TYPES OF STRUCTURE IN THE ACID ROCKS--BOSSES
Returning now to the consideration of the acid rocks as these manifest
themselves in the volcanic areas of Britain, I would remark that
three distinct types of structure may be noted among them, viz. (1)
bosses, (2) sills or intrusive sheets, (3) veins and dykes. These
types, as above remarked, belong entirely to the underground operations
of volcanism, for though the rhyolitic fragments in the tuffs and
agglomerates of the plateaux prove that acid lavas existed near the
surface, no undoubted case of superficial lava belonging to the acid
series has yet been observed.[386]
[Footnote 386: The rhyolites of Tardree in Antrim have recently been
claimed by Professor Cole as true lavas grouped round an eruptive vent.
For reasons to be given in the next chapter I regard them as intrusive
masses, though they may not improbably have been connected with streams
of lava now entirely removed.]
The bosses of acid material in the British Tertiary volcanic series
are irregular protrusions, varying in size from knobs only a few
square yards in area up to huge masses many square miles in extent,
and comprising groups of lofty hills. As a rule, their outlines are
markedly irregular. Beneath the surface they plunge down almost
vertically through the rocks which they traverse, but in not a few
instances their boundaries are inclined to the horizon, so that the
contiguous rocks seem to rest against them, and sometimes lie in
outliers on their sides and summits. From the margins of these bosses
apophyses are given off into the surrounding rocks, sometimes only
rarely and at wide intervals, in other places in prodigious numbers.
Sometimes the acid material has been injected in thousands of veins and
minute threads, which completely enclose fragments of the surrounding
rock.
The rock of which the bosses consist is generally granophyric in
texture, passing on the one hand, particularly in the central parts,
into granite, and on the other, and especially towards the margin, into
various more compact felsitic varieties, and sometimes exhibiting along
the outer edge more or less developed spherulitic and flow-structures.
Decided contact metamorphism is traceable round the bosses, but is
by no means uniform even in the same rock, some parts being highly
altered, while others, exposed apparently to the same influences, have
undergone little change. The most marked examples of this metamorphism
are those in which the Cambrian limestone of Skye has been converted
into a pure white saccharoid marble. But the most interesting to the
student of volcanic action are those where the altered rocks are older
parts of the volcanic series. As the bosses of each volcanic area offer
distinctive peculiarities they will here be described geographically.
i. THE ACID BOSSES OF SKYE
It is in the island of Skye that the granophyre and granite bosses
attain their largest dimensions and afford, on the whole, the most
complete evidence of their structures and their relations to the other
parts of the volcanic series (Map VI.). They cover there a total area
of about 25 square miles, and form characteristic groups of hills from
2000 to 2500 feet in height. On the south-east side, three conspicuous
cones (the Red Hills) rise from the valley of Strath (Beinn Dearg
Mhor, Beinn Dearg Bheag and Beinn na Caillich). A solitary graceful
pointed cone (Beinn na Cro) stands between Strathmore and Strathbeg,
while to the north-west a continuous chain of connected cones runs from
Loch Sligachan up into the heart of the Cuillin Hills. Their conical
outlines, their smooth declivities, marked with long diverging lines
of screes, and their pale reddish or reddish-yellow hue, that deepens
after a shower into glowing orange, mark off these hills from all the
surrounding eminences, and form in especial a singular contrast to the
black, spiry, and rugged contours of the gabbro heights to the west of
them.
Besides this large continuous mass, a number of minor bosses are
scattered over the district. Of these the largest forms the ridge of
Beinn an Dubhaich, south of Loch Kilchrist. Several minor protrusions
lie between that ridge and the flank of Beinn Dearg. Others protrude
through the moory ground above Corry; several occur on the side of
the Sound of Scalpa, about Strollamus; and one, already referred to,
lies at the eastern base of Blath Bheinn. In the neighbouring island
of Raasay, a large area of granophyre likewise occurs, which will be
described with the Sills in later pages.
In so extensive a district there is room for considerable diversity
of composition and texture among the rocks. As already stated, in
some places, more particularly in the central parts of the hills,
the acid material assumes the character of a granite, being made up
of a holocrystalline aggregate of quartz, orthoclase, plagioclase,
hornblende and biotite, without granophyric structure, and thus becomes
a hornblende-biotite-granite (quartz-syenite, granite-syenite of
Zirkel, or amphibole-granitite of Rosenbusch). By the development of
the micropegmatitic structure and radiated spherical concretions, it
passes into granophyre. By the appearance of a felsitic groundmass, it
shades off into different varieties of quartz-porphyry or rhyolite,
sometimes with distinct bi-pyramidal crystals of quartz.[387] This
change, which here and there is observable along the edge of a boss,
is sometimes accompanied with an ample development of spherulitic and
flow-structures. As it is convenient to adopt some general term to
express the whole series of varieties, I have used the word granophyre
for this purpose.
[Footnote 387: The best account yet published of these varieties in Skye
is that by Prof. Zirkel, _Zeitsch. Deutsch. Geol. Gesellsch._ xxiii.
(1871) p. 88.]
[Illustration: Fig. 346.--View of Glamich, 2537 feet, Glen Sligachan.
(From a photograph by R. J. A. Berry, M.D., lent by the Scottish
Mountaineering Club).]
