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ANCIENT VOLCANOES
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* D.C.L. OxF., D.Sc. Camb., Dubi>. ; LL.D. St. And., Edinb.
DIRKCTOR-OENERAL OP THE GEOUKilOAl. RURVEY OF GREAT BRITAIN AND IRELAND; (CORRESPONDENT OP THE INSTITUTE OP PRANCE ;
op THE ACADEMIES OF BERLIN, VIENNA, MirSICH, TURIN, BlSLCilUM, STOCKHOLM, OOITINOEN, NEW YORK ; OF THE IMPERIAL MlNERALOniCAL SOCIE'lN AND SOCIETY OF NATURALISTS, KT. I'ETERHBURC *, NATURAL THSTORV SOCIETY, MOSCOW ; SCIENTIFIC SOCIETY, CHRISTIANIA; AMERICAN PHILOSOPHICAI. SOCIETY ; OF I'HF, OEOLOCICAL SOCIETIES OF LONDON, FRANCE, BEI/i ! CM, STOCKHOLM, ETC.
WITH SEVEN MAPS AND NUMEKOUS ILLUSTRATIONS
IN TAVO VOLUMES
VOL. I
Hanfton
MACMILLAN AND CO., Limited.
NEW YORK : THE MACMILLAN COAIFANY.
1897
All rights reserved
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MEMBER OF 'I'llE* INSTITUTE DIRECTOR OF THE OEOLOOTCAL SURVEY OF FRANCE IHSTINOITISHED REPRESENTATIVES OF THAT FREN(m S(HlOOL OF OEOLOGY WITTOH BY THE HANDS OF DESMAREST FOUNDED THE STUDY OF ANCIENT VOLCANOES AND HAS SrN(0'’i DONE SO MUCH TO PRC»MOl'E ITS PROISRESS THESE VOLUMES ARE INSCRIBED WITH THE HIGHEST ADMIRATION AND
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A
PREFACE
In 110 depiirtiiieiit of science is the slow and chequered progress of investiga- tion more conspicuous than in that branch of Geology which treats of volcanoes. Although from tlie 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. Ihirning 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 con- nection 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 liecn vigorous and prolonged in many countries, where no remains of volcanoes can now be seen.
I’o France, which has led the way in so many ilepartments of huinaii 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 leserved tor 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 carctirl 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
THE ANCIENT VOLCANOES OF GREA T BRITAIN
viii
of Central France were not all of one age, but had made their appearance in a long series, whereof the individual ineinhers 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, Desuiarest traced the volcanic series backward in time, through successive stages of the decay and degradation wrought upon them by the iiiilueuee of the atmosphere, rain and running water. He was thus able, as it were, to watch the gradual obliteration of the cones, tlie removal of the ashes and scoripe, and the erosion of the lava-streams, until lie 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 coidd 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 subairial erosion in the excavation of valleys and tlie trans- formation 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 liiiewise 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 observa- tions, eonfiriued 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 tlieir 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
PREFACE
IX
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 G-erman. 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 irrecon- cilable with the tenets enunciated from the Freiberg Chair, were now either Ignored or contemptuously rejected. AVerner’s disciples loved to call themselves Fy their teacher’s term “geognosts,” and claimed that they confined them- selves 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.
AVith 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 l^eptunists, while their opponents, equally stubborn in defence of the potency of volcanic fire, were known as Aiulcanists 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 AYeruerians.
VOL. I j
X
THE ANCIENT VOLCANOES OF GREAT BRITAIN
Cuvier tells us that wheu 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.
Daiibuisson, a favourite pupil of the Freiberg professor, had written and published at Paris in 180:1 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 raptiu’es 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 tliat while we may infer what takes place at Vesuvius, we can actually see what has transpired at the Buy de Pariou. With the enthusiasm of a convert he rushed into the discussion of the phenomena, but somehow omitted to make any mention of Des- marest, who had taught the truth so many years before.
Impressed by the example of such men as Daubuisson and Yon 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 Ediubiu'gh the most implicit acceptance of the tenets of the Saxon school, and continued to maintain the aqueous origin of basalt for many years
PREFACE
XI
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 ot 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. T)e la Beche, Murchison and Sedgwick led the way in recognizing, even among the most ancient stratified forma- tions 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. Eamsay, Selwyn, and Jukes, following these pioneers, were the first to map out a Palajozoic volcanic region in ample detail. Sorby, apply- ing to the study of rocks the method of microscopic examination by thin slices, devised by William Nicol of Edinbirngh 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 vol- canisin. 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.
01 n 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 plienomena.
Fioni time to time during a period of almost forty years I liave com- municated chiefly to the Geological Society of London and the Eoyal Society of Ediubiu'gh the residts of my researches. As materials accumulated, the esire 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 esign into execution occurred when, as President of the Geological Society, le duty devolved upon me of giving the Annual Addresses in 1891 and 89..J. Within the limits permissible to such essays, it was not possible to Piesent more than a full summary of the subject. Since that time I have ontinued my researches in the field, especially among the Tertiary vol-
THE ANCIENT VOLCANOES OF GREA T BRITAIN
xii
cailic 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 know- ledge 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 Eoyal Society of Edinburgh and the G-eological Society for their permission to use a number of the illustra- tions which have accompanied my papers published in their Trmisactions and Joimial. 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 informa- tion 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. Eobert Lunn for the Geological Survey among the volcanic districts of Central Scotland.
Geologioai, Survey Office,
28 Jeemyx Stu,ef.t, London, Jamuiry 1897.
CONTENTS
BOOK I
GENEEAL PEINCIPLES AKD METHODS OF INYESTIGATIOK
CHAPTEE 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 . . . .
CHAPTEE H
The Nature
and Causes of Volcanic Action — Modern Volcanoes
10
CHAPTEE HI
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
CHAPTEE IV
Materials erupted at the Surface — Extrusive Series — continu-ed. iii. Types of old Volcanoes — 1. The Vesuvian Type ; 2. The Plateau or Fi.ssure Type; 3. The Puy Tjqie. iv. Determination of the relative Geological Dates of Ancient Volcanoes, v. How the Physical Geography associated with Ancient Volcanoes is ascertained . . 39
CHAPTEE 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. Metainorphism in and around Volcanic Cones, Sol-
XIV
THE ANCIENT VOLCANOES OF GREAT BRITAIN
fataric Action; v. Inward Dip of Rocks towards Necks; vi. Influence of contem- poraneous Denudation upon Volcanic Cones ; vii. Stages in the History of old Volcanic Vents
CHAPTER VI
Under^onnd Phases of Volcanic R<,tiou-conUmied. 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 Ten-estrial Crust
CHAPTER VII
Influence of Volcanic Rooks on the Scenery of the Laud— Effects of Denudation
BOOK II
VOLCANIC ACTION IN PRE-CAMBRIAN TIME
CHAPTER VHI
Pbe-Oaiibrian Volcanoes
The Beginnings of Geological History — Diiflciilties in fixing on a generally applicable Terminology— i. The Lewisian (Archaean) 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
BOOK III
THE CAMBRIAN VOLCANOES CHAPTER IX
Characteristics of the Cambrian System in Britain
The Physical Geography of the Cambrian Period— The Pioneers of Palaiozoic Geology in Britain— Work of the Geological Survey in Wales— Subdivisions of the Cambrian System in Britain ......
CHAPTER X
The C.ambrian Volcanoes of South Wales ....
CHAPTER XI
The Cambrian Volcanoes of North Wales, the Malvern Hills Warwickshire
AND
. 159
CONTENTS
XV
BOOK IV
THE SILUEIAK VOLCANOES CHAPTER XH
Characters of the Silurian System in Britain. The Arenig Volcanoes
PAUE
The Land and Sea of Silurian time — Classification of the Silurian System — General Petro-
graiihy of the Silurian Volcanic Rooks — I. The Eruptions of Arenig Age . . 173
CHAPTER XIH
The Eruptions of Llandeilo and Bala Age
1- 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. Uj)per Silurian (?) Volcanoes of Gloucestershire . . . 202
CHAPTER XIV
The Silurian Volcanoes op 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
XVI
THE ANCIENT VOLCANOES OF GREAT BRITAIN
CHAPTEE XVII
PAGE
Distribution op the Volcanic Centres in the Lower Old Red Sandstone
— Characters op the Materials Erupted bv the Volcanoes . .271
CHAPTER XVIII
Structure and Arrangement op the Lower Old Red Sandstone Volcanic
Rocks in the Field . . . , . .281
CHAPTER XIX
Volcanoes op the Lower Old Red Sandstone op “Lake Caledonia”
De.soription 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 Centi'c ......
CHAPTER XX
Volcanoes op the Lower Old Red Sandstone op “Lake Caledonia” — continued
The Southern Chain— Tlie Pentland Volcano— The Biggar Centre— The Duneaton Centre
— The Ayrshire Volcanoes 317
CHAPTER XXI
Volcanoes op the Lower Old Red Sandstone op the Cheviot Hills, Lorne, “Lake Orcadie” and Killarney .....
336
CHAPTER XXH
Volcanoes op the Upper Old Red S.andstone THE North op Scotland
— The South-IVest op Irel.and,
. 348
BOOK VI
THE CARBONIFEROUS VOLCANOES CHAPTER XXIH
The Carboniferous Sy'stem op Britain and its Volcanic Records
Geography and Scenery of the Carboniferous Period — Range of V’olcanic Eruptions during that time — I. The Carboniferous Volcanoes of Scotland — Di.stribution, Arrangement and Local Characters of the Carboniferous Sy.steni in Scotland — Sketch of the Work of previous Observers in this Subject ....... 355
CONTENTS
xvu
CHAPTER XXIV
Caeboniferous Volcanic Plateaux of Scotland
PACE
I. The Plateau-ty]ie 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 op Scotland
!• 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
CHAPTER XXVI
The Carboniferous Puys op 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 .
CHAPTER XXVH
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 Subaerial Character of some of the Cones or Puys of Tuff— Entombment of the Volcanic Cones and their Relation to the Superficial Ejections. 2. Bedded Tufts and Lavas — Effects of Subsequent Dislocations. 3. Sills, Bosses, and Dykes
CHAPTER XXVITI
Illustrative Examples of the Carboniferous Puys of Scotland The Basin of the Firth of Forth — North Ayrshire — Liddesdale . . . •
LIST OF ILLUSTRATIONS
fig.
