In most tropical regions, where there is a long dry season, followed by a wet season of violent rainfall, the manner of decay is characteristically different. The felspars and other aluminous silicates lose nearly all of their silica, so that the residue is the hydrated oxide of aluminium, not the silicate, and the iron is oxidized, forming nodules and masses and staining the whole a deep red. This characteristic warm-country soil is called laterite.

Very many igneous rocks weather into heaps and masses of rounded boulders, which are often mistaken for the deposits made by glaciers. The spheroidal shape is due to the more rapid decay of the edges of the original joint blocks, which are attacked on both sides at once. As the edges and angles are removed more quickly than the broad faces of the blocks, a rounded form results. Once acquired, the round shape is long retained, because then decay penetrates at a nearly equal rate from all parts of the surface.

Rocks which are themselves composed of substances derived from the decay of older rocks are attacked in their turn and yield material for new formations. These derivative rocks, such as sandstones, slates, and limestones, are affected in characteristic ways by the rain.

Sandstones are composed of grains of sand (quartz, Si02) cemented together; the cementing substance may be silica itself, some compound of iron, such as Fe203, or carbonate of lime (CaC03), and the dissolving away of the cement causes the rock to crumble into sand. In a sandstone with siliceous cement the action is excessively slow, atmospheric waters having very little effect upon silica, but underground it is slowly attacked. Ferric oxide (Fe203) is likewise unchanged by rain-water, but beneath the soil it is converted into the soluble carbonate and removed. The uppermost layers of red sandstone are often thus completely disintegrated into loose sand, bleached by the removal of the iron which gave it its colour. Carbonate of lime is very soluble in water containing carbon dioxide, as all rain-water does, and in sandstones with calcareous cement, disintegration is rapid. In sandstones and slates it is the cementing substance which is removed, leaving the grains of sand or particles of clay unchanged.

This is because the materials of these rocks were, for the most part, originally derived from the decomposition of the igneous rocks, and the minerals which compose them are already of a very simple and stable character.

The sandstones are largely employed for building materials, and their value and permanence for such purposes depend principally upon the character of the cementing substances in them. For this reason, the siliceous and ferruginous sandstones are the most Slates and shales, by removal of their soluble constituents, crumble down into clay.

Fig. 34. - Soil originating in place by the decomposition of sandstone durable, those with calcareous cements usually yielding with comparative rapidity to the attacks of the weather.

Limestones are among the few rocks which are chiefly or entirely made up of soluble material, the carbonate of lime (CaC03). This is attacked by the rain-water, dissolved and carried away in solution, while the insoluble impurities contained in the rock remain to form soil. The proportion of such impurities varies greatly in different limestones, and hence the residual soil will vary, but it is generally a clay, since that is much the commonest of the impurities in limestone. Sand also occurs in limestones, either with or without clay. When the sand forms a coherent mass, out of which the calcareous material has been dissolved, it is called rotten stone.

The gradual formation of soil by the disintegration of rock may be easily observed in excavations, even shallow ones, such as cellars, wells, railroad cuttings, and the like. At the surface is the true soil, which is usually dark-coloured, due partly to the admixture of vegetable mould, partly to the complete oxidation and hydration of its minerals. Next follows the subsoil, which, owing to the absence of vegetable matter and the less complete oxidation and hydration, is of a lighter colour. The subsoil is frequently divided into distinct layers, and often contains unaltered masses of the parent rock, which have resisted decomposition, while the surrounding parts have become entirely disintegrated. By imperceptible gradations the subsoil shades into what looks like unaltered rock, but is friable and crumbles in the fingers; this is rotten rock. From this to the firm, unchanged rock the passage is equally gradual.

In the northern portions of the United States the soil is, in most localities, of only moderate depths, because at a late period (geologically speaking) this region was covered with a great ice-sheet, which swept away much of the accumulations of ancient rock decay. In the parts of the country where the ice-sheet did not come, the soil is much deeper, and in tropical lands it attains remarkable depths. In our Southern States the felspathic rocks are often found thoroughly disintegrated to depths of 50 or 100 feet while in Brazil the soil is often 200 to 300 feet deep.

The mechanical effect of rain is less extensive, perhaps, than its chemical work of disintegration, but is very important, nevertheless. Under ordinary conditions, this mechanical work consists in the washing of soil from higher to lower levels. How considerable is the movement of soil that has thus been brought about, may be imagined when one sees, after a heavy rain, the rills which run over the slopes, muddy and charged with sediments, and how turbid the streams become with the soil which the rain washes into them. Bare soil is rapidly torn up and washed away by the action of rain, but a covering of vegetation, and especially of the elastic and matted stems and roots of grasses, much retards the action.