The Island of Mull, for the most part, is an extinct volcano. It furnishes
the most wonderful example of dissected volcano kown to science. Of its two
hundred and thirty-five thousand acres, only thirteen thousand are arable.
An examination of its structure exhibits the action of terrific forces which
have wrought strange changes, in forming the present surface. The action of
water has been almost as great as the flow of molten masses. The base of the
volcano has a circumference of about forty miles, with the highest point
(Ben More) reaching an altitude of three thousand one hundred and sixtv-nine
feet. If the rocks that came from Ben More were restored to that mountain,
its elevation would be not less than six thousand feet, and the plateaux, of
which it forms a part, would reach a much greater altitude. While there are
many bens closely connected yet, originally there was but one. Only
fragments of the great volcano remain, which give striking evidences of the
paroxysms undergone during the changes. During these periods millions of
tons of material would be blown from the crater, and within a comparatively
short time a new cone of a different form would result; and yet it may be
strange that so many mountains should be formed by volcanic action, in so
restricted an area.
The present appearance exhibits a gradual increase of the inclination of the
lava beds on the higher slope of the main mountain, and the rugged surface
shows great faults that contributed towards producing the central
The enormous amount of denudation, which the volcanic products have
undergone, represents a remarkable series of events, the latest dating to a
very remote period, and in turn, the previous epochs were separated by
untold ages. The proofs of extensive denudation between the outflow of great
masses appear to be of a very positive character. The basaltic lavas and
their accompanying piles of scoriae, in many cases, rest directly on the
intrusive felsites and granites which formed the interior and deeply seated
portions. The felspatic lavas, before they were buried under the
overwhelming products of the period of basaltic eruptions, experienced
considerable movements and suffered extensively from denudation* It is also
probable that the basaltic streams may frequently lie in hollows eroded in
The discovery of Miocene beds intercolated with the basalt, in Mull,
culminated in elucidating the history of the Tertiary period existing in the
Highlands of Scotland. The discovery presents the changes which have taken
place in the construction and distribution of basaltic rocks. In this field
Mull is unrivalled, for here may be found a complete and correct
illustration of the revelation which volcanic rocks may bear to one another.
Plainly stated the Mull plateaux district embraces nearly the whole of the
island, crosses over to Morvern and Loch Sunart and includes a part of
Ardnamurchan, and extends westward as far as the Treshnish Isles. The whole
of this great plateaux has been subjected to demunition and has suffered by
waste along its margin and deeply trenched by the excavation of glens and
arms of the sea. Mull is so pene<-trated by sea-lochs and divided by deep
valleys that a comparatively slight depression would turn it into a group of
islands. Besides its enormous denudation it has been subjected to disruption
and perhaps to subsidence from subterranean movements. In its southern part
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. The isle may be said to be composed of layers of basaltic lava piled
upon one another and that to a depth of two thousand feet, though now broken
up by denuding agencies. These layers, on the south, through the agency of a
great fault, are brought into abrupt contact with palaeozoic rock.
As already noted, in the center of the plateaux rises a group of mountains
which exhibit a number of intrusive masses composed of rocks varying in
hardness, and associated with vast deposits of volcanic agglomerates and
breccias which irregularly yield to denuding forces.
The basis of the group of mountains consists of masses of highly siliceous
intrusive rock. In the great valleys these masses are seen to be granite,
usually of the hornblendic variety, but passing into ordinary granite with
mica in the deepest and most central portions. As the masses are traced
upwards and outwards the granite gradually passes into
The outlying mountains are made up of lava streams, nearly horizontally
deposited, presenting thick masses of fel-stone, which alternate with beds
of scoriae, lapelli, and ashes, containing included blocks of stratified
rocks. This rim of the mountains represents the lavas and fragments ejected
from a volcanic vent.
Masses of granitic and felsitic rocks constitute the bulk of Ben Buy, Ben
Varriach, and Ben Talla, together with a great abundance of gabbro rock,
from which proceed veins, sheets and intrusive masses of irregular form,
that traverse the whole of the highly siliceous rocks in every direction.
veins and dykes proceed to great distances from the central mass and
intersect the lavas of the great plateaux, and also the various Primary and
Overlapping the edges of the felstone-lava series, may be found the
basaltic-lavas of the great surrounding plateaux. Surrounding the central
masses of eruptive rocks patches qf volcanic agglomerates and breccias,
which alternate with lava sheets, and traversed by innumerable veins and
dykes. These patches constitute the last remaining vestiges of the great
The Mull plateaux affords a striking example of 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 dolorite, gab-bro and granophyre, with
various allied kinds of rocks. They have been disturbed in their continuity
and undergone considerable metamorphism.
The most extensive development of lavas which are readily distinguishable
from the group of plateaux-basalts occur in Mull. These rocks form part of a
group of pale lavas which overlie the main mass of the plateau-basalts and
More, together with several of the lofty contiguous mountains. These rocks
are called andesites and trachytes.
The slopes above Fishnish Bay show a group of basalts, which die out
southward, and are overlapped by a younger group that has been poured over
their ends. A more striking example occurs beyond the west end of Glen More,
where one series of basalts has been tilted up, during some volcanic
disturbance, and there had a later series banked up against its edges.
From the western base of the headland of Gribun, the basalts, in almost
horizontal beds, rise in one vast sweep of precipice and terraced slope to a
height of over sixteen hundred feet, and then stretch eastward to pass under
the higher part of Ben More, at a distance of eight miles. They have a
slight easterly inclination, so that the basement sheets, at the sea-1
level, at the mouth of Loch Scridain, gradually sink below that level as
they move eastward.
