Gold dredging has won an important place in the industry of mining, and it has done so by right of hard-earned merit. Probably no branch of mining has come so prominently to the front in so short a time; by proving its commercial value it has opened to the financier an opportunity for safe and large investments.
Ten years ago there were no dredges working in California. Today there are fifty, either in operation or building in the districts of Oroville, Yuba, and Folsom, and it is probable that in no single instance is money being lost, while many of the dredges are clearing net profits of from $2000 to $5000 per month.
It is estimated that over $5,000,000 is invested in the business of dredging in the three districts of Oroville, Yuba, and Folsom, and the combined producing capacity in these three districts is estimated at an annual output of about 30,000,000 cubic yards.
Naturally in such a young and growing field there is tremendous diversity of opinion as to the relative advantage and value of the different methods in the practice and design of the machines and their parts. In the account presented herewith I have endeavored to trace the progress of the industry in California, and to give as many actual examples from practical experience as possible.
Geological and Historical
The explorations of the United States Geological Survey have, defined three great mineral belts in California. Three parallel lines are to be observed in the structure of the region : the first is coincident with the summit of the Sierras, the second is along their approximate base, say, from Visalia to Red Bluff and about fifty miles west of the first or main axial line, and the third, equidistant, drawn from the neighborhood of Clear lake to Kern lake, marks the eastern edge of the Coast Range, the shores of the Pacific forming its western edge at about the same distance. These lines divide the State geologically as well as physically. The Sierra Nevada is a belt of intrusive granite of ante-Cretaceous elevation, but of Triassic and Jurassic age, partially covered by important Pliocene river deposits. These mountains are of great height and are the legion of precious metal (and some iron and copper) mining, while the Coast Range, formed of strata chiefly of Cretaceous or Tertiary age and of post-Cretaceous elevation, yield principally quicksilver ores and carbonaceous minerals.
In the Sierra Nevada, volcanic activity is supposed to have ceased, while solfataric and seismic disturbances are still apparent in the Coast Range, the effect of the latter having been terribly illustrated in the recent earthquake and its disastrous consequences. The great valley lying between them is covered by recent alluvial deposits laid down on the bed of a fresh-water sea. It is to a part of the western border of this region that attention is to be drawn. The gravel deposits occur in every variety of texture from fine pipe-clay through sand and gravel to rolled pebbles and boulders, sometimes weighing tons. It is now generally accepted that they have been laid down by the action of a system of late Tertiary rivers which had nearly the same general course as the present streams on the west slope of the Sierra, but whose channels were wide and slopes greater.
The waters of these rivers, eroding the auriferous slates with their included quartz veins, concentrated the precious metals in deposits often 350 to 400 ft. wide at the bottom and sometimes several thousand feet wide at the top. Their depth now varies from a few yards up to six or seven hundred feet. Volcanic eruptions have, in places, covered these deposits with lava and tufa hundreds of feet thick. Denudation and erosion ensued, and the products of volcanic activity have sometimes been covered in turn with gold-bearing detritus.