That the large area of these rocks in Skye was the result of many
separate protrusions from distinct centres of emission may be inferred,
I think, not only from the varieties of petrographical character in
the material, but also from the peculiar topography of the ground,
and perhaps from the curious relation which seems, in some instances
at least, to be traceable between the external features and apparent
internal structure of the hills. It will be seen from the Map (No. VI.)
that in the area lying to the east of Strath More the granophyre is
broken up into nearly detached portions by intervening patches of older
rocks. There can be little doubt that the mass of Beinn na Caillich
and the two Beinn Deargs is the product of a distinct orifice, if not
of more than one. Beinn na Cro, lying between its two deep bounding
glens, is another protrusion. The western cones stand so closely
together that their screes meet at the bottoms of the intervening
valleys. Yet each group is not improbably the result of emission from
an independent funnel, like the separate domite puys of Auvergne.
But, though I believe this large area of granitoid rock to have
proceeded not from one but from many orifices, I have only here and
there obtained, from the individual hills themselves, indications of an
internal structure suggestive of distinct and successive protrusions
of material from the same vent of discharge. On the outer declivities
of some of the cones we may detect a rudely bedded structure, which
will be subsequently referred to as well displayed in Rum (p. 403).
This structure is specially observable along the east side of Glen
Sligachan. Down the northern slopes of Marsco the granophyre (here in
part a hornblende-biotite-granite) is disposed in massive sheets or
beds that plunge outwards from the centre of the hill at angles of 30°
to 40°. On the southern front of the same graceful cone, as well as on
the flanks of its neighbour, Ruadh Stac, still plainer indications of
a definite arrangement of the mass of the rock in irregular lenticular
beds may be noticed. These beds, folding over the axis of the hill, dip
steeply down as concentric coats of rock. The external resemblance of
the red conical mountains of Skye to the trachyte puys of Auvergne was
long ago remarked by J. D. Forbes,[388] and in this internal arrangement
of their materials, indefinite though it may be, there is a further
resemblance to the onion-like coatings which Von Buch and Scrope
remarked in the structure of the interior of the Grand Sarcoui.[389]
[Footnote 388: _Edin. New Phil. Jour._ xl. p. 78.]
[Footnote 389: Von Buch, _Geognostische Beobachtungen auf Reisen durch
Deutschland und Italien_, vol. ii. (1809) p. 245; Scrope, _Geology
and Extinct Volcanoes of Central France_, 2nd edit. p. 68. Von Buch
regarded the external form of this Puy as having been determined by its
internal structure.]
Where the contour of the cones is regular, and the declivities are not
marked by prominent scars and ribs of rock, this monotony of feature
betokens a corresponding uniformity of petrographical character. But
where, on the other hand, the slopes are diversified by projecting
crags and other varieties of outline, a greater range of texture and
composition in the material of the hills is indicated. This relation
is well brought out on the western front of Marsco, where numerous
alternations of granitoid and felsitic textures occur. On many
declivities also, which at a distance look quite smooth, but which
are really rough with angular blocks detached from the parent mass
underneath, an occasional basalt-dyke will be observed to rise as a
prominent dark rib. A good example of this structure is to be seen on
the south front of Beinn na Caillich. Where a group of dark parallel
dykes runs along the sides of one of these pale cones, it sometimes
produces a curiously deceptive appearance of bedding. A conspicuous
illustration may be noticed on the southern front of Beinn Dearg
Meadhonach, north from Marsco. When I first saw that hillside I could
not realize that the parallel bars were actually dykes until I had
crossed the valley and climbed the slopes of the hill.[390]
[Footnote 390: The difference of contour and colour between the ordinary
reddish smooth-sloped "syenite" and the black craggy "hypersthene rock"
and "greenstone" in the Glamaig group of hills caught the eyes of Von
Oeynhausen and Von Dechen (Karsten's _Archiv_, i. p. 83).]
Good evidence of successive protrusions of the acid rock within the
great area of the Red Hills may be found on the south side of Meall
Dearg at the head of Glen Sligachan, where the granophyre is traversed
by a younger band or dyke of fine-grained spherulitic material
about ten feet broad. The rock exhibits there the same beautiful
flow-structure with rows of spherulites as is to be seen along the
contact of the main granophyre mass with the gabbro on the same hill,
which will be afterwards described. This dyke, vein or band, though
possibly belonging to the same epoch of protrusion as the surrounding
granophyre, must obviously be later than the consolidation of the rock
which it traverses.
Occasionally round the margin of the granophyre a singular brecciated
structure is to be seen. I have found it well marked on weathered
faces, along the flanks of Glamaig and of Marsco, and Mr. Harker has
observed many examples of it on the north side of the granophyre
mass of the Red Hills. When the rock is broken open, it is less easy
to detect the angular and subangular fragments from the surrounding
matrix, which is finely crystalline or felsitic.
The actual junction of the eruptive mass with the surrounding rocks
through which it has ascended is generally a nearly vertical boundary,
but the granophyre sometimes plunges at a greater or less angle under
the rocks that lie against or upon it. On the north side of Glamaig,
for instance, the prophyritic and felsitic margin of the great body
of eruptive rock descends as a steeply inclined wall, against which
the red sandstones and marls at the base of the Secondary formations
are sharply tilted. On the south side of the area a similar steep
face of fine-grained rock forms the edge of the granophyre of the
great southern cones, and plunges down behind Lias limestone and
shale, Cambrian limestone and quartzite, or portions of the Tertiary
volcanic series. Where the granophyre cuts vertically through the
gabbro, the latter rock being more durable is apt to rise above the
more decomposable granophyre as a crag or wall, and thus the deceptive
appearance arises of the basic overlying the acid rock. As above
mentioned, there seems every reason to believe that this peculiarity of
weathering has given rise to or confirmed the mistaken impression that
the granophyre is older than the gabbro.
