THE ACID BOSSES OF MULL, SMALL ISLES, ST. KILDA, ARRAN AND
THE NORTH-EAST OF IRELAND
ii. THE ACID BOSSES OF MULL
Though of comparatively small extent, the granophyre bosses of the
island of Mull afford to the geologist a large amount of instruction in
regard to the relations of the different members of the volcanic series
to each other. Especially important is the evidence which they contain
of the connection between the acid and basic groups of rocks. They have
been laid bare in many natural sections, some of which, forming entire
hillsides, are among the most astonishing in the whole wonderful series
which, dissected by denudation, reveal to us the structure of these
volcanic regions. They lie in two chief areas. One of these extends
along the northern flanks of the mountainous tract from the western
side of Beinn Fhada across Loch Ba' to the west side of Glen Forsa. The
other occupies for over three miles the bottom of Glen More, the deep
valley which, skirting the southern side of the chief group of hills,
connects the east side of the island by road with the head of the great
western inlet of Loch Scridain. There are other minor areas. One of
these extends for about a mile along the declivities to the south of
Salen, across the valley of the Allt na Searmoin; another occurs at
Salen; a third runs along the shore at Craignure. In the interior also,
many isolated areas of similar rocks, besides thousands of veins, occur
in the central group of hills and valleys which form the basins of the
Glencannel and Forsa rivers (Map VI.).
The chief northern boss, which for the sake of convenience of reference
may be called that of Loch Ba', has a length of nearly six miles,
with a breadth varying from a quarter of a mile to about a mile and a
quarter. It descends to within 50 feet of the sea-level, and is exposed
along the crest of Beinn Fhada at a height of more than 1800 feet.
It chiefly consists of a grey crystalline rock which might readily
be identified as a granite, but which when examined microscopically
is found to possess the granophyric structure. With this distinctly
granular-crystalline rock are associated various porphyritic and
felsitic masses, which pass into it, and are more specially observable
along its border. An exceedingly compact black quartz-felsite or
rhyolite forms its southern boundary, runs as a broad dyke-like ridge
from the head of the Scarrisdale Water north-eastward across Loch
Ba' (Fig. 352), and spreads out eastward into a mass more than a
mile broad on the heights above Kilbeg in Glen Forsa. The sharp line
of demarcation of this felsite, and its mass and extent, point to a
different period of extravasation.
[Illustration: Fig. 352.--View of the hills on the south side of the
head of Loch na Keal, showing the junction of the granophyre and the
bedded basalts.
One bird, the bedded basalts of the Gribon plateau; two birds,
the bedded dolerites and basalts of Beinn a' Chraig adhering to
the northern slope and capping the hill; three birds, summit of
Ben More, with A'Chioch to the left and the top of Beinn Fhada
appearing in the middle distance between them; four birds, the
granophyre slopes of Beinn a' Chraig with the great dyke-like mass
of felsite on the left.
]
The geologist, who approaches this district from the north-east, has
his attention arrested, even at a distance of several miles, by the
contrast between the outer and inner parts of the hills that lie to the
south-west of Loch Ba'. He can readily trace from afar the dark bedded
basic rocks rising terrace above terrace, from the shores of Loch na
Keal, to form the seaward faces of the hills along the southern side
of that fjord. But he observes that immediately behind these terraces
the mass of the rising ground obviously consists of some amorphous
rock, which weathers into white debris. Nothing can be sharper than
the contrast of colour and form between the two parts of the hills.
The bedded plateau-rocks lie as a kind of wall or veneer against a
steep face of the structureless interior (Fig. 352). Seen from the
other or hilly side, the contrast is perhaps even more striking.
But the astonishment with which it is beheld at a distance becomes
intensified when one climbs the slopes, and finds that the sheets of
dolerite and basalt (which from some points of view look quite level,
yet dip towards the north-east at a gentle angle) are immediately
behind the declivity abruptly truncated by a mass of granophyre. Of
all the junction-lines between the acid bosses and the lavas of the
plateaux, those exposed on these Mull hillsides are certainly the
most extraordinary. So little disturbed are the lavas, that one's
first impulse is to search for pebbles of the granophyre between the
basalts, for it seems incredible that the inner rock should be anything
but a central core of older eruptive material, against and round which
the younger basic rocks have flowed. But, though the granophyre is so
decomposing and covers its slopes with such "screes" of debris, that
had the basalts been poured round it, they must infallibly have had
some of its fragments washed down between their successive flows, not a
single pebble of it is there to be found. This might not be considered
decisive evidence, but it is extended and confirmed by the fact that
the acid rock gives off veins which ramify through the basalts.
Before examining the actual contact of the two rocks, however, the
geologist will not fail to observe here an admirable example of the
gradual change which was described in the foregoing chapter as coming
over the bedded basalts near the acid bosses. As he approaches the
nucleus of white rock, the basalts assume the usual hard indurated
character, not decaying into brown sand as on the plateaux, but often
standing out as massive crags with vertical clean-cut joint-faces.
This metamorphosed condition extends in some cases to a considerable
distance from the main body of acid rock, especially where knobs of
that material, protruding through the more basic lavas, show that it
must extend in some mass underneath. Thus along the shore at Saline
the bedded basalts succeed each other in well-defined sheets, some
being solid, massive and non-amygdaloidal, others quite vesicular, and
recalling the black scoriform surfaces of recent Vesuvian lavas; yet
they are all more indurated than in the normal plateau-country, and
they break with a hard splintery fracture. Immense numbers of dykes
cut these rocks, and they are likewise pierced by occasional felsitic
intrusions.
If we cross to the other side of the island and trace the bedded
basalts away from the central masses of acid rock we meet with so
gradual a diminution of the induration that no definite boundary-line
for the metamorphism can be drawn. As we recede from the centre of
alteration, the rocks insensibly begin to show brown weathered crusts,
with spheroidal exfoliation, the reticulations of epidote and calcite
become much less abundant, the amygdaloids gradually assume their
normal earthy character, and eventually we find ourselves on the
familiar types of the plateau. This transition is well seen along the
shores of Loch na Keal.[401]
[Footnote 401: Some of the thick massive sheets of basic rock along the
south side of this inlet may possibly be altered sills.]
These proofs of the alteration of the plateau-basalts are accompanied
in Mull as in Skye by further abundant evidence that the acid rocks are
of younger date than the basic. In particular, dykes and veins may be
traced proceeding from the former and intersecting the latter. Thus, in
the bed of the south fork of the Scarrisdale stream, a separate mass
of granophyre (which under the microscope exhibits in perfection the
characteristic structure of this rock) protrudes through the basalts
in advance of the main mass, and a little higher up on the outskirts
of that mass narrow ribbons of the granophyre run through the basic
rocks. The contrast of colour between the pale veins of the intrusive
rock and the dark tint of the basalts is well shown in the channel of
the water. Similar sections may be seen on the flanks of Beinn Fhada,
especially in the great corry north of Ben More, where the granophyre
sends a tongue of finer grain between the beds of basalt. On the east
side of Loch Ba' numerous proofs of similar intrusion may be observed.
Thus at the east end of Loch na Dàiridh, where the granophyre has been
intruded into the basalts, hand-specimens may be obtained showing the
two rocks welded together. On the slopes of Cruach Tòrr an Lochain,
where the granophyre has a felsitic selvage, the bedded basalts are
traversed by veins of the latter material (Fig. 353). A little further
east, at the head of the Allt na Searmoin, the bedded basalts, some of
which are separated by slaggy scoriaceous surfaces, are intersected by
another protrusion from the compact felsitic porphyry (Fig. 354).[402] A
mile lower down the same valley a separate mass of granophyre sends out
veins into the basalt, which as usual is dark bluish-grey in colour,
indurated and splintery.
[Footnote 402: This rock appears to the eye as a black finely
crystalline-granular felsite. Under the microscope, it was found by
Dr. Hatch to "present a markedly granulitic structure, consisting
mainly of small rounded grains of dirty brown turbid felspar, with
isolated granules of colourless quartz. Scattered through the rock, or
accumulated in patches, are small spherical or drop-like granules of a
bright green augite (coccolite)."]
[Illustration: Fig. 353.--Section on south side of Cruach Tòrr an
Lochain, Mull.
_a_, bedded basalts and dolerites; _b_, granophyre; _c_, marginal
finer-grained band; _d_ _d_, veins from the granophyre traversing the
basic rocks.]
[Illustration: Fig. 354.--Section at head of Allt na Searmoin, Mull.
_a_, basalts and dolerites, with slaggy upper surfaces; _b_, felsite.]
As the posteriority of the Mull granophyre and felsites to the basalts
is thus proved, the further question remains as to their manner of
intrusion. Here and there, especially on the south-eastern side,
between the head of the Scarrisdale river and Loch Ba', the line of
junction between the two rocks is nearly vertical, but a body of black
felsite intervenes as a huge wall between the ordinary granophyre and
the basalt. On Beinn Fhada and Beinn a' Chraig the line of separation,
as I have above remarked, is inclined outwards, and plunges under the
basalts at an angle of 30° to 40°. The terraced basalts and dolerites
are not sensibly disturbed, but end off abruptly against the steep
face of intrusive rock. We might suppose that in this case the younger
rock had merely carried upward the continuation of the beds that are
truncated by it, as if an orifice had been punched out for its ascent.
But on the top of the ridge of Beinn a' Chraig we find that the
outliers which there remain are not portions of the lower basalts, but
of the upper "pale group" of Ben More. The same rocks are prolonged
on the other side of the Scarrisdale Glen, sweep over the summit of
Beinn Fhada, and run on continuously into the crest of A'Chioch and the
upper part of Ben More. The granophyre has usurped the place of the
lower dolerites and basalts, but has left the more felspathic lavas of
the "pale group" in their proper position. And to make this remarkable
structure still more clear, sections may be seen on the southern flanks
of Beinn Fhada, where the upper surface of the granophyre comes down
obliquely across the edges of the lavas, and allows the junction of the
basalts and the "pale group" to be seen above it (Fig. 355). As in the
case of Beinn an Dubhaich, it is as if the granophyre had eaten its way
upward and dissolved the rocks which it has replaced.
