34 SOUTHWESTERN NEVADA AND EASTERN CALIFORNIA. 
Before the deposition of the Siebert lake beds ceased rhyolites and 
siliceous latites and dacites outflowed. In some of the ranges these 
are folded with the lake beds; in others they lie upon the tilted 
and eroded surfaces of the sediments. During and immediately after 
the extrusion of these siliceous rocks tuffaceous sediments were 
deposited in several of the ranges in the northwestern part of the 
area, and these are in part probably equivalent to the Humboldt 
series, the deposits of the Shoshone Lake (late Pliocene) of King." 
The older alluvium, so widely distributed, particularly in the inclosed 
vallev>. represents ancient playa deposits and alluvial slopes of what 
was probably the Pleistocene residuum of this lake. The Sarly stages 
of Shoshone Lake 1 were followed immediately by basalt flows which 
cover wide areas in southwestern Nevada and eastern California. 
In several of the ranges these basalts were preceded by basic pyr- 
oxene andesites. While the basalt is thus largely of late Pliocene 
age, it- extrusion began during the deposition of the Siebert lake 
Vn^\>. basalt flows occurring near the top of these sediments in the 
Amargosa Range and the Goldfield hills.'' and probably in the Moni- 
tor Hills. Some of the basalt cones an 1 remarkably fresh and are 
certainly of Pleistocene age. and it is by no means impossible that 
volcanism has not ceased in this portion of the Basin Range. 
The succession of the lavas of the Great Basin lias recently been 
treated at length by Spurr. c His succession for the petrographic 
province of the Great Basin and that of the writer for the portion of 
that province here treated are practically identical. Both assign 
the first rhyolite to the end of Cretaceous and to Eocene time. It is 
believed, however, from the length of time indicated by the complex 
history of extrusion and erosion intervening between this rhyolite and 
the deposition of the Sieberl lake \hh\>* that the monzonites and acidic 
andesites may well also be in part Eocene, while the second rhyolite is' 
believed to be largely of early Miocene age. The andesite and dacite 
are also of comparatively early Miocene age. while the third rhyolite 
covers the middle and late Miocene and early Pliocene. The basalts 
range in age from late Miocene to Pleistocene, the major extrusions 
occurring in late Pliocene and Pleistocene time. As to the genetic 
relations of the magmas, the writer is wholly in accord with the views 
of Spurr in the article already cited. He believes that the Tertiary 
lavas of the Great Basin are the representatives of two complete 
cycles of the differentiation of a magma of medium composition into 
acidic and basic lavas and that probably the end of a still earlier cycle 
is also represented. 
"Kin-, Clarence, U. S. Geol. Explor. 4<Uh Tar., vol. 1, 1878, pp. 4:54, 456. 
» Oral communication from Mr. l\ L. Ransome. 
c Spurr, J. E., Jour. Oeol., vol. 8, 1900, pp. 621 646. 
