VOLCANOES OF THE LOWER OLD RED SANDSTONE OF "LAKE CALEDONIA"
Description of the several Volcanic Districts: "Lake Caledonia,"
its Chains of Volcanoes--The Northern Chain: Montrose Group,
Ochil and Sidlaw Hills, the Arran and Cantyre Centre, the Ulster
Centre.
I now propose to give some account of each of the districts which have
been separate areas of volcanic action during the time of the Lower
Old Red Sandstone, tracing its general structure, the arrangement and
sequence of its volcanic rocks and the history of its eruptions. As by
far the most varied development of the Old Red Sandstone is to be found
in the great Midland Valley of Scotland, and as it is there that the
remarkable volcanic phenomena of the system have been most abundantly
displayed and are most clearly recorded, I shall begin my description
of the volcanic eruptions of the Lower Old Red Sandstone with a
detailed account of the different centres of volcanic activity in that
region. The phenomena are so fully displayed there that a more summary
treatment of the subject will suffice for the other regions.
Under the designation of "Lake Caledonia," as already remarked, I
include the whole of the Midland Valley of Scotland between the
Highlands and the Southern Uplands, likewise the continuation of the
same ancient hollow by Arran and the south of Cantyre across the
north of Ireland to Lough Erne.[349] Throughout most of the area thus
defined, the present limits of the Lower Old Red Sandstone are sharply
marked off by large parallel faults. On the north-west side one, or
rather a parallel series, of such dislocations runs from Stonehaven
along the flank of the Highland mountains to the Clyde, thus traversing
the whole breadth of the island. On the south-east side another
similar series of faults, which there skirts the edge of the Silurian
tableland, has nearly the same effect in precisely defining the margin
of the Old Red Sandstone. As thus limited, the tract has a breadth of
about 50 miles in Scotland, while the portion of it now visible in the
British Isles has an extreme length of about 280 miles (Map III.).
[Footnote 349: My own investigations of this region have been continued
over an interval of forty years. Besides personally traversing every
portion of it, I have mapped in detail, for the Geological Survey, many
hundreds of square miles of its area from the outskirts of Edinburgh
south-westwards into Lanarkshire, in Ayrshire, and in the counties of
Fife, Perth and Kinross. The Geological Survey maps of the volcanic
tracts of the Sidlaw Hills have been prepared by my brother, Prof.
James Geikie, and Messrs. H. M. Skae and D. R. Irvine. The Western
Ochils were mapped chiefly by Mr. B. N. Peach, partly by Prof. J.
Young, Mr. R. L. Jack and myself; the Eastern Ochils were surveyed
mainly by Mr. H. H. Howell; while the volcanic belt between the tracts
mapped by me in Lanarkshire and in Ayrshire was chiefly traced out by
Mr. Peach. As a rule, each of these geologists has described in the
Survey Memoirs the portions of country surveyed by him.]
But though the boundary-faults determine, on the whole, the present
limits of the tract of Old Red Sandstone, they do not necessarily
indicate the shore-lines of the sheet of water in which that great
series of deposits was laid down. They point to an enormous subsidence
of the tract between them--a prolonged and extensive sagging of the
strip of country that stretches across the Midland Valley of Scotland
into the north of Ireland.[350] This downward movement began as far
back as the close of the Silurian period, but the marginal fractures
and the disruption and plication of the thick masses of sandstone and
conglomerate which were accumulated in the lake chiefly took place
after the close of the period of the Lower Old Red Sandstone. I think
we may reasonably connect these movements with the general sinking of
the area consequent upon the enormous outpouring of volcanic materials
during that period.
[Footnote 350: In some of the dislocations along the Highland border,
the Old Red Sandstone is bent back upon itself, and the older schists
are thus made to recline upon it, as if there had been a push over from
the Highland area.]
Along both the northern and southern margins of the basin there occur,
on the farther side of the boundary faults, outlying patches of Lower
Old Red Sandstone that rest unconformably on the rocks forming the
flanks of the hills. These areas possess a peculiar interest, inasmuch
as they reveal some parts of the shore-line of the lake, and show the
relation between the earlier rocks and the sediments of the Old Red
Sandstone. We learn from them that the shore-line was indented with
wide bays, but nevertheless ran in a general north-easterly direction.
It thus corresponded in trend with the present Midland Valley, with the
axes of plication among the schists of the Highlands as well as among
the Silurian rocks of the Southern Uplands, and with the subsequent
faulting and folding of the Old Red Sandstone.
[Illustration: Fig. 73.--Section at the edge of one of the bays of
Lower Old Red Sandstone along the northern margin of Lake Caledonia,
near Ochtertyre.
_a_, slates and phyllites; _b_, volcanic conglomerates; _c_,
andesite-lava.]
I may remark in passing that the conglomerates and other associated
materials which have been preserved in these bays and hollows beyond
the lines of the great faults, though they lie unconformably on the
rocks beneath, are not the basement portions of the Old Red Sandstone.
On the contrary, where their probable stratigraphical horizons can be
recognized or inferred, they are found to belong to parts of the series
considerably above the base of the whole. They point to the gradual
sinking of the basin and the creeping of the waters with their littoral
shingles further and further up the slopes of the hills on either side
(Fig. 73).
But this is not all the evidence that can be adduced to show that
the limits of the lake extended considerably beyond the lines of
dislocation between which the present area of Old Red Sandstone mainly
lies. No one can look at the noble escarpments of the Braes of Doune
on the one side (Fig. 74), or walk over the upturned conglomerates and
andesites which flank the Lanarkshire uplands on the other, without
being convinced that if the effects of the boundary faults could be
undone, so as to restore the original structure of the ground, the
prolongations of the rocks, now removed by denudation, would be found
sweeping far into the Highlands on the north and into the Silurian
Uplands on the south.
[Illustration:
Fig. 74.--Craig Beinn nan-Eun (2067 feet), east of Uam Var, Braes
of Doune. Old Red Conglomerate, with the truncated ends of the
strata looking across into the Highlands; moraines of Corry
Beach in the foreground.
]
If the area of "Lake Caledonia" were taken to be defined by the
boundary faults, it covered a space of about 10,000 square miles.
But, as we know that it certainly stretched beyond the limits marked
by these faults, it must have been of still greater extent. We shall
probably not exaggerate if we regard it as somewhat larger than the
present Lake Erie, the superficies of which is about 9900 square miles.
In this long narrow basin the remarkable volcanic history was enacted
of which I now proceed to give some account.
The Lower Old Red Sandstone of Central Scotland may be conveniently
divided into three great groups, each of which marks a distinct epoch
in the history of the basin wherein they were successively accumulated.
The lowest of these groups indicates a time of quiet sedimentation
during which the basin was defined by plication of the terrestrial
crust, and when, by the same subterranean movements, some parts of
the floor of the lake were pushed upward above water, and were then
denuded and buried. The middle group consists largely of volcanic
rocks. It points to the existence of lines of active volcanic cones
situated along the length of the lake. The uppermost group records the
extinction of volcanic action and the gradual obliteration of the lake,
partly by the pouring of sediment into it, and partly no doubt by the
continued terrestrial movements which had originally produced the basin.
