assayers.
There are always three parcels made, one for the check assay, and the
other two for the two assayers, who will test their parcels either by
the humid or fire assay, or both, as the custom may be. It may be
observed that this assaying of minerals or ores is held in very high
esteem, and it has come under my personal notice of mine
superintendents obtaining leave of absence for the express purpose of
their acquiring, from competent assayers, the knowledge of testing
their own ores, and that the fees for a quarter's tuition ranged from
150 to 300 dollars in these cases.
Section VII.
(a.)—HYDRAULIC MINING OF AURIFEROUS GRAVELS.
In
reporting upon the manner in which the ancient rivers of California are
operated on for the gold these deposits are charged with, it may be
stated that these rivers of Pliocene age are, in most cases, located
above the present bottom of modern watersheds. To get at the gravels
necessitates, therefore, the sinking a series of prospecting shafts,
and upon the proper levels having been ascertained, the driving of
tunnels through the banks or " rim-country" to afford both drainage and tail-races, in order to dispose, by means of such tunnels, of the debris. And,
as the ordinary puddling and sluicing of such deposits from 150 to 380
feet in height, gold-bearing throughout—the North Bloomfield Company's,
Nev. Co., Cal., average yield of gold was only at .the rate of 5-1/2
cents, per yard of gravel moved—was out of the question, the present
very perfect hydraulic sluicing was introduced, and gives very
satisfactory results. Hydraulic sluicing, or " hydraulicking," as it is
termed in Californian mining camps, necessarily includes the
following:—Construction of large storage and distributing reservoirs
in suitable localities, generally in the vicinity of the glaciers of
the Sierra Nevada Mountains ; cutting of races (ditches) ; construction
of flumes bridging the valleys ; laying of surface pipe tracks ;
water-gauges and nozzles." In this portion of my report I would again
take a leading gravel mine as a guide for a general description, as
such will furnish all the necessary data controlling the others.
At
the North Bloomfield mine, the Bowman dam is built to a height of 97
feet, for a length of 400 feet, with a gangway of 2 feet in width on
the top ; it consists of rough-hewn stone, and is three times the width
at the base of its total height, neither puddle-wall or mortar having
been used in its construction. Regular courses of horizontal bearers of
pine-wood are laid in the stone-work, and these project a few inches at
the water side of the dam. On these bearers, strong ribs are screwed,
which are lined with 4 to 6 inch planking, tongued and grooved, which
is all the precaution taken to make these reservoirs water-tight for
quite a number of years.
This
reservoir holds 920,000,000 cubic feet of water, and it cost only
135,000 dollars, or about £27,000, and it is altogether a very
substantial and fine piece of work. Repeated failures with dams
constructed of earth with puddle trenches, owing chiefly to the
different shrinkage of the clays used on the one hand, and the earth in
the bank on the other, have led to the general adoption of rough-hewn
stone in the construction of the dam described. This reservoir
supplies a main-race, or ditch, having a fall of from 12 feet to 20
feet per mile, for a sectional water area of 30 square feet, being 9
feet on top and 5 feet 6 inches wide in the bottom, and it delivers at
the rate of 44,000,000 gallons of water per diem, or about, in
Californian parlance = 3,200 miners' inches, i.e., one "miner's inch" is equal to a
