242 



Penfield and Ford—Calaverite. 



from his letter :— " Unless, however, the crystals are regarded 

 as triclinic twins it is impossible to obtain simple indices. 

 This, I allow, is the only argument in favor of this view; 

 there are no reentrant angles. I do not know whether I shall 

 persuade any one to share this view. If the crystals are 

 twinned there must be extraordinarily intimate penetration." 



The problem may, perhaps, resolve itself as follows : — Either 

 in the assumption of monoclinic symmetry (which the crystals 

 apparently possess) with complex symbols, or triclinic sym- 



metry (wholly obscured by twinning and resulting in pseudo- 

 monoclinic symmetry) with simple indices. 



When the present article was nearly completed we learned 

 from Dr. Palache of Cambridge that he and Professor Moses 

 of New York were also engaged upon the study of calaverite 

 crystals. They both have found the same forms as observed 

 by us, with few exceptions, and also have crystals which are 

 far more complex. It may be expected that the results of 

 their investigation will appear in due time. 



