PETROGRAPHICAL EVIDENCE IN SOUTH INDIA. 23I 



course, make the discrimination of these phenomena amongst the 

 crystalline schists a doubtful matter. 



In the case of the charnockite series the evidence of distinct 

 contact action is extremely limited. There is the case of alteration of 

 the old biotite-gneiss near its contact with the charnockite tongues 

 in the neighbourhood of Salem (p. 226 ). This might be considered 

 the result of contact action if we had first established the intrusive 

 character of the charnockite ; but in itself it is insufficient proof. 



In Coorg we have perhaps more satisfactory evidence. The 



Western Gh&t range is composed of the char- 

 Altered forms of the ... 



Mercara series. nockite series which march for some thirty miles 



with a group of schists and gneisses distinguished as the Mercara 

 series. The Mercara series includes a very complex succession of 

 highly metamorphosed rocks, many of which, judging from their 

 composition, are almost certainly altered argillaceous sediments, 

 whilst others, quartzites for instance, are probably altered sandstones 

 Bands of "greenstones " are fairly common in the Mercara series 

 and probably represent original dykes, sheets, or flows of diabase. 

 The point of immediate importance to us, however, is the very com- 

 posite nature of the Mercara series, which indicates that its constitu- 

 ents represent a variety of origins although they are grouped together 

 as one formation for stratigraphical purposes. The adjoining large 

 formation of the charnockite series, on the other hand, presents no 

 greater variations than are usually met with in a great intrusive 

 " stock." 



The zone separating these two series of rocks has, therefore, 

 more probably derived its "contact" features by the action of 

 the charnockite on the Mercara series than the converse. Every 

 continuous section across the junction of the two shows the 

 peculiar rocks which characterise the zone of contact ; and al- 

 though on a small-scale map the junction would be represented 

 by a sharp line, it is impossible to define in the field the exact 

 point which would separate the endogenous from the exogenous 

 phenomena. Besides the graphite, sillimanite (or kyanite) whicl 



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