Page 75 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

74 GEOLOGY AND ORE DEPOSITS OF SIERRA CO., N. M.
basal beds of the overlying Chupadera (Permian) formation. Palomas gravel of Quaternary age abuts against the older sediments low down on the eastern slope of the range, and con- stitutes part of the bolson deposits that fill the valley to a width of 5 or 6 miles between the Black Range and the Sierra Cuchillo
to the east.
EXTRUSIVE ROCKS
The following paragraphs, which give an excellent descrip- tion of the extrusive andesites of the Black Range, are quoted from Gordon's description37 of the Chloride district.
A great thickness of andesite, consisting of flows, tuffs, and breccias, extends all along the top of the Black Range, and except where removed by erosion, reaches down into the valley, being there covered by the Palomas gravel. The higher ridges are capped usually by rhyolite and rhyolite tuffs, deposited evidently on the eroded surface of the andesite. The lower por- tions of the andesite where revealed by erosion along the bottom of the can- yon above Chloride are altered to a grayish-green rock, but at the top of the range the rock is notably fresher and darker in appearance.
The constituents of the altered andesite determinable under the micro- scope are plagioclase and augite in a groundmass composed mostly of cal- cite and chlorite, with some epidote and very little serpentine. There is us- ually a notable amount of magnetite, though it is in many places changed to pyrite. Veinlets composed of calcite, pyrite, epidote, and adularia are abundant.
These propylitic andesites are characteristic of the Black Range region and have a large development along the eastern face of the range. The same alteration has affected the tuffs and breccias, which present a characteristic grayish-green mottled appearance on exposed surfaces and constitute vast deposits over the north end of the range.
A period of erosion intervened between the extrusion of the last of the andesites and the first of the rhyolites, as shown by the presence of rhyolite in the deep erosional channels cut into the andesite tuff west of Chloride, and by the presence of lenses of detrital matter here and there between the top of the andesite and the base of the rhyolite. The rhyolitic material consists largely of tuffs, though some breccia and flow rocks are present. The tuff is notably porphyritic, showing small fragments of crys- tals of orthoclase and larger phenocrysts of corroded and broken quartz, along with fragments of plagioclase (andesine) and a few of biotite. The groundmass for the most part is glassy, in places with spherulitic development, but here and there it is partly crystalline. In places the phenocrysts predominate over the groundmass, and the rock in hand specimens has a gran- ular appearance. The rhyolite flows are less extensive on the east side of the range than are the andesites. As a result of faulting and stream erosion, rhyolite remains only as cappings on the ridges or as masses filling old depressions in the surface of the andesite.
Remnants of Quaternary basalt occur in the valley to the east of Chloride, this rock forming residual masses capping small buttes and mesas which have been carved out of the original bol-
37 Gordon, C. H., op. cit. (U. S. G. S. Prof. Paper 68), p. 262.