152 GEOLOGY 



during a wet season only. The maximum inequality of distribution 

 would occur if all the rainfall of a given period were concentrated 

 in a single shower. With such concentration the volume of water 

 flowing off over the surface immediately after the down-pour 

 would be greater than under any other conditions of precipitation, 

 and since velocity is increased with volume, and erosive power 

 with velocity, it follows that the erosive power of a given amount 

 of water would be greatest under these circumstances. Further- 

 more a larger proportion of the precipitation would run off over the 

 surface under these circumstances than under any other, for less 

 of it would sink beneath the surface, and less would be evaporated. 



If erosive power and rate of erosion were equal terms, the maxi- 

 mum concentration of rainfall would be the condition for greatest 

 erosion; but erosive power and rate of erosion do not always cor- 

 respond. If the water falling in this way could get all the material 

 it could carry, erosion would be at a maximum; but if the amount 

 of available material for transportation is slight, a large part of the 

 force of the water could not be utilized in erosion. In this case, 

 that distribution of precipitation which most favored weathering 

 would lead to greatest erosion. Temperature favoring, the uni- 

 form distribution of moisture through the year would allow the 

 growth of vegetation. This would favor some processes of weather- 

 ing, but it would retard erosion in general. While, therefore, it is 

 hardly possible to say what distribution of rainfall favors most 

 rapid erosion without knowing the nature of the surface on which 

 it is to fall, enough has been said to show that the problem is not a 

 simple one. Some of the most striking phases of topography 

 developed by erosion, such as those of the Bad Lands (Figs. 99 ! 

 and 100), are developed where the rainfall is unequally distributed 

 in time, and too slight or too infrequent to support abundant 

 vegetation. 



Except in dry regions, where wind-work sometimes excrnN 

 water-work, the movements of the atmosphere are of less import ance 

 directly than precipitation in determining the rate of erosion. Hut 

 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. 



