al emergence during geologic time. During the periods of
overflow, great portions of the low-lying parts of the continents were
submerged, and formed extensive but comparatively shallow seas. The
mountains through long continued erosion were reduced to gentle and
uniform slopes of comparatively slight elevation. Their materials were
brought down by rivers to the sea-coast, and distributed as
sedimentary formations over the shallow interior seas or along the
margins of the continents. But this load of sediments, transferred
from the dry land to the ocean margins and shallow seas, disturbed the
balance of weight (isostasy) which normally keeps the continental
platforms above the level of the ocean basins (which as shown by
gravity measurement are underlain by materials of higher specific
gravity than the continents). In due course of time, when the strain
became sufficient, it was readjusted by earth movements of a slowness
proportioned to their vastness. These movements while tending upon the
whole to raise the continents to or sometimes beyond their former
relief, did not reverse the action of erosion agencies in detail, but
often produced new lines or areas of high elevation.

[Illustration: Fig. 2.--North America in the Later Cretacic
Period. Map outlines after Schuchert.]

_Geologic Periods._ A geologic period is the record of one of these
immense and long continued movements of alternate submergence and
elevation of the continents. It begins, therefore, and ends with a
time of emergence, and includes a long era of submergence.

These epochs of elevation are accompanied by the development of cold
climates at the poles, and elsewhere of arid conditions in the
interior of the continents. The epochs of submergence are accompanied
by a warm, humid climate, more or less uniform from the equator to the
poles.

The earth has very recently, in a geologic sense, passed through an
epoch of extreme continental elevation the maximum of which was marked
by the "Ice Age." The continents are still emerged for the most part
almost to the borders of the "continental shelf" which forms their
maximum limit. And in the icy covering of Greenland and Antarctica a
considerable portion still remains of the great ice-sheets which at
their maximum covered large parts of North America and Europe. We are
now at the beginning of a long period of slow erosion and subsidence
which, if this interpretation of the geologic record be correct,