File:Annual report of the United States Geological Survey to the Secretary of the Interior (1895) (14778239931).jpg

Identifier: annualreportofun1711geol (find matches)
Title: Annual report of the United States Geological Survey to the Secretary of the Interior
Year: 1880 (1880s)
Authors: Geological Survey (U.S.)
Subjects: Geological Survey (U.S.) Geology Water-supply Forest reserves Mines and mineral resources
Publisher: Washington : G.P.O.
Contributing Library: University of Illinois Urbana-Champaign
Digitizing Sponsor: University of Illinois Urbana-Champaign

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area mapped as uralite gabbro may not be a strict geological unit, butthe general decomposed condition of the rock would render any furtherseparation extremely tedious and unsatisfactory at best. lYHOXEXITE. Typical pyroxenite, i. e., a rock with granular structure, composedchiefly of pyroxene, was not noted within the area studied. About 3miles northeasterly from Coulterville, just north of the road to Dudleys,there is a small area of interesting rock (No. 1174 S. N.) made up almostentirely of hornblende and augite. The hornblende forms conspicuousrounded crystals of dark-greenish color, lying in a gray-green granularmatrix of augite and hornblende. On fresh tracture the larger hornblendes show brilliant cleavage faces, and are sharply separated fromthe finely granular matrix. The microscope shows that the large hornblendes are pale-brown by transmitted light, with rather faint pleo-chroism, and are full of inclusions of the same colorless augite whichmakes up most of the groundmass.
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TTJRNrK.) DIABASE AND PORPHYRITE. 671 The hornblende in the grounduiass frequently shows sharply idio-morphic cross sections, as do also the augites included in the porphyritichornblende. A small amount of clear quartz was observed, filling spacesbetween the partly idiomorphic crystals of augite and hornblende. PERIDOTITE. No areas of peridotite are indicated upon the geological map, but itoccurs occasionally in small isolated masses or loose fragments in thelarge serpentine area west of Chinese Camp. A specimen (No. 1054S. N.) taken from a small outcrop in the serpentine, about half a milesouthwest of the latter town, is a dark gray-green, heavy rock whichthe microscope shows to be a fresh granular aggregate of olivine anddiallage, forming a wehrlite. PRE-CRETACEOUS EFFUSIVE IGNEOUS EOCKS.DIABASE AXD PORPHYRITE (GREENSTONE SERIES). A very characteristic feature of the foothills of the Sierra Nevada,especially in the area of the Sonora, Oakdale, and Jackson sheets, areprojecting slabs

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