# The South Staffordshire Coalfield/Chapter 14

CHAPTER XIV.
On the Formation of Coal.
Coal is universally allowed to be of vegetable origin; it is a mass of the débris of trees and plants, that having been buried under mud and sand, has been subsequently converted into coal, the muds and sands being similarly converted into shales, and clays, and sandstones. There are, however, two opinions as to how the vegetables got into the situation we now find them in. The first, and at one time the more general opinion was, that trees and plants were drifted into large lakes, estuaries, and shallow seas, and there becoming water-logged, sank to the bottom, and were subsequently covered up there by the other accumulations. The second opinion, and perhaps the most generally entertained now, is, that the plants entering into the composition of the coal were not drifted, but grew and perished on the very spot we now find them forming coal, just as our own peat bogs at the present day would form coal if buried for a vast series of years under a great accumulation of earthy matters.
Botanists tell us that all the plants entering into the composition of coal, so far as they have been able to trace and verify any part of their structure, appear to have been not aquatic but terrestrial plants. For the formation of the beds of coal, therefore, in situ, it is necessary to suppose that the water in which the shales and sandstones were deposited became filled up, and the space converted into dry land, or, at all events, into a marsh at or above the level of the water; that on this dry land or marsh, plants accumulated in sufficient quantity to form a bed of coal; that then a depression took place, and the space became again covered by water, in which more shale and sandstone materials accumulated, again filling it up to the level of the water, and then another marsh, and so on.
I by no means intend to range myself among the advocates of either one or the other opinion, but I think there are certain difficulties in the way of the latter, which in spite of all the evidence as to the roots and upright stems of trees, (or whatever the plants called Sigillaria, Stigmaria, &c, may have been,) would make me hesitate to embrace it exclusively. Some of these difficulties arise from facts observed in the South Staffordshire coal-field, and these it is now proposed to lay before the reader.
1st. The "rolls," "swells," or "horse's backs."—We have seen, page 52, Fig. 9, that these are long ridge-like accumulations ​of clunch or clay rising for sometimes at least 8 feet above the floor of the coal around them. If they were accumulated under water, but the surrounding coal that abuts against their base was begun to be formed at or above the level of the water, we must necessarily suppose an elevation of the ground to have taken place to the amount of 8 feet, between the time of the formation of the "swells," and that of the coal, or else that they were formed during seasons of flood when the waters were 8 feet higher than usual. The structure of these "swells" seems to me a very small foundation on which to build the hypothesis either of a partial eight-foot elevation of the land, or for an eight-foot rise of the water. If, however, both "swell" and coal were formed under water, there is no difficulty in the case.
2nd. The rock faults in the Thick coal.—These were described pp. 45 to 51. Figs. 5 to 8. The most obvious questions to put respecting them, when we are inquiring into the origin of coal, are, if the sandstone was deposited in water, and the coal is so intimately and minutely interstratified with the sandstone, how comes it that the coal was not itself deposited in water? or, if the coal is of terrestrial origin, must not the sandstone be so too? The only possible origin for the sandstone I can imagine on the latter alternative is, that the sand was brought up in among the vegetable matter by means of a strong spring or springs, but whether such an imagination be allowable to account for a mass of sandstone of 250 by 400 yards in extent, at least, must be left to the decision of the reader. There is, perhaps, at first a little difficulty in understanding this local accumulation of sand over a comparatively small area, surrounded by so much almost unmixed coal, even on the supposition of their both being drifted into the place we now find them, and deposited under some considerable depth of water, but in this case it is nothing more than the local occurrence of a cake of sandstone among wide spread beds of clay, or other material, a case which we know frequently occurs in nature.
More recent examination of the sand patches in the Thick coal of the Causeway Green colliery, and accounts of similar occurrences in many other parts of the neighbourhood, and the ending of the Thick coal in beds of sandstone in various directions, have only confirmed me in my belief in the entirely subaqueous deposition of those coals.
The way in which thick and thin seams of sandstone and coal alternated occasionally, with little seams of perfectly bright pure coal in the regularly stratified sandstone, while thick beds of pure bright coal are often flaked by little partings of clean sand throughout their thickness over considerable areas, seems to me to render it impossible to suppose otherwise than that their deposition and stratification was produced by the same agent.
