. 1G9. OF THE STRUCTURE OF MINERALS. 



pressed by the combination (Pr 4- oo) 3 . Pr + 09. Pr + 09 

 which is imperfect, that is to say, unlimited in the direc- 

 tion of its axis, except by faces of crystallisation, or by 

 faces of cleavage different from those contained in the repre- 

 sentative sign of the compound form, or at last by faces 

 not produced by cleavage at all. 



, 168. FORMS OF CLEAVAGE, DISTINGUISHED AC- 

 CORDING TO THE QUALITY OF THEIR FACES. 



Faces of cleavage, which belong to one and the 

 same simple form, are homologous (. 163.). Faces 

 of cleavage which are not homologous, belong to 

 different simple forms of cleavage. 



If we find an opportunity ol comparing the form of 

 cleavage of an individual with its form of crystallisation, it 

 will not be difficult to decide which of the faces of cleavage 

 belong to one, and which to different simple forms ; and 

 indeed the certainty and generality of the present propo- 

 sition depends upon observations of this kind. 



In many instances, however, we only can examine forms 

 of cleavage, belonging to minerals which are not crystal- 

 lised, or such individuals as present no regular external 

 forms, and in which therefore the forms of cleavage are of 

 still higher value for the natural-historical determination. 

 Here the above mentioned proposition becomes of the