ro THE ORIGIN OF SOILS [chap, 



barren lands of the United States, and many parts of 

 both North and South Africa, are formed in this way ; 

 because of the dryness of the atmosphere, radiation is 

 extreme, and the temperature of the rock surface will 

 rise to 6o° C. in the day and fall below zero at night. 

 Crystalline rocks soon disintegrate under such alterna- 

 tions of temperature, and the fine angular dust thus 

 formed is transported by wind into the plains and valleys, 

 giving rise to soils largely wind-borne. Richthoven has 

 supposed that the immense loess deposits of China are in 

 the main dust that has been blown from the Central 

 Asian deserts. Even in a humid country like our own the 

 wind plays a considerable part in forming soil, material 

 being constantly removed from any bare surface and 

 deposited elsewhere as dust When all the country 

 was in its natural state and clothed with vegetation, 

 the amount of transport as dust must have been con- 

 siderably smaller than at present, but even then worm 

 casts brought up in the spring would crumble in dry 

 weather, and be moved to lower levels by the wind. 

 The thickness of the dust deposit may be gauged 

 by the rapidity with which shingle beds newly won 

 from the sea become covered with vegetation ; in 

 the neighbourhood of Dungeness shingle beds known 

 to be less than fifty years old are already clothed 

 with a scanty flora. On scraping away a few inches 

 of the shingle the interstices between the stones are 

 found to be filled with a fine black sand, which 

 can only have been wind-borne ; this rapidly increases 

 as the first vegetation checks the velocity of the 

 wind above the stones and arrests the dust, till at last 

 it reaches the surface and the grass begins to spread 

 over the stones. Exact dates are difficult to obtain, 

 but probably considerably less than a century is suffi- 

 cient to form a thin turf over a bare shingle bed. 



