STRUCTURE AND ARRANGEMENT OF THE LOWER OLD RED SANDSTONE VOLCANIC ROCKS
IN THE FIELD
We have now to consider the manner in which the various volcanic
products have been grouped around and within the orifices of discharge.
The first feature to arrest the eye of a trained geologist who
approaches them as they are displayed in one of the ranges of hills in
Central Scotland is the bedded aspect of the rocks. If, for example,
he looks eastward from the head of the Firth of Tay, he marks on the
right hand, running for many miles through the county of Fife, a
succession of parallel escarpments, of which the steep fronts face
northwards, while their long dip-slopes descend towards the south. On
his left hand a similar but higher series of escarpments, stretching
far eastwards into Forfarshire, through the chain of the Sidlaw Hills,
repeats the same features, but in opposite directions. If he stands on
the alluvial plain of the Forth, near Stirling, and looks towards the
north, he can trace bar after bar of brown rock and grassy slope rising
from base to summit of the western end of the Ochil Hills. If, again,
from any height on the southern outskirts of the city of Edinburgh,
he lets his eye range along the north-western front of the chain of
the Pentland Hills, especially towards evening, he can follow the same
parallel banding as a conspicuous feature on each successive hill that
mounts above the plain. Or if, as he traverses the west of Argyllshire,
he comes in sight of the uplands of Lorne, he at once recognizes the
terraced contours of the hills between Loch Awe and the western sea,
presenting so strange a contrast to the rugged and irregular outlines
of the more ancient schist and granite mountains all around (see Fig.
99).
i. BEDDED LAVAS AND TUFFS
On a nearer inspection, the dominant topographical features are found
to correspond with a well-marked stratification of the whole volcanic
series. Where two sheets of andesite are separated by layers of tuff,
sandstone or conglomerate, a well-marked hollow will often be found to
indicate the junction-line; but even where the lavas follow each other
without such interstratifications, their differences of texture and
consequent variations in mode and amount of weathering usually suffice
to mark them off from each other, and to indicate their trend along the
surface in successive terraces. Even where the angles of inclination
are high, the bedded arrangement can generally be detected.
It is in the picturesque and instructive coast-sections, however, that
the details of this bedded structure are most clearly displayed. On
both sides of the country, along the shores of Ayrshire on the west,
and those of Kincardineshire and Forfarshire on the east, the volcanic
group has been admirably dissected by the waves. The lava-beds have
been cut in vertical section, so that their structure and their mode
of superposition, one over another, can be conveniently studied, while
at the same time, the upper surfaces of many of the flows have been
once more laid bare as they existed before they were buried under the
sedimentary accumulations of the waters in which they were erupted.
Though distinctly bedded, the Lavas show little of the regularity and
persistence so characteristic of those of Carboniferous and of Tertiary
time. Some of them are not more than from four to ten feet thick, and
generally, on the coast-cliffs, they appear to be less than fifty feet.
A continuous group of sheets can sometimes be traced for ten miles or
more from the probable vent of discharge.
That many of these lavas were erupted in a markedly pasty condition
may be inferred from certain of their more prominent characteristics.
Sometimes, indeed, they appear as tolerably dense homogeneous masses,
breaking with a kind of prismatic jointing; but more frequently they
are strongly amygdaloidal, and sometimes so much so that, as already
stated, the amygdales form the larger proportion of their bulk. Where
the secondary infiltration-products have weathered out, the rough
scoriform rock looks as if it might only recently have been erupted.
In a few instances I have observed an undulating rope-like surface,
which reminded me of well-known Vesuvian lavas. Usually the top and
bottom of each sheet assume a strikingly slaggy aspect, which here and
there is exaggerated to such an extent that between the more solid and
homogeneous parts of two consecutive flows an intermediate band occurs,
ten or twelve feet thick, made up of clinker-like lumps of slag, the
interspaces being filled in with hardened sand. In some cases these
agglomeratic layers may actually consist in part of ejected blocks; but
the way in which many of the lavas have cooled in rugged scoriaceous
surfaces is as conspicuous as on any modern _coulée_. The loosened
slags, or the broken-up cakes and blocks of lava, have sometimes been
caught up in the still moving, pasty current, which has congealed with
its vesicles drawn out round the enclosed fragments, giving rise to a
mass that might be taken for a breccia or agglomerate. Now and then
we may observe that the upper slaggy portion of a sheet has assumed
a bright red colour from the oxidation of its ferruginous minerals;
and from the contrast it thus presents to the rest of the rock we
may perhaps legitimately infer that the disintegration took place
before the outflow of the next succeeding lava. If this inference
be well founded, and it is confirmed by other evidence which will be
subsequently adduced, it points to the probable lapse of considerable
intervals of time between some of the outflows of lava.
