Survey.[357] In that fine group of mountains, rising to an extreme height
of 5000 feet above the surrounding plateau, and 11,000 feet above
the level of the sea, masses of trachyte have been injected between
sedimentary strata belonging to the Jura-Triassic and Cretaceous
systems. These masses, thirty-six in number, have consolidated in
dome-shaped bodies, termed by Mr. Gilbert "laccolites," which have
arched up the overlying strata, sending sheets, veins and dykes into
them, and producing in them the phenomena of contact metamorphism.
There is no proof that any of these protrusions communicated with the
surface, and there is positive evidence that most if not all of them
did not. The progress of denudation has laid bare the inner structure
of this remarkable type of hill, and yet has left records of every
stage in its sculpture. In one place are seen only arching strata,
the process of erosion not having yet cut down through the dome of
stratified rocks into the trachyte that was the cause of their uprise.
In another place, a few dykes pierce the arch; in a third, where a
greater depth has been bared away, a network of dykes and sheets is
revealed; in a fourth, the surface of the underlying "laccolite" is
exposed; in a fifth, the laccolite, long uncovered, has been carved
into picturesque contours by the weather, and its original form is more
or less destroyed.[358]

[Footnote 357: See the remarks and diagram, _ante_, p. 86.]

[Footnote 358: "Geology of the Henry Mountains," by Mr. G. K. Gilbert,
_U.S. Geographical and Geological Survey of the Rocky Mountain Region_,
1877.]

The gabbro "laccolites" of the West of Scotland belong to an older
geological period than those of Utah, and have, therefore, been longer
subject to the processes of denudation. They have been enormously
eroded. The overlying cover of basalt has been stripped off from
them, though from the escarpments beyond them it is not difficult in
imagination to restore it. In Rum it has been so completely removed,
that only a few fragments remain at some distance from the core of
gabbro, which now stands isolated. In Ardnamurchan, and still more
in Skye, the surrounding plateau of basalt remains in contact with
the gabbro bosses. But in Mull, where the plateau-basalts reach now,
and perhaps attained originally a greater thickness than anywhere
else, they have protected the intrusive sheets, which are therefore
less deeply cut away than in any of the other districts, and no great
central core of gabbro has yet been uncovered.

                              CHAPTER XLV

                            THE ACID ROCKS

  Their Petrography--Their Stratigraphical Position and its
  Analogies in Central France

We now come to the examination of another distinct phase of volcanic
action during Tertiary time in Britain. The igneous rocks that have
been under consideration in the foregoing chapters, whether poured
out at the surface or injected below ground, have been chiefly of
basic, partly indeed, like the peridotites, of ultra-basic character.
Some, however, have shown an andesitic or intermediate composition.
Reference has also been made to the probable eruption of acid rhyolites
in the long interval between the outflow of the lower and the upper
basalts in Antrim. But we now encounter a great series, decidedly acid
in composition, in the more largely crystalline members of which the
excess of silica is visible to the eye in the form of free quartz.
While there is a strong contrast in chemical composition between this
series and the rocks hitherto under discussion, there are also marked
differences in structure and mode of occurrence. Like the gabbros,
all the masses of acid rock now visible appear to be intrusive. They
have been injected beneath the surface, and therefore record for us
subterranean rather than superficial manifestations of volcanic action.

The existence of rocks of this class in the midst of the basic
masses has long been recognized. They were noticed by Jameson, who
described the hills between Loch Sligachan and Broadford as composed
of "a compound of felspar and quartz, or what may be called a
granitel, with occasional veins of pitchstone."[359] Macculloch gave a
fuller account of the same region, and classed the rocks as chiefly
"syenite" and "porphyry."[360] In Antrim, also, even in the midst of the
basalt-tableland, masses of "pitchstone-porphyry "pearlstone-porphyry,"
"clay-porphyry," and "greystone" were observed and described.[361] In
more recent years Professor Zirkel has given a brief account of the
so-called "syenite and porphyry" of Mull and Skye,[362] and the late
Professor Von Lasaulx fully described the "trachyte" or rhyolite of
Antrim.[363]

[Footnote 359: _Mineralogical Travels_, ii. 90.]

[Footnote 360: _Western Isles_, see the descriptions of Skye, Mull and
Rum.]

[Footnote 361: Berger, _Trans. Geol Soc._ iii. (1816), p. 190; Portlock,
_Journ. Geol. Soc. Ireland_, vol. i. (1834), p. 9.]

[Footnote 362: _Zeitsch. Deutsch. Geol. Gesellsch._ xxiii. (1871), pp.
54, 77, 84, 88.]

[Footnote 363: Tschermak's _Min. und Petrog. Mittheilungen_, 1878, p.
412. The chemical composition of this rock and its place among the
rhyolites had already been determined by E. T. Hardman from analysis,
_Journ. Geol. Soc. Ireland_, vol. iii. (1871), p. 32.]

This interesting series of rocks embraces a greater variety of
petrographical characters than any other portion of the British
Tertiary volcanic rocks. On the one hand, it presents thoroughly
vitreous masses, some of which in their colour, lustre and microscopic
structure remind us of recent obsidians. On the other hand, it affords
coarsely crystalline compounds, to which no other name than granite
can be assigned, and which, did we not know their geological position,
might almost be classed with some of the most ancient eruptive rocks.
Between these two extremes abundant gradations may be found, including
beautiful spherulitic rocks, felsites and rhyolites.

In dealing with such a series of intrusive rocks, we again encounter
the difficulty of reaching certainty as to their relative dates of
eruption, since in each case all that can usually be affirmed is that
the intrusive mass is younger than that into which it is injected. It
is quite possible that protrusions of acid rocks occurred at intervals
during the accumulation of the basic masses, as may perhaps be inferred
from the rhyolite-tuffs and conglomerates of Antrim and from the
occurrence of fragments of siliceous lavas in the gravels near the base
of the basalt-plateau of Mull, and in the agglomerates of that island
as well as of other districts.[364] It is probable, therefore, that at
the time when the basalts of the plateaux were emitted, there existed,
within reach of volcanic explosions, masses of granophyric, felsitic
or rhyolitic rocks, fragments from which were shot up the funnels of
discharge. That portions of these rocks were actually intruded into
the basalt-sheets before the building up of the plateaux was completed
appears to be proved in Antrim. Elsewhere, however, no evidence has
yet been obtained of any such intrusion until after the close of the
plateau-period. On the contrary, in every case where the relative
ages of the rocks can be fixed, the acid are younger than the basic
protrusions.

[Footnote 364: Reference may also again be made to the agglomerates of
Strath, Skye, which contain in some parts abundant fragments of acid
rocks that closely resemble some of the masses of granophyre which
disrupt these agglomerates.]

The only known exceptions to this rule are the latest basalt-dykes.
Hence, while amid the large and varied series of acid rocks, which
no doubt represents a wide interval of time, some may belong to
comparatively early epochs in the protracted volcanic period, the
actual available evidence places the emission of these rocks, as a
whole, towards the end of the volcanic history. This evidence I shall
bring, forward in full detail, since it necessitates an abandonment of
what has been the general belief in regard to the relative ages of the
rocks.

i. PETROGRAPHY OF THE ACID ROCKS

The classification of the rocks which best harmonizes the
field-evidence and the detailed study of their mineralogical
composition, is one that arranges these volcanic protrusions into two
series. In the one, the orthoclase is sanidine, and the rocks range
from the most vitreous pitchstone through perlitic and spherulitic
varieties to rhyolite ("quartz-trachyte"). In the other series, which
embraces by far the largest proportion of the whole, the orthoclase is
always turbid, and in this respect as well as in many others the rocks
remind us rather of ancient eruptive masses than of those which have
appeared in Tertiary time. They range from flinty felsitic varieties,
which are obviously devitrified glasses, through different textures of
quartz-porphyry into granophyre, and finally into granite. As I have
been unable to recognize any essential difference of structure and
composition between these acid Tertiary rocks and those of far earlier
geological time, I give them the names which no petrographer would
hesitate to apply to them if they were of Palæozoic age. It has long
appeared to me that these rocks furnish conclusive evidence of the
misleading artificiality of any petrographical nomenclature in which
relative antiquity is made an essential element of discrimination.

_Granite._--That true granites form part of the Tertiary volcanic
series of the British Isles has now been completely established. They
occur as bosses and sills which have been intruded into the gabbros
and all older rocks. They are thus proved not only to belong to the
Tertiary period, but to one of the latest phases of its volcanic
history. But besides these granites, the relative age of which can be
definitely fixed, there occur others which, standing alone and at some
distance from the basaltic plateaux, can only be inferentially classed
in the Tertiary series. To this group belong the granite masses of the
Isle of Arran and the Mourne Mountains in north-eastern Ireland.

