Chapter I: 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