Page 62 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

MINING DISTRICTS 61
writer is inclined toward the belief, without being able to prove the point, that it is the top of an elongated cupola or small stock, which may enlarge rapidly with depth. Monzonite is present in the Tierra Blanca district as an intrusion cutting Paleozoic sedi- ments.
In the Carpenter mining district the intrusive rock is a granite porphyry, which forms dikes of large size near the mines. About half a mile west of the Grandview mine, an elongated stock of granite porphyry forms a prominent ridge which extends in a northerly direction for nearly 3 miles. North of the mine a dike of granite porphyry 20 feet wide and striking N. 40° W. cuts across the limestone. A few dikes of a diabasic rock are known in the district, but these appear to be much younger than the period of metallization.
Although the proportion of the total surface area occupied by intrusive rocks in the Black Range is very small, there is con- siderable evidence indicating that much larger bodies of these rocks have come to rest at relatively shallow depths below the present surface, and that the ore deposits of the region formed during the final stages of the solidification of these masses and are closely related to them.
EXTRUSIVE ROCKS
The Black Range, except for its eastern edge, is almost entirely covered with a thick series of Tertiary flow rocks, which in places is as much as 3,000 feet thick. The sequence of these flows is, in general, brecciated andesite, andesite flows, and alter- nating thin layers of andesitic ash 'and of sedimOnts derived from the andesites in the intervals between the various flows. Within this series of andesites are numerous dikes and sills of latite and latite porphyry. Following the andesites and latites came a thick series of rhyolite flows, tuffs, and breccias, which varied in the order of their occurrence from place to place along the range. Near Kingston and to the north, tuffs and breccias were apparently laid down first upon the andesite. Flows of rhyolite followed, and finally these were covered with thick beds of white rhyolitic ash. South of Kingston and down to Lake Valley, wherever erosion has developed canyons that cut deep into the flow series, it was noted that the first of the rhyolite series is a flow with eutaxitic to finely porphyritic texture, having a glassy obsidian base that varies from 2 to 15 feet in thickness. Following this came an alternation of tuffs, agglom- erates and flows, and finally at the top a thick series of flows, these surmounted by a thick bed of white ash. Dikes of rhyolite, possibly the feeders for these flows, cut the andesites. Many of these dikes have a glassy texture, and glassy borders are a characteristic feature. A few basic dikes cutting the flow rocks are considered to be of Quaternary age and associated with the basaltic flow rocks that are intercalated with or cover the Ter-