382 THE POPULAR SCIENCE MONTHLY. 



tion 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 com- 

 posed 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 val- 

 leys and canons the mouths of which have been crossed by the em- 

 bankments of gravel built along the shores of these Quaternary lakes. 

 The material forming playas of this nature is always of a light yel- 

 lowish 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 disinte- 

 gration 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 thick- 

 ness 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 frag- 

 ments, 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. 



