Hall. "*42 |.]an. J, 



phia rocks, and, by so doing, assign the Philadelphia series to a higher 

 group than the Hudson river. 



The relation of the Philadelphia schists to the schists of the sixth group 

 is not fully determined, but they bear a great resemblance to them, and in 

 many respects are identical. The syenitic rocks of Delaware county which 

 belong to the second group, cut off the connection between them. 



To all appearances the serpentine belts, which are visible on the Schuyl- 

 kill river at Lafayette station, Montgomery county, and at a point just 

 north of them, are above the mica schists of Philadelphia. The southern 

 belt extends in an almost unbroken line from Chestnut Hill, Philadelphia, 

 to Bryn Mawr, in Montgomery county. A' less prominent belt extends 

 from the Schuylkill river to the neighborhood of Rosemont station, on the 

 Pennsylvania R. R., in a parallel line to the first belt. 



The serpentines of Delaware county are on a general line of strike with 

 these belts, and without doubt represent the same horizons. 



There is no evidence in this section of the Oneida conglomerate or 

 Medina sandstone, as far as I can observe, but there are sandstones in the 

 south-eastern portion of Chester county I have classified with the sixth 

 group, which may prove to belong to a sandstone formation succeeding the 

 Hudson river slates. 



LIMESTONE. 

 MICA SCHISTS. 

 f SERPENTINES. 

 | GARNETIFEROUS SCHISTS. 

 HYDROMICA SLATES. 

 CHLORITIC SLATES. 

 Alternations of Slate 

 and Limestone. 



MARBLE. 

 II. -j SLATE. 



1 DOLOMITES. 

 I. POTSDAM sandstone. 



A QUARTZITE. 



SYENITIC AND 

 GRANITIC ROCKS. 



There is no doubt that magncsian beds may be altered into serpentines 

 wherever they may be, and the mere fact of serpentine existing at any 

 place is not proof of a given horizon, but it is in all probability confined to 

 definite horizons within limited areas. 



The whole question of structure would be easily solved could we prove 

 whal is everywhere indicated, viz, a gradual subsidence of the formations 

 north-west of the line of junction between the South Valley Hill slates and 

 the syenitic rock of tin; second group, which change in level, at, the close of 

 the Hudson river group, allowed the upper magnesian beds of that group 

 to lie deposited over the edges of and encircle the second group. 



The evident nonconformity between the Philadelphia schists and the 



in. { 



! 