The precipices of Gribun, at McGorry’s Head, expose a succession of beds of
columnar amorphous and amygdaloidal basalt, which attains a thickness of
twenty-five hundred feet before they are overlain by the higher group of
pale lavas in Ben More.
On the east side of Mull, 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 Ben Meadhon (2087 feet),
Dun-da-Gha-oithe (2512 feet), and Mainnir nam Fiadh (2483 feet).
Above the ordinary compact and amygdaloidal basalt comes the higher group of
pale lavas forming the uppermost part of Ben More, and then stretching
continuously along the pointed ridge of A’Chioch, and thence northward into
Ben Fhada. The same lavas are formed in two outliers, capping Ben Craig, a
mile farther north.
The highest and youngest group of lavas consists of mere isolated patches
about eight hundred feet thick, consisting of bedded andesitic lavas, which
alternate with and follow continuously and conformably upon the top of the
Everywhere inclined sheets are met with in the terraced basalts which do not
thicken out individually and collectively in any given direction, except as
the result of unequal denudation. They afford no evidence of any volcanic
cone from which they originated. Their present inclinations are due to
movements subsequent to the formation of the plateau.
An abundant rock named gabbro, but commonly called greenstone, was early
noticed on Mainnir nam Fiadh (2483 feet) near Loch Don. This rock is
regularly interstratified with the basalt. It is well displayed in beds and
sheets. The district in which it is well placed lies between Loch na Keal
and Loch Spelve. The sheets are specially prominent along the higher parts
of the ridge that run northwards from the northern end of Loch Spelve, and
along the west side of Glen Forsa, but extending out into the thickest mass
in the southwestern part of the hilly ground, where, from above Craig, in
Glen More, they cross the valley, and from the rugged ridge that rises into
Ben Buy (2354 feet), and stretches eastward to near Ardara. On the declivity
to the west of Ben Buy and Loch Fhuaran is a fine line of the rock. The main
mass of gabbro sets in on Ben Buy at a height of seventeen hundred feet, and
become a huge overlying sheet, which reaches a thickness of eight hundred
feet. It is enormously thicker than any other sheet on the island.
The granophyre bosses, which form some of the hillsides, are among the most
astonishing of the wonderful series, which, dissected by denudation reveal
the structure of the volcanic region. They lie in two chief areas, one of
which extends along the northern flanks of the mountains tract from the
western side of Ben Fhada, across Loch Ba, to the west of Glen Forsa, while
the other occupies for over three miles the bottom of Glen More, the deep
valley which skirting the southern side of the chief group of hills,
connects the east side of the island by road with the head of the great
western inlet of Loch Scridain. Among the lesser areas, one extends about a
mile along the declivities to the south of Salen, across the valley of the
Allt na Searmoin; another occurs at Salen; a third extends along the shore
of Craignure. In the interior are isolated areas, besides thousands of veins
in the central group of hills and valleys which form the basins of Glen
Cannel and Forsa Rivers.
The chief northern boss is that of Loch Ba, having a length of nearly six
miles, with a breadth varying from a quarter of a mile to a mile and a
quarter. It descends to within fifty feet of the sea-level, and is exposed
along the crest of Ben Fhada at a height of eighteen hundred feet. Along its
southern boundary is a compact black quartz-felsite, which runs as a broad
dyke-like ridge from the head of the Scarrisdale water north eastward across
Loch Ba, and spreads out eastward into a marl more than a mile broad, on the
heights above Killbeg, in Glen Forsa.
From a distance of several miles a contrast may be seen between the outer
and inner parts of the hills that lie to the southwest of Loch Ba. From afar
there may be readily traced the dark bedded basic rocks rising terrace above
terrace, from the shore of Loch na Keal, to form the seaward faces of the
hills along the southern side of that body of water. Perhaps the contrast is
more striking from the hilly side, where astonishment becomes intensified by
the sheets of dolerite and basalt which dip towards the northeast. Of all
the junction lines between the acid bosses and the lavas of the plateau,
those exposed on the Mull hillsides are the most extraordinary.
Along the shore at Salen the bedded basalts succeed each other in well
defined sheets, some being solid, massive and non-amygdaloidal1, and others
quite vesecular. Immense numbers of dykes cut those rocks, and they are
likewise pierced by occasional felsitic intrusions. On the opposite side of
the island, away from the central masses of acid rock, may be traced the
bedded basalts with so gradual a diminution of the induration that no
definite boundary line for the metamoiV phism can be drawn.
In the bed of the south fork of the Scarrisdale stream, a separate mass of
granophyre protrudes through the basalts in advance of the main mass, and a
little higher up on the outskirts of that mass, narrow ribbons of the
granophyre run through the basic rocks. Similar sections may be seen on the
flanks of Ben Fhada, especially in the great corry of Ben More. On the east
side of Loch Ba are numerous similar intrusions. At the east end of Loch na
Dairdh, where the granophyre has been intruded into the basalts, specimens
may be obtained showing the two rocks welded together. On the slopes of
Cruach Torr an Lochain, where the granophyre has a felsitic selvage, the
bedded basalts are traversed by veins of the latter material. A little
further east, at the head of Allt na Sear-moin, the bedded basalts are
intersected by another protrusion from the compact felsitic porphyry.
