THE BOSSES AND SHEETS OF GABBRO
Petrography of the Rocks--Relations of the Gabbros to the other
members of the Volcanic series--Description of the Gabbro
districts--Skye
In singular contrast to the nearly flat basalts of the plateaux,
another series of rocks rises high and abruptly above these tablelands
into groups of dome-shaped, conical, spiry, and rugged hills. It is
these heights which, more than any other feature, relieve the monotony
of the wide areas of almost horizontal stratification so characteristic
of the volcanic region of the north-west. Their geological structure
and history are much less obvious than those of the bedded basalts.
Their mountainous forms at once suggest a wholly different origin. Some
portions of them have even been compared with the oldest or Archæan
rocks.[325] That they are really portions of the Tertiary volcanic
series, and that they reveal a wholly distinct phase in the history of
volcanic action, is now frankly admitted. Whether we regard them from
the petrographical or structural point of view, they naturally arrange
themselves into two well-defined groups. Of these one consists of
highly basic compounds, of which olivine-gabbro is the most prominent.
The other comprises numerous varieties--granite, granophyre, felsite,
quartz-porphyry, pitchstone and others--all of them being more or less
decidedly acid, and some of them markedly so. For reasons which will
appear in the sequel, the former group must be considered as the older
of the two, and it will therefore be described first.
[Footnote 325: This was my own first impression, when I began, as a
boy, to ramble among them. The remarkable resemblance of some parts
of them to ancient gneisses will be afterwards dwelt upon. Macculloch
had correctly grouped them with the other overlying rocks, and this
conclusion was afterwards confirmed by Prof. Zirkel.]
i. PETROGRAPHY OF THE GABBRO AREAS
Since the publications of Macculloch, the occurrence of beautiful
varieties of highly basic rocks among the igneous masses of the
Western Isles has been familiar to geologists. They were named by him
"hypersthene rock" and "augite rock,"[326] names which continued in use
until 1871, when my friend Professor Zirkel published the results
of his tour through the West of Scotland, and showed that the rocks
in question were mostly true gabbros.[327] Since his observations were
published some of these rocks have formed the subject of important
papers by Professor Judd.[328]
[Footnote 326: _Western Islands_, vol. i. pp. 385, 484.]
[Footnote 327: _Zeitschrift. Deutsch. Geol. Gesellsch._ xxiii. (1871), p.
1.]
[Footnote 328: _Quart. Jour. Geol. Soc._ xli. (1885), p. 354; xlii.
(1886), p. 49.]
The general petrographical characters of the gabbro areas of Western
Scotland may be summarized as follows:--A very considerable variety of
petrological structure and chemical composition is observable among the
rocks. At the one end of the series are compounds of plagioclase and
augite, which, though wanting in olivine, have the general structure
and habit of dolerites. At the other end are mixtures wherein felspar
is scarce or absent, and where olivine becomes the chief constituent.
Between these two extremes are many intermediate grades, of which the
most important are those containing the variety of augite known as
diallage and also olivine. These are the olivine-gabbros, which form so
marked a feature in the central parts of the great basic bosses. That
some of these varieties of rock pass into each other cannot be doubted.
Their distinctive composition and structure appear to have been
largely determined by their position in the eruptive mass. The outer
and thinner sheets are in great measure dolerites, with little or no
olivine. Coarse gabbros are abundant in the inner portions. Rocks rich
in olivine, however, occur at the outer and especially the lower part
of the gabbro masses of Rum and in some parts of Skye. The following
leading varieties may be enumerated:--
Dolerite.--This rock varies from an exceedingly close grain (when
it approaches and graduates into basalt) up to a coarse granular
crystalline texture, in which the component minerals are distinctly
visible to the naked eye. An average sample is found to consist of
plagioclase, usually lath-shaped, and crystals or grains of augite
with or without olivine. Under the microscope, the different varieties
are distinguished by the presence of more or less distinct ophitic
structure, the felspar being enveloped in the augite. For the most
part they are holocrystalline, but occasionally show traces of a
glassy base. Ilmenite is not infrequent, with its characteristic
turbid decomposition product (leucoxene). In other cases, the iron-ore
is probably magnetite. Between the dolerites and gabbros no line of
demarcation can be drawn in the field, nor can a much more satisfactory
limitation be made even with the aid of the microscope. As a rule, the
thickest and largest intrusive masses or bosses are gabbro, those of
less size are dolerite, while the smallest (and sometimes the edges of
the others) assume externally the aspect of basalts.
Gabbro.--Under this term I arrange, as proposed by Professor Judd,
all the coarse-grained granitoid basic rocks of the region without
reference to the variety of augite present in them. Under the
microscope, they are found to be holocrystalline, but with a granitic
or granulitic rather than an ophitic structure, though traces of the
latter are by no means rare. To the naked eye their component minerals
are usually recognizable. Professor Zirkel, from his examination
of the Mull gabbros, believed them to consist of three parts of
plagioclase, two parts of olivine, and one part of diallage.[329]
Olivine, however, is not invariably present.[330] The pyroxene also does
not always show the peculiar fibrous structure of diallage. Professor
Judd, indeed, maintains that the diallagic form is due to a deep-seated
process of alteration (schillerization), and that the same crystal may
consist partly of ordinary augite and partly of diallage.[331] Ilmenite
(with leucoxene), magnetite, apatite, biotite, and epidote are not
infrequent constituents.
[Footnote 329: _Zeitschr. Deutsch. Geol. Gesellsch._ xxiii. (1871), p.
59.]
[Footnote 330: Professor Judd (_Quart. Jour. Geol. Soc._ xlii. p. 62)
believes that originally all the gabbros contained olivine, and
that where it is now absent, it has been altered into magnetite or
serpentine. But in some coarse massive gabbros this mineral does not
appear to have been an essential constituent. See _op. cit._ vol. l. p.
