1890.] NATURAL SCIENCES OF PHILADELPHIA. 83 



granite veins, or more properly segregations, are chiefly ortlioclase 

 which contains at times G<ithite in minute crystals ; muscovite occurs 

 in the granite, but not in large quantities, also a highly lustrous 

 black mica, which Prof. Lewis determined to be lepidomelane.^ 

 The gneiss is considerably jointed, and many of these joints are filled 

 with stilbite in radiated crystallizations. Heulandite and apophyllite, 

 the latter in very fine specimens, also occur in these joints. 



Calcite occurs in small quantity apparently filling joint fissures, and 

 in the calcite, fine crystals of epidote. Copper minerals occur, 

 also molybdenite, at times abundantly, and sometimes in remark- 

 ably fine crystals showing the planes of the dome as well as the 

 prismatic planes, perfectly bright and sharp. Sphene and uranium 

 minerals occur, but the Autunnite so common in the Fairmount and 

 Chester gneiss has not been detected, while sphene and molybdenite 

 have not been found in the Fairmount gneiss.^ The strike of this 

 gneiss is nearly due west, and outcrops of it occur at Wayne station, 

 Germantown, and at McKinney's quarry on the Wissahickon. The 

 late Prof. H. Carvill Lewis regarded this gneiss as " a highly met- 

 amorphosed intrusive dyke of Lower Silurian*age." ^ 



There is another rather insignificant outcrop of a rock which does 

 not fall within the description of any of the group, exposed on the 

 Pennsylvania Railroad at 59tli Street. It is chiefly a very compact 

 felsite when not weathered, but some portions seem to be a mica schist. 

 It is peculiar in containing a large percentage of pyrite, in some of the 

 beds causing rapid disintegration on exposure, while others equally 

 pyritiferous, but more compact, are perfectly stable. It apparently 

 occupies a synclinal just southeast of the porphyritic gneiss, the axis 

 of the synclinal pitches downward northeastward about thirty 

 degrees. 



The rocks of Rogers' second group, Mr. Hall's Chestnut Hill 

 schists, are in certain areas very well marked, especially the garnet- 

 iferous variety, and that in which the wood-like structure is most 

 developed, but there are areas difficult indeed to distinguish from the 

 Manayunk schists. This may be well seen along the Wissahickon 

 where the exposures are excellent yet no line can be drawn between 

 the two, and in the Chestnut Hill schists, at Middleton's quarry, about 



1 Pioc. Min. and GeoJ. Sec. A. N. S. No. 1, p. 11. 



2 Molybdenite however occurs at Upland near Chester, perhaps in the Fair- 

 mountChester gneiss. 



3 Nature, Oct. 8, 1885, p. 560. 



