THE ACID SILLS, DYKES AND VEINS
i. THE SILLS
Not only have the acid rocks been protruded in small and large bosses,
they have also been injected as sills between the bedding-planes of
stratified rocks, between the surfaces of the basalt-beds, and between
the bottom of the plateau-basalts or of the gabbros and the platform
of older rock on which the volcanic series has been piled up. Every
gradation of size may be observed, from mere partings not more than
an inch or two in thickness, up to massive sheets, which now, owing
to the removal of their original covering of rock by denudation, form
minor groups and ranges of hills. Where the sheets are numerous, they
are usually small in size; where, on the other hand, they are few in
number, they reach their greatest dimensions.
It is not always possible to discriminate between bosses and large
irregular sills. A good illustration of the connection between these
two forms of intrusion will be cited from the island of Raasay, where a
widespread intrusive sheet is in part connected with a true boss.
In Mull, sills of acid eruptive rocks are profusely abundant throughout
the central mountainous tract between Loch na Keal and Loch Spelve. If
we ascend the slopes from the Sound of Mull, for instance, we have not
gone far before some of these sheets make their appearance. They are
usually dull granular quartz-porphyries, or granophyres, often only two
or three feet in thickness, and interposed between the beds of basalt
that form the mass of the hills. Along the crest of the ridge that
stretches through Beinn Chreagach Mhor to Mainnir nam Fiadh they take
a prominent place among the ledges of basalt, basalt-conglomerate and
dolerite. The largest sheet in Mull is probably that which has thrust
itself between the base of the basalts and the underlying Jurassic
strata and crystalline-schists on the shore of the Sound of Mull at
Craignure. The porphyry of this sheet is referred to by Professor
Zirkel as only a finer-grained variety of the same quartziferous rock,
with hornblende and orthoclase crystals, which in Skye breaks through
the Lias.[430] On the south coast also, at the base of the thick basalt
series, similar porphyries have been injected into the underlying
strata; and under the great gabbro mass of Ben Buy similar protrusions
occur. But as we retire from the mountainous tract into the undisturbed
basalts of the plateau, these acid intercalations gradually disappear.
[Footnote 430: _Zeitsch. Deutsch. Geol. Gesellsch._ xxiii. p. 54.]
In the islands of Eigg and Rum, excellent examples occur of the
tendency which the sheets of porphyry or granophyre manifest to appear
at or about the base of the bedded basalts. I have already alluded to
the boss or sheet at the north end of the former island. A still more
striking illustration occurs in Rum. All along the base of the great
mass of gabbro, protrusions of various kinds of acid rock have taken
place. The great mass of Orval, already described, is one of these.
Below Barkeval and round the foot of the hills to the south-east of
that eminence an interrupted band of quartz-porphyry may be traced,
from which veins proceed into the gabbros and dolerites.
But it is in Skye and Raasay that the intrusive sheets of the acid
group of rocks reach their chief development. They have been most
abundantly injected underneath the bedded basalts, particularly among
the Jurassic strata. A band or belt of them, though not continuous,
can be traced round the east side of the main body of granophyre, at
a distance of from a mile and a half to about three miles. Beginning
near the point of Suisnish, this belt curves through the hilly ground
for some five miles, until it dies out on the slopes above Skulamus.
It may be found again on the west side of the ridge of Beinn Suardal,
and on the moors above Corry, till it reaches the shore at the Rudh'
an Eireannich (Irishman's Point). It skirts the west side of Scalpa
Island, and runs for some miles through Raasay. Another series of sills
occurs below the basalts and gabbros in the Blaven group of hills.
Over a large part of their course, the rocks of the eastern belt rest
in great overlying sheets upon the Jurassic strata, which may almost
everywhere be seen dipping under them. From the analogy of other
districts, we may, I think, infer that the position of these sills here
points to their having been intruded at the base of the plateau-basalts
which have since been removed from almost the whole tract. Fortunately,
a portion of the basalts remains in Raasay, and enables us to connect
that island with the great plateau of Skye of which it once formed a
part. There can be no doubt that the basalts of the Dùn Caan ridge once
extended westwards across the tract of granophyre which now forms most
of the surface between that ridge and the Sound of Raasay. A thin sheet
of quartz-porphyry, interposed among the Oolitic strata, may be seen
a little inland from the top of the great eastern cliff and below the
position of the bedded basalts.
