VOLCANOES OF THE UPPER OLD RED SANDSTONE--THE SOUTH-WEST OF IRELAND,
THE NORTH OF SCOTLAND
In the northern half of Britain, where the Old Red Sandstone is so
well displayed, the two great divisions into which this series of
sedimentary deposits is there divisible are separated from each other
by a strongly marked unconformability. The interval of time represented
by this break must have been of long duration, for it witnessed
the effacement of the old water-basins, the folding, fracture, and
elevation of their thick sedimentary and volcanic accumulations, and
the removal by denudation of, in some places, several thousand feet
of these rocks. The Upper Old Red Sandstone, consisting so largely as
it does of red sandstones and conglomerates, indicates the return or
persistence of geographical conditions not unlike those that marked
the deposition of the lower subdivision. But in one important respect
its history differs greatly from that which I have sketched for the
older part of the system. Though the Upper Old Red Sandstone is well
developed across the southern districts of Scotland from the Ochil
to the Cheviot Hills, and appears in scattered areas over so much of
England and Wales, no trace has ever been there detected in it of any
contemporaneously erupted volcanic rocks. The topographical changes
which preceded its deposition must have involved no inconsiderable
amount of subterranean disturbance, yet the volcanic energy, which had
died out so completely long before the close of the time of the Lower
Old Red Sandstone, does not appear to have been rekindled until the
beginning of the Carboniferous period.
Two widely separated tracts in the British Isles have yielded traces of
contemporaneous volcanic rocks in the Upper Old Red Sandstone. One of
these lies in the south-west of Ireland, the other in the far north of
Scotland.
THE SOUTH-WEST OF IRELAND
The Irish locality is situated a few miles to the south of the town
of Limerick, where the Carboniferous Limestone has been thrown into
long folds, and where, along the anticlines, strips of the underlying
red sandstones have been brought up to the surface. Two such ridges
of Upper Old Red Sandstone bear, each on its crest, a small but
interesting relic of volcanic activity[394] (Map I.).
[Footnote 394: See Sheet 153 of the Geological Survey of Ireland, and
Explanation to that Sheet (1861), by Messrs. G. H. Kinahan and J.
O'Kelly. The account of the ground above given is from notes which I
made during a personal visit.]
The more northerly ridge rises in the conical eminence of Knockfeerina
to a height of 949 feet above the sea. Even from a distance the
resemblance of this hill (Fig. 102) to many of the Carboniferous necks
of Scotland at once attracts the eye of the geologist. The resemblance
is found to hold still more closely when the internal structure of the
ground is examined. The cone consists mainly of a coarse agglomerate,
with blocks generally somewhat rounded and varying in size up to two
feet in length. The most prominent of these, on the lower eastern
slopes, are pieces of a fine flinty felsite weathering white, but there
also occur fragments of grit and baked shale. The matrix is dull-green
in colour, and among its ingredients are abundant small lapilli of a
finely vesicular andesite or diabase. These more basic ingredients
increase in number towards the top of the eminence, where much of
the agglomerate is almost wholly made up of them. No marked dip is
observable over most of the hill, the rock appearing as a tumultuous
agglomerate, though here and there, particularly near the top and on
the south side, a rude bedding may be detected dipping outwards. On the
west side the agglomerate is flanked with yellow sandstone baked into
quartzite, so that the line of junction there between the two rocks not
improbably gives us the actual wall of the vent. The induration of the
surrounding sandstones is a familiar feature among the Carboniferous
vents. Some intrusive dark flinty rock traverses the agglomerate near
the top on the north side.
[Illustration: Fig. 102.--View of Knockfeerina, Limerick, from the
north-east--a volcanic neck of Upper Old Red Sandstone age.]
Retiring eastwards from the cone, the observer finds evidence of the
intercalation of tuff among the surrounding Upper Old Red Sandstone.
At the east end of the village of Knockfeerina a red nodular tuff,
with rounded pieces of andesite, grit and sandstone, sometimes 12
inches long, is seen to dip under yellow, grey and red sandstones and
shales, while other shales and sandstones underlie this tuff and crop
out between it and the agglomerate. There is thus evidence of the
intercalation of volcanic tuff in the Upper Old Red Sandstone of this
district. And there seems no reason to doubt that the tuff was ejected
from the adjoining vent of Knockfeerina.
On the next ridge of Old Red Sandstone, which runs parallel to that
of Knockfeerina at a distance of little more than a mile to the
south, another mass of volcanic material rises into a prominence at
Ballinleeny. On the north side it consists of agglomerate like that
just described, and is flanked by sandstone baked into quartzite. Here
again we probably see the edge of a volcanic funnel. There may possibly
be more than one vent in this area. But well-bedded tuffs can be
observed to dip away from the centre and to pass under sandstones and
shales which are full of fine ashy material. Gradations can be traced
from the tuff into ordinary sediment. In this instance, therefore,
there is additional proof of contemporaneous volcanic action in the
Upper Old Red Sandstone. There can be no uncertainty as to the horizon
of the strata in which these records have been preserved, for they
dip conformably under the shales and limestones at the base of the
Carboniferous Limestone series. They are said to have yielded the
characteristic fern _Palæopteris_ of Kiltorcan.[395]
[Footnote 395: There may be some other examples of Upper Old Red
Sandstone volcanic rocks in Ireland which I have not yet been able
personally to examine. On the maps of the Geological Survey (Sheet
198, and Explanation, pp. 8, 17) contemporaneous rocks of this age are
marked as occurring at Cod's Head and Dursey Island, on the south side
of the mouth of the Kenmare estuary.]
THE NORTH OF SCOTLAND
The only district in England or Scotland wherein traces of volcanic
action during the time of the Upper Old Red Sandstone have been
observed lies far to the north among the Orkney Islands, near the
centre of the scattered outliers which I have united as parts of the
deposits of "Lake Orcadie"[396] (Map I.). The thick group of yellow and
red sandstones which form most of the high island of Hoy, and which,
there can be little doubt, are correctly referred to the Upper Old Red
Sandstone, rest with a marked unconformability on the edges of the
Caithness flagstones (Fig. 103). At the base of these pale sandstones,
and regularly interstratified with them, lies a band of lavas and tuffs
which can be traced from the base of the rounded hills to the edge of
the cliffs at the Cam, along the face of which it runs as a conspicuous
feature, gradually sloping to a lower level, till it reaches the sea.
At the Cam of Hoy it is about 200 feet thick, and consists of three or
more sheets of andesite. The upper 50 or 60 feet show a strongly slaggy
structure, the central portion is rudely columnar, and the lower part
presents a kind of horizontal jointing or bedding. There can be no
question that this rock is not a sill but a group of contemporaneous
lava-flows. Beneath it, and lying across the edges of the flagstones
below, there is a zone of dull-red, fine-grained tuff, banded with
seams of hard red and yellow sandstone. This tuff zone dies out to the
eastward of the Cam.
[Footnote 396: First noticed in _Geol. Mag._ February 1878; and _Trans.
Roy. Soc. Edin._ xxviii. (1878), p. 411.]
[Illustration:
Fig. 103.--Section of the volcanic zone in the Upper Old Red
Sandstone, Cam of Hoy, Orkney.
1. Caithness flagstones; 2. Dull-red tuff and bands of sandstone;
3. Lava zone in three bands; 4. Yellow and red sandstone.
]
[Illustration: Fig. 104.--Section of the volcanic zone in the Upper Old
Red Sandstone at Black Ness, Rackwick, Hoy.]
A few miles south of the Cam the volcanic zone appears again as the
platform on which the picturesque natural obelisk of the Old Man of Hoy
stands. Here the lava runs out as a promontory from the base of the
cliff, and on this projection the Old Man has been left isolated from
the main precipice. The cliffs of Hoy are traversed by numerous small
faults which have shifted the volcanic zone. But on the great face of
rock behind the Old Man there appears to be a second volcanic zone
lying several hundred feet above that just described. It is probably
this upper zone which emerges from under the hills a mile and a half
to the south at Black Ness in the bay of Rackwick. A good section is
there visible, which is represented in Fig. 104. The ordinary red and
yellow sandstones (_a_) appear from under the volcanic rocks at this
locality, and stretch southwards to the most southerly headland of Hoy.
The lowest volcanic band in the section is one of red sandy well-bedded
tuffs (_b_). Some of the layers are coarse and almost agglomeratic,
while others are fine marly and sandy, with dispersed bombs, blocks
and lapilli of diabase and andesite. Hard ribs of a sandy tufaceous
material also occur. These fragmental deposits are immediately overlain
by a dark-blue rudely prismatic diabase with slaggy top (_c_). It is
about 150 feet thick at its thickest part, but rapidly thins away in a
westerly direction. It passes under a zone of red tuff (_d_) like that
below, and above this highest member of the volcanic group comes the
great overlying pile of yellow and reddish sandstone of Hoy. Followed
westward for a short distance, the whole volcanic zone is found to die
out and the sandstones below and above it then come together.
The interest of this little volcanic centre in Hoy is heightened by
the fact that the progress of denudation has revealed some of the
vents belonging to it. On the low ground to the east of the Cam, and
immediately to the north of the volcanic escarpment, the flagstones
which there emerge from under the base of the unconformable upper
sandstones are pierced by three volcanic necks which we may with little
hesitation recognize as marking the sites of vents from which this
series of lavas and tuffs was discharged (Fig. 105). The largest of
them forms a conspicuous hill about 450 feet high, the smallest is only
a few yards in diameter, and rises from the surface of a flagstone
ridge. They are filled with a coarse, dull-green, volcanic agglomerate,
made up of fragments of the lavas with pieces of hardened yellow
sandstone and flagstone. Around the chief vent the flagstones through
which it has been opened have been greatly hardened and blistered. The
most easterly vent, which has been laid bare on the beach at Bring, due
east of Hoy Hill, can be seen to pierce the flagstones, which, although
their general dip is westerly at from 10° to 15°, yet at their junction
with the agglomerate are bent in towards the neck, and are otherwise
much jumbled and disturbed.
[Illustration: Fig. 105.--Section across the volcanic band and its
associated necks, Hoy, Orkney.
1. Caithness flagstones; 2. Volcanic band lying on red sandstones
and conglomerates and dying out eastwards; 3 3. Two vents
between the base of the hills and the sea; their connection with
the volcanic band is shown by dotted lines; 4. Overlying mass of
Upper Old Red Sandstone forming the hills of Hoy.
]
On the northern coast of Caithness I have described a remarkable
volcanic vent about 300 feet in diameter, which rises through the
uppermost group of the Caithness flagstones. It is filled with a coarse
agglomerate consisting of a dull-greenish diabase paste crowded with
blocks of diabase, sometimes three feet in diameter, and others of red
sandstone, flagstone, limestone, gneiss and lumps of black cleavable
augite (Fig. 106).[397] The sandstones around it present the usual
disrupted, indurated and jointed character, and are traversed by a
small diabase dyke close to the western margin of the neck. Another
similar neck has since been found by the officers of the Geological
Survey on the same coast. That these volcanic orifices were active
about the same time with those in the opposite island of Hoy may be
legitimately inferred.
[Footnote 397: See _Trans. Roy. Soc. Edin._ xxviii. (1878), p. 405;
also p. 482 of the same volume for an account of the cleavable augite.]
[Illustration: Fig. 106.--Ground-plan of volcanic neck piercing the
Caithness Flagstone series on the beach near John o' Groat's House.]
These northern volcanoes made their appearance in a district where
during the preceding Lower Old Red Sandstone period there had been
several widely separated groups of active volcanic vents. So far as the
fragmentary nature of the geological evidence permits an opinion to
be formed, they seem to have broken out at the beginning, or at least
at an early stage, of the deposition of the Upper Old Red Sandstone,
and to have become entirely extinct after the lavas of Hoy were poured
forth. No higher platform of volcanic materials has been met with in
that region. With these brief and limited Orcadian explosions the
long record of Old Red Sandstone volcanic activity in the area of the
British Isles comes to an end.[398]
[Footnote 398: There appear to be traces of volcanic eruptions
contemporaneous with the Upper Old Red Sandstone of Berwickshire,
but as they merely formed a prelude to the great volcanic activity
of Carboniferous time, they are included in the account of the
Carboniferous plateau of Berwickshire in Chapters xxiv. and xxv.]
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.