Taking first the unquestionably Tertiary granites which occur as
bosses and intrusive sheets, we have to note that the more coarsely
crystalline granophyres are hardly to be distinguished externally from
granite. As the dark ferro-magnesian constituent of these rocks was
generally believed to be hornblende, they were called by the older
petrographers "syenite"; that is, granite with hornblende instead of
mica. The peculiar micropegmatitic groundmass, which constitutes the
distinguishing feature of the granophyres, may occasionally be observed
so reduced in amount as only to appear here and there between the
other minerals, which are grouped in a granitic structure. From this
condition, one step further carries us into a true granite, from which
all trace of the granophyric character has disappeared. Such gradations
may be traced even within short distances in the same boss of rock.
Thus, in the hornblende-biotite-granite boss of Beinn-an-Dubhaich,
Skye, a thoroughly granitic arrangement of the component minerals is
observable in the centre, while a specimen taken from near the edge
on the shore of Camas Malag shows the development of a granophyric
groundmass. But, though the large bosses are usually somewhat coarsely
crystalline in the centre, and tend to assume finer felsitic textures
around their borders, as was observed long ago by Oeynhausen and Von
Dechen,[365] the granitic structure is sometimes exhibited even at the
very edge, and not only so, but in the dykes that protrude from the
bosses into the surrounding rocks. Thus the Beinn-an-Dubhaich mass, at
its margin in Camas Malag, sends a vein into the surrounding limestone,
but though more close-grained than the main body of the rock, this vein
is neither felsitic nor granophyric, but truly granitic in structure.

[Footnote 365: Karsten's _Archiv_, i. p. 89.]

So far as I have observed, the true granites contain a brown mica and
also a little hornblende, both visible to the naked eye, but generally
somewhat decomposed. These rocks are thus hornblende-biotite-granites
(amphibole-granitites of Rosenbusch). They may be defined as
medium-grained aggregates of quartz, orthoclase (also plagioclase),
biotite and hornblende, with sometimes magnetite, apatite, epidote and
zircon. Dr. Hatch found that in some instances (Beinn-an-Dubhaich) the
quartz contains minute inclusions (glass?), bearing immovable bubbles
with strongly-marked contours; while in others (Beinn-na-Chro, Skye)
this mineral is full of liquid inclusions with bubbles, sometimes
vibratile, sometimes fixed. He remarked that the quartz and felspar
have consolidated almost simultaneously, but that in some instances
(Marsco, Glen Sligachan) there are isolated roughly idiomorphic
crystals, of a white, less turbid orthoclase, which belong to a
slightly earlier consolidation than that of the more kaolinized felspar
of the rest of the rock.

The granite of the island of Arran, in the Birth of Clyde, which is
here included in the Tertiary volcanic series, has long been recognized
as consisting of two distinct portions, an eastern or coarse-grained,
and a western or fine-grained variety. The latter sends veins into the
former. These granites contain orthoclase, plagioclase, quartz and dark
mica, the quartz being often idiomorphic with respect to the felspar,
and a tendency towards a micropegmatitic structure being sometimes
observable. A distinguishing characteristic of the Arran granite is
the cavernous or drusy structure which it presents, the cavities being
often lined with well-crystallized orthoclase and smoky quartz.[366] The
granite of the Mourne Mountains in Ireland closely resembles that of
Arran. Its druses, with their beautifully terminated minerals, have
long been well known.

[Footnote 366: See Mr. Teall's _British Petrography_, p. 328.]

_Microgranite._--This term is applied to certain intrusive masses,
which megascopically may be classed with the quartz-porphyries
and felsites, but which microscopically are found to possess a
holocrystalline granitic groundmass of quartz and orthoclase, through
which are scattered porphyritic crystals of the same two minerals,
sometimes also with plagioclase, augite, magnetite or apatite. Rocks of
this type do not appear to be abundant. They occur as dykes and bosses,
but occasionally also as sheets. I have collected them from Skye, Rum
and Ardnamurchan.

_Granophyre._--Under this name may be grouped the large majority of
the acid rocks which play an important part in the geology of the West
of Scotland. They are typically developed in the islands of Mull and
Skye. Generally pale grey or buff in colour, they range in texture
from the true granites, into which, as above stated, they graduate,
to exceedingly close-grained varieties like the felsites of Palæozoic
formations. In the great majority of them the micrographic intergrowth
of quartz and felspar, known as micropegmatite, is their conspicuous
structure, and even constitutes most of their substance. They may
thus be classed generally as granophyres, in the sense in which this
term is employed by Rosenbusch, but without his limitation of it to
pre-Tertiary rocks.

The specific gravity of these rocks has been determined from a series
of specimens by Mr. A. Harker to range from about 2·3 among the
felsites to 2·7 among the granites. No chemical analyses of these rocks
have yet been made, but they have been subjected to microscopical
examination, and their general structure and composition are now known.

The typical granophyre of the Inner Hebrides outwardly closely
resembles an ordinary granite of medium grain, in which the component
dull felspar and clear quartz can be readily distinguished by the naked
eye. Throughout all the varieties of texture there is a strong tendency
to the development of minute irregularly-shaped drusy (miarolitic)
cavities, which here and there give a carious aspect to the rock.
That these cavities, however, are part of the original structure
of the rock, and are not due to mere weathering, is shown by the
well-terminated crystals of quartz and felspar which project into them.
On a small scale, it is the same structure so characteristic of the
granite of the Mourne Mountains and of parts of that of Arran.

Examined under the microscope, a normal specimen of the granophyre of
the Western Isles presents a holocrystalline groundmass, which fills
all the interspaces between the crystals of earlier consolidation.
This groundmass consists of an aggregate of clear quartz and turbid
orthoclase, arranged as micropegmatite, but also in more or less
idiomorphic crystals. In some parts, the two dominant minerals are
grouped in alternate parallel fibres, diverging from the surface of the
enclosed crystals, which are thus more or less completely surrounded by
a radially fibrous mass. The felspathic portion of the micropegmatite
which usually surrounds the orthoclase crystals, when viewed between
crossed Nicols, is found to extinguish simultaneously with the central
crystal.[367] In other parts, the felspar forms a kind of network, the
meshes of which are filled up with quartz. Through the groundmass,
besides the clear quartz and dull orthoclase, some ferro-magnesian
or other additional constituent is generally distributed, but
usually somewhat decomposed. In certain varieties Dr. Hatch found an
abundant brown mica, as in the rock at Camas Malag, Skye. In others,
a pyroxene occurs, which he observed in minute greenish grains,
sometimes completely enclosed in the quartz. In a third variety, the
dark constituent is hornblende, the most remarkable example of which
is one to be seen at Ishriff, in the Glen More of Mull, where the
ferro-magnesian mineral takes the form of long dirty-green needles,
conspicuous on a weathered surface of the rock. A fourth variety is
distinguished by containing plagioclase in addition to or instead of
orthoclase. In the rock of the sheet forming Cnoc Carnach, near Heast,
in Skye, Dr. Hatch observed both orthoclase and plagioclase scattered
through a fine micropegmatitic groundmass, and in a part of the boss at
Ishriff he found the rock to be composed mainly of plagioclase, in a
micropegmatitic groundmass of quartz and felspar, with a few scattered
grains of a pale brown augite and grains of magnetite. A fifth variety
is marked by the prominence of the crystals of quartz and felspar of
earlier consolidation, and by the fineness of grain in the surrounding
micropegmatitic groundmass, whereby a distinct porphyritic structure
is developed. Rocks of this kind are megascopically like ordinary
quartz-porphyries. Still another variety has been detected by Mr. Teall
in the rock of Meall Dearg, at the head of Glen Sligachan, Skye, in
which, besides irregular patches which may represent decayed biotite,
and others which are possibly ilmenite, the rare mineral riebeckite is
present.[368]

[Footnote 367: Mr. Teall, _Quart. Journ. Geol. Soc._ vol. 1. (1894) p.
219. See also his _British Petrography_, p. 327.]

[Footnote 368: _Quart. Journ. Geol. Soc._ vol. 1. (1894), p. 219.]

