VOLCANOES OF THE LOWER OLD RED SANDSTONE OF "LAKE
CALEDONIA"--_continued_
The Southern Chain--The Pentland Volcano--The Biggar Centre--The
Duneaton Centre--The Ayrshire Volcanoes.
We have now to note the leading features of the groups of volcanic
rocks distributed along the southern line of vents already described.
At least four different centres of eruption may be observed on that
line. Their mutual limits are, on the whole, better seen than those of
the northern line, for from the north-eastern to the south-western end
of the volcanic belt the Old Red Sandstone and rocks of older date are
almost continuously exposed at the surface. The encroaching areas of
Carboniferous formations in Lanarkshire and Ayrshire interrupt but do
not entirely conceal the volcanic tracts.
II. THE SOUTHERN CHAIN OF VOLCANOES IN "LAKE CALEDONIA"
5. _The Pentland Volcano_
Beginning at the north-east end of the line we first come upon
the classic area of the Pentland Hills, for the study of which
the geologist is prepared by the admirable description of Charles
Maclaren,[363] and the earlier geognostical papers of Jameson.[364]
The area mapped in detail is represented in Sheet 32 of the Geological
Survey of Scotland, published in 1859, and described in the Memoir
accompanying that sheet.
[Footnote 363: _A Sketch of the Geology of Fife and the Lothians_,
1839. The detailed descriptions in this work are accompanied with a
map and two plates of sections. In the map all the volcanic rocks are
represented by one colour. In the sections the bedding of the rocks
is shown, and an indication is given of the succession of their chief
varieties.]
[Footnote 364: See specially _Mem. Wernerian Soc._ vol. ii.; also
MacKnight in vol. i. The account of the Pentland Hills by Hay
Cunningham in vol. vii. (1838) is clear but brief.]
When in these early days I surveyed this ground I found it extremely
difficult to understand. Being then myself but a beginner in geology,
and the study of old volcanic rocks not having yet advanced much beyond
its elementary stage, I failed to disentangle the puzzle. Not until
after more than twenty years, largely spent in the investigation of
volcanic rocks elsewhere, had I an opportunity of resurveying the
ground and bringing to its renewed study a wider knowledge of the
subject. A new edition of the map was issued in 1892, and I shall here
embody in my summary the chief results obtained in the course of this
revision.
The most obvious features in the Pentland area are the marked
development of the volcanic rocks at the north end of the chain,
their rapid diminution and disappearance towards the south-west, the
abrupt truncation of the bedded masses by the line of craggy declivity
which forms the northern termination of the hills, and lastly, the
continuation of the volcanic series northward in a totally different
form in the lower eminences of the Braid Hills.
The length of the whole volcanic tract is about eleven miles; its
breadth at the widest northern part is four miles, but from that
maximum it dwindles southwards and dies out in seven miles. Its western
side is in large measure flanked by the unconformable overlap of the
Upper Old Red Sandstone and Lower Carboniferous formations, though
in some places the base of the volcanic series is seen. The eastern
boundary is chiefly formed by a large fault which brings down the
Carboniferous rocks against the volcanic ridge. At the northern end,
this ridge plunges unconformably under the Upper Old Red Sandstone of
the southern outskirts of Edinburgh.
The bedded aspect of the truncated end of the Pentland chain, as seen
from the north, has been already alluded to (p. 281). The rocks dip to
the south-east, hence the lower members of the series are to be found
along the north-west side of the hills.
[Illustration: Fig. 86.--Section across the north end of the Pentland
Hills, from Warklaw Hill to Pentland Mains. Length about five miles.
1. Upper Silurian grits and shales, not seen where the line of
section crosses; 2 2. Andesites and diabases in numerous
interstratified sheets; 2 _s_. Intercalated sandstones and
conglomerates; 3. Felsitic tuffs and breccias and orthophyre
sheets; _n_, Volcanic neck; 4. Lower Carboniferous strata
lying unconformably on and overlapping the volcanic series;
5. Calciferous Sandstones and Carboniferous Limestone series
brought down against the volcanic series by a fault (_f_).
]
It will be noticed from the Geological Survey map that the volcanic
rocks of the main body of the Pentland Hills are arranged in
alternations of somewhat basic and more acid bands. The most basic
sheets are some amygdaloidal diabases at the bottom of the whole
series which make their appearance in Warklaw Hill (Fig. 86). The
greater number of the dark lavas are varieties of andesite, sometimes
tolerably compact, sometimes highly cellular and amygdaloidal. But
interstratified with these are thick sheets of what used to be
called "claystone," a term which here comprised decayed felsites
(orthophyres), and also felsitic tuffs and breccias. The remarkably
acid nature of some of these rocks has been already pointed out.
The total thickness of the volcanic series at the north end of the
hills is about 7000 feet, but as neither the top nor the bottom is
there visible, it may be considerably greater. At these maximum
dimensions the rocks form the high scarped front of the Pentland Hills,
which rises into so prominent a feature in the southern landscape of
Edinburgh. A series of transverse sections across the chain from north
to south will illustrate its structure and history. These I shall here
describe, reserving for subsequent consideration the great vent of the
Braid Hills.
A section taken through the north end of the chain, where the maximum
depth of volcanic material is exposed, presents the arrangement
represented in Fig. 86. It will be seen that the base of the series is
here concealed by the unconformable overlap of the Lower Carboniferous
rocks on the west side, while the top is cut off by the great fault
which on the east side brings down the Midlothian Coal-field.
[Illustration: Fig. 87.--View of the lava-escarpments of Warklaw Hill,
Pentland chain, from the north-west.]
