26 University of Kansas Geological Survey. 



the early divisions, but the tendency has been to recognize the 

 same general groupings. There are, however, some questions 

 of nomenclature which it may be well to mention here. 



The divisions of the Dakota by Logan are the only ones so 

 far proposed. As he remarks, there seem to be no well-marked 

 dividing lines. The divisions were made for convenience, and 

 may prove to be valid. 



The division of the Benton into Upper and Lower is upon 

 lithological grounds. The names used by Logan for the lower 

 group must stand ; but his division of Blue Hills is precisely 

 identical with the Victoria shales previously proposed by Cragin, 

 and must be given up on the ground of priority. Cragin's Rus- 

 sell group does not correspond with the Lower, since it does not 

 include the Ostrea shales. 



The division line between the Benton and Niobrara is in ac- 

 cordance with Gilbert's eastern Colorado section. (17th Ann. 

 Rep. U. S. G. S., 1896.) 



The Niobrara is divided into the Fort Hays and Pteranodon 

 beds. The name Fort Hays was limited by Williston in 1893. 

 The Pteranodon beds are further divided by Williston into the 

 Hesperornis and Rudistes, on paleontological grounds. Cra- 

 gin's Trego and Norton zones may correspond to these divisions, 

 based upon lithological grounds, and scarcely recognizably 

 characterized. Nevertheless, it is a question whether or not 

 these terms should take precedence over Williston's. 



The Fort Pierre was first reported by Williston . Cragin has 

 since proposed the name Lisbon shales for certain outcrops, 

 but has not in any way differentiated them or given any char- 

 acters by which they may be distinguished from the rest of the 

 Fort Pierre. 



The argument for the use of the Arickaree beds rests upon 

 similar grounds. They were first reported by Hay to be Fort 

 Pierre. Such being the case, the names Lisbon shales and 

 Arickaree shales may be with propriety used for the two 

 horizons. 