[Illustration: Fig. 355.--Section on south side of Beinn Fhada, Mull.
_a_, bedded basalts and dolerites; _b_, "pale group" of Ben More; _c_,
granophyre.]
The usual kind of contact-metamorphism has been produced around this
intrusive boss. It is most marked in the outliers that cap Beinn a'
Chraig and on the two ridges to the south-west, where it is seen to
consist in a high degree of induration, the production of a shattery,
irregularly-jointed structure, and the effacement of the obvious
bedding which characterizes the unaltered rocks.
The position of this eruptive mass, quite a mile broad, breaking
through, without violently tilting, more than 1800 feet of the bedded
basalts, and then stopping short about the base of the "pale group,"
presents a curious problem to the student of geological physics. It at
once reminds him of many sections among Palæozoic granites where an
eruptive boss has ascended and taken the place of an equivalent volume
of the surrounding rocks, which, though more or less metamorphosed,
are not made to dip away from it as from a solid wedge driven upwards
through them. In this Mull case, however, there are some peculiar
features that deserve consideration, for they seem to show that here,
as elsewhere, passages for the uprise of the intrusive rock were
already provided by the presence of volcanic pipes, which, even if
filled up with fragmentary materials, would no doubt continue to be
points of weakness. Round the flanks of the Loch Ba' boss, and here
and there on its surface, patches of intensely indurated volcanic
agglomerate may be detected. A little to the south of the tarn called
Loch na Dàiridh, the granophyre is succeeded by the black, flinty
felsite or rhyolite already referred to. This rock in some places
exhibits a beautiful flow-structure, with large porphyritic felspars,
and encloses a great many fragments of dolerite and gabbro, varying
from the size of a pea up to blocks several inches in diameter. Lying
on its surface are detached knolls of much altered dolerite, basalt,
and coarse breccia or agglomerate. On its southern margin one of these
patches of agglomerate contains abundant fragments of various felsitic
rocks, among which are pieces of a compact rock with flow-structure
like that found in place immediately to the north; also rounded pieces
of quartzite, and of compact and amygdaloidal basalt wrapped up in a
very hard matrix which seems to consist largely of basalt-dust. No
bedding can be made out in this rock, and the mass looks like part of a
true neck. Further down the slope the bedded basalts appear. The actual
junctions of the different rocks cannot be satisfactorily traced, but
the structure of the ground appears to me to be as shown in Fig. 356.
A patch of similar agglomerate appears a little to the south-west of
the last section in front of a cliff of the felsite, and seems to
be enclosed in the latter rock, and other exposures of agglomerate,
underlain and intensely indurated by the felsite, may be noticed on the
ground that slopes towards Loch Ba'.
[Illustration: Fig. 356.--Section to south of Loch na Dàiridh, Mull.
_a_, basalts; _b_, dolerite; _c_, volcanic agglomerate; _d_, black
felsite; _e_, granophyre.]
That these agglomerates do not belong to the period of the eruption of
the granophyre and felsite, but to that of the bedded basalts, may be
inferred from their intense induration next the acid rocks, and also
from the fact that similar breccias are actually found here interposed
between the bedded basalts. This is well shown on the hill above the
Coille na Sròine, where the accompanying section can be seen (Fig.
357). The broad dyke-like mass of black flinty felsite already referred
to runs as a prominent rib over the southern end of Beinn a' Chraig
into the head of the Scarrisdale glen (see Fig. 352). It cuts across
the bedded basalts, and immediately to the south of where these appear,
a thin intercalated bed of breccia crops out, of the usual dull-green
colour, with abundant fragments of basalt and many of yellow and grey
felsite.
From these various facts we may, I think, conclude that along the
strip of ground now occupied by the Loch Ba' boss of granophyre and
felsite, there once stood a line or group of vents, from which, besides
the usual basalt-debris, there were ejected many pieces of different
felsitic or rhyolitic rocks, and that these eruptions of fragmentary
material took place during the accumulation of the plateau-basalts.
These volcanic funnels occasioned a series of points or a line of
weakness of which, in a long subsequent episode of the protracted
volcanic period, the acid rocks took advantage, forcing themselves
upwards therein, and leaving only slight traces of the vents which
assisted their ascent. The mingling of acid and basic fragments in the
material ejected from these vents is another proof of the existence
of acid rocks in the volcanic reservoirs before the advent of the
great granophyre intrusions. The evidence thus entirely confirms the
conclusions deduced from the Skye area.
[Illustration: Fig. 357.--Section of junction of south side of Loch Ba'
granophyre boss, with the bedded basalts, Mull.
_a_, bedded basalts; _b_ _b_, basalt-tuff and breccia; _c_, granophyre;
_d_, black felsite; _e_, coarse dolerite dyke, 30 or 40 feet wide.]
The second or Glen More boss, instead of rising into hilly ground, is
confined to the bottom of the main and tributary valleys, and has only
been revealed by the extensive denudation to which these hollows owe
their origin. It begins nearly a mile below Torness and extends up to
Loch Airdeglais--a distance of almost four miles. Though singularly
devoid of topographical feature, it exhibits with admirable clearness
the relation of the granophyres to the gabbros, and thus deserves an
important place among the tracts of acid rocks in the Western Islands.
Its petrographical characters change considerably from one part of its
body to another. For the most part, it is a true granophyre, sometimes
with orthoclase, sometimes with plagioclase as its predominant
felspar. At Ishriff, as already stated, it is sprinkled with long
acicular decayed crystals of hornblende; but at the watershed the
ferro-magnesian mineral is augite. The surrounding rocks are mainly the
plateau-basalts, with their sills of dolerite and gabbro.
[Illustration: Fig. 358.--Mass of dark gabbro about two feet in
diameter traversed by pale veins of granophyre, lying on north slope of
Creag na h-Iolaire, Mull.]
[Illustration: Fig. 359.--Section at Creag na h-Iolaire, Glen More,
Mull, showing basalts and gabbros resting on and pierced by granophyre.
_a_, much indurated and altered basalts and dolerites; _b_ _b_, gabbro;
_c_, granophyre; _d_ _d_, basalt dykes.]
This strip of granophyre sends abundant apophyses from its mass into
the dark basic rocks around it. Some of the best sections to show the
nature of these offshoots are to be found on the steep hillslope which
mounts from the watershed in Glen More southward into the Creag na
h-Iolaire (Eagle's Crag), and thence up into the great gabbro ridge of
Ben Buy. From the main body of granophyre a multitude of veins ascends
through the basalts and gabbros from two feet or more in breadth down
to mere filaments (Fig. 358). Even at a height of 300 feet up the hill
some of these veins are still three inches broad, and present the
usual granophyric structure, though rather finer in grain than the
general mass of the boss, and sometimes assuming a compact felsitic
and spherulitic texture at the immediate contact with the surrounding
rock. One of the most striking proofs of the posteriority of these
veins is furnished by the perfect flow-structure they not infrequently
exhibit along their margins, their long felspar crystals being arranged
parallel to the walls in lines that follow the sinuosities of the
boundary between the two rocks. Patches of gabbro and of the indurated
basalts may be seen lying on the granophyre, from which veins and
strings ramify through them (Fig. 359). Similar veins can be traced
upward into the main body of coarse gabbro, forming the ridge of Ben
Buy. Some of them are of the usual granular granophyric texture, others
are dull and fine-grained (claystones of the older authors).
Hence it is evident that the granophyres of Mull have been protruded
not only after the accumulation of the plateau-basalts, but after
these were traversed by the sheets and veins of gabbro. The amount
of acid rock injected into these older rocks over the mountainous
part of the island is enormous; but I reserve further reference to it
for the section on acid Dykes and Veins, for these are the forms in
which it chiefly occurs in that region. It should be added, that in
the localities here referred to basalt-veins and dykes are generally
abundant, cutting through all the other rocks (Fig. 359). So numerous
are they that the geologist ceases to take note of them when his
thoughts are engaged upon the problems presented by the masses through
which they rise.
iii. THE ACID BOSSES OF SMALL ISLES
In the island of Eigg three small bosses or sheets of acid rock occur.
That at the northern end rises through the Jurassic sedimentary rocks,
and forms a bold cliff from 150 to 200 feet high. It is a light grey
granophyric porphyry, with rounded blebs of quartz in a micropegmatic
base of quartz and felspar. The other two masses, of smaller size, cut
through the bedded basalts[403] (Map VI.).
[Footnote 403: _Quart. Journ. Geol. Soc._ xxvii. (1871) p. 294.]
In the opposite island of Rum, the acid protrusions play a much more
important part. On the east side of the hills, they occur in sheets at
the base of the gabbros; on the west side, they form a large tract of
hilly ground, which, stretching along the coast-line for about three
and a half miles from the headland of A' Bhrideanach to Harris, forms
there a range of shattered sea-cliffs, that tower for 1000 feet above
the Atlantic breakers that beat about their base. The area extends
inland to the slopes on the west side of Loch Sgathaig, a distance of
about three and a half miles, descending in a range of precipices along
its northern front, and reaching in its culminating summit, Orval, a
height of 1868 feet above the sea. The rocks of which this triangular
area consists resemble those of the Mull bosses. They are chiefly
quartz-porphyries, becoming felsitic in texture towards their contact
with adjacent rocks. In some places, as was noticed by Macculloch on
the sea-cliffs,[404] they have a rudely bedded structure. Thus on the
north-west front of Orval, this structure is shown by parallel planes
that dip outwards or north-west at 30° to 40°, and which are made
still more distinct by an occasional intrusive dyke or sheet of basalt
between their surfaces. I have already alluded to indications of an
internal arrangement in the granitoid bosses of Skye (p. 381).
[Footnote 404: _Western Islands_, vol. i. p. 487.]
[Illustration: Fig. 360.--Section on north side of Orval, Rum.
_a_, Torridon sandstones; _b_, bedded basalts of Fionn Chro; _c_,
dolerite; _d_, quartz-porphyry.]
[Illustration: Fig. 361.--Junction of Quartz-Porphyry (Microgranite)
and Basic Rocks, south-east side of Orval, Rum.