Ages ago, long before the advent of man upon this planet, the western coast of the continent lay approximately along the line of the present Sierra Nevada range, then included in flat beds of shale and slate and on a level with the present great interior plains. The present buried river-beds had their origin in immense sluggish streams or sloughs of great length and extremely tortuous. These probably included watersheds as far north as the present Columbia river; they extended east into Utah and emptied into the ocean. Presently, however, a change occurred in affairs terrestrial and an uplift began to produce the Sierra Nevada mountains. These sloughs with their increased grades became roaring torrents, eroding deep channels through the embryo mountains and tending to straighten their sinuosities in the long and turbulent journey to the sea. Though enormous bodies of detritus must have been washed away, none of it remained in the river-beds owing to the fierce rush of the current. The upheaval which lifted the Sierras to an altitude greater than even that of the present magnificent peaks, abated and finally ceased; a period of erosion commenced. Up to this point it is probable that the river’s burden was non-auriferous, but from the interior magma the slate and schist were penetrated by intrusive lava, followed by thermal activity that produced veins,
the source of the gold in the placer mines of today. The Glacial epoch followed and during the rainy period that ensued, with the renewed influence of the sun’s heat, tremendous erosion occurred; the peaks were worn down, and the ravines and valleys were formed. Simultaneously another phenomenon occurred that had an all-important bearing on mining. The valley of the Sacramento river, lying between the Sierra and the Coast Range, which had meanwhile become a vast inland fresh-water sea, began to rise. This uplift including the foothill country and its total vertical effect is variously estimated at from 800 to 1000 ft. The immediate result was a decrease in the grade of the great river system and the consequent slowing of the current, the channels becoming gradually filled with gravel, sand, and silt, containing the concentrated gold from the eroded portions of the veins over which they coursed.
Contemporaneously, man appeared, and no written papyrus is necessary to record the event. Certain it is that he did not exist during the immediate post-Glacial period, but at this stage, when the rivers were filled with gravel, we have abundant evidence that he had even progressed in the arts. His prehistoric stone mortars for pulverizing maize or other cereals are constantly being brought to light in the hydraulic and drift mines, and piles of faggots and fire wood stacked by hand and exposed in the bench workings of the ancient rivers suggest the site of aboriginal habitations.
The final destruction, by burial, of the life of these channels came about through volcanic activity, which was at work only periodically during a comparatively short geological period; up to the time of the final outburst of molten lava, it appears to have produced mainly ash and dust that were swept down the streams in form of sand and slime. Subsequently this ejected material was compressed into the various layers of rhyolite and other tufaceous rock of different coarseness. Between these there were often beds of alluvium produced from detritus of non-volcanic origin.
There seems only to have been one general flow of actual melted lava, remnants of which may now be seen in the basaltic cap forming the isolated table mountains existing at several points in the foothills, notably on the Feather river and in Stanislaus county. This cap was in turn worn down by the later rivers, often flowing in an entirely different direction to the buried channels; these are thus found today at varying elevations up to several hundred feet above the existing rivers, which, cutting the ancient channels at many points, are locally indebted to them, as well as to the quartz lodes, for their gold. Indeed, it is probable that to this secondary concentration is due most of the enrichment at those favorable spots that provided the gold of the pioneers, and in fact all of the gold obtained in the pan, rocker, sluice, and other river workings. These streams of later origin continually changed their channels, though continuing to flow approximately in the same direction, and at each change they cut deeper. The result was that a series of terraces, bars and benches were formed, gradually rising from the present channel. A striking example of this action may be seen during almost any month in the year in the Yuba, but more emphatically shortly after the winter rains. Long sections of the river shift as much as several hundred feet in a few weeks, leaving gravel bars deposited in positions that a few days before represented the channel.
On the American river half a dozen distinct benches may be observed rising from the south side of the river for several hundred feet in height, and each of these contain consecutive auriferous channels—the former courses of the present river. It is these later river channels that in most cases form the richest portions of the dredging areas of today in almost every instance, though of course, quantities of gold-bearing matter must have been deposited by the ancient rivers previous to their becoming choked by the flow of volcanic material. That the gold from this source does actually exist at greater depths than it is possible to dredge (with present apparatus) has been proved by several comparatively deep bore-holes that penetrated below one or more beds of volcanic ash and also, I believe, by drift-mining operations in a few cases. Most of these drift mines worked by shaft in the valley are probably on the beds of the recent channels.
