PENNSYLVANIA. 



467 



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PENNSYLVANIA. 



The views of Prof. H, D. Rogers, former State Geologist of Pennsyl- 

 Tania, in regard to the lower formation in the region embraced within 

 his Survey, have already been briefly mentioned.* Two years later 

 than the date of the publieation to whieh reference ia there made 

 1858, namely —his Final Kcport appeared. In this Report, Professor 

 Rogers adheres, essentially, to the ideas maintained by himself in the 

 article in Johnson's Physical Atlas. In the introductory chapter to Part I. 

 of the Final Report, under the heading of " Classification of the Metaitior- 

 phic Strata of the Atlantic Slope of the Middle and Southern States," 

 he discusses certain questions which pertain to the subject of this 

 paper. He remarks, that, previous to the light thrown upon the older 

 rock formations of the Atlantic Slope by the Geological Surveys of Penn- 

 sylvania and Virginia, these rocks were supposed to constitute but one 

 group, and were included under the name of " Primary," and he then 

 proceeds to make the following statement : 



" 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, including much of the Blue Ridge and of the Green Mountains, were 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 gen- 

 eral difference in the composition of the two sets of strata ; secondly, a marked 

 diil^rcncc in their conditions of metamorphism ; and thirdly, and more espe- 

 cially, a striking contrast in the direction and manner of their uplift, the pli- 

 cations and undulations of the less metamorphic series dii:)ping almost invariably 

 southeastward, while the gneiss in many localities has no synnnctrical foldings, 

 but only a broad outcrop dipping to a different quarter." (/. c, pp. 62, 03.) 



From the above statement it would appear that there could have 

 been no difficulty, in the course of the Survey, of drawing the line thus 

 said to be perfectly well defined by ^^ an obvious and verij general differ- 

 ence in ike com2')osition of the tivo sets of strata,^* by " marked, differences in 

 their condition of metamorphismy^ and finally, l)y "a striking contrast in 

 the manner and direction of their uplift^ One is surprised to find, how- 

 ever, that all the above-stated difterences were not realities, but *'iuduc- 

 tions," and that it was not "w?^^' a, relatively late date in the prosecutio7i 

 of the Geological Survey of Pennsylvania, thai the geologist of that State 

 detected jjositive evidences of this physical break, and of a lap}se of time be- 



* Sec mitCj pp. 440, 441 



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