88 PROCEEDINGS OF THE ACADEMY OF [1890. 



wholly accounted for by the greater hardness and want of decomposi- 

 tion of the rocks where deep erosion has taken place. 



The 'question of the relative age of these rocks seems most difficult. 

 Prof. Rogers' view is given as follows, p. 79 : — 



" If we now review these interesting features in the structure of this broad zone 

 of gneiss, we can hardly resist the conclusion, that in the three belts passed over 

 by our section, there are really but two groups of rocks, a lower and a higher; that 

 the entire zone, viewed broadly, constitutes but one wide synclinal wave or basin, 

 tiie harder felclspathic and hornblendic gneiss dipping northward throughout the 

 whole southern belt or outcrop and reappearing in steep and multiplied contortions 

 on the other side of the trough, and compressed into the lesser foldings which it 

 exhibits, by the lateral force of the wide crust undulation, within which it has been 

 caught and folded." 



That is, that the Fairmount and Manayunk gneisses formed the 

 southeasterly leg of the synclinal, the Laurentian the northwesterly,, 

 with the Chestnut Hill schists overlying. Dr. T. Sterry Hunt refers 

 to the identification of the rocks of Rogers' third belt with the Lau- 

 rentian and, in his valuable contribution to American Geology 

 published by the Second Geological Survey, Vol. E., seems to regard 

 the Chestnut Hill schists as Huronian, the Manayunk and Fair- 

 mount gneiss and schists as Montalban overlying them, and in these 

 views the lamented Prof. Lewis who made considerable study of 

 these rocks is well known to have coincided,^ though later^ he con- 

 cludes that they are of purely eruptive origin, that a re-crystalliza- 

 tion of the old material under the influence of pressure-fluxion has 

 taken place, by which the feldspar has been re-crystallized, the biotite 

 made out of the hornblende, garnets have been developed and the 

 quartz granulated and optically distorted by the pressure. 



Dr. Hunt's description of the Montalban as given in IMin. 

 Physiology and Physiography, p. 411, will be found to accord in 

 every particular except the " dichroite gneiss and crystalline lime- 

 stone, " with the rocks described in the Manayunk and Chestnut 

 Hill groups. It is as foUoAvs : — 



"As regards this younger gneissic or Montalban series in North 

 America, it need here only be said that it contains fine-grained 

 white gneisses, sometimes porphyritic, but distinct from the granitoid 

 gneisses of the Laurentian, and passing into granulites, on the one 

 hand, and into very quartzose coarse-grained mica schists, on the 

 other. It also includes hornblendic gneisses and black hornblende 

 schists, together with serpentine chrysolite rocks, dichroite-gneiss 



1 Geol. of Phila., a lecture before the Franklin Inst., Jan. 12, 1882, p. 18. 



2 Proc. British Assn. in Nature, Oct. 1st, 1885, p. 560. 



