SOIL-FORMATION, ATMOSPHERE 17 



and the change of the mineral pyrite is typical of the 

 process. 



2Fe S 2 + 70 2 + 2H 2 = 2Fe S0 4 + 2H 2 S0 4 



Fe S0 4 + 2H 2 = Fe(OH) 2 + H 2 S0 4 



All of these changes of iron compounds under the 

 action of moist atmosphere are imperfectly understood, 

 but it is agreed that the above products may result from 

 the process. Since the sulfate is much more soluble than 

 the sulfid, the mineral is in this way easily removed. 



The purely chemical action of the atmosphere is less 

 pronounced in its effects than its mechanical action. 

 As wind, it exerts some pressure upon projecting masses 

 tending to push them over, but its great work is accom- 

 blished when the wind carries solid particles of dust and 

 sand and when it acts on vegetation as a lever. In arid 

 and semi-arid regions, particularly, the amount of solid 

 material carried in the atmosphere is very large at 

 some seasons. There frequently occur dust storms, 

 when the atmosphere is so filled with wind-driven par- 

 ticles as to obscure the sun and all objects, at even a 

 short distance away. In the region of western Nebraska 

 and Kansas these dust storms are well known, and 

 on certain soils it is unwise to plow in the fall, because 

 by spring the soil will have been blown away to the 

 depth of the furrow, and indeed this sometimes results 

 from plowing at any other season of the year. Further 

 west in the mountain region this wind-blown material 

 is most effective, where the particles may be driven 

 against the bare rock faces. It then becomes a titanic 

 sand blast to drill away the rock. It eats into the rock 

 surface with remarkable rapidity, carving fantastic 



