CHAPTER II.
THE ATMOSPHERE AS A GEOLOGICAL AGENT.
While it is convenient to regard the lithosphere as the earth proper, and the atmosphere as its envelope, the latter is as truly a part of the planet as the former, and its activities and its history are as truly subjects of geological study as the formation of the rocks. This view is in no way vitiated by the fact that the special study of the atmosphere is set apart under the name Meteorology, for in the same way the special study of rocks is set apart under the name Petrology, that of ancient life under the name Paleontology, and that of other phases of the subject under other names. The atmosphere is one of the three great formations of the earth, and as a geological factor takes its place beside the hydrosphere and the lithosphere. It has played a part in the history of the earth comparable to that of the water, though its mass is less and its record more elusive. Unsubstantial as the atmosphere seems when contrasted with the liquid and solid portions of the earth, its extreme mobility and its chemical activity compensate for its lightness and tenuity, and give it a function of the first order of importance.
The atmosphere plays a direct part as (1) a mechanical and (2) a chemical agent, and at the same time serves an indirect function in furnishing favorable conditions under which (3) solar radiation produces temperature effects, and (4) evaporation gives origin to precipitation and stream effects, and furnishes the necessary conditions for land plants and animals, and the important influences that spring from them.
This chapter is devoted to the work of the atmosphere in these and some less notable phases. The consideration of the origin and history of the atmosphere will receive attention later.