again are crossed at right angles by similar bords, so as to leave square pillars of support, which vary in size according to the tenderness of the roof and coal, or the softness of the floor. When these galleries have reached the boundaries of the field, or as far as it is expedient to carry them, the next step is to remove the pillars, or as much of them as can be safely accomplished, by working back to the shaft and allowing the roof to subside the excavated and deserted portion, now filled with debris constituting the waste, goaf or gob of the miner (G). In the latter method, the main-ways are carried out to the boundary, and then the whole of the coal is removed by working the face backwards to the shaft, care being taken to maintain a free and accessible face by packing or propping, and then ultimately to allow the roof to subside. In some cases, the working face is carried back from the shaft, and the ways maintained by build

ing, packing, and planking; but generally speaking, the former plan is preferred. In some coal-fields a combination of longwall and bord-and-pillar is adopted-much depending, of course, upon the nature of the seam, roof, and floor. Again, where the seam lies at a high inclination, the main-ways are carried obliquely to the rise, in order to reduce the difficulty of ascent and descent-a matter of main importance when the angle of inclination exceeds 10 or 12 degrees.

Speaking of the comparative merits of the two methods, Mr Warrington Smyth, in his Treatise on Coal and Coal-Mining,' appropriately observes: "The great advantages of the long

:

work method are, simplicity of plan (and consequently of ventilation), and the entire removal of all the coal,-added to which, under most circumstances, are greater safety to the men, and a larger proportion of round coal in comparison to small or slack-a matter which, considering the prices, is of vital importance in the selection of the mode of working. It has been mostly practised where the seams are thin, or where they contain a band of refuse; but neither condition is indispensable for, on the one hand, coals of 6, 8, or 9 feet thick are at the present moment worked advantageously in this manner; and on the other, we have seen bind or stone-debris carried from one seam to another, or even taken down from the surface to assist in the packing where it was needful. Nor is it necessary that the roof be good, although the expense will be very different according to its fragility; but if the operations be carried on with sufficient smartness to push the working-place daily under a fresh or green" roof, it may be managed upon this system, even when composed of mere fire-clay with slippery joints. Only a few years have passed since the long-wall was much decried, except in a few localities; but its manifest economy is gradually introducing it elsewhere; and even in some of the deepest Durham collieries it is successfully applied to the working off of their gigantic pillars; whilst in a few of the pits near Dudley it has been employed for removing bodily first the upper and afterwards the lower half of the 10yard coal, with greatly increased yield of coal and security to life." To these remarks we may add the applicability of coalcutting machines to the long-wall system, as one of its chief recommendations-these machines coming more and more into use alike on the score of economy and of freeing the miner from the most laborious and dangerous part of his duties.

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And here it may be further observed, that with the yearly increasing consumption of mineral produce, the gradual increase of prices (coal in particular), and the certainty that the supply is limited, and must sooner or later come to an end, it should be the effort of every mining engineer to raise the largest possible amount from any one seam, or, what is the same thing, to leave the smallest possible portion of it beneath ground. Where the long-wall system can be practised with safety, nothing is left, and nothing more can be desired; but in the bord-and-pillar method, a large portion, even with the most daring and dexterous harrying or removing the pillars, is always left beyond reclaim; and it is certainly worth while to adopt the other method wherever it possibly can, with due regard to the safety of the miner.

But whatever system may be adopted to remove the seambe it coal, fire-clay, shale, or ironstone-care should always be taken to study not only its own structure and texture, but the nature of the roof and floor by which it is bounded, so as at once to insure safety to the workman and prevent undue waste of the material. Some floors are soft, and rise or "creep" under the adjacent pressure, and may thus interfere with the ventilation. Some roofs of shale are slippery, or full of "slicks ;" and others of sandstone are jointed and shivery, and thus require frequent proppings and packings to prevent falls. The whole secret of successful mining lies in raising the largest amount of material in the best condition from any given space, and that at the cheapest rate, and with the greatest safety to the miner. We say the largest amount of material in the best condition; for it must be admitted that in many of our coal-fields the structure of the seams is too little studied, and the result is a large amount of small coal and slack, which might be avoided by the adoption of more skilful methods.

