part 4] sttdi' or the st. atjstell granite. 569 



give the compositions of the granites of Rubislaw, Mountsorrel, 



and Sliap. The values for the last-named are quoted from 



Dr. A. Holmes, and slides of the others were measured for the 

 purpose. 



Field relations. — Consider first the relation of the biotite- 

 muscovite-granite of the Luxullyan area to the lithionite-granite 

 of the adjacent Hensbarrow area. In the Pentruff series of 

 quarries lithionite-granite is found in Mr. Vivian's quarries, and 

 the Luxullyan type at Cam Grey Quarry ; but, unfortunately, no 

 contact of the two types could lie found, and these quarries do not 

 provide continuous sections. Nevertheless, no matter how the rocks 

 of the two districts are compared, they are strikingly different. 

 Topographically, the Luxullvan granite forms tors, while the 

 lithionite-granite is characterized by rounded slopes and flat 

 plateaux (such as Longstone Downs), covered by a mantle of 

 decomposed rock. Mineralogically, the Luxullvan rock contains 

 biotite and museovite (with the characters usual in granite and 

 abundant haloes), also a strongly-zoned acid oligoclase ; while, 

 structurally, it is always coarse in grain. In the lithionite-granite 

 the plagioclase is smaller, un-zoned, and nearer albite in composition. 

 Quantitatively, the proportions of the minerals are different, and 

 there is a relatively sudden change, wherever the boundary is 

 crossed, as is indicated clearly in the space-variation diagram of 

 Jig. 10 (p. 570). In fact, these two rocks have clearly crystal- 

 lized under very different conditions, and are best regarded as 

 separate intrusions. 



The micas of the Cam Grey rock are interesting. Muscovite 

 and biotite, the latter indistinguishable from that of the Luxullyan 

 rock, are present; but, in some slides, the paler biotite, grading in 

 one and the same crystal into lithionite, is present. In fact, all 

 three types of mica, so far as optical properties can distinguish 

 them, are found in the Cam Grey rock. With respect to the type 

 of mica present, then, this rock is transitional to the lithionite 

 type ; but its other characters, both mineral and structural, place 

 it unmistakably in the Luxullyan area. 



The lithionite- and gilbertite-granites evidently grade one into 

 the other. As we approach the St. Stephen's area no radical 

 differences are discernible, and, moreover, the lithionite type 

 reappears gradually on the other side of that area. Structurally 

 and mineralogically, the two types are similar. The plagioclase is 

 rather closer to albite in the St. Stephen's area, and the grain 

 is somewhat coarser. The quantitative boundaries overlap to a 

 considerable extent. Undoubtedly, the two rock-types are to be 

 regarded as transition;)! and nearly, if not quite, simultaneously 

 intruded. 



Quantitative variations. — Leaving the mutual relations 

 of the types for further consideration later, let us examine the 

 quantitative variations of the different areas with respect to the 

 boundaries assigned to them. In order to define the quantitative 



2q2 



