THE BASALT-PLATEAUX OF SKYE AND OF THE FAROE ISLES
iv. THE SKYE PLATEAU
This largest and geologically most important of all the Scottish
plateaux comprises the island of Skye, at least as far south as Loch
Eishort, and the southern half of Raasay, but is shown by its sills
to stretch as far as the Shiant Isles on the north, and the Point of
Sleat on the south (see Map VI.). It may be reckoned to embrace an area
of not less than 800 square miles. The evidence that its limits, like
those of the other plateaux, are now greatly less than they originally
were, is abundant and impressive. The truncated edges of its basalts,
rising here and there for a thousand feet as a great sea-wall above the
breakers at their base, and presenting everywhere their succession of
level or gently inclined bars, are among the most impressive monuments
of denudation in this country. But still more striking to the geologist
is the proof, furnished beyond the margins of the plateau, that the
Jurassic and other older rocks there visible were originally buried
deep under the basalt-sheets, which have thus been entirely stripped
off that part of the country.
Throughout most of the district, wherever the base of the basalts can
be seen, it is found to rest upon some member of the Jurassic series,
but with a complete unconformability. The underlying sedimentary strata
had been dislocated and extensively denuded before the volcanic period
began. On the southern margin, however, the red (Torridon) sandstones
emerge from under the basalts of Loch Scavaig, and extending into
the island of Soay are prolonged under the sea into Rum. This ridge
probably represents the range of the ancient high ground of the latter
island already referred to.
Nowhere are the distinctive topographical features and geological
structure of the basalt-plateaux better displayed than in the northern
half of the island of Skye. The green terraced slopes, with their
parallel bands of brown rock formed by the outcrop of the nearly flat
basalt-beds, rise from the bottoms of the valleys into flat-topped
ridges and truncated cones (Fig. 283). The hills everywhere present
a curiously tabular form that bears witness to the horizontal sheets
of rock of which they are composed.[262] And along the sea-precipices,
each excessive sheet of basalt can be counted from base to summit,
and followed from promontory to promontory (Figs. 284, 286). In the
district of Trotternish, the basalt hills reach a height of 2360 feet.
Further west, the singular flat-topped eminences, called "Macleod's
Tables" (Fig. 283) ascend to 1600 feet.
[Footnote 262: These features are more fully described in my _Scenery of
Scotland_, 2nd edit (1887), pp. 74, 145, 216.]
[Illustration: Fig. 283.--Terraced Hills of Basalt Plateau (Macleod's
Tables), Skye.]
Along the western side of Skye, the basalts descend beneath the level
of the Atlantic, save at Eist in Duirinish, where the Secondary strata,
with their belt of intrusive sills, rise from underneath them, and at
the Sound of Soa, where they rest on the Torridon Sandstone. Along
the eastern side, their base runs on the top of the great Jurassic
escarpment, whose white and yellow sandstones rise there, and on the
east side of Raasay, into long lines of pale cliffs. To the south-east,
the regularity of the volcanic plateau is effaced, as in Mull and
Ardnamurchan, by the protrusion of extensive masses of eruptive rocks
constituting the Cuillin and Red Hills, east of which the basalts have
been almost entirely removed by denudation, so as to expose the older
rocks which they once covered, and through which the younger eruptive
bosses made their way. This is undoubtedly the most instructive
district for the study of that late phase in the volcanic history of
Britain comprised in the eruptive bosses of basic and acid rocks.
The magnificent plateau of this island has been so profoundly cut down
into glens and arms of the sea, and its component layers are exposed
along so many leagues of precipice, that its structure is perhaps more
completely laid open than that of any of the other Tertiary volcanic
areas in Britain. It is built up of a succession of basalts and
dolerites of the usual types, which still reach a thickness of more
than 2000 feet, though in this instance, also, denudation has left
only a portion of them, without any evidence by which to reckon what
their total original depth may have been. In rambling over Skye, the
geologist is more than ever struck with the remarkable scarcity and
insignificance of the interstratifications of tuff or of any other
kind of sedimentary deposit between the successive lava-sheets. One
of the thickest accumulations of volcanic tuff and conglomerate has
already been referred to as occurring on the south side of Portree
Harbour, where it attains a depth of about 200 feet. As it is in
immediate connection with its parent vent, it will be more fully
alluded to in Chapter xli. Here, as is so generally observable among
the basalt-plateaux, traces of vegetation are plentiful among the
stratified intercalations, even forming thin seams of lignite and coal,
one of which was formerly worked. That volcanic eruptions, though
possibly of a feebler kind, continued during the interval between the
basalt-outflows at this locality, is shown by the thick accumulation
of tuff and by the occurrence of abundant lapilli of fine basic pumice
among the shales, even to a distance of several miles from the vent.