654.]
[Footnote 331: _Op. cit._ xli. In a later paper he insists on
the gradation of the coarse granitoid varieties (gabbros) into
holocrystalline compounds, where the felspar appears in lath-shapes
with crystals or rounded grains of augite and olivine (dolerites), and
thence into true basalts, magma-basalts, and tachylytes (_op. cit._
xlii. p. 62).]
In a recent study of the gabbros of the Cuillin Hills of Skye by
Mr. J. J. H. Teall and myself, four characteristic types have been
recognized.[332]
[Footnote 332: _Quart. Journ. Geol. Soc._ vol. 1. (1894), pp. 645-659,
and Plates xiii. xxvi.-xxviii. See also Prof. Judd's paper, _op. cit._
(1886), p. 49.]
(1) _Granulitic Gabbros._--These are dark, fine-grained rocks which
externally resemble some of the altered basalts of the plateau-series.
They occur in bands or sheets which, so far as can be made out, are the
oldest portions of the whole gabbro mass. Under the microscope they
are found to possess a finely granulitic structure, and to consist
of grains of pyroxene (augite, but more usually with the inclusions
characteristic of diallage and pseudo-hypersthene), and of felspar
allied to labradorite, with green pseudomorphs agreeing in form and
size with the pyroxene-grains, but made of minute prisms and fibres of
green hornblende and a little chlorite.
(2) _Banded Gabbros._--These are characterized by a remarkable
arrangement in parallel bands of different mineral composition
like the banding of ancient gneisses. This structure will be more
particularly described in later pages. They are coarse-grained rocks
composed of pyroxene, plagioclase, olivine and magnetite. But these
minerals are not distributed equally through the mass. The pale bands
contain much felspar; the dark bands are largely composed of the
ferro-magnesian minerals and magnetite. The pyroxene, occurring as
ordinary augite, not uncommonly shows a tendency to ophitic structure.
The felspar, a variety closely allied to labradorite, occurs as
grains, as irregular ophitic patches, and also in forms that give
broad rectangular sections. Olivine in an unaltered condition has
been detected by Mr. Teall in only one specimen, and he thinks that
this mineral probably never played an important part in the original
constitution of these rocks. Its rounded grains may be observed to
have the other minerals moulded round them, whence it may be inferred
to be of older consolidation. Magnetite is generally present, either
in rounded grains or in large irregular masses. Though it occurs also
in strings traversing the other minerals as a secondary product, it
must undoubtedly have entered largely into the original composition
of these rocks. It is found enclosing the augite grains and behaving
like a groundmass between the felspars. Among the dark bands there
occur narrow lenticular black layers ('schlieren') composed entirely of
augite and iron-ore.
The extraordinary differences between the composition of the pale
felspathic and the dark ultra-basic bands are well brought out in
the following analyses by Mr. J. Hort Player, No. 1 being from a
light-coloured band consisting mainly of labradorite with some augite,
uralitic hornblende and magnetite; No. 2 from a dark band composed of
augite, magnetite and labradorite; and No. 3 from a thin ultra-basic
layer mainly formed of augite and magnetite. All these specimens were
taken from the ridge of Druim an Eidhne, on the eastern side of the
Cuillin Hills, Skye.[333]
I. II. III.
Silica 52·8 40·2 29·5
Titanic acid ·5 4·7 9·2
Alumina 17·8 9·5 3·8
Ferric oxide 1·2 9·7 17·8
Ferrous oxide 4·8 12·2 18·2
Ferric sulphide ··· ·4 ·4
Oxide of manganese ··· ·4 ·3
Lime 12·9 13·1 10·0
Magnesia 4·8 8·0 8·7
Soda 3·0 ·8 ·2
Potash ·5 ·2 ·1
Loss by ignition 1·2 ·5 1·0
---- ---- ----
99·5 99·7 99·2
---- ---- ----
Spec. grav. 2·91 3·36 3·87
==== ==== ====
[Footnote 333: _Quart. Journ. Geol. Soc._ vol. 1. (1894), p. 653. Banded
structures have been recognized in many gabbros of different ages. See
the references in this paper; also Mr. W. S. Bayley, _Journ. Geol._
Chicago, ii. (1895), p. 814, and vol. iii. p. 1.]
(3) _Coarse-grained massive Gabbros._--These rocks, so abundant among
the great basic bosses of the Inner Hebrides, are characterized by
their coarse granitic structure, their component crystals being
sometimes more than an inch long. They occur as sheets, veins and
irregular masses traversing the varieties of gabbro already mentioned.
They consist of the same minerals as the banded forms, and indeed are
themselves sometimes banded. They are more uniform in composition than
the typical banded gabbros, though showing also some variation in the
relative proportions of their constituents. The specific gravity of
three specimens was found to be 2·82, 2·97, and 3·06.
[Illustration: Fig. 330.--Granulitic and coarsely foliated gabbro
traversed by later veins of felspathic gabbro, Druim an Eidhne, Cuillin
Hills, Skye.]
(4) _Pale Gabbros of the Veins._--These occur abundantly as irregular
branching veins, from less than an inch to several yards in width,
and cross all the other varieties (Fig. 330[334]). Their whiteness on
weathered surfaces makes them conspicuous by contrast with the dark
brown or black hue of the rocks which they traverse, and shows at
once that they must be poorer in bases than these. They are found on
microscopic examination to consist of the same minerals as the more
coarsely crystalline gabbros, but with a much greater abundance of
the felspar. They contain also apatite, and hornblende appears to
predominate in them over augite. They are to be distinguished from the
pale veins that form apophyses from the intrusive granophyres.