_Felsite._--The close-grained rocks into which the ordinary granophyres
frequently graduate may be conveniently grouped under the general name
of Felsite. They differ in no essential feature from the felsites of
the Palæozoic formations. They are more particularly developed, as
might be expected, in those places where the conditions have been most
favourable for rapid cooling, while the more coarsely crystalline
granophyres occur where the material may be supposed to have
consolidated most slowly. Where the acid magma has been injected into
chinks and fissures so as to take the form of veins or dykes, it is
sometimes felsitic, sometimes granophyric, in texture. Along the margin
of large bosses, like those of Mull and Skye, it frequently though
not invariably has assumed a fine texture, with even spherulitic and
flow-structures. But in the centre of large bosses it usually appears
as coarse granophyre or as granite.

The felsites vary in texture from flinty or horny to dull
finely-granular, and in colour from white through shades of grey,
buff and lilac, to black, generally with porphyritic felspars and
blebs of quartz. Where these porphyritic enclosures increase in size
and number, the rocks cannot be distinguished externally from ancient
quartz-porphyries. In general the groundmass of these rocks has been
completely devitrified. But in some dykes enough of the glassy base
remains to show their original vitreous condition. A gradation can thus
be traced from thoroughly glassy pitchstone into completely lithoid
felsites and crystalline granophyres.

A characteristic feature of the felsitic varieties of acid rock is
their flow-structure, which they often display in great perfection.
Sometimes, indeed, this structure has been so strongly developed as to
cause the rock to weather along the planes of flow and to break up into
thin slabs.

Many of these rocks also present admirably developed spherulitic
structures, varying from microscopic minuteness up to large round or
egg-shaped balls nearly two inches in diameter, and often distributed
in lines along those of flow-structure. They likewise exhibit a
frequent development of micropegmatite. No line indeed can be drawn
between these felsites and the granitoid varieties, for the same
characteristic granophyric intergrowth of felspar and quartz runs
through them all.

_Pitchstone._--This name is applied to the glassy varieties apart from
their chemical composition, and specially denotes the possession of
a vitreous structure. Some of the rocks to which it has been applied
are probably glassy varieties of andesite, others are dacites, while
some may be as acid as the most acid felsites and granophyres. The
pitchstones are found in veins or dykes which traverse different
geological formations up to and including the great granophyre bosses
of the Inner Hebrides. They vary in colour from a deep jet-black or
raven-black to a pale bottle-green, and in lustre from an almost
glassy obsidian-like to a dull resinous aspect. Occasionally they
assume a felsitic texture, owing to devitrification, and also a finely
spherulitic structure. Some varieties appear to the naked eye to be
perfectly homogeneous, others become porphyritic by the appearance of
abundant sanidine crystals.

The microscopic structure of the British pitchstones has not yet been
fully worked out. The beautiful feathery microlites of the Arran dykes,
first made known by David Forbes, and subsequently described by Zirkel,
Allport and others, are well known objects to geological collectors.
Dr. Hatch, in whose hands I placed my tolerably large collection of
specimens and their thin slides, furnished me with some preliminary
notes on the slides, from which the following generalized summary is
compiled.

At the one end of the pitchstone group we have a nearly pure glass,
with no microlites, and only a few scattered crystals of sanidine,
quartz, augite or magetite. The glass in thin slices is almost
colourless, but generally inclines to yellow, sometimes to dark-grey.
Some varieties of the rock are crowded with microlites, in others these
bodies are gathered into groups, the glass between which is nearly
free from them. Among the minerals that have been observed in this
microlitic form are sanidine, augite, hornblende (forming the beautiful
green feathery or fern-like aggregates in the Arran pitchstones, Fig.
3) and magnetite. Sometimes the rudimentary forms appear as globulites,
or as belonites, but more commonly as dark trichites. Among the more
definite mineral forms are grains of sanidine, quartz and augite. The
porphyritic crystals are chiefly sanidine, augite and magnetite, but
plagioclase occasionally occurs. The development of spherulites is well
seen in a few of the slides, and occasionally perlitic structure makes
its appearance.

The interesting rhyolitic areas of Antrim include several varieties of
pitchstone. One of these is described by Professor Cole as "a glassy
pyroxene-rhyolite, on the verge of the rhyolitic andesites." Another is
a blue-black porphyritic obsidian.[369]

[Footnote 369: _Scientif. Trans. Roy. Dublin Soc._ vol. vi. (ser. ii.)
1896, p. 77.]

_Rhyolite (Quartz-Trachyte)._--This rock has been abundantly erupted
in north-east Ireland, where it rises in occasional bosses among the
plateau-basalts.[370] It is best exposed at the Tardree and Carnearny
Hills, where it has long been quarried. Its petrographical characters
at that locality were described by Von Lasaulx as those of a typical
quartz-trachyte rich in tridymite, and containing large crystals
of glassy sanidine, isolated narrow laths of plagioclase (probably
andesine), grains of smoky-grey quartz, partly bounded by dihexahedral
faces, and a few scattered flakes of a dark-coloured mica. The
groundmass is microgranitic, and under a high power is resolvable
into a confused aggregate of minute microlites of felspar, with
interstitial quartz-granules.[371] More recently a detailed investigation
of the petrography of the Antrim rhyolites has been conducted by
Professor Cole, who has called attention to their remarkable varieties
of structure, ranging from perfect volcanic glass to a thoroughly
lithoidal texture, and exhibiting flow, perlitic and spherulitic
structures.[372]

[Footnote 370: Fragments of acid rock were detected by Prof. Cole in the
gravel among the Ardtun basalt of Mull, as already noticed on p. 212.]

[Footnote 371: Tschermak's _Min. und Pet. Mittheil._ 1878, p. 412.]

[Footnote 372: _Scientif. Trans. Roy. Dublin Soc._ vol. vi. (ser. ii.)
1896, p. 77. This paper gives an excellent account of the microscopical
character and mineralogical and chemical compositions of these rocks.]

Intrusive masses of rhyolite are also found in the Carlingford region.
One of these, seen at Forkhill, is a velvet-black almost resinous rock
with abundant quartz and felspar, and sometimes displaying beautiful
flow-structure. It will be more particularly described in Chapter
xlvii. Some of the acid dykes and sills of the Inner Hebrides are
varieties of rhyolite. No undoubted example has yet been observed of a
superficial rhyolite-lava, though such not improbably appeared in the
interval between the lower and upper basalts of Antrim.

ii. STRATIGRAPHICAL POSITION.--ANALOGIES FROM CENTRAL FRANCE

In the history of opinion regarding the relative position of the
Tertiary eruptive rocks, no feature is so remarkable as the universal
acceptance of the misconception regarding the place of the acid
protrusions. In tracing this mistake to its source, we find that
it probably arose from the fact that along their line of junction
the granitoid masses generally underlie the basic. This order of
superposition, which would usually suffice to fix the age of two groups
of stratified rocks, is obviously not of itself enough to settle the
relative epochs of two groups of intrusive rocks. Yet it has been
assumed as adequate for this purpose, and hence what can be proved
to be really the youngest has been placed as the oldest part of the
Tertiary volcanic series.

Macculloch, who showed that his "syenites" and "porphyries" had invaded
the Secondary strata of the Inner Hebrides, and must therefore be of
younger date than these, left their relations to the other igneous
rocks of the region in a curiously indefinite position. He was disposed
to regard them all as merely parts of one great series; and seems to
have thought that they graduate into each other, and that any attempt
to discriminate between them as to relative age is superfluous. Yet
he evidently felt that the contrasts of topography which he described
could hardly fail to raise the question of whether rocks so distinct
in outward form did not differ also in relative antiquity. But he
dismissed the question without answering it, remarking that if there
is any difference of age between the two kinds of rock, "there appears
no great prospect of discovering it."[373] He records an instance of
a vein of "syenite" traversing the "hypersthene rock" in the valley
of Coruisk. "If this vein," he says, "could be traced to the mass of
syenite, it might be held a sufficient ground of judgment, but under
the present circumstances it is incapable of affording any assistance
in solving the difficulty."[374] Instead, however, of being a solitary
instance, it is only one of hundreds of similar intrusions which can be
connected with the general body of granitic and granophyric masses, and
which put the relative ages of the several groups of rock beyond any
further doubt.

[Footnote 373: _Western Islands_, i. p. 368; see also pp. 488, 575, 578.]

[Footnote 374: _Op. cit._ p. 370.]

