Vast lapse of time between the close of the Palæozoic and
beginning of the Tertiary Volcanic Eruptions--Prolonged Volcanic
Quiescence--Progress of Investigation among the Tertiary Volcanic
Series of Britain.
From the evidence which has been led in the foregoing chapters it
is clear that during the later stages of the Palæozoic period there
was a gradual enfeeblement of volcanic vigour over the area of the
British Isles. When the last puys of the Permian series became extinct
a remarkable volcanic quiescence settled down on the region. This
interval of rest lasted throughout the whole of the long succession
of the Mesozoic ages. Though the geological record of this section of
geological time is singularly complete in Britain, not a single vestige
has yet been found in it of any contemporaneous eruption. And what is
true of this country is, on the whole, true of the entire European
continent. With some trifling exceptions there were no volcanoes in
Europe, so far as we know, during the enormous lapse of time between
the last of the Palæozoic and the earliest of the Tertiary eruptions.
When the geologist attempts to form an estimate of the chronological
value of this interval of time he is soon lost in bewilderment over its
obvious vastness, and the impossibility of discovering any standards of
measurement by which to reckon its duration. On the one hand, he sees
that it lasted long enough to admit of the gradual elaboration of many
thousands of feet of various sedimentary deposits, which, from their
remarkable diversities of character, were evidently accumulated, on the
whole, with extreme slowness and amidst many geographical vicissitudes.
On the other hand, he perceives that the interval sufficed to bring
about an entire change in the fauna and flora of the globe. Indeed, the
more he investigates the details of this biological transformation,
the more he is impressed with the length of time that it must have
required. For it is not merely one complete change, but a multifold
succession of changes. The stratigraphical records of the long array
of geological periods over which it was spread show that the biological
evolution advanced through a vast series of species, genera and orders
which one by one appeared and disappeared.
The ages that elapsed between the final dying out of the Palæozoic
volcanoes and the outburst of those of Tertiary time were so protracted
that many revolutions of the geography of Europe were comprised within
them. Land and sea changed places again and again. First came the
singular topography of the Trias, which prolonged and accentuated the
characteristics of the closing Palæozoic ages. Next arose the more
genial climate and more varied geography of the Jurassic period, when
comparatively shallow seas overspread the site of most of the European
continent, and tracts of old land stretched away to the west and north.
Another crowded succession of changes in the disposition of land and
sea filled the long Cretaceous period, at the close of which a more
rapid and complete transformation in European geography took place.
Yet during all these transitions and vicissitudes, so far as we know,
volcanic energy remained quiescent throughout Western Europe. It was
not until some time after the great terrestrial movements that raised
so much of the Cretaceous sea-floor into land, and laid the foundations
of the modern continent, that the subterranean fires once more awoke to
vigorous action.
The renewal of eruptions in the early ages of Tertiary time was as
widespread as it was energetic. Over many regions of the European
continent volcanoes broke out either in new areas or on old sites. For
the most part they appeared as scattered puys or as Vesuvian vents,
generally not of the first magnitude, like those of Central France,
Hungary, Würtemberg and Italy. But in the north-west they assumed more
colossal proportions, and took the form of fissure-eruptions by which
many thousands of square miles of country were deluged with lava.
From the South of Antrim all along the West of Scotland to the north
of the Inner Hebrides remains of these basalt-floods form striking
features in the existing scenery. The same kind of rocks reappear
in the Faroe Islands and in Iceland, so that an enormous tract of
North-western Europe, much of it now submerged under the sea, was the
scene of activity of the Tertiary volcanoes. In entering, therefore,
upon a consideration of the British Tertiary volcanic rocks, we are
brought face to face with the records of the most stupendous succession
of volcanic phenomena in the whole geological history of Europe.
Fortunately these records have been fully preserved in the British
Isles, so that ample materials remain there for the elucidation of this
last and most marvellous of all the volcanic epochs in the evolution of
the continent.
As the remains of the Tertiary series of volcanic eruptions are the
youngest of all the volcanic records of Britain, they are naturally
the freshest and most abundantly preserved. They consequently reveal
with singular clearness multitudes of volcanic phenomena that are
less distinctly recognizable, or not to be found at all, among the
Palæozoic systems. Hence they will be discussed in greater detail in
the following chapters.
