INTRODUCTION 47 



6. Extinction periods of certain nintnvials. — World-wide predominance 

 has its counterpart in the world-wide cHsappearance or extinction of cer- 

 tain forms, correlated with f>rand geologic and physiographic changes. 

 An example of this kind is the very general extinction of l)rowsing types of 

 Herbivora during the Oligocene. Among rodents, the beaver-like Steneo- 

 fihcr disappears at the same time both in Europe and North America, or is 

 rei)laced by modified forms. 



This general comparison of the evolution stages of the Old World and the 

 New World will naturally become precise and final only after the time in 

 the Old- World stages and in the New has been separately established and 

 defined. Thus there are correlation problems, as follows: 



European or Eurasiatic Correlation. 

 American Correlation. 

 American and Eurasiatic Correlation. 

 North and South American Correlation. 



When these four broad problems of American-Asiatic-European-African 

 correlation and of the broader New and Old World correlation are worked 

 out we shall be able to establish a complete and very accurate geologic 

 time scale for the entire Age of Mammals, and to speak with precision re- 

 garding the time of successive migrations, appearances, and extinctions. 

 It is even possible that we shall be able in the New and Old World to em- 

 ploy the same stages or subdivisions of the Epochs of time. 



Geologic Formations and Life Zones 



The earth's crust is made up of a vast series of separate deposits which 

 are technically known as 'formations.' The formation is the geologic unit. 

 It may var}' in thickness or in extent of geographic distribution; it may 

 be laid down in many ways, such as by the transporting power of water or of 

 wind or through falls of volcanic ash, but it is of the essence of a 'forma- 

 tion' that the conditions of deposition remain more or less uniform; when 

 the conditions change, as from fresh-water to marine, for example, we pass 

 into a new formation. 



If animal remains are varied in the formation, we may select among the 

 number a very conspicuous or abundant or unique mammal as (^specially 

 distinctive of the whole formation or of a certain level in the formation as 

 marking off a life zone. The word 'beds' previously used in a similar 

 sense is liable to cause confusion b(H;ause it has also been applied to geologic 

 formations. It is clear that while the geologic formation may be limited 

 in extent, the life zone, owing to the wider geographic range of the mam- 

 mal from which it takes its name, is not limited, but may be found else- 

 where. 



For example, in southwestern Wyoming there is a very thick, more or 

 less uniform deposit of volcanic ash, or tuff, which has been named the 



