Page 129 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

128 GEOLOGY AND ORE DEPOSITS OF SIERRA CO., N. M.
which measure as much as three-quarters of a millimeter across, differential weathering along the basal parting planes has devel- oped a radial design of alternating dull and glistening triangular segments. The plagioclase crystals are white, glistening and stri- ated, and are smaller than the augites, being about half a milli- meter in length. In general the rock is dark gray to greenish gray, but where brecciated, and especially along fractures, pro- pylitizing solutions have converted the groundmass in large part to green chlorite. Some of the breccia fragments have a purple color. These andesites vary in thickness, and in places they have been deeply trenched by stream action, but it is probable that originally they averaged 1,500 feet or more in thickness. A period of quiescence and slight erosion appears to have inter- vened between successive flows of the andesite, as is indicated at two places in the district by the presence of thin beds of sedimen- tary material composed entirely of fragments of andesite, which occur as irregular lenses filling old erosional depressions on the surface of these flows.
Latite and Latite Porphyry. — These rocks include the "birdseye porphyry" flows, remnants of which cap the hilltops north of the Rattlesnake and Opportunity mines and the hilltop near the Caballero workings ; the radiating dikes of the region with which the mineralization is so closely associated ; and the deeper seated portions of these dikes, which have a distinctly porphyritic appearance and much resemble the monzonite por- phyry stocks of the area. They are classed as latites and latite porphyries in order to distinguish them from the more basic rocks which enclose and underlie them, and which have been re- ferred to above as andesites.
In hand specimens the latitic rocks have distinctive appear- ances. In the dikes at the higher geologic elevations in the south- ern part of the district, the rock is gray to salmon colored and contains a few sparsely scattered small phenocrysts of flesh- colored feldspar in a dense aphanitic groundmass. Oxidation has altered the rock and stained it with bands of yellow limonite, so that the dikes have a distinctive light yellow color as viewed from a distance, in contrast with the darker colors of the enclos- ing andesites. Where these dikes have spread out into small sill- like masses within the andesites and where they have formed surface flows, the appearance is different, due to more massive form, greater thickness, protective covering, and consequent slower cooling and crystallization. In these masses the rock is distinctly porphyritic and predominantly brown to gray in color. Phenocrysts consist of white to flesh-colored orthoclase with per- fect crystal outlines measuring as much as an inch in length, corroded almond-shaped phenocrysts of white plagioclase from which the name "birdseye porphyry" is derived, numerous small specks and crystals of hornblende, and in some phases a few glistening black books of biotite. The groundmass is dense and