Chap. V.] crystalline rocks. 107 



1st. — Those aggregated in the valley of the Vellaui- Proper, or the 

 Principal series of Volcondapuram region, and— 



^ ^^" 3nd. — Those in or about the valley of the 



Pereyanr, more properly belonging to a region of trappean intrusion lying 

 among the Pythoor hills, south of Ahtoor, in the Salem district. 



In the first series the general direction is north by west — south by 



Dykes of Vellaiu- ^^^^' ^^^^^ "^ ^^'^^ °^' ^^^ ^^^^^ ^ north-east by east 

 ^'^ ^^* direction has been followed. Here there are at 



least seven well-marked and long dykes of basaltic trap, all of which are 

 exposed for a length of 4 or 5 miles, bat the fui'ther extension of 

 which it is impossible to make out with certainty, on account of their 

 being so much covered up in different places by the thick cotton soil of 

 that neighbourhood. These may be specified as follows : — 



1st. — The Thavyur dyke, which crosses the Madras road about 

 2 miles north-east of Volcondapuram in a north-west — south-east 

 direction. 



2nd. — The Pulioor dyke, about half a mile east of the former, rims for 

 about 4 miles north by west — south by east. 



3rd. — Further east, about 8 miles from Pulioor, are two fine 

 dykes near the village of Nunnay. 



4th.— -Where the Madras road crosses the Vellaur by a ford at Too- 

 loodoor Tuckurij there is a regular assemblage of small basaltic trap-dykes. 

 5th. — A large dyke, about 5 miles long, crosses the Vellaur at 

 Agrarum (Korawadi of map), about 4 miles west of Tooloodoor. A re- 

 ference to the map will, however, show the true position and extent of 

 these better than any lengthened details, and it is only necessary to treat 

 separately of those which are remarkable for some peculiarity of mineral 

 composition, mode of occurrence, or great size. 



Westward of the dykes just described, are several groups of others 

 worthy of notice because of their size or the complicated relations of the 

 several members of the groups. 



(329) 



