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NEW YORK STATE MUSEUM 



The aeons are subdivided into periods as follows : 



.^Eons 

 Cenozoic 



Mesozoic 



Periods 



j Quaternary or Pleistocene 

 I Tertiary 



Cretaceous 



Jurassic 



Triassic 



Palaeozoic 



Carboniferous 

 Devonian 

 ^ Upper Silurian 

 Lower Silurian or Ordovician 

 Cambrian 



Prevailing types of 

 animal life 



Mammals 



Reptiles 



Fishes 



Mollusks 

 Crustaceans 



Keeweenawan 

 Huronian 



Proterozoic or 



Agnotozoic 



Archaean, 



i t • "f Laurentian 

 not yet sub-di vided 



Not known in New York 



The rock formations of the aeons are called series and of the 

 periods, systems. 



The systems may be described in general terms as those divi- 

 sions of the series which are world-wide in their differentiation. 

 The subdivisions of the systems which are called groups are 

 chiefly local and variable. The groups are divided into stages. 



