56 THE POTOMAC OR YOUNGER MESOZOIC FLORA. 



other cases it is impossible to separate the material carrjnng- these dicoty- 

 ledonous plants from that containing the seemingly older ones. There is 

 no break between the beds, and the general geological features are the 

 same in all. An attempt will be made in another connection to give the 

 significance of these apparently younger elements, If anv more argilla- 

 ceous upper member was ever generally present in the lower Potomac 

 it is synchronous with it, and the two form one unbroken series 



A good type of the upper member, whose probable wide extent has 

 been noticed above, may be found in the southeast slope of Federal Hill, 

 Baltimore. The spot is located on Covington street, near Ellicott's iron 

 furnace. The exposures seen at the time of my visit (August, 1886) were 

 necessarily temporary, as they were made in the grading of a street and 

 in slight excavations formed for the purpose of obtaining sand from the 

 sandy member of the lower Potomac. Professor Uhler writes that the 

 aspect of the place has since been totally changed. The hill has been dug 

 away and the pits have been filled up. 



I was informed by intelligent persons who had much experience in 

 excavating the Potomac beds at Baltimore, and whose business compelled 

 them to make a close study of the different kinds of rocks found there, 

 that the features seen in the section given below might be seen at a num- 

 ber of other points in and near the city. In the descending order we 

 have : 



(4) Variegated Clay to the top of the hill Upper Potomac. 



(3) Iiiterbedded argillaceous sand aud plastic lead-gray clay C ) 



to S feet I Lower 



(2) Layer of ferrugiuous sand and ocher 4 to 6 inches j Potomac. 



(1) Current-bedded sand, maximum thickuess shown 8 feet ( 



The Variegated Clay (No. 4) has already been descril)ed in its gen- 

 eral character. It descends from the top of the hill to near its base. It 

 is mostly a tenacious gray clay, quite plastic towards the base, and when 

 obtained from that horizon it is used for making tiles, etc. It has no bed- 

 ding and no cleavage, is much contorted and affected witli slickensides, 

 contrasting strongly in these points with the cla}' of No. 3. While gray 

 is the normal color, it is in places variegated in large blotches with red 



