Geological Aspects of the Question
The Coal Question: Geological Aspects of the Question
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Historical E-Book: The Coal Question: An Inquiry Concerning the Progress of the Nation, and the Probable Exhaustion of Our Coal-Mines
Author: William Stanley Jevons
Edition Used: London: Macmillan and Co., 1866. (Second edition, revised)
First Published: 1865
Chapter III
I CANNOT pretend to do more, as regards the geological aspects of this question, than to give some brief account of the way in which geologists argue concerning it. At the most I must only try to point out what is clear and easy, and what is yet involved in doubt. In the first place, when we know the extent and thickness of a coal seam, we easily calculate its contents by weight. Coal varies in specific gravity, from about 1.25 to 1.33, or is from one and a quarter to one and a third times as heavy as an equal bulk of water (The Coal Question: Geological Aspects of the Question) . A cubic yard of solid coal therefore weighs from 2,103 lbs. to 2,243 lbs. And since 2,240 lbs. make one ton, it is quite exact enough to say that a cubic yard is a ton in weight.
Supposing a seam, then, to be exactly a yard thick, an acre of it will contain 4,840 tons of coal, and a square mile 3,097,600 tons. We may say in round numbers that a coal seam gives a million tons of coal per foot thick per square mile.
Our task is now reduced to that of defining the area and thickness of the coal seams of any district. The manner, however, in which the seams have been formed and disposed in the crust of the earth gives rise to several difficulties.
- The seams are of very different thickness and quality, some workable and others unworkable; we are not certain how many we may count upon.
- The area of the seams in a district is not uniform, some having been much more denuded or swept away by aqueous agency than others.
- Coal seams are more or less broken up by faults and hitches, and a greater or less quantity of coal must be sacrificed to the necessities of mining.
- Coal seams on one side often sink to unexplored depths, and we are uncertain how far we can follow them. There are reasons, too, for supposing that coal measures may exist where they have never yet been reached.
The first question, of the thickness of workable seams, will be more fitly discussed in the next chapter. The fact is sufficient here, that, under the present prices of coal, seams of less than eighteen or twenty-four inches do not repay the cost of working.
We have next to consider the superficial extent of coal seams. It is obvious that so far as seams lie one above the other co-extensively, we may lump them together in our estimate. Thus, in the Newcastle field, there are ten seams of more than two feet thickness, and in workable condition. Of these the High main and Low main coal seams are each six feet thick, and the intermediate Bensham seam is nearly three feet. Adding in the seven other less valuable seams, we have a total thickness of coal of thirty-six feet. As the area of the field, according to Mr. Hull, is 460 square miles, we might be inclined to reckon the total contents according to the rule at 460 × 36 millions of tons, or 16,560 millions. But we should here commit a considerable error, because the seams are not co-extensive. The quantity assumed by Mr. Hull, "corrected for denudation," is only 8,548 millions of tons.
The origin of the difference is very easily explained, though overlooked by many early and some late estimators. It arises from the very large portions of the upper seams that have been swept away or denuded during geological ages. The coal measures consist of many alternated beds of sand, mud, coal, and ironstone, deposited during a long interval of time in estuaries, great swamps, fresh-water lakes, deltas, or flat shores, which gradually sank as the beds were added. As first deposited, the strata must have been nearly level, but they are seldom so now. They lie at every angle from the horizontal to the vertical. Nowhere have we such good opportunities as in our coal mines of observing the upraisals, the downfalls, the dislocations, contortions, and denudations which rocks (Rock cycle) have suffered. The Scotch coal-fields must, at one time, have formed a nearly continuous and level sheet, but are now broken up into many separate irregular basins, and the seams are sometimes, as in the Mid-Lothian mines, turned up quite vertically on their edge. In the French fields the beds are sometimes folded in and out in a highly complicated and troublesome manner.
In general the coal measures have only been tilted up on one side in sloping plains, or bent into gentle curves and basin-like depressions. These movements could not take place without destroying the continuity of the strata; for though rocks seem to us solid and immovable, they are in comparison with volcanic forces but as thin and incoherent crusts. Accordingly, the beds are transversed in every direction by cracks, fissures, faults, where the whole mass of strata many thousand feet thick has been cloven through, one side comparatively to the other being thrown up. The great ninety fathom dyke, for instance, which crosses the Newcastle field, in a somewhat curved line to the north of the River Tyne, has caused the downthrow of the strata on the north side to the depth of 540 feet, and has had curious influences upon the progress of the English coal trade. On the whole, the Newcastle field is one of the least disturbed, and presents few great difficulties to the miner.
