Penfield and Fofd — Calaverite. 



237 



Twinning. — Two laws of twinning have been determined 

 which may be stated as follows : 1. The twinning plane is in 

 the ortho-dome zone, at 90° to 101. 2. The twinning plane is 

 the ortho-dome 101. According to both laws the symmetry 

 axis and the positions of the p faces are the same for both the 

 normal and twin positions. Penetration twins have also been 

 observed with the symmetry axes, or the striated zones, crossed 



24 



25 



26 



27 



at angles of about 90°, but no crystals have been found from 

 which sufficiently accurate measurements could be obtained for 

 determining the exact law of twinning. 



The first law of twinning has been observed only on crystals 

 from the Prince Albert mine (Occurrence Ko. 5). The twin- 

 ning is partly of the nature of two individuals meeting along 

 the twinning plane and partly of the nature of twin lamellae, 

 revealed by a series of striations crossing the terminal faces at 

 right angles to the intersection of two p faces. Figures 24 to 

 26 represent three individuals which illustrate this law of twin- 

 ning, and although the figures do not show the twinning at all 

 distinctly, it is doubtful whether any other kind of illustration 



