212 'EARTH FEATURES AND THEIR MEANING 



the desert plants, like the yuccas, have so adapted themselves to 

 desert conditions that they may grow upward with the sand for 

 many feet and so keep their crowns above its surface. 



The cloudburst in the desert. Such clouds as enter the 

 desert through its mountain ramparts, and those derived from 

 evaporation from the hot desert soil, usually precipitate their 

 moisture before passing out of the basin. Above the highly 

 heated floor the heavy rain clouds are unable to drop their bur- 

 den. The rain can sometimes be seen descending, but long 

 before it has reached the ground it has again passed into vapor, 



FIG. 223. Ideal section across the rising mountain wall surrounding a desert 

 and a part of the neighboring slope (after R. W. Pumpelly). 



and through repetition of this process the clouds become so charged 

 with moisture that when they encounter a mountain wall and 

 are thus forced to rise, there is a sudden downpour not equaled 

 in the humid regions. Desert rains are rare, but violent beyond 

 comparison. Often for a year or more there is no rainfall upon 

 the loose sand or porous clay, and the few plants which survive 

 must push their roots deep down until they have reached the 

 zone of ground water. When the clouds burst, each small canon 

 or wed (pi. wadi) within the mountain wall is quickly occupied 

 by a swollen current which carries a thick paste of sediment and 

 drowns everything before it. Ere it has flowed a mile, it may 

 be that the water has disappeared entirely, leaving a layer of mud 

 and sand which rapidly dries out with the reappearance of the sun. 

 As the mountains upon seacoasts are generally rising with 

 reference to the neighboring sea bottom, so the mountains which 

 hem in the deserts are generally growing upward with reference 

 to the inclosed desert floor. The marginal dislocations which 

 separate the two are often in evidence at the foot of the steep 

 slope (Fig. 223), and these may even appear as visible earth- 