The Lower Carboniferous conglomerates (4) creep over the edge and up
the slopes of the volcanic series of the Pentland Hills. They contain
abundant pebbles of the lavas, and were evidently laid down along a
shore from which the Pentland rocks rose steeply into land. Though
the actual base of the lavas is not seen here, two miles further to
the south highly-inclined Upper Silurian shales and mudstones are
found emerging unconformably from under the volcanic pile, and similar
strata probably underlie Warklaw Hill as indicated in the figure.
The Upper Silurian strata pass up into a lower group of the Lower
Old Red Sandstone, which has also been covered unconformably by the
volcanic series. In these underlying deposits we have evidence of the
pre-volcanic accumulations of the lake, which were broken up and tilted
at the beginning of the volcanic eruptions.
The lowest lavas, consisting of well-marked beds of diabase (2),
present their escarpments to the north-west and dip into the rising
ground, as sketched in Fig. 87. Their characters have been already
noticed in the general petrography of the Old Red Sandstone volcanic
rocks. Dark solid compact portions of them pass rapidly into coarsely
cellular slag, especially along the upper and under parts of the
several sheets. No tuff has been noticed between these basic flows, but
here and there thin lenticular layers of sandstone, lying in hollows
of the lava-sheets, are connected with vertical or highly-inclined
ramifying veins of similar material, with the plains of stratification
passing across the breadth of the veins. These features are an
exact reproduction of those above described in Forfarshire and
Kincardineshire. The amygdales consist of chalcedony, crystallized
quartz and calcite.
Torduff Hill, which rises to the east of Warklaw, consists of a mass of
coarse volcanic breccia or agglomerate (_n_), markedly felsitic in its
materials. It probably forms a neck marking a small volcanic vent, like
some others at the north end of the chain to be afterwards referred to.
In the lower part of Capelaw Hill, the next eminence in an easterly
direction, bedded andesites, with an intercalated band of sandstone and
conglomerate (2_s_), appear and pass under rocks of so decomposing a
kind that no good sections of them are to be found. The hill is covered
with grass, but among the rubbish of the screes pieces of felsite-like
rocks and breccias may be observed. Some of these blocks show an
alternation of layers of felsitic breccia with a fine felsite-like
material which may be a tuff. These rocks, conspicuous by the light
colours of their screes, alternate further up with other dark andesitic
lavas, and run south-westward for about five miles.
Beyond Capelaw Hill, upon a band of these pale rocks, comes a thick
group of sheets of dark andesite, which form the main mass of Allermuir
Hill. They are well seen from the south side and likewise from the
north, dipping towards the south-east at angles of from 35° to 40°, and
weathering along the crest of the hills into a succession of scars and
slopes which show the bedded character of the lavas.
At Caerketton Hill another band of pale material forms the conspicuous
craggy face so familiar in the aspect of the Pentland Hills as seen
from Edinburgh. This band consists of pale felsitic breccia, and
amorphous, compact, much-decayed rock, regarding which it is difficult
to decide whether it should be considered as a fine felsitic tuff,
or as a decomposed felsite. The band is better seen when traced
southwards. The light colour of its screes makes it easily followed by
the eye even from a distance along the hill-tops and declivities.
On the next hill to the south-west, known as Castlelaw Hill, this pale
band of rock is exposed in a few crags and quarries, and its debris,
protruding through the scanty herbage, slips down the slopes. On its
north side the screes display the same felsitic breccias and compact,
decayed felsitic rocks, occasionally showing a structure like the
flow-structure of rhyolite. The breccia which projects in blocks from
the summit of the hill has been quarried immediately below the crest on
the south side, where it overlies a thin intercalated band of a dull,
much-decomposed porphyry.
The breccias are composed almost entirely of thoroughly acid
rock-fragments, as may be judged from the percentage of silica shown
to occur in them. These fragments vary from the finest lapilli up to
angular pieces several inches long. They not infrequently display a
fine and extremely beautiful flow-structure. It is thus quite certain
that there are acid breccias intercalated among the more basic lavas
of the northern Pentlands, and that among the constituents of these
breccias are fragments of felsite or perhaps even lithoid rhyolite.
We may therefore be prepared to find that actual outflows of
felsitic lava accompanied the discharge of these highly-siliceous
tuffs. Unfortunately the manner in which the rocks decay and conceal
themselves under their own debris makes it difficult to separate the
undoubtedly fragmental bands from those which may be true lavas. But an
occasional opening, and here and there a scattered loose block, serve
to indicate that the two groups of rock certainly do coexist in this
pale band, which can be followed through the chain for upwards of six
miles until it is cut off by the eastern boundary fault.
At the south-west end of Castlelaw Hill, where a quarry has been
opened above the Kirk Burn, blocks of felsite may be observed showing
flow-structure on a large scale. The bands of varied devitrification
are sometimes a quarter of an inch broad, and weather out in lighter
and darker tints. Some of them have retained their felsitic texture
better than others, which have become more thoroughly kaolinized. That
these are not deceptive layers of different texture in fine tuffs is
made quite clear by some characteristic rhyolitic structures. The
bands are not quite parallel, but, on the contrary, are developed
lenticularly, and may be observed to be occasionally puckered, and to
be even bent back and folded over as in ordinary rhyolites. There is no
contortion to be observed among the stratified tuffs of the hills. This
irregularity in the layers is obviously original, and can only be due
to the flow of a moving lava.
On the east side of Castlelaw Hill, as shown in Fig. 86, dull reddish
andesites overlie the pale belt of felsitic rocks. Their lower bands
are marked by the presence of well-formed crystals of a dark green
mica. Their central and higher portions consist of porphyrites of the
prevalent type, both compact and vesicular. These lavas continue as far
as any rock can be seen. Beyond the boundary fault, the Burdiehouse
Limestone and oil-shales of the Lower Carboniferous series are met
with, inclined at high angles against the hills. It is impossible to
say how much of the volcanic series has here been removed from sight by
the dislocation.
