872 BULLETIN UNITED STATES GEOLOGICAL SURVEY. 



Its general lithological characteristics are similar to those of the Fox 

 Hills Group, a known marine formation, but its fauna, as has been shown, 

 is mainly of brackish-, but partly of fresh-water origin, and not marine. 

 Furthermore, the brackish-water species are distributed throughout 

 its entire thickness and its whole geographical extent. These facts, 

 together with the absence from all the strata yet examined of any true 

 estuary characters, show that the Laramie Group was deposited in a 

 great brackish-water sea. This being the case, it must have received 

 its peculiar character as well as its boundaries by having been sepa- 

 rated from the great open sea by an encircling elevation of land. The 

 final act of the inclosing movements was the elevation of land at both 

 the northern and southern end of the intercontinental Mesozoic sea, 

 which connected the two great continental factors, so that that sea 

 became a land-locked one, without material change of its status in its 

 principal portion as regards the continued accumulation of sediments 

 upon its bottom. 



Whether the brackish saltness of the Laramie Sea was sustained 

 throughout the period by limited communication of its waters with 

 those of the great open sea, or whether such communication was 

 entirely cut off and the supply of salt, above that which was retained 

 of its original marine saltness, came by adjacent continental drainage 

 in amount suflicient to balance the waste by overflow, can probably 

 never be known, but the latter seems probable. If the former condi- 

 tion existed, one of the places of communication was no doubt at the 

 southeastern border of the Laramie Sea, and some fortunate exposure 

 of strata* in the region between Western Kansas and the Gulf of Mexico 

 may yet reveal the true relations of the Laramie Group with the Cre- 

 taceous and Eocene deposits of the Gulf border. If tide-level com- 

 munication between the Laramie Sea and the open ocean was entirely 

 cut off, as there is much reason to believe it was, the question of such 

 relationship or contemporaneousness of deposition must ever remain 

 an open one. 



It is evident that the movements which caused the inclosure of the 

 Laramie Sea did not materially interrupt the continuity of sedimenta- 

 tion within at least a very large part of its area, although the effects of 

 those physical changes were such as to cause a total change in at least 

 the molluscan fauna. The wide geographical distribution and great 

 vertical range of many of the molluscan species of the Laramie Group, 

 and the great uniformity of its lithological characters, show that the 

 period was one of comparative quiet within the region which was occu- 

 pied by its waters. There were, however, some comparatively slight 



* In Professor Powell's Report on the Geology of the Uinta Mountains, aud in the 

 American Journal of Science, vol. xi, 3cl series, p. 161, 1 airuouuced, on the authority of 

 Professor Powell, the existence of marine Tertiary fossils in the strata of the valley of 

 Bijou Creek, 40 miles east of Denver, Colo. A personal examination of that region 

 in 1877 failed to confirm that reported discovery, as I have shown in my report for 

 that year. See An. Rep. U. S. Geol. Surv. Terr, for 1877. 



