INTRODUCTOEY REIMARKS. 33 



conditions in this region had taken place a Httle before, instead of exactly at, 

 the close of the Cretaceous period, this change alone would be amply suffi- 

 cient to account for the destruction of the marine Cretaceous forms. Still, 

 this would not account for the paucity of strictly Cretaceous types here 

 through a considerable thickness of marine sandstones below, nor for the 

 striking Tertiary and more modern affinities of several of the forms in the 

 estuary-beds above.* 



While willing to admit that facts may yet be discovered showing that 

 some of the brackish-water beds so widely distributed in this internal region 

 of the continent belong more properly to the Cretaceous than to the Ter- 

 tiary, I still think, from all the light we now have on the subject, that the 

 Bear and Judith River fresh- and brackish- water deposits represent the oldest 

 Eocene Lignites of the Paris basin. At least, if they are Cretaceous, there 

 is little or nothing in the molluscan remains yet obtained from them to sup- 

 port such a conclusion, f 



The probability is, as I have elsewhere remarked, that, as the continent 

 was rising, toward the close of the Cretaceous epoch, the Rocky Mountains, 

 in part at least, existed as islands in the Cretaceous Sea. Still later, as the 

 process of elevation continued, considerable areas that had been occupied 

 by the sea became at first partly isolated, so as to form bodies of brackish 

 water, that gradually became fresh-water lakes, as further elevation com- 

 pletely isolated them from the influx of the sea. In the deposits formed in 

 these waters, we might natui'ally expect to find, at the base, brackish-water 

 types, and, further up, wholly fresh-water forms, just as is the case in the 

 beds referred to the Tertiary in that region. 



Whether this change from marine to fresh- water conditions was exactly 

 contemporaneous with the close of the Cretaceous, and the introduction of 

 the Tertiary epochs elsewhere, perhaps we shall never know; but that it 



* For iustance, compare Corbicida DurJceei (fig. 6 a-g, plate 16) with G. antiqiia, 

 Ferrussac, and C. Forbesii, Desbayes, from the Lower Lignites of the Paris basin, as 

 well as Pi/rguli/era humerosa (figs. 19 and 19 a, plate 17) with the so-called Melania 

 armata of Matheron, from the same horizon at the mouth of the Rhone. 



t These remarks were written in 1870; since that time, however, additional facts 

 have come to light, as already suggested, rather favoring the conclusion that these 

 Bear River estiiary-beds may belong to the latest Cretaceous. — F. B. M., "Nov., 1874. 



