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 brachydomes i (021) and x (012), and the pyra* 

 mid 2J (111) (see plate 7, fig. 1). They vary from four to 

 eight millimetres in length. The forms observed upon 

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



i, 2Pao (021). 



X, ^Pao (012). 



r, VPx (0.25.4).* 

 P, P (111). 



&, ocPoo (010). 

 m, QoP (110). 



r, odP3 (130). 



The above enumerated forms were determined by means 

 of the following measurements : 



