undamental and physical aspect of this
revolution, the upheaval of the land. It began about the close of the
Jurassic period. Western and Central Europe emerged considerably from
the warm Jurassic sea, which lay on it and had converted it into an
archipelago. In North-western America also there was an emergence of
large areas of land, and the Sierra and Cascade ranges of mountains were
formed about the same time. For reasons which will appear later we must
note carefully this rise of land at the very beginning of the Cretaceous
period.

However, the sea recovered its lost territory, or compensation for it,
and the middle of the Cretaceous period witnessed a very considerable
extension of the waters over America, Europe, and southern Asia. The
thick familiar beds of chalk, which stretch irregularly from Ireland to
the Crimea, and from the south of Sweden to the south of France, plainly
tell of an overlying sea. As is well known, the chalk consists mainly
of the shells or outer frames of minute one-celled creatures
(Thalamophores) which float in the ocean, and form a deep ooze at its
bottom with their discarded skeletons. What depth this ocean must have
been is disputed, and hardly concerns us. It is clear that it must have
taken an enormous period for microscopic shells to form the thick masses
of chalk which cover so much of southern and eastern England. On the
lowest estimates the Cretaceous period, which includes the deposit of
other strata besides chalk, lasted about three million years. And as
people like to have some idea of the time since these things happened,
I may add that, on the lowest estimate (which most geologists would at
least double), it is about three million years since the last stretches
of the chalk-ocean disappeared from the surface of Europe.

But while our chalk cliffs conjure up a vision of England lying deep--at
least twenty or thirty fathoms deep--below a warm ocean, in which
gigantic Ammonites and Belemnites and sharks ply their deadly trade,
they also remind us of the last phase of the remarkable life of the
earth's Middle Ages. In the latter part of the Cretaceous the land
rises. The chalk ocean of Europe is gradually reduced to a series of
inland seas, separated by masses and ridges of land, and finally to a
series of lakes of brackish water. The masses of the Pyrenees and Alps
begin to rise; though it will not be until a much later date that they
reach anything like their present elevati