[Footnote 334: Figs. 330, 336 and 337 are from photographs taken for the
Geological Survey by Mr. R. Lunn.]
Troctolite (Forellenstein).--This beautiful variety of
plagioclase-olivine rock occurs as a conspicuous feature on the east
side of the gabbro-area of the island of Rum. It forms a sill on the
side of the mountain Allival, in which the component minerals are drawn
out parallel with the upper and under surfaces of the bed (Fig. 341).
So marked is this flow-structure that hand-specimens might readily
be taken at the first glance for ancient schistose limestone. "The
felspathic ingredient (probably labradorite or anorthite) is white,
and its lath-shaped crystals have ranged themselves with their long
axes parallel to the line of flow. The olivine occurs in perfectly
fresh grains, which in hand-specimens have a delicate green tint.
Under the microscope they appear colourless, and are penetrated by
the felspar prisms in ophitic intergrowth. There is a small quantity
of a pale brownish augite, which not only occurs in wedge-shaped
portions between the felspars, but also as a narrow zone round the
olivines."[335] Considerable differences are visible in the development
of the flow-structure, and with these there appear to be accompanying
variations in the microscopic structure. Dr. Hatch, to whom I submitted
my specimens, informed me that in one of them, where the flow-structure
is so marked as to give a finely schistose aspect to the rock, "there
is a larger proportion of augite, some of which exhibits a distinct
diallagic striping; the olivine grains show no ophitic structure, but
are sometimes completely embedded in the augite." To this remarkable
flow-structure I shall again refer in connection with the light it
throws on the bedded character of much of the gabbro bosses.
[Footnote 335: MS. of Dr. Hatch.]
Between the different basic intrusive igneous rocks of the Inner
Hebrides, as Professor Judd has shown, there are many gradations
according to the varying proportions of the chief component minerals.
Thus from the olivine-gabbros, by the diminution or disappearance of
the augite we get such rocks as troctolite; where the plagioclase
diminishes or vanishes, we have different forms of picrite; where the
olivine is left out, we come to compounds, like eucrite; while by the
lessening or disappearance of the felspar and augite, we are led to
ultra basic compounds, consisting in greatest part of olivine, like
lherzolite, dunite and serpentine. To some of the features and probable
origin of these chemical and mineralogical diversities in the same
great eruptive mass further reference will be made in later pages.
ii. RELATIONS OF THE GABBROS TO THE OTHER MEMBERS OF THE VOLCANIC SERIES
Various opinions have been expressed regarding the connection between
the amorphous eruptive rocks of the hill-groups and the level
basalt-sheets of the plateaux. Jameson, though he landed at Rudh' an
Dunain, in Skye, where this connection can readily be found, does not
seem to have made any attempt to ascertain it. He noticed that the
lower grounds were formed of basalt, and that the mountains "appeared
to be wholly composed of syenite and hornblende rock, traversed
by basalt veins."[336] Macculloch, in many passages of his _Western
Islands_, alludes to the subject as one which he knew would interest
geologists, but about which he felt that he could give no satisfactory
information, and with characteristic verbiage he refers to the
impossibility of determining boundaries, to the transition from one
rock into another, to the inaccessible nature of the ground, to the
almost insuperable obstacles that impede examination, to the distance
from human habitation, and to the stormy climate,--a formidable list of
barriers, in presence of which he leaves the relative position and age
of the rocks unsettled.[337]
[Footnote 336: _Mineralogical Travels_ (1813), vol. ii. p. 72.]
[Footnote 337: See his _Western Islands_, vol. i. pp. 368, 374, 385,
386. With much admiration for the insight and zeal, amounting almost
to genius, which Macculloch displayed in his work among the Western
Islands, at a time when, with poor maps and inadequate means of
locomotion, geological surveying was a more difficult task than it
is now, I have found it impossible to follow in his footsteps with
his descriptions in hand, and not to wish that for his own fame he
had been content to claim credit only for what he had seen. His
actual achievements were enough to make the reputation of half a
dozen good geologists. It was unfortunate that he did not realize how
inexhaustible nature is, how impossible it is for one man to see and
understand every fact even in the little corner of nature which he may
claim to have explored. He seems to have had a morbid fear lest any one
should afterwards discover something he had missed; he writes as if
with the object of dissuading men from travelling over his ground, and
he indeed tacitly lays claim to anything they may ascertain by averring
that those who may follow him "will find a great deal that is not
here described, although little that has not been examined" (p. 373).
Principal Forbes long ago exposed this weak side of Macculloch and his
work (_Edin. New Phil. Journ._ xl. 1846, p. 82).]
Von Oyenhausen and Von Dechen, who wrote so excellent an account of
their visit to Skye, and who traced much of the boundary-line between
the gabbros and the other mountainous eruptive masses ("syenite"), seem
to have made no attempt to work out the connection between the former
and the rest of the volcanic rocks.[338]
[Footnote 338: Karsten's _Archiv_, i. p. 99. They frankly admit that
"the relation of the hypersthene rock to the other trap rocks was not
ascertained."]
J. D. Forbes, in his able sketch of the _Topography and Geology of
the Cuchullin Hills_, was the first to recognize the superposition
of the "hypersthene rock" upon the "common trap rocks"--that is, the
plateau-basalts. He was disposed to consider the "hypersthene mass as
a vast bed, thinning out both ways, and inclined at a moderate angle
towards the S.E."[339]
[Footnote 339: _Edin. New Phil. Journ._ xl. (1846), pp. 85, 86.]
