48 



THE AGE OF MAMMALS 



Bridger Formation, from its proximity to the famous old Fort Bridger.^ 

 The entire formation is 1800 fe(>t in thickness. The upper half of it 

 is distinguished by the sudden appearance of a very large and distinc- 

 tive quadruped, Uintatherium, named after the adjoining range of 

 Uintah Mountains, which are on the boundary between Wyoming and 

 Utah. This animal is so very distinctive that we may speak of the Upper 

 half of the Bridger formation as the Uintatherium Zone. One hundred 

 miles east of the Bridger is a deposit known as the Washakie, and in the 

 lower half of this we find the same quadruped, Uintatherium, very abun- 

 dant and characteristic. Thus the Lower Washakie is also in the Uinta- 

 therium Zone. From the presence not only of Uintatherium but many 

 other animals in common we are able to correlate these two formations, as 

 follows : 



Formations 



Upper 

 Washakie 



Zones 

 Eobasileus zone 



Upper 

 Bridger 

 Lower 

 Bridger 



f Lower 

 1 Washakie 



Uintatherium zone 

 Orohippus zone 



This single example illustrates how all fossil-bearing formations may 

 be correlated with each other where they contain similar life zones. This 

 furnishes a simple key to the elaborate correlations which the reader will 

 find in the later pages of this work. The above is a strildng example of an 

 overlapping in time; that is, while the upper half of the Bridger Forma- 

 tion was being deposited, the deposition of its more or less distant neigh- 

 bor, the Washakie Formation, began. In this case the two formations 

 happened to be somewhat similar in their rock composition, both being 

 composed of volcanic ash; but another Uintatherium life zone might be 

 found in a formation of river sand or clay. Thus the life zone enables us 

 to synchronize geological formations of many different kinds which may 

 be widely distributed geographically, and may vary greatly in thickness. 



It is obvious that the correlation of innumerable fossil-bearing formations 

 of the Old and New World respectively can be made much closer and more 

 exact than the correlation of the Old and New World combined; yet the 

 method of investigation is in each case the same. It should be based on: 



1. Comparisons of animals of similar mutative, specific, and generic stages. 



2. Evidences of similar local evolution. 



3. Dominance or scarcity of similar animals in the fauna as a whole. 



4. Diminution, disappearance, or apparent extinction of similar forms. 



5. First appearance of similar forms, apparently by migration or invasion 

 from some other region. 



' See Osborn, H. F., pp. 50 ff., Cenozoic Mammal Horizons of Western North America. 

 U.S. Geol. Surv., Bull. 3G1, 1909. 



