8 THE COLLECTION OF FOSSIL VERTEBRATES 
GEOLOGICAL ERAS, PERIODS AND AGES. 
Eras. Periods. Ages. 
Age of Man 
Quaternary 8g : 
50,000 years 
Cenozoic 
; Age of Mammals, 
Tertiary 
2 3,000,000 years 
Cretaceous 
; c of Reptiles 
Mesozoic Jurassic Age e , 
7,000,000 years 
Triassic 
Permian Age of Amphibians and Coal 
Plants, 
Carboniferous 5,000,000 years 
’ : Age of Fishes, 
Paleozoic Devonian 
2,000,000 years 
Silurian 
Age of Invertebrates, 
4 I0,000,000 years 
Cambrian 
Algonkian 
Eozoic (No fossils) 
Archean 
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. 5. 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. 
