560 C. R. KEYES THE GEOGRAPHIC CYCLE IN AN ARID CLIMATE 



There appears to be, as recenth' shown/^ a downward limit even to 

 desert-leveling- and eolic excavation. The ground-water level in each 

 structuralh' inclosed basin must finally put a stop to wind-scour by keep- 

 ing the surface above it moist^ gi^^iiig rise either to salinas or forming a 

 basin into whicli sporadic storm waters find a long resting place.'* 



In illustration, many of the salinas of the dry region of western 

 America might be enumerated. The lakes of Death and Imperial val- 

 leys, both below sealevel, in southern California ; the basin of Lake Eyre, 

 in Australia, the Sea of Aral, and other similar bodies of water in arid 

 Asia are most notable. On a somewhat smaller scale are many of the 

 lakelets of the Mexican tableland. Of these the Sandoval, the Hueco, 

 the Casa Grande, and the Mapimi bolsons are best known.'^ The first 

 of these'^ is the highest and driest bolson of the Mexican tableland 

 within the boundaries of the United States. Its surface is 6,000 feet 

 above the sea. Its center is occupied by a great chain of dry and bitter 

 lakes. In all of its vast area and during a period of iOO years since the 

 earliest occupation of the country by Europeans only two small springs 

 of potable water were known within its confines. Eecently it was in- 

 ferred from the general character of the broad basin, its geologic struc- 

 ture, and the location of the two springs, that ground-water level at cer- 

 tain places must be very close to the surface. Proceeding on this hy- 

 pothesis, several test wells were put down and the inferences found to be 

 correct. At once there was excavated an area of several acres in extent 

 for reservoir purposes. Xow there stands a fine large body of soft water, 

 the surface of which comes within a few feet of that of the surrounding 

 plain. Around the lakelet a prosperous town has sprung up. 



Depressions of the Saharan region appear to be downwardly arrested 

 by ground-water level. Beadnell,'^' in describing the Kharga oasis, ex- 

 plains the presence of lake beds in the hollow by the uncovering of 

 impervious clay strata and the consequent exposure of the surface-water 

 sandstone with its artesian supplies. Long ago Lyons'^ called attention 

 to similar phenomena in the Xile Yalley, but the springs thus let loose 

 were regarded by him as increasing local erosion. 



Normal Water Action in Desert Eegions 



The derivation of the larger relief features of the arid regions through 

 means of deflation does not necessarily preclude all normal sculpturing 



"American Journal of Science (4), vol. xix, 1905, p. 167. 



" Journal of Geology, vol. xvii, 1909, p. 6G1. 



^•» American Journal of Science (4), vol. xvi. 1903. p. 377. 



^= Bull. Geol. Soc. America, vol. 19. 1908. p> 91. 



'« Journal of Geology, vol. xvi. 1908, p. 434. 



'' Geological Magazine, n. s., decade v, vol. vi. 1909. p. 476. 



"Quart. Jour. Geol. Soc. London, vol. 1, 1894, p. 531. 



