Cole — On the Growth of Crystals. 



121 



the production of brown mica, which is so frequently a contact- 

 product of amphibole, and the addition to the mass of quartz and 

 repeatedly twinned oligoclase. Ill-defined knots of the original 

 amphibolite remain in the composite or reconstructed mass ; but the 

 typical hornblende of the latter is a recrystallised product, and is thus 

 an essential part of the new quartz -diorite that has been formed. 

 This composite rock retains as high a specific gravity as 2*93, which 

 is partly due to addition of pyrite, and preserves a very basic aspect 

 when viewed with the naked eye. Where traversed by granite 



Fig. 2.— Section of composite rock (fine-grained specimen) formed by intrusion of 

 granite into amphiboHte, Cor, Co. Donegal. The large crystal of horn- 

 blende on the right shows two stages of secondary growth outward from 

 the dusky central region. Completely recrystallised idiomorphic horn- 

 blende is abundant in the rock. Brown biotite has arisen freely in the 

 hornblende, x 9. 



veins, however, it becomes an obviously "dioritic" mass in Hauy's 

 sense. Its composite character is still suggested by the spread 

 of quartz and felspar from the granite in sporadic patches, and 

 not in uniform distribution ; but the hornblende has become still 

 further regenerated, and has shot out into crystals often 10 mm. and 

 sometimes 18 mm. long. The introduction of alkali-felspar, largely 

 orthoclase, from the granite magma makes this handsome marginal 

 rock virtually a hornblende-granite. Its specific gravity is 2'85, and 