Boué, who knew the geology of some of the extinct volcanic regions of
Europe, recognized the similarity of the Scottish masses to those of
the Continent, and classed the acid rocks as "trachytes." He saw in
each of the volcanic areas of the West of Scotland a trachytic centre,
and supposed that the more granitoid parts might represent the centres
in the European trachytic masses. He traced in imagination the flow of
the lava-streams from these foci of volcanic activity, distinguishing
them as products of different epochs of eruption, among the last
of which he thought that the trachytic porphyries might have been
discharged. He admitted, however, that his restoration could not be
based on the few available data without recourse to theoretical notions
drawn from the analogy of other regions.[375]

[Footnote 375: _Essai Géologique sur l'Écosse_, pp. 291, 322, 327.]

In the careful exploration of the central region of Skye made by Von
Oeynhausen and Von Dechen, these able observers traced the boundary
between the "syenite" and the "hypersthene rock"; and as they found
the former lying underneath the latter, they seem naturally to have
considered it to be the older protrusion of the two.[376] Principal
Forbes came to a similar conclusion from the fact that he found the
dark gabbro always overlying the light-coloured felspathic masses.[377]
Professor Zirkel also observed the same relative position, and adopted
the same inference as to the relative age of the rocks.[378] Professor
Judd followed these writers in placing the acid rocks before the basic.
He supposed the granitoid masses to form the cores of volcanic piles
probably of Eocene age, through and over which the protrusions of
gabbro and the eruptions of the plateau-basalts took place.[379]

[Footnote 376: Karsten's _Archiv_, i. p. 82. It will be shown in later
pages that the apparent infraposition of the granophyre is often
deceptive, the real junction being vertical.]

[Footnote 377: _Edin. New Phil. Jour._ xl. (1846) p. 84.]

[Footnote 378: _Zeitsch. Deutsch. Geol. Gesellsch._ xxiii. (1871) pp. 90,
95. He says that the gabbro seems to be the younger rock, so far as
their relations to each other can be seen.]

[Footnote 379: _Quart. Jour. Geol. Soc._ xxx. (1874) p. 255.]

The evidence for the posteriority of the acid rocks will be fully
detailed in later pages. Before entering upon its consideration,
however, I would remark that the uprise of the British granophyres
presents so many points of resemblance to that of the trachytes and
phonolites among the basalt-plateaux of Auvergne and the Velay in
Central France, that a brief account of the acid protrusions of these
regions may be suitably given here as an introduction to the account of
those of the Inner Hebrides. A succession of stages in the progress of
denudation allows us to follow the gradual isolation and dissection of
the French volcanic groups. The youngest examples occur in the chain of
cones and craters, in the region of the Puy de Dôme. These may be of
Pleistocene, or even of more recent date. Older and more deeply eroded
than these are the numerous domes and cones in the territory of Haute
Loire. Yet more ancient and still more stupendously denuded come the
bosses, sills and dykes of Britain. Nevertheless, the geologist, by
the methods so admirably devised by Desmarest, may follow the chain of
relationship through these different regions and trace a remarkable
continuity of structure. The younger rocks serve to illustrate the
original condition of the more ancient, while the latter, by their
extensive denudation, permit points of structure to be seen which in
the former are still concealed.

No feature in the interesting volcanic district of Auvergne has
attracted more attention than the trachytic protrusions.[380] Rising
conspicuously along the chain of puys, they claim notice even from
a distance owing to the topographical contrast which their pale
rounded domes offer to the truncated, crater-bearing cones of dark
cinders around them. They consist of masses of a pale variety of
trachyte (domite), which in ground-plan present a circular or somewhat
elliptical outline. They vary in size from the nearly circular dome of
the Grand Sarcoui, which measures about 400 yards in diameter, to the
largest mass of all--that of the Puy de Dôme, which extends for some
1500 yards from north to south with a breadth varying from 500 to 800
yards. They are likewise prominent from their height; in the Puy de
Dôme they form the highest elevation of the whole region (1465 metres),
and even in the less conspicuous hills they rise from 500 to 600 feet
above the surrounding plateau.

[Footnote 380: The admirable Map and Memoirs of Desmarest on Auvergne
are classics in geology. Scrope's work, vol. i. p. 45, gives still
the best published account of this district. See also the work of
Lecoq (_ibid._). The results of more detailed petrographical research
regarding the rocks will be found in the essays of M. Michel Lévy
(_Bull. Soc. Géol. France_, 1890, p. 688) and in the Clermont sheet of
the Geological Survey Map of France (Feuille, 166). A bibliography of
the district up to the year 1890 is given in the volume of the _Bull.
Soc. Géol. France_ just cited, p. 674.]

Five such dome-shaped protrusions of trachyte have made their
appearance among the cinder-cones in a space of about five English
miles in length by about two miles in extreme breadth. Though opinions
have varied as to the mode of formation of these domes, there has been
a general agreement that their present topographic contours cannot be
far from the original outlines assumed by the masses at the time of
their production. The position of the trachyte bosses among the puys
serves to show that they were not deep-seated masses which have been
entirely uncovered by denudation, but were essentially superficial,
and were protruded to the surface at various points along the plateau
in the midst of already existing cinder-cones. In some cases, they have
risen on or near the position of the vents of these cones. Thus the Puy
de Chopine is half encircled by the crater of the Puy de la Goutte,
and the Grand Sarcoui stands in a similar relation to the fragmentary
crater-wall of the Petit Sarcoui.

M. Michel Lévy, in pointing out the superficial character of the
domitic protrusions, has forcibly dwelt on the evidence that these
rocks have undergone a comparatively trifling denudation, and that
they could never have extended much beyond their present limits.[381] As
Scrope pointed out, they were obviously protruded in a pasty condition,
not flowing out in streams like the other lavas of the district, but
consolidating within their chimneys and rising from these in rounded
domes.

[Footnote 381: _Op. cit._ p. 711.]

[Illustration: Fig. 344.--Section through the Puy de la Goutte and Puy
de Chopine.

1, Mica-schist; 2 2, Granite; 3 3, Tuffs; 4, Trachyte; 5, Basalt dyke.]

Undoubtedly denudation, cannot have left them altogether unaffected,
but must have removed some amount of material from their surface.
There is reason to believe that the material so removed may have been
in large part of a fragmental character, and that it was under a
covering of loose pyroclastic debris that the upward termination of the
trachyte column assumed its typical dome-form. Thus in the crater-wall
of the Puy de la Goutte, layers of buff-coloured trachytic tuff dip
gently away from the central domite mass of the Puy de Chopine. That
this material was thrown out from the vent previous to the uprise of
the domite may be inferred from the way in which the latter rock has
obliterated the northern half of the crater. The relations of the rocks
are somewhat obscured by talus and herbage, but when I last visited
the locality in the spring of 1895 the structure seemed to me to be as
expressed in the accompanying diagram (Fig. 344).[382]

[Footnote 382: Compare M. Michel Lévy, _ibid._]

The relative date of the protrusion of the trachytic domes cannot
be very precisely defined. There can, indeed, be no doubt that it
belongs to a late phase of the volcanic history. It came long after the
outpouring of the older basaltic plateaux, of which large fragments
emerge from beyond the limits of the younger lavas on both sides of the
great ridge of the puys, and not only long after that outpouring, but
even after the widespread sheets of basalt had been deeply trenched by
valleys and isolated into outliers capping the hill-tops. Yet there is
good evidence also that the uprise of the comparatively acid trachytes
was not the last volcanic episode of the district. The abundance of
dark slags and fragments of basalt lying on the domite hills shows that
discharges of more basic detritus occurred after these hills had taken
their place in the landscape.

Since the latest eruptions, a gradual alteration of the topographical
features by denudation has been slowly but continuously going on. The
Grand Sarcoui, possibly from having originally had a considerable
covering of fragmentary material, shows least the effects of this
waste. Its remarkably regular form, like that of an inverted cauldron
(the "Chaudron," as it is called in the district), presents, in a
distant view, a smooth grassy surface which slopes steeply down into
the great volcanic plain. But on a nearer examination these declivities
are found to be seamed with trenches which the rain-storms of centuries
have dug out. The covering of loose debris has been largely washed
away, though many fragments of dark slag are still strewn over the
slopes, and the scars are now being cut into the domite below. A more
advanced stage of decay may be seen on the Puy de Dôme, where, from
greater elevation and exposure, the domite is already deeply gashed
by gullies and ravines, while the slopes below are strewn with its
detritus.

