NATURAL SCIENCES OP PHILADELPHIA. 301 



It is, liowever, most probable that this is a distortion of cc P oo. 



In the cutting of the Pennsylvania Railroad, northwest of the 

 wire bridge, an orthoclase was found, somewhat decomposed and 

 chalky in aspect, cleaving into rhombohedra of the second kind, 

 and also into six-sided prisms, ter- 

 minated by the OP plane, and from 

 a slight distortion in angle the ^^ff- ^ 

 prism angles each measuring ex- <xPx 



actly 120° (Fig. .3). 



In Frankford, the orthoclase oc- 

 curs in a granitic vein on the south- 

 ern portionof thebedof hard horn- 

 blendic gneiss, the IST. E. outcrop of '^^ ^ 



which is at this point, and which is 

 exposed also near Wajnie St. on 



the Germantown Railroad, and on Rittenhouse Lane near the 

 Wissahicon, and which crosses the Schuylkill at the hill through 

 which the Flat Rock Tunnel on the Reading Railroad is pierced. 

 This gneiss throughout is very hard, and in its fissures occur 

 several species of zeolites, with calcite, and at Frankford crys- 

 tallized epidote in fine specimens, and also fluor spar in a vein 

 of calcite, and well-crystallized molybdenite in a vein of ortho- 

 clase. The calcite contains also j-ellow crystals so minute as to 

 be detected only by examination with the microscope, of the por- 

 tion insoluble in hydrochloric acid. Their nature has not been 

 determined, the quantity being verj^ small. There also occurs a 

 yellow hyaline coating, in veins of the gneiss, which is probably 

 a hyalite, colored by uranium. 



This gneiss, except near Wayne Station, is bedded with great 

 regularity, and affords a most excellent curb and building stone ; 

 its fracture in one direction, owing to cleavage, is smooth and 



1872.] 



