76 SOUTHWESTERN NEVADA AND EASTERN CALIFORNIA. 
of iron- stained glassy olivine. Vesicular facies occur at the top and 
bottom of the two main Aoavs that compose the 350 feet of basalt. 
With the introduction of vesicles much of the rock takes on a 
magenta-red color. The vesicles, which reach a maximum length of 
2 inches, are usually elongated parallel to the flow bedding. Some 
of them arc 1 Idled by white granular calcite and others by chalcedony. 
Certain bands contain in a basalt matrix many fragments of basalt, 
indicating that sonic portions of the rock solidified before cessation of 
movement in the magma. Surface- of Hows are often ropy. 
Under the microscope the groundmass is usually seen to be holo- 
crystalline and composed of plagioclase laths, angite columns, and 
magnetite grains. Rarely a little dark glass is also present, and in 
other thin sections large areas of augite inclose plagioclase laths in 
the manner of the ophitic texture. A reddish-brown serpentine is 
developed at the expense of olivine, and a little calcite is formed from 
plagioclase. 
The basalt forms a typical mesa, and buttes isolated in the desert 
gravels have been carved from it by erosion. This rock is to be con-e- 
lated with other basic (low- widely distributed throughout the area 
surveyed, which are presumably of late Pliocene or early Pleisto- 
cene age. 
STRUCTURE. 
As Ransome has shown, the Goldfield hills have a domical struc- 
ture, the oldest formations being confined to the central portion of 
the hills. He further -how-" that the older Tertiary rocks have 
been subjected to considerable folding and some faulting. The Sie- 
bert lake beds usually dip away from the central portion of the 
Goldfield hills. Even the later basalt has 1 >;-*mi subjected to tilting, in 
the main away from the center of the dome, and also to some normal 
faulting. These faults have usually an east-west or north-south 
strike. 
ECONOMIC GEOLOGY. 
The ore deposits of Goldfield have been treated at some length in 
the bulletin already mentioned.'' Outside of the special area few pros- 
pects exist. The andesite and rhyolite 3 miles southeast of Preble 
Mountain have been altered similarly to those of the productive 
territory in Goldfield, and the area has been located by prospectors. 
ALKALI SPRING FLAT. 
The broad valley north of Montezuma Peak is separated from Big 
Smoky Valley by a gravel -covered divide. The valley has gently 
sloping sides, except on the west, where it rises rather steeply to the 
Lone Mountain foothills. The lowest portion, 4,850 feet above sea 
« Ransome, F. L., Bull. U. S. Geol. Survey No. 303, p. 21. 6 Idem, pp. 22-.'?9. 
