Page 128 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

MINING DISTRICTS 127
stricted to two or three small residual patches on the dip slope of the Fusselman limestone, and to a larger area about half a mile south of the Rio Percha and about due south of the S. J. Macy vanadium mine. Where this formation overlies the silici- fied Fusselman limestone, the base is silicified and converted to a form of jasper, probably by the same solutions that silicified the underlying limestone. North of the area mapped, Percha shale is exposed in a faulted block of Paleozoic sediments, but this occurrence was not studied in detail.
Lake Valley Limestone.--The Lake Valley limestone of Mis- sissippian age, occurs in only one place. South of Percha Creek a small patch of it rests unconformably on Percha shale, but only the Crinoidal beds, the topmost of the three beds distinguished at Lake Valley (see page 174) have been found. The absence of the Nodular and Blue limestone may be due to erosion prior to the deposition of the Crinoidal beds. North of the area mapped the Lake Valley limestone overlies the Percha shale with about normal thickness.
Tertiary Gravels and Agglomerates.—Gravels and agglom- erates of Tertiary age, showing pronounced stratification and in places interlayered with thin flows of glassy rhyolite, fill the de- pressions and make the terraces between the Rattlesnake mine and the town of Hillsboro, and along the main fault of the dis- trict they lie in contact with the high scarp composed of Ordovi- cian and Silurian limestone.
Palomas Gravel.—On the outskirts of the district Palomas gravel is rather sparingly developed as alluvial fans and as thin coverings over the high terraces. In the arroyos and creek beds recent sediments have accumulated, and in the flood plains of the Percha and the Animas they are quite extensive.
IGNEOUS ROCKS
Andesite Breccia and Andesite.—Resting upon the Lower Paleozoic sediments is a mass of brecciated rock called andesite in this report, although Gordon 47 reports an analysis by George Steiger, showing that it is high enough in potash to be classed as a latite. In part this rock is brecciated and in part it preserves the flow characteristics, but the contact between the two phases is very irregular and indistinct. The impression was gathered in the field that the extrusion occurred in several surges, and that at each surge the cooler portions were cracked and brecciated, but that the last portion to be extruded maintained in part its original massive structure. Under the microscope these later por- tions of the andesite are seen to vary in texture between a typical flow structure with glassy groundmass, and a microcrystalline trachytic groundmass. The phenocrysts are chiefly plagioclase and augite. In some parts of the field the phenocrysts are easily visible to the naked eye, and on some of the augite cross-sections,
47Gordon. C. H., op. cit. (U. S. G. S. Prof. Paper 681, p. 275.