46 GEOLOGICAL BIOLOGY. 



Wherever this happened, on one side rocks were forming, and 

 on the other erosion and degradation were obhterating them 

 as time-records. The Appalachian and the Rocky Mountain 

 revolutions constitute the two grander revolutions. The first 

 closed the Paleozoic life period, the fossils being chiefly 

 marine until the Devonian, and being associated with marine 

 forms up to the close of the Carboniferous. The deposits are 

 distributed across the continent, with local interruptions. 

 After the Appalachian revolution the eastern half of the con- 

 tinent, except its Atlantic and Gulf borders, became perma- 

 nently above the sea-level. The period between the Appa- 

 lachian and Rocky Mountain revolutions is the period of the 

 Mesozoic life. In the faunas and floras of this period, land 

 and fresh-water species take a prominent part. The marine 

 life is distributed over the western half of the continent and 

 alone a narrow line of formations on the Atlantic and Gulf 

 borders. After the beginning of the Rocky Mountain revo- 

 lution, 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. 



Time-scale and the Geological Revolutions of the American Con- 

 tinent. — Thus the grander revolutions recorded in the devel- 

 opment of the American continent break up the geological 

 time-scale, as expressed in the systems of stratified rocks, into 

 a few natural sub-divisions, as may be illustrated by the dia- 

 gram on the opposite page : 



Revolutions made Interruptions in the Record. — In the use of 

 the time-scale for the study of the history of organisms, the 

 places marked by the revolutions 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 in- 

 tervals of revolution the records of life-history are relatively 

 continuous. There were series of successive faunas or even 

 sub-faunas in which were expressed the general features of 

 the evolution of life on the globe. The species preserved 



