THE BOSSES AND SHEETS OF GABBRO IN THE DISTRICTS OF RUM,
ARDNAMURCHAN, MULL, ST. KILDA AND NORTH-EAST IRELAND. HISTORY OF
THE GABBRO INTRUSIONS
2. _The Island of Rum_
The mountains of the island of Rum, rising as they do from a wide
expanse of open sea, present one of the most prominent and picturesque
outlines in the West Highlands (Map VI.). More inaccessible than
most of the other parts of the volcanic region, they have been less
visited by geologists. They were described by Macculloch as composed
of varieties of "augite rock." He noticed in this rock "a tendency
to the same obscurely bedded disposition as is observed in other
rocks of the trap family," and found at one place that it assumed "a
regularly bedded form, being disposed in thin horizontal strata, among
which are interposed equally thin beds of a rock resembling basalt
in its general characters."[350] Professor Judd repeats Macculloch's
observation, that "the great masses of gabbro in Rum often exhibit that
pseudo-stratification so often observed in igneous rocks." He regards
these masses, like those of Skye and Mull, as representing the core
of a volcano from which the superficial discharges have been entirely
removed, and he gives a section of the island in which the gabbro is
represented as an amorphous boss sending veins into a surrounding mass
of granite.[351] In a subsequent paper he gave an excellent detailed
account of the mineralogical composition of some of the remarkably
varied and beautiful basic rocks constituting the hills of Rum, but
added no further information regarding the geological structure of the
island.[352]
[Footnote 350: _Western Islands_, i. p. 486.]
[Footnote 351: _Quart. Journ. Geol. Soc._ xxx. p. 253.]
[Footnote 352: _Op. cit._ xli. (1885) p. 354. See also his paper in vol.
xlii. of the same Journal.]
Even from a distance of eight or ten miles, the hills of Rum are seen
to be obviously built up of successive nearly horizontal tiers of rock.
As the summer tourist is carried past the island, in that wonderful
moving panorama revealed to him by the "swift steamer" of modern days,
these great dark cones remind him of colossal pyramids, and as the
ever-varying lights and shadows reveal more prominently the alternate
nearly level bars of crag and stripes of slope, the resemblance to
architectural forms stamps these hills with an individuality which
strikes his imagination and fixes itself in his memory. If choice or
chance should give him a nearer view of the scene, he would not fail
to notice that it is among the northern hills of the island that the
bedded character is so conspicuous, and that it ceases to be prominent
in the southern heights, though here and there, as in the upper part
of Scuir na Gillean, it may in certain lights be detected even from
a distance. Crossing over from Eigg, he would recognize each of the
features represented in the sketch reproduced in Fig. 339. Along the
shore, red sandstones rise in naked cliffs, from the top of which the
ground slopes upward in brown moors to the bare rocky declivities. A
deep valley (Glen Dibidil) is seen to run into the heart of the hills,
between the bedded group to the north and the structureless group to
the south. If the weather is favourable, some eight or more prominent
parallel bars of rock may be counted on the two higher cones to the
right. These bars are not quite level, but slope gently from right to
left. They remind one of the terraced basalts of the plateaux, but
present a massiveness and a breadth of intervening bare talus-slope
such as are not usual among those rocks.
[Illustration: Fig. 339.--Outline of the Hills of the Island of Rum,
sketched from near the Isle of Eigg.]
Nor is this impression of regularity and bedded arrangement lessened
when we actually climb the slopes of the hills. I had for years been
familiar with the outlines of Rum as seen from a distance, and had
sketched them from every side, but I shall never forget the surprise
and pleasure when my first ascent of the cones revealed to me the
meaning of these parallel tiers of rock. I found it to be the structure
of the Cuillin Hills repeated, but with some minor differences which
are of interest, inasmuch as they enlarge our conceptions of the
process by which the gabbro-bosses were formed.
The northern half of the island of Rum consists almost entirely of red
sandstone, which, as already stated, is a continuation of the same
formation (Torridonian) so well developed in the south-east of Skye,
Applecross and Loch Torridon, and traceable between the Archæan gneiss
and the Cambrian strata up as far as Cape Wrath. The sandstones, though
full of false bedding, show quite distinctly their true stratification,
which is inclined with singular persistence towards W.N.W., at angles
averaging from 15° to 20°. If they are not repeated by folds or faults,
they must reach in this island a thickness of some 10,000 feet. Their
red or rather pinkish tint seems mainly to arise from the pink felspar
so abundant in them, for in many places they really consist of a kind
of arkose. Pebbly bands with rounded pieces of quartz are of common
occurrence throughout the whole formation. Dykes and veins of basalt
are profusely abundant. Sometimes these run with the bedding, and might
at a distance be taken for dark layers among the pink sandstones. They
often also strike obliquely up the face of the cliffs like ribbons.