If now we move three miles further to the south-west and take a second
section across the Pentland Hills, it will be found to expose the
arrangement of rocks represented in Fig. 88. At the western end the
Upper Old Red Sandstones (4) and Lower Carboniferous series (5) are
seen lying unconformably on the upturned edges of the Upper Silurian
shales (1). North Black Hill consists of a large intrusive sheet of
pale felsite (F) that has broken through the Silurian strata and has
in places thrust itself between them and the conglomerates of the
Lower Old Red Sandstone which lie unconformably upon them. In the
neighbouring Logan Burn, at the bottom of the Habbie's Howe Waterfall,
the felsite can be seen injected into the conglomerate. The felsitic
sill of North Black Hill runs for a mile and a half along the western
base of the volcanic series, and has a breadth of about half a mile. It
is the only important intrusive mass in the Pentland Hills.
[Illustration: Fig. 88.--Section across the Pentland Hills through
North Black Hill and Scald Law (length about three miles).]
To the south of the Silurian shales that lie against the southern
flank of North Black Hill, pale felsitic tuffs (3) occur, which are a
continuation of those already referred to as running southwards from
Capelaw Hill. Above them a series of andesites (2), with intercalated
bands of tuff, sandstone and conglomerate (2_s_), occupy the bottom
of the Logan valley and part of the slopes on both sides. In the
thickest band of tuffs, which is well-exposed along the road by the
side of the Loganlee Reservoir, a group of well-bedded strata occurs
from less than an inch to a foot or more in thickness. Generally
they are pale in colour, and are made up of white felsitic detritus,
but with a sprinkling of dull purplish-red fragments, and occasional
larger rounded pieces of different andesites. Some of the rocks might
be called felspathic sandstones. Other bands in the group are dark
purplish-red in tint, and consist mainly of andesitic debris, with
a dusting of white felsitic grains and fragments. There would thus
seem to have been showers both of felsitic and of andesitic ashes and
lapilli.
The dark lavas that overlie the tuffs are likewise well displayed
along the same road-section. They vary rapidly from extremely compact
homogeneous dark blue rocks, that weather with a greenish crust, to
coarse, slaggy masses and amygdaloids.
[Illustration: Fig. 89.--Section from the valley of the Gutterford Burn
through Green Law and Braid Law to Eight-Mile Burn.]
These more basic lavas are a continuation of those of Allermuir Hill,
and, as at that locality, they plunge here also under the same band
of white tuffs, breccias and felsites (3), which has been referred to
as stretching southward from Caerketton Crags. This band must here
be at least 500 feet thick. It forms Scald Law (1898 feet) and the
surrounding summits, and thus occupies the highest elevations in the
Pentland chain. It dips beneath the uppermost group of andesites,
which, as before, are here truncated by the eastern fault (_f_), the
Calciferous Sandstones and Carboniferous Limestone series (6) being
thrown against them.
A third section (Fig. 89), taken two miles still further south, shows a
remarkable attenuation of the volcanic series, and the appearance of a
thick group of conglomerates (2) lying conformably below that series,
but resting on the upturned edges of the upper Silurian shales (1). The
thick Allermuir porphyrites are here reduced to a few thin beds (3)
intercalated among the conglomerates and sandstones, amidst which the
whole volcanic series dies out southward. A detailed section of the
rocks exposed on the western front of Braid Law shows the following
succession:--
White felsitic rocks of Braid Law (4 in Fig. 89).
Coarse conglomerate passing down into sandstone. About 20 feet
visible.
Dark andesite, 4 feet.
Parting of yellow felspathic grit, 8 or 10 inches.
Andesite, 10 feet.
Hard felspathic grit, 6 feet.
Dark green amygdaloidal andesite, 2 feet.
Yellow felspathic sandstone and grit, 2 feet.
Dark green amygdaloidal andesite, 6 feet.
Felspathic grit and red and brown sandstone, 4 feet.
Dark andesite, perhaps 6 or 8 feet.
Great conglomerate with alternating courses of sandstone, rapidly
increasing in thickness southwards.
Above these dwindling representatives of the northern andesitic lavas
comes the continuation of the white band of tuffs and breccias of
Caerketton and Scald Law (4), which in turn dips under the highest
group of andesites. The Carboniferous strata (5) are brought in by the
fault (_f_). In little more than two miles beyond this line of section
the volcanic series disappears, and the Old Red Sandstone for a brief
space consists only of sedimentary deposits.
Besides the remarkable alternation of basic and acid ejections, there
is a further notable feature in the geology of the Pentland Hills.
This volcanic centre presents us with one of the most remarkable vents
anywhere to be seen among the volcanic rocks of Britain. The full
significance of this feature may best be perceived if we advance along
the hills from their south-western end. As has now been made clear, the
volcanic materials which begin about the line of the North Esk near
Carlops rapidly augment in thickness until, in a distance of not more
than seven miles, they attain a thickness of about 7000 feet, and then
form the great scarped front of the hills that look over Edinburgh. But
at the base of that wall their continuity abruptly ceases. The lower
ground, which extends thence to the southern suburbs of Edinburgh, and
includes the group of the Braid Hills, is occupied by another and more
complex group of rocks in which the parallelism and persistence so
marked in the Pentland chain entirely disappear.
This abrupt truncation of the bedded lavas and tuffs marks
approximately the southern margin of a large vent from which at
least some, if not most, of these rocks were probably ejected. The
size of this vent cannot be precisely ascertained on account of the
unconformable overspread of Lower Carboniferous strata. But that it
must have been a large and important volcanic orifice may be inferred
from the fact that the visible area of the materials that fill it up
measures two miles from north-east to south-west, and a mile and a
half from south-east to north-west, thus including a space of rather
more than two square miles. Its original limits towards the north and
south can be traced by help of the bedded lavas that partially surround
it, but on the two other sides they are concealed by the younger
formations. We shall probably not over-estimate the original area of
the vent if we state it at about four square miles.
