134 GEOLOGY AND PALEONTOLOGY, 



with many of the species of the Boundary Survey collections, and from the persistence of Inoce- 

 ramus problematicus, 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 specimens of the cephalopoda 

 enumerated above, which characterize the upper cretaceous strata, and the few fossils which are 

 common to Texas and New Mexico and New Jersey, render it probable that the higher beds of the 

 formation have thinned out in that direction to a degree which renders them subordinate in 

 importance to the lower beds of the system. The few specimens identical with species known 

 in New Jersey, Alabama, and Tennessee, appear, from their color and the character of the 

 associated rock, to have been obtained in a different bed from that of the greater number of 

 specimens in the collection, which are associated with a more calcareous rock. At the same 

 time, the absence of sections of strata leaves us without positive information in this respect.* 



In the present state of our knowledge^ it would appear that the beds 2 and 3 of the cretaceous 

 formation of Nebraska have gradually increased in thickness and importance in a southwesterly 

 direction, and, at the same time, have become more fossil iferous. In tracing the same beds 

 through Arkansas, we find, in addition to the Inoceramus problematicus, and associated with 

 that fossil, Holectypus planatus, I'oxaster elegans, Holaster simplex, Cardiummultistriatum, Inoce- 

 ramus confertim-annulatus, Gryplicea Pitcheri, and others, which occur also among the Boundary 

 collections. These facts clearly show that the beds have become much more fossiliferous than 

 on the Missouri, or on the Kansas and Blue rivers, and we must regard the greater part of the 

 Boundary collections as derived from the horizon of these beds. 



From the great vertical range of the characteristic cephalopods, above enumerated, in New 

 Jersey, and their wide geographical distribution, and from the marked distinction in the types 

 of fossils holding the lower position, we shall probably find it convenient to subdivide the 

 cretaceous formation into three great groups : 



3. The upper division, comprising the first and second marl beds of New Jersey, with the 

 intermediate ferruginous sand, and the clay below the first greensand bed, (Nos. Ill to VI of 

 the section,) parallel to the beds 4 and 5 of Nebraska. 



2. The middle division, equivalent to the beds 2 and 3 of Nebraska, and the calcareous beds 

 of the southwest, Arkansas, Texas, and New Mexico, containing the numerous Echinoderms, 

 Inoceramus problematicus, Gryplicea Fitcheri, Hippurites, Gaprina, Nerinea, Ammonites Texanus, 

 and numerous other fossils. 



1. The lower division, represented by No. 1 of Nebraska, and probably equivalent to the 

 lower clay beds of New Jersey, in which the only fossils yet known are of vegetable origin. 



It is not unlikely that the medial division may prove, in many localities, to be divisible into 

 distinct beds beyond those recognized in Nebraska ; or, that as the formation expands to the 

 southward, other beds not known on the Missouri will come in, or that the two there known 

 will be found to become much modified in character. 



* A single observation in the notes accompanying the specimens leads me to infer that Exogyra costaia, and one or two 

 species besides, were collected from a higher position in the cliff than the other fossils. Since, however, it is probable that 

 many species, not known in the same association in New Jersey and Alabama, may occur in connexion with Exogyra costata 

 in the southwest, we cannot at this time separate the species belonging to the upper and lower divisions of the formation. 



I learn, also, from Dr. Parry, since tliese pages were written, that he regards the bed containing Exogyra costata and some 

 other species, as holding a higher position than the calcareous beds of Leon Springs, and other localities along the route. 