But, notwithstanding their apparent continuity, there can be no doubt
that these sandstones have suffered from those powerful terrestrial
disturbances which have affected all the older rocks of the North-West
Highlands. On the west side, where they plunge steeply into the sea,
they have undergone a change into fine laminated rocks, which might at
first be mistaken for shales, but which owe their fissility to shearing
movements. Along their southern border, from a point on the east coast
near Bagh-na-h-Uamha, south of Loch Scresort, to the head of Kilmory
Glen, they are abruptly truncated against a group of dark, flaggy and
fissile schists and fine quartzites or grits, which in some places are
black and massive like basalt, and in others are associated with coarse
grey gneiss. That some of these rocks are portions of the Lewisian
series can hardly be doubted, and their structure and relations are
probably repetitions of those between the Lewisian gneiss and Torridon
sandstone of Sleat in Skye. I found also on the northern slopes of Glen
Dibidil a patch of much altered grey and white limestone or marble,
which reminded me of the Cambrian limestone of Skye. The red sandstones
in a more or less altered condition are prolonged to the south-east
promontory of the island.
In passing over the zone of these more ancient rocks, we find them
to present increasing signs of alteration as they are traced up the
slopes towards the great central mass of erupted material. The pink
sandstones gradually lose their characteristic tint, and grow much
harder and more compact, while the veins and dykes of basalt and sheets
of dolerite intersecting them increase in number. The zone of black
compact quartzite, which lies to the south of the sandstones, and which
at one point reminds us of basalt, at another of the flinty slate of
the schistose series, likewise displays increasing induration. Its
bedding, not always to be detected, is often vertical and crumpled.
But the most remarkable point in its structure is the intercalation in
it of bands of breccia. These vary from less than an inch to several
yards in diameter; they run mostly with the bedding, but occasionally
across it. The stones in them are fragments of the surrounding rock
embedded in a matrix of the same material, but also with pieces of a
somewhat coarser grit or quartzite. A band of coarse breccia forms
the southern limit of this zone along the northern base of Barkeval
and Allival. In general character it resembles the thinner seams of
the same material just referred to. The matrix so closely agrees with
the black flinty quartzite, that but for the included stones it could
hardly be distinguished; so greatly has the mass been indurated that
the stones seem to shade off into the rest of the rock. But here and
there its true brecciated nature is conspicuously revealed by prominent
blocks of hardened sandstone. This band of breccia must in some places
be 150 or 200 feet broad. It has no distinct bedding, but seems to
lie as a highly inclined bed dipping into the hill. It may possibly
be a crush-breccia belonging to a period earlier than the volcanic
eruptions. It is at once succeeded by a black flinty felsite like
that of Mull. The groundmass of this rock, so thickly powdered with
magnetite grains as to be almost opaque under the microscope, displays
good flow-structure round the turbid crystals of orthoclase and the
clear granules of quartz. Further up the hill, the rock becomes lighter
in colour and less flinty in texture--a change which is found to arise
from more complete devitrification, the groundmass having become a
crystalline granular aggregate of quartz and felspar with scattered
porphyritic crystals of these minerals (microgranite). In some places,
the felsite incloses fragments of other rocks. A specimen of this
kind, taken from the head of Coire Dubh, shows under the microscope a
brown micro-felsitic groundmass, with crystals of felspar and augite,
inclosing a piece of basalt, composed of fine laths of plagioclase,
abundant magnetite and a smaller proportion of granules of augite.
[Illustration: Fig. 340.--View of Allival, Rum, sketched from the base
of the north-east side of the cone.]
This band of felsite and microgranite may be traced continuously from
Loch Gainmich along the base of Barkeval and Allival, and similar
rocks appear at intervals on the same line round the eastern base of
the hills. Immediately above this belt of felsitic protrusions comes
the great body of gabbro. It will be observed that here, as in Skye,
the base of the gabbro mass presents a horizon on which injections of
acid rocks have been particularly abundant. Whether the breccias be
regarded as the result of earlier rock-crushings, or as due to volcanic
explosions during the Tertiary period, they are evidently older than
the eruption of the gabbros. In that respect they may be compared
with the agglomerates through which the youngest eruptive bosses of
Skye have made their way; but their component materials have been
derived from the surrounding platform of ancient rocks, and not from
subterranean lavas.
[Illustration: Fig. 341.--Section of foliated gabbros in the Tertiary
volcanic series of Allival, Rum.
