288 B. T. Hill — Texas Section of American Cretaceous, 



limits which I have marked, a great similarity of molluscan 

 fauna, both specifically and generic-ally, although the lithologic 

 and stratigraphic characters vary in each. To the local forma- 

 tions of this collection of strata many different names have 

 been given, and nearly every horizon has been termed a "group" 

 regardless of any specific definition of that word, which has 

 been used with a indefinite meaning in all the nomenclature, of 



J?eatyMt. Rsaiorr. 











Gvzr STATES 



















Texas Aza.m/sS? 





NORTH ATLANTIC 





A-UFPER 



[Tex Sill 





Fox [Rip- 



Tom- 

 btgb*. 





Rotten lime- 

 stone. 





'^PTRaiN'tA. NxwJBR£E7> 

 i 



* " ■ 



A -UPPER. 





Tcmbi$oee. 







Jfiotraro. 

 Benton. 



/ 



EajjleTord. 

 Shales. 



/ 



Eulaw 





Potomac 





Tiarilan. 



c-zowzb. 









PRB-CRSTA.CEOUS. 



B-MTDDLE 



Lower Croii 

 Ti'mbefS 





/ j 











Dakota. 



Washita. 

 Div. 



\ Pred.- 

 n rickS- 



1 Div 



%Baeal 







c 



''-LOWER 







PRE- CRB TJL CEOUS 



the American Cretaceous. Instead of indicating a plurality of 

 related phenomena, it has usually been applied indiscriminately 

 to single characteristics based upon lithologic or specific culmi- 

 nations. For instance, in the Alabama regions the " Tombig- 

 bee sands," the "Eotten limestone," and the Ripley beds have 

 each been called " Groups," when in reality together they con- 

 stitute but a single group, as interpreted by the present ac- 

 cepted meaning of that word, their lithologic features being the 

 fluctuations of an unbroken sedimentation, and the molluscan 

 fauna continuous or interwoven by connective species from top 

 to bottom. The " groups" so-called are really " horizons," rep- 

 resenting the culmination of species or sedimental variations. 

 The same can probably be said of the New Jersey Cretaceous 

 above the Raritan clays, and the Fox hills and Pierre " Groups 

 of the Northwest." The entire collection of strata of this 

 upper portion of the American Cretaceous together constitutes 

 a group which has been correlated with the Upper Cretaceous 

 of Europe, but until our stratigraphic studies are completed, 

 can only be called the " Upper Cretaceous." 



In the Rocky Mountain and Texas regions other marine for- 

 mations exist, but they have not as yet been traced east of 

 central Texas. These include the Benton, Niobrara and 

 Dakota sandstone groups of Meek and Hayden, which the for- 

 mer geologists correlated, upon good reasons, with the Lower 

 Chalk or Upper Greensand of Europe. The state of knowledge 