1. Vesicular structure, Lava from Ascension Island, slightly less than natural size
2. Elongation and branching of steam-vesicles in a lava, Kilninian, Isle of Mull, a little less
than natural size
3. Microlites of the Pitchstoiie of Arran (magnified 70 diameters) . . . •
4. PeiTitic structure in Felsitic Glass, Isle of Mull (magnified) . . . •
0. Spherulitic structure (magnified)
0. Micropegmatitic or Granophyric structure in Granophyre, Mull (magnified)
7. Ophitic structure in Dolerite, Gortaeloghan, Co. Derry (magnified)
8. Variolitic or orbicular structure, Napoleonite, Corsica (nat. size)
9. Flow-structure in Rhyolite, Antrim, slightly reduced . . ■ ■ •
10. Lumpy, irregular traohytic lava-streams (Carboniferous), East Linton, Haddingtonsliire
11. View at the entrance of the Svinofjord, Eiroe Islands, illustrating the terraced forms
assumed by basic lavas
12. Sack-like or pillow-form structure of basic lavas (Lower Silurian), Bennan Head,
Ballantrae, Ayrshire
13. Alternations of coanser and finer Tull' ..•••'
14. Alternations of Tuff with nou-volcanic sediment . . . • •
15. Ejected block of basalt which has fallen among Carboniferous shales and limestones
shore, Pettycur, Fife
Ifi. Effects of denudation on a Vesuviau cone . • • • *
17. Section to illustrate the structure of the Plateau type . . • •
18. Diagram illustrating the structure and denudation of Puys
19. Section illustrating submarine eruptions ; alternations of lavas and tuffs with lime
stones and shales full of marine organisms . . ■ • ■
20. Diagram illustrating volcanic eruptions on a river-plain . • • •
21. Diagram illustrating volcanic eruptions on a land-surface
22. Ground-plans of some volcanic vents from the Carboniferous districts of Scotland
23. View of an old volcanic “Neck” (The Knock, Largs, Ayrshire, a vent of Lower Car
boniferoiis age)
24. Section of neck of agglomerate, rising through sandstones and shales
25. Neck filled with stratified tuff
26. Section of neck of agglomerate with plug of lava . • ■ ■
27. Section of agglomerate neck with dykes and veins • • ■ •
28. Section of neck filled with massive rock ...•••
29. Successive shiftiiigs of vents giving rise to double or triple cones
30. Section to show the connection of a neck with a cone and surrounding bedded tuffs
PA op:
15
17
19
19
19
20 20 22
23
24
25
26
34
35
37
40
43
45
48
49
50
55
56 58
64
65
66 68
70
71
XX
THE ANCIENT VOLCANOES OF GREA T BRITAIN
KIO.
PAGE
31. Diagram illustrating the gradual emergence of buried volcanic cones through the in-
fluence of jjrolonged denudation . . . . . . .75
32. Dyke, Vein, and Sill ....... go
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-sehist series between Loch Tay and
Amulree ....... ^^24
38. Sketch of crushed basic igneous rook among the schists, E. side of Porth-tywyn-mawr,
E. side of Holyhead Straits ...... ]^28
39. Section across the Uriconian series of Caer Caradoc . X32
40. Map of the volcanie district of St. David’s .... 145
41. Section .showing the interstratifioation of tuff and conglomerate above Lower Mill,
St. David’s ....... 454
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-becldod 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 rooks. 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 Moelvvyn 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 Kuockdolian 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
65. 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 Forth Wen, w'est of Amlwch . . 223
59. Section of intercalated black shale in the volcanic series at Forth 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 Forth Padrig, near Carmel
. 225
62. Section of felsites in the Coniston Limestone group, west of Stockdale . ' . . 232
63. Fine tuff with coarser bauds 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
LIST OF ILLUSTRATIONS
XXI
FIG.
67.
68.
69.
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Section across the volcanic serie.s of Forlarsliire ...■•■ Section across two necks above Tillicoultry, Ocliil Hills ..... Section of the granite core between Merrick and Corscrine .... Section across the thi'ee Dirrington Laws, Berwickshire ..... Section of Papa Stour, Shetlands, showing sill of spherulitic felsite traversing Old Red Sandstone and bedded porphyrites (Messrs. Peach and Horne) .... Section across Korthinaviue, from Okrea Head to Skea Ness, Shetland, showing dykes and connected sill of granite and felsite (Messrs. Peach and Horne)
Section at the edge of one of the bays of Lower Old Red Sandstone along the northern margin of Lake Caledonia, near Ochtertyre
Craig Beinn-nan-Eun (2067 feet), east of Uam Var, Braes of Douue. Old Red Con- glomerate, with the truncated ends of the strata looking across into the Highlands , moraines of Corry Beach in the foreground ...••■ Section showing the top of the volcanic series at the foot of the precipice of the Red Head, Forfarshire
Andesite with sandstone veiuings and overlying conglomerate. Todhead, south of ■ Caterline, coast of Kincardineshire
Section across the Boundary-fault of the Highlands at Glen Turrit, Perthshire .
Section across the chain of the Sidlaw Hills near Kilspindie .... Section across the Eastern Ochil Hills from near Newburgh to near Auchtermuchty Generalized section across the heart of the Ochil Hills from Dunning on the north to the Fife coal-field near Saline on the south ...••• Diagram of the volcanic scries of the Western Ochil Hills . - . •
View of Cnoc Gurbh, Southend, Campbeltown. A volcanic neck of Lower Old Red Sandstone age, about 400 yards wide in its longer diameter . . . ■
Section of volcanic series on beach, Southend, Campbeltown Section of the base of the volcanic series, Reclain, five miles south of Pomeroy .
Section of shales and breccias at Crossna Chapel, north-east of Boyle Section across the north end of the Pentland Hills, from Warklaw Hill to Pentland Mains. Length about five miles ..••••• View of the lava-escarpments of Warklaw Hill, Pentland chain, from the north-west . Section across the Pentland Hills through North Black Hill and Scald Law. Length about three miles
Section from the valley of the Gutterford Burn through Green Law and Braid Law to Eight-Mile Burn
Section across the north end of the Pentland Hills, and the southern edge of the Braid Hill vent. Length about two miles
Section across the northern end of the Biggar volcanic group, from Fadden Hill to beyond Mendick Hill
Section across the southern part of the Biggar volcanic group from Covington to Culter Section from Thankerton Moor across Tinto to Lamington .... Section across the Dnneaton volcanic district from the head of the Duneaton Water to
Kirklea Hill . . . • • • ' '
Cavernous spaces in andesite, filled in with sandstone, John o’ Groat’s Port, Turnberry,
Ayrshire
Section of andesites, Turnberry Castle, Ayrshire
Lenticular form of a brecoiated andesite (shown in Fig. 96), Turnberry, Ayrshire Section across the volcanic area of St. Abb s Head (after Prof. J. Geikie)
View of terraced andesite hills resting on massive conglomerate, south of Oban .
Section of lava-escarpment at Beiun Lora, north side of mouth of Loch Etive, Argyllshire
PAGE
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334 334 339
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xxii THE ANCIENT VOLCANOES OF GREAT BRITAIN
PAGI<:
101. Section across Strathbogie, below Rliynie, 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. A-’icw 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 Garloton 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 Hilt— a fragment of the Mhllothian Plateau, seen from the north . 373
112. View of Arthur Seat from Calton Hill to the north . . . .374
113. View of Arklotou Fell, part of the Solway Plateau, from the south-west . . 370
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 . . , , . . . _ gg»
117. Section of the top of the Midlothian Plateau in the Muriestou 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 we.st 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-veii t.s 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 . . . . . . . . . 39^
126. Ground-plan of tutf-neck, shore east of Dunbar ...... 393
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 ..... 499
129. Diagrammatic section across the central vent of the Clyde plateau in Renfrewshire . 499
130. Section across Southern Berwickshire, to show the relation of the volcanic plateau to the
vents lying south from it , . . . . . . , . 49]^
131. Section of south end of Dumbuck Hill. East of Dumbarton .... 493
132. Section across the East Lothian plateau, to show the relative position of one of the necks 403
133. View of Trapraiu Law from the south, a phonolite neck of the Garleton Plateau . 495
134. Veins and dykes traversing the agglomerate and tuff of the great Renfrewshire vent . 498
135. “ The Yellow Man,” a dyke in volcanic tuff and conglomerate on the shore a little east
of North Berwick ......... 499
LIST OF ILLUSTRATIONS
XXIll
riG.
PAGE
136. Trachytic sills, Kiiookvadie, Kilpatrick Hills
137. Section across the edge of the Clyde plateau, south-east of Beith
138. Section across the upper part of the Clyde plateau at Kilbirnie, Ayrshire
139. Section across the upper surface of the Clyde volcanic plateau, Burnhead, noith-west of
Kilsyth
140. Section across the upper surface of the Clyde volcanic plateau at Campsie
141. Section across western edge of the Garlton plateau . • ■ • •
142. Section across the Solway plateau ..•••••
143. Section of volcanic vent at East Grange, Perthshire coal-field, constructed by Mr. B. K.
Peach from the rocks e.xposed in a railway-cutting, and from plans of ironstone- and coal-pits
144. View of the Binn of Burntisland — a volcanic neck of agglomerate
145. View of part of the cliffs of vertical agglomerate, Binn of Burntisland .
146. Diagram of buried volcanic cone near Dairy, Ayrshii'e. Constructed from information
obtained in mining operations
147. Diagram to illustrate how Volcanic Necks may be concealed and exposed
148. Section across the Saline Hills, Fife ..-•••■
149. Section across the Binn of Burntisland, in an east and west direction
150. Section in old quarry, west of Wester Ochiltree, Linlithgowshire, Calciferous Sand-
stone aeries
151. Ejected volcanic block in Carboniferous strata, Burntisland . . ■ ■
152. View of volcanic agglomerate becoming finer above east end of Kingswood Craig, two
miles east from Burntisland
153. Alternations of basalt and tuff, with shale, etc., of Kingswood Craig, Burntisland
154. Section of the upper surface of a diabase (“ leckstone ”) sheet, Skolie Burn, south-east of
Bathgate
155. Section across the volcanic ridge of the Linlithgow and Bathgate Hills, shorving the
intercalation of limestones that mark important stratigraphical horizons
156. Section in Wardlaw Quarry, Linlithgowshire .■•••■
15^. Section from Linlithgow Loch to the Firth of Forth . . . • •
158. Section across the Campsie Fells illustrating the contrast between the sills below and
above the plateau-lavas
159. Section showing the position of the basic sills in relation to the volcanic series at
Burntisland, Fife ....■■•••
160. Sills between shales and sandstones. Hound Point, Linlithgowshire
161. Section of Sill, Cramond Railway, Barn ton, near Edinburgh ....
162. Intrusive dolerite sheet enclosing and sending threads into portions of shale, Salisbury
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Crags, Edinburgh
163. Intrusive sheet invading limestone and shale, Dodhead Quarry, near Burntisland
164. Spheroidal weathering of dolerite .sill, quarry east of North Queeusferry, Fife .
165. Two thin sills of “white trap” injected into black carbonaceous shale overlying the
Hurlet Limestone, Hillhouse Quarry, Linlithgow .....