Within the area of the British Isles, the geological record is
comparatively full and continuous from the base of the Upper Old Red
Sandstone to the top of the Coal-measures. We learn from it that the
local basins of deposit in which the later portion of the Old Red
Sandstone was accumulated sank steadily in a wide general subsidence,
that allowed the clear sea of the Carboniferous Limestone ultimately
to spread for some 700 miles from the west coast of Ireland into
Westphalia. Over the centre of England this Carboniferous Mediterranean
had a breadth of at least 150 miles, gradually shallowing northwards
in the direction of land in Scotland and Northern Ireland. The gentle
sinking of the floor of the basin continued until more than 6000 feet
of sediment, chiefly composed of the remains of crinoids, corals and
other marine organisms, had been piled up in the deeper parts. Traces
of the southern margin of this sea, or at least of a long insular ridge
that rose out of its waters, are to be seen in the protuberances of
older rocks which appear at intervals from under the Coal-measures
and later formations between the borders of Wales and the heart of
Leicestershire, and of which the crags of Charnwood Forest are among
the few peaks that still remain visible. To the south of this ridge,
open sea extended far southward and westward over the site of the
Mendip Hills and the uplands of South Wales.
The Carboniferous period, as chronicled by its sedimentary deposits,
was a time of slow submergence and quiet sedimentation, terrestrial
and marine conditions alternating along the margins of the sinking
land, according as the rate of depression surpassed or fell short of
that of the deposition of sediment. There is no trace of any general
disturbance among the strata, such as would be marked by an important
and widely extended unconformability. But many indications may be
observed that the rate of subsidence did not continue uniform, if,
indeed, the downward movement was not locally arrested, and even
exchanged for a movement in the opposite direction. It is difficult,
for instance, to believe the ancient ridge of the Midlands to have
been so lofty that even the prolonged subsidence required for the
accumulation of the whole Carboniferous system was insufficient to
carry its highest crests below the level of the coal-jungles. More
probably the depression reached its maximum along certain lines or
bands running in a general north-easterly direction, the intervals
between these lines sinking less, or possibly even undergoing some
measure of uplift. One of the subsiding tracts, that of the wide
lowlands of Central Scotland, was flanked on the south by a ridge
which, while its north-eastern portion was buried under the Upper Old
Red Sandstone and Lower Carboniferous rocks, remained above water
towards the south-west, and does not appear to have been wholly
submerged there even at the close of the Carboniferous period.
So abundant and varied are the sedimentary formations of Carboniferous
time, and so fully have they preserved remains of the contemporary
plants and animals, that it is not difficult to realise in some measure
the general aspect of the scenery of the time, and the succession of
changes which it underwent from the beginning to the end of the period.
The land was green with a luxuriant if somewhat monotonous vegetation.
Large pine trees flourished on the drier uplands. The lower grounds
nourished dense groves of cycads or plants allied to them, which rose
as slim trees twenty or thirty feet high, with long hard green leaves
and catkins that grew into berries. The swamps and wetter lands bore a
rank growth of various gigantic kinds of club-moss, equisetaceous reeds
and ferns.
Nor was the hum of insect-life absent from these forests. Ancestral
types of cockroaches, mayflies and beetles lived there. Scorpions
swarmed along the margins of the shallow waters, for their remains,
washed away with the decayed vegetation among which they harboured, are
now found in abundance throughout many of the dark shales.
The waters were haunted by numerous kinds of fish quite distinct from
those of the Old Red Sandstone. In the lagoons, shoals of small ganoids
lived on the cyprids that peopled the bottom, and they were in turn
preyed on by larger ganoids with massive armature of bone. Now and
then a shark from the opener sea would find its way into these more
inland waters. The highest types of animal life yet known to have
existed at this time were various amphibians of the extinct order of
Labyrinthodonts.
The open sea, too, teemed with life. Wide tracts of its floor supported
a thick growth of crinoids whose jointed stems, piled over each other
generation after generation, grew into masses of limestone many
hundreds of feet in thickness. Corals of various kinds lived singly and
in colonies, here and there even growing into reefs. Foraminifera,
sponges, sea-urchins, brachiopods, gasteropods, lamellibranchs and
cephalopods, in many genera and species, mingled their remains with the
dead crinoids and corals to furnish materials for the wide and thick
accumulation of Carboniferous Limestone.
Looking broadly at the history of the Carboniferous period, and bearing
in mind the evidence of prolonged depression already referred to, we
can recognize in it three great eras. During the first, the wide clear
sea of the Carboniferous Limestone spread over the centre and south
of Britain, interrupted here and there by islands that rose from long
ridges whereby the sea-floor was divided into separate basins. Next
came a time of lessened depression, when the sea-bottom was overspread
with sand, mud and gravel, and was even in part silted up, as has been
chronicled in the Millstone Grit. The third stage brings before us the
jungles of the Coal-measures, when the former sea-floor became a series
of shallow lagoons where, as in the mangrove-swamps of our own time, a
terrestrial vegetation sprang up and mingled its remains with those of
marine shells and fishes.
Such a state of balance among the geological forces as is indicated
by the stratigraphy of the Carboniferous system would not prepare us
for the discovery of the relics of any serious display of contemporary
volcanic activity. And, indeed, throughout the Carboniferous rocks
of Western Europe there is for the most part little trace of
contemporaneous volcanic eruptions. Yet striking evidence exists that,
along the western borders of the continental area, in France as well
as over much of Britain, which had for so many previous geological
ages been the theatre of subterranean activity, the older half of
Carboniferous time witnessed an abundant, though less stupendous and
prolonged, renewal of volcanic energy.
From the very commencement of the Carboniferous period to the epoch
when the Coal-measures began to be accumulated, the area of the British
Isles continued to be a scene of active volcanism. In the course of
that prolonged interval of geological time the vents shifted their
positions, and gradually grew less energetic, but there does not
appear to have been any protracted section of the interval when the
subterranean activity became everywhere entirely quiescent.
The geologist who traces, from older to younger formations, the
progress of some persistent operation of nature, observes the evidence
gradually to increase in amount and clearness as it is furnished
by successively later parts of the record. He finds that the older
rocks have generally been so dislocated and folded, and are often
so widely covered by younger formations, that the evidence which
they no doubt actually contain may be difficult to decipher, or may
be altogether concealed from view. In following, for instance, the
progress of volcanic action, he is impressed, as he passes from the
older to the younger Palæozoic chronicles, by the striking contrast
between the fulness and legibility of the Carboniferous records and the
comparative meagreness and obscurity of those of the earlier periods.
The Carboniferous rocks have undergone far less disturbance than the
Cambrian and Silurian formations; while over wide tracts, where their
volcanic chapters are fullest and most interesting, they lie at the
surface, and can thus be subjected to the closest scrutiny. Hence
the remains of the volcanic phenomena of the later Palæozoic periods
present a curiously modern aspect, when contrasted with the fragmentary
and antique look of those of older date.
The history of volcanic action during the Carboniferous period in
Britain is almost wholly comprised in the records of the earlier half
of that period, that is, during the long interval represented by the
Carboniferous Limestone series and the Millstone Grit. It was chiefly
in the northern part of the region that volcanic activity manifested
itself. In Scotland there is the chronicle of a long succession of
eruptions across the district of the central and southern counties,
from the very beginning of Carboniferous time down to the epoch when
the Coal-measures began to be accumulated. In England, on the other
hand, the traces of Carboniferous volcanoes are confined within a
limited range in the Carboniferous Limestone, while in Ireland they
appear to be likewise restricted to the same lower division of the
system. During the whole of the vast interval represented by the
Coal-measures volcanic energy, so far as at present known, was entirely
dormant over the region of the British Isles.
These general statements will be more clearly grasped from the
accompanying table, which shows the various sections into which the
Carboniferous system of Britain has been divided, and also, by black
vertical lines, the range of volcanic intercalations in each of the
three kingdoms.
+-----------------------------------------+----------+-----------+----------+
| | England. | Scotland. | Ireland. |
+-----------------------------------------+----------+-----------+----------+
|Coal-measures. | | | |
| { Upper Red Sandstones with | | | |
| { _Spirorbis_-limestone. | | | |
| { Middle or chief coal-bearing | | | |
| { measures. | | | |
| { Gannister group. | | | |
| | | | |
|Millstone Grit. | | || | |
| } Grits, flagstones and shales with | | || | |
| } thin coals. | | || | |
| | | || | |
|Carboniferous Limestone. | | || | |
| { Yoredale group of shales and grits | | || | |
| { with limestones. | | || | |
| { Thick (Scaur or Main) Limestone | || | || | |
| { of England, with sandstones | || | || | || |
| { and coals in Scotland. | || | || | || |
| { Lower Limestone Shale (Calciferous | | || | |
| { Sandstones of Scotland). | | || | |
+-----------------------------------------+----------+-----------+----------+
Such being the general range in time of the Carboniferous volcanic
phenomena, it may be convenient, in this preliminary survey, to take
note of the general distribution of the volcanic districts over the
British Isles, as in this way we may best realise the extent and
grouping of the eruptions, which will then be considered in further
detail (see Map I.).
Not only were the Carboniferous volcanoes most abundant and persistent
in Scotland, but they attained there a variety and development which
give their remains an altogether exceptional interest in the study of
volcanic geology. They were distributed over the wide central valley,
from the south of Cantyre to beyond the mouth of the estuary of the
Forth. On the southern side of the Silurian Uplands, they were likewise
numerous and active. There is thus no considerable tract of Lower
Carboniferous rocks in Scotland which does not furnish its evidence of
contemporaneous volcanic action.
Although some portions of the Scottish Carboniferous igneous rocks run
for a short distance into England, it is remarkable that, when these at
last die out southwards, no other relics of contemporaneous volcanic
energy take their place. Along the Pennine chain, from the Border into
the heart of England, though natural sections are abundant, no trace of
included volcanic rocks appears until we reach Derbyshire. The whole of
that wide interval of 150 miles, so far as the present evidence goes,
remained during Carboniferous time entirely free from any volcanic
eruption. But from the picturesque country of the Peak southwards, the
sea-floor of the Carboniferous Limestone, in what is now the heart
of England, was dotted with vents whence the sheets of "toadstone"
were ejected, which have so long been a familiar feature in English
geology. Beyond this limited volcanic district the Carboniferous
formations of the south-west of England remain, on the whole, devoid
of contemporaneous volcanic intercalations, traces of Carboniferous
volcanic action having been recognized only in West Somerset and
Devonshire. In the Mendip district and in the ridges of limestone near
Weston-super-Mare bands of cellular lava and tuff have been observed.
To the west of Dartmoor, Brent Tor and some of the surrounding igneous
masses may mark the positions of eruptive vents during an early part of
the Carboniferous period.
At the south end of the Isle of Man relics remain of a group of vents
among the Carboniferous limestones. Passing across to Ireland, where
these limestones attain so great a thickness and cover so large
a proportion of the surface of the island, we search in vain for
any continuation of the abundant and varied volcanic phenomena of
Central Scotland. So far as observation has yet gone, only two widely
separated areas of Carboniferous volcanic rocks are known to occur
in Ireland.[399] One of these shows that a little group of vents
probably rose from the floor of the Carboniferous Limestone sea, near
Philipstown, in King's County. The other lies far to the west in the
Golden Vale of Limerick, where a more important series of vents poured
out successive streams of lava with showers of ashes, from an early
part of the Carboniferous period up to about the beginning of the time
of the Coal-measures.
[Footnote 399: The supposed Carboniferous volcanic rocks of Bearhaven
on the coast of Cork are noticed on p. 49, vol. ii.]
The total area within which the volcanic eruptions of Carboniferous
time took place was thus less than that over which the volcanoes of
the Lower Old Red Sandstone were distributed, yet they were scattered
across the larger part of the site of the British Isles. From the vents
of Fife to those of Limerick is a distance of above 300 miles; from the
latter eastward to those of Devonshire is an interval of 250 miles;
while the space between the Devonshire volcanoes and those of Fife is
about 400 miles. In this triangular space volcanic action manifested
itself at each of the apices, to a slight extent along the centre of
the eastern side, but with much the greatest vigour throughout the
northern part of the area.
Since the volcanic phenomena of Carboniferous time are exhibited on
a much more extensive scale in Scotland than in any other region of
the world yet studied, it will be desirable to describe that area in
considerable detail. The other tracts in Britain where volcanic rocks
of the same age occur need not be so fully treated, except where they
help to a better comprehension of the general volcanic history.
