Page 70 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

MINING DISTRICTS 69
posures this flow banding dips at angles which indicate consid- erable movement subsequent to the extrusion. Much erosion of the rhyolite is also indicated prior to the laying down of the later tuffs, agglomerates and sandstones. In some places beds of rhyolite stand vertically and are surrounded by horizontal beds of the tuffs and other rocks ; in other places the vertical beds of rhyolite are surrounded by tuff that is itself slightly dipping. In the caves of Taylor Creek, there is evidence that they were formed by the arching of successive rhyolite flows.
White, buff and red sandstones, white rhyolite tuff, and conglomerate composed of purplish rhyolite pebbles overlie the rhyolite flows and are visible over large areas on the surface and in the walls of the canyons.
Small areas of the region are covered by flows of basalt that lie either directly on rhyolite or upon the later tuff series. This rock is a black olivine basalt, which in some exposures is dense and in others vesicular. The material is largely glassy, although generally a few crystals of long, slender plagioclase and other minerals may be seen.
STRUCTURAL RELATIONS
There was apparently much folding and faulting of the rhyolites prior to the deposition of the tuff and sandstone. From Indian Peaks in a south-southeast direction the rhyolite forms a compressed fold about a mile wide, which can apparently be traced as far as the Inman ranch in the southeast corner of the district. In general the tuffs and related rocks are essentially horizontal and have not been disturbed to any considerable extent, except along the flanks of the folded rhyolite, where some movement subsequent to their deposition has taken place along the axis of the fold. The tuff series was deeply eroded before the basalts were extruded, along lines that are parallel to the courses of the present streams in the area. When the basalt was extruded it completely filled a deep canyon along the present Beaver Canyon and sent tongues up the old canyons now known as Railroad, Kennedy, and Corduroy canyons. In places it spilled out over the mesas in thin flows, as between Indian and Taylor creeks. No folding or faulting later than the extrusion of the basalts is known to have occurred.
ORE DEPOSITS
Cassiterite, accompanied by specular hematite, occurs in
veinlets in the soft, altered rhyolite at several localities in the district, particularly in the belt of folding which extends between Indian Peaks and Inman's ranch. Another area is in Taylor and Whitetail creeks, in which over 500 feet of older rhyolite and younger tuff and sandstone are visible in the walls of the canyons. The rhyolite is cut by a network of narrow nonper- sistent fractures in all of the exposures noted. Where the frac-