56 GRAVEL AND PLACER MINING IN ALASKA. [bull. 263. 
fig. 5. Two men are necessary to use the rocker properly, while 
only 3 to 5 cubic yards of gravel can be washed in ten hours. 
At no place was the long torn seen in use in the north, although it 
was formerly employed to some extent in washing the Nome beach 
placers. 
SHOVELING INTO SLUICE BOXES. 
CONDITIONS FOR SHOVELING. 
Conditions throughout many of the northern placer districts favor 
this well-known, simple, and cheaply installed method of placer min- 
ing, for in many localities the pay streaks are thin, ranging from 2 to 
4 feet in thickness, rarely exceeding 5 feet. It is often applicable 
where conditions of transportation prohibit the installation of more 
elaborate plants. 
When the total depth to the bottom of the pay streak does not 
exceed 12 feet, the overlying barren material can generally be 
" ground sluiced" oil', even where the grade does not exceed 1 per 
cent, at an expense varying from 7 to 20 cents per cubic } r ard. The 
frozen muck which composes this overburden is from 50 to 75 per 
cent water, and the solid residue of silt or fine clay is easily carried 
away at any time of slight rise in the stream. The method of ground- 
sluicing the muck is described under the heading "Hydraulic min- 
ing," on page 141. PL IV, A, shows the ordinary method of setting 
up a string of sluice boxes on a creek placer. This method is used 
in an elaborate form on Anvil Creek, where 5 strings of boxes and 
120 shovelers give a twenty-hour capacity of 1,080 cubic yards. 
The handling of the water is important. Even when the gravel has 
been solidly frozen .previous to stripping and sun thawing, there are 
generally from 15 to 25 miners' inches of seepage water, which work 
into the pit floor. This must be disposed of either by draining or by 
pumping. If sufficient water (50 miners' inches) is available, a ' c China 
pump" can be rigged up, as shown in PI. IV, B. This device is 
not economical in its use of water, for the water is generally of more 
value for other purposes in Alaska mining. 
DAMS. 
As in all other forms of creek mining, great care must be taken to 
keep the creek water out of the working pits Iry means of dams. 
Dams constructed of sod walls lined with sacks have been found 
cheaper in the Klondike than those built of sod and brush. Pile 
driving is not advisable in the north, but in case timber is abundant 
cribbing is desirable, as illustrated in PI. V, B, showing a dam built 
across Discovery Fork of American Creek to impound water for 
" booming." The ends of the logs were set in frozen ground on both 
sides and the muck was allowed to ref reeze around them. The dam, 
40 feet long, consists of 12-inch timbers, from 9 to 18 feet long, laid 
up in two rows, 5 feet apart, earth and rock filled, and braced with 
