DESCRIPTION OF THE CHIEF TYPES. 14* 



The rhombic pyroxene seldom presents any noticeable approach 

 to idiomorphic outlines. Whenever definite 



Hypersthene. ■ . ... 



cleavage cracks are exhibited, the extinction is 

 always straight. I have never found a monoclinic pleochroic pyroxene 

 in the mass of charnockite near St. Thomas' Mount {vide supra t 

 p. 126, foot-note). The pleochroism is very distinct, similar to that 

 of hypersthene. 



& t reddish brown or bright pink. 



b, reddish yellow. 



C, green with a bluish tinge. 



A greenish-yellow, fibrous, pleochroic mineral resembling deless- 

 ite is often developed along irregular fissures in the hypersthene and 

 is evidently the same as that noticed by Lacroix in the pyroxene of a 

 rock described by him as a H pyroxenic leptynite " from Ceylon. 

 The maximum absorption is parallel to the fibres which is also the 

 axis of minimum optical elasticity. 



The opaque iron-ores appear to be referable chiefly to magne- 

 tite ; titaniferous varieties, however, occur very 



Accessory minerals. 



commonly in the basic associates of charnockite. 

 I have never found a garnet in the unaltered type mass at St. Thomas' 

 Mount, but, as shown below, this mineral is an invariable constituent 

 of the varieties which have suffered from marked dynamo-metamor- 

 phism. 



Chemical analysis shows that the type-mass of charnockite agrees 

 with normal granite in the predominance of 



Chemical composition. 



potash amongst the alkalies and in the general 

 proportions of the other constituents. The following results (I and II), 

 obtained from specimens collected near St. Thomas' Mount, Madras, 

 are compared with the hypersthene granite of the hkersund area, 



( 23 ) 



