82 X Hall on the Cretaceous Strata of the United States. 



r 



wLicli is coEsidered identical with L prohlematicus of Sclilotlieim. 

 la addition to this, we have seen, in Mr. Marcou^s collection, 

 specimens of Ostrea congesta from Gralisteo, between Fort Smith 

 and Santa Fe, where it probably holds the same geological po- 

 sition as the so called Oryphaea dilatata. 



'' The formations from which the above named fossils were ob- 

 tained in the sonthvfcstern territories, appear, from the statements 

 of the various ex2:)lorers of that region, to repose on a series of 

 red, yellow^, and w^hitish 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 Nebraslca, by our formation Xo. 1, which, as pre- 

 viously stated, w^e 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 Eivi:?r, Poblazon etc. 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 ovatus^ Nautilus DeKayi and 

 Ammonites placenta. ' 



The reasons for this conclusion are obvious from what has 

 preceded. The most conspicuous know^n fossils of beds 2 and 3 

 in Nebraska,- arc found in Arkansas and elsewhere associated 

 with many of the species of the Boundary Survey Collections, 

 and from the persistence of Inoceramus prohlematicns^ and the 

 almost uniform character of the rock in which it occurs from 

 Nebraska to New Mexico, we can have no doubt that the beds 

 containing this fossil everywhere occupy the same horizon. 



The collections from the southwest have never furnished spe- 

 cimens of tlic cephalopods enumerated above, which characterize 

 the upper cretaceous strata, and the few fossils which are com- 

 mon to Texas and New Mexico and New Jersey, render it prob- 

 able that the higher beds of the formation have thinned out in 

 that direction to a degree Avhich renders them subordinate in 

 importance to the lower beds of the system. The few specimens 

 identical with species known in New Jersey, Alabama and Ten- 

 nessee, appear, from their color and the character of the associa- 

 ted rock, to have been obtained in a different bed from that of 

 the greater number of specimens in the collection, wdiich are 

 associated with a more calcareous rock. At the same time, the 

 absence of sections of strata leaves us without positive informa- 

 tion in this respect."^ 



* A single observation in the notes accompanying the ppecimens leatls me to infer 

 that Exogyra costafa^ and one or two species besides, were collected from a higher 

 position in the cliff, tlian the other fossils. Since however it is probable that many 



