22 BORAX OF DEATH VALLEY AND MOHAVE DESERT. [bill. 200. 
The data gathered during the present reconnaissance shows that 
such is not the case. From the character and arrangement of the 
material composing these beds, it is apparent that most of the lakes 
occupied valleys, the bounding walls of which may be seen to-day with 
the coarse detrital material of the lake beds at their feet. As pre- 
viously mentioned, the lakes undoubtedly existed at different geologic 
epochs, and from data now at hand it seems probable that lake- 
forming conditions existed practically throughout the Tertiary period 
(Eocene, Miocene, and Pliocene); but sedimentation was interrupted 
at intervals by violent crustal movement that tilted the earlier sedi- 
ments and materially changed the topography of the country. 
The conditions which led to the formation of many lakes in the past 
were certainly climatic. The climate was less arid, for at the present 
time lakes exist in this region only where they receive drainage from 
extensive mountain masses that receive more moisture than the aver- 
age for the region. The aridity of the climate appears to have varied 
from time to time, producing variations in the character of the sedi- 
ments being deposited. At times the water seems to have evaporated, 
leaving its mineral constituents in the form of beds of salt, soda, 
gypsum, and borax. Again, the lake was tilled and beds of sand or 
mud covered the mineral deposits and preserved them for future use. 
It is possible that the existence of such mineral deposits does not 
mean the complete evaporation of the lake, but the conditions were 
similar to those prevailing to-day about Owens Lake and Great Salt 
Lake, where extensive alkaline deposits are accumulating. 
The conditions which prevailed in this region during Pleistocene 
time have not been satisfactorily determined. They are evidently 
recorded in the great accumulations of unconsolidated gravel that 
occur at many points in Mohave Desert and the Death Valley region. 
These deposits have been regarded as indicative of lake conditions, as 
have also the salt fields of Death Valley and adjacent localities, but the 
evidence is far from conclusive. 
It is well known that large lakes, like Bonneville and Lahontan, 
existed in the Great Basin in Pleistocene time, but these lakes, although 
transient, left unmistakable evidence of their existence in the form of 
sediments and terraces around the margins of their basins. 
In Death Valley no such features have been recognized. If they 
were formed they have been so masked and disarranged by recent 
earth movements as to have escaped detection. It seems possible to 
explain the salt field of this valley by the evaporation of saline water 
brought in by Amargosa River in the period of floods which occasion- 
ally affect that stream, but which are not of so frequent occurrence as 
to produce a lake in Death Valleys 
The evidence at hand is too meager to warrant the writer to attempt 
to present an outline of the Pleistocene history of this region; it can be 
done only after a more detailed study of the.geology and physiography 
of the region. 
