468 



THE AZOIC SYSTEM AND ITS SUBDIVISIONS. 



twaen the two groups of strata^ and established j by ocular proof, the correct- 

 ness of the previous induction^ 



In accordance witli thia latter statement, and not with the former, we 

 do not find on the gcijcral geological map of the State accompaiijing 

 the Final Eeport any indication of the two groups in question. All the 

 crystalline rocks seem to be indicated by a single color, and included 

 under the term " hypozoic." 



Eeading still farther In Professor Eogers's Eeport {I. c, p. 63), we are 

 even more surprised than before, when we learn that 



" Assembling all the evidence which wc now possess, we have in the Atlan- 

 tic Slope, by actual demonstration, but one physical break or horizon of un- 

 conformity throughout the whole immense succession of altered crystalline 

 sedimentary strata, and within this region but one Palaiontolugical horizon, -^ 

 that, namely, of the already discovered dawn of life among the American 

 strata. This latter plane or limit, marking the transition from the non-fossil* 

 iferous or azoic deposits to those containing organic remains, lies within the 

 middle of the primal sciries or group of the Pennsylvania survey, that is to say, 

 in the primal white sandstone The Primal slates beneath the sand- 

 stone, and in intimate alternation with it, possess not a vestige of organic life, 

 nor has any such been yet discovered anywhere within the limits of the Allan* 

 tic Slope, or on the northern or \vestern borders Of the Great Appalacliian 

 Basin of North ATnerlca." 



Farther on in his Eeport, Professor Eogers remarks concerning his 

 two systems of rocks, — the Hypozoic or Gncissic, and the Azoic, --^ 

 that "the members of the two groups often simulate each other so 

 closely, and are indeed so identicalin mineral aspect and structure, as to 

 baffle all attempts at distinguishing them lithologically T Again, wc arc 

 informed that these systems are nevertheless distinct from each other, 

 and ''susceptible of delineation on the geological map." But, it is im- 

 mediately added, "the State geologist did not venture to dcfme them 



on the map." 



Before proceeding to notice the results attained by the Second Geo- 

 logical Survey of Pennsylvania, in connection with the investigation and 

 classification of the crystalline rocks of that State, it will be well to refer 

 to the views published by Dr. Hunt at various times between the com- 

 pletion of the First Survey and the beginning of the Second. This is 

 the more desirable, because ho was ertiploycd by the State Geologist 

 (Professor Lesley) 'Ho collate all the Icuovm, supposed, and suspected facts 

 of American Azoic Geology'' for publication by that Survey, apparently 

 as a sort of manual or guide to the mysteries of that department of the 

 science, for the use of future investigators. Professor Lesley considers 



