156 proceedings: geological society 



in the plain. It seems plausible that they were formed while the plain 

 was being developed and during a climatic cycle more favorable than 

 the present one; i.e., under milder and more humid conditions. 



Discussion: Sidney Paige inquired as to the source of the manganese. 

 Hewett replied that there had been considerable speculation on this 

 matter; it seems likely that it was derived from basic silicates in cer- 

 tain shale beds. T. Wayland Vaughan spoke of the climate of the 

 Eocene in the Gulf States, as shown by marine fossils. In the early 

 Eocene the climate was tropical, in the middle Eocene somewhat 

 cooler, and in the late Eocene tropical again. 



REGULAR PROGRAM 



L. W. Stephenson: Correlation of the Upper Cretaceous deposits 

 of the Atlantic and Gulf Coastal Plain. (Illustrated.) 



The Upper Cretaceous sediments of the Atlantic and Gulf Coastal 

 Plain are chiefly medium to fine-grained clays, sands, chalks, and marls, 

 ranging in origin from those laid down on low coastal plains, in estuaries 

 or in very shallow seas, to those formed in waters perhaps exceeding 

 100 fathoms in depth. In our present state of progress the fossils 

 most usable in determining the age relations of the marine sediments 

 formed in waters ranging in depth from moderately shallow to 50 

 fathoms or more are the representatives of the genus Exogyra, which 

 were adapted for life in all but the very shallowest of the Upper Cre- 

 taceous marginal seas and which underwent evolutionary changes 

 with sufficient rapidity to form faunal zones traceable through con- 

 temporaneous formations. 



Three principal zones have been differentiated, which, in ascending 

 order, are: (1) The zone of Exogyra upatoiensis, which is at the base 

 of the Eutaw formation in the Chattahoochee region; (2) the zone of 

 Exogyra ponderosa, which has been traced from New Jersey to the Rio 

 Grande. On the basis of fossils other than Exogyra this zone is sepa- 

 rable into two parts, — lower and upper. The former is traceable from 

 Georgia to the Rio Grande, and the latter from New Jersey to the Rio 

 Grande; (3) the zone of Exogyra costata, which has been traced from 

 New Jersey to Mexico. This zone is roughly separable into three 

 parts: a lower, characterized by a variety of Exogyra costata with 

 broad costae; a middle, characterized by a variety with costae of 

 medium breadth ; and an upper, characterized by a variety with narrow 

 costae. 



The correlations based on the Exogyra zones and subzones are sup- 

 ported by many other molluscan species of restricted stratigraphic 

 range and more or less extended geographic range. 



Tables and charts were shown illustrating the physical and age re- 

 lations of the formations, the stratigraphic position of the species and 

 varieties of Exogyra, and the correlation of the formations with each 

 other and with the Upper Cretaceous deposits of the Western Interior 

 and of Europe. 



