134 W. J. McGee — Three Formations of 



part has a drainage independent of the varying obduracy of 

 the terrane and therefore evidently superimposed by a forma- 

 tion (which could only have been the Potomac) now generally 

 removed by erosion ; this conclusion has since been verified by 

 the discovery of isolated remnants of the Potomac formation 

 on many parts of the area of supposed superimposed drainage ; 

 and it may be confidently inferred from the configuration of 

 the Piedmont plain that about half of its area north of the 

 James River (but not much more than half) was submerged 

 beneath the Potomac sea and at one time covered by its de- 

 posits. A great extension of the formation eastward may also 

 be inferred from the Fort Monroe boring, fifty miles from the 

 nearest outcrops, in which undoubted Potomac deposits were 

 found in considerable volume. The thickness of the forma- 

 tion, either original or present, has not been accurately ascer- 

 tained ; the upper member must approach 350 feet and the 

 lower 250 feet near Baltimore and Acquia creek respectively, 

 from which points both attenuate gradually, giving an aggre- 

 gate (the maxima being nearly 100 miles apart) of perhaps 

 500 feet ; and while the loss by erosion at the localities of 

 maximum thickness has not been great, it cannot be readily 

 evaluated. 



Stratigraphic Relations. — South of the James river, the 

 Potomac deposits rest upon an irregularly eroded and some- 

 times deeply ravined surface of highly inclined crystalline 

 rocks, the inequality in altitude of the base within a mile on 

 the same meridian (the formation and its subsurface having a 

 considerable eastward inclination) reaching 100 feet or more. 

 On that river the formation rests upon the irregular surface of 

 the Richmond granite at the " fall-line," and upon the trun- 

 cated edges of highly tilted gneisses and schists down the river 

 for 20 or 30 miles; while to the westward outlying gravel 

 patches repose unconformably on the tilted and faulted 

 Rhaetic* beds forming the Richmond coal field. Near Han- 

 over Junction, the upturned Piedmont crystallines and the 

 faulted, tilted, and diked strata of the Rhaetic are alike 

 overlain by the Potomac arkose which fills deep ravines in, and 

 conceals irregularities of, the subjacent surface ; and thence 

 northward to the river from which they take their name, the 

 Potomac sandstones repose unconformably upon irregularly 

 eroded surfaces of gneiss, except at Drainesville (25 miles west 

 of "Washington), where a gravelly outlier rests on the planed 

 edges of tilted Triassic sandstones. At Washington the in- 

 equality in altitude of the base of the formation, which evi- 

 dently filled the valley of the Mesozoic progenitor of the Po- 



* Fontaine, "Older Mesozoic Flora of Virginia," Monog. U. S. Geol. Surv., VI. 

 1883, 2, 96, 128. 



