420 PROCEEDINGS OF THE ACADEMY OF 



Geologists will understand that we do not regard the several rocks to which 

 we have applied the names "Dakota Group, Fort Benton Group," &c, as being 

 always separately and individually recognizable at widely distant parts of the 

 world, nor even in all cases throughout North America. They are, merely, 

 convenient subdivisions, presenting more or less marked lithological and pa- 

 laeontological peculiarities, due to physical agencies that were probably, in 

 some instances, comparatively local in their action; though in other cases we 

 have been able to identify the equivalents of some of them, as we have shown 

 on former occasions, in New Jersey, Alabama and New Mexico.* When we 

 wish to draw parallels between these rocks and those of the Cretaceous System 

 of the Old World, however, we find it necessary to group them together, so far 

 as our present knowledge extends, into two, or at any rate, not more than three 

 principal series. 



Dakota Group. Although we still retain this as a distinct rock, our present 

 impression is, that it is probably only a subdivision or member of the Fort Benton 

 Group. Still, until more fossils can be obtained from it in the region of the typi- 

 cal localities, the question whether or not it should rank as a distinct formation 

 must remain an open one. That it is at least as old as the Fort Benton Group, 

 however, is clearly demonstrated by its position beneath some two hnndred feet 

 of that reck near the mouth of Big Sioux River, where the latter formation is 

 seen to pass beneath the Niobrara Division. This order of superposition is also 

 known to prevail throughout a considerable area in north-eastern Kansas, and 

 south eastern Nebraska, as may be seen at hundreds of localities where all the 

 strata lie in a nearly horizontal position. 



That this rock cannot be referred to any older epoch than the Cretaceous, is 

 also equally clear fromthe modern affinities of numerous fossil leaves embedded 

 in it. Amongst these remains Dr. Newherryand Prof. Heer, have identified the 

 genera Populux, Salix, Alnus, Platanus, Liriodendron, &c, &c.\ The few animal 

 remains yet found in it are merely casts of shells referable to the genera Pha- 

 rella, Axinaca, Mactra and Cyprina. 



Along the Missouri, in the region of the mouth of Big Sioux River, and below, 

 this rock consists mainly of yellowish and reddish sandstones, in rather thick 

 beds, interstratified, however, at places, with beds of yellow and ash-colored 

 clays, and impure lignite. In this region it is generally quite distinct from the 

 Fort Pierre Group above, but farther south, in Kansas, the two rocks seem to 

 be less distinct, while at the Black Hills and along the Rocky Mountains west of 

 there, the whole appears to be represented at places by a series of alternating 

 sandstones and clays. 



Below the mouth of Big Sioux River, this formation is seen at some localities 

 resting directly upon the limestones of the Coal Measures ; but in north-eastern 

 Kansas it usually reposes on a series of reddish and various colored clays, 

 probably of Jurassic age. Near the Black Hills, and along the Rocky Mountains 

 west of there, apparently representative beds were usually seen to be immedi- 

 ately underlaid by well marked Jurassic strata, excepting at the localities 

 where the brackish-water deposits already mentioned, appear to intervene be- 

 tween them and the Jurassic. J 



This rock has not yet been satisfactorily recognized on the upper Missouri, 

 though there is a similar formation near the mouth of Judith River and below 

 there, which we have sometimes thought may possibly represent it. Until we 

 can know more, however, in regard to the relations of the Judith River bed to 

 the other rocks of the series in that region, and have an opportunity to examine 

 more of its fossils, it would be unsafe to refer it to the Dakota Group. The 



See Proceed Acad. Nat. Sci., Phila., Nov., 1856, p. 267, and May, 1857. 



t Proceed. Acad., Dec, 1858 ; Am. Journ. Sci., New Ser., vol, xxvii, 1859 ; also March, 

 186D. 



\ This brackish-water bed we think probably belongs to the formation under consid- 

 eration. 



[Dec. 



