The Devonian period had opened with a rise of the land, but the
sea eat steadily into it once more, and, with some inconsiderable
oscillations of the land, regained its territory. The latter part
of the Devonian and earlier part of the Carboniferous were
remarkable for their great expanses of shallow water and
low-lying land. Except the recent chain of hills in Scotland we
know of no mountains. Professor Chamberlin calculates that
20,000,000, or 30,000,000 square miles of the present continental
surface of Europe and America were covered with a shallow sea. In
the deeper and clearer of these waters the earliest Carboniferous
rocks, of limestone, were deposited. The "millstone grit," which
succeeds the "limestone," indicates shallower water, which is
being rapidly filled up with the debris of the land. In a word,
all the indications suggest the early and middle Carboniferous as
an age of vast swamps, of enormous stretches of land just above
or below the sea-level, and changing repeatedly from one to the
other. Further, the climate was at the time--we will consider the
general question of climate later--moist and warm all over the
earth, on account of the great proportion of sea-surface and the
absence of high land (not to speak of more disputable causes).
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