GRANITIC AND T1UPPEAN BOOKS. 57 



felspar bluisk-wliite quartz and light-brown silvery mica, largely crys- 

 tallised and charged with garnets. It often assumes the form of 

 graphic granite. The granite veins do not often contain garnets, or 

 indeed any foreign minerals, but the association with garnetiferous . 

 schists is remarkable. In one instance about a mile and half east of 

 Turamulla (on the southern tributary of the Kandleru) there is a string 

 of tourmaline and garnet crystals occurring in one of the larger granite 

 reefs, the garnets of the adjacent schists being large and perfect dodeca- 

 hedrons. Every now and then along the road from Ogili to Gudur 

 there are frequent outcrops of this granite, the road metal having been 

 obtained from decomposed masses on either side, and sometimes the 

 mica is so strong that the road and the adjacent fields shine out in the 

 sun light. At the junction of the Madras and Dugarazupatam roads, 

 the plates of mica are 3 or 4 inches in diameter. 



The commonest igneous rocks in the district are greenstones or 



diorites, which are, however, more particularly de- 

 Trappean rocks in de- 

 finite and ill-defined out- veloped in the Kandra and Gelacapad areas already 



specialised on account of their quartzite outcrops. 



These trap rocks may be very conveniently classified in the present 

 connection as definite and ill-defined outbursts ; the former being, of 

 course, the well-marked dykes and intrusive sheets, while the latter are 

 irregular masses of obscure origin, often presenting the characteristics of 

 both dykes and sheets, but of great extent as compared with the true and 

 obvious igneous intrusions. 



A few large dykes occur in the south-west corner of the field, which are 



the dyings-out of a tremendous development in the 

 Dykes in the gneiss. ... 



South Arcot country. They are striking either 



west-by-south to east-by-north, or about east-west, and others run north- 

 west or again nearly with the general strike of the foliation. The rock 

 is usually a heavy massive somewhat coarse-granular diorite of dark-green 

 or nearly black colours, and it is occasionally porphyritic with large 

 crystals of pale-green felspar. In the Swarnamukhi valley, to the east- 

 south-east of Kirkumbadi fort, there is a large west-by-south to east-by- 



( 165 ) 