There can be no doubt, however, that along many parts of the
boundary-line the acid eruptive mass extends underneath the surface
far beyond the actual base of the cones, for projecting knobs as
well as veins and dykes of it rise up among the surrounding rocks.
This is well seen along the northern foot of Beinn na Caillich. But
of all the Skye bosses none exhibits its line of junction with the
surrounding rocks so well and continuously as Beinn an Dubhaich. This
isolated tract of eruptive material lies entirely within the area
of the Cambrian limestone, and its actual contact with that rock,
and with the basalt-dykes that traverse it, can be examined almost
everywhere. The junction is usually vertical or nearly so, sometimes
inclining outwards, sometimes inwards. It is notched and wavy, the
granite sending out projecting spurs or veins, and retiring into little
bays, which are occupied by the limestone. The subdivisions of the
latter rock have recently been traced by Mr. Harker up to one side of
the granite and recognized again on the other side, with no apparent
displacement, as if so much limestone had been punched out to make way
for the uprise of the acid boss. The older dykes, too, are continuous
on either side of the ridge. The granite is massive and jointed,
splitting up into great quadrangular blocks like an ancient granite,
and weathering into rounded boulders. Its granitic composition and
texture are best seen where the mass is broadest, south of Kilbride.
Towards its margin, on the shore of Camas Malag, the granophyric
structure appears, especially in narrow ribbons or veins that run
through the more granitic parts of the rock. These may be compared with
the much larger dyke of spherulitic rock above noticed as traversing
the granophyre of Meall Dearg.
[Illustration: Fig. 347.--Section across the north slope of Beinn an
Dubhaich, Skye.
_a_ _a_, Cambrian limestone; _b_ _b_, basalt dykes; _c_, granite.]
Immediately to the south of Camas Malag the junction with the limestone
is well displayed, and the eruptive rock, which is there granitic
in character, sends out into the limestone a vein or dyke about two
feet broad, of closer grain than the main body of the boss, but still
distinctly granitic in structure. The junction on the north side
is equally well seen below the crofts of Torran. Here the rock of
the boss, for a few yards from its margin, assumes a fine-grained
felsitic aspect, and under the microscope presents a curious brecciated
appearance, suggestive of its having broken up at the margin before
final consolidation. Portions of the already crystallized granite seem
to be involved in a microgranitic base. The rock has here truncated a
number of basalt-dykes which intersect the Cambrian limestone. To one
of these further reference will be made in the sequel.
On the surface of the mass of Beinn an Dubhaich, a few little patches
of limestone occur to the south of Kilchrist Loch. Considering the
nearly vertical wall which the granophyre presents to the adjacent
rock all round its margin, we may perhaps reasonably infer that these
outliers of limestone are remnants of a once continuous limestone sheet
that overlay the eruptive rock, and hence that, with due allowance for
considerable denudation, the present surface of the boss represents
approximately the upper limit to which the granophyre ascended through
the limestone. The actual facts are shown in Fig. 347.
All round the margin of this boss, the limestone has been converted
for a variable distance of a few feet or many yards into a granular
crystalline marble. The lighter portions of the limestone have become
snowy white; but some of the darker carbonaceous beds retain their
dark tint. The nodules of chert, abundant in many of the limestones,
project from the weathered faces of the marble. The dolomitic portions
of the series have likewise undergone alteration into a thoroughly
crystalline-granular or saccharoid rock. The most thorough metamorphism
is exhibited by portions of the limestone which are completely
surrounded by and rest upon the granite. The largest of these overlying
patches was many years ago quarried for white marble above the old
Manse of Kilchrist. I have shown by lithological, stratigraphical and
palæontological evidence that this limestone, instead of belonging
to the Lias, as was formerly believed, forms a part of the Cambrian
or possibly the very lowest Silurian series, being a continuation of
the fossiliferous limestone of western Sutherland and Ross-shire.[391]
Mr. Clough and Mr. Harker, in the progress of the Geological Survey
in Skye, have ascertained that the distinctive characters of the
three groups of strata into which the limestone can be divided may be
recognized even through the midst of the metamorphism.[392]
[Footnote 391: _Quart. Journ. Geol. Soc._ vol. xliv. (1888) p. 62.]
[Footnote 392: _Annual Report of Director-General of the Geological
Survey for 1895._]
The generally vertical line of separation between the rock of Beinn
an Dubhaich and the contiguous limestone has been taken advantage of
for the segregation of mineral veins. On the southern boundary at
Camas Malag, a greenish flinty layer, from less than an inch to two
or three inches in width, consisting of a finely-granular aggregate
of some nearly colourless mineral, which polarizes brilliantly, coats
the wall of the granophyre, and also both sides of the vein which
proceeds from that rock into the limestone. But the most abundant
and interesting deposits are metalliferous. Fragments of a kind of
"gossan" may be noticed all along the boundary-line of the boss, and
among these are pieces of magnetic iron-ore and sulphides of iron and
copper. The magnetite may be seen in place immediately to the south of
Kilbride. A mass of this ore several feet in diameter sends strings
and disseminated particles through the surrounding granophyre, and is
partially coated along its joints with green carbonate of copper.
