there, in the extended surfaces of the cracks, the rock substances are exposed to the action, and the compounds forming the rocks become oxidized. The simplest case of this is the oxidation of the blue colouring matter of rocks, which is a sulphide of iron; on the surface of the earth these rocks are usually coloured red or yellow, the blue sulphide having been converted to the red or yellow oxide of iron.
In a large number of cases oxidation of compounds in the rocks renders them soluble, so that when the rain descends on the weathered rock it washes away a certain proportion of the substance in solution, and the rest, becoming loosened by the removal of the cementing material, crumbles away and is washed down the hillside as sand.
Nitrogen is an extremely inert gas, but under the influence of the bacteria a large amount is fixed as a compound of oxygen. Organic life could not go on without nitrogen compounds, but the geological action of nitrogen is insignificant. Saltpetre, formed by decomposing animal matter, is really an organic product, and the only original mineral nitrogen compound of any importance is the ammonium chloride which escapes explosively from liquid lavas and forms the dense white cloud that accompanies volcanic eruptions.
Carbon dioxide is of extreme importance. It is expelled in enormous volumes in all volcanic explosions, but it is absorbed rapidly by the plants, which are able to use it for building out of it a more complex carbon compound called cellulose, which is the basis of wood and indeed of all vegetable tissue, and hence of all the coal, anthracite, and graphite of the rocks. The waste products of cell activity in plants are got rid of by oxi-