_a_, basalts and dolerites; _b_, dolerite and gabbro veins; _c_,
quartz-porphyry cutting _a_ and _b_.]
As in the other islands, the granophyres, porphyries and felsites
of Rum have been intruded at the base of the volcanic series. Over
much, if not all, of their area they lie directly on the red Torridon
sandstone. That the bedded basalts once covered them is indicated
by the position of the three outliers of the basalt-plateau already
noticed. But a fourth outlier still lies upon the porphyry of Orval
as a cake that dips gently northward. It consists of a bedded, dark,
finely-crystalline, ophitic dolerite, porphyritic in places, with a
rudely prismatic or columnar structure (Fig. 360). It has undergone
contact-metamorphism, and tongues from the underlying rock project
up into it. On the south-eastern side of the same hill, still more
striking evidence is presented of the posteriority of the acid to
the basic rocks. The porphyry shows here the same tendency to assume
a bedded structure, the parallel "beds" again dipping outward or
south-east at 40°. They plunge under the body of gabbro, dolerite and
other intrusive masses which from this point stretch eastward into the
great cones of Allival and its neighbours. The rock at the junction
is a fine microgranite with traces of micropegmatite. It is composed
of a holocrystalline base of quartz and orthoclase, with porphyritic
crystals of microcline, blebs of quartz and scattered granules of
augite. The rocks that rest immediately next it are basalt and
dolerite, into which it has sent an intricate network of veins (Fig.
361).[405] It has also pushed long tongues down the slope into them,
which may be seen traversing the dolerite and gabbro veins that cut
the basalts. The basic rocks next the porphyry have been intensely
altered. They seem in places as if they have been shattered by some
explosive force, and had then been invaded by the mass that rushed into
all the rents thus caused. This remarkable structure is still better
displayed on St. Kilda, and is more fully described in the following
account of the geology of that island.
[Footnote 405: In a thin slice cut from a specimen showing the junction,
there is a minute vein of the porphyry penetrating the basalt which is
much altered, while the porphyry becomes much finer in grain than at a
distance from the contact.]
iv. THE ROCKS OF ST. KILDA
Brief allusions to St. Kilda and its rocks have already been made (pp.
173, 358). We may now enter more fully upon the consideration of its
geological structure and history.
When the weather is clear there may be seen from the western headlands
of the Outer Hebrides a small blue cone rising above the Atlantic
horizon at a distance of about 60 miles. As the voyager approaches
this distant land it gradually shapes itself into a group of islets of
which St. Kilda, the largest and only inhabited, has an extreme length
of about four miles, a breadth of less than two miles, and a height of
1262 feet above the sea. Four miles to the north-east Borrera, about
one square mile in extent, rises with precipitous sides to a height
of 1000 feet. Off the north-western promontory of St. Kilda the huge
rock of Soay, half a square mile in area, towers from 600 to 800 feet
above the waves. Borrera has two attendant rocks--Stack Li and Stack
an Armin--huge pyramidal masses several hundred feet high, and the
home of thousands of gannets. St. Kilda possesses two less imposing
islets between its north-western headland and Soay, and a third to the
south-east known as Levenish.
The scenery of this picturesque group affords a good indication of its
geological structure. It displays two distinct types of topographical
form. In Borrera the marvellous combination of spiry ridges, deep
gullies and clefts, notched crests and splintered pinnacles, at once
reminds the visitor of the outlines of the Cuillin Hills of Skye. The
same features are repeated on a less magnificent scale in Soay and
along the whole of the south-western precipitous coast-line of St.
Kilda.
In marked contrast to these varied outlines, the eastern half of St.
Kilda rises with a smooth green surface, varied with sheets of grey
screes, up to the rounded summit of Conagher, the highest point in the
island. If the dark crags of the rest of the island group remind one of
the Cuillins, this eastern tract recalls at once the form and colour
of the Red Hills of Skye. A closer examination shows that in each case
the topography arises from the influence of the very same rocks and
geological structure as in that island.
There is, however, one aspect in which St. Kilda has no rival
throughout the Western Isles. Its russet-coloured cone, though rising
on the west side with gentle green slopes from the central valley,
plunges on the eastern side in one vast precipice from a height of
1000 feet or more into the surge at its base. Nowhere among the Inner
Hebrides, not even on the south-western side of Rum, is there any such
display of the capacity of the youngest granite to assume the most
rugged and picturesque forms. It is hardly possible to exaggerate the
variety of outline assumed by the rock as it yields along its system
of joints to the influence of a tempestuous climate. It has been carved
into huge projecting buttresses and deep alcoves, the naked stone
glowing with tints of orange and fawn colour, veiled here and there
with patches of bright green slope, or edged with fringes of sea-pink
and camomile. Every outstanding bastion is rent with chasms and split
into blocks, which accumulate on the ledges like piles of ruined
walls. To one who boats underneath these cliffs the scene of ceaseless
destruction which they present is vividly impressive.
The geology of St. Kilda was sketched by Macculloch, who recognized
the close resemblance of its two groups of rock to the "augite-rock"
(gabbro) and "syenite" (granophyre) of Skye and other islands of
the Inner Hebrides. But he left the relations of the two groups to
each other undetermined.[406] Professor Heddle has published a brief
reference to the rocks of St. Kilda, without, however, offering any
definite opinion as to the geological structure of the islands.[407]
The best account of the geology has been given by Mr. Alexander Ross,
who obtained evidence that the acid sends veins into the basic rock.
He brought away specimens clearly showing this relation, but in his
description left the question open for further inquiry.[408] To some of
the observations in these papers reference will be made in the sequel.
The following account is based on the results of two visits paid by
me to St. Kilda in the summers of 1895 and 1896, during which I was
enabled to examine the rocks on land, and to sail several times round
the islands, boating along those parts of the cliffs which presented
features of special geological importance.
[Footnote 406: _Description of the Western Isles_, vol. ii. p. 54.]
[Footnote 407: In an article on the general geological features of
the Outer Hebrides contributed to _A Vertebrate Fauna of the Outer
Hebrides_, by J. A. Harvie-Brown and T. E. Buckley, 1888.]
[Footnote 408: _British Association Report_, 1885, p. 1040, and a much
fuller paper in the _Proceedings of the Inverness Field Club_, vol.
iii. (1884), p. 72.]
In the St. Kilda islets three groups of rock differing from each other
in age may be recognized. 1st, A series of gabbros, dolerites and
basalts which have been intruded through and between each other as
sills; 2nd, a mass of granophyre which invades these sills; and 3rd,
abundant dykes and veins of basalt which occur both in the basic and
acid masses.
From the extension of the basalt-dykes across the Outer Hebrides it is
clear that the Tertiary volcanic region reached at least to within 60
miles of St. Kilda. Whether or not it stretched over the intervening
space now overflowed by the Atlantic must be matter for conjecture.
There can be no doubt that the intrusive rocks of St. Kilda are in
age and origin the equivalents of those of the Inner Hebrides. The
remnants left of them were assuredly not superficial extrusions, but
are characteristic examples of the more deep-seated intrusions of the
Tertiary volcanic period. Down to the most minute details of structure
they reproduce the features so well displayed by the gabbros and
granophyres of Skye, Rum and Mull. If it is demonstrable in the case
of these islands that the intrusions have taken place under a deep
cover of basalt-sheets, now in large part removed, the inference may
legitimately be drawn that at St. Kilda a basalt-plateau once existed
which has been more completely destroyed than in the other regions.
Not a fragment of such a plateau has survived, unless we may perhaps
be allowed to recognize it in some of the basalts enclosed among the
gabbro-sills. Placed far amid the melancholy main and exposed to the
full fury of the Atlantic gales, these islets must be regarded as the
mere fragmentary cores of a once much more extensive volcanic area.
The geologist who visits them is deeply impressed at every turn by the
evidence of the active and unceasing destruction which their cliffs
are undergoing. Nothing now remains save the deep-seated nucleus of
intrusive sills, bosses and dykes.
1. _The Gabbro Sills._--The rudely-bedded arrangement of these rocks is
conspicuous along the west side of St. Kilda, in Soay and in Borrera.
They consist of coarse and fine varieties disposed in successive sheets
which dip at angles varying from as little as 15° up to as much as 60°
or even more. In St. Kilda they form the picturesque promontory of
the Dune, and extend thence along the western side of the island to
its extreme northern end. Their escarpments face the ocean, and their
dip-slopes descend towards the north-east in grassy declivities to the
south bay and the long verdant glen which runs thence across to the
north bay. The same strike is prolonged into Soay, but further east in
Borrera the direction curves so as to present vast escarpments towards
the west and shelving sheets of rock towards the east.
None of the gabbros seen by me are as coarse as the large-grained
varieties of Skye, nor does there appear ever to be such a marked
banded structure among them as that displayed by the Cuillin rocks.
Faint banding, however, may be noticed. A series of specimens which
I collected from the west side of the island has been sliced for
microscopic examination, and Mr. Harker has furnished me with the
following notes regarding them.
"An olivine-gabbro from the west side of St. Kilda [7107] is a
dark, heavy, medium-grained rock, in which augite and felspar are
conspicuous. The microscope shows, in addition, plentiful grains of
olivine, with but little original iron-ore, and some apatite-needles.
The structure is ophitic, the plates of pale-brown augite enveloping
both olivine and felspar. A little brown hornblende and red-brown mica
are probably original, the rock showing little sign of alteration. The
felspar is labradorite, with albite- and Carlsbad-twinning, and forms
elongated rectangular crystals.
"Another specimen [7108] is a rock of similar appearance but somewhat
coarser texture, and structurally is a more typical gabbro than the
preceding, the felspar having little of the 'lath' shape, while the
augite, though still moulded on the felspar, scarcely assumes an
ophitic habit. A striking feature in this rock is the way in which
the augite is crowded with 'schiller'-inclusions, in places so
closely as to be almost opaque. A high magnification shows that these
inclusions are dark, linear in form, and disposed along two directions
intersecting at a high angle. The labradorite has unusually close
twin-lamellation on both albite and pericline laws, and it is possible
that this is a strain-effect.
"A third specimen [7109] is from a rock in every respect identical
with the preceding, except that the olivine is rather more plentiful,
and in some grains is partially serpentinized."
