John MacCulloch

“ The interior of the country is too much encumbered with peat and with vegetation to permit of any decision on a subject so obscure as is the point of change between the gneiss series and the rocks which follow it, and I must therefore limit myself to the appearances which occur on the sea shore, where every change can be traced in the most minute manner. Here there will be found a transition, unexpected and improbable as it may at first seem, between the two series, that of the gneiss, and that of the graywacké and sandstone which follows it ”

“ If they are disjointed in position, or if they appear promiscuously scattered, they still retain their natural connection, while the identity of their mineral structure is every where consistent. In one place only some strata of a quartz rock are to be seen, which might lead us to hesitate did we not recollect that in other instances the same causes which have converted shale into siliceous schist have also been found to change sandstone into quartz. ”

“ As we find analogous causes producing daily and visible changes of the same nature in the courses of rivers, the supposition is not incompatible with facts, since the narrowness of the Kylehaken channel and the rapidity of its tide, give it in this respect all the characters of an inland river as far as the contraction extends. We may perhaps indulge our conjectures still further in supposing that Sky was once united to the main land by means of this alluvium, and that the gradual effect of the tides circulating through the bay on each side had at length produced the effect in question ”

“ Approaching the point of Sleat the schist gradually becomes green and glossy, thus passing into chlorite slate, and here we arrive by an insensible gradation to the variety of gneiss which I formerly described as found there. In thus pursuing the red sandstone on the western side of Sleat, the reason appears why I could not when speaking of that rock, define the boundary of the gneiss on this side of the island, and I need now scarcely repeat, that any attempt to examine rocks so constituted, in the interior country, would leave nothing but doubt and uncertainty ”

“ It is among such dispersed portions of strata that the appearances of coal are observed. They are not infrequent, but are always extremely scanty, both in their thickness and in their apparent horizontal extent, since the strata which contain them are every where cut off by veins or by masses of trap. They are interposed, as we might expect, among the shale and sandstone, and, as we may conclude from the general bearings of the strata already described, occupy the upper beds of this formation. ”

“ The principal variations of appearance are produced by the greater or less proportion of the hypersthene, by the varieties of its colour, and by the unequal magnitude of the crystals. While on the subject of Coruisk I ought to add, that the sonorous rock formerly described is either a compact felspar or a compound of that substance and augit in minute admixture, and not a greenstone; a term which has been too indiscriminately lavished on many of the obscurer members of the trap family. ”

“ The remainder of the error consisted in mistaking an irregular lump of a very anomalous kind of sandstone which is entangled among the marble and the trap veins now described, for the syenite which is in the immediate vicinity similarly interfering with the limestone, and of which, pieces detached by the workmen were lying upon this sandstone as if they had been recently separated from it. If care and caution are required in examining the most simple appearances among the regular rocks, a tenfold portion is necessary when we are engaged in investigating the irregular ones. ”

“ Independently of this connection, the identity of the whole is proved by the correspondence of the organic remains, which at Broadford however are more abundant than on the opposite side. The principal difference at the two extremities consists in the numerous beds of shale and sandstone that alternate with the limestone strata on the Broadford side, and in the inferior solidity and thickness of the calcareous beds; while at the same time the harder schist, which divides them on the south-western shore, is absent, the one appearing to be a substitute for the other. ”

“ In attempting to trace the red sandstone below, or rather beyond the white quartz rock, it is not found to terminate on the north-western shore of Sleat, at the place where, according to the line of direction prolonged from Isle Oransa, it should end. Instead of that, the alternations of sandstone and schist continue. Gradually however they increase in frequency, and becoming at length undulated and contorted, they cannot at a distance be distinguished from gneiss in their general aspect and disposition. ”

“ The shale is a mixture of black clay, sand, and mica, thickly and imperfectly fissile, and the sandstone which is of different colours, but generally brownish, contains much clay and calcareous earth, the organic remains being found in each of these beds just as they are in the limestone. ”

“ Beds of the stratified limestone and shale, succeeding a narrow portion of the red sandstone, and precisely resembling the strata of Broadford, extend from the portions of irregular limestone before mentioned to the shore of the loch. These also dip to the north-west, but at a considerable angle, and the direction of their elevated edges, like that of all the regular rocks of Sky, is to the north-east, or thereabouts. It is evident that this limestone is a portion of the same series which occupies Strath, the intermediate parts having been either displaced or overwhelmed by the syenite. ”