The great sheet, or rather series of sheets, which stretches
north-eastwards from Suisnish at the mouth of Loch Eishort in
Skye, consists of a rock which for the most part may readily be
distinguished in the field from the granitoid material of the
bosses. It appears to the naked eye to be a rather close-grained or
finely crystalline-granular quartz-porphyry, with scattered blebs
or bi-pyramidal crystals of quartz and crystals of orthoclase. At
the contact with adjacent rocks, the texture becomes more felsitic,
sometimes distinctly spherulitic (west side of Carn Nathragh, next
Lias shale). Under the microscope the rock is seen to be a fine-grained
granophyric porphyry or porphyritic granophyre. It caps Carn Dearg (636
feet) above Suisnish, where it covers a space of nearly a square mile,
and reaches at its eastern extremity (Beinn Bhuidhe), a height of 908
feet above the sea (Fig. 249). This rock rests upon a sill of dolerite,
and is apparently split up by it. But, as I have already stated, the
basic rock is probably the older of the two, and the granophyre seems
to have wedged itself between two earlier doleritic sheets. To the
north-west of Carn Dearg, above the northern end of the crofts of
Suisnish, the same sill, or one occupying a similar position, crops out
between masses of granophyre, and is intersected by narrow veins from
that rock.
Though severed by denudation, the large sheets of granophyre to the
east of Beinn Bhuidhe are no doubt continuations of the Carn Dearg
mass, or at least occupy a similar position. That they are completely
unconformable to the Jurassic strata is shown by the fact, that while
at Suisnish they lie on sandstones which must be fully 1000 feet above
the bottom of the Lias, only two miles to the east they are found
resting on the very basement limestones, within a few yards from the
underlying quartzite and Torridon sandstone. I do not think that this
transgression can be accounted for by intrusion obliquely across the
stratification. I regard it as arising from the eruptive rock having
forced its way between the bottom of the now vanished basalt-plateau
and the denuded surface of Jurassic rocks, over which the basalts were
poured. The platform underneath these granophyre sills thus represents,
in my opinion, the terrestrial surface before the beginning of the
volcanic period.
But there is abundant proof that though the intruded granophyre sills
followed generally this plane of separation, they did not rigidly
adhere to it, but burrowed, as it were, along lower horizons. Thus
on the south-east front of Beinn a' Chàirn, which forms so fine an
escarpment above the valley of Heast, the base of the granophyre,
after creeping upward across successive beds of limestone, sends out
a narrow tongue into these strata, and continues its course a little
higher up in the Lias. The same rock, after spreading out into the
broad flat tableland of Beinn a' Chàirn (983 feet), rapidly contracts
north-eastwards into a narrow strip which forms the crest of the ridge,
and at once suggests a much-weathered lava-stream. The resemblance
to a _coulée_ is heightened by the curious thinning off of the rocks
where the two streams emerge from the Heast lochs; it looks as if the
igneous mass were a mere superficial ridge which had been cut down by
erosion, so as to expose the shales beneath it. But that the granophyre
is really a sill becomes abundantly clear at its eastern end, where we
find that it consists of two separate sheets with intervening Liassic
shales. The structure of this interesting locality is shown in Fig.
372. In this instance also, there is evidence that the acid sills are
younger than the basic, for the upper sheet of granophyre sends up
into the overlying dark basaltic rock narrow vertical felsitic veins,
a quarter of an inch to an inch in width, which being more durable,
stand out above the decomposable surface of the containing rock, and
show their quartz-blebs and felspar crystals on the weathered surface.
Perhaps the most striking feature of the granophyre sills of Skye is
their general association with thinner basic intrusive sheets between
which they have insinuated themselves. This characteristic structure,
pointed out by me in 1888, has recently been more minutely mapped
in the progress of the Geological Survey. Mr. Harker has found the
typical arrangement to be the occurrence of a thick sill of granophyre
interposed between two sills of basalt, each of which is usually not
more than six or eight feet thick. Where the granophyre has been
intruded independently among the Lias formations, it does not assume
the regularity and persistence which mark it where it has followed the
course of basic sills.