While the gabbros of St. Kilda are not a mere uniform boss, but a
series of sills and irregular masses which have been successively
injected into each other, they have subsequently been cut through by
many basalt-dykes and veins. These, which are sometimes as abundant
as in the gabbro of the Cuillin Hills, traverse the rocks both in
the line of bedding and also at many different angles across it. As
they generally weather faster than the gabbros, they give rise to
deep narrow clefts which may be traced up the whole height of the
precipices, occasioning sea-caves below and sharp notches on the crests
above.
These scenic features, so indicative of the geological structure that
causes them, are specially well seen on the western face of the Dune or
south-western promontory of the island, and likewise in the strangely
rifted precipices further north and in Soay. They are, however, most
impressively displayed around the naked walls of Borrera, which in
their marvellous combination of spiry ridges, deep straight gullies,
and splintered crests, remind one at every turn of the scenery of
Blaven and the Cuillin Hills.
2. _The Granophyre Boss and its Apophyses._--The eastern half of the
island of St. Kilda consists of a pale rock which Macculloch long ago
identified with the granophyre of Skye, and which, as he pointed out,
has much resemblance to parts of the granite of Arran.[409] Not only does
it give rise to topographical forms like those of the Red Hills, but it
weathers, like the Skye granophyre and the Arran granite, into thick
bed-like sheets divided by transverse joints into large quadrangular
blocks. On closer inspection it is found to resemble still more
precisely the acid rocks of the Inner Hebrides. It possesses the same
drusy micropegmatitic structure as the granophyres of Skye, Rum and
Mull. The ferro-magnesian constituents are present in small quantity,
hence the pale hue of the stone. The quartz and felspar project in
well-terminated crystals into the drusy cavities, which are sometimes
further adorned with delicate tufts of clear crystallized epidote. In
these and other respects the rock displays the familiar external forms
of the younger or Tertiary granites of Britain.
[Footnote 409: _Description_, vol. ii. p. 54.]
Mr. Harker's notes on the microscopic structure of this granophyre are
as follows:--"The prevailing felspar is orthoclase, often very turbid
from secondary products. Even what appear to be distinct crystals are
sometimes seen in the slices to be invaded on the margin by quartz in
rough micrographic intergrowths, and much of the finer intergrowth
occurs as a fringe to the crystals. In this case the felspar of the
micropegmatite can often be verified to be in crystalline continuity
with the crystal which has served as a nucleus [6624]. Quartz occurs
in distinct crystals and grains as well as in the micropegmatite.
There is a more granitoid variety of the rock, in which only a very
rude approach to micrographic intergrowths is seen [6623]. In both
varieties there is but little trace of any ferro-magnesian mineral; the
more typical granophyre has what seems to be destroyed augite, while
the granitoid rock contains a little deep-brown biotite. Scattered
crystal-grains of magnetite occur in both."
Narrow ribbon-like veins of a finer material, sometimes only an inch in
breadth, traverse the ordinary granophyre. Similar veins run through
the rock of the Red Hills in Skye; they are sharply defined from
the enclosing rock, as if the latter had already solidified before
their intrusion. With regard to the microscopic structure of some
thin slices prepared from these veins, Mr. Harker remarks that "the
material of the veins is of a type intermediate between granophyre
and microgranite [6622, 6623]. The chief bulk is a finely-granular
aggregate of quartz and felspar, the latter very turbid; but in this
aggregate are imbedded numerous patches of micropegmatite, often of
perfect and delicate structure. These areas of micropegmatite show
some approach to a radiate or rudely spherulitic structure, and, in
some cases, are clustered round a crystal of felspar or quartz. Some
granules of magnetite and rare flakes of brown biotite are the only
other constituents of the rock. Although they must be of somewhat later
date, there is evidently nothing in the petrographical characters of
these fine-textured veins to separate them widely from the ordinary
granophyres of the region."
These veins may be compared with the spherulitic dyke that traverses
the granophyre of Meall Dearg at the head of Glen Sligachan (described
at p. 381), which, though undoubtedly somewhat younger than the rock
that contains it, yet presents the very same structures as are visible
at the margin of that rock.[410] The material of this dyke and of the
finer veins of St. Kilda and the Red Hills probably belongs to a later
period of protrusion from a deeper unconsolidated portion of the same
acid magma as at first supplied the general body of granophyre.
[Footnote 410: _Quart. Journ. Geol. Soc._ vol. 1. (1894), p. 220.]
Undoubtedly the most interesting feature in the granophyre of St.
Kilda is its junction with the mass of basic rock to the west of it.
This junction-line runs from about the middle of the chief or south
bay (where, however, its precise position is concealed under detritus)
across the island to the north shore, where it descends the face of the
precipice and plunges under the sea. Important as the actual contact
of the two rocks obviously is in regard to their relative date, it has
not hitherto been observed or described. Macculloch noticed "numerous
fragments of trap penetrated by veins of syenite," but he did not see
these rocks in place, and, in spite of their apparent testimony to the
posteriority of the acid intrusions, he was inclined to believe that
the veins were not real veins, but that the "trap" and "syenite" had a
common origin and would be found to pass into each other, as he thought
also occurred in Mull and Rum. In recent years Mr. Alexander Ross,
during his visit to St. Kilda, collected specimens illustrating the
varieties of gabbro, dolerite and basalt, and showing the intrusion of
the acid into the basic rocks. As already stated, he was disposed to
regard the "granite" as of younger date than the gabbros, but left the
question undecided.[411]
[Footnote 411: In his paper, _Proceed. Inverness Field Club_, vol.
iii. (1884), p. 78, Mr. Ross quotes a letter from Prof. Judd, who
there states that the rock supposed to be granite "is seen under the
microscope to be a quite different rock--a quartz-diorite." Some of
the specimens from St. Kilda collected by Mr. Ross were exhibited at
a meeting of the Geological Society on 25th January 1893. With regard
to these Prof. Judd, in the course of the discussion on his paper on
"Inclusions of Tertiary Granite in the Gabbro of the Cuillin Hills,"
remarked:--"They show a dark rock traversed by veins of a light one,
but the dark rock is not a gabbro and the light one is not a granite"
(_Quart. Journ. Geol. Soc._ vol. xlix. (1893), p. 198).]
The best locality for the examination of the junction of the main
granophyre mass with the gabbros is inaccessible save by boat, and only
in the calmest weather. It occurs in the great cliff on the northern
side of the island between the north bay and the sea-stack known as
the Bragstack. The line of contact emerges from below the sea-level,
and ascends the cliff with a westward inclination of from 60° to 80°.
Here, as in Skye, the acid rock underlies the basic masses, which are
rudely bedded and much jointed. About 150 feet above the sea-level,
the nearly vertical cliff breaks up into an exceedingly rocky and
rugged acclivity, across which the junction seems to slope at a lower
angle. But the place is hardly reachable, save perhaps by the intrepid,
barefooted cragsmen of St. Kilda.
[Illustration: Fig. 362.--Junction of granophyre and gabbro, north side
of St. Kilda.]
Along the sharply defined line of contact the granophyre is
close-grained, and sends a network of veins into the dark sheets of
gabbro. The general features of the junction are represented in Fig.
362. The veins are narrow, those nearest the main body of granophyre
diverging from it at a still more acute angle than those from the
mass of Meall Dearg (Fig. 376), and then branching so as to enclose
masses of the gabbro and to run across them in long parallel veins.
A characteristic feature of many of these veins, besides their
narrowness, is their tendency to split up at the ends into mere fingers
and threads as represented in Fig. 363.
Owing to the depth of soil on the cultivated land, and of boulders and
sand on the beach, the actual junction of the main body of granophyre
with the gabbro is not seen on the southern shore. But a few yards to
the westward of where it must lie, the beach is cumbered with large
blocks of rock broken up from the mass, which can be seen _in situ_
a little further south forming a line of low cliff with a rugged
foreshore. These rocks consist of various gabbros and basalts of rather
fine grain, profusely traversed with veins of white granophyre. Some
of these veins are two feet or more in breadth, and, when of that
size, show the distinctive granular texture and drusy structure of
the main part of the acid rock. But from these dimensions they can
be traced through every stage of diminution until they become mere
threads. When they are only an inch or two broad, they assume a finely
granular texture like that of the veins that run through the body of
the granophyre.
[Illustration: Fig. 363.--Veins of granophyre traversing gabbro and
splitting up into thin threads, north side of St. Kilda.]
The amount of injected material in the dark basic rocks is here and
there so great as to form a kind of breccia (Fig. 364), which, from
the contrast of tone between its two constituents, makes a conspicuous
object on the shore. Here, as in the example already cited from Rum,
the basic rocks seem to have been shattered into fragments, and the
acid material to have been injected into the minutest interstices
between them. The enclosed fragments are of all sizes from mere grains
up to blocks a foot or more in length. They are generally angular, like
rock-chips from a quarry. Moreover, they are not all of the same kind
of material. While at this locality most of them consist of basalt,
they include also a few large and small pieces of rather coarse gabbro.
There has evidently been a certain amount of transport of material, as
well as an extensive disruption of the rocks _in situ_. The granophyre
here and there assumes a darker or greener tint, as if it had dissolved
and absorbed some portion of the older rock.
Still more astonishing are the sections to be seen on the western
cliffs and rocky declivities of the ridge to the north of the Dune,
at a distance of perhaps 500 or 600 yards westwards from those of
the South Bay. Here the gabbro-sheets are traversed by a number of
conspicuous white bands, which on examination prove to be veins or
dykes of granophyre. As viewed from the sea, the general disposition
of the two groups of rocks is represented in Fig. 366. The broadest
mass of granophyre breaks out towards the bottom of the precipice, and
slants upward as a sheet intercalated between the gabbro sills, with
a breadth of about 40 or 50 feet, but rapidly thinning away in its
ascent. One of the bands below it has a breadth of about 15 feet. The
material of these intrusions is a pale fine-grained granophyre like
that of the South Bay, I did not detect, either here or anywhere else
in St. Kilda, a definite spherulitic structure such as is so common in
the granophyre dykes of Skye.
