THE ERUPTIVE VENTS OF THE BASALT-PLATEAUX
Vents filled with Basalt or other Lava-form Rock--Vents filled
with Agglomerate
It is one of the most interesting points in the Tertiary volcanic
history that, in spite of the enormous geological revolutions that
have passed since they became extinct, the sites of many scattered
vents can still be recognized. A far greater number must lie buried
under the basalts, and of others the positions are concealed by
the sea, which now covers so large an area of the old lava-fields.
Nevertheless, partly within the area of the plateaux, but still more
on the surrounding tracts from which the basalts have been removed
by denudation, the traces of unmistakable vents of discharge may be
recognized amid the general wreck.
In Britain and the Faroe Isles, it is chiefly along the coast-line
that the process of denudation has revealed the volcanic vents of
Tertiary time. The interior of the country is often loaded with peat,
covered with herbage, or strewn with glacial detritus: and even where
indications of the vents are to be detected, it is not always possible
to ascertain their true limits and connections. But where the structure
of the plateaux has been laid bare along ranges of rocky precipice, the
vents have sometimes been so admirably dissected by the sea that every
feature of their arrangements can be satisfactorily determined.
As the actual physical connexion of these volcanic orifices with the
plateaux has been in most cases removed by denudation, we can usually
only by inference place them in what was probably their true relation
to the plateau-eruptions. Those which project from the surface of the
plateaux must, of course, be younger than the basalts through which
they rise; how much younger we cannot tell. They may possibly be later
than any of the plateau-sheets; they may even belong to a subsequent
and waning condition of volcanic action. On the other hand, the vents
which can now be traced outside of the present limits of the edges
of the plateaux may, like those just mentioned, be younger than the
basalt-sheets, or, on the contrary, they may be records of a period
of eruptivity anterior to the emission of any of the rocks of the
plateaux, and may have been deeply buried under a mass of basalt-beds
subsequently removed. Positive demonstration is, from the nature of
the case, impossible in these instances. But examples will be cited
from the Western Isles and from Faroe, where the vents can be proved
to belong to the time of the plateau-eruptions, for they are seen to
have broken through some of the basalt-sheets and to have been buried
under others. With this clear evidence of relationship in some cases,
there need be little hesitation in believing that in other instances
where no such positive connexion can be found, but where the vents are
obviously such as the general structure of the plateaux would have led
us to expect, they may be confidently regarded as part of the phenomena
of the plateau-eruptions.
Sometimes the vents can be linked with lines of fissures or dykes. This
is especially the case where they are small in size. More usually,
however, no such relation can be demonstrated. It will be remembered
that among the modern Icelandic eruptions, some eruptive vents, like
the later cinder-cones of Laki, are ranged in a linear direction
along the great fissure, while others, of an older series in the same
district, almost engulphed amidst the more recent lavas, are clustered
irregularly in groups. A similar diversity of arrangement has been
observed among the volcanic cones of the Velay in Central France.
Considering as a whole the volcanic necks or eruptive vents which rise
from the older rocks around the Tertiary basalt-plateaux, and sometimes
even from the surface of these plateaux themselves, we may conveniently
follow the same classification as was adopted in dealing with those
of Palæozoic age, and, according to the nature of the material that
now fills them, arrange them in two series: (1) Those occupied by some
form of crystalline eruptive rock, and (2) those filled with volcanic
agglomerate.
i. VENTS FILLED WITH DOLERITE, BASALT, ETC.
These, as the composition of the plateaux would lead us to anticipate,
are numerous. They perhaps attain their most conspicuous development
in Antrim, either on the tableland or among the underlying rocks
round its edges. The finest example in that district is undoubtedly
furnished by the lofty eminence called Slemish, which rises above the
surrounding basalt-terrace, to a height of 1437 feet above the sea
(Fig. 294). It is elliptical in ground-plan, measuring some 4000 feet
in length by 1000 in breadth. Seen from the north, it appears as a
nearly perfect cone. The material of which it consists is a coarsely
crystalline olivine-dolerite, presenting under the microscope a nearly
holocrystalline aggregate, in which the lath-shaped felspars penetrate
the augite, with abundant fresh olivine, and wedge-shaped patches
of interstitial matter. The rock is massive and amorphous, except
that it is divided by parallel joints into large quadrangular blocks
like a granitic rock, and wholly different from the character of the
surrounding basalts. The latter, which possess the ordinary characters
of the rocks of the plateaux, can be followed to within 80 yards of
this neck, which rises steeply from them, but their actual junction
with it is concealed under the depth of talus.
[Illustration: Fig. 294.--Slemish, a Volcanic Neck or Vent on the
Antrim Plateau, seen from the north.]
[Illustration: Fig. 295.--Section of Volcanic Vent at Carnmony Hill (E.
Hull).
T, Lower basalt; C, Cretaceous strata; L, Lower Lias; M, Triassic
marls; V, Vent.]
At the nearest point to which the two rocks are traceable, the basalts
appear somewhat indurated, break with a peculiar splintery fracture,
and weather with a white crust. These characters are still better shown
on abundant fragments which may be picked up among the debris further
up the slope. There can be no doubt, I think, that a ring of flinty
basalt, differing considerably in texture from the usual aspect of that
rock in the district, surrounds the neck. The meaning of this ring
will be more clearly seen from the description of another example in
Mull. About four miles to the north-east of Slemish, a smaller and less
conspicuous neck rises out of the plateau-basalts. The rock of which
it consists is less coarsely crystalline than that of Slemish, but its
relations to the surrounding volcanic rocks are obviously the same. On
the west side of Belfast Lough a boss of similar rock, about 1200 feet
in diameter, rises at the very edge of the basalt escarpment into the
eminence known as Carnmony Hill (Fig. 295). On its northern side it
presents along its wall a mass of interposed volcanic agglomerate.[295]
On visiting with Mr. M'Henry the quarry opened on the eastern face of
this vent, I was much struck with the remarkable cellular structure
of some parts of the dolerite. Many of the vesicles are lined with a
thin pellicle of black glass, and the same substance occurs in minute
patches in the body of the rock. A thin slice exhibiting this structure
was found by Mr. Watts to possess the following characters:--"The
rock is an ophitic dolerite consisting of plagioclase, augite, and
iron ores, without olivine, enclosing one or two patches of finer
basalt. The vesicles in the latter, and certain angular spaces between
the crystals of the former, have been wholly or partially filled with
brown glass, the outer part of which has been converted into radiating
crystals of a brown mineral." The occurrence of patches of glass which
seem to have been squeezed into vesicles or cracks in the body of a
dolerite or andesite has been noticed in some of the Tertiary dykes.
But in the present case the glass occurs as a mere coating on the walls
of the larger spheroidal vesicles, the interior of which generally
remains empty.
[Footnote 295: This neck was recognised by Du Noyer in 1868 as "one of
the great pipes or feeders of the basaltic flows." See Prof. Hull,
Explanation of Sheets 21, 28 and 29, _Geol. Survey of Ireland_ (1876),
p. 30.]
Of the other doleritic necks scattered over the surface of the Antrim
plateau, I will refer to only one which occurs on the hillslopes
between Glenarm and Larne. It forms a prominence known as the Scawt
Hill, and consists of a boss of basalt, which, in rising through a vent
in the plateau-sheets, has carried up with it and converted into marble
a large mass of chalk which is now exposed along its eastern wall (Fig.
296).
[Illustration: Fig. 296.--Section of the east side of Scawt Hill, near
Glenarm.
_a_, bedded basalt; _b_, mass of chalk; _c_, basalt neck.]
[Illustration: Fig. 297.--Section of Neck of Basalt, Bendoo, Ballintoy.
_a_ _a_, Chalk; _b_, neck.]
As examples of similar necks which have been exposed by denudation
outside the present limits of the same plateau, I may allude to those
which rise through the Cretaceous and other Secondary strata on the
northern coast near Ballintoy. One of the most striking of these may
be seen at Bendoo, where a plug of basalt, measuring about 1400 feet
in one diameter and 800 feet in another, rises through the Chalk, and
alters it around the line of contact (Fig. 297). Another remarkably
picturesque example is to be seen near Cushendall, where a prominent
doleritic cone rises out of the platform of Old Red Sandstone, some
distance to the north of the present edge of the volcanic escarpment
(Fig. 298).