[Illustration: Fig. 90.--Section across the north end of the Pentland
Hills, and the southern edge of the Braid Hill vent. Length about two
miles.
1 1. Andesites; 2. Fine tuffs, etc., of the Braid Hill vent; 3 3 3.
Agglomerate in lateral necks with felsitic intrusions (4).]
The materials that now fill this important orifice consist mainly of
"claystones," like those of the Pentland series--dull rocks, meagre to
the touch, varying in texture from the rough porous aspect of a sinter
through stages of increasing firmness till they become almost felsitic,
and ranging in colour from a dark purple-red, through shades of lilac
and yellow, to nearly white, but often strikingly mottled. A more or
less laminar structure is often to be observed among them, indicating
a dip in various directions (but especially towards the north) and at
considerable angles. Throughout this exceedingly fine-grained material,
lines of small lapilli may occasionally be detected, also bands of
breccia, consisting of broken-up tuff of the same character, and of
fine "hornstone" and felsite, with delicate flow-structure. Exhibiting
on the whole so little structure, this tract may be regarded as
consisting largely of fine volcanic dust derived from the explosion of
felsitic or orthophyric lavas. Some portions indeed are not improbably
composed of decayed felsites, like those which present so many
difficulties to the geologist who would try to trace their course among
the other lavas and tuffs of the Pentland chain. Various veins, dykes
and small bosses of felsite, andesite and even more basic material,
such as fine dolerite, have been intruded into the general body of the
mass.
On the outskirts of the main vent some subordinate necks may be
observed (3, 3 in Fig. 90), perhaps, like Torduff Hill, already
noticed (Fig. 86), marking lateral eruptions from the flanks of the
great cone. Three of these occur in a line more than half a mile long,
possibly indicating a fissure on the side of the old volcano, running
in a south-westerly direction from the southern edge of the vent. The
smallest of them measures about 500 feet in diameter; the largest is
oblong in shape, its shorter diameter being about 500 feet, and its
longer about 1000 feet. The materials that fill these lateral vents are
coarse agglomerates, traversed by veins and irregular intrusions of a
fine horny or flinty felsite.
From the acid character of most of the rocks that now fill the wide
vent of the Braid Hills it may be inferred that at least the last
eruptions from it consisted chiefly of acid tuffs and lavas. The upper
portion of the volcanic series being everywhere concealed, there are no
means left to verify this inference from an examination of the ejected
material. It may be remarked, however, that the pale yellow sandstones
which lie on the east side of the fault and are exposed in the Lyne
Water above West Linton are in great measure composed of fine felsitic
material.[365] They certainly belong to a higher horizon than the most
southerly lavas of the Pentland Hills, and if they have not derived
their volcanic detritus from the Biggar volcanic area, it may be
assumed that they obtained it from the vent of the Braid Hills. In any
case they show that after the lavas of the southern end of the Pentland
Hills were buried, acid volcanic detritus continued to be abundantly
distributed over this part of the floor of Lake Caledonia.
[Footnote 365: Explanation to Sheet 24 of the Geological Survey of
Scotland, pp. 10, 12.]
6. _The Biggar Centre_[366]
[Footnote 366: This area is included in Sheets 23 and 24 of the
Geological Survey of Scotland. It was mapped and described by myself.
(Explanations of Sheets 23 and 24.) Various parts of it have been
referred to by earlier writers, particularly Maclaren, _Geology of
Fife_, etc., p. 176.]
Another distinct group of volcanoes had its centre about 25 miles
south-westward from the Braid vent, and on the same line as those of
the Pentland Hills. In no part of the basin can the isolation of the
different volcanic clusters be so impressively observed as in the
area to the south-west of these hills. On the one hand, the lavas and
tuffs from the Braid vent die out, and on the other, as we follow the
conglomerates south-westwards, a new volcanic series immediately makes
its appearance.
The space between the last extremity of the Pentland lavas and the
beginning of the Biggar series does not exceed some 500 yards. It will
be remembered that the lower half of the Pentland volcanic series dies
out long before it reaches the southern end of the hills, and that it
is by lavas on the horizon of some of the dark andesites of Allermuir
Hill that the volcanic band is finally prolonged to its extreme
southern limit. The most northerly extension of the Biggar lavas lies
somewhere on the same general platform. But whereas, at the north
end of the Pentland chain, the volcanic sheets rest on the edges of
the Upper Silurian shales, at the south end, several hundred feet of
coarse conglomerate and sandstone intervene between the Silurian shales
and the porphyrites. So rapidly does the bulk of these sedimentary
formations increase that in the course of two miles they must be 3000
feet in thickness below the most northerly of the Biggar lavas just
referred to. But after that point, when they cross the Lyne Water,
they begin to be more and more interstratified with thin sheets of
andesite. These lavas, the beginning of the Biggar series, soon number
nine or ten distinct bands, and so quickly do they usurp the place of
the sedimentary materials that in a distance of not more than twelve
miles they form, where traversed by the river Clyde, the whole breadth
of the visible tract of Old Red Sandstone, to the exclusion of the
conglomerates.
Unfortunately, soon after the lavas make their appearance at the
north end they are in great measure overlapped unconformably by the
red sandstones at the base of the Carboniferous system, but where the
Medwin Water has cut through this covering, they can be seen here and
there underneath on their southerly course.
A section through the northern end of the Biggar series, where the
successive lavas are dying out northwards among the conglomerates,
shows the structure given in Fig. 91. The sedimentary strata consist
largely of debris of andesite, and the lavas include dark red or purple
andesites and also pale felsites, both having the same characters as
those of the Pentland Hills.
