

CLASSIFICATION OF THE ROCKS. 131 



original differentiation. The most basic types contain olivine, 

 and show an excess of enstatite over augite ; the members of the 

 next division are without olivine and show a tendency for the augite 

 to replace the enstatite, whilst in the third division the augite exceeds 

 the enstatite in quantity and the rocks then always contain a certain 

 quantity of micropegmatite. It is to this last, the most acid division 

 of the dyke-rocks, that these three exposures in the neighbour- 

 hood of Salem belong. 



In all their original characters these three exposures are 

 in perfect agreement with one another, and 



Relationship of the dykes. ........ 



with the usual type of this division of the 

 South Indian dykes. 1 They consist of pyroxene and plagioclase 

 with micropegmatite. The pyroxene crystals are pale-coloured, 

 and are often composed of intergrowths of the rhombic and mono- 

 clinic forms ; the plagioclase (near labradorite) is generally brown 

 through included dust, and the micropegmatite often contains lumps 

 or skeletons of opaque black iron-ores. In the immediate neigh- 

 bourhood of the micropegmatite the augite has been changed to a 

 green hornblende ( and sometimes biotite ), as is commonly the 

 case with these rocks. So far as our experience goes, these 

 points are peculiarities which distinguish what we regard as 

 Cuddapah dykes from some others, like the augite-plagioclase dykes 

 of Coorg, for instance, which are probably representatives of the 

 younger Deccan traps. 



Whilst the dyke near Karipatti shows all these peculiar 



Differences due to features in the most typical manner, the 



secondary causes, exposure in the Shevaroys, and the one near 



the Chalk hills, show certain peculiarities which I am inclined to 



regard as secondary. For instance, the plagioclase has lost its 



brown colour, but is instead crowded with distinct black rods 



1 See " Augite-diorite group" in the paper " On some norite and associated basic dykes 

 and lava-flows in Southern India." Rec, Geol. Surv., fnd., Vol. XXX (1897), p, 31. Also 

 Quart. Journ., Geol, Soc*, Vol. LIII (1897), p. 405. 



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