MEDIEVAL OR MESOZOIC TIMES. 173 



CHAPTER II. 



MEDIEVAL OR MESOZOIG TIMES IN NEBRASKA. 



Absence of Deposits of the Triassic and Jurassic Periods. — Cause of this 

 Absence. — Length of these Periods in Nebraska. — Cretaceous Period. — How 

 it Originated. — Divisions of the Cretaceous. — Dakota Group. — Its Character, 

 Extent and Remarkable Flora — Origin of this Flora.— Climate of the Dakota 

 Group Epoch. — Fort Benton Group. — Its General Character. — Length of this 

 Epoch, and its Vegetable and Animal Life. — Niobrara Group Epoch. — Extent 

 of its Deposits, and General Character. — Vegetable Remains. — Animal life 

 of this Epoch. — Rhizopods, Mollusks and Fishes. — Reptiles, their Great 

 Abundance and Peculiar Character. — Final Disappearance of this Reptile 

 Fauna. 



Triassic axd Jurassic Periods. 



''INHERE are no known deposits of the Triassic and Jurassic 

 JL periods in Nebraska. The deposits of the next or Cretaceous 

 period rest directly on the Permian. Two explanations of this fact 

 are possible. First, the Triassic and Jurassic deposits may once 

 have been here, and were removed before the Cretaceous was laid 

 down by denudation. Or, second, this region may have been a land 

 surface during these periods. This latter view seems to be the most 

 probable, and best explains all the facts of this portion of our geo- 

 logical history. 



We have already seen that the Carboniferous Age was brought 

 to a close by an upward movement of the continent, and that this 

 movement continued through the Permian, until much of the pre- 

 vious water surface was drained, and made it impossible to preserve 

 the memorials of its latter history. The same events that prevented 

 the preservation of the memorials of the Permian, would, if con- 

 tinued, prevent the deposition of Triassic and Jurassic rocks. With 

 a large degree, therefore, of certainty, we may rest assured that 

 during these periods Nebraska was an extended land surface, and 

 if so, there must have flourished here for countless centuries the 

 peculiar vegetable and animal life of those times. 



Length of the Trio-Juro Periods. — The length of the Trio-Juro 

 periods can be ascertained only relatively. Not even an approxi- 

 mate estimate can be made, but all geologists admit that they were 



