290 



THE JOURNAL OF GEOLOGY. 



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 excep- 

 tions) 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 sub- 

 divisions, as may be illustrated by the following diagram : 



Cenozoic 



Mesozoic ( 



Palaeozoic 



? Pre -Cambrian 



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 mag- 

 nitude. 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 



