io THE ORIGIN OF SOILS [chap. 



and chemical, the complex effects of solution and oxida- 

 tion that are brought about by water, especially when 

 charged with carbonic acid. 



In dry climates the alternations of temperature 

 between day and night set up sufficient strain to fracture 

 even large rocks, and eventually reduce them to dust. 

 The dust and sand of the deserts of Central Asia, the 

 barren lands of the United States, and of 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 