[Illustration: Fig. 298.--Volcanic Neck of Dolerite near Cushendall.]
The greater coarseness of grain of the material filling these pipes,
compared with that of the sheets in the terraces, is only what the very
different conditions of cooling and consolidation would lead us to
expect. There is no essential difference of composition between the two
rocks. Where the erupted material has been poured out at the surface,
it has assumed a finely crystalline texture, while, where it has slowly
solidified within a volcanic pipe at some depth beneath the surface,
and where consequently its component crystals have had more time for
development, the resulting structure is much more largely crystalline,
with a more or less complete development of the ophitic structure.
[Illustration: Fig. 299.--Section of Volcanic Neck at 'S Airde Beinne,
near Tobermory, Mull.
_a_ _a_, bedded basalts; _b_ _b_, bedded basalts altered along the side
of vent; _c_ _c_, dolerite.]
In the island of Mull, another instance of the same kind of vent has
been observed and described by Professor Judd.[296] It rises in the
conspicuous hill, 'S Airde Beinne (Sarta Beinn), about two miles
south-west from Tobermory, and consists of a coarsely crystalline
dolerite, which becomes finer in grain towards the outer margin
(Fig. 299). No bedding, or structure of any kind beyond jointing, is
perceptible in it. Examined in thin sections under the microscope,
this rock is found to be another typical ophitic dolerite, consisting
of lath-shaped felspars embedded in augite, with here and there
wedge-shaped portions of interstitial matter and grains of olivine. Dr.
Hatch found the felspars to contain spherical inclusions of devitrified
glass, filled with black granules and trichites, and he observed that,
under a high power, the interstitial matter is seen to consist mainly
of a greenish-brown isotropic substance, in which are inclosed small
crystals of augite, skeleton-forms and microlites of felspar, sometimes
in stellate aggregates, as well as club-shaped, cruciform, arrow-headed
and often crested microlites of magnetite.
[Footnote 296: _Quart. Jour. Geol. Soc._, xxx. (1874), p. 264.]
[Illustration: Fig. 300.--Interior of the Volcanic Neck of 'S Airde
Beinne, near Tobermory, Mull.]
Towering prominently above the flat basalt sheets, this neck has an
oval form, measuring about half a mile in length by a quarter of a
mile in breadth. Its central portion, however, instead of rising into
a rugged hill-top, as is usually the case, sinks into a deep hollow,
which is filled with water, and reminds one of a true crater-lake
(Figs. 299, 300). The middle of the neck is thus concealed from view,
and we can only examine the hard prominent ring of dolerite that
surrounds the tarn. The material occupying the hollow may be softer
than that of the ring, and may have been scooped out by denudation.
What we now see may not be the original surface, but may have been
exposed after the removal of possibly hundreds of feet of overlying
material. On the other hand, it is conceivable that the hollow is
really a crater-lake which was filled up with detritus and may have
been overspread with basalt, since removed. It may be suggestively
compared with the crater-hollows revealed by denudation on the cliffs
of Stromö and Portree Harbour, which will be described in a later part
of this chapter. Possibly some more easily removable agglomerate,
representing an eruption later than that of the dolerite, may occupy
the centre of the volcanic pipe.
One of the most interesting features of this vent is to be found in
its relation to the surrounding basalts. The marginal parts of the
rock along the line of contact are much finer in grain than the rest,
and have obviously cooled more rapidly. The contrast between them and
the ordinary dolerite nearer the centre, however, cannot be properly
understood, except in thin sections under the microscope. Dr. Hatch, to
whom I submitted my specimens, observed that, in place of the structure
above described, the marginal parts show an absence of the ophitic
grouping except in small isolated patches. Instead of occurring in
large grains or plates enveloping the felspars, the augite is found in
numerous small roundish grains, together with grains of magnetite, in
equal abundance and of similar size. The felspars are speckled over
with opaque particles; olivine has not been detected.
For miles around the vent, the plateau-rocks are of the usual
type--black, compact, sometimes amygdaloidal, alternating with more
coarsely crystalline decomposing bands, the separation between
different sheets being often marked by the ordinary red ferruginous
partings. But around the margin of the neck, they have undergone a
remarkable metamorphism. The portions of them which adhere to the outer
wall of the neck have lost their distinct bedding, and have been, as
it were, welded together into an indurated compact, black to dull-grey
rock, so shattery and jointed that fresh hand-specimens, three or four
inches in length, are not easily obtainable. Especially marked is one
set of joints which, running approximately parallel, cause the rock to
split into plates or slabs. These joints are sometimes curved. Yet, in
spite of the alteration from its normal character, the basalt retains
in places some of its more usual external features, such, for instance,
as its amygdaloidal structure, the amygdales consisting of calcite,
finely acicular mesotype, and other minerals.
Examined under the microscope, this altered basalt presents "a confused
aggregate of colourless microlites (felspar?) and innumerable minute
granules of magnetite, these two constituents being very unequally
distributed. Sometimes the colourless portions preponderate, in other
places the opaque granules are heaped together in black patches, which
may possibly mark the position of fused augites."[297]
[Footnote 297: Notes by Dr. Hatch.]
In the zone of contact-metamorphism around some of the volcanic pipes
in the plateaux, we see changes analogous to, but less developed than,
those which have been superinduced on so large a scale round the great
eruptive bosses of gabbro, granophyre, etc., that have broken up the
terraced basalts along the west coast of Scotland. I shall accordingly
return to this subject in connection with phenomena presented by these
younger rocks (p. 386).
ii. VENTS FILLED WITH AGGLOMERATE
While the necks of dolerite or basalt cannot always be satisfactorily
discriminated from bosses which may never have established a connection
with the surface, there is no room for any doubt in this respect in
the case of those filled with fragmentary materials. As has been
already pointed out, the occurrence of true volcanic agglomerate
may be accepted as evidence of the existence of an eruptive vent
communicating with the surface of the earth. The agglomerate in the
vents associated with the basalt-plateaux, like that of the Palæozoic
vents, is generally exceedingly coarse, and without any trace of
structure. Blocks of all sizes up to masses some yards in length, and
of the most diversified materials, both volcanic and non-volcanic, are
dispersed confusedly through a granular paste of similar miscellaneous
composition.
[Illustration: Fig. 301.--Diagram to show the probable relation of the
Neck at Carrick-a-raide, Antrim, to an adjacent group of tuffs.
_a_ _a_, Chalk; _b_ _b_, lower group of bedded basalts; _c_, vent
of Carrick-a-raide, filled with coarse volcanic agglomerate; _d_
_d_, bedded tuffs; _e_ _e_, large veins of basalt traversing the
agglomerate; _f_ _f_, zone of tuffs and pisolitic iron ore; _g_ _g_,
upper group of bedded basalts.]
An instructive example of the general characteristics of
agglomerate-vents, and of the relation of these vents to the
surrounding tuffs and basalts, is to be found at the island of
Carrick-a-raide, on the north coast of Antrim, and on the opposite
mainland. The visible mass of this neck is about 1000 feet in diameter,
but the boundaries, except on the land side, are concealed by the sea.
The material filling up the vent is a coarse agglomerate, in which
blocks and bombs of basalt, with pieces of chalk and flint, are stuck
at all angles in a dull dirty-green granular tuff. Some large and small
intrusions of basalt rise through it. Owing partly to these intrusions,
and partly to the grass-covered slope that separates it from the line
of cliff, the actual contact of this neck with the volcanic beds of the
escarpment cannot be seen. I have no doubt, however, that the tuff,
which has already been referred to as so conspicuous a member of the
series here, was discharged from this vent.[298] The materials are as
usual coarser in the pipe than beyond it, but the finer portion or
matrix of the agglomerate is similar to many bands of the tuff. The
structure of the locality may be diagrammatically represented as in
Fig. 301. The bedded tuff is thickest in the neighbourhood of the vent,
and gradually dies away on either side of it.
[Footnote 298: See Explanation of Sheets 7 and 8, _Geol. Survey of
Ireland_ (1888), p. 31.]
