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 tracheae 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.