22 GEOLOGICAL SURVEY OF THE TERRITORIES. 



only serve to show the views we entertained at that time after a careful 

 study and discussion pf the invertebate fossils, but incidentally the 

 opinions of other eniinent paleontologists : 



" 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 question 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 

 deposits have .yet been discovered in all the Eocky Mountain region of 

 *Nebraska, nor, so far as known, 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 

 ISrackish 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 remains. All the facts indicate a gradual 

 change from the marine conditions of the Cretaceous ; at first to brack- 

 ish, and then to the fresh-water conditions of the Tertiary. The pre- 

 dominance of Gasteropoda and Lamellibra^icJiiata, and the comjiarative 

 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 of this region, indicate that the waters were grow- 

 ing more shallow as the land on the east encroached on the sea, and 

 islands were rising where the Eocky Mountains now stand, while the 

 close of the Cretaceous period seems to have been attended by the grad- 

 ual 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 Cretaceous fauna. 



"After this, extensive tracts of country in the region of the Eocky- 

 Mountains, and east of them in Nebraska and other northwestern Ter- 

 ritories, were occupied by bays, inlets, estuaries, etc., of brackish water, 

 inhabited by Mollusca of the genera Ostrea, Unio, Pisidium, Gorbmda, 

 Potamomya, Mela7iia, Melampus, Vivijjara, etc., all of Tertiary types. 

 As the gradual elevation of the country continued, the salt and brack- 

 ish waters receded and gave place to lakes and other bodies of fresh- 

 water, in which most of the Tertiary rocks of the Northwest 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 

 member 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 clima,tic 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 portion of the species of fresh-water Mol- 

 lusca ranging through this whole lower member. 



" The principal difference between the fossils of its upper and lower 

 beds consists of the gradual disappearance of strictly brackish-water 

 types as we ascend from the inferior strata. The entire series of Ne- 



* The old Territory of Nebraska is here referred to. 



