

CLASSIFICATION OF THE ROCK?. I IQ 



hand-specimen, although under the microscope the low powers are 

 sufficient to show that differentiation has taken place on a small 

 scale, the ferromagnesian silicates being concentrated in some areas 

 and cleared away from others. 



In some parts of South India the separation of acid from 

 More complete basic forms has resulted, as in the hill near 



segregation. gt Thomas > Mount, Madras, in a formation of 



a distinct and large mass having the specific gravity and chemical 

 composition of a granite, associated with other masses uniformly basic 

 in composition, with a constant specific gravity of 3*03 and the mineral 

 constitution of the augite-norites. This formation of large distinct 

 masses of the acid and basic forms of the charnockite series is, 

 however, comparatively rare ; the exposures most frequently met 

 with belong to the " intermediate " group in which the differenti- 

 ation into acid and basic phases has proceeded to a very limited 

 degree, sufficient merely to produce a blotchy character in the rock. 

 The significance of this constancy in average composition and of the 

 limited range of variation are discussed more fully in the separate 

 paper on the charnockite series. 



The charnockite series of the Shevaroy hills are even- 

 grained, blue-grey, or greenish-grey rocks in which it is generally 

 difficult in hand-specimen to distinguish the quartz from the 

 felspar, both being very fresh and presenting the characteristic 

 colour of the series. 



This colour is probably due to the innumerable, minute, acicular 

 D1 inclusions which are found, on microscopic 



Blue quartz. ' r 



examination, to be included in the felspars as 

 well as in the quartz. The acicular inclusions have apparently a 

 definite crystallographic disposition within both minerals, and the 

 simple optical characters of the uniaxial quartz permit the determi- 

 nation of the main crystallographic directions, from which the positions 

 of three of the sets of needles have been obtained. The needles 



( >7 ) 



