THE SUPPLY AND WASTE OF COAL.

BY PROFESSOR D. T. ANSTED, M.A., F.R.S.

Or all subjects connected with geology, there is not one that possesses more varied interest than coal or mineral fuel. Of its origin and history as a mineral we know little, but that little is extraordinary and unfamiliar, constantly exciting inquiry and attention, and left undecided, notwithstanding the most repeated and the closest investigation. The structure of coal is still a puzzle to the microscopist, to the botanist, and to the geologist. Certainly of organic origin, there is still doubt as to whether some varieties were derived from vegetable or animal life. Certainly vegetable, as much at any rate is, no one has been able to say positively what part was tree vegetation, what was leaf, and what woody tissue, while a good deal may have been once peat-bog, or mere accumulation of sea-weed.

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And if the origin of coal is so open to discussion, its geological history is no less so. Who can examine a good natural section in a shaft, or even the rock laid bare by a railway cutting in a coal country, without being struck by the marvellously frequent alternation of seams of coal, shales, and sandstones? A hundred distinct coal seams, of thickness varying from a tenth of an inch to a dozen feet, in a thickness of a thousand feet of strata, is the smallest complication presented to us. a large majority of the seams it will perhaps be found that rootlets of plants extend down from the coal into a bed of clay below. In others, there will be absolutely no connection of any kind between the coal and the underlying or overlying rock, whether sandstone or shale. Not unfrequently, a band, called a parting, of black clay, will be found in the middle of a bed or seam of coal. Now and then the roof or overlying bed of a coal seam is made up entirely of clayey material, in the form of leaves, twigs, and branches of trees, while sometimes a similar appearance will be visible in sands where there is no coal whatever. So much for the association and the mode of accumulation; but what shall we say to the mechanical position in which it is found? In England we are so accustomed in all our coal-fields to find the coal-seams broken and crushedlifted here out of place; dropped there fifty or a hundred fathoms and lost sight of; re-appearing, where least expected, by another fault; dipping at a high angle in one place, and horizontal in another; the dip considerable, but in the opposite direction, in a third-that we are apt to fancy this to be the normal, or, at least, a necessary condition. An English coal-miner hardly believes that a coal-field can be in proper order without it, and some of our well-meaning but somewhat

hasty writers on this subject have regarded the breaking-up of the coal in this way by fault, as a special and providential arrangement, to bring within range portions of a valuable material which would otherwise have been useless, at least to the present generation. It needs but little knowledge of the coal-fields of other parts of the world to show, in the first place, that vast areas of coal may exist in a workable state, and in the most convenient position, without any faults or fractures at all; secondly, that the faulting is anything but an advantage, even in England; and, thirdly, that the temptation to abuse and waste this valuable material-to throw away more than half the natural supply in order to obtain the remainder at a somewhat earlier period, and at a somewhat cheaper price-has been systematically yielded to by the people supposed to be worthy of this miraculous interposition. Certainly a different distribution of the beds, and a somewhat firmer roof, would have been a simpler and more effectual means of securing a permanent supply.

But without entering into this question, which is not one here to be discussed, there can be no doubt that the extreme complication manifest in the coal measures, the multitude of faults, and the numerous thinnings-out of the seams, as well as the impossibility of identifying beds in distant parts of the country, is connected in an important manner with the general geology of England, and the special structure of the country at the time when the vegetable matter was accumulated, but not at all to the nature of coal itself, or to the removal of coal once formed to a convenient distance from the surface.

The coals of different parts of the world agree in essential characters, but differ so considerably in detail, that no practical difficulty is experienced in distinguishing them. To the uninstructed, there are but three or four recognizable varieties, such as stone-coal, or anthracite, burning without flame; caking-coal, burning with long flame, and cementing into a compact mass as it burns; hard-coal, burning to a white or red powdery ash without caking; and cannel-coal, which does not soil the fingers. Few but those interested in the coal trade, or using coal in large quantities for manufacturing purposes, are aware of the extreme difference in value, and the facility of recognizing the kinds from various localities, or from the several scams of the same mining district.

The points of chief importance in a coal-field, speaking solely with reference to economic value, are the following:1. The quality of the coal. 2. The thickness of the seam. 3. The depth at which the workings must be carried on. 4. The nature of the roof and floor. 5. The state of the measures with regard to water. 6. The degree to which the measures

are affected by faults and the dip of the strata. A brief notice concerning each of these conditions will prepare the reader for a consideration of the inquiries suggested by the title of this article, namely the probable extent of the supply of coal in our own country, and the degree of economy exercised in getting and using the mineral fuel.

1. The quality of the coal. This depends to some extent on the uses to which it is to be applied, for although all fair coal is usable for most purposes, and saleable, the value differs enormously. Thus an excellent household coal may be but a poor gas coal, and the converse; a good steam coal adapted for locomotives may be indifferent for furnace-work; and so with regard to all. Coals consist of carbon, with a certain admixture of hydrogen, and some oxygen and nitrogen. There is also a certain quantity of siliceous and earthy matter forming the ash. Coals that consist of pure or nearly pure carbon, require so strong a draft to keep up combustion, and give out heat so intensely at a short distance, that they require very peculiar contrivances for using them economically, and thus anthracite, as such coal is called, has but a local reputation. Cannel-coal, again, yielding an enormous quantity of gas, is by no means economical fuel for the open fire of an ordinary grate, or for a steam furnace. The heat is not sufficiently concentrated. Coals yielding a large quantity of ash are, in like manner, undesirable, for much of the caloric produced by combustion is wasted in keeping the earthy ash at a red heat. Thus the value of a coal must be estimated locally, and is strictly relative, though in general no coal could be considered bad in which the quantity of ash is not more than six or seven per cent., and where the per-centage of volatile substances does not exceed twenty-five per cent., and amounts to ten per cent. With less volatile matter than ten per cent. it is anthracite, and with more than twenty-five it passes into cannel coal. In either case the uses are special. Most of the English and Scotch coals are of sufficiently good quality to be available for general purposes, but the money value varies. The Welsh coals are to a large extent anthracites.

