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, will in the course of time reduce the mountains to plains and submerge great parts of the lowlands beneath the ocean.