112 



BULLETIN OF THE UNIVERSITY OF WISCONSIN. 



usual forms m, r, and z, the crystals are modified by the 

 trigonal pyramids s and s', the right and left forms being 

 often observed together on the crystal. Examination with 

 a lens reveals the fact that some crystals are further 

 modified by the occurrence of a positive right trapezohe- 

 dron between s and z. Measurement with the goniometer 

 shows this plane to be the form r',, + ?Pf r, (7187), which 

 I think has not before been observed, though the comple- 

 mentary negative left form r^, - ?Pf 1, (1787) is given in 

 the lists of Dana and Goldschmidt. In the zone of mr on 

 another crystal a faint but distinct reflection was also ob- 

 tained from a plane which has the symbol UB, (47. 0. 47. 30) 

 and which has not before been described on quartz. The 

 habit and development of the crystals is represented in 

 figure 1. 



Fig. 1.— Quartz crystals from Devil's Lake. 



The following list gives the forms observed in the order 

 of their amounts of development: 



m, odR, (1010). 

 r, R, (1011). 



z, — R, (0111). 



2P2 

 s, -4- r, (1121). 



. 2P2 



s. 



1, (2111). 



r\, + ?P| r, (7187).* 

 a, I5R, (47. 0. 47. 30). 



Irby and Goldschmidt has been adopted. For new planes discovered German letters have 

 been used wherever this is possible. 



