in solid ground on part of three sides. After the manways were thus completed to within a few feet of the back of the stope, and equipped with air lines, the stope was ready for active stoping.

Stoping Operations.-Usually, in working a stope, a crew consisting of a machine man and a pick man worked from each manway, so that ordinarily five machines were working in each main-level stope. Stoping was done on but one shift a day, so the same crew was responsible for conditions at a given manway. The pickman trimmed down the back of the stope, so as to make it safe for the machine man. In a shift of 8 hr., each stope crew was expected to put in from 14 to 20 holes, 6 ft. deep, depending on the ground, and load and fire them. A round of 15 holes ordinarily broke 150 tons. The motor crew on the following shift pulled about 75 tons, representing the swell of the ore, so that the stope crew

FIG. 4. SECTION SHOWING THREE STAGES OF STOPES FROM THE MOTOR

HAULAGE LEVEL.

would have sufficient room to work on top of the broken ore. The ideal condition for efficiency and safety in a stope was to keep the top of the broken ore within 9 ft. of the back of the stope. With five machines in a stope, a separate crew of crib men was employed to build the manways, and keep them at all times well above the level of broken ore in the stope. The stopes from the main tunnel were carried to capping, or abandoned 10 ft. below the second level, if the ground above the second level had already been stoped. As soon as a stope was abandoned, the chutes were nailed up or otherwise obstructed, so that no ore could be pulled from them (Fig. 4).

Upper Levels

As already mentioned the upper levels consist of main drifts directly above the motor drifts forming the limits of the stope blocks. Crosscuts were driven every 120 ft. at right angles to the main drifts, that is, paral

lel to the center line of the stopes. These were driven on a 1 per cent. grade from each main drift, meeting in the center of the block. Each crosscut was connected to the main-haulage level by two raises which came out in the floor of the crosscut 100 ft. from each of the main drifts. On both sides of these crosscuts, stope drifts were driven, at 30-ft. intervals, for a length of 30 ft., or 11 ft. beyond the center line of the stopes. They were then enlarged sufficiently to accommodate the timbers for stope chutes.

Chute Timbering.-The chute timbering consisted of three tunnel sets spaced at 42-ft. centers, except in every other drift where an additional set was put in at 31⁄2-ft. centers to accommodate the manway. Tunnel sets were of 10 by 10-in. timbers, with posts cut so as to leave 7 ft. between

FIG. 5.-STOPE CHUTE TIMBERS ON HAND-TRAMMING LEVELS.

top of rail and bottom of cap. Collar braces were of 8 by 8-in. timbers, or round poles not less than 7 in. in diameter. Caps for manway sets were 11 ft. long to form the sill for manway cribbing. The chute lip was 3 ft. 9 in. above the top of rail and was made of 3 by 12-in. plank. Chutes were equipped with two gates so as to control the flow of ore easily. Top and sides were lagged with 2 by 12-in. plank, or split round poles (see Fig. 5).

Stope Preparation.-After the stope-chute timbers were in place, the ground above the timbers was drilled with a stope machine; but before blasting, the top was lagged with short split round poles, so that the ore could be loaded direct into a car without shoveling. An 8 by 8-ft. drift was then started on top of the chute timbers with the center line of the

2'x12'

stope as one side of the drift, and extended the full length of the stope. Later this drift was widened to 16 ft.

Manways. Manways were put up in every second stope drift built of the same dimensions and material as the manways from the main level, and carried to the third level or to capping if struck before the third level was reached. Where the distance to capping was greater than 100 ft. but less than 200 ft., connection was made with the stope on the third level, the old manways abandoned, and new ones started from the third level. Where the ore thickness was greater than 200 ft. above the second level, stopes were started from the third level, run to capping, and abandoned before the stopes from the second level were started.

ORDER OF WORKING STOPES

In a given block it was the custom to have the stopes on the top level worked out considerably in advance of the stopes started from the middle

FIG. 6-SECTION SHOWING A STOPE STARTED FROM THE UPPER LEVEL, AND RAISES THROUGH WHICH THE ORE IS DROPPED TO THE MOTOR-HAULAGE LEVEL.

level. In this way the upper stopes were abandoned before the stopes from below began to disturb the tramming drifts and stope manways. This was necessary also to avoid cutting off the raises through which the ore was dumped from the upper to the main-haulage level, before the upper stopes were worked out. For the same reasons the stopes started

from the middle level were kept well in advance of those started from the main level (Fig. 6).

HAND TRAMMING ON Upper Levels

On the middle and upper levels all ore was hand trammed from the stope chutes to the nearest raise. The raises were so spaced that the maximum length of tram was 150 ft., and the average 90 ft. for all stope ore. Tracks for hand tramming were 18-in. gage, laid with 16-lb. rails and a grade of 1 per cent. in favor of the loaded cars. The stope crosscuts were double-tracked for short distances near the raises, to permit several cars to run to the same raise without delaying or interfering with each other. Tram cars measured 2 by 2.5 by 4.6 ft. in the clear, and held about 1 ton of porphyry ore. Two trammers were used on a car, and the average tonnage trammed by each crew from a stope was about 65 tons per shift. A tallyman, stationed at each active stope, credited each car dumped to the proper crew.

Data from an Average Monthly Statement

GENERAL COSTS

During the 3-year period, 1911 to 1913 inclusive, 2,824,439 dry tons of ore was drawn from stopes at a cost of 56.6c. per ton loaded into railroad cars at the ore bin. In addition, 102,719 ft. of development work yielded 247,280 dry tons at a cost of $4.95 per foot of development or $2.05 per ton of ore. As the development was carried on entirely in average grade of ore, it about paid for itself from the ore broken. Counting every item of expense, the cost of producing a ton of ore from all sources by underground mining averaged 68.7c. By the system in use, with 16-ft. stopes and 44-ft. pillars, less than 27 per cent. of the ground developed was actually stoped, and as the stopes were full of broken ore when abandoned, the actual production represented less than 50 per cent. of the ore broken, or about 13 per cent. of the ore blocked out by the development work. The cost of production would have been considerably less, if the pillars could have been mined and the broken ore in the stopes pulled. Manway drifts and raises, and stope drifts were charged direct to stoping, all other drifts and raises being charged to development.

Cost of Development Work

Motor-haulage drifts (7 by 7.5 ft. in the clear); No. 9
water Leyner used (progress 5 ft. per round):

Tramming drifts (5.5 by 6.5 ft. in the clear) 2.5-in. piston

Raises (4 by 5 ft.); hammer machines used, (prog