Professor Judd regarded the bosses of basic and acid rocks that rise
out of the bedded basalts as the basal cores of enormously denuded
volcanic cones. He believed the granitoid rocks to have been first
erupted, and that after a long interval the basic masses were forced
through them, partly consolidating underneath and partly appearing at
the surface as the plateau-basalts.[340] That the order of appearance
of the several rocks has been exactly the reverse of this supposed
sequence was fully established by me in the year 1888, and has since
been amply confirmed.[341] Professor Zirkel recognized that the gabbros
are a dependence of the basalts, that they overlie them, and that
on the naked flanks of the mountains they are regularly bedded with
them.[342]
[Footnote 340: _Quart. Journ. Geol. Soc._ xxx. (1874), p. 249.]
[Footnote 341: _Trans. Roy. Soc. Edin._ xxxv. (1888), pp. 122 _et seq._;
_Quart. Journ. Geol. Soc._ vol. 1. (1894), pp. 216, 645; vol. lii.
(1896), p. 384, and Mr. Harker, _ibid._ p. 320.]
[Footnote 342: _Zeitschrift. Deutsch. Geol. Gesellsch._ xxiii. (1871),
pp. 58, 92.]
Up to the time of the publication of my memoir in 1888 no one had
traced out in more detail the actual boundaries of the several rocks on
the ground, so as to obtain evidence of their true relations to each
other as regards structure and age. Some of the numerous impediments
recorded by Macculloch no doubt retarded the investigation. But, as
Forbes so well pointed out, there is really no serious difficulty in
determining the true structural connection of the amorphous rocks
with each other and with the bedded basalts of the plateaux. I have
ascertained them in each of the districts,[343] and have found that there
cannot be the least doubt that the amorphous bosses, both basic and
acid, are younger than the surrounding bedded basalts, and that the
acid protrusions are on the whole younger than the basic, I shall now
proceed to show how these conclusions are established by the evidence
of each of the areas where the several kinds of rock occur.
[Footnote 343: In two of my excursions in Mull, and once in Skye, I was
accompanied by my former colleague Mr. H. M. Cadell, who rendered me
great assistance in mapping those regions.]
iii. DESCRIPTIONS OF THE SEVERAL GABBRO-DISTRICTS
1. _The Gabbro of Skye_
The largest, most picturesque, and to the geologist most important area
of Tertiary gabbro in Britain, is that of Skye (Map. VI.). Though, like
every other portion of the Tertiary volcanic districts, it has suffered
enormous denudation, and has thereby been trenched to the very core,
it reveals, more conspicuously and clearly than can be seen anywhere
else, the relation of the gabbro to the bedded basalts on the one hand,
and to the acid protrusions on the other. Its chief portion is that
which rises into the group of the Cuillin Hills, which for blackness of
hue, ruggedness of surface, jaggedness of crest, and general grimness
of aspect, have certainly no rivals within the limits of the British
Isles (Fig. 331). It has long been known to extend eastwards into
Blath Bheinn (Blaven) and its immediate northern neighbours. There is,
indeed, no break whatever between the rock of the Cuillins and that of
the hills on the east side of Strath na Creitheach. In Strath More the
gabbro is interrupted by the granitoid mass of the Red Hills. Patches
of it, however, occur further to the east, even as far as the Sound of
Scalpa.
[Illustration: Fig. 331.--Scuir na Gillean, Cuillin Hills, shewing the
characteristic craggy forms of the Gabbro. (From a photograph by Mr.
Abraham, Keswick.)]
If we throw out of account the invading granitoid rocks, and look
upon the whole tract within which the gabbro occurs as originally
one connected area, we find that it covered an elliptical space
measuring about nine miles from south-west to north-east and six
miles from north-west to south-east, and embracing at least 40 square
miles.[344] But that its original size was greater is strikingly shown
more particularly on the western margin, which like that of the
basalt-escarpments, has obviously been determined by denudation, for
its separate beds present their truncated ends to the horizon all along
the flanks of the Cuillins, from the head of Glen Brittle round to Loch
Scavaig (Fig. 332), and from Strath na Creitheach round the southern
flanks of Blath Bheinn to Loch Slapin and Strath More.
[Footnote 344: Though this and the other bosses are here spoken of
as consisting of gabbro, it will be understood that this rock only
constitutes the larger portion of their mass, which includes also
dolerites and other more basic compounds, together with involved
portions of the plateau-basalts and masses of agglomerate which
probably mark the position of older vents.]
[Illustration: Fig. 332.--Section across Glen Brittle, to show the
general relations of the Bedded Basalts (_a_) and the Gabbros (_b_).]
The first point to be ascertained in regard to the gabbro and
associated basic rocks of the mountainous tract is their connection in
geological structure and age with the bedded basalts of the plateau.
This initial and fundamental relation, as Forbes long ago said, can
be examined along the whole western and southern flank of the Cuillin
Hills, from the foot of Glen Sligachan round to the mouth of Loch
Scavaig. Even from a distance, the observer, who is favoured with clear
weather, can readily trace the almost level sheets of basalt till they
dip gently under the darker, more massive rock of the hills. Tourists,
who approach Skye by way of Loch Coruisk, have an opportunity, as
the steamer nears the island of Soay, of following with the eye the
basalt-terraces of the promontory of Rudh' an Dunain until they
disappear under the gabbro of the last spur of the Cuillins that guards
the western entrance to Loch Scavaig.
What is so evident at a distance becomes still more striking when
viewed from nearer ground. Nowhere can it be more impressively seen
than at the head of Glen Brittle. Looking westwards, the traveller
sees in front of him only the familiar level terraces and green slopes
of the basalt-plateau, rising platform above platform to a height of
nearly 1500 feet above the sea. But turning to the east, he beholds
the dark, gloomy, cauldron-like Corry na Creiche, from which rise some
of the ruggedest and loftiest crests of the Cuillins. On the hills
that project from either side of this recess and half enclose it,
the bedded basalts mount from the bottom of the valley, with their
lines of parallel terrace dipping gently inward below the black rugged
gabbro that crowns them and sweeps round to form the back or head of
the corry. Down the whole length of Glen Brittle the same structure
conspicuously governs the topographical features. On the right hand,
the ordinary terraced basalts form the slopes; and they rise for some
500 or 600 feet up the eastern side, until they pass under the darker,
more rugged, and less distinctly bedded rocks of the mountains (Fig.
