THE SEVERAL BASALT-PLATEAUX AND THEIR GEOLOGICAL HISTORY,
ANTRIM, MULL, MORVEN AND ARDNAMURCHAN
There are five districts in North-western Europe where the original
widespread Tertiary lava-fields have been less extensively eroded
than elsewhere, or at least where they have survived in larger and
thicker masses. Whether or not each of them was an isolated area of
volcanic activity cannot now be determined. Their several outflows of
lava within the area of the British Isles may have united into one
continuous volcanic tract, and their present isolation there may be due
entirely to subterranean movements and denudation. There is a certain
convenience, however, in treating the districts separately. They
are--1. Antrim; 2. Mull, Morven and Ardnamurchan; 3. Small Isles; 4.
Skye; 5. The Faroe Islands.
i. ANTRIM[230]
[Footnote 230: The basalts of Antrim are the subject of an abundant
literature. I may refer particularly to the papers of Berger and
Conybeare (_Trans. Geol. Soc._ iii.), the Geological Report of
Portlock, and the Explanations of the Sheets of the Geological Survey
of Ireland. Other papers will be afterwards cited. The general features
of the Antrim plateau are shown on Map VII.]
The largest of the basalt-plateaux of Britain is that which forms so
prominent a feature in the scenery and geology of the North of Ireland,
stretching from Lough Foyle to Belfast Lough, and from Rathlin Island
to beyond the southern margin of Lough Neagh. Its area may be roughly
computed at about 2000 square miles. But, as its truncated strata rise
high along its borders, and look far over the surrounding low grounds,
it must be regarded as a mere fragment of the original volcanic plain.
It may be described as an undulating tableland, which almost everywhere
terminates in a range of bold cliffs, but which, towards the centre and
south, sinks gently into the basin of Lough Neagh. The marginal line of
escarpment, however, presents considerable irregularity both in height
and form, besides being liable to frequent local interruptions. It is
highest on the west side, one of its crests reaching at Mullaghmore,
in County Londonderry, a height of 1825 feet. It sinks down into the
valley of the Bann, east of which it gradually ascends, forming the
well-known range of cliffs from the Giant's Causeway and Bengore Head
to Ballycastle. It then strikes inland, and making a wide curve in
which it reaches a height of more than 1300 feet, comes to the sea
again at Garron Point. From that headland the cliffs of basalt form a
belt of picturesque ground southwards beyond Belfast, interrupted only
by valleys that convey the drainage of the interior of the plateau
to the North Channel. Above the valley of the Lagan the crest of the
plateau rises to a height of more than 1500 feet.
Throughout most of its extent the basalt-escarpment rests on the white
limestone or Chalk of Antrim, beneath which lie soft Lias shales and
Triassic marls. Here and there, where the substratum of Chalk is thin,
the action of underground water on the crumbling shales and marls below
it has given rise to landslips. The slopes beneath the base of the
basalt are strewn with slipped masses of that rock, almost all the way
from Cushendall to Larne, some of the detached portions being so large
as to be readily taken for parts of the unmoved rock. On the west side
also, a group of huge landslips cumbers the declivities beneath the
mural front of Benevenagh.
I have found some difficulty in the attempt to ascertain what was the
probable form of surface over which the volcanic rocks of this plateau
began to be poured out. The Chalk sinks below the sea-level on the
north coast, but, in the outlier of Slieve Gallion, three miles beyond
the western base of the escarpment, it rises to a height of 1500 feet
above the sea. On the east side also, it shows remarkable differences
of level. Thus, below the White Head at the mouth of Belfast Lough,
it passes under the sea-level, but only 16 miles to the south, where
it crops out from under the basalt, its surface is about 1000 feet
above that level. If these variations in height existed at the time
of the outpouring of the basalt, the surface of the ground over which
the eruptions took place was so irregular that some hundreds of feet
of lava must have accumulated before the higher chalk hills were
buried under the volcanic discharges. But it seems to me that much
of this inequality in the height of the upper surface of the Chalk
is to be attributed to unequal movements since the volcanic period,
which involved the basalt in their effects, as well as the platform
of Chalk below it. Had the present undulations of that platform been
older than the volcanic discharges, it is obvious that upper portions
of the basalt-series would have overlapped lower, and would have come
to rest directly on the Chalk. But this arrangement, so far as I am
aware, never occurs, except on a trifling scale. Wherever the Chalk
appears, it is covered by sheets of the lower and not of the upper of
the two groups into which the Antrim basalts are divisible. We have
actual proof of considerable terrestrial disturbance, subsequent to the
date of the formation of the volcanic plateau. Thus, near Ballycastle,
a fault lets down the basalt and its Chalk platform against the
crystalline schists of that district. On the east side of the fault,
the Chalk is found far up the slope, circling round the base of the
beautiful cone of Knocklayd--an outlier of the basalt which reaches a
height of 1695 feet (Fig. 263). The amount of vertical displacement of
the volcanic sheets is here 700 feet.[231] Many other displacements, as
shown by the mapping of my colleagues in the Geological Survey, have
shifted the base of the escarpment from a few inches up to several
hundred feet. Besides actual dislocations, the Antrim plateau has
undergone some marked subsidences of which the most notable is that of
Lough Neagh.[232]
[Footnote 231: Explanatory Memoir of Sheets 7 and 8, Geological Survey,
Ireland, by Messrs. Symes, Egan, and M'Henry (1888), p. 37.]
