MEXICAN BOUND ART LINE. 133 



know, of all tlie other species of the lowest and upper two Nehraska Cretaceous formations in the 

 rocks from which Roemer and others collected so many species in Texas, and other south- 

 western localities, renders it highly probable that if the latter occur at all in Nebraska, they 

 must be represented by the beds Nos. 2 and 3 of our section. This conclusion is further 

 strengthened by the fact that the only Nebraska species yet found in the southwest, so far as 

 we know, are Inoceramus problematicus and Ostrea congesta, both of which are unknown in the 

 northwest, excepting in the above named beds, and are mainly restricted to the latter. The 

 well marked specific characters of these two fossils and their limited vertical range, together 

 with their extensive geographical distribution, render the bed in which they occur a horizon as 

 the highest importance in the identification of strata at remotely separated localities in these 

 far western Territories. 



"That these beds, or formations of the same age, are widely distributed over a vast area of 

 country, extending from near the great bend of the Missouri, in latitude 44° 15', longitude 

 99° 20', westward to, and perhaps beyond, the eastern slope of the Rocky Mountains, and far 

 south into Texas and New Mexico, is highly probable, from the occurrence of their character- 

 istic fossils at many widely separated localities in this region. At any rate we know, from 

 information obtained through Mr. Henry Pratten, of the geological survey of Illinois, that 

 Inoceramus problematicus is found in a light-colored limestone overlying a red sandstone on 

 Little Blue river, a tributary of Kansas river. Colonel Fremont also collected specimens of the 

 same shell from a similar rock on Smoky Hill river, in latitude 39°, longitude 98°, and at other 

 localities between there and the rocky mountains.* More recently Lieut. Abert found the 

 same, or a closely allied species, at a point as far southwest as latitude 35° 3' N., longitude 

 107° 2' W., and apparently on the western declivity of the anticlinal axis of the Rocky 

 Mountains. t Roemer likewise collected in Texas specimens of a shell he refers to Inoceramus 

 mytiloides of Mantell, which is considered identical with /. problematicus of Schlotheim. In 

 addition to this we have seen, in Mr. Marcou's collection, specimens of Ostrea congesta, from 

 Galisteo, between Fort Smith and Santa Fe, where it probably holds the same geological 

 position as the so-called Gryphaa dilatata. 



" The formations from which the above named fossils were obtained in the southwestern Ter- 

 ritories, appear, from the statements of the various explorers of that region, to repose on a 

 series of red, yellow, and whitish sandstones, and various colored clays, which are referred by 

 Mr. Marcou to the Jurassic and Triassic systems. These lower beds, we think, are represented 

 wholly or in part in Nebraska, by our formation No. 1, which, as previously stated, we regard 

 as probably belonging to the lower part of the Cretaceous system, though it may be older." 



Finally, in reference to the relative position in the series of a large part of the cretaceous 

 fossils of the Boundary Survey, I have already, in a previous communication, stated that I 

 regard them as occurring in the same geological horizon with the beds of Smoky Hill river, 

 Poblazon, &c. I am now prepared to fix their position in the same parallel with beds Nos. 2 

 and 3 of the Nebraska section, and below those beds in New Jersey and Alabama, which contain 

 Baculites ovatua, Nautilus De Kayi, and Ammonites placenta. 



The reasons for this conclusion are obvious from what has preceded. The most conspicuous 

 known fossils of beds 2 and 3 in Nebraska, are found in Arkansas and elsewhere, associated 



• See Prof. Hall's figures and remarks in Fremont's report, p. 174, pi. 4. 

 f Lieut. Abort's report "of explorations in New Mexico and California, p. 547. 



