T?IE ATMOSPHERE AS A GEOLOGICAL AGENT. 



41 



face in such a way as to make loose earth more readily accessible to 

 wind and water. The uprooting of trees on steep slopes often causes 

 the descent of considerable quantities of loose rock and soil. Again, 

 organisms of various sorts (certain types of seeds, germs, etc.), as well 

 as dust and sand, are extensively transported by the wind. While 

 this is important biologically its geological effects are remote. 



Fig. 25. — Shows the disturbance of surface earth and rocks by upturning uf trees. 



(Darton, U. S. Geol. Surv.) 



Indirect effects of the wind. — Other dynamic processes are called 

 into being or stimulated by the atmosphere. Winds generate both 

 waves and currents, and both are effective agents in geological work. 

 The results of their activities are discussed elsewhere. 



//. The Chemical Work of the Atmosphere. 



The chemical work of the atmosphere (including solution and 

 precipitation from solution) is principally accomplished in connection 

 with water, a dry atmosphere having relatively little direct chemical 

 effect on rock or soils. 



Precipitation from solution. — The water in the soil is constantly 

 evaporating. Such substances as it contains in solution are deposited 

 where the water evaporates, and where evaporation is long continued 



