SUN AND WIND IN LANDS OF INFREQUENT RAINS 207 



windward over the great depression, his field of view is generally 

 obscured by the yellow haze of the dust clouds moving across 

 the margins. Upon the mountain 

 flanks and extending far outside 

 the borders, this cloud of dust 

 settles as a shrouding mantle of 

 impalpable yellow powder, which 

 is known as loess. These deposits 

 are continually deepening, and 

 have sometimes accumulated until 

 they are hundreds or even thou- 

 sands of feet in thickness. Before 

 reaching its final resting place the 

 dust of this deposit may have 

 settled many times, and has cer- 

 tainly been in part redistributed FlG> 2 16. Cliffs in loess 200 feet in 



by the Streams near the desert height which exhibit the characteris- 



margin. In it are the ingredients tic vertical jointing (after von Rich ' 



..T tofen). 



which are necessary for the nour- 

 ishment of plants, and it constitutes the most important of natural 

 soils. Continually fed by new deposits from 

 the desert, and refertilized from below by a 

 natural process so soon as the upper layers 

 become impoverished, it requires no artificial 

 fertilization. Without artificial aids the loess 

 of northern China has been tilled for thousands 

 of years without any signs of exhaustion. 



Though easily pulverized between the fingers, 

 loess is none the less characterized by a perfect 

 vertical jointing and stands on vertical faces 

 as does the solid rock (Fig. 216), but it is ab- 

 solutely devoid of layers or bedding. Its ca- 

 pacity of standing in vertical cliffs the loess owes 

 to a never failing content of lime carbonate 

 which acts as a cement, and to a peculiar porous 

 structure caused by capillary canals that run 

 vertically through the mass, branching like 

 rootlets and lined with carbonate of lime. This texture once 

 destroyed, loess resolves itself into a common sticky clay. 



FIG. 217. A canon 

 in loess worn by 

 traffic and wind. A 

 highway in north- 

 ern China (after 

 von Richtofen). 



