84 
GRAVEL AND PLACER MINING IN ALASKA. 
t 
[BULL. 263. 
timbered either with sets, where ground is heavy, with single posts 
and caps in moderately firm ground, or left entirely un timbered, as 
in solidly frozen ground. Fig. 15 shows the manner of timbering 
closely and rock filling behind when breasting out faces of loose 
gravel in the Hidden Treasure gravel mine in California. The 
sketch is made up in part from report of Mr. Ross E. Browne/ 
and in part from the writer's own observations. The system oi 
filling behind with quartz stones is impracticable for Alaska min- 
ing; instead of this timbers should be pulled and the ground allowed 
to cave. This form of timbering is in use in the drift mines of Solo- 
mon Hill, Klondike, except that instead of separate base blocks sills 
are used. Fig. 16 shows a portion of the drift operations which were 
prosecuted on Solomon Hill in the early years of the Klondike exploita 
tion. The plan of the workings is here reproduced by courtesy of Mr, 
George T. Coffey, manager of the Anglo-Klondike Mining Company'} 
Floor of channel 
Fig. 15.— Hidden Treasure system of timbering drift mine. 
operations. The drifting operations in the southern portion of th 
ground are still in progress. PL XIII, B, shows the mouth of ori 
of the adits leading to the operations. 
The main adits are timbered with 4-inch timbers, in sets with 5i-fo< 
centers, at a cost of $1.75 for framing and putting in per foot of tui j 
nel run. The entire cost of driving the tunnel 5J feet high, includir 
steam thawing, excavating, tramming, timbering, and laying track < 
12-pound rail, was $6.25 per foot. Transverse drifts were run fro j 
the main adits, and the ground was always breasted out in the directk f 
of the adit mouth. The irregularity of the workings in a portion 
the ground is accounted for by the fact that a certain amount of unsy 
tematic work had been done previous to the purchase of the ground 1 
the present company. 
The ground is solidly frozen and had to be thawed by steam point 
The rate of speed with which the work was carried on may be judg 
from the fact that two men, working a shift in running a straig 
a The ancient river beds of the Forest Hill divide: Tenth Ann. Rept. State Min. Cal., 1890, p. 4 
