CHEMICAL WORK 





Fig. 15. Wind erosion. Casa Colorado, Dry Valley, Utah, between Aba jo 

 . and La Sal Mountains. La Plata (Jurassic) sandstone. (Cross, U. S. Geol. Surv.) 



Indirect effects. Other dynamic processes are called into being 

 by the atmosphere. Winds generate both waves and currents, 

 which are effective agents in geologic work. The results of their 

 activities are discussed elsewhere. 



( HI MICAI. WORK 



The chemical work of the atmosphere is accomplished prin- 

 cipally in connection with water. Dry air has little chemical effect 

 on rocks or soils. The important chemical changes wrought by the 

 atmosphere are oxidation, carbonation, and hydration. Oxidation, 

 as used in this connection, is the union of oxygen with some con- 

 stituent of the rock, forming an oxide. Carbonation is a union of 

 carbon dioxide of the air with constituents of the rock, forming 

 carbonates. Hydration, similarly, is the union of water with con- 

 stituents of the rock. Oxidation and hydration may go on at the 

 same time. Thus when iron rusts, oxygen and water both enter 

 into combination with the iron. In most cases these chemical 

 changes result in breaking up the rock, much as steel or iron is 



