and the elements,
_bb_, of the wood itself.

[Illustration: 12h: 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.

[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 tracheae 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 tracheae; 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