126 GEOLOGY AND PALiEONTOLOGY. 



OBSERVATIONS UPON THE CRETACEOUS STRATA OF THE UNITED STATES, 

 WITH REFERENCE TO THE RELATIVE POSITION OF THE FOSSILS COL- 

 LECTED BY THE BOUNDARY COMMISSION. 



The list of fossil species from the localities visited by the Boundary Commission shows so 

 large a number identical with those described and figured by Dr. Roomer in his Kreideblldungen 

 von Texas, that we cannot doubt the occurrence of the same beds throughout the whole extent 

 surveyed, as far as the neighborhood of El Paso and Frontera. These collections, made at 

 intervals over so wide an extent, would be likely to give us some representative species from 

 different and successive beds of the formation, should it there exhibit similar subdivisions as are 

 elsewhere known in this formation, in other parts of the country. With the exception of two 

 species, they are all distinct from those known in the cretaceous formation of New Jersey and 

 Alabama, where the fossils have been most carefully studied. They are equally distinct 

 from the species occurring in Nebraska ; while those from the last-named region present 

 so many species in common with New Jersey and Alabama, that we cannot doubt the general 

 equivalency of the beds in these distant points. The species known from Tennessee are likewise 

 identical with New Jersey species to a great extent, leaving no doubt as to the exact equivalency 

 of the formation in the two localities. 



The cretaceous formation, as known in New Jersey, can therefore be traced by the Atlantic 

 coast to Alabama, and thence into Tennessee, and even southern Illinois; and though not yet 

 followed continuously to the northward, it is nevertheless recognized in Nebra.ska by numerous 

 identical species of fossils. 



When we carry forward our investigations in a southwesterly direction, however, we soon 

 lose, to a great extent, the evidence of identity in the fossils ; and in Arkansas the Exogyra 

 costata, Ostrea vesicularis, and Trigonia ihoracica, are almost the only species identical with 

 those known on the east of the Mississippi river, and in Nebraska. At tlie same time, other 

 species occur in considerable abundance, which are of decidedly cretaceous character, leaving no 

 doubt of the existence of that formation, though we have lost the evidences which guide us in 

 more eastern localities. 



Since this change in the character of the fossils is quite observable as far north as Fort 

 Washita, in Arkansas ; and since tlie tyjies of the green sand of New Jersey and Alabama 

 extend as far north as Tennessee and Illinois, it is clear that the change is not due to climatic 

 influence or to geographical distance. It would moreover be unreasonable to suppose that such 

 a change in the nature of the sediment had taken place as to destroy within this short distance 

 all the forms of life so well known further east, and replace them with others adapted to the 

 different condition. Indeed, we are not informed that there is any great change in the litho- 

 logical character of the strata ; thotigh it is true that the cretaceous beds of Arkansas, Texas, 

 and New Mexico, (as we judge from the specimens,) are more calcareous than those of New 

 Jersey and Nebraska. But they are not more so than in Alabama, where the " Rotten lime- 

 stone' ' attains a thickness of 400 feet^ and contains species common to tlie regions just referred to. 



It is not due therefore to difference of latitude, or to a change of conditions in the sediment, 



