Thus it is said that the grade is 6 inches, meaning 6 inches to 12 feet or inch to the foot. The run of sluices may be several hundred or several thousand feet long. The false-bottoms for sluices are called "riffles." The wood preferred for the block-riffles is that of the "digger" pine (Pinus sabiniana). Longitudinal riffles are made of poles, wooden rails covered with strips of iron, or iron rails, In New Zealand the riffles are sometimes made of transverse bars of angle-iron, riveted to angle-iron or placed in a wooden frame, which enables them to be reversed when worn. The sluice-boxes are lined with thin sheet iron, and sacking or cocoa-nut matting is placed under the riffles to assist in retaining the gold. 66 In order to catch its gold more effectually, the finer material is taken out and treated separately in broad sluices called "undercurrents," at the side of the main one. A grating of bars of iron, 1 inch apart, called a grizzly," is fixed across the main sluice, and the fine gravel and sand which drop through are led to a broad, shallow, sloping box, eight or ten times as wide as the sluice itself, and paved like it with stones, wooden blocks, or longitudinal riffles. The finer portions of the gravel, after passing over the "undercurrent" and depositing much of their gold, are once more turned into the main sluice lower down. The big boulders rushing down the sluice are of service at first by breaking up gravel which is much cemented together, but at the same time they naturally wear out the sides and the pavement. It is therefore advisable to get rid of them, as soon as they have done all the useful work they are capable of performing. This is effected by arranging a "grizzly" or grating which will deliver the boulders into a ravine or gully, and so dispose of them without any further cost. Mercury is added several times a day at the head of the sluice; and the upper part, say, the first 1000 feet, is cleaned up every two or three weeks. At the time of the clean-up the washing down of the gravel bank is stopped, or the current is diverted into a parallel line of sluices. A small quantity of water is turned into the sluice which is to be cleaned up, the blocks are then taken out, washed, and put on one side. All the amalgam is picked up with iron scoops, washed, and squeezed through canvas or leather, and the amalgam is retorted. The spongy gold remaining behind in the retorts is then finally melted into bars. The mercury recovered by condensation is used over again. When the bed-rock is below the drainage level, the hydraulic elevator may be employed. A jet of water under heavy pressure * * Rickard, "The Gold-fields of Otago and Alluvial Mining in Otago," Trans. Amer. Inst. M. E., vol. xxi. 1892, p. 443 and 455; Parliamentary Reports on the Mining Industry of New Zealand, Well.ngton, 1891, p. 67, with plates. is brought by a pipe A (Figs. 345 to 348) to the nozzle B, and rushes up the pipe D, producing a powerful suction in the hopper" C. The water and gravel are carried up against the cast-iron striking plate S, and then run down the sluice-boxes. 66 Fig. 349 explains the method of using the elevator for treating an immense accumulation of tailings at the Blue Spur, Otago, N.Z. On the left hand side is a huge nozzle playing upon the face of the tailings, 59 feet high, and washing down the gravel and sand of which they are composed. To the right is the first elevator, which raises the stuff 15 feet into a set of sluice-boxes, and further to the right is a second elevator lifting it 56 feet vertically into another run of sluice-boxes. The quantity of water used is measured by a unit called the "miner's inch," which unfortunately is not invariably the same. The term means the quantity of water discharged per square inch of sectional area of an orifice cut through a vertical board, forming one side of a box. The discharge will necessarily vary with the height of the surface of the water above the orifice, the thickness of the board, and the shape and nature of the orifice; as these factors of the problem are not the same in all localities, it is impossible to give one definite value for the miner's inch of water. The orifice is usually rectangular, but it may differ in height and width. However, the quantity represented by the miner's inch may be taken as varying from 2000 to 2600 cubic feet per FIG. 349. THE BLUE SPUR 24 hours; in some cases the outflow is reckoned for 10 hours only, and is spoken of as the "ten hours miner's inch." Under these circumstances it is evident that great care must be taken to ascertain precisely what inch is meant, before making any estimates based upon this uncertain unit, the miner's inch. The "duty" of the miner's inch is "the quantity of material washed by an inch of water in 24 hours." As might be expected, the duty varies very considerably, indeed from 1 to 4 cubic yards. The duty necessarily depends upon the pressure of the jet of water, and upon other causes, such as "character of the material washed, height of banks, use of explosives, size and grade of sluices, and class of riffles. The sluice affects the duty of the inch in so far as its capacity regulates the quantity washed." + Under favourable conditions at Cherokee Flat,+-viz., fine "The Auriferous Gravels of California," Ninth Annual Report of the State Mineralogist for the year ending December 1, 1889; Sacramento, 1890, p. 122; and Bowie, op. cit. p. 124. Bowie, op. cit. pp. 268, 269. + Bowie, op. cit. p. 268. + 1 material, high banks, head of 300 to 350 feet, and grade 3, 55 cubic yards are said to be the duty of the miner's inch. At Osceola,* in Nevada, the average washing in 1890 was 1.62 cubic yards to the inch of water and it was expected that the duty would be raised eventually to 2 cubic yards. It is hardly necessary to say that the yield of the gravel varies between very wide limits, and it is consequently impossible to lay down any average for the hydraulic mines of California or any other country. But the accompanying table gives the results of actual work, and will at all events show that poor gravel, containing gold worth only 10 or 15 cents, say, 5d. to 74d. per cubic yard, can sometimes be made to pay good profits. With the exception of Osceola, the works were all in California; the figures are borrowed from Mr. Bowie, and many other examples of the yield of auriferous gravel will be found in his work and in Mr. Hammond's report. A cubic yard of gravel is estimated by Mr. Hammond to weigh from 1 to 1 tons. One of the great difficulties with which the hydraulic miner has to contend is getting rid of the enormous quantities of refuse produced by his washings. Some idea of these quantities will be gathered from the statement that one working alone, the Gold Run Ditch and Mining Company, was for a period of eight years discharging 4000 to 5000 cubic yards of sand, gravel and boulders daily into a tributary of the Sacramento. As a natural con sequence banks were formed in the river, obstructing the navigable channels, rendering overflows more frequent and destructive, and causing valuable land to be destroyed by deposits of sand. Litigation ensued, and some years ago the Superior Court of Sacramento decided that the hydraulic mining companies must build dams to impound the coarse and heavy débris, or take other means to prevent their being washed down the rivers. The consequence of this decision was a great diminution of the amount of hydraulic mining carried on in the State; but quite lately an Act of Congress has been passed which will allow work to be resumed at many of the mines. (2) EXCAVATION OF MINERALS UNDER WATER. -In Chapter IV. mention was made of dredges of various types, which are employed for the purpose of extracting gold-bearing sand and gravel from the beds of rivers. Gold is not the only mineral worked in this fashion; in South Carolina phosphate of lime is dredged up from river-bottoms, and in Prince Edward Island a shell-marl obtained in a similar manner is sold as a fertiliser. Lastly, on the coast of Germany, between Dantzig and Memel, two forms of subaqueous work are applied to the getting Eng. Min. Jour., vol li., 1891, p. 630. |