68 



plane parallel to the third pinacoid, and one of the halves is 

 revolved 60° about the vertical axis (Fig. 1, Plate IV). The two 

 halves generally fit in perfectly with each other, the prism 7i and the 

 second pinacoid Ъ forming an almost regular hexagon, and the 

 fundamental prism, w, coinciding with the first pinacoid a. The 

 congruity is so perfect that, if the two halves are of equal size, 

 the twinning line may easily escape notice. Often, however, 

 the two halves project beyond each other, the wide composition 

 faces become visible, and it looks as if the two sub-individuals 

 lay loose on each other. 



With regard to clearness and translucency the crystals of 

 type III surpass those of the other two types. The latter are 

 generally traversed by a number of cracks or present a somewhat 

 opaque appearance, whereas those of the third type almost al- 

 ways are quite clear. 



The mineral is , in the form now described , comparatively 

 rare, as only a few specimens have been found. They were 

 met with at the locality No. 2 lying loose on the ground, before 

 any digging was undertaken. The crystals are not attached by 

 one end like the needle-shaped individuals, but they are im- 

 bedded in a porous mass, which chiefly consists of very small 

 calcite individuals and elpidite and a brown alteration product 

 with earthy texture. 



The optical investigation of epididymite presents no small 

 difficulties , owing partly to the feeble double refraction of 

 the mineral, partly to its cleavages, which render the pre- 

 paration of crystal siections suitable for determining the optic 

 axial angles impossible. The epididymite material found in the 

 Lützen collection was very scanty, and the optical data gained 

 by examining it have now proved to be very unsatisfactory, 

 which justifies the doubts previously M suggested by me as to 

 the accuracy of that examination. The renewed investigation 



') Zeitschr. f. Kryst. 1894, Vol.23, p. 357. 



