AGE OF THE AEAPAHOB AND DKNVKK. 249 



Eocene formation is found resting unconformably on the Laramie or older 



.strata, it must still be a matter for proof as to whether the movement thus 

 recorded took place before or after the post-Laramie epochs which have 

 been described above. In other words it appears that at present there is 

 no distinct evidence of an especially important earth movement succeeding 

 the Cretaceous period, it' the Denver beds are assigned to the Mesozoic, 

 while that preceding the Arapahoe seems from present knowledge to have 

 been of the character and magnitude usually assumed for the disturbance 

 closing the Mesozoic. 



As the stratigraphic data now available do not satisfactorily determine 

 the preeminent importance of the post-Denver interval, the evidence of 

 fossils must be relied upon to justify the reference of the Arapahoe and 

 Denver formations to the Cretaceous. While an exhaustive discussion of 

 this question can not be entered upon in this place, the writer wishes to 

 briefly state the inferences which seem to him justifiable from a considera- 

 tion of the facts already presented. 



Evidence of fossil plants — It is shown by Mr. Kiiowlton that the Denver flora 

 is remarkably distinct from that of the Laramie of the Denver Basin, only 

 1") species being now known in both formations out of a total flora of 1 IS 

 species. An equally satisfactory comparison of the Denver flora with that 

 of the Fort Union can not be made until the latter has been revised, hut 

 Mr. Knowlton informs the writer that at least 13 species seem common to 

 the Denver and Fort Union floras. The Middle Park and Livingston floras 

 are included with that of the Denver beds in these statements. These 

 figures do not indicate a much greater break in plant life between the 

 Denver and Fort Union epochs than is found between Laramie and Denver. 



In general character the Fort Union and Laramie floras are so closely 

 related that for ,-i long time these formations were assigned to the same 

 geologic epoch. It does not appear then that the climatic influences of 

 the post-Denver interval modified plant life to a superlative degree. 



invertebrate fossils. — The Laramie se;is have cotnnioiilv been regarded as 

 transitional in character between the true marine waters of the Fox Hills 

 Cretaceous and the fresh-water lakes of the Eocene. Inharmony with this 

 idea the invertebrate fossils represent brackish-water or fresh-water tonus. 