It is evident from these records that though volcanic activity
continued vigorous for a vast period of time, it had entirely ceased
in "Lake Caledonia" long before the last sediments of the Lower Old
Red Sandstone were laid down. The great cones of the Ochil Hills, for
example, sank below the waters of the lake in which they had long been
a conspicuous feature, and so protracted was the subsidence of the
lake-bottom that the site of these volcanoes was buried under 8000 or
9000 feet of sandstones and conglomerates, among which no trace of any
volcanic eruptions has yet been found. The sagging of the terrestrial
crust over an area from which such an enormous amount of volcanic
products had been discharged would doubtless be a protracted process.
Long after the subsidence of the lake-bottom and the accumulation of
its thick mass of sediments, after even the entire effacement of the
topography and the deposition of the thick Carboniferous formations
over its site, the downward movement showed itself in the production of
gigantic north-east faults, and the sinking of the Carboniferous rocks
for several thousand feet. These dislocations, as was natural, have run
through the heart of some of the volcanic groups, carrying much of the
evidence of the ancient volcanoes out of sight, and leaving us only
fragments from which to piece together the records of a volcanic period
which is by no means the least interesting in the geological history of
this country.
Confining our attention for the present to the records of the middle or
volcanic group, we find evidence of a number of distinct clusters of
volcanoes ranged along the whole length of the basin. The independence
of these volcanic districts may be inferred from the following
facts:--1st, The actual vents of discharge may in some cases be
recognized; 2nd, Even where these vents have been buried, we may often
observe, as we approach their probable sites, a marked increase in the
thickness of the volcanic accumulations, as well as a great development
of agglomerates and tuffs; 3rd, Traced in opposite directions, the
volcanic materials are found to thin away or even to disappear. Those
from one centre of discharge may be observed now and then to overlap
those from another, but the two series remain distinct.
Reasoning from these data and studying the distribution of the various
volcanic areas, we are led to recognize the former existence of two
parallel chains of vents, running along the length of the lake at a
distance from each other of somewhere about twenty miles. They may be
conveniently distinguished as the northern and the southern chain.
The northern band runs from the coast-line near Stonehaven
south-westward through the Sidlaw and Ochil Hills. It is then abruptly
truncated by a large fault and by the unconformable superposition of
the Carboniferous formations. But 60 miles further to the south-west,
where the Old Red Sandstone comes out on the west side of the Firth of
Clyde, a continuation of the volcanic band has recently been detected
by Mr. W. Gunn of the Geological Survey in the Island of Arran.
Twenty-five miles still further in the same direction a much ampler
development of the volcanic rocks occurs to the south of Campbeltown
in Cantyre. If we cross the 22 miles of sea that separate the
Argyllshire coast-sections from those of Red Bay in Ireland, we find
near Cushendall a repetition of the Scottish volcanic conglomerates,
while still further along the same persistent line, some 50 miles into
the interior, the hills of Tyrone include sheets of lava precisely
like those of Central Scotland. The total length of this northern
chain of volcanoes is thus not much less than 250 miles, and as its
north-eastern end is now cut off by the North Sea it must have been
still longer. It ran parallel to the north-western coast-line of the
lake, at a distance which, over the site of the Midland Valley of
Scotland, seems to have varied from 10 to 20 miles, but which greatly
lessened further to the south-west.
At a distance of some twenty miles to the south of the northern belt,
the second parallel chain of volcanoes ran in a nearly straight
line, which is now traceable from the southern suburbs of Edinburgh
to the coast of Ayrshire, a distance of about 75 miles, but as its
north-eastern end is concealed by Carboniferous formations, and its
south-western passes under the sea, its true length is probably
considerably more.
If the areas which present evidence of distinct and independent vents
are grouped according to their positions on these two lines, they
naturally arrange themselves as in the following list:--
I. Northern Chain of Volcanoes
1. The Montrose Centre.
2. The Sidlaw and Ochil Group.
3. The Arran and Cantyre Centre.
4. The Ulster Centres.
II. Southern Chain of Volcanoes
5. The Pentland Volcano.
6. The Biggar Centre.
7. The Duneaton Centre.
8. The Ayrshire Group.
The distribution of these various volcanic areas will be most easily
understood from an examination of Map III. accompanying this volume.
I. THE NORTHERN CHAIN OF VOLCANOES IN "LAKE CALEDONIA"
1. _The Montrose Centre_
Beginning at the north-eastern end of the area, we first encounter
a series of volcanic rocks which attain their maximum thickness in
Forfarshire around the town of Montrose. The main vents probably lay
somewhere to the east of the present coast, under the floor of the
North Sea; at least no clear indication of their existence either on
the coast or inland has been detected. From Montrose, both to the
north-east and south-west, the lavas thin away, becoming intercalated
among the sandstones, flagstones and conglomerates, and gradually dying
out. The total length of the volcanic belt is about 18 miles, that is
nine miles from the central thick mass in a north-easterly and the same
distance in a south-westerly direction.[351] The volcanic pile must be
several thousand feet thick, but owing to the prolongation of the great
Ochil anticline, the lavas roll over and do not allow their base to be
seen. The axis of the fold must pass out to sea, through the hollow
on which the town of Montrose stands. The volcanic series consists
of andesite-sheets with volcanic conglomerates. It contains little
ordinary tuff, but the conglomerates no doubt partly represent ejected
fragmental material, as well as the waste of exposed lavas. A section
across the anticlinal fold from Forfar to Panbride, a little to the
south-west of Montrose, would reveal the structure shown in Fig. 67.
[Footnote 351: The south-western part of this area from Arbroath to
Johnshaven was mapped for the Geological Survey by the late Mr. H. M.
Skae, the north-eastern part by Mr. D. R. Irvine. My account of it is
mainly taken from notes made by myself on the ground preliminary to the
commencement of the mapping of the Survey.]
In the north-eastern prolongation of the volcanic series from the
Montrose centre, successively lower members are exposed along the
coast-line. But the lavas are dying out in that direction, and
sometimes many hundreds of feet of ordinary sediment intervene between
two successive flows. It was in one of these long pauses near the
top of the whole pile of lavas that the strata of Canterland were
deposited, to which reference has already been made. South-west from
Montrose the thick volcanic mass rapidly diminishes, and is prolonged
to the end only by three or four bands separated by sandstones and
flagstones. It is in these intercalated groups of sedimentary material
that the "Forfarshire flags" occur.
Nowhere can the details of the Old Red Sandstone volcanic rocks be more
conveniently studied than along the coast-section in this district
from the Red Head to Stonehaven. The rocks have not only been cut into
vertical cliffs, but along many parts of the shore they have been also
laid bare in ground-plan, so that a complete dissection of them is
presented to the geologist. At the south end, the top of the volcanic
series appears at the bold promontory of the Red Head. There, at the
base of the cliffs of red sandstone, the accompanying section may be
seen. Beneath the red false-bedded and sometimes pebbly sandstones
(_e_), which form nearly the whole precipice, lies a band of dull
purplish ashy conglomerate (_d_), composed almost wholly of fragments
of different andesites, imbedded in a paste of the same comminuted
material. Towards the south, this rock rapidly becomes coarser, until
it passes into a kind of agglomerate, in which the andesite blocks are
sometimes a yard or more in diameter. It includes bands of sandstone,
which increase in number and thickness towards the north, and sometimes
intervene underneath the conglomerate. The lowest rocks here visible
are sheets of andesite or "porphyrite" (_a_), separated from each other
by irregular bright red layers of tufaceous sand and agglomerate. These
lavas are dull purplish-grey to green, some of them being tolerably
compact, others highly amygdaloidal, with large steam-cavities often
drawn out in the direction of flow.