[Illustration: Fig. 284.--"Macleod's Maidens" and part of Basalt Cliffs
of Skye.]
Another conspicuous intercalation of sedimentary materials in the Skye
plateau occurs on the Talisker cliffs at the mouth of Loch Bracadale,
where, on the face of the great precipice of Rudha nan Clach, some
conspicuous bands of lilac and red are interspersed among the basalts.
These bands were noticed by Macculloch, who described them as varieties
of "iron-clay."[263] I have not had an opportunity of examining them
except from the sea at a little distance. But they suggest a similarity
to some of the variegated clays between the upper and lower basalt
series of Antrim.
[Footnote 263: _Western Islands_, vol. i. p. 376.]
Though good coal is not well developed in the Tertiary volcanic
plateaux of the British Isles, it has already been pointed out that
coaly layers are abundant, and that as the vegetable matter may
confidently be assumed always to indicate terrestrial vegetation, the
presence of the carbonaceous bands may be regarded as good evidence
of some lapse of time between the eruption of the basalts which they
separate. I have also called attention to the fact that the vegetable
material is more especially observable in the highest parts of a
group of intercalated sediments between two sheets of basalt. This
relation, so strikingly exhibited in the isle of Canna, as already
observed, is also to be remarked in the Skye plateau. I may here cite
an interesting example which occurs at the base of the lofty sea-cliff
of An Ceannaich, to the south of Dunvegan Head, on the west coast of
Skye (Fig. 285). At the base of the precipice, ledges of a highly
cellular basalt (_a_) show a singularly scoriaceous and amygdaloidal
structure, with abundant and beautiful zeolites, the hollows of the
upper surface of the sheet being filled in with dark brown carbonaceous
shale, forming a layer from one to fourteen inches thick, marked by
coaly streaks and lenticles (_b_). A band of green and yellow sandstone
(_c_) next supervenes, which, from its pale colour, attracts attention
from a distance, and led me, while yachting along the coast, to land
at the locality in the hope that it might prove to be a plant-bearing
limestone. This sandy stratum is only some three or four inches thick
at the north end of the section, but increases rapidly southward to a
thickness of as many feet or more, when, owing to the cessation of the
underlying shale, it comes to lie directly on the amygdaloid and to
enclose slaggy portions of that rock. Immediately above the sandstone
two or three feet of fissile shale, black with plant-remains (_d_),
include brown layers that yield to the knife like some oil-shales. The
next stratum is a seam of coal (_e_) about a foot thick, of remarkable
purity. It is glossy, hard, and cubical, including layers that break
like jet. It has been succeeded by a deposit of green sand (_f_), but
while this material was in course of deposition another outpouring of
lava (_g_) took place, whereby the terrestrial pool or hollow of the
lava-field, in which the group of sedimentary materials accumulated,
was filled up and buried. This lava is about 20 feet thick, and
consists of a coarsely-crystalline, jointed dolerite with highly
amygdaloidal upper and under surface. Its slaggy bottom has caught
up or pushed aside the layer of green sand, so as to lie directly on
the coal, and has there been converted into the earthy modification
so familiar under the name of "white trap" among our coal-fields. It
is interesting to find that this kind of alteration, where molten
rock comes in contact with carbonaceous materials, is not confined to
subterranean sills, but may show itself in lavas that have flowed over
a terrestrial surface.
[Illustration:
Fig. 285.--Intercalated group of strata between Basalts, An
Ceannaich, western side of Skye.
]
From the frequent intercalation of such local deposits of sedimentary
material between the basalts, we may reasonably infer that during older
Tertiary time the rainfall in North-Western Europe was copious enough
to supply many little lakes and streams of water. As the surface of
the lava-fields decayed into soil, vegetation spread over it, so that,
perhaps for long intervals, some tracts remained green and forest-clad.
But volcanic action still continued to show itself, now from one vent,
now from another. These wooded tracts were buried under overflows of
lava, and, the water-courses being filled up, their streams were driven
into new channels, and other pools and lakes were formed.