[Footnote 232: These inequalities in the level of the base of the Antrim
plateau will be more particularly discussed in Chapter xlix., in
connection with the subsidences and dislocations which have affected
the region since the close of the volcanic period.]
It is evident, therefore, that the present position of the Chalk
platform is far from agreeing with that which it presented to the
outflow of the sheets of basalt. But, on the other hand, there can be
no doubt that its surface at the beginning of the volcanic outbursts
was not a level plain. It was probably a rolling country of low bare
chalk-downs, like parts of the South-east of England. The Irish Chalk
attains its maximum thickness of perhaps 250 feet at Ballintoy. But it
is liable to rapid diminution. On the shore at Ballycastle about 150
feet of it can be seen, its base being concealed; but only two and a
half miles to the south, on the outlier of Knocklayd, the thickness is
not quite half so much. On the west side of the plateau also, there are
rapid changes in the thickness of the Chalk. Such variations appear
to be mainly attributable to unequal erosion before the overflow of
the basalts. So great indeed had been the denudation of the Cretaceous
and underlying Secondary formations previous to the beginning of the
volcanic eruptions, that in some places the whole of these strata had
been stripped off the country, so that the older platform of Palæozoic
or still more ancient masses was laid bare. Thus, on the west side of
the escarpment, the basalt steals across the Chalk and comes to rest
directly upon Lower Carboniferous rocks.
The authors who have described the junction of the Chalk and basalts in
Antrim have generally referred to the uneven surface of the former rock
as exposed in any given section. The floor on which the basalt lies is
remarkably irregular, rising into ridges and sinking into hollows or
trenches, but almost everywhere presenting a layer of earthy rubbish
made of brown ferruginous clays, mixed with pieces of flint, chalk,
and even basalt.[233] The flints are generally reddened and shattery.
The chalk itself has been described as indurated, and its flints as
partially burned by the influence of the overlying basalt. But I have
not noticed, at any locality, evidence of alteration of the solid
chalk, except where dykes or intrusive sheets have penetrated it.[234]
There can be no doubt that the hardness of the rock is an original
peculiarity, due to the circumstances of its formation. The irregular
earthy rubble, that almost always intervenes between the chalk and the
base of the basalt, like the "clay with flints" so general over the
Chalk of Southern England, no doubt represents long-continued subærial
weathering previous to the outflow of the basalt. Even, therefore, if
there were no other evidence, we might infer with some confidence from
this layer of rubble, that the surface over which the lavas were poured
was a terrestrial one. Here and there, too, we may detect traces of
the subsidence of the basalt into swallow-holes dissolved in the chalk
subsequent to the outflow of the basalt-sheets.
[Footnote 233: Portlock, _Report on Geology of Londonderry_, etc.
(Geological Survey), p. 117.]
[Footnote 234: See Portlock, _op. cit._ p. 116.]
The Antrim plateau is not only the largest in the British Islands, it
is also the most continuous and regular. It may be regarded, indeed,
as one unbroken sheet of volcanic material, not disrupted by any such
mountainous masses of intrusive rock as in the other plateaux interrupt
the continuity of the horizontal or gently inclined sheets of basalt.
Around its margin, indeed, a few outliers tower above the plains, and
serve as impressive memorials of its losses by denudation. Of these,
by much the most picturesque and imposing, though not the loftiest, is
Knocklayd already referred to, which forms so striking a feature in the
north-east of Antrim (Fig. 263).
[Illustration: Fig. 263.--Section of Knocklayd, an outlier of the
Antrim basalt-plateau lying on Chalk.
1. Crystalline schists; 2. Cretaceous strata; 3. Lower basalts; 4.
Group of tuffs, clays and iron-ore; 5. Upper basalts; _f_. Fault.]
The total thickness of volcanic rocks in the Antrim plateau exceeds
1000 feet; but, as the upper part of the series has been removed by
denudation, the whole depth of lava originally poured out cannot now be
told. A well-marked group of tuffs and clays, traceable throughout a
large part of Antrim, forms a good horizon in the midst of the basalts,
which are thus divisible into a lower and upper group (Fig 264).