But another important inference may be drawn from this locality. I have
already pointed out that the lower basalts here reach their minimum
thickness. Their basement beds thin away towards the vent as markedly
as the tuff thickens. Obviously they cannot have proceeded from that
point of eruption. Yet, that they had begun to be poured out before
the discharge of the tuff is shown by their underlying as well as
overlying that rock, though westward, owing to the thinning away of
the undermost basalts, the tuff comes to lie directly on the Chalk.
Hence, we may legitimately infer that in this neighbourhood one or more
other vents supplied the sheets of the lower basalts.
In the island of Mull a number of detached bosses or patches of
agglomerate much obscured by invasions of granophyre probably mark
the sites of volcanic vents. They will be more particularly noticed
in Chapter xlvii. One of their most interesting features is the large
number of fragments of felsitic or rhyolitic rocks which they contain.
In the promontory of Ardnamurchan, where the basalt-plateau has been
invaded and displaced by later intrusions of crystalline rocks, and has
likewise been reduced to such a fragmentary condition by denudation,
some interesting examples of agglomerate necks have been laid bare.
One of the largest of these occurs on the north shore at Faskadale.
Cut open by the sea for more than a quarter of a mile, this neck
is seen to be filled with a coarse agglomerate, composed mainly of
basalt-blocks and debris, but crowded also with angular and subangular
pieces of different close-grained andesitic, felsitic and porphyritic
rocks belonging to the acid series to be afterwards described.[299] Some
of these stones exhibit a very perfect flow-structure, and closely
resemble certain fine-grained, flinty, intrusive rocks in Mull, to
which allusion will subsequently be made. The matrix of the agglomerate
is of the usual dull dirty-green colour, but is so intensely indurated
that on a fresh fracture it can hardly be distinguished from some of
the crystalline rocks of the locality. The neck is pierced in all
directions with dykes and veins of basalt, dolerite, andesite, gabbro,
and felsitic rocks. Similar intrusions continue and increase in numbers
farther west until the cliffs become a labyrinth of dykes and veins
running through a mass of rocks which appears to consist mainly of dull
dolerites and fine gabbros. Though the relations of this vent to the
plateau-basalts are not quite plain, the agglomerate seemed to me to
rise out of these rocks. At least the basalts extend from Achateny to
Faskadale, but, as they are followed westwards, they are more and more
invaded by eruptive sheets, and assume the indurated character to which
I have already referred.
[Footnote 299: One of these felsites when viewed under a high magnifying
power is seen to present an abundant development of exceedingly minute
micropegmatite arranged in patches and streaks parallel with the
lines of flow-structure in the general cryptocrystalline groundmass.
The close relationship between the felsites, quartz-porphyries, and
granophyres will be afterwards pointed out in the description of the
acid rocks. It is remarkable that, though these rocks occur abundantly
in fragments in the volcanic necks and agglomerates of the plateaux,
not a single instance has been observed of their intercalation as
contemporaneous sheets among the basic lavas. The analogous case of the
interstratification of felsitic tuffs among basic lavas in the volcanic
series of the Old Red Sandstone of Central Scotland has been described
(vol. i. p. 279). It is interesting to note that liparitic pumice and
dykes have been erupted by some of the basaltic craters of Iceland, for
example at Askja, Öræfajökull and Snaefellsjökull. (Mr. Thoroddsen,
_Dansk. Geograf. Tidsskrift_, vol. xiii. 7th and 8th parts.)]
On the south side of the peninsula of Ardnamurchan, another
agglomerate, noticed by Professor Judd,[300] rises into the bold headland
of Maclean's Nose, at the mouth of Loch Sunart, and affords better
evidence of its relation to the bedded basalts. It measures about
1000 yards in length by 300 in breadth, and its summit rises more
that 900 feet above the sea, which washes the base of its southern
front. It is filled with an agglomerate even coarser than that on
the northern coast. The blocks are of all sizes, up to eight or ten
feet in diameter. By far the largest proportion of them consists of
varieties of basalt and andesite, slaggy and vesicular structures
being especially conspicuous. There are also large blocks of different
andesitic porphyries and felsitic rocks like those just referred to,
a porphyry with felspar crystals two inches long being particularly
abundant. All the stones are more or less rounded, and are wrapped
up in a dull-green compact matrix of basalt-debris. There is no
stratification or structure of any kind in the mass. Numerous dykes
or veins of basalt, of andesite, and of a porphyry, resembling that
of Craignure, in Mull, traverse the agglomerate. Some of the narrow
basalt-dykes cut through the others.
[Footnote 300: _Quart. Journ. Geol. Soc._ xxx. (1874), p. 261. Professor
Judd has subsequently (_op. cit._ xlvi. 1890, pp. 374 _et seq._) given
a map, section and description of what he believes to be the structure
of this ground, with numerous details as to the petrography of the
rocks. The geological structure of this area is more fully referred to
on pp. 318 _et seq._]
[Illustration: Fig. 302.--Section of agglomerate Neck at Maclean's
Nose, Ardnamurchan.
_a_ _a_, quartzites and schists; _b_, bedded basalts lying partly
on the schists and partly on patches of Jurassic sandstones that
occupy hollows of the older crystalline rocks; _c_, agglomerate;
_d_ _d_, dykes and veins traversing the agglomerate; _e_, dolerite
sheets of Ben Hiant.
]
The position of the vent, with reference to the surrounding rocks, will
be understood from the accompanying section (Fig. 302). On the eastern
side, the agglomerate can be seen to abut against the truncated ends
of the flat beds of the plateau-basalts, which are of the usual bedded
compact and amygdaloidal character. There can be no doubt, therefore,
that the vent has been opened through these basalts. But it will be
observed that the latter belong to the lower part of the volcanic
series. These lowest sheets are exposed on the slope, resting upon
yellowish and spotted grey sandstone, with seams of jet and a reddish
breccia, which, lying in hollows of the quartzites, quartz-schists, and
mica-schists, form no doubt the local base of the Jurassic rocks of
the district. Hence, the vent, though younger than the older sheets of
the plateau, may quite well be contemporaneous with some of the later
sheets.[301]
[Footnote 301: It may here be remarked that there is evidence of great
differences in the level of the base of the Jurassic series and
the bottom of the volcanic plateau in this district. On the south
and west sides of Ben Hiant the Jurassic conglomerates may be seen
lying on the edges of the crystalline schists only a little above
high-water mark, while on the north side, the schists, with their
overlying unconformable cake of limestones, rise several hundred feet
above sea-level. The surface on which the basalts were poured out was
probably very uneven, but there may also have been some considerable
displacements of these basalts either before or during the injection of
the dolerite sills of Ben Hiant.]
An interesting feature at this locality is the peculiar grouping of
some of the large dykes in the area around the agglomerate. They run
in the direction of the vent, and one or other of them may represent
the fissure or fissures on which the volcanic orifice was blown open
to the surface. Another notable element in the geological structure
of the ground is the vast amount of intrusive material, both in dykes
and sheets, which has been erupted. The intrusive sheets of Ben
Hiant form the most prominent eminence in this part of Ardnamurchan.
Reserving them for description in the following Chapter (p. 318), I
will only remark here that they partly overlie the agglomerate, and
are therefore, to some extent at least, younger than the vent. They
belong to that late stage in the history of the basalt-plateaux when
the molten material, no longer getting ready egress to the surface,
forced its way among the rocks about the base of the bedded basalts,
and more especially on the sites of older vents, which were doubtless
weak places, where it could more easily find relief.
The large neck now described is only one of a group scattered around
it in the ground to the north. Two of these may be seen rising through
a detached area of Jurassic limestones and shales at the northern base
of Ben Hiant. A third, almost obliterated by the intrusive sheets, may
be traced at the western end of that mountain above Coiremhuilinn.
Two others rising through the schists on either side of Beinn na
h-Urchrach, have been much invaded by the sills of that eminence
(Fig. 326). It is doubtless owing to the extensive denudation of the
basalt-plateau, and the consequent uncovering of the rocks underneath
it, that this series of vents has been laid bare.[302]
[Footnote 302: Professor Judd has united these scattered vents into a
continuous platform of volcanic agglomerates, which he represents as
underlying the supposed lavas of Ben Hiant. Since the publication
of his map and description, I have re-examined the ground without
being able to discover any trace of this platform. All the visible
agglomerates are separate necks, their actual walls being sometimes
exposed, as in the neck immediately north of the base of Ben Hiant,
where the limestone in contact is marmorised, though twelve yards of it
is an ordinary dull blue rock.]