[Illustration: Fig. 75.--Section showing the top of the volcanic series
at the foot of the precipice of the Red Head, Forfarshire.
_a_, Top of slaggy andesite; _b_, coarse volcanic conglomerate;
_c_, Red sandstone; _d_, Tuff and volcanic conglomerate; _e_, Red
sandstones.]
One of the most striking features in the andesites of this coast is the
remarkable manner in which they include the veinings of pale green and
red sandstone already described (see Figs. 65, 66). Some of the sheets
have in cooling cracked into rude polygons. They are likewise traversed
by large cavernous spaces and intricate fissures or steam-cavities.
Into all these openings the sand has been washed, filling them up and
solidifying into well-stratified sandstone, the bedding of which is
generally parallel with that of the rocks that enclose it, the dip of
the whole series of strata being gently seawards. But a still more
intimate mixture of the sand with the lava-sheets is to be remarked
where these rocks assume their most slaggy character. In some of them
the upper part, to a depth of ten or twelve feet, consists of mere
rugged lumps of slag which, while the mass was in motion, were probably
in large measure loose, and rolled over each other as they were borne
onward. The sand has found its way into all the interstices of these
clinker-beds, and now binds the whole mass firmly together. At first
sight, these bands might be taken for agglomerates of ejected blocks,
and as already suggested, some of the slags may have been thrown out
as loose pieces, but a little examination will show that in the main
the rough scoriaceous lumps are pieces of the lava underneath. In
these instances, also, it is clear that the blocks were in position
before the fine sand was sifted into their interspaces, for the pale
green sandstone is horizontally stratified through its intricate
ramifications among the pile of dark clinkers.
The seaward inclination of the rocks allows the succession of lavas
to be seen as the coast is followed westward into Lunan Bay. On the
further side of that inlet, after passing over a group of sandstones
that underlie the volcanic series of the Red Head, the observer
meets with a second and lower succession of lavas which in the five
miles northward to Montrose Harbour are admirably exposed both along
coast-cliffs and on the beach. They resemble those of the Red Head,
being made up of alternations of highly vesicular andesite with more
compact varieties, and showing similar sandstone veinings. Here
and there, as at Fishtown of Usar, the sea has cut them down into
a platform from which the harder parts rise as fantastic half-tide
stacks. In some cases, the more durable rock consists of the slaggy
upper portions of the flows, and in one case this material stands
up as a rude pillar twelve feet high, composed of clinkers firmly
cemented with veinings of sandstone. The geologist who wanders over
this coast-line is arrested at every turn by the marvellously fresh
volcanic aspect of many of the lavas. Their upper parts are so cellular
that if the calcite, chalcedony and other infiltrated minerals were
removed from their vesicles, they would be transformed into surfaces of
mere slag. In one respect would their antiquity still be evident. These
slaggy bands are generally a good deal reddened, as if they had been
long exposed to oxidation before being covered by the overlying sheets
of lava--a feature already cited, as probably indicating the lapse of
some considerable interval of time between successive outflows.
Along this coast-section the absence of intercalated tuffs is soon
remarked. The volcanic ejections seem to have consisted almost entirely
of andesitic lavas, though it is possible that here and there the very
slaggy bands between the more solid parts of the sheets may include a
little pyroclastic material. The lowest portion of the volcanic group
here visible is reached at Montrose Harbour, where, in the flagstones
and shales of Ferryden, the late Rev. Hugh Mitchell obtained some of
the fossil-fishes of the formation.
A space of more than three miles now intervenes where the rocks are
concealed by blown sand and other superficial accumulations. It is
through this hollow, as already stated, that the great Ochil anticline
runs out to sea. On the north side of the North Esk River, we again
come upon the same band of lavas as to the south of Montrose, but with
a dip to the north-west. This inclination, however, soon bends round
more westerly, and the result of the change is to expose a slowly
descending section all the way to the Highland fault at Stonehaven.
A picturesque line of high inland cliff, running northwards beyond
St. Cyrus, reveals with great clearness the bedded structure of
the andesites. But as one moves northward, owing to the change in
the direction of dip, one finally passes out of this volcanic belt
and begins gradually to descend into the thick Kincardineshire Old
Red Sandstone. The amount of conglomerate exposed along this part
of the coast-line probably considerably surpasses in thickness
any other conglomerate series in the Lower Old Red Sandstone of
Britain. Throughout the enormous depth of sedimentary material,
the conglomerates are well-bedded, consisting of a dull green
paste, composed in large degree of comminuted andesitic debris, and
interstratified with green felspathic sandstones. They are often
remarkably coarse, the pebbles sometimes measuring three feet in
length. Interposed among them are some ten or twelve bands, probably
often single outflows of andesite, sometimes compact and porphyritic,
at other times highly amygdaloidal. Such is the succession of rocks
for many miles along the shore; and as the inclination varies from
a little north of west to west, or even west by south, the observer
gradually passes over a thickness of rather more than 2000 feet from
the base of the St. Cyrus andesites to Gourdon. In this accumulation
of coarse, well water-worn material, with abundant intercalations of
finer sandstone and occasional sheets of lava, there is the record of
prolonged and powerful denudation with intermittent volcanic activity.
Dykes of a quartziferous porphyry cut the conglomerates, and at Gourdon
they are pierced by the intrusion of serpentine above referred to.
The proportion of andesite fragments in the conglomerates of this part
of the coast varies, but is generally much lower than that of the rocks
from the Highlands. Thus at Johnshaven, out of 100 blocks, broken
promiscuously from the conglomerate, I found that only 8 per cent were
of andesite, while 44 per cent were of quartzite, and the remainder
consisted of various quartz-porphyries, granites and schists. It is
evident, therefore, that some area of crystalline rocks was subjected
to enormous waste, and that its detritus was strewn over the floor of
Lake Caledonia, at the same time that from the Montrose volcanic vents
many streams of andesitic lava were poured forth.
A vast mass of coarse conglomerate intervenes between Gourdon and
Dunnottar, and forms a nearly continuous line of precipices which in
some places rise 200 feet above the waves. The bedding is everywhere
distinctly marked, so that there is no difficulty in following the
succession of the strata, and estimating their thickness. From the
last of the lavas at Gourdon to the base of the conglomerates near
Stonehaven, there lies an accumulation of conglomerate at least 8000
feet thick. The boulders and pebbles in these deposits are generally
well-rounded, and vary up to four feet or more in length. I observed
one of quartz-porphyry at Kinneff which measured seven feet long
and six feet broad. The proportion of andesite fragments in these
conglomerates continues to be small. I ascertained that in the coarsest
mass at Kinneff they numbered only 14 per cent; at Todhead Point, a
mile and a half to the north, 20 per cent, and at Caterline, three
quarters of a mile further in the same direction, 21 per cent.
