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.
We now enter upon the consideration of the records of a notable era in
the geological evolution of north-western Europe. Up to the close of
the Silurian period the long history embodied in the rocks presents
a constant succession of slowly sinking sea-floors. Wide tracts of
ocean stretched over most of Europe, and across the shifting bottom,
sand and mud, washed from lands that have long vanished, spread in
an ever-accumulating pile. Now and then, some terrestrial movement
of more than usual potency upraised this monotonous sea-bed, but the
old conditions of ceaseless waste continued, and fresh sheets of
detritus were thrown down upon the broken-up heaps of older sediment.
All through the vast cycles of time denoted by these accumulations of
strata, generations of sea-creatures came and went in long procession,
leaving their relics amidst the ooze of the bottom. Genera and
families, once abundant, gradually died out, and gave place to others,
the onward march of life being slow but uninterrupted. Of the land of
the time or of the plants and animals that lived on its surface, hardly
anything is known. The chronicles that have come down to us are almost
wholly records of the vicissitudes of the ocean-bed.
Over the centre and south of Europe, the marine conditions of Silurian
time were prolonged, as we have seen, into the next period, when the
Devonian formations were deposited. In that wide region, no marked
break has been traced between either the sedimentation or the animal
life of the Silurian and Devonian periods. But in the north-west of
Europe a striking departure took place from the protracted monotony of
marine conditions. By a series of terrestrial movements that affected
the area lying to the north of the line of the Bristol Channel, and
extended not only to the furthest limit of the British Isles, but
probably as far as Norway, and perhaps even into northern Russia,
the previous widespread conditions of marine sedimentation were
entirely altered. Instead of the fine oceanic silts and sands with
their abundant organic remains, and the thick limestones with their
masses of coral and crowds of crinoids, there were now laid down, over
these northern regions, vast piles of deep red sediment, from which
traces of animal life are almost wholly absent. The shelving land
against which these ferruginous sands and gravels gathered can still
in part be recognized. As the observer follows its margin, notes the
varying local peculiarities of its sediment, and detects, sometimes
in great abundance, remains of the vegetation which clothed it, the
conviction grows in his mind that the remarkable contrast between these
deposits, known as the Old Red Sandstone, and those of the Silurian and
Devonian systems is not to be accounted for by any mere rearrangement
of the sea-bottom, or redistribution of the land that supplied that
sea-bottom with sediment. It has long been the general belief among
geologists that the subterranean movements which, over the greater part
of Britain, brought the deposition of the Upper Silurian formations
to a close, led to a total alteration of the geography of the region
affected, that the sea-floor was elevated, and that, over the upraised
tract, large lakes or inland seas were eventually formed, in which the
peculiar sediments of the Old Red Sandstone were accumulated.
The records of this series of geographical changes are too fragmentary
to enable us to follow, except in a very general way, the sequence of
events in the transformation of the Silurian sea into the peculiar
topographical conditions in which the Old Red Sandstone was laid down.
While there was a widespread elevation of the sea-floor, and of such
ridges of insular land as may have risen above sea-level, the upheaval
appears to have been of a somewhat complicated kind, and to have been
combined with many local subsidences. The area of disturbance was
probably thrown into a series of parallel ridges and troughs, the
former continuing to be pushed upward, while the latter tended to
subside. The ridges thus became land surfaces, and their prolonged
elevation may have more or less compensated for the denudation to
which, on their emergence, they were necessarily exposed. The troughs,
on the other hand, which sank down, may in many cases have subsided
below the sea. But where the general upheaval of the crust was most
pronounced, some of the depressions would be isolated above sea-level
and become lake-basins in the terrestrial areas.
Of some of these water-basins the outlines can still in some measure
be defined. The rocks that rose into hills around them, and from which
their enormous accumulations of detritus were derived, still partially
survive. We can explore these piles of sediment, and from them can form
some idea of the condition of the water in the lakes, and the nature
of the vegetation on the surrounding land. The frequent occurrence
and exceeding coarseness of the conglomerates, which appear on many
successive horizons throughout the deposits of these basins, probably
indicate contemporaneous terrestrial disturbances. The same causes that
led to the wrinkling of the crust into parallel ridges and troughs
no doubt still continued in operation. From time to time the ridges,
much worn down by prolonged denudation, were pushed upward, either by
gradual uprise or by more rapid jerks. The troughs may in like manner
have been still affected by their old tendency to subsidence. Hence, in
spite of the effects of degradation and deposition, it is possible that
the ridges may not, on the whole, have varied much in height, nor the
basins in depth, during the time when thousands of feet were stripped
off the land and strewn in detritus over the bottoms of the lakes.