_a_, massive gabbro with rude lamination parallel to bedding, only seen
in some weathered surfaces; _b_, laminated troctolite; _c_, massive
coarsely crystalline gabbro rudely laminated.]
For my present purpose, however, the chief point of importance is the
structure of the gabbro mass that springs from that platform into
the great conical hills of Rum. The accompanying sketch (Fig. 340)
will convey a better idea of this structure than a mere description.
At the base, immediately above the felsite just referred to, bedded
dolerites make their appearance, much intersected with veins from
the siliceous rock. Veins and dykes of basalt also cut all the rocks
here, the newest being those which run in a north-west direction. The
lowest sheets of dolerite are succeeded by overlying sills of coarser
dolerites, gabbros, troctolites, etc., which are as regular in their
thickness and continuity as the ordinary basalts of the plateaux. The
band of light-coloured troctolite, in particular (Fig. 341), about 20
to 30 feet thick, which has been already referred to for its remarkable
laminar structure, can be followed for some distance along the base
of the hill as a marked projecting escarpment. This rock at once
arrests attention by its platy or fissile structure, parallel to the
bedding-surfaces of the sheet. Indeed hand-specimens of it, as I have
said, might readily pass for pieces of schistose limestone, especially
if taken from the upper part. It consists of successive layers, which
on the weathered surface divide it into beds almost as regular as those
of a flagstone, each bed being further separated into laminæ marked
off by the darker and lighter tints of their mineral constituents. The
darker layers consist of olivine, and the lighter of plagioclase. This
segregation here and there takes the form of rounded masses, where the
minerals are more indefinitely gathered together. The affinity of the
rock with intrusive sheets is further displayed by the occurrence of
abundant nut-like aggregates of pale green olivine. Examined under the
microscope, flow-structure is admirably seen, the lath-shaped felspars
being drawn out parallel to the planes of movement, and giving thereby
the peculiarly schistose structure which is so deceptive.
The massive and coarsely crystalline gabbros below and above this
troctolite are all more or less affected by the same laminar structure.
Some of those in higher parts of the mountain are quite massive in
part, but also include bands of lamination. Banding like that of the
Skye gabbros is generally developed among them, the individual bands
varying from less than an inch to a foot or more in thickness. This
structure, like the lamination, is parallel to the general bedding of
the sheets. As in the Cuillin Hills, the bands differ from each other
in the relative proportions of the constituent minerals, especially
the predominant pyroxene and olivine. The crystals or crystalline
aggregates are often from a quarter of an inch to an inch in diameter,
and in these large forms are crowded together in certain bands.
Magnetite, on the whole, is rather less conspicuous than in the Cuillin
gabbro: at least, it is not so prominently aggregated in special
layers. In one or two instances I have observed curvature of the
banding, but no example so striking as that cited from the Cuillin area
(Fig. 337).
On weathered surfaces, where the felspars decay into a creamy white
and the ferro-magnesian minerals assume tints of green, brown and red,
the resemblance of the rocks to schists is striking. This external
likeness is combined with a tendency to split into thin plates parallel
to the lamination, which still further increases their schistose
appearance. Though less developed than in Skye, the banding appears
to be of the same kind and origin; but in Rum it is combined with the
remarkable lamination above mentioned, produced by the arrangement of
the component minerals with their longer axes parallel to the planes
of bedding, as in flow-structure--a combination which I have not yet
observed in Skye.
The bedded arrangement of the gabbros of Rum, so conspicuous in
the great eastern cones (Figs. 339 and 340), is emphasized by the
fact that some sheets, of a more durable kind, stand out boldly as
prominent ribs, while the softer crumble into a kind of sand, which
forms talus-slopes between the others. Alternations of this nature are
continued up to the very top of the mountains. The beds are nearly
flat, but dip slightly into the interior or towards the south-west.
On the west side of the island also, beyond Loch Sgathaig, a distinct
bedding may be traced, the inclination being here once more inwards
or to the east. But from Glen Harris and the base of Askival this
structure becomes less marked, and gradually disappears. There is thus
a central or southern more amorphous region, while round the margin
towards the north and east the rock appears in frequent alternating
beds.
