HOBBS MINERALOGY OF WISCONSIN. 151 



ber supplied the writer by Dr. Kremers included any crys- 

 tals of sphalerite, and these were few and small. These 

 crystals were twinned according to the common law, as a 

 result of which they were flattened parallel to a face of 

 the tetrahedon. The faces are not bright enough for meas- 

 urement, but the forms Q (111), ocO (101). an icositetra- 



u 



hedron, and a hexoctahedron, can be made out upon them. 



As is frequently observed on this mineral, the faces of 

 the tetrahedron show triangular markings. The faces of 

 the icositetrahedron truncate the edges of the hexoctahe- 

 dron, so that it is very probable that the icositetrahedron 

 is 2O2 and the hexoctahedron 3O|. In order to determine 

 with certainty, however, it would be necessary to secure 

 much better material. 



Marcasite. While this paper was in the printer's hands, 

 I had an opportunity to visit the Milwaukee cement quar- 

 ries and collect there specimens of marcasite and celestite. 

 I also had an opportunity to make a hasty examination of 

 the valuable mineral collection owned by Howard Green, 

 Esq., in which are contained very beautiful millerite crys- 

 tals from the cement works. So far as I know, none of 

 these minerals have been reported from the locality. 



The marcasite is found in very well formed, brightly iri- 

 descent crystals of lath-like habit, with the axis of devel- 

 opment the brachy-diagonal. The largest crystals found 

 had a length of only a few millimetres. The forms devel- 

 oped are apparently c, u, and ra, with other smaller faces. 

 They have not yet been carefully studied. 



Celestite. This mineral occurs in masses as large as a 

 man's head composed of inter-grown crystals, the individ- 

 uals of which are sometimes several inches in length. 

 These masses are found in the layer of rock immediately 

 underlying the one quarried for cement. The crystals 

 seem quite pure and have a white color with irregular 

 areas of pale celestial blue. A qualitative determination 