The Lower Basalts have a thickness of from 400 to 500 feet. But,
as already mentioned (p. 194), they die out in about six miles to
no more than 40 feet at Ballintoy. They are distinguished by their
generally cellular and amygdaloidal character, and less frequently
columnar structure. The successive flows, each averaging perhaps above
15 feet in thickness, are often separated by thin red ferruginous
clayey partings, sometimes by bands of green or brown fine gravelly
tuff. The most extensive of these tuff-bands occurs in the lower part
of the group at Ballintoy, and can be traced along the coast for
about five miles. In the middle of its course, near the picturesque
Carrick-a-raide, it reaches a maximum thickness of about 100 feet and
gradually dies out to east and west. The neck of coarse agglomerate at
Carrick-a-raide, is doubtless the vent from which this mass of tuff was
discharged (see Fig. 301). Owing to the thinning out of the sheets of
basalts, as they approach the vent, the tuff comes to rest directly on
the Chalk, and for some distance westwards forms the actual base of
the volcanic series.[235] Occasional seams of carbonaceous clays, or of
lignite, appear in different horizons among the basalts. Beneath the
whole mass of basalt, indeed, remains of terrestrial vegetation here
and there occur. Thus, near Banbridge, County Down, a patch of lignite,
four feet ten inches thick, underlies the basalt, and rests directly on
Silurian rocks. Such fragmentary records are an interesting memorial
of the wooded land-surface over which the earliest outflows of basalt
spread.
[Footnote 235: See Explanation of Sheets 7 and 8 of the Geological Survey
of Ireland (1888), p. 23.]
[Illustration: Fig. 264.--Diagram-Section of the Antrim Plateau.
1. Triassic series; 2, 3. Rhaetic strata and Lias; 4. Greensand; 5.
Chalk; 6. Gravel and soil; 7. Lower group of basalts; 8. Group of
tuffs, clays and iron-ore; 9. Upper group of basalts.]
In looking at the great basalt-escarpments of Antrim, the Inner
Hebrides or the Faroe Islands, and in following with the eye the
successive sheets of lava in orderly sequence of level bands from
the breaking waves at the base to the beetling crest above, we are
apt to take note only of the proofs of regularity and repetition
in the outflows of molten rock and to miss the evidence that these
outflows did not always rapidly follow each other, but were separated
by intervals of varying, sometimes even of long duration. One of the
most frequent and conspicuous proofs of such intervals is to be found
in the red layers or partings above referred to which, throughout all
the basalt-plateaux, so commonly intervene between successive sheets
of basalt. These red streaks cannot fail to arrest the eye on the
coast-precipices where by their brilliant contrast of colour, they help
to emphasize the bedded character of the whole volcanic series.
Examined more closely, they are found to consist of clay or bole
which shades into the decomposed top of the bed whereon it lies, and
is usually somewhat sharply marked off from that which covers it.
This layer has long, and I think correctly, been regarded as due to
the atmospheric disintegration of the surface of the basalt on which
it rests, before the eruption of the overlying flow. It varies in
thickness from a mere line up to a foot or more, and it passes into the
tuffs and clays which are sometimes interposed between the sheets of
basalts. It may be looked upon as probably furnishing evidence of the
lapse of an interval sufficiently extended to permit a considerable
subserial decay of the surface of a lava-sheet before the outflow
of the next lava. But an attentive study of the plateaux discloses
other and even more remarkable indications that the pauses between
the consecutive basalt-beds were frequently so prolonged as to allow
extensive topographical changes to be made in a district. Nowhere is
the long duration of some of these intervals more impressively taught
than in the central zone of sedimentary strata in Antrim.
This persistent group of tuffs, clays, and iron-ore is generally from
30 to 40 and sometimes as much as 70 feet thick. From the occurrence
of the ore in it, it has been explored more diligently in recent years
than any other group of rocks in the district, and its outcrop is now
known over most of the plateau. The iron-ore bed varies from less
than an inch up to 18 inches in thickness, and consists of pisolitic
concretions of hæmatite, from the size of a pea to that of a hazel
nut, wrapped up in a soft ochreous clayey matrix.[236] Where it is
absent, its place is sometimes taken by an aluminous clay, worked as
"bauxite," which has yielded stumps of trees and numerous leaves and
cones. Beneath the iron-ore or its representative, lies what is called
the "pavement,"--a ferruginous tuff, 8 to 10 feet thick, resting on
"lithomarge,"--a lilac or violet mottled aluminous earth sometimes
full of rounded blocks or bombs of basalt. The well-known horizon for
fossil plants at Ballypallidy is a red tuff in this zone. The section
of strata between the two basalt-groups at this locality may serve as
an illustration of the nature and arrangement of the deposits.[237]
[Footnote 236: Consult a good essay on the Iron-ore and Basalts of
North-east Ireland by Messrs. Tate and Holden, _Quart. Journ. Geol.
Soc._ xxvi. (1870), p. 151. In this paper the nature, composition and
modes of origin of the iron-ore and its associated strata are fully
discussed.]
[Footnote 237: A. M'Henry, _Geol. Mag._ (1895), p. 263.]
