2« 8 Tlseory of CryJlaUifatioti. 



but parallel to lines fituated between the diagonals and the 

 edges. This is the cafe when the. fubtractions are made by 

 ranges of double, triple &c. molecular. Fig. 47, (Plate IX.) 

 exhibits an in 'lance of the fubtracYions in queftion •, and it 

 is fcen that the molecuiae which compofe the range repre- 

 fented by that figure, are aflbrted in fuch a manner as if of 

 two there were formed only one ; fo that we need only to 

 conceive the cryita! con. poled of paralielopipedons, having 

 their bafes equ d to the fmall rectangles abed, edfg, 

 hgil, &c. to reduce this cafe under that of the common 

 decrements on the -.ng'es. I give the name of intermediate 

 decrtment to chis partici ir kind of decreafe, the progrefs of 

 which will be' b.e<.^r 1! unrated by the following example. 



Bjiftatli Iron Ore (Fig- 48). 



De l'lfle Cryjluil g <■;'.'/., torn. iii. p 198 and 199. var. 9 

 and 10. 



Geomet. eharacl. RefpecHve inclination of the trapeziums 

 hc g°> »ng°y of the' rifir.g pyramid^ 135 34' 31"; 

 of the edges eg, g 7, 129 31' 16". Angles of the tra- 

 pezium bego, b or c = 103° 48' 35''; or g = 76 1 1' 

 25". _ 



This variety of iron ore, which for the moft part appears un- 

 der the formof twooppofite pyramid?, riling from a common 

 bafe, is found at Framcnt in les Vofges. There are fome 

 groupes the furface of which, like the iron ore of the ifland of 

 Elba, reflects the moft lively prifmatic colours. The cryftals 

 are often fo fmall, that they might be taken for fimple tetra- 

 gonal laminas; but, on dole infpeclion, the fmall fpots which 

 form the faces of the riling pyramids may be feen. 



Thefe cryftals, which hi. de l'lfle clafl^d among the modi- 

 fications of the clodecaedron with ifofceles triangular planes, 

 have for nucleus a cube which performs the functions of the 

 rhomboid, as in the ore cf the ifland of Elba. The two 

 regular hexagons, by which they are terminated, arife from 

 a decrement by a fingle range of cubic moleculx on the 

 angles c, n, (Jig. 46) of the nucleus. 



To 