From the Skye area important evidence is obtainable in regard to the
relation of the acid eruptions to (1) earlier eruptive vents filled
with agglomerate; (2) the bedded basalts of the plateaux; (3) the
bosses, sills and dykes of gabbro and dolerite; and (4) the great
system of basic dykes.
(1) _Relation of the Granophyre to older Eruptive Vents._--The
granophyre of Beinn na Caillich and the two Beinn Deargs has invaded
on its north-eastern side the Cambrian limestone and quartzite, and has
truncated the sheets of intrusive dolerite and gabbro that have there
been injected into them. But to the south-west it rises through the
great Strath agglomerate already described, and continues in that rock
round to the entrance into Strath Beg. The eruptive mass is in great
part surrounded with a ring of agglomerate, as if it had risen up a
huge volcanic chimney and solidified there, though probably there were
more than one vent in this agglomerate area. Again the thick mass of
agglomerate north of Belig is interposed between the bedded lavas and
the great granophyre mass which extends northwards to Loch Sligachan.
On the west side of the Blaven ridge, a number of masses of agglomerate
are found on both sides of Glen Sligachan, along the border of the same
great tract of acid rock.
[Illustration: Fig. 348.--Section from Beinn Dearg to Beinn an
Dubhaich, Skye.
_a_ _a_, Cambrian limestone; _b_ _b_, volcanic agglomerate; _c_ _c_
_c_, basalt-dykes older than granophyre; _d^1_, granophyre of Beinn
Dearg; _d^2_, granophyre in the agglomerate neck; _d^3_, granite of
Beinn an Dubhaich; _e_, basalt-dyke younger than granite.
]
With regard to the relation of the granophyre of the Red Hills to the
great agglomerate of Strath, we may infer that the granophyre has
not risen exactly in the centre of the old funnel, but rather to the
north of it, unless we suppose, as already suggested, that some of
the agglomerate belongs to the cone that gathered round the eruptive
orifice. It is interesting to observe, however, that granophyre, from
the same or from another centre of protrusion, has likewise risen along
the outer or southern margin of the agglomerate, generally between that
rock and the limestone, but sometimes entirely within the agglomerate.
The distance between the nearest part of this ring of eruptive rock
and the edge of the boss of Beinn an Dubhaich is under 400 yards,
the intervening space being occupied by limestone (or marble), much
traversed by north-west basalt-dykes. Most of these dykes do not enter
the rocks of the vent, and are abruptly truncated by the mass of Beinn
an Dubhaich. The probable structure of this locality is shown in Fig
348.
The masses of agglomerate which further westward so curiously follow
the margin of the great granophyre bosses, and those which are
entangled in that rock and in the gabbro, probably indicate, as already
suggested, the position of a group of older volcanic funnels which
provided facilities for the uprise of the basic and acid magmas.
The group of vents which, as we have seen, probably rose out of the
plateau-basalts, and first served for the rise of the masses of gabbro,
has by the subsequent protrusion of the granophyres been still further
destroyed and concealed.
The granophyre intrusions in the great Strath agglomerate have lately
been mapped and described by Mr. Harker. As regards their internal
structure and composition, this observer remarks that compared with the
normal granophyres of the Red Hills and other bosses of the district,
these smaller intrusive masses are darker and manifestly richer in the
iron-bearing minerals, and have a slightly higher specific gravity. But
in their general characters they agree with the other granophyres. The
most interesting feature in them is the evidence they afford that they
have enclosed and partially dissolved fragments of basic rocks. To this
evidence further reference will be made on a later page (see p. 392).
(2) _Relation of the Granophyre to the Bedded Basalts of the Plateaux.
Metamorphism of the Basalts._--On the north-west side, the granophyre
of Glamaig and Glen Sligachan mounts directly out of the bedded
basalts. These latter rocks, which rise into characteristic terraced
slopes on the north side of Loch Sligachan, appear on the south side
immediately to the west of Sconser, and stretch westwards round the
roots of Glamaig into the Coire na Sgairde. As they approach that hill
they assume the usual dull, indurated, splintery, veined character of
their contact metamorphism, and weather with a pale crust. Some of them
are highly amygdaloidal, and between their successive beds thin bands
of basalt-breccia, also much hardened, occasionally appear. Veins of
granophyre become more numerous nearer the main mass of that rock.
The actual line of junction runs into the Coire na Sgairde and slants
up the Druim na Ruaige, ascending to within a few feet of the top of
that ridge. A dark basic rock lies on the granophyre, the latter being
here finer grained and greenish in colour, and projecting up into the
former.[393] There is so much detritus along the sides and floor of
Glen Sligachan that the relations of the two groups of rock cannot be
well examined there. But the basalts, which present their ordinary
characters to the north of the Inn, are observed to become more and
more indurated, close-grained, dull and splintery, as they draw nearer
to the granophyre of Marsco. This part of the district furnishes the
clearest evidence of the posteriority of the great cones of Glamaig and
its neighbours to the plateau-basalts which come up to the very base of
these hills.[394]
[Footnote 393: I think it probable that some of the greenish portions of
the granophyre along this part of the junction-line will be found to
have had their structure and composition altered by having incorporated
into their substance a proportion of the bedded basalts through which
they have been disrupted.]
[Footnote 394: The dykes of granophyre in these basalts are referred to
at p. 444.]
