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/. Hall on the Cretaceous Strata of the United States. 73 



is not due to climatic influence or to geograpliical distance. It 

 would moreover be unreasonable to suppose that sucli 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 far- 

 ther east, and to replace them with others adapted to the differ- 

 ent condition. Indeed we are not informed that there is an 

 great change in the hthological character of the strata; thoug 

 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 "Eotten limestone" attains 

 a tliickness of 400 feet and contains species common to the re- 

 gions just mentioned. 



Therefore it is not due either to difference of latitude or to 

 change of conditions in the sediment, that we have this differ- 

 ence in the organic remains of the formation ; but it is doubtless 

 true that this region of the cretaceous formation of the south- 

 west which has yielded nearly all the fossils, represents a differ- 

 ent epoch in the cretaceous period^ from those beds farther cast, 



and in the northwest, of which the organic contents are better 

 known. 



The relations of that part of the cretaceous formation, which 

 is developed in Texas and New Mexico, to the same formation 

 as known on the east of the Mississippi river, and in Nebraska, 

 becomes a matter of much interest and importance. 



The various examinations in Texas and in Arkansas, as well 

 as along several lines of survey, do not give us any sections of 

 these beds showing their relations with other formations ; or in- 

 dications that there ma}' be more than a single member of the 

 cretaceous formation from which all these fossils have been de- 

 rived. Before attempting to theorize in regard to the probable 

 cause of this difference in the fossils of the cretaceous strata at 

 these distant points^ we may bring together in a general manner 

 the results of investigations made at various points and at dif- 

 ferent times, which may serve to throw some light upon this 

 question. 



In the earlier investigations of the cretaceous formations of 

 New Jersey and other parts of the United States, Dr. Morton 

 subdivided the whole into three groups or divisions. 



FIRST GROUP.— UprER Cretaceous Strata. 



SECOND GROUP.— Medial Cretaceous Strata. 



THIRD GROUP. — Lower Cretaceous Strata. 



The upper division embraced the Xummulitic limestone of 

 Alabama ; being especially characterized by the presence of Fla- 



SECOND SERIES, VOL. XXIV, NO. TO. JULY, 1857. 



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