1903] 



Ami Correspondence. 



V. Laraniie 



IV. Pierre-Fox Hills. 



III. Belly River series. 



If. Niobrara-Benton. 



I. Dakota.- 



There is now no reason to doubt the position of the Belly 

 River series in the succession of sediments in the reg^ion of the 

 Great Plains; nor its position above the Niobrara-Benton, which 

 in turn is underlaid by the Dakota formation, which latter is 

 recognized by all to be of Upper Cretaceous age. The only 

 conclusion, therefore, that one can reach is that the Belly River 

 series is high up in the Upper Cretaceous. This is the place 

 where it has stood for years, and the large percentage of Tertiary 

 invertebrates, found within its upper and lower limits, which have 

 a close affinity and remarkable resemblance to Eocene Tertiary 

 forms (many forms being identical) must lead one to conclude 

 that it cannot be in the interest of chronological geology to place 

 the Belly River series in the Middle Cretaceous as we would be 

 led to believe from the recent writings above referred to. It may 

 well be that many forms of the vertebrata occurring in the Belly 

 River series are primitive in their character, i.e., show traits which 

 are older than are exhibited by their successors or descendants in 

 later times; nevertheless, when the age of a geological horizon or 

 series of strata has ,to be determined, the whole biologic assem- 

 blage must be taken into consideration. When such is taken, it is 

 impossible to arrive at any other conclusion either on stratigra- 

 phical or palaeontological grounds than that the Belly River series 

 is not Middle Cretaceous but Upper Cretaceous, and well up in 

 that portion of the Time-scale. As to its occurrence as an inter- 

 calated or " mid-Cretaceous" formation I have no doubt. How- 

 ever, the term "Mid-Cretaceous" appears to bean ambiguous 

 one and may lead to further confusion. 



To anyone who has considered the flora and fauna of the 

 Belly River series as a whole, as well as the flora of the Dakota, 

 and compared them with European equivalents it is easy to see 

 their Senonian or Upper Cretaceous age clearly. 



H. M. A. 



