. Bulletin of the University of Wisconsin. Science. HOBBS—MINERALOGY OF WISCONSIN. 125 the component individuals at the ends of the principal axes a diameter of about one to two centimeters. On specimens from Highland are found interesting arbo- rescent forms not unlike the forms assumed by native cop- per (see figure 6). The individual cubes are much elongated in the direction of a principal axis and otherwise distorted, and the arborescent groups of crystals are often attached by the end of a single crystal, so that their resemblance to trees is very Fig. 6.—Arborescent Form op Ga
. Bulletin of the University of Wisconsin. Science. HOBBS—MINERALOGY OF WISCONSIN. 125 the component individuals at the ends of the principal axes a diameter of about one to two centimeters. On specimens from Highland are found interesting arbo- rescent forms not unlike the forms assumed by native cop- per (see figure 6). The individual cubes are much elongated in the direction of a principal axis and otherwise distorted, and the arborescent groups of crystals are often attached by the end of a single crystal, so that their resemblance to trees is very Fig. 6.—Arborescent Form op Galena prom Highland. Polysynthetic tv/in lamellae like those described by Cross ' have been observed on crystals from several of these localities. On crystals from Highland the twinning plane of the lamellae is the octahedron, which corresponds to the second law mentioned by Cross. The crystals on which the lamellae are observed, are cubes with small trunca- tion by the octahedron. The faces of the cube are divided along lines parallel to their own edges into four sectors, in each of which pronounced striations run parallel to the adjacent octahedral face. All the cubic faces are affected in the same manner, but the octahedral faces are quite plane. This structure is indicated in plate 6, fig. 7. Lamellar twinning according to the other law deter- mined by Cross, where the twinning plane is a plane of the dodecahedron, is well shown by crystals from Mineral Point. The appearance of these crystals is shown in plate » Whitman Cross, Note on Slipping Planes and Lamellar Twinning in Galena. Proc CoL Sci. Soc, 2. p. 171-174. (1887).. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original University of Wisconsin. Madison, Wis. : The University
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