e 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 jus