Fig. 16.—Mass of Coal with Coal Balls, A and B both enclosing part of the same stem L

Still a further witness may be found in the structure of the “coal balls” described in the last chapter. These stony masses, lying in the pure coal, might well be considered as apart from it and bearing no relation to its structure; but recent work has shown that they were actually formed at the same time as the coal, developing in its mass as mineral concretions round some of the plants in the soft, saturated, peaty mass which was to be hardened into coal later on.[3] All “coal balls” do not show their concretionary structure so clearly, but sometimes it can be seen that they are made with concentric bands or markings like those characteristic of ordinary mineral concretions (see fig. 15). Concretions are formed by the crystallization of minerals round some centre, and it must have happened that in the coal seams in which the coal-ball concretions are found that this process took place in the soft plant mass before it hardened. Recent research has found that there is good evidence that those seams[4] resulted from the slow accumulation of plant dÉbris under the salt or brackish water in whose swamps the plants were growing, and that as they were collecting the ground slowly sank till they were quite below the level of the sea and were covered by marine silt. At the same time some of the minerals present in the sea water, which must have saturated the mass, crystallized partly and deposited themselves round centres in the plant tissues, and by enclosing them and penetrating them preserved them from decay till the mineral structure entirely replaced the cells, molecule by molecule. Evidence is not wanting that this process went on without disturbance, for in fig. 16 is shown a mass of coal in which lie several coal balls, two of which enclose parts of the same plant. This means that round different centres in the same stem two of the concretions were forming and preserving the tissues; the two stone masses, however, did not enlarge enough to unite, but left a part of the tissue unmineralized, which is now seen as a streak of coal. We have here the most important proof that the coal balls are actually formed in the coal and of the plants making the coal, for had those coal balls come in as pebbles, or in any way from the outside into the coal, they could not have remained in such a position as to lie side by side enclosing part of the same stem. There are many other details which may be used in this proof, but this one illustration serves to show the importance of coal balls when dealing with the theories of the origin of coal, for they are perfectly preserved samples of what the whole coal mass was at one time.

There are but few seams, however, which contain coal balls, and about those in which they do not occur our knowledge is very scanty. It is often assumed that the plant impressions in the shales above the coal seams can be taken as fair samples of those which formed the coal itself; but this has been recently shown to be a fallacious argument in some cases, so that it is impossible to rely on it in general. The truth is, that though coal is one of the most studied of all the geological deposits, we are still profoundly ignorant of the details of its formation except in a few cases.

The way in which coal seams were formed has been described often and variously, and for many years there were heated discussions between the upholders of the different views as to the merits of their various theories. It is now certain that there must have been at least four principal ways in which coal was formed, and the different seams are illustrations of the products of different methods. In all cases more or less water is required, for coal is what is known as a sedimentary deposit, that is, one which collects under water, like the fine mud and silt and dÉbris in a lake. It will be understood, however, that if the plant remains were collecting at any spot, and the water brought in sand and mud as well, then the deposit could not have resulted in pure coal, but would have been a sandy mixture with many plant remains, and would have resulted in the formation of a rock, such as parts of the millstone grit, where there are many streaks of coal through the stone.

Among various coal seams, evidence for the following modes of coal formation can be found:—

(a) In fresh water.—In still freshwater lakes or pools, with overhanging plants growing on the banks, twigs and leaves which fell or were blown into the water became waterlogged and sank to the bottom. With a luxuriant growth of plants rapidly collecting under water, and there preserved from contact with the air and its decaying influence, enough plant remains would collect to form a seam. After that some change in the local conditions took place, and other deposits covered the plants and began the accumulations which finally pressed the vegetable mass into coal.

To freshwater lakes of large size plants might also have been brought by rivers and streams; they would have become waterlogged in time, after floating farther than the sand and stones with which they came, and would thus settle and form a deposit practically free from anything but plant remains.

(b) As peat.—Peat commonly forms on our heather moors and bogs to-day to a considerable thickness. This also took place long ago in all probability, and when the level of the land altered it would have been covered by other deposits, pressed, and finally changed into coal.

