Popular Science Monthly/Volume 23/May 1883/How the Ancient Forests Became Coal
|HOW THE ANCIENT FORESTS BECAME COAL.|
THE carboniferous formation represents the most wonderful episode in the history of our globe. It gives us an impression comparable in strangeness to that produced by those wonderful civilizations which blossomed out so suddenly and so splendidly in the infancy of mankind. Only a rare concurrence of circumstances could have brought on the expansion of plant-growth which characterized its epoch. The world of plants was still young and imperfect. Vegetation was characterized by the abundance of green parts susceptible of rapid growth, and of an almost indefinite development. It was, however, destitute of two characters which have been acquired by the later plants: those of the periodical and gradual increase of parts destined to endure, and of an absolute specialization of the reproductive apparatus. The vegetable kingdom was the first factor in the production of coal, but not the only one; and two other factors must be taken into account in studying its genesis. One of these related to the conditions of the environment, the climate, and the temperature; the other to the situations in which the plants that were converted into coal were placed. Had either of these conditions been essentially different or left out, we would have had no coal. The influence of situation is shown by the fact that the coal-beds are always intermittent; that they are limited in extent, and pass laterally into shales and sandstones, so as to show that there was nothing universal in the phenomenon, and that it was liable to interruption by physical changes. It is also easy to conceive that the formation of coal could not have gone on unless the vegetation was adapted to the process 3 and the conditions of the climate were suitable. The coal-plants could never have grown and flourished as they did in the present climates of the North; and our hard-wood trees, with their firm foundations in the ground, and their slow, periodical growth, could never by decaying in the open air have produced the peculiar and rich combinations we find in the coal-beds. The nature and bearing of these three concurrent factors have been carefully studied out by M. L. Grand' Eury, who has for that purpose spent many years in personal inspection of various mines and their surroundings, and has presented, in his "Mémoire sur la formation de la houille" ("Memoir on the Formation of Coal," Paris, 1882), a complete theory on the subject, including a review of the details of the process as taught by his observations of the phenomena.
The plants of the coal-measures, so far as their nature has been revealed to us by their remains, were great ferns, gigantic lycopodiums, called by the geologists lepidodendrons, and calamites and asterophyllites, allied to existing Equisetaceæ; all referable to the class of cryptogams. Besides these was another group, the character of which was long problematical, composed of the sigillarias and stigmarias. It now appears to be established that the stigmarias were a kind of rhizoma which had the faculty of persisting for a long time under the mud unchanged, growing and multiplying by stolons, but incapable in that condition of producing sexual organs; while under favorable circumstances they formed enormous buds whence shot up to the height of a hundred or a hundred and fifty feet the huge leaf-clad stems whose fossils, marked with the beautiful scars representing the leaf attachments, have been called sigillarias. Gymnosperms were also quite plentiful, of one class of which, the cordaites, M. Grand' Eury has made some happy restorations. He has found their leaves and seeds in considerable abundance at Saint-Étienne, and he has observed at the same place visible traces of their carbonized trunks, still standing erect and traversing the sandstone strata of some of the quarries.
A peculiar feature of these plants was the extraordinary predominance of the cellular or succulent tissues in them, and the corresponding rarity of the hard or fibro-ligneous parts, which appear reduced to insignificant cylinders. It was certainly not the destiny of these parts to increase with time, after the manner of the wood of our trees; and, in examining the mature stems of the ancient plants, we never find any more than an extremely thin ring of real wood. The rest is all pith, and even the bark, except on the outside, frequently presents an open or spongy texture. Such structure is similar to that of the aquatic plants of the present time, which can not exist at all in the air, and wither as soon as they are taken out of the water. An atmosphere saturated with moisture was therefore essential to their vigor; and in such an atmosphere, according to M. Grand' Eury, they grew continually, without interruption by changes of season, without rest or alternations, to exhaustion; then to fall to the ground, and give place to other similar growths. So luxuriant a vegetation could only have been produced by the combination of an ultra-tropical heat with an excessive humidity, under no other changes of seasons than those distinguished by intervals of relative calm and of torrential rains. At the same time, the superabundance of green parts, which characterized even the trunks of the trees, presupposes a considerable intensity of light; and all the phenomena point to a strong diffused light, the direct rays of the sun being tempered by the interposed veil of vapors, as that under the influence of which these growths were produced.