Several ingenious theories have been advanced to account for the source of the gold in the channels, but in no case has a reasonable argument been put forth. The ‘marine idea’ holds that these beds were deposited on the floor of a sea, but that this is altogether fallacious is shown by the fact that no marine remains have been detected and terrestrial signs such as trees and even human implements and (in some of the older beds) remnants of mammals, such as the mastodon, are found. The famous ‘blue lead’ theory pre-supposed the existence of an ancient river flowing from north to south and roughly parallel with the crest of the Sierras (some said this could be traced from Alaska to Mexico) and containing a characteristic blue gravel. In the first place, it has been proved that no such ancient channel exists and, secondly, that the ‘blue lead’ forms the bottom stratum of practically every buried channel, of Pliocene age, that has been opened. By analysis the ‘blue lead’ gravels contain iron pyrite, and these strata are similar in every way to the ‘red leads’ or ‘rotten boulder’ leads as they are called, which generally overlie them, except that in the latter case the iron has become decomposed and changed to an oxide.
That the gold is evidently derived from the bedrock traversed by the channel systems seems practically certain. The argument has frequently been brought forward that the veins throughout many of these districts are too poor to pay for working even under present economic conditions. This does not necessarily justify the conclusion that they were too poor to furnish gold to the channels. Some of the quartz veins have shown gold enough to warrant the investment of capital and they have even paid a profit. The $2 to $5 per ton that the average of these deposits contain is an extremely fine concentrate and represents the product of a small fraction of the mass of quartz broken and sluiced down these old rivers. Moreover, some of these veins have been eroded to depths approximating 1000 ft. and, incidentally, many fabulously rich pockets must have been treated by nature’s concentrator—the river.
Placer deposits have been the earliest sources of gold throughout the world and since 1848, the date of the first important find in California, it is estimated that about four-fifths of the total output has been produced by the different forms of alluvial mining in this State. The total production from all sources up to date has been $1,450,000,000.
The successive steps in placer mining were the miner’s pan, the cradle or rocker, the long tom, the riffle-box or sluice, the ground-sluice, booming or gouging, drift mining, hydraulic mining, the hydraulic elevator, and dredging.
The pan, rocker, and long tom are almost too well known to mention and may be passed over with but a word of comment. The pan was the earliest implement used in separating the precious metal from the accompanying gravel in California and is still necessary to the prospector, mill-man, and assayer. It is made of the best quality of Russia iron, generally stamped out of a single sheet, with the edge turned over a stout wire. The usual dimensions are: Diam. 10 in. at the bottom, 16 in. at top, and 214 in. deep. The angle of the sides is 37°. The method of use is as follows: The pan is filled with the gravel and sand and then carefully lowered under water; the fine and light material are gradually washed off, care being taken not to allow any gold particles to escape; the pebbles and coarser material, after examination, are removed by hand ; washing continues until only magnetic sand and the gold remains. The pan being tilted and the water carefully manipulated, the gold forms a fringe at the top of the sand and is thus collected.
The ‘rocker’ is a box about 4 ft. long and 2 ft. wide, and is mounted on semicircular pieces of wood and worked by a handle to give it a side motion ; and it is also inclined so as to carry the material down to the lower end, which is open. At the upper end is a small hopper that may be removed and which has a sheet-iron bottom perforated with ½-in. holes. Under the hopper is a canvas apron or tray inclined toward the head of the box but touching neither end of the hopper-box. A wooden riffle is placed across the box at the centre and another at the end. The material is fed into the hopper and screened through by water poured on top; the lighter material is carried over the end, while the riffles catch the gold and magnetic sand. This residue is panned at the end of the operation.
The ‘tom’ was originally a rough wooden box, 14 ft. long, and 2 ft. wide at the upper end, and 3 ft. at the lower end. The sides were about 10 in. high and the bottom had six or more cleats or riffles. The water was fed in a continual stream and the material treated was in larger quantities than the rocker. The next step was the ‘sluice-box’, which, as it could be lengthened indefinitely, had a much larger capacity. It is generally made in sections 12 ft. long and from 1 to 2 ft. wide. Riffles are placed both across and lengthwise with the box; mercury is introduced and the gold amalgamated.