Round the eastern group of cones some interesting fragments of the
once continuous sheet of plateau-basalts remain, and show the same
relation of the acid protrusions on that side. One of these lies on
the granophyre of the flanks of Beinn na Caillich, a little to the
west of the loch at the northern base of that hill. Another of larger
size forms a prominent knob about three-quarters of a mile further
west, and is prolonged into the huge dark excrescence of Creagan Dubha,
which rises in such striking contrast to the smooth red declivities
of the granophyre cones around it. This prominence at its eastern
and northern parts consists of highly indurated splintery basalt in
distinct beds, some of which are strongly amygdaloidal. The bedding is
nearly vertical, but with an inclination inwards to the hill. Towards
the south-west end a thin band of basalt-breccia makes its appearance
between two beds of basalt. Its thickness rapidly increases southward
until it is the only rock adhering to the granophyre. Beyond the foot
of the hill, limestone and quartzite occupy for some distance the
bottom of Strath Beg, much invaded by masses of quartz-porphyry. At the
summit of Creagan Dubha abundant veins run into the basic rocks from
the granophyre, which is here finer grained towards the margin; and
there are likewise veins of quartz-porphyry which, though their actual
connection with the main mass of granophyre cannot be seen, are no
doubt apophyses from it.
This outlier of altered basalt and breccia appears to me to be a
fragment of the plateau-basalts which once overlay the Cambrian and
Jurassic rocks of Strath Beg, and were disrupted by the uprise of the
granophyre. It continues to adhere to the wall of the eruptive mass
that broke up and baked its rocks. Its breccia, passing southward into
a coarse agglomerate, may be a product of the same vent or group of
vents that discharged the great agglomerate mass above Kilbride and
Kilchrist. I have already (p. 282) referred to what appears to be
another outlier of the basalts on the south side of Beinn Dearg.
On the northern and southern flanks of Beinn na Cro, similar evidence
may be observed of the posteriority of the granophyre to the basic
rocks. Round the northern base of the hill a continuous tract of
plateau-basalts, dolerites and gabbros forms the ridge between
Strathmore and Strathbeg. There is an admirable section of the relation
of the two groups of rock on the eastern side of the western glen.
Along the lower part of the declivity, coarsely-crystalline gabbros,
like some of those in the Cuillin Hills, are succeeded by sheets of
dolerite and basalt, the whole forming an ascending succession of beds
to the summit of the ridge. The edges of these beds are obliquely
truncated by the body of granophyre, which slants up the hill across
them and sends veins into them. They are further traversed by basalt
dykes, which here, as almost everywhere, abound (Fig. 349). On the
south side of Beinn na Cro, highly indurated black and grey Lias
shales and sandstones have been tilted up steeply and indurated by the
eruptive rock of the hill; and at one place some 800 feet above the
sea, a little patch of altered basalt, lying on the shale, but close
up against the steep declivity of granophyre, forms a conspicuous
prominence on the otherwise featureless slope.
Reference has already been made to the mass of fine-grained
hornblende-granite which runs for several miles at the base of the
volcanic series on the eastern side of the Blaven group of hills. Mr.
Harker has traced a great development of granophyre on the west side of
these hills, where the acid rock sends apophyses both into the bedded
basalts and into the gabbros.
[Illustration: Fig. 349.--Section at north end of Beinn na Cro, Skye.
_a_, basalt, dolerite and gabbro; _b_, granophyre of Beinn na Cro;
_b′_, dyke of granophyre; _c_ _c_, basalt dykes.]
Combining the results of observations made not only in Skye but in
Mull, Rum and Ardnamurchan, I shall here give a fuller account of the
metamorphism of the basalts, to which frequent allusion has been made
as one of the evidences of the posteriority of the eruptive bosses of
rock round which it occurs.[395] The field-geologist observes that the
basalts, as they are traced towards these bosses, lose their usual
external characters. They no longer weather into spheroidal blocks with
a rich brown loam, but project in much jointed crags, and their hard
rugged surface shows when broken a thin white crust, beneath which the
rock appears black or dark bluish-grey, dull and splintery. They are
generally veined with minute threads or strings of calcite, epidote and
quartz, which form a yellowish-brown network that projects above the
rest of the weathered surface. Where they are amygdaloidal, the kernels
no longer decay away or drop out, leaving the empty smooth-surfaced
cells, but remain as if they graduated into the surrounding rock by
an interlacing of their crystalline constituents. They then look at a
distance more like spots of decoloration, and even when seen close at
hand would hardly at first betray their real nature.
[Footnote 395: Many years ago I was much struck with the evidence of
alteration in the igneous rocks of Mull, and referred to it in several
papers, _Proc. Roy. Soc. Edin._ (1866-67) vol. vi. p. 73; _Quart.
Journ. Geol. Soc._ xxvii. (1871) p. 282, note. The subject was more
fully discussed in my memoir in the _Trans. Roy. Soc. Edin._ vol. xxxv.
(1888) p. 167, from which the account in the text is taken. Prof.