Upper Basalt, compact and often columnar sheets.
Brown laminated tuff and volcanic clays.
Laminated brown impure earthy lignite, 2 feet 3 inches.
Brown and red variegated clays, tuffs and sandy layers, with irregular
seams of coarse conglomerate composed of rounded and subangular
fragments of rhyolite and basalt, 3 feet 4 inches.
Brown, red and yellowish laminated tuffs, mudstones, and bole, with
occasional layers of fine conglomerate (rhyolitic and basaltic),
pisolitic iron-ore band and plant-beds, 8 feet 10 inches.
Lower basalt, amygdaloidal.
In some of the Ballypallidy tuffs the most frequent lapilli are pieces
of green and brown glass, which Mr. Watts compares with the pitchstone
of Sandy Braes, though rarely containing phenocrysts as that rock does.
He has found also in these strata a smaller proportion of lithoidal
rhyolites and occasionally fragments of basic rock.
The pale and coloured clays that occur in this marked sedimentary
intercalation have doubtless been produced by the decomposition of
the volcanic rocks and the washing of their fine detritus by water.
Possibly this decay may have been in part the result of solfataric
action. From true bauxite or aluminium-hydrate, the sediments vary in
composition and specific gravity and pass into aluminous silicates
and iron-ores. They seem to indicate a prolonged interval of volcanic
quiescence when the lavas and tuffs already erupted were denuded and
decomposed.[238]
[Footnote 238: See a note on Bauxite by Professor G. A. Cole, _Scientif.
Trans. Royal Dublin Soc._ vol. vi. series ii. (1896), p. 105.]
The area over which this interesting series of stratified deposits now
extends is obviously much less than it was originally. It has indeed
been so reduced by denudation into mere scattered patches that it
probably does not exceed 170 square miles. But the group can be traced
from Divis Hill, near Belfast, to Rathlin Island, a distance of 50
miles, and from the valley of the Bann to the coast above Glenarm, more
than 20 miles. There can be little doubt that it was once continuous
over all that area, and that it probably extended some way further on
each side. If the so-called Pliocene clays of Lough Neagh be regarded
as parts of this group of strata, its extent will be still further
increased. Hence the original area over which the iron-ore and its
accompanying tuffs and clays were laid down can hardly have been less
than 1000 square miles. This extensive tract was evidently the site of
a lake during the volcanic period, formed by a subsidence of the floor
of the lower basalts. The salts of iron contained in solution in the
water, whether derived from the decay of the surrounding lavas or from
the discharges of chalybeate springs, were precipitated as peroxide in
pisolitic form, as similar ores are now being formed on lake-bottoms
in Sweden. For a long interval, quiet sedimentation went on in this
lake, the only sign of volcanic energy during that time being the dust
and stones that were thrown out and fell over the water-basin, or were
washed into it by rains from the cones of the lava-slopes around.
It may here be remarked that the tendency to subsidence in the Antrim
plateau seems to have characterized this region since an early part
of the volcanic period. The lake in which the deposits now described
accumulated was entirely effaced and overspread by the thick group
of upper basalts. But long after the eruptions had ceased, a renewed
sinking of the ground gave rise to the sheet of water which now forms
Lough Neagh.[239]
[Footnote 239: This subject will be discussed in Chapter xlix.]
Nowhere else among the Tertiary basalt-plateaux of Britain has any
trace been found of so marked and prolonged a pause in the volcanic
activity as is indicated by the Antrim zone of tuffs and clays.
Throughout the Inner Hebrides, indeed, numerous intercalations of
sedimentary material occur among the basalts, but these consist
mainly of tuffs and volcanic conglomerates with less frequent shales
and coal-seams, and they never suggest so distinct and lengthened an
interval as is indicated by the Antrim deposit.
It is not improbable that this interval was marked by the outbreak
of rhyolitic eruptions somewhere in the region. The abundance of
rhyolite fragments in some of the tuffs is striking evidence that
acid rocks were in one way or other brought to the surface at this
time. At Glenarm one of the members of the stratified series is a
marked rhyolitic conglomerate, composed of rounded pebbles of a rock
not unlike the well-known rhyolite of Tardree and Carnearny. These
fragments, obviously of local origin, must either have been derived
from a surface of acid rock laid bare by denudation, or from rhyolite
ejected in lapilli or poured out in streams. I formerly believed that
all the Antrim rhyolites had been injected into the basalts after the
close of the plateau-period. But the proved abundance and wide extent
of the rhyolitic detritus among the sediments associated with the
iron-ore point to a possible outflow of acid lavas with accompanying
tuffs during the sedimentary interval between the two groups of
basalt. The characters of the Antrim rhyolites, however, will be more
particularly discussed in Chapter xlvii., in connection with the acid
rocks of the Tertiary volcanic series.