* * * * *
It is in the southern half of Scotland that the Carboniferous system is
developed (Map IV.). A line drawn from Machrihanish Bay, near the Mull
of Cantyre, north-eastward across Arran and Bute to the south end of
Loch Lomond, and thence eastward by Bridge of Allan, Kinross and Cupar
to St. Andrews Bay, forms the northern limit of this system. South of
that line Carboniferous volcanic intercalations are to be met with in
nearly every county across into the borders of Northumberland.
That we may follow intelligently the remarkably varied volcanic history
of this region, it is desirable to begin by taking note of the nature
and sequence of the sedimentary formations among which the volcanic
rocks are intercalated, for these serve to bring before us the general
conditions of the geography of the period. The subjoined table exhibits
the subdivisions into which the Carboniferous system in Scotland has
been grouped:--
Coal-measures.
{ Upper Red Sandstone group, nearly devoid of coal-seams.
{
{ Coal-bearing, white, yellow and grey sandstones, dark shales and
{ ironstones (Upper Coal series).
Millstone Grit.
{ Thick white and reddish sandstones and grits.
Carboniferous Limestone series.
{ Sandstones, shales, fireclays, coal-seams, ironstones and three seams
{ of marine limestone, of which the uppermost is known as the
{ Castlecary seam, the second as the Calmy or Arden, and the lowest
{ as the Index (Lower Coal series).
{
{ Bands of marine limestone intercalated among sandstones, shales and
{ some coal-seams. A thick band of limestone lying at or near the
{ bottom of the group, traceable all over Central Scotland, is known
{ as the Hurlet or Main Limestone. Some higher and thinner seams are
{ called Hosie's (see Fig. 155).
Calciferous Sandstones.[400]
{ In the basin of the Firth of Forth, below the Hurlet Limestone, comes
{ a varied series of white and yellow sandstones, black shales
{ (oil-shales), cyprid shales and limestones (Burdiehouse), and
{ occasional coal-seams (Houston), having a total depth of about 3000
{ feet. This local group abounds in fossil plants, entomostraca and
{ ganoid fishes. It passes down into the Cement-stone group, which,
{ however, is feebly developed in this district, unless it is partly
{ represented by the sandstones, shales, limestones and coals just
{ mentioned.
{ Cement-stone group consisting of red, blue and green marls and
{ shales, red and grey sandstones, and thin bands of cement-stone:
{ fossils scarce.
{ Reddish and grey sandstones and shales, with occasional plant-
{ remains, passing down into the deep red (sometimes yellow)
{ sandstones, red marls and cornstones of the Upper Old Red Sandstone.
[Footnote 400: The Calciferous Sandstones are the stratigraphical
equivalents of the Limestone Shale and lower portion of the
Carboniferous Limestone of England.]
From this table the gradual geographical evolution of the Carboniferous
period in Scotland may be gleaned. We observe that at the beginning,
the conditions under which the Old Red Sandstone had been accumulated
still in part continued. The great lacustrine basins of the Lower Old
Red Sandstone had indeed been effaced, and their sites were occupied
by comparatively shallow areas of fresh or brackish water in which
the Upper Old Red Sandstone was laid down. Their conglomerates and
sandstones had been uplifted and fractured. Their vast ranges of
volcanic material, after being deeply buried under sediment, had
been once more laid bare, and extended as ridges of land, separating
the pools and lagoons which they supplied with sand and silt. This
singular topography had not been entirely effaced at the beginning of
the Carboniferous period, for we find that many of the ridges which
bounded the basins of the Upper Old Red Sandstone remained as land
until they sank beneath the waters in which the earliest Carboniferous
strata accumulated. Thus, while no trace of an unconformability has
yet been detected at the top of the Upper Old Red Sandstone, there is
often a strong overlap of the succeeding deposits. At the south end of
the Pentland Hills, for example, the Upper Old Red Sandstone attains a
thickness of 1000 feet, but only three miles further south it entirely
disappears, together with all the overlying mass of Calciferous
Sandstones, and the Carboniferous Limestone then rests directly on the
Lower Old Red Sandstone. Again, at the north end of the same chain the
upper division of the Old Red Sandstone dies out against the lower,
which is eventually overlapped by the Calciferous Sandstones.
The change from the physical conditions of the Scottish Old Red
Sandstone to those of the Carboniferous system was no doubt gradual
and slow. The peculiar red sandy sediment continued to be laid down
in basins that were apparently being gradually widened by access of
water from the open sea. Yet it would seem that in Scotland these
basins still for a long time continued saline or, from some other
cause, unfavourable to life; for the red, blue and green shales or
marls, and occasional impure limestones or cement-stones and gypseous
layers, which were deposited in them, are in general unfossiliferous,
though drifted plants from the neighbouring land are here and there
common enough. The sediments of these early Carboniferous waters are
met with all over the southern half of Scotland, but in very unequal
development, and constitute what is known as the "Cement-stone Group."
It was while these strata were in course of deposition that the
earliest Carboniferous volcanoes broke into eruption. In some
localities a thickness of several hundred feet of the Cement-stone
group underlies the lowest lavas. In other places the lavas occur in
and rest on the Upper Old Red Sandstone and have the Cement-stone group
wholly above them; while in yet other districts the volcanic rocks seem
entirely to take the place of that group. So vigorous was the earliest
display of volcanic action in Carboniferous times that from the borders
of Northumberland to the uplands of Galloway, and from the slopes of
the Lammermuirs to Stirlingshire and thence across the estuary of the
Clyde to Cantyre, innumerable vents were opened and large bodies of
lava and ashes were ejected.
The Cement-stone group, save where succeeded by volcanic
intercalations, passes up conformably into the lowest crinoidal
limestones of the Carboniferous Limestone series. In the basin of the
Firth of Forth, however, the cement-stones, feebly represented there,
are overlain by a remarkable assemblage of white sandstones, black
carbonaceous shales, or "oil-shales," cyprid limestones, occasional
marine limestones and thin seams of coal, the whole having a thickness
of more than 3000 feet. These strata, unlike the typical Cement-stone
group, abound in fossils both vegetable and animal. They prove that,
over the area of the Forth, the insalubrious basins wherein the red and
green sediments of the Cement-stone group were laid down, gave place
to opener and clearer water with occasional access of the sea. The
peculiar lagoon-conditions which favoured the formation of coal were
thus developed in Central Scotland earlier than elsewhere in Britain.
We shall see in later pages that these conditions were accompanied by a
fresh outbreak of volcanic activity, in a phase less vigorous but more
enduring and extensive than that of the first Carboniferous eruptions.
The Carboniferous Limestone sea over the site of the southern half of
Scotland appears never to have reached the depth which it attained in
England and Ireland. To the north of it lay the land from which large
quantities of sand and mud were carried into it, as shown by the deep
accumulations of sandstone and shale, which far surpass in thickness
the few comparatively thin marine limestones intercalated in them.
There is thus a striking contrast between the thick masses of limestone
in central and south-western England and their dwindled representatives
in the north. Another marked difference between the Scottish and
English developments of this formation is to be noticed in the abundant
proof that the comparatively shallow waters of the northern basin were
plentifully dotted over with active volcanoes. The eruptions were
especially vigorous and prolonged in the basin of the Firth of Forth.
They continued at intervals, even after the peculiar geographical
conditions of the Carboniferous Limestone had ceased. But they had died
out by the time of the beginning of the Coal-measures.
Owing to the number and variety of the natural sections, the
Carboniferous volcanic rocks of Scotland have been the subject of
numerous observations and descriptions, from the early days of geology
down to the present time. The mere enumeration of the titles of the
various publications regarding them would make a long list. These
rocks formed the subject of some of Hutton's early observations,
and furnished him with facts from which he established the igneous
origin of "whinstone."[401] They supplied Playfair with numerous apt
illustrations in support of Hutton's views, and he seems to have made
himself thoroughly familiar with them.[402] In the hands of Sir James
Hall they became the groundwork of those remarkable experiments on the
fusion of whinstone which may be said to have laid the foundation of
experimental geology.[403] In the controversies of the Neptunian and
Plutonian schools these rocks were frequently appealed to by each side
in confirmation of its dogmas. The appointment in 1804 of Jameson to
the Chair of Natural History in the Edinburgh University gave increased
impetus to the study of the igneous rocks of Scotland. Though he did
not himself publish much regarding them, we know that he was constantly
in the habit of conducting his class to the hills, ravines and quarries
around Edinburgh, and that the views which he taught were imbibed
and extended by his pupils.[404] Among the early writers the names
of Allan,[405] Townson,[406] Lord Greenock,[407] and Ami Boué,[408]
deserve especial mention.
[Footnote 401: Hutton's _Theory of the Earth_, vol. i. p. 155 _et seq._]
[Footnote 402: Playfair's _Illustrations of the Huttonian Theory_, §
255 _et seq._]
[Footnote 403: _Trans. Roy. Soc. Edin._ (1805), vol. v. p. 43.]
[Footnote 404: _Mem. Wern. Soc._ ii. 178, 618; iii. 25; _Edin. Phil.
Journ._ i. 138, 352; xv. 386.]
[Footnote 405: Trans. Roy. Soc. Edin. (1811), vi. p. 405.]
[Footnote 406: _Tracts and Observations in Natural History and
Physiology_, 8vo, Lond. 1799.]
[Footnote 407: _Trans. Roy. Soc. Edin._ (1833), xiii. pp. 39, 107.]
[Footnote 408: _Essai géologique sur l'Écosse._ Paris; no date,
probably 1820.]
The first broad general sketch of the Carboniferous igneous rocks of a
large district of the country was that given by Hay Cunningham in his
valuable essay on the geology of the Lothians.[409] He separated them
into two series, the Felspathic, including "porphyry" and "clinkstone,"
and the Augitic or Trap rocks. To these he added "Trap-tufa," which
he considered to be identical in origin with modern volcanic tuff. It
was the eruptive character of the igneous rocks on which he specially
dwelt, showing by numerous sections the effects which the protrusion
of the molten masses have had upon the surrounding rocks. He did not
attempt to separate the intrusive from the interstratified sheets, nor
to form a chronological arrangement of the whole.
[Footnote 409: _Mem. Wern. Soc._ vii. p. 1. Published separately, 1838.]
Still more important was the sketch given by Maclaren, in his classic
_Geology of Fife and the Lothians_.[410] This author clearly recognized
that many of the igneous rocks were thrown out contemporaneously with
the strata among which they now lie. He constantly sought for analogies
among modern volcanic phenomena, and presented the Carboniferous
igneous rocks of the Lothians not as so many petrographical varieties,
but as monuments of different phases of volcanic action previous to
the formation of the Coal-measures. His detailed descriptions of Arthur
Seat and the rocks immediately around Edinburgh, which alone the work
was originally intended to embrace, may be cited as models of exact and
luminous research. The portions referring to the rest of the basin of
the Forth did not profess to be more than a mere sketch of the subject.
[Footnote 410: Small 8vo, Edin. 1838, first partly published as
articles in the _Scotsman_ newspaper. A second edition, which was
little more than a reprint of the first, appeared in 1866.]
Various papers of more local interest, to some of which allusion will
be made in the sequel, appeared during the next quarter of a century.
But no systematic study of the volcanic phenomena of any part of
Scotland was resumed until the extension in 1854 of the Geological
Survey to the north of the Tweed by A. C. Ramsay. The volcanic rocks
of the Lothians and Fife were mapped by Mr. H. H. Howell and myself.
The maps of that district began to be published in the year 1859, and
the Memoirs two years later. In 1861, in a chronological grouping of
the whole of the volcanic phenomena of Scotland, I gave an outline of
the Carboniferous eruptions.[411] By degrees the detailed mapping of
the Geological Survey was pushed across the whole of the rest of the
south of Scotland, and the Carboniferous volcanic rocks of each area
were then for the first time carefully traced and assigned to their
various stratigraphical horizons. In the following pages reference will
be given to the more important features of the Survey maps and Memoirs.
