126 BULLETIN OF THE UNIVERSITY OF WISCONSIN." 



6, fig. 6, which has been drawn from one of the crystals 

 examined. The top cubic face of this individual is stri- 

 ated in the same manner as the crystals from Highland, 

 but the structure is different on the adjacent faces. Here 

 the course of the lamellae is parallel to the cleavage lines, 

 showing clearly that the twinning plane is the dodecahe- 

 dron. On one of these faces a division of the face into 

 sectors is noticed, but in this case the lines of division run 

 parallel to the diagonals of the face. A few of the stri- 

 ations take their course across the direction of the pre- 

 vailing ones at such an angle as to suggest that they rep- 

 resent a third law of twinning for the mineral such as has 

 been described by Sadebeck,' in which the twinning plane 

 is a trisoctahedron. In all cases the sectors into which 

 the striations divide the faces constitute vicinal planes, all 

 of which slope away from the center of the face.* 



An exceptionally beautiful instance of the dodecahedral 

 twinning above described is afforded by a large crystal 

 from Yellowstone. This specimen is the broken end of 

 what was apparently one of the elongated crystals with 

 tetragonal habit which are common from this locality. The 

 crystal had a breadth of about five and a thickness of some 

 four centimeters. Figures 3, 4, and 5 of plate 6, repre- 

 sent three parallel sections of this crystal along the cleav- 

 age planes, those of figures 4 and 5 being separated by 

 about three centimeters and those of figures 3 and 4 by 

 about a centimeter and a half. The central portion of the 



a Zeitsch. d. D. geol. Gesellsch., Vol. XXVI, p. 631 (1874). 



*NoTB.— That these twin lamellae have been noticed before on Galena from the upper 



Mississippi valley, is evident from the following extract from a paper bj' James T. Hodge 



entitled, "On the Wisconsin and Missouri Lead Region," which was published in Sllliman's 



Journal in 1842 (Vol. 43, p. 38); 



"The smelters thinl:: they can distinguish the ores that are found iu different fissures — 

 that from an east and west fissure being perfectly crystallized, of a smooth surface, striae 

 indistinct; that from a nortti and south fi>sure, of crystalline structure, with two sets of 

 strife very distinct, crossing each other at right angles; and the ore from a quartering fis- 

 sure crystalline, with many sets of strias crossing each other obliquely; and to some ex- 

 tent I had opportunity of proving their observations correct." 



Supposing this idea of the smelters to be correct, it is difficult to account for this differ- 

 ence unless the crystals of galena have a uniform orientation with reference to the walls 

 of the fissure. 



