DESCRIPTION OF THE PRINCIPAL EXPOSURES. 1 79 



between the charnockite series and the remarkable rock which has 

 been referred to by the older members of the survey as " the 

 granitoid gneiss of South Arcot "- 1 



This latter rock is distinguished by the white colour of its 

 quartz and the inclusion of grains of pink felspar, accompanied by 

 Only a feeble ^development of gneissose and banded structures. 

 Inclusions of hornblendic and micaceous gneiss and schist are ex- 

 tremely numerous and are often arranged with their planes of 

 foliation at various angles to that of the imperfect foliation which 

 the "granitoid gneiss" displays. So far these rocks agree with the 

 granitoid gneisses which are so largely developed in the Hosur and 

 Krishnagiri taluks of the Salem District ; but their geological and 

 petrological characters have not been studied sufficiently to justify 

 more than a mere suggestion as to their correlation. The large tors 

 and piles of loose blocks of this rock are very conspicuous along the 

 Madras Railway at several points between Arkonam and Uili 

 stations, a distance of about 57 miles. 



The specimens of the charnockite series collected by Dr. Warth 

 in the South Arcot District belong to the " intermediate group," 

 and their microscopic characters have already been described. As 

 in other localities they form low, rounded hills isolated by stretches 

 of alluvium which frustrate all attempts to trace out any character 

 over more than very limited areas. 2 



Salem District. 



The most conspicuous exposure of this series of rocks in the 



Salem District forms the main mass of the 

 Shevaroy Hills. ... . . . . 



Shevaroy hills, which cover an area of about 

 100 square miles, and form a mass 16 miles long from north-east to 

 south-west and 10 miles wide from south-east to north-west. The 

 highest portions exceed 5,000 feet above sea-level. The southern 



1 See King and Foote, Mem. G. S. I., Vol. IV, p. 298, and Foote, Mem. G, S. /., Vol. 

 X, p. 129. Dr. Warth's report on this area has not been published, 



2 My attention was first called in 1892 to the existence of the charnockite series in South 

 Arcot District by Mr. F. G. Brock-Fox, F.G.S., Executive Engineer, P. W. D., who col- 

 lected specimens near Mailam, and from slides prepared by himself, recognised their resem- 

 blance to the rocks of Pallavaram. 



( 61 ) 



