At the close of the Devonian period, several upheavals occurred of great significance for the future history of America. One in Ohio raised the elevated ground on which Cincinnati now stands; another hill lifted its granite crest in Missouri, raising with it an extensive tract of Silurian and Devonian deposits; while a smaller one, which does not seem, however, to have disturbed the beds about it so powerfully, broke through in Arkansas. At the same time, elevations took place toward the East,—the first links, few and detached, in the great Alleghany chain which now raises its rocky wall from New England to Alabama.
In the Ohio hill, the granite did not break through, though the force of the upheaval was such as to rend asunder the Devonian deposits, for we find them lying torn and broken about the base of the hill; while the Silurian beds, which should underlie them in their natural position, form its centre and summit. This accounts for the great profusion of Silurian organic remains in that neighborhood. Indeed, there is no locality which forces upon the observer more strongly the conviction of the profusion and richness of the early creation; for one may actually collect the remains of Silurian Shells and Crustacea by cart-loads around the city of Cincinnati. A naturalist would find it difficult to gather along any modern sea-shore, even on tropical coasts, where marine life is more abundant than elsewhere, so rich a harvest, in the same time, as he will bring home from an hour's ramble in the environs of that city.
These elevations naturally gave rise to depressions between themselves and the land on either side of them, and caused also so many counter-slopes dipping toward the uniform southern slope already formed at the north. Thus between the several new upheavals, as well as between them all and the land to the north of them, wide basins or troughs were formed, inclosed on the south, west, and east by low hills, (for these more recent eruptions were, like all the early upheavals, insignificant in height,) and bounded on the north by the more ancient shores of the preceding ages.
These were the inland seas of the Carboniferous period. Here, again, we must infer the successive stages of a history which we can read only in its results. Shut out from the ocean, these shallow sea-basins were gradually changed by the rains to fresh-water lakes; the lakes, in their turn, underwent a transformation, becoming filled, in the course of centuries, with the materials worn away from their shores, with the débris of the animals which lived and died in their waters, as well as with the decaying matter from aquatic plants, till at last they were changed to spreading marshes, and on these marshes arose the gigantic fern-vegetation of which the first forests chiefly consisted. Such are the separate chapters in the history of the coal-basins of Illinois, Missouri, Pennsylvania, New England, and Nova Scotia. First inland seas, then fresh-water lakes, then spreading marshes, then gigantic forests, and lastly vast storehouses of coal for the human race.
Although coal-beds are by no means peculiar to the Carboniferous period, since such deposits must be formed wherever the decay of vegetation is going on extensively, yet it would seem that coal-making was the great work in that age of the world's physical history. The atmospheric conditions, so far as we can understand them, were then especially favorable to this result. Though the existence of such an extensive terrestrial vegetation shows conclusively that an atmosphere must have been already established, with all the attendant phenomena of light, heat, air, moisture, etc., yet it is probable that this atmosphere differed from ours in being very largely charged with carbonic acid.
We should infer this from the nature of the animals characteristic of the period; for, though land-animals were introduced, and the organic world was no longer exclusively marine, there were as yet none of the higher beings in whom respiration is an active process. In all warm-blooded animals the breathing is quick, requiring a large proportion of oxygen in the surrounding air, and indicating by its rapidity the animation of the whole system; while the slow-breathing, cold-blooded animals can live in an air that is heavily loaded with carbon. It is well known, however, that, though carbon is so deadly to higher animal life, plants require it in great quantities; and it would seem that one of the chief offices of the early forests was to purify the atmosphere of its undue proportion of carbonic acid, by absorbing the carbon into their own substance, and eventually depositing it as coal in the soil.
Another very important agent in the process of purifying the atmosphere, and adapting it to the maintenance of a higher organic life, is found in the deposits of lime. My readers will excuse me, if I introduce here a very elementary chemical fact to explain this statement. Limestone is carbonate of calcium. Calcium is a metal, fusible as such, and, forming a part of the melted masses within the earth, it was thrown out with the eruptions of Plutonic rocks. Brought to the air, it would appropriate a certain amount of oxygen, and by that process would become oxide of calcium, in which condition it combines very readily with carbonic acid. Thus it becomes carbonate of lime; and all lime deposits played an important part in establishing the atmospheric proportions essential to the existence of the warm-blooded animals.
Such facts remind us how far more comprehensive the results of science will become when the different branches of scientific investigation are pursued in connection with each other. When chemists have brought their knowledge out of their special laboratories into the laboratory of the world, where chemical combinations are and have been through all time going on in such vast proportions,—when physicists study the laws of moisture, of clouds and storms, in past periods as well as in the present,—when, in short, geologists and zoologists are chemists and physicists, and vice versa,—then we shall learn more of the changes the world has undergone than is possible now that they are separately studied.
It may be asked, how any clue can be found to phenomena so evanescent as those of clouds and moisture. But do we not trace in the old deposits the rainstorms of past times? The heavy drops of a passing shower, the thick, crowded tread of a splashing rain, or the small pinpricks of a close and fine one,—all the story, in short, of the rising vapors, the gathering clouds, the storms and showers of ancient days, we find recorded for us in the fossil rain-drops; and when we add to this the possibility of analyzing the chemical elements which have been absorbed into the soil, but which once made part of the atmosphere, it is not too much to hope that we shall learn something hereafter of the meteorology even of the earliest geological ages.
The peculiar character of the vegetable tissue in the trees of the Carboniferous period, containing, as it did, a large supply of resin drawn from the surrounding elements, confirms the view of the atmospheric conditions above stated; and this fact, as well as the damp, soggy soil in which the first forests must have grown, accounts for the formation of coal in greater quantity and more combustible in quality than is found in the more recent deposits. But stately as were those fern forests, where plants which creep low at our feet to-day, or are known to us chiefly as underbrush, or as rushes and grasses in swampy grounds, grew to the height of lofty trees, yet the vegetation was of an inferior kind.