[Illustration:
Fig. 65.--Veins and nests of sandstone due to the washing of
sand into fissures and cavities of an Old Red Sandstone lava.
Turnberry Point, Ayrshire.
]
But perhaps the most singular structure displayed by these lavas is
to be seen in the manner in which they are traversed by and enclose
portions of sandstone. Since I originally observed this feature on
the Ayrshire coast, near Turnberry Point, many years ago,[342] I have
repeatedly met with it in the various volcanic districts of the Lower
Old Red Sandstone across the whole of the Midland Valley of Scotland.
The first and natural inference which a cursory examination of it
suggests is that the molten rock has caught up and carried along pieces
of already consolidated sandstone. But a little further observation
will show that the lines of stratification in the sandstone, even
in what appear to be detached fragments, are marked by a general
parallelism, and lie in the same general plane with the surface of the
bed of lava in which the sandy material is enclosed. In a vertical
section the sandstone is seen to occur sometimes in narrow dykes
with even, parallel walls, but more usually in irregular twisting
and branching veins, or even in lumps which, though probably once
connected with some of these veins, now appear as if entirely detached
from them (Fig. 65). Frequently, indeed, the nodular slaggy andesite
and the sandstone are so mixed up that the observer may hesitate
whether to describe the mass as a sandstone enclosing balls and blocks
of lava, or as a scoriaceous lava permeated with hardened sand. A
close connection may be traced between these sandstone-inclosures and
the beds of sandstone interstratified between the successive lavas.
We can follow the sandy material downwards from these intercalated
beds into the andesites below them. On exposed upper surfaces of the
lava, an intricate reticulation of sandstone veins may be noticed,
in each of which the stratification of the material runs across the
veins, showing sometimes distinct current-bedding, but maintaining
a general parallelism with the bedding of the volcanic sheets and
their fragmentary accompaniments (Fig. 66). If we could remove the
sandstone-veinings and aggregates, we should find the upper surfaces
of these igneous masses to present a singularly fissured and slaggy
appearance, reminding us of the rugged, rent and clinker-loaded slopes
of a modern viscous lava, like some of those in the Atrio del Cavallo
on Vesuvius. There cannot, therefore, be any doubt that the sandstone,
so irregularly dispersed through these lavas, was introduced originally
as loose sand washed in from above so as to fill the numerous rents and
cavernous interspaces of the volcanic rock. A more striking proof of
the subaqueous character of the eruptions could hardly be conceived.
This interesting feature in lavas erupted under water is not confined
to the volcanic series of the Old Red Sandstone. We shall find that it
is hardly less distinct among the basic lavas of the Permian series
both in Scotland and in Devonshire.
[Footnote 342: See Jukes' _Manual of Geology_, 3rd edit. (1872), Fig.
111, p. 276.]
[Illustration: Fig. 66.--Ground-plan of reticulated cracks in the upper
surface of an Old Red Sandstone lava filled in with sandstone. Red
Head, Forfarshire.]
A remarkable exception to the general type of dark basic and
intermediate lavas is furnished by the pale, decomposing felsites of
the Pentland and Dolphinton Hills. Those which issue from the great
eruptive centre of the Braid Hills, alternate with the andesites and
the diabases, gradually diminishing like these in a southward direction
and dying out in some six or seven miles. Beyond the limits of these
lavas, another similar thick group was erupted from a separate vent
at the northern end of the Biggar district near Dolphinton. The same
occurrence has been ascertained also in the area of the Ochil chain.
Fuller reference will be made to these interesting rocks in the
descriptions to be afterwards given of the structure and history of the
volcanic areas of the Pentland Hills, the Biggar centre and the Ochil
Hills.
It is certainly a notable feature in the volcanism of Old Red Sandstone
time that from the same, or from closely adjoining vents, lavas should
be alternately poured forth, differing so much from each other, alike
in chemical composition and petrographical characters, as andesites
and diabases on the one hand, and felsites on the other. Additional
examples, from widely different geological systems, will be cited
in subsequent pages. It will be shown that even in the very latest
volcanic period in Britain, that of older Tertiary time, highly basic
and markedly acid materials were ejected from the same centres of
eruption.
