distinguished in the generality of hand specimens from genuine granite: at the same time I must add, that it bears but little resemblance to veins. The neighbouring rocks furnish abundant examples of ordinary granite veins, which are readily distinguished from the present appearances by the decision of their junctions, while in this case a regular gradation exists between the granite and the schist, and no boundary to the former can be assigned. The appearance which I have now described is not limited, as I have just said, to this place: other examples of it may be found. Besides the loose specimens of it which are to be observed among the transported blocks on the hills which bound the left side of the valley, it is also of frequent occurrence through a large tract which extends from Blair to Dalwhinnie, and thence to the head of the Spey. The specimens seen in this direction frequently exhibit various and repeated alternations of granite and schist, from the thickness of three or four inches down to that of a quarter of an inch. The tract in question is wholly, or in a great measure, composed of hard argillaceous schist, graduating into schistose quartz rock and more rarely into micaceous schist, with granite veins dispersed here and there through it. Between the lamina of these schists the granite is found graduating into the schist, many varieties of coarser or finer texture appearing in rapid succession in the space of a few inches. It is only by the comparison of a number of specimens that the true nature of the rock is discovered, and it affords a curious example of the facility with which false conclusions may be formed on this intricate subject. However parallel the laminæ of granite may be to those of the schist for a certain space, they invariably quit that direction when the specimen is of sufficient magnitude, and may be seen holding an oblique course through these laminæ, often indeed becoming true granite
Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/300
