﻿GEOLOGY AXD PALEONTOLOGY OB" THE CANAL ZONE. 575 



CHOPTANK AND ST. MARYS FORMATIONS. 



Miss Julia Gardner contributes the following statement on these for- 

 mations: "Because of faunal similarity with the Calvert formation, 

 both the Choptank and the St. Marys formations are also correlated 

 with the Tortonian of Europe, though, of course, they represent hori- 

 zons slightly higher than that of the Calvert. The Choptank fauna is 

 little more than a sandy bottom facics of the Calvert and is the 

 biologic expression of the physical conditions attending its close. 

 About 60 per cent of the Choptank species are present in the under- 

 lying formation, while approximately 30 per cent persist into the 

 overlyiiig St. Marys. 



"The St. Marys fauna, though similar to those of the lower forma- 

 tions of the Chesapeake group in the general make-up, is differentiated 

 from them by an influx of new forms and by the absence of those 

 species peculiar to the cooler waters of the Calvert and the sands of 

 the Choptank. The more modern element includes not far from 

 35 per cent of the entire St. Marys fauna." 



YORKTOWN FORMATION AND DUPLIN MARL. 



Miss Gardner has kindly prepared the following statement: 

 "The change in the paleontologic character at the close of the 

 St. Marys is much more significant than that preceding it. Although 

 the percentage of now forms in the Yorktown is not remarkably 

 large, the general facies shows a distinct advance over the St. Marys. 

 The more primitive types, such as Ostrea compressirostra, had become 

 extinct or they show an abrupt decrease in prominence, while a 

 number of more advanced types such as Area lienosa, which con- 

 stitute conspicuous elements in the later faunas, are initiated at 

 this horizon. 



"The views advanced by Dall ^ on the approximate synchroneity 

 of the Yorktown and Duplin faunas have been verified by subsequent 

 investigations. Doctor Dall, in his discussion of Tertiary conditions 

 along the East Coast, suggested the elimination of the cool inshore 

 current of the earlier Miocene and the reestablishment of a Tertiary 

 Gulf Stream as the probable cause of the subtropical aspect of tho 

 Duplin fauna. This late Miocene warm current apparently hugged 

 the North Carolina shore even more closely than does the present 

 Gulf Stream, but swung off into the open sea in the vicinity of Hatteras 

 so that its influence upon the Yorktown fauna was almost negUgiblo. 

 The sea floor, on which the Dulphin marl, as at present known, was 

 deposited, was apparently more sandy than that on which the St. 

 Marys and Yorktown formations were laid down, as the conspicuous 

 abundance in Virginia and northern North Carolina of such a form 

 as Mulinia congesia indicates dominantly muddy bottom in some 



> Dall, W. H., Contribntions to the Tertiary fauna of Florida, Wagner Free Inst. Sci. Phila. Trans. 



vol.3, pt. 6, p. 1598, 1903. 



37149— 19— Bull. 103 3 



