DESCRIPTION OF THE CHIEF TYPES. 133 



CHAPTER IV. 

 Description of the chief types. 



As already stated, the members of the charnockite series vary 

 from acid types, having the mineralogical and chemical composition 

 of granites, to ultra-basic forms or pyroxenites. For convenience 

 of description, however, they may be divided into four groups which 

 are not sharply marked off from one another : — 



(1) Acid division, represented by charnockite, a hypersthene- 



granite having a constant specific gravity of 2-67 and 

 silica percentage of about 75. The type-mass forms 

 the central portion of the hill near St. Thomas' Mount 

 (fig. 1). The type-mass is cut through by contempor- 

 aneous veins of coarse quartz-felspar rock. The gar- 

 netiferous forms resemble leptynites in composition. 



(2) Intermediate varieties are by far the most abundant 



and are characterised by an apparently composite 

 structure, all the minerals of the series being frequently 

 found in one hand-specimen with a tendency for the 

 coloured minerals to gather into groups. The average 

 specific gravity is 2*77, with a silica percentage of 

 about 64. Acid contemporaneous veins and basic fine- 

 grained schlieren are common, The Shevaroy mass is 

 a typical exposure. 



(3) Basic forms, mineralogically equivalent to the norites 



and composed of pyroxene (hypersthene and augite), 

 plagioclase and iron-ores, often with hornblende. The 

 type-mass forms the flanks of the hill near St. Thomas' 

 Mount (fig. 1). Specific gravity, 3*03 and silica per- 

 centage, 50 to 52. Garnetiferous forms are common 

 near the outskirts of mountain masses, and form large, 

 lenticular bodies in the older gneisses and schists. 

 Garnetiferous as well as non-garnetiferous varieties 

 in Coorg form dykes having chilled selvages. 



( 15 ) 



