CHAPTER XVI. 

 THE EOCENE PERIOD. 



The Cenozoic Era. — The remaining periods of geological history con- 

 stitute «the Cenozoic era, the era of modern life. The era is commonly 

 divided into two principal parts, the Tertiary and the Quaternary. 

 These principal divisions are variously subdivided, as shown below: 



Recent, or Human. Post-glacial formations. 

 Quaternary \ Pleistocene, or Glacial. Glacial formations and non-glacial 



[ deposits of glacial age. 



(Pliocene Pliocene \ AT 



[ Eocene Eocene Eocene. 



The threefold subdivision of the Tertiary is the one which seems 

 to best fit the phenomena of our continent as now understood, though 

 there is a growing tendency toward the recognition of the Oligocene. 

 This tendency seems to mean that beds are found in our continent 

 which carry fossils similar to those of the Oligocene of Europe, rather 

 than that the Oligocene of this continent constitutes a natural and 

 major subdivision of the Tertiary. 



The nominal basis of the Cenozoic classification and nomenclature is a radi- 

 cal departure from that used for earlier eras. Here, stages of approach to exist- 

 ing types of life are made the basis, at least nominally. Originally, Eocene 

 (dawn of the recent) formations were defined by the presence of a few fossils 

 of living species, specifically 3^ per cent, generalized to 5 per cent or less; Mio- 

 cene (less recent, i.e. less than half the fossils represent living species), defined 

 by about 17 per cent, generalized to mean a minority, of living species; and Pli- 

 ocene (more recent) by 36 to 95 per cent, interpreted as a majority. 



On its face, this classification seems as artificial as the Linnsean classification 

 of plants by the number of their stamens, though it has a somewhat more natu 



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