TOPOGBAPHIC FEATURES OP THE DESERT BASINS. 5 



drawing to a close and which was marked by extensive and repeated 

 flows of rhyolites and basalts, and by the discharge of enormous 

 quantities of fragmental material. This period was characterized by 

 the existence of a number of scattered and successive lakes, often 

 quite extensive but probably shallow, in which the fragmentary vol- 

 canic material found a resting place. Apparently the region was then 

 cut off partially or completely from the sea, and while most of these 

 lakes probably overflowed, the occurrence of salt and gypsum among 

 their deposits indicates that some of them were sahne. 



The division between this Tertiary period and the present is not a 

 sharp one. With the lapse of time vulcanism has decreased, move- 

 ment has disturbed the Tertiary lake beds, and erosion has doubtless 

 been active; but conditions are essentially the same now as then 

 and the Tertiary lakes find their direct descendants ia the present 

 "dry lakes" or playas and in the great lakes of the recent past. 



In summary, the history of the Great Basin region begins at the 

 close of the Jurassic with crustal movements which have continued 

 ever since. At first these movements did not interfere with seaward 

 drainage or normal erosion, but early in the Tertiary the separation 

 from the sea began to be effective and the "Great Basin" (perhaps 

 then drained by overflow) was produced. Since that time rising 

 walls and increasing aridity have joined hands to make the imprison- 

 ment of the drainage more effective. 



So much for the general outline of the history. It is now necessary 

 to examine its most recent section a little more closely. In a time 

 which is usually correlated with the Glacial Epoch many of the 

 inclosed valleys of the Great Basin contained large and persistent 

 lakes. The beginnings, the early history, even the exact chro- 

 nology of the lakes remain unknown. They were probably preceded 

 by a period of aridity and they probably rose very slowly. All this 

 is yet uncertain and need not be pursued. Starting with these lakes, 

 we find that they were subject to extreme variations of level, probably 

 in response to the climatic fluctuations, now coming to be recognized 

 as both incessant and world-wide.^ These fluctuations are not yet 

 worked out in detail, but they seem to indicate two main periods of 

 lake expansion separated by a long period of contraction, probably 

 to complete desiccation. The second expansion was followed by a 

 second desiccation and contraction to the present condition. Since the 

 beginning of this double-lake period the structural movements, 

 though continuing, have been slight and have not affected the 

 topography. 



The detailed history of this lake period — its precedent conditions, 

 its chronology, its various physiographic and chemical relict a — is 



I See the books and papers of Ellsworth Huntington, especially the Pulse of Asia (1907) and Palestine 



and its Transformation (1910j. 



