THE WORK OF THE ATMOSPHERE 



103 



FIG. 98. Talus slope in the Snake River Canon, opposite Enterprise, 



Idaho. (Russell, U.S. Geol. Surv.) 



What can be inferred from the picture concerning the char- 

 acter of the topbeds? 



CHEMICAL WORK 



The oxygen, carbon dioxide, and water vapor of the atmos- 

 phere are very active chemically. Chief among the rock sub- 

 stances with which oxygen unites is iron. This process 

 (oxidation) is illustrated familiarly by the rusting of iron ob- 

 jects exposed in damp weather, the rust being a chemical 

 combination of iron, oxygen, and water. The brick-red and 

 yellow colors of many soils and rocks are due to the oxidized 

 condition of their iron. Among the common minerals affected 

 by the process are mica, hornblende, and augite, all complex 

 silicates containing iron. The union of the carbon dioxide 

 (C0 2 ) of the atmosphere with certain rock materials (carbon- 

 atiori), is also an important and common process. For ex- 

 ample, carbon dioxide may unite with the calcium and with 

 the iron of minerals containing these elements, to form cal- 

 cium carbonate and iron carbonate. The chemical union of 



