GENERAL CONSIDERATIONS. 197 



up into a more basic garnet and a more silicious bye-product, 

 quartz or felspar — a kind of liquation process facilitating the 

 segregation of basic and acid extremes from a body of intermediate 

 composition. It is to such a cause that the prevalence of garnets 

 in these old pyroxenic rocks should be ascribed. 



The means by which the pyroxenic and garnetiferous rocks 

 have been brought to the surface have been in action for long 

 geological ages, during which Peninsular India has suffered from no 

 serious earth-movements. The deep-seated rocks have thus been 

 brought to the surface without undergoing any form of crushing 

 at intermediate levels, and so the original characters of our charnoc- 

 kite series are probably preserved in an unusually perfect manner. 



Limited degree of Hydration. 



Besides the freedom from crushing due to absence of earth- 

 movements since Cuddapah times, the peninsular rocks have 

 escaped general hydration in a most remarkable way. The freshness 

 of South Indian rocks is not confined to the charnockite series ' 

 the olivines in the basic dykes of Cuddapah age scarcely ever show 

 a sign of serpentinization, and even in the large number of dunite 

 areas serpentine occurs only in small quantity. 1 



Another striking example is offered by the elaeolite-syenites 

 recently discovered in the Coimbatore District, where the mineral 

 elaeolite, quite as susceptible to hydration as olivine, has been pre- 

 served in a perfectly fresh condition although the rock is probably as 

 old as any sedimentary formation in South India, and was even 

 described as a member of the crystalline schists. 



And yet these rocks, which are internally so remarkably fresh* 

 are, in common with all rocks exposed to the moist, warm climate 

 of tropical countries, changed near the surface into a soft clay 

 to depths of 50 feet or more. Near Coonoor in the Nilgiri Hills both 



1 Dunites and other peiidotites of South Inr'ia are frequently changed in large 

 quantities to magnesite. In another paper reasons are given for ascribing this change to 

 the deep-seated action of carbonic acid and not to subaerial agencies. 



( 79 ) 



