Page:Transactions of the Geological Society, 1st series, vol. 4.djvu/275

 masses are sometimes truly porphyritic, containing crystals of felspar, while at other times they consist of an uniform reddish compact felspar, the usual basis of those porphyries which are found associated with the primary rocks. There is much difficulty in determining the true nature of these porphyritic masses. Occasionally they seem to cut the strata, while in other places there can be no question that they are parallel to them. Nevertheless I am still inclined to think that even those which are occasionally seen conforming to the position of the strata in which they lie, are, like the masses of trap which have a direction parallel to the associated beds, only veins, of which the direction is so far parallel to the position of the beds in which they are found.

From Loch Rannoch the quartz rock is seen crossing the mountains to Loch Ericht, accompanied in the same way with the various schists, and finally terminating in different places round the outskirts of Ben Vualach, where its junction with the granite of this mountain is visible. It may then be traced along the southern border of Loch Ericht, where it joins with a similar series of rocks that will be found extending from Dalwhinnie along the course of the Truim, and then along that of the Garry all the way to Blair. A fact of some importance is visible at Loch Ericht which I will mention here, although not particularly connected with the history of quartz rock. About halfway between the top and bottom of the lake on the south side, a large slide of the mountain is to be seen; the ruin is still so entire, of such magnitude, and so little encumbered with recent accumulations of soil, that there is no difficulty in tracing the fallen masses to the broken summits whence they were detached. The quartz rock here, as in all the outskirts of this granitic country, is traversed by granite veins. A few fallen stones have formed a sort of cave capable of containing three or four persons, and known