1880.] 441 [Hall. 



The lower portion of Pequea creek flows along an anticlinal in which 

 the limestone is exposed along the creek to the neighborhood of Martic- 

 ville P. O. A tongue of slates extends eastward between the Pequea and 

 Conestoga creeks as far as Willow Street P. O., which is on a line of a 

 synclinal axis shown to end at Compassville, and along which the Pequea 

 creek flows from Compassville to Wheatland Mills P. O. An anticlinal 

 having its axis about on a line between Petersville P. O. and the mouth of 

 Conestoga creek. A synclinal of Chloritic slates ending somewhere near 

 Indian town. An anticlinal exposing the limestone, extending from Pros- 

 pect Furnace P. O., in York county, to Lancaster. 



A synclinal of slates extending from a point south-east of Montville P. O. 

 to Washington Manor P. O., and crossing the river, is evident from the 

 distribution of slates in York county, beyond the line of the Peach Bottom 

 R. R. Lastly, we have a monoclinal flanking the Chickis Potsdam, extend- 

 ing from Hempfield, which is at the extreme eastern end of the Potsdam 

 anticlinal through Columbia in Lancaster county, and Wrightsville, York, 

 Springforge to Hanover in York county. North of York the limestone 

 is not continuous across to the northern arm of the anticlinal, which is 

 principally covered by the Trias, but has a considerable exposure in 

 Lancaster county. The slates, therefore, south-east of the limestones of 

 Chester county, are of Hudson river age. 



The slates of York and Lancaster counties, which includes the roofing 

 slates of Peach Bottom, are a continuation of those of Chester county, 

 which flank the limestone on the south, and are referable to the same group. 



The serpentines of Radnor township, Delaware county, and those of 

 Easttown, Willistown, East and West Goshen, are undoubtedly altered 

 beds of the South Valley Hill slates, or Hudson river slates. They lie un- 

 conformably upon the syenitic rocks of the second group. 



The probability is that all the serpentines of Chester county will be 

 found to belong to the Hudson river group, and are possibly pretty nearly 

 on the same horizon as the limestones of West Bradford, Merlin, east and 

 west Marlborough, London Grove, Kennet, and other townships of south- 

 ern Chester county, although I have spoken of this limestone group as 

 being possibly of Helderberg age. The serpentines of southern Lancaster 

 county are undoubtedly altered beds of the Hudson river group, and from 

 their relative position to the roofing slates of Peach Bottom, would be in 

 their proper place. 



Dr. T. Sterry Hunt insists that the serpentines of the Schuylkill are 

 below the Philadelphia schists. If they are, the structure would be even 

 more simple than otherwise. Placing them below the Philadelphia schists 

 they would be on a horizon with the serpentine beds of Chester county, 

 and these Philadelphia rocks equivalent to those which they resemble in 

 southern Chester county : but if the serpentines of Montgomery and Dela- 

 ware counties are above the Philadelphia series, they necessarily belong to 

 a later age than those of the Hudson river group. 



At present I am inclined to place these serpentines -above the Philadel- 



PROC. AMER. PHII.0S. SOC. XVIII. 105. 3e. PRINT! ID MARCH 2, 1880. 



