Transcriber Note
Text emphasis denoted by _Italics_ and =Bold=.
THE ANCIENT VOLCANOES OF GREAT BRITAIN
[Illustration]
THE
ANCIENT VOLCANOES
OF
GREAT BRITAIN
BY
SIR ARCHIBALD GEIKIE, F.R.S.
D.C.L. Oxf., D. Sc. Camb., Dubl.; LL.D. St. And., Edinb.
DIRECTOR-GENERAL OF THE GEOLOGICAL SURVEY OF GREAT BRITAIN AND IRELAND;
CORRESPONDENT OF THE INSTITUTE OF FRANCE;
OF THE ACADEMIES OF BERLIN, VIENNA, MUNICH, TURIN, BELGIUM, STOCKHOLM,
GÖTTINGEN, NEW YORK; OF THE IMPERIAL MINERALOGICAL SOCIETY AND SOCIETY
OF NATURALISTS, ST. PETERSBURG; NATURAL HISTORY SOCIETY, MOSCOW;
SCIENTIFIC SOCIETY, CHRISTIANIA; AMERICAN PHILOSOPHICAL SOCIETY;
OF THE GEOLOGICAL SOCIETIES OF LONDON, FRANCE, BELGIUM, STOCKHOLM, ETC.
WITH SEVEN MAPS AND NUMEROUS ILLUSTRATIONS
IN TWO VOLUMES
VOL. I
London
MACMILLAN AND CO., Limited.
NEW YORK: THE MACMILLAN COMPANY.
1897
_All rights reserved_
TO
M. Ferdinand Fouqué
MEMBER OF THE INSTITUTE
PROFESSOR OF THE NATURAL HISTORY OF INORGANIC BODIES
IN THE COLLÈGE DE FRANCE
AND
M. Auguste Michel-Lévy
MEMBER OF THE INSTITUTE
DIRECTOR OF THE GEOLOGICAL SURVEY OF FRANCE
DISTINGUISHED REPRESENTATIVES
OF THAT FRENCH SCHOOL OF GEOLOGY
WHICH BY THE HANDS OF DESMAREST FOUNDED THE
STUDY OF ANCIENT VOLCANOES
AND HAS SINCE DONE SO MUCH TO
PROMOTE ITS PROGRESS
THESE VOLUMES ARE INSCRIBED
WITH THE HIGHEST ADMIRATION AND
ESTEEM
PREFACE
In no department of science is the slow and chequered progress of
investigation more conspicuous than in that branch of Geology which
treats of volcanoes. Although from the earliest dawn of history, men
had been familiar with the stupendous events of volcanic eruptions,
they were singularly slow in recognizing these phenomena as definite
and important parts of the natural history of the earth. Even within
the present century, the dominant geological school in Europe
taught that volcanoes were mere accidents, due to the combustion of
subterranean beds of coal casually set on fire by lightning, or by the
decomposition of pyrites. Burning mountains, as they were called, were
believed to be only local and fortuitous appearances, depending on the
position of the coal-fields, and having no essential connection with
the internal structure and past condition of our planet. So long as
such fantastic conceptions prevailed, it was impossible that any solid
progress could be made in this branch of science. A juster appreciation
of the nature of the earth's interior was needed before men could
recognize that volcanic action had once been vigorous and prolonged in
many countries, where no remains of volcanoes can now be seen.
To France, which has led the way in so many departments of human
inquiry, belongs the merit of having laid the foundations of the
systematic study of ancient volcanoes. Her groups of Puys furnished the
earliest inspiration in this subject, and have ever since been classic
ground to which the geological pilgrim has made his way from all
parts of the world. As far back as the year 1752, Guettard recognised
that these marvellous hills were volcanic cones that had poured forth
streams of lava. But it was reserved for Desmarest twelve years later
to examine the question in detail, and to establish the investigation
of former volcanic action upon a broad and firm basis of careful
observation and sagacious inference. His method of research was as
well conceived as the region of Auvergne was admirably fitted to be
the field of exploration. He soon discovered that the volcanoes of
Central France were not all of one age, but had made their appearance
in a long series, whereof the individual members became less perfect
and distinct in proportion to their antiquity. Beginning with the
cones, craters, and lava-streams which stand out so fresh that they
might almost be supposed to have been erupted only a few generations
ago, Desmarest traced the volcanic series backward in time, through
successive stages of the decay and degradation wrought upon them by
the influence of the atmosphere, rain and running water. He was thus
able, as it were, to watch the gradual obliteration of the cones, the
removal of the ashes and scoriæ, and the erosion of the lava-streams,
until he could point to mere isolated remnants of lava, perched upon
the hills, and overlooking the valleys which had been excavated through
them. He showed how every step in this process of denudation could be
illustrated by examples of its occurrence in Auvergne, and how, in this
way, the various eruptions could be grouped according to their place in
the chronological sequence. To this illustrious Frenchman geology is
thus indebted, not only for the foundation of the scientific study of
former volcanic action, but for the first carefully worked out example
of the potency of subærial erosion in the excavation of valleys and the
transformation of the scenery of the land.
While these fruitful researches were in progress in France, others of
hardly less moment were advancing in Scotland. There likewise Nature
had provided ample material to arrest the attention of all who cared
to make themselves acquainted with the past history of our globe.
Hutton, as a part of his immortal _Theory of the Earth_, had conceived
the idea that much molten material had been injected from below into
the terrestrial crust, and he had found many proofs of such intrusion
among the rocks alike of the Lowlands and Highlands of his native
country. His observations, confirmed and extended by Playfair and Hall,
and subsequently by Macculloch, opened up the investigation of the
subterranean phases of ancient volcanic action.
Under the influence of these great pioneers, volcanic geology would
have made steady and perhaps rapid progress in the later decades
of last century, and the earlier years of the present, but for the
theoretical views unfortunately adopted by Werner. That illustrious
teacher, to whom volcanoes seemed to be a blot on the system of
nature which he had devised, did all in his power to depreciate their
importance. Adopting the old and absurd notion that they were caused
by the combustion of coal under ground, he laboured to show that they
were mere modern accidents, and had no connection with his universal
formations. He proclaimed, as an obvious axiom in science, that the
basalts, so widely spread over Central and Western Europe, and which
the observations of Desmarest had shown to mark the sites of old
volcanoes, were really chemical precipitates from a primeval universal
ocean. Yet he had actually before him in Saxony examples of basalt
hills which entirely disprove his assertions.
Fortunately for the progress of natural knowledge, Werner disliked the
manual labour of penmanship. Consequently he wrote little. But his
wide range of acquirement, not in mineralogy only, his precision of
statement, his absolute certainty about the truth of his own opinions,
and his hardly disguised contempt for opinions that differed from them,
combined with his enthusiasm, eloquence and personal charm, fired
his pupils with emulation of his zeal and turned them into veritable
propagandists. Misled as to the structure of the country in which
their master taught, and undisciplined to investigate nature with an
impartial mind, they travelled into other lands for the purpose of
applying there the artificial system which they had learnt at Freiberg.
The methodical but cumbrous terminology in which Werner had trained
them was translated by them into their own languages, where it looked
still more uncouth than in its native German. Besides imbibing their
teacher's system, they acquired and even improved upon his somewhat
disdainful manner towards all conclusions different from those of the
Saxon Mining School.
Such was the spirit in which the pupils of Werner proceeded to set the
"geognosy" of Europe to rights. The views, announced by Desmarest,
that various rocks, far removed from any active volcano, were yet of
volcanic origin, had been slowly gaining ground when the militant
students from Saxony spread themselves over the Continent. These views,
however, being irreconcilable with the tenets enunciated from the
Freiberg Chair, were now either ignored or contemptuously rejected.
Werner's disciples loved to call themselves by their teacher's term
"geognosts," and claimed that they confined themselves to the strict
investigation of fact with regard to the structure of the earth, in
apparent unconsciousness that their terminology and methods were
founded on baseless assumptions and almost puerile hypotheses.
With such elements ready for controversy, it was no wonder that before
long a battle arose over the origin of basalt and the part played by
volcanoes in the past history of the globe. The disciples of Werner,
champions of a universal ocean and the deposition of everything from
water, were dubbed Neptunists, while their opponents, equally stubborn
in defence of the potency of volcanic fire, were known as Vulcanists or
Plutonists. For more than a generation this futile warfare was waged
with extraordinary bitterness--dogmatism and authority doing their best
to stop the progress of impartial observation and honest opinion.
One of the most notable incidents in the campaign is to be found in
the way in which the tide of battle was at last turned against the
Wernerians. Cuvier tells us that when some of the ardent upholders of
the Freiberg faith came to consult Desmarest, the old man, who took
no part in the fray, would only answer, "Go and see." He felt that in
his memoir and maps he had demonstrated the truth of his conclusions,
and that an unprejudiced observer had only to visit Auvergne to be
convinced.
By a curious irony of fate it was from that very Auvergne that the
light broke which finally chased away the Wernerian darkness, and it
was by two of Werner's most distinguished disciples that the reaction
was begun.
Daubuisson, a favourite pupil of the Freiberg professor, had written
and published at Paris in 1803 a volume on the Basalts of Saxony,
conceived in the true Wernerian spirit, and treating these rocks, as he
had been taught to regard then, as chemical precipitates from a former
universal ocean. In the following year the young and accomplished
Frenchman went to Auvergne and the Vivarais that he might see with
his own eyes the alleged proofs of the volcanic origin of basalt.
Greatly no doubt to his own surprise, he found these proofs to be
irrefragable. With praiseworthy frankness he lost no time in publicly
announcing his recantation of the Wernerian doctrine on the subject,
and ever afterwards he did good service in making the cause of truth
and progress prevail.
Still more sensational was the conversion of a yet more illustrious
prophet of the Freiberg school--the great Leopold von Buch. He too had
been educated in the strictest Wernerian faith. But eventually, after
a journey to Italy, he made his way to Auvergne in 1802, and there, in
presence of the astonishing volcanic records of that region, the scales
seem to have fallen from his eyes also. With evident reluctance he
began to doubt his master's teaching in regard to basalt and volcanoes.
He went into raptures over the clear presentation of volcanic phenomena
to be found in Central France, traced each detail among the puys, as
in the examination of a series of vast models, and remarked that while
we may infer what takes place at Vesuvius, we can actually see what
has transpired at the Puy de Pariou. With the enthusiasm of a convert
he rushed into the discussion of the phenomena, but somehow omitted to
make any mention of Desmarest, who had taught the truth so many years
before.
Impressed by the example of such men as Daubuisson and Von Buch, the
Wernerian disciples gradually slackened in zeal for their master's
tenets. They clung to their errors longer perhaps in Scotland than
anywhere else out of Germany--a singular paradox only explicable by
another personal influence. Jameson, trained at Freiberg, carried
thence to the University of Edinburgh the most implicit acceptance of
the tenets of the Saxon school, and continued to maintain the aqueous
origin of basalt for many years after the notion had been abandoned by
some of his most distinguished contemporaries. But the error, though it
died hard, was confessed at last even by Jameson.
After the close of this protracted and animated controversy the study
of former volcanic action resumed its place among the accepted subjects
of geological research. From the peculiarly favourable structure
of the country, Britain has been enabled to make many important
contributions to the investigation of the subject. De la Beche,
Murchison and Sedgwick led the way in recognizing, even among the most
ancient stratified formations of England and Wales, the records of
contemporaneous volcanoes and of their subterranean intrusions. Scrope
threw himself with ardour into the study of the volcanoes of Italy
and of Central France. Maclaren made known the structure of some of
the volcanic groups of the lowlands of Scotland. Ramsay, Selwyn, and
Jukes, following these pioneers, were the first to map out a Palæozoic
volcanic region in ample detail. Sorby, applying to the study of
rocks the method of microscopic examination by thin slices, devised
by William Nicol of Edinburgh for the study of fossil plants, opened
up a new and vast field in the domain of observational geology, and
furnished the geologist with a key to solve many of the problems of
volcanism. Thus, alike from the stratigraphical and petrographical
sides, the igneous rocks of this country have received constantly
increasing attention.
The present work is intended to offer a summary of what has now been
ascertained regarding the former volcanoes of the British Isles. The
subject has occupied much of my time and thought all through life.
Born among the crags that mark the sites of some of these volcanoes,
I was led in my boyhood to interest myself in their structure and
history. The fascination which they then exercised has lasted till now,
impelling me to make myself acquainted with the volcanic records all
over our islands, and to travel into the volcanic regions of Europe and
Western America for the purpose of gaining clearer conceptions of the
phenomena.
From time to time during a period of almost forty years I have
communicated chiefly to the Geological Society of London and the
Royal Society of Edinburgh the results of my researches. As materials
accumulated, the desire arose to combine them into a general narrative
of the whole progress of volcanic action from the remotest geological
periods down to the time when the latest eruptions ceased. An
opportunity of partially putting this design into execution occurred
when, as President of the Geological Society, the duty devolved upon
me of giving the Annual Addresses in 1891 and 1892. Within the limits
permissible to such essays, it was not possible to present more than
a full summary of the subject. Since that time I have continued my
researches in the field, especially among the Tertiary volcanic areas,
and have now expanded the two Addresses by the incorporation of a large
amount of new matter and of portions of my published papers.
In the onward march of science a book which is abreast of our knowledge
to-day begins to be left behind to-morrow. Nevertheless it may serve
a useful purpose if it does no more than make a definite presentation
of the condition of that knowledge at a particular time. Such a
statement becomes a kind of landmark by which subsequent progress may
be measured. It may also be of service in indicating the gaps that have
to be filled up, and the fields where fresh research may most hopefully
be undertaken.
I have to thank the Councils of the Royal Society of Edinburgh and
the Geological Society for their permission to use a number of the
illustrations which have accompanied my papers published in their
_Transactions_ and _Journal_. To Colonel Evans and Miss Thom of Canna
I am indebted for the photographs which they have kindly taken for me.
To those of my colleagues in the Geological Survey who have furnished
me with information my best thanks are due. Their contributions are
acknowledged where they have been made use of in the text.
The illustrations of these volumes are chiefly from my own note-books
and sketch-books. But besides the photographs just referred to, I have
availed myself of a series taken by Mr. Robert Lunn for the Geological
Survey among the volcanic districts of Central Scotland.
Geological Survey Office,
28 Jermyn Street, London,
_1st January 1897_.
CONTENTS
BOOK I
GENERAL PRINCIPLES AND METHODS OF INVESTIGATION
CHAPTER I
PAGE
Earliest Knowledge of Volcanoes--Their Influence on Mythology and
Superstition--Part taken by Volcanic Rocks in Scenery--Progress
of the Denudation of Volcanoes--Value of the Records of former
Volcanoes as illustrating Modern Volcanic Action--Favourable
Position of Britain for the Study of this Subject 1
CHAPTER II
The Nature and Causes of Volcanic Action--Modern Volcanoes 10
CHAPTER III
Ancient Volcanoes: Proofs of their existence derived from the
Nature of the Rocks erupted from the Earth's Interior. A.
Materials erupted at the Surface--Extrusive Series. i. Lavas,
their General Characters. Volcanic Cycles. ii. Agglomerates,
Breccias and Tuffs 14
CHAPTER IV
Materials erupted at the Surface--Extrusive Series--_continued_.
iii. Types of old Volcanoes--1. The Vesuvian Type; 2. The
Plateau or Fissure Type; 3. The Puy Type. iv. Determination
of the relative Geological Dates of Ancient Volcanoes. v. How
the Physical Geography associated with Ancient Volcanoes is
ascertained 39
CHAPTER V
Underground Phases of Volcanic Action. B. Materials injected or
consolidated beneath the Surface--Intrusive Series: I. Vents
of Eruption--i. Necks of Fragmentary Materials; ii. Necks of
Lava-form Materials; iii. Distribution of Vents in relation
to Geological Structure-Lines; iv. Metamorphism in and around
Volcanic Cones, Solfataric Action; v. Inward Dip of Rocks
towards Necks; vi. Influence of contemporaneous Denudation upon
Volcanic Cones; vii. Stages in the History of old Volcanic Vents 52
CHAPTER VI
Underground Phases of Volcanic Action--_continued_. II.
Subterranean Movements of the Magma: i. Dykes and Veins; ii.
Sills and Laccolites; iii. Bosses (Stocks, Culots), Conditions
that govern the Intrusion of Molten Rock within the Terrestrial
Crust 77
CHAPTER VII
Influence of Volcanic Rocks on the Scenery of the Land--Effects of
Denudation 100
BOOK II
VOLCANIC ACTION IN PRE-CAMBRIAN TIME
CHAPTER VIII
Pre-Cambrian Volcanoes
The Beginnings of Geological History--Difficulties in fixing on
a generally applicable Terminology--i. The Lewisian (Archæan)
Gneiss; ii. The Dalradian or Younger Schists of Scotland;
iii. The Gneisses and Schists of Anglesey; iv. The Uriconian
Volcanoes; v. The Malvern Volcano; vi. The Charnwood Forest
Volcano 109
BOOK III
THE CAMBRIAN VOLCANOES
CHAPTER IX
Characteristics of the Cambrian System in Britain
The Physical Geography of the Cambrian Period--The Pioneers of
Palæozoic Geology in Britain--Work of the Geological Survey in
Wales--Subdivisions of the Cambrian System in Britain 139
CHAPTER X
The Cambrian Volcanoes of South Wales 145
CHAPTER XI
The Cambrian Volcanoes of North Wales, the Malvern Hills and
Warwickshire 159
BOOK IV
THE SILURIAN VOLCANOES
CHAPTER XII
Characters of the Silurian System in Britain. The Arenig Volcanoes
The Land and Sea of Silurian time--Classification of the Silurian
System--General Petrography of the Silurian Volcanic Rocks--I.
The Eruptions of Arenig Age 173
CHAPTER XIII
The Eruptions of Llandeilo and Bala Age
i. The Builth Volcano--ii. The Volcanoes of Pembrokeshire--iii. The
Caernarvonshire Volcanoes of the Bala Period--iv. The Volcanic
District of the Berwyn Hills--v. The Volcanoes of Anglesey--vi.
The Volcanoes of the Lake District; Arenig to close of Bala
Period--vii. Upper Silurian (?) Volcanoes of Gloucestershire 202
CHAPTER XIV
The Silurian Volcanoes of Ireland 239
BOOK V
THE VOLCANOES OF DEVONIAN AND OLD RED SANDSTONE TIME
CHAPTER XV
The Devonian Volcanoes 257
CHAPTER XVI
The Volcanoes of the Old Red Sandstone
Geological Revolutions at the close of the Silurian Period Physical
Geography of the Old Red Sandstone--Old Lake-basins, their Flora
and Fauna--Abundance of Volcanoes--History of Investigation in
the Subject 263
CHAPTER XVII
Distribution of the Volcanic Centres in the Lower Old Red
Sandstone--Characters of the Materials Erupted by the Volcanoes 271
CHAPTER XVIII
Structure and Arrangement of the Lower Old Red Sandstone Volcanic
Rocks in the Field 281
CHAPTER XIX
Volcanoes of the Lower Old Red Sandstone of "Lake Caledonia"
Description of the several Volcanic Districts: "Lake Caledonia,"
its Chains of Volcanoes--The Northern Chain: Montrose
Group--Ochil and Sidlaw Hills--the Arran and Cantyre Centre--the
Ulster Centre 294
CHAPTER XX
Volcanoes of the Lower Old Red Sandstone of
"Lake Caledonia"--_continued_
The Southern Chain--The Pentland Volcano--The Biggar Centre--The
Duneaton Centre--The Ayrshire Volcanoes 317
CHAPTER XXI
Volcanoes of the Lower Old Red Sandstone of the Cheviot Hills,
Lorne, "Lake Orcadie" and Killarney 336
CHAPTER XXII
Volcanoes of the Upper Old Red Sandstone--The South-West of
Ireland, the North of Scotland 348
BOOK VI
THE CARBONIFEROUS VOLCANOES
CHAPTER XXIII
The Carboniferous System of Britain and its Volcanic Records
Geography and Scenery of the Carboniferous Period--Range of
Volcanic Eruptions during that time--I. The Carboniferous
Volcanoes of Scotland--Distribution, Arrangement and Local
Characters of the Carboniferous System in Scotland--Sketch
of the Work of previous Observers in this Subject 355
CHAPTER XXIV
Carboniferous Volcanic Plateaux of Scotland
I. The Plateau-type restricted to Scotland--i. Distribution in the
Different Areas of Eruption--ii. Nature of the Materials Erupted 367
CHAPTER XXV
Geological Structure of the Carboniferous Volcanic
Plateaux of Scotland
1. Bedded Lavas and Tuffs; Upper Limits and Original Areas and
Slopes of the Plateaux; 2. Vents; Necks of Agglomerate and Tuff;
Necks of Massive Rock; Composite Necks; 3. Dykes and Sills; 4.
Close of the Plateau-eruptions 383
CHAPTER XXVI
The Carboniferous Puys of Scotland
i. General Character and Distribution of the Puys; ii. Nature of
the Materials Erupted--Lavas Ejected at the Surface--Intrusive
Sheets--Necks and Dykes--Tuffs 414
CHAPTER XXVII
Geological Structure of the Carboniferous Puys of Scotland
1. Vents: Relation of the Necks to the Rocks through which they
rise--Evidence of the probable Subærial Character of some of
the Cones or Puys of Tuff--Entombment of the Volcanic Cones and
their Relation to the Superficial Ejections. 2. Bedded Tuffs and
Lavas--Effects of Subsequent Dislocations. 3. Sills, Bosses, and
Dykes 424
CHAPTER XXVIII
Illustrative Examples of the Carboniferous Puys of Scotland
The Basin of the Firth of Forth--North Ayrshire--Liddesdale 462
LIST OF ILLUSTRATIONS
FIG. PAGE
1. Vesicular structure, Lava from Ascension Island, slightly less
than natural size 15
2. Elongation and branching of steam-vesicles in a lava, Kilninian,
Isle of Mull, a little less than natural size 17
3. Microlites of the Pitchstone of Arran (magnified 70 diameters) 19
4. Perlitic structure in Felsitic Glass, Isle of Mull (magnified) 19
5. Spherulitic structure (magnified) 19
6. Micropegmatitic or Granophyric structure in Granophyre, Mull
(magnified) 20
7. Ophitic structure in Dolerite, Gortacloghan, Co. Derry
(magnified) 20
8. Variolitic or orbicular structure, Napoleonite, Corsica
(nat. size) 22
9. Flow-structure in Rhyolite, Antrim, slightly reduced 23
10. Lumpy, irregular trachytic lava-streams (Carboniferous), East
Linton, Haddingtonshire 24
11. View at the entrance of the Svinofjord, Faroe Islands,
illustrating the terraced forms assumed by basic lavas 25
12. Sack-like or pillow-form structure of basic lavas (Lower
Silurian), Bennan Head, Ballantrae, Ayrshire 26
13. Alternations of coarser and finer Tuff 34
14. Alternations of Tuff with non-volcanic sediment 35
15. Ejected block of basalt which has fallen among Carboniferous
shales and limestones, shore, Pettycur, Fife 37
16. Effects of denudation on a Vesuvian cone 40
17. Section to illustrate the structure of the Plateau type 43
18. Diagram illustrating the structure and denudation of Puys 45
19. Section illustrating submarine eruptions; alternations of lavas
and tuffs with limestones and shales full of marine organisms 48
20. Diagram illustrating volcanic eruptions on a river-plain 49
21. Diagram illustrating volcanic eruptions on a land-surface 50
22. Ground-plans of some volcanic vents from the Carboniferous
districts of Scotland 55
23. View of an old volcanic "Neck" (The Knock, Largs, Ayrshire, a
vent of Lower Carboniferous age) 56
24. Section of neck of agglomerate, rising through sandstones and
shales 58
25. Neck filled with stratified tuff 64
26. Section of neck of agglomerate with plug of lava 65
27. Section of agglomerate neck with dykes and veins 66
28. Section of neck filled with massive rock 68
29. Successive shiftings of vents giving rise to double or triple
cones 70
30. Section to show the connection of a neck with a cone and
surrounding bedded tuffs 71
31. Diagram illustrating the gradual emergence of buried volcanic
cones through the influence of prolonged denudation 75
32. Dyke, Vein, and Sill 80
33. Section of Sill or Intrusive Sheet 83
34. Ideal section of three Laccolites. (After Mr. Gilbert) 86
35. Diagram illustrating the stratigraphical relations of the
pre-Cambrian and Cambrian rocks of the North-west Highlands
of Scotland 112
36. Map of a portion of the Lewisian gneiss of Ross-shire 118
37. Section showing the position of sills in the mica-schist series
between Loch Tay and Amulree 124
38. Sketch of crushed basic igneous rock among the schists, E.
side of Porth-tywyn-mawr, E. side of Holyhead Straits 128
39. Section across the Uriconian series of Caer Caradoc 132
40. Map of the volcanic district of St. David's 146
41. Section showing the interstratification of tuff and
conglomerate above Lower Mill, St. David's 154
42. Basic dyke traversing quartz-porphyry and converted into a
kind of slate by cleavage. West side of Llyn Padarn 162
43. Section of well-cleaved tuff, grit and breccia passing up into
rudely-cleaved conglomerate and well-bedded cleaved fine
conglomerate and grit. East side of Llyn Padarn 163
44. Section of Clegyr on the north-east side of Llyn Padarn,
near the lower end 164
45. Section across the Cambrian formations of the Malvern Hills,
showing the position of the intercalated igneous rocks.
After Phillips 170
46. Section across Rhobell Fawr 178
47. Section at the Slate Quarry, Penrhyn Gwyn, north slopes of
Cader Idris 180
48. Sketch-section across Cader Idris 182
49. Section across the Moelwyn Range 185
50. Section across the anticline of Corndon 190
51. Structure in finely-amygdaloidal diabase lava, south of mouth
of Stinchar River, Ayrshire 193
52. View of Knockdolian Hill from the east 194
53. Section across the Lower Silurian volcanic series in the south
of Ayrshire (B. N. Peach) 197
54. Section of part of the Arenig volcanic group, stream south of
Bennane Head, Ayrshire 198
55. Flow-structure in the lowest felsite on the track from
Llanberis to the top of Snowdon 211
56. Section of Snowdon 212
57. Section across the Berwyn Hills. (Reduced from Horizontal
Section, Geol. Surv. Sheet 35) 219
58. Section of the strata on the shore at Porth Wen, west of Amlwch 223
59. Section of intercalated black shale in the volcanic series at
Porth yr hwch, south of Carmel Point, Anglesey 224
60. Green slates overlain with volcanic breccia, Carmel Point 224
61. Blue shale or slate passing into volcanic breccia east of Porth
Padrig, near Carmel Point 225
62. Section of felsites in the Coniston Limestone group, west of
Stockdale 232
63. Fine tuff with coarser bands near Quayfoot Quarries, Borrowdale 234
64. Diagram of the general relations of the different groups of
rock in the Lower Silurian volcanic district along the
western shore of Lough Mask 253
65. Veins and nests of sandstone due to the washing of sand into
fissures and cavities of an Old Red Sandstone lava. Turnberry
Point, Ayrshire 283
66. Ground-plan of reticulated cracks in the upper surface of an
Old Red Sandstone lava filled in with sandstone. Red Head,
Forfarshire 284
67. Section across the volcanic series of Forfarshire 286
68. Section across two necks above Tillicoultry, Ochil Hills 288
69. Section of the granite core between Merrick and Corscrine 290
70. Section across the three Dirrington Laws, Berwickshire 291
71. Section of Papa Stour, Shetlands, showing sill of spherulitic
felsite traversing Old Red Sandstone and bedded porphyrites
(Messrs. Peach and Horne) 292
72. Section across Northmavine, from Okrea Head to Skea Ness,
Shetland, showing dykes and connected sill of granite and
felsite (Messrs. Peach and Horne) 292
73. Section at the edge of one of the bays of Lower Old Red
Sandstone along the northern margin of Lake Caledonia,
near Ochtertyre 295
74. Craig Beinn-nan-Eun (2067 feet), east of Uam Var, Braes of Doune.
Old Red Conglomerate, with the truncated ends of the strata
looking across into the Highlands; moraines of Corry Beach
in the foreground 296
75. Section showing the top of the volcanic series at the foot of
the precipice of the Red Head, Forfarshire 300
76. Andesite with sandstone veinings and overlying conglomerate.
Todhead, south of Caterline, coast of Kincardineshire 303
77. Section across the Boundary-fault of the Highlands at Glen
Turrit, Perthshire 305
78. Section across the chain of the Sidlaw Hills near Kilspindie 306
79. Section across the Eastern Ochil Hills from near Newburgh to
near Auchtermuchty 307
80. Generalized section across the heart of the Ochil Hills from
Dunning on the north to the Fife coal-field near Saline on
the south 308
81. Diagram of the volcanic series of the Western Ochil Hills 309
82. View of Cnoc Garbh, Southend, Campbeltown. A volcanic neck of
Lower Old Red Sandstone age, about 400 yards wide in its
longer diameter 312
83. Section of volcanic series on beach, Southend, Campbeltown 313
84. Section of the base of the volcanic series, Reclain, five miles
south of Pomeroy 316
85. Section of shales and breccias at Crossna Chapel, north-east
of Boyle 316
86. Section across the north end of the Pentland Hills, from
Warklaw Hill to Pentland Mains. Length about five miles 318
87. View of the lava-escarpments of Warklaw Hill, Pentland chain,
from the north-west 319
88. Section across the Pentland Hills through North Black Hill and
Scald Law. Length about three miles 322
89. Section from the valley of the Gutterford Burn through Green
Law and Braid Law to Eight-Mile Burn 322
90. Section across the north end of the Pentland Hills, and the
southern edge of the Braid Hill vent. Length about two miles 324
91. Section across the northern end of the Biggar volcanic group,
from Fadden Hill to beyond Mendick Hill 326
92. Section across the southern part of the Biggar volcanic group
from Covington to Culter 328
93. Section from Thankerton Moor across Tinto to Lamington 328
94. Section across the Duneaton volcanic district from the head
of the Duneaton Water to Kirklea Hill 330
95. Cavernous spaces in andesite, filled in with sandstone,
John o' Groats Port, Turnberry, Ayrshire 333
96. Section of andesites, Turnberry Castle, Ayrshire 334
97. Lenticular form of a brecciated andesite (shown in Fig. 96),
Turnberry, Ayrshire 334
98. Section across the volcanic area of St. Abb's Head (after Prof.
J. Geikie) 339
99. View of terraced andesite hills resting on massive conglomerate,
south of Oban 341
100. Section of lava-escarpment at Beinn Lora, north side of mouth
of Loch Etive, Argyllshire 342
101. Section across Strathbogie, below Rhyme, showing the position
of the volcanic band 344
102. View of Knockfeerina, Limerick, from the north-east--a volcanic
neck of Upper Old Red Sandstone age 349
103. Section of the volcanic zone in the Upper Old Red Sandstone,
Cam of Hoy, Orkney 351
104. Section of the volcanic zone in the Upper Old Red Sandstone at
Black Ness, Rackwick, Hoy 351
105. Section across the volcanic band and its associated necks,
Hoy, Orkney 352
106. Ground-plan of volcanic neck piercing the Caithness Flagstone
series on the beach near John o' Groat's House 353
107. View of the escarpment of the Clyde Plateau in the Little
Cumbrae, from the south-west 368
108. View of the edge of the Volcanic Plateau south of Campbeltown,
Argyllshire 370
109. View of North Berwick Law from the east, a phonolite neck
marking one of the chief vents of the Garleton Plateau.
(From a photograph) 371
110. The Bass Rock, a trachytic neck belonging to the Garleton
plateau, from the shore at Canty Bay 372
111. Corston Hill--a fragment of the Midlothian Plateau, seen from
the north 373
112. View of Arthur Seat from Calton Hill to the north 374
113. View of Arkleton Fell, part of the Solway Plateau, from the
south-west 376
114. Vertical sections of the escarpment of the Clyde plateau from
north-east to south-west 384
115. Section of Craiglockhart Hill, Edinburgh 387
116. Section of the bottom of the Midlothian Plateau, Linnhouse
Water above Mid-Calder Oilworks 387
117. Section of the top of the Midlothian Plateau in the
Murieston Water 388
118. Section of Calton Hill, Edinburgh 389
119. Cliff of tuff and agglomerate, east side of Oxroad Bay, a
little east from Tantallon Castle, East Lothian 391
120. Section across part of the Clyde Plateau to the west of Bowling
(reduced from Sheet 6 of the Horizontal Sections of the
Geological Survey of Scotland) 392
121. Diagram illustrating the thinning away southwards of the lavas
of the Clyde Plateau between Largs and Ardrossan. Length
about 10 miles 393
122. Diagram illustrating the thinning away eastwards of the lavas of
the Clyde Plateau in the Fintry Hills. Length about 12 miles 394
123. View of the two necks Dumgoyn and Dumfoyn, Stirlingshire,
taken from the south 395
124. Ground-plan of Plateau-vents near Strathblane, Stirlingshire,
on the scale of 6 inches to a mile 395
125. Ground-plans of double and triple necks in the Plateau series,
on the scale of 6 inches to a mile 396
126. Ground-plan of tuff-neck, shore east of Dunbar 398
127. Section across the vents Dumgoyn and Dumfoyn, and the edge of
the Clyde plateau above Strathblane, Stirlingshire 400
128. Section through the large vent of the Campsie Hills 400
129. Diagrammatic section across the central vent of the Clyde
plateau in Renfrewshire 400
130. Section across Southern Berwickshire, to show the relation
of the volcanic plateau to the vents lying south from it 401
131. Section of south end of Dumbuck Hill. East of Dumbarton 403
132. Section across the East Lothian plateau, to show the relative
position of one of the necks 403
133. View of Traprain Law from the south, a phonolite neck of the
Garleton Plateau 405
134. Veins and dykes traversing the agglomerate and tuff of the
great Renfrewshire vent 408
135. "The Yellow Man," a dyke in volcanic tuff and conglomerate on
the shore a little east of North Berwick 409
136. Trachytic sills, Knockvadie, Kilpatrick Hills 410
137. Section across the edge of the Clyde plateau, south-east
of Beith 411
138. Section across the upper part of the Clyde plateau at
Kilbirnie, Ayrshire 411
139. Section across the upper surface of the Clyde volcanic
plateau, Burnhead, north-west of Kilsyth 412
140. Section across the upper surface of the Clyde volcanic
plateau at Campsie 412
141. Section across western edge of the Garlton plateau 412
142. Section across the Solway plateau 413
143. Section of volcanic vent at East Grange, Perthshire coal-field,
constructed by Mr. B. N. Peach from the rocks exposed in a
railway-cutting, and from plans of ironstone- and coal-pits 426
144. View of the Binn of Burntisland--a volcanic neck of agglomerate 428
145. View of part of the cliffs of vertical agglomerate, Binn of
Burntisland 431
146. Diagram of buried volcanic cone near Dalry, Ayrshire.
Constructed from information obtained in mining operations 434
147. Diagram to illustrate how Volcanic Necks may be concealed
and exposed 434
148. Section across the Saline Hills, Fife 435
149. Section across the Binn of Burntisland, in an east and
west direction 436
150. Section in old quarry, west of Wester Ochiltree,
Linlithgowshire. Calciferous Sandstone series 437
151. Ejected volcanic block in Carboniferous strata, Burntisland 438
152. View of volcanic agglomerate becoming finer above east end
of Kingswood Craig, two miles east from Burntisland 439
153. Alternations of basalt and tuff, with shale, etc., of
Kingswood Craig, Burntisland 441
154. Section of the upper surface of a diabase ("leckstone") sheet,
Skolie Burn, south-east of Bathgate 443
155. Section across the volcanic ridge of the Linlithgow and
Bathgate Hills, showing the intercalation of limestones that
mark important stratigraphical horizons 444
156. Section in Wardlaw Quarry, Linlithgowshire 445
157. Section from Linlithgow Loch to the Firth of Forth 446
158. Section across the Campsie Fells illustrating the contrast
between the sills below and above the plateau-lavas 447
159. Section showing the position of the basic sills in relation
to the volcanic series at Burntisland, Fife 448
160. Sills between shales and sandstones, Hound Point,
Linlithgowshire 449
161. Section of Sill, Cramond Railway, Barnton, near Edinburgh 450
162. Intrusive dolerite sheet enclosing and sending threads into
portions of shale, Salisbury Crags, Edinburgh 452
163. Intrusive sheet invading limestone and shale, Dodhead Quarry,
near Burntisland 452
164. Spheroidal weathering of dolerite sill, quarry east of North
Queensferry, Fife. 455
165. Two thin sills of "white trap" injected into black
carbonaceous shale overlying the Hurlet Limestone, Hillhouse
Quarry, Linlithgow 456
166. Dyke cutting the agglomerate of a neck. Binn of Burntisland 457
167. Boss of diabase cutting the Burdiehouse Limestone and sending
sills and veins into the overlying shales. Railway cutting,
West Quarry, East Calder, Midlothian 458
168. Side of columnar basalt-dyke in the same agglomerate as in
Fig. 166 459
169. Dyke rising through the Hurlet Limestone and its overlying
shales. Silvermine Quarry, Linlithgowshire 460
170. Junction of amygdaloidal basalt with shales and limestone,
shore, half a mile east from Kinghorn, Fife 464
171. Columnar basalt, Pettycur, Kinghorn, Fife 469
172. Section across the Fife band of Sills 473
173. Section across the upper volcanic band of north Ayrshire.
Length about four miles 474
174. Section showing the connection of the two volcanic bands
in Liddesdale 476
175. Diagram to show the position of a mass of Upper Old Red
Sandstone which has fallen into the great vent near Tudhope
Hill, east of Mosspaul 476
MAPS
I. General map of the Volcanic districts of the British
Isles--_At the end of the volume_
II. Map of the Cambrian and Silurian volcanic region of
North Wales _To face p. 256_
III. Map of the Old Red Sandstone volcanic region of "Lake
Caledonia" in Central Scotland and North Ireland _To face p. 334_
IV. Map of the Carboniferous volcanic districts of
Scotland _To face p. 476_
BOOK I
GENERAL PRINCIPLES AND METHODS OF INVESTIGATION
CHAPTER I
Earliest Knowledge of Volcanoes--Their Influence on Mythology and
Superstition--Part taken by Volcanic Rocks in Scenery--Progress
of the Denudation of Volcanoes--Value of the Records of former
Volcanoes as illustrating Modern Volcanic Action--Favourable
Position of Britain for the Study of this Subject.
Among the influences which affected the infancy of mankind, the most
potent were those of environment. Whatever in outer nature stimulated
or repressed courage, inventiveness, endurance, whatever tended to
harden or to weaken the bodily faculties, whatever appealed to the
imagination or excited the fancy, became a powerful factor in human
development.
Thus, in the dawn of civilization, the frequent recurrence of
earthquakes and volcanic eruptions throughout the basin of the
Mediterranean could not but have a marked effect on the peoples that
dwelt by the borders of that sea. While every part of the region was
from time to time shaken by underground commotion, there were certain
places that became specially noteworthy for the wonder and terror of
their catastrophes. When, after successive convulsions, vast clouds
of black smoke rose from a mountain and overspread the sky, when the
brightness of noon was rapidly replaced by the darkness of midnight,
when the air grew thick with stifling dust and a rain of stones and
ashes fell from it on all the surrounding country, when streams of what
looked like liquid fire poured forth and desolated gardens, vineyards,
fields and villages--then did men feel sure that the gods were angry.
The contrast between the peacefulness and beauty of the ordinary
landscape and the hideous warfare of the elements at these times of
volcanic fury could not but powerfully impress the imagination and give
a colour to early human conceptions of nature and religion.
It was not only in one limited district that these manifestations of
underground convulsion showed themselves. The islands of the Ægean
had their volcanoes, and the Greeks who dwelt among them watched
their glowing fires by night and their clouds of steam by day,
culminating now and then in a stupendous explosion, like that which, in
prehistoric time, destroyed the island of Santorin. As the islanders
voyaged eastward they would see, on the coast of Asia Minor, the black
bristling lavas of the "Burnt Country," perhaps even then flowing from
their rugged heaps of cinders. Or when, more adventurously still, they
sailed westward into the Tyrrhenian waters, they beheld the snowy cone
of Etna, with its dark canopy of smoke and the lurid nocturnal gleam of
its fires; while from time to time they witnessed there on a still more
stupendous scale the horrors of a great volcanic eruption.
From all sides, therefore, the early Greek voyagers would carry back to
the mother-country marvellous tales of convulsion and disaster. They
would tell how the sky rapidly darkened even in the blaze of mid-day,
how the land was smothered with dust and stones, how over the sea there
spread such a covering of ashes that the oarsmen could hardly drive
their vessels onward, how red-hot stones, whirling high overhead,
rained down on sails and deck, and crushed or burnt whatever they fell
upon, and how, as the earth shook and the sea rose in sudden waves and
the mountain gave forth an appalling din of constant explosion, it
verily seemed that the end of the world had come.
To the actual horrors of such scenes there could hardly fail to be
added the usual embellishments of travellers' tales. Thus, in the end,
the volcanoes of the Mediterranean basin came to play a not unimportant
part in Hellenic mythology. They seemed to stand up as everlasting
memorials of the victory of Zeus over the giants and monsters of an
earlier time. And as the lively Greek beheld Mount Etna in eruption,
his imagination readily pictured the imprisoned Titan buried under the
burning roots of the mountain, breathing forth fire and smoke, and
convulsing the country far and near, as he turned himself on his uneasy
pallet.
When in later centuries the scientific spirit began to displace the
popular and mythological interpretation of natural phenomena, the
existence of volcanoes and their extraordinary phenomena offered
a fruitful field for speculation and conjecture. As men journeyed
outward from the Mediterranean cradle of civilization, they met with
volcanic manifestations in many other parts of the world. When they
eventually penetrated into the Far East, they encountered volcanoes on
a colossal scale and in astonishing abundance. When they had discovered
the New World they learnt that, in that hemisphere also, "burning
mountains" were numerous and of gigantic dimensions. Gradually it was
ascertained that vast lines of volcanic activity encircle the globe.
By slow degrees the volcano was recognized to be as normal a part of
the mechanism of our planet as the rivers that flow on the terrestrial
surface. And now at last men devote themselves to the task of
critically watching the operations of volcanoes with as much enthusiasm
as they display in the investigation of any other department of nature.
They feel that their knowledge of the earth extends to little beyond
its mere outer skin, and that the mystery which still hangs over the
vast interior of the planet can only, if ever, be dispelled by the
patient study of these vents of communication between the interior and
the surface.
If, however, we desire to form some adequate idea of the part which
volcanic action has played in the past history of the earth, we should
be misled were we to confine our attention to the phenomena of the
eruptions of the present day. An attentive examination of any modern
volcano will convince us that of some of the most startling features of
an eruption no enduring memorial remains. The convulsive earthquakes
that accompany a great volcanic paroxysm, unless where they actually
fissure the ground, leave little or no trace behind them. Lamentably
destructive as they are to human life and property, the havoc which
they work is mostly superficial. In a year or two the ruins have been
cleared away, the earth-falls have been healed over, the prostrated
trees have been removed, and, save in the memories and chronicles of
the inhabitants, no record of the catastrophe may survive. The clouds
of dust and showers of ashes which destroyed the crops and crushed
in the roofs of houses soon disappear from the air, and the covering
which they leave over the surface of a district gradually mingles with
the soil. Vegetation eventually regains its place, and the landscape
becomes again as smiling as before.
Even where the materials thrown out from the crater accumulate in much
greater mass, where thick deposits of ashes or solid sheets of lava
bury the old land-surface, the look of barren desolation, though in
some cases it may endure for long centuries, may in others vanish in
a few years. The surface-features of the district are altered indeed,
but the new topography soon ceases to look new. Another generation of
inhabitants loses recollection of the old landmarks, and can hardly
realize that what has become so familiar to itself differs so much from
what was familiar to its fathers.
But even when the volcanic covering, thus thrown athwart a wide
tract of country, has been concealed under a new growth of soil and
vegetation, it still remains a prey to the ceaseless processes of
decay and degradation which everywhere affect the surface of the land.
No feature of a modern volcano is more impressive than the lesson
which it conveys of the reality and potency of this continual waste.
The northern slopes of Vesuvius, for example, are trenched with deep
ravines, which in the course of centuries have been dug out of the
lavas and tuffs of Monte Somma by rain and melted snow. Year by year
these chasms are growing deeper and wider, while the ridges between
them are becoming narrower. In some cases, indeed, the intervening
ridges have been reduced to sharp crests which are split up and
lowered by the unceasing influence of the weather. The slopes of such
a volcanic cone have been aptly compared to a half-opened umbrella. It
requires little effort of imagination to picture a time, by no means
remote in a geological sense, when, unless renovated by the effects
of fresh eruptions, the cone will have been so levelled with the
surrounding country that the peasants of the future will trail their
vines and build their cots over the site of the old volcano, in happy
ignorance of what has been the history of the ground beneath their
feet.
What is here predicted as probable or certain in the future has
undoubtedly happened again and again in the past. Over many districts
of Europe and Western America extinct volcanoes may be seen in every
stage of decay. The youngest may still show, perfect and bare of
vegetation, their cones and their craters, with the streams of lava
that escaped from them. Those of older date have been worn down into
mere low rounded hills, or the whole cone has been cleared away, and
there is only left the hard core of material that solidified in the
funnel below the surface. The lava-sheets have been cut through by
streams, and now remain in mere scattered patches capping detached
hills, which only a trained eye can recognize as relics of a once
continuous level sheet of solid rock.
By this resistless degradation, a volcanic district is step by step
stripped of every trace of its original surface. All that the eruptions
did to change the face of the landscape may be entirely obliterated.
Cones and craters, ashes and lavas, may be gradually effaced. And yet
enough may be left to enable a geologist to make sure that volcanic
action was once rife there. As the volcano marks a channel of direct
communication between the interior of the earth and the atmosphere
outside, there are subterranean as well as superficial manifestations
of its activity, and while the latter are removed by denudation, the
former are one by one brought into light. The progress of denudation
is a process of dissection, whereby every detail in the structure of a
volcano is successively cut down and laid bare. But for this process,
our knowledge of the mechanism and history of volcanic action would be
much less full and definite than happily it is. In active volcanoes
the internal and subterranean structure can only be conjectured; in
those of ancient date, which have been deeply eroded, this underground
structure is open to the closest examination.
By gathering together evidence of this nature over the surface of
the globe, we learn that abundantly as still active volcanoes are
distributed on that surface, they form but a small fraction of the
total number of vents which have at various times been in eruption. In
Italy, for example, while Vesuvius is active on the mainland, and Etna,
Stromboli and Volcano display their vigour among the islands, there are
scores of old volcanoes that have been silent and cold ever since the
beginning of history, yet show by their cones of cinders and streams of
bristling lava that they were energetic enough in their day. But the
Italian volcanic region is only one of many to be found on the European
Continent. If we travel eastward into Hungary, or northward into the
Eifel, or into the heart of France, we encounter abundant cones and
craters, many of them so fresh that, though there is no historical
record of their activity, they look as if they had been in eruption
only a few generations ago.
But when the geologist begins to search among rocks of still older
date than these comparatively recent volcanic memorials, he meets
with abundant relics of far earlier eruptions. And as he arranges the
chronicles of the earth's history, he discovers that each section of
the long cycle of geological ages has preserved its records of former
volcanoes. In a research of this kind he can best realize how much
he owes to the process of denudation. The volcanic remains of former
geological periods have in most cases been buried under younger
deposits, and have sunk sometimes thousands of feet below the level of
the sea. They have been dislocated and upheaved again during successive
commotions of the terrestrial crust, and have at last been revealed by
the gradual removal of the pile of material under which they had lain.
Hence we learn that the active volcanoes of the present time, which
really embrace but a small part of the volcanic history of our planet,
are the descendants of a long line of ancestors. Their distribution
and activity should be considered not merely from the evidence they
themselves supply, but in the light derived from a study of that
ancestry. It is only when we take this broad view of the subject that
we can be in a position to form some adequate conception of the nature
and history of volcanoes in the geological evolution of the globe.
In this research it is obvious that the presently active volcano
must be the basis and starting-point of inquiry. At that channel of
communication between the unknown inside and the familiar outside of
our globe, we can watch what takes place in times of quiescence or of
activity. We can there study each successive phase of an eruption,
measure temperatures, photograph passing phenomena, collect gases and
vapours, register the fall of ashes or the flow of lavas, and gather a
vast body of facts regarding the materials that are ejected from the
interior, and the manner of their emission.
Indispensable as this information is for the comprehension of volcanic
action, it obviously affords after all but a superficial glimpse of
that action. We cannot see beyond the bottom of the crater. We cannot
tell anything about the subterranean ducts, or how the molten and
fragmental materials behave in them. All the underground mechanism
of volcanoes is necessarily hidden from our eyes. But much of this
concealed structure has been revealed in the case of ancient volcanic
masses, which have been buried and afterwards upraised and laid bare by
denudation.
In yet another important aspect modern volcanoes do not permit us to
obtain full knowledge of the subject. The terrestrial vents, from which
we derive our information, by no means represent all the existing
points of direct connection between the interior and the exterior of
the planet. We know that some volcanic eruptions occur under the sea,
and doubtless vast numbers more take place there of which we know
nothing. But the conditions under which these submarine discharges
are effected, the behaviour of the outflowing lava under a body of
oceanic water, and the part played by fragmentary materials in the
explosions, can only be surmised. Now and then a submarine volcano
pushes its summit above the sea-level, and allows its operations to be
seen, but in so doing it becomes practically a terrestrial volcano, and
the peculiar submarine phenomena are still effectually concealed from
observation.
The volcanic records of former geological periods, however, are in
large measure those of eruptions under the sea. In studying them we
are permitted, as it were, to explore the sea-bottom. We can trace
how sheets of coral and groves of crinoids were buried under showers
of ashes and stones, and how the ooze and silt of the sea-floor were
overspread with streams of lava. We are thus, in some degree, enabled
to realize what must now happen over many parts of the bed of the
existing ocean.
The geologist who undertakes an investigation into the history of
volcanic action within the area of the British Isles during past
time, with a view to the better comprehension of this department
of terrestrial physics, finds himself in a situation of peculiar
advantage. Probably no region on the face of the globe is better fitted
than these islands to furnish a large and varied body of evidence
regarding the progress of volcanic energy in former ages. This special
fitness may be traced to four causes--1st, The remarkable completeness
of the geological record in Britain; 2nd, The geographical position of
the region on the oceanic border of a continent; 3rd, The singularly
ample development to be found there of volcanic rocks belonging to a
long succession of geological ages; and 4th, The extent to which this
full chronicle of volcanic activity has been laid bare by denudation.
1. In the first place, the geological record of Britain is singularly
complete. It has often been remarked how largely all the great periods
of geological time are represented within the narrow confines of these
islands. The gaps in the chronicle are comparatively few, and for the
most part are not of great moment.
Thanks to the restricted area of the country and to the large number
of observers, this remarkably full record of geological history has
been studied with a minute care which has hardly been equalled in any
other country. The detailed succession of all the formations has been
so fully determined in Britain that the very names first applied here
to them and to their subdivisions have in large measure passed into
the familiar language of geology all over the globe. Every definite
platform in the stratigraphical series has been more or less fully
worked out. A basis has thus been laid for referring each incident in
the geological history of the region to its proper relative date.
2. In the second place, the geographical position of Britain gives
it a notable advantage in regard to the manifestations of volcanic
energy. Rising from the margin of a great ocean-basin and extending
along the edge of a continent, these islands have lain on that critical
border-zone of the terrestrial surface, where volcanic action is apt
to be most vigorous and continuous. It has long been remarked that
volcanoes are generally placed not far from the sea. From the earliest
geological periods the site of Britain, even when submerged below
the sea, has never lain far from the land which supplied the vast
accumulations of sediment that went to form the Palæozoic and later
formations, while, on the other hand, it frequently formed part of the
land of former geological periods. It was thus most favourably situated
as a theatre for both terrestrial and submarine volcanic activity.
3. In the third place, this advantageous geographical position is
found to have been attended with an altogether remarkable abundance
and persistence of volcanic eruptions. No tract of equal size yet
known on the face of the globe furnishes so ample a record of volcanic
activity from the earliest geological periods down into Tertiary
time. Every degree of energy may be signalized in that record, from
colossal eruptions which piled up thousands of feet of rock down to
the feeblest discharge of dust and stones. Every known type of volcano
is represented--great central cones like Etna or Vesuvius, scattered
groups of small cones like the _puys_ of France, and fissure- or
dyke-eruptions like those of recent times in Iceland.
Moreover, the accurate manner in which the stratigraphy of the country
has been established permits each successive era in the long volcanic
history to be precisely determined, and allows us to follow the whole
progress of that history stage by stage, from the beginning to the end.
These characteristics may be instructively represented on a map, such
as that which accompanies the present volume (Map I.). The reader will
there observe how repeatedly volcanic eruptions have taken place, not
merely within the general area of the British Isles, but even within
the same limited region of that area. The broad midland valley of
Scotland has been especially the theatre for their display. From the
early part of the Lower Silurian period, through the ages of the Old
Red Sandstone, Carboniferous and Permian systems, hundreds of volcanic
vents were active in that region, while in long subsequent time there
came the fissure-eruptions of the Tertiary series.
4. In the fourth place, the geological revolutions of successive ages
have made this long volcanic chronicle fully accessible to observation.
Had the lavas and ashes of one period remained buried under the
sedimentary accumulations of the next, their story would have been lost
to us. We should only have been able to decipher the latest records
which might happen to lie on the surface. Fortunately for the progress
of geology, the endless vicissitudes of a continental border have
brought up the very oldest rocks once more to the surface. All the
later formations of the earth's crust have likewise been upraised and
exposed to denudation during long cycles of time. In this manner, the
rocky framework of the country has been laid bare, and each successive
chapter of its geological history may be satisfactorily deciphered.
The singularly complete volcanic chronicle, after being entombed under
younger deposits, has been broken up and raised once more into view.
The active vents of former periods have been dissected, submarine
streams of lava have been uncovered, sheets of ashes that fell over the
sea-bottom have been laid bare. The progress of denudation is specially
favoured in such a variable and moist climate as that of Britain,
and thus by the co-operation of underground and meteoric causes the
marvellous volcanic records of this country have been laid open in
minutest detail.
There is yet another respect in which the volcanic geology of Britain
possesses a special value. Popular imagination has long been prone to
see signs of volcanic action in the more prominent rocky features
of landscape. A bold crag, a deep and precipitous ravine, a chasm in
the side of a mountain, have been unhesitatingly set down as proof of
volcanic disturbance. Many a cauldron-shaped recess, like the corries
of Scotland or the cwms of Wales, has been cited as an actual crater,
with its encircling walls still standing almost complete.
The relics of former volcanoes in this country furnish ample proofs
to dispel these common misconceptions. They show that not a single
crater anywhere remains, save where it has been buried under lava;
that no trace of the original cones has survived, except in a few
doubtful cases where they may have been preserved under subsequent
accumulations of material; that in the rugged tracts, where volcanic
action has been thought to have been most rife, there may be not a
vestige of it, while, on the other hand, where the uneducated eye would
never suspect the presence of any remnant of volcanic energy, lavas
and ashes may abound. We are thus presented with some of the most
impressive contrasts in geological history, while, at the same time,
this momentous lesson is borne in upon the mind, that the existing
inequalities in the configuration of a landscape are generally due
far less to the influence of subterranean force than to the action of
the superficial agents which are ceaselessly carving the face of the
land. Those rocks which from their hardness or structure are best able
to withstand that destruction rise into prominence, while the softer
material around them is worn away. Volcanic rocks are no exception to
this rule, as the geological structure of Britain amply proves.
In the following chapters, forming Book I. of this work, I propose to
begin by offering some general remarks regarding the nature and causes
of volcanic action, so far as these are known to us. I shall then
proceed to consider the character of the evidence that may be expected
to be met with respecting the former prevalence of that action at any
particular locality where volcanic disturbances have long since ceased.
The most telling evidence of old volcanoes is naturally to be found
in the materials which they have left behind them, and the reader's
attention will be asked to the special characteristics of these
materials, in so far as they give evidence of former volcanic activity.
As has been already remarked, many of the most prominent phenomena of
a modern volcano are only of transient importance. The earthquakes and
tremors, and the constant disengagement of steam and gases, that play
so conspicuous a part in an eruption, may leave no sensible record
behind them. But even the cones of ashes and lava, which are piled up
into mountainous masses, have no true permanence: they are liable to
ceaseless erosion by the meteoric agencies of waste, and every stage in
their degradation may be traced. In successive examples we can follow
them as they are cut down to the very core, until in the end they are
entirely effaced.
We may well, therefore, ask at the outset by what more enduring
records we may hope to detect the traces of former volcanic action.
The following introductory chapters will be devoted to an attempt to
answer this question. I shall try to show the nature and relative
importance of the records of ancient volcanoes; how these records,
generally so fragmentary, may be pieced together so as to be made to
furnish the history which they contain; how their relative chronology
may be established; how their testimony may be supplemented in such
wise that the position of long vanished seas, lands, rivers, and lakes
may be ascertained; and how, after ages of geological revolution,
volcanic rocks that have lain long buried under the surface now
influence the scenery of the regions where they have once more been
exposed to view.
From this groundwork of ascertained fact and reasonable inference, we
shall enter in Book II. upon the story of the old volcanoes of the
British Isles. It is usual to treat geological history in chronological
order, beginning with the earliest ages. And this method, as on the
whole the most convenient, will be adopted in the present work. At
the same time, the plan so persistently followed by Lyell, of working
backward from the present into the past, has some distinct advantages.
The volcanic records of the later ages are much simpler and clearer
than those of older times, and the student may, in some respects,
profitably study the history of the Tertiary eruptions before he
proceeds to make himself acquainted with the scantier chronicles of the
eruptions of the Palæozoic periods. But as I wish to follow the gradual
evolution of volcanic phenomena, and to show how volcanic energy has
varied, waxing and waning through successive vast intervals of time, I
will adhere to the chronological sequence.
CHAPTER II
The Nature and Causes of Volcanic Action--Modern Volcanoes.
A volcano is a conical or dome-shaped hill or mountain, consisting of
materials which have been erupted from an orifice leading down from
the surface into the heated interior of the earth. Among modern and
recent volcanoes three types may be recognized. In the first and most
familiar of these, the lavas and ashes ejected from the central vent
have gathered around it by successive eruptions, until they have built
up a central cone like those of Etna and Vesuvius. As this cone grows
in height and diameter, lateral or parasitic cones are formed on its
flanks, and may become themselves the chief actively erupting vents.
This type of volcano, which has been so long well known from its
Mediterranean examples, was until recently believed by geologists to be
the normal, or indeed the only, phase of volcanic energy on the face of
the earth.
A modification of this type is to be found in a few regions where
fragmentary discharges are small in amount and where the eruptions are
almost wholly confined to the emission of tolerably liquid lava. A vast
dome with gently sloping declivities may in this way be formed, as in
the Sandwich Islands and in certain parts of Iceland.
The second type of volcano is at the present day extensively developed
only in Iceland, but in Tertiary time it appears to have had a wide
range over the globe, for stupendous memorials of it are preserved
in North-Western Europe, in Western America, and in India. It is
distinguished by the formation of numerous parallel fissures from which
the lava gushes forth, either with or without the formation of small
cinder-cones along the lines of the chasms.
The third type is distinguished by the formation of groups of
cinder-cones or lava-domes, which from their admirable development
in Central France have received the name of _Puys_. From these vents
considerable streams of lava have sometimes been discharged.
Without entering here into a detailed inquiry regarding the nature and
causes of Volcanic Action, we may with advantage consider briefly the
two main factors on which this action appears to depend.
1. Much uncertainty still exists as to the condition and composition
of the earth's interior. The wide distribution of volcanoes over the
globe, together with the general similarity of materials brought
by them up to the surface, formerly led to the belief that our
planet consists of a central mass of molten rock enclosed within a
comparatively thin solid crust. Physical arguments, however, have
since demonstrated that the earth, with such a structure, would have
undergone great tidal deformation, but that in actual fact it has a
greater rigidity than if it were made of solid glass or steel.
From all the evidence obtainable it is certain that the temperature
of the earth's interior must be high. The rate of increase of this
temperature downward from the surface differs from place to place; but
an increase is always observed. At a depth of a few miles, every known
substance must be much hotter than its melting point at the surface.
But at the great pressures within the earth, actual liquefaction is no
doubt prevented, and the nucleus remains solid, though at a temperature
at which, but for the pressure, it would be like so much molten iron.
Any cause which will diminish the pressure may allow the intensely hot
material within the globe to pass into the liquid state. There is one
known cause which will bring about this result. The downward increment
of temperature proves that our planet is continually losing heat. As
the outer crust is comparatively cool, and does not become sensibly
hotter by the uprise of heat from within, the hot nucleus must cool
faster than the crust is doing. Now cooling involves contraction. The
hot interior is contracting faster than the cooler shell which encloses
it, and that shell is thus forced to subside. In its descent it has to
adjust itself to a constantly diminishing diameter. It can do so only
by plication or by rupture.
When the terrestrial crust, under the strain of contraction, is
compressed into folds, the relief thus obtained is not distributed
uniformly over the whole surface of the planet. From an early
geological period it appears to have followed certain lines. How these
came to be at first determined we cannot tell. But it is certain
that they have served again and again, during successive periods of
terrestrial readjustment. These lines of relief coincide, on the whole,
with the axes of our continents. The land-areas of the globe may be
regarded as owing their existence above sea-level to this result of
terrestrial contraction. The crust underneath them has been repeatedly
wrinkled, fractured and thrust upward by the vast oceanic subsidence
around them. The long mountain-chains are thus, so to speak, the crests
of the waves into which the crust has from time to time been thrown.
Again, the great lines of fracture in the crust of the earth probably
lie in large measure within the land-areas, or at least parallel with
their axes and close to their borders. Where the disposition of the
chief ruptures and of the predominant plications can be examined,
these leading structural features are found to be, on the whole,
coincident. In the British Islands, for instance, the prevalent trend
of the axes of folding from early Palæozoic to Tertiary time has
been from south-west to north-east. How profoundly this direction
of earth-movement has affected the structure of the region is shown
by any ordinary map, in the long hill-ranges of the land and in the
long inlets of the sea. A geological map makes the dependence of the
scenery upon the building of the rocks still more striking. Not only
have these rocks been plicated into endless foldings, the axes of
which traverse the British Islands with a north-easterly trend: they
have likewise been dislocated by many gigantic ruptures, which tend on
the whole to follow the same direction. The line of the Great Glen,
the southern front of the Highlands, and the northern boundary of the
Southern Uplands of Scotland, are conspicuous examples of the position
and effect of some of the greater fractures in the structure of this
country.
The ridging up of any part of the terrestrial crust will afford
some relief from pressure to the parts of the interior immediately
underneath. If, as is probable, the material of the earth's interior is
at the melting point proper for the pressure at each depth, then any
diminution of the pressure may allow the intensely heated substance to
pass into the liquid state. It would be along the lines of terrestrial
uplift that this relief would be given. It is there that active
volcanoes are found. The molten material is forced upward under these
upraised ridges by the subsidence of the surrounding regions. And where
by rupture of the crust this material can make its way to the surface,
we may conceive that it will be ejected as lava or as stones and ashes.
Viewed in a broad way, such appears to be the mechanism involved in
the formation and distribution of volcanoes over the surface of the
earth. But obviously this explanation only carries us so far in the
elucidation of volcanic action. If the molten magma flowed out merely
in virtue of the influence of terrestrial contraction, it might do so
for the most part tranquilly, though it would probably be affected
by occasional sudden snaps, as the crust yielded to accumulations of
pressure. Human experience has no record of the actual elevation of a
mountain-chain. We may believe that if such an event were to happen
suddenly or rapidly, it would be attended with gigantic catastrophes
over the surface of the globe. We can hardly conceive what would
be the scale of a volcanic eruption attending upon so colossal a
disturbance of the terrestrial crust. But the eruptions which have
taken place within the memory of man have been the accompaniments
of no such disturbance. Although they have been many in number and
sometimes powerful in effect, they have seldom been attended with any
marked displacement of the surrounding parts of the terrestrial crust.
Contraction is, of course, continuously and regularly in progress, and
we may suppose that the consequent subsidence, though it results in
intermittent wrinkling and uplifting of the terrestrial ridges, may
also be more or less persistent in the regions lying outside these
ridges. There will thus be a constant pressure of the molten magma
into the roots of volcanoes, and a persistent tendency for the magma
to issue at the surface at every available rent or orifice. The energy
and duration of outflow, if they depended wholly upon the effects of
contraction, would thus vary with the rate of subsidence of the sinking
areas, probably assuming generally a feeble development, but sometimes
bursting into fountains of molten rock hundreds of feet in height, like
those observed from time to time in Hawaii.
2. The actual phenomena of volcanic eruptions, however, show that a
source of explosive energy is almost always associated with them, and
that while the transference of the subterranean molten magma towards
the volcanic vents may be referred to the results of terrestrial
contraction, the violent discharge of materials from those vents must
be assigned to some kind of energy stored up in the substance of the
earth's interior.
The deep-seated magma from which lavas ascend contains various vapours
and gases which, under the enormous pressure within and beneath the
terrestrial crust, are absorbed or dissolved in it. So great is the
tension of these gaseous constituents, that when from any cause the
pressure on the magma is suddenly relieved, they are liberated with
explosive violence.
A volcanic paroxysm is thus immediately the effect of the rapid escape
of these imprisoned gases and vapours. With such energy does the
explosion sometimes take place, that the ascending column of molten
lava is blown into the finest impalpable dust, which may load the air
around a volcano for many days before it falls to the ground, or may be
borne in the upper regions of the atmosphere round the globe.
The proportion of dissolved gases varies in different lavas, while the
lavas themselves differ in the degree of their liquidity. Some flow
out tranquilly like molten iron, others issue in a pasty condition and
rapidly congeal into scoriæ and clinkers. Thus within the magma itself
the amount of explosive energy is far from being always the same.
It is to the co-operation of these two causes--terrestrial contraction
and its effects on the one hand, and the tension of absorbed gases and
vapours the other--that the phenomena of volcanoes appear to be mainly
due. There is no reason to believe that modern volcanoes differ in any
essential respect from those of past ages in the earth's history. It
might, indeed, have been anticipated that the general energy of the
planet would manifest itself in far more stupendous volcanic eruptions
in early times than those of the modern period. But there is certainly
no geological evidence in favour of such a difference. One of the
objects of the present work is to trace the continuity of volcanic
phenomena back to the very earliest epochs, and to show that, so far as
the geological records go, the interior of the planet has reacted on
its exterior in the same way and with the same results.
We may now proceed to inquire how far volcanoes leave behind them
evidence of their existence. I shall devote the next two or three
chapters to a consideration of the proofs of volcanic action furnished
by the very nature of the materials brought up from the interior of
the earth, by the arrangement of these materials at the surface, by
the existence of the actual funnels or ducts from which they were
discharged above ground, and by the disposition of the masses of rock
which, at various depths below the surface, have been injected into and
have solidified within the terrestrial crust.
CHAPTER III
Ancient Volcanoes: Proofs of their existence derived from the
Nature of the Rocks erupted from the Earth's Interior. A.
Materials erupted at the Surface--Extrusive Series. i. Lavas,
their General Characters. Volcanic Cycles. ii. Volcanic
Agglomerates, Breccias and Tuffs.
The materials brought by volcanic action from the earth's interior
have certain common characters which distinguish them from other
constituents of the terrestrial crust. Hence the occurrence of these
materials on any part of the earth's surface affords convincing proofs
of former volcanic eruptions, even where all outward trace of actual
volcanoes may have been effaced from the topographical features of the
ground.
Volcanic products may be classed in two divisions--1st, Those which
have been ejected at the surface of the earth, or the Extrusive series;
and 2nd, Those which have been injected into the terrestrial crust at
a greater or less distance below the surface, and which are known as
the Intrusive series. Extrusive rocks may be further classified in two
great groups--(i.) The Lavas, or those which have been poured out in a
molten condition at the surface; and (ii.) The Fragmental Materials,
including all kinds of pyroclastic detritus discharged from volcanic
vents.
Taking first the Extrusive volcanic rocks, we may in the present
chapter consider those characters in them which are of most practical
value in the investigation of the volcanic phenomena of former
geological periods.
i. LAVAS
The term Lava is a convenient and comprehensive designation for all
those volcanic products which have flowed out in a molten condition.
They differ from each other in composition and structure, but their
variations are comprised within tolerably definite limits.
As regards their composition they are commonly classed in three
divisions--1st, The Acid lavas, in which the proportion of silicic acid
ranges from a little below 70 per cent upwards; 2nd, The Intermediate
lavas, wherein the percentage of silica may vary from 55 to near 70;
and 3rd, The Basic lavas, where the acid constituent ranges from 55 per
cent downwards. Sometimes the most basic kinds are distinguished as a
fourth group under the name of Ultrabasic, in which the percentage of
silica may fall below 40.
The structures of lavas, however, furnish their most easily appreciated
characteristics. Four of these structures deserve more particular
attention: 1st, Cellular, vesicular or pumiceous structure; 2nd, The
presence of glass, or some result of the devitrification of an original
glass; 3rd, Flow-structure; and 4th, The arrangement of the rocks in
sheets or beds, with columnar and other structures.
[Illustration: Fig. 1.--Vesicular structure, Lava from Ascension
Island, slightly less than natural size.]
1. The CELLULAR, VESICULAR, SCORIACEOUS or PUMICEOUS STRUCTURE of
volcanic rocks (Fig. 1) could only have arisen in molten masses from
the expansion of imprisoned vapours or gases, and is thus of crucial
importance in deciding the once liquid condition of the rocks which
display it. The vesicles may be of microscopic minuteness, but are
generally quite visible to the naked eye, and are often large and
conspicuous. Sometimes these cavities have been subsequently filled up
with calcite, quartz, agate, zeolites or other mineral deposition. As
the kernels thus produced are frequently flattened or almond-shaped
(_amygdales_), owing to elongation of the steam-holes by movement of
the lava before its consolidation, the rocks containing them are said
to be _amygdaloidal_.
This structure, though eminently characteristic of superficial lavas,
is not always by itself sufficient to distinguish them from the
intrusive rocks. Examples will be given in later chapters where dykes,
sills and other masses of injected igneous material are conspicuously
cellular in some parts. But, in such cases, the cavities are generally
comparatively small, usually spherical or approximately so, tolerably
uniform in size and distribution, and, especially when they occur in
dykes, distributed more particularly along certain lines or bands,
sometimes with considerable regularity (see Figs. 90, 91, and 236).
Among the superficial lavas, however, such regularity is rarely to be
seen. Now and then, indeed, a lava, which is not on the whole cellular,
may be found to have rows of vesicles arranged parallel to its under
or upper surface, or it may have acquired a peculiar banded structure
from the arrangement of its vesicles in parallel layers along the
direction of flow. The last-named peculiarity is widely distributed
among the Tertiary lavas of North-Western Europe, and gives to their
weathered surfaces a deceptive resemblance to tuffs or other stratified
rocks (see Figs. 260, 310 and 311). It will be more particularly
referred to a few pages further on. In general, however, we may say
that the steam-cavities of lavas are quite irregular in size, shape
and distribution, sometimes increasing to such relative proportions
as to occupy most of the bulk of the rock, and in other places
disappearing, so as to leave the lava tolerably compact. When a lava
presents an irregularly vesicular character, like that of the slags of
an iron-furnace, it is said to be _slaggy_. When its upper surface is
rugged and full of steam-vesicles of all sizes up to large cavernous
spaces, it is said to be _scoriaceous_, and fragments of such a rock
ejected from a volcanic vent are spoken of as _scoriæ_.
Attention to the flattening of the steam-vesicles in cellular lavas,
which has just been alluded to as the result of the onward movement
of the still molten mass, may show, by the trend and grouping of
these elongated cavities, the probable direction of the flow of the
lava before it came to rest. Sometimes the vesicles have been drawn
out and flattened to such a degree that the rock has acquired in
consequence a fissile structure. In other instances, the vesicles have
been originally formed as long parallel and even branching tubes, like
the burrows of Annelids or the borings of _Teredo_. Some remarkable
examples of this exceptional structure have been obtained from the
Tertiary plateau-basalts of the Western Isles, of which an example is
represented in Fig. 2.
In many cases the vesicles extend through the whole thickness of a
lava. Frequently they may be found most developed towards the top and
bottom; the central portion of the sheet being compact, while the top
and bottom are rugged, cavernous or scoriaceous.
Though originally the vesicles and cavernous spaces, blown open by the
expansion of the vapours dissolved in molten lava, remained empty on
the consolidation of the rock, they have generally been subsequently
filled up by the deposit within them of mineral substances carried in
aqueous solution. The minerals thus introduced are such as might have
been derived from the removal of their constituent ingredients by the
solvent action of water on the surrounding rock. And as amygdaloids
are generally more decayed than the non-vesicular lavas, it has been
generally believed that the abstraction of mineral material and its
re-deposit within the steam-vesicles have been due to the influence of
meteoric water, which at atmospheric temperatures and pressures has
slowly percolated from the surface through the cellular lava, long
after the latter had consolidated and cooled, and even after volcanic
energy at the locality had entirely ceased.
[Illustration: Fig. 2.--Elongation and branching of steam-vesicles in a
lava, Kilninian, Isle of Mull, a little less than natural size.]
Examples, however, are now accumulating which certainly prove that, in
some cases, the vesicles were filled up during the volcanic period.
Among the Tertiary basalt-plateaux of the Inner Hebrides, for instance,
it can be shown that the lavas were already amygdaloidal before the
protrusion of the gabbros and granophyres which mark later stages of
the same continuous volcanic history, and even before the outpouring
of much of the basalt of these plateaux. Not improbably the mineral
secretions were largely due to the influence of hot volcanic vapours
during the eruption of the basalts. This subject will be again referred
to in the description of the Tertiary volcanic series.
Vesicular structure is more commonly and perfectly developed among the
lavas which are basic and intermediate in composition than among those
which are acid.
While the existence of a highly vesicular or scoriaceous structure may
generally be taken as proof that the rock displaying it flowed out at
the surface as a lava, other evidence pointing to the same conclusion
may often be gathered from the rocks with which the supposed lava is
associated. Where, for example, a scoriaceous lava is covered with
stratified deposits which contain pieces of that lava, we may be
confident that the rock is an interstratified or contemporaneous sheet.
It has been erupted after the deposition of the strata on which it
rests, and before that of the strata which cover it and contain pieces
of it. In such a case, the geological date of the eruption could be
precisely defined. Illustrations of this reasoning will be given in
Chapter iv., and in the account of the volcanic series of Carboniferous
age in Central Scotland, where a basic lava can sometimes be proved to
be a true flow and not an intrusive sill by the fact that portions of
its upper slaggy surface are enclosed in overlying sandstone, shale or
limestone.
2. The presence of GLASS, or of some result of the devitrification of
an original glass, is an indication that the rock which exhibits it has
once been in a state of fusion. Even where no trace of the original
vitreous condition may remain, stages in its devitrification, that is,
in its conversion into a stony or lithoid condition, may be traceable.
Thus what are called spherulitic and perlitic structures (which will
be immediately described), either visible to the naked eye or only
observable with the aid of the microscope, afford evidence of the
consolidation and conversion of a glassy into a lithoid substance.
Striking evidence of the former glassy, and therefore molten, condition
of many rocks now lithoid is to be gained by the examination of thin
slices of them under the microscope. Not only are vestiges of the
original glass recognizable, but the whole progress of devitrification
may be followed into a crystalline structure. The primitive
crystallites or microlites of different minerals may be seen to have
grouped themselves together into more or less perfect crystals, while
scattered crystals of earlier consolidation have been partially
dissolved in and corroded by the molten glass. These and other
characteristics of once fused rocks have to a considerable extent been
imitated artificially by MM. Fouqué and Michel Lévy, who have fused the
constituent minerals in the proper proportions.
Since traces of glass or of its representative devitrified structures
are so abundantly discoverable in lavas, we may infer the original
condition of most lavas to have been vitreous. Where, for instance, the
outer selvages of a basic dyke or sill are coated with a layer of black
glass which rapidly passes into a fine-grained crystalline basalt, and
then again into a more largely crystalline or doleritic texture in the
centre, there can be no hesitation in believing that glassy coating to
be due to the sudden chilling and consolidation of the lava injected
between the cool rocks that enclose it. The part that solidified first
may be regarded as probably representing the condition of the whole
body of lava at the time of intrusion. The lithoid or crystalline
portion between the two vitreous outer layers shows the condition which
the molten rock finally assumed as it cooled more slowly.
Some lavas, such as obsidians and pitchstones, have consolidated
in the glassy form. More usually, however, a lithoid structure
has been developed, the original glass being only discoverable by
the microscope, and often not even by its aid. Two varieties of
devitrification may be observed among lavas, which, though not marked
off from each other by any sharp lines, are on the whole distinctive of
the two great groups of acid and basic rocks.
(1) Among the acid rocks, what is called the Felsitic type of
devitrification is characteristic. Thus, obsidians pass by intermediate
stages from a clear transparent or translucent glass into a dull
flinty or horny mass. When thin slices of these transitional forms are
examined under the microscope, minute hairs and fibres or trichites,
which may be observed even in the most perfectly glassy rocks, are seen
to increase in number until they entirely take the place of the glass.
Microlites of definite minerals may likewise be observed, together with
indefinite granules, and the rock finally becomes a rhyolite, felsite
or allied variety (Fig. 3).
[Illustration: Fig. 3.--Microlites of the Pitchstone of Arran
(magnified 70 diameters).]
At the same time it should be observed that, even in the vitreous
condition of a lava, definite crystals of an early consolidation were
generally already present. Felspars and quartz, usually in large
porphyritic forms, may be seen in the glass, often so corroded as to
indicate that they were in course of being dissolved in the magma at
the time of the cooling and solidification of the mass. In obsidians
and pitchstones such relics of an earlier or derived series of
crystallized minerals may often be recognized, while in felsites and
quartz-porphyries they are equally prominent. Where large dispersed
crystals form a prominent characteristic in a rock they give rise to
what is termed the _Porphyritic_ structure.
[Illustration: Fig. 4.--Perlitic structure in Felsitic Glass, Isle of
Mull (magnified).]
[Illustration: Fig. 5.--Spherulitic structure (magnified).]
Accompanying the passage of glass into stone, various structures
make their appearance, sometimes distinctly visible to the naked
eye, at other times only perceptible with the aid of the microscope.
One of these structures, known as _Perlitic_ (Fig. 4), consists in
the formation of minute curved or straight cracks between which the
vitreous or felsitic substance, during its contraction in cooling,
assumed a finely globular form.
Another structure, termed _Spherulitic_ (Fig. 5), shows the development
of globules or spherules which may range from grains of microscopic
minuteness up to balls two inches or more in diameter. These not
infrequently present a well-formed internal fibrous radiation, which
gives a black cross between crossed Nicol prisms. Spherulites are more
especially developed along the margins of intrusive rocks, and may be
found in dykes, sills and bosses (see Figs. 375 and 377). Where the
injected mass is not thick it may be spherulitic to the very centre, as
can be seen among the felsitic and granophyric dykes of Skye.
Some felsitic lavas possess a peculiar nodular structure, which was
developed during the process of consolidation. So marked does this
arrangement sometimes become that the rocks which display it have
actually been mistaken for conglomerates. It is well exhibited among
the Lower Silurian lavas of Snowdon, the Upper Silurian lavas of
Dingle, and the Lower Old Red Sandstone lavas near Killarney.
[Illustration: Fig. 6.--Micropegmatitic or Granophyric structure in
Granophyre, Mull (magnified).]
[Illustration: Fig. 7.--Ophitic structure in Dolerite, Gortacloghan,
Co. Derry (magnified).]
A marked structure among some intrusive rocks, especially of an acid
composition, is that called _Micropegmatitic_ or _Granophyric_. It
consists in a minute intergrowth of two component minerals, especially
quartz and felspar, and is more especially characteristic of certain
granitic or granitoid rocks which have consolidated at some distance
from the surface and occur as bosses, sills and dykes. It is also met
with, however, in some basic sills. Examples of all these and other
structures will occur in the course of the following description of
British volcanic rocks.
(2) The second type of devitrification, conspicuous in rocks of
more basic composition, is marked by a more complete development of
crystallization. Among basic, as among acid rocks, there are proofs of
the consolidation of definite minerals at more than one period. Where
the molten material has suddenly cooled into a black glass, porphyritic
felspars or other minerals are often to be seen which were already
floating in the magma in its molten condition. During devitrification,
however, other felspars of a later period of generation made their
appearance, but they are generally distinguishable from their
predecessors. Probably most basic and intermediate rocks, when poured
out at the surface as lavas, were no longer mere vitreous material,
but had already advanced to various stages of progress towards a stony
condition. These stages are still to some extent traceable by the aid
of the microscope.
Microlites of the component minerals are first developed, which, if the
process of aggregation is not arrested, build up more or less perfect
crystals or crystalline grains of the minerals. Eventually the glass
may be so completely devitrified by the development of its constituent
minerals as to be wholly used up, the rock then becoming entirely
crystalline, or to survive only in scanty interstitial spaces. In the
family of the basalts and dolerites the gradual transition from a true
glass into a holocrystalline compound may be followed with admirable
clearness. The component minerals have sometimes crystallized in their
own distinct crystallographic forms (idiomorphic); in other cases,
though thoroughly crystalline, they have assumed externally different
irregular shapes, fitting into each other without their Proper
geometric boundaries (allotriomorphic).
A specially characteristic feature of many basic rocks is the presence
of what is termed an _Ophitic_ structure (Fig. 7). Thus the component
crystals of pyroxene occur as large plates separated and penetrated
by small needles and crystals of felspar. The portions of pyroxene,
divided by the enclosed felspar, are seen under the microscope to be in
optical continuity, and to have crystallized round the already formed
felspar. This structure is never found in metamorphic crystalline
rocks. It has been reproduced artificially from fusion by Messrs.
Fouqué and Michel Lévy.
The name _Variolitic_ is applied to another structure of basic rocks
(Fig. 8), in which, especially towards the margin of eruptive masses,
abundant spheroidal aggregates have been developed from the size of a
millet-seed to that of a walnut, imbedded in a fine-grained or compact
greenish matrix into which the kernels seem to shade off. These kernels
consist of silicates arranged either radially or in concentric zones.
3. Flow-structure is an arrangement of the crystals, vesicles,
spherulites, or devitrification-streaks in bands or lines, which sweep
round any enclosed object, such as a porphyritic crystal or detached
spherulite, and represent the curving flow of a mobile or viscous mass.
Admirable examples of this structure may often be observed in old
lavas, as well as in dykes and sills, the streaky lines of flow being
marked as distinctly as the lines of foam that curve round the boulders
projecting from the surface of a mountain-brook.
Flow-structure is most perfectly developed among the obsidians,
rhyolites, felsites and other acid rocks, of which it may be said to
be a frequently conspicuous character (Fig. 9). In these rocks it is
revealed by the parallel arrangement of the minute hair-like bodies and
crystals, or by alternate layers of glassy and lithoid material. The
streaky lines thus developed are sometimes almost as thin and parallel
as the leaves of a book. But they generally show interruptions and
curvatures, and may be seen to bend round larger enclosed crystals, or
to gather into eddy-like curves, in such a manner as vividly to portray
the flow of a viscous substance. These lines represent on a minute
scale the same flow-structure which may be traced in large sheets among
the lavas. The porphyritic crystals and the spherulites are also drawn
out in rows in the same general direction. Sometimes, indeed, the
spherulites have been so symmetrically grouped in parallel rows that
they appear as rod-like aggregates which extend along the margin of a
dyke.