[Illustration: Fig. 286.--Escarpment of Plateau-basalts, Cliffs of
Talisker, Skye.]
In no part of the Tertiary volcanic area of Britain can the characters
of the lavas and the structure of the plateaux be better seen than
along the west side of Skye, north of Loch Bracadale. The precipices
rise sheer out of the sea, to heights of sometimes 1000 feet, and from
base to summit every individual bed may be counted. Some particulars
have already been given (p. 192) regarding the average thickness of the
basalt-sheets on this coast-line. The general aspect of these cliffs
and the arrangement of their component lavas is shown in Fig. 286.
As a further detailed illustration of the general succession of the
basalts in the Skye plateau, I give a diagrammatic view of the largest
of Macleod's Maidens--the three weird sea-stalks that rise so grandly
in front of the storm-swept precipice at the mouth of Loch Bracadale.
The height of the stack must be at least 150 feet (Figs. 284 and 287).
About ten distinct sheets of igneous rock can be counted in it, which
gives an average thickness of 15 feet for the individual beds. It will
be observed that there is a kind of alternation between the compact,
prismatic basalts and the more earthy amygdaloids, but that the former
are generally thickest.[264]
[Footnote 264: A striking and illustrative contrast between the relative
thickness of the beds of the two kinds of rock is supplied by the fine
sections of this district. The amygdaloids range from perhaps 6 or 8
to 25 or 30 feet; but the prismatic basalts, while never so thin as
the others, sometimes enormously exceed them in bulk. In the island of
Wiay, for example, a bed of compact black basalt, with the confused
starch-like grouping of columns, reaches a thickness of no less than
170 feet. Its bottom rests upon a red parting on the top of a dull
greenish earthy amygdaloid. It is possible, however, that some of these
columnar sheets of basalt are really sills.]
[Illustration: Fig. 287.--Section of the largest of Macleod's Maidens.]
These features, which are repeated on cliff after cliff, may be
considered typical for all the plateaux. Another characteristic point,
well displayed here, is the intervening red parting between the
successive beds. If the occurrence and thickness of this layer could
be assumed as an indication of the relative lapse of time between
the different flows of lava, it would furnish us with a rude kind of
chronometer for estimating the proportionate duration of the intervals
between the eruptions. It is to be noticed on the top both of the
compact prismatic and of the earthy amygdaloidal sheets; but it is more
frequent and generally thicker on the latter than on the former, which
may only mean that the surfaces of the cellular lavas were more prone
to subærial decay than those of the compact varieties. Nevertheless,
I am disposed to attach some value to it, as an index of time. In the
present instance, for example, it seems to me probable that the lavas
in the lower half of Macleod's Maiden, where the red layers are very
prominent, were poured out at longer intervals than those that form the
upper half. The remarkable banded arrangement of the vesicles in one of
the cellular lavas of this sea-stack has been already referred to (p.
191).
Another characteristic plateau-feature is admirably displayed in
Skye--the flatness of the basalts and the continuity of their level
terraces (though not of individual sheets) from cliff to cliff and
hillside to hillside. This feature may be followed with almost tiresome
monotony over the whole of the island, north of a line drawn from Loch
Brittle to Loch Sligachan. Throughout that wide region, the regularity
of the basalt-plateau is unbroken, except by minor protrusions of
eruptive rock, which, as far as I have noticed, do not seriously affect
the topography. But south of the line just indicated, the plateau
undergoes the same remarkable change as in Rum, Ardnamurchan and Mull.
Portions of it which have survived indicate with sufficient clearness
that it once spread southwards and eastwards over the mountainous
district, and even farther south into the low parts of the island. Its
removal from that tract has been of the utmost value to geological
research, for some of the subterranean aspects of volcanism have
thereby been revealed, which would otherwise have remained buried under
the thick cover of basalt. Denudation has likewise cut deeply into the
eruptive bosses, and has carved out of them the groups of the Red Hills
and the Cuillins, to whose picturesque forms Skye owes so much of its
charm.
