HOBBS MINERALOGY OF WISCONSIN. 127 



crystal is an unstriated nucleus whose former crystal 

 boundaries are indicated by a line of tarnished mineral. 

 The nucleus contains irregular-shaped cavities in which 

 cerussite has crystallized. The striations in the outer por- 

 tion of the crystal are high ridges near the periphery, but 

 these diminish in prominence as they take their course to- 

 ward the nucleus, dying out completely before they reach 

 it, and as they do so often taking a sharp curve to a di- 

 rection which suggests a sudden change to the law of 

 Sadebeck. It is evident that this structure has not been 

 produced by any stress to which the crystal has been sub- 

 jected since its formation, but it is in some way to be con- 

 nected with the strains induced in the outer portion of the 

 crystal, as it accommodated itself to the nucleal crystal in 

 growing about this and being oriented by it. Lamellse 

 have also been observed on crystals from Platteville, Wis. 



Some very interesting forms of galena come from Galena, 

 111. Many specimens indicate that there have been for the 

 limestone cavities at least two periods in which galena crys- 

 tallized out, separated by a period when crystallized marca- 

 site was deposited. Cubes of galena, having an edge of 

 three to five centimeters, are found coated with marcasite 

 and studded with numerous nearly perfect octahedrons of 

 galena, having a diameter of about two millimeters. The 

 octahedron is usually absent from the crystals of the first 

 separation of galena though it sometimes appears as a very 

 minute truncation of the solid angles; while the cube, 

 though always very small, is usually present on the octa- 

 hedral crystals of the later generation. 



One crystal in the collection has some interest from its 

 twisted form. It is a simple cube elongated in the direc- 

 tion of a principal axis so that its length is about six inches 

 and its breadth and thickness only about two inches. One 

 end of the crystal is nearly or quite parallel to the other but 

 occupies the position it would have if it had been rotated 

 thirty degrees from its normal position about the long 



