Fig. 67.—Annularia orifolia.
In front of this group we see two trunks broken and overthrown. These are a Lepidodendron and Sigillaria, mingling with a heap of vegetable débris in course of decomposition, from which a rich humus will be formed, upon which new generations of plants will soon develop themselves. Some herbaceous Ferns and buds of Calamites rise out of the waters of the marsh.
A few Fishes belonging to the period swim on the surface of the water, and the aquatic reptile Archegosaurus shows its long and pointed head—the only part of the animal which has hitherto been discovered ([Fig. 68]). A Stigmaria extends its roots into the water, and the pretty Asterophyllites, with its finely-cut stems, rises above it in the foreground.
A forest, composed of Lepidodendra and Calamites, forms the background to the picture.
Fig. 68.—Head and neck of Archegosaurus minor.
Formation of Beds of Coal.
Coal, as we have said, is only the result of a partial decomposition of the plants which covered the earth during a geological period of immense duration. No one, now, has any doubt that this is its origin. In coal-mines it is not unusual to find fragments of the very plants whose trunks and leaves characterise the Coal-measures, or Carboniferous era. Immense trunks of trees have also been met with in the middle of a seam of coal. In the coal-mines of Treuil,[44] at St. Etienne, for instance, vertical trunks of fossil trees, resembling bamboos or large Equiseta, are not only mixed with the coal, but stand erect, traversing the overlying beds of micaceous sandstone in the manner represented in the engraving, which has been reproduced from a drawing by M. Ad. Brongniart ([Fig. 69]).