AGE OF THE ARAPAHOE AM) DENVER. 251 



The dinosaurs of the Ceratops beds are highly modified and special- 

 ized forms unknown as yet in other parts of the world, except, perhaps, in 

 the Gosau formation of Austria, and the conclusion that they necessarily 

 indicate a Mesozoic age implies some reason why they may not have 

 survived into the early Tertiary. 



In the light of the facts which have been presented concerning the 

 several epochs succeeding the Laramie, it is not clear u> the writer wh) this 

 belief that the dinosaurs, or, indeed, the whole vertebrate fauna, surely 

 indicate a Mesozoic age should be bo positively maintained as is done by 

 the vertebrate paleontologists. 



[f the dinosaurs of the Ceratops fauna did actually live in the Lara- 

 mie epoch of Colorado the] survived a great orographic movemenl and its 

 accompanying climatic changes, and continued through the Arapahoe and 

 Denver epochs so lhtle modified that Professor Marsh has not detected 

 any changes corresponding to the stratigraphic Time divisions. This is ;ill 

 the more remarkable since the fossil plants show a great modification during 

 this time, and it has been commonly claimed that enormous and highly 

 specialized vertebrate animals are particularly sensitive to conditions of 

 environment. It' the Laramie vertebrates were unaffected by the known 

 dynamic phenomena of the Colorado region in post-Laramie times, it may 

 well !><• asked what caused their extermination in the post-Denver interval, 

 where as vet no evidence of orographic movements comparable with that 

 of the |ire-Ara]iahoe have been fraud. And if their extinction was due 

 in large measure to other causes than those associated with dynamic 



phenomena, may that extinction not have been deferred until the Eocene? 

 These considerations seem to the writer ample ground tor the demand 

 that the causes leading to tin' extinction of the Ceratops fauna should he 

 definitely connected with some orographic disturbance at the close of the 

 Denver epoch before their presence in the Arapahoe ami Denver beds can 

 lie admitted as full proof ot' the Mesozoic age of these formation-. 



Is a dual nomenclature desirable? The Vertelll'ate f;|||lia of' the p. .-t - La HI IJlic 



beds is said by paleontologists to be strongly .Mesozoic in it- affinities. 

 The post-Laramie formations are later than the beginning of the great 

 Rocky .Mountain revolution which has heretofore been considered to mark 



