOES OF SOUTH WALES - - -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.[85] - -[Footnote 85: For Dr. Hicks' views, see especially his papers in the -_Quart. Journ. Geol. Soc._ vols. xxxi. xxxiii. xxxiv. xl. My criticism -of them will be found in _op. cit._ vol. xxxix. (1883), subsequently in -the main confirmed by Prof. Lloyd Morgan, _op. cit._ xlvi. p. 241. See -also Prof. Blake, _op. cit._ xl. (1884). Dr. Hicks in his more recent -papers has merely reiterated his previously published opinions.] - -[Illustration: _Walker & Boutall sc._ - -Fig. 40.--Map of the volcanic district of St. David's.] - -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 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. - -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. - -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.[86] A few additional -observations, from the subsequent exploration of Professor Lloyd -Morgan,[87] are incorporated in the narrative. - -[Footnote 86: _Quart. Journ. Geol. Soc._ vol. xxxix. (1883), p. 294 _et -seq._ 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.] - -[Footnote 87: _Op. cit._ vol. xlvi. (1890), p. 241.] - -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. - -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. - -1. _Bedded Tuffs and Lavas._--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 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. - -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. - - +-----------------------------+--------+--------+--------+ - | | I. | II. | III. | - +-----------------------------+--------+--------+--------+ - |SiO_{2} | 51·25 | 48·11 | 61·54 | - |Al_{2}O_{3} | 20·41 | 13·30 | 16·30 | - |Fe_{2}O_{3} | 3·02 | 3·70 | 4·40 | - |FeO | 3·91 | 8·10 | 3·66 | - |MnO | 0·21 | 1·43 | 0·32 | - |CaO | 4·53 | 8·48 | 3·08 | - |MgO | 7·22 | 9·51 | 2·99 | - |K_{2}O | 2·93 | 1·57 | 1·62 | - |Na_{2}O | 1·82 | 1·96 | 2·81 | - |H_{2}O and Loss on Ignition. | 5·02 | 4·21 | 2·99 | - |Total. | 100·32 | 100·37 | 99·71 | - |Specific Gravity. | 2·84 | 2·92 | ... | - +-----------------------------+--------+--------+--------+ - - I. Purplish-red shaly tuff from below olivine-diabase, Crag Rhosson. - Analysis by Mr. J. S. Grant Wilson. - - II. Dull purple and green tuff from the lowest group of tuffs between - Pen-maen-melyn and Pen-y-foel. Analysis by Mr. Wilson. - - III. Greenish shaly finely granular tuff, from road-side, north of Board - Schools, St. David's. Analysis by Prof. A. Renard of Ghent. - -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.). - - +-----------------------------+--------+--------+--------+ - | | IV. | V. | VI. | - +-----------------------------+--------+--------+--------+ - |SiO_{2} | 80·59 | 73·42 | 72·63 | - |Al_{2}O_{3} | 11·29 | 12·09 | 16·23 | - |Fe_{2}O_{3} | 0·28 | 0·91 | 2·70 | - |FeO | 1·41 | 3·13 | 0·48 | - |MnO | trace | 0·25 | ... | - |CaO | 0·52 | 2·94 | 0·18 | - |MgO | 0·95 | 1·12 | 1·36 | - |K_{2}O | 2·98 | 1·67 | 3·35 | - |Na_{2}O | 0·72 | 3·88 | 0·15 | - |H_{2}O and Loss on Ignition | 1·96 | 1·28 | 3·00 | - |Total. |100·70 | 100·69 | 100·12 | - |Specific Gravity. | 2·55 | 2·74 | ... | - +-----------------------------+--------+--------+--------+ - - IV. Greenish felsitic breccia, Clegyr Hill; angular fragments of various - felsites in a greenish base. Analysis by Mr. J. S. Grant Wilson. - - V. Grey granular felsitic tuff, Bridge over Allan River north from St. - David's Board Schools. Analysis by Mr. Wilson. - - VI. Pale pinkish-white, finely schistose tuff--a characteristic sample - of the "Porth-lisky schists." Analysis by Prof. Renard. - -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. - -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 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. - -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. - -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. - -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 Fig. 41). - -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:-- - -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. - -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. - -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: (_a_) lapilli of a -minutely-cellular pumice with spherical cells; (_b_) lapilli with -well-developed flow-structure; (_c_) lapilli consisting of a pale green -serpentinous substance resembling altered palagonite and probably -originally glass; (_d_) lapilli derived from the destruction of older -tuffs; and (_e_) lapilli consisting of ejected crystals, especially of -felspars, sometimes entire, often broken. - -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. - -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. - -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 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. - -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 Chapter -xx. 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. - -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. - -Excluding the granites and porphyries (which are described at p. 155), -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. - -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. - -The eruptive rocks thicken towards the south-west, as if the main -vents 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. - -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:-- - - +-----------------------------+---------+--------+ - | | VII. | VIII. | - +-----------------------------+---------+--------+ - | SiO_{2} | 45·92 | 45·38 | - | Al_{2} O_{3} | 18·16 | 16·62 | - | Fe_{2} O_{3} | 1·18 | 4·06 | - | FeO | 9·27 | 8·63 | - | MnO | 0·19 | 0·14 | - | CaO | 7·19 | 8·19 | - | MgO | 10·07 | 9·41 | - | K_{2}O | 1·78 | 0·71 | - | Na_{2}O | 2·12 | 2·20 | - | H_{2}O and Loss on Ignition.| 4·22 | 4·34 | - | Insoluble Residue. | 0·04 | 0·08 | - | Total. | 100·14 | 99·76 | - | Specific Gravity. | 2·96 | 2.99 | - +-----------------------------+---------+--------+ - -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 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. - -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. - -[Illustration: - - Fig. 41.--Section showing the interstratification of tuff and - conglomerate above Lower Mill, St. David's. -] - -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 Fig. 41. On the left side -vertical layers of fine reddish tuff (_a_) are succeeded by a band -of quartz conglomerate (_b_) of the usual character. Parallel to this -conglomerate comes a band, about six inches thick, of fine tuff (_c_), -followed by ashy sandstone (_d_), which graduates into conglomerate -(_e_). 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. - -Not less striking is the evidence of the contemporaneous eruption of -fine volcanic dust in the overlying shales and sandstones.[88] 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. - -[Footnote 88: These are a portion of Dr. Hicks' "Caerfai group" in the -Lower Cambrian series. They have yielded Lower Cambrian fossils.] - -2. _Intrusive Granite and Quartz-Porphyry._--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."[89] -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. - -[Footnote 89: See the papers cited on p. 145 and my discussion of the -relations of this granite in _Quart. Journ. Geol. Soc._ vol. xxxix.; -also Prof. Lloyd Morgan, _op. cit._ vol. xlvi. (1890).] - -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 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.[90] - -[Footnote 90: These apophyses from the granite constitute the -"Arvonian" formation of Dr. Hicks' pre-Cambrian series of St. David's.] - -These spherulitic rocks have been fully described.[91] 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. - -[Footnote 91: See, for example, J. Davies, _Quart. Journ. Geol. Soc._ -vol. xxiv. p. 164, xxxv. p. 203; also the paper already referred to, -_op. cit._ xxxix. p. 315; and Mr. Teall's _British Petrography_, p. -334.] - -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. - -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. - -3. _Diabase Dykes and Sills._--The latest rocks of the St. David's -district are dykes and intrusive sheets of diabase, which traverse -all the other 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.[92] - -[Footnote 92: The occurrence of these dykes is paralleled by that of -the similar intrusions in the quartz-felsite of Llyn Padarn to be -afterwards described.] - -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. - -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. - -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;[93] consequently they must be -posterior to the development of the system of jointing in that rock. - -[Footnote 93: _Proc. Nat. Hist. Soc. Bristol_, vol. ii. part ii. -(1879), p. 115.] - -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. - -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 stratigraphical order.[94] 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. - -[Footnote 94: 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 (_Quart. Journ. Geol. Soc._ 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.] - - - - -CHAPTER XI - -THE CAMBRIAN VOLCANOES OF NORTH WALES, THE MALVERN HILLS AND -WARWICKSHIRE - - -NORTH WALES - -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.[95] 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.[96] In the Pass -of Llanberis the series of grits that overlies the purple slates is -estimated to be about 1300 feet thick.[97] 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 in the formation of -granites, syenites and other rocks, commonly called intrusive."[98] -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."[99] 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. - -[Footnote 95: _Mem. Geol. Surv._ vol. iii. 2nd edit. "Geology of North -Wales," p. 21. It is possible that this thickness has been somewhat -overestimated. Dr. Hicks (_Geol. Mag._ 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.] - -[Footnote 96: _Ibid._ p. 24.] - -[Footnote 97: _Ibid._ p. 173.] - -[Footnote 98: _Mem. Geol. Surv._ vol. iii. 2nd edit. p. 173.] - -[Footnote 99: _Ibid._ p. 174.] - -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. - -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. - -These assertions and inferences do not seem to me to be warranted. They -have, in my judgment, been disproved by Mr. Blake,[100] 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.[101] - -[Footnote 100: In an excellent memoir read before the Geological -Society in 1888, with the main conclusions of which I agree.] - -[Footnote 101: _Quart. Journ. Geol. Soc._ vol. xliv. p. 271. For -subsequent papers by Mr. Blake, see _op. cit._ vols. xlviii. (1892) p. -243, xlix. (1893) p. 441.] - -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 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. - -It is generally agreed that the rocks variously termed -quartz-porphyries, felsites or rhyolites form the oldest members -of this volcanic series.[102] 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. - -[Footnote 102: 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.] - -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;[103] while, on the other hand, the rock generally presents -the tolerably uniform flinty texture so familiar in intrusive sheets of -similar material.[104] 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. - -[Footnote 103: 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.] - -[Footnote 104: 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 (_Quart. Journ. Geol. -Soc._ vol. xliv. (1888), p. 283). But the rock supposed by him to be -altered slate has been shown to be intrusive "greenstone" (Miss Raisin, -_op. cit._ vol. xlvii. (1891), p. 336).] - -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.[105] 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.[106] - -[Footnote 105: See for example, Prof. Bonney, _Quart. Journ. Geol. -Soc._ vol. xxxv. (1879), p. 312; Mr. Blake, _op. cit._ 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.] - -[Footnote 106: Mr. Blake, _ibid._ p. 277.] - -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.[107] - -[Footnote 107: _Op. cit._ vol. xxxv. p. 312.] - -[Illustration: Fig. 42.--Basic dyke traversing quartz-porphyry and -converted into a kind of slate by cleavage. West side of Llyn Padarn. - -_p_ _p_, quartz-porphyry; _d_ _d_, dyke and connected veins.] - -Reference may be made here to the remarkable influence of the intense -cleavage of the district upon this rock.[108] 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, 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 (Fig. 42). 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. - -[Footnote 108: The secondary planes due to cleavage must not be -confounded with the original flow-structure.] - -[Illustration: Fig. 43.--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.] - -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.[109] -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.[110] These may -be well studied both on the south and the north side of the porphyry -ridge at the lower 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. - -[Footnote 109: 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.] - -[Footnote 110: I can find no evidence of unconformability beneath any -of the conglomerates. The section described by Professor Green, _Quart. -Journ. Geol. Soc._ 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 Fig. 43. 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 (_Quart. Journ. Geol. Soc._ vol. xlix. (1893), p. 445). -The correct interpretation is given by Professor Bonney and Miss Raisin -(_op. cit._ vol. l. p. 592).] - -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 _in situ_ 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.[111] - -[Footnote 111: On the composition of the conglomerates or breccias, see -Professor Bonney and Miss Raisin, _Quart. Jour. Geol. Soc._ vol. l. -(1894), p. 598.] - -[Illustration: Fig. 44.--Section of Clegyr on the north-east side of -Llyn Padarn, near the lower end.] - -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 -(Fig. 44), near the lower end of Llyn Padarn, on the north-east side, -consists mainly of cleaved tuffs (_t_) and slates with conglomerates -(_c_), overlying the quartz-porphyry (_p_). Near the summit a band of -felsite is intercalated in these rocks. - -Still more striking are the sections on the south-west side of the -lake.[112] 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.[113] 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. - -[Footnote 112: For various readings of these sections, compare Mr. -Blake (_Quart. Jour. Geol. Soc._ vol. xlix. (1893), p. 450) with -Professor Bonney and Miss Raisin (_op. cit._ vol. l. (1894), p. 581).] - -[Footnote 113: See Professor Bonney and Miss Raisin, _op. cit._ p. 593 -_et seq._] - -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.[114] - -[Footnote 114: 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 _in situ_. -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, _op. cit._ p. 596, _footnote_.] - -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. - -The tuffs, with their interstratified bands of porphyry, become more -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. - -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.[115] And in his section he represents them as undulating towards -the east and passing under the great mass of the Caernarvonshire slates -and porphyries. - -[Footnote 115: _Proc. Geol. Soc._ vol. iv. p. 212; _Quart. Journ. Geol. -Soc._ vol. iii. (1847), p. 136.] - -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.[116] He has been followed -in this view by subsequent writers;[117] 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.[118] - -[Footnote 116: _Quart. Journ. Geol. Soc._ vol. xxxiv. (1878), p. 137.] - -[Footnote 117: Prof. Bonney, _op. cit._ vol. xxxv. (1879), p. 316; Dr. -Hicks, _ibid._ p. 296.] - -[Footnote 118: _Op. cit._ vol. xliv. (1888), p. 278.] - -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 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.[119] - -[Footnote 119: 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.] - -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. - -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. - -No satisfactory proof has yet been obtained of any interstratified -lavas among the tuffs of the Bangor district. Some rocks, indeed, can -be seen 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. - -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 Chapter xii., the Cambrian volcanic eruptions -appear to have been on the whole less acid, and to have begun with the -outpouring of andesitic lavas. - -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.[120] - -[Footnote 120: See Mr. Blake on this point, _Quart. Journ. Geol. Soc._ -vol. xlviii. (1892), p. 252, _note_. I retain the opinion expressed -above.] - -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 everything older than themselves as they are traced -northwards.[121] 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.[122] - -[Footnote 121: _Mem. Geol. Surv._ vol. iii. 2nd edit. p. 252.] - -[Footnote 122: 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 _Annual -Report of the Geological Survey for 1895_, p. 27 of reprint.] - -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. - - -THE MALVERN HILLS - -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. - -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 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. - -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.[123] The position of these eruptive rocks is shown in Fig. 45. - -[Footnote 123: _Mem. Geol. Survey_, 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 _Mem. Geol. Surv._ -pp. 34, 38.] - -[Illustration: Fig. 45.--Section across the Cambrian formations of the -Malvern Hills, showing the position of the intercalated igneous rocks -(_p_ _p_). After Phillips.] - -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.[124] - -[Footnote 124: _Quart. Journ. Geol. Soc._ vol. xxi. (1865), pp. 87-91.] - -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. - - -WARWICKSHIRE - -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.[125] 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.[126] 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.[127] These shales are -immediately overlain by the Coal-measures. - -[Footnote 125: Their antiquity was recognized by Yates as far back as -1825 (_Trans. Geol. Soc._ 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.] - -[Footnote 126: _Geol. Mag._ (1882), p. 563.] - -[Footnote 127: _Op. cit._ (1886), p. 319.] - -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[128] and -Mr. F. Rutley,[129] and have been traced upon a revised edition of -the Geological Survey map by Mr. A. Strahan.[130] 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. - -[Footnote 128: _Op. cit._ p. 323.] - -[Footnote 129: _Op. cit._ p. 557.] - -[Footnote 130: _Geol. Mag._ (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," _Mem. Geol. Survey_, "South Staffordshire Coal-field" -(1859), p. 134.] - -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. - - - - -BOOK IV - -THE SILURIAN VOLCANOES - - - - -CHAPTER XII - -CHARACTERS OF THE SILURIAN SYSTEM IN BRITAIN. THE ARENIG VOLCANOES - - 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. - - -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. - -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 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. - -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. - -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. - -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. - -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 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. - -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. - -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.[131] - -[Footnote 131: For references to the older literature see _ante_, p. -142.] - -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:-- - - +---------------------------------------+-------+-------+--------+-------+ - | |England| Wales |Scotland|Ireland| - +---------------------------------------+-------+-------+--------+-------+ - | { Ludlow Group | ... | ... | ... | ... | - |Upper Silurian { Wenlock Group | ... | ... | ... | --- | - | { Llandovery Group | ? | ... | ... | ... | - | | | | | | - | { Bala and Caradoc Group| --- | --- | --- | --- | - |Lower Silurian { Llandeilo Group | --- | --- | --- | --- | - | { Arenig Group | --- | --- | --- | --- | - +---------------------------------------+-------+-------+--------+-------+ - -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. - - -I. THE ERUPTIONS OF ARENIG AGE - - -i. MERIONETHSHIRE - -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. - -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. - -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.[132] 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 _Geology of North Wales_.[133] 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. - -[Footnote 132: _Mem. Geol. Surv._ vol. iii. 2nd edit. p. 95, note.] - -[Footnote 133: 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, _Quart. Journ. -Geol. Soc._ vol. xlv. (1889), p. 436.] - -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.[134] 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.[135] 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.[136] 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.[137] -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.[138] - -[Footnote 134: _Mem. Geol. Survey_, vol. iii. 2nd ed., p. 96.] - -[Footnote 135: 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, _Geol. Mag._ (1890), p. 451.] - -[Footnote 136: _Op. cit._ p. 72.] - -[Footnote 137: He was disposed to regard Rhobell Fawr as one of the -great centres of eruption of the district. See _Memoir of A. C. -Ramsay_, p. 81, and _Geology of North Wales_, 2nd edit. p. 98.] - -[Footnote 138: _Op. cit._ pp. 71, 96, 105.] - -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 _Survey Memoirs_, and my own examination of the ground led me to -accept their conclusion.[139] 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. - -[Footnote 139: _Quart. Journ. Geol. Soc._ vol. xlv. (1889), p. 436; -_Geol. Mag._ (1890), p. 447. _Pres. Address Geol. Soc._ 1890, p. 107.] - -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.[140] - -[Footnote 140: _Quart. Journ. Geol. Soc._ vol. lii. (1896), p. 511.] - -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.[141] - -[Footnote 141: Prof. Cole, _Geol. Mag._ (1893), p. 337.] - -[Illustration: Fig. 46.--Section across Rhobell Fawr.[142] - -L L, Lingula flags; _t_, tuffs and ashy slates; _s_, slates and grits; -F F, Arenig volcanic series; D, dolerite.] - -[Footnote 142: After Messrs. Cole and Holland, _Geol. Mag._ (1890), p. -450.] - -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. - -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.[143] There can be no doubt, however, that the volcanic -intercalations continue far above that horizon in the southern part of -the district. - -[Footnote 143: _Mem. Geol. Survey_, vol. iii. 2nd edit. pp. 249, 250.] - -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 as they are traced southwards from Cader -Idris and north-westwards from Tremadoc.[144] 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. - -[Footnote 144: _Op. cit._ p. 96.] - -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.[145] - -[Footnote 145: 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.] - -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. - -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 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. - -[Illustration: - - Fig. 47.--Section at the Slate Quarry, Penrhyn Gwyn, north slopes - of Cader Idris. -] - -The accompanying section (Fig. 47) represents the arrangement of the -rocks exposed at the Slate Quarry of Penrhyn Gwyn. About 50 feet of -black slate (_a_) 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 (_b_) 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 (_c_), 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 (_d_), containing abundant -felspar crystals, pass under a flinty porphyritic felsite (_e_) 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. - -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. - -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. - -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."[146] 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 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.[147] 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 Fig. 48). 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. - -[Footnote 146: _Quart. Journ. Geol. Soc._, vol. xlv. (1889), p. 424; -_Geol. Mag._ (1890), p. 447.] - -[Footnote 147: _Op. cit._ p. 429. A tuff lying below the ironstone near -Cross Foxes, east of Dolgelly, likewise contains fragments of trachytic -lavas.] - -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 _in situ_, -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. - -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. - -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_ in Fig. 48). 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 _Lingulæ_ 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.[148] - -[Footnote 148: Messrs. Cole and Jennings, _op. cit._ p. 426.] - -Above the ironstone some more blue and black shale and grit pass under -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. - -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 Fig. 48). 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. - -[Illustration: Fig. 48.--Sketch-section across Cader Idris. - -_st_, slates and tuffs and ashy slates; _s_, slates and grits; _i_, -ironstone; _b_, volcanic breccias; _a_, slaggy andesitic and more basic -lavas; _e_, microgranite or eurite; _f_, felsites; _d_, "greenstone" -(dolerites, diabases, etc.).] - -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 _Descriptive Catalogue of Rock-Specimens -in the Museum of Practical Geology_ (3rd edit., 1862) the rocks -that form the "lava-streams of Llandeilo age," in Merionethshire, -are named "felstone," "felspar-porphyry," "felstone-porphyry," -"felspathic-porphyry," and "calcareous amygdaloid." - -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."