the
tunnel by means of self-acting separators of the coarser from the finer
gravels, and the repeated subjection of the latter to a manipulation in
the "under-currents," which are also charged with mercury, lodged in
the irregular ripples, formed of the hardest boulders that can be
obtained. Much of the gravel being cemented together, the continuous
attrition such suffers in this torrent of water and boulders with
gravel, tends to liberate more and more gold, until even the "
under-currents " fail to save any more, whereupon the process is
considered complete.
Resume of Hydraulic Mining of Auriferous Gravels.
Reservoirs
constructed of rough squared blocks of stone, rendered watertight by
means of wooden casings ; ditches (races), 30 feet sectional area ;
fluming; wrought sheet-iron pipes made from iron 3/8-inch to 5/10-inch
thick ; wrought-iron pipes from 30 inches to 15 inches in diameter ;
nozzles from 4 inches to 9 inches diameter, throwing streams of water
to a distance of from 400 feet to 500 feet; main tunnel and tail-race 5
per cent, grade or incline ; depth of tunnel below gravel 54 feet;
height of gravel from 340 to 400 feet, all gold-bearing. Modes of
saving the gold : in the crevices of the bed-rock ; in the wood
pavements with mercury in the tunnel; and, lastly, by numerous "
under-currents " outside.
In
concluding this portion of the paper, I would call attention to a
difficulty that was experienced until recently with the working of the
nozzles, inasmuch as the water, discharged in some cases with a
pressure exceeding 375 lbs. per square inch through a smooth-bore
nozzle, obtained a gyrating motion, which destroyed, in a very great
measure, the effects of the jets against tenacious gravels. By riveting
four " tee " pieces of iron, 2 feet in length each, and equi-distant
from each other, inside the nozzle, the water issued with unabated
force direct to the point sought to be undermined, thus resembling the
bore of a rifle, which acts in a similar manner.
(b.) DEIFT MINING BY MEANS OF SHAFTS AND TUNNELS.
With
regard to the mining of the ancient rivers, in localities where the "
capping of basalts " interferes with the action of the nozzles on
account of the too fiirge percentage of these volcanic rocks in the
gravels, drift mining, as it is termed in California, has been resorted
to in some places (Capt. Godfrey's mine, Grass Valley; Watts' mine,
Bloomfield) ; but in no case has this kind of work been carried on to
such perfection or extent as here, at Ballarat, and other districts.
For these reasons it is more judicious, on the score of saving time, to
describe their tunnelling operations, as meriting our attention; and
for that purpose the Bald Mountain Company, Sierra County, California,
will serve as a very good example.
That
company work their mine by means of a large tunnel, timbered
close—4-foot caps, 8-foot legs, with an extraordinary "spread" at the
bottom of 12 feet, on account of the bed-rock (serpentine), largely
impregnated with lime, swelling continuously in the open air,
frequently filling in the whole drive. Two parallel lines of rails (22
lbs. per yard), are laid down in this tunnel, with the necessary
sidings, lay-offs, and turns provided. Owing to the rapid ascent of the
river-bed, at the rate of 200 feet per mile, neither manual nor animal
power could be used in working the tunnel, even if the " cars," i.e., trucks,
were geared with the strongest of breaks. And, as they had then already
driven 5,000 feet under the adjaeent basaltic plateau, which rises
1,100 feet above the main
