Transactions of the Geological Society, 1st series, vol. 4/Observations on the Mountain Cruachan
THE geological history of this mountain being, as far as I know, unrecorded, I shall relate the few observations which I made on it, as they are sufficiently numerous to form at least a basis for future and more accurate investigation.
It is evident to any eye in approaching through the vales of
Glenorchy or Glenara to the head of Loch Awe, that the nature of
the country has changed. The rugged forms and rocky faces characterizing
those hills of mica slate which bound Loch Lomond,
Loch Long, Loch Fyne, or Strath Fillan, have disappeared; the
mountains assume a more uniform flowing line, their sides are more
completely covered with herbage, and exhibit fewer denuded rocks;
their summits are less serrated, and are almost the only parts which
exhibit the naked rock, while at the same time they are strewed with
heaps of fragments, a character from which the hills of mica slate
are almost always free. On approaching nearer to their bases, the
red colour of the fragments which have fallen down from their sides,
and the rounded pebbles of granite and porphyry which are met
with in the beds of the torrents, give the mineralogist pretty plain
intimation of the causes of this change of feature.
Except in the rolled fragments however, no appearance of granite is visible as we skirt the sides of Cruachan from Dalmally to Inverawe.
Every where we observe schistose rocks, which have been laid bare, sometimes by the operation of natural causes, but more generally by the process of making the road. This schistus possesses in different places different features. Sometimes it is a compact mica slate, where the mica and the quartz predominate by turns, but its more general tendency is toward clay slate, of which it often exhibits very well characterized examples of various colours, varying from dark lead-grey to pale greyish-green. These in many places, and particularly where in contact with the veins about to be described, assume an extreme degree of hardness, putting on the aspect of that which is called Lydian stone and flinty slate. It is impossible to assign the breadth of the zone occupied by these schistose rocks, as, independently of the turf upon the sides of the hills, the lower skirt is clothed to a very considerable elevation with an impenetrable covering of mixed coppice and Underwood. It is probable however, that the incumbent schistus does not any where extend very high, as it may be found terminating at a low elevation on that part of the hill which is naked and accessible, and which impends over the course of the river Awe.
Numerous veins, of very variable but generally large dimensions, appear traversing this schistus throughout the course I have described. They are of various composition, but consist in general of different coloured porphyries. Their directions are as various as their colours, but their position is generally perpendicular, or nearly so. Together with these veins of porphyry, a few veins of trap, exhibiting the several aspects of greenstone, greenstone-porphyry, and even of genuine basalt, may be observed; and, from some fragments of amygdaloid which I picked up by the road side, it is probable that more numerous varieties which did not come under my observation may be found to exist.
Arriving at the high bridge which crosses the river Awe, the mineralogist will be surprised to meet with a secondary stratum in a situation certainly as unexpected as can well be conceived. A small portion of it only is denuded by the action of the river, and exhibits two distinct beds, the lowermost consisting of the well known red sandstone spotted with white, which occurs near Dumbarton, in Arran, and elsewhere, and the uppermost of a coarse grained white calcareous grit. This stratum is elevated at a small angle, and shelves away towards the side of Cruachan as if it was about to dip under the mountain: its junction with the primary schistus is not visible. I could not trace it upwards on the opposite side of the river in the direction of the stratum, although it is probable that from the red colour of the soil some portion of it exists where the road is made. It may be traced a little way down the course of the river, but the ground being much encumbered and difficult to examine, it soon disappears. I attempted in vain to find it on the flat tract at Bunawe, nor did I succeed any where from Tyanuilt to the sea in recovering any portion of this stratum; the low land being always covered with alluvial matter, and the denuded rocks appearing every where to consist of schist and granite traversed by veins of porphyry. It affords a remarkable example of a deposit of secondary rock, not many hundred yards square, entangled in the middle of a primary district, and separated, apparently by many miles, from any other similar rock. I shall hereafter however attempt to give a general sketch of the connection of Cruachan with the surrounding country, when the recurrence of the same circumstance will be described, which, if it diminishes the surprise at first excited by its solitary and unconnected appearance here, increases its general interest, while it naturally leads the geologist to inquire into the state of things under which so remarkable a dismemberment of these strata has taken place.
