Chap. III.] trichinopoly district — crystalline rocks. 37 



The foliation of the gneiss in the Northern part of the Trichinopoly 



district presents many contortions and irregularities, 

 Foliation of the Gneiss. . __ 



out its general tendency is to an E. N. E, and 



W. S. W. strike, the dip being sometimes to the North — sometimes to the 



South, of this prevalent axis. 



Opposite Yolcondah, which is situated on the water-shed of the 



Cauvery and Vellaur, the hill country extends to 

 Hills to the Westward. 



the North-west, gradually approaching the more 



Northerly groups of the muUays as far as Ahtoor. From this point 



both groups trend off to the Westward, enclosing between them a 



narrow pass through which flows the Vellaur, and which extends for 30 



miles between the ahove and the adjoining groups of hills, opening out 



at Salem into the broad plain, 80 miles across, which is drained by the 



Cauvery and its tributaries, and extends up to the foot of the Nilghiris 



and the Western Ghats. 



In the neighbourhood of Yolcondah and throughout the country to 



rp. , , ^ y^j_ the North, greenstone dykes occur at distant 



condah. . intervals, some of them of considerable size, and 



running sometimes in the direction of the foliation, but more frequently 



at right angles to it. 



Between Purawoy and Vapoor two of these dykes extend up to 



the boundary of the Cretaceous rocks, the Ootatoor, or lowest group 



of which, here rests on the gneiss. As in the case of the green- 



_ . <v. ^ /-< i. stone dykes near Ootatoor, the Cretaceous rocks 



Do not anect Lretace- "^ 



ous rocks. g^j-Q q^ite unaffected by these basaltic* intrusions, 



and although I have never found pebbles of the latter in the congio- 



,^ ^ ,,, . merates of the Cretaceous rocks, their absence 



No Trap pebbles m 



Cretaceous roclss. jjj^y i^q satisfactorily explained by the rarity of 



any local conglomerates in the beds immediately resting on the 

 gneiss, the insignificance of the total mass of the trap as compared 



* The rock is apparently Anamesite. 