2. Number and thickness of the seams. Coal exists in beds of all degrees of thickness, from a tenth of an inch to at least fifty yards. It is evident that there must be a practical limit to economic working, and that under certain circumstances it may be expedient and profitable to work seams that under other circumstances could not possibly pay. When worked by shafts in the ordinary way, it is sometimes worth while to remove coal not more than a foot thick; but this is rarely the case, for so much stone and rubbish has to be moved to mine a seam of this thickness, that the work is not profitable. In

most districts, however, a two-foot seam, if of fair quality, and not accompanied by very indifferent roof and floor, can be worked, and thus all seams of two feet and upwards are generally calculated among the available coals of a district.

On the other hand, very thick seams involve waste, especially when the roof above the coal is weak, for then coal, often a foot thick, is left, and ultimately lost. In very thick seams, where the coal is of good quality, the works are often conducted in a very wasteful manner, and thus seams from three to six or eight feet thick are perhaps the most profitable, and those least wastefully worked. The number of workable seams in a coalfield varies exceedingly in the Newcastle coal-fields it is sixteen, in the Lancashire seventy-five.

3. Depth of workings. In most coal districts in England the coal seams, although they may once have approached the surface, have long since been worked out to all small depths, and everything that can be got by horizontal tunnels or drifts into a hill-side has been already removed. Pits or shafts have then to be sunk, at a cost and risk varying according to circumstances, and varying so greatly as to give a character of speculation to coal mining that would not otherwise belong to it. When the coal measures (the rocks known to belong to the coal-bearing series) are at the surface, or only covered by gravel and soil, the depth of any particular seam can be calculated pretty closely, especially in a district where there are many pits already sunk. Where this is not the case, and rocks of newer geological date have to be pierced, there is, however, great doubt. This arises from the fact that the exposed surface of the coal series, as of other rocks, has been worn down by air and water, and pared off to a great but unknown extent, during the lapse of the ages that have passed away since they were formed; whereas in those cases in which the measures are protected by a newer rock, they have escaped this, and are probably much thicker. There is also in such cases a possibility of disturbances having altered the dip or tilt of the beds, though generally the deeply-covered beds are more likely to be horizontal.

The present limit of workings in coal mines is not much more than 2000 feet. At that depth in England the temperature is already so high as to be inconvenient, and it is at least probable that, at greater depths, it would be still hotter. It has been found possible to work in mines where the temperature is at least fifteen degrees of Fahrenheit higher than in the celebrated Monkwearmouth pit; but this is by slave labour, and might be impracticable in England. At any rate, it is not likely that, even with improved contrivances for lighting and ventilation, a greater depth than from 3000 to 4000 feet will be attained in coal mines.

4. The roof and floor. When a continuous bed, or stratum of coal is removed from the mass of the earth, it is as certain that the rest will come together as that the ceiling of a room would fall if the four walls were taken away. By the ordinary mode of mining this is checked for a time, by leaving walls and props of coal, stone, or timber; but the check is only temporary. It is clear that the amount of fracture of the overlying strata will depend on their plasticity and thickness. Thus, a very thick, hard roof of sandstone may, perhaps, squeeze up a soft floor of clay into the spaces between the walls, and any beds of coal in the overlying strata may not be crushed. If, however, there is the smallest irregularity in the sinking, the coal above will be crushed to powder and destroyed. When the roof and floor are both good, the works of a mine may also be carried on more regularly and more profitably than when they are indifferent; and, as in many cases the most valuable seam of a series lies below two or three others that are somewhat inferior, it may happen that the question of the preservation or destruction of these may depend on the nature of the associated beds.

5. Water. In almost every coal district a certain quantity of water, draining naturally to the deepest point, will accumulate in the mines, and require removal by artificial means. Every stratum of the coal series that allows water to percolate will act as a channel; and faults, though sometimes they are "close," and cut off the water, in many instances act as conduits, and help to bring into the mine all the rain-fall of a district.

It is evident that, the deeper the mines, the more will they be subjected to this cause of trouble and expense; but, as all the strata cut through in sinking the shafts can be effectually stopped from entering by a water-tight iron casing to the shaft, it is only the quantity brought in by the coal strata themselves that is troublesome. Perhaps it may safely be assumed that improvements in pumping machinery will render this part of the work of mining as easy hereafter in deeper mines as it is now in those already worked.

6. Faults and the dip of strata. The main faults in some of the most important British coal-fields are tolerably well known; but there are many, affecting parts of the coal not yet reached, that may prove very serious. The dip of the coal may also render more difficult, though it may make much easier, the working of the deep coal under the New Red Sandstone. There is reason to believe that none of the coal-fields of Europe are really very extensive, and even the adjoining districts in our own island, though but a few score of square miles in extent, are not capable of any exact comparison with one another. On the other hand, the American coal district of Western Virginia is certainly