ON THE ORIGIN AND STRUCTURE OF COAL.
The origin of coal, that combustible which is distributed over the earth in all latitudes, from the frozen regions of Greenland to Zambesi in the tropics, utilized by the Chinese from the remotest antiquity for the baking of pottery and porcelain, employed by the Greeks for working iron, and now the indispensable element of the largest as well of the smallest industries, is far from being sufficiently clear. The most varied hypotheses have been offered to explain its formation. To cite them all would not be an easy thing to do, and so we shall recall but three: (1) It has been considered as the result of eruptions of bitumen coming from the depths, and covering and penetrating masses of leaves, branches, bark, wood, roots, etc., of trees that had accumulated in shallow water, and whose most delicate relief and finest impressions have been preserved by this species of tar solidified by cooling. (2) It has also been considered as the result of the more or less complete decomposition of plants under the influence of heat and dampness, which has led them to pass successively through the following principal stages: peat, lignite, bituminous coal, anthracite. (3) Finally, while admitting that the decomposition of plants can cause organic matter to assume these different states, other scientists think that it is not necessary for such matter to have been peat and lignite in order to become coal, and that at the carboniferous epoch plants were capable of passing directly to the state of coal if the conditions were favorable; and, in the same way, in the secondary and tertiary epochs the alteration of vegetable tissues generally led to lignite, while now they give rise to peat. In other words, the nature of the combustible formed at every great epoch depended upon general climatic conditions and local chemical action. Anthracite and bituminous coal would have belonged especially to primary times, lignites to secondary and tertiary times, and peat to our own epoch, without the peat ever being able to become lignites or the latter coal.
As for the accumulation of large masses of the combustible in certain regions and its entire absence in others belonging to the same formation, that is attributed, now to the presence of immense forests growing upon a low, damp soil, exposed to alternate rising and sinking, and whose debris kept on accumulating during the periods of upheaval, under the influence of a powerful vegetation, and now to the transportation of plants of all sorts, that had been uprooted in the riparian forests by torrents and rivers, to lakes of wide extent or to estuaries. Not being able to enter in this place into the details of the various hypotheses, or to thoroughly discuss them, we shall be content to make known a few facts that have been recently observed, and that will throw a little light upon certain still obscure points regarding the formation of coal.
(1) According to the first theory, if the impressions which we often find in coal (such as the leaves of Cordaites, bark of Sigillarias and Lepidodendrons, wood of Cordaites, Calamodendrons, etc.) are but simple and superficial mouldings, executed by a peculiar bitumen, formerly fluid, now solidified, and resembling in its properties no other bitumen known, we ought not to find in the interior any trace of preservation or any evidence of structure. Now, upon making preparations that are sufficiently thin to be transparent, from coal apparently formed of impressions of the leaves of Cordaites, we succeed in distinguishing (in a section perpendicular to the limb) the cuticle and the first row of epidermic cells, the vascular bundles that correspond to the veins and the bands of hypodermic libers; but the loose, thin-walled cells of the mesophyllum are not seen, because they have been crushed by pressure, and their walls touch each other. The portions of coal that contain impressions of the bark of Sigillaria and Lepidodendron allow the elongated, suberose tissue characteristic of such bark to be still more clearly seen.
Were we to admit that the bitumen was sufficiently fluid to penetrate all parts of the vegetable debris, as silica and carbonates of lime and iron have done in so many cases, we should meet with one great difficulty. In fact, the number of fragments of coal isolated in schists and sandstone is very large, and without any communication with veins of coal or of bitumen that could have penetrated the vegetable. We cannot, then, for an instant admit such a hypothesis. Neither can we admit that the penetration of the plants by bitumen was effected at a certain distance, and that they have been transported, after the operation, to the places where we now find them, since it is not rare to find at Commentry trunks of Calamodendrons, Anthropitus, and ferns which are still provided with roots from 15 to 30 feet in length, and the carbonized wood of which surrounds a pith that has been replaced by a stony mould. The fragile ligneous cylinder would certainly have been broken during such transportation.
