102 HOLLAND: CHARNOCKITE SERIES, 



CHAPTER VI. 

 General Considerations. 



On account of the large are acovered by the charnockite 

 series in the southern portion of the Madras Presidency, the pre- 

 dominant features which they present must necessarily affect 

 our generalisations concerning South Indian petrography and 

 geology. 



Four such points considered below deserve special mention : — 



(i) The abundance of magnesian minerals. 



(2) The preservation of old pyroxenic rocks. 



(3) The limited amount of hydration suffered by the rock- 



constituents. 



(4) The nature of the so-called (( trap-shotten " gneiss. 



Abundance of Magnesian Minerals. 



The most remarkable feature which has been revealed by 

 recent microscopic study of the crystalline rocks in South India is 

 the very great predominance of the pyroxenes (and especially of the 

 rhombic forms of that group) amongst the ferro-magnesian silicates. 

 In addition to the series described in this paper, which is pro- 

 bably the most abundant of all the rock- groups in the southern 

 portion of Peninsular India, and in every variety of which hyper- 

 sthene is a constant and characteristic constituent, the central and 

 eastern parts of the Madras Presidency are cut through by an 

 enormous number of basic dykes, in which rhombic pyroxene also is 

 a leading mineral. Judging thus by the mineralogical composi- 

 tion of the rocks, which is confirmed by the few chemical analyses 

 that have so far been made, the bases magnesia and ferrous oxide 

 must take an unusually prominent place amongst the chemical 

 constituents of the southern portions of the Peninsula. Besides 



( 74 ) 