Judd has more recently referred the alteration to solfataric action
(_Quart. Journ. Geol. Soc._ xlvi. 1890, p. 341). As already mentioned,
I have been unable to detect evidence of such action. The alteration
is always intimately connected with the presence of intrusive masses,
and it affects indifferently any part of the basalt-plateaux which may
chance to lie next to these masses. The bedded lavas can be traced step
by step from their usual unaltered condition in the plateaux to their
metamorphosed state next to the eruptive rocks. The nature or degree
of the metamorphism has doubtless somewhat varied with the composition
and structure of the rocks affected, and with the character and mass
of the eruptive material; but it is certainly not confined to the
older parts of the plateaux, nor to any supposed pre-basaltic group of
andesites. I have found no evidence that such a group anywhere preceded
the plateau-basalts. The andesites, so far at least as my observations
go, were erupted at intervals during the plateau period, and alternate
with the true basalts. The greatest accumulation of them lies not below
but above the general body of the basalts, in the "pale group" of Mull.
Nor even if the term "propylite" be adopted for these altered rocks,
can it be applied to any special horizon in the volcanic series. The
alteration of the basic rocks by the granophyre of St. Kilda will be
described in the account of that island in Chapter xlvii.]
From the specimens collected by me among the Inner Hebrides up to the
year 1888, I selected two dozen which seemed to be fairly typical of
these altered rocks, and placed thin slices of them for microscopic
examination in Dr. Hatch's hands. His notes may be condensed into the
following summary. One of the most frequent features in the slides is
the tendency in the component minerals to assume granular forms. In
one specimen from Loch Spelve, Mull, the rock, probably originally
a dolerite, shows only a few isolated recognizable crystals of
plagioclase and augite, the whole of the rest of the rock consisting of
roundish granules embedded in a felspathic matrix. The felspar crystals
are sometimes broken up into a mosaic, though retaining their external
contours. Besides the granules, which are no doubt augite, a few grains
of magnetite are scattered through the rock, aggregated here and there
into little groups. In another specimen, taken from the junction with
the granophyre in Glenmore in the same island, parts of the augite
crystals are converted into granular aggregates associated with large
grains and patches of magnetite. The latter mineral also assumes in
some of the rocks granular and even globular shapes suggestive of
fusion.
The felspars, which in most of the basic rocks are usually remarkably
clear and fresh, show marked kaolinization in some of these altered
masses. Minute dusky scales of kaolin are developed, sometimes also
with the separation of minute grains of quartz. The augite shows
frequent alteration to hornblende, proceeding as usual from the
exterior inward. In some cases only an envelope of uralite appears
round the augite, while in others only a kernel of the original mineral
is left, or the whole crystal has been changed. In many cases the
altered substance appears as minute needles, blades and fibres of
actinolite. Occasionally, besides the green hornblende, shred-like
pieces of a strongly pleochroic brown hornblende make their appearance.
Serpentinous and chloritic substances are not infrequent. Epidote is
sometimes abundant. The titaniferous iron has commonly passed more
or less completely into leucoxene. Here and there a dark mica may be
detected.
Since the year 1888 I have continued the investigation of this subject,
and have especially studied the metamorphism of the bedded basalts
on the western shores of Loch Scavaig, where, as already described,
they are truncated by vertical beds of gabbro, and are traversed by
basalt-dykes and by abundant veins of fine-grained granophyre. The
alteration here effected affords excellent materials for study, as the
very same sheets of basalt can be followed from the normal conditions
outside to the altered state within the influence of the metamorphic
agent. The alternations of amygdaloidal and more compact sheets
can still be recognized, although their enclosed amygdales have in
places been almost effaced. They show the dull, indurated, splintery
character, with the white weathered crust, so distinctive of this type
of contact-metamorphism. They are traversed by numerous sills and veins
of gabbro. As has been already suggested, although no large mass of
granophyre appears here at the surface, the alteration of the basalts
is probably to be attributed not so much to the influence of the
gabbro, as to the abundant acid sills, dykes and veins, for there may
be a considerable body of granophyre underneath the locality, the dykes
and veins being indications of its vicinity.
In the summer of 1895 I examined the locality with much care, and
collected some typical specimens illustrative of the conditions of
metamorphism presented by different varieties of the bedded basalts.
Thin slices cut from these specimens were placed in Mr. Harker's hands
for microscopical examination, and he furnished me with the following
notes regarding them.
"In hand-specimens the bedded basalts from the neighbourhood of the
gabbro of Loch Scavaig [6613-6618] do not appear very different from
the normal basalts of this region. The most conspicuous secondary
mineral is yellowish-green epidote in patches, and especially in the
amygdales.
"The texture of the rocks varies, and the slices show that the
micro-structure also varies, the augite occurring sometimes in small
ophitic plates, sometimes in small rounded granules. The chief
secondary change in the body of the rock is shown by the augite, which
is seen in various stages of conversion to greenish fibrous hornblende.
Some round patches seem also to consist mainly of the latter mineral,
and are probably pseudomorphs after olivine. Here the little fibres are
confusedly matted together, without the parallelism proper to uralite
derived from augite. No fresh olivine has been observed. The felspar
and magnetite of the basalts show little or no sign of metamorphic
processes, unless a rather unusual degree of clearness in the felspar
crystals is to be regarded in that light.
"The contents of the metamorphosed amygdales are not always the same.
Epidote is usually present in some abundance, and in well-shaped
crystals. It has a pale citron tint in the slices, with marked
pleochroism; but a given crystal is not always uniform in its optical
characters. Frequently the interior is pale, and has a quite low
birefringence. This is probably to be regarded as an intergrowth of
zoisite in the epidote, and there are a few distinct crystals of
zoisite seen in some places.