In this, as in each of the other plateaux, there is no trace of
any thickening of the basalts towards a supposed central vent of
eruption. The nearly level sheets may be followed up to the very
edge of the great mountainous tract of eruptive rocks, retaining all
the way their usual characters; they do not become thicker there
either collectively or individually, nor are they more abundantly
interstratified with tuffs or volcanic conglomerates. On the contrary,
their very base is exposed around the mountain ground, and the thickest
interstratifications of fragmentary materials are found at a distance
from that area. So far as regards the structure of the remaining part
of the plateau, the eruption of the gabbros and granitoid rocks might
apparently have taken place as well anywhere further north.
v. THE FAROE ISLANDS[265]
[Footnote 265: For references to the recent geological literature
connected with these islands see the footnote _ante_, p. 191.]
Though these islands lie beyond the limits of the region embraced by
the present work, I wish to cite them for the singular confirmation
and extension they afford to observations made among British Tertiary
volcanic rocks. Over a united extent of coast-cliffs which may be
roughly estimated at about 500 English miles, the nearly level sheets
of basalts, with their occasional tuffs, conglomerates, leaf-beds and
coals, can be followed with singular clearness. Although the Faroe
Islands have been so frequently visited and so often described that
their general structure is sufficiently well known, they present in
their details such a mass of new material for the illustration of
volcanic action that they deserve a far more minute and patient survey
than they have yet received. They cannot be adequately mapped and
understood by the traveller who merely sails round them. They must be
laboriously explored, island by island and cliff by cliff.
While I cannot pretend to more than a mere general acquaintance
with their structure, I have learnt by experience that one may sail
near their precipices and yet miss some essential features of their
volcanic structure. In the summer of the year 1894 I passed close to
the noble range of precipices on the west side of Stromö, at the mouth
of the Vaagöfjord, and sketched the sill which forms so striking a
part of the geology of that district (Figs. 312, 328 and 329). But I
failed to observe a much more remarkable and interesting feature at
the base of the same sea-cliffs. The following summer, probably under
better conditions of light, I was fortunate enough to detect with my
field-glass, from the deck of the yacht, what looked like a mass of
agglomerate, and found on closer examination the interesting group of
volcanic vents described in Chapter xli. The magnificent precipices
of Faroe, which in Myling Head reach a height of 2260 feet, present
a series of natural sections altogether without a rival in the rest
of Europe. They are less concealed with verdure than those of Mull
and Skye, and therefore display their geological details with even
greater clearness than can be found either in Scotland or in Ireland.
I would especially refer to the bare precipitous sides of the long
narrow islands of Kalsö and Kunö, as admirable sections wherein the
characters of the plateau-basalts are revealed as in a series of
gigantic diagrams. The scarcity of vegetation, and the steepness of
the declivities which prevents the abundant accumulation of screes
of detritus, enable the observer to trace individual beds of basalt
with the eye for several miles. Thus on the west side of Kunö, one
conspicuous dark sheet in the lower part of the section can be followed
from opposite Mygledahl in Kalsö to the southern end of the island.
There is one concealed space at the mouth of the corrie behind Kunö
village, but the same, or at least a similar band of rock at the same
level, emerges from the detritus on the further side, and may possibly
run into the opposite promontory of Bodö. It extends in Kunö for at
least six geographical miles.
[Illustration: Fig. 288.--Dying out of Lava-beds, east side of Sandö,
Faroe Isles.]
These vast escarpments of naked rock show, with even greater clearness
than the precipices of the Inner Hebrides, how frequently the basalts
die out, now in one direction now in another. The two sides of the
Kalsöfjord exhibit many examples of this structure, and some striking
instances of it are to be seen on the west side of Haraldsfjord. In
these cliffs, which must be about 2000 feet high, upwards of forty
distinct flows can sometimes be traced from the sea-level to the crest.
The average thickness of each bed is thus somewhat less than 50 feet.
Such vast escarpments, with wide semicircular corries scooped out of
their sides, such serrated crests and dark rifts in the precipices,
such deep fjords winding through nearly horizontal basalts, of which
the parallel sheets can be followed by the eye from island to island,
fill the mind with a vivid conception at once of the enormous scale
of the volcanic eruptions and of the stupendous denudation which this
portion of North-Western Europe has undergone since Tertiary time.
