=Geologic activity.=—The atmosphere is the most mobile and active of the three great subdivisions of the earth, and when its indirect effects through the agency of water, as well as its direct effects, are considered, it is to be regarded as one of the most effective agencies of change. It acts chemically upon the rock substance of the earth, causing induration in some instances, but more often inducing disintegration and change of composition by means of which rock is reduced to soil, or soil-like material, and rendered susceptible of easy removal by winds and waters. When in motion the atmosphere acts mechanically on the surface of the earth, transporting dust and sand, and by the friction of these it abrades the surface. It is chiefly effective, however, in furnishing the conditions for water action. Partly by its mechanical aid, but chiefly by securing the right temperature, it is a necessary factor in the action of rains, streams, glaciers, and the various forms of moving water upon land. So also, on the ocean, wave action is essentially dependent on the winds. In the absence of atmospheric propulsion, wave action would be chiefly confined to the tides and to occasional earthquake impulses, and would lose nearly all its efficiency. Stream action and wave action, which are the most declared of the geological agencies, are therefore to be credited as much to the atmosphere as to the hydrosphere, since the action is a joint one to which both envelopes are essential. =A thermal blanket.=—A function of the atmosphere of supreme importance is the thermal blanketing of the earth. In its absence the heat of the sun would reach the surface with full intensity, and would be radiated back from the surface almost as rapidly as received, and only a transient heating would result. During the night an intensity of cold would intervene scarcely less severe than the temperature of space. In penetrating the atmosphere certain portions of the radiant energy of the sun are absorbed. Of the remainder which reaches the surface of the earth, a part is transformed into vibrations of lower intensity, which are then more effectively retained by the atmosphere. The air thus distributes and equalizes the temperature. The two constituents of the atmosphere which are most efficient in this work are aqueous vapor and carbon dioxide, and the climate of the earth is believed to have been very greatly affected by the varying amounts of these constituents in the atmosphere, as well as by the total mass of the atmosphere. The function of the atmosphere in sustaining life and promoting all that depends on life is too obvious to need comment. The special geological action of the atmosphere will be discussed in the next chapter. _II. The Hydrosphere._