The third element of the problem, that of the material disposition of the places in which the coal was formed, is the one that has offered the most difficulties. Two theories have been held on this subject. One is, that the materials were carried by ocean-currents or rivers from considerable distances to the places of deposit. Naturalists, however, who have applied themselves specially to the study of the carboniferous flora, have not been able to reconcile the orderly arrangement of the fragments, in which the specimens are so delicately posed, mingled without confusion, and often distributed uniformly in collections of leaves of the same species, with the confused drifts which are the almost invariable results of such a method of transportation. Moreover, in all coal-regions, recognizable trunks of calamites, tree ferns, sigillarias, and other types of the carboniferous flora are found in the neighborhood of the coal, vertically crossing the strata of sandstone that accompany and separate the coal-beds in such a manner as to show that they grew over the ground of the whole region, and to indicate that their transformation was dependent upon some special or local phenomenon which may have been quite simple, or at least natural, and were probably resultant from the physical conditions of the land at that epoch. The other theory, that coal originated in the decomposition of trees and plants that grew on the spot, is insufficient to account for all the phenomena and circumstances, and raises new difficulties.
M. Grand' Eury, in whose theory transportation, but of a different character from that presumed in the first of these two theories, forms an important element, has been enabled, through his investigations at Saint-Étienne, to form a clear idea of the nature of coal and the processes to which we owe it, and also to enter into the details of the matter, to go back to the true causes of the processes, and to describe with remarkable precision how they must have taken place. The land of the carboniferous formations appears, after an intelligent examination of the stratigraphy, to have been frequently covered by the sea, and therefore in its immediate neighborhood. The coal-beds themselves were an essentially terrestrial formation, peculiar to the recently emerged land of the period. In Belgium and England they rest upon a marine deposit, which forms their floor, and which reappears in the course of the formation, alternating several times with the strata of land growth. We learn from this that the sea was retiring from these spots before the extension of the continental area, leaving a broader strip of land after each fitful inundation, and that the carboniferous vegetation was developed on the ground which the marine waters had just abandoned. This phenomenon acquires great force with its frequent recurrence and repetition in various places.
None of the carboniferous plants except the stigmarias, whose peculiarities we have noticed, appear to have been especially aquatic; but they could all endure the immediate neighborhood and occasional contact of water without being hurt by it, and could live and grow, even when partially inundated. They grew around the borders and on the slopes of the lagoons with which the shore was studded, the smaller ones thickly matted under the cover of the larger trees, in groups characterized by the predominance of single species, as is shown by the distribution of the fossils. The coal was deposited in the lacustrine beds at the center of these forest-covered depressions; and the extent of the deposits is measured by the area of the basins that were fitted to receive them. One condition was essential, without which no seam of combustible matter could have been formed. It was, that the water flowing over the ground should bring with it and leave in the bottom of the basin where the carboniferous matter was destined to accumulate, only the remains of plants, to the exclusion of every other form of sediment. This condition may have been more easily realized in the Carboniferous epoch than at any other time, because the flora was more abundant and its extension more favored by the climate. It is conceivable also that, after having been once established, it might have been liable to interruption at any time; for a slight oscillation of the ground, a change in the direction of the currents, the washing down of a bluff, or the removal of some impediment, may have been enough to furnish an opportunity for the introduction of sand, mud, or rock-dust, into the deposits. We may also affirm as essential that there should be no real affluent coming down to the place of deposit, or current of running water, for that would bring down mud, and leave in the bed some other sediment than one of coal. The flow of water must have been a gentle trickling over the soil, bathing it without washing it, but strong enough to carry along the vegetable matter which it finally deposited. Whenever the flow became more violent, the formation of coal was interrupted to give place to deposits of shale or sandstone, according to the character of the mineral elements brought down, or, if they were in relatively small proportion to the vegetable fragments, of schistose laminæ marked with impressions of plants.