The region of the Velay displays on a far more extensive scale the
protrusion of trachytic and phonolitic bosses, but as its volcanic
history goes back beyond the time of the Puys of Auvergne, its
volcanic monuments have consequently been more extensively affected
by denudation.[383] A series of basaltic eruptions forming extensive
sheets can there be traced, the oldest dating from Miocene time, the
youngest coming down to the age of the mammoth, cave-bear and early
man. During this prolonged outpouring of basic lavas there were several
intervals during which materials of a more acid nature--trachytes and
phonolites--were erupted. These rocks occur partly as extensive tracts,
covering five or six square miles, like those of the Mezenc, the Megal,
the Pic de Lizieux, and the Rand, and partly in isolated conical or
dome-shaped prominences, sometimes only a few hundred feet in diameter.
Upwards of one hundred distinct eruptions of phonolite have been
observed in the Velay. Even in the tracts where they cover the largest
space, several prominent eminences may usually be observed, not unlike
in general shape the isolated cones and domes of Auvergne. In these
wider areas there appears to be evidence of the outcome of the lava
from one or more vents, either as superficial streams or as underground
intrusive sheets. M. Boule has expressed his opinion that most of the
masses of trachyte and phonolite have been the result of local and
limited eruptions, the pasty rock having risen in and accumulated
around its pipe, without flowing far in any direction. A section across
one of these masses would present a somewhat mushroom-shaped form.[384]

[Footnote 383: In addition to the work of Scrope, the student of this
important volcanic district will find an invaluable guide in the Le
Puy Sheet (No. 186) of the Geological Survey Map of France, and in the
_Bulletins_ of the Survey, particularly those by MM. Termier and Boule,
No. 13 (1890) and No. 28 (1892).]

[Footnote 384: _Bull. Carte. Géol. France_, No. 28 (tome iv.) p. 125.]

That fragmentary ejections accompanied the protrusion of these rocks,
though probably on a very limited scale, is shown by the occasional
survival of portions of trachyte tuff around them. One of the most
notable of these deposits occurs in the hollow between the Suc du
Pertuis and the next dome to the south. It consists of fine and coarse,
trachytic detritus, which in one place is rudely bedded and appears to
dip away from the phonolite dome behind it at an angle of 30°. This
material and its inclination are what might be expected to occur round
an eruptive vent, and may be compared with those of the crater-wall
of the Puy de la Goutte in relation to the domite boss of the Puy de
Chopine.

The denudation of Velay has undoubtedly advanced considerably further
than that of the Puys of Auvergne. The pyroclastic material which may
have originally covered the domes of trachyte and phonolite has been
in great part swept away. The surrounding rocks, too, both aqueous and
igneous, have been extensively removed from around the necks of more
enduring material. Hence the trachyte and phonolite bosses stand out
with so striking a prominence as to arrest the eye even for a distance
of many miles.

[Illustration: Fig. 345.--View of the Huche Pointue and Huche Platte
west of Le Pertuis.

The cone is one of the trachytic domes, while the flat plateau to the
left is a denuded outlier of the basalt sheets.]

There cannot be any doubt that these necks have pierced the older
basalts, and therefore belong to a later epoch in the volcanic history.
The approximately horizontal sheets of basalt have been deeply eroded
and reduced to mere fragments, and in some instances their existing
portions owe their survival to the protection afforded to them by the
immense protrusions of more acid material. But there is here, as well
as in Auvergne, evidence of the uprise of a later more basic magma, for
sheets of basalt are found overlying some parts of the trachytes and
phonolites.

While the external forms of these Velay necks recall with singular
vividness the features of many more ancient necks in Britain, an
examination of the internal structure of some of them affords some
further interesting points of resemblance. The slabs into which, by
means of weathering along the joints, the rock is apt to split up are
sometimes arranged with a general dip outwards from the centre of the
hill, so that their flat surfaces roughly coincide with the hillslopes.
In other cases the peculiar platy structure, so characteristic of
phonolite, is disposed vertically or dips at a steep angle into the
hill, so that the edges of the slabs are presented to the declivities,
which consequently become more abrupt and rugged.

Though none of the volcanic series in Auvergne or the Velay is so acid
in composition as the more acid members of the Tertiary volcanic series
of Britain, the manner in which the trachytes and phonolites of the
French region make their appearance presents some suggestive analogies
to that of the corresponding rocks in this country. We see that they
were erupted long after the outpouring of extensive basaltic plateaux,
that they belonged to successive epochs of volcanic activity, that they
were protruded in a pasty condition to the surface, where, more or less
covered with fragmentary ejections, they terminated in dome-shaped
hills or spread out to a limited distance around the vents, and
lastly, that they were succeeded by a still later series of more basic
eruptions, which completed the long volcanic history. We shall see in
the following pages how closely the various stages in this complex
record of volcanic activity may be paralleled in the geological records
of Tertiary time in Britain.[385]

[Footnote 385: The phonolite necks of Bohemia, which form so prominent
a feature in the Tertiary geology of that country, might likewise be
cited here in illustration of the acid domes and bosses of the British
Isles.]

                             CHAPTER XLVI

             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.]

  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.

                             CHAPTER XLVII

      THE ACID BOSSES OF MULL, SMALL ISLES, ST. KILDA, ARRAN AND
                       THE NORTH-EAST OF IRELAND

ii. THE ACID BOSSES OF MULL

Though of comparatively small extent, the granophyre bosses of the
island of Mull afford to the geologist a large amount of instruction in
regard to the relations of the different members of the volcanic series
to each other. Especially important is the evidence which they contain
of the connection between the acid and basic groups of rocks. They have
been laid bare in many natural sections, some of which, forming entire
hillsides, are among the most astonishing in the whole wonderful series
which, dissected by denudation, reveal to us the structure of these
volcanic regions. They lie in two chief areas. One of these extends
along the northern flanks of the mountainous tract from the western
side of Beinn Fhada across Loch Ba' to the west side of Glen Forsa. The
other occupies for over three miles the bottom of Glen More, the deep
valley which, skirting the southern side of the chief group of hills,
connects the east side of the island by road with the head of the great
western inlet of Loch Scridain. There are other minor areas. One of
these extends for about a mile along the declivities to the south of
Salen, across the valley of the Allt na Searmoin; another occurs at
Salen; a third runs along the shore at Craignure. In the interior also,
many isolated areas of similar rocks, besides thousands of veins, occur
in the central group of hills and valleys which form the basins of the
Glencannel and Forsa rivers (Map VI.).

The chief northern boss, which for the sake of convenience of reference
may be called that of Loch Ba', has a length of nearly six miles,
with a breadth varying from a quarter of a mile to about a mile and a
quarter. It descends to within 50 feet of the sea-level, and is exposed
along the crest of Beinn Fhada at a height of more than 1800 feet.
It chiefly consists of a grey crystalline rock which might readily
be identified as a granite, but which when examined microscopically
is found to possess the granophyric structure. With this distinctly
granular-crystalline rock are associated various porphyritic and
felsitic masses, which pass into it, and are more specially observable
along its border. An exceedingly compact black quartz-felsite or
rhyolite forms its southern boundary, runs as a broad dyke-like ridge
from the head of the Scarrisdale Water north-eastward across Loch
Ba' (Fig. 352), and spreads out eastward into a mass more than a
mile broad on the heights above Kilbeg in Glen Forsa. The sharp line
of demarcation of this felsite, and its mass and extent, point to a
different period of extravasation.

[Illustration: Fig. 352.--View of the hills on the south side of the
head of Loch na Keal, showing the junction of the granophyre and the
bedded basalts.

  One bird, the bedded basalts of the Gribon plateau; two birds,
  the bedded dolerites and basalts of Beinn a' Chraig adhering to
  the northern slope and capping the hill; three birds, summit of
  Ben More, with A'Chioch to the left and the top of Beinn Fhada
  appearing in the middle distance between them; four birds, the
  granophyre slopes of Beinn a' Chraig with the great dyke-like mass
  of felsite on the left.
]

The geologist, who approaches this district from the north-east, has
his attention arrested, even at a distance of several miles, by the
contrast between the outer and inner parts of the hills that lie to the
south-west of Loch Ba'. He can readily trace from afar the dark bedded
basic rocks rising terrace above terrace, from the shores of Loch na
Keal, to form the seaward faces of the hills along the southern side
of that fjord. But he observes that immediately behind these terraces
the mass of the rising ground obviously consists of some amorphous
rock, which weathers into white debris. Nothing can be sharper than
the contrast of colour and form between the two parts of the hills.
The bedded plateau-rocks lie as a kind of wall or veneer against a
steep face of the structureless interior (Fig. 352). Seen from the
other or hilly side, the contrast is perhaps even more striking.
But the astonishment with which it is beheld at a distance becomes
intensified when one climbs the slopes, and finds that the sheets of
dolerite and basalt (which from some points of view look quite level,
yet dip towards the north-east at a gentle angle) are immediately
behind the declivity abruptly truncated by a mass of granophyre. Of
all the junction-lines between the acid bosses and the lavas of the
plateaux, those exposed on these Mull hillsides are certainly the
most extraordinary. So little disturbed are the lavas, that one's
first impulse is to search for pebbles of the granophyre between the
basalts, for it seems incredible that the inner rock should be anything
but a central core of older eruptive material, against and round which
the younger basic rocks have flowed. But, though the granophyre is so
decomposing and covers its slopes with such "screes" of debris, that
had the basalts been poured round it, they must infallibly have had
some of its fragments washed down between their successive flows, not a
single pebble of it is there to be found. This might not be considered
decisive evidence, but it is extended and confirmed by the fact that
the acid rock gives off veins which ramify through the basalts.

