of Crystallized Bodks. 89 



by R —- 1. R + 1. R + od, 2 [f }. The composition of 

 some of those called heart-shaped twins, first described by 

 Count Bournon, may be explained according to the same law. 

 Mr. Allan possesses a beautiful crystal of this kind, which is re- 

 presented in Fig. 6. Its crystallographic sign isR — 1. (P) 3 . 

 R + goj { t}' Another equally interesting crystal is the one 

 of Fig. 7, likewise in Mr. Allan's collection, and denoted by 

 (P) 3 . R -f oo, [x] - ^ n a regular composition of the simple 

 pyramid according to this law, Fig. 8, the edges x and x 

 will include an angle of 14-4° 32'. It is an acute terminal edge 

 of each crystal, which terminates here in the re-entering angle. 

 The crystals of this and the preceding variety are generally 

 a little flattened, as represented in the figures. The angle 

 included by the edges xo and w is= 141° 44'. 



The most common, however, of all the regular composi- 

 tions in Calcareous spar, is that parallel to one 4 or even pa- 

 rallel to all the faces of R — J. Of the first of these cases, 

 Figs. 9. 10. and 11. represent interesting varieties. Fig. 9. 

 is expressed by the sign R — oo. R — 1. R + 1. R -|- go, 

 [~ j. It refers to a variety from the Hartz, in the posses- 

 sion of Mr. Sack of Bonn. Fig. 10. is from the mine of Him- 

 melsfiirst near Freiberg. I have been indebted for a speci- 

 men of it to Mr. Euler of Deuxponts. Its crystallographic 

 sign isR — 1. It + od, { R ^}- The variety, Fig. 11, ex- 

 pressed by (P) 11 , [-^p 1 ]* has been discovered by Mr. Allan 

 in a vein eighteen inches wide, and consisting only of this 

 species, near Westmanhaven in Stromoe, one of the Faroe 

 islands, and first described by Count Bournon. The crystals 

 are perfectly transparent, and generally lengthened in the di- 

 rection of a b, as the figure indicates. The variety Fig. 19, 

 from Chamouni, in the collection of Mr. Allan, is expressed by 

 R — oo. R, 2 {^^}. The two individuals do not terminate 

 at the face of composition, but they reach beyond it, which 

 produces the cruciform aspect of the whole, and the parallel- 

 ism of the face P in one, with P' in the other individual. The 

 incidence of o on o' is = 127° 29'. The lace of composition in 

 Fig. J 3. is perpendicular to one of the terminal edges of R. 

 This is the supplemental composition of that according to 

 which Figs. 9. 10. and 11. are grouped. The variety Fig. 12. 



