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THE AUEIFEEOUS GEAVELS OF THE SIEREA NEVADA. 



whether it be of volcanic or sedimentary origin, can be sluiced off without 

 more than counterbalancing by the expense of the operation the profit to be 

 got from working the productive portion of the deposit. When the cover of 

 volcanic material or of unproductive gravel becomes too thick, so that the 

 work no longer pays, the locality has to be abandoned, unless the channel is 

 sufficiently well marked and rich enough to make it worth while to follow 

 and work it by the method called "tunnel mining." 



Tunnel mining was formerly extensively employed in California, although 

 now quite overshadowed in importance by the hydraulic method. Indeed, 

 the present knowledge of the extent and value of the hydraulic claims is 

 largely due to the other and older method of attacking the auriferous depos- 

 its. Some localities, as, for instance, the Sonora Table Mountain, have been 

 exclusively worked by tunnels. This method consists, simply, in driving or 

 tunnelling in the channel and bringing out the pay dirt in cars, or otherwise, 

 to be sluiced, exactly as any auriferous material obtained by river or bar 

 mining would be. The channel must first be found by drifting or tunnelling 

 to it, except in those rare cases where the ground has been so eroded as to 

 bring the pay gravel to the surface. When a u lead " has been struck suf- 

 ficiently rich to pay for working, it will be followed by a drift, so run as to 

 keep in the best ground, or in the pay streak, as long as the operation 

 proves remunerative. In case of necessity one or more shafts may be sunk 

 from the surface to the workings, to furnish ventilation, and some claims 

 have been and still are worked by means of shafts and drifts, the pay dirt 

 being hoisted with a windlass through the shaft. This is the method chieJIy 

 followed in the Australian gravel mines, where the conditions are rarely 

 favorable for using the hydraulic process. Shafts have also, in former times, 

 been extensively sunk for the purpose of ascertaining the position of the 

 channels, in order to fix the level at which the tunnel should be carried in. 



The above general description of the mode of occurrence of the gravel 

 deposits and of the methods by which they are worked seems to be suffi- 

 cient to enable the reader to understand the detailed descriptions which are 

 to follow in the next section of this chapter, and to which we now proceed. 

















Section II. — Detailed Description of Portions of the Auriferous Gravel Region. 



The Geological Survey of the State of California, after a suspension of two 

 years' duration, in consequence of the failure of the Legislature of 1869- 70 

 to make any appropriation for its continuance, began work again in the 



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