GEOLOGICAL SURVEY OF THE TERRITORIES. 377 



Camptonectes hellistriata^ with beds at the Eed Buttes on North Platte, 

 and near the southwest base of the Black Hills, containing well-marked 

 Jurassic forms ; while the other forms found associated with this Camp- 

 tonectes are quite unlike species found in any other than the Jurassic 

 beds of the far West. 



The Cretaceous fossils of the collection are, like the others, generally 

 in a bad state of preservation. Those from Coalville, however, are quite 

 interesting, because they come from very near the junction of apparently 

 the Upper Cretaceous and the Lower Tertiary, and seem to consist of a 

 mixture of Cretaceous and Tertiary forms, or at least of species most 

 nearly allied to forms belonging to these horizons; while a few of them 

 appear to be fresh and brackish water types, directly associated with 

 InoceramuSj Ostrea, Anoniia, Uuspira, or JSfatica, and other marine tyi)es. 

 The univalves are unfortunately too much broken and imbedded in the 

 hard, gritty matrix to show clearly the forms of their apertures ; but some 

 of them seem to resemble very closel}^ species of Goniohasis found in the 

 fresh- water Tertiary beds of that region. These, however, may possibly 

 prove to belong to some marine genus. There are also among them 

 some very imperfect specimens that have much the general appearance 

 of Melanopsis, and a bivalve very like Corhicula cytlieriformis^ M. & H. 

 Without a more careful study, and the devotion of more time to work- 

 ing these specimens out of the matrix than circumstances will just now 

 permit, it would be unsafe to speak positively in regard to the af&nities 

 of the species that do not seem to be strictly marirus types. If any of 

 them are fresh or brackish water forms, of course they must have been 

 carried by streams, or other agencies, from their proper stations, and 

 deposited in neighboring bodies of salt-water in which the marine forms 

 lived and died. It might be urged, however, that the deposit is really 

 a brackish- water Tertiary formation, of the earliest Eocene age, and that 

 the Cretaceous marine forms belong properly to accidentally inter- 

 mingled fragments of Cretaceous beds. The nature of the matrix and 

 the exactly similar state of preservation of all of these fossils, as well as 

 the proportionally larger number of the marine species, seem to me to 

 show that they really belong to one and the same geological formation 

 and lived at the same time. 



I can see no good reason why there might not have been living in the 

 streams and estuaries of the closing period of the Cretaceous age, and 

 while Cretaceous types were still existing in the seas, a few fresh and 

 brackish water species that continued to live and multiply during the 

 earlier part of the Tertiary age. It is evident, however, from these and 

 other collections brought by Mr. King from these beds, that there is a 

 gradual passage from the Upper Cretaceous into the Lower Tertiary in 

 this region ; and that, unless the fossils from each subordinate seam or 

 layer are kept carefully separated, and very minutely detailed local sec- 

 tions are taken, it will not always be easy to determine, at localities 

 where the two groups meet, exactly where the line should be drawn 

 between the Cretaceous and Tertiary ; or, in other words, to separate, 

 in all cases, the Cretaceous from the Lower Tertiary forms collected at 

 such places. 



All the undoubted Tertiary species in the collection jire fresh and 

 brackish water types of the oldest Eocene age, being s})ecies previously 

 known from rocks of that age in this region. 



In addition to the few new species that are here indicated, there are 

 some other undescribed si)ecies in the collection that I have not yet had 

 time to study with suilicient care to determine their ailinities; and 

 these have been lelt for future consideration. 



