G. II. Fldridge — Formations of the Middle Cretaceous. 31> 



Art. XLIII. — Some suggestions upon the method of grouping 

 the formations of the middle Cretaceous and the employ- 

 ment of an additional term in its nomenclature ; by Geo. 

 H. Eldridge. 



In recent studies in Colorado, in earlier work in many parts 

 of Montana, and in a general examination of the results ob- 

 tained by the leading workers in the Cretaceous geology of the 

 West, it has several times occurred to the writer that there 

 existed a demand for a reconsideration and revision of the 

 methods of grouping the formations comprising the middle 

 portions of the Cretaceous, — that is, the Fort Benton, the 

 Niobrara, the Fort Pierre, and the Fox Hills. 



Granting the desirability of such revision, there arises with 

 it a parallel necessity for the reconsideration of the nomencla- 

 ture of this part of the Cretaceous, and for the suggestion of 

 a group name for one of the two more general divisions into 

 which it is still proposed to cast the formations mentioned. 



The object to be attained by the revision here advocated is : 

 first, the creation out of the formations of the middle Cre- 

 taceous of two divisions, each of such stratigraphical and pale- 

 ontological weight as shall rank it with either the Dakota 

 below or the Laramie above ; secondly, the assignment to these 

 divisions of appropriate names, of a taxonomic value equal 

 to that of the ones already given to the formations with 

 which they are to rank. 



The plan of grouping, suggested here, includes in the lower 

 of the two more general divisions the formations of the Fort 

 Benton and Niobrara ; in the upper, the Fort Pierre and Fox 

 Hills : for the former no better name can be found than that 

 already in use — Colorado ; for the latter the name — Montana — 

 is now, for the first time, proposed. 



To take up the foregoing points at somewhat greater length : 

 first, a brief exposition of the leading characteristics of the 

 formations involved is quite in order. 



The Fori Benton. — This is essentially a formation of black, 

 argillaceous shales, passing by transitional beds into the forma- 

 tions both above and below, though into the latter always in a 

 manner more or less abrupt. Its thickness, like that of the 

 other marine Cretaceous formations in the West, varies consid- 

 erably from point to point, but is generally between four and 

 eight hundred feet, approximating the smaller figure oftener 

 than the larger. In addition to its leading characteristics — its 

 black or leaden-hue color, argillaceous composition, and shaly 

 nature — it is marked by the occurrence, here and there, of a 

 few narrow and intermittent bands of fossiliferous limestone, 



