THE GEEEN EIVER COAL BASIN. 465 



why tliey would contain none of tlie mollusks, usually regarded as char- 

 acteristic of the Cretaceous, since the latter are, almost without exception, 

 marine forms, while these beds were evidently deposited in brackish waters. 

 Still, this would not account for the fact that among a considerable number 

 of specimens, found ranging through some hundreds of feet of the marine 

 sandstones, immediately below the brackish- wa+,er beds of Utah, strictly 

 Cretaceous, or older types, are only known to be represented by the genus 

 Inoceramus and possibly Anchura. Again, most of the mollusks yet known, 

 fi-om these brackish-water beds, at the several widely separated (and some 

 intermediate) localities, seem to be nearly all either very closely allied to 

 species found at the base of the Eocene, or lower lignite beds of France, or 

 to existing East Indian or Chinese species.^ Several of the species of Vivi- 

 para, found in the Judith River estuary beds, are exceedingly like existing 

 East Indian and Chinese species, while some of these, and one "or two of 

 Melantho^ as well as other associated forms, can scarcely be distinguished 

 from species found in the higher fresh-water deposits of Dakota, acknowl- 

 edged by all to be Tertiary, not only from position and from the affinities of 

 their animal remains, but from the affinities of the numerous plants found in 

 the same. 



The probability is, that toward the close of the Cretaceous epoch, this 

 region was gradually rising, and that portions of the Rocky Mountains, and 

 other elevations farther west, existed as islands surrounded by the sea, and 

 that as the gradual elevation of the continent continued, considerable areas 

 that had been entirely occupied by the sea were more or less isolated, so as 

 to become at first brackish and then fresh-water lakes. Whether or not this 

 change from marine to estuary conditions was exactly contemporaneous with 

 the close of the Cretaceous and the commencement of the Tertiary of 

 Europe, we may perhaps never know, but that it corresponded in the 



^ It is a very interesting fact that we have from these brackish- water beds in Utah 

 several of those sub trigonal corbiculas, very similar to C.foriesii, C cuneiformis, C. anti- 

 qua, and other forms scarcely distinguishable from species from the lower lignites of the 

 Paris basin ; and a Tiara equally as nearly allied to a form figured by Matheron from 

 the same horizon at the mouth of the Ehone. The most remarkable fact in this con- 

 nection is, that the genus Tiara, although represented by species now living in the 

 East Indies, has no representatives among the existing fauna of America. 

 59 