166. Dyke cutting the agglomerate of a neck. Binn of Burntisland ....
167. Boss of diabase cutting the Burdiehouse Limestone and sending sills and veins into
the overlying shales. Railway cutting, West Quarry, East Calder, Midlothian
168. Side of columnar basalt-dyke in the same agglomerate as in Fig. 166
169. Dyke rising through the Hurlet Limestone and its overlying shales. Silvermine
Quarry, Linlithgowshire
1/0. Junction of aniygdaloidal ba.salt with shales and limestone, shore, hall a mile east from
452
452
455
456
457
458
459
460
Kinghorn, Fife -
464
XXIV
THE ANCIENT VOLCANOES OF GREAT BRITAIN
FIG. PAGE
171. Columnar basalt, Pettycur, Kinglioim, 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 bauds in Liddesdale . . 476
175. Diagram to show the position of a mass of Upper Old Red Sandstone which has fallen
into the gi'eat vent near Tudhope Hill, east of Mosspaul .... 476
MAPS
I. General map of the Volcanic districts of the British Isles. — At the eiwL 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 ...... Tofacep.ZZi
IV. Map of the Carboniferous volcanic districts of Scotland ... To face p. 476
BOOK I
gexeeal peinciples and methods of investigation
CHAPTER I
Earliest Knowledge of Volcanoes — Tlieir Influence on Mytliolog}- and Superstition — Part taken by Volcanic Rocks in Scenery — Progress of tbe 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 developmeirt.
Thus, in the dawn of civilization, the frequent recurrence of earthquakes and volcanic eruptions throughout the basin of the Mediterranean could not 1>ut have a marked effect on the peoples that dv’elt 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. AVhen, 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 rani 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 iury 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 ^Egean had their volcanoes, and the Greeks who dwelt among them watched their VOL. I n
I
\
\
INTRODUCTION
BOOK 1
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 eastw’ard they would see, on the coast of Asia Minor, the black bristling lavas of the “ Burnt Country,” perliaps 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 gleain 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 Ifiaze of mid-day, how the land was smothered with dust and stones, how over the sea there spread such a covering of ashes that tlie oai'smen could hardly drive their vessels onw'ard, 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 siidden 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 tlie 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 tlie burning roots of the mount- ain, 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 Medi- terranean cradle of civilization, they met with volcanic manifestations in many other parts of the w'orld. When they eventually penetrated into the Far East, they encountered volcanoes on a colossal scale and in astonishing aliundance. Wlien 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 ci'itically 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
CHAP. I
EFFACEMENT OF OLD VOLCANOES
3
planet can only, if ever, be disi^elled by the patient stndy of these vents of coniniunication between the interior and the surface.
If, however, we desire to fortu some adequate idea of the part which '"olcanic action has played in the past history of the earth, we should be misled were we to coniine orir attention to the phenomena of the eruptions ol 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 tliat 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. Ill a year or two the ruins have been cleared aw'ay, the earth-falls have been healed over, the prostrated trees have been removed, and, save in the memories mid chronicles of the inhabitants, no record of the catastrophe may survive. Ihe clouds of dust and show'ers of ashes which destroyed the crops and crushed in the roofs of houses soon disappear from the air, and the covei- itig which they leave over the surface of a district gradually mingles with the soil. Vegetation eventually regains its place, and the landscape becomes ugain as smiling as befoi'e.
Even where the materials thrown out Ifom the crater accumulate in much greater mass, where thick deposits of ashes or solid sheets of la^'a 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, ljut the new topograjdiy soon ceases to look new*. Another generation of inhabitants loses recollec- tion of the old landmarks, and can hardly realize that what has become so familiar to itself differs so much from what w'as familiar to its fathers.
But even wiien the volcanic covering, thus throwm 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 tlie surface of the land. Ko feature of a modern volcano is more impressive than the lesson which it conveys of the reality ^nd potency of this continual waste. The northern slopes of Vesuvius, for example, are trenched with deep ravines, which in the course of centuries liave been dug out of the lavas and tuffs of Monte Somma by rain and melted snowx Year by year these chasms are growing deeper and widei', while the ridges between them are becoming narrower. In some cases, ludeed, the intervening ridges have been reduced to sharp crests which are split up and lowered by the unceasing inffuence of the weather. The slopes of such a volcanic cone have been aptly compared to a half-opened umbrella. If reqirires little effort of imagination to picture a time, by no means 1 emote in a geological sense, when, unless renovated Ijy 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 m site of the old volcano, in happy ignorance of what has been the history of the ground beneath their feet.
4
INTRODUCTION
BOOK I
Wliat 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 efiaced. And yet enough may be left to enable a geologist to make sure that volcanic action was once rife there. As the volcano rnarics a channel of direct communication between the interior of the earth and the atmo.sphere 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. Tlie 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 bn 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 Hun- gary, 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 abund- ant relics of fiir 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
CHAP. I
MODERN AND ANCIENT VOLCANOES
5
he can best realize how mucli 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 arradual removal of the pile of material under which they liad 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, ijut in the light derived from a study of that ancestry. It is only when we take this broad view of the sidjject 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 tbe basis and starting-point of inquiry. At that channel of commuiucation between the unknown inside and the familiar outside of our globe, we can rvateh 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 ol ashes or the How of lavas, and gather a vast body ot facts regarding the materials that are ejected from the interior, and tlie 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 cairnot tell anything about the subterranean ducts, or how the molten and fragmental materials behave in them. All the underground mechanism of volcanoes is neces- sarily 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 ftdl 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 con- ditions under which these submarine discharges are ellected, the be- haviour of the out-flowing lava under a body of oceanic water, and the part played by fragmentary materials in tlie explosions, can only be surmised. How and then a submarine volcano pushes its summit above the sea-level, ^nd allows its operations to 1)6 seen, but in so doing it becomes practically a terrestrial volcano, and the peculiar submailne phenomena are still effec- tually concealed from observation.
The volcanic records of former geological periods, however, are in large
6
INTRODUCTION
BOOK I
ineasiu’e those ot eruptions under the sea. In studying them we are per- mitted, as it were, to explore the sea-bottom. We can trace how sheets of coral and groves of crinoids were buried under showers of aslies 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 wlio undertakes an investigation into tlie 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 phy-sics, finds himself in a situation of peculiar advantage. I’robably no region on tlie 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 re- markable completeness of the geological record in Britain ; 2nd, The geographical }iosition of the region on the oceanic border of a continent; :5rd. The singu- larly 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 com- plete. It has often been remai'ked 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 anil to the large number of observers, this remarkably full record of geological history has been studied with a minute care which lias hardly been equalled in any other country. The detailed succession of all tlio formations has been so fully determined in Britain that the very names first applied here to them and to their sub- divisions have in large measure passed into the familiar language of geology all over the globe. Every definite platform in the stratigraphical series lias been more or less fully worked out. A basis has thus been laid for referring each incident in the geological liistory 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. Basing 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 jilaced not far from the sea. From the earliest geological periods the site of Britain, even when submerged below the sea, has never Iain far from the land which supplied the vast accumulations of sediment that went to form the Bakeozoic 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 lioth terrestrial and submarine volcanic activity.
chap. 1
GEOLOGICAL ADVANTAGES OF BRITAIN
7
3. In the third place, this advantageous geographical position is found to have been attended with an altogether remarkable alnindaiice and per- sistence of volcanic eruptions. No tract of equal size yet known on the face of the globe furnishes so ample a record of volcanic actirnty from the eai’liest geological periods down into Tertiary time. Nvery 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 Vesuviirs, scattered groups of small cones like the puys of France, and fissure- or dyke-eruptions like those of recent times in Iceland.
iloreover, 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 us that which accompanies the present volume (Map I.). The reader will there observe how repeatedly volcanic eruptions have taken place, not merely within tlie 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. Erom the early part ot the Lower Silurian period, through the ages of the Old Bed Sandstone, Carboniferous and Permian systems, hundreds of volcanic vents were active hi that region, while in long subsequent time there came the fissure- uruptions of the Tertiary series.
d. In the fourth place, the geological revolutions of successive ages have made this long volcanic chronicle fully accessible to observation. Had tile lavas and ashes of one period remained buried under the sedimentary accumulations of the next, their story would have been lost to us. e ehould oidy have been able to decipher the latest records which might happen to lie on the sui’face. Eortunately for the progress of geology, the endless vicissitudes of a continental border have brought up the very eldest rocks once more to the surface. All the later lormations 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 he satisfactorily deciphered. The singularly complete volcanic chronicle, after heing 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 de- mxdation 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 fhe 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. Popixlar imagination has long been prone to see
INTROD UCTION
BOOK I
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 mount- ain, have been unhesitatingly set down as proof of volcanic disturbance. IMany 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 i)roofs 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 tlie face of the land. I hose rocks which from their liardness or structure are best aide to with- stand 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. Tlie most telling evi- dence of old volcanoes is naturally to Ije found in the materials whicirthey 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 aie 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 endurino- records W'e may hope to detect the traces of fonner volcanic action. The following introductory chapters will be devoted to an attempt to answer
CHAP. I
PLAN OF THE WORK
9
this question. I shall try to show the nature and relative import- ance of the records of ancient volcanoes ; how these records, generally so fragmentary, may he pieced together so as to he made to furnish the history which they contain ; how their relative chronology may bo estab- lished ; how their testimony may be supplemented in such wise that the position of long vanished seas, lands, rivers, and lakes may be ascertained ; *ind how, after ages of geological revolution, volcanic rocks that have lain long buried under the surface now inlluence 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 11. 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 per- sistently followed by Lyell, of working backward from the present into the past, has some distinct advantages. The volcanic records of the later are much simpler and clearer than those of older times, and the student may, in some respects, profitably study the history of the Tertiary ®>’uptions before he proceeds to make himself acquainted with the scantier chronicles of the eruptions of the Balfcozoic periods. But as I wish to follow the gradual evolution of volcanic phenomena, and to show how vol- canic 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 clome-sliaped hill or nioniitain, consisting of tnaterials whicli have been ernpte.cl from an orifice leading down from the surface into the heated interior of the earth. Among modern and recent volcanoes three types may he recognized. In the first and most familiar of these, the lavas and ashes ejectetl from the central vent have gathered around it by successive eruptions, until they have huilt 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 Hanks, and may become themselves the chief actively erujrting vents. This type of volcano, which has been so long w'oll 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 frag- mentary discharges are small in amount and where the eruptions are almost wholly confined to the emission of tolerably licpiid lava. A vast dome with gently sloping declivities may in this way he 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, hut 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 wliich 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 Pays. From these vents considerable streams of lava have sometimes been discharged.
