TOPOGEAPHIC FEATURES OF THE DESERT BASINS. 57 



mum in early spring to a minimum in late autumn. Most of the 

 lakes do not overflow and many of them are brackish. All are un- 

 questionably recent and due to sand accumulations which the drain- 

 age has never since been vigorous enough to clear away. 



Farther south, where the rainfall is less, are playas instead of lakes, 

 but otherwise conditions are the same. Everywhere the outposts of 

 the drainage system have retreated and their channels have been 

 barred. The bars are sometimes alluvial, sometimes eolian, more 

 often both. The result is the same. 



Of course none of the basins thus created could ever become the 

 place of accumulation of any large quantity of salt. It is an essential 

 of the process outlined that large basins can not be created, since their 

 concentrated drainage would be suflS.cient to sweep away the dam. 

 Where a stream of any size is permanently dammed by sand or allu- 

 vium it must be dammed in many places and split into many basins 

 in order that evaporation may be sufficient to balance or overbalance 

 the inflow. In places ponds may become quite saline, but the total 

 amount of salt accumulated is always small. 



It is of special interest in the present connection to note that some 

 small saline ponds in western Nebraska have been found to contain 

 very large proportions of potassium carbonate undoubtedly derived 

 from the concentration of the run-off of burnt-over prairies. Were 

 there a place where concentration of this kind had occurred for a long 

 time or from a considerable area, a workable deposit of potassium 

 salts might have accumulated. No such place has been discovered 

 and it is probable that none exists. 



LOCAL BASINS OF UNUSUAL ORIGIN. 



For the sake of completeness it is necessary to note briefly a few 

 areas of inclosed drainage which have originated from local and 

 unusual causes. These are of two types — volcanic and eolian. The 

 craters of extinct volcanos frequently contain inclosed lakes and there 

 is at least one example of this in the United States — Crater Lake, 

 Oreg. The Ragtown Soda Lakes, near Fallon, Nev., noted on page 

 15, are probably of similar origin, though the vulcanism was far less 

 vigorous. Apparently the Zuni Salt Lake on the plateau of north- 

 western New Mexico is of the same type.^ In both the latter cases 

 the salinity of the inclosed lake is due to the concentration of water 

 received from seepage or springs. 



Basins due to ''deflation," or eolian erosion, have only one promi- 

 nent example in the United States. West of Laramie, Wyo., are three 

 or four isolated depressions, one of which, Bates Hole, is of consider- 

 able size and depth. These have been studied by Blackwelder ^ who 



1 Darton, U. S. Geol. Survey, Bui. 260, 565 (1905). 



2 Jour. Geol., 17, 443(1909).. , ... .....-, = .. 



