As we have already seen, the disintegration and decay of rock results in the formation of soil, which is the residuum after the removal of more or less of the parent rock:. In humid climates" there is usually a distinct subsoil which is less thoroughly oxidized and hydrated and is lighter in colour and much less fertile than the top soil, which is largely due to the washing downward of the fine clay particles by percolating rainwater. In arid climates there is less kaolinization of the aluminous silicates, a much deeper top soil, and little or no distinct subsoil. Under the influence of wind, rain, and other agencies, the soil travels down the slopes and accumulates, often to great depths, in valleys and depressions, and is carried in enormous volume by the rivers. Very little soil, as such, is built into the rocks of the earth's crust, but sometimes it is buried under a lava stream or depressed beneath the sea or a lake in such a manner as not to be washed away, but immediately covered by other deposits.

Such an old soil, or "dirt bed," may be recognized by its texture and appearance and by the fossil roots and stems of land plants with which it is apt to be filled.

Laterite is a peculiar soil very widely spread in the tropics and of a deep red colour, caused by the presence of Fe203. It differs from ordinary soils in the fact that much of the alumina is present as an oxide, instead of the silicate, and is frequently characterized by lumps and nodules of Fe203, which are produced by a chemical process of concentration.