3S W. Cross — Pod- Laramie Deposits of Colorado. 



tions, they certainly cannot be used as diagnostic fossils for 

 any horizon. 



The facts of stratigraphy and lithology demand that the in- 

 terval between the Laramie and the lake-bed deposits should 

 be recognized as a very important one. The known fossils do 

 not aid us in determining the importance of that interval 

 because their distribution with reference to it is so imperfectly 

 established. Such seems to be the necessary verdict from the 

 examination of the direct evidence available at present. Much 

 collateral testimony might be introduced into this discussion, 

 from observations in remote districts, but it seems undesirable 

 and premature to consider such evidence at this time. Some 

 of the reasons for this conclusion will appear in the more 

 general discussion to follow. 



Assuming that the lake-beds described in this article should 

 be separated from the Laramie, the question as to whether they 

 are Tertiary or Cretaceous is quite another problem. It was 

 argued in the article on the Denver beds that the establish- 

 ment of a profound orographic movement in the period suc- 

 ceeding the Laramie, followed by extensive and long-continued 

 volcanic outbursts, indicated that the deposits of the Arapahoe 

 and Denver epochs should be assigned to the Eocene. This 

 was in harmony with the ideas to be found in all previous 

 speculations as to what actually closed the Cretaceous period. 

 At the time the article in question was written a few bones 

 had been identified bv Professor Marsh as belong-ino; to Dino- 

 saurs and other vertebrate forms of Mesozoic types, but the 

 fact that these animals had survived a period of various 

 dynamic disturbances of great magnitude seemed then to 

 indicate that they were straggling survivors into earliest 

 Eocene time rather than that the movement should be placed 

 in the Cretaceous. 



The facts presented in this paper make it plain that the dis- 

 cussion of this question involves now a discussion of the 

 broader one as to the character and position of the line to be 

 drawn between Mesozoic and Cenozoic deposits in the Rocky 

 Mountain region, and as to the criteria to be used. And at 

 present there is such a decided conflict between the results 

 reached in applying different criteria that the necessity for 

 more information on many points is clear. In illustration of 

 this necessity a few of the recently expressed opinions upon 

 this subject will be quoted. 



Professor Marsh* in a recent publication on the Cretaceous 

 mammalia and their associated vertebrate fossils, occurring 

 in the Ceratops beds — " the Laramie of Wyoming " — says : 



* This Journal, vol. xliii, p. 249. March, 1892. 



