286 STANTON 



with several identical forms is found at a much higher horizon at 

 Black Buttes,afew miles away, it is a fair inference that the Black 

 Buttes fauna is directly descended from that at Point of Rocks and 

 that both are of Cretaceous age. When the whole region is studied 

 and it is learned that no marine Tertiary fauna ever entered it and 

 that the Tertiary sea did not approach closely enough to the region 

 to have been connected with these brackish waters, and when it is 

 further learned that no such assemblage of non-marine forms is anv- 

 where found intercalated in or closely associated with marine Tertiary 

 rocks the truth of the inference that the brackish-water fossils of 

 Black Buttes are Cretaceous is practically demonstrated and it may 

 be confidently asserted that in this area all higher rocks in which such 

 fossils occur are also Cretaceous. Of course isolated occurrences of 

 Corhula and perhaps a few other genera that are known to range into 

 purely fresh waters far from the ocean must be treated with caution. 

 It is for these reasons that I have given much importance to the oyster 

 bed reported by Leonard in southwest North Dakota above the Tri- 

 ceratops horizon and above an unconformity. It is in itself proof that 

 the rocks are Cretaceous and that the unconformity has no significance 

 in the matter of separating Cretaceous from Tertiary. The brackish- 

 water fauna of the Upper Cretaceous has the same general character 

 and many specific identities throughout its great vertical and geogra- 

 phic range. 



The fresh-water mollusca include a few species that actually pass 

 up into the overlying formation, but the majority of the species are 

 confined to this horizon or occur in either identical or closely related 

 forms in the Mesaverde, Judith River, and other Cretaceous forma- 

 tions. This element of the fauna is best developed in the Hell Creek 

 and Converse County areas, where they are not associated with the 

 brackish-water beds, but the distribution of some of the species is 

 wide-spread in association with the dinosaur fauna, and a large pro- 

 portion of them, including some of the more striking and characteris- 

 tic forms, occur at Black Buttes in beds that contain intercalated 

 brackish-water strata. These facts together with the absence of 

 types similar to most of the forms from the Eocene and other Tertiary 

 rocks of the region justify the reference of the frcsh-watcr fauna to the 

 Cretaceous. 



Evidence from the flora. — Doctor Knowlton's statistics show a close 



