1 53 Theory of Cryjlallifation. 



variety at Coufon near Lyons. It is obferved alfo in vmal! 

 yellowifh cryftals, often thrown together in confufed groups 

 in the calcareous banks near Paris. The freeftone (lapis 

 arenarius) of Fontainbleau, which is nothing elfe than 

 calcareous fpar accidentally mixed with quartzy particles, 

 exhibits the fame form. The cryftals of this ftone yield to 

 the mechanical divifion, and have their natural joinings, like 

 thofe of the cryftals of pure fpar, fituated in the planes 

 parallel to the edges p n, py, pu, he. (fig. 43), and which 

 would pafs at an equal diftance from thefe edges. 



"Rhomho'idal Iron Ore {fig. 45) • 



Mine de fer lenticulaire. Daubenton, Tab. Miner, edit. 

 J793. p. 30, n°. 3. 



Geomet. CharaB. Inclination of B C R P to B C O A or 

 OCRS, 146 26' 2,3"; Angles of the rhombus BCRP, 

 Cor P = ii7°2'9"; B or R = 62° 57 '51". 



The laminae which compofe this rhomboid decreafe by 

 two ranges on the angles b c r, ocr, bco, &c. (fig. 46), 

 which concur to the formation of the two folid angles c, n } 

 of a cubic nucleus. The faces produced, inftead of being on 

 a level, three and three, around thefe angles, ; S in the cale 

 of a decrement by a fingle range, incline one to the other, 

 and extend above the faces of the nucleus in fuch a manner 

 that their diagonals are parallel to the horizontal diagonals 

 of the fame faces. 



By this it is feen that the cube here anfwers the purpofe 

 of a rhomboid, which ihould have its fummits in c and n, \i\ 

 which cafe there would be only one axis paffing through the 

 fummits in queftion. In the dodecaedron, on the other 

 hand, with pentagonal planes (fig. 19), the cube performs 

 the functions of a rectangular parallelopipedon, and then 

 we may conceive three different axes, each of which 'panes 

 through the middle of the two oppofite faces. I have 

 :d that when the cube began to perform the one or 



other 



