Page:Popular Science Monthly Volume 22.djvu/396

382 of small lakes; others by the evaporation of lakes after a term of years when the season was unusually dry; there are also other playas that are only covered with water during exceptionally wet seasons. One might perhaps include in the list of playa-lakes the great lakes of the Quaternary, whose fluctuations extended through geological periods, and whose desiccation has left the largest of all the playas. Lake Bonneville and Lake Lahontan, however, can be called playa-lakes only during the closing chapters of their history; in the earlier portions of their existence, they were fresh-water lakes of great depth, and one of them, at least, overflowed.

When we examine the material composing playas more critically, we find that they are formed of at least two varieties of sediments. In the broad, open playas, like Great Salt Lake Desert and the two desert regions of the Lahontan Basin in Nevada, the surface is composed of soft, fine, greenish, saline clay, that is commonly saturated with alkaline water at a depth of a few feet, and becomes tenacious and difficult to handle. These clays are, without question, simply lake-beds that were deposited by sedimentation at the bottom of the great lakes that once occupied the valleys where they are found.

The second variety of playa-beds occurs in restricted basins and in valleys that are without outlets. These are found very commonly behind shore-bars of Lake Bonneville and Lake Lahontan, and in valleys and canons the mouths of which have been crossed by the embankments of gravel built along the shores of these Quaternary lakes. The material forming playas of this nature is always of a light yellowish color, becoming almost white when dry; is extremely fine, and readily crumbles into dust between the fingers; near the surface, the beds are full of small globular vesicles that were apparently once filled with gas or water. These characteristics hold good even when the playas are surrounded on all sides by dark basalts, from the disintegration of which the playa-beds must have been formed.

True playa-beds, composed of light-colored material as described above, have been penetrated to the depth of five to six feet without revealing any change in the composition of the deposit. The thickness that may be reached by these slowly accumulating beds depends on the nature of the basin in which they are deposited; in some cases they can not be less than twenty or thirty feet in thickness. The coarse material swept down the sides of these inclosed basins by the infrequent rains is invariably left at the edge of the playa, and in a section of the beds appears as thin wedges of gravel and angular fragments, that thin out and become lost as we trace them away from the shore. Playa-beds may become covered with lake-beds, thus forming a peculiar light-colored stratum, in reality a fossil playa, that bears record of a time of desiccation; and, when lake-beds occur below, it is evidence that a dry period has intervened between two periods of more abundant precipitation.