Without entering here 'into a detailed imjuiry 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
^Hap. II
THE INTERIOR OF THE EARTH
I
surface, fornieiiy 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 mrdergone great tidal deformation, hut that in actual tact 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 t't the earth’s interior must l)e high. The rate of increase of this tem- perature downward from the surface differs from place to place ; but an increase is always observed. At a depth of a few miles, every known sub- stance must be much hotter than its melting point at the surface. Hut at the great pressures within the earth, actual liquefaction is no doubt pre- sented, and the nucleus remains solid, though at a temperature at which, Fut 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 licprid state. There is one known cause which will bring about this result. The downward increment of temperature proves that our planet is continually losing lieat. As the owter 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 orust is doing. Now cooling involves contraction. The hot interior is con- tracting faster than the cooler shell which encloses it, and that shell is tlius forced to subside. In its descent it has to adjust itself to a constantly 'fiminishing 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 imiformly 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 ®'mcessive periods of terrestrial readjustment. These lines of relief coincide, cn the whole, with the axes of our continents. The land-areas of the globe '‘^ay be regarded as owing their existence above sea-level to this result of ferrestrial contraction. The crust underneath them has been repeatedly Wrinkled, fractured and thrust upward by the vast oceanic subsidence around fhein. Xpe 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 111 large measure within the land-areas, or at least parallel with their axes niid close to their borders. Where the disposition of the chief ruptures and cf the predominant plications can be examined, these leading structural matures are found to be, on the whole, coincident. In the British Islands, °r instance, the prevalent trend of the axes of folding from early Paheozoic kertiary time has been from south-west to north-east. How profoundly is 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 I'l'g inlets of the sea. A geological map makes the dependence of the Scenery upon the building of tlie rocks still more striking. Not only have
12
NATURE AND CAUSES OF VOLCANIC ACTION
BOOK I
these rocks lieen plicated into endless foldings, the axes of which traverse the British Islands with a north-easterly trend ; they have likewise been dislocated l)y many gigantic ruptures, which tend on the whole to follow the same direction. Tlie 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.
"STewed 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 ^■olcanic action. If the molten magma flowed out merely in virtue of the influence of terrestrial contraction, it might do so for the most part tran- quilly, though it would probably be affected l)y 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 W’e may suppose that the consequent subsidence, though it results in intermittent wrinkling and ^iplifting 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 assundng generally a feeble development, Imt sometimes Imrsting into fountains of molten I'oek hundreds of feet in height, like those observed from time to time in Hawaii.
'-Hap. n
VOLCANIC ERUPTIONS
13
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 ''olcanic vents may be referred to the results of terrestrial contraction, the violent discharge of materials from those vents must be assigned to some idnd 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 loagma is suddenly relieved, they are liberated with explosive violence.
A volcanic paroxysm is thus immediately the efiect of the rapid escape 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 '^'Pper 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 scoriae and clinkers. Thus within the magma itself the amount 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 On the other — that the phenomena of v'olcanoes appear to be mainly due. There is no reasoir to believe that modern volcanoes differ in any essential respect fronr 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 favonr of such a difference. One of the objects of the present work is to trace the continuity of volcanic phenonrena 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 w’ay 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 lo a consideration of the proofs of volcanic action furnished by the very iiature 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 I^jnnels or ducts from which they were discharged above ground, and by the *^^isposition of the masses of rock which, at various depths below the surface, Jiave been injected into and have solidified within the terrestrial crust.
CHAPTER III
Ancient Volcanoes ; Proofs of their existence derived from the Nature of the Hocks 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 Ttrffs.
The materials brought by volcanic action from the earth’s interior Imve 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 topograpliical features of the ground.
Volcanic products may lie 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 tlie 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 arc commonly classed in three di-vi- sions— 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.
chap, m
STRUCTURES OF LAVAS
15
Tile structures of lavas, however, furnish their most easil)’ appreciated characteristics. Four of these structures deserve more particular attention : Istj Cellular, vesicular or pumiceous structure ; 2nd, The presence ot glass, or some result of the devitrification of an original glass ; Mrd, h low-structure ; ^od 4th, The arrangement of the rocks in sheets or beds, with columnar and ether structures.
1. The CELLULAK, VESICULAR, SCORIACEOUS Or PUMICEOUS STRUCTURE of ''olcanic rocks (Fig. 1) could only have arisen in molten masses from the
Pig. I. — Ve.sicul.ir structure, Lava front Ascension Isltind, slightly less than natural size.
expansion of imprisoned vapours or gases, and is thus of crucittl importance deciding the once liquid condition of the rocks which display it. The ^esicles may Ite of microscopic minuteness, Imt are generally quite visible ° the naked eye, and are often large and conspicuous. Sometimes these '^9'Vities have been subsequently tilled up with calcite, quartz, agate, zeolites a *^ther mineral deposition. As the kernels thus produced tire frequently eiied or almond-shaped (ainj/f/dales), owing to elongation of the steam- o es by movement of the lava hefoi’e its consolidation, the rocks containing ®i^i^are said to he aniyfjdaloidal.
This structure, though eminently characteristic of superficial lavas, is
i6
VOLCANIC PRODUCTS
BOCK I
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. F)Ut, 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 Pigs. 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 accpiired 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 propor- tions as to occupy most of the bulk of the rock, and in other places dis- appearing, so as to leave the lava tolerably compact. When a lava presents an irregularly vesicular character, lilce that of the slags of an iron-furnace, it is said to be datfjy. AVhen its upper surface is rugged and full of steam- vesicles of all sizes up to large cavernous spaces, it is said to be ^coriaceous, and fragments of such a rock ejected from a volcanic vent are spoken of as scoriae.
xittention to the llattening 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. Some- times 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 tliis exceptional structure have been obtained from the Tertiary plateau-basalts of the Western Isles, of which an example is represented in Eig. 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 vapoixrs dissolved in molten lava, remained empty on the consolidation of the rock, they have generally been subsequently filled up by the deposit withirr them of mineral substances carried in aqueous solu- tion. The minerals thus introduced are such as might have been derived
chap. III
STRUCTURES OF LAVAS
17
from the removal of their constituent ingredients by the solvent action of 'vater 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 tlie steam-vesicles have been due to the influence of meteoric water, which at atmospheric temperatures and pressures has slowly percolated from the surhice through the cellular lava, long after the latter had consolidated and cooled, and even *frter volcanic energy at the locality had entirely ceased.
Examples, however, are now accumulating which certainly prove that, in ®ome cases, the vesicles were filled up during the volcanic period. Among the Tertiary basalt-jdateaux of the Inner Hebrides, for instance, it can be ®frown that the lavas were already amygdaloidal before the protrusion of the
Flo. 2.
Elongation and brandling of .steam-veside.s in a lava, Kilniiiian,
than natural size.
Lsle of Mull, a little less
gabbros and granophyres which mark later stages of the same continuous ^olcanic history, and even before the outpouring of much of the basalt of plateaux. Not improbably the mineral secretions were largely due to I'm of hot volcanic vapours during the eruption of the basalts.
's subject will be again referred to in the description of the Tertiary ’^’olcanic series.
V esicular structure is more commonly and perfectly developed among lavas which are basic and intermediate in composition than among ose which are acid.
Idle the existence of a highly vesicular or scoriaceous structure may honerally be taken as proof that the rock displaying it flowed out at the ®'irtace as a lava, other evidence pointing to the same conclusion may often gathered from the rocks with which the supposed lava is associated. ''’OL. I c
i8
VOLCANIC PRODUCTS
BOOK I
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 some- times be proved to be a true How 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 spheru- litic 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 foimer 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 oidy are vestiges of the original glass recognizable, but the whole progress of tlevitrification 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. Eouqmi and Michel Levy, who have fused the constituent minerals in the proper proportions.
Since traces of glass or of its representative devitrified stnictures are so abundantly discoverable in lavas, we may infer tlie original condition of most lavas to have been vitreou.s. 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 consolida- tion 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 crystal- line 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
CHAP. Ill
STRUCTURES OF LAVAS
19
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 devitri- fication 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 in- orease in number until they entirely take the place of the glass. JMicrolites of definite minerals may likewise be observed, together With indefinite granules, and the rock finally lieeomes a rhyolite, felsite or allied variety (Fig. 3).
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 he 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 solidi- fication of the mass. In obsidians and pitchstones such relics of an earlier cr derived series of crystallized minerals may often be recognized, while in Iclsites and quartz-porphyries they are equally prominent. Where large dispersed crystals form a prominent characteristic in a rock they give rise what is termed the Porphyritic structure.
Accompanying tlie passage of glass into stone, various structures make
Fig. 3. — Microlites of tlie Pltclistone of Arran (magnified 70 diameters).
^•'“t’erlitic structure in Felsitic ^lass, Isle of Mull (magnified).
Fig. 5. — Spliemlitic structure (inagnitied).
mr appearance, sometimes distinctly visible to the naked eye, at other mes only perceptible with the aid of the microscope. One of these struc- known as Perlitic (Fig. 4), consists in the formation of minute cui'ved
20
VOLCANIC PRODUCTS
BOOK I
or straight cracks between which the vitreous or felsitic substance, during its contraction in cooling, assumed a finely globular form.
Another structure, termed Spliervlitic (Fig. 5), shows the development of globules or spherules which may range from grains of microscopic minute- ness 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 ISlicol 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 sti'ucture, which was developed during the process of consolidation. So marked does this arrange- ment 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 tapper Silurian lavas of Dingle, and the Lower Old Eed Sandstone lavas near Killarney.
A marked structure among some intrusive rocks, especially of an acid composition, is that called Micropeg'inatitic or Granopliyric. It consists in a minute intergrowth of two component minerals, especially (j^uartz and felspar.
Fig. 6. — Mieropegiuatitic or Granopliyric structure in Grauopliyre, Mull (magnified).
Fig. 7. — Opliitic structure in Dolerite, Gortacloglian, Co. Derry (niagnitted).
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 crystal- lization. Among basic, as among acid rocks, there are proofs of the consoli- dation 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
chap, in
STRUCTURES OF LA VAS
21
magma iu its molten condition. During devitritication, however, other fel- spars of a later period of generation made their appearance, but they are generally distinguishable from their predecessors. I’robably most liasic and intermediate rocks, when poured orit 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 ioy the aid of the microscope.
