54 EVOLUTION OF THE EARTH 



a greater abundance of both, and especially of aqueous vapor, 

 there results a thickened atmospheric blanket that not only 

 holds in more of the earth's warmth but takes up greater 

 amounts of the sun's radiations, and when there is less of this 

 gas and of water-vapor the climates are thinner and cooler. 

 The amount of carbon dioxide and water-vapor in the atmos- 

 phere has varied much throughout the geologic ages. 



Huxley in his interesting book Physiography has well said 

 that without the sun "There could neither be rain nor rivers. 

 . . . Rain is dependent for its distribution upon currents in 

 the atmosphere, but these currents are due to disturbances of 

 equilibrium which are brought about by means of solar heat. 

 Without the sun, then, there could be no winds." The currents 

 of the sea in their final analysis are also due to the heat of 

 the sun. Therefore through the radiant action of the sun, 

 water-vapor is added to the atmosphere, and the amount of 

 it is at present and always has been the latter's most variable 

 constituent, and, as rain, is of the greatest importance geologi- 

 cally. Clarke states 2 that water "is not merely a solvent and 

 disintegrator of rocks, but it is also a carrier, distributing other 

 substances and making them more active. To the circulation 

 of atmospheric moisture we owe our rivers, and through them 

 erosion is effected. The process of erosion is partly chemical 

 and partly mechanical, and the two modes of action reenforce 

 each other. By flowing streams the rocks are ground to sand, 

 and so new surfaces are exposed to chemical attack. On the 

 other hand, chemical solution weakens the rocks and renders 

 them easier to remove mechanically. . . . It is through the 

 agency of rain or snow that the atmosphere produces its great- 

 est geological effects. . . . Aqueous vapor dissolves and con- 

 centrates the other ingredients of air, and brings them to the 

 ground in rain." 



2 Clarke, F. W., "The Data of Geochemistry." U. S. Geol. Survey, Bull. 491, 

 1911, p. 48. 



