^viiiTE.] RELATIONS OF THE CRETACEOUS GROUPS. 187 



tlic Tort Benton Gronp (Cretaceous ISTo. 2) and the Niobrara Group 

 (Cretaceous No. 3) in a single group, under the name of Colorado Group. 

 Also the consolidation of the Fort Pierre Group (Cretaceous No. 4) and 

 the Fox Hills Group (Cretaceous No. 5) under the single name of Fox 

 Hills Group. It is in this sense that the latter name will be used in all 

 references to the Cretaceous strata of Colorado and adjacent Territories ; 

 but for the Upper Missouri Eiver region it will continue to be used in 

 the restricted sense applied to it by its authors. This consolidation will 

 reduce the Cretaceous groups as recognized in Colorado and Territories 

 adjacent to three, the names of which are, in the ascending order, Da- 

 kota, Colorado, and Fox Hills Groups. 



The Dakota Group (Cretaceous No. 1) is so constant in its lithological 

 and paleontological characteristics over the great Western region as to 

 separate it distinctly from all the others. No species of any kind, so 

 far as I am aware, has been found to pass up from it into the Fort Ben- 

 ton Group or equivalent strata. In Colorado and Territories adjacent, 

 neither the lithological nor paleontological characterivstics of the equiva- 

 lents of the Fort Pierre and Fox Hills Groups, respectively, are such as 

 to afford any satisfactory ground for a separation, such as has been 

 made in the Upper Missouri Biver region ; and even in that region a 

 blending of the fossils of each has been frequentlj' found. Precisely 

 similar remarks may be made concerning the equivalents of the Niobrara 

 and Fort Benton Groups. Between the equivalents of these two groux)s 

 on the one hand and those of the Fort Pierre and Fox Hills Groups on 

 the other there is, however, a vrell-marked paleontological difference ; 

 in some places v^ith a corresponding lithological change, but in other 

 places with no change of the latter character to separate the two con- 

 solidated groui)s. It is to this fact that are largely due the discrepancies 

 between the surface limits assigned by different geologists to the Fox 

 Hills and Colorado Groups respectively in Colorado and Territories ad- 

 jacent, some of whom appear to have given little attention to the pale- 

 ontological characteristics of the strata they examined or ignored their 

 imjiortance in the grouping of strata. With this statement and defini- 

 tion of terms, I return to the consideration of the fossils of the foregoing 

 lists. 



Of the forty-two species or entries embraced in the foregoing notes, 

 fifteen are either at present uidcnown in Upper Missoiui strata, or they 

 are otherwise irrelevant in the comparison here proposed, between the 

 fossils obtained from the consolidated Fox Hills Group east of the Eocky 

 Mountains in Colorado, and those of the Fort Pierre and Fox Hills 

 Groups together, of the Upper Missoiui Eiver region. Twenty-six spe- 

 cies of that list were first described from either one or the other or both 

 of those groups in that region. In that northern region eight of these 

 species are common to both the Fox Hills and Fort Pierre Groups ; 

 five are confined, so far as knowTi, to the Fort Pierre Groui>, eight to 

 the Fox Hills Group, and five of them, not including any of the others, 

 are there knowii only in the uppermost strata of the Fox HiUs Group, 

 a series which Mr. Meek was at one time disposed to separate as a sub- 

 diAision of the Fox Hills Group i)roper, if not to make it a group coor- 

 dinate with the others. 



The strata of Fossil Ridge are among the lowest of the series in ques- 

 tion that found in Colorado east of the mountains, and they are no 

 doubt equivalent with the Fort Pierre Group of the Upper Missouri, 

 and yet they are plainly not separable from the Fox Hills Group in Col- 

 orado. The following is a statement of the relations of its list of thir- 

 teen species of fossils, as given on a previous page, with those of the 