It seems to me absolutely necessary to suppose that the vegetable matter was strewed out in regular thin laminæ at the bottom of ​some water, and that occasionally little clouds of fine sand or silt were carried into that water, and likewise sank to the bottom in fine layers. I have no doubt of the process of accumulation being a very slow and gradual one, for I have long been accustomed to look upon the time required for the formation of any wide spread bed of stratified rock whatever, even a single foot in thickness, as one to be measured only by the lapse of scores of years, perhaps by that of centuries. Each bed of coal is certainly made up of thin laminæ, which are obviously laminæ of deposition, every tenth or every hundredth of an inch requiring a distinct period for its production, as in the case of all other laminated rocks. The variations that take place in the quality and character of coals, sometimes inch by inch in their different laminæ, one being less and another more earthy, &c., the separation of the laminæ by little films of shale, or by thicker "partings" of substances that are distinctly argillaceous earth, more or less mingled with carbonaceous matter, those partings occasionally thickening out into substantial beds, and the occasional occurrence of nearly pure quartzose or micaceous sand, sometimes quite free from carbonaceous admixture, either in the thinnest films or in thicker beds, will all then be naturally accounted for by one process, namely, the gradual deposition of laminæ and strata of different kinds of substances, with different degrees of mingling at the bottom of some water.
3rd. The phenomenon of the Flying reed.—If the reader will turn to page 39. Fig. 4, he will see that to account for the structure there described, on the principle of all coal-beds being of terrestrial growth, the following suppositions are necessary:— Firstly, the water was filled up to its surface, and on that level plain the Heathen coal was formed, then a depression took place, and a subsequent refilling of the water with earthy deposits, and then the Thick coal was formed. When, however, the accumoulation of the vast quantity of vegetable matter necessary for the production of the Thick coal was nearly completed, a very partial and local subsidence took place in one or two localities, bending down the previously formed beds into a hollow or pool of water, sufficiently deep for 128 feet of shale and sandstone to be accumulated, and thus another level floor formed for the growth of the upper bed of coal, which grew partly on the shale and sandstone, and partly on the undepressed coal. Then another depression took place, which was itself very unequal, and had this peculiarity, that over the part previously depressed it only amounted to 38 feet, while over the part not so depressed, and over the district generally, it was from 120 to 180 feet.
4th. The expansion of the Thick and other coals towards the north.—When, however, we extend our observation beyond the ​mere phenomenon of the Flying reed, and embrace that of the expansion of the whole Coal-measure series to the north, and the splitting up and separation of the coals in that direction. I must confess the idea of those coals being formed only at or above the surface of the water, while all the beds between the coals were deposited below the surface of the water, becomes one that I find it impossible to entertain. It will scarcely be worth while to go into an elaborate argument on this part of the subject, since I think an inspection of the diagram on Plate 1, p. 25, will be of itself sufficient.
It has been formerly pointed out from facts observed in this district, which are in perfect harmony with the well-known facts elsewhere, that single beds of stratified rocks have generally an area greater, and a thickness less, in proportion to the fineness of the materials of which they are composed. When the- materials are coarse they are usually heaped together co as to form a thick bed in one place, which gets rapidly thinner in different directions, and often soon ends altogether. When several beds of the same kind of substance rest on each other, so as to form a group or set of beds, the above statement is almost equally true. When, on the other hand, the materials of which a single bed, or any set of beds, is composed are of very fine grain, those materials are generally equally diffused over a very wide area, changes in thickness are rare and occur very gradually, and the bed or set of beds where it comes to an end does so very gradually, and sometimes almost insensibly. When beds of coarse material are interstratified with beds of fine, the changes that take place in the grouping of the beds is often the result, solely or chiefly, of the changes in the coarser members, their sudden terminations, or their sudden thickenings and thinnings; the apparent changes in the thickness of the finer beds is due to single continuous beds or single continuous layers or laminæ being either separated by the partial interposition, or brought together by the partial absence, of the coarser over certain spaces. Now, it is clear that this law of the area of beds and laminæ being greater and their thickness less, in proportion to their fineness of grain, means nothing else than that the materials were spread over a larger space in consequence of their comparatively light specific gravity, or at least of their being more easily and therefore more widely transported by water, and being more generally diffused through it before finally coming to rest at the bottom. It was pointed out before too, that beds of coal so far from forming any exception to this general rule, are its most marked example at the one extreme, while coarse sandstones and conglomerates form the most striking example at the other.
In no Coal-measures that I ever examined in any part of the world, either in the British Islands. Newfoundland, or Australia could I ever detect anything but the most perfect conformity and blending between beds of coal and the stratified aqueous rocks in which they lay, the whole apparently forming one series of deposits produced by one agent acting in one way.
​The diagram on Plate 1 confirms these views in perhaps as striking a fashion as could be imagined. The beds of coal are evidently continuous from the northern to the southern part of the field. Each layer of carbonaceous matter has been widely and generally diffused throughout the water on the bottom of which it was ultimately deposited. How it came there, whether it was altogether of extraneous origin and transported from a greater or less distance, or whether it was the result of the death and decomposition of vegetables that grew in the water and were rooted in its bed, or whether some of them grew and some of them were transported thither, or how else the carbonaceous matter came there, is a part of the subjection which I offer no opinion. I wish merely to say as the result of an experience of a good many years, confirmed by the particular instance under examination, that it appears to me that the phenomena of the lamination and stratification of beds of coal, and their interstratification and association with other stratified rocks, are explicable solely by the relation of the specific gravity of their materials to the action of moving water, and the consequent diffusion of those materials through the mass of that water.
