120 W. J. McGee — Three Formations of 



treme variability of the ammonites, the most marked changes 

 are shown from stratum to stratum. Hence it is scarcely pos- 

 sible that a well-marked form could be preserved through any 

 considerable extent of time or any marked change in the con- 

 ditions of life. In other classes of animals many cases can be 

 cited where forms have continued unchanged for long periods 

 of time, but when such is observed among the ammonites it is 

 certainly a proof of the faunal affinity of the formations con- 

 sidered and a strong reason for uniting them most closely in 

 the geological system. 



Before closing this brief contribution it will not perhaps be 

 irrelevant to refer in a word to the general positions held by 

 geologists of different countries upon this question, as shown 

 in their reports to the Committee for the Unity of Nomencla- 

 ture at the Geological Congress at Berlin. In this report the 

 opinion of French and English geologists was decidedly in favor 

 of according a closer relationship of the Rhsetic beds to Lias 

 than to Trias, while the weight of evidence obtained from Ger- 

 man sources plainly pointed to the opposite conclusion. The 

 inability and folly of endeavoring to correlate the strata of 

 widely separated regions is thus most forcibly shown, since 

 facts which in certain localities warrant the close association of 

 conformable beds, in others preclude the union of apparently 

 synchronous horizons. 

 Geological Laboratory, Johns Hopkins University, Oct. 1887. 



Aet. XI. — Three Formations of the Middle Atlantic Slope ; 

 by W J. McGee. With Plate II. 



Introduction. 



The Middle Atlantic Slope may be described as that portion 

 of Eastern United States which sheds its waters directly into 

 the Atlantic Ocean and in which the principal rivers rise within 

 the Appalachian mountain system. Thus defined, it extends 

 from near the Mohawk and Hudson on the north to the 

 Roanoke (called the Staunton in the middle part of its course) 

 on the south, or from southern New York to northern North 

 Carolina. Its principal rivers, in addition to those mentioned, 

 are the Delaware, the Susquehanna (including its continuation, 

 Chesapeake Bay), the Potomac, and the James ; and its smaller 

 but yet notable streams extending to tide water are the Raritan, 

 the Schuylkill, the Brandywine, the Patapsco, the Patuxent, 

 the Occoquan, the Mattaponi, the North Anna and South Anna 

 (which unite to form the Pamunkey), the Appomattox, the 

 Nottoway, and the Meherrin. 



