Vol. 6] Baker: Cenozoic History of the Mohave Desert. 379 



mountain ridges. The first lava to outflow was a rhyolite, which was 

 followed by monzonites and acidic andesites, in part intrusive bodies. 

 Next was an important and probably long erosion interval, followed by 

 a mighty extrusion of rhyolite, which equals or exceeds in bulk the basalt 

 outflow of late Pliocene and early Pleistocene time. The ryholite 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 this general period that 

 the most important ore deposits in the Tertiary rocks were formed, pre- 

 sumably by waters heated by the still-hot magmas. Between the out- 

 flow of the rhyolite and the formation of the Pahute Lake erosion was 

 probably continuous, partially accounting for the formation of the lake 

 basin which in the main, however, originated through orogenie sinking 

 that was possibly, in part, consequent on adjustments due to the extru- 

 sion of immense bodies of lava, a hypothesis suggested by Spurr. 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 sur- 

 face 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 fossilized wood in the lake beds shows that trees 

 flourished near its shores. While the lake was thus for the most j)art 

 fresh, periods of aridity alternated with those of comparative humidity, 

 and the lake or portions of it were partially dissected, permitting the 

 local precipitation of limestone, gypsum, and boron minerals. Volcanic 

 flows and explosive eruptions of rhyolitic materials 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 defor- 

 mation, which was accompanied and immediately followed by the extru- 

 sion of rhyolite. By this deformation the whole area was uplifted with 

 attendant southward tilting, which accounts for the relatively low alti- 

 tudes occupied by the Siebert lake beds in the southern portion of south- 

 western Nevada. Furthermore, the deformation, by differential uplift, 

 blocked out the mountain ranges as they now appear and formed many 

 of the inclosed valleys by broad folding and warping. Death Valley was 

 at this time first outlined, though it was depressed later, probably in the 

 late Pliocene or early Pleistocene time, by block faulting. 



Extensive erosion followed and before the end of Pliocene time it 

 had reduced the surface to mature and comparatively gentle topography. 

 In restricted areas the Pliocene-Pleistocene basalt appears to have flowed 

 upon a local peneplain. In late Pliocene time the climate was moist and 

 a shallow lake probably covered a considerable area in the vicinity of 

 Goldfield. The older alluvium, a formation widely distributed over the 

 area, may be considered as deposits of the waning stages of this lake 

 period in the inclosed valleys. Towards the end of this period the basalt 

 extrusion reached its climax. Uplift accompanied by normal faulting 

 and folding followed the deposition of the older alluvium, and at this 

 time erosion appears to have been very active. 



In recent time the erosion characteristic of arid lands has partially 