Such formations are of frequent occurrence in the coal-measures, in alternation with the seams of coal and the marine deposits left by the overflow of the sea, and this was doubtless their origin. The fossil forests which have been discovered in the same regions may be similarly accounted for. The trees growing around the perimeters of the lagoons would be partially submerged by the overflowing water, and the sections of them buried in its muddy deposit would be left to decay and fossilize in it. The persistent root-stocks of the sigillarias and calamites, unharmed by the flood, would send up new aërial stems; and most of the other plants, having the power of sending out adventitious roots from their trunks, would be able to live and continue to grow by that means, leaving their old lower parts to die, while they lifted themselves, as it were, bodily up with the ascensional movement of the soil. Several examples of such successive emissions of roots are figured in the "Mémoire." M. Grand' Eury has assumed that the concurrence of two principal circumstances, acting coincidently and in combination with each other, contributed essentially to the formation of coal. One was the transportation by water for short distances of all the vegetable matter of a region to be spread out flat and stratified at the bottom of the lagoon destined to receive it; the other was the exposure of the matter, previous to this process, in the open air to a certain amount of decay, of the nature and effects of which he has made a patient analysis. From these principles he has deduced a theory which may be summarized thus: The water which served as the vehicle for the vegetable matter, which must have been perfectly clean, because it was free from all mud, strong enough to carry along its drift, and plentiful enough to sweep all the points of the wooded region, could not have been any other than rain-water shed upon slopes pronounced enough to make it easy for it to run and carry the vegetable residues along with it, yet level enough not to allow the ground to be cut up. The land over which the water flowed must have been covered with a mass of plants and accumulated fragments abundant enough to furnish much flotsam matter, and matted enough to prevent its eroding the subjacent soil. The water must have been intermittent, else the fallen trunks of trees and the fragments of every kind which lay scattered over the ground would not have had time to undergo the partial decomposition and disaggregation of their tissues which necessarily preceded complete submersion. There must have been, then, if not real seasons, intervals of relative calm, in which the decomposition could have taken place, to be succeeded by times of protracted and extremely violent precipitation. The fact that a transportation and deposition of the parts took place is attested by the stratified structure of the coal. In both the coal and the schisto-carbonaceous laminæ, all of the fragments, down to the most delicate isolated organs, are always, with only the rarest exceptions, spread out flat, and cemented one over another, lying together like the leaves of a book. A close examination of coal and its texture under the microscope will show that only water could have taken all the fragments of such different sizes and consistencies and arranged them in this way. The disposition is the same as is always shown when leaves and fragments of plants, having become thoroughly soaked, sink and form stratifications at the bottom of ponds. In the coal, the elements continue visible, and their arrangement in superposed lamina? is evident, while the interstices between the planes of junction appear to have been filled up in the course of the formation of the beds. These facts, with the perfect condition of the organization of many of the fragments; their agglomeration in an amorphous pulp, the resultant of the previous maceration of a host of fragments; and the reduction of the whole mass by compression to half of its primitive thickness—all parts of one and the same phenomenon—point to the action and weight of the bed of water at the bottom of which the stratification took place. The perfect uniformity which reigned over the formation of coal has given it generally a schistous structure, in thin leaves disposed in a parallel fashion and fissile in the direction of the plane of deposition. It is also found, on attentive examination, to be somewhat varied in constituency, according to the varying character of the elements of which it is composed, and the different stages of freshness and maturity in which they were deposited. One kind, which M. Grand' Eury calls "fusaine," from its resemblance to a stick-charcoal, comes from the decomposition of stems from which the anatomical structure has disappeared while they preserve their form. The green parts appear as crystalline laminæ, or scales, or black particles in the amorphous mass. This mass, the result of the maceration of wholly decomposed particles, constitutes the amorphous coal in which, besides "fusaine," we can always discover some remains of vegetable structure testifying to the common origin of all the coal products. Within these differences of type are innumerable variations passing from one to another, the existence of which prevents our establishing a clear distinction between the coals most homogeneous in appearance and those which show the multifarious and manifest traces of hardly altered organized elements.
