LARGER RELATIONS OF DELTAS 411 



as shown by the overlap of the Patapsco formation on the ancient rock 

 floor north of Philadelphia. Erosion was therefore weakening and the 

 sea was gaining more power. Consequently by the beginiiiiig of* Upper 

 Cretaceous time the shoreline had advanced inland, oscilhiting over the 

 present limits of the Coastal Plain and laying down the Magothy forma- 

 tion. Then the sea, gaining headway, planed still farther inland and 

 gave rise to the marine formations of the Upper Cretaceous and Eocene. 

 These, if they could be restored to their original shoreward limits, might 

 possibly be found to contain at certain liorizons restricted terrestrial 

 deposits. The marls, glauconitic sands and clays, and diatoniaceous 

 earths suggest, however, that sedimentation became so slack that the sea 

 probably planed inland and closed completely the delta cycle. The oscil- 

 lations and subsidence shown by the stratigraphic record follow the 

 outline previously given for the theoretic delta cycle, but there is here 

 imposed as a further condition a tilting; uplift accentuating erosion in- 

 land, while toward the margin of the continent subsidence made more 

 room for deposition of terrestrial beds. This condition Is a not uncom- 

 mon one in crust movements, tilting without regional subsidence acting 

 to prolong the life of the delta cycle. In thi$ case it lasted through late 

 Jurassic and all of Lower Cretaceous time. 



If the interpretation here offered shall be found to be an advance on 

 previous conceptions it will justify to that degree the use of the theory 

 of the delta cycle as a criterion of interpretation. 



CONTRAST OF MESOZOIC AND PALEOZOIC DELTA CONDITIONS IX THE 



APPALACHIAN PROVINCE 



During the Paleozoic the zone of uplift of the Appalachians, rejuve- 

 nated by successive movements, lay over the present Piedmont Plateau, 

 the Coastal Plain, and doubtless still farther east over a region now cov- 

 ered by the sea. The regional uplifts were doubtless varied in nature 

 but were several times of an orogenic character, mountain axes being 

 separated by such intermontane depressions as the Narragansett and New 

 Brunswick basins. To the west of this ancient land of Appalachia lay a 

 great axis of subsidence which, as a result of the transformations wrought 

 through geologic time, has since the Paleozoic, and especially in the 

 Cenozoic, assumed the position of maximum uplift. No evidence is 

 available as to the distance to which the land extended toward what is 

 now the open ocean, and the sediments swept from it in that direction 

 are lost to observation. The formations which are accessible are those 

 laid down to the west and represent that portion of the Appalachian 

 detritus which was swept inland, largely trapped within the adjacent 



