142 



striking contrast in their directions and manner of uplift, the pli- 

 cations and undulations of the less metamorphic series, dipping 

 almost invariably southeastward, while the gneiss presents in 

 many localities, no symmetrical foldings, but only a broad out- 

 crop, dipping to a different quarter. These structural dissimili- 

 tudes imply essential differences in the direction and date of the 

 crust movements, lifting and transforming the respective groups, 

 and led the geologists of Pennsylvania and Virginia to a convic- 

 tion, that over at. least many tracts, a physical unconformity, both 

 in strike and dip, would be yet discovered. It was not, however, 

 till a relatively late date in the prosecution of the geological sur- 

 vey of Pennsylvania, that the geologists of that State detected 

 there positive evidences of this physical break, and interval in 

 time between the two groups of strata, and established by ocular 

 proof the correctness of the previous induction. This unconform- 

 ity, reflecting so much light on the whole geology of the Atlantic 

 slope, was first clearly discerned in tracing the common boundary 

 of the two formations from the Schuylkill to the Brandy wine, 

 and the Susquehanna, but it was quickly afterwards recognized on 

 the borders of the gneissic district, north of the Chester County 

 limestone valley, and again, soon after, in the Lehigh Hills at 

 their intersection with the Delaware. 



Prior to the suspension of the geological survey from 1843 to 

 1851, the true Paleozoic Age of the non-fossiliferous crystaUine 

 marbles, and semi-crystalline talcoid slates, and vitreous sand- 

 stones of the Chester and Montgomery Valley, had been clearly 

 demonstrated by the State geologist, through a comparison of the 

 strata with their corresponding formations in a less altered con- 

 dition further north ; but it was not until the resumption of field 

 research, upon the revival of the survey in 1851, that any dis- 

 tinctive fossils were detected in these greatly changed rocks, 

 ■which even in their original state seem to have been almost des- 

 titute of their usual organic remains. 



Assembling all the evidence which we now possess, we have 

 in the Atlantic slope by actual demonstration but one physical 

 break or horizon of unconfoi'mity throughout the whole immense 

 succession of altered crystalline, sedimentary strata, and within 

 this region but one paleontological horizon, that, namely, of the 

 already-discovered dawn of life among the American strata. 