Let us try to realize a little more definitely the general aspect of
the region in which the Old Red Sandstone water-basins lay. As the
axes of the folds into which the crust of the earth was thrown ran in
a north-east and south-west direction, they gave this trend to the
lakes and to the tracts of land that separated them. These intervening
ridges must in some instances have been hilly or even mountainous.
Thus, the Scottish Highlands rose between two of the lakes, and poured
into them an abundant tribute of gravel, sand and silt. The terrestrial
vegetation of the time has been partially preserved. The hills seem
to have been clothed with conifers, while the lower slopes and swamps
were green with sigillariæ, lepidodendra and calamites. One of the most
characteristic plants was _Psilophyton_, of which large matted sheets
were drifted across the lakes and entombed in the silt of the bottom.
A grass-like vegetation, with long linear leaves, seems to have grown
thickly in some of the shallows of the lakes.
Of the land animals we have still less knowledge than of the
vegetation. Doubtless various forms of insect life flitted through the
woodlands, though no relics of their forms have yet been recovered. But
the remains of myriapods have been found in Forfarshire.[309] These
early relics of the animal life of the land inhabited the woodlands,
like our modern gally-worms, and were swept down into the lakes,
together with large quantities of vegetation.
[Footnote 309: Mr. B. N. Peach, _Proceedings of Royal Physical Society
of Edinburgh_, vol. vii. (1882).]
Some of the lakes, especially in the earlier part of their history,
abounded in eurypterid crustacea. These animals inhabited the seas
in Upper Silurian time, and appear to have been isolated in the
water-basins of the Old Red Sandstone. Certain species of Pterygotus,
a Silurian genus found also in the Lower Old Red Sandstone, reached a
length of six feet. But the most abundant forms of animal life were
fishes. These furnish additional evidence in favour of the lacustrine
nature of the waters in which they lived. Such characteristically
marine forms as the sharks and rays of the Silurian seas were replaced
by genera of Acanthodians, Ostracoderms, Dipnoids, Teleostomes,
Placoderms, and Palæoniscids, which abounded in the more northerly
waters. The distinctive outward characters of many of these early
vertebrates were their bony scales and plates. Some of them had their
heads encased in an armature of bone, of large size and massive
thickness. In several genera the bone was coated with a layer of
glittering enamel. Even now, after the vast lapse of time since their
day, the cuirasses and scale-armour of these fishes keep their bright
sheen in the hardened sand and mud from which they are disinterred.
A difference is observable between the faunas of the different
water-basins. Even where the same genus occurs in two adjacent
areas, the species are often distinct. Two large lakes, separated
by the tract of the Scottish Highlands, had each its own assemblage
of fishes, not a single genus being common to the two basins. Such
contrasts, whether the two lakes were geologically contemporaneous,
or the northern arose later than the southern, undoubtedly indicate
long-continued isolation and the gradual evolution of new forms under
different conditions of environment.[310]
[Footnote 310: In my memoir "On the Old Red Sandstone of Western
Europe" (_Trans. Roy. Soc. Edin._ vol. xxviii. 1878), I argued for the
probable geological contemporaneity of the conglomerates, sandstones
and flagstones on either side of the Grampian chain, even although
their organic contents were so unlike. The stratigraphical evidence
favours this view. In each case a thick series of strata is covered
unconformably by Upper Old Red Sandstone, containing _Holoptychius
nobilissimus_ and other fishes. The question cannot perhaps be
definitely settled by the data available in Scotland. It is quite
possible that the basin on the northern side of the Grampians, which
I have termed "Lake Orcadie," came into existence after that on the
southern side. But I do not think the differences in their respective
faunas are to be accounted for simply by lapse of time and the gradual
organic evolution in progress over one continuous region. The more the
Old Red Sandstone is studied, the more local do its various fish-faunas
appear to have been. These strongly-marked diversities appear to me
rather to point to prolonged isolation of the basins from each other,
as stated above. Dr. Traquair has drawn attention to the remarkable
fact that, even in what appears to be one continuous series of strata
of no great thickness forming the Upper Old Red Sandstone of the Moray
Firth basin, the fishes found about Nairn are entirely different from
those met with in the rest of the region.]