[Illustration: Fig. 91.--Section across the northern end of the Biggar
volcanic group, from Fadden Hill to beyond Mendick Hill.
1. Conglomerates and sandstones; 2. Lavas, the lowest being an
olivine-diabase or basalt, the main mass being andesites; 3. Felsites
and tuffs; 4. Upper Old Red Sandstone. _f_, Fault.]
In one important respect the volcanic series in the northern part of
the Biggar area differs from that of the Pentland Hills, for whereas
the uppermost parts of the latter are concealed by faults which bring
down the Carboniferous strata against the base of the hills, the lavas
at the north end of the Biggar district pass conformably under a thick
group of Lower Old Red conglomerates and sandstones. We thus learn
that here the volcanic eruptions ceased long before the close of the
deposition of the Lower Old Red Sandstone. The overlying sedimentary
series is disposed in a long synclinal trough, corresponding in
direction with the general north-easterly strike of the volcanic rocks
which reappear from under the sandstones and conglomerates along its
south-eastern border, where they are abruptly truncated by the fault
(_f_, Fig. 92), which brings them against the flanks of the Silurian
Uplands. It is interesting to note that by this dislocation the lavas
of the Lower Old Red Sandstone are placed almost in immediate contact
with those of the Lower Silurian series, which appear here on the
crests of numerous anticlinal folds that are obliquely cut off by the
fault.
There is yet another feature of interest in the northern part of
the Biggar volcanic centre. While the lowest visible lava is an
olivine-diabase not unlike parts of the Warklaw group of the Pentland
Hills, those which occur above it are partly andesites and partly
orthoclase-felsites. The latter form, among the hills near Dolphinton,
an important group which reaches its greatest development in the Black
Mount (1689 feet). These rocks cover a breadth of more than a mile of
ground, and probably attain a thickness of not less than 2000 feet.
They so closely resemble in their general characters the corresponding
rocks of the Pentland Hills that a brief description of them may
suffice. As in that chain of hills, they are so prone to decomposition
that they are in large part concealed under a covering of their own
debris and of herbage, though their fragments form abundant screes, and
numerous projecting knobs of rock suffice to show the main features of
the lavas and their accompaniments.
The felsites weather into pale yellow and greyish "claystones," but
where fresher sections can be procured they often show darker tints of
lilac and purple. They are close-grained, sometimes flinty, generally
porphyritic with scattered highly-kaolinized white felspars, but
without quartz, often presenting beautiful flow-structure, and not
infrequently showing a brecciated appearance, which in the usual
weathered blocks is hardly to be distinguished from the breccia of
interstratified tuffs.
A locality where some of these features may be satisfactorily examined
is a dry ravine in the farm of Bank, on the south-east side of the
Black Mount. Here the felsite possesses such a perfectly developed
flow-structure as to split into slabs which, dipping S.E. at about 25°,
might deceive the observer into the belief that it is a sedimentary
rock. A fresh fracture shows the laminæ of flow, many of which are
as thin as sheets of paper, to be lilac in colour, some of the more
decomposed layers assuming tints of grey. The felspars and micas are
arranged with their long axes parallel to the lines of flow. The rock
is not vesicular, but it breaks up here and there into the brecciated
condition just referred to. Below the sheet which displays the most
perfect flow-structure, what is probably a true volcanic breccia makes
its appearance. It consists of angular fragments of a similar lilac
felsite, of all sizes up to pieces two or three inches in length,
cemented in a matrix of the same material stained reddish-brown. In
this breccia the stones show little or no flow-structure.
Above the group of felsites and felsitic breccias, grey andesites make
their appearance, like some of those in the Pentland Hills. They are
sometimes extraordinarily vesicular, the vesicles in the body of the
rock being filled with calcite, agate, etc. Such lavas must have been
originally sheets of rough slag. The elongated steam-vesicles have been
partly filled up with micaceous sand and fine red mud that were washed
into crannies of the lava in direct communication with the overlying
water. It is evident that in the northern part of the Biggar centre the
succession of volcanic events followed closely the order observable in
the Pentland Hills, but on a feebler scale. We may suppose that the
lower diabases and andesites are the equivalents of those of Warklaw
and Allermuir, that the felsites and breccias were contemporaneous
with those of Capelaw, Caerketton and Castlelaw, and that the last
andesites made their appearance together with those which form the
highest lavas of the Pentland chain.
[Illustration: Fig. 92.--Section across the southern part of the Biggar
volcanic group from Covington to Culter.
1. Lower Silurian strata; 2. Lower Old Red Sandstone (pre-volcanic
group); 3. Andesite lavas with intercalated sandstones and
conglomerates; 4. Felsite neck. _f_, The boundary-fault on northern
edge of Southern Uplands.]
A section across the southern end of the Biggar volcanic belt shows
less diversity of structure (Fig. 92). The lavas (3) are there found
to flatten out and to spread unconformably over the older part of the
Lower Old Red Sandstone (2), which, as already stated, passes down
into the Upper Silurian shales. A few intercalations of conglomerate,
mainly made up of volcanic detritus, are here and there to be detected
among these lavas. But the chocolate sandstones and conglomerates that
lie unconformably below them contain no such detritus, for they belong
to the pre-volcanic part of the history of Lake Caledonia, and were
here locally upraised, perhaps as an accompaniment of the terrestrial
disturbances that preceded or attended the first outburst of volcanic
energy. Followed south-westwards, the stratigraphical break in the
Lower Old Red Sandstone disappears, and, as will be shown in the
account of the Duneaton centre, a continuous succession can there be
traced from the Upper Silurian shales up into the volcanic series.
