i?ukington.] HYDRAULIC MINING. 101 
which hydraulic operations may safely be undertaken is 220 feet to 
the mile. 
The above statements regarding hydraulic mining are as worthy of 
consideration to-day as when they were written. Bench deposits, as 
already defined, are those above all others to which the hydraulic 
method is most likely to be successfully applied, because adequate 
grade for the disposal of tailings can be secured. Deposits in the 
beds of present streams, on the other hand, are exploitable by 
hydraulicking to a far less extent. In Alaska extensive bench depos- 
its do not occur, even in the alpine districts of the south coast, and, 
moreover, where auriferous benches are found they are not backed by 
high mountains from which the water under pressure can be obtained. 
The bench deposits of Alaska are not characterized by great thickness 
as are those of California. A depth of 10 feet of auriferous gravel 
with 20 feet of overlying barren silt, or a total of 30 feet, is a not 
uncommon condition in Alaska, while in California gravel banks of 75 
to 500 feet in thickness occur and have been worked by hydraulick- 
ing. The hydraulic operation shown in PI. XV, B (p. 92), is on a lake 
bed, locally formed in a portion of Silver Bow basin, Juneau district. 
The hydraulic principle implies the breaking down of all the allu- 
vial material by water under pressure, the cheapest power. It is evi- 
dent that unless the gold is distributed throughout the vertical section 
of the alluvial deposit, as well as on and in the bed rock, the greatest 
amount of material is hydraulicked off at a loss. In general the pa}^ 
dirt in Alaska composes less than 15 per cent of the bank, measured 
vertically. In California gravels, on the other hand, much more fre- 
quently a small amount of gold occurs throughout the whole bank. 
In Alaska attempts have been made to work very shallow ground, 
in some cases of 5 feet section, by hydraulic methods involving the 
construction of ditches and installation of machinery at a cost of 
$100,000 and over. Unless the gravels worked were much richer than 
previous workings in the vicinity, it was impossible to see how the 
enterprise could by any means be profitable. Simple mechanical 
plants, requiring a comparatively small investment, could have handled 
the small body of gravel fully as fast and in the long run more cheaply. 
It is undoubtedly true that many men are deterred from installing 
really economical plants of a mechanical nature by the desire to apply 
the spectacular methods of the hydraulic miner, without regard to the 
existing conditions. 
A word should be said about the application of power to produce 
pressure for hydraulicking." Several plants where this principle is 
applied have been visited in Alaska and the neighboring British terri- 
tory. Nine plants were seen and it was not clear that any plant pump- 
a These statements do not refer to the hot- water method of hydraulicking frozen gravel. 