[Illustration: Fig. 372.--Section across the Granophyre Sills at Loch
a' Mhullaich, above Skulamus, Skye.
_a_, Jurassic sandstones and shales; _b_, Jurassic dark brown
sandy shales; _c_, sills of basalt, some bands highly cellular;
_c´_, basalt-sill with veins of felsite rising into it from the
granophyre below; _d_ _d_, intrusive sheets or sills of granophyre.
]
"The acid rock," Mr. Harker observes, "is invariably the later
intrusion, for it sends narrow veins into the basalts, metamorphosing
them to some extent and frequently enclosing fragments of them. These
fragments are always rounded by corrosion, and show various stages
of dissolution down to mere darker patches as seen by the naked
eye. Such inclusions and patches are found in the marginal part of
a granophyre, where no continuous basalt occurs, but where the acid
magma has evidently in places completely destroyed the earlier basic
sheets between which it was forced. It seems probable that in all cases
a certain amount of solution of the basalt by the granophyre magma
took place at their contact, facilitating the injection of the later
intrusion and accounting for its persistent choice of the contact-plane
of two basalt-sills as the surface offering least resistance to its
injection."
These observations throw fresh light on the remarkable original
regularity and persistence of the basic sills. Where one of these
sills disappears above or below a granophyre sheet its probable former
presence is often indicated by corroded fragments of the basic in the
acid rock. Mr. Harker remarks that the acid magma seems to have been
"in itself less adapted than the basic to follow accurately a definite
horizon and to maintain a uniform thickness in its intruded sheets, but
could do both when guided by a pre-existing basalt-sill, or especially
when insinuated between contiguous basalt-sills." The corrosive action
of the acid magma on the surface of the basalt, which enabled it to
force its way more readily between the basic sills, might proceed so
far as partially or wholly to destroy these sills.
This solvent action may serve to explain some of the irregularities
of the granophyre intrusions. According to the same observer, such
irregularities are found "where the granophyre sheet and its encasing
basalt-sills are not co-extensive, or again where the two basalt-sills
separate, owing to one of them cutting obliquely across the bedding.
In the latter case, which is not common, the granophyre follows one of
the basalt-sills, necessarily parting from the other. When one of the
two guiding basalt-sills dies out, the granophyre may still continue,
following the sill which persists. If the latter also dies out, while
the granophyre is still in some force, the acid magma seems to have
been reluctant to travel beyond the limit of the basalt, but has drawn
towards it, and the granophyre presents a blunt laccolitic form, which
contrasts with the acutely tapering edge of a granophyre which dies out
before reaching the limit of its basalt-sills. If, on the other hand,
on reaching the limit of the basalt, the acid magma has been in such
force as to be driven further, it is usually found to lose something
of its regularity and to depart from the exact horizon which it has
hitherto followed. This seems to happen, for instance, in the Beinn
a' Chàirn sheet, which, when traced westward, is found to behave as a
'boss' and is obviously transgressive, having cut across the bedding of
the strata so as to enter the limestones, where it no longer behaves
in any degree as a sill. The district affords many examples of the
tendency of intrusive masses in general to cut sharply across the beds
when they enter a group of limestones."
More complex examples of acid sills are to be found where there have
been three or more basic sheets together. The great granophyre sheet
already referred to at Suisnish affords the best illustration of this
structure. Mr. Harker has noticed that "round most of its circumference
there is seen merely a single basalt-sill passing under the granophyre.
Probably there has been another similar sheet over the acid rock, but
if so, it has been removed by erosion, the granophyre itself forming
everywhere the surface of the plateau. On the southern side, however,
we see that the original basalt must have been at least triple, or
counting the uppermost member, now removed, quadruple. The granophyre
has forced its way in between the several members of the multiple
basalt-sill, the intermediate ones being thus completely enveloped.
They are evidently metamorphosed as well as veined by the granophyre,
and when traced onward they give place to detached portions which,
floating as it were in the acid rock, are soon lost."
It is seldom easy to determine where lay the vent or vents from which
the granophyre sills proceeded. Those of the Skye platform just
described may be chiefly concealed under some of the larger areas of
the rock, such as the sheets of Carn Dearg or Beinn a' Chàirn. But
in several places, in close association with the compound sills of
granophyre and basalt, Mr. Harker has found large dyke-like bodies of
the acid rock, which may with considerable probability be regarded
as marking the position of the channels by which the material of the
sills ascended. "These bodies," he remarks, "either occur isolated by
erosion, the sills or the parts of the sills presumed to have been
in connection with the dykes having been removed, or are only very
partially exhibited in direct connection with sills still remaining.