It is clear that in the broad features of their architecture the hills
of Rum follow closely the plan shown in the Cuillin Hills of Skye. But,
unfortunately, in the former island denudation has gone so far that
no connection can be traced on the ground between the gabbros and the
plateau-basalts. As already stated, the latter rocks have been almost
entirely stripped off from the platform of sandstones and schists
which they undoubtedly at one time covered, and the few outliers of
them that remain lie at some little distance from the margin of the
gabbro area (_ante_, p. 216). Nevertheless, we are not without some
indications of them underneath the gabbros. I have alluded to the
basalts that lie at the base of the eastern cones. As we follow the
bottom of the gabbro southward round the flanks of the hills, dull
compact black shattery basalts, with a white crust, appear from under
the more crystalline sheets. These at once remind one of the altered
basalts of Skye and Mull. On the west side also, beds of basalt emerge
from under the gabbro, but they have been so veined and indurated by
the granophyre of that district, that their relations to the gabbro
are somewhat obscured. If we could restore the lost portions of the
plateau, I believe we should find the gabbros of Rum resting on part of
the volcanic plateau, and some of the gabbro-beds prolonged as sills
between the sheets of basalt.
3. _The Gabbro of Ardnamurchan_
The promontory of Ardnamurchan reveals as clearly as the flanks of
the Cuillin Hills, though in a less imposing way, the relations of
the gabbros to the plateau-basalts (Map VI.). From the southern shore
at Kilchoan to the northern shore at Kilmory, bedded basalts, of the
usual type, amygdaloidal and compact, weathering into brown soil, may
be followed along the eastern slopes of the hills, resting upon the
schists and Jurassic series of western Argyleshire. These rocks are a
continuation of those that cap the ridges further to the south-east and
cross Loch Sunart into Morven. They dip westwards, and followed upwards
in that direction, they soon present the usual marks of alteration.
They weather with a white crust and become indurated and splintery.
Sheets of dolerite with many veins and dykes of basalt run between and
across them. Bands of gabbro make their appearance, and these, as we
advance westwards, increase in number and in coarseness of grain until
this rock, in its rudely bedded form, constitutes practically the whole
of the promontory from Meall nan Con to the light-house. Many admirable
sections may be seen on the coast-cliffs and in the rugged interior,
showing the irregular bedding of the gabbro, and how prone this rock is
to develop its component minerals in bands or ribbons, sometimes made
up of large crystals, as in Skye, Rum and Mull.
4. _The Gabbro of Mull_
In the island of Mull, the conclusions to which the geology of the
other volcanic districts leads us as to the position of the gabbros
in the series of volcanic phenomena, are further confirmed. The first
geologist who appears to have observed the relation of these rocks
in that island was Jameson, who classed them under the old name of
"greenstone," including in the same designation rocks now termed
dolerites and gabbros. He ascended one of the hills above Loch Don,
probably Mainnir nam Fiadh (2483 feet), which he found to consist of
"strata of basalt and greenstone," with some basalt-breccia or tuff and
a capping of basalt. He speaks of the "singular scorified-like aspect"
of the weathered greenstone--a description which applies to some of
the coarser gabbro bands of that locality. But he appears to have
recognized the general bedded arrangement of the rocks up even to the
summit of the hill.[353]
[Footnote 353: _Mineralogy of the Scottish Isles_ i. p. 205.]
It was not, however, until the visit of Professor Zirkel in 1868,
that the true petrographical characters of the gabbro of Mull were
recognized. This observer remarked that the rock is regularly
interstratified with the basalt.[354] Professor Judd, as already stated,
has supposed the gabbros to be the deep-seated portion of the masses
which when poured out at the surface became the plateau-basalts, and he
represents them in his map and sections of Mull as ramifying through
the granitic rocks.[355]
[Footnote 354: _Zeitsch. Deutsch. Geol. Gesellsch._ xxiii. (1871) p. 58.]
[Footnote 355: _Quart. Jour. Geol. Soc._ xxx. (1874).]
In Mull the disposition of the gabbro in beds, sheets or sills is well
displayed, for there is here no great central complicated mass of
interlacing banded and amorphous sheets. We have seen that a higher
group of plateau-basalts has survived in this island better than in the
other plateaux, and it would seem that denudation has not yet succeeded
here in cutting down so deeply into the gabbro core as in Skye, Rum
and Ardnamurchan. Only the upper or outer fringe of intrusive sheets
among the bedded basalts has been laid bare. The district within which
this fringe may be observed is tolerably well-defined by the difference
of contour between the long terraced uplands of the ordinary basalts
and the more conical forms of the southern group of gabbro hills
between Loch na Keal and Loch Spelve. The number and thickness of the
gabbro-sheets increase as we proceed inwards from the basalt-plateau.