In quitting this part of the skirt of Cruachan it is necessary either to rise to a considerable elevation, crossing above the woods of Inverawe, or to avoid it altogether and make a circuit by Bunawe. We then reach the shore of Loch Etive, and may continue our observations on the base of the mountain. As we ascend to cross the foot of the hill granite appears exclusively, generally in the shape of loose blocks accompanied by similar pieces of porphyry, but sometimes in situ. On reaching the shore beyond Bunawe the appearances become more interesting. The junction of the granite with the schistus is here clearly seen. Large veins may be observed proceeding from the great mass of the mountain, and ramifying into innumerable small divisions penetrating and traversing the schistus in every direction. No where can this appearance which has excited so much attention, be more distinctly perceived, and it is moreover attended here by some interesting circumstances, which as far as I have observed do not occur in the junctions at Loch Ranza, in Glen Tilt, or at Locheil. Two distinct varieties of granite appear in this place; the one a granite according to the strictest acceptation of the term, consisting of reddish felspar, quartz, and mica; the other a syenitic granite, or a compound of white felspar, mica, and hornblende. These are coexistent in every respect and seem to pass into each other, while both of them ramify in a similar manner through the schist, a sufficient proof, if any were wanting, of the geological identity of these two rocks which have so improperly been distinguished by the accidental presence or absence of the single ingredient hornblende. Although the granite veins sometimes run through the schist in a distinct form, just as they do in the junction of Loch Ranza, yet in many places they are intermingled with it in a very remarkable manner. Crystals of the hornblende may be observed shooting far into the body of the schist, so as to render it often difficult to assign the limits of each rock. In a less degree the quartz and felspar exhibit the same appearance. Together with this, the schist is singularly contorted, being bent, broken, and intermingled in a most confused manner with the rock that traverses it, while distinct detached fragments are often involved in the mass of granite. In many instances these fragments either exhibit at their edges a change into a substance resembling basalt, or are actually converted into a black matter which has at first sight the aspect of a fine grained hornblende rock or a basalt of the blackest hue, and which only an accurate inspection discovers to be modified fragments of schistus. The schistus in general which lies in the immediate vicinity of the granite vein is highly indurated and gives fire readily with steel: in other respects it retains its general character, a laminated structure and alternating stripes of colour.
Independently of the peculiarities now described which characterize the junction of the granite with the schist at this place, the passage of the porphyry veins may be observed in great perfection, and as they form an important feature of the character of Cruachan and occur in all parts of it, I shall here describe them at some length.
These veins are of different sizes as well as aspects, varying from the breadth of three or four feet to that of fifty, or more: they are all very erect, and in a general view appear perpendicular. They traverse both the schist and the granite, and are to be observed in this part of the hill, cutting the vein equally with the schist in all directions, as may be seen in the accompanying sketches. They are uniformly well defined, neither intermixing in any respect with the granite nor with the schist, nor apparently producing any disturbance in the course or direction of these rocks. They are, as I before remarked, of various colours and compositions, and two veins may often be seen running parallel and in absolute contact with each other, without interference or disturbance, the one of a dark red, and the other of a light grey or some other colour, as represented in one of the sketches. They are so numerous that perhaps a fortieth or fiftieth part of that region of Cruachan which I examined consists of porphyry veins. The principal varieties both of colour and composition which I remarked, are the following, and their basis consists of that rock which is now by general consent, called compact felspar, but which has at times been designated by the term hornstone.
These latter varieties appear to form a regular series of a transition from porphyry to trap, of which I shall immediately speak; first remarking that besides these leading varieties which I have now described, there are many others which it would be superfluous to notice, as the variations of colour, aspect, and composition, are endless. Together with the veins of decided porphyry, various parts of the mountain are intersected by veins of grey rock, having as I have just remarked, the general character of the trap rocks, and sometimes porphyritic. Veins of perfectly characterized basalt also occur in some places, and these, or fragments of them may in particular be observed in great quantity strewing the top of the first summit, (that one which is marked by two cairns,) and laying fair claim to an equal antiquity with the veins of porphyry. It is easy to procure detached specimens of the junction between the granite and basalt in great variety and abundance. The line of junction is in all cases clear and well defined, but does not admit of ready separation even after long exposure to weather. My observations on the base of the hill were terminated here, some way before the place where the foot of Ben Starive interferes with that of Cruachan.