Where they can be examined in detail they are seen to be dykes varying
up to about 100 feet in width, but of no great longitudinal extent.
Between Suisnish and Cnoc Carnach they bear E.N.E., that is, at right
angles to the ordinary basic dykes of the district and parallel to the
general direction of the axes of folding, though further north they
change this trend, but still remain parallel to the strike of the Lias.
"These dykes are composed essentially of granophyre, identical with
that of the sills. In some cases, they are flanked with basalt-dykes
on one or both sides, or the former existence of such lateral dykes
is indicated by partly-destroyed inclusions of the basic rock in the
granophyre. The basalt found in these cases is identical with that
of the basic sills, and shows the same relation to the granophyre.
Discontinuity and failure of the basalt are commoner, however, in the
dykes than in the sills--a difference presumably attributable to more
energetic destructive action of the acid magma when it was hotter and
fresher. These supposed feeders of the granophyre sills are certainly
in some cases, and have possibly been in all, double or triple dykes.
The acid magma thus appears not only to have spread laterally along the
same platforms as the earlier basalts, but to have reached these levels
by rising through the same fissures which had already given passage to
the basic magma."[431]
[Footnote 431: MS. notes supplied by Mr. Harker.]
The granophyre sills which, as already stated, can be followed as an
interrupted band from Suisnish Point to the Sound of Scalpa, emerge
again beyond Loch Sligachan and also in the island of Raasay, where
a great sheet of the acid rock covers an area of about five square
miles. This tract has recently been mapped for the Geological Survey by
Mr. H. B. Woodward, who has found it to have been intruded across the
Jurassic series, a large part of its mass coming in irregularly about
the top of the thick white sandstones of the Inferior Oolite. But it
descends beneath the Secondary rocks altogether, and in some places
intervenes between the base of the Infra-liassic conglomerates and the
Torridon sandstone. Its irregular course transgressively across the
Mesozoic formations is probably to be regarded as another example of
the intrusion of the acid material preferentially along the line of
unconformability between the older rocks and the Tertiary basalts, now
nearly all removed from Raasay by denudation, though the intrusion does
not rigidly follow that line of division, but sometimes descends below
it.
The central portions of this Raasay granophyre possess the ordinary
structures of the corresponding rocks in Skye. They show a finely
crystalline-granular, micropegmatitic base, through which large
felspars and quartzes are dispersed. But at the upper and under
junction with the sedimentary rocks, beautiful spherulitic structures
are developed. This is well seen on the shore near the Point of
Suisnish (Raasay), where, below the Lias Limestones, the top of the
granophyre appears, and where its bottom is seen to lie on the Torridon
sandstone.
This granophyre sheet presents a further point of interest inasmuch as
it appears to have preserved one of the dyke-like masses which may mark
channels of escape from the general body of the acid magma below. Near
the Manse the section represented in Fig. 373 may be observed. Owing
to great denudation, the massive sheet of granophyre has been cut into
isolated outliers which cap the low hills, and the rock may be seen
descending through the Jurassic sandstones, which in places are much
indurated. It is observable that the amount of contact-metamorphism
induced by the granophyre sills upon the rocks between which they have
been injected is, in general, comparatively trifling. It is for the
most part a mere induration, sometimes accompanied with distortion and
fracture.
[Illustration: Fig. 373.--Section to show the connection of a sill of
Granophyre with its probable funnel of supply, Raasay.
_a_ _a_, Jurassic sandstones; _b_, granophyre.]
[Illustration: Fig. 374.--Granophyre sill resting on Lower Lias shales
with a dyke of basalt passing laterally into a sill, Suisnish Point,
Isle of Raasay.]