By far the largest mass of agglomerate in any of the Tertiary volcanic
areas of Britain is that which occurs on the north side of the main
valley of Strath, in Skye.[303] Unfortunately, it has been so seriously
invaded by the eruptive rocks of the Red Hills, that its original
dimensions and its relations to the surrounding rocks, especially
to the bedded basalts, are much obscured (see Fig. 348). It can be
followed continuously from the lower end of Loch Kilchrist along the
southern slopes of Beinn Dearg Bheag round to the western roots of
Beinn Dearg Mhor--a distance of more than two miles in a straight
line, and from Kilbride to the flank of Beinn na Caillich above
Coire-chat-achan--a direct distance of two miles and a quarter. A
similar rock, possibly a portion of the same mass, appears in Creagan
Dubha, on the north side of the Red Hills. If the whole of this
agglomerate forms part of one originally continuous mass, it must have
been upwards of two miles in diameter. There may, however, have been
two or three closely adjacent vents. The Beinn na Caillich patch, for
example, appears to belong to a different area, and that of Creagan
Dubha is also probably distinct. But there seems no reason to doubt
that the mass which forms Cnoc nam Fitheach, and all the long declivity
on the southern flank of Beinn Dearg Bheag, occupies part of the site
of a single volcano. Owing to the absence of sufficient sections, it
is hardly possible to determine how much of this fragmentary material
should be assigned to the actual chimney. The diameter of the whole
mass is almost two miles. But possibly a considerable proportion of
this accumulation belongs to the external cone which gathered round
the vent, so that the eruptive pipe might thus be of much smaller
dimensions than the superficial area of the agglomerate. The subsequent
invasion of so much granophyre, not only that of the Red Hills, but
that of numerous smaller intrusions, has indurated the agglomerate and
made the investigation of its structure somewhat unsatisfactory.
[Footnote 303: This extensive mass was not separated from the "syenite"
of the Red Hills by Macculloch. Von Oeynhausen and Von Dechen noticed
it as a conglomerate with quartz pebbles, but did not realise its
volcanic nature (_Karsten's Archiv_, i. p. 90). In my map of Strath
(_Quart. Jour. Geol. Soc._ xiv. plate i.) I distinguished it from the
rock of the Red Hills, but no name for it appears in the legend of
the map, nor is it referred to in the text. Its character as a true
volcanic agglomerate was recognised by Professor Judd, _op. cit._ p.
255. See _postea_, pp. 384 _et seq._]
It might be supposed that the mere existence of intrusive bosses and
veins rather furnishes an argument in favour of considering the visible
agglomerate to belong to a deeper-seated part of the erupted material
than the external cone. But, as will be afterwards shown, there is
some reason to regard the present conical or dome-shaped outlines of
the granophyre hills as not far from their original forms, and to
believe that, like the trachytic Puys of Auvergne, they were much more
superficial than plutonic eruptions. A study of the cinder cones of
Central France shows that even these superficial accumulations have
been invaded not only by bosses but by dykes.[304]
[Footnote 304: The existence of a small dyke of andesite on the northern
rim of the well-known crater of the Puy Parion has already been
noticed.]
The agglomerate of the great Strath vent is a coarse tumultuous
assemblage of blocks and bombs, imbedded in the usual dull, dirty-green
matrix. Among the stones, grit and sandstone, together with
scoriaceous, vesicular and amygdaloidal basalts are specially abundant;
also pieces of various quartz-porphyries and granophyres, among which
a black felsite like that of Mull may often be recognised. In some
places, large masses of altered limestone and quartzite (Cambrian)
are included; in others, pieces of yellow sandstone and dark shale
(Jurassic), or of the bedded lavas. Some of these masses may be 100
yards or more in length. Occasionally a breccia, mainly made up of acid
materials--granophyre or granite,--has been noticed by Mr. Harker along
the north side of the Red Hills, which he thinks may rather be of the
nature of a crush-breccia than a part of the true agglomerate.
The agglomerate of this district is wholly without stratification
or structure of any kind. On the north-west side of Loch Kilchrist,
indeed, it weathers into large tabular forms, the parallel surfaces
of which dip to south-west; but this is probably due only to
jointing. Here and there, dykes of basalt cut the rock in a general
north-westerly direction, but their number is remarkably small when
compared with the prodigious quantity of them in the limestone at the
bottom and opposite side of the valley, some of which may possibly mark
the fissure on which the vent was placed. More abundant and extensive
are the masses of granophyre that rise particularly along the outer
margin of the agglomerate near Loch Kilchrist. These may be connected
with the great boss that forms the Red Hills, of which further details
will be given in Chapter xlvi.[305]
[Footnote 305: The granophyre intrusions in this agglomerate have been
found by Mr. Harker to have taken up and dissolved a considerable
proportion of fragments of gabbro, Chapter xlvi. p. 392.]
The important question of the relation of this agglomerate to the
plateau-basalts does not admit of satisfactory treatment, owing to
destruction of the evidence by the intrusion of the granophyre, and
likewise to enormous denudation. Nevertheless, some traces still remain
to indicate that the basalts once stretched over the site of the vent,
which probably rose through them. Looking westward from the Hanks of
Beinn Dearg Bheag to the other side of Loch Slapin, the geologist sees
the bold basalt-escarpment of Strathaird presenting its truncated beds
to him at a distance of only two miles. That these lavas were once
prolonged eastwards beyond their present limits is obvious, and that
they stretched at least over these two intervening miles can hardly
be doubted. But we can still detect relics of them on the flanks of
Beinn Dearg. As we follow the agglomerate round the margin of the
granophyre that mounts steeply from it, we lose it here and there under
beds of amygdaloidal basalt. The rocks next the great eruptive mass of
the mountain are so indurated and shattered that it is difficult to
separate them from each other and determine their relative positions.
But, so far as I could ascertain, these basalts are fragments of beds
that overlie the agglomerate (Fig. 303). This is not the only place
along the flanks of the Red Hills where portions of the bedded basalts
have survived. Other localities will be subsequently alluded to.
[Illustration:
Fig. 303.--Diagram to show the probable relations of the rocks on
the southern flank of Beinn Dearg Bheag.
_a_, agglomerate; _b_, amygdaloidal and compact basalt-rocks; _c_,
granophyre.
]
The Strath vent has been drilled through the Cambrian limestone, and
as the result of protracted denudation it now towers steeply 500 or
600 feet above that formation on the floor of the valley. Of the
material discharged from it over the surrounding country no certain
trace now remains. We may infer from the nature of the rock which fills
it that towards the end, if not from the beginning of its activity,
its discharges consisted mainly of dust and stones. A cone, of which
the remains are two miles in diameter, must surely have sent its
fragmentary materials far and wide over the surrounding region. But on
the bare platform of older rocks to the south, beyond the bottom of
the agglomerate declivities, not a vestige of these erupted materials
can now be found. Westward the escarpment of Strathaird remains to
assure us that no thick showers of ashes fell at even so short a
distance as two miles, either before or during the outpouring of the
successive basalt sheets still remaining there. We may therefore
conclude with some confidence that here, as at Ardnamurchan, the
vent is younger than at least the older parts of the basalt-plateau.
Unfortunately the uprise of the large bosses of granophyre that stretch
from the Red Hills to Loch Sligachan has entirely destroyed the vent
and its connections in that direction. There is no certain proof that
any molten rock ever issued from this orifice, unless we suppose the
fragmentary patches of amygdaloid on the southern flank of Beinn Dearg
Bheag to be portions of flows that proceeded from this centre of
eruption. The basalt-plateau which still remains in Strathaird no doubt
formerly extended eastwards over Strath and northwards across the site
of the Red Hills and Cuillins, joining on to the continuous tableland
north of Lochs Brittle and Sligachan. How much of the plateau had been
built up here before the outburst of the vent cannot be ascertained.