Before examining the actual contact of the two rocks, however, the
geologist will not fail to observe here an admirable example of the
gradual change which was described in the foregoing chapter as coming
over the bedded basalts near the acid bosses. As he approaches the
nucleus of white rock, the basalts assume the usual hard indurated
character, not decaying into brown sand as on the plateaux, but often
standing out as massive crags with vertical clean-cut joint-faces.
This metamorphosed condition extends in some cases to a considerable
distance from the main body of acid rock, especially where knobs of
that material, protruding through the more basic lavas, show that it
must extend in some mass underneath. Thus along the shore at Saline
the bedded basalts succeed each other in well-defined sheets, some
being solid, massive and non-amygdaloidal, others quite vesicular, and
recalling the black scoriform surfaces of recent Vesuvian lavas; yet
they are all more indurated than in the normal plateau-country, and
they break with a hard splintery fracture. Immense numbers of dykes
cut these rocks, and they are likewise pierced by occasional felsitic
intrusions.

If we cross to the other side of the island and trace the bedded
basalts away from the central masses of acid rock we meet with so
gradual a diminution of the induration that no definite boundary-line
for the metamorphism can be drawn. As we recede from the centre of
alteration, the rocks insensibly begin to show brown weathered crusts,
with spheroidal exfoliation, the reticulations of epidote and calcite
become much less abundant, the amygdaloids gradually assume their
normal earthy character, and eventually we find ourselves on the
familiar types of the plateau. This transition is well seen along the
shores of Loch na Keal.[401]

[Footnote 401: Some of the thick massive sheets of basic rock along the
south side of this inlet may possibly be altered sills.]

These proofs of the alteration of the plateau-basalts are accompanied
in Mull as in Skye by further abundant evidence that the acid rocks are
of younger date than the basic. In particular, dykes and veins may be
traced proceeding from the former and intersecting the latter. Thus, in
the bed of the south fork of the Scarrisdale stream, a separate mass
of granophyre (which under the microscope exhibits in perfection the
characteristic structure of this rock) protrudes through the basalts
in advance of the main mass, and a little higher up on the outskirts
of that mass narrow ribbons of the granophyre run through the basic
rocks. The contrast of colour between the pale veins of the intrusive
rock and the dark tint of the basalts is well shown in the channel of
the water. Similar sections may be seen on the flanks of Beinn Fhada,
especially in the great corry north of Ben More, where the granophyre
sends a tongue of finer grain between the beds of basalt. On the east
side of Loch Ba' numerous proofs of similar intrusion may be observed.
Thus at the east end of Loch na Dàiridh, where the granophyre has been
intruded into the basalts, hand-specimens may be obtained showing the
two rocks welded together. On the slopes of Cruach Tòrr an Lochain,
where the granophyre has a felsitic selvage, the bedded basalts are
traversed by veins of the latter material (Fig. 353). A little further
east, at the head of the Allt na Searmoin, the bedded basalts, some of
which are separated by slaggy scoriaceous surfaces, are intersected by
another protrusion from the compact felsitic porphyry (Fig. 354).[402] A
mile lower down the same valley a separate mass of granophyre sends out
veins into the basalt, which as usual is dark bluish-grey in colour,
indurated and splintery.

[Footnote 402: This rock appears to the eye as a black finely
crystalline-granular felsite. Under the microscope, it was found by
Dr. Hatch to "present a markedly granulitic structure, consisting
mainly of small rounded grains of dirty brown turbid felspar, with
isolated granules of colourless quartz. Scattered through the rock, or
accumulated in patches, are small spherical or drop-like granules of a
bright green augite (coccolite)."]

[Illustration: Fig. 353.--Section on south side of Cruach Tòrr an
Lochain, Mull.

_a_, bedded basalts and dolerites; _b_, granophyre; _c_, marginal
finer-grained band; _d_ _d_, veins from the granophyre traversing the
basic rocks.]

[Illustration: Fig. 354.--Section at head of Allt na Searmoin, Mull.

_a_, basalts and dolerites, with slaggy upper surfaces; _b_, felsite.]

As the posteriority of the Mull granophyre and felsites to the basalts
is thus proved, the further question remains as to their manner of
intrusion. Here and there, especially on the south-eastern side,
between the head of the Scarrisdale river and Loch Ba', the line of
junction between the two rocks is nearly vertical, but a body of black
felsite intervenes as a huge wall between the ordinary granophyre and
the basalt. On Beinn Fhada and Beinn a' Chraig the line of separation,
as I have above remarked, is inclined outwards, and plunges under the
basalts at an angle of 30° to 40°. The terraced basalts and dolerites
are not sensibly disturbed, but end off abruptly against the steep
face of intrusive rock. We might suppose that in this case the younger
rock had merely carried upward the continuation of the beds that are
truncated by it, as if an orifice had been punched out for its ascent.
But on the top of the ridge of Beinn a' Chraig we find that the
outliers which there remain are not portions of the lower basalts, but
of the upper "pale group" of Ben More. The same rocks are prolonged
on the other side of the Scarrisdale Glen, sweep over the summit of
Beinn Fhada, and run on continuously into the crest of A'Chioch and the
upper part of Ben More. The granophyre has usurped the place of the
lower dolerites and basalts, but has left the more felspathic lavas of
the "pale group" in their proper position. And to make this remarkable
structure still more clear, sections may be seen on the southern flanks
of Beinn Fhada, where the upper surface of the granophyre comes down
obliquely across the edges of the lavas, and allows the junction of the
basalts and the "pale group" to be seen above it (Fig. 355). As in the
case of Beinn an Dubhaich, it is as if the granophyre had eaten its way
upward and dissolved the rocks which it has replaced.

[Illustration: Fig. 355.--Section on south side of Beinn Fhada, Mull.

_a_, bedded basalts and dolerites; _b_, "pale group" of Ben More; _c_,
granophyre.]

The usual kind of contact-metamorphism has been produced around this
intrusive boss. It is most marked in the outliers that cap Beinn a'
Chraig and on the two ridges to the south-west, where it is seen to
consist in a high degree of induration, the production of a shattery,
irregularly-jointed structure, and the effacement of the obvious
bedding which characterizes the unaltered rocks.

The position of this eruptive mass, quite a mile broad, breaking
through, without violently tilting, more than 1800 feet of the bedded
basalts, and then stopping short about the base of the "pale group,"
presents a curious problem to the student of geological physics. It at
once reminds him of many sections among Palæozoic granites where an
eruptive boss has ascended and taken the place of an equivalent volume
of the surrounding rocks, which, though more or less metamorphosed,
are not made to dip away from it as from a solid wedge driven upwards
through them. In this Mull case, however, there are some peculiar
features that deserve consideration, for they seem to show that here,
as elsewhere, passages for the uprise of the intrusive rock were
already provided by the presence of volcanic pipes, which, even if
filled up with fragmentary materials, would no doubt continue to be
points of weakness. Round the flanks of the Loch Ba' boss, and here
and there on its surface, patches of intensely indurated volcanic
agglomerate may be detected. A little to the south of the tarn called
Loch na Dàiridh, the granophyre is succeeded by the black, flinty
felsite or rhyolite already referred to. This rock in some places
exhibits a beautiful flow-structure, with large porphyritic felspars,
and encloses a great many fragments of dolerite and gabbro, varying
from the size of a pea up to blocks several inches in diameter. Lying
on its surface are detached knolls of much altered dolerite, basalt,
and coarse breccia or agglomerate. On its southern margin one of these
patches of agglomerate contains abundant fragments of various felsitic
rocks, among which are pieces of a compact rock with flow-structure
like that found in place immediately to the north; also rounded pieces
of quartzite, and of compact and amygdaloidal basalt wrapped up in a
very hard matrix which seems to consist largely of basalt-dust. No
bedding can be made out in this rock, and the mass looks like part of a
true neck. Further down the slope the bedded basalts appear. The actual
junctions of the different rocks cannot be satisfactorily traced, but
the structure of the ground appears to me to be as shown in Fig. 356.
A patch of similar agglomerate appears a little to the south-west of
the last section in front of a cliff of the felsite, and seems to
be enclosed in the latter rock, and other exposures of agglomerate,
underlain and intensely indurated by the felsite, may be noticed on the
ground that slopes towards Loch Ba'.

