HOBBS MINERALOGY OF WISCONSIN. 143 



Type 6. The crystals of marcasite from Galena which 

 are in the University 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 s. 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 I faces are adjacent, and opposite 

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

 fig. fib. ). 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 : 101 54 57 56 34 



I : I, Oil ;011 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, iPoo 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. 



