18 COLORADO FORMATION ANJ) ITS INVERTEBRATE FAUNA Dm 100. 



the upper, but thi a is exceptional. The great majority of the species 

 and all of the abundant ones are distinct, consequently Prof. Meek's 

 argument for making the primary division at the top of the Niobrara 

 is not impaired. 



. In 1878j when describing the Cretaceous formations of northwestern 

 Colorado, Dr. 0. A. White ' used the nomenclature of King, dividing 

 the section into the Dakota, Colorado, and Pox Hills groups, but for 

 reasons similar to those jusl enumerated tie applied the term Colorado 

 group only to the equivalents oldie Fort Benton and the Niobrara, 

 while the Fort Pierre was embodied in the Pox Bills group. These two 

 groups as thus defined are equivalent respectively to the marine por- 

 tions of the Lower Series and the Upper Series of Meek and Bayden, 

 and are characterized by distinct marine faunas, though there may also 

 be in either of them local intercalations of fresh-water or brackish- 

 water deposits. 



This method of grouping the formations was adopted by some of the 

 other geologists of the Survey of the Territories, and it was used on 

 the maps of the Atlas of Colorado. 



Mr. George II. Eldridge 2 has recently published an excellent genera] 

 review and discussion of the classification and nomenclature of the 

 Western Cretaceous, in which he adopted the divisions as defined by 

 Dr. White, but he proposed the new name Montana group for the com- 

 bined Fort Pierre and Fox Hills. In this manner the confusion caused 

 by using the name Fox Hills in two different senses is avoided, and the 

 original names of the minor subdivisions may slill be used in local 

 sections where it is desirable. 



After describing each member of the Upper Cretaceous section, espe- 

 cially as developed along the eastern base of the Front range in Col- 

 orado, Mr. Eldridge sums up the evidence thus: 



From the foregoing details, the following relations of the several formations 

 to each other may be clearly and legitimately deduced: First, the component strata 

 of the Fort Benton and Niobrara, frequently, and fche lite generally shade into each 

 other; second, this is again even more forcibly paralleled in the sedimentation and li l'v, 

 of the Fort Pierre and Fox Hills; third — and on the contrary — between the Nioluara 

 and Fort Pierre, or, which is the same thing, between fche Colorado and Montana 

 groups (as was suggested they should be designated early in the paper), I lie).- is ;i 

 differentiation of both sediments and life, greatly in excess of any similarities in 

 these respects that may be noticed from time to time in any particular locality. 



Some of the detailed evidence in support of these views will be given 

 in subsequent paragraphs. 



Mr. Eldridge has also described 3 a local nonconformity at the top of 

 the Colorado formation at Golden, Colo. 



In the present paper the term Colorado formation is accepted in its 

 restricted sense as defined by White and Eldridge. The species de- 



1 Ann. Rept. r. s. Geol. Sur. Terr, for 1870, pi-, l 00. 

 •Am. Jour.Sci..3d ser., vol. XXXVHT, 1880, p. 313 321. 

 •Bull. Philoe. Soc. oi Washington, vol. xi, 1890, p. 204. 