The part taken by the Tuffs in the structure of the ground agrees with
what might have been expected in the accompaniments of extremely slaggy
and viscid lavas. These pyroclastic intercalations are, in most of the
volcanic districts, comparatively insignificant in amount, by far the
largest proportion of solid material ejected from the various vents
having consisted of streams of lava. Round or within some of the vents
the fragmentary materials attain a remarkable coarseness, as may be
seen in the great agglomerates of Dumyat, near Stirling, the largest
of which is more than 700 feet thick. These massive accumulations
doubtless represent a long series of explosive discharges from the
summit of the lava column in one or more adjacent vents. Traced away
from the orifices of emission, the tuffs rapidly grow finer in grain,
less in thickness, and more mixed with ordinary detritus, until they
pass into ordinary non-volcanic sediment or die out between the
lava-sheets.
Good sections, showing the nature and arrangement of the thin
intercalations of andesite-tuff between the successive outpourings
of lava, may be examined on the coast. Thus, near Turnberry Point,
in Ayrshire, upwards of a dozen successive flows of lava, with their
sandy and ashy intervening layers, are exposed in plan upon the beach,
and partly also in section along the cliffs on which the ruins of the
historic castle of Turnberry stand. (Figs. 95, 96, 97). Again, along
the coast of Forfarshire, from the Red Head to Montrose, the numerous
sheets of andesite are separated by layers of dull purplish tuff
passing into conglomerate, with blocks of porphyrite a yard or more in
diameter.
The most remarkable interstratified tuffs in the Lower Old Red
Sandstone are the felsitic varieties. Those which proceed from the
great vent of the Braid Hills, extend south-westwards for eight or nine
miles, and their peculiar materials, mixed with ordinary sediment,
may be traced several miles further. They occur in successive sheets,
which, from a maximum thickness and number at the north end, gradually
thin away southwards, like the felsitic lavas which they accompany, and
from the explosion of which they no doubt were derived. They consist
to a large extent of extremely fine volcanic dust, and since they are
generally much decomposed, it is often, as already remarked, hardly
possible to distinguish between them and the equally decayed felsites.
In some parts of the hills they present a distinct fissile bedding; but
still more satisfactory is the occasional fine brecciated structure
which they assume, when they are seen to consist of angular lapilli of
different felsites.
The amount of volcanic material ejected from the more important vents
was much greater than the height of the present hills would lead us to
suppose. The rocks have generally been tilted into positions much more
inclined than those which they originally occupied, so that to measure
their actual thickness we must take a line approximately perpendicular
to the dip. In this way we ascertain that the accumulated mass of lavas
and tuffs immediately outside the vent at the north end of the Pentland
Hills must be at least 7000 feet thick, for the base of the series is
concealed under the unconformable overlap of the Lower Carboniferous
Sandstones, while the top is cut off by a fault which brings down
the Carboniferous formations against the eastern flank of the hills.
Probably not less voluminous is the pile of ejected material in the
Ochil Hills, where, though the base of the whole is concealed by the
fault which throws down the coal-field, some 6500 feet of lavas, tuffs
and conglomerates can be seen. There were thus, during the time of the
Lower Old Red Sandstone, more than one volcano in Central Scotland
which might be compared in bulk of ejected material to Vesuvius.
[Illustration: Fig. 67.--Section across the volcanic series of
Forfarshire. _a_, conglomerates, sandstones and flagstones; _b_, sheets
of andesitic lava.]
That the eruptions were mainly subaqueous is indicated, as I have
shown, by the intercalated bands of sandstone and conglomerate between
the successive lavas, as these are traced away from the centres of
discharge, and likewise, even more impressively, by the hardened sand
which has been washed into former fissures and crevices in the lava.