The Lancashire field is more troubled. The new map of the Geological Survey, prepared by Mr. Hull, a complete copy of which may be seen in the Museum of Practical Geology, represents it as scored and broken by a number of cracks, small and great, interlacing in a very complex manner. In short, a sheet of coal measures, to use Dr. Buckland's expression, is like a sheet of ice broken into numerous irregular pieces, but soldered together again without any bit being wholly lost.
Now, when all these disturbances took place, the surface of the ground must have been affected as well as the underground strata. We might expect to find on the south side of the ninety fathom dyke at Newcastle, a perpendicular rocky cliff of corresponding height. But no such thing is known on any of the coal-fields. The surface of our English coal-fields is either quite flat, or only swelling in one direction into round topped hills, showing no conformity to the underground disturbances. We cannot mistake the reason. While earthquakes and intrusions of lava were breaking up the strata, winds and rains and streams, or perhaps the tides of a shallow estuary, were wearing away all prominences, and carrying off great masses of rock. It has been shown, for instance, by Professor Ramsay, that the whole body of the coal measures between the South Wales field and that of the Forest of Dean, has been swept away; and the missing portion, far larger than mountains in mass, is conjecturally restored in the plates to one of the earlier memoirs of the Geological Survey.
During this process the upper beds of course would be soonest carried off. And when the beds are thrown up on one side into an inclined plane, we find the seams of coal more and more cut away as they are nearer the surface. Thus the coal measures, as they usually appear to us, successively crop up to the surface, like the layers of a piece of wood that has been planed off obliquely to its grain.
Thus it happens that the High main seam of coal at Newcastle is quite near the surface, and of comparatively limited extent; while the lower seams crop up to the surface at successively greater distances from the centre of the field, and the lowest crow coals not included in the true measures appear far away.
It is obvious, therefore, that in estimating the contents of a coal-field as we find it, we ought to lay down on a map the line of out-crop of each seam, that line at which it is cut by the surface of the ground. Then we should measure separately the area of each seam, and multiply each area by the thickness of the seam. On many of the maps of the Geological Survey the out-crop of the seams is beautifully shown in series of devious curves, sharply dislocated here and there by the faults. But I am not aware that any person has yet estimated the seams separately. The subject has hardly required so much nicety as yet, and Mr. Hull arrives at a corresponding result by what he calls a "correction for denudation," or an allowance for the large part of the upper beds worn away in the Newcastle field. How he estimates this "correction," almost amounting to half, I do not know.
But the amount of coal ascertained by multiplying the area into the thickness of a seam must not be taken as the amount available. Some part of a seam is always broken up, burnt, or spoiled by the faults and dykes which traverse it. Another considerable part is always lost in mining. Up to the end of last century it was not usual to extract more than four-tenths of the coal in a seam, when working at a greater depth than 100 fathoms; the rest was left in the form of thick pillars to keep the roof from falling in. The free use of timber to support the roof, and the introduction of long-wall, and panel working, has allowed the extraction of nearly all the coal in favourable positions. Still, in unfavourable circumstances, the highest mining skill will probably be unable to get the whole coal; and besides this it is always necessary to leave thick barriers of coal around the limits of the property in order to shut out the water, or the foul air of neighbouring works. A clause to this effect is always introduced into a mining lease, and if not observed, the mine may easily become unworkable. If to these barriers and the wasted pillars of coal, we add the small coal burnt at the pit mouth, or consumed in the ventilating furnaces and engines, we cannot estimate the coal available for commerce at more than two-thirds of that which the continuous seams would contain. Accordingly, Mr. Hull allows one-third for waste.
The contents of a coal-field may then be estimated with some certainty, provided that the boundaries of the seams on every side be known. This is the case in a perfect coal basin like that of the Forest of Dean. In the case of fields abutting on the sea, like those of Newcastle and Whitehaven, we have only the uncertainty concerning the distance to which coal can be worked under the sea. From one to three miles is the greatest distance we can conceive possible, except under a rise of price, which would constitute the scarcity of coal to be apprehended.
