THE COLLECTION OF FOSSIL VERTEBRATES 



GEOLOGICAL ERAS, PERIODS AND AGES. 



Eras. 



Periods. 



Ages. 



Cenozoic 



Quaternary 



Age of Man, 



50,000 years 



Tertiary 



Age of Mammals, 

 3,000,000 years 



Mesozoic 



Cretaceous 



Age of Reptiles, 

 7.000,000 years 



Jurassic 



Triassic 



Palaeozoic 



Permian 



Age of Amphibians and Coal 



Plants, 



5,000,000 years 



Carboniferous 



Devonian 



Age of Fishes, 

 2,000,000 years 



Silurian 



Age of Invertebrates, 

 10,000,000 years 



Cambrian 



Eozoic 



Algonkian 



(No fossils) 



Archaean 



The estimates in years of the geological periods given in this 

 table, which is arranged in descending order from the most recent 

 to the most ancient time, must be understood to be merely very 

 rough approximations. There is no known method of finding any- 

 exact equivalent in years of any geological period, although the 

 relative length of each to each is much more nearly known. The 

 estimates given herewith are based on the careful study of the 

 subject made by C. D. Walcott, Director of the U.S. Geological 

 Survey. In concluding his discussion Dr. Walcott stated his 

 belief that the duration of geological time (the entire period 

 included in this table) might be measured by tens of millions of 

 years, but not by single millions or by hundreds of millions. 



