288 PROCEEDINGS 'of THE ACADEMY OF [1880. 



during its degradation, offer the material for the pebbles and 

 boulders of that drift. In Triassic times some such barrier may 

 have formed the southern shore of the Triassic waters. It has 

 been interesting to discover that most of the pebbles belonging to 

 the sub-Cretaceous plastic clays of the Delaware are formed of 

 Potsdam sandstone, and that therefore during lower Cretaceous 

 times also, some such mountain of Potsdam must have offered 

 itself to eroding agencies. Again, it is found that Tertiary* gravels, 

 both in Pennsyh'ania and New Jersey, contain an abundance of 

 Potsdam pebbles. The hypothesis that the materials for the sub- 

 Cretaceous plastic clays and the Tertiary gravels were furnished by 

 hills now sunk beneath the Atlantic Ocean ^ is not sustained by 

 what is known of the configuration of the sea-bottom. The theory 

 now offered is supported by numerous facts concerning the power 

 of erosion, which geological considerations in other fields have 

 presented. 



In a former paper on "The Surface Geology of Philadelphia and 

 vicinity," the writer showed that, in addition to the clays, four 

 separate gravels of different ages can be distinguished in that 

 region. These are (1) ''The River Gravel," the newest of all the 

 gravels; (2) "The Philadelphia Eed Gravel," of Champlain age; 

 (3 ) '' The Fossiliferous Gravel," recently proved by the writer to 

 be of upper Tertiary, perhaps Pliocene age, and now called the 

 " Glassboro Gravel ; " (4) " The Br^ai Mawr Gravel," the oldest 

 of the gravels, also oceanic, aud conjectured to be of upper 

 .^Miocene agfi. This last gravel, and this only, agrees in its char- 

 s.'Sicters with the valley drift now under consideration. In the 

 «;absence of .all Triassic fragments, in the presence of Potsdam 

 'boulders, and in the amount of erosion, these two gravels are iden- 

 :tieal, and it saems probable that the " Drift Iron Ore " of the one 

 is only a very ferruginous variety of the "Mt, Holly Conglom- 

 ero^te " of the olher. This being the case, we have here a formation 

 wh.icii, notwithstanding its boulders, suggestive of floating ice, ap- 

 pears ,to be older than an oceanic Pliocene gravel. There is per- 

 haps no good reason wh}'^ a glacier might not have existed in 

 upper Tertiary tim«s, boulders formed by which ma^^ still be 

 .fouit(i. However tikis may be, it appears that there are strong 

 grourjds for assiguiiag an upper Tertiary age to the drift ore and 

 :gra,vel «of the Montgoiiaer^^ County Valley. 



Retui'Aing, finall}', to the lignite and associated strata, shown to 



•^ .Report,ffla Clay Deposits of N. J., 1878, pp. 20.31. 



