Page 100 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

MINING DISTRICTS 99
and are not greatly faulted. To the east of the axis of the range, however, they are much broken by faulting and in general have prevailing dips to the east. The dip of the basal andesite coin- cides closely with that of the underlying sediments, indicating that the early extrusions had been poured out before much faulting had taken place in the region. On the other hand, the dips of the rhyolitic flows may or may not coincide, depending upon the amount and direction of movement that had occurred before these had been poured out. The sequence, thickness and general character of the various flows, tuffs and breccias is quite similar to that found elsewhere along the east slope of the Black Range.
A monzonite porphyry intrusion cuts the sedimentary rocks west of the town of Kingston. This mass of porphyry is 400 feet wide in the canyon just west of the town and can be traced to the north and south for 2 miles or more in each direction. In general outline the mass resembles a dike of very irregular shape, but in the field the writer gained the impression that it was a small, elongated stock, and that with depth it would increase in size. The monzonite porphyry is very similar in appearance to that of the Hillsboro district, 9 miles to the east, and it is almost cer- tainly of this same general period of activity, if not a part of the same deep-seated magma.
STRUCTURAL RELATIONS
The crest of the Black Range is about 3 miles west of Kings- ton, and the various sedimentary formations and overlying ex- trusive rocks west of the crest dip gently to the west and appar- ently are not faulted. To the east of the crest, the range•presents a fault scarp that has exposed the entire sequence of rocks from the pre-Cambrian basement complex to the last of the rhyolite extrusives. East of this fault scarp the east limb of the original arch is cut by a series of north-south faults, forming many long, narrow slices, all of which have been depressed relative to the west limb of the arch, but which have been tilted and moved among themselves into an intricate pattern of small fault blocks. The faulting occurred after the arching of the range, and it was apparently accompanied and preceded by flows of andesite and the intrusion of the monzonite porphyry. The locus of these flows and intrusions appears to have been where the east flank of the main arch had been further warped into a number of minor folds, which finally developed lines of weakness when com- pressional stresses were relieved and along which normal faults developed, and it was through these fractures that the first ande- sitic extrusions reached the surface. In this favorably prepared zone, the later intrusions of granite-porphyry and monzonite por- phyry forced their way upward to near the base of the still- heated andesite flows, and in places to within the lower part of the extrusive rock. With the crystallization of these deep-seated