INTRODUCTION. 13 
In 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. Supetior 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,t 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 areas, 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 isto 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. 
} Thalassiques of Al, Brogniart. 