These sheets are specially prominent along the higher parts of the
ridge that runs northwards from the northern end of Loch Spelve, and
along the west side of Glen Forsa. But they swell out into the thickest
mass in the south-western part of the hilly ground, where, from above
Craig, in Glenmore, they cross that valley, and form the rugged ridge
that rises into Ben Buy (2354 feet), and stretches eastward to near
Ardara (Map VI.). It is in this southern mass that the Mull gabbro
approaches nearest in general characters to that of Skye. But even here
its true intercalation above a great mass of bedded basalt may readily
be ascertained in any of the numerous ravines and rocky declivities.
One of the best lines of section for exhibiting the relations of
the rocks is the declivity to the west of Ben Buy and Loch Fhuaran.
Ascending from the west side, we walk over successive low escarpments
and terraces of the plateau-basalts with a gentle inclination towards
north-east or east. These rocks weather in the usual way, some into
a brown loam, others into spheroidal exfoliating masses. But as we
advance uphill they gradually assume the peculiar indurated shattery
character already referred to. The soft earthy amygdaloids become dull
splintery rocks, in which the amygdales are no longer sharply defined
from the matrix, but rather seem to shade off into it, sometimes
with a border of interlacing fibres of epidote. The compact basalts
have undergone less change, but they too have become indurated, and
generally assume a white or grey crust, and none of them weather out
into columnar forms. Strings and threads full of epidote run through
much of these altered rocks. Abundant granophyric and felsitic veins
traverse them. Sheets of dolerite likewise make their appearance
between the basalts, followed further up the slope by sheets of gabbro
until the latter form the main body of the hill.
On the north side of the same ridge similar evidence is obtainable,
though somewhat complicated by the injections of granophyric and
felsitic veins and bosses, to which more detailed reference will
afterwards be made. But the altered basalts with their amygdaloidal
bands and their intercalated basalt-tuffs and breccias, can be followed
from the bottom of the glen up to a height of some 1700 feet, above
which the main gabbro mass of Ben Buy sets in. Many minor sheets of
dolerite and gabbro make their appearance along the side of the hill
before the chief overlying body of the rock is reached. Some of these
can be distinctly seen breaking across or ending off between the bedded
basalts which here dip gently into the hill (Fig. 342). A conspicuous
band of coarse basalt-agglomerate, containing blocks of compact and
amygdaloidal basalt a yard or more in diameter, shows by the excessive
induration of its dull-green matrix the general alteration which the
rocks of the basalt-plateau have here undergone. An almost incredible
number of veins of fine basalt, porphyry and felsite has been injected
into these rocks--a structure which is precisely a counterpart of what
occurs under the main body of gabbro in Skye, Ardnamurchan and Rum.
[Illustration:
Fig. 342.--Altered Plateau-Basalts invaded by Gabbro, and with a
Dyke of prismatic Basalt cutting both rocks, north slope of Ben
Buy, Mull.
_a_ _a_, amygdaloidal basalt, much altered; _b_, gabbro; _c_, finely
prismatic basalt.]
The gabbro mass of the Ben Buy ridge is thus undoubtedly a huge
overlying sheet, which probably reaches a thickness of at least 800
feet. It seems to descend rather across the bedding into the hollow of
Glen More, and possibly its main pipe of supply lay in that direction.
Being enormously thicker than any other sheet in the island, it
exhibits the crystalline peculiarities which are so well developed in
the central portions of the larger bosses of gabbro. It presents more
coarsely crystalline varieties than appear in the thinner sheets, some
portions showing crystals of diallage and felspar upwards of an inch in
length. It likewise contains admirable examples of banded structure,
which, as in Skye and elsewhere, is best developed where the texture
becomes especially coarse. Veins or bands, in which the constituent
minerals have crystallized out in more definite and conspicuous forms,
here and there succeed each other so quickly as to impart a bedded or
foliated look to the body of rock, recalling, as in Skye, the aspect
of some coarsely crystalline granitoid gneiss. In these respects the
Mull gabbro closely resembles that of the Cuillin Hills. Occasionally,
on the exposed faces of crags, portions of such bands or veins are
seen to be detached and enveloped in a finer surrounding matrix. The
thick belts or bands of coarser and finer texture alternate, and give
an appearance of bedding to the mass. Nevertheless they are really
intrusive sills, which run generally parallel with beds of finer
gabbro or with sheets of highly indurated basalt, that may be detached
portions of the ordinary rocks of the plateau. The thick sheet of Ben
Buy, like the mass of the Cuillin Hills, is thus the result not of one
but of many uprises of gabbro.
