HOBBS MINERALOGY OF WISCONSIN. 141 



tion, than of tabular plates. Simple twins with twinning' 

 plane the prism are common on these specimens. The 

 prism was determined as m by the following measure- 

 ments: 



Measured. Calculated. 



m : m, 110 : 110, 105 2' 105 5' 



On twinned crystals, 



m : m, 110 : 110, 30 34' 30 10' 



The face c is striated parallel to the brachy diagonal 

 axis, which causes a slight rounding of the face near the 

 edge 110 : 110. The color of this type of crystals is 

 brown. Parallel growths of a simple character produce 

 the "cockscomb" aggregates. 



Type 5. The second type of crystals from Hazel Green 

 (plate 8, fig. 5) is quite different from the first, but al- 

 most identical with a marcasite from Schemnitz which is 

 figured by Dana. 1 These crystals have a diameter of 

 4-5 mm . and are quite symmetrically developed. They are 

 bounded by the following forms: 



m, ooP (110). 



I, Poo (Oil). 



e, Pc (101). 



s, P (111). 



x, mPoo (Olh). 



The faces are bright with a brassy lustre and in most 

 cases reflect but a single image of the signal, so that the 

 probable error of reading is very small. Notwithstanding 

 this fact the measurements of corresponding angles not 

 only vary much from the values determined by Sadebeck 

 for this mineral but they vary much from one another. 

 This variation must therefore be explained by actual vari- 

 ations in the angles themselves, though the faces do not 



i System, 6th Ed., p. 95, fig. 2. 



