DISPLACEMENT IN THE POTOMAC BEDS 209 



very definite evidence was adduced in demonstration of the same. 

 Other writers, including Fontaine, however, believe that we have to do 

 merely with sedimentation across a pre-Potomac escarpment. In the 

 opinion of the authors of this paper, the Fall Line phonomena in Mary- 

 land, and elsewhere, afford considerable evidence of monoclinal flexures 

 merging into simple and compound faulting. A number of carefully 

 constructed vertical sections on a large scale have been made across the 

 Fall Line zone, and these show in nearly every instance evidence of one 

 or the other of the above mentioned phenomena, along with the marked 

 increase in the dip of the Potomac deposits already mentioned. 



Evidence of the actual displacement in the Potomac beds is most 

 clearly defined in the vicinity of Relay, Maryland, and the evidence is 

 strengthened by the fact that the Miocene beds of Catonsville lie consid- 

 erably higher than the normal dip of the main body of the Miocene 

 deposits calls for. 



At the openings of the Maryland Clay Company, at Northeast, Mary- 

 land, there occurs a well defined example of an anticline in the Patuxent 

 beds, which is believed by Ries to have been produced by the hydration 

 of the subjacent feldspathic rock in the process of its decomposition into 

 the residual koalin mined at this point. Though the scale on which the 

 folding occurs is small, the phenomena afford a suggestion as to the 

 possible causes of some of the lesser irregularities in the Patuxent beds 

 which lie near the crystalline floor. 



Surface Configuration of Crystalline Floor and its Relation to 

 Potomac Basin of Deposition 



The basal beds of the Potomac group rest on a more or less uneven 

 surface of crystalline rocks, in which certain of the more important 

 drainage lines of the present day were already established, as is shown 

 both by the marginal contacts and by the well borings near the land- 

 ward border of the formations. 



The great increase in the dip of the Patuxent and succeeding forma- 

 tions along the Fall line has already been alluded to, as well as the 

 evidence that it represents in part at least a fault scarp. 



It is significant, however, that there is a marked though less pro- 

 nounced decline in the dip of the strata eastward of the Fall line all 

 the way to the seaward margin of the Coastal plain. The evidence for 

 this is furnished by the deep-well borings in Delaware, Maryland, and 

 Virginia, the number of which is not as great as could be desired, 

 although they all show, without exception, a progressively lessened dip 

 of the beds as the distance from the landward margin increases. 