"In the slide which best exhibits these features [6613] the crystals
of epidote are in part enwrapped and enclosed by what are doubtless
zeolitic minerals. At least two of these are to be distinguished. One,
very nearly isotropic, and with a pale-brownish tint, is probably
analcime. Associated with this is a colourless mineral with partial
radiate arrangement and with twin lamellation; the birefringence is
somewhat higher than that of quartz, and the γ-axis of optic elasticity
makes a small angle with the twin-line. These characters agree with
those of epistilbite. In other parts of the same large amygdale, the
epidote crystals are embedded in what seems to be a felspar. This
latter mineral is rather obscure, and twin-lamellation is rarely to
be detected; but it seems highly probable that felspar has here been
developed by metamorphic agency at the expense of zeolites which once
occupied the amygdale. I have observed undoubted examples of this in
metamorphosed basalts from other parts of Skye, _e.g._ from Creagan
Dubha, near the granophyre mass of Beinn Dearg.[396] The felspar occurs
there in the same fashion, and in the same relation to epidote [2700,
2701]. In the specimens now described the chief minerals in the
metamorphosed amygdales are those already named: others occur more
sparingly, associated with them. In some cases there is a grass-green,
strongly pleochroic, actinolitic hornblende, accompanied by a little
iron pyrites [6615].
[Footnote 396: Compare _Trans. Roy. Soc. Edin._ vol. xxxv. p. 166.]
"Epidote and various hornblendic and augitic minerals are
characteristic products in the metamorphism of amygdaloidal basalts
in other regions: felspar with this mode of occurrence I have not
seen except in Skye, where it seems to connect itself naturally with
the abundance of zeolites in the amygdales of the non-metamorphosed
lavas. It is to be observed that in these basalts from Loch Scavaig the
alteration is shown especially in the amygdales, the body of the rock
not being greatly affected: this indicates a not very advanced stage
of metamorphism. The production of uralitic hornblende, rather than
brown mica, from the augite and its decomposition-products, seems to be
characteristic of the metamorphism of basaltic as distinguished from
andesitic rocks, and is well illustrated by a comparison of the two
sets of lavas near the Shap granite."[397]
[Footnote 397: _Quart. Journ. Geol. Soc._ vol. xlix. (1893) p. 361.]
Mr. Harker, who is at present engaged in mapping the central region of
Skye, has had occasion to go over a number of the localities (Creagan
Dubha, etc.) originally cited by me, and, while corroborating my
general conclusions regarding them, has been able to obtain much fresh
evidence regarding the nature and extent of the metamorphism which the
bedded basalts have undergone. The results of his investigations will
be published when the Geological Survey of Skye is further advanced.
(3) _Relation of the Granophyre to the Gabbros._--That the granophyres
invade the gabbros has been incidentally illustrated in the foregoing
pages. But as the mutual relations of the two rocks in the island of
Skye have been the subject of frequent reference in previous writings
of geologists, it is desirable to adduce some detailed evidence from a
region which has been regarded as the typical one for this feature in
the geological structure of the Inner Hebrides. No geological boundary
is more easily traced than that between the pale reddish granophyre and
the dark gabbro. It can be followed with the eye up a whole mountain
side, and can be examined so closely that again and again the observer
can walk or climb for some distance with one foot on each rock. That
there should ever have been any doubt about the relations of the two
eruptive masses is possibly explicable by the very facility with which
their junction can be observed. Their contrasts of form and colour make
their boundary over crag and ridge so clear that geologists do not
seem to have taken the trouble to follow it out in detail. And as the
pale rock undoubtedly often underlies the dark, they have assumed this
infraposition to mark its earlier appearance.
I will only cite one part of the junction line, which is easily
accessible, for it lies in Glen Sligachan immediately to the south of
the mouth of Harta Corry. The rounded eminence of Meall Dearg, which
rises to the south of the two Black Lochs, belongs to the granophyre,
while the rugged ground to the west of it lies in the gabbro. The
actual contact between the two rocks can be followed from the side of
Harta Corry over the ridge and down into Strath na Creitheach, whence
it sweeps northward between the red cone of Ruadh Stac and the black
rugged declivities of Garbh Beinn. There is no more singular scene
in Skye than the lonely tract on the south side of Meall Dearg. The
ground for some way is nearly level, and strewn with red shingle from
the decomposing granophyre underneath. It reminds one of some parts of
the desert "Bad lands" of Western America. Grim dark crags of gabbro,
with veins from the granophyre, rise along its western border, beyond
which tower the black precipices of the Cuillins, while the flaming
reddish-yellow cones of Glen Sligachan stand out against the northern
sky.
Having recently described in some detail the relations of the boss
of granophyre at this interesting locality, I will only here offer a
brief summary of the chief features.[398] The granophyre of Meall Dearg
forms a marginal portion of the great mass of the Red Hills. It has
broken across the banded gabbros, and also cuts an isolated boss of
agglomerate in the ridge of Druim an Eidhne. Its line of junction is
nearly vertical, but along part of its course the wall of gabbro rises
higher than that of the more decomposable granophyre. Hence the origin
of the black crags that crown the red slopes of granophyre debris. Seen
from a distance the basic rock seems to rest as a great bed upon the
acid mass.
[Footnote 398: See _Quart. Journ. Geol. Soc._ vol. 1. (1894) p. 212.]