[149] - -[Footnote 149: _Mem. Geol. Surv._ vol. iii. 2nd edit. p. 36; see also -pp. 31, 32.] - -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 (_b_ -in Fig. 48), 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 (_a_ in Fig. 48). 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. - -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. - -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. - -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. - -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.[150] 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. - -[Footnote 150: 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 p. 26.] - -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.[151] The petrography of these rocks still remains to be worked -out. - -[Footnote 151: Messrs. Cole and Jennings, _Quart. Journ. Geol. Soc._ -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."] - -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 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,[152] 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. - -[Footnote 152: _Quart. Journ. Geol. Soc._ xlvii. (1891), p. 368.] - -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. - -[Illustration: Fig. 49.--Section across the Moelwyn Range.[153] - - 1, Tremadoc Group; 2, Garth or Arenig grit (base of Arenig group); - 3, Arenig slates, etc.; 3^1, Lower slate band; 3^2, Middle slate - band; 3^3, Upper slate band; 4^1, Lower agglomerate; 4^2, Middle - agglomerate; 4^3, Upper agglomerate; 5, Llandeilo group; G, - Granite boss of Moel tan y Grisiau. -] - -[Footnote 153: After Messrs. Jennings and Williams, _Quart. Journ. -Geol. Soc._ vol. xlvii. (1891), p. 371, and Horizont. Sect. Geol. Surv. -Sheet 28.] - -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 _Lingula_, _Orthis Carausii_, and a _Tetragraptus_, 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 (Fig. 49). 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 upper agglomerate, fragments -of cleaved slate containing _Lingula_ have been observed. - -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. - -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. - -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.[154] 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. - -[Footnote 154: _Mem. Geol. Surv._ vol. iii. 2nd edit. p. 98; see also -pp. 44, 54, 58, 71.] - -If we are justified, on stratigraphical and petrographical grounds, in -connecting the lowest volcanic rocks of the Berwyn range with those -of 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.[155] - -[Footnote 155: The Berwyn Hills, however, will be described in later -pages as a distinct volcanic district.] - -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."[156] 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. - -[Footnote 156: _Mem. Geol. Surv._ vol. iii. p. 26.] - -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. - -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.[157] 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. - -[Footnote 157: _Quart. Journ. Geol. Soc._ vol. xlv. (1889), p. 432.] - -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 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. - -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."[158] - -[Footnote 158: Mr. Harker speaks of the rock as a granophyre.] - -A similar rock occurs at a lower horizon among the Lingula Flags at -Gelli-llwyd-fawr, two miles south-west of Dolgelly,[159] and much -microgranite has been injected along the slopes above Tyddyn-mawr. - -[Footnote 159: Messrs. Cole and Jennings, _op. cit._ p. 435.] - -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.[160] -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 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. - -[Footnote 160: _Quart. Journ. Geol. Soc._ vol. xlvii. (1891), p. 379.] - -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. - -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. - - -ii. SHROPSHIRE - -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.[161] The ground has in recent years been more carefully worked -over by Mr. G. H. Morton[162] and Professor Lapworth.[163] - -[Footnote 161: _Silurian System_ (1839), chap. xix.; _Siluria_, 4th -edit. (1867), pp. 26, 49.] - -[Footnote 162: _Proc. Liverpool Geol. Soc._ x. (1854), p. 62.] - -[Footnote 163: _Geol. Mag._ (1887), p. 78.] - -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.[164] -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 the -Geological Survey they are thickest towards the west, and become more -split up with intercalated sediments as they range eastward. - -[Footnote 164: Prof. Lapworth and Mr. W. W. Watts, _Proc. Geol. Assoc._ -xiii. (1894), pp. 317, 337.] - -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,[165] 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."[166] 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. - -[Footnote 165: _Silurian System_, p. 229.] - -[Footnote 166: Messrs. Lapworth and Watts, _op. cit._ p. 318.] - -[Illustration: Fig. 50.--Section across the anticline of Corndon.[167] - -A, Arenig flags and shales; B, andesites and tuffs; C, intrusive -dolerite.] - -[Footnote 167: After Prof. Lapworth and Mr. Watts, _op. cit._ p. 342.] - -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.[168] 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 Fig. 50. - -[Footnote 168: _Op. cit._ p. 339.] - -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" 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.[169] - -[Footnote 169: See Mr. W. W. Watts on the Igneous and Associated Rocks -of the Breidden Hills, _Quart. Journ. Geol. Soc._ vol. xli. (1885), p. -532.] - - -iii. SCOTLAND - -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.[170] 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. - -[Footnote 170: _Trans. Roy. Soc. Edin._ xxii. (1860), p. 636.] - -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. 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. - -Without anticipating details which will properly appear in the -official _Memoirs_ 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. - -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.[171] - -[Footnote 171: _Quart. Journ. Geol. Soc._ vol. xxxiv. (1878), p. 769.] - -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 _Memoirs_ 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 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. - -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. - -[Illustration: Fig. 51.--Structure in finely-amygdaloidal diabase lava, -south of mouth of Stinchar River, Ayrshire. The fine dots and circles -mark the lines of amygdales.] - -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 Fig. 12, 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 Fig. 51 -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. - -This singular structure has already (p. 184) 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.[172] 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,[173] and likewise in some modern -lavas, as in that of Acicastello already referred to (_ante_, p. 26). - -[Footnote 172: Mr. J. J. H. Teall, _Roy. Geol. Soc. Cornwall_, 1894, p. -3. Mr. L. Ransome, _Bull. Depart. Geol. University of California_, vol. -i. p. 106.] - -[Footnote 173: Messrs. Cole and Gregory, _Quart. Journ. Geol. Soc._ -vol. xlvi. (1890), p. 311.] - -[Illustration: Fig. 52.--View of Knockdolian Hill from the east.] - -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 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. - -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. - -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. - -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. - -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 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.[174] 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. - -[Footnote 174: _Geol. Mag._ 1889, p. 22.] - - Llandovery. - }Pentamerus grit. - }Conglomerate (Mulloch Hill). - - Caradoc. - {Shales, sandstones, grits, etc. (Ardmillan, Balcletchie). - {Thick conglomerate (Byne Hill, Bennane, etc.). - {Thick fossiliferous limestone (Stinchar, Girvan). (On this horizon - { come the perlitic felsites and soda-felsites of Winkstone and Wrae.) - {Sandstone (_Orthis confinis_) passing down into thick conglomerate. - - [Unconformability.] - - Upper Llandeilo. - }Green mudstones, grits and greywackes. - }Thin band of dark mudstone with Upper Llandeilo graptolites. - - Arenig and Lower and Middle Llandeilo. - {Group of Radiolarian cherts (about 70 feet) with alternating tuffs. - {Tuff or volcanic conglomerate, with occasional lava-flows. - {Black shale (10 feet) with Arenig graptolites. - {Volcanic breccias around local centres (Knockdolian, etc.). - {Thick group of porphyrite and diabase lavas. - {Red flinty mudstones with Arenig graptolites. - {Porphyrites, etc. - {Fine tuffs, etc., with Lower Arenig fossils. - {Diabase lavas, etc. (base not seen). - -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. - -Among the lowest lavas there are interstratified courses of fine -tuffs, 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 _Tetragraptus bryonoides_, _T. fruticosus_, _T. quadribrachiatus_, -and _T. Headi_ together with _Caryocaris Wrightii_. 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. - -[Illustration: Fig. 53.--Section across the Lower Silurian volcanic -series in the south of Ayrshire (B. N. Peach). - -B, Interstratified lavas in Arenig group; _t_, tuffs; _r_, radolarian -cherts; _b_^2, Llandeilo group; _b_^3 Caradoc group. Σ, Serpentine. -G, Gabbro.] - -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 volcanic outburst, and that such an occurrence took place several -times in succession over the same area. - -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.[175] - -[Footnote 175: _Ann. Mag. Nat. Hist._ (1890), 6th ser. vi. p. 40.] - -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.[176] - -[Footnote 176: _Annual Report of the Geol. Surv. for 1895_, p. 27 of -reprint.] - -[Illustration: - - Fig. 54.--Section of part of the Arenig volcanic group, stream - south of Bennane Head, Ayrshire. -] - -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 (Fig. 54) is inserted. In -descending order we come first upon a group of greywackes and grey -shattery mudstones (_a_), 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 (_c_). 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 (_d_) 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 of lava six inches or more in diameter. Below the belt -of lavas come black cherts and shales (_e_) succeeded below by volcanic -breccias and tuffs (_f_) 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. - -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. - -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. - -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. - -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 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. - -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. - -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. - -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. - -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. - -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. - -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. - -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 Chapter xiv., where the whole of the Silurian volcanic rocks -of the island are discussed. - - - - -CHAPTER XIII - -THE ERUPTIONS OF LLANDEILO AND BALA AGE - - 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. - - -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. - -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. - -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 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. - -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. - - -i. THE VOLCANO OF BUILTH AND ITS NEIGHBOURS - -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. - -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."[177] These igneous rocks -were mapped in some detail by the Geological Survey, and their general -relations were expressed in lines of horizontal section.[178] They were -likewise described by Ramsay in the _Catalogue of the Rock-specimens -in the Jermyn Street Museum_, specimens of them being displayed in -that collection.[179] The tuffs and lavas were distinguished, and -likewise the intrusive "greenstones." But no attempt was made towards -petrographical detail. - -[Footnote 177: _Silurian System_, 1839, p. 330. The occurrence of -"trappean ash" with fossils in the Builth district was noticed by De la -Beche, _Mem. Geol. Surv._ vol. i. (1846), p. 31.] - -[Footnote 178: See Sheet 56 of the one-inch map and Sheets 5 and 6 of -the Horizontal Sections.] - -[Footnote 179: _Catalogue of Rock Specimens_, 3rd edit. 1862, p. 36 _et -seq._] - -This interesting district has recently been studied by Mr. Henry -Woods,[180] 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. - -[Footnote 180: _Quart. Journ. Geol. Soc._ vol. l. (1894), p. 566.] - -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 (_Orthis -calligramma_, _Leptæna sericea_, _Serpulites dispar_, etc.), and as -it is overlain with shales containing _Ogygia Buchii_, 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. - -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 section runs across this ground, and the band of -rock does not appear to have been described.[181] - -[Footnote 181: The locality is referred to by De la Beche, _Mem. Geol. -Surv._ vol. i. p. 31, and by Ramsay in the _Descriptive Catalogue of -Rock-specimens in the Museum of Practical Geology_, 3rd edit. p. 38, -but no specimens from it are in the collection.] - -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 _Horizontal Section_ (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 _Catalogue -of Rock-specimens_ 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.[182] - -[Footnote 182: _Op. cit._ p. 38.] - -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.[183] - -[Footnote 183: One of the patches was shown by J. Phillips in -_Horizontal Section_, 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.] - - -ii. THE VOLCANOES OF PEMBROKESHIRE - -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.[184] 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. - -[Footnote 184: See _Silurian System_, p. 401; Sheet 40 of the -Geological Survey Map; _Memoir of A. C. Ramsay_, p. 232 _et seq._; De -la Beche, _Trans. Geol. Soc._ 2nd series, vol. ii. part i. (1826), p. -3.] - -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. - -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.[185] 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 _Didymograptus Murchisoni_, 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. - -[Footnote 185: _Quart. Journ. Geol. Soc._ vol. li. (1895), p. 149.] - -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.[186] - -[Footnote 186: 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.] - -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. - -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.[187] 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.[188] - -[Footnote 187: Mr. Cowper Reed, _op. cit._ p. 180.] - -[Footnote 188: _Op. cit._ p. 193.] - -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.[189] But the -volcanic history of this area still remains to be properly investigated. - -[Footnote 189: _Quart. Journ. Geol. Soc._ xxxi. (1875), p. 177.] - -In southern Pembrokeshire two conspicuous bands of eruptive rocks -have long been known and described. Their general characters and -distribution were sketched by De la Beche,[190] and further details -were afterwards added by Murchison.[191] 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. - -[Footnote 190: _Trans. Geol. Soc._, 2nd ser. vol. ii. (1823), p. 6 _et -seq._] - -[Footnote 191: _Silurian System_, p. 401 _et seq._] - -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.[192] 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.[193] - -[Footnote 192: _Mem. Geol. Survey_, vol. i. p. 231.] - -[Footnote 193: _Silurian System_, p. 404.] - -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.[194] 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."[195] - -[Footnote 194: "The Felsitic Lavas of England and Wales," _Mem. Geol. -Survey_ (1885), pp. 16, 18.] - -[Footnote 195: _British Petrography_, pp. 224, 284, 336.] - -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.[196] - -[Footnote 196: _Rep. Brit. Assoc._ 1893, p. 766; _Geol. Mag._ 1896, p. -481.] - - -iii. THE CAERNARVONSHIRE VOLCANOES OF THE BALA PERIOD - -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,[197] 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 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 -(Map II.). - -[Footnote 197: Estimated at from 6000 to 7000 feet, _Mem. Geol. Surv._ -vol. iii. 2nd edit. p. 131.] - -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. - -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.[198] 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. - -[Footnote 198: _Mem. Geol. Surv._ vol. iii. 2nd edit. pp. 126, 128, -131, 139, etc.] - -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 Glyder-Fach, which, according to the -Survey measurements, are about 1500 feet thick, while still lower -lie the ancient _coulées_ 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. - -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 _puy_-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.[199] 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,"[200]--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. - -[Footnote 199: _Op. cit._ pp. 137, 220.] - -[Footnote 200: This was the Sedgwick Prize Essay for 1888, and was -published in 1889.] - -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 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.[201] 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."[202] - -[Footnote 201: _Op. cit._ p. 148.] - -[Footnote 202: _Bala Volcanic Rocks_, p. 25.] - -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. - -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. - -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 and enclosed one within the other (Fig. 55). -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. - -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.[203] - -[Footnote 203: _Quart. Journ. Geol. Soc._ vol. xxxv. (1879), p. 508.] - -[Illustration: Fig. 55.--Flow-structure in the lowest felsite on the -track from Llanberis to the top of Snowdon. Length about 4 feet, height -2½ feet.] - -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).[204] - -[Footnote 204: 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.] - -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.[205] 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.[206] 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.[207] 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 Fig. 56). 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. - -[Footnote 205: Mr. Harker, _op. cit._ p. 127.] - -[Footnote 206: _Mem. Geol. Surv._ vol. iii. 2nd edit. pp. 141, 144, -145, 147, 161.] - -[Footnote 207: _Bala Volcanic Series_, 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.] - -[Illustration: Fig. 56.--Section of Snowdon.[208] - -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."] - -[Footnote 208: After the Geological Survey Section (Horizont. Sect. -Sheet 28), slightly modified.] - -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 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. - -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. - -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. - -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 -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. - -The tuffs in the upper part of Snowdon are well-bedded deposits made up -partly of volcanic detritus and partly of ordinary muddy sediment.[209] -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. - -[Footnote 209: See the interesting account of these tuffs given by Sir -A. Ramsay, _Mem. Geol. Survey_, vol. iii. 2nd edit. p. 142.] - -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."[210] 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.[211] 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."[212] - -[Footnote 210: _Mem. Geol. Survey_, vol. iii. 2nd edit. pp. 137, 139.] - -[Footnote 211: _Bala Volcanic Series_, pp. 41, 71, 72, 123.] - -[Footnote 212: 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.] - -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. - -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.[213] 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. - -[Footnote 213: The geological relations and petrographical characters -of these various rocks are given by Mr. Harker in the fourth and fifth -sections of his Essay.] - -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.[214] 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.[215] 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.[216] 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 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. - -[Footnote 214: Mr. Harker speaks of some of them as laccolites.] - -[Footnote 215: _Op. cit._ pp. 57, 72.] - -[Footnote 216: See especially pp. 9, 120 _et seq._, and fig. 6 of his -Essay.] - -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,[217] 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. - -[Footnote 217: This rock is referred to in the _Geological Survey -Memoir_ as "a short thick band of conglomeratic ash, which strikes -northwards about half a mile and then disappears" (p. 134).] - -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. - -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 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. - -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. - -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. - -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 _Memoir_ as hornblendic.[218] 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.[219] - -[Footnote 218: _Op. cit._ p. 156.] - -[Footnote 219: Mr. Harker, _Bala Volcanic Series_, p. 83.] - -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 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.[220] 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. - -[Footnote 220: _Mem. Geol. Sur._ vol. iii. 2nd edit. p. 326. See also -Mr. Harker's _Bala Volcanic Series_, p. 76.] - - -iv. THE VOLCANIC CENTRE OF THE BERWYN HILLS - -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.[221] 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. - -[Footnote 221: See Sheet 74 of the one-inch map; Sheets 32, 35, 37 and -38 of the Horizontal Sections; and chapter xxxi. of the _Memoir_ on the -Geology of North Wales.] - -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. - -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 _Memoir_ 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 thickness of 300 feet -and separated from the next band by sometimes 1000 or 1500 feet of -non-volcanic sediment. - -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." - -[Illustration: Fig. 57.--Section across the Berwyn Hills. (Reduced from -Horizontal Section, Geol. Surv., Sheet 35). - -_L_, Llandeilo Flags; _B_, Bala group; _B L_, Bala Limestone; _t_ _t_, -volcanic tuffs; _D_, intrusive "greenstones."] - -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. - - -V. THE VOLCANOES OF ANGLESEY - -We now turn to another part of the country, about which much has 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.[222] Dr. Callaway regards them as pre-Cambrian,[223] while -Professor Blake places them in his "Monian system."[224] When I went -over them some years ago, I accepted the view that they are volcanic -agglomerates.[225] 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.[226] - -[Footnote 222: _Proc. Camb. Phil. Soc._ vol. iii. (1880), p. 347.] - -[Footnote 223: _Quart. Journ. Geol. Soc._] - -[Footnote 224: _Op. cit._] - -[Footnote 225: _Presidential Address Geol. Soc._ vol. xlvii. (1891), p. -130.] - -[Footnote 226: _Quart. Journ. Geol. Soc._ vol. li. (1895), p. 563. See -_Geol. Mag._ 1896, p. 481.] - -But though their present coarse, agglomerate-like structure is, I -think, entirely due to the mechanical crushing of the rocks _in situ_ -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. - -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 _in situ_. 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. - -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 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. - -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. - -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.[227] 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."[228] - -[Footnote 227: 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, _Geol. Mag._ 1891, p. 483.] - -[Footnote 228: 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.] - -It is no part of my aim to disprove the existence of faults along the -line 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. - -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.[229] 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. - -[Footnote 229: 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 _Geol. Mag._ 1891, p. 483.] - -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.[230] 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 (Fig. 58). -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 _Orthis Bailyana_, 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 _Orthis_ and other fossils form an integral part of the so-called -"Amlwch slates."[231] - -[Footnote 230: _Quart. Journ. Geol. Soc._ vol. xliv. (1888), p. 517. -See his further remarks in _Geol. Mag._ 1891, p. 483.] - -[Footnote 231: 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, _British Assoc. -Report_ (1885), pp. 839, 840.] - -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 given (Fig. 58). 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 _Orthis_ -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. - -[Illustration: Fig. 58.--Section of the strata on the shore at Porth -Wen, west of Amlwch.] - -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. - -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."