Immediately above the iron-ore of Antrim, or separated from it in
places by only a few inches of tuff, comes the group of Upper Basalts,
which varies up to 600 feet in thickness, though as the upper portion
has been everywhere removed by denudation, no measure remains of what
may have been the original depth of the group. The general character
of these basalts is more frequently columnar, black and compact, and
with fewer examples of a strongly amygdaloidal structure than in the
lower group. But this distinction is less marked in the south than in
the north of Antrim, so that where the intervening zone of tuffs and
iron-ore disappears, no satisfactory line of division can be traced
between the two groups of basalt. The occurrence of that zone, however,
by giving rise to a hollow or slope, from which the upper basalts rise
as a steep bank or cliff, furnishes a convenient topographical feature
for mapping the boundary of these rocks. Among the upper basalts, also,
there is perhaps a less frequent occurrence of those thin red partings
of bole between successive flows, so conspicuous in the lower group.
But the flows are not less distinctly marked off from each other.
Nowhere can their characteristic features be better seen than along the
magnificent range of cliffs from the Giant's Causeway eastwards. The
columnar bed that forms the Causeway is the lowest sheet of the upper
group, and may be seen resting directly on the zone of grey and red
tuffs. It is about 60 or 70 feet thick; and, while perfectly regular in
its columnar structure at the Causeway and the "Organ," assumes further
eastward the confusedly starch-like arrangement of prisms already
referred to. But in the great cliff section of the "Amphitheatre,"
the more regular structure is resumed, the bed swells out to about
80 feet in thickness, and columns of that length run up the face
of the precipice, weathering out at the top into separate pillars,
which, perched on the crest of an outstanding ridge, are known as the
"Chimneys." The basalt-beds that succeed the lowest one are each only
about 10 to 15 feet thick (Fig. 265).
[Illustration: Fig. 265.--View of Basalt escarpment, Giant's Causeway,
with the Amphitheatre and Chimneys. (From a photograph by Mr. R.
Welch.)]
Between the successive sheets of the Upper Basalts thin seams of red
ferruginous clay though, as I have said, less frequent perhaps than in
the lower group, continue to show that the intervals between successive
eruptions were of sufficient duration to admit of some subærial decay
of the surface of a lava before the outflow of the next bed. Occasional
thin layers of tuff also, and even of pisolitic iron-ore, have been
observed among these higher basalts. But the most interesting and
important intercalations are inconstant seams of lignite. One of the
most conspicuous of these lies immediately above the basalt of the
"Causeway," where it was long worked for fuel, and was found to be
more than six feet thick. But it is quite local, as may be seen at the
"Organ" over which it lies, having a thickness of only 12 inches and
rapidly dying out so as to allow the basalts above and below it to come
together. The removal of the upper portion of the basalts by denudation
has destroyed the records of the latest part of the volcanic history of
the Irish plateaux.
It is obvious that nowhere in Antrim does any trace exist of a central
vent or cone from which the volcanic materials were discharged. There
is no perceptible thickening of the individual basalt-sheets, nor of
the whole series in one general direction, in such a manner as to
point to the site of some chief focus of eruption. Nor can we place
reliance on the inclination of the several parts of the plateau. I have
pointed out that the varying dip of the beds must be attributed mainly
to post-volcanic movements, or at least to movements which, if not
later than all the phases of volcanic action, must have succeeded the
outpouring of the plateau-basalts. There has been a general subsidence
towards the central and southern tracts now occupied by the valley of
the Bann and Lough Neagh. But nowhere in the depression is there any
trace of the ruins of a central cone or focus of discharge.
The Antrim plateau, in these respects, resembles the others. But as
has already been remarked, it differs from them in one important
particular. It has nowhere been disrupted by huge bosses of younger
rocks, such as have broken up the continuity of the old lava-fields
further north. Yet it also is not without its memorials of younger
protrusions. It contains not a few excellent examples of true volcanic
vents, and, as above stated, it includes some small acid bosses that
may represent the great protrusions of the Inner Hebrides, and may have
been connected with superficial outflows of rhyolitic lava and showers
of rhyolitic tuff.
ii. MULL, MORVEN AND ARDNAMURCHAN
This plateau covers nearly the whole of the island of Mull, embraces a
portion of Morven on the Argyleshire mainland, and, stretching across
Loch Sunart, includes the western part of the peninsula of Ardnamurchan
(Map VI.). That these now disconnected areas were once united into a
continuous lava-field which extended far beyond its present limits
is impressively indicated by their margin of cliffs and fringe of
scattered islands and outliers. The plateau went west, at least, as far
as the Treshnish Isles, which are composed of basalt. On its eastern
border, a capping of basalt on the top of Beinn Iadain (1873 feet) in
Morven, and others further north, prove that its volcanic sheets once
spread into the interior of Argyleshire (Fig. 266). On the south, its
fine range of lofty cliffs, with their horizontal bars of basalt, bear
witness to the diminution which it has undergone on that side; while,
on the north, similar sea-walls tell the same tale. Not only has it
suffered by waste along its margin, it has also been deeply trenched
by the excavation of glens and arms of the sea. The Sound of Mull
cuts it in two, and the mainland portion is further bisected by Loch
Sunart, and again by Loch Aline. The island of Mull is so penetrated
by sea-lochs and divided by deep valleys that a comparatively slight
depression would turn it into a group of islands. But, besides its
enormous denudation, this plateau has been subjected to disruption,
and perhaps also to subsidence, from subterranean movements. In the
southern portion of the island of Mull it has been broken up by the
intrusion of large bosses and sheets of gabbro, and by masses as
well as innumerable veins of various granitoid and felsitic rocks.
