TOPOGEAPHIC FEATURES OP THE DESEET BASINS. 53 



in a group of email and variable lakes. The inclosed condition of the valley is un- 

 doubtedly very recent and due only to stream decay. The area of the present basin 

 is about 2,800 square miles. 



THE RED DESERT BASIN. 



The Red Desert Basin, or group of basins, lies in south-central Wyoming, on the very 

 crest of the Rocky Mountains, occupying a broad plain bordered on three sides by 

 mountain ranges but essentially open toward the south. At a timeby no means remote 

 this southern divide was nonexistent and the basin drained, probably freely but at least 

 by overflow, into the Little Snake River and thence to the Colorado. The present 

 barrier is a series of low divides which are superficially alluvial and probably entirely 

 so. The basin is by no means a unit but is cut by alluvial or structural divides into 

 a complex series of smaller basins each with its playa and its greater or lesser drainage. 

 The past and present relations of these basins are not known in detail, but it is im- 

 probable that their discharge ever concentrated in a single basin or a single channel 

 of escape. The region is more a decayed drainage system than a single basin. 



None of these basins is ancient, and none would have .any importance were it not 

 for the fact that part of the western slope of the area is formed by the Leu cite Hills, 

 a zone of volcanic activity in which are large masses of leucitic rocks containing con- 

 siderable proportions of potash.^ How fully the drainage of these hills has been 

 localized and retained can not be determined from present data. The writer inclines 

 to the opinion that both retention and concentration have been comparatively slight, 

 but the evidence is far from conclusive, and the region can not be disregarded. It 

 should be noted that the presence of extensive deposits of sodium salts in the basins 

 of the Red Desert and in other small basins both west and east of it is no proof of 

 long-continued concentration. The shales and sandstones which make up the greater 

 portion of the areas tributary to these basins contain large quantities of occluded 

 sodium salts, which rapidly find their way into the drainage and to the places where 

 it concentrates. 



West of the Red Desert and on the westward slope of the Leucite HUls are several 

 small and local basins now without overflow and which share the topography and 

 geochemical characteristics of the western part of the Red Desert proper. 



The total area of the Red Desert Basin is approximately 3,600 square miles, but it 

 is apparent from the above discussion that importance lies not in the total area, but 

 in the areas and topographies of the various subsidiary basins and in what propor- 

 tion of leucitic country chanced to be tributary to each. These facts can not be 

 determined from the information now available. 



THE GREAT VALLEY OF CALIFORNIA. 



Through the heart of California, between the Sierra Nevada and 

 the Coast Range, runs a great filled trough which differs from the 

 "basin troughs" east of the Sierra only in its greater size and in the 

 fact that its western wall is breached by the Golden Gate, giving free 

 egress to the sea. Southward through the north end of this valley 

 flows the Sacramento River, and northward from the south end comes 

 the San Joaquin, both rivers uniting to form the Straits of Suisan 

 and entering the sea through San Francisco Bay and the Golden 

 Gate. In essence both the Sacramento and San Joaquin Valleys are 

 regions of free seaward drainage, but rainfall is low, and is insufficient 

 to keep the valleys entirely clear. Local playas and ''alkali" spots 



1 See Schultz and Cross, U. S. Geol. Sur., Bull. 512 (1912). 



