Page 176 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

MINING DISTRICTS 173
which in many places is drusy and lined with clear quartz crystals. In the main the quartz is milky and granular, and where the breccia fragments have been replaced by this material the rock is locally called quartzite, which it greatly resembles. Brecciation of the original limestone has extended for some dis- tance from the fault along the top of the beds, and this part of the formation has also been recemented and replaced with silica. Occasional fossils are found, notably one or more species of Favosites that usually have been converted to silica, the original interior open spaces being filled with this material while the walls have been dissolved away. Where erosion has exposed the Fus- selrnan limestone, great boulders of the silicified rock are strewn over the surface, and rough, craggy outcrops occur on the dip slopes or project through the shallow detrital material that covers the lower slopes.
Percha Shale.—This formation is 160 feet thick in the Lake Valley district and is separated from the underlying and over- lying formations by breaks in sedimentation, although accordant with them in attitude. It consists of a lower part of dark gray to black limonite-stained thin-bedded shale 100 feet thick, and an upper part of greenish-gray shale 60 feet thick, which is fossiliferous in places and contains thin layers of slabby and nodular limestone. The fauna of the upper part occurs princi- pally in the limy beds and is regarded as being of late Devonian age. No fossils have been found in the lower beds, and it is not possible to say what proportion of the entire Devonian system these beds represent. Erosion of the soft shale has formed a valley from 500 to 1,000 feet wide on the dip-slope side of the underlying Fusselman limestone, along the contact of which the highway northward to Hillsboro follows for a distance of about 3 miles. In places the base of the Percha shale has been silicified and converted to a red, jaspery rock containing many vugs lined with clear quartz crystals. It is believed that the same solutions that silicified and replaced the topmost brecciated bed of the Fusselman limestone also replaced and jasperized the lower beds of the Percha shale. At other places the base of the shale member appears to be unaltered, but below it and above the silicified limestone, there is red limestone 12 feet thick, which may be Devonian. In a shaft which was sunk through this formation, nodules of iron pyrite about the size and shape of potatoes were observed near the base of the shale member.
Lake Valley Limestone.—The Lake Valley limestone is 210 feet thick in the local section, and although accordant in attitude with the Percha shale, the two formations are separated by an erosional unconformity. The known Lake Valley ore deposits occur almost exclusively in the Lake Valley limestone. In the district this formation can be divided into three members. 53 At
53Clark, Ellis, op. cit., pp. 140-142.