As a consequence of their greater freshness and wider extent, and
largely also because of the way in which they have been exposed along
many leagues of picturesque sea-cliffs in the North of Ireland and the
West of Scotland, they attracted attention at an earlier time than the
less obvious volcanic memorials of older ages. The gradual development
of opinion regarding the nature and history of volcanic rocks is thus
in no small measure bound up with the progress of observation and
inference in regard to the Tertiary volcanic series. I shall therefore
begin this narrative by offering a rapid sketch of the history of
inquiry respecting the Tertiary volcanic areas of the British Isles.
The basaltic cliffs of Antrim and the Inner Hebrides had attracted the
notice of passing travellers, and their striking scenery had become
more or less familiar to the reading public, before any attention
was paid to their remarkable geological structure and history. In
particular, the wonders of the Giant's Causeway and the Antrim coast
had already begun to draw pilgrims, even from distant countries, at a
time when geology had not come into existence. The scientific tourist
of those days who might care to look at rocks was, in most cases, a
mineralogist, for whom their structural relations and origin were
subjects that lay outside of the range of his knowledge or habits of
thought. In the year 1772 Sir Joseph Banks, together with Solander and
a party, visited Staffa and brought back the earliest account of the
marvels of that isle as they appeared to the sober eyes of science.
His narrative was communicated to Pennant, together with a number of
drawings of the cliffs and of Fingal's Cave. These were inserted by
that geographer in his _Second Tour_, published in 1774, and from their
careful measurements of the basaltic pillars and their delineation
of the basaltic structure, are of special interest in the history of
volcanic geology.
An intelligent appreciation of some of the geological interest of the
region is to be found in the writings of Whitehurst,[125] who gave a
good account of the basalt-cliffs of Antrim, and regarded the basaltic
rocks as the results of successive outflows of lava from some centre
now submerged beneath the Atlantic. More important are the observations
contained in two letters of Abraham Mills.[126] This writer had been
struck with the dykes on the north coast of Ireland, and was led to
examine also those in some of the nearer Scottish islands. He believed
them to be of truly volcanic origin, and spoke of them as veins of
lava. A few years later, Faujas St. Fond made his well-known pilgrimage
to the Western Isles. Familiar with the volcanic rocks of Central
France, he at once recognized the volcanic origin of the basalts of
Mull, Staffa and the adjoining islands.[127] His account of the journey,
published in Paris in 1797, may be taken as the beginning of the
voluminous geological literature which has since gathered round the
subject. Three years afterwards (1800) appeared Jameson's _Outline
of the Mineralogy of the Scottish Isles_. Fresh from the teaching of
Werner at Freiberg, the future distinguished Professor of Natural
History in the Edinburgh University naturally saw everything in the
peculiar Wernerian light. He gave the first detailed enumeration of
some of the eruptive rocks of the Hebrides, but of course ridiculed
the idea of their igneous origin. Having heard of a reported "crater
of a volcano" near Portree, he ironically expressed a hope that "there
may be still sufficient heat to revive the spirits of some forlorn
fire-philosopher, as he wanders through this cold, bleak country."[128]
[Footnote 125: _Inquiry into the Original State and Formation of the
Earth_, 2nd edit. 1786.]
[Footnote 126: _Philosophical Transactions for 1790._]
[Footnote 127: _Voyage en Angleterre, en Écosse et aux Îles Hébrides._
Paris, 1797.]
[Footnote 128: It will be shown in a later chapter that there is a
remarkably perfect volcanic vent near Portree, but the supposed crater
referred to by Jameson was probably some little corry among the sheets
of basalt.]
The advent of Jameson to Edinburgh gave a fresh impetus to the warfare
of the Plutonists and Neptunists, for he brought to the ranks of the
latter a mineralogical skill such as none of their Scottish opponents
could boast. The igneous origin of basalt, which the Plutonists stoutly
maintained, was as strongly denied by the other side. For some years
one of the most telling arguments against the followers of Hutton was
derived from the alleged occurrence of fossil shells in the basalt of
the north coast of Ireland. Kirwan[129] quoted with evident satisfaction
Richardson's observation of "shells in the basalts of Ballycastle," and
Richardson[130] himself, though the true explanation, that the supposed
basalt is only Lias shale altered by basalt, had been stated in 1802
by Playfair,[131] continued for ten years afterwards to reiterate his
belief in the aqueous origin of basalt. Thus the Tertiary volcanic
rocks furnished effective weapons to the combatants on both sides.