Although the intrusion of the granophyre sills has been subsequent to
that of the basalt-sheets with which they are so generally associated,
we may expect that as there is a series of post-granophyre basic dykes,
so there may be some basic sills later than the injections of the acid
sheets. The Raasay granophyre appears to furnish an example of such a
later basic intrusion. At the Point of Suisnish on that island I have
observed the relations shown in Fig. 374. There the dark shales of the
Lower Lias (_a_ _a_) are immediately overlain by the granophyre sill
(_b_), and are cut by a basalt-dyke which, when it rises to the base
of the granophyre, turns abruptly to one side, and then pursues its
course as a sill (_c_) between the granophyre and the shales. There can
be little doubt that this intrusion is later than the granophyre. Here
a basic sill is interposed at the bottom of the acid sheet; and is
visibly connected with the actual fissure up which its molten material
was impelled.
ii. THE ACID DYKES AND VEINS
Besides bosses and sills, the acid rocks of the Inner Hebrides take the
form of Dykes and Veins which have invaded the other members of the
volcanic series. Some of these have already been referred to; but a
more particular description of the venous development of the acid rocks
as a whole is now required.
As regards their occurrence and distribution, they present two phases,
which, however, cannot always be distinguished from each other. On the
one hand, they are found abundantly either directly proceeding from the
bosses (more rarely from the sills), or in such immediate proximity
and close relationship to these as to indicate that they must be
regarded as apophyses from the larger bodies of eruptive material. On
the other hand, they present themselves as solitary individuals, or in
groups at a distance of sometimes several miles from any visible boss
of granophyre. In such cases, it is of course obvious that though not
exposed at the surface, there may be a large mass of the acid magma at
no great distance beneath, and that these isolated dykes and veins do
not essentially differ in origin from those of which the relations to
eruptive bosses can be satisfactorily observed or inferred.
Considered as a petrographical group, these Dykes and Veins are marked
by the following characters. At the one extreme, we have thoroughly
vitreous rocks in the pitchstones. From these, through various degrees
of devitrification, we are led to completely lithoid felsites,
quartz-porphyries or rhyolites. Micropegmatitic structure is commonly
present, and as it increases in development, the rocks assume the
ordinary characters of granophyre. Occasionally the structure becomes
microgranitic in the immediate periphery of a boss wherein a granitic
character has been assumed. Viewed as a whole, however, it may be said
that the dull lithoid rocks of the dykes and veins can generally be
resolved under the microscope into some variety of granophyric porphyry
or granophyre.
A characteristic feature in the granophyric, felsitic or rhyolitic
dykes and veins is the presence of spherulitic structure (Figs. 375,
377). In some cases this structure is hardly traceable save with
the aid of the microscope, but from these minute proportions it may
be followed up to such a strong development that the individual
spherulites may be an inch or two in diameter, and lie crowded
together, like the round pebbles of a conglomerate. The structure is
a contact phenomenon, being specially marked along the margin of the
dykes, as it is on the edge of sills and bosses. In the Strath district
of Skye, Mr. Clough and Mr. Harker have observed that the spherulites
are apt to be grouped in parallel lines so as to form rod-like
aggregates along the walls, and that where the rock is fairly fresh the
centre of the dyke sometimes consists of glassy pitchstone, so that the
spherulitic felsite or granophyre is probably devitrified pitchstone.
Frequently flow-structure is admirably developed in these dykes, the
streaky layers of devitrification flowing round the spherulites and any
enclosed fragments as perfectly as in any rhyolitic lava (Fig. 378).
[Illustration: Fig. 375.--Weathered surface of spherulitic granophyre
from dyke in banded gabbros, Druim an Eidhne, Meall Dearg, Glen
Sligachan, Skye. Natural size.]
In regard to their modes of occurrence, the dykes of acid material
differ in some important respects from those of basic composition. More
especially they are apt to assume the irregular venous form, rather
than the vertical wall-like character of ordinary dykes. They take
the form of dykes, particularly where their material has been guided
in its uprise by one or more already existent basic or intermediate
dykes, as in the compound dykes, already described. The conditions for
their production must thus have been essentially different from those
of the great body of the basic dykes. Their intrusion was not marked
by any general and widespread fissuring of the earth's crust, such as
prepared rents for the reception of the basalt and andesite dykes.
They were rather accompaniments of the protrusion of large masses of
acid magma into the terrestrial crust. This magma, as we have seen,
was often markedly liquid, and was impelled, sometimes with what
might be called explosive violence, into the irregular cracks of the
shattered surrounding rocks or into pre-existing dyke-fissures. Hence
long straight dykes of the acid rocks are much less common than short
irregular tortuous veins and strings.
