In such mines ore is usually transferred with wheelbarrows or with ore buckets on small trucks that run on narrow-gauge tracks. Hoisting is nearly always by buckets, which are lifted by a handpower windlass, small gasoline engine, or horse whim. At the surface the ore is either washed in log washers or roughly hand picked (if hard ore) and hauled to a custom mill.

OTHER SYSTEMS

The square-set system of mining is used to some extent in the surface sections of the residual gravel fluorspar deposits in the western Kentucky district. At the Yandell mine the following system was in use in 1922.

The deposit, in blocks 150 feet in length along the vein, was being worked by contractors. In the middle of each block a small cribbed

Vein

Hanging wall

FIGURE 3.-Modified square-set system used in Yandell mine, western Kentucky

shaft was sunk about 50 feet; from its bottom drifts were run in each direction to the end of the block. These drifts were timbered by a modified square-set system. Regular four-piece drift sets separated by plank dividers were used.

Where the vein was wide, long caps reaching to the footwall were used as shown in Figure 3.

At each end of the block, raises timbered with the same modified square-set system were put up to the top of the ore. Then the ore was stoped by successive vertical slices parallel to the raises, retreating from each end of the block toward the shaft, and the openings made were all supported by square-sets. Stoping was continued until all the ore in the block above the 50-foot level was withdrawn.

If a second horizontal slice was necessary-that is, if solid rock was not reached at 50 feet-the shaft was deepened 20 or 30 feet and a second lift taken in the same way, the timber mat from the workings above being caught up as it was encountered. Only a 20 or 30 foot slice can be taken on the second lift because of the weight of the ground. Successive layers were removed in this way until solid rock was reached.

If the ore was so wide that single caps would not reach from wall to wall a different method was used. Formerly only part of the vein was taken in the first mining, and the workings and shaft were allowed to cave. Later, a new shaft was sunk and another strip adjacent and parallel to the first strip mined out in the same way.

In some places the vein was so wide that it was necessary to remove the ore in three or more such parallel strips. This method was so slow, inefficient, and wasteful that another method was devised.

Wide places in the vein (some of them as much as 40 feet wide) were mined thus: A shaft was sunk to a depth of 50 feet in the center of the block, and drifts were run down the middle of the vein in each direction to the ends of the block. From the ends of these drifts crosscuts were driven to the foot and hanging walls, and from the ends of the crosscuts raises were put up to the top of the vein. The ore was then stoped out of the vertical slice over the crosscuts back to the drift, then other crosscuts were driven to the walls next to those just mined out and the process was repeated until all of the ore was mined back to the shaft, as Figure 4 shows.

In all this work the ground was supported by square sets like those described.

When all of the "soft ore" down to the solid rock has been removed it is planned to leave a horizontal pillar of solid vein filling

(locally called an arch) 5 to 15 feet or more thick to support the surface and walls. The thickness of this pillar will depend on the width of the vein, the strength of the ore and walls, and the nature of the overburden. Below this pillar it is planned to remove the ore by one of the regular methods of "hard-ore" mining common in the district.

Square-set timbering is also used to some extend in places in some of the hard-ore mines where the walls are broken and difficult to hold.

Shrinkage stoping is the standard method of mining hard ore in the three large mines on the Rosiclare vein in Illinois, at the Franklin mine in Kentucky, and at a few other mines in the IllinoisKentucky district. This method, as it is applied at the mines of the Fairview Fluorspar & Lead Co. (now Franklin Fluorspar Co.), near Rosiclare, Ill., is described in detail later.

Overhand stoping on stulls is similar to the shrinkage-stoping method, except that no ore is allowed to accumulate in the stopes,

and stulls are set between the walls to provide a footing for the miners close to the working face above. This method, as applied at the Mary Belle mine, is described later.

Underhand stoping into raises has been used at a few Kentucky mines. In this method the ore may be blocked out, as in shrinkage stoping or in overhand stoping on stulls, into blocks or stopes of convenient length, and at the center of each block a raise is put through connecting the adjacent levels. Then, beginning at the upper level, the ore around the raise is drilled, broken into the raise, and drawn off through chutes at the bottom of the raise. As the ore is removed stulls may be set in the open stopes to hold the walls. This method is illustrated in the sketch, Figure 5.