The agglomerate may possibly, of course, belong to the very latest
period of the plateau-eruptions, or even to a still younger phase of
Tertiary volcanic history. The impression, however, made on my mind by
a study of the evidence from the Western and Faroe Isles is that the
necks of agglomerate, like those of dolerite and basalt, really belong
to different epochs of the plateau period itself; and mark some of
the vents from which the materials of the plateaux were successively
emitted.
The example of Carrick-a-raide (p. 277) is peculiarly suggestive when
we regard it in connexion with the great Strath vent. Already the
progress of denudation has removed at least half of the layer of dust
and stones which, thrown out from that little orifice, fell over the
bare chalk-wolds and black basalt-fields of Antrim. The neck that
marks the position of the volcanic funnel has been largely cut away
by the waves, and is almost entirely isolated among them. The vents
at Canna, Portree and the Faroe Isles, to be afterwards described,
unquestionably belong to the eruptions of the plateau-period, for their
connection with the basalts can be clearly established. At the Strath
vent, however, the march of destruction has been greater. The connexion
between this vent and the materials ejected from it has been entirely
removed, and we can only guess from the size of the remaining neck what
may have been the area covered by the discharges from this largest of
all the volcanic cones of the Inner Hebrides.
Other masses of similar agglomerate are observable in the same region
of Skye, where they not improbably mark the sites of other vents.
Unfortunately their original limits and relations to the rocks through
which the eruptive orifices were drilled have been much obscured by
the uprise of the great masses of gabbro and granophyre of the Cuillin
Hills. Several of these isolated intrusions occur in the midst of the
gabbro, as in Harta Corry and on the west side of the Blaven ridge.
Another mass is interposed between the gabbro and granophyre on Druim
an Eidhne and at the base of the lavas between Druim an Eidhne and
the Camasunary valley. Mr. Harker has found a huge mass of agglomerate
underlying the bedded basalts to the north and west of Belig, one of
the hills on the west side of the large valley that runs from the head
of Loch Slapin to Loch Aynort. This mass has its bottom concealed by
the granophyre which underlies it; but it reaches a maximum thickness
of perhaps 1000 feet, rapidly thinning out and disappearing. It
generally resembles the Strath agglomerate, but is distinguished by
including a large proportion of fragments of gabbro. Mr. Harker remarks
that "a study of these agglomerates points to the existence of both
gabbros and granophyres older than the volcanic series, and therefore
distinct from the gabbros and granophyres now exposed at the surface."
It is a suggestive fact that so many detached masses of agglomerate
should occur around and within the areas of the great eruptive bosses
of gabbro and granophyre. They seem to indicate the former existence
of groups of volcanic vents in these tracts, and may thus account for
the uprise of such large bodies of intrusive material through what must
have been a weakened part of the terrestrial crust.
Further north in Skye a much smaller but more perfectly preserved
vent has been laid open by denudation on the south side of Portree
Bay--a deep inlet which has been cut out of the plateau-basalts and
their underlying platform of Jurassic sandstones and shales. The great
escarpment of the basalts has, at the recess of Camas Garbh, been
trenched by a small rivulet, aided by the presence of two dykes. The
gully thus formed exposes a section of a neck of agglomerate that
underlies the basalts of the upper half of the cliff. This neck is
connected with a thick deposit of volcanic conglomerate and tuff which,
lying between the basalts, extends from the neck to a considerable
distance on either hand. The general relations of the rocks at this
locality are represented in Fig. 304.
[Illustration: Fig. 304.--Section of Volcanic Vent and connected lavas
and tuffs, Scorr, Camas Garbh, Portree Bay, Skye.
_a_, Rudely-bedded dull green tuff; _b_, coarse agglomerate;
_c_, prismatic basalt; _d_, massive jointed basalt; _e_, red
banded decomposing rock, probably of detrital origin; _f_,
plateau-basalts, prismatic and rudely columnar; _g_, dyke of
dolerite, somewhat vesicular, five to six feet broad; _h_, basalt
dyke two to three feet broad; _i_, dyke or sill of similar basalt
to _h_, and possibly connected with it.
]
The agglomerate (_b_) is quite tumultuous, and here and there
strikingly coarse. Some of its included blocks measure five feet in
length. These fragments represent most of the varieties of the lavas
of the district. Large slaggy masses are abundant among them, and
sometimes exhibit the annelide-like elongation of the vesicles which
I have referred to as occasionally displayed by the plateau-basalts.
More than 60 feet of agglomerate are visible in vertical height from
where its base is concealed by debris and vegetation to where its
upper surface passes under a banded rock to be afterwards described.
That this unstratified mass of volcanic detritus marks the site of a
vent can hardly be doubted, although denudation has not revealed the
actual walls of the chimney. The steep grassy slopes do not permit
the relations of the rocks to be everywhere seen, but the agglomerate
appears to pass laterally into finer, rudely-stratified material of a
similar kind, which extends towards east and west as a thick deposit
between the bedded basalts. Possibly denudation has only advanced far
enough to lay bare the crater and its surrounding sheets of fragmentary
material, while the chimney lies still buried underneath.
To the east or left of the agglomerate the detritus becomes less
coarse, and shows increasing indications of a bedded arrangement. Close
to the agglomerate the dip of the coarse tuff is towards that rock at
about 10°. A few yards further east a sheet of very slaggy basalt is
seen to lie against the tuff, which it does not pierce. The vesicles
in this adhering cake of lava have been pulled out in the direction of
the slope till they have become narrow tubes four or five inches long
and parallel to each other. Some parts of this rock have a curved ropy
surface, like that of well-known Vesuvian lavas, suggestive of the
molten rock having flowed in successive thin viscous sheets down the
slope, which has a declivity of about 30°. This part of the section may
possibly preserve a fragment of the actual inner slope of the crater
formed of rudely-bedded tuffs.
Continuing still eastward, we find the feebly stratified tuff (_a_) to
be perhaps 200 feet thick. It forms a grassy declivity that descends
from the basalt-escarpment above to the grass-covered platform which
overlies a lower group of basalts. The visible portion of this tuff
presents a thoroughly volcanic character, being made up of the usual
dull dirty-green granular paste, through which are dispersed angular
and rough lumps of slag and pieces of more solid basalt, varying up
to a foot or two feet in length. These stones are generally disposed
parallel to the indistinct bedding, but are sometimes placed on end,
as if they had assumed that position on falling from an explosive
shower. Among the smaller stones, pieces of a finely vesicular basic
pumice are frequent and are among the most strikingly volcanic products
of the deposit. From a characteristic sample of these stones, a thin
slice was prepared and placed in Mr. Harker's hands. The following are
his observations on it:--"A very compact dark grey rock, amygdaloidal
on a minute scale. The lighter grey crust is probably due merely to
weathering, and the specimen seems to be a distinct fragment, not a
true bomb. The slice shows it to be essentially a brown glass with only
occasional microscopic crystals of a basic plagioclase. It has been
highly vesicular, and the vesicles are now filled by various secondary
products, including a chloritic mineral, nearly colourless and singly
refracting in thin section, and a zeolite."
Tracing now the tuff from the west or right side of the vent, we can
follow it to a greater distance. No abrupt line can be detected here,
any more than on the other side, between the agglomerate and the tuff.
The latter rock extends under the overlying plateau of basalt, at least
as far west as Portree Loch, a distance of fully a mile, but rapidly
diminishes in thickness in that direction. Traces of what is probably
the same tuff can be detected between the basalts at Ach na Hannait,
more than three miles to the south (Fig. 305). It is thus probable that
from the Portree vent fragmentary discharges took place over an area of
several square miles.
Above the agglomerate of this vent two lavas may be seen to start
towards opposite directions. One of these (_c_), already referred to,
is a dull prismatic basalt with a slaggy bottom, its vesicles being
pulled out in the direction of the general bedding of the section. It
descends by a twist or step, and then lies on the inclined surface of
the tuff which dips towards the agglomerate and seems to pass into
that rock. Further east this basalt increases in thickness and forms
the lowest of the basalt-sheets of the cliff. The lava that commences
on the west side of the agglomerate (_d_) is a massive jointed basalt,
which, though not seen at the vent, appears immediately to the west of
it and rapidly swells out so as to become one of the thickest sheets of
the locality. It lies upon the rudely-bedded tuff, and is covered by
the other basalts of the cliff.
