John MacCulloch

“ It is also obvious that the boundary of almost the whole northern side, as far at least as from Meggarney to Fortingall, consists of a sort of compact sandstone or “granular quartz,” a rock in no way differing from many of the varieties of quartz rock already described. This is the ridge which declines from the south of Schihallien, and which is intimately connected with the main ridge of that mountain. The sandstone itself exhibits a texture of infinite variety, but in all cases it possesses the same aspect of antiquity and induration which characterizes the rocks of Jura and Schihallien. ”

“ As we approach the summit of the mountain, the quartz rock becomes established to the exclusion of the schist, and it continues to the top, where it offers that aspect of complete disintegration before described under the heads of Jura and Assynt, which covers the tops of the quartz mountains with ruins, and gives them that acute apex and regularly conoidal declivity, which, as far as I have yet observed in Scotland, is sufficient even at a distance to indicate the nature of the rock of which they are composed. ”

“ It is perfectly true that many of the beds in Jura contain large tracts of a granular quartz, often very pure and compact, and which from its crystalline texture might in the hand be supposed a primitive and chemical deposit; but the occurrence of blunted fragments, and of rolled pebbles, suffice to shew, that like many other rocks it possesses at least the compound structure both of a chemical and of a mechanical deposit. Neither is the perfect crystalline texture of quartz any proof of a primitive formation ”

“ Proofs however are not wanting of mechanical attrition in the component parts of many beds of this rock, although the fragments do not absolutely reach the size of a pebble. A rock resembling ferruginous sandstone, of a highly compacted nature, is found in beds, alternating with the finer and more compact quartz rock, the grains of sand and gravel bearing most evident marks of attrition; and the whole mass being indurated to such a degree, that a hammer makes no more impression on it than it would on a mass of iron. ”

“ At times the felspar and quartz are so nearly equal in quantity, that the whole forms a pink coloured granular mass, often so loose as to crumble in the hand; and in this case the grains seldom bear any mark of crystallization, but are rounded as if they had undergone attrition previously to their aggregation: occasionally, but rarely, it contains imbedded fragments of quartz. It is not often traversed by quartz veins, but wherever these occur they are much confounded at their edges with the mass of the stone. ”

“ Specimens from Arisaig, from Assynt, Mar, Balahulish, Airds, Sky, Schihallien, and numerous other places, which consist of variable mixtures of reddish felspar, and transparent quartz, sometimes appearing to be nearly all quartz, and at others to contain a large proportion of felspar. The grains are of various dimensions, and the texture of the stone is consequently various. At times it has the aspect of a fine reddish sandstone, at others of a coarse grit; and where the grains are very unequal in dimensions it resembles a breccia. The grains have never a defined form. ”

“ This quartz rock is stratified, although vertical, in beds which, as far as they can be seen, appear absolutely and nicely parallel, their lines of separation being defined with a most mechanical exactness. They have a tendency to break into rhombic and prismatic fragments, by fissures perpendicular to their stratification, and being thus broken, they have fallen on all sides around the summit, producing the same appearances as those exhibited by the Paps of Jura, and the mountains of Assynt ”

“ This rock is succeeded by quartz rock in large masses. Some beds of a hard or compact schistus, of a character often approximating to hornblende slate, are also found to all appearance alternating with it, and I have already noticed in the paper above alluded to, [2] that granite veins are found to traverse this particular schist. The hill is, on its accessible face, so covered with soil, that no very positive evidence can easily be procured of the alternation which I suspect; and the precipitous faces are generally inaccessible, always hazardous of access. ”

“ Circumstantial evidence renders the latter probable. In the description of Jura, I have remarked that mica slate does, according to Professor Jameson, lie above the quartz rock, and Williams considers the rock I have described, which he also calls granular quartz, to be, like granite, the rock on which micaceous schist usually rests. Professor Playfair also in his Illustrations, (P. 166,) speaks of a “granitic sandstone in vertical beds,” and mentions its alternation with “micaceous and other varieties of primitive schist.” ”

“ An extremely compact granular stone, consisting of grains of quartz, of unequal sizes, united without cement.The same, containing grains of clay, which appear to be decomposed felspar.The same, with more numerous grains of felspar, which appear on examination generally to consist of rounded fragments.These rocks are traversed by veins of quartz, of which the aspect is also granular, but they are distinguished from the body of the rock by their snow white colour. They often appear so incorporated with the rock that the line of separation cannot be distinguished. ”