In the year 1879, availing myself of the large amount of information
which my own traverses and the work of the Survey had enabled me to
acquire, I published a Memoir on the geology and petrography of the
volcanic rocks of the basin of the Firth of Forth;[412] and lastly,
in my Presidential Address to the Geological Society in 1892, I gave
a summary of all that had then been ascertained on the subject of the
volcanic rocks of Carboniferous time in the British Isles.[413]
[Footnote 411: _Trans. Roy. Soc. Edin._ vol. xxii.]
[Footnote 412: _Ibid._ vol. xxix. (1879), p. 437.]
[Footnote 413: _Quart. Journ. Geol. Soc._ xlviii. (1892), p. 104. This
summary, with additional details and illustrations, is embodied in the
text.]
Two well-marked types of volcanic accumulations are recognizable in the
British Isles, which may be conveniently termed Plateaux and Puys.
1. Plateaux.--In this type, the volcanic materials were discharged
over wide tracts of country, so that they now form broad tablelands
or ranges of hills, reaching sometimes an extent of many hundreds
of square miles and a thickness of more than 1000 feet. Plateaux of
this character occur within the British area only in Scotland, where
they are the predominant phase of volcanic intercalations in the
Carboniferous system.
It is noteworthy that the Carboniferous plateaux appeared during a
well-marked interval of geological time. The earliest examples of them
date from the close of the Upper Old Red Sandstone. They were all in
vigorous eruption during the time of the Calciferous Sandstones, but in
no case did they survive into that of the Hurlet and later limestones.
They are thus eminently characteristic of the earliest portion of the
Carboniferous period.
2. Puys.--In this type, the ejections were often confined to the
discharge of a small amount of fragmentary materials from a single
solitary vent, and even where the vents were more numerous and the
outpourings of lava and showers of ash more copious, the ejected
material usually covered only a small area round the centres of
eruption. Occasionally streams of basic lava and accumulations of
tuff were piled up into long ridges. Volcanoes of this character
were specially abundant in the basin of the Firth of Forth, and more
sparingly in Ayrshire and Roxburghshire. They form the persistent type
throughout the rest of the British Isles.
The Puys also occupy a well-defined stratigraphical position. They did
not begin until some of the volcanic plateaux had become extinct. From
the top of the Cement-stone group up into the Carboniferous Limestone
series, their lavas and tuffs are met with on many platforms, but none
occur above that series save in Ayrshire, where some of the eruptions
appear to have been as late as about the beginning of the Coal-measures.
Arranged in tabular form the stratigraphical and geographical
distribution of the two great volcanic types of the Carboniferous
system in Scotland will be more easily followed. I have therefore drawn
up the accompanying scheme:--
Location Key:
A. Ayrshire and Renfrewshire.
B. Stirlingshire.
C. West Lothian.
D. Midlothian.
E. East Lothian.
F. Fife.
G. Berwick & Roxburghshire.
+--------------------------------------------------++--------------------+
| Plateau-type. || Puy-type. |
+-----------------------------------+--+--+--+--+--++--+--+--+--+--+--+--+
| |A.|B.|D.|E.|G.||A.|B.|C.|D.|E.|F.|G.|
|Coal Measures |..|..|..|..|..||..|..|..|..|..|..|..|
| | | | | | ||‖ | | | | | | |
|Millstone Grit |..|..|..|..|..||‖ |..|..|..|..|..|..|
| | | | | | ||‖ | | | | | | |
|Carboniferous Limestone Series. | | | | | ||‖ | | | | | | |
| { Castlecary Limestone |..|..|..|..|..||‖ |..|..|..|..|..|..|
| { | | | | | ||‖ | | | | | | |
| { Calmy " |..|..|..|..|..||‖ |..|..|..|..|..|..|
| { | | | | | ||‖ | |‖ | | |‖ | |
| { Index " |..|..|..|..|..||‖ |..|‖ |..|..|‖ |..|
| { | | | | | || | |‖ | | |‖ |‖ |
| { Hurlet " |..|..|..|..|..||..|..|‖ |..|..|‖ |‖ |
| |‖ |‖ | |‖ | || | |‖ | | |‖ |‖ |
|Calciferous Sandstone Series. |‖ |‖ | |‖ | || | |‖ | | |‖ |‖ |
| { Burdiehouse Limestone |‖ |‖ | |‖ | || | |‖ | | |‖ |‖ |
| { and Oil-shale Group |‖ |‖ |..|‖ |..||..|..|‖ |..|..|‖ |‖ |
| { |‖ | |‖ |‖ | || | |‖ |‖ | |‖ |‖ |
| { Cement-stone Group |..|..|..|..|‖ ||..|..|‖ |‖ |..|‖ |‖ |
| { | | | | |‖ || | | | | | | |
| { Red Sandstones passing down |..|..|..|..|‖ ||..|..|..|..|..|..|..|
| { into Upper Old Red Sandstone|..|..|..|..|‖ ||..|..|..|..|..|..|..|
+-----------------------------------+--+--+--+--+--++--+--+--+--+--+--+--+
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.
In the division of the Plateaux I group all the more copious eruptions
during the Carboniferous period, when the fragmentary materials
generally formed but a small part of the discharges, but when the
lavas were poured out so abundantly and frequently as to form
lava-fields sometimes more than 2000 square miles in area, and to
build up piles of volcanic material sometimes upwards of 3000 feet
in thickness. As already remarked, this phase of volcanic action,
especially characteristic of the earlier part of the Carboniferous
period across the south of Scotland, but not found elsewhere in the
same system in Britain, preceded the type of the Puys. Its eruptions
extended from about the close of the Old Red Sandstone period through
that section of Carboniferous time which was marked by the deposition
of the Calciferous Sandstones, but they entirely ceased before the
accumulation of the Main or Hurlet Limestone, at the base of the
Carboniferous Limestone Series of Scotland. Its stratigraphical
limits, however, are not everywhere the same. In the eastern part of
the region, the lavas appear to be intercalated with, and certainly
lie directly upon, the Upper Old Red Sandstone containing scales of
_Bothriolepis_ and other characteristic fishes, and they are covered by
the Cement-stone group of the Calciferous Sandstones. In the western
district a considerable thickness of Carboniferous strata sometimes
underlies the volcanic sheets. On the other hand, the type of the Puys,
although it appeared in Fife, Linlithgowshire and Midlothian during
the time of the Calciferous Sandstones, attained its chief development
during that of the Carboniferous Limestone, and did not finally die out
in Ayrshire until the beginning of the deposition of the Coal-measures.
i. DISTRIBUTION OF THE PLATEAUX
Notwithstanding the effects of many powerful faults and extensive
denudation, the general position of the Plateaux and their independence
of each other can still be traced. They are entirely confined, as I
have said, to the southern half of Scotland (see Map IV.). In noting
their situations we are once more brought face to face with the
remarkable fact, so strikingly manifested in the geological history of
Britain, that volcanic action has been apt to recur again and again
in or near to the same areas. The Carboniferous volcanic plateaux
were poured out from vents, some of which not impossibly rose among
the extinct vents of the Old Red Sandstone. Another fact, to which
also I have already alluded as partially recognizable in the records
of Old Red Sandstone volcanism, now becomes increasingly evident--the
tendency of volcanic vents to be opened along lines of valley rather
than over tracts of hill. The vents that supplied the materials of
the largest of the Carboniferous volcanic plateaux broke forth, like
the Old Red Sandstone volcanoes, along the broad Midland Valley of
Scotland, between the ridge of the Highlands on the north and that of
the Southern Uplands on the south. Others appeared in the long hollow
between the southern side of these uplands, and the Cheviot Hills
and hills of the Lake District. It is not a question of the rise of
volcanic vents merely along lines of fault, but over broad tracts of
low ground rather than on the surrounding or neighbouring heights. It
can easily be shown that this distribution is not the result of better
preservation in the valleys and greater denudation from the higher
grounds, for, as has been already remarked in regard to the volcanoes
of the Old Red Sandstone, these higher grounds are singularly free
from traces of necks which, had any vents ever existed there, would
certainly have remained as memorials of them. The following summary of
the position and extent of the Plateaux will afford some idea of their
general characters:--
[Illustration: Fig. 107.--View of the escarpment of the Clyde Plateau
in the Little Cumbrae, from the south-west.]
1. The Clyde Plateau.--The chief plateau rises into one of the most
conspicuous features in the scenery of Central Scotland. Beginning at
Stirling, it forms the tableland of the Fintry, Kilsyth, Campsie and
Kilpatrick Hills, stretching westwards to the Clyde near Dumbarton.
It rises again on the south side of that river, sweeping southwards
into the hilly moorlands which range from Greenock to Ardrossan, and
spreading eastwards along the high watershed between Renfrewshire,
Ayrshire, and Lanarkshire to Galston and Strathavon. But it is not
confined to the mainland, for its prolongation can be traced down the
broad expanse of the Firth of Clyde by the islands of Cumbrae to the
southern end of Bute, and thence by the east of Arran to Campbeltown in
Cantyre. Its visible remnants thus extend for more than 100 miles from
north-east to south-west, with a width of some thirty-five miles in
the broadest part. We shall probably not exaggerate if we estimate the
original extent of this great volcanic area as not less than between
2000 and 3000 square miles.
It is in this tract that the phenomena of the plateaux are most
admirably displayed. Ranges of lofty escarpments reveal the succession
of the several eruptions, and the lower ground in front of these
escarpments presents to us, as the result of stupendous denudation,
many of the vents from which the materials of the plateau were ejected,
while in the western portion of the area admirable coast-sections lay
bare to view the minutest details of structure.[414]
[Footnote 414: This plateau is represented in Sheets 12, 21, 29, 30, 31
and 39 of the Geological Survey, and is described in the accompanying
Memoirs as far as published. The eastern part of the Campsie Hills was
surveyed by Mr. B. N. Peach, the western part by Mr. R. L. Jack, who
also mapped the rest of the plateau to the Clyde, and a portion of
the high ground of Renfrewshire and Ayrshire; the rest of the area,
south to Ardrossan, was surveyed by myself. The tract from Stewarton
to Strathavon was surveyed by Mr. James Geikie, the Cumbraes and Bute
by Mr. W. Gunn, and southern Cantyre by Mr. R. G. Symes. The Campsie
Hills have been partly described by Mr. John Young in the first volume
of the _Transactions of the Glasgow Geological Society_. The occurrence
of plants in the tuffs of the east coast of Arran was discovered by Mr.
E. Wunsch. The Campbeltown igneous rocks were described by J. Nicol,
_Quart. Journ. Geol. Soc._ viii. (1852), p. 406. See also J. Bryce's
_Arran and Clydesdale_.]
It will be seen from the map (No. IV.), that the Clyde plateau extends
in a general north-east and south-west direction. It is inclined on
the whole towards the east, where, when not interrupted by faults, its
highest lavas and tuffs may be seen to pass under the Carboniferous
Limestone series. Its greatest elevations are thus towards its
escarpment, which, commencing above the plains of the Forth a little
to the west of Stirling, extends as a striking feature to the Clyde
above Dumbarton. On the south side of the great estuary the escarpment
again stretches in a noble range of terraced slopes for many miles
into Ayrshire. It is well developed in the Little Cumbrae Island (Fig.
107), and in the south of Bute, where its successive platforms of lava
mount in terraces and green slopes above the Firth. Even as far as the
southern coast of Cantyre the characteristic plateau scenery reappears
in the outliers which there cap the hills and descend the slopes (Fig.
108).
While the escarpment side of this plateau is comparatively unfaulted,
so that the order of succession of the lavas and their superposition
in the sedimentary rocks can be distinctly seen, the eastern or dip
side is almost everywhere dislocated. Innumerable local ruptures have
taken place, allowing the limestone series to subside, and giving to
the margin of the volcanic area a remarkably notched appearance. To the
effects of this faulting may be attributed the way in which the plateau
has been separated into detached blocks with intervening younger
strata. Thus a complex series of dislocations brings in a long strip
of Carboniferous Limestone which extends from Johnston to Ardrossan,
while another series lets in the limestone that runs from Barrhead to
near Dalry. In each of these instances, the continuity of the volcanic
plateau is interrupted. To the same cause we owe the occasional
reappearance of a portion of the plateau beyond the limits of the main
mass, as for instance in the detached area which occurs in the valley
of the Garnock above Kilwinning.
[Illustration: Fig. 108.--View of the edge of the Volcanic Plateau
south of Campbeltown, Argyllshire.
