MOUNT JACKSON AND HILLS NORTHEAST. 
Siebert lake heels. — White, finely bedded tuffaceous sandstones 
and clays, 200 to 300 feet thick, underlie the rhyolite of Mount Jack- 
son and that of the isolated butte 10 miles east of north of it. Small 
areas of the same formation, not shown on the map. occur with the 
rhyolite northeast of Mount Jackson. Rhyolite pebbles occur in 
these sediments. At the more northerly butte they contain also 
thin rhyolite flows and lie unconformably upon the folded Cambrian 
rocks, which were evidently low hillocks in the lake bottom on which 
the sandstones were deposited. On lithologic and structural grounds 
these sandstones are correlated with the Siebert lake tuffs at Tonopah, 
described by Spurr. 
IGNEOUS ROCKS. 
Diorite porphyry. — A dike of dark greenish-gray diorite porphyry 
cuts the Cambrian limestone one-fifth of a mile northwest of the 
Groklfield-Mid way-Bullfrog mining camp already mentioned. The 
like is 20 feet wide and courses N. 35° E. The dike rock, much 
iltered, weathers into iron-stained spheroidal masses. The limestone 
it the contact is slightly indurated, approaching a hornfels. Macro- 
>copically this dike rock resembles pre-Tertiary diorite prophyry of 
he Silver Peak Range. 
Rhyolite. — Rhyolite flows cover a large portion of these hills. 
The rhyolite contains small and scarce phenocrysts of quartz, glassy 
eldspar, and biotite, which hoAvever, in some facies are lacking, 
lie groundmass ranges from pumiceous to compact glasses of con- 
hoidal fracture and varies in color from white through gray to black, 
^erlitic cracks traverse the glass in all directions and reddish-brown 
pherulites are common. The smaller spherulites show both radiate 
nd concentric structure, while the larger, which reach a diameter 
f 8 inches, are traversed by fine wavy lines parallel to a diameter, 
lthough they break into sectors of a sphere. Some of the spherulites 
re simple, although as a rule two or more are linked together. Chal- 
edony is abundantly developed in joint cracks. Thin sections under 
le microscope show spherulites, cavities rilled with gas, perlitic 
racks which may extend through the phenocrysts, flow lines, and 
ther phenomena typical of rhyolites. 
The flow beds are from 2 inches to 50 feet thick, the upper surface 
ping in some instances corrugated like the ropy surface of basalt 
lows. The maximum determined thickness of the rhyolite flows 
400 feet, although the total thickness may be somewhat greater, 
mall isoclinal folds were formed during the outflow of the lava, 
Ihich must have been rather viscous and which on meeting an ob- 
ruction was crumpled. In one case a glassy facies folded with thin 
lands of semiglassy phenocryst-bearing rhyolite exhibits interesting 
° Spurr, J. E., Prof. Paper U. S. Geol. Survey No. 42, 1905, pp. 51-55. 
