1883.] Crystallography of Miargyrite. 369 



of his measurements. He seems not to have measured many 

 crystals, and, however definite the images he may have obtained 

 from the faces s, s t , g, one feels somewhat doubtful as to the 

 reliability of elements so obtained, when the other measurements 

 on even the same crystal diverge largely from their calculated 

 values. Moreover the angles he found differ much from those found 

 on any of the crystals examined by me. Prof, vom Rath gives 

 ss t = 77° 4' ; gs = 69° 54' ; and gs / = 53° 33'. From this triangle it 

 is possible to obtain the angles between three theoretical planes in 

 the zone of symmetry, and hence the elements of the crystal. 

 The elements are therefore obtained in a very indirect way, and 

 any errors in the angles will run great risk of being multiplied in 

 the course of the calculations. L have seldom found the planes s 

 give definite images, and the planes g rarely gave images suffi- 

 ciently definite to be observed with a telescope. 



The crystallography of the mineral is now seen to be much 

 simpler than that given by Miller. I find the principal zones to be 

 [AoOm], [Afdst], [opg], [Ahgg], and [fizkto-g]. The crystals, how- 

 ever, vary much in appearance according as some planes pre- 

 ponderate over others or not, and the actual planes which so 

 preponderate. As far as they can be placed under types I should 

 be inclined to class them as follows : 



(1) A type common at Braunsdorff in which the planes a, o, c 

 are large, and d, s, t appear in diminishing magnitude giving the 

 crystals at the edges in which the two adjacent zones [dst] meet a 

 sharp wedge-shaped appearance. This type may be subdivided into 

 one in which the planes d, s, t, &c. are well developed, and a second 

 subdivision in which the striations are so prominent that the suc- 

 cessive planes seem to flow into one another, and we have a couple 

 of apparently curved wedges terminating the crystals on opposite 

 sides of the zone of symmetry. Figs. 2 and 3 represent crystals of 

 these forms. (2) Crystals resembling the figure given in Naumann's 

 Mineralogy in which the planes in the zone of symmetry and in 

 [dst] are all well developed ; and the plane £ (213) has been found 

 by me to be prominent (see Fig. 4). (3) A type in which with 

 the planes a, o, c large, the planes d and g are about equally 

 developed, giving the crystals a somewhat rhombic appearance as is 

 shown in Fig. 5. (4) Crystals with numerous planes in [dst] and 

 in [fizkt], all about equally developed. The crystals of this type 

 are common and somewhat difficult to decipher without measure- 

 ment. They differ moreover considerably the one from the other, 

 inasmuch as in some crystals the planes x will be largely developed, 

 in others the planes /3, and again in others one large x plane will 

 be associated with a large /? plane lying on opposite sides of the 

 [dst] zone not containing the large x plane. Figs. 6, 7 and 8 are 

 representations of crystals of this kind. (5) Crystals in which c is 



