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University of California. 



[Vol. i. 



II. THE POST-CRETACEOUS OK ROCKY-MOUNTAIN REVOLUTION. 



The next in order, going backward, is that which separates the 

 Mesozoic from the Cenozoic, or more definitely, the Cretaceous from 

 the Eocene. The physical changes which occurred at this time in 

 America, are, (i) the unification of the American continent by the 

 final abolition of the great interior Cretaceous sea which previously 

 divided the continent into two parts, and (2) the formation of the 

 Colorado, the Uinta, and Wahsatch Mountains. In America, this 

 may therefore be called the Rocky Mountain Revolution. Similar, 

 and even greater movements seem to have occurred in other conti- 

 nents, for the unconformities are even greater there than in America. 



Here again we have widespread crust oscillations accompanied 

 by (0 great changes in physical geography, shown by very general, 

 almost universal, unconformity of Tertiary on Cretaceous or on 

 still lower rocks, in every part of the world. Only in a few places 

 on the American continent, e. g., in the plateau region and in Cali- 

 fornia, is the stratification doubtfully continuous. (2) By rapid 

 changes of life-forms even where the sedimentation is continuous. (3) 

 The introduction of a new and higher dominant class, the Eutheria. 

 (4) Wide migrations partly permitted by changes in physical geog- 

 raphy, and partly enforced by changes of climate; and consequent 

 great and rapid changes in organic forms, partly by pressure of a 

 new environment, partly by mingling of faunas and floras, and 

 partly by necessary readjustment to a new and higher dominant 

 class. 



The apparent suddenness of the change in organic forms — for 

 example, in this case the appearance of Eutheria — is the result 

 partly of loss of record (unconformity of strata), but mainly, I 

 believe, of migrations (unconformity of faunas and floras). This 

 suddenness, however, has in this case been probably exaggerated. 

 Until recently, after the Jurassic Metatheria, no mammals were 

 known until the Tertiary. But then without apparent transition an 

 entirely different and higher type suddenly appeared in great num- 

 bers. The discovery by Marsh of mammals in the uppermost Cre- 

 taceous was therefore hailed as probably filling the great gap. But 

 according to Marsh these are all of pure Mesozoic types, and there- 