On the southeastern side, between the head of the Scarrisdale River and Loch
Ba, the line of junction between the granophyre and basalt is nearly
vertical, but a body of black felsite intervenes as a huge wall between the
ordinary granophyre and the basalt. On Ben Fhada and Ben a’Chraig the line
of separation is inclined, outwards. On the top of the ridge of Ben a’Chraig
the outliers are parts of the upper basalt of Glen More. The same rocks,
prolonged on the other side of Scarrisdale Glen, sweep over the summit of
Ben Fhada, and run on continuously into the crest of A’Chioch and the upper
part of Ben More. There are sections on the southern flanks of Ben Fhada,
where the upper surface of the granophyre comes down obliquely across the
edges of the lavas,, and allows the junction of the basalts and the pale
group to be seen above it. As in the case of Ben an Dubhaich, it is as if
the granophyre had eaten its way upward and dissolved the rocks which it has
The usual contact metamorphism has been produced around this intrusive boss.
It is the most marked in the out-lyers that cap Ben a’Chraig, and are the
two ridges to the southwest, where it consists in a high degree of
induration, the production of an irregularly joined structure, and the
ef-facement of the obvious bedding which characterizes the unaltered rocks.
The position of this eruptive mass, a mile broad, breaking through without
violently tilting, more than eighteen hundred feet of the bedded basalts,
and then stopping short about the base of the pale group, presents a curious
problem in geological physics.
A little to the south of the tarn called Loch na Dairidh, the granophyre is
succeeded by the black flinty felsite. Lying on its surface are detached
knolls of much altered dolerite, basalt, and agglomerate. That the
agglomerates do not belong to the period of the eruption of the granophyre
and felsite but to that of the bedded basalts, may be inferred from their
intense induration next the acid rocks, and also from the fact that similar
breccias are accurately found here interposed between the bedded basalts.
This is well proved on the hill above the Coille na Sroine.’
Along the strip of ground now occupied by the Loch Ba boss of granophyre and
felsite, there once stood a line of vents, from which, besides the usual
basalt debris, there were ejected many pieces of different felsitic rocks,
and there eruptions of fragmentary material took place during the
accumulation of the plateau basalts. These volcanic funnels occasioned a
series of points or a line of weakness of which, in a long subsequent
episode of the protracted volcanic period, the acid rocks took advantage,
forcing themselves upwards therein and leaving only slight traces of the
vents which arrested! their ascent.
The second or Glen More boss, instead of rising into hilly ground, is
confined to the bottom of the main and tributary valleys and has only been
revealed by the extensive denudation to which these hollows owe their
origin. It begins nearly a mile below Torness, and extends up to Loch
Airdeglais, a distance of four miles. Some of the best sections to show
offshoots of this rock are along the steep hill slope which mounts from the
water-slip in Glen More southward into the Creag na h-Iolaire (eagle’s
Crag), and thence up into the great gabbro ridge of Ben Buy.
One of the most remarkable examples of breccias of non-volcanic materials
occur near the summit of Sgurr Dearg, northwest of Loch Spelve. The bedded
basalt encloses a lenticular band of exceedingly coarse breccia consisting
mainly of angular pieces of quartzite with fragments of amygdaloidal basalt.
In the midst of the breccia lies a huge mass of erupted mica-schist, at
least one hundred yards long by thirty yards wide. To the west, owing to
thinning out of the breccia, this piece of schist comes to lie between two
beds of basalt. A little higher up, other smaller, but still large blocks of
similar schist, are involved in the basalt. The visible mica-schist measures
fifteen thousand cubic yards, which weighs about thirty thousand tons.. A
remarkable feature consists in its being located two thousand feet up, in
the basalt. A similar breccia lies near Carsaig.
The tertiary lavas of eastern Mull are divisible into por-phoritic and
non-porphoritic basalts. Towards the center of the island, which appears to
have a general basin structure, the upper lavas are covered by a thick
breccia charged with lava, schist, granophyre, gabbro, and sandstone
fragments, all heaped together without assortment. Beginning at Sgurr Dearg,
this breccia can be followed on to a mountain of schist rising up through
the lavas, and is here particularly crowded with schist debris; it also
locally rests upon a highly eroded surface of gabbro which is intrusive in
the schist. No lavas have been formed connected with the breccia, and it?
formation may well mark a respite in the volcanic history of Mull.
The Mull dykes have a phase almost peculiar to the isle. They are of
frequent occurences. Along the shore of the Sound of Mull, between Pennygown
and Fishnish Bay, are numerous dykes running north to north-east, which are
distinct from those of the north-west. At Leth Thorcaill, a mile and a half
east of Fishnish Bay, is a longential dyke, and another on the shore west of
Pennygown, and still another on the hillside south of the burial ground.at
Knock. More than one class of dykes occur to the south of Ben More. The same
occurs to the west of Dererach, where massive erosion leaves them standing
out like a wall. A northwest dyke on the southeast flank, of Ben More has so
indurated the lavas near it that a trachytic intrusion has been unable to
The center of Mull is also characterized by numerous ring-structures, as may
be evidenced by the occurrence of inclined sheets of basic and acid rocks
that have risen through fissures dipping inwards towards the middle of the
isle. This outcrop describes curves which are greater, or less segments of
circles. The center of the series of intrusions was not constant in its
position, but shifted in course of time along a ’ northern line for a
distance of two miles. Verticle intrusions of circular shape are also found,
especially, in the district of Loch Ba.
