INTRODUCTOKY REMARKS. 11 



no Ammonites, ScapUtcs, Baculites, Eamites, TurrilUes, Ilelicoccras, or any 

 of tlie numerous other types generally believed to have died out at tlie 

 close of tlio Cretaceous period (excepting the very few forms already 

 mentioned), would appear to indicate that this formation was deposited at 

 near the close of the physical conditions most favoralde to the existence of 

 types of life peculiar to the Cretaceous epoch; and hence that it belongs to 

 some of the latest, if not to the latest, deposits of that period.* 



The fossils from the Bear River fresh- or brackish-Avater strata, and 

 those from later and undoubted Tertiary beds, have been illustrated 

 together on plates 16 and 17. The species from these different horizons 

 might more appropriately have been separated on distinct plates ; but this 

 could not well be done without arranging the figures on three plates; while 

 there are not quite enough of them to be so distributed. Those from the 

 oldest upheaved brackish-water beds associated with the light-colored 

 marine Cretaceous sandstone already mentioned, at the Bear River locality, 

 are Unio prisms, U. hellipUcatus, and Corhicula Diirkeei of plate IG; and Cor- 

 hida Engelmanni, C. pynformis, Limnma nitida, Rlujtipliorus priscus, Campe- 

 hma {Mclantho) occidentaUsf, C. macrospira, Vivipams Conradi, and Punjii' 

 lifcra hmierosa of plate 1 7. 



This fonnatlon seems, at Bear River, to be associated with the Creta- 

 ceous sandstone of that region in such a manner as to give the Impression 

 that it probably immediately succeeded the latter in point of time. It Is 

 evident, however, from the striking contrast In the fossils characterizing the 

 two formations, that marked physical changes had taken place here between 

 the deposition of the last of the undoubted Cretaceous sandstones and the 

 commencement of what appears to be the oldest Tertiary, since nearly or 

 quite all the fossils found in the former are marine types, while those of the 

 latter are fresh- and brackish- water, or, in some few instances, perhaps terres- 

 trial forms. This strongly-marked change in the fossils in passing from 

 tlie Cretaceous sandstone below into the fresh- and brackish-water beds 

 apparently above, and the close affinities of most of the S[)ecles in the latter 

 * Later personal cxaiiiiuations of tliese beds at Coalville, and tbe equivalent 

 marine Cretaceous strata at Bear River, as stated in Dr. Hayden's Report of 1872, 

 liave satisfied me that this formation, at the former locality, represents probably nearly 

 the whole uf the Upper Jlissouri (Jietaceons seiies.— F. B. M., Nov., lS7t. 



