Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/486

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Dr. Mac Culloch on Quartz Rock.

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. 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,[1] 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. 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. The laminated or stratified structure is by no means so evident in this example as in those I have before adduced from Jura, Schihallien, and Assynt; but it still bears the marks of a broken and disturbed stratification. Such being the case, and such the infinite variety of position, direction, and elevation, assumed by this rock, I think it unnecessary to enter into any details on these heads. I may only remark generally, that it stands at an angle not far deviating from