179 



mineral in some of these beds, and its existence in greater or 

 less degree in all, besides the obvious identity between most of 

 the fossil genera in this and the European group, suggest the 

 propriety of adopting the above name, as characterizing the 

 peculiar composition of the deposit, and its relationship to the 

 rocks of a well known system in other countries. But much yet 

 remains to be ascertained respecting the organic remains of the 

 two deposits in the respective continents, before we can determine 

 more than approximately the tiue amount of aj^w/Z?/ subsisting be- 

 tween them. The existence of but one or two species common to 

 the European and American formations, renders it indeed a ques- 

 tion of much uncertainty, to what part of the cretaceous period as 

 defined in Europe, we are to refer the origin of these deposits of 

 New Jersey, nor are we able positively to decide, merely by the 

 relationship of the genera, whether the cretaceous period em- 

 braces both the commencement and termination of the American 

 greensand series. 



SECTION I. 



Of the Potters^ Clay Formation; Geology of Middlesex and part 

 of Mercer counties. 



Geographical Range. — The southern edge of the middle se- 

 condary district marks the northwestern boundary of this forma- 

 tion, from the mouth of Lawrence's brook to the Shipetaukin 

 creek, between which and the Delaware, the gneiss rocks of the 

 Assunpink form its limit. Advancing southwestward, the forma- 

 tion occupies a narrow belt along the Delaware, from Borden- 

 town to a point opposite New Castle, the southeast bank of the 

 river being its northwest boundary. Its inferior beds do not 

 appear in Pennsylvania, having been washed away by the cur- 

 rents which excavated this part of the valley of the Delaware. 



In some places the river in its meanderings cuts across the belt 

 almost to the southeastern margin, where it is bordered by the 

 lower beds of the greensand deposit. This is the case at Borden- 

 town, where the greensand approaches the Delaware within half 

 a mile. Where the river on the other hand recedes towards the 

 northwest, as for example opposite Tullytown, the lower beds of 



