Surface indications, or the strata near the surface, are always a good guide in regular basins; but, in irregular formations, experience and geological knowledge are required to locate mining operations properly. In the following table will be found some inte resting and valuable information. The data were prepared by Mr. John Holmes, a practical miner of St. Clair, and the calculations made by General II. Pleasants, of Pottsville.
This table gives the distances of both shaft and tunnel from one seam to the other under different degrees of dip, and the relative cost of sloping, tunnelling, and shafting, from one seam to the other, through the intervening measures. It also gives the vertical or right-angle distances from one seam to the other; and we have added our nomen clature, or alphabetical letters, to the common names of the seams as accepted in the Pottsville district.
We think the distances given in this table are the minimum distances for the Potts ville district, and will not conform to the distances general in the anthracite regions. We find the distance at Scranton from A to J to be only 455 feet vertical; at Pottsville the distance is over 1000 feet; while at Locustdale, near Ashland, in the Mahanoy region, it is considerably greater, as shown in figure 59. But by taking the vertical or right-angle distances, as given in the various vertical sections of the several regions and their relative dips, an approximate tunnel and shaft distance can be obtained by the relative computation given in the table.
We find the distance from M to L, or from the Lewis to the Little Tracy, to be 150 feet at right angles to the dip, and the horizontal or tunnel distance, at 15° of dip, to be 579.5 feet, while the shaft distance is only 155.3 feet. As the dip increases,'the tunnel distance decreases, and the shaft distance increases: therefore, at 75° of dip, the tunnel or horizontal distance is only 155.3 feet, and the shaft or vertical distance 579.5 feet, which is the reverse of 15°, because the 75° from the vertical is the same in reverse as 15° from the horizontal. This rock is very hard, and we find the cost to be $60 per lineal yard for tunnels, $250 for shafts, and $30 for slopes, when labor is at $1.50 per day for miners; the shaft being about 10X 20 feet in diameter, and the tunnel 7 X 10 feet. The slopes vary with the size of the seams, and are made in proportion: there fore the cost of sinking on a small seam is from $25 to $30 per yard, while on the Mammoth it is $50 per yard. These figures include only labor ; the outlays for material, timber, machinery, cars, and hoisting and pumping, are additional.
In order to find the depth of a basin by this table, it will be necessary first to get the average dip of some well-known seam; find the distance from that to the basin of the seam required, and then the distance from the same point to the centre of the basin as nearly as can be approximated by surface indications, taking care to allow for any difference in dip that may exist between the opposite sides of the basin, as described in figure 149.
We wish, for instance, to find the depth of the Mammoth at a certain point where this seam does not outcrop; but we find the outcrop of the Primrose, which we know is from 290 to 300 feet above the Mammoth. We wish to start a shaft on the north side of the basin, 150 feet south of the outcrop of the Primrose, which we find to be near the outcrop of the Orchard. The average dip is 15°, and we find the vertical distance to be about 455 feet, which is verified by the St. Clair shaft, commenced and sunk under such circumstances. To find the basin of the Mammoth under a dip of 25°, we find the out crop of one of the upper seams—say the Big Tracy—and measure the distance to the centre of its basin. We find the distance to be 638.9 feet, the perpendicular 270, or the distance at right angles through the measures, since the dip is 0° in the centre of the basin. If the dip has been preserved at 25° the whole distance of 638.9 feet, the per pendicular will be 297.9 feet. We find, therefore, the depth to K, or the Big Tracy, to be 270 feet in the centre of the basin, where the distance from the outcrop of the same is 638.9 feet, and the dip 25° on an average. This being demonstrated, we find by the table that the distance from K to E, or from the Big Tracy to the Mam moth, is 902 feet: consequently, the whole distance is 1172 feet. This we consider a minimum. The probability is that the distance would be rather over this calcula tion.
The approximate distances are given in the vertical sections accompanying our de scriptions of the respective coal regions, and the approximate depth may be obtained in the same manner by finding the basin of some upper seam and then adding the distance as given from the seam found and known to the seam wanted, in the centre of any basin. But if the shaft is located on the dip, the average angle of the dip must be demonstrated, and the perpendicular depth found from the accompanying table, allowing for the increased or decreased thickness of the measures, as found on comparing the vertical column of the region with the distances in the table.