Page:South African Geology - Schwarz - 1912.djvu/28

 we see the rocks crumbled and worn by weathering, and this is due to the action of the atmosphere.

The atmosphere consists of

The amount of water vapour is extremely variable, being very slight in desert regions and very high in the moist tropical regions. Besides these there are the minor constituents, the gases argon, krypton, xenon, neon, helium; solids, ammonium chloride from volcanic eruptions, sodium chloride from the salt spray of the oceans, nitrates due to the combination of nitrogen and oxygen by the electric spark (lightning), and various floating particles, volcanic dust, dust from the burning of falling stars, dust swept up by the wind from deserts, and even microscopic plants and the spores of plants.

The oxygen is the prime agent in the atmosphere; it has a chemical affinity for most things and will unite with them, forming oxides. The simplest case is that of iron, which rusts or oxidizes on exposure to the atmosphere; the most important case, however, is its union with carbon. This may be brought about directly with the aid of heat, as when coal burns, but it also takes place if the carbon or a compound of carbon is exposed in minute particles to the action of the oxygen, as in the case of the blood in animals, and the tissues of plants, in both of which the waste carbon products are burnt off at a comparatively low temperature. The surface of the earth breathes much as an animal does; down all the little cracks and crevices on the surface there is carried water containing a certain proportion of oxygen, and