It is only when coal seams sink down beyond our knowledge on one side, as in the Yorkshire field, that we are in thorough uncertainty as to the quantity of available coal. The question here becomes a two-fold one. Firstly, how far may the coal measures be supposed to dip and extend under more modern formations? Secondly, how far can we follow them with profit, considering the growing costs and difficulty of deep mining?
Leaving the second question for discussion in the Next chapter (The Coal Question: Geological Aspects of the Question), there is but little that can be said concerning the first.
If the science of geology had no other claims upon our attention, it would repay all the labour spent upon it, many times over, by showing where coal may reasonably be looked for. By fixing the geological date of each rock, it points out in what interval the coal measures must appear, if they appear at all. One-third of the whole kingdom, it is said, is excluded from the search by being formed of rocks older than the coal-bearing age. On the other hand, there are large areas of country under which coal may reasonably be expected to occur, although there are no signs of it at the surface: and geology may enable us even to fathom the thickness of overlying rocks and tell with some certainty the depth at which coal will probably occur, if at all.[1]
Mr. Hull includes in his estimate 932 square miles of such country. Of these 225 square miles occur at the south-east corner of the Durham field, where the coal measures dip under the Magnesian Limestone and the New Red Sandstone. Another 400 square miles occur similarly on the eastward dip of the great Yorkshire and Derbyshire field. Wirral and other parts of the Cheshire New Red Sandstone are probably underlain by bands or sheets of coal measures, connecting the Flintshire and Denbighshire fields with the great Lancashire field. The North and South Stafford, Warwick, Coal-brookdale, and Forest of Wyre fields are more or less completely connected. On the other sides the fields are definitely terminated by the appearance of the carboniferous or mountain limestone, that great basement rock which in nearly every part of the kingdom bears the coal measures.
As these sunken coal-fields are continuous with those now worked, there can be little or no doubt as to their existence. But while they can hardly contain better seams than those already known, the seams may very possibly thin out if followed far. And in many cases the overlying Permian and New Red Sandstone rocks may contain so much water and swell to such a thickness as to be quite impenetrable.
A band of coal seams connecting the Durham and Yorkshire fields is of a more conjectural character. In the country between these two fields the Magnesian Limestone, which is above the coal, lies directly upon the millstone grit and carboniferous limestone below the coal. As there is no sign of coal measures at the junction, coal cannot now exist at the point. If it ever existed in the interval, it must have been swept away before the era of the Permian or Magnesian Limestone.
Noticing, then, the rectangular direction in which the northerly edge of the Yorkshire coal, and the southerly edge of the Durham coal run under Permian beds, it seems to be wholly a matter of uncertainty how far the denudation, or absence of the coal measures, may extend.
Another possible position of coal measures is beneath the cretaceous and Wealden beds of Wilts, Berks, Surrey, and Kent. In 1855, Mr. Godwin-Austen published a remarkable argument, showing that a range of rocks, an underground ridge of mountains, as it were, probably stretched from the Mendip Hills to the Ardennes in Belgium; and "we have strong à priori reasons for supposing that the course of a band of coal measures coincides with, and may some day be reached, along the line of the valley of the Thames, whilst some of the deeper-seated coal, as well as certain overlying and limited basins, may occur along and beneath some of the longitudinal folds of the Wealden denudation." His deductions were partially verified immediately after publication by the actual discovery of old rocks in the boring of wells at Kentish Town and Harwich. But Mr. Whitaker, to whose able memoir[2] and kind aid I am indebted, remarks on the uncertainty of such deductions concerning coal. "It must not be supposed that because there is almost a certainty of there being a ridge of old rocks at some depth below the surface along part of the valley of the Thames, and a likelihood of some of those old rocks belonging to the coal measures, therefore coal will be found at a workable depth in parts of the London District; for the alternations of sandstone, shale, &c., that so generally contain workable beds of coal, and are therefore known as the 'coal measures,' are sometimes almost without that mineral."
In short, all that is shown is a bare possibility of finding coal. But as it is uncertain whether the coal measures are there at all—whether, if there, they contain good coal—and if so, whether they are within workable depth and circumstances, it must still be held very unlikely that coal will ever be got in this tract.
And on the principle that "a bird in the hand is worth two in the bush," we should avoid putting too much reliance on possible coal-fields. Their existence is doubtful—they cannot well contain better coal than that we now enjoy, and may contain much worse, and they are very probably at depths, and in conditions, where they are commercially out of the question, as regards competition with foreign coals. There is plenty of coal known to exist out of our reach without resorting to coal that may or may not exist, but is in any case perhaps out of reach.
