the Middle Atlantic Slope. 135 



tomac river, is fully 200 feet within a mile and a half. JSTorth 

 of the Potomac river, the deposits, both in the main body and 

 in the outliers, generally repose upon the eroded surface of the 

 Piedmont crystallines, the depth of local ravining reaching 

 some 250 feet about the mouth of the Susquehanna and pro- 

 portionately less depths on the smaller streams ; but the great 

 outlier north of the Schuylkill in Pennsylvania, which exhib- 

 its all of the characteristics of the main body of the formation, 

 rests in part upon crystallines, in part upon folded Silurian 

 limestones, shales, and quartzites, and in part upon degraded 

 Triassic sandstones and shales. On the Delaware the " Yellow 

 Pocks," believed to represent the formation, rest upon a planed 

 surface of tilted Triassic sandstone and highly inclined gneiss ; 

 and the still more doubtfully identified outlier forming the 

 " Sand Hills '' near the Raritan, rests upon an apparently eroded 

 Triassic trap dike. In short, the formation everywhere reposes 

 on a deeply degraded surface of upturned Piedmont - crystal- 

 lines, folded Silurian strata, tilted, faulted, and diked Triassic 

 sandstones, and diked and displaced Rhsetic beds ; and it is 

 significant that this surface is a generally uniform plain, in- 

 clined seaward and deeply incised by great Waterways, coin- 

 ciding closely with those of the present. 



South of the Rappahannock the Potomac formation is over- 

 lain by the Appomattox formation, the fossiliferous Miocene, 

 and the Eocene ; between the Rappahannock and the Patapsco 

 it is overlain by the Eocene ; north of the Patapsco it is over- 

 lain so far as known by the upper Cretaceous ; and in the ab- 

 sence of these, and up to certain altitudes indicated later, it 

 is overlain by the Columbia formation ; the relation gener- 

 ally, but not invariably, being one of visible unconformity. 

 The unconformity between the Columbia and the Potomac is 

 well exhibited in representative sections at Fredericksburg, Wash- 

 ington, Baltimore, and the head of Chesapeake hay, but over 

 the interfluvial plains and on certain slopes about Washington 

 the formations, though widely diverse in age, intergraduate so 

 imperceptibly that it is impossible to demark them ; there is 

 notable unconformity between the Appomattox and Potomac 

 formations on the Roanoke and in some sections on the Appo- 

 mattox and James .rivers, but in other sections on the last 

 named rivers (at Fredericksburg and elsewhere), there are de- 

 posits of composite character certainly belonging to one or both 

 of these formations which cannot be discriminated even in the 

 same section ; the Miocene rests on the Potomac in the local 

 absence of the Eocene at Petersburg and Richmond, but no 

 noteworthy unconformities have been observed ; the Eocene is 

 unconformable to the Potomac on the ISTottoway and generally 

 about Richmond, and notably in a railway cutting near Brooke 