Such, in brief, were the aspects of the physical geography of the
time on the further consideration of which we are now to enter. The
subterranean disturbances, so characteristic of the period, were
accompanied by a display of volcanic activity more widespread, perhaps,
than any which had yet taken place in the geological history of
Britain. Nevertheless, it is worthy of remark that this manifestation
of underground energy did not begin with the commencement of these
displacements of the crust. The earliest eruptions only took place
after the geography of the region had been completely changed; at least
no trace of them is to be found in the earliest portions of the Old Red
Sandstone. After the last lingering Silurian volcanoes in the west of
Ireland had died out, a protracted quiescence of the subterranean fires
ensued. In the latest ages of Silurian time there was not in Britain,
so far as at present known, a single volcanic eruption. Not until after
the inauguration of the Old Red Sandstone topography, when the lakes
had taken shape and had begun to be filled with sediment from the
surrounding hills, did a series of new volcanoes burst into activity
over the northern half of Britain. Rising in the midst of the lakes
in groups of separate cones, these vents poured out floods of lava,
together with clouds of ashes and stones. Their sites, the history of
their eruptions, and the piles of material ejected by them, can still
be ascertained, and I shall now proceed to give some account of them.
The thick mass of sedimentary material known as the Old Red Sandstone,
lying between the top of the Silurian and the base of the Carboniferous
system, has been divided into two sections, which, however, are of
unequal dimensions, and doubtless represent very unequal periods of
time. The older series, or Lower Old Red Sandstone, is by far the more
important and interesting in its extent, thickness, palæontological
riches, and, what specially concerns us in the present inquiry, in
its volcanic records. Wherever its true base can be seen, this series
passes down conformably into Upper Silurian strata. It sometimes
reaches a thickness of 15,000 and even 20,000 feet. There is generally
a marked break between its highest visible strata and all younger
formations. Even the upper division of the Old Red Sandstone rests
unconformably upon the lower.[311] Such a hiatus undoubtedly points to
a considerable lapse of geological time, and to the advent of important
geographical changes that considerably modified the remarkable
topography of the older part of the period.
[Footnote 311: _Quart. Journ. Geol. Soc._ vol. xvi. (1860), p. 312. In
Wales no break has actually been discovered between the two divisions
of the Old Red Sandstone, though it is suspected to exist there also.]
The younger division or Upper Old Red Sandstone passes upward
conformably into the base of the Carboniferous system. Its red and
yellow sandstones, conglomerates and breccias, covering much more
restricted areas, and attaining a much less thickness than those of
the lower division, indicate the diminution and gradual effacement of
the lakes of the older time, and the eventual return of the sea to the
tracts from which it had been so long excluded. So vast an interval
elapsed between the time recorded in the deposits respectively of the
two sections of the Old Red Sandstone that the characteristic forms of
animal life in the earlier ages had entirely passed away, and their
places had been taken by other types when the diminished lake-basins of
the second period began to be filled up. Volcanic action also dwindled
to such a degree that in contrast to the abundant vents of the older
period, only one or two widely scattered groups of vents are known to
have existed in the area of the British Isles during the later period,
and these, after a feeble activity, gave way to a prolonged volcanic
quiescence, which lasted until the earlier ages of the succeeding or
Carboniferous period.
Although geologists are in the habit of grouping the Old Red Sandstone
and the Devonian rocks as equivalent or homotaxial formations,
deposited in distinct areas under considerably different conditions
of sedimentation, the attempt to follow out the sequence of strata in
Devonshire, and to trace some analogy between the Devonian succession
and that of the Old Red Sandstone, presents many difficulties for
which no obvious solution suggests itself. Into these problems it is
not needful to enter further than was done in the last chapter. We may
assume that not improbably some of the eruptions now to be described
were coeval with those of Devonian time in the south-west of England,
though we may hesitate to decide which of them should be brought into
parallelism.
