24 THE POTOMAC OR YOUNGKR MESOZOIC FLORA. 



Vernon. No prolonged search has been made at either of these places for 

 plant-impressions, and it is quite possible that good fossils may be found at 

 both. Mount Vernon is noteworthy for showing on the sand of the lower 

 Potomac a gray laminated argillaceous material similar to that seen in 

 Federal Hill, Baltimore. The Fort Washington bluff shows at its base 

 lower Potomac, consisting of sand with pockets of disturbed clay. Over 

 this comes upper Potomac, composed of the Variegated Clay, or so-called 

 "iron-ore clay," so conspicuous in portions of Maryland. Next above we 

 have Eocene with impressions of marine shells ; and over all. Quaternary. 

 The disturbed clay in the top of the lower Potomac shows masses and 

 particles of various sizes embedded in the sand. Some of the larger par- 

 ticles retain their original lamination, and in these fragments several spe- 

 cies of plants may be found. The most important of these are Frenclopsis 

 ramosissima, Dioonites Buchianus, and an undetermined dicotyledon. 



The lower part of the Potomac formation appears at Washington with 

 the features characterizing it in Virginia. In the clays exposed in the ex- 

 cavation for the reservoir a few obscure fragments of leaves occur, being 

 found in pale reddish clay. In the dark-gray clay, which forms pockets 

 in the sand composing the greater part of the formation here, a good 

 deal of lignite is found. This seems to be formed by the alteration of 

 drift-wood which has accumulated in certain places. 



In Virginia the Potomac formation is composed of only one group of 

 beds, tlie lower one, which is mostly sand or sandstone, with here and 

 there sandy clay. In these materials come irregularly subordinate strata 

 of more or less pure clay, containing the plants found in this formation. 

 In Maryland the case is different, for here we find a series of beds overlying 

 the lower or sandstone member, and underlying the Eocene, which as yet 

 cannot be separated from the sandstone or sand member by any clearly 

 defined universally present features. For this reason this group is retained 

 in tlie Potomac formation, forming its upper member. 



The Potomac formation extends from Washington to Baltimore, a dis- 

 tance of forty miles. In this space the lower or sand member is rarely 

 seen, for it is too deeply buried under the upper member to be visible, 

 except in the lowest natural depressions and the deepest artificial excava- 