The uppermost of the three zones is the volcanic series with its
lava-ridges. The central band is the Upper Old Red Sandstone, lying
conformably beneath the lavas, with its cornstone which has been
quarried. The lowest band, tinted dark, is the Lower Old Red Sandstone,
on which the other rocks rest unconformably.]
Denudation has likewise come into play, not only in reducing the area
of the plateau, but in isolating portions of it into outliers, with or
without the assistance of faults. The site of the Cumbraes and Bute
was no doubt at one time covered with a continuous sheet of volcanic
material, and there appears to be no reason for refusing to believe
that this sheet formed part of that which caps the opposite uplands of
Ayrshire. From the southern end of Bute it is only about seven miles
across to the shore of Arran near Corrie, where the lavas and tuffs
reappear. They are so poorly represented there, however, that we are
evidently not far from the limit of the plateau in that direction. So
vast has been the denudation of the region that it is now impossible to
determine whether the volcanic ejections of Campbeltown, which occupy
the same geological platform as those of Arran, Bute and Ayrshire, were
also actually continuous with them. But as the distance between the
denuded fragments of the volcanic series in Arran and in Cantyre is
only about 20 miles it is not improbable that this continuity existed,
and thus that the volcanic accumulations reached at least as far as the
southern end of Argyllshire, where they now slip under the sea.
[Illustration: Fig. 109.--View of North Berwick Law from the east, a
trachyte neck marking one of the chief vents of the Garleton Plateau.
(From a photograph.)
This illustration and Figs. 119, 133 and 135 are from photographs taken
by Mr. Robert Lunn for the Geological Survey.]
2. The East Lothian or Garleton Plateau.--Some 50 miles to the east
of the Clyde volcanic district, and entirely independent of it, lies
the plateau of the Garleton Hills in East Lothian, which, as its
limits towards the east and north have been reduced by denudation, and
towards the west are hidden under the Carboniferous Limestone series
of Haddington, covers now an area of not more than about 60 square
miles.[415] That the eruptions from this area did not extend far to
the north is shown by the absence of all trace of them among the Lower
Carboniferous rocks of Fife. A relic of them occurs above Borthwick,
in Midlothian, about twelve miles to the south-west of the nearest
margin of the plateau. The area over which the lavas and tuffs were
discharged may not have exceeded 150 square miles. Small though this
plateau is, it possesses much interest from the remarkable variety of
petrographical character in its lavas, from the size and composition of
its necks, and from the picturesque coast-line where its details have
been admirably dissected by the waves. In many respects it stands by
itself as an exception to the general type of the other plateaux.
[Footnote 415: This plateau is represented in Sheets 33 and 41 of the
Geological Survey of Scotland, and is described in the Explanation to
accompany Sheet 33.]
[Illustration: Fig. 110.--The Bass Rock, a trachytic neck belonging to
the Garleton plateau, from the shore at Canty Bay.]
From its proximity to Edinburgh this volcanic area has been often
studied and described. The memoirs of Hay Cunningham and Maclaren
gave the fullest account of it until its structure was mapped by
the Geological Survey. Its scenery differs from that of the other
plateaux chiefly in the absence of the terraced contour which in them
is so characteristic. The peculiar lavas of the Garleton Hills form
irregularly-uneven ground, rising to not more than 600 feet above the
sea. They slope gradually down to the coast, where a succession of
fine sections of the volcanic series has been laid bare for a distance
of altogether about ten miles. Nowhere, indeed, can the phenomena of
the plateau-tuffs and their association with the Carboniferous strata
be so well studied as along the coast-line from North Berwick to
Dunbar. Among the necks of this plateau distinguished for their size,
conspicuous prominence and component materials, the most important are
those that form the conical eminences of North Berwick Law (Fig. 109),
Traprain Law (Fig. 133), and the Bass Rock (Fig. 110).
3. The Midlothian Plateau.--On the same general stratigraphical horizon
as the other volcanic plateaux, a narrow band of lavas and tuffs can
be followed from the eastern outskirts of the city of Edinburgh into
Lanarkshire, a distance of about 23 miles. It is not continuously
visible, often disappearing altogether, and varying much in thickness
and composition. This volcanic tract, which may be conveniently termed
the Midlothian Plateau, is the smallest and most fragmentary of all the
series. Its most easterly outliers form Arthur Seat and Calton Hill at
Edinburgh.[416] Three miles to the south-west a third detached portion
is known as Craiglockhart Hill. After another interval of ten miles,
the largest remaining fragment forms the prominent ridge of Corston
Hill (Fig. 111), whence a discontinuous narrow strip may be traced
nearly as far as the River Clyde.
[Footnote 416: I formerly classed these eminences with the Puys, but I
am now of opinion that they ought rather to be regarded as fragments
of a long and somewhat narrow plateau. Their basic lavas and overlying
sheets of porphyrite repeat the usual sequence of the plateaux, which
is not met with among the Puys. But, as will be pointed out in the
sequel, Arthur Seat in long subsequent time became again the site of a
volcanic vent.]
[Illustration: Fig. 111.--Corston Hill--a fragment of the Midlothian
Plateau, seen from the north.
The volcanic rocks form a cake on the top, the slopes lying across the
edges of the Calciferous Sandstones.]
The well-known Arthur Seat and Calton Hill have been fully described
by Maclaren, and have been the subject of numerous observations by
other geologists.[417] They have been likewise mapped in detail on a
large scale by the Geological Survey, and have been described in the
Survey Memoirs. The rest of the plateau to the south-west is much less
familiar.
[Footnote 417: Maclaren's _Geology of Fife and the Lothians_, 1839, pp.
1-67; and Hay Cunningham, _Mem. Wer. Soc._ vii. pp. 51-62. The plateau
is represented in Sheets 24 and 32 of the Geological Survey, and Arthur
Seat and Calton Hill will be found on Sheet 2 of the Geological Survey
map of Edinburghshire on the scale of 6 inches to a mile.]
In Fig. 112 the great escarpment which descends from the right
towards the centre is the sill of Salisbury Crags. The long dark
crag (Long Row) rising between the two valleys is the lowest of the
interstratified lavas. The slope that rises above it has been cut
out of well-bedded tuffs, on which lie the basalts and andesites in
successive sheets that form all the eastern or left side of the hill.
The rocks around the summit belong to a much later period of volcanic
eruption, and are referred to in Chapter xxxi.
[Illustration: Fig. 112.--View of Arthur Seat from Calton Hill to the
north.]
The rocks of this plateau are comparatively limited in thickness,
and have a much more restricted vertical range than those of other
districts. At Arthur Seat and Corston Hill they begin above the
cement-stones and cease in a low part of the great group of white
sandstones and dark shales which form the upper half of the Calciferous
Sandstones of Midlothian. They do not ascend as high as the Burdiehouse
Limestone, which to the west of Corston Hill is seen to come on above
them. One of their most remarkable features is the manner in which
they diminish to a single thin bed and then die out altogether,
reappearing again in a similar attenuated form on the same horizon.
This impersistence is well seen in the south-western part of the area,
between Buteland, in the parish of Currie, and Crosswood, in the
parish of Mid-Calder. The lowest more basic band may there be traced
at intervals for many miles without the overlying andesitic group. Yet
that andesites followed the basalts, as in other plateaux, is well
shown by large remnants of these less basic lavas left in Arthur Seat
and Calton Hill. On the extreme southern margin of the area also a thin
band of porphyrite with a group of overlying tuffs is seen above the
red sandstones near Dunsyre.[418] The eruptions over the site of this
plateau seem to have been much more local and limited than in the other
plateaux. They appear to have gathered chiefly around two centres of
activity, one of which lay about the position of Edinburgh, the other
in the neighbourhood of Corston Hill. It is worthy of remark that this
tract of volcanic material flanks the much older range of lavas and
tuffs of the Pentland Hills and wraps round the south-western end
of this range, thus furnishing another illustration of the renewal
of volcanic activity in the same region during successive geological
periods.
[Footnote 418: _Explanation, Geol. Surv. Scotland_, Sheet 24, p. 13
(1869).]
4. The Berwickshire Plateau.--Another and entirely disconnected area
occurs in the broad plain or Merse of the lower portion of the valley
of the Tweed.[419] The northern limit of its volcanic tuff occurs in
the River Whitadder above Duns, whence the erupted materials rapidly
widen and thicken towards the south-west by Stitchell and Kelso, until
they die out against the flanks of the Cheviot Hills. The eastern
extension of the area is lost beneath the Cement-stone group which
covers the Merse down to the sea. Its western boundary must once have
reached far beyond its present limits, for the low Silurian ground in
that direction is dotted over with scattered vents to a distance of ten
miles or more from the present outcrop of the bedded lavas, extensive
denudation having cleared away the erupted materials and exposed the
volcanic pipes over many square miles of country. Among the more
prominent of these old vents are the Eildon Hills, Minto Crags and
Rubers Law, as well as many other eminences familiar in Border story.
[Footnote 419: This plateau is shown on Sheets 17, 25, 26 and 33 of the
Geological Survey Map of Scotland. It was chiefly mapped by Prof. James
Geikie and Mr. B. N. Peach.]
The bedded volcanic rocks of this area form a marked feature in the
topography and geology of the district. They rise above the plain of
the Merse as a band of undulating hills, of which the eminence crowned
by Hume Castle, about 600 feet above the sea, is the most conspicuous
height. In the geological structure of this part of Scotland they are
mainly interposed between the Upper Old Red Sandstone and the base of
the Carboniferous system, which they thus serve to divide from each
other. But their lowest sheets appear to be in some places intercalated
in the Old Red Sandstone, so that their eruption probably began before
the beginning of the Carboniferous period. They form a band that curves
round the end of the great Carboniferous trough at Kelso and skirts the
northern edge of the andesites of the Lower Old Red Sandstone in the
Cheviot Hills.
5. The Solway Plateau.--The last plateau, that of the Solway basin,
though its present visible eastern limits approach those reached by
the lavas from the Berwickshire area, was quite distinct, and had
its chief vents at some distance towards the south-west.[420] On
the north-western flanks of the Cheviot Hills, the Upper Old Red
Sandstone is overlain by the lowest Carboniferous strata, without the
intercalation of any volcanic zone, so that there must have been some
intermediate ground that escaped being flooded with lava from the
vents of the Merse on the one hand, and of the Solway on the other.
The Solway lavas form a much thinner group than those of Berwickshire.
From the wild moorland between the sources of the Liddell and the Rule
Water, they run in a narrow and much-faulted band south-westward across
Eskdale and the foot of Annandale, and are traceable in occasional
patches on the farther side of the Nith along the southern flanks of
Criffel, even as far as Torrorie on the coast of Kirkcudbright--a total
distance of about 45 miles. It is probable that this long outcrop
presents merely the northern edge of a volcanic platform which is
mainly buried under the Carboniferous rocks of the Solway basin. Yet it
exhibits many of the chief characters of the other plateaux, and even
occasionally rivals them in the dignity of the escarpments which mark
its progress through the lonely uplands between the head of Liddesdale
and the Ewes Water (Figs. 113, 142).
[Footnote 420: For a delineation of the distribution and structure of
this plateau see Sheets 5, 6, 10, 11 and 17 of the Geological Survey of
Scotland. In the upper part of Liddesdale, Ewesdale and Tarras it was
mapped by Mr. B. N. Peach; in lower Liddesdale and Eskdale by Mr. R. L.
Jack and Mr. J. S. Grant Wilson; from Langholm to the Annan by Mr. H.
Skae; and in Kirkcudbright by Mr. John Horne.]
[Illustration: Fig. 113.--View of Arkleton Fell, part of the Solway
Plateau, from the south-west.
The lower slopes below the single bird, round to the left side of
the sketch, are on the Upper Old Red Sandstone; the line of crag
below the two birds marks the volcanic group above which lies an
outlier of the Calciferous Sandstone series, forming the upper
part of the hill (three birds). The knobs under the four birds
are bosses of andesite.