But that, in some cases, the volcanic cones were built up above the
surface of the lake may be legitimately inferred from the remarkable
volcanic conglomerates which occur, more particularly in the great
chain of the Ochil and Sidlaw Hills. These thick accumulations of
well-rounded and water-worn blocks are interspersed between sheets of
andesite, and are mainly made up of andesite fragments. Impressive
sections of them may be seen along the Kincardineshire coast. The
conglomerates are sometimes so remarkably coarse, many of their blocks
exceeding two feet in diameter, and so rudely bedded, that it is only
by noting the position of oblong boulders that one can make out the
general direction of the stratification. In their smooth rounded forms,
these blocks resemble the materials of storm-beaches on an exposed
coast. The trituration of the andesite fragments has given rise to a
certain amount of green paste, which firmly wraps round the stones,
and retains casts of them after they have dropped out. It is further
deserving of remark that while in some districts, as in the central
Ochils, the materials were entirely derived from the destruction of
volcanic rocks, in others a large proportion of non-volcanic materials
is mingled with the debris of the lavas. South of Stonehaven, for
example, large boulders of quartzite form a conspicuous feature in the
conglomerates, of which in places they make up quite half of the total
constituents. There can be little doubt, I think, that the materials
of these coarse detrital accumulations were gathered together as
shingle-beaches, and were derived in part from volcanic cones which had
risen above the level of the lake. They seem to suggest considerable
degradation of these cones by breaker-action, whereby blocks of rock a
yard or more in diameter could be rounded and smoothed.
Another inference deducible from such conglomerates, and to which
I have already alluded, is that considerable intervals of time
took place between some of the eruptions. Round the vents, indeed,
where the successive sheets of volcanic material follow each other
continuously, it is perhaps impossible to form any definite opinion
as to the relative chronological value of the lines of separation
between different ejections. But where some hundreds of feet of
coarse conglomerate, chiefly composed of well-rounded andesite
blocks, intervene between two streams of lava, we may conclude that
the interval between the outpouring of these streams must have been
of considerable duration. Other evidence of a similar tendency may
be recognized in the intercalation of groups of varied sedimentary
accumulations, such as those which were deposited over the site of
Eastern Forfarshire and Kincardineshire during the time that elapsed
between two successive floods of lava. In the Den of Canterland, for
example, in the midst of the volcanic sheets we find interesting
evidence of one of these intervals of quiescence, during which
layers of fine olive shales were laid quietly down, while macerated
vegetation, drifting over the lake-bottom, was buried with remains of
fishes, and abundant gally-worms (_Kampecaris_, _Archidesmus_), washed
from the neighbouring land.[343] So undisturbed were the conditions
of deposition that calcareous sediment gathered round some of the
organisms and encased them in limestone nodules.
[Footnote 343: An abundant organism in some of these deposits, named
_Parka_, was first regarded as a plant, was afterwards believed to
be the egg-packets of crustacea, and is now pronounced by competent
authorities to belong to an aquatic plant with creeping stems, linear
leaves and sessile sporocarps.]
In some of the districts the discharges of volcanic material were so
abundant or so continuous that no recognizable deposition of ordinary
sediment has taken place between them. Thus, at the north end of
the Pentland Hills the rocks are entirely of volcanic origin, and
though, as we trace them southwards away from the centre of eruption,
they diminish in thickness, they include hardly any interstratified
sandstones and conglomerates until they finally begin to die out.
The distances to which the lavas and tuffs have been erupted from the
chief vents of a district vary up to 15 or 20 miles. Those of the
Pentland Hills extend from the Braid Hill vent for 10 miles to the
south-west. Those of the Biggar centre stretch for about 16 miles to
the north-east. Those of the Ochil Hills, which probably came from a
number of distinct vents, can be traced for nearly 50 miles.
ii. VENTS
On the whole the actual vents of the volcanoes of Lower Old Red
Sandstone time are less clearly distinguishable than those of
subsequent volcanic periods. This deficiency doubtless arises from the
geological structure of the districts in which the formation is chiefly
developed. Thus, in the great Midland Valley of Scotland, where the Old
Red Sandstone covers a large part of the surface, the vents seem to
have been placed along the central parts of the long trough rather than
among the older rocks on either margin. Hence they are in large measure
buried either under the volcanic and sedimentary accumulations of their
own period or under Carboniferous strata.
[Illustration: Fig. 68.--Section across two necks above Tillicoultry,
Ochil Hills.
1 1, Andesite lavas; 2 2, Tuffs and volcanic conglomerates; 3 3, The
two necks; 4 4, Dykes of felsite, etc.; 5, Coal-measures; _f_, Fault.]
