72 LA TOUCHE: GEOLOGY OF WESTERN RAJPUTANA. 



large dykes of olivine dolerite. One of these, which is quite 200 feet 

 wide, may be traced at intervals along the base of the hill to the south 

 of Jalor town. The dolerite is greatly decomposed, and weathers 

 more easily than the granite, so that the latter stands up like a wall 

 on either side of it. Several dykes also run through the granite of 

 the hill itself, their course being marked by deep chasms weathered 

 out. Another very large one occurs in the pass between Jalor fort 

 and Roza hill. The dolerite in this is in a better state of preservation, 

 and in the centre of the dyke is rather coarse grained. At the northern 

 end of the pass this dyke splits up into a number of smaller parallel 

 dykes, and in contact with the granite the dyke rock becomes very 

 fine grained and has a vertical platy structure parallel to the walls. 

 All the dykes run in a north-north-west, south south-east direction. 



The large hills to the south and south-east of Jalor were surveyed 

 and mapped by Mr. Hacket. They lie for the most part in the State 

 of Sirohi and are not included in the area which I surveyed. To the 

 north of Erinpura, however, there is a considerable mass of hills lying 

 in Jodhpur territory, and as the conclusions I came to regarding the 

 rocks exposed in them differ somewhat from those of Mr. Hacket, it 

 is perhaps worth while to give such information as I can about them. 



The relations of the rocks composing these hills are well seen in 

 the detached hill at Sanderao, a village about 13 miles north-east of 

 Erinpura, on the old road from Ahmedabad to Ajmere. The main 

 mass of the hill consists of quartzite with schistose bands, nearly 

 vertical and striking in a general north-east, south-west direction. On 

 the east side of the hill a large mass of gneissose granite is exposed, 

 which was evidently originally intrusive, since it sends off veins into 

 the quartzites. The intrusion must have taken place prior to the dis- 

 turbance and folding of the quartzites, for the granite has become 

 foliated, and ihe fragments of schist included in it, which are very 

 numerous in places near the boundary, have been rolled out into len- 

 ticular patches. The granite of some of the veins is also foliated, 

 especially where they run parallel to the bedding of the quartzites. 

 ( 7* ) 