[Illustration: Fig. 8.--Variolitic or Orbicular structure, Napoleonite,
Corsica (nat. size).]
Among lavas of more basic composition flow-structure is not so often
well displayed. It most frequently shows itself by the orientation
of porphyritic felspars or of lines of steam-vesicles. Occasionally,
however, sheets of basalt may be found in which a distinct streakiness
has been developed owing to variations in the differentiation of the
original molten magma. A remarkable and widespread occurrence of such a
structure is met with among the Tertiary basalt-plateaux of the Inner
Hebrides and the Faroe Islands. In the lower parts of these thick
accumulations of successive lava-sheets, a banded character is so
marked as to give the rocks the aspect of truly stratified deposits.
The observer, indeed, can hardly undeceive himself as to their real
nature until he examines them closely. As a full description of this
structure will be given in a later chapter, it may suffice to state
here that the banding arises from two causes. In some cellular lavas,
the vesicles are arranged in layers which lie parallel with the upper
and under surfaces of the sheets. These layers either project as
ribs or recede into depressions along the outcrop, and thus impart a
distinctly stratified aspect to the rock. More frequently, however,
the banded structure is produced by the alternation of different
varieties of texture, and even of composition, in the same sheet of
basalt. Lenticular seams of olivine-basalt may be found intercalated
in a more largely crystalline dolerite. These differences appear to
point to considerable variations in the constitution of the magma from
which the lavas issued--variations which already existed before the
discharge of these lavas, and which showed themselves in the successive
outflow of basaltic and doleritic material during the eruption of what
was really, as regards its appearance at the surface, one continuous
stream of molten rock. It is impossible to account for such variations
in the same sheet of lava by any process of differentiation in the
melted material during its outflow and cooling. Analogous variations
occur among the basic sills and bosses of the Tertiary volcanic series
of Britain. These, as will be more fully discussed in later chapters,
indicate a considerable amount of heterogeneity in the deep-seated
magma from which the intrusive sheets and bosses were supplied (see
vol. ii. pp. 329, 342).
[Illustration: Fig. 9.--Flow-structure in Rhyolite, Antrim, slightly
reduced.]
It is a common error to assume that flow-structure is a distinctive
character of lavas that have flowed out at the surface. In reality some
of the most perfect examples of the structure occur in dykes and sills,
both among acid and basic rocks. Innumerable instances might be quoted
from the British Isles in support of this statement.
Although, in the vast majority of cases, the presence of flow-structure
may be confidently assumed to indicate a former molten condition of
the rock in which it occurs, it is not an absolutely reliable test for
an igneous rock. Experiment has shown that under enormous pressure
even solid metals may be made to flow into cavities prepared for their
reception. Under the vast compression to which the earth's crust is
subjected during terrestrial contraction, the most obdurate rocks are
crushed into fragments varying from large blocks to the finest powder.
This comminuted material is driven along in the direction of thrust,
and when it comes to rest presents a streakiness, with curving lines
of flow round the larger fragments, closely simulating the structure
of many rhyolites and obsidians. It is only by attention to the local
surroundings that such deceptive resemblances can be assigned to their
true cause.
[Illustration: Fig. 10.--Lumpy, irregular trachytic Lava-streams
(Carboniferous), East Linton, Haddingtonshire.]
4. The DISPOSITION OF LAVAS IN SHEETS OR BEDS is the result of
successive outflows of molten rock. Such sheets may range from only a
yard or two to several hundred feet in thickness. As a rule, though
with many exceptions, the basic lavas, such as the basalts, appear
in thinner beds than the acid forms. This difference is well brought
out if we compare, for instance, the massive rhyolites or felsites of
North Wales with the thin sheets of basalt in Antrim and the Inner
Hebrides. The regularity of the bedded character is likewise more
definite among the basic than among the acid rocks, and this contrast
also is strikingly illustrated by the two series of rocks just referred
to. The rhyolites and felsites, sometimes also the trachytes and
andesites, assume lumpy, irregular forms, and some little care may be
required to trace their upper and under surfaces, and to ascertain
that they really do form continuous sheets, though varying much in
thickness from place to place (Fig. 10). Like modern acid lavas, they
seem to have flowed out in a pasty condition, and to have been heaped
up round the vents in the form of domes, or with an irregular hummocky
or mounded surface. The basalts, and dolerites, and sometimes the
andesites, have issued in a more fluid condition, and have spread out
in sheets of more uniform thickness, as may be instructively seen in
the sea-cliffs of Antrim, Mull, Skye, and the Faroe Islands, where the
horizontal or gently-inclined flows of basalt lie upon each other in
even parallel beds traceable for considerable distances along the face
of the precipices (Figs. 11, 265, and 286). The andesites of the Old
Red Sandstone (Figs. 99, 100) and Carboniferous series (Figs. 107, 108,
111, 112, 113, 123) in Scotland likewise form terraced hills.
The length of a lava-stream may vary within wide limits. Sometimes an
outflow of lava has not reached the base of the cone from the side of
which it issued, like the obsidian stream on the flanks of the little
cone of the island of Volcano. In other cases, the molten rock has
flowed for forty or fifty miles, like the copious Icelandic lava-floods
of 1783. In the basalt-plateaux of the Inner Hebrides a single sheet
may sometimes be traced for several miles.
[Illustration: Fig. 11.--View at the entrance of the Svinofjord, Faroe
Islands, illustrating the terraced forms assumed by basic lavas.
The island on the left is Borö, that in the centre Viderö, and that on
the right Svinö.]
Some lavas, more especially among the basic series, assume in cooling
a _Columnar structure_, of which two types may be noticed. In one of
these the columns pass with regularity and parallelism from the top
to the bottom of a bed (Figs. 171, 225). The basalt in which Fingal's
Cave, in the isle of Staffa, has been hollowed out may be taken as a
characteristic example (Fig. 266a). Not infrequently the columns are
curved, as at the well-known Clam-shell Cave of Staffa. In the other
type, the columns or prisms are not persistent, but die out into
each other and have a wavy, irregular shape, somewhat like prisms of
starch. These two types may occur in successive sheets of basalt,
or may even pass into each other. At Staffa the regularly columnar
bed is immediately overlain with one of the starch-like character.
The columnar structure in either case is a contraction phenomenon,
produced during the cooling and shrinking of the lava. But it is
difficult to say what special conditions in the lava were required for
its production, or why it should sometimes have assumed the regular,
at others the irregular form. It may be found not only in superficial
lavas but in equal perfection in some dykes and intrusive sills or
injections, as among the Tertiary volcanic rocks of the island of Canna
(Figs. 307 and 308).
The precipitation of a lava-stream into a lake or the sea may cause
the outer crust of the rock to break up with violence, so that the
still molten material inside may rush into the water. Some basic lavas
on flowing into water or into a watery silt have assumed a remarkable
spheroidal sack-like or pillow-like structure, the spheroids being
sometimes pressed into shapes like piles of sacks. A good instance
of this structure occurs in a basalt at Acicastello in Sicily.[1] A
similar appearance will be described in a later chapter as peculiarly
characteristic of certain Lower Silurian lavas associated with
radiolarian cherts in Britain and in other countries (Fig. 12).
[Footnote 1: See Prof. G. Platania in Dr. Johnston-Lavis' _South
Italian Volcanoes_, Naples (1891), p. 41 and plate xii.]
[Illustration: Fig. 12.--Sack-like or pillow-form structure of basic
lavas (Lower Silurian), Bennan Head, Ballantrae, Ayrshire.]
It probably seldom happens that a solitary sheet of lava occurs among
non-volcanic sedimentary strata, with no other indication around it
of former volcanic activity. Such an isolated record does not seem
to have been met with in the remarkably ample volcanic register of
the British Isles. The outpouring of molten rock has generally been
accompanied with the ejection of fragmentary materials. Hence among the
memorials of volcanic eruptions, while intercalated lavas are generally
associated with sheets of tuff, bands of tuff may not infrequently
be encountered in a sedimentary series without any lava. Instances
in illustration of these statements may be culled from the British
Palæozoic formations back even into the Cambrian system.
A characteristic feature of some interest in connection with the flow
of lava is the effect produced by it on the underlying rocks. If these
are not firmly compacted they may be ploughed up or even dislocated.
Thus the tuffs of the Velay have sometimes been plicated, inverted,
and fractured by the movement of a flowing current of basalt.[2] The
great heat of the lava has frequently induced considerable alteration
upon the underlying rocks. Induration is the most common result, often
accompanied with a reddening of the altered substance. Occasionally
a beautifully prismatic structure has been developed in the soft
material immediately beneath a basalt, as in ferruginous clay near the
village of Esplot in the Velay, in which the close-set columns are 50
centimetres long and 4 to 5 centimetres in diameter.[3] Changes of this
nature, however, are more frequent among sills than among superficial
lavas. Many examples of them may be gathered from the Scottish
Carboniferous districts.
[Footnote 2: M. Boule, _Bull. Cart. Géol. France_, No. 28, tom. iv.
(1892), p. 235.]
[Footnote 3: M. Boule. _Op. cit._ p. 234.]
Variations of structure in single lava-sheets.--From what has been said
above in regard to certain kinds of flow-structure among basic rocks,
it will be evident that some considerable range of chemical, but more
particularly of mineralogical, composition may be sometimes observed
even within the same sheet of lava. Such differences, it is true,
are more frequent among intrusive rocks, especially thick sills and
large bosses. But they have been met with in so many instances among
superficial lavas as to show that they are the results of some general
law, which probably has a wide application among the surface-products
of volcanic action. Scrope expressed the opinion that in the focus of
a volcano there may be a kind of filtration of the constituents of a
molten mass, the heavier minerals sinking through the lighter, so that
the upper portions of the mass will become more felspathic and the
lower parts more augitic and ferruginous.[4]
[Footnote 4: _Volcanoes_, p. 125.]
Leopold von Buch found that in some of the highly glassy lavas of the
Canary Islands the felspar increases towards the bottom of the mass,
becoming so abundant as almost to exclude the matrix, and giving rise
to a compound that might be mistaken for a primitive rock.[5]
[Footnote 5: _Description Physique des Isles Canaries_ (1836), p. 190.]
Darwin observed that in a grey basalt filling up the hollow of an
old crater in James Island, one of the Galapagos group, the felspar
crystals became much more abundant in the lower scoriaceous part, and
he discussed the question of the descent of crystals by virtue of their
specific gravity through a still molten lava.[6]
[Footnote 6: _Geological Observations on Volcanic Islands_ (1844), p.
117.]
Mr. Clarence King during a visit to Hawaii found that in every case
where he broke newly-congealed streamlets of lava, "the bottom of the
flow was thickly crowded with triclinic felspars and augites, while
the whole upper part of the stream was of nearly pure isotropic and
acid glass."[7] This subject will be again referred to when we come to
discuss the characters of intrusive sills and bosses, for it is among
them that the most marked petrographical variations may be observed.
Examples will be cited both from the intrusive and extrusive volcanic
groups of Britain.
[Footnote 7: _U.S. Geol. Exploration of the Fortieth Parallel_, vol. i.
(1878), p. 716.]
Volcanic cycles.--Closely related to the problem of the range of
structure and composition in a single mass of lava is another problem
presented by the remarkable sequence of different types of lava which
are erupted within a given district during a single volcanic period.
Nearly thirty years ago Baron von Richthofen drew attention to the
sequence of volcanic materials erupted within the same geographical
area. He showed, more especially from observations in Western America,
that a definite order of appearance in the successive species of lava
could be established, the earliest eruptions consisting of materials
of an intermediate or average composition, and those of subsequent
outflows becoming on the whole progressively more acid, but finishing
by an abrupt transition to a basic type. His sequence was as follows:
1. Propylite; 2. Andesite; 3. Trachyte; 4. Rhyolite; 5. Basalt.[8]
This generalisation has been found to hold good over wide regions of
the Old World as well as the New. It is not, however, of universal
application.[9] Examples are not uncommon of an actual alternation of
acid and basic lavas from the same, or at least from adjacent vents.
Such an alternation occurs among the Tertiary eruptions of Central
France and among those of Old Red Sandstone age in Scotland.
[Footnote 8: _Trans. Acad. California_, 1868. Prof. Iddings' _Journ.
Geol._, vol. i. (1893), p. 606.]
[Footnote 9: See Prof. Brögger, "Die Eruptivgesteine des
Kristianiagebietes," part ii. (1895), p. 175; _Zeitsch. Kryst. und
Mineral_, vol. xvi. (1890) p. 83. This author would, from this point
of view, draw a distinction between rocks which have consolidated deep
within the earth and those which have flowed out at the surface, since
he thinks that we are not justified in applying our experience of the
order of sequence in the one series to the other. Yet there can be
no doubt that in many old volcanic districts the masses that may be
presumed to have consolidated at a great depth have been in unbroken
connection with masses that reached the surface. These latter, as Prof.
Iddings has urged, furnish a much larger body of evidence than the
intrusive sheets and bosses.]
The range of variation in the nature of the eruptive rocks during the
whole of a volcanic period in any district may be termed "a volcanic
cycle." In Britain, where the records of many volcanic periods have
been preserved, a number of such cycles may be studied. In this way
the evolution of the subterranean magma during one geological age may
be compared with that of another. It will be one of the objects of the
following chapters to trace out this evolution in each period where
the requisite materials for the purpose are available. We shall find
that back to Archæan time a number of distinct cycles may be observed,
differing in many respects from each other, but agreeing in the general
order of development of the successive eruptions. Leaving these British
examples for future consideration, it may be useful to cite here a few
from the large series now collected from the European continent and
North America.[10]
[Footnote 10: Prof. M. Bertrand in a suggestive paper published in
1888 dealt with the general order of appearance of eruptive rocks in
different provinces of Europe. But the materials then at his command
probably did not warrant him in offering more than a sketch of the
subject, _Bull. Soc. Geol._, France, xvi. p. 573. In the same volume
there is a paper by M. Le Verrier, who announces his opinion that the
eruption of the basic rocks takes place in times of terrestrial calm,
while that of the acid rocks occurs in periods of great disturbance,
_op. cit._ p. 498. Compare also Prof. Brögger, _Die Eruptivgesteine des
Kristianiagebietes_, ii. p. 169.]
Among the older rocks of the European continent, Prof. Brögger
has shown that in the Christiania district the eruptive rocks
which traverse the Cambrian and Silurian formations began with the
outburst of basic material such as melaphyre, augite-porphyrite, and
gabbro-diabase, having from about 44 to about 52 per cent of silica.
These were followed by rocks with a silica-percentage ranging from
about 50 to 61, including some characteristic Norwegian rocks, like the
rhomben-porphyry. The acidity continued to increase, for in the next
series of eruptions the silica-percentage rose to between 60 and 67,
the characteristic rock being a quartz-syenite. Then came deep-seated
protrusions of highly acid rocks, varieties of granite, containing
from 68 to 75 per cent of silica. The youngest eruptive masses in the
district show a complete change of character. They are basic dykes
(proterobase, diabase, etc.).[11]
[Footnote 11: _Eruptivgest. Kristianiageb._, 1895.]
The same author institutes a comparison between the post-Silurian
eruptive series of Christiania and that of the Triassic system in the
Tyrol, and believes that the two cycles closely agree.[12]
[Footnote 12: _Op. cit._ He supposes in each case the pre-existence of
a parent magma from which the eruptive series started and which had
a silica-percentage of about 64 or 65. In this difficult subject it
is of the utmost importance to accumulate fact before proceeding to
speculation.]
During Tertiary time in Central France more than one cycle may be
made out in distinct districts. Thus in the Velay, during the Miocene
Period, volcanic activity began with the outpouring of basalts,
followed successively by trachytes, labradorites and augitic andesites,
phonolites and basalts. The Pliocene eruptions showed a reversion to
the intermediate types of augitic andesites and trachytes, followed by
abundant basalts, which continued to be poured forth in Pleistocene
time.[13]
[Footnote 13: M. Boule, "Description Géologique du Velay," _Bull.
Carte. Géol. France_, 1892. This author draws special attention to the
evidence for the alternation of basic and more acid material in the
Tertiary eruptions of Central France.]
Further north, in Auvergne, where the eruptions come down to a later
period, the volcanic sequence appears to have been first a somewhat
acid group of lavas (trachytes or domites), followed by a series of
basalts, then by andesites and labradorites, the latest outflows again
consisting of basalts.[14]
[Footnote 14: M. Michel Lévy, _Bull. Soc. Géol. France_, 1890, p. 704.]
Not less striking is the succession of lavas in the Yellowstone
region, as described by Mr. Iddings. The first eruptions consisted
of andesites. These were followed by abundant discharges of basalt,
succeeded by later outflows of andesite, and of basalt like that
previously erupted. After a period of extensive erosion, occupying a
prolonged interval of time, volcanic energy was renewed by the eruption
of a vast flood of rhyolite, after which came a feebler outflow of
basalt that brought the cycle to a close, though geysers and fumaroles
show that the volcanic fires are not yet entirely extinguished
below.[15]
[Footnote 15: _Journal of Geology_, Chicago, i. (1893) p. 606. See
also this author's excellent monograph on "Electric Peak and Sepulchre
Mountain," _12th Ann. Rep. U.S. Geol. Survey_ (1890-91), and Mr. H.
W. Turner on "The Succession of Tertiary Volcanic Rocks in the Sierra
Nevada of North America," _14th Ann. Rep. U.S. Geol. Survey_ (1892-93),
p. 493.]
But not only is there evidence of a remarkable evolution or succession
or erupted material within the volcanic cycle of a single geological
period. One of the objects of the present work is to bring forward
proofs that such cycles have succeeded each other again and again,
at widely separated intervals, within the same region. After the
completion of a cycle and the relapse of volcanic energy into repose,
there has been a renewal of the previous condition of the subterranean
magma, giving rise ultimately to a similar succession of erupted
materials.
If we are at a loss to account for the changes in the sequence of
lavas during a single volcanic cycle, our difficulties are increased
when we find that in some way the magma is restored each time to
somewhat the same initial condition. Analogies may be traced between
the differentiation which has taken place within a plutonic intrusive
boss or sill and the sequence of lavas in volcanic cycles. It can be
shown that though the original magma that supplied the intrusive mass
may be supposed to have had a fairly uniform composition deep down in
its reservoir, differentiation set in long before the intrusive mass
consolidated, the more basic constituents travelling outwards to the
margin and leaving the central parts more acid. If some such process
takes place within a lava-reservoir, it may account for a sequence of
variations in composition. But this would not meet all the difficulties
of the case, nor explain the determining cause of the separation of the
constituents within the reservoir of molten rock, whether arising from
temperature, specific gravity, or other influence. This subject will be
further considered in connection with intrusive Bosses.
Another fact which may be regarded as now well established is
the persistence of composition and structure in the lavas of all
ages. Notwithstanding the oft-repeated cycles in the character of
the magma, the materials erupted to the surface, whether acid or
basic, have retained with wonderful uniformity the same fundamental
characteristics. No part of the contribution of British geology to the
elucidation of the history of volcanic action is of more importance
than the evidence which it furnishes for this persistent sameness of
the subterranean magma. An artificial line has sometimes been drawn
between the volcanic products of Tertiary time and those of earlier
ages. But a careful study of the eruptive rocks of Britain shows that
no such line of division is based upon any fundamental differences.
The lavas of Palæozoic time have of course been far longer exposed
to alterations of every kind than those of the Tertiary periods, and
certain superficial distinctions may be made between them. But when
these accidental differences are eliminated, we find that the oldest
known lavas exhibit the same types of structure and composition
that are familiar in those of Tertiary and recent volcanoes. Many
illustrations of this statement will be furnished in later chapters.
As a particularly striking instance, I may cite here the most ancient
and most modern lavas of the Grand Cañon of the Colorado. Mr. Walcott
and Mr. Iddings have shown that in the Lower Cambrian, or possibly
pre-Cambrian, formations of that great gorge, certain basic lavas
were contemporaneously interstratified, which, in spite of their vast
antiquity, are only slightly different from the modern basalts that
have been poured over the surrounding plateau.[16]
[Footnote 16: _14th Annual Report U.S. Geol. Survey_ (1892-93).]
The chief lavas found in Britain.--Of the lavas which have been
poured out at the surface within the region of the British Isles, the
following varieties are of most frequent occurrence. In the acid series
are Rhyolites and Felsites, but the vitreous forms are probably all
intrusive. The intermediate series is represented by Trachytes and
Andesites (Porphyrites), which range from a glassy to a holocrystalline
structure. The basic series includes Dolerites, Diabases, Basalts,
Limburgites (or Magma-basalts, containing little or no felspar), and
Picrites or other varieties of Peridotites. The intrusive rocks display
a greater variety of composition and structure.
ii. VOLCANIC AGGLOMERATES, BRECCIAS AND TUFFS
The coarser fragmentary materials thrown from volcanic vents are known
as Agglomerates where they show no definite arrangement, and especially
where they actually fill up the old funnels of discharge. When they
have accumulated in sheets or strata of angular detritus outside an
active vent they are termed Breccias, or if their component stones have
been water-worn, Conglomerates. The finer ejected materials may be
comprehended under the general name of Tuffs.
Although these various forms of pyroclastic detritus consist as a
rule of thoroughly volcanic material, they may include fragments of
non-volcanic rocks. This is especially the case among those which
represent the earliest explosions of a volcano. The first efforts to
establish an eruptive vent lead to the shattering of the terrestrial
crust, and the consequent discharge of abundant debris of that crust.
The breccias or agglomerates thus produced may contain, indeed, little
or no truly volcanic material, but may be made up of fragments of
granite, gneiss, sandstone, limestone, shale, or whatever may happen to
be the rocks through which the eruptive orifice has been drilled. If
the first explosions exhausted the energy of the vent, it may happen
that the only discharges from it consisted merely of non-volcanic
debris. Examples of this kind have been cited from various old volcanic
districts. A striking case occurs at Sepulchre Mountain in the
Yellowstone Park, where the lower breccias, the product of the earliest
explosions of the Electric Peak volcano, and attaining a thickness of
500 feet, are full of pieces of the Archæan rocks which underlie the
younger formations of that district.[17] These non-volcanic stones do
not occur among the breccias higher up. Obviously, however, though
most abundant at first, pieces of the underlying rocks may reappear in
subsequent discharges, wherever by the energy of explosion, fragments
are broken from the walls of a volcanic chimney and hurled out of the
crater. Illustrations of these features will be given in the account of
the British Carboniferous, Permian and Tertiary volcanic rocks.
[Footnote 17: Prof. J. P. Iddings, _12th Ann. Rep. U.S. Geol. Survey_
(1890-91), p. 634.]
It will be obvious that where the component materials of such
fragmentary accumulations consist entirely of non-volcanic rocks, great
caution must be exercised in attributing them to volcanic agency. Two
sources of error in such cases may here be pointed out. In the first
place, where angular detritus has been precipitated into still water,
as, for instance, from a crag or rocky declivity into a lake, a very
coarse and tumultuous kind of breccia may be formed. It is conceivable
that, in course of time, such a breccia may be buried under ordinary
sediments, and may thereby be preserved, while all trace of its parent
precipice may have disappeared. The breccia might resemble some true
volcanic agglomerates, but the resemblance would be entirely deceptive.
In the second place, notice must be taken of the frequent results
of movements within the terrestrial crust, whereby rocks have not
only been ruptured but, as already pointed out, have been crushed
into fragments. In this way, important masses of breccia or
conglomerate have been formed, sometimes running for a number of
miles and attaining a breadth of several hundred feet. The stones,
often in huge blocks, have been derived from the surrounding rocks,
and while sometimes angular, are sometimes well-rounded. They are
imbedded in a finer matrix of the same material, and may be scattered
promiscuously through the mass, in such a way as to present the closest
resemblance to true volcanic breccia. Where the crushed material has
included ancient igneous rocks it might deceive even an experienced
geologist. Indeed, some rocks which have been mapped and described as
volcanic tuffs or agglomerates are now known to be only examples of
"crush-conglomerates."[18]
[Footnote 18: For an account of "crush-conglomerates," see Mr.
Lamplugh's paper on those of the Isle of Man, _Quart. Journ. Geol.
Soc._, li. (1895), p. 563. Mr. M'Henry has pointed to probable cases of
mistake of this kind in Ireland, _Nature_, 5th March 1896. A. Geikie,
_Geol. Mag._ November 1896.]
Not only have vast quantities of detritus of non-volcanic rocks been
shot forth from volcanic vents, but sometimes enormous solid masses of
rock have been brought up by ascending lavas or have been ejected by
explosive vapours. Every visitor to the puys of Auvergne will remember
the great cliff-like prominence of granite and mica-schist which, as
described long ago by Scrope, has been carried up by the trachyte of
the Puy Chopine, and forms one of the summits of the dome (Fig. 344).
The same phenomenon is observable at the Puy de Montchar, where large
blocks of granite have been transported from the underlying platform.
Abich has described some remarkable examples in the region of Erzeroum.
The huge crater of Palandokän, 9687 feet above the sea contains, in
cliff-like projections from its walls as well as scattered over its
uneven bottom, great masses of marmorised limestone and alabaster,
associated with pieces of the green chloritic schists, serpentines
and gabbros of the underlying non-volcanic platform. These rocks,
which form an integral part of the structure of the crater, have been
carried up by masses of trachydoleritic, andesitic and quartz-trachytic
lavas.[19] Examples will be given in a later chapter showing how
gigantic blocks of mica-schist and other rocks have been carried many
hundred feet upwards and buried among sheets of lava or masses of
agglomerates during the Tertiary volcanic period in Britain (Fig. 262).
[Footnote 19: Abich, _Geologie des Armenischen Hochlandes_ (Part ii.,
western half), 1882, p. 76.]
In the vast majority of cases, the fragmentary substances ejected
by ancient volcanic explosions, like those of the present day, have
consisted wholly or mainly of material which existed in a molten
condition within the earth, and which has been violently expelled to
the surface. Such ejected detritus varies from the finest impalpable
dust or powder up to huge masses of solid rock. These various materials
may come from more than one source. Where a volcanic orifice is blown
out through already solidified lavas belonging to previous eruptions,
the fragments of these lavas may accumulate within or around the vent,
and be gradually consolidated into agglomerate or breccia. Again,
explosions within the funnel may break up lava-crusts that have there
formed over the cooling upper surface of the column of molten rock. Or
the uprising lava in the chimney may be spurted out in lumps of slag
and bombs, or may be violently blown out in the form of minute lapilli,
or of extremely fine dust and ashes.
Although in theory these several varieties of origin may be
discriminated, it is hardly possible always to distinguish them among
the products of ancient volcanic action. In the great majority of cases
old tuffs, having been originally deposited in water, have undergone
a good deal of decomposition, and such early alteration has been
aggravated by the subsequent influence of percolating meteoric water.
Where disintegration has not proceeded too far, the finer particles of
tuffs may be seen to consist of minute angular pieces of altered glass,
or of microlites or crystals, or of some vitreous or semi-vitreous
substance, in which such microlites and crystals are enclosed. It has
already been stated that the occurrence of glass, or of any substance
which has resulted from the devitrification of glass, may be taken as
good evidence of former volcanic activity.
Most commonly, especially in the case of tuffs of high antiquity, like
those associated with the Palæozoic formations, the fresh glassy and
microlitic constituents, so conspicuous in modern volcanic ashes, are
hardly to be recognised. The finer dust which no doubt contained these
characteristic substances has generally passed into dull, earthy,
granular, or structureless material, though here and there, among basic
tuffs, remaining as palagonite. In the midst of this decayed matrix,
the lapilli of disrupted lavas may endure, but it may be difficult or
impossible to decide whether they were derived from the breaking up of
older lavas by explosion, or from the blowing out of the lava-crusts
within the funnel.
The cellular structure, which we have seen to be a markedly volcanic
peculiarity among the lavas, is not less so in their fragments among
the agglomerates, breccias and tuffs; indeed it may be said to be
eminently characteristic of them. The vesicles in the lapilli, bombs,
and blocks are sometimes of large size, as in masses of ejected slag,
but they range down to microscopic minuteness. The lapilli of many old
tuffs are sometimes so crowded with such minute pores, as to show that
they were originally true pumice.
The composition of tuffs must obviously depend upon that of the lavas
from which they were derived. But their frequently decayed condition
makes it less easy, in their case, to draw definite boundary-lines
between varieties. In a group of acid lavas, the tuffs may be expected
to be also acid, while among intermediate or basic lavas, the tuffs
will generally be found to correspond. There are, however, exceptions
to this general rule. As will be afterwards described in detail,
abundant felsitic tuffs may be seen among the andesitic lavas of Lower
Old Red Sandstone age in Scotland, and rhyolitic tuffs occur also among
the Tertiary basalts of Antrim.
As a rule, basic and intermediate tuffs, like the lavas from which
they have been derived, are rather more prone to decomposition than
the acid varieties. One of their most characteristic features is the
presence in them of lapilli of a minutely vesicular pumice, which will
be more particularly described in connection with volcanic necks, of
which it is a characteristic constituent. Occasional detached crystals
of volcanic minerals, either entire or broken, may be detected in
them, though perhaps less frequently than in the agglomerates. The
earthy matrix is generally greenish in colour, varying into shades of
brick-red, purple and brown.
The acid tuffs are, on the whole, paler in colour than the others,
sometimes indeed they are white or pale grey, but graduate into
tones of hæmatitic red or brown, the varying ferruginous tints being
indicative of stages in the oxidation of the iron-bearing constituent
minerals. Small rounded lapilli or angular fragments of felsite or
rhyolite may be noticed among them, sometimes exhibiting the most
perfect flow-structure. As typical examples of such tuffs, I may refer
to those of the Pentland Hills, near Edinburgh, and those that lie
between the two groups of basalt in Antrim.
[Illustration: Fig. 13.--Alternations of coarser and finer Tuff.]
Thrown out promiscuously from active vents, the materials that form
tuffs arrange themselves, on the whole, according to relative size
over the surface on which they come to rest, the largest being
generally grouped nearest to the focus of discharge, and the finest
extending farthest from it. As the volcanoes of which records have been
preserved among the geological formations were chiefly subaqueous, the
fragmentary substances, as they fell into water, would naturally be to
some extent spread out by the action of currents or waves. They would
thus tend to take a more or less distinctly stratified arrangement.
Moreover, as during an eruption there might be successive paroxysms
of violence in the discharges, coarser and finer detritus would
successively fall over the same spot. In this way, rapid alternations
of texture would arise (Fig. 13). A little experience will enable the
observer to distinguish between such truly volcanic variations and
those of ordinary sedimentation, where, for instance, layers of gravel
and sand repeatedly alternate. Besides the volcanic nature of the
fragments and their non-water-worn forms, he will notice that here and
there the larger blocks may be placed on end--a position the reverse
of that usual in the disposal of aqueous sediments, but one which is
not infrequently assumed by ejected stones, even when they fall through
some little depth of water. Further, the occurrence of large pieces of
lava, scattered at random through deposits of fine tuff, would lead him
to recognize the tumultuous discharges of a volcanic focus, rather than
the sorting and sifting action of moving water.
Admirable illustrations of these various characteristics may be
gathered in endless number from the Palæozoic volcanic chronicles of
Britain. I may especially cite the basin of the Firth of Forth as a
classical region for the study of Carboniferous examples.
[Illustration: Fig. 14.--Alternations of Tuff (_t_, _t_,) with
non-volcanic sediment (_l_, _l_).]
When the conditions of modern volcanic eruptions are considered, it
will be seen that where ejected ashes and stones fall into water, they
will there mingle with any ordinary sediment that may be in course
of deposition at the time. There will thus be a blending of volcanic
and non-volcanic detritus, and the transition between the two may be
so insensible that no hard line of demarcation can be drawn. Such
intermingling has continually taken place during past ages. One of the
first lessons learnt by the geologist, who begins the study of ancient
volcanic records, is the necessity of recognizing this gradation of
material, and likewise the frequently recurring alternations of true
tuff with shale, sandstone, limestone or other entirely non-volcanic
detritus (Fig. 14). He soon perceives that such facts as these furnish
him with some of the most striking proofs of the reality and progress
of former eruptions. The intermingling of much ordinary detritus
with the volcanic material may be regarded as indicative either of
comparatively feeble activity, or at least of considerable distance
from the focus of discharge. It is sometimes possible to trace such
intermixtures through gradually augmenting proportions of volcanic dust
and stones, until the deposit becomes wholly volcanic in composition,
and so coarse in texture as to indicate the proximity of the eruptive
vent. On the other hand, the gradual decrease of the volcanic ejections
can be followed in the upward sequence of a series of stratified
deposits, until the whole material becomes entirely non-volcanic.
The occurrence of thin partings of tuff between ordinary sedimentary
strata points to occasional intermittent eruptions of ashes or stones,
the vigour and duration of each eruptive interval being roughly
indicated by the thickness and coarseness of the volcanic detritus.
The pauses in the volcanic activity allowed the deposit of ordinary
sediment to proceed unchecked. The nature of such non-volcanic
intercalations gives a clue to the physical conditions of sedimentation
at the time, while their thickness affords some indication of the
relative duration of the periods of volcanic repose.
A little reflection will convince the observer that in such a section
as that represented in Fig. 14 the volcanic intercalations must be
regarded as a mere local accident. Evidently the normal conditions
of sedimentation at the time these strata were accumulated are
indicated by the limestone bands (_l_, _l_). Had there been no volcanic
eruptions, a continuous mass of limestone would have been deposited,
but this continuity was from time to time interrupted by the explosions
that gave rise to the intercalated bands of tuff (_t_, _t_).
The application of these rules of geological evidence will be best
understood from actual examples of their use. Many illustrations of
them will be subsequently given, more especially from the volcanic
records of the Carboniferous period.
One of the most interesting peculiarities of interstratified tuffs is
the not infrequent occurrence of the remains of plants and animals
imbedded in them. Such remains would have been entombed in the ordinary
sediment had there been no volcanic eruptions, and their presence in
the tuffs is another convincing proof of contemporaneous volcanic
action during the deposition of a sedimentary series. But they may
be made to furnish much more information as to the chronology of the
eruptions and the physical geography of the localities where the
volcanoes were active, as will be set forth farther on.
Tuffs, as already remarked, frequently occur without any accompaniment
of lava, although lavas seldom appear without some tuff. We thus
learn that in the past, as at present, discharges of fragmentary
materials alone were more common than the outflow of lava by itself.
The relative proportions of the lavas and tuffs in a volcanic series
vary indefinitely. In the Tertiary basalt-plateaux of Britain,
the lavas succeed each other, sheet above sheet, for hundreds of
feet, with few and trifling fragmental intercalations. Among the
Carboniferous volcanic ejections, on the other hand, many solitary or
successive bands of tuff may be observed without any visible sheets
of lava. Viewed broadly, however, in their general distribution
during geological time, the two great groups of volcanic material may
be regarded as having generally appeared together. In all the great
volcanic series, from the base of the Palæozoic systems up to the
Tertiary plateaux, lavas and tuffs are found associated, much as they
are among the ejections of modern volcanoes. They often alternate, and
thus furnish evidence as to oscillations of energy at the eruptive
vents.
Now and then, by the explosions from a volcano at the present day, a
single stone may be ejected at such an angle and with such force as to
fall to the ground at a long distance from the vent. In like manner,
among the volcanic records of former periods, we may occasionally
come upon a single block of lava imbedded among tuffs or even in
non-volcanic strata. Where such a stone has fallen upon soft sediment,
it can be seen to have sunk into it, pressing down the layers beneath
it, and having the subsequently deposited layers heaped over it. An
ejected block of this nature is represented among the tuffs shown in
Fig. 13. Another instance from a group of non-volcanic sediments is
given in Fig. 15, and is selected from a number of illustrations of
this interesting feature which have been observed among the Lower
Carboniferous formations of the basin of the Firth of Forth. A solitary
block, imbedded in a series of strata, would not, of course, by itself
afford a demonstration of volcanic activity. There are various ways
in which such stones may be transported and dropped over a muddy
water-bottom. They may, for example, be floated off attached to
sea-weeds, or wrapped round by the roots of trees. But where a block
of basalt or other volcanic rock has obviously descended with such
force as to crush down the deposits on which it fell, and when lavas
and tuffs are known to exist in the vicinity, there can be little
hesitation in regarding such a block as having been ejected from a
neighbouring vent, either during an explosion of exceptional violence
or with an unusually low angle of projection.
[Illustration: Fig. 15.--Ejected block of Basalt which has fallen among
Carboniferous shales and limestones, shore, Pettycur, Fife.]
In conclusion, reference may conveniently be made here to another
variety of fragmental volcanic materials which cannot always be
satisfactorily distinguished from true tuffs, although arising from a
thoroughly non-volcanic agency. Where a mass of lava has been exposed
to denudation, as, for instance, when a volcanic island has been
formed in a lake or in the sea, the detritus worn away from it may be
spread out like any other kind of sediment. Though derived from the
degradation of lava, such a mechanical deposit is not properly a tuff,
nor can it even be included among strictly volcanic formations. It may
be called a volcanic conglomerate, rhyolitic conglomerate, diabase
sandstone, felsitic shale, or by any other name that will adequately
denote its composition and texture. But it may not afford proof of
strictly contemporaneous volcanic activity. All that we are entitled
to infer from such a deposit is that, at the time when it was laid
down, volcanic rocks of a certain kind were exposed at the surface and
were undergoing degradation. But the date of their original eruption
may have been long previous to that of the formation of the detrital
deposit from their waste.
Nevertheless, it is sometimes possible to make sure that the
conglomerate or sandstone, though wholly due to the mechanical
destruction of already erupted lavas, was in a general sense
contemporaneous with the volcanoes that gave forth these lavas.
The detrital formation may be traced perhaps up to the lavas from
which it was derived, and may be found to be intercalated in the
same sedimentary series with which they are associated. Or it may
contain large bombs and slags, such as most probably came either
directly from explosions or from the washing down of cinder-cones or
other contemporaneously existing volcanic heaps. Examples of such
intercalated conglomerates will be given from the Lower Old Red
Sandstone of Central Scotland and from the Tertiary volcanic plateaux
of the Inner Hebrides.
CHAPTER IV
Materials erupted at the Surface--Extrusive Series--_continued_.
iii. Types of old Volcanoes--1. The Vesuvian Type; 2. The
Plateau or Fissure Type; 3. The Puy Type. iv. Determination
of the relative Geological Dates of ancient Volcanoes. v. How
the Physical Geography associated with ancient Volcanoes is
ascertained.
Having now taken note of the various materials ejected to the surface
from volcanic orifices, we may pass to the consideration of these
orifices themselves, with the view of ascertaining under what various
conditions volcanic action has taken place in the geological past. We
have seen that modern and not long extinct volcanoes may be grouped
into three types, and a study of the records of ancient volcanoes shows
that the same types may be recognized in the eruptions of former ages.
The following chapters will supply many illustrations of each type from
the geological history of the British Isles. In dealing with these
illustrations, however, we must ever bear in mind the all-powerful
influence of denudation. We ought not to expect to meet with the
original forms of the volcanoes. Some little reflection and experience
may be required before we can realize under what aspect we may hope
to recognize ancient and much-denuded volcanoes. It may therefore be
of advantage to consider here, in a broad way, which of the original
characters are most permanent, and should be looked for as mementoes of
ancient volcanoes after long ages of denudation.
iii. TYPES OF OLD VOLCANOES
The three forms of ancient volcanoes now to be discussed are--1st, the
Vesuvian type; 2nd, the Plateau or Fissure type; and 3rd, the Puy type.
1. _The Vesuvian Type._--In this kind of volcano, lavas and fragmental
ejections are discharged from a central vent, which is gradually built
up by successive eruptions of these materials. As the cone increases
in size, parasitic cones appear on its sides, and in the energy and
completeness of their phenomena become true volcanoes, almost rivalling
their parent mountain. Streams of lava descend upon the lower grounds,
while showers of dust and ashes are spread far and wide over the
surrounding country.
If a transverse section could be made of a modern Vesuvian cone, the
volcanic pile would be found to consist of alternations of lavas and
tuffs, thickest at the centre, and thinning away in all directions.
At some distance from the crater, these volcanic materials might be
seen to include layers of soil and remains of land-vegetation, marking
pauses between the eruptions, during which soil accumulated and plants
sprang up upon it. Where the lavas and ashes had made their way into
sheets of fresh water or into the sea, they would probably be found
interstratified with layers of ordinary sediment containing remains of
the animal or vegetable life of the time.
[Illustration: Fig. 16.--Effects of denudation on a Vesuvian cone.]
Conceive now the effects of prolonged denudation upon such a pile of
volcanic rocks. The cone will eventually be worn down, the crater will
disappear, and the only relics of the eruptive orifice may be the
top of the central lava-column and of any fragmental materials that
solidified within the vent (Fig. 16). The waste will, on the whole, be
greater at the cone than on the more level areas beyond. It might, in
course of time, reach the original surface of the ground on which the
volcano built up its heap of ejected material. The central lava-plug
might thus be left as an isolated eminence rising from a platform of
older non-volcanic rocks, and the distance between it and the dwindling
sheets of lava and tuff which came out of it would then be continually
increased as their outcrop receded under constant degradation.
This piece of volcanic history is diagrammatically illustrated in Fig.
16. The original forms of the central volcano and of its parasitic
cones are suggested by the dotted lines in the upper half of the
Figure. All this upper portion has been removed by denudation, and the
present surface of the ground is shown by the uppermost continuous
line. The general structure of the volcanic pile is indicated
underneath that line--the lenticular sheets of lava and tuff (_l_,
_l_), the dykes (_d_, _d_), and the lavas (_p_, _p_) and agglomerates
(_a_, _a_) of the central vent and of the subordinate cones.
The waste, though greatest on the higher ground of the great cone,
would not stop there. It would extend over the flatter area around
the volcano. Streams flowing over the plain would cut their way down
through the lavas and tuffs, eroding ravines in them, and leaving
them in detached and ever diminishing outliers on the crests of the
intervening ridges. We can easily picture a time when the last of
these relics would have been worn away, and when every vestige of
the volcanic ejections would have been removed, save the lava-column
marking the site of the former vent.
Every stage in this process of effacement may be recognized in actual
progress among the extinct volcanoes of the earth's surface. Probably
nowhere may the phenomena be more conveniently and impressively studied
than among the volcanic districts of Central France. On the one hand,
we meet there with cinder-cones so perfect that it is hard to believe
them to have been silent ever since the beginnings of history. On the
other hand, we see solitary cones of agglomerate or of lava, which have
been left isolated, while their once overlying and encircling sheets
of ejected material have been so extensively worn away as to remain
merely in scattered patches capping the neighbouring hills. Valleys
many hundreds of feet in depth have been cut by the rivers through the
volcanic sheets and the underlying Tertiary strata and granite since
the older eruptions ceased. And yet these eruptions belong to a period
which, in a geological sense, is quite recent. It is not difficult to
contemplate a future time, geologically not very remote, when in the
valley of the Loire not a trace will remain of the wonderfully varied
and interesting volcanic chronicle of that district, save the plugs
that will mark the positions of the former active vents.
In the British Islands, ancient volcanoes of the Vesuvian type are well
represented among the Palæozoic systems of strata. Their preservation
has been largely due to the fact that they made their appearance in
areas that were undergoing slow subsidence. Their piles of erupted
lava and ashes were chiefly heaped up over the sea-floor, and were
buried under the sand, silt and ooze that gathered there. Thus
covered up, they were protected from denudation. It is only in much
later geological ages that, owing to upheaval, gradual degradation
of the surface, and removal of their overlying cover of stratified
formations, they have at last been exposed to waste. The process of
disinterment may be observed in many different stages of progress. In
some localities, only the tops of the sheets of lava and tuff have
begun to show themselves; in others, everything is gone except the
indestructible lava-plug.
These inequalities of denudation arise not only from variations in the
durability of volcanic rocks, but still more from the relative position
of these rocks in the terrestrial crust, and the geological period at
which, in the course of the general lowering of the surface, they have
been laid bare. Not only are volcanic rocks of many different ages,
and lie, therefore, on many successive platforms within the crust of
the earth: their places have been still further dependent upon changes
in the arrangement of that crust. Fracture, upheaval, depression,
curvature, unconformable deposition of strata, have contributed to
protect some portions, while leaving others exposed to attack. Hence
it happens that the volcanic record varies greatly in its fulness of
detail from one geological system or one district to another. Some
chapters have been recorded with the most surprising minuteness, so
that the events which they reveal can be realized as vividly as those
of a modern volcano. Others, again, are meagre and fragmentary, because
the chronicle is still for the most part buried underground, or because
it has been so long exposed at the surface that only fragments of it
now remain there.
In the descriptions which will subsequently be given of ancient
British volcanoes of the Vesuvian type, it will be shown that at many
successive periods during Palæozoic time, and at many distinct centres,
lavas and tuffs have been piled up to a depth of frequently more
than 5000 feet--that is to say, higher than the height of Vesuvius.
Sometimes the vent from which these materials were ejected can be
recognized. In other places, it is still buried under later formations,
or has been so denuded as to be represented now merely by the column of
molten or fragmental rock that finally solidified in it. Examples will
be quoted of such ancient vents, measuring not less than two miles in
diameter, with subsidiary "necks" on their flanks, like the parasitic
cones on Etna.
I shall show that while the ejected volcanic products have accumulated
in greatest depth close to the vent that discharged them, they die
away as they recede from it, sometimes so rapidly that a volcanic pile
which is 7000 feet thick around its source may entirely thin out and
disappear in a distance of not more than ten or twelve miles. I shall
point out how, as the lavas and tuffs are followed outwards from their
centre, they not only get thinner, but are increasingly interstratified
among the sedimentary deposits with which they were coeval, and that in
this way their limits, their age, and the geographical conditions under
which they were accumulated can be satisfactorily fixed.
As illustrations of the Vesuvian type in the volcanic history of
Britain, I may refer to the great Lower Silurian volcanoes of Cader
Idris, Arenig, Snowdon and the Lake District, and to the Old Red
Sandstone volcanoes of Central Scotland.
2. _The Plateau_ or _Fissure type_ is, among modern volcanoes, best
developed in Iceland, as will be more fully detailed in Chapter xl.
In that island, during a volcanic eruption, the ground is rent open
into long parallel fissures, only a few feet or yards in width, but
traceable sometimes for many miles, and descending to an unknown depth
into the interior. From these fissures lava issues--in some cases
flowing out tranquilly in broad streams from either side, in other
cases issuing with the discharge of slags and blocks of lava which are
piled up into small cones set closely together along the line of the
rent. It was from a fissure of this kind that the great eruption of
1783 took place--the most stupendous outpouring of lava within historic
time.
By successive discharges of lava from fissures, or from vents opening
on lines of fissure, wide plains may be covered with a floor of rock
hundreds or thousands of feet in thickness, made up of horizontal beds.
The original topography, which might have been undulating and varied,
is completely buried under a vast level lava-desert.
The rivers which drained the country before the beginning of the
volcanic history will have their channels filled up, and will be driven
to seek new courses across the lava-fields. Again and again, as fresh
eruptions take place, these streams will be compelled to shift their
line of flow, each river-bed being in turn sealed up in lava, with
all its gravels, silts and drift-wood. But the rain will continue to
fall, and the drainage to seek its way seaward. When the last eruption
ceases, and the rivers are at length left undisturbed at their task
of erosion, they will carve that lava-floor into deep gorges or open
valleys. Where they flow between the lavas and the slopes against which
these ended, they will cut back the volcanic pile, until in course of
time the lavas will present a bold mural escarpment to the land that
once formed their limit. The volcanic plain will become a plateau,
ending off in this vertical wall and deeply trenched by the streams
that wind across it. And if the denudation is continued long enough,
the plateau will be reduced to detached hills, separated by deep and
wide valleys.
[Illustration: Fig. 17.--Section to illustrate the structure of the
Plateau type.]
This geological history is illustrated by the diagram in Fig. 17.
The stippled ground underneath (_x_, _x_) represents the original
undulating surface of the country on which the plateau eruptions were
poured out. The lavas of these eruptions are shown by the horizontal
lines to have entirely buried the heights and hollows of the old land,
and to have risen up to the upper dotted line, which may be taken to
mark the limit reached by the accumulation of volcanic material. The
dark lines (_d_, _d_) which come up through the bedded lavas indicate
the dykes with their connected vents. Denudation has since stripped off
the upper part of the volcanic series down to the uppermost continuous
black line which represents the existing surface of the ground. The
level sheets of lava have been deeply trenched, and in one instance the
valley has not only been cut through the volcanic pile, but has been
partly eroded out of the older rocks below. To the right and left, the
lavas end off abruptly in great escarpments.
The succession of events here depicted has occurred more than once in
Britain. The Plateau type is chiefly developed in this country among
the great Tertiary basalt districts of Antrim and the Inner Hebrides,
which reappear in the Faroe Islands, and again still farther north in
Iceland. But it also occurs among the volcanic rocks of the Old Red
Sandstone and Carboniferous periods.
As compared with the other volcanic types, that of the Plateaux is
distinguished by the wide extent of surface which its rocks cover, by
the great preponderance of lavas over tuffs, and by the regularity
and persistence of the individual sheets of rock. In Britain, the
plateau-lavas are even still often approximately horizontal, and lie
piled on each other in tolerably regular beds to a thickness of 1000,
and in one place to more than 3000 feet. They form wide level or
gently undulating tablelands, which rise in bold escarpments from the
surrounding country and have been deeply carved into valleys. The sides
of their cliffs and slopes are marked by parallel lines of terrace,
arising from the outcrop of successive sheets of lava (Figs. 11, 265).
With the Tertiary basalt-plateaux are connected thousands of dykes,
that follow each other along nearly parallel lines in a general
north-westerly direction, and mark the position of fissures up which
the molten lava ascended. Occasional necks of agglomerate or basalt
indicate the sites of some of the eruptive vents.
The Carboniferous volcanic plateaux have been more extensively denuded
than those of Tertiary age, so that a large number of their vents have
been laid bare. In general these vents are of comparatively small size,
though larger than those of the Carboniferous Puys. In some districts,
abundant dykes traverse the rocks on which the plateaux rest, though
the fissures seem to have been less numerous than in Tertiary time.
3. _The Puy type_, as before remarked, takes its name from the
well-known _puys_, or volcanic cones, of Central France. Volcanoes
of this type form conical hills, generally of small size, consisting
usually of fragmental materials, sometimes of lava. Where a cone is
partially effaced by a second, and even by a third, successive slight
shiftings of the vent are to be inferred (see Figs. 29 and 214). In
many cases, no lava has issued from such cones, nor were the ashes
and cinders dispersed far from the vent. Hence, in the progress of
denudation, cones of this kind are easily effaced.
From the uniformity of composition of their materials, the simplicity
and regularity of their forms, and their small size, it may be inferred
that many of these cones were the products of single eruptions. They
may conceivably have been thrown up in a few days, or even in a single
day. The history of Monte Nuovo, in the Bay of Naples, which was formed
within twenty-four hours in the year 1538, is a memorable example of
the rapidity with which a cone more than 400 feet high may be thrown up
at some distance from a central vent.
The smallest independent volcanoes are included in the Puy type. In
many instances the diameter of the funnel has not exceeded a few yards;
the largest examples of the type seldom exceed 1000 feet in width.
Where lavas have been discharged, as well as ashes and stones, a more
vigorous activity is indicated than where merely cones of tuff were
formed. The lavas may come from more than one side of a cone, and
may flow in opposite directions for a distance of several miles. It
is observable that considerable streams of lava have issued from the
base of a cinder-cone without disturbing it. The molten rock has found
a passage between the loose materials and the surface on which they
rest,[20] though, in some cases, the cone may have been thrown up after
the emission of the lava.
[Footnote 20: M. Boule, _Bull. Carte Géol. France_, No. 28, tome iv. p.
232.]
In the history of a puy there is commonly a first discharge of
fragmentary material; then lava may flow out, followed by a final
discharge of loose stones and ashes. Hence the products of such a vent
group themselves into three layers--two of breccia separated by an
intervening sheet of lava.[21]
[Footnote 21: M. Boule, _Bull. Carte Géol. France_, No. 28, tome iv.]
Great changes are wrought on puys and their connected lavas and tuffs
during the progress of denudation. The cones are eventually destroyed,
and only a stump of agglomerate or lava is left to mark its place.
The connection of a lava-stream with its parent vent may likewise be
effaced, and the lava itself may be reduced to merely a few separate
patches, perhaps capping a ridge, while the surrounding ground has
been hollowed into valleys. If the waste continues long enough, even
these outliers will disappear, and nothing but the neck or stump of the
little volcano will remain.
[Illustration: Fig. 18.--Diagram illustrating the structure and
denudation of Puys.]
The accompanying diagram (Fig. 18) may help to make these changes more
intelligible. The upper dotted lines show the original forms of three
puys with the covering of loose materials discharged by them over the
surrounding ground. The lower shaded portion represents the surface
as left by denudation, and a section of the three vents beneath that
surface. The whole of the cones and craters has here been swept away,
and only the volcanic "neck" is in each case left. In the vent to the
right, the material that fills it up is a coarse agglomerate, which
projects as a rounded dome above the surrounding country. The central
pipe is filled with fragmentary materials, through which molten rock
has risen, giving off dykes and veins. In the vent to the left hand,
only lava is seen to occupy the orifice, representing the column of
molten rock which solidified there and brought the activity of this
little volcano to an end. It will be observed that in each of these
volcanic hills the present outlines are very far from being those
of the original volcano, and that the eminence projects because of
its greater resistance to the forces of denudation that have not
only removed the superficial volcanic material, but have made some
progress in lowering the level of the ground on which that material was
accumulated.
The typical area for the study of Puys is the extraordinarily
interesting volcanic region of Central France. There the volcanic
cones are clustered in irregular groups, sometimes so close as to be
touching each other; elsewhere separated by intervals of several miles.
They may be traced in all stages of decay, from the most perfect cones
and craters to the isolated eminence that marks the site of a once
active chimney. Their lavas, too, may be seen as detached fragments of
plateaux, many hundred feet above the valleys that have been excavated
since they flowed.[22]
[Footnote 22: See Desmarest's classic map and his papers in _Mem. Acad.
Roy. Sciences_, Paris, 1774, 1779; _Journ. de Physique_, 1779; Scrope's
_Geology of Central France_, 1827, and _Extinct Volcanoes of Central
France_, 1858; Lecoq's _Époques Géologiques de l'Auvergne_, 1867; M.
Michel Lévy, _Bull. Soc. Géol. France_, 1890, p. 688; M. Boule, _Bull.
Carte Géol. France_, No. 28, tome iv. 1892.]
Another well-known region of modern Puys is that of the Eifel, where
the cones and craters are often so fresh that it is difficult to
believe them to be prehistoric.[23] One of the most remarkable denuded
puy-regions in Europe covers a wide territory in the Swabian Alps
of Würtemberg. No fewer than 125 denuded necks filled with tuff,
agglomerate and basalt have there been mapped and described. They
are of higher antiquity than the Upper Miocene strata, and have thus
probably been exposed to prolonged denudation. In external aspect
and internal structure they present the closest parallel to the
Carboniferous and Permian "necks" of Britain described in Books VI. and
VII. of the present work.[24]
Among the Palæozoic volcanoes of Britain many admirable illustrations
of the Puy type are to be found. Their cones are almost always entirely
gone, though traces of them occasionally appear. The "necks" that show
the position of the vents are in some districts crowded together as
thickly as those of Auvergne. During the Carboniferous and Permian
periods in Central Scotland, clusters of such little volcanoes must
have risen among shallow lagoons and inland sheets of water, casting
out their ashes and pouring forth their little streams of lava over the
water-bottom around them and then dying out. As these eruptions took
place in a region that was gradually subsiding, the cones and their
ejected ashes and lavas were one by one submerged, the looser materials
being washed down and spread out among the silt, sand or mud, and
enveloping the remains of any plants or animals that might be strewn
over the floor of the lake or sea. Hence the Puys of Palæozoic time in
Britain have been preserved with extraordinary fulness of detail. They
have been dissected by denudation, both among the hills of the interior
and along the margin of the sea. Their structure can thus be in some
respects made out even more satisfactorily than that of the much
younger and more perfect cones of Central France.
[Footnote 23: The Eifel district has been fully described by Hibbert,
Von Dechen, and other writers. Von Dechen's little handbooks to the
Eifel and Siebengebirge are useful guides.]
[Footnote 24: These Würtemberg vents have been elaborately described
and discussed by Professor W. Branco of Tübingen in his _Schwabens 125
Vulkan-Embryonen und deren tufferfülte Ausbruchsröhren, das grösste
Gebiet chemaliger Maare auf der Erde_, Stuttgart, 1894.]
iv. DETERMINATION OF THE RELATIVE GEOLOGICAL DATES OF ANCIENT VOLCANOES
In themselves, accumulations of volcanic materials do not furnish any
exact or reliable evidence of the geological period in which they
were erupted. The lavas of the early Palæozoic ages may, indeed, on
careful examination, be distinguished from those of Tertiary date, but,
as we have seen, the difference is rather due to the effects of age
and gradual alteration than to any inherent fundamental distinction
between them. In all essential particulars of composition and internal
structure, the lavas of the Cambrian or Silurian period resemble
those of Tertiary and modern volcanoes. The igneous magmas which
supply volcanic vents thus appear to have been very much what they are
now from early geological epochs. At least no important difference,
according to relative age, has yet been satisfactorily established
among them.
But although the rocks themselves afford no precise or trustworthy clue
to their date, yet where they have been intercalated contemporaneously
among fossiliferous stratified formations, of which the geological
horizon can be determined from included organic remains, it is easy
to assign them to their exact place in geological chronology. A
determination of this kind is only an application of the general
principle on which the sequence of the geological record is defined. A
few illustrations will suffice to make this point quite obvious.
Among the volcanic tuffs in the upper part of Snowdon various fossils
occur, which are identical with those found in the well-known Bala
Limestone. As the accepted reading of such evidence, we conclude
that these tuffs must therefore be of the same geological age as
that limestone. Now the position of this seam of rock has been well
established as a definite horizon in the series of Lower Silurian
formations. And we consequently without hesitation place the eruptions
of the Snowdon volcano on that same platform, and speak of them as
belonging to the Bala division of the Lower Silurian period.
Again, in West Lothian the tuffs and lavas ejected from many scattered
puys were interstratified among shales and limestones in which the
characteristic fossils of the Carboniferous Limestone are abundant.
There cannot, therefore, be any doubt that these eruptions were much
younger than those of Snowdon, and that they took place at the time
when the Carboniferous Limestone was being deposited. We thus speak of
them as belonging to volcanoes which were active in that early part
of the Carboniferous period to which the thick Mountain Limestone of
Ireland and Derbyshire belongs.
As yet another illustration of the determination of geological age,
an example from the plateau-type of eruption may be given. The great
basalt-plateaux of Antrim and the Inner Hebrides are built up of lavas
that lie unconformably on the Chalk. They are thus proved to be later
than the Cretaceous system, and this deduction would hold true even if
no organic remains were found associated with the volcanic rocks. But
here and there, intercalated between the basalts, lie layers of shale,
limestone and tuff containing well-preserved remains of plants which
are recognizable as older Tertiary forms of vegetation. This fossil
evidence definitely places the date of the eruptions in older Tertiary
time.
It is clear that, proceeding on this basis of reasoning, we may arrange
the successive volcanic eruptions of any given district, make out their
order of sequence in time, and thus obtain materials for a consecutive
history of them. Or, proceeding from that district into other regions,
we may compare its volcanic phenomena with theirs, determine the
relative dates of their respective eruptions, and in this way compile
a wider history of volcanic action in past time. It is on these
principles that the general and detailed chronology of the volcanic
rocks of the British Isles has been worked out, and that the following
chapters have been arranged.
v. HOW THE PHYSICAL GEOGRAPHY ASSOCIATED WITH ANCIENT VOLCANOES IS
ASCERTAINED
While the materials erupted from old volcanic vents tell plainly enough
their subterranean origin, they may leave us quite in the dark as to
the conditions under which they were thrown out at the surface. Yet a
careful examination of the strata associated with them may throw much
light on the circumstances in which the eruptions took place. Many of
the results of such examination will be given in subsequent chapters. I
will here submit illustrations of how four different phases of physical
geography during former volcanic eruptions may be satisfactorily
determined.
[Illustration:
Fig. 19.--Section illustrating submarine eruptions; alternations
of lavas and tuffs with limestones and shales full of marine
organisms.
]
1. _Submarine Eruptions._--As by far the largest accessible part of the
crust of the earth consists of old marine sediments, it is natural that
the volcanic records preserved in that crust should be mainly those of
submarine eruptions. That many lavas during the geological past were
poured out upon the sea-bottom is plainly shown by the thick beds of
marine organisms which they have overspread and which lie above them
(Fig. 19). In Central Scotland, for example, sheets of basalt have
flowed over a sea-bottom on which thick groves of crinoids, bunches
of coral and crowds of sea-shells were living. Not less striking is
the evidence supplied by bands of tuff. Around Limerick, for instance,
the thick Carboniferous Limestone is interrupted by many thin layers
of tuff marking intervals when showers of volcanic dust fell over
the sea-bottom, killing off the organisms that lived there. But the
limestone that overlies these volcanic intercalations is again crowded
with fossils, proving that the crinoids, corals and shells once more
spread over the place and flourished as abundantly as ever above the
tuff.
The accompanying diagram (Fig. 19) illustrates these statements. At
the bottom a thick mass of limestone (_l_) full of crinoids, corals,
brachiopods and other marine organisms bears witness to a long time of
repose, when the clear sea-water teemed with life. At last a volcanic
explosion took place, which threw out the first seam of tuff (_t_).
But this was only a transient interruption, for the accumulation of
calcareous sediment was immediately resumed, and the next band of
limestone was laid down. Thereafter a more prolonged or vigorous
eruption ejected a larger mass of dust and stones, which fell over the
bottom and prevented the continuation of the limestone. But that the
sea still abounded in life is shown by the numerous organisms imbedded
in the second stratified band of tuff. At last an access of volcanic
vigour gave vent to a stream of slaggy lava, which rolled over the
sea-bottom and solidified in the thick sheet of amydaloidal basalt
marked B. This outflow was followed by a further discharge of ashes and
stones, which, from their absence of stratification, may be supposed to
have been the result of a single explosion, or at least to have fallen
too rapidly for the marine currents to rearrange them in layers. When
the water cleared, the abundant sea-creatures returned, and from their
crowded remains limestone once more gathered over the bottom. Yet the
volcanic history had not then reached its close, for again there came a
discharge of ashes, followed by the outpouring of a second lava, which
consolidated as a sheet of columnar basalt (B').
It is not necessary, in order to prove the eruptions to have been
submarine, that organic remains should be found in the tuffs or between
them. If the volcanic ejections are intercalated among strata which
elsewhere can be proved to be marine, their discharge must obviously
have taken place under the sea. The vent that discharged them may have
raised its head above the sea-level, but its lavas and tuffs were
spread out over the adjoining sea-floor.
2. _Lacustrine Eruptions._--The same line of evidence furnishes
proof that some volcanoes arose in inland sheets of water. If their
products are interstratified among sandstones, gravels and shell-marls,
wherein the remains of land-plants, insects and lacustrine shells, are
preserved, we may be confident that the eruptions took place in or
near to some lake-basin. The older lavas and tuffs of Central France
supply an instructive example of such an association. In Britain, the
abundant and extensive outpouring of lavas and tuffs during the time
of the Lower Old Red Sandstone probably occurred in large lakes. Among
the sediments of these bodies of water, interstratified between the
volcanic sheets, remains of land-plants are abundant, together with,
here and there, those of myriapods washed down from the woodlands, and
of many forms of ganoid fishes.
[Illustration: Fig. 20.--Diagram illustrating volcanic eruptions on a
river-plain.]
3. _Fluviatile Eruptions._--Volcanoes have sometimes arisen on
river-plains or on the edges of valleys and gorges, and have poured out
their lavas and discharged their ashes over the channels or alluvial
lands of the streams. Volcanic materials, usurping the water-channels,
bury or are interstratified with fluviatile sand or shingle, containing
perhaps remains of the vegetation or animal life of the surrounding
land. There may thus be a constant shifting of the river-courses,
and a consequent deposit of fluviatile sediment at many successive
levels among the lavas and tuffs. In Fig. 20 some of these changes
are indicated in a series of bedded lavas (_l_). The lower part of the
diagram shows the dying out of a bed of river gravel (_g_) against the
sloping end of a lava-stream, and the sealing up of this intercalation
by a fresh outpouring of lava. Higher up in the diagram a section is
shown of a gully or ravine which has been cut out of the lavas by a
stream, and has become choked up with water-worn detritus. Subsequent
outflows of lava have rolled over this channel and sealed it up.
Examples of such intercalations of lava with old river deposits, and
of the burying of water-courses, will be cited in the account of the
Tertiary volcanic plateaux of Britain in Chapter xxxviii.
4. _Terrestrial Eruptions._--That volcanoes in former times broke
out on land as well as in water may readily be expected. But it is
obvious that the proofs of a terrestrial origin may not be always
easy to obtain, for every land-surface is exposed to denudation; and
thus the relics of the eruptions of one age may be effaced by the
winds, rains, frosts and rivers of the next. In assigning any volcanic
group to a terrestrial origin, we may be guided partly by negative
evidence, such as the absence of all trace of marine organisms in
any of the sedimentary layers associated with the group. But such
evidence standing by itself would not be satisfactory or sufficient.
If, however, between the sheets of lava there occur occasional
depressions, filled with hardened sediment full of land-plants, with
possibly traces of insects and other terrestrial organisms, we may with
some confidence infer that these silted-up hollows represent pools or
lakes that gathered on the surface of the lava-sheets, and into which
the vegetation of the surrounding ground was blown or washed. Rain
falling on the rugged surface of a lava-field would naturally gather
into pools and lakes, as the bottoms of the hollows became "puddled"
by the gradual decay of the rock and the washing of fine silt into the
crevices of the lava.
[Illustration: Fig. 21.--Diagram illustrating volcanic eruptions on a
land-surface.]
Again, it may be expected that prolonged exposure to the air would give
rise to disintegration of the lava and to the consequent formation of
soil. Terrestrial vegetation would naturally spring up on such soil;
trees might take root upon it. Hence, if another lava-flood deluged the
surface, the soil and its vegetable mantle would be entombed under the
molten rock.
These geological changes are represented diagrammatically in Fig. 21.
Two hollows among the lavas are there shown to have been filled with
silt, including successive layers of vegetation now converted into
coal. One of the soils (_s_) is marked between the lavas, and the
charred stump of a tree with its roots still in another layer of soil
higher up is seen to have been engulphed in the overlying sheet of
melted rock.
Admirable illustrations of this succession of events are to be
encountered among the great Tertiary basaltic plateaux which cover so
large an area in the north-west of Europe. Not only has no trace of
any marine organism been found among their interstratified sedimentary
layers, but they have yielded a terrestrial flora which is preserved
in hollows between the successive sheets of basalt. A full account of
these rocks will be given in Book VIII.
CHAPTER V
Underground Phases of Volcanic Action. B. Materials injected or
consolidated beneath the Surface--Intrusive Series: I. Vents
of Eruption--i. Necks of Fragmentary Materials; ii. Necks of
Lava-form Materials; iii. Distribution of Vents in relation
to Geological Structure-Lines; iv. Metamorphism in and around
Volcanic Cones, Solfataric Action; v. Inward Dip of Rocks
towards Necks; vi. Influence of contemporaneous Denudation upon
Volcanic Cones; vii. Stages in the History of old Volcanic Vents.
In our profound ignorance of the nature of the earth's interior, we
know as yet nothing certain regarding the condition and distribution
there of those molten materials which form the prime visible source of
volcanic energy. By the study of volcanoes and their products we learn
that the fused substances are not everywhere precisely the same and
do not remain absolutely uniform, even in the same volcanic region.
But in what manner and from what causes these variations arise is
still unknown. We are further aware that the molten magma, under a
centre of volcanic disturbance, manifests from time to time energetic
movements which culminate in eruptions at the surface. But what may
be the exciting cause of these movements, to what depth they descend,
and over what extent of superficies they spread, are matters regarding
which nothing better than conjecture can as yet be offered. It is true
that, in some cases, a magma of fairly uniform composition has been
erupted over a vast tract of the earth's surface, and must have had
a correspondingly wide extent within the terrestrial crust. Thus in
the case of the older Tertiary volcanic eruptions of North-Western
Europe, basalt of practically the same composition was discharged
from thousands of fissures and vents distributed from the south of
Antrim northward beyond the Inner Hebrides, through the chain of the
Faroe Islands and over the whole breadth of Iceland. Under the British
Isles alone, the subterranean reservoirs of molten lavas must have
been at least 40,000 square miles in united area. If they stretched
continuously northwards below the Faroe Islands and Iceland, as is
highly probable, that is, for 600 miles further, their total extent may
have been comparable to such a region as Scandinavia.
Was this vast underground body of lava part of a universal liquid mass
within the globe, or was it rather of the nature of one or more lakes
or large vesicles within the crust? We can only offer speculation for
answer. On the other hand, there seems to be good proof that in some
districts, both now and in former geological periods, such differences
exist between the materials ejected from vents not far distant from
each other as to show the existence of more limited distinct reservoirs
of liquid rock underneath.
Some of the questions here asked will be further dealt with in later
pages in connection with such geological evidence as can be produced
regarding them. But it will be found that at every step in the
endeavour to ascertain the origin of volcanic phenomena difficulties
present themselves which are now and may long remain insoluble.
I. Vents of Eruption
It is a general belief that the first stage in the formation of a
volcano of the Vesuvian type by the efforts of subterranean energy
is the rending of the terrestrial crust in a line of fissure. Some
of the most remarkable groups of active volcanoes on the face of the
globe are certainly placed in rows, as if they had risen along some
such great rents. The actual fissure, however, is not there seen, and
its existence is only a matter of probable inference. Undoubtedly the
effect of successive eruptions must be to conceal the fissure, even if
it ever revealed itself at the surface.
What is supposed to have marked the initial step in the formation of a
great volcano is occasionally repeated in the subsequent history of the
mountain. During the convulsive shocks that precede and accompany an
eruption, the sides of the cone, and even sometimes part of the ground
beyond, are rent open, occasionally for a distance of several miles,
and on the fissures thus formed minor volcanoes are built up.
It is in Iceland, as already stated, that the phenomena of fissures
are best displayed. There the great deserts of lava are from time to
time dislocated by new lines of rent, which ascend up to the surface
and stretch for horizontal distances of many miles. From these long
narrow chasms lava flows out to either side; while cones of slag and
scoriæ usually form upon them. This interesting eruptive phase will be
more fully described in the chapters dealing with the Tertiary volcanic
rocks of Britain.
There can be no doubt, however, that in a vast number of volcanic
vents of all geological periods no trace can be discovered of their
connection with any fissure in the earth's crust. Such fissures may
indeed exist underneath, and may have served as passages for the
ascent of lava to within a greater or less distance from the surface.
But it is certain that volcanic energy has the power of blowing out
an opening for itself through the upper part of the crust without the
existence of any visible fissure there. What may be the limits of depth
at which this mode of communication with the outer air is possible we
do not yet know. They must obviously vary greatly according to the
structure of the terrestrial crust on the one hand, and the amount
and persistence of volcanic energy on the other. We may suppose that
where a fissure terminates upward under a great depth of overlying
rock, the internal magma may rise up to the end of the rent, and even
be injected laterally into the surrounding parts of the crust, but
may be unable to complete the formation of a volcano by opening a
passage to the surface. But where the thickness of rock above the end
of the fissure is not too great, the expansive energy of the vapours
absorbed in the magma may overcome the resistance of that cover, and
blow out an orifice by which the volcanic materials can reach the
surface. In the formation of new cones within the historic period at a
distance from any central volcano, the existence of an open fissure at
the surface has not been generally observed. When, for example, Monte
Nuovo was formed, it rose close to the shore among fields and gardens,
but without the appearance of any rent from which its materials were
discharged.
That in innumerable instances during the geological past, similar vents
have been opened without the aid of fissures that reached the surface,
will be made clear from the evidence to be drawn from the volcanic
history of the British Isles. So abundant, indeed, are these instances
that they may be taken as proving that, at least in the Puy type of
volcanoes, the actual vents have generally been blown out by explosions
rather than by the ascent of fissures to the open air.
In cases where, as in Iceland, fissures open at the surface and
discharge lava there, the channel of ascent is the open space
between the severed walls of the rent. Within this space the lava
will eventually cool and solidify as a _dyke_. It is obvious that a
comparatively small amount of denudation will suffice to remove all
trace of the connection of such a dyke with the stream of lava that
issued from it. Among the thousands of dykes belonging to the Tertiary
period in the British Islands, it is probable that many may have
served as lines of escape for the basalt at the surface. But it is now
apparently impossible to distinguish between those which had such a
communication with the outer air and those that ended upward within
the crust of the earth. The structure of dykes will be subsequently
discussed among the subterranean intrusions of volcanic material.
In an ordinary volcanic orifice the ground-plan is usually irregularly
circular or elliptical. If that portion of the crust of the earth
through which the vent is drilled should be of uniform structure, and
would thus yield equally to the effects of the volcanic energy, we
might anticipate that the ascent and explosion of successive globular
masses of highly heated vapours would give rise to a cylindrical
pipe. But in truth the rocks of the terrestrial crust vary greatly in
structure; while the direction and force of volcanic explosions are
liable to change. Hence considerable irregularities of ground-plan are
to be looked for among vents.
Some of these irregularities are depicted in Fig. 22, which represents
the ground plan of some vents from the Carboniferous volcanic districts
of Scotland. They are all drawn on the same scale. Other examples will
be cited in later chapters from the same and other parts of the British
Isles.
Some of the most marked departures from the normal and simple type of
vent occur where two orifices have been opened close to each other,
or where the same vent has shifted its position (Figs. 29, 125, 205,
and 214). Curiously irregular or elongated forms may thus arise in the
resultant "necks" now visible at the surface. Many striking examples
of these features may be seen among the Carboniferous and Permian
volcanoes to be afterwards described. Occasionally where an open
fissure has served as a vent it has given rise to a long dyke-like mass
(No. 1 in Fig. 22).
[Illustration: Fig. 22.--Ground-plans of some Volcanic vents from the
Carboniferous districts of Scotland.
1. Linhope Burn, near Mosspaul, Roxburghshire; the shaded parts
are intrusions of trachytic material. 2. Hazelside Hill, two
miles W. from Newcastleton, Roxburghshire. 3. St. Magdalen's,
Linlithgow. 4. South-west side of Coom's Fell (see Fig. 174).
5. Neck on Greatmoor, Roxburghshire. 6. Pester Hill, Tarras
Water. 7. Head of Routing Burn, S.E. side of Hartsgarth Fell,
Liddesdale. 8. Hartsgarth Flow, Liddesdale.
]
The size of a volcanic vent may vary indefinitely from a diameter of
not more than a yard or two up to one or two or more miles. As a rule,
the smaller the vents the more numerously are they crowded together. In
the case of large central volcanoes like Etna, where many subsidiary
vents, some of them forming not inconsiderable hills, may spring up
along the sides of the parent cone, denudation will ultimately remove
all the material that was heaped up on the surface, and leave the
stumps or necks of the parasitic vents in groups around the central
funnel.
Each volcanic chimney, by which vapours, ashes or lava are discharged
at the surface, may be conceived to descend in a more or less nearly
vertical direction until it reaches the surface of the lava whence the
eruptions proceed. After the cessation of volcanic activity, this pipe
will be left filled up with the last material discharged, which will
usually take the form of a rudely cylindrical column reaching from the
bottom of the crater down to the lava-reservoir. It will be obvious
that no matter how great may be the denudation of the volcano, or how
extensive may be the removal of the various materials discharged over
the surrounding ground, the pipe or funnel with its column of solid
rock must still remain. No amount of waste of the surface of the land
can efface that column. Successively lower and yet lower levels may be
laid bare in it, but the column itself goes still further down. It will
continue to make its appearance at the surface until its roots are laid
bare in the lava of the subterranean magma. Hence, of all the relics of
volcanic action, the filled-up chimney of the eruptive vent is the most
enduring. Save where it may have been of the less deep-seated nature
of a "hornito" upon a lava-stream, we may regard it as practically
permanent. The full meaning of these statements will be best understood
from a consideration of the numerous illustrations to be afterwards
given.
The stumps of volcanic columns of this nature, after prolonged
denudation, generally project above the surrounding ground as rounded
or conical eminences known as "Necks" (Fig. 23. See also Figs. 52,
82, 102, 109, 123, 133, 144, 178, 192, 195, 203, 204, 209, 294, 298,
306 and 310). Their outlines, however, vary with the nature of their
component materials. The softer rocks, such as tuffs and agglomerates,
are apt to assume the form of smooth domes or cones, while the harder
and especially the crystalline rocks rise into irregular, craggy
hills. Occasionally, indeed, it may happen that a neck makes no
prominence on the surface of the ground, and its existence may only
be discoverable by a careful examination of the geological structure
of the locality. Now and then an old vent will be found not to form a
hill, but to sink into a hollow. Such variations, however, have little
or no reference to original volcanic contours in the history of the
localities which display them. They arise mainly from the differing
hardness and structure of the materials that have filled the vents, and
the consequent diversity in the amount of resistance which they have
offered to the progress of denudation.
[Illustration: Fig. 23.--View of an old volcanic "Neck" (The Knock,
Largs, Ayrshire, a vent of Lower Carboniferous age).]
The materials now found in volcanic funnels are of two kinds: 1st,
Fragmentary, derived from volcanic explosions; and 2nd, Lava-form,
arising from the ascent and consolidation of molten rock within the
funnel.
i. _Necks of Fragmentary Materials_
By far the most satisfactory evidence of a former volcanic orifice
is furnished by a neck of fragmentary materials. Where "bosses" of
crystalline rock rise to the surface and assume the outward form of
necks, we cannot always be certain that they may not have been produced
by subterranean intrusions that never effected any connection with the
surface. In other words, such bosses may not mark volcanic orifices
at all, though they may have been part of the underground protrusions
of volcanoes in their neighbourhood. But where the chimney has been
filled with debris, there can be no doubt that it truly marks the site
of a once active volcano. The fragmentary material is an eloquent
memorial of the volcanic explosions that drilled the vent, kept it
open, and finally filled it up. These explosions could not have taken
place unless the elastic vapours which caused them had found an escape
from the pressure under which they lay within the crust of the earth.
Now and then, indeed, where the outpouring of lava or some other cause
has left cavernous spaces within the crust, there may conceivably
be some feeble explosion there, and some trifling accumulation of
fragmentary materials. But we may regard it as practically certain that
the mass of tumultuous detritus now found in volcanic necks could not
have been formed unless where a free passage had been opened from the
molten magma underneath to the outer surface of the planet.
Considerable diversity may be observed in the nature and arrangement of
the fragmentary materials in volcanic necks. The chief varieties may
be arranged in four groups: (1) Necks of non-volcanic detritus; (2)
Necks of volcanic agglomerate or tuff; (3) Necks of agglomerate or tuff
with a central plug of lava; and (4) Necks of agglomerate or tuff with
veins, dykes or some lateral irregular mass of lava.
(1) _Necks of non-volcanic Detritus._--During the first convulsive
efforts of a volcanic focus to find a vent at the surface, the
explosions that eventually form the orifice do so by blowing out in
fragments the solid rocks of the exterior of the terrestrial crust. Of
the detritus thus produced, shot up the funnel and discharged into the
air, part may gather round the mouth of the opening and build up there
a cone with an enclosed crater, while part will fall back into the
chimney, either to accumulate there, should the explosions cease, or
to be thrown out again, should they continue. In the feeblest or most
transient kinds of volcanic energy, the explosive vapours may escape
without any accompanying ascent of the molten magma to the surface,
and even without any sensible discharge of volcanic "ashes" from that
magma. In such cases, as I have already pointed out, the detritus of
the non-volcanic rocks, whatever they may be, through which volcanic
energy has made an opening, accumulate in the pipe and eventually
consolidate there. Examples of this nature will be adduced in later
chapters from the volcanic districts of Britain.
Where only non-volcanic materials fill up a vent we may reasonably
infer that the eruptions were comparatively feeble, never advancing
beyond the initial stage when elastic vapours made their escape with
explosive violence, but did not lead to the outflow of lava or the
discharge of ashes. In the great majority of necks, however, traces of
the earliest eruptions have been destroyed by subsequent explosions,
and the uprise of thoroughly volcanic fragments. Yet even among these
fragments, occasional blocks may be detected which have been detached
from the rocks forming the walls of the funnel.
The general name of Agglomerate, as already stated, is given to all
accumulations of coarse, usually unstratified, detritus in volcanic
funnels, irrespective of the lithological nature of the materials.
For further and more precise designation, when an agglomerate is
mainly made up of fragments of one particular rock, the name of that
rock may be prefixed as sandstone-agglomerate, granite-agglomerate,
basalt-agglomerate, trachyte-agglomerate. Volcanic agglomerate is a
useful general term that may include all the coarser detritus ejected
by volcanic action.
Where volcanic explosions have been of sufficient violence or long
continuance, the upper part of the funnel may be left empty, and on the
cessation of volcanic activity, may be filled with water and become a
lake. The ejected detritus left round the edge of the orifice sometimes
hardly forms any wall, the crater-bottom being but little below the
level of the surrounding ground. Explosion-lakes are not infrequent in
Central France and the Eifel (Maare). A more gigantic illustration is
afforded by the perfectly circular crater of Coon Butte in Arizona,
about 4000 feet in diameter and 600 feet deep. It has been blown out in
limestone, the debris of which forms a rampart 200 feet high around it.
Examples will afterwards be cited from the Tertiary volcanic plateaux
of North-Western Europe. Vents may also be formed by an engulphment
or subsidence of the material, like that which has taken place at the
great lava cauldron of Hawaii, still an active volcano. The picturesque
Crater Lake of Oregon is an admirable instance of this structure.
(2) _Necks of Agglomerate or Tuff._--In the vast majority of cases, the
explosions that clear out a funnel through the rocks of the upper part
of the crust do not end by merely blowing out these rocks in fragments.
The elastic vapours that escape from the molten lava underneath are
usually followed by an uprise of the lava within the pipe. Relieved
from the enormous pressure under which it had before lain, the lava as
it ascends is kept in ebullition, or may be torn into bombs which are
sent whirling up into the air, or may even be blown into the finest
dust by the sudden expansion of the imprisoned steam. If its ascent is
arrested within the vent, and a crust is formed on the upper surface of
the lava-column, this congealed crust may be disrupted and thrown out
in scattered pieces by successive explosions, but may re-form again and
again.
[Illustration: Fig. 24.--Section of neck of agglomerate, rising through
sandstones and shales.]
In many vents, both in recent and in ancient times, volcanic progress
has never advanced beyond this early stage of the ejection of stones
and dust. The column of lava, though rising near enough to the surface
to supply by its ebullition abundant pyroclastic detritus, coarse and
fine, has not flowed out above ground, nor even ascended to the top
of the funnel. It may have formed, at the surface, cones of stones
and cinders with enclosed craters. But thereafter the eruptions have
ceased. The vents, filled up with the fragmentary ejected material,
have given passage only to hot vapours and gases. As these gradually
ceased, the volcanoes have become finally extinct. Denudation has
attacked their sides and crests. If submerged in the sea or a lake,
the cones have been washed down, and their materials have been strewn
over the bottom of the water. If standing on the land, they have been
gradually levelled, until perhaps only the projecting knob or neck of
solidified rubbish in each funnel has remained to mark its site. The
buried column of compacted fragmentary material will survive as the
only memorial of the eruptions (Fig. 24. For views of necks formed of
agglomerate or tuff see Figs. 23, 82, 102, 123, 144, 178, 192, 203,
204, 209, 210, 212, 216).
