to earlier and later Formations. 437 



The species referred to were gill-bearing mollusca, and to 

 have survived they must have had a continuously congenial 

 habitat. That is, they were in part fresh-water and in part 

 brackish-water forms, and those respective conditions of the 

 waters in which they lived must have been somewhere contin- 

 uous to have made the survival of those species possible. It is 

 therefore probable that the Belly River and Laramie faunas 

 somewhere became blended together as one, upon the final 

 retirement of the marine Cretaceous waters ; although no such 

 blending of the,, strata of those formations has yet been dis- 

 covered. Whatever may have been the facts in the case, the 

 specific identity of those Belly River and Laramie mollusca 

 makes it necessary to assume that at least a considerable part 

 of the Laramie molluscan fauna began its existence long be- 

 fore the close of the Cretaceous period as it is represented by 

 marine formations. This faunal relationship between the Belly 

 River and Laramie formations also strongly connects the latter 

 formation with the Cretaceous. 



The two categories of facts relating to stratigraphical rela- 

 tions of the Laramie which have been presented in the preced- 

 ing paragraphs of this article, and which are strongly sug- 

 gestive of its Cretaceous age, have not before been publicly 

 discussed in that connection. There are however two other 

 categories, one relating to physical, and the other to paleonto- 

 logical phenomena which have been much discussed, both of 

 which have been held by many persons to prove conclusively 

 the Cretaceous age of the Laramie. The paleontological fact 

 which has most influenced the views referred to, and the only 

 one that need be mentioned here, is the occurrence of dino- 

 saurian remains in the Laramie, extending even to some of its 

 uppermost strata. 



The physical phenomena referred to pertain to certain of the 

 orogenic and epirogenic* movements which have taken place 

 within the great region occupied by the Laramie Group. The 

 movements referred to are those which on the one hand have 

 resulted in the present elevation of that great western portion 

 of North America, and on the other, in such great folds, for 

 example, as those out of which the Uinta, and Rocky* Mountains 

 have been carved. In at least the greater part, and apparently 

 all, of those movements the Laramie Group is found to have 

 been fully involved together with all the formations beneath 

 it ; while the later formations were not so fully involved. 

 Thus there appears to have been within that region no phys- 

 ical break in the continuous accumulation of material compos- 

 ing the true marine Cretaceous formations, and none of 

 importance until the close of the Laramie period, if we ex- 



* Etym. H7retpof ; mainland, or continent. 



