79 



present and always very narrow. In general the faces of the 

 vertical zone are strongly striated longitudinally; on some in- 

 dividuals, however, they are even enough to allow of fairly 

 accurate measurements. The termination of the crystals gene- 

 rally consists of the third pinacoid alone. The faces of this 

 form are a little less brilliant than the vertical faces. With a 

 high magnifying power one perceives on them very minute, 

 papillary elevations close to one another. In most cases, how- 

 ever, the faces give well defined reflections. The faces of the 

 prism of the first order г, and those of the prism of the third 

 order r occur comparatively seldom; they are small and not 

 very bright. 



The values obtained by measurements made on six different 

 crystals , compared with the values calculated according to 

 Brögger's axial ratios, are given in the following 



Table of angle s. 





1 



2 



3 



4 



5 



6 



Calcu- 

 lated 



(110):(110) 

 (no) • (010) 



56° 3' 

 61° 59' 



56° 5' 

 61° 40' 



56° 7' 

 61° 52' 





56° 1' 



62° 

 28° 18' 



56° 5' 

 61° 58' 



(no) ■ (100) 







28° 3' 



(no) • (310) 







29° 39' 



77° 56' 



102° 19' 



29° 45' 







29° 56' 



(]10):(00l) 

 (110): (001) 

 (021): (001) 

 (111) : (001) 



102° 29' 

 34° 23' 



77° 31' 



102° 51' 



28° 56' 



35° 4' 



102° 45' 

 104° 13' 



77° 27' 



102° 33' 



29° 28' 



34° 39' 



(l"00) : (001) 









104° 15' 

















A twin crystal of arfvedsonite was found in the Lützen 

 collection. It is twinned according to the common law: the 

 twinning-plane = the first pinacoid, the twinning 

 axis normal to this, the second individual revolved 

 180°. Among the material now examined I have not found any 

 twins. They must, therefore, be of very rare occurrence. 