It must be further noticed that a remarkable central subsidence took place
in the volcano. The five sections of lavas of the basaltic plateaux exposed
along the shores of the deep fiords of Loch Scridain and Loch na Keal,
instead of sloping away from the great central masses of eruptive rock,
reverse the order, and for many miles dip towards them at angles varying
from two to. three degrees, the inclination increasing towards the volcano.
From this it is evident that a great subsidence took place in the great
central mass of the island. This is also sustained by the inclination of the
whole of the lavas surrounding the volcano. Besides all this, in the valley
between Kilfinichen and Gribun, where the cretaceous rocks underlying the
lavas, are suddenly cut off, there ,is the evidence of the existence of
faults, the downthrow of which is in all cases towards the great central
mass. So, it has been observed, that a great caldera has been observed in
the neighborhood of Loch Ba, and also to its south. The southern and eastern
margins can be traced for a considerable distance, with the volcanic
breccia. In the interior of the caldera there are great lava flows,
aggregating from two thousand to three thousand feet in thickness, which
frequently exhibit “pillow-structure” in great perfection. This structure is
found only in lavas that have congealed under water, and leads to the
conclusion that the caldera was at times occupied by a lake.
Other rocks found in various localities deserve special consideration but
must be passed over; and the fragments of, flint and chalk, which are of
common occurrence, should court profound investigation.
Some of the bens afford the clearest insight into the relations of the
different rocks which constitute them. It is well to particularize some of
these bens, with accompanying illustrations, in order that the subject might
be simplified. Almost any ben might exhibit the facility with which certain
of the volcanic rocks yield to disintegrating forces, owing to the extremely
Ben More:—Beginning at the highest, most commanding and widely known peak,
it is found fairly to illustrate the relation which the several volcanic
products bear to one another. The lower parts of Ben More, with its
surrounding spurs, present the granites, felsites, felstone lavas, felspatic
agglomerates, with the intersecting dykes of gabbro, dolerite and basalt.
Uoon these is a mass resting unconformably, many hundreds of feet in
thickness, and constituting the whole of the higher portions of the
mountain, composed of basaltic scoriae, tuffs, and ashes, alternating with
lava sheets and intersected by a plexus of dykes. The masses of agglomerate
having been thinned out, the basaltic lavas came together and formed the
peninsula of Burg or Gribun, which is made up of lava sheets piled on one
another to the depth of sixteen hundred feet. The beds, formerly the summit
of Ben More, composed of alternations of lavas and agglomerates, constitute
the last vestage of a volcanic cone, formed during the period at which the
basaltic lavas were ejected. The agglomerates exhibit, in the fissures of
ejected blocks, many beautiful minerals of the same kind as are found in
similar portions of existing volcanoes. The entire absence of ejected blocks
of the stratified rocks in the later agglomerates is ,a significant feature
in that while the older eruptions of acid rocks broke through masses of
earlier strata, the later basaltic masses forced their way through the midst
of the former.
Although the great mass of the lavas, constituting the great plateaux, are
of basaltic composition, they vary greatly among themselves in many minor
features, and especially in sheets of clinkstone, usually called porphyrite.
The number of exceptional lavas greatly increases toward their origin
passing up Ben More.
For geological exposures, the west and south sides of Ben More, afford the
most favorable field. On the west side, about one mile above Craig, is a
course grained basic plutonic mass which has a close resemblance to the
plutonic masses in Coir a’ Mhaim and Coir’ant-Seilein. A shaly bed on the
south side contains traces of plant remains. A few flows of more basic
character occur high up in the andesitic series, forming the summit of Ben
A’Chioch forms a face of Ben More. On it is a group of inclined sheets,
composed of basic intrusions, very irregular, and often follow the bedding
of the lavas. They are numerous on Ben More, but die out along the strike,
about a mile to the south, starting abruptly in the Choir Odhar a rock known
as mugearite forms a flat sill whose out-crop may be traced almost entirely
to the Ben More-A’Chioch mass, wjth a maximum thickness of about three
hundred feet. It dies out abruptly on the eastern slope of A’Chioch.
Ben Talaidh. In altitude Ben Talla exceeds all others in Mull, save Ben
More. It is situated in the central portion of the plutonic area, resting in
the forks of the upper source of Glen Cannel River. It consists of an
abundance of the minor basic intrusions. The more numerous are the inclined
sheets that dip towards a point lying a little to the west, on the southern
slopes of Ben na Duatharach. The southwestern slopes of Ben Talla are
largely composed of acid or subacid lavas. On its upper slopes is a great
abundance of inclined sheets. The summit is largely composed of fine-grained
rocks of lava.
Ben Buy. Forming the south-by-east section of the volcanic plateau is Ben
Buy. The great Tertiary breccia forms a conspicuous feature of southeastern
Mull, and consists of blended blocks and fragments of granophyre, gneiss,
gabbro, basalt and sandstone. Its lower half of the southern slope is
composed of granophyre breccia, which is cut by a large number of inclined
sheets of dolerite, and of a subacid rock with a cicular auzite. The flat
top and northern slope of the Ben consist of a large mass of gabbro cut by
numerous inclined sheets.
