BELTED RANGE, IGNEOUS ROCKS. 123 
Chalcedonic quartz, in considerable quantities, is deposited in the 
cavities of the sediments in this vicinity. The conglomerate near 
Wheelbarrow Peak incloses many bowlders of earlier rhyolite from 
3 to 4 feet in diameter. Such bowlders protect the softer sandstone 
immediately beneath from erosion, and in consequence they form 
in places the caps of columns 20 feet high. 
These beds appear to be contemporaneous with those of Pahute 
Mesa and Shoshone Mountain, and like them are considered the 
equivalent of the Siebert lake beds." of Miocene age, at Tonopah. 
IGNEOUS BOCKS. 
Monzonite. — Included in the granite near Oak Spring are frag- 
ments of a fine-grained, dark-gray, granular igneous rock. On micro 
scopic examination this proves to be a hornblende-biotite monzonite. 
Titanite and magnetite are accessory minerals. 
Granite. — A stock of granite, approximately three-fourths of a mile 
in diameter, cuts the Pennsylvanian limestone -2\ miles south of Oak 
Spring and sends many apophyses into it. The granite, forming a 
dome, set with many exposures in blocklike masses, rises above the 
near-lying limestone. In the eastern portion of the area it has a 
sheeting dipping eastward, parallel to the limestone contact. The 
granite tends to weather into spheroidal masses, although some of the 
joint blocks have their corners but slightly blunted. 
This rock is a light-gray porphyritic granite, characterized by 
unusually large feldspar and quartz phenocrysts, which lie in 
a medium- to coarse-grained matrix of white feldspar and biotite. 
Quartz is an unimportant constituent of the groundmass, the silica 
having largely separated out in the older crystals. Phenocrysts of 
pale-pink feldspar, of perfect outline, reaching a maximum length 
of 4 inches, are the most prominent, in places forming one-third of the 
jrock surface. The feldspar phenocrysts inclose minute blades of bio- 
tite zonally arranged. The slightly smoky quartz phenocrysts are 
perhaps more numerous, though smaller than those of feldspar, their 
maximum diameter being one-half inch. Phenocrysts of mica 1 inch 
in diameter are rare. Both feldspar and quartz phenocrysts pro- 
trude on weathering and finally drop out. The granite is in places 
deeply weathered. The smaller feldspars are more kaolinized than 
the large crystals, while the biotites are bronze-brown in color and are 
surrounded by hematitic stain which has separated from them. Un- 
der the microscope this rock proves to be a biotite-hornblende gran- 
ite, containing considerable plagioclase and hence leaning toward 
quartz monzonite. The texture is granular and hypidiomorphic, on 
account of the presence of plagioclase laths in the groundmass and of 
" Spurr, J. E., Prof. Paper U. S. Geol. Survey No. 42, 1905, p. 51. 