I have already said that in ascending Cruachan the schist soon disappears: at the same time the accumulation of soil and the covering of plants are so great, that it is scarcely possible to meet with the natural rock for at least half the ascent. Many masses which appear to be in their places, turn out on examination to be only large rolled stones, and these invariably consist of granite and porphyry. It is not till we have attained about two thirds of the height of the mountain, that the natural rock makes its appearance in an unquestionable manner. From this part to both the summits there are abundant opportunities of examination, as immense faces of it are left uncovered even on the southern side where the acclivity is easiest. To the north it presents a range of nearly perpendicular precipices extending many hundred feet down the mountain. The mass of the mountain is easily seen from this point to consist of granite. This appears to be of uniform composition on both the summits, and to be formed of a very equal mixture of reddish felspar and white quartz with very little mica, nearly resembling the granite of Cairn Gorm. The porphyry veins which traverse it are here also as visible as they are below, but if there is any difference they appear of greater magnitude, and the red variety predominates. There are immense fissures on the northern side, which seem to have resulted from the wasting of these veins. It is on the lower of the two summits, as I before said, that the junction of the basaltic veins with the granite is visible.
I have remarked in a former paper  that the magnet is much affected by the granite on the summit of Goattield. This is still more strikingly the case in Cruachan, its affections being indeed stronger here than they are even on many of the basaltic rocks of Canna. Both the porphyry veins and the granite affect it, but the former in the greatest degree. Having had occasion to observe these two instances of a fact but little noticed, I think it right to add, that in both cases I found this property confined to the masses which occupy the summits of Goatfield and Cruachan, and that I did not perceive it either in the rocks or detached masses at the foot of these mountains. I do not mean to say that this is likely to prove general, but I merely point it out as an accompanying circumstance, to be confirmed, or perhaps contradicted, by future and more numerous observations.
Having thus determined the nature of Cruachan itself, it will be useful to trace its connection with the surrounding mountains, and in defect of more satisfactory observations, to conjecture by their external aspect and by analogy, the nature of their composition. It forms the highest point of a complicated group, which to the south-east is bounded by Loch Awe, to the south-west by Mid Lorn, and to the north-west by Loch Etive, but which extends towards the north-east in a continuous line, uniting itself with the ridges of Schihallien and Ben Lawers. The part of this group which the great elevation of Cruachan brings almost immediately under the eye, is coextensive on the three quarters first enumerated with the boundaries there mentioned. To the north it does not extend further than Buachaille Etive, while it is in some measure separated from the eastern mountains by the lower land of the Black Mount over which the military road passes. Within this space the whole of the mountains, including both boundaries of Loch Etive, appear to consist of granite, and to be of the same composition with Cruachan; there being no perceptible difference either to the naked eye, or when seen through the telescope, between their general outline, fracture, mode of disintegration, colour, or form. How far this conjecture may be confirmed by actual examination, or to what extent they may resemble it in the minor circumstances, the intersections of porphyritic veins, can only be known by inspection of the rocks themselves, a task not likely to be soon accomplished, since it is scarcely possible to discover a mode of traversing this region, which, in addition to its extent and difficulty, is nearly void of inhabitants. Whether it be well founded or not, the rocks which form this mountain can be traced, under certain modifications, beyond the boundary here mentioned, and as they add useful illustrations to the subject, while they also serve the purpose of determining a difficult tract in the geological topography of Scotland, I shall make no apology for describing them.
Granite is found at Balahulish, extending round the shore of Loch Leven on its southern side towards Appin, assuming during this course, various aspects; but it is generally grey and small grained, and of the most ordinary appearance and composition, consisting of quartz, mica, and felspar. Superadded to these, it frequently contains hornblende, or else it consists of quartz, felspar, and hornblende, a circumstance which as I have before remarked forms no distinction geologically considered.
