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 debris 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.