332). The dip of the whole series is here at a gentle angle towards
south-east, that is, into or under the main mass of the Cuillin group.
When, however, we proceed to examine the junction between the two
rocks we find it to be less simple than it appears. It is not an
instance of mere superposition. The gabbro unquestionably overlies
the basalts, and is therefore of younger date. But it overlies them,
not as they rest on each other, in regular conformable sequence of
eruption, but intrusively, as a sill does upon the rocks on which it
appears to follow in the unbroken order of accumulation. This important
structure may be ascertained in almost any of the many sections cut
by the torrents which have so deeply trenched with gullies the flanks
of the hills. Starting from the ordinary bedded basalts, we observe,
in mounting the slopes and approaching the gabbro, that the rocks
insensibly assume that indurated shattery character, which has been
referred to as characteristic of them round the margins of vents,
and which will be shown to be not less so in contact with large
eruptive masses of basic or acid rock.[345] Beds of dolerite make their
appearance among the basalts, so distinctly crystalline, and so similar
in character to the rocks of the sills, that there can be little
hesitation in regarding them as intrusive. These sills increase in size
and number as we ascend, though hardened amygdaloidal basalts may still
be observed. True gabbros then supervene in massive beds, and at last
we find ourselves entirely within the gabbro area, where, however, thin
bands of highly altered basalt may still for some distance appear. One
further fact will generally be noticed, viz. that before reaching the
main mass of gabbro, veins and sills of basalt, as well as of various
felsitic and porphyritic members of the acid group, come in abundantly,
crossing and recrossing each other in the most intricate network. The
base of the thick gabbro-sheets is thus another horizon on which, as on
that below the plateau-basalts, intrusive masses have been especially
developed. Through all these rocks numerous parallel basalt-dykes,
running in a general persistent N.N.W. direction, with a later N.E.
series, rise from below the sea-level up even to the very crests of the
Cuillins (Fig. 333).
[Footnote 345: This indurated, altered character of the bedded basalts
near the intrusive bosses and sills will be more particularly described
in a later chapter in connection with the granophyre intrusions (see p.
386). The metamorphism induced by the basic rocks has generally been
less pronounced than that effected by the acid masses.]
The sections on the western side of the gabbro area of Skye thus prove
that this rock inosculates with the bedded basalts by sending into
them, between their bedding planes, sheets which vary in texture from
fine dolerites at the outside into coarse gabbros further towards the
central mass, and that this intrusion has been accompanied by a certain
amount of induration of the older rocks.
[Illustration: Fig. 333.--View of the crest of the Cuillin Hills,
showing the weathering of the gabbro along its joints, and of a
compound basic dyke which rises through it. (From a photograph by Mr.
Abraham, Keswick.)]
On the eastern side, the same structure can be even more distinctly
seen, for it is not only exposed in gullies and steep declivities, but
can be traced outward into the basalt-plateau. In the promontory of
Strathaird, Jurassic sandstones and shales, which form the coast-line
and lower grounds, are surmounted by the bedded basalts. Denudation
has cut the plateau into two parts. The smaller of these makes the
outlier that rises into Ben Meabost (1128 feet). The larger stretches
continuously from Glen Scaladal and Strathaird House northward into
Blath Bheinn. Hence from the ordinary terraced basalts, with their
amygdaloids, thin tuffs, red partings, and seams of lignite, every
step can be followed into the huge gabbro mountain. Starting from the
black Jurassic shales on which the lowest basalt lies, we walk over
the successive terraces up into the projecting ridge of An da Bheinn.
But as we ascend, sheets of dolerite and gabbro make their appearance
between the basalts, which gradually assume the altered aspect already
noticed. The dip of the whole series is at a low angle northwards, and
the beds can be followed round the head of the Glen nan Leac into the
southern slopes of Blath Bheinn. Seen from the eastern side of this
valley, the bedded character of that mountain is remarkably distinct,
but it becomes less marked towards the upper part of the ridge where
the gabbros preponderate. One of the most striking features of the
locality is the number and persistence of the dykes, which strike
across from the ordinary unaltered basalts of the plateau up into the
highest gabbros of the range. Where less durable than the intractable
gabbro, they have weathered out on the face of the precipices, thereby
causing the vertical rifts and gashes and the deep notches on the crest
that form so marked a feature in the scenery. On the other hand, they
are often less destructible than the plateau-basalts, and hence in the
Glen nan Leac they may be seen projecting as low dams across the stream
which throws itself over them in picturesque waterfalls. The youngest
dykes in the Blath Bheinn group of hills, have been found by Mr. Harker
to have a north-easterly trend, and a north-westerly hade of about 40°,
and to give a stratified appearance to the gabbro when viewed from a
distance.
The deep dark hollow of the Coire Uaigneich has been cut out of the
very core of Blath Bheinn, and lays bare the structure of the east part
of the mountain in the most impressive as well as instructive way (Fig.
