92 Mr. Haidinger on the Regular Composition, §c. 



hatsch of Freiberg, the outside and inside of which are form- 

 ed by smooth faces of crystallization. This shape depends 

 entirely upon regular composition, the sides of the hexangu- 

 lar tube being formed by a tissue of small crystals, all 

 aggregated, according to the above-mentioned law. Also the 

 complement to this law occurs, though less frequently, if as 

 in Fig. 24. the face of composition is parallel to one of the 

 faces of R — 2. 



While at Freiberg I also observed a composition similar 

 to the preceding one in Rhombohedral Lead-bary te (the arseni- 

 ate oflead) from Johanngeorgenstadt in Saxony, upon a spe- 

 cimen in the possession of Count Lubiensky. Two individuals 

 of the form R — co. P. P-f oo, Fig- 25. are compound in a 

 plane perpendicular to one of the terminal edges of the iso- 

 sceles pyramid, as represented in Fig. 26. 



Besides the twin-crystals described in the last number of 

 this Journal, in Rhombohedral Iron-ore, Figs. 20. and 22. 

 as heing produced by the union of two individuals with 

 parallel axes, there exists still another law of regular com- 

 position in that series, according to which the axis of re- 

 volution is perpendicular, the face of composition paral- 

 lel to a face of the fundamental rhombohedron R = 85* 

 58'. A group thus formed of a small crystal joined to a 

 large one, is represented in Fig. 27, and refers to a bright 

 specimen of the specular iron-ore, from Stromboli, in Mr. 

 Allan's collection. The faces P and P' fall into one and 

 the same plane, o and o' produce an angle of 115° 17'. The 

 crystals from Elba very often present traces of this compo- 

 sition. The lines upon their surface in a direction agreeing 

 with that which a plane parallel to a face of R would pro- 

 duce if intersecting the crystal, originate in thin films of the 

 substance being engaged in them in a reversed position, like 

 the portion abed, in the rhombohedron, Fig. 28. The 

 angle P' P" is = 171° 56'. P P = 188° 4'. We find this 

 not only in crystals, but also in massive varieties ; and 

 those from Sweden, in particular, which appear to be cleav- 

 able with greater facility than others, owe the even planes, 

 which may be obtained in the direction of the faces of R, 

 not so much to cleavage as to their being composed in that 

 direction. The same applies to the green varieties of Rhom- 



