THE CENOZOIC EISA 



CHAPTER XXVII 

 THE EOCENE PERIOD 



FORMATIONS AND PHYSICAL HISTORY 



The remaining periods of geological history constitute tl 

 Cenozoic era, or the era of modern life. The era is variously sul 

 divided, as shown below: 



f 



Cenozoic 

 Era 



Quaternary 



Tertiary 



I 



Recent or Human. Post-glacial formations 

 Pleistocene or Glacial. Glacial formations 



and non-glacial deposits of glacial age 

 Pliocene Pliocene % 



Miocene 

 Oligocene ) 

 Eocene \ 



Miocene 

 Eocene 



Neocene 

 Eocene 



The threefold subdivision of the Tertiary (Eocene, Miocei 

 Pliocene) is the one which seems to fit the phenomena of our con- 

 tinent best, but there is a growing tendency toward the recognition 

 of the Oligocene. This means that beds are found in our continent 

 with fossils similar to those of the Oligocene of Europe, rut her than 

 that the Oligocene of this continent constitutes a natural and 

 major subdivision of the Tertiary. 



Eocene formations are found in widely separated parts of North 

 America (Fig. 524), though they do not appear at the surface over 

 large areas. They include beds laid down in the seas and bays. 

 and on the land. The former include- formations of marine and 

 brackish-water origin, and the latter those of lacustrine and sub- 

 aerial (fluvial, pluvial, eolian) origin. The subaerial formations 

 are probably more important than has commonly been recogniy.nl. 

 The marine and brackish-water beds are confined to the borders 

 of the continent, while the terrestrial deposits are found in many 

 places in the Great Plains and farther west. Many of the format ions 

 are not indurated, but locally they are even metamorphosed 



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