DESCRIPTION OF THE PRINCIPAL EXPOSURES. I 77 



Many of the features presented by the rocks at Pallavaram and 

 , . . St. Thomas' Mount were noticed by Dr. P. M. 



Previous descriptions. J 



Benza as long ago as 1836, who recorded an 

 abundance of accurate information in his H Notes on the Geology of 

 the Country between Madras and the Neilgherry Hills, via Bangalore 

 and via Salem". 1 The garnetiferous rock, the trap-dykes, the de- 

 composed, kaolinized form of the rock herein referred to as 

 charnockite-pegmatite were noticed by Benza. The rocks forming 

 the hills and here included in the charnockite series he speaks of as 

 hornblende rock, and considered them to be overlying the funda- 

 mental granite. 2 



The rocks of St. Thomas' Mount and the Pallavaram Hills are 

 referred to by Mr. R. B. Foote as " hornblendic gneiss of a very 

 compact character," whilst the more acid forms of the charnockite 

 series in this neighbourhood are spoken of as n quattzo-felspathic 

 gneiss ", 8 



The Seven Pagodas. 



Along the East Coast south of Madras, the crystalline rocks 



rise up like islands in the midst of the cultivated alluvium, the great 



prevalence of which reduces the value of the 



The Seven Pagodas. . , . 



necessarily isolated geological observations. 



The only exposure of crystalline rock in the Chingelput District south 

 of Pallavaram, which I have been able to examine carefully, occurs 

 near the village of Mahavalipuram, or the Seven Pagodas (lat. 

 12 36' 55 "; long. 8o° 13' 55") which is 35 miles south of Madras and 

 on the coast. 



The Seven Pagodas are well known on account of the in- 

 teresting rock-cut caves and temples, for the production of which 

 nearly every exposure of rock for about two miles along the 

 coast has been elaborately carved. The rock is referred to in 

 Mr. Foote's memoir* as a " quartzo-felspathic gneiss" in which the 



» Madras Journ. Lit. Sci., Vol. IV (1836), pp. I— 27. 



3 For remarks by Capt. J. Allardyce, see section 12. 

 » Mem. Geol. Surv. Ind., Vol. X, p. 127 (1871). 



4 Op. cit.y p. 127. 



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