The volcanic agglomerates of such vents sometimes include, among their
non-volcanic materials, pieces of rock which bear evidence of having
been subjected to considerable heat (see vol. ii. p. 78). Carbonaceous
shales, for instance, have had their volatile constituents driven off,
limestones have been converted into marble, and a general induration
or "baking" may be perceptible. In other cases, however, the fragments
exhibit no sensible alteration. Fossiliferous limestones and shales
often retain their organic remains so unchanged that specimens taken
out of the agglomerate cannot be distinguished from those gathered from
the strata lying _in situ_ outside. Some stones have evidently been
derived from a deeper part of the chimney, where they have been exposed
to a higher temperature than others, or they may have been lain longer
within the influence of hot ascending vapours.
The volcanic materials in agglomerate range in size from the finest
dust to blocks several yards in length, with occasionally even much
larger masses. The proportions of dust to stones vary indefinitely, the
finer material sometimes merely filling in the interstices between the
stones, at other times forming a considerable part of the whole mass.
The stones of an agglomerate may be angular or subangular, but are
more usually somewhat rounded. Many of them are obviously pieces that
have been broken from already solid rock and have had their edges
rounded by attrition, probably by knocking against each other and
the walls of the chimney as they were hurled up and fell back again.
Their frequently angular shapes negative the supposition that they
could have been produced by the discharge of spurts of still liquid
lava. As already stated, they have probably been in large measure
derived from the violent disruption of the solidified cake or crust
on the top of the column of lava in the pipe. Many of them may have
been broken off from the layer of congealed lava that partially coated
the rough walls of the funnel after successive uprises of the molten
material. Among them may be observed many large and small blocks that
appear to have been derived from the disruption of true lava-streams,
as if beds of lava had been pierced in the formation of the vent, or
as if those that congealed on the slopes of the cone had been broken
up by subsequent explosions. These fragments of lava are sometimes
strongly amygdaloidal. A characteristic feature, indeed, of the blocks
of volcanic material in the agglomerates is their frequent cellular
structure. Many of them may be described as rough slags or scoriæ.
These have generally come from the spongy crust or upper part of the
lava where the imprisoned steam, relieved from pressure, is able to
expand and gather into vesicles.
Less frequently evidence is obtainable that the blocks were partially
or wholly molten at the time of expulsion. Sometimes, for example, a
mass which presents on one side such a broken face as to indicate that
it came from already solidified material, will show on the other that
its steam-vesicles have been pulled out in such a way as to conform to
the rounded surface of the block. This elongation could only take place
in lava that was not yet wholly consolidated. It seems to indicate that
such blocks were derived from a thin hardened crust lying upon still
molten material, and that they carried up parts of that material with
them. As each stone went whirling up the funnel into the open air, its
melted part would be drawn round the gyrating mass, and would rapidly
cool there.
In other cases, we encounter true volcanic bombs, that is, rounded or
bomb-shaped blocks of lava, with their vesicles elongated all round
them and conforming to their spherical shape. Sometimes such blocks are
singularly vesicular in the centre, with a more close-grained crust
on the outside. Their rapid centrifugal motion during flight would
allow of the greater expansion of the dissolved steam in the central
part of each mass, while the outer parts would be quickly chilled, and
would assume a more compact texture. Bombs of this kind are met with
among ancient volcanic products, and, like those of modern volcanoes,
have obviously been produced by the ejection of spurts or gobbets of
lava from the surface of a mass in a state of violent ebullition.
Occasionally they are hollow inside, the rotation in these cases having
probably been exceptionally rapid.
Passing from the larger blocks to the smaller fragments, we notice
the great abundance of nut-like subangular or rounded pieces of lava
in the agglomerates. These include lumps of fine grain not specially
vesicular, and probably derived from the disruption of solidified
rock. But in many agglomerates, especially those associated with the
outpouring of basalts or other basic lavas (as those of Carboniferous
and Tertiary age described in later chapters), they comprise also vast
numbers of very finely cellular material or pumice. These pumiceous
lapilli have been already alluded to as ingredients of the stratified
tuffs. But they are still more characteristic of the necks, and reach
there a larger size, ranging from the finest grains up to lumps as
large as a hen's egg, or even larger.
The peculiar distinctions of this ejected pumice are the extreme
minuteness of its vesicles, their remarkable abundance, their prevalent
spherical forms, and the thinness of the walls which separate them.
In these respects they present a marked contrast to the large
irregularly-shaped steam-cavities of the outflowing lavas, or even of
the scoriæ in the agglomerates.
This characteristic minutely vesicular pumice is basic in composition.
Where not too much decayed, it may be recognized as a basic glass.
Thus among the remarkable agglomerates which fill up the Pliocene or
Pleistocene vents of the Velay, the fragments consist of a dark very
basic glass, which encloses such a multitude of minute steam-cavities
that, when seen under the microscope, they are found to be separated
from each other by walls so thin that the slice looks like a pattern
of delicate lace.[25] In necks of earlier date, such as those of older
Tertiary, and still more of Palæozoic, time, the glass has generally
been altered into some palagonitic material.
[Footnote 25: M. Boule, _Bull. Cart. Géol. France_, No. 28, tome iv.
(1892) p. 193.]
This finely pumiceous substance appears to be peculiar to the vents and
to the deposits of tuff immediately derived from them. It is not found,
so far as I know, among any of the superficial lavas, and, of course,
would not be looked for among intrusive rocks. It was evidently a
special product of the volcanic chimney, as distinguished from the mass
of the magma below. We may perhaps regards it as in some way due to a
process of quiet simmering within the vent, when the continual passage
of ascending vapours kept the molten lava there in ebullition, and gave
it its special frothy or finely pumiceous character.
The compacted dust, sand or gravelly detritus found in necks, and
comprised under the general name of Tuff, consists partly of the
finer particles produced during the violent disruption of already
solidified rocks, partly of the detritus arising from the friction and
impact of stones ascending and descending above an active vent during
times of eruption, and partly of the extremely light dust or ash into
which molten lava may be blown by violent volcanic explosions. In
old volcanic necks, where the rocks have long been subjected to the
influence of percolating meteoric water, it is not perhaps possible
to discriminate, except in a rough way, the products from these three
sources. The more minutely comminuted material has generally undergone
considerable alteration, so that under the microscope it seldom reveals
any distinctive structures. Here and there in a slide, traces may
occasionally be detected of loose volcanic microlites, though more
usually these can only be found in lapilli of altered glass or finely
pumiceous lava.
The composition of the detritus in a neck of agglomerate or tuff has
almost always a close relation to that of any lavas which may have been
emitted from that vent. If the lavas have been of an acid character,
such as rhyolites, felsites or obsidians, the pyroclastic materials
will almost always be found to be also acid. Where, on the other hand,
the lavas have been intermediate or basic, so also will be the tuffs
and agglomerates. Occasionally, however, as has already been pointed
out, from the same or closely adjoining vents lavas of very different
chemical composition have been successively erupted. Felsites or
rhyolites have alternated with diabases, basalts or andesites. In such
cases, a commingling of acid and basic detritus may be observed, as,
for example, among the volcanoes of the Old Red Sandstone. It has even
happened sometimes that such a mixture of material has taken place when
only one class of lavas has been poured out at the surface, as in the
agglomerates that fill vents among the basalts of the Inner Hebrides.
But we may be sure that, though not discharged at the surface, the
lavas of which pieces are found in the tuffs must have risen high
enough in the vents to be actually blown out in a fragmentary form.
The occurrence of felsitic fragments among the otherwise basic
agglomerates of Mull and Skye will be described in subsequent pages,
likewise the intercalation of rhyolitic detritus between the basalts of
Antrim. A similar association occurs among the modern vents of Iceland.
Among the contents of the tuffs and agglomerates that occupy old
volcanic vents, some are occasionally to be observed of which the
source is not easily conjectured. Detached crystals of various
minerals sometimes occur abundantly which were certainly not formed
_in situ_, but must have been ejected as loose lapilli with the other
volcanic detritus. Where these crystals belong to minerals that enter
into the composition of the lavas of the district in which they are
found, they may be regarded as having probably been derived from the
explosion of such lavas in the vents, the molten magma being blown into
dust, and its already formed crystals being liberated and expelled
as separate grains. But it seems to be extremely rare to find any
neighbouring lava in which the minerals in question are so largely and
so perfectly crystallized as they are in these loose crystals of the
neck. The beautifully complete crystals of augite found in the old
tuffs of Vesuvius and on the flanks of Stromboli may be paralleled
among Palæozoic tuffs and agglomerates in Britain. Thus the necks
belonging to the Arenig and Llandeilo volcanoes of southern Scotland
are sometimes crowded with augite, varying from minute seed-like
grains up to perfectly formed crystals as large as hazel nuts. The
conditions under which such well-shaped idiomorphic minerals were
formed were probably different from those that governed the cooling and
consolidation of the ordinary lavas.
But besides the minerals that may be claimed as belonging to the
volcanic series of a district, others occur not infrequently in some
tuff-necks, the origin of which is extremely puzzling. Such are the
large felspars, micas, garnets and the various gems that have been
obtained from necks. The large size of some of these crystals and
their frequently perfect crystallographic forms negative the idea that
they can, as a rule, be derived from the destruction of any known
rocks, though they may sometimes be conceivably the residue left after
the solution of the other constituents of a rock by the underground
magma, like the large residual felspars enclosed in some dykes. The
crystals in question, however, seem rather to point to some chemical
processes still unknown, which, in the depths of a volcanic focus,
under conditions of pressure and temperature which we may speculate
about but can perhaps hardly ever imitate in our laboratories, lead to
the elaboration of the diamond, garnet, sahlite, smaragdite, zircon and
other minerals.[26] Examples of such foreign or deep-seated crystals
will be described from the probably Permian necks of Central Scotland.
[Footnote 26: For lists of the minerals found in the diamond-bearing
necks of Kimberley, see M. Boutan in Frémy's _Encyclopédie Chimique_
(1886), vol. ii. p. 168; Dr. M. Bauer's _Edelsteinkunde_ (1895), p.
223.]
Whatsoever may be the source and nature of the fragmentary materials
that fill old volcanic vents, they present, as a general rule, no
definite arrangement in the necks. Blocks of all sizes are scattered
promiscuously through the agglomerate, just as they fell back into
the chimney and came to rest there. The larger masses are placed at
all angles, or stand on end, and are sometimes especially conspicuous
in the centre of a neck, though more usually dispersed through the
whole. Such a thoroughly tumultuous accumulation is precisely what
might be expected where explosions have taken place in still liquid
and in already consolidated lavas, and where the materials, violently
discharged to the surface, have fallen back and come finally to rest in
the chimney of the volcano.
Nevertheless, this absence of arrangement sometimes gives place to
a stratification which becomes more distinct in proportion as the
material of the vent passes from coarse agglomerate into fine tuff. It
is possible that the existence and development of this structure depend
on the depth at which the materials accumulate in the funnel. We may
conceive, for instance, that in the lower parts of the chimney, the
stones and dust, tumultuously falling and rebounding from projections
of the rugged walls, will hardly be likely to show much trace of
arrangement, though even there, if the explosions continue to keep an
open though diminishing passage in the vent, alternations of coarser
and finer layers, marking varying phases of eruptivity, may be formed
in the gradually heightening pile of agglomerate. Rude indications of
some such alternations may sometimes be detected in what are otherwise
quite unstratified necks.
In the upper part of a volcanic funnel, however, close to and even
within the crater, the conditions are not so unfavourable to the
production of a stratified arrangement. As the pipe is filled up, and
the activity of eruption lessens, explosions may occur only from the
very middle of the orifice. The debris that falls back into the vent
will gather most thickly round the walls, whence it will slide down
to the central, still eruptive hole. It will thus assume a stratified
arrangement, the successive layers lying at the steepest angles of
repose, or from 30° to 35°, and dipping down in an inverted conical
disposition towards the centre. If the process should continue long
enough, the crater itself may be partially or completely filled up with
detritus (Fig. 25).
Of this gradual infilling of a volcanic chimney with stratified
agglomerate and tuff, examples belonging to different geological
periods will be cited in subsequent chapters. I may here especially
allude to one of the most recently observed and best marked
illustrations, which occurs on the west side of Stromö, in the Faroe
Islands (see Figs. 310, 311, 312). A neck has there been filled up
with coarse agglomerate, which is rudely stratified, the layers
dipping steeply into the centre, where the tumultuous assemblage of
large blocks no doubt points to the final choking up of the diminished
orifice of explosion. The walls of the neck are nearly vertical, and
consist of the bedded basaltic lavas through which the vent has been
opened. They terminate upward in a conical expansion, evidently the old
crater, which has subsequently been filled up by the inroads of several
lava-streams from adjacent vents. It is here manifest that the bedded
agglomerate belongs to the uppermost part of the volcanic funnel.
[Illustration: Fig. 25.--Neck filled with stratified tuff. A. ground
plan; B. transverse section.]
Where vents have been filled up with tuff rather than with agglomerate,
the stratified structure is best developed. Alternations of coarser and
finer detritus give rise to more or less definite layers, which, though
inconstant and irregular, serve to impart a distinctly stratified
character to the mass. Where there has been no subsequent disturbance
within a vent, these layers show the same inward dip towards the centre
just referred to, at the ordinary angles of repose. Now and then, where
a neck with this structure has been laid bare on a beach, its denuded
cross-section presents a series of concentric rings of strata from the
walls towards the centre. Good illustrations of these features are
supplied by the probably Permian necks of eastern Fife (Figs. 25 A and
217).[27]
[Footnote 27: See also the sections of vents on the west coast of
Stromö Faroes, above referred to.]
It has frequently happened, however, that, owing to subsidence of the
materials filling up the vents or to later volcanic disturbances, the
compacted tuffs have been broken up and thrown into various positions,
large masses being even placed on end. Among the Carboniferous and
Permian necks of Central Scotland such dislocated and vertical tuffs
are of common occurrence (see Figs. 145, 218). If, as is probable, we
are justified in regarding the stratified parts of necks as indicative
of the uppermost parts of volcanic funnels, not far from the surface,
the importance of this inference will be best understood when the
Carboniferous and Permian volcanoes are described.
(3) _Necks with a central Lava-plug._--Some vents of agglomerate or
tuff are pierced by a plug of lava, as may be instructively seen in
many of the Carboniferous and Permian necks of the centre and south of
Scotland (Fig. 26; compare also Figs. 148, 174, 207, and 226). Where
this structure shows itself, the contrast in hardness and durability
between the more destructible fragmentary material and the solid
resisting lava leads to a topographical distinction in the outer forms
of necks. The smooth declivities of the friable tuffs are crowned or
interrupted by more craggy features, which mark the position of the
harder intrusive rock.
[Illustration: Fig. 26.--Section of neck of agglomerate (_a_ _a_) with
plug of lava (_b_).]
The plug, like the pipe up which it has risen, is in general
irregularly circular in ground-plan. It may be conceived to be a column
of rock, descending to an unknown depth into the interior, with a
casing of pyroclastic debris surrounding it. It may vary considerably
in the proportion which its cross-section bears to that of the
surrounding fragmental material. Sometimes it does not occupy more than
a small part of the whole, often appearing in the centre. In other
cases, it more than equals all the rest of the material in the vent,
while instances may be noted where only occasional patches of tuff
or agglomerate are visible between the lava-plug and the wall of the
pipe. From these we naturally pass to the second type of vent, where
no fragmentary material is to be seen, but where the chimney is now
entirely filled with some massive once-molten rock.
A neck with a lava-plug probably contains the records of two stages
in volcanic progress, the first of which, indicated by the tuff or
agglomerate, was confined to the discharge of fragmentary materials;
while the second, shown by the lava-plug, belonged to the time when,
after the earlier explosions, lava ascended in the vent and solidified
there, thus bringing the eruptions from that particular orifice to
an end. Where a small central column of lava rises through the tuff,
we may suppose that the funnel had been mainly choked up by the
accumulation in it of ejected detritus, which was compacted to a solid
mass adhering to the wall of the funnel, but leaving a central orifice
to be kept open by the gradually waning energy of the volcano. By a
final effort that impelled molten rock up that duct and allowed it to
consolidate there, the operations of the vent were brought to a close.
Where, on the other hand, only occasional strips of tuff or agglomerate
are to be found between the lava-plug and the wall of the pipe, the
last uprise of lava may be supposed to have been preceded by more
vigorous explosions which cleared the throat of the volcano, driving
out the accumulated detritus and leaving only scattered patches
adhering to the sides of the funnel.
There is, no doubt, some downward limit to the production of
fragmentary material, and if we could lay bare successive levels in the
chimney of a volcano we should find the agglomerate eventually replaced
entirely by lava.
The materials of the lava-plugs vary widely in composition. Sometimes
they are remarkably basic, and present rocks of the picrite or
limburgite type; in other cases they are thoroughly acid rocks such
as felsite and granophyre. Many intermediate varieties may be found
between these extremes. It is noteworthy that, in districts where the
lavas erupted to the surface have been andesitic or basaltic, the
material which has finally solidified in the vents is often more acid
in composition, trachytic rocks being specially frequent.
[Illustration: Fig. 27.--Section of agglomerate neck (_a_ _a_) with
dykes and veins (_b_ _b_).]
(4) _Necks with Dykes, Veins, or irregular intrusions of Lava._--While
the presence of a central plug of lava in a neck of fragmental material
may indicate that the vent was still to some extent open, there is
another structure which seems to point to the ascent of lava after
the funnel has been choked up. Numerous instances have been observed
where lava has been forced upward through rents in a mass of tuff
or agglomerate, and has solidified there in the form of dykes or
veins (Fig. 27). Illustrations of this structure abound among the
Carboniferous and Permian necks of Britain. Here, again, though on a
less marked scale, the contrast in the amount and character of the
weathering of the two groups of rock gives rise to corresponding
topographical features, which are especially observable in cliffs and
coast-sections, where the dykes and veins project out of the tuffs as
dark prominent walls (Figs. 135, 149, 166, 168, 219, 221, 222).
These intrusive injections are generally irregular in their forms, the
lava having evidently been driven through a mass of material which, not
having yet consolidated sufficiently to acquire a jointed structure,
afforded few dominant lines of division along which it could ascend.
Now and then, however, sharply defined dykes or veins, which at a
distance look like dark ribbons, may be seen running vertically or at
a high angle, and with a straight or wavy course, through the fine
compacted tuff of a vent. Frequently the injected material has found
its readiest line of ascent along the walls of the funnel, between the
tuff and the surrounding rocks. Occasionally it has made its way into
rents in these rocks, as well as into the body of the neck.
It is worthy of remark in passing that complete consolidation of the
fragmentary material does not appear to be always requisite in order
to allow of the formation of such fissures as are needed for the
production of dykes. A singularly interesting illustration of this fact
may be seen on the northern crest of the outer crater of the Puy Pariou
in Auvergne. A dyke of andesite 8 or 10 feet broad may there be traced
running for a distance of about 300 yards through the loose material
of the cone. The rock is highly vesicular, and the vesicles have been
elongated in the direction of the course of the dyke so as to impart a
somewhat fissile structure to the mass.
There can be little doubt that the dykes and veins which traverse
necks of agglomerate belong to one of the closing phases in the
history of the vents in which they occur. They could only have been
injected after the pipes had been so choked up that explosions had
almost or entirely ceased, and eruptions had consequently become
nearly or quite impossible. They show, however, that volcanic energy
still continued to manifest itself by impelling the molten magma
into these extinct funnels, while at the same time it may have been
actively discharging materials from other still open vents in the same
neighbourhood.
With regard to the composition of these dykes and veins, it may be
remarked that in a district of acid lavas they may be expected to be
felsitic or rhyolitic, sometimes granophyric. Where, on the other
hand, the lavas poured out at the surface have been intermediate or
basic, the veins in the necks may be andesites, basalts or other still
more basic compounds. But it is observable, as in the case of the
lava-plugs, that the injections into the necks may be much more acid
than any of the superficial lavas. The advent of acid material in the
later part of a volcano's history has been already alluded to, and many
examples of it will be given in this work.
After all explosions and eruptions have ceased, heated vapours may
still for a long period continue to make their way upward through the
loose spongy detritus filling up the vent. The ascent of such vapours,
and more particularly of steam, may induce considerable metamorphism of
the agglomerate, as is more particularly noticed at p. 71.
ii. _Necks of Lava-form Material_
The second type of neck is that in which the volcanic pipe has been
entirely filled up with some massive or crystalline rock. As already
remarked, it is not always possible to be certain that bosses of rock,
having the external form of necks of this kind, mark the sites of
actual volcanic orifices. Eruptive material that has never reached
the surface, but has been injected into the crust of the earth, has
sometimes solidified there in forms which, when subsequently exposed
by denudation, present a deceptive resemblance to true volcanic necks.
Each example must be examined by itself, and its probable origin must
be determined by a consideration of all the circumstances connected
with it. Where other evidence exists of volcanic activity, such, for
instance, as the presence of bedded tuffs or intercalated sheets of
lava, the occurrence of neck-like eminences or bosses of felsite,
andesite, dolerite, basalt or other eruptive rock, would furnish a
presumption that these marked the sites of some of the active vents of
the period to which the tuffs and lavas belonged.
If a neck-like eminence of this kind were found to possess a circular
or elliptical ground-plan, and to descend vertically like a huge pillar
into the crust of the earth; if the surrounding rocks were bent down
towards it and altered in the manner which I shall afterwards describe
in detail; if, moreover, the material composing the eminence were
ascertained to be closely related petrographically to some parts of the
surrounding volcanic series, it might with some confidence be set down
as marking the place of one of the active vents from which that series
was ejected.
The chief contrast in external form between this type of neck and
that formed of fragmentary material arises from differences in the
relative durability of their component substance. The various kinds of
lava-form rock found in necks are, as a whole, much harder and more
indestructible than agglomerates and tuffs. Consequently bosses of them
are apt to stand out more prominently. They mount into higher points,
present steeper declivities, and are scarped into more rugged crags.
But essentially they are characterized by similar conical outlines, and
by rising in the same solitary and abrupt way from lower ground around
them (see Figs. 109, 133, and 195, 294).
[Illustration: Fig. 28.--Section of neck filled with massive rock.]
Various joint-structures may be observed in these necks. In some cases
there is a tendency to separate into joints parallel to the bounding
walls, and occasionally this arrangement goes so far that the rock has
acquired a fissile structure as if it were composed of vertical strata.
In other instances, the rock shows a columnar structure, the columns
diverging from the outer margin, or curving inwards, or displaying
various irregular groupings. More usually, however, this jointing is so
indefinite that no satisfactory connection can be traced between it and
the walls of the orifice in which the rock has solidified.
Some of the most remarkable examples of necks ever figured and
described are those to which attention was called by Captain Dutton as
displayed in the Zuni plateau of New Mexico, where, amid wide denuded
sheets of basalt, numerous prominent crags mark the sites of eruptive
vents. The basalt of these eminences is columnar, the columns standing
or lying in all sorts of attitudes, and in most cases curved.[28] In
the Upper Velay, in Central France, numerous conspicuous domes and
cones of phonolite rise amidst the much-worn basalt-plateau of that
region (Fig. 345). Many instances will be cited in later chapters from
the British Isles.
[Footnote 28: _U.S. Geol. Survey, 6th Annual Report_, 1884-85, p. 172.]
iii. _Distribution of Vents in Relation to Geological Structure-lines_
Where the positions of true volcanic necks can be accurately
determined, it is interesting to study their distribution and their
relation to the main lines of geological structure around them.
Sometimes a distinct linear arrangement can be detected in their
grouping. Those of the Lower Old Red Sandstone of Central Scotland,
for instance, can be followed in lines for distances of many miles
(Map No. III). Yet when we try to trace the connection of such
an arrangement with any known great lines of dislocation in the
terrestrial crust, we can seldom establish it satisfactorily. In the
case of the Scottish Old Red Sandstone just cited, it is obvious that
the vents were opened along a broad belt of subsidence between the
mountains of crystalline schist on the north, and those of convoluted
Silurian strata on the south, either margin of that belt being
subsequently, if not then, defined by lines of powerful fault. No vents
have risen along these faults, nor has any relation been detected
between the sites of the volcanic foci and dislocations in the area of
ancient depression.
Indeed, it may be asserted of the vents of Britain that they are
usually entirely independent of any faults that traverse at least the
upper visible part of the earth's crust. They sometimes rise close to
such lines of fracture without touching them, but they are equally well
developed where no fractures are to be found. Now and then one of them
may be observed rising along a line of fault, but such a coincidence
could hardly fail occasionally to happen. From the evidence in the
British Isles, it is quite certain that if volcanic vents have, as is
possible, risen preferably along lines of fissure in the terrestrial
crust, these lines are seldom those of the visible superficial faults,
but must lie much deeper, and are not generally prolonged upward to the
surface. The frequent recurrence of volcanic outbursts at successive
geological periods from the same or adjacent vents seems to point
to the existence of lines or points of weakness deep down in the
crust, within reach of the internal molten magma, but far beneath the
horizon of the stratified formations at the surface, with their more
superficial displacements.
While sometimes running in lines, old volcanic vents of the Vesuvian
and Puy types often occur also in scattered groups. Two or three may
be found together within an area of a few hundred yards. Then may come
an interval where none, or possibly only a solitary individual, may
appear. And beyond that space may rise another sporadic group. These
features are well exhibited by the Carboniferous and Permian series of
Scotland, to the account of which the reader is referred.
A large neck may have a number of smaller ones placed around it, just
as a modern Vesuvian cone has smaller parasitic cones upon its flanks.
An instructive example of this arrangement is to be seen at the great
vent of the Braid Hills belonging to the Lower Old Red Sandstone
and described in Chapter xx. Other instances may be cited from the
Carboniferous and Permian volcanic series (see Figs. 90, 148, 213).
Not infrequently the irregularities in the ground-plan of a neck, as
already remarked, may be accounted for on the supposition that they
mark the site of more than one vent. Sometimes, indeed, it is possible
to demonstrate the existence of two or even more vents which have
been successively opened nearly on the same spot. The first orifice
having become choked up, another has broken out a little to one side,
which in turn ceasing to be effective from the same or some other
cause, has been succeeded by a third (Fig. 29). The three cones and
craters of the little island of Volcanello supply a singularly perfect
recent instance of this structure (Fig. 214). Here the funnel has
twice shifted its position, each cone becoming successively smaller
and partially effacing that which preceded it. In Auvergne, the Puy de
Pariou has long been celebrated as an example of a fresh cinder-cone
partially effacing an earlier one. In the much denuded Palæozoic
volcanic tracts of Britain, where the cones have long since disappeared
and only the stumps of the volcanic cylinders are left, many
illustrations occur of a similar displacement of the funnel, especially
among the volcanoes of the Carboniferous system.
Among the irregularities of necks that may indicate a connection with
lines of fissure, reference may be made here to dykes or dyke-like
masses of agglomerate which are sometimes to be seen among the volcanic
districts of Britain. In these cases the fragmentary materials, instead
of lying in a more or less cylindrical pipe, appear to fill up a long
fissure. We may suppose that the explosions which produced them did
actually occur in fissures instead of in ordinary vents. The remarkable
Icelandic fissures with their long rows of cinder cones are doubtless,
at least in their upper parts, largely filled up with slag and scoriæ.
Some illustrations of this structure will be given in the account of
the Carboniferous volcanic rocks of Scotland (see No. 1 in Fig. 22).
[Illustration: Fig. 29.--Successive shiftings of vents giving rise to
double or triple cones. A, ground-plan; B, vertical section.]
There is yet another consideration in regard to the form and size of
necks which deserves attention. Where the actual margin of a neck and
its line of vertical junction with the rocks through which it has been
drilled can be seen, there is no room for dispute as to the diameter
of the original funnel, which must have been that of the actual neck.
But in many cases it is impossible to observe the boundary; not merely
because of superficial soil or drift, but occasionally because the
volcanic detritus extends beyond the actual limits of the funnel.
In such cases the necks have retained some portion of the original
volcanic cone which accumulated on the surface around the eruptive
vent. It may even chance that what appears to be a large neck would be
considerably reduced in diameter, and might be shown to include more
than one pipe if all this outer casing could be removed from it. In
Fig. 30, for example, a section is given of a neck (_n_) from which on
the right-hand side all the cone and surrounding tuffs (_t_) have been
removed by denudation, the original form of the volcano being suggested
by the dotted lines. On the left side, however, the tuffs which were
interstratified with the contemporaneous sediments are still connected
with the neck, denudation not having yet severed them from it. The
overlying strata (_l_, _l_) which originally overspread the extinct
volcano have been bent into an anticline, and the neck of the vent has
thus been laid bare by the removal of the crest of the arch.
[Illustration: Fig. 30.--Section to show the connection of a neck with
a cone and surrounding bedded tuffs.]
The instances where a structure of this kind is concealed are probably
fewer in number in proportion to their antiquity. But among Tertiary
cones they may perhaps not be so rare. The possibility of their
occurrence should be kept in view during the investigation of extinct
volcanoes. The term Neck ought not properly to be applied to such
degraded volcanic cones. The true neck still remains preserved in the
inside of them. As illustrative of the structure here referred to, I
may cite the example of the Saline Hill (Fig. 148) and of Largo Law
(Fig. 226), both in Fife.
iv. _Metamorphism in and around Volcanic Vents--Solfataric Action_
The prolonged ascent of hot vapours, stones, dust and lava, in the
funnel of a volcano must necessarily affect the rocks through which
the funnel has been driven. We may therefore expect some signs of
alteration in the material forming the walls of a volcanic neck. The
nature of the metamorphism will no doubt depend, in the first place,
on the character and duration of the agents producing it, and in the
second, on the susceptibility of the rocks to undergo change. Mere heat
will indurate rocks, baking sandstone, for instance, into quartzite,
and shales into porcellanite. But there will almost invariably be
causes of alteration other than mere high temperature. Water-vapour,
for instance, has probably always been one of the most abundant and
most powerful of them. The copious evolution of steam from volcanoes
is one of their most characteristic features at the present day,
and that it was equally so in past time seems to be put beyond
question by the constantly recurring vesicular structure in ancient
lavas and in the lapilli and ejected blocks of old agglomerates and
tuffs. Direct experiment has demonstrated, in the hands of various
skilful observers, from the time of Sir James Hall to that of
Professor Daubrée, how powerfully rocks are acted upon when exposed
to superheated vapour of water under great pressure. But the steam of
volcanoes often contains other vapours or mineralizing agents dissolved
in it, which increase its metamorphic influence. The mineral acids, for
instance, must exert a powerful effect in corroding most minerals and
rocks. At the Solfatara of Naples and at other volcanic orifices in
different parts of Italy, considerable alteration is seen to be due to
this cause.
Bearing these well-known facts in mind, we may be prepared to find
various proofs of metamorphism around and within old volcanic vents.
The surrounding rocks are generally much hardened immediately
contiguous to a neck, whether its materials be fragmental or massive.
Sandstones, for example, are often markedly bleached, acquire the
vitreous lustre and texture of quartzite, lose their usual fissility,
break irregularly into angular blocks, and on an exposed surface
project above the level of the unaltered parts beyond. Shales are
baked into a kind of porcelain-like substance. Coal-seams are entirely
destroyed for economic purposes, having been burnt into a kind of
cinder or fused into a blistered slag-like mass. Limestones likewise
lose their usual bluish-grey tint, become white and hard, and assume
the saccaroid texture of marble.
The distance to which this metamorphism extends from the wall is,
among the exposed necks in Britain, smaller than might be anticipated.
Thus I have seldom been able to trace it among those of Carboniferous
or Permian age for more than 15 or 20 yards in ordinary arenaceous
and argillaceous strata, even where every detail of a neck and its
surroundings has been laid bare in plan upon a beach. The alteration
seems to reach furthest in carbonaceous seams, such as coals.
It is evident that the element of time must enter into the question
of the amount of metamorphism produced in the terrestrial crust
immediately surrounding a volcanic pipe. A volcano, of which the
eruptions begin and end within an interval of a few days or hours,
cannot be expected to have had much metamorphic influence on the rocks
through which its vent was opened. On the other hand, around a funnel
which served for many centuries as a channel for the escape of hot
vapours, ashes or lava to the surface, there could hardly fail to be
a considerable amount of alteration. The absence or comparatively
slight development of metamorphism at the Carboniferous and Permian
necks of Scotland may perhaps be regarded as some indication that these
volcanoes were generally short-lived. On the other hand, more extensive
alteration may be taken as pointing to a longer continuance of eruptive
vigour.
The same causes which have induced metamorphism in the rocks
surrounding a volcanic vent might obviously effect it also among the
fragmentary materials by which the vent may have been filled up. When
the eruptions ceased and the funnel was left choked with volcanic
debris, hot vapours and gases would no doubt still continue for a time
to find their way upward through the loose or partially compacted mass.
In their ascent they would permeate this material, and in the end
produce in it a series of changes similar to, and possibly even more
pronounced than, those traceable in the walls of the vent. Instances of
this kind of metamorphism will be cited in the following chapters (see
in particular p. 404).
v. _Inward Dip of Docks towards Necks_
One concluding observation requires to be made regarding the relation
of old volcanic necks to the rocks which immediately surround them.
Where a vent has been opened through massive rocks, such as granite,
felsite, andesite or basalt, it is generally difficult or impossible
to determine whether there has been any displacement of these rocks,
beyond the disruption of them caused by the explosions that blew out
the orifice. But where the pipe has been drilled through stratified
rocks, especially when these still lie nearly flat, the planes of
stratification usually supply a ready test and measure of any such
movement. Investigation of the volcanic rocks of Britain has shown me
that where any displacement can be detected at a neck, it is almost
invariably in a downward direction. The strata immediately around
the vent tend to dip towards it, whatever may be their prevalent
inclination in the ground beyond (Fig. 24). This is the reverse of the
position which might have been expected. It is so frequent, however,
that it appears to indicate a general tendency to subsidence at the
sites of volcanic vents. After copious eruptions, large cavernous
spaces may conceivably be left at the roots of volcanoes, and the
materials that have filled the vents, losing support underneath, will
tend to gravitate downwards, and if firmly welded to their surrounding
walls may drag these irregularly down with them. Examples of such
sagging structures are abundantly to be seen among the dissected vents
of the Carboniferous and Permian volcanic series of Scotland.
vi. _Influence of Contemporaneous Denudation upon Volcanic Cones_
It must be remembered that former vents, except those of the
later geological periods, are revealed at the surface now only
after extensive denudation. As a rule, the volcanoes that formed
them appeared and continued in eruption during periods of general
subsidence, and were one by one submerged and buried beneath subaqueous
deposits. We can conceive that, while a volcanic cone was sinking
under water, it might be seriously altered in form and height by waves
and currents. If it consisted of loose ashes and stones, it might be
entirely levelled, and its material might be strewn over the floor of
the sea or lake in which it stood. But, as has been already pointed
out, the destruction of the cone would still leave the choked-up pipe
or funnel from which the materials of that cone had been ejected.
Though, during the subsidence, every outward vestige of the actual
volcano might disappear, yet the agglomerate or lava that solidified
in the funnel underneath would remain. And if these materials had
risen some way within the cone or crater, or if they reached at least
a higher level in the funnel than the surrounding water-bottom or
land-surface, the destruction of the cone might leave a projecting knob
or neck to be surrounded and covered by the accumulating sediments of
the time. It is thus evident that the levelling of a cone of loose
ashes during gradual subsidence, and the deposition of a contemporary
series of sedimentary deposits, might give rise to a true neck, which
would be coeval with the geological period of the volcano itself.
In practice it is extremely difficult to decide how far any now visible
neck may have been reduced to the condition of a mere stump or core of
a volcano before being buried under the stratified accumulations of its
time. In every case the existence of the neck is a proof of denudation,
and perhaps, in most cases, the chief amount of that denudation is
to be ascribed not to the era of the original volcano, but to the
comparatively recent interval that has elapsed since, in the progress
of degradation, the volcanic rocks, after being long buried within the
crust, were once more laid bare by the continuous waste and lowering of
the level of the land.
vii. _Stages in the History of old Volcanic Vents_
Let us now try to follow the successive stages in the history of a
volcano after its fires had quite burnt out, and when, slowly sinking
in the waters of the sea or lake wherein it had burst forth, it was
buried under an ever-growing accumulation of sedimentary material.
The sand, mud, calcareous ooze, shell-banks, or whatever may have
been the sediment that was gathering there, gradually crept over the
submerged cone or neck, and would no doubt be more or less mixed with
any volcanic detritus which waves or currents could stir up. If the
cone escaped being levelled, or if it left a projecting neck, this
subaqueous feature would be entombed and preserved beneath these
detrital deposits. Hundreds or thousands of feet of strata might
be laid down over the site of the volcano, which would then remain
hidden and preserved for an indefinite period, until in the course of
geological revolutions it might once again be brought to the surface.
These successive changes involve no theory or supposition. They must
obviously have taken place again and again in past time. That they
actually did occur is demonstrated by many examples in the British
Isles. I need only refer here to the interesting cases brought to light
by mining operations in the Dairy coal-fields of Ayrshire, which are
more fully described in Chapter xxvii. (p. 433). In that district a
number of cones of tuff, one of which is 700 feet in height, have been
met with in the course of boring and mining for ironstone and coal.
The well-known mineral seams of the coal-field can be followed up to
and over these hidden hills of volcanic tuff which in the progress of
denudation have not yet been laid bare (Fig. 146).
The subsidence which carried down the water-bottom and allowed the
volcanic vents to be entombed in sedimentary deposits may have been in
most cases tolerably equable, so that at any given point these deposits
would be sensibly horizontal. But subsequent terrestrial disturbances
might seriously affect this regularity. The sedimentary formations,
piled above each other to a great depth, and acquiring solidity by
compression, might be thrown into folds, dislocated, upheaved or
depressed. The buried volcanic funnels would, of course, share in the
effects of these disturbances, and eventually might be so squeezed
and broken as to be with difficulty recognizable. It is possible
that some of the extreme stages of such subterranean commotions are
revealed among the "Dalradian" rocks of Scotland. Certain green schists
which were evidently originally sediments, and probably tuffs, are
associated with numerous sills and bosses of eruptive material. The way
in which these various rocks are grouped together strikingly suggests
a series of volcanic products, some of the crushed bosses recalling
the forms of true necks in younger formations. But they have been so
enormously compressed and sheared that the very lavas which originally
were massive amorphous crystalline rocks have passed into fissile
hornblende-schists.
[Illustration: Fig. 31.--Diagram illustrating the gradual emergence of
buried volcanic cones through the influence of prolonged denudation.]
Among the Palæozoic systems of Britain, however, where considerable
fracture and displacement have taken place, examples of successive
stages in the reappearance of buried volcanic cones and necks may be
gathered in abundance. As an illustrative diagram of the process of
revelation by the gradual denudation of an upheaved tract of country,
Fig. 31 may be referred to (compare also Fig. 147).
Here three volcanic vents are represented in different stages of
re-emergence. In the first (A) we see a cone and funnel which, after
having been buried under sedimentary deposits (_s_, _s_,) have been
tilted up by subterranean movements. The overlying strata have been
brought within the influence of denudation, and their exposed basset
edges along the present surface of the land (_g_, _g_) bear witness
to the loss which they have suffered. Already, in the progress of
degradation, a portion of the volcanic materials which, ejected from
that vent, were interstratified with the contemporaneous sediments of
the surrounding sea-floor, has been exposed at _t_. A geologist coming
to that volcanic intercalation would be sure that it pointed to the
existence of some volcanic vent in the neighbourhood, but without
further evidence he would be unable to tell whether it lay to right or
left, whether it was now at the surface or lay still buried under cover
of the stratified deposits which were laid down upon it.
In the second or central example (B) we have a pipe and cone which have
been similarly disturbed. But in this case denudation has proceeded
so far as to reveal the cone and even to cut away a portion of it, as
shown by the dotted lines to the right hand. Owing, however, to the
general inclination of the rocks towards the left, that side of the
cone, together with the tuffs or lavas connected with it, still lies
buried and protected under cover of the sedimentary formations (_s_,
_s_).
The third example (C) shows a much more advanced stage of destruction.
Here the whole of the cone has been worn away. All the lavas and tuffs
which were ejected from it towards the right have likewise disappeared,
and strata older than the eruptions of this vent now come to the
surface there. To the left, however, a little portion of its lavas
still remains at _l_, though all the intervening volcanic material has
been removed. That solitary fragment of the outpourings of this volcano
once extended further to the left hand, but the occurrence of the
large dislocation (_f_) has carried this extension for down below the
surface. The vent in this instance, owing to its position, has suffered
more from denudation than the other two. Yet, judged by the size of
its neck, it was probably larger than either of them, and threw out a
more extensive pile of volcanic material. Its funnel has been filled
with agglomerate (_a_), through which a central plug of lava (_p_) has
ascended, and into which dykes or veins (_d_, _d_), the last efforts of
eruption, have been injected.
This diagram will serve to illustrate the fact already so often
insisted on, that although denudation may entirely remove a volcanic
cone, and also all the lavas and tuffs which issued from it, the actual
filled-up pipe cannot be so effaced, but is practically permanent.
CHAPTER VI
Underground Phases of Volcanic Action--_continued_. II.
Subterranean Movements of the Magma: i. Dykes and Veins; ii.
Sills and Laccolites; iii. Bosses (Stocks, Culots)--Conditions
that govern the Intrusion of Molten Rock within the Terrestrial
Crust.
II. Subterranean Movements of the Magma
In the foregoing pages attention has been more specially directed to
those aspects of volcanic energy which reveal themselves above ground
and in eruptive vents. We have now to consider the various ways in
which the molten magma is injected into the crust of the earth.
Such injection must obviously take place during the expulsion of
volcanic materials to the surface. If the explosive violence of an
eruption, or the concomitant movements of the earth's crust, should
lead to ruptures among the subterranean rocks, the molten magma will
be forced into these rents. It is evident that this may happen either
with or without any discharge of lava at the surface. It may be either
entirely a plutonic, that is, a deep-seated phenomenon, or it may be
part of a truly volcanic series of events.
It is clear that, by the study of old volcanoes that have had their
structure laid bare by denudation, we may hope to obtain fresh light in
regard to some of the more deeply-seated features of volcanic energy,
which in a modern volcano are entirely concealed from view. A little
reflection will convince us that the conditions for consolidation
within the crust are so different from those at the surface that we may
expect them to make themselves visible in the internal characters of
the rocks.
An essential distinction between underground propulsions of molten
rock and superficial outflows of the same material lies in the fact
that while the latter are free to take any shape which the form and
slope of the ground may permit, the subterranean injections, like metal
poured into a mould, are always bounded by the walls of the aperture
into which they are thrust. According, therefore, to the shape of this
aperture a convenient classification of such intrusions may be made.
Where the molten material has risen up vertical fissures or irregular
cracks, it has solidified as Dykes and Veins. Where it has been thrust
between the divisional planes either of stratified or unstratified
rocks, so as to form beds, these are conveniently known as Sills,
Laccolites or Intrusive Sheets. Where it has taken the form of large
cylindrical masses, which, ascending through the crust, appear at the
surface in rounded, elliptical or irregularly-shaped eminences, these
are called Bosses (Stocks, Culots).
Further contrasts between the superficial and subterranean
consolidation of molten material are to be found in the respective
textures and minute structures of the rocks. The deep-seated intrusions
are commonly characterized by a general and markedly greater coarseness
of crystallization than is possessed by lavas poured out at the
surface. This difference of texture, obviously in great measure the
result of slower cooling, shows itself in acid, intermediate, and basic
magmas. A lava which at the surface has cooled as a fine-grained,
compact black basalt, in which neither with the naked eye nor with
the lens can the constituent minerals be distinctly determined, may
conceivably be represented at the roots of its parent volcano by a
coarse-textured gabbro, in which the felspars and pyroxenes may have
grown into crystals or crystalline aggregates an inch or more in
length. Mr. Iddings has pointed out that the various porphyrites which
form the dykes and sills of Electric Peak are connected with a central
boss of coarsely crystalline diorite.[29] Examples of the same relation
from different volcanic centres in Britain will be cited in later
chapters.
[Footnote 29: _12th Ann. Rep. U.S. Geol. Survey_ (1890-91), p. 595.]
This greater coarseness of texture is shown by microscopic examination
to be accompanied by other notable differences. In particular, the
glassy residuum, or its devitrified representatives, which may be so
frequently detected among the crystals of outflowing lavas, is less
often traceable in the body of subterranean intrusive rocks, though it
may sometimes be noticed at their outer margins where they have been
rapidly chilled by contact with the cool upper part of the crust into
which they have been impelled. Various minerals, the constituents of
which exist in the original magma, but which may be hardly or not all
recognisable in the superficial lavas, have had leisure to crystallize
out in the deep-seated intrusions and appear sometimes among the
components of the general body of the rock, or as well-terminated
crystals in its drusy cavities.
Considerable though the variations may be between the petrographical
characters of the intrusive and extrusive rocks of a given district
and of the same eruptive period, they appear generally to lie within
such limits as to suggest a genetic relation between the whole series.
Conditions of temperature and pressure, and the retention or escape of
the absorbed vapours which play so large a part in volcanic activity,
must exercise great influence on the crystallization of constituent
minerals, and on the consolidation and ultimate texture of the rocks.
Slow cooling under great pressure and with the mineralizing vapours
still largely retained seems to be pre-eminently favourable for the
production of a holocrystalline texture in deep-seated portions of the
magma, while rapid cooling under merely atmospheric pressure and with
a continuous disengagement of vapours, appears to be required for the
finer grain, more glassy structure, and more vesicular character of
lavas poured out at the surface.
Besides these differences, however, there is evidence of a migration
of the constituent minerals in the body of large intrusive masses
before consolidation. In particular, the heavier and more basic
constituents travel towards the cooling margin, leaving the central
portions more acid. This subject will be more fully considered in
connection with the internal constitution of Bosses, and some British
examples will then be cited.
Reference, however, may here be made to one of the most exhaustive
and instructive studies of the relations of the subterranean and
superficial erupted rocks of an old volcano, which will be found in the
monograph by Mr. Iddings on Electric Peak and Sepulchre Mountain in the
Yellowstone Park of Western America. From the data there obtainable he
draws the deduction that one parent magma, retaining the same chemical
composition, may result in the ultimate production of rocks strikingly
different from each other in structure and mineralogical constitution,
yet chemically identical. Electric Peak includes the central funnel
filled up with coarsely crystalline diorite, and having a connected
series of sills and dykes of various porphyrites. Sepulchre Mountain,
separated from its neighbouring eminence by a fault of 4000 feet,
displays some of the superficial discharges from the vent--coarse
breccias with andesite-lavas. These rocks are not chemically
distinguishable from the intrusive series, but the lavas are, on the
whole, more glassy, while the materials of the bosses, sills and dykes
are more crystalline. The latter display much more visible quartz and
biotite.[30]
[Footnote 30: _12th Ann. Rep. U.S. Geol. Survey_, 1890-91. As already
stated, the eruptions of this volcanic centre became progressively more
acid, and this change appears to be exhibited by the extrusive lavas as
well as by the intrusive rocks.]
By practice in the field, supplemented by investigation with the aid
of the microscope, a geologist acquires a power of discriminating
with fair accuracy, even in hand specimens, the superficial from the
subterranean igneous rocks of an old volcanic district.
Denudation, while laying bare the underground mechanism of an ancient
volcano, has not always revealed the evidence of the actual structural
relations of the rocks, or has first exposed and then destroyed it.
Sometimes a mass of eruptive rock has been worn down and left in such
an isolated condition that its connection with the rest of the volcanic
network cannot be determined. So far as its position goes, it might
perhaps be either a remnant of a lava-stream or the projecting part of
some deeper-seated protrusion. But its texture and internal structure
will often enable a confident opinion to be expressed regarding the
true relations of such a solitary mass.
i. _Dykes and Veins_
For the study of these manifestations of volcanic energy, the British
Isles may be regarded as a typical region. It was thence that the word
"dyke" passed into geological literature. Thousands of examples of both
dykes and veins may be seen from the Outer Hebrides southwards across
the length and breadth of the southern half of Scotland, far into the
north of England and towards the centre of Ireland. They may be found
cutting the crests of the mountains and extending as reefs below the
level of the sea. They are thus exposed in every conceivable divergence
of position and in endless varieties of enclosing rock. Moreover, they
can be shown to represent a vast range of geological time. One system
of them belongs to some remote part of the Archæan periods, another is
as young as the older Tertiary ages.
[Illustration: Fig. 32.--Dyke, Vein and Sill.
The dyke (_d_) rises along a small fault among sandstones, shales, and
ironstones (_sh_), and gives off a vein (_v_) and an intrusive sheet or
sill (_b_).]
Full details regarding these interesting relics of volcanic activity
will be given in later chapters, especially in Chapters xxxiv. and
xxxv. It may suffice here to note that each of the three types of
old volcanoes above described has, in Britain, its accompaniment
of dykes and veins. The plateaux, however, present by far the most
abundant and varied development of them. The dykes of this series are
characterized not only by their prodigious numbers in and around some
of the plateaux, but by the long distances to which they may be traced
beyond these limits. They are chiefly found in connection with the
Tertiary basalt-plateaux, though the Carboniferous andesite-plateaux
present a feebler display of them. The Tertiary dykes are pre-eminently
distinguished by their persistent rectilinear lines, sometimes for
distances of many miles, and their general north-westerly direction.
They form a vast system extending over an area of some 40,000 square
miles. Throughout that wide region their persistence of direction and
of petrographical characters point to the former existence of one or
more reservoirs of an andesitic and basaltic magma underneath the
northern half of Britain, and to the rupture of the crust overlying
this subterranean reservoir by thousands of parallel fissures. They
thus constitute perhaps the most astonishing feature in the volcanic
history of Tertiary time.
The dykes and veins connected with the puys are mainly to be found at
or close to the vents. Not infrequently they traverse the agglomerates
of the necks, and are sometimes to be traced to a central pipe or core
of basalt.
The larger cones are likewise intersected with similar vertical,
inclined or tortuously irregular walls of intruded lava. Occasionally
a radiate arrangement may be observed in such cases, like that
noticeable at some modern volcanoes, the dykes diverging from the
eruptive centre.
Many dykes exist regarding which there is no evidence to connect them
with any actual volcanic rocks. They have been injected into fissures,
but whether this took place during volcanic paroxysms, or owing to some
subterranean movements which never culminated in any eruption, cannot
be decided.
The question of the age of dykes, like that of intrusive masses of
all kinds, is often difficult or impossible to decide. A dyke must of
course be younger than the rocks which it traverses, and a limit to
its antiquity is thus easily fixed. But we cannot always affirm that
because a dyke stops short of a particular rock, or series of rocks,
it is older than these. The Hett Dyke, in the north of England, rises
through the Coal-measures, but stops at the Magnesian Limestone; yet
this cessation does not necessarily imply that the dyke was in place
before the deposition of that limestone. The structure may have arisen
from the dyke-fissure having ended at the bottom of the limestone.
Where dykes rise up to the base of an unconformable formation without
in any single case entering it, and where fragments of them are
enclosed in that formation, they must be of higher antiquity, and must
have been laid bare by extensive denudation before the unconformable
strata were deposited upon them. The great system of dykes in the
Lewisian Gneiss of the north-west of Scotland is in this way proved to
be much more ancient than the Torridon Sandstones under which it passes
(Figs. 35, 36).
Where two dykes cross each other, it is sometimes not difficult
to decide upon their relative antiquity. In intrusive rocks, the
finest-grained parts are those which lie nearest the outer margin,
where the molten material was rapidly chilled by coming in contact
with cool surfaces of rock. Such "chilled margins" of closer grain are
common characteristics of dykes. Wherever a dyke carries its chilled
margin across another dyke, it must be the younger of the two, and
wherever such a margin is interrupted by another dyke, it must belong
to the older.
As a rule, the uprise of molten material in a fissure has so
effectually sealed it up that in the subsequent disturbances of the
terrestrial crust the fissure has not been reopened, though others
may have been produced near it, or across it. Sometimes, however, the
enormous tension to which the crust was exposed opened the fissure
once more, sometimes even splitting a dyke along its centre, and a
new ascent of molten rock took place within the rent. Hence double
or treble or compound dykes have been produced. The second or later
infillings are generally somewhat different from the original dyke.
Occasionally, indeed, they present a strong contrast to it. Thus,
among the dykes of Skye examples occur where the centre is occupied by
an acid granophyre, while the sides are occupied by dykes of basalt.
Instances of this compound type of dyke will be given in the account of
the Tertiary volcanic rocks of Britain.
It is obvious that in a wide fissure the central portion may remain
molten for some time after the sides have consolidated. If the fissure
served as a channel for the ascent of lava to the surface, it is
conceivable that the central still fluid part might be driven out and
be replaced by other material from below, and that this later material
might differ considerably in composition from that which first filled
the opening. Such, according to Mr. Iddings, has been the probable
history of some of the dykes at the old volcano of Electric Peak.[31]
But we can hardly suppose that this explanation of compound dykes can
have any wide application. It could only hold good of broad fissures
having an outlet, and is probably inadmissible in the case of the
numerous compound dykes not more than 10 or 15 feet in diameter, where
the several bands of rock are sharply marked off from each other.
The abrupt demarcation of the materials in these dykes, their closer
texture along their mutual boundaries, the indications of solution
of the older parts of the group by the younger, and of injection of
the latter into the former, show that they belong to separate and
unconnected intrusions. These questions will be again referred to in
the account of the British Tertiary dykes (Chapter xxxv. vol. ii. p.
159).
[Footnote 31: _12th Ann. Rep. U.S. Geol. Survey_ (1890-91), p. 587.]
Another kind of compound dyke has arisen from the manner in which the
original fissure has been produced. While, in general, the dislocation
has taken the form of a single rectilinear rent, which on opening has
left two clean-cut walls, cases occur where the rupture has followed
several parallel lines, and the magma on rising into the rents appears
as two or more vertical sheets or dykes, separated by intervening
partitions of the surrounding rock. Examples of this structure are
not infrequent among the Tertiary dykes of Scotland. One of these may
be noticed rising through the cliffs of Lewisian gneiss on the east
coast of the island of Lewis, south of Stornoway. One of the most
extraordinary instances of the same structure yet observed is that
described by Professor A. C. Lawson from the Laurentian rocks at the
mouth of White Gravel River, on the N.E. coast of Lake Superior. In
a breadth of only about 14 feet no less than 28 vertically intrusive
sheets or dykes of diabase, from 1 inch to 6½ inches broad, rise
through the granite, which is thus split into 27 thin sheets. The
diabase undoubtedly cuts the granite, some of the sheets actually
anastomosing and sending veins into the older rock.[32]
[Footnote 32: _American Geologist_ (1894), p. 293.]
From the evidence supplied by the modern eruptions of Iceland, it is
evident that gaping fissures, which are filled by ascending lava and
thereby converted into dykes, in many instances serve as channels by
which molten rock escapes to the surface. It would be interesting if
any test could be discovered whereby those dykes could be distinguished
which had ever established a connection with the outer air. If the lava
continued to ascend in the fissures, and to pour out in superficial
streams for a long time, the rocks on either side would be likely
to undergo considerably more metamorphism than where there was only
one rapid injection of the magma, which would soon cool. Possibly in
the much greater alteration of the same rocks by some dykes than by
others, a sign of such a connection with the surface may survive. This
subject will be again referred to in the account of the Tertiary dykes
of Britain in Book VIII., where the whole of the phenomena of this
phase of volcanic action will be fully discussed (see vol. ii. p. 163).
ii. _Sills and Laccolites_
The word "sill," derived from a remarkable sheet of eruptive rock in
the north of England, known as the Great Whin Sill (Chapter xxix.),
is now applied as a convenient general term to masses of intrusive
material, which have been injected between such divisional planes as
those of stratification, and which now appear as sheets or beds (Fig.
33). These masses are likewise called Intrusive Sheets, and where the
injected material has accumulated in large blister-like expansions,
these are known as Laccolites (Fig. 34).
[Illustration: Fig. 33.--Section of Sill or Intrusive Sheet.]
Sills vary from only an inch or two up to 500 feet or more in
thickness. Lying, as they frequently do, parallel with strata above and
below them, they resemble in some respects true lava-sheets erupted
contemporaneously with the series of sediments among which they are
intercalated. And, indeed, cases occur in which it is hardly possible
to decide whether to regard a given mass as a sill or as a superficial
lava. In general, however, sills exhibit the coarser texture above
referred to as specially characteristic of subterranean eruptive
masses. Moreover they are usually, though not always, free from the
vesicular and amygdaloidal structures of true surface-lavas. Their
under and upper surfaces, unlike the more scoriaceous parts of lavas,
are commonly much closer in grain than the general body of the mass;
in other words, they possess chilled borders, the result of more rapid
consolidation by contact with cooler rock. Again, instead of conforming
to the stratification of the formations among which they lie, as truly
interstratified lavas do, they may be seen to break across the bedding
and pursue their course on a higher or lower platform. The strata that
overlie them, instead of enclosing pieces of them and wrapping round
irregularities on their surface, as in the case of contemporaneously
erupted lava-sheets, are usually indurated, sometimes even considerably
altered, while in many cases they are invaded by veins from the
eruptive sheet, or portions of them are involved in it, and are then
much hardened or metamorphosed.
The petrographical character of the sills in a volcanic district
depends primarily on the constitution of the parent magma, whence
both they and the outflowing lavas have issued. Where the lavas are
rhyolites or felsites the sills are acid, where basalts have been
erupted the sills are basic, though there has often been a tendency
towards the appearance of more acid material, such as trachyte. As
we have seen, considerable differences in petrographical characters
may arise between the intrusive and extrusive offshoots from the same
parent magma during the course of a volcanic cycle. This question will
be more appropriately discussed together with the leading characters of
Bosses.
Between the upper and under surface of a thick sill considerable
petrographical variation may sometimes be observed, especially where
the rock is of basic constitution. Differences both of texture and even
to some extent of composition can be detected. Sometimes what have
been called "segregation veins" traverse the mass, consisting of the
same minerals as the general body of the rock, but in larger crystals
and in somewhat different proportions. That these veins belong to the
period of original consolidation appears to be shown by the absence of
fine-grained, chilled margins, and by the way in which the component
crystals of the veins are interlocked with those of the body of the
rock. Other veins of finer grain and more acid composition probably
belong to a later phase of consolidation, when, after the separation
and crystallization of the more basic minerals, the more acid mother
liquor that remained was, in consequence of terrestrial movements,
injected into cracks in the now solidified, though still highly heated,
rock. Examples of these features will be cited from various geological
formations in the following chapters.
Reference has already been made to the difference occasionally
perceptible between the constitution of the upper and that of the
under portions of superficial lavas. A similar variation is sometimes
strongly marked among sills, especially those of a basic character,
the felspars remaining most abundant above, while the olivines and
augites preponderate below. Mr. Iddings has observed some excellent
illustrations of this character in the great series of sills connected
with the volcanic pipe of Electric Peak in the Yellowstone country.[33]
Some examples of the same structure will subsequently be cited from the
Carboniferous volcanic series of Central Scotland.
[Footnote 33: "Electric Peak and Sepulchre Mountain," _12th Ann. Rep.
U.S. Geol. Survey_ (1890-91), p. 584.]
The greatest extreme of difference which I have observed in the
petrographical characters of any group of sills is that displayed by
the Tertiary gabbros of Skye. These rocks occur as sheets interposed
among the bedded basalts, and injected between each other in such
a manner as to form thick piles of rudely stratified sills. They
possess a remarkable banded structure, due to the aggregation of their
component minerals in distinct layers, some of which are dark in
colour, from the abundance of their iron-ore, pyroxene and olivine;
while others are light-coloured, from the predominance of their
felspar. From the manner in which the component minerals of one band
interlace with those of the contiguous bands, it is quite certain that
the structure is not due to successive injections of material among
already consolidated rocks, but belongs to the original conditions
of expulsion of the gabbro as a whole. It seems to indicate that
the magma which supplied the sills was at the time of its extrusion
heterogeneous in composition, and that the banding arises from the
simultaneous or rapidly successive protrusion of different portions of
this variously-constituted magma. The details of the structure will be
described in the general account to be given of the Tertiary volcanic
rocks (Chapters xliii. and xliv.).
Besides such visible differences in the composition of sills, others
much less obtrusive may occasionally be detected with the aid of
microscopic or chemical research. The outer parts of some sills are
thus discovered to be more basic or more acid than the inner portions.
Or evidence may be obtained pointing to the probable melting down of
surrounding rocks by the erupted magma, with a consequent local change
in the chemical and mineralogical constitution of the mass.
In regard to their position in the geological structure of an old
volcanic district I may here remark that sills, seldom entirely absent,
are more especially developed either among the rocks through which the
volcano has driven its vent, or about the base of the erupted lavas
and tuffs. Many illustrations of this distribution will be described
from the various volcanic areas of Britain belonging to Palæozoic and
Tertiary time. At the base of the great Cambrian and Lower Silurian
volcanic series of Merionethshire, sills are admirably developed, while
among the basaltic eruptions which closed the long volcanic record in
the north of Ireland and the Inner Hebrides, they play a notable part.
From the frequent place which sills take at the base of a volcanic
series, it may be inferred that they generally belong to a late phase
in the history of an eruptive episode or cycle, when the orifices of
discharge had become choked up, and when the volcanic energy found an
easier passage laterally between the strata underneath the volcanic
pile or between the sheets of that pile itself, than upward through the
ever-increasing thickness of ejected material.
While there is an obvious relation between most sills and some eruptive
centre in their neighbourhood, cases occur in which no trace of any
contemporaneous volcano can be found, but where the intrusive sheet
remains as the sole evidence of the movements of the subterranean
magma. The Great Whin Sill, one of the most extensive intrusive sheets
in the British Isles, is an instance of this kind. Though this large
mass of injected material can be traced for a distance of about 80
miles, and though the strata beneath and above it are well exposed in
innumerable sections, no evidence has yet been detected to show that
it was connected with any vent that formed a volcano at the surface
(see vol. ii. p. 2). The absence of this evidence may, of course,
arise from the failure of denudation to uncover the site of the vent,
which may possibly still remain buried under the Carboniferous strata
that overlie the sill towards the south-east. But it may be due to the
non-existence of any such vent. We can quite conceive that volcanic
energy should sometimes have failed to complete the formation of
an actual volcano. Aided by subterranean movements, it might have
been potent enough to disrupt the lower parts of the terrestrial
crust, to propel the molten magma into fissures, even to inject it
for many miles between the planes of stratification, which would be
lines of least resistance, and yet in default of available rents,
might have been unable to force its way through the upper layers and
so reach the surface. Examples of such incompleted volcanoes are
perhaps to be recognized among solitary sills, which not infrequently
present themselves in the geological structure of Britain. But the
positive decision of this question is almost always frustrated by the
imperfection of the evidence, and the consequent possibility that a
connected vent may still lie concealed under overlying strata.
Besides the more usual intrusions of molten material in the form of
sheets of which the vertical thickness bears but a small proportion
to the horizontal extent, there occur also large and thick cakes of
intruded material in which the vertical thickness may approach, or
perhaps even surpass, the horizontal diameter. These dome-shaped or
irregular expansions form a connecting link between ordinary sills and
the bosses to be subsequently described. They have received the name of
_Laccolites_ from Mr. G. K. Gilbert, who worked out this peculiar type
of structure in the case of the Henry Mountains in southern Utah[34]
(Fig. 34). The same type has since been found distributed over Arizona
and Colorado, and it has been recognized as essentially that of many
eruptive masses or bosses in all parts of the world.
[Footnote 34: "Geology of the Henry Mountains," _U.S. Geog. and Geol.
Survey of the Rocky Mountain Region_, 1877. For a review of the
whole subject of laccolites in Western America see a paper by Mr.
Whitman Cross, in the _14th Annual Report of the Director of the U.S.
Geological Survey_, 1892-93 (pub. 1895), p. 157.]
[Illustration: Fig. 34.--Ideal section of three Laccolites. (After Mr.
Gilbert.)]
In Western America, owing in large measure to the previously
undisturbed condition of the sedimentary formations, the relations of
the injected igneous material to these formations can be satisfactorily
ascertained. The geological structure of the various isolated
laccolites thus clearly presented, helps to explain the structure of
other intrusive bodies which, having been injected among plicated and
dislocated rocks, do not so readily admit of interpretation.
In Colorado, Utah and Arizona the eruptive magma, usually a porphyrite,
diorite or quartz-porphyry, has risen in one or more pipes, and has
then intruded itself laterally between the planes of the sedimentary
formations which, over the centre of intrusion, have been pushed upward
into a vast dome-shaped or blister-like elevation. The horizon on which
this lateral and vertical expansion of the intruded material took
place would seem to have lain several thousand feet below the surface.
It ranges from the Cambrian to the Tertiary formations. Subsequent
denudation has cut down the upraised mantle of sedimentary layers, and
has revealed more or less of the igneous rock underneath, which is
thus allowed to protrude and to be affected by atmospheric erosion.
In this way, wide plains of horizontal or gently undulating Secondary
and Tertiary strata have been diversified by the appearance of cones,
detached or in groups, which have become more peaked and varied in
outline in proportion as their original sedimentary covering has been
removed from them. The largest of the laccolitic masses in the Henry
Mountains is about 7000 feet deep and about 4 miles in diameter. Less
than one-half of the cover of overarching strata has been removed, and
denudation has cut deeply into the remaining part.
That the type of structure, so well exhibited among the Henry
Mountains, has not been more abundantly recognized elsewhere probably
arises from the fact not that it is rare, but that the conditions
for its development are seldom so favourable as in Western America.
Obviously where stratified rocks have been much disturbed, they cease
to furnish definite or regular platforms for the reception of eruptive
material, and to afford convenient datum-lines for estimating what
was probably the shape of the intruded magma. We may believe that the
effect of the propulsion of eruptive material is usually to upheave
the overlying crust, and thus to give rise to a laccolitic form of
intrusion. The upheaval relatively to the surrounding country will
be apt to be practically permanent, the intruded body of rock being
welded to the surrounding formations, and forming in this way a solid
and resisting core directly united by pipes or funnels with the great
magma-reservoir underneath. On the other hand, where the molten rock,
instead of consolidating underground, has been copiously discharged at
the surface, its emission must tend towards the production of cavernous
spaces within the crust. The falling in of the roofs of such caverns
will give rise to shocks of earthquakes. Subsequent uprisings of the
magma may fill these spaces up, and when the rock has solidified in the
form of laccolites or bosses, it may effectually put an end there to
further eruptions.
Some contact metamorphism may be observed along the upper and under
surfaces of large sills. The rocks over the American laccolites have
sometimes been highly altered. But as the change is the same in kind as
that attendant upon Bosses, though generally less in degree, it will
be considered with these intrusive masses. The problems in terrestrial
physics suggested by the intrusion of such thick and persistent
masses of eruptive material as those which form sills and laccolites
will likewise be discussed in connection with the mechanism of the
remaining intrusive masses which have now to be described.
iii. _Bosses (Stocks, Culots)_
The term Boss has been applied to masses of intrusive rock which form
at the surface rounded, craggy or variously-shaped eminences, having a
circular, elliptical or irregular ground-plan, and descending into the
terrestrial crust with vertical or steeply-inclined sides (Fig. 28).
Sometimes they can be seen to have pushed the surrounding rocks aside.
In other places they seem to occupy the place of these rocks through
which, as it were, an opening has been punched for the reception of the
intrusive material.
Occasionally, more especially in the case of large bosses, like
those in which granite so frequently appears, the eruptive mass may
be observed to rise here and there in detached knobs through the
surrounding rocks, or to enclose patches of these, in such a manner as
to indicate that the large body of eruptive material terminates upward
in a very irregular surface, of which only the more prominent parts
project through the cake of overlying rocks. In true bosses, unlike
sills or laccolites, we do not get to any bottom on which the eruptive
material rests. Laccolites, indeed, may be regarded as intermediate
between the typical sill and the typical boss. The difference between
a laccolite and a boss lies in the fact that the body of the laccolite
does not descend into an unknown depth in the crust, but lies upon a
platform on which it has accumulated, the magma having ascended by one
or more ducts, which generally bear but a small proportion in area to
the mass of the laccolite. The boss, on the other hand, is not known
to lie on any horizon, nor to proceed from smaller ducts underneath,
but plunges as a great pillar or irregular mass, which may frequently
be noticed to widen downwards into the crust. There can be no doubt,
however, that many masses of eruptive rock, which, according to the
definition here given, should be called bosses, would be found to be
truly laccolites if their structure below ground could be ascertained.
It is obvious that our failure to find any platform on which the body
of a boss lies, may arise merely from denudation having been as yet
insufficient to lay such a platform bare. It is hardly probable that
a boss several miles in diameter should descend as a column of that
magnitude to the magma-reservoir from which its material came. More
probably it has been supplied through one or more smaller ducts. The
large boss now visible at the surface may thus be really a laccolitic
expansion on one or more horizons. M. Michel Lévy lays stress on the
general widening of granitic bosses as they descend into the crust.[35]
While his observations are supported by many illustrations from all
parts of the globe, and are probably true of the deeper-seated masses
of granite, it is no less true that numerous examples have been met
with where a granite boss is sharply marked off from the rocks which it
has invaded and on which it may be seen to lie. Apart from the cases
where granite seems to form part of a vast internal, once molten mass,
into which its encircling gneisses seem to graduate, there are others
in which this rock, as now visible, has been injected into the crust as
a boss or as a laccolite. Instances will be described in later chapters
where such bosses have risen through Cambrian, Silurian, Devonian and
Carboniferous formations. It may be said that between such granitic
intrusions and volcanic operations no connection can be traced. But
reasons will be brought forward in later chapters to regard some of
the granitic bosses as parts of the mechanism of Palæozoic volcanoes.
It will also be shown that among the intrusive rocks of the Tertiary
volcanic series of Britain there occur bosses of truly granophyric and
granitic material. Hence, though mainly what is called a "plutonic"
rock, granite has made its appearance among the subterranean
protrusions of volcanoes.
[Footnote 35: M. Michel Lévy, _Bull. Carte Géol. France_, No. 35, tome
v. (1893), p. 32. The view stated in the text is also that adopted
by Prof. Brögger with reference to the granite of the Christiania
district. "Die Eruptivgesteine des Kristianiagebietes."]
It is no doubt true that many intrusive masses, which must be included
under the general name of bosses, have probably had no connection
whatever with volcanic action properly so called. They are plutonic
injections, that is, portions of the subterranean magma which have been
intruded into the terrestrial crust during its periods of disturbance,
and have not been accompanied with any superficial discharges, which
are essential in truly volcanic energy. It has been proposed to draw
a distinction between such deep-seated intrusions and those which
represent volcanic funnels.[36] If this were always practicable it
would certainly be desirable. But the distinction is not one that can
in every case be satisfactorily drawn. Even in regard to granitic
bosses, which may generally be assumed to be plutonic in origin, the
British examples just referred to have in all likelihood been connected
with undoubted volcanic outbursts. Without, therefore, attempting here
to separate the obviously volcanic necks of eruptive material from the
probably plutonic bosses, I propose to describe briefly the general
characters of bosses considered as a group of intrusive rocks, together
with the phenomena which accompany them, and the conditions under which
they may have been injected.
[Footnote 36: M. Michel Lévy, _Bull. Carte Géol. France_, No. 35, tome
v. (1893).]
Bosses, whether of plutonic or volcanic origin, are frequently not
merely single masses of eruptive rock, but are accompanied with a
system of dykes and veins, some of which can be traced directly into
the parent-mass, while others traverse it as well as the surrounding
rocks. Hence the history of a boss may be considerably more complex
than the external form of the mass might suggest.
The petrographical characters of bosses link them with the other
underground injections of igneous material, more especially with sills
and laccolites. Indeed, on mere lithological grounds no satisfactory
line could be drawn between these various forms of intrusive rocks. The
larger the mass the more coarsely crystalline it may be expected to be.
But the whole range of structure, texture and composition, from those
of the narrowest vein to those of the widest boss, constitutes one
connected series of gradations.
Acid, intermediate and basic rocks are abundantly displayed among the
bosses. Huge masses of granite, granophyre, quartz-porphyry, felsite or
rhyolite, represent the acid series. Intermediate varieties consist of
trachyte, phonolite, diorite, andesite or other rock. The basic bosses
include varieties of gabbro, dolerite, basalt, picrite, and other
compounds.
In a boss of large size, a considerable range of texture, composition
and structure may often be observed. The rock is generally much
coarser in grain than that of thin sills or dykes. Sometimes it
exhibits a finer texture along the margin than in the centre, though
this variation is not usually so marked as in sills and dykes. The
rapidly-chilled and therefore more close-textured selvage seems to
have been developed much more fully in small than in large masses of
eruptive material. The latter, cooling more slowly, allowed even their
marginal parts to retain their heat, and sometimes perhaps even their
molten condition, longer than small injections. Some influence must
also have been exercised by the temperature of the rocks into which the
eruptive material was intruded. Where this temperature was high, as
in deep-seated parts of the crust, it would allow the intrusive magma
to cool more slowly, and thus to assume a more coarsely crystalline
condition. The absence of a close grain round the margins of granitic
bosses may be due to this cause.
But a much more important distinction may be traced between the central
and marginal parts of some large bosses and thick sills. I have already
alluded to the fact that while the middle of a large intrusive mass
may be decidedly acid, taking even the form of granite, the outer
borders are sometimes found to be much more basic, passing into such a
rock as gabbro, or even into some ultra-basic compound. Between these
extremes of composition no sharp division is sometimes discoverable,
such as might have been expected had the one rock been intruded into
the other. The differences graduate so insensibly into each other
as to suggest that originally the whole mass of the rock formed one
continuous body of eruptive material. It is possible that in some cases
the magma itself was heterogeneous at the time of intrusion.[37] But
the frequency of the distribution of the basic ingredients towards
the outer margin, and the acid towards the centre, points rather to a
process of differentiation among the constituents of the boss before
consolidation. In some instances the differentiation would appear to
have taken place before crystallization to any great extent had set in,
because the minerals ultimately developed in the central parts differ
from those at the sides. In other cases, the transference of material
would seem to have been in progress after the component minerals had
crystallized out of the magma, for they are the same throughout the
whole intrusive mass, but differ in relative proportions from centre to
circumference.[38]
[Footnote 37: The Tertiary gabbros of the Inner Hebrides have already
been cited, and will be more fully described in a later chapter as
exhibiting the heterogeneousness of an eruptive magma.]
[Footnote 38: See Messrs. Dakyns and Teall, _Quart. Journ. Geol. Soc._
xlviii. (1892), p. 104; Prof. Brögger, _op. cit._ 1. (1894), p. 15; Mr.
A. Harker, _op. cit._ p. 320; Prof. Iddings, _Journ. Geol. Chicago_,
i. (1893), p. 833; _Bull. Phil. Soc. Washington_, ii. (1890), p. 191;
1892, p. 89.]
As illustrations of these features I may cite two good examples, one
from Scotland and one from England. The mass of Garabol Hill, in the
Loch Lomond district, consists mainly of granite, occupying an area
of about 12½ square miles. Messrs. Dakyns and Teall have shown that
while the central portions consist of granite, the south-eastern
margin affords a remarkable series of intermediate rocks, such as
hornblende-biotite-granite, tonalite (quartz-mica-diorite), diorite and
augite-diorite, which lead us outwards into highly basic compounds,
including wehrlites (olivine-diallage rocks), picrites (olivine-augite
rocks), serpentine (possibly representing dunites, saxonites, and
lherzolites), and a peculiar rock consisting essentially of enstatite,
diallage, brown hornblende and biotite. The authors regard the whole
of these widely different rocks as the products of one original magma,
the more basic marginal area having consolidated first as peridotites,
followed by diorites, tonalites and granites in the order of increasing
acidity. The most acid rock in the whole series consists of felspar
and quartz, is almost devoid of ferro-magnesian minerals, and occurs
in narrow veins in the granite and tonalite. It indicates that after
the segregation and consolidation of the whole boss, ruptures occurred
which were filled in by the ascent of the very latest and most acid
remaining portion of still fluid magma.[39]
[Footnote 39: Messrs. Dakyns and Teall, _Quart. Journ. Geol. Soc._
xlviii. (1892), p. 104.]
The case of Carrock Fell in Cumberland has been described by Mr.
A. Harker, who has ascertained that the gabbro of this boss has in
its central portions a specific gravity of less than 2·85 and a
silica-percentage sometimes as high as 59·46, whilst its marginal
zone gives a specific gravity above 2·95 and a silica-percentage as
low as 32·50. The migration of the heavy iron ores towards the margin
is readily apparent to the naked eye, and is well established by
chemical analysis, the oxides of iron amounting in the centre to 6·24
(Fe_{2}O_{3} 3·60, FeO 2·64), and at the margin to 25·54 (Fe_{2}O_{3}
8·44, FeO 17·10).[40] Neither in this instance nor in that of Garabol
Hill has any evidence been noticed which would suggest that the basic
and acid rocks belong to different periods of intrusion. They pass so
insensibly into each other as to form in each case one graduated mass.
[Footnote 40: Mr. A. Harker, _op. cit._ p. 320.]
From these and other examples which have been observed, it is difficult
to escape the conclusion that the differences between the basic
margin and the acid centre are due to some process of segregation or
differentiation while the mass was still in a liquid condition, and
its constituents could pass from one part of the boss to another.
According to Professor Brögger, it may be stated as a general law that
differentiation sets in during consolidation, and is determined by,
and dependent on, the laws of crystallization in a magma, in so far as
the compounds which, on given conditions, would first crystallize out,
diffuse themselves towards the cooling margin so as to produce in the
contact-stratum a peculiar chemical composition in the still liquid
material before crystallization takes place.[41]
[Footnote 41: This general conclusion is stated by Professor Brögger
from his investigation of the rocks of Gran, _Quart. Journ. Geol. Soc._
l. (1894), p. 36.]
If during the process of differentiation, and before consolidation,
injections of the magma occur, they may be expected to differ in
character according to the portion of the magma from which they are
derived. Professor Brögger believes that among the basic eruptive
rocks of Gran in the Christiania district, one and the same magma has
in the bosses solidified as olivine-gabbro-diabases, and in the dykes
as camptonites, bostonites, pyroxenites, hornblendites, and more acid
augite-diorites.[42]
[Footnote 42: _Quart. Journ. Geol. Soc._ l. (1894), p. 35.]
Various opinions have been propounded as to the cause or causes of this
so-called differentiation, but none of them are entirely satisfactory.
We must await the results of further exploration in the field and of
continued research in the laboratory.
What appears to have taken place within a subterranean molten magma
which has been propelled into the earth's crust as a boss or laccolite,
with or without a connected system of dykes, may possibly be made to
throw some light on the remarkable changes in the characters of lavas
successively erupted from the same vent during the continuance of a
volcanic cycle. Whether or not any such process of differentiation can
be proved to take place within a subterranean volcanic reservoir, the
sequence of erupted lavas bears a curious resemblance to the order in
which the constituents of some large bosses succeed each other from
margin to centre. The earliest lavas may be of an intermediate or
even basic character, but they generally tend to become more acid.
Nevertheless alternations of basic and acid lavas which have been noted
in various districts would seem to show that if there be a process of
differentiation in the magma-basins, it is not regular and continuous,
but liable to interruption and renewal. The return to basic eruptions,
which so often marks the close of a volcanic cycle, is likewise not
easily explicable on the supposition of continuous differentiation.
Where no sensible evidence of differentiation is traceable in the
general body of a large intrusive mass, indications that some such
process has there been in progress are perhaps supplied by the more
acid dykes or veins, and the so-called "segregation veins," which have
been already alluded to as traversing large intrusive masses. Though
these portions differ to a greater or less extent in texture and
composition from the main substance of the boss, the differences are
not such as to prevent us from regarding them as really parts of the
same parent magma. The veins, which are more acid than the rock that
they traverse, may be regarded as having emanated from some central
or deeper-seated part of a boss, which still remained fluid after the
marginal or upper portion had consolidated sufficiently far to be
capable of being rent open during subterranean disturbance. But that
the mass, though coherent enough to be fissured, still remained at a
high temperature, may be inferred from the general absence of chilled
edges to these veins. The evidence of differentiation supplied by
"segregation veins" has been referred to in the case of Sills.
The study of the petrographical variations in the constitution of large
eruptive bosses has a twofold interest for the geologist. In the first
place, it affords him material for an investigation of the changes
which a volcanic magma undergoes during its eruption and consolidation,
and thereby provides him with some data for an elucidation of the
cause of the sequence of erupted products during a volcanic cycle.
In the second place, it yields to him some interesting analogies
with the structures of ancient gneisses, and thus helps towards the
comprehension of the origin and history of these profoundly difficult
but deeply fascinating rocks.
Bosses, like sills, occur in the midst of volcanic sheets, and also
as solitary protrusions. Where they rise amidst interstratified lavas
and tuffs they may often be recognized as occupying the position
of volcanic vents. They are then necks, and their characters in
this connection have already been given. Where, however, as so
frequently happens, they appear among rocks in which no trace of any
contemporaneous volcanic material is to be detected, their relation to
former volcanic activity remains uncertain.
Of this doubtful nature some of the most notable examples are supplied
by the great granitic bosses which occur so frequently among the
older Palæozoic rocks of Britain. The age of these can sometimes
be approximately fixed, and is then found to correspond more or
less closely with some volcanic episode. Thus the granite-bosses of
Galloway, in the south of Scotland, disrupt Upper Silurian strata, but
are older than the Upper Old Sandstone. Hence they probably belong
to the period of the Lower Old Red Sandstone, which was eminently
characterized by the vigour and long continuance of its volcanoes. The
granite of Arran and of the Mourne Mountains can be shown by one line
of reasoning to be younger than surrounding Carboniferous formations,
by other arguments to be probably later than the Permian period, and
by a review of the whole evidence to form almost certainly part of the
volcanic history of Tertiary time.
But even where it can be shown that the uprise of a huge boss of
eruptive material was geologically contemporaneous with energetic
volcanic action, this coincidence may not warrant the conclusion that
the boss therefore marks one of the volcanic centres of activity. Each
example must be judged by itself. There have, doubtless, been many
cases of the intrusion of molten material in bosses, as well as in
sills, without the establishment of any connection with the surface.
Such incompleted volcanoes have been revealed by denudation after the
removal of a great thickness of superincumbent rock. The evidence which
would have decided the question to what extent any of them became true
volcanic vents has thus been destroyed. We can only reason tentatively
from a careful collation of all the facts that are now recoverable.
Illustrations of this kind of reasoning will be fully given in
subsequent chapters.
It has been supposed that a test for the discrimination of a
subterranean protrusion from a true volcanic chimney may be found
in the condition of the surrounding rocks, which in the case of the
prolonged flow of molten matter up a vent would be likely to undergo
far more metamorphism than would be the case in the injection of a
single eruptive mass.[43] But, as has been already pointed out, no
special or excessive metamorphism of the encircling rocks is noticeable
around many vents. There is certainly no more alteration contiguous
to numerous true necks than around bosses, which there is no reason to
suppose ever communicated directly with the surface, and which were
probably the result of a single intrusion. We must always remember
that the denudation which has revealed these bosses has generally
removed the evidence of their upward termination and of their possible
connection with any volcanic ejections. Many of them may mark the sites
of true vents from which only single eruptions took place. The opening
of a volcanic vent does not necessarily imply a prolonged ascent of
volcanic material. In a vast number of cases the original eruption was
the first and last effort of the volcano, so that in such circumstances
there seems no more reason for much alteration of the walls of the
chimney than for the metamorphism of the rocks round a boss, laccolite,
sill or dyke.
