CHAPTER XV 



SUN AND WIND IN THE LANDS OF INFREQUENT 



RAINS 



The law of the desert. It is well to keep ever in mind that 

 there is no universal law which dominates Nature's processes in 

 all the sections of her realm. Those changes which, because often 

 observed, are most familiar, may not be of general application, 

 for the reason that the areas habitually occupied by highly civi- 

 lized races together comprise but a small portion of the earth's 

 surface. In the dank tropical jungle, upon the vast arid sand 

 plains, and in the cold white spaces near the poles, Nature has 

 instituted peculiar and widely different processes. 



The fundamental condition of the desert is aridity, and this 

 necessitates an exclusion from it of all save the exceptional rain 

 cloud. Thus deserts are walled in by mountain ranges which 

 serve as barriers to intercept the moisture-bringing clouds. They 

 are in consequence saucer-shaped depressions, often with short 

 mountain ranges rising out of the bottoms, and such rain as falls 

 within the inclosure is largely upon the borders. Of this rainfall 

 none flows out from the desert, for the water is largely returned 

 to the atmosphere through evaporation. 



The desert history is thus begun in isolation from the sea from 

 which the cloud moisture is derived, a balance being struck be- 

 tween inflow and evaporation. Yet if deserts have no outlets, 

 it is not true that they have no rivers. These are occasionally 

 permanent, often periodic, but generally ephemeral and violent. 

 The characteristic drainage of deserts comes as the immediate 

 result of sudden cloudburst. As a consequence, the desert stream 

 flows from the mountain wall choked with sediment, and entering 

 the depressed basin, is for the most part either sucked down into the 

 floor or evaporated and returned to the atmosphere. The dis- 

 solved material which was carried in the water is eventually left 



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