vals, 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
_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
s