464 MINING INDUSTRY. 



The above-mentioned facts, and the general conformability of these es- 

 tuary beds to the marine sandstones of that region, referred to the Creta- 

 ceous, and their apparent unconformability to later deposits in the same coun- 

 try referred to the Tertiary, may suggest the inquiry whether we ought not 

 to carry up the line between the Cretaceous and Tertiary here, so as to in- 

 clude these estuary beds also in the Cretaceous. This view would certainly 

 seem to receive some support, from the fact that there is a formation on the 

 Upper Missouri, near the mouth of Judith River, the exact age of which has 

 long been regarded as somewhat doubtful, though Dr. Hayden and the writer 

 have generally placed it in the Tertiary, that contains an exactly similar 

 brackish-water group of fossils, some of which are identical with those found 

 in these Bear River estuary beds ; while some Saurian remains, discovered 

 by Dr. Hayden at the Judith River localities, and at least supposed to be 

 from the brackish-water beds, are regarded by Professor Leidy as being most 

 nearly allied to Wealden forms. 



The strata of the Judith River locality are upheaved and confusedly 

 mingled together, and I have always thought that the Saurian remains found 

 there may belong properly to undoubted Cretaceous strata that exist there 

 beneath the estuary beds, rather than to the latter. 



In studying Colonel Simpson's collections from the Bear River region, in 

 1860, I also identified an oyster from the marine coal-bearing sandstones 

 there, referred to the Cretaceous, and holding a position directly beneath 

 the estuary beds, with O. glabra, M. & H. brought in, enveloped in an 

 exactly similar matrix, from near the mouth of Judith River, and stated that 

 the last-mentioned specimens, also, probably came from the Cretaceous. Mr. 

 Pope likewise informed me that some Saurian remains, found by Dr. Hayden 

 on the upper branches of Moreau River, in Dakota Territory, and supposed to 

 be associated with some Corbiculas, and one or two other brackish- water 

 shells, are Cretaceous types. 



Supposing the estuary deposits, at all of these widely-separated locali- 

 ties, to belong to the very highest beds of the Cretaceous, (the fact that they 

 certainly hold a position above beds clearly equivalent to the latest chalk, if 

 not even to the Maestrich beds, would preclude the supposition that they 

 could belong to the horizon of the Wealden) we might readily understand 