334). By ascending into this recess from Loch Slapin, we pass over the
whole series of rocks, and can examine them in an almost continuous
section in the bed of the stream and on the bare rocky slopes on
either side. Sandstones and shales of the Jurassic series extend up
the Allt na Dunaiche for nearly a mile, much veined with basalt and
quartz-porphyry, by which the sandstones are locally indurated into
quartzite. At last these strata are overlapped by the basalts of the
Strathaird plateau, which with a marked inclination to N.N.W., here dip
towards the mountains. But by the time these rocks have reached the
valley, they have already lost their usual brown colour and crumbling
surfaces, and have assumed the indurated splintery character, though
still showing their amygdaloidal structure. They are much traversed
by felsitic veins and strings which proceed from a broad band of
fine-grained hornblende-granite that runs up the bottom of the Coire
Uaigneich and, ascending the col, crosses it south-westwards into the
Glen nan Leac. On the left or south-eastern side of this intrusive
mass, a portion of Lias shales and limestone (here and there altered
into white marble) is traceable for several hundred yards up the
stream.[346]
[Footnote 346: This limestone was formerly identified by me with the
Cambrian strata of the district. It was noticed by Von Oyenhausen and
Von Dechen, who, as Mr. Harker has recently ascertained, correctly
believed it to be a portion of the Lias torn off and carried upward by
the eruptive rocks (Karsten's _Archiv_, i. p. 79).]
The bedded basalts of Strathaird, after dipping down towards the
N.N.W., bend up where they are interbanded with dolerites and gabbros,
and form the prominence called An Stac, which rises as the eastern
boundary of the Coire Uaigneich. Their steep dip away from the mountain
is well seen from the east side, and their outward inclination is
continued into the ridge to the southward. Similar rocks appear on the
other flank of the band of granite, and form the base of Blath Bheinn.
They are likewise continued in the mountains further north called
Sgurr nan Each and Belig, where they dip in a northerly direction
away from Blath Bheinn, which seems to be the centre of uprise, with
the gabbro-sheets dipping away from it. The bedded basalts have been
traced by Mr. Harker up to a height of well over 2000 feet on the Blath
Bheinn range. They are of the usual altered, indurated, and splintery
character. The intrusive sheets interposed between them become thicker
and more abundant higher up, until they constitute the main mass of
the mountain. But that they are in separate sheets, and not in one
amorphous mass, can be recognized by the parallel lines that mark their
boundaries. The junction of the gabbro sills and the lavas is a very
irregular one, portions of the latter rocks being enveloped in the
intrusive sills.
The granite which sends out veins into the surrounding rocks is
obviously the youngest protrusion of the locality, except of course
the basalt-dykes which cross it, and which are nowhere seen in a more
imposing display than round the flanks of Blath Bheinn. A section
across the corry shows the structure represented in Fig. 334.
It is thus demonstrable that when its line of junction with the
surrounding plateau-basalts is traced in some detail, the gabbro is
found to overlie them as a whole, but also to be intercalated with
them in innumerable beds, bands, or veins which rapidly die out as
they recede outwards from the main central mass; that these interposed
beds are intrusive sheets or sills from that mass which have cut off
and enveloped portions of the basalts, and that the contiguous bedded
basalts show more or less marked metamorphism.
We have now to consider the structure of the interior of the gabbro
area of the Cuillin Hills. The first impression of the geologist
who visits that wild district is that the main mass of rock is as
thoroughly amorphous as a core of granite. Yet a little further
examination will reveal to him many varieties of texture, sometimes
graduating into, sometimes sharply marked off from, each other, and
suggesting that the rock is not the product of one single protrusion.
He will notice further indications of successive discharges or
extravasations of crystalline material during probably a protracted
period of time, and in the intricate network of veins crossing each
other and the general body of the rock in every direction, as well as
in the system of basalt-dykes that traverse all the other rocks, he
will recognize the completion of the evidence of repeated renewals of
subterranean energy.
[Illustration: Fig. 334.--Section across the Coire Uaigneich, Skye.
_a_, _b_, Jurassic sandstones and shales; _c_ _c_, bedded basalts
and dolerites; _d_ _d_, gabbros and dolerites with indurated
basalts; _e_, fine-grained hornblende-granite sending veins into
surrounding rocks; _f_ _f_, basalt-dykes running through all the
other rocks.
]
But the observer will be struck with the absence of the more usual
proofs of volcanic activity in such forms as vesicular lavas and
abundant masses of slag, bombs and tuffs, which are commonly associated
with the idea of the centre of a volcanic orifice, though he will meet
with isolated masses of coarse volcanic agglomerate within the gabbro
area and along some parts of its junction with the granophyre. The
general characters of the rocks around him suggest that he stands, as
it were, far beneath that upper part of the earth's crust which is
familiar to us in the phenomena of modern volcanoes; that he has been
admitted into the heart of one of the deeper layers, where he can study
the operations that go on at the very roots of an active vent.
When the geologist begins a more leisurely and systematic examination
of the interior of the gabbro area of Skye he soon sees reason to
modify the impression he may at first have received that this rugged
region presents the characters of one single eruptive mass. The more
he climbs among the hills the more will he meet with evidence of
long-continued and oft-repeated extravasation, one portion having
solidified before another broke through it, and both having been
subsequently disrupted by still later protrusions.
But if by chance he should begin his examination of the ground upon
some of the more typically banded varieties of rock, he may for a time
almost refuse to admit that these can be either of volcanic origin or
of Tertiary age.[347] He will find among them such startling counterparts
of the structure of the ancient Lewisian gneiss of the North-West
of Scotland that he may well be pardoned if for a time he seeks for
evidence that they really do belong to that primeval formation, and
have only been accidentally involved among the Tertiary volcanic rocks.
If, for instance, he should land in Loch Scavaig, and first set foot
upon the gabbros as they appear around Loch Coruisk, he would find
himself upon masses of grey coarsely crystalline, rudely banded rock,
like much of the old gneiss of Sutherland and Ross. Ascending over
the ice-worn domes, he would notice that the banding becomes here and
there more definitely marked by strong differences in texture and
colour, while elsewhere it disappears and is replaced by a granitoid
arrangement of the crystals, which are often as large as walnuts.