[232] Professor -Hughes was the first to suggest that the whole of these rocks should be -referred to the Bala group.[233] - -[Footnote 232: _Mem. Geol. Surv._ vol. iii. 2nd edit. p. 242.] - -[Footnote 233: _Proc. Camb. Phil. Soc._ vol. iii. (1880), pp. -341-348.] [Illustration: - - Fig. 59.--Section of intercalated black shale in the volcanic - series at Porth yr hwch, south of Carmel Point. -] - -[Illustration: Fig. 60.--Green slates overlain with volcanic breccia, -Carmel Point, Anglesey.] - -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 Fig. 59, where some yellow decomposing breccias -(1), cut by a fault (_f_), 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 -(Fig. 60) a similar structure may be observed to that at Llyn Padarn -already referred to (p. 163). The cleavage, which is well developed in -the green slates (_a_), is much more faintly marked in the overlying -breccia (_b_), but the bedding can still be detected in both rocks -running parallel to their mutual boundary-line. Beyond Porth Padrig, -which lies east from Carmel Point, the section may be seen which is -shown in Fig. 61. Here the blue or lead-coloured shale or slate (_a_) -marked as Silurian on the Geological Survey map passes up into a mass -of fine yellowish felsitic tuff and breccia (_b_). The shale at the -junction intercalates in thin leaves with the tuff. - -[Illustration: Fig. 61.--Blue shale or slate passing into volcanic -breccia east of Porth Padrig, near Carmel Point, Anglesey.] - -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. - -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.[234] - -[Footnote 234: Presidential Address, _Quart. Journ. Geol. Soc._ vol. -xlvii. p. 134; _Rep. Brit. Assoc._ 1896, Section C; _Geol. Mag._ 1896, -p. 481.] - -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 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. - -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. - -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. - -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 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. - -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. - - -vi. THE VOLCANOES OF THE LAKE DISTRICT (ARENIG TO CLOSE OF BALA PERIOD) - -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 _Geological Survey Memoirs_, wherein he -embodied the results of his minute investigation and mapping of the -northern portion of the district.[235] 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 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. - -[Footnote 235: Sheet 101 S. E. of the Geological Survey of England -and Wales and Explanation illustrating the same; and papers by him -in _Quart. Journ. Geol. Soc._ vols. xxxi. xxxii. (1875-76). See also -Messrs. Aveline and Hughes, _Mem. Geol. Survey_, Sheet 98 N.E. (Kendal, -Sedbergh, etc.).] - -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,[236] 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. - -[Footnote 236: 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. _Quart. Journ. Geol. Soc._ vol. -xlvii. (1891), pp. 500, 512.] - -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.[237] Professors -Harkness and Nicholson, on the other hand, gave the average thickness -as not more than 5000 feet.[238] 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 thickest -accumulation of volcanic material, belonging to a single geological -period, anywhere known to exist in Britain. - -[Footnote 237: Ward, _op. cit._ p. 46.] - -[Footnote 238: _Brit. Assoc. (1870) Sectional Reports_, p. 74.] - -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,[239] 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. - -[Footnote 239: Mr. Dakyns has expressed his belief that the volcanic -group lies unconformably on the Skiddaw Slate (_Geol. Mag._ 1869, -pp. 56, 116), and Professor Nicholson has formed the same opinion -(_op. cit._ pp. 105, 167; _Proc. Geol. Assoc._ vol. iii. p. 106). Mr. -Goodchild, however, has shown that in the Cross Fell inlier the oldest -tuffs are interstratified with the Skiddaw Slates (_Proc. Geol. Assoc._ -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, _op. cit._ p. 48.] - -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.[240] 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 -felspar with a brownish glassy groundmass, and with some chloritic -material probably representing augite, but with no trace of quartz.[241] - -[Footnote 240: _Quart. Journ. Geol. Soc._ vol. xxxi. (1875) p. -408, vol. xxxii. (1876) p. 24. Geology of Northern Part of Lake -District (_Mem. Geol. Survey_), p. 22. In a subsequent paper the more -basic lavas of Eycott Hill are compared with dolerites (_Monthly -Microscopical Journ._ 1877, p. 246).] - -[Footnote 241: These rocks were mapped as tuffs by Mr. Ward. Their -microscopic characters have been described by Messrs. Harker and Marr, -_Quart. Journ. Geol. Soc._ xlvii. (1891), p. 292; by Mr. Harker, _op. -cit._ p. 517; and by Mr. W. M. Hutchings, _Geol. Mag._ 1891, p. 537; -1892, pp. 227, 540.] - -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.[242] Gradations from these rocks to -the ordinary more coarse-grained andesites may be observed. - -[Footnote 242: Mr. Hutchings, _Geol. Mag._ 1891, p. 539. This observer -describes a quartz-andesite or dacite from near Dunmail Raise.] - -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.[243] - -[Footnote 243: _Op. cit._ p. 543.] - -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.[244] -The microscopic characters of the group have been more recently -determined by Mr. Hutchings[245] and Messrs. Harker and Marr.[246] - -[Footnote 244: _Monthly Microscopical Journal_, 1877, p. 246.] - -[Footnote 245: _Geol. Mag._ 1891, p. 538.] - -[Footnote 246: _Quart. Journ. Geol. Soc._ vol. xlix. (1893), p. 389. -Mr. Harker, _op. cit._ vol. xlvii. (1891).] - -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.[247] One of the flows from this locality was -found by Dr. Hatch, under the microscope, to belong to the more basic -series. It 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. - -[Footnote 247: 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 _Mem. Geol. -Survey_, Explanation to Sheet 98 N.E. 2nd edit. p. 8 (1888).] - -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.[248] - -[Footnote 248: The metamorphism of all the rocks, aqueous and igneous, -around the Shap granite has been well worked out by Messrs. Harker -and Marr, _Quart. Journ. Geol. Soc._ vol. xlvii. (1891) p. 266, xlix. -(1893) p. 359.] - -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 (Chapter xxxvi.) 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. - -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.[249] A fine series of -felsites is interbedded in the lower 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[250] (Fig. -62). 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. - -[Footnote 249: See Mr. F. Rutley, "The Felsitic Lavas of England and -Wales," _Mem. Geol. Surv._ 1885, pp. 12-15; also the description of -Messrs. Harker and Marr, _Quart. Journ. Geol. Soc._ xlvii. (1891), p. -301.] - -[Footnote 250: Unfortunately these acid lavas are not distinguished -from the others in the Geological Survey maps.] - -[Illustration: Fig. 62.--Section of felsites on the Coniston Limestone -group, west of Stockdale. - -_a_, Felsites more or less cleaved; _b_, Calcareous shales with -fossils, much cleaved; _c_, Cleaved felsite; _d_, Coniston Limestone; -_e_, Stockdale Shales (with graptolites).] - -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.[251] -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.[252] 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.[253] - -[Footnote 251: Compare the structure described by Mr. Harker from the -Cross Fell inlier, _Quart. Journ. Geol. Soc._ xlvii. (1891), p. 518.] - -[Footnote 252: "Geology of Kendal," etc., _Mem. Geol. Survey_, Sheet 98 -N.E. 2nd edit. p. 9.] - -[Footnote 253: Messrs. Harker and Marr, _op. cit._ p. 302.] - -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 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. - -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. - -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,[254] 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. - -[Footnote 254: 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" (_op. cit._ 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 (_Geol. Mag._ 1891, p. 544).] - -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 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 (Fig. 63). - -[Illustration: Fig. 63.--Fine tuff with coarser bands near Quayfoot -quarries, Borrowdale. - -The highly-inclined fine lines show the cleavage. The more gently -dipping bands and lines mark the bedding.] - -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. - -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.[255] - -[Footnote 255: The microscopic and chemical characters of the -Ash-Slates of the Lake District have been investigated by Mr. -Hutchings, _Geol. Mag._ 1892, pp. 155, 218.] - -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. -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,"[256] 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. - -[Footnote 256: _Op. cit._ p. 70.] - -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. - -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.[257] - -[Footnote 257: Mr. Harker, _Quart. Journ. Geol. Soc._ vol. l. (1894) p. -312, li. (1895) p. 125. _Geol. Mag._ 1894, p. 551.] - -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 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. - -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. - -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.[258] -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.[259] 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,[260] while that of the volcanic -group by the Shap granite has been carefully worked out by Mr. Harker -and Mr. Marr.[261] - -[Footnote 258: _Quart. Journ. Geol. Soc._ vol. li. (1895), p. 144.] - -[Footnote 259: _Op. cit._ p. 126.] - -[Footnote 260: "Geology of Northern Part of the English Lake District," -_Mem. Geol. Surv._ 1876, chap. iii. The metamorphism around the -diorites and dolerites, and the granophyres and felsites, is described -in the same chapter.] - -[Footnote 261: _Quart. Journ. Geol. Soc._ xlvii. (1891) p. 266, xlix. -(1893) p. 359.] - -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. - -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 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. - -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. - -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.[262] These ingredients seem to indicate that some -part of the volcanic group was above water when the Coniston Limestone -was deposited. - -[Footnote 262: Messrs. Harker and Marr, _Quart. Journ. Geol. Soc._ vol. -xlvii. (1891), p. 310.] - -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 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. - -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.[263] 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. - -[Footnote 263: For a description of the dykes around the Shap granite -see the paper by Messrs. Harker and Marr, _Quart. Journ. Geol. Soc._ -vol. xlvii. (1891), p. 285.] - - -vii. UPPER SILURIAN (?) VOLCANOES OF GLOUCESTERSHIRE - -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.[264] Murchison regarded -them as intrusive sheets;[265] Phillips looked on them as partly -intrusive and partly interstratified.[266] They consist largely of -coarsely-amygdaloidal basalts, some of which have been microscopically -examined.[267] 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. - -[Footnote 264: _Trans. Geol. Soc._ 2nd ser. vol. i. (1819), pp. -324-334.] - -[Footnote 265: _Silurian System_ (1839), p. 457.] - -[Footnote 266: _Mem. Geol. Surv._ vol. ii. part i. (1848), p. 194.] - -[Footnote 267: "Geology of East Somerset," etc., in _Mem. Geol. Surv._ -(1876), p. 210; descriptions by Mr. F. Rutley.] - - - - -CHAPTER XIV - -THE SILURIAN VOLCANOES OF IRELAND - - -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. - - -I. The Lower Silurian Series - - -i. _Eruptions probably of Arenig Age_ - -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. - -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. - -The remarkable zone of crushed cherts, igneous rocks and sandstones, -probably of Lower Silurian age, which I have referred to (p. 201) 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.[268] 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. - -[Footnote 268: This area was mapped by Mr. J. Nolan for the Geological -Survey, and was described by him in the _Geol. Mag._ 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 (_Pres. Address Geol. Soc._ 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.] - -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. - -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. - -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. - -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 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. - -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. - -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. - -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. - -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. - -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. - - -ii. _Eruptions of Llandeilo and Bala Age_ - -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 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. - -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 _facies_ 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.[269] 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. - -[Footnote 269: 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.] - -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.[270] - -[Footnote 270: 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.] - -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.[271] 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.[272] - -[Footnote 271: 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.] - -[Footnote 272: _Guide to the Collection of Rocks and Fossils belonging -to the Geological Survey of Ireland_, by Messrs. M'Henry and Watts, -Dublin, 1895, p. 74.] - -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.[273] In one locality south of the -Drumcalpin Loughs a large exposure of this ash is visible: "brown -crumbly beds, with small 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." - -[Footnote 273: Sheet 69 Geol. Survey, Ireland, and Explanation of -Sheets 68 and 69, pp. 9, 13, 15.] - -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.[274] 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.[275] -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 _Didymograptus Murchisoni_ 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. - -[Footnote 274: _Ibid._ 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 (_Guide to Irish Rock-Collection_, p. 36).] - -[Footnote 275: _Ibid._ Sheets 91 and 92 and Explanation to these -Sheets (1871), p. 10; _Guide to Irish Rock-Collection_, 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.] - -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 of the Chair of Kildare, has long been known to have -igneous rocks associated with its abundantly fossiliferous Bala -limestone.[276] 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. - -[Footnote 276: See Explanation to Quarter Sheet 35 N.E. (Sheet 119 of -newer numeration) of Geol. Survey Ireland (1858), p. 16. (See note, p. -256.)] - -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. - -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. - -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 -an inch in thickness, lie among the shales. In some of the tuffs the -lapilli are fragments of trachytic or andesitic rocks. - -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.[277] 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. - -[Footnote 277: See Explanation of Sheets 167, 168, 178 and 179, Geol. -Surv. Ireland, p. 56.] - -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."[278] 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.[279] -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. - -[Footnote 278: Explanation of Sheets 129, 130, p. 13 (1869).] - -[Footnote 279: Explanation of Sheets 138, 139.] - -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 lenticular bands -so common in Caernarvonshire. In like manner the nodular structure, -though not entirely absent, is rare.[280] - -[Footnote 280: 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.] - -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.[281] 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. - -[Footnote 281: Explanation of Sheets 138, 139, p. 49; and _Geol. Mag._ -1889, p. 545.] - -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.[282] - -[Footnote 282: _Guide to Irish Rock-Collections_, pp. 34, 35.] - -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 of making himself -acquainted with some of the conditions of volcanism during older -Palæozoic time. - -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. - -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. - -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. - -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. - -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.[283] 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; they have been tossed -about, crushed and invaded by dykes and veins of felsite. - -[Footnote 283: But see the _Geol. Survey Memoir_ 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.] - -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. - -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. - -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.[284] 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.[285] - -[Footnote 284: Explanation of Sheets 138 and 139, p. 53.] - -[Footnote 285: 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.] - -The basic sills, or "greenstones," consist largely of diabase, -frequently altered into epidiorite; they include also varieties of -diorite.[286] 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. - -[Footnote 286: Dr. Hatch, _op. cit._ p. 49.] - -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 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. - -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.[287] - -[Footnote 287: Explanation of Sheets 167, 168, 178 and 179 of the -Geological Survey of Ireland (1865), pp. 10, 59.] - -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. - -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 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.[288] - -[Footnote 288: 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.] - -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. - -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.[289] 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.[290] - -[Footnote 289: 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.] - -[Footnote 290: 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.] - -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.[291] 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 -_Illænus Bowmanni_ and _Orthis elegantula_ 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. - -[Footnote 291: See the list of fossils as determined by Mr. Baily in -_Explanatory Memoir_ to accompany Sheets 73, 74, 83 and 84 of the -Geological Survey of Ireland, p. 68 (1876).] - -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. - -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. - -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 volcanic rocks.[292] -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. - -[Footnote 292: 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.] - -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. - -[Illustration: Fig. 64.--Diagram of the general relations of the -different groups of rock in the Lower Silurian volcanic district along -the western shore of Lough Mask. - - _a_, Llandeilo shales, cherts and grits; _b_, Volcanic breccias; - _c_, Felsites and andesites; _d_, Tuffs and ashy grits and - shales; _e_, Limestone with Bala fossils; _f_, Calcareous - tuffs and thin bands of ashy limestone with fossils; _g_, - Coarse conglomerate and grits; _h_, Wenlock strata resting - unconformably on the Bala rocks and passing southwards from - these to overlie an older series of schists; *, Fault. -] - -The general structure of the ground occupied by the Lough Mask volcanic -rocks is diagrammatically represented in Fig. 64. 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 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. - -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. - -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. - - -II. The Upper Silurian Series - -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. - -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.[293] -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. - -[Footnote 293: Sheets 160 and 171 of the one-inch map, and Memoir on -Sheets 160, 161, 171 and 172.] - -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. - -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. - -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. - -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 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. - -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.[294] - -[Footnote 294: 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 (_Quart. Journ. Geol. Soc._ 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.] - -[Illustration] - - TO ACCOMPANY SIR ARCHIBALD GEIKIE'S "ANCIENT VOLCANOES OF BRITAIN" - Map II - - MAP OF THE SILURIAN VOLCANIC DISTRICTS OF NORTH WALES - - Reduced from the Maps of the Geological Survey. - - The Edinburgh Geographical Institute -- Copyright -- J. G. - Bartholomew. - - - - -BOOK V - -THE VOLCANOES OF DEVONIAN AND OLD RED SANDSTONE TIME - - - - -CHAPTER XV - -THE DEVONIAN VOLCANOES - - -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. - -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 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. - -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. - -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. - -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. - -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. - -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 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. - -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. - -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. - -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.[295] - -[Footnote 295: _Trans. Geol. Soc._ 2nd ser. vol. vi. (1842), pp. 465, -470, 473.] - -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, 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."[296] 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."[297] -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.[298] - -[Footnote 296: "Report on the Geology of Cornwall, Devon and West -Somerset," _Mem. Geol. Survey_, 1839, p. 37.] - -[Footnote 297: _Op. cit._ p. 57.] - -[Footnote 298: _Op. cit._ pp. 57, 61.] - -Since these early researches many geologists have studied the igneous -rocks of Devonshire. I would especially refer to the labours of Mr. -Allport,[299] the late J. A. Phillips,[300] Mr. Rutley,[301] the late -Mr. Champernowne,[302] Mr. W. A. E. Ussher,[303] Mr. Hobson,[304] and -General M'Mahon.[305] 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. - -[Footnote 299: _Quart. Journ. Geol. Soc._ xxxii. (1876), p. 418.] - -[Footnote 300: _Op. cit._ xxxi. (1875) p. 325, xxxii. (1876) p. 155, -xxxiv. (1878) p. 471.] - -[Footnote 301: "Brent Tor," _Mem. Geol. Surv._ p. 18; _Quart. Journ. -Geol. Soc._ lii. (1896), p. 66.] - -[Footnote 302: See in particular his last paper "On the Ashprington -Volcanic Series of South Devon," _Quart. Journ. Geol. Soc._ vol. xlv. -(1889), p. 369.] - -[Footnote 303: 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 _Transactions of the Devonshire -Association_ and of the _Royal Cornwall Geological Society_, in the -_Proceedings of the Somersetshire Archæological and Natural History -Society_, and of the _Geologists' Association_, in the _Geological -Magazine_, and the _Quarterly Journal of the Geological Society_. -Reference may especially be made to his Memoir in the last named -journal, vol. xlvi. (1890), p. 487.] - -[Footnote 304: _Quart. Journ. Geol. Soc._ xlviii. (1892), p. 496.] - -[Footnote 305: _Op. cit._ xlix. (1893), p. 385.] - -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.[306] 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. - -[Footnote 306: See Memoir cited in a previous note.] - -In the meantime, it has been established that while there is a -singular 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. - -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.[307] Subsequently they were studied by Dr. -Hatch, who found the fresher specimens generally to possess an ophitic -structure, while some are granular, others porphyritic.[308] - -[Footnote 307: See especially _Quart. Journ. Geol. Soc._ vols. xxxii. -and xxxiv.] - -[Footnote 308: 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 (_Neues Jahrb._ 1896, p. -57).] - -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 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. - -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. - -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. - -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 -Chapter xxix.). - - - - -CHAPTER XVI - -THE VOLCANOES OF THE OLD RED SANDSTONE - - 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. - - -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. - -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 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. - -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. - -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 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. - -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 _Psilophyton_, 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. - -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.