It is curious that no remains of any land-inhabiting animals have been discovered in this system—whether in the Old Red or the Devonian formations (the Lower and Upper Red Sandstones). We can only distinguish the remains of aquatic animals or plants. We may picture the great cuttle-fish, the nautilus, and the dipterus, with various orders of mollusca, and the gradual approach to the crustacea, but no terrestrial animals have been discovered. We may take it for granted then that the Old Red Sandstone and Devonian systems are different—the former being found in Scotland and parts of England, and formed of deposits in fresh or brackish water, while in the Devonian system the marine deposits are corals, and all the indications of ocean life, separated from the great inland lakes by a range of hills. Neither of the terms (Old Red Sandstone or Devonian) limit geographically or descriptively the formations of this system. All the rocks are clearly distributed between the Silurian and the Carboniferous. The invertebrates in this last system have not developed very much, but corals are very abundant, and fish of some armoured species are plentiful and curious, while the crustacea were enormous.
Fig. 659.—Palæozoic Fish. Trilobites, Brachiopods, Coral and Graptolite.
We now arrive at the most important of all the rock formations, the one to which we owe our national prosperity—we mean the Coal System.
The Carboniferous Formation.
While the foregoing depositions were being made the earth was still undergoing changes. The sandstones were deposited, and the corals making use of the lime carried into the waters began to build and form masses of limestone under the sea, pushing back the water and changing the forms and positions of land and water. All this went on apparently very quietly—volcanic action was not very frequent—the water was warm. But sometimes earthquakes would heave up the submarine formations into mountains, and therefore we find the fossils of the tiny sea-animals on the hills. Extensive swamps were formed by partially retreating sea-water, and their vegetation became luxuriant. Tree ferns and all the floral appearance of the tropics grew up and formed dense forests, far thicker than any we know of at the present time. It will readily be understood that the condition of the atmosphere must have been particularly favourable to the growth of plants, and therefore not suitable for air-breathing animals. Heat and carbonic acid must have been greatly developed.
Fig. 660.—Tree Ferns.
We can now perceive how the gradual filling up of the earth for man’s reception was taking place. The rain was taking carbonic acid from the air, for the dead plants gave it out in enormous volumes. The carbonate of lime dissolved in the water-springs, etc., was carried to the sea for polypi to build shells from. The trees were absorbing carbonic acid, too, and while purifying the air, were retaining the carbon in their stems and leaves and branches, which (when they decayed) remained untouched, and accumulated in thick layers to sink down, and by pressure be turned into coal. This great effect was carried out several times; and it is a remarkable fact that we find coal, limestone, and ironstone so near together, all useful to us and to each other in the course of the working of the minerals—so we come to the Carboniferous system.
