UNDERGROUND DEPOSITS. In the whining of the useful minerals from underground deposits com plications are introduced which are not met in open cuts. The overlying rock is always to be supported as long as that portion of the mine is being operated or is used as a passageway. This may require the leaving of much of the useful mineral as pillars to support 'the overhanging wall or roof, or the use of heavy timbering or even of masonry. Ventilation also becomes an important. item, and all these charges. it must be appreciated, have to he borne by the product be fore any profit• is realized.
The mining of metallic ores and minerals occurring in fiat or slightly inclined beds or deposits does not differ materially from the methods pursued in the mining of coal -\l utmost all metal mining is concerned with stc(iply inclined beds, veins, and irregular masses. Lt the past history of the earth. especially in mountainous regions, and where eruptive rocks have come up from the depths below, era CkS of greater or less size have been formed in the solid rocks, and often in numbers. Up through these have mine waters. as a rule at elevated temperatures and charged with minerals. Where they have brought in metallic ores they have often deposited them in the fissures, along with more or less harren material called gam:me, and in this way ha ye produced 'veins' or 'lodes,' Where, coming through a crack as a channel of supply. they have met some soluble rock like lime stone, they have often replaced it with valuable ore, the limestone acting like a precipitant upon the dissolved metals. If a porous rock has been met the solutions have at times impregnated it with ore. Ore bodies of great size and of more or less irregular charaeter have thus resulted. and problems of varying degree.: of eomplexity are met In• the mining engineer in developing them. The ore is seldom uniformly distributed through out a vein or other deposit. hut, on the contrary. oceurs in rich portions vaned (dunes or bonanzas, with intervening spaces of barren ground. It is advisable therefore to keep the mine well opened up ahead of actual extraction of ore, so as to average the rich and lean portion, and make the enterprise It permanent one. Veins often fork and send off stringers into the walls; they pinch and swell along their length and depth. They usually run out at their ends into
small ramifications and finally cease. They may be cut ott sharply by other cross-fractures and disturbances. They extend to eonsiderable depths, having been followed in sonic cases as deep as 3000 feet or more.
As a typical case by which to illustrate the usual methods of procedure, we may assume that a vein has been located on the surface, that it extends a considerable distance, say a half-mile o• mile, and dips at 60 degrees into the earth. Test pits and shallow shafts have indicated its value. The engineer, in opening a new deposit like this, would select as suitable a place as pos sible for his surface works. such as engine house. ore bins, and dump for waste rock, all in con nection with a spot where the vein showed good ore. lie would then sink a shaft or slope on the vein. and if it held good. would start drifts or levels at each GO to 100 feet of descent. As soon as a level had advanced some distance from the shaft, say 100 feet or more, another party would be started near the shaft. working on the vein in the roof of the level. At first propped up on timbers, they would excavate a space, and clear away a working face, so that while the level was being driven ahead they could follow a short dis tance hack, taking off a slice. Now in order that the loose rock and ore that are blasted down should not block the passageway, timbers would be set across the top of the level as at first run. The timbers called shills would fit into sockets in the walls and on them would be laid rough plank or lagging, with taps or little hatehways at intervals for tapping out into ears the ore that would be blasted down upon them. This method is called 'overhand stoping? and is the one usual ly adopted. When the first party of stopers had advanced far enough to warrant it. a second, and later a third. would be set at work following them up on other and higher slices. As soon as the levels had gone some distance. another shaft would be sunk to co meet with them, not alone for hoisting, but to afford ventilation after blast ing and for a safe line of escape for the men in ease of accident.