When the seams have a limited dip, say 25° or less, and where cars can be used in the breasts, shafts are more available than slopes, since the length of the slope is proportionately increased, while the depth of the shaft is decreased; and the objec tions to the length of breasts do not hold good under these conditions, because in seams of 25° or less the cars can go into the breasts, and the lift is not of so much im portance. Still, it is a question of some doubt at what angle of dip slopes become more expensive than shafts ; and some practical men, of much experience, are of the opinion that slopes on the underlay of the seam are the most available so long as the dip is sufficient to allow the car to drag the chain or rope down the incline by its gravity. For the purpose of drawing coal, the incline principle is to be preferred whenever a mine can be slanted on the dip of a seam, since more coal can be raised on an incline than through a shaft, even if the distance be ten times greater. An engine of 100 horse-power will draw up a train of cars with 20 tons of coal over an incline of 10° with greater ease than 2 tons of coal up a shaft. But generally we would prefer shafts to slopes when the dip is under 20°, except for the first two lifts near the surface. There are, however, instances of frequent occurrence where shafts are the only available means of reaching the coal, even when the seams have high angles of dips. This may happen when the boundary lines of estates cannot be located on the outcrops of the beds, or when the outcrops of the seams are very high on the mountain-sides, and positions for machinery cannot be obtained. In the first case, shafting must be re sorted to, and in the latter, tunnels may be used to cut the seams above water-level, and slopes below that point, though sometimes slopes are used from the end of the tunnels on the dip of the seam, in the same manner as when started from the surface; and this plan is available, and may be advantageously made use of, under certain conditions.
In other instances, the seams undulate in such a manner as to form basins whose out crops do not come near the surface, and on which slopes cannot be used; for instance, the Mammoth bed' undulates in four or five successive basins between the Mine Hill and the Sharp Mountain, in the Pottsville district, yet the outcrops come to the surface only in one or two of these basins near the Mine Hill: consequently, this seam can be reached only by shafting ; and the same may be said of many of the overlying and of all the underlying seams.
Sometimes the anticlinals of these concealed formations come comparatively near the surface. When these anticlinals can be correctly located, a shaft on their axis may be made available at a limited distance and cost, and from the point of intersection slopes may be put down the respective dips to the basins, on each side of the anticlinal. By this arrangement the coal of two basins may be obtained through one shaft, not only from a single seam, but from all the seams whose anticlinal axis is penetrated by the shaft. To make such a shaft fully available, it ought to be very large, in order to admit of several hoisting apartments, and of a separate compartment for the pumping apparatus and upcast shaft, or air-course, since a steady current cannot be maintained in a shaft where the cars are constantly passing up and down. It would be also desirable, in case a large amount of coal is required, to have separate hoisting-ways from each dip of the Mammoth. The lower seams could be cut by tunnel, and the coal raised through the same ways, while the upper seams would be more available after the lower ones are exhausted.
The cost of sinking shafts varies considerably in the measures of the anthracite regions, Where the strata are composed of slate, shale, and soft sandstones, the cost of sinking is less than where they are composed of harder materiah—sandstones, rocky conglomerates, &c. In the vicinity of the Tracy's, or between the Little Tracy and the Lewis, or Gate, we find some very massive and flinty rocks of great hardness. We also find a few hard strata between the Diamond and the Big Tracy, and between the Orchard and the Diamond; while a very massive and compact coarse sandstone exists between the Holmes F and the Seven-Feet, above the Mammoth. Below the Mammoth the measures are generally hard, with the exception of a few thin strata of slate; but above the Gate bed a rock of peculiar hardness exists, which may be equally expen sive to cut.