Page 25 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

24 GEOLOGY AND ORE DEPOSITS OF SIERRA CO., N. M.
Associated with the granite are numerous dikes of fine-grained granite, aplite and pegmatite ; other dikes are of a more basic character. Good exposures of fairly fresh granite occur along the western base of the Fra Cristobal Range and the Sierra Caballos and in the Black Range near Kingston. In places the granite is undergoing rapid disintegration to a coarsely granular surface deposit of sedentary arkose. This is particularly true where the exposures have been eroded down to gently sloping surfaces. Near Pittsburg the granite is associated with much more hornblende schist and gneiss. It is cut by numerous acidic and basic dikes and is intersected at short intervals by quartz veins, most of which have a northwest trend. West of Kingston granite outcrops for nearly 12 miles and is but little decomposed or disintegrated. It is cut in different directions by dikes of greenstone, aplite, pegmatite, etc.
SEDIMENTARY ROCKS
CAMBRIAN SYSTEM
Bliss Sandstone.—Lying unconformably upon the eroded
and irregular surface of the pre-Cambrian rocks is quartzite or sandstone of Cambrian age. In the type section in the Franklin Mountains north of El Paso, Tex., the formation is known as the Bliss sandstone, but in the Sierra Caballos in Sierra County it was named the Shandon quartzite. The name Bliss has prece- dence and is used in this report. Near Pittsburg (Shandon) the Bliss sandstone has a thickness of 55 feet. Here the usual basal bed of dark-brown quartzite is missing from the section. In other parts of the range the formation is 1.00 feet or more in thickness. Near Kingston the granite is overlain by 75 feet of dark red quartzite with a 3-foot bed of shale near the middle. In general the basal beds are mostly quartzite, with conglomerate locally present, while the upper beds are softer and finer grained.
ORDOVICIAN SYSTEM
The strata of Ordovician age comprise the Lower Ordo-
vician El Paso limestone and the Upper Ordovician Montoya limestone.
El Paso Limestone.—The El Paso limestone in the lower part is somewhat similar to the underlying Bliss sandstone. At