“ Here then, as in other instances, we see that it exhibits the ambiguous or rather double appearance of a mechanical and of a crystallized deposit, a mixture of character, which, as I have elsewhere remarked, [3] is not the least important difficulty which yet remains for geologists to explain.
I have reason to think that this rock will be found to occupy a very considerable space among the mountains in the vicinity of Glenco. In many places the fragments assume a dirty reddish appearance, which at a distance gives them the aspect of granite. ”

“ And thus, as in other instances, a connexion in structure and habits is established between certain rocks of the primary and of the secondary classes. In all cases the mica is disposed in lamellæ parallel to the strata or laminæ of the rock; it is never as in gneiss partially disposed in different directions, still less as in granite indifferently placed. ”

“ Grey quartzy rock, with a basis of a compact splintery appearance, containing imbedded grains of transparent quartz.Small grained breccia, of which the basis is an earthy-looking mixture of felspar and quartz, resembling clay stone, or compact felspar, and uniting angular and rounded fragments of quartz.An uniform aggregate of large grains of felspar and of transparent quartz, without the aspect of granite, since all the grains appear to have been mechanically rounded and re-united. ”

“ Its mechanical structure deprives it of a right to the latter title, and its connexions and disposition prevent us equally from arranging it with the latest stratified or “flœtz” rocks. If it is a necessary condition of the “transition” rocks to contain organic remains, then this rock is also excluded from the intermediate or transition class, as much by this as by its position in respect of the mica slate, universally esteemed among the primitive, or to speak more properly among the most ancient rocks. ”

“ Mr. Pennant too, although not an authority in a case of this nature, describes quartz about Loch Broom, and more cursory observers have given us reason to think that the generality of the higher mountains of this northern tract of Scotland are composed of “quartz.” There is little doubt that the whole of this elevated tract is of the same formation as that under consideration, and it ought therefore perhaps (according to the remarks I have already made on Jura) to be ranked with the Transition rocks, if we adopt the divisions of Werner. ”

“ Pure white quartz, formed of semitransparent amorphous grains, but the mass has a fracture intermediate between the granular and splintery. From Sky, and alternating with micaceous schist. A similar rock from Balahulish, of a pink colour, but with an aspect more harsh and dry, resembling common secondary sandstone. In the same situation. Pure granular quartz in the same situation, but the grains of large size, and the fracture consequently very coarse. ”

“ I also suspect that greater simplicity of composition and more perfect induration will be found to be leading characters of the breccias of this class, the causes of which must be sufficiently obvious, and that portions of breccia or large grained mechanical formation, will generally be found accompanying the older schistose strata whenever these exhibit the smaller grained mechanical disposition. It is to these that the unfortunately constructed term of primitive breccias has been applied. ”

“ Those who have been occupied in similar investigations well know how difficult it is to ascertain the disposition of large tracts of country unless observed from situations so elevated as to raise the spectator above all the obstructions which the varying forms of high ground throw in the way of this great natural perspective. ”

“ The mountain itself forms the most elevated part of a ridge which may be considered as rising near Fascally, and extending in a south-westerly direction till it meets that complicated group from which the ridge bounding the southern side of Glenco takes its rise. ”

“ But I have little doubt that many instances even of this occurrence, will be found by those who may have more time to bestow on the investigation, and who with better fortune may collect such specimens as might easily have eluded my cursory search. ”

“ From the former (perhaps indispensable) details, future observers will be enabled to satisfy themselves of the truth of these fundamental remarks, and from the latter, opportunity will be afforded for a more easy to those who may wish to assign its place in a system. ”

“ The same rock occurs in the district of Arisaig. It occupies there a considerable portion of the shore, and may be traced forming low hills and interrupted projections, from the point of Arisaig, nearly to Loch Morrer. It seems here to alternate with mica slate, as I have already mentioned in another place, where I have quoted Professor Playfair's remarks on it; but possesses much less of the appearance of stratification than in the other instances which I have enumerated. ”

“ Coarse graywacke slate.Fine ditto.Perfectly fine and uniform clay slate, of a dark blue colour. It is important to remark, that the beds which are found at the foot of the mountain, are of a coarser texture than those at the summit, and these are probably the uppermost beds of the deposit. Many of them are nearly black from the quantity of clay they contain; in others are found grains of mica, and in some there are imbedded large fragments of clay slate and chlorite slate. ”