XLIV INTRODUCTORY REMARKS. 



Dr. Isaac Lea, of Philadelphia, in an interesting paper read before the 

 Academy in December, 1858, likewise favored the conclusion that the New 

 Jersey Green-sands represent the Senonien of d'Orbigny, but, at the same 

 time, he suggested some reasons for thinking that Ihey may possibly belong 

 to a lower horizon. 



From a careful review of the whole subject, and an attentive study 

 of extensive collections from the various Upper Missouri Cretaceous rocks 

 throughout wide areas, the author was led to adopt (in a joint paper with 

 Dr. Hayden, published in the Proceedings of the Academy in December, 

 18G1) the views of d'Orbigny, so far as regards the middle and upper beds 

 of the New Jersey Cretaceous, and their equivalents in the Upper Missouri 

 country and districts farther southward, both east and west of the Mississippi; 

 that is to say, his opinion that these upper members represent the Upper or 

 White Chalk, and probably the Maestricht beds, was fully adopted. But all 

 of the older Upper Missouri Cretaceous beds, and their equivalents, so far 

 as elsewhere represented in this country, almost certainly belong to the 

 horizon of the Lower or Gray Chalk ; or perhaps in part to that of the Upper 

 Green-sand, of the Old World, as shown in the paper above cited. 



The evidence respecting the exact part of the European Cretaceous 

 series to which the Dakota group belongs is not entirely satisfactory, the 

 few animal remains yet known from it being mainly casts, and, so far as 

 determined, not such forms as can be regarded as especially characteristic 

 of any particular horizon in the Cretaceous of Europe. Up to this time, we 

 also know of no single species being common to it and any of the beds 

 above; but then we, as yet, know comparatively only a few species of animal 

 remains from this rock. One of these, however, belongs to the Cretaceous genus 

 Leptosolen; while the other shells are allied to Cretaceous species and unlike 

 Jurassic forms. In addition to this, the modern affinities of the numerous 

 leaves of the higher types of dicotyledonous trees found in it, present a 

 strong objection to the adoption of the conclusion that it may belong to a 

 lower horizon than the Upper Green-sand of British geologists; while its 

 position directly below beds almost beyond doubt representing the Lower or 

 Gray Chalk, precludes its reference to any higher stratigraphical position. 

 Consequently, we have long regarded it as most probably representing, in 

 part, if not the whole, of the Upper Green-sand, ami therefore cannot agree 

 with d'Orbigny in referring its equivalent in New Jersey, Alabama, etc., 

 along with the later \>f(\s, In the Senonien or While Chalk horizon. 