Certain bosses of massive rocks lying well within the volcanic area may
with some confidence be regarded as the sites of eruptive centres. They
occur either singly or in groups, and may be specially noticed along
the chain of the Ochil and Sidlaw Hills. Yet it seems to me probable
that these visible bosses, even if we are correct in regarding them as
marking the positions of true vents, do not indicate the chief orifices
of discharge. If we consider their size and their distribution with
reference to the areas of lava and tuff discharged at the surface,
we are rather led to look upon them as subsidiary vents, the more
important orifices, from which the main bulk of the eruptions took
place, being still concealed under the Carboniferous rocks of the
Midland Valley. The bosses which rise through different portions of the
volcanic series are obviously not the oldest or original vents. The
great felsitic mass of Tinto in Lanarkshire (Fig. 93), indeed, pierces
strata which lie near the base of the Lower Old Red Sandstone, but the
smaller cone of Quothquan in its neighbourhood appears in the midst
of the lavas (Fig. 92). In the south-western part of the Ochil chain
the bosses or necks are chiefly small in size, seldom exceeding half
a mile in diameter. They have been filled sometimes with crystalline,
sometimes with fragmental materials. Two of them, containing the
remarkable granophyric quartz-diorite already referred to, emerge
from among the tuffs in a low part of the series, immediately above
the village of Tillicoultry in Clackmannan (Fig. 68). Two or three
more, which are occupied by orthophyres and quartz-felsites, pierce
the volcanic group a few miles to the west of Loch Leven. The whole of
the visible bosses of the Ochil Hills may be regarded as one connected
group, subsidiary to the main orifices which lay rather further to the
south and west. More particular reference to this district will be made
in the following chapter (p. 303).
Vents which have been filled up with agglomerate, and which thus
furnish the most obvious proofs of their connection with the eruptions
of the volcanic series, though not frequent, may be observed in a
number of the volcanic districts. Their fragmentary materials generally
consist mainly of the detritus of andesites or diabases like those
which form the bedded lavas. But where more acid lavas have risen to
the surface, fragments of felsite may occur more or less abundantly.
In the great vent of the Braid Hills the tuffs and breccias are
almost wholly acid. Non-volcanic materials may often be found in the
agglomerates, and occasionally even to the exclusion of volcanic
detritus. Thus, in the far north of Scotland several examples occur
among the Shetland Isles of necks filled entirely with blocks of
the surrounding flagstones and sandstones. Such cases, as has been
already pointed out, probably represent incompleted volcanoes, when
the explosive vapours were powerful enough to drill orifices in the
crust of the earth and eject the shattered debris from them, but were
not sufficiently vigorous or lasting to bring up any solid or liquid
volcanic material to the surface. These Shetland examples are further
noticed on p. 345.
Necks of agglomerate in the Lower Old Red Sandstone vary in size from
a great orifice measuring two miles across to little plugs only a few
yards in diameter. They may be found in limited numbers in most of
the volcanic districts. No examples have been observed rising through
older rocks than the Old Red Sandstone, all the known instances being
eruptive through some part of the volcanic series or of the sandstones,
and therefore not belonging to the earliest eruptions.
The largest, and in some respects the most interesting, vent in the
Lower Old Red Sandstone, that of the Braid Hills near Edinburgh,
described in Chapter xx., covers an area of more than two square miles,
and is filled with felsitic breccias and tuffs, through which bosses
and veins of acid and basic rocks have been injected. It completely
truncates the bedded lavas and tuffs of the Pentland Hills, and not
improbably marks the chief centre from which these rocks were erupted.
Several smaller necks rise a little beyond its southern margin,
marking, perhaps, lateral cones on the main volcano.
In the small area of Lower Old Red Sandstone lying between Campbeltown
and the Mull of Cantyre, several necks of agglomerate occur, which have
been partly dissected by the waves along the shore, thus revealing
their internal structure and their relation to the surrounding
conglomerates. An account of them will be found at p. 311. One of the
series, which lies back from the coast-line, forms a prominent rounded
hill measuring about 400 yards in its longest diameter. Its general
contour is represented in Fig. 82.
