238 KNOWLTON 



usually distinct lithologically, cannot be separated structurally, 

 sedimentation having been uninterrupted, except locally. 



7. The paleontological elements of the lower members are con- 

 sidered at length, beginning with the plants. It is shown that of the 

 84 known species, 61 are common to the upper member, and only 

 II species to the Laramie of Colorado, while 15 species are com- 

 mon to other American Eocene and 9 species to the Miocene. The 

 Eocene age of the Fort Union is fixed by tying its flora to that of 

 various Old World beds of known Eocene position. 



8. The invertebrate evidence is shown to be in substantial accord 

 with that of the plants, there being only 4 of the 49 species common 

 to the Colorado Laramie. All, with a single possible exception, are 

 fresh-water forms. 



9. It is shown that the vertebrates afford no positive evidence of 

 Cretaceous age. That the dinosaurs exhibit Cretaceous affinities 

 is not denied, since, being without known descendants, it is pos- 

 sible to compare them only with their progenitors. It has been 

 proved beyond question that they survived the profound orogenic 

 movement and attendant physical break at the top of the Laramie 

 in the Denver Basin of Colorado, and lived on in Arapahoe and 

 Denver time, and it is shown that in the areas considered in this 

 paper they passed over a similar erosional interval and are found in 

 association with the Fort Union flora, which is of Eocene age. 



10. The mammals of the lower Fort Union show very little rela- 

 tionship with Jurassic or Cretaceous forms, but find their closest 

 affinities with those of the Puerco and Torrejon, which are of 

 acknowledged Eocene age. 



11. The chelonians are shown to be of little value in their bearing 

 on the age of the lower Fort Union, especially when compared with 

 the Judith River forms, which are evidently in confusion. 



12. It is held that the line between Cretaceous and Tertiary 

 should be drawn at the top of the true Laramie. 



13. The final conclusion is reached that the beds here considered 

 ("Hell Creek beds," ''somber beds," "Ccratops beds," "Laramie" 

 of many writers) are stratigraphically, structurally, and palcontologi- 

 cally inseparable from the Fort Union, and are Eocene in age. 



Note. — For assistance in the preparation of this paper, both in the field 

 and in the office, I wish to acknowledge my great indebtedness to Dr. A. C. 

 Peale of the U. S. National Museum. 