[Illustration: Fig. 364.--Pale granophyre injected into dark basalt,
South Bay, St. Kilda.
The crags on the further side of the bay are the gabbro sheets of the
Dune. (From a photograph by Colonel Evans.)]
Though the acid intrusions are somewhat irregular both in thickness
and direction, they lie generally parallel to each other in the line
of strike of the bedding of the gabbros. They are no doubt apophyses
from the main body of granophyre, which emerges to the surface about a
third of a mile to the eastward, but may of course be at no great depth
underneath.
[Illustration:
Fig. 365.--Veins of granophyre traversing a line-grained gabbro and
scarcely entering a coarse-grained sheet, west side of Rueval, St.
Kilda.
]
Besides the broader bands of acid rock, and diverging from them, a
complicated network of veins ramifies in all directions through the
gabbros, as at the South Bay. The extraordinary degree to which the
basic rocks have been shattered into fragments is strikingly displayed
here, likewise the extreme liquidity of the acid magma, whereby it
was able to insinuate itself into every chink and cranny. But the
observer notices that this condition of excessive disruption is not
shared by all the basic sills, and is not attendant upon all the acid
dykes. As an example of this irregular distribution of the structure,
I give the accompanying sketch (Fig. 365), where a fine-grained gabbro
has been completely broken up and intersected with granophyre veins,
while the coarser sheet overlying it has almost entirely escaped.
The dark basalt-like sheets appear generally to have been much more
disrupted than the more largely-crystalline varieties. It is noticeable
here, also, that the fragments entangled in the network of granophyre
veinings do not entirely belong to the rock that has been shattered,
but sometimes include large and small lumps of different gabbros,
showing some transference of material with the inrush of the acid magma.
Though closer in grain where it comes in contact with the gabbro,
the granophyre never assumes any vitreous texture along its margin.
A series of thin slices, prepared from specimens collected by me in
the South Bay in the summer of 1895, was examined by Mr. Harker, who
furnished the following notes regarding them:--"The basalt traversed
by the granophyre is a fine-textured variety with small porphyritic
felspars. These latter seem to be usually unaltered, retaining the
glass cavities which in some of the crystals are abundant. The
groundmass, however, shows minerals of metamorphic origin which must
be derived mainly from the original augite. A brown mica is the most
conspicuous; but with it are associated some brownish-green hornblende
and certain chloritic and perhaps serpentinous substances. It is
chiefly near the margin of a fragment of basalt that the mica gives
place to these minerals. The basalt still retains plenty of unaltered
granules of augite in the central parts of a fragment. It is not
certain that the secondary minerals named come exclusively from the
augite of the basalt; from their form and mode of occurrence they may
in part have replaced olivine or even rhombic pyroxene.
[Illustration: Fig. 366.--View of sills and veins of pale granophyre
traversing dark sheets of gabbro, west side of St. Kilda.
(From a photograph by Colonel Evans.)]
"The acid rock, though styled granophyre above, belongs to a granitoid
variety of that group of rocks, and has but little indication of
micrographic structures. Compared with the other granophyres from
St. Kilda, sliced and examined, these examples show a less acid
composition. This is expressed mineralogically in the presence of
a somewhat larger proportion of ferro-magnesian minerals and of
soda-lime felspar. These features might indeed be matched in many
normal granophyres among the Western Isles, but in the present case it
can hardly be doubted that they are to be explained, at least in some
degree, by the acid magma having taken up a certain amount of material
from the basalt. Many of these Tertiary granophyres have undoubtedly
been modified by the incorporation of pieces of basalt and gabbro,
and a collection made in the Strath district of Skye will furnish
examples for future study. Professor Sollas's description of similar
phenomena in the Carlingford district has already proved the importance
of this kind of action.[412] In the present instance, both brown mica
and hornblende occur plentifully in the granophyre, and especially
round the basalt fragments. This latter point is conclusive as to the
derivation of the basic material, and further proves a certain degree
of viscosity in the acid magma at the time of its intrusion."
[Footnote 412: _Trans. Roy. Irish Acad._ vol. xxx. (1894), pp. 477-572.]
Another series of specimens which I collected in the following year
was submitted to Mr. Harker for petrographical determination, and his
observations on two of the microscopic slices are as follow: "A breccia
from the South Bay, St. Kilda [7105], consists of angular fragments up
to two inches in diameter set in a matrix of grey granophyre of medium
texture. The fragments belong to two types--one of very close texture
(basalt), the other more evidently crystalline (diabase). Both are cut
by the slice.
"The basalt shows very evident metamorphism, its augite being wholly
transformed into greenish-brown hornblende. The little felspar-laths
and granules of iron-ore seem to be unaltered, though the latter may
perhaps have contributed to the formation of the hornblende. Another
fragment of basalt has some larger crystal-grains of augite, and these
are not converted into hornblende.
"The diabase shows a less marked boundary under the microscope, but
otherwise has similar characters to the preceding. The striated
felspar-crystals and grains of iron-ore have not been re-crystallized.
A considerable amount of pale augite remains, but there is also plenty
of deeply-coloured hornblende, both fibrous and compact. This diabase
is certainly an intrusive rock, but the basalt, from its petrographic
character, might be from a lava-flow or from a dyke.
"The granophyre is of somewhat coarse texture, the micrographic
structure being only of a rude type. It is notably richer in the darker
constituents than is usual in such rocks. Further, the hornblende and
magnetite tend to cluster in little patches which suggest destroyed
fragments of basic rocks. A grain or two of sphene occur, a mineral
foreign to the normal granophyres.
"Another similar specimen [7106] from the same locality shows a basic
rock of coarser texture, approaching some of the gabbros in appearance
and with boundaries in places not very sharply defined. The grey matrix
is again relatively rich in the dark elements, and the manner in which
they occur in little patches, like nearly obliterated 'xenoliths,'
points unmistakably to a certain amount of absorption of basic material
by the acid magma, with consequent enrichment in the ferro-magnesian
minerals.
"The slice cuts only the acid rock, which is seen to be of granitoid
rather than granophyric structure, though the tendency of the felspar
to enclose quartz-grains is unlike a typical granite. Oligoclase, with
combined albite- and Carlsbad-twinning, is well represented in addition
to orthoclase, and some zoned crystals seem to be of albite with a
border of oligoclase. Brown hornblende and a little brown mica are the
coloured constituents. Magnetite and apatite are also observed."
The testimony of the rocks of St. Kilda to the posteriority of the
granophyre to the gabbros and basalts is thus clear and emphatic.
It entirely confirms my previous observations regarding the order
of sequence of these rocks in Mull, Rum and Skye. But the St. Kilda
sections display, even more strikingly than can be usually seen in
these islands, the intricate network of veins which proceed from the
granophyre, the shattered condition of the basic rocks which these
veins penetrate, the remarkable liquidity of the acid magma at the time
of its intrusion, and the solvent action of this magma on the basic
fragments which it enveloped.
3. _The Basic Dykes._--Reference has already been made to the numerous
dykes by which the gabbros of the St. Kilda group of islets is
traversed. Similar dykes occur also, though less plentifully, in the
granophyre. It remains for future observation to determine whether
there is one series older and another later than the intrusion of the
acid rock. In any case, it is quite certain that the dykes in the
gabbro do not all belong to one period of injection, for frequent
examples of intersection may be noticed, especially on the cliffs of
Borrera, and also cases of double and even treble dykes which have been
formed by successive infillings within the same fissure. The remarkably
varied precipices of that island are marked by the long narrow rifts
left by the weathering of vertical dykes, which, as above remarked, may
be followed with the eye from the sea-level to the sky-line, ascending
obliquely across the bedding of the gabbro sheets. Another group of
dykes may be traced sloping upward at low angles along the face of
the cliffs and affording admirable ledges with overarching roofs
for innumerable gannets, kittywakes and guillemots. Other dykes and
ribbon-like veins may be seen traversing the gabbro in many different
directions, precisely as among the Cuillin Hills. As no similar network
of dykes and veins is to be observed in the granophyre, I am disposed
to regard a large number of these intrusions as older than that rock.
But I did not observe any actual example of a basic dyke truncated by
the granophyre.
There can be no doubt, however, that an injection of similar dykes and
veins took place after the invasion of the granophyre. These later
intrusions are conspicuously displayed along the cliffs that extend
from the gabbro junction on the north side of St. Kilda round the
eastern coast into the South Bay. They maintain a general parallelism
and ascend from the sea-level at varying angles of inclination, running
up the pale sea-wall as dark bands. They consist of basalt-rocks, and
may often be seen to branch and to die out. Like those in the gabbro,
they are not infrequently compound, being made up of two or three or
even more distinct dykes. This is well seen on the great precipice
below Conacher, where the section given in Fig. 367 is displayed. Here
in a vertical height of about 800 or 900 feet, there must be at least
seven dykes, simple and compound. A little further south a triple dyke
may be seen to be composed of a thick central zone and two thinner
marginal bands, of which the lower strikes off from the others and
maintains an independent course through the granophyre (Fig. 368).
[Illustration: Fig. 367.--Section of the sea-cliff below Conacher, St.
Kilda, showing basic dykes in granophyre.]
[Illustration: Fig. 368.--Triple basic dyke, sea-cliff, east side of
St. Kilda.]
V. THE GRANITE OF ARRAN
The northern half of the island of Arran is mainly occupied by one
of the most compact and picturesque groups of granite mountains in
Scotland.[413] These heights, rising out of the Firth of Clyde to a
height of 2866 feet, present, in their spiry and serrated crests, a
contrast to the smoother contours of the older granitic elevations of
this country. The granite is surrounded by a ring of schistose rocks,
belonging to the metamorphic series of the Southern Highlands, save for
a short distance on the eastern margin, where it comes in contact with
and indurates the Lower Old Red Sandstone. Macculloch long ago pointed
out that no pebbles of the granite are to be found in the surrounding
conglomerates and red sandstones of Carboniferous and younger age.[414]
Geologists accordingly came to the conclusion that the protrusion of
the granite took place after Carboniferous time, and hence that it had
no connection with the appearance of the far older granites of the
Highlands. In the year 1873 I gave reasons for believing the granite
to be not only younger than the Carboniferous formations, but to be
referable with most probability to the Tertiary volcanic series.[415]
The progress of inquiry has tended to confirm this inference, though
no direct proof of its correctness has been obtained. Two lines of
investigation may be pursued, and each leads to the conclusion of the
probability of the Tertiary age of the granite. One of these proceeds
on a comparison of the petrographical characters of the Arran rocks
with those of undoubted members of the Tertiary series among the
Western Isles. The other inquiry deals with the relation of the rocks
to each other in the general geological structure of Arran itself.
