TOPOGRAPHIC FEATURES OF THE DESERT BASINS. - D : 
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 saline. 
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 in 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 contimuing, 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 relicta—is 
1See the books and papers of Ellsworth Huntington, especially the Pulse of Asia (1907) and Palestine 
and its Transformation (1910). 
