134 BULLETIN OF THE UNIVERSITY OF WISCONSIN. 



of sphalerite much over a centimeter in diameter have 

 been observed by the writer on Wisconsin specimens. 



Sphalerite is found in exactly the same association at 

 Galena. These o.rystals have much the same form, but some 

 have been found on which the faces are much more 

 perfect. They nearly always exhibit one form in addi- 

 tion to those observed on crystals from the other localities. 



FIG. 7. SPHALERITE FROM GALENA, ILL. 



This is the tetragonal tristetrahedron or hemi-trisoctahe- 

 dron A, 1O (775) which has not, I believe, been before ob- 

 served on this mineral. On one crystal in particular the 

 faces d, o, and A are very perfect and give excellent reflec- 

 tions of the signal (Fig. 7). On this crystal the following 

 measurements were made: 



Measured. Calculated. 



o : d, 111 : 110 35 18' 35 16' 



d : A, 110 : 775 26 49 26 48 



On the same crystal the faces of the icositetrahedron 

 (probably m) and the hexoctahedron are too much rounded 

 to admit of measurement. On less perfect crystals the 

 face A was measured with results varying 11' to 12' from 

 those obtained on the most perfect crystal. 



