FLORA OF OLDEK POTOMAC FOKMATION. 'Ml 



then examined the eastern niarijin of the heh all the way lo the Poto- 

 mac Kiver and fonnd the ai'gillaceoiis sands and white stratihed clays 

 uniformly ovei'lyin,<i; llu> sandstone and overlain in tm-n hy the marls. 

 The conclusion became iiresistible that for this entire iv<^um this is 

 th(> normal order of deposition. This vicnv is abundantly confirmed 

 by the plant remains found respectively in the li,<,niitic clays on Potomac 

 Creek west of the Telejrraph road and in the uppei- clays neai' A(iuia 

 Creek, which differ widely in character and iiulicate a fiix'at time inter- 

 val between the earlier and the later deposits. The beds in this reijion 

 are thicker than in the valley of the Rappahaimock, the erosion having 

 been less. Measuring as carefully as possibl(\ we arrived at, the follow- 

 ing approximate section for an.\- line drawn across tlie belt perpendicular 

 to the strike, as, for example, on Potomac Creek or Accokeek Creek: 



Section of Accokeek Vretk. 



3. Loose sands interstnitiCicd witli wliilc Imiiinated days can-yiiit; plants of liigli raiil<. siicli as Sapiii- 



dopsis and otlior uiidoiiliU'd ilk-olyledoiious gonora "^'^ 



2. C'oai-so fcldspatliic sandstone Ijecoiiiing workable freestone '•''^ 



I . Lignitie clays carrying the older type-< of plants (ferns, <-ycuds, conifers, and archaic dicotyledons) . . . . 50 



Total exposure '-^^^ 



Attention was next turned to the northern extension of the 

 Potomac beds, and two months were spent in their systematic stud>-. 

 Following first the landward margin in the District of Columbia and 

 Maryland, we soon discovered that less difference exists between the 

 beds here and those of Virginia than had been supposed. The old 

 idea of an "Upper Clay member" in Maryland, as opposed to a "Lower 

 Sandstone member" in Virginia, was now wholly dispelled, the Virginia 

 beds having been found to begin and end as clays and th(> sandstone 

 to occupy an intermediate position. It was now found that in Mary- 

 land also wherever the tleposition is normal (i. e., no transgression of 

 higher beds) the basal ntember is clay and the succeeding one is, if not 

 sandstone, at least very arenaceous and often lithified. Moreover, this 

 second member in Maryland, although usually reddish fiom iron oxida- 

 tion that has filtered into it from the overlying iron ci?s or from iron 

 constituents of its own, usually contains casts and molds of stems, logs, 

 and plants wholly similar to those f the Rappahanno('k sandstone, 

 and these beds must be stratigraphically the same in both States. This 

 condition of things with slight variations extends entirely across the 