In the midst of this gigantic accumulation of the very coarsest
water-worn detritus, there are still records of contemporaneous
volcanic action. Near Kinneff the beautiful andesite, with large
tabular crystals of plagioclase, alluded to on p. 274, occurs in the
conglomerate.[352] South of Caterline two flows, lying still lower in
the system, project into the sea. One of these presents a section of
much interest. It shows a central solid portion, jointed into rudely
prismatic blocks, with an indefinite platy structure, which gives it a
roughly-bedded aspect. Its upper ten or twelve feet are sharply marked
off by their slaggy structure, ending upwards in a wavy surface like
that of the Vesuvian lava of 1858. Into its fissures, steam-cavities
and irregular hollows, fine sand has been washed from above, as at Red
Head, while immediately above it comes a coarse conglomerate of the
usual character (Fig. 76). Still lower down, beneath some 900 feet of
remarkably coarse conglomerate, another group of sheets of andesite
abuts at Crawton upon the coast, with which, at a short distance
inland, it runs parallel for more than two miles, coming back to the
sea at Thornyhive Bay and at Maidenkaim. We have then to pass over
about 5000 feet of similar conglomerates, until, after having crossed
several intercalated sheets of andesite, we meet with the last and
lowest of the whole volcanic series of this region in the form of some
bands of porphyrite at the Bellman's Head, Stonehaven. The peculiar
geographical conditions that led to the formation of the coarse
conglomerates appear to have been established at the same time that the
volcanic eruptions began, for as we descend in the long coast section,
we find that the coarse sediment and the intercalated lavas cease on
the same general horizon. Below that platform lie some 5000 feet of red
sandstones and red shales, yet the base of the series is not seen, for
the lowest visible strata have been faulted against the schists of the
Highlands. It is thus obvious that more than 5000 feet of sediment had
been laid down over this part of the floor of Lake Caledonia before the
first lavas were here erupted.
[Footnote 352: For an analysis of the felspar in this rock, see Prof.
Heddle's paper, _Trans. Roy. Soc. Edin._ xxviii. (1879), p. 257.]
[Illustration: Fig. 76.--Andesite with sandstone veinings and overlying
conglomerate. Todhead, south of Caterline, coast of Kincardineshire.]
2. _The Sidlaw and Ochil Group_
The volcanoes which poured out the masses of material that now form
the chain of the Ochil and Sidlaw Hills appear to have been among the
most vigorous in the whole region of Lake Caledonia. Their chief vents
probably lay towards the south-west in the neighbourhood of Stirling,
where the lavas, agglomerates and tuffs discharged from them reach a
thickness of not less than 6500 feet, without revealing their bottom.
From that centre the lavas range continuously for nearly fifty miles
to the north-east, until they reach the sea at Tayport; but they are
prolonged on the north side of the Firth of Tay from Broughty Ferry
to near Arbroath, so as to overlap those of the Montrose group. They
thus attain a total length of nearly sixty miles in a north-easterly
line. How far they stretched south-west cannot now be ascertained,
for they have been dislocated and buried in that direction under the
Carboniferous formations of the Midland Valley.
It will be observed from the map (No. III.) that the great volcanic
ridge of the Ochil Hills continues unbroken for twenty-two miles, from
Stirling to Bridge of Earn. Thereafter it branches into two divergent
portions, one of which runs on through the north of Fife to the
southern promontory of the estuary of the Tay, while the other, after
sinking below the alluvial plains of the Earn and the Tay, mounts once
more into a high ridge near Perth, and thence stretches eastward into
Forfarshire as the chain of the Sidlaw Hills. This bifurcation is due
to the opening out and denudation of the great anticlinal fold above
mentioned. The rocks in the northern limb dip north-westward, those in
the southern limb dip south-eastward. The lower members of the Old Red
Sandstone, underlying the volcanic series, ought to be seen beneath
them along the crest of the anticline. Unfortunately, however, partly
by the action of faults along the boundaries of the volcanic bands, but
chiefly from the unconformable overspread of Upper Old Red Sandstone
and Lower Carboniferous rocks across the plains of the Carse of Gowrie
and of the Earn, the lower parts of the system are there concealed (see
Fig. 78). As already remarked, this important anticlinal fold runs
to the north-east across Forfarshire, and passes out to sea north of
Montrose.
Through the Ochil chain the fold runs obliquely in a south-westerly
direction, until it is truncated by the great fault which lets down the
Clackmannan coal-field. The total traceable length of this anticline
is thus about sixty miles. It flattens down towards the south-west;
consequently the rocks in the western part of the Ochil Hills are so
gently inclined that the same bands may be followed winding round the
sides of the valleys, and giving to the steep declivities the terraced
contours to which allusion has already been made (see Fig. 68). Another
result of this structure is that the base of the volcanic series is
entirely concealed by its higher portions.
From an examination of the map it will be further obvious that the
whole wide plain of Strathmore--that is the great hollow, more than 80
miles long and about ten or twelve miles broad, which stretches between
the base of the Highland mountains and the north-western slopes of the
Ochil and Sidlaw chain--is underlain with volcanic rocks of Lower Old
Red Sandstone age. This plain lies on a broad synclinal fold, along
the south-east side of which the lavas, tuffs and conglomerates of the
Ochil and Sidlaw Hills dip under a thick accumulation of red sandstone
and flagstone. On the north-west side similar lavas and tuffs rise
again to the surface, both on the southern side of the great boundary
faults, and also in the little bays which here and there survive on the
northern side of the dislocations (Fig. 77). I have already alluded to
these interesting relics of the shore-line of Lake Caledonia, and to
the fact that though they lie unconformably on the Highland schists,
they do not belong to the actual basement members of the Old Red
Sandstone (_ante_, p. 295, and Fig. 73). We have seen that below the
bottom of the volcanic series a thickness of 5000 feet of sandstones
and shales emerges on the Stonehaven coast, and yet that even there the
base of the whole system is not visible, owing to the effect of the
Highland boundary fault.
It is thus evident that over the bottom of Lake Caledonia a very thick
deposit of tolerably fine sedimentary material was spread before the
commencement of the Ochil and Sidlaw eruptions,--that when the lavas
were poured out and the coarse conglomerates began to be formed, these
materials overlapped the older deposits and gradually encroached upon
the subsiding area of the Highlands. The lavas rolled across the floor
of the lake and entered the successive bays of the northern coast-line,
where their outlying patches may still be seen.
[Illustration: Fig. 77.--Section across the Boundary-fault of the
Highlands at Glen Turrit, Perthshire. _s_, Crystalline schists of
the Highlands; _c_ _c_, conglomerates and sandstones (Lower Old Red
Sandstone) with interstratified volcanic rocks (_v_ _v_); _f_, fault.]
From these facts it is clear that to the actually visible area of
volcanic material in the Ochil and Sidlaw region, and to the anticlinal
tract whence the andesites have been removed by denudation, we have
to add the area that lies under the plain of Strathmore, which may be
computed to be at least 800 square miles, making a total of probably
not less than 1300 square miles. But it will be remembered that
practically only one side of the anticlinal fold is accessible to
observation. We cannot tell how far in a southerly direction the lavas
of the Ochil Hills may extend. It is quite possible that not a half of
the total area covered by the eruptions of this volcanic group is now
within reach, either of observation or of well-founded inference.
