One of the most noted drift-mines in the State is the Bald Mountain, Sierra County, where there is every facility for economical working. There steam locomotives are used for transporting men and material through the tunnel, which is over one and one-fourth miles long.

The following sketches of the workings of the Sunny South Mine, in Placer County, will give a general idea of the method of drift-mining. At this place the main tunnel is below the channel, allowing the mine to be opened and worked in a very convenient manner.

Drifting was at one time the most extensively used method of deep mining, and through it a very large amount of information has been obtained as to the nature of the ancient river channels.

Hydraulic Mining.-Hydraulic mining is that method of gold-mining in which the ground is excavated by means of water discharged against it under pressure (hydraulicked).

The term in its limited sense, as generally applied, presupposes the existence of, 1st, water, in sufficient quantity, which can be used under pressure for mining; 2d, gravel deposits containing gold which can be worked profitably by the application of water in the manner above mentioned.

The application of the mining of auriferous gra The pressing necessity of

Origin in California. science of hydraulics to the vels originated in California. a more economical process of gold-washing became evident as the rich surface deposits were exhausted, and led to the adoption of this method, which was favored by the topography of the country.

Hydraulic vs. Drift Mining.-Deep placers, if suffi ciently rich, can be, and for various reasons generally are, worked by drifting. But the results of actual practice in Nevada County and elsewhere demonstrate that hydraulic mining, compared with drifting, employs twice

the number of men and extracts four to six times the amount of gold per lineal foot of channel.

The yield of the North Bloomfield channel by drifting has been $150 per lineal foot of channel, while hydraulicking the entire deposit in this locality has given a yield of $750 per foot.

Requirements for Financial Success. From a financial point of view it is essential for profitable hydraulic mining that there should be ample facilities for grade and dump and a sufficient head and an abundant supply of cheap water, all of which requirements vary in importance inversely with the richness and extent of the gravel. Economical management may be considered in all classes of mining a sine qua non to success; but it is especially requisite here, as the value of this method is based on the great facility with which profitable results can be obtained at trifling cost from expeditiously and skilfully washing vast areas of ground which contain relatively insignificant amounts of precious metal.

Strictly speaking, in hydraulic mining, water does all the work, but in the application of this process to the washing of cemented gravel and masses of volcanic products, it has been found that water alone has little effect on banks composed of such material, and to overcome this difficulty recourse is had to blasting in order to shatter the bank before water can be advantageously em ployed.

CHAPTER VII.

PRELIMINARY INVESTIGATIONS.

IN the investigation of all hydraulic-mining enterprises the first problem which presents itself to the engineer is the ascertaining of the value of the gravel deposits. This involves the determining of the course of the channel; the depth and position of the bed-rock, generally covered by hundreds of feet of detritus; the available area for washing; and the estimates of the yield of the ground, with the calculations of the cost of the work. Accurate information on these points is necessary. But without the assistance of underground explorations few of them can be definitely determined. A study of the geology and topography of the deposit and of its surroundings aids in determining the course of the channel, the depth of the bed-rock, and the facilities for dump. The value of the gravel can be approximated by sinking small pits, washing the material obtained from them and from such other places as good judgment dictates.

Where the prosecution of an enterprise involves the expenditure of large sums, it is advisable to thoroughly explore the ground by means of prospecting shafts and drifts. Should the results of this work warrant the opinion that the ground would pay to hydraulic, then the water-supply and the facilities for dump should be accurately determined, with close estimates of the costs.

Indications. The colors red and blue, with their varying shades, as seen in gravel deposits, are generally considered by miners to be good indications of gold in the different localities. While it is true that these different colored sands often accompany gold, it by no means follows that gold always accompanies them.

Ferruginous colored spots, so well marked in “upper or top gravel," are not, as a rule, so productive of gold in

California as they are generally found to be in the Ural Mountains.

A black sand, composed chiefly of glancing grains of magnetic iron, generally accompanies the precious metal, though it occurs also without it.

Dr. T. Sterry Hunt, speaking of the impressions which prevail in reference to the presence of black sand in auriferous alluvions, very appropriately remarks that "similar black sand residues, consisting chiefly of various ores of iron (sometimes oxide of tin and other minerals), may be obtained from the washing of almost all sands and gravels derived from crystalline rocks, and the occurrence of a black sand, therefore, in no way indicates the presence of gold. When, however, this metal is present in gravel, it, from its great weight, remains behind with the black sand and dense matters in the residue after washing."*

Explorations at Malakoff.-The explorations of the North Bloomfield Company furnish a remarkable instance of the extent to which preliminary work has been successfully carried on. To determine the value of their claims and the feasibility of working them, four prospect shafts were sunk to ascertain the value of the gravel, the position of the channel, and the depth to the. bed-rock. No. 1 shaft struck the bed-rock of the main channel at a depth of two hundred and seven feet, one hundred and thirty-five feet of which was in blue gravel averaging 41 cents per cubic yard. Drifts were driven from the bottom of this shaft a distance of twelve hundred feet on the course of the channel, the width of which was estimated at five hundred feet. The aggregate length of the channel explorations was over two thousand feet. The samples of the various drifts indicated a value of $2.01 per cubic yard. The actual yield of 21,614 tons of gravel extracted from these drifts was $33,053.69, or $1.53 per ton, or about $2.75 per cubic yard.

The gross cost of the entire prospecting work, includ. ing the four shafts, was $63,956.20.

"Geological Survey of Canada, Report of Progress, 1863-66," p. 36.