“ As the strata rarely exceed an inch in thickness, it is equally evident that those which are visible are worthless in an economical view, while the certainty of a speedy interruption from the intrusion of trap removes all temptation to penetrate to greater depths, or to expend capital in a more effectual research. ”

“ The connections of the western islands with each other and with the main land are so intimate, and the light obtained from one portion is so necessary for the elucidation of others, that the separate description of any individual of the group must always be imperfect.
The account of the limestone which is found near Ord on the southern shore of Loch Eishort was in the original paper imperfect, as well in respect to its topography as its mineralogical description. ”

“ To add to the difficulty, the limestone when weathered puts on the grey colour and aspect of the sandstone so perfectly, that it is often difficult to distinguish them, when even within reach, without the assistance of a recent fracture. ”

“ The interruptions, to which I have here alluded, that prevent us from tracing the limestone over the hills that bound the southern side of Strath, arise partly from the boggy and covered nature of the ground, and partly from the intrusion of a hill of syenite, which extends far from the portion formerly noticed, towards Broadford, and which can in many places be distinctly traced overlying the limestone, shale, or sandstone, as either of these happens to be present at the point where the contact is exposed. ”

“ If we proceed to the western side of this promontory for the purpose of recovering these strata, we find them at Duntulm, from which place they extend interruptedly for a few miles along this shore, when they finally disappear. The same organic remains, the same shale, the same limestone, calcareous sandstone, and siliceous schist, mark the identity of these with the strata on the eastern side, an identity still further confirmed by the prevailing correspondence of their inclinations. ”

“ The directions of the strata are marked by prolonged lines extending to the neighbouring shores or islands wherever they presented the same substances, and on these the tendency as well as the quantity of the dip is occasionally pointed out by an appropriate index and numbers, wherever it seemed necessary to specify them.
Finally, whatever corrections may be found in this map are merely intended to serve the purpose of this paper; the formation of a correct map of Sky must yet be considered a work far distant. ”

“ From this place they continue to form high cliffs, covered in a similar manner by trap, and extending along the coast for a considerable space towards Holme. Limestone occurs together with the sandstone in the interval last described, but the shore is so difficult of access on account of its rocky boundary, the want of creeks or harbours where a boat may land, and the general prevalence of a heavy sea, that it is not possible to trace every point, nor, consequently, to perceive where the changes of the strata take place ”

“ It is possible to reconcile them only by supposing that the red sandstone series belongs to the primary rocks, and the whole of these strata which extend from the point of Sleat to the limestone of Strath, will thus form a succession of rocks alternating with and graduating into each other. That the gneiss is a primary rock can, I imagine, admit of no dispute. ”

“ These masses have fallen from the cliffs above, and lie detached on the shore. They consist principally of a confused crystallization, but cavities are also found in them in which the mineral has crystallized at liberty and in its regular form. These crystals exceed a quarter of an inch in length, and the substance is here invariably of a white colour. There is nothing more remarkable in this mineral than the contrast between its present and its original state with respect to hardness. ”

“ That portion however is important, and I shall here describe it, although much remains to be done before the history of this division of Sky can be considered complete.
I remarked in the former paper that hypersthene was found united to felspar and hornblende in the rocks which surround Coruisk, but in the same place I also stated that a large portion of these rocks consisted of common greenstone. I have now reason to think this observation incorrect, and that the only greenstones (formed of felspar and hornblende) are found in veins. ”

“ In one or more instances I have observed this to happen after more than a mile in extent, throughout all which space not the minutest irregularity had appeared to indicate any thing else than a perfectly conformable and alternating stratification. I have no doubt that, could such extensive exposure be oftener procured, all the instances of supposed alternation between the trap rocks and the stratified ones would prove similar to these. ”

“ The lumps which I have described sometimes exceed twenty pounds in weight, yet they remain entire on the beach notwithstanding they must have fallen, together with the other rocks which are here found, from an elevation of many hundred feet. At the present time the slightest contact causes them to crumble into atoms. ”

“ Their height reaches from 200 to 300 feet and upwards, a dimension, however large, not sufficient to overpower the due proportion which should exist between the aggregate structure and the parts of which it is composed, since the magnitude of the columns is proportioned to their height, and the total effect therefore similar to that of Staffs, where the proportions are so nicely adapted for beauty. ”