28 



FOSSIL FORESTS OF YELLOWSTONE PARK. 



(1,000 years) ^^e shall have 12,000 years as the possible time during 

 Avhich these forests flourished. It is possible that the truth lies some- 

 where betAveen these extremes. 



Era. 



Period. 



Epoch. 





Qiiaternarj-. 



Recent. 





Pleistocene (glacial). 



renozoic. 





Pliocene. 





*Tertiar3'. 



*Miocene. 





Oligocene. 







Eocene. 





Cretaceous. 





Mesozoic. 



Jurassic. 







Triassic. 







Carboniferous. 







Devonian. 





Paleozoic. 



Silurian. 







Ordovician. 







Cambrian. 





Protero- 



Algonkian. 





zoic. 



Archean. 





Fig. 16. — Geologic divisions. 



CLIMATE DURING THE LIFE OF THE FOSSIL TREES. 



A final word may be added regarding the probable climate of the 

 region during the lifetime of these fossil forests. It is obvious that 

 the present flora of the YelloAvstone National Park has compara- 

 tively little relation to the Tertiary flora and can not be considered 

 the descendant of it. It is also clear that the climatic conditions 

 must have greatly changed since Tertiary time. The Tertiary flora 

 appears to have come from the south, but the present flora is evi- 

 dently of more northern origin. The climate during Tertiary time, as 

 indicated by the vegetation, Avas temperate or warm-temperate, not 

 unlike that of Virginia or the Carolinas at the present time, and the 



