1890.] NATURAL SCIENCES OK PHILADELPHIA. 121 



Valley, reaching the Laurentian at Devon, south westward of that 

 seeming to retain no longer its rectilinear aspect. 



I believe, therefore, that there are in this region certainly 

 tw'o, and perhaps four horizons of serpentine ; one in the Laurentian, 

 a second, perhaps in, but certainly if not in, almost immediately 

 above, the Laurentian, a third of altered schists, probably a synclinal 

 in mica schists of the Chestnut Hill series and the fourth altered horn- 

 blende-like rocks in the Manayunk schists. It may be that the first 

 should be groui)ed with the second, and the third with the fourth. 

 It would seem probable that at the close of the Laurentian period 

 in this region magnesian silicates were abundant, possibly derived 

 from the trap rocks so common in the Laurentian of Delaware 

 County, or from volcanoes whose existence the trap dykes indicate ; 

 that subsequently, portions of the rocks were eroded and probably 

 sorted by wave action, and owing to their high specific gravity 

 formed beds in the more recent schists similar to the magnetite beds 

 of the Laurentian. 



As to the age of these schists much more must be learned before 

 we can feel any certainty. Lithologically they are Montalban and 

 Huronian, but if there is no fault and the sandstone at Waverly 

 Heights and vicinity is Potsdam, the schists adjacent to the Lauren- 

 tian are of that age or more recent. This the exposure on the 

 Paper Mill road at Chestnut Hill seems to confirm. I have seen 

 no evideuce of the supposed fault and much to lead me to believe 

 that it does not exist — at least that it does not exist as an extensive 

 fault. Minor faults are common. 



The belief that the Potsdam is absent west of the Schuylkill has, 

 I believe, led to this error and to the connecting of the North 

 (Chester) Valley Hill with the Barren Hill ridge i ; whereas they are,, 

 apparently, as shown on the map in C*^, parts of the opposing legs of a 

 synclinal underlying the Chester-Montgomery limestone vallev,, 

 and my observations make the structure west of the Schuylkill 

 almost precisely that shown in C**, at Spring Mill, viz., going 

 northwest from the 1 -Laurentian ; 2-Rogers' altered primal ; 8-Pots- 

 dam ; 4-limestone No. 2 of Cream Valley ; 5-Hydromica schist (South 

 Valley Hill); 6-Limestone No. 2 (of Chester Valley) ; 7-Potsdamof 



1 Prof. Lesley, Notes of the Geol. of the Schuylkill river, 1884, p. 6, says: 

 " On the east side of the river at Spring Mills the limestone is evidently faulted 

 against the azoic rocks of the Philadelphia belt." But the visible succession is 

 southtast, 1-Laurentian, 2-Roger's altered primal, 3-Poisdam sand>tone, 4- 

 limestone, 5-hydromica (with trapj, 6-limestone, 7-Potsdam northwest. 



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