‘Ground-sluicing’ came next; it consisted in bringing water through a ditch to a point above the claim high enough to produce a strong current. The bottom, if possible, is on bedrock and large stones form an artificial riffle. Occasionally a wooden sluice with riffles is placed at the end. Large quantities of material are shoveled from the sides into the ditch. The stones are finally removed and the concentrated material at the bottom is taken out and put through a rocker or long tom.
‘Booming’ or ‘gouging’ is somewhat similar to, ground- sluicing, except that a large quantity of water is collected in a temporary dam above the workings and allowed to rush down suddenly, cutting away large quantities of ‘pay dirt’ which is conducted through ground-sluices.
The gravels were first tested by pan and rocker. The latter was introduced from Georgia, and toward the end of 1850 the long tom was brought into use. The dry bars of the rivers that were easy of access at low water, were first worked out; then the bottoms were worked, by using wing-dams; and finally entire
streams were deflected from their course by flumes and ditches. The lower ‘bench gravel’ was then worked by ‘tom’ and other sluices, booming, and gouging, etc. Gradually these benches and low alluvium supplies also becoming short, attention was turned in 1852 to the high hill deposits—the buried rivers. These immense alluvial banks, often capped with basalt, sometimes have a depth (or ‘face’) of over 600 ft., and as the richest stratum usually lies near the bottom, the grade of material was found to be, as a rule, lower in gold content than the deposits already described. A larger scale of operations and more economic methods became necessary. This was done by hydraulicking, the directing of jets of water under heavy heads against the bank, which is thus disintegrated and washed over sluices; it was also done by drift mining, following the deposits by ‘tunnels’ under the hills or reaching them by shafts on the flat lands.
Another method of working these deeper deposits, particularly where they do not contain water, is by the hydraulic elevator, an upright pipe into the bottom of which the gravel is drawn by a jet of water under head that also raises it to the top (sometimes 40 ft.), where it is sluiced in the usual manner. It is not, however, proposed in this article to enter into a discussion of the relative conditions surrounding the most advantageous use of each of the methods mentioned.
In the later seventies hydraulic mining had become one of the most important industries of California, in which more than $100,000,000 had been invested, in mines, reservoirs, canals, and equipment, but owing to damage done to farming lands in the valleys adjacent to the Sacramento river and its tributaries, this method of mining was inhibited in 1880 by Congressional act, and hydraulicking was almost entirely discontinued, until the passage of the so-called Caminetti Act, early in the ‘eighties, which permitted the resumption of hydraulic mining under the permit and restrictions of a corps of United States engineers, known as the Debris Commission.
The Debris Commission’s work is directed entirely to controlling the natural detrital flow, and although appointed for this work about 1883 and at present employing conscientious and efficient engineers, their whole work to date has been confined to the Yuba river, where practical results were only accomplished in 1904. A point was selected a few miles above the present site of Hammon City, where it was proposed to place a dam, the duty of which was to arrest the burden of sediment in the river and collect it there. The height of the dam was to be sufficient for a year’s accumulation and at the end of that period it was to be added to year by year.
It is commonly supposed that the efforts of the Debris Commission are to restore the practice of hydraulic mining throughout the State, but such is unfortunately not the case, nor does there
appear to be the slightest hope for a largely increased output from this source in the near future. It is true that under certain conditions—the chief of which is that the miners provide impounding dams or settling basins for the tailing they produce—some approved mines may continue to work, but the formality to be gone through and the expense of this work as a rule prohibit large workings and the output from such mines as look after their own tailing is inconsiderable compared with the product of former hydraulic mining or what might be produced under the favorable mining conditions of today.