[Footnote 413: The rocks of Arran have often been described. Besides the
work of Macculloch above quoted, reference may be made to the paper by
Sedgwick and Murchison, _Trans. Geol. Soc._ 2nd Ser. vol. iii. p. 21;
A. C. Ramsay's _Geology of the Island of Arran_, 1841, the paper of
Necker de Saussure quoted on p. 412; J. Bryce's _Geology of Clydesdale
and Arran_, 3rd edit. 1865. The island is at present being surveyed for
the Geological Survey by Mr. W. Gunn.]
[Footnote 414: _Description of the Western Islands of Scotland_, vol. ii.
p. 388.]
[Footnote 415: _Trans. Edin. Geol. Soc._ vol. ii. part iii.]
Macculloch first remarked the strong lithological resemblance of
the Arran granite to the "syenite," or granophyre, of Skye and St.
Kilda.[416] More recent petrographical investigation, as already stated,
has furnished additional proofs of the connection between the acid
rocks of these islands. So closely indeed are these rocks linked by
megascopic and microscopic characters, that the petrologist has no
hesitation in placing them together as probably products of the same
period of igneous activity.
[Footnote 416: _Description_, vol. ii. p. 352.]
From the general geological structure of Arran, a further strong
argument may be deduced in favour of the late date of the eruptions
of granite. Good reasons have been given for classing as Permian the
bright red sandstones which occupy much of the central and southern
parts of this island, and include the little volcanic group already
referred to. These sandstones have been invaded by a complex series of
eruptive rocks which would thus be later than the Permian period. No
igneous masses posterior to this period are certainly known in Britain
save those of Tertiary age. The larger body of granite in the northern
half of the island nowhere comes into direct contact with the newer red
sandstones, but these strata are pierced by smaller bodies of granite.
Hence, both by the evidence of their internal structure and by the
stratigraphy of the ground, the later igneous rocks of Arran may be
reasonably grouped together as one important and consecutive series,
comparable in age and general characters with those of Tertiary date in
the Inner Hebrides.
[Illustration: Fig. 369.--Jointed structure of the granite near the top
of Goatfell Arran.
(From a photograph by Mr. W. Douglas, lent by the Scottish
Mountaineering Club.)]
The igneous rocks of Arran, later than the probably Permian sandstones,
range from acid to basic in composition. Besides the northern granite,
there are in the southern part of the island acid rocks that include
granite, coarse-grained quartz-porphyry and fine-grained felsite. Where
the relations of these rocks to each other can be seen, the felsite is
found by Mr. Gunn to be newer than the porphyry, into which it sends
sills and dykes.
A feature observed by the same geologist in Arran offers a further
point of resemblance to the acid sills and dykes of Skye. He has
noticed that accompanying the quartz-porphyry of Drumadoon and Bennan,
a mass of basic rock forms a kind of fringe or selvage round it,
sometimes with what appears to be a rock of intermediate character
between them. Basic sills are abundant south of Glen Ashdale, though
to the west of Whiting Bay most of the intrusive sheets are of acid
material.
Some of the quartz-porphyry sheets are markedly columnar. One of
them, near Corriegills, displays a divergent grouping of the prisms,
not unlike parts of the pitchstone sheets of Eigg and Hysgeir, and
suggestive of the rock having flowed along a hollow like that of a
valley. No certain trace, however, has been found of any Tertiary
lava-stream in Arran, nor has evidence of tuffs been detected in
any part of the younger igneous series. All the rocks appear to be
intrusive, though so abundant and varied are they as to indicate that
they belong to a vigorous eruptive centre, which may have poured out at
the surface lavas and ashes, since entirely removed by denudation.
The numerous basic dykes for which the south end of Arran has long
been celebrated have a general northerly trend, and appear to be all
of the same or nearly the same age. They undoubtedly cut through the
quartz-porphyries and the coarse-grained basic sills, but are less
numerously visible in the finer-grained basic sills, while in the
felsitic sheets they are seldom to be seen. In several places dykes
running in an E.N.E. direction cut the others, and are therefore of
later date.[417] The compound dykes of Tormore on the west side of the
island have been already noticed (p. 161).
[Footnote 417: _Ann. Rep. of Geol. Surv._ for 1894, p. 286.]
VI. THE NORTH-EAST OF IRELAND
In the north-eastern counties of Ireland there are two regions which
afford ample material for discussion in connection with the protrusion
of acid rocks during the Tertiary volcanic period. One of these, which
for distinction may be called the Carlingford region, embraces the
tract of country which includes the Mourne Mountains on the north-east
side of Carlingford Lough and the ranges of Slieve Foye and Slieve
Gullion on the south-west side. The other lies mainly within the
basaltic plateau, the largest of its scattered portions forming parts
of the hills of Carnearny and Tardree in the county of Antrim (Map
VII.).
1. The Carlingford Region
a. _The Mourne Mountains._--This compact and picturesque group of
hills, about twelve miles long and six miles broad, and reaching a
height of 2798 feet in Slieve Donard, presents a comparatively simple
geological structure, since it consists almost entirely of granitic
rocks which pierce, overlie and underlie Upper Silurian grits
and shales. So far as regards the contact of these rocks with the
disrupted sedimentary formations, all that can be asserted is that
the granite must be later than at least the older part of the Upper
Silurian period. But for at least two reasons, the eruptive rocks
may be regarded with some confidence as part of the Tertiary series.
In the first place, there is a strong petrographical resemblance
between the Mourne Mountain granite and that of the Island of Arran
and the granitic parts of the granophyre of the Western Isles. And
this resemblance is so close as to furnish a cogent argument in favour
of grouping all these rocks together as parts of one geologically
contemporaneous series. In the second place, the Mourne Mountain
granite abruptly cuts off a large number of basic dykes which, running
in a general N.N.W. direction, may be looked upon as almost certainly
members of the Tertiary system of protrusions.
The manner in which the granite of the district behaves towards certain
detached areas of Silurian strata with their accompanying dykes is one
of the most astonishing features in the whole assemblage of intrusive
rocks in Britain. As has been excellently shown in the Geological
Survey Map and sections by Mr. W. A. Traill, the granite has carried up
on its surface broad cakes of vertical Silurian strata, together with
all their network of dykes.[418] A cake of this kind, from 50 to about
200 feet thick and nearly two miles broad, has been bodily uplifted
from the rest of the mass and carried upward by the granite, so that
the truncated ends of the beds of grit and shale with their system of
dykes stand upon a platform of granite, from which also numerous veins
penetrate them. There can be little doubt that the basic dykes thus
broken through are parts of the great Tertiary system, and if so, the
granite which disrupts them cannot be older than Tertiary time.
[Footnote 418: See Sheets 60, 61 and 71 of the one-inch map of the
Geological Survey of Ireland, and Sheets 22, 23 and 24 of the
Horizontal Sections. The Explanation to these Sheets of the map was
written by Professor Hull, Mr. Traill having previously retired from
the service. The Mourne Mountain area is now undergoing critical
revision by Prof. Sollas for the Geological Survey, and important
additional material for the elucidation of this district may be
expected from him.]
Besides the older basic dykes disrupted by the granite, a younger but
much less abundant series traverses that rock, and also follows a
general north-westerly direction. These later dykes in some cases cross
more acid dykes which have risen through the granite. There is no trace
of any superficial discharge from the Mourne Mountain area. But from
the analogy of other districts we may easily conceive that the granite
represents the underground parts of volcanic material which has now
been entirely removed.
b. _Slieve Foye and Barnavave District._--This area embraces the
mountainous ground lying between Carlingford Lough and Dundalk Bay, and
culminating in Slieve Foye (1935 feet). It measures roughly about six
miles in extreme length and four miles in breadth.
The remarkable assemblage of basic and acid materials in this area
has received considerable attention from geologists. The relative
order of the two groups of rocks was first clearly recognized by
Griffith, who showed that the granite (granophyre) is intruded into the
gabbro.[419] Professor Haughton subsequently confirmed this observation,
and proved the post-Carboniferous date of the intrusive materials,
which he compared with those of Skye.[420] The general distribution of
the rocks was traced out in some detail by the Geological Survey, and
described in the official _Memoirs_.[421] More recently the district
has been examined by Professor Sollas, who, bringing the photographic
camera and the microscope to the aid of field-geology, has elucidated
the structure and relations of the rocks, and has obtained abundant
evidence that the acid and basic rocks maintain there the same relative
order as among the Inner Hebrides.[422]
[Footnote 419: _Journ. Geol. Soc. Ireland_ (1843), p. 113.]
[Footnote 420: _Quart. Journ. Geol. Soc._ vol. xii. (1856), p. 171; xiv.
p. 300; and _Journ. Geol. Soc. Ireland_ (1876), p. 91.]
[Footnote 421: Sheet 71 of the Geol. Surv. Ireland, and accompanying
Explanation. These were the work of Mr. W. A. Traill.]
[Footnote 422: _Trans. Roy. Irish Acad._ vol. xxx. (1894), p. 477. This
is part i. of what is intended to be a series of papers.]
One of the first features in this tract of country to arrest the eye of
the geologist is the situation of this centre of protrusion and that
of Slieve Gullion along a north-west line, coincident with the general
direction of the numerous basic dykes of the region. Whether or not the
successive intrusions took place contemporaneously in the two areas,
they have followed each other in the same order. In the Barnavave
district the igneous rocks occupy an area of about 20 square miles.
They consist of a central and chief mass composed of acid materials,
which have risen through the basic rocks now found as an interrupted
ring round them.
