286 Dr Gustavus Rose on Epistiibite, 



The formulae of these faces, after the method of Professor 

 Weiss, expressing the ratio of their axes, are — 



The following are the measures of the angles : 

 M on M = 135* 10'. 

 M on r = 112° 25'. 

 M on t = 122° 9'. 

 t on t =109° 46 / . 

 t on m = 154° 51 ; . 

 * on * =141° 47'. 

 s on * =147° 40 7 . 

 Simple crystals are rare ; generally the epistilbitc is found 

 in twins, resembling, in some respect, those of carbonate of 

 lead, and joined parallel to one of the faces of the prism M, 

 as represented in Fig. 23, of which Fig. 24 is a horizontal pro- 

 jection. The faces of the horizontal prism s, in both the in- 

 dividuals, form an angle, which is salient at the edge x, and 

 re-entering at the edge a/. The truncations of the acute 

 lateral edges of the prism M form an angle of 135* 10 7 , equal 

 to the angle of the prism itself. 



The crystals are implanted on a mass of the same species, 

 consisting of granular particles, in the cavities of amygdaloi- 

 dal rocks, and are found in Iceland, and the Faroe Islands, 

 along with heulandite. 



The cleavage of epistilbite is highly perfect, and easily ob- 

 tained parallel to the face r, which replaces the acute lateral 

 edge of the prism M. In other directions there is uneven 

 fracture. The faces M are shining, but uneven, and do not 

 admit of measurement by means of the reflective goniometer ; 

 the faces s are dull, t and r are both smooth and shining. 

 The lustre of M and t is vitreous, and that of r is pearly and 

 bright ; the colour of the crystals is white, their transparency 

 sometimes perfect ; often they are only translucent on the 

 edges. 



