Intensive placering is now the order of things and the marvelous increase in the use of dredges attests the success which these "gold ships" have attained. It is very interesting to watch the operations of these huge boats loaded with ponderous machines, especially when they are installed in inland regions or up in high mountain gulches. Yet numbers of them are thus in steady use. Wherever suitable beds with a tolerably uniform size of boulders and gravel are found, dams are built to retain the flows of streams until ponds are created of sufficient size to contain and float the barges.
Dredges of Yuba Consolidated Goldfields, Hammonton, California.
Continual improvements are being made in the construction of these mammoth machines with a view to economy in operations that will result from greater capacities. All costs of placering are reckoned per cubic yard washed. Costs have been rapidly dropping during the past decade until now some companies, with extensive operations, are handling dirt at not to exceed three cents per cubic yard for excavating, washing, wasting the refuse, maintenance, repairs, labor, taxes, interest on investment, and the depreciation of equipment. Such figures will hold good only under very favorable natural conditions of ground and climate such as prevail in California; they have not been attained in the frigid regions of Alaska nor in the torrid South American interior. In view of the wonderful improvements brought forth by mechanical engineers, it is improper to deny that the future will bring still further reductions in placer costs. On the contrary, the signs are good for material reductions.
Dredges are very costly in their installation. They are usually designed to handle so many thousands of cubic yards per day. It has been stated, as a fair but rough rule, that "bucket" dredges will average, in initial cost, one dollar for every cubic yard the boats will handle per month. Thus, if a dredge of this type is built to treat fifty or seventy thousand cubic yards in a month, working steadily, the costs will be respectively $50,000 or $70,000. Other types of dredges, known as the "dipper" and the "suction," will cost less than the bucket type, but have not gained general usage.
"Hydraulicking" is extensively practiced. This term signifies the working of placer deposits by water which is conducted through flumes and pipe-lines and, by means of nozzles called "giants" or "monitors," is directed, in huge jets, against the banks of gravel. These banks or walls are thus torn down and, by the same water, the loosened, disintegrated materials are caused to flow into and through long, wooden, box-like troughs known as "sluices." The floors of these sluices are paved with ribs, cleats or other obstructions termed "riffles" whose function it is to retard and collect the heavy particles which may, later, during the process of cleaning up, be removed as the valuable product. The word "sluicing" is frequently used quite synonymously with hydraulicking.
Costs of this latter sort of placering are considerably higher than those of dredging; but there are many deposits not adapted to dredging operations that may be nicely worked by sluicing, so that there will always be a field for this scheme. Average costs are difficult to obtain since it happens that most of the companies now operating hydraulically are secretive in their accounts. More labor is entailed, more time is required, greater delay is occasioned in cleaning up, and the amount of water used is much greater. Where water is abundant, this last item need not be considered. It is well to remember that even a very large dredge, while requiring a continual and large flow of water through its devices, can still operate with just the water in which it floats, this water being pumped and used repeatedly; whereas, in the case of hydraulic mining, the water may be used but once and, consequently, there must be a large supply and at a good head or pressure.
But, in spite of these disparaging points, we find instances in which, under peculiarly favorable conditions, hydraulicking has been carried on at very low figures. E. B. Wilson says: "The yield of the gravel at North Bloomfield was 7.75 cents per cubic yard; the cost of mining, 4.1 cents per cubic yard. The yield per cubic yard of gravel at La Grange was 10.19 cents, the cost of mining, 6 cents. The costs of mining at these two mines would analyze about as follows: Labor, 60 per cent; supplies, 17 per cent; water, 13 per cent; office, 10 per cent. Ground carrying but 3.99 cents per cubic yard has been worked at a profit at the first mine. With such a small margin to work on, it is evident that skill and executive ability must be provided from the pipemen up." It is claimed that an Idaho mine was worked profitably with less than two cents value in the dirt, but this is to be regarded with some doubt.
The Snowstorm Placer, Fairplay, Colorado.
A typical Hydraulic Mine.