DETAILED DESCRIPTIONS OF ROCKS. 53 



overlying flow of rhyolite, as they occur in patches like heaps or 

 mounds surrounded by the lava. They may possibly be the remnants 

 of an ancient neck filled with agglomerate. The rhyolite immediately 

 surrounding them has an altered appearance, yellow and earthy look- 

 ing, perhaps due to the passage of heated vapours through the body 

 of the rock. 



The rocks comprising the range are traversed by numerous dykes 

 of basic rock, of a dark green colour, consisting principally of a plagio- 

 clase felspar with a good deal of augite and olivine. These dykes 

 run along the joint planes of the rhyolites, the most usual direction 

 being due north and south, another system cutting these at right 

 angles. Generally their course is well marked by a depression in the 

 surface, since the material of which they are composed weathers more 

 rapidly than the rhyolites. A large chasm formed by the weathering 

 out of one of these dykes is seen in the hillside immediately to the 

 south of Nagar village (PI. V, fig. 1) ; this dyke is 18 feet wide. About 

 a mile and a half to the east of Nagar is a narrow pass through the 

 range, the bottom of which is occupied by a broad dyke, 155 feet 

 wide, to the weathering out of which the formation of the pass is due. 

 Another pass close to the east end of the range has been formed in 

 a similar manner. Again, near the western end of the range, on the 

 northern slope beneath the survey mark, are two broad dykes crossing 

 each other at right angles, and bounded by lofty vertical walls of 

 rhyolite. The north-south dyke is 45 feet and the other 30 feet wide. 

 Some of the dyke£ split up into a number of narrow parallel branches, 

 running in among the rhyolites ; one of these, on the hillside a 

 little to the east of Nagar, is altogether about go feet wide, and 

 includes about a dozen alternations of dolerite and rhyolite. The latter 

 has lost it= colour, but does not appear to be otherwise altered. 



About eight miles to the south-east of Jodhpur, near the village of 

 Rassida, and again at Salawas, 12 miles south of the city, a coarse 

 granite is exposed forming bosses rising abruptly from the surrounding 



( 53 ) 