The carbonized specimens were never fluid or pasty, since there are some that have left their impressions with the finest details in the schists and sandstones, but none of the latter that has left its traces upon the coal. The surface of the isolated specimens is well defined, and their separation from the gangue (which has never been penetrated) is of the easiest character.
The facts just pointed out are entirely contrary to the theory of the formation of coal by way of eruption of bitumen.
(2) The place occupied by peats, lignites, and bituminous and anthracite coal in sedimentary grounds, and the organic structure that we find less and less distinct in measure as we pass from one of these combustibles to one more ancient, have given rise to the theory mentioned above, viz., that vegetable matter having, under the prolonged action of heat and moisture, experienced a greater and greater alteration, passed successively through the different states whose composition is indicated in the following table:
H. C. O. N. Coke. Ashes. Density.
Peat 5.63 57.03 29.67 2.09 ---- 5.58 ----
Lignite 5.59 70.49 17.2 1.73 49.1 4.99 1.2
Bitumin. coal 5.14 87.45 4 1.63 68 1.78 1.29
Anthracite 3.3 92.5 2.53 ---- 89.5 1.58 1.3
Aside from the fact that anthracite is not met with solely in the lower coal measures, but is found in the middle and upper ones, and that bituminous coal itself is met with quite abundantly in the secondary formations, and even in tertiary ones, it seems to result from recent observations that if vegetable matter, when once converted into lignites, coal, etc., be preserved against the action of air and mineral waters by sufficient thick and impermeable strata of earth, preserves the chemical composition that it possessed before burial. The coal measures of Commentry, as well as certain others, such as those of Bezenet, Swansea, etc., contain quite a large quantity of coal gravel in sandstone or argillaceous rocks. These fragments sometimes exhibit a fracture analogous to that of ordinary coal, with sharp angles that show that they have not been rolled; and the sandstone has taken their exact details, which are found in hollow form in the gangue. In other cases these fragments exhibit the aspect of genuine shingle or rolled pebbles. These pebbles of coal have not been misshapen under the pressure of the surrounding sandstone, nor have they shrunk since their burial and the solidification of the gangue, for their surface is in contact with the internal surface of their matrix. Everything leads to the belief that they were extracted from pre-existing coal deposits that already possessed a definite hardness and bulk, at the same time as were the gravels and sand in which they are imprisoned. It became of interest, then, to ascertain the age to which the formation of these fragments might be referred, they being evidently more ancient than those considered above, which, as we have seen, could not have been transported in this state on account of their dimensions and the fragility of made coal. Thanks to the kindness of Mr. Fayol, we have been enabled to make such researches upon numerous specimens that were still inclosed in their sandstone gangue and that had been collected in the coal strata of Commentry. In some of their physical properties they differ from the more recent isolated fragments and from the ordinary coal of this deposit. They are less compact, their density is less, and a thin film of water deposited upon their surface is promptly absorbed, thus indicating a certain amount of porosity. Their fracture is dull and they are striped with shining coal, and can be more easily sliced with a razor.
From a fresh fracture, we find by the lens, or microscope, that some of them are formed of ordinary coal, that is, composed of plates of variable thickness, brilliant and dull, with or without traces of organization, and others of divers bits of wood whose structure is preserved. When reduced to thin, transparent plates, these latter show us the organization of the wood of Arthropitus, Cordaites, and Calamodendron, and of the petioles of Aulacopteris, that is to say, of the ligneous and arborescent plants that we most usually meet with in the coal measures of Commentry in the state of impression or of coal.
In a certain number of specimens the diminution in volume of the tracheæ is less than that that we have observed in the same organs of corresponding genera. The quantity of oxygen and hydrogen that they contain is greater, and seems to bring them near the lignites.
We cannot attribute these differences to the nature of the plants converted into coal, since we have just seen that they are the same in the one case as in the other. Neither does time count for anything here, since, according to accepted ideas, the burial having been longer, the carbonization ought to have been more perfect, while the contrary is the case.
