10 



SOILS. 



brought out in this way, especially in the case of " mush- 

 room " rocks, where a hard stratum has remained as a covering 

 while softer layers underneath have been worn away. The 

 illustration annexed shows such a case on the plains of Wyom- 

 ing as figured in the Report of the U. S. Geological Survey, 

 on the Central Great Plains, by N. H. Darton. Striking ex- 

 amples of the same effects are seen on the shores of Lake 

 Michigan in the Grand Traverse region, where the rocky cliffs 

 are visibly worn away and carved under the influence of the 

 regular " sand-blasts " of northwest winds. On a smaller 

 scale the effects of these sand-blasts mav be noted in the cob- 



O 



FIG. 4. "Mushroom rocks." produced by Wind action, Wyoming. (Darton.) 



ble-deserts, where we frequently find the cobbles worn away 

 on the windward side in a very characteristic manner; the lee 

 side remaining rounded and smooth, while the structure of the 

 rock is strongly outlined on the windward side. 



CLASSIFICATION OF SOILS. 



The physical Constituents of soils are thus, in the most gen- 

 eral terms, first, rock pozvder (" sand ") more or less changed 