After several abortive attempts, the Yuba dam was built of piles and rubble, and capped with a sheet of concrete 18 in. thick. The horizontal distance between top of apron to end of toe of this first portion is 36 ft. and the height is 6 ft. It was estimated that the annual increase necessary would be about 8 ft.; but this has proved to be an over-estimate. In 1906 eight feet were added to the upper side, making the present total height from original base 14 ft. and the width from top of present apron to toe 56 ft. The length of the dam is 1200 ft. and a spill-way is now being constructed around the south end. Since the last addition the basin has filled up to within a few feet of the top with precipitated material, the water at the deepest point (near the spill-way) being about five feet.
Work of great magnitude too, in connection with this impounding problem is being carried on by the dredges and contract work at Hammon City, a few miles below the barrier dam. These settling basins, especially the one behind the barrier dam, should prove a valuable experimental ground for the State with regard to ascertaining the annual accumulation of gold brought down by the river. A series of borings, at close intervals across this and the other basins, through the material deposited there since their construction, would certainly be extremely interesting; at any rate it would give some data of the amount of concentration that had taken place in the early gravel flows.
An ambitious, though seemingly impracticable, suggestion has been made with regard to assisting the resumption of work by the hydraulic mines. It is proposed to direct the tailing toward the worthless tide lands, which cover a vast acreage in the Sacramento valley, and it has been contended that on filling these swamps with sand and gravel, they may become useful for agricultural purposes. This point has not been satisfactorily settled as yet, though at the present time it is understood experiments are being made with that end in view.
As stated above, the districts to be treated in this article are Oroville, Folsom, and the Yuba, as exemplifying the most modern dredging practice, and, as few mining camps are surrounded by such pleasant living conditions, a descriptive note may be appreciated in passing.
To the stranger fresh from the East, the first view of the Sacramento valley is a new sensation. In his long trip across the continent he has no doubt seen prairie, desert, and magnificent mountain peak, but looking westward from the vantage point of the Sierras is a scene that for breadth and beauty cannot be surpassed. The prairie is usually rolling and its horizon is but a few miles away; the desert is deadly monotonous; here, however, is a refreshing change.
From the lava-capped heights rising abruptly near the headwaters of the Feather and Yuba rivers, one looks over a broad expanse of valley 50 to 60 miles wide. In the near foreground are miles of orchard land, bearing the orange, lemon, pommelo, peach, olive, and almond, interspersed with far-stretching vineyards or squares of wheat and grazing land. Silver streaks and patches shimmering in the bright sunlight denote rivers and over-flowed levees. Halfway across the valley the jagged peaks of the Marysville buttes, a clear blue silhouette 20 miles long, thrust themselves abruptly from the level plain to a height of 1800 ft. Further to the west, the Coast Range, a long low hand of still fainter blue, capped occasionally by glint of sun on ice, may be just discerned, fading from sight as the eye traces it to the north. Where the Feather river escapes from the mountains and immediately below
the foothills, nestling among its orchards, lies Oroville. The name means “city of gold,” and it is doubly applicable to the perennial color of its fruit and the precious metal contained in the soil on which it stands.
Marysville is the chief town and distributing point for the Feather and Yuba river dredging districts. It is one of the oldest towns in the valley and has the somnolent atmosphere of one of those communities in the Southern States not yet recovered from the effects of the war. Though this impression may belie the real commercial activity of its inhabitants, it is strengthened by the sight of the only public transportation facility afforded by the town—the mule cars—another ante-bellum reminder from the South. On the Yuba, 28 miles almost due south from Oroville, lies Hammon City, also situated in the foothills, and named from its founder, W. P. Hammon, the acknowledged leader of the dredging industry. The site of this settlement was only established about two years ago and though comprising but a handful of buildings, occupied by the dredging population, and temporary shops for repair work, it is growing fast and care is being taken to ensure its future beauty and convenience. The main street has been metalled with gravel and planted on either side with rows of black walnut and palm trees. Such older trees as were originally on the ranch have been left standing as far as is possible.
The town of Folsom, best known as the site of a State penitentiary and for its squalidness amid beautiful surroundings, is on the American river and marks the northeastern limit of the dredging ground on that stream.