THE ATMOSPHERE AS A GEOLOGICAL AGENT. 51 



tion of water. Atmospheric 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. 



The average amount of annual precipitation on the land is variously 

 estimated at from forty to sixty inches, the lesser figure being probably 

 more nearly correct. Since much of this water falls at high altitudes, 

 the work which it accomplishes in getting back to the sea is great. 

 The water which falls on the land, if withdrawn wholly from the ocean, 

 would exhaust that body of water in 10,000 to 15,000 years if none 

 of it returned. The work of evaporation is of course not done by the 

 atmosphere, though the atmosphere determines the effect of the solar 

 energy which does the work.^ 



The precipitation is distributed with great inequality, and this 

 inequality affects both the rain and the snow. Some regions have 

 heavy precipitation and some light; some regions have much rain and 

 little snow; others have much snow and little rain; others have rain 

 and no snow, and still others have snow and little or no rain. The 

 amount and distribution of rain and snow determine the size and dis- 

 tribution of streams and glaciers, and streams and glaciers are the most 

 important agencies modifying the surface of the land. 



It is impossible to separate sharply the geologic work of the water 

 of the atmosphere from that of other waters; but so long as moisture ia 

 in the atmosphere (including the time of its precipitation) its effects 

 are best considered in connection with the atmosphere. 



The mechanical work of the rain. — In falling the rain washes the 

 atmosphere, taking from it much of the dust, spores, etc., 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, and perhaps particles of soluble solid matter. When there- 

 fore the falling water reaches the surface of the land it is no longer 

 pure, and some of the gases it has' taken up in its descent enable it 

 to dissolve various mineral matters on which pure Avater has little 

 effect. 



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

 effects of a mechanical nature. In the first place, it leaves on the 



^ On the assumption that condensation takes place at an average elevation of 

 3000 feet, it has been estimated that the force necessary to evaporate and diffuse 

 the moisture which falls as rain and snow would be equivalent to 300,000,000,000 

 horse-power constantly in operation. (Strachey, Lectures on Geography, p. 145.) 



