STRATIGRAPHY OF THE POTOMAC GROUP 497 
formation from the two basal members of the Potomac group 
points to elevation after the close of the Arundel epoch and toa 
prolonged period of denudation. A striking feature of this 
erosion is the greater resistance which the Arundel clays pre- 
sented when compared with the deposits of the Patuxent forma- 
tion. The partially eroded clay lenses which project above the 
com.non line of contact can readily be explained upon these 
grounds. It was over this uneven surface that the deposits of 
the Patapsco formation were spread, reaching far beyond their 
western limits until they rested directly upon the crystalline rocks 
of the Piedmont Plateau. Their materials, so largely arkosic in 
the vicinity of the feldspathic rocks (gneiss and granite), point 
to the rapid stripping off of the disintegrated surface materials, 
and their deposition along the continent border. To how large 
an extent subsequent disintegration has kaolinized these mate- 
rials it is difficult to determine, but that they had been exten- 
sively weathered prior to their removal seems justified. The 
highly colored and variegated clays which were evidently formed 
in the quieter and deeper waters of the Patapsco epoch bear 
some relation to the great belts of basic eruptive rocks which lie 
to the west and north of them. This phase of sedimentation is 
much more marked in central Maryland where the rocks of this 
character are most highly developed and where the proximity to 
the eastern margin of the Piedmont belt is most apparent. It is 
also probable that these highly colored clays were in part 
derived from the weathered surface of the Arundel iron-ore 
beds. 
The unconformity separating the Raritan from the under- 
lying deposits is less prominent than that which has been above 
described at the base of the Patapsco formation, although the 
gradual thinning of the Patapsco along its western margin and 
the overlapping of the Raritan points strongly toward a struc- 
tural break. The Raritan deposits also obliquely transgress the 
other materials of the Potomac group northward until in the 
Delaware valley they come to rest directly upon the crystalline 
rocks of the Piedmont belt. The thick deposits of sand and 
