Per* field and Ford — Calaverite. 



239 



The crystal represented by figure 26 appears for the most 

 part like a normal individual, crossed, however, by twin lamel- 

 lae as indicated by the striations. The form lettered s seems 

 to be in twin rather than in normal position. 



The second law of twinning, where 101 is the twinning 

 plane, has been observed on two crystals, one from the Monu- 

 ment the other from the Prince Albert Mine. These are illus- 

 trated by figures 28 and 29, respectively. In the crystal rep- 



28 



resented by figure 28 the faces o, m and part of s above are in 

 twin position. The twinning plane separates the two o faces, 

 runs through s, and disappears in that portion, stippled in the 

 figure, where the crystallization has been interrupted. The m 

 faces make a reentrant angle. The s faces in normal ,and twin 

 position fall almost together, making, according to calculation, 

 an angle of only 0° 44' with one another. In the crystal 

 under consideration s was not a very good face. The meas- 

 ured angles are as follows: 



Symbols. 



m 110 



m of twin 



o I3-22-10 



o of twin 



r 10-44-15 



s probably of twin 



1 



Measured. 





Tertical. 



Horizontal. 



00 c 



00' 



30° 



52' 



70 



33 



31 



37 



47 



3 



28 



13 



117 



40 



28 



13 



113 



59 



18 



34 



146 



32 



8 



8 



Calculated. 



Yertical. 

 00° 00' 

 70 26 



Horizontal. 

 31° 30£' 



31 30^ 

 47 30-1 28 18 



117 56 

 114 54 

 147 20 



18 

 50 



In the crystal from the Prince Albert Mine, figure 29, both 

 laws of twinning are combined. The first law is indicated by 

 twin lamellae crossing the lower portion of the crystal. The 

 faces in the lower left-hand portion of the figure may be 

 regarded as belonging to a single individual in normal position. 

 To the right the two m faces meet along the twinning plane, 

 which seems to run for a ways through s and then disappears. 

 The salient angle made by the two m faces, measured over the 



Am. Jour. Sci. — Fourth Series, Yol. XTI, No. 69— September, 1901. 

 1-7 



