the Middle Atlantic Slope. 141 



proportion of Blue Eidge quartzite, vein quartz, etc., as appear 

 to be mingled with it, suggesting that the portions of the for- 

 mation now exposed were littoral, and that the liner materials 

 were swept into deeper off-shore waters. 



The History recorded in the Formation. — The conditions of 

 deposition of the lower member of the Potomac formation may 

 be inferred from its structure and composition : the coarseness 

 of the predominant materials is proof of the prevalence of pow- 

 erful currents or violent waves ; the local accumulations of arkose 

 and of finely laminated clay are indicative of quiescent periods, 

 of slack- water eddies, or of sheltered spots on a stormy coast ; the 

 frequent alternation of coarse and fine deposits, the broken up 

 and re-deposited clay beds, and the local unconformities, all 

 suggest repeated alternations of slack water and strong currents 

 throughout the area of deposition ; the distribution of the 

 quartzite pebbles proves that this material was brought down 

 from the easternmost Appalachian mountains by rivers coinci- 

 dent with the great rivers of to-day, and the unequal altitude 

 of the base of the formation along the rivers and the prevalent 

 coarseness and inconstant structure of the deposits there indi- 

 cate that the ancient rivers embouched into deep turbulent 

 estuaries, while the interosculation of some of the estuarine 

 deltas and the coarseness of the deposit connecting others 

 prove violent wave action along the intermediate shore; the 

 dearth of remains of marine and estuarine animals is suggestive 

 of turbulent waters ; and the peculiar distribution and preser- 

 vation of. the plant remains suggests encroachment of the 

 Potomac ocean upon lands covered with a luxuriant flora. 

 The conditions of deposition of the upper member appear to 

 have been similar but quieter. 



From the relations of the formation to the foundation upon 

 which it rests, from structure and composition and indirectly 

 from the conditions of deposition indicated thereby, the physio- 

 graphic conditions attending the deposition of the Potomac 

 formation may be inferred : The surface upon which the de- 

 posits rest is formed of dislocated strata of Archean, Cambrian, 

 Silurian, Triassic and Rhsetic age, all degraded to a plain as 

 uniform as the Piedmont zone of to-day — a plain destitute of 

 noteworthy eminences despite the great heterogeneity of the 

 rocks, and one which accordingly must have been reduced to 

 base level ; yet the unequal altitude of the deposits about the 

 waterways indicate that this plain was ravined as deeply as is 

 the present Piedmont plain ; and the slight sinuosity of the 

 shore line, despite the depth of the ravines, is proof of pro- 

 nounced seaward inclination of the surface. 



Thus the structure, composition, and stratigraphic relations of 



