44 THE POTOMAC OE YOUNGER MESOZOIC FLORA. 



gives a description of it.^ It is surprising to find here, so far to the east, 

 the heterogeneous composition, coarse character, and indications of the 

 action of agitated waters so characteristic of the Potomac formation in its 

 exposures far to the west. Professor Rogers speaks of a conglomerate of 

 cLay, sand, and pebbles, which reminds us of the features seen in the 

 erosion horizons of the Potomac lie mentions coarse sandy clay with 

 brownish or reddish blotches, clay and sand with some coarse pebbles, 

 reddish mottled clay with quartz pebbles, and other features common in 

 tlie Potomac, but found in no other formation under the Eocene of the 

 Atlantic slope. 



Fortress Monroe, at the mouth of James River, is on an air line in the 

 direction of the declination of the strata, sixty miles distant from the last 

 exposures seen on the James and the Appomattox. The average declina- 

 tion of the surface of the Potomac formation is from these data about four- 

 teen feet to the mile. The strata in this, as in the Fredericksburg area, 

 sink in a southeast direction. As no regular dip can be determined, we can 

 note only the depression of the surface of the formation taken as a whole. 



South of Petersburg no detailed examination has been made in. 

 search of Potomac outcrops, and hence too mucli stress should not be laid 

 on the apparent absence of tliem in given places. South of this town, 

 along the eastern margin of the Azoic rocks, no Potomac has been seen 

 until the Nottoway River, twenty miles distant, is reached. In this inter- 

 val the Potomac, if present, is buried under a thick mass of the Appo- 

 mattox formation, and there is no stream capable of cutting down through 

 it. The country or dirt road from Petersburg to Weldon, in North Car- 

 olina, runs near the margin of the crystalline terrane, and hence outcrops 

 of Potomac, if they exist, would appear near this road. As stated, none 

 are seen for twenty miles. When we reach the Nottoway the formation 

 again appears. 



At Boiling's Bridge, where the road above mentioned crosses this 

 stream, a few feet of typical Potomac sand with clay balls may be seen in 

 the base of the bank. This is capped by a thin stratum of Eocene with 

 obscure impressions of marine shells. This exposure was apparently not 



' Geology of the Virgiuias, p. 735. 