]
The plateaux of the Merse and the Solway illustrate in a striking
manner the distribution of the volcanic eruptions along valleys and low
plains. The vents from which the lavas and tuffs proceeded are chiefly
to be found on the lower grounds, though these bedded volcanic rocks
rise to a height of 1712 feet (the Pikes) to the west of the Cheviot
Hills. Between the Silurian uplands of Selkirkshire and Berwickshire on
the north and the ridge of the Cheviot Hills on the south, the broad
plain was dotted with volcanic vents and flooded with lava, while to
the south-west the corresponding hollow between the uplands of Dumfries
and Galloway on the one side, and those of Cumberland on the other,
was similarly overspread. The significance of these facts will be more
apparent when the grouping of the vents has been described. We shall
then also be better able to realize the validity of the inference that
the present plateaux are mere fragments of what they originally were,
wide areas having been removed from the one side of them by denudation,
and having been concealed on the other under later portions of the
Carboniferous system.
The same two plateaux likewise supply further illustrations of the
outflow of similar volcanic materials in the same locality at widely
separated intervals of time. They may be traced up to and round the
margin of the great pile of andesites of Lower Old Red Sandstone age
forming the Cheviot Hills.
ii. NATURE OF THE MATERIALS ERUPTED
The volcanic materials characteristic of the plateau-type of eruptions
consist mainly of lavas in successive sheets, but include also various
tuffs in frequent thin courses, and less commonly in thick local
accumulations. The lavas are chiefly andesites in the altered condition
of porphyrites. They vary a good deal in the relative proportions of
silica. Some of them are decidedly basic and take the form of dolerites
and olivine-basalts. With these rocks are occasionally associated
"ultra-basic" varieties, where the felspar almost disappears and the
material consists mainly of ferro-magnesian minerals. The more basic
rocks are generally found towards the bottom of the volcanic series,
where they appear as the oldest flows. In the Garleton Hills lavas of
a much more acid nature are met with--true sanidine-trachytes, which
overlie the porphyrites and basalts of the earlier eruptions.
No adequate investigation has yet been made of the chemical and
microscopic characters of these various rocks, regarded as a great
volcanic series belonging to a definite geological age, though many of
the individual rocks and the petrography of different districts have
been more or less fully described. I cannot here enter into much detail
on the subject, but must content myself with such a summary as will
convey some idea of the general composition and structure of this very
interesting volcanic series.
(_a_) Augite-olivine Rocks (Picrites and Limburgites).--Towards the
bottom of the plateaux there are found here and there sheets of
"ultra-basic" material, some of which appear to be bedded with the
other rocks and to have flowed out as surface-lavas, though it may be
impossible to prove that they are not sills. Thus at Whitelaw Hill, on
the south side of the Garleton Hills, a dark heavy rock is found to
contain hardly any felspar, but to be made up mainly of olivine and
augite. Dr. Hatch has published a description and drawing of this rock,
together with the following analysis by Mr. Player:[421]--
Silica 40·2
Titanic oxide 2·9
Alumina 12·8
Ferric oxide 4·0
Ferrous oxide 10·4
Lime 10·4
Magnesia 11·9
Potash 0·8
Soda 2·7
Loss by ignition 3·4
----
Spec. grav. 3·03. 99·5
[Footnote 421: _Trans. Roy. Soc. Edin._ vol. xxxvii. (1893), p. 116.]
(_b_) Dolerites and Basalts.[422]--These rocks are found both as
interstratified lavas and as intrusive masses. In the former condition
they take a conspicuous place among the sheets of the plateaux,
but especially in the lower parts of the series. They are dark,
often black, usually more or less porphyritic, with large felspars,
frequently also large crystals of augite or olivine, and may be
described as porphyritic olivine-dolerites and olivine-basalts, more
rarely as olivine-free dolerites and basalts. Their groundmass consists
of short laths or microlites of felspar (probably labradorite) and
granules or small crystals of augite and magnetite, with sometimes a
little fibrous brown mica. The large porphyritic felspars are striped
(probably labradorite), the augites are frequently chloritized, and the
olivines are generally more or less serpentinized. But in some cases
all these minerals are as fresh as in a recent basalt. The rocks are
sometimes beautifully columnar, as at Arthur Seat.
[Footnote 422: A general classification of the whole series of Scottish
Carboniferous dolerites and basalts, including both the plateau and
puy examples, will be given in the account of the rocks of the puys in
Chapter XXVI. (p. 418).]
Of these basic lavas conspicuous examples may be seen at Arthur Seat,
Calton Hill and Craiglockhart Hill. The eastern part of Arthur Seat,
known as Whinny Hill, furnishes examples of olivine-dolerites of the
Jedburgh type (p. 418). The beautiful basalt of Craiglockhart with its
large porphyritic olivines and augites has afforded a distinct type
of Carboniferous basalt (p. 418). The same type occurs on the Calton
Hill in the cliff below the gaol. Similar basic lavas are especially
abundant and remarkable in the Clyde plateau near Campbeltown in
Argyllshire, and at the south end of Bute and in the Cumbraes, where
they are associated with an interesting series of dykes and sills.
But even where, as in the Garleton Hills, the lavas are for the most
part somewhat acid in composition, those first poured out, which form
the lowest band, include some typical olivine-basalts, of which a
characteristic example occurs at Kippie Law at the base of the Garleton
plateau (p. 418). It has been described by Dr. Hatch as exhibiting
under the microscope porphyritic crystals of felspar and olivine lying
in a groundmass composed of lath-shaped felspars, granular olivine and
magnetite, and microlitic augite. The olivine, originally the most
abundant constituent, has been converted into a fibrous aggregate
of serpentine. All the minerals are more or less idiomorphic, but
especially the augite, which crowds the groundmass in delicately-shaped
prisms, most of which are terminated at both ends by faces of the
hemi-pyramid. The analysis of this rock is given in the accompanying
table of analyses of Garleton basalts. The Kippie Law type of basalt
was recognized by Dr. Hatch among the Geological Survey collections
from other districts, as in the intrusive bosses of Neides Law and
Bonchester near Jedburgh, and from the Campsie plateau a mile and a
half north of Lennoxtown.[423]
[Footnote 423: _Trans. Roy. Soc. Edin._ vol. xxxvii. (1893), pp.
117-119.]
At Hailes Castle, in the Garleton plateau, the lower basic lavas
include another olivine-basalt somewhat more felspathic than that just
described, and at Markle quarry the rock is still more felspathic and
contains the olivine only in small sporadic grains. The composition of
these basic rocks of the Garleton plateau is shown in the subjoined
table of analyses by Mr. J. S. Grant Wilson:--
+------------+--------------+----------------+----------------+
| | Kippie Law, | Hailes Castle, | Markle Quarry, |
| | specific | specific | specific |
| | gravity 2·8 | gravity 2·76 | gravity 2·7 |
+------------+--------------+----------------+----------------+
|SiO_{2} | 46·01 | 49·07 | 49·54 |
|Al_{2}O_{3} | 19·19 | 19·43 | 22·23 |
|Fe_{2}O_{3} | 5·91 | 10·58 | 9·55 |
|FeO | 6·75 | 2·35 | 1·12 |
|MnO | 0·19 | 0·32 | 0·08 |
|CaO | 8·68 | 7·87 | 7·19 |
|MgO | 6·81 | 4·36 | 2·80 |
|K_{2}O | 1·20 | 0·98 | 1·81 |
|Na_{2}O | 3·27 | 3·31 | 4·56 |
|H_{2}O | 3·07 | 2·26 | 2·42 |
|Total | 101·08 | 100·53 | 101·30 |
+------------+--------------+----------------+----------------+
Olivine-dolerites are more especially developed in the district around
Jedburgh, where they form some of the most prominent bosses, such
as Dunian and Black Law. They show a sub-ophitic groundmass, with
inconspicuous porphyritic crystals, among which those of olivine are
more prominent than the felspars (p. 418).
(_c_) Andesites (Porphyrites).--These are the most abundant lavas
of the plateaux. They occur in every district, and usually form the
main constituents of the pile of volcanic material. They vary in
colour from a pale pinkish grey, through many shades of red, purple,
brown and yellow, to sometimes a dark green or nearly black rock.
Their texture ranges from almost semi-vitreous, through different
degrees of compactness, to open, cellular, slaggy masses. Generally
through their base porphyritic felspars are abundantly disseminated,
sometimes in large, flat, tabular forms, like those of the Lower Old
Red Sandstone already referred to. The amygdaloidal kernels consist of
calcite, zeolites, chalcedony or quartz. It is from the amygdaloids on
either side of the Clyde that the fine examples of zeolites have been
chiefly obtained for which the south of Scotland has long been famed.
Occasionally, as at the south end of Bute, the andesitic lavas display
a marked columnar structure.
Under the microscope these rocks present the usual fine felted
aggregate of felspar microlites, with granules or crystals of magnetite
and sometimes pyroxene. The porphyritic felspars, often large and well
defined, generally contain inclusions of the groundmass. Occasionally
some of the large porphyritic constituents are augite, or pseudomorphs
after that mineral. The alteration of the rocks has oxidized some of
the iron-ore and given rise to the prevalent purplish and reddish tints.
(_d_) Trachytes.--Some of the most remarkable lavas to be found in
any of the plateaux are those which constitute a large part of the
Garleton Hills. They overlie the lower andesite and basalt platform,
which surrounds them as a narrow belt, while they occupy the central
and much the largest part of the area. They have been included among
the porphyrites, but are pale rocks, generally with a yellowish crust,
presenting when quite fresh a grey, compact, felsitic base with large
porphyritic crystals of unstriped felspar.
A number of specimens selected as illustrative of the different
varieties have been analyzed and the results are stated in the
subjoined table.[424] The specific gravity of the rocks is about 2·6.
[Footnote 424: The first two analyses are by Mr. J. S. Grant Wilson,
the last two by Mr. A. Dick jun., and that from Hopetoun Monument by
Mr. G. Barrow. _Trans. Roy. Soc. Edin._ vol. xxxvii. p. 122.]
+------------+--------+--------+----------+-------------+----------+
| | Pepper | Kae | Hopetoun | Phantassie | Bangley |
| | Craig | Heughs | Monument | | Quarry |
+------------+--------+--------+----------+-------------+----------+
| SiO_{2} | 62·61 | 61·35 | 62·50 | 59·50 | 58·50 |
| Al_{2}O_{3}| 18·17 | 16·88 | 18·51 | 18·25 | 21·12 |
| Fe_{2}O_{3}| 0·32 | 0·41 | } 4·39 | 4·81 | 4·68 |
| FeO | 4·25 | 5·01 | } | 2·34 | ... |
| MnO | 0·21 | 0·26 | ... | ... | ... |
| CaO | 2·58 | 2·39 | 2·00 | 2·10 | 3·70 |
| MgO | 0·74 | 0·44 | 0·61 | 0·70 | 0·93 |
| K_{2}O | 4·02 | 6·12 | 6·31 | 6·30 | 5·84 |
| Na_{2}O | 6·49 | 5·26 | 3·44 | 5·03 | 3·90 |
| H_{2}O | 0·80 | 1·70 | 2·10 | 1·60 | 2·00 |
| Total | 100·19 | 99·82 | 99·86 | 100·63 | 100·67 |
+------------+--------+--------+----------+-------------+----------+
The microscopic characters of these rocks, as worked out by Dr. Hatch,
show them to be well-marked and wonderfully fresh sanidine-trachytes.
Some of them are porphyritic, with large crystals of perfectly
unaltered sanidine, sometimes also oligoclase. Small but well-formed
crystals of yellowish-green augite, in addition to the porphyritic
felspars, are imbedded in a fine groundmass composed chiefly of
microlites of sanidine, but with granules of augite and magnetite
plentifully interspersed, and occasionally prisms of apatite. There
is a group in which the porphyritic felspars are scarce or absent.
In these there is little or no ferro-magnesian constituent. Other
trachytes, rather less basic than the augite-bearing varieties here
referred to, occur as bosses in the Garleton Hills district, and are
referred to in the following section (_e_).[425]
[Footnote 425: For fuller petrographical details consult Dr. Hatch's
paper above cited.]
(_e_) Rocks of the Necks.--In the necks connected with the plateaux
other types of massive rock are to be found. Among these perhaps the
most frequent are trachytes, grey to pink in colour, but apt to weather
yellow, exceedingly compact, sparingly porphyritic, and with a peculiar
platy structure and waxy lustre. Rocks of this character also appear as
sills and dykes. Other varieties that occur in similar positions are
more basic in composition, including dark, coarse, granular diabases.
In the Jedburgh district the most frequent rocks are beautiful
varieties of olivine-dolerite and olivine-basalt, which form most of
the prominent hills of the neighbourhood. These bosses are sometimes
associated with agglomerates as at Rubers Law.
