150 BULLETIN OF THE UNIVERSITY OF WISCONSIN. 



terminated. The common scalenohedron R3, (2131) some- 

 times appears as a replacement of the solid angle formed 

 by two faces of — i R and one of 18 R. The forms ob- 

 served are therefore 



P, 18R (18.0.18.1) 



e, — iR (0112) 



V, R3 (2131) 

 the two latter having been determined by the following 

 measurements : 



Measured. Calculated, 



e : e, 0112 : 1102 44^^ 35' 45° 3' 



V : V, (Y), 8121 : 2131 36 29 35 36 



The faces are all somewhat rounded, particularly the 

 faces P and v. One specimen in the collection shows a 

 larger wine yellow calcite which is a parallel growth of 

 several simple rhombohedrons, about which and on which 

 the colorless crystals with habit Pe have formed. This 

 larger crystal is thus shown to belong to an older genera- 

 tion. 



Fig. 10.— Pyrite from Milwaukee. 



Pyrite. — The cavities in the Milwaukee Cement Rock 

 which contain the calcite crystals just described, are 

 often coated with pyrite, so that the calcite is formed on 

 the pyrite. The pyrite appears in good crystals having a 

 diameter of 1-2'""" and is bounded by the octahedron and 

 cube. The habit is octahedral, the cube usually having a 

 small though a quite constant development (Fig. 10.). 



Sphalerite. — This mineral w^ould seem to be much less 

 abundant in the cavities of the cement rock than either 

 calcite or pyrite. Only one specimen out of quite a num- 