Of the thinner sheets of dolerite and gabbro in Mull little need here
be said. I have referred to their great abundance in the range of
eastern hills that rise from the Sound of Mull between Loch Spelve
and Fishnish Bay. Though obviously intrusive, they lie on the whole
parallel to the bedding of the basalts. The latter rocks exhibit the
usual dull indurated shattery character which they assume where large
bosses of gabbro have invaded them, and which gradually disappears as
we follow them down hill away from the intrusive sheets to the shores
of the Sound. They dip towards the centre of the hill group, that is,
to south-west in the ridge of Mainnir nam Fiadh, Dùn da Ghaoithe, and
Beinn Meadhon, the angle increasing southwards to 15°-20°, and at the
south end reaching as much as 35°-40°. Some fine crags of gabbro and
dolerite form a prominent spur on the east side of the ridge of Ben
Talaidh, in the upper part of Glen Forsa. These consist of successive
sheets bedded with the basalts, and dipping south-west. A large sheet
stands out conspicuously on the north front of Ben More, lying at the
base of the "pale lavas," and immediately above the ordinary basalts.
It circles round the fine corry between Ben More and A'Chioch, some
of its domes being there beautifully ice-worn. This is the highest
platform to which I have satisfactorily traced any of the intrusive
sheets of Mull. Another dyke-like mass emerges from beneath the talus
slopes of A'Chioch, on the southern side, and runs eastward across the
col between the Clachaig Glen and Loch Scridain.
5. _The Gabbros of St. Kilda and North-east Ireland_
Sixty miles to the westward of the Outer Hebrides lies the lonely
group of islets of which St. Kilda is the chief. As the main feature
of geological interest in this group is the relation of the acid
protrusions to the other rocks, the account of the geology will be
more appropriately given as a whole in Chapter xlvii. I need only
remark here that the predominant rocks of these islands are dark basic
masses, chiefly varieties of gabbro, but including also dolerites and
basalts. Reasons will be afterwards brought forward for regarding
these rocks as parts of the Tertiary volcanic series. They present a
close parallel to the gabbros and associated rocks of Skye. But in one
important respect they stand alone. No certain trace remains of any
basalt-plateau at St. Kilda such as those through which the gabbros of
Skye, Mull and Ardnamurchan have been injected. In regard to their mode
of production they have doubtless been intruded at some considerable
depth beneath the surface. But no relic appears to have survived of the
overlying cover of rock under which they consolidated, and into which
they were injected.
In the remarkable volcanic district of the north-east of Ireland a
series of basic rocks appears, which in its mode of occurrence and its
relation to the other members of the series presents many points of
resemblance to the gabbros of the Inner Hebrides. The Irish gabbros are
well developed in the Carlingford district, where they form intrusive
bosses and sheets which have been erupted through the Palæozoic rocks
(Map VII.). They are themselves pierced by later masses of granophyre
and other acid rocks. Further reference will be made to these gabbros
in later pages, where an account will be given of the granite masses of
Mourne, Barnavave and Slieve Gullion.
* * * * *
It is interesting to observe that, while in St. Kilda no relic of any
basaltic plateau has been preserved, in the Faroe Islands, on the
other hand, no sign has been revealed by denudation that the volcanic
plateau of that region is pierced by any eruptive core of gabbro or of
granophyre. During my cruises round these islands and through their
channels, I was ever on the outlook for any difference in topography
that might indicate the presence of some eruptive boss like the gabbro
and granophyre masses of the Inner Hebrides. But nothing of that nature
could be discerned. Everywhere the long level lines of the bedded
basalts were seen mounting up to the crests of the ridges and the tops
of the highest peaks. Though I cannot assert that no intrusions of
gabbro or of granophyre exist among the Faroe Islands, I feel confident
that any such masses which may appear at the surface must be of quite
insignificant dimensions, and do not make the important feature in
geology and topography which they do among the Inner Hebrides. It is,
of course, possible that, vast as the denudation of these islands has
undoubtedly been, it has not yet trenched the plateau deeply enough
to expose any great intrusive bosses and sills which may underlie and
invade the basalts.
iv. HISTORY OF THE GABBRO INTRUSIONS
We are now in a position to draw, from the observations which have been
given in this and the preceding chapter regarding the different areas
of gabbro in the Tertiary volcanic region of Britain, some general
conclusions with respect to the type of geological structure and the
phases of volcanic energy which they illustrate.
1. No evidence exists to show that the masses of gabbro ever
communicated directly with the surface. They never exhibit the
cellular, slaggy and other structures so characteristic of
surface-flows. They are, on the whole, free from included pyroclastic
material, though masses of agglomerate are enclosed in, and have
probably been invaded by, the gabbro of the Cuillin Hills. If the
gabbro-bosses ever were continuous with sheets of rock emitted above
ground, all such upward continuations have been entirely removed. In
any case, we may be quite certain that in an outburst at the surface,
the rock would not have appeared in the form of a coarsely crystalline
or granitoid gabbro.