[Footnote 43: See, for example, Mr. Harker, _Quart. Journ. Geol. Soc._
l. (1894), p. 329.]
The metamorphism produced by intrusions of molten material upon the
rocks with which they have come in contact has long been studied.
Its amount varies so greatly in different cases that the conditions
on which it has specially depended are not easily determined. Three
factors have obviously been of great importance--first, the bulk
of the intruded material; secondly, the chemical composition and
lithological texture and structure of the rocks affected; and thirdly,
the constitution and temperature of the invading magma.
1. It is clear that a huge boss of eruptive material will be likely to
effect much more alteration of the surrounding rocks than a small boss,
sill or dyke. Its initial temperature will probably be higher at the
time of its assuming its final place than that of the same material
after it has found its way into the narrower space of a thin sill or
dyke. It will likewise take much longer to cool. Hence the influence of
its heat and its vapours will continue to act long after those of the
dyke or sill have ceased to manifest themselves.
2. It is equally evident that much of the resultant metamorphism will
depend on the susceptibility of the rocks to change. An obdurate
material such as pure quartz-sand, for example, will resist further
alteration than mere hardening into quartzite. Shales and mudstones may
be indurated into cherty substances of various textures. Limestones
and dolomites, on the other hand, may become entirely crystalline,
and may even have new minerals, such as garnet, tremolite, pyroxene,
etc., developed in them. Hence in comparing the amount of metamorphism
attendant on two separate bosses we must always take into account the
nature of the rocks in which it has been induced.
3. But perhaps the most effective cause of variation in the nature
and amount of contact metamorphism has been the constitution of the
eruptive magma. A broad distinction may be drawn between the alteration
produced by basic and by acid rocks. The intrusion of basic material
has often produced singularly little change, even when the eruptive
mass has been of considerable size. The greatest amount of alteration
is to be found where the basic boss has caught up and enveloped
portions of the surrounding rocks. Thus where the gabbro of Carrock
Fell has invaded the basic Lower Silurian lavas of the Lake District,
the enveloped portions of the latter show considerable modification.
Their groundmass becomes darker and more lustrous, the felspars assume
a clearer appearance and lose some of their conspicuous inclusions,
the pyroxenic constituents are converted into pale amphibole, and the
glassy base disappears. At the actual line of contact the felspars of
the lavas have become disengaged from their original matrix, which
seems to have been dissolved and absorbed in the gabbro-magma. Brown
mica has been exceptionally developed in the altered lava. At the same
time, a change is noticeable in the character of the gabbro itself near
the contact. Brown mica is there to be seen, though not a constituent
of the rock elsewhere. The eruptive material has incorporated the basic
groundmass of the lavas, leaving the felspars undissolved.[44]
[Footnote 44: Mr. Harker, _Quart. Journ. Geol. Soc._ vol. l. (1894), p.
331.]
Much more serious are the changes produced by intrusions of acid
material, though here again the metamorphism varies within wide limits,
being sometimes hardly perceptible, and in other cases advancing
so far as to convert mere sedimentary material into thoroughly
crystalline rocks. Small sills and dykes of felsite and granophyre
may produce very slight change even upon shales and limestones, as
may be seen among the eruptive rocks of Skye and Raasay. Large bosses
of granophyre, and still more of granite, have been accompanied with
the most extensive metamorphism. Round these eruptive masses every
gradation may be traced among sandy and argillaceous sediments, until
they pass into crystalline mica-schists, which do not appear to be
distinguishable from rocks of Archæan age. Admirable examples of this
extreme alteration may be observed around the great granite bosses of
Galloway.[45] Again, among calcareous rocks a transition may be traced
from dull grey ordinary fossiliferous limestones and dolomites into
pure white crystalline marbles, full of crystals of tremolite, zoisite,
garnet and other minerals. The alteration of the fossiliferous Cambrian
limestones of Strath in Skye by the intrusive bosses of Tertiary
granite well illustrates this change.[46]
[Footnote 45: See Explanation to Sheet 9 of the _Geological Survey of
Scotland_, p. 22; Prof. Bonney and Mr. Allport, _Proc. Roy. Soc._ xvi.
(1889); Miss Gardiner, _Quart. Journ. Geol. Soc._ vol. xlvi. (1890), p.
569.]
[Footnote 46: Macculloch, _Trans. Geol. Soc._ vol. iii. (1816), p. 1;
_Description of the Western Isles_, vol. i. p. 322. See also _Quart.
Journ. Geol. Soc._ vol. xiv. (1857), p. 1; and vol. xliv. (1888), p.
62.]
Without entering further here into the wide subject of contact
metamorphism, to which a large literature has now been devoted, we may
note the effects which have been produced in the eruptive material
itself by its contact with the surrounding rocks. Not only have these
rocks been altered, but very considerable modifications have likewise
taken place in the active agent of the change.
Sometimes the alteration of the invading material has been effected
without any sensible absorption of the mineral constituents of the
rocks invaded. This appears to be the case in those instances where
sheets of basalt, intruded among coals or highly carbonaceous shales,
have lost their compact crystalline character and have become mere
clays. In the coal-fields of Britain, where many examples of this
change have been noted, the igneous material is known as "white trap."
The iron oxides have been in great part removed, or, together with the
lime of the component minerals, have been converted into carbonates.
Traces of the original felspar crystals may still be detected, but the
groundmass has been changed into a dull, earthy, friable and decomposed
substance.
Nearly always, however, the alteration of the intrusive magma has
resulted from the incorporation of portions of the surrounding rocks.
Reference has been made above to the alteration of the Carrock Fell
gabbro by the absorption of some of the basic lavas around it. But
still more remarkable is the change produced in some acid rocks by the
incorporation of basic material into their substance. Professor Sollas
has described in great detail a remarkable instance of this effect in
the probably Tertiary eruptive rocks of the Carlingford district in
the north-east of Ireland. He has ascertained that the eruptive gabbro
of that district is older than the granite, for it is traversed by
granophyre dykes which enclose pieces of it. The granophyre dykes,
on the other hand, often show a lithoidal or chilled margin, which
is not visible in the gabbro. He believes that the gabbro is not
only older than the acid protrusions, but was already completely
solid, traversed by contraction-joints, and probably fractured by
earth-movements, before the injection of the granophyric material,
which at the time of its intrusion was in a state of extreme fluidity,
for it has found its way into the minutest cracks and crevices. He has
especially studied the alteration produced by the granophyre upon the
enclosed pieces of basic rock. The diallage, isolated from the other
constituents of the gabbro, may commonly be seen to have broken up into
numerous granules, like the augite grains of basalt, while in some
cases biotite and hornblende have been developed with the concomitant
excretion of magnetite. The acid rock itself has undergone considerable
modification owing to the incorporation of basic material into its
substance. Professor Sollas distinguishes the following varieties
of the rock:--Biotite-granophyre, biotite-amphibole-granophyre,
augite-granophyre, diallage-amphibole-augite-granophyre.[47]
[Footnote 47: _Trans. Roy. Irish Acad._ xxx. (1894), part xii. p. 477.]
Similar phenomena have been described by Mr. Harker as occurring
where granophyre has invaded the gabbro of Carrock Fell.[48] The same
observer has more recently detected some interesting examples furnished
by injections of Tertiary granophyre in the agglomerates of Skye. The
acid rock is roughly estimated by him to have taken up about one-fourth
of its bulk of gabbro fragments. He has investigated the minute
structure of the rock thus constituted, and has been able to recognize
the augite of the original gabbro, in various stages of alteration and
completely isolated, the other minerals having been dissolved in the
acid magma.[49]
[Footnote 48: _Quart. Journ. Geol. Soc._ li. (1895), p. 183.]
[Footnote 49: _Op. cit._ lii. The metamorphism produced upon fragments
of different kinds of foreign material enclosed within various igneous
rocks has in recent years been studied in great detail by Professor
Lacroix--_Les Enclaves des Roches Volcaniques_, Macon, 1893.]
It is not easy to comprehend the conditions under which large masses
of molten material have been injected into the crust of the earth. The
two main factors in volcanic action--terrestrial contraction and the
energy of the vapours in the magma--have no doubt played the chief part
in the process. But the relative share of each and the way in which
the enormous load of overlying rock has been overcome are not readily
intelligible.
Let us first consider for a moment the pressure of the superincumbent
crust under which the injection in many cases took place. The Whin Sill
of England may serve as a good illustration of the difficulties of the
problem. This notable mass of intrusive rock has been forced between
the stratification planes of the Carboniferous Limestone series in
one, or sometimes more than one, sheet. It stretches for a horizontal
distance of not less than 80 miles with an average thickness of between
80 and 100 feet. From the area over which it can be traced its total
extent underground must be at least 400 square miles (see Chapter
xxix.).
In any single section the Whin Sill might be supposed to be a truly
interstratified sheet, so evenly does it seem to be intercalated
between the sedimentary strata. But here and there it diverges
upward or downward in such a way as to prove it to be really a vast
injected sheet. The age of the injection cannot be precisely fixed.
It must be later than the Carboniferous Limestone. There is no trace
of any stratigraphical break in the Carboniferous system of the
region traversed by the sill. If the injection took place during the
Carboniferous period, it does not appear to have been attended with any
local disturbance, such as we might suppose would have been likely to
accompany the extravasation of so enormous a mass of igneous material.
If the date of injection be assigned to the next volcanic episode in
the geological history of Britain--that of the Permian period--it will
follow that the Whin Sill was intruded into its present position under
the superincumbent weight of the whole of the Carboniferous system
higher than the platform followed by the injected rock. The overlying
body of strata would thus exceed 5000 feet in thickness, or in round
numbers would amount at least to an English mile. The pressure of this
mass of superincumbent material, at the depth at which the injected
magma was forced between the strata, must have been so gigantic that it
is difficult to believe that the energy of the magma would have been
able to achieve of itself so stupendous a task as the formation of the
Great Whin Sill.
The volume of injected material is likewise deserving of special
attention. Many sills exceed 300 or 400 feet in thickness; and some
laccolites must enormously surpass these limits. The intrusion of so
vast a body of new material into the terrestrial crust will necessitate
either a corresponding elevation of that part of the crust overlying
the injected magma or a subsidence of that part underlying it, or some
combination of both movements. It is conceivable that, where the body
of protruded magma was large and the thickness of overlying crust was
small, the expansive force of the vapours under high tension in the
molten rock may have sufficed for the uplift. This result will be most
likely to be effected around a volcanic chimney where the magma has
the least amount of overlying load, and encounters that relief from
pressure which enables it to become a powerful agent in terrestrial
physics.
But in the case of the larger bodies of injected rock, especially
where they do not seem to have been accompanied by the opening of any
volcanic vents, the propulsion of the igneous material into the crust
has probably been effected as a consequence of disturbance of the
terrestrial crust. When the strain of contraction leads to the pushing
upward of the terrestrial areas intervening between wide regions of
subsidence, even though the differential movement may be slight,
the isogeotherms undergo deformation. The intensely hot nucleus is
squeezed upward, and if in the process of compression ruptures take
place in the crust, and cavities in it are consequently opened, the
magma will at once be forced into them. Such ruptures may be expected
to take place along lines of weakness. Rocks will split along their
stratification-planes, and the tendency to separation along these lines
may be aided by the readiness of the energetic magma to find its way
into and to enlarge every available opening. Hence we may expect that,
besides vertical fractures, leading to the production of dykes and
bosses, there will often be horizontal thrusts and ruptures, which will
give rise to the formation of sills.
There is still another feature of terrestrial contraction which
may help us to follow the behaviour of the magma within the crust.
Plication of the crust is one of the most characteristic results of the
contracting strain. Where a great series of sedimentary formations has
been violently compressed so that its component strata have been thrown
into rapid folds and squeezed into a vertical position, the portion
of the crust thus treated may possibly be on the whole strengthened
against the uprise of molten material through it. But the folding is
often accompanied with dislocation. Not only are the rocks thrown into
endless plications, but portions of them are ruptured and even driven
horizontally over other parts. Such greatly disturbed areas of the
crust are not infrequently found to have been plentifully injected with
igneous rocks in the form of dykes, veins, sills, laccolites and bosses.
The elevation of a mountain-chain is known to be accompanied with
a diminution of density in the crust underneath. Mr. O. Fisher has
suggested that along such lines of terrestrial uplift there may be a
double bulge in the crust, one portion rising to form the upheaved
land and the other sinking down into the hot nucleus. If the lighter
descending crust were there melted it might form a magma ready to be
poured out as lava on the opening of any vent. The lava thus ejected
would be of the lighter kinds. It has been remarked as certainly a
curious fact that the lavas which issue from high mountain ranges
are generally much more acid than the heavy basic lavas which are so
characteristic of volcanoes close to the level of the sea.
But even where no actual mountain-chain is formed, there are gentle
undulations of the crust which no doubt also affect the isogeotherms.
If any series of disturbances should give rise to a double system of
such undulations, one crossing the other, there would be limited
dome-shaped elevations at the intersections of these waves, and if
at the same time actual rupture of the crust should take place, the
magma might find its way upward under such domes and give rise to the
formation of laccolitic intrusions. Cessation of the earth-movements
might allow the intruded material slowly to solidify without ever
making an opening to the surface and forming a volcano. Doubtless many
sills, laccolites and bosses represent such early or arrested stages in
volcanic history.
Propelled into the crust at a high temperature, and endowed with great
energy from the tension of its absorbed vapours and gases, the magma
will avail itself of every rent which may be opened in the surrounding
crust, and where it has succeeded in reaching the surface, its own
explosive violence may enable it to rupture the crust still further,
and open for itself many new passages. Thus an eruptive laccolite or
boss is often fringed with veins, dykes and sills which proceed from
its mass into the rocks around.
The question how far an ascending mass of magma can melt down its
walls is one to which no definite answer can yet be given. Recent
observations show that where the difference in the silica percentage
between the magma and the rock attacked is great, there may be
considerable dissolution of material from this cause. Allusion has
already been made to Mr. Harker's computation that some of the acid
granophyres of Skye have melted down about a fourth of their bulk of
the basic gabbros. If such a reaction should take place between the
magma of a boss, sill or laccolite and the rocks among which it has
been intruded, great changes might result in the composition of the
intruded rock. We are not yet, however, in possession of evidence
to indicate that absorption of this kind really takes place on an
extensive scale within the earth's crust. If it did occur to a large
extent, we should expect much greater varieties in the composition of
eruptive rocks than usually occur, and also some observable relation
between the composition of the igneous material and that of the rocks
into which it has been injected. But enough is not yet known of this
subject to warrant any decided opinion regarding it.
CHAPTER VII
Influence of Volcanic Rocks on the Scenery of the Land--Effects of
Denudation.
As considerable popular misapprehension exists respecting the part
which volcanism has played in the evolution of the existing topography
of the earth's surface, and as the British Isles, from their varied
geological structure, offer special facilities for the discussion of
this subject, it may not be out of place to devote a final section of
the present Introduction to a consideration of the real topographical
influence of volcanic action.
With modern, and especially with active, volcanoes we need not here
concern ourselves. Their topographical forms are well known, and give
rise to no difficulty. The lofty cones of the Vesuvian type, with their
widespread lavas and ashes, their vast craters and their abundant
parasitic volcanoes; the crowded, but generally diminutive, cones
and domes of the puy type, so well displayed in Auvergne, the Eifel
and the Bay of Naples; and the vast lava deserts of the plateaux, so
characteristically developed in Iceland and Western America, illustrate
the various ways in which volcanic energy directly changes the contours
of a terrestrial surface.
But the circumstances are altered when we deal with the topographical
influence of long extinct volcanoes. Other agencies then come into
play, and some caution may be needed in the effort to disentangle the
elements of the complicated problem, and to assign to each contributing
cause its own proper effect.
Reference has already been made to the continuous denudation of
volcanic hills from the time that they are first erupted. But the
comparative rapidity of the waste and the remarkable topographical
changes which it involves can hardly be adequately realized without
the inspection of an actual example. A visit to the back of Monte
Somma, already alluded to, will teach the observer, far more vividly
than books can do, how a volcanic cone is affected by daily meteoric
changes. The sides of such a cone may remain tolerably uniform slopes
so long as they are always being renewed by deposits from fresh
eruptions. But when the volcanic activity ceases, and the declivities
undergo no such reparation, they are rapidly channelled by the descent
of rain-water, until the furrows grow by degrees wide and deep ravines,
with only narrow and continually-diminishing crests between them. If
unchecked by any fresh discharge of volcanic material, the degradation
will at last have removed the whole cone.
It is thus obvious that purely volcanic topography, that is, the
terrestrial scenery due directly to the eruption of materials from
within the earth, can never become in a geological sense very old.
It can only endure so long as it is continually renewed by fresh
eruptions, or where it is carried down by subsidence under water and is
there buried under a cover of protecting sediments. When, therefore,
we meet with volcanic rocks of ancient date exposed at the surface,
we may be quite certain that their present contours are not those of
the original volcano, but have been brought about by the processes of
denudation.
It is true that, in the general erosion of the surface of the land,
volcanic rocks of ancient date sometimes rise into wonderfully craggy
heights, including, perhaps, cones and deep crater-like hollows, which
to popular imagination betoken contours left by now extinguished
volcanic fires. Examples of such scenery are familiar in various
parts of Britain; but the resemblance to recent volcanic topography
is deceptive. There are, indeed, a few hills wherein the progress of
denudation seems not as yet to have entirely removed the lavas and
tuffs that gathered round the original vents. Some of the tuff-cones
of eastern Fife, for example, present cases of this kind. Again, the
great granophyre domes and cones of the Tertiary volcanic series of the
Inner Hebrides, though they have undoubtedly been extensively denuded,
may possibly retain contours that do not greatly differ from those
which these protruded bosses originally assumed under the mass of rock
which has been removed from them. Nevertheless, putting such doubtful
exceptions aside, we may confidently affirm that hills composed of
ancient volcanic material give no clue to the forms of the original
volcanoes.
It can hardly be too often repeated that the fundamental law in
the universal decay and sculpture of the land is that the waste is
proportioned to the resistance offered to it: the softer rocks are
worn down with comparative rapidity, while the harder varieties
are left projecting above them. As a general rule, volcanic rocks
are more durable than those among which they are interstratified,
and hence project above them, but this is not always the case. No
universal rule can, indeed, be laid down with regard to the relative
durability of any rocks. While, therefore, topographic contours afford
a valuable indication of the nature and disposition of the rocks below
the surface, they cannot be relied upon as in all circumstances an
infallible guide in this respect. No better proof can be offered of the
caution that is needed in tracing such contours back to their origin
than is furnished by the old volcanic rocks of Britain. These eruptive
masses, consisting usually of durable materials and ranging through a
vast cycle of geological time, usually rise into prominent features and
thus support the general law. But they include also many easily eroded
members, which, instead of forming eminences, are worn into hollows.
They include, in short, every type of scenery, from featureless plains
and rolling lowlands to craggy and spiry mountains.
The first point, then, which is established in an investigation of
the topographical influence of old volcanic rocks is that their
prevailing prominence arises from relative durability amidst universal
degradation. When we proceed further to inquire why they vary so
much from each other in different places, and how their complicated
details of feature have been elaborated, we soon learn that such local
peculiarities have arisen mainly from variations in the internal
structure and grouping of the rocks themselves.
Here again the general law of sculpture comes into play. The local
features have depended upon the comparative resistance offered to the
sculpturing agents by the different portions of a volcanic series. Each
distinct variety of rock possesses its own characteristic internal
structure. The lines along which atmospheric disintegration will
most effectually carry on its carving work are thus already traced
in the very substance and architecture of the rock itself. Each rock
consequently yields in its own way to the processes of disintegration,
and thus contributes its own distinctive share to topographical feature.
Among the massive rocks abundant examples of such special types of
weathering may be cited, from the acid and basic series, and from
superficial lavas as well as from intrusive bosses and sills. Acid
bosses, such as those of granite, granophyre and quartz-porphyry, tend
to weather into blocks and finally into sand, and as this tendency
is somewhat uniformly distributed through the rocks, they are apt to
assume rounded, dome-shaped or conical forms which, at a distance, may
seem to have smooth declivities, but on examination are generally found
to be covered with a slowly-descending sheet of disintegrated blocks
and debris (Fig. 346). When less prone to decay, and especially where
traversed by a strongly-defined system of vertical joints, they may
shoot up into tower-like heights, with prominent spires and obelisks.
Basic bosses, when their materials decay somewhat rapidly, give rise to
analogous topographical forms, though the more fertile soils which they
produce generally lead to their being clothed with vegetation. Where
they consist of an obdurate rock, much jointed and fissured, like the
gabbro of the Inner Hebrides, they form exceedingly rugged mountains,
terminating upward in serrated crests and groups of aiguilles (Figs.
331, 333).
Acid lavas that have been superficially erupted weather into
irregularly craggy hills, like the flanks of Snowdon. Those of
intermediate composition, where they have accumulated in thick masses,
are apt to weather into conical forms, as may be seen among the
Cheviot, Pentland and Garleton Hills (Figs. 109, 110, 133); but where
they have been poured out in successive thin sheets they have built
up undulating plateaux with terraced sides, as among the Ayrshire and
Campsie Fells and the hills of Lorne (Figs. 99, 107). Basic lavas have
issued in comparatively thin sheets, frequently columnar or slaggy,
forming flat-topped hills and terraced escarpments, such as are
typically developed among the Tertiary basalt-plateaux of the Inner
Hebrides and the Faroe Islands (Figs. 11, 265, 283, 284, 286).
One of the most frequent causes of local peculiarities of topography
among old volcanic rocks is the intercalation of very distinct
varieties of material in the same volcanic series. Where, for instance,
lavas and tuffs alternate, great inequalities of surface may be
produced. The tuffs, being generally more friable, decay faster and
give rise to hollows, while the lavas, being more durable, project in
bold ridges or rise into mural escarpments (Fig. 265). Again, where
dykes weather more readily than the rocks which they traverse, they
originate deep narrow clefts, while where they weather more slowly than
the rocks around them, they project as dark ribs. Thus in Skye some
dykes which rise through the obdurate gabbro are marked by chasms which
reach up even to the highest crests of the mountains (Fig. 333), while
of those which run in the pale crumbling granophyre, some stand up as
black walls that can be followed with the eye across the ridges even
from a long distance.
Many further illustrations of these principles might be cited here from
the old volcanic districts of Britain. But they will present themselves
successively in later chapters. For my present purpose it is enough
to show that the scenery of these districts is not directly due to
volcanic action, but is the immediate result of denudation acting upon
volcanic rocks, modified and directed by their geological structure.
It may, however, be useful, in concluding the discussion of this
subject, to cite some typical volcanic regions in the British Isles as
illustrations of the relations between geology and topography, which,
besides impressing the main lesson here enforced, may serve also to
show some of the striking contrasts which geology reveals between the
present and former conditions of the surface of the globe. Among these
contrasts none are more singular than those offered by tracts where
volcanic action has once been rife, and where the picture of ancient
geography presented in the rocks differs so widely from the scenery of
the same places to-day as to appeal vividly to the imagination.
The first district to which I may refer where ancient volcanic rocks
are well developed is that of Devonshire. The story of the Devonian
volcanoes will be told in some detail in later chapters, when it will
be shown that the eruptions were again and again renewed during a long
course of ages. Yet, abundant as the intercalated lavas and tuffs are,
they can hardly be said to have had any marked effect on the scenery,
though here and there a harder or larger mass of diabase rises into a
prominent knoll or isolated hill. When the amount of volcanic material
in this region is considered, we may feel some surprise at the trifling
influence which it has exerted in the general denudation of the surface.
To one who wanders over the rich champaign of southern Devonshire, and
surveys from some higher prominence the undulating tree-crowned ridges
that slope down into orchard-filled hollows, and the green uplands
that sweep in successive waves of verdure to the distant blue tors of
Dartmoor, the scene appears as a type of all that is most peaceful,
varied and fertile in English landscape. In the trim luxuriance that
meets the eye on every side, the hand of man is apparent, though from
many a point of vantage no sound may be heard for a time to show that
he himself is anywhere near us. Yet ever and anon from the deep lanes,
hidden out of sight under their canopy of foliage, there will come the
creak of the groaning waggon and the crack of the waggoner's whip, as
evidence that there are roads and human traffic through this bosky
silent country.
Amid so much quiet beauty, where every feature seems to be eloquent
of long generations of undisturbed repose, it must surely stir the
imagination to be told that underneath these orchards, meadows and
woodlands lie the mouldering remnants of once active and long-lived
volcanoes. Yet we have only to descend into one of the deep lanes to
find the crumbling lavas and ashes of the old eruptions. The landscape
has, in truth, been carved out of these volcanic rocks, and their
decomposition has furnished the rich loam that nourishes so luxuriant a
vegetation.
Not less impressive is the contrast presented between the present
and former condition of the broad pastoral uplands of the south of
Scotland. Nowhere in the British Islands can the feeling of mere
loneliness be more perfectly experienced than among these elevated
tracts of bare moorland. They have nothing of the grandeur of outline
peculiar to mountain tracts. Sometimes, for miles around one of their
conspicuous summits, we may see no projecting knob or pinnacle. The
rocks have been gently rounded off into broad featureless hills, which
sink into winding valleys, each with its thread of streamlet and its
farms along the bottom, and its scattered remnants of birch-wood or
alder-copse along its slopes and dingles. Across miles of heathy
pasture and moorland, on the summits of this great tableland, we may
perchance see no sign of man or his handiwork, though the bleating of
the sheep and the far-off barking of the collie tell that we are here
within the quiet domain of the south-country shepherd.
In this pastoral territory, also, though they hardly affect the
scenery, volcanic rocks come to the surface where the foldings of the
earth's crust have brought up the oldest formations. Their appearance
extends over so wide an area as to show that a large part of these
uplands lies on a deeply-buried volcanic floor. A whole series of
submarine volcanoes, extending over an area of many hundreds of square
miles, and still in great part overlain with the accumulated sands and
silts of the sea-bottom, now hardened into stone, underlies these quiet
hills and lonely valleys.
A contrast of another type meets us in the broad midland valley of
Scotland. Around the city of Edinburgh, for instance, the landscape
is diversified by many hills and crags which show where harder rocks
project from amidst the sediments of the Carboniferous system. On some
of these crags the forts of the early races, the towers of Celt and
Saxon, and the feudal castles of the middle ages were successively
planted, and round their base clustered for protection the cots of
the peasants and the earliest homesteads of the future city. Beneath
these crags many of the most notable events in the stormy annals of
the country were transacted. Under their shadow, and not without
inspiration from their local form and colour, literature, art and
science have arisen and flourished. Nowhere, in short, within the
compass of the British Isles has the political and intellectual
progress of the people been more plainly affected by the environment
than in this central district of Scotland.
When now we inquire into the origin and history of the topography
which has so influenced the population around it, we find that its
prominences are relics of ancient volcanoes. The feudal towers are
based on sills and dykes and necks. The fields and gardens, monuments
and roadways, overlie sheets of lava or beds of volcanic ashes. Not
only is every conspicuous eminence immediately around of volcanic
origin, but even the ranges of blue hills that close in the distant
view to south and north and east and west are mainly built up of lavas
and tuffs. The eruptions of which these heights are memorials belong to
a vast range of geological ages, the latest of them having passed away
long before the advent of man. But they have left their traces deeply
engraven in the rocky framework of the landscape. While human history,
stormy or peaceful, has been slowly evolving itself during the progress
of the centuries in these fertile lowlands, the crags and heights have
remained as memorials of an earlier history when Central Scotland
continued for many ages to be the theatre of vigorous volcanic activity.
As a final illustration of the influence of volcanic rocks in scenery,
and of the contrast between their origin and their present condition,
I may cite the more prominent groups of hills in the Inner Hebrides.
In the singularly varied landscapes of that region three distinct
types of topography attract the eye of the traveller. These are best
combined and most fully developed in the island of Skye. Throughout
the northern half of that picturesque island, the ground rises into a
rolling tableland, deeply penetrated by arms of the sea, into which it
slopes in green declivities, while along its outer borders it plunges
in ranges of precipice into the Atlantic. Everywhere, alike on the
cliffs and the inland slopes, long parallel lines of rock-terrace
meet the eye. These mount one above another from the shores up to the
flat tops of the highest hills, presenting level or gently-inclined
bars of dark crag that rise above slopes of debris, green sward and
bracken. It is these parallel, sharply-defined bars of rock, with their
intervening strips of verdure, that give its distinctive character to
the scenery of northern Skye. On hillside after hillside and in valley
after valley, they reappear with the same almost artificial monotony.
And far beyond the limits of Skye they are repeated in one island after
another, all down the chain of the Inner Hebrides.
In striking contrast to this scenery, and abruptly bounding it on the
south, rise the Red Hills of Skye--a singular group of connected cones.
Alike in form and in colour, these hills stand apart from everything
around them. The verdure of the northern terraced tableland here
entirely disappears. The slopes are sheets of angular debris,--huge
blocks of naked stone and trails of sand, amidst which hardly any
vegetation finds a footing. The decay of the rock gives it a pale
yellowish-grey hue, which after rain deepens into russet, so that in
favourable lights these strange cones gleam with a warm glow as if
they, in some special way, could catch and reflect the radiance of the
sky.
Immediately to the west of these pale smooth-sloped cones, the dark
mass of the Cuillin Hills completes the interruption of the northern
tableland. In almost every topographical feature these hills present
a contrast to the other two kinds of scenery. Their forms are more
rugged than those of any other hill-group in Britain (Fig. 331). Every
declivity among them is an irregular pile of crags, every crest is
notched like a saw, every peak is sharpened into a pinnacle. Instead of
being buried under vast sheets of their own debris, these hills show
everywhere their naked rock, which seems to brave the elements as few
other rocks can do. Unlike the pale Red Hills, they are dark, almost
black in tone, though when canopied with cloud they assume a hue of
deepest violet.
Each of these three distinct types of topography owes its existence
to the way in which a special kind of volcanic rock yields to the
influences of denudation. The terraced tableland of the north is built
up of hundreds of sheets of basaltic lava, each of the long level
ledges of brown rock marking the outcrop of one or more of these once
molten streams. The black rugged mass of the Cuillin Hills consists
of a vast protruded body of eruptive material, which, in the form
of endless sills and bosses of gabbro and dolerite, has invaded the
basalt-plateau, and has now been revealed by the gradual removal of the
portion of that plateau which it upraised. The pale cones and domes
of the Red Hills mark the place of one of the last protrusions in the
volcanic history of Britain--that of large masses of an acid magma,
which broke through the basalt-plateau and also disrupted the earlier
gabbro.
In no part of North-Western Europe has volcanic activity left more
varied and abundant records of its operations than in these three
contiguous tracts of Skye. It is interesting therefore to note the
striking contrast between the former and the present landscapes of the
region. The lavas of the basaltic tableland crumble into a rich loam,
that in the mild moist climate of the Hebrides supports a greener
verdure than any of the other rocks around will yield. The uplands
have accordingly become pasture-grounds for herds of sheep and cattle.
The strips of lowland along the valleys and in the recesses of the
coast-line furnish the chief tracts of arable land in the island, and
are thus the main centres of the crofter population. The bays and
creeks of the much-indented shores form natural harbours, which in
former days attracted the Norse sea-rovers, and supplied them with
sites for their settlements. Norse names still linger on headland and
inlet, but the spirit of adventure has passed away, and a few poor
fishing-boats, here and there drawn up on the beach, are usually the
only token that the islanders make any attempt to gather the harvest of
the sea.
The mountain groups which so abruptly bound the basalt-plateau on the
south, and present in their topographical features such distinctive
scenery, comprise a region too lofty, too rugged and too barren for
human occupation. The black Cuillins and the pale Red Hills are
solitudes left to the few wild creatures that have not yet been
exterminated. The corries are the home of the red deer. The gabbro
cliffs are haunts of the eagle and the raven. Where patches of soil
have gathered in the crannies of the gabbro, alpine plants find their
home. In the chasms left by the decay of the dykes between the vertical
walls of their fissures, the winter snows linger into summer, and
conceal with their thick drifts the mouldering surface of the once
molten rock beneath them. On every side and at every turn a mute appeal
is made to the imagination by the strange contrasts between the quiet
restfulness of to-day, when the sculpture-tools of nature are each
busily carving the features of the landscape, and the tumult of the
time when the rocks, now so silent, were erupted.
* * * * *
The general discussion of the subject of Volcanism in this Introduction
will, I hope, have prepared the reader who has no special geological
training for entering upon the more detailed descriptions in the
rest of this treatise. As already stated, the chronological order
of arrangement will be followed. Beginning with the records of the
earliest ages, we shall follow the story of volcanic action down to the
end of the latest eruptions.
Each great geological system will be taken as a whole, representing a
long period of time, and its volcanic evolution will be traced from
the beginning of the period to the close. Some variety of treatment
is necessarily entailed by the wide range in the nature and amount of
the evidence for the volcanic history of different ages. But where
practicable, an outline will first be given of what can be gathered
respecting the physical geography of each geological period in Britain.
In the description which will then follow of the volcanic phenomena,
an account of the general characters of the erupted rocks will precede
the more detailed narrative of the history of the volcanic eruptions in
the several regions where they took place. References to the published
literature of each formation will be given in the first part of each
section, or will be introduced in subsequent pages, as may be found
most convenient.
BOOK II
VOLCANIC ACTION IN PRE-CAMBRIAN TIME
CHAPTER VIII
PRE-CAMBRIAN VOLCANOES
The Beginnings of Geological History--Difficulties in fixing on
a generally-applicable Terminology--i. The Lewisian (Archæan)
Gneiss; ii. The Dalradian or Younger Schists of Scotland;
iii. The Gneisses and Schists of Anglesey; iv. The Uriconian
Volcanoes; v. The Malvern Volcano; vi. The Charnwood Forest
Volcano.
The early geological history of this globe, like the early history
of mankind, must be drawn from records at once scanty and hardly
decipherable. Exposed to the long series of revolutions which the
surface of the planet has undergone, these records, never perhaps
complete at the first, have been in large measure obliterated.
Even where they still exist, their meaning is often so doubtful
that, in trying to interpret it, we find little solid footing,
and feel ourselves to be groping, as it were, in the dimness of
mythological legend, rather than working in the light of trustworthy
and intelligible chronicles. These primeval records have been more
particularly the objects of sedulous study during the last twenty years
all over Europe and in North America. A certain amount of progress in
their decipherment has been made. But the problems they still present
for solution are numerous and obscure. Fortunately, with many of
these problems the subject of the present treatise is not immediately
connected. We need only concern ourselves with those which are related
to the history of primeval volcanic activity.
To the earliest and least definite division of the geological annals
various names have been applied. Some writers, believing that this
period preceded the first appearance of plants or animals upon the
globe, have named it Azoic--the lifeless age of geological history. But
the absence of any hitherto detected trace of organic existence among
the oldest known rocks cannot be held to prove that these rocks were
formed before the advent of living things on the surface of the earth.
The chance discovery of a single fossil, which might at any moment be
made, would show the name "Azoic" to be a misnomer. Other geologists,
believing that, as a matter of fact, organic structures of low types do
actually occur in them, have called these old rocks "Eozoic," to denote
that they were deposited during the dawn of life upon our planet. But
the supposed organisms have not been everywhere accepted as evidence of
former life. By many able observers they are regarded as mere mineral
aggregates. Another term, "Archæan," has been proposed for the primeval
ages of geological history, which are recorded in rocks that carry us
as far as may ever be possible towards the beginnings of that history.
In choosing some general term to include the oldest known parts of the
earth's crust, geologists are apt unconsciously to assume that the
rocks thus classed together represent a definite section of geological
time, comparable, for instance, to that denoted by one of the Palæozoic
systems. Yet it is obvious that, under one of these general terms of
convenient classification, a most multifarious series of rocks may be
included, representing not one but possibly many, and widely separated,
periods of geological history.
In many countries the oldest sedimentary accumulations, whether
fossiliferous or not, are underlain by a series of crystalline rocks,
which consist in great part of coarse massive gneisses and other
schists. All over the world these rocks present a singular sameness of
structure and composition. What might be found below them no man can
say. They are in each country the oldest rocks of which anything is yet
known, and whatsoever may be our theory of their origin, we must, at
least for the present, start from them as the fundamental platform of
the terrestrial crust.
But though crystalline rocks of this persistent character are widely
distributed, both in the Old World and in the New, they in themselves
furnish no means of determining their precise geological age. No method
has yet been devised whereby the oldest gneiss of one country can be
shown to be the true stratigraphical equivalent of the oldest gneiss of
another. Palæontology is here of no avail, and Petrology has not yet
provided us with such a genetic scheme as will enable us to make use of
minerals and rock-structures, as we do of fossils, in the determination
of geological horizons. All that can be positively affirmed regarding
the stratigraphical relations of the rocks in question is that they
are vastly more ancient than the oldest sedimentary and fossiliferous
formations in each country where they are found. The "Lewisian" gneiss
of the north-west of Scotland, the "Urgneiss" of Central Europe, and
the "Laurentian" gneiss of Canada occupy similar stratigraphical
positions, and present a close resemblance in lithological characters.
We may conveniently class them under one common name to denote this
general relationship. But we have, as yet, no means of determining how
far they belong to one continuous period of geological history. They
may really be of vastly different degrees of antiquity.
From the very nature of the case, any name by which we may choose to
designate such ancient rocks cannot possess the precise stratigraphical
value of the terms applied to the fossiliferous formations. Yet the
convenience of possessing such a general descriptive epithet is obvious.
Until much more knowledge of the subject has been gained, any
terminology which may be proposed must be regarded as more or less
provisional. The comprehensive term "pre-Cambrian" may be usefully
adopted as a general designation for all rocks older than the base of
the Cambrian system, irrespective of their nature and origin. Already
it is well known that under this term a vast series of rocks, igneous
and sedimentary, is included. In some regions several successive
formations, or systems of formations, may be recognized in this
series. But until some method has been devised for determining the
stratigraphical relations of these formations in different regions,
it would seem safest not to attempt to introduce general names for
universal adoption, but to let the sequence of rocks in each distinct
geological province be expressed by a local terminology. This caution
is more especially desirable in the case of sedimentary deposits.
We may surmise as to the equivalence of the rocks called Huronian,
Torridonian and Longmyndian, but whilst so much is mere conjecture, it
is certainly injudicious to transfer the local names of one province to
the rocks of another.
The only relaxation of this general precaution which I think may at
present be made is the adoption of a common name for the oldest type of
gneisses. The term "Archæan" has been applied to these rocks, and if
it is used simply to express a common petrographical type, occupying
the lowest horizon in the stratigraphical series of a country, it
has obvious advantages. But I would still retain the local names as
subordinate terms to mark the local characteristics of the Archæan
rocks of each province. Thus the "Laurentian" rocks of Canada and the
"Lewisian" rocks of Scotland are widely-separated representatives of
the peculiar stratigraphical series which is known as Archæan.
The pre-Cambrian rocks of Britain include several distinct systems or
groups. How far those of even one part of this comparatively limited
region are the proper equivalents of those of another and distant part
is a problem still unsolved. Hence each distinct area, with its own
type of rocks, will here be treated by itself. The following rock-types
will be described: I. The Lewisian (Archæan) Gneiss; II. The Younger
(Dalradian) Schists of Scotland; III. The Gneisses and Schists of
Anglesey; IV. The Uriconian Group; V. The Malvern Group; VI. The
Charnwood Forest Group (see Map I.).
i. THE LEWISIAN (ARCHAÆN) GNEISS
The British Isles are singularly fortunate in possessing an admirable
development of pre-Cambrian rocks. These ancient masses rise up in
various parts of the islands, but the region where they are most
extensively displayed, and where their stratigraphical position
and sequence are most clearly shown, lies in the north-west of
Scotland.[50] In that territory they form the whole chain of the
Outer Hebrides, and likewise extend as an irregular selvage along the
western margin of the counties of Sutherland and Ross. The lowest known
platform of the fossiliferous formations has there been discovered
and has been traced for a distance of more than 100 miles. From this
definite horizon, the high antiquity of all that lies below it is
impressively demonstrated. The accompanying diagram (Fig. 35) will
explain the general relations of the various geological formations of
the region.
[Footnote 50: These rocks have been the subject of much discussion, but
geologists are now agreed as to their succession and structure. A full
summary of the literature of the controversy regarding them will be
found in the _Quarterly Journal of the Geological Society_, vol. xliv.
(1888), p. 378.]
In certain dark shales (_b_) which occupy a well-defined and
readily-traceable position among the rocks of Sutherland and Ross,
numerous specimens of the trilobite genus _Olenellus_, together with
other fossils, have been found. By common consent among geologists, the
zone of rock in which this genus appears is taken as the lowest stage
of the Cambrian system. In Britain it marks the oldest known group of
fossiliferous strata--the platform on which the whole of the Palæozoic
systems rest.
[Illustration: Fig. 35.--Diagram illustrating the stratigraphical
relations of the pre-Cambrian and Cambrian rocks of the North-west
Highlands of Scotland.
_c_, Durness Limestones, with Upper Cambrian and perhaps Lower Silurian
fossils, 1500 feet, top nowhere seen. _b_, Serpulite grit and "fucoid"
shales, 70 to 80 feet, containing the _Olenellus_-zone. _a_, Quartzite,
with abundant annelid tubes, about 600 feet. II. Red Sandstones and
Conglomerates, sometimes 8000 feet or more (Torridonian). I. Gneiss
with dykes, etc. (Lewisian).]
From the definite geological epoch indicated by this platform, we can
go backward into pre-Cambrian time, and realize in some measure how
prodigious must be the antiquity of the successive groups of rock which
emerge from beneath the base of the Palæozoic systems. Nowhere is
this antiquity more impressively proclaimed than in the north-west of
Scotland. From below the _Olenellus_-zone with its underlying sheets of
quartzite (_a_), a thick group of dull red sandstones and conglomerates
(II.) rises into a series of detached conical or pyramidal mountains,
which form one of the most characteristic features in the scenery of
that region. As this detrital formation is well developed around Loch
Torridon, it has been termed Torridonian. It attains a thickness of
at least 8000 or 10,000 feet, and is traceable all the way from the
extreme northern headlands of Sutherland to the southern cliffs of the
island of Rum.
In judging of the chronological significance of the geological
structure of the north-west of Scotland, we are first impressed by the
stratigraphical break between the base of the Cambrian system and the
Torridonian deposits below. This break is so complete that here and
there the thick intervening mass of sandstones and conglomerates has
been nearly or wholly removed by denudation before the lowest Cambrian
strata were laid down. Such a discordance marks the passage of a
protracted interval of time.
Again, when the composition of the Torridonian rocks is considered,
further striking evidence is obtained of the lapse of long periods.
The sandstones, conglomerates and shales of this pre-Cambrian system
present no evidence of cataclysmal action. On the contrary, they bear
testimony that they were accumulated much in the same way and at the
same rate as the subsequent Palæozoic systems. In that primeval period,
as now, sand and silt were spread out under lakes and seas, were
ripple-marked by the agitation of the water, and were gradually buried
under other layers of similar sediment. The accumulation of 10,000 feet
of such gradually-assorted detritus must have demanded a long series of
ages. Here, then, in the internal structure of the Torridonian rocks,
there is proof that in passing across them, from their summit to their
base, we make another vast stride backward into the early past of
geological history.
But when attention is directed to the relations of the Torridonian
strata to the rocks beneath them, a still more striking proof of an
enormously protracted period of time is obtained. Between the two
series of formations lies one of the most marked stratigraphical breaks
in the geological structure of the British Isles. There is absolutely
nothing in common between them, save that the conglomerates and
sandstones have been largely made out of the waste of the underlying
gneiss. The denudation of the crystalline rocks before the deposition
of any of the Torridonian sediments must have been prolonged and
gigantic. The more, indeed, we study the gneiss, the more do we feel
impressed by the evidence for the lapse of a vast interval of time,
here unrecorded in rock, between the last terrestrial movements
indicated by the gneiss and the earliest of the Torridonian sediments.
In this manner, reasoning backward from the horizon of the
_Olenellus_-zone, we are enabled to form some conception of the
vastness of the antiquity of the fundamental rocks of the North-west
Highlands. The nature and origin of these rocks acquire a special
interest from a consideration of their age. They contain the chronicles
of the very beginnings of geological history, in so far as this history
is contained in the crust of the earth. No part of the geological
record is so obscure as this earliest chapter, but we need not here
enter further into its difficulties than may be necessary for the
purpose of understanding what light it can be made to throw on the
earliest manifestations of volcanic action.
Under the term Lewisian Gneiss (I. in Fig. 35) a series of rocks is
comprised which differ from each other in composition, structure
and age, though most of them possess such crystalline and generally
foliated characters as may be conveniently included under the
designation of gneiss. The complexity of these ancient crystalline
masses was not recognized at the time when Murchison called them the
"Fundamental" or "Lewisian" gneiss. It is only since the Geological
Survey began to study and map them in full detail that their true
nature and history have begun to be understood.[51]
[Footnote 51: See the Report of this Survey work by Messrs. Peach,
Horne, Gunn, Clough, Cadell and Hinxman, _Quart. Journ. Geol. Soc._
vol. xliv. (1888), pp. 378-441; and Annual Reports of Director-General
of the Geological Survey in the _Report of The Science and Art
Department for 1894_, p. 279, and 1895, p. 17 of reprint. The general
area of the gneiss is shown in Map I.]
The researches of the Survey have shown the so-called Lewisian
gneiss to comprise the following five groups of rock: 1. A group of
various more or less banded and foliated rocks which form together
the oldest and chief part of the gneiss (Fundamental complex); 2.
Highly basic dykes cutting the first group; 3. Dykes and sills of
dolerite, epidiorite and hornblende-schist; 4. A few dykes of peculiar
composition; 5. Gneissose granite and pegmatite.
The first of these groups, forming the main body of the gneiss, has
been critically studied on the mainland from Cape Wrath to Skye.
But its development in the Outer Hebrides has not yet been worked
out, although the name "Lewisian" was actually taken from that chain
of islands. So far as at present known, however, the gneiss of the
Hebrides repeats the essential characters of that of the mainland.
Mr. Teall, as the result of a careful investigation in the field and
with the microscope, has ascertained that on the mainland between Skye
and Cape Wrath the rocks of the "fundamental complex" are essentially
composed of olivine, hypersthene, augite (including diallage),
hornblende, biotite, plagioclase, orthoclase, microcline and quartz. He
has further observed that these minerals are associated together in the
same manner as in peridotites, gabbros, diorites and granites. Treating
the rocks in accordance with their composition and partly with their
structure, but excluding theoretical considerations, he has arranged
them in the following five subdivisions:--
1. Rocks composed of ferro-magnesian minerals, without felspar or
quartz--Pyroxenites, Hornblendites.
2. Rocks in which pyroxenes are the dominating ferro-magnesian
constituents, felspar always being present, sometimes quartz: A,
Without quartz, Hypersthene-augite-rocks (pyroxene granulites;
rocks of the Baltimore-gabbro type) and augite-rocks (gabbros);
B, With quartz, Augite-gneiss.
3. Rocks in which hornblende is the prevalent ferro-magnesian
constituent: A, Without quartz, or containing it only in
small quantity; rocks basic in composition: (_a_) massive or
only slightly foliated (Amphibolites, as epidote-amphibolite,
zoisite-amphibolite, garnet-amphibolite); (_b_) foliated
(Hornblende-schist). B, With quartz; rocks intermediate or acid
in composition: (_a_) with compact hornblende and a granular
structure (Hornblende-gneiss proper); (_b_) with hornblende
occurring in fibrous or other aggregates; (_c_) with compact
hornblende and a more or less granulitic structure (Granulitic
hornblende-gneiss).
4. Rocks in which biotite is the predominant ferro-magnesian
constituent; felspar and quartz both present: (_a_) Biotite
occurring as independent plates or in aggregates of two or three
large individuals (Biotite-gneiss); (_b_) Biotite occurring
in aggregates of numerous small individuals (rare type);
(_c_) Biotite occurring as independent plates in a granulitic
structure.
5. Rocks in which muscovite and biotite are present, together with
felspar and quartz--Muscovite-biotite-gneiss. These, though not
forming a well-defined natural group, are placed together for
purposes of description. They are all foliated, some having the
aspect of mica-schists, others being typical augen-gneisses, or
light grey gneisses with abundant oligoclase and inclusions of
microlitic epidote.
The rocks of each of these types are usually restricted to relatively
small areas, and they succeed each other with much irregularity all
the way from Skye to Cape Wrath. Their chemical and mineralogical
composition proves them to have decided affinities with the plutonic
igneous masses of the earth's crust.
The only exceptions to this prevalent igneous type occur in the
districts of Gairloch and Loch Carron, where the gneiss appears to be
associated with a group of mica-schists, graphitic-schists, quartzites
and siliceous granulites, limestones, dolomites, chlorite-schists and
other schists. That these are altered sedimentary formations can hardly
be doubted. What their precise relations to the fundamental complex
of the gneiss may be has not yet been satisfactorily determined. They
are certainly far older than the Torridon sandstone which covers them
unconformably. Possibly they may represent a sedimentary formation
still more ancient than the gneiss.
Save these obscure relics of a pre-Torridonian system of strata, the
gneiss never presents any structure which suggests the alteration of
clastic constituents. Everywhere its mineral composition points to
a connection with the subterranean intrusions of different igneous
magmas, while the manner in which its different rock-groups are
associated together, and the internal structure of some of them, still
further link it with phenomena which will be described in succeeding
chapters as parts of the records of volcanic action.
An interesting feature of the fundamental complex, as bearing on the
origin of the gneiss, is to be found in the occurrence of bosses and
bands which are either non-foliated or foliated only in a slight
degree. These comparatively structureless portions present much of the
character of bosses or sills of true eruptive rocks. They occur in
various parts of Sutherland and Ross. Their external margins are not
well defined, and they pass insensibly into the ordinary gneiss, the
dark basic massive rocks shading off into coarse basic gneisses, and
the pegmatites of quartz and felspar which traverse them merging into
bands of grey quartzose gneiss.
So far, therefore, as present knowledge goes, the main body or
fundamental complex of the Lewisian gneiss in the North-west Highlands
of Scotland consists of what may have been originally a mass of
various eruptive rocks. It has subsequently undergone a succession
of deformations from enormous stresses within the terrestrial crust,
which have been investigated with great care by the Geological Survey.
But it presents structures which, in spite of the abundant proofs of
great mechanical deformation, are yet, I venture to think, original, or
at least belong to the time of igneous protrusion before deformation
took place. The alternation of rocks of different petrographical
constitution suggests a succession of extravasations of eruptive
materials, though it may not be always possible now to determine the
order in which these followed each other. In the feebly foliated or
massive bands and bosses there is a parallel arrangement of their
constituent minerals or of fine and coarse crystalline layers which
recalls sometimes very strikingly the flow-structure of rhyolites and
other lavas. This resemblance was strongly insisted on by Poulett
Scrope, who believed that the laminar structure of such rocks as gneiss
and mica-schist was best explained by the supposition of the flow of a
granitic magma under great pressure within the earth's crust.[52]
[Footnote 52: _Volcanoes_, pp. 140, 283, 299.]
The conviction that these parallel structures do, in some cases, really
represent traces of movements in the original unconsolidated igneous
masses, not yet wholly effaced by later mechanical stresses, has been
greatly strengthened in my mind by a recent study of the structures of
various eruptive bosses, especially those of gabbro in the Tertiary
volcanic series of the Inner Hebrides. The banded structure, the
separation of the constituent minerals into distinct layers or zones,
the alternation of markedly basic with more acid layers, and the
puckering and plication of those bands, can be seen as perfectly among
the Tertiary gabbro bosses of Skye as in the Lewisian gneiss (see
Figs. 336 and 337). It cannot be contended that such structures in the
gabbro are due to any subsequent terrestrial disturbance and consequent
deformation. They must be accepted as part of the original structure
of the molten magma.[53] It seems to me, therefore, highly probable
that the parallel banding in the uncrushed cores of the Lewisian gneiss
reveals to us some of the movements of the original magma at the
time of its extrusion and before it underwent those great mechanical
stresses which have so largely contributed to the production of many of
its most characteristic structures.
[Footnote 53: See A. Geikie and J. J. H. Teall, _Quart. Journ. Geol.
Soc._ vol. 1. (1894), p. 645.]
While the material of the oldest gneiss presents many affinities to
plutonic rocks of much younger date, a wide region of mere speculation
opens out when we try to picture the conditions under which this
material was accumulated. Some geologists have boldly advanced the
doctrine that the Archæan gneisses represent the earliest crust that
consolidated upon the surface of the globe. But these rocks offer no
points of resemblance to the ordinary aspect of superficial volcanic
ejections. On the contrary, the coarsely-crystalline condition even
of those portions of the gneiss which seem most nearly to represent
original structure, the absence of anything like scoriæ or fragmental
bands of any kind, and the resemblances which may be traced between
parts of the gneiss and intrusive bosses of igneous rock compel us
to seek the nearest analogies to the original gneiss in deep-seated
masses of eruptive material. It is difficult to conceive that any rocks
approaching in character to the gabbros, picrites, granulites and other
coarsely-crystalline portions of the old gneiss could have consolidated
at or near the surface.
When the larger area of gneiss forming the chain of the Outer Hebrides
is studied, we may obtain additional information regarding the probable
origin and the earliest structures of the fundamental complex of the
Lewisian gneiss. In particular, we may look for some unfoliated cores
of a more acid character, and perhaps for evidence which will show that
both acid and basic materials were successively protruded. We may even
entertain a faint hope that some trace may be discovered of superficial
or truly volcanic products connected with the bosses which recall those
of later date and obviously eruptive nature. But up to the present time
no indication of any such superficial accompaniments has been detected.
If any portions of the old gneiss represent the deeper parts of columns
of molten rock that flowed out at the surface as lava, with discharges
of fragmentary materials, all this superincumbent material, at least in
the regions which have been studied in detail, had disappeared entirely
before the deposition of the very oldest part of the Torridonian rocks,
unless some trace of it may remain among the pebbles of the Torridonian
conglomerates, to which reference will be immediately made.
So far, then, as the evidence now available allows a conclusion to
be drawn, the Lewisian gneiss reveals to us a primeval group of
eruptive rocks presenting a strong resemblance to some which in later
formations are connected, as underground continuations, with bedded
lavas and tuffs that were erupted at the surface; and although no
proof has yet been obtained of true volcanic ejections associated with
the fundamental complex, the rocks seem to be most readily understood
if we regard them as having consolidated from igneous fusion at some
depth, and we may plausibly infer that they may have been actually
connected with the discharge of volcanic materials at the surface. The
graphite-schists, mica-schists, and limestones of the Gairloch and Loch
Carron may thus be surviving fragments of the stratified crust into
which these deep-seated masses were intruded, and through which any
volcanic eruptions that were connected with them had to make their way.
The limited areas occupied by the several varieties of rock in the
fundamental complex suggests the successive protrusion of different
magmas, or of different portions from one gradually changing magma.
Mr. Teall has ascertained that whenever in this series of rocks the
relative ages of two petrographical types can be clearly ascertained,
the more basic is older than the more acid.
But besides all the complexity arising from original diversity of area,
structure and composition among the successive intrusions, a further
intricacy has been produced by the subsequent terrestrial disturbances,
which on a gigantic scale affected the north-west of Europe after
the formation of the fundamental complex of the old gneiss, but long
before the Torridonian period. By a series of terrestrial stresses that
came as precursors of those which in later geological times worked
such great changes among the rocks of the Scottish Highlands, the
original bosses and sheets of the gneiss were compressed, plicated,
fractured and rolled out, acquiring in this process a crumpled,
foliated structure. Whether or not these disturbances were accompanied
by any manifestations of superficial volcanic action has not yet
been determined. But we know that they were followed by a succession
of dyke-eruptions, to which, for extent and variety, there is no
parallel in the geological structure of Britain, save in the remarkable
assemblage of dykes belonging to the Tertiary volcanic period[54] (Fig.
36).
[Footnote 54: _Quart. Journ. Geol. Soc._ vol. xliv. (1888), p. 389 _et
seq._]
[Illustration: _Walker & Boutall sc._
Fig. 36.--Map of a portion of the Lewisian gneiss of Ross-shire.
Taken from Sheet 107 of the Geological Survey of Scotland on the scale
of one inch to a mile. The white ground (A) marks the general body of
the Lewisian gneiss. This is traversed by dykes of dolerite (B), which
are cut by later dykes of highly basic material (peridotite, picrite,
etc., P). The gneiss and its system of dykes is overlain unconformably
by the nearly horizontal Torridon Sandstone (_t_), which is injected by
sheets of oligoclase-porphyry (F).]
For the production of these dykes a series of fissures was first opened
through the fundamental complex of the gneiss, having a general trend
from E.S.E. to W.N.W., running in parallel lines for many miles, and so
close together in some places that fifteen or twenty of them occurred
within a horizontal space of one mile. The fissures were probably not
all formed at the same time; at all events, the molten materials that
rose in them exhibit distinct evidence of a succession of upwellings
from the igneous magma below.
Considered simply from the petrographical point of view, the materials
that have filled the fissures have been arranged by Mr. Teall in the
following groups: 1. Ultrabasic dykes, sometimes massive (peridotites),
sometimes foliated (talcose schists containing carbonates and sometimes
gedrite); 2. Basic dykes which where massive take the forms of dolerite
and epidiorite, and where foliated appear as hornblende-schist, the
same dyke often presenting the three conditions of dolerite, epidiorite
and hornblende-schist; 3. Dykes of peculiar composition, comprising
microcline-mica rocks and biotite-diorite with macro-poikilitic
plagioclase; 4. Granites and gneissose granites (biotite-granite with
microcline); 5. Pegmatites (microcline-quartz rocks with a variable
amount of oligoclase or albite).[55]
[Footnote 55: _Annual Report of Geological Survey for 1895_, p. 18 of
reprint.]
Distinct evidence of a succession of eruptions can be made out among
these rocks. By far the largest proportion of the dykes consists
of basic materials. The oldest and most abundant of them are of
plagioclase-augite rocks, which, where uncrushed, differ in no
essential feature of structure or composition from the dolerites and
basalts of more modern periods, though they have been plentifully
changed into epidiorite and hornblende-schist.[56] They present,
too, most of the broad features that characterize the dykes of later
times--the central more coarsely-crystalline portion, the marginal band
of finer grain, passing occasionally into what was probably a basic
glass, and the transverse jointing. They belong to more than one period
of emission, for they cross each other. They vary in width up to nearly
200 feet, and sometimes run with singular persistence completely across
the whole breadth of the strip of gneiss in the west of Sutherland and
Ross. Dozens of dykes have been followed by the Geological Survey for
distances of ten or twelve miles.
[Footnote 56: See Mr. Teall, _Quart. Journ. Geol. Soc._ vol. xli.
(1885), p. 133.]
Later in time, and much less abundant, are certain highly basic
dykes--peridotites with schistose modifications--which cut across the
dolerites in a more nearly east-and-west direction. There are likewise
occasional dykes of peculiar composition, which, as above stated,
have been distinguished by Mr. Teall as microcline-mica rocks and
biotite-diorite.
Last of all comes a group of thoroughly acid rocks--varieties of
granite and pegmatite--which form intrusive sheets and dykes. The
granites contain biotite with microcline, and are sometimes gneissose.
The pegmatites are microcline-quartz rocks with a variable amount
of oligoclase or albite. These dykes coincide in direction with the
basalts and dolerites, but they are apt to run together into belts of
granite and pegmatite, sometimes 1500 feet broad.
Up to the present time no evidence has been found of any superficial
outpouring of material in connection with this remarkable series of
dykes in the Lewisian gneiss. That they may have been concomitant
with true volcanic eruptions may be plausibly inferred from the close
analogy which, in spite of their antiquity and the metamorphism they
have undergone, they still present to the system of dykes that forms
a part of the great Tertiary volcanic series of Antrim and the Inner
Hebrides. The close-set fissures running in a W.N.W direction, the
abundant uprise into these fissures of basic igneous rocks, followed by
a later and more feeble extravasation of acid material, are features
which in a singular manner anticipate the volcanic phenomena of
Tertiary time.
There can be no question as to the high antiquity of these dykes. They
were already in place before the advent of those extraordinary vertical
lines of shearing which have so greatly affected both the gneiss and
the dykes; and these movements, in turn, had long been accomplished
before the Torridon Sandstone was laid down, for the dykes, with their
abundant deformation, run up to and pass beneath the sandstone which
buries them and all the rocks with which they are associated. Though
later than the original fundamental complex, the dykes have become so
integral and essential a part of the gneiss as it now exists that they
must be unhesitatingly grouped with it.
With so wide an extension of the subterranean relics of volcanic
energy, it is surely not too much to hope that somewhere there may
have been preserved, and may still be discovered, proofs that these
eruptive rocks opened a connection with the surface, and that we
may thus recognize vestiges of the superficial products of actual
Archæan volcanoes. Among the pebbles in the conglomerates of the
Torridon Sandstone there occur, indeed, fragments of felsites which
possess great interest from the perfection with which they retain
some of the characteristic features of younger lavas. Mr. Teall has
described their minute structure. They are dark, purplish, compact
rocks, consisting of a spherulitic micropegmatitic, micropoikilitic or
microcrystalline groundmass, in which are imbedded porphyritic crystals
or crystal-groups of felspar, often oligoclase. These spherulitic rocks
occasionally show traces of perlitic structure. They bear a striking
resemblance to some of the Uriconian felsites of Shropshire, pebbles
from which occur in the Longmynd rocks.[57] These fragments suggest
the existence of volcanic materials at the surface when the Torridon
Sandstone was deposited. Possibly they may represent some vanished
Lewisian lavas. But the time between the uprise of the dykes and the
formation of the Torridonian series was vast enough for the advent of
many successive volcanic episodes. The pebbles may therefore be the
relics of eruptions that took place long after the period of the dykes.
[Footnote 57: _Annual Report of Geological Survey for 1895_, p. 21 of
reprint.]
Among the Torridonian strata no undoubted trace of any contemporaneous
volcanic eruptions has been met with.[58] The only relics of volcanic
rocks in this enormous accumulation of sediments are the pebbles just
referred to, which may be referable to a time long anterior to the very
oldest parts of the Torridonian series.
[Footnote 58: The supposed tuff referred to in _Quart. Journ. Geol.
Soc._ vol. xlviii. (1892), p. 168, is probably not of truly volcanic
origin.]
That Archæan time witnessed volcanic eruptions on a considerable scale,
and with great variety of petrographical material, has recently been
shown in detail by Mr. Otto Nordenskjöld from a study of the rocks
of Småland in Sweden. He has described a series of acid outbursts,
including masses of rhyolite and dacite, together with agglomerates and
tuffs, likewise basic eruptions, with dioritic rocks, augite-porphyrite
and breccia. He refers these rocks to the same age as most of the
Scandinavian gneisses, and remarks that though they have undergone
much mechanical deformation and metamorphism, they have yet here and
there retained some of their distinctive volcanic structures, such as
the spherulitic.[59] When the large area of Lewisian gneiss forming
the chain of the Outer Hebrides is investigated it may possibly supply
examples of a similar series of ancient volcanic masses.
[Footnote 59: "Über Archæische Ergussgesteine aus Småland," _Sveriges
Geol. Undersökn_, No. 135 (1894).]
ii. THE DALRADIAN OR YOUNGER SCHISTS OF SCOTLAND
We now come to one of the great gaps in the geological record. The
Lewisian gneiss affords us glimpses of probable volcanic activity at
the very beginning of geological history. An enormous lapse of time,
apparently unrepresented in Britain by any geological record, must be
marked by the unconformability between the gneiss and the Torridon
Sandstone. Another prodigious interval is undoubtedly shown by the
Torridonian series. Neither this thick accumulation of sediment nor
the Cambrian formations, which to a depth of some 2000 feet overlie
the Torridon Sandstone, have yielded any evidence of true superficial
eruptions, though they are traversed by numerous dykes, sills and
bosses. The age of these intrusive masses cannot be precisely fixed; a
large proportion of them is certainly older than the great terrestrial
displacements and concurrent metamorphism of the North-West Highlands.
While from the Lewisian gneiss upward to the highest visible Cambrian
platform in Sutherland, no vestige of contemporaneous volcanic rocks
is to be seen, the continuity of the geological record is abruptly
broken at the top of the Durness Limestone. By a series of the most
stupendous dislocations, the rocks of the terrestrial crust have there
been displaced to such a degree that portions have been thrust westward
for a horizontal distance of sometimes as much as ten miles, while
they have been so crushed and sheared as to have often lost entirely
their original structures, and to have passed into the crystalline
and foliated condition of schists. Portions of the floor of Lewisian
gneiss, and large masses of the Torridon Sandstone, which had been
buried under the Cambrian sediments, have been torn up and driven over
the Durness Limestone and quartzite.
Though much care has been bestowed by the officers of the Geological
Survey on the investigation of the complicated mass of material which,
pushed over the Cambrian strata, forms the mountainous ground that
lies to the east of a line drawn from Loch Eribol, in the north of
Sutherland, to the south-east of Skye, some uncertainty still exists as
to the age and history of the rocks of that region. For the purposes of
this work, therefore, the rest of the country eastwards to the line of
the Great Glen--that remarkable valley which cuts Scotland in two--may
be left out of account.
To the east of the Great Glen the Scottish Highlands display a vast
succession of crystalline schists, the true stratigraphical relations
of which to the Lewisian gneiss have still to be determined, but which,
taken as a whole, no one now seriously doubts must be greatly younger
than that ancient rock. Murchison first suggested that the quartzites
and limestones found in this newer series are the equivalents of
those of the North-West. This identification may yet be shown to be
correct, but must be regarded as still unproved. Traces of fossils
(annelid-pipes) have been found in some of the quartzites, but they
afford little or no help in determining the horizons of the rocks.
In Donegal, where similar quartzites, limestones and schists are
well developed, obscure indications of organic remains (corals and
graptolites) have likewise been detected, but they also fail to supply
any satisfactory basis for stratigraphical comparison.
Essentially the schists of the Scottish Highlands east of the Great
Glen consist of altered sedimentary rocks. Besides quartzites and
limestones, there occur thick masses of clay-slate and other slates
and schists, with bands of graphitic schist, greywacke, pebbly grit,
quartzite, boulder-beds and conglomerates. Among rocks that have been
so disturbed and foliated it is necessarily difficult to determine the
true order of succession. In the Central Highlands, however, a certain
definite sequence has been found to continue as far as the ground has
yet been mapped. Were the rocks always severely contorted, broken and
placed at high angles, this sequence might be deceptive, and leave
still uncertain the original order of deposition of the whole series.
But over many square miles the angles of inclination are low, and the
successive bands may be traced from hill to hill, across strath and
glen, forming escarpments along the slopes and outliers on the summits,
precisely as gently-undulating beds of sandstone and limestone may be
seen to do in the dales of Yorkshire. It is difficult to resist the
belief, though it may, perhaps, be premature to conclude, that this
obvious and persistent order of succession really marks the original
sequence of deposition. In Donegal also a definite arrangement of
the rock-groups has been ascertained which, when followed across the
country, gives the key to its geological structure.[60]
[Footnote 60: _Geol. Survey Memoirs: Geology of N.W. Donegal_, 1891.]
In the order of succession which has been recognized during the
progress of the Geological Survey through the Central and Southern
Highlands, it is hard in many places to determine whether the sequence
that can be recognized is in an upward or downward direction. Two bands
of limestone, which appear to retain their relative positions across
Scotland for a distance of some 230 miles, may afford a solution of
this difficulty, and if, as is probable, they are to be identified with
the similar limestones of Donegal, Mayo and Galway, their assistance
will thus be available across a tract of more than 400 miles. What is
regarded as the lower zone of limestone is particularly well seen about
Loch Tay; what is believed to be the upper is typically displayed in
the heart of Perthshire, about Blair-Athol.
From under the Loch Tay Limestone a great thickness of mica-schists,
"green schists," schistose grits and conglomerates, slates and
greywackes, emerges up to the border of the Highlands. Above that
calcareous band thick masses of mica-schist and sericite-schist are
succeeded by a well-marked zone of quartzite, which forms the mountains
of Ben-y-Glo and Schihallion, and stretches south-westward across
Argyllshire into Islay and Jura. The second or Blair-Athol Limestone
lies next to this quartzite. If the limestones are identical with those
of Donegal, Mayo and Galway, the quartzites may doubtless be also
regarded as continued in those of the same Irish counties, where they
form some of the most conspicuous features in the scenery, since they
rise into such conspicuous mountains as Erigal, Slieve League, Nephin,
and the twelve Bins of Connemara.
The age of this vast system of altered rocks has still to be
determined. It is possible that they may include some parts of the
Torridonian series, or even here and there a wedge of the Lewisian
gneiss driven into position by gigantic disruptions, like those of
the North-West Highlands. But there can be no doubt that the schists,
quartzites and limestones form an assemblage of metamorphosed
sedimentary strata which differs much in variety of petrographical
character, as well as in thickness, from the Torridonian sandstone,
and which has not been identified as the equivalent of any known
Palæozoic system or group of formations in Britain. It may conceivably
embrace the Cambrian series of the North-West Highlands, and also the
sedimentary deposits that succeeded the Durness Limestone, of which no
recognizable vestige remains in Sutherland or Ross.
That the metamorphic rocks east of the line of the Great Glen are at
least older than the Arenig formation of the Lower Silurian system
may be inferred from an interesting discovery recently made by the
officers of the Geological Survey. A narrow strip of rocks has been
found which, from their remarkable petrographical characters, their
order of sequence and their scanty fossil contents (_Radiolaria_), are
with some confidence identified with a peculiar assemblage of rocks on
the Arenig horizon of the Silurian system in the Southern Uplands of
Scotland, to which fuller reference will be made in Chapter xii. This
strip or wedge of probably Lower Silurian strata intervenes between
the Highland schists and the Old Red Sandstone in Kincardineshire,
Forfarshire and Dumbartonshire. It has been recognized also, occupying
a similar position, in Tyrone in Ireland. The schists in some places
retain their foliated character up to the abrupt line of junction
with the presumably Lower Silurian strata, while in other districts,
as at Aberfoyle, they have been so little affected that it is hardly
possible to draw a line between the Highland rocks and those of this
border-zone, which indeed are there perhaps more metamorphosed than the
Highland grits to the north of them. The metamorphism of the schists
may have been mainly effected before the final disturbances that wedged
in this strip of Silurian strata along the Highland border, though
some amount of crushing and schist-making seems to have accompanied
these disturbances. No trace of any similar strip of Palæozoic rocks
has ever been detected among the folds of the schists further into the
Highlands. But some of the Highland rocks in the region of Loch Awe
lose their metamorphosed character, and pass into sedimentary strata
which, so far as petrographical characters are concerned, might well be
Palæozoic.
Until some clue is found to the age of the Younger or Eastern schists,
quartzites and limestones of the Highlands, it is desirable to have
some short convenient adjective to distinguish them. As a provisional
term for them I have proposed the term "Dalradian," from Dalriada, the
name of the old Celtic kingdom of the north of Ireland and south-west
of Scotland.[61]
[Footnote 61: _Presidential Address to Geological Society_, 1891, p.
39.]
The special feature for which this Dalradian series is cited in
the present volume is the evidence it furnishes of powerful and
extensive volcanic action. In a series of rocks so greatly dislocated,
crumpled and metamorphosed, we cannot look for the usual clear proofs
of contemporaneous eruptions. Nevertheless all over the Scottish
Highlands, from the far coast of Aberdeenshire to the Mull of Cantyre,
and across the west of Ireland from the headlands of Donegal into
Galway, there occurs abundant evidence of the existence of rocks which,
though now forming an integral part of the schists, can be paralleled
with masses of undoubtedly volcanic origin.
[Illustration: Fig. 37.--Section showing the position of Sills in the
mica-schist series between Loch Tay and Amulree.
_a_, Mica-schist; _b_, _b_, Sills.]
Intercalated in the vast pile of altered sediments lie numerous sheets
of epidiorite and hornblende-schist, which were erupted as molten
materials, not improbably as varieties of diabase-lava. Most of these
sheets are doubtless intrusive "sills," for they can be observed to
break across from one horizon to another. But some of them may possibly
be contemporaneous lava-streams. A sheet may sometimes be followed
for many miles, occupying the same stratigraphical platform. Thus a
band of sills may be traced from the coast of Banffshire to near Ben
Ledi, a distance of more than 100 miles. Among the hornblendic sills
of this band some occur on a number of horizons between the group of
Ben Voirlich grits and the Ben-y-Glo quartzite. One of the most marked
of these is a sheet, sometimes 200 feet thick, which underlies the
Loch Tay Limestone. Another interesting group in the same great band
has been mapped by the Geological Survey on the hills between Loch Tay
and Amulree, some of them being traceable for several miles among the
mica-schists with which they alternate (Fig. 37).
In Argyllshire also, between Loch Tarbert and Loch Awe, and along the
eastern coasts of the islands of Islay and Jura, an abundant series of
sheets of epidiorite, amphibolite and hornblende-schist runs with the
prevalent strike of the schists, grits and limestones of that region.
Similar rocks reappear in a like position in Donegal, where, as in
Scotland, the frequency of the occurrence of these eruptive rocks on
the horizons of the limestones is worthy of remark. The persistence,
number and aggregate thickness of the sills in this great band mark it
out as the most extensive series of intrusive sheets in the British
Isles.
In addition to the sills there occur also bosses of similar material,
which in their form and their obvious relation to the sheets recall the
structure of volcanic necks. They consist of hornblendic rocks, like
the sills, but are usually tolerably massive, and show much less trace
of superinduced foliation.
Besides the obviously eruptive masses there is another abundant
group of rocks which, I believe, furnishes important evidence as
to contemporaneous volcanic action during the accumulation of the
Dalradian series. Throughout the Central and South-Western Highlands
certain zones of "green schist" have long occupied the attention of
the officers of the Geological Survey. They occur more especially on
two horizons between the Loch Tay Limestone and a much lower series
of grits and fine conglomerates, which run through the Trossachs and
form the craggy ridges of Ben Ledi, Ben Voirlich and other mountains
near the Highland border. In the lower group of "green schists," thick
hornblendic sills begin to make their appearance, increasing in number
upwards. The upper group of "green schists" lies between two bands of
garnetiferous mica-schist, above the higher of which comes the Loch
Tay Limestone. The peculiar greenish tint and corresponding mineral
constituents of these schists, however, are likewise found diffused
through higher parts of the series.
So much do the "green schists" vary in structure and composition that
no single definition of them is always applicable. At one extreme
are dull green chlorite-schists, passing into a "potstone," which,
like that of Trondhjem, can be cut into blocks for architectural
purposes.[62] At the other extreme lie grits and quartzites, with a
slight admixture of the same greenish-coloured constituent. Between
these limits almost every stage may be met with, the proportion of
chlorite or hornblende and of granular or pebbly quartz varying
continually, not only vertically, but even in the extension of the same
bed. The quartz-pebbles are sometimes opalescent, and occasionally
larger than peas. An average specimen from one of the zones of "green
schists" is found, on closer examination, to be a thoroughly schistose
rock, composed of a matrix of granular quartz, through which acicular
hornblende and biotite crystals, or actinolite and chlorite, are ranged
along the planes of foliation.
[Footnote 62: From such a rock, which crosses the upper part of Loch
Fyne, the Duke of Argyll's residence at Inveraray has been built.]
That these rocks are essentially of detrital origin admits of no
doubt. They differ, however, from the other sedimentary members of the
Dalradian series in the persistence and abundance of the magnesian
silicates diffused through them. The idea which they suggested to my
mind some years ago was that the green colouring-matter represents fine
basic volcanic dust, which was showered out during the accumulation of
ordinary quartzose, argillaceous and calcareous sediments, and that,
under the influence of the metamorphism which has so greatly affected
all the rocks of the region, the original pyroxenes and felspars
suffered the usual conversion into hornblendes, chlorites and micas.
This view has occurred also to my colleagues on the Survey, and is now
generally adopted by them.
Not only are these "green schists" traceable all through the Central
and South-Western Highlands, rocks of similar character, and not
improbably on the same horizons, reappear in the north-west of Ireland,
and run thence south-westward as far as the Dalradian rocks extend.
If we are justified in regarding them as metamorphosed tuffs and
ashy sediments, they mark a widespread and long-continued volcanic
period during the time when the later half of the Dalradian series was
deposited.
Besides the extensive development of basic sills which, though probably
in great part later than the "green schists," may belong to the same
prolonged period of subterranean activity, numerous acid protrusions
are to be observed in the Dalradian series of Scotland and Ireland.
That these masses were erupted at several widely-separated intervals
is well shown by their relation to the schists among which they occur.
Some of the great bosses and sills of granite were undoubtedly injected
before the metamorphism of the schists was completed, for they have
shared in the foliation of the region. Others have certainly appeared
after the metamorphism was complete, for they show no trace of having
suffered from its effects. Thus some of the vast tracts of newer
granite in the Grampian chain, which cover many square miles of ground,
must be among the newest rocks of that area. They have recently been
found by Mr. G. Barrow, of the Geological Survey, to send veins into
the belt of probably Lower Silurian strata which flanks the Highland
schists. They are thus later than the Arenig period. Not impossibly
they may be referable to the great granite intrusions which formed so
striking a feature in the history of the Lower Old Red Sandstone.
iii. THE GNEISSES AND SCHISTS OF ANGLESEY
In the island of Anglesey an interesting series of schists and
quartzites presents many points of resemblance to the Dalradian or
younger schists of the Highlands. At present the geologist possesses
no means of determining whether these Welsh rocks are the equivalents
of the Scottish in stratigraphical position, but their remarkable
similarity justifies a brief allusion to them in this place. Much
controversy has arisen regarding the geology of Anglesey, but into
this dispute it is not necessary for my present purpose to enter.[63]
I will content myself with expressing what seems to me, after several
traverses, to be the geological structure of the ground.
[Footnote 63: The literature of Anglesey geology is now somewhat
voluminous, but I may refer to the following as the chief authorities.
The island is mapped in Sheet 78 of the Geological Survey of England
and Wales, and its structure is illustrated in Horizontal Sections,
Sheet 40. A full account of its various formations and of their
relations to each other is given in vol. iii. of the _Memoirs of the
Geological Survey_, "The Geology of North Wales," by Sir A. C. Ramsay,
2nd edit. 1881. The subject has been discussed by Professor Hughes,
_Quart. Journ. Geol. Soc._ vols. xxxiv. (1878) p. 137, xxxv. (1879) p.
682, xxxvi. (1880) p. 237, xxxviii. (1882) p. 16; _Brit. Assoc. Rep._
(1881) pp. 643, 644; _Proc. Camb. Phil. Soc._ vol. iii. pp. 67, 89,
341; by Professor Bonney, _Quart. Journ. Geol. Soc._ vol. xxxv. (1879)
pp. 300, 321; _Geol. Mag._ (1880) p. 125; by Dr. H. Hicks, _Quart.
Journ. Geol. Soc._ vols. xxxiv. (1878) p. 147, xxxv. (1879) p. 295;
_Geol. Mag._ (1879) pp. 433, 528 (1893) p. 548; by Dr. C. Callaway,
_Quart. Journ. Geol. Soc._ vols. xxxvii. (1881) p. 210, xl. (1884) p.
567; and by the Rev. J. F. Blake, _Quart. Journ. Geol. Soc._ vol. xliv.
(1888) p. 463. Further references to the work of these observers in
Anglesey are given in Chapter xiii. p. 220 _et seq._ The Pre-Cambrian
areas of Anglesey are shown in Map II.]
There are two groups of rocks in Anglesey to which a pre-Cambrian age
may with probability be assigned. In the heart of the island lies a
core of gneiss which, if petrographical characters may be taken as
a guide, must certainly be looked upon as Archæan. In visiting that
district with my colleague Mr. Teall I was much astonished to find
there so striking a counterpart to portions of the Lewisian gneiss of
the north-west of Sutherland and Ross. The very external features of
the ground recall the peculiar hummocky surface which so persistently
characterizes the areas of this rock throughout the north-west of
Scotland. If the geologist could be suddenly transported from the
rounded rocky knolls of Sutherland, Ross-shire or the Hebrides to those
in the middle of Anglesey, south of Llanerchymedd, he would hardly be
aware of the change, save in the greater verdure of the hollows, which
has resulted from a more advanced state of decomposition of the rocks
at the surface, as well as from a better climate and agriculture.
When we examine these rocky hummocks in detail we find them to consist
of coarse gneisses, the foliation of which has a prevalent dip to
N.N.W. Some portions abound in dark hornblende and garnets, others are
rich in brown mica, the folia being coarsely crystalline and rudely
banded, as in the more massive gneisses of Sutherland. Abundant veins
of coarse pegmatite may here and there be seen, with pinkish and white
felspars and milky quartz. Occasionally the gneiss is traversed by
bands of a dark greenish-grey rock, which remind one of the dykes
of the north-west of Scotland. There are other rocks, some of them
probably intrusive and of later date, to be seen in the same area; but
they require more detailed study than they have yet received.
The relation of this core of gneiss and its associated rocks to the
second group of pre-Cambrian rocks has not hitherto been satisfactorily
ascertained. The core may conceivably be an eruptive boss in that
group, and may have acquired its foliation during the movements that
produced the foliation of the surrounding schists. But it seems more
probable that the gneiss is much older than these schists, though
it would undoubtedly participate in the effects of the mechanical
movements which gave rise to their deformation, cleavage and foliation.
The second group of rocks occupies a large area in the west and in the
centre and south of Anglesey. The schists of which it consists are
obviously in the main a clastic series. One of their most conspicuous
members is quartzite, which, besides occurring sporadically all over
the island, forms the prominent mass of Holyhead Mountain. There
are likewise flaggy chloritic schists, green and purple phyllites
or slates, and bands of grit, while parts of the so-called "grey
gneiss" consist of pebbly sandstones that have acquired a crystalline
structure. That some order of sequence among these various strata may
yet be worked out is not impossible, but the task will be one of no
ordinary difficulty, for the plications and fractures are numerous,
and much of the surface of the ground is obscured by the spread of
Palæozoic formations and superficial deposits.
These Anglesey schists are so obviously an altered sedimentary series
that it is not surprising that they should have been regarded as
metamorphosed Cambrian strata. All that can be positively affirmed
regarding their age is that they are not only older than the lowest
fossiliferous rocks around them--that is, than Arenig or even Tremadoc
strata--but that they had already acquired their present metamorphic
character before these strata were laid down unconformably upon them.
There is no actual proof that they include no altered Cambrian rocks.
But when we consider their distinctly crystalline structure, and the
absence of such a structure from any portion of the Cambrian areas
of the mainland; when, moreover, we reflect that the metamorphism
which has affected them is of the regional type, and can hardly have
been restricted to merely the limited area of Anglesey; we must agree
with those observers who, in spite of the absence of positive proof
of their true geological horizon, have regarded these rocks as of
much higher antiquity than the Cambrian strata of the neighbourhood.
No one familiar with the Dalradian rocks of Scotland and Ireland can
fail to be struck with the close resemblance which these younger
Anglesey schists bear to them, down even into the minutest details.
Petrographically they are precisely the counterparts of the quartzites
and schists of Perthshire and Donegal, and a further connection may be
established of a palæontological kind. The upper part of the Holyhead
quartzite was found by Mr. B. N. Peach and myself in the autumn of
the year 1890 to be at one place crowded with annelid-pipes, and I
subsequently found the same to be the case with some of the flaggy
quartzites near the South Stack.
