CAMBRO-ORDIVICIAN KOCKS 299 



which lies on the west side of the Schuylkill and follows the southeast 

 flank of the south Chester Valley hill, there are three exposures of the 

 limestone — in West Conshohocken, at the head of the Gulf ravine, and 

 in the bed of Gulf creek 11 miles southwest — while a series of sink holes 

 in line with the strike of these outcrops attest the presence of the lime- 

 stone near the surface. This line of outcrops is continued southwest in 

 Chester county, where limestone comes to the surface at some seven 

 localities and is farther exposed to the south of this series of outcrops at 

 nine isolated localities. 



Character of the formation. — The limestone is highly silicious and mag- 

 nesian, but there are no analyses of it which give a sufficiently high 

 percentage of magnesium carbonate to warrant calling the formation a 

 dolomite. 



Limestones from Mogeetovm to Conshohocken 



1 2 3 



CaCO a 60.18 55.09 41.59 



Fe 2 C0 3 Not determined. 



Insoluble resi- Not 

 due ! det. 2.59 26.48 46.10 37.60 2.65 7.85 9.83 5.89 6.60 5.63 4.79 3.15 



West Conshohocken 



CaC0 3 40.27 



MgC0 3 31.24 



SiO, 24.23 



A1 2 3 1.12 



FeA 1.06 



MgO 0.11 



CaO 0.55 



Alkalies 1.42 



4 



5 6 



7 



8 



9 10 11 



12 



13 



39.95 



48.04 63.36 



91.62 



53.27 



58.02 61.43 93.32 



85.00 



54.11 



2.55 



Not deter. 













100.00 



Analyses made by F. A. Geuth, volume C6, Second Geological Survey of Penn- 

 sylvania, pp. 126, 127. 



The limestone is always crystalline and increasingly so from west to 

 east. Associated with increasing crystallinity is a lighter color, though 

 blue and white marble may occur in the same quarry. 



It is sometimes quite micaceous and is always so in the neighborhood 

 of the overlying mica-schist. The beds immediately underlying the 

 mica-schist are silicious, micaceous, and schistose, and are to be char- 

 acterized as calcareous schist. It is abundantly traversed by calcite and 

 quartz veins. Quartz, feldspar, phlogopite, graphite, pyrite, siderite, and 

 limonite are accessory constituents. 



Stratigraphic relations. — The limestone lies above the Cambrian quartz - 

 ite in an overturned synclinorium. The prevailing strike is north 60 to 