If we admit (1) that vegetable remains alter more and more through maceration in ordinary water and in certain mineral waters; (2) that, beginning with their burial in sufficiently thick strata of clay and sand, their chemical composition scarcely varies any further; and (3) that these are important changes only as regards their physical properties, due to loss of water and compression, we succeed quite easily in learning what has occurred.
In fact, when, as a consequence of the aforesaid alteration, the vegetable matter had taken the chemical composition that we find in the less advanced coal of the pebbles, it was in the first place covered with sand and protected against further destruction, and it gradually acquired the physical properties that we now find in it. At the period that channels were formed, the coal was torn from the beds in fragments, and these latter were rolled about for a time, sometimes being broken, and then covered anew, and this too at the same time as were the plants less advanced in composition that we meet with at the same level. These latter, being like them protected against ulterior alteration, we now find less advanced in carbonization (notwithstanding their more ancient origin) than the other vegetable fragments that were converted into coal after them, but that were more thoroughly altered at the time of burial.
There are yet a few other important deductions to be made from the foregoing facts: (1) the same coal basin may, at the same level, contain fragments of coal of very different ages; (2) its contour may have been much modified owing to the ravines made by the water which transported the ancient parts into the lowest regions of the basin; and (3) finally, since the most recent sandstones and schists of the same basin may contain coal which is more ancient, but which is formed from the same species of plants that we find at this more recent level, we must admit that the conversion of the vegetable tissues into coal was relatively rapid, and far from requiring an enormous length of time, as we are generally led to believe.
If, then, lignites have not become soft coal, and if the latter has not become anthracite, it is not that time was wanting, but climatic conditions and environment. Most analyses of specimens of coal have been made up to the present with fragments so selected as to give a mean composition of the mass; it is rare that trouble has been taken to select bits of wood, bark, etc., of the same plant, determined in advance by means of thin and transparent sections in order to assure the chemist of the sole origin and of the absolute purity of the coal submitted to analysis. This void has been partially fitted, and we give in the following table the results published by Mr. Carnot of analyses made of different portions of plants previously determined by us:
Carbon Hydrogen Oxygen Nitrogen
1. Calamodendron (5 specimens) 82.95 4.78 11.89 0.48
2. Cordaites (4 specimens) 82.94 4.88 11.84 0.44
3. Lepidodendron (3 specimens) 83.28 4.88 11.45 0.39
4. Psaronius (4 specimens) 81.64 4.80 13.11 0.44
\----v----/
5. Ptychopteris (1 specimen) 80.62 4.85 14.53
6. Megaphyton (1 specimen) 83.37 4.40 12.23
As seen from this table, the elementary composition of the various specimens is nearly the same, notwithstanding that the selection was made from among plants that are widely separated in the botanical scale, or from among very different parts of plants. In fact, with Numbers 1 and 2 the analysis was made solely of the wood, and with No. 3 only of the prosenchymatous and suberose parts of the bark. Here we remark a slight increase in carbon, as should be the case. With No. 4 the analysis was of the roots and the parenchymatous tissue that descends along the stem, and with No. 6 of the bark and small roots. One will remark here again a slight increase in the proportion of carbon, as was to be foreseen. The elementary composition found nearly corresponds with that of the coal taken from the large Commentry deposit.
Carbon. Hydrogen. Oxygen and
Nitrogen.
Regnault 82.92 5.39 11.78
Mr Carnot 83.21 5.57 11.22
Although the chemical composition is nearly the same, the manner in which the different species or fragments of vegetables behave under distillation is quite different.
In fact, according to Mr. Carnot, the plants already cited furnish the following results on distillation:
Volatile Fixed Coke.
matters. residue.
Calamodendron 35.5 64.7 Well agglomerated.
Cordaites 42.1 57.8 Quite porous.
Lepidodendron 34.7 55.3 Well agglomerated.
Psaronius 29.4 60.5 Slightly porous.
Ptychopteris 39.4 60.5
Megaphyton 35.5 64.5 Well agglomerated.
Coal of the Great Bed 40.5 59.5 Slightly porous.