To secure these conditions of economy and safety, the mining engineer should make himself thoroughly acquainted with the structure of his field, noting every fault and dislocation, its amount of throw and direction-every dyke, its direction and effects on the adjacent strata, whether faulting or altering them-every thinning and thickening of strata—the nature of the beds at points where "blowers of gas occurand, in fine, every irregularity and unusual appearance that presents itself. These, carefully and distinctly noted on his working plans, will be guides and directions in all his future winnings; and it is simply for want of such plans that so many accidents occur, by breaking into old workings full of gas or water, and so much money is spent in trial-drifts, cautionborings, and the like, where nothing of the kind would have been needed had former experiences been properly plotted and registered as they ought to have been. No doubt things are now in a better condition in this respect than they were thirty or forty years ago; but much yet remains to be done and enforced as a national necessity.

Our coal-fields are limited, and, with an annual output of between 120,000,000 and 130,000,000 tons, are gradually becoming poorer, and seams now neglected, and boundaries between estates now left unworked, must sooner or later be fallen back upon. Under such circumstances, it would be no undue interference with private rights on the part of the Legislature not only to enforce minute plans and records of every working, but to regulate the extent or removal of boundaries, as well as

the pumping of water, that may pervade two or more adjacent properties. What might never be done where individual interests come in conflict can be readily accomplished by general enactment. The law which regulates the agricultural drainage of contiguous estates in fens and levels, could surely be as advantageously applied to the drainage of contiguous mines.

Obstacles to be overcome.

We say nothing about main-ways, water-levels, air-courses, ventilation, pumping, haulage, the necessity of having access from seam to seam where more than one are worked from the same shaft, and the like-for these belong to mining in its purely technological aspects; but it may be remarked, that by prudent foresight, and the noting of mineral changes in the strata-many contingencies may be anticipated, and precautions taken to meet them successfully. There are generally some premonitory indications of the presence of water, of blowers of gas, of dykes, faults, thickenings and thinnings of strata-changes, for example, from cannels to ordinary coals, or from cannels to blackbands, and similar phenomena; and the engineer watchful of these indications must ever stand on a higher platform than one who fails to note them, or who is ignorant of their teachings even when observed.

The most frequent interruptions to coal-mining (and the same remarks apply to shales, ironstones, fire-clays, &c.), are fissures, faults or dislocations, which throw the strata out of their usual position; soft dykes filled in with debris from above; hard dykes injected with igneous rocks from below; wash-outs, nips-out, or dead-grounds, where the seam disappears for a certain space, and its place is taken by clayey, stony, or other debris ; fouls from the infiltration of some foreign matter through rents and fissures; or natural pits, (puits naturelles) as in the Belgian

coal-field, where the strata disappear for circumscribed areas, their place being occupied by miscellaneous debris from above.

In the preceding diagram w represents a wash-out cutting deeply into the strata, and evidently an old pre-glacial streamcourse; n n nips-out, apparently caused by runnels through the vegetable growth which formed the coal-seam; and pa natural pit or local subsidence of the strata, which may have been caused either by the removal of some subjacent bed or by the disintegration of some eruptive neck of trap-rock. Several of these give no indication of their presence till struck by the pick of the miner; some may be anticipated from changes in the texture of the coal, and the occurrence of strings and veins of calcareous spar, iron-pyrites, heavy spar, and the like; and others, again, as throws and trap-dykes, which traverse the country in linear directions for many miles, should be prepared for either from previous surveys of the field or from the plans of adjacent workings.

Still more frequent than the preceding, perhaps, are the "slips" and "throws" of the miner step-faults, which at intervals throw the strata nearer and nearer the surface, till some are thrown out altogether and denuded; trough-faults,

d

d Dyke-fault; sss Step-faults; Trough-fault; Reversed fault.

where a portion of the strata is thrown down between two adjacent faults; overlap or reverse-faults, where the strata are broken, and thrown, as it were, above themselves; and dykefaults, where a dyke not only intersects the strata, but throws them up or down out of their usual position.

The practice of mining is no doubt surrounded by numerous difficulties; but it is astonishing how many of these may be overcome by an engineer thoroughly acquainted with the geological structure of his field, and ever on the alert to notice the nature, position, and direction of any changes that may occur in the strata through which he is driving.

In the preceding pages we have alluded chiefly to coalmining; but the same remarks are more or less applicable to the winning of the shales, ironstones, fire-clays, and limestones