[Illustration: Fig. 38.--Sketch of crushed basic igneous rock among the
schists, E. side of Porth-tywyn-mawr, E. side of Holyhead Straits.]
For the purpose of the inquiry which forms the theme of this work, the
feature of greatest interest about these younger schists of Anglesey is
the association of igneous rocks with them. They include bands of dark
basic material, the less crushed parts of which resemble the diabases
of later formations, while the sheared portions pass into epidiorites
and true hornblende-schists. As in other regions where eruptive rocks
have been crushed down and changed into the schistose modification,
it is frequently possible to see groups of uncrushed cores round
which, under severe mechanical stresses, the rock has undergone this
conversion. Lines of movement through the body of the rock may be
detected by bands of schist, the gradation from the solid core to the
hornblende-schist being quite gradual. The accompanying figure (Fig.
38) represents a portion of one of these crushed basic igneous rocks on
the east side of Holyhead Straits.
As in the Dalradian series of the Highlands, many, perhaps most, of
these igneous bands are probably intrusive sills, but others may be
intercalated contemporaneous sheets. They occur across the whole
breadth of the island from the Menai Strait to the shores of Holyhead.
Besides these undoubtedly igneous rocks, the green chloritic slates
of Anglesey deserve notice. They are well-bedded strata, consisting
of alternations of foliated fine grit or sandstone, with layers more
largely made up of schistose chlorite. The gritty bands sometimes
contain pebbles of blue quartz, and evidently represent original layers
of sandy sediment, but with an admixture of chloritic material. The
manner in which this green chloritic constituent is diffused through
the whole succession of strata, and likewise aggregated into bands with
comparatively little quartzose sediment, reminds one of the "green
schists" of the Central Highlands and Donegal, and suggests a similar
explanation. Taken in connection with the associated basic igneous
rocks, these chloritic schists seem to me to represent a thick group
of volcanic tuffs and interstratified sandy and clayey layers. If this
inference is well founded, and if we are justified in grouping these
Anglesey rocks with the Dalradian schists of Scotland and Ireland,
a striking picture is presented to the mind of the wide extent and
persistent activity of the volcanoes of that primeval period in
Britain.[64]
[Footnote 64: Mr. E. Greenly, late of the Geological Survey of
Scotland, has recently established himself on the Menai Strait for
the purpose of working out in detail the geological structure of this
interesting and complicated region. We may therefore hope that some of
the still unsolved problems presented by the rocks of Anglesey will
before long be satisfactorily explained.]
iv. THE URICONIAN VOLCANOES
Along the eastern borders of Wales a ridge of ancient rocks, much
broken by faults and presenting several striking unconformabilities,
has long been classic ground in geology from the descriptions and
illustrations of Murchison's _Silurian System_.[65] The main outlines
of the structure of that district, first admirably worked out by
this great pioneer, were delineated on the maps and sections of
the Geological Survey, wherein it was shown that in the Longmynd
an enormously thick group of stratified rocks, which, though
unfossiliferous, were referred to the Cambrian system, rose in the very
heart of the country; that to the east of these rocks lay strata of
Caradoc or Bala age; that by a great hiatus in the stratigraphy the
Upper Silurian series transgressively wrapped round everything below
it; that yet again the Coal-measures crept over all these various
Palæozoic formations, followed once more unconformably by Permian and
Triassic deposits.[66] Besides all this evidence of extraordinary
and repeated terrestrial movement, it was found that the region was
traversed by some of the most powerful dislocations in this country,
while to complete the picture of disturbance, many protrusions of
igneous rocks were recognized.
[Footnote 65: See especially chap. xix. vol. i. p. 225.]
[Footnote 66: The area is embraced in Sheet 61 of the Geological
Survey, and is illustrated by Nos. 33 and 36 of the sheets of
Horizontal Sections. In the early editions of the Survey maps the
"felspathic traps" and the "greenstones" of the Wrekin district were
distinguished by separate colours, but unfortunately this useful and so
far correct discrimination was given up in subsequent editions, where
all the acid and basic rocks are merged into one.]
In a territory so complicated, though it had been sedulously and
skilfully explored, there could hardly fail to remain features of
structure which had escaped the notice of the first observers. In
particular, the igneous rocks had been dealt with only in a general
way, and they consequently offered a favourable field for more detailed
study; while by a more searching examination of some of the rocks for
fossils, important corrections of the earlier work might yet be made.
A notable step towards a revision of the received opinions regarding
the igneous rocks of this region was taken by Mr. Allport, who showed
that the so-called "greenstone" included masses of devitrified
spherulitic pitchstones and perlites, together with indurated volcanic
breccias, agglomerates and ashes.[67] Subsequently Professor Bonney
described more fully the petrographical characters of the Wrekin
igneous rocks, confirming and extending the observations of Mr.
Allport.[68]
[Footnote 67: _Quart. Journ. Geol. Soc._ vol. xxxiii. (1877) p. 449.]
[Footnote 68: _Op. cit._ vol. xxxv. (1879) p. 662; vol. xxxviii. (1882)
p. 124.]
But the correction of the prevalent error as to the geological age of
these rocks was due to Dr. Callaway, who, after spending much time and
labour in ascertaining, by a careful search for fossils, the position
of the superincumbent rocks (wherein he discovered Cambrian organisms),
and in a detailed investigation of the structure and relationships of
the igneous masses themselves, was led to regard them as part of an
ancient pre-Cambrian ridge; and he proposed for the volcanic group
the name of Uriconian, from the name of the former Roman town which
stood not far to the west of them.[69] He has shown how essentially
volcanic this ancient series of rocks is, how seldom they present
any clearly-marked evidence of stratification, and how small is the
proportion of sedimentary material associated with them.[70]
[Footnote 69: _Quart. Journ. Geol. Soc._ vols. xxx. (1874) p. 196,
xxxiv. (1878) p. 754, xxxv. (1879) p. 643, xlii. (1886) p. 481. For a
criticism of Dr. Callaway's views as to the order of succession among
the rocks of this district, see Prof. Blake, _op. cit._ vol. xlvi.
(1890) p. 386, and Dr. Callaway's reply, vol. xlvii. (1891) p. 109.]
[Footnote 70: _Op. cit._ vol. xlvii. (1891) p. 123.]
Subsequently Professor Lapworth, by his discovery of the
_Olenellus_-fauna, marking the lowest known fossiliferous Cambrian
zone in the Wrekin district, and his recognition of Cambrian fossils
under the Coal-measures of Warwickshire, supplied valuable evidence for
the discussion of the geological position of the older rocks of the
Midlands. He has mapped in minute detail the rocks of the Wrekin, and
has exhausted all the evidence that is at present obtainable on the
subject. But unfortunately the publication of his researches is still
delayed.[71]
[Footnote 71: _Geol. Mag._ (1882) p. 563, (1886) p. 319, (1887) p. 78,
(1888) p. 484; and a joint paper with Mr. W. W. Watts on the Geology of
South Shropshire, _Proc. Geol. Assoc._ vol. xiii. (1894) pp. 302, 335.]
It is now recognized that the core of the ancient ridge, extending
from near Wellington through the Wrekin, Caer Caradoc and other hills,
until it sinks beneath the Upper Silurian formations, is formed of
igneous rocks that consist partly of lavas, partly of volcanic breccias
and fine tuffs. The lavas are thoroughly acid rocks of the felsitic
or rhyolitic type. One of them, about 100 feet thick, which forms
a prominent feature on the flanks and crest of Caer Caradoc, shows
abundant finely-banded flow-structure, often curved or on end, while
its bottom and upper parts are strongly amygdaloidal, the cavities
being occasionally pulled out in the direction of flow and lined with
quartz or chalcedony. Some of the detached areas of eruptive rocks show
the beautiful spherulitic and perlitic structures first noticed in this
region by Mr. Allport. More recently the structures of these acid rocks
have been described by Mr. F. Rutley.[72]
[Footnote 72: _Quart. Journ. Geol. Soc._ vol. xlvii. (1891) p. 540. Mr.
Rutley more particularly describes those of Caradoc Hill.]
The breccias and tuffs appear to consist mainly of felsitic material.
In the coarser varieties, fragments of finely-banded felsite may
be noticed, while the finer kinds pass into a kind of hornstone
(hälleflinta), which in hand-specimens could hardly be distinguished
from close-grained felsite. In some places, these pyroclastic rocks
are well stratified, but elsewhere no satisfactory bedding can be
recognized in them. Various other rocks, which are probably intrusive,
occur in the ridge. At either end of the Wrekin there is a mass of pink
microgranite, while at Caer Caradoc numerous sheets of "greenstone,"
intercalated in the fine tuffs, sweep across the hill. Mr. Rutley
has published an account of these basic rocks, which he classes as
"melaphyres," or altered forms of basalt or andesite.[73] That at least
some of them are intrusive is manifest by the way in which they ramify
through the surrounding strata. But others are so strongly amygdaloidal
and slaggy that they may possibly be true interbedded lavas, though
there may be some hesitation in admitting that such basic outflows
could be erupted in the midst of thoroughly acid ejections.[74] Leaving
these doubtful flows out of account, we have here a group of undoubted
volcanic rocks represented by acid lavas and pyroclastic materials,
by intrusive bosses of acid rocks, and by younger basic sills. The
general lithological characters of these masses and the sequence of
their appearance thus strongly resemble those of subsequent Palæozoic
volcanic episodes.
[Footnote 73: _Op. cit._ p. 534.]
[Footnote 74: This difficulty, however, need not be in itself
insuperable, as is evident from the remarkable alternation of basic and
acid lavas and tuffs in the Cambrian volcanic group of St. David's and
in the Old Red Sandstone series of the Pentland Hills.]
The geological age of this volcanic group is a question of much
interest and importance in regard to the history of volcanism in this
country. An inferior limit to the antiquity of the group can at once be
fixed by the fact that, as originally pointed out by Dr. Callaway, the
quartzite which overlies the volcanic rocks passes under a limestone
containing Cambrian fossils in which Professor Lapworth has since
recognized _Olenellus_, _Paradoxides_ and other Lower Cambrian forms.
The eruptions, therefore, must be at least as old as the earlier part
of the Cambrian period. But it is affirmed that the quartzite rests
with a complete unconformability on the volcanic rocks. If this be so,
then the epoch of eruption must be shifted much farther back.
[Illustration: Fig. 39.--Section across the Uriconian series of Caer
Caradoc.
S3, Upper Silurian; S2, Bala group; S1, Arenig group; C, Cambrian; L,
Longmyndian; _u_, Uriconian; _f_ _f_, faults.]
The evidence adduced in favour of this great break appears to me to be
threefold. In the first place, the quartzite contains fragments of the
volcanic rocks. I do not think much stress can be laid on this fact.
When I visited the ground, what struck me most in the composition of
the quartzite was its singularly pure quartzose character, and the
comparative scarcity of felsite-pebbles in it. Any deposit laid down
conformably upon the top of the breccias and tuffs might obviously
contain some of these materials, while, if laid down unconformably, it
might reasonably be expected to be full of them. In the second place,
this quartzite is alleged to pass transgressively across the edges
of successive sheets of the volcanic group, and thus to have a quite
discordant dip and strike. I failed to find satisfactory evidence of
this unconformability in the northern part of the district. But in
the Caer Caradoc area the quartzite does appear to steal across the
outcrops of the older rocks, which plunge at nearly right angles in an
opposite direction. In the third place, the felsitic volcanic group is
believed by Professor Lapworth to pass upwards into the Longmynd rocks.
Obviously, if this group lies at the very bottom of the vast Longmynd
series, the discordance between it and the quartzite must be enormous,
and the date of the volcanic eruptions must be placed vastly farther
back in geological antiquity. Though the evidence does not seem to me
to amount to clear proof, I am disposed, in the meantime, to accept it
as affording the most probable solution of the difficulties presented
by the structure of the ground.
The sequence of the rocks around Caer Caradoc is partly concealed by
surface accumulations, but if these could be cleared away the structure
of the ground would be, according to Messrs. Lapworth and Watts, as
shown in Fig. 39.[75]
[Footnote 75: _Proc. Geol. Assoc._ vol. xiii. (1894), pp. 314, 315.]
If, then, this volcanic group underlies the whole of the Longmynd
series, and if, as it now appears, that series is older than the
_Olenellus_-zone of the Lower Cambrian rocks, we can hardly include the
volcanic rocks of the Wrekin and Caer Caradoc in the Cambrian system.
They must belong to a still older geological formation, and I think we
cannot do better than adopt for them Dr. Callaway's name, Uriconian.
There are still, however, many problems to be solved before the
geological history of that region is completely understood. The rocks
of the Longmynd must be more fully worked out. It is improbable that
strata which look so likely to yield fossils should for ever prove
barren. The lower half at least may be hopefully searched, although the
upper massive reddish sandstones and conglomerates offer less prospect
of success. On the west side of the Longmynd, above Pontesbury,
there occurs a small area of volcanic rocks like those of the Wrekin
district, including a well-marked nodular felsite and fine tuffs.
These rocks have been regarded by Dr. Callaway as another axis of the
Uriconian series. It is very difficult, however, by any combination
of geological structures, to bring up a portion of the very bottom
of the Longmynd series and place it apparently at the top. This is
a feat which a detailed study of the region, and the detection of
unconformabilities in the Longmynd, may possibly accomplish. In the
meantime, however, I would venture to suggest whether it is not more
probable that we have here a detached area of much younger volcanic
rocks, like those which, in various districts, may be included in
the Cambrian system, and which will be referred to in some detail in
subsequent pages.
V. THE MALVERN VOLCANO
Regarding the age and origin of the oldest rocks of the Malvern Hills
some controversy has arisen, and no general agreement has yet been
reached.[76] It is clear that the core of crystalline rocks which
is overlain unconformably by the Hollybush Sandstone must be older
than the Upper Cambrian rocks. There is no good evidence of any
stratigraphical break in the Cambrian system of England or Wales, and
it may be reasonably inferred that the break seen at the base of the
Hollybush Sandstones indicates that the rocks underneath that horizon
are pre-Cambrian. Some portions of these certainly very ancient rocks
are gneisses or schists; others have been described as "felsites,"
and have been regarded as passing into schists, and as the original
material from which portions of the foliated series of the range
have been produced by mechanical deformation. Not improbably the
whole series of rocks is of igneous origin, but has been subsequently
rendered more or less schistose.
[Footnote 76: There is no room here for a full bibliography of the
geological literature devoted to this locality. In the monograph by J.
Phillips in vol. ii. part i. of the _Memoirs of the Geological Survey_,
a list of writings is given up to the time of its publication in 1848.
Since that year many additional papers have appeared. I may especially
refer to H. B. Holl, _Quart. Journ. Geol. Soc._ xxi. (1865) p. 72; J.
H. Timins, _op. cit._ xxii. (1867); Mr. F. Rutley, _op. cit._ xliii.
(1887) p. 481; Dr. Callaway, _op. cit._ xliii. (1887) p. 525, xlv.
(1889) p. 475, xlix. (1893) p. 398; Prof. Green, _op. cit._ li. (1895)
p. 1; Mr. H. D. Acland, _Geol. Mag._ 1894, p. 48.]
There is one area where the rocks have escaped metamorphism, and where
they present some of the well-known features of ancient volcanic
materials. This tract was first indicated by Dr. H. B. Holl as one
occupied by "altered primordial rocks and post-primordial trap."
Its evidently igneous materials have been examined and described by
different observers, among whom Dr. Callaway has contributed some
detailed papers on the subject. More recently Professor Green, who
had the advantage of sections exposed in the excavations for the
construction of a reservoir for supplying water to Great Malvern, came
to the conclusion that the rocks consist mainly of felsites, having
many of the characters of rhyolites. With these are associated felsitic
tuffs, while bands of dolerite, probably intrusive, form likewise
part of the series. So far as the somewhat meagre evidence allows an
opinion to be formed, there appears to be an alternation of felsites,
lavas and tuffs placed in a more or less vertical position, striking
in a northerly direction, and traversed by several sheets of intrusive
dolerite.
No junction has been found between these unfoliated volcanic rocks and
the schists that form the core of the range. Judging merely from their
present relative condition, one would naturally infer that the volcanic
rocks must be the younger of the two groups. But, as Professor Green
has pointed out, it is conceivable that the latter may have locally
escaped crushing, and yet be of the same age as the felsites and
epidiorites of the neighbouring Raggedstone Hill, which have been in
part considerably affected by mechanical movements.[77]
[Footnote 77: _Op. cit._ p. 7. The metamorphism of the igneous rocks of
the Malvern Hills into schists has been especially investigated by Dr.
Callaway.]
For our present inquiry it is perhaps sufficient to take note that
in the heart of the Malvern Hills there lies a remnant of a volcanic
district, probably of pre-Cambrian age, the rocks of which had been
raised up into a vertical position so as to form islets or reefs in the
sea in which the Upper Cambrian strata (Hollybush Sandstone and Upper
Lingula shales) were deposited. Until some more precise evidence is
obtained as to the geological age of these rocks it may be convenient
to place them provisionally with the volcanic Uriconian series.
vi. THE CHARNWOOD FOREST VOLCANO
In the heart of England the great Triassic plain is diversified by
the uprise through it of the peaks and crests of an old Triassic
land-surface, which are embraced in the district known as Charnwood
Forest. These scattered eminences consist of materials not only
immensely older than the Trias, but once doubtless buried under
thousands of feet of Palæozoic strata. They had been laid bare by
denudation and carved into picturesque crags and pinnacles before the
New Red Sandstone was deposited around and above them.
To these vestiges of an early Mesozoic land, still half buried among
Triassic strata, a peculiar interest attaches from the obviously high
antiquity of their rocks and their uprise in the very centre of the
island. Various opinions have been expressed as to the age of their
component rocks. When they were mapped by the Geological Survey they
were recognized to be as old as any group of rocks then known, and they
were accordingly placed in the Cambrian system. More recent research
has suggested that they may be still more ancient, and may be regarded
as pre-Cambrian.
The rocks of Charnwood Forest have been the subject of an exhaustive
research by the Rev. E. Hill and Professor Bonney, to whom most of our
knowledge regarding them is due. These observers first pointed out the
truly volcanic nature of the coarse clastic rocks of the district.
They have traced their relations in the field, and have likewise
described their structure and composition as shown by the microscope.
Subsequently the district has been re-mapped on the scale of six
inches to a mile by Mr. Fox Strangways for the Geological Survey,
while Mr. W. W. Watts, another member of the Survey, has studied the
petrography of the ground, and has traced the boundaries of the several
rock-groups so far as these can be determined. Confirming generally the
stratigraphical arrangement sketched by Messrs. Hill and Bonney, Mr.
Watts has proposed the following classification of the rocks:--[78]
7. Groby and Swithland slates. }
}
6. Hanging Rocks conglomerate and } The Brand series.
Bradgate quartzite. }
5. Woodhouse beds (ashy grits). }
}
4. Slate-agglomerate of Roecliffe. } The Maplewell series (volcanic
} tuffs and agglomerates).
3. Hornstone beds of Beacon Hill. }
}
2. Felsitic agglomerate of Benscliffe. }
1. Quartzose, felspathic and felsitic grits. The Blackbrook series.
[Footnote 78: Annual Report of Director-General of the Geological
Survey, in the _Report of Science and Art Department for 1895_.]
Under any computation or measurement, the total thickness of detrital
material in this series of formations must amount to several thousand
feet. The chief interest centres in the middle series, which consists
largely of fragmental volcanic rocks, with intercalations of slate
and grit. As was first shown by Mr. Hill and Professor Bonney, these
volcanic materials vary from exceedingly coarse agglomerates to fine,
ashy or felspathic slates. In most cases distinct bedding can be
recognized in them, but more particularly in the fine-grained material.
Yet even among the massive agglomerates a tendency may be seen towards
an orientation of the blocks with their long axes parallel. That
this arrangement is not entirely due to the effects of cleavage may
be inferred from the many exceptions to it, which would hardly have
occurred had such powerful cleavage affected the whole district, as
would be needed to rearrange the large blocks in the agglomerates.
Besides, the coarser parts often intercalate with fine felspathic
grits, which distinctly mark the stratification of the whole.
The remarkably coarse breccia of Benscliffe is mainly made up of blocks
of quartz-porphyry, felsite or rhyolite, with slate fragments. The
Roecliffe agglomerate, another extraordinarily coarse rock, consists
of slate fragments imbedded in an andesitic matrix, some of the blocks
of slate being six feet long. The finer tuffs have been ascertained
to consist of felsitic or andesitic detritus, sometimes forming
exceedingly compact flinty rocks or hornstones.
In this thick accumulation of detrital rocks we are presented with
a series of alternations of coarser and finer pyroclastic material,
interstratified among green, grey and purple slates and grits, which
probably represent the non-volcanic sediments of the time of eruption.
The succession of strata bears witness to a long series of eruptions
of varying intensity, but culminating at two distinct periods in
the discharge of huge blocks of rock (Benscliffe and Roecliffe
agglomerates).
After some search I have been unable to detect a single vesicular
fragment among the stones in the breccias and tuffs, and Messrs. Hill
and Bonney were not more successful. Not a trace of anything in the
least degree scoriaceous is anywhere to be found. The paste in which
the blocks lie consists of such fine material as would result from the
trituration of felsite and slate. It contains many broken crystals of
felspar, with grains of clear quartz. A gradation can be traced from
the coarser into the finer bands of volcanic and non-volcanic material,
fine slates being also interleaved with highly-felspathic partings of
grit.
Having looked with some care for a trace of a true volcanic neck in
the district, I have not seen anything that could be unhesitatingly so
designated. Even in the north-western part of the district, where the
breccias are coarsest, and there is least trace of ordinary sediments,
some signs of bedding can usually be detected in the position of the
imbedded stones and the partings of finer tuff. Both the coarser and
finer detritus suggest the kind of material discharged from vents
before the uprise of any lava. The entire absence of scoriaceous
fragments is noteworthy, and the abundance of slate blocks rather
points to the early eruptions of a volcanic focus. Possibly, while the
chief centre of eruption lay towards the north-west, numerous vents may
have been opened all over the district, discharging abundant showers
of dust and stones, but seldom or never culminating in the actual
outpouring of lava.
No indubitable lava-sheet has, in my judgment, been yet recognized in
Charnwood Forest. Various opinions have been expressed as to some of
the more compact close-grained rocks, and even the verdicts of the same
observers have varied from time to time, the rocks once considered as
felsites being afterwards regarded as tuffs, and subsequently placed
with the felsites or andesites after all. It is not necessary for my
present purpose to enter into these questions, which are rather of
local interest. I will only say that, in my opinion, the rocks of
Sharpley, Peldar, and Bardon Hill are massive rocks, as they have
finally been classed by Messrs. Hill and Bonney. But I cannot look upon
them as lavas, at least I have seen no evidence to lead me to believe
that they were ever erupted at the surface. I have fully considered
the arguments of Mr. Hill and Professor Bonney on this point.[79]
There can, I think, be no doubt of the close association of these
felsitic rocks and the breccias, but the structure of the rocks in
the field seems to me to be decidedly in favour of the view expressed
above. The microscope affords no assistance in the question.[80] The
doubtful rocks seem to me rather to be intrusive masses which have
been protruded into the volcanic sedimentary series among which they
rise. They are acid, fine-grained, porphyritic rocks, which would
formerly have been included under the general name of felsites or
quartz-porphyries. Their coarse porphyritic parts rapidly pass into
close-grained felsitic material. Many of the blocks in the breccias are
precisely like parts of these rocks. It might hence be asserted that
these fragmental deposits are later than the eruptive bosses. At least
it is obvious that rocks of the same type as those of Sharpley, Peldar,
and Bardon Hill must have been disrupted to produce the coarse breccias.
[Footnote 79: _Quart. Journ. Geol. Soc._ xlvii. (1891), pp. 80-88.]
[Footnote 80: See Messrs. Hill and Bonney, _op. cit._ xxxiv. (1878), p.
211.]
Later eruptive rocks, consisting of masses of syenite and granite, with
still younger dykes of dolerite, andesite, diorite and felsite, have
successively made their appearance, and add to the diversity of the
igneous phenomena of this district.
The question of the age of this isolated volcanic series is one of
much interest, but of great perplexity. Though a resemblance may be
admitted to exist between some of the slates and parts of the Cambrian
system of North Wales, the difference between the Charnwood rocks and
the undoubted Cambrian series of Warwickshire, only thirteen miles to
the south-west, is such as to indicate that the former are probably
older than the latter. While the Charnwood rocks have been intensely
cleaved and crushed, those of Warwickshire have undergone no such
change. The argillaceous strata in the one region have been converted
into slates, in the other they remain mere shales. Though cleavage is
sometimes irregularly developed, its rapid disappearance in so short a
distance as the interval between Charnwood Forest and Nuneaton seems
most explicable if we suppose that the rocks at the more easterly
locality were cleaved before those towards the west were deposited. If
this inference be well grounded the pre-Cambrian age of the Charnwood
volcanoes would be established. But the argument is not conclusive.
No fossils of any kind have yet been found in any of the old rocks of
Charnwood.[81] Merely lithological resemblances or differences are all
that can be used as a guide to the geological age of these masses.
Mr. Watts has suggested that possibly the quartzite of Bradgate (No.
6 of the Charnwood groups) may be the equivalent of the quartzite
which in Shropshire and Warwickshire forms the base of the sedimentary
Cambrian formations. If that correlation could be established, the
volcanic series below the quartzite in Charnwood might be regarded as
representing the Uriconian volcanic series of Shropshire.
[Footnote 81: Since this page was in type, Professor Lapworth has found
a worm-burrow low down in the Brand Series, and one or two additional
examples have since been obtained by Mr. J. Rhodes of the Geological
Survey. These are the first undoubted organisms from the Charnwood
Forest rocks. Mr. Watts, _Geol. Mag._ 1896, p. 487.]
BOOK III
THE CAMBRIAN VOLCANOES
CHAPTER IX
CHARACTERISTICS OF THE CAMBRIAN SYSTEM IN BRITAIN
The Physical Geography of the Cambrian Period--The Pioneers of
Palæozoic Geology in Britain--Work of the Geological Survey in
Wales--Subdivisions of the Cambrian System in Britain.
In leaving the investigation of the pre-Cambrian rocks and entering
upon that of the Palæozoic systems, that is, the great series of
sedimentary formations which include the earliest records of organized
life upon the surface of the globe, the geologist feels much as the
historian when, quitting the domain of legend and tradition, he can
tread firmly in the region of documentary evidence. From the bottom
of the Cambrian system upward through the long series of geological
formations, the chronicle, though often sadly incomplete, is usually
clear and legible. As we follow the lowest fossiliferous strata across
a territory, we recognize that they bear witness to the same processes
of denudation and deposition which have been going on uninterruptedly
on the face of the globe ever since. The beds of conglomerate represent
the gravels and shingles of old coast-lines and river-beds. The
sandstones recall the familiar features of sandy sea-bottoms not far
from land. The shales bear witness to the fall of fine sediment in
stiller water, such as now takes place in the deeper parts of seas and
lakes. Notwithstanding their vast antiquity, the strata themselves
exhibit no exceptional peculiarities of origin. They seem to be just
such familiar deposits as are gathering under fitting conditions at the
present time.
Some writers have speculated on the far greater intensity of all
geological activities in the early times of the planet's history. But
if we may interpret the record of the stratified formations by the
phenomena of to-day, there is for these speculations no confirmation in
the sedimentation of the oldest stratified deposits. It is of course
quite intelligible, if not probable, that many geological forces
may have been more vigorous in primeval times than they afterwards
became. But of the gigantic tides, prodigious denudation and violent
huddling together of the waste of the earth's surface, which have been
postulated for the early Palæozoic ages, there is assuredly nowhere
any indication among the stratified formations. In those vast orderly
repositories, layer succeeds layer among thinly-laminated shales, as
gently and equably as the fine silt of each tide sinks to-day over the
floor of a sheltered estuary. At the primeval period of which these
sediments are the memorial, the waters receded from flat shores and
left tracts of mud bare to the sky, precisely as they do still. Then as
now, the sun shone and dried such mud-flats, covering their surfaces
with a network of cracks; the rain fell in heavy drops, that left their
imprints on the drying mud; and the next tide rose so gently as to
overflow these records of sunshine and shower without effacing them,
but spreading over them a fresh film of sediment, to be succeeded by
other slowly-accumulating layers, under which they have lain preserved
during the long cycles of geological history.
That organized creatures had already appeared upon the earth's surface
before the beginning of the Cambrian period cannot be doubted. The
animal remains in the lowest Cambrian strata are far from being the
simple forms which might be expected to indicate the first start of
animal life upon the surface of the earth. On the contrary, though they
are comparatively scanty in types, and often rare or absent throughout
a thick mass of sedimentary deposits, they show beyond dispute that,
when they flourished, invertebrate life had already reached such a
stage of advancement and differentiation that various leading types had
appeared which have descended, in some cases with generic identity,
down to our own day. There must have been a long pedigree to these
organisms of the oldest known fossiliferous rocks. And somewhere on
the earth's surface we may yet hope to find the remains of their
progenitors in pre-Cambrian deposits.
The researches of many explorers in Europe and North America have
brought to light an interesting series of organic remains from the
Cambrian system. Of the plants of the time hardly any traces have
survived, save some markings which have been referred to sea-weeds. The
earliest known sponges and corals occur in this system, likewise the
ancestors of the graptolites, which played so prominent a part in the
life of the next or Silurian period. There were already representatives
of crinoids and star-fishes, besides examples of the extinct group of
cystideans. Sea-worms crawled over the muddy and sandy sea-bottom, for
they have left their trails and burrows in the hardened sediments.
Molluscs had by this time appeared in their four great divisions of
Brachiopods, Lamellibranchs, Gasteropods and Cephalopods, though the
forms yet discovered among Cambrian rocks are comparatively few. The
most abundant and characteristic inhabitants of the Cambrian seas were
the trilobites, of which many genera have been disinterred from the
strata. In the lowest fossiliferous Cambrian group the trilobitic genus
_Olenellus_, already referred to, is the characteristic form. Higher
up _Paradoxides_ is predominant, while towards the top of the system
the most characteristic genus is _Olenus_.
From the organic remains which have been preserved, we may legitimately
infer the existence of others which have entirely disappeared. There
seems no reason to doubt that the leading grades of invertebrate life
which are wanting in the known Cambrian fauna were really represented
in the Cambrian seas. The chance discovery of a band of limestone may
any day entirely alter our knowledge as to the relative proportions
of the several divisions of the animal kingdom in the earliest
Palæozoic rocks. Sand is rather adverse to the preservation of a varied
representation of the organisms of the overlying sea-water. Mud is
generally favourable, but calcareous accumulations are greatly more
so, and they usually consist almost entirely of organic remains. Thus
in the Cambrian series of the north-west of Scotland the quartzites
that form the lower group, though sometimes crowded with worm-burrows,
contain hardly any other sign of organisms. The overlying shales,
besides their abundant worm-castings, have yielded perfect specimens of
_Olenellus_ and other fossils. But in the uppermost group, consisting
of limestones, every particle of the sediment appears to have passed
through the intestines of worms, and as it gathered on the sea-bottom
it enclosed and has preserved a varied and abundant assemblage
of organisms, including trilobites, gasteropods and a number of
cephalopods. While in the Cambrian rocks of Europe calcareous bands are
comparatively rare, in those of North America they are not infrequent.
Hence it is largely from American deposits that our knowledge of the
Cambrian fauna has been derived.
Not a vestige of any vertebrate organism has yet been detected among
the earlier Palæozoic sediments. So far as we know, there were no
fishes in the Cambrian seas. The highest organisms then existing were
chambered shells, a once abundant and singularly varied class, of which
the living Nautilus is now the sole representative.
In trying to realize the general geographical conditions of Cambrian
time, the geologist finds himself entirely without any evidence as
to the character of the terrestrial vegetation. We can hardly doubt
that the land was clothed with plants, probably including lycopods
and ferns, possibly even cycads and conifers. But no remains of this
flora have yet been recovered. Nor have any traces of land-animals
been detected. All that we yet know of the life of the period has
been gleaned from marine sediments, which show that the invertebrate
population by which the sea was then tenanted embraced some of the
leading types of structure that have survived through all the long
vista of geological time down to our own day.
Some of the shore-lines of the Cambrian waters may still be traced,
and it is possible to say where the land of the time stood and where
lay the sea. In the British area the largest relic of Cambrian land is
found in the far north-west of Scotland. Formed partly of the Lewisian
Gneiss and partly of the Torridon Sandstone, it takes in the whole
chain of the Outer Hebrides and likewise part of the present western
seaboard of Sutherland and Ross. Along the margin of that northern land
the white sand was laid down which now gleams in sheets of snow-like
quartzite on most of the higher mountains from Cape Wrath to Skye. The
sea lay to the east and, so far as we know, may have stretched across
the rest of Scotland, and the north and centre of England. Another
vestige of the land of this ancient era occurs in Anglesey. There, and
likewise over scattered tracts in the Midlands, and in the south-west
of England, the geologist seems to descry the last relics of islets
that rose out of the Cambrian sea, and are now surrounded with its
hardened sediments.
While such was the general aspect of the region of the British Isles
during Cambrian time, volcanic action manifested itself at various
localities over the area, breaking out on the sea-bottom, and pouring
forth sheets of lava and showers of ashes, which mingled with the sand
and silt that were settling there at the time. In the northern or
Scottish tract no trace of this subterranean activity has been found;
but in the English Midlands and over much of Wales abundant evidence
has been obtained to show that in those districts the Cambrian period
was marked by frequent and prolonged eruptions.
As its name denotes, the Cambrian system is typically developed in
Wales. It was there that Sedgwick first worked out the stratigraphical
relations of its ancient sediments, and that Murchison demonstrated the
succession of organic remains contained in them, applying to them the
principles of classification laid down by William Smith in regard to
the Secondary formations. It was there too that some of the earliest
and most memorable achievements were made in the investigation of
ancient volcanic rocks. Sedgwick and Murchison, besides the admirable
work which they accomplished in establishing the stratigraphy of
the older Palæozoic formations, clearly recognized that among these
formations there were preserved the records of contemporaneous
submarine eruptions. Sedgwick showed that the mountainous masses
of eruptive rock in North Wales were really lavas and ashes, which
had been discharged over the sea-floor at the time when the ancient
sediments of that region were deposited, while Murchison established
the same fact by numerous observations in the east and south of Wales,
and in the bordering English counties. De la Beche had found similar
evidence among the "grauwacke" rocks of Devonshire.[82]
[Footnote 82: For early researches on the older Palæozoic volcanic
rocks of Britain, see Sedgwick, _Proc. Geol. Soc._ vols. ii. (1838) pp.
678, 679, iii. (1841) p. 548, iv. (1843) p. 215; _Quart. Journ. Geol.
Soc._ vols. i. (1845) pp. 8-17, iii. (1847) p. 134. Murchison, _Proc.
Geol. Soc._ vol. ii. (1833-34) p. 85; _Silurian System_ (1839) pp. 225,
258, 268, 287, 317, 324, 401; _Siluria_, 4th edit. (1867) p. 76 _et
seq._ De la Beche, _Mem. Geol. Survey_, vol. i. (1846) pp. 29-36. A. C.
Ramsay in the Maps and Horizontal Sections of Wales published by the
Geological Survey; also Descriptive Catalogue of the Rock-Specimens in
the Museum of Practical Geology, 1st edit. (1858), 2nd edit. (1859),
3rd edit. (1862); "The Geology of North Wales," forming vol. iii. of
_Memoirs of the Geological Survey_, 1st edit. (1866), 2nd edit. (1881).]
Following in the track thus opened up by these great masters, the
officers of the Geological Survey were enabled to unravel, as had never
before been attempted, the complicated structure of the old volcanic
regions of Wales. At the outset of the following discussion I wish to
express my admiration of the labours of the early pioneers who thus
laid for us the foundation of our knowledge of volcanic action in the
Palæozoic periods. To De la Beche and his associates in the Survey a
special measure of gratitude is due from all who have followed in their
steps and profited by their work. When we consider the condition of
geological science, and especially of the department of petrography, in
this country at the time when these early and detailed investigations
were carried on, when we remember the imperfection of much of the
topography on the old one-inch Ordnance maps (which were the only maps
then available), when we call to mind the rugged and lofty nature of
the ground where some of the most complicated geological structures
are displayed, we must admit that at the period when these maps and
sections were produced they could not have been better done; nay,
that as in some important respects they were distinctly in advance
of their time, their publication marked an era in the progress of
structural, and especially of volcanic, geology. The separation of
lavas and tuffs over hundreds of square miles in a mountainous region,
the discrimination of intrusive sheets and eruptive bosses, the
determination of successive stratigraphical zones of volcanic activity
among some of the oldest fossiliferous formations, were achievements
which will ever place the names of Ramsay, Selwyn, Jukes and their
associates high in the bede-roll of geological science. No one ever
thinks now of making a geological excursion into Wales without carrying
with him the sheets of the Geological Survey map. These form his guide
and handbook, and furnish him with the basis of information from which
he starts in his own researches.
But science does not stand still. The most perfect geological map that
can be made to-day will be capable of improvement thirty or forty years
hence. The maps of the Geological Survey are no exception to this rule.
In criticizing and correcting them, however, let us judge them not by
the standard of knowledge which we have now reached, but by that of the
time when they were prepared. It is easy to criticize; it is not so
easy to recognize how much we owe to the very work which we pronounce
to be imperfect.
The ancient volcanoes of Wales, thanks mainly to the admirable labours
of my former friend and chief, Sir Andrew C. Ramsay, have taken a
familiar place in geological literature. But a good deal has been
learnt regarding them since he mapped and wrote. The volcanic history,
as he viewed it, began in the Arenig period. The progress of subsequent
inquiry, however, has shown that there are volcanic rocks in Wales of
much older date. I shall show that the Cambrian period, both in South
and North Wales, was eminently volcanic.
Much controversy having arisen as to the respective limits and
nomenclature of the older Palæozoic rocks, let me state, at the outset
of the inquiry into the volcanic eruptions of Cambrian time, that under
the term "Cambrian" I class all the known Palæozoic rocks which lie
below the bottom of what is termed the Arenig group. It was maintained
by Sir Andrew Ramsay and his colleagues on the Geological Survey that
on the mainland of Wales no base is ever found to the Cambrian system.
More recently certain conglomerates have been fixed upon as the true
Cambrian base, both in South and North Wales, and endeavours have been
made to trace an unconformability at that line, all rocks below it
being treated as pre-Cambrian. But conglomerates do not necessarily
mark a stratigraphical discordance, and in South Wales there is no
trace of any unconformability between the strata above and below the
supposed line of break.[83] Professor Bonney has shown that in North
Wales several zones of conglomerate have been erroneously identified
as the supposed basal platform of the Cambrian series, and more
recently Mr. Blake has pointed out that some of these conglomerates are
unquestionably Lower Silurian.
[Footnote 83: See a discussion of this subject in _Quart. Journ. Geol.
Soc._ vol. xxxix. (1883), p. 305.]
My own examination so far confirms the conclusions arrived at by
these observers. Like my predecessors in the Geological Survey,
however, I have been unable to detect anywhere in Caernarvonshire
or Merionethshire a base to the Cambrian system, and I am compelled
to agree with them in regarding as Cambrian (partly even as Lower
Silurian) all the rocks from Bangor to Llanllyfni, which have more
recently been classed as pre-Cambrian. But though thus supporting their
general stratigraphy, I am bound to acknowledge that they failed to
recognize the existence of a great volcanic series below the Arenig
horizon. The existence of this series, noticed by Sedgwick, was first
definitely stated by Professor Hughes,[84] and his statements have been
confirmed and extended by subsequent observers, notably by Professor
Bonney and Mr. Blake. The Cambrian period is thus proved to have been
perhaps even more continuously volcanic than the Lower Silurian period
was in Wales.
[Footnote 84: _Proc. Camb. Phil. Soc._ vol. iii. (1877), p. 89. The
Cambrian volcanic areas of North Wales are represented in Map II.]
The following table shows the subdivisions of the Cambrian system now
recognized in Britain:--
+----------------------+----------------------+-------------------------+
| Wales. | | |
| (Ranging up to | Western England. | N.W. Scotland. |
| 12,000 feet | (About 3000 feet.) | (About 2000 feet.) |
| or more.) | | |
+----------------------+----------------------+-------------------------+
|Upper or _Olenus_ Zones. | |
| Tremadoc Slates | Shineton Shales | Limestones, about 1500 |
| Lingula Flags | (_Dictyograptus_ | feet thick, divisible |
| (_Lingulella_, | or _Dictyonema_, | into seven groups |
| _Olenus_, etc.). | _Olenus_, etc.). | (_Archæocyathus_, |
| | | _Maclurea_, _Ophileta_,|
|Middle or _Paradoxides_ Zones. | _Murchisonia_, |
| Menevian group | Conglomerates and | _Orthoceras_, |
| (_Paradoxides_). | limestones (Comley),| and vast quantities of |
| | (Comley), with | annelid castings). |
| | _Paradoxides_, etc. | |
| | | |
|Lower or _Olenellus_ Zones. | |
| Harlech and | Thin quartzite | Shales ("fucoid beds"), |
| Llanberis | passing up into | with _Olenellus_, |
| group with | green flags, grits, | _Salterella_, etc. |
| basement volcanic | shales | Quartzites with annelid |
| rocks; bottom not | and sandstone | burrows. The base of |
| seen. | (Comley Sandstone), | the series lies |
| | containing | unconformably on |
| | _Olenellus_. | pre-Cambrian rocks. |
+----------------------+----------------------+-------------------------+
CHAPTER X
THE CAMBRIAN VOLCANOES OF SOUTH WALES
In the southern part of the Principality of Wales a remarkably varied
display of British Cambrian volcanic rocks has been preserved. The
district around St. David's has the distinction of being the first
in which volcanic rocks of such high antiquity were recognized. As
far back as the year 1842, Ramsay found that "felspathic volcanic
ash" was associated with other proofs of igneous action, and this
fact was recorded by him on the published Horizontal Sections of the
Geological Survey. Unfortunately he afterwards came to regard the
rocks as "altered Cambrian," thus following certain hypothetical views
which, as will be further alluded to in the sequel, he had adopted
in explanation of the phenomena in Caernarvonshire and in Anglesey.
The volcanic nature of these ancient materials was subsequently
rediscovered by Dr. Hicks, who has devoted much time and labour to
their study. Distinguishing the volcanic series of St. David's by
the name "Pebidian," he has contended that it forms a pre-Cambrian
system separated by an unconformability from the base of the Cambrian
formations. He likewise endeavoured to show that an older system
of rhyolitic lavas, felsitic breccias and hälleflintas could be
distinguished, which he termed "Arvonian"; and more ancient still,
a core of granitoid or gneissic rocks, which he separated under the
name of "Dimetian." My own investigation of the ground thoroughly
convinced me that there are no pre-Cambrian rocks at St. David's;
that the "Arvonian" and "Dimetian" series are merely intrusive rocks
(quartz-porphyry, granite, etc.) which have invaded the volcanic
series; and that the "Pebidian," instead of being a pre-Cambrian
formation on which the Cambrian base rests unconformably, is a group of
volcanic rocks into which the Cambrian strata pass down conformably,
and which in the St. David's district constitutes the lowest group of
the Cambrian system.[85]
[Footnote 85: For Dr. Hicks' views, see especially his papers in the
_Quart. Journ. Geol. Soc._ vols. xxxi. xxxiii. xxxiv. xl. My criticism
of them will be found in _op. cit._ vol. xxxix. (1883), subsequently in
the main confirmed by Prof. Lloyd Morgan, _op. cit._ xlvi. p. 241. See
also Prof. Blake, _op. cit._ xl. (1884). Dr. Hicks in his more recent
papers has merely reiterated his previously published opinions.]
[Illustration: _Walker & Boutall sc._
Fig. 40.--Map of the volcanic district of St. David's.]
The volcanic geology of St. David's possesses a special interest
inasmuch as it embraces a tolerably full development of various
features which characterize the volcanic groups of later Palæozoic
systems. Though the rocks are chiefly tuffs, they include also sheets
of lava, as well as sills, dykes and bosses. They show a remarkable
range in chemical composition from quite basic to highly acid
materials. They present the amplest proofs of having been erupted
and spread out over the sea-bottom, and they likewise afford clear
evidence of alternation with the ordinary non-volcanic sediment of the
time to which they belong. In these respects they are particularly
noteworthy, for they prove that in the earliest Palæozoic ages the
essential features of volcanic action were already as well developed as
in any subsequent epoch of geological history.
The volcanic group of St. David's attains a visible thickness of about
1800 feet. Its upper part graduates upward into purple and green
Lower Cambrian sandstones. The base of the group is not seen owing to
the plicated structure of the district. Hence the total thickness of
volcanic material cannot be determined, neither can we tell on what it
rests, whether on a still lower sedimentary series or on some platform
of pre-Cambrian rocks.
The structure of the group, notwithstanding all that has been written
about it, has never yet been adequately worked out. The unfortunate and
barren controversy about supposed pre-Cambrian rocks at St. David's
has tended to obscure the real importance of these rocks as the oldest
well-preserved record of volcanic action in Britain. They deserve to
be carefully surveyed on maps of a large scale, in the same detailed
manner as has been so successfully applied to the elucidation of
younger volcanic tracts. Until such detailed investigation is made,
any account of them which is given can be little more than a general
outline of the subject. The following description is the result of
my examination of the ground in company with my colleague Mr. B. N.
Peach, and afterwards with the late Mr. W. Topley.[86] A few additional
observations, from the subsequent exploration of Professor Lloyd
Morgan,[87] are incorporated in the narrative.
[Footnote 86: _Quart. Journ. Geol. Soc._ vol. xxxix. (1883), p. 294 _et
seq._ While the essential parts of the investigation are given in the
following pages, I would refer the reader to this paper for details not
transferred to the present volume.]
[Footnote 87: _Op. cit._ vol. xlvi. (1890), p. 241.]
The geologist who traces these St. David's rocks in the field cannot
fail to be struck with their general resemblance to volcanic masses
of later Palæozoic date. Many of the lavas and tuffs are in outward
characters quite indistinguishable from those of the Lower Old Red
Sandstone and Carboniferous systems of Britain. So many points of
detail may be observed to be common to the Palæozoic eruptive rocks
all over the country from the Cambrian to the Permian periods as to
indicate that volcanic phenomena must have recurred under much the same
conditions throughout Palæozoic time.
By far the larger part of the Cambrian volcanic group of St. David's
consists of bedded tuffs, though a few lavas are interstratified in it,
particularly towards the top. The whole has subsequently been invaded
by acid protrusions, and lastly by basic dykes.
1. _Bedded Tuffs and Lavas._--The tuffs, which are the predominant
members of the volcanic group, present many varieties of colour, from
dark purple, through tints of brick-red and lilac, to pale pink, yellow
and creamy white, but not unfrequently assume various shades of dull
green. They vary likewise in texture from somewhat coarse breccias
or agglomerates, through many gradations, into fine silky schists in
which the tuffaceous character is almost lost. Generally they are
distinctly granular, presenting to the naked eye abundant angular and
subangular lapilli, among which broken crystals of a white, somewhat
kaolinized, felspar and fragments of fine-grained felsite are often
conspicuous. The greater part of the tuffs, particularly the purple,
red and dark-green varieties, which constitute so large a proportion of
the whole, has been derived from the explosion of basic rocks similar
in character to the diabases now found associated with them. On the
other hand, the paler varieties, both in the form of fine tuffs and of
breccias, have probably resulted mainly from the destruction of more
siliceous lavas, probably felsites (rhyolites) or other acid rocks.
That many of the tuffs are due to the destruction of diabase-lavas
may be surmised from their close general external resemblance to
these rocks, and from the way in which they are associated with the
contemporaneous sheets of diabase. Some of the dull dark-purple tuffs
might almost at first sight be mistaken for truly eruptive rocks. The
analyses of two typical examples of these basic tuffs (Nos. I. and
II.), and one (No. III.) of an intermediate variety containing an
admixture of acid fragments, are given in the subjoined table.
+-----------------------------+--------+--------+--------+
| | I. | II. | III. |
+-----------------------------+--------+--------+--------+
|SiO_{2} | 51·25 | 48·11 | 61·54 |
|Al_{2}O_{3} | 20·41 | 13·30 | 16·30 |
|Fe_{2}O_{3} | 3·02 | 3·70 | 4·40 |
|FeO | 3·91 | 8·10 | 3·66 |
|MnO | 0·21 | 1·43 | 0·32 |
|CaO | 4·53 | 8·48 | 3·08 |
|MgO | 7·22 | 9·51 | 2·99 |
|K_{2}O | 2·93 | 1·57 | 1·62 |
|Na_{2}O | 1·82 | 1·96 | 2·81 |
|H_{2}O and Loss on Ignition. | 5·02 | 4·21 | 2·99 |
|Total. | 100·32 | 100·37 | 99·71 |
|Specific Gravity. | 2·84 | 2·92 | ... |
+-----------------------------+--------+--------+--------+
I. Purplish-red shaly tuff from below olivine-diabase, Crag Rhosson.
Analysis by Mr. J. S. Grant Wilson.
II. Dull purple and green tuff from the lowest group of tuffs between
Pen-maen-melyn and Pen-y-foel. Analysis by Mr. Wilson.
III. Greenish shaly finely granular tuff, from road-side, north of Board
Schools, St. David's. Analysis by Prof. A. Renard of Ghent.
Although the majority of the tuffs are more or less basic, they
frequently contain evidence in the form of small felsitic lapilli that
acid lavas were present in the eruptive vents, while the pale tuffs
show that at the time of their discharge it was these acid lavas and
not the diabases that were blown out by the explosions. Appended are
three analyses of the acid tuffs (Nos. IV. V. and VI.).
+-----------------------------+--------+--------+--------+
| | IV. | V. | VI. |
+-----------------------------+--------+--------+--------+
|SiO_{2} | 80·59 | 73·42 | 72·63 |
|Al_{2}O_{3} | 11·29 | 12·09 | 16·23 |
|Fe_{2}O_{3} | 0·28 | 0·91 | 2·70 |
|FeO | 1·41 | 3·13 | 0·48 |
|MnO | trace | 0·25 | ... |
|CaO | 0·52 | 2·94 | 0·18 |
|MgO | 0·95 | 1·12 | 1·36 |
|K_{2}O | 2·98 | 1·67 | 3·35 |
|Na_{2}O | 0·72 | 3·88 | 0·15 |
|H_{2}O and Loss on Ignition | 1·96 | 1·28 | 3·00 |
|Total. |100·70 | 100·69 | 100·12 |
|Specific Gravity. | 2·55 | 2·74 | ... |
+-----------------------------+--------+--------+--------+
IV. Greenish felsitic breccia, Clegyr Hill; angular fragments of various
felsites in a greenish base. Analysis by Mr. J. S. Grant Wilson.
V. Grey granular felsitic tuff, Bridge over Allan River north from St.
David's Board Schools. Analysis by Mr. Wilson.
VI. Pale pinkish-white, finely schistose tuff--a characteristic sample
of the "Porth-lisky schists." Analysis by Prof. Renard.
Many varieties of texture can be traced among the tuffs, from coarse
breccias or agglomerates, with blocks a yard or more in length,
to fine schistose mudstones or sericitic schists. One of the most
remarkable of the finer kinds, found near Pen-y-foel, is externally
dirty-green, compact and tolerably homogeneous, but with distinct
evidence of its clastic character. Under the microscope it is
found to be composed mainly of lapilli of a peculiar rock, which
is characterized by the abundance and freshness of its plagioclase
(an unusual feature in the volcanic group of St. David's); by the
large, well-defined crystals (one of which measured 0·022 inch
by 0·0125 inch) of augite; by large crystals replaced by green
decomposition-products, but having the external form of olivine; by
the absence or scantiness of any base or groundmass; and, in many of
the lapilli, by the abundance of spherical cells, either empty or
filled up as amygdales with decomposition-products. These spherical
vapour-vesicles, so characteristic of the basic or palagonitic lapilli
in many Palæozoic volcanic vents, were found in one fragment, where
they were particularly abundant, to range from a minimum of 0·0008 inch
to a maximum of 0·0033 inch, with a mean of about 0·0018. The rock
from which these lapilli have been derived comes nearest to one of the
diabases from the same part of the district (which will afterwards be
referred to), but shows a closer approach to basalt rocks.
Another interesting tuff is that of which the analysis (No. II.)
has been given. It occurs not far from the horizon of the rock just
described. Under a low power, it is seen to be composed mainly of
fragments of diabase like the rocks of Rhosson and Clegyr Foig. These
fragments are subangular, or irregular in shape, and vary considerably
in size. They are sometimes finely cellular--the cavities, as in the
case just referred to, being spherical. The plagioclase crystals
in the diabase-lapilli are everywhere conspicuous; so also is the
augite, which occurs in larger forms than in the rock of Rhosson or
Clegyr Foig. Next in abundance to these basic fragments are rounded
or subangular pieces of felsite. These weather out in conspicuous
grey rough projections on the exposed face of the rock; under the
microscope they are seen to consist of fine granular felsite, which
shows a groundmass remaining dark between crossed nicols, but with
luminous points and filaments, and an occasional spherulite giving
the usual cross in polarized light. Lapilli of an older tuff may here
and there be detected. A few angular and subangular grains of quartz
are scattered through the rock. The lapilli are bound together by a
finely-granular dirty-green substance.
As a typical illustration of the minute structure of the felsitic
tuffs, I may refer to the rock No. V. of the foregoing analyses. It is
composed mainly of fragments of various felsites, many of which show
good flow-structure. Large, and usually broken, crystals of orthoclase
are dispersed among the other ingredients. Here and there a fragment
of diabase may be detected; but I could find no trace of pieces of the
peculiar microcrystalline spherulitic quartz-porphyries of St. David's.
There is but little that could be called matrix cementing the lapilli
together. The presence of fragments of diabase may possibly reduce the
proportion of silica and increase that of magnesia, as compared with
what would otherwise have been present in the rock.
Some of the tuffs appear to have been a kind of volcanic mud. A
specimen of this nature collected from the road-side section, north of
the Board School, presents a finely-granular paste enclosing abundant
angular and subangular lapilli of diabase, a smaller proportion
of felsite (sometimes displaying perfect flow-structure), broken
plagioclase crystals, and a greenish micaceous mineral which has been
subsequently developed out of the matrix between the lapilli.
Though they lie in the sedimentary series above the main volcanic
group, I may refer here to certain thin bands of tuff at Castell, on
account of their interest in relation to the true Cambrian age of the
volcanic group. They are not quite so fresh as the tuff that occurs in
thicker masses, but their volcanic origin is readily observable. One
band appears to be made up of the debris of some basic rock, like the
diabase of the district, through which detached plagioclase crystals
are scattered. The lapilli are subangular; and around their border a
granular deposit of hæmatite has taken place, giving a red colour to
the rock. Another band presents small angular lapilli, almost entirely
composed of a substance which to the naked eye, or with a lens, is
dull, white and clay-like, easily scratched, and slightly unctuous to
the touch. Under the microscope, with a low power, it becomes pale
greyish-green and transparent, and is seen to consist in large part of
altered felspar crystals, partially kaolinized and partially changed
into white mica and calcite. These scattered crystals are true volcanic
lapilli, and have not been derived from the mechanical waste of any
pre-existing volcanic rock. In the tuffs interstratified with the
conglomerate, at the quarry above Porth-clais, though much decomposed,
crystals of plagioclase can likewise still be traced. These strata are
also true tuffs, and not mere detritus due to mechanical degradation
(see Fig. 41).
The general result of the study of the microscopic structure of the
Cambrian tuffs of St. David's may be briefly summed up as follows:--
1. These pyroclastic deposits are almost wholly composed of fragments
of eruptive rocks, sometimes rounded, but usually angular or
subangular. In the more granular varieties very little matrix is
present; it consists of fine debris of the same materials. No detached
microlites have been noted, such as are common among modern volcanic
ashes; but there are abundant ejected crystals. In these respects
the Cambrian tuffs resemble those of the other Palæozoic systems. A
mingling of grains of quartz-sand may indicate the intermixture of
ordinary with volcanic sediment.
2. They may be divided into two groups--one composed mainly of
fragments of diabase or other similar basic rocks, the other of
felsite. The former group has doubtless been derived from the explosion
of such rocks as the diabase-sheets of the district. The felsitic tuffs
have not been observed to contain any fragments of the microcrystalline
quartz-porphyries of St. David's. They have been derived from true
fine-grained felsites or rhyolites. There are various intermediate
varieties of tuff, due to the mingling in various proportions of the
two kinds of debris.
3. They are marked by the presence of some characteristic features
of the volcanic vents of later Palæozoic time, and in particular
by presenting the following peculiarities: (_a_) lapilli of a
minutely-cellular pumice with spherical cells; (_b_) lapilli with
well-developed flow-structure; (_c_) lapilli consisting of a pale green
serpentinous substance resembling altered palagonite and probably
originally glass; (_d_) lapilli derived from the destruction of older
tuffs; and (_e_) lapilli consisting of ejected crystals, especially of
felspars, sometimes entire, often broken.
4. They frequently show that they have undergone metamorphism, by the
development of a pale greenish micaceous mineral between the lapilli,
the change advancing until the fine tuffs occasionally pass into fine
silky schists.
In my study of the St. David's district, I was unable to observe any
evidence that the basic and siliceous tuffs characterize two distinct
periods of volcanicity. From the foregoing analyses it appears that
some of the oldest visible tuffs which are seen between Pen-maen-melyn
and Pen-y-foel contain only 48·11 per cent of silica; while a specimen
from Porth-lisky yielded 72·63 per cent of that ingredient. Specimens
taken even from adjacent beds show great differences in the percentage
of silica, as may be seen in the analyses Nos. III. and V.
This alternation of basic and siliceous fragmental materials has
its parallel in the neighbouring eruptive rocks, some of which are
olivine-diabases containing only 45 per cent of silica, while others
are highly siliceous quartz-porphyries. But all the siliceous eruptive
rocks, so far as I have been able to discover, are intrusive, and
belong, I believe, to a later period than that of the volcanic group;
in no single instance do they appear to me to be true superficial
lava-flows. On the other hand, the basic eruptive rocks occur both
as contemporaneous sheets and as intrusive masses. The presence of
both siliceous and basic lavas in the Cambrian volcanic reservoirs,
however, is proved by the character of the tuffs. It would appear from
the evidence at present known, that while the basic lavas were most
abundant in the vents during the volcanic period recorded by the rocks
of St. David's, furnishing the material for most of the fragmental
eruptions, and issuing in occasional superficial streams of molten
rock, the siliceous lavas did not flow forth at the surface, though
their debris was copiously discharged in the form of dust and lapilli.
The rise of both basic and acid lavas at different periods in the
same or adjoining vents, so familiar in recent volcanic phenomena,
thus appears to have also characterized some of the oldest examples
of volcanic action. An interesting parallel may be traced between the
succession of events at St. David's and that which occurred in the
volcanic group of the Lower Old Red Sandstone of the Pentland Hills,
near Edinburgh, of which a detailed account will be given in Chapter
xx. of this volume. It is also worthy of remark that in the latest of
the volcanic episodes in British geology a remarkable similarity to the
St. David's volcanic group may be observed. Some of the older Tertiary
agglomerates are full of pieces of acid rocks (felsites, rhyolites or
granophyres), while the lavas poured out at the surface were mainly
basalts.
In the volcanic group of St. David's the tuffs contain evidence that
ordinary sedimentation was not entirely interrupted by the volcanic
discharges. Thus, in the Allan valley, west from the Cathedral, one of
the schistose tuffs is full of well-rounded pebbles of white quartz.
Occasional shaly bands indicate the deposit of mud with the tuffs.
Excluding the granites and porphyries (which are described at p. 155),
two kinds of eruptive rocks are associated with the volcanic group. One
of these is certainly intrusive and of late date, viz. dykes and veins
of diabase, to be afterwards referred to. The other kind occurs in long
parallel sheets, some of which, if not all, are true contemporaneous
lava-streams, erupted at intervals during the accumulation of the
volcanic group. They form prominent crags to the west of St. David's,
such as Clegyr Foig, Rhosson, and the rocky ground rising from the
eastern shores of Ramsey Sound. Their dip and strike coincide with
those of the tuffs above and below them. It is possible that some of
these sheets may be intrusive sills intercalated along the bedding of
the tuffs; and in one or two cases I have observed indications of what,
on further and more careful exploration, may prove to be disruption
across the bedding.
But it is the interbedded sheets that possess the chief interest as
superficial lava-streams of such venerable antiquity. They present many
of the ordinary features of true lava-flows. In particular a slaggy
structure may be detected at the bottom of a sheet, the vesicles being
here and there lengthened in the direction of flow. Some of the sheets
are in part amygdaloidal. The alternation of these sheets with tuffs,
evidently derived from lavas of similar character, is another argument
in favour of their contemporaneous date. One of the best localities for
studying these features lies between Clegyr Foig and the coast, west of
Rhosson.
The eruptive rocks thicken towards the south-west, as if the main
vents had lain in that direction. There are doubtless intrusive as
well as contemporaneously interbedded masses in the rough ground
between Pen-maen-melyn and Treginnis. To separate these out would be
a most interesting and beautiful piece of mapping for any competent
geologist in possession of a good map on a sufficiently large scale.
The interbedded lavas, so far as I have had an opportunity of studying
them, appear to present remarkable uniformity of petrographical
characters. Megascopically they are dull, fine-grained to compact,
sparingly porphyritic, ranging in colour from an epidote-green to
dull blackish-green and dark chocolate-brown. Some of them are finely
porphyritic from the presence of small glistening surfaces which
present the colour and metallic lustre of hæmatite and yield its
characteristic streak. Obviously basic rocks, they present, as I
have said, a close external resemblance to many of the lavas of the
Lower Old Red Sandstone and Carboniferous districts of Scotland. From
their chemical composition and microscopic structure they may be most
appropriately ranged among the diabases. The analyses of two of the
most conspicuous diabases of this class in the district, those of
Rhosson (VII.) and Clegyr Foig (VIII.), by Mr. J. S. Grant Wilson, are
shown in the following table:--
+-----------------------------+---------+--------+
| | VII. | VIII. |
+-----------------------------+---------+--------+
| SiO_{2} | 45·92 | 45·38 |
| Al_{2} O_{3} | 18·16 | 16·62 |
| Fe_{2} O_{3} | 1·18 | 4·06 |
| FeO | 9·27 | 8·63 |
| MnO | 0·19 | 0·14 |
| CaO | 7·19 | 8·19 |
| MgO | 10·07 | 9·41 |
| K_{2}O | 1·78 | 0·71 |
| Na_{2}O | 2·12 | 2·20 |
| H_{2}O and Loss on Ignition.| 4·22 | 4·34 |
| Insoluble Residue. | 0·04 | 0·08 |
| Total. | 100·14 | 99·76 |
| Specific Gravity. | 2·96 | 2.99 |
+-----------------------------+---------+--------+
The two rocks here analyzed, likewise that from the crag south of
Castell and that from the cliffs at the southern end of the promontory
between Ramsey Sound and Pen-y-foel, show under the microscope a
general similarity of composition and structure. They present a
variable quantity of a base, which under a ⅕ objective is resolved
into ill-defined coalescent globulites and fibre-like bodies, that
remain dark when rotated between crossed nicols. In some varieties,
as in part of Rhosson Crag, the base is nearly lost in the crowd
of crystalline constituents; in others, as in the crag south of
Castell, it forms a large part of the whole mass, and may be seen in
distinct spaces free from any crystalline particles. Through this
base are diffused, in vast numbers, irregularly-shaped grains of
augite, seldom showing idiomorphic forms. These grains, or granules,
may perhaps average about 0·003 inch in diameter. Plagioclase is
generally hardly to be recognized, though here and there a crystal
with characteristic twinning may be detected in the base. Magnetite
occurs abundantly--its minute octahedra, with their peculiar colour and
lustre, being apparent with reflected light on the fresher specimens,
though apt to be lost as diffused ferruginous blotches in the more
decomposed varieties. But perhaps the most remarkable ingredient is
olivine. Red hæmatitic crystals are visible, even to the naked eye,
dispersed through the groundmass of the rocks. With a lens these
may be observed to be orthorhombic in form, and to be evidently
pseudomorphs after some mineral which has been converted chiefly
into hæmatite. Such red pseudomorphs are common in Carboniferous and
Old Red Sandstone lavas, where in some cases they appear to be after
hornblende, and in others after augite, but occasionally are suggestive
of olivine, though with no trace of the original substance of that
mineral. In the lava associated with the tuffs at the south end of
the promontory between Ramsey Sound and Pen-y-foel, however, certain
large, well-developed pseudomorphs are undoubtedly after olivine.
They have the characteristic contour of that mineral and its peculiar
transverse curved and irregular fractures. The average length of these
pseudomorphs was found, from the measurement of six examples, to be
0·023 inch, the largest being 0·034, and the smallest 0·014. Seen by
transmitted light they present a structureless pale-green material
nearly inert in polarized light, round the borders and across fissures
in which an opaque substance has been developed, as serpentine and
magnetite have been grouped in the familiar alteration of olivine.
The opaque material is bright brick-red in reflected light, and is
evidently now chiefly oxidized into hæmatite. Every stage may be
traced, from orthorhombic forms with the incipient development of
transverse fissures filled with iron-oxide, to others of distorted
shapes in which the ferruginous matter occupies the whole, or nearly
the whole, of the mould of the original crystal.
The rocks now described differ from the Palæozoic andesites or
"porphyrites," with which I am acquainted, in their more basic
composition, in the less abundance of their microscopic base, in the
comparatively inconspicuous development of their felspars of later
consolidation, in the absence of large porphyritic felspars of earlier
consolidation, in the extraordinary prominence of the granular augite,
and in the presence of olivine. In composition and structure they are
essentially forms of olivine-diabase.
[Illustration:
Fig. 41.--Section showing the interstratification of tuff and
conglomerate above Lower Mill, St. David's.
]
Above the volcanic group of St. David's lies a band of
quartz-conglomerate which has been taken by Dr. Hicks to mark the base
of the Cambrian system. This rock, though mainly composed of quartz
and quartzite, contains fragments of the underlying volcanic rocks.
But that it does not mark any decided break in the sedimentation, much
less the violent unconformability and vast interval of time which
it has been erroneously supposed to do, is well illustrated by the
occurrence of bands of tuff, as well as diffused volcanic dust, in
the conglomerate and also in the green and red shales and sandstones
which conformably overlie it. An example of this intercalation
of volcanic material is represented in Fig. 41. On the left side
vertical layers of fine reddish tuff (_a_) are succeeded by a band
of quartz conglomerate (_b_) of the usual character. Parallel to this
conglomerate comes a band, about six inches thick, of fine tuff (_c_),
followed by ashy sandstone (_d_), which graduates into conglomerate
(_e_). No more complete evidence could be desired of the perfect
inosculation of the conglomerate with the volcanic group. On the coast
at Nun's Chapel similar evidence presents itself. The conglomerate
there contains some thin seams of tuff, and is intercalated between a
series of dull green agglomerates and tuffs and some greenish shales
and sandstones with layers of tuff.
Not less striking is the evidence of the contemporaneous eruption of
fine volcanic dust in the overlying shales and sandstones.[88] Some
of the red shales are full of this material, which here and there
is gathered into the thin seams or ribs of which the microscopic
characters have already been described. This diffused volcanic detritus
marks, no doubt, the enfeebled discharges of fine dust towards the
close of the volcanic episode in the Lower Cambrian period at St.
David's. It would be difficult to find an instance of a more perfect
transition from a series of thoroughly volcanic masses into a series of
ordinary mechanical sediments.
[Footnote 88: These are a portion of Dr. Hicks' "Caerfai group" in the
Lower Cambrian series. They have yielded Lower Cambrian fossils.]
2. _Intrusive Granite and Quartz-Porphyry._--The heart of the
volcanic group is pierced by a mass of granite which also cuts the
conglomerate and overlying shales and sandstones on the east side.
The age of this intrusive boss cannot be more definitely fixed than
by saying that it must be later than the volcanic group. This rock
has been the subject of a remarkable amount of description, and has
been dignified by being actually elevated into a distinct Archæan
"formation" composed of "highly crystalline gneissic rocks," with
"bands of limestone, hornblende, chlorite, and micaceous schists."[89]
Into this somewhat dreary chapter of English geological literature
it is fortunately not necessary to enter here. I will only say that
the rock is unquestionably a granite, with no essential differences
from many other bosses regarding which there has been no controversy.
It is a holocrystalline rock with a thoroughly granitic texture, and
composed of the ordinary minerals of granite--quartz, orthoclase
and plagioclase, among which a green chloritic mineral, more or
less plentiful, probably represents original hornblende, biotite or
augite. Sometimes the quartz and felspar in the body of the rock show
a micropegmatitic arrangement, and the same structure occurs in veins
that traverse it. This structure gives the rock some resemblance to the
Tertiary granites and granophyres of Ireland and Scotland.
[Footnote 89: See the papers cited on p. 145 and my discussion of the
relations of this granite in _Quart. Journ. Geol. Soc._ vol. xxxix.;
also Prof. Lloyd Morgan, _op. cit._ vol. xlvi. (1890).]
This granite has undergone a good deal of decomposition, for its
felspars are turbid, and its original ferro-magnesian constituents are
always replaced by green chloritic aggregates, while epidote is also
present. The rock tends to become finer in grain towards the margin,
and then sometimes assumes a more decidedly pegmatitic structure,
like graphic granite. At the northern end of the granite ridge, a
gradation can be traced from the ordinary texture through increasingly
fine-grained varieties until we pass into a microcrystalline
spherulitic porphyry. After a careful examination of the ground I
satisfied myself that the spherulitic quartz-porphyries, which form a
conspicuous feature in the geology of St. David's, are really offshoots
from this granitic core.[90]
[Footnote 90: These apophyses from the granite constitute the
"Arvonian" formation of Dr. Hicks' pre-Cambrian series of St. David's.]
These spherulitic rocks have been fully described.[91] They consist
of a base composed of a microcrystalline aggregate of quartz and
orthoclase, which is distributed between the spherulites. These
have been developed in remarkable beauty and perfection. While the
microcrystalline structure is everywhere recognizable, the spherulites
occasionally disappear. But their absence is merely local, and they may
be found both in large dykes and narrow veins. A further porphyritic
structure is given to the rocks by the presence in them of abundant
quartz, which takes the form of conspicuous rounded blebs or worn
crystals sometimes distinctly dihexihedral, but with somewhat blunted
angles. Porphyritic plagioclase is also common. Flow-structure is
occasionally traceable. Some parts of these rocks where the porphyritic
elements are locally absent might be cursorily classed as felsites;
but they all possess a microcrystalline and not a felsitic base. They
cannot be confounded with the true felsites of which fragments occur in
the tuffs.
[Footnote 91: See, for example, J. Davies, _Quart. Journ. Geol. Soc._
vol. xxiv. p. 164, xxxv. p. 203; also the paper already referred to,
_op. cit._ xxxix. p. 315; and Mr. Teall's _British Petrography_, p.
334.]
In addition to the parallelism that may be traced between the earliest
Palæozoic agglomerates and those of the youngest volcanic series
of Britain, a close analogy may also be noticed between the acid
intrusive rocks of the two widely-separated periods. In both cases we
have a granitic core sending out apophyses which assume a spherulitic
structure and traverse earlier volcanic products of the district.
These spherulitic quartz-porphyries of St. David's occur as bosses,
dykes (elvans) or veins, cutting through all horizons of the volcanic
group, and in one case apparently, if not actually, reaching the quartz
conglomerate. One of the best exposures of this intrusive character may
be seen in the cliff below Nun's Chapel, where the elvan runs along the
face of the cliff through the uppermost zone of the volcanic group,
cutting the strata somewhat irregularly. Apparently in connection with
this dyke, a network of intrusions of decomposed quartz-porphyry may be
observed in the shales along the face of the cliff immediately below
Nun's Chapel. On the whole, the intruded material has forced its way
along the bedding-planes of the shales, but has also broken across
them, sending out finger-like branches.
3. _Diabase Dykes and Sills._--The latest rocks of the St. David's
district are dykes and intrusive sheets of diabase, which traverse
all the other formations. The dykes are specially abundant in the
granite. One or two may be detected in almost every artificial opening
which has been made in that rock; while on the coast-section they are
here and there profusely abundant. They are likewise frequent in the
quartz-porphyries, and occur also in the volcanic group as well as in
the sandstones and shales above the conglomerate, but become fewer in
number as they recede from the granite centre.[92]
[Footnote 92: The occurrence of these dykes is paralleled by that of
the similar intrusions in the quartz-felsite of Llyn Padarn to be
afterwards described.]
In external characters, the rock composing these dykes and sheets may
be described as usually a dull dirty-green or yellowish-brown mass, to
which the old name of "wacke" might appropriately be given. It exhibits
the texture and mode of weathering of the more distinctly crystalline
members of the basalt family. It is occasionally amygdaloidal or
cellular, the kernels or cavities being arranged parallel with the
sides of the dyke. Here and there a rudely prismatic structure extends
between the walls.
The microscopic structure of this rock has been described by Professor
Judd, Mr. Davies and Mr. Tawney. It is a diabase, but more allied in
structure to true basalt than the olivine-diabase of the volcanic
group. It especially differs from the older rock in the abundance and
freshness of its felspars, in the comparative scarcity of its augite,
and in the absence of olivine. The magnesian silicates are very
generally replaced by green decomposition-products diffused through
the mass. An occasional crystal of hornblende, recognizable by its
cleavages and dichroism, may be detected. Some of the diabase dykes
present excellent examples of flow-structure. A beautiful instance
occurs in a dyke that cuts the shales, in a small cove to the east of
Nun's Chapel. The shale and eruptive rock are in contact; and the small
acicular prisms of felspar, besides ranging themselves in line parallel
to the side of the dyke, stream round the larger felspar crystals.
Some of the dykes or veins are only three inches broad. They send
out fingers, and sometimes break abruptly across from one line to
another. They appear generally to have followed the lines of joint in
the granite, as Mr. Tawney has observed;[93] consequently they must be
posterior to the development of the system of jointing in that rock.
[Footnote 93: _Proc. Nat. Hist. Soc. Bristol_, vol. ii. part ii.
(1879), p. 115.]
Besides the abundant dykes, there has been a more limited extrusion
of the same material in sheets parallel (or approximately so) to the
bedding of the sandstones and shales. These sheets are well displayed
at St. John's Point, where evidence of their being intrusive, and not
truly bedded, may be seen along the fine cliffs which have been cut by
the waves on this part of the coast-line.
The sedimentary series which overlies the volcanic group of St.
David's, and contains the fossils of the lower part of the Cambrian
system, gradually loses all trace of volcanic material, as its members
are followed upward in stratigraphical order.[94] We thus learn that
the eruptions of this district came to an end in an early part of the
Cambrian period. But as we shall see in the following pages, volcanic
activity was subsequently renewed at no great distance in the next or
Silurian period.
[Footnote 94: Dr. Hicks has noted the occurrence of "volcanic tuff"
in the Lower Lingula Flags of Porth-y-Rhaw, a little to the east of
St. David's (_Quart. Journ. Geol. Soc._ vol. xx. 1864, p. 240). This
intercalation is marked as a "dyke" in the MS. notes of Sir A. C.
Ramsay on a copy of the Geological Survey map of the district.]
CHAPTER XI
THE CAMBRIAN VOLCANOES OF NORTH WALES, THE MALVERN HILLS AND
WARWICKSHIRE
NORTH WALES
The Cambrian volcanic rocks in the northern part of the Welsh
Principality have their main development in Caernarvonshire. Southwards
from that tract, though the Lower Cambrian strata form a vast pile of
sedimentary material in the Harlech anticline, which is estimated by
the Geological Survey to be from 6000 to 7000 feet thick, they have
yielded no trace of any contemporaneous volcanic rocks.[95] The purple
slates that rise along the centre of the anticline dip below the grits
and conglomerates on either side without disclosing a glimpse of the
base of the system. This enormous accumulation of sedimentary deposits
seems to diminish in thickness as it is traced northwards, for towards
the Menai Strait it does not reach more than a fourth part of the depth
which it is said to display in the Harlech anticline.[96] In the Pass
of Llanberis the series of grits that overlies the purple slates is
estimated to be about 1300 feet thick.[97] This gradual thinning away
of the Cambrian series towards the north was, in the opinion of Sir
Andrew Ramsay, accompanied by an increasing metamorphism of the lower
portions of the system. In his view, the long ridge of quartz-porphyry
which crosses the lower end of Llyn Padarn represents the result of
the extreme alteration of the stratified rocks. He believed that
he could trace an insensible passage from the slates, grits and
conglomerates into the porphyry, and he was led to the "conviction that
the solid porphyry itself is nothing but the result of the alteration
of the stratified masses carried a stage further than the stage of
porcellanite, into the condition of that kind of absolute fusion that
in many other regions seems to have resulted in the formation of
granites, syenites and other rocks, commonly called intrusive."[98]
Certain structural lines in the quartz-porphyry he looked upon
as indicating "traces of stratification in a rock, the original
felspathic and quartzose material of which has been metamorphosed into
true porphyry."[99] In conformity with these ideas, the remarkable
felspathic strata which lie nearest the porphyry were regarded as
metamorphosed Cambrian rocks, and where similar rocks reappear over
a large area near Bangor they were coloured on the map with the same
tint and lettering as were used for the so-called "altered Cambrian" of
Anglesey.
[Footnote 95: _Mem. Geol. Surv._ vol. iii. 2nd edit. "Geology of North
Wales," p. 21. It is possible that this thickness has been somewhat
overestimated. Dr. Hicks (_Geol. Mag._ 1880, p. 519) has referred to
certain "highly felsitic rocks, for the most part a metamorphic series
of schists, alternating with harder felsitic bands, probably originally
felsitic ashes," lying at the bottom of the whole pile, and he has
claimed them as pre-Cambrian. But I have not found any evidence of such
rocks, nor any trace of igneous materials save dykes and sills, acid
and basic, such as are indicated on the Survey map.]
[Footnote 96: _Ibid._ p. 24.]
[Footnote 97: _Ibid._ p. 173.]
[Footnote 98: _Mem. Geol. Surv._ vol. iii. 2nd edit. p. 173.]
[Footnote 99: _Ibid._ p. 174.]
No one who has examined this Caernarvonshire ground can have failed
to find the sections which doubtless led my predecessor to form the
convictions to which he gave expression in the passages I have just
quoted. It is easy to see how these sections, wherein it is certainly
difficult to draw a sharp line between the igneous rock and the clastic
materials derived from it, would be welcomed as appearing to offer
confirmation of the ideas concerning metamorphism which were then in
vogue. There cannot, however, be any doubt that my friend was mistaken
in his interpretation of the structure of that part of the country. It
is to me a subject of keen regret that in his later years, when the
subject was revived, he was no longer able to re-examine this ground
himself, for no one would have confessed more frankly his error, and
done more ample justice to those who, coming after him, have been able
in some parts to correct his work.
The quartz-porphyry, felsite or rhyolite of Llyn Padarn, as well as
that of Llandeiniolen, is not a metamorphic but an eruptive rock, as
has been demonstrated by Professors Hughes and Bonney. There is no
true passage of the sedimentary rocks into it; on the contrary, the
conglomerates which abut against it are in great part made out of its
fragments, so that it was already in existence before these Cambrian
strata were deposited upon it. These conclusions must be regarded
as wholly indisputable. But most of the critics of the work of the
Geological Survey have proceeded to certain further deductions. They
have maintained that the presence of fragments of the porphyry in the
overlying conglomerate marks an unconformability between the two rocks,
that the conglomerate shows the base of the Cambrian system, and that
the porphyry is therefore pre-Cambrian.
These assertions and inferences do not seem to me to be warranted. They
have, in my judgment, been disproved by Mr. Blake,[100] who shows that
there is no break in the Cambrian series, that the various porphyries
and their accompaniments are parts of that series, and that there is no
certain proof of the existence of any pre-Cambrian rocks in the whole
district.[101]
[Footnote 100: In an excellent memoir read before the Geological
Society in 1888, with the main conclusions of which I agree.]
[Footnote 101: _Quart. Journ. Geol. Soc._ vol. xliv. p. 271. For
subsequent papers by Mr. Blake, see _op. cit._ vols. xlviii. (1892) p.
243, xlix. (1893) p. 441.]
That the igneous rocks of the Llyn Padarn area mark a volcanic period
has been recognized by most writers since Professor Bonney pointed
out the flow-structure of the quartz-porphyry, and other proofs of
active volcanic eruptions have been traced by him, as well as by
Professor Hughes and Mr. Blake, in the stratified rocks which stretch
north-eastwards to Bangor. The extent and persistence of these ancient
volcanic phenomena, and their probable connection with the remarkable
northward attenuation of the Cambrian sedimentary rocks, deserve
special attention.
It is generally agreed that the rocks variously termed
quartz-porphyries, felsites or rhyolites form the oldest members
of this volcanic series.[102] They come to the surface in two long
ridges, one running from Caernarvon to near Bangor, the other from
Llanllyfni to Ann's Chapel, at the mouth of Nant Francon (Map. II.).
Whether the materials of these two ridges are parts of one originally
continuous sheet or group of sheets, or, if different protrusions,
whether they belong to the same geological horizon, or whether, as Mr.
Blake believes, they are distinct masses, separated by a considerable
thickness of detrital material, cannot in the present state of our
knowledge be positively decided. It seems to me probable that they are
connected underground, as a continuous platform beneath the overlying
pyroclastic materials.
[Footnote 102: Whether the granitic rock of Twt Hill, Caernarvon,
is connected with the porphyry or belongs to an older eruption is
immaterial for my present purpose.]
These acid rocks have been regarded by some observers as intrusive
sheets, by others as lava-streams that were poured out at the surface.
If account be taken simply of their petrographical characters, they
find their nearest analogies among the intrusive quartz-porphyries
of older geological periods. The presence of flow-structure in them
has been thought to indicate that they were superficial streams. But
this structure may be found in dykes and intrusive sheets as perfectly
as in lava-flows, so that it cannot by itself be taken as proof of
a surface-discharge of lava. It must be confessed that, both in the
main mass of quartz-porphyry and in the abundant fragments of it in
the overlying conglomerates and breccias, there is an absence of such
scoriform portions as one would naturally look for in a superficial
lava-stream;[103] while, on the other hand, the rock generally presents
the tolerably uniform flinty texture so familiar in intrusive sheets of
similar material.[104] My own impression is that these igneous masses
were probably erupted to the surface as long banks which rose above the
waves; that they were thus exposed to prolonged subærial and marine
denudation; that by this means any upper more cellular portions of
the lava which may have existed were broken up and pounded down into
detritus, and thus that what is now visible is a part of the eruptive
rock which originally lay at some depth within its body. This view
is confirmed by a study of other lavas which are found on different
platforms in the detrital deposits that overlie the Llyn Padarn
quartz-porphyry.
[Footnote 103: But the Llyn Padarn rock, like many acid lavas, may
never have possessed a vesicular structure in any portion of its mass.
The sheets of felsite which occur among the overlying tuffs are not
cellular, but present the closest resemblance to the main mass below.]
[Footnote 104: Mr. Blake brought forward the evidence of a section on
the north or under side of the Llyn Padarn ridge to show that the rock
has there been intruded into the Cambrian strata (_Quart. Journ. Geol.
Soc._ vol. xliv. (1888), p. 283). But the rock supposed by him to be
altered slate has been shown to be intrusive "greenstone" (Miss Raisin,
_op. cit._ vol. xlvii. (1891), p. 336).]
