34 GEOLOGY OF THE SON VALLEY, ETC. 



cither side to the foot of the bounding hills, or with a slight slope 

 downwards from the centre, they form a shallow concave trough- 

 shaped surface, rising on either side towards the hills ; or in the 

 larger plains, such as those of the main valley of the Son, they have 

 a gently undulating surface, in whose hollows rainwater accumulates. 

 The difficulties in the way of an aqueous, or alluvial, origin 

 . of the deposits disappear if we regard them 



as dust deposits formed from the air. The com- 

 position is that of the fine particles into which the surrounding rocks 

 disintegrate, the fine grains which would only settle with extreme 

 slowness in water would settle rapidly in air, and the absence of 

 lamination would be accounted for by the action of vegetation, which 

 would also serve to catch and retain the dust settling on the ground, 

 and protect it from being removed by subsequent gusts of wind. 

 The absence of lamination may also be partly due to the gusty 

 nature of the winds and especially to the numerous small whirlwinds 

 which traverse the country in every direction during the hot weather. 

 These pick up the loose soil and dust on the surface of the ground, 

 and after whirling it high into the air, allow it to settle tumultously 

 and without any sorting into widespread layers. They are also doubt- 

 less the means by which the occasional coarser grains are carried 

 out from the hills into the plains, and these deposited among the 

 vastly preponderating bulk of fine-grained dust. 



The deposit is, besides, by no means confined to the valleys, but 

 is found on the sides and tops of the hills, 

 wherever a sheltered hollow exists, in which 

 dust and sand, swept from the surrounding hills, can settle. The 

 strong winds of the hot weather, sweeping over a parched and 

 friable soil, strip the rocky elevations bare of soil, or of all but a 

 thin layer of soil retained by a scanty growth of grass, and the roots 

 of trees speading over the surface of the rock. But where there is 

 a sheltered nook, or on the broad and open plains where there is no 

 prominence to oppose and so increase the force of the wind, the 

 dust blown from the surrounding hills settles and accumulates. 



In accordance with this mode of origin is the variation in the 

 ( 34 ) 



