"DRIFTS," OR WATER-LEVELS.
The first mode of mining coal was by open quarry, or by uncover ing the coal-beds where they ap proached the surface at their out crops, and extracting as much of the coal as possible until prevented by water or the increasing thickness of the covering earth. The most re markable instance of mining by "open quarry" was at the old Lehigh Summit mines, during the early days of the anthracite coal trade (see figure 15), where the out crops of the Mammoth presented a mass of coal nearly one hundred feet thick, covered by a small amount of earthy surface.
In the Richmond coal-field numerous excavations of this kind are found along the outcrops of the seams, and, in some cases, immense quantities of earth have been removed to obtain a small amount of coal. In Alabama the writer witnessed this mode oof obtaining coal within the last five years, where the negroes were removing thirty feet of cover to obtain four feet of coal; while the seam was finely exposed in the side of the hill, where the coal could have been obtained with one-tenth the labor by the ordinary process of "drifting" on the seam. But in all countries this style is always the first adopted. We presume, however, the development of the Southern coal-fields. to be behind that of all other countries, as there neither skill nor science has aided brute force with any degree of intelligence.
The first regular system of mining generally adopted is "drifting" or tunnelling on the "strike" of the seam, in all coal-fields where the seams are found outcropping in the bills above water-level. But this mode can be used only when the beds are exposed by denudation, beneath their horizon when flat, or beneath their line of strike when pitching. Drifts differ from tunnels, inasmuch as the former enter the seam and follow it; while the latter generally cross the measures at right angles with the seam and penetrate the rocky strata to the coal.
In the anthracite regions, drifts are generally used above water-level, entering the seams where cut by the water-courses; but where the seams are not thus exposed, they are cut by tunnels, and the coal worked at right angles to the same by "gangways."
This term (gangway) is also applied to the drift after it has entered the bed a sufficient distance to admit of chambers or "breasts" being "turned." All the main avenues of the mine through which the coal is conveyed from the chambers are known as gang ways. The English miners call them main-ways, rolley-ways, &c.
To explain more clearly to the inexperienced, we give the following illustration, figure 137, showing the outcrop, strike, water-level, and dip, which, in connection with the accompanying representations, will fully express the position of coal-beds and the modes of entering them by drift, tunnel, slope, or shaft.
In figure 137 the coal above the water-level line may be reached by drift from either side of the ravine or water-course denoted, and on a level with the stream. The strike of the outcrop, or course of the seam, is indicated by the straight arrow; while the actual outcrop is shown by the bent arrows in the coal. The intervening space is denuded by the action of water,—both coal and coal measures having been swept away. The dip of the coal is denoted by the word in the engraving, but is more clearly expressed in the following cut,—figure 138,—which shows the outcrops from an end view, instead of the surface view in the preceding.
The action of the water, as shown in the foregoing figure, enables the miner to enter the coal on its strike at water-level. If not thus exposed, it could only be opened by tunnel across the measures, in order to effect its drainage and obtain the coal without the use of steam-power.
This form of exposure is peculiar to pitching seams and hilly districts. In districts or fields where the coal is horizontal or nearly so,—for instance, as represented in figure 123, Kanawha region,—the effect of denudation is quite different, and the seams can be entered at almost any point on the sides of a mountain. Tunnels cannot be used; and, when the denuding waters have not cut down the measures and exposed the seams, shafting alone can be resorted to.