40 



and the few fossils found in it there seem to be nearly or quite all dis- 

 tinct from those occurring in the several known horizons, we have been 

 at a loss to decide in regard to its exact place in the Cretaceous section. 

 I have long suspected, however, from the affinities of its organic 

 remains, that its position is near the horizon of the top of the Fox 

 Hills group ; and although the illustrations of its fossils have been ar- 

 ranged by themselves on separate plates, in the unpublished report on 

 the invertebrate palaeontology of the Upper Missouri, I had, some time 

 back, numbered the plates of this work so as to bring those containing 

 the illustrations of the species from this rock, in the ascending series, 

 directly in after those on which the fossils from the Fox Hills group are 

 arranged. 



The correctness of this view, in regard to the position of these beds, 

 seems to be sustained by the Colorado collections under consideration. 

 This will be the more clearly understood by reference to the foregoing 

 list of species. This list, it will be seen, contains some seventeen deter- 

 mined species of Cretaceous molluscan remains, four of which (those 

 marked with an asterisk) are certainly identical with forms that have, 

 in the Upper Missouri, only been found in the rock mentioned at the 

 mouth of Judith Eiver ; while they are also very characteristic species 

 too of that rock and locality. At the same time, the other thirteen 

 identified species of Mollusca mentioned in the list, all occur at numer- 

 ous Upper Missouri localities in the Fox Hills group; and, with the 

 exception of some three or four of the species that also occur in the Fort 

 Pierre group below, they seem to be all highly characteristic species of 

 the Fox Hills division. 



From this blending- together of the Fox Hills species, and those of the 

 marine Cretaceous beds found at the mouth of Judith Eiver ou the 

 Upper Missouri, it is evident, I think, that we cannot be far wrong in 

 regarding the latter beds as holding a position at the horizon of the top 

 of the Fox Hills group. Whether they may, however, in the Upper 

 Missouri country, be distinct enough from the Fox Hills group to form a 

 sixth subdivision of the Cretaceous series, holding a position just above 

 the latter, or whether they ought rather to be regarded as merely an 

 upi)er member of the Fox Hills group, may admit of some doubt in the 

 present state of our knowledge ; though I strongly incline to the latter 

 opinion. It is true, however, that they might really be properly distinct, 

 as a subdivision of the Cretaceous, from the Fox Hills group, and still 

 be so intimately related to the latter that some of their characteristic 

 species of fossils might range down into the same at the Colorado local- 

 ities (just as some of the Fox Hills types also occur in the Fort Pierre 

 group below, at many localities), without necessarily proving that these 

 two subdivisions should not be treated as distinct rocks. 



In another point of view, the collections under consideration will 

 doubtless prove- to be of some interest — that is, from their bearing on 

 the mooted question respecting the age of the Brown-coal formation 

 aloug the eastern base of the Eocky Mountains, in Colorado, and at 

 other localities farther westward. Not having myself visited the par- 

 ticular localities at which these fossils were collected, I, of course, cannot 

 speak from personal observation in regard to the stratigraphical rela- 

 tions of these marine Cretaceous beds to the brackish-water coal-bear- 

 ing strata alluded to ; though I am assured by the gentlemen who 

 collected the specimens that, although they came from the very ui)X)er 

 beds of well-defined marine Cretaceous, they, nevertheless, in all cases, 

 hold a position below the horizon of all the coal-bearing strata of that 

 region; although, in some instances, they were found not far below the 