An interesting feature in this district is the felsitic boss of
Quothquan already alluded to (p. 288) as rising up through the
andesites, and possibly marking one of the vents of the district. It is
one of a number of felsitic intrusions in this neighbourhood, of which
the most important is Tinto.
[Illustration: Fig. 93.--Section from Thankerton Moor across Tinto to
Lamington.
1_a_. Lower Silurian; 1. Upper Silurian strata; 2. Lower Old Red
Sandstone with two marked bands of conglomerate; 3. Lower Old
Sandstone (pre-volcanic chocolate sandstones); 4. Andesite lavas
with sandstones, conglomerates and tuffs lying unconformably
on No. 3; 5. Felsite sill of Tinto with the smaller sill of
the Pap Craig (6). _f_, Fault bounding the Silurian uplands on
the north. A small patch of the unconformable Lower Old Red
conglomerate is seen on the south side of the fault.
]
A third section taken across Tinto, from Thankerton Moor on the north
side to Lamington on the south, will serve further to illustrate the
great unconformability in the Lower Old Red Sandstone of this district,
and to show the relation of the largest felsitic intrusion to the
surrounding rocks (Fig. 93). The conglomerates and sandstones that
appear on the south slopes of Tinto lie near the base of the Old Red
Sandstone, and if we could bore among the overlying andesites we should
probably meet with the Upper Silurian shales among or conformably
beneath the red passage-beds, as in the Lesmahagow district.
The andesitic lavas creep over the upturned denuded edges of these
strata and sweep round the flanks of Tinto. This conspicuous hill
reaches a height of 2335 feet above the sea, and consists of the
felsitic rocks already described (p. 278). Seen from many points of
view it rises as a graceful cone, distinguished from all the other
eminences around it by the pinkish colour of its screes. In reality it
forms a continuous ridge which runs in an east and west direction for
about five and a half miles, with a breadth of about a mile. Some part
at least, and possibly the whole of this oblong mass, is in the form
of a sill or laccolite which dips towards the north. Conglomerates and
sandstones plunge under it on the southern side, and similar sandstones
overlie it on the north. If there be a neck in this mass, as one might
infer from the shape of the hill, its precise limits are concealed. The
rock does not break through the andesites, and may belong to an earlier
period of eruptivity than the lavas immediately around it. There were
other, though smaller, vents in the immediate neighbourhood. Besides
the cone of Quothquan just referred to, another may be marked by the
felsite boss which overlooks the village of Douglas, four miles to the
south-west of the Tinto ridge, while a third rises into a low rounded
hill close to the village of Symington.
The lavas spread out again to the south-west of Tinto in a group of
hills, until they are interrupted by a fault which brings in the
Douglas coal-field.[367] This dislocation abruptly terminates the
Biggar volcanic band in a south-westerly direction, after extending for
a length of 26 miles, with a breadth of sometimes as much as five miles.
[Footnote 367: See Explanation to Sheet 23 of the Geological Survey of
Scotland (1873), p. 15. This ground was mapped and described by Mr. B.
N. Peach.]
7. _The Duneaton Centre_
Among the high bleak muirlands on the confines of the three counties of
Lanark, Ayr and Dumfries, traversed by the Duneaton Water, a distinct
volcanic area may be traced.[368] Its boundaries, however, cannot be
satisfactorily fixed. It is overspread with Carboniferous rocks both
to the north-east and south-west, so that its rocks are only visible
along a strip about seven miles long and two miles broad. On the
north-western side its lower members are seen lying interstratified
among the sandstones and conglomerates which thence pass down
conformably into the Upper Silurian series (Fig. 94). But although we
thus get below the volcanic series we meet with no vents or sills among
the lower rocks. On the south-east side the highest lavas and tuffs are
overlain by some 5000 feet of red sandstones and conglomerates (2, 3),
which completely bury all traces of the volcanic history.
[Footnote 368: This area was mapped by Mr. B. N. Peach in Sheet 15
of the Geological Survey of Scotland, and is described by him in the
accompanying Memoir.]
The volcanic series in this limited district reaches an estimated
thickness of 4000 feet, built up of purple and green slaggy
andesites, dark heavy diabases (melaphyres) and tuffs, with abundant
interstratification of sandstone, especially towards the base. One of
its chief features of interest is the manner in which it exhibits,
better, perhaps, than can be found in any of the other volcanic areas,
the frequent and rapid alternations of lavas and tuffs with sandstone
and conglomerate. In this part of the region the volcanic discharges
were obviously frequent and intermittent, while at the same time the
transport and deposition of sediment were continuous. This sediment
consisted largely, indeed, of volcanic detritus mixed with ordinary
sand and silt. That these conditions of sedimentation were not wholly
inimical to animal life is shown by the occasional occurrence of
worm-burrows in the ashy sandstones.[369]
[Footnote 369: Memoir on Sheet 15 Geol. Surv. Scotland (1871), p. 22.]
[Illustration: Fig. 94.--Section across the Duneaton volcanic district
from the head of the Duneaton Water to Kirklea Hill.
1. Silurian strata; 2. Lower Old Red Sandstone and conglomerate; 3.
Coarse conglomerate; 4. Andesite-lavas; 5. Stratified tuffs; 6. Spango
granite; 7. Upper Old Red Sandstone.]
The thick accumulation of sandstones and conglomerates above the main
mass of lavas has been derived almost wholly from the waste of the
volcanic rocks (3). Blocks of andesite, well rounded and often from
six to twelve inches in diameter, may be seen in the remarkable band
of coarse conglomerate which runs as a nearly continuous ridge from
the Nith to the Clyde--a distance of more than twenty miles. Nothing
impresses the geologist more, as he wanders over this district,
than the evidence of the prodigious waste which the volcanic series
underwent before it was finally buried. Some part of the detritus may
have been supplied, indeed, by occasional discharges of fragmental
matter, as has already been suggested in the case of the Ochil and
Montrose conglomerates. But the nature of the pebbles in these masses
of ancient shingle shows them to be not bombs, but pieces worn away
from sheets of lava.
