72 



MR. E. E. WALKER ON THE GARXET-BEARIXG [Feb. 1904, 



of Blea Crag an intermediate rock is cut by a more acid rock — the 

 junctions being well denned ; so it is probable that there were succes- 

 sive periods of intrusion and intermixture. That differentiation 

 went on almost simultaneously with intrusion, is well shown 

 by a specimen obtained from the summit of Blea Crag, in which 

 the black fine-grained rock encloses completely a portion of coarse 

 pink rock, the latter being an acid segregation. Naturally, under 

 these circumstances, xenoliths are exceedingly abundant and of 

 great variety. Usually the xenolith is more basic than the 

 enclosing mass, but the reverse also occurs. 



Fig. 1. — Diagrammatic sketch of an exposure north- nortli-iv est of 



Blea Crag, Lang strath. 



F = Fine-grained black rock. 



C = Coarse pink rock. 



C'= The same, with pink felspars. 



G = Dark-green porphyritic rock, 

 with greenish-white porphy- 

 ritic felspars. 



<-X-> = 1| yards. * 



The dark fine-grained rock bears a very great resemblance to the 

 dark-green fine ash into which these rocks are intruded ; and, so 

 far as it is possible to judge, Ward mapped this rock as an ash. 



A section (3750) ! across the specimen mentioned above shows 

 that the pink rock is a granophj-re. The structure is entirely a 

 micropegmatitic intergrowth of quartz and orthoclase. Oblong 

 sections of turbid felspar are seen to be continuous with the 

 felspar of the intergrowth, but it is only these idiomorphic felspars 

 that are altered to white mica. Sometimes they show signs of 

 corrosion by the granophyric material. Garnet occurs in rounded 

 sections, ilmenite and chlorite occurring at the border as products 

 of its decomposition. Chlorite-scales are abundant throughout the 



1 The numbers in parentheses throughout this paper refer to the numbers 

 on the slides which are in the Sedgwick Museum, Cambridge. 



