54 THE POTOMAC OR YOUS(iER MESOZOIG FLORA. 



argillaceous member in the top of the lower Potomac, like that seen at 

 Federal Hill, Baltimore, resting- on the sand and apparently extending 

 over a wide area. As this member, from its position, was the portion of 

 the lower Potomac most exposed to erosion, and as it was soft and easily 

 destroj^ed, we might expect to find it preserved less commonly than the 

 underlying sand. If the lower Potomac has lost much from erosion, this 

 member, if it existed, would suffer most. There is good reason to think 

 that ill the long period of time that has elapsed since the deposition of the 

 lower Potomac it has lost much of its thickness. Allusion has already 

 been made to the trenched and eroded condition of its surface where, as 

 in Virginia, it is covei'ed by the Eocene. In Virginia the outcrops of the 

 Potomac occur at very different altitudes, so that it may in some places 

 stand much higher than the Eocene and younger formations, even when 

 found in the inunediate vicinity. An example of this maA' be seen at 

 Point of Rocks, on the Appomattox. Here the Eocene occurs only a mile 

 away. It is exposed in a ravine much below the level of the surface of 

 the Potomac at that point. The two pre-Eocene ravines near Brooke 

 have been mentioned. They give another example of a worn surface. 

 In Maryland, between Relay and Baltimore, where the Variegated Clay 

 rests on the lower Potomac, much irregularity is shown in the height 

 of the top of the latter formation. Of course, where the Quaternary 

 rests on the lower Potomac the greatest variation is shown in the height 

 of its surface. A striking case is at White House Bluff, where eighty 

 feet of the formation seen in the point shows on each side Quaternary to 

 the level of the water. Many other localities on the Potomac River and 

 elsewhere might be mentioned where the lower Potomac rises high above 

 the surrounding Quaternary. 



While from this extensive erosion of the lower Potomac we would 

 expect to find the former uppermost portions of it generally missing, we 

 find in some places remnants which appear to represent it. These have 

 been already mentioned, being the clay bed near the railroad in the 

 vicinity of Brooke, the upper argillaceous portion of the lower Potomac 

 at Mount Vernon, and the plant-bearing argillaceous material occurring 

 in the top of the formation at Federal Hill. This latter may, I think, be 



