PETROGRAPHICAL EVIDENCE IN SOUTH INDIA, 237 



tonic masses. For the time, therefore, these corundiferous bodies 

 must be looked upon as doubtfully, though for theoretical reasons, pro- 

 bably, foreign material enclosed in and metamorphosed by the char« 

 nockites. Their position near the margin of the large charnockite 

 masses also favours this conclusion. Unfortunately this boundary 

 line between the charnockite series and the adjoining biotite-gneisses 

 was not traced out during the course of the survey by Mr. Middle- 

 miss. Besides these examples found near Palakod (12 18' ; jB°S') 

 in the Salem District, similar instances, with beautiful ruby-coloured 

 corundum, have recently been found by Captain Campbell, R.A., 

 at Badavadi (u°5o'; 77 6') in the Mysore State, where the 

 xenoliths occur near the margin of the charnockite mass forming the 

 Bargur hills. A fairly close parallel to the phenomena observed in 

 these corundiferous xenoliths occurs in connection with the well 

 known Klausen norites and enstatite diorites described by Teller 

 and von John. These authors have described foreign inclusions in 

 the Klausen rocks composed of andalusite, corundum, biotite, garnet 

 and apparently orthoclase. 



Chemical and Microscopical Evidence. 



To the petrographer the chemical composition and the micro- 

 scopical characters of rocks are criteria which rank with the larger 

 features recognisable in the field for importance and reliability. 

 In an area where the deformation of rocks has been carried to a 

 high degree, these features are often the only ones left which give 

 any unequivocal indication of origin. The chemical is more valuable 

 than the microscopical method ; for a rock may be so profoundly 

 deformed that all the minute structures have been destroyed and 

 even new minerals formed by internal re-arrangement of the chemical 

 compounds, but the bulk analysis of the rock might still be not far 

 removed from that of the original material. In the following pages 

 these two features are treated in order. 



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