32 SOUTHWESTERN NEVADA AND EASTERN CALIFORNIA. 
The oldest of the Tertiary rocks is a rhyolite which occurs in 
Stonewall Mountain. This occupies a similar stratigraphic posi- 
tion with rhyolites in the Panamint Range, in the Randsburg dis- 
trict, and near Daggett in the Mohave Desert, which, according to 
Spurr/ 1 lie beneath " lake beds which are probably, in part at least, 
Upper Eocene/" This rhyolite in Stonewall Mountain is cut by 
dikes of quartz-monzonite porphyry and quartz syenite, which are 
probably contemporaneous with monzonite porphyry, quartz-mon- 
zonite porphyry, hornblende-biotite lathe, and biotite andesite of 
other ranges. These rocks are in part intrusive masses and in part 
Hows. Prior to the succeeding extrusion of rhyolite these rocks of 
siliceous andesitic and monzonitic composition were eroded, uncon- 
formities being observed in the Kawich, Amargosa, and Cactus 
ranges, in the Bullfrog Hills, and in Shoshone and Skull mountains. 
Then followed an extrusion of rhyolite, with minor siliceous lathes 
and dacites, which was attended by insignificant basalt flows. This 
period of extrusion of rhyolitic lavas, the most important in this 
portion of the Great Basin, was not, however, strictly contempora- 
neous over the whole area: it is believed, for example, that the 
earlier rhyolite of Gold-field represents only the base of the rhyolite 
series of the Bullfrog Mills, and that the upper portions of the Pull- 
frog rnass may have been extravasated while the andesites and 
dacites of (loldlield were solidifying. This rhyolite is everywhere 
separated from the Siebert lake beds by a marked erosional uncona 
formity, and in the Kawich and possibly in the Cactus Range it is 
separated by an erosional unconformity from the succeeding andesi 
ites. The next younger igneous rocks, which are confined to the 
northern half of the area mapped, are basic andesites and dacites. 
These two rocks were found in contact only in the area covered by 
the Goldfield special map, and here Air. F. L. Ransome b found the 
dacites intrusive into the andesites. The succeeding igneous rocks 
are rhyolites and siliceous hit ites and dacites which occur only in 
the Goldfield and Southern Klondike hills and the Silver Peak 
Range. In the latter two localities at least these rocks are inter- 
bedded with the Siebert lake beds without erosional unconformity. 
Thick masses of sediments occur in the majority of the ranges of 
the area, and on lithologic and stratigraphic grounds are correlated 
with the Siebert lake beds of Miocene age at Tonopah described by 
Spurr.' These tuffaceous sandstones and conglomerates, largely com- 
posed of rhyolitic material, reach an observed maximum thickness (in 
the Amargosa Range) of 1,150 feet. 
"«* Spun*, J. E., Jour. Geol., vol. 8, 1900, p. 033. 
h Oral communical inn. 
c Spurr, J. E., Prof. Paper !'. S. Geol. Survey No, 42, 1905, pp. 51 .">.%. 0'.)-70. 