[Footnote 347: See _Quart. Journ. Geol. Soc._ vol. l. pp. 217, 657, and a
paper by the author, "Sur la Structure rubannée des plus anciens Gneiss
et des Gabbros Tertiaires," _Compt. rend. Cong. Géol. Internat._ 1894,
p. 139.]
[Illustration: Fig. 335.--Banded and puckered gabbro, Druim an Eidhne,
Glen Sligachan, Skye.]
Nowhere is the gneissoid banding more beautifully developed than on
the east side of the Cuillin group near the head of Glen Sligachan
along the ridge of Druim an Eidhne. It was at this locality that the
four typical structures were observed which have already been referred
to (p. 329). The varieties of colour and composition depend upon the
exceedingly irregular distribution of the component minerals. The
paler bands, rich in felspar, lie parallel with dark brown bands full
of pyroxene, olivine and magnetite, in which, moreover, thin ribs of
glistening black consist in large part of the iron ore. These layers
vary in thickness from mere pasteboard-like laminæ to beds a yard
or more in thickness. Within a space of a few square yards their
parallelism reminds one of stratified deposits (Fig. 336), but traced
over a wider space they are found to be more or less irregular in
thickness and lenticular in form.
[Illustration: Fig. 336.--Banded structure in the Gabbro, from the
ridge of Druim an Eidhne between Loch Coruisk and Glen Sligachan.]
The resemblance to gneisses, and sometimes to the flow-structure of
coarse rhyolites, is still further sustained by occasional undulations
or minute puckerings (Fig. 335). Still more extraordinary are the
examples of the actual plication of a group of successive bands, as
shown in Fig. 337, wherein such a group about ten feet thick is shown
to have been doubly folded between parallel bands above and below. This
structure is not due to any deformation of the gabbro long subsequent
to the consolidation of the mass. It belongs to the phenomena of
protrusion and solidification. An examination of thin slices of
these rocks under the microscope reveals no evidence of crushing. On
the contrary, the minerals of one band interlock with those of the
band adjoining, in such a manner as to prove that the differences of
composition cannot be due to crushing and shearing or to successive
intrusion, but must have been present before the final consolidation of
the whole rock.[348]
[Footnote 348: Mr. J. J. H. Teall and A. G., _Quart. Journ. Geol. Soc._
vol. 1. (1894), p. 652.]
The conclusion which seems most consonant with the facts is that the
magma which supplied the visible masses of gabbro in Skye existed
below in a heterogeneous condition, that portions of it, differing
considerably from each other in composition, were simultaneously
intruded, and that by the deformation of these portions during
their intrusion their present plicated structures were produced. A
careful study of these banded gabbros offers many suggestive points
of comparison with the gneisses and anorthosite (Norian) rocks of
pre-Cambrian age. It seems in the highest degree probable that the
banded structures and peculiar mineral aggregation in these ancient
rocks arose under conditions closely analogous to, if not identical
with, those in which the Tertiary gabbros of Skye originated.[349]
[Footnote 349: Consult the Memoirs cited in the footnote on p. 342.]
Similar structures are found to be widely developed through the gabbros
of the Cuillin Hills. Not only are these rocks disposed in distinct
beds, but many of the beds display the most perfect banding. Thus
the mountains that surround the head of Loch Scavaig and sweep round
Loch Coruisk up to the great splintered crests of Sgurr na Banachdich
display on their bare black crags a distinct bedded structure. On the
east side of Loch Scavaig the rock presents a rudely-banded character,
the bands or beds being piled over each other from the sea-level up
to the summits of the rugged precipices, and dipping into the hill
at angles of 25° to 35°. Abundant dykes and veins of various basic,
intermediate and acid rocks cut this structure. The individual layers
here show sometimes the wavy and puckered condition already referred to.
Even from a distance the alternating lighter and darker bands can
readily be seen, so that this structure, with the variations in
its inclination, can be followed from hill to hill (Fig. 338). The
regularity of the arrangement, however, is often less pronounced on
closer inspection. While the gabbro is rudely disposed in thick beds,
indicative of different intrusive sheets or sills, with which the
banding is generally parallel, considerable irregularities may be
observed in the arrangement of the structure of individual sheets.
These sheets may be parallel to each other, and yet, while in some the
banding is tolerably regular in the direction of the planes of the
sheets, in others it is much twisted or inclined at various angles.
[Illustration: Fig. 337.--Banded and doubly-folded Gabbro, Druim an
Eidhne, 10 feet broad.]
On the west side of the Coruisk river the banding is vertical;
southward from that stream it inclines slightly towards the south, but
soon again becomes vertical, and continues conspicuously so at the
junction of the gabbro with the Torridon sandstones and plateau-basalts
on the west side of Loch Scavaig.
Thus, instead of being one great eruptive boss, the gabbro of this
district is in reality an exceedingly complicated network of sills,
veins and dykes. While the general inclination of the bedding sometimes
continues uniform in direction and amount from one ridge to another,
it is apt to change rapidly, as if the complex assemblage of intruded
masses had been disrupted and had subsided in different directions. For
example, after overlying the bedded basalts of the plateau all the way
from Glen Brittle to the west side of Loch Scavaig, the gabbro descends
abruptly across these basalts and also across the Torridon sandstones,
on which they unconformably rest. These two groups of rocks are not
only truncated by the gabbro, but are traversed by the intricate system
of sills, dykes and veins already referred to. Where it abuts against
the sandstones and basalts in Loch Scavaig, the gabbro is arranged in
vertical bands of different mineral composition and texture. Much of it
is remarkably coarse, some bands displaying pyroxene crystals more than
an inch in length. There is no fine-grained selvage here, indicative
of more rapid cooling. So coarse, indeed, is the rock close up against
the sandstone, that the junction-line can hardly be supposed to be the
normal contact of the intrusive rock. This inference is confirmed by
the existence of a singular kind of breccia between the gabbro and the
sandstones. It is a tumultuous mass of fragments of coarse and fine
gabbro, Torridon sandstone and shale, and plateau-basalts, embedded
in a pale crystalline matrix of fine granular granophyre; veins from
this acid intrusion run off into the gabbro on the one side as well
as into the Torridon sandstones on the other. It would seem that this
junction-line has been one of great movement, that the gabbro-sheets
have subsided against a fault-wall of plateau-basalt and Torridon
sandstone, and that subsequently an intrusion of finely granular
granophyre has come up the fissure, involving in its ascent fragments
of all the materials around.
