DEVITRIFIED ROCKS PROM BEDDGELERT AND SNOWDON. 



413 



consequent on the passage of a glassy into a crystalline substance of 

 sensibly the same composition ; and instanced the inverse case of the 

 fusion of felspar, where the increase of volume, in passing to a glass, 

 fissures the crystal, in the direction of its principal cleavage, into 

 lanrinse, which are kept together by the glassy cement. 



The Author said that the Arran rocks, so far as he knew, were 

 intrusive ; he quite agreed it would be well to use the term felstone 

 in a wide sense. Mr. Blake's question had been answered by Mr. 

 Bauerman, whose remarks on the fused felspar were of much im- 

 portance. 



