HOBBS MINERALOGY OF WISCONSIN. 133 



All the faces with the exception of m are dull and conse- 

 quently give no image of the signal. The face m is very 

 bright but is rounded through bevelment by a considerable 

 number of hexoctahedrons having large indices. The angle 

 m : m (over a), was measured by reflection and found to be 

 50 53', the theoretical value being 50 28'. The angle d : d, 

 (101 : Oil) was roughly measured with a hand goniometer 

 as 61, the calculated angle being 60. A hexoctahedron 

 which has a development about equal to m, and whose faces 

 occur on either side of the m face, could not be determined 

 owing to the lustreless character of its faces. 



A smaller crystal from the same specimen is bounded by 

 the forms o (111) and d (110) and is twinned according to 

 the common law for this mineral, the twinning plane a face 

 of o. Other specimens from Shullsburg are light wine yel- 

 low in color to nearly colorless and transparent. 



An analysis by Mr. E. B. Skinner of the darker crys- 

 tals first described gave the following : 



Zn 66.67 



S 32.48 



Fe 37 



99.52 



Sphalerite also occurs in crystals at Platteville and at 

 Mineral Point, the individuals obtained from the former 

 locality being a centimeter or more in diameter, while those 

 from the latter locality are small though often with toler- 

 ably bright faces. The forms d, o, and m were observed 

 upon both. Frequently, however, those found at Mineral 

 Point are coated with a layer of white mineral which is 

 probably smithsonite. Dana 1 states that crystals of this 

 mineral three inches or more in diameter are found altered 

 to smithsonite at Mineral Point. No such specimens are 

 included in the University collection, though spheroidal ag- 

 gregates of crystals as large as that are not uncommon, 

 and these are sometimes altered to smithsonite. No crystals 



i System, 6th Ed., p. 62. 