Jlicrolites of the component minerals are first developed, which, if the process of aggi’egation is not arrested, build up more or less perfect crystals or crystalline grains of the minerals. Eventually the glass may be so com- pletely devitrified by the development of its coustitutent 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 holocrystalliue compound be followed with admirable clearness. The component minerals have sometimes crystallized in their own distinct crystallographic forms (idio- morphic) ; iu other cases, though thoroughly crystalline, they have assumed externally different irregular shapes, fitting into each other without their proper geometric boundaries (allotriomorphic).
-b specially characteristic featiire of many basic rocks is the presence of "’hat is termed an Ophitie structure (Fig. 7). Thus the component crystals pyroxene occur as large plates separated and penetrated by small needles and crystals of felspar. The portions of pyroxene, divided by the enclosed ^olspar, 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. Foiupui and Michel Levy.
The name Variolitic is applied to another structure of basic rocks ifig. 8), in which, especially towards the margin of eruptive masses, abund- ant spheroidal a<Tgregates have been developed from the size of a millet-seed cn 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, spheru- lites, or devitrification-streaks in bands or lines, which sweep round any enclosed object, such as a porphyritic crystal or detached spherulite, and ^'epresent the curving fiow of a mobile or viscous mass. Admirable examples ef tills structure may often be observed iu old lavas, as well as in dykes and ®^lls, the streaky lines of flow being marked as distinctly as the lines of foam tlint curve round the boulders projecting from the surface of a mountain-brook.
Flow-structure is most perfectly developed among the obsidians, rhyolites, tulsites 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 ^irangement of the minute hair-like bodies and crystals, or by alternate ^yers of glassy and lithoid material. The streaky lines thus developed are •^onietimes almost as thin and parallel as the leaves of a book. But they
22
VOLCANIC PRODUCTS
BOOK I
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 Auvidly to portray the flow of a viscous substance. These lines repi'eseut on a minute scale the same flow-structure which may he 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.
Among lavas of more basic composition flow-sti-uctuie is not so often well displayed. It most frequently shows itself by the orientation of porphyritic felspars or of lines of steam -vesicles. Occasionalh", however, sheets of basalt may Ije found in which a distinct streakiness has been developed owing to variations in the differentiation of the original molten
Fig. 8. — Variolitk' or Orbicular structm-c, Napoleoiiite, Corsica (iiat. size).
magma. A remarkable and wide-spread 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 suc- cessive 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
chap. Ill
STRUCTURES OF LA VAS
23
tile constitution of the magma from which the la^'as issued — variations "diich already existed before the discharge of these lavas, and which showed themselves in the successi\'e 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 ior such '’ariations 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 lertiary volcanic series of Ih'itain. These, as will be more fully discussed in later chapters, indicate ^ Considerable amount of heterogeneity in the deep-seated magma from which the intrusive sheets and bosses were supplied (see vol. ii. pp. 329, .t42).
Fig. 9. — Flow-struftiirt in Rhyolite, Aiitriiii, .slightly reduced.
It is a common error to assume that flow-structure is a distinctive ^^araeter of lavas that have flowed out at the surflice. In reality some ^ the most perfect examples of the structure occur in dykes and sills, both acid and basic rocks. Innumerable instances might be quoted from Ih'itish Isles in support of this statement.
Altliough, in the vast majority of cases, tbe presence of flow -structure ^^ay be confidently assumed to indicate a former molten condition of the in which it occurs, it is not an absolutely reliable test for an igneous K.xperiment has shown that under enormous pressure even solid ’Petals niay be made to flow into cavities iirepared for their reception.
24
VOLCANIC PRODUCTS
BOOK I
Under tlie vast compression to which the earth’s crust is subjected during terrestrial contraction, the most obdurate rocks are crushed into frag- ments varying from large blocks to the huest powder. Tliis comminuted mateiial is driven along’ in the direction of thrust, and when it comes to rest presents a streakiness, with curving lines of Ilow round the larger fragments, closely simulating the structirre ot many rhyolites and obsidians. Tt is only by attention to the local surroundings that such deceptive re- semblances can be assigned to their true cause.
4. lire LUSPO.siTioN of lavas in .sheets or eeij.s 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 Xorth 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
Fig. 10.— Lumpy, irregular tracliytie Lava-streams (Carboniferous), East Linton, Haddingtonshire.
the trachytes and andesites, assume lumpy, irregular forms, and some httle care may be recpdred to trace their upper and under surfaces, and to ascertain that they really do form continuous sheets, though varyiim 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 Hows of basalt lie upon each other in even parallel beds traceable for considerable distances along the face of the precipices (Fig.s. 11, 265, and 286). The ande- stes of the Old Eed Sandstone (Figs. 99, 100) and Carboniferous series (Fig.s.
I Of, 108, 111, 112, 113, 123) m 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 Hanks 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 ba.salt- plateaux of the Inner Hebrides a single sheet may sometimes be traced for several miles.
'^hap. in
STRUCTURES OF LA VAS
2
Some lavas, more especially among the basic series, assume in cooling a ^olurnnar structure, of which two types may be noticed. Tn one of these the 'Columns pass with regularity and parallelism from the top to the bottom of ^ bed (Figs. l7l, 225). The basalt in which Fingal’s Cave, in the isle of ^taffa, has been hollowed out may be taken as a characteristic example 266a). Not infrecpiently the columns are curved, as at the well-known ^'lam-shell Cave of Statfa. 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 twm types may occur in successive slieets of basalt, or may even pass into each other. At Staffa the regularly
FiCi. ll._View at the entrance of the Sviiiofjord, Faroe Islande, illustrating the terraced forms
a.s.sunied hy basic lavas.
The islanil on the left is Borii, that in the centre Viderii, and that on the right Svino.
^joliuinyjj^j. lied is immediately overlain with one of the starch-like character.
he columnar structure in either case is a contraction phenomenon, produced ' niiiig 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 sometimes have assumed the regular, at othei’s the irregular form. ^ 'nay be found not only in superficial lavas but in equal pei’fection in ®°nre dykes and intrusive sills or injections, as among the Tertiary volcanic ’■’neks of the island of Canna (Figs. llOV and 308).
^he precipitation of a lava-stream into a lake or the sea may cause the nuter crust of the rock to break up with violence, so that the still molten naterial inside may rush into the water. Some basic lavas on flowing into "'ater or into a watery silt have assumed a remarkable spheroidal sack -like
26
VOLCAN/C PRODUCTS
BOOK I
or pillow-like structure, the spheroids being sometimes pressed into shapes like piles of sacks. A good instance of this structure occurs in a liasalt at Acicastello in Sicily.^ 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).
It piobably seldom happens that a solitary sheet of lava occurs among non-volcanic sedimentary strata, with no otlier 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 lias generally been accompanied with the ejection
Fig. 12.— Sack-like or pillow-form structure of basic lavas (Lower Silurian), Beunaii Head,
Ballantrae, Ayrshire.
of fragmentary materials. Hence among the memorials of volcanic erup- tions, while intercalated lavas are generally associated with sheets of tuff, bands of tuft may not infreipiently be encountered in a sedimentary series without any lava.^ Instances in illustration of these statements may be culled from the British I’aheozoic formations back even into the Cambrian system.
A cbaracteristic feature of some interest in connection with the flow of lava is the efiect 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
1 See Prof. G. Platania in Dr. Johnston-Lavis’ South ltalia7i Volcanoes, Naples (1891) ii 41 and plate xii. ' ^
Chap. I] I
STRUCTURES OF LAVAS
movement of a flowing current of basalt.^ The great heat of tlie lava frequently induced considerable alteration upon the underlying rocks, ^iiduration is the most common result, often accompanied with a reddening the altered suljstance. Occasionally a beautifully prismatic structure has ^een developed in the soft material immediately beneath a basalt, as in ferruginous clay near the village of Esplot in the \ elay, in which the close- ®6t columns are 50 centimetres long and 4 to 5 centimetres in diameter, tlhauges 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.
Variations of structure in single lava-sheets. — From what has said above in regard to certain kinds of flow-structure among basic ^ouks, 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 diflerences, it is true, are more frequent U'uong intrusive rocks, especially thick sills and large bosses, tut thev ^uve 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 ‘Af the constituents of a molten mass, the heavier minerals sinking through the lighter, so that the upper portions of the mass will become more felspatlnc ^'Ud the lower parts more augitic and ferruginous.®
Leopold von Ihieh found that in some of the highly glassy lav as of the Lanary Islands the felspar increases towards the liottoni of the mass, becom- 'AAg so abundant as almost to exclude the matrix, and giving rise to a com- pound that might be mistaken for a primitive rock.A
Ilarwin oliserved that in a grey basalt filling up the hollow of an old crater in dames Island, one of the Clalapagos group, the felspar crystals ^'ccanre much more abundant in the lower scoriaceous part, and he discussed ^'Ae question of the descent of crystals by virtue of their specific gravuty through a still molten lavm.®
Mr. Clarence King during a visit to Hawaii found that in every case "'here he broke newly-congealed streamlets of lava, “ the bottom of the flow ""us thickly crowded with triclinic felspars and augites, while the whole "Pper part of the streain was of nearly pure isotropic and acid glass.” This ®Aihject will be again referred to when we come to discuss the characters of Autrusive sills and bosses, for it is among them that the most marked petro- "Aaphical variations may be observed. Examples will be cited both from intrusive and extrusive volcanic groups of Ilritain.
Volcanic cycles. — Closely related to the problem of the range of struc-
1 M. Boule, BvXl. Cart. CM. France., No. 28, tom. iv. (1892), j). 235.
- M. Boule. Op. cit. p. 234. * Volcanoes, p. 125.
Description Fhysi<iue des Isles Ca'narics (1836), p. 190.
Geological Observations mi Volcanic Islaiuls (1844), p. 117.
“ U.S. Gcol. E.rploralion of the Fortieth Parallel, vol. i. fl878), p. 716.
28
VOLCANIC PRODUCTS
BOOK I
ture and composition iu a single mass of lava is another problem presented by the remarkable secpxence of different types of lava which are erupted within a given district during a single volcanic period. Nearly thirty years ago Tlaron von Pdchthofen drew attention to the sequence of volcanic materials erujited within the same geographical area. He showed, more especially from observations in Western America, that a definite order of appearance iu 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. 1’ropylite ; 2. Andesite ; 3. Trachyte ; 4. Ehyolite ; 5. Basalt.^ 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 appli- cation.^ Examples are not uncommon of an actual alternation of acid and ■ basic lavas from the same, or at least from adjacent vents. Such an alterna- tion occurs among the Tertiary eruptions of Central France and among those of Old lied Sandstone age iu Scotland.
