"CERATOPS beds" OF WYOMING AND MONTANA 28 1 



time there must have been local subsidence about equalling the rate of 

 deposition, because these thick formations seem throughout to have 

 been formed near sea level. Locally, as in part of Bighorn Basin, this 

 non-marine sedimentation may have been almost continuous until the 

 end of the Cretaceous, but over most of the area there was a more 

 important subsidence which brought the marine sediments represented 

 by the Lewis and Bearpaw shales over the coal-bearing formations 

 above-mentioned. Again the uplift was resumed and there was 

 another transition from marine to land conditions. In some areas 

 this was gradual with alternations of brackish- and fresh-water beds 

 through a considerable thickness, as in southern Wyoming. There 

 is a similar transition with less fresh-water sedimentation in the 

 Laramie of the Denver Basin, where marine conditions had prevailed 

 continuously since Benton time, and there is such a transition beneath 

 the " Ceratops beds" of Converse County. In other areas the change 

 is more abrupt and the marine rocks are followed directly by land and 

 fresh-water deposits. 



The idea seems to be still prevalent that this latest Cretaceous sea 

 was uplifted as a whole, its direct intercommunication with the ocean 

 cut off, and its waters then gradually freshened so that its strictly 

 marine animals were killed while those that could endure brackish 

 waters survived. It has even been suggested that the change was 

 so gradual that the brackish-water fauna may have adjusted itself 

 to the changing conditions and continued to live after the waters 

 became entirely fresh. Such a history seems to be implied in the 

 statement of F. V. Hay den quoted on p. 250, and similar statements 

 may be found in the writings of C. A. White. There is no evidence, 

 however, in favor of this view. As far up in the series as brackish- 

 water fossils are found they occur in usually thin beds intercalated 

 amongst the fresh-water strata, showing that the two elements of the 

 fauna had separate habitats. There is no more admixture of the 

 two kinds of forms than is to be expected where slight oscillations of 

 level alternately bring them over the same area and where currents 

 may easily carry the fresh-water shells into brackish or even marine 

 waters. "The intimate association of brackish- and fresh- water 

 forms" may be found in printed lists and possibly in museum collec- 

 tions but it does not exist in the rocks, and this is as true of the brack- 

 ish-water shells of the highest horizon at which they occur in the Inte- 



