WEATHERING. Of an entirely opposite char acter to metamorphism is that change in rocks which results from contact with the air. In the processes of metamorphism the materials of rocks are rearranged, and in most cases bound more closely together ; in the processes of weath ering the materials are weakened and the rock caused to disintegrate and fall apart. Weather ing, like other geological processes, is a complex phenomenon resulting from a cooperation of vari ous agencies. Most of the agencies of weathering operate both chemically and mechanically.
Air aids in the weathering of rocks by sup plying oxygen, carbon dioxide, and other sub stances for chemical changes. Through the wind it performs mechanical work. Heat and cold, by causing contraction and expansion, aid in the breaking up of the rocks. Percolating waters cause many chemical changes, especially by the aid of oxygen from the air, carbon dioxide from air and decaying vegetation, and organic acids, derived from plant decay. Mechanically, water is important when the rain-drop strikes the ground; and when frost is formed in soil and rocks, the expansive force rends the materials apart with great effect. Plants are also important, both chemiCally and mechanically. Chemically they work by obtaining plant food from the earth, mechanically by the intrusion of their roots in soil and rock. Burrowing animals are likewise effective agents of weathering, especially the ants and earthworms, which bring fresh materials to the surface, and make the soil more porous.
The effectiveness of the agencies of weathering varies with the nature and situation of the rock. All rocks are entered by water; but some are far more porous than others. Some minerals are easily soluble, some insoluble; some decay with ease, others are quite indestructible. But even the densest rock, made of the most indestructible of minerals, will crumble, though slowly, in the weather. On steep slopes, as on mountain tops and cliffs, the bare rock is exposed to the weather by the aid of gravity which removes the frag ments as they fall; but on more level ground some of the weathered material remains as a blanket, protecting the rock from some of the agencies of weathering. Arid lands are unfavor able places for weathering, because of the general absence of water. A forested country is pro tected by the forest cover, and it is probable that this protective effect is of more importance than its destructive effect. In damp tropical regions
rock decay is of most importance; in cold cli mates frost is one of the most important agencies.
Of the effects of weathering, by far the most. important is the disintegration of the rock to. form soil. Whenever the slope is not too steep the disintegrated fragments accumulate as soil Such a soil of rock decay is called a residual soil, because it is a residuum of mineral decay after all the easily soluble portions have been removed. By far the greater part of the land has a soil cover of this origin. A second highly important effect of weathering is the preparation of rock for transportation, and were it not for weathering the geological history would have been far different. The rock waste falls or is washed into the streams which use it as tools for carving valleys, as the material for flood-plains and deltas, and as contributions to the deposits of sediment being made in the sea. See WEATHERING.
Wilco WORK. As an agent of geological change the importance of the wind is not fully recog nized by dwellers in humid regions. Aside from its influence in weathering mentioned above, the wind does effective work in two classes of re gions, namely, in arid lands and on seacoasts. In both places the protective covering of vegetation is absent; and in both places fine-grained rock fragments are dried and exposed to the wind. In these positions the sand is borne about by the wind and piled into irregular hills or dunes. The friction of the sand particles over one an other grinds them down; and, when blown against rocks and cliffs, a natural sand-blast is in operation, with the result that the rocks are worn away. An additional effect of wind action is the construction of land in the sea. Where sand-bars are thrown up by the waves, or where coral beaches are built on coral reefs, the wind completes the construction of land by building dunes of the fragments washed ashore. The blowing of sand and dust out to sea adds to the sediments gathering there. The distribution of volcanic ash over wide areas is another im portant geological effect of the wind. Indirectly the wind is exceedingly potent as the transporter of vapor for rain, and as the force which causes the waves and currents in the ocean. See WIND; DUNE; LOESS.