24 fHOCEEDINGS OF MADISON MEETING^. 



Vice-President Chamberlin assumed the chair, and thcLnext paper was : 



CENOZOIC HISTORY OF EASTERN VIRGINIA AND MARYLAND 



BY N. H. DARTON 



This paper was not offered for publication. Remarks upon it were 

 made by W J McGee, J. A. Holmes and R. D. Salisbury as follows : 



Mr McGee: 



Mr Darton's paper is an important contribution to knowledge of coastal plain 

 geology. Despite the px'esumptive unity of the Columbia formation, certain diver- 

 sities have been recognized since it was first discriminated, and in the original de- 

 scription two phases, designated respectively the " fluvial" phase and the "inter- 

 fluvial" phase, w^ere characterized. It has long been recognized, too, that the 

 topographic configuration of the phases is dissimilar, the high-level or interfluvial 

 phase displaying topographic adolescence or maturity, while the low-level or fluvial 

 phase is so nearly free from subaerial structure as to stand for topographic youth. 

 This diversity was tentatively and hypothetically explained as the result of unequal 

 altitude above base-level ; but, in view of the recent observations, the tentative 

 explanation seems less probable than before. Perhaps the most significant of Mr 

 Darton's conclusions is that relating to the inequality in coastal w^arping during 

 the respective periods of depression which he has described. This may perhaps 

 be regarded as the most nearly conclusive line of evidence of genetic bipartition of 

 the Columbia formation. 



Should Mr Darton's tentative division of the Columbia formation be supported 

 by later work in other portions of the Coastal plain, especially in New Jersey, it 

 will of course become necessary to modify the original nomenclature, perhaps by 

 retaining the old name for one division and applying a new term to the other, 

 possibly by abandoning the term Columbia and giving distinctive names to the 

 different deposits. This is, however, a matter for the future ; the application of 

 the name Columbia served as a milestone marking progress in geologic research in 

 the Coastal plain ; but when the important vicissitudes in continental evolution 

 for which it stood are generally recognized and come to be more clearly pictured, 

 perhaps the occasion for preserving the monument will disappear. 



Professor Salisbury : 



I am interested to observe the twofold divisions of the Columbia formation, 

 which Mr Darton has clearly brought out. I am glad to say, also, that north of 

 Mr Darton's territory, in New Jersey, a twofold division of the Columbia seems 

 to exist. These divisions will perhaps be found to correspond to his " high-level " 

 and ' ' low-level ' ' phases. I think it may prove to be necessary to separate these 

 two divisions of the Columbia, and to assign to one portion a new^ name, though 

 we perhaps know too little of their genetic and chronologic relations to make 

 this subdivision advisable at present. Evidence is accumulating in New Jersey 

 which seems to point to the separation of the later from the earlier Columbia by a 

 considerable time-interval, accompanied by considerable orographic movements- 

 This statement is made on the supposition that the reference of beds to the Co. 

 lumbia has been correct. I am sure we have not made this formation include 

 more than has been included in it further south. 



