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.