188 GEOLOGY. 



THE NIOBRARA GROUP EPOCH. 



A still further subsidence of the continent, especially towards the 

 north and west, inaugurated the Niobrara Group Epoch. Hayden 

 gave it this name because of the great development of its deposits 

 below the mouth of the Niobrara in northeastern Nebraska. Here 

 its deposits consist of an impure chalk rock, varying from a gray- 

 ish white to a pink bluish and yellow hue. Below the mouth of 

 the Niobrara many of the chalk bluffs are several hundred feet 

 high, with a perpendicular face often excavated beneath by atmos- 

 pheric agencies. These chalk rocks are seen through Knox, Cedar, 

 in many places in Dixon County, and in places on the lower Re- 

 publican. Elsewhere the deposits, especially those beneath the 

 stratum of chalk, are mostly of an impure limestone, which often 

 shade imperceptibly into a silicate of lime. This stratum is often 

 called the Inoceramus bed, from the immense numbers of this mol- 

 lusk which frequently compose it. Under the Inoceramus bed 

 there is in many places toward the southwest, a stratum varying 

 from a few inches to fifteen feet in thickness, of an impure, yellow- 

 ish, silicious limestone. According to Prof. Mudge, it is the char- 

 acteristic feature of this group in Kansas. It can be observed at 

 Milford, in Seward County, in places in Harlan County, and at 

 many other . points between these stations. Lately a chalk bed of 

 this deposit was found near Red Cloud, in the Republican Valley. 

 It is pure white, soft, easily worked, and contains little besides car- 

 bonate of lime and a small amount of iron carbonate, but not 

 sufficient to color it. Judging from microscopic and chemical tests, 

 it is as pure as the best European chalks. 



The Niobrara is the most widely extended of all the Cretaceous 

 groups in Nebraska. In southern Nebraska, from the western line 

 of the Dakota Group to Harlan County where it is overlaid by 

 the Pliocene, it is over 100 miles wide. In north Nebraska, from 

 Dakota County where it begfns to overlie the Dakota Group, it 

 extends westward for over 150 miles. In general, the area on the 

 geological map marked Cretaceous is all Niobrara Group, except a 

 border from sixty to one hundred miles wide on the eastern rim, 

 from the Omaha Reservation southward, which mainly belongs to the 

 Dakota Group. As before intimated, it was mostly a period when 

 deep seas overspread a large part of the area now covered by its 

 deposits. Southeastern Nebraska was also a land surface during 