Ben Gliraig. This mountain rises near the western end of Loch Ba. Viewed
from the north it presents a very striking appearance. Its face is almost
precipitous; and here its much jointed granite and felsite, which constitute
the greater part of its mass, have crumbled down, thus exposing its
perpendicular side. The lower part of the ben is composed-of a well formed
typical granite, principally of the hornblendic kind. This kind of rock may
be seen exposed on its south side in the deep ravines which divide it from
Ben y Chat and Ben Gabhar. Ascending the mountain the granite is seen to
pass by insensible gradations with a quartziferous felsite, the hornblend
being replaced by the decay of minerals which greatly facilitates the
disintegration of the rock. Higher up the rock becomes finely crystalline.
From the coarsest granite to the finest grained felsite the prophyritic
structure is displayed.
From a short distance removed, the northern face of the ben has the
appearance of being made up of concentrically curved beds. The granite and
felsite are traversed by innumerable veins, and appear to be composed of
similar material to that of the mass itself, but differing for the most part
only in the degree of fineness of grain, color, &c. In a few instances thin
veins of almost pure quartz, and others made up of crystallized felspar are
found. Lying upon the summit and flank of the eruptive rocks are sheets of
lava highly vesicular and amygdaloidal in structure, which alternate with
great masses of ash, lapilli, and scoriaceous fragments. These rocks have
been thrust upwards by the vast intrusive masses below them, causing a dip
in both directions. On the west they are intersected by the shores of Loch
na Keal, where the character may be observed.
Ben Sarsta. About two miles southwest of Tobermory is a conspicuous hill,
presenting striking features. Its height above the sea level is about eight
hundred feet. It is a prominent object owing to the peculiarity of the mode
of weathering of the rock masses which compose it, as compared with the
surrounding tubular basalts. This rocky mass, standing up abruptly in the
midst of the basaltic plateau, is composed of coarsely crystalline dolerite,
which becomes finer in grain towards the outer margin of the vent, but in
its lower portions passes into gabbro. The surface of these rocks are rugged
and of a rusty brown. They resist denudation and the growth of vegetation.
They form a striking contrast with the grassy tabular masses of basaltic
lavas in the midst of which they are formed. This hill v/as upheaved through
the older basaltic lava sheets. As shown in the plan and section of the
accompanying illustrations, the contrast between these highly crystalline
rocks and basalts, in the midst of which they lie, is marked by a belt of
metamorphosed rock. The basalts near their junction with the intrusive
dolerites and gabbro have acquired a harder texture, a splintery fracture,
and a peculiar platy mode of weathering, often in concentrically curved
planes. From the central mass a number of dykes and veins can be traced
intersecting the surrounding lavas.
Sarsta Ben sets forth every evidence of an extinct volcano. Its date is
subsequent to the eruption of the great sheets of basaltic lavas. It was of
great size and surmounted by a volcanic cone, and from its vent, lava
streams flowed, fragments of which still remain. The vent measures about a
half mile in length, by a quarter of a mile in breadth, presenting an oval
form. The central portion is a deep hollow, which is filled with Water of
great depth, thus forming a crater lake.
Every ben has certain features peculiar to itself, thosei already detailed
must suffice. However interesting and important the many bens may be, it .is
necessary to pass them over, and take up other geological data, although
more or less connected with the mountain formations.
The masses of volcanic matter did not proceed from one cone. The series of
volcanic upheavels represent different volcanic cones, or vents, and all did
not belong to the same epoch. At a number of points the basalt was broken
through by similar eruptive masses, sometimes composed of basic rocks, and
at others of felspatic materials, and in still others of both these
varieties. It is more than probable that there was one main vent, and the
others may be termed accompanying volcanoes, although all were not
immediately contemporaneous. At times the eruptions were very violent as
proved by the distance covered by the flow of the lava.
The Ross of Mull is characterized by some very interesting geological
features. From Peterhead, on the north-east .coast of Scotland, to the Ross
of Mull, there occur, along the whole of the‘Grampian Mountains, a series of
masses of crystalline and igneous rocks which have been protruded through
the contorted and metamorphosed strata of the Lower Silurian. The
undulating, and, sometimes almost level tracts of the Ross of Mull, where
through extensive denudation, the lower and deeper portions of these masses
are exposed, may be found to be composed of an almost uniform mass of
typical granite. In the coast sections may be found the most complex
entanglements of the granitic and stratified rocks. On its extreme point, by
the Sound of Iona, and up the country, reaching nearly to Bunessan, the rock
is entirely granite. The reed or grain of the stone is horizontal, which
would indicate that the hills, by some great convulsion, have been tilted
upon edge. The general direction of the reed is north-east and south-west.
On cutting the stone it is seen that the quartz and felspar are large and
flaky. There are two varieties of granite exposed—gray^and red—the latter
The granite of the Ross of Mull extends over twenty square miles, and varies
very little in composition and texture, except along its line of junction
with the schists,where mica is more marked. Inclusions containing
sillimanite, an-dalusite and cordierite are very abundant near the boundary.
The sillimanite, in the rocks, occurs in larger crystals than elsewhere in
Scotland. Small masses of diorite are met with on the west of the Ross of
Mull, which was formed earlier than the granite. The coast sections of the
granite contain numerous minor intrusions, which latter consist of three
sections, the oldest of which is pre-granite. The next, cutting the granite,
is of mica trap, sometimes reaching ten feet in thickness. Lastly, there is
a set of north-west dykes, which cut the sheets of mica trap. This appears
in the coast section on the west side of Port Gart an Fhithrich. The
pregranite intrusion crops out near Rudha na Traighe-maoraich.
