GEOLOGY. 301 



Discarding from our present survey the newer deposits of the region, or 

 those long, narrow, superficial troughs of unconformably overlying red and 

 gray shales and sandstones of mesozoic, or middle secondary age, which 

 partially cover the older or crystalline, and semi-crystalline strata, and 

 restricting our attention to these, we shall find that, when carefully studied, 

 they rank themselves, so far as they admit of subdivision at all, into three 

 natural physical groups. All the sedimentary mineral masses, without 

 exception, are in a condition, more or less, of metamorphism or transforma- 

 tion from the earthy to the crystalline state by heat, and therefore, using the 

 term in a critical sense, all of them are Metamorphic Rocks. In the more 

 current conventional application of this word, only some of them, however, 

 pertain to the usually recognized Metamorphic or Gneissic series; others 

 belong unequivocally to the Paleozoic, or ancient life-representing system, 

 while others again constitute an extensive, intermediate group, not typically 

 gneissic or granitoid in their degree of crystalline structure or metamorphism 

 on the one hand, nor yet fossiliferous on the other, at least so far as the 

 closest scrutiny can discover. For a long while, indeed, from the commence- 

 ment of geological researches in this district of the Atlantic slope, until the 

 geological surveys of Pennsylvania and Virginia had unravelled the com- 

 position and structure of the region, all of these ancient, and more or less 

 altered strata of the Atlantic slope, from its summit in the Blue Ridge and 

 South Mountain, to its base at the margin of tide water, were regarded and 

 designated alike as primary rocks, and were supposed to constitute but one 

 group, and that the oldest known to geologists. Early, however, in the 

 course of those surveys, it came to light that by far the larger portion of the 

 rocky masses of at least the middle and northwestern tracts of the Atlantic 

 slope, including much of the Blue Ridge and of the Green Mountains, was 

 of a different type and age from the oldest metamorphic, or true gneissic 

 system. The evidence in support of this conclusion was, first, an obvious 

 and very general difference in the composition of the two sets of strata; 

 secondly, a marked difference in their conditions of metamorphism, and 

 thirdly and more especially, a striking contrast in their directions and man- 

 ner of uplift, the plications 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 outcrop, dipping to a 

 different quarter. These structural dissimilitudes 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 conviction, that over at least many tracts, a physical unconformity, both 

 in stripe and dip, would be yet discovered. It was not, however, till a 

 relatively late date in the prosecution of the geological survey of Pennsyl- 

 vania, 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 unconformity, 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 Brandywine, and the Sus- 

 quehanna; 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 geologic-til survey from 1843 to 18-51, the 

 true Paleozic Age of the non-fossil iferous crystalline marbles and semi- 



20 



