THE FEATURES IN DESERT LANDSCAPES 



217 



trine deposits of clay and separated salts (Fig. 230 and Fig. 207, 

 p. 201). The lake deposits fill in all the original irregularities 

 of the desert floor, out of which the tops of isolated ranges of 

 mountains now project like islands out of the surface of the sea. 

 The several zones of de- 

 posits in their order from 

 the margin to the center 

 of the desert are given 

 schematically in Fig. 231. 

 The zone of vegetation, 

 as already stated, lies near 

 the foot of the alluvial 

 bench, so that here are 

 found the oases about 

 which have clustered the 

 cities of the desert from 

 the earliest records of antiquity until now. Just without the line 

 of oases is the wall of dunes held back from further advance only 

 by the vegetation which in turn is dependent upon the rains in 

 the neighboring mountains. With every diminution in the water 

 supply, the dunes advance and encroach upon the oases (plate 7 B) ; 

 while with every considerable increase in this supply of moisture 



FIG. 230. Billowy surface of the salt crust on 

 the central sink in the Lop Desert of central 

 Asia (after Ellsworth Huntington). 



FIG. 231. Schematic diagram to show the zones of deposition in their order from 

 the margin to the center of a desert. 



the alluvial bench advances over the dunes and acquires a strip 

 of their territory. Thus with varying fortunes a war is con- 

 tinually waged between the withering river and the flying sand, 

 and the alternations of climate are later recorded in the dove- 