It appears here to be the basis on which micaceous schistus and
quartz rock repose, both of which conjoined form the group of
mountains called Ben na vear, which rises above the house of
Balahulish. It is not necessary for the purpose I have now in view
to inquire into the further extent of these rocks. Branches of the
same granite in no way altered in character pass through the schist,
and probably the quartz rock, although I did not detect their actual
junction with this last. I can only conclude that as the schist and
quartz rock alternate, that vein which traverses the one must also
traverses the other: The schist which is traversed by the granite
often much indurated, and approaches by various undefinable gradations
to a sort of hornblende slate. Masses of a similar substance
may be found imbedded in many parts of the granite. Occasionally
these masses appear on close examination to be only irregular
spots of hornblende, such occur not unfrequently in those granites of which this mineral forms an ingredient. More often however
their shape is perfectly defined, and they appear to be laminæ
of which the edges are truncated or broken at angles with the plane.
In some places this appearance of fracture is so precise, that when
two fragments occur together in the granite the imagination as
easily replaces the separated parts as it does in the brecciated marbles
or agates: nay, further, the fragment will sometimes be
found to consist of an argillaceous or slightly micaceous schistus,
maintaining this character with scarcely a perceptible alteration,
and sometimes only approaching to hornblende schist at its exterior
parts. It is also worthy of remark that these fragments sometimes
exhibit at their edges stripes of different colours and degrees
of hardness, arising from the varying texture of the laminæ which
compose them. The masses vary in size from an inch to a foot
and upwards, but whatever their size may be they have almost invariably
parallel sides. The examples of this appearance are very
numerous both at Balahulish and in the rolled fragments of granite
which are spread over the Black mount to the eastward of Glenco,
and we shall presently see that the same granite with similar connections
occupies a very large tract of country. The frequency
of the occurrence also enables an observer to examine the specimens
without difficulty, and to compare their various aspects and circumstances.
From these I have no scruple in saying, that the granite
now described contains fragments of schist imbedded in its mass,
generally so altered in their original appearance by their connection
with the granite, as to approach to, or partially to assume the
character of hornblende slate, but often also possessing the characters
of micaceous schist unchanged, and under all the varieties of aspect
by which it is characterized in the surrounding country.
I shall continue to describe this rock as far as I have traced it, since if not strictly a part of the professed object of this paper, it will, in addition to the contribution which it forms to the mineral topography of the country, either assist future geologists in connecting their own observations, or present them, in a tract among the most desert and difficult of access in Scotland, with a point of departure from whence they may prolong their observations over the adjoining country.
In leaving Balahulish to proceed westward we almost immediately lose sight of the granite, which occupies here only the lowest position, and is scarcely to be found above the level of the sea, The schistose rocks which cover it do not however accompany us long, being succeeded at the mouth of Glenco by a mass of rocks appertaining to the porphyry family, which I shall recur to when I have traced the granite. This becomes again visible as we approach the King's house, and, quitting the rugged hills which separate Glenco from Loch Etive and from Loch Leven, enter upon the wide, trackless, and solitary moor of Rannoch. This extensive and barren tract is elevated at a very considerable height above the sea, and, although unascertained, it probably does not fall much short of a thousand feet. Its surface in a general sense is flat, when compared with the ordinary aspect of a Highland moor, yet it is diversified by low rocky hills and undulations, covered with a deep peat which conceals the subjacent rock. There are notwithstanding abundant indications of its nature in the beds of the streams which flow over it, by which the naked rock is completely exposed to view, while every protuberance which time or accident has laid bare, and the detached blocks that are strewed over its surface, confirm its identity with the granite of Glenco. It can be thus traced to the head of Loch Rannoch, a distance estimated at 24 miles, with no variation of character; but of its breadth between north and south I am unable to speak, the country being absolutely trackless and uninhabited.
It appears to me however that it extends, perhaps with some interruption, from the schistose rocks to Ben Vualach by which it is connected with the granite district of Loch Ericht; and there is equal reason to suspect that to the westward of this it will be found similarly connected with the granite of Ben Nevis.
Where it terminates, at the head of Loch Rannoch, it forms hills of moderate elevation, and these are immediately followed and covered by a succession of schistose rocks, consisting of quartz rock, micaceous schist, and an obscure variety of gneiss which I have already had occasion to describe in another place. The junction of the two may be observed in different places, in all of which it is invariably accompanied by the appearances already noticed at Balahulish, namely, fragments of the different schists imbedded in the granite.