Here I may notice the differences of opinion that have arisen concerning the amount of accessible coal in the Great South Wales coal tract. For a long time it was considered an inexhaustible store, to which we might have final recourse some centuries hence.
Mr. H. H. Vivian, a great land and coal owner of that district, Member of Parliament for Glamorganshire, yet insists upon its being regarded in this light. During the discussions on the French Treaty of Commerce in 1860, some opposition having been raised to the 11th clause, on the ground that free exportation of coals must accelerate the exhaustion of our mines, Mr. Vivian roundly asserted that the South Wales field alone would serve the whole consumption of England for 500 years, and it would sustain its own present consumption for 5,000 years. "It was perfectly absurd," he said, "to talk of the exhaustion of coal in this country."
Now, when Mr. Hull came to estimate the amount of available coal in this field, he found it to be only 2,000 times its present yield, or two-fifths as much as Mr. Vivian's estimate.
Having the accuracy of his statement then called in question, Mr. Vivian published a small pamphlet containing, in addition to a reprint of his speech, and of a lecture on coal, a brief critique on Mr. Hull's calculations. "Mr. Hull," he says, "takes the total thickness of strata at 10,000 feet, containing 84 feet of valuable coal; he then deducts for denudation 48,000 millions of tons; he next deducts one half the remainder, or 24,000 millions of tons, for those seams which lie below 4,000 feet; he further deducts one third for waste, and the quantity already extracted, leaving a balance of 16,000 millions of tons out of his original quantity, which he does not state, but which I calculate from his data at 78,000 millions of tons, as the quantity likely to be available for man's use, equal to the present rate of the consumption of South Wales for 2,000 years, my estimate having been 5,000 years." Mr. Vivian then objects to the first of these deductions, that it is wholly arbitrary, and beyond the power of any person, however intimate his local knowledge, to estimate. The second deduction he considers opposed to fact.
But when Mr. Vivian defends and explains his own estimate, what has he to urge? "I took the thickness of coal," he says, "after the most careful consideration, at 60 feet. I had mainly in view the 'Great Lower Veins,' varying from 50 feet on the northern to 100 feet on the southern upcrop, and upwards of 70 feet on the central upheave. I looked at the area over which now, and ages hence, those beds might probably be won. I considered the comparatively limited area under which they would lie too deep, but where the 'Upper Vein,' to some extent, supplied their place, and I concluded that I might fairly take 60 feet as an average workable thickness over the entire area. I then took the produce at 40 per cent. less than the actual contents, that is to say, I calculated the cubic yard at 1,500 tons instead of 1,613 tons, or 6.66 per cent. (less), and I allowed one-third, equal to 33.33 per cent. for waste, faults, quantity already worked, &c., together 40 per cent.; and upon these data I arrived at the conclusion that South Wales could supply all England for 500 years, and her own consumption for 5,000; to that I adhere in spite of the calculations which Mr. Hull has adduced."
Now this sort of argument may be very satisfactory to Mr. Vivian's own mind, and, in a Parliamentary debate, a confident assertion by a man of local knowledge and influence has a good deal of weight, and rightly so. But will Mr. Vivian's views bear a moment's criticism? Would Mr. Vivian accept such an estimate from a witness before him on a Parliamentary Committee? Would he be satisfied with taking the thickness of coal, "after the most careful consideration, at 60 feet?" Why, what are the facts? Geologists of the highest standing—Sir T. De La Beche and Sir W. E. Logan, after a long geological survey, most admirably conducted, proved that the coal measures of South Wales are 10,000 or 12,000 feet thick, and contain altogether 84 feet of coal in seams of workable thickness, the most of which lie near the base. Mr. Vivian assumes, apparently, by nothing more than conjecture, that 60 out of the 84 feet on an average may be taken as available over the whole area!
Mr. Hull may have deducted too much for denudation, and possibly too much for depth; but Mr. Hull's is an estimate—Mr. Vivian's is no more than a guess. And, of course, when Mr. Vivian asserts that South Wales can supply all England for 500 years, he means at the present rate of consumption, which is quite beside the question. The question is, how long will South Wales supply us at the present price with the present growing demand?
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