Of the eruptive bosses of massive rock outside the limits of the Old
Red Sandstone which may be plausibly referred to the volcanic phenomena
of the period, though they cannot be proved to be actually part of
them, the most notable are the bosses of granite and other acid
material which rise through the Silurian strata of the Southern Uplands
of Scotland.[344] The largest are the well-known masses of Galloway
(Fig. 69), with which must be grouped the bosses near New Cumnock,
that of the Spango Water (Fig. 94), and those of Cockburn Law and
Priestlaw in Lammermuir, together with a number of masses of felsitic
material scattered over the same region, such as the Dirrington Laws of
Berwickshire (Fig. 70). These bosses present some points of structure
in common with true vents. They come like great vertical columns
through highly-folded and puckered strata, and, as they truncate the
Llandovery and Wenlock formations, they are certainly younger than the
greater part of the Upper Silurian series. They must be later, too,
than the chief plication and cleavage of these strata; but they are
older than the Upper Old Red Sandstone or basement Carboniferous rocks
which contain pebbles of them. Their date of eruption is thus narrowed
to the interval between the later part of the Upper Silurian period
and the beginning of the Upper Old Red Sandstone. I have myself little
doubt that they are to be associated with the volcanic epoch we are now
considering, as it was the only known great episode of igneous activity
in this region during the interval within which the protrusion of these
granites must have taken place. In the Cheviot Hills, indeed, we have
evidence of the eruption of a large mass of augite-granitite through
the porphyrite-lavas of the Lower Old Red Sandstone, with abundant
veins projecting from it into them, as will be narrated in later
pages.[345]
[Footnote 344: I suggested this possible connection many years ago in
_Trans. Geol. Soc. Edin._ vol. ii. (1874) p. 21.]
[Footnote 345: The volcanic geology of the Cheviot Hills is described
by Mr. Teall, _Geol. Mag. for 1883_, p. 106; and by Mr. Clough, _Mem.
Geol. Survey_, "Geology of the Cheviot Hills," Sheet 108 N.E., 1888, p.
24.]
[Illustration: Fig. 69.--Section of the granite core between Merrick
and Corscrine.
_a_, Silurian greywackes, grits and shales; _b_, granite.]
Not improbably many other granite protrusions throughout the British
Isles are to be referred to the volcanic operations of the Lower Old
Red Sandstone. Such are those of the Lake District, notably that
of Shap,[346] the granites of Newry and Leinster in the east of
Ireland, which are later than the Silurian rocks and older than the
Carboniferous Limestone, and the younger Grampian granites, which
pierce the presumably Arenig belt along the Highland border. Whether
or not these granitic protrusions were connected with superficial
volcanic discharges of which no remains have survived, they seem to
indicate the wide extent and remarkable vigour of the subterranean
igneous action of this geological period.
[Footnote 346: See the descriptions of the Shap granite by Messrs. Marr
and Harker, _Quart. Journ. Geol. Soc._ xlvii. (1891) p. 266, and xlix.
(1893) p. 359.]
[Illustration: Fig. 70.--Section across the three Dirrington Laws,
Berwickshire.
_a_, Upper Silurian strata; _b_, Necks probably of Lower Old Red
Sandstone age; _c_, Upper Old Red Sandstone lying unconformably both on
_a_ and _b_.]
Viewed as a whole, the materials which now occupy the vents of the
volcanic chains in the Lower Old Red Sandstone of the British Isles
are more acid than the lavas erupted at the surface. In the Pentland
district, indeed, and in some other areas this acid material was
ejected at intervals in abundant discharges of dust and lapilli and in
outflows of felsitic lavas, while between these successive discharges
copious streams of diabasic and andesitic lavas, either from the same
or from some closely-adjoining vent, were poured out. Throughout the
whole region, however, as a closing phase of the volcanic history,
the acid magma rose after the outpouring of the more basic lavas and
filled such chimneys of the volcanoes as were not already blocked with
agglomerate. It was probably after these pipes were plugged that the
final efforts of volcanic energy were expended in the protrusion of the
acid material as sills between the bedding-planes of the surrounding
rocks, and as dykes and veins in and around the vents.
iii. SILLS AND DYKES
Nowhere throughout the volcanic tracts of the Lower Old Red Sandstone
is there any such development of sills as may be seen beneath the
Silurian volcanic sheets of North Wales. Those which occur are most
abundant in the Lanarkshire district, to the north-west and south-west
of Tinto, and in the south of Ayrshire. From the village of Muirkirk to
the gorge of the Clyde, below the Falls, the Upper Silurian and Lower
Old Red Sandstone strata are traversed by numerous intrusive sheets
of pink and yellow felsite, quartz-porphyry, minette, lamprophyre and
allied rocks, which are no doubt to be regarded as part of the volcanic
phenomena with which we are here concerned. In the south of Ayrshire,
between the villages of Dalmellington and Barr, there is a copious
development of similar sills, especially along one or more horizons
near the base of the Old Red Sandstone. Garleffin Fell, Glenalla Fell,
Turgeny and other heights are conspicuous prominences formed of these
rocks; above the sills lie thick conglomerates and sandstones on which
the great andesite-sheets rest.
