104 GEOLOGICAL SUEVRY OF THE TERRITOEIES. 



ter of these upper beds has caused them to be rather poorly exposed, 

 and the mountain work left not sufficient time for their i^roper study. 



Before turning to the consideration of the overlying beds it will be 

 well, now that their lithological characters have been described, to ex- 

 amine the reasons for separating the preceding beds into the divisions in- 

 dicated, viz, the Triassic, Jurassic, and Cretaceous, IsTos. 1, 2, 3, 4, and 5. 



In 1869 Dr. Hayden passed along the base of the mountains, and in 

 his report for that year described these beds at the many points at which 

 he touched them. Having traced them from regions where they were 

 j typically developed he easily assigned them to their proper horizons, 

 and my classification is but a reflex of his. The reasons for his classifi- 

 cation he also gives, but at scattered points through the report. I hope 

 I may be pardoned, therefore, if I undertake to throw the evidence which 

 bears on the age of these particular beds in Colorado into a little more 

 definite form. 



The Cretaceous age of the groups numbered 1, 2, 3, 4, and 5 cannot be 

 questioned. 



From Nos. 3 and 4 especially the characteristic Biocerimi, Ostrece, hacu- 

 liies, sca/pJdtes, ammonites, &c., determine their Cretaceous age conclu- 

 sively. The western Cretaceous was- first satisfactorily studied by Dr. 

 F. y. Hayden and Mr. F. B. Meek, in the Upper Missouri country, and 

 afterward by the former in the Black Hills, and five well-marked divis- 

 ions were there clearly made out, having well-defined Jurassic below 

 and Tertiary above. 



These divisions, with their lithological and paleontological characters, 

 were first given in the proceedings of the Academy of Natural Sciences 

 of Philadelphia, May, 1857, and subsequently reproduced in tabular 

 form in Dr. Hayden's first annual report, 1867, reprint, page 49. They 

 may be exi)ressed briefly as follows : 



General section of the Cretaceous rocks of NebrasTca. 



No. 5. Fox Hill heds. — Gray, ferruginous, and yellow sandstones, and arenaceous 

 clays. , 



No. 4. Fort Pierre groups. — Dark-gray and bluish plastic clays. 



No. 3. Niolrara division. — Lead-gray calcareous marls, and massive layers of chalky 

 limestone. 



No. 2. Fort Benton group. — Dark-gray or black laminated clays, sometimes with lime- 

 stones near top and sandstones near base. 



No. 1. Dakota group. — Yellowish, reddish, and occasionally white sandstones, with, 

 at places, some clayey beds and dicotylodenous leaves. 



The general resemblance between these five divisions and the cor- 

 resi3onding ones in Colorado, but for the great distance separating the 

 two localities, would naturally suggest their direct co-relation and 

 identity. 



Dr. Hayden has, however, directly traced them, except some easily- 

 passed gaj)s, across the intervening region, thus establishing their 

 identity. 



In his third annual report, 1869, page 121, he says : 



Quite marked changes occur in the sediments of these divisions in different parts of 

 the West, but by following them continuously, in every direction, from their typical 

 appearance on the Upper Missouri, the changes are so gradual that I have never lost 

 sight of them for a mile, unless concealed by more recent deposits. 



Again, page 114, in speaking of the Cretaceous in Colorado, he says : 



At various localities all along the margin of the mountain ranges these divisions of 

 the cretaceous are far less distinctly separated, and vary more or less in their structure 

 and composition, and yet in tracing them carefully and continuously from the Mis- 