The dispute regarding the black fossiliferous rocks of Portrush had
the effect of drawing special attention to the geology of the North
of Ireland. Among the more noted geologists who were led to examine
them, particular reference must be made to Conybeare and Buckland, who,
in the year 1813, studied the interesting coast-sections of Antrim.
The report of their observations gives an excellent summary of the
arguments for the truly igneous origin of basalt, and a statement of
opinion in favour of the view that the bedded basalts are the products
of submarine volcanoes. Berger also about the same time described in
fuller detail the geology of the Antrim district, and showed the rocks
of the basalt-plateau to be younger than the Chalk. He likewise made a
study of the basalt-dykes of the North of Ireland, and was the first
to point out their prevalent north-westerly direction. The memoirs
of these geologists[132] may justly be regarded, to quote the words
of Portlock, as "the first effectual step made in Irish geology."
Portlock's own description is still the most complete summary of the
geology of that interesting region.[133]
[Footnote 129: _Geological Essays_, 1799, p. 252, _footnote_.]
[Footnote 130: Richardson lived on the Antrim coast, and had daily
opportunities of examining the admirable rock-sections there exposed.
It was he who found the shells in supposed basalt, and led the
geologists of his day astray on this subject. He made a clever but
irrelevant reply to Playfair's plain statement of facts (_Trans. Roy.
Irish Acad._ vol. ix. 1803, p. 481). His elaborate attack on "the
Volcanic Theory" will be found in _Trans. Roy. Irish Acad._ vol. x.
(1806), pp. 35-107. Though lively enough as a specimen of controversial
writing, it forms, when seriously considered, rather a melancholy
chapter in geological literature.]
[Footnote 131: _Illustrations of the Huttonian Theory_, § 252.]
[Footnote 132: They are contained in the third volume of the
_Transactions of the Geological Society_.]
[Footnote 133: "Report on the Geology of the County of Londonderry and
parts of Tyrone and Fermanagh," _Mem. Geol. Survey_, 1843.]
While such advances were being made in the knowledge of the structure
of the volcanic rocks of the North of Ireland, the geologist had
already appeared who was the first to attempt a systematic examination
of the Western Islands, and whose published descriptions are still a
chief source of information regarding the geology of this extensive
region. Dr. Macculloch seems to have made his first explorations among
the Hebrides some time previous to the year 1814, for in that year he
published some remarks on specimens from that district transmitted to
the Geological Society.[134] For several years in succession he devoted
himself with great energy and enthusiasm to the self-imposed task of
geologically examining and mapping in a generalized way all the islands
that lie to the westward of Scotland, from the remote St. Kilda even
as far as the Isle of Man. From time to time, notices of parts of his
work were given in the _Transactions of the Geological Society_. But
eventually in 1819 he embodied the whole in his _Description of the
Western Islands of Scotland, including the Isle of Man_.
[Footnote 134: _Trans. Geol. Soc._ vol. ii. 1814.]
This great classic marks a notable epoch in British geology. Properly
to estimate its value, we should try to realize what was the state
of the science in this country at the time of its appearance. So
laborious a collection of facts, and so courageous a resolution
to avoid theorizing about them, gave to his volumes an altogether
unique character. His descriptions were at once adopted as part of
the familiar literature of geology. His sections and sketches were
reproduced in endless treatises and text-books. Few single works of
descriptive geology have ever done so much to advance the progress of
the science in this country. With regard to the special subject of the
present memoir, Macculloch showed that the basalts and other eruptive
rocks of the Inner Hebrides pierce and overlie the Secondary strata
of these islands, and must therefore be of younger date. But though
he distinguished the three great series of "trap-rocks," "syenites"
and "hypersthene-rocks" or "augite-rocks," and indicated approximately
their respective areas, he did not attempt to unravel their relations
to each other. Nor did he venture upon any speculations as to the
probable conditions under which these rocks were produced. He claimed
that those who might follow him would find a great deal which he
had not described, but little that he had not examined. Subsequent
observers have noted many important facts, of which, had he observed
them, he would at once have seen the meaning, and which he certainly
would not have passed over in silence. But as a first broad outline
of the subject, Macculloch's work possesses a great value, which is
not lessened by the subsequent discovery of details that escaped his
notice, and of important geological relations which he failed to detect.
