Page 116 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

MINING DISTRICTS 115
rhyolite occur at several places cutting limestone. Several small masses of rhyolite in the region (see figure 8) consist of small intrusions, or residual patches left by erosion of flows, or the tops of dikes, only the uppermost irregular parts of which have as yet been bared by erosion.
A large mass of monzonite porphyry, the largest in the county, judged by the extent of its surface exposure, has intruded across and between the beds of Pennsylvanian age in the Sierra Cuchillo. From the northern end of the sedimentary series for a distance of nearly 8 miles along the range, it appears in the western face as a thick sill that separates the Pennsylvanian limestone into an upper and a lower part approximately equal in thickness. At the south end the sill thickens in a short distance to laccolithic proportions, and here it has broken up through the upper beds of the Magdalena formation and the Permian rocks and forms the crest of the range. As a result of its greater re- sistance to erosion, the monzonite remains as a bold rocky sum- mit and is a prominent landmark for miles around. The sill where exposed at the base has the same dip to the east as the sedi- mentary strata. The sill-like portion of the monzonite is not everywhere of equal thickness, and near the Black Knife mine in the saddle north of Cuchillo Peak, a depression in the upper sur- face of the sill has preserved a residual mass of limestone. (See figure 8.) At the base of the sedimentary column and just visible in the bottom of the arroyo for a distance of 150 feet or more, an outcrop of fine to moderate-grained dark gray to greenish-gray basalt was noted. The nature of this occurrence is uncertain, al- though it is probably a dike.
STRUCTURAL FEATURES
The Sierra Cuchillo is a monoclinal block having the scarp face to the west and a gentle dip slope of approximately 25° to the east. This large faulted block is probably the local expres- sion of a continuous line of breaks along which elevation and tilt- ing has occurred, and which extends from the north end of the county through the Hillsboro and Lake Valley areas to the south. At Palomas River in the southern prolongation of the Sierra Cu- chillo, a small block of Magdalena limestone and Abo sandstone projects through the extrusives with dip and strike similar to that of the main mass of the range. Locally, cross-fracturing has occurred in the tilted block along tear faults or planes of ad- justment within the block. One of these cross faults marks the course of the Hot Springs-Fairview highway across the range, and several others are known, especially in the southern part of the range where the sill thickens into a laccolith, and where the thickening has caused a series of east-west faults to develop in the limestone above and to some extent below the intrusive rock. It is probable also that the contact between the Abo sandstone and the younger Chupadera beds is marked by a northwest-