In his more recent examination, Prof. Sollas has devoted special
attention to the influence of the solvent action of the acid magma
upon the basic rocks and upon its own composition and structure.
Besides confirming the work of previous observers as to the order of
appearance of the two kinds of material, he has obtained evidence that
the gabbro had not only completely solidified, but was traversed by
contraction-joints, possibly even fractured by earth-movements, before
the injection of the granophyric material. He found that this material,
like that of the Inner Hebrides and St. Kilda, must have been in a
state of great fluidity at the time of its intrusion, and made its way
into the minutest cracks and crevices. In observing the solvent action
of the granophyre, he ascertained that this action took place even in
comparatively narrow dykes, which probably consolidated at no great
depth beneath the surface.[423]
[Footnote 423: _Op. cit._]
c. _The Slieve Gullion District._--This area is separated from that
just described by a narrow strip of Silurian strata, so that its
isolation as a separate igneous district is complete. It will be
observed from the map to continue the same north-westerly line as the
Slieve Foye tract, the two together running in that direction for a
distance of some 16 miles. It is interesting to note the adoption of
this predominant north-westerly trend even by eruptive masses which
were mainly of acid material.
This district measures about ten miles in length by from one to five
miles in breadth. The rocks are, on the whole, similar to those in the
area south of Carlingford Lough, and bear the same relation to each
other, the acid being intrusive in the basic series. It is worthy of
remark that the Tertiary eruptive rocks have made their appearance in
the midst of the older granite of Newry. This granite has been already
alluded to as disrupting Upper Silurian strata, and being probably
of the age of the Lower Old Red Sandstone (vol. i. p. 290). In long
subsequent ages, after protracted denudation, during which its cover
of Silurian and Carboniferous formations was stripped off and it was
laid bare, it was broken through by the whole series of basic and acid
protrusions of Slieve Gullion.
This district is portrayed on Sheets 59, 60, 70 and 71 of the
Geological Survey of Ireland, which show a central core of basic and
acid material piercing the Newry granite.[424] Round this core and
touching it at its north-western and south-eastern end, but elsewhere
separated from it by a space of several miles, runs a curiously
continuous band of igneous material which is marked as "quartziferous
porphyry" and "felstone-porphyry" on the Survey maps.
[Footnote 424: The ground was chiefly mapped and described by Mr. Joseph
Nolan and Mr. F. W. Egan.]
The south-western portion of this elliptical ring possesses a peculiar
interest from its including certain remarkable masses of breccia
or agglomerate. These rocks have been mapped by Mr. Nolan, and are
described by him in the official _Explanation_, but in more detail in
two separate papers.[425] Having had an opportunity of paying a brief
visit to the ground, I can confirm the general accuracy of his mapping
and description, and am able to add a few further particulars to the
facts enumerated by him.
[Footnote 425: Sheet 70 of the Geol. Surv. Map of Ireland and Explanation
thereto; also _Journ. Roy. Geol. Soc. Ireland_, vol. iv. (1877), p.
233; _Geol. Mag._ 1878.]
The tract of ground where these agglomerates appear forms a prominent
ridge which rises several hundred feet above the lower country on
either side, and extends in a W.N.W. direction for about seven miles,
nearly along the line of junction between the Newry granite and the
Silurian strata. The ridge has a breadth varying from a few hundred
yards to upwards of a mile. It is separated from the main igneous mass
of the Slieve Gullion area by an intervening strip of lower ground from
three-quarters of a mile to about a mile and a half in width, which is
occupied by the Newry granite. At the north-west end of the ridge the
newer eruptive rocks lie within the area of that granite, while at the
south-east end they rise entirely amongst the Silurian strata.
Beginning at the south-eastern extremity, we find the agglomerate
occupying several detached eminences and surrounded by altered
Silurian grits and shales. Further west the rock occurs in larger and
more continuous masses, appearing at intervals, especially along the
southern borders of the quartz-porphyry which forms by much the greater
part of the ridge. Actual junctions of the agglomerate with the older
rocks around seem to be seldom visible. I found one, however, above
the gamekeeper's house on the southern flanks of the hill called
Tievecrom. The Upper Silurian grits and shales, in a much indurated and
shattered condition, are there traceable for several hundred feet up
the slope, until they are abruptly cut off by the agglomerate. The line
of separation appears to be nearly vertical, the truncated ends of the
strata being wrapped round by the mass of fragmental material.
The most remarkable features of this agglomerate, which has been well
described by Mr. Nolan, are the notable absence of truly volcanic
stones in it, and the derivation of its materials from the rocks
around it. I found only one piece of amygdaloid, but not a single lump
of slag, no bombs, no broken fragments of lava-crusts, and no fine
volcanic dust or enclosed lapilli. The rock may be said to consist
entirely of fragments of Silurian grits and shales where it lies
among these strata, and of granite where it comes through that rock.
Blocks of these materials, of all sizes up to two feet in breadth, are
confusedly piled together in a matrix made of comminuted debris of the
same ingredients.
The agglomerate on the ridge of Carrickbroad has no definite boundary,
but seems to graduate into an andesitic rock, and then into a
quartz-felsite or rhyolite. This apparent gradation is one of the
most singular features of the ridge. The andesite resembles some of
the "porphyrites" of the Old Red Sandstone. It is close-grained,
with abundant minute felspar-laths, and numerous large porphyritic
felspars, which latter are sometimes aggregated in patches, as in the
old porphyries of Portraine, Lambay Island and the Chair of Kildare.
This rock has undoubtedly been erupted at the time of the formation of
the agglomerate, or at least before the loose materials were compacted
together; for it is full of separate stones of the same materials, and
becomes so charged with them as to become itself a kind of agglomerate,
with a small proportion of andesitic matrix cementing the blocks.
A thin slice prepared from one of the specimens obtained by me
from this hill has been studied by Mr. Watts, who reports that the
fine-grained andesitic matrix in which the stones are imbedded has
often been injected into their minute fissures, and that the minute
fragments enclosed in this matrix consist here of a trachyte-like
porphyry, felsite, andesites, basalts of various degrees of fineness
and olivine-basalt, together with isolated grains of felspar, such as
might have been derived from the breaking up of some of these fragments.
Westward from Carrickbroad, the chief eruptive rock is a dark,
sometimes nearly velvet-black, flinty, occasionally almost resinous,
quartz-porphyry or rhyolite, with abundant quartz and large felspars
and occasional well-marked flow-structure. This material, near the
much smaller protrusion of andesite, is curiously mixed up with that
rock, as if the two had come up together. Sometimes they seem to
pass into each other, at least the separation between them cannot
be sharply drawn. There can be little doubt, however, that the acid
magma continued to ascend after the other, for it sends veins and
strings into the more basic material, and encloses blocks of it. This
thoroughly acid porphyry plays the same part as the andesite in regard
to the stones of the agglomerate. Throughout its whole extent, it is
found to enclose these stones, which here and there become so numerous
as to form the main bulk of the mass, leaving only a limited amount of
quartz-porphyry (rhyolite) matrix to bind the whole into an exceedingly
compact variety of breccia. Occasionally the acid rock cuts through the
ordinary clastic agglomerate, as may be well seen on the southern face
of Tievecrom.
A specimen of this porphyry with its enclosed fragments, which was
collected by me from above the old tower at Glendovey, Carrickbroad,
has been sliced and examined by Mr. Watts under the microscope, and is
thus described by him: "The large fragment in this slide consists of
ophitic olivine-dolerite full of large phenocrysts of olivine. It is
broken up and penetrated by veins of quartz-porphyry, rich in quartz,
which exhibits a beautiful flow-structure. The felspars and augite of
the dolerite do not appear to have suffered much alteration at the
margin of the fragment, but the olivines are much serpentinized, the
serpentine passing into a border of actinolite which runs in veins into
the neighbouring rock and even passes out into the quartz-porphyry at
the junction, impregnating it with actinolite and chlorite for some
distance. A few particles of basalt also occur and a portion of a
granite-fragment comes into the slide, from the edge of which a piece
of biotite has floated off into the quartz-porphyry."
The essentially non-volcanic material of the agglomerate shows, as Mr.
Nolan pointed out, that it was produced by æriform explosions, which
blew out the Silurian strata and granite in fragments and dust. These
discharges probably took place either from a series of vents placed
along a line of fissure running in a north-westerly line, or directly
from the open fissure itself. Possibly both of these channels of escape
were in use; detached vents appearing at the east end and a more
continuous discharge from the fissure further west.
After the earliest explosions had thrown out a large amount of
granitic and Silurian detritus, andesitic lava rose in the fissure,
and solidifying there enclosed a great deal of the loose fragmentary
material that fell back into the chasm. Subsequently, and on a more
extensive scale, a much more acid magma ascended from below, likewise
involving and carrying up a vast quantity of loose stones, among which
are pieces of basalt and dolerite.
No evidence remains as to the extent of the material discharged
over the surface from this fissure. Denudation has removed all the
surrounding fragmental sheets as well as any lava that may have flowed
out upon or become intercalated among them. There remains now only
the cores of the little necks at the east end, and the indurated
agglomerate and lava that consolidated along the mouth of the fissure
or vents.
This is the only example of such a line of fissure-eruption which
has yet been met with in the British Isles. Its connection with the
eruptive masses of Slieve Gullion and Carlingford links it with
the Tertiary volcanic series. But no evidence appears to remain
regarding the epoch in the long volcanic period when the eruptions
from it took place. They may possibly date back to the time of the
plateau-basalts; but the abundant acid magma, which constitutes one of
their distinguishing characteristics, suggests that they more probably
belong to the later time when the main protrusions of acid material
took place. They suggest that coeval with the uprise of the great domes
of Slieve Gullion, Carlingford and the Mourne Mountains there may have
been many superficial eruptions of which, after prolonged denudation,
all trace has now been effaced.
