HOW THE FIELD WOEK WAS ARRANGED. 



79 



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spring of 1870. From that time on, during the ensuing three years and a 

 half, a considerable portion of the resources at our command were devoted to 

 detailed work on the western slope of the Sierra Nevada. Naturally, among 

 the subjects to be investigated, that of the mode of occurrence of the aurif- 

 erous gravels was one of prime importance. The principal difficulty in the 

 way of executing satisfactorily this part of our task was the necessity of 

 accurate maps on a large scale of the region to be geologically surveyed. It 

 was evident that without such maps as a basis for the observations the re- 

 sults would be very far from meeting the expectations of those pecuniarily or 

 scientifically interested in the gravel deposits. But the peculiar topography 

 of the Sierra,, together with the fact that its western slope is so densely 

 wooded, render the accurate mapping of the region a work of great labor 

 and expense. As much was done as was possible with the very limited means 

 at our command, and we received essential aid from the Central Pacific Rail- 

 road Company, which furnished us with detailed maps of the region adjacent 

 to their line, embracing a belt of perhaps ten or fifteen miles in width, run- 

 ning across the Sierra in the heart of the gravel mining region.* With this 

 assistance, and with such additional information as could be derived from the 

 very incomplete and erroneous surveys of the United States Land Office, 

 combined with the topographical work executed by the Geological Survey, a 

 map of the mining counties was constructed, on the same scale as that of the 

 Bay Map, namely, two miles to an inch. This map covered an area of about 

 nine thousand square miles, extending from Knight's Ferry on the Stanislaus 

 to Quincy in Plumas County; it was never published, the survey having 

 been discontinued before it could be made ready for the engraver. 



In arranging for the detailed exploration of the mining counties, the 

 western slope of the Sierra was divided into two portions by the North Fork 

 of the American River. The region to the south of this river was assigned to 

 Mr. W. A. Goodyear, with special instructions to make as thorough an in- 

 vestigation as possible of the auriferous gravel deposits, not neglecting, how- 

 ever, other facts of general geological interest. Mr. Goodyear was in the 

 field, without assistance, from May 15 to December 17, 1871, and he went 

 over all the important gravel mining districts between the North Fork and 

 the Middle Fork of the Middle Fork of the American River, a region in 

 which the gravels are very extensive, quite various in character, and where 



* This cartographic work, as wo were informed, cost the company more than the total appropriation for 

 the Geological Survey during the two years of its continuance from 1870 to 1872. 



