58 THE POTOMAC OR YOUNGER MESOZOIC FLORA. 



This group of clays appears to be greatly developed northeast of 

 Baltimore, and from the statement of Professor Rogers, previously quoted, 

 it extends into Delaware. 



Mr. McGee has found in this clay formation at the head of Chesa- 

 peake Bay some poorly preserved plant-impressions, which are nearly all 

 dicotyledons. These impressions are too few in number and too imperfect 

 to throw any clear light on the question of the age of the formation. In 

 the absence of any evidence that shows the Variegated Clay group to be 

 distinct from and decidedly younger than the lower Potomac formation, 

 its close association with the latter and the difficulty of separating the 

 two make it necessary to group them together as forming one series of 

 beds. 



As the Variegated Clay group furnishes none of the plants described 

 in this memoir, no furtlier description of it is called for. 



GEOLOGICAL POSITION AND ORIGIN OF THE LOWER POTOMAC BEDS. 



In what will be said under this head the evidence of the fossil plants 

 will not be used, but the horizon of the lower Potomac will be fixed so 

 far as it can be done on evidence independent of them. 



Throughout most of the area occupied by the lower Potomac the 

 base when seen has been found to be composed of the crystalline rocks, 

 whose age must be placed so far back that they give us no aid in deter- 

 mining the lower limit of the horizon of the Potomac. But in the vicinity 

 of Hanover Junction this formation may be seen resting on the eroded 

 surface of the older Mesozoic, with evidence of a decided break between 

 them. This older Mesozoic in this part of Virginia, as I have shown in 

 Monograph U. S. Geological Survey, No. VI,* is Rha^tic in age ; at least 

 the plant-bearing portion of it. If the strata of this formation overlying 

 the plant-bearing part differ from the rest in age, they are of course 

 younger. 



The striking differences generally found in the lithologic and struct- 

 ural features of the two formations indicate that the difference in their 

 ages is considerable, the Potomac being decidedly the younger. Time 



' Coul. to the Knowledge of the Older Mesozoic Flora of Virginia, 