That the lavas underlie these piles of detritus and extend southwards
even up to the very edge of the Silurian Uplands is shown by the
rise of a number of successive beds from under the trough into which
the conglomerate has been thrown. These lavas, however, are almost
immediately cut off by the great boundary fault (_f_) which flanks the
Silurian territory. That they are not met with now to the south-east
of the dislocation, where they must once have lain, is an evidence of
the great denudation which the district has undergone. Fig. 94, which
gives a section across the broadest part of the area, from the edge
of the Muirkirk coal-field to the Silurian uplands, shows the general
structure of the ground.
No satisfactory evidence regarding the position of any of the vents of
the period has been met with in this district. The rocks to the south
of the boundary-fault are older than the Old Red Sandstone, and as
they must have been for some distance overspread by the conglomerates,
sandstones and volcanic series, we might hope to find somewhere among
them traces of necks or bosses. The only mass of eruptive rock in that
part of the district is the tract of Spango granite which has been
already referred to in connection with the subject of the vents and
granite protrusions of Old Red Sandstone time. This mass, about four
miles long and two miles broad, rises through Silurian strata, and
by means of the boundary fault is brought against the higher group
of conglomerates and sandstones. The Silurian shales and sandstones
around the granite have undergone contact-metamorphism, becoming highly
micaceous and schistose. The ascent of this granite must have taken
place between the upheaval and contortion of the Upper Silurian strata,
and the deposition of the higher parts of the Lower Old Red Sandstone
of this region. Its date might thus come within the limits of the
volcanic period. But one must frankly own that there is no positive
evidence to connect its production with the volcanic history.
8. _The Ayrshire Group of Vents_
The original limits of the volcanic districts in the remaining portion
of the Old Red Sandstone area on the mainland of Scotland, from
the valley of the Nith to the Firth of Clyde, can only be vaguely
indicated.[370] There is a difficulty in ascertaining the south-western
termination of the Duneaton area, and in deciding whether the lavas and
tuffs of Corsincone in Nithsdale should be assigned to that district or
be placed with those further to the south-west. Between Corsincone and
the next visible volcanic rocks of the Lower Old Red Sandstone there
intervenes a space of six miles, along which, owing to the effect of
the great fault that flanks the north-western margin of the Southern
Uplands, the Carboniferous Limestone and even the Coal-measures are
brought against the Silurian formations, every intermediate series of
rocks being there cut out. It may therefore be, on the whole, better to
include all the volcanic rocks on the left side of the Nith as part of
the Duneaton series. There will still remain a tract of five miles of
blank intermediate ground before we enter upon the volcanic rocks of
Ayrshire.
[Footnote 370: The mapping of the Old Red Sandstone volcanic areas of
Ayrshire for the Geological Survey was thus distributed:--The district
east of Dalmellington was surveyed by Mr. B. N. Peach, that between
Dalmellington and Straiton by Prof. James Geikie, and all from the line
of the Girvan Valley south of Straiton westward to the sea by myself.
The ground is embraced in Sheets 8, 13 and 14 of the Map of Scotland,
and is described in accompanying Explanations.]
Owing to complicated faults, extensive unconformable overlaps of
the Carboniferous formations, and enormous denudation, the volcanic
tracts of Old Red Sandstone age in Ayrshire have been reduced to mere
scattered patches, the true relations of which are not always easily
discoverable. One of these isolated areas flanks the Silurian Uplands
as a belt from a mile to a mile and a half in breadth and about six
miles long, but with its limits everywhere defined by faults. A second
much more diversified district extends for about ten miles to the
south-west of Dalmellington. It too forms a belt, averaging about four
miles in breadth, but presenting a singularly complicated geological
structure. Owing to faults, curvatures and denudation, the volcanic
rocks have there been isolated into a number of detached portions,
between some of which the older parts of the Old Red Sandstone, and
even the Silurian rocks, have been laid bare, while between others the
ground is overspread with Carboniferous strata. A third unbroken area
forms the Brown Carrick Hills, south of the town of Ayr, and is of
special interest from the fact that its rocks have been exposed along
a range of sea-cliffs and of beach-sections for a distance of nearly
four miles. Other detached tracts of volcanic rocks are displayed on
the shore at Turnberry and Port Garrick, on the hills between Mochrum
and the vale of the Girvan, and on the low ground between Dalrymple and
Kirkmichael.
The isolation of these various outliers and separated districts is
probably not entirely due to the effects of subsequent geological
revolutions. More probably some of the areas were always independent
of each other, and their igneous rocks were discharged from distinct
volcanic centres. We may conjecture that one of these centres lay
somewhere in the neighbourhood of New Cumnock, for the lavas between
that town and Dalmellington appear to diminish in thickness and number
as they are traced south-westward. Another vent, or more probably a
group of vents, may have stood on the site of the present hills to the
right and left of the Girvan Valley, south of the village of Straiton.
A third probably rose somewhere between Dailly and Crosshill, and
poured out the lavas of the ridges between Maybole and the Dailly
coal-field. The important centre of eruption that produced the thick
and extensive lavas of the Brown Carrick Hills may be concealed under
these hills, or may have stood somewhere to the west of Maybole. Still
another vent, perhaps now under the sea, appears to be indicated by the
porphyrites of the coast-section between Turnberry and Culzean Bay.
Owing to the complicated structure of the ground, several important
points in the history of the Old Red Sandstone of this region have not
been established beyond dispute. In particular, the unconformability
which undoubtedly exists in that system in the south-west of Ayrshire
has not been traced far enough eastwards to determine whether it
affects the volcanic belt east of Dalmellington, or whether the
break took place before or after the eruption of that belt. West of
Dalmellington it clearly separates a higher group of sandstones,
conglomerates and volcanic rocks from everything older than themselves.
