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: