THE DEVONIAN VOLCANOES
Throughout the whole region of the British Isles, wherever the
uppermost strata of the Silurian system can be seen to graduate into
any later series of sedimentary deposits, they are found to pass up
conformably into an enormous accumulation of red sandstones, marls,
cornstones, and conglomerates, which have long been grouped together
under the name of "Old Red Sandstone." In England and Wales, in
Scotland and in Ireland, this upward succession is so well shown that
at first British geologists were naturally disposed to believe it to
represent the normal order of the geological record. When, however,
Sedgwick and Murchison demonstrated that in the counties of Devon
and Cornwall a very different group of strata contained an abundant
assemblage of organic remains, including types which Lonsdale showed
to be intermediate between those of the Silurian and the Carboniferous
systems; when, moreover, this palæontological facies of the south-west
of England, termed by its discoverers "Devonian," was found to be
abundantly developed on the Continent, and to be there indeed the
prevalent stratigraphical type of the formations intervening between
Silurian and Carboniferous, geologists began to perceive that the
Old Red Sandstone must be regarded as the record of peculiar local
conditions of sedimentation, while the Devonian type was evidently the
more usual development of the same geological period.
From the remote Shetland Isles, across the whole of Scotland and
England, down to the northern shores of the Bristol Channel, the Old
Red Sandstone maintains its general characters. Nowhere, indeed, are
these characters more typically developed than in South Wales, where
many thousands of feet of red sediments, almost entirely devoid of
organic remains, emerge from under the escarpments of Carboniferous
Limestone, and stretch into broad uplands until they are lost at the
top of the Silurian system.
But when the geologist crosses the Bristol Channel to the opposite
shores of North Devon, he encounters a remarkably different assemblage
of rocks. It is true that he has not yet been able to detect there any
equivalents of the uppermost Silurian strata of Glamorganshire, nor
does he find any conspicuous band of Carboniferous Limestone, such as
that which encircles the Welsh Coal-field. He is thus unable to start
from a known definite horizon in the attempt to work out the order of
succession, either in an upward or downward direction. Lithological
characters likewise afford him no means of establishing any
satisfactory parallelism. As he follows the Devonian strata, however,
he finds them to disappear conformably under the Culm-measures, which,
though strangely unlike the Carboniferous strata on the opposite coast,
are yet proved by their fossils to belong to the Carboniferous system.
Hence the Devonian type, like the Old Red Sandstone, is proved to be
immediately anterior to, and to graduate into, the Carboniferous rocks.
There is no stratigraphical change in Britain so rapid and complete as
that from the Old Red Sandstone on the one side of the Bristol Channel
to the Devonian series on the other. No satisfactory explanation has
yet been found for this sudden transformation, which still remains one
of the unsolved problems in British geology.
As the observer follows the Devonian assemblage across the land to
the southern coast-line, he is conscious that its general characters,
both lithological and palæontological, depart more and more from the
type of the Old Red Sandstone, and approach more closely to the common
Devonian facies of the Continent. He is forced to admit that the Old
Red Sandstone, notwithstanding its extensive development in Britain,
must be regarded as an exceptional type of sedimentation, while the
Devonian facies represents that which is most widely prevalent, not
only in Europe, but generally over the world.
The broad estuary of the Bristol Channel unfortunately conceals from
view the tract which lies between the typical Old Red Sandstone of
Glamorganshire and the typical Devonian formations of Devonshire.
Whether this intervening space of some fifteen miles was occupied by
a physical barrier, which separated the respective areas of deposit
of these two types, or the circumstances of sedimentation in the one
region merged insensibly into those of the other, must remain matter
for speculation.
The geographical conditions betokened by the Old Red Sandstone will
be considered in the next chapter. There can be no doubt that those
indicated by the Devonian system were marine. The organic remains
so plentifully distributed through the argillaceous and arenaceous
sediments of that system, and so crowded together in its limestones,
were obviously denizens of the open sea. Yet the only tract of Britain
over which this sea can be shown to have spread was the south of
England. To the north of that belt, the site of Britain during Devonian
time appears to have been partly land and partly wide water-basins in
which the Old Red Sandstone was deposited.
In that half terrestrial half lacustrine territory that stretched
northwards to beyond the Shetland Isles, many volcanoes were active,
of which the chronicles will be described in later pages. The most
southerly of these centres of eruption yet known was the district of
the Cheviot Hills. Throughout the rest of England and Wales no trace
of any contemporary volcanic action has been detected in the Old Red
Sandstone. It is true that over most of that region rocks of this age
have been concealed under younger formations. Yet throughout Wales,
where the Old Red Sandstone attains so vast a thickness, and covers so
wide an area, it has not yet yielded a vestige of any contemporaneous
volcanic eruptions.
But over the sea-floor that covered the south of England, and stretched
thence into the heart of Europe, abundant volcanoes have left behind
them proofs of their activity. The first geologist who recognized these
proofs and traced their extent on the ground appears to have been De la
Beche, who, by his detailed maps and careful description of the igneous
rocks of Devonshire, did so much to advance the study of ancient
volcanic action. This great pioneer not only determined the former
existence of Devonian volcanoes, but he was likewise the first to
detect and map the volcanic rocks associated with the Carboniferous and
"New Red Sandstone" formations of the same region. The broad outlines
traced by him among the volcanic products of these three geological
periods in the south-west of England still remain but little changed.
