Page:Transactions of the Geological Society, 1st series, vol. 2.djvu/485

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475
Dr. Mac Culloch on Quartz Rock.

On the borders of Loch Leven, and on its northern side, it is most abundant, and appears to constitute large portions of the mountains, having an evident similarity of direction and disposition, to those masses of it which form the principal parts of the mountains of Ben-na-vear on the opposite or southern side of this lake, and which I am now about to describe.

I have already had occasion to mention in a note supplementary to the account of Cruachan, that granite is found at the base of the group of mountains called Ben-na-vear, extending from Balahulish towards Glenco in one direction, and for a considerable space along the Appin road in the other. On ascending this group, the granite continues for some way. This is followed by a rock, which, although it forms here as in many other places, considerable portions of mountain masses, has scarcely received a name and an establishment in the system of rocks. It is a sort of schistose quartz; it is not graywacke, nor is it micaceous schist, though it contains grains of quartz, and mica, and clay: neither is it gneiss, although it contains grains of felspar. Previous prejudices might perhaps find it a place among either of these, according as the predominant system dictated. But the best idea of its general structure may be conveyed by saying, that if it was a flœtz rock it would be called a schistose sandstone. There is in many cases so strong an affinity between the rocks of the primary and of the secondary, or flœtz, classes, that it can scarcely lead to error, if, pursuing this analogy, I should call it a schistose quartz rock. It will in fact be found to form the same connexion between the micaceous or clay slate and the quartz rock, as the schistose sandstone which alternate with thin lamina of clay slate do between the latest clay slates or shales and common sandstone. And thus, as in other

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