Page:The American Cyclopædia (1879) Volume XI.djvu/590

572 MINE as levels or drifts in vein mining, and gang- ways in coal mining. When a mine is opened by a vertical shaft, the vein is sometimes cut by a crosscut level run fr6m the shaft through barren rock, at a point higher than the inter- section of the shaft and the vein. From the point where the crosscut enters the vein, levels are then run in both directions horizontally on the vein. After the main shaft has reached the vein and has been carried through it, the distance between vein and shaft of course grows larger with increasing depth, and the vein must be again opened by crosscuts from the shaft at different levels. The levels opened in the vein are so many parallel roads on the vein, succeeding each other every GO to 100 ft. in depth. The winzes connecting them serve both in ventilation and in extraction, besides affording convenient access to different parts of the mine. The running of drifts to make connection with old and abandoned workings is sometimes dangerous, when the old work- ings are full of water and their exact position is not known by surveys. In such a case the approach is made cautiously, and a bore-hole is kept in advance, to tap the accumulated waters in such a way as to avoid an excessive flow or give the workmen time to escape. An accident of this kind at the Gouley mine, near Aix-la-Chapelle, in 1835, which caused the drowning of 63 miners, gave rise to the publi- cation by the government of the Khenish province of exact regulations, which constitute an excellent guide to the mining engineer. III. EXTRACTION OF MINERALS. To perform the work of regular extraction with due economy and safety, the following circumstances must be considered : the shape of the deposit, as a tabular or sheet deposit, a mass or stockwork, regular or irregular, &c., and if a tabular de- posit, like a fissure vein or a bed, then its course and dip, its folds, basins, faults, and breaks; the thickness and inner structure of the deposit, or, in ore veins, the nature and distribution of the ore bodies, the amount of barren gangue, and in coal beds and other deposits the pro- portion of marketable to waste material ; the character of the "country" or wall rock, as making a solid or a precarious roof, and re- quiring more or less support; the number, relation, and distance apart of several deposits which it may be desirable to work at once or successively, as for instance seams of coal, lying one under the other ; the conditions of ventila- tion, particularly where explosive gases are to be feared; the conditions of drainage; the character, abundance, and price of materials for underground supports (timber, masonry, iron pillars, loose rock, or earth) ; the size and shape of the pieces of material to be extracted (commercially important in coal and quar- ried stone) ; the method of excavation to be employed (picking, shovelling, fire setting, hydraulic sluicing, leaking, blasting, &c.) ; and finally, in a subordinate degree, the nature of the mineral itself, as for instance very rich and brittle silver ore, which is liable to be lost in fine particles among the piles of waste, or some kinds of coal which deteriorate by standing too long in the mine after they have been exposed and drained, or clays which become like quick- sands in contact with water. Any one of the foregoing conditions may, under certain cir- cumstances, be decisive as to the choice of a method of extraction. Hydraulic mining is an application of the power of water, under high pressure and at great velocity, to move great masses of material, separate the heavier and more valuable portions, and convey away the lighter waste. (See GOLD MINING.) In its modern form it was invented in California about 1852, and is mainly employed in that state. It has been introduced at Iron mountain, Missouri, for the purpose of removing and con- centrating the surface ore of that locality. For hydraulic mining, it is necessary to have, be- sides a deposit suitable to operate upon, a supply of water at a proper altitude, and an escape for the turbid stream, at a level below all the workings. The waste material is thus removed by the current at small cost, while the gold is precipitated to the bottom and caught in crevices, " riffles," &c. As most " hydrau- lic-gravel" deposits have been accumulated in basins of ancient river channels, they are sur- rounded by hard rock, called "bed rock" be- low, and "rim rock" where it comes to the surface at the edge of the deposit. For hy- draulic exploitation, the bottom of the deposit must be underrun by a tunnel through the bed rock, and a short shaft from the overlying deposit must be connected with this tunnel. Then the bank may be " hydraulicked " down, and everything except the large bowlders sluiced into the shaft and tunnel, and so away to lower grounds. The method of extraction by leach- ing is peculiar to certain rock-salt mines, in which the mineral is too much mixed with earthy and stony matter to be quarried. Cham- bers are excavated in the mass, and filled with water, which is allowed to stand until it has extracted from walls and roof sufficient salt to render it a concentrated brine. It is then drawn off, and conveyed in pipes to the boil- ing works. A layer from the roof of the chamber, disintegrated to a certain distance by this leaching, falls, covering the floor with ma- terial from which the salt has been extracted, and leaving a new roof exposed for a repeti- tion of the process. Thus the chambers slowly rise through the mountain. This method is in use at Berchtesgaden in S. E. Bavaria, and Hal- lein in Salzburg. Other modes of extraction are divided into two classes : those in which the space excavated is refilled wholly or partially with waste material, and those in which no such " packing " or " gobbing up " is employed. The former class is subdivided, according to the direction in which the work proceeds, into overhand stoping, underhand stoping, cross stoping, and long-wall working. (The latter method and its modifications, used chiefly in