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Title: The Tertiary Gravels of the Sierra Nevada of California by Waldemar Lindgren (1911) Collection Type: Books and Periodicals Source(s): Google Books # of Pages: 301 Download PDF: User Login Required Download OCR Text: User Login Required

Date Scanned: 2024-07-14 06:26:22 Item Sourced From: PDF First Created: 2025-03-17 05:53:20 Last Updated: 2025-03-17 05:53:20

Original Held At: Searls Historical Library Location: 161 Nevada City Hwy, Nevada City, CA 95959

Page: of 301  

126 TERTIARY GRAVELS OF THE SIERRA NEVADA OF CALIFORNIA. or bluish color and contains much secondary pyrite or marcasite, locally auriferous; streaks of reddish gravel also occur in the deeper parts of the mass. Nearer the surface the gravel is generally reddish. Fluviatile stratification is of extremely common occurrence. Very little gravel occurs in the Banner Hill area, though the lower parts of now largely eroded Neocene stream channels doubtless contained much of it. The Grass Valley area and the southern part of the Nevada City area also contain little gravel. The largest accumulations are found north of Nevada City, in the deepest parts of the ancient stream system, where they reach & maximum thickness of 175 feet at the Manzanita hydraulic cut (Pl. II, A, p. 20). The banks of Cement Hill show 60 feet of well-washed gravel, with excellent fluviatile structure. RHYOLITIC TUFFS. Above the auriferous gravels lie, in the deeper parts of the depressions, a series of lightcolored or white clayey or sandy rocks, more or less perfectly consolidated, commonly described as pipe clay and sand. These are largely rhyolitic tuffs, more or less pure. Certain of the beds consist almost exclusively of minute fragments of glass; others are so admixed with mainly granitic detritus as nearly to mask their tuffaceous character. The fragments both of glass and of granitic minerals are generally very sharp and angular. Bodies of gravel are also included in the tuffaceous series, and, on the whole, it is impossible to draw a distinct line between the auriferous gravels and the rhyolitic tuffs. On the southern face of Cement IIill the line between the two formations is fairly sharp, separating 60 feet of gravel from over 200 feet of rhyolitic tuff. A little rhyolitic material is found in the sands of the main channels down to a distance of 40 feet, or even less, from the bedrock. The rhyolitic tuff is practically confined to the northern part of the Banner Hill and Nevada City tracts. The purest tuff has very nearly the composition of a rhyolite. Grains and flakes of a brownish, translucent mineral, with faint double refraction, are abundantly developed, especially in the rocks poor in alkalies. This is undoubtedly the kaolin mineral recognized by H.-W. Turner in his Ione sandstone.! , At the Cement Hill diggings, in the northwest corner of the Nevada City area, sandstones and gravel occur cemented by an almost pure, yellowish opal. ANDESITIC TUFFS. The high, gently sloping ridges of these districts are covered by andesitic flows, generally tuffs and tuffaceous breccias. These flows consist mainly of a detrital mass well cemented and made up of andesitic grains. Abundant angular or roughly rounded fragmegts of andesite of all sizes up to a foot or more in diameter are inclosed in this finer-grained mass. This andesite is of a gray to brown or reddish color, rarely greenish, and is in general distinctly porphyritic, with small crystals of white feldspar and black augite or hornblende. As a rule it has a rough, trachytic appearance. Mica is rarely found. Pyroxene (both augite and hypersthene) is almost invariably present. Black basaltic hornblende commonly occurs with the pyroxene, usually in larger crystals. The groundmass is partly glassy, or of a very fine-grained, holocrystalline structure. The thickness of the volcanic flows ranges from 400 feet in the Banner Hill district to about 200 feet in the Nevada City district. The easily distintegrating cement renders the exposures unsatisfactory, and a deep reddish soil usually covers the tops of the ridges. This disintegration and the tendency of the decomposed material and residual andesitic bowlders to slide downhill makes the contacts with the underlying formations in many places obscure and difficult to trace. Good exposures are found in the vicinity of the Harmony gravel mines. The best exposure, though practically inaccessible, is in the bluff of the Manzanita hydraulic pit, north of Nevada City, where resisting unconformably on the sloping surface of the white clays and sands there are at least four distinct flows of andesitic tuff, each 20 to 30 feet thick, separated by irregular, worn surfaces. The amount of angular andesitic bowlders is not constant, and some flows consist entirely of the fine, detrital cementing tuff. Of such character are the tuffs overlying the clays and gravels exposed in the hydraulic pit just north of Grass Valley. ‘1 Fourteenth Ann. Rept. U. 8. Geol. Survey, pt. 2, 1804, p. 464.