Page:Transactions of the Geological Society, 1st series, vol. 3.djvu/342

 with clay slate, nor that it has any immediate connection with granite, much as it resembles it in composition. But in mica slate, as well as in the micaceous quartz rock which occurs in the vicinity of granite, a real gneiss is not infrequently seen near the points of junction; the schistose rock which in other parts consists of quartz and mica in various proportions, having felspar superadded, and preserving the same parallelism in the disposition of the mica which it possessed before the addition of that ingredient. This change is of various extent, in some cases not reaching many inches beyond the line of contact with the granite, and gradually disappearing even in the lateral progress of the laminæ, while in other instances it occupies a more considerable space. But in no case that I have witnessed is it prolonged through the whole extent of a bed so as to allow us to say that beds of gneiss alternate with or precede the mica slate. I conceive the appearance of gneiss which I have described in Glen Tilt to be of this partial nature, and in all probability owing to similar causes. I would also remark that as quartz rock is evidently recomposed from the ruins of ancient granites, it often contains all the ingredients of that substance. In the extensive remarks which I have made on this rock, as yet so little observed, I have noticed instances in which it has been mistaken for granite. In the same way it may be mistaken for gneiss, since with a stratified structure it may contain all the ingredients of that substance. Nay, even the mica may be disposed in the same parallel form in the triple compound as it is in the more ordinary mixtures of quartz and mica. Even here however an experienced eye may detect differences which words are inadequate to define, and the true connections and nature of the rock may be known, by tracing its connections with, and gradation into, the