PETROGRAPHICAL EVIDENCE IN SOUTH INDIA. 225 



Important light is thrown on the nature of this series by a section 

 exposed near the Namakkal road, 3! miles 



Transgressive tongues. * r o 1 ti. 



south of the town or Salem. The quarry in 

 which this section is exposed occurs at the junction of the char- 

 nockite series, which forms the great mass of the Jarugamalais lying 

 to the east, and the old biotite-gneisses which stretch away to the 

 west and crop up at intervals in the well cultivated plain. On the 

 freshly exposed rock surface, tongues of the charnockite series, 

 proceeding from the direction of the great Jaruga hill mass, are 

 seen to protrude into the biotite-gneiss, running obliquely to the 

 foliation planes of the latter. The charnockite forming these 

 tongues is slightly more basic than the ordinary " intermediate " 

 form, having a specific gravity of 2*8o. The biotite-gneiss contains 

 much quartz and is distinctly more acid in composition. In peno- 

 logical characters, also, the two rocks are quite distinct : the char- 

 nockite is a compact, blue-grey, fresh-looking rock, whilst the biotite- 

 gneiss is mottled by patches of a dark-green micaceous mineral lying 

 in dirty-white felspar and pale-blue quartz, with, frequently, lumps of 

 pyrite. Under the microscope the charnockite is found to be 

 composed of hypersthene, pale blue-green augite, felspar and a little 

 quartz with lumps of magnetite — all showing a type of rock quite 

 common amongst the charnockite series, and displaying practically 

 no signs whatever of dynamo-metamorphism. The biotite-gneiss, 

 on the other hand, is not only highly crushed, but its minerals all 

 show signs of alteration of a kind not seldom found in definite 

 contact cases : epidote and muscovite are formed in the felspars, 

 pyrite and rutile are fairly abundant, whilst the ferromagnesian 

 silicates have completely lost their individuality, being replaced by 

 an indeterminate felsitic product, patches of which are surrounded 

 by a radiate fringe of green micaceous and hornblendic minerals, now 

 far gone in the processes of chloritization. 



If these two rock-masses, the biotite-gneiss and the charnockite, 

 merely existed as adjacent formations, it is possible that one of them 

 might suffer dynamo-metamorphism without noticeable alteration of 



( i°7 ) 



