36 tup: roTOMAC or yoonger ]\[esozoic flora. 



present under the Variegated Clays, but in most cases it is too deeply 

 buried under the latter to be seen, except in a few spots, where erosion has 

 been carried deep enough to reach it. 



At Baltimore the only exposures seen are within the city limits. Bal- 

 timore is evidently near the original shore line of the Potomac waters, for 

 at this place the Potomac abuts against hills composed of crystalline rock, 

 and the latter sends tongues into the Potomac terrane. 



The Potomac at Baltimore has suffered much and unequally from 

 erosion. In places the Quaternary alone is found above, resting immedi- 

 ately on the sands of the lower member. In other places the Variegated 

 Clay forms the surface, resting on this sand ; and in still others, the typical 

 sand of the lower member has interposed between it and the Variegated 

 Clays, fossiliferous, argillaceous sand and laminated clay belonging to the 

 lower member. 



Going south towards Washington, the lower Potomac is covered by 

 a thick mass of the Variegated Clays. It is exposed at Relay Station, nine 

 miles from Baltimore. At Relay Station also the Potomac must be near 

 its original western limit, for we find a high bluff of cr3'stalline rock 

 abutting against and rising much above it. The hills of crystalline rock 

 at Relay Station appear never to have been covered by the Potomac. 



From Relay Station to Washington the Baltimore and Washington 

 turnpike runs parallel to the Baltimore and Ohio Railroad and a mile or 

 more west of it. No exjjosures of the lower Potomac were seen on this 

 pike, for this portion is covered by the Variegated Clays. The margin 

 must lie to the west of this i"oad. That it is present under this clay is 

 shown by its occurrence at Hanover, about six miles south of Relay 

 Station. This point is some distance east of the western limit of the for- 

 mation. These are the onl}^ points between Baltimore and Washington 

 where exposures of the lower Potomac have been seen. No detailed 

 search, however, for exposures in this interval has been made. 



At Washington, both in the city and near it, the lower member may 

 be seen in a number of places We may, for the sake of distinction, 

 call the belt from Baltimore to Washington the Baltimore area. It is, 

 howevei', continuous with the Fredericksburg area of Virginia. In the 



