126 HOLLAND : GEOLOGY OF THE NEIGHBOURHOOD OF SALEM* 



form complete rings surrounding a perfectly granulitic fine-grained 

 mass of quartz (fig. 3). 



Fig. 3— Coronal ring of spongy garnet surrounding granular quarts, 

 Nagaramalai {No. irgos). 



The disposition of these garnet crowns around the hypersthenes, 

 and the peculiar structure they exhibit, suggest their secondary 

 formation at the expense of the ferromagnesian silicate ; in this 

 case the colourless quartz would represent the supplementary 

 silica relieved by the formation of the less siliceous garnet from 

 pyroxene. 



Some pyroxenic rocks in Bengal described elsewhere 1 show 

 a similar formation of spongy garnet with concomitant separation 

 of quartz ; but in the Bengal rocks the formation of the garnet is 

 preceded by amphibolization of the pyroxene, and the details of the 

 change can be more perfectly traced than in these Salem rocks. 



The garnetiferous basic members of the charnockite series 



Conditions favourable to in the neighbourhood of Salem are often quite 

 the formation of garnets, massive, and show no signs of crushing after 



consolidation. There are many features in them, however, which 



indicate that the garnets are not simple primary constituents, and 



of course we have many other instances in which there is no 



possible doubt about the garnets being of secondary origin. 



Simple dynamo-metamorphism is evidently not essential to their 



1 Holland. " On the origin and growth of garnets and of their micro pegmatitic inter- 

 growths in pyroxenic rocks." Rec, Geol. Surv., lnd.> Vol. XXIX (1896), p. 20. 



( *4 ) 



