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 falls. In this method of mining no pillars need be left under the levels, as the rock-filling gives sufficient support to the roof. This method of mining affords the maximum of safety to the miners.

In the working of thick deposits the block of ground between two levels is divided into horizontal sections or floors which are worked either from above downward or from the bottom upward; in the first case the separate floors are worked by one of the caving systems; in the second, generally with the aid of filling. Fig. 9 illustrates

the working of a block of ground by the top-slice caving system. Above, the ground has been completely worked out from the surface, and the space formerly occupied by ore is now filled with the débris of the overlying strata which has caved in above the block of ore now being worked. There is considerable thickness of old timber left from the working of the upper levels. This mat of timber forms a roof under the protection of which the mining of the ore proceeds downward floor by floor. The working-floors are connected by winzes with the main haulage roads below. These winzes serve for ventilation, for the passage of the workmen, and for chutes through which the ore is dumped to the level below. The working out of each floor is conducted much as if it were a bed of corresponding thickness. Haulage roads are driven in the ore so as to divide the floor into areas of convenient size. These separate areas are then mined in small rooms, each room being timbered as in mining under a weak roof rock. The room is driven in this way from one haulage road to another or to the boundary of the ore body. On completion of any room the timbers are withdrawn and the overlying mass of timber and rock is allowed to fall and a new room is started immediately alongside of the one just completed. In this way the whole floor is worked out and the mat of timber and overlying rock is gradually lowered and rests upon the top of the ore forming the floor below. Before abandoning a room it is usual to cover the bottom of the working-place with lagging-poles, which facilitate the mining of the floor below. In this manner one floor after another is worked until the floor containing the main haulage roads of the level below is reached. In the meantime a new level and a system of haulage roads have been driven a hundred feet below, and winzes have been driven upward to connect with the old level which is to be abandoned. The floor containing these old haulage roads now becomes the top slice of the one hundred-foot block of ground below and is mined out as described.

Several floors may be mined simultaneously, the workings in the upper floor being kept in advance of those below, so as to allow the broken mass above to become consolidated before it is again disturbed by the working places of the next floor. This system permits the complete extraction of the ore at moderate cost and without danger to the men.

The subdrift caving system, fig. 10, differs from the top-slice system mainly in the greater thickness given to the working floors, which may be from 12 to 40 ft. in thickness, whereas in the top-slice system the height of the floor is limited by the length of the timbers used in the working-rooms, rarely over 8 or 10 ft. The subdrift system requires a smaller amount of narrow work in excavating the necessary haulage roads, and is therefore better adapted to hard ores in which such narrow work is expensive. The mining of each floor is carried on in sections with small working-places which are first driven of moderate height to their full length and width, leaving a back of ore above and pillars of ore between to support the upper portion of the upper layer or floor.

These pillars and the back of ore above are then mined in retreating back towards the haulage road. The subdrift system is somewhat cheaper than the top-slice system, the output per man being greater. The bottom-slice caving system of mining begins at the bottom of a hundred-foot block of ground, a floor being excavated under the whole area, leaving pillars of sufficient size to support the ground above. These pillars are then filled with blast holes which are fired simultaneously, permitting the whole block of ground to the level above to drop. A floor is then reopened in this fallen ore, leaving pillars for temporary support which are blasted out as before. This is the cheapest of the three caving systems, but is applicable only when the deposit lies between walls of very solid rock, as otherwise wall rock is liable to cave with and become mixed with ore, which adds greatly to the expense of handling.

When rock filling is available, as when the ore contains much barren material to be left behind in mining, the ore body is divided into blocks of convenient height as above, and these blocks are divided into floors, the bottom floor of each block however being attacked. Each floor is opened up by subsidiary haulage roads and worked out in small rooms which are timbered and filled with broken rock when completed. An adjoining room is next excavated and filled, and thus the whole floor is worked out and replaced with rock-filling. Work is then. started on the floor above, the upper floors being connected with the main haulage roads by winzes which are maintained through the filled ground. Several floors can be mined simultaneously, the work in the lower floors being kept well in advance. Instead of mining in horizontal floors the filling method permits the ore to be mined in vertical chambers or slices which extend from one level to the next above and from one wall of the deposit to the other. When a chamber has been excavated and completely filled the slice adjoining is mined out, or at times a block of ground may be left untouched between two filled chambers and then mined out. In the latter case the top-slice caving method will usually be employed for the working of such intervening pillars. In order to lessen the cost of handling the rock-filling, the excavation sometimes takes the form of inclined working-places, parallel to the slope naturally taken by the rock when dumped from above into the working