CHAPTER XV. TAILINGS AND DUMP. Tailings. The refuse material thrown aside in quartz, drift, hydraulic, or other mines, after the extraction of the precious metal, is called "tailings." The tailings from hydraulic mines are called "débris " also. The number of cubic yards of débris from the various gravel mines discharged in 1880-1 into the streams and valleys of California, between Chico Creek on the north and the Merced River on the south, has been estimated at about 46,000,000. To this amount, according to Professor Price, there should be added 1,000,000 cubic yards from the tailings from the working of 1,500,000 tons of quartz by 12,546 stamps in mills. Composition of Tailings.-The tailings from mills consist of pulverized quartz particles. The refuse from gravel-washing is of all forms and dimensions, and is composed of the most diversified materials. The light particles of soil, loam, and sand are easily carried forward by running water, while the rocks and boulders, though readily transported through sluices, lodge and distribute themselves, when discharged therefrom, in the creeks and streams in accordance with their size, shape, and specific gravity, and for their further removal the agencies of time and flood are necessary. Cemented material and pipe-clay are more or less disintegrated and ground down in the process of sluicing. When subjected to the action of running water further pulverization and disintegration ensue, the actual amount of which is unknown. Wear in Running Water.- The wearing down of solid cobbles and boulders by running water after lodgment in the beds of large streams, at a distance from the mine, is not great. When these materials are carried further forward by floods or torrents they move along the bottom until they find permanent lodgment, consequent upon a decrease in the grade of the bed of the stream or from some other cause. In water the weight of rocks is materially lessened, and the friction which would be due to their weight is correspondingly decreased. The constant collision and rubbing of the harder rocks against each other smooths and polishes them, somewhat changes their form and lessens their surface, and, to a certain extent, reduces them to fine powder but not to sand. Experiments made to ascertain the wear due to erosion of solid materials transported by rivers or streams tend to establish the fact that no perceptible deposit can be attributed to such cause, as the sediment from such wear is found to be a very fine powder, which immedi ately passes off in suspension. The distribution of gravels along the course of any stream will be found to be in accordance with their size, forin and specific gravity, and distance from the source. Thus the material composing the bed of a stream, which may at its source consist entirely of large boulders and cobbles, will become finer and finer through the successive stages of gravel, pebbles, and sand, until it is finally discharged as muddy water into the ocean. Effects of Hydraulic Débris.-The working of hydraulic mines in California has here and there given rise to disputes with farmers. These disputes have, unfortunately, been carried into the domain of local politics, and thereby not only brought into undue prominence, but also exaggerated, and an equitable settlement prevented. Meantime manipulators have taken advantage of the situation to the detriment of both the farming and the mining interests. The navigable waters affected by the mines are the bays of Suisun and San Pablo and the Sacramento, San Joaquin, and Feather rivers. The smaller and non-navigable streams which receive more or less of the sands are (besides the Trinity and Klamath rivers, where so little washing is done that they need not be considered): the American (tributary of the Sacramento) in the north; and the Merced, the Tuolumne, the Stanislaus, the Calaveras, the Mokelumne, and the Cosumnes (tributaries of the San Joaquin) in the south. The quantity of débris which has been washed into these streams is unknown, and data based on reconstructed topography in the mining regions are, from the nature of the case, simply guesses. The only available method of estimating with any approach to accuracy the amounts of material mined seems to be that of taking the water used and averaging the duties of the inch, as surveys of the washings are kept up only in exceptional cases. The inch differs as much as 20 per cent., the nature of the ground mined continually changes, and the character of the sluices varies not only in every district but in almost every claim. These estimates, therefore, must be consid ered as the mean of many conjectures. It can be safely stated that only in a few instances do any of the ditches discharge the quantity of water which they are rated to deliver according to official statements or in the assessors' returns, from which sources chiefly the cubic yards mined have been estimated. The following tables, XXIX. and XXX., are based on this method. Table XXIX. is from William Hammond Hall, State Engineer, Report of 1880, part iii. p. 24. Table XXX. is from Lieutenant-Colonel G. H. Mendell's Report upon Mining Débris in California Rivers, 1882, p. 15: In the region south of the American River Mendell's Report shows the discharge of tailings to be 7,414,465 cubic yards. The differences in the above tabulated estimates, which were undoubtedly prepared with care, show how difficult it is to arrive at exact data. In view of the fact that the details on which the calculations are made are not given, it is impossible to criticise with fairness. It would appear that the duty of the inch is rather too large.‡ By far the greater part of the material washed remains comparatively near the ends of the sluices in the cañons until removed by heavy freshets. "In the Polar Star and Southern Cross mines, at Dutch Flat, I have estimated that nearly 50 per cent. of the material mined is of a character which need never be carried a mile below the dumps; it is of heavy rock and cobble-stones, and probably not over 45 per cent. of the whole need ever become sandy and sedimentary in character if reservoired before being transported very far; so that all but about 15 per cent. could be held readily behind dams and other obstructions in the cañons." S The State Engineer's estimate of quantities washed is based upon the returns of the amount of water used, made by mining superintendents or secretaries, on blank forms furnished from the State Engineer's office. + Colonel Mendell's estimate is based upon returns of water used in mining, made by the county assessors to the State Engineer, as provided by law. The average duty of the inch for the region draining into the Sacramento Valley is (according to the tables) 3.6 cubic yards, and for the region south of the American River 2.2 cubic yards. The latter is certainly, and the former probably, too great. § Report of the State Engineer, 1830, p. 23. The coarse detritus which gets into the streams and is subjected to the action of floods is moved along when the grades are over 40 feet to the mile, and is deposited mostly when the grade is lessened to between 30 and 20 feet. "The sands predominate greatly " when the grade is reduced to 10 feet and less.* The finest and lightest material is held in suspension until the velocity of the water carrying it is greatly reduced. The amount of material suspended in the California rivers has been estimated from tests made of these waters, but these tests have not been continued for a sufficient length of time to afford any reliable results. The deposition of this material on lands overflowed during high water was one of the original causes of the disputes mentioned above. Up to the year 1880, the total area in the Sacramento Basin thus affected is estimated by the State Engineer at 43.546 acres, a large portion of which was of little value and had always been subject to overflow. The catchment area on the east side of the Sacramento Valley is very large, and the descent from the high sierra to the valley is very abrupt and precipitous. During the stormy seasons immense quantities of water, caused by rainfall and melting snows, are rapidly discharged into the lowlands, where the river channels, having but small areas † and light grades, are unable to carry them off, and floods invariably follow. The reservoirs which have been constructed by the hydraulic mining companies in the mountains partially mitigate the evils arising from this source. THE DUMP. It is impossible to lay too much stress on the importance of the dump, as without it hydraulic mining could not be carried on. Where thousands of cubic yards of * Report of Lieutenant-Colonel Mendell, pp 33 and 34. |