By GEORGE O'BRIEN.
Our knowledge of the primitive operations of the aboriginal inhabitants of
the globe in pursuit of gold is barely traditional, as we are only aware
that from very early times the precious metal was collected and highly
prized by them, and that they chiefly extracted the visible gold, which
existed in prodigious quantities on or closely beneath the surface of the
earth, and of its being particularly abundant in Asia and Africa. But we
can draw more positive conclusions as we survey remains of the rude but
effective contrivances used by them in later, but still remote, periods,
with full evidence as to the extent of their operations, in the numerous
perpendicular shafts located at short distances from each other, over
large areas of auriferous gravel in India, as well as from precisely
similar memorials of ancient workings which remain also further
demonstrations, in the abandoned "hill diggings," and shifted beds, and
beds of rivers, in Peru South America, flowing between the sea and coast
ranges of the Andes, descending in a northeasterly direction to the river
Amazon, and that their much coveted and enormous productions were the
accumulated riches of the Incas, transferred as spoils of war to their
Spanish conquerors in the sixteenth century. And for similar explorations
in the same class of depositions we have the experiences of our own times,
and which explain by comparison all the previous operations alluded to.
Thus in the year 1849, after the cession of the northern portion of Mexico
to the United States of North America, the rich mineral district of
California was at once invaded by hardy and intelligent bands of mining
adventurers from all parts of the world, who, with little other means at
their disposal but pick, shovel, and pan, soon fell on the productive bars
of rivers and rich ravines where the gold was trapped, derived from its
original birthplaces, where it had been sparsely disseminated, to be
dispersed by the subsequent disintegrations and denudations of the
mountains themselves, and deposited in a disengaged form for the first
comer; and so perfect were sometimes these concentrations, in certain
localities where water once streamed, that, divested of its earthy matrix,
the cleansed pure metal was found deposited, detained by its superior
specific gravity, on the bare rock, and only hidden from vision by a
slight covering of vegetable mould. In this manner, as an example of such
concentration, a "pot" or "find" (in mining parlance) to the value of
£10,000 was collected in a space of 15 square yards, or within the limits
of a particular "mining claim," at the foot of Mokulumne Hill, in a
southern county of California, soon after the territorial transfer from
Mexico. And in search of such locations we must account for the numberless
shafts which still exist both in India and Peru, and sometimes sunk within
a few feet of each other, passing through the alluvium to a depth of 40
feet to the bed rock.
These mining adventurers soon extended their explorations over the other
recently acquired territories, and built Virginia City, the capital of
Montana, with the gold derived from the alluvium of a river channel which
they excavated; and its inhabitants were the founders of an institution
called the Vigilance Committee, with "Lynch law," and by it ruled
supremely for many years. But their surface diggings, by the manual
operations alone of multitudes, were soon exhausted in every direction,
and then their energies and powers of invention were dedicated to discover
and explore deeper and more permanent depositions, along the western
slopes of the Sierra Nevada, the Andes of the Western Territories, and
which originally were without doubt several miles higher than they are at
the present time--probably 20,000 feet above the sea-level--and of which,
or whatever superior elevation they formerly had, the greater portion of
it has already been removed, by the continuous natural action of
centuries, to form there, as elsewhere, the plains and prairies of the
earth, burying and diverting by the mutation the ancient river system,
whose sources of supply were consequently extinguished by the removal of
these altitudes. These denudations and subsequent depositions have been
caused by alternations of temperature and combined action of air, water,
and time since the creation of the world; and powerful demonstrations of
these transformations instruct us in all directions, if we care to observe
them. Thus in "Little Cottonwood" ravine, in the Wahsatch range of
mountains in Utah Territory, lie isolated in the center of the valley huge
masses of metamorphic granite, some blocks of which weigh individually
thousands of tons, and were dislodged from the hills--which on either side
are of limestone formation--with no visible granite in them, having been
undermined by the removal of their pulverized basis by denudation, and
which is the material now forming the tablelands, the foundation, of Salt
Lake City. The blocks of granite, having alone resisted the atmospheric
changes, were precipitated into the valley beneath, and the Mormons are
now constructing their cathedral church from these granitic remains.