2. The Antrim Region
Reference was made in Chapter xxxvii. to the occurrence of rhyolitic
conglomerate and tuff between the lower and upper series of basalts in
the Antrim plateau, and to the evidence furnished by these detrital
deposits either that masses of rhyolite appeared at the surface, or
that rhyolitic ashes were discharged from volcanic vents in the long
interval that elapsed between the two groups of basalt. The further
consideration of this question, and an account of the rhyolite bosses,
were reserved for the present chapter, that they might be taken in
connection with the other acid eruptions of Tertiary time in Britain.[426]
[Footnote 426: For an early account of the Antrim trachytic rocks, see
Berger, _Trans. Geol. Soc._ iii. (1816), p. 190. Professor Hull has
described the Tardree rock in the Explanation to Sheets 21, 28 and 29,
_Geol. Survey of Ireland_ (1876), p. 17, and has supposed it to be
older than the basalts, referring it to the Eocene period (_Physical
Geology and Geography of Ireland_, 2nd edit. (1891), pp. 87, 95).
Duffin (quoted by Mr. Kinahan) believed that "the trachytes occur at
the centre of eruption, and were probably poured out at the end of the
outburst." Du Noyer also (quoted by the same writer) thought them to be
newer than the plateau-basalts, and to have lifted up masses of these
rocks. Mr. Kinahan himself (_Geology of Ireland_, p. 172) has pointed
to the absence of any rhyolitic fragments between the basalts as an
argument against the supposed antiquity of the acid protrusions. A
petrographical account of the Tardree rock is given by Von Lasaulx in
the paper already cited, Tschermak's _Min. Pet. Mittheil._ (1878), p.
412. A more elaborate discussion of the petrography by Prof. Cole will
be found in the Memoir above referred to (_Scientif. Trans. Roy. Dublin
Soc._ vol. vi. 1896), and the geological relations of the rocks are
discussed by him in another shorter paper, _Geol. Mag._ (1895), p. 303.
See also Mr. M'Henry on the trachytic rocks of Antrim, _Geol. Mag._
(1895), p. 260, and _Proc. Geol. Assoc._ vol. xiv. (1895), p. 140.]
With one exception, all the known protrusions of acid material in the
Antrim area lie within the limits of the basalt-plateau (see Map. No.
VII.). They occur along a line at intervals for a distance of about 17
miles, from Templepatrick to a point four miles north of Ballymena.
It is worthy of remark that here again the line of protrusion has a
north-west trend. It not improbably indicates the position of a fissure
up which the acid material rose at various points.
The petrography of the rocks has been frequently discussed. They
include several varieties of rhyolite, generally rather coarsely
crystalline, but sometimes becoming compact, and even passing into
dark obsidian. No undoubted tuff occurs associated with them in any of
the exposures, nor do the rhyolites anywhere display structures that
point to their having flowed out at the surface.[427] That the masses now
visible may have communicated with the surface is quite conceivable,
but what we now see appears in every case to be a subterranean and not
a superficial part of the protrusion.
[Footnote 427: At Sandy Braes an exposure is visible of what at first
might be thought to be a volcanic conglomerate, but closer examination
shows the rock to consist of obsidian, which decomposes into a
clay, leaving round sharply-defined glassy cores enclosed in the
decayed material. The "banded rhyolites" do not exhibit any kind of
flow-structure that may not be met with in dykes and bosses. Nor have
any satisfactory traces been found of vesicular or pumiceous bands such
as might mark the upper surfaces of true lava-streams. Professor Cole
has described what he calls "The Volcanoe of Tardree" (_Geol. Mag._
July 1895). If the Tardree mass ever was a volcano, which is far from
improbable, its superficial ejections have long ago disappeared. At
least, after the most diligent search, I have been unable to discover
any trace of them, all that now remains appearing to me to be the neck
or core of protruded material.]
[Illustration: Fig. 370.--Intrusive rhyolite in the Lower Basalt group
of Antrim, Templepatrick.
1 1, Chalk; 2 2, Gravel; 3 3, Bedded basalt; 4 4, Rhyolite, intrusive.]
Most of the rhyolitic exposures are extremely limited in area--mere
little knobs, sometimes rising in the middle of a bog, and never
forming conspicuous features in the landscape. The relation of these
rocks to the basalts are generally concealed, but the isolation of
the small rhyolitic patches leaves no doubt that they are intrusive
as regards the surrounding basalts. This relation is well seen at
Templepatrick, where it was first observed by Mr. M'Henry of the
Geological Survey (Fig. 370). The rhyolite there forms a sill which has
been thrust between the basalts and the gravel that underlies them, the
basalts being bent back and underlain by the acid rock.[428]
[Footnote 428: The progress of quarrying operations during the last eight
years has somewhat destroyed the section as exposed in 1888. But we now
see that the basalt has not only been bent back but is underlain by the
acid rock.]
The largest and most interesting of the Antrim rhyolite tracts covers a
space of about ten square miles in the heart of the basalt-plateau to
the north-east of the town of Antrim. It rises to about 1000 feet above
the sea, and forms a few featureless hills, some of which are capped
with basalt. The best known localities in this tract are Tardree and
Carnearny. The rock is chiefly a somewhat coarse-textured lithoidal
rhyolite, but includes also vitreous portions.
[Illustration: Fig. 371.--Section across the southern slope of
Carnearny Hill, Antrim.
_a_ _a_ _a_, bedded basalts; _b_, rhyolite.]
Owing to the cover of soil and turf, the junction of this mass with
the surrounding basalts cannot be so clearly seen as in the sections
of the Inner Hebrides, and hence the stratigraphical relations of
the two groups are apt to be misunderstood. What is actually seen is
represented in Fig. 371. The lithoidal rhyolite emerges from underneath
the basalts which abut against its sloping surface, forming on the
north side of Carnearny Hill a steep bank about 150 feet above the more
gently inclined slope below. The basalts consist of successive nearly
level sheets of compact and amygdaloidal rock.
It is obvious that only two explanations of this section are possible.
Either the rhyolite was in existence before the basalts which flowed
round it and gradually covered it, or it has been erupted through these
rocks, and is therefore of later date.
The former supposition has been the more usually received. The rhyolite
has been supposed to form the summit of an ancient volcanic dome,
perhaps of Eocene age, which had been worn down before the outflow
of the plateau-basalts under which it was eventually entombed. Had
this been the true history of the locality, it is inconceivable that
of a rock which decays so rapidly as this rhyolite, and strews its
slopes with such abundance of detritus, not a single fragment should
occur between the successive beds of basalt which are supposed to have
surrounded and buried it. Though the several beds of basalt are well
exposed all round, I could not, on my first visit, find a trace of any
rhyolitic fragments between them, nor had Mr. Symes, who mapped the
ground in detail for the Geological Survey, been more successful. I
have since made a second search with Mr. M'Henry, but without detecting
a single pebble of the acid rock among the basalts. Yet it is clear
from the upper surfaces of some of these lavas that a considerable
interval of time separated their successive outflows, so that there was
opportunity enough for the scattering of rhyolite-debris had any hill
of that rock existed in the vicinity.
Again, little more than a mile to the east of Carnearny Hill, an
outlier of the basalts forming the prominent height of the Brown Dod
lies upon and is completely surrounded by the rhyolite, which along the
east side of the hill can be traced as it passes under the level sheets
of basalt. The line of junction ascends and descends on that flank of
the outlier, so that successive flows of basalt are truncated by the
acid rock. But I could find no rhyolitic debris between them.
It appears to me, therefore, that the relations between the two groups
of rock in this area are similar to those between the granophyres and
bedded basalts on the south side of Loch na Keal in Mull (p. 396). In
other words, the rhyolites have risen through the basalts, and are
therefore younger than these lavas. This conclusion is corroborated
by the actual proofs of the intrusion of rhyolite into the basalts at
Templepatrick.
All the known rhyolitic masses in Antrim are confined to the Lower
group of basalts.[429] And as they traverse some of its highest
members, they may be regarded as certainly younger than that group.
Mr. M'Henry, who first indicated this relation, suggested that the
rhyolites were erupted in the interval between the two basaltic
series, and he connected with their eruption the rhyolitic detritus
found in association with the iron-ore at so many places in Antrim. It
appears to me that this suggestion carries with it much probability.
The rhyolitic conglomerate of Glenarm proves that, in the long period
represented by the iron-ore and its associated group of sedimentary
deposits, there were masses of rhyolite at the surface, the waste of
which could supply such detritus. The resemblance between the material
of that conglomerate and the rhyolites now visible at Tardree and
elsewhere is so close that we cannot doubt that, if not derived from
some of the known rhyolitic protrusions, this material certainly came
from exposed masses that had the same general petrographic characters.
[Footnote 429: The only exception to this rule was believed to be that
of the mass at Eslerstown, four miles east of Ballymena, which, as
originally mapped, was shown as crossing from the Lower into the Upper
basalts. Mr. M'Henry, however, has recently ascertained that the acid
rock is entirely restricted to the area of the older group.]
While the rhyolite pebbles in the Glenarm conglomerate are distinctly
rounded and water-worn, showing that some prominences of acid rock were
undergoing active denudation at the time when this conglomerate was
laid down, the finer rhyolitic detritus in the tuffs of Ballypallidy
rather suggests the actual discharge of rhyolitic ashes during the same
period. But it would appear that the superficial outbursts of rhyolitic
material, whether in the form of lava or of tuff, were only of trifling
extent, or else that the interval between the eruption of the two
basalt-groups was so prolonged that any such superficial material was
then removed by denudation. The varieties of lithological character to
be met with among the acid protrusions of Antrim suggests a succession
of uprises of rhyolites differing from each other more or less in
composition and structure. Unfortunately the ground is generally so
covered with superficial accumulations, and the exposures of rock are
so poor and limited, that no sequence has yet been determined among the
several kinds of acid rock. The only locality where I have observed
clear evidence of such a sequence is on the old quarries half a mile
west of Shankerburn Bridge, and three miles north-west of Dromore,
County Down. A small boss of rhyolite there rises through the Silurian
strata. It consists partly of a coarse-grained lithoidal rhyolite, with
large smoky quartzes and felspars, and partly of a much finer textured
variety. The latter, on the south side of the small brook which
separates the quarries, can be seen to ascend vertically through the
coarse-grained rock into which it sends a projecting vein. Its margin
shows a streaky flow-structure parallel with its vertical wall and is
in places spherulitic. Here the closer-grained rock is certainly later
than the rest of the mass.