In Ardnamurchan, it has suffered so much disturbance from the same
cause that its original structure has been almost obliterated over a
considerable area. Moreover, it has been dislocated by many faults,
by which different portions have been greatly shifted in level. The
most important of these breaks is one noticed by Professor Judd, and
visible to every tourist who sails up the Sound of Mull. It traverses
the cliffs on the Morven side, opposite Craignure, bringing the basalts
against the crystalline schists, and strikes thence inland, wheeling
round into the long valley in which Lochs Arienas and Teacus lie.
On its western side, the base of the basalt-series is almost at the
sea-level; on its eastern side, that platform rises high into the
outliers of Beinn na h-Uamha (1521 feet) and Beinn Iadain. The amount
of displacement is probably more than 1000 feet. Many other minor
faults in the same district show how much the crust of the earth has
been fractured here since older Tertiary time.
[Illustration: Fig. 266.--Basalt-capping on top of Beinn Iadain, Morven.
The hummocky ground to the right consists of the Highland schists
against which the basalts are brought by lines of dislocation.[240]]
[Footnote 240: There are no fewer than three faults in the basalt-capping
on the summit of Beinn Iadain. By bringing the basalts and schists into
juxtaposition, they have given rise to topographical features that can
be seen even from a distance.]
A little to the west of Mull, and belonging originally to the same
plateau, lies the isle of Staffa, the famous columnar basalts of which
first attracted the attention of travellers, and gave to the Tertiary
volcanic rocks of Scotland their celebrity (Fig. 266_a_).
[Illustration:
Fig. 266_a_.--View of the south side of Staffa, showing the bedded
and columnar structure of the basalt. The rock in which the cave to
the left hand has been eroded is a conglomeratic tuff underlying
the basalt; to the right is Fingal's Cave. These caverns bear
witness to the enormous erosive power of the Atlantic breakers.
]
In spite of the extent to which it has suffered from denudation and
subterranean disturbance, and indeed in consequence thereof, the Mull
plateau presents clear sections of many features in the history of
the basalt-outflows and of the subsequent phases of Tertiary volcanic
action which cannot be seen in the more regular and continuous
tableland of Antrim. Moreover, it still possesses in its highest
mountain, Ben More (3169 feet), a greater thickness, and probably
a higher series, of lavas than can now be seen in any other of the
plateaux.
The difficulties, already referred to in regard to Antrim, of tracing
the probable form of ground on which the volcanic eruptions began, are
even greater in the case of the Mull plateau. We can dimly perceive
that the depression in the crystalline rocks of the Highlands which
had, from at least the older part of the Jurassic period, stretched in
a N.N.W. direction along what is now the western margin of Argyleshire,
lay beneath the sea in Jurassic time, and was then more or less filled
up with sedimentary deposits. The hollow appears thereafter to have
become a land-valley, whence the Jurassic strata were to a large extent
cleared out by denudation before its subsequent submergence under the
sea in which the upper Cretaceous deposits accumulated. Professor
Judd has shown that relics of these Cretaceous strata appear on both
sides of the plateau from under the protecting cover of basalt-sheets.
But, before the volcanic eruptions began, the area had once again
been raised into land, and the youngest Secondary formations had been
extensively eroded.
In their general aspect the basalts of Mull agree with those of
Antrim, and the circumstances under which they were erupted were no
doubt essentially the same. But considerable differences in detail are
observable between the succession of rocks in the two areas. When I
first visited the island in 1866, the only available maps, with any
pretensions to accuracy, were the Admiralty charts; but, as these do
not give the interior except in a generalized way, it was difficult
to plot sections from them, and to arrive at satisfactory conclusions
as to the thickness of different groups of rock. Accordingly, as the
successive nearly flat flows of basalt can be traced from the sea-level
up to the top of Ben More, I contented myself with the fact that the
total depth of lava-beds in Mull was at least equal to the height of
that mountain, or 3169 feet. The publication of the Ordnance Survey
Maps now enables us to make a nearer approximation to the truth. From
the western base of the magnificent headland of Gribon, the basalts in
almost horizontal beds rise in one vast sweep of precipice and terraced
slope to a height of over 1600 feet, and then stretch eastwards to
pass under the higher part of Ben More, at a distance of some eight
miles. They have a slight easterly inclination, so that the basement
sheets seen at the sea-level, at the mouth of Loch Scridain, gradually
sink below that level as they go eastward. It is not easy to get a
measurement of dip among these basalts, except from a distance. If we
take the inclination at only 1°, the beds which are at the base of the
cliff on the west, must be about 700 feet below the sea on the line
of Ben More, which would give a total thickness of nearly 3900 feet
of bedded lava below the top of that mountain. We shall not probably
overestimate the thickness of the Mull plateau if we put it at 3500
feet.
