TREATMENT OF SULPHUEET OKES AT CHAKACTERISTIC MINES. 133 This raise serves afterwards as a chute (mill-hole). Drifts are then run below this pillar until the limit of the stope in length (about 30 to 40 feet in all) is reached, leaving a vertical pillar 15 to 20 feet in thickness between the stopes. The ground is then cut away between the foot- and hanging walls, completely exposing as roof the bottom of the chain pillar above, which is sprung in the shape of an arch, with its heavier toe in the foot-wall and a minimum thickness of 15 feet. This, as well as all other work in tight ground, is done by air-drills. Stoping is then carried downward by hand-drilling in circular steps, arranged in such a manner as to allow the broken ore to drop into the chute, without further handling. The angle of 45° given to the latter allows a steady flow of
	the material down the foot-wall without completely choking it. At the bottom of the chute is a rough grizzly (a, fig. 24) made of logs, which holds back the larger boulders and prevents them from choking the smaller loading pocket below. This grizzly is easily accessible from the drift, and the larger pieces of ore are here sledged. The loading-chute and grizzly are kept up as long as possible, until the stope is finally broken through to the drift-level below, the ore being shoveled into cars. As far as possible, the pillars are left in poor ore, the diabase dike fulfilling this purpose admirably. Iso timber whatever is used, and although chambers 100 by 100 by 40 feet have been cut out, there seems to be no danger of a fall, the country-slate being very tough and self-supporting. The stopes from the 100- and 200-foot levels are connected with the surface by raises, so that at a future date the worked-out