FIGURE 3.-Elevation showing underhand stoping into raise: 0-a, b-b, c-c, Successive slices of ore broken into raise; p. pillar to be left on lines of stulls to be set to hold walls

MISCELLANEOUS MINING PRACTICE AND EQUIPMENT

Shafts. Almost without exception vertical shafts are used in fluorspar mines in the United States. At small mines, worked by contractors or small leasing companies, the proper location of shafts has received little attention; the shafts almost invariably are sunk on the vein regardless of the nature of the ground. They are cut as small as possible consistent with ease and speed in sinking (many are 4 by 4 feet or smaller), and are lightly timbered. Such shafts are too small for efficient work, they are not strong enough to stand much earth or rock pressure, and many of them cave before the sections of the deposits they serve are mined out.

Shafts sunk on the vein have other serious disadvantages. Much ore must be left standing in pillars to protect the shaft, or there is the constant danger of its collapsing. Even if pillars are left there is still some danger of movement if the vein is wide and large open stopes are left.

The sinking of small, single-compartment shafts in the vein is, of course, permissible for exploration or development. If the vein proves to be narrow and nonpersistent in depth, to use the smali

prospect shafts for extracting the small tonnage recoverable may be economical. Often, however, development work is not carried deep enough to determine conclusively the downward extent of the vein. There is little excuse, except lack of ready capital, and this should not be allowed to govern, for the mining of a wide vein through numerous small, temporary, ill-equipped shafts scattered irregularly along it is not good engineering practice.

The best practice, and that which is now followed by all of the large companies, is to sink a main vertical shaft or shafts in the footwall and so far from it that no settling of the vein or hanging wall will affect the shaft. If the vein has an appreciable dip, the shaft may perhaps be sunk only a few feet from the footwall side of

FIGURE 6.-Typical small contractor's headframe and log washer, Yandell mine, near Mexico, Ky.

the vein at the surface. If the vein is almost vertical the distance should be 50 to 100 feet or even more, depending upon the strength and character of the footwall rock.

At the larger mines in Illinois it is now standard practice to sink well-timbered, 3-compartment shafts that measure 5 by 14 feet to 6 by 20 feet in the clear. Of the 3 compartments 2 are for hoisting and 1 for pipes and ladders. At several mines that part of some of the shafts that is above solid rock is concreted. In many sections through solid rock timber is unnecessary except for skip guides and ladderways.

Headframes. At fluorspar mines head frames range from light, low timber structures suitable only for prospecting to those built of heavy timber and structural steel. Figures 6 and 7 show these two extremes. At a few mines the headframes have been so light and poorly constructed that hoisting accidents seemed inevitable. Both safety and efficiency demand that headframes shall be well built.

Levels. In the largest mines levels are spaced at about 100-foot intervals vertically. In some of the smaller mines 50-foot intervals are used, and at others no regular system is followed. At most mines the 50-foot interval is too small, unless the ground is hard to hold, and working drifts are kept open for long periods with difficulty. If 50-foot intervals are used instead of 100-foot the cost of drifting, timbering, and tracklaying is doubled.

Haulage. Hand haulage is common in the smaller mines, but in the largest mines mules are used. In one mine mules spot cars beneath ore chutes and make short hauls, and a gasoline locomotive is used for the long hauls. Usually gasoline locomotives are not to be recommended for underground use because of the danger from poi

FIGURE 7.-Headframe and mill, Hillside Fluor Spar Mines, near Rosiclare, Ill.

sonous gases in the exhaust. In the mines of the Rosiclare Lead & Fluorspar Mining Co. three storage-battery locomotives do most of the underground hauling; each can haul 8 to 10 steel cars holding 1 ton each down a 1 per cent grade.

The ore is usually hauled to the shaft in steel or wooden cars, bottom or side dump, holding about 1 ton each. In the large mines in Illinois the ore is dumped from the cars over grizzlies with bars. spaced about 8 inches apart into large storage bins beneath the shaft station, from which it is drawn off into skips and hoisted to the surface. At the smaller Illinois mines, and almost without exception at mines in western Kentucky, there are no ore storage bins underground. The ore is dumped from the cars into buckets that are hoisted to the surface. Where a regular production, even if of moderate size, is to be maintained the use of underground storage bins and skips is strongly recommended.