In the Garleton Hills district, some of the necks present another
petrographical type which directly connects them with the remarkable
lavas of the higher part of that plateau. Thus the rock of Traprain Law
was ascertained by Dr. Hatch to be a true phonolite. In its general
platy structure and sonorous ring under the hammer it reminds one of
typical phonolites. Under the microscope the rock is found to consist
mainly of small lath-shaped crystals of sanidine arranged in a marked
minute flow-structure, but with few porphyritic crystals. It contains
small crystals and ophitic patches of a light green soda-augite,
with practically no magnesia in it. A small quantity of iron-ore and
isolated granules of apatite are also present, together with patches of
nepheline which, though generally decomposed and replaced with zeolitic
products, occasionally display six- and four-sided crystal-contours. An
analysis of the Traprain phonolite by Mr. Player is subjoined:--[426]
Silica 56·8
Titanic acid 0·5
Alumina 19·7
Ferric oxide 2·2
Ferrous oxide 3·5
Manganous oxide 0·2
Lime 2·2
Magnesia 0·4
Soda 4·3
Potash 7·1
Loss by ignition 2·5
----
Spec. grav. 2·588 99·4
[Footnote 426: _Trans. Roy. Soc. Edin._ vol. xxxvii. p. 125.]
The neck of North Berwick Law was found by Dr. Hatch to be a trachyte,
showing a plexus of lath-shaped sanidines that diminish in size
to minute microlites, but with no porphyritic or ferro-magnesian
constituent. The Bass Rock, though its geological relations are
concealed by the sea, is in all probability another neck of this
district. It is likewise a mass of trachyte, composed almost entirely
of lath-shaped crystals of sanidine, with no ferro-magnesian
constituent, but a good deal of iron ore. It shows none of the large
porphyritic felspars so characteristic of the Garleton Hills lavas, but
it closely resembles the non-porphyritic varieties, particularly the
lavas of Score Hill, Pencraig, Lock Pit Hill, and Craigie Hill.[427]
[Footnote 427: The composition of the rocks of North Berwick Law and
the Bass closely resembles that of the trachytic lavas of the plateau.
For analyses, see Dr. Hatch's Paper, _ibid._ pp. 123, 124.]
(_f_) Tuffs.--The fragmentary ejections of the plateaux vary in texture
from the finest-grained tuffs to coarse agglomerates.[428] As they have
been derived from the explosion of andesite-lavas, they consist mainly
of the debris of these rocks. They are often deep red in colour, as for
example those of Dunbar, but are most frequently greenish. They have
a granular texture, due to the small lapilli of various porphyrites
imbedded in a fine dust of the same material. Grains of quartz,
frequently to be detected even in the finer tuffs, may either have
been ejected from the volcanic vents, or may have been grains of sand
in the ordinary sediment of the sea-bottom. Both at the base and at
the top of the plateau-series, the tuffs are interstratified with and
blend into sandstones and shales, so that specimens may be collected
showing a gradual passage from volcanic into non-volcanic detritus.
In many of the tuffs of the necks fragments of sandstone and other
stratified rocks occur, representing the strata through which the vents
were drilled. In the tuffs of the Eaglesham district pieces of grey
and pink granite have been met with which, if they are portions of an
old granite mass below, must have come from a great depth.[429] In the
coarser tuffs and agglomerates a larger variety of lava-form rocks is
to be found than can be seen among the bedded lavas of the Plateaux.
They include felsites and quartz-porphyries, and more rarely basic
lavas (diabases, etc.).
[Footnote 428: For accounts of these rocks, see Explanation of Sheet 33
_Geol. Surv. Scot._ p. 32; Sheet 22, pp. 11-14; Sheet 31, pp. 14-17.]
[Footnote 429: Explanation of Sheet 22 _Geol. Surv. Scot._ p. 12.]
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.
The structure of the various plateaux presents a general similarity,
with many local variations. Each plateau is built up entirely, or
almost entirely, of sheets of volcanic material, the intercalations
of ordinary sedimentary layers being, for the most part, few and
unimportant, and usually occurring either towards the base or the top
of the volcanic series, though at a few localities interstratifications
of shale and sandstone, marking pauses in the eruptions, occur
throughout that series. The vents of eruption are in some instances
still to be recognized on the plateaux themselves. More usually they
occur on the lower ground flanking the volcanic escarpments, where they
have been laid bare by denudation. Dykes, though seldom abundant, are
associated with the plateaux, while the sills which may mark the latest
manifestations of volcanic energy, though not developed on so large a
scale as among the Cambrian and Silurian volcanoes, can nevertheless be
distinctly recognized.
It is a question of some interest to determine the geological date
of the commencement of the plateau-eruptions by fixing the precise
stratigraphical horizon on which the base of the volcanic series rests.
I have already referred to the fact that this base does not always
lie on the same platform among the Lower Carboniferous formations. In
Berwickshire, as above mentioned, the earliest eruptions appear to have
taken place before the close of the Upper Old Red Sandstone period.
These are the earliest of the whole series. In Cantyre, the lowest
lavas and tuffs come directly upon the sandstones, marls and cornstones
of the Upper Old Red Sandstone. In Stirlingshire, Renfrewshire and
Ayrshire several hundred feet of the Cement-stone group are sometimes
interposed between the bottom of the volcanic rocks and the top of
the Old Red Sandstone. This divergence doubtless indicates that the
eruptions began earlier in some districts than in others. But there
were also probably unequal terrestrial movements preceding, and
perhaps accompanying, the volcanic outbursts. In the case of the Clyde
plateau, for example, if we examine its base in the neighbourhood of
Fintry, we find that it lies upon some 500 feet of Carboniferous white
sandstone, red and green marls and cement-stones, which rest on the
Upper Old Red Sandstone. Yet only eight miles to the eastward, this
considerable mass of strata disappears, and the bottom of the lavas
comes down upon the red sandstones. Five miles still further in the
same direction the volcanic masses likewise die out, and then the
Carboniferous Limestone series is found at Abbey Craig to lie, with
scarcely any representative of the Cement-stone group, on the Upper
Old Red Sandstone (Fig. 114). Again, to the south-west of Fintry, the
zone of cement-stones below the volcanic series continues to vary
considerably in thickness and sometimes almost to disappear, while in
Ayrshire the lavas lie immediately on the red sandstones.
[Illustration: Fig. 114.--Vertical sections of the escarpment of the
Clyde plateau from north-east to south-west.
I. Section at the east end of the Campsie Hills, four miles west
from Stirling. II. Section above Glins, six miles west from
No. I. III. Section at Strathblane Hill, eight miles further
south-west. IV. Section at Lang Craig, east from Dumbarton,
eight miles south-west from No. III. V. Section above Fort
Matilda, Greenock, eleven miles from the previous section and on
the south side of the Clyde.
1. Lower Old Red Sandstone; 2. Upper Old Red Sandstone; 3.
Carboniferous shales, sandstones and cement-stones (the
"Ballagan beds"); 4. Thick white sandstone which comes in above
the Ballagan beds; 5. Andesite lava-sheets; 6. Interstratified
tuffs. The dotted lines connect the base of the volcanic series.
]
These irregularities, not improbably indicative of inequalities
of subsidence and of deposition, may have been connected with the
subterranean disturbances which culminated in the abundant outbreak
of volcanic action. But though the volcanic rocks of the plateaux may
be traced overlapping the underlying strata, no evidence has anywhere
been detected of an unconformability between them and the Lower
Carboniferous or Upper Old Red Sandstone series.
1. BEDDED LAVAS AND TUFFS
The successive sheets of lava in a plateau usually form thin and
widespread beds which are only occasionally separated by intercalations
of tuff or of red marl. In this, as well as in other respects, they
present much resemblance to the lavas of the Tertiary plateaux of
Antrim and the Inner Hebrides. They are generally marked off from each
other by the slaggy upper and under portions of the successive flows,
and this structure gives a distinctly bedded aspect to the escarpments,
as in the Campsie and Largs Hills, or still more conspicuously in
Little Cumbrae (Fig. 107) and the southern end of Bute. Considerable
diversity of structure may be noticed among these sheets. Some
present a compact jointed centre passing up and down into the slaggy
material just referred to; others have assumed a vesicular character
throughout, the vesicles being often elongated in the direction of
flow. Where, as usually occurs, the vesicular is replaced by the
amygdaloidal structure, some of the rocks have long been famous for
the minerals found in their cavities. The beautiful zeolites of the
Kilpatrick and Renfrewshire Hills, for example, may be found in every
large mineralogical collection in the country. Well-developed columnar
structure occasionally appears among the lavas of the plateaux, but
chiefly, so far as I have observed, in the lower or more basic group,
as in the basalts along the east side of the Dry Dam at Arthur Seat.
In each plateau the lavas may be observed to thicken in one direction,
or more usually towards more than one, and this increase no doubt
indicates in which quarters the chief centres of discharge lay. Thus
in the Clyde plateau, several areas of maximum development may be
detected. In the Kilpatrick Hills the total thickness of lavas and
tuffs exceeds 3000 feet (Fig. 120). Above Largs it is more than 1500
feet, rapidly thinning away towards the south. The continuation of the
plateau far to the north-east in the Campsie Fells reveals a thickness
of about 1000 feet of lavas at Kilsyth, which become thicker further
west, but eastward rapidly diminish in collective bulk, until in about
twelve or thirteen miles they disappear altogether, and then, as
already remarked, the Calciferous Sandstone series closes up without
any volcanic intercalation.
In the Solway plateau, the lavas attain a maximum development about
Birrenswark, whence they diminish in bulk towards the north-east and
south-west. The Berwickshire plateau reaches its thickest mass about
Stitchill, whence it rapidly thins away towards the north-east, until
at a distance of some twelve miles it disappears altogether, the last
trace of it in that direction being a band of tuff which dies out in
the Calciferous Sandstones to the north of Duns.
In the Midlothian Plateau, the development of the volcanic series is
more irregular than in any of the others. As already remarked, there
appear to have been at least two chief centres of discharge in this
region, one at Edinburgh and one some fourteen miles to the south-west.
At the former, the volcanic materials attain in Arthur Seat and Calton
Hill a thickness of about 1100 feet. In Craiglockhart Hill, three miles
distant, they are still about 600 feet thick. But beyond that eminence
they cease to be traceable for about eight miles, either because they
entirely die out, or because their dwindling outcrops are concealed
under superficial deposits. As we approach the south-western centre
of eruption around Corston Hill a new volcanic group begins and soon
increases in bulk.
A distinguishing feature of the plateaux is found in the difference
between the lavas that were first erupted and those which followed
them. The earlier eruptions, as above remarked, were generally basic,
sometimes highly so. Thus at Arthur Seat the thick series of lavas
which form the eastern part of the hill have at their base several
sheets of columnar basalt, over which come the andesites that make up
the main mass of the erupted material. In the Calton Hill the same
sequence may be observed. Underneath the andesites of Campbeltown comes
a well-marked and persistent band of olivine-dolerite. Still more basic
are some portions of the earliest lavas of the Garleton plateau where,
as already stated, rocks present themselves composed mainly of olivine
and augite.
It is worthy of notice that where the lavas of a plateau diminish
greatly in thickness or become impersistent, the lowest basic group may
continue while the overlying andesites disappear. This feature has been
already mentioned as well seen in the Midlothian plateau. The thick
group of andesites in Arthur Seat and Calton Hill is not to be found
in the next volcanic eminence, Craiglockhart Hill; but the basalts
with their underlying tuffs continue. In the south-western tract from
Harper Rig to Hare Law in Lanarkshire, the thin lava-band, which can
be found only at intervals along the line of outcrop of the volcanic
series for about nine miles, is a dolerite often highly slaggy in
structure. Again, at Corrie in Arran, the lavas which appear upon the
shore, apparently at the extreme western limits of the Clyde plateau,
are basic rocks.
But whether or not the lowest and more basic lavas appear in any
plateau, the main mass of the molten material erupted has usually
consisted of varieties of andesite. The successive discharges of
these intermediate lavas have flowed out in sheets, some of which
must have been little more than heaps of clinkers and scoriæ, while
others were more fluid and rolled along with a ropy or slaggy surface.
