(H. R. Knipe.)
It would hardly seem unreasonable to suppose that these depositions, and the changes that brought them about, might have occupied as much time as the formation of the coal beds. This would double the figures, and make this period last something between two million and five million years.
In the coal as we know it are the remains of great forest trees; gigantic tree-ferns for the most part, and of many small plants forming a close thick sod, partially buried in whole countries of marsh land.
Every one knows those marsh plants, which bear the vulgar name of "mare's-tail." These humble plants were represented during the coal period by trees from twenty to thirty feet high and four to six inches in diameter. Their trunks have been preserved to us: they bear the name of Calamites.
The Lycopods of our age are humble plants, scarcely a yard in height, and most commonly creepers; but those of the ancient world were trees of eighty or ninety feet in height. Their leaves were sometimes twenty inches long, and their trunks a yard in diameter. Such are the dimensions of some specimens which have been found. Another Lycopod of this period attained dimensions still more colossal. The Sigillarias sometimes exceeded 100 feet in height. Herbaceous Ferns were also exceedingly abundant and grew beneath the shade of these gigantic trees. It was the combination of these lofty trees with the undergrowth of smaller vegetation which formed the forests of the Carboniferous period. "What could be more surprising," exclaims Figuier, "than the aspect of this exuberant vegetation, these immense trees, these elegant arborescent ferns with airy foliage, fine cut, like delicate lace. Nothing at the present day can convey to us an idea of the prodigious and immense extent of never-changing verdure which clothed the earth, from pole to pole, under the high temperature which everywhere prevailed over the whole terrestrial globe. In the depths of these inextricable forests parasitic plants were suspended from the trunks of the great trees, in tufts or garlands, like the wild vines of our tropical forests. They were nearly all pretty, fern-like plants, they attached themselves to the stems of the great trees, like the orchids of our times." The margin of the waters would also be covered with various plants with light and whorled leaves, belonging, perhaps, to the Dicotyledons. Before leaving the subject of the plants of the coal measures, we should perhaps mention as one of the most interesting discoveries of the present generation that whereas the links between the fern-like trees of those days and the cycads, or early group of seed-bearing plants, were for long missing, they have been found by the researches of Professor F. W. Oliver, F.R.S., who has identified in the Lyginodendron a seed-bearing fern from the coal measures.
We must now turn to the less interesting but not less important topic of the animal life of the Carboniferous period. At the beginning of the period when only a small portion of the British Isles was above the waters, and an ocean rolled from Ireland to China, the life of which the most important relics were left was that of the sea. In the early Carboniferous seas the rhizopods, some small as dust, laid down with their tiny shells the foundations of mountains yet to be; the "sea lilies" were at the height of their pride, occupying vast areas in the flowing tide; forms like the present-day nautilus began to appear, and the "lamp-shells" attained their greatest size. The trilobites, hitherto the most conspicuous and noticeable animals of the earth's childhood, were beginning to die out, vanquished in the struggle for life by more adaptable forms, and the big sea scorpions were waning fast. The king crabs and the water fleas still throve, and the fishes, though most of them not very large, were growing larger, some of them taking the appearance of the dog-fish, some of the ray, some of the shark; and, what is more important than the fact of size, the fishes were growing speedier and more capable of attacking weaker creatures.
In the course of these ages the sea invaded the land; and shores where land-snails and millipedes and centipedes, beetles and scorpions, spiders and cockroaches had found a home became entirely changed, not only in their appearance and character, but in the type which subsisted on them. It is possible (for something of the kind has been noticed in our own days in the West Indies, where a sea-crab species is showing signs of becoming a land animal) that some of the forms of water animals became used to living in shallower and shallower water as the generations went on till they became partly land and partly water animals—amphibians, as they are called. Thus small newt-like beings, moving clumsily through the swamps, made their appearance, and others with stronger limbs pushed onwards through the forest. Others in form resembling snakes crept through the mud and lived among the swamps by the side of the sea. Not much is known of the food and life-habits of any of these amphibians. From their teeth we may perhaps judge that they lived on fish, crustaceans, insects, and on one another, and their predatory life sometimes led them to climb trees in search of food. What, however, is most important about the amphibian is that they were the pioneers of the march of those creatures which had backbones—the vertebrates—from the sea to the land.
CHAPTER XIX
THE AGE OF REPTILES
We have already said that in the many hundreds of thousands of years which went by during Carboniferous times the sea sometimes advanced and sometimes receded, and nothing shows this better than the great thickness of the deposits in which the coal lies in seams. In America, as in Europe, Asia, Africa, Australia, and New Zealand, South and Central America, the Carboniferous system is found. In Arkansas, in North America, the coal measures attain the remarkable thickness of 18,000 feet; in the Wasatch Mountains the Carboniferous strata have been estimated to be 13,000 feet thick, and in silver-bearing Nevada 10,000 feet. The formations of the Western European coal measures, like those of Eastern North America, consist principally of shales and clays, with smaller amounts of sandstone and limestone. They attain great thickness, and, including 5500 feet of the Millstone Grit, are 13,500 feet thick in Lancashire and several thousand feet thick in many parts of Great Britain and Ireland. The extraordinary thicknesses show that near our islands must have been a very extensive and lofty area of land. In Germany the same strata, thickly seamed with coal, are 10,000 feet thick. There must also have been considerable volcanic or earthquake action as we know, because in Germany, near the Hartz Mountains and elsewhere, there are many igneous rocks thrust into the strata, and also because in Belgium and in France the coal strata are very much twisted and contorted. The same or similar beds are found in Siberia, in Japan, and in China, where the coal beds are said to be thicker than anywhere else in the world. The Carboniferous system is also found in Africa, in the north, south-east, and south of the continent; and in Australia and New Zealand Carboniferous strata to the thickness of 10,000 feet are indicated.