141 



derived chiefly from a study of the formations of this portion of 

 Pennsylvania. 



Discarding from our present survey tlie newer deposits of the 

 region, or those long, narrow, superficial troughs of unconform- 

 ably 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 atten- 

 tion 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 

 metaraorphism or transformation from the earthy to the crystal- 

 line state by heat, and therefore using the term in a critical sense, 

 all of them are Metamorphic Rocks. In the more current con- 

 ventional 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-rep- 

 resenting 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 

 commencement of geological researches in this district of the 

 Atlantic slope, until the geological surveys of Pennsylvania and 

 Virginia had unravelled the composition 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 tar 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 verj^ general difference in the composition 

 of llie two sets of strata; secondly, a marked difference in their 

 conditions of metamorphism, and thirdly and more especially, a 