The range of variation 'in the natrrre of the eruptive rocks during the whole of a v'olcanic period in any district may be termed “ a volcanic cycle.” In Britain, where the records of many volcanic periods have been preserved, a numlrer of such cycles may be studied. In this way the evolutioir 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 iu each period where the requisite materials for the purpose are available. We shall find that back to Archaean 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 hero a few from the large series now collected from the European continent and North America.^
Among the older rocks of the European continent. Prof. Brogger has shown that in the Christiania district the eruptive rocks which traverse the Cambrian and Silurian formations began with the outburst of basic material
^ Tran.i. Acad. California., 1868. Prof. Iddings’ Jcnirn. Geol., vol. i. (1893), p. 606.
- See Prof. Brogger, “ Die Eruptivgesteine des Kristianiagebietes, ” part ii. (1895), ji. 175 ; Zcitsch. Kri/st. mid Mineral, vol. xvi. (1890) p. 83. This author wouhl, from thus ]iohit 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 he no doubt that in many old volcanic districts the masses that may be jwesuined to have consolidated at a gi'cat depth have lieen iu unbroken connection with nia.sses that reached the surface. These latter, a.s Prof. Iddings has urged, fiu-nish a much larger body of evidence than the intrusive sheets and bosses.
^ Prof. M. Bertrand iu a suggestive jtaper published in 1888 dealt with the general order of ajipearanoe 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 JI. Le Veirier, who announces his opinion that the eruption of the basic rocks takes jdacc in times of terrestrial calm, while that of the acid rocks occurs in periods of great disturbance, up. cU. p. 498. Compare also Prof. Briigger, I>ie Eruptivgesteine dcs Kristiaiiiagchicles, ii. p. 169.
^Hap. hi
VOLCANIC CYCLES
29
such as inelaphyre, 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 ^^orwegiau rocks, like the rhomben-porpliyry. 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 ipiartz-syenite. Then «aine deep-seated protrusions of liighly acid rocks, varieties of granite, containing from 68 to 75 per cent of silica. The youngest eruptive ’basses in the district show a complete change of character. They are
basic dykes (proterobase, diabase, etc.).^
The same author institutes a comparison between the post-biliirian eruptive series of Christiania and that of the Triassic system in the Tyrol, *rnd believes that the two cycles closely agree."
Durino’ Tertiary time in Central Prance more than one cycle may le loade 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, phonohtes ®rui basalts. The Pliocene eruptions showed a reversion to the inter- ruediate types of augitic andesites and trachytes, followed by abundant basalts, which continued to be poured forth in Pleistocene time.'*
Further north, in Auvergne, where the eruptions come down to a later period, the volcanic sequence appears to have been first a s^omewhat acn g’-oup of lavas (trachytes or domites), followed by a_ series^ of basalts, then andesites and labradorites, the latest outflows again consisting oi basalts. Not less striking is the succession of lavas in the Yellowstone region, described by Mr. Iddings. The first eruptions consisted of andesites. These were followed by abundant discharges of basalt, succeeded by later o*itflows of andesite, and of basalt like that previously erupted. After a Puriod of extensive erosion, occupying a prolonged interval of tune, volcanic energy was renewed by the eruption of a vast flood of rhyolite, after winch eanie a feebler outflow of basalt that brought the cycle to a close, thougli §eysers and fumaroles show that the volcanic fires are not yet entirely
extinguished below.®
But not only is there evidence of a remarkable evolution or succession erupted material within the volcanic cycle of a single geological period, ^ne of the objects of the present work is to bring forward proofs that such
* Eruplivqest. Kristianmjeb., 1895. , . ,
' Op. cit. He supposes in eaeli case the pre-existenco of a parent magma ^
'"'■aptive series started and wliich had a silica-percentage of about 64 or 65. In this difficult it is of the utmost iiiiportanee to accumulate fact before proceeding to speculation , ■' M, Boiile, “Description Geologique du Velay,” -Buff. Carle. OM. Fram’c, 1892. Hus authoi ^raws special attention to the evidence for the alternation of basic and mure acid mateiial in tl Mtiary eruptions of Central France.
■* M. Michel Levy, LnU. Soc. Giol. France, 1890, p. 704. ,1,
Journal of rjloqy. Chicago, i. (1893) p. 606. See also this author s “Electric Peak and Sepulchre Mountain,” mh Ann. Rep. U.S. Gcol. Surrey (1890-91X and 7' H. W. Turner on “The Succession of Tertiary Volcanic Rocks 111 the Sierra Nevada oi IN ■^’Uorica,” 14ft Ann. Itcp. U.S. Gcol. Surrey (1892-93), p. 493.
' North
3°
VOLCANIC PRODUCTS
BOOK I
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 lise ultimately to a similar succession of erupted materials.
If we are at a loss to account for the changes in the secpience 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 lietween the differentiation which has taken place within a plutonic intrusive boss or sill and the secpience of lavas in volcanic cycles. It can be shown that though the original magma that supplied tlie 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 con- stitutents 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 sec[uence of variations in composition. Hut this would not meet all the difficulties of the case, nor explain the determining cause of the separation of the constituents within tlie 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. Notwith- standing 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 contribu- tion 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 pi’oducts 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 Palaeozoic 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 strilcing instance, I may cite here the most ancient and most modern lavas of the Grand Canon of the Colorado. Mr. Walcott and Air. 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.^
’ \ith Animal Jleport V.S. Ccol. Survey (1892-93).
CHAP, in
AGGLOMERATES, BRECCIAS AND TUFFS
3'
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 llhyolites a-nd Felsites, but the vitreous forms are probably all intrusive. The inter- mediate series is represented by Trachytes and Andesites (Porphyrites), which range from a glassy to a holoerystalline structure. The basic series includes Dolerites, Diabases, Basalts, Limburgites (or Magma-basalts, con- faining little or no felspar), and Fieri tes 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 Agents are known as Agglomerates where they show no definite arrangement, and sspecially where they actually fill up the old funnels of discharge. MTien iiiey have accumulated in sheets or strata of angular detritus outside an f'Ctive vent they are termed Breccias, or if their component stones have keen water-worn. Conglomerates. The finer ejected materials may be com- prehended 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 pro- Tueed may contain, indeed, little or no tiuly volcanic material, but may be *iiade 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, h' may happen that the only discharges from it consisted merely of non- ^oleanic debris. Examples of this kind have been cited from various old ''oleauic 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 l®et, are full of pieces of the Arclnean rocks which underlie the younger formations of that district.^ These non-volcanic stones do not occur aimTiig fhe breccias higher up. Obviously, however, though most abundant at krst, pieces of the underlying rocks may reappear in subsequent discharges, ''vherever by the energy of explosion, fragments are broken from the walls a volcanic chimney and hurled out of the crater. Illustrations of these features will be given in the account of the British Carboniferous, Permian Tertiary volcanic rocks.
ft will be obvious that where the component materials of such frag- ^‘lentary accumulations consist entirely of non-volcanic rocks, great caution be exercised in attributing them to volcanic agency. Two sources of ’ Prof. J. P. Iddiiigs, XWiAnn. Rep. U.S. Geol. Survey (1890-91), p. 634.
32
VOLCANIC PRODUCTS
BOOK I
error in such cases may here be pointed out. In the first place, where angular detritus has been precipitated into still w'ater, as, lor instance, from a crag or rocky declivity into a lake, a very coarse and tumultuous l^-ind of breccia may be formed. It is conceivalde 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 w’hich have been mapped and described as volcanic tuffs or agglomerates are now known to be only examples of “ crush-conglomerates.” ^
Not only have vast quantities of detritus of non-volcanic rocks been sliot forth from volcanic vents, but sometimes enormous solid masses of rock have been brought up by ascending lavas or have been ejected by ex- plosive vapours. .Every visitor to the puys of Auvergne will remember the oreat cliff-like prominence of granite and mica-schist which, as described long ago by Scrope, lias been carried up by the trachyte of the I’uy Chopine, and forms one of the summits of the dome (Fig. 344). The same pheno- menon is observable at the Euy de Montchar, wdiere large blocks of gianite have been transported from the underlying platform. Abich has descriljed some remarkable examples in the region of Erzeroum. The huge crater ot Palandokiin, 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 gabl.u'os of the underlying non-volcanic platform. These rocks, wdiich form an integral part of the structure of the crater, have been carried up by masses of trachydoleritic, andesitic and quartz-trachytic lavas.'^ Examples will be given in a later chapter showing- how gigantic blocks of mica-schist and other rocks have been carried man} hundred feet irpwnrds and buried among sheets of lava or masses of agglo- merates dui’ing the Tertiary volcanic period in liritaiir (iig. 262).
' For an account of “ crush -conglomerates, ” sec Mr. Lainplngli’s paper on those of the Isle of Man, Quart. Journ. <leol. Soc., 11. (189.5), p. 563. Mr. M ‘Henry has pointed to probable cases of mistake of this kind in Ireland, Natii.ir, 5th March 1896. A. Gcikie, Gcol. Mag. November 1896.
2 Abich, Geologic des Armmwclien Moch/cmdcs (Part ii., -vvestern hall'), 1882, p. 76.
chap, in
ORIGIN OF TUFFS
33
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 varioris 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 he 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 i'e spurted out in lumps of slag and bombs, or may he violently Idown out ill the form of minute lapilli, or of extremely fine dust and ashes.
Althougli in theory these several varieties of origin may lie 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 decom- position, 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 inicrolites or crystals, or of some vitreous or semi-vitreous substance, in which such inicrolites 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 he taken as good evidence of former volcanic activity.
i\Iost commonly, especially in the case of tuffs of high anticpiity, like those associated with the Pakeozoic formations, the fresh glassy and micro- htic constituents, so conspicuous in modern volcanic ashes, are hardly to he Recognised. The finer dust which no doubt contained these ehai’acteristic substances has generally passed into dull, earthy, granular, or structureless material, though here and there, among basic tuffs, remaining as palagonite. hi the midst of this decayed matrix, the lapilli of disrupted lavas may uudure, hut it may lie 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 maiPedly volcanic peculiarity among the lavas, is not less so in their fragments among the '''■ogloinerates, breccias and tuffs ; indeed it may he said to be eminently eharacteristic of them. The vesicles in the lapilli, bombs, and blocks are sometimes of large size, as in masses of ejected slag, hut tliey range down to niicroscopic minuteness. The lapilli of many old tuffs are sometimes ‘30 crowded with such minute pores, as to show that they were originally true pumice.
