128 BULLETIN OF THE UNIVERSITY OF WISCONSIN. 



axis of the crystal. The long faces of the crystal present 

 a nearly perfect warped surface with only a single impor- 

 tant interruption near one end. 



On closer examination very slight interruptions are no- 

 ticed at intervals of three or four centimeters, which indi- 

 cate that the apparent individual is an aggregate of sev- 

 eral individuals nearly, though not quite, parallel. Each 

 individual is rotated through a small angle from its nearest 

 neighbors about the axis of the aggregate, the rotation 

 being always in the same direction. But this is not the 

 only attempt which the crystal has made to assume a spiral 

 form, for the faces of the sub-individuals are warped, and 

 though the greater part of the aggregate is coated with 

 marcasite, a cleavage surface is exposed at one point and 

 exhibits the polysynthetic twin lamellae. It is thus prob- 

 able that the crystal has been subjected to some force from 

 without which has inclined it to the direction it has taken. 



CERUSSITE FROM HIGHLAND, MINERAL POINT, AND GALENA. 



This mineral always appears on the surface of Galena 

 crystals where it has doubtless been formed through the 

 action of carbonated waters. The best specimens in the 

 University Collection were found at Highland and Mineral 

 Point. Larger but less perfect crystals are common on 

 specimens from Mineral Point and from Galena, 111., the 

 latter place being located but a few miles from the state 

 boundary. The Galena to which the Highland cerussite is 

 attached occurs on the hopper-faced octahedrons composed 

 of a great number of sub-individuals, which have been de- 

 scribed under Galena (Fig. 5). Two strikingly dif- 

 ferent types of Cerussite crystals are found upon the 

 same specimens, the one being long columnar in the direc- 

 tion of the brachydiagonal axis, and the other pseudo- 

 hexagonal in habit, from the nearly equal development of 

 pyramid and brachydome. Both types are to be found in 



