The gold-bearing valleys in the Western States of North America are dredged by machines each of which may cost as much as £100,000 to build. The dredges recover enormous quantities of gold, and they are able to work in waterlogged soil where mining operations are impossible
THE ENDLESS CHAIN OF BUCKETS in a Californian gold dredge will bring as much as 500 cubic yards an hour of gold-bearing gravel from depths as great as 100 feet. Each of the buckets of the dredge illustrated weighs 1,750 lb. and has a capacity of six cubic feet.
ON the western side of the North American, continent, by the action of water, gold has been washed towards the Pacific by the swift rivers coursing down from the great chain known as the Rocky Mountains. For thousands of miles, from Alaska down to Mexico, gold is found almost everywhere.
Towards the middle of the nineteenth century gold was discovered in California and the world’s first great gold rush began. The red-shirted miner’s only implements were a shallow circular pan and a shovel. He filled the pan with a shovelful of “dirt”, dipped it in water and, by clever manipulation, washed the dirt away, leaving the heavy flakes of gold at the bottom. The surface gravel was so rich that men made as much as a thousand dollars (£200) a day. Between 1850 and 1870 the world’s stock of gold increased at the rate of 30 to 40 million pounds sterling yearly, nearly all of which was the result of this rough placer mining. That early gold was found just under the grass roots and was skimmed off within four or five years. When it had gone the miners began to use rockers, cradles and other mechanical devices which would treat a leaner type of pay dirt. All these methods were similar. The gravel dumped in by the barrow load was washed over “riffles”, when it was often collected by mercury. In some methods, however, mercury was not used. The gold concentrated by gravity and by the “boiling” action of the riffle. Having apparently exhausted the surface ground, the pioneers went to fresh regions and the patient Chinese arrived and panned the whole ground over again. Content with mere wages, they worked long hours and made from two to five dollars a day.
The third stage in the game of gold gaining was hydraulicking — in other words, gold mining with a hose. This process is described in the chapter “How Gold is Mined”. Near Grenoble in France is a factory which uses water from a reservoir situated 1,600 feet up in the mountains above the town. The water is brought to the factory by a vertical pipe of this length and with a diameter of only
about 1 in. If this jet were allowed to escape, a man with a sword might hack at it until he was exhausted or his sword broken without making the slightest impression upon it. He might as well try to cut through a 1-in. bar of tempered steel. The speed at which the jet moves is about 100 yards a second — less than one-tenth the speed of a shell fired from a gun — yet a jet of water moving at this speed is as rigid as the finest steel and, if it hit a man’s body, would instantly go through it.
This example shows how easy it is to break up gold-bearing rock with water. The jet from the monitor may have a diameter as much as 6 in. and it roars out with a fury and force that is sufficient to tear asunder the everlasting hills. Played upon the base of a bank or cliff, it rapidly eats its way inwards, undermining the cliff until presently hundreds of tons of material come crashing down.
The cheapness of this method of mining is amazing. It will wash a ton of rock for about threepence. On the other hand its results are deplorable. It has turned fertile valleys into deserts of boulders and sun-cracked mud. The debris flowing down the streams has filled the channels, killed the fish and caused every freshet to become a flood which covers farms with a sticky blanket of slime. Even worse, the sand and mud have been washed into navigable rivers until steamers have stranded on new bars a hundred miles or more from the spot where the hydraulickers have been at work.
A PLACER DREDGE carries out all the operations of the old-fashioned placer methods, but one dredge does the work of 5,000 men. Some thirty tons of gold-bearing sand and gravel are continually being taken on board by the immense ladder of steel buckets. The fine dirt sifts through perforated screens and falls on to tables, where the particles of gold amalgamate with mercury.
The method, however, was profitable. In California alone it is said that gold to the value of a hundred million pounds sterling was taken out by this process, but it is an open question whether even that vast sum would pay for the damage done to the State. The whole of Northern California is covered with hideous scars caused by this misuse of Nature’s gifts.
The farmers protested vigorously. They represented to the State Legislature that they were being ruined and that was no more than the truth. There ensued a war between the farmers and the miners, and the whole of the great State of California was rent by the controversy. It was the rivers rather than the farmers that eventually swung the scale against the hydraulickers. River traffic was becoming paralysed. A law was passed enacting that hydraulic mining should be placed under the control of a Board of State Engineers, and that the miners should in future build dams to impound their slime and boulders or that they should form
canals to lead their rivers of waste on to desert lands. The cost of all this virtually killed hydraulic mining, though it is still practised in a few places where conditions are favourable.
Meanwhile some inventive genius in New Zealand had been using a dredge to draw up gold-bearing sand and mud from the bottom of a river. This paid well and the idea was imported to California. Who it was history does not relate, but whoever he was he had vision. He knew that there were many valleys in the West where seepage and underground water had so far made the recovery of gold-bearing gravel impossible. No amount of pumping was of any use.
100 Feet Below the Surface
If he could build a dredge on such a site, he hoped to make use of the underground water for the purpose of recovering the pay dirt below. Yet even he could scarcely have visualized the tremendous industry that was to grow from such small beginnings. The first small dredges used in America cost £8,000 to £10,000, but those at present in use cost ten times that sum.
The earliest gold dredge was built and set up in the Oroville Valley, in Northern California, on ground occupied by one of the most famous fruit farms in the State. The profits on the pear trees were £50 an acre yearly, and the orchard was so famous that people came from long distances to see it. Yet the soil was found to be so rich in fine gold that the gold-seekers offered a huge price for the land. The beautiful orchard was grubbed up and its site is now a desert. Work went on steadily. The dredge was built of timber on dry land close to a small creek, and was similar in shape to a large scow. The machinery was hauled over miles of rough country and put in place by skilled engineers. Then a dam was thrown across the creek and, as soon as there was water enough to float the gold ship, dredging began.
