operations of such an organization in the Leadville basin may

"This association includes nearly all the leasing companies, as well as the owners of territory embraced in the Leadville basin. All mines operating within the association territory bear the cost of pumping in proportion to their output, based on net smelter returns less cost of haulage. By means of counters on these pumps the amount pumped is computed in gallons and charged to the association at the rate of 10 cents per 1,000 gallons. Those mines which pump are credited with the amount of water they have raised. Taking the entire district, investigation shows that the flow of water which must be handled is not less than 15,000,000 gallons a day. Comparing this amount of water with the average daily tonnage of the district for the past year, we find that 28.6 tons of water are raised for every ton of ore raised. Careful estimates of the cost of pumping have been compiled and show that it costs 4 cents to pump each ton of water to the surface. Hence, the cost of pumping referred to the ore makes a charge of $1.14 per ton extracted."

Ventilation. As a usual thing the ventilation of metal and especially precious metal mines is easily accomplished. There are special cases, however, where ventilation is exceedingly difficult, one of the most prominent cases in the history of mining in the

1 Special Rept. Census Bureau, Mines and Quarries, 1902, p. 575, and Rept. Director of Mint on the Production of the Precious Metal, 1900, p. 118.

United States was that of the mines of the Comstock lode, but the conditions of excessive heat encountered there were exceptional and cannot be cited as other than unusual in practically every respect.

With the introduction of compressed air drills the problem of mine ventilation has been materially simplified, as the exhaust of a number of drills is often sufficient, especially when supplemented by natural ventilation. Under ordinary conditions, then, gold and silver mines may be ventilated by two shafts, preferably with the mouth of one elevated somewhat above the other, or by one shaft in which is placed a vertical partition or dividing extending from the surface to the workings below. When tunnels or drifts are employed in opening a deposit, ventilation can readily be maintained by running a wooden or metal pipe into the workings along the line of the drift and blowing air in with a blower or forcing it in with a jet of compressed air. Tunnel ventilation is most advantageously accomplished when the inner end of the tunnel or the workings are connected with the foot of a shaft sunk from the surface above. The one essential condition is that there must be a motive column of air, i.e., that one of the two columns of air necessary for a movement of air currents, must be heavier than the other, which may result from greater length or density of the column.

In mining districts where considerable development-work has been done the ventilation of the mines is disposed of as soon as connection is made with other workings in the immediate vicinity a usual and legitimate method of procedure. However, when the atmosphere of the mine is close and warm or hot, owing to chemical change in the rock-walls or heat emanating from uncooled masses of igneous rocks, the problem of providing a sufficient volume of cool and fresh air becomes more difficult. The usual method of procedure under such circumstances is to install a blower at one of the mine openings thus aiding natural ventilation, or placing small fans, usually of the high speed type, at or near the localities where the greatest inconvenience is experienced. These fans are driven by air or electricity and have practically solved the difficulty of poor ventilation in metal mines. They are also used quite extensively in coal mines.

CHAPTER VI.

EXTRACTION OF VALUES.

THE extraction of values from gold and silver ores may be accomplished by mechanical or chemical means, the choice of the process depending largely upon the occurrence and condition of the ore. In general it may be said that free-gold and silver may be extracted by the former, while when chemically combined, chemical means must be resorted to.

Another important consideration in the extraction of values from ores is their reduction to a size suitable for the removal of the individual particles of useful metal or mineral. It is evident then that the size of the particles and their matrix are important factors in the economical working of a given ore.

The mechanical treatment of ores may be considered as milling, while the chemical treatment is usually spoken of as metallurgy. However, there is no sharp line dividing the two processes and they are probably more often used together, as supplementary processes, than separately, although in smelting there is often no connection, except in some instances a slight hand treatment, as sorting.

As between mining and milling the latter is probably of the most importance as a factor in the growth of the mineral industry. Means of winning ores from the earth naturally antedated the extraction of values therefrom, for which reason attention was first turned to improvements in mining the ores. Another important factor was the condition of the metal sought in the ore - little or no attention was at first paid to those deposits in which the metals occurred other than in the free or native state. The extraction of the values of such ores meant only a simple reduction, but, then as now, the methods of extracting the ores from the ground were largely independent of the after-treatment and were the immediate object of improvement. It is not surprising, then, that mining should have reached a comparatively high state of perfection at an early date, while the advance made in extraction of values has been by slow and devious ways, and even yet is wanting in many details. However, it has been largely through the overcoming of apparently

impossible conditions in extraction of values that the remarkable and phenomenal growth of mining in all its phases has been due.