As we trace the shore-lines of the ancient basins of the Lower Old
Red Sandstone, and walk over the shingle of their beaches, or as we
examine the silt of their deeper gulfs, and exhume the remains of
the plants that shaded their borders, and of the fishes that swarmed
in their waters, we gradually learn that although the sediments
which accumulated in some of these basins amount to many thousand
feet in thickness; yet from bottom to top they abound in evidence of
shallow-water conditions of deposit. The terrestrial disturbances
above referred to continued for a vast interval, and while, as already
suggested, the floors of the basins sank, and the intervening tracts
were ridged up, as the results of one great movement of the earth's
crust, the denudation of the surface of the land contributed to the
basins such a constant influx of sediment as, on the whole, compensated
for the gradual depression of their bottoms.
We need not suppose that these movements of subsidence and upheaval
were uninterrupted and uniform. Indeed, the abundant coarse
conglomerates, which play so prominent a part in the materials thrown
into the basins, suggest that at various intervals during the prolonged
sedimentation subterranean disturbances were specially vigorous. But
the occurrence of strong unconformabilities among the deposits of the
basins sets this question at rest, by proving that the terrestrial
movements were so great as sometimes to break up the floor of a lake,
and to place its older sediments on end, in which position they were
covered up and deeply buried by the succeeding deposits.[312]
[Footnote 312: An unconformability of this kind occurs between the
south end of the Pentland Hills and Tinto in Lanarkshire, and another
in Ayrshire.]
It is not surprising to discover, among these evidences of great
terrestrial disturbance, that eventually groups of volcanoes rose in
long lines from the waters of most of the lakes, and threw out enormous
quantities of lava and ashes over tracts hundreds of square miles in
extent. So vast, indeed, were these discharges, across what is now the
Midland Valley of Scotland, that the portions of sheets of lava and
tuff visible at the surface form some of the most conspicuous ranges
of hills in that district, stretching continuously for 40 or 50 miles
and reaching heights of more than 2000 feet above the sea. Exposed in
hundreds of ravines and escarpments, and dissected by the waves along
both the eastern and western coasts of the country, these volcanic
records may be studied with a fulness of detail which cannot be found
among earlier Palæozoic formations.
It might have been supposed that a series of rocks so well displayed
and so full of interest, would long ere this have been fully examined
and described. But they can hardly be said to have yet received, as a
whole, the attention they deserve. Without enumerating all the writers
who, each in his own measure, have added to the sum of our knowledge of
the subject, I may refer to the labours of Jameson,[313] Macknight[314]
and Fleming,[315] among the observers who began the investigation.
But of the early pioneers, by far the most important in regard to the
igneous rocks of the Old Red Sandstone was Ami Boué. While attending
the University of Edinburgh, where he took the degree of M.D. in the
year 1816, he imbibed from Jameson a love of mineralogy and geognosy,
and for several years spent his leisure time in personally visiting
many parts of Scotland, in order to study the geological structure of
the country. Probably in 1820 he published in French his now classic
_Essai_.[316] The value of this work as an original contribution to
the geology of the British Isles has probably never been adequately
acknowledged. For this want of due recognition the author himself
was no doubt in some measure to blame. He refers distinctly enough
to various previous writers, notably to Jameson and Macculloch, but
he mingles the results of his own personal examinations with theirs
in such a way that it is hardly possible to ascertain what portions
are the outcome of his own original observations. Less credit has
accordingly been given to him than he could fairly have claimed for
solid additions to the subjects of which he treated. In the later years
of his life I had opportunities of learning personally from him how
extensive had been his early peregrinations in Scotland, and how vivid
were the recollections which, after the lapse of half a century, he
still retained of them. Judged simply as a well-ordered summary of all
the known facts regarding the geology of Scotland, his _Essai_ must be
regarded as a work of very great value. Especially important is his
arrangement of the volcanic phenomena of the country, which stands far
in advance of anything of the kind previously attempted. Under the head
of the "Terrain Volcanique," he treats of the basaltic formations,
distinguishing them as sheets (_nappes_, _coulées_) and dykes; and
of the felspathic or trachytic formations, which he subdivides into
phonolites, trachytes, porphyries (forming mountains and also sheets)
and felspathic or trachytic dykes. In the details supplied under each
of these sections he gives facts and deductions which were obviously
the result of his own independent examination of the ground, and he
likewise marshals the data accumulated by Jameson, Macculloch and
others, in such a way as to present a more comprehensive and definite
picture of the volcanic phenomena of Scotland than any previous writer
had ventured to give.
[Footnote 313: _Memoirs of the Wernerian Society_, vol. ii. (1811), pp.
178, 217, 618; vol. iii. (1820), p. 220, 225.]
