. 174. OF SURFACE. 235 



CHAPTER III. 



OF SURFACE. 

 . 174. SURFACE IN GENERAL. 



The Faces of Crystallisation (. 28.) arc the 

 most interesting and useful among all those faces 

 which limit the forms occurring in the productions 

 of the mineral kingdom, 



Beside the faces of crystallisation, Terminology con- 

 siders the Faces of Cleavage ', of Fracture^ and of Composition. 

 An explanation of the three first has been given in . 28. 

 163. 172. Faces of Composition are those in \ihich several 

 individuals touch one another ; they belong to tli indi- 

 viduals (. 158.), and therefore require to be considered in 

 this part of the work. Besides, it is necessary to attend to 

 the difference existing between these faces of composition, 

 and those of crystallisation, of cleavage, and of fracture, 

 in order to distinguish forms contained only under faces of 

 this kind, from such as are contained under faces of crys- 

 tallisation, of cleavage, or of fracture. This will in parti- 

 cular be necessary if we have to consider individuals, 

 limited merely by faces of composition, by themselves or 

 singly, and in their original connexion with others. 



Of all sorts of faces, the most interesting ones are thof e 

 which are even, since, in the mineral kingdom, the uneven 

 faces are not subject to any constant law, not being curved 

 like the surface of the geometrical solids, the sphere, the 

 cone, or the cylinder. The only even faces are the faces 

 of crystallisation and the faces of cleavage. The latter 

 exhibit very little remarkable differences in their quality ; 

 while the former shew a peculiarity, the more deserving of 

 consideration, because it is closely connected with the 

 phenomenon of crystallisation itself. The faces of crystal* 