[309] 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. - -[Footnote 309: Mr. B. N. Peach, _Proceedings of Royal Physical Society -of Edinburgh_, vol. vii. (1882).] - -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. - -A difference is observable between the faunas of the different -water-basins. Even where the same genus occurs in two adjacent -areas, the 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.[310] - -[Footnote 310: In my memoir "On the Old Red Sandstone of Western -Europe" (_Trans. Roy. Soc. Edin._ 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 _Holoptychius -nobilissimus_ 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.] - -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. - -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 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.[311] 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. - -[Footnote 311: _Quart. Journ. Geol. Soc._ 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.] - -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. - -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. - -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 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. - -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.[312] - -[Footnote 312: An unconformability of this kind occurs between the -south end of the Pentland Hills and Tinto in Lanarkshire, and another -in Ayrshire.] - -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. - -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,[313] Macknight[314] -and Fleming,[315] 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 structure of -the country. Probably in 1820 he published in French his now classic -_Essai_.[316] 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 _Essai_ 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 (_nappes_, _coulées_) 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. - -[Footnote 313: _Memoirs of the Wernerian Society_, vol. ii. (1811), pp. -178, 217, 618; vol. iii. (1820), p. 220, 225.] - -[Footnote 314: _Op. cit._ vol. ii. pp. 123, 461.] - -[Footnote 315: _Op. cit._ vol. i. (1808), p. 162; vol. ii. (1811), pp. -138, 339.] - -[Footnote 316: _Essai géologique sur l'Écosse_ (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 -_Wernerian Memoirs_ 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 _Geology of the Lothians_, p. 59, -footnote. Compare an article on Boué, _Edinburgh Review_ for May 1823 -(vol. xxxviii. p. 413).] - -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. - -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 _Geology of Fife and the Lothians_ may still be read with -pleasure and instruction.[317] Boué had indicated roughly on the little -sketch-map accompanying his _Essai_ 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. - -[Footnote 317: _Geology of Fife and the Lothians_, 1839. More detailed -reference will be made in later pages to this classic.] - -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.[318] A first sketch of the volcanic history of the Old -Red Sandstone was given by me in 1861,[319] which was subsequently -enlarged and filled in with more detail in 1879.[320] 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.[321] 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. - -[Footnote 318: "On the Old Red Sandstone of the South of Scotland," -_Quart. Journ. Geol. Soc._ xvi. (1860), p. 312.] - -[Footnote 319: "On the Chronology of the Trap-Rocks of Scotland," -_Trans. Roy. Soc. Edin._ vol. xxii. (1861), p. 63.] - -[Footnote 320: Article "Geology," in Ninth Edition of the _Encyclopædia -Britannica_, vol. x. (1879), p. 343. Reprinted in my _Text-Book of -Geology_, of which the first edition appeared in 1882.] - -[Footnote 321: "Presidential Address to the Geological Society," -_Quart. Journ. Geol. Soc._ vol. xlviii. (1892).] - -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. - - - - -CHAPTER XVII - -DISTRIBUTION OF THE VOLCANIC CENTRES IN THE LOWER OLD RED -SANDSTONE--CHARACTERS OF THE MATERIALS ERUPTED BY THE VOLCANOES - - -i. DISTRIBUTION OF VOLCANIC CENTRES - -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 (Map I.). 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:-- - -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."[322] - -[Footnote 322: _Trans. Roy. Soc. Edin._ vol. xxviii. (1878), p. 354.] - -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. - -3. The great central basin of Scotland, which, for the sake of -distinctness, I have called "Lake Caledonia,"[323] 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 Map III.). - -[Footnote 323: _Op. cit._] - -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. - -5. The Killarney tract, including the hills lying around Lough Guitane -in the east of County Kerry. - -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,[324] 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. - -[Footnote 324: _Trans. Roy. Soc. Edin._ vol. xxix. (1879), p. 454.] - -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.[325] 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. - -[Footnote 325: 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.] - - -ii. CHARACTERS OF THE MATERIALS ERUPTED BY THE VOLCANOES - -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. - -1. _Bedded Lavas._--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. - -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;[326] 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. - -[Footnote 326: The only examples known to me are those of Benaun More -and other hills in County Kerry.] - -(_a_) The Diabase-lavas 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:[327]-- - -[Footnote 327: For analyses of some Shetland diabases of Old Red -Sandstone age, see Mr. R. R. Tatlock, _Trans. Roy. Soc. Edin._ vol. -xxxii. (1887), p. 387.] - - +------------+-----------+---------+-----------+---------+-----------+ - | | Carnethy | Buiselaw. | Warklaw Hill. | - | | Hill[328]| Sp. grav. 2·80. | Sp. grav. 2·77. | - +------------+-----------+---------+-----------+---------+-----------+ - | | | Soluble | Insoluble | Soluble | Insoluble | - | | | in HCl | in HCl | in HCl | in HCl | - +------------+-----------+---------+-----------+---------+-----------+ - | SiO_{2} | 51·16 | ... | 52·00 | ... | 47·77 | - | Al_{2}O_{3}| 22·27 | 1·30 | 17·46 | 5·23 | 13·08 | - | Fe_{2}O_{3}| 2·94 | 1·53 | 7·85 | 7·32 | 0·84 | - | FeO | 4·02 | 1·14 | ... | ... | ... | - | CaO | 5·61 | 2·43 | 6·80 | 7·88 | 4·07 | - | MgO | 3·46 | 0·98 | 1·06 | 3·65 | 0·30 | - | K_{2}O | 2·42 | ... | 1·66 | ... | 1·17 | - | Na_{2}O | 2·58 | ... | 4·17 | ... | 2·30 | - | H_{2}O | 3·42 | ... | 2·68 | ... | 2·48 | - | P_{2}O_{5} | 0·48 | 0·32 | ... | 0·12 | ... | - | CO_{2} | 1·28 | ... | ... | 5·01 | ... | - +------------+-----------+---------+-----------+---------+-----------+ - -[Footnote 328: There was a trace of manganous oxide in this specimen.] - -(_b_) The Andesites, or, as they were formerly called, Porphyrites, -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 _verde antique_ of the ancients.[329] -One of the Cheviot andesites lying at the bottom of the series is -distinguished by its large and abundant plates of black mica.[330] - -[Footnote 329: An instance of this rock occurs in Kincardineshire, from -which the large flat twins of labradorite have been analyzed by Dr. -Heddle (_Trans. Roy. Soc. Edin._ vol. xxviii. (1879), p. 257).] - -[Footnote 330: C. T. Clough, "The Cheviot Hills," _Mem. Geol. Survey_ -(1888), p. 12.] - -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.[331] 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 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. - -[Footnote 331: _Ibid._ p. 11.] - -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:-- - -+-----------+-----+-------+-------+--------+-------+--------+-----------+-------+ -| |Scawd|Rennie-|Cunrie-|Duncan's|Whitton|Cuddies'| Cocklaw- |More- | -| | Law | ston | ston | Dubs | Hill | Tops | foot | battle| -|-----------+-----+-------+-------+--------+-------+--------+-----------+-------+ -|SiO_{2} |59·29| 62·81 | 63·38 | 59·44 | 60·70 | 60·58 | 62·29 | 59·82 | -|Al_{2}O_{3}|16·30| 16·40 | 15·77 | 16·15 | 17·98 | 12·25 | 17·03 | 16·96 | -|Fe_{2}O_{3}| 1·77| ·55 | ·73 | 1·05 | ·66 | 1·01 | ·93 | ·20 | -|FeO | 3·70| 3·27 | 2·65 | 2·83 | 2·58 | 4·13 | 2·44 | 6·57 | -|MnO | ·41| ·81 | ·08 | ·37 | ·20 | ·15 | ·21 | ·15 | -|CaO | 4·81| 4·46 | 4·44 | 6·70 | 7·07 | 4·40 | 3·92 | 4·73 | -|MgO | 3·15| 1·64 | 1·88 | 2·46 | 2·20 | 2·86 | 2·71 | 2·84 | -|K_{2}O | 4·19| 3·60 | 1·88 | 3·18 | 3·57 | 2·19 | 1·14 | 2·63 | -|Na_{2}O | 3·44| 3·02 | 4·54 | 3·70 | 2·95 | 3·61 | 3·20 | 3·04 | -|H_{2}O | 3·84| 4·04 | 4·69 | 3·35 | 3·45 | ... | ·29[332]| ... | -|H_{2}SO_{4}| ...| ... | ... | ... | ... | ·55 | ·37 | trace | -|Loss. | ...| ... | ... | ... | ... | 2·15 | 4·81 | 1·98 | -+-----------+-----+-------+-------+--------+-------+--------+-----------+-------+ - -[Footnote 332: This is CO_{2}.] - - -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[333] and by Dr. Petersen,[334] 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 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.[335] - -[Footnote 333: _Geol. Mag. for 1883_, pp. 100, 145, 252.] - -[Footnote 334: _Mikroskopische und chemische Untersuchungen am -Enstatit-porphyrit aus den Cheviot Hills_, 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, _Quart. Journ. Geol. Soc._ vol. xlii. (1886), p. -418.] - -[Footnote 335: Dr. F. H. Hatch supplied notes on microscopic structure -which are incorporated in the text, together with particulars -afterwards furnished by Mr. Watts.] - -(_c_) The lavas which may be separated as Trachytes 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:-- - - +-----------------------+ - | N.W. of Whitton Hill, | - | Jedburgh (No. 1938) | - | Sp. gr. 2·55. | - +-------------+---------+ - | SiO_{2} | 62·44 | - | Al_{2}O_{3} | 18·99 | - | Fe_{2}O_{3} | 3·35 | - | FeO | 1·8 | - | MnO | ·25 | - | CaO | 1·84 | - | MgO | 1·37 | - | K_{2}O | 5·02 | - | Na_{2}O | 2·65 | - | H_{2}O | 2·48 | - +-------------+---------+ - | Total. | 100·19 | - +-------------+---------+ - -(_d_) Acid rocks such as Felsites and Rhyolites 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 Fig. 9. - -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, when they undergo weathering, pass into the -varieties formerly comprised under the name claystone. - -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. - -2. _Intrusive Bosses, Sills and Dykes._--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.[336] 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.[337] - -[Footnote 336: _Geol. Mag._ for 1883, pp. 100, 145, 252; and _British -Petrography_, pp. 272, 278.] - -[Footnote 337: Notes by Dr. Hatch.] - -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.[338] - -[Footnote 338: 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 _British Petrography_, p. 286).] - -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 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. - -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:[339]-- - -[Footnote 339: Two analyses of rhyolites from Shetland by Mr. Tatlock -will be found in _Trans. Roy. Soc. Edin._ 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.] - - +-----------+------------+------------+-------------+-----------+ - | | | |Tinto, | | - | |"Hornstone."| | Lanarkshire:| | - | | Torgeith |"Hornstone."| Soluble in | | - | | Knowe, | Braid | hydrochloric|Insoluble | - | | Pentlands | Hills[340] | acid | in ditto. | - +-----------+------------+------------+-------------+-----------+ - |SiO_{2} | 73·91 | 64·73 | ·04 | 70·28 | - |Al_{2}O_{3}| 14·41 | 17·01 | 1·01 | 12·54 | - |Fe_{2}O_{3}| ·76 | 2·35 | 1·24 | ·43 | - |MnO | ·07 | ·24 | ... | ... | - |CaO | 1·21 | 4·19 | ·92 | ·91 | - |MgO | 4·90 | ·66 | ·52 | ... | - |K_{2}O | 3·36 | 3·27 | ... | 3·92 | - |Na_{2}O | 1·57 | 3·75 | ... | 5·84 | - |P_{2}O_{5} | ... | ·26 | ·16 | ... | - |H_{2}O | ·90 | 2·78 | ... | 1·99 | - +-----------+------------+------------+-------------+-----------+ - -[Footnote 340: This specimen also yielded 0·13 of ferrous oxide, and 2·42 -of carbon dioxide.] - -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[341] (see Fig. -93). 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. - -[Footnote 341: 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.] - -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 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." - -3. _Tuffs and Agglomerates._--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. - -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. - -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:-- - - Silica - percentage. - - 1. Quarry above Woodhouselee 63·3 - 2. South-west side of Castlelaw Hill 73·15 - 3. Quarry on road, ½ mile N.E. of Swanston (Braid Hill vent) 74·1 - 4. South-west side of Castlelaw Hill 75·0 - 5. Castlelaw Hill 76·00 - 6. South side of White Hill Plantation 90·00 - -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. - -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. - - - - -CHAPTER XVIII - -STRUCTURE AND ARRANGEMENT OF THE LOWER OLD RED SANDSTONE VOLCANIC ROCKS -IN THE FIELD - - -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 Fig. -99). - - -i. BEDDED LAVAS AND TUFFS - -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 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. - -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. - -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. - -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 _coulée_. 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 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. - -[Illustration: - - Fig. 65.--Veins and nests of sandstone due to the washing of - sand into fissures and cavities of an Old Red Sandstone lava. - Turnberry Point, Ayrshire. -] - -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,[342] 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 (Fig. 65). 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 (Fig. 66). If we could remove the -sandstone-veinings and aggregates, we should find the upper surfaces -of these igneous masses to present a singularly fissured 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. - -[Footnote 342: See Jukes' _Manual of Geology_, 3rd edit. (1872), Fig. -111, p. 276.] - -[Illustration: Fig. 66.--Ground-plan of reticulated cracks in the upper -surface of an Old Red Sandstone lava filled in with sandstone. Red -Head, Forfarshire.] - -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. - -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. - -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. - -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. 95, 96, 97). 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. - -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. - -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 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. - -[Illustration: Fig. 67.--Section across the volcanic series of -Forfarshire. _a_, conglomerates, sandstones and flagstones; _b_, sheets -of andesitic lava.] - -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 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. - -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 (_Kampecaris_, _Archidesmus_), washed -from the neighbouring land.[343] So undisturbed were the conditions -of deposition that calcareous sediment gathered round some of the -organisms and encased them in limestone nodules. - -[Footnote 343: An abundant organism in some of these deposits, named -_Parka_, 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.] - -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. - -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. - - -ii. VENTS - -On the whole the actual vents of the volcanoes of Lower Old Red -Sandstone time are less clearly distinguishable than those of -subsequent volcanic 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. - -[Illustration: Fig. 68.--Section across two necks above Tillicoultry, -Ochil Hills. - -1 1, Andesite lavas; 2 2, Tuffs and volcanic conglomerates; 3 3, The -two necks; 4 4, Dykes of felsite, etc.; 5, Coal-measures; _f_, Fault.] - -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 (Fig. 93), 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 (Fig. 92). 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 (Fig. 68). 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 further to the -south and west. More particular reference to this district will be made -in the following chapter (p. 303). - -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. - -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. - -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 Chapter xx., 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. - -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 Fig. 82. - -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 notable are the bosses of granite and other acid -material which rise through the Silurian strata of the Southern Uplands -of Scotland.[344] The largest are the well-known masses of Galloway -(Fig. 69), with which must be grouped the bosses near New Cumnock, -that of the Spango Water (Fig. 94), 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 (Fig. 70). 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.[345] - -[Footnote 344: I suggested this possible connection many years ago in -_Trans. Geol. Soc. Edin._ vol. ii. (1874) p. 21.] - -[Footnote 345: The volcanic geology of the Cheviot Hills is described -by Mr. Teall, _Geol. Mag. for 1883_, p. 106; and by Mr. Clough, _Mem. -Geol. Survey_, "Geology of the Cheviot Hills," Sheet 108 N.E., 1888, p. -24.] - -[Illustration: Fig. 69.--Section of the granite core between Merrick -and Corscrine. - -_a_, Silurian greywackes, grits and shales; _b_, granite.] - -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,[346] 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 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. - -[Footnote 346: See the descriptions of the Shap granite by Messrs. Marr -and Harker, _Quart. Journ. Geol. Soc._ xlvii. (1891) p. 266, and xlix. -(1893) p. 359.] - -[Illustration: Fig. 70.--Section across the three Dirrington Laws, -Berwickshire. - -_a_, Upper Silurian strata; _b_, Necks probably of Lower Old Red -Sandstone age; _c_, Upper Old Red Sandstone lying unconformably both on -_a_ and _b_.] - -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. - - -iii. SILLS AND DYKES - -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. - -In the Pentland Hills, as will be described in Chapter xx., a massive -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. - -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. 71, 72). 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.[347] - -[Footnote 347: Messrs. B. N. Peach and J. Horne, _Trans. Roy. Soc. -Edin._ xxxii. (1884), p. 359.] - -[Illustration: Fig. 71.--Section of Papa Stour, Shetlands, showing -sill of spherulitic felsite traversing Old Red Sandstone and bedded -porphyrites (Messrs. Peach and Horne). - -1. Red sandstones and flagstones; 2. Purple diabase-porphyrites; 3. -Great sheet of pink spherulitic felsite.] - -[Illustration: Fig. 72.--Section across Northmavine, from Okrea Head to -Skea Ness, Shetland, showing dykes and connected sill of granite and -felsite (Messrs. Peach and Horne). - -1. Schists, etc.; 2. Serpentine; 3. Granite and quartz-felsite; 4. -Breccia of serpentine fragments; 5. Bedded andesites and tuffs. _f_, -Fault.] - -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.[348] 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. - -[Footnote 348: See Sheet 56 of the Geological Survey of Scotland.] - -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. - -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 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. - -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 Fig. 68). 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 Chapter xxi. Another remarkable development of dykes -occurs in Shetland (Fig. 72), where they consist of granite, felsite -and rhyolite, and are associated with the acid sills above referred to. - - - - -CHAPTER XIX - -VOLCANOES OF THE LOWER OLD RED SANDSTONE OF "LAKE CALEDONIA" - - 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. - - -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. - -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.[349] 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 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 (Map III.). - -[Footnote 349: 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.] - -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.[350] 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. - -[Footnote 350: 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.] - -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. - -[Illustration: Fig. 73.--Section at the edge of one of the bays of -Lower Old Red Sandstone along the northern margin of Lake Caledonia, -near Ochtertyre. - -_a_, slates and phyllites; _b_, volcanic conglomerates; _c_, -andesite-lava.] - -I may remark in passing that the conglomerates and other associated -materials which have been preserved in these bays and hollows beyond -the 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 -(Fig. 73). - -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 (Fig. 74), 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. - -[Illustration: - - Fig. 74.--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. -] - -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 remarkable volcanic history was enacted -of which I now proceed to give some account. - -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. - -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. - -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 now and then to overlap -those from another, but the two series remain distinct. - -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. - -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. - -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. - -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:-- - - I. Northern Chain of Volcanoes - - 1. The Montrose Centre. - 2. The Sidlaw and Ochil Group. - 3. The Arran and Cantyre Centre. - 4. The Ulster Centres. - - II. Southern Chain of Volcanoes - - 5. The Pentland Volcano. - 6. The Biggar Centre. - 7. The Duneaton Centre. - 8. The Ayrshire Group. - -The distribution of these various volcanic areas will be most easily -understood from an examination of Map III. accompanying this volume. - - -I. THE NORTHERN CHAIN OF VOLCANOES IN "LAKE CALEDONIA" - - -1. _The Montrose Centre_ - -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.[351] 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 Fig. 67. - -[Footnote 351: 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.] - -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. - -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 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 -(_e_), which form nearly the whole precipice, lies a band of dull -purplish ashy conglomerate (_d_), 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" (_a_), 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. - -[Illustration: Fig. 75.--Section showing the top of the volcanic series -at the foot of the precipice of the Red Head, Forfarshire. - -_a_, Top of slaggy andesite; _b_, coarse volcanic conglomerate; -_c_, Red sandstone; _d_, Tuff and volcanic conglomerate; _e_, Red -sandstones.] - -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. 65, 66). 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 stratified through its intricate -ramifications among the pile of dark clinkers. - -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. - -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. - -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. - -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 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. - -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. - -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. - -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 p. 274, occurs in the -conglomerate.[352] South of Caterline 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 (Fig. 76). 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. - -[Footnote 352: For an analysis of the felspar in this rock, see Prof. -Heddle's paper, _Trans. Roy. Soc. Edin._ xxviii. (1879), p. 257.] - -[Illustration: Fig. 76.--Andesite with sandstone veinings and overlying -conglomerate. Todhead, south of Caterline, coast of Kincardineshire.] - - -2. _The Sidlaw and Ochil Group_ - -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 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. - -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 -Fig. 78). As already remarked, this important anticlinal fold runs -to the north-east across Forfarshire, and passes out to sea north of -Montrose. - -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 Fig. 68). Another -result of this structure is that the base of the volcanic series is -entirely concealed by its higher portions. - -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 (Fig. 77). I have already alluded to -these interesting relics of the shore-line 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 (_ante_, p. 295, and Fig. 73). 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. - -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. - -[Illustration: Fig. 77.--Section across the Boundary-fault of the -Highlands at Glen Turrit, Perthshire. _s_, Crystalline schists of -the Highlands; _c_ _c_, conglomerates and sandstones (Lower Old Red -Sandstone) with interstratified volcanic rocks (_v_ _v_); _f_, fault.] - -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. - -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 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 (Fig. 78). - -[Illustration: Fig. 78.--Section across the chain of the Sidlaw Hills, -near Kilspindie. - -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; _f_, Fault; _d_, Basic dyke.] - -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[353] (Fig. 78). 