LOCATION OF POTOMAC BEDS. 35 



of small width. There is, however, g-ooil reason to tiiiiik that this isolation 

 of the areas is rather apparent than real, ami that the formation extends 

 far to the east and seaward under the later depo.sits. 



A <>'eneral account only can he given here of the location and dimen- 

 sions of the several Potomac areas. In this account 1 will begin at the 

 north and proceed southward. 



My e.xaniination of the distribution of the foinnation north ol" the 

 Potomac River in Maryland and the District of Columbia has not been 

 sufficiently detailed to enable me to do more than indicate approximately 

 the location of the area in which exposures are found. 



I have not made any search for Potomac strata north of ]5altimore, 

 but Mr. McGee has found the upper member, or the Variegated Clays, as 

 stated before, in force near the head of Chesapeake Bay. 



Prof William B. Rogers* si)eaks of the Variegated Clays which occur 

 between Washington and lialtimore as extending to Wilmington in Dela- 

 ware, stating that toward Delaware, from the pi'eponderance of ferru- 

 ginous coloring, they have been called by Professor Booth the "red clay 

 formation." How far they extend in that direction, and what relation 

 these incoherent beds bear to the plastic clays of New Jersey, are points 

 that remain to be determined. 



The upper member, the Variegated Clay, is shown greatly developed at 

 Baltimore and all along the lines of the Baltimore and Ohio and Baltimore 

 and Potomac Railroads. The eastern limit is to be found several miles east 

 of these railroads,- the formation being covered in that quarter by a vai-ying 

 thickness of the Tertiary, by the Quaternary, or by the ruins of these for- 

 mations. 



The western limit lies some miles west of the above-mentioned rail- 

 roads. The farthest jjoint south at which the Variegated Clay mendjer has 

 been certainly identified is Fort Washington. Possibly it may form a 

 portion of the hills at Mount Vernon, but this member is certainly absent 

 beyond this point. 



As stated before, the lower Potomac has not been identified north of 

 Baltimore. Between Baltimore and Washington it is evidently genei'ally 



' Geology of tlio Virginias, p. 712. 