It has already been pointed out that some of the earliest and ablest
observations among the volcanic rocks of this country, especially in
Scotland, were made by foreigners. Students who had repaired from
abroad to Edinburgh for education sometimes caught the geological
enthusiasm, then so marked in that city, and made numerous journeys
through the country in search of further knowledge of Scottish rocks
and minerals. In other instances, geologists of established reputation,
attracted by the interest which the published accounts of the geology
of Scotland had excited, were led to visit the country and to record
their impressions of its rock-structure. Of the first class of
observers the two most noted were Ami Boué and L. A. Necker; of the
second, special acknowledgment is due to Faujas St. Fond and to Von
Oyenhausen and Von Dechen.
The labours of Boué[135] have already been referred to in connection with
the literature of the Scottish Old Red Sandstone (vol. i. p. 269). In
his treatment of the Tertiary Volcanic series of Scotland he appears to
have relied mainly on the then recently published volumes of Macculloch.
[Footnote 135: _Essai géologique sur l'Écosse._ Paris, 1820.]
L. A. Necker, as the grandson of the illustrious De Saussure, had
strong claims on the friendly assistance of the School of Geology
at Edinburgh when he went thither in 1806, at the age of twenty, to
prosecute his studies. He was equally well received by the Plutonists
and Neptunists, and devoted some time to the exploration of the geology
not only of the Lowlands, but of the Highlands and the Inner Hebrides.
Most of his observations appear to have been made in the year 1807,
but it was not until fourteen years afterwards that he published the
account of them.[136] The geological part of this work must be admitted
to be somewhat disappointing. The author's caution not to commit
himself to either side of the geological controversy then waging makes
his descriptions and explanations rather colourless. He adds little to
what was previously known. Even as regards the origin of the basalts
of the Western Islands, he could not make up his mind whether or not
to regard them as volcanic, but contented himself by referring them to
"the trappean formation." Yet these islands had so fascinated him that
eventually he returned to them as his adopted home, passed the last
twenty years of his life among them, and died and was buried there.
Besides his _Voyage_, he published in French an account of the dykes of
the Island of Arran.[137]
[Footnote 136: _Voyage en Écosse et aux Îles Hébrides._ See also
biographical notice of L. A. Necker, by Principal J. D. Forbes, _Proc.
Roy. Soc. Edin._ v. (1862), p. 53.]
[Footnote 137: _Trans. Roy. Soc. Edin._ vol. xiv. (1840), p. 667.]
Among the foreign geologists who have been drawn to the Scottish
mountains and islands by the interest of their Tertiary volcanic rocks,
I have already spoken of Faujas St. Fond. Much more important, however,
were the observations made some thirty years later by two German men
of science, Von Oyenhausen and Von Dechen. Their careful descriptions
of the geology of Skye, Eigg and Arran added new materials to the
knowledge already acquired by native geologists.[138] To some of the more
interesting parts of their work reference will be made in later pages.
[Footnote 138: Karsten's _Archiv_ (1829), vol. i. p. 56.]
The numerous trap-dykes of Northumberland, Durham and Northern
Yorkshire at an early date attracted the attention of geologists.
As far back as 1817, they had been the subject of a memoir by N. J.
Winch,[139] who gave an account of their effects on the adjacent rocks.
More important were the subsequent papers on the same subject by
Sedgwick, who, discussing the lithological characters, probable origin
and geological age of the dykes, pointed out that while the Cleveland
dyke was undoubtedly younger than a large part of the Jurassic rocks,
there was no direct evidence to determine whether dykes farther north
were earlier or later than the time of the Magnesian Limestone.[140]
Subsequent accounts of the dykes of the same region were given by
Buddle,[141] M. Forster,[142] N. Wood,[143] H. T. M. Witham,[144] Tate
[145] and others, while in more recent years important additions to our
knowledge of these dykes and of their effects have been made by Sir J.
