HOBBS MINERALOGY OF WISCONSIN. 145 



are bright but have a purplish iridescence due to incipient 

 alteration. The pyritohedron was determined with suffi- 

 cient accuracy as the common form e, . (210) by meas- 

 uring the edge A on the reflecting goniometer, the result 

 obtained being 52 32', and the calculated value 53 8'. 



Pyrite occurs at Mineral Point in cubes apparently un- 

 modified and as much as three-quarters of a centimeter on 

 an edge. Associated with these symmetrically developed 

 cubes are interesting groups composed of long columnar in- 

 dividuals arranged radially about a centre located a little be- 

 low the point of attachment (plate 8, fig. 9.). These crystals 

 have a length of a centimeter or a little less, and a thick- 

 ness of a little more than a millimeter. They are cubes 

 developed in the direction of one of the principal axes. 

 The individuals composing the group do not come in con- 

 tact, except perhaps at the point of attachment. Other 

 specimens from this locality are similar in character but of 

 much larger dimensions, the pyrite needles having a length 

 of several inches and a diameter of only a few millimeters. 

 The space between the pyrite needles is occupied by crys- 

 tallized sphalerite, which is in part coated with smithsonite. 

 The large radial sheaves of iron sulphide which occur at 

 Galena are for the most part pyrite. 



A perfect unmodified octahedron of pyrite a half centi- 

 meter along the axes was collected by Mr. Cheney at Dem- 

 ocrat, near Etna, Wis. 



AZURITE FROM MINERAL POINT. 



Both the carbonates of copper are found at Mineral 

 Point attached to massive chalcopyrite, where they have 

 doubtless been formed through the action of carbonated 

 waters. The azurite is quite well crystallized, but the in- 

 dividuals are rarely over 2 ram long. In a few specimens 

 they attain dimensions of 3-4 ram The crystals have either 

 a domal habit with the ortho-diagonal the axis of great- 



