2G GEOLOGY OF ARRAN. 



the older granite. Formed under such conditions, this later 

 granite might be expected to differ in structure, if not also 

 in composition, from the older granite or other rocks invaded 

 and displaced by it. Such differences we know do actually 

 exist between granites in the Alps, Andes, and other 

 localities, which can be clearly proved to be of different 

 ages. Now, we have two such granites within the area of 

 the mountain nucleus, the coarse or under-lying, and the 

 fine-grained or over-lying (Art. 6). "With this latter, Mr. 

 Ramsay's Ploverfield and our Craig-Dhu granite exactly 

 agree (Art. 11, 12). Is it not, then, probable that these 

 three, the fine-grained of the nucleus, and those of Plover- 

 field and Craig-Dhu, belong to one period of disturbance 

 that they were simultaneously injected amid the rocks which 

 now enclose them, at a period subsequent to the deposit of 

 the carboniferous strata? This conclusion is rendered highly 

 probable by the character of the contact between the fine 

 granite and the slate, on the south boundary near Loch 

 Ghnuis, discovered by us in 1864, and to which reference 

 has been made (Art. 6). The two rocks adhere firmly, the 

 slate is altered in the usual way, and invaded by ramifying 

 veins, in the same manner as the coarse variety invades 

 the slate in other places. At several yards back from the 

 junction a few masses or large blocks of the coarse-grained 

 variety are seen, and in two or three cases were observed to 

 be penetrated by veins of the fine kind ; in othei-s the two 

 kinds were irregularly associated in one block, in a way 

 suggesting either injection of one into the other, or a simul- 

 taneous crystallization under varying conditions, the true 

 relations being masked by the extensive decomposition which 

 lias affected both. The junction here was traced for upwards 

 of a mile, and the coarse and fine kind found to be irregularly 

 associated throughout, the fine predominating, and the coarse 

 often occurring in such positions that it was difficult to 

 determine whether it was in situ or transported. The 

 contortions of the slate are very striking along the boundary. 