The reduction of working costs of the Comstock ores of from $50 to $30 was considered both remarkable and extraordinary. The cost has, however, been still further reduced to $10 and even $7 and $6 per ton, which under the present conditions of low-grade ore is considered high, but could it have been effected during the production of the high-grade ores it would have meant not millions but billions saved.

The economical working of low-grade ores is becoming of more and more importance and will continue to draw the attention of mill men and metallurgists until a point has been reached when ore containing values worth $1 and even less per ton can be worked with profit. Such a process would be tantamount to the discovery of new mines of the precious metals and would be a potent factor in maintaining if not materially increasing the production of gold and silver.

For convenience of discussion in this connection, extraction is considered under the two general heads of milling and metallurgy, but it is obviously impossible to treat of them exhaustively, and therefore typical examples are given of the various methods and processes which have proven to be especially suited to the classes of ores mined.

HISTORICAL SKETCH.

The washing of gravel for gold dust and nuggets constitutes probably the first attempt at extraction of values from their natural and crude surroundings. Further, this may be considered the first method employed in each new locality in which gold, especially, is discovered.

Where free-gold occurs in the upper oxidized portions of veins its extraction from the loosely associated matrix of weathered rock is a comparatively easy operation and does not involve extensive and costly appliances. Later, when considerable depth is reached and the free metal gives way to combined forms, as auriferous sulphides, in which the gold and silver occur in smaller quantities and from which it is more difficult to recover, the problem of extraction becomes more complicated. Improved methods both mechanical and chemical must be employed, thus elaborate and costly equipments are necessary, which are planned as much with an idea of preventing loss as ensuring rapid and economical work.

If, at the present time, auriferous deposits are found and give

evidence of permanence, the crude methods are soon replaced by highly efficient and labor-saving devices. This was not, however, the case in the early days of the mining industry and that, too, even as late as the discovery of gold and silver in the United States, with which this treatise is particularly confined. Nevertheless the history of mining and milling and metallurgy of South America, Mexico, and the United States is so closely connected and intimately interwoven that casual mention of the two former countries is not out of place here.

The reduction of ore was first accomplished by hand mortars which were used in the Southern goldfields of the Appalachian region as early as 1825, or about the time that vein-mining began, and were employed in reducing the silver sulphurets of the Comstock lode in order that the gold might more readily be extracted. The hand mortar was replaced by the Mexican drag-stone mills and wooden stamps.

Another Mexican method of reducing ore, which in many instances was employed in place of hammers, was the use of a boulder attached to a balanced pole, the ore being placed upon a large stone and pounded by the spring-pole hammer.

The following interesting description is given of the early methods of reduction employed in 1835 by the United States Mining Company, at their mine on the Rappahannock River, Virginia: 1

"The plant consists of a crushing-mill (rolls) and a vertical mill (stamp-mill) in a building 26 by 36 feet. Both mills are located on the ground floor, and are propelled by a water-wheel 11 feet in diameter, with a 11-foot face. The crushing mill has three sets of cylinders 2 feet in length and 15 inches in diameter, the first or upper set fluted, the other smooth. The ore is thrown into a hopper on the upper floor, from which it is conducted over an inclined shaking table to the fluted cylinders by which it is crushed to a size fromto 1 inch in diameter. The crushed material is equally divided, and goes to the two sets of smooth cylinders. By them it is further greatly reduced, ranging from impalpable powder to grains as large as coarse hominy. From these cylinders it falls into a sifter having the fineness and motion of the common meal sifter, from whence the material which passes through is conducted to 12 amalgamators, constructed upon the principle of the Tyrolese bowls, making from 90 to 100 revolutions per minute. They perform the office of washing and amalgamating. The sand discarded

T. A. I. M. E., Vol. 25, p. 682.