102 GEOLOGY 



Again, organisms of various sorts (certain types of seeds, germs, 

 etc.), as well as dust and sand, are extensively transported by the 

 wind. 



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 OP THE ATMOSPHERE 



The chemical work of the atmosphere (including solution and 

 precipitation from solution) is principally accomplished in connec- 

 tion with water, a dry atmosphere having relatively little direct 

 chemical effect on rocks or soils. The important chemical changes 

 wrought by the atmosphere are oxidation, carbonation, and hydra- 

 tion. The nature and the results of these processes have already 

 been noted in connection with the decay of rocks (p. 32). A few 

 other effects of the atmosphere may be noted. 



Precipitation from solution. The water in the soil is constantly 

 evaporating. Such substances as it contains in solution are depos- 

 ited where the water evaporates, and where evaporation is long 

 continued without re-solution of the substances deposited, the 

 surface becomes coated with an efflorescence of mineral matter. 

 An example is found in the alkali plains of certain areas in the 

 western part of the United States. Since the alkaline efflorescence 

 is the result of evaporation, it is connected with the atmosphere, 

 but the material of the efflorescence was brought to its present 

 position by water. Other substances deposited when the water 

 which held them in solution is evaporated, coat the pebbles and 

 stones of arid plains in many places. Gravel is thus cemented 

 into conglomerate not infrequently. 



Conditions favorable for chemical changes. Conditions are 

 not everywhere equally favorable for the chemical work of the 

 atmosphere. In general, high temperatures facilitate chemical 

 action, and, other things being equal, rocks are more readily decom- 

 posed by the chemical action of the atmosphere in warm than in 

 cold regions. Chemical activity is probably greater where the 



