BARITE IN LEAD AND ZINC DISTRICT 
239 
is their splendid play of colors. Many of the crystals are very 
iridescent and the light reflected from their surfaces is of many 
colors, due to the fine cleavage-lamellae, in the light reflected 
from which interference of the incident and reflected waves 
takes place. 
Crystallography of the harite . — The general characteristics of 
the barite have been given. The faces represented on the crys- 
tals are as follows : 
Pinacoids 
Macropinacoid 
Brachypinacoid 
Basal pinacoid 
Prism 
Brachyprism 
Domes 
Macrodomes 
Brachydomes 
Pyramid 
(not common) 
These faces occur in several common combinations. The 
macropinacoid, brachydome, macrodome, and brachyprism forms 
are the most common combinations. Another form consists of 
the macropinacoid, brachypinacoid, brachyprism, macrodome, 
and brachydome. The macropinacoid, brachyprism, and macro- 
dome forms also occur. Other more uncommon forms are pres- 
ent. The habit of the crystals is very variable. There seems to 
be no relation between the size of the faces and the size of the 
crystals. 
G-eological Laboratory, 
University op Iowa. 