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 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. - -[Footnote 353: 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.] - -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. - -The structure of the Sidlaw range is repeated among the hills of east -Fife on the southern side of the great anticlinal fold.[354] Thus -a section from near Newburgh on the Firth of Tay southward to near -Auchtermuchty in Stratheden gives the arrangement of rocks shown in -Fig. 79. 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. - -[Footnote 354: The eastern part of the Ochils was mapped for the -Geological Survey by Mr. H. H. Howell and Mr. B. N. Peach.] - -[Illustration: Fig. 79.--Section across the Eastern Ochil Hills from -near Newburgh to near Auchtermuchty. - -1. Lower Old Red Sandstones and conglomerates; 2. Andesite lavas; 3. -Volcanic conglomerates; 4. Upper Old Red Sandstone.] - -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 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. - -[Illustration: Fig. 80.--Generalized section across the heart of the -Ochil Hills, from Dunning on the north to the Fife Coal-field near -Saline on the south. - -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); _f_, Fault.] - -A section across the centre of the Ochil chain,[355] from Dunning -in Strathearn to the Crook of Devon and the Fife Coal-field, gives -the structure which is generalized in Fig. 80. 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. - -[Footnote 355: 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.] - -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. - -The best sections to show the nature and sequence 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 Fig. 80, 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 Fig. -81. The total thickness of volcanic material amounts to more than 6500 -feet. - -[Illustration: Fig. 81.--Diagram of the volcanic series of the Western -Ochil Hills. - -The bands with vertical lines are various lavas (_a_); the tuffs and -volcanic breccias are shown by the dotted bands (_b_); the uppermost -portion of the section above the last thick group of lavas consists of -conglomerates and sandstones (_c_) with a sheet of lava.] - -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. - -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 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. - -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, Fig. 81. It was from -this locality that the specimens of _Eucephalaspis_, _Pteraspis_ and -_Scaphaspis_ were obtained which were described by Professor Ray -Lankester.[356] - -[Footnote 356: _Palæontographical Society_, vols. xxi. (1867) and -xxiii. (1869).] - -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 (_c_ in Fig. 81). -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. - -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 Fig. 68), 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.[357] - -[Footnote 357: 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 (_ante_, p. 272).] - -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. - - -3. _The Arran and Cantyre Centre_ - -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.[358] 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. - -[Footnote 358: 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.] - -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." - -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.[359] -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 Fig. 82; -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. - -[Footnote 359: 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 (_Quart. Journ. -Geol. Soc._ vol. viii. (1852), p. 406).] - -[Illustration: Fig. 82.--View of Cnoc Garbh, Southend, Campbeltown. A -volcanic neck of Lower Old Red Sandstone age, about 400 yards wide in -its longer diameter.] - -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 (_a_ in Fig. 83) dipping towards the east are found to pass -upward into coarse volcanic breccias (_b_), which, followed eastwards, -lose almost all trace of stratification, and are then abruptly -succeeded by a neck of coarse agglomerate (_c_) 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 neck, except by -the rude bedding of the former which pass down into the well-bedded -sandstones. - -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. - -[Illustration: Fig. 83.--Section of volcanic series on beach, Southend, -Campbeltown. - - _a_, Fine reddish and purplish highly felspathic sandstones, - largely composed of porphyry-debris and passing up into coarse - breccias; _b_, 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; _c_, coarse unstratified agglomerate - forming a neck. -] - -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. - -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. - -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 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. - -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. - - -4. _The Ulster Centres_ - -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. - -A picturesque section has been laid bare between the Coastguard Station -south of Cushendall and Cushendun Bay.[360] 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. - -[Footnote 360: For descriptions of this district see J. Bryce, _Proc. -Geol. Soc._ i. (1834) p. 396, v. (1837) p. 69; J. Kelly, _Proc. Roy. -Irish Acad._ 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.] - -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 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. - -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.[361] This interesting westward -prolongation of the volcanic record consists of a number of outlying -patches confined to the eastern part of the district. - -[Footnote 361: 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.] - -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. - -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 Fig. -84 may be seen. At the base lies a coarse conglomerate (_a_) 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 (_b_), followed by an exceedingly -coarse conglomerate (_c_), formed mainly of the debris of andesites, -especially lumps of slag. Some of the stones measure 18 inches in -diameter, and all are well water-worn. Immediately over this mass of -detritus lies the lowest sheet of andesite-lava (_d_). - -[Illustration: Fig. 84.--Section of the base of the volcanic series, -Reclain, five miles south of Pomeroy.] - -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. - -[Illustration: - - Fig. 85. Section of shales and breccias at Crossna Chapel, - north-east of Boyle. - - _a_ _a_, Green and grey shales; _b_ _b_, green and grey hard - sandstones and grits, some bands strongly felspathic; _c_, fine - compact felspathic breccia, with angular chips of different - felsites and andesites, etc. -] - -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. - -Further south-west, near Boyle, in the county of Roscommon, certain -curious felspathic breccias in the Old Red Sandstone have been mapped -as "felstone."[362] 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. - -[Footnote 362: See Sheet 66 Geological Survey of Ireland, and -Explanation to that sheet (1878), p. 15. The rocks were previously -described by Jukes and Foot, _Journ. Roy. Geol. Soc. Ireland_, vol. i. -(1866), p. 249.] - -The accompanying section (Fig. 85) 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. - - - - -CHAPTER XX - -VOLCANOES OF THE LOWER OLD RED SANDSTONE OF "LAKE -CALEDONIA"--_continued_ - - The Southern Chain--The Pentland Volcano--The Biggar Centre--The - Duneaton Centre--The Ayrshire Volcanoes. - - -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. - - -II. THE SOUTHERN CHAIN OF VOLCANOES IN "LAKE CALEDONIA" - - -5. _The Pentland Volcano_ - -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,[363] and the earlier geognostical papers of Jameson.[364] -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. - -[Footnote 363: _A Sketch of the Geology of Fife and the Lothians_, -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.] - -[Footnote 364: See specially _Mem. Wernerian Soc._ vol. ii.; also -MacKnight in vol. i. The account of the Pentland Hills by Hay -Cunningham in vol. vii. (1838) is clear but brief.] - -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 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. - -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. - -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. - -The bedded aspect of the truncated end of the Pentland chain, as seen -from the north, has been already alluded to (p. 281). 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. - -[Illustration: Fig. 86.--Section across the north end of the Pentland -Hills, from Warklaw Hill to Pentland Mains. Length about five miles. - - 1. Upper Silurian grits and shales, not seen where the line of - section crosses; 2 2. Andesites and diabases in numerous - interstratified sheets; 2 _s_. Intercalated sandstones and - conglomerates; 3. Felsitic tuffs and breccias and orthophyre - sheets; _n_, 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 (_f_). -] - -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 (Fig. 86). 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. - -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 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. - -A section taken through the north end of the chain, where the maximum -depth of volcanic material is exposed, presents the arrangement -represented in Fig. 86. 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. - -[Illustration: Fig. 87.--View of the lava-escarpments of Warklaw Hill, -Pentland chain, from the north-west.] - -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. - -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 Fig. 87. 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 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. - -Torduff Hill, which rises to the east of Warklaw, consists of a mass of -coarse volcanic breccia or agglomerate (_n_), 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. - -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_s_), 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. - -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. - -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. - -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. - -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 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. - -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. - -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. - -On the east side of Castlelaw Hill, as shown in Fig. 86, 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. - -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 Fig. 88. 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. 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. - -[Illustration: Fig. 88.--Section across the Pentland Hills through -North Black Hill and Scald Law (length about three miles).] - -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_s_), 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. - -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. - -[Illustration: Fig. 89.--Section from the valley of the Gutterford Burn -through Green Law and Braid Law to Eight-Mile Burn.] - -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 the uppermost group of andesites, -which, as before, are here truncated by the eastern fault (_f_), the -Calciferous Sandstones and Carboniferous Limestone series (6) being -thrown against them. - -A third section (Fig. 89), 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:-- - - White felsitic rocks of Braid Law (4 in Fig. 89). - - Coarse conglomerate passing down into sandstone. About 20 feet - visible. - - Dark andesite, 4 feet. - - Parting of yellow felspathic grit, 8 or 10 inches. - - Andesite, 10 feet. - - Hard felspathic grit, 6 feet. - - Dark green amygdaloidal andesite, 2 feet. - - Yellow felspathic sandstone and grit, 2 feet. - - Dark green amygdaloidal andesite, 6 feet. - - Felspathic grit and red and brown sandstone, 4 feet. - - Dark andesite, perhaps 6 or 8 feet. - - Great conglomerate with alternating courses of sandstone, rapidly - increasing in thickness southwards. - -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 (_f_). 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. - -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. - -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, 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. - -[Illustration: Fig. 90.--Section across the north end of the Pentland -Hills, and the southern edge of the Braid Hill vent. Length about two -miles. - -1 1. Andesites; 2. Fine tuffs, etc., of the Braid Hill vent; 3 3 3. -Agglomerate in lateral necks with felsitic intrusions (4).] - -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. - -On the outskirts of the main vent some subordinate necks may be -observed (3, 3 in Fig. 90), perhaps, like Torduff Hill, already -noticed (Fig. 86), 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 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. - -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.[365] 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. - -[Footnote 365: Explanation to Sheet 24 of the Geological Survey of -Scotland, pp. 10, 12.] - - -6. _The Biggar Centre_[366] - -[Footnote 366: 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, _Geology of -Fife_, etc., p. 176.] - -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. - -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 -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. - -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. - -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 Fig. 91. 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. - -[Illustration: Fig. 91.--Section across the northern end of the Biggar -volcanic group, from Fadden Hill to beyond Mendick Hill. - -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. _f_, Fault.] - -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 -(_f_, Fig. 92), 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. - -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 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. - -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. - -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. - -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 last -andesites made their appearance together with those which form the -highest lavas of the Pentland chain. - -[Illustration: Fig. 92.--Section across the southern part of the Biggar -volcanic group from Covington to Culter. - -1. Lower Silurian strata; 2. Lower Old Red Sandstone (pre-volcanic -group); 3. Andesite lavas with intercalated sandstones and -conglomerates; 4. Felsite neck. _f_, The boundary-fault on northern -edge of Southern Uplands.] - -A section across the southern end of the Biggar volcanic belt shows -less diversity of structure (Fig. 92). 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. - -An interesting feature in this district is the felsitic boss of -Quothquan already alluded to (p. 288) 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. - -[Illustration: Fig. 93.--Section from Thankerton Moor across Tinto to -Lamington. - - 1_a_. 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). _f_, 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. -] - -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 (Fig. 93). The conglomerates and sandstones that -appear on the south slopes of Tinto 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. - -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 (p. 278). 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. - -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.[367] 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. - -[Footnote 367: 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.] - - -7. _The Duneaton Centre_ - -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.[368] 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 (Fig. 94). 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. - -[Footnote 368: 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.] - -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.[369] - -[Footnote 369: Memoir on Sheet 15 Geol. Surv. Scotland (1871), p. 22.] - -[Illustration: Fig. 94.--Section across the Duneaton volcanic district -from the head of the Duneaton Water to Kirklea Hill. - -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.] - -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. - -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 (_f_) which flanks the -Silurian territory. That they are not met with now to the south-east -of the dislocation, where they must once have lain, is an evidence of -the great denudation which the district has undergone. Fig. 94, 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. - -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. - - -8. _The Ayrshire Group of Vents_ - -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.[370] 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. - -[Footnote 370: 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.] - -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. - -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. - -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 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. - -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. - -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. - -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. - -[Illustration: Fig. 95.--Cavernous spaces in andesite, filled in with -sandstone, John o' Groat's Port, Turnberry, Ayrshire.] - -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 Fig. 95 may be seen in -one of the lavas at John o' Groat's Port. - -[Illustration: Fig. 96.--Section of andesites, Turnberry Castle, -Ayrshire.] - -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 (Fig. 96) was taken. The -lower andesite (_a_) 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 (_b_), 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 (_c_) 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 (_d_) 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 (_e_), which further -north becomes amygdaloidal and much veined with sandstone, passing into -a breccia of lava fragments and sandstone. - -[Illustration: Fig. 97.--Lenticular form of a brecciated andesite -(shown in Fig. 96), Turnberry, Ayrshire.] - -[Illustration: Map III.] - - MAP OF THE VOLCANIC DISTRICTS OF THE LOWER OLD RED SANDSTONE - OF "LAKE CALEDONIA" IN CENTRAL SCOTLAND & NORTH EAST IRELAND - -The remarkable brecciated band (_d_) 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 -_c_ and _e_ to come together, as shown in Fig. 97. 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. - -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. - - - - -CHAPTER XXI - -VOLCANOES OF THE LOWER OLD RED SANDSTONE OF THE CHEVIOT HILLS, LORNE, -"LAKE ORCADIE" AND KILLARNEY - - -THE CHEVIOT AND BERWICKSHIRE DISTRICT - -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,"[371] to distinguish it from the -other basins of the same geological period (Map I.). - -[Footnote 371: _Trans. Roy. Soc._ Edin. xxviii. (1878), p. 354.] - -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.[372] No -good horizontal section, however, has yet been constructed to show this -structure--a deficiency which, it is hoped, may before long be supplied. - -[Footnote 372: 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 (_Mem. Geol. Surv._ 1888); "Geology of Otterburn -and Elsdon," by H. Miller and C. T. Clough (_Mem. Geol. Surv._ 1887); -"Geology of Part of Northumberland between Wooler and Coldstream," by -W. Gunn and C. T. Clough, with Petrographical Notes by W. W. Watts -(_Mem. Geol. Surv._ 1895). Other descriptions have been published by -Professor James Geikie, _Good Words_, vol. xvii. (1876), reprinted in -_Fragments of Earth-lore_ (1893), and by Prof. Lebour, _Outlines of -the Geology of Northumberland_, 2nd edit. 1886. For the petrography -of the rocks consult Mr. J. J. H. Teall, _Geol. Mag._ 1883, pp. 100, -145, 252, 344; 1884, p. 226; 1885, p. 106; _Proc. Geol. Assoc._ ix. -(1886) p. 575; and his _British Petrography_, 1888; Dr. J. Petersen, -_Mikroskopische und chemische Untersuchungen am Enstatit-porphyrit aus -den Cheviot Hills_, Inaugural Dissertation, Kiel, 1884.] - -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. - -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."[373] 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. - -[Footnote 373: Clough, _Geology of the Cheviot Hills_, p. 15.] - -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. - -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 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."[374] Mr. Teall also, from a chemical and -microscopical examination of the rocks, drew a similar conclusion.[375] - -[Footnote 374: _Op. cit._ p. 24.] - -[Footnote 375: _Geol. Mag._ 1885, p. 106.] - -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.[376] - -[Footnote 376: _Op. cit._ pp. 26-28.] - -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. - - * * * * * - -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. - -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 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.[377] - -[Footnote 377: 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, _Proc. Roy. Soc. Edin._ xiv. (1887).] - -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. - -[Illustration: Fig. 98.--Section across the volcanic area of St. Abb's -Head (after Prof. J. Geikie). - - 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. _f_, Fault. -] - -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. - -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. - -The bedded tuffs are usually well stratified deposits. The most -important 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. - -The lavas, tuffs and agglomerates have been abundantly invaded by -intrusive rocks, chiefly andesites.[378] - -[Footnote 378: See Prof. J. Geikie, _op. cit._] - -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 _Pterygotus_ -in some of the green shales interstratified among fine tuffs and ashy -sandstones.[379] 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. - -[Footnote 379: "Geology of Eastern Berwickshire," _Mem. Geol. Surv. -Scotland_ (1864), pp. 26, 27, 57.] - -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.[380] Five miles still further in the same direction -the group of the beautiful cones of Dirrington (Fig. 70) overlooks -the wide Merse of Berwickshire,[381] and six miles to the north of -these hills, in the very heart of Lammermuir, lies the solitary -boss of the Priestlaw granite.[382] 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 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. - -[Footnote 380: See "The Geology of Eastern Berwickshire" (Sheet 34), -_Mem. Geol. Surv. Scotland_ (1864), p. 29.] - -[Footnote 381: These hills are chiefly represented in Sheet 25. But see -"The Geology of East Lothian," _Mem. Geol. Surv. Scotland_ (1866), p. -26.] - -[Footnote 382: "Geology of East Lothian," _Mem. Geol. Surv. Scotland_, -p. 15, and authorities there cited.] - - -"THE LAKE OF LORNE" - -[Illustration: Fig. 99.--View of terraced andesite hills resting on -massive conglomerate, south of Oban.] - -The basin of Lorne has not yet been carefully examined and described, -though various writers have referred to different parts of it (Map I.). -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 (Fig. 99), 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 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. - -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. - -[Illustration: Fig. 100.--Section of lava-escarpment at Beinn Lora, -north side of mouth of Loch Etive, Argyllshire. - -1. Phyllites; 2. Thick conglomerate; 3. Successive sheets of andesite.] - -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 -(Fig. 100). 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. - -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 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. - -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.[383] - -[Footnote 383: _Ann. Report Geol. Surv._ (1895), p. 29 of reprint.] - -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. - - -"LAKE ORCADIE" - -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" (Map I.). 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, _Psilophyton_ 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. - -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.[384] Since -that time Messrs. 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. - -[Footnote 384: _Trans. Roy. Soc. Edin._ xxviii. (1878).] - -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.[385] This -latter band, as shown in Fig. 101, 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. - -[Footnote 385: See Sheet 76 of the Geological Survey of Scotland.] - -[Illustration: Fig. 101.--Section across Strathbogie, below Rhynie, -showing the position of the volcanic band. - -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. _f_, Fault.] - -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 -(Fig. 101). - -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.[386] 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. - -[Footnote 386: See Sheet 95 of the Geological Survey of Scotland and -_Trans. Roy. Soc. Edin._ vol. xxviii. (1878), p. 435.] - -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.[387] 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 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. - -[Footnote 387: Messrs. B. N. Peach and J. Horne, _Proc. Roy. Phys. Soc. -Edin._ vol. v. (1879), p. 80.] - -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,[388] 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.[389] 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. - -[Footnote 388: _Trans. Roy. Soc. Edin._ vol. xxviii. (1878), p. 418.] - -[Footnote 389: _Ibid._ vol. xxxii. (1884), p. 359.] - -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. - -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, p. 292) 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 intrusion of a series of bosses -and dykes of basic rocks (diabases) which traverse the sills. - - -The Killarney District - -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. - -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;[390] 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. - -[Footnote 390: 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.] - -The felsite was long ago brought into notice by Dr. Haughton, -who published an analysis of it.