200 GEOLOGY. 



ditional species of chambered shells there were seven of Heteroceras 

 and one Placenticeras; This last was a form of exceptional beauty. 

 P. Placenta was equally fine. The three partially uncoiled scaphites 

 ;show the beginning of a return to the original form of chambered 

 shells. The three species of Nautilus are as perfect as the finest 

 from existing seas. Fish life was abundant. Reptiles were pres- 

 ent, but not in such amazing numbers as in the preceding era. The 

 commonest kind being species of Mosassaurs. 



This era was closed by a further elevation of the country in Ne- 

 braska, on the Upper Missouri and wherever this group now con- 

 stitutes the upper rock of a region. 



THE Fox HILLS GROUP. 



No deposits of this group are exposed in Nebraska, and it is un- 

 certain whether any exist in the State. If they are present, they 

 underlie the Tertiary in the northwestern part of the State. As 

 this group constitutes the surface or upper rock in the Fox Hills, 

 from which it was so named by Hayden, above Moreau River, on 

 the upper Missouri, and near Long Lake, above Fort Pierre, it is 

 possible that it also runs in a southwesterly direction, and underlies 

 the Tertiary in northwestern Nebraska, as stated above. In doing 

 so it would follow the law of the preceding groups in Nebraska, 

 each of the newer following after the preceding on its northwestern 

 side. This group is also found along the base of the Big Horn 

 Mountains, on the North and South Platte Rivers, and at other 

 points in the mountains. Its thickness is about five hundred feet. 

 (Meek and Hayden.) It is largely composed of gray ferruginous 

 and yellowish sandstone and arenaceous clays. During the depo- 

 sition of these deposits, the greater part of Nebraska *was an ex- 

 tended land surface. Nebraska doubtless drained into this Fox 

 Hills sea, but the sediments that filled it up were derived mainly 

 from land surfaces on the west and north, as is indicated by their 

 character. That it was also a long period, is evident from the 

 thickness of the deposits 500 feet in the region of the upper Mis- 

 souri. According to Clarence King, (Systematic Geology, p 349), 

 east of the North Platte and north of the Union Pacific Railroad 

 in Wyoming, its maximum thickness is 4,000 feet. At the most 

 rapid rate of deposition, the time involved in laying down such a 

 mass of sediment is beyond calculation. According to Hayden, 

 Meek and Lesquereux and others, it was the closing portion of 



