636 J. E. SPURR 



In the Sweetwater Range, near Wellington, pyroxene ande- 

 site flows are found intercalated with the sediments of the great 

 Pliocene Shoshone Lake. 



In the Schell Creek, Antelope, and Snake ranges the-pyroxene 

 aleutite, which overlies glassy biotite rhyolite, has filled up val- 

 leys which are probably early Pliocene, and has suffered only 

 slight erosion, resulting in the development of a narrow Pleisto- 

 cene valley, since that time. It can hardly, therefore, be older 

 than late Pliocene. 



On the whole the main period of eruption of the basic inter- 

 mediate lavas appears to have been during the Pliocene, and 

 chiefly the late Pliocene. Most of the great andesitic breccias, 

 indicating widely distributed explosive eruptions, seem to belong 

 to this period. 



5. Basic. — In the Eureka district it is suggested that the 

 latest outbursts, which were of olivine-basalt, occurred in the 

 Pleistocene. 1 



Near Steamboat Springs, which is just west of the southern 

 end of the Virginia Range and in the immediate district of the 

 Comstock lode, the writer observed that the olivine-basalt is 

 probably early Pleistocene, since it has filled the valleys and 

 covered the scarps eroded by the late Pliocene and early 

 Pleistocene Shoshone Lake. 



On the borders of the Pine Nut Range the basalt appeared 

 after the Shoshone Lake had shrunk to the later Pleistocene 

 Lake, and after the country exposed by this recession had been 

 partly dissected into canyons. It is therefore plainly of Pleisto- 

 cene age. 



In the Ralston Desert and in the Reveille and Pancake ranges 

 the basalt overlies the Pliocene sediments, supposed to belong 

 to the Shoshone Lake period. In the Quinn Canyon and Grant 

 ranges the basalt has been poured out into valleys which were 

 probably formed in the late Pliocene period. 



In most of the other localities where this lava has been found 

 there is little doubt that the age is Pleistocene, although some 

 of the eruptions may date back to the late Pliocene. 



1 Mon. U. S. Geol. Surv., Vol. XX, p. 232. 



