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. [Illustration: 12g: 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. [Illustration: 12i: FIG. 9.--_Calamodendron,_ Commentry; prosenchymatous portion of the wood carbonized, X200.]