The younger date and intrusive nature of the granophyre are well shown
by the change in the texture of the mass as it approaches the rocks
against which it has cooled. The ordinary granophyric characters
rapidly pass into a fine-grained felsitic texture, and this change
is accompanied with the development of a remarkably well-defined
flow-structure and of rows of spherulites which run parallel to the
boundary wall. In a ravine on the west side of Meall Dearg, the lines
of flow-structure and rows of large spherulites are seen to be arranged
vertically against the face of gabbro.
Further proof of the later date of the protrusion of the granophyre
is supplied by abundant felsitic dykes and veins which traverse the
gabbro, and some of which can be seen to proceed from the main body of
granophyre. These intrusions will be described in the next chapter, in
connection with the dykes and veins of the acid rocks.
Additional evidence as to the posteriority of the granophyre to the
gabbro has recently been obtained by Mr. Harker from a study of the
internal structure and composition of the masses of these rocks which
have been intruded into the agglomerate above Loch Kilchrist in
Strath. He has found that the granophyre has there caught up from some
subterranean depth portions of gabbro, and has partially dissolved
them, thereby undergoing a modification of its own composition. "The
gabbro-debris," he remarks, "has been for the most part completely
disintegrated by the caustic or solvent action of the acid magma on
some of its minerals. Those constituents which resisted such action
have been set free and now figure as xenocrysts [foreign crystals],
either intact or more or less perfectly transformed into other
substances. At the same time the material absorbed has modified the
composition of the magma, in the general sense of rendering it less
acid." Mr. Harker has traced the fate of each of the minerals of the
gabbro in the process of solution and isolation in the acid magma,
which, where this process has been most developed, is believed by him
to have taken up foreign material amounting to fully one-fourth of its
own bulk, derived not from the rocks immediately around, but from a
gabbro probably at a considerable depth beneath.[399]
[Footnote 399: _Quart. Journ. Geol. Soc._ vol. lii. (1896) p. 320.]
[Illustration:
Fig. 350.--Ground-plan of basic dyke in Cambrian Limestones
truncated by granophyre which encloses large blocks of the dyke,
Torrin, Skye.
]
(4) _Relation of the Granophyre to the Basic Dykes and
Veins._--Reference has already been made to the fact that the "syenite"
bosses of Skye cut off most of the basalt-dykes, but are themselves
traversed by a few others.[400] The locality that furnished me with
the evidence on which this statement was originally made nearly
forty years ago affords in small compass a clearer presentation of
the facts than I have elsewhere met with. The sections described by
me are visible at the eastern end of the boss of Beinn an Dubhaich,
Strath; but similar and even better examples may be cited from the
whole northern and southern margins of that eruptive mass. On the
north side an extraordinary number of dykes may be traced in the
Cambrian limestone from the shores of Loch Slapin eastwards. They
have a general north-westerly trend, but one after another, as I have
already remarked, is abruptly cut off by the granophyre. As an example
of the way in which this truncation takes place, I may site a single
illustration from the northern margin of the eruptive mass, near
Torrin. It might perhaps be contended that the numerous dykes which
traverse the limestone and stop short at the edge of the acid rock,
are not necessarily older than the granophyre, but may actually be
younger, their sudden termination at the edge of the acid boss being
due to their inability to traverse that rock. That this explanation
is untenable is readily proved by such sections as that given in Fig.
350, where a basic dyke (_b_) 9 or 10 feet broad running through the
Cambrian Limestone (_a_ _a_) is abruptly cut off by the edge of the
great granophyre boss. Not only is the dyke sharply truncated, but
numerous pieces of it, from an inch to more than a foot in length,
are enclosed in the granophyre. The latter is well exposed along the
shore of Loch Slapin in an almost continuous section of nearly a mile
in length. The contrast therefore between the development of dykes
within and beyond its area cannot but arrest the attention of the
observer. Though I was on the outlook for dykes in the granophyre, I
found only one. Yet immediately beyond the eruptive boss they at once
appear on either side up to its very edge, where they suddenly cease.
The conclusion cannot be resisted that the protrusion of the acid rock
took place after most of the dykes of the district had been formed, but
before the emission of the very latest dykes, which pursue a north-west
course across the boss (Fig. 348).
[Footnote 400: _Ante_, p. 173, and _Quart. Journ. Geol. Soc._ vol. xiv.
(1857) p. 16.]
Some sections on the southern margin of Beinn an Dubhaich complete
the demonstration that such has been the order of appearance of the
rocks. Near the head of the Allt Lèth Slighe (or Half-way Burn), where
the granite has pushed a long tongue into the limestone, a north-west
basalt-dyke is abruptly cut off by the main body of the boss and by the
protruded vein (Fig. 351). Besides this truncation, the acid rock sends
out strings and threads of its own substance into and across the dyke,
these injected portions being as usual of an exceedingly fine felsitic
texture.
[Illustration: Fig. 351.--Section on south side of Beinn an Dubhaich,
Skye, showing the truncation of a basalt-dyke (_b_), in Cambrian
Limestone (_a_), by the granite (_c_) of Beinn an Dubhaich, Skye.]
Similar evidence may be gathered from the area of the great granophyre
cones further north. The profusion of basalt-dykes in the surrounding
rocks stops short at the margin of that area. The comparatively few
dykes which cross the boundary pursue a general north-west course
through the granophyre, and, as already remarked, from their dark
colour, greater durability and straightness of direction, stand out as
prominent ribs on the flanks of the pale cones which they traverse.