58 LA TOUCHE: GEOLOGY OF WESTERN RAJPUTANA. 



the south or south south west and form a bare dip slope on the 

 southern side of the ridge inclined at an angle of about 2o°. Nearer the 

 western end the dip curves round to south-south-east at about the same 

 angle, and the rhyolites both above and below the trappoid band are 

 of the ordinary grey or reddish brown porphyritic variety. 



A dyke or boss of dolerite rises through the rhyolites on a low pass 

 to the east of the road through the ridge from Siwana to Pachpadra. 

 It is 70 feet wide and runs from north-west to south-east, but is 

 only exposed for a short distance on either side of the pass. The 

 material is very much decomposed ; at the edges it has caught up large 

 fragments of the rhyolite, which weather out in rounded ellipsoidal 

 masses, each surrounded by concentric shells of dolerite. 



The southern ridge, when looked at from a distance, appears at first 

 sight to be a repetition of that to the north, but on closer inspection 

 the rocks composing it are found to be quite distinct. The detached 

 hills at the east end are all porphyritic rhyolite of the ordinary type, 

 with some vesicular bands dipping to the south-south-west at 30 to 35 

 degrees. The upper portion of the main ridge is of similar rhyolite, but 

 at the base on the northern side, which is steeply 'scarped, some very 

 different beds are exposed. These are sandy and gravelly, containing 

 rolled pebbles of rhyolite, and form a band from 20 to 30 feet thick 

 with sheets of rhyolite above and below. Further to the west, at a 

 pass due north of the village of Kuip, and on the west side of the peak 

 marked 1,319 feet, these beds are better seen, and are altogether 

 about 100 feet thick, consisting of pebble beds interstratified with 

 breccias and felsitic tuffs. In the uppermost pebble bed rolled frag- 

 ments of other crystalline rocks, gneiss and quartzite, as well as the 

 rhyolites, are found. One of the rhyolite pebbles found was a variety 

 containing large red crystals of felspar similar to that in the northern 

 ridge. At the pass these beds are broken through by a dyke of 

 strongly porphyritic rhyolite, 60 feet wide, running directly across the 

 ridge ; near the contact with this dyke the pebble beds are highly in- 

 durated. The pebble beds and tuffs extend along the base of the 

 ( 58 ) 