That these two basalts came out of this vent cannot be affirmed. If
they did so at different times, their emission must have been followed
by the explosion which cleared the funnel and left the central mass
of agglomerate there. But that some kind of saucer-shaped depression
was still left above the site of the vent is indicated by the curious
elliptical mass of rock (_e_) that lies immediately above the
agglomerate, from which it is sharply marked off. This is one of the
most puzzling rocks in the district, probably in large measure owing to
its advanced state of decay. It is dull-red in colour, and decomposes
into roughly parallel layers, so that at a short distance it looks like
a bedded tuff, or like some of the crumbling varieties of banded lavas.
I could not obtain specimens fresh enough to put its nature and origin
beyond dispute. Whatever may have been its history, this ferruginous
rock rests in a flat basin-shaped hollow directly above the agglomerate
of the vent. The form of this depression corresponds fairly well with
what we may suppose to have been the final position and shape of the
crater of the little volcano. The rock that occupies the bowl dies out
towards the east on the face of the cliff, and the prismatic basalt
(_c_) is then immediately covered by the rest of the basalt-sheets
of the plateau (_f_). On the west side its precise termination is
concealed by grass. But it must rapidly dwindle in that direction
also, for not many yards away it is found to have disappeared, and the
basalts (_d_ and _f_) come together.
Though the decayed state of this rock does not warrant any very
confident opinion regarding its history, I am inclined to look upon
it as a deposit of much disintegrated volcanic detritus washed into
the hollow of the old crater when it had become filled with water, and
had passed into the condition of a _maar_. The peculiarly oxidized
condition of its materials points probably to long atmospheric
exposure, and an examination of the surrounding parts of the district
furnishes more or less distinct evidence that a considerable lapse of
time did actually intervene between the cessation of the eruptions of
the Portree volcano and the next great basalt-floods of this part of
Skye.
That volcanic eruptions from other vents continued after the Portree
vent had become extinct is proved by the great sheets of basalt (_f_)
that overspread it, and still bury a large tract of the fragmentary
material which it discharged. At a later time a fissure that was
opened across the vent, allowed the uprise of a basalt dyke (_g_), and
subsequently another injection of similar material took place along the
same line of weakness (_h_).
Before leaving this interesting locality we may briefly take note
of the distribution of the ashes and stones ejected by the volcano,
and the evidence for the relative length of the interval between
the outflow of the lavas below and that of those above the tuff
and volcanic conglomerate. These deposits may be traced in clear
sections along the base of the cliffs for a mile to the west of the
vent. They thin away so rapidly in that direction that at a distance
of three-quarters of a mile they do not much exceed fifty feet in
thickness. At Camas Bàn they consist mainly of a fine, dull-green,
granular, rudely-stratified basalt-tuff, through which occasional
angular pieces of different lavas and rough slags are irregularly
dispersed. These stones occur here and there in rows, suggestive of
more vigorous discharges, the layers between the platforms of coarser
detritus being occupied by fine tuff. Some of the ejected blocks are
imbedded on end--an indication of the force with which they were
projected so as to fall nearly a mile from the crater.
The upper parts of the tuff pass upward into fine yellow, brown, and
black clays a few feet in thickness, the darker layers being full of
carbonaceous streaks. On this horizon the coal of Portree was formerly
mined. The workings, however, have long been abandoned, and, owing to
the fall of large blocks from the basalt-cliff overhead, the entrance
to the mine is almost completely blocked up. One wooden prop may still
be seen keeping up the roof of the adit, which is here a slaggy basalt.
To the east and south-east of the Portree vent, extensive landslips
of the volcanic series and of the underlying Jurassic formations make
it hardly possible to trace the continuation of the tuff-zone in that
direction. To the south, however, at a distance of rather more than
three miles, what is probably the same stratigraphical horizon may be
conveniently examined from Ach na Hannait for some way to the north
of Tianavaig Bay. At the former locality the calcareous sandstones
of the Inferior Oolite are unconformably covered by the group of
rocks represented in Fig. 305. At the bottom of the volcanic series
lies a sheet of nodular dolerite with a slaggy upper surface (_a_).
Wrapping round the projections and filling up the depressions of this
lava comes a thin group of sedimentary strata from an inch or two to
eighteen inches or more in thickness (_b_). These deposits consist of
hardened shale charged with macerated fragments of linear leaves and
other plant-remains, including and passing into streaks of coal, which
may be looked upon as probably occupying the same horizon with the
coal of Portree. But here, instead of reposing on a mass of stratified
tuff, the carbonaceous layers lie on one of the bedded lavas. The tuff
has died out in the intervening three miles, yet that some of the
discharges of volcanic detritus reached even to this distance, and
that they took place during the accumulation of these layers of mud
and vegetation, is shown by the occurrence in the shales of pieces of
finely amygdaloidal basalt, from less than an inch to six inches in
length, likewise lapilli of a fine minutely cellular basic pumice, like
some varieties of palagonite. The overlying dolerite (_c_) becomes
finely prismatic at its junction with the sedimentary layers and has
probably indurated them.
[Illustration:
Fig. 305.--Section of the Volcanic Series at Ach na Hannait, south
of Portree, Skye.
]
This intercalation of a shaly and coaly band among the lavas can be
followed northward along the coast. In some places it has been invaded
by dykes, sills, and threads of basalt on the most remarkably minute
scale, of which I shall give some account in Chapter xlii. (see Fig.
321). North of Tianavaig Bay--that is, about three-quarters of a mile
nearer to the Portree vent--a perceptible increase in the amount of
volcanic material is observable among the shales and leaf-beds. Not
only are lapilli of basic pumice abundant, but the volcanic detritus
has accumulated here and there in sufficient amount to form a band of
dull greenish-brown tuff.
These coast-sections in the neighbourhood of Portree afford additional
illustrations of the characteristic fact, on which I have already
insisted, that the interstratifications of sedimentary material in the
basalt-plateaux frequently terminate upward in leaf-beds, thin coals,
or layers of shale, full of indistinctly preserved remains of plants.
As I have endeavoured to show, this vegetation, which was undoubtedly
terrestrial, probably grew not far from the sites where its remains
have been preserved. Leaves and seeds would naturally be blown or
washed into pools on the lava-fields, and would gather there among
the mud and sand carried by rain from the surrounding ground. Such a
topography and such a sequence of events point to intervals of longer
or shorter duration between the successive outpourings of basalt. It
was probably during one of these intervals of quietude that the crater
of the Portree volcano became a _maar_ and was finally silted up.
Reference has already been made to a conspicuous mass of agglomerate
which occurs at the east end of the island of Canna, and marks the site
of an important volcanic vent belonging to the Small Isles plateau. A
portion of it projects from the grassy slopes, and rises vertically
above the beach as a picturesque crag, in front of the precipice of
Compass Hill (Fig. 306). But the same rock may be traced southward to
the Coroghon Mòr, and north-westward in the lower part of the cliffs to
a little beyond the sea-stack of An Stòll. It has thus a diameter of at
least 3000 feet. Westward it passes under the conglomerate described in
Chapter xxxviii. Its eastern extension has been concealed by the sea.
[Illustration: Fig. 306.--View of part of a Volcanic Neck at the
eastern end of the island of Canna. (From a photograph by Miss Thom.)]
The materials that fill this vent consist of a typical agglomerate
composed entirely, or almost entirely, of volcanic detritus. The
embedded blocks vary up to eight feet in diameter or even more. They
are chiefly fragments of various basalts and andesites, generally
vesicular or amygdaloidal. Some of these, which have evidently been
broken off from already consolidated lavas, are angular or subangular
in shape, and their steam-holes are cut across by the outer surfaces
of the stones. Where they consist of calcite, zeolite, etc., the
amygdales so exactly resemble those of the bedded basalts of the
plateaux that, as already remarked, we must believe them to have been
already filled by infiltration before the disruption of the rocks by
volcanic explosions. Other blocks are true bombs, with a fine-grained
crust outside and a more cellular texture inside, the vesicles of the
outer crust being sometimes dragged round the surface of the stone.
