64 



THE AGE OF MAMMALS 



Knight 1899 



Sollas 1900 



Penck 



1909 



1908 



4,000,000 years 



(Eocene-Miocene = 

 2,500,000 app. 



Plio.-Pleistocene = 

 1,584,000) 



4,200,000 years 



(Tertiary = 

 3,800,000 



Quaternary = 

 400,000) 



6,380,000 years 



Quaternary = 500,000 

 to 1,000,000 years 



Based on the rate of denudation or erosion 

 as measured by the amount of ex- 

 posure of roots of j)ine trees of known 

 age (1 foot in 100 years). 



Based on the rate of accunmlation esti- 

 mated at 1 foot in 100 years. The 

 estimated thickness of sedimentary 

 rocks (Eocene to Recent) is 42,000 

 feet. 



The thickness of sedimentarj' rocks 

 (Eocene to Recent) estimated at 

 63,800 feet. 



Based on the average rate of denudation 

 of the present land surface (a^Vo foot 

 in 1 year). 



VI. The World Supply of Mammals 



The source of the world's supply of mammals, the great homes, centers, 

 or continents from which the orders evolved and took on their distinctive 



Fig. 16. — Late Cretaceous and Basal Eocene. Period of extinction of the great Reptilia. 

 A time of elevation, favoring an interchange of archaic life between South and North America, 

 also between Nortli America and Europe. South America probably united with Australia via 

 Antarctica, allowing an interchange of carnivorous and herbivorous marsupials. A partial 

 community of fauna between North America and Eurasia with Africa. Rearranged from 

 W. D. Matthew, 1908. 