Lowthian Bell[146] and Mr. J. J. H. Teall.[147]
[Footnote 139: _Trans. Geol. Soc._ vol. iv. (1817), p. 21. See also
Tilloch's _Phil. Mag._ vols. xlix. and l.]
[Footnote 140: _Cambridge Phil. Trans._ vol. ii. (1827), pp. 21, 139.]
[Footnote 141: _Trans. Nat. Hist. Soc. Northumberland_, i. (1831), p. 9.]
[Footnote 142: _Op. cit._ i. p. 44.]
[Footnote 143: _Op. cit._ i. pp. 305, 306, 308, 309.]
[Footnote 144: _Op. cit._ ii. (1838), p. 343.]
[Footnote 145: _Trans. Northumberland and Durham_, ii. (1868), p. 30.]
[Footnote 146: _Proc. Roy. Soc._ xxiii. (1875), p. 543.]
[Footnote 147: _Quart. Journ. Geol. Soc._ xl. (1884), p. 209.]
The geological age of the great series of Tertiary volcanic rocks has
only been determined district by district, and at wide intervals.
That some part of the Antrim basalts is younger than the Chalk of
that region was clearly shown by Berger, Conybeare and Buckland.
Portlock, however, referred to the occurrence of detached blocks of
basalt which he supposed to be immersed in the Chalk near Portrush,
and which inclined him to believe that "the basaltic flows commenced
at a remote period of the Cretaceous system."[148] Macculloch showed
that the corresponding basaltic plateaux of the Inner Hebrides were
certainly younger than the Oolitic rocks of that region. But no
nearer approximation to their date had yet been made when in the year
1850 the Duke of Argyll announced the discovery of strata containing
fossiliferous chalk-flints and dicotyledonous leaves, lying between
the bedded basalts of Ardtun Head, in the Isle of Mull.[149] In the
following year these fossil leaves were described by Edward Forbes, who
regarded them as decidedly Tertiary, and most probably Miocene. This
was the first palæontological evidence for the determination of the
geological age of any portion of the basalt-plateaux, and it indicated
that the basalts of the south-west of Mull were of older Tertiary date.
Taken also in connection with the occurrence of lignite-beds between
the basalts of Antrim, it suggested that these volcanic plateaux
were not due to submarine eruptions, as the earlier geologists had
supposed, but were rather the result of the subærial outpouring of lava
at successive intervals, during which terrestrial vegetation sprang up
upon the older outflows.
[Footnote 148: _Report on the Geology of Londonderry_, p. 93. There
can be no doubt that this was an error of observation. The Antrim
basalts are all certainly younger than the Chalk. The supposed "lumps
of basalt" were probably the ends of veins intruded into the Chalk,
and perhaps partially disconnected from the main parts of the veins.
Such apparently detached masses of intrusive rock are not infrequent
occurrence in connection with the Tertiary intrusive sills. An example
will be found represented in Fig. 321.]
[Footnote 149: _Brit. Assoc. Report_, 1850, Sections, p. 70; and _Quart.
Jour. Geol. Soc._ vii. (1851), p. 87.]
While Forbes brought forward palæontological proofs of the Tertiary
age of the volcanic rocks of the south-west of Mull, he at the same
time laid before the Geological Society a paper on the Estuary Beds
and the Oxford Clay of Loch Staffin, in Skye, wherein, while admitting
the existence of appearances which might be regarded as favourable to
the view that the intercalated basalts of that region were of much
later date than the Oolitic strata between which they might have been
intrusively injected, he stated his own belief that they were really
contemporaneous with the associated stratified rocks, and thus marked
an outbreak of volcanic energy at the close of the Middle Oolitic
period.[150] The Duke of Argyll, in the paper which he on the same
occasion communicated to the Geological Society, adopted this view
of the probable age of most of the basalts of the Western Islands.
He looked upon the Tertiary volcanic rocks of Mull as occupying a
restricted area, the great mass of the basalt of that island, like that
of Skye, being regarded by him as probably not later than some part of
the Secondary period.
[Footnote 150: _Quart. Journ. Geol. Soc._ vol. vii. (1851), p. 104.]