[Illustration: Fig. 356.--Section to south of Loch na Dàiridh, Mull.

_a_, basalts; _b_, dolerite; _c_, volcanic agglomerate; _d_, black
felsite; _e_, granophyre.]

That these agglomerates do not belong to the period of the eruption of
the granophyre and felsite, but to that of the bedded basalts, may be
inferred from their intense induration next the acid rocks, and also
from the fact that similar breccias are actually found here interposed
between the bedded basalts. This is well shown on the hill above the
Coille na Sròine, where the accompanying section can be seen (Fig.
357). The broad dyke-like mass of black flinty felsite already referred
to runs as a prominent rib over the southern end of Beinn a' Chraig
into the head of the Scarrisdale glen (see Fig. 352). It cuts across
the bedded basalts, and immediately to the south of where these appear,
a thin intercalated bed of breccia crops out, of the usual dull-green
colour, with abundant fragments of basalt and many of yellow and grey
felsite.

From these various facts we may, I think, conclude that along the
strip of ground now occupied by the Loch Ba' boss of granophyre and
felsite, there once stood a line or group of vents, from which, besides
the usual basalt-debris, there were ejected many pieces of different
felsitic or rhyolitic rocks, and that these eruptions of fragmentary
material took place during the accumulation of the plateau-basalts.
These volcanic funnels occasioned a series of points or a line of
weakness of which, in a long subsequent episode of the protracted
volcanic period, the acid rocks took advantage, forcing themselves
upwards therein, and leaving only slight traces of the vents which
assisted their ascent. The mingling of acid and basic fragments in the
material ejected from these vents is another proof of the existence
of acid rocks in the volcanic reservoirs before the advent of the
great granophyre intrusions. The evidence thus entirely confirms the
conclusions deduced from the Skye area.

[Illustration: Fig. 357.--Section of junction of south side of Loch Ba'
granophyre boss, with the bedded basalts, Mull.

_a_, bedded basalts; _b_ _b_, basalt-tuff and breccia; _c_, granophyre;
_d_, black felsite; _e_, coarse dolerite dyke, 30 or 40 feet wide.]

The second or Glen More boss, instead of rising into hilly ground, is
confined to the bottom of the main and tributary valleys, and has only
been revealed by the extensive denudation to which these hollows owe
their origin. It begins nearly a mile below Torness and extends up to
Loch Airdeglais--a distance of almost four miles. Though singularly
devoid of topographical feature, it exhibits with admirable clearness
the relation of the granophyres to the gabbros, and thus deserves an
important place among the tracts of acid rocks in the Western Islands.
Its petrographical characters change considerably from one part of its
body to another. For the most part, it is a true granophyre, sometimes
with orthoclase, sometimes with plagioclase as its predominant
felspar. At Ishriff, as already stated, it is sprinkled with long
acicular decayed crystals of hornblende; but at the watershed the
ferro-magnesian mineral is augite. The surrounding rocks are mainly the
plateau-basalts, with their sills of dolerite and gabbro.

[Illustration: Fig. 358.--Mass of dark gabbro about two feet in
diameter traversed by pale veins of granophyre, lying on north slope of
Creag na h-Iolaire, Mull.]

[Illustration: Fig. 359.--Section at Creag na h-Iolaire, Glen More,
Mull, showing basalts and gabbros resting on and pierced by granophyre.

_a_, much indurated and altered basalts and dolerites; _b_ _b_, gabbro;
_c_, granophyre; _d_ _d_, basalt dykes.]

This strip of granophyre sends abundant apophyses from its mass into
the dark basic rocks around it. Some of the best sections to show the
nature of these offshoots are to be found on the steep hillslope which
mounts from the watershed in Glen More southward into the Creag na
h-Iolaire (Eagle's Crag), and thence up into the great gabbro ridge of
Ben Buy. From the main body of granophyre a multitude of veins ascends
through the basalts and gabbros from two feet or more in breadth down
to mere filaments (Fig. 358). Even at a height of 300 feet up the hill
some of these veins are still three inches broad, and present the
usual granophyric structure, though rather finer in grain than the
general mass of the boss, and sometimes assuming a compact felsitic
and spherulitic texture at the immediate contact with the surrounding
rock. One of the most striking proofs of the posteriority of these
veins is furnished by the perfect flow-structure they not infrequently
exhibit along their margins, their long felspar crystals being arranged
parallel to the walls in lines that follow the sinuosities of the
boundary between the two rocks. Patches of gabbro and of the indurated
basalts may be seen lying on the granophyre, from which veins and
strings ramify through them (Fig. 359). Similar veins can be traced
upward into the main body of coarse gabbro, forming the ridge of Ben
Buy. Some of them are of the usual granular granophyric texture, others
are dull and fine-grained (claystones of the older authors).

Hence it is evident that the granophyres of Mull have been protruded
not only after the accumulation of the plateau-basalts, but after
these were traversed by the sheets and veins of gabbro. The amount
of acid rock injected into these older rocks over the mountainous
part of the island is enormous; but I reserve further reference to it
for the section on acid Dykes and Veins, for these are the forms in
which it chiefly occurs in that region. It should be added, that in
the localities here referred to basalt-veins and dykes are generally
abundant, cutting through all the other rocks (Fig. 359). So numerous
are they that the geologist ceases to take note of them when his
thoughts are engaged upon the problems presented by the masses through
which they rise.

iii. THE ACID BOSSES OF SMALL ISLES

In the island of Eigg three small bosses or sheets of acid rock occur.
That at the northern end rises through the Jurassic sedimentary rocks,
and forms a bold cliff from 150 to 200 feet high. It is a light grey
granophyric porphyry, with rounded blebs of quartz in a micropegmatic
base of quartz and felspar. The other two masses, of smaller size, cut
through the bedded basalts[403] (Map VI.).

[Footnote 403: _Quart. Journ. Geol. Soc._ xxvii. (1871) p. 294.]

In the opposite island of Rum, the acid protrusions play a much more
important part. On the east side of the hills, they occur in sheets at
the base of the gabbros; on the west side, they form a large tract of
hilly ground, which, stretching along the coast-line for about three
and a half miles from the headland of A' Bhrideanach to Harris, forms
there a range of shattered sea-cliffs, that tower for 1000 feet above
the Atlantic breakers that beat about their base. The area extends
inland to the slopes on the west side of Loch Sgathaig, a distance of
about three and a half miles, descending in a range of precipices along
its northern front, and reaching in its culminating summit, Orval, a
height of 1868 feet above the sea. The rocks of which this triangular
area consists resemble those of the Mull bosses. They are chiefly
quartz-porphyries, becoming felsitic in texture towards their contact
with adjacent rocks. In some places, as was noticed by Macculloch on
the sea-cliffs,[404] they have a rudely bedded structure. Thus on the
north-west front of Orval, this structure is shown by parallel planes
that dip outwards or north-west at 30° to 40°, and which are made
still more distinct by an occasional intrusive dyke or sheet of basalt
between their surfaces. I have already alluded to indications of an
internal arrangement in the granitoid bosses of Skye (p. 381).

[Footnote 404: _Western Islands_, vol. i. p. 487.]

[Illustration: Fig. 360.--Section on north side of Orval, Rum.

_a_, Torridon sandstones; _b_, bedded basalts of Fionn Chro; _c_,
dolerite; _d_, quartz-porphyry.]

[Illustration: Fig. 361.--Junction of Quartz-Porphyry (Microgranite)
and Basic Rocks, south-east side of Orval, Rum.

_a_, basalts and dolerites; _b_, dolerite and gabbro veins; _c_,
quartz-porphyry cutting _a_ and _b_.]