Phe composition of tuffs must obviously depend upon that of the lavas VOL. 1 D
34
VOLCANIC PRODUCTS
liOOK I
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 tuff's may be expected to he also acid, while among intermediate or basic lavas, tlie tuffs will generally be found to correspond. There are, however, exceptions to this general rule. As will he afterwaixls described in detail, abundant felsitic tuffs may be seen among the andesitic lavas of Lower Old Bed 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 lirick-red, purple and brown.
The acid tuffs are, on tlie whole, paler in colour than the others, some- times indeed they are white or pale grey, but graduate into tones of hicmatitic 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 how-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.
Thrown out promiscuously from active vents, the materials that form
tub's 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 hnest extending farthest from it. As the volcanoes of which records have been preserved among the geo- logical formations were chiehy subaqueous, the fragmentary substances, as they fell into water, w'ould 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 stratihed arrangement.
Fkj 13. — Alternations of coarser ami liner waVCS Tuff.
Moreover, as during an eruption there might be successii'e paroxysms of violence in the discharges, coarser and finer detritus would successively fall over the same spot. In this way, rapid iilternatious 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 inst;ince, layers of gravel and sand repeatedly alternate. Besides the volcanic nature of tlie fragments and their non-
CHAP. Ill
NATURE OF TUFFS
35
water-worn forms, he will notice that here and there the larger blocks may he 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 I’aheozoic volcanic chronicles of Britain. 1 may especially cite the liasin of the Firth of Forth as a classical region for the study of Carboniferous examples.
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 witli any ordinary sediment that may be in course of deposition at the time. There will thus l)e a blending of volcanic and non - volcanic detritus, and the transition between the two may l)e so insensible that no hard hire of demarcation can lie 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 tliis gradation of material, and likewise the frequently recurring alternations of true tuff with shale, sandstone, limestone or other entirely lion-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 ac- tivity, or at least of considerable distance from the focus of discharge. It IS sometimes possible to trace such intermixtures through gradually augment- ing proportions of volcanic dust and stones, until the deposit becomes wholly volcanic in composition, and so coarse in texture as to indicate the prox- imity 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 entii'ely non-volcanic.
The occurrence of thin partings of tulf 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 tlie relative duration of the periods of volcanic repose.
Tn^rriiTOiniTn ^
a Q ^ ® “■»» OT O • „ I" • . ^
v>. O O*^- •* -O
Fig. 14. — Alternations of Tuff (/, ^,) with nou-volcanic sediment (/, V).
36
VOLCANIC PRODUCTS
BOOK 1
A little reflection will convince the observer that in such a section as that represented in Fig. 14 the volcanic intercalations must he 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 (/, 1). 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 tlie intercalated bands of
tuff ($, t).
The application of these rules of geological evidence will be best under- stood from actual examples of their use. Many illustrations of tliem^ will be subsequently given, more especially from the volcanic records of the Carboniferous peifod.
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 sedi- ment 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 fortli farther on.
Tuffs, as already remarked, frecpiently occur without any accompani- ment 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 intercala- tions. 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 Baheozoic 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
CHAP. Ill
VOLCANIC CONGLOMERATES
37
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 (jf 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
Fig. 15. — Ejected block of Basalt wliicli has fallen among Carboniferous shales and limestones,
shore, Pettycur, Fife.
which it fell, and when la^'as and tuff's 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.
In conclusion, reference may conveniently be made here to another Variety of fragmental r'olcanic materials which cannot always be satisfac- torily distinguished from true tuffs, although arising from a thoroughly non-volcanic agency. Where a mass of lava has been exposed to denuda- tion, 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 conglomer- ate, 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, wms 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 inter-
38
VOLCANIC PRODUCTS
ISOOK I
calated in the same sedimentary series with wliich 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 contemporaneoxisly existing volcanic heaps. Examples of such intercalated conglomerates will be given from the Lower Old Tted Sandstone of Central Scotland and from the Tertiary volcanic plateaux of the Inner Hebrides.
CHAPTER IV
Materials erupted at the Surface— Extrusive Series— cowtmMed. iii. Types of old Volcanoes
1. The Vesnviaii Type ; 2. The Plateau or Fissure Type ; 3. The Pny Tyire. iv.
Determination of the I'elative 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 tliese orifices them- selves, 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 Jlritish 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 redeetion 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 foi 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 Vesiivian type ; 2nd, the I’lateaii or Fissure type ; and drd, the Fiiy type.
1. The Vesneian 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, liarasitic 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
40
TYPES OF VOLCANOES
KOOK I
distance from the crater, these volcanic materials might he 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 fje found interstratilied with layers of ordinary sediment containing remains of the animal or vegetable life of the time.
f’ozrceive 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, he 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
l a I (I pad/ p / p
Fig. 16. — EtTects of d«TiiuUition on a Vesuvian cone.
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 diagranimatically 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 (/, /), the dykes id,d), and the lavas and agglomer-
ates (a, a) of the central vent and of the subordinate cones.
I’he 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. AVe 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.
CHAP. IV
VESUVIAN TYPE
41
Every stage in this process of eflaceinent may he 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 Prance. 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. X^alleys 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 Palieozoic systems ot 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 w’ere 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 oidy 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 diflerent ages, and lie, therefore, on many successive platforms within the crust of the earth : their places ha^ e been still further de])endent upon changes in the arrangement of that crust. Fracture, upheaval, depression, curvature, unconforruable 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 chapteivs have been recorded with the most surprising minuteness, so that the events which they reveal can be realized as vividly as those ot a modern volcano. Others, again, are meagi’e and fragmentary, because the chronicle is still for the most part buried underground, or because it has been so long e.xposed at the surface that only fragments of it now remain there.
42
TYPES OF VOLCANOES
HOOK I
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 Paheozoic time, and at many distinct centres, lavas and tufts 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 volcaidc 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 accumu- lated 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 Bed Sandstone volcanoes of Central Scotland.
2. The ricUeau or Fmure 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 bur’ied 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 How, each river-bed being in turn sealed up in lava, with all its gravels.
CHAP. IV
PLATEAU OR FISSURE TYPE
43
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 cvit 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. Ihe 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.
This geological history is illustrated by the diagram in Fig. 17. The stippled ground underneath (a:, .*) 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 ac- cumulation of volcanic material. Tire 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 wdiich 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 Bed 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 ai’e even still often approximately horizontal, and lie piled on each other in tolerably regulai beds to a thickness of 1000, and in one place to more than 3000 feet.
44
TYPES OF VOLCANOES
BOOK I
They form wide level or gently undulating tablelands, which rise in bold escarpments from the surrounding coimtry and have been deeply carved into valleys. The sides of their clihs and slopes are marked by parallel lines of terrace, arising from the outcrop of successive sheets of lava (kigs. 11, :165).
With the Tertiary basalt-plateaux are connected thousands of dykes, that follow each other along nearly parallel lines in a general north-westerly direc- tion, 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 tliose 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.
a. The Puy type, as before remarked, takes its name from the well- known or volcanic cones, of Central Prance. 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 Pigs. 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 eflaced.
Prom 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, &nd 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 I'est,^ though, in some cases, the cone may have been thrown up after the emission of the lava.
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 1 M. Boule, Bull. Carle Gcol. France, Ifo. 28, tome iv. p. 232.
CHAP. IV
PUY TYPE
45
and ashes. Hence the products of such a vent group themselves into three layers— two of breccia separated by an intervening sheet of lava.^
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 he effaced, and the lava itself may he reduced to merely a few separate patches, perhaps capping a rid"e, while the surrounding ground has been hollowed into valleys. It the waste continues long enough, even these outliers will disappear, and nothing but the neck or stump of the little volcano will remain.
H’he accompanying diagram (Tig. lH) 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 sur- roundino- 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 super- ficial volcanic material, but have made some progress in lowering the level of the ground on which that material was accumulated.
The tyyjical area for tlie 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 ; else- where 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 e.xcavated since they flowed."
1 M. Boiile, Bull. CaHc Giul. France, No. 28, tome iv.
^ Sec DeHiaarest’s classic map anil his pajiers in Mcm. Acad. Jouni. dc Physiqiic^ l77^^ ; Hcrope’s (k.olugy of VeMtral l^rancc.
Jloil. Sciences^ Paris, 1774, 1/79 ;
, 1827, and E-ctinct Volcanoes of
46
VOLCANIC CHRONOLOGY
BOOK 1
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^ One of the most remarkable denuded puy-regions in Europe covers a wide territory in the Swabian Alps of Wtirtemberg. 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 Tipper Miocene strata, and have thus probably been exposed to prolonged denudation. In external a,spect and internal structure they present the closest parallel to the Carboniferous and Permian “necks ” of Britain described in Books VI. and VI 1. of the present work."
Among the Pakcozoic 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 appeal-. 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 w-ater, 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 lava.s were one by one submei-ged, 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 Paheozoic 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.
iv. DETERMINATION OF THE RELATIVE UEOLOGICAL DATES OF ANCIENT
VOLCANOES
In themselves, accumulation.s 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 Palieozoic ages may, indeed, on careful examination, be distinguished from those of Tertiary date, but, as w'e 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
Ventral Frmiee, 1858 ; Leoiwps jijwqncs Giologiqucs de V Av.rergnc, 1867 ; M. Michel Levy, BnlL Roc. Oiol. France, 1890, p. 688 ; iM. Honle, Ihill. Va.rte lUol. France, No. 28, tome iv. 1892.
' Tile Eifel district lias lieoii fully deswibed by llibbert. Von Decheii, and other writers. You Uecheu’s little handbooks to the Eifel and Siebengebirge are useful guides.
These Wilrtomberg vent.s have been elaborately described and discussed by Professor VV. Hraiico of Tubingen in his Rchwahens 7^7 Vulkan-Eiiihnjoneti vnd dcren tufe.rfuUc Ansbruche- ruhren, das grOssle (lebiel ehcnutligcr Aaare anf der Frdc, Stuttgart, 1894.
CHAP. IV
VOLCANIC CHRONOLOGY
47
igneovis magmas whicli supply volcanic vents thus appear to have been very much what they are now from early geological epochs. At least no im- portant 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 w’hich 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 tufts in the upper part of Snowdon various fossils occur, which are identical with those found in the well-lviiown Bala Lime- stone. 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 establisheil 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 tuft's and lavas ejected from many scattei-ed pays were interstratified among shales and limestones in which the charac- teristic 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 w^ere found associated with the volcanic rocks. But here and there, intercalated between the basalts, lie layers of shale, limestone and tuff con- taining well-preserved remains of plants which are recognizable as older Tertiary forms of vegetation. TIris fossil evidence definitely places the date of the eruptions in older Tertiary time.
