CHAPTER XH. 



THE HORNERSTOWN MARL. 



Clark proposed the name Sewell for the old "middle marl" 

 formation of Cook, omitting the limesand portion, but since this 

 term was previously applied to a formation in the Carboniferous 

 of Virginia and West Virginia, the name "Hornerstown," first 

 applied by Knapp in unpublished manuscript, is used instead. 

 The formation can be traced across the State from Monmouth 

 to Salem counties, and is uniformly a dark, nearly pure, greeni- 

 sand marl, although the glauconite content diminishes some- 

 what in its southwestward extension. In Monmouth County 

 the total thickness of the formation is about 30 feet, and it is 

 sharply differentiated from the Tinton beds below and from the 

 Vincentown formation above. Further south, however, where 

 the Red Bank and Tinton formations are no longer typically 

 developed, their time interval being occupied by the more highly 

 glauconitic beds which seem to be the southern continuation of 

 the Navesink marl of the north, it becomes impossible to recog- 

 nize any definite base for the Hornerstown marl. 



FAUNA OF THE HORNERSTOWN MARL. 



The main portion of the Hornerstown marl is only sparsely 

 fossiliferous, but at the very summit of the formation a distinct 

 shell layer 5 feet or less in thickness is uniformly present, com- 

 posed of vast numbers of a species of Gryphaea and usually with 

 very many examples of Terebratula harlani. Collections have 

 been made from the formation from the following localities : 



Locality 152. This locality is ^4 mile southwest of Horners- 

 town, and lies southwest of that point in the State where the 

 Tinton beds can be differentiated as a distinct formation, and as 



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