Page:The American Cyclopædia (1879) Volume XI.djvu/589

 MINE 571 surest but most expensive method of explora- tion. It may be resorted to when the exist- ence and value of the deposits are well known, and the topography is such that the entry may subsequently be valuable for drainage and transportation. Finally, new deposits may be discovered under ground by driving experimen- tal openings from mines already in operation. The value of deposits is tested by shafts and drifts, usually excavated within the deposit itself. The construction of such works in the barren rock is seldom undertaken until the vein is found to be worthy of the expenditure. In the case of coal, building stone, iron ore, and, in general, all materials which occur in extensive and tolerably uniform deposits, and the value of which is small in comparison with their bulk, the test of quality is not difficult. But minerals of more concentrated value usual- ly occur mixed with such variable proportions of "gangue" or barren matter, and when in fissure veins are subject to such variations in width and course, as to render it necessary to expose considerable bodies of vein matter, and to make tests either by thorough sampling or by actual reduction of large quantities, before the economical value of the deposits can be ascertained. By connecting with the shafts or inclines sunk upon the dip of the vein lon- gitudinal drifts run upon its course, this object can be measurably secured. Alluvial deposits, such as those of gold and stream tin, are tested by actual working with pan, sluice, &c. When large operations, like those of hydraulic mi- ning, are contemplated, the body of earthy gravel, cement, &c., if its value is not already known, is tested by shafts sunk to the bed rock at a sufficient number of points to give an indication of its average contents. II. APPROACHES TO MINES. Access to mineral deposits for permanent exploration is estab- lished, first by suitable wagon or tram roads on the surface, and secondly by either strip- ping the overlying soil and rock from the de- posit itself, as is done in quarries, clay banks, and some iron mines, or by sinking a shaft or running a drift or crosscut from the surface into the deposit. In the case of beds or veins which dip at a convenient and uniform angle, the shaft may be carried down upon the de- posit itself, and is then usually called a slope or an incline. For less regular deposits, and for those in which the angle of inclination is inconvenient or variable, or the vein matter is too valuable to permit the leaving of it in pillars to protect the shaft, it is better to drive a vertical shaft at some distance from the out- crop, in the hanging wall, so as to strike the vein at a considerable depth. A gallery run from the surface in a nearly horizontal line, to effect access and drainage, is called an adit or entry, and in some situations, as at the base of steep hills, this may be made the principal feature at the mine, the main workings being carried on through it until the vein is ex- hausted above its level. Sometimes the nature of the shafts permits the opening of mines at different levels, by means of adits. This sys- tem was most highly esteemed before the im- provements in machinery and the introduction of steam favored the economy of mining in deep shafts. When adits must be driven for long distances through hard and barren rocks, it is sometimes difficult to decide whether the cost of their construction will be repaid by the saving in hoisting, drainage, and mechani- cal ventilation. Adits are usually called tun- nels by miners of the Pacific states and territo- ries, but this is a misnomer, as a tunnel proper extends entirely through a hill. Mining shafts are generally rectangular in section, and range in size from 3 or 4 ft. to 6 ft. on the shorter sides, and from 6 ft. to 20 ft., or even more, on the longer sides. This form facilitates timbering, and at the same time permits the best utilization of space, through the division of the shaft by partitions into separate com- partments for pumps, hoisting, ladder ways, &c. Adits are placed with reference to se- curing the greatest depth below the surface by running as short a distance as possible, par- ticularly in barren rock ; with reference to the presence of a good place for a " dump " at the adit mouth; and also with reference to easy escape of waters, freedom from flooding by freshets, and facility of natural ventilation when the adit is to be connected with a shaft. For the latter purpose it is well that the adit mouth should not be in a narrow ravine or in the corner of a valley. Dimensions of adits depend upon the amount of water expected to run in them and the other purposes to which they are to be put. When in barren rock, it is an object to make them as small as prac- ticable ; 7 ft. high and 5 to 6 ft. wide is a con- venient size. But when transportation is to be carried on and double tracks are to be laid, the dimensions must be increased. The height of the adit available for passage is diminished by the water channel, which usually runs under the floor or in a ditch at one side. The grade of adits is determined with reference to the amount and character of the water flowing in them and the speed which it is desirable to give to the current. The ancient mining regu- lations of Prussia required of deep adits a grade of from 1 in 800 to 1 in 400. Some of the adits at the coal mines of Saarbriick rise at the rate of 1 in 1,600 ; others at the rate of 89 in 64,000. According to the Saxon law, the grade may vary between 3 in 10,000 and 1 in 1,000. The long Ernst August adit in the Hartz has, for a length of nine miles, an average grade of 0-67 in 1,000. Here the water in the adit is itself used for transportation, and the current is intentionally kept slow. Access is further obtained to the different parts of the mineral deposit by subordinate shafts and galleries ex- cavated in the deposit. These interior shafts not extending to the surface are known as win- zes, and usually serve to connect the galleries on different levels. The galleries are known