210 



1 



^enfield a? 



id Ford — Calaverite. 



twinning plane, is 35° 2'. 



calculated 



35° 4/. The measured 



angles are as follows : 











Measured. 



Calculated. 





Symbols. 



Vertical. 



Horizontal. 



Vertical. Horizontal. 



m 



no 



00° 00' 



31° 39' 



00 00 31° 30^' 



m 



of twin 



109 41 



31 40 



109 38 31 30i 



t 



13-20-4 



24 14 



23 19 



23 38| 23 324- 



t 



of twin 



86 1 



23 31 



85 57 23 32| 







13-22-10 



47 14 



28 11 



47 30^ 28 18" 



P 



111 



54 49 



46 49 



54 48 46 54i 



<1 



11-29-5 



32 9 



15 21 



32 52 15 30| 



s 



of twin 



147 4 



7 52 



147 20 7 50 



As far as the angles of the crystals are concerned, the sec- 

 ond law (twinning plane 101) might be considered as alone 

 sufficient for explaining the two kinds of twinning. Instead 

 of the first law, as stated above, it might then be said that the 

 crystals were twinned about 101, but were united by a compo- 

 sition face at right angles to the twinning plane. Since we 

 have in figure 29 a combination of the two kinds of twinning, 

 it has seemed to us simplest to explain it as according to two 

 laws. 



Summary. — There have thus far been presented in the 

 tables of, angles sufficient data to indicate that there are certain 

 planes which occur on calaverite crystals repeatedly, and with 

 such constancy in their angles that they can not be regarded 

 as in any way accidental. There are no forms more frequent 

 in their occurrence and more prominent in their development 

 than those designated as m (110) and^> (111), unless it is, per- 

 haps, o (13-22-10). If it be accepted that m (110) and jp (111) 

 have been well chosen as fundamental forms, then the crystal - 

 lographic relations between calaverite and sylvanite may be 

 expressed as follows: — Both are closely related not only in 

 chemical composition, but also in their axial ratios and in their 

 axial inclinations /3. — Both are alike in crystallizing in the 

 monoclinic system, and in having the dome (101) as twinning 

 plane. — Calaverite differs from sylvanite in having no distinct 

 clino-pinacoid cleavage, and in having, with few exceptions, 

 different forms, most of which must be designated by unusually 

 complex indices. 



The symbols assigned by us to the calaverite forms are those 

 which seem to correspond most nearly with the results of our 

 measurements. No one can appreciate more fully than the 

 writers that many of the symbols are so complicated that it is 

 almost impossible to believe that they are true. On the other 

 hand, we fail to n'nd any way of simplifying them which does 



