Page:The Journal of geology (IA journalofgeology11893univ).pdf/306

nent part. The marine life is distributed over the western half of the continent and along a narrow line of formations on the Atlantic and Gulf borders. After the beginning of the Rocky Mountain revolution, the deposits of marine origin and their faunas were distributed on the marine borders of the continent as it now is, and fresh-water and land deposits were accumulated over the plains and plateaus of the western half (with few exceptions) of the continent.

Thus the grander revolutions recorded in the development of the American continent break up the geological time-scale expressed in the systems of stratified rocks into a few natural subdivisions, as may be illustrated by the following diagram:

In the use of the time-scale for the study of the history of organisms, the places marked by the revolution are those in which are found the grander interruptions to the continuity of the record. They may represent periods of great relative magnitude. They do represent periods of marked change in the faunas and floras over extensive regions. Between the grander intervals of revolution the records of life-history are relatively