The schist which is here imbedded in the granite is often composed of black scaly mica with a high lustre. Towards the junction of the fragment with the surrounding rock it generally contains crystals of hornblende. The fragments vary much in size, and I must add that they differ completely in aspect from those accumulated plates of mica which are found in the granite of Aberdeen, as well as in many other granites. In other cases the imbedded fragments consist of the same quartz rock and gneiss which form the general body of these schistose rocks. If any mineralogists are unwilling to consider them as imbedded fragments, it can only be said that if they were really detached fragments they could possess no other aspect than that which they now have. The head of Loch Spey, a tract far removed, yet possibly not unconnected with this, is also composed of granite; and among this are found perfect granitic conglomerates, in which fragments of mica schist, equalling in quantity the substance which connects them, are seen imbedded in a paste of granite.
Occasionally the fragments are confounded with the mass at their edges, but at times they are so defined, and even so separable, that I procured a specimen with the distinct vacant impression of a rectangular fragment which had probably been detached. Cavities left in this way by the wearing out of the schist occur frequently in the rocks throughout the moor of Rannoch. The union between granite and the schists which it touches when passing through them in the form of veins, is known to be subject to similar variations. I may here add that the same appearances, though more rarely, may be found in Mar, and in the granite which occurs near Comrie.
It is well known that the passage of granite veins through schist
is commonly well defined, and that the two are generally easily separable
by the action of the weather. But the district of Rannoch,
offers a multiplicity of veins which are so confounded with each
other and with the rocks which they traverse, that their appearances
cannot be described. They frequently vanish so imperceptibly both
in the quartz rock and the mica slate, that a perfect passage from the
one to the other is visible, while the accessions of additional veins,
traversing and often shifting the already intricate structure, increase
the unexampled confusion which reigns among them. The granite
is often found imbedded in detached lumps in the schist, and I
must remark of these lumps and veins, however minute they may
be, that contrary to the granite veins and detached masses of Glen-Tilt
or Corpach, their character is perfect even to the minutest division.
I shall add but one remark more on this subject. Although the schistose rocks are seen only at the two ends of this prolonged tract of granite, the imbedded fragments can be traced throughout the whole. Hence it might be supposed that they were independent of the vicinity of the great schistose masses, whereas in the view which I have taken of them I consider them as connected with and dependent on them. It is plain that the absence of schist at present from the surface of the granite which forms the moor of Rannoch, proves no negative in this case, as we have abundant examples every where of the removal of great tracts of rock from parts of the earth's surface, and of the consequent denudation of the inferior substances; a change which may easily be conceived to have occurred here, thus leaving exposed that very surface of the granite which once was in contact with the superincumbent beds of schist.
Having thus traced the granite district of which Cruachan forms the most conspicuous portion as far as means of observation were afforded, I shall point out, under similar limitations, the porphyry which fell under my notice, since it will tend to illustrate the great predominance of the porphyritic veins in that mountain, as already described. I must however say that I consider the few following remarks, like those on the granite of Balahulish and Rannoch, in no other light, since they cannot be viewed even as a sketch of one of the most complicated and interesting districts in the whole range of Scottish geology. It will presently be seen that the great extent, the divided disposition, and the almost impracticable nature of the country throughout which these rocks are scattered, would require a very different investigation from that which a few distant and rapid visits permitted me to bestow on it.
The various schistose rocks, consisting of quartz rock, mica slate, clay slate, and limestone, cease at the place where Glenco first begins to contract its dimensions as we proceed from Balahulish eastward. From this point till we arrive at the King's house, or near it, the mountains on each side consist of porphyry, or of the different simple substances which form its bases, these simple rocks being indeed much more prevalent than those which are, strictly speaking, porphyritic. As these hills subside at their eastern end in the moor of Rannoch, the mass of porphyry disappears, and is succeeded by the granite already described; but veins of all the different varieties are found connected with it, every where intersecting the latter rock, and although gradually diminishing in frequency as we recede from the great mass, still admitting of being traced even to its very extremity. I have no means of assigning the boundary of this porphyry towards the north, but as the same rock is found to form the summit of Ben Nevis, it is probable there is some connection, more or less interrupted, between them. It is easily seen that towards the south it forms both the mountains which go by the name of Buachaille Etive, the conoidal and acute forms of which are visible from a great distance throughout the surrounding country. From these it extends along the eastern side of Corrich y bae, but as the southern sides of their declivities have not been examined, it is impossible at present to state the limits here between the porphyry and the granite which I have conjectured in a former part of this paper to extend from those mountains to Cruachan.
