GEOLOGICAL FEATURES. 15 



In addition to the minerals already enumerated, garnets also are 



„ .... CL . occasionally found. They are, however, greatly 



Decomposition of the r J ' ° J 



garnet. decomposed, and in many cases consist merely 



of minute fragments of garnet lying in a confused mass of chlorite 

 or limonite (PI. 2, fig. 4), or a mixture of both. This secondary 

 alteration of the garnet is not improbably due to the charnockite which, 

 as previously stated, is found in many places on this ridge. No actual 

 contact between it and the quartzite has been observed, but it is 

 probable that if sufficient time could be devoted to a careful search 

 along the ridge, such a contact would be found. This, however, 

 would involve a post-Dharwar age for the charnockite, and in view of 

 the absence of direct evidence bearing on this question, we are not at 

 present in a position to assume this. Yet the advanced state of 

 alteration of the garnets in both the gneiss and the quartzite, taken 

 in conjunction with the absence of alteration of those of the charnock- 

 ite, would tend to show that such alteration was not produced 

 subsequently to the intrusion of the latter rock. The similarity in 

 the products and degree of alteration of the garnets of both the 

 gneiss and the quartzite on the Marpanmudi ridge is favourable to the 

 supposition that the alteration was in both cases due to the same or 

 a similar cause, while the fact, that in the neighbourhood of the 

 charnockite intrusions the alteration of the garnets is much more 

 complete than elsewhere, points to the charnockite as being that 

 cause. 



The presence of such a small isolated strip of Dharwar rocks in 

 the midst of the gneissic area of south-east Waina'd is somewhat 

 surprising, the nearest known outliers of that age occurring far to 

 the north-east in Mysore and also in North Coimbatore. 

 (4) Intrusives other than the charnockites. 



These include four series of intrusives, viz.,—* 



(a) Older basic. 



(b) Younger basic. 



(c) Biotite granite. 



(d) Pegmatite veins. 



( 15 ) 



