154 HOLLAND: CHARNOCKITE SERIES. 



weakness of our system of rock classification, however, is strongly 

 emphasised by a study of these rocks. In the charnockite series we 

 have large masses of rock composed essentially of rhombic pyroxene, 

 augite and plagioclase, a composition precisely similar to that of 

 some of our black dyke-rocks, and yet no one who has studied these 

 rocks could fail to notice that the norites now under description 

 differ far more from the norite dykes than they do from the acid rock 

 charnockite. To group together a basic rock and an acid one as we 

 do here is contrary to the usual practice of petrographical classifica- 

 tion, and yet there can be no more doubt about the close genetic 

 relationship of these two types in South India than there is about 

 the consanguinity of those described by Vogt in the Ekersund 

 area. 



The norites are almost always granulitic (panidiomorphic) in 

 structure, neither the ferromagnesian silicates 



Granulitic structure. , . . 



nor the plagioclase showing any noticeable ap- 

 proach to idiomorphic outlines ; but when quartz occurs, as it some- 

 times does in small quantities, it is generally irregularly developed 

 around the other minerals as if it were the last of the constituents 

 to crystallize. Consequently the granulitic structure is more per- 

 fectly developed in the varieties free of quartz. The fact that many 

 rocks which, like marble, have taken on a granulitic structure as the 

 result of metamorphism naturally favours the idea that the pyroxene 

 granulites are metamorphic rocks. But we now know, however, 

 that a granulitic structure may result from disturbance of the magma 

 during the process of consolidation, so the phenomena displayed by 

 these norites belong as much to igneous rocks as to those formed by 

 metamorphism (see p. 239). The dykes of pyroxenite which are 

 associated with, and cut, these norites at Pallavaram are as perfectly 

 granulitic in structure as the norites are and there is no reason to 

 doubt their igneous origin. 



The norites of the Pallavaram area are very uniform in their 



specific gravity and by it are sharply marked 

 Specific gravity. off from the associated charnockite. The fol- 



( 3*3 ) 



