198 GEOLOGY. 



brownish sandstone, underlaid by dark gray plastic clay, calcareous 

 shales, sometimes containing sulphuret of iron, and more rarely 

 carbonaceous matter. A large amount of gypsum is present, which 

 often has the form of selenite. The star-like shapes which it fre- 

 quently assumes, makes it desirable for cabinets. The masses of 

 selenite scattered over these deposits, on the Missouri" bluffs, beyond 

 the Niobrara, has given them the name of shining hills. From the 

 occasional presence of scales of fishes, and still more rarely of am- 

 monites and other chambered shells, I conclude that only the lower 

 member of this group is present in Nebraska. 



On the Upper Republican this group in many places lies beneath 

 the Tertiary, and can only be seen in cuts and canyons, and the 

 sides of bluffs and ravines. It almost certainly extends from near 

 the mouth of the Niobrara in a southwesterly direction across the 

 State. Passing beneath the Tertiary, it is not seen again until the 

 western Republican region is reached in Hitchcock and Dundy 

 counties. It runs, therefore, proximately parallel to the Niobrara 

 group, and on its northwestern side. 



Clarence King unites this group with the preceding Niobrara 

 and Fort Benton group, to constitute the Colorado group. Hayden 

 and White, on the other hand, attach it to the next above, or Fox. 

 Hills group. King's reason for this reference is lithological. That 

 is, in the character of its rocks and other deposits it is much like 

 the preceding groups. Hayden and White refer it to the Fox Hills 

 groups or palasontological grounds, its animal life being more nearly 

 like that of the next era. 



The Fort Pierre sea that extended diagonally across the State 

 from the mouth of the Niobrara and beyond represents a depression 

 left or made after the elevation of the Niobrara group area above 

 the old oceans. As already intimated, it is questionable whether 

 this interior sea of Nebraska was connected with the ocean, except 

 for a very brief period. The rarity of organic remains in this ter- 

 ritory in this group is indicative of that unfitness for life which 

 characterizes a sea that is losing more water by evaporation than it 

 gains. Gypsum, which is so abundant in this deposit, is also formed 

 under the same circumstances. Hence the vegetable and animal life 

 that it here at first possessed gradually but surely was exterminated. 



Elsewhere, however, it was very different. While this region 

 was steadily rising, further northward and westward it was for a 

 time slowly sinking, and had direct communication with the ocean. 



