=Weathering.=—The result of all atmospheric processes, whether physical or chemical, by which surface rock is disrupted, decomposed or in any way loosened, is _weathering_. This convenient term also includes similar results effected by ground water, plants, etc. The tendency of weathering is to produce a mantle of residuary earth over solid rock. Weathering by mechanical means tends to produce material which, though in a finer state of division, is still like the original rock in chemical composition. Weathering by chemical means tends to produce a mantle made up chiefly of the less soluble parts of the rock from which it was derived. All processes of weathering prepare material for transportation by wind and water. [Illustration: +Fig 35.+—Details of a weathered rock surface, due partly to wind work and partly to solution. The particular phase of weathering illustrated by this figure is known as “honeycomb” weathering. (Gilbert, U. S. Geol. Surv.)] Many considerations determine the thickness which the mantle of weathered rock (mantle rock) attains. Some of these considerations have to do with the atmosphere, and some with drainage. Since the latter are, on the whole, more important, this matter will be discussed in connection with the work of water (Chapters III and IV). CHAPTER III. THE WORK OF RUNNING WATER. Familiar phenomena, both of land and sea, reveal the constant activity and importance of water as a geologic agent. Even when there is no precipitation the moisture in the air influences its activity in certain ways. Just as iron “rusts” more readily in moist air than in dry, so changes in other mineral substances are influenced by atmospheric humidity. Where precipitation takes place the results are more obvious. The passing shower works changes in the surface of the land, striking in proportion to the rate and amount of precipitation. The rains feed the streams, and every stream is modifying its bed, and with increasing rapidity as its current is swollen. Even the moisture which is precipitated as snow works its appropriate results. Before it melts it protects the surface against other agents of change; but if it accumulates in sufficient quantity in appropriate situations, it may give rise to avalanches and glaciers, which, like running water, degrade the surface over which they pass.