It i.s clear that, proceeding on this basis of reasoning, we may arrange the successive volcanic eruptions of any given district, make out their order of se([uence in time, and thus obtain materials for a consecutive history oi 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
48
GEOGRAPHY OF OLD VOLCANOES
BOOK I
chronology of the volcanic rocks of the British Isles has been worked out, and that the following chapters luive been arranged.
V. HOW THE PHYSICAL GEOGRAPHY ASSOCIATED WITH ANCIENT VOLCANOES IS ASCERTAINED
Wliile 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 he 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.
1. Siihinarine 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 i)lainly 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
of ijivas tiiid tuffs with liuic- and houiTshed as abundantly as evei above the
•stones and shales full of marine
organisms. ’ .
The accompanying diagram (lig. 19) illustrates these statements. At the bottom a thick mass of limestone (1) 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 <lust and stones, which fell over the bottom find prevented the continuation
mnvhiA priiTifinn.>< • {ilfi‘riifi.t.ifvii.>i
CHAP. IV
LACUSTRINE AND FLUVIATILE ERUPTIONS
49
of the limestone, lint 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 streani of slaggy lava, which rolled over the sea-bottom and solidified in the thick sheet of amydaloidal basalt marked B. This outliow 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 tlie bottom. Yet the volcanic history had not then reached its close, for again there came a discharge of ashes, followed by tlie outpouring of a second lava, which consolidated as a sheet of columnar basalt (!’/).
It is not necessary, in order to prove the eruptions to have been sub- marine, 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 adjoin- ing sea-fioor.
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 con- fident 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 Bed 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.
3. Fluviatik EnqMons. — Volcanoes have sometimes arisen on river- plains or on the edges of valleys ( .dFLJjjH
I ^
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 fiuviatile sand or shingle, containing per- haps 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 fiuviatile sediment at many successive levels among the lavas and tuffs. In Fig. 20 some VOL. I E
Eig. 20. — Diagram illu.stratiiig volcanic eruptions on a river-plain.
5°
GEOGRAPHY OF OLD VOLCANOES
BOOK I
of these changes are indicated in a series of bedded lavas (1). 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 inter- calation 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 outhows 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 plateau.x of Britain in Chapter xxxviii.
4. Terreatrml 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. Tn 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 sediineirt full of land-plants, with possiblj' traces of insects and other terrestrial organisms, we nray with some confidence infer that these silted-up hollows represeirt pools or lakes that gathered on the surface of the lava-sheets, and into which tire vegetation of the surrorrirdirrg ground was blowir or washed. Bain fallirrg orr tire rugged sirrface of a lava-field worrld 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.
Again, it nray be expected that prolonged exposure to the air would give
rise to disiirtegratioir of the lava and to the consequeirt fortrra- tion of soil. Terrestrial vegeta- tion would naturally spriirg up on such soil ; trees might take root upoir it. Heirce, if airother lava-flood deluged the surface, the soil and its vegetable mantle would be eirtonrbed under the molten rock.
These geological changes are represented diagramirratically in Fig. 21. Two hollows among the lavas are there showir to have beerr filled with silt, iircludiirg srrccessive layers of vegetation now corrverted irrto coal. Oire of the soils (.S-) is marked between the lavas, aird the charred strrrrrp 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
Fig. 21. — Duagrain illustrating volcanic eruptions on a land-surface.
CHAP. IV TERRESTRIAL ERUPTIONS 51
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 Hook VIII.
CHAPTER V
Underground Hiases of Volcanic Action. B. Materials injected or consolidated lieneath tlie 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. Metamorpbism in and around Volcanic Cones, Solfataric Action ; V. Inward Dip of Rocks towards Necks ; vi. Influence of contemporaneous Denuda- tion 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 sub- stances 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 composi- tion 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 iissures and vents distributed from the south of Antrim north- ward beyond the Inner Hebrides, through the chain of the Faroe Islands and over the whole breadth of Iceland. Ij nder 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 witliin 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'
CHAP. V
ERUPTIVE VENTS
53
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.
Home 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. Texts of Euuptjon
It is a general belief that the first stage in the formation of a \’olcano 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. Tlie actual fissure, however, is not there seen, and its existence is only a matter of pi'obable inference. Undoubtedly the effect of successive eruptions must be. to con- ceal 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 sulisequent history of the moun- tain. During the convulsive shocks that precede and accompany an erup- tion, the sides of the cone, and even sometimes part of the ground Iteyond, 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 scoria? usually form upon them. Tliis 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. Wliat 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 liand, and the amount and persistence of volcanic energy on the other. We may suppose that where a fi.ssure 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 com-
/
54
SUBTERRANEAN VOLCANIC ACTION
liOOK I
plete 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 orihee 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 opep fissure at the surface has not been generally observed. When, for example, Monte Nnovo 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 readied 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 Buy 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 solidity 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 belong- ing 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 circrdar or elliptical. If that portion of the crust of the earth througli 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 ot the terrestrial crust vary greatly iu 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 Garboniferous 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 tlie 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
CHAP.
VOLCANIC VENTS
55
“ 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).
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 to- gether. In the case of large central volcanoes like Etna, where many subsidiary vents, some of them form- ing not inconsiderable hills, may spring up along the sides of the parent cone, denudation will ulti- mately remove all the material tliat was heaped up on the surface, and leave the stumps or necks of the pai’asitic vents in groups around the central funnel.
Each volcanic chimney, by whicli va])ours, 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 erup- tions 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 rro matter how great may be the derrudation of the volcarro, or how extensive rrray be the removal of the various nraterials discharged over the surrounding grourrd, the pipe or funnel with its column of solid rock must still renrain. No amount of waste of the surface of the land can efface that colurirn. Successively lower and yet lower levels may be laid bare in it, but the colurrrn itself goes still fui’ther down. It will contirrue to rrrake its appeararrce at the surface until its roots are laid bare in the lava of the subterranean magma. Hence, of all the relics of volcarric action, the filled-up chunney 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.
Tlie stumps of volcanic columns of this nature, after prolonged denuda- tion, generally project above the surrounding ground as rounded or conical
1. Linhope Burn, near Mosspaul, Roxburghshire ; the shaded parts are intrusioufi of trachytie luaterial. 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. Heail of Routing Burn, S.E. side of Harts- garth Fell, Liddesdale. 8. Hart.sgarth Flow, Liddes- tlale.
56
SUBTERRANEAN VOLCANIC ACTION
BOOK I
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 he found not to
Fig. 23. — View of aii old voleanie “ Neek ” (The Knock, Largs, Ayrshire, a vent of Lower Carboniferous age).
form a hill, but to sink into a hollow. Such variations, liowever, have little or no reference to original volcanic contours in the history of the localities which display them. They arise mainly from the differing hard- ness and structure of the materials that have filled the vents, and the con- sequent diversity in the amount of resistance which they have offered to the progress of denudation.
Tlie materials now found in volcanic funnels are of two kinds : 1st, Fragmentary, derived from volcanic explosions; and 2nd, Lava-form, arising from tlie ascent and consolidation of molten rock within the funnel.
i. JVecks 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 tlie underground protrusions of volcanoes in their
CHAP. V
VOLCANIC NECKS
57
neighbourliood. 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, wliere the outpouring of lava or some other cause has left cavernous spaces within the crust, there may conceivably he some feeble explosion there, and some trilling accumula- tion 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 hail 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. Tlie 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 tlie first convulsive eflorts of a volcanic focus to find a. vent at the surface, the explosions that eventu- ally form tlm 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 hack into the chimney, either to accumulate there, should the ex- plosions cease, or to lie 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 withorit any sensible discharge of volcanic “ ashes ” from that magma. In such cases, as I have already pointed out, tlie 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 he 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 he 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.
58
SUBTERRANEAN VOLCANIC ACTION
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 tlie 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 tire 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 he formed by an engulphment or subsidence of the material, hke that which has taken place at the great lava caiddron of Hawaii, still an active volcano. The picturesque Crater Lake of Oregon is an admirable instance of this structure.
(2) Nechn of Agglomurate or Tuff. — In the vast majority of cases, the ex- plosions 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 uAially followed by an uprise of the lava within the pipe. Eelieved 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 e.xpansion 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-foran again and again.
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 al)undant 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 erup- tions 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
Fig. 24. — Section of neck of agglomerate, rising tlirougli sandstones aiul sliales.
CHAP. V
AGGLOMERATE NECKS
59
and crests. If submerged in the sea or a lake, the cones have been washed down, and tlieir 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 (Tig. 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. 7 8). 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 tlie agglomerate cannot be distinguished from those gathered from the strata lying in sitw 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 l)y 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. Jlany of them may have been broken ott' from the layer of congealed lava that partially coated the rough walls of the funuel 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 explo.sions. 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 scoriae. These have generally come from the spongy crust or upper part of the lava where the imprisoned steam, relieved from pressui'c, is able to expand and gather into vesicles.
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SUBTERRANEAN VOLCANIC ACTION
BOOK I
Less fi'equeiitly 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 lace 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 Idocks 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 ilrawn round the gyrating inass, and would rapidly cool there.
In other cases, we encounter true vf>lcanic bombs, that is, rounded or Ijomb-shaped blocks of lava, with their vesicles elongated all round them and conforming to theii’ spherical shajie. Sometimes such blocks are singularly vesicular in the centre, with a more close-grained crust on the outside. Their rapid centrifugal motion during llight 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-lik'e subangular or rounded pieces of lava in the agglomerates. These include lumps of line grain not specially vesicular, and probably derived from the disruption of solidified rock. But in many agglomerates, esp(icially those associated with the o^itpouring 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 t\iffs. 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 e.xtreme minute- ness 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 outllowiug lavas, or even of the scoria3 in the agglomerates.
'This characteristic minutely vesicular pumice is basic in composition. tVhore 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 A'elay, the fragments consist of a dark very basic glass, which encloses such a nmltitude of minute steam-cavities that, when seen under the microscope, they are found to he separated from each other hy walls so thin
CHAP. V
AGGLOMERATE NECKS
6i
that the slice looks like a pattern of delicate laced In necks of earlier date, such as those of older Tertiary, and still more of Palaeozoic, time, the glass has generally been altered into some palagonitie material.
This finely puniiceous 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 1 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.
Tlie 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 neclc of agglomerate or tuff has almost always a close relation to that of any lavas which may have been emitted from tliat vent. If the lavas have been of an acid character, such as rhyolites, felsites or obsidians, the pyroclastic materials will almost always be foiuid 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 ob.served, as, for example, among the volcanoes of the Old Hed 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 1