near Mullan, Idaho, with Crystallographie notes. 411 



In conclusion the writer desires to tender his thanks to Mr. 

 Clark, who so generously furnished the material for study and 

 the data for the description of its occurrence. 



Crystallographie JVoles. 



From the specimens of plattnerite submitted to the writer 

 by Mr. W. S. Teates the following results were obtained. 

 The crystals of plattnerite (PbO„), which look very much like 

 cassiterite, are very minute, varying from -^ to -xoVo" °^ an mcn 

 in length, and are found tilling crevices in the massive platt- 

 nerite. Moreover, they are tetragonal and isomorphous with 

 rutile (Ti0 2 ), cassiterite (Sn0 2 ), and polianite (MnO- 2 ), all 

 minerals which have a similar chemical composition. 



The color is black, and where the planes are smooth, the 

 crystals are very bright, having a nearly adamantine luster. 

 No cleavage could be distinguished and the crystals are very 

 brittle. 



/ / V \l 



a 



: a 



The prevailing form is that shown in figures 1 and 2, the 

 basal plane being very common ; many of the smaller crystals, 

 however, have simply the steep pyramid v (301) terminating 

 the prisms as seen in tig. 1. The crystal, which gave much the 

 best results, was about the T -^ ¥ of an inch long and of the form 

 and about the proportions shown in fig. 3. The forms ob- 

 served, referred to the common axial ratio of this group of 

 dioxides, are : 



a (100, i-i) 

 c (001, 0) 



e (101, l-i) 

 v (301, S-i) 



x (332, |). 



The plane x, (332), bevelling the edge between the pyramids v, 

 (301) was observed in a few instances. 



The prismatic faces are badly rounded and considerably 

 striated, and though bright, could not be depended on for any 

 correct measurements since their reflection extended over a 