M. Grand' Eury's sketches introduce us to the depths of the carboniferous forests, into regions of dense moisture, at the feet of gentle slopes where are accumulating in stagnant ponds immense drifts of the remains of constantly active, exuberant, and quickly exhausted vegetation. Masses of this kind may even now be observed in the midst of the virgin forests of hot countries; how much more might we have expected to find them in ancient epochs, when the trees made no wood, but sent up spontaneous, ungainly shoots, sudden growths in green columns, the function of which was as ephemeral as their texture was weak! Most of the carboniferous stems, hollow or filled with pith only, fell by the sheer exaggeration of their growth; the tree-ferns were crowned with fronds of marvelous dimensions; the stems of the sigillarias shed their leaves rapidly; and the remains of all these rank growths were incessantly accumulating in a sultry shade on a water soaked soil. We can conceive the enormous production of humus. Decomposition was accelerated by every rain, and the whole mass was reduced, down to the very bottom, to a black pulp; and this is why, notwithstanding we have such abundant materials, we meet so many difficulties in reconstructing the types. The fallen trunks seldom remained whole, but swelled and burst. The soft and porous parts gave way first, then the dense and fibrous parts were detached from the cortical mass; that, more tenacious and firm, spread out and resisted longer than the rest. Nothing remained of the fern-stems but the peripheric sheath or the disaggregated interior fibers; of the cordaites, sigillarias, and lepidodendrons only the cortical regions. The detached leaves formed other accumulations; and all these heaps, standing as obstructions in different places, were waiting for the arrival and passage of the water to yield to it innumerable fragments in very unequal degrees of decomposition. When the great rains came on, the waters, filtering in from every side, trickling down all the slopes, gathered here and there in temporary lakes, and finally overcame all the dams of organic matter they met—an immense mass of détritus going down to the lacustrine center. With these old and disorganized residues, the rains, which we must imagine to have been torrential, brought down also everything that would yield to their impulsion—tree-trunks, leaves, young shoots, and at times entire plants. It is these remains, so fresh in condition, these leaves so delicate, and clearly defined, these organs so whole which we see in our collections distinguishable in their slightest details, and lying spread out in the leaves of the great herbarium of which it is our privilege to turn the pages.
M. Grand' Eury's theory does not appear to offer anything that is discordant either with ancient phenomena or with those of more recent periods. It possibly has its place marked even now among the grand scenes of contemporary nature. We read in the narratives of the travelers who have ascended the great rivers of the interior of Africa, the Nile, for example, how their boats have been stopped for days at a time by submerged remains and the accumulations of plants hiding the river on which they were floating. In the face of such pictures, which show us sedges, water-lilies, and immense colonies of floating plants, under which the river has disappeared, while its eddies, its lagoons, and its deep basins are temporarily flooded after having been dry for months, we can not escape being carried back in mind to the phenomena, doubtlessly not quite parallel, but assuredly of the same order, to which was due the formation of the coals and lignites in ancient epochs. These were certainly not accidental or episodical phenomena, produced by circumstances which, once realized, were never to appear again, but occurred in the course of a series of analogous combinations of conditions which may have been frequently repeated, and involve nothing incompatible with what is going on on the surface of the globe in our own day. In speaking thus, we do not regard Europe, but the interior of tropical countries, and the parts of those lands where water, heat, and an exuberant vegetation are combined upon a ground the configuration of which is agreeable to the material conditions that have been postulated in this sketch.
- Translated and condensed for "The Popular Science Monthly" from the "Revue des Deux Mondes."