432 PROCEEDINGS OF THE ACADEMY OE 



and e, of the New Jersey Section, belong to the horizon of the Upper or White 

 Chalk and Maestricht Beds of Europe. 



TERTIARY ROCKS. 



It would extend these remarks beyond the limits assigned them, to attempt 

 any detailed account of the Tertiary rocks of Nebraska, or to discuss at length 

 the questions respecting their relations to those of the Atlantic coast, or of the 

 Old World. We must therefore limit ourselves here to a few brief statements 

 of leading facts, and leave all details for another occasion. 



In the first place, we would remark, that no strictly marine Tertiary depo- 

 sits have yet been discovered in all the Rocky Mountain region of Nebraska, 

 nor, so far as we know, in any other portion of Nebraska, Kansas or Utah. 

 Throughout all this great central area of the Continent, wherever the oldest 

 Tertiary deposits have been seen, they give evidence of fresh and brackish 

 water origin ; and where observed resting upon the most recent Cretaceous beds, 

 the two have been found conformable, and sometimes blended together, so as 

 to render it difficult to draw a line between them, in the absence of organic re- 

 mains. 



All the facts indicate a gradual change from the marine conditions of the 

 Cretaceous at first to brackish, and then to the fresh water conditions of the 

 Tertiary. The predominance of Gasteropoda and Lamellibranchiata, and the 

 comparative paucity of types usually considered characteristic of deeper water 

 deposits, as well as the coarser nature of the sediments, near the end of the 

 Cretaceous epoch in this region, indicate that the waters were growing more 

 shallow as the land on the east encroached on the sea, and islands were 

 rising where the Rocky Mountains now stand ; while the close of the Creta- 

 ceous period seems to have been attended by the gradual elevation of large 

 areas of country here above the ocean level. This and other contemporaneous 

 changes of physical conditions, caused the total destruction of the whole Cre- 

 taceous fauna. 



After this, extensive tracts of country in the region of the Rocky Mountains, 

 and east of there in Nebraska, and other North-western Territories, were oc- 

 cupied by Bays, Inlets, Estuaries, &c, of brackish water, inhabited by mol- 

 lusca of the genera Oztrea, Unto, Pisidium, Corbicula, Potamomya, Melania, 

 Melampus, Vivipara, &c. &c, all of Tertiary types. As the gradual elevation 

 of the country continued, the salt and brackish waters receded, and gave 

 place to Lakes and other bodies of fresh water, in which most of the Tertiary 

 rocks of the North-west were deposited ; so that in all, excepting the earliest 

 Tertiary beds of this region, we find only the remains of strictly fresh water 

 and terrestrial animals. 



The passage from the brackish to the fresh water beds in the oldest mem- 

 ber of the Tertiary of this region, seems not to be marked by any material 

 alteration in the nature of the sediments. Nor have we, so far as is yet known, 

 any reasons for believing that any climatic or other important physical changes 

 beyond the slow rising of the land, and the consequent recession of the salt 

 and brackish water, took place during the deposition of the whole of the oldest 

 member of the Tertiary here ; since we find a considerable proportion of the 

 species of fresh water mollusca ranging through this whole lower member. 

 The principal difference between the fossils of its upper and lower beds, con- 

 sists in the gradual disappearance of strictly brackish water types, as we as- 

 cend from the inferior strata. 



The entire series of Nebraska Tertiary rocks consists of three or four groups, 

 three of which at least, (and probably four), evidently belong to separate and 

 distinct epochs. They usually occur in isolated basins, but have with one 

 exception, all been seen in such connection as to leave no doubts in regard to 

 their order of superposition. Their prevailing lithological characters, esti- 

 mated maximum thickness, and order of succession, will be seen in the sec- 

 tion given below. 



[Dec. 





