334 Dr, Mac Culloch on the Geology of Glen Tilt. 



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 unfrequently 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 pro- 

 gress of the laminse, 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 ob- 

 served, 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 de- 

 fine, and the true connections and nature of the rock may be 

 known, by tracing its connections with, and gradation into, the 



