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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anged in layers, as on the eastbank of the Middle Fork of the Feather River, northwest of MohawkValley. There is a small patch of andesite tuft on the south slope of theSierra Buttes at an altitude of about 7,000 feet, and as the toll may bepresumed to rest on a bit of the old Neocene surface, there is here e\ ideuce that Sierra Buttes formed a mountain mass in Neocene time, thepresent altitude being 8,615 feet, or 1,(515 feet higher than the tuft. There are granite bowlders on the backbone of a spur of ClermontHill at about the 5,100-foot contour. This spur extends easterly fromLimestone Point. Near its terminus, west of the road to Quincy aminorth of the Nelson Point bridge, t here are also granite bowlders. Theirposition is anomalous, as the nearest area from which they could havebeen derived appears to be that at Happy Valley. 10 miles or more tothe east. There is also a large granite bowlder on andesitic breccia on U. S. GEOLOGICAL SURVEY SEVENTEENTH ANNUAL REPORT PART I PL. XXVI
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B A. ANDESITE-BRECCIA, SHOWING THE ENTIRE LACK OF STRATIFICATION B. DIKE OF HYPERSTHENE ANDESITE IN THE ANDESITE-BRECCIA TURNER.) ANDESITE. 619 the west slope of the Grizzly Mountains, on a spur 4 miles due east ofSpring Garden ranch. Possibly at all these places the granite bowlderswere formerly embedded in the andesite breccia, and were broughtdown with the volcanic material. This origin has already been sug-gested under Glaciers for the granite bowlders on the ridge 4 miles duewest of the summit of Mount In galls. Forming a flat-topped hill on the ridge 2 miles northwest of Downie-ville is a massive andesite (No. 626 S. N.) containing about 60 per centof silica. The rock is remarkable as containing corroded quartzeswhich are surrounded by shells composed of augite-microlites, in thisrespect being like the quartzes of basalts described by Iddings andDiller.1 There are also corroded feldspars in the rock and phenocrystsof a clear, nonpleochroic, rhombic mineral, probably enstatite. I

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