FLORA OF OLDER POTOMAC FORMATION. 383 



The gulch or deep ravine, at the bottom of which there is running 

 water, with a spring near the plant localit}^ makes southward. On the 

 left or east bank there is a fine exposure consisting chiefly of Rappahan- 

 nock sand, but with clay seams, and in one of these, a foot above the 

 stream bed, fossil plants were found in abundance. There is also much 

 lignite, and this extends below the bottom of the ravine. A short dis- 

 tance below the principal plant bed is a bluff of typical cross-bedded 

 Rappahannock sand. Well up in this bluff, about 20 fee,t above the 

 stream bed, is a thin vein of fine buff-colored clay containing abundant 

 remains of coniferous twigs with small cones and male aments, seeds, etc. 

 A bivalve shell was also found here. A rather large collection was 

 made at both the horizons. These plants are fully treated in a later 

 part of this paper (see pp. 509-515). 



On May 14, assisted by Mr. William Hunter and Mr. Victor Mason, 

 I made a much larger and better collection than any of the previous 

 ones from the Mount Vernon clays, both at the original locality discovered 

 by me on October 16, 1892, and also in White House Bluff, at a locaHty 

 discovered on December 5, 1892. 



Nearly the whole month of July was devoted to a critical reexamina- 

 tion by Professor Fontaine and myself of the Potomac l^eds of the District 

 of Columbia, Maryland, and Virginia from Baltimore to Fredericksburg. 

 We confirmed my previous observation that the conditions to the north- 

 west and north of Washington closely approaches those of Virginia, the 

 principal difference being that the Ptappahannock sands are not lithified 

 and are somewhat less feldspathic, being generally white and cross-bedded. 

 But in passing eastward on the strike these sands are soon overlain by the 

 dull-red clays characteristic of the Maryland exposures. This condition 

 begins at Eckington, within the city, and is well shown in numerous 

 cuttings on the Metropolitan branch of the Baltimore and Ohio Railroad, 

 notably at Terra Cotta. Typical basal clays may be seen at Lamond, 

 also on the railroad. The plant bed at the bottom of the new reservoir 

 was found to be in lignitiferous clays of the same age underlying the 

 sands, which here hold large quantities of silicified wood. At many 

 points northeast of Washington the dull-red clays that overlie the white 

 (here often yellowish and ferruginous) sands are in turn overlain by a 



