3* LA TOUCHE: GEOLOGY OF WESTERN RAJPUTAfU. 



but are separated from them by a deep ravine kept clear by the 

 drainage from the hill -sides, and probably to some extent also by 

 eddies of wind. When, however, the drainage is thrown off to either 

 side by a rocky spur, the sand runs up to the rock and forms a 

 gently sloping ramp on which one may climb the hill without having 

 to scramble over the loose angular talus with which the lower slopes 

 are covered. These sloping ramps of sand perform a certain amount 

 of work in distributing the rock debris falling from the hill sides, the 

 creep of the sand carrying the fragments outwards and downwards. 

 Thus they are sometimes transported to considerable distances from 

 the hill, much in the same way that a moraine is carried by a glacier. 



The sand of which the dunes are formed presents several features 

 of interest. That which is found in the dunefi out in the plain^ where 

 the sorting action of the- wind has had full play, is very uniform in 

 grain, the diameter of the individual grains averaging about one-fifth 

 of a millimetre, though much of the sand is still finer than this. But 

 on the lee side of outcrops of solid rock, especially of the sandstones 

 and granites, a greater variety of size is observed. For example, on 

 the north side of the broad plateau of sandstone north-west of Jodh- 

 pur, in the neighbourhood of the village of Teori, the ordinary fine 

 yellowish buff sand of the dunes is mingled with grains derived from 

 the sandstones, easily distinguishable from the former by their bright 

 red colour. Owing to their size and weight these grains have a 

 tendency to collect along the crests and in the furrows of the ripple 

 marks formed by the wind, and their strongly contrasting colour gives 

 a peculiar streaky appearance to the surface of the ground. 



The composition of the sand, judging from numerous samples 

 collected from widely separated localities, appears to be very uniform. 

 Quartz grains predominate, and flakes of hornblende and felspar, as 

 well as chips of the lavas are common. All of these might be derived 

 from the rocks locally present in the desert, and it is a significant fact 

 that flakes of mica are very rare and appear to be almost confined to 

 ( 3S ) 