(c) In salt or brackish water, growing in situ.—Trees and undergrowth growing thickly together in a salt or brackish marsh supplied a large quantity of dÉbris which fell into the mud or water below them, and were thus shut off from the air and partly preserved. When conditions favoured the formation of a coal seam the land level was slowly sinking, and so, though the dÉbris collected in large quantities, it was always kept just beneath the water level. Finally the land sank more rapidly, till the vegetable mass was quite under sea water, then mud was deposited over it, and the materials which were afterwards hardened to form the roof rocks were deposited. This was the case in those seams in which “coal balls” occur, and the evidence of the sea water covering the coal soon after it was deposited lies in the numerous sea shells found in the roof immediately above it.

(d) In salt water, drifted material.—Tree trunks and large tangled masses of vegetation drifted out to sea by the rivers just as they do to-day. These became waterlogged, and finally sank some distance from the shore. (Those sinking near the shore would not form pure coal, for sand and mud would be mixed with them, also brought down by rivers and stirred up from the bottom by waves.) The currents would bring numbers of such plants to the same area until a large mass was deposited on the sea floor. Finally the local conditions would have changed, the currents then bringing mud or sand, which covered the vegetable mass and formed the mineral roof of the resulting coal seam. There is a variety of what might be called the “drifted coals”, which appears to have been formed of nothing but the spores of plants of a resinous nature. These structures must have been very light, and possibly floated a long distance before sinking.

If we could but obtain enough evidence to understand each case fully we should probably find that every coal seam represents some slightly different mode of formation, that in each case there was some local peculiarity in the plants themselves and the way they accumulated in coal-forming masses, but the above four methods will be found to cover the principal ways in which coal has arisen.

Coal, as we now know it, has a great variety of qualities. The differences probably depend only to a small extent on the varieties among the plants forming it, and are almost entirely due to the many later conditions which have affected the coal after its original formation. Some such conditions are the various upheavals and depressions to which the rocks containing the coal have been subjected, the weight of the beds lying over the coal seams, and the high temperatures to which they may have been subjected when lying under a considerable depth of later-deposited rocks. The influence on the coal of these and many other physical factors has been enormous, but they are purely cosmical and belong to the special realm of geological study, and so cannot be considered in detail now.

To return to our special subject, namely, the plants themselves which are now preserved in the coal. Their nature and appearance, their affinities and minute structure, can only be ascertained by a detailed study, to which the following chapters will be devoted, though in their limited space but an outline sketch of the subject can be drawn.

It has been stated by some writers that in the Coal Measure period plants were more numerous and luxuriant than they ever were before or ever have been since. This view could only have been brought forward by one who was considering the geology of England alone, and in any case there appears to be very little real evidence for such a view. Certainly in Europe a large proportion of the coal is of this age, and to supply the enormous masses of vegetation it represents a great growth of plants must have existed. But it is evident that just at the Carboniferous period in what is now called Europe the physical conditions of the land which roughly corresponded to the present Continent were such as favoured the accumulation of plants, and the gradual sinking of the land level also favoured their preservation under rapidly succeeding deposits. Of the countless plants growing in Europe to-day very few stand any chance of being preserved as coal for the future; so that, unless the physical conditions were suitable, plants might have been growing in great quantity at any given period without ever forming coal. But now that the geology of the whole world is becoming better known, it is found that coal is by no means specially confined to the Coal Measure age. Even in Europe coals of a much later date are worked, while abroad, especially in Asia and Australia, the later coals are very important. For example, in Japan, seams of coal 14, 20, and even more feet in thickness are worked which belong to the Tertiary period (see p. 34), while in Manchuria coal 100 feet thick is reported of the same age. When these facts are considered it is soon found that all the statements made about the unique vegetative luxuriance of the Coal Measure period are founded either on insufficient evidence or on no evidence at all.

The plants forming the later coals must have had in their own structure much that differed from those forming the old coals of Britain, and the gradual change in the character of the vegetation in the course of the succeeding ages is a point of first-rate importance and interest which will be considered shortly in the next chapter.