34 THE 1'OTOiMA.G OR YOUiS^GER MESOZOIC FLORA. 



ports on the geology of Virginia, the " Upper Secondary." But Professor 

 Rogers under this designation included the strata extending in Hanover 

 County from the North Aima River to the Chickahominy. This series 

 belongs almost wholly to the Older Mesozoic, or Rhtnetic formation. It is 

 true that in the vicinity of Hanover Junction the Potomac appears in 

 some jjlaces, but it is inconspicuous. 



It is to be understood that in Virginia the A'ariegated Clays, or upper 

 member of the Potomac formation, has not been recognized. Tiie Potomac 

 of this State is the lower member. Only when we cross the Potomac River 

 going north is a distinction to be made between lower and upper Potomac. 



The Potomac strata show themselves in disconnected areas along a 

 narrow belt which extends from the Nottoway River twenty miles south of 

 Petersburg in a northerly direction past the cities of Petersburg, Rich- 

 mond, Fredericksburg, and Alexandria. This formation continues from 

 Virginia into the District of Columbia and Maryland, including Baltimore. 

 The direction of this belt is almost due north and south until Fredericks- 

 burg- is passed. Then it curves around so as to take a northeast course 

 by Alexandria and Washington to Baltimore. Beyond Baltimore the lower 

 member has not been identified with certainty, but judging from the char- 

 acter of the material seen at its last exposures it is not probable that it 

 ends at that city. The name Potomac was selected for the formation by 

 Mr. McGree on account of its comparatively great development and its typi- 

 cal character along, the Potomac River below Washington. 



The belt in which the Potomac areas lie is situated just outside or east 

 of the terrane of crystalline rocks. This, in Virginia and Maryland, extends 

 to the head of tide in the principal rivers. The Potomac was buried 

 deeply beneath the more recent deposits which form the coastal and tide- 

 water region of the Atlantic slope, and hence it appears only where these 

 have been removed by erosion. We accordingly find the exposures of this 

 formation near the junction of the post-Potomac formations with the crys- 

 talline rocks, and for the most part near the head of tide in the largest 

 rivers. 



The exposures of the formation in the belt above mentioned are so 

 arranged as to indicate the existence of at least four disconnected areas, all 



