19 



extremely smooth and brilliant. A few of them, which have 

 been more closely examined, will here be described. 



Crystal 1 (Fig. 1, Plate I) is 14'"'° in length, 6'"'° in thickness 

 and, as usual, developed only at one end. It is a left-handed 

 crystal, and shows the following forms: 



m = {lOTo}, r = {lOTl}, z = {on l}, 6- = {2ГГ1}, M = {ЗОЗ1} , 

 ^ = {7181}, and Б = {8081}. 



The prismatic faces below the positive rhombohedral faces 

 are narrower than those below the negative. They are, all of 

 them, without any striation. The positive rhombohedral faces 

 are larger on an average than the negative. The lustre is the 

 same on both. The bipyramid s is represented by two distinct 

 faces, small, but very brilliant. The positive rhombohedron Ж 

 is also represented by two faces, which are comparatively large. 

 The trapezohedral form v is, strictly speaking, represented only 

 by one determinable face , but on the back of the crystal 

 (Fig. I) there is a plane that probably also belongs to this 

 form. It belongs to the same zone, but is strongly curved 

 cylindrically round the zonal axis and gives on the goniometer 

 an unbroken band of light extending over about 10 degrees. 

 The values of the angles by which the forms M and v are 

 determined, are as follow^s 



Measured. Calculated. 



(303 1) : (10Г0) = 14°42'| 



(033 1) : (0110) = 14° 41'/ 



(7181) : (iTOO) == 8° 48' 



14° 42' 



The form В is fully determined by the zones [7181 :0110] 

 and [303"l : lOTo]. But the face is deeply striated parallel 

 to the latter zone, and on the goniometer a large number of 

 vicinal images are obtained , none of which is more distinct 

 than the others. 



Crystal 2 (Fig. 2, Plate 1) is 6'°°^ in length and, like the 

 last, a left-handed crystal. It shows the following forms 



2" 



