This mining method was developed in the Michigan iron mines. The principles of the method are il lustrated in fig. 8. Main levels are from ioo to 1 so ft. apart, and are connected by raise. At intervals of 20 to 25 ft. vertically, sub-levels 7 to 8 ft. high are driven from the raise to the end of the stope. These sub-levels are commonly a single drift down the centre of the stope. By driving cross-cuts at the end, sub-level No. i is for a short distance widened to the limits of the stope. Inclined holes are drilled around the chute raises C which are put up from the haulage level at intervals of 25 ft. or even less. These holes are blasted to give a funnel-shaped top to the chutes. Holes are now drilled in the back over the sub-level, and a slice in the centre of the stope from level to level and the sub-levels is shot down, the broken ore falling into the chutes. Sub-level No. 2 is taken back by first cross-cutting to the full width of the stope, and then drilling down holes in the floor to remove part of the lower pillar and "uppers" in the back to take out half of the upper pillar. Thus miners in No. 2 sub-level retreat toward the
raise, drilling down holes in the slice B and up holes in the slice D. The ore falls into the chutes. The ore should be firm and free from slips. The sub-levels are carried back in steps, one working face being some 3o ft. horizontally ahead of that above.
If the walls are soft some waste will fall and mix with the ore. If the ore is hard, the cost of driving the sub-levels and drilling the ore is increased. In long stopes a vertical cut may be made in the centre of the stope from level to level and the sub-levels are carried back each way from this centre point. In wide stopes,
to Ioo ft. in width, two parallel haulage drifts may be driven. These are connected at intervals by cross cuts as at A. This method entails a high cost for development, but requires little timber and gives a large output from each stope because of the several working places. The ore can be drawn out at once. The method is limited to deposits having a dip of not less than
since the ore should work freely down the footwall.