20 



w = {l0T0}, r = {l0Tl}, z=-{oni], s = {2ni}, and 

 л: = {OlT2}, 



The last-mentioned negative rhombohedron is one of the 

 rarest forms of the quartz. On this crystal it is represented, 

 as the figure shows, by two comparatively large faces. By its 

 position as a symmetrical truncation of the combination-edge 

 between two positive rhombohedral planes it is fully determ- 

 ined. 1 can also give the following values of angles. 



Measured. Calculated. 



(OlT2i : (lOTo) = 57° 34' 57° 35' 



{OIT2) : (lT02) = 55° 18' 55° 20'. 



Crystal 3 (Fig. 3, Plate 1) is also a left-handed individual of 

 about the same size as the last. On it the following forms 

 have been determined: 



^г = {|0Т0}, r = {l0Tl}, Ä = {oni}, s = {2Tri}, if={303l} 

 / = {2021}, and (a) = {2T. 9. 12. 7}. 



Of the positive rhombohedron I only one very narrow face is 

 present. However, it gives a distinct reflection on the gonimeter. 

 Of the form (a) only a single face is present. It has deep, appar- 

 ently irregular depressions and givis only a shimmering reflection. 

 That it belongs to the zone [3031, Olio] is obvious; its posi- 

 tion in another zone could not be ascertained. It has, therefore, 

 been determined through a shimmer-measurement of no great 

 accuracy. The indices proposed are unusual for quartz , but 

 no indices agreeing with previously known forms can be ob- 

 tained, unless the zonal relations are given. As, however, only 

 one face occurs, the form may, for the present, be regarded 

 as somewhat uncertain. 



Measured. Calculated. 



(2021) : (10Г0)= 21° 37 21° 29' 



(21. 9. 12. 7) : (0lT0)= 86° 36' 85° 39' 