We know that at all events the materials of the clays and sandstones were transported into that water at intervals, and it appears that in the case of the South Staffordshire coal-field, the principal and most abundant source of these materials lay to the north of the coal-field, and that these materials which were largely and frequently deposited on the north, sometimes failed to reach the more southern part of the area, while the coals were diffused equally, or at least more equally, over the whole area.
If, for instance, we were to examine the constitution of the Bottom coal, commencing on the north and following it to the south, we should learn the following facts respecting,—
First of all about the latitude of the Rising Sun at the Brown Hills, a great number of layers of carbonaceous matter were deposited until eventually enough was accumulated to form a thickness of 5 feet of coal. In whatever way this took place we cannot conceive it to have been otherwise than a long process. No one perhaps has yet formed an adequate conception of the vast length of time and great growth of vegetable life, either on the spot or in the neighbourhood, required for the formation of 5 feet of coal over an area of many square miles. There was then a gradual and successive deposition of several hundred layers of fine black mud forming more than 3 feet of dark shale. This bed is only 2} feet thick at Pelsall, a mile or two farther south, but in one place becomes 9 feet thick in consequence either of some change in the currents or of some undulation in the bottom, or some other arrestation of the suspended materials. Farther south this bed is not mentioned at all, either because it does not exist at all or because it is too thin to notice. Near the Rising Sun we find over it another series of coal layers forming the Roof coal, which in one pit is a 10-inch and in another a 13-inch coal. At Pelsall, where the ​bed below becomes thin, this coal swells out to 3 feet for a short space, while at High Bridge, where the swelling out of the bed below to 9 feet takes place, the coal thins again to 1 foot 9 inches. The deposition, like all other aqueous depositions, was evidently thicker where there was a hollow, and thinner where there was a rise,in the bottom. South of this point where the intermediate bed ceases altogether, the united coal layers continue southwards with a uniform aggregate thickness of 7 feet. Returning to the Rising Sun we find over the Roof coal (which together with the coal below and the intervening bed is spoken of there as the Deep coal) an accumulation of shales and sandstones varying greatly in the details of their grouping and thickness even in closely adjoining pits, but making an aggregate thickness of 41 feet 3 inches in one pit, and 48 feet 10 inches in another. This is in each case covered by the series of layers of coal called the Shallow coal, having an aggregate thickness of 6 feet. At Pelsall Wood the Shallow coal is only 4 feet thick, while the beds between it and the Deep coal are 45 feet, but at High Bridge the coal is 5 feet 2 inches, and the beds below are only 37 feet 9 inches.
From this point the two coals retain their thickness of 5 feet and 7 feet as they range southwards, but the beds between them rapidly thin out, and finally disappear about a mile and a half south of High Bridge. The two coals then come together to form a twelve-foot coal, and continue under the name of the Bottom coal with that thickness for a mile or two to the southward, when they also begin to get thinner and thinner, and finally die out in the district south of Dudley.
This instance, of which the details could be traced, if necessary, through various contiguous collieries in far greater detail, is only one instance of the many similar changes occurring throughout the Coal-measure series. These changes are distinctly referable to the action of water in transporting materials of different kinds that have been committed to it, and cannot, so far as I can see, be referred to any other agency.
If we look at the diagrams on Plate I., with the knowledge of these and similar facts fresh in our minds, and not with the view of extracting a merely possible explanation of it from conceivable circumstances, I think we cannot fail to be struck with the obvious "delta-like" or "bank-like" form which the Coal-measures of South Staffordshire must have originally possessed, and the perfect resemblance they must have had to an undisturbed subaqueous accumulation.
It seems to me then impossible to suppose otherwise than that the whole series of the Coal-measures, coals included, were deposited by one connected operation of the same forces acting in obedience to the same physical laws on similar but slightly differing materials, through an indefinite but immensely long period of time.
* ↑ Among other evidences is the case described by my friend Mr. Beckett, of Wolverhampton, in the first vol. of the Geological Journal. I afterwards accompanied him to the open work at Parkfields, where these stumps of plants occurred in the coal. They certainly looked as if they had grown there, and perhaps they may have done so, but even so, it by no means settles the whole question.
* ↑ It is clearly impossible it can have been "blown" sand.
* ↑ It seems necessary to suppose that the depressions were unequal, and the irregularities levelled up to the surface of the water, by the accumulation of materials, because, if the depressions were equal over large spaces, and ¢he accumulations of unequal thickness, we should be obliged to call in the aid of unequal elevations or depressions again, to form a level surface "à fleur d'eau," for the growth of the next bed of coal.