Occasionally the upper part of an andesite shows the reddened and
decomposed character that suggests some degree of disintegration or
weathering before the next lava-stream buried it. The intervals between
successive outflows of these lavas are not, as a rule, defined by any
marked breaks or by the intercalation of other material. In general,
the plateaux are mainly built up of successive sheets of lava which
have followed each other at intervals sufficiently short to prevent
the accumulation of much detritus between them. Thus the Campsie Hills
have the upper 600 feet of their mass formed of admirably-well-defined
sheets of andesite, separated sometimes by thin partings of tuff, but
more usually only by the slaggy vesicular surfaces between successive
flows.
Where the lavas consisted of trachytes they were apt to assume more
irregular forms. Of this tendency the rocks of the Garleton Hills
supply an excellent example. As already stated, their lumpy character
gives to these hills an outline which offers strong contrast to the
ordinary symmetrical terraced contours of the andesitic plateaux.
[Illustration: Fig. 115.--Section of Craiglockhart Hill, Edinburgh.
1. Red sandstones and clays; 2. Green stratified tuffs; 3. Columnar
basalt; 4. Dark shales, ironstones and sandstones, with plants.]
[Illustration: Fig. 116.--Section of the bottom of the Midlothian
Plateau, Linnhouse Water above Mid-Calder Oilworks.
1. Shales and cement-stones; 2. Sandstones; 3. Highly vesicular lava;
4. Tuffs and sandstone bands. _f_, Fault.]
Although tuffs play, on the whole, a comparatively unimportant part
among the constituents of the plateaux, they attain in a few localities
an exceptionally great development, and even where they occur only
as thin partings between the successive lava-flows, they are always
interesting memorials of the volcanic activity of a district. In many
portions of the plateaux, the lowest members of the volcanic series
are tuffs and agglomerates, showing that the eruptions often began
with the discharge of fragmentary materials. Thus in the Midlothian
plateau at Arthur Seat, though the lowest interbedded volcanic sheet
is a dolerite, it is immediately followed by a series of bedded tuffs,
before the main mass of the lavas of that hill make their appearance.
At Craiglockhart Hill, three miles distant (Fig. 115), this lowest lava
is absent, and a group of tuffs about 300 feet thick rests immediately
on the red Carboniferous sandstones and shales, and is overlain by
sheets of columnar basalt. The scoriaceous bottom of the latter rock
may here and there be seen to have cut out parts of the tuff as it
rolled over the still unconsolidated material. In the same district, a
few miles further to the south-west, some interesting sections of the
Midlothian plateau are laid bare in the streams which descend from the
western slopes of the Pentland Hills. I may cite, in particular, those
exposed in the course of the Linnhouse Water. At the railway viaduct
near the foot of Corston Hill, a good section is displayed of the
Cement-stone group--thick reddish, purplish, and greenish-blue marly
shales or clays, with thin ribs and bands of cement-stone and grey
compact cyprid-limestone, as well as lenticular seams and thicker beds
of grey shaly sandstone, sometimes full of ripple-marks and sun-cracks.
These strata, which exactly reproduce the typical lithological
characters of the Cement-stone group of Stirlingshire (Ballagan Beds),
Ayrshire and Berwickshire, are surmounted by a group of reddish,
yellow and brown sandstones, sometimes pebbly and containing a band of
conglomerate. Among the stones in this band, pieces of the radiolarian
cherts of the Lower Silurian series of the Southern Uplands are
conspicuous, likewise pieces of andesite which may have come from the
neighbouring Pentland Hills.
Above these strata lie the lavas of Corston Hill. These are highly
vesicular in some parts, and include bands of tuff which are well
exposed further down the same stream, immediately above the railway
bridge near the Mid-Calder oilworks (Fig. 116). There the lavas, though
much decomposed, show a highly vesicular structure with a rugged upper
surface, in the hollows and over the prominences of which fine flaky
and sandy tuffs have been deposited, while thin seams of vesicular lava
are intercalated among these strata.
[Illustration: Fig. 117.--Section of the top of the Midlothian Plateau
in the Murieston Water.]
The upper part of the same plateau, as exposed in the course of the
Murieston Water, contains evidence that the last eruptions consisted
of tuff. The highly slaggy lava (1 in Fig. 117) is there surmounted
by a thick mass of grey and greenish-white well-bedded granular tuff
(2) including occasional lumps of the basic lava, and passing up into
black shale (3). But that the volcanic eruptions continued during
the accumulation of the shale is proved by the intercalation of thin
partings and thicker layers of tuff in the black sediment. A short way
higher up the Burdiehouse Limestone comes in.
The great lava-escarpment of the Kilpatrick Hills rests on a continuous
band of tuff which is thickest towards the west, near the group of
vents above Dumbarton, while it thins away eastward and disappears in
Strathblane, the lavas then forming the base of the volcanic series.
But perhaps the most remarkable group of basal tuffs is that which
underlies the lavas of the Garleton plateau, to which further reference
will be immediately made.
Extensive accumulations of tuff form in one or two localities a large
proportion of the thickness of the whole volcanic series of a plateau.
Thus in the north-eastern part of Ayrshire, between Eaglesham and the
valley of the Irvine, the lavas die out for a space and give place to
tuffs. During the discharge of the fragmentary materials over that
ground no lava seems to have flowed out for a long period. Ordinary
sediment, however, mingled with the volcanic detritus, and there were
even pauses in the eruptions when layers of ironstone were deposited,
together with thin impure limestone that inclosed shells of _Productus
giganteus_.[430]
[Footnote 430: Explanation of Sheet 22 _Geol. Surv. Scotland_, p. 12.]
In some of the plateaux, particularly within the older part of the
volcanic series, intercalations of ordinary sediment among the tuffs
and lavas show that eruptions occurred only occasionally, and that
during the long intervals between them the deposition of sand and mud
went on as before. Thus the lower 400 feet of the Campsie Fells are
built up of slaggy andesites and thick beds of fine-grained stratified
tuff, with bands of red, green and grey clays and cement-stone and
a zone of white sandstone. The Calton Hill at Edinburgh (Fig. 118)
affords an excellent illustration of the interstratification both of
tuffs and ordinary sediments among the successive outflows of lava.
In the total thickness of about 1100 feet of volcanic material in
this hill, at least eight intervals in the discharge of the lavas
are marked by the intercalation of as many bands of nodular tuff,
together with seams of shale and sandstone more or less charged with
volcanic detritus. The highest lava is immediately covered by the white
sandstones and black shales of the Calciferous Sandstone series.
[Illustration: Fig. 118.--Section of Calton Hill, Edinburgh.
1. Lower Carboniferous sandstones; 2. Basic lava at the bottom
of the volcanic series; 3. Tuff often interstratified with
sandstones and shales; 4. Sheets of andesite-lava frequently
separated by layers of tuff; 5. Shale passing into tuff; 6.
White sandstone and black carbonaceous shales overlying the
volcanic series.
]
The tuffs, as might be expected, are coarsest in texture and thickest
in mass where they approach most nearly to some of the vents of
eruption, and, on the other hand, become finer as they recede from
these. As a rule, they are distinctly stratified, and consist of layers
varying in the size of their component lapilli. Here and there, near
the centres of discharge, the bedding becomes hardly traceable or
disappears, and the fragmentary materials take the form of agglomerate.
In the admirable range of coast-cliffs which extend from North Berwick
to Dunbar, we learn that above the red sandstones at the base of the
Carboniferous system, a thick pile of volcanic ashes was accumulated
by numerous discharges from vents in the immediate neighbourhood. Some
of the explosions were so vigorous that blocks of different lavas,
sometimes a yard or more in length, were thrown out and heaped up in
irregular mounds and hollows. Others discharged exceedingly fine dust,
and between these two extremes every degree of coarseness of material
may be recognized.
As an illustration of the remarkable alternation of coarse and fine
materials, according to the varying intensity of the volcanic paroxysm,
Fig. 119 is here introduced. It represents a portion of the tuff-cliffs
east of Tantallon Castle, and shows at the bottom fine well-stratified
tuff, over which a shower of large blocks of lava has fallen. Fine
detritus is seen to cover the deposits of this shower, and successive
discharges of large stones may be noticed higher up on more or less
well-defined horizons.
The space over which this pyroclastic material can now be traced,
large though it is, does not represent the whole of the original area
included within the range of the discharges of ash and stones, for
much has been removed by denudation. During pauses of various length
between the eruptions, waves and currents washed down the heaps of
volcanic material and distributed ordinary sediment over the bottom
of the water. Hence, abundantly interstratified in some parts of the
tuff, seams of sandstone, blue and green shale, cement-stone and
limestone occur. One thick band of limestone may be traced from near
Tynningham House to Whittinghame, a distance of about four miles;
another patch appears near Rockville House; and a third at Rhodes, near
North Berwick. No fossils have been noticed in these limestones. The
calcareous matter, together sometimes with silica, appears to have been
supplied, at least in part, by springs, which may have been connected
with the volcanic phenomena of the district. The North Berwick
limestone, in particular, has the peculiar carious wavy structure with
minute mamillated interstices so common among sinters. It contains
grains of pyrites, flakes of white kaolin, which probably represent
decayed prisms or tufts of natrolite, and cavities lined with dog-tooth
spar. Some portions give out a strongly fœtid odour when freshly broken.
After the tuffs of the Garleton plateau had accumulated to a depth
of perhaps 200 feet or more, lavas began to be poured out. First
came basic outflows (olivine-basalts with picrites) and andesites
(porphyrites), which form a thin but continuous sheet all over the
area. These were succeeded by the series of trachytes which distinguish
this area. Although the observer remarks the absence there of the usual
terraced arrangement, yet from some points of view, particularly from
the westward, a succession of low escarpments and longer dip-slopes can
be detected among the trachytes of the Garleton Hills, while there can
be no doubt that, in spite of their irregular lumpy contours, these
lavas lie as a great cake above the lower platform of more basic flows
(Fig. 10). There is evidence that during the emission of the trachytes
occasional eruptions of andesite took place. Not the least striking and
interesting feature of this plateau is the size and distribution of its
necks, to which reference will be made in the sequel.
The latest eruption in the Garleton area had ceased and the cones
and lava sheets had probably been buried under sediment before the
commencement of the deposition of the Hurlet or thick Main Limestone of
the Carboniferous Limestone series which lies immediately to the west
of the plateau.
The tuffs of the plateaux are seldom fossiliferous, probably for
the same reason that fossils are scarce in the Cement-stone group
which the plateau volcanic rocks overspread and with which they are
interstratified. Occasional stems and other fragments of vegetation
occur in the plateau-tuffs, as in those of North Berwick, where I have
found a decayed coniferous trunk three feet in length. The green tuff
at the base of the volcanic group of Arthur Seat contains abundant
macerated plant-remains, together with scales of _Rhizodus_ and other
fishes. In some places the plants are represented by trunks or roots,
which appear to remain in their positions of growth. A remarkable
instance of this nature occurs in some bands of tuff in the volcanic
group of the east coast of the Isle of Arran, first brought to notice
by Mr. E. Wunsch,[431] and of which the plants have been so fully
investigated by Professor Williamson.[432]
[Footnote 431: _Trans. Geol. Soc. Glasgow_, vol. ii. (1867) p. 97.]
[Footnote 432: _Phil. Trans._ 1871-1883.]
[Illustration: Fig. 119.--Cliff of tuff and agglomerate, east side
of Oxroad Bay, a little east from Tantallon Castle, East Lothian.]
Plant-remains also occasionally occur in the stratified layers
intercalated among the lavas and tuffs of the plateaux. Some of the
best examples of their occurrence are to be found in the shales and
tuffs interstratified among the enormous pile of volcanic material near
Bowling. Not only does abundant vegetable debris occur distributed
through the detrital strata in the volcanic series at that locality,
but it is even aggregated into thin seams of coal which have been
examined and described by various observers.[433] It may be remarked
that the plant remains thus found intercalated in the volcanic series,
especially when they have been entombed in tuff, have often had their
internal structure admirably preserved, the organic tissues having
been delicately replaced by calcite or other petrifying medium.
The remarkably perfect structure of some of these plants has been
demonstrated by Professor Williamson, especially in the case of the
Arran deposit just referred to. Mr. John Young has also found the
structure well preserved among the _Sigillariæ_ and _Stigmariæ_ that
occur in the stratified intercalations between the lavas near Bowling.