238 DISINTEGKATION Chap. V. 



countries, worms aid in the work of denuda- 

 tion in several ways. The vegetable mould 

 which covers, as with a mantle, the surface 

 of the land, has all passed many times 

 through their bodies. Mould differs in ap- 

 pearance from the subsoil only in its dark 

 colour, and in the absence of fragments or 

 particles of stone (when such are present in 

 the subsoil), larger than those which can pass 

 through the alimentary canal of a worm. 

 This sifting of the soil is aided, as has already 

 been remarked, by burrowing animals of 

 many kinds, especially by ants. In countries 

 where the summer is long and dry, the 

 mould in protected places must be largely 

 increased by dust blown from other and more 

 exposed places. For instance, the quantity 

 of dust sometimes blown over the plains of 

 La Plata, where there are no solid rocks, is 

 so great, that during the " gran seco," 1827 

 to 1830, the appearance of the land, which 

 is here unenclosed, was so completely changed 

 that the inhabitants could not recognise the 

 limits of their own estates, and endless law- 

 suits arose. Immense quantities of dust are 

 likewise blown about in Egypt and in the 