These differences in the proportions of volatile substances, of fixed residua, and of density in the coke obtained seem to be in harmony with the primitive organic nature of the carbonized tissues. We know, in fact, that the wood of the Calamodendrons is composed of alternately radiating bands formed of ligneous and thick walled prosenchymatous tissue, while the wood of Cordaites, which is less dense, recalls that of certain coniferæ of the present day (Araucariæ).
We have remarked above that the portions of Lepidodendron analyzed belonged to that part of the bark that was considerably thickened and lignefied. So too the portion of the Megaphyton that was submitted to distillation was the external part of the hard bark, formed of hypodermic fibers and traversed by small roots. The Psaronius, on the contrary, was represented by a mixture of roots and of parenchymatous tissue in which they descend along the trunk.
It results from these remarks that we may admit that those parts of the vegetable that are ordinarily hard, compact, and profoundly lignefied furnish a compact coke and relatively less volatile matter, while the tissues that are usually not much lignefied, or are parenchymatous, give a bubbly, porous coke and a larger quantity of gas. The influence of the varied mode of grouping of the elements in the primitive tissues is again found, then, even after carbonization, and is shown by the notable differences in the quantities and physical properties of the products of distillation.
The elementary chemical composition, which is perceptibly the same in the specimens isolated in the sandstones and in those taken from the great deposit, demonstrates that the difference in composition of the environment serving as gangue did not have a great influence upon the definitive state of the coal, a conclusion that we had already reached upon examining the structure and properties of the coal pebbles.
We may get an idea of the nearly similar composition of the coal produced by very different plants or parts thereof, in remarking that as the cells, fibers, and vessels are formed of cellulose, and some of them isomeric, the difference in composition is especially connected with the contents of the cells, canals, etc., such as protoplasm, oils, resins, gums, sugars, and various acids, various incrustations, etc. After the prolonged action of water that was more or less mineralized and of multiple organisms, matters that were soluble, or that were rendered so by maceration, were removed, and the organic skeletons of the different plants were brought to a nearly similar centesimal composition representing the carbonized derivatives of the cellulose and its isomers. The vegetable debris thus transformed, but still resistant and elastic, were the ones that were petrified in the mineral waters or covered with sand and clay. Under the influence of gradual pressure, and of a desiccation brought about by it, and by a rising of the ground, the walls of the organic elements came into contact, and the physical properties that we now see gradually made their appearance.
The waters derived from a prolonged steeping of vegetables, and charged with all the soluble principles extracted therefrom, have, after their sojourn in a proper medium, deposited the carbonized residua that have themselves become soluble, and have there formed masses of combustibles of a different composition from that resulting from the skeletons of plants, such as cannel coal, pitch coal, boghead, etc.
A thin section of a piece of Commentry cannel coal shows that this substance consists of a yellowish-brown amorphous mass holding here and there in suspension very different plant organs, such as fragments of Cordaites, leaves, ferns, microspores, macrospores, pollen grains, rootlets, etc., exactly as would have done a gelatinous mass that upon coagulating in a liquid had carried along with it all the solid bodies that had accidentally fallen into it and that were in suspension.
It is evident (as we have demonstrated) that other cannel coals may show different plant organs, or even contain none at all, their presence appearing to be accidental. The composition itself of cannel coal must be, in our theory, connected with the chemical nature of the materials from whence it is derived, and that were first dissolved and then became insoluble through carbonization. Several preparations made from Australian (New South Wales), Autun, etc., boghead have shown us merely a yellowish-brown amorphous mass holding in suspension lens-shaped or radiating floccose masses which it is scarcely possible to refer to any known vegetable organism.
Among the theories that we have cited in the beginning, the one that best agrees with the facts that we have pointed out is the third, which would admit, then, two things in the formation of coal. The first would include the different chemical reactions which cannot yet be determined, but which would have brought the vegetable matter now to the state of soft coal (with its different varieties), and now to the state of anthracite. The second would comprehend the preservation, through burial, of the organic matter in the stage of carbonization that it had reached, and as the result of compression and gradual desiccation, the development of the physical properties that we now find in the different carbonized substances.