It must be granted that the appearances of contemporaneous
intercalation of the basalt among the Secondary strata are singularly
deceptive. When, several years after the announcement of the Tertiary
age of the basalts of Ardtun, I began my geological work in the Inner
Hebrides, I was led to the same conclusion as Edward Forbes, and
expressed it in an early paper.[151] All over the north of Skye I traced
what appeared to be evidence of the contemporaneous interstratification
of basalts with the Jurassic rocks and I concluded (though with some
reservation) that the whole of the vast basaltic plateaux of that
island were not younger than some late part of the Jurassic period. In
that same paper the attention of geologists was called to the probable
connection of the great system of east-and-west dykes traversing
Scotland and the North of England, with the basalt-plateaux of the
Inner Hebrides, and as I believed the latter to be probably of the
age of the Oolitic rocks, I assigned the dykes to the same period in
geological history. But subsequent explorations enabled me to correct
the mistake into which, with other geologists, I had fallen regarding
the age of the volcanic phenomena of the Western Islands. In 1867 I
showed that instead of being confined to a mere corner of Mull, the
Tertiary basalts, with younger associated trachytic or granitic rocks,
covered nearly the whole of that island, and that in all likelihood
the long chain of basaltic masses, extending from the North of Ireland
along the west coast of Scotland to the Faroe Islands, and beyond
these to Iceland, was all erupted during the Tertiary period. At the
same time I drew special attention to the system of east-and-west
dykes as proofs of the vigour of volcanic action at that period, and
I furnished evidence that this action was prolonged through a vast
interval of time, during which great subærial denudation of the older
lavas took place before the outflow of the younger.[152] Later in the
same year, in an address to the Geological Section of the British
Association, I reiterated these views, and more particularly emphasized
the importance of the system of dykes, which in my opinion was possibly
the most striking manifestation of the vigour of Tertiary volcanic
action.[153] In 1871, after further explorations in the field, I gave a
detailed account of the structure which had led to the mistake as to
the age of the Tertiary volcanic rocks of the Western Islands; and in
a description of the island of Eigg, I brought forward data to show
the enormous duration of the Tertiary volcanic period in the west of
Britain.[154]
[Footnote 151: "On the Chronology of the Trap-rocks of Scotland,"
_Trans. Roy. Soc. Edin._ xxii. (1861), p. 649.]
[Footnote 152: _Proc. Roy. Soc. Edin._ vi. (1867), p. 71.]
[Footnote 153: _Brit. Assoc. Report_ (Dundee), 1867, Sections, p. 49.]
[Footnote 154: _Quart. Journ. Geol. Soc._ xxvii. (1871), p. 279.]
Three years later Mr. J. W. Judd read before the Geological Society
a paper "On the Ancient Volcanoes of the Highlands."[155] The most
novel feature of this paper was the announcement that the author had
recognized the basal wrecks of five great central volcanoes in the
Western Islands, among which that of Mull was inferred by him to have
been at least 14,500 feet high. He was led to the conclusion that the
volcanic period in these regions was divisible into three sections--the
first marked by the outburst of acid rocks (felspathic lavas and ashes,
connected with deeper and more central granitic masses); the second by
the extrusion of basic lavas and tuffs (the basaltic plateaux); the
third by the appearance of small sporadic volcanic cones ("felspathic,
basaltic, or intermediate in composition") after the great central
cones had become extinct. It will be seen in the following pages
that these conclusions of Professor Judd are not supported by a more
detailed study of the region.
[Footnote 155: _Quart. Journ. Geol. Soc._ xxx. (1874), p. 220.]