[391] It is also referred to by -Mr. Teall for its spherulitic structure.[392] 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. - -[Footnote 391: _Trans. Roy. Irish Acad._ vol. xxiii. (1859), p. 615.] - -[Footnote 392: _British Petrography_, p. 349.] - -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 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. - -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.[393] 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. - -[Footnote 393: 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.] - - - - -CHAPTER XXII - -VOLCANOES OF THE UPPER OLD RED SANDSTONE--THE SOUTH-WEST OF IRELAND, -THE NORTH OF SCOTLAND - - -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. - -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. - - -THE SOUTH-WEST OF IRELAND - -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 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[394] (Map I.). - -[Footnote 394: 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.] - -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 (Fig. 102) 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. - -[Illustration: Fig. 102.--View of Knockfeerina, Limerick, from the -north-east--a volcanic neck of Upper Old Red Sandstone age.] - -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 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. - -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 _Palæopteris_ of Kiltorcan.[395] - -[Footnote 395: 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.] - - -THE NORTH OF SCOTLAND - -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"[396] (Map I.). 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 (Fig. 103). 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 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. - -[Footnote 396: First noticed in _Geol. Mag._ February 1878; and _Trans. -Roy. Soc. Edin._ xxviii. (1878), p. 411.] - -[Illustration: - - Fig. 103.--Section of the volcanic zone in the Upper Old Red - Sandstone, Cam of Hoy, Orkney. - - 1. Caithness flagstones; 2. Dull-red tuff and bands of sandstone; - 3. Lava zone in three bands; 4. Yellow and red sandstone. -] - -[Illustration: Fig. 104.--Section of the volcanic zone in the Upper Old -Red Sandstone at Black Ness, Rackwick, Hoy.] - -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 Fig. 104. The ordinary red and -yellow sandstones (_a_) 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 (_b_). 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 (_c_). 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 (_d_) 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 -westward for a short distance, the whole volcanic zone is found to die -out and the sandstones below and above it then come together. - -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 (Fig. 105). 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. - -[Illustration: Fig. 105.--Section across the volcanic band and its -associated necks, Hoy, Orkney. - - 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. -] - -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 (Fig. 106).[397] 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. That these volcanic orifices were active -about the same time with those in the opposite island of Hoy may be -legitimately inferred. - -[Footnote 397: See _Trans. Roy. Soc. Edin._ xxviii. (1878), p. 405; -also p. 482 of the same volume for an account of the cleavable augite.] - -[Illustration: Fig. 106.--Ground-plan of volcanic neck piercing the -Caithness Flagstone series on the beach near John o' Groat's House.] - -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.[398] - -[Footnote 398: 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 xxiv. and xxv.] - - - - -BOOK VI - -THE CARBONIFEROUS VOLCANOES - - - - -CHAPTER XXIII - -THE CARBONIFEROUS SYSTEM OF BRITAIN AND ITS VOLCANIC RECORDS - - 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. - - -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. - -The Carboniferous period, as chronicled by its sedimentary deposits, -was a time of slow submergence and quiet sedimentation, terrestrial -and marine 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. - -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. - -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. - -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. - -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, 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. - -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. - -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. - -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. - -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 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. - -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. - -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. - -+-----------------------------------------+----------+-----------+----------+ -| | England. | Scotland. | Ireland. | -+-----------------------------------------+----------+-----------+----------+ -|Coal-measures. | | | | -| { Upper Red Sandstones with | | | | -| { _Spirorbis_-limestone. | | | | -| { Middle or chief coal-bearing | | | | -| { measures. | | | | -| { Gannister group. | | | | -| | | | | -|Millstone Grit. | | || | | -| } Grits, flagstones and shales with | | || | | -| } thin coals. | | || | | -| | | || | | -|Carboniferous Limestone. | | || | | -| { Yoredale group of shales and grits | | || | | -| { with limestones. | | || | | -| { Thick (Scaur or Main) Limestone | || | || | | -| { of England, with sandstones | || | || | || | -| { and coals in Scotland. | || | || | || | -| { Lower Limestone Shale (Calciferous | | || | | -| { Sandstones of Scotland). | | || | | -+-----------------------------------------+----------+-----------+----------+ - -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 Map I.). - -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. - -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. - -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.[399] 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. - -[Footnote 399: The supposed Carboniferous volcanic rocks of Bearhaven -on the coast of Cork are noticed on p. 49, vol. ii.] - -The total area within which the volcanic eruptions of Carboniferous -time took place was thus less than that over which the volcanoes of -the 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. - -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. - - * * * * * - -It is in the southern half of Scotland that the Carboniferous system is -developed (Map IV.). 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. - -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:-- - - Coal-measures. - { Upper Red Sandstone group, nearly devoid of coal-seams. - { - { Coal-bearing, white, yellow and grey sandstones, dark shales and - { ironstones (Upper Coal series). - - Millstone Grit. - { Thick white and reddish sandstones and grits. - - Carboniferous Limestone series. - { 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). - { - { 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 Fig. 155). - - Calciferous Sandstones.[400] - { 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. - { Cement-stone group consisting of red, blue and green marls and - { shales, red and grey sandstones, and thin bands of cement-stone: - { fossils scarce. - { 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. - -[Footnote 400: The Calciferous Sandstones are the stratigraphical -equivalents of the Limestone Shale and lower portion of the -Carboniferous Limestone of England.] - -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. - -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 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." - -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. - -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. - -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. - -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."[401] They supplied Playfair with numerous apt -illustrations in support of Hutton's views, and he seems to have made -himself thoroughly familiar with them.[402] 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.[403] 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.[404] Among the early writers the names -of Allan,[405] Townson,[406] Lord Greenock,[407] and Ami Boué,[408] -deserve especial mention. - -[Footnote 401: Hutton's _Theory of the Earth_, vol. i. p. 155 _et seq._] - -[Footnote 402: Playfair's _Illustrations of the Huttonian Theory_, § -255 _et seq._] - -[Footnote 403: _Trans. Roy. Soc. Edin._ (1805), vol. v. p. 43.] - -[Footnote 404: _Mem. Wern. Soc._ ii. 178, 618; iii. 25; _Edin. Phil. -Journ._ i. 138, 352; xv. 386.] - -[Footnote 405: Trans. Roy. Soc. Edin. (1811), vi. p. 405.] - -[Footnote 406: _Tracts and Observations in Natural History and -Physiology_, 8vo, Lond. 1799.] - -[Footnote 407: _Trans. Roy. Soc. Edin._ (1833), xiii. pp. 39, 107.] - -[Footnote 408: _Essai géologique sur l'Écosse._ Paris; no date, -probably 1820.] - -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.[409] 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. - -[Footnote 409: _Mem. Wern. Soc._ vii. p. 1. Published separately, 1838.] - -Still more important was the sketch given by Maclaren, in his classic -_Geology of Fife and the Lothians_.[410] 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 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. - -[Footnote 410: Small 8vo, Edin. 1838, first partly published as -articles in the _Scotsman_ newspaper. A second edition, which was -little more than a reprint of the first, appeared in 1866.] - -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.[411] 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;[412] 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.[413] - -[Footnote 411: _Trans. Roy. Soc. Edin._ vol. xxii.] - -[Footnote 412: _Ibid._ vol. xxix. (1879), p. 437.] - -[Footnote 413: _Quart. Journ. Geol. Soc._ xlviii. (1892), p. 104. This -summary, with additional details and illustrations, is embodied in the -text.] - -Two well-marked types of volcanic accumulations are recognizable in the -British Isles, which may be conveniently termed Plateaux and Puys. - -1. Plateaux.--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. - -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. - -2. Puys.--In this type, the ejections were often confined to the -discharge of a small amount of fragmentary materials from a single -solitary 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. - -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. - -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:-- - - Location Key: - A. Ayrshire and Renfrewshire. - B. Stirlingshire. - C. West Lothian. - D. Midlothian. - E. East Lothian. - F. Fife. - G. Berwick & Roxburghshire. - - +--------------------------------------------------++--------------------+ - | Plateau-type. || Puy-type. | - +-----------------------------------+--+--+--+--+--++--+--+--+--+--+--+--+ - | |A.|B.|D.|E.|G.||A.|B.|C.|D.|E.|F.|G.| - |Coal Measures |..|..|..|..|..||..|..|..|..|..|..|..| - | | | | | | ||‖ | | | | | | | - |Millstone Grit |..|..|..|..|..||‖ |..|..|..|..|..|..| - | | | | | | ||‖ | | | | | | | - |Carboniferous Limestone Series. | | | | | ||‖ | | | | | | | - | { Castlecary Limestone |..|..|..|..|..||‖ |..|..|..|..|..|..| - | { | | | | | ||‖ | | | | | | | - | { Calmy " |..|..|..|..|..||‖ |..|..|..|..|..|..| - | { | | | | | ||‖ | |‖ | | |‖ | | - | { Index " |..|..|..|..|..||‖ |..|‖ |..|..|‖ |..| - | { | | | | | || | |‖ | | |‖ |‖ | - | { Hurlet " |..|..|..|..|..||..|..|‖ |..|..|‖ |‖ | - | |‖ |‖ | |‖ | || | |‖ | | |‖ |‖ | - |Calciferous Sandstone Series. |‖ |‖ | |‖ | || | |‖ | | |‖ |‖ | - | { Burdiehouse Limestone |‖ |‖ | |‖ | || | |‖ | | |‖ |‖ | - | { and Oil-shale Group |‖ |‖ |..|‖ |..||..|..|‖ |..|..|‖ |‖ | - | { |‖ | |‖ |‖ | || | |‖ |‖ | |‖ |‖ | - | { Cement-stone Group |..|..|..|..|‖ ||..|..|‖ |‖ |..|‖ |‖ | - | { | | | | |‖ || | | | | | | | - | { Red Sandstones passing down |..|..|..|..|‖ ||..|..|..|..|..|..|..| - | { into Upper Old Red Sandstone|..|..|..|..|‖ ||..|..|..|..|..|..|..| - +-----------------------------------+--+--+--+--+--++--+--+--+--+--+--+--+ - - - - -CHAPTER XXIV - -CARBONIFEROUS VOLCANIC PLATEAUX OF SCOTLAND - - I. The Plateau-type restricted to Scotland--i. Distribution in the - Different Areas of Eruption--ii. Nature of the Materials erupted. - - -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 -_Bothriolepis_ 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. - - -i. DISTRIBUTION OF THE PLATEAUX - -Notwithstanding the effects of many powerful faults and extensive -denudation, the general position of the Plateaux and their independence -of each other can still be traced. They are entirely confined, as I -have said, to the southern half of Scotland (see Map IV.). 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:-- - -[Illustration: Fig. 107.--View of the escarpment of the Clyde Plateau -in the Little Cumbrae, from the south-west.] - -1. The Clyde Plateau.--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 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. - -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.[414] - -[Footnote 414: 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 _Transactions of the Glasgow Geological Society_. 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, -_Quart. Journ. Geol. Soc._ viii. (1852), p. 406. See also J. Bryce's -_Arran and Clydesdale_.] - -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 (Fig. -107), 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 (Fig. -108). - -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 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. - -[Illustration: Fig. 108.--View of the edge of the Volcanic Plateau -south of Campbeltown, Argyllshire. - -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.] - -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. - -[Illustration: Fig. 109.--View of North Berwick Law from the east, a -trachyte neck marking one of the chief vents of the Garleton Plateau. -(From a photograph.) - -This illustration and Figs. 119, 133 and 135 are from photographs taken -by Mr. Robert Lunn for the Geological Survey.] - -2. The East Lothian or Garleton Plateau.--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.[415] 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. - -[Footnote 415: 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.] - -[Illustration: Fig. 110.--The Bass Rock, a trachytic neck belonging to -the Garleton plateau, from the shore at Canty Bay.] - -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 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 (Fig. 109), -Traprain Law (Fig. 133), and the Bass Rock (Fig. 110). - -3. The Midlothian Plateau.--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.[416] 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 (Fig. 111), whence a discontinuous narrow strip may be traced -nearly as far as the River Clyde. - -[Footnote 416: 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.] - -[Illustration: Fig. 111.--Corston Hill--a fragment of the Midlothian -Plateau, seen from the north. - -The volcanic rocks form a cake on the top, the slopes lying across the -edges of the Calciferous Sandstones.] - -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.[417] They have been likewise mapped in detail on a -large scale by the Geological Survey, and have been described in the -Survey Memoirs. The rest of the plateau to the south-west is much less -familiar. - -[Footnote 417: Maclaren's _Geology of Fife and the Lothians_, 1839, pp. -1-67; and Hay Cunningham, _Mem. Wer. Soc._ 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.] - -In Fig. 112 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 Chapter xxxi. - -[Illustration: Fig. 112.--View of Arthur Seat from Calton Hill to the -north.] - -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.[418] 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 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. - -[Footnote 418: _Explanation, Geol. Surv. Scotland_, Sheet 24, p. 13 -(1869).] - -4. The Berwickshire Plateau.--Another and entirely disconnected area -occurs in the broad plain or Merse of the lower portion of the valley -of the Tweed.[419] 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. - -[Footnote 419: 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.] - -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. - -5. The Solway Plateau.--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.[420] 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 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. 113, 142). - -[Footnote 420: 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.] - -[Illustration: Fig. 113.--View of Arkleton Fell, part of the Solway -Plateau, from the south-west. - - 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. -] - -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. - -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. - - -ii. NATURE OF THE MATERIALS ERUPTED - -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. - -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. - -(_a_) Augite-olivine Rocks (Picrites and Limburgites).--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:[421]-- - - Silica 40·2 - Titanic oxide 2·9 - Alumina 12·8 - Ferric oxide 4·0 - Ferrous oxide 10·4 - Lime 10·4 - Magnesia 11·9 - Potash 0·8 - Soda 2·7 - Loss by ignition 3·4 - ---- - Spec. grav. 3·03. 99·5 - -[Footnote 421: _Trans. Roy. Soc. Edin._ vol. xxxvii. (1893), p. 116.] - -(_b_) Dolerites and Basalts.[422]--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. - -[Footnote 422: 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 -Chapter XXVI. (p. 418).] - -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 (p. 418). The beautiful basalt of Craiglockhart with its -large porphyritic olivines and augites has afforded a distinct type -of Carboniferous basalt (p. 418). 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 (p. 418). 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.[423] - -[Footnote 423: _Trans. Roy. Soc. Edin._ vol. xxxvii. (1893), pp. -117-119.] - -At Hailes Castle, in the Garleton plateau, the lower basic lavas -include another olivine-basalt somewhat more felspathic than that just -described, and 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:-- - - +------------+--------------+----------------+----------------+ - | | Kippie Law, | Hailes Castle, | Markle Quarry, | - | | specific | specific | specific | - | | gravity 2·8 | gravity 2·76 | gravity 2·7 | - +------------+--------------+----------------+----------------+ - |SiO_{2} | 46·01 | 49·07 | 49·54 | - |Al_{2}O_{3} | 19·19 | 19·43 | 22·23 | - |Fe_{2}O_{3} | 5·91 | 10·58 | 9·55 | - |FeO | 6·75 | 2·35 | 1·12 | - |MnO | 0·19 | 0·32 | 0·08 | - |CaO | 8·68 | 7·87 | 7·19 | - |MgO | 6·81 | 4·36 | 2·80 | - |K_{2}O | 1·20 | 0·98 | 1·81 | - |Na_{2}O | 3·27 | 3·31 | 4·56 | - |H_{2}O | 3·07 | 2·26 | 2·42 | - |Total | 101·08 | 100·53 | 101·30 | - +------------+--------------+----------------+----------------+ - -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 (p. 418). - -(_c_) Andesites (Porphyrites).--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. - -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. - -(_d_) Trachytes.--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. - -A number of specimens selected as illustrative of the different -varieties have been analyzed and the results are stated in the -subjoined table.[424] The specific gravity of the rocks is about 2·6. - -[Footnote 424: 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. _Trans. Roy. Soc. Edin._ vol. xxxvii. p. 122.] - - +------------+--------+--------+----------+-------------+----------+ - | | Pepper | Kae | Hopetoun | Phantassie | Bangley | - | | Craig | Heughs | Monument | | Quarry | - +------------+--------+--------+----------+-------------+----------+ - | SiO_{2} | 62·61 | 61·35 | 62·50 | 59·50 | 58·50 | - | Al_{2}O_{3}| 18·17 | 16·88 | 18·51 | 18·25 | 21·12 | - | Fe_{2}O_{3}| 0·32 | 0·41 | } 4·39 | 4·81 | 4·68 | - | FeO | 4·25 | 5·01 | } | 2·34 | ... | - | MnO | 0·21 | 0·26 | ... | ... | ... | - | CaO | 2·58 | 2·39 | 2·00 | 2·10 | 3·70 | - | MgO | 0·74 | 0·44 | 0·61 | 0·70 | 0·93 | - | K_{2}O | 4·02 | 6·12 | 6·31 | 6·30 | 5·84 | - | Na_{2}O | 6·49 | 5·26 | 3·44 | 5·03 | 3·90 | - | H_{2}O | 0·80 | 1·70 | 2·10 | 1·60 | 2·00 | - | Total | 100·19 | 99·82 | 99·86 | 100·63 | 100·67 | - +------------+--------+--------+----------+-------------+----------+ - -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 (_e_).[425] - -[Footnote 425: For fuller petrographical details consult Dr. Hatch's -paper above cited.] - -(_e_) Rocks of the Necks.--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. - -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 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:--[426] - - Silica 56·8 - Titanic acid 0·5 - Alumina 19·7 - Ferric oxide 2·2 - Ferrous oxide 3·5 - Manganous oxide 0·2 - Lime 2·2 - Magnesia 0·4 - Soda 4·3 - Potash 7·1 - Loss by ignition 2·5 - ---- - Spec. grav. 2·588 99·4 - -[Footnote 426: _Trans. Roy. Soc. Edin._ vol. xxxvii. p. 125.] - -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.[427] - -[Footnote 427: 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, _ibid._ pp. 123, 124.] - -(_f_) Tuffs.--The fragmentary ejections of the plateaux vary in texture -from the finest-grained tuffs to coarse agglomerates.[428] 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 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.[429] 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.). - -[Footnote 428: For accounts of these rocks, see Explanation of Sheet 33 -_Geol. Surv. Scot._ p. 32; Sheet 22, pp. 11-14; Sheet 31, pp. 14-17.] - -[Footnote 429: Explanation of Sheet 22 _Geol. Surv. Scot._ p. 12.] - - - - -CHAPTER XXV - -GEOLOGICAL STRUCTURE OF THE CARBONIFEROUS VOLCANIC PLATEAUX OF SCOTLAND - - 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. - - -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. - -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 -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 (Fig. 114). 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. - -[Illustration: Fig. 114.--Vertical sections of the escarpment of the -Clyde plateau from north-east to south-west. - - 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. - - 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. -] - -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. - - -1. BEDDED LAVAS AND TUFFS - -The successive sheets of lava in a plateau usually form thin and -widespread beds which are only occasionally separated by intercalations -of tuff 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 (Fig. 107) 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. - -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 (Fig. 120). 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. - -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. - -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 the south-western centre -of eruption around Corston Hill a new volcanic group begins and soon -increases in bulk. - -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. - -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. - -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. - -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. - -[Illustration: Fig. 115.--Section of Craiglockhart Hill, Edinburgh. - -1. Red sandstones and clays; 2. Green stratified tuffs; 3. Columnar -basalt; 4. Dark shales, ironstones and sandstones, with plants.] - -[Illustration: Fig. 116.--Section of the bottom of the Midlothian -Plateau, Linnhouse Water above Mid-Calder Oilworks. - -1. Shales and cement-stones; 2. Sandstones; 3. Highly vesicular lava; -4. Tuffs and sandstone bands. _f_, Fault.] - -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 (Fig. 115), 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. 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. - -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 (Fig. 116). 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. - -[Illustration: Fig. 117.--Section of the top of the Midlothian Plateau -in the Murieston Water.] - -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 Fig. 117) 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. - -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. - -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 _Productus -giganteus_.[430] - -[Footnote 430: Explanation of Sheet 22 _Geol. Surv. Scotland_, p. 12.] - -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 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 (Fig. 118) -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. - - -[Illustration: Fig. 118.--Section of Calton Hill, Edinburgh. - - 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. -] - -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. - -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. - -As an illustration of the remarkable alternation of coarse and fine -materials, according to the varying intensity of the volcanic paroxysm, -Fig. 119 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. - -The space over which this pyroclastic material can now be traced, -large 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. - -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 -(Fig. 10). 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. - -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. - -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 _Rhizodus_ 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,[431] and of which the plants have been so fully -investigated by Professor Williamson.[432] - -[Footnote 431: _Trans. Geol. Soc. Glasgow_, vol. ii. (1867) p. 97.] - -[Footnote 432: _Phil. Trans._ 1871-1883.] - -[Illustration: Fig. 119.