[Illustration:
Fig. 376.--Plan of portion of the ridge north of Druim an Eidhne,
Glen Sligachan, Skye, showing three dykes issuing from a mass of
granophyre.
_a_, gabbros; _b_, granophyre; I. II. III., three dykes proceeding
from the granophyre. The arrows show the direction of dip of the
bands of gabbro.
]
Much difference may be noticed among the granophyre bosses in regard to
their giving off a fringe of apophyses. Thus, along the well-exposed
boundary of Beinn-an-Dubhaich in Skye, though the edge of the boss is
remarkably notched, hardly any veins deserving the name diverge from
it. On the other hand, the ridge of Meall Dearg at the head of Glen
Sligachan, already referred to, is distinguished by the number and
variety of the dykes and veins which proceed from the granophyre and
traverse the banded gabbros. As this locality has been elsewhere fully
described, I will give here only the leading structural features which
it presents.[432]
[Footnote 432: Professor Judd (_Quart. Journ. Geol. Soc._ vol. xlix.
(1893), p. 175) described the granophyre dykes of this locality as
inclusions of Tertiary granite in the gabbro, and cited them in proof
of his contention that the acid eruptions of the Western Isles are
older than the basic. Their true character was shown by me in a paper
published in the _Quart. Journ. Geol. Soc._ vol. 1. (1894), p. 212.]
[Illustration: Fig. 377.--Weathered surface of spherulitic granophyre
from dyke in banded gabbros, Druim an Eidhne, Meall Dearg, Glen
Sligachan, Skye. Natural size.]
Within a horizontal distance of less than 100 yards three well-marked
dykes issue from the spherulitic edge of the Meall Dearg granophyre,
and run in a south-easterly direction in the handed gabbros (Fig. 376).
The most northerly of these is traceable in a nearly straight line for
800 feet. The central dyke, which can be followed for 200 feet or more,
rises as a band six to ten feet broad between the dark walls of gabbro
as represented in Fig. 379.
These dykes are marked by the most perfectly developed spherulitic
and flow-structures (Figs. 375, 377). Numerous detached portions of
other dykes and also irregular veins are to be observed cutting the
banded gabbros all over the ridge of Druim an Eidhne for a distance of
a mile or more. Many of these exhibit the same exquisitely beautiful
spherulitic and flow-structure displayed by the dykes which can
actually be traced into the main body of granophyre. The lines of
flow conform to every sinuosity in the boundary-walls of gabbro, and
sometimes sweep round and enclose blocks of that rock. The example
of this structure, given in Fig. 378, shows how these lines, curving
round projections and bending into eddy-like swirls, exhibit the
motion of a viscous lava flowing in a cleft between two walls of solid
rock. Sometimes the laminæ of flow have been disrupted, and broken
portions of them have been carried onward and enveloped in the yet
unconsolidated material. Certain portions of this dyke are richly
spherulitic, the spherulites varying from the size of small peas up to
that of tennis-balls. Occasionally two large spherulites have coalesced
into an 8-shaped concretion, and it may be observed in some cases that
the spherulites are hollow shells.
[Illustration: Fig. 378.--Plan of pale granophyric dyke, with
spherulitic and flow-structure, cutting and enclosing dark gabbro,
Druim an Eidhne.]
[Illustration: Fig. 379.--Dyke (six to ten feet broad) proceeding
from a large body of granophyre and traversing gabbro, from the same
locality as Figs. 375 and 377.]
A remarkable feature has been recently observed by Mr. Harker among the
abundant granophyre dykes and veins which intersect the gabbros and
older rocks, along the eastern flanks of the Red Hills of Skye between
Broadford and the Sound of Scalpa. Broad dykes of granophyre which
traverse the Cambrian limestone of that district might be supposed
at first sight to be cut off by the intrusions of gabbro. But closer
examination proves that their apparent truncation arises from their
suddenly breaking up into a network of small veins where they abut
against the basic rock. This structure evidently belongs to the same
type as that of the St. Kilda granophyre.
[Illustration: Fig. 380.--Section of intruded veins of various acid
rocks above River Clachaig, Mull.
