28 GEOLOGIC WORK OF ATMOSPHERE 



The disruption of rock by changes of temperature is one phase 

 of weathering. It tends to the formation of a mantle of rock 

 waste, which, were it not removed, would soon completely cover 

 the solid rock beneath and protect it from further disruption by 

 heating and cooling; but the loose material thus produced becomes 

 an easy prey to running water, so that the work of the atmosphere 

 prepares the way for that of other eroding agencies. 



A thermal blanket. The atmosphere is a thermal blanket to 

 the rest of the earth. Without it the heat of the sun would reach 

 the earth with far greater intensity than now, and it would be radi- 

 ated back from the surface almost as rapidly as received. During 

 the night the earth would be far colder than any part of the earth is 

 now. In passing through the atmosphere, parts of the radiant 

 energy of the sun are absorbed. Of the remainder which reaches 

 the surface of the earth, a part is radiated back into the air by which 

 it is absorbed and retained. The air thus distributes and equalizes 

 the temperature. The constituents of the atmosphere which are 

 most efficient in this work are water vapor and carbon dioxide, and 

 the climate of the earth is believed to have been greatly affected by 

 the varying amounts of these constituents, as well as by variation 

 in the total mass of the atmosphere. 



Evaporation and precipitation. Perhaps the most important 

 work of the atmosphere as a geologic agent lies in its relation to 

 the evaporation, circulation, and distribution of water. Atmos- 

 pheric temperature is the primary factor governing evaporation, an 

 important factor in the circulation of the vapor after it is formed, 

 and controls its condensation and precipitation. 



Mechanical effects of rain. In falling, the rain washes the 

 atmosphere, taking from it much of the dust which the winds 

 have lifted from the surface of the dry land. Not only this, but 

 in passing through the atmosphere, the water dissolves some of 

 its gases, so that when the rain reaches the land, the water is no 

 longer pure. The dissolved gases enable it to dissolve various 

 mineral matters on which pure water has little effect. 



As it falls on the surface of the land, the rain produces various 

 effects of a mechanical nature, (i) It leaves on the surface the solid 

 matter taken from the air. (2) Clayey soils, baked under the influ- 

 ence of the sun, are softened by the rain, and more easily eroded by 

 running water. (3) Under the influence of the expansion and con- 

 traction caused by wetting and drying, the soils and earths on 



