Page:Popular Science Monthly Volume 30.djvu/488

468 The difficulties caused by the reef have been at last overcome without having first to remove the accumulation of material by the adoption of a system of shafts and tunnels which is called, after its inventor, "Jones's system." A shaft was sunk through the fallen reef within the mine by letting down a series of caissons till the solid "blue ground" was reached. A hundred feet of loose reef was thus penetrated, after which the shaft could be extended to any desired depth in the "blue," and tunnels driven in all directions, so as to continue the excavation of the mine underground. The scheme had the merit of entailing little initial outlay, while, as soon as the "blue" was reached, the work of opening up the galleries more than paid for itself in the value of the ground removed. Other shafts have been sunk outside of the mine, one being more than five hundred and twenty feet deep, and others going down to the hard rock, and connected by tunnels and cross-tunnels, so built that the cross-tunnels have a wall of hard rock on one side, while the roof and other side are solid "blue." The excavation, says the report, "is then continued by cutting down the 'blue' from the roof overhead; but instead of trucking away the 'blue' as it falls, the rails in the tunnel are taken up and the 'blue' is allowed to pack underfoot, the miners therefore being continually climbing to a higher level, while the height of the tunnel remains uniform, just enough for the miners to be 'in touch' with the roof." In order to preserve safe means of access and egress for the workers in this continually rising chamber, a couple of cross-headings are driven parallel with it, five feet high by four wide, branching out at right angles from either side of the main tunnel, and leaving a solid wall of "blue" ten feet thick between the cross-headings and the main chamber. "A number of inclined passes are then driven at a sharp angle through this wall of blue, connecting the cross-heading with the working-chamber, and where they strike the latter vertical 'pass-pits' are carried up, rising simultaneously with the chamber, and separated from it by a three-inch plank, which prevents the loose pack of blue from filling the passes. At last the overhead excavation has proceeded so far as nearly to strike the fallen reef which at present covers all the open workings in the mine. The crown of the chamber is then broken through at either end, and the loose reef allowed to enter and pack on the top of the excavated blue. A sliding-door in the planks at the bottom of the pass-pits is then opened and the excavated blue drawn off, sliding down the inclined passes into trucks in the cross-headings, which convey it through the main tunnel and shaft to the surface. As the blue is drawn off, the loose reef above it subsides and takes its place till the chamber is entirely emptied of blue and filled with reef. The sliding-doors are then closed, and the excavation of that chamber is complete." This process is repeated, with modifications in the several chambers as they are successively excavated; while in another part of the mine an opposite system of excavation is