That the material of each of the two main ridges is the result of more
than one eruption has been inferred from the supposed intercalation
of bands of slate and of breccia in the rock.[105] Considerable
lithological differences may be detected in each mass, but they are not
greater than may be observed in single sills and bosses. In some parts
of the Llyn Padarn porphyry a distinct nodular structure appears which
shades off into bands and lenticular streaks, reminding one of the
characters of some of the Bala rhyolites. Other portions are markedly
brecciated, the separated fragments being surrounded in a matrix of
the rock, which shows flow-structure sweeping past them. On Moel Gronw
angular fragments of a dark pinkish tint are scattered through the
general mass. Again, some parts are crowded with quartz-grains, while
others are comparatively free of these, and occasionally a spherulitic
structure has been observed.[106]
[Footnote 105: See for example, Prof. Bonney, _Quart. Journ. Geol.
Soc._ vol. xxxv. (1879), p. 312; Mr. Blake, _op. cit._ vol. xliv.
(1888), pp. 277, 287. But some at least of the supposed "slates," as
stated in a previous footnote, have been since shown to be dykes.]
[Footnote 106: Mr. Blake, _ibid._ p. 277.]
The microscopic structure of this ancient eruptive rock has been
studied by Professor Bonney, who found that the general type was a
compact dull grey felsite, with porphyritic crystals of felspar and
grains of quartz, closely resembling some modern rhyolites. Though
unable to detect any actual glass in the base, he had no doubt that the
rock was originally vitreous, and he found abundant and fresh examples
of the most perfect flow-structure.[107]
[Footnote 107: _Op. cit._ vol. xxxv. p. 312.]
[Illustration: Fig. 42.--Basic dyke traversing quartz-porphyry and
converted into a kind of slate by cleavage. West side of Llyn Padarn.
_p_ _p_, quartz-porphyry; _d_ _d_, dyke and connected veins.]
Reference may be made here to the remarkable influence of the intense
cleavage of the district upon this rock.[108] Along its southern
margin, where it has been exposed to pressure from the south-east, the
quartz-porphyry has been so crushed that it passes here and there into
a fine unctuous slate or almost a schist. Nowhere can this change be
more clearly seen than on the slopes of Mynydd y Cilgwyn. The cleavage
planes strike about N. 40° E., with an inclination to dip towards
the N.W. Within a space of a few yards a series of specimens may be
collected showing at one end an ordinary or only slightly-sheared
quartz-porphyry with abundant quartz-blebs, and at the other a fine
greenish sericitic slate or phyllite, wherein the quartz has been
almost entirely crushed down. Lines of shearing may be detected across
the breadth of the porphyry ridge, each of them coinciding with
the prevalent trend of the cleavage. Sometimes also certain basic
dykes, which traverse the porphyry in some numbers, have undergone
considerable deformation from the same cause. Their thinner portions
are so well cleaved that they have been mistaken for included bands
of green slate (Fig. 42). But these cleaved branches may sometimes be
traced into a thicker and more solid dyke, whose uncrushed cores still
preserve the original character of the rock and prove it to be eruptive.
[Footnote 108: The secondary planes due to cleavage must not be
confounded with the original flow-structure.]
[Illustration: Fig. 43.--Section of well-cleaved tuff, grit and breccia
passing up into rudely-cleaved conglomerate and well-bedded cleaved
fine conglomerate and grit. East side of Llyn Padarn.]
The rocks which succeed the porphyry in the Valley of Llanberis are
of great interest, for they contain abundant proof of contemporaneous
volcanic activity, and they show that, so far from there being
any marked hiatus here, there is evidence of the persistence
of eruptions even into the time of the Llanberis Slates.[109]
Considerable misapprehension has arisen from the attempt to make one
of the conglomerates the base of the Cambrian series, and the real
significance of the volcanic detrital strata in association with it
was consequently missed. The conglomerate does not lie on one definite
horizon. In truth, there are several zones of conglomerate, each with
some difference of composition, thickness or extent.[110] These may
be well studied both on the south and the north side of the porphyry
ridge at the lower end of Llyn Padarn. They are intercalated among
fine tuffs, grits, volcanic breccias and purple slates, sometimes full
of fine ashy material. On the south-east side of the ridge, where the
rocks have suffered intense cleavage, they assume a fissile unctuous
character, and then resemble parts of the cleaved Cambrian tuffs at
St. David's. But on the north-west side, where they have in large
measure escaped the effects of the cleavage-movements, their original
structures are well preserved.
[Footnote 109: The sections in the Vale of Llanberis on either side
of Llyn Padarn have been again and again described and fought over.
Some of the papers are cited in the following pages, but it would
be impossible in this volume to find room for a full discussion of
the differences of opinion. What is stated in the text is the result
of my own study of the rocks on the ground, coupled with a careful
consideration of the work of other observers.]
[Footnote 110: I can find no evidence of unconformability beneath any
of the conglomerates. The section described by Professor Green, _Quart.
Journ. Geol. Soc._ vol. xli. (1885), p. 74, merely shows the difference
between the effects of cleavage on the fine tuffs and the more massive
resisting conglomerate which overlies them. This section is represented
in Fig. 43. At first sight the conglomerate appears to be lying on the
vertical edges of an older group of slates, but any one acquainted with
cleavage can trace this structure from the tuffs into the conglomerate
and resuming its course again in the finer sediments above. The whole
series of deposits in the section is continuous and conformable.
The section on the slate railway has deceived Mr. Blake as well as
Professor Green (_Quart. Journ. Geol. Soc._ vol. xlix. (1893), p. 445).
The correct interpretation is given by Professor Bonney and Miss Raisin
(_op. cit._ vol. l. p. 592).]
One of the first features of these detrital deposits to arrest
attention is the amount and variety of the fragments of igneous rocks
in them. Some of the conglomerates, though enclosing pebbles of quartz,
quartzite, granite and other rocks not found _in situ_ in the immediate
district, are mainly composed of the debris of the quartz-porphyry of
the ridge. Indeed, this latter material appears to have contributed a
large proportion of the detritus of which the general body of strata
here is made up. But there are to be noticed among the contents of
the conglomerates and breccias pieces of many volcanic rocks not
to be found on the porphyry ridge. Among these, besides felsites
showing sometimes beautiful flow-structure (rhyolites) and various
quartz-porphyries, there occur abundant fragments of less acid lavas
(andesites) and pieces of older tuffs. Some of the fragmental rocks are
green in colour, probably from the abundance of fine basic volcanic
dust in them. Certain bands are full of large angular pieces of shale,
similar in character to the Cambrian slates, and doubtless due to the
disruption of pre-existing Cambrian strata by volcanic explosions. It
is clear that from vents in this neighbourhood there continued to be
an abundant discharge of dust and various andesitic and other lapilli,
which, falling on the sea-floor, mingled there with the ordinary
mechanical sediment that was being deposited at the time.[111]
[Footnote 111: On the composition of the conglomerates or breccias, see
Professor Bonney and Miss Raisin, _Quart. Jour. Geol. Soc._ vol. l.
(1894), p. 598.]
[Illustration: Fig. 44.--Section of Clegyr on the north-east side of
Llyn Padarn, near the lower end.]
But we have evidence that, during the period when these showers of
volcanic detritus were thrown out, streams of lava, though on a greatly
diminished scale, continued to be poured forth. The hill of Clegyr
(Fig. 44), near the lower end of Llyn Padarn, on the north-east side,
consists mainly of cleaved tuffs (_t_) and slates with conglomerates
(_c_), overlying the quartz-porphyry (_p_). Near the summit a band of
felsite is intercalated in these rocks.
Still more striking are the sections on the south-west side of the
lake.[112] Starting from the porphyry of the ridge, we cross a zone of
conglomerate and grit largely composed of the debris of the porphyry,
until we reach a band of felsite or quartz-porphyry, which at its
eastern end is about ten feet thick, while it seems to increase in
dimensions westwards.[113] In the centre the rock is dark purplish-red,
exceedingly compact or flinty, sprinkled with a variable proportion of
quartz-blebs and felspar crystals. Towards its southern or upper edge
(for the rocks, though nearly vertical, dip southwards) it has been
cleaved into a variety of purple slate, and would there at once be
classed among the ordinary slates of the neighbourhood. But the fissile
character is merely a marginal structure which the rock shares with
the highly-cleaved tuffs that follow it. Traced westwards, this bed is
found to enclose a core of quartziferous porphyry, which, though it has
escaped from the most complete results of crushing, is nevertheless
cleaved along its margin as well as partially in its interior. It would
not be possible to distinguish parts of this intercalated less crushed
core from portions of the porphyry of the main ridge. The difference of
colour does not count for much, for even in this core the purple tint
gives place to greenish grey, and what in the centre at the east end
is a solid dark purplish-red felsite passes westward into a greenish
slate, like that already noticed on Mynydd y Cilgwyn.
[Footnote 112: For various readings of these sections, compare Mr.
Blake (_Quart. Jour. Geol. Soc._ vol. xlix. (1893), p. 450) with
Professor Bonney and Miss Raisin (_op. cit._ vol. l. (1894), p. 581).]
[Footnote 113: See Professor Bonney and Miss Raisin, _op. cit._ p. 593
_et seq._]
The microscopical examination of this rock shows it to be a true
felsite of the rhyolitic type, which in the central uncleaved part
exhibits a wavy flow-structure like that found in the quartz-porphyry
of the ridge. So intense has been the cleavage in its upper part that
the original structure of the rock is there effaced. The immediately
overlying tuffs, which are likewise so thoroughly cleaved that it
is not easy to draw a sharp and continuous line between them and
the intercalated lava, precisely resemble those found below the
conglomerate on the opposite side of the lake. They include bands of
coarse volcanic breccia as well as fine compact material, showing the
varying intensity of the volcanic discharges. Their included stones
consist of various felsites, andesites and slates.[114]
[Footnote 114: I see no reason to doubt that the less acid igneous
fragments were ejected during the closing phases of volcanic action,
even though no such rocks have been found at the surface _in situ_.
We must remember how frequently mixtures of acid and basic materials
are to be found in the same continuous series of volcanic ejections
and even in the same vent, of which illustration will be given in
subsequent pages. Nor should we forget what a mere fragment of a
volcanic group is exposed at the surface in the Llanberis district. See
Professor Bonney and Miss Raisin, _op. cit._ p. 596, _footnote_.]
The thin sheet of interstratified quartz-porphyry here described is
not the only one to be found in the section. Others thinner and more
intensely cleaved lie among the tuffs higher up. They have been sheared
into mere pale unctous slates, but the remains of their quartz-blebs
may still be detected in them.
The tuffs, with their interstratified bands of porphyry, become more
and more mingled with ordinary argillaceous and sandy sediment as they
are followed in upward succession. Among them occur bands of grit and
fine conglomerate containing pebbles of porphyry and pieces of slate.
Some of these grits are mainly composed of white felspar, felsite and
clear grains of quartz, evidently derived from the disintegration of a
rock like the porphyry of the main ridge. As the ordinary sediment of
the Llanberis group sets in, the tuffs are restricted to thinner and
more widely-separated bands. Some thin layers of felspathic breccia,
seen among the slates close to the Glyn Peris Hotel, probably mark the
last discharges of the slowly-expiring vents of this region. Here,
as at St. David's, from the most ancient of our volcanic records,
striking evidence is furnished of the gradual extinction of volcanic
action. Through many hundreds of feet of strata which now supervene,
representing the closing ages of the Cambrian and the earlier ages of
the Silurian period, no trace of volcanic material has been found in
this district until we reach the Bala lavas and agglomerates of Snowdon
and the Pass of Llanberis.
In the neighbourhood of Bangor another area of similar rocks
wraps round the northern end of the western porphyry ridge. The
Geological Survey map, in conformity with the ideas that governed its
representation of the older rocks of Anglesey and Caernarvon, colours
these as altered Cambrian. That this error should have been made, or,
when made, should not have been speedily corrected, is all the more
surprising when we consider the thorough mastery which the surveyors
had acquired of the aspects and the interpretation of ancient volcanic
rocks in Wales, and when, moreover, we remember that as far back
as 1843, long before the Survey of Caernarvonshire was published,
Sedgwick had pointed out the true volcanic nature of the rocks. That
great pioneer recognized the presence of "trappean conglomerates" and
"trappean shales (Schaalstein)" among these deposits at Bangor; but
he could not separate them from the Cambrian series of the rest of
Wales.[115] And in his section he represents them as undulating towards
the east and passing under the great mass of the Caernarvonshire slates
and porphyries.
[Footnote 115: _Proc. Geol. Soc._ vol. iv. p. 212; _Quart. Journ. Geol.
Soc._ vol. iii. (1847), p. 136.]
This interpretation, which I believe to be essentially accurate, was
modified by Professor Hughes, who, fixing on a conglomerate as the base
of the Cambrian system, regarded all the rocks below it, or what he
termed his "Bangor group," as pre-Cambrian.[116] He has been followed
in this view by subsequent writers;[117] but Mr. Blake has argued that
here, as in the Llanberis district, there is no evidence to separate
the volcanic detrital deposits above the porphyry from the Cambrian
system.[118]
[Footnote 116: _Quart. Journ. Geol. Soc._ vol. xxxiv. (1878), p. 137.]
[Footnote 117: Prof. Bonney, _op. cit._ vol. xxxv. (1879), p. 316; Dr.
Hicks, _ibid._ p. 296.]
[Footnote 118: _Op. cit._ vol. xliv. (1888), p. 278.]
A little southward from Bangor the quartz-porphyry is overlain by
a most interesting group of fragmental rocks, the "Bangor group"
of Professor Hughes. Largely of volcanic origin, they must be some
hundreds of feet thick, and pass under the dark shales and grits
of the Lower Silurian (Arenig) series. Some of the most persistent
bands among them are conglomerates, which differ from each other in
composition, but most of which consist largely of fragments of various
igneous rocks. Some of the coarser masses might be termed agglomerates,
for they show little or no trace of bedding, and are essentially made
up of blocks of volcanic material. There are abundant beds of grit,
sometimes pebbly or finely conglomeratic, alternating with tuffs and
with bands of more ordinary sediment. Courses of purple shale and
sandstone, green shale and dark grey sandy shale occasionally occur
to mark pauses in the volcanic explosions. Perhaps the most striking
feature in the pyroclastic materials is the great abundance of very
fine compact pale tuffs (hälleflintas of some writers), sometimes
thinly laminated, sometimes occurring in ribbon-like bands, each of
which presents internally a close-grained, almost felsitic or flinty
texture.[119]
[Footnote 119: The occurrence of flinty or cherty deposits, in
association with volcanic rocks of Lower Silurian age, is well
established in Britain, and will be more particularly referred to in
the sequel.]
A cursory examination of the contents of the conglomerates, breccias
and grits shows them to consist largely of different felsites, with
fragments of more basic lavas. Some of these might obviously have been
derived from the rock of the porphyry ridge, but, as at Llyn Padarn,
there is a far greater variety of material than can be found in that
ridge. Some of the fragments show perfect flow-structure. Professor
Bonney has described the microscopic characters of some of these
fragments, and has especially remarked upon their glassy character.
Among the slides prepared from specimens collected by myself, besides
the abundant fragments of felsite (rhyolite), there are also numerous
pieces of different andesitic lavas and fine tuffs, as well as grains
of quartz and felspar, and sometimes a good deal of granular iron-ore.
That a large proportion of the material of the so-called "Bangor beds"
was directly derived from volcanic explosions can hardly be doubted.
There appears to have been a prolonged succession of eruptions, varying
in intensity, and somewhat also in the nature as well as in the
relative fineness of the material discharged. On the one hand, coarse
massive agglomerates were probably accumulated not far from the active
vents, as the result of more violent or transient explosions; on the
other hand, exceedingly fine and well-stratified tuffs, which attain
a great thickness, serve to indicate a phase of eruptivity marked by
the long-continued discharge of fine volcanic dust. Ordinary sediment
was doubtless drifted over the sea-bottom in this district during
the volcanic episode, but the comparative infrequence of distinct
interstratifications of shale or sandstone may be taken to imply that
as a rule the pauses between the eruptions were not long enough to
allow any considerable accumulation of sand or mud to take place.
No satisfactory proof has yet been obtained of any interstratified
lavas among the tuffs of the Bangor district. Some rocks, indeed, can
be seen on the road between the George Hotel and Hendrewen, which, if
there were better exposures, might possibly furnish the required proof;
but at present little can be made of them, for their relations to the
surrounding rocks are everywhere concealed.
From what I have now adduced, it is obvious that while both felsitic
and andesitic lavas existed within the volcanic foci, and were ejected
in fragments to form the tuffs and breccias, the lavas poured out at
the surface during the Cambrian period in Caernarvonshire were mainly,
if not entirely, felsites (rhyolites) in which the chief porphyritic
constituent was quartz. These lavas thus stand entirely by themselves
in the volcanic history of Wales. Though felsites of various types were
afterwards poured out, nothing of the same quartziferous kind, so far
as we yet know, ever again appeared. Further south, in Merionethshire,
as will be shown in Chapter xii., the Cambrian volcanic eruptions
appear to have been on the whole less acid, and to have begun with the
outpouring of andesitic lavas.
I have now to consider the relation of the volcanic group of Bangor to
the strata which overlie it. The geological horizon of these strata
is not, perhaps, very definitely fixed. It may be Arenig, possibly
even older. But for my present purpose it will be sufficient to
consider the strata in question as lying at the bottom of the Lower
Silurian series. Professors Hughes and Bonney have taken as their base
a marked but impersistent band of conglomerate. Mr. Blake, however,
has more recently shown that, as this band is succeeded by tuffs like
those below it, it cannot be claimed as marking the upper limit of
the volcanic group. He therefore classes it in that group and traces
what he thinks is an overlap or unconformability at the bottom of the
Lower Silurian strata to the east. Mr. B. N. Peach, who accompanied
me in an examination of this ground, agreed with me in confirming Mr.
Blake's observation as to the position of the conglomerate, which is
undoubtedly overlain by the same flinty felsitic tuffs as are found
below it. But we were unable to trace any unconformability. According
to the numerous observations which we made, there does not seem to
be any discordance in strike or dip between the flinty tuffs and the
overlying shales and grits. The two groups of rock appeared to us to be
conformable and to pass into each other, as at Llyn Padarn.[120]
[Footnote 120: See Mr. Blake on this point, _Quart. Journ. Geol. Soc._
vol. xlviii. (1892), p. 252, _note_. I retain the opinion expressed
above.]
An unconformable junction here would, in some respects, have been
welcome, for it would at once have accounted for the superposition of
Lower Silurian strata directly upon the Cambrian volcanic series, and
for the disappearance of the Llanberis slates and grits which form
so conspicuous a feature above the tuffs and conglomerates at Llyn
Padarn. In the absence of such a structure we must accept the order of
succession as apparently unbroken, and rely on some such explanation
as was proposed by Sir Andrew Ramsay to account for the overlap of the
Arenig rocks on everything older than themselves as they are traced
northwards.[121] But this explanation will not entirely remove the
difficulties of the case. The inosculation of the volcanic group of
Bangor with the base of the Lower Silurian series cannot be accounted
for by any such overlap; it seems only explicable on the supposition
that the volcanic activity, which ceased in the Llyn Padarn district
about the time that the Llanberis Slates were deposited, was continued
in the Bangor area until Arenig time, or was then renewed. The thick
volcanic group of Bangor would thus be the stratigraphical equivalent
not only of the thin volcanic group of Llyn Padarn, but of the
overlying mass of strata up to the Arenig rocks. In confirmation of
this view, I shall show in a later chapter that volcanic action seems
to have been prolonged in Anglesey to a still later geological period,
that it appeared during the deposition of the Arenig strata, and that
it attained a great development throughout the time of the Bala group.
That a series of volcanic rocks, with associated cherty strata, may
be the stratigraphical equivalent of a great thickness of ordinary
sediments in other districts will be dwelt upon in the description
of the Lower Silurian volcanic geology of the Southern Uplands of
Scotland.[122]
[Footnote 121: _Mem. Geol. Surv._ vol. iii. 2nd edit. p. 252.]
[Footnote 122: A group of cherts and mudstones not more than 60 or 70
feet thick appear in that region to be stratigraphically equivalent to
the great depth of sedimentary material which elsewhere constitutes the
Upper Arenig and Lower and Middle Llandeilo formations. See _Annual
Report of the Geological Survey for 1895_, p. 27 of reprint.]
In the areas of North Wales which have now been described, volcanic
action appears to have begun and ended within the limits of the
Cambrian period. Southwards, in the district of Dolgelly, another
distinct and, in some respects, very different development of Cambrian
volcanic activity may be recognized. In that district there is evidence
that the volcanoes which distinguished the earlier part of the Silurian
period had already begun their eruptions during Cambrian time. As their
records, however, are intimately linked with those of Silurian age, an
account of them is deferred to the next chapter.
THE MALVERN HILLS
Although the chief surviving records of Cambrian volcanic action in
Britain are found in Wales, there is no evidence that the volcanoes of
the period lay chiefly in that region. It is certainly a suggestive
fact that, in the few districts where Cambrian strata appear from under
younger formations in England, they are generally accompanied with
igneous rocks, though the age of the latter may be older or later than
the Cambrian period. If the oldest Palæozoic rocks could be uncovered
over the English counties, a more abundant development of volcanic
materials might be laid bare than is now to be seen in Wales.
Taking, however, the extremely limited exposures of Cambrian strata,
we find two tracts that specially deserve attention. Reference has
already been made to the ancient eruptive rocks of the Malvern Hills,
the antiquity of which is proved by the position of the Cambrian
fossiliferous strata that overlie them. But these strata themselves
include certain igneous rocks which point to a recrudescence of
eruptive energy in a far later geological period.
Nearly half a century has passed away since John Phillips mentioned
the intercalation of igneous rocks in the series of strata which is
now classed as Upper Cambrian in the Malvern Hills. Since that date
hardly anything has been added to the information which he collected.
The existence of a group of rocks of such high antiquity, asserted to
be of truly volcanic origin, and the precise horizon of which could
be fixed by the stratigraphical aid of organic remains, seems to have
almost dropped out of sight. Phillips noted the occurrence of what he
regarded as truly volcanic materials in the Hollybush Sandstone and
the overlying dark (Lingula) shales, and he clearly recognized that a
wide difference of age separated them from the far more ancient igneous
rocks of the central core of the chain. The Hollybush Sandstones were
observed by him to have "often a trappean aspect and to be traversed
with felspathic dykes." He found the overlying black shales to include
"layers of trappean ashy sandstone." But it was at the top of these
shales that he obtained what he regarded as the most conspicuous
evidence of contemporaneous volcanic action. He there encountered a
zone of "interposed trap rocks" varying up to 50 feet in thickness,
consisting of "porphyritic and greenstone masses, which, erupted from
below, have flowed in limited streams over the surface of the black
shales." He recognized amygdaloidal and prismatic structures among
them.[123] The position of these eruptive rocks is shown in Fig. 45.
[Footnote 123: _Mem. Geol. Survey_, vol. ii. part i. pp. 52, 55; also
Horizontal Sections of the Geol. Survey, Sheet 13, No. 8, and Sheet
15. Reference to the igneous rocks of this area will be found in the
remarkable essay by De la Beche in vol. i. of the _Mem. Geol. Surv._
pp. 34, 38.]
[Illustration: Fig. 45.--Section across the Cambrian formations of the
Malvern Hills, showing the position of the intercalated igneous rocks
(_p_ _p_). After Phillips.]
These rocks were afterwards observed and described by Dr. Holl, who
found what he considered to be four true lava sheets interstratified in
the Hollybush Sandstones. He noted the intercalation of "numerous beds
of volcanic ash, grit and lava" in the black shales.[124]
[Footnote 124: _Quart. Journ. Geol. Soc._ vol. xxi. (1865), pp. 87-91.]
So far as I am aware, no more recent account of these rocks has
been published. Their true stratigraphical and petrographical
relations require to be more precisely determined. If they are really
contemporaneous lavas, they point to volcanic eruptions at the time
when the middle division of the Cambrian system was being deposited.
If, on the other hand, they should prove to be intrusive, they would
indicate probable volcanic activity in this part of England at some
time later than the middle of the Cambrian period.
WARWICKSHIRE
Some fifty miles to the north-east of the Malvern Hills, in the
heart of the rich Midlands, and among the coal-fields and the
New Red Sandstone to which these Midlands owe so much of their
manufacturing industry and their agricultural fertility, another
little tract of Cambrian rocks rises to the surface on the east side
of the Warwickshire coal-field between Nuneaton and Atherstone.
So unobtrusively do these ancient strata take their place among
their younger peers, that their venerable antiquity was for a long
time undetected.[125] They were actually regarded as parts of the
Carboniferous series, which at first sight they seem to underlie
conformably. It was not until 1882 that the mistake was corrected by
Professor Lapworth, who proved the rocks to be Cambrian by finding
undoubted Upper Cambrian fossils in them.[126] Subsequent investigation
enabled him to work out the detailed sequence of these strata. He found
that the supposed "Millstone Grit" is a thick-bedded quartzite perhaps
1000 feet in thickness, and resembling the well-known quartzites of
the Lickey and Caer Caradoc. The "Coal-shales" proved to be a series
(possibly 2000 feet thick) of purple, green, grey and black shales,
which from their fossils could be paralleled with the dark shales of
the Upper Cambrian series of the Malvern Hills.[127] These shales are
immediately overlain by the Coal-measures.
[Footnote 125: Their antiquity was recognized by Yates as far back as
1825 (_Trans. Geol. Soc._ 2nd series, vol. ii. p. 261). They had been
confounded with "Millstone Grit" and "Coal-shale" by Conybeare and
Phillips, and this mistake was adopted on the maps and memoirs of the
Geological Survey.]
[Footnote 126: _Geol. Mag._ (1882), p. 563.]
[Footnote 127: _Op. cit._ (1886), p. 319.]
For our present inquiry, however, the chief feature of interest in
these discoveries is the recognition of a group of volcanic rocks
underneath the quartzite. This group was named the "Caldecote Volcanic
Rocks" by Professor Lapworth, who first recognized its nature and
relations. Its rocks have been studied by Mr. T. H. Waller[128] and
Mr. F. Rutley,[129] and have been traced upon a revised edition of
the Geological Survey map by Mr. A. Strahan.[130] They consist of a
thin series of well-stratified tuffs apparently derived from andesitic
lavas. Their base is not seen owing to the fault which brings down the
New Red Sandstone against them. They are surmounted by the quartzite,
which at its base is conglomeratic and contains blocks of the tuff. A
mass of quartz-felsite is possibly intrusive in these strata, and is
associated with a diabase-porphyrite. In these rocks, but still more in
the shales which overlie them, numerous sills of diorite and diabase
occur. The total thickness of rocks from the lowest visible part of the
Caldecote volcanic series to the base of the Coal-measures is probably
between 2000 and 3000 feet.
[Footnote 128: _Op. cit._ p. 323.]
[Footnote 129: _Op. cit._ p. 557.]
[Footnote 130: _Geol. Mag._ (1886), p. 540. In this paper full
references will be found to the previous papers on the geology of the
district. Jukes had recognized that the rocks below the coal-bearing
strata were "older than the Upper Silurian, perhaps older than any
Silurian," _Mem. Geol. Survey_, "South Staffordshire Coal-field"
(1859), p. 134.]
There can be no doubt as to the geological position of the dark
fossiliferous shales and their underlying quartzite. The fact that
the basement conglomerate of the quartzite is partly made up of the
underlying volcanic series may possibly mark a wide difference of age
between them, and may indicate that the eruption of the tuffs took
place long before Upper Cambrian time. On the other hand, the tuffs
have the same strike and angle of dip with the quartzite, and as
Professor Lapworth admits, the break between them may not be of great
moment. It is at least certain that the intrusive sills of the district
are later than the tuffs, and later also than the sedimentary Cambrian
groups.
BOOK IV
THE SILURIAN VOLCANOES
CHAPTER XII
CHARACTERS OF THE SILURIAN SYSTEM IN BRITAIN. THE ARENIG VOLCANOES
The Land and Sea of Silurian time--Classification of the Silurian
System--General Petrography of the Silurian Volcanic Rocks--I.
The Eruptions of Arenig Age.
The next great geological period, to which Murchison gave the name of
Silurian, has in Britain a fuller record than the period which preceded
it. The rocks that tell its history are more varied in origin and
structure. They are displayed at the surface over a far wider area,
and, what gives them special interest and value, they contain a much
larger assemblage of organic remains. For the immediate subject of the
present volume, they have likewise the additional attraction that they
include a singularly complete and widespread volcanic chronicle. They
display in many admirable sections the piled-up lavas and tuffs of
scores of volcanoes, scattered all over the three kingdoms, from the
headlands of Kerry to the hills of Lammermuir. They thus enable us to
form a truer conception of what the early Palæozoic volcanoes were than
is possible from the more limited evidence furnished by the Cambrian
system.
At the beginning of the Silurian period most of the area of the British
Isles lay under the sea. But if we may judge from the sedimentary
strata which represent the floor of that sea, the water, during most
of the time, was of no great depth. There is evidence, indeed, that
during a part of the period the sea was deep enough to admit of the
accumulation of wide tracts of radiolarian ooze, with but little
admixture of mechanical sediment. But, for the most part, sand and
mud were drifted from neighbouring lands, the more important of
which probably lay to the north, over what are now the Highlands
of Scotland and the north and north-west districts of Ireland. No
general change in topography or in physical conditions took place at
the close of Cambrian time. The older era glided insensibly into
the newer, unmarked by any such catastrophe as was once supposed to
have intervened at the end of each great geological period. There are
traces, indeed, of slight local disturbances, but these only make the
general gradual transition more marked.
Of the vegetation which covered the Silurian lands hardly anything
is known. Traces of lycopods and ferns have been detected, and these
probably formed the chief constituents in what must have been rather a
sombre and monotonous flora. The character of the terrestrial fauna is
still hidden from us, though we do know that insects winged their way
through those green flowerless forests, and that scorpions likewise
harboured there. That these primeval arachnoids were air-breathers
is shown by their breathing stigmata; and from the fact that they
possessed a well-developed poison-gland and sting, we may believe that
there were already living at the same time other land-animals, possibly
of higher grade, on which they preyed. But of these ancestral types no
actual relics have yet been discovered.
It is the life of the sea-floor that has mainly been chronicled among
the sedimentary formations. Taking the Silurian system as a whole,
we find it to be the repository of a remarkably varied assemblage
of organisms. Among the simpler forms, Radiolaria deserve especial
notice, from their wide range in space and time, and the comparative
indestructibility of the highly-siliceous, fine-grained, flinty
strata, which have preserved them in abundance and have a wide
distribution over the British Isles. The Graptolites, so specially
characteristic of the system, range entirely through it, and by their
successive differences of specific and generic forms, furnish a basis
for the division of the whole series of rocks into more or less
definite stratigraphical zones. Hardly less important for purposes of
correlation are the Trilobites which in the Silurian period reached
the culmination of their development in regard to number of species
and genera. These interesting extinct types of crustacean life must
have swarmed over some parts of the sea-bottom, for their remains
abound in its hardened silts. The Brachiopods are likewise numerously
represented among Silurian strata; and since the vertical range of the
species is generally not great, they serve as useful guides in fixing
stratigraphical horizons. Lamellibranchs, Gasteropods, and Cephalopods
become increasingly numerous and varied as we follow the succession of
strata from the base to the summit of the Silurian system. That there
were fishes also in the Silurian seas is proved by the occurrence of
their remains, more particularly in the higher formations.
From the organic remains which have been preserved in the rocks, it may
be inferred that the animal life of the globe became more varied in
Silurian time; higher types made their appearance, until vertebrates
took the place of pre-eminence which they have ever since maintained.
The volcanic activity that had marked the passage of Cambrian time
in Britain was prolonged into the Silurian period. In North Wales,
indeed, it is clear that though the eruptions began in the earlier era
of geological history they continued to be comparatively feeble until
they broke out into full activity in the succeeding epoch. There is no
hiatus or essential difference between the volcanic phenomena, any more
than there is between the sedimentary deposits, of the two periods.
Although it may be only owing to the fact that the Silurian formations
come much more extensively to the surface of the land than the
underlying Cambrian are permitted to do, yet it is at least noteworthy
that the relics of Silurian volcanoes are spread over a far wider area
of the British Isles than those of the earlier period. Throughout a
large part of Wales they form some of the most prominent mountains,
such as Cader Idris, the Arans, Arenig Fawr, Moel Wyn, Moel Siabod,
and Snowdon. They rise into the picturesque hill-groups of the Lake
District, they appear at many detached places throughout the south of
Scotland, and form conspicuous eminences in Carrick. In Ireland they
abound all down the east side of the island, and even reappear on the
far western headlands of the Dingle coast-line.
To the same pioneers, by whom the foundations of our knowledge of the
Cambrian volcanoes were laid, we are indebted for the first broad
outlines of the history of volcanic action in Silurian time. The
writings of Sedgwick and Murchison, but still more the detailed mapping
of De la Beche, Ramsay, Selwyn, Jukes, and the other members of the
Geological Survey, have given to the Silurian volcanic rocks of Wales
a classic interest in the history of geology. To these labours further
reference will be made in subsequent pages.[131]
[Footnote 131: For references to the older literature see _ante_, p.
142.]
The amount of material being so ample for the compilation of a
record of volcanic action in Britain during Silurian time, it will
be desirable to arrange it in stratigraphical order. For this
purpose invaluable assistance is afforded by the evidence of organic
remains, whereby the whole Silurian system has been subdivided into
sections, each characterized throughout the whole region by certain
distinctive fossils. The following tabular statement exhibits the chief
stratigraphical divisions of the system, and the short black lines in
it mark the positions of separate volcanic platforms in each of the
three kingdoms:--
+---------------------------------------+-------+-------+--------+-------+
| |England| Wales |Scotland|Ireland|
+---------------------------------------+-------+-------+--------+-------+
| { Ludlow Group | ... | ... | ... | ... |
|Upper Silurian { Wenlock Group | ... | ... | ... | --- |
| { Llandovery Group | ? | ... | ... | ... |
| | | | | |
| { Bala and Caradoc Group| --- | --- | --- | --- |
|Lower Silurian { Llandeilo Group | --- | --- | --- | --- |
| { Arenig Group | --- | --- | --- | --- |
+---------------------------------------+-------+-------+--------+-------+
It will be most convenient, following the combined stratigraphical and
geographical arrangement of this table, to discuss first the volcanic
history of the Lower Silurian period as recorded in each of the three
kingdoms, and then that of the Upper Silurian.
I. THE ERUPTIONS OF ARENIG AGE
i. MERIONETHSHIRE
Placing the upper limit of the Cambrian system at the top of the
Tremadoc group, we pass into the records of another series of volcanic
eruptions which marked various epochs during the Silurian period over
the area of the British Isles. The earliest of these volcanic episodes
has left its memorials in some of the most impressive scenery of North
Wales. To the picturesque forms sculptured out of the lavas and ashes
of that early time, we owe the noble range of cliffs and peaks that
sweeps in a vast semicircle through the heights of Cader Idris, Aran
Mawddwy, Arenig, and Moel Wyn. To the east other volcanic masses,
perhaps in part coeval with these, rise from amidst younger formations
in the groups of the Berwyn and Breidden Hills, and the long ridges of
the Shelve and Corndon country. Far to the south, traces of Silurian
volcanoes are met with near Builth, while still more remote are the
sheets of lava and tuff interstratified among the Lower Silurian rocks
of Pembrokeshire, and those which extend into Skomer Island.
The most important of these districts is unquestionably that of
Merionethshire. In this area, as was pointed out in the last chapter,
the eruptions certainly began before the close of the Cambrian period,
for traces of them occur in the Tremadoc and Lingula Flag groups.
But below these strata, in the vast pile of grits and conglomerates
of the Harlech anticline, there does not appear to be any trace of
contemporaneous volcanic action.
At the time when the Geological Survey maps of this region were
prepared, the Cambrian and Lower Silurian rocks had not been subdivided
into the various palæontological groups which are now recognized.
Nor had any attempt been made to separate the various kinds of
contemporaneous igneous masses from each other and from the tuffs in so
extensive and complicated a mountain-region. The task undertaken by the
Survey was beset with difficulties, some of which geologists, furnished
with the advantages of a later time, can hardly perhaps realize. The
imperfections of the mapping were long ago recognized by the original
surveyors, and various corrections of them were made from time to time.
First of all, the volcanic rocks, which originally had been all massed
under one colour, were traced out separately on the ground, according
to their structure and mode of origin, and were distinguished from each
other on the maps.[132] Subsequently divisional lines were followed
out between some of the larger stratigraphical groups, the maps and
sections were still further modified, and the results were summed up in
the volume on the _Geology of North Wales_.[133] But short of actually
resurveying the whole of that rugged tract, it was impossible to bring
the maps abreast of the onward march of science. They consequently
remain, as a whole, very much as they were some thirty or forty years
ago.
[Footnote 132: _Mem. Geol. Surv._ vol. iii. 2nd edit. p. 95, note.]
[Footnote 133: Some of the modifications introduced are, I think, to
be regretted, for the earlier editions of the maps and sections are in
certain respects more accurate than the later. On this point I concur
with the criticism made by Messrs. Cole and Jennings, _Quart. Journ.
Geol. Soc._ vol. xlv. (1889), p. 436.]
Sir Andrew Ramsay, in his great Monograph on the geology of North
Wales, has described the Merionethshire volcanic district in
considerable detail. He seems finally to have come to the conclusion
that the eruptions of that area were included within the Arenig
period.[134] He shows, indeed, that on Rhobell Fawr the ejected
materials lie directly on disturbed Lingula Flags without the
intervention of the Tremadoc group, which is nevertheless present
in full development in the near neighbourhood.[135] And in trying
to account for this remarkable fact he evidently had in his mind
the possibility that volcanic eruptions had taken place long before
as well as after the beginning of the deposition of the Arenig grit
and slates.[136] He seems eventually, however, to have looked on the
Rhobell Fawr sections as exceptional and possibly to be accounted
for by some local disturbance and intrusion of eruptive rock.[137]
He clearly recognized that there were two great epochs of volcanic
activity during the Silurian period in Wales, one belonging to the
time of the Arenig, the other to that of the Bala rocks, and he
pointed out that the records of these two periods are separated by
a thick accumulation of sedimentary strata which, being free from
interstratifications of contemporaneous igneous rocks, may be taken
to indicate a long interval of quiescence among the subterranean
forces.[138]
[Footnote 134: _Mem. Geol. Survey_, vol. iii. 2nd ed., p. 96.]
[Footnote 135: The ashes and agglomerates of Rhobell Fawr can be seen
in various places to rest on the highest members of the Lingula Flags.
See Messrs. Cole and Holland, _Geol. Mag._ (1890), p. 451.]
[Footnote 136: _Op. cit._ p. 72.]
[Footnote 137: He was disposed to regard Rhobell Fawr as one of the
great centres of eruption of the district. See _Memoir of A. C.
Ramsay_, p. 81, and _Geology of North Wales_, 2nd edit. p. 98.]
[Footnote 138: _Op. cit._ pp. 71, 96, 105.]
The lower limit of the Arenig rocks has been fixed at a band or bands
of grit or conglomerate (Garth grit) which can be followed with some
slight interruptions all round the great dome of Cambrian strata from
Llanegrin on the south to the shore at Criccieth on the north. The
volcanic group doubtless lies, generally speaking, above that basement
platform. But, besides the sections at Rhobell Fawr just referred
to, where the volcanic materials lie on the Lingula Flags, the same
relation may, I think, be observed on the north flank of Cader Idris.
Messrs. Cole, Jennings, and Holland have come to the conclusion that
the eruptions began at a rather earlier date than that assigned to them
in the _Survey Memoirs_, and my own examination of the ground led me to
accept their conclusion.[139] I inferred that the earliest discharges
in the southern part of the region took place in Cambrian time, at or
possibly before the close of the deposition of the Lingula Flags, and
that intermittent outbursts occurred at many intervals during the time
when the Tremadoc and Arenig rocks were deposited.
[Footnote 139: _Quart. Journ. Geol. Soc._ vol. xlv. (1889), p. 436;
_Geol. Mag._ (1890), p. 447. _Pres. Address Geol. Soc._ 1890, p. 107.]
Important confirmation of this view of the Cambrian age of the earlier
volcanic eruptions of the Cader Idris region has recently been obtained
by Messrs. P. Lake and S. H. Reynolds who, in the ground intervening
between the lower slopes of Cader Idris and Dolgelly, have ascertained
the existence of a marked band of andesitic lava traceable for some
distance in the upper Lingula Flags. They have also observed a higher
volcanic group reposing upon the Tremadoc strata at the top of the
Cambrian system, and consisting of rhyolite with rhyolite-tuffs.[140]
[Footnote 140: _Quart. Journ. Geol. Soc._ vol. lii. (1896), p. 511.]
Some of the most stupendous memorials of the earlier eruptions are to
be seen in the huge mountain mass of Rhobell Fawr (2403 feet). They
consist mainly of agglomerates and tuffs, one of the most remarkable
varieties of which is distinguished by its abundant scattered crystals
of hornblende and of augite. The fragments of rock included in these
rocks are scoriæ and lumps of various lavas, especially basaltic and
trachytic andesites. The tuffs become finer towards the top of the
mountain where they are interleaved with grits. Among the pyroclastic
materials occasional lavas (basaltic andesites) occur which may be
contemporaneous streams, but most of the lava-form rocks appear to
be intrusive. They include dolerites (augite-aphanites), basaltic
andesites, and trachytic andesites.[141]
[Footnote 141: Prof. Cole, _Geol. Mag._ (1893), p. 337.]
[Illustration: Fig. 46.--Section across Rhobell Fawr.[142]
L L, Lingula flags; _t_, tuffs and ashy slates; _s_, slates and grits;
F F, Arenig volcanic series; D, dolerite.]
[Footnote 142: After Messrs. Cole and Holland, _Geol. Mag._ (1890), p.
450.]
The materials from the Rhobell Fawr volcano are clearly distinguishable
from those of the Arenig volcanoes in the neighbourhood. The latter
begin to make their appearance among the black slates at the base of
the northern declivities of Cader Idris, and extend upward through that
mountain into the country beyond.
An upper limit to this volcanic group is not easily traceable; partly,
no doubt, from the gradual cessation of the eruptions and partly from
the want of any marked and persistent stratigraphical horizon near the
top of the group. Sir Andrew Ramsay, indeed, refers to the well-known
band of pisolitic iron-ore as lying at or near to the top of the
Arenig rocks.[143] There can be no doubt, however, that the volcanic
intercalations continue far above that horizon in the southern part of
the district.
[Footnote 143: _Mem. Geol. Survey_, vol. iii. 2nd edit. pp. 249, 250.]
In spite of the extent to which the volcanic masses of the Arenig
period have been covered by later Palæozoic formations, it is still
possible to fix approximately the northern, western, and southern
limits of the district over which the ashes and lavas were distributed.
These materials die out as they are traced southwards from Cader
Idris and north-westwards from Tremadoc.[144] The greatest diameter
of ground across which they are now continuously traceable is about
twenty-eight miles. They attain their greatest thickness, upwards
of 5000 feet, in Aran Mawddwy, which rises from their most easterly
escarpment. We may therefore infer that the main vent or vents lay
somewhere in that direction. The noble range of precipices facing
westwards shows how greatly the limits of the volcanic rocks have been
reduced by denudation. There can be little doubt that at least the
finer tuffs extended westwards as far as a line drawn from Tremadoc to
Llanegrin--that is, some fifteen miles or more beyond the cliffs of
Aran Mawddwy, thus stretching across much of the site of what is now
the great Harlech anticline.
[Footnote 144: _Op. cit._ p. 96.]
This compact, well-defined volcanic area, in spite of the faults
which traverse it and the disturbed positions into which its rocks
have been thrown, is, in many respects, one of the simplest and most
easily studied among the Palæozoic formations of this country. Its main
features have been delineated on the maps of the Geological Survey
and have been described in Sir Andrew Ramsay's monograph. But these
publications cannot be regarded as more than a first broad, though
masterly, outline of the whole subject. There is an ample field for
further and more minute research wherein, with the larger and better
Ordnance maps now available, and with the advantage of the numerous
modern petrographical aids, a more exhaustive account may be given of
the district. The whole volcanic succession from base to summit is laid
bare in innumerable magnificent natural sections along ranges of hills
for a distance of some forty miles, and a careful study and re-mapping
of it could not fail to add greatly to our knowledge of the early
history of volcanic action.[145]
[Footnote 145: The excellent papers of Professor Cole, Mr. Jennings,
Mr. Holland, Mr. G. J. Williams, Mr. P. Lake and Mr. S. H. Reynolds are
illustrations of how the published work of the Geological Survey may be
modified and elaborated.]
According to the observations of the Geological Survey, the Arenig
volcanic rocks of Merionethshire naturally arrange themselves in
three great bands, each of which is described as tolerably persistent
throughout the whole district:--1st, a lower series of ashes and
conglomerates, sometimes 3300 feet thick (Aran Mawddwy); 2nd, a
middle group of "felstones" and "porphyries," consisting partly of
true contemporaneous lava-streams and partly of intrusive sheets, and
reaching a thickness of 1500 feet; 3rd, an upper series of fragmental
deposits like that beneath, the extreme thickness of which is 800 feet
(Arenig mountain). A re-mapping of the ground on the six-inch maps
would, no doubt, show many local departures from this general scheme.
The pyroclastic members of this volcanic series present many features
of interest both to the field-geologist and the petrographer; but
they have as yet been only partially studied. At the southern end of
the district it is remarkable to what a large extent the earliest
eruptions must have been mere gaseous explosions, with the discharge
of comparatively little volcanic material. Many of the tuffs that are
interstratified with black slates (? Lingula Flags) at the foot of
the long northern slope of Cader Idris, consist mainly of black-slate
fragments like the slate underneath, with a variable proportion of grey
volcanic dust.
[Illustration:
Fig. 47.--Section at the Slate Quarry, Penrhyn Gwyn, north slopes
of Cader Idris.
]
The accompanying section (Fig. 47) represents the arrangement of the
rocks exposed at the Slate Quarry of Penrhyn Gwyn. About 50 feet of
black slate (_a_) are there seen, the bedding in which dips S. at
20°, while the cleavage is inclined towards S.W. at a slightly higher
angle. The next 20 feet of slate (_b_) are distinguished by many
intercalations of slate-tuff or breccia, varying from less than an inch
to three feet in thickness. An intrusive sheet of andesite (_c_), which
varies from two or three to ten feet in thickness, and is strongly
cellular in the centre, interrupts the slates and hardens them. Above
this sill the indurated slate and tuff (_d_), containing abundant
felspar crystals, pass under a flinty porphyritic felsite (_e_) or
exceedingly fine tuff, enclosing a band of granular tuff. Beyond this
band the black slates with their seams of tuff continue up the hill and
include a sheet of slaggy felsitic lava 8 or 10 feet thick.
This section, affording as it does the first glimpse of the volcanic
history of Cader Idris, indicates a continued series of feeble gaseous
discharges, probably from one or more small vents, whereby the black
clay on the sea-floor was blown out, the fragments falling back again
to be covered up under a gradual accumulation of similar dark mud. By
degrees, as the vigour of eruption increased, lava-dust and detached
felspar crystals were ejected, and eventually lava rose to the surface
and flowed over the sea-bottom in thin sheets.
But elsewhere, and likewise at a later period in this same southern
part of the district, the fragmental discharges consisted mainly
of volcanic material. Sir Andrew Ramsay has described the coarse
conglomerates composed of subangular and rounded blocks of different
"porphyries," sometimes 20 inches in diameter, embedded in a fine
matrix of similar materials. The true nature of the component fragments
in these rocks has still to be worked out.
Messrs. Cole and Jennings have noticed that the grey volcanic dust of
the older slate-tuff of Cader Idris is seen under the microscope "to
abound in particles of scoriaceous andesite-glass, now converted into
a green palagonite."[146] Their investigations show that while the
same kinds of volcanic rocks continue to be met with from the bottom
to the top, nevertheless there is an increase in the acid character of
the lapilli as the section is traced upwards. Some of the fragments
consist of colourless devitrified glass, with pieces of pumice, as
if derived from the breaking up of previously-formed tuffs. Others
resemble quartz-andesites, rhyolites, or trachytes, while in at
least one instance, somewhat low down in the section, quartz-grains
with intruded material point to the existence of some fairly acid and
vitreous lava.[147] On the south side of Llyn Cau, that is towards the
top of the volcanic group, I found a coarse agglomerate with blocks of
felsitic lavas, sometimes three feet across (see Fig. 48). This gradual
increase of acidity in the lapilli of the tuffs finds an interesting
confirmation in the contemporaneous lava-sheets to which I shall
afterwards allude.
[Footnote 146: _Quart. Journ. Geol. Soc._, vol. xlv. (1889), p. 424;
_Geol. Mag._ (1890), p. 447.]
[Footnote 147: _Op. cit._ p. 429. A tuff lying below the ironstone near
Cross Foxes, east of Dolgelly, likewise contains fragments of trachytic
lavas.]
One of the most noticeable features in the tuffs of this volcanic
group is the great abundance of entire and broken crystals dispersed
through them. These crystals have certainly not been formed _in situ_,
but were discharged from the vents as part of the volcanic dust. They
usually consist of felspar which, at least in the southern portion of
the district, appears generally to be plagioclase. Frequent reference
to these crystals as evidence of volcanic explosions may be found in
the publications of the Survey. Nowhere can they be better seen than
in the black slate-tuffs of Cader Idris. They are there white, more or
less kaolinized, and as they lie dispersed through the black base, they
give the rock a deceptive resemblance to some dark porphyry. The large
crystals of hornblende and augite abundantly scattered through much of
the tuff of Rhobell Fawr have been already referred to.
In the central parts of the district thick bands of ashes were mapped
by the Survey, and described as consisting almost wholly of volcanic
materials, but containing occasional thin bands of slate which suffice
to mark pauses in the eruptions, when ordinary sediment was strewn
over the sea-bottom. In the Cader Idris ground, on the other hand,
interstratifications of non-volcanic material are of such frequent
recurrence as to show that there, instead of constant and vigorous
discharges accumulating a vast pile of ashes, the eruptions followed
each other after intervals of sufficient duration to allow of the usual
dark sediment spreading for a depth of many feet over the sea-bottom.
One of the most interesting deposits of these interludes of quiescence
is that of the pisolitic ironstone and its accompanying strata on
the north front of Cader Idris (_i_ in Fig. 48). A coarse pumiceous
conglomerate with large slag-like blocks of andesite and other rocks,
seen near Llyn-y-Gadr, passes upward into a fine bluish grit and shale,
among which lies the bed of pisolitic (or rather oolitic) ironstone
which is so widely diffused over North Wales. The finely-oolitic
structure of this band is obviously original, but the substance was
probably deposited as carbonate of lime under quiet conditions of
precipitation. The presence of numerous small _Lingulæ_ in the rock
shows that molluscan life flourished on the spot at the time. The
iron exists in the ore mainly as magnetite, the original calcite or
aragonite having been first replaced by carbonate of iron, which was
subsequently broken up so as to leave a residue of minute cubes of
magnetite.[148]
[Footnote 148: Messrs. Cole and Jennings, _op. cit._ p. 426.]
Above the ironstone some more blue and black shale and grit pass under
a coarse volcanic conglomerate like that below, lying at the base of
the high precipice of Cader Idris. Hence this intercalated group of
sedimentary strata marks a pause in the discharge of ashes and lavas,
during which the peculiar conditions of sedimentation indicated by the
ironstone spread over at least the southern part of the volcanic area.
Some few miles to the east, where the ironstone has been excavated near
Cross Foxes, the band is again found lying among tuffs and grits full
of volcanic lapilli.
Between a lower and an upper band of tuff in the Arenig volcanic group
the Maps and Memoirs of the Geological Survey distinguish a central
zone of "felspathic porphyry," which attains a maximum thickness of
1500 feet (see Fig. 48). From Sir Andrew Ramsay's descriptions, it is
clear that he recognized in this zone both intrusive and extrusive
sheets, and that the latter, where thickest, were not to be regarded
as one mighty lava-flow, but rather as the result of successive
outpourings, with occasional intervals marked by the intercalation of
bands of slate or of tuff. To a certain extent the intruded sheets are
separated on the map from the contemporaneous lavas; but this has been
done only in a broad and sketchy way. One of the most important, and
at the same time most difficult, tasks yet to be accomplished in this
district is the separation of the rocks which were probably poured
out at the surface from those that were injected underneath it. My
own traverses of the ground have convinced me that good evidence of
superficial outflows may be found in tracts which have been mapped as
entirely intrusive; while, on the other hand, some of the so-called
"lavas" may more probably be of the nature of sills.
[Illustration: Fig. 48.--Sketch-section across Cader Idris.
_st_, slates and tuffs and ashy slates; _s_, slates and grits; _i_,
ironstone; _b_, volcanic breccias; _a_, slaggy andesitic and more basic
lavas; _e_, microgranite or eurite; _f_, felsites; _d_, "greenstone"
(dolerites, diabases, etc.).]
The petrography of the rocks, moreover, still requires much study.
Among the so-called "felspathic porphyries" of the Survey maps a
considerable variety of texture, structure and composition will
doubtless be detected. In the _Descriptive Catalogue of Rock-Specimens
in the Museum of Practical Geology_ (3rd edit., 1862) the rocks
that form the "lava-streams of Llandeilo age," in Merionethshire,
are named "felstone," "felspar-porphyry," "felstone-porphyry,"
"felspathic-porphyry," and "calcareous amygdaloid."
The most interesting feature which my own slight personal acquaintance
with the region has brought before me is the clear evidence of a
succession from comparatively basic lavas in the lower part of the
group to much more acid masses in the higher part. In the Survey map
numerous sheets of intrusive "greenstone" are shown traversing the
Lingula Flags, Tremadoc slates, and lower part of the volcanic group
along the northern slopes of Cader Idris. The true intrusive nature of
much of this material is clearly established by transgressive lines of
junction and by contact-metamorphism, as well as by the distinctive
crystalline texture of the rocks themselves. But the surveyors were
evidently puzzled by some parts of the ground. Sir Andrew Ramsay
speaks of "the great mass of problematical vesicular and sometimes
calcareous rock which is in places almost ashy-looking." After
several oscillations of opinion, he seems to have come finally to the
conclusion that this vesicular material, which occurs also in the upper
part of the mountain, passes into, and cannot be separated from, the
undoubted intrusive "greenstones."[149]
[Footnote 149: _Mem. Geol. Surv._ vol. iii. 2nd edit. p. 36; see also
pp. 31, 32.]
The true solution of the difficulty will be found, I believe, in the
recognition of a group of scoriaceous lavas among these greenstones.
The presence of a cellular structure might not be sufficient to
demonstrate that the rocks in which it appears are true lava-beds,
for such a structure is far from unknown both among dykes and sills.
But in the present case there is other corroborative testimony that
some of these Cader Idris amygdaloids were really poured out at the
surface. Below Llyn-y-Gadr--the dark tarn at the foot of the vast
wall of Cader Idris--the beds of coarse volcanic conglomerate (_b_
in Fig. 48), to which I have already alluded, are largely composed
of blocks of the vesicular "greenstones" on which they lie. These
"greenstones," moreover, have many of the most striking characteristics
of true lavas (_a_ in Fig. 48). They are extraordinarily cellular;
their upper surfaces sometimes present a mass of bomb-like slags with
flow-structure, and the vesicles are not infrequently arranged in rows
and bands along the dip-planes.
A microscopic examination of two slides cut from these rocks shows them
to be of a trachytic or andesitic type, with porphyritic crystals of a
kaolinized felspar embedded in a microlitic groundmass. The rocks are
much impregnated with calcite, which fills their vesicles and ramifies
through their mass.
A few miles to the east some remarkable felsitic rocks take the place
of these vesicular lavas immediately below the pisolitic iron ore. I
have not determined satisfactorily their relations to the surrounding
rocks, and in particular am uncertain whether they are interbedded
lavas or intrusive sheets. Dr. F. H. Hatch found that their microscopic
characters show a close resemblance to the soda-felsites described by
him from the Bala series of the south-east of Ireland.
The slopes of Cader Idris are partly obscured with debris, from above
which rises the great precipitous face formed by the escarpment of
"porphyry," here intrusively interposed among the Arenig volcanic
rocks. This enormous sill will be referred to a little further on in
connection with the other intrusive sheets of the region.
The remarkably cellular rock which forms the peak of Cader Idris is
coloured on the Survey map as an intrusive sill of "greenstone," which
in the Memoir is said to alter the contiguous slates and to appear to
cut across them diagonally. I am disposed, however, to think that these
appearances of intrusion are deceptive. On the southern declivity of
the mountain this rock presents one of the most curious structures
to be seen in the whole district. Its surface displays a mass of
spheroidal or pillow-shaped blocks aggregated together, each having
a tendency to divide internally into prisms which diverge from the
outside towards the centre.[150] Some portions are extremely slaggy,
and round these more solid portions finely crystalline parts are drawn,
suggestive rather of free motion at the surface than of the conditions
under which a subterranean sill must be formed. The idea occurred to
me on the ground that while the band of rock marked as "greenstone" on
the map is probably, in the main, an interstratified lava, there may
nevertheless be basic intrusions along its course, as in the lower part
of the mountain. The minute structure of this amygdaloid, as revealed
by the microscope, shows it to be an epidiorite wherein the hornblende,
paramorphic after augite, has been again partially altered along the
margins into chlorite.
[Footnote 150: This peculiar structure of the more basic Arenig
lavas, where the rock looks as if built up of irregularly-spheroidal,
sack-like or pillow-shaped blocks, will be again referred to in
connection with the Arenig (and Llandeilo) lavas of Scotland and
Ireland. It appears to be widely distributed, and especially in
connection with the occurrence of radiolarian cherts. The black slate
above the Cader Idris amygdaloid would, in a similar position in
Scotland, be associated with such cherts, but these have not yet been
noticed at this locality. With the spheroidal internally-radiating
prismatic structure of the Cader Idris rock, compare that of the lava
at Acicastello already noticed on p. 26.]
The highest lavas of Cader Idris, forming the ridge to the south of
Llyn Cau, are separated from the amygdaloid just described by a thick
zone of black slate with thin ashy intercalations, beyond which comes
the coarse volcanic agglomerate already referred to as containing
blocks of felsite a yard or more in diameter. These lavas are true
felsites, sometimes beautifully spherulitic and exhibiting abundant
flow-structure, like some of the felsites of the next or Bala volcanic
period.[151] The petrography of these rocks still remains to be worked
out.
[Footnote 151: Messrs. Cole and Jennings, _Quart. Journ. Geol. Soc._
vol. xlv. (1889), p. 430. From the examination of slices prepared from
a few of the felsites of the Dolgelly district, Dr. Hatch observed a
"striking difference between their characters and those of the Cambrian
felsites of Caernarvonshire. The porphyritic constituent is now no
longer quartz, but felspar (plagioclase), and the rocks belong, not to
the rhyolitic, but rather to the less acid trachytes, perhaps even to
the andesites."]
The volcanic series of Cader Idris sweeps northward through the chain
of Aran and Arenig, and then curves westward through the group of
Manod and Moelwyn, beyond which it rapidly dies out. In its course
of about 45 miles it undergoes considerable variation, as may be
seen by comparing a section through Moelwyn with that through Cader
Idris already given. According to the researches of Mr. Jennings and
Mr. Williams,[152] the main mass of volcanic material in the northern
part of the region consists of fragmentary rocks varying in texture
from agglomerates into fine tuffs, but showing some differences in the
succession of beds in different localities.
[Footnote 152: _Quart. Journ. Geol. Soc._ xlvii. (1891), p. 368.]
The Tremadoc group of strata clearly underlies the volcanic series of
these more northerly tracts. But it contains, so far as appears, no
intercalation of volcanic material. The inference may thus be drawn
that the eruptions began in the Cader Idris district, and did not
extend into that of Manod and Moelwyn until after the beginning of
the Arenig period. Above the Tremadoc group lies the well-marked and
persistent band, about 13 feet thick, known as the Garth grit, which
has been already referred to as a convenient base-line to the Arenig
group.
[Illustration: Fig. 49.--Section across the Moelwyn Range.[153]
1, Tremadoc Group; 2, Garth or Arenig grit (base of Arenig group);
3, Arenig slates, etc.; 3^1, Lower slate band; 3^2, Middle slate
band; 3^3, Upper slate band; 4^1, Lower agglomerate; 4^2, Middle
agglomerate; 4^3, Upper agglomerate; 5, Llandeilo group; G,
Granite boss of Moel tan y Grisiau.
]
[Footnote 153: After Messrs. Jennings and Williams, _Quart. Journ.
Geol. Soc._ vol. xlvii. (1891), p. 371, and Horizont. Sect. Geol. Surv.
Sheet 28.]
In this northern district, among the sediments which overlie the
Garth grit, layers of fine tuff begin to make their appearance, which
north of Cwm Orthin thicken out into a considerable mass between
the grit and the lowest of the great agglomerates. These tuffs,
which mark the beginning of the volcanic eruptions of the district,
are followed by a band of slate which in some places has yielded
a _Lingula_, _Orthis Carausii_, and a _Tetragraptus_, and points
to an interval of quiescence in the volcanic history. We now enter
upon an enormous thickness of agglomerates and tuffs separated by
several bands of slate. Taking advantage of the slaty intercalations,
Messrs. Jennings and Williams have divided this great accumulation of
fragmentary volcanic material into three beds (Fig. 49). The matrix
of the agglomerates is compact and pale, so as to resemble and to
have been called "felstone," but showing its fragmentary nature on
weathered surfaces. The blocks imbedded in this paste range up to
sometimes as much as 11 feet in length by 4 feet in width. Their minute
petrographical characters have not been studied, but the blocks are
stated to consist for the most part of "slaty and schistose fragments
mixed with rounded pebbles of fine-grained 'felstone.'" They are heaped
together as in true agglomerates. In the upper agglomerate, fragments
of cleaved slate containing _Lingula_ have been observed.
The name of "felstone" is restricted by Messrs. Jennings and Williams
to certain fine-grained varieties of rock, of which a thin band lies
at the base of the lower agglomerate, while another of considerably
greater importance occurs in the middle of the upper agglomerate. These
bands consist of a fine compact greenish base, and weather with a dull
white crust; sometimes, as in the thicker sheet, a columnar structure
shows itself. Whether these rocks are to be regarded as lavas or sills,
or even as finer varieties of tuff, is a question that awaits further
inquiry. But it is clear, from the investigation of the two observers
just cited, that the pyroclastic constituents must vastly preponderate
in the volcanic series over the northern part of the region. All these
rocks, whether coarse or fine-grained, appear to be rather acid in
composition, and no evidence has yet been obtained of a sequence among
them from a more basic to a more acid series, as in Cader Idris.
The highest agglomerate bed of the Manod and Moelwyn area is covered by
slates which contain Llandeilo graptolites. In this way, by means of
palæontological evidence, the upward and downward limits of the Arenig
volcanic series in this part of Wales are definitely fixed.
Hardly any information has yet been obtained as to the situation and
character of the vents from which the lavas and ashes of Merionethshire
were discharged. In the course of the mapping of the ground, the
Geological Survey recognized that, as the greatest bulk of erupted
material lies in the eastern and south-eastern parts of the region,
the chief centres of emission were to be looked for in that quarter,
and that possibly some of the intrusive masses which break through the
rocks west of the great escarpment may mark the site of vents, such as
Tyddyn-rhiw, Gelli-llwyd-fawr, Y-Foel-ddu, Rhobell Fawr, and certain
bosses near Arenig.[154] The distribution of the volcanic materials
indicates that there were certainly more than one active crater. While
the southward thickening of the whole volcanic group points to some
specially vigorous volcano in that quarter, the notable thinning away
of the upper tuffs southward and their great depth about Arenig suggest
their having come from some vent in this neighbourhood. On the other
hand, the lower tuffs are absent at Arenig, while on Aran Mawddwy, only
nine miles to the south, they reach a depth of 3000 feet. Still farther
to the south these volcanic ejections become more and more divided by
intercalated bands of ordinary sediment. One of the most important
volcanoes of the region evidently rose somewhere in the neighbourhood
of what is now Aran Mawddwy. There seems reason to surmise that the
sites of the chief vents now lie to the east and south of the great
escarpment, buried under the thick sedimentary formations which cover
all that region.
[Footnote 154: _Mem. Geol. Surv._ vol. iii. 2nd edit. p. 98; see also
pp. 44, 54, 58, 71.]
If we are justified, on stratigraphical and petrographical grounds, in
connecting the lowest volcanic rocks of the Berwyn range with those
of Merionethshire, we may speculate on the existence of a group of
submarine vents, coming into eruption at successive intervals, from
some epoch during the period of the Lingula Flags up to that of the
Bala rocks, and covering with lavas and ashes a space of sea-bottom
at least forty miles from east to west by more than twenty miles from
north to south, or roughly, an area of some 800 square miles.[155]
[Footnote 155: The Berwyn Hills, however, will be described in later
pages as a distinct volcanic district.]
Besides the materials ejected to the surface, the ancient volcanic
region of Merionethshire was marked by the intrusion of a vast amount
of igneous rock between and across the bedding-planes of the strata
deep underground. One of the most prominent features of the Geological
Survey map is the great number of sills represented as running with
the general strike of the strata, especially between the top of the
Harlech grits and the base of the volcanic series. On the north side of
the valley of the Mawddach, between Barmouth and Rhaiadr Mawddach, in
a distance of twelve miles the Survey mapped "more than 150 intrusions
varying from a few yards to nearly a mile in length."[156] This zone
of sills is equally marked on the south side of the valley. It may be
traced all round the Harlech anticline until it dies out, as the bedded
masses also do, towards Towyn on the south and about Tremadoc on the
north.
[Footnote 156: _Mem. Geol. Surv._ vol. iii. p. 26.]
The presence of such a zone of intrusive sheets at the base of an
ancient volcanic series is a characteristic feature in the geology of
Britain. It is met with again and again among the Palæozoic systems,
and appears on a striking scale in association with the Tertiary
basaltic plateaux of Antrim and the Inner Hebrides. But nowhere,
perhaps, is it more strongly developed than beneath the Arenig group of
lavas and tuffs in North Wales. Abundant as are the protrusions marked
on the Geological Survey map, they fall short of the actual number to
be met with on the ground. Indeed, to represent them as they really are
would require laborious surveying and the use of maps on a far larger
scale than one inch to a mile.
The vast majority of these sills are basic rocks, or, in the old
and convenient terminology, "greenstones." Those of the Cader Idris
district have been examined by Messrs. Cole and Jennings, who found
that, notwithstanding the considerable alteration everywhere shown
by the abundant epidote and calcite, the coarser varieties may be
recognized as having originally been dolerites approaching gabbro, with
a well-developed ophitic character, the general range of structure
being from dolerites without olivine and aphanites to andesitic
rocks with an originally glassy matrix.[157] Dr. Hatch confirmed
this diagnosis from slides prepared from my specimens. The ophitic
structure is usually characteristic and well preserved, in spite of the
alteration indicated by epidote, chlorite, uralite, and leucoxene.
[Footnote 157: _Quart. Journ. Geol. Soc._ vol. xlv. (1889), p. 432.]
That this zone of "greenstone" sills belongs to the period of the
Merionethshire volcanoes may be reasonably concluded. The way in which
they follow the line of the great escarpment, their almost entire
absence from the Cambrian dome to the west, their cessation as the
overlying lavas and tuffs die out laterally, and their scarcity above
the lower part of the volcanic group, seem to indicate their close
relationship to that group. Moreover, that they must have been as a
whole later than the main part of the lavas and tuffs may be inferred
from their position. The molten material of which they were formed
could hardly have forced its way between and across the strata unless
egress to the surface had been impeded by some thick overlying mass.
The "greenstones" may therefore be regarded as lateral emanations from
funnels of more basic lava towards the close of the volcanic period.
Possibly some at least of the highly slaggy and vesicular bands to
which I have referred may represent portions of this material, which
actually flowed out as streams of lava at the surface.
But there is likewise evidence of extensive intrusion of more
siliceous rocks. On the Geological Survey map, besides the numerous
"greenstones," various sheets of "felspathic porphyry" are represented
as running with the general strike of the region, but here and there
breaking across it. One of the most remarkable of these acid sills is
that which, in the noble precipice of Cader Idris, has a thickness of
about 1500 feet and a length of three or four miles. It is shown on the
map to be transgressive across other rocks, and, as seen on the ground,
it maintains the uniformity of texture which is characteristic rather
of sheets that have solidified underneath than of those which have
congealed with comparative rapidity at the surface. On a fresh fracture
the rock presents a pale bluish-grey tint, becoming yellowish or
brownish as the result of weathering. Its texture is finely granular,
with occasional disseminated felspars. Under the microscope a section
of it was found by Dr. Hatch to exhibit the characteristic structure
of a microgranite, a confused holocrystalline aggregate of quartz and
felspar, with a few porphyritic felspars. Messrs. Cole and Jennings
have proposed to revive for this rock Daubuisson's name "Eurite."[158]
[Footnote 158: Mr. Harker speaks of the rock as a granophyre.]
A similar rock occurs at a lower horizon among the Lingula Flags at
Gelli-llwyd-fawr, two miles south-west of Dolgelly,[159] and much
microgranite has been injected along the slopes above Tyddyn-mawr.
[Footnote 159: Messrs. Cole and Jennings, _op. cit._ p. 435.]
The chronological relation of these acid sheets and bosses to the more
basic intrusions has not yet been definitely determined. That some of
them may have solidified in vents and may have been directly connected
with the protrusion of the later or more highly siliceous lavas is not
at all improbable. Others again would seem to belong to a much later
geological period than the Arenig volcanoes. In this late series the
well-known boss of Tan-y-grisiau near Festiniog should probably be
included. This mass of eruptive material was mapped by the Geological
Survey as "intrusive syenite." It has been more recently examined
and described by Messrs. Jennings and Williams as a granitite.[160]
These observers have noticed not only that it intrusively traverses
and alters the Tremadoc group, but that its intrusion appears to have
taken place subsequent to the cleavage which affects the Llandeilo
as well as older formations. This granitic boss has thus probably no
connection with the Arenig volcanoes, but belongs to a later period in
the volcanic history of the Principality.
[Footnote 160: _Quart. Journ. Geol. Soc._ vol. xlvii. (1891), p. 379.]
The remarkable scarcity of dykes in the volcanic districts of Wales
has been noticed by more than one observer. Among the intrusive
"greenstones" of Merionethshire some occasionally assume the dyke
form, and through the agglomerates and tuffs of Rhobell Fawr dykes
of olivine-diabase have worked their way. In the Festiniog district
various altered andesitic dykes have been noted. But there has been no
widespread fissuring of the ground and uprise of lava in the rents,
such as may be seen in the Archæan gneiss, and in the later Palæozoic,
but still more in the Tertiary volcanic regions. This feature becomes
all the more notable when it is viewed in connection with the great
development of sills, and the evidence thereby afforded of widespread
and extremely vigorous subterranean volcanic action.
In the Merionethshire region there certainly was a long period of
quiescence between the close of the Arenig and the beginning of the
Bala eruptions. Moreover, no evidence has yet been found that active
vents ever again appeared in that district, the subterranean energy
at its next outburst having broken out farther to the east and north.
In Anglesey, however, where, as I shall point out, there is proof of
contemporaneous tuffs among the Arenig rocks, it is possible that a
continuous record of volcanic action may yet be traced from Arenig well
onward into Bala time.
ii. SHROPSHIRE
About 35 miles to the south-east of the great volcanic range of
Merionethshire a small tract of Arenig rocks rises from amidst younger
formations, and forms the picturesque country between Church Stoke and
Pontesbury. Murchison in his excellent account of this district clearly
recognized the presence of both intrusive and interstratified igneous
rocks.[161] The ground has in recent years been more carefully worked
over by Mr. G. H. Morton[162] and Professor Lapworth.[163]
[Footnote 161: _Silurian System_ (1839), chap. xix.; _Siluria_, 4th
edit. (1867), pp. 26, 49.]
[Footnote 162: _Proc. Liverpool Geol. Soc._ x. (1854), p. 62.]
[Footnote 163: _Geol. Mag._ (1887), p. 78.]
At the top of the Arenig group of this district lies a zone of
well-stratified andesitic tuff and breccia (Stapeley Ash), with
frequent intercalations of shales, and occasionally fossiliferous.[164]
There is thus satisfactory proof of contemporaneous eruptions at
intervals during the accumulation of the later Arenig sediments. That
there were also outflows of lava is shown by the presence of sheets
of augite- and hypersthene-andesite. These volcanic intercalations
form marked ridges, having a general northerly trend. They are folded
over the broad laccolitic ridge of Corndon, on the east side of which
they are thrown into a synclinal trough, so that successive parallel
outcrops of them are exposed. According to the mapping of the
Geological Survey they are thickest towards the west, and become more
split up with intercalated sediments as they range eastward.
[Footnote 164: Prof. Lapworth and Mr. W. W. Watts, _Proc. Geol. Assoc._
xiii. (1894), pp. 317, 337.]
Volcanic eruptions in this Shropshire region continued from the Arenig
into the Bala period. They are marked among the Llandeilo strata by
occasional tuffs and by two massive beds of "volcanic grit," described
by Murchison,[165] but they appear to have been rather less vigorous in
the interval represented by this subdivision of the Silurian system.
Those of Bala time gave forth abundant discharges of ash, of which
the lowest accumulation, locally known as the Hagley Ash, consists of
andesitic detritus. Occasional layers of tuff are intercalated in the
overlying Hagley Shales, above which comes an important band called
the Whittery Ash, "consisting of andesitic and rhyolitic breccias
and conglomerates, fine ashes with curious spherulitic or pisolitic
structures, and bands of shale often fossiliferous."[166] It is evident
that the eruptions of the Shelve district came from independent
vents in that neighbourhood, and never reached the importance of the
great volcanoes of Arenig age in Montgomeryshire or of Bala age in
Caernarvonshire.
[Footnote 165: _Silurian System_, p. 229.]
[Footnote 166: Messrs. Lapworth and Watts, _op. cit._ p. 318.]
[Illustration: Fig. 50.--Section across the anticline of Corndon.[167]
A, Arenig flags and shales; B, andesites and tuffs; C, intrusive
dolerite.]
[Footnote 167: After Prof. Lapworth and Mr. Watts, _op. cit._ p. 342.]
Numerous dykes and sills traverse the rocks of this district. They
consist chiefly of hypersthene-dolerite. They appear to belong to a
much later period than the interstratified volcanic series; at least
some of them are found altering the Pentamerus limestones, and these
must be later than the Llandovery rocks.[168] The most important sill
is that which forms Corndon, the central igneous mass of the district.
This body of dolerite was ascertained by Mr. Watts not to be a boss
but a laccolite, which wedges out both towards the north-west and
south-east, as shown in Fig. 50.
[Footnote 168: _Op. cit._ p. 339.]
Six miles to the north of the Shelve and Corndon district the Breidden
Hills rise on the border of Shropshire and Montgomeryshire, and include
a mass of volcanic material belonging to a distinct area of eruption.
In the ridge that extends for about three and a half miles through
Moel-y-golfa and Middletown Hill, a synclinal trough of volcanic rocks
lies upon shales, which from their fossils have been placed in the Bala
group. The volcanic series appears to exceed 1000 feet in thickness.
The lowest part of it on Moel-y-golfa consists of andesitic lavas about
400 feet thick, followed by tuffs and volcanic conglomerates. The lavas
resemble some of the "porphyrites" of the Old Red Sandstone, and
contain two forms of pyroxene--one rhombic, probably enstatite, and
the other monoclinic augite. There are likewise considerable masses of
intrusive rock, which are varieties of diabase or dolerite.[169]
[Footnote 169: See Mr. W. W. Watts on the Igneous and Associated Rocks
of the Breidden Hills, _Quart. Journ. Geol. Soc._ vol. xli. (1885), p.
532.]
iii. SCOTLAND
From the centre of England we must in imagination transport ourselves
into the Southern Uplands of Scotland, where a widely distributed
series of Silurian volcanic rocks has been preserved. It was, until
recently, supposed that the Silurian system north of the Tweed contains
no contemporaneously erupted volcanic rocks. Yet, as far back as the
year 1860, I pointed to the abundant existence of volcanic detritus in
these strata throughout the southern counties as a probable indication
of volcanic activity at the time and in the area within which the
strata were deposited.[170] Some years later, when the microscope
had been introduced as an aid to field-geology, I sliced some of
the Silurian sediments of that region and found them, particularly
certain shales and grits of Moffatdale, to contain a large admixture
of perfectly fresh unworn felspar crystals, which I felt tolerably
certain had been supplied by volcanic explosions. As no trace, however,
had then been detected of an intercalated volcanic group in any part
of the Silurian series of the south of Scotland, I used at that time
to speculate on the possibility of the volcanic detritus having been
wind-borne from the volcanoes of the Lake District. I had at that time
no suspicion that its source was rather to be sought under my feet.
The presence of volcanic rocks underneath the uplands of the south
of Scotland would have been a welcome explanation of the frequent
felspathic composition of many of the Silurian greywackes and shales of
that region, and particularly the abundance of andesitic and felsitic
fragments in them.
[Footnote 170: _Trans. Roy. Soc. Edin._ xxii. (1860), p. 636.]
It had been long known that the Scottish Silurian formations, besides
having undergone extensive plication, have also been injected by
protrusions of igneous material of various kinds. The intrusive
character of many of these is so obvious that a similar origin was
attributed even to those bosses which could not be proved to be
intrusive. Recent work of the Geological Survey, however, and more
especially the numerous and careful traverses of my friend and
colleague Mr. Peach, have revealed the unlooked-for and important fact
that a large number of these supposed intrusions are really portions of
a volcanic group brought up on the crests of anticlinal folds, and laid
bare by denudation. This group can be traced for at least 100 miles
from north-east to south-west over a belt of country sometimes 30 miles
broad. Its original limits cannot be ascertained, but they obviously
exceeded those within which the rocks can now be seen. Nevertheless
the present boundaries embrace an area of nearly 2000 square miles.
This Palæozoic volcanic region is thus one of the most extensive in
the British Isles. Owing, however, to the constant plication of the
strata, and the wide space which the overlying sedimentary deposits
are thus made to cover, the volcanic group only comes occasionally
into view, and thus occupies but a mere fraction of the superficial
extent of the region over which its scattered outcrops appear. These
exposures, sometimes only a few square yards in extent, may always
be looked for where the anticlinal folds bring up a sufficiently low
portion of the Silurian system; they prove that a vast volcanic floor
underlies the visible Lower Silurian grits and shales over the length
and breadth of the Southern Uplands of Scotland.
Without anticipating details which will properly appear in the
official _Memoirs_ of the Geological Survey, I may briefly indicate
the visible boundaries of the volcanic group, and refer to some of the
localities where it may best be seen. The most easterly points where
it has been recognized by Mr. Peach stand on the crests of some sharp
anticlinal folds near St. Mary's Loch and near Leadburn and Winkstone
in Peeblesshire. Farther westwards it appears at many places along the
northern border of the Silurian territory, as at Romanno Bridge, Wrae,
Kilbucho, Culter Water and Abington, the length and breadth of each
exposure depending partly on the breadth of the anticline and partly on
the depth to which it has been cut down by denudation. Near Sanquhar
the volcanic series opens out for a breadth of more than a mile, and
is seen at intervals across the wild moorlands of Carrick, until from
the Stinchar valley it widens out seaward and occupies much of the
coast-line of Ayrshire between Girvan and the mouth of Loch Ryan. It
probably rises again along a fold near Portpatrick, and it is seen at
various points along the southern borders of the Silurian uplands, as
near Castle-Douglas, at Glenkiln, Bell Craig near Moffat, and the head
of Ettrickdale.
The best sections are those exposed along the coast to the north
and south of Ballantrae. When that ground was first examined by the
Geological Survey, the hypothetical views in regard to metamorphism
already referred to were in full ascendant, and the rocks were mapped
on the same general principles as those which had been followed in
Wales. Professor Bonney, however, a few years later recognized the true
igneous nature of many of the rocks. He found among them porphyrite
lavas and agglomerates which he regarded as of Old Red Sandstone age,
likewise intrusive serpentines and gabbros.[171]
[Footnote 171: _Quart. Journ. Geol. Soc._ vol. xxxiv. (1878), p. 769.]
The volcanic rocks of this wide district include both lavas and
their pyroclastic accompaniments, as well as intrusive sills and
bosses of various materials. They have recently been studied by Mr.
J. J. H. Teall, and full descriptions of them by him will appear
in a forthcoming volume of the _Memoirs_ of the Geological Survey.
He has ascertained that though generally more or less decomposed,
the lavas would be classed by German petrographers as diabases and
diabase-porphyrites. The former are compact dark-green non-porphyritic
rocks, often containing numerous small spherical amygdales; while the
latter are markedly porphyritic, enclosing large phenocrysts of more
or less altered plagioclase, often measuring half an inch across.
These two groups of rock are connected by transitional varieties.
They were probably, in the first instance, composed of plagioclase,
augite, iron-ores, and a variable quantity of imperfectly crystallized
interstitial matter.
Some of these rocks closely resemble in outward appearance the
andesites ("porphyrites") of the Old Red Sandstone of the district
not many miles to the north, that is, fine purplish-red rocks with
a compact base through which porphyritic felspars are abundantly
scattered. Occasionally they are markedly slaggy, and show even a ropy
surface, while the breccias associated with them contain blocks of
similar slag.
[Illustration: Fig. 51.--Structure in finely-amygdaloidal diabase lava,
south of mouth of Stinchar River, Ayrshire. The fine dots and circles
mark the lines of amygdales.]
But the most characteristic external feature of these lavas is their
tendency to assume irregularly-elliptical, sack-like or pillow-shaped
forms. On a weathered face they sometimes look like a pile of
partially-filled sacks heaped on each other, the prominences of one
projecting into corresponding hollows in the next. The general aspect
of this structure is shown in Fig. 12, which represents a face of rock
about eight feet high and six feet broad. The rocks exhibiting this
peculiarity are usually finely amygdaloidal, and it may be observed
that the vesicles are grouped in lines parallel to the outer surface
of the pillow-like block in which they occur. The diagram in Fig. 51
represents in ground-plan a surface about twelve feet square on the
shore immediately to the south of the mouth of the River Stinchar.
In the heart of the spheroids enclosed fragments of other lavas are
sometimes observable.
This singular structure has already (p. 184) been referred to as
strikingly displayed in a rock at the top of Cader Idris. It is found
in dark basic lavas probably of Arenig age, which will be afterwards
referred to as occurring along the southern flanks of the Scottish
Highlands and also in the north of Ireland. It has been observed by
Mr. Teall among the rocks of the Lizard, and has been described as
occurring in Saxony and California.[172] In these different localities
it is associated with jaspers and cherts, some of which contain
abundant Radiolaria. The same structure has been found among the
variolitic diabases of Mont Genèvre,[173] and likewise in some modern
lavas, as in that of Acicastello already referred to (_ante_, p. 26).
[Footnote 172: Mr. J. J. H. Teall, _Roy. Geol. Soc. Cornwall_, 1894, p.
3. Mr. L. Ransome, _Bull. Depart. Geol. University of California_, vol.
i. p. 106.]
[Footnote 173: Messrs. Cole and Gregory, _Quart. Journ. Geol. Soc._
vol. xlvi. (1890), p. 311.]
[Illustration: Fig. 52.--View of Knockdolian Hill from the east.]
