48o 



SAIITHSOXIAN MISCELLANEOUS COLLECTIONS 



VOL. 52- 



(2) Galena laminated parallel to the octahedron (m). About 

 •one-fourth of the mineral is developed in this way and is arranged 

 with its octahedral surface usually parallel, though at times slightly 

 inclined, to the cube faces of the pyrite. Natural etching has given 

 a triangular and hexagonal outline to the plates, as is shown in the 

 lower right-hand corner of figure 2, plate lix. This contour is ex- 

 plained by the fact that an octahedral plane alone is an equilateral 

 triangle, and, when truncated by cube faces, forms a surface of six- 

 sided outline. The strongly laminated nature of the galena may be 

 due to polysynthetic twinning parallel to (m). 



(3) Galena laminated parallel to the cube (100). This habit, 

 which comprises about two-thirds of the galena, shows a varied 

 orientation in respect to the pyrite. (a) The most common appear- 

 ance is shown in the central portion of figure 2, plate lix, where the 

 laminae are parallel to the surface of the pyrite. This is explained 



Fig. 83. — Greatly enlarged cleavage 

 fragment of galena, showing eminent 

 cubic cleavage modified bj' octahedral 

 cleavage. 



Fig. 84. — Fragment of galena, made 

 up of laminae, parallel to a cube face, 

 bounded by octahedral slopes. 



by the accompanying drawing (figure 84), which figures a fragment 

 made up of plates parallel to the cube a (100), bounded by octa- 

 hedral (hi) slopes, this combination giving a square outline to the 

 plates, (b) Occasionally there occur long, narrow strips, likewise 

 made up of laminae parallel to a (100) and bounded by elongated 

 (ill) faces. These may have their a (100) faces parallel to 

 a (too) of pyrite; or less often the long o (11 1) faces may have 

 this relation. Crystals of like distortion, but without such platy 

 structure or appearance, from Yellow,stone, Wisconsin, have been 

 described by Hobbs.^ (c) Finally, the laminae and the two cube 



* Hobbs, W. H. Die krystallisirten Mineralicn aus dem "Galena Limestone" 

 des siidlichen Wisconsin und des nordlicben Illinois. Zcitsch. fiir Kryst.. vol. 

 25, i895-'96, pp. 257-275. Especially plate 4, figure 10, and p. 263. 



