238 



Pen field and Ford — Calaverite. 



would answer the purpose better. The stereographic projection, 

 figure 27, is better adapted for the_purpose. The diameter of 

 the circle, passing through 101, 101 and the poles lettered p, 

 represents the twinning plane, and symmetrically on either 

 side of it are the poles of the two n and two o faces, making 

 small angles with one another. The poles of the four m faces, 

 two in normal and two in twin position, serve to make the 

 orientation clear. 



In figure 24 the faces in the upper right-hand portion are in 

 normal position, except for the occasional crossing of twin 

 lamelloe, as indicated by the striations. The faces n and o 

 below and to the left are in twin position and make reentrant 

 angles with the corresponding planes of the normal crystal. 

 The two p faces are common to both individuals. The angles 

 n/\n and '0/\o, measured over the twinning plane, are as 

 follows : 



Measured. Calculated. 



3° 21' 3° 14' 



7ly\H 



o^ o 



6 54 



The form co was observed only on this crystal and appears 

 as a very narrow, somewhat tapering face, intersecting p and 

 disappearing against a twin lamella. The measured and cal- 

 culated angles of this crystal are as follows : 









Measured 







Calculated 





Symbols. 



Vertical. Horizontal. 



Vertical. Horizontal. 



p 



Ill 



54° 



48' 46° 



50' 



54° 



48' 46° 54J' 



p 



III 



it 



" 46 



50 



i( 



" 46 54^ 



n 



11-14-10 



52 



11 38 



34 



52 



12 38 Hi 



n 



of twin 



57 



10 38 



34 



57 



24 38 17i 







13-22-10 



47 



26 28 



6 



47 



30J 28 18" 







of twin 



62 



28 



2 



62 



5 28 18 



CO 



20-15-12 



40 



16 47 



12 



40 



26 47 8 



i 



10-7-11 



57 



5 58 



4 



57 



18 58 29 



On the crystal represented by figure 25 only the small and 

 m faces to the left are in twin position, the faces making a 

 reentrant angle at the twinning plane. A few of the angles 

 of this crystal are as follows : 





Symbols. 





Measured 



mtal. 





Calculated. 





Vertical. Horizc 



Vertical. Horizontal. 



p 



Ill 



54° 



50' 46° 



47' 



54° 



48' 46° 54f 



p 



111 



« 



" 46 



47' 



a 



" 46 54| 







13-22-10 



47 



6 28 



5 



47 



30J 28 18 







of twin 



62 



28 28 



5 



62 



5 28 18 



m 



110 



00 



00 31 



34 



00 



00 31 30^ 



m 



of twin 



109 



45 31 



35 



109 



35 31 30| 



