GEOLOGICAL IMPORTANCE OE SEDIMENTATION 535 



about some of the hot springs, and are then apt to contain borax in addition to 

 the sulphate and carbonate of soda, common salt, etc., which make up the bulk 

 of such incrustations 



North Carson and South Carson lakes are of the playa type, but are more 

 persistent than the lakes of Black Rock and Smoke Creek deserts. They sometimes 

 hold their integrity for a succession of years, but evaporate to dryness during 

 seasons of more than usual aridity. North Carson Lake is rudely elliptical 

 in outline, and is from 20 to 25 miles across from east to west, and about 14 

 miles broad from north to south. That its depth is never over a few feet, has 

 been shown by examining its bed when dry 



Hundreds of other inclosed basins, particularly in southern Nevada, are 

 partially flooded in winter in a similar manner to those already enumerated, 

 and become desert plains of hardened mud in summer. Various portions of the 

 region surrounding Nevada, and especially those embraced within the boun- 

 daries of Utah, Arizona, and California, experience changes similar to those 

 just described, and illustrate some of the most striking peculiarities of a region 

 where the topographic and climatic conditions favor the existence of temporary 

 lakes. 



Numerous playa lakes are also found in Australia and in iVfrica, 

 especially in the Kalahari, and may be looked upon as common 

 features of desert regions where the regolith is not sufficiently deep 

 and sandy to absorb all of the occasionally precipitated water. Playa 

 formations are not necessarily accbmpanied by conspicuous saline 

 deposits since the clay washed in, and subsiding each year prevents 

 re-solution of the buried salts and may largely exceed them in quan- 

 tity. The amount of salt will also depend upon the area of the 

 playa to the catchment area and the extent to which ground water 

 contributes. In old desert regions such as the Kalihari there may 

 be thus wide playa surfaces where water stands for a longer or 

 shorter period. Speaking of the Kalihari, Brewer states that "Lake 

 Ngami is fresh in the rainy season, but covers much less surface in 

 the dry season, and is then brackish; and the other lakes of this 

 desert are described as brackish rather than salt."^ 



Nature of the geological records. — The preceding is a description 

 of playa surfaces. In the absence of descriptions of partially eroded 

 playa deposits seen in cross-section the following statement of the 

 characteristics which they would presumably show when incorporated 

 into the geological record must be to some extent deductive and open 

 to corroboration by observation. 



I Wm. H. Brewer, Warren's New Physical Geography. 



