GENERAL CONSIDERATIONS. 193 



occurring as a constituent of the enstatites, which are so widely 

 distributed through the Madras rocks, magnesia occurs even in 

 larger proportions in the peridodites, which, either as dunites, 

 saxonites, picrites or their decomposed forms magnesite, serpentine 

 and steatite, are now known to be far more abundant than was 

 suspected at the time of the first recognition of these highly magne- 

 sian, ultra-basic rocks near Salem in 1892. 



Prevalence of Pyroxene. 



More remarkable than the prevalence of magnesian compounds 

 is great predominance of the pyroxenes amongst the rock-forming 

 minerals of the south. The pyroxenes are essential constituents 

 of the charnockite series which make up the chief mountain 

 masses and cover large areas in the low-lands; they characterise the 

 basic dykes which cut the older crystalline rocks in all direc- 

 tions, and the soda-bearing members of the group have recently been 

 found in the augite-syenites of the Yelagiri hills and associated 

 intrusives. 



These are all geologically old rocks, probably not later than 

 lower palaeozoic in any case, and the perfect preservation of such 

 unstable minerals as the pyroxenes forms an interesting corro- 

 boration of now established conclusions as to the long quies- 

 cence of Peninsular India, a geological quiescence which was infer- 

 red by the older geological surveyors from stratigraphical evidence 

 alone. 



Although the dyke-rocks of supposed Cuddapah age are pre* 

 served with perfect freshness, there is abundant evidence to show 

 that the previously formed Dharwars suffered from dynamo-meta- 

 morphism, and the pyroxenic dykes and flows of the earlier period 

 have been largely changed to hornblendic schists. 



Knowledge of the profound dynamo-metamorphism suffered by 

 the Dharwar transition rocks] naturally provokes a search for 

 evidences of similar action on the associated and presumably 

 older Archaean formations in the same area ; and here the enquiry 



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