GEOLOGICAL FEATURES. I? 



{<:) Intrusive masses of biotite granite occur at Sultan's Battery 

 and Kalpeta of Mottumalai, a prominent hill 



Biotite granite. _, . 



near Meppadi. These granite masses were 

 described by Dr. King 1 as " great rocky cores, around and over which 

 the foliated gneisses were laid down." The granite of Kalpeta 

 strongly resembles, both in its composition and in its relation to the 

 surrounding rocks, the so-called " dome gneiss " of the H^zaribcigh 

 district of Bengal, 3 and in the light of more recent researches by 

 Mr. Holland and the present writer, in Bengal, and by the writer, in 

 Waindd, it seems more probable that this rock is in reality younger 

 than the surrounding gneisses and is intrusive in them. 



(^) Pegmatite veins are very common throughout south-east and 

 south Waindd, and occur in both the gneiss and 



Pegmatite. 



the charnockite : in the former, they run 



parallel to the strike of foliation of the gneiss. They are composed 



of quartz and felspar with muscovite and occasionally biotite and 



garnet — no other minerals have been identified in them. 



The quarts occurs in great masses, and has in some cases been 

 mistaken for reef quartz: but its granular and sugary character is 

 almost invariably a safe means of distinguishing it from the clear 

 uncrushed quartz of the true reefs. 



The felspar is always decomposed to kaolin, usually free from 

 impurities. 



Muscovite occurs in large crystals of very fine quality (best ruby 

 mica) and of fair size, and is of considerable economic value. 

 Mining is being carried on at Cheramb&di, and offers every prospect 

 of success. The other minerals, biotite and garnet, are merely ac- 

 cessory, and are both somewhat rare. 



The presence of great masses of quartz in the pegmatites led to 

 this being confounded with the auriferous reefs, and mining operations 

 were in some instances carried on for a considerable time before it 

 was realised that they were not auriferous. The fact that these 



1 Rec. Geol. Surv. Ind., Vol. VIII, p. 38. 



2 General Report, Geological Survey of India, 1898-99, p. 29. 



C ( 17 ) 



