60 LA TOUCHE : GEOLOGY OF WESTERN RAJPUTANA. 



small quantity of porphyritic rock with red felspar crystals exposed at 

 the base. 



The lower part of the scarped hill on which the fort at Siwana is 

 built is composed of tuffs of various colours, pink, buff, purple and 

 green, with thin bands of rhyolite interbedded among them. At the 

 base they are conglomeratic, containing rolled pebbles of rhyolite, and 

 rounded blocks a foot or more in diameter. These beds are about 

 40 feet thick on the southern side of the hill, resting on red rhyolites 

 without porphyritic crystals, and are overlaid by a thick flow of dark 

 brown rhyolite, with a roughly columnar structure, on which the fort 

 is built. The tuff beds extend all round the hill at the foot of 

 the scarp below the fort, and on the north side are from 80 to 

 100 feet thick, dipping in slightly towards the hill on all sides. On 

 the west side they are either banked up or faulted against the rhyolites 

 of which the western portion of the hill is composed. 1 he tuffs have 

 every appearance of having been laid down under water. 



Beneath the high peak at the western end of the hill a bed of 

 conglomerate, consisting entirely of well rounded pebbles of the rhyo- 

 lites, is intercalated between the flows. This band is from 20 to 30 feet 

 thick. It does not appear to correspond with the conglomerate band 

 below the fort, for the tuff beds are wanting. Some sandy looking ash 

 beds are exposed immediately above the conglomerate band on the 

 southern face of the peak, and are hollowed out into a deep gallery 

 running horizontally along the hillside beneath the scarp. The con- 

 o-lomerate band dies out before reaching the western end of the hill, and 

 is replaced by ash beds and thin bedded rhyolites. 1 he rhyolites above • 

 and below are of the red or reddish brown non-porphyritic variety. 

 They dip, with the conglomerates, at 20° to the north. 



The structure of the large hill, 1,790 feet high, to the north-east of 



Siwana, is very similar to that of the hill already described to the 



south-west of Kuip. The beds are inclined to the south, dipping on 



the north side at 27 or 30 degrees, and becoming more horizontal 



( 60 ) 



