BULLFROG HILLS, STRUCTURE. 
Earlier basalt. — Interbedded with the rhyolite -cries in the Bull- 
frog district are several thin flows of basalt, and the latest of these 
is also found in the sin-rounding hills near the boundary of the special 
area. Dacite usually lies on the basalt and is evidently a Inter flow of 
a practically continuous period of extrusion. The basalt « a dark- 
gray or blue-black dense rock containing numerous lath- of glassy 
striated feldspar and here and there a suggestion of an olivine or 
pyroxene phenocryst. Vesicular facies are common, and with the 
introduction of vesicles much of the rock takes on a red tinge. A 
single thin section examined proved to be an olivine basalt, with 
plagioclase, augite, and olivine phenocrysts in a groundmass com- 
posed of glass and tiny crystals of plagioclase, pyroxene, and iron 
ore (hyalopilitic). 
■ Dacite is well exposed 1 mile southwest of Indian Spring. It is a 
purplish-gray rock, with dense groundmass and abundant pheno- 
crysts, including glassy striated feldspar, black mica, and quartz. 
The feldspars very in size from one-sixteenth to 1 inch, the larger 
ones usually showing well-developed zonal growth. The quartz 
crystals reach a maximum diameter of one-fourth inch. Under the 
microscope the groundmass is seen to be a glass containing many tiny 
plagioclase laths (hyalopilitic). Besides the phenocrysts already 
mentioned, augite is present, while aggregates of heavily iron-stained 
serpentine may possibly be altered olivine. 
Later basalt. — Large areas of later basalt cap the rhyolite in the 
northern portion of the hills ami pass into northeastward-dipping 
mesas, which extend toward Pahute Mesa. The basalt, which is a 
dense black rock, locally vesicular, lies upon the eroded surface of 
the early Miocene rhyolite. It is similar lithologically to and ap- 
pears to be an extension of the Pahute mesa basalt, and is, in conse- 
quence, probably of late Pliocene age. 
STRUCTURE. 
The isolated inliers of Ordovician-Silurian rocks arc gently folded 
and somewhat faulted. Upon the major folds minor flexures, in some 
cases overturned isoclinals, are superimposed. The folding of the 
Paleozoic rocks probably occurred largely prior to Tertiary time. 
although the supposed Siebert lake beds appear to he conformable in 
dip with the Lone Mountain limestone. The rhyolite throughout the 
Bullfrog Hills is a mosaic intersected by numerous normal fault-. 
These faults have been worked out in detail by Messrs. Emmons and 
Garrey in the area covered by the Bullfrog special map. The work 
carried on under the direction of Mr. F. L. Ransome -how- that 
predominant faults trend northeast, while a second set trends north 
and south. The later basalt is slightly tilted to the northeast. 
