HOBBS MINERALOGY OF WISCONSIN. 



129 



twins according to the common law, the twinning plane a 

 face of the fundamental prism. Stellate forms of the sec- 

 ond type are frequently only to be distinguished from the 

 individual crystals by a search for the twinning lines, or by 

 measurement of the angles on the goniometer. 



Type 1. Crystals columnar in the direction of the brachy- 

 diagonal axis: Crystals of this type usually have a length 

 several times their breadth, and have their habit condi- 

 tioned by the brachy domes i (021) and x (012), and the pyra- 

 mid p (111) (see plate 7, fig. 1). They vary from four to 

 eight millimetres in length. The forms observed upon 

 them are the following, y being new to the species: 



i, 2Poo (021). 

 x, iPoo (012). 

 y, -Poo (0.25.4).* 

 P, P (HI). 

 b, ooPoo (010). 

 m, ooP (110). 



r, ooP3 (130). 



The above enumerated forms were determined by means 

 of the following measurements : 



Measured. Calculated. 



62 46' 62 46' 



57 35 57 19 



58 37 58 42 

 35 46 35 46 

 69 13 69 20 

 39 51 39 45 

 34 31 34 40 

 12 304- 12 29 



