the Middle Atlantic Slope. 385 



ment from the James to beyond the Schuylkill ; there is not a 

 fossil nor a rock-variety in one that cannot be duplicated in 

 the other ; the materials are sometimes absolutely undistin- 

 guishable in hand-specimens or in extensive sections except by 

 general structural features ; and in some cases the high-level 

 gravels of the Potomac outliers may be traced continuously 

 into the low-level gravels of the Columbia. The lower mem- 

 ber of the Potomac formation is the great gravel source of the 

 Middle Atlantic slope, and has furnished materials for upper 

 Potomac, Cretaceous, Eocene, and Miocene gravel beds ; but 

 during the Columbia period of delta-deposition the waves of 

 the ocean beat upon its unprotected outliers between the Rap- 

 pahannock and the Raritan, and especially between the Sus- 

 quehanna and the Delaware, and its contributions were larger 

 than ever before. By petrography, by paleontolog} r , by struc- 

 tural continuity, and by physiographic relations, it is proved 

 that the source of the mysterious gravels of the northern 

 Coastal plain are derived from the Mesozoic gravel-heaps of 

 the adjacent Piedmont margin. 



3. In a general way the fine gravel and sand may be traced 

 by petrographic similarity to the immediate sub-terrane and to 

 the terranes traversed by the nearest great water-ways ; and in 

 some cases — e. g., where they consist of redeposited Potomac 

 arkose with little admixture of foreign matter — the exact 

 source may be ascertained. 



4. The amount of loam found in any part of the formation 

 is roughly proportional to the proximity of deltas ; and its origin 

 is evidently the same as that of the predominant element in 

 the upper division of the fluvial phase of the formation, i. e., 

 it is redeposited residuary debris from the Piedmont region. 



5. The clays and silts appear to be made up of the finest 

 and farthest-transported debris from rocks in situ and from 

 the coarser materials, both local and erratic. 



Genesis. — It is impossible to convey definite conceptions of 

 geologic structure or topographic configuration by verbal de- 

 scription ; and it is impracticable to prove the sub-aqueous origin 

 of the interfluvial phase of the Columbia formation by mechani- 

 cal reproduction of structural aspect, as in the fluvial phase ; but 

 neither is necessary (1) since the sub-aqueous deltas of the fluvial 

 phase graduate into and are stratigraphically continuous with 

 the interfluvial phase, (2) since marine fossils have been found 

 within it by a dozen eminent geologists and paleontologists 

 (enumerated later), (3) since no other agency or agencies 

 known to geologic science are competent to produce such de- 

 posits, (4) since its margin is marked by unmistakable shore- 

 lines and beaches, and (5) since every geologist who has ever 

 investigated any considerable part of the formation, including 



