Upon this follow a number of sketches introducing us to the middle periods, where the land is higher and more extensive, covered chiefly with Pine forests, beneath which grows a thick carpet of underbrush, consisting mostly of Grasses, Rushes, and Ferns. Here and there one of the gigantic reptiles of the time may be seen sunning himself on the shore. One of these sketches shows us such a creature hungrily inspecting a pool where Crinoids, with their long stems, large, closely-coiled Chambered Shells, and Brachiopods, the Oysters and Clams of those days, offer him a tempting repast. Here and there a Pterodactyl, the curious winged reptile of the later middle periods, stretches its long neck from the water, and birds also begin to make their appearance.
After these come the Tertiary periods: the Eocene first, where the landscape is already broken up by hills and mountains, clothed with a varied vegetation of comparatively modern character. Lily-pads are floating on the stream which makes the central part of the picture; large herds of the Palaeotherium, the ancient Pachyderm, reconstructed with such accuracy by Cuvier, are feeding along its banks; and a tall bird of the Heron or Pelican kind stands watching by the water's edge. In the Miocene the vegetation looks still more familiar, though the Elephants roaming about in regions of the Temperate Zone, and the huge Salamanders crawling out of the water, remind us that we are still far removed from present times. Lastly, we have the ice period, with the glaciers coming down to the borders of a river where large troops of Buffalo are drinking, while on the shore some Bears are feasting on the remains of a huge carcass.
It is, however, with the Carboniferous age that we have to do at present, and I will not anticipate the coming chapters of my story by dwelling now on the aspect of the later periods. To return, then, to the period of the coal, it would seem that extensive freshets frequently overflowed the marshes, and that even after many successive forests had sprung up and decayed upon their soil, they were still subject to submergence by heavy floods. These freshets, at certain intervals, are not difficult to understand, when we remember, that, beside the occasional influx of violent rains, the earth was constantly undergoing changes of level, and that a subsidence or upheaval in the neighborhood would disturb the equilibrium of the waters, causing them to overflow and pour over the surface of the country, thus inundating the marshes anew.
That such was the case we can hardly doubt, after the facts revealed by recent investigations of the Carboniferous deposits. In some of the deeper coal-beds there is a regular alternation between layers of coal and layers of sand or clay; in certain localities, as many as ten, twelve, and even fifteen coal-beds have been found alternating with as many deposits of clay or mud or sand; and in some instances, where the trunks of the trees are hollow and have been left standing erect, they are filled to the brim, or to the height of the next layer of deposits, with the materials that have been swept over them. Upon this set of deposits comes a new bed of coal with the remains of a new forest, and. above this again a layer of materials left by a second freshet, and so on through a number of alternate strata. It is evident from these facts that there have been a succession of forests, one above another, but that in the intervals of their growth great floods have poured over the marshes, bringing with them all kinds of loose materials, such as sand, pebbles, clay, mud, lime, etc., which, as the freshets subsided, settled down over the coal, filling not only the spaces between such trees as remained standing, but even the hollow trunks of the trees themselves.
Let us give a glance now at the animals which inhabited the waters of this period. In the Radiates we shall not find great changes; the three classes are continued, though with new representatives, and the Polyp Corals are increasing, while the Acalephian Corals, the Kugosa and Tabulata, are diminishing. The Crinoids were still the most prominent representatives of the class of Echinoderms, though some resembling the Ophiurans and Echinoids (Sea-Urchins) began to make their appearance. The adjoining wood-cut represents a characteristic Crinoid of the Carboniferous age.
[Illustration]
Among the Mollusks, Brachiopods are still prominent, one new genus among them, the Productus, being very remarkable on account of the manner in which one valve rises above the other. The wood-cut below represents such a shell, looked at from the side of the flat valve, showing the straight cut of the line of juncture between the valves and the rising curve of the opposite one, which looks like a hooked beak when seen in profile.
[Illustration]
Other species of Bivalves were also introduced, approaching more nearly our Clams and Oysters, or, as they are called in scientific nomenclature, the Lamellibranchiates. They differ from the Brachiopods chiefly in the higher character of their breathing-apparatus; for they have free gills, instead of the net-work of vessels on the lining skin which serves as the organ of respiration in the Brachiopods. We shall always find, that, in proportion as the functions are distinct, and, as it were, individualized by having special organs appropriated to them, animals rise in the scale of structure. The next class of Mollusks, the Gasteropods, or Univalves, with spiral shells, were numerous, but, from their brittle character, are seldom found in a good state of preservation.
The Chambered Shells, or the Cephalopods, represented chiefly in the earlier periods by the straight Orthoceratites described in a previous article, are now curled in a close coil, and the internal structure of their chambers has become more complicated. The subjoined wood-cut represents a characteristic Chambered Shell of the Carboniferous age. Goniatites is the scientific name of these later forms. If we had looked for them in the Devonian period, we should have found many with looser coils than these, and some only slightly curved in the shape of a horn. These, as well as the perfectly straight forms, still exist in the coal period, but the Goniatites with close whorls are the more numerous and more characteristic.