One further general characteristic of this volcanic district will be
obvious from an inspection of the map. While the thickest mass of lavas
and tuffs, lying towards the south-west, points to the existence of
the most active vents in that part of the area, the actual positions
of these vents have not been detected. Probably they lie somewhere to
the south of the edge of the Ochil chain, under the tract which is
overspread with the coal-field. But other and possibly minor orifices
of eruption appear to have risen at irregular intervals towards the
north-east along the length of the lake. Thus there are numerous bosses
of felsitic and andesitic rocks among the central Ochils, some of
which may mark the positions of active vents. For some miles to the
east of that area an interval occurs, marked by the presence of only
a few small intrusive masses. But as the broad anticline of the Firth
of Tay opens out and allows the lower or pre-volcanic members of the
Old Red Sandstone to approach the surface, another group of bosses
emerges from the lower sandstones and flagstones. Some of these cover
a considerable space at the surface, though a portion of their visible
area may be due to lateral extravasation from adjacent pipes, the
true dimensions of which are thereby obscured. Some of the masses are
undoubtedly sills. In the case of Dundee Law we probably see both the
pipe and the sill which proceeded from it; the prominent, well-defined
hill marking the former, while the band of rock which stretches from
it south-westwards to the shore belongs to the latter. The material
that forms the bosses and sills in this neighbourhood is generally a
dark compact andesite. The rock of Dundee Law was found by Dr. Hatch
to show under the microscope "striped lath-shaped felspars abundantly
imbedded in a finely granular groundmass, speckled with granules of
magnetite, but showing no unaltered ferro-magnesian constituents." Here
and there in the same district a solitary neck may be observed filled
with agglomerate (Fig. 78).
[Illustration: Fig. 78.--Section across the chain of the Sidlaw Hills,
near Kilspindie.
1. Lower Old Red Flagstones and Sandstones; 2. Andesite lavas; 3.
Volcanic tuff; 4. Volcanic conglomerates and sandstones; N, Volcanic
neck; 5. Upper Old Red Sandstone under Carse of Gowrie, lying
unconformably on the lower division; _f_, Fault; _d_, Basic dyke.]
The variations in the structure of the Ochil and Sidlaw volcanic group
will be most easily understood from a series of parallel sections.
Beginning on the north-eastern or Sidlaw branch of the volcanic
band, we find the arrangement of the rocks to be as is shown in the
accompanying figure[353] (Fig. 78). As is usually the case in this
region, the base of the volcanic series is here concealed by the fault
which brings down the Upper Old Red Sandstone under the alluvial
deposits of the Carse of Gowrie. The total thickness of the series
in this section is about 2500 feet. The rocks consist of successive
sheets of andesite of the familiar types, varying in colour through
shades of blue, purple and red, and in texture from a dull compact
almost felsitic character to more coarsely crystalline varieties. They
are often amygdaloidal, especially in the upper and lower portions
of the individual flows. They are not infrequently separated from
each other by courses of conglomerate or ashy sandstone and grit. Of
these intercalations four are of sufficient thickness and persistence
to be mapped, and are shown on the Geological Survey Sheet 48. The
stones in the conglomerates vary up to blocks two feet in diameter,
and consist chiefly of andesites, but include also some pink felsites
and pieces of greenish hardened sandstone. Generally they are more or
less well-rounded; but occasionally they become angular like those of
volcanic agglomerates.
[Footnote 353: This section and the notes accompanying it have been
supplied by Prof. James Geikie, who mapped the western half of the
Sidlaw range for the Geological Survey. The eastern half was mapped by
the late Mr. H. M. Skae.]
One of the most interesting features in this section is the neck which
at Over Durdie rises through the volcanic series. Oval in form, it
measures 630 yards in one diameter and 350 in another, and is filled
with pinkish granular tuff, full of andesitic lapilli and blocks. A
much smaller neck of similar material lies about 100 yards further to
the south-west. There seems no reason to doubt that these necks mark
two of the volcanic vents belonging to a late part of the volcanic
history of the district.
The structure of the Sidlaw range is repeated among the hills of east
Fife on the southern side of the great anticlinal fold.[354] Thus
a section from near Newburgh on the Firth of Tay southward to near
Auchtermuchty in Stratheden gives the arrangement of rocks shown in
Fig. 79. In this traverse a thick mass of fragmental material occurs
in the higher part of the series of volcanic rocks. Though on the
whole stratified and forming a group of conglomerate-beds between the
lavas, the material is in places an amorphous agglomerate of volcanic
blocks varying in size up to two feet in diameter. These portions show
abundant angular and subangular blocks, many of which, after having
undergone some attrition, have been finally broken across before
reaching their present resting-places. Sharply fractured surfaces can
be picked out of the felspathic ashy matrix. The stones are chiefly
varieties of andesite, but they include also pink felsites and pieces
of some older fine-grained tuff.
[Footnote 354: The eastern part of the Ochils was mapped for the
Geological Survey by Mr. H. H. Howell and Mr. B. N. Peach.]
[Illustration: Fig. 79.--Section across the Eastern Ochil Hills from
near Newburgh to near Auchtermuchty.
1. Lower Old Red Sandstones and conglomerates; 2. Andesite lavas; 3.
Volcanic conglomerates; 4. Upper Old Red Sandstone.]
These fragmental materials form a local deposit about nine miles
long, and probably not less than 1700 feet thick. They are partly
interstratified with flows of andesite. Though, from the rounded forms
of some of the pebbles, wave-action may be inferred to have been
concerned in their accumulation, they seem to be mainly due to volcanic
explosions. No trace, however, has been found of the vent from which
the eruptions took place. Not improbably its site lies somewhere to the
south in the area now concealed under the Upper Old Red Sandstone and
Carboniferous formations. The large size of many of the blocks suggests
that they do not lie far from their parent focus of discharge. It is
impossible to tell how much of the volcanic series is here concealed by
the unconformable overlap of the younger formations.
[Illustration: Fig. 80.--Generalized section across the heart of the
Ochil Hills, from Dunning on the north to the Fife Coal-field near
Saline on the south.
1. Volcanic tuffs and agglomerates; 2. Andesite lavas; 3. Lower Old
Red Sandstone and conglomerate; 4. Necks of felsitic rocks; 5. Upper
Old Red Sandstone and Calciferous Sandstones; 6. Representative of the
Plateau lavas and tuffs of the Lower Carboniferous series; 7. Hurlet
(Carboniferous) Limestone; 8. Dolerite sill; 9. Sandstones, shales and
coals of the Carboniferous Limestone series; 10. Neck of the Puy series
(Carboniferous); _f_, Fault.]
A section across the centre of the Ochil chain,[355] from Dunning
in Strathearn to the Crook of Devon and the Fife Coal-field, gives
the structure which is generalized in Fig. 80. At the north end the
volcanic series is found to be gradually split up into separate
lava-sheets until it dips under the red sandstones of Strathearn.
Traced southwards the rocks become entirely volcanic. Some of their
most conspicuous and interesting members are pale felsitic tuffs, which
occupy a considerable tract of ground about Craig Rossie, south-east of
Auchterarder. As the dip gradually lessens the harder lavas are able
to spread over wider tracts of ground, capping the hills and ridges,
while underneath them thick masses of tuff and conglomerate are laid
bare in the valleys. A number of bosses of orthophyre rise through
these rocks and are accompanied by many veins and dykes of similar
material. It is not improbable that some of these bosses, as already
suggested, may represent vents. They are especially prominent among
the hills due south of Auchterarder. One of these eminences, known as
the Black Maller, is composed of a typical orthoclase-felsite without
mica. Another, about four and a half miles further south, forms the
conspicuous summit of Ben Shee overlooking Glen Devon, and consists of
a similar rock with a characteristic platy structure.
