212 CLARK AND BIBBINS — GEOLOGY OF THE POTOMAC GROUP 



Crisfleld, Md. 



North End 



Point, Va. 



Gloucester 



C. H., Va. 



Tilghman's 



island, Md. 

 Tunis mills, 



Md. 



Claiborne, Md. 



Rock Hall, Md. 



Middletown, 



Del. 



Indian Head, 

 Md. 



Quantico, Va. 



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above been indicated may well have served 

 as the seaward buttress in such deformation. 

 Whether there may be more than one of these 

 axial prominences in the Potomac surface is 

 a question of much interest, but which can 

 not be answered with the data at hand. 



The lessening in the descent of the Potomac 

 surface far to the seaward, as indicated by 

 the borings at North End point and Crisfleld, 

 is in general in harmony with the relations 

 of the subjacent crystalline floor above de- 

 scribed. 



Age of the Potomac Deposits 



There has been much discussion as to the 

 age of the Potomac group. Most geologists, 

 particularly those who have studied the 

 floras, have believed the entire group to be 

 of Cretaceous age, while a few investigators, 

 notably the late Professor Marsh * of Yale 

 University, have regarded it of Jurassic age. 

 The authors of this paper in an earlier publi- 

 cation pointed out this difference of view, 

 and clearly showed that the dicotyledonous 

 floras were practically confined to the two 

 upper formations, while the dinosaurs on 

 which Professor Marsh based the Jurassic 

 age of the Potomac group were found in the 

 Arundel formation. As the result of these 

 observations, and without attempting to de- 

 cide finally regarding the paleontologic evi- 

 dence, they placed the two lower formations 

 of the Potomac group questionably in the 

 Jurassic. Since the publication of the above 

 paper the authors have made a very exhaust- 

 ive examination of the several formations 

 and collected large numbers of animal and 

 plant remains. As the result of this work 

 a considerable dicotyledonous flora has been 

 found to exist in the Arundel, although 



*0. C. Marsh : "Jurassic Formation of the Atlantic Coast." 

 Amer. Jour. Sci., Aug., 1896, pp. 105-115. 