2. The crystalline structures of the gabbros point unmistakably to
slow cooling and consolidation at some depth beneath the surface. The
most coarsely-crystalline varieties, and those with the best developed
banded structure, occur in the largest bodies of rock, where the
cooling and consolidation would be most prolonged.[356]
[Footnote 356: On this subject, see the papers by Professor Judd already
cited.]
3. The remarkable differences in composition between the dark and pale
layers in the banded gabbros cannot be accounted for by segregation
or successive intrusion, but seem to point to the existence of a
heterogeneous magma from which these distinct varieties of material
were simultaneously intruded.
4. From the prevalence of a bedded structure and the occurrence of
bands and more irregular portions of considerably different texture and
even mineralogical composition which intersect each other, it may be
confidently inferred that even what appears now as one continuous mass
was produced by more than one intrusion.
5. In every case there would necessarily be one or more pipes up which
the igneous material rose. These channels might sometimes be wider
parts of fissures, such as those filled by the dykes. In other places,
they may have been determined by older vents, which had served for the
emission of the plateau-basalts and their pyroclastic accompaniments.
There can be no doubt that some of these vents afforded egress for the
subsequent eruption of granitoid rocks, as will be pointed out in the
following chapters. In the case of the gabbros, however, the position
of the vents seems to have been generally concealed by the tendency
of these rocks to spread out laterally. Denudation has cut deeply
into the gabbro-masses, but apparently not deep enough to isolate any
of the pipes from the larger bodies of material which issued from
them, and thus to leave solitary necks like those in and around the
basalt-plateaux. In Skye, where the central core of gabbro is largest
and most completely encircled, we cannot tell how much of it lies above
the true pipe or pipes, and has spread out on all sides from the centre
of eruption. The prevalence of rude bedding and a banded structure
indicate that most of the visible rock occurs in the form of sills,
successively injected not only into the plateau-basalts, but between
and across each other. Round the margin of the gabbro we undoubtedly
reach horizons below that rock, and see that it lies as a cake or
series of cakes upon the plateau-basalts. The actual pipe or fissure of
supply must in each case lie further inward, away from the margin, and
may be of comparatively small diameter.
6. From the central pipe or group of pipes or fissures which rose
from the platform of older rocks into the thick mass of the
basalt-plateaux, successive sheets of dolerite and gabbro were forced
outward between the layers of basalt. This took place all round the
orifices of supply, on many different horizons, and doubtless at many
different times. In some cases, the intrusive sheets were injected
into the very bottom of the basalts, and even between these rocks
and the older surface on which they rested. This is particularly
the case in Rum, where the gabbro-cones spring almost directly from
the ancient grits, schists and sandstones on which they rest. The
intrusive sheets have likewise found egress at every higher platform
in the basalt-series, up at least to the base of the "pale group" in
Mull--that is, through a continuous pile of more than 2000 feet of
bedded basalt. But the intrusion did not proceed equally all round an
orifice. At all events, the progress of denudation has revealed that on
one side of a gabbro area the injected portions may occur on a lower
stratigraphical level than they do on the opposite side. At the Cuillin
Hills, for example, the visible sheets of dolerite and gabbro to the
north of Coire na Creiche begin about 1600 feet above the sea, which
must be much more than that distance above the bottom of the basalts.
On the south-east side, however, they come down to near the base of the
basalts at Loch Scavaig; that is to say, their lowest members lie at
least 1600 feet below those on the opposite margin.
7. The uprise of so much igneous material in one or more funnels, and
its injection between the beds of plateau-basalt, would necessarily
elevate the surface of the ground immediately above, even if we
believe that surface to have been eventually disrupted and superficial
discharges to have been established. If no disruption took place, then
the ground would probably be upraised into a smooth dome, the older
lavas being bent up over the cone of injected gabbro until the portion
of the plateau so pushed upward had risen some hundreds of feet above
the surrounding country. The amount of elevation, which would of course
be greatest at the centre of the dome, might be far from equable all
round, one side being pushed up further or with a steeper slope than
another side. But even in the case of the Cuillin Hill area, it is
conceivable that the total uplift produced at the surface a gentle
inclination of no more than 8° or 10°.
