Fig. 661.—Limestone made up of corals (Favosites polymorpha).
Coal we have already treated of in Mineralogy, and coal looks at times very different from our preconceived ideas of a sedimentary rock, which we know is regularly deposited in layers. But when we split or break the coal we find its cleavage in a certain direction. Coal is wood squeezed and petrified by ages between enormous layers of sedimentary rocks, and the coal-seam rests upon the soil in which the plants once grew—perhaps more than six hundred thousand years ago!
Of course coal, as we burn it, was not all made at once. We can trace it from the swamp as Peat, on to “Lignite,” or woody coal, through the Tertiary and New Red Sandstone to the coal measures themselves. Even lower down we find the remains in more or less pure carbon forms—the anthracite and the graphite of the primary formations.
Coal appears not to have been formed equally in all places during the period in which it originated. The remains of plants found in these strata lead us to infer, that, during that period there existed an exceedingly vigorous and crowded vegetation, consisting principally of tree ferns and equisetaceæ, of which the Sphenopteris Hœninghausii (fig. 662), Pecopteris aquilina (fig. 663), and Neuropteris Loshii (fig. 664), are amongst the most beautiful that have been found, and the flora and fauna of this period were of a more or less primitive kind or low order, but very luxurious. The former display a decided advance, and reptiles of aquatic forms appear with large and predaceous fishes. Mountain limestone, which is usually found in the coal formations, includes metallic deposits, and organic remains are very abundant in it. The following are specimens of the fossils—
Fig. 662. Fig. 663. and Fig. 664.
Fig. 665-1. Bellerophon costatus. 2. Spirifer glaber. 3. Productus Martini. 4. Orthoceras lateralis.
The Carboniferous system is a very important one, as may be seen. In these beds we have coal, limestone, sandstone, and shale. The Coal Measures consist of grit and sandstone and shale, with coal seams. The Carboniferous Limestone, or “Mountain” Limestone, has no coal in it. The sandstone has been termed “Millstone Grit,” because millstones are made from it. We then have limestone shales, and the sandstone beds. Each coal seam indicates a subsidence of the land and a regular series of underclay or soil in which the plants grew,—the plants themselves,—iron, coal, and then the shale, and so on again, indicating frequent changes and a long lapse of ages.
