that the rays of light are reflected from it at the same angle over a wide space, leading to its detection at a distance, as I have also noticed under the head of gneiss. We might, in such a case, imagine the whole vein to be a single crystal, deficient in external form only because there was not a vacuity in which that could have displayed itself. Though the variety of granite called graphic is not limited to the veins which traverse gneiss, it is most frequent in those, though seldom occupying the whole of one. I need not describe what belongs to the O business of mineralogy; only remarking, that it is sometimes the quartz, and at others the felspar, which has first crystallized, and thus determined the figure of the other mineral. A third kind of granite veins has been mentioned, independent of any central mass. There is no reason to presuppose this; and I have traced to the parent granite all that were pointed out as such in Scotland. Their great size and long persistence seem to have led to the notion that they differed from the veins in gneiss; but while it is evident that no small vein could occupy a great range, they are analogous to the similarly persistent veins of trap, as their connexions must thus be equally difficult to ascertain. That granite veins traverse masses of granite, is a fact that I have used to prove distinct ages in this rock; while they are often marked by the same peculiarities of character as when in gneiss. In those which traverse this substance, the texture is uniform through out ; hut in others, it very often differs in the middle and at the sides. Yet they are often so amalgamated with the gneiss that the boundary is undefinable; the mica or hornblende losing their parallel directions, VOL. II. ii