Page 36 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

GENERAL FEATURES 35
than a monzonite. In the Sierra Cuchillo a large sill of mon- zonite porphyry has invaded beds of Magdalena limestone and is exposed for a distance of approximately 9 miles along the west- ern escarpment of this faulted block of sediments. This sill thickens toward the south to almost laccolithic proportions, and as a result of erosion now appears as the crest of the range. At Hillsboro several bodies of intrusive rock have been exposed by erosion. These are intrusive into the lower part of the Tertiary andesitic flows that cover the district. Northeast of the town is a prominent hill composed in large part of a coarse-grained mon- zonite which grades into a granodiorite along its eastern border. Numerous small, irregular stringers of aplite traverse this intrusion. Approximately a mile east of this cupola is a dike of medium-grained monzonitic rock. Two miles north of these out- crops, in the Copper Flat basin, is an area of intrusive porphy- ritic monzonite, which is undoubtedly a stock and probably con- nected at no great depth with the other outcrops in the district. This rock is coarse grained and contains large feldspar crystals, many of which are visibly twinned according to the Carlsbad law. Plagioclase is present in smaller grains but in slightly greater quantity than the orthoclase. Hornblende is the prominent dark mineral with some biotite and augite, and locally quartz. Other accessory minerals are magnetite, apatite and zircon. A small area of porphyry was noted in the west face of the Fra Cristobal Range adjoining one of the faults of that region, and west of Cutter on the east slope of the Sierra Caballos a small mass of similar rock has penetrated the Cretaceous beds. The "porphy- rite" of the Lake Valley district is ,considered by the writer to be possibly of monzonitic derivation, and at numerous places in the county small dikes of monzonite and latite suggest the presence at no great depth of other large masses of monzonite or related intrusive rocks.
STRUCTURAL FEATURES
During the period of Tertiary igneous activity and perhaps for a considerable length of time before and after, faulting occurred in the region on a large scale. Lindgren20 states that this was a period of general continental uplift, which was accompa- nied by the formation of general north-south breaks in the earth's crust of sufficient magnitude to outline the principal mountain ranges of the county. With the exception of the Black Range and the San Mateo Mountains, all the mountains of Sierra County give the general appearance of being monoclinal blocks with steep fault scarps on the western front and with gentle dip slopes to the east. In the western part of the county the Black Range forms a broad northward-trending anticlinal dome with the strata dipping away on each side. This anticline has been
20 Lindgren, Waldemar, op. cit. (U. S. G. S. Prof. Paper 68), p. 25.