ElvISHA MlTCHElvIv SCIENTIFIC SOCIETY. 45 



overlie the Potomac. During- later Cretaceous then the 

 shore line returned nearly to its former position. Close 

 study of the different members of each of these forma- 

 tions would probably reveal signs of shore migrations 

 of comparatively small magnitude besides the sweep- 

 ing changes herein mentioned. Moreover it is not defi- 

 nitely made out that the inner border of existing Cre- 

 taceous deposits is the limit of encroachments of the 

 sea in Cretaceous time. Great denudation, going on in 

 Tertiary time, caused a second base-level to spread over 

 a considerable part of the ancient land area which had 

 been uplifted at the close of the Triassic; and it is to 

 be supposed that much of the denudation took place in 

 the relatively soft Cretaceous beds, by which large 

 areas may have been entirely removed. 



Coming now to the Tertiary, we find that while there 

 were undoubtedly many oscillations of level during this 

 period, the principal Tertiary shore-line corresponds 

 closely to that of the Cretaceous from their most north- 

 erly occurrence (off Cape Cod) as :^r south as Virginia. 

 Between these points the two border lines are never 

 more than twenty miles apart. In Southern Virginia, 

 North Carolina, South Carolina, and part of Georgia, 

 the Tertiary border overlays that of the Cretaceous; 

 but from Georgia northwestward into the Mississippi 

 embayment, the Tertiary lies further out, allowing an 

 exposure of the Cretaceous beds in a strip perhaps fifty 

 miles wide. 



In the Tertiary of the Atlantic coast province. Eocene, 

 Miocene, and probably Pliocene, sediments occur, 

 though in the case of one or more formations it is diffi- 

 cut to discriminate between Miocene and Pliocene. 

 Hence the term Neocene is often used to describe the 

 Lafayette (Appomattox, Orange Sand) formation of the 