It is along the periphery of a gabbro area that we may most hopefully
search for traces of this uplift. But unfortunately it is just there
that the work of denudation has been most destructive. There appears
also to have been a general tendency to sagging subsequent to the
gabbro protrusions, and the inward dip thereby produced has probably
been instrumental in effacing at least the more gentle outward
inclinations caused by the uprise of the eruptive rock. In one striking
locality, however, to which I have already referred, the effects
of both movements are, I think, preserved. The basalt-plateau of
Strathaird, which in its southern portion exhibits the ordinary nearly
level bedding, dips in its northern part at an unusually steep angle
to the north-west, towards the gabbro mass of Blath Bheinn. But before
reaching that mountain the basalts, much interbanded with sheets of
dolerite and gabbro, suddenly bend up to form the prominent eminence
of An Stac, where they dip rapidly towards south-east and south (Fig.
334). This steep dip away from the central mass of gabbro, is repeated
in the hills to the north, where the beds are inclined to north-east,
the angle gradually lessening northwards till they are truncated by the
granophyre of Strathmore. The mass of Blath Bheinn thus occupies the
centre of the dome or anticline. The theoretical structure of one of
the gabbro bosses is represented in Fig. 343. It will be understood,
however, that what for the sake of clearness is here represented in one
uniform tint of black in reality consists of an exceedingly complex
network of sheets and dykes differing from each other in texture and
structure, as well as in the relative dates of their intrusion.
8. The injection of so much igneous material among the bedded basalts
has induced in these rocks a certain amount of contact metamorphism.
I have referred to it as showing itself in the field as a marked
induration, the rocks becoming closer grained, dull, splintery, and
weathering, with a grey or white crust, while their amygdales lose
their definite outlines, and epidote and calcite run in strings, veins
and patches through many parts of the rocks. As already remarked,
it is difficult to determine how much of this change should be
referred to the influence of the gabbro, and how much to that of the
numerous intrusions of granophyre which may be apophyses of much
larger bodies of that rock lying not far underneath. On account of
this difficulty, the more detailed description of the metamorphism
of the plateau-basalts is reserved for Chapter xlvi., where it will
find a place in connection with the effects produced by the intruded
granophyres, which have undoubtedly been more extensive than those
effected by the gabbros.
[Illustration: Fig. 343.--Theoretical representation of the structure
of one of the Gabbro Bosses of the Inner Hebrides.
_a_ _a_, platform of older rock on which the bedded basalts (_b_ _b_)
have been poured out; _c_, gabbro.]
The structure and history of the gabbro bosses of the Inner Hebrides
find a close parallel in those of the Henry Mountains of Southern Utah,
so well described by Mr. G. K. Gilbert of the United States Geological
Survey.[357] In that fine group of mountains, rising to an extreme height
of 5000 feet above the surrounding plateau, and 11,000 feet above
the level of the sea, masses of trachyte have been injected between
sedimentary strata belonging to the Jura-Triassic and Cretaceous
systems. These masses, thirty-six in number, have consolidated in
dome-shaped bodies, termed by Mr. Gilbert "laccolites," which have
arched up the overlying strata, sending sheets, veins and dykes into
them, and producing in them the phenomena of contact metamorphism.
There is no proof that any of these protrusions communicated with the
surface, and there is positive evidence that most if not all of them
did not. The progress of denudation has laid bare the inner structure
of this remarkable type of hill, and yet has left records of every
stage in its sculpture. In one place are seen only arching strata,
the process of erosion not having yet cut down through the dome of
stratified rocks into the trachyte that was the cause of their uprise.
In another place, a few dykes pierce the arch; in a third, where a
greater depth has been bared away, a network of dykes and sheets is
revealed; in a fourth, the surface of the underlying "laccolite" is
exposed; in a fifth, the laccolite, long uncovered, has been carved
into picturesque contours by the weather, and its original form is more
or less destroyed.[358]
[Footnote 357: See the remarks and diagram, _ante_, p. 86.]
[Footnote 358: "Geology of the Henry Mountains," by Mr. G. K. Gilbert,
_U.S. Geographical and Geological Survey of the Rocky Mountain Region_,
1877.]
The gabbro "laccolites" of the West of Scotland belong to an older
geological period than those of Utah, and have, therefore, been longer
subject to the processes of denudation. They have been enormously
eroded. The overlying cover of basalt has been stripped off from
them, though from the escarpments beyond them it is not difficult in
imagination to restore it. In Rum it has been so completely removed,
that only a few fragments remain at some distance from the core of
gabbro, which now stands isolated. In Ardnamurchan, and still more
in Skye, the surrounding plateau of basalt remains in contact with
the gabbro bosses. But in Mull, where the plateau-basalts reach now,
and perhaps attained originally a greater thickness than anywhere
else, they have protected the intrusive sheets, which are therefore
less deeply cut away than in any of the other districts, and no great
central core of gabbro has yet been uncovered.