The melting of the snow which formerly capped all these ranges of
mountains furnished the water that once flowed in the extinguished
channels of ancient rivers, and whose now diverted waters were also the
powerful agent to assist in causing these marvelous alternations; and by
the means of hydraulic mining we can advance our feeble knowledge on the
subject.
These mighty changes have gradually been accomplished, and the accumulated
denudations of the mineral zones have defended themselves by strata of
crystallized silicates of quartz of various thicknesses, and thus in
places beneath such system of defense, or by their own concretion, have
preserved in many localities a thickness of from 500 to 600 feet of
conglomerate, but without this necessary cementation its further removal
is very certain when again attacked by water. An example of this
continuous process is very observable in "Death Valley," Lower California,
where a width of about 100 miles has been filled up from the hills to the
gulf of same name, invading and occupying its former bed; and this
activity is still proceeding, and a temporary formation of tableland
above it is in course of removal, although already overgrown with forest
trees, which are toppling over the side which is being attacked. But
eternal snow now only covers a small portion of these Sierras, and a
period of comparative repose may be expected, as the distribution has
already been far advanced by the excessive reduction of the mountains.
The deep and extensive depositions which I now attempt to describe
attracted the early attention of the mining adventurers, and were called
"hill diggings," but not being properly understood were therefore not
immediately operated upon, and remained in abeyance, while the lower,
richer, and more manifest alluvials endured. They were designated "blue
gravel," the color being due to the action of sulphuret of iron and other
salts, the cementing auxiliaries requisite to form the hard conglomerate,
and on exposure to the atmosphere changes color to yellow and violet,
losing also its firmness by oxidation.
The "great blue lead" is another important mining term and designates the
alluvium found reposing in a well-defined channel on the bed rock, being
the well-worn path of an ancient river; and it is obvious that the
material in these channels should be richer than the general mass beyond
their limits.
"Rim rock" is the boundary line of the banks of the old channel, and, like
the bottom, is well worn and corrugated by the running water into cavities
and "pot holes," where the force of the stream eddied. The width of these
channels varies from 60 to 400 feet, and the cement near the rim and
bottom is always richer than elsewhere. The wider and deeper channels
generally course from N. to N.W. The richest and most explored belt of
gold-bearing alluvium in California lies between the South and Middle Yuba
Rivers, commencing near Eureka, in Nevada county, and extends downwards to
Smartsville and Timbuctoo, in Yuba county, a distance of 40 miles; and
from among snowy mountains the country falls gradually from where the
ravines or canons are cut by the actual rivers, which are 2,000 feet
beneath the auriferous gravel and region near Smartsville, and 2,000 feet
above the Yuba River, where snow is unknown, and near its terminus the
ancient river bed courses more westerly than it does above it, and crosses
Yuba below Timbuctoo, where the auriferous depositions disappear. The
whole distance of 40 miles has been ransacked by the earlier adventurers,
and around the village of Timbuctoo was a center famed for its wonderful
yield of gold, obtained chiefly in the ravines, in holes, and depressions
in the bed rock. These hollows detained the concentrations of the
denudated alluvium from the altitudes, and were generally closely beneath
the surface, and by such guidance and means of discovery the miners traced
the gold up the ravines to their sources in the lofty mounds and deposits,
or hills of cemented conglomerate, near Eureka in Nevada county; and by
constructing canals from a higher level began the new system of "hydraulic
mining" and washing, and gradually extended their operations over the area
of the metallic zone mentioned, of 40 miles long by 20 wide, using the
Yuba River below Timbuctoo to receive and discharge the tailings, or
refuse from their operations. The result in gold was considerable, but the
system is from its violent nature difficult to control, by presuming to
handle and remove such huge depositions in order to collect the richest
material. The idea was bold, being an anticipation of Nature's operations;
but the equitable disposal of the "tailings" in a cultivated country is
impossible, as the silt runs down the rivers, creating banks and bars in
their channels, obstructing navigation and agricultural arrangements.