In the Pentland Hills, as will be described in Chapter xx., a massive
felsitic sill forms a conspicuous feature along the north side of the
chain, and there are probably others which have not yet been separated
from the felsitic tuffs and orthophyres which they so much resemble.
Perhaps the most remarkable acid sills in the Old Red Sandstone of
Britain are those which occur at the extreme northern end of the region
among the volcanic phenomena of the Shetland Isles (Figs. 71, 72). The
largest of them, consisting mainly of granite and felsite, is believed
to reach a length of 20 and a breadth of from three to four miles.[347]
[Footnote 347: Messrs. B. N. Peach and J. Horne, _Trans. Roy. Soc.
Edin._ xxxii. (1884), p. 359.]
[Illustration: Fig. 71.--Section of Papa Stour, Shetlands, showing
sill of spherulitic felsite traversing Old Red Sandstone and bedded
porphyrites (Messrs. Peach and Horne).
1. Red sandstones and flagstones; 2. Purple diabase-porphyrites; 3.
Great sheet of pink spherulitic felsite.]
[Illustration: Fig. 72.--Section across Northmavine, from Okrea Head to
Skea Ness, Shetland, showing dykes and connected sill of granite and
felsite (Messrs. Peach and Horne).
1. Schists, etc.; 2. Serpentine; 3. Granite and quartz-felsite; 4.
Breccia of serpentine fragments; 5. Bedded andesites and tuffs. _f_,
Fault.]
A group of sills composed of a bright red quartz-porphyry has been
traced along the southern flanks of the Highlands for upwards of
18 miles.[348] This rock, already referred to as the "Lintrathen
porphyry," lies chiefly among the conglomerates and sandstones, but
also intersects the lavas, and may be later than the Old Red Sandstone
(p. 277). An extension of it is found even on the north side of the
boundary fault, cutting the andesites which there lie unconformably on
the schists.
[Footnote 348: See Sheet 56 of the Geological Survey of Scotland.]
Examples, however, occur of sills much less acid in composition. In
the Dundee district, for instance, the intrusive sheets are andesites
and diabases. They send veins into and bake the sandstones among which
they have been intruded, and are sometimes full of fragments of such
indurated sandstone, as may be well seen on the northern shore of the
Firth of Tay, west of Dundee.
A conspicuous characteristic of most of the volcanic tracts of the
Lower Old Red Sandstone is the comparative scarcity of contemporaneous
dykes. In the band of acid sills between Muirkirk and the Clyde, a
considerable number of dykes have been mapped, which must be regarded
as due to the same series of movements and protrusions of the magma
that produced the adjacent sills. Throughout the length of the Southern
Uplands dykes of felsite, minette, lamprophyre, vogesite and other
varieties, which may also be connected with the volcanic phenomena of
the Lower Old Red Sandstone, not infrequently occur among the Silurian
rocks. On the Kincardineshire coast, south of Bervie, a number of dykes
of pink quartz-porphyry traverse the conglomerates and sandstones. The
coast south of Montrose displays some singularly picturesque sections,
where a porphyry dyke running through andesitic lavas and agglomerates
stands up in wall-like and tower-like projections. On the shore at
Gourdon, as well as inland, intrusive dykes of serpentine occur. A line
of these, possibly along the same fissure, has been traced for more
than a dozen of miles from above Cortachy Castle to near Bamff. But
there is no evidence to connect them with the volcanic phenomena of the
Old Red Sandstone. Not improbably they belong to a later geological
period.
One would expect to meet with a network of dykes in and around the
volcanic vents; but even there they are usually not conspicuous either
for number or size. In the great vent of the Braid Hills only a few
have been noticed. In the Ochil Hills groups of dykes of felsite and
andesite may be observed, especially near the necks. They are fairly
numerous in the neighbourhood of Dollar (see Fig. 68). One of the most
abundant series yet observed traverses the tract around the granite
boss of the Cheviot Hills, from which many dykes of granite, felsite,
quartz-porphyry and andesite radiate. This district will be more fully
referred to in Chapter xxi. Another remarkable development of dykes
occurs in Shetland (Fig. 72), where they consist of granite, felsite
and rhyolite, and are associated with the acid sills above referred to.