As the lenticular character of the basalts, and the evidence they
supply of having been discharged from many small local vents are of
great importance in the comprehension of the volcanic history of
the plateaux, some further illustrations of these features may with
advantage be given here. Thus the traveller who skirts the western
precipices of Suderö will notice some good examples to the north of
the highest part of the cliffs. On Stromö he will detect other cases
of the same structure. Similar features will arrest his attention on
the precipices of Sandö, where, though at first sight the basalts
seem to be regular and continuous, a nearer view of them reveals such
sections as that shown in Fig. 288, where a group of sheets rapidly
dies out towards the north against a thicker band that thins away in
the opposite direction. Further north he will come upon other examples
in the range of low cliffs between Kirkebonaes and Thorshaven, and more
impressive still in the rugged precipices that front the Atlantic on
the western front of Hestö (Fig. 289), where the disappearance is in a
northerly direction.
[Illustration: Fig. 289.--Lenticular lavas, western front of Hestö,
Faroe Isles.]
But it is in the northern part of the Faroes, where the basalt-plateau
has been so deeply trenched by parallel fjords as to be broken up into
a group of long, narrow, lofty, and precipitous insular ridges, that
the really local and non-persistent character of the lavas can best be
seen. The eastern cliffs of Svinö present admirable examples, where
in the same vertical wall of rock some of the basalts die out to the
south, others to the north, while occasionally a shorter sheet may be
seen to disappear in both directions as if it were the end of a stream
that flowed at right angles to the others (Fig. 290).
[Illustration: Fig. 290.--Lenticular lavas east side of Svinö, Faroe
Isles.]
The more the basalt-plateaux of Britain and the Faroe Islands are
studied, the more certain does the conclusion become that these
widespread sheets of lava never flowed from a few large central
volcanoes of the type of Etna or Vesuvius, but were emitted from
innumerable minor vents or from open fissures. In a later chapter an
account will be given of the vents, which may still be seen under the
overlying sheets of basalt, and, in particular, a remarkable group in
the Faroe Islands will be described.
[Illustration: Fig. 291.--Section at Frodbonyp, Suderö, Faroe.]
The occurrence of tuffs, leaf-beds and thin coals between the
plateau-basalts of the Faroe Islands has long been known. These
stratified deposits are well seen in the island of Suderö, where they
serve to divide two distinct series of basalts, like the iron-ore and
its accompaniments in Antrim. As a characteristic illustration of the
same diversity of deposits observable between the lava-sheets of the
basalt-plateaux of the British Isles I give here a section exposed on
the east side of this island--a locality often visited and described
in connexion with its coal-seams (Fig. 291). At the base lies a sheet
of basalt (_a_) with an irregularly lumpy upper surface. It may be
remarked that the lower group of basalts is marked by the occurrence of
numerous columnar sheets, some of them possibly sills, and also more
massive, solid, and durable basalts than the sheets above. The lowest
of the intercalated sediments are light-coloured clays, passing down
into dark nodular mudstone and dark shale, the whole having a thickness
of at least 20 feet (_b_). These strata are succeeded by (_c_) pale
clays with black plant-remains, about three feet thick. Immediately
above this band comes the coal or coaly layer (_d_), here about six
inches thick, which improves in thickness and quality further inland,
where it has been occasionally worked for economic purposes. A deposit
of green and brown volcanic mudstone (_e_), twelve feet in thickness,
overlies the coal and passes under a well-bedded granular green tuff
and mudstone three feet thick (_f_). The uppermost band is another
volcanic mudstone (_g_) four feet in thickness, dark green in colour,
and more or less distinctly stratified, with irregular concretions, and
also pieces of wood. Above this layer comes another thick overlying
group of basalts (_h_) distinguished by their abundantly amygdaloidal
character, and by their weathering into globular forms which at a
little distance give them a resemblance to agglomerates.
We have here an intercalated group of strata upwards of 40 feet thick,
consisting partly of tuffs and partly of fine clays, which may either
have been derived from volcanic explosions or from the atmospheric
disintegration of basaltic lavas. Through some of these strata abundant
carbonaceous streaks and other traces of plants are distributed,
while among them lies a band almost wholly composed of compressed
vegetation. Unfortunately none of the strata at this locality seem
to have preserved the plant-remains with sufficient definiteness
for identification. There can be no doubt, however, that they were
terrestrial forms like those of Mull and Antrim.
This coal, with its accompanying sedimentary deposits, has been traced
through Suderö, and another outcrop, possibly of the same horizon,
occurs on Myggenaes, the extreme western member of the group of
islands, at a distance of some 40 miles.[266]
[Footnote 266: See in particular Prof. J. Geikie, _Trans. Roy. Soc.
Edin._ vol. xxx. (1880), p. 229.]