_a_ _a_, basalt, dolerite, etc.; _b_ _b_, granophyre.]
Compound dykes and sills, where one or more of the injections has
consisted of acid material, have been already noticed as intimately
associated together in Skye (p. 162). Dykes of this nature are more
particularly abundant in Strath, especially along its eastern side. In
addition to the examples cited already from that district, I may refer
to other two which intersect the Middle Lias shales and limestones
in the island of Scalpa. They are both compound dykes, but the more
basic marginal bands are not always continuous, having possibly been
here and there dissolved by the acid invasion. Though they do not show
any distinct spherulitic forms, the presence of flow-structure is
indicated by the thin slabs into which the rocks weather parallel to
the dyke-walls. The rock in each case is a fine-grained felsitic mass,
with bi-pyramidal crystals of quartz. It is observable that where these
dykes come directly against the Liassic strata, the latter are more
seriously indurated than where they are traversed by the ordinary basic
dykes.
In the central mountainous tract of the island of Mull veins of
acid material are extraordinarily abundant. They probably proceed
from a much larger subterranean body of granophyre than any of the
comparatively small bosses of this rock which appear at the present
surface of the ground. They show themselves partly at the margins of
the visible bosses, but much more profusely in that tract of altered
basalt, with intrusive sheets and dykes of basalt, dolerite and gabbro,
which lies within the great ring of heights between Loch na Keal and
Loch Spelve. In some areas, the amount of injected material appears to
equal the mass of more basic rock into which it has been thrust. Pale
grey and yellowish porphyries and granophyres, varying from thick dykes
down to the merest threads, ramify in an intricate network through the
dark rocks of the hills, as shown in the accompanying illustration
(Fig. 380), which represents a portion of the hillside between Beinn
Fhada and the Clachaig River. Such a profusion of veins probably
indicates the existence here of some large mass of granophyre or
granite, at no great depth beneath the surface.
In Mull, as in the other islands of the Inner Hebrides, two horizons
on which protrusions of acid materials have been specially abundant,
are the base of the bedded basalts of the plateau and the bottom of the
thick sheets of gabbro. Dykes and veins of granophyre, quartz-porphyry,
felsite and other allied rocks are sometimes crowded together along
these two horizons, though they may be infrequent above or below them.
Illustrations of solitary veins in the midst of unaltered
plateau-basalts or in older rocks may be gathered from many parts of
the Western Isles. Some remarkable instances are to be seen among
the basalts that form the terraced slopes on the north side of Loch
Sligachan. Several thick dykes of granophyre run up the declivity,
cutting across hundreds of feet of the nearly level basalt-beds. Some
of them can be seen on the shore passing under the sea. They trend in a
S.S.E. direction towards Glamaig, and they are not improbably apophyses
from that huge boss, the nearest edge of which is three-quarters of
a mile distant. Another example may be cited from the basalt-outlier
of Strathaird, where two veins of felsite, one of them a pale flinty
rock showing flow-structure parallel to the walls, may be seen on the
west front of Ben Meabost. In this case, the veins are three miles and
a half from the granophyre mass of Strath na Creitheach to the north,
four miles from that of Beinn an Dubhaich to the north-east, and nearly
three miles from that of Coire Uaigneich at the foot of Blath Bheinn.
A special place must be reserved for the pitchstone-veins. Ever
since the early explorations of Jameson and Macculloch, the West of
Scotland has been noted as one of the chief European districts for
these vitreous rocks. From Skye to Arran, and thence to Antrim, many
localities have furnished examples of them, but always within the
limits of the Tertiary volcanic region. That all of the pitchstones
are of Tertiary age cannot, of course, be proved, for some of them are
found traversing only Palæozoic rocks, and of these all that can be
absolutely affirmed is that they must be younger than the Carboniferous
or even the Permian system. But, as most of them are unquestionably
parts of the Tertiary volcanic series, they are probably all referable
to that series. Not only so, but there is, I think, good reason to
place them among its very youngest members. It is a significant fact
that they almost always occur either in or close to granophyre or
granite bosses, the comparatively late origin of which has now been
proved.
[Illustration:
Fig. 381.--Pitchstone vein traversing the bedded basalts, Rudh an
Tangairt, Eigg.
