PL AY AS AND PLAYA-LAKES. 381 



miles broad, composed principally of fine, tenacious, greenish clay. 

 Other areas that now exist as great playas occur on the Carson Desert 

 and on the Black Rock and Smoke Creek Deserts of Northwestern 

 Nevada : these are portions of the bottom of Lake Lahontan that have 

 been laid bare by the desiccation of the former lake. Playas of smaller 

 extent, but which are yet typical examples of the deserts left by the 

 withdrawal or evaporation of Quaternary lakes, are found in Diamond 

 Valley, White-Pine Valley, Gabb's Valley, and Osobb Valley. All of 

 these examples are in Nevada ; but hundreds of others, of greater or 

 less extent, might be enumerated that are scattered throughout the 

 length and breadth of the Great Basin. As already mentioned, many 

 of these playas are covered with water during the rainy season. Others 

 exist as lakes, excepting when the season is unusually dry. They then 

 become mud-flats that can not be distinguished from the playas that 

 become dry and hard every summer. 



The lakes that cover playas at certain times, and appear and disap- 

 pear as the wet and dry seasons alternate, and are sometimes born of 

 a single shower, and become many square miles in area during a single 

 night, may with convenience be designated as playa-lakes, as they have 

 many features peculiar to themselves. These lakes are without out- 

 lets, are seldom more than a few feet deep, and usually hold no 

 more than a few inches of water ; they are commonly alkaline or 

 brackish, and are always, so far as the observations of the writer ex- 

 tend, of a peculiar yellowish or greenish-yellow color. The character- 

 istic tint is due to the extremely fine mud, and probably chemical pre- 

 cipitates, that the waters hold in suspension. Owing to the extreme 

 shallowness of these lakes, the fine mud at the bottom is agitated by 

 every breeze, and thus the clearing of the water by the subsidence of 

 the material held in suspension is prevented. 



Playa-lakes, that form in the wet season and vanish again during 

 the summer months, occur in a great number of the desert valleys 

 and small inclosed basins to be found in the arid region between the 

 Sierra Nevadas and the Wahsatch range. Some of these annual lakes 

 are of considerable dimensions. On the northern part of the Black 

 Rock Desert, Nevada, where Quin's River enters the desert from the 

 northeast, a shallow lake is formed during the rainy season that is 

 reported to be from ten to fifteen miles broad and as much as forty 

 or fifty miles in length. When the dry season comes on, this lake 

 evaporates, and the river that formed it shi'inks back seventy-five or a 

 hundred miles, leaving its channel a dry water-course, with perhaps a 

 few water-holes to indicate its former extent. 



Examples of playa-lakes that reach desiccation only during excep- 

 tionally dry seasons are furnished by Eagle Lake, Worth Carson Lake 

 (" Carson and Humboldt Sink "), and Yellow-water Lake, in Nevada, 

 and by Honey Lake and the lakes of Surprise Valley in California. 



We have spoken of playas as being formed by the annual evapora- 



