300 



PROCEEDINGS OP THE ACADEMY OP 



do not admit of accurate measurements, but in one specimen it 

 graduates into oligoclase, and it is perliaps but an alteration of 

 that species. It lias distinct triclinic striations. 



Cassinite, from Blue Hill, Providence Township, Delaware 

 County, measures 00°. 



Lennilite, Lenni, Delaware County, a very perfect cleavage 

 gave 8G-87, but the absence of triclinic striae would indicate that 

 this is an orthoclase. 



Bluish transparent veined Feldspar, from Yan Arsdale's quarrj'-, 

 near Feisterville, Bucks County, Pa., two cleavages, highly per- 

 fect, and giving finely polished surfaces, 90°. The opalescent 

 feldspar from the same quarry, called labradorite, is probably the 

 same. 



Orthoclase, from Frankford, Philadelphia, nearly pure white in 

 color, and translucent. Besides the ordinary cleavages, OP and 

 00 P oc, the more unusual, oo P co and co P, are occasionally well 

 developed, giving rise to two rhombohedra, one with the faces 

 parallel to the axes, the other derived from this by the develop- 

 ment of the plane oo P, as shown in the annexed section on a 

 plane parallel to OP (Fig. 1). Rhombohedra of the first form 

 have been found also at the quarries on the Pennsylvania Rail- 

 road, near the west end of the Fairmount dam. 



(X>Pab 



Po 



P^ 



00 J.-" oo 



oo J? oc 



In three specimens of the Frankford orthoclase, a fifth cleavage 

 appears, quite imperfect, truncating the angle between co P, OP 

 and GO P ex, or the edge between OP and oo P oo forming with OP 

 an angle of about 135, and with oo P (» nearly 90°, Fig. 2. If this 

 is not a mere accident, it is not easy to explain. 



Rhombs from the quarries mentioned on the Pennsylvania Rail- 

 road have been found, with the following angles for the face of 

 nnusual cleavage on OP and oo P c^: — 



[February 13, 