The volcanic agglomerates and breccias, in the south-west of Ayrshire,
attain a great development in several centres probably at or near the
original volcanic vents. They present several distinct petrographical
types. The remarkable neck-like hill of Knockdolian in the Stinchar
Valley is made of a coarse breccia composed mainly of angular pieces of
dull greyish-green fine-grained diabase. The breccias and agglomerates
of Bennane Head in some parts consist largely of broken-up shales,
flinty mudstone, black radiolarian flint or chert, and abundant
fragments of andesites and felsites. In other parts the volcanic
material predominates, including angular and subangular fragments of
various somewhat basic lavas, lumps of vesicular slag and pieces of
pumice. Here and there much calcite is diffused through the matrix in
strings, veins and patches, which enclose the lapilli. The agglomerate
north of Lendalfoot possesses a greenish, somewhat serpentinous
matrix, through which immense numbers of tabular felspar crystals are
scattered. Similar crystals also occur abundantly in embedded blocks
of one of the purplish diabase-porphyrites, which occurs in mass on
the shore and inland, and closely resembles the rock of Carnethy in the
Old Red Sandstone volcanic series of the Pentland Hills.
Yet another and very distinct type of agglomerate is to be seen on the
Mains Hill south-east of Ballantrae. It is a coarse rock, enclosing
blocks up to a yard or more in diameter, of a fine compact purplish
porphyrite, with large crystals of plagioclase and smaller ones of
augite. In some places immense numbers of the small lapilli in the
matrix consist of an extremely fine vesicular pumice. Small perfect and
larger broken crystals of augite are likewise abundant in some of the
greenish, more basic parts of the mass. These greenish serpentinous
parts and the numerous augite crystals point to the explosion of some
tolerably basic pyroxenic lava. A similar dark green, almost black,
rock, with augite crystals, which sometimes measure a quarter of an
inch in diameter, occurs near Sanquhar in Nithsdale. It presents a
close resemblance to the agglomerate of Rhobell Fawr, already alluded
to. So far as these Scottish agglomerates have yet been microscopically
examined, they have been found to be composed of crystals,
crystal-fragments, and lapilli derived partly from lavas similar to
those above described, and partly from felsitic and other rocks which
have not yet been observed here in the form of lavas.
The finer tuffs show likewise a considerable range of composition.
According to Mr. Peach's observations along the south-eastern parts of
the volcanic area, the ejected materials have consisted largely of fine
dust (probably in great measure felsitic), which towards the north-east
is gradually interleaved with ordinary sediment till the ashy character
disappears. As I have already remarked, there is reason to believe that
the overlying greywackes and shales derived part of their material
either directly from volcanic explosions or from the attrition of banks
of lavas and tuffs exposed to denudation.
But besides the interstratified lavas and fragmental rocks there occur
numerous intrusive masses which are so intimately associated with the
volcanic series that they may with little hesitation be regarded as
forming part of it. They consist of various gabbros and serpentines,
which are especially developed where the volcanic series comes out in
greatest force in the south-west of Ayrshire. They also include more
acid intrusions which, as in the case of the rock of Byne Hill, near
Girvan, even assume the characters of granite.
The dying out of the volcanic material towards the north-east probably
indicates that the vents of the period lay rather in the central
or south-western parts of the district. Unfortunately, the limited
extent of the exposures of the rocks makes it a hopeless task to
search for traces of these vents over by far the largest part of the
area. There are two localities, however, where the search may be
made with better prospect of success. One of these is a tract to the
north of Sanquhar in Nithsdale, which still requires to be studied in
detail with reference to the sequence and structure of its volcanic
rocks. The other area is that south-western part of Ayrshire which
has been already cited as displaying so large a development of the
volcanic series. Here the coast-sections reveal the intercalation of
fossiliferous bands which show the true stratigraphical horizons of the
lavas and tuffs. Under Bennane Head, Professor Lapworth some years ago
found, in certain hardened black shales, a group of graptolites which
mark an undoubted Arenig platform.[174] Recently the ground has been
carefully re-examined by Messrs Peach and Horne, who have detected a
number of other fossiliferous zones which confirm and extend previous
observations. They have also been able to unravel the complicated
structure of the volcanic series, and to represent it on the 6-inch
maps of the Geological Survey, of which a reduction on the scale of 1
inch to a mile is now in course of preparation. The following tabular
summary, taken partly from notes made by myself during a series of
traverses of the ground with Mr. Peach when the revision was begun, and
partly from memoranda supplied by that geologist himself, may suffice
as a general outline of the volcanic history of this exceedingly
interesting and important region.
[Footnote 174: _Geol. Mag._ 1889, p. 22.]
Llandovery.
}Pentamerus grit.
}Conglomerate (Mulloch Hill).
Caradoc.
{Shales, sandstones, grits, etc. (Ardmillan, Balcletchie).
{Thick conglomerate (Byne Hill, Bennane, etc.).
{Thick fossiliferous limestone (Stinchar, Girvan). (On this horizon
{ come the perlitic felsites and soda-felsites of Winkstone and Wrae.)
{Sandstone (_Orthis confinis_) passing down into thick conglomerate.
[Unconformability.]
Upper Llandeilo.
}Green mudstones, grits and greywackes.
}Thin band of dark mudstone with Upper Llandeilo graptolites.
Arenig and Lower and Middle Llandeilo.
{Group of Radiolarian cherts (about 70 feet) with alternating tuffs.
{Tuff or volcanic conglomerate, with occasional lava-flows.
{Black shale (10 feet) with Arenig graptolites.
{Volcanic breccias around local centres (Knockdolian, etc.).
{Thick group of porphyrite and diabase lavas.
{Red flinty mudstones with Arenig graptolites.
{Porphyrites, etc.
{Fine tuffs, etc., with Lower Arenig fossils.
{Diabase lavas, etc. (base not seen).
It will be noticed from this table that the bottom of the volcanic
series is not reached, so that no estimate can be formed of its full
thickness, nor on what geological platform it begins. Possibly its
visible portions represent merely the closing scenes of a long volcanic
history, which, over the area of the south of Scotland, extended into
Cambrian time, like the contemporary series of Cader Idris.
Among the lowest lavas there are interstratified courses of fine
tuffs, flinty shales and thin limestones, which sometimes fill in the
hollows between the pillow-like blocks above referred to. Among the
characteristic Lower Arenig graptolites of these intercalated layers
are _Tetragraptus bryonoides_, _T. fruticosus_, _T. quadribrachiatus_,
and _T. Headi_ together with _Caryocaris Wrightii_. Considerable
variation is to be seen in the development of the upper part of the
volcanic series. In some places the lavas ascend almost to the top;
in others, thick masses of breccia or agglomerate take their place.
These fragmentary materials are locally developed round particular
centres, which probably lie near the sites of active vents whence
large quantities of pyroclastic material were discharged. One of the
volcanic centres must have been situated close to the position of
Knockdolian Hill already referred to. The exceedingly coarse breccia
of that eminence is rudely stratified in alternations of coarser and
finer material, which was probably to some extent assorted under water
around the cinder-cone that discharged it. The date of the explosions
of this hill has been ascertained by Mr. Peach from the intercalation
of black shales containing Arenig graptolites among the breccias.
Another vent lay somewhere in the immediate neighbourhood of the Mains
Hill agglomerate, if not actually on part of the site of that rock.
Though probably not more than a mile from the Knockdolian volcano, and
belonging to the same epoch of eruption, this vent, to judge from the
peculiarities of its ejected material, must have been quite distinct
in its source. A third vent lay somewhere in the immediate vicinity of
Bennane Head, and threw out the extraordinary masses of agglomerate and
the sheets of lava seen on the coast at that locality. A fourth may be
traced by its separate group of fine tuffs on the coast three miles
south of Ballantrae.
[Illustration: Fig. 53.--Section across the Lower Silurian volcanic
series in the south of Ayrshire (B. N. Peach).
B, Interstratified lavas in Arenig group; _t_, tuffs; _r_, radolarian
cherts; _b_^2, Llandeilo group; _b_^3 Caradoc group. Σ, Serpentine.
G, Gabbro.]
A feature of singular interest in the material erupted from these
various centres of activity consists in the evidence that the
explosions occurred at intervals during the deposition of the Lower
Silurian formations, and that these formations were successively
disrupted by submarine explosions. Mr. Peach has found, for example,
abundant pieces of the peculiar and easily recognized radiolarian
cherts imbedded in the volcanic series. That these cherts were
deposited contemporaneously with the volcanic eruptions is proved by
their intercalation among the breccias. Yet among these very breccias
lie abundant fragments of chert which must have already solidified
before disruption. It is thus evident that this siliceous ooze not only
accumulated but set into solid stone on the sea-floor, between periods
of volcanic outburst, and that such an occurrence took place several
times in succession over the same area.
These facts derive further interest from the organic origin of the
chert. It is now some years since Mr. Peach and his colleagues observed
that between the Glenkiln Shale with its Upper Llandeilo graptolites
and the top of the volcanic group in the central part of the Silurian
uplands, alternations of green, grey or red shaly mudstones and flinty
greywackes are interleaved with fine tuffs, and are specially marked
by the occurrence in them of nodules and bands of black, grey and
reddish chert. This latter substance, on being submitted to Dr. Hinde,
was found by him to yield twenty-three new species of Radiolaria
belonging to twelve genera, of which half are new. It thus appears that
during the volcanic activity there must have been intervals of such
quiescence, and such slow, tranquil sedimentation in clear, perhaps
moderately deep water, that a true radiolarian ooze gathered over the
sea-bottom.[175]
[Footnote 175: _Ann. Mag. Nat. Hist._ (1890), 6th ser. vi. p. 40.]
That the deposition of this ooze probably occupied a prolonged lapse
of time seems clearly indicated by the evidence of the fossils that
occur below and above the cherts. The graptolites underneath indicate
a horizon in the Middle Arenig group, those overlying the cherts are
unmistakably Upper Llandeilo. Thus the great depth of strata which
elsewhere constitute the Upper Arenig and Lower and Middle Llandeilo
subdivisions is here represented by only some 60 or 70 feet of
radiolarian cherts. These fine siliceous, organic sediments probably
accumulated with extreme slowness in a sea of some depth and over a
part of the sea-floor which lay outside the area of the transport and
deposit of the land-derived sediment of the time.[176]
[Footnote 176: _Annual Report of the Geol. Surv. for 1895_, p. 27 of
reprint.]
[Illustration:
Fig. 54.--Section of part of the Arenig volcanic group, stream
south of Bennane Head, Ayrshire.
]
As an illustration of some of the characteristic features in the
succession of deposits in the volcanic series of the south-west
of Ayrshire, the accompanying section (Fig. 54) is inserted. In
descending order we come first upon a group of greywackes and grey
shattery mudstones (_a_), followed by grey-green and dark banded
cherts, containing Radiolaria and much plicated. Next comes a group
of dark-grey, black and red cherts, with numerous partings and thin
bands of tuff and volcanic conglomerate (_c_). The siliceous bands were
certainly deposited during the volcanic eruptions, and they are moulded
round the rugose, slaggy upper surface of the band of lavas (_d_) on
which they directly lie. These lavas have the sack-like or pillow
structure already described, and they enclose lumps of chert containing
Radiolaria. A few yards to the west of the line of section bands of
nodular tuff are interposed between the top of the lavas and the
overlying cherts, with which also they are interstratified. These tuffs
contain blocks of lava six inches or more in diameter. Below the belt
of lavas come black cherts and shales (_e_) succeeded below by volcanic
breccias and tuffs (_f_) alternating with shales in thin inconstant
courses. These coarse detrital rocks are thoroughly volcanic in origin,
and they contain fragments of the black cherts which lie still lower in
the series. The whole depth of strata represented in this section does
not amount to much more than 100 feet.
While in some parts of the Ayrshire district the coarse breccias
that accumulated around their parent vents form most of the upper
part of the volcanic series, in others the lavas are succeeded by
fine tuffs which are intercalated among the ordinary sediments, and
show a gradual decline and cessation of volcanic energy. South of
Ballantrae, for example, the lavas occupy more than two miles of coast,
in which space they display hardly any intercalations of sedimentary
material, though they show more or less distinctly that they consist
of many separate flows. Where they at last end, bands of nodular and
fine tuff make their appearance, together with bands of ashy shale
and the characteristic zone of the red radiolarian cherts or flints.
Above these, in conformable sequence, come bands of black shale,
containing abundant Upper Llandeilo graptolites, overlain by greenish
or olive-coloured shaly mudstones, which pass upward into a thick
overlying group of greywackes.
In this section the alternation of fine pyroclastic with ordinary
sediment shows that the volcanic eruptions in the southern part of the
Ballantrae district came to an end by a slowly-lessening series of
explosions. The ashy material gradually dies out, and does not reappear
all through the thick group of sandy and muddy sediments which here
overlies the volcanic series.
We thus learn from the evidence of the Ayrshire sections that volcanic
action was in full vigour in the south-west of Scotland during the
Arenig period, but gradually died out before the end of the Llandeilo
period. The rocks in which this volcanic history is chronicled have
been very greatly disturbed and plicated, so that though from their
frequently vertical position they might be thought to attain a vast
depth, they very possibly do not exceed 500 feet in thickness.
As the volcanic series is followed north-eastwards it exhibits a
gradual diminution in extent and variety, but this may be at least
partly due to the much less depth of it exposed on the crests of the
narrow anticlines that bring it to the surface. There is evidence in
that region that the eruptions did not everywhere terminate in the
Llandeilo period, but were in some districts prolonged into the age
of the Bala rocks. Thus in the neighbourhood of Sanquhar volcanic
breccias, tuffs and lavas have been found by Messrs. Peach and Horne
intercalated in strata apparently belonging to the Bala group. Again,
in the district of Hartfell, a moderately coarse volcanic agglomerate
occurs in the heart of the so-called "barren mudstones" of the Hartfell
black-shale group, which, from its graptolites, is placed on the
horizon of the Bala rocks. At Winkstone, Hamilton Hill, and Wrae in
Peeblesshire, perlitic felsites and soda-felsites have been detected
by Messrs. Peach and Horne and determined by Mr. Teall. They are
associated with the Bala limestone, which in some of its conglomeratic
bands contains pebbles of felsite.
The intrusive rocks which accompany the Lower Silurian volcanic series
of the south of Scotland are best displayed in the south-west of
Ayrshire, between Girvan and Ballantrae, where they appear to be on the
whole later than at least the great mass of the interstratified lavas
and tuffs. The most abundant rocks and the earliest to be injected are
complex basic masses which include serpentine, olivine-enstatite rock,
troctolite, gabbro and other compounds, all which may be different
modifications of the same original basic magma. They do not show a
finer texture where they respectively meet, nor any other symptom
of having been subsequently intruded into each other, though they
do exhibit such structures along their lines of contact with the
surrounding rocks, into which they are intrusive. These more basic
masses have subsequently been invaded by irregular bosses and dyke-like
protrusions, which, when small, are fine-grained dolerites, but when in
larger bodies take the form of gabbro, sometimes exhibiting a mineral
banding and foliated structure. These banded varieties much resemble
the banded Tertiary gabbros of Skye and some parts of the Lewisian
gneiss.
At the Byne Hill, near Girvan, a large intrusive boss or ridge displays
on its outer margin a fine-grained texture, where it comes in contact
with the serpentine. Further inwards it becomes a fine dolerite,
passing into gabbro and increasing in coarseness of grain as well as
in acidity of composition, through stages of what in the field would
be called diorite and quartz-diorite, into a central granitic rock,
whereof milky or blue quartz forms the prominent constituent. The
intrusive rocks of this district have generally been injected parallel
to the stratification-planes, and take on the whole the form of sills.
Some time after the close of the volcanic episode in the Silurian
period of the south of Scotland, the rocks were locally subjected to
considerable disturbance and elevation, whereby parts of the volcanic
series were exposed to extensive denudation. Hence the overlying
unconformable Caradoc conglomerates are in some places largely made
up of the detritus of the volcanic rocks. It is interesting to find
this evidence of waste during the very next stage of the Silurian
period, for it affords good evidence that the extensive sheets of
intrusive material could not have had any large amount of overlying
strata resting upon them at the time of their injection. Pieces of
these intrusive rocks, such as the serpentine, occur abundantly in the
Caradoc conglomerates, some of which indeed are almost wholly composed
of their detritus. Probably the total thickness of the overlying cover
of rock under which the sills were injected did not amount to as much
as 200 or 300 feet. Yet we see that among the sills were coarse gabbros
and granitoid rocks. We may therefore infer that for the injection of
such intrusive masses, great depth and enormous superincumbent pressure
are possibly not always necessary.
During the progress of the Geological Survey along the southern borders
of the Highlands, a remarkable group of rocks has been observed,
intervening as a narrow interrupted strip between the schistose
masses to the north and the great boundary-fault which brings the Old
Red Sandstone in vertical strata against them. Between Cortachy in
Forfarshire and Stonehaven on the east coast, these rocks have been
mapped by Mr. G. Barrow, who has carefully worked out their relations.
They appear again between Callander and Loch Lomond, where their extent
and structure have been mapped by Mr. C. T. Clough. For the purpose of
our present inquiry two chief features of interest are presented by
these rocks. They include a group of sedimentary strata among which
occur bands of jasper or chert containing radiolaria, and one of their
most conspicuous members is a series of volcanic rocks consisting
chiefly of dolerites and basalts, some of which have been much crushed
and cleaved, but in which vesicular structures can still occasionally
be recognized.
The striking resemblance of both the aqueous and igneous members of
this marginal strip of rocks along the Highland border to the Arenig
cherts and their accompanying lavas in the south of Scotland, the
remarkable association of the same kinds of material in the same order
of sequence, the occurrence of radiolaria in the siliceous bands in
both regions, furnish strong presumptive evidence that a strip of
Arenig rocks has been wedged in against the Highland schists.
In many respects, these dull green diabasic lavas of the Highland
border resemble those of the Ayrshire coast. In particular, the same
peculiar sack-like or pillow-shaped masses are conspicuous in the
Forfarshire ravines. As in Ayrshire, igneous materials underlie the
cherts which are doubled over and repeated by many successive folds.
Unfortunately, it is only a narrow strip of these probably Arenig lavas
that has been preserved, and no trace has been detected of tuffs,
agglomerates or necks. If, however, we may regard the rocks as truly
of Arenig age, they furnish interesting additional proof of the wide
extent of the earliest Silurian volcanoes. The distance between the
last Arenig volcanic outcrop in the Southern uplands and the band of
similar lavas along the margin of the Highlands is about 50 miles. If
the volcanic ejections were continuous across the intervening tract,
the total area over which the lavas and tuffs of the Arenig volcanoes
were distributed must be increased by at least 6000 square miles in
Scotland.
But it is in the north of Ireland that this northern extension of what
may probably be regarded as an Arenig series of volcanic rocks attains
its greatest development. Of this Irish prolongation a brief account is
given in Chapter xiv., where the whole of the Silurian volcanic rocks
of the island are discussed.
CHAPTER XIII
THE ERUPTIONS OF LLANDEILO AND BALA AGE
i. The Builth Volcano--ii. The Volcanoes of Pembrokeshire--iii. The
Caernarvonshire Volcanoes of the Bala Period--iv. The Volcanic
District of the Berwyn Hills--v. The Volcanoes of Anglesey--vi.
The Volcanoes of the Lake District; Arenig to close of Bala
Period--vii. Upper Silurian (?) volcanoes of Gloucestershire.
The stratigraphical subdivisions of geology are necessarily more
or less arbitrary. The sequence in the sedimentary deposits of one
region always differs in some degree from that of adjoining regions.
In drawing up a table of stratigraphical equivalents for separate
countries, we must be content to accept a general parallelism, without
insisting on too close an identity in either the character of the
strata or the grouping of their organic remains. We need especially to
guard against the assumption that the limit assigned to a geological
formation in any country marks a chronological epoch which will
practically agree with that denoted by the limit fixed for the same
formation in another country. The desirability of caution in this
respect is well shown by the vagueness of the horizons between the
several subdivisions of the Lower Silurian system. So long as the
areas of comparison are near each other, no great error may perhaps
be committed if their stratigraphical equivalents are taken to have
been in a broad geological sense contemporary. But in proportion as
the element of distance comes in, there enters with it the element of
uncertainty.
Even within so limited a region as the British Isles, this difficulty
makes itself strongly felt. Thus, in the typical regions of Wales, the
several subdivisions of the Lower Silurian strata are tolerably well
marked, both by lithological nature and by fossils. But as they are
followed into other parts of the country, they assume new features,
sometimes increasing sometimes diminishing in thickness, changing their
sedimentary character, and altering the association or range of their
organisms. The subdivisions into which the geologist groups them may
thus be vaguely defined by limits which, in different parts of the
region, may be far from representing the same periods of time.
Hence, in trying to ascertain how far the volcanic eruptions of one
area during the Silurian period may have been contemporary with those
of another area, we must be content to allow a wide margin for error.
It is hardly possible to adhere strictly to the stratigraphical
arrangement, for the geological record shows that in the volcanic
districts the sedimentary formations by which the chronology might have
been worked out are not infrequently absent or obscure. It will be more
convenient to treat the rest of the Lower Silurian formations as the
records of one long and tolerable definite section of geological time,
without attempting in each case to distinguish between the eruptions
of the successive included periods, so long as the actual volcanic
sequence is distinctly kept in view. I will therefore take the history
of each district in turn and follow its changes from the close of the
Arenig period to the end of Upper Silurian time. The stages in the
volcanic evolution of each tract will thus be clearly seen.
Above the Arenig group with its voluminous volcanic records comes the
great group of sediments known as the Llandeilo formation, in which
also there are proofs of contemporaneous volcanic activity over various
parts of the sea-floor within the site of Britain. We have seen that
in the south of Scotland the eruptions of Arenig time were probably
continued into the period of the Llandeilo rocks, or even still later
into that of the Bala group. But it is in Wales that the history of the
Llandeilo volcanoes is most fully preserved. A series of detached areas
of volcanic rocks, intercalated among the Llandeilo sediments, may be
followed for nearly 100 miles, from the northern end of the Breidden
Hills in Montgomeryshire, by Shelve, Builth, Llanwrtyd and Llangadock,
to the mouth of the Taf river. But some 35 miles further west another
group of lavas and tuffs appears on the coast of Pembrokeshire,
from Abereiddy Bay to beyond Fishguard. The want of continuity in
these scattered outcrops is no doubt partly due to concealment by
geological structure. But from the comparative thinness of the volcanic
accumulations and their apparent thinning out along the strike it may
be inferred that no large Llandeilo volcano existed in Wales. There
would rather seem to have been a long line of minor vents which in
the south-east part of the area appear to have only discharged ashes.
Certainly, if we may judge from their visible relics, these eruptions
never rivalled the magnitude of the discharges from the Arenig
volcanoes that preceded, or the Bala volcanoes that followed them.
i. THE VOLCANO OF BUILTH AND ITS NEIGHBOURS
So far as the available evidence goes, the most important volcanic
centre down the eastern side of Wales during the Llandeilo period
was one which lay not far from the centre of the long line of vents
just referred to. Its visible remains form an isolated tract of
hilly ground, some seven miles long, and four or five miles broad,
immediately north from the town of Builth. This area is almost entirely
surrounded by unconformable Upper Silurian strata, so that its total
extent is not seen, and may be much more considerable than the area now
laid bare by denudation.
The volcanic rocks of Builth were first described in the "Silurian
System." Murchison clearly recognized that they included some which
were "evolved from volcanic apertures during the submarine accumulation
of the Lower Silurian rocks," and also "unbedded volcanic masses
which had been intruded subsequently, dismembering and altering all
the strata with which they came in contact."[177] These igneous rocks
were mapped in some detail by the Geological Survey, and their general
relations were expressed in lines of horizontal section.[178] They were
likewise described by Ramsay in the _Catalogue of the Rock-specimens
in the Jermyn Street Museum_, specimens of them being displayed in
that collection.[179] The tuffs and lavas were distinguished, and
likewise the intrusive "greenstones." But no attempt was made towards
petrographical detail.
[Footnote 177: _Silurian System_, 1839, p. 330. The occurrence of
"trappean ash" with fossils in the Builth district was noticed by De la
Beche, _Mem. Geol. Surv._ vol. i. (1846), p. 31.]
[Footnote 178: See Sheet 56 of the one-inch map and Sheets 5 and 6 of
the Horizontal Sections.]
[Footnote 179: _Catalogue of Rock Specimens_, 3rd edit. 1862, p. 36 _et
seq._]
This interesting district has recently been studied by Mr. Henry
Woods,[180] who has grouped the igneous rocks in probable order of
appearance, as follows:--1st, Andesites; 2nd, Andesitic ash; 3rd,
Rhyolites; 4th, Diabase-porphyrite; and 5th, Diabase.
[Footnote 180: _Quart. Journ. Geol. Soc._ vol. l. (1894), p. 566.]
Some of the andesites are described as intrusive in the Llandeilo
strata. The ash in its lower part contains numerous well-rounded
pebbles of andesite, usually five or six inches in diameter, but
sometimes having a length of two feet. It contains fossils (_Orthis
calligramma_, _Leptæna sericea_, _Serpulites dispar_, etc.), and as
it is overlain with shales containing _Ogygia Buchii_, it may be
regarded as probably of Lower Llandeilo age. The rhyolites are feebly
represented, and some of them may possibly be intrusive. Among them a
nodular variety has been noticed, the nodules being solid throughout,
varying up to two inches in diameter, and formed of microcrystalline
quartz and felspar, with no trace of any radial or concentric internal
arrangement. The diabase-porphyrite, the most conspicuous rock of the
district, is intrusive in the andesites and ashes, and occurs in four
separate masses or sills. The diabases are all intrusive and of later
date than any of the other igneous rocks, and as they traverse also
the Llandeilo shales, they are probably considerably later than the
previous eruptions. But as they do not enter the surrounding Llandovery
and Wenlock strata, they are regarded by Mr. Woods as of intermediate
age between the time of the Llandeilo and that of the Upper Silurian
formations.
About nine miles in a west-south-westerly direction from the southern
extremity of the Builth volcanic area, another much smaller exposure of
igneous rocks has been mapped by the Geological Survey at the village
of Llanwrtyd. This tract is only about three miles long and half a mile
broad. The volcanic rocks are represented as consisting of three or
more bands of "felspathic trap" interstratified in the Lower Silurian
strata, and folded into an anticline along the ridge of Caer Cwm. No
published line of section runs across this ground, and the band of
rock does not appear to have been described.[181]
[Footnote 181: The locality is referred to by De la Beche, _Mem. Geol.
Surv._ vol. i. p. 31, and by Ramsay in the _Descriptive Catalogue of
Rock-specimens in the Museum of Practical Geology_, 3rd edit. p. 38,
but no specimens from it are in the collection.]
Seventeen miles to the south-west a still feebler display of
intercalated volcanic material occurs in the Llandeilo formation near
the village of Llangadock. The Geological Survey map represents one
or more bands of ash associated with limestone, and thrown into a
succession of folds. In the _Horizontal Section_ (Sheet III. Section
3) a band, 100 to 200 feet thick, of "trappean ash" with fossils is
shown among the shales, limestones and grits, and in the _Catalogue
of Rock-specimens_ the same rock is referred to as brecciated ash in
connection with specimens of it in the Museum, which are described
as not purely ashy, but containing many slate-fragments and broken
felspar-crystals together with organic remains.[182]
[Footnote 182: _Op. cit._ p. 38.]
About twenty-four miles still further in the same south-westerly
direction, two patches of "ash" are shown upon the Survey map, near the
mouth of the river Taf. No description of these rocks is given.[183]
[Footnote 183: One of the patches was shown by J. Phillips in
_Horizontal Section_, Sheet III. Section 6, as a "felspathic trap,"
near which the shales are bleached. The map, however, was subsequently
altered, so as to make the igneous rocks pyroclastic.]
ii. THE VOLCANOES OF PEMBROKESHIRE
In north-western Pembrokeshire, the observations of Murchison, De
la Beche and Ramsay showed the existence of an important volcanic
district, where numerous igneous bands are interstratified among the
Lower Silurian rocks, over an area extending from St. David's Head
for thirty miles to the eastward.[184] On the maps of the Geological
Survey, lavas, tuffs, sills and bosses were discriminated, but no
description of these rocks was published. Since the publication of the
Survey map very little has yet been added to our information on the
subject.
[Footnote 184: See _Silurian System_, p. 401; Sheet 40 of the
Geological Survey Map; _Memoir of A. C. Ramsay_, p. 232 _et seq._; De
la Beche, _Trans. Geol. Soc._ 2nd series, vol. ii. part i. (1826), p.
3.]
There appear to have been at least three principal groups of vents.
One may be indicated by the bands of "felspathic trap" which have been
mapped as extending from near St. Lawrence for fourteen miles to the
east. Another must have existed in the neighbourhood of Fishguard. A
third is shown to have lain between Abereiddy Bay and Mathry, by the
abundant bands of lava and tuff and intrusive sills there to be seen.
Of these areas the only one which has yet been examined and described
in some detail is that of Fishguard, of which an account has recently
been published by Mr. Cowper Reed.[185] This observer has shown that
the eruptions began there during the deposition of the Lower Llandeilo
rocks, and continued intermittently into the Bala period. The earliest
consisted of felsites and tuffs intercalated between Lower Llandeilo
black slates containing _Didymograptus Murchisoni_, the tuffs
themselves being sometimes fossiliferous. A second great volcanic belt,
composed of felsitic lavas, breccias and tuffs, lies at the base of the
Upper Llandeilo strata and shows the maximum of volcanic energy. The
breccias are partly coarse agglomerates, which probably represent, or
lie not far from, some of the eruptive vents of the time. A higher band
of lavas and breccias appears to be referable to the Bala formation.
The whole volcanic series is stated to thin out towards the south-west,
so that the chief focus of eruption probably lay somewhere in the
neighbourhood of Fishguard.
[Footnote 185: _Quart. Journ. Geol. Soc._ vol. li. (1895), p. 149.]
The lavas may all be included under the general term felsite. Their
specific gravity ranges from 2·60 to 2·76, and their silica percentage
from 68 to 72. Mr. Cowper Reed observed among them three conspicuous
types of structure. Some are characterized by a distinct arrangement in
fine light and dark bands which rapidly alternate, and are sometimes
thrown into folds and convolutions. A second structure, observed only
at one locality, consists in the development of pale grey or whitish
ovate nodules, about half an inch in length, with a clear quartz-grain
in their centre, or else hollow. The third type is shown by the
appearance of perlitic structure on the weathered surface.[186]
[Footnote 186: Mr. Cowper Reed enters into a detailed account of the
microscopic structures and chemical composition of these rocks. They
have rather a high percentage of alumina, potash and soda, and are
obviously akin to the keratophyres of other districts.]
The tuffs and breccias are chiefly developed at the base and top of
each volcanic group. Some of them contain highly vesicular fragments,
as well as pieces of slate and broken crystals of quartz and felspar.
A characteristic feature of this volcanic district is the occurrence
in it of sills and irregularly-intruded masses of "greenstone." Under
that name are comprised basalts, dolerites, andesitic dolerites with
tachylitic modifications, as well as diabases and gabbros.[187] Some of
these rocks exhibit a variolitic structure. As regards age, some of the
intrusions appear to have taken place before the tilting, cleavage and
faulting of the strata. They have not been noticed in the surrounding
Upper Silurian strata, and we may perhaps infer that here, as at
Builth, they are of Lower Silurian date. Mr. Cowper Reed, however, is
inclined to regard the large Strumble Head masses as later than the
tilting and folding of the rocks.[188]
[Footnote 187: Mr. Cowper Reed, _op. cit._ p. 180.]
[Footnote 188: _Op. cit._ p. 193.]
A few miles to the south-west of the Fishguard district, on the coast
of Abereiddy Bay, good sections have been laid bare of the volcanic
rocks of this region. Dr. Hicks has shown that the bands of tuff
there displayed are intercalated among the black slates of the Lower
Llandeilo group, and that there was probably a renewal of volcanic
activity during the deposition of the upper group.[189] But the
volcanic history of this area still remains to be properly investigated.
[Footnote 189: _Quart. Journ. Geol. Soc._ xxxi. (1875), p. 177.]
In southern Pembrokeshire two conspicuous bands of eruptive rocks
have long been known and described. Their general characters and
distribution were sketched by De la Beche,[190] and further details
were afterwards added by Murchison.[191] As traced by the officers
of the Geological Survey, they were represented as consisting of
"greenstone," "syenite" and "granite." The more northerly band was
shown to run in a nearly east and west line from Lawrenny to the Stack
Rock, west of Talbenny, a distance of about fourteen miles. The second
band, placed a short way farther south, stretches in the same general
line, from Milford Haven at Dall Road into Skomer Island, a distance of
about seven miles.
[Footnote 190: _Trans. Geol. Soc._, 2nd ser. vol. ii. (1823), p. 6 _et
seq._]
[Footnote 191: _Silurian System_, p. 401 _et seq._]
The relations of these rocks to the surrounding formations and their
geological age have been variously interpreted. De la Beche regarded
the different masses as intrusive, and probably later than even the
adjoining Coal-measures.[192] Murchison, on the other hand, considered
the bedded eruptive rocks of Skomer Island to be undoubtedly lavas
contemporaneous with the strata among which they are intercalated.[193]
[Footnote 192: _Mem. Geol. Survey_, vol. i. p. 231.]
[Footnote 193: _Silurian System_, p. 404.]
The rocks have been studied petrographically by various observers. Mr.
Rutley gave a full description of the remarkable nodular and banded
felsites of Skomer Island.[194] Mr. Teall has also noticed these rocks,
likewise "a magnificent series of basic lava-flows" in the same island,
and a number of "porphyrites." The basic lavas seemed to him to contain
too much felspar and too little olivine to be regarded as perfectly
typical olivine-basalts, and he found them to lie sometimes in very
thin and highly vesicular sheets. The "porphyrites" he placed "on the
border-line between basic and intermediate rocks."[195]
[Footnote 194: "The Felsitic Lavas of England and Wales," _Mem. Geol.
Survey_ (1885), pp. 16, 18.]
[Footnote 195: _British Petrography_, pp. 224, 284, 336.]
More recently this southern district of Pembrokeshire has been examined
by Messrs. F. T. Howard and E. W. Small, who have obtained further
evidence of the interbedded character of the igneous series. Below
an upper basalt they have noted the occurrence of bands of felsitic
conglomerate, sandstone, shale and breccia lying upon and obviously
derived from a banded spherulitic felsite, below which comes a lower
group of basalts. The age of this interesting alteration of basic and
acid eruptions has not been precisely determined, but is conjectured to
be that of the Bala or Llandovery rocks.[196]
[Footnote 196: _Rep. Brit. Assoc._ 1893, p. 766; _Geol. Mag._ 1896, p.
481.]
iii. THE CAERNARVONSHIRE VOLCANOES OF THE BALA PERIOD
Owing to the effects partly of plication and partly of denudation, the
rocks of the next volcanic episode in Wales, that of the Bala period,
occupy a less compact and defined area than those of the Arenig group
in Merionethshire. From the latter they are separated, as we have seen,
by a considerable depth of strata,[197] whence we may infer, with
the Geological Survey, that the eruptions of Arenig, the Arans and
Cader Idris were succeeded by a long period of repose, the Llandeilo
outbreaks described in the foregoing pages not having extended
apparently into North Wales. When the next outbreaks took place, the
vents are found to have shifted northwards into Caernarvonshire, where
they fixed themselves along a line not much to the east of where the
Cambrian porphyries and tuffs now appear at the surface. The lavas and
ashes that were thrown out from these vents form the highest and most
picturesque mountains of North Wales, culminating in the noble cone
of Snowdon. They stretch northwards to Diganwy, beyond Conway, and
southwards, at least as far as the neighbourhood of Criccieth. They die
out north-eastwards beyond Bala Lake, and there can be but little doubt
that they thin out also eastwards under the Upper Bala rocks. The lavas
and tuffs that rise up on a similar horizon among the Bala rocks of the
Berwyn Hills evidently came not from the Snowdonian vents, but from
another minor volcanic centre some miles to the east, while still more
remote lay the vents of the Breidden Hills and the sheets of andesitic
tuff that probably spread from them over the ground east of Chirbury
(Map II.).
[Footnote 197: Estimated at from 6000 to 7000 feet, _Mem. Geol. Surv._
vol. iii. 2nd edit. p. 131.]
The Caernarvonshire volcanic group extends from north to south for
fully thirty miles, with an extreme breadth of about fifteen miles;
while, if we include the rocks of the Lleyn peninsula, the area will be
prolonged some twenty miles farther to the south-west.
The general stratigraphical horizon of this volcanic group has been
well determined by the careful mapping of Ramsay, Selwyn and Jukes on
the maps of the Geological Survey. These observers brought forward
ample evidence to show that the lavas and tuffs were erupted during
the deposition of the Bala strata of the Lower Silurian series, that
the Bala Limestone is in places full of ashy material, and that this
well-marked fossiliferous band passes laterally into stratified
volcanic tuffs containing the same species of fossils.[198] But the
progress of stratigraphical geology, and the increasing value found
to attach to organic remains as marking even minor stratigraphical
horizons, give us reason to believe that a renewed and still more
detailed study of the Bala rocks of North Wales would probably furnish
data for more precisely defining the platforms of successive eruptions,
and would thus fill in the details of the broad sketch which Sir
Andrew Ramsay and his associates so admirably traced. Besides the
Bala Limestone there may be other lithological horizons which, like
the Garth grit and the pisolitic iron-ore of the Arenig group, might
be capable of being followed among the cwms and crests as well as the
opener valleys of Caernarvonshire. Until some such detailed mapping is
accomplished, we cannot safely advance much beyond the point where the
stratigraphy was left by the Survey.
[Footnote 198: _Mem. Geol. Surv._ vol. iii. 2nd edit. pp. 126, 128,
131, 139, etc.]
From the Survey maps and sections it is not difficult to follow the
general volcanic succession, and to perceive that the erupted materials
must altogether be several thousand feet in thickness from the lowest
lavas in the north to the highest on the crest of Snowdon. In that
mountain the total mass of volcanic material is set down as 3100 feet.
But this includes only the higher part of the whole volcanic group.
Below it come the lavas of Y Glyder-Fach, which, according to the
Survey measurements, are about 1500 feet thick, while still lower
lie the ancient _coulées_ of Carnedd Dafydd and those that run north
from the vent of Y-foel-frâs, which must reach a united thickness of
many hundred feet. We can thus hardly put the total depth of volcanic
material at a maximum of less than 6000 to 8000 feet. The pile is, of
course, thickest round the vents of discharge, so that no measurement,
however carefully made at one locality, would be found to hold good for
more than a short distance.
Though little is said in the Survey Memoir of the vents from which this
vast amount of volcanic material was erupted, the probable positions
of a number of these orifices may be inferred from the maps. From
the shore west of Conway a series of remarkable eminences may be
traced south-westwards for a distance of nearly forty miles into the
peninsula of Lleyn. At the northern extremity of this line stands the
prominent boss of Penmaen-mawr, while southward beyond the large mass
of Y-foel-frâs, with the smaller knobs west of Nant Francon, and the
great dome of Mynydd-mawr, the eye ranges as far as the striking group
of _puy_-like cones that rise from the sea around Yr Eifl and Nevin.
Some of these hills, particularly Y-foel-frâs, were recognized by the
Survey as vents.[199] But the first connected account of them and of
their probable relation to the volcanic district in which they occur
has been given by Mr. Harker in his exceedingly able essay on "The
Bala Volcanic Series of Caernarvonshire,"[200]--the most important
contribution to the volcanic history of Wales which has been made since
the publications of the Geological Survey appeared. I shall refer to
these vents more specially in the sequel. I allude to them here for the
purpose of showing at the outset the marvellous completeness of the
volcanic records of Caernarvonshire. So great has been the denudation
of the region that the pile of lavas and tuffs which accumulated
immediately around and above these orifices has been swept away. No
trace of any portion of that pile has survived to the west of the
line of bosses; while to the east, owing to curvature and subsequent
denudation, the rocks have been dissected from top to bottom, until
almost every phase of the volcanic activity is revealed.
[Footnote 199: _Op. cit._ pp. 137, 220.]
[Footnote 200: This was the Sedgwick Prize Essay for 1888, and was
published in 1889.]
The volcanic products discharged from these vents consist of a
succession of lava-streams separated by bands of slate, tuff,
conglomerate and breccia. These fragmental intercalations, which vary
from a few yards to many hundred feet in thickness, are important not
only as marking pauses in the emission of lava or in the activity of
the volcanoes, but as affording a means of tracing the several lavas to
their respective vents. Essentially, however, the volcanic materials
consist of lava-flows, the intercalations of fragmentary materials,
though numerous, being comparatively thin. The thickest accumulation
of tuffs is that forming much of the upper part of Snowdon. It is set
down by my predecessor at 1200 feet in thickness, but I should be
inclined to reduce this estimate. I shall have occasion to show that
the summit and upper shoulders of Snowdon are capped with andesites
interstratified among the tuffs. Sir Andrew Ramsay has referred with
justice to the difficulty of always discriminating in the field between
the fine tuffs and some of the lavas.[201] Yet I am compelled to admit
that, if the ground were to be re-mapped now, the area represented
as covered by fragmental rocks would be considerably restricted. Mr.
Harker is undoubtedly correct when he remarks that, taken "as a whole,
the Bala volcanic series of Caernarvonshire is rather remarkable for
the paucity of genuine ashes and agglomerates."[202]
[Footnote 201: _Op. cit._ p. 148.]
[Footnote 202: _Bala Volcanic Rocks_, p. 25.]
The lavas of the Bala volcanic group, like those of the Arenig series,
were mapped by the Survey as "porphyries," "felstones," or "felspathic
traps." They were shown to be acid-lavas, having often a well-developed
flow-structure comparable with that of obsidian and pitchstone, and to
consist of successive sheets that were poured out over the sea-floor.
Their petrography has subsequently been studied more in detail by many
observers, among whom I need only cite Professor Bonney, Professor
Cole, Mr. Rutley, Mr. Teall, and Miss Raisin; the most important recent
additions to our knowledge of this subject having been made by Mr.
Harker in the Essay to which I have just referred.
The great majority of these lavas are thoroughly acid rocks, and
present close analogies of composition and structure to modern
rhyolites, though I prefer to retain for them the old name of
"felsites." Their silica-percentage ranges from 75 to more than 80.
To the naked eye they are externally pale greyish, or even white, but
when broken into below the thick decomposed and decoloured crust, they
are bluish-grey to dark iron-grey, or even black. They break with a
splintery or almost conchoidal fracture, and show on a fresh surface
an exceedingly fine-grained, tolerably uniform texture, with minute
scattered felspars.
One of their most striking features is the frequency and remarkable
development of their flow-structure. Not merely as a microscopic
character, but on such a scale as to be visible at a little distance on
the face of a cliff or crag, this structure may be followed for some
way along the crops of particular flows. The darker and lighter bands
of devitrification, with their lenticular forms, rude parallelism and
twisted curvature, have been compared to the structure of mica-schist
and gneiss. One aspect of this structure, however, appears to have
escaped observation, or, at least, has attracted less notice than it
seems to me to deserve. In many cases it is not difficult to detect,
from the manner in which the lenticles and strips of the flow-structure
have been curled over and pushed onward, what was the direction in
which the lava was moving while still a viscous mass. By making a
sufficient number of observations of this direction, it might in some
places be possible to ascertain the quarter from which the several
flows proceeded. As an illustration, I would refer to one of the
basement-felsites of Snowdon, which forms a line of picturesque crags
on the slope facing Llanberis. The layers of variously-devitrified
matter curl and fold over each other, and have been rolled into balls,
or have been broken up and enclosed one within the other (Fig. 55).
The general push indicated by them points to a movement from the
westward. Turning round from the crags, and looking towards the west,
we see before us on the other side of the deep vale of Llyn Cwellyn,
at a distance of little more than three miles, the great dome-shaped
Mynydd-mawr, which, there is every reason to believe, marks one of the
orifices of eruption. It might in this way be practicable to obtain
information regarding even some of the vents that still lie deeply
buried under volcanic or sedimentary rocks.
That these felsites were poured forth in a glassy condition may be
inferred from the occurrence of the minute perlitic and spherulitic
forms so characteristic of the devitrification of once vitreous rocks.
Mr. Rutley was the first who called attention to this interesting
proof of the close resemblance between Palæozoic felsites and modern
obsidians, and other observers have since confirmed and extended his
observations.[203]
[Footnote 203: _Quart. Journ. Geol. Soc._ vol. xxxv. (1879), p. 508.]
[Illustration: Fig. 55.--Flow-structure in the lowest felsite on the
track from Llanberis to the top of Snowdon. Length about 4 feet, height
2½ feet.]
Another remarkable aspect of the felsites is that nodular structure so
often to be seen among them, and regarding the origin of which so much
has already been written. I agree with Professor Cole and Mr. Harker
in looking upon the "nodules" as derived from original spherulites by
a process of alteration, of which almost every successive stage may be
traced until the original substance of the rock has been converted into
a flinty or agate-like material. If this be the true explanation of the
structure, some of the original lavas must have exhibited perlitic and
spherulitic forms on a gigantic scale. There can, I think, be little
doubt that this peculiar structure was very generally misunderstood
by the earlier observers, who naturally looked upon it as of clastic
origin, and who therefore believed that large beds of rock consisted
of volcanic conglomerate, which we should now map as nodular felsite
(pyromeride).[204]
[Footnote 204: Another source of error may probably be traced in
the occasional brecciated structure of the felsites, which has
been mistaken for true volcanic breccia, but which can be traced
disappearing into the solid rock. Sometimes this structure has resulted
from the breaking up of the lenticles of flow, sometimes from later
crushing.]
While by far the larger proportion of the Caernarvonshire lavas
consists of thoroughly acid rocks, the oldest outflows are much less
acid than those erupted at the height of the volcanic activity, when
the rocks of Snowdon were poured forth.[205] But towards the close
of the period there was apparently a falling off in the acidity of
the magma, for at the top of the group the andesitic lavas to which
I have already alluded are encountered. Sir Andrew Ramsay has shown
the existence of an upper "felstone" or "felspathic porphyry," almost
entirely removed by denudation, but of which outliers occur on
Crib-goch, Lliwedd, and other crests around Snowdon, and likewise on
Moel Hebog.[206] Mr. Harker alludes to these remnants, and speaks of
them as less acid than the older lavas, but he gives no details as to
their structure and composition.[207] In an examination of Snowdon
I was surprised to find that the summit of the mountain, instead of
consisting of bedded ashes as hitherto represented, is formed of a
group of lava-sheets having a total thickness of perhaps from 100 to
150 feet (6 in Fig. 56). The apex of Yr Wyddfa, the peak of Snowdon,
consists of fossiliferous shale lying on a dull grey rock that weathers
with elongated vesicles, somewhat like a cleaved amygdaloid, but a
good deal decomposed. A thin slice of this latter rock shows under the
microscope irregular grains and microlites of felspar, with a few grams
of quartz, the whole much sheared and calcified. Below this bed comes
a felsite, or devitrified obsidian, showing in places good spherulitic
structure, and followed by a grey amygdaloid. The latter is a markedly
cellular rock, and, though rather decayed, shows under the microscope a
microlitic felspathic groundmass, through which granules of magnetite
are dispersed.
[Footnote 205: Mr. Harker, _op. cit._ p. 127.]
[Footnote 206: _Mem. Geol. Surv._ vol. iii. 2nd edit. pp. 141, 144,
145, 147, 161.]
[Footnote 207: _Bala Volcanic Series_, pp. 10, 23, 125. He refers also
to lavas occupying a similar position at Nant Gwynant and Moel Hebog;
but he adds that he had not had an opportunity of studying them.]
[Illustration: Fig. 56.--Section of Snowdon.[208]
1. Grits and slates; 2. Felsite with good flow-structure; 3. Volcanic
tuffs; 4. Felsite; 5. Tuffs with sheets of felsite and andesite;
6. Group of andesitic lavas on summit of Snowdon; 7. Intrusive
"greenstones."]
[Footnote 208: After the Geological Survey Section (Horizont. Sect.
Sheet 28), slightly modified.]
Underneath this upper group of lavas lie the tuffs for which Snowdon
has been so long celebrated. But, as I have already stated, there
does not appear to me to be such a continuous thickness of fragmental
material as has been supposed. There cannot, I think, be any doubt that
not only at the top, but at many horizons throughout this supposed
thick accumulation of tuff, some of the beds of rock are really
lava-flows. Some of these lavas have suffered considerably from the
cleavage which has affected the whole of the rocks of the mountain,
while the results of centuries of atmospheric disintegration, so active
in that high exposed locality, have still further contributed to alter
them. They consequently present on their weathered faces a resemblance
to the pyroclastic rocks among which they lie. Where, however, the
lavas are thicker and more massive, and have resisted cleavage better,
some of them appear as cellular dull grey andesites or trachytes, while
a few are felsites. Many instructive sections of such bands among the
true tuffs may be seen on the eastern precipices of Snowdon above
Glas-lyn.
It thus appears that the latest lavas which flowed from the Snowdonian
vent were, on the whole, decidedly more basic than the main body of
felsites that immediately preceded them. They occur also in thinner
sheets, and are far more abundantly accompanied with ashes. At the
same time it is deserving of special notice that among these less
acid outflows there are intercalated sheets of felsite, and that
some of these still retain the spherulitic structure formed by the
devitrification of an original volcanic glass.
Far to the south-west, in the promontory of Lleyn, another group
of volcanic rocks exists which may have been in a general sense
contemporaneous with those of the Snowdon region, but which were
certainly erupted from independent vents. Mr. Harker has described
them as quartzless pyroxene-andesites, sometimes markedly cellular,
and though their geological relations are rather obscure, he regards
them as lava-flows interbedded among strata of Bala age and occurring
below the chief rhyolites of the district. If this be their true
position, they indicate the outflow of much less highly siliceous lavas
before the eruption of the acid felsites. In the Snowdon area any such
intermediate rocks which may have been poured out before the time of
the felsitic outflows have been buried under these.
The tuffs of the Bala series in Caernarvonshire have not received the
same attention as the lavas. One of the first results of a more careful
study of them will probably be a modification of the published maps by
a reduction of the area over which these rocks have been represented.
They range from coarse volcanic breccias to exceedingly fine compacted
volcanic dust, which cannot easily be distinguished, either in the
field or under the microscope, from the finer crushed forms of felsite.
Among the oldest tuffs pieces of dark blue shale as well as of felsite
may be recognized, pointing to the explosions by which the vents were
drilled through the older Silurian sediments already deposited and
consolidated. Sometimes, indeed, they recall the dark slate-tuffs of
Cader Idris, like which they are plentifully sprinkled with kaolinized
felspar crystals. The beds of volcanic breccia intercalated between
the lower felsites of Snowdon include magnificent examples of the
accumulation of coarse volcanic detritus. The blocks of various
felsites in them are often a yard or more in diameter. Among the
felsite fragments smaller scattered pieces of andesitic rocks may
be found. This mixture of more basic materials appears to increase
upwards, the highest ashes containing detritus of andesitic lavas like
those which occur among them as flows.
The tuffs in the upper part of Snowdon are well-bedded deposits made up
partly of volcanic detritus and partly of ordinary muddy sediment.[209]
Layers of blue shale or slate interstratified among them indicate
that the enfeebled volcanic activity marked by the fine tuffs passed
occasionally into a state of quiescence. As is well known, numerous
fossils characteristic of the Bala rocks occur in these tuffs. The
volcanic discharges are thus proved to have been submarine and to have
occurred during Bala time.
[Footnote 209: See the interesting account of these tuffs given by Sir
A. Ramsay, _Mem. Geol. Survey_, vol. iii. 2nd edit. p. 142.]
I have already alluded to some of the probable vents from which the
lavas and tuffs were discharged, and to their position along a line
drawn from Penmaen-mawr into the peninsula of Lleyn. It will be
observed that they lie outside the area of the bedded volcanic rocks
and rise through parts of the Silurian system older than these rocks.
The largest and most important of them is unquestionably that formed by
Y-foel-frâs and its neighbouring heights. As mapped by the Geological
Survey, this mass of igneous rock is irregularly elliptical, measures
about six square miles in area, and consists mainly of intrusive
"felstone-porphyry" passing into "hornblendic greenstone."[210] Mr.
Harker, however, has made an important correction of this petrography,
by showing that a large part of the area consists of augitic
granophyre, while the so-called "greenstone" is partly diabase and
partly andesitic ashes and agglomerates. He suggests that an older vent
has here been destroyed by a later and larger protrusion of igneous
matter.[211] This high and somewhat inaccessible tract of ground is
still in need of detailed mapping and closer study, for undoubtedly
it is the most important volcanic vent now visible in North Wales. My
former colleague in the Geological Survey, Mr. E. Greenly, spent a
week upon it some years ago, and kindly supplied me with the following
notes of his observations:--"The central and largest area of the neck
is mainly occupied with diabases and andesites, while the ashes and
agglomerates, which are intimately connected with them, seem to run as
a belt or ring round them, and to occur in one or more patches in the
midst of them. Portions of green amygdaloid run through the pyroclastic
masses. Outside the ring of agglomerate and ashes an interrupted border
of felsite can be traced, which may be presumed to be older than
they, for a block of it was observed in them. The granophyre, on the
other hand, which is interposed between the fragmental masses and the
surrounding rocks on the western wall of the vent, seems to be of later
date. Dykes or small bosses of diabase, like the material of the sills,
pierce both the agglomerates and the rocks of the centre."[212]
[Footnote 210: _Mem. Geol. Survey_, vol. iii. 2nd edit. pp. 137, 139.]
[Footnote 211: _Bala Volcanic Series_, pp. 41, 71, 72, 123.]
[Footnote 212: Mr. Greenly has made a sketch map of this interesting
locality. As he has now established his home in North Wales, I trust he
may find an opportunity of returning to Y-foel-frâs and completing his
investigations.]
No agglomerate appears to have been noticed by any observer among the
other supposed vents along the line that runs south-westwards from
Penmaen-mawr, to the promontory of Lleyn. These bosses are rudely
circular in ground-plan and rise vertically out of the Lower Silurian
or Cambrian strata, or partake more of the nature of lenticular sheets
or laccolites which have been thrust between the planes of bedding.
There is usually an observable alteration of the surrounding rocks
along the line of contact.
The material of these bosses is sometimes thoroughly acid, as is the
granophyre of Y-foel-frâs, the microgranite of Mynydd-mawr with its
riebeckite crystals, the augite-granite-porphyry of Clynog-fawr, and
the granophyric and rhyolitic quartz-porphyries of the Rivals. In other
cases the rock is of an intermediate grade, as in the enstatite-diorite
of Penmaen-mawr, the pyroxene-andesite of Carn Boduan, and the
quartz-augite-syenite of Llanfoglen.[213] A few bosses of still more
basic material occur in the Sarn district, including hornblende-diabase
and hornblende-picrite. Sometimes both the acid and the more basic
rocks are found in the same boss, as in the large mass of Y-foel-frâs.
[Footnote 213: The geological relations and petrographical characters
of these various rocks are given by Mr. Harker in the fourth and fifth
sections of his Essay.]
It must be confessed that there is no absolute proof that any of
these masses mark the actual sites of eruptive vents, except probably
the boss of Y-foel-frâs. Some of them may have been intruded without
establishing any outlet to the surface.[214] But that a few of them
really represent orifices from which the Bala volcanic group was
erupted may be plausibly inferred from their neck-like form, from
their positions with reference to the volcanic district, from the
obvious thickening of the lavas and tuffs in the direction of these
bosses, and from the petrographical relation that exists between their
component materials and rocks that were discharged at the surface. This
last-named feature has been well pointed out by Mr. Harker, who has
established, by a study of microscopic slides, a gradation from the
granophyric material of the bosses into structures greatly resembling
those of the bedded felsites, and likewise a close similarity between
the intermediate rocks of the other bosses and the andesites which
have elsewhere been poured out at the surface.[215] But perhaps the
most impressive evidence as to the sites of the chief centres of
eruption is supplied by the lavas and tuffs themselves as they thicken
in certain directions and thin away in others. This feature of their
distribution has been well expressed in the maps and sections of the
Survey, and has been clearly summarized by Mr. Harker.[216] The oldest
lavas now visible lie at the northern end of the district, and the
vents from which they proceeded may, with considerable probability, be
placed somewhere in the tract which includes the chain of bosses of
Penmaen-mawr, Y-foel-frâs, and Y Drosgl. The chief centre of eruption
no doubt lay somewhere in the Snowdon tract, where the lavas and
tuffs attain their greatest thickness, and whence they thin away in
all directions. The Mynydd-mawr boss may be presumed to have been one
of the main vents. But there were not improbably others, now concealed
under the deep cover of their own ejections.
[Footnote 214: Mr. Harker speaks of some of them as laccolites.]
[Footnote 215: _Op. cit._ pp. 57, 72.]
[Footnote 216: See especially pp. 9, 120 _et seq._, and fig. 6 of his
Essay.]
More diligent search, with a special eye to the discovery of such
vents, might indeed be rewarded, even in the midst of the volcanic
district itself. To the north-east of Capel Curig, for example, there
is a prominent knob of agglomerate,[217] which I visited with Mr. B.
N. Peach, and which we regarded as probably marking one of the minor
vents. The material of this eminence has a base which by itself would
probably be regarded by the field-geologist as a felsite. But through
this compact matrix are dispersed abundant stones of all sizes up to
six inches or more in diameter. They are mostly subangular or somewhat
rounded-off at the edges, and generally markedly cellular. Among
them may be observed pieces of trachyte, felsite, and a rock that is
probably a devitrified pitchstone or obsidian. The vesicles in these
stones are sometimes lined with an acicular zeolite. Traces of rude
bedding can be detected, dipping at high angles. On the north-east side
of the hill finer agglomerate is seen to alternate with ashy grits and
grey shales, which, dipping E.N.E. at 20°-30°, pass under a group of
felsites, one at least of which retains a very fine perlitic structure
and evidently flowed as a true glass. Some of these lavas are full of
enclosed pieces of various flinty cellular and porphyritic felsites and
andesites or trachytes, like the stones which occur abundantly in the
agglomerate. The connection of these bedded lavas and tuffs with the
agglomerate-neck seems obvious.
[Footnote 217: This rock is referred to in the _Geological Survey
Memoir_ as "a short thick band of conglomeratic ash, which strikes
northwards about half a mile and then disappears" (p. 134).]
The Caernarvonshire volcanic area furnishes another admirable example
of the intrusion of basic sills as a final phase of eruptivity. These
masses have been carefully separated out on the maps of the Geological
Survey, which present a striking picture of their distribution and
their relation to the other igneous rocks. An examination of the maps
shows at once that the basic sheets tend to lie parallel with the
bedding along certain horizons. In the southern and western portions
of the area they have forced themselves among the Lower Silurian
sedimentary strata that underlie the Bala volcanic group--a position
analogous to that taken by the corresponding sills of the Arenig
series. But they likewise invade the volcanic group itself. Along the
eastern borders of the district they abound, especially in the higher
parts of the volcanic pile, where they have been injected between the
flows, and have subsequently participated in the abundant plication of
the rocks between the mountains and the line of the River Conway.
The curvatures into which the rocks of the region have been thrown,
and the consequent breadth of country over which the volcanic sheets
can now be examined, furnish a much better field than Merionethshire
for the attempt to trace the probable centre or centres from which the
basic magma of the sills was protruded. A study of the Survey maps soon
leads to a conviction that the intrusions were not connected, except
perhaps to a trifling extent, with the great line of western vents.
It is remarkable that the older strata which emerge from under the
volcanic group on its western outcrop are, on the whole, singularly
free from sills, though some conspicuous examples are shown opposite to
Mynydd-mawr, while a few more occur further north along the same line.
Their lenticular forms, their short outcrops, and their appearance on
different horizons at widely separated points seem to indicate that the
sills probably proceeded from many distinct subterranean pipes. Their
greater abundance along the eastern part of the district may be taken
to indicate that the ducts lay for the most part considerably to the
eastward of the line of western vents. They may have risen in minor
funnels, like that of Capel Curig.
It is noteworthy that so abundant an extravasation of basic material
should have taken place without the formation of numerous dykes.
We have here a repetition of the phenomena that distinguished the
preceding Arenig volcanic period in Merionethshire, and it will be
remembered that the Llandeilo eruptions of Builth were likewise
followed by the injection of large bodies of basic rock. As an enormous
amount of igneous magma may thus be impelled into the Earth's crust
without the formation of dykes, it is evident that the conditions for
the production of sills must be in some important respects different
from those required for dykes.
No evidence has yet been obtained that any one of these sills
established a connection with the surface. Not a trace can be found of
the outpouring of any such basic lava-streams, nor have fragments of
such materials been met with in any of the tuffs. On the other hand,
there is abundant proof of the usual contact-metamorphism. Though the
sills conform on the whole to the bedding, they frequently break across
it. They swell into thick irregular masses, and thin out rapidly. In
short, they behave as true intrusive sheets, and not as bedded lavas.
In regard to their internal character, they show the customary
uniformity of texture throughout each mass. They are mapped under
the general name of "greenstones" by the Geological Survey, and are
described in the _Memoir_ as hornblendic.[218] The more precise modern
methods of examination, however, prove them to be true diabases, in
which the felspar has, as a rule, consolidated before the augite,
giving as a result the various types of diabasic structure.[219]
[Footnote 218: _Op. cit._ p. 156.]
[Footnote 219: Mr. Harker, _Bala Volcanic Series_, p. 83.]
The date of the intrusion of these basic sills can be fixed by the same
process of reasoning as was applied to those of the Arenig volcanic
group. Their connection with the other igneous rocks of Caernarvonshire
is so obvious that they must be included as part of the volcanic
history of the Bala period. But they clearly belong to a late stage,
perhaps the very latest stage, of that history. They probably could not
have been injected into their present positions, unless a considerable
mass of rocky material had overlain them. Some of them are certainly
younger than the tuffs of Snowdon and Moel Hebog, which belong to a
late part of the volcanic period. On the other hand, they had been
intruded before the curvature and compression of the region, for
they share in the foldings and cleavage of the rocks among which they
lie. The terrestrial movements that produced this disturbance have
been proved to have occurred after the time when the uppermost Bala
rocks were deposited, and before that of the accumulation of the Upper
Silurian formations.[220] The epoch of intrusion is thus narrowed
down to some part of the Upper Bala period. With this subterranean
manifestation, volcanic action in this part of the country finally died
out.
[Footnote 220: _Mem. Geol. Sur._ vol. iii. 2nd edit. p. 326. See also
Mr. Harker's _Bala Volcanic Series_, p. 76.]
iv. THE VOLCANIC CENTRE OF THE BERWYN HILLS
Among the thick group of sedimentary formations which overlies the
great volcanic ridge of the Arans and Arenig, and undulates eastwards
across the Bala Valley, occasional thin intercalations of tuff point
to the existence of another centre of volcanic activity which lay
somewhere in the region of the Berwyn Hills. The structure of this
ground, first indicated by Sedgwick, was investigated in detail by
J. B. Jukes and his colleagues, whose work was embodied in the Maps,
Sections and Memoirs of the Geological Survey.[221] The distinguishing
characteristics of the volcanic rocks of this district are the
occurrence of both lavas and tuffs as comparatively thin solitary bands
in the midst of the ordinary sediments, and the persistence of these
bands for a distance of sometimes more than 24 miles. The position of
the vent or vents from which this extensive outpouring of volcanic
material took place has not been revealed. As the bands tend to thin
away eastwards, it may be surmised that the chief focus of eruption lay
rather towards the west, perhaps under the trough of Upper Silurian
strata somewhere in the neighbourhood of Llandderfel. There was
probably another in the Hirnant district.
[Footnote 221: See Sheet 74 of the one-inch map; Sheets 32, 35, 37 and
38 of the Horizontal Sections; and chapter xxxi. of the _Memoir_ on the
Geology of North Wales.]
The mapping of the officers of the Survey showed that in the Berwyn
Hills there are representatives of both the great volcanic periods of
North Wales. A lower series of "felstones and greenstones" probably
belongs to the older period, which began towards the end of Cambrian
time and lasted in some districts even into the time of the Llandeilo
formation. An upper group of tuffs, lying among the Bala rocks, is
evidently equivalent, on the whole, to the much thicker volcanic series
of the Snowdon region.
The lowest visible volcanic rocks occur among the hills to the
north-west of Llanrhaiadr yn Mochnant. They are described as consisting
of felstone of a pale greenish-grey colour and compact texture,
like those of Arenig, and ashes distinctly interstratified with the
slates. No exact petrographical examination of these rocks has yet
been made. From the account given in the Survey _Memoir_ there appears
to be here a group of lavas and tuffs intercalated in Llandeilo
perhaps partly in Upper Arenig, strata which form the broken dome
of the Berwyn anticline. The lavas are represented as lying on four
or five platforms, a single band reaching a thickness of 300 feet
and separated from the next band by sometimes 1000 or 1500 feet of
non-volcanic sediment.
These lower lavas, according to the measurements of Jukes, are overlain
by more than 4000 feet of sedimentary strata before the upper or Bala
volcanic series is reached. Three successive "ash-beds" constitute
this upper series. Of these the lowest band, about 50 or 60 feet
thick, was named a "greenstone ash" in contradistinction to a felstone
ash, and was not traceable for more than a short distance. Above it,
after an intervening thickness of several hundred feet of sedimentary
strata, comes a second and much more continuous band of tuff, known
as the "Lower ash-bed," about 100 feet thick on the west front of the
Berwyn range. Still higher, after an interval of about 1500 feet of
slates, lies the "Upper ash-bed," which on the same line of section
has a thickness of about 200 feet. This is the most persistent of all
the volcanic horizons, for it can be followed continuously round the
whole range of the Berwyns until it is overlain by the Carboniferous
Limestone near Selattyn, a distance of not less than twenty-four miles.
The same band, but much more feebly developed, has been traced through
the faulted country on both sides of Bala Lake, where it formed a
useful platform in the investigation of the complicated geological
structure of that area. Along the north side of the Berwyn Hills
another thin band of tuff lies from 150 to 200 feet still higher up in
the series, and has been traced for a distance of about twelve miles.
The Bala limestone comes in about 800 or 1000 feet above the "Upper
ash-bed."
[Illustration: Fig. 57.--Section across the Berwyn Hills. (Reduced from
Horizontal Section, Geol. Surv., Sheet 35).
_L_, Llandeilo Flags; _B_, Bala group; _B L_, Bala Limestone; _t_ _t_,
volcanic tuffs; _D_, intrusive "greenstones."]
Besides the rocks now enumerated, the Survey maps show the
intercalation of four or five sheets of "greenstone," which are
represented as following with marked regularity the strike of the
strata. Until these sheets have been more precisely examined it is
impossible to decide regarding their true petrographical character,
or to determine whether they are sills, or interstratified lavas, or
include rocks of both these types.
V. THE VOLCANOES OF ANGLESEY
We now turn to another part of the country, about which much has been
written and keen controversy has arisen. In the centre of Anglesey,
among the rocks grouped together by the Geological Survey as "altered
Cambrian," there occur masses of breccia, the probable volcanic
origin of which was, so far as I know, first suggested by Professor
Hughes.[222] Dr. Callaway regards them as pre-Cambrian,[223] while
Professor Blake places them in his "Monian system."[224] When I went
over them some years ago, I accepted the view that they are volcanic
agglomerates.[225] Subsequent examination, however, has convinced me
that notwithstanding their remarkable resemblance to true agglomerates
they are not really of volcanic origin, but are essentially
"crush-conglomerates," like those in the Isle of Man, so well described
by Mr. Lamplugh.[226]
[Footnote 222: _Proc. Camb. Phil. Soc._ vol. iii. (1880), p. 347.]
[Footnote 223: _Quart. Journ. Geol. Soc._]
[Footnote 224: _Op. cit._]
[Footnote 225: _Presidential Address Geol. Soc._ vol. xlvii. (1891), p.
130.]
[Footnote 226: _Quart. Journ. Geol. Soc._ vol. li. (1895), p. 563. See
_Geol. Mag._ 1896, p. 481.]
But though their present coarse, agglomerate-like structure is, I
think, entirely due to the mechanical crushing of the rocks _in situ_
and not to volcanic explosions, it does not follow that the rocks
which have been broken up do not contain evidence of volcanic action
contemporaneous with their original formation. Obviously, pyroclastic
materials may be subjected to deformation and disruption as well as any
other components of the earth's crust, and may be equally converted
into crush-conglomerates. And in Anglesey it can, I think, be shown
that some of the rocks which have been broken up were originally tuffs
and volcanic breccias.
Throughout Anglesey the stratified rocks present evidence of having
undergone very great compression, deformation and rupture. Thus
at Llanerchymedd thick-bedded Lower Silurian grits, with their
intercalations of shale, have been broken up by numerous small faults,
and have been pushed over each other in large irregular blocks, the
shales being now pinched out, and now pressed up into the interstices
between the dislocated harder and more resisting grits. This condition
of rupture may be regarded as one of the stages towards the formation
of a conglomerate by the crushing together of rocks _in situ_. A
few miles further south at the beginning of the railway cuttings of
Llangefni, green, red and purple slates and grits appear in a rather
more crushed state, and immediately beyond these strata come the
coarse breccias. Neither in their composition nor in their structural
condition do these Llangefni strata appear to be marked off from the
undoubted Lower Silurian rocks as parts of a different system.
The railway cuttings at Llangefni reveal a series of rocks which appear
to have been originally shales, with thin bands of siliceous grit.
The argillaceous portions of this series are now green and phyllitic,
and remind one of the finer parts of some basic tuffs among the older
Palæozoic systems. They include, however, pale flinty bands, such as
might have been derived from fine felsitic dust. The grits are for the
most part fine-grained and highly siliceous, but they include also
coarser varieties with clear quartz-grains. The enormous deformation
which these strata have undergone is at once apparent. They seem to
have been plicated, ruptured and thrust over each other, the harder
parts surviving longest, but being eventually broken into small
fragments. Every stage may be traced from a recognizable band of grit
down to the rounded or elliptical pebbles of the same material entirely
isolated in this phyllitic matrix of crushed shale.
But while the volcanic origin of these coarsely-fragmental masses
cannot be maintained, there is elsewhere evidence that the older
Palæozoic rocks of Anglesey include relics of contemporaneous volcanic
eruptions. Seven miles to the south-east of Holyhead, in the basal
Lower Silurian conglomerates which, as before referred to, Mr. Selwyn
found lying unconformably on the green schists, there occur abundant
fragments of volcanic rocks, besides the prevalent detritus of the
schists of the neighbourhood. Some of the bands have somewhat the
character of volcanic breccias or tuffs, and they show an evident
resemblance to portions of the Bangor group and the rocks of Llyn
Padarn, though they are doubtless of much later age. That these
volcanic fragments were not derived from the waste of rocks of a much
earlier period is made tolerably certain by the intercalation of true
tuffs among the black shales higher up in the order of succession.
Here, then, we have evidence of contemporaneous volcanic action in the
very basement Lower Silurian strata of Anglesey, which by their fossil
contents are shown to be on the horizon of the lowest Arenig or even
Tremadoc group.
But still further and fuller evidence of Silurian volcanism is to be
obtained by an examination of the northern coast-line. I have already
referred to the elliptical fault which is marked on the Geological
Survey map as running from the north-western headland to the eastern
coast beyond Amlwch. The necessity for inserting this fault, apart
from any actual visible trace of its occurrence, arose when the
conclusion was arrived at that the rocks of the extreme north of
Anglesey were essentially altered Cambrian strata.[227] For immediately
to the south of these rocks black shales, obviously Silurian, were
seen to dip to the north--a structure which could only be accounted
for by a dislocation letting them down into that position. The same
necessity for a fault has of course been felt by all writers who
have subsequently treated the northern area as pre-Cambrian. But it
is deserving of notice that in the original mapping of the Survey no
continuous abrupt hiatus is shown by the line which was afterwards
marked as a continuous line of fault. On the contrary, on one of
the field-maps in, I believe, Mr. Selwyn's handwriting the remark
occurs:--"The gradual passage from the black shale to the upper green
gritty slates of Llanfechell is best seen at Bothedd, on road from
Llanfaethlu to Llyn-llygeirian."[228]
[Footnote 227: I have fully considered the evidence adduced by Dr.
Callaway and Professor Blake, and have examined the ground, and can
come to no other conclusion than that stated in the text. But see Mr.
Blake's remarks, _Geol. Mag._ 1891, p. 483.]
[Footnote 228: There is no continuous section now visible at this
place, but the two groups of rock can be traced to within a few feet of
each other, both inclined as usual in the same direction, and the black
shales appearing to pass under the others.]
It is no part of my aim to disprove the existence of faults along the
line referred to. These may quite well exist; but there is assuredly
no one gigantic displacement, such as the theory I am combating would
require; while any faults which do occur cannot be greatly different
from the others of the district, and do not prevent the true relations
of the rocks from being discoverable.
Where the supposed elliptical fault reaches the shore at Carmel Point,
the two groups of rock seem to me to follow each other in unbroken
sequence.[229] The black slates, which are admittedly Lower Silurian,
dip underneath a breccia and greenish (Amlwch) slates. Not only so,
but bands of similar black slates occur higher up, interstratified
with and shading-off into tuffs and greenish slates. Further, bands
of coarse volcanic breccia occur among the black slates south of the
supposed break. These, in accordance with the exigencies of theory,
are represented as separated by a network of faults from the black
slates amid which they lie. But good evidence may be found that they
are truly interbedded in these slates. In short, the whole of the rocks
in that part of Anglesey form one great series, consisting partly of
black slates, partly of greenish slates, with abundant intercalations
of volcanic detritus. The age of the base of this series is moreover
determined by the occurrence of Bala fossils in a band of limestone
near Carmel Point.
[Footnote 229: I cannot admit that there is any evidence of a
thrust-plane here. To insert one is merely to modify field-evidence to
suit theory. See _Geol. Mag._ 1891, p. 483.]
The rocks which extend eastward along the coast from the north-western
headland of Anglesey are marked on the Survey map as "green, grey
and purple slates with conglomeratic and siliceous beds." The truly
volcanic nature of a considerable proportion of these strata has been
clearly stated by Mr. Blake.[230] As they dip in a general northerly
direction, higher portions of the series present themselves as far as
the most northern projection of the island near Porth Wen (Fig. 58).
They have been greatly crumpled and crushed, so that the slates pass
into phyllites. They include some thick seams of blue limestone and
white quartzite, also courses of black shale containing Lower Silurian
graptolites. Among their uppermost strata several (probably Bala)
fossils, including _Orthis Bailyana_, have been obtained by Professor
Hughes. It has been supposed that the higher bands of black shale may
also have been brought into their present positions by faults, and that
they do not really belong to the series of strata among which they lie.
But this suggestion is completely disproved by the coast-sections,
which exhibit many thin interstratified leaves of black shale,
sometimes less than an inch thick. These and the ashy layers containing
the _Orthis_ and other fossils form an integral part of the so-called
"Amlwch slates."[231]
[Footnote 230: _Quart. Journ. Geol. Soc._ vol. xliv. (1888), p. 517.
See his further remarks in _Geol. Mag._ 1891, p. 483.]
[Footnote 231: The Amlwch slates exhibit on a great scale the puckering
that points to intense compression. This "gnarled" structure, as Prof.
Hughes called it, has been illustrated by Mr. Harker, _British Assoc.
Report_ (1885), pp. 839, 840.]
As evidence of the regular intercalation of the black shales and tuffs
in this sedimentary series, a portion of the coast section at Porth
Wen is here given (Fig. 58). The lowest member (1) of the series is
a white quartzite much jumbled in its bedding, but yet distinctly
interstratified with the other sediments, and containing intercalated
courses of green tuff and highly carbonaceous shale. Markings like
worm-pipes are here and there to be seen. The next group of strata (2)
consists of black shale followed by yellow conglomeratic sandstone
and pebbly tuffs. The shales enclose rounded and angular fragments
of quartzite. The sandstone passes upward into pinkish and yellowish
conglomerate (3), with an abundant lustrous phyllitic matrix, which
when free from pebbles closely resembles some of the tuffs of Llyn
Padarn. The next band (4) is one of yellow, sandy, felspathic grit,
quartz-conglomerate and fine tuffs, with leaves of dark shale towards
the base. It was in the lower part of this band that the _Orthis_
above mentioned was found. The black shales contain markings which are
probably graptolites. Reddish quartzite and quartz-conglomerate (5)
next succeed. These strata have the same phyllitic base just noticed.
The highest group here shown is one of black, yellow and green shales
mixed with patches and bands of volcanic breccia and tuff, the whole
being greatly disturbed, cleavage and bedding seeming as it were to
be struggling for the mastery. These last strata look as if they were
about to pass up vertically into the ordinary dark Lower Silurian
shales or slates.
[Illustration: Fig. 58.--Section of the strata on the shore at Porth
Wen, west of Amlwch.]
There can be no doubt regarding the serious amount of crushing which
the rocks of this coast-line have undergone. Some of the bands might
even be described as "crush-conglomerates." Yet the intercalation of
seams of black shale and limestone, and the occurrence of the exactly
similar but thicker group of black shales at Porth Prydd, which are
admitted to be Lower Silurian, unite the whole series of strata as
parts of one formation.
It thus appears that the area coloured "altered Cambrian" on the Survey
map, and regarded as pre-Cambrian by some later observers, is proved by
the evidence of fossils at its base, towards its centre and at its top,
to belong to the Lower Silurian series, probably to the Bala division.
That this was the geological horizon of part at least of the area
was recognized by Sir A. Ramsay, though he confessed himself unable
"precisely to determine on the north coast of Anglesey how much of the
strata are of Silurian and how much of Cambrian age."[232] Professor
Hughes was the first to suggest that the whole of these rocks should be
referred to the Bala group.[233]
[Footnote 232: _Mem. Geol. Surv._ vol. iii. 2nd edit. p. 242.]
[Footnote 233: _Proc. Camb. Phil. Soc._ vol. iii. (1880), pp.
341-348.] [Illustration:
Fig. 59.--Section of intercalated black shale in the volcanic
series at Porth yr hwch, south of Carmel Point.
]
[Illustration: Fig. 60.--Green slates overlain with volcanic breccia,
Carmel Point, Anglesey.]
I have dwelt on the determination of the true geological age of the
rocks of the north of Anglesey because of the diversity of opinion
respecting them, and because of their great interest in regard to the
history of volcanic action in Wales. These rocks contain a record of
volcanic eruptions, probably contemporaneous on the whole with those
of the Bala period in Caernarvonshire, yet independent of them and
belonging to a different type of volcanic energy. Some of the vents
probably lay in the north-western part of Anglesey. The materials
ejected from them were, so far as we know, entirely of a fragmentary
kind. Vast quantities of detritus, largely in the form of fine dust,
were thrown out; but no trace has yet been found of the outflow of
any lava. The lower part of this volcanic series consists of bedded
breccias which are sometimes remarkably coarse. Their included stones,
ranging up to six inches or more in diameter, are usually more or
less angular, and consist mainly of various felsites. Layers of more
rounded pebbles occasionally occur, while the bedding is still further
indicated by finer and coarser bands, and even by intercalations of
fine tuffs and ashy shales. Towards their upper limits some of these
volcanic bands shade off into pale grey or greenish ashy shale,
followed by black sandy shale of the usual kind. The relation of the
peculiar greenish shale of the Amlwch type to these tuffs and breccias
is well shown east of Carmel Point. This shale is interleaved with tuff
and contains frequent repetitions of finer or coarser volcanic breccia,
as well as occasional seams of black shale. An illustration of this
structure is given in Fig. 59, where some yellow decomposing breccias
(1), cut by a fault (_f_), are overlain by about 40 or 50 feet of black
shale (2), above which lies a flinty felsitic rock (3) that appears
to run in bands or dykes through the agglomerate. At Carmel Point
(Fig. 60) a similar structure may be observed to that at Llyn Padarn
already referred to (p. 163). The cleavage, which is well developed in
the green slates (_a_), is much more faintly marked in the overlying
breccia (_b_), but the bedding can still be detected in both rocks
running parallel to their mutual boundary-line. Beyond Porth Padrig,
which lies east from Carmel Point, the section may be seen which is
shown in Fig. 61. Here the blue or lead-coloured shale or slate (_a_)
marked as Silurian on the Geological Survey map passes up into a mass
of fine yellowish felsitic tuff and breccia (_b_). The shale at the
junction intercalates in thin leaves with the tuff.
[Illustration: Fig. 61.--Blue shale or slate passing into volcanic
breccia east of Porth Padrig, near Carmel Point, Anglesey.]
The breccias south of Carmel Point, though they are chiefly made up
of felsitic detritus, sometimes show a preponderance of fragments
of shale. They vary also rapidly in texture and composition. These
variations may indicate that the vent or vents from which their
materials were derived stood somewhere in the near neighbourhood, if
indeed they are not to be recognized in some of the boss-like eminences
that rise above the shore. At the same time, the enormous amount of
crushing and shearing which the rocks of this region have undergone
has doubtless introduced crush-conglomerates into the structure of the
ground. And some patient labour may be required before the nature and
origin of the different fragmental masses are determined.
Certain remarkably coarse, tumultuous breccias, exposed on the coast
at Mynyddwylfa and Cemmaes, were formerly regarded by me as volcanic
agglomerates. But more recent examination has satisfied me that these,
like the breccias at Llangefni, are not of volcanic origin but are
crush-conglomerates.[234]
[Footnote 234: Presidential Address, _Quart. Journ. Geol. Soc._ vol.
xlvii. p. 134; _Rep. Brit. Assoc._ 1896, Section C; _Geol. Mag._ 1896,
p. 481.]
While the lower breccias are sometimes tolerably coarse, the volcanic
detritus becomes much finer in the higher parts of the Amlwch slates.
Above the limestones and black shales of Cemmaes volcanic breccias
and ashes, with limestone, quartzite, conglomerate and thin seams of
black shale, continue to the extreme northern headlands. The amount
of fine volcanic detritus distributed through these strata is very
great. We can clearly make out that while ordinary sedimentation was
in progress, an almost constant but variable discharge of fragmental
materials took place from the vents in the neighbourhood. Sometimes
a special paroxysm of explosion would give rise to a distinct band
of breccia or of tuff, but even where, during a time of comparative
quiescence, the ordinary sand or mud predominated, it was generally
mingled with more or less volcanic dust.
Some bands of conglomerate in this group of strata deserve particular
notice. The most conspicuous of these, already referred to as seen at
Porth Wen, is made up of quartz and quartzite blocks, embedded in a
reddish matrix largely composed of ashy material, and recalling the red
spotted tuffs of Llyn Padarn. The occurrence of strong conglomerates
near the top of a volcanic series has been noted at St. David's, Llyn
Padarn and Bangor. In none of these localities, as I have tried to
show, do the conglomerates mark an unconformability or serious break
between two widely-separated groups of rock. The Anglesey section
entirely supports this view, for the conglomerates are there merely
intercalations in a continuous sequence of deposits; they are succeeded
by tuffs and shales like those which underlie them. The interposition
of such coarse materials, however, may undoubtedly indicate local
disturbance, connected, perhaps, in this and the other localities, with
terrestrial readjustments consequent upon the waning of volcanic energy.