The structure is similar to that in the Pentland Hills, a marked
disturbance having taken place here as well as there after a
considerable portion of the Lower Old Red Sandstone had been deposited.
These earlier strata were upraised, and on their denuded ends another
group of sandstones and conglomerates was laid down, followed by an
extensive eruption of volcanic materials.
It is the upper unconformable series that requires to be considered
here, as it includes all the volcanic rocks of the Old Red Sandstone
lying to the west of the meridian of Dalmellington. The position of
these rocks on their underlying conglomerates is admirably exposed
among the hills between the valleys of the Doon and the Girvan, as well
as on Bennan Hill to the south of Straiton. The andesites rise in a
craggy escarpment crowning long green slopes that more or less conceal
the conglomerates and sandstones below.
Along the coast-sections the structure of the volcanic rocks may be
most advantageously studied. The shore from the Heads of Ayr to Culzean
Castle affords a fine series of exposures, where every feature in
the succession of the lavas may be observed. Still more instructive,
perhaps, is the mile and a half of beach between Turnberry Bay and
Douglaston, of which I shall here give a condensed account, for
comparison with the coast-sections of Kincardineshire and Forfarshire
already described.
The special feature of this part of the Ayrshire coast-line is the
number of distinct andesite sheets which can be discriminated by means
of the thin layers of sandstone and sandy tuff that intervene between
them. In the short space of a mile and a half somewhere about thirty
sheets can be recognized, each marking a separate outflow of lava.
It was in this section that I first observed the sandstone-veinings
which have been described in previous pages, and nowhere are they more
clearly developed. Almost every successive stream of andesite has been
more or less fissured in cooling, and its rents and irregular cavernous
hollows have been filled with fine sand silted in from above. The
connection may often be observed between these sandstone partitions or
patches and the bed of the same material, which overspread the surface
of the lava at the time that the fissures were being filled up.
[Illustration: Fig. 95.--Cavernous spaces in andesite, filled in with
sandstone, John o' Groat's Port, Turnberry, Ayrshire.]
The andesites of the Turnberry shore are of the usual dark purplish-red
to green colours, more or less compact in the centre and vesicular
towards the top and bottom. They display with great clearness the large
empty spaces that were apt to be formed in such viscous slaggy lavas as
they moved along the lake-bottom. These spaces, afterwards filled with
fine sand, now appear as irregular enclosures of hard green sandstone
embedded in the andesite. The example shown in Fig. 95 may be seen in
one of the lavas at John o' Groat's Port.
[Illustration: Fig. 96.--Section of andesites, Turnberry Castle,
Ayrshire.]
From the arrangement of the veins of sandstone it is evident that
irregularly divergent, but often more or less stellate, fissures opened
in the lavas as they cooled. Sometimes, indeed, the molten rock appears
to have broken up into a shattered mass of fragments, as must often
have happened when lavas were poured over the lake-floor. What may be
an instance of this effect is to be seen on the cliff under the walls
of Turnberry Castle, whence the annexed sketch (Fig. 96) was taken. The
lower andesite (_a_) is highly amygdaloidal towards the top, and is
traversed in all directions with irregular veins and nests of sandstone
which can be traced upward to the bed (_b_), consisting of sandstone,
but so full of lumps or slags of amygdaloidal andesite that one is here
and there puzzled whether to regard it as a sedimentary deposit, or as
the upper layer of clinkers of a lava-stream strewn with sand. Above
this fragmentary layer lies another bed of andesite (_c_) of a coarsely
amygdaloidal structure, which encloses patches of the underlying
sandstone. It passes upward, in a space of from four to six feet, into
a mass of angular scoriaceous fragments (_d_) of all sizes up to blocks
18 inches in length cemented in a vein-stuff of calcite, chalcedony and
quartz. This brecciated structure ascends for about 13 or 14 feet, and
is then succeeded by a greenish compact andesite (_e_), which further
north becomes amygdaloidal and much veined with sandstone, passing into
a breccia of lava fragments and sandstone.
[Illustration: Fig. 97.--Lenticular form of a brecciated andesite
(shown in Fig. 96), Turnberry, Ayrshire.]
[Illustration: Map III.]
MAP OF THE VOLCANIC DISTRICTS OF THE LOWER OLD RED SANDSTONE
OF "LAKE CALEDONIA" IN CENTRAL SCOTLAND & NORTH EAST IRELAND
The remarkable brecciated band (_d_) in this cliff, though 13 or 14 feet
in the centre, immediately thins out on either side, until in the
course of a few yards it completely disappears and allows the lavas
_c_ and _e_ to come together, as shown in Fig. 97. We may suppose that
this section reveals the structure of the terminal portion of a highly
viscous lava which was shattered into fragments as it moved along under
water.
No clear evidence of the sites of any of the volcanic vents has yet
been detected in the Old Red Sandstone of Ayrshire. Possibly some
of the numerous felsitic bosses to the south-west of Dalmellington
may partly mark their positions. But the sills connected with the
volcanic series are well exposed in the 12 miles of hilly ground
between Dalmellington and Barr. Two groups of intrusive sheets may
there be seen. The most numerous consist of pale or dark-pink felsite,
often full of crystals of mica. They form prominent hills, such as
Turgeny, Knockskae and Garleffin Fell. The second group comprises
various diabase-sheets which have been intruded near the base of the
red sandstones and conglomerates, over a distance of seven miles on
the north side of the Stinchar Valley above Barr. They attain their
greatest development on Jedburgh Hill, where they form a series of
successive sills, the largest of which unite northwards into one thick
mass and die out southward among the sandstones and conglomerates.