In the year 1879, during a traverse of some portions of the volcanic
region of Wyoming, Montana and Utah, I was vividly impressed by
the identity of structure between the basaltic plateaux of these
territories and the youngest volcanic areas of Britain. It then
appeared to me that some of the puzzling features in the Tertiary
volcanic series of the Inner Hebrides might be explained by the
structures so admirably displayed in these lava-fields of the Far
West.[156] Riding over the great basalt-plains of the Snake River and
looking at the sections cut by the river through the thick series
of horizontal basalt-beds, I appreciated for the first time the
significance of Baron von Richthofen's views regarding "massive"
or "fissure" eruptions, as contradistinguished from those of great
central cones of the type of Etna or Vesuvius, and I gathered so
many suggestions from my examination of these American regions that
I renewed with increased interest the investigation of the Tertiary
volcanic tracts of Britain. At last, after another interval of nine
years, during which my weeks of leisure were given to the task, I was
able to complete a discussion of the whole history of Tertiary volcanic
action in this country, which was communicated to the Royal Society
of Edinburgh in the early summer of 1888.[157] Since that time I have
continued the research, and have from time to time communicated my
results to the Geological Society. These various memoirs are combined
with hitherto unpublished details in the following account of the
British Tertiary Volcanic Rocks.
[Footnote 156: _Geological Essays at Home and Abroad_ (1882), pp. 271,
274; _Nature_, November 1880.]
[Footnote 157: _Trans. Roy. Soc. Edin._ vol. xxxv. part ii. (1888), pp.
23-184.]
Professor Judd has also prosecuted the investigation of the petrography
of the rocks, and has published his observations in the _Quarterly
Journal of the Geological Society_.[158] To these papers by him more
detailed reference will be made in later Chapters.
[Footnote 158: _Quart. Journ. Geol. Soc._ vols. xlv. (1889), xlvi.
(1890), xlix. (1893). In the first of these volumes Professor Judd
offered a detailed criticism of my views as to the order of succession
and history of the volcanic rocks of the Inner Hebrides. Subsequent
investigation having entirely confirmed my main conclusions, it is
not necessary to enter here upon matters of controversy. Reference,
however, will be made in subsequent Chapters to some of the points in
dispute.]
In describing the geological history of a great series of rocks,
chronological order is usually the most convenient method of treatment.
Where, however, the rocks are of volcanic origin, and do not always
precisely indicate their relative age, and where moreover the same
kinds of rock may appear on widely-separated geological horizons, it
is not always possible or desirable to adhere to the strict order
of sequence. With this necessary latitude, I propose to follow the
chronological succession from the older to the newer portions of the
series. I shall treat first of the system of dykes, by which so large a
part of Scotland and of the north of England and Ireland is traversed.
Many of the dykes are undoubtedly among the youngest members of the
volcanic series, and in no case has their age been as yet determined
except relatively to the antiquity of the rocks which they traverse.
They must, of course, be posterior to these rocks, and hence it would
be quite logical to reserve them for discussion at the very end of the
whole volcanic phenomena. My reason for taking them at the beginning
will be apparent in the sequel. After the dykes, I shall describe the
great volcanic plateaux which, in spite of vast denudation, still
survive in extensive fragments in Antrim, the Inner Hebrides and the
Faroe Islands. The eruptive bosses of basic rocks that have broken
through the plateaux will next be discussed. An account will then be
given of the protrusions of acid rocks which have disrupted these basic
bosses. The last chapters will contain a sketch of the subsidences
and dislocations which the basalt-plateaux have suffered, and of the
denudation to which they have been subjected.
As has been explained in Chapter iii., the volcanic cycle of any
district, during a given geological period, embraces the whole range of
erupted products from the beginning to the end of a complete series of
eruptions. Reference was made in Book I. to the remarkable variation
in the character of the lavas successively poured out from the same
volcanic reservoir during the continuance of a single cycle, and it was
pointed out that Richthofen's law generally holds good that while the
first eruptions may be of a basic or average and intermediate nature,
those of succeeding intervals become progressively more acid, but are
often found to return again at the close to thoroughly basic compounds.
This law is well illustrated by the volcanic history of Tertiary time
in Britain. We shall find that the earliest eruptions of which the
relative date is known consisted generally of basic lavas (dolerites
and basalts), but including also more sparingly andesites, trachytes
and rhyolites; that the oldest intrusive masses consisted of bosses,
sills and dykes of dolerite and gabbro; that these intrusions
were followed by others of a much more acid character--felsites,
pitchstones, quartz-porphyries or rhyolites, granophyres and granites;
that the latest lava is a somewhat acid rock, being a vitreous form of
dacite; and that the most recent volcanic products of all are dykes of
a thoroughly basic nature, like some of the earlier intruded masses.