THE COLLECTION OF FOSSIL VERTEBRATES 



GEOLOGICAL ERAS, PERIODS AND AGES. 



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. 