The modern gold dredge is not only larger than its predecessors, but it is also much more efficient. Again, instead of operating with steam as the first dredges did, it is now generally worked by electricity. Gold dredges are in use not only in California but also in Idaho, Oregon, New Mexico and Nevada. Alaska has at least fifty of them.
ENDLESS CHAIN OF BUCKETS on one of the largest gold dredges operating in the Western States of North America. Each bucket has a capacity of 15 cubic feet, and weighs about two tons. The buckets are strongly built, being fitted with lips of special steel. There may be as many as a hundred buckets on the chain. Sometimes masses of rock come on board in the buckets and the machinery has to be stopped while the rock is rolled overboard again.
As a rule, a gold dredge is built and worked by a private company or corporation. It is not often that the small investor is asked to buy shares. The capital needed is considerable, but profits are so certain that most of these dredges pay for themselves in less than two years. Everything depends upon having a sufficient area of land on which to work and a sufficient depth of gold-bearing soil. The newest dredges can work down to more than 100 feet below the surface. The area to be worked is carefully prospected beforehand by boreholes, and the area pre-empted will probably vary from 300 to 1,000 acres. A company aims at seeing at least twenty years of work ahead; for, though a dredge can be taken to pieces and moved, this is a costly business. The machinery alone weighs it least 300 tons.
This machinery differs a good deal from that used in the ordinary dredger which may be seen clearing out a harbour or deepening a river. The river dredger generally has to deal only with soft mud, but the gold dredge will be rooting among gravel, pebbles or even large boulders. All kinds of dredging machinery have been tried — steam shovel, dipper and suction — but the California gold dredges have evolved a type of their own which consists of an endless chain of from fifty to one hundred large steel buckets which revolve on a swinging girder.
The buckets are massively built, for they have to withstand heavy shocks. When great masses of rock come up in the buckets the machinery must be stopped while they are rolled overboard. Working in gravel or ordinary earth the capacity of a dredge is 400 to 500 cubic yards an hour. The worst kind of soil from the gold dredger’s point of view is clay, which coats the buckets and working parts with a sticky mass and is difficult to remove.
As the material comes on board the dredge it is dumped by the buckets into a revolving screen which separates the fine stuff from the coarse. The coarse stuff, with the stones and boulders, rolls into a chute and is dumped behind the ship. Water drawn from overside is poured into the screen, which revolves so as to winnow the pay dirt in the same way as is done by pans or cradles in ordinary placer mining.
The whole principle of the gold dredge is merely an elaboration of the old-fashioned placer work, but one dredger does the work of at least 5,000 men. Some thirty tons of sand and gravel are constantly climbing the steel ladder. The fine dirt sifts through the perforated screens and passes to riffled tables. In the riffles is quicksilver which amalgamates with the gold particles. The process is so effective that scarcely one-thousandth part of the gold escapes.
Floating Gold Dust
What is lost is chiefly dust too fine to be secured by any known process. So fine is this dust that, if some of it is caught in a bottle and allowed to stand, at the end of three hours it has not settled but is still floating in the water.
Other small losses are caused, too, by the slight tilting of the ship as she works. This disturbs the level of the gold tables. There is also what is called “rusty” gold — particles of gold coated with iron or silica. These will not amalgamate with the mercury and are thus lost. Methods of recovery, however, are constantly improving and the percentage lost grows ever less.
Other rare metals besides gold are sometimes brought up. Platinum, iridium and osmium are found in the gold-bearing dirt, but, as a rule, in infinitesimal quantities. Sometimes fossils or implements which were made by prehistoric man are found in the dirt
and, where the ground being worked is old placer ground, odd relics such as coins, buttons and bullets may frequently be dredged up. A dredge can be made to pay if the gravel it handles contains as little as sixpennyworth of gold to the cubic yard.
A gold dredge is provided with two long steel legs which are aptly named “spuds”. These reach to the bottom of the water, and on one of them the ship swings as on a pivot. She is anchored to the shores by cables. When the dredge has exhausted one spit the second leg is lowered and the dredge takes a step forward. As the ship progresses her “sea” is constantly opened out in front but, behind, is filled in with boulders and debris.
The damage done by gold dredges, if not so widespread as that caused by hydraulicking, is almost irreparable. Before a ship lies fertile land, perhaps orchards similar to that at Oroville or fine pasture or fields of waving grain. Behind is a boulder-strewn waste. Some dredges, however, are fitted with resoiling devices which enable the land to be reclaimed.
At present some 15 per cent of all the gold recovered in Western America is obtained by the gold dredges, and it could never have been recovered in any other way. For the dredge, besides being able to make a profit out of pay dirt so poor it could never be worked in any other way, is also able to burrow in waterlogged soil where any form of mining is out of the question.
THE TAILINGS AND OTHER DEBRIS which a gold dredge leaves in its wake utterly destroy the value and the beauty of the surrounding country. The loss is not so widespread as that caused by hydraulicking.
The modern gold dredge is not only larger than its predecessors, but it is also much more efficient. Again, instead of operating with steam as the first dredges did, it is now generally worked by electricity. Gold dredges are in use not only in California but also in Idaho, Oregon, New Mexico and Nevada. Alaska has at least fifty of them.