MEXICAN BOUNDARY LINE. 131 



Missouri which was examined by Mr. Nicollet. Although admitting of several suhdivisions 

 from changes in lithological character, the beds of No. IV do not present any groups of fossil 

 species restricted within the physical or lithological limits designated, and they can scarcely, 

 therefore, be regarded as of importance in the classification of the formation, or valuable in 

 tracing the limits of its members over a wide extent of country. 



The subdivisions A and B, corresponding to Nos. II and III of our section, are more important; 

 and, although yielding so few fossils on the Missouri, they become well marked in other parts 

 of the country. The " Inoceramua Barahini," represented by Mr. Nicollet as found in great 

 numbers at Dixon's Bluif, very much compressed and so arranged as to give the rock " a slaty 

 structure," is undoubtedly the laoceramus prohlematicus, which is known to occur in this position, 

 and does not occur in the higher beds of the formation upon the Missouri, so far as known at the 

 present time. The Ostrea congesta, and all the other fossils from beds Nos. II and III of the 

 section, are unlike species from New Jersey or Alabama, and appear to be restricted to these 

 beds. At the same time the species identical with or analogous to species of New Jersey and 

 Alabama occur in beds Nos. IV and V, which may perhaps be regarded as subdivisions of one 

 group. 



We are warranted, therefore, in referring the beds above No. Ill to the fossiliferous beds of 

 New Jersey and Alabama, while we have yet no evidence that Nos. II and III do occur in either 

 of these States. 



The beds Nos. Ill, IV, V, and VI of the New Jersey section, given on a preceding page, 

 correspond in their fossils with Nos. IV and V of the Nebraska section ; leaving the third green 

 sand of New Jersey (No. VIII of that section) unrepresented in the northwest, so far as known 

 at the present time. 



The New Jersey beds, Nos. I and II, which are marked only by fossil wood and impressions 

 of leaves, appear to be represented by No. I of the Nebraska section, judging from the general 

 character of the remains yet known in the two. Should this inference prove to be correct, the 

 beds Nos. II and III of the Nebraska section will hold a position between Nos. II and III of 

 the New Jersey section ; but I do not regard this question as yet determined. 



The relations of the beds Nos. II and III of the Nebraska section, and their characteristic 

 fossils, become very important when we undertake the comparison of the cretaceous formation 

 of Texas and New Mexico with that of Nebraska, Alabama, and New Jersey. 



The wide extent and persistence of Inoceramus prohlematicus, and its restriction to beds Nos. 

 2 and 3, and their equivalents, so far as at present known, render it of great value in deter- 

 mining a geological horizon. This species was first brought from the Missouri river by Mr. 

 Nicollet.* It was collected by Captain, now Colonel, Fremont j upon the Smoky Hill Fork, 

 where it occurs in a gray or bufi' color, and also in a blue, slaty limestone in great numbers, 

 and being extremely flattened, gives to the rock a slaty structure, as described by Mr. Nicollet. 



" Report on the Upper Mississippi River, by J. N. Nicollet, 1853. 



f Report of the Exploring Expedition to the Rocky Mountains, by Captain J. C. Fremont, 1854. Appendix, geological 

 formations and organic remains, by James Hall. At the time of my examination of Captain Fremont's collections, I had 

 an opportunity of comparing the specimens of Inoceramus with those brought from the Jlissouri i^y Mr. Nicollet, smd iden- 

 tified the specimens in the two collections as the same species. The collections of Mr. Nicollet were, at that time, broken 

 up, and I saw pome of them in Professor Ducatels possession, in Baltimore, and others in Georgetown. The information 

 given me was, that they were from near the Great Bend of the Missouri ; but by the examination of Mr. Nicollet's report, 

 it is very clear, from his statements, page 35, that this Inoceramat occurs at Dixon's Bluff, and not at Great Bend, since 

 Mr. Nicollet refers to the former locality as exhibiting the base of the formation. 



