33 



with full certainty. Besides them there, however, occur several 

 others which it has not been possible to identify. Thus, both 

 positive and negative rhombohedrons, hexagonal bipyramids, 

 and perhaps also scalenohedral forms occur that are undeter- 

 mined. Owing to the strong horizontal striation or step-like 

 structure of the crystals the zonal relation between the different 

 forms also remains uncertain. The hexagonal prism of the 

 second order, w, whose faces likewise always are dull, could, 

 however, be determined by its zonal position (Fig. 5, PI. II). 



As has already been mentioned, also the small unaltered cry- 

 stals and the unaltered end-portions of the larger crystals are not 

 well adapted for accurate angular measurements. Sometimes the 

 faces certainly are tolerably brilliant, but owing to the striation 

 the reflections are in most cases multiplied and consequently 

 uncertain. Of the great number of angular values obtained by 

 numerous measurements on about twenty crystals the majority 

 had to be rejected as not leading to simple symbols for the 

 various forms. Only such forms as have been more regularly 

 met with on several crystals have been regarded as certain and 

 put down in the above list. 



The crystals generally present a very distinct rhombohedral 

 development; this is especially true of the small individuals. 

 On the larger individuals the positive and negative forms are 

 not infrequently of nearly equal development. It is then doubt- 

 ful which are of one and which of the other kind. In most 

 cases, however, the form « predominates and can serve to 

 decide the question. No other means of distinguishing between 

 positive and negative forms can be given. 



With regard to the different forms the following remarks 

 may be added. The base occurs constantly on every crystal, 

 not infrequently with both faces , especially on the small 

 crystals , on which it forms small triangular planes. It is the 

 only form whose faces are always brilliant; they are also mostly 

 quite even. On the larger crystals, which are often thickly 

 XXIV. 3 



