24 



broken up when it rusts. 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 



Fig. 1 6. Erosional forms characteristic of dry regions where erosion by the 

 wind is effective. Fissure Canyon, north slope of the La Sal Mountains, Utah. 

 The rock is Peimian. (Cross, U. S. Geol. Surv.) 



continued, without re-solution of the substances deposited, the 

 surface becomes coated with an efflorescence of mineral matter. 

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

 western part of the United States. Certain substances, deposited 

 when the water which held them in solution is evaporated, coat the 

 pebbles and stones of some arid plains. In some places gravel is 

 thus cemented into conglomerate. 



Conditions favorable. Conditions are not everywhere equally 

 favorable for the chemical work of the atmosphere. Since high 



