358 G. H. Williams — Hornblende and its Gliding Planes. 



see that, if they were present, they must form nearly a contin- 

 uation of the pyroxene parting. On the left of the figure 

 inclusions of calcite in the hornblende are indicated. 



Fig. 6 shows a remarkable growth of dark green hornblende 

 from Russell, St. Lawrence County, around a crystal of pale 

 Both crystals have their clinopinacoids parallel, 



green salite 



6. 



while the parting planes, which are here present in both, are 

 as nearly as possible parallel. This specimen is about three 

 inches in length, and belongs to the collection of Mr. Clarence 

 Benient, in Philadelphia. 



Another long crystal of green hornblende from Somerville, 

 St. Lawrence County, loaned by Mr. Joseph Wilcox, of Phila- 

 delphia, is a twin, according to the common law (twinning- 

 plane the orthopinacoid), and has the transverse parting _well 

 developed in both individuals parallel to the face Poo (101). 

 Both surfaces give good reflections, and the angle between 

 them was found to be 150° 10' (calc. v. Koksch., 150° 4'). 

 This specimen offers strong evidence against the possibility of 

 a gliding _in hornblende parallel to both the planes, OP (001), 

 and Pa© (101), whose inclinations to the vertical axis are nearly 

 the same. 



In view of all this evidence we must, therefore, conclude 

 that an alteration of the symbols for the terminal planes of 

 hornblende is necessary to show its analogy to pyroxene ; 

 furthermore that this change must be made in accordance with 

 the assumption that the gliding plane, now called the ortho- 

 dome, Poo (101), is the basal pinacoid, OP (001), as tirs*t sug- 

 gested by Tschermak in his Lehrbuch der Mineralogie, in 1884. 



Petrographical Laboratxny of the Johns Hopkins University, 

 Baltimore, Feb., 1890. 