[Footnote 355: The central portion of the Ochils was mapped for the
Geological Survey by Mr. B. N. Peach, Prof. James Geikie, Prof. J.
Young, Mr. R. L. Jack and myself.]
No necks of agglomerate have been observed in this part of the chain.
It will be seen from the section that the lowest visible parts of the
Ochil volcanic series are here truncated by a fault which brings in
the lower part of the Carboniferous system. By a curious conjuncture,
immediately on the south side of this fault, a band of tuff appears,
lying on the platform of the Carboniferous "plateau-lavas," to be
hereafter considered, and passing below the well-known Hurlet seam
of the Carboniferous Limestone, while through these strata rises one
of the puys belonging to the second phase of volcanic activity in
Carboniferous time in Scotland.
The best sections to show the nature and sequence of the volcanic
series of the Ochil Hills are to be observed at the west end of the
chain. But as the whole succession of rocks cannot conveniently be
obtained along one line, it is better to make several traverses,
starting in each case from a known horizon. In this way, by means
of three parallel sections, we may obtain the whole series of lavas
and tuffs in continuous order. The first line of section starts in
the lowest part of the tuffs represented at the bottom of the group
in Fig. 80, and runs up to the first thick ashy intercalation among
the lavas. Following this bed south-westward to the Burn of Sorrow,
we make from that horizon a second traverse across the strike to the
summit of King's Seat Hill (2111 feet above the sea), where we meet
with a well-marked lava which can be traced south-westwards, gradually
descending the southern escarpment of the hills until it reaches the
boundary fault near the village of Menstrie. Starting again from this
definite horizon, we take a third line across the top of Dumyat (1373
feet) to the plain of Sheriffmuir, and there pass beyond the volcanic
series into the overlying red sandstones. Arranged thus in continuous
vertical sequence the succession is found to be as represented in Fig.
81. The total thickness of volcanic material amounts to more than 6500
feet.
[Illustration: Fig. 81.--Diagram of the volcanic series of the Western
Ochil Hills.
The bands with vertical lines are various lavas (_a_); the tuffs and
volcanic breccias are shown by the dotted bands (_b_); the uppermost
portion of the section above the last thick group of lavas consists of
conglomerates and sandstones (_c_) with a sheet of lava.]
In this vast pile of volcanic ejections the lavas are almost entirely
andesites of the usual characters. They include many slaggy and
amygdaloidal varieties, some beautiful porphyries with large tabular
felspars, likewise the resinous or glassy variety already referred to
as occurring above Airthrey Castle. Their upper and under surfaces show
the same structure as already described in those of the coast-sections
in the Montrose tract. They include also more acid lavas, like the pale
pink decomposing felsites of the Pentland Hills.
The tuffs and conglomerates occur on many platforms throughout the
succession of lava-sheets. They form the lowest visible part of the
whole volcanic series, but they are most abundant towards the top, and
are best displayed at the western end of the hills. In Dumyat they form
a conspicuous feature. The whole of that hill consists of a constant
alternation of lavas (chiefly slaggy andesites, but including also
one felsitic flow) with bands of coarse and finer tuff and volcanic
conglomerate. The greatest continuous mass of this fragmental material
is 600 or 700 feet thick. From the extraordinary size of its included
blocks it obviously must have been formed of ashes, stones and huge
pieces of lava ejected from some vent in the near neighbourhood. Some
of the individual blocks in this mass are as large as a Highland
crofter's cottage.
The uppermost lavas of Dumyat dip under a still higher series of
coarse volcanic conglomerates entirely made up of andesitic debris and
reaching a thickness of about 1000 feet. This enormous accumulation was
probably due partly to the abrasion of exposed cones and lava-ridges,
and partly to volcanic discharges of fragmentary materials. Yet it is
worthy of note that even amidst these evidences of the most vigorous
volcanic activity we have also proofs of quiet sedimentation and traces
of the fishes that lived in the waters of the lake. This particular
zone of coarse conglomerate as it extends in a south-westerly direction
becomes finer, and its upper part passes into a chocolate-coloured
sandstone which has been quarried at Wolfe's Hole, Westerton, Bridge
of Allan, at a distance of about three miles from where the line of
section runs, which is embodied in the diagram, Fig. 81. It was from
this locality that the specimens of _Eucephalaspis_, _Pteraspis_ and
_Scaphaspis_ were obtained which were described by Professor Ray
Lankester.[356]
[Footnote 356: _Palæontographical Society_, vols. xxi. (1867) and
xxiii. (1869).]
Above the last-named thick group of coarse volcanic conglomerates a
solitary sheet of dark slaggy andesite may be observed. This lava
is then overlain by the great depth of chocolate-coloured and red
sandstones and marls of the plain of Strathmore (_c_ in Fig. 81).
Nevertheless a few hundred feet up in these sedimentary deposits we
meet with yet one further thin sheet of lava--the last known eruption
of the long volcanic history of this district.
Before quitting the Ochil range I may refer to the evidence there
obtainable as to the horizontal extent of separate sheets of lava. The
western end of this range affords great facilities for following out
individual beds of andesite along the bare terraced front of the great
escarpment. Thus, the easily recognizable porphyrite which caps King's
Seat Hill, above Tillicoultry (see Fig. 68), can be traced winding
along the hill-slopes until it descends to the plain, and is then lost
under the great fault, at the foot of Dumyat--a distance of more than
six miles. There is, therefore, no difficulty in supposing that from
the Ochil line of vents streams of lava should have rolled along the
floor of the lake across to the base of the Highland slopes, 10 or 12
miles distant. We cannot tell, of course, whether any buried vents lie
below the plain of Strathmore, but certainly no unquestionable trace of
vents has yet been found among the crystalline rocks along the borders
of the Highlands.[357]
[Footnote 357: Allusion has already been made to the possible
connection of the younger Highland granites with the volcanic series
of the north-eastern part of Lake Caledonia; also to the occurrence of
isolated masses of breccia piercing the crystalline schists near Loch
Lomond (_ante_, p. 272).]
Reference has already been made to the comparative scarcity of sills
in this region, and to the occurrence of the acid group of Lintrathen
porphyry and the more basic sheets between the Firth of Tay and Forfar.
This scarcity no doubt arises in part from the extent to which the
rocks that underlie the volcanic series are concealed. Yet it is
noteworthy that along the coast-section of these rocks near Stonehaven
hardly any intrusive sheets are to be seen.
3. _The Arran and Cantyre Centre_
It is unfortunate that the Ochil chain should be broken across and
buried under younger formations at the very place where some of the
most interesting vents in the whole area of the Old Red Sandstone might
have been looked for.[358] We have to pass westwards across the Firth
of Clyde to the Isle of Arran before we again meet with rocks of the
same age and character.
[Footnote 358: The Ochil area is not the only example of the abrupt
termination of a volcanic band near its centre owing to faults or
overlaps. The sudden disappearance of the Pentland lavas and tuffs on
the northern side of the Braid Hills is another striking illustration.]