We annex to this article a number of figures made from preparations of various coals. These preparations were obtained by making the fragments sufficiently thin without the aid of any chemical reagent, so as to avoid the reproach that things were made to appear that the coal did not contain. This slow and delicate method is not capable of revealing all the organisms That the carbonaceous substance contains, but, per contra, one is riot absolutely sure of the pre-existence of everything that resembles organs or fragments of such that he distinguishes therein by means of the microscope.
Our researches, as we have above stated, have been confined to different cannel coals, anthracite, boghead, and coal plants isolated either in coal pebbles, or in schists and sandstones.
FIG. 1.—Lancashire cannel coal; longitudinal section, X200.
FIG. 2.—Lancashire cannel coal; transverse section, X200.
Figs. 1 and 2 (magnified two hundred times) represent two sections, made in rectangular planes, of fragments of Lancashire cannel coal. In a certain measure, they remind one of Figs. 4 and 5, Pl 11, of Witham's "Internal Structure of Fossil Vegetables," and which were drawn from specimens of cannel coal derived likewise from Lancashire, but which are not so highly magnified. There is an interesting fact to note in this coincidence, and that is that this structure, which is so difficult to explain in its details, is not accidental, but a consequence of the nature of the materials that served to produce the coal of this region. In the midst of a mass of blackish debris, a, organic and inorganic, and immersed in an amorphous and transparent gangue, we find a few recognizable fragments, such as thick-walled macrospores, b, of various sizes, bits of flattened petioles, c, pollen grains, d, debris of bark, etc. In Fig. 2 all these different remains are cut either obliquely or longitudinally, and are not very recognizable. It is not rare to meet with a sort of vacuity, e, filled with clearer matter of resinoid aspect, without organization.
FIG. 3.—Commentry cannel coal, X200.
In Fig. 3, which represents a section made from Commentry cannel coal, the number of recognizable organs in the midst of the mass of debris is much larger. Thus, at a we see a macrospore, at b a fragment of the coat of a macrospore, at c another macrospore having a silicified nucleus, such as has been found in no other case, at d we have a transverse section of a vascular bundle, at e a longitudinal section of a rootlet traversed by another one, at f we have a transverse section of another rootlet, at g an almost entire portion of the vascular bundle of a root, and at h we see large pollen grains recalling those that we meet with in the silicified seeds from Saint Etienne.
Cannel coal, then, shows that it is formed of a sort of dark brown gangue of resinoid aspect (when a thin section of it is examined) holding in suspension indeterminable black organic and inorganic debris, which are arranged in layers, and in the midst of which (according to the locality and the fragment studied) is found a varying number of easily recognized vegetable organs.
FIG. 4.—Pennsylvania anthracite, X200.
It is very rare that anthracite offers any discernible trace of organization. Preparations made from fragments of Sable and Lamore coal could not be made sufficiently thin to be transparent; the mass remained very opaque, and the clearest parts exhibited merely amorphous, irregular granulations. Still, fragments of anthracite from Pennsylvania furnished, amid a dominant mass of dark, yellow-brown, structureless substance, a few organized vegetable debris, such as a fragment of a vascular bundle with radiating elements (Fig. 4, a), a macrospore, b, and a few pollen grains or microspores, c.
FIG. 5.—Boghead from New South Wales, X500.
From what precedes it seems to result, then, that anthracite is in a much less appreciable state of preservation than cannel coal, and that it is only rarely, and according to locality, that we can discover vegetable organs in it. Soft coal comes nearer to amorphous carbon. Boghead appears to be of an entirely different character (Fig. 5, magnified X300). It is easily reduced to a thin transparent plate, and shows itself to be formed of a multitude of very small lenses, differing in size and shape, and much more transparent than the bands that separate them. In the interior of these lenses we distinguish very fine lines radiating from the center and afterward branching several times. The ramifications are lost in the periphery amid fine granulations that resemble spores. We might say that we here had to do with numerous mycelia moulded in a slightly colored resin. Preparations made from New South Wales and Autun boghead presented the same aspect.