--Cliff of tuff and agglomerate, east side -of Oxroad Bay, a little east from Tantallon Castle, East Lothian.] - -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.[433] 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 _Sigillariæ_ and _Stigmariæ_ that -occur in the stratified intercalations between the lavas near Bowling. - -[Footnote 433: See in particular J. Young, _Trans. Geol. Soc. Glasgow_, -vol. iv. (1874) p. 123.] - -[Illustration: Fig. 120.--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). - - 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. -] - -_Upper Limits and Original Areas and Slopes of the Plateaux._--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. -Examples of this gradation may be seen in many natural sections along -the flanks of the Ayrshire plateau from above Kilbirnie to Strathavon. - -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 _coulées_ which flowed from the Solway vents did not quite join -with those from the Berwickshire volcanoes. - -[Illustration: Fig. 121.--Diagram illustrating the thinning away -southwards of the lavas of the Clyde Plateau between Largs and -Ardrossan. Length about 10 miles. - -1. Upper Old Red Sandstone; 2. Sandstones, shales, etc., with "Ballagan -Beds"; 3. Tuffs; 4. Andesite lavas; 5. Carboniferous Limestone series.] - -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 (Fig. 121). 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 (Fig. -122). - -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 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. - -[Illustration: Fig. 122.--Diagram illustrating the thinning away -eastwards of the lavas of the Clyde Plateau in the Fintry Hills. Length -about 12 miles. - - 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. -] - - -2. VENTS - -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. - -[Illustration: Fig. 123.--View of the two necks Dumgoyn and Dumfoyn, -Stirlingshire, taken from the south. - -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 Fig. 124.] - -[Illustration: Fig. 124.--Ground-plan of Plateau-vents near -Strathblane, Stirlingshire, on the scale of 6 inches to a mile.] - -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. 23 and 123. In rare 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 (Fig. 128). - -[Illustration: Fig. 125.--Ground-plans of double and triple necks in -the Plateau series, on the scale of 6 inches to a mile. - -A. Barwood Hill and Ravenscraig, east of Dumbarton, double vent. B. The -Knock Hill, Largs, Ayrshire, double vent (see Fig. 23). C. Dumbowie and -Dumbuck, east of Dumbarton, triple vent.] - -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 Fig. 124, 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 -view in Fig. 123.[434] 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 (Fig. 125, B) 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 (Fig. -125, A).[435] 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 C, Fig. 125, 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 (Fig. 22, No. 1). The most striking -example of these, however, occur in association with the puys and will -be described in later pages. - -[Footnote 434: The illustrations in Figs. 124 and 125 are taken from -the field-maps of the Geological Survey on the scale of 6 inches to a -mile. The ground represented in Fig. 124 was mapped by Mr. R. L. Jack.] - -[Footnote 435: 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.] - -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 (Fig. 129). 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.[436] - -[Footnote 436: 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.] - -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 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. 123, 124, 127). 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[437] (see Fig. 130). - -[Footnote 437: 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 p. 475.] - -[Illustration: Fig. 126.--Ground-plan of tuff-neck, shore east of -Dunbar. - -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," _Mem. Geol. -Survey_, p. 44.] - -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. - -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 Fig. 126, 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. - -The materials which have filled up the vents connected with the -plateau-eruptions generally consist of (_a_) agglomerates or tuffs, but -occasionally of (_b_) some kind of lava, and frequently (_c_) of both -these kinds of rock combined. - -(_a_) _Necks of Agglomerate or Tuff._--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 p. 382, 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 (p. 476). - -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 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. - -[Illustration: Fig. 127.--Section across the vents Dumgoyn and Dumfoyn, -and the edge of the Clyde plateau above Strathblane, Stirlingshire. - - 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. _f_, Fault. _D._ Late dolerite dyke. -] - -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. - -[Illustration: Fig. 128.--Section through the large vent of the Campsie -Hills. - -1. Andesite lavas; 2. Agglomerate and tuff; 3. Trachytic and andesitic -intrusive rocks.] - -[Illustration: Fig. 129.--Diagrammatic section across the central vent -of the Clyde plateau in Renfrewshire. - -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.] - -Illustrations of the varying structure of these vents are given in the -accompanying figures. In Fig. 127, 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. Fig. 128 gives a section taken -through the great vent of the Campsie Hills, with the minor adjacent -necks of Dungoil, Bin Bairn, and the Meikle Bin. - -The diagram in Fig. 129 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 Fig. 130. - -[Illustration: Fig. 130.--Section across Southern Berwickshire to show -the relation of the volcanic plateau to the vents lying south from it. - -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.] - -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 Fig. 23, and the ground-plan in Fig. -125, B. 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. - -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 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. - -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. - -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. - -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. - -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 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 -(Fig. 131). In general form this basalt resembles such intrusions as -that of Largo Law, to be afterwards described (Fig. 226), where what -may have been the hollow or bottom of the crater is filled with basalt. - -[Illustration: Fig. 131.--Section of south end of Dumbuck Hill. East of -Dumbarton.] - -[Illustration: Fig. 132.--Section across the East Lothian plateau to -show the relative position of one of the necks. - -1. Lower Carboniferous sandstones and shales; 2. Red and green tuffs -with a seam of limestone (_l_); 3. Band of basic sheets at the base of -the lavas; 4. Trachytes; 5. Phonolite neck.] - -(_b_) _Necks of Andesite, Trachyte, Dolerite, Diabase, or other massive -Rock._--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. 132, 133), North -Berwick Law (Fig. 109), and the Bass Rock (Fig. 110)--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.[438] - -[Footnote 438: See "Geology of East Lothian," _Geological Survey -Memoir_, p. 40.] - -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, Fig. 128). 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 -(Fig. 130). This more basic composition of itself suggests that -these bosses may be connected rather with the puy- than with the -plateau-eruptions. - -(_c_) _Necks of Composite Character._--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. 124, 125, 127 and 128). 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 (Fig. 130). - -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 Fig. 129). 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.[439] Another large -mass of tuff and agglomerate lies a little to the south-west of the -main area. - -[Footnote 439: 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, _Geological Survey of Scotland_.] - -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 (Fig. 128).[440] 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. - -[Footnote 440: See Explanation to Sheet 31, _Geological Survey of -Scotland_, par. 21 (1878).] - -[Illustration: Fig. 133.--View of Traprain Law from the south, a -phonolite neck of the Garleton Plateau.] - - -3. DYKES AND SILLS - -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. - -Dykes.--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. - -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. - -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 -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. - -"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. - -"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. - -"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 other dykes and sills on the mainland, which traverse -the Coal-measures, and would thus be not older than late Carboniferous -or Permian time. - -"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 -xxxiv. and xxxv. - -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. - -[Illustration: Fig. 134.--Veins and dykes traversing the agglomerate -and tuff of the great Renfrewshire vent.] - -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 -(Fig. 129), furnishes some admirable examples of this characteristic -volcanic feature. An illustration from that locality forms the subject -of Fig. 134. The agglomerate which fills the large hollow among the -Campsie Hills may be quoted as another illustration (Fig. 128). Further -instances will be found in some of the sections given in preceding -pages (see Figs. 124, 125, 127). The general aspect of a dyke in the -volcanic series is shown in Fig. 135. - -[Illustration: Fig. 135.--"The Yellow Man," a dyke in volcanic tuff -and conglomerate on the shore a little east of North Berwick.] - -The Sills 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 (Fig. 136), 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. - -[Illustration: Fig. 136.--Trachytic sills, Knockvadie, Kilpatrick -Hills. - -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).] - - -4. Close of the Plateau-eruptions - -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. - -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 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 (Fig. 137), 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. - -[Illustration: Fig. 137.--Section across the edge of the Clyde plateau, -south-east of Beith. - -1. Plateau-lavas; 2. Tuffs and volcanic conglomerates; 3. Hurlet -Limestone; 4. Coal-bearing strata above the limestone; 5. Dolerite -dyke.] - -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.[441] Further -west, in the Kilbirnie district, the limestone lies directly on the -tuffs that rest upon the andesites (Figs. 137, 138). - -[Footnote 441: Explanation of Sheet 22, _Geol. Surv. Scotland_, p. 12.] - -[Illustration: Fig. 138.--Section across the upper part of the Clyde -plateau at Kilbirnie, Ayrshire. - -1 1. Plateau-lavas; 2 2. Tuffs; 3 3. Hurlet Limestone; 4. Black-band -Ironstone. _f_ _f_. Faults.] - -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 (Fig. 139). 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 (Fig. 140). 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. - -[Illustration: Fig. 139.--Section across the upper surface of the Clyde -volcanic plateau, Burnhead, north-west of Kilsyth. - -1. Lavas of the plateau; 2. Tuffs; 3. Hurlet Limestone; 4. Hosie's -Limestone; _f_, Fault.] - -[Illustration: Fig. 140.--Section across the upper surface of the Clyde -volcanic plateau at Campsie. - -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; _f_. Fault.] - -[Illustration: Fig. 141.--Section across western edge of the Garleton -plateau. - -1. Trachyte lavas of the plateau; 2. Calciferous Sandstones; 3. Hurlet -Limestone.] - -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 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 (Fig. 141). 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. - -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 (Fig. 142). The equivalent of the Hurlet -Limestone of Central Scotland must lie many hundred feet above them. - -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. - -[Illustration: Fig. 142.--Section across the Solway plateau from -Birrenswark to Kirtlebridge. - -1. Upper Silurian strata; 2. Upper Old Red Sandstone; 3. Plateau-lavas; -4. Calciferous Sandstones and Carboniferous Limestone series; 5. -Trias.] - - - - -CHAPTER XXVI - -THE CARBONIFEROUS PUYS OF SCOTLAND - - 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. - - -i. GENERAL CHARACTER AND DISTRIBUTION - -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 _Puys_, 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. - -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 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. - -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. 137, 138, 139 and 140, 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. - -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 (Fig. 140) 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 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. - -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 (Map IV.). 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. - -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.[442] 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.[443] - -[Footnote 442: A means of definitely placing some of these vents in the -series of puy-eruptions is stated further on, at p. 476.] - -[Footnote 443: 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 p. 46. 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.] - - -ii. NATURE OF THE MATERIALS ERUPTED - - -A. The Lava-form Rocks - -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 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. - -Dolerites and Basalts.--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.[444] -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 (p. 418), 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." - -[Footnote 444: This classification was given in my Presidential Address -to the Geological Society, 1892, _Quart. Journ. Geol. Soc._ vol. -xlviii. p. 129. See Report of Geological Survey for 1896.] - -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_b_ and 2_c_; the latter -again shades into 2_d_ and 2_e_ by the decrease of the felspar. - -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 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." - - - I. The Olivine-bearing Series - - 1. _Olivine-Dolerites_ - - 1_a_. Porphyritic elements inconspicuous, olivine being } - the principal, and felspar of secondary importance; } Jedburgh Type. - groundmass sub-ophitic. } - - 1_b_. Strongly porphyritic; felspar-phenocrysts large; } Kilsyth Type. - olivine smaller; groundmass sub-ophitic. } - - 1_c_. Porphyritic olivine, but not felspar; groundmass } Gallaston Type. - sub-ophitic. } - - 2. _Olivine-Basalts_ - - 2_a_. Porphyritic olivine, augite and felspar; groundmass} Lion's Haunch - of felspar-laths imbedded in granules of augite. } Type. - } (See Fig. 207.) - - 2_b_. Porphyritic olivine and augite; groundmass of } - felspar-laths imbedded in granules of augite. More } Craiglockhart - rarely the groundmass is made of idiomorphic augite } Type. - imbedded in felspar-substance. } - - 2_c_. Porphyritic olivine abundant, augite much less } - common, and felspar very rare or absent; groundmass } Dalmeny Type. - with granular or idiomorphic augite (one of the most } - common types). } - - 2_d_. Porphyritic olivine more common than augite; } - groundmass of granules of augite set amongst lath-like } Picrite Type. - felspars which are much fewer in number than in 2_c_. } - - 2_e_. Porphyritic olivine more common than augite; } - groundmass of idiomorphic augite imbedded in } Limburgite - felspathic material which is not abundant. } Type. - - 2_f_. Porphyritic olivine and felspar, without augite; } Kippie Law Type. - groundmass of granular or idiomorphic augite, with } (For analysis - lath-shaped felspars. } see p. 379). - - II. The Non-olivine-bearing Series - - 3. _Olivine-free Dolerites_ - - Felspar, augite, magnetite in coarse-grained aggregate usually ophitic - or sub-ophitic; groundmass not plentiful. - - 3_a_. Groundmass absent Ophitic Type. - - 3_b_. Groundmass micropegmatitic Ratho Type. - - 3_c_. Groundmass an unstriated felspar (not orthoclase) } - and occasionally some interstitial altered glass or a } Burntisland - little quartz. } Sill Type. - - 4. _Doleritic Basalts_ - - Felspar, augite and magnetite in coarse-grained aggregate; groundmass - rather more plentiful and often in large patches. - - 4_a_. Felspar and augite, related sub-ophitically where } - together, but augite showing crystalline contours in } Bowden - contact with the groundmass; some interstitial quartz } Hill Type. - and unstriated felspar. } - - 5. _Basalts_ - - Finer-grained rocks, generally with a porphyritic ingredient and much - scattered interstitial matter in the groundmass. - - 5_a_. Porphyritic felspar, and occasionally a little } - augite; groundmass of granular augite, felspar needles } Binny Craig - and magnetite with some interstitial matter. } Type. - - 5_b_. Porphyritic felspars not conspicuous and small; the } - rock mainly made up of a mesh of fine felspar-laths set } Tholeiite - amongst granular augite, magnetite and base. } Type. - - 5_c_. Similar to the last but even finer-grained, and } Crypto- - with the base in a cryptocrystalline condition. } crystalline - } Type. - -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.[445] 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 p. 443, 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. - -[Footnote 445: _Trans. Roy. Soc. Edin._ vol. xxix. (1879) p. 506.] - -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. 153, 170, 171). These external differences are an -index to the corresponding variations in composition and microscopic -structure enumerated in the foregoing tabular arrangement. - -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. - -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.[446] 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. - -[Footnote 446: _Trans. Roy. Soc. Edin._ vol. xxix. (1879) p. 506. -Teall, _British Petrography_, p. 94.] - -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. - -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 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.[447] - -[Footnote 447: Dr. Stecher, _Tschermak's Mineralog. Mittheil._ vol. ix. -(1887) p. 193. _Proc. Roy. Soc. Edin._ vol. xv. (1888) p. 162.] - -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. - -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. - -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.[448] - -[Footnote 448: _Geol. Survey Memoir on Geology of Edinburgh_ (Sheet 32, -Scotland), p. 46.] - -2. Andesites.--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. - -3. Trachytes and Quartz-bearing Rocks.--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. -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. - - -B. Tuffs - -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. - -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. - -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. - -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. - -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 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.[449] 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. - -[Footnote 449: _Ante_, p. 36, and Figs. 15 and 151. See also _Geol. -Mag._ i. (1864), p. 22; _Trans. Roy. Soc. Edin._ vol. xxix. (1879) p. -515.] - -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.[450] 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. - -[Footnote 450: Memoir on Sheet 32 _Geol. Surv. Scotland_ (1861), p. -42. The stratigraphical position of these "Houston Marls," as they are -locally called, is indicated in Fig. 155.] - -Palagonitic-tuff is of frequent occurrence. It is met with in the -Firth of Forth district,[451] and Mr. Watts has detected fragments of -palagonite among the tuffs of the Limerick basin. - -[Footnote 451: _Trans. Roy. Soc. Edin._ vol. xxix. (1819) p. 515.] - - - - -CHAPTER XXVII - -GEOLOGICAL STRUCTURE OF THE CARBONIFEROUS PUYS OF SCOTLAND - - 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. - - -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. - -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. - - -i. VENTS - -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 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. - -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.[452] - -[Footnote 452: 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 (Fig. 155); 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.] - -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 Fig. 22). 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.[453] - -[Footnote 453: These facts were ascertained by Mr. Peach in mapping the -ground for the Geological Survey. See Sheet 17, Scotland.] - -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. - -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. - -1. _Necks of Non-volcanic Debris._--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.[454] - -[Footnote 454: Prof. Jas. Geikie, _Mem. Geol. Surv. Scotland_, -Explanation of Sheet 23, p. 39.] - -[Illustration: Fig. 143.--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. - -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.] - -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 (Fig. 143). 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 of -abundant vertical rents, which have been filled partly with the same -material that forms the nodules, and partly with sandstone. - -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 _maar_ 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.[455] - -[Footnote 455: The vent is shown in Sheet 39, _Geol. Surv. Scotland_.] - -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. - -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. - -2. _Necks of Tuff and Agglomerate._--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 - -[Illustration: Fig. 144.--View of the Binn of Burntisland--a volcanic -neck of agglomerate. (This illustration and Figs. 145, 152, 153, -164, 166 and 168 are from photographs taken by Mr. Robert Lunn -for the Geological Survey.)] 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 (Fig. 145). - -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. - - Diabase and basalt 57 per cent. - Older tuff 3 " - Andesite (probably from Old Red Sandstone - volcanic series of the neighbourhood) 14 " - Limestone (cement-stone, etc.) 20 " - Shale 3 " - Sandstone 2 " - Fossil wood 1 " - ---- - 100 - -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. 166, 168) 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. - -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 Fig. 144, 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 -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 -Fig. 145, which represents a portion of the south front of the hill. -Sections of this neck are given in Figs. 149 and 159. - -(3) _Necks of Tuff or Agglomerate with a Central Plug of Basalt or -other Lava._--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. 26, 148, 149 and 174). - -(4) _Necks of Basalt, Dolerite, etc._--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. 195, -196). 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. - -_Relation of the Necks to the Rocks through which they rise._--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 - -[Illustration: Fig. 145.--View of part of the cliffs of vertical -agglomerate, Binn of Burntisland.] 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. 24, 143, 147, 148 and 149) -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. 126 and 210), -and serves to mark the position of the necks from a distance. - -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.[456] - -[Footnote 456: _Quart. Journ. Geol. Soc._ 1860, vol. xvi. p. 245.] - -_Evidence for the probable subærial Character of some of the Cones or -Puys of Tuff._--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 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 (p. 426). - -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.[457] 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 _Stigmaria_, -_Lepidodendron_, 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. - -[Footnote 457: 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.] - -_Entombment of the Volcanic Cones and their relation to the bedded -Lavas and Tuffs._--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 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[458] (Fig. 146). - -[Footnote 458: Explanation of Sheet 22, _Geol. Surv. of Scotland_, p. -16.] - -[Illustration: - - Fig. 146.--Diagram of buried volcanic cone near Dalry, Ayrshire. - Constructed from information obtained in mining operations. - - 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. -] - -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 (Fig. 147) may serve to illustrate this process -of gradual re-emergence. - -[Illustration: Fig. 147.--Diagram to illustrate how Volcanic Necks may -be concealed and exposed. - -1, Neck, still buried under the succeeding sedimentary accumulations; -2, Neck uncovered and denuded.] - -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 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 Fig. 148. 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. - -[Illustration: Fig. 148.--Section across the Saline Hills, Fife. - - 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; _t_, 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. -] - -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 Fig. 149. 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 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. - -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. - -[Illustration: Fig. 149.