The vertical structure of this rock explains the cause of the abrupt and perpendicular faces which give the peculiar character to the dark, solemn, romantic Glenco. Together with that it presents the same laminar tendency which is so remarkable in the rock of Devar, described in a former paper. In many places there is an appearance of veins proceeding from the mass, rendered conspicuous by their projection and superior durability, but in examining the surrounding rocks where in contact with them, no difference of structure or composition is perceptible. I did not any where perceive a tendency to the columnar form.
Although the naked surfaces of these rocks might lead us to suppose they were inimical to vegetation, this effect must rather be attributed to the steepness of their declivities, which prevents the accumulation of soil. From whatever cause it may arise, they seem particularly subject to be destroyed by the action of the mountain torrents, whence the enormous piles of fragments which annually overwhelm the road and are fast raising the level of the valley.
The simple rock, which, as I have already said, prevails over the porphyritic varieties, is at one extreme a claystone, and at the other a compact felspar, varying through several intermediate stages of hardness. In the intermediate stages of transition to porphyry, a single crystal only of felspar will sometimes be found in a large fragment, the ultimate accumulation of which produces porphyries of an infinite variety of aspects. Every variety of this substance indeed, whether in colour or composition, which occurs in veins throughout Scotland, is here found mixed together in the mass, sometimes placed side by side with a sudden and decided transition, at others graduating into each other by imperceptible degrees. The colours graduate into each other in these cases just as do the different structures, and among these gradations the most striking are those where black passes into red. It would be an useless task to describe the varieties of colour which occur, but the different shades of grey, purple, and red, are the predominant ones. In some cases dark blueish specimens are found veined with red, producing very beautiful and remarkable varieties: in others, the red colour is so bright, and the texture so compact, that they can scarcely be distinguished from jasper, a rock which, I may remark, although hitherto but little examined in its geological relations, possesses a very near affinity to the family of porphyry, as well in the extensive independence of its position among the regular rocks, as in its appearance and composition.
The occasional minerals which are found in these rocks are hornblende, quartz, and epidote, all of them entering into the composition of some of the varieties, and the latter in particular forming a very conspicuous feature among them, being disposed either in the form of veins, or in amygdaloidal cavities, or else in occasional grains. Hard breccias, of which the structure can scarcely be detected except on the weathered surfaces, and exactly resembling those so conspicuous on Ben Nevis, also occur dispersedly among the more simple rocks, the fragments consisting only of different varieties of the same substance.
Before quitting these rocks it will not be uninteresting to mark the principal circumstances in which they differ from the analogous rocks which occur in Arran, in the Ochil hills, and in many other parts of Scotland. They all consist alike of claystone and compact felspar, simple or porphyritic. But they differ in situation, the hills of Glenco reposing on granite and the older schists, while the former lie above the red sandstone. They also differ in their general features, since the former assume a spiry shape, while the others present a succession of tame and rounded outlines. They appear equally to differ in durability, since although the hills of Glenco are destroyed by the effects of the mountain torrents, they are not like those of Arran subject to decompose by the ordinary action of the atmosphere. In the variety of composition there is also a conspicuous difference, the infinite number of hard, coloured, and compounded porphyries that occur in Glenco and correspond with those which are every where found in veins among the older rocks, being entirely absent in those which lie above the red sandstone. Are we to attribute these diversities to a different era of formation? Unfortunately our knowledge of these rocks is as yet so limited that this question cannot be answered; but the few remarks which precede may for the present remain as slender contributions towards their history, independently of the local interest they may possess in illustrating the description of Cruachan.