]
The first pitchstone observed in Skye was found by Jameson on the
flanks of the great granophyre cone of Glamaig. Another rises on the
side of the porphyry mass of Glas Bheinn Bheag, in Strath Beg. Several
occur at the foot of Beinn na Callich. In Rum, I found a pitchstone
vein traversing the western slopes of the wide granophyre boss of
Orval. In Eigg, the well-known veins of this rock intersect the
plateau-basalts (Fig. 381), but they are accompanied, even within the
same fissure, with granophyre, and in their near neighbourhood lie the
masses of this rock already alluded to.[433] In Antrim, pitchstone and
obsidian occur in the midst of the rhyolite. The only marked exceptions
to the general rule, with which I am acquainted, are those of the
island of Arran. Most of the pitchstone-veins in that district traverse
the red sandstones which may be Permian. But none of them are far
removed from the great granite boss of the northern half of the island,
while large masses of quartz-porphyry, which strikingly resemble some
of those of Skye and Mull, lie still nearer to them. It is also worthy
of notice that pitchstone-veins rise through the Arran granite boss
itself, the probably Tertiary date of which has been already discussed.
[Footnote 433: For an account of the pitchstone veins of Eigg, see
_Quart. Journ. Geol. Soc._ xxvii. p. 299.]
This common association of pitchstone-veins with the Tertiary eruptive
bosses of acid rocks can hardly be a mere accidental coincidence.
It seems to prove a renewed extravasation of acid material, now in
vitreous form, from the same vents that had supplied the granitoid,
granophyric, porphyritic and felsitic varieties of earlier protrusions.
We must remember that the pitchstone-veins are not mere local glassy
parts of the larger bodies of granophyre or granite in which they
lie. Their margins are sharply defined; they are indeed in all
respects as manifestly intruded, and therefore later masses, as are
the basalt-dykes. Their occurrence, therefore, within the acid bosses
proves them to be younger than these members of the Tertiary volcanic
series. Whether they are also later than the latest basalt-dykes cannot
yet be decided, for I have never succeeded in finding an example of the
intersection of these two groups of veins and dykes. But, with this
possible exception, the pitchstones are the most recent of all the
eruptive rocks of Britain.
As a rule, the intrusive pitchstones occur as veins which cannot be
traced far, and which vary from a few yards to less than an inch in
width. They generally show considerable irregularity in breadth and
direction, sometimes sending out strings into the surrounding rock
(Fig. 381). The outer portions are not infrequently more glassy and
obsidian-like than the interior. Occasionally the vitreous character
disappears by devitrification, and the rock assumes the texture of a
compact felsite or of a spherulitic rock.
Among the later movements of the acid magma account must be taken here
of the pale fine-grained veins which have already been referred to as
traversing the granophyre bosses. These intrusions, so well seen in
the bosses of Skye and St. Kilda, are often so close in texture that
they may be called quartz-felsites. Their sharply-defined edges and
felsitic character suffice to separate them from what are termed "veins
of segregation." In at least one instance, that of Meall Dearg, already
cited, a mass of typical granophyre which has developed spherulitic
and flow-structures along its margin, and which sends out dykes having
the very same structures for a distance of several hundred feet across
the banded gabbros, is itself traversed by a dyke of precisely similar
character. Here we see that after the intrusion of its apophyses,
and after its own consolidation in the upper parts, the granophyric
magma that rose into rents in the solidified portion retained the same
tendency to produce large spherulites as it had shown at first.
The fine felsitic veins that traverse the granophyre of the Red
Hills are now being mapped by Mr. Harker during the progress of the
Geological Survey. He has not yet obtained evidence of the age of these
veins in relation to the latest basic dykes. He has observed that they
appear to be on the whole rather less acid than the material of the
surrounding bosses, though they were probably all connected with the
same underlying acid magma from which the bosses were protruded. A
somewhat similar relation has been noticed between older granites and
their surrounding dykes, as in Cornwall and Galloway.
[Illustration: TO ACCOMPANY SIR ARCHIBALD GEIKIE'S "ANCIENT VOLCANOES
OF BRITAIN"
Map VII MAP OF THE TERTIARY VOLCANIC DISTRICT OF NORTH EAST IRELAND
The Edinburgh Geographical Institute Copyright J. G. Bartholomew]