150 GRAVEL AND PLACER MINING IN ALASKA. [btjll. 263] 
In the elevator operations on Ophir Creek and on Basin Creek, in 
the Nome district, it is said that 75 per cent of the gold is caught 
before the elevator, and in one case no gold was caught below the firs! 
four boxes of the tail flume. The experience in Seward Peninsula 
has shown that tin 1 gold is for the most part rough, and even crystal- 
line, and even when finely divided is saved with ease. One marked 
exception was seen at Anvil Creek, where a 500-foot string of boxes 
on tti-inch grade caught gold for the entire length. 
TAILINGS. 
It has been said that the most important part of the equipment of 
a hydraulic mine is the dump. It has been found that the amount of 
material which can be moved is governed primarily by the grade of the 
sluice boxes. Reference to table 11 (p. 139), shows that there is a fairly 
constant ratio between the grade of the tail sluice and the amount of 
gravel moved through it per twenty-four hours per miner's inch of 
water used. 
Since it is a prime necessity in hydraulic mining operations that as 
large an amount of gravel as possible shall be delivered through the 
tail sluice and disposed of on the dump, it is evidently of the highest 
importance that a steep grade, 12 inches in 12 feet if possible, and on 
no account less than 6 inches to 12 feet, should be available for the 
sluice. It is manifestly impossible to secure this grade if the natural 
slope of the ground or creek bottom over wdiich the material is run is 
less than that required for the sluices. The grade of the surface must 
in fact be more than that of the sluice, in order that there may be 
ample vertical space below the end of the lowest box for the tailings. 
In hydraulic operations on bench gravels, as, for example, in the 
Klondike region, the conditions for obtaining grade for the sluices are 
more favorable than in any other Irydraulic operation of the North. 
The gravels of the Klondike White Channel benches are easily moved 
after they are thawed. This is fortunate, as on account of the small 
amount of water available hydraulic mining on the benches would 
otherwise be impossible. The expense of keeping up dams for 
impounding tailings, however, is considerable. Naturally the barren 
tailings, consisting of white quartz stones of the size of paving stones, 
can not be allowed to wash and slide down to the creek bottom. Were 
this permitted, the creek miners would soon find their cuts flooded and 
their ground covered with thousands of tons of useless debris. There- 
fore impounding dams for tailings must be constructed of the strongest 
available materials, in order to resist the weight of the stones. The 
cost of building tailings dams of brush and poles, 20 feet high, in the 
Klondike, is $5 per linear foot. Retaining dams along Bonanza Creek 
in the Klondike are shown on PI. XXV, B. 