The base of the volcanic series of Mull can best be seen on the
south coast at Carsaig, and at the foot of the precipices of Gribon.
As already stated, it is there found resting above Cretaceous and
Jurassic rocks. The lowest beds are basalt-tuffs, of the usual dull
green colour. They are in places much intermingled with sandy and
gravelly sediment, as if the volcanic debris had fallen into water
where such sediment was in course of deposition. One of the most
interesting features, indeed, in this basement part of the series, is
the occurrence of bands of non-volcanic material which accumulated
after the tuffs and some of the lavas had been erupted, but before the
main mass of basalts. Those at Carsaig include a lenticular bed, 25
feet thick, of rolled flints, which, with some associated sandy bands,
lies between sheets of basalt. On the opposite side of the promontory
is the well-known locality of Ardtun, from which the first land-plants
in the volcanic series were determined. The actual base of the basalts
is not there seen, being covered by the sea. The "leaf-beds," with
their accompanying sandstones, gravels, and limestone, lie upon a
sheet of basalt, which in some parts is exceedingly slaggy on the
top, passing down into a black compact structure, and assuming at
the base of the cliff a columnar arrangement, with the prisms curved
and built up endways towards each other. Some of the gravels exceed
30 feet in thickness, and consist of rolled flints, bits of chalk,
and pieces of basalt and other basic igneous rocks. But some of
their most interesting ingredients are pebbles of sanidine lavas,
which have been recognized in them by Prof. G. Cole.[241] No known
protrusions of such lavas occur anywhere beneath or interstratified
with the plateau-basalts of this district. As will be afterwards
shown, all the visible acid rocks, the geological relations of which
can be ascertained, are here of younger date than these basalts. I am
disposed to regard the fragments found in the Ardtun conglomerates as
probably derived from some of the basalt-conglomerates of the plateau,
in which fragments of siliceous igneous rocks do occur. Though there
is no evidence that any lavas of that nature were here poured out at
the surface before or during the emission of the basalts, the contents
of these fragmental volcanic accumulations suggest that such lavas,
already consolidated, lay at some depth beneath the surface, and that
fragments were torn off from them during the explosions that threw out
the materials of the basalt-conglomerates to the surface.
[Footnote 241: _Quart. Jour. Geol. Soc._ xliii. (1887) p. 277.]
The succession of strata at the Ardtun headland varies considerably in
a short distance, some of the sedimentary deposits rapidly increasing
or diminishing in thickness. The section as measured by Mr. Starkie
Gardner is as follows[242]:--
Columnar basalt, 40 feet.
Position of first leaf-bed, obscured by grass, about 2 feet.
Gravel varying from about 25 feet to a maximum of nearly 40 feet.
Black or second leaf-bed, 2-1/2 feet.
Gravel about 7 feet.
Grey clay, 2 feet.
Laminated sandstone, 6 inches, with 3 inches of fine limestone,
containing leaves at the base.
Clay, with leaves at base, 1 foot.
Clunch, with rootlets, 7 inches.
Amorphous basalt, becoming columnar at base, about 60 feet.
[Footnote 242: _Op. cit._ p. 280.]
Mr. Starkie Gardner has called attention to the extraordinarily fresh
condition of the vegetation in some of the layers of the Ardtun
section. One of the leaf-beds he has found to be made up for an inch or
two of a pressed mass of leaves, lying layer upon layer, and retaining
almost the colours of dead vegetation. Among the plants represented is
a large purple _Ginkgo_ and a fine _Platanites_, one leaf measuring
15-1/2 inches long by 10-1/2 broad. The characteristic dicotyledonous
leaves at this locality possessed relatively large foliage.[243]
[Footnote 243: For fuller local details regarding the Ardtun leaf-beds,
I may refer to the original paper by the Duke of Argyll (_Quart. Jour.
Geol. Soc._ vii. p. 89), and to the memoir by Mr. Starkie Gardner (_op.
cit._ xliii. (1887), p. 270).]
