THE CENOZOIC ERA 



CHAPTER XXVI 

 THE EOCENE AND OLIGOCENE PERIODS 



The remaining periods of geological history constitute the 

 Cenozoic era, or the era of modern life. The earlier part of the era 

 is called the Tertiary, and the later the Quaternary. The Tertiary 

 is variously subdivided, as shown below: 



[ Recent or Human. Post-glacial formations 

 Quaternary J Pleistocene or Glacial. Glacial formations 



and non-glacial deposits of glacial age 



Cenozoicl I II m 



Era fPliocene Pliocene I 



Miocene Miocene f Neocene 



lertiary -\ ,-.,. 



,-.,. 



Ohgocene 



Eocene Eocene 



FORMATIONS AND PHYSICAL HISTORY 



There is much to be said for a two-fold division of the Tertiary, 

 the first including the Eocene, Oligocene, and Early Miocene, and 

 the second the later Miocene and Pliocene. This division differs 

 from that of the right-hand column above only in putting the lower 

 part of the Miocene in the lower division. 



Eocene formations appear in widely separated parts of North 

 America (Fig. 471), though they do not appear at the surface over 

 large areas. They include marine formations, brackish- water forma- 

 tions (made in bays and estuaries), and land (lacustrine and subae- 

 rial) formations. The marine and brackish- water beds are confined to 

 the borders of the continent, while the terrestrial deposits are found 

 in the Great Plains and farther west. Many of the formations are 

 not indurated, but locally they are even metamorphosed. 



The eastern coast. 1 Eocene formations appear at the surface 



1 Ball, i8th Ann. Rept., U. S. Geol. Surv., Pt. II. 



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