But we must follow the original development of coal further, before explaining the more advanced modes.
Figure 138 explains more fully the outcrops of pitching seams, and the mode in which they may be found and proved on the surface by means of a "trial-pit." The indica tions of coal are unmistakable to the practised eye, and any good practical miner should know the difference between coal measures and ordinary strata; but even the most expe rienced cannot always tell the exact spot in which a seam of coal may be found within those measures. The surface is generally covered to a considerable depth by debris or wash from other material than that near the coal, and it requires some skill to locate the positions of the seams, and frequently some digging ; but when once located and proved, as shown in the preceding illustration, the tracing of the respective seams along their outcrop or strike is not a difficult matter. In the anthracite regions the lower series, or white-ash beds, can generally be traced by the accompanying rocks, which are peculiar and always in place. The conglomerate always lies below the lower bed, and frequently between the lower coal-strata ; while a heavy, coarse sandrock overlies the Mammoth But this large bed, lying between those massive rocks and ex posing its slates and outcrops frequently, is not easily concealed from the miner. The
upper seams are more difficult to find; but, knowing their respective distances from some well-known rock or bed, it is only a matter of patience and time. They can always be opened by the pick and shovel with a little labor; and the tracing from point to point can then be done by the accompanying surface slates or rocks, by noting the dip and course, and following by compass, by tracing with an auger, by light surface-shafting, or simply by the eye and mind.
But in some coal-fields there is more difficulty in finding the coal and tracing the seams. In the Richmond coal-field, where the surface is covered with sand and the debris of distant and foreign strata, where the coal measures consequently are con cealed, and where the seams are irregular in both dip and strike, nothing but actual proof by shaft or auger will be available. But in the Western coal-fields, where the measures are cut down by streams, the discovery and tracing of the coal-seams are matters which require little experience or skill.