1880.] 437 [Hall. 



first, second, third and fourth groups and arc somewhat different in charac- 

 ter from the fifth group, though they resemhle portions of the sixth group. 



There are besides these groups probably two serpentine horizons, which 

 are undoubtedly unconformable deposits above the second group. I think 

 the northern belt of serpentine may be considered as altered Hudson river 

 rock ; while the southern belts are doubtful. 



The first and second groups are the oldest rocks, overlaid by the Pots- 

 dam sandstone unconformably. The Potsdam is flanked on the south by 

 the first from Willow Grove to Chestnut Hill, where this group seems to 

 be succeeded by the second. It may be only the upper part of the first, 

 however, the sandstone rests on both. The first group is flanked on the 

 south by the Edge Hill rock or Potsdam S. S. from the Delaware river to 

 the Pennepack creek in Montgomery county. To the north the upturned 

 edges of these rocks are overlaid by the New Red Sandstone. West of the 

 Pennepack creek the structure is plainly a synclinal, the axis of which 

 would be just south of Willow Grove; and an anticlinal, the axis being 

 about on a line from Abington to Attleboro. 



The syenitic rocks flank the Potsdam on the north of the synclinal 

 north of Willow Grove, encircle the end of the synclinal and are exposed 

 along the anticlinal to a point near Chestnut Hill. The Potsdam sand- 

 stone is not found between Huntingdon Valley on the Pennepack and 

 Waverly Heights, south-west of Edge Hill P. O., along the south side of 

 the anticlinal. 



The overlying limestone occurs just south of Huntingdon Valley, over- 

 lying the sandstone, and extends westward beyond the Pennepack creek 

 some distance, lying immediately above the gneisses of the first group. 



The unconformity is evident between the gneisses of the first group and 

 the limestones, and inasmuch as the limestone occurs almost on the line of 

 strike of the sandstone which again appears at Waverly Heights, it would 

 seem to be sufficient proof of its unconformity to the sandstone. 



Resting on the Potsdam sandstone from the neighborhood of the Dela- 

 ware river to a point near Huntingdon Valley, and on the limestones be- 

 tween Huntingdon Valley and Waverly Heights, and also on the Gneisses 

 of the first group, we have the micaceous, garnetiferous schists of the 

 Philadelphia group. These are unconformable, resting upon and against 

 these lower rocks. 



The sandstone along the south margin of the synclinal, which I spoke 

 of as extending from Willow Grove to the Schuylkill is, most of it, very 

 different in character to that along the north side. The difference in 

 character on the north and south sides of the valley may be due to the in- 

 filtration of ferruginous matter derived from the New Red sandstones 

 which flank the group and overlap it on the north. 



The dips are high, the rocks are sandy, light colored and very free from 

 iron as a rule ; the ridge is known as Edge Hill. Towards the Schuylkill 

 it dies down rapidly, and disappears below the limestones at Spring Mill, 

 not only swallowed up by a fault, but unconformably overlapped as well. 



On the north side of this synclinal valley, we find the quartzites and 



