_H 



474 



THE AZOIC SYSTEM AND ITS SUBDIVISIONS. 



" Since 1858 the district in question has been closely and repeatedly investi- 

 gated by Prof. Frazer and Mr. Hall, the one approaching it from the west, the 

 other from the east. They differ radically in their views of the order and 

 superposition of the formations, not only from Prof. Rogers, but from each 

 other ; so that this report will leave several points of geology in almost as great 

 obscurity as ever." 



Finally, the following may be commended to the consideration of the 



reader (/. c, p. 53) : 



« It will be seen from the summary of Pr(*f. H. D, Rogers' observations in 

 1851, given above, that he was guided everywhere by a theory of parallel anti- 

 clinal folds in the great Azoic or Ilypozoic strata ; and of internuidiate synclinal 

 trowjlis, some of them wide and deep, others narrow and shallow, but all of 

 them containing preserved remnants of more micaceous strata, of a later age, 

 but probably older than the hydro-mica slates of the South Valley hill. In 

 no respect however does he settle the great (pcPtion of what the true relation- 

 ship may be between the older and newer gneisses, — between the newer 

 gneisses and the talc-mica-schists, — between the talc-mica-schists and the 

 sandstone (quartzite) — between the quartzite and limestone — between the 

 limestone and serpentine — in the southern townships of Chester county. 

 After an apparently copious and precise array of facts the geology of the 

 whole district remains as confused and obscure as ever. The section along 

 the Schuylkill is the key to the lock ; but the key will not turn in the lock ; 



the door remains closed We travel to and fro across the hills and find 



no clue to guide us out the labyrinth of infinitely various and yet strangely 

 similar deposits, the strike of which is everywhere more or less doubtful to the 

 eye and tempting to the imagination." 



vmomiA. 



Prof. W. M. Fontaine states that certain coarse sycnitea and granites 

 in the vicinity of Balcony Falls and the Peaks of Otter, ''from their 

 stratigraphical relations and composition," are plainly of Laurentian 

 age. These rocks appear to be regarded by him as being eruptive in 

 many places. Since be assumes without any apparent evidence, except 

 the " look " of the rocks, the passage of sedimentary rocks into remark- 

 able metamorphic forms, and also that other rocks are eruptive from the 

 same evidence (their " looks "), his statements relating to crystallme 

 rocks need in most cases to be accepted with caution until the proof is 

 presented. It is doubtful if the supposed primordial rocks are of that 



