Note on Cerussite from Illinois and Wisconsin. 
399 
NOTE ON CERUSSITE FROM ILLINOIS AND WISCONSIN. 
By WM. H. HOBBS. 
A few years since the University of Wisconsin obtained by purchase 
the mineral collections of W. T. Henry, of Mineral Point, Wis. They 
consist largely of the minerals associated in the zinc and lead deposits 
of southwestern Wisconsin and northwestern Illinois, and comprise the 
best series existing from that region. Among the carbonates cerussite 
is found, the best localities being Galena, Ill., and several points in Iowa 
County, Wis. 
In a number of specimens in the University of Wisconsin collections 
the mineral is quite well crystallized and is always found on the surface 
of galenite, where its presence is best explained by the action of car¬ 
bonated waters on the galenite. Cubes of galenite attaining in some 
cases dimensions of several inches are half covered by yellowish white 
cerussite crystals, which vary in size from a millimetre to more than a 
centimetre in diameter. They are stoutly columnar and translucent, with 
a color varying from yellowish white to light steel-gray. The faces are 
generally somewhat rounded, especially the terminal ones. The brachy- 
axis is the one of principal development, the columnar habit being 
given by the planes s, |Pob (012), and u, 2P,oo (021), which have about 
equal development. The fundamental prism M terminates the crystals. 
The pyramid t, P (111) appears as a rounding of the combination edge 
M : s. Twins parallel to M were frequently observed. Measurements of 
the interfacial angles with the goniometer of the Fness Universal- 
apparat gave the following results: 
u : u (2Pc6 : 2P5o ) 
s : s (£Pc6 : iPoo ) . 
M : M (ooP : ooP).. 
t : t (P : P). 
Measured Calculated. 
111° 2' 110° 40' 
139° 54' 140° 15' 
117° 25' 117° 14' 
130° 129° 30' 
The face t gave no image, the value being the average of a number of 
measurements by the shimmer seen when the lens was in place before the 
telescope. 