The next rock of which it is necessary to take a somewhat more extended view, for the purpose of illustrating the structure of this mountain, is the red sandstone, together with the white calcareous sandstone which lies above it. As the former is of frequent occurrence while the latter is only occasionally present, and as there is no difficulty respecting the consecutive position of these rocks, I shall neglect it in the short notice here to be given.
It has been shewn that these sandstones occur at the foot of the mountain, occupying a very small space, and that no continuation of them can be traced to the immediate vicinity. But in traversing a larger portion of this tract of Argyleshire a similar phenomenon is found frequently occurring, which, if it diminishes the suprize at first excited by this very limited extent of the secondary strata in the spot above described, adds a much greater interest to the fact, while it naturally leads the geologist to enquire into the circumstances under which so remarkable a dismemberment of these strata has taken place. Their continuity and extent as they occur on the eastern and southern skirts of the highland mountains are too well known to require notice, and they will be found to occupy an extent similarly continuous, as far as geographical circumstances will permit, on the north-western coast of Scotland and in the islands connected with it. The interval between these two principal masses of sandstone may be said, in a general way, to extend from near the Mull of Cantyre to Kintail, and the predominant rocks throughout this space are gneiss, micaceous schist, quartz rock, and a variety of analogous substances which it would be out of place to enumerate here.
It is in this interval that the scattered fragments of the sandstone strata are to be occasionally found, sometimes like that near the foot of Cruachan, connected only with the more ancient rocks, in other places associated with and covered by a variety of rocks more or less appertaining to the trap family, or to the porphyritic rocks which accompany them. It would lead to a length of description unfit for this paper to describe the places where they are to be seen, but I may mention two which are remarkable on account of the narrow space which the sandstone occupies, still more limited than even in the spot which has led to this discussion. These are the island of Seil, and Inish capel, in the latter of which their total extent only amounts to a very few yards. It is remarkable that in all these cases, as far at least as I have examined them, their dip is toward the west, however limited this may be, and that this is also the dip of the leading masses both at the southern side of the interruption above quoted and at its northern extremity where the same strata are found occupying parts of many of the islands, and extending for a considerable space between Kintail and the Ru Storr in Assynt.
The uniformity of their dip proves that these independent masses have not been separated by any disturbance from below, and we have therefore to chuse only between two explanations; either that they had been originally independent deposits, or that they had formed one mass subsequently disjoined either by the operations of water or of other destroying forces acting on the surface, or else by the intrusion of some other rock. The uniformity of their dip seems a sufficient reason to reject the former explanation, and their present appearance is more probably derived from both the last mentioned causes acting on different points. Concerning the action of water or other similar causes we can only conjecture, but of the latter we have occasional proof in the actual existence of masses of trap rock overwhelming them in some places, and doubtless concealing them entirely in others. It may perhaps be owing to this rock and to its subsequent destruction only, that their present state is to be referred. I shall therefore conclude these illustrative remarks by a few words respecting the trap rocks which occur in the vicinity of Cruachan, and which are in many places so intimately connected with these detached portions of the secondary strata.
The nearest mass of these substances is a long mountain ridge which occupies part of the northern shore of Loch Etive, descending towards the western sea and skirting the plain of Connel. It is in this place well known to all who have travelled the west Highlands, since the road passes under large rocks of the conglomerate which is connected with it. It is equally familiar to those who visit Oban, since the surrounding country and the neighbouring islands are covered with more or less extensive masses of it, in some cases reposing on the older schists, which form the visible basis of this country, in others upon the sandstone strata already described. It is in this latter case that it produces the effect already alluded to of partially concealing the masses of sandstone, so as to give an appearance of separation where no real one exists. In some cases it is also probable that it has actually dislocated and separated them, intruding among them as all the rocks of this family do, from below. Hence it assists us in explaining the state of the sandstone formerly described, although, from the circumstances already mentioned, that separation is in many cases independent of it. It is far beyond the bounds of this paper to pursue further the very interesting circumstances under which the whole of this formation of trap appears; and I shall probably take some future opportunity of entering at large into its history. I shall here therefore terminate these miscellaneous remarks, which appeared to me necessary to illustrate the description of Cruachan.
- Plate 6, fig. 2.
- Vol. II, page 430.
- Vide Paper on Quartz Rock, Vol. 4.
- Geol. Trans. Vol. 2.