To the early observations of Macculloch we are indebted for the
record of an interesting fact in connection with the vegetation of
the land-surface over which the first lava-flows spread. He figured a
vertical tree trunk, imbedded in prismatic basalt, and rightly referred
it to some species of fir.[244] This relic may still be seen under the
basalt precipices of Gribon. Mr. Gardner found it to be "a large trunk
of a coniferous tree, five feet in diameter, perhaps _Podocarpus_,
which has been enveloped, as it stood, in one of the flows of trap to
the height of 40 feet. Its solidity and girth evidently enabled it to
resist the fire, but it had decayed before the next flow passed over
it, for its trunk is a hollow cylinder filled with debris, and lined
with the charred wood. A limb of another, or perhaps the same tree, is
in a fissure not far off."[245]
[Footnote 244: _Western Islands_, vol. i. p. 568, and plate xxi. Fig. 1.]
[Footnote 245: _Quart. Jour. Geol. Soc._ xliii. p. 283.]
At different levels in the volcanic series of Mull, beds of lignite and
even true coal are observable. These seem to be always mere lenticular
patches, only a few square yards in extent. The best example I have
met with lies among the basalts near Carsaig. It is in part a black
glossy coal, and partly dull and shaly. Some years ago it was between
two and three feet thick, but now, owing to its having been dug away by
the shepherds, only some six or eight inches are to be seen. It lies
between two basalt-flows, and rapidly disappears on either side.
More frequent than these inconstant layers of fossil vegetation are
the thin partings of tuff and layers of red clay, sometimes containing
iron-ore, which occur at intervals throughout the series between
different flows of basalt. But even such intercalations are of trifling
thickness, and only of limited extent. The magnificent precipices of
M'Gorry's Head and Gribon expose a succession of beds of columnar
amorphous and amygdaloidal basalt, which must attain a thickness of at
least 2500 feet, before they are overlain by the higher group of pale
lavas in Ben More. On the east side of the island, thin tuffs and bands
of basalt-conglomerate occur on different horizons among the bedded
basalts, from near the sea-level up to the summit of the ridge which
culminates in Beinn Meadhon (2087 feet), Dùn-da-Ghaoithe (2512 feet),
and Mainnir-nam-Fiadh (2483 feet). Reference has already been made to
the remarkably coarse character of some of the breccias intercalated
among the basalts in this part of Mull, and to the enormous dimensions
of some of the masses of mica-schist and quartzite which have been
carried up from a depth of 2000 feet or more by volcanic agency (see
_ante_, p. 196, and Fig. 262).
Above the ordinary compact and amygdaloidal basalt comes the higher
group of pale lavas already referred to as forming the uppermost
part of Ben More, whence it stretches continuously along the pointed
ridge of A'Chioch, and thence northwards into Beinn Fhada. The same
lavas are likewise found in two outliers, capping Beinn a' Chraig,
a mile further north, and I have found fragments of them on some of
the loftier ridges to the south-east. This highest and youngest group
of lavas in the plateaux has been reduced to mere isolated patches,
and a little further denudation will remove it altogether. Yet it is
not less than about 800 feet thick, and consists of bedded andesitic
or trachytic lavas, which alternate with and follow continuously
and conformably upon the top of the ordinary plateau-basalts. These
dull, finely crystalline or compact, light-grey rocks weather with a
characteristic platy form, which has been mistaken for the bedding
of tuffs. The fissility, however, has none of the regularity or
parallelism of true bedding, and may be observed to run sometimes
parallel with the bedding of the sheets, sometimes obliquely or even
at right angles to it. Even where this structure is best developed,
the truly crystalline nature of the rocks can readily be detected.
Some of them are porphyritic and amygdaloidal, the very topmost bed
of the mountain being a coarse amygdaloid. Intercalated with these
curious rocks there are others in which the ordinary characters of
the dolerites and basalts of the plateaux can be recognised. The
amygdaloids are often full of delicate prisms of epidote.
In Mull, as in the other areas of terraced basalts, we everywhere meet
with gently inclined sheets, which do not thicken out individually or
collectively in any given direction, except as the result of unequal
denudations. So far as I have been able to discover, they afford
no evidence of any great volcanic cone from which they proceeded.
Their present inclinations are unquestionably due, as in Ireland, to
movements subsequent to the formation of the plateau. In Loch-na-keal
they dip gently to the E.N.E.; in Ulva and the north-west coast to
N.N.E.; near Salen to W.S.W. on the one side, and N.W. on the other.
Round the southern and eastern margins of the mountainous tract of the
island, they dip generally inwards to the high grounds.
The Mull plateau presents a striking contrast to that of Antrim, in
the extraordinary extent to which it has been disrupted by later
protrusions of massive basic and acid rocks over a rudely circular
area, extending from the head of Loch Scridain to the Sound of Mull,
and from Loch-na-keal to Loch Buy. The bedded basalts have been invaded
by masses of dolerite, gabbro, and granophyre, with various allied
kinds of rock. They have not only been disturbed in their continuity,
but have undergone considerable metamorphism.
Again, further to the north, in the promontory of Ardnamurchan,
the plateau has been disrupted in a similar way, and only a few
recognisable fragments of it have been left. These changes will be more
appropriately discussed in connection with similar phenomena in the
other plateaux further north.