90 Mr. Haidinger on the Regular Composition 



contains them both, and thus demonstrates that the two ap- 

 parently different laws of composition enter in fact within a 

 single one. The sign of Fig. 13. is R — 1 . R . R + 1. (P) s . 

 R_|_ oo? {— ^}. It has been found atBieiberg, in Carinthia. 

 The angle at which the acute terminal edges x and a/ of the 

 two individuals meet, is = 106° 16'. Fig. 14. represents the 

 simple pyramid (P) 3 , composed according to this law, per- 

 pendicular to an edge of R, and forms an interesting point of 

 comparison with Fig. 15, which contains two individuals pos- ' 

 sessing the same simple form, but joined according to the 

 above-mentioned supplemental law, parallel to one of the 

 faces of R — 1. The inclination of the two obtuse edges y 

 and y is =171° 18'. Both these kinds of composition are 

 united in Fig. 16, which will serve to illustrate the mode of 

 their formation. A variety similar to Fig. 15. is preserved 

 in the Wernerian collection at Freiberg. 



In Fig. 17, which represents a group of crystals of the 

 form R — 1. R -j- 1, {r-i.r}, the composition takes place 

 perpendicularly to all the terminal edges of R at once, so that 

 the group seems to consist of a central crystal, round which 

 the others are aggregated. Generally, however, in this case 

 each of the crystals joined to the central one, again has crystals 

 attached to it, according to the same law. This variety has 

 been found at Moldawa, in the Bannat, where it occurs with 

 malachite and brown iron ore. 



If R, the fundamental rhombohedron of the species itself, 

 be composed parallel to one of the faces of R — 1, the result 

 will be like Fig. 18, a form which it is very common to ob- 

 tain among the cleavages of this species. Among many ex- 

 amples, some of the most distinct are found in the varieties 

 from the Pfaffenberg mine, near Harzgerode. Generally, 

 however, the reversed situation of one of the parts of the 

 rhombohedron in respect to the other, is soon interrupted by 

 another part of the first joining in a plane parallel to the for- 

 mer composition, which again makes room for part of the se- 

 cond, and so on, and thus produces a succession of laminae 

 belonging to two individuals, Fig. 19. If these plates are 

 thin enough, they produce stria? upon two opposite faces of 

 a rhombohedron parallel to the horizontal diagonals, which 

 will be observable upon all the faces of cleavage, if the com- 



