72 /- Hall on the Cretaceous Strata of 



Ap.t, YII. — Ohservations upon the Cretaceous Strata of the Uni- 

 ted States tvith reference to tJie Relative Position of Fossils col- 

 lected hy the Boundary Commission ;^'' by James Hall. 



The list of fossil species from localities visited b}^ the Mexican 

 boundary commission sbows so large a number identical with 

 those described and figured by Dr. Roemer in his Kreideh'ildun- 

 gen von Texas ^ that we cannot doubt the occurrence of the same 

 beds throughout the whole extent surveyed, as far as the neigh- 

 borhood 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 dif- 

 ferent and successive beds of the formation, should it there ex- 

 hibit similar subdivivisions as are elsewhere known in this sys- 

 tem, in other parts of the country. With the exception of two 

 species, they are all distinct from those kno\vn in the cretaceous 

 formation of New Jersey and Alabama, where the fossils have 

 been most carefully studied. They are eq^ually distinct likewise, 

 fi*om 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 equiva- 

 lency of the beds in these distant localities. 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 avS known in New Jersey, can there- 

 fore 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 recog- 

 nized in Nebraska by numerous identical species of fossils. 



When we carry forward our investigations in a southwesterly 

 direction however, we soon lose to a gTcat extent the evidence 

 of identity in the fossils ; and in Arkansas the Exogyra costata^ 

 Ostrea versicularis^ and Trigonia thoracica^ are almost the onl}^ 

 ;species identical with those known on the east of the Mississippi 

 river. At the 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 ^ob- 

 servable as far north as Foi-t Washita in Arkansas ; and since 

 the types of the greensand of New Jersey and Alabama extend 

 jas far north as Tennessee and Illinois, it is clear that the change 



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m BounJaiy 

 Commissione 



