Collection: Books and Periodicals

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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  

DOWNIEVILLE QUADRANGLE. 105 About the headwaters of Slate and Canyon creeks important volcanic eruptions of andesite, andesitic tuff, and basalt took place. The observer finds many places where masses of basalt have been intruded in the gravels, and several examples are cited by Turner and Pettee. An excellent illustration of this is furnished by a hydraulic bank at Port Wine, a photograph of which is reproduced in Plate XVII, A. It shows an irregular sheet of basalt which has been injected in the fine quartz gravel some 20 or 30 feet above the bedrock. Gravels composed almost wholly of volcanic pebbles and representing the latest fluviatile deposits of the Neocene epoch are found in smaller amounts at several places, as the Scales diggings, La Porte, Bells Bar, Spring Garden Ravine; andthe American Valley. But these can not readily be connected with definite river channels,.and most of them are poor in gold. MAIN CHANNEL FROM HEPSIDAM TO SCALES. In the description of the Bidwell Bar quadrangle (p. 100) the main channel is traced from Poverty Hill at Scales down to Camptonville, with an average grade of 100 feet to the mile, the smallest grade, 60 feet to the mile, being recorded from Poverty Hill to the Rock Creek outlet at Scales, where the channel has a general southerly direction and is filled with quartz gravel to a depth of 120 feet. A probable distance of 4 miles along Slate Creek carries it up to the Secret diggings, a small patch remaining on the northwest side of Slate Creek with the rim yising steeply behind it. A distance of about a mile carries it up from this place to the lower end of the La Porte diggings, which have a north-northwesterly direction and are 1} miles long, the bedrock having been uncovered for the whole of that distance. From La Porte to Hepsidam the channel is deeply covered by andesitic tuff and clays, the total distance being about 10 miles. The greatest thickness of the volcanic covering is 800 feet. Much mining has been done on this ridge, stimulated by the known richness of the channel. The bedrock is mainly amphibolite, with some serpentine between Gibsonville and Hepsidam and a narrow belt of clay slate at La Porte. At the Dutch diggings, at the northwest end of the La Porte gravel area, the main channel, as exposed, is about 500 feet wide with sharply rising rims. On the southwest side the amphibolite rim rises several hundred feet probably without being influenced by faulting. The northeast rim also rises sharply, as shown by the fact that bedrock appears on the main ridge along the road northeast and southeast of Bald Mountain, several hundred feet above the diggings, in spite of the fact that, as shown below, the channel has suffered a downthrow between these exposures and the Dutch diggings. The banks show 80 feet of almost clean quartz gravel even next to the bedrock there are few cobbles over 6 inches in diameter. Above the gravel lie 50 feet of sands and clays, the latter partly carbonaceous, rather evenly stratified, and conformable upon the gravels. Above the clays is the heavy cap of andesitic tuff. The gold was practically on the bedrock or in the gravel within 2 feet of it. What gold is contained in the upper gravel is fine and flaky. The Halsey bore hole, 2 miles north of La Porte, is evidently in the channel and shows the following section: Log of Halscy bore hole near La Porte. Feet. Surface lava... cc... eee ee eee ee ee ee ee ee eee eee n eee cece eee e eee eeeeeeeeeeeeee 26 Hard gray lava....... ee ee eee eee ene cee eee eee eeeee ved c eee c eee e scence eeeeees 119 Cla vsrweumewewsureey os 2s $5 vycarmucan oa as ys anmsamnnae 2 2 9s paUyENY ¢ ed oe EG SIRI 2 2 2 2 es eww eEIEE £ eee 10 Black laviiess seuesws xs so 3 2 seemiees i sss 1S GIRGWIREGrS 2 5 FS SRURWRING.S ¢ 1 gS EERE Gd £25 25 REESE E5554 120 Volcanic sand....... 2.02. eee ne ene eee nee e eee e nsec e eee eeeeeeeeneee 2 Lava... ee ene een eee beeen ee nee teen nee ene eeneee .. 39 Quartz gravel. 2.0... ee eee een e ee ee nec eee e eee e ee eeeeeee 8 Quartz gravel. 2.2... cee ee enn eee een e ee nee n eee e eee e eee eeeennee 90 Gravel and Clayieccss ¢s sss smemwes xo 23 6 peeReee 2s = 3s BETES ee 2 oe SRR ng 5a Ramee eae 14 CAVE] sce S555 tak 05 3 SReRen 8E 3 5 § BSBIRoncdabd « « « 2 Gobodhearseunce = oe eormeuzentesens #14 4. no ssenmocrannenmase cee ee 2 Gravel and clay... 0.2.0... ec ee ee eee eee eee etree eee eee eee eeeeeeee 10 Quartz gravel... ee eee eben eee een nee been een e eee eeeeeeeeeenee 5 Bedrock.