DESCRIPTION OF THE PRINCIPAL EXPOSURES. 175 



railway occur to the north of the leptynite and charnockite. The 

 most abundant variety is that in which both augite and basaltic 

 hornblende accompany the hypersthene. In some cases the hornblende 

 crystals are distinctly larger than the other constituents, and by their 

 more rapid decomposition on weathered surfaces give the rock a 

 spotted appearance (Nos. 8*752 and 9*661). Sometimes, however, the 

 hornblende is practically absent, whilst augite and hypersthene are 

 associated in about equal proportions with the plagioclase and opaque 

 iron-ores (No. 9*397). 



On the west side of the railway station the charnockite 

 series is represented in the Christian village, Isa Pallavaram, by a 

 garnetiferous and micaceous variety, resembling, apparently, the 

 peculiar biotite-bearing rock referred to on p. 159. 



The sections exposed in the wells near the village show 

 the prevalence of a coarse-grained quartz-felspar rock resembling 

 that which veins the charnockite at St. Thomas' Mount. Although 

 the rock is generally much decomposed, the quartz crystals retain 

 the characteristic blue colour of this series. Sometimes fragments of 

 garnet and biotite occur in this rock. Further exposures appear 

 rising like islands through the alluvium of the paddy-fields to the west. 

 Rounded lumps of black rock mark the outcrop of a dyke of augite- 

 diorite (diabase) with micropegmatite, whose microscopic characters 

 have been described elsewhere. 1 



The hill which rises to about 200 feet above the alluvium 

 near the village of Pammal, 2 miles west of Pallavaram, is composed 

 principally of hornblende-augite norite (No. 9*673), with less horn- 

 blende in the central portions of the hill (9*676). On the western 

 and north-western margin of the hill considerable quantities of biotite 

 and quartz appear (9*675) sometimes with garnet (9*674). The 

 rocks here include numerous veins of the acid forms, generally much 

 coarser in grain than the average sample of norite. 



The most interesting feature in this hill is the occurrence 



1 Rec. Geol. Surv. Ind., Vol. XXX (1897), p. 3*> Quart. Journ % GeoL Soc, Vol. LIII 

 (1897)* P. 405. 



( 57 ) 