--Section across the Binn of Burntisland, in an -East and West direction. - -1, Sandstones; 2, Limestone (Burdiehouse); 3, Shales, etc.; _b_, _b_, -Interstratified basalts; _t_ _t_, Bedded tuff, etc.; T, Tuff of the -great neck of Burntisland; B, Basalt veins.] - - -ii. BEDDED TUFFS AND LAVAS - -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. - -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. - -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 (p. 423), which remind -one 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. - -[Illustration: - - Fig. 150.--Section in old quarry, west of Wester Ochiltree, - Linlithgowshire. Calciferous Sandstone series. -] - -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 Fig. 150, 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.[459] - -[Footnote 459: See _Geol. Surv. Memoir of Edinburgh_, p. 45. These -tuffs are further described on pp. 465 _et seq._] - -The second illustration may be taken from the admirable coast-section -between Burntisland and Kinghorn, where the number of intercalations -of 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:-- - - Tuff 1·5 inch. - Limestone 0·2 " - Tuff 0·5 " - Shale 0·2 " - Tuff 0·1 " - Shale and tuff 1·0 " - Shale 0·2 " - Limestone 0·5 " - Shale full of volcanic dust 3·5 " - Shaly limestone 1·5 " - Laminated tufaceous limestone 2·0 " - Limestone in thin bands, with thin laminæ of tuff 0·8 " - Granular tuff 0·6 " - Argillaceous limestone, with diffused tuff 0·9 " - Fine granular tuff 0·7 " - Argillaceous limestone, with diffused tuff 1·5 " - Laminated limestone 0·1 " - Limestone, with parting of granular tuff in middle 0·9 " - Tufaceous shale 2·0 " - Limestone 0·4 " - Shaly tuff 1·25 " - Laminated limestone 0·1 " - Tuff 1·2 " - ----- - 21·65 inches. - -[Illustration: Fig. 151.--Ejected volcanic block in Carboniferous -strata, Burntisland. - - 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. -] - -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 (Fig. 151).[460] - -[Footnote 460: _Geol. Mag._ vol. i. p. 22. This Fife coast-section is -given in full at p. 470.] - -[Illustration: Fig. 152.--View of volcanic agglomerate becoming finer -above. East end of Kingswood Craig, two miles east from Burntisland.] - -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 Fig. 152, 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. - -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. - -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. - -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. - -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 - -[Illustration: Fig. 153.--Alternations of basalt and tuff with shale, -etc., Kingswood Craig, Burntisland.] 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 (Fig. 171). 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 _coulées_ is -sometimes remarkably striking. The full details of this most -interesting section will be given in later pages (p. 470). But some of -its more characteristic external features may be understood from the -views which are presented in Figs. 152, 153, 170, 171. - -The general bedded character of the volcanic series is well shown in -Fig. 153, 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 Fig. 170, 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 Fig. 171, which shows -further how the columns sometimes merge into an amorphous part of the -same sheet. - -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.[461] 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 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 Fig. -154. Immediately above the diabase, which is highly cellular, lies a -green felspathic sandstone or shale containing detached fragments of -the amygdaloid together with _Lingulæ_ and other shells. There seems no -reason to doubt that this is a true interstratified lava.[462] - -[Footnote 461: _Trans. Roy. Soc. Edin._ xxix. (1879) p. 504.] - -[Footnote 462: _Trans. Roy. Soc. Edin._ xxix. (1879), pp. 505-507.] - -[Illustration: Fig. 154.--Section of the upper surface of a diabase -("leckstone") sheet, Skolie Burn, south-east of Bathgate. - - 1. Slaggy diabase; 2. Green sandy shale and shaly sandstone - containing _Lingulæ_, 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 - _Spiriferæ_, etc.; 5. Bed of blue crinoidal limestone; 6. Clays - and thin coal; 7. Black and blue calcareous shales and thin - limestones. -] - -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. - -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 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. - -[Illustration: Fig. 155.--Section across the volcanic ridge of the -Linlithgow and Bathgate Hills, showing the intercalation of limestones -that mark important stratigraphical horizons. - - 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). -] - -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 (Fig. 155). 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. - -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 (Fig. 156). 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 -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 _Lithostrotion -irregulare_. 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 -_Calamites_, _Producti_, 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.[463] - -[Footnote 463: _Geol. Surv. Mem._ "Geology of Edinburgh," p. 58.] - -[Illustration: Fig. 156.--Section in Wardlaw Quarry, Linlithgowshire.] - -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 (p. -390), 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.[464] 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.[465] - -[Footnote 464: _Ibid._ p. 49, _et seq._] - -[Footnote 465: _Proc. Roy. Soc. Edin._ 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.] - -_Subsequent Dislocation of Bedded Lavas and Tuffs._--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 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 Fig. 157, -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. - -[Illustration: Fig. 157.--Section from Linlithgow Loch to the Firth of -Forth.] - - -iii. SILLS, BOSSES AND DYKES - -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. - -Sills.--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. - -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. - -[Illustration: Fig. 158.--Section across the Campsie Fells illustrating -the contrast between the sills below and above the plateau-lavas. - -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. _f_, Fault.] - -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 (Fig. 158). 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. - -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. - -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 (Fig. 159). - -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 (Fig. 158) may quite possibly be connected with -a group of vents lying 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. - -[Illustration: Fig. 159.--Section showing the position of the basic -sills in relation to the volcanic series at Burntisland, Fife. - - 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. -] - -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. - -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 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"). - -[Illustration: Fig. 160.--Sills between shales and sandstones, Hound -Point, Linlithgowshire.] - -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 (Fig. -160), 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.[466] 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. - -[Footnote 466: See Hay Cunningham's "Essay," p. 66, and plate ix.; and -_Geol. Survey Memoir_ on "Geology of Edinburgh," p. 114.] - -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.[467] 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.[468] - -[Footnote 467: _British Petrography_, p. 208.] - -[Footnote 468: Mr. H. W. Monckton. _Quart. Journal Geol. Soc._ vol. li. -(1895), p. 482.] - -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[469] (Fig. 161). The intrusive nature of the -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. - -[Footnote 469: This rock has been described by Mr. J. Henderson and -Mr. Goodchild, _Trans. Geol. Soc. Edin._ vi. (1893) pp. 297, 301, and -by Mr. H. W. Monckton, _Quart. Journ. Geol. Soc._ 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.] - -[Illustration: Fig. 161.--Section of Sill, Cramomd Railway, Barnton, -near Edinburgh. - - 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. -] - -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. - -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. - -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 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. - -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.[470] -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. - -[Footnote 470: See, for instance, Maclaren's _Geology of Fife and -the Lothians_, 1839; Hay Cunningham's _Essay_, previously cited; -_Geological Survey Memoir on the Geology of Edinburgh_ (Sheet 32), -1861; Mr. Allport, _Quart. Journ. Geol. Soc._ vol. xxx. (1874) p. 553; -Teall, _British Petrography_, p. 187; E. Stecher, _Contacterscheinungen -an schottischen Olivindiabasen_, Tschermak's _Mineralog. Mittheil._ -vol. ix. (1887) p. 145; _Proc. Roy. Soc. Edin._ vol. xv. (1888) p. 160.] - -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. - -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 (Fig. 162) are paralleled by -scores of other instances in different parts of the same region. - -Moreover, sills do not always remain on the same horizon; that is, -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 (Fig. 163), this rock breaks through the limestone -and then spreads out among the overlying shales, across which it passes -obliquely. - -[Illustration: Fig. 162.--Intrusive dolerite sheet enclosing and sending -threads into portions of shale, Salisbury Crags, Edinburgh.] - -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.[471] - -[Footnote 471: Explanation of Sheet 31, _Geological Survey of -Scotland_, §§ 43 and 83.] - -[Illustration: Fig. 163.--Intrusive sheet invading limestone and shale, -Dodhead Quarry, near Burntisland.] - -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 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. - -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,[472] 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. - -[Footnote 472: _Mikroskopische Beschaffenheit der Mineralien und -Gesteine_, p. 273; Vogelsang's _Krystalliten_.] - -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 (Fig. 160). - -Another instructive example is furnished by the small threads which -proceed from the dolerite of Salisbury Crags, and traverse enclosed -fragments of shale (Fig. 162). 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 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.[473] 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. - -[Footnote 473: 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.] - -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. - -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.[474] - -[Footnote 474: 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 p. 451.] - -[Illustration: Fig. 164.--Spheroidal weathering of dolerite sill, -quarry east of North Queensferry, Fife.] - -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. - -[Illustration: Fig. 165.--Two thin sills of "White Trap" injected into -black carbonaceous shale overlying the Hurlet Limestone, Hillhouse -Quarry, Linlithgow. - - 1. Hurlet Limestone; 2. Black shales; 3 3. Two sills of "White - Trap"; 4. Columnar Basalts. -] - -As a good illustration of the behaviour of such thin sills among -carbonaceous shales I give here a section (Fig. 165) 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. - -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. - -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 (Fig. 164). - -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 - -[Illustration: Fig. 166.--Dyke cutting the agglomerate of a neck. Binn -of Burntisland.] 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. - -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. - -[Illustration: Fig. 167.--Boss of diabase cutting the Burdiehouse -Limestone and sending sills and veins into the overlying shales. -Railway cutting, West Quarry, East Calder, Midlothian. - -1. Burdiehouse Limestone; 2. Shales; 3. Diabase.] - - * * * * * - -Bosses.--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 Fig. 157). 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 - -[Illustration: Fig. 168.--Side of columnar basalt-dyke in the same -agglomerate as in Fig. 166.] Carboniferous formations. The area -between Edinburgh and Linlithgow and the hills of the north of Fife -furnish many examples. - -The connection between bosses and intrusive sheets is instructively -exhibited in a railway cutting to the west of Edinburgh, where the -section shown in Fig. 167 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. - - * * * * * - -[Illustration: Fig. 169.--Dyke rising through the Hurlet Limestone and -its overlying shales. Silvermine Quarry, Linlithgowshire.] - -Dykes 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. 149 and 159). Two additional illustrations from this -locality are here given. In Fig. 166 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 Fig. 168. - -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 Fig. 169 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 (Fig. 155). - -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.[475] 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. - -[Footnote 475: These are fully described in Chapters xxxiv. and xxxv.] - - - - -CHAPTER XXVIII - -ILLUSTRATIVE EXAMPLES OF THE CARBONIFEROUS PUYS OF SCOTLAND - - The Basin of the Firth of Forth--North Ayrshire--Liddesdale. - - -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. - - -1. BASIN OF THE FIRTH OF FORTH - -Reference has already been made to the remarkable peculiarity in the -development of the lower part of the Carboniferous system in this -district.[476] 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 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. - -[Footnote 476: See Maclaren's "Geology of Fife and the Lothians," the -_Memoirs of the Geological Survey of Scotland_, on Sheets 31 and 32, -and my Memoir, already cited, _Trans. Roy. Soc. Edin._ vol. xxix. -(1879) p. 437.] - -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. - -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 _Orthoceras_, _Bellerophon_ and _Discina_, which -suffice to prove the strata to be stratigraphical equivalents of the -Lower Limestone shale, and part of the Carboniferous Limestone of -England (Fig. 170). 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. - -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 (Map IV.). 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 - -[Illustration: Fig. 170.--Junction of amygdaloidal basalt with shales -and limestone, Shore, half a mile east from Kinghorn, Fife. (From a -photograph by Mr. R. Lunn.)] 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. - -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. - -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. - -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. - -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. -148, 149, 159, 166, 168). - -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. - -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. 437, 438. 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. - -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. - -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 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. - -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. - -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. - -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 (Fig. 155). 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. - -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. 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. - -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. - -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. - -[Illustration: Fig. 171.--Columnar basalt, Pettycur, Kinghorn, Fife. -(From a photograph taken for the Geological Survey by Mr. R. Lunn.)] - - -Section of the Volcanic Series below the Hurlet or Main Limestone on -the Coast of Fife, west of Kinghorn, in descending order[477] - -[Footnote 477: The succession of rocks in this interesting -coast-section was briefly given by Dr. P. Neill in his translation of -Daubuisson's _Basalts of Saxony_, Edinburgh, 1814, note _f_, 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.] - - 75. Reddish and white sandstones. - - 74. Shale with hard ribs of limestone and ironstone nodules. - Fossils abundant. - - 73. Limestone, crinoidal, 8 or 9 feet. - - 72. Blue shale, becoming calcareous towards the top, where shells - are plentiful. - - 71. Reddish false-bedded sandstones, with bands of reddish and blue - shale. - - 70. Basalt in two sills separated by 2 or 3 feet of sandstone and - shale. - - 69. Dark fissile sandy shale, passing up into white shaly - sandstone, and including a thin parting of impure coal. - - 68. Limestone (Hurlet or Main Seam) in a number of bands having a - united thickness of 25 feet. Abundant fossils. - - 67. Black shale becoming calcareous at top, and then enclosing - abundant _Productus_, etc., 8 or 10 feet. - - 66. Red and green tufaceous marl and tuff. About 30 feet. - - 65. Basalt, the lower part strongly amygdaloidal. - - 64. Tufaceous red marl and tuff; comparatively coarse below, - becoming finer above, 3 or 4 feet. - - 63. Basalt, earthy and amygdaloidal, with an irregular bottom - involving masses of the shales below. - - 62. Dark calcareous shale and dull green tufaceous marly shale, 2 - or 3 feet. - - 61. Crinoidal limestone in several bands with a united thickness of - 10 feet. - - 60. Shale, 1 foot. - - 59. Fine green sandy tuffs in a number of bands of varying - coarseness, about 6 feet. - - 58. Dark shale with abundance of _Aviculopecten_ immediately under - the tuffs above, 1½ feet. - - 57. Soft, light, marly shale with fragmentary plants, 1½ feet. - - 56. Dark fissile shale, full of fish-scales, plants, etc., 3 feet. - - 55. Basalt, rudely columnar, dark fine-grained in centre, becoming - highly amygdaloidal and scoriaceous at bottom and top. - - 54. Basalt, like the sheet above, vesicular at top and bottom, with - a parting of red clay on top. - - 53. Fissile rippled sandy shale, with plants, having a red and - green marly parting at the top, 12 or 14 feet. - - 52. Basalts; a group of beds, probably in part sills, involving - three bands of sandstone or quartzite. - - 51. Quartzite--a hard white altered sandstone, 2 to 3 feet. - - 50. Basalt, light green, earthy, amygdaloidal. - - 49. Sandstones and shales with plants, 25 feet. - - 48. Basalt, with a highly amygdaloidal central band. There may be - several sheets here. - - 47. Green tufaceous shale and marl, 1 foot. - - 46. Basalt, dark, firm and amygdaloidal. - - 45. Sandstones and shales with plants. - - 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. - - 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. - - 42. Basalt, firm, compact and highly amygdaloidal throughout, 15 - feet. - - 41. Basalt, earthy, amygdaloidal and scoriaceous in the upper part, - compact below. - - 40. Red tufaceous marl, clay or bole, a few inches thick. - - 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. - - 38. Basalt more or less scoriaceous throughout, especially at the - bottom, the vesicles being drawn out round the slag-like blocks. - - 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. - - 36. Basalt, with an upper, earthy and highly amygdaloidal portion, - 30 feet. - - 35. Tufaceous sandstone and shale, 6 to 8 feet. - - 34. Basalt, in a thick bed, having an earthy, slaggy top and a - scoriaceous bottom. - - 33. Basalt, very slaggy below with a compact centre. - - 32. Basalt, like that below it. - - 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. - - 30. Green shale like that below the underlying limestone, a few - inches in thickness. - - 29. Coarse, green, sandy tufaceous limestone, averaging 1 foot in - thickness. - - 28. Black shale with plants, 12 or 14 feet, becoming green and - tufaceous at the top. - - 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 Fig. 171.) - - 26. Black shale with fragmentary plants, 3 feet. - - 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. - - 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. - - 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. - - 22. Green tuff, 12 feet, well stratified and fine-grained, with - minute lapilli of highly vesicular basic lavas; becomes shaly at - the bottom. - - 21. Basalt, compact, amygdaloidal, with highly vesicular upper - surface, 20 feet. - - 20. Basalt, hard, black and full of olivine; an irregular bed 3 to - 6 feet thick. - - 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. - - 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. - - 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 p. 438, - as an illustration of the feeble intermittent character of many - of the volcanic explosions between successive outflowings of - lava. - - Owing 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 - (Fig. 153). 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 (Fig. 152). - - 16. Black and grey shales forming a thin band at the summit of King - Alexander's Crag. - - 15. Basalt, dark compact rock, with an upper and lower highly - scoriaceous and amygdaloidal band, 15 feet. - - 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 Fig. 151.) - - 13. Basalt, dull, earthy and highly amygdaloidal, with abundant - calcite in kernels and veins; about 15 feet, but varying in - thickness. - - 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. - - 11. Black shale, varying up to 6 inches, but sometimes entirely - removed by the overlying lava-stream. - - 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. - - 9. Basalt, hard black, with abundant olivine, and a columnar - structure. - - 8. Green shale, 6 inches to 1 foot, much baked and involved in the - overlying basalt. - - 7. Basalt, dull-green, earthy, amygdaloidal, varying from 10 to 40 - feet in thickness. - - 6. Blue shale, disappearing where the basalt above it unites with - that below. - - 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. - - 4. Sandstones forming a thick group of beds which have long been - quarried for building-stone at the Grange and elsewhere. - - 3. Black shales. - - 2. Limestone (Burdiehouse). - - 1. Sandstones, shales and thin limestones forming the strata at - Burntisland through which the sills of that district have been - injected (Fig. 159). - -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, and apart from the geological horizons on -which they lie, they form a wide ring surrounding the Falkirk and -Stirlingshire coal-field. - -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. - -[Illustration: Fig. 172.--Section across the Fife band of Sills. - -1. Upper Old Red Sandstone; 2. Calciferous Sandstones; 3. Carboniferous -Limestone series; 4. Millstone Grit; 5. Coal-measures; 6. Dolerite -Sills. _f_, Fault.] - -But it is in Fife that the sills reach their greatest development among -the Carboniferous rocks of Scotland (Fig. 172). 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 (Fig. 159). - -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. - -Another question of importance in dealing with the history of these -sills is their stratigraphical position. By far the larger proportion -of them lies 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. - -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 Chapter xxxi. - -[Illustration: Fig. 173.--Section across the Upper Volcanic Band of -north Ayrshire. Length about four miles. - - 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. -] - - -2. NORTH OF AYRSHIRE - -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 (Map IV.). 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 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. - -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 (p. -434). 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.[478] - -[Footnote 478: See Explanation of Sheet 22, _Geol. Surv. of Scotland_, -pars. 29, 33, 45.] - - -3. LIDDESDALE - -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 (Map IV.). - -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 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.[479] 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 (Fig. 175). - -[Footnote 479: They have been recognized and mapped by Mr. B. N. -Peach for the Geological Survey. See Sheets 11 and 17, _Geol. Surv. -Scotland_.] - -[Illustration: Fig. 174.--Section showing the connection of the two -volcanic bands in Liddesdale. - - 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. -] - -[Illustration: Fig. 175.--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. - -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).] - -The materials ejected from the Liddesdale vents include both basaltic -lavas and tuffs. The former are sometimes highly vesicular, especially -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 -(Fig. 174). 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. - - -END OF VOL. I. - - -_Printed by_ R. & R. Clark, Limited, _Edinburgh_. - -[Illustration: - - TO ACCOMPANY SIR ARCHIBALD GEIKIE'S "ANCIENT VOLCANOES OF BRITAIN" - - Map IV. - -MAP OF THE CARBONIFEROUS VOLCANOES OF SCOTLAND - -English Miles - -EXPLANATION OF COLOURING - - _Basalts and thin Tuffs_ } _Puy Series_ - _Thicker Sheets of Tuff_ } - - _Lavas and thin Tuffs_ } _Plateau Series_ - _Thicker Sheets of Tuff_ } - - _Vents filled with Agglomerate - or Tuff_ - - _Basic Sills and Bosses_ - - _Intermediate and acid Sills - and Bosses_ - - The Edinburgh Geographical Institute - -Copyright - - J. G. Bartholomew. -] - -[Illustration: MAP 1 - - MAP OF THE - - VOLCANIC DISTRICTS - - OF THE - - BRITISH ISLES - - BY - - Sir ARCHIBALD GEIKIE, D.C.L., F.R.S.] - - -BY THE SAME AUTHOR. - - TEXT-BOOK OF GEOLOGY. With Illustrations. Third Edition. Revised - and Enlarged. Medium 8vo. 28s. - - CLASS-BOOK OF GEOLOGY. Illustrated with woodcuts. Second Edition. - Crown 8vo. 4s. 6d. - - ELEMENTARY LESSONS IN PHYSICAL GEOGRAPHY. 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