the Middle Atlantic Slope. 133 



posited. At the same time it is too unlike the well defined 

 Cretaceous and Tertiary formations found a few miles to the 

 southeast to warrant correlation with, and is therefore evi- 

 dently older than, any of these formations. In short, it appears 

 to be a remnant of a formation largely removed by erosion 

 before the later Cretaceous submergence, just as are the Poto- 

 mac outliers in Virginia and Maryland ; and on this indirect 

 evidence of its chronology together with that of its petro- 

 graphy it may be provisionally referred to the lower member 

 of the Potomac, although nearly twenty miles from its^nearest 

 supposed, and thirty-five miles from its nearest known, homo- 

 logue, 



It should be noted that the Karitan clays, which are'perhaps 

 equivalent to the superior member of the Potomac, and almost 

 certainly newer than the sands and gravels of the two expo- 

 sures just mentioned, are tentatively regarded as Jurassic by 

 Whitfield* 



Briefly, the Potomac formation consists of two perhaps un- 

 conformable members, of which the upper is an inconstantly 

 bedded and protean clay of variegated colors, either clean or 

 sandy and pebbly, and the lower a generally friable sandstone, 

 arkose, or gravel of irregular and inconstant structure. The 

 upper member extends from the Rappahannock at least to the 

 Delaware and probably to the mouth of the Hudson, either 

 forming or closely approaching the surface over a somewhat 

 sinuous zone 275 miles long and only ten miles or less in maxi- 

 mum width, overlooked throughout by the Piedmont escarp- 

 ment. As a continuous terrane the lower member forms a 

 still narrower zone flanking the Piedmont escarpment from the 

 Appomattox to beyond the Susquehanna, and reappears in iso- 

 lated exposures southward to the Roanoke and northward to 

 beyond Delaware, over a total length of 300 miles ; while as a 

 series of insulated remnants crowning the hills of circumdenu- 

 dation toward the coastward margin of the Piedmont region it 

 occurs occasionally from near the James certainly to the Dela- 

 ware and probably to the Raritan, over a zone from five to forty 

 miles in width. The tenuity of these zones of outcrop is but 

 an accident of degradation and deposition. Studies about the 

 head of Chesapeake Bay led to the conclusion that the former 

 westward extension of the formation can be reliably inferred 

 from the topographic configuration of the Piedmont region, 

 the western part of which has a drainage evidently determined 

 by lateral heterogeneity of the vertically-bedded terrane and a 

 characteristic topography resulting therefrom, while the eastern 



* Brachiopoda and Lamellibrachiata of the Raritan Clays and Greensand 

 Marls of N. J., Monog. U. S. Geol. Surv., ix, 1885 (Mem. Geol. Surv. N. J., 1885), 

 23. 



