576 THE ST. AUSTELL GKANITE. [vol. lxxix, 



by additions to crystals already present. In this region also, 

 crystal-sorting according to the velocity of the incoming stream 

 again took place, so that the larger plates of the perthite had a 

 central deposition. Final stages of intrusion, or rather of crystal- 

 lization, took place in the St. Stephen's area, accompanied by a 

 great concentration of mineralizers, which helped towards the 

 production of a uniform grain, removed the iron compounds, and 

 retained the lime in solution to a late stage. 



Discussion. 



Dr. J. S. Flett said that, in his studies of the Cornish granites, 

 he had been impressed rather with their uniformity than with their 

 differentiation. It was the remarkable pneumatolytie facies that 

 constituted their chief attraction from a petrologieal standpoint. 

 It was interesting, consequently, to see that by the detailed 

 methods of modern investigation distinct types could be identified 

 and their distribution mapped. The process employed was certainly 

 laborious, but promised to yield useful results. 



Dr. A. Bbamjiall expressed appreciation of the Author's 

 methods of investigation, and enquired whether, before rejecting 

 pneumatolysed types for the purposes of this work, he had tested 

 the possibility of an approximate mass-equivalence between 

 primary minerals such as biotite, ilmenite, sphene, etc., on the one 

 hand, and secondary minerals such as tourmaline, brookite, anatase, 

 on the other ; this equivalence was observed in the case of the 

 Dartmoor Granite. He also asked whether zoned tourmalines of 

 the type shown were regarded by the Author as definitely primary: 

 similar occurrences on Dartmoor were certainly secondary. 



The Author, in reply to Dr. Brammall, stated that the investi- 

 gation indicated merely a very slight correlation between tour- 

 maline and biotite ; but the general relation could only be investi- 

 gated by considering granites as a whole. Much of the zoned 

 tourmaline in the normal granite appears to be of the same date as 

 the unzoned, and is regarded as primary. 