If boghead was derived from the carbonization of parts that were soluble, or that became so through maceration, and were made insoluble at a given moment by carbonization, we can understand the very peculiar aspect that this combustible presents when it is seen under the microscope.
The following figures were made in order to show the details of anatomical structure that are still visible in coal, and to permit of estimating the shrinkage that the organic substance has undergone in becoming converted into coal.
It is not rare in coal mines to find fragments of wood, of which a portion has been preserved by carbonates of iron and lime, and another portion converted into coal. This being the case, it was considered of interest to ascertain whether the carbonized portion had preserved a structure that was still recognizable, and, in such an event, to compare this structure with that of the portion of the specimen that was preserved in all its details by mineralization.
FIG. 6.—Arthropitus gallica, St. Etienne; transverse section, X200.
Fig. 6 shows a transverse section of a specimen of Arthropitus Gallica found under such conditions. The region marked c is carbonized; the organic elements of the wood-cells, tracheæ, etc., have undergone but little change in shape. Moreover, no change at all exists in the internal parts of another specimen (Fig. 8), where we easily distinguish by their form and dimensions the ligneous cells, aa, and the elements, bb, of the wood itself.
FIG. 8.—Arthropitus gallica, St. Etienne; transverse section through the carbonized part.
In the region, b, of Fig. 6, the ligneous elements have undergone an evident change of form, and the walls have been broken. This region, already filled by petrifying salts, but not completely hardened, has not been able to resist, as the region, a, an external pressure, and has become more or less misshapened. As for the not yet mineralized external portion, c, it has completely given way under the pressure, the walls of the different organic elements have come into contact, the calcareous or other salts have been expressed, and this region exhibits the aspect of ordinary coal, while at the same time preserving a little more hardness on account of the small quantity of mineral salts that has remained in them despite the compression.
From the standpoint of carbonization there seems to us but little difference between the organic elements that occupy the region, a, and those that occupy b. If the former had not been filled with hardened petrifying matter, they would have been compressed and flattened like those of region c, and would have given a compact and brilliant coal, having very likely before petrifaction reached the same degree of carbonization as the latter. The layer of coal in contact with the carbonized or silicified part of the specimens is due, then, to a compression of the organic elements already chemically carbonized, but in which the mineral matter was not yet hardened and was able to escape.
FIG. 7.—Arthropitus gallica, St. Etienne; tangential longitudinal section.
If this be so, we ought to find the remains of organic structure in this region c. In fact, on referring to Fig. 7, which represents a tangential, longitudinal section of the same specimen, we perceive at ab a ligneous duct and some unchanged tracheæ situated in the carbonized region, and then at c the same elements, though flattened, in which, however, we still clearly distinguish the bands of the tracheæ; at d is found a trachea whose contents were already solidified, and which has not been flattened; then, near the surface, in the region, e, the pressure having been greater, it is no longer possible to recognize traces of organization in a tangential section. In a large number of cases, the fact that the coal does not seem to be organized must be due to the too great compression that the carbonized cells and vessels have undergone when yet soft and elastic, at the time this slow but continuous pressure was being exerted.
It also became of interest to find out whether, through the very fact of carbonization, the dimensions of the organic elements had perceptibly varied—a sort of research that presents certain difficulties. At present we have no living plant that is comparable, even remotely, with those that grew during the coal epoch. Moreover, the organic elements have absolutely nothing constant in their dimensions.
Still, if we limit ourselves to a comparison of the same carbonized wood, preserved on the one hand by petrifaction, and on the other hand non-mineralized, we find a very perceptible diminution in bulk. The elements have contracted in length, breadth, and thickness, but principally in the direction of the compression that they have undergone in the purely carbonized specimens.
In the vicinity of the carbonized portions, those of the tracheæ that have not done so have perceptibly preserved their primitive length, which has, so to speak, been maintained by their neighbors, but their other dimensions have become much smaller—a quarter in thickness and half in length.