Nor are they likely to be much improved until the ground is resurveyed
on a larger and more accurate map, and with better petrographical
equipment than were available in his day.
Not long after the observations of De la Beche came those of A. C.
Godwin-Austen, who devoted much time to a sedulous exploration of
the rocks of South Devon, and satisfied himself that contemporaneous
volcanic sheets were intercalated among the limestones of that
district. "The coral limestones," he says, "are in many places
superincumbent on great sheets of volcanic materials, with which, in
some instances, as at North Whilborough, they alternate." He pointed
out that the interstratified volcanic rocks are of two periods, one
Devonian and the other Carboniferous.[295]
[Footnote 295: _Trans. Geol. Soc._ 2nd ser. vol. vi. (1842), pp. 465,
470, 473.]
In his Geological Maps of Devon and Cornwall, which are to the present
time those issued by the Geological Survey, De la Beche made no attempt
to discriminate between the varieties of igneous rocks, save that
the basic "greenstones" were distinguished from the acid bosses of
granite and the elvans. But in his classic "Report" much more detail
was inserted, showing that he clearly recognized the existence both
of volcanic ashes and of lavas, as well as of intrusive sheets. At
the outset of his account of the "Grauwacke," he remarks that the
sedimentary deposits are accompanied with igneous products, "a portion
of which may also be termed sedimentary, inasmuch as it would seem
to have been deposited in beds among contemporaneous rocks of the
former description by the agency of water, after having been ejected
from fissures or craters in the shape of ashes and cinders, precisely
as we may now expect would happen with the ashes and cinders ejected
from volcanoes, particularly insular and littoral volcanoes, into the
sea."[296] Again he speaks of "two kinds of trappean rocks having
probably been erupted, one in the state of igneous fusion, and the
other in that of ash, during the time that the mud, now forming slates,
was deposited, the mixtures of volcanic and sedimentary materials
being irregular from the irregular action of the respective causes
which produced them; so that though the one may have been derived
from igneous action, and the other from the ordinary abrasion of
pre-existing solid rocks, they were geologically contemporaneous."[297]
He recognized the origin of the amygdaloidal varieties of rock, and by
dissolving out the calcite from their cells showed how close was their
resemblance to modern pumice.[298]
[Footnote 296: "Report on the Geology of Cornwall, Devon and West
Somerset," _Mem. Geol. Survey_, 1839, p. 37.]
[Footnote 297: _Op. cit._ p. 57.]
[Footnote 298: _Op. cit._ pp. 57, 61.]
Since these early researches many geologists have studied the igneous
rocks of Devonshire. I would especially refer to the labours of Mr.
Allport,[299] the late J. A. Phillips,[300] Mr. Rutley,[301] the late
Mr. Champernowne,[302] Mr. W. A. E. Ussher,[303] Mr. Hobson,[304] and
General M'Mahon.[305] Mr. Champernowne in particular has shown the
abundance of volcanic material among the rocks of Devonshire, and the
resemblance which in this respect they offer to the Devonian system of
North Germany.
[Footnote 299: _Quart. Journ. Geol. Soc._ xxxii. (1876), p. 418.]
[Footnote 300: _Op. cit._ xxxi. (1875) p. 325, xxxii. (1876) p. 155,
xxxiv. (1878) p. 471.]
[Footnote 301: "Brent Tor," _Mem. Geol. Surv._ p. 18; _Quart. Journ.
Geol. Soc._ lii. (1896), p. 66.]
[Footnote 302: See in particular his last paper "On the Ashprington
Volcanic Series of South Devon," _Quart. Journ. Geol. Soc._ vol. xlv.
(1889), p. 369.]
[Footnote 303: This geologist has spent many laborious years in
the investigation of the geology of Devonshire, and has published
numerous papers on the subject, in the _Transactions of the Devonshire
Association_ and of the _Royal Cornwall Geological Society_, in the
_Proceedings of the Somersetshire Archæological and Natural History
Society_, and of the _Geologists' Association_, in the _Geological
Magazine_, and the _Quarterly Journal of the Geological Society_.
Reference may especially be made to his Memoir in the last named
journal, vol. xlvi. (1890), p. 487.]
[Footnote 304: _Quart. Journ. Geol. Soc._ xlviii. (1892), p. 496.]
[Footnote 305: _Op. cit._ xlix. (1893), p. 385.]
Unfortunately the geological structure of the Palæozoic rocks of the
South-west of England has been complicated to an amazing extent by
plication and fracture, with concomitant cleavage and metamorphism.
Hence it is a task of extreme difficulty to trace out with any
certainty definite stratigraphical horizons, and to determine the range
of contemporaneous volcanic action. Mr. Ussher has shown with what
success this task may be accomplished when it is pursued on a basis of
minute mapping, combined with a sedulous collection and determination
of fossils.[306] But years must necessarily elapse before such detailed
work is carried over the whole Devonian region, and probably not till
then will the story of the volcanic history of the rocks be adequately
made out.
