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inhabitants. For sufficient reasons we divide this history into principal 

 and subordinate portions, each having its own characteristics. Each con- 

 tinent has had its own course of development, physical and biological, this 

 course sometimes agreeing only in a general way with that of other conti- 

 nents, being perhaps ahead of them or behind them, possibly sometimes 

 only different In order to compare and describe contemporary conditions 

 in different lands there must be a few fixed dates from which to reckon 

 the march of time and progress. These dates are found in the limits be- 

 tween the primary divisions, as that between the Silurian and the De- 

 vonian or that between the Cretaceous and the Tertiary. In a similar 

 way we orient the history of even a savage people with reference to such 

 dates as the founding of Rome and the birth of Christ. 



?>. The Primary Divisions of Geological Time Are Not Usually Indi- 

 cated by Great Unconformities. 



Inasmuch as those geologists and paleontologists who favor the refer- 

 ence of the Arapahoe and the Denver beds of Colorado, the Lance Creek 

 beds of Wyoming and the Hell Creek beds of Montana to the Eocene, give 

 as their principal reason therefor the existence of a great unconformity 

 between the Arapahoe and the formation immediately below it, while there 

 appears to be no similar unconformity below the Fort Union, it may be 

 worth while to examine the adequacy of the reason. 1 believe that it is 

 fallacious. 



It is possible that, as Chamberlin and Salisbury suggest in their gen- 

 eral work on geology (Geology, iii, p. 192), there is a natural basis for the 

 larger divisions of geological history; that this basis is to be found in 

 the profounder changes in the earth's crust ; and that this basis is of 

 world-wide application. This suggestion may be accepted as valuable 

 without its arousing the expectation that a great stratigraphical break 

 will be discovered everywhere between each great rock system and its 

 predecessor and its successor. As a matter of fact, as geological history 

 is now understood and now divided, such breaks are not commonly found. 

 I will quote from Geikie's Text-book of Geology, ed. 4. 1003, p. 10S1 : 



Though no geologist now admits the abrupt lines of division which 

 were at one time believed to mark off the limits of geological systems and 

 to bear witness to the great terrestial revolutions by which these systems 

 were supposed to have been terminated, nevertheless the influence of the 

 ideas which gave life to these banished beliefs is by no means extinct. 



