292 STANTON 



It is obvious from the testimony of the paleobotanists, and this 

 testimony could be indefinitely increased by similar examples, that 

 the modern flora in its essential features was well developed in Creta- 

 ceous time at least as early as the Dakota epoch (or approximately the 

 Cenomanian of European geologists). The changes since that time 

 have been mostly due to minor differentiations of the types then intro- 

 duced. The floras therefore do not furnish competent evidence for 

 discriminating between Cretaceous and Tertiary. These later floras 

 are useful in local stratigraphy and in provincial or regional correlation 

 in much the same way that the non-marine faunas are, though they 

 are usually more important than the non-marine faunas because they are 

 more numerous and more highly differentiated. They cannot be 

 used in determining the boundary between Cretaceous and Eocene 

 until the stratigraphic range of the species is thoroughly known and 

 the evidence of the plants is checked by means of other criteria. The 

 differences between late Cretaceous and early Tertiary floras are chiefly 

 specific characters and modifications in the grouping of the types, both 

 of which must have varied from place to place as well as from time to 

 time. The method, often adopted by paleobotanists, of determining 

 the relative stratigraphic positions of local fossil floras by ignoring 

 all the undescribed and unidentified species, and reckoning percent- 

 ages of identified species that are recorded in various horizons, 

 is based on the unwarranted assumption that there were no variations 

 in topography, soil, or other conditions affecting plants, and that 

 the distribution and preservation of the species was uniform through- 

 out their range. That this method should lead to errors in correla- 

 tion is inevitable. 



Conclusions. 



In the Interior Region of North America the formations between 

 the uppermost marine Cretaceous and the Wasatch together constitute 

 a real transition from the Cretaceous to the Tertiary. 



Notwithstanding the fact that there are several local unconformities 

 at various horizons and perhaps some of more general distribution 

 there is no conclusive evidence that any one of these represents a very 

 long period of erosion not represented by sediments elsewhere in the 

 region. 



