142 BULLETIN OF THE UNIVERSITY OF WISCONSIN. 



show vicinal planes, and with the exception of I and x are 

 not striated. The variation in the angles of marcasite is 

 well known and the values obtained by Sadebeck are not 

 supposed to be very accurate. The measurements made 

 on the crystals from Hazel Green are as follows, the gen- 

 eral correctness of the average determination being con- 

 firmed by the zone relations: 



Measured. Calculated. 



s : s (over /) 111 : 111 88° 57' 90° 48' 



s : s (over e) 111 : 111 65 35 66 7 



s : s (over w) 111 : 111 56 7 52 28 



I : I, Oil : Oil 99 56 ) 



99 13 [ 100° 26' 101 58 

 102 9 3 



e : e, 101 : 101 113 48 \ 



112 30 M12 47 116 20 

 112 3 3 



I : X, . Oil : Oih 54 16 



54 25 



X : V, Oih : Oil 44 38 



44 48 



Beside the form x, which occurs with only half its 

 planes on these crystals, the edge IV is occupied by a 

 large number of indeterminate brachydomes, so that the 

 edges Ix and xV are well rounded. If the readings Ix and 

 I'x could be relied upon the form x would have the sym- 

 bol tVPqo , and it is in any case a very flat dome. That it 

 is not the basal pinacoid, which occurs on the Schemnitz 

 crystals, is apparent even by examination with the naked 

 eye. A number of simple twins and several polysyn- 

 thetic trillings were observed on these specimens (plate 8, 

 fig. 5a). The twinning plane is shown to be m, the unit 

 prism, by the twinning line following the edge le and then 

 traversing the face m parallel to the edge mm. Trillings 

 are easily determined from the zigzag forms assumed by 

 the X plane. 



