' DEFLATION OF ARID LANDS i 585 



the surface of the ground during one of these storms must be enormous. 

 Compared with the amount of sediments carried along by the Mississippi 

 Eiver in time of flood, it is estimated that in the lower 20 feet of the de- 

 flative stream there are equal amounts of rock waste moving in like 

 cross-sections of the great river and of the air current of the desert. The 

 air stream moves 40 miles an hour instead of 4, as in the case of the 

 water stream, and in place of being only a mile wide, the path of the 

 sand-storm is several hundreds of miles wide. The lower 6 inches of the 

 air stream is almost wholly moving sand and fine gravel. The finer dust 

 soars upwards thousands of feet, darkening the sun as by a heavy thunder- 

 cloud. Little wonder is it that the harder rock surfaces of the desert are 

 constantly swept clean. 



After a sand-storm in the desert the changes affected excite no more 

 attention than those after a rain storm in the moist land. 



IMPORTANCE OF DEFLATIVE PROCESS 



The recent observations made in the arid regions of the West point 

 strongly to the wind as the chief erosive agent, water action having dis- 

 tinctly only a minor role in the sculpturing of the country. According 

 to the conclusions thus reached, the wind must be regarded in a dry cli- 

 mate as being fully as effective in general erosion and leveling as is water 

 in a wet climate. In the arid region wind is not only the most potent of 

 the gradational agencies, but its efficiency as an erosive force is probably 

 greater than all other geologic processes combined. Its main activities 

 are strongly degradational in character; its constructional effects are 

 local, relatively unimportant, and mainly extralimital. 



In its broader effects of reducing a region to a lower plains level, wind 

 scour is not so very unlike stream action. The less resistant rocks are 

 removed faster than the more indurated ones, dividing the country into 

 belts of highland and belts of lowland. The latter become plains very 

 early in the cycle of general lowering of the land surface ; the former re- 

 solve themselves into mountain ranges, finally attaining a stage of rem- 

 nantal eminences comparable to monadnocks. The main difference be- 

 tween general leveling in a moist climate and in a dry one is that in the 

 one the greater part of each geographic cycle is spent in attaining a 

 plains surface which is baselevel, while in the other the plains surface is 

 the dominant relief feature from the beginning. 



Comparative Effects of Corrasion and Deflation 



The origin of the mountain ranges of the Great Basin and of arid 

 America generally has been considered mainly only from the side of pure 