As in the other islands, the granophyres, porphyries and felsites
of Rum have been intruded at the base of the volcanic series. Over
much, if not all, of their area they lie directly on the red Torridon
sandstone. That the bedded basalts once covered them is indicated
by the position of the three outliers of the basalt-plateau already
noticed. But a fourth outlier still lies upon the porphyry of Orval
as a cake that dips gently northward. It consists of a bedded, dark,
finely-crystalline, ophitic dolerite, porphyritic in places, with a
rudely prismatic or columnar structure (Fig. 360). It has undergone
contact-metamorphism, and tongues from the underlying rock project
up into it. On the south-eastern side of the same hill, still more
striking evidence is presented of the posteriority of the acid to
the basic rocks. The porphyry shows here the same tendency to assume
a bedded structure, the parallel "beds" again dipping outward or
south-east at 40°. They plunge under the body of gabbro, dolerite and
other intrusive masses which from this point stretch eastward into the
great cones of Allival and its neighbours. The rock at the junction
is a fine microgranite with traces of micropegmatite. It is composed
of a holocrystalline base of quartz and orthoclase, with porphyritic
crystals of microcline, blebs of quartz and scattered granules of
augite. The rocks that rest immediately next it are basalt and
dolerite, into which it has sent an intricate network of veins (Fig.
361).[405] It has also pushed long tongues down the slope into them,
which may be seen traversing the dolerite and gabbro veins that cut
the basalts. The basic rocks next the porphyry have been intensely
altered. They seem in places as if they have been shattered by some
explosive force, and had then been invaded by the mass that rushed into
all the rents thus caused. This remarkable structure is still better
displayed on St. Kilda, and is more fully described in the following
account of the geology of that island.

[Footnote 405: In a thin slice cut from a specimen showing the junction,
there is a minute vein of the porphyry penetrating the basalt which is
much altered, while the porphyry becomes much finer in grain than at a
distance from the contact.]

iv. THE ROCKS OF ST. KILDA

Brief allusions to St. Kilda and its rocks have already been made (pp.
173, 358). We may now enter more fully upon the consideration of its
geological structure and history.

When the weather is clear there may be seen from the western headlands
of the Outer Hebrides a small blue cone rising above the Atlantic
horizon at a distance of about 60 miles. As the voyager approaches
this distant land it gradually shapes itself into a group of islets of
which St. Kilda, the largest and only inhabited, has an extreme length
of about four miles, a breadth of less than two miles, and a height of
1262 feet above the sea. Four miles to the north-east Borrera, about
one square mile in extent, rises with precipitous sides to a height
of 1000 feet. Off the north-western promontory of St. Kilda the huge
rock of Soay, half a square mile in area, towers from 600 to 800 feet
above the waves. Borrera has two attendant rocks--Stack Li and Stack
an Armin--huge pyramidal masses several hundred feet high, and the
home of thousands of gannets. St. Kilda possesses two less imposing
islets between its north-western headland and Soay, and a third to the
south-east known as Levenish.

The scenery of this picturesque group affords a good indication of its
geological structure. It displays two distinct types of topographical
form. In Borrera the marvellous combination of spiry ridges, deep
gullies and clefts, notched crests and splintered pinnacles, at once
reminds the visitor of the outlines of the Cuillin Hills of Skye. The
same features are repeated on a less magnificent scale in Soay and
along the whole of the south-western precipitous coast-line of St.
Kilda.

In marked contrast to these varied outlines, the eastern half of St.
Kilda rises with a smooth green surface, varied with sheets of grey
screes, up to the rounded summit of Conagher, the highest point in the
island. If the dark crags of the rest of the island group remind one of
the Cuillins, this eastern tract recalls at once the form and colour
of the Red Hills of Skye. A closer examination shows that in each case
the topography arises from the influence of the very same rocks and
geological structure as in that island.

There is, however, one aspect in which St. Kilda has no rival
throughout the Western Isles. Its russet-coloured cone, though rising
on the west side with gentle green slopes from the central valley,
plunges on the eastern side in one vast precipice from a height of
1000 feet or more into the surge at its base. Nowhere among the Inner
Hebrides, not even on the south-western side of Rum, is there any such
display of the capacity of the youngest granite to assume the most
rugged and picturesque forms. It is hardly possible to exaggerate the
variety of outline assumed by the rock as it yields along its system
of joints to the influence of a tempestuous climate. It has been carved
into huge projecting buttresses and deep alcoves, the naked stone
glowing with tints of orange and fawn colour, veiled here and there
with patches of bright green slope, or edged with fringes of sea-pink
and camomile. Every outstanding bastion is rent with chasms and split
into blocks, which accumulate on the ledges like piles of ruined
walls. To one who boats underneath these cliffs the scene of ceaseless
destruction which they present is vividly impressive.

The geology of St. Kilda was sketched by Macculloch, who recognized
the close resemblance of its two groups of rock to the "augite-rock"
(gabbro) and "syenite" (granophyre) of Skye and other islands of
the Inner Hebrides. But he left the relations of the two groups to
each other undetermined.[406] Professor Heddle has published a brief
reference to the rocks of St. Kilda, without, however, offering any
definite opinion as to the geological structure of the islands.[407]
The best account of the geology has been given by Mr. Alexander Ross,
who obtained evidence that the acid sends veins into the basic rock.
He brought away specimens clearly showing this relation, but in his
description left the question open for further inquiry.[408] To some of
the observations in these papers reference will be made in the sequel.
The following account is based on the results of two visits paid by
me to St. Kilda in the summers of 1895 and 1896, during which I was
enabled to examine the rocks on land, and to sail several times round
the islands, boating along those parts of the cliffs which presented
features of special geological importance.

[Footnote 406: _Description of the Western Isles_, vol. ii. p. 54.]

[Footnote 407: In an article on the general geological features of
the Outer Hebrides contributed to _A Vertebrate Fauna of the Outer
Hebrides_, by J. A. Harvie-Brown and T. E. Buckley, 1888.]

[Footnote 408: _British Association Report_, 1885, p. 1040, and a much
fuller paper in the _Proceedings of the Inverness Field Club_, vol.
iii. (1884), p. 72.]

In the St. Kilda islets three groups of rock differing from each other
in age may be recognized. 1st, A series of gabbros, dolerites and
basalts which have been intruded through and between each other as
sills; 2nd, a mass of granophyre which invades these sills; and 3rd,
abundant dykes and veins of basalt which occur both in the basic and
acid masses.

From the extension of the basalt-dykes across the Outer Hebrides it is
clear that the Tertiary volcanic region reached at least to within 60
miles of St. Kilda. Whether or not it stretched over the intervening
space now overflowed by the Atlantic must be matter for conjecture.
There can be no doubt that the intrusive rocks of St. Kilda are in
age and origin the equivalents of those of the Inner Hebrides. The
remnants left of them were assuredly not superficial extrusions, but
are characteristic examples of the more deep-seated intrusions of the
Tertiary volcanic period. Down to the most minute details of structure
they reproduce the features so well displayed by the gabbros and
granophyres of Skye, Rum and Mull. If it is demonstrable in the case
of these islands that the intrusions have taken place under a deep
cover of basalt-sheets, now in large part removed, the inference may
legitimately be drawn that at St. Kilda a basalt-plateau once existed
which has been more completely destroyed than in the other regions.
Not a fragment of such a plateau has survived, unless we may perhaps
be allowed to recognize it in some of the basalts enclosed among the
gabbro-sills. Placed far amid the melancholy main and exposed to the
full fury of the Atlantic gales, these islets must be regarded as the
mere fragmentary cores of a once much more extensive volcanic area.
The geologist who visits them is deeply impressed at every turn by the
evidence of the active and unceasing destruction which their cliffs
are undergoing. Nothing now remains save the deep-seated nucleus of
intrusive sills, bosses and dykes.

1. _The Gabbro Sills._--The rudely-bedded arrangement of these rocks is
conspicuous along the west side of St. Kilda, in Soay and in Borrera.
They consist of coarse and fine varieties disposed in successive sheets
which dip at angles varying from as little as 15° up to as much as 60°
or even more. In St. Kilda they form the picturesque promontory of
the Dune, and extend thence along the western side of the island to
its extreme northern end. Their escarpments face the ocean, and their
dip-slopes descend towards the north-east in grassy declivities to the
south bay and the long verdant glen which runs thence across to the
north bay. The same strike is prolonged into Soay, but further east in
Borrera the direction curves so as to present vast escarpments towards
the west and shelving sheets of rock towards the east.

None of the gabbros seen by me are as coarse as the large-grained
varieties of Skye, nor does there appear ever to be such a marked
banded structure among them as that displayed by the Cuillin rocks.
Faint banding, however, may be noticed. A series of specimens which