﻿136 
  

  

  NEW 
  YORK 
  STATE 
  MUSEUM 
  

  

  The 
  aeons 
  are 
  subdivided 
  into 
  periods 
  as 
  follows: 
  

  

  ^ons 
  

  

  Cenozoic 
  

   Mesozoic 
  

  

  Palaeozoic 
  

  

  Periods 
  

  

  j 
  Quaternary 
  or 
  Pleistocene 
  

   ( 
  Tertiary 
  

  

  Cretaceous 
  

  

  Jurassic 
  

  

  Triassic 
  

  

  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, 
  , 
  y 
  ,. 
  

  

  , 
  ,. 
  . 
  , 
  , 
  "( 
  Laurentian 
  

   not 
  yet 
  sub-divided 
  

  

  Not 
  known 
  in 
  JSTew 
  York 
  

  

  The 
  rock 
  formations 
  of 
  the 
  aeons 
  are 
  caviled 
  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. 
  

  

  