Should the vein be found to contain ore of good quality and in sufficient quantity, both laterally and in depth, the various operations which have now been described may proceed indefinitely. The shaft will continue to be sunk, cross-cuts driven to the vein at every ten fathoms or thereabouts, levels extended in both directions from them (the upper level being always of course considerably more advanced than the lower, from having been longer in progress), and the ground between them subdivided by winzes as before described. The excava tions will now have assumed a regular form, and become what is properly termed a mine, the objects for which they were undertaken having been accomplished, or the contents of the vein thoroughly explored, and its produce, where of sufficient value, rendered capable of being economically and expeditiously extracted. In the accompanying sketch,Fig.,1,the state of the works is shown, the shaded portion repre renting the parts of the mine whence the ore has been extracted as described below. Fig. 2 gives a cross section of the mine, and with the former will completely illustrate the foregoing description.
Raising of Ore.—Tho operation last named, or the extraction of the ore, will not however have waited the advanced state of the works which we are now contemplating : a certain quantity will have been produced by driving the' upper levels and sinking winzes below them, although not forming the primary object of these works; and wherever orey ground was seen in the " back " or upper part of the level first driven, it will early have been pursued upwards towards the surface, and will yield the first returns of the mine. When by the further progress of the works the vein has been divided into the solid rectangular masses before described, the mine will have been brought into an effective state of working, and parties of men will be set to raise ores from all the most productive points. Where the vein is not very hard, the ore may be broken down with the " pick " only, but it is generally necessary to blast it with powder, by which process large quantities are detached from the vein by every shot. In raising ores the men generally work upwards from the " hack " or upper part of one level towards the " bottom " of another, and the excavations are so arranged that the ore may readily fall down to the level below them, whence it is carried in tram-waggons to the shaft, and thence raised to the surface. As in all large and well-regulated mines it is desirable to keep the quantity of ore raised as nearly as possible to a uniform standard, the process of "opening ground," as above described, will generally either keep pace with or even exceed the rate of exhaustion, so that a portion of the ore thus laid open may always be held as a reserve to keep up the returns of the mine during periods of temporary depression, when only poor and unproductive ground may be presented by the works of discovery. This judicious mode of proceeding is
justly regarded as one of the greatest modern improvements in the economy of mining : it provides a compensation for those great irregularities and fluctuations to which all mineral deposits are con tinually subject; it keeps up the returns of the mine during temporary intervals of poverty, and enables those trials to be effected which may again place it in a productive state.
Extension of the Works.—As the workings of a mine become more extended, they gradually lose the simplicity which we have been sup posing, and, from the multiplication of shafts and other excavations, become highly complex, a result to which the irregularity of mineral deposits greatly contributes. The nature of these ulterior operations will now be briefly described, still considering the mine as a mere system of excavations, and reserving till hereafter the subject of drainage, support, &c. When the levels have been extended to a con sidemblo distance from the shaft, the ventilation will again become defective, notwithstanding their communication by winzes ; the current of air, from the greater distance it is carried, becomes more and more feeble ; and this evil is still further augmented by the increasing number of men now employed in the works, the number of candles, and the frequent process of blasting. The expense of the transport of ore and masses of rock and rubbish to the shaft also becomes con siderable ; and if the prospects of the mine continue such as to warrant the expense, a new shaft must now be sunk on one or both sides of the former. Whether one or two shafts will be necessary will depend on the direction in which the ore is found to extend, and the indica tions exhibited by the vein ; and they will be so placed as to command that portion of it which, for reasons before stated, is without the reach of the former one, which by this time also may have become inade quate for the various purposes of extraction, drainage, and descent, to which it is applied.
In order to avoid the expense and delay of unnecessary cross-cuts, the new shaft will be so placed as to intersect the vein much deeper than the former, and this point will be so arranged as to correspond either with one of the deepest levels, or some proposed level deeper still. The new shaft may have been commenced in anticipation, while the levels were yet distant from the point where it is sunk ; and in this case its communication with the mine will have been accelerated by driving levels to meet those which are proceeding from thence towards it. When this communication has been effected, its benefits will be immediately felt, both in the thorough ventilation of the mine and the increased facilities afforded for the extraction of ore and stuff.