As previously noted the Isle of Mull is volcanic, yet there are patches of
older rock, which occur at short intervals around the shores of the island.
Previously to the volcanic eruptions nearly, the entire isle was of Tertiary
formation, only patches of which remain.
Carsaig1. The exposure, near the outlet of Loch Buy, on its west side, was
early noticed. The exposure of the Miocene, at this point, was the beginning
of the investigation which elucidated the history of the Tertiary period in
the Highlands of Scotland. A fine field was offered in Mull. The Lower Lias
crops out, but is much disturbed and altered by igneous intrusions. This
series may be traced towards Loch Buy, where it is completely and abruptedly
cut off by the intrusion of volcanic masses of Miocene age. In this
formation are well marked zones of Ammonites semicostatus and A. bucklandi.
At the foot of the cliff at Carsaig House, when the tide was out, I picked
up some very fine specimens, in the exposed rock. They were quite small, and
of a very dark color.
Overlying the lower Lias is the Middle Lias, composed of two marked members
(called the Pabba and Scalpa series), the former represented. The lower
member or Pabba .series, consists of more or less sandy and very micaceous
shales, with limestone nodules. At many points in the cliff the beds are
sometimes concealed by a talus of fallen rock. But there is no difference in
studying the beds collecting fossils, which are abundant, and met with in a
fine state of preservation. Over the Pabba are the Scalpa beds, which are
less calcareous, and consist of greenish and yellow sandstone containing
fewer fossils. In the upper part of the series the beds graduate into white
sandstones and grits.
The bottom bed of the Tertiary igneous rocks is a thin mudstone, which is
overlain by a thick mass of basalt lavas rising to a height of about fifteen
hundred feet. Although the base of the lavas dips to the westward near
Carsaig, yet the rock terraces near the shore of Loch Scridain have a slight
dip to the east, towards the center of the island.
The masses of chalk at Carsaig do not form bedded layers but are deposited
without order in a sandstone matrix.
Sapphire has been found in two localities to the west of Carsaig. In the
first of these (near Loch Scridain) large xen-olithic blocks of baked
sandstone and shale are seen to be involved in an igneous matrix which is
probably intrusive, and which encloses large numbers of little hexagonal
plates of clear blue corundum. The second locality is on the coast of
Rudh’a’Chromac’n where it is found in an irregular composite sill consisting
partly of andesitic felsite, and partly of trachy-tic granophyre.
The above localities are the same spoken of as along the course of the Allt
a’Mhuchaidh, near the foot of the Abhuinn Bail’ a’Mhuilinn, east and west of
Loch Buy District. Near Glenbyre, and also on the south coast of Laggan, the
base of the lava occurs, and in some parts of Laggan, between the lavas and
the underlying Mesozoic rocks, is a thin layer of Tertiary sediments. To the
west of Port Ohirnie the layer is about twelve feet thick. To the eastward
the bed dies out, and near Glen Libidil is replaced by sandstone with
fragments of igneous rocks. Above the Tertiary sediments comes a great
thickness of lavas of which the bottom members are mostly basic in
character. The latter thin down from about eight hundred feet, near
Glen-byre, to four hundred feet near Glen Libidil. Above these there are
various zone lavas, some of which are of great thickness.
A great number of intrusions occur around Loch Buy, the largest being the
granophyre, which extends from Loch Buy along both sides of Loch Uisg and
into Laggan as far as the head of Glen Libidil. The granophyre is older than
two large gabbro masses, one of which is exposed between Loch Uisg and the
sea at Loch Buy, and the other in Glen Libidil. Numerous dykes met with run
west and east-north-east.
A small volcanic neck on the coast about three hundred feet east of Glen
Libidil, consists of an outer intrusion of a coarsely crystalline basic rock
full of xenoliths, with central patch of volcanic agglomerates.
A short distance southwest of Glenbyre is a small patch of Jurassic rock,
well exposed in the cliff. In Glen Libidil is a patch of Jurassic rock
exposed by denudition, and, on the northern side, is bounded a fault with a
downthrow to the north of about four hundred feet.
Loch Spelve District. This district embraces the southeast coast. A
calcareous sandstone of the upper portion of the Lower Lias lies on the
coast between Port Donain and Port nam Marbh, and in the Oakbank burn. On
the coast of the peninsula, stretching south of Loch Spelve, the same
formation contains two varieties of ammonites. The same formation at Port
nam Marbh is characterized by the ammonites gryphaea. The Upper Lias, in the
Loch Don district, along the coast from Port nam Marbh to Port Donain, is
marked by thirty feet of indurated dark shale. At Port Donain this shale
yields an ammonite closely allied to Dactylioceras commune. A small patch of
similar shale occurs at Port na Muice Duibhe, some two miles south of Loch
The Lower Oolite occurs at Port Donain and Port nam Marbh, which is about
seventy feet in thickness.
In Glen Ardnadrochet the Lower Oolites are in contact with the Middle Lias
sandstone, and represented in two beds of limestone separated by thirty feet
from calcareous sandstone. Beds of the same age are found on the west side
of the Loch a’Ghleannain anticline. The Upper Greensand, crowded with a
crushed ostreiform shell occurs at Port Donain and north of Port na
Craignure. This section forms a part of the northeast of Mull, bordering on
the Sound of Mull. Immediately to the southeast of Craignure is a compact
limestone of a white color, which passes up into a shelly limestone, a few
feet thick, on the shore between Duard and Craignure, and nearly forty feet
in Glen Ardnadrochet. This stone contains bands crowded with the fossil
Grypliaea arcuata. On the Duard-Craignure shore, and in Glen Arnadrochet,
the stone is followed by a rusty colored shaly micaceous sandstone, with
poorly preserved ammonites.