In the course of the recent work of the Geological Survey in that
island, Mr. W. Gunn has discovered that the Lower Old Red Sandstone
includes some interstratified volcanic rocks on the north side of
North Glen Sannox, and he has supplied me with the following notes
regarding them. "The area in which the volcanic intercalations occur
is much faulted and only a part of it has been mapped in detail, but
the position of the interbedded igneous rocks is quite clear. The Old
Red Sandstone here consists of three distinct members, the lowest of
which is made up of coarse, well-rounded conglomerates, alternating
with sandstones and purple mudstones. Above this, and apparently
unconformable to it, is a middle series of light coloured conglomerates
and sandstones, the pebbles in which are mainly of quartz. Finally
comes an upper series of red sandstones and conglomerates, which
occupy nearly the whole of the coast section, and it is this series
which has generally been taken as the typical Old Red Sandstone of the
island. The volcanic series is intercalated between the middle and
upper divisions given above, and may be seen in several places on the
hillside between the shepherd's house at North Sannox and Laggan. It
consists mainly of old lava-beds of a dull reddish or purplish colour,
often soft, and in places much decomposed. It seems basic in character.
A specimen from near the Fallen Rocks, examined by Mr. Teall, was found
to be too much altered for precise determination, but was probably a
basalt originally. These rocks do not occur on the coast."
In the southern extremity of Cantyre some important relics of the
volcanic rocks of the Lower Old Red Sandstone have been recently
detected and mapped for the Geological Survey by Mr. R. G. Symes.[359]
This division of the system has been ascertained by him to be
extensively developed to the south of Campbeltown, and to include some
small but interesting remains of the volcanic action which was so
marked a feature in the areas of Lake Caledonia, lying further to the
east. To the student of volcanic geology, indeed, this small tract at
the extreme southern end of Argyllshire has a peculiar interest, for in
no other part of the British Isles have the phenomena of the eruptive
vents of the Lower Old Red Sandstone been more admirably laid bare. Not
only are there necks in the interior like that represented in Fig. 82;
but others have been dissected by the waves along the southern shore,
and their relations to the deposits of fragmentary material showered
over the bottom of the lake have been more or less clearly exposed.
[Footnote 359: The late Prof. James Nicol published in 1852 an account
of the geology of the southern portion of Cantyre. He grouped all the
igneous rocks of the district as one series, which he regarded as later
than the Coal-formation and possibly of the same age as those of the
north-east of Ireland. He made no distinction between the Lower Old Red
Sandstone and the younger unconformable conglomerates (_Quart. Journ.
Geol. Soc._ vol. viii. (1852), p. 406).]
[Illustration: Fig. 82.--View of Cnoc Garbh, Southend, Campbeltown. A
volcanic neck of Lower Old Red Sandstone age, about 400 yards wide in
its longer diameter.]
At Keil Point, a little to the east of the most southerly headland
of the Mull of Cantyre, some reddish and purplish highly felspathic
sandstones (_a_ in Fig. 83) dipping towards the east are found to pass
upward into coarse volcanic breccias (_b_), which, followed eastwards,
lose almost all trace of stratification, and are then abruptly
succeeded by a neck of coarse agglomerate (_c_) measuring 25 yards
from north to south, where its limits can be seen, and at least 12
yards from west to east. It is hardly possible to distinguish between
the breccias to the west and the agglomerate of the neck, except by
the rude bedding of the former which pass down into the well-bedded
sandstones.
The agglomerate is a thoroughly volcanic rock. The materials consist
chiefly of angular blocks of a pale purplish or lilac highly
porphyritic mica-porphyrite, with large white felspars and hexagonal
tables of black mica. These blocks might sometimes be mistaken for
slags from their cavernous, weathered surfaces, but this rough aspect
is found on examination to be due to the decay of their felspars.
[Illustration: Fig. 83.--Section of volcanic series on beach, Southend,
Campbeltown.
_a_, Fine reddish and purplish highly felspathic sandstones,
largely composed of porphyry-debris and passing up into coarse
breccias; _b_, volcanic breccias, coarse and only rudely
stratified, formed of blocks of porphyry, sandstone fine tuff
and andesite, together with water-worn quartzite pebbles derived
from some conglomerate; _c_, coarse unstratified agglomerate
forming a neck.
]
Perhaps the most singular feature among the contents of this neck
is the number of well-rounded and smoothed pebbles and boulders of
quartzite. These are dispersed at random through the mass, and are
often placed on end. There can be no doubt that they are water-worn
stones, but the contrast of their smooth surfaces and rounded forms
with the rough angular blocks of igneous material is so striking as
to lead at once to the conclusion that they cannot have acquired
their water-worn character in the deposit where they now lie. Their
positions and their occurrence with ejected volcanic blocks suggest
that they too were discharged by volcanic explosions. They so exactly
resemble the quartzite boulders and pebbles in the neighbouring Old Red
Conglomerates that there can be little hesitation in regarding them as
derived from these conglomerates. They seem to me to have come from a
lower part of the Old Red Sandstone, which was shattered by volcanic
energy either before the conglomerates were firmly consolidated or
afterwards by such violent explosions as served to separate the pebbles
from the matrix of the rock.
There occur also in the agglomerate blocks of fine tuff and ashy
sandstone sometimes four feet long, and often stuck on end, showing
that the deposits of earlier eruptions were broken up during the
drilling of this little vent.
A few hundred yards further east a larger neck rises on the beach,
immediately to the south of the old Celtic chapel of St. Columba. It
consists also of exceedingly coarse agglomerate, with andesite blocks
three and four yards in diameter. It is about 125 yards broad from east
to west, on which sides it is seen to be flanked by coarse volcanic
breccias and conglomerates, resembling in composition the materials
of the neck, but showing an increasingly definite stratification as
they are traced eastward in the ascending succession of deposits.
Following the section in still the same easterly direction along
the coast, we find that bands of fine felspathic sandstone, marking
probably intervals of quiescence, are again and again succeeded by
coarse brecciated conglomerates of igneous materials, which may be
inferred to have been due to a renewal of violent eruptions. By degrees
the evidence of stratification and of attrition among the volcanic
materials becomes more pronounced as the ascending section is followed;
blocks of andesite, even 18 inches or two feet in diameter, assume
well-rolled, rounded, water-worn forms, like the pebbles of quartzite
associated with them, and eventually the strata return to the usual
aspect of the conglomerates of the district.
I have never seen anywhere better proofs of volcanic explosions,
contemporaneous with a group of strata, and of the distribution of
volcanic fragmentary material round the vents. A further point of much
interest is the additional evidence furnished by this shore-section
of considerable wave-action during the accumulation of the coarse
conglomerates. To give to blocks of porphyrite two feet in diameter a
smoothed and rounded form must have required the action of water in
considerable agitation.
4. _The Ulster Centres_
From the volcanic breccias and conglomerates of the Mull of Cantyre
to the coast of Antrim in a straight line is a distance of little
more than twenty miles. On a clear day the Old Red Sandstone of Cross
Slieve, and the range of cliffs in which it abruptly descends to the
sea between Cushendall and Cushendun, can be distinctly seen from the
Argyllshire shore. The geologist who passes from the Scottish to the
Irish sections cannot fail to be impressed with the resemblance of the
rocks in the two countries, and with the persistence of the types of
conglomerate in Lake Caledonia.