The rocks for a considerable distance to the south of the gabbro are
intensely altered. The Torridon sandstone has been so indurated as to
pass into a bleached white quartzite, while the shales interstratified
with it have been converted into a kind of porcellanite. But the
most interesting alterations are those to be observed in the
plateau-basalts, which at a height of about 300 feet above the sea,
are to be seen in nearly horizontal sheets that lie immediately on the
upturned edges of the Torridon sandstones. These lavas have suffered
great metamorphism, to which more particular reference will be made in
Chapter xlvi. in connection with the action of the granophyre. Whether
this alteration has been produced by the intrusion of the gabbro or of
some concealed mass of granophyre underneath, of which only projecting
dykes and veins reach the surface, must remain a matter of doubt. On
the whole, as the gabbro is here undoubtedly thrown against the basalts
and Torridon sandstone by a fault, it seems most probable that the
change has been mainly due to the influence of the acid rock.
In the Blath Bheinn group of hills the relations of the gabbro to the
bedded basalts have recently been mapped in detail by Mr. Harker during
the progress of the Geological Survey of Skye. He has observed that,
allowing for irregularities of form, the mass of gabbro obliquely
overlies the basalts as a great sheet, not necessarily due to a
single intrusion, which dips towards the west. He has found the rock
to vary from a coarse gabbro to a diabasic type, and to vary also in
mineralogical constitution, becoming in places very rich in olivine,
though the banded structure is here only exceptionally developed. North
of Garbh Bheinn the gabbro is much crushed and the outlying patch to
the north of Belig is in part a crush-breccia. Mr. Harker remarks that
similar brecciated structures are common among the granophyres of the
Red Hills, and that it is sometimes difficult to distinguish their
structure from that of the true volcanic agglomerates.
[Illustration: Fig. 338.--Sketch of Banded Structure in the Gabbros of
the hills at the head of Loch Scavaig.]
Besides the main area of gabbro in Skye, a great many small detached
bosses, sills and dykes lie further east on the flanks of the Red
Hills. One of the best marked of these detached areas forms a
conspicuous crag on the east side of Strath More, immediately to the
north of Beinn na Cro. It consists of beds of coarse gabbro, with
others of dolerite intercalated in an outlier of the plateau-basalts,
and is traversed by veins from the granophyre of the glen, as well as
by the usual north-west basalt dykes (Fig. 349). It appears to be a
marginal portion of the main gabbro area separated by the intrusion of
the great granitoid boss of the Red Hills. On the north-eastern side
of Beinn na Caillich numerous intrusive sheets of gabbro and dolerite
traverse the quartzite and limestone, and extend down to the sea-margin
in the Sound of Scalpa.
There is an important feature in the main gabbro area of Skye not yet
clearly understood, and which only a minute and patient survey can
elucidate. Though I have found among the Cuillin Hills no distinct
proof that the mass of gabbro ever gave rise to discharges of material,
either lava-form or fragmentary, which reached the surface, the
gabbro area, as already remarked, contains unquestionable evidence
of explosions and the production of pyroclastic masses. Among the
moraine-mounds of Harta Corry, blocks of basalt-agglomerate are strewn
about, full of angular fragments of altered basalt, sometimes highly
amygdaloidal, and also boulders in which lumps of coarse gabbro are
enveloped in a matrix of finer material. I did not find the parent
rocks from which these glacier-borne masses had been derived, but there
can be no doubt that they exist among the gabbro crags that surround
that deep glen. Reference has already been made to the similar rock
found _in situ_ on the opposite side of the Cuillin ridge at the head
of the great cauldron of Corry na Creich; likewise to the mass of
coarse agglomerate which forms a group of knolls and crags on the east
side of Druim an Eidhne above the head of Glen Sligachan. This rock
contains abundant blocks of various slaggy lavas like those of the
basalt-plateau, and runs for some distance along the eastern limit of
the gabbro, between that rock and the granophyre. It is intersected by
numerous basalt-veins. Mr. Harker, as above mentioned, has recently
found some considerable strips of agglomerate which, like that which I
traced round the west side of Beinn Dearg, are interposed between the
gabbro and the bosses of granophyre, or lie at the base of the volcanic
series (p. 284).
There does not, however, appear to be any evidence to connect these
isolated masses of agglomerate with the phenomena attending the uprise
of the gabbro. They seem to be more probably related to the plateau
eruptions, and may be compared with those of Strath, Ardnamurchan and
Mull (pp. 278, 280, 384). That the huge gabbro mass of Skye, besides
invading and altering the bedded basalts, may have communicated
eventually with the surface, and have given rise to superficial
discharges, is not at all improbable, but of any such outflows not
a vestige appears now to remain. We must remember, however, that
the gabbro no doubt in many places found its readiest upward ascent
in vents belonging to the plateau-period, and that portions of the
agglomerates of these earlier vents may be expected to be found
involved in it, as the agglomerate of the great vent of Strath has been
invaded by the granophyre.