DETAILED DESCRIPTIONS OF ROCKS. 57 



denuded and covered up with sand. To the west the granite ridge 

 slopes very gradually down to the plain, the end of it being rounded 

 off on either side like the crown of a dome. 



3. Quarter Sheet 20 S. E. 



The ridge just described forms the northern edge of a very large 

 group of hills, about the centre of which is situated the town of Siwana 

 at the foot of a steeply scarped hill on which is built an ancient fort. 

 From about a mile south of the westerly termination of the granite 

 ridge, two others run more or less continuously for about 10 miles in a 

 west-south-west direction to near the village of Thapan. Both of these 

 are steeply scarped to the north, but present sloping surfaces, corres- 

 ponding to the inclination of the rhyolite flows composing them, to the 

 south. 



At the base of the more northerly of the two ridges, near the east 

 end, a dark grey vesicular rock, very trap-like in appearance, is 

 exposed, and seems to be intrusive along the strike of the rhyolites. 

 The relations of this rock with the lavas are, however, somewhat 

 obscure. In one place, about a mile from the end of the ridge, it is 

 certainly intrusive, being thrust up vertically through the rhyolites 

 which stand up on either side of it like a wall, and a little further to 

 the west it runs in among them in narrow bands and appears to alter 

 them. Further west again the same rock seems to be interbedded 

 with the rhyolites, and is associated with dark grey ashy looking beds. 



Towards the east end of the ridge the rhyolites forming the 

 upper part of it are a dark brown variety with large crystals of red 

 felspar often in strings drawn out in the direction of flow, dipping to 

 south-south-west at about 20°. Further west a thick band of rhyolite 

 with well developed flow-structure, frequently vesicular in its upper part, 

 crops out in places beneath the porphyritic variety. In one place a 

 narrow dyke of rhyolite in which the lines of flow are vertical runs 

 fromcast to west through these rocks. The upper lava flows clip to 



( 57 ) 



