DESCRIPTION OF THE PRINCIPAL EXPOSURES. 



>73 



number of low rounded hills which lie on the east side of the line, 

 forming an irregular range not exceeding 500 feet in height. The 

 rocks immediately east of the line are garnetiferous leptynite, which 

 are here regarded as the result of the metamorphism of the charnoc- 

 kite. Whilst the normal charnockite is a compact, dark-grey rock 

 which breaks with a semi-conchoidal fracture, the garnetiferous lepty- 

 nite is dirty-white in colour, is more distinctly foliated and crushes 

 easily under the hammer. The blue quartz, however, which is so 

 characteristic of the charnockite series, retains its colour in this 

 leptynite, but instead of hypersthene the rock is sprinkled with 

 garnets. Microscopic examination shows that this rock, like char- 

 nockite, is composed largely of quartz, microcline and opaque iron- 

 ores, whilst pink garnets appear to replace the original pyroxene. 

 The crushed character of the rock bears evidence to the dynamic 

 action to which it has been subjected (No. 9*665). 



The leptynite is limited on the north and south sides by two 

 rocky ridges, which mark its junction with the normal charnockite. 

 These ridges are generally rust-coloured through the decompo- 

 sition of pyrite, which occurs in large quantities near the margins 

 of the leptynite. A depression between the exposures of leptynite 

 shows masses of norite cropping out in a gully. 



(&Z* * * * *\ CHARNOCKITE 



-CARNETIFEROJJS LEPTYNITE 



'ililllj|pf%?, . _.;,.. : ..^,, 



..-NORITE 







100 



Scale of Yards 



Fig. 4. Plan of hill near Pallavaram, showing the zones of leptynite between 

 the norite and unaltered charnockite. 



( 55 ) 



