In the tropics, which so largely have a regular alternation of rainy and dry seasons, and in arid regions, where the rain often falls in torrential showers, followed by long periods of drought, the movement of water through the soil is frequently reversed in direction. During the rains the movement is downward; in the dry period evaporation from the surface and capillarity cause a slow ascent of the water through the soil. Often this ascending water is charged with material in solution and this material is deposited on or near the surface as the water evaporates. In deserts and semi-deserts the surface is often white with salt, the sulphate or carbonate of soda, borax, and other soluble compounds. The iron nodules of laterite are produced in this manner, and sometimes these nodules are cemented into continuous sheets of crude haematite. Where the soil and underlying rocks contain the carbonate of lime abundantly, the water concentrates them at the surface, it may be, as in. South Africa, in very extensive sheets of hard limestone.

These terrestrial chemical deposits may cover very wide areas, but never in any great thickness.

Mechanical Deposits are made on land surfaces by various agencies and form quantitatively much the most important series of the' class.

Talus And Breccia

As has been pointed out (pp. 114 and 118), great masses of angular blocks of all sizes accumulate at the foot of cliffs and on mountain slopes as talus, which shows an imperfect division into layers and is slowly but continually creeping downward. By the deposition of some cementing material (usually CaC03) in the interstices of the talus the blocks may be bound into a solid mass, called breccia, of which the peculiarity is that the fragments composing it are angular, not rounded.