FIG. 9.—Calamodendron, Commentry; prosenchymatous portion of the wood carbonized, X200.
If the two fragments of the same wood are, one of them silicified and the other simply carbonized and preserved in sandstone, the diminution in volume will have occurred in all directions in the latter of the two.
FIG. 10.—Calamodendron, fragment of the vascular portion of the wood carbonized.
Figs. 9 and 11, which represent a portion of the fibrous region of Calamodendron wood, may give an idea of the shrinkage that has taken place therein. In Figs. 11 and 12, which show a few tracheæ and medullary rays of the ligneous bands of the same plant, we observe the same phenomenon. We might cite a large number of analogous examples, but shall be content to give the following: Figs. 13 and 15 represent radial and tangential sections of the bark of Syringodendron pes-capræ. This is the first time that one has had before his eyes the anatomical structure of the bark of a Syringodendron, a plant which has not yet been found in a petrified state. It is coal, then, with its structure preserved, that allows of a verification of the theory advanced by several scientists that the often bulky trunks of Syringodendron are bases of Sigillariæ.
FIG. 11.—Calamodendron, from Autun; prosenchymatous portion of the wood silicified, X200.
FIG. 12.—Calamodendron, from Autun; vascular portion of the wood silicified.
If we refer to Fig. 13, which represents a radial vertical section running through the center of one of the scars that permitted the specimen to be determined, we shall observe, in fact, a tissue formed of rectangular cells, longer than wide, arranged in horizontal series, and very analogous in their aspect to those that we have described in the suberose region of the bark of Sigillariæ. Fig. 15 shows in tangential section the fibrous aspect of this tissue, which has been rendered denser through compression. Fig. 14 shows it restored. In Fig. 13, the external part of the bark is occupied by a thick layer of cellular tissue that exists over the entire surface of the trunk, but particularly thick near the scars, exactly as in the barks of the Sigillariæ that we have formerly described. Finally, at b, we recognize the undoubted traces of a vascular bundle running to the leaves. If the bundle appears to be larger than that of the Sigillariæ, this is due to the flattening that the trunk has undergone, the effect of this having been to spread the bundle out in a vertical plane, although its greatest width in the first place was in a horizontal one.
FIG. 13.—Syringodendron pes-capræ; from Saarbruck; radial vertical section, X200.
FIG. 14.—Suberose cells restored.
In anatomical structure, the barks of the Syringodendrons are, then, analogous to those of the Sigillariæ. If, now, we compare the dimensions of the tissues of these barks with the same silicified tissues of the barks of Sigillariæ, we shall find that there was likewise a diminution in the dimensions, but yet a less pronounced one than in the woods that we have previously spoken of. The corky nature of this region of the bark was likely richer in carbonizable elements than the wood properly so called, and had, in consequence, to undergo much less shrinkage.—Dr. B. Renault (of Paris Museum) in Le Genie Civil.
FIG. 15.—Syringodendron pes-capræ; tangential vertical section in the corky part of the bark, X200.
DESCRIPTION OF THE FIGURES.—Fig. 1, Lancashire cannel coal; longitudinal section, X200. Fig. 2, Lancashire cannel coal; transverse section, X200. Fig. 3. Commentry cannel coal, X200. Fig. 4, Pennsylvania anthracite, X200. Fig. 5, Boghead from New South Wales, X500. Fig. 6, Arthropitus gallica, St. Etienne; transverse section, X200. Fig. 7, same; tangential longitudinal section. Fig. 8, same; transverse section through the carbonized part. Fig. 9. Calamodendron, Commentry; prosenchymatous portion of the wood carbonized, X200. Fig. 10, same; fragment of the vascular portion of the wood carbonized. Fig. 11, same, from Autun; prosenchymatous portion of the wood silicified, X200. Fig. 12, same, Autun; vascular portion of the wood silicified. Fig. 13, Syringodendron pes-capræ; from Saarbruck; radial vertical section, X200. Fig. 14, Suberose cells restored. Fig. 15. Syringodendron pes-capræ; tangential vertical section in the corky part of the bark, X200.