240 TERMINOLOGY. . 177. 



haloide, homogeneous matter of the crystal to fill up the 

 interstices between the faces of those minute hexahedrons, 

 so that the formation is terminated ; no character remains 

 by which a crystal thus formed could be distinguished from 

 a simple mineral. We may imagine that, in the progress 

 of the formation, a simple mineral may give rise to a com- 

 pound one ; but it is utterly impossible that a simple one 

 could be formed out of a compound mineral. Crystals 

 with drusy faces may consequently be simple minerals. 



. 177. FACES OF COMPOSITION. 



The quality of the faces of composition is acci- 

 dental. 



The faces of composition sometimes are even, yet this is 

 very rare. Even faces of composition may easily be dis- 

 tinguished from faces of cleavage, because those particles 

 which are contained between two faces of composition, can 

 no more be cleaved in the same direction ; provided they 

 do not possess besides a cleavage of that kind ; in this case, 

 however, the quality of the two kinds effaces would suffice 

 for their distinction. 



They are rarely smooth ; and if this happens, we find it 

 only in single, not continuous parts of the faces. More 

 commonly they are streaked ; but the striae are irregular, 

 without any determined or constant direction. Very often 

 we meet with rough faces of composition, their lustre 

 being of a very low degree, or even sometimes entirely 

 wanting ; this may be used as a distinctive character be- 

 tween the faces of cleavage and those of composition, if, in 

 a mineral, these two kinds of faces should happen to be pa- 

 rallel. Lastly, they often are uneven, or contain more or 

 less considerable elevations and depressions. Faces of this 

 kind must not be confounded with uneven faces of fracture ; 

 this, however, may be very easily avoided by comparing 

 them with real faces of fracture in the same individual. 



The character by which the faces of composition essen^ 



