Ford — Stephanite Crystals from Arizpe, Mexico. 245 



sumably formed by a combination of the basal plane c(001) 

 with the prism, m(110), and the pinacoid, o (010). 



Occasionally on the basal plane radial striation lines were 

 observed which suggested that the crystal form might be in 

 part at least due to twinning. The crystals average between 

 two and three centimeters across the base, and in one or two 

 as much as four centimeters, while the height of the 



cases 



crystals is quite uniformly one and one-half centimeters. The 

 prism faces are invariably vertically striated and frequently 

 show a strong tendency to round into each other, making it 



impossible to obtain even approximate measurements of their 

 angles. The basal planes are rough and etched, showing small 

 irregular angular pits, the unetched surfaces however being 

 smooth and with a good luster. In general no other faces 

 beside the base and those in the prism zone Avere to be 

 observed, but in one or two instances small truncations were 

 seen on the edges between but in such positions as not to 

 admit of measurements. 



The smaller type of crystals were more interesting crystal- 

 lographically. Their faces were brilliant and in most cases 

 gave on the goniometer angular readings with almost theoret- 

 ical values. In all twelve different forms were identified, 



