130 W. J. McGee — Three Formations of 



(where two deep ravines in the Potomac are filled with fossil- 

 iferous Eocene deposits), though in some sections at Richmond 

 the two cannot be discriminated, and at Good Hope Hill, 

 near Washington, Eocene fossils occur in dejDOsits made up of 

 Potomac materials supposed to be in situ before the discovery 

 of the fossils ; and on Sassafras river there is a marked uncon- 

 formity between the pyritous clays and greensands of the 

 upper Cretaceous and the subjacent Potomac clays, while in 

 the best section about the head of Chesapeake bay (Maulden's 

 Mountain) the sequence from massive Cretaceous greensand 

 above to iron-bearing Potomac cla}^ below is unbroken save by 

 arbitrary lines. In short, it is evident that the Potomac ter- 

 rane has ever lain near sea-level, and has alternately suffered 

 less by denudation and accretion by sedimentation during oft- 

 repeated oscillations, has been deeply ravined during one epoch 

 only to have the ravines filled largely with its own materials but 

 with some foreign matter and more recent fossils during the 

 next, and has contributed material to each newer formation of 

 the Coastal plain. Its characteristic pebbles have indeed been 

 successively transferred to the newer Mesozoic beds, to several 

 of the Cenozoics, and finally incorporated in the Quaternary. 



There is an apparent unconformity between the two mem- 

 bers of the Potomac, marked by beds and pockets of gravel 

 superimposed upon erosion planes, as in a. noteworthy section 

 three miles southeast of Washington ; but since local unconfor- 

 mities, including both erosion planes and accumulations of 

 gravel and bowlders, occur at various horizons in each member, 

 there is some doubt as to the importance of this apparent un- 

 conformity. 



Fossils. — Until recently animal remains were believed to be 

 exceedingly rare in the formation : fragments of a rib of a 

 cetacean and part of the teeth and bones of a reptile supposed 

 to be related to the Iguanodon are recorded by Tyson from the 

 upper member in Maryland,* the latter being the Astrodon 

 Johnstonii of Leidy ;f six undescribed species of the fresh 

 water genera TJnio and Anodonta and several " ctenoid fish 

 scales " associated with " leaves of dicotyledonous trees " are 

 noted from the upper member in western New Jersey, and 

 identical Umos " from the banks of the Potomac " by Cope \% 

 Conrad described from the plastic clays of New Jersey (which 

 are either equivalent to or newer than the upper division of 

 the Potomac formation, but which he regarded as Triassic), 

 two lamellibranchiates, Astarte veta, and A. annosa ;§ and 



* First Report Maryland Agricultural Chemist, 1860, 42 ; cf. Uhler, Johns Hop- 

 kins University Circulars, vol. ii, No. 21, 1883, 53. 



f Cretaceous Reptiles of the U. S., Smithsonian Contributions to Knowledge. 

 vol. xiv, 1865, 102. 



% Proc. Acad. Nat. Sci., xx. 1868, 157. § Am. Jour. Conch., iv, 1868, 279. 



