36 CROSS 



The idea of transitional deposits is, of course, not new. Numer- 

 ous sections have been thought to represent consecutive sedimenta- 

 tion but the investigations of the last twenty years have shown 

 breaks in many such sections, while in no case has a transitional 

 series been established. The Hayden survey geologists believed that 

 Laramie and Wasatch beds were, in some places at least, connected 

 by an unbroken series of strata. Aside from apparent conformity 

 the existence of certain species of fresh water shells in both Laramie 

 and Wasatch has been taken as evidence of continuity of deposition. 

 Dr. C. A. White, whose observations during the Hayden survey were 

 of much weight in influencing the opinion of his colleagues, again 

 expressed this idea while reviewing the relations of the Laramie to the 

 Denver and Arapahoe formations. ^^ The presence of identical fresh 

 water species in the Laramie and Wasatch of any given section does 

 not, however, prove the continuity of conditions at the site of that sec- 

 tion during the whole time involved. 



In the light of observations at Golden, Carbon, and many other 

 localities, apparent conformity between the Laramie and beds above 

 it cannot be regarded as proof of continuous sedimentation. The 

 only evidence which can show conformity to mean continuity of 

 deposition in this case would appear to be gradual change in fauna 

 or flora of the epoch in question. This evidence has not been fur- 

 nished at any locality. 



Referring only to the established facts of the Denver district, a 

 section of some other region, supposed to represent continuous sedi- 

 mentation between the Laramie and Fort Union epochs, must 

 contain beds representing: — 



1. The long interval of uplift and erosion preceding the Arapahoe 

 deposition. 



2. The Arapahoe. 



3. The interval of erosion and volcanic eruption preceding the 

 Denver. 



4. The Denver. 



5. Interval between the Denver and Fort Union, 



No section even approximating such a record has been established. 



'* The North American Mesozoic. Vice-Presidential address, Proc. A.A.A.S., 

 Vol. XXXVIII, 1889, p. 14. 