[Footnote 306: See Memoir cited in a previous note.]
In the meantime, it has been established that while there is a
singular absence of igneous rocks in North Devon, a strip of country
extending from Newton Abbot and Torquay westwards by Plymouth across
Cornwall to Penzance contains abundant records of volcanic action. It
has not yet been possible to map out, among what were formerly all
grouped together as "greenstones," the respective limits of the bedded
lavas and the tuffs, to distinguish the true sills, and to fix on the
position of the chief vents of eruption. So intense have been the
compression and shearing of the rocks that solid sheets of diabase have
been crushed into fissile schists, which can hardly be distinguished
from tuffs. Moreover, owing perhaps in large measure to the mantle
of red Permian (or Triassic) strata, which has been stripped off by
denudation from large tracts of this region once overspread by it, the
Devonian rocks have been deeply "raddled," or stained red. But probably
one of the main sources of difficulty in studying the petrography of
the area is to be found in the results of atmospheric weathering.
Devonshire lies in that southern non-glaciated strip of England, where
the rocks have been undergoing continuous decay since long before the
Ice Age. No ploughshare of ice has there swept off the deep weathered
crust, so as to leave hard surfaces of rock, fresh and bare, under a
protecting sheet of boulder-clay. It is seldom that a really fresh
piece of any igneous rock can be procured among the lanes and shallow
pits of Devon, where alone, for the most part, the materials are
exposed.
Much, therefore, remains to be done, both in the stratigraphy and
petrography of the Devonian volcanic rocks of this country. To the late
J. A. Phillips geology is indebted for the first detailed chemical and
microscopical investigation of these rocks. He clearly showed the truly
volcanic origin of many of the so-called "greenstones." He believed
that certain "slaty blue elvans," which he found to have a composition
identical with that of altered dolerites, might be highly metamorphosed
tuffs, and that others might have been originally sheets of volcanic
mud. After studying the chemical composition and minute structure
of a large collection of "greenstones," he demonstrated that in all
essential particulars they were dolerites, though somewhat altered from
their original character.[307] Subsequently they were studied by Dr.
Hatch, who found the fresher specimens generally to possess an ophitic
structure, while some are granular, others porphyritic.[308]
[Footnote 307: See especially _Quart. Journ. Geol. Soc._ vols. xxxii.
and xxxiv.]
[Footnote 308: A few of the eruptive rocks of Devonshire have recently
been studied by K. Busz. He finds most of his specimens (chiefly from
the Torquay district) to be varieties of diabase, but describes a
palæopicrite from Highweek near Newton Bushel, and a kersantite from
South Brent on the south-east edge of Dartmoor (_Neues Jahrb._ 1896, p.
57).]
Although the rocks have undergone so much crushing, solid cores of
them, showing the original structure, may be obtained, also examples
of the amygdaloidal, vesicular or slaggy character. They occur
in sheets either singly or in groups, and appear generally to be
regularly interstratified in the slates and grits. While some of
these intercalations, especially the amygdaloidal sheets, may be
true superficial lavas, it can hardly be doubted that others are
sills, especially those which assume the crystalline structure and
composition of gabbros, and show an entire absence of the vesicular
structure. But no one has yet attempted to separate the two types from
each other.
With these rocks are associated abundant diabase-tuffs (schalstein),
frequently mingled with ordinary non-volcanic detrital matter, and
shading off into the surrounding grits and slates. There is thus clear
evidence of the outpouring of basic lavas and showers of ashes during
the Devonian period in the south-west of England, under conditions
analogous to those which characterized the deposition of the Devonian
system in Nassau and the Harz.
The exact range of these eruptions in geological time has still to be
ascertained. So far as at present determined, volcanic activity was not
awakened during the accumulation of the Lower Devonian formations. It
was not until the sporadic coral-reefs and shell-banks had grown up,
which form the basement limestones of the Middle Devonian group, that
the first eruptions took place. As Godwin-Austen, Champernowne and Mr.
Ussher have shown, some of these reefs were overwhelmed with streams
of lava or buried under showers of ashes. The volcanic discharges,
however, were peculiarly local, probably from many scattered vents,
and never on any great scale. Some districts remained little or not at
all affected by them, so that the growth of limestone went on without
interruption, or at least with no serious break. In other areas again
the place of the limestone is taken by volcanic materials.
The chief epoch of this volcanic action, marked by the "Ashprington
Volcanic Series," appears to have occurred about midway in the Middle
Devonian period. But in certain districts it extended into Upper
Devonian time. Intrusive sills of diabase may mark the later phases
of the volcanic history. But the occurrence of such sills even in the
Upper Devonian rocks, and the alteration of the strata in contact with
them (spilosite), point to the continuance or renewal of subterranean
disturbance even in the later Devonian ages, if not in subsequent
geological time. That volcanic activity accompanied the deposition of
the Carboniferous rocks of Devonshire has long been well known (see
Chapter xxix.).