lhiii.] 4db [j an , 2, 



Third. Potsdam sandstone, conglomerate, quartzite, and occasional 

 schistose hcils. In this group is included the Edge Hill lock which ex- 

 tends in an unbroken ridge from the Delaware river at Trenton to Hunt- 

 ingdon Valley in Montgomery county, and another ridge of the same rock 

 from a point south of Willow Grove to Spring Mill, Montgomery county, 

 near the Schuylkill river. The Edge Hill sandstone is identical with the 

 quartzites flanking the north side of the limestone valley of Montgomery 

 and Chester counties, and merges into them about Willow Grove. This, 



the Potsdam sandstone, rests unconformably 

 upon the preceding two groups. The un- 

 conformity is seen at points east of Willow 

 Grove, where the lower conglomerates con- 

 tain fragments of the syenitic rocks. 



Fourth. Dolomites, schistose or slaty micaceous beds, limestone, mar- 

 ble, hydromica schists and bastard marble. This group of limestones and 

 schists rest upon the above group, and are the equivalent of the Cambrian 

 limestones of the Great Valley. Trenton fossils have been found in the 

 upper part of this group at Buckingham, Bucks county, by Mr. Ash. This 

 Bucks county belt of limestone is cut off from the limestones of Chester 

 Valley by the New Red Sandstone. There is no apparent unconformity 

 throughout the limestone group. The lower beds are Dolomites ; there 

 are occasional alternations of shale throughout the mass. The marbles are 

 all confined to the upper horizon and are followed by alternations of shale 

 and shaly limestone. 



Fifth. Hydromica schists, quartzose schists, chloritic schists, and occa- 

 sional beds of quartzites and sandy beds and serpentines. It is difficult to 

 draw a line between J.his group and the limestones, which pass into it by 

 alternations exactly as the Trenton limestone passes into the shales of the 

 Hudson river group in Central Pennsylvania. These are the Hudson river 

 shales and flank the Chester Valley on the south from some point not far 

 east of the Schuylkill river throughout the entire length of the valley. 

 They extend south to the syenitic rocks of the second group, and west of 

 the Schuylkill to the neighborhood of the Brandywine creek in Chester 

 county, and gradually widening out to the south-west. 



Sixth. Micaceous, garnetiferous schists, limestone in beds which rapidly 

 thin out to the eastward, mica schists, and sandstones. The area of this 

 group I have not determined, but it is principally confined to the southern 

 central portion of Chester county, resting upon the Hydromica schists of 

 the group above-mentioned. The eastern boundary is about the line of the 

 Brandywine creek, although a tongue extends cast of the creek to the 

 neighborhood of Dillworthtown. This group rests unconformably upon 

 the western extension of the second group. 



Seventh. The mica schists of Philadelphia, mica schists, hornblendic, 

 garnetiferous, talcose schists with soapstone and serpentine. These rocks 

 lie to the south of the first and second groups of rocks, and are cut off on 

 the west and south by the rocks of the second group from any connection 

 with the rocks of Chester county. They rest unconformably upon the 



