igS HOLLAND: CHARNOCK1TE SERIES. 



the charnockites and the olivine-norite dykes are changed to consi- 

 derable depths into a red or yellow clay, and yet when the clay is 

 removed and the hard rock revealed, a microscopic section taken from 

 quite close to the weathered surface shows that even such a suscept- 

 ible mineral as olivine scarcely ever shows any signs of alteration 

 to serpentine (see Nos. ir35°> ii'35*> I i'353i J1 '356). It seems 

 therefore, that mere subaerial action is insufficient to account for 

 the marked hydration which olivine generally shows in rocks that 

 are not quite recently formed, and the only reason for accounting 

 for the remarkable freshness with which our South Indian rocks 

 have been preserved since lower palaeozoic times is based on the 

 fact that they have not been depressed below the sea-level. The 

 action of water would naturally be accentuated by the greatly 

 increased pressure following depression below the sea, and the action 

 of the water itself would be accentuated by the presence in solution 

 of carbonic acid and salts of lime, magnesia aud the alkalies. 

 Except for narrow marginal portions along the coast, we have no 

 evidence of any great changes of level in South India since 

 Cuddapah times, and it is extremely unlikely that the central por- 

 tions of the Madras Presidency have, since that time, been depressed 

 below the sea-level. Our rocks have, therefore, been exposed in 

 all probability to undisturbed subaerial action for many geological 

 ages. Peridotites occurring in other parts of India — the Anda- 

 mans, Burma and the North- West Himalayas — which have been 

 submerged below the sea in Tertiary times, are, like the common 

 instances in Europe, largely changed to serpentine, and although 

 there may be some other reason for such hydro-metamorphism, the 

 only apparent difference between Peninsular India and the ether 

 areas lies in the circumstance that the former area has not been 

 subject to submarine conditions. 



M Trap-shotten " gneiss. 



Messrs. King and Foote were the first to recognise this 

 peculiar phenomenon, which is now known in several exposures of 



( 80 ) 



