130 



GEOLOGY. 



developed by erosion, such as those of the Bad Lands (Figs. 75 to 78, and 

 108), are developed where the rainfall is unequally distributed in time, 

 and too slight or too infrequent to support abundant vegetation. 



Fig. 108. — Bad-land topography developed under conditions of aridity and unequal 

 distribution of rainfall. Slope of Pinal Mountains, Ariz. (Ransome, U. S. Geol. 

 Surv.) 



During its fall, and immediately after, rain is more effective than 

 an equal amount of snow; but the snow may be accumulated through 

 a considerable period of the year, and then melted rapidly, when it 

 has an effect comparable to that which would be produced by the con- 

 centration of the rainfall into a hmited period of the year. If the 

 ground beneath be frozen when the snow melts (and this is often the 

 case) the erosion accomphshed by the resulting water will be diminished. 



Except in dry regions, where wind-work sometimes exceeds water- 

 work, the movements of the atmosphere are of less importance directly 

 than precipitation in determining the rate of erosion. But even in 

 regions which are not arid the winds have much to do with the rate of 

 evaporation and the distribution of rainfall, so that their indirect effect 

 is great. Even their direct effects in moist climates are not to be 

 lost sight of, for even here the surface is sometimes dry enough to 

 yield dust and sand, and the uprooting of trees so disturbs the surface 

 as to make earthy debris more accessible to wind and water. Where 

 trees gain precarious footholds on steep slopes, as they often do, they 

 are likely to be overturned as soon as they are large enough to offer 



