2<J t Theory of Cryjlallifatwii. 



The numerous analogies by which this variety Is con- 

 nected with different crystalline forms, whether we confider 

 certain angles formed by planes, as the angle heg of 90 , 

 the angle c eg of 6o 3 , or certain triangles obtained by draw- 

 ing the diagonals of the trapezoids, have induced me to give 

 it the name of analogical f par. It is derived from three other 

 varieties mentioned before, viz. very obtufe (par by the tra- 

 pezoids emihyfihty &c. ; metaftatic fpar by the trape- 

 zoids emdc, ehogy ohtz, Sec. and the prifmatic fpar by 

 the trapezoids bdek, cegp, &c. which are confequently 

 parallel to the axis. 



It often happens that the trapezoids imeh, ft h t, &c. 

 are feparated, by an intermediary ridge, from the vertical 

 trapezoids cegpygozr, he. In that cafe the trapezoids 

 c dme, ge he, &c. are changed into pentagons. I have here 

 fuppofed the cryflal brought back to the mod fymmetric 

 figure, that is to fay, having its furface compofed only of 

 quadrilaterals, as fometimes happens. This variety is found 

 in Derbyfhire. 



Icofaedral Stilfure of Iron {Fig. 55- )• 

 Pyrite ferrugineufe polyedre a vingt facts triangulairts. 

 Daubenton Tab. Miner, edit. 1792, p. 30. De l'lfle Cryf- 

 tallographiey torn. iii. p. 231, var. 22. 



Geomet. characl. Refpective inclinations of the ifofccles 

 triangles P LR, P S R, 126 52' 1 1"; of any one P NL of 

 the equilateral triangles, to each adjacent ifofceles triangle 

 P L R orLNK 140 46 7 1 '/'. Angles of the ifofceles tri- 

 angle PLK, L = 4 b 1 1' 20" ; P or R = 65 54' 2c". 



This variety refults from a combination of the law which 

 produces the o£t,aedron originating from a cube (fg. 42.), 

 with that which takes place for the dodecaedron with pen- 

 tagonal planes (fg. 19 and 2c). The firft law gives birth 

 to the eight equilateral triangles which correfpond with the 

 folid angles of the nucleus, and the fecond to twelve ifofceles 



triangles, 