The detailed geological structure of Anglesey is still far from being
completely understood. Besides the serious crushing here referred to,
there is reason to suspect that considerable plication, perhaps even
inversion, of the strata has taken place, and that, by denudation,
detached portions of some of the higher groups have been left in
different parts of the island. The occurrence of Upper Silurian
fossils in several localities adds to the perplexity of the problem
by indicating that, among the folds and hardly distinguishable from
the older slates, portions of Upper Silurian formations may have been
caught and preserved. These difficulties, moreover, involve in some
obscurity the closing phases of volcanic activity in Wales; for until
they are, to some extent at least, removed, we shall be left in doubt
whether the vents in the north of Anglesey, which were in eruption
probably during Bala time, were the last of the long succession of
Welsh volcanoes. If the black shales of Parys Mountain are really
referable to the horizon of the Mayhill Sandstone, the two great
igneous bands between which they lie would seem to mark an outbreak
of volcanic energy during Upper Silurian time. No other indications,
however, of eruptions of that age having been met with in Great Britain
(though they occur in the south-west of Ireland and possibly in
Gloucestershire), more careful investigation is required before such a
position can be safely assigned to any rocks in Anglesey.
Putting these doubtful rocks aside for the present, we may, in
conclusion, contrast the type of eruption in Anglesey with that of
the great Snowdonian region. While the Caernarvonshire volcanoes were
pouring forth their volumes of felsitic lava, and piling them up for
thousands of feet on the sea-floor, the northern Anglesey vents, not
more than some five-and-twenty miles away, threw out only stones and
dust, but continued their intermittent explosions until they had strewn
the sea-bottom with detritus to a depth of many hundred feet.
There is yet another feature of interest in this independent group of
submarine vents in Anglesey. Their operations appear to have begun
before the earliest eruptions of the Bala volcanoes in Caernarvonshire.
Their first beginnings may, indeed, have been coeval with the
explosions that produced the older Arenig tuffs of Merionethshire;
their latest discharges were possibly the last manifestations of
volcanic energy in Wales. They seem thus to bridge over the vast
interval from Tremadoc to Upper Bala, possibly even to Upper Silurian
time. But we may, perhaps, connect them with the still earlier period
of Cambrian volcanism. I have referred to the evidence which appears
to show that the vents whence the lavas and tuffs of Moel Trefan and
Llyn Padarn were erupted gradually moved northwards, and continued
in eruption until after the beginning of the deposition of the black
slates that are generally regarded as Arenig. The Anglesey tuffs
and breccias may thus be looked upon as evidence of a still further
shifting of the active orifices northward. In this view, while the Aran
and Cader Idris volcanoes broke out in Upper Cambrian and continued
through Arenig time, and the Snowdonian group was confined to Bala
time, a line of vents opened to the north-west in the Cambrian period
before the epoch of the Llanberis slates, and, dying out in the south,
continued to manifest a minor degree of energy, frequently discharging
fragmental materials, but no lava, over the sea-bottom, until, towards
the close of the Bala period, possibly even in Upper Silurian time,
they finally became extinct.
vi. THE VOLCANOES OF THE LAKE DISTRICT (ARENIG TO CLOSE OF BALA PERIOD)
From the time of the appearance of Sedgwick's classic letters to
Wordsworth, no volcanic area of Britain has probably been so well known
in a general sense to the ordinary travelling public as the district
of the English Lakes. Many geologists have since then visited the
ground, and not a few of them have published additions to our knowledge
respecting what is now known as the Borrowdale Volcanic Series. The
most elaborate and detailed account of any part of it is that given by
the late Mr. J. C. Ward in the _Geological Survey Memoirs_, wherein he
embodied the results of his minute investigation and mapping of the
northern portion of the district.[235] Notices of the petrography of
some of the more interesting rocks have subsequently been published by
Mr. Rutley, Professor Bonney, Mr. Harker, Mr. Marr, Mr. Hutchings and
others. But up to the present time no complete memoir on the volcanic
geology of the Lake District as a whole has appeared. The sheets
of the Geological Survey map present a graphic view of the general
distribution of the rocks, but so rapid has the progress of certain
branches of geology been since these sheets were published, that the
map is even now susceptible of considerable improvement.
[Footnote 235: Sheet 101 S. E. of the Geological Survey of England
and Wales and Explanation illustrating the same; and papers by him
in _Quart. Journ. Geol. Soc._ vols. xxxi. xxxii. (1875-76). See also
Messrs. Aveline and Hughes, _Mem. Geol. Survey_, Sheet 98 N.E. (Kendal,
Sedbergh, etc.).]
In estimating the area over which the volcanic rocks of the Lake
District are spread, geologists are apt to consider only the tract
which lies to the south of Keswick and stretches southward to a line
drawn from the Duddon Sands to Shap. But it can easily be shown that
this area falls far short of the extent of that wherein the rocks can
still be traced, and yet further short of that over which the lavas
and ashes originally spread. For, in the first place, the volcanic
group can be followed round the eastern end of the mountain-group which
culminates in Skiddaw, and along the northern base of these heights to
Cockermouth, though only a narrow fringe of it emerges from underneath
the Carboniferous series. It is thus manifest that the volcanic rocks
once stretched completely across Skiddaw and its neighbours, and that
they extend northwards below the Whitehaven Coal-field. But, in the
next place, far beyond these limits, volcanic rocks, which there can
be little doubt were originally continuous with those of the Lakes,
emerge from beneath the base of the Cross Fell escarpment,[236] and
still further to the east a prolongation of the same group rises for
a brief space to the surface from under the great limestone sheets of
Upper Teesdale. Between the north-western and south-eastern limits
within which the rocks can now be seen there intervenes a distance of
some 11 miles, while the extreme length of the tract from south-west
to north-east is about 50 miles. Even if we take these figures as
marking the approximate boundaries of the region covered by the
volcanic ejections, it cannot be less than 550 square miles. But this
is probably much less than the original area.
[Footnote 236: For an account of the Cross Fell inlier of Silurian
rocks see the paper by Professor Nicholson and Mr. Marr, with the
petrographical appendix by Mr. Harker. _Quart. Journ. Geol. Soc._ vol.
xlvii. (1891), pp. 500, 512.]
The thickness of the accumulated volcanic materials is proportionate
to the large tract of country over which they have been spread. From
various causes, it is difficult to arrive satisfactorily at any precise
statement on this question. In a volcanic series bedding is apt to be
obscure where, as in the present case, there are no interstratified
bands of ordinary sedimentary strata to mark it off. It tends,
moreover, to vary considerably and rapidly within short distances, not
only from subsequent unequal movements of subsidence or elevation, but
from the very conditions of original accumulation. Mr. Ward considered
that the maximum thickness of the volcanic group of the Lake District
might be taken to range from 12,000 to 15,000 feet.[237] Professors
Harkness and Nicholson, on the other hand, gave the average thickness
as not more than 5000 feet.[238] My own impression is that the truth is
to be found somewhere between these two estimates, and that the maximum
thickness probably does not exceed 8000 or 9000 feet. In any case
there cannot, I think, be much doubt that we have here the thickest
accumulation of volcanic material, belonging to a single geological
period, anywhere known to exist in Britain.
[Footnote 237: Ward, _op. cit._ p. 46.]
[Footnote 238: _Brit. Assoc. (1870) Sectional Reports_, p. 74.]
The geological age of this remarkable volcanic episode is fortunately
fixed by definite palæontological horizons both below and above. The
base of the volcanic group rests upon and is interstratified with the
upper part of the Skiddaw Slate,[239] which from the evidence of its
fossils is paralleled with the Arenig rocks of Wales. The highest
members of the group are interstratified with the Coniston Limestone,
which, from its abundant fauna, can without hesitation be placed on
the same platform as the Bala Limestone of Wales, and is immediately
followed by the Upper Silurian series. Thus the volcanic history
comprises the geological interval that elapsed between the later part
of the Arenig period and the close of the Bala period. It begins
probably not so far back as that of the Arenig group of Merionethshire,
and its termination was perhaps coincident with the dying out of the
Snowdonian volcanoes. But it contains no record of a great break or
interval of quiescence like that which separated the Arenig from the
Bala eruptions in Wales.
[Footnote 239: Mr. Dakyns has expressed his belief that the volcanic
group lies unconformably on the Skiddaw Slate (_Geol. Mag._ 1869,
pp. 56, 116), and Professor Nicholson has formed the same opinion
(_op. cit._ pp. 105, 167; _Proc. Geol. Assoc._ vol. iii. p. 106). Mr.
Goodchild, however, has shown that in the Cross Fell inlier the oldest
tuffs are interstratified with the Skiddaw Slates (_Proc. Geol. Assoc._
vol. xi. (1889), p. 261). Mr. Ward in mapping the district inserted a
complex series of faults along the junction-line between the volcanic
series and the Skiddaw Slates. When I went over the ground with him
some years before his death I discussed this boundary-line with him
and could not adopt his view that it was so dislocated. More recent
re-examination has confirmed me in my dissent. A large number of the
faults inserted on the Geological Survey map to separate the Skiddaw
Slates from the Borrowdale volcanic series cannot be proved, and
probably do not exist. Others may be of the nature of "thrust-planes."
But see Mr. Ward's explanation of his views, _op. cit._ p. 48.]
The materials that form this enormous volcanic pile consist entirely
of lavas and ashes. No intercalations of ordinary sedimentary material
have been met with in it, save at the bottom and at the top. The lower
lavas, well seen among the hills to the south of Keswick, were shown by
Mr. Ward to be intermediate between felsites and dolerites in regard
to their silica percentage, and he proposed for them the name of
felsi-dolerites. They are comprised in the group of the andesites or
"porphyrites." From the analyses published by Mr. Ward, the amount of
silica appears to range up to about 60 per cent.[240] They are usually
close-grained, dull dark-grey to black rocks, breaking, where fresh,
with a splintery or conchoidal fracture, showing a few minute striated
felspars, apt to weather with a pale-brown or yellowish-grey crust,
and sometimes strongly vesicular or amygdaloidal. They present many
external resemblances to some of the "porphyrites" or altered andesites
of the Lower Old Red Sandstone of Scotland. A microscopic examination
of specimens collected by Dr. Hatch and myself from the hills to the
south of Keswick showed the rocks to be true andesites, composed of a
multitude of slender laths (sometimes large porphyritic crystals) of
felspar with a brownish glassy groundmass, and with some chloritic
material probably representing augite, but with no trace of quartz.[241]
[Footnote 240: _Quart. Journ. Geol. Soc._ vol. xxxi. (1875) p.
408, vol. xxxii. (1876) p. 24. Geology of Northern Part of Lake
District (_Mem. Geol. Survey_), p. 22. In a subsequent paper the more
basic lavas of Eycott Hill are compared with dolerites (_Monthly
Microscopical Journ._ 1877, p. 246).]
[Footnote 241: These rocks were mapped as tuffs by Mr. Ward. Their
microscopic characters have been described by Messrs. Harker and Marr,
_Quart. Journ. Geol. Soc._ xlvii. (1891), p. 292; by Mr. Harker, _op.
cit._ p. 517; and by Mr. W. M. Hutchings, _Geol. Mag._ 1891, p. 537;
1892, pp. 227, 540.]
Another type of andesite has been found by Mr. Hutchings to occur
abundantly at Harter Fell, Mardale, between the Nan Bield Pass and
High Street, and in the cliffs on the right side of the Kentmere
Valley. It consists of rocks mostly of a grey-green or grey-blue
colour with resinous lustre and extremely splintery fracture. They are
augite-andesites of a much more vitreous nature than the dominant type
of lavas of the Lake District. Their groundmass under the microscope is
seen to have originally varied from a wholly glassy base to an intimate
mixture of glass and exceedingly minute felspar-microlites. This
groundmass is permeated with chlorite in minute flakelets, and encloses
numerous porphyritic sharply-defined felspar-crystals, together with
chlorite-pseudomorphs after augite.[242] Gradations from these rocks to
the ordinary more coarse-grained andesites may be observed.
[Footnote 242: Mr. Hutchings, _Geol. Mag._ 1891, p. 539. This observer
describes a quartz-andesite or dacite from near Dunmail Raise.]
Some of the andesites appear to have a trachytic facies, where the
felspars of the groundmass consist largely of untwinned laths and
appear to be mainly orthoclase.[243]
[Footnote 243: _Op. cit._ p. 543.]
Among the lavas of the Lake District there occur many which are
decidedly more basic than the andesites, and which should rather be
classed among the dolerites and basalts, though they do not appear to
contain olivine. These rocks occur at Eycott Hill, above Easedale Tarn,
Scarf Gap Pass, Dale Head, High Scawdell, Seatoller Fell and other
places. Analyses of those from Eycott Hill were published by Mr. Ward,
and their silica percentage was shown to range from 51 to 53·3.[244]
The microscopic characters of the group have been more recently
determined by Mr. Hutchings[245] and Messrs. Harker and Marr.[246]
[Footnote 244: _Monthly Microscopical Journal_, 1877, p. 246.]
[Footnote 245: _Geol. Mag._ 1891, p. 538.]
[Footnote 246: _Quart. Journ. Geol. Soc._ vol. xlix. (1893), p. 389.
Mr. Harker, _op. cit._ vol. xlvii. (1891).]
The andesitic and more basic lavas are particularly developed in
the lower and central part of the volcanic group. They rise into
ranges of craggy hills above the Skiddaw Slates, and form, with their
accompanying tuffs, the most rugged and lofty ground in the Lake
District. They extend even to the southern margin of the volcanic
area at one locality to the south-west of Coniston, where they
may be seen with their characteristic vesicular structure forming
a succession of distinct flows or beds, striking at the Coniston
Limestone which lies upon them with a decided, though probably very
local, unconformability.[247] One of the flows from this locality was
found by Dr. Hatch, under the microscope, to belong to the more basic
series. It approaches a basalt, containing porphyritic crystals of
fresh augite instead of the usual felspars, and showing a groundmass
of felspar microlites with some granules of augite and dispersed
magnetite. This local increase of basic composition is interesting as
occurring towards the top of the volcanic group. A porphyritic and
somewhat vesicular andesite, with large crystals of striated felspar in
a dark almost isotropic groundmass, occurs under the Coniston Limestone
near Stockdale.
[Footnote 247: This unconformability has been described and discussed
by various observers. The general impression has been, I think, that
the break is only of local importance. Mr. Aveline, however, believed
it to be much more serious, and he regarded the volcanic rocks which
were ejected during the deposition of the Coniston Limestone series as
much later in date than those of the Borrowdale group. See _Mem. Geol.
Survey_, Explanation to Sheet 98 N.E. 2nd edit. p. 8 (1888).]
Mr. Ward was much impressed with the widespread metamorphism which
he believed all the volcanic rocks of this region had undergone,
and as a consequence of which arose the difficulty he found in
discriminating between close-grained lavas and fine tuffs. There is, of
course, a general induration of the rocks, while cleavage has widely,
and sometimes very seriously, affected them. There is also local
metamorphism round such bosses as the Shap granite, but the evidence of
any general and serious metamorphism of the whole area does not seem to
me to be convincing.[248]
[Footnote 248: The metamorphism of all the rocks, aqueous and igneous,
around the Shap granite has been well worked out by Messrs. Harker
and Marr, _Quart. Journ. Geol. Soc._ vol. xlvii. (1891) p. 266, xlix.
(1893) p. 359.]
With regard to the original structure and subsequent alteration of
some of the andesitic lavas, an interesting section has recently
been cut along the road up Borrowdale a little south of the Bowder
Stone. Several bands of coarse amygdaloidal lava may there be seen
interstratified among tuffs. The calcite amygdales in these rocks are
arranged parallel to the bedding and therefore in the planes of flow,
while those lined with chlorite are more usually deformed parallel to
the direction of the cleavage. This difference suggests that before the
cleavage took place, not improbably during the volcanic period, the
rocks had been traversed by heated water producing internal alteration
and rearrangements, in virtue of which the vesicles along certain
paths of permeation were filled up with calcite, so as then to offer
some resistance to the cleavage, while those which remained empty, or
which had been merely lined with infiltrated substance, were flattened
and pulled out of shape. Messrs. Harker and Marr have shown that the
amygdaloidal kernels had already been introduced into the cellular
lavas before the intrusion of the Shap granite. In the account to be
given of the Tertiary plateau-basalts (Chapter xxxvi.) evidence will
be adduced that this filling up of the steam-cavities of lava may take
place during a volcanic period, and that it is probably connected with
the passage of heated vapours or water through the rocks.
Though acid lavas are not wholly absent from the central and lower
parts of the volcanic group, it is at the top that their chief
development appears to occur. These rocks may be grouped together as
felsites or rhyolites. They probably play a much larger part in the
structure of the southern part of the volcanic area than the published
maps would suggest, and a detailed survey and petrographical study
of them would well reward the needful labour.[249] A fine series of
felsites is interbedded in the lower part of the Coniston Limestone,
and spreads out underneath it along the southern margin of the volcanic
district from the Shap granite south-westward for some miles[250] (Fig.
62). Between the valleys of the Sprint and Kent these felsites (which
farther east are said to be 700 feet thick) may be seen interposed
between the limestone and the fossiliferous calcareous shales below it,
while from underneath the latter other sheets rise up into the range of
hills behind.
[Footnote 249: See Mr. F. Rutley, "The Felsitic Lavas of England and
Wales," _Mem. Geol. Surv._ 1885, pp. 12-15; also the description of
Messrs. Harker and Marr, _Quart. Journ. Geol. Soc._ xlvii. (1891), p.
301.]
[Footnote 250: Unfortunately these acid lavas are not distinguished
from the others in the Geological Survey maps.]
[Illustration: Fig. 62.--Section of felsites on the Coniston Limestone
group, west of Stockdale.
_a_, Felsites more or less cleaved; _b_, Calcareous shales with
fossils, much cleaved; _c_, Cleaved felsite; _d_, Coniston Limestone;
_e_, Stockdale Shales (with graptolites).]
These acid lavas are generally grey, cream-coloured, or pink, with a
white weathered crust. Their texture when fresh is flinty or horny, or
at least extremely fine-grained and compact. They are seldom markedly
porphyritic. They frequently display good flow-structure, and sometimes
split up readily along the planes of flow. Occasionally the flow-lines
on the outer crust have broken up in the movement of the rock, giving
rise to irregular fragments which have been carried forward. Short,
extremely irregular, branching veins of a fine cherty felsitic
substance, which occasionally shows a well-marked flow-structure
parallel to the walls, traverse certain parts of a dark-grey felsite,
near Brockstones, between the valleys of the Kent and Sprint.[251]
Occasionally a distinct nodular structure may be observed in these acid
lavas, sometimes minute, like an oolite, in other parts, as on Great
Yarlside, presenting large rounded balls. This nodular structure is not
confined to the lava-flows, but has been detected by Messrs. Harker
and Marr in what appears to be an intrusive rock near Shap Wells. The
microscopic characters of some of the Lake District rhyolites were
described by Mr. Rutley, who found them to exhibit beautiful perlitic
and spherulitic structures.[252] That such rocks as these were poured
out in a vitreous condition, like obsidian or pitchstone, cannot be
doubted. Chemical analysis shows that the Lake District rhyolites agree
exactly with those of North Wales in their composition. They contain
about 76 per cent of silica.[253]
[Footnote 251: Compare the structure described by Mr. Harker from the
Cross Fell inlier, _Quart. Journ. Geol. Soc._ xlvii. (1891), p. 518.]
[Footnote 252: "Geology of Kendal," etc., _Mem. Geol. Survey_, Sheet 98
N.E. 2nd edit. p. 9.]
[Footnote 253: Messrs. Harker and Marr, _op. cit._ p. 302.]
The rhyolitic lavas have been seriously affected by the general
cleavage of the region. In some places they have been so intensely
cleaved as to become a kind of fissile slate, and there seems good
reason to believe that in this altered condition they have often been
mistaken for tuffs. Where they assume a nodular structure, the nodules
have sometimes been flattened and elongated in the direction of the
prevalent cleavage.
The abundance and persistence of thoroughly acid lavas along the
southern edge of the volcanic area where the youngest outflows are
found, is a fact of much interest and importance in the history of
the eruptions of this region. It harmonizes with the observations
made in Wales, where in the Arenig, and less distinctly in the Bala
group, a marked increase in acidity is noticeable in the later volcanic
products. At the same time, as above mentioned, there is evidence also
of the discharge of more basic materials towards the close of the
eruptions, and even of the outflow of a lava approaching in character
to basalt.
According to the Geological Survey maps, by far the largest part of
the volcanic district consists of pyroclastic materials. When my
lamented friend, the late Mr. Ward, was engaged in mapping the northern
part of the district, which he did with so much enthusiasm, I had an
opportunity of going over some of the ground with him, and of learning
from him his ideas as to the nature and distribution of the rocks and
the general structure of the region. I remember the difficulty I had
in recognizing as tuff much of what he had mapped as such, and I felt
that had I been myself required, without his experience of the ground,
to map the rocks, I should probably have greatly enlarged the area
coloured as lava, with a corresponding reduction of that coloured as
tuff. A recent visit to the district has revived these doubts. It is
quite true, as Mr. Ward maintains, that where the finer-grained tuffs
have undergone some degree of induration or metamorphism, they can
hardly, by any test in the field, be distinguished from compact lavas.
He was himself quite aware of the objections that might be made to his
mapping,[254] but the conclusions he reached had been deduced only
after years of unremitting study in the field and with the microscope,
and in the light of experience gained in other volcanic regions.
Nevertheless I think that he has somewhat exaggerated the amount of
fragmental material in the northern part of the Lake District, and that
the mapping, so consistently and ably carried out by him, and followed
by those members of the Survey who mapped the rest of the ground, led
to similar over-representation there. Some portions of the so-called
tuffs of the Keswick region are undoubtedly andesites; other parts in
the southern tracts include intercalated bands of felsite as well as
andesite.
[Footnote 254: He says: "I shall be very much surprised if my mapping
of many parts of the district be not severely criticized and found
fault with by those who examine only one small area and do not take
into consideration all the facts gathered together, during the course
of several years, from every mountain flank and summit" (_op. cit._ p.
25). Mr. Hutchings has expressed his agreement with the opinions stated
in the text. He likewise coincides in the belief that there are many of
these Lake District volcanic rocks, regarding which it is impossible to
decide whether they are lavas or ashes (_Geol. Mag._ 1891, p. 544).]
But even with this limitation, the pyroclastic material in the Lake
District is undoubtedly very great in amount. It varies in texture
from coarse breccia or agglomerate, with blocks measuring several
yards across, to the most impalpable compacted volcanic dust. In
the lower parts of the group some of the tuffs abound in blocks
and chips of Skiddaw Slate. Some good examples of this kind may be
seen in Borrowdale, below Falcon Crag and at the Quayfoot quarries.
Where the tuff is largely made up of fragments of dark blue slate,
it much resembles the slate-tuffs of Cader Idris. Some of the pieces
of slate are six or eight inches long and are now placed parallel to
the cleavage of the rock. Among the slate debris, however, felspar
crystals and felsitic fragments may be observed. Bands of coarser and
finer green tuff show very clearly the bedding in spite of the marked
cleavage (Fig. 63).
[Illustration: Fig. 63.--Fine tuff with coarser bands near Quayfoot
quarries, Borrowdale.
The highly-inclined fine lines show the cleavage. The more gently
dipping bands and lines mark the bedding.]
But throughout the whole volcanic group the material of the tuff is
chiefly of thoroughly volcanic origin, and its distribution appears to
agree on the whole with that of the bedded lavas. In the older portions
of the group it is probably mainly derived from andesitic rocks, though
with an occasional intermingling of felsitic or rhyolitic detritus,
while in the higher parts many of the tuffs are markedly rhyolitic.
Among the lapilli minute crystals of felspar, broken or entire, may be
detected with the microscope. Some of the ejected ash must have been an
exceedingly fine dust. Compacted layers of such material form bands of
green slates, which may occasionally be seen to consist of alternations
of coarser and finer detritus, now and then false-bedded. Such tuffs
bring vividly before the mind the intermittent explosions, varying
a little in intensity, by which so much of the fabric of the Lake
mountains was built up.
Breccias of varying coarseness are likewise abundant, composed of
fragments of andesite and older tuffs in the central and lower parts
of the volcanic group, and mainly of felsitic or rhyolitic detritus
in the upper parts. Some of these rocks, wherein the blocks measure
several yards across, are probably not far from the eruptive vents,
as at Sourmilk Gill and below Honister Pass. Generally the stones are
angular, but occasionally more or less rounded. Stratification can
generally be detected among these fragmental rocks, but it is apt to be
concealed or effaced by the cleavage, while it is further obscured by
that widespread induration on which Mr. Ward has laid so much stress.
The extreme state of comminution of the volcanic dust that went to form
the tuffs has probably caused them to be more liable to metamorphism
than the lavas.[255]
[Footnote 255: The microscopic and chemical characters of the
Ash-Slates of the Lake District have been investigated by Mr.
Hutchings, _Geol. Mag._ 1892, pp. 155, 218.]
Little has yet been done in identifying any of the vents from which the
vast mass of volcanic material in the Lake District was ejected. Mr.
Ward believed that the diabase boss forming the Castle Head of Keswick
marks the site of "one of the main volcanic centres of this particular
district,"[256] whence the great lava sheets to the southward flowed
out. There are obviously two groups of bosses on the northern side of
the district, some of which may possibly mark the position of vents. A
few of them are occupied by more basic, others by more acid rocks. It
is not necessary to suppose that the andesitic lavas ascended only from
the former and the felsites from the latter. While the felsites on the
whole are younger than the more basic lavas, they may have been erupted
from vents which had previously emitted andesites, so that the present
plug may represent only the later and more acid protrusions.
[Footnote 256: _Op. cit._ p. 70.]
Besides the boss of Castle Head there are numerous smaller basic
intrusions farther down the Derwent Valley on either side of
Bassenthwaite Lake. Among these are the highly basic rocks forming
the picrite on the east side of the Dash Beck and the dykes on
Bassenthwaite Common. All these bosses, sills, and dykes rise through
the Skiddaw Slates, but there is no positive proof that they belong to
the Lower Silurian volcanic series; they may possibly be much later.
The most important and most interesting of all the intrusive masses
of basic material is that which constitutes a large part of the
eminence that culminates in Carrock Fell. The remarkable variations
in the composition of this mass have been already referred to. Mr.
Harker has shown that while the centre of the mass is a quartz-gabbro,
it becomes progressively more basic towards the margin. Through the
gabbro a mass of granophyre has subsequently made its way, and along
the line of junction has incorporated into its own substance so much
of the basic rock as to undergo a marked modification in its structure
and composition. Whether these intruded bodies of basic and acid
material have ascended in one of the old volcanic funnels and have been
injected laterally in laccolitic fashion has not been ascertained. Mr.
Harker, indeed, is rather inclined to refer the intrusions to a time
not only later than the Borrowdale volcanoes, but later even than the
terrestrial movements that subsequently affected the district and gave
the rocks their present cleaved and faulted structures. Besides the
gabbro and granophyre of this locality, igneous activity has manifested
itself in the uprise of numerous later dykes and veins, intermediate
to basic in composition. Some of these are glassy (tachylyte) and
spherulitic or variolitic.[257]
[Footnote 257: Mr. Harker, _Quart. Journ. Geol. Soc._ vol. l. (1894) p.
312, li. (1895) p. 125. _Geol. Mag._ 1894, p. 551.]
Throughout the Lake District a considerable number of bosses of more
acid rocks rise through the Skiddaw Slates, and likewise through the
volcanic group even up to its highest members. Some of these bosses may
possibly indicate the site of volcanic vents. Two of them, which form
conspicuous features on either side of the Vale of St. John, consist of
microgranite, and rise like great plugs through the Skiddaw Slates, as
well as through the base of the volcanic group. The view of the more
eastern hill, as seen from the west, is at once suggestive of a "neck."
These masses measure roughly about a square mile each.
With the acid intrusions may possibly be associated some of the other
masses of granophyre, microgranite and granite (felsite, felstone,
quartz-felsite, syenitic granite, quartz-syenite, elvanite), which
have long attracted attention in this region. The largest of these
intrusions is the tract of granite which stretches from Eskdale down to
near the sea-coast as a belt about eleven miles long and from one to
three miles broad. Another large mass is the granophyre or "syenite" of
Ennerdale. Numerous other intrusions of smaller dimensions have been
mapped.
To what extent any of these eruptive masses were associated with the
volcanic phenomena remains still to be worked out. There seems to be
little doubt that a number of them must belong to a much later period.
Mr. Harker has expressed his belief that the intrusion of some of
these igneous rocks was intimately associated with the post-Silurian
terrestrial movements of which cleavage is one of the memorials.[258]
The Skiddaw granite, though it does not touch any part of the volcanic
group, but is confined to the underlying Skiddaw Slates, was erupted
after the cleavage of the district, which affects the volcanic as
well as the sedimentary series. In other instances also, as in that
of Carrock Fell, the intrusion seems to have been later than the
disturbances of the crust.[259] The amount of metamorphism around some
of the bosses of granite is considerable. That of the Skiddaw region
has been well described by J. C. Ward,[260] while that of the volcanic
group by the Shap granite has been carefully worked out by Mr. Harker
and Mr. Marr.[261]
[Footnote 258: _Quart. Journ. Geol. Soc._ vol. li. (1895), p. 144.]
[Footnote 259: _Op. cit._ p. 126.]
[Footnote 260: "Geology of Northern Part of the English Lake District,"
_Mem. Geol. Surv._ 1876, chap. iii. The metamorphism around the
diorites and dolerites, and the granophyres and felsites, is described
in the same chapter.]
[Footnote 261: _Quart. Journ. Geol. Soc._ xlvii. (1891) p. 266, xlix.
(1893) p. 359.]
The Shap granite comes through the very highest member of the volcanic
series, and even alters the Upper Silurian strata. It must thus be of
much younger date than the volcanic history of the Lake District. It
presents some features in common with the granite bosses of the south
of Scotland. Like these, it is later than Upper Silurian and older than
Lower Carboniferous or Upper Old Red Sandstone time. Its protrusion may
thus have been coeval with the great volcanic eruptions of the period
of the Lower Old Red Sandstone. It will accordingly be again referred
to in a later chapter.
It must be confessed that none of the large bosses of massive rocks,
whether diabases, gabbros, felsites, granophyres, or granites, appear
to afford any satisfactory proof of the position of the vents which
supplied the lavas and tuffs of the Lake District. Nor can such a
decided accumulation of the volcanic materials in certain directions
be established as to indicate the quarters where the centres of
eruption should be sought. On the contrary, the confused commingling
of materials, and the comparative shortness of the outcrop of the
several sheets which have been traced, rather suggest that if any one
great central volcano existed, its site must lie outside of the present
volcanic district, or more probably, that many scattered vents threw
out their lavas and ashes over no very wide area, but near enough to
each other to allow their ejected materials to meet and mingle. The
scene may have been rather of the type of the Phlegræan fields than
of Etna and Vesuvius. If this surmise be true, we may expect yet to
recognize little necks scattered over the volcanic district and marking
the positions of some of these vanished cones.
What appears to have been one of these small vents stands near Grange
at the mouth of Borrowdale, where I came upon it in 1890. In the little
Comb Beck, the Skiddaw Slates are pierced by a mass of extremely coarse
agglomerate, forming a rudely-circular boss. The slates are greatly
disturbed along the edges of the boss, so much so, indeed, that it is
in some places difficult to draw a line between them and the material
of the agglomerate. That material is made up of angular blocks, varying
in size up to three feet long, stuck in every position and angle in an
intensely-indurated matrix formed apparently of comminuted debris like
the stones. The blocks consist of a finely-stratified shale, which is
now hardened into a kind of hornstone, with some felsitic fragments. I
could see no slags or bombs of any kind. There is no trace of cleavage
among the blocks, nor is the matrix itself sensibly cleaved. I believe
this to be a small volcanic neck and not a "crush-conglomerate." It has
been blown through the Skiddaw Slates, and is now filled up with the
debris of these slates. Its formation seems to have taken place before
the cleavage of the strata, and its firm position and great induration
enabled it to resist the cleavage which has so powerfully affected the
slates and many members of the volcanic group.
It was the opinion of my predecessor, Sir Andrew Ramsay, and likewise
of Mr. Ward, that the Cumbrian volcanic action was mainly subærial.
This opinion was founded chiefly on the fact that, save at the bottom
and top of the series, there is no evidence of any interstratified
sediment of non-volcanic kind. The absence of such interstratification
may undoubtedly furnish a presumption in favour of this view, but,
of course, it is by no means a proof. Better evidence is furnished
by the unconformability already mentioned between the Coniston
Limestone and the lavas on which it lies. Besides angular pieces of
lava, probably derived from direct volcanic explosion, this limestone
contains fragments of amygdaloidal andesite, and also rolled crystals
of striated felspar.[262] These ingredients seem to indicate that some
part of the volcanic group was above water when the Coniston Limestone
was deposited.
[Footnote 262: Messrs. Harker and Marr, _Quart. Journ. Geol. Soc._ vol.
xlvii. (1891), p. 310.]
The absence of interstratifications of ordinary non-volcanic sediment
in the Borrowdale group might conceivably arise from the eruptions
following each other so continuously on the sea-floor, and at so
great a distance from land that no deposition of sand or mud from the
outside could sensibly affect the accumulation of volcanic material.
Certainly some miles to the east at the Cross Fell inlier, as already
mentioned, there is evidence of the alternation of tuffs with the sandy
and muddy sediment of the sea-bottom. Here, at the outer confines of
the volcanic district, the ejected materials evidently fell on the
sea-floor, mingled there with ordinary sediment, and enclosed the same
organic remains. The well-defined stratification of many of the fine
tuffs is rather suggestive to my mind of subaqueous than of subærial
accumulation. At the same time, there seems no reason why, here and
there at least, the volcanic cones should not have risen above the
water, though their materials would be washed down and spread out by
the waves.
One of the most marked points of contrast between the Cumbrian and
the Welsh volcanic districts is to be found in the great paucity of
sills in the former region. A few sheets of diorite and diabase have
been mapped, especially in the lower parts of the volcanic group and
in the underlying Skiddaw Slates. On the other hand, dykes are in some
parts of the district not unfrequent, and certainly play a much more
prominent part here than they do in the Welsh volcanic districts. The
majority of them consist of felsites, quartz-porphyries, diorites, and
mica-traps. But there is reason to suspect that where they are crowded
together near the granite, as around Shap Fells, they ought to be
connected with the uprise of the post-Silurian granitic magma rather
than with the history of the volcanic group.[263] If this series of
dykes be eliminated, there remain comparatively few that can with any
confidence be associated with the eruption of the Borrowdale rocks.
[Footnote 263: For a description of the dykes around the Shap granite
see the paper by Messrs. Harker and Marr, _Quart. Journ. Geol. Soc._
vol. xlvii. (1891), p. 285.]
vii. UPPER SILURIAN (?) VOLCANOES OF GLOUCESTERSHIRE
A remarkable group of igneous materials has long been known to rise
among the Silurian rocks of the Tortworth district at the north
end of the Bristol coal-field. They were believed to be aqueous
deposits in the Wernerian sense by Weaver.[264] Murchison regarded
them as intrusive sheets;[265] Phillips looked on them as partly
intrusive and partly interstratified.[266] They consist largely of
coarsely-amygdaloidal basalts, some of which have been microscopically
examined.[267] But their field-relations as well as their petrography
have not yet been adequately determined. They are represented on the
Geological Survey Map as forming a number of parallel bands in strata
classed as Upper Llandovery. If, as seems probable, some of them are
really interstratified, they form the youngest group of Silurian
volcanic rocks in England, Scotland, or Wales.
[Footnote 264: _Trans. Geol. Soc._ 2nd ser. vol. i. (1819), pp.
324-334.]
[Footnote 265: _Silurian System_ (1839), p. 457.]
[Footnote 266: _Mem. Geol. Surv._ vol. ii. part i. (1848), p. 194.]
[Footnote 267: "Geology of East Somerset," etc., in _Mem. Geol. Surv._
(1876), p. 210; descriptions by Mr. F. Rutley.]
CHAPTER XIV
THE SILURIAN VOLCANOES OF IRELAND
Abundant as are the volcanic records of the Silurian period in England,
Wales and Scotland, the description of them would be incomplete without
an account of those of Ireland. The eruptions of Arenig, Llandeilo and
Bala time, which we have followed from the south of Caermarthenshire to
the borders of the Scottish Highlands, had their counterparts all down
the east of Ireland. The Irish register of them, however, supplies some
details which are less clearly preserved in the sister island. But the
most distinctive feature of the Silurian volcanic history in Ireland is
the preservation of memorials of eruptions during the Upper Silurian
period. In no part of Great Britain has any unquestionable trace been
found of volcanic activity during that part of the geological record,
the last eruptions of which the age is known being those of the Bala
rocks. But in the south-west of Ireland there is evidence that for a
time active vents appeared over the sea-floor on which the earlier
deposits of Upper Silurian time were laid down.
I. The Lower Silurian Series
i. _Eruptions probably of Arenig Age_
It is in that part of Ireland which lies east of a line drawn from
Strabane to Dungarvan Harbour that the records of Lower Silurian
volcanic activity are to be found. In the north the development of
volcanic rocks resembles that in Scotland, in the south it corresponds
rather with the volcanic districts of Wales.
The Irish Silurian volcanic rocks have been traced with more or less
detail on the maps of the Geological Survey. Since these maps were
published, however, great advances have been made in the study of the
petrography of volcanic rocks, as well as in the art of tracing their
structure upon maps. Much, therefore, now remains to be done to bring
our knowledge of the older volcanic history of Ireland abreast of that
of the rest of the British Isles. In the following summary I have had
to rely mainly on my own traverses of the ground, guided by the maps
and memoirs of the Survey, and with the personal assistance of some of
my colleagues.
The remarkable zone of crushed cherts, igneous rocks and sandstones,
probably of Lower Silurian age, which I have referred to (p. 201) as
wedged in between the schists and the Old Red Sandstone along the
southern margin of the Highlands of Scotland, reappears in Ireland.
It occupies an area in the County Tyrone, about 24 miles long and
about 9 miles broad at the broadest part, but disappearing towards the
north-east and south-west.[268] Lying between the Palæozoic formations
on the south and the schists on the north, it occupies a similar
position to the Scottish belt, but presents a much broader area, and
thus affords greater facilities for examining the rocks. It presents
the same indefinite or faulted boundaries as in Scotland, so that its
relations to the rocks along its flanks have not been satisfactorily
determined. That the rocks of this area are older than the Silurian
strata to the south of them seems to be established by the occurrence
of fragments of them in these strata, and that they are younger than
the schists may be inferred from their non-foliated character. But
they have undoubtedly undergone considerable crushing by powerful
terrestrial movements which have placed them in their present position.
[Footnote 268: This area was mapped by Mr. J. Nolan for the Geological
Survey, and was described by him in the _Geol. Mag._ for 1879. I
visited it in company with my colleagues, Mr. B. N. Peach and Mr. A.
M'Henry, in 1890 and again in 1894. My first conclusion was that the
volcanic rocks should be regarded as part of the schistose series lying
to the north of them (_Pres. Address Geol. Soc._ 1891, p. 77). But on
the second visit, after having studied the rocks of the border of the
Scottish Highlands, I formed the opinion stated in the text.]
The special feature of interest in this Irish area is the remarkable
development of volcanic materials which is there to be seen, spreading
over a far wider area than in Scotland. The rocks include lavas
associated with tuffs and agglomerates, likewise a varied series of
intrusive masses.
The lavas are chiefly dull greenish, fine-grained rocks, having the
general character of diabases and "porphyrites." They are sometimes
quite slaggy, and where the amygdaloidal kernels remain, these are
usually of calcite. Under the microscope, the diabases show in some
parts that their lath-shaped felspars, and the augite which these
penetrate, are tolerably fresh, while in other parts fibrous chlorite,
granular epidote and veins of calcite bear witness to the metamorphism
which they have undergone.
One of the most conspicuous features in some of these lavas is the
occurrence of the same sack-like or pillow-shaped structure which
has been already referred to as so marked among the Arenig lavas of
Scotland. Though the vesicles of these rocks are often quite uncrushed,
showing that there has been no general subsequent deformation of the
whole mass, there occur local tracts where evidence of considerable
movement may be noticed. Thus close to a mass of gneiss, and elsewhere
along their margin, the lavas are apt to be much jointed and broken
with numerous lines of shear, along which the crushed material assumes
more or less of a schistose structure. Yet in the solid cores between
these bands of crushing the original forms of the vesicles are retained.
These greenish lavas are occasionally interleaved with grey flinty
mudstones, cherts and red jaspers, which are more particularly
developed immediately above. In lithological character, and in their
relation to the diabases, these siliceous bands bear the closest
resemblance to those of Arenig age in Scotland. But no recognizable
Radiolaria have yet been detected in them.
Besides the more basic lavas, there occur also, but less abundantly,
platy felsitic rocks which have suffered much from shearing, and
consequently have acquired a fissile slaty structure.
The agglomerates are made up of angular, subangular and rounded
fragments imbedded in a matrix of similar composition. This matrix
has in places become quite schistose, and then closely resembles
some parts of the "green schists" of the Scottish Highlands.
Of the inclosed stones the great majority consist of various
felsites, which, weathering with a thick white opaque crust, are
internally close-grained, dull-grey or even black, sometimes showing
flow-structure, and of all sizes up to eight inches in diameter or
more. There are also fragments of the basic lavas, and likewise pieces
of chert and jasper. On many of the rocky hummocks no distinct bedding
can be made out in the agglomerate, but in others the rock is tolerably
well stratified.
The tuffs are fine silky schistose rocks, and seem to have been largely
derived from basic lavas. They have suffered more than any of the other
rocks from mechanical deformation, for they pass into green chloritic
schists. Some portions of them are not unlike the slaty tuffs of Llyn
Padarn in Caernarvonshire.
Accompanying the fragmental volcanic rocks, some ordinary sedimentary
intercalations are to be found--red shales and pebbly quartzites, that
seem to have escaped much crushing. The true order of succession in the
volcanic series has not yet been determined. But apparently above this
series come some dark shales, such as might yield graptolites, pale
grits and occasional limestones.
Later than the lavas and the pyroclastic material are various intrusive
masses, which in bands and bosses form numerous craggy hills throughout
the area. So far as I have been able to observe, these rocks include
two groups. Of these the older consists of basic injections, such as
gabbros and allied rocks, some of which remind me of the so-called
"hypersthene-rock" of Lendalfoot, in Ayrshire. The coarser varieties,
as at Carrickmore or Termon rock, are sometimes traversed by
fine-grained veins from an inch to several feet in breadth. Portions
of the slaggy diabases may be observed inclosed in these intrusive
masses. The younger group is of more acid composition (granite,
quartz-porphyry, etc.), and sends veins into the older.
ii. _Eruptions of Llandeilo and Bala Age_
Into the east of Ireland the Lower Silurian rocks are prolonged from
Scotland, from the Lake District and from Wales. Though greatly
concealed under younger formations across the breadth of the island,
and occasionally interrupted by what are regarded as older strata of
Cambrian age, they nevertheless occupy by much the larger part of the
maritime counties from Belfast Lough to the southern coast-line of
Waterford, even as far as Dungarvan Harbour. With the same lithological
types of sedimentary deposits as in other parts of the United
Kingdom, they carry with them here also their characteristic records
of contemporaneous volcanic action. Though nowhere piled into such
magnificent mountain-masses as in Westmoreland and North Wales, these
records become increasingly abundant and interesting as they are traced
southwards, until they are abruptly terminated by the coast-line along
the south of the counties of Wexford and Waterford.
While much remains to be done, both in the field and in the laboratory
and microscope-room, before our acquaintance with the Irish Silurian
volcanic rocks is as complete as our knowledge of their equivalents in
other portions of the United Kingdom, a serious preliminary difficulty
must be recognized in the fact that the several geological horizons
of these rocks have only been approximately fixed. Great difficulty
was experienced by the Geological Survey in drawing any satisfactory
line between the Llandeilo and Bala formations. This arose not so
much from deficiency of fossil evidence as from the way in which the
fossils of each group seemed to occur in alternating bands in what were
regarded as a continuous series of strata. Indeed, in some localities
it almost appeared as if the occurrence of one or other _facies_ of
fossils depended mainly on lithological characters indicative of
original conditions of deposit, for the Llandeilo forms recurred
where black shales set in, while Bala forms made their reappearance
where calcareous and gritty strata predominated.[269] More recent
work among the Silurian formations in England and Scotland, however,
indicates that the parallel repetition of the two types of fossils
is due to rapid and constant plication of the rocks, whereby the two
formations, neither of them, perhaps, of great thickness, have been
folded with each other in such a way that without the evidence of an
established sequence of fossils, or the aid of continuous sections,
it becomes extremely difficult to make out the stratigraphical order
in any district. When the ground is attacked anew in detail, with
the assistance of such palæontological and lithological horizons as
have permitted the complicated structure of the southern uplands of
Scotland to be unravelled, we may be enabled to tabulate the successive
phases of the volcanic history of the region in a way which is for
the present impossible. We have as yet no palæontological evidence
that in the Silurian region of the east of Ireland, which extends
from Belfast Lough to the south coast of County Waterford, any of the
anticlinal folds bring up to the surface a portion of the Lower Arenig
formation, though possibly some of the lowest visible strata may be of
Upper Arenig age. A considerable part of the region must be referred
to the Llandovery and other Upper Silurian formations, but the precise
limits of the two divisions of the Silurian system have not yet been
determined, except for the region north of Dublin, which has recently
been re-examined for the Geological Survey by Mr. F. W. Egan and Mr. A.
M'Henry.
[Footnote 269: Jukes was disposed to regard the two faunas as
essentially coeval, but inhabiting different kinds of sea-bottom. See
his note, Explanation of Sheets 167, 168, 178, 179, p. 30.]
These observers have ascertained that, as in Southern Scotland, by far
the larger part of the Silurian region of the north-east of Ireland is
occupied by strata belonging to the upper division of the system. The
Lower Silurian formations, including the Llandeilo and Bala groups,
form a belt varying up to six miles in breadth, which stretches from
the coast of Down, between the mouth of Belfast Lough and Copeland
Island, in a south-westerly direction to near the valley of the Shannon
in County Longford. South of this belt the Lower Silurian rocks
rise to the surface only here and there on the crests of anticlinal
folds, and it is in these scattered "inliers" that the volcanic and
intrusive rocks are found. So far as the available evidence goes, the
volcanic history of this part of Ireland is entirely to be assigned to
Lower Silurian time, and more especially to the interval between the
beginning of the Llandeilo and the close of the Bala period. I must
for the present content myself with this general limit of geological
chronology, and make no attempt to trace the relative antiquity
of the igneous rocks in the several districts in which they are
distributed.[270]
[Footnote 270: The task of revising the Irish maps and tracing out
the respective areas of Upper and Lower Silurian rocks over the whole
island is now in progress by the Geological Survey, Mr. Egan and Mr.
M'Henry being entirely engaged on it.]
Viewing the volcanic region of Eastern Ireland as a whole, we are
first struck by the feebleness of the manifestations of eruptivity in
the north, and their increasing development as we advance southwards.
At the northern end of the Silurian area in County Down, thin bands
of "felstone" and "ash" have been mapped by the Geological Survey
as interstratified with the highly inclined and plicated Silurian
rocks.[271] As the latter are plainly a continuation of the strata
which have been mapped out zone by zone in the south of Scotland, their
igneous intercalations may be looked upon as probably equivalents
of some of those in the Silurian districts of Wigtonshire and
Kirkcudbrightshire. But in County Down no representative has yet been
detected of the Arenig and Llandeilo volcanic series of the southern
uplands of Scotland. Nor has more precise petrographical examination
confirmed the reference of any of the igneous rocks in the Silurian
area of that district to truly contemporaneously intercalated volcanic
rocks. All the eruptive material appears to be of an intrusive
character. It occurs in the form of dykes of lamprophyre or mica-trap
belonging to the groups of minettes and kersantites. Nothing definite
is known of the age of these intrusions: they are possibly referable to
the time of the Lower Old Red Sandstone.[272]
[Footnote 271: See Sheet 49 Geol. Survey, Ireland, and Explanation
thereto (1871), pp. 16, 37, 39. The so-called "ashes" of the
Explanation are probably parts of dykes which have been more or less
crushed.]
[Footnote 272: _Guide to the Collection of Rocks and Fossils belonging
to the Geological Survey of Ireland_, by Messrs. M'Henry and Watts,
Dublin, 1895, p. 74.]
Far in the interior several bands of "felspathic ash" and "massive
agglomerate" are shown on the Survey map as running through the
counties of Monaghan and Cavan.[273] In one locality south of the
Drumcalpin Loughs a large exposure of this ash is visible: "brown
crumbly beds, with small rounded pebbles, give place to a massive bed
of agglomerate, the enclosed blocks of which are always of one species
of felstone, sometimes measuring 10 × 12 × 18 inches, and not always
rounded." South of Carrickatee Lough, and a few miles farther to the
south-west, near Lackan Bridge, considerable exposures of these rocks
occur. One crag in particular displays a thickness of more than 70 feet
of "tough flaky breccias," "thick agglomerates with small and large
blocks of felstone," and "thin beds of fine pale green compact grit
without pebbles, and a few flags." "One of the flaky beds contains
numerous white worn crystals of felspar"; "the imbedded blocks of
felstone are of the usual kind--pale compact matrix showing dark oblong
patches, vesicular and amygdaloidal, the cavities being filled with
chlorite."
[Footnote 273: Sheet 69 Geol. Survey, Ireland, and Explanation of
Sheets 68 and 69, pp. 9, 13, 15.]
Further south a more extensive area of igneous rocks has been mapped
on the borders of Louth and Meath, where, according to the Geological
Survey map, a group of lavas and tuffs extends for about twelve miles
near Slane.[274] Other bands of "ash" and "felstone" have been mapped
in the Silurian area south of Drogheda. Thus at Hilltown, west from the
racecourse, a "bluish crystalline felstone, showing in places lines
of viscous flow," is stated to be overlain by "indurated felspathic
ash and tuff, felstone, and indurated shale" in alternating beds.[275]
On a recent visit to this locality I found that the "porcellanite or
indurated shale" is a greenish-grey chert, full of Radiolaria and
finely-diffused volcanic dust. This association of radiolarian chert
with contemporaneous volcanic activity is of much interest, as showing
the extension of the same physical conditions of the Lower Silurian sea
from Scotland into Ireland. The Lower Llandeilo age of the volcanic
intercalations in County Meath is further indicated by the occurrence
of _Didymograptus Murchisoni_ in grey shales in the same neighbourhood
with the radiolarian cherts. In the Lower Silurian district of
Balbriggan numerous intrusive bosses and sills have been mapped by
the Geological Survey. I have found, however, that among these rocks
there occur bands of volcanic breccia, containing abundant angular
fragments of a minutely-vesicular pumice, and also that some of the
diabase-masses display the pillow-structure and amygdaloidal texture.
Hence, though most of the igneous rocks are no doubt intrusive, they
appear to include lavas and tuffs of Bala age.
[Footnote 274: _Ibid._ Sheets 81 and 91. These rocks are chiefly
augitic andesites, a few are basalts, and some seem related to
felstones. Probably many of them are intrusive sills of uncertain
age. The "ashes" contain fragments of felsite and porphyrite often of
considerable size (_Guide to Irish Rock-Collection_, p. 36).]
[Footnote 275: _Ibid._ Sheets 91 and 92 and Explanation to these
Sheets (1871), p. 10; _Guide to Irish Rock-Collection_, p. 36. Some
of these lavas are andesites, others are felsites. Mr. M'Henry has
contended that certain "ashes" and "agglomerates," particularly
those exposed on the coast at Portraine, opposite Lambay Island, are
"crush-conglomerates" due to terrestrial disturbances, which have
affected both intrusive igneous rocks and the sedimentary series into
which these have been injected.]
When the numerous Silurian cores of the mountain-groups in the interior
of Ireland shall have been searched for traces of contemporaneous
volcanic action, it is not improbable that these will be found. One of
the smaller Silurian inliers which diversify the great Carboniferous
plain, that of the Chair of Kildare, has long been known to have
igneous rocks associated with its abundantly fossiliferous Bala
limestone.[276] On recently visiting this locality I found that,
besides the amygdaloidal and porphyritic andesites and basalts
described by Jukes and Du Noyer, the fossiliferous conglomerates
contain pebbles of rocks like those of the Chair, together with worn
crystals of felspar, while intercalated with them are thin courses of
volcanic tuff. There is thus evidence here of contemporaneous volcanic
activity during the accumulation of the Bala group of strata. The
limited area over which the rocks are exposed, however, affords merely
a glimpse of this volcanic centre.
[Footnote 276: See Explanation to Quarter Sheet 35 N.E. (Sheet 119 of
newer numeration) of Geol. Survey Ireland (1858), p. 16. (See note, p.
256.)]
Crossing over the broad belt of Carboniferous Limestone through which
the Liffey flows into Dublin Bay, we come to the great continuous
tract of older Palæozoic rocks which stretches southward to the cliffs
of Waterford. Through this tract runs the huge ridge of the Wicklow
and Carlow granite. On the west side of this intrusive mass, bands
of "greenstone-ash," as well as "felspathic ashes," have been traced
among the Silurian rocks by the Geological Survey. But it is on the
south-east side of the granite that the volcanic intercalations are
best displayed. Indeed, from Wicklow Head to Dungarvan Harbour there is
an almost continuous development of igneous rocks, rising into rocky
eminences, trenched into ravines by the numerous streams, and laid bare
by the waves in fine coast-cliffs. It is in this south-eastern region,
comprising the counties of Wicklow, Wexford and Waterford, that the
Irish Lower Silurian igneous rocks can best be studied.
There are obviously various distinct centres of eruption in this long
belt of country. The Rathdrum and Castletimon tract forms one of these.
Another of less size culminates in Kilpatrick Hill, a few miles to
the southward. Arklow Head marks the position of a third. The lavas
and tuffs which set in a few miles to the south of that promontory,
and may be said to extend without interruption to the south coast,
were probably thrown out by a series of vents which, placed along a
north-east and south-west line, united their ejections into one long
submarine volcanic bank. There can be no doubt that the most active
vents lay at the southern end of the belt, for there the volcanic
materials are piled up in thickest mass, and succeed each other with
comparatively trifling intercalations of ordinary sedimentary material.
Some of these vents, as I shall relate in the sequel, have been cut
open by the sea along a range of precipitous cliffs.
The comparatively feeble character of the volcanic energy during Lower
Silurian time over the greater part of the south-east of Ireland is
shown by the great contrast between the thickness of the volcanic
intercalations there and in Wales and the Lake country, but still more
strikingly by innumerable sections where thin interstratifications of
fine tuff or volcanic breccia occur among the ordinary sedimentary
strata, and are sometimes crowded with Bala fossils. Some interesting
illustrations of this feature are to be seen in the Enniscorthy
district, where layers of fine felsitic tuff, sometimes less than
an inch in thickness, lie among the shales. In some of the tuffs the
lapilli are fragments of trachytic or andesitic rocks.
A striking example of rapid alternations of pyroclastic material with
ordinary sediment lies far to the south in County Waterford, close
to Dunhill Bridge, where a group of fine volcanic breccias and grits
has been laid bare by quarrying.[277] These strata consist of coarser
and finer detritus, enclosing angular fragments of felsites and grey
and black shale. The felsite-lapilli vary in texture, some of them
presenting beautiful flow-structure. The stones are stuck at random
through each bed, the largest being often at the bottom. The beds of
breccia vary from a few inches to a foot or more in thickness. There
can, I think, be little doubt that each of these breccia-bands points
to a single volcanic explosion, whereby felsitic fragments were thrown
out, mingled with pieces of the Silurian strata through which the vents
were drilled. In a vertical thickness of some fifty feet of rock there
must thus be a record of ten or twelve such explosions.
[Footnote 277: See Explanation of Sheets 167, 168, 178 and 179, Geol.
Surv. Ireland, p. 56.]
Nearer the active vents the fragmental deposits become, as usual,
coarser and thicker. But I have not observed any thick masses of
tuff like those of North Wales. So far as my examination has gone,
the tuffs are mainly felsitic. The so-called "greenstone-ash" of the
Survey maps is certainly in many cases not a true tuff. This term was
proposed by Jukes for certain apple-green to olive-brown flaky fissile
rocks only found "in association with masses of greenstone."[278] Some
years ago I had occasion to make a series of traverses in Wicklow and
Wexford, and then convinced myself that in that part of the country
the "greenstone-ashes" were probably crushed bands of basic sills. Dr.
Hatch has proved this to be their origin from a series of microscopic
slides prepared from specimens collected by himself on the ground.[279]
In other cases the "greenstone-ashes" seem to be excessively-cleaved
or sheared felsites, which have acquired a soapy feel and a dull green
colour; but they also do include true tuffs. Thus, in one instance,
at Ballyvoyle cross-roads, in the south of County Waterford, a
"greenstone-ash" is a dull green tuff full of fragments of felspar
(chiefly plagioclase) and pieces of dark andesitic lavas. Another
example may be found to the west of the Metal Man, near Tramore, where
the tuff is full of fragments of felspar and shale cemented in a
greenish-yellow material which may be palagonite.
[Footnote 278: Explanation of Sheets 129, 130, p. 13 (1869).]
[Footnote 279: Explanation of Sheets 138, 139.]
The felsites of the south-east of Ireland form by much the largest
proportion of the whole volcanic series. They occur as lenticular
sheets from a few feet to several hundred feet in thickness, and
occasionally traceable for some miles. On the whole, they are compact
dull grey rocks, weathering with a white crust. A geologist familiar
with the contemporary lavas of North Wales cannot fail to be struck
with the absence of the coarse flow-structure so often characteristic
of the felsites in that region. This structure, indeed, is not entirely
absent from the Irish rocks, but it occurs, so far, at least, as I have
seen, rather as a fine streakiness than in the bold lenticular bands
so common in Caernarvonshire. In like manner the nodular structure,
though not entirely absent, is rare.[280]
[Footnote 280: In Waterford nodular felsites occur with concretions
varying from the size of a pea to several inches in diameter.
Explanation to Sheets 167, 168, 178 and 179, p. 11.]
Until these felsites have been subjected to more detailed
investigation, little can be said as to their petrography, and as to
the points of resemblance or difference between them and those of other
Lower Silurian districts in the United Kingdom. An important step,
however, in this direction was taken by Dr. Hatch, who studied them
on the ground, in the laboratory, and with the microscope. He found
that some of them were soda-felsites or keratophyres (with albite as
their felspar), that others were potash-felsites (with orthoclase as
their felspar), while a third group contained both soda and potash, the
last-named greatly preponderating.[281] The existence of soda-felsites
had not been previously detected among British volcanic rocks, and it
remains to be seen how far they may occur in the large and somewhat
varied group of rocks combined under the general term "felsites." Dr.
Hatch believed that these rocks probably graduate into the normal or
orthoclase felsites; but it has not yet been possible to test this
view on the ground, nor to ascertain whether there is any essential
difference between the mode of occurrence of the two types.
[Footnote 281: Explanation of Sheets 138, 139, p. 49; and _Geol. Mag._
1889, p. 545.]
Besides the more abundant felsites, occasional bands of andesite have
been detected. Various other eruptive rocks occur, probably in most or
all cases intrusive. Such are quartz-mica-diorites, quartz-diorites,
augite-diorites or proterobases, dolerites, gabbros, diabases and
epidiorites.[282]
[Footnote 282: _Guide to Irish Rock-Collections_, pp. 34, 35.]
I have said that the chief theatre of eruption lay towards the
south-west end of the volcanic belt of the south-east of Ireland. The
coast-line of County Waterford, from Tramore westward to Ballyvoyle
Head--a distance of nearly fifteen miles--presents, perhaps, the
most wonderful series of sections of volcanic vents within the
British Islands. No one coming from the inland is prepared for either
the striking character of the cliff scenery or the extraordinary
geological structure there presented, for the country is, on the whole,
rather featureless, and much of it is smoothed over and obscured by
a covering of drift, through which occasional knobs of the harder
felsites protrude. The cliffs for mile after mile range from 100 to
150 or 200 feet in height, and present naked vertical walls of rock,
trenched by occasional gullies, through which a descent may be made
to the beach. Throughout the whole distance agglomerates and felsites
succeed each other in bewildering confusion, varied here and there by
the intercalation of Lower Silurian shales and limestones involved
and pierced by the igneous rocks. Hardly any bedded volcanic material
is to be recognized from one end to the other. The sea has laid bare
a succession of volcanic vents placed so close to each other that it
will be difficult or impossible to separate them out. A careful study
and detailed mapping of this marvellous coast-section, however, is a
task well worthy of the labour of any one desirous of making himself
acquainted with some of the conditions of volcanism during older
Palæozoic time.
At the east end of the section, black shales containing Llandeilo
graptolites, and calcareous bands full of Bala fossils, dip westward
below a group of soda-felsites and felsitic tuffs, which seem to lie
quite conformably on these strata. Here, then, we start with proof that
the volcanic eruptions of this locality began during some part of the
Bala period. But immediately to the west, these bedded igneous rocks
are broken through by a neck of coarse agglomerate stuck full of chips
and blocks of shale, some of them a foot long, with abundant fragments
of scoriform and flinty felsites. Some columnar dykes of dolerite cut
through the neck, and a larger intrusion seems to have risen up the
same funnel. The bedded tuffs appear again for a short distance, but
they are soon replaced by a tumultuous mass of agglomerate. And from
this part of the coast onwards for some distance all is disorder.
The agglomerates are crowded with blocks of various felsites and
micro-granites sometimes 18 inches in diameter, many of them presenting
the most exquisite streaky flow-structure. The angularity of these
stones and the abrupt truncation of their lines of flow prove that they
were derived from the shattering of already consolidated rocks. In
other places the ejected materials consist almost wholly of black shale
fragments, but with an intermixture of felsite-lapilli.
It is difficult to convey an adequate idea of the way in which the
agglomerates are traversed by dykes, veins and bosses of various
felsites, and of how these break in endless confusion through each
other. Some of the intrusive rocks are compact and amorphous, others
are vesicular, others close-grained and columnar. Again and again they
present the most perfect flow-structure, and it is noticeable that the
lines of flow follow the inequalities of the walls of the fissure up
which the rock has ascended, and not only so, but even of the surfaces
of detached blocks of shale or felsite which have been caught up and
enclosed in the still moving mass.
A few of these intrusive rocks were examined in thin slices by Dr.
Hatch. Most of them appear to be soda-felsites, but they include also
rather decomposed rocks, some of which are probably diorites and
quartz-diorites. Occasionally, thoroughly basic dykes (dolerite) may be
observed.
In the midst of this tumultuous assemblage of volcanic masses,
representing the roots of a group of ancient vents, there occur
occasional interspaces occupied by ordinary stratified rocks. In the
eastern part of the section these consist mainly of black shale,
sometimes with calcareous bands, from which a series of Bala fossils
has been obtained.[283] A very cursory examination suffices to show
that these intercalations do not mark pauses in the volcanic eruptions.
They are, in fact, portions of the marine accumulations under the
sea-floor through which the vents were blown; they have been tossed
about, crushed and invaded by dykes and veins of felsite.
[Footnote 283: But see the _Geol. Survey Memoir_ on Sheets 167,
168, 178 and 179, Ireland (1865), p. 28, for a description of the
association of Bala and Llandeilo fossils on that coast-line.]
But certain other intercalated strips of stratified rocks present a
special interest, for they bring before us examples of volcanic ashes
that gathered on the sea-floor, but which were disrupted by later
explosions. Thus, at the Knockmahon headland, well-bedded felspathic
grits and ashy shales occur, thrown in among the general mass of
eruptive material. As I have already remarked, it is difficult or
impossible to fix the horizons of the stratified patches that are
involved among the igneous ejections of this coast-section, save where
they contain recognizable fossils, but the intercalation of true bedded
tuffs among them is a proof that volcanic action had been in operation
there long before the outbreak of the vents which are now laid bare
along the cliffs.
In the south-east of Ireland there is the usual association of acid
and basic sills with the evidence of a superficial outpouring of lavas
and ashes. But these intrusive masses play a much less imposing part
than in Wales. They may be regarded, indeed, as bearing somewhat the
same proportion to the comparatively feeble display of extrusive rocks
in this region that the abundant and massive sheets of Merionethshire
and Caernarvonshire do to the enormous piles of lavas and tuffs which
overlie them.
Among the acid intrusive sheets the most conspicuous are those mapped
by the Survey as "elvans." These rocks, as they occur in Wicklow
and Wexford, have been examined by Dr. Hatch, who finds them to be
micro-granitic in structure, occasionally exhibiting micropegmatitic or
granophyric modifications.[284] The true stratigraphical relations of
these rocks have not yet been adequately investigated. Those of them
which occur on the flanks of the great granite ridge are not improbably
connected with that mass, and if so are much younger than the Lower
Silurian volcanoes.[285]
[Footnote 284: Explanation of Sheets 138 and 139, p. 53.]
[Footnote 285: The Leinster granite is certainly later than the Lower
Silurian rocks and older than the Carboniferous rocks of the south-east
of Ireland. It may belong to the great epoch of granite protrusion
during the Old Red Sandstone period.]
The basic sills, or "greenstones," consist largely of diabase,
frequently altered into epidiorite; they include also varieties of
diorite.[286] That they were intruded before the plication and cleavage
of the rocks among which they lie is well shown by their crushed and
sheared margins where they are in thick mass, and by their cleaved
and almost schistose condition where they are thinner. The intense
compression and crushing to which they have been subjected are well
shown by the state of their component minerals, and notably by the
paramorphism of the original augite into hornblende.
[Footnote 286: Dr. Hatch, _op. cit._ p. 49.]
The scarcity of dykes associated with Silurian volcanic action is as
noticeable in the south-east of Ireland as it is in Wales. I have
observed a considerable number, indeed, but they are confined to the
line of old vents on the Waterford coast, and, but for the clear
cliff-sections cut by the sea, they would certainly have escaped
observation, for they make no feature on the ground in the interior.
They are sometimes distinctly columnar, and vary from less than a foot
to many yards in width. They traverse both the agglomerates and the
intrusive felsites. Most of them are of felsite, sometimes cellular;
but in some cases they are dolerites. There is obviously no clue to the
dates of these dykes.
That some at least of the vents along the south coast of County
Waterford may be vastly younger than the Lower Silurian rocks through
which they have forced their way is suggested, if not proved, by
a section which is in some respects the most extraordinary of the
whole of this remarkable series. The occurrence of a group of red
strata was carefully noted by the late Mr. Du Noyer at Ballydouane
Bay, when he was engaged in carrying on the Geological Survey of that
part of the country. At first he regarded them as belonging to the
Old Red Sandstone, which comes on in great force only a few miles to
the west; but he subsequently arrived at the belief that they are
really an integral part of the Lower Silurian rocks of the district.
Professor Jukes had previously expressed himself in favour of this
latter idea, which was thought to receive support from the occurrence
of some reddish strata in the Lower Silurian rocks of Tagoat, County
Wexford.[287]
[Footnote 287: Explanation of Sheets 167, 168, 178 and 179 of the
Geological Survey of Ireland (1865), pp. 10, 59.]
The occurrence of red rocks among Silurian strata, which are not
usually red, might quite reasonably be looked for in the neighbourhood
of Old Red Sandstone, Permian or Triassic deposits. If these deposits
once spread over the Silurian formations, a more or less decided
"raddling" of the latter may have taken place. But in the present
instance, though the Old Red Sandstone begins not many miles to the
west, no such explanation of the colour of the strata is possible.
The cliffs of Ballydouane Bay consist of red sandstone, red sandy
shale and conglomerate. The red tint is of that dull chocolate tone so
characteristic of the Lower Old Red Sandstone. The conglomerates are
immense accumulations of ancient shingle, consisting largely of pieces
of white vein-quartz and quartzite, sometimes a foot long and often
well water-worn. Some of the sandy beds are full of large scales of
white mica, as if derived from some granitic or schistose region at no
great distance. Taken as a whole, the strata are much less indurated
and broken than the Silurian grits and shales of the district; some of
them, indeed, weather into mere incoherent sand that crumbles under the
fingers. There does not appear to be any positive proof that the red
rocks are truly bedded with the ordinary Silurian strata, the junctions
being faulted or obscured by intrusive igneous masses.
Nowhere in the British Islands, so far as I am aware, is there a
similar group of strata among the Lower Silurian rocks. If they belong
to so ancient a series, they show that in the south of Ireland, during
Lower Silurian time, there arose a set of peculiar physical conditions
precisely like those that determined the accumulation of the Old Red
Sandstone in the same region at a later geological period. And in that
case it is hardly possible to conceive that these conditions could
have been confined to the extreme south of Ireland. We should certainly
expect to meet with evidence of them elsewhere, at least in the same
Silurian region.[288]
[Footnote 288: The nearest approach of any Silurian group of strata to
the character of these conglomerates is furnished by the remarkably
coarse conglomerates, boulder-beds and pebbly grits of the Bala and
Llandovery series in the region between Killary Harbour and Lough Mask,
to which further reference is made in a later part of this chapter.]
While I hesitate to express a decided opinion in opposition to the
conclusions of such experienced observers as Jukes and Du Noyer, I
incline to believe that the rocks in question really belong to the
Old Red Sandstone. If such shall finally be determined to be their
geological position, they will supply evidence that some at least of
the volcanic vents of the coast-line cannot be older than the Old
Red Sandstone. They are pierced by masses of soda-felsite and by a
coarse red agglomerate containing abundant pieces of felsite. These
volcanic rocks belong to the same type as those which break through the
undoubted Silurian rocks on either side. They may thus come to prove a
recrudescence of volcanic energy in this same district at a much later
geological period; and a new problem will arise to task the skill of
the most accomplished field-geologist and petrographer--to unravel
the structure and history of this chain of volcanic vents, and, in so
doing, to detect and separate the eruptions of Lower Silurian time from
those of the Lower Old Red Sandstone.
In the far west of Ireland, another group of Lower Silurian volcanoes
has left its remains in the mountainous tract of country between the
western shores of Lough Mask and Killary Harbour.[289] There appear
to have been at least three separate centres of eruption along a line
stretching in a north-easterly direction for about 16 miles from the
western end of Lough Nafooey to the hamlet of Derrindaffdery beyond
Tourmakeady, where the older rocks are unconformably overlain by the
lower Carboniferous strata. As shown by the mapping of the Geological
Survey, the most northerly area, which may be called the Tourmakeady
centre, has a breadth of about a mile, and dies out southward after
a course of nearly six miles. About a mile to the south-west of the
last visible prolongation of its rocks, we encounter a second volcanic
centre which occupies an area of about a square mile in the valley of
Glensaul. The third centre stretches from the western shores of Lough
Mask across Lough Nafooey, where it forms a mass of high rugged ground,
and reaches a length of some six or seven miles before it finally dies
out.[290]
[Footnote 289: This group was placed in the Upper Silurian series
by the officers of the Geological Survey who mapped the region
(see Sheets 84, 85, 94 and 95 of the Geological Map of Ireland
and accompanying Explanation), and on their testimony I formerly
referred to the volcanic rocks as of Upper Silurian age. Mr. Baily,
however, had pointed out that the limestone associated with the
lavas and agglomerates contains Bala fossils. Yet, in spite of
this palæontological testimony, the fossils were considered to be
"derivative," and the rocks were removed from the series of formations
to which they would naturally be assigned. A recent examination of the
ground, in company with Mr. J. R. Kilroe of the Geological Survey,
has satisfied me that the volcanic rocks are interstratified with
sedimentary deposits of Bala age, and must consequently be grouped with
the rest of the Lower Silurian series of Ireland. The results of this
examination are given in the text.]
[Footnote 290: These areas were carefully mapped for the Survey by Mr.
Nolan, and the lines of division marked by him fairly represent the
general distribution of the rocks.]
The rocks in each of these three areas are similar. One of their
distinguishing features is the intercalation among them of a
fossiliferous limestone and calcareous fossiliferous tuffs, which
contain well-preserved species of organisms characteristic of the Bala
division of the Lower Silurian rocks.[291] There cannot be any question
that these organisms were living at the time the strata in which their
remains occur are found. The most delicate parts of the sculpture on
_Illænus Bowmanni_ and _Orthis elegantula_ are well preserved. Nor
have the limestones been pushed into their present places by volcanic
agency, or by faults in the terrestrial crust. They are not only
regularly intercalated among the volcanic rocks, but the limestone in
some places abounds in volcanic dust, while above it come calcareous
tuffs, also containing the same fossils. It is thus clearly established
that the volcanic series now to be described has its geological age
definitely fixed as that of the Bala period.
[Footnote 291: See the list of fossils as determined by Mr. Baily in
_Explanatory Memoir_ to accompany Sheets 73, 74, 83 and 84 of the
Geological Survey of Ireland, p. 68 (1876).]
The lavas of the Lough Mask region consist of felsites and andesites
with rocks of probably more basic composition. The felsites are
generally quartziferous porphyries, which occupy a considerable space
in each of the three districts. To what extent they are intrusive
rather than interstratified remains for investigation. Some of them
have undoubtedly invaded other members of the volcanic series. But, on
the other hand, fragments of similar quartz-porphyries and felsites
abound in the intercalated bands of volcanic breccia.
The andesites and more basic lavas are finely-crystalline or
compact, dull-green to chocolate-purple rocks, often resembling the
"porphyrites" of the Old Red Sandstone. Some of them are strongly
vesicular, the cavities being filled with calcite on fresh fracture,
though empty on weathered surfaces. The sack-like or pillow structure,
already referred to as characteristic of many Lower Silurian lavas,
appears conspicuously among some of these rocks. At Bohaun, nine miles
south from Westport, where a prolongation of the volcanic series
rises to the surface from under the overlying coarse conglomerates,
I observed that, owing to the compression which the rocks have there
undergone, the pillow-shaped blocks have been squeezed together into
rudely polygonal forms, while their vesicles have been greatly drawn
out in the direction of tension. Where the rocks have been still
more sheared, the distinct pillow-shaped blocks with their vesicular
structure disappear, while the more fine-grained crusts that surround
them have been broken up and appear as fragments involved in a matrix
of green schist.
Intercalated with the lavas are numerous bands of volcanic breccia and
fine tuff. The stones in these breccias consist chiefly of various
felsites with andesites and more basic lavas. But pieces of jasper,
chert, shale and grit are not infrequent. In some places abundant
blocks of black shale are to be noticed, probably derived from the
Llandeilo group which exists below, and which has here and there been
ridged up to the surface in the midst of the volcanic rocks.[292]
Near Shangort I noticed in one of these breccias one block measuring
12 feet, another 20 feet in length and 3 or 4 feet thick, composed of
alternating bands of grit and slate. It is interesting to note that
these strata had already undergone cleavage before disruption, the
bands of slate being strongly cleaved obliquely to the bedding. None
of the Llandeilo or other rocks in the neighbourhood display this
structure. The blocks seem to have been derived from some deeper group
of strata. They are laid down parallel with the rude bedding of the
breccia in which they lie.
[Footnote 292: In re-examining this region, Mr. Kilroe has found in the
stream west of the monastery, Tourmakeady, an uprise of graptolitic
black shale containing forms belonging to the very lowest Llandeilo or
Upper Arenig strata, and a similar band above Leenane, Killary Harbour.]
The fine tuffs and thin ashy limestones associated with the thicker
band of limestone show the renewal of volcanic explosions after the
interval marked by the calcareous deposit which is sometimes 20 or
40 feet thick. In many places this limestone is brecciated and much
mingled with volcanic dust and lapilli. At Shangort, for example, the
thick tolerably pure limestone is truncated on the west and north sides
by a coarse agglomerate probably filling a volcanic vent. A few hundred
yards further north, beyond the interrupting agglomerate, the limestone
reappears on the same line of strike, but is then found to be nodular
and brecciated and much mingled with volcanic detritus. It lies among
ashy grits and tuffs.
[Illustration: Fig. 64.--Diagram of the general relations of the
different groups of rock in the Lower Silurian volcanic district along
the western shore of Lough Mask.
_a_, Llandeilo shales, cherts and grits; _b_, Volcanic breccias;
_c_, Felsites and andesites; _d_, Tuffs and ashy grits and
shales; _e_, Limestone with Bala fossils; _f_, Calcareous
tuffs and thin bands of ashy limestone with fossils; _g_,
Coarse conglomerate and grits; _h_, Wenlock strata resting
unconformably on the Bala rocks and passing southwards from
these to overlie an older series of schists; *, Fault.
]
The general structure of the ground occupied by the Lough Mask volcanic
rocks is diagrammatically represented in Fig. 64. The thickness of the
volcanic series must amount to many hundred feet, but it has not been
precisely determined. The uppermost parts of the series pass under a
great thickness of coarse conglomerates and pebbly grits which form
the ridge of Formnamore, and stretch thence westwards along Killary
Harbour and through the Mweelrea mountains. These strata are classed
as the Upper Silurian on the Geological Survey map. Since, however,
they conformably overlie rocks containing Bala fossils, and in the
Killary district include green shales which have yielded fossils of the
same age, they doubtless belong in large part to the Lower Silurian
division. The remarkable coarseness of these conglomerates towards the
south, and their rapid passage into much finer grits and shales towards
the north, probably indicate that they were formed close to the shores
of a land composed of schistose rocks, quartzite and granite, of which
the mountainous tracts of Connemara are the last relics.
A base to the volcanic series is found in the occasional uprise of
a short axis of Llandeilo, or perhaps even upper Arenig strata,
containing bands of dark chert and black graptolitic shales.
Unfortunately the relations of these underlying rocks to the volcanic
masses are not very clear, being obscured by superficial accumulations
and also by faulting. It is thus hardly possible to be certain whether
they pass up conformably into the base of the volcanic series, or are
covered by it unconformably.
The position of this isolated volcanic district in the far west of
Ireland, the abundance, variety and thickness of the erupted materials,
and the definite intercalation of these materials in the Bala or
highest division of the Lower Silurian series, acquire a special
interest from the history of the nearest Silurian volcanic area which
has now to be described--that of the western shores of the Dingle
promontory.
II. The Upper Silurian Series
The latest volcanic eruptions of Silurian time yet definitely known
took place during the accumulation of the Wenlock and Ludlow rocks in
the far west of Ireland. No satisfactory record of any contemporaneous
phenomena of a like kind has yet been met with in any other Upper
Silurian district in the British Isles, unless at Tortworth in
Gloucestershire, as above described. So far as at present known, only
one centre of activity has been preserved. It lies among the headlands
of Kerry, where the land projects furthest west into the stormy
Atlantic. The occurrence of volcanic rocks in this remote area and
their geological horizon have been clearly indicated on the maps of the
Geological Survey. More than thirty years, however, have elapsed since
some of the mapping was done, and we must therefore be prepared to find
it, more especially in its petrography, capable of modification and
improvement now.
In the country known as the Dingle promontory, these traces of
contemporaneous volcanic rocks are to be observed at various localities
and on several horizons. To the east, near Anascaul, on the northern
shore of Dingle Bay, some tuffs occur in what are believed to be
Llandovery strata. But it is on the western coast, among the headlands
and coves that lie to the north and south of Clogher Head, that the
best sections are to be seen. The succession of the rocks in this
locality was well worked out by Du Noyer, and the Memoir prepared by
him, with the general introduction by Jukes, is an invaluable guide to
the geologist who would explore this somewhat inaccessible region.[293]
The most important correction that will require to be made in the work
arises from a mistake as to the true nature of certain rocks which were
described as pisolitic tuffs, but which are nodular felsites.
[Footnote 293: Sheets 160 and 171 of the one-inch map, and Memoir on
Sheets 160, 161, 171 and 172.]
By far the most striking geological feature of this singularly
interesting and impressive coast-line is to be found in the
interstratification of lavas with bands of tuff among abundantly
fossiliferous strata which, from their organic contents, are
unmistakably of the age of the Wenlock group. These lavas occur in
a number of sheets, separated from each other by tuffs and other
fragmental deposits. They thus point to a series of eruptions over a
sea-bottom that teemed with Upper Silurian life. They consist for the
most part of remarkably fine typical nodular felsites. The nodules vary
in dimensions from less than a pea to the size of a hen's egg. They
are sometimes hollow and lined with quartz-crystals. They vary greatly
in number, some parts being almost free from them and others entirely
made up of them. The matrix, where a fresh fracture can be obtained, is
horny in texture, and often exhibits an exceedingly beautiful and fine
flow-structure. On weathered faces there may be seen thick parallel
strips and lenticles of flow-structure like those of the Snowdon
lavas. The upper portions of some of the sheets enclose fragments of
foreign rocks. The microscopic examination of a few slices cut from
these lavas shows them to be true felsites (rhyolites) composed of
a microcrystalline aggregate of quartz and felspar, with layers and
patches of cryptocrystalline matter, and only occasional porphyritic
crystals of orthoclase and plagioclase.
The pyroclastic rocks associated with these lavas vary from exceedingly
fine tuff to coarse agglomerate. Some of the finer tuffs contain
pumiceous fragments and pieces of grey and red shale; they pass into
fine ashy sandstones and shales, crowded with fossils, and into
gravelly breccias made up of fragments of different volcanic rocks.
But the most extraordinary of these intercalated fragmental strata is
a breccia or agglomerate, about 15 feet thick, which lies in a thick
group of fossiliferous dull-yellow, ashy and ochreous sandstones. The
stones of this bed consist chiefly of blocks of different felsites,
varying up to three feet in length. Some of them show most perfect
flow-structure; others are spongy and cellular, like lumps of pumice.
The calcareous sandstone on the top of the breccia is crowded with
fossils chiefly in the form of empty casts, and the same material,
still full of brachiopods, crinoids, corals, etc., fills up the
interstices among the blocks down to the bottom of the breccia, where
similar fossiliferous strata underlie it.
Nowhere has the volcanic history of a portion of Palæozoic time been
more clearly and eloquently recorded than in this remote line of
cliffs swept by the gales of the Atlantic. We see that the ordinary
sedimentation of Upper Silurian time was quietly proceeding, fine mud
and sand being deposited, and enclosing the remains of the marine
organisms that swarmed over the sea-bottom when volcanic eruptions
began. First came discharges of fine dust and small stones, which
sometimes fell so lightly as not seriously to disturb the fauna on the
sea-floor, but at other times followed so rapidly and continuously as
to mask the usual sediment and form sheets of tuff and volcanic gravel.
Occasionally there would come more paroxysmal explosions, whereby
large blocks of lava were hurled forth until they gathered in a thick
layer over the bottom. But the life that teemed in the sea, though
temporarily destroyed or driven out, soon returned. Corals, crinoids
and shells found their way back again, and fine sediment carried their
remains with it and filled up the crevices. The ejected volcanic blocks
are thus enclosed in a highly fossiliferous matrix.
A succession of lava-streams, of which the strongly-nodular sheet of
Clogher Head is the thickest and most conspicuous, mark the culmination
of the volcanic energy, and show how at this late part of the Silurian
period felsites that reproduce some of the most striking peculiarities
of earlier time were once more poured out at the surface. A few more
discharges of tuff and the outflow of a greenish flinty felsite brought
this series of eruptions to an end, and closed in Britain the long and
varied record of older Palæozoic volcanic activity.[294]
[Footnote 294: As this sheet is passing through the press, the
interesting paper by Messrs. S. H. Reynolds and C. J. Gardiner, "On the
Kildare Inlier" has appeared (_Quart. Journ. Geol. Soc._ vol. lii. p.
587). These authors give petrographical details regarding the lavas,
which they show to be andesites and basalts of Bala age, associated
with highly fossiliferous tuffs.]
[Illustration]
TO ACCOMPANY SIR ARCHIBALD GEIKIE'S "ANCIENT VOLCANOES OF BRITAIN"
Map II
MAP OF THE SILURIAN VOLCANIC DISTRICTS OF NORTH WALES
Reduced from the Maps of the Geological Survey.
The Edinburgh Geographical Institute -- Copyright -- J. G.
Bartholomew.
BOOK V
THE VOLCANOES OF DEVONIAN AND OLD RED SANDSTONE TIME