A picturesque section has been laid bare between the Coastguard Station
south of Cushendall and Cushendun Bay.[360] At the south side of the
little inlet of Cushendall, a compact dull quartz-porphyry is exposed
in crags along the shore. This rock ranges in colour from dark brown
and purple to pale-green and buff. Its texture also varies, as well as
the proportion of its felspar-crystals and quartz-blebs. Some parts
have a cavernous structure, like that of an amygdaloid, the small
globular cavities being filled with green decomposition products.
[Footnote 360: For descriptions of this district see J. Bryce, _Proc.
Geol. Soc._ i. (1834) p. 396, v. (1837) p. 69; J. Kelly, _Proc. Roy.
Irish Acad._ x. (1868), p. 239. The area is contained in Sheet 14 of
the Geological Survey of Ireland, and was mapped by Mr. A. M'Henry and
described by him in the accompanying Explanatory Memoir (1886), pp. 12,
25.]
The stratigraphical relations of this rock are not quite clear, but
it is certainly older than the Old Red conglomerates which lie to
the north of it, for these are largely made up of its fragments. The
matrix of these detrital masses consists mainly of the comminuted
debris of the porphyry. The pebbles include all the varieties of that
rock, and are tolerably well-rounded. There is no distinct evidence
of volcanic action among these conglomerates. They resemble, however,
many of the conglomerates in the Midland Valley of Scotland, which,
as in the case of those on the Forfarshire and Kincardineshire coast,
are in great part made of the detritus of andesitic lavas. The
Cushendall rocks become coarser as they are traced northwards into
lower members of the series, while at the same time the proportion of
porphyry-debris in their constitution diminishes, and materials from
the metamorphic series take its place. Thus at Cushendun the percentage
of quartz-pebbles rises to 70 or 80. These blocks, of all sizes up to
two feet or more in diameter, are admirably rounded and smoothed, like
those in the Stonehaven section and those among the conglomerates at
the south end of Cantyre. Fragments of the porphyry, however, still
continue to appear, and the matrix shows an admixture of the finer
detritus of that rock. I may remark in passing that no conglomerates
of the Old Red Sandstone show more strikingly than these at Cushendun
the effects of mechanical crushing subsequent to deposition and
consolidation. In many parts of the rock it is hardly possible to find
a rounded block that has not been fractured. Some of them, indeed, may
be seen cut into half a dozen slices, which have been pushed over each
other under the strain of strong lateral or vertical pressure.
In the interior of the country, after passing over the broad Tertiary
basaltic plateau of Antrim, we come upon a large area of Lower Old Red
Sandstone in Tyrone. It stretches from Pomeroy to Loch Erne, a distance
of about 30 miles, and is about 12 miles broad. In lithological
character the strata of this tract exactly resemble parts of the
deposits of Lake Caledonia in Central Scotland. They include also a
volcanic series which, down to the smallest points of detail, may
be paralleled in the sister island.[361] This interesting westward
prolongation of the volcanic record consists of a number of outlying
patches confined to the eastern part of the district.
[Footnote 361: This area of Old Red Sandstone is represented on Sheets
33, 34, 45 and 46 of the Geological Survey of Ireland, and the igneous
rocks are described in the Memoirs on Sheets 33 (1886, p. 17) by Mr. J.
R. Kilroe, and 34 (1878, p. 16) by Mr. J. Nolan.]
The largest of these patches lies to the south of Pomeroy, where it
forms a line of hills about four miles long, and covers an area of
some five square miles. The rocks consist of successive sheets of
andesite-lavas. These, as a rule, are not markedly cellular, though
they include some characteristic amygdaloids. A distinguishing
feature of some of the sheets is their remarkably well-developed
flow-structure. Thus on Sentry Box, at the north-western end of the
ridge, the fissility resulting from this structure so perfectly divides
the rock into parallel flags that the material might easily be mistaken
for a bedded rock. Where this structure has been produced in a cellular
lava, the cavities have been drawn out and flattened in the direction
of flow.
I have not observed true tuffs in any of the sections traversed by me
in this district. But the conglomerates furnish abundant evidence of
the contemporaneous outpouring of the lavas. Thus, in a brook a little
west of Reclain, five miles south of Pomeroy, the section shown in Fig.
84 may be seen. At the base lies a coarse conglomerate (_a_) largely
composed of andesite-debris, the stones being here, as elsewhere in
the district, well rounded. Then comes a series of green and reddish
highly-felspathic sandstones (_b_), followed by an exceedingly
coarse conglomerate (_c_), formed mainly of the debris of andesites,
especially lumps of slag. Some of the stones measure 18 inches in
diameter, and all are well water-worn. Immediately over this mass of
detritus lies the lowest sheet of andesite-lava (_d_).
[Illustration: Fig. 84.--Section of the base of the volcanic series,
Reclain, five miles south of Pomeroy.]
Some sections visible in the neighbourhood of Omagh afford further
evidence of volcanic action at the time of the deposition of the Old
Red Sandstone of this region. At Farm Hill, a little to the east of
the town, felspathic sandstones and breccias enclose angular and
subangular pieces of various andesites, and occasionally even pieces
of tuff. Near these strata a decayed andesite occurs in the bed of
a stream, and a fresher variety is quarried at Farm Hill. A little
further south another variety of andesite is exposed in two quarries
at Recarson Meeting-House--a fine granular purplish-grey rock, with
abundantly-diffused hæmatite pseudomorphs, probably after a pyroxene,
and sometimes strongly amygdaloidal.
[Illustration:
Fig. 85. Section of shales and breccias at Crossna Chapel,
north-east of Boyle.
_a_ _a_, Green and grey shales; _b_ _b_, green and grey hard
sandstones and grits, some bands strongly felspathic; _c_, fine
compact felspathic breccia, with angular chips of different
felsites and andesites, etc.
]
There can thus be no doubt that this region of Ulster included
several centres of volcanic activity during the deposition of the
red sandstones and conglomerates, and that the lavas and volcanic
conglomerates belonged to precisely the same types as those of the same
geological age which occur so abundantly in Scotland.
Further south-west, near Boyle, in the county of Roscommon, certain
curious felspathic breccias in the Old Red Sandstone have been mapped
as "felstone."[362] So far as I have been able to examine them,
however, they are entirely of fragmental origin. They contain pieces
of andesitic and felsitic rocks, with fragments of devitrified glass,
which undoubtedly point to the occurrence of volcanic eruptions during
their deposition, though no tuffs and lavas appear to crop out in the
narrow strip of the formation there exposed.
[Footnote 362: See Sheet 66 Geological Survey of Ireland, and
Explanation to that sheet (1878), p. 15. The rocks were previously
described by Jukes and Foot, _Journ. Roy. Geol. Soc. Ireland_, vol. i.
(1866), p. 249.]
The accompanying section (Fig. 85) may be seen on the hills to the
north-east of Boyle. Where quarried on the road-side to the north of
Boyle, the series of deposits here represented contains a bed of coarse
and exceedingly compact breccia, similar to that just referred to, but
containing angular and subangular fragments six or eight inches long.
The joints of these compact strata are remarkably sharp and clean cut,
so that where the fragmentary character is not very distinct the rocks
might easily be mistaken on casual inspection for felsites.