82 TERMINOLOGY. 



between two faces of composition, can no more be cleaved in the 

 same direction after their separation, unless they possess a cleavage 

 of that kind, in which event, the quality of the two kinds of faces, 

 would suffice for their distinction. 



Faces of composition are rarely smooth ; and if this happens, we 

 find it only in single, and not in continuous parts of the faces. If 

 they are streaked, the markings are irregular, and without any con- 

 stant direction. Very often the faces of composition are rough, 

 their lustre being of a low degree, or wholly wanting, which ena- 

 bles us to distinguish them readily from the faces of cleavage, where 

 these two kinds happen to be parallel. They are sometimes un- 

 even, or contain more or less considerable elevations and depressions. 

 In such cases, we must avoid confounding them, with uneven faces 

 of fracture, by comparing them with real faces of fracture in the 

 same individual.* 



SECTION II. 



COMPOUND MINERALS. 



$. 71. REGULAR AND IRREGULAR COMPOSITION. 



The mode of composition in which individuals occur is 

 said to be regular, if the form produced by their union is 

 a regular one ; if the contrary takes place, the composition 

 is said to be irregular. 



If two or more homogeneous individuals join in a compound form, 

 regularly and symmetrically, the composition is as much determina- 

 ble as the form of a single crystal. For we may indicate with the 

 utmost precision, in which faces of the simple forms, or in which 

 plane the individuals cohere, even though this plane should not be 

 parallel to a face of any simple form of that species to which the 

 individuals belong. A composition of this kind is said to be regular. 



* The character by which the faces of composition essentially differ 

 from those of crystallization and of cleavage, consists in the circum- 

 stance, that generally they preserve no determined direction, and do not 

 produce any regular forms. 



