DETAILED DESCRIPTIONS OF ROCKS. 59 



ridge to the west of the pass through which the road to Pachpadra 

 runs, but in that direction they are^more or less concealed by sand- 

 hills. The rock below them -crops out about a mile beyond the pass 

 and is a red porphyritic rhyolite. Above the pebble beds come thick 

 sheets of a very dark grey variety of rhyolite only slightly porphyritic. 

 The pebble beds and the sheets of rhyolite above and below are quite 

 conformable to each other, dipping to south-south-east at about 25°. 

 The southern side of the ridge is a steep dip slope formed of bare 

 sheets of rhyolite. 



In the hill rising to 1,634 leet > about three miles west-south-west 

 of the village of Kuip, some greenish ash beds, about 30 feet thick, are 

 exposed near the base of the spur on the north-east side, dipping to 

 the south at about 20 . These are pierced by a dyke or rather boss 

 of red rhyolite forming a low dome surrounded on all sides by the ash 

 beds. The latter are succeeded above by thick flows of vesicular 

 rhyolite, and these by successive sheets of the ordinary variety of 

 porphyritic rhyolite, forming the mass of the hill, and steeply scarped 

 on all sides, rising above each other like the steps of a staircase 

 (PI. Ill, fig. 1). Towards the south side of the hill these flows become 

 nearly horizontal. 



The large scarped hill rising to 1,601 feet to the south of that last 

 described, and about two miles north of Garah, is also composed 

 entirely of rhjolite. The main mass of the hill at its western end 

 consists at the base of brick red rhyolite full of porphyritic crystals of 

 red felspar, extending for about 300 feet up the scarp. This is 

 succeeded by thick flows of red rhyolite without porphyritic crystals. 

 The junction between the two is quite abrupt, and they are evidently 

 distinct flows. Towards the east the rocks are inclined to east-south- 

 east and the non-porphyritic rhyolite is brought down to the level 

 of the plains. Here a well developed flow-structure is seen in some 

 portions of the rock. 



The small detached group of hills between this and the hill at 

 Siwana also consists of the brick red non-porphyritic rhyolite, with a 



( 59 ) 



