FLORA OF OLDER POTOMAC FORMATION. 359 



myself. After two days spent in the immediate vicinity of Washington 

 we commenced the journey southward into Virginia. The plan was to 

 follow the Younger Mesozoic belt, zigzagging frequentl}* across it to 

 examine its landward and coastward margins and the contact with 

 both the underlying crystalline and Paleozoic rocks and the overlying 

 Tertiary deposits. Thus by easy stages Fredericksburg was reached 

 on August 4 and Richmond on the 8th. From Richmond an excursion 

 was made in a rowboat down the James and up the Appomattox to 

 Petersburg. Eight days were spent in this region, and on the 18th the 

 southward journey was resumed in the attempt to trace the formation 

 in that direction. The locality mentioned by Rogers on the Nottoway 

 River at Boilings Bridge" as the most southerly point at which he had 

 been able to find an exposure of the formation was visited, and it was 

 found with characteristic clay balls in the bed of the river underlying 

 the Eocene. The best exposure was close to Boilings Bridge, but it 

 was also found at Fields Bridge, 4 miles above, which is undoubtedly 

 Rogers's localit3^ The reconnaissance was continued to the Roanoke 

 at Weldon, opposite which, near the railroad bridge, beds were found 

 closely resembling those of the Older Potomac, but the exposures were 

 not decisive on this point. The expedition then returned to Peters- 

 burg, Richmond, Fredericksburg, and Washington, varying the route 

 as much as possible and examining many additional localities and 

 exposures. The party reached Washington on the 25th, but resumed 

 operations on the 27th and traversed the State of Maryland, following 

 the Potomac outcrop as before. The iron-ore region was carefully 

 examined, the only plant remains found being pyritized lignite and 

 silicified wood. The cycadean trunks collected by Tyson, and then 

 at the Maryland Academy of Sciences, were shown us by Prof. P. R. 

 Uhler, president of the academy, and permission was given us to have 

 photographs made of them, which was done a little later. Professor 

 Uhler accompanied us to many localities with which he was familiar. 

 Considerable time was spent on the coastward margin on the forma- 

 tion, where its relations to the overlying marine beds were studied. 

 The reconnaissance was concluded on the 31st, a clear conception hav- 

 ing been acquired by all the members of the party of the general nature 

 of the Potomac formation. 



<i Report for 1839, p. 17. 