Tobermory. The Pabba shales are rich in fossils about Tobermory. The general
association of the fossils there appear to prove that they belong to the
base of the Pabba shales, representing a mixture of the Lower and Middle
Lias species. The country west of Tobermory is chiefly built up of a
succession of almost flat flows of fine grained olivine basalt.
Bloody Bay. Near the northwestern extremity of Mull is Bloody Bay, where
beds of sandstone of a bright and red color are found. This has been
quarried to build the lighthouse of Hu na Gal, near by. The eroded surface
of these beds is covered by Miocene basaltic lavas. It is probable these
sandstones, are an altered condition of the Scalpa series. No trace of
fossil remains has here been found.
Gribun. The beds at Gribun, on the shores of Loch na Keal, arouse the
profound interest of the geologist. Here, the Upper Greensand rests directly
upon the Poikilitic beds. At two different points the peculiar beds of the
Scottish chalk, converted into a siliceous material, are crushed out from
beneath the overwhelming masses of basaltic lava that covers all the
Secondary strata to a great depth. The accompanying illustration gives the
general relations of the beds seen at Gribun.
Ardtun. A mud stream, that accompanied one of the eruptions of the volcano,
is revealed at Ardtun, near Bunessan, Ross of Mull. This stream overwhelmed
and buried deposits of fine sediment, with leaves which had accumulated in
pond-like hollows. The streams had formed over a disintegrated surface of
the chalk rocks of the district, and thus swept up chalk flints and angular
fragments of the highly indurated Scottish chalk.
Some of the vegetable matter presents a fresh appearance. One of the leaf
beds is made up of a pressed mass of leaves, lying layer upon layer. Among
the plants there found, are a purple Ginkgo and a Platanites.
Coal. From time to time much attention has been given to coal, existing in
Mull. There have been mining operations and much expense incurred. That coal
exists in Mull has long been known, and it was positively mined as early as
1587. In that year James IV. rented to Hector MacLean the Ross of Mull, and
made a charge for the coal. Early in the 18th century efforts were made to
reach the coal beds in the west of Mull. A considerable sum of money was
spent in coal mining at Ben an Sonaich, near Carsaig, where a seam was known
to be three feet in thickness. In 1910 another attempt was made to ascertain
the extent of the coal beds, and the entire summer was spent prospecting.
Seams of various thickness were discovered occupying ’large areas amongst
the Brolas hills and at Ardtun. At the latter place, close to the seashore,
and under thick layers of rock, a twelve inch seam, containing high
calorific quality was mined. So far as the explorations extended the product
was not found to be in paying quantities.
Tertiary coal is found in various places west of Ben an Aoinidh. One of the
seams locally called Cadh’ Easan Coal, is near the top of a cliff about one
half mile south of Airidh Mhic Cribhain. It is a bright coal lying nearly
horizontal. Another is near the bottom of a waterfall, called Eas Dubh, a
half mile east of Sheaba. Other veins of coal have been found at Gowan brae
(Bunessan), and on the coast of Torr, Ardtun Head. These appear to be merely
lenticular patches, and of an inferior quality.
At different levels, in the volcanic series, beds of lignite and true coal
are observed. These appear to be always lenticular patches, only a few
square yards in extent. The best example is among the beds at Carsaig. It is
in part a black glossy coal, and partly dull and shale. Some years ago it
was between two and three feet thick, but now is about eight inches. It lies
between two basalt flows and rapidly disappears on either side.
Glacial Period. The ice-flow that moved along Loch Linnhe was split on the
eastern coast of Mull and sent one stream to the north and the other to the
south of the main mountain range of Mull. The northern branch proceeded
west, north-west, along the Sound of Mull, as far as Salen, leaving very
remarkable grooves, and severing of the rocks along its course. At Salen the
glaciation made a sudden change, the great mass of ice having swept round
this point to the southwest, and passed over into Loch na keal. The
wonderful grooving between Salen and Loch na keal evidences the great power
and persistence of the current which overflowed in that direction. It is
probable that this opening was the outlet to the west for the passage of the
main body during the greater part of the glacial period.
During a later stage of the ice-flow the local glaciers of the mountains
appear to have spread out over ground formerly occupied by mainland ice.
This is true in the case of Glen Forsa glacier, the striae of which cross
the grooves and flutings of the earlier glaciation almost at right angles.
On the coast to the west of the river Forsa the two ice mover ments may be
seen. A farther confirmation is exhibited of the striae of the local glacier
passing up and down over trough and crest of the earlier groovings. The ice
from the mainland had vanished in that region at the time when the Glen
Forsa glacier had attained its maximum and spread out into the Sound of
Mull. The striae of the last named glacier follow the direction of the Glen,
showing no sign of deflection, and pass out at the shores of the Sound.
The deposits of the Glen Forsa glacier indicate the nature of the moraine,
and contrast strikingly with the boulder-clay of the main ice. In a section
near Pennygown is shown the relation of this drift to the deltaic deposits
of the Forsa. Here the exposed delta gravels overlie the moraine of the Glen
The cause of the sudden change in the direction of the ice towards Loch na
keal, from Salen, was owing to the pressure of a powerful ice-stream, which
set in from the northeast, off Morvern. The congestion caused by the
conjunction of these two ice streams formed the very exceptional
ice-moulding of the Salen pass.
