CH. iv] Effect on Mineral Matter 49 



ceeds in widely different fashion in places where the 

 climatic conditions are very different and cases have 

 been observed where the differences in soil of two 

 regions are greater than could be expected from the 

 rocks alone; these differences are therefore attributed 

 to climate. For example, in this country the rocks 

 break down to yield enormous quantities of silica, the 

 cliief constituent of sand, and of various complex sili- 

 cates, containing combinations of iron and aluminium, 

 which occur largely in clay; iron and aluminium com- 

 pounds, however, form only relatively small propor- 

 tions of the soil. But in parts of the tropics, where the 

 disintegration processes have gone on under wholly 

 different conditions, the rocks have broken down to 

 yield soils containing only small amounts of silica and 

 relatively large quantities of aluminium and iron oxides. 

 These soils differ entirely from ours and have received 

 a special name, Laterite soils. In subtropical regions 

 another type of disintegration has gone on, giving rise 

 to considerable areas of a distinct type of red soil, in 

 which again there is only relatively little silica. The 

 study of these changes is very incomplete, and it is not 

 supposed that the original rocks were identical in all 

 cases. But it is very significant that under these three 

 sets of climatic conditions three distinct varieties of soil 

 have arisen, all differing in character and requiring 

 different treatment. 



There is a second direction in which climate regulates 

 the composition of the soil. As we have already seen, 

 the particles formed from the rocks do not remain where 

 they are but get carried away by various climatic 

 agencies such as running water, ice, or wind. Usuallj^ 

 there has been some selection and the particles became 



R s. 4 



