American Association of Museums 



5i 



This chart is based upon (1), an assumed age for the earth of 

 72,000,000 years, which is a fair average of the numerous estimates 

 which have been made by geologists and physicists in this connection, 

 and (2), the ratios between the several geologic time divisions as esti- 

 mated by eminent geological authorities. 



The clock dial, representing the cosmic day, is divided into twenty- 

 four hours, hence each hour is equivalent to 3,000,000 years of geo- 

 logic time. 



The estimates of the geologic time division ratios are : 



Pre-Paleozoic (Azoic and Eozoic) Time 12 



Paleozoic Time 8 



Mesozoic Time 3 



Neozoic Time 1 



