HOBBS MINERALOGY OF WISCONSIN. 139 



face m, so that crystals have a hexagonal appearance when 

 observed in the direction of the c axis. The specimen 

 which shows the most perfect crystals of this type is 

 without a label, but it was obtained from the W. T. Henry 

 Collection with material from the Linden Mine, and was 

 probably found at that locality. These crystals have a 

 diameter of 5-8 raiu . The following measurements were 

 made upon them : 



Measured. Calculated. 



m : m, 110 : 110, 75 22' 74 55' 



I : I, Oil : Oil, 100 5 101 58 



The prism m is sometimes beveled for a short distance 

 by a prism which gives a reading of 78 1', the reflection 

 of the signal being as sharp and bright as that from m. In 

 one instance also a brachyprism gave a reading of 99 52'. 

 These two forms would correspond to the symbols of the 

 brachyprisms MPoo and f^Poo , but in view of the wide 

 variations of the angles exhibited by crystals of marcasite, 

 the correctness of these symbols can not be relied upon. 

 The face I furnishes but poor images of the signal 

 and c and v are altogether unsuited to measurement. The 

 greater number of specimens of this type are parallel 

 growths, each individual varying slightly from perfect 

 parallelism with the individuals adjacent. This produces 

 the usual "cockscomb" forms. Twins, with the twinning 

 plane the prism m are quite common, but so far as observed 

 they are composed of but two individuals (plate 8, fig. la). 

 On one such twin the angle between adjacent prism faces 

 lying on either side of the twinning plane was measured 

 with the following result : 



Measured. Calculated. 

 m : m, 110 : 110, 30 1' 30 10' 



Pseudomorphs of limonite after marcasite have been 

 found at Mineral Point as well as at other localities. Ex- 



