78 THE AURIFEROUS GRAVELS OF THE SIERRA 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 “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 chiefly 
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 LH. — 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 
