YUCCA MOUNTAIN, IGNEOUS ROCKS. 
Post- Jurassic pegmatite. — A single fragment of quartz-feldspar 
pegmatite was noted in the basalt of the cone S miles north of Rose's 
Well. The basalt in its passage to the surface evidently passed 
through a mass of pegmatite. 
Rhyolite and latite. — The mountain mass, with the exception of a 
number of rather small basalt areas, is composed wholly of rhyolite 
and latite. The series includes siliceous volcanic rocks of varying 
aspect. The brown groundmass in many instances is dense, aphanitic, 
and opaque, and from the presence of flow banding, perlitic parting, 
spherulites, and lithophysa^, is evidently of a glassy nature. Gray 
and black semitransparent glasses also occur. The more altered facies 
have a gray or white dull lithoidal groundmass and the microscope 
proves that this also was once a glass that is now devitrified. The 
phenocrysts are medium sized and usually subordin te to the ground- 
mass; in certain instances they are practically lacking. Slightly 
smoky rounded quartz individuals and glassy unstriated feldspar 
crystals are present in equal development; smaller black or bronze 
biotite crystals are less abundant. In some facies the phenocrysts are 
all orthoclase, but microscopic examination shows enough quartz in 
the groundmass to determine their rhyolite nature. 
Latites a are interbedded with the rhyolites. They are slightly 
darker in color and contain both striated and unstriated feldspar 
phenocrysts in approximately equal development. The single speci- 
men examined microscopically proves to be a clear glass, the brown- 
ish color of the hand specimen being due to abundant curved 
trichites radiating from a common center. Perlitic parting is beauti- 
fully developed. Phenocrysts of orthoclase and plagioclase of 
medium composition are each more abundant than biotite. Wedge- 
shaped titanite crystals, magnetite, and a little hornblende are also 
present. 
Interbedded with the rhyolite and latite flows are porous semi- 
tuffaceous facies. Flow breccias also occur, the inclusions being of 
a slightly different rhyolite than that of the main mass. The cen- 
tral portion of the main east-west ridge is probably the site of a 
vent from which siliceous volcanic rocks flowed. The flows, which 
extended particularly to the south and west, reach a minimum thick- 
ness of 1,000 feet, The series is a continuation of similar rocks of 
the Bare Mountain group. Presumably it is to be correlated with 
the rhyolites of the Bullfrog Hills and the Kawich, Belted, and 
Reveille ranges, and is probably of early Miocene age. 
Basalt.— -In Crater Flat are two large and one small volcanic cone 
and a number of low mesa ridges of basalt, A fourth cone is situated 
a The use of the term latite is here tentative, since no chemical analyses of the 
have been made. 
