CORRELATION AND PALEOGEOGRAPHY 



23' 



several levels of mammalian life zones enables us to institute exact com- 

 parisons with other life zones. 



Middle and Upper Eocene 



The first formation to be analyzed in this way was the classic Bridger 

 of southwestern Wyoming, a pivotal formation in the Middle Eocene. 

 Under the instruction of the writer, the American Museum parties, guided 

 by Matthew and Granger, especially undertook the problem of ascertain- 

 ing whether the Bridger was divisible into a series of life zones. After 

 four years (1902-1905) of very careful geologic work on the ground, ac- 

 companied by the level-record of every specimen, the work reached a 

 highly^ successful conclusion.^ 



The Bridger was found to subdivide geologically and faunistically into 

 five levels, designated as A, B, C, D, E. Of these levels A, B form the 

 Lower Bridger, characterized by the absence of Uintatheriuni, and C, D, 

 and E the Upper Bridger,. distinguished by the sudden appearance of 

 JJintatlierium, a genus so characteristic as to give the distinctive zonal 

 name (see table, page 240). 



The next problem w^as to correlate the Washakie formation, lying 50 

 miles east of the Bridger. This, by careful lithological and paleonto- 

 logical analysis by Granger and Matthew, was found to split into Was- 

 hakie A, identical in its mammalian life with Bridger D, and Washakie B 

 presenting a higher life zone, containing EohasUeus and Dolichorhinus. 

 The Washakie formation (Granger, 1909)^" was found to couple in a 

 precise and beautiful manner the Bridger and Uinta formations, as shown 

 in the accompanying diagram. 



Life Zones. 



C. Upper. 







Diplacodon. 



B. Middle. 



B. Upper 





Eobasileus. 



A. Lower. 



Upper D. 

 C. 



Uintatheriuni 



; I 





Lower B. 

 A. 



Orohippus. 



Uinta. 



Washakik. 



BiiinoKK. 



Formations. 



" See Matthew : Carnivora and Insectivora of the Bridger Rasin, Middle Eocene. Me- 

 moirs of the American Museum of Natural History, vol. ix. .\u,i;;ist. liMtO. pj). 'JOirtdl. 



1° Walter Granger : Faunal Horizons of the Washakie Formation of Southern Wyoming. 

 Bulletin of the Au.ericun Museum of Natural History, vol. xxvi, art, iii, January 19, 

 1909, pp. 13-23. 



