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956 FIG. i.

Inspiration Mine, Arizona. The ore-body is a massive deposit of disseminated sulphide of copper (chalcocite), containing about ipo,- 000,000 tons of low-grade ore, and overlaid by a valueless capping, 30 to 350 ft. thick. A variation of the caving system is used, known as " block-caving." The ore-body is intersected at vertical intervals of 150 ft. by main haulage levels, connecting with the hoisting shafts (fig. i). Above the haulage levels are long chute-raises, inclined at. 50 to the horizontal, from which numerous short secondary raises (" finger-raises ") are driven to a system of sub-level drifts, 35 ft. apart vertically and 50 ft. horizontally. The ore developed by each series of sub-level drifts is thus divided into relatively small " blocks," which are undercut and broken by blasting out the sup- porting pillars between the sub-level drifts. The broken ore is drawn down through the branching finger-raises underneath, into the main raises, and thence to the haulage levels, the flow being controlled by gates. As the upper part of the ore-body is thus removed, the overlying capping caves in on top of the solid unmined ore below. (For full details, see Trans. Amer. Inst. Mining Engs., vol. lv.,_p. 218.) Block-caving, conducted similarly to that of the Inspiration mine, is also used by the Ohio Copper Co., Bingham, Utah, and the Nevada Consolidated Copper Co., Ely, Nev. (Peele, Mining Engineers' Handbook, pp. 640-2).

LONGITUDINAL SECTION FIG. 2.

VERTICAL CROSS SECTION

Arizona Copper Co. A top-slicing method (fig. 2) is applied to large bodies of soft ore, carrying 2 to 4% copper. A main haulage- way is driven near the bottom of the ore-body, and above it are inter- mediate working levels, 50 to 60 ft. apart vertically. These comprise a rectangular system of tramming drifts and crosscuts, from which chute-raises are made into the ore above at 25 to 30 ft. intervals. Starting from the tops of these raises, horizontal slices of ore, 7 to 15 ft. thick, are blasted out and the broken ore is run down through the raises to the tramming level. Thence it is conveyed to the near- est main raise, connecting with the haulage-way below. (For details of the slicing operations and the manner in which the overlying capping caves as the successive slices are removed, see 18.532.) The modifications of the top slicing system noted above are chiefly in the mode of laying out the main haulage-ways and intermediate tram- ming levels, and in making raises from the latter at such short inter- vals as to minimize the labour cost of handling the broken ore mined in the slices. In one of the Arizona Copper Co.'s mines, a further saving has been effected by omitting the tramming levels and the small raises from them to the slices. A main drift is driven longi- tudinally through the axis of the ore-body, just below the capping, and from it at right angles are crosscuts, 40 ft. apart, to the walls. The panels of ore between the crosscuts are sliced back from the walls of the ore-body towards the main drift. On each side of the

latter a pillar is left, which is finally sliced back from its end, in com- pleting a floor. While one floor is being mined, the next, 11 ft. below, is in preparation. This method of panel slicing has recently been adopted successfully in the Herman gold mine, California. Over hand sloping, with square-set timbering, was formerly employed. The vein dips 45 to 60, and the panels are laid out at an inclination of 52, across the ore-body. Top slicing is also used in many other copper-mines containing massive deposits of low-grade ore, for example: Cumberland-Ely, Nev.; Cananea, Mexico; Miami, Ariz.; Bingham, Utah, and mines of the Calumet & Arizona Mining- Arizona. At the last-named property, the older caving method has been replaced by a modification called the Mitchell top-slicing sys- tem, which is found economical in reducing the amount of shovelling required. (For illustrated details, see Peele, Mining Engineers' Hani- book, pp. 619, 620.)

Other Variations. In a number of important mines workin,, ore-bodies, special conditions have been dealt with by comb two or more of the methods referred to above. Examples of i practice are to be found in the mines of the Braden Copper ' Chile; New Jersey Zinc Co., Franklin, N.J.; Utah Copper (Boston mine); Ray Consolidated Copper Co., Ariz.; Homes! Gold Mining Co., S.D., and of the De Beers Mining Co., S. / In most cases, operations begin by shrinkage stoping, after the intervening pillars are mined by top slicing, block-caving or t, level caving. The aim is to obtain a high tonnage extraction fn the ore-body; that is, to minimize loss of ore left in permanent | '" or through mixture with waste material.

Stripping Superficial Ore Deposits. Though this old mode. attacking shallow deposits of large horizontal area underwent little change, it was oftener resorte-f to in the decade 1910-20 previously, and, with good organization and plant, was being ap r. to deeper ore-bodies than formerly were considered minable stripping operations.

Standard Methods. Standardization of methods of < and working mines would undoubtedly promote efficiency economy of operation. Beds of coal, owing to their comparative uniformity of geological occurrence, can be worked to a con- siderable degree by standardized methods. The same is true of some regular bedded deposits of the base metals. But, as metalliferous deposits in general vary greatly in their physical characteristics, standards of practice in working them are less readily attainable, and any rules that may be formulated are subject to many exceptions. (To illustrate this matter, see a useful discussion by C. A. Mitke of the " Mining Methods of the United Verde Extension Mining Company, Arizona," Trans. Amer. Inst. Mining Engs., vol. La., p. 188.)

In many of the particular operations of mining, there has been of late a stronger tendency toward standardization of details. A standard, in this sense, may be denned as a well- tested and approved mode of doing a certain piece of work. For example, some large mining companies have adopted standard " rounds " of holes for shaft-sinking, drifting, cross- cutting, raising and stoping. The foremen and shift bosses are furnished with instruction sheets which specify the position, angle to the drilling face, depth, and charge of explosive, for each hole of the round. The miners are required to follow these instructions instead of doing their work in accordance with their own individual ideas. The adoption of standardized methods has been assisted by the greater attention now given to efficiency engineering. Many large mines maintain " efficiency engineers," whose duty it is to study in detail the performance of both men and machinery, thus aiming to improve the quality and amount of work done, and so reduce cost. Directed toward this end also are the recent movements inaugurated by several large mining companies to give their employees systematic instruction in the best methods of doing their work. This benefits the employer as well as the employee. Thus, an education department has been organized by the Phelps-Dodge Corp., of New York, which operates a number of mines in the south-western part of the United States. Series of lectures are delivered on practical mining topics, followed by examinations. The North Butte Mining Co., of Montana, has also given much attention in the last few years to instructing its employees and standardizing the details of many of the underground operations. The cost of maintaining these education departments is considerable, but it is amply justified by the results obtained. A wholesome spirit of rivalry is encouraged amongst the miners and' their ambition is aroused; hence, better work is done, the moral of