The variety of materials included among the ejected blocks and the
abundance of pieces of the red bole which so generally separates the
plateau-basalts indicate that a considerable thickness of bedded lavas
has probably been broken through by the vent.
Beside the volcanic materials, occasional angular pieces of red
(Torridon) sandstone may be observed in the agglomerate. The paste is a
comminuted mass of the same material as the blocks, tolerably compact,
and entirely without any trace of stratification.
The actual margin of this vent has nowhere been detected by me. We
never reach here the base of the volcanic series, for it is sunk under
the sea-level. On the other hand, the upper limits of the agglomerate
have been partially effaced or obscured by the conglomerates which
overlie it. From the breadth of ground across which the agglomerate
can be followed along the shore, the vent might be regarded as having
been perhaps not less than three-quarters of a mile in diameter. But
there is the same difficulty here as at the Strath vent in Skye in
determining the actual limits of the volcanic funnel. Possibly there
may have been more than one vent in close proximity. Even if there was
only one, the existing agglomerate may include not only what filled the
chimney, but also a portion of what had accumulated round the orifice
and formed the external cone. That the volcano continued for some time
in vigorous eruption may be judged from the amount of material ejected
from it, the large size of its blocks, and the distance to which they
were sometimes thrown.
The pieces of Torridon Sandstone were no doubt derived from the
extension of that formation underneath Canna. On the opposite island of
Rum, where these pre-Cambrian red sandstones are copiously developed,
they form the platform through which the Tertiary volcanic series has
been erupted. The several remaining outliers of the bedded basalts,
referred to in a previous chapter (p. 215 and Fig. 267) as visible on
the west side of this island, show that the basalt-plateau of Small
Isles, which once covered that area, rested immediately on the inclined
edges of the Torridon Sandstones. Probably the same structure stretches
westward under Canna and Sanday. No traces of any Jurassic strata
have been detected beneath the volcanic rocks of Rum, though they
are so well developed a few miles to the east in the island of Eigg.
Either they were not deposited over the pre-Cambrian rocks of Rum, or
they had been removed from that ancient ridge before the beginning
of the Tertiary volcanic period. Certainly I have not detected a
single recognizable fragment of any Jurassic sedimentary rock in the
agglomerate of Canna.
This Canna vent exhibits, better than is usually shown, the occurrence
of dykes and irregular injections of lava through the agglomerate.
A large mass of a finely columnar basalt runs up from the beach at
Garbh Asgarnish. A similar rock forms several detached crags a little
further south, particularly in the headland of Coroghon Mòr and the
island of Alman. Here the basalt is beautifully columnar, its slender
prisms curving from a central line until their ends abut against the
agglomerate. The truly intrusive character of this basalt is well shown
on the southern front of Coroghon Mòr, and on the northern face of
Alman, as represented in the accompanying diagrams (Figs. 307 and 308).
[Illustration:
Fig. 307.--Columnar Basalt invading Agglomerate of Volcanic Vent,
Coroghon Mòr, Isle of Canna. (Height above 20 feet.)
]
[Illustration:
Fig. 308.--Columnar Basalt invading Volcanic Conglomerate, north
side of Alman Islet, Canna.
]
Although there is no conclusive evidence that these intrusions belong
to the time of the activity of the vent, yet they differ so much from
the ordinary dykes (one of which also cuts the agglomerate and ascends
through the conglomerates and basalts above), are confined so markedly
to the vent and its immediate proximity, and resemble so closely the
basalt-injections of other vents, such as those of the Carboniferous
and Permian necks of Scotland, that they may with probability be
regarded as part of the mechanism of the Canna volcano.
Though the form and size of the vent of this volcano cannot be
precisely defined, the upper part of its agglomerate, as we have seen
(_ante_, p. 219), is dovetailed in the most interesting way with the
series of coarse conglomerates, which indicate strong river-action in
this part of the volcanic area during the time of the eruption of the
plateau-basalts.
The agglomerate vents described in the foregoing pages as occurring
in Antrim and among the Inner Hebrides all appear either in the midst
of the plateau-basalts or in close proximity to them. Before quitting
the Scottish examples, I may refer to some that rise through much more
ancient formations at a distance from any portion of the volcanic
plateaux, and yet may with probability be assigned to the Tertiary
volcanic period.
During the progress of the Geological Survey through the district of
Applecross, in the western part of the mainland of Ross-shire, and far
away from the basalt-plateau of Skye, Mr. John Horne[306] has found two
small necks rising on each side of a line of fracture, through gently
inclined Torridon Sandstones. They are conspicuous from a distance by
the verdure of their slopes, in contrast with the brown tints of the
surrounding moorland. The larger of the two necks measures about 180 by
150 feet, and abruptly truncates the beds of Torridon Sandstone, which
as they approach it assume a bleached aspect and become indurated.
The material filling this vent is an agglomerate made up mainly of
pieces of Torridon Sandstone and grit which, though generally small,
occasionally measure a foot across, and in one case were found to reach
a length of four feet. They are not as a rule markedly altered, but
some of them have acquired a glazed or vitreous texture. Besides these
fragments of the general rock of the district, there occur abundant
lapilli of a basic volcanic rock, found by Mr. Teall to consist of
porphyritic felspar, extremely minute acicular microlites of felspar,
somewhat irregular transparent spaces now occupied by a yellowish-green
substance, and interstitial matter. At the south end of the vent a
small mass of decayed basalt appears to pierce the agglomerate.
[Footnote 306: _Trans. Geol. Soc. Edin._ vii. (1894), p. 35.]
[Illustration: Fig. 309.--View of neck-like mass of breccia, Brochel,
Raasay.]
Though there is no indication of the age of these necks, they agree
so closely in general character with known vents of the Tertiary
volcanic plateaux that there cannot be much hesitation in regarding
them as dating from the same great period of basalt-eruption. But no
relic now exists anywhere around of lavas or tuffs ejected from them.
They rise on the bare Applecross hills, 1000 feet above sea-level, two
miles from the shore, and about ten miles from the nearest outlier of
the basalt-plateau in the Dùn Can of Raasay. If they once discharged
streams of lava that united with the rest of the plateau, the total
destruction of this lava affords another impressive picture of the
waste which the volcanic rocks of the Inner Hebrides have undergone.
The large proportion of Torridon Sandstone blocks in these two
Applecross necks suggests, however, that the orifices never became
active volcanic vents. They may have been mere spiracles, or
blow-holes, where the funnels drilled by explosive vapours were filled
up with the debris of the rocks that were blown out. But that lava did
rise within them is shown by the basic lapilli in the agglomerate, and
by the basalt which in both vents has found its way up the chimney.
In the island of Raasay Mr. Teall, during the summer of 1894,
observed a group of curious neck-like masses of breccia which pierce
the Torridon Sandstone near Brochel (Fig. 309). The blocks in them
are large angular unaltered pieces of the surrounding sandstones
and shales, sometimes ten feet or more in length, and the matrix is
sometimes pure crystalline calcite like Iceland spar. The breccia is
generally coarsest towards the outer margin. But though the Lewisian
gneiss exists immediately below the thin cake of Torridonian strata,
not a fragment of it could either Mr. Teall or I, when I visited the
locality with him, find among the components of the breccia. Nor did
we detect any trace of volcanic material. The general ground-plan of
these masses is elliptical, the most northerly measuring 30 yards in
diameter. Where the junction of the breccia with the Torridon strata
can be seen it is a nearly vertical one, the sandstones and shales
being much jumbled and broken, but not sensibly indurated. This little
cluster of patches of breccia can hardly be due to local crushing of
the rocks. Their definite outlines and composition seem rather to
indicate spiracles of Tertiary time, which never became vents erupting
lava or ashes. The absence of fragments of the underlying gneiss may be
accounted for if we suppose that the orifices were completely cleared
out by the violence of the explosions and were afterwards filled up
by the falling in of the walls of the higher parts now removed by
denudation, which consisted of Torridon Sandstone and shale.[307]
[Footnote 307: It is on one of these neck-like patches of breccia that
Brochel Castle stands, of which Macculloch gave so sensational a
picture in one of the plates of his _Western Isles_.]
