MEXICAN BOUNDARY LINE. 127 



that we have this difference ia the organic remains of the formation ; but it is ilouhtless true 

 that this region of the cretaceous formation of the southwest, which has yielded nearly all the 

 fossils, represents a, different epoch in i\\e cretaceous period ix ova. those beds further east 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 

 indications that there may be more than a single member of the cretaceous formation from 

 which all these fossils have been derived. 



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 tlie 

 results of investigations made at various points and at different 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. — Upper cretaceous strata. 



Second Group. — Medial cretaceous strata. 



Third Group. — Lower cretaceous strata. 



The upper division embraced the Nummulite limestone of Alabama, being especially charac- 

 terized by the presence of Plagiostoma dumosum, Nummulites Mantellii, (Orhitoides Mantdli.) 

 This rock is now regarded as belonging to the older Tertiary formations. 



The medial division was regarded as contemporaneous with the white chalk of Europe. 



The lower division embraced the ' ' ferruginous sand deposits of the Atlantic States, extending 

 from Martha's Vineyard to South Carolina and Alabama, and into Mississippi, Arkansas and 

 Missouri." 



These strata were at that time regarded as contemporaneous with those which lie between the 

 white chalk and oolite in Europe. 



The foregoing subdivisions were proposed by Dr. Morton in some " additional observations" 

 appended to his " Synopsis of the organic remains of the cretaceous group of the United States," 

 and published in the Journal of the Academy of Natural Sciences in 1842. Accompanying this 

 classification of the cretaceous formation is a list of fossils from each of the subdivisions, 

 including all those which had been described up to that period. 



Professor Rogers, in his report upon the geology of New Jersey, in 1810, proposed a division 

 of the cretaceous formation of the State into five members. These subdivisions, however useful 

 they may have been topographically, are not accompanied by the palajontological evidence 

 necessary to enable us to determine their value as distinctive groups, or to aid us in a compari- 

 son with the sequence in other localities. 



More recently the investigations made during the geological survey of the State of New 

 Jersey have thrown further light upon the order of succession, and the lithological character of 

 the members composing the green sand formation of New Jersey. The section given by 

 Professor Cook, which has been verified by borings in several places, leaves no doubt that we 

 have now arrived at a knowledge of the true relations of the different members of this period as 



