Page 177 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

174 GEOLOGY AND ORE DEPOSITS OF SIERRA CO., N. M.
the base is the Nodular limestone, 48 feet thick, which consists of nodules of limestone up to 3 inches in diameter surrounded by a soft limestone matrix. Next above this is the Blue limestone or "Footwall Limestone," so called because it immediately underlies the ore bodies. This member is 24 feet thick, and according to Clark it contains 3 to 5 per cent silica and 45 to 50 per cent lime. It is broken in places by faults, which have been filled with a ferruginous, manganiferous and silver-bearing flint. The upper part of this bed consists of a layer of the same kind of flint 1 to 2 feet thick, which is present in practically the entire area of min- ing operations. A few nodules of flint are found in some of the layers of the limestone, but these are more like the flint or chert generally distributed throughout the Lower Paleozoic rocks in this region and not at all like the flint of the faults and of the topmost bed. The top member is a shaly limestone called the Crinoidal limestone, which is about 140 feet thick, and which may be further divided into a lower part 90 feet thick, an intermediate part 25 feet in thickness, and an upper part 25 feet thick. This limestone is very siliceous, containing 35 per cent silica and 30 per cent lime. In it are nodules of flint which can be distin- guished only with difficulty in weathered-out specimens from the flint nodules of the underlying Blue limestone. This limestone is known in the district as the "Hanging-wall Limestone" because it directly overlies the ore bodies, and as the "Crinoidal
Lime- stone" because of the abundant crinoids found •in it at Lake
Valley and elsewhere.
Intrusive Igneous Rocks.—Until recently no cross-cutting
intrusive rocks had been observed in the district, but when the writer last visited it (January, 1933), he observed in two recent underground workings igneous rocks that cut across the sedi- mentary beds and may be parts of dikelike connections with the overlying "Porphyrite" described by Clark. One of these intru- sions is almost directly under the old rock house northeast of the Emporia incline. The sediments above it have been domed up into a small, rounded hill of perhaps an acre in extent, from which a large quantity of ore has been mined. It seems probable° that the doming in this area has been caused by the intrusion of a small mass of igneous rock that came to rest at greater depth than the mine openings have penetrated, and that the dikelike mass and also the surface porphyrite are offshoots from it. The other intrusion is in the Daly drift of the Carolina claim, and it also appears to be a direct feeder for the overlying porphyrite mass. These rocks are too greatly altered to be classified defi- nitely, but they are probably monzonite porphyry or a closely related rock.
Tertiary Lavas.—In the vicinity of Lake Valley, as else- where in the county, great quantities of lava flowed over the sur- face in Tertiary times. These flows were mainly andesite in the