The drift deposit of Glen Forsa and that of the valley of Loch Ba present
unusual features. The glaciers which occupied these valleys, though forming
the usual type of morainic drift in the upper reaches,, yet near the termini
of the glens, have deposited extensive masses of gravel which form terraces,
fan-like sheets and mounds, and also several well-marked linear eskers. At
Calla^hally, in Glen Forsa, one of these eskers terminates in a clearly
defined fan, which slopes away in all directions from the end of the esker,
and drops, with an unbroken surface, to a level of about thirty-five feet,
where it passes beneath the gravels of a twenty-five foot beach. A half mile
east of Callachally, along the side of the river, is a small but distinct
esker; and at Pennygown is a remnant of another outwash fan. The striae on
the shore prove that the glacier extended out into the Sound of Mull, which
would indicate that this gravel formation was not deposited at this period
of its greatest extension. It is a product of the retreat of the ice. This
position is probably marked out by the steep fan of gravel bounding the peat
moss of Blar Mor. This moss and that of Dail Bhaite, on the east side of the
river occupy a hollow which is a cast of the glaciers at this stage.
The mounds which rise above the peat for a distance) of a mile up the glen,
are composed of gravel. A sudden change sets in at Killbeg, where the gravel
gives place to a clayey moraine, forming the hummocky topography so
characteristic of. Highland glens. From there to the head of Glen Forsa.
there appears to be no deposit of gravel.
The valley of Loch Ba contains similar phenomena. During its maximum, when
the glacier occupied this valley, it appears to have extended into Loch na
keal, as indicated by cross striae on the shore, similar to that of Glen
Forsa. The first stage of retreat is indicated by gravel formation, and the
position of the ice front, during the principal halt, was the present limits
of Loch Ba. The gravel belt is marked by mounds, eskers, and outwash fans,
similar to those of Glen Forsa.
About a mile and a half east by northeast of Craig, in the valley that
extends north from the sharp bend of Glen More, is a stiff dark gray
boulder-clay, well exposed, and forms the top deposit over a considerable
The west part of the floor of Glen More, above Craig, is crossed by a series
of fine terminal moraines. The origin of these is due to the glaciers moving
down the valley in a southwest direction; and above every one of the
moraines, is an alluvial flat, where the point of ice, which joined the
moraine, was situated.
A remarkable series of striated hollows and winding grooves occur along the
shores of Loch na keal, between Knock and Scarrisdale.
Numerous glacial striae prove there was a flow of ice down Loch Scridain,
and also down the valley of the Leidle. Raised beaches are seen on the
shores of Loch Scridain and at Carsaig. From the north shore of Loch
Scridain to the head of Loch Beg, a twenty-five foot raised beach can be
followed, and this marks the limit of certain fine gravelly deposits. North
of Loch Beg there is also a trace of a higher feature, at about fifty feet
above sea-level, while still higher can readily be seen from a distance,
which has been followed for nearly a mile. This appears to have been
connected with the preglacial shore line on the west coast of the island.
At one time a field of ice moved in a southwestern direction over the pass
of Coir’a’Mhaim, between Ben a Meadhoin and Corra-Ben, and also over the
next pass to the west, near
Coir’ant-sailein, between Corra-Ben and Cruachan Dearg. The height of the
first pass lies a little below the twelve hundred and fifty foot contour,
but that of the second is about seventeen hundred and fifty feet. The pass
of Mam Chachaig, nearly a mile west of Cruachan Dearg, was crossed by ice
moving in a southwest direction. The height of this pass is about ten
hundred and eighty-eight feet.
North of Loch Don are great masses of glacial sand and gravel, probably
produced by confluent glaciers from the hills to the west.
Between Loch Spelve and Loch Uisg is a high level fan of gravel over-lying
shelly boulder clay, save where the latter protrudes in well formed moraine
ridges. These ridges are the terminal moraines of a glacier which for a time
occupied the site of Loch Spelve, and the moraine material was derived from
the bottom of the loch, having been shoved forward by the ice. The surface
level of the granite was connected with the sea at the date of the one
hundred foot period. If the glacial deposit of the Kinlochspelve Isthmus was
removed, then the sea would extend into the deeper portion of the Loch Uisg
Near Loch Buy are patches of the one hundred foot raised beaches, and a
twenty-five foot beach forms a well marked shelf on which stands Loch Buy
plain and Laggan flats. Between Loch Buy and Loch Uisg is an exposure of
five feet of blue boulder-clay, containing Morvern granite resting on
moraine gravel. The glacial striae in Laggan show a general westerly trend
of the ice movement.
The Loch Buy district, east of Allt Mhic Slamhaich, does not contain very
large areas of glacial drift, but the work of ice is shown by numerous
erratic blocks and striae.
North of Loch Fuaran the striae indicate a southerly ice flow, while in
Gleann a Chaiginn Mhoir the flow was west by south.
The Ross of Mull contains no moraines and the drift der posits are few, and
chiefly confined to small hollows. Glacial striae have been observed only in
two places,—one resting on a dolerite sheet near the farm of Scoor, and the
other on the gneiss, near by.
Peat. The fuel used on the island is peat, deposits of which are found in
abundance in various parts of the island.