Page:The American Cyclopædia (1879) Volume XI.djvu/592

574 MINE opened. The ore has to be hoisted, and the waste rock has to be lifted by hand and packed on stulls behind the miner, as shown in fig. 3. This system permits an earlier beginning of extraction, and gives the workman a firm foot- ing on the solid vein and an easier and safer direction of working (viz., downward instead of upward). Moreover, there is less chance of losing small pieces of rich ore, which in over- hand stoping get into the waste rock under foot and cannot be recovered. But overhand stoping has two great advantages : first, the con- venience of rolling and dropping rock and ore, instead of hoisting them ; and second, the saving of timber, which in most mining districts soon becomes expensive. The great amount of tim- ber used in an underhand stope is not merely lost ; it may give rise by its decay to slides in the packing, or the necessity of expensive repairs to prevent them. Both overhand and under- hand stoping are variously modified, as for in- stance in their application to any thick vein in which cross stoping is not desirable. In such cases, the vein is worked in successive layers or zones, parallel with the walls, each layer be- ginning with that on the foot wall, being stoped out by itself, as a separate vein; 12 ft. is usu- ally as great a thickness as can be stoped at one time with safety or convenience. Cross stoping is common in working thick veins. In this method, the vein material is removed in layers, not parallel with the walls, but ex- tending from the foot to the hanging wall; and in each layer the exploitation takes place by driving breasts across the vein, leaving pillars between them ; supporting the roof of the breast, 6 to 12 ft. wide, with timbers until FIG. 4. Cross Stoping. it has reached the hanging wall; then with- drawing the timbers and packing the exca- vation with waste rock ; and then extract- ing the pillars and replacing them also with waste rock. A cross layer of the vein, 6 or 7 ft. in vertical height, having been thus re- moved and the space packed, the operation is repeated with the layer next above. Fig. 4 shows this method by a vertical cross section. It is employed at the quicksilver mine of Idria, Carniola, and in various modifications at the zinc mines near Aix, the coal mines of Le Creuzot and St. Etienne, in France, the mines of roofing slate near the Rhine, and the lignite mines in Lower Styria. Long-wall working is employed on nearly horizontal deposits, usu- ally coal beds. It may be classed as retreating or advancing, according to whether the extrac- tion begins at the borders of the field or section of the bed to be worked, and retreats toward the. main shaft, or begins at the shaft and advances toward the limits. In the latter case roadways are kept through the ground al- ready worked out. Varieties of this method are employed in the copper schist beds of Mansfeld, and at many foreign coal mines. The methods of extraction without packing are : those in which the roof or hanging wall is supported by timbering, masonry, or pillars of the original material, left standing until the workings are to be abandoned; and those in which the roof is allowed to come down imme- diately after extraction. In the mines of the Comstock vein in Nevada, the spaces are kept open with elaborate timbering, framed as for immense houses.- This is a great expense, be- sides being a source of loss and danger in case of fire. A conflagration in the Yellow Jack- et, Kentuck, and Crown Point mines on that lode, which began April 7, 1869, not only cost many lives, but continued to burn, from 600 to 900 ft. underground, for many months, being sustained by the great quantity of dry timber in the stopes. The system of extraction by breasts or chambers and pillars is practised chiefly in coal mining. It is wasteful of coal, since the pillars of that material left standing are but partially recovered by "robbing," when the breasts are worked out. It is esti- mated that from 30 to 40 per cent, of the coal in the anthracite mines of Pennsylvania is thus lost. To the department of extraction belong also the various methods of transporting work- men and material. Where an adit or a slope of gentle inclination leads to the underground workings, the ore and rock are brought out in cars or wagons. For horizontal transpor- tation men or boys, horses, mules, stationary engines, and locomotives are employed. Hoist- ing through shafts is performed by windlass, horse whim, or water or steam power. When the material extracted has to be lowered, as for instance to deliver it from breasts or stopes to the main roads of transportation under- ground, or from the shaft or adit mouth to a loading place at a lower level, gravity tram- ways may be employed, on which the loaded cars, descending, pull up the empty ones. The entrance and exit of workmen through shafts is effected by ladders or stairs, or by lowering