280 W. Cross — Denver Tertiary Formation. 



tion began again and volcanic outbreaks are frequently men- 

 tioned as characterizing the beginning of the Tertiary Era. 



As to the fossil flora, it is well known that Lesquereux and 

 others have referred the entire Laramie Group to the Eocene, 

 or Miocene, from the evidence of fossil plants. It remains a 

 task for competent hands to ascertain whether the Table Moun- 

 tain plants have influenced opinions as to what constitutes the 

 " typical Laramie Flora," or not, — and to what extent. 



The few invertebrate fossils of the Denver beas are declared 

 by Dr. White to have no positive weight against the conclu- 

 sion adopted. 



As to the vertebrate fossils of the Denver strata the Dino- 

 saurian remains certainly present an element of evidence, 

 which, judged by current belief, is strongly opposed to the 

 idea of a Tertiary age for the Formation. The doctrine that 

 the Dinosauria became extinct in the Cretaceous period is gen- 

 erally accepted, yet it has been characterized as a "dogma" by 

 Heer and Lesquereux.* How far is this doctrine supported 

 by a knowledge of the actual conditions which led to the sup- 

 posed extinction of this interesting group of peculiar animals ? 

 Are the facts of experience competent to establish the extinc- 

 tion at the time mentioned, independently of a knowledge of 

 conditions? 



The best record of accomplished extinction of the Dinosauria 

 is found in the absence of their remains in the great Eocene 

 Formations of the basin area west of the Rocky Mountains. 

 The causes of this extinction are not as yet known. As Dr. 

 C. A. White pointed out some years ago, " The climate and 

 other physical conditions which were essential to the existence 

 of the Dinosaurians of the Laramie period having evidently 

 been continued into the Tertiary epochs that are represented 

 by the Wasatch, Green River and Bridger Groups, they might, 

 doubtless, have continued their existence through those epochs 

 as well as through the Laramie period but for the irruption of 

 the mammalian hordes to which they probably soon succumbed 

 in the unequal straggle for existence."'}' That a group of ani- 

 mals with such highly specialized characteristics as are pos- 

 sessed by the Dinosauria could not adapt themselves to sudden 

 changes of environment is no doubt true, but it is an assump- 

 tion that the orographic movements causing or following the 

 close of the Laramie produced sudden changes of more than 

 local influence. In the Denver strata, whatever their age, is 

 the proof that certain types of Dinosauria did survive the 

 changes of conditions attending a period of folding and an- 

 other period of great volcanic activity. 



* Heer quoted by Lesquereux in " Cretaceous and Tertiary Floras," p. 112. 

 f Bull. U. S. G. and G-. S., vol. iv, No. 4, p. 876. 