_General Description of Hydraulic Mining._
The first work to be accomplished, after calculating that the amount or
value of the material to be operated upon is sufficient to guarantee the
cost of the undertaking in general, is the construction of a canal or
canals, to convey the requisite volume of water from the fountain-head,
and of sufficient elevation to command the ground to be worked upon,
having also in view the levels of the necessary tunnels and shafts as
outlets for the discharge of the gravel through them, these being
engineering operations requiring much skill and labor to avoid useless
after-cost.
Aqueducts of considerable elevation have to be constructed across deep
valleys, and the speculation is at all times problematical, as the ground
cannot be properly tested until the water arrives upon it, and disputes
may arise between the shareholders of the canal and the mining company,
ending frequently in the one devouring the other, unless the two interests
be quickly amalgamated.
The starting point should be the lowest level, or "bed rock," on the white
cement in the ancient channel, which is probably the original silt
collected in it, and is harder than the conglomerate above it, which is
more easily removed. The courses of these beds can be easily traced by
landmarks and undulations, and occasional exposures of the bed rock at low
levels; also trial shafts are sunk in various places in search of it, to a
depth of 100 feet, passing through blue gravel. The grades of these beds
are not steep, being from 10 to 40 feet per mile as of an ordinary river,
and the calculated thickness of the alluvial conglomerate is about 600
feet in many places across the ridge between the South and Middle Yuba
River across the Columbia.
The power of the water for the operation is dependent on a given volume
deposited in a reservoir, and at sufficient elevation above the points of
discharge, as on this depends effectivity to tear down the gravel. It is
delivered to the miner by huge pipes made of wrought iron, and laid down
to follow the curvatures of the surface of the ground; and the pipe I now
treat of, belonging to the Excelsior Water Company, has a diameter of 40
inches on a length of 6,000 feet, and 20 inches on the rest of its length
of 8,000 feet, being 9,000 feet in all; and this large pipe forms an
inverted siphon across a valley, following on the gravel, to the top of
the hill into the reservoir.
These pipes offer advantages over wooden aqueducts for spanning chasms,
and also to avoid coursing the sides of valleys; being also cheaper to
construct in general, and less liable to accidents from fire and storms,
and have the convenience for conveying the water from point to point, as
the work of excavation advances, necessitating the removal of portions of
the aqueduct forward. The watershed, or reservoir, of the Excelsior
Company embraces the valley of the South Yuba and its affluents, and the
entire cost of its eight amalgamated canals was 750,000 dollars.
The rainfall during three years in the mountains averaged 49 inches
annually, while the medium in the same period did not exceed 20 inches in
the plains beneath. The height of the reservoir above the tailing, or Yuba
River, is 393 feet: and the height of the head above the floor, or outlet
sluice-tunnel, of the Blue Gravel Mining Company was 197 feet.
The exact quantity of water required to wash every class of gravel is
difficult to estimate, but no quantity or pressure would be excessive if
properly arranged. The measurement of water is effected by miner's
inches, by allowing it to flow from the reservoir of the seller to the
purchaser through a box 10 or 12 feet square, with divisions to obtain a
quiet head, with a slide or opening capable of adjustment to any required
measure; thus an opening of 25 inches by 2 inches, with a quiet head of 6
inches above the middle of the orifice, would give 50 inches, or about
89,259 cubic feet of water, flowing during ten hours per day, being an
amount necessary for a first-class operation. The capability of the
Excelsior Canal in rainy seasons reached to a delivery in twenty-four
hours, to the various mining companies, of 21,120,000 cubic feet of water,
or 8,000 miner's inches, and the value of the water paid for by the Blue
Gravel Company in forty-three months ending November 9, 1867, was 157,261
dollars, being at the rate of 15 cents of a dollar per miner's inch; and
the proportion of water used to wash down 989,165 cubic yards of gravel
was 17,074,758 cubic yards, or 17¼ cubic yards of water to 1 cubic yard of
gravel; and when at work the quantity of gravel daily moved was 1,298
cubic yards, and the estimated cost to move one cubic yard of gravel was 5
and 7/10 cents of a dollar. But in the face of contingencies the Blue
Gravel Company moved 1,000,000 cubic yards of gravel in four years, or at
the rate of 250,000 cubic yards per annum, and the cost of washing each
cubic yard stands thus:
Cents.
