WHITE.J MESOZOIC AGE. ■ 31 



tegrated and eroded has produced a marked efifect upon the topography 

 of the region in which they exist. Where the strata are considerably 

 flexed, the space occupied by those of this group is a hollow or valley 

 between lines of hogbacks which are composed of the harder strata that 

 belong above and beneath this group. But where the flexure is more 

 gentle, and especially upon a broad anticlinal axis, the strata of this 

 group are often so eroded as to form a basin or park, such as have been 

 described in Chapter II. 



Near the base of the Colorado Group there is almost always to be found 

 a bed of dark, fissile shales, containing the remains of teliost fishes. This 

 bed not only occurs at all places in this district where the base of the 

 Colorado Group is exposed, but it is equally characteristic of the lower 

 part of the group in the adjoining districts. Overlying the bed of shales 

 containing the remains of teliost fishes, there is often to be found a 

 bed of coal. The only place in this district that I observed this bed of 

 coal is a couple of miles south of the west end of Lily's Park, and south- 

 ward from Junction Mountain. It is there three or four feet in thickness. 



In this district the strata of the Colorado Group occupy the surface of 

 Agency and Eaven Parks; of Midland Basin ; of, at least the eastern half 

 of. Axial Basin, and also a considerable space bordering the mountain 

 uplifts of the northwestern portion of tlie district, 



THE FOX HILLS GROUP. 



In accordance with the modified classification of the Cretaceous strata 

 adopted in this report, the Fox Hills Group includes not only the strata 

 that are understood to be exactly equivalent with those of that group, 

 as it was originally defined by Meek and Hayden, in the Upper Missouri 

 Eiver region, but also those that are equivalent with the Fort Pierre 

 Group, as it was originally defined in that region. As already stated, 

 Mr. King does not on his geological map include with the Fox Hills 

 Group those strata that he, as well as all other geologists, regards as 

 equivalent with the Fort Pierre Group ; but he includes the latter in the 

 Colorado Group, together with the equivalents of the Niobrara and 

 Fort Benton Groups. In the eastern portion of this district, and also to 

 a less extent in other portions, the strata that I regard as probably 

 equivalent with the Fort Pierre Group are more readily eroded and dis- 

 integrated than are those which overlie them, and constitute there 

 the upper portion of the Fox Hills Group. In this respect they more 

 nearly resemble in general aspect the strata of the group beneath than 

 those above them; but for paleontological reasons, as before stated, I 

 include the equivalent strata of the Fort Pierre with those of the Fox 

 Hills Group, designating the whole by the latter name alone, rather 

 than place it with the Colorado Group, as Mr. King has done, appar- 

 ently on lithological grounds. 



The fossils obtained from the equivalents of the Fort Pierre and Fox 

 Hills Groups are not only so similar as to ally them closely with each 

 other, but many of the species range through the whole series of strata 

 that are understood to represent both of these groups. It is probable 

 that some of the fossils of the Colorado Group range up through the 

 remainder of the Cretaceous series; but, so far as the fossils of that 

 group are now known, they seem to be so generally restricted to its 

 own limits as to give it quite definite paleontological characteristics,. 

 and to leave the Fox Hills Group quite as clearly characterized. 



In the western portion of this district the lithological difference be- 

 tween the upper and lower portions of the Fox Hills Group is not so 