[Footnote 314: _Op. cit._ vol. ii. pp. 123, 461.]
[Footnote 315: _Op. cit._ vol. i. (1808), p. 162; vol. ii. (1811), pp.
138, 339.]
[Footnote 316: _Essai géologique sur l'Écosse_ (Paris; no date, but
probably about 1820). He acknowledges his indebtedness to Jameson,
whose demonstrations of the geology of the Edinburgh district he
partly reproduced in his book. Jameson's early writings in the
_Wernerian Memoirs_ and in separate works were mere mineralogical or
"geognostical" descriptions. His later lectures became more valuable
but were never published, save indirectly in so far as they influenced
the opinions of his pupils who published writings on the same subjects.
See, for instance, Hay Cunningham's _Geology of the Lothians_, p. 59,
footnote. Compare an article on Boué, _Edinburgh Review_ for May 1823
(vol. xxxviii. p. 413).]
The account which Boué wrote of the Old Red Sandstone and its
associated igneous rocks marked the first great forward step in the
investigation of this section of the geological record. He was the
earliest observer to divide what he calls the "roches feldspathiques
et trappéennes" into groups according to their geological position and
mineralogical character, and to regard them as of igneous origin and of
the age, or nearly of the age, of the red sandstone of Central Scotland.
Of later writers who have treated of the volcanic rocks of the Old Red
Sandstone, my old friend Charles Maclaren deserves special recognition.
His survey and description of the Pentland Hills embodied the first
detailed and accurate investigation of any portion of these rocks,
and his _Geology of Fife and the Lothians_ may still be read with
pleasure and instruction.[317] Boué had indicated roughly on the little
sketch-map accompanying his _Essai_ the chief bands of his felspathic
and trappean rocks of the Old Red Sandstone, but their position and
limits were more precisely defined in Macculloch's "Geological Map of
Scotland," which was published in 1840, five years after the sudden
and tragic death of its author. The observers who have more recently
studied these rocks have been chiefly members of the Geological Survey,
and to some of the more important results obtained by them I shall
refer in the sequel.
[Footnote 317: _Geology of Fife and the Lothians_, 1839. More detailed
reference will be made in later pages to this classic.]
For many years I have devoted much time to the investigation of the Old
Red Sandstone and its volcanic rocks. In the year 1859 I ascertained
the existence of the great hiatus between the Lower and Upper divisions
of the system.[318] A first sketch of the volcanic history of the Old
Red Sandstone was given by me in 1861,[319] which was subsequently
enlarged and filled in with more detail in 1879.[320] But it was not
until 1892 that I published a somewhat detailed outline of the whole
subject, tracing the history of volcanic action during the period of
the Old Red Sandstone, the distribution of the volcanoes, and the
character of the materials erupted by them.[321] This outline I now
proceed to amplify, filling in details that were necessarily omitted
before, though there are still several districts regarding which
information is scanty.
[Footnote 318: "On the Old Red Sandstone of the South of Scotland,"
_Quart. Journ. Geol. Soc._ xvi. (1860), p. 312.]
[Footnote 319: "On the Chronology of the Trap-Rocks of Scotland,"
_Trans. Roy. Soc. Edin._ vol. xxii. (1861), p. 63.]
[Footnote 320: Article "Geology," in Ninth Edition of the _Encyclopædia
Britannica_, vol. x. (1879), p. 343. Reprinted in my _Text-Book of
Geology_, of which the first edition appeared in 1882.]
[Footnote 321: "Presidential Address to the Geological Society,"
_Quart. Journ. Geol. Soc._ vol. xlviii. (1892).]
In arranging the treatment of the subject I shall divide the record
into two main sections, the first and much the more important being
devoted to the Lower and the second to the Upper Old Red Sandstone.
In the first of these divisions it will be convenient to begin by
taking note of the distribution of the various districts over which
the geological evidence is spread. We may then proceed to consider the
general character of the volcanic rocks and the manner in which they
are arranged in the stratigraphy of the country, taking in consecutive
order (1) the superficial lavas and tuffs; (2) the vents; (3) the
dykes and sills. From these general considerations we may pass to the
detailed history of events in each of the separate volcanic areas, and
thus obtain, as far as the evidence at present permits, a broad view of
the progress of volcanic action during the time of the Lower Old Red
Sandstone in Britain.