GEOLOGIC HISTORY. 
flow of late Pliocene and early Pleistocene time. The rhyolite in 
turn was followed by andesites and dacites, and these by unimportant 
rhyolites, immediately preceding the deposition of the Siebert lake 
beds in the Pahute Lake. It was probably during- | H general period 
that the most important ore deposits in the Tertian were 
formed, presumably by waters heated by the still-hot magmas. 
Between the outflow of the rhyolite and the formation of the Pahi 
Lake erosion was probably continuous, partially accounting for the 
formation of the lake basin which in the main, however, originated 
through orogenic sinking that was possibly, in part, consequent on 
adjustments due to the extrusion of immense masses of lava, a hypoth- 
esis suggested by Spurr. a The Pahute Lake covered practically the 
whole area surveyed, although the presence of coarse conglomerates in 
the Kawich and Amargosa ranges and the Bullfrog Hills indicates 
that rugged islands rose above the surface of the lake. Wherever the 
former shores of the lake are now seen it is evident that the surface 
of the older rocks was uneven and that each island was surrounded 
by islets. The climate must have been moist and the presence of fos- 
silized Avood in the lake beds shows that trees flourished near its 
shores. While the lake was thus for the most part fresh, periods of 
aridity alternated with those of comparative humidity, and the lake 
or portions of it were partially desiccated, permitting the local pre- 
cipitation of limestone, gypsum, and boron minerals. Volcanic flows 
and explosive eruptions of rhyolitic material occurred at various 
times during the existence of the lake. The Pahute Lake was 
destroyed in part by the increasing aridity of the climate and in part 
by deformation, which was accompanied and immediately followed 
by the extrusion of rhyolite. By this deformation the whole area 
was uplifted with attendant southward tilting, which accounts for 
the relatively low altitudes occupied by the Siebert lake beds in the 
southern portion of southwestern Nevada. Furthermore, the defor- 
mation, by differential uplift, blocked out the mountain ranges as they 
now appear and formed many of the inclosed valleys by broad fold- 
ing or warping. Death Valley was at this time first outlined, though 
it was depressed later, probably in the late Pliocene or early Pleisto 
ene time, by block faulting. 
Extensive erosion followed and before the end of Pliocene time it 
lad reduced the surface to mature and comparatively gentle topogra- 
phy. In restricted areas the Pliocene-Pleistocene basalt appears i 
iavc flowed upon a local peneplain. In late Pliocene time the cli 
nate was moist and a shallow lake probably covered a considerable 
irea in the vicinity of Goldfield. The older alluvium, a formation 
videly distributed over the area, may be considered as deposits of the 
vaning stages of this lake period in the inclosed valleys. Toward th< 
a Spun-, J. E., Trof. Paper U. S. Geol. Survey No. 42, 1905, pp. 52-53. 
