nOBBS — MINERALOGV OF WISCONSIN, 148 



Type 6. The crystals of marcasite from Galena which 

 are in the Universitj'^ Collection bear most resemblance to 

 the crystals of type 5 from Hazel Green. One variety has 

 a distinctly prismatic habit conditioned by the large devel- 

 opment of the prism m, the nearly equal development of 

 the domes I and e, and the absence of the base c, the dome 

 X, and the pyramid .<?. Crystals of this type are built up of 

 a large number of sub-individuals and are nearly always 

 twinned with a face of m the twinning plane (plate 8, 

 fig. 6.). Such twins are either simple twins or polysyn- 

 thetic trillings, in which latter case the middle individual 

 is quite thin. A trilling is easily distinguished from a 

 simple twin by the striations on the I faces. In a simple 

 twin the larger striated / faces are adjacent, and opposite 

 to a pair of adjacent and glistening e faces (plate 8, 

 fig. 6b.). In a trilling two e faces appear opposite one an- 

 other as in a simple individual (plate 8, fig. 6a), and the 

 only evidence that the crystal is twinned may be the re- 

 entrant angle along the edge le and on the face m. Al- 

 though a majority of the crystals are composed of small 

 sub-individuals which appear as projections on the faces 

 of the aggregate crystal, many of the composite individuals 

 are terminated by one large simply twinned crystal. On 

 such a twinned individual the following measurements were 



made: 



Measured. Calculated. 

 I : e, Oil : 101 68° 18' 70^ 49' 



e:e, 101 :m 54 57 56 34 



I : /, Oil : Oil 84 36 85 4 



A specimen of marcasite collected at Galena by Mr. 

 L. S. Cheney has a somewhat different development. The 

 fundamental prism is large, giving the crystals their 

 columnar form, while v, ^Pco and subordinate 1, Poo serve 

 as terminal planes. These crystals are in part attached to 

 larger octahedrons of pyrite.* 



* Note. — Some quite interesting arborescent aggregates of marcasite crystals were col- 

 lected by Mr. Cheney at Cuba City, Wis. These were received too late for study in connec- 

 tion with the other specimens of this mineral which are here described. 



