54 



THE AGE OF MAMMALS 



ing of levels. In strata like those of our American Oligocene, where depo- 

 sition has been extremely slow, every foot of level may mark a long period 

 of time ; fifteen or twenty feet, or even less, may mark a time during which 

 one species passed by mutation into another. 



Fig. 11. — Chief areas of deposition in the Cffinozoic of North America: black = littoral 

 depositions of the Atlantic and the Pacific coasts ; dotted = ' continental ' formations of the 

 Great Plains and of the Mountain Regions. After W. B. Scott. 



