82 PROCEEDINGS OF THE ACADEMY OF [1890. 



other minenil so far as I have observed, thus differing greatly from 

 the granite of the Fairmount gneiss. 



Some narrow bands of this gneiss are not porphyritic and are a 

 very fine grained hard gneiss. Near Merion Station, Pennsylvania 

 Railroad, one stratum closely resembles the helleflinta of Danne- 

 mora. In the old quarry at the Falls of Schuylkill, immediately 

 west of the stone bridge of the Reading Railroad there was quar- 

 ried a gneiss, somewhat resembling the Fairmount gneiss, but por- 

 phyritic, which the Fairmount gneiss is not. The Falls rock con- 

 tains more feldspar and much less mica, and is much more compact. 



In a quarry on the west bank of the Schuylkill, about a quarter 

 of a mile above the Park bridge, at the Falls, a great variety of 

 rock may be seen, and it is not certain whether this quarry is in the 

 porphyritic belt, or in the Manayunk gneiss. There is much coarse 

 granite like that of the porphyry, also highly feldspathic gneiss 

 like that of the lower quarry, but some beds show the mica so abun- 

 dant as to make a true mica schist, and the curved and twisted feld- 

 spathic layers hereafter referred to are found. In this quarry, pyrite 

 and magnetopyrite occur with epidote which appears to be chang- 

 ing into hornblende. 



On both sides of the porphyry, at the Schuylkill and west of it, 

 the rocks are not essentially different. On the southeast they are 

 somewhat softer and more evenly bedded and hornblende schist is 

 more abundant, but in both there is the same evidence of folding and 

 contortion, particularly exhibited by certain narrow feldspathic 

 veins or beds, compressed sometimes to less than one-sixth their orig- 

 inal length. So far as this region is concerned, this seems to be con- 

 fined to the schists lying on each side of the porphyritic gneiss, and, on 

 Cobb's creek, within it. It is in these schists that the serpentine 

 rocks of Cresheim creek on the Wissahickon occur and also proba- 

 bly all, or nearly all, of the southerly Delaware county outcrops. 

 While the ]\Ianayunk schists contain garnets they are not abundant, 

 but often large. 



There is one gneiss in Philadelphia which presents many points of 

 difference from the other schists and gneisses of the region. This is 

 the Frankford gneiss, best exposed at the quarries on Frankford 

 creek. This gneiss is distinctly stratified. It contains compara- 

 tively little mica and hornblende, some specimens are strictly felsite 

 and granulite. It is extremely hard and of even texture making ex- 

 cellent paving stone when properly broken for that purpose. Its 



