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 disappear 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 outlets, 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 extend, of a peculiar yellowish or greenish-yellow color. The characteristic tint is due to the extremely fine mud, and probably chemical precipitates, 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 shrinks 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 exceptionally 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-