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.
Pages:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25