Further research may detect at still greater distances from the
basalt-plateaux ancient volcanic necks that might, with more or less
probability, be referred to the Tertiary period. As an instance of this
kind, I refer to the neck at Bunowen, County Galway, recently described
by Mr. M'Henry and Professor Sollas. Though so remote from the Tertiary
basalt-plateaux, the rock of this boss is an olivine-basalt presenting
a close resemblance to some of the rocks of Antrim.[308]
[Footnote 308: _Trans. Roy. Irish Acad._, 1896].
As a final illustration of Tertiary volcanic vents I will now describe
the Faroe group already alluded to (vol. i. p. 63, vol. ii. p. 256).
It was almost by a kind of happy accident that these vents were
discovered. Noticing at a distance of a mile or more from the deck of
a steam-yacht that the base of the great basalt cliffs on the west
side of Stromö were varied by what looked like agglomerate, I steamed
inshore, and was delighted to find, as the vessel drew near to the
cliff, that the agglomerate assumed definite boundaries and occurred in
several distinct patches, until at last it presented the unmistakable
outlines of a group of vents underlying and overspread by the bedded
basalts of the plateau. Favoured by an unusually calm sea, I was
enabled to boat into every nook and round every buttress and islet of
this part of the coast-line.
[Illustration: Fig. 310.--View of Volcanic Neck piercing and overlain
by the Plateau-Basalts, Stromö, entrance of Vaagöfjord, Faroe Islands.
(From a photograph by Colonel Evans.)]
The basalt-plateau here presents to the western ocean a nearly vertical
escarpment which must reach a height of at least 1000 feet (see Fig.
328), and displays a magnificent section of the bedded lavas. The
lower part of this section shows chiefly the banded structure already
described, the layers of different consistency being etched out by the
weather in such a way as to give them the look of stratified rocks.
In the upper part of the precipice columnar and jointed or prismatic
sheets are more common, but the most prominent band is the great sill,
to which further reference will be made in the next Chapter.
In the course of the gradual retreat of the cliff, as the waves tunnel
its base, and slice after slice is detached from its vertical front,
a group of at least five small vents has been uncovered lying along a
nearly north and south line. Of two of these a segment remains still
on the cliff-wall and passes under the basalts; the others have been
dissected and half cut away from the cliff, while groups of stacks and
rocky islets of agglomerate may mark the position of others almost
effaced. The horizontal distance within which the vents are crowded
is probably less than half a mile, but the lofty proportions of the
precipice tend to lead the eye to underestimate both heights and
distances.
[Illustration: Fig. 311.--Section of the same Neck as that shown in
Fig. 310.]
The agglomerate is a thoroughly volcanic rock, consisting of large and
small blocks of various basalts, among which large slags are specially
conspicuous, the whole being wrapped in a granular matrix of comminuted
volcanic detritus. The arrangement of this material is best seen in the
fourth vent (Figs. 310 and 311). In this characteristic volcanic neck
(_b_ in Fig. 311) the boundary walls, as laid bare on the face of the
precipice, are vertical, and are formed of the truncated ends of the
banded lavas (_a_ _a_) which have been blown out at the time of the
formation of the orifice. The visible diameter of the vent was roughly
estimated by me to be about 100 yards. No appreciable alteration was
observed in the ends of the lavas next the vent.
The agglomerate is coarsest in the centre, where huge blocks of
slaggy lava lie imbedded in the amorphous mass of compacted debris.
On either side of this structureless central portion the agglomerate
is distinctly stratified from the walls towards the middle, at angles
of 30° to 35°. Even from a distance it can be observed that the upper
limit of the agglomerate is saucer-shaped, the sloping sides of the
depression dipping towards the centre of the neck at about the same
angle as the rudely-stratified agglomerate underneath. From the bottom
of this basin to the sea-level may be a vertical distance of some 30
yards. The basin itself has been filled up by three successive flows
of basalt, of which the first (_c_) has merely overflowed the bottom,
the second (_d_), entering from the northern rim of the basin, extends
across to the southern slope, while the third (_e_), also flowing from
the north, has filled up the remainder of the hollow and extended
completely across it. The next succeeding lava (_f_) stretched over the
site in such a way as to bury it entirely, and to provide a level floor
for the piling up of the succeeding sheets of basalt.
[Illustration: Fig. 312.--Volcanic Neck close to that shown in Figs.
310 and 311.]
[Illustration:
Fig. 313.--Section of wall of another Neck of agglomerate in the
same group with those represented in Figs. 310, 311, and 312.
]
The second vent, which is represented in Fig 312, exhibits the same
features, but with some additional points of interest. It measures
roughly about 20 yards in diameter at the sea-level, rises through the
same group of banded basalt (_a_ _a_), and is filled with a similar
agglomerate (_b_). Its more northerly wall is now coincident with a
line of fault (_h_) which ascends the cliff, and probably marks some
subsidence after the eruptions had ceased. The southern wall shows
that a dyke of basalt (_g_) has risen between the agglomerate and the
banded basalts, and that a second dyke (_g´_) traverses the latter at
a distance of a few feet. In this instance, also, the upper surface of
the agglomerate forms a cup-shaped depression which has been filled
in by two successive streams of lava (_c_, _d_). Among the succeeding
lavas (_e_) the prominent sill (_f_) has been intruded, to which
further allusion is made on p. 323.
These necks are obviously volcanic vents belonging to the time of the
basaltic eruptions. They have been drilled through the basalts of
the lower part of the cliff, but have been buried under those of the
central and higher parts. The arrangement of their component materials
in rude beds dipping towards the middle of each vent shows that the
ejected dust and stones must have fallen back into the orifice so as
to be rudely stratified towards the centre of the chimney, which was
finally closed by its own last discharges of coarse detritus. The
saucer-shaped upper limit of the agglomerate seems to indicate, as has
been suggested above in the case of the Portree volcano, that after
the eruptions ceased each vent remained as a hollow or _maar_ on the
surface of the lava-fields. And the manner in which they are filled
with successive sheets of basalt shows that in course of time other
eruptions from neighbouring orifices gave forth streams of lava which,
in flowing over the volcanic fields, eventually buried and obliterated
each of the vents.
[Illustration: TO ACCOMPANY SIR ARCHIBALD GEIKIE'S "ANCIENT VOLCANOES
OF BRITAIN"
Map VI MAP OF THE TERTIARY VOLCANIC REGION OF THE INNER HEBRIDES
The Edinburgh Geographical Institute Copyright J. G. Bartholomew]
In the destruction of the precipice some of the vents have been so much
cut away that only a small part of the wall is left, with a portion of
the agglomerate adhering to it. The third neck, for instance, affords
the section represented in Fig. 313, where the horizontal sheets of
basalt (_a_) have still a kind of thick pellicle of the volcanic
detritus (_b_) adhering to what must have been part of the side of the
orifice of eruption. The waves have cut out a cave at the base, so that
we can, by boat, get behind the agglomerate and see the margin of the
volcanic funnel in the roof overhead.
The fragment of geological history so picturesquely laid bare on the
Stromö cliffs presents a significant illustration of what seems to
have been a frequent, if not the normal type of volcanic vent in the
Tertiary basalt-plateaux. By the fortunate accident that denudation has
not proceeded too far, we are able to observe the original tops of at
least two of the vents, and to see how such volcanic orifices, which
were doubtless abundant all over these plateaux, came to be entombed
under the ever-increasing pile of accumulating basalt.
There is still one feature of interest in these cliff-sections which
deserves notice here. Every geologist who has studied the composition
of the basalt-plateaux has remarked the comparatively insignificant
part played by tuffs in these volcanic accumulations. Hundreds of
feet of successive basalt-sheets may often be examined without the
discovery of any intercalation of fragmental materials, and even where
such intercalations do occur they are for the most part quite thin and
extremely local. I found it impossible to scale the precipice for the
purpose of ascertaining whether around the Stromö vents, and connected
with them, there might not be some beds of tuff interstratified between
the basalts. If such beds exist, they can only be of trifling thickness
and extent. Here, then, are examples of once active vents, the funnels
of which are still choked up with coarse fragmentary ejections, yet
from which little or no discharge of ashes and stones took place over
the surrounding ground. They seem to have been left as crater-like
hollows on the bare surface of the lava-fields.