Cost of water, at 15 cents per miner's inch 5.77
Cost of labor, gunpowder, sluices, and
superintendence 16.10
-----
21.87
Or 21¾ cents of a dollar per cubic yard.
Thus the gravel should contain gold to the value of 22 cents of a dollar
per cubic yard to cover cost, and the value of the gravel referred to
ranged from 20 to 45 cents per cubic yard; and the cost of work done in
shafts and tunnels, in the said Blue Gravel Company's Mining claim,
reached 100,000 dollars. But with the cost of the necessary canals paid
for by the Excelsior Water Company apart, the total cost amounted to about
1,000,000 dollars, and we must note that the latter company sold water to
other mining companies.
The gross yield in gold of the Blue Gravel Company in four years was
837,399 dollars, and in the year 1866 the returns from the Blue Gravel
Company paid all the costs of the developments; but in 1867 assessments
were paid by the owners to meet the deficiency arising from the cost of
sinking two new shafts, and driving fresh tunnels on the lowest levels,
which evidently contain on the bed rock the richest concentrations.
In smaller mining adventures of this description, involving less capital,
large profits have been made in the gold-bearing zone treated of, by also
not having invested in costly canals, which would not have repaid the
latter investment; and thus it is evident that the water companies are
dependent blindly on the prosperity of the miners.
I will now more minutely describe the actual mining operations. The mining
ground being selected, a tunnel is projected from the nearest and most
convenient ravine, so that the starting-point on the bed rock toward the
face of the ravine shall approach the center of the material to be removed
at a gradient of 1 in 10 to 1 in 30. The dimensions of such tunnels are
usually 6 feet in width by 7 in height, and continuing in contact with the
hard river-bed, for the greater ease of excavation, collection of gold,
and conservation of quicksilver amalgam.
These tunnels vary in length from a few hundred feet to a mile, and some
of the longer ones occupying from one to seven years in execution, at a
cost of from 10 to 60 dollars per foot of frontage. The tunnel of the Blue
Gravel Company, with length of 1,358 feet, cost in labor alone 70,000
dollars, but it could now be driven for 35,000 dollars, as skilled labor
is cheaper now than then. The grade in this tunnel is about 12 per cent.,
and the end of the tunnel is designed to be 170 feet of elevation, and
reaching to a point beneath the surface of the gravel which is being
operated upon, and where a shaft or incline is sunk to or through the bed
rock or gravel, until it intersects the tunnel.
The object of this laborious operation is obvious, as the long tunnel
becomes a sluiceway, and through the whole length of which sluice boxes
are laid, for the double motive of carrying off the material and saving
the gold, and for this purpose a trough of strong planks is placed in the
tunnel, 2½ feet wide, and with sides high enough to contain the stream.
The pavement of the trough is generally laid of blocks of wood 6 inches in
thickness, cut across the grain, and placed on their ends, to the width of
the sluiceway. The wooden blocks are usually alternated with sections of
stone pavement, the stones being set endwise, and in the interstices
between the stones and wooden blocks quicksilver is distributed, and as
much as 2 tons of this metal is required to charge a long sluice. The
water in the canal is brought by aqueducts, or other means, to the head of
the mining ground, having an elevation of 100 to 200 ft. above the lowest
level of the mining ground, and is finally conveyed to it by iron pipes,
sometimes sustained on a strong incline of timber.
These pipes are of sheet iron, of adequate strength, riveted at the
joints, and measure from 12 to 20 inches in diameter, and communicate at
the bottom with a strong prismatic box of cast-iron, on the top and sides
of which are openings for the adaptation of flexible tubes, made of very
strong fabric of canvas, strengthened by cording, and terminating in
nozzles of metal of 2½ to 3 inches in diameter. From these nozzles the
streams of water are directed against the face of the gravel to be washed,
exercising incredible effectivity.
The volume of water employed varies of course with the work to be done;
but it is not uncommon to see four such streams acting simultaneously on
the same bank, each conveying from 100 to 600 inches of water per
hour--1,000 miner's inches being equal to 106,600 cubic feet of water per
hour, constantly exerting its force under a pressure of 90 to 200 pounds
to the square inch, varying with the height of the column.
Under the continuous action of this enormous force, aided by the softening
power of the water, large sections of the gravelly mass are dislodged, and
fall with great violence, the _debris_ speedily disintegrating and
disappearing under the resistless force of the water, and is hurried
forward in the sluices to the mouth of the shaft, down which it is
precipitated with the whole volume of turbid water. Bowlders of 100 to 200
lb. in weight are dislodged and shot forward by the impetuous stream,
accompanied by masses of the harder cement which meet in the fall, and by
the concussion from the great bowlders the crushing and pulverizing agency
required is found to disintegrate it. The heavy banks, of 80 feet and
upward, are usually worked in two benches, the upper never being so rich
as the lower, and also less firm, and therefore worked away with greater
rapidity.
The lower section is much the more compact, as this stratum on the bed
rock being strongly cemented resists great pressure, and even sometimes
the full force of the streams of water, until it has been loosened by
gunpowder or other explosives. For this purpose adits are driven in on its
foundation-point of from 40 to 70 feet and more from the face of the bank,
and drifts are extended at right angles therefrom to a short distance on
each side of the adit, and in these drifts a large quantity of gunpowder
is placed (from 1 to 3 tons), and fired at one blast, having been
previously built in with masonry. And in this manner the compact
conglomerate is broken up, and then the water easily completes its work.
Sometimes in the soft, upper strata the systems of tunnel is extended, as
in a coal-mine, by cross alleys, leaving blocks which are afterward washed
away, and then the whole mass settles, and is disintegrated under the
influence of water. The wooden sluices in the tunnels already described
are often made double for the convenience of "cleaning up" one of them,
while the other remains in action. The process of cleaning up is performed
according to the quantity and richness of the material worked upon, at
intervals of twenty to forty days, and consists in removing the pavement
and blocks from the bed of the sluice, and then gathering all the amalgam
of gold and rich dirt collected, and replacing the locks in the same way
as at first. Advantage is taken on this occasion to reverse the position
of the blocks and stones when they are worn irregularly, or substitute new
ones for those which are worn through. The mechanical action of the
washing process on the blocks is of course very rapid and severe,
requiring complete renewal of them once in eight to ten weeks. Some miners
prefer a pavement of egg-shaped stones set like a cobble-stone flooring,
the gold being deposited in the interstices. Most of the sluiceways are,
however, paved with rectangular wooden blocks, with or without stones as
described. Standing at the mouth of one of the long tunnels in full
action, any person unaccustomed to the process is struck with
astonishment, amounting almost to terror, as the muddy mass sweeps onward,
bearing in its course the great rolling bowlders, which add their din to
the roar of the water, the whole being precipitated down a series of
falls, at each of which it is caught up again by new sluices of timber,
lined like the first, and so onward and downward many hundreds of feet
until the level of the river is reached, at a distance of about a half
mile or more from the mouth of the first tunnel.
At each of these new falls of 25 to 50 feet the process of comminution
begun in the first shaft is carried on, and a fresh portion of gold
obtained. Rude as this plan of saving gold appears to be, more gold is
procured by it than by any other method of washing yet devised for this
process of work, and the economical advantages obtained by it cannot be
surpassed, as it would be impossible to handle such vast quantities of
material in any other way, and we can compare the cost of washing and
handling a cubic yard of auriferous gravel by it as follows:
Dollars.
By manual labor with the pan 15.00
" " with rocker 3.75
" " with the long tom .75
By the hydraulic process .15
But this process, even if effective or profitable as a mining operation,
may be prejudicial to the interests of the general public, if conducted on
a large scale, as the vast quantity of material which it so suddenly
removes is merely shifted into the shallows beneath, to be redistributed
by every freshet to points lower and lower down until it reaches the
sea-coast, creating bars at the mouths of rivers in its course, and
changing the hydrography of harbors--as it has done with the Bay of San
Francisco by its silt.
The hills behind, torn up and washed by the gold miner, are abandoned as
desolate and irredeemable; and the costly canals, constructed with
peculiar conveniences for mining purposes, eventually fall into disuse
from being too expensive to maintain or alter for general agricultural
uses.--_Journal of Science._
* * * * *