INTRODUCTION. 13 



[n the Wealden formation, overlying the last, and be- 

 longing to the Cretaceous group* of De la Beche, we 

 find such organic remains as prove it to be of fluviatile 

 origin. The shells found, are those which are known to 

 exist only in rivers, (Uniones, &c.) and those genera which 

 are considered as estuary shells. In it the reptilia are 

 numerous. Superior to this formation, we find in the same 

 group, the Lower green sand, the Gault, Upper green sand, 

 and the Chalk, which to the geologist of this country are of 

 great interest, as we find in these deposits generally known 

 as " marl of New-Jersey and Delaware," a part of their 

 equivalents, f 



The Supercretaceous group, | which is next in superpo- 

 sition, contains organic remains of the highest interest. 

 Consisting of various deposits, and many localities in Eu- 

 rope, as it does in this country, much attention has been 

 given to it there, and more recently, it has attracted the 

 notice of our geologists. 



The observation of Mr Lyell, that the Tertiary groups 

 of Europe have detached and isolated positions, while the 

 Secondary period extends over great aread, applies equally 

 to our country. In these groups we, for the first time, find 

 those species of shells which we are able to identify with 

 living species, while those genera which existed in such 

 abundance in the inferior formations, have here nearly 

 disappeared. Of the genus Ammonites, two only of the 

 one hundred and eighty-three described in Great Britain 



* Pelagiques of Al. Brogniart includes this and the Oolitic group. 



t It is to Prof. Vanuxem (Jour, of the Acad. of Nat. Sci. of Phil., vol. 

 6, p. 59) we are indebted for the identification of this formation with the 

 Cretaceous group of Europe. This geologist having collected littoral shells 

 from the Burr-stone of Georgia, brought to this city during our late war, 

 when the foreign importation ceased, and having collected the pelagian 

 remains of New Jersey, was led to conclude, while in France, in 1818, 

 examining the Paris basin, that the alluvial of M'Clure consisted of 

 Secondary, Tertiary and Alluvial masses. 



t Thalassiques of Al. Brogniart. 



