INSTITUTE OF SCIENCE, PHILADELPHIA. 123 



which constitutes the distinctive faunal feature of the underlying cherty- 

 rock of the Hillsboro, and of the tough blue rock which crops out at 

 Ballast Point, is wholly absent from the rock with orbitolites ; similarly, 

 the orbitolite appears to be wanting in the Cerithium rock. What the 

 precise age of the latter deposit may be cannot be determined from its 

 faunal features alone, since the Cerithium, which, as far as my own experi- 

 ence goes, constitutes the only clearly definable species among the num- 

 erous molluscan impressions, has thus far not been met with in any other 

 formation, and consequently gives no clue as to the horizon represented 

 by it ; but from the position occupied by the rock stratigraphically under- 

 lying the Miocene (probably the lowest member of the Miocene) and 

 geographically wedged in between the Oligocene and Miocene from 

 both of which it differs widely in faunal characters I think it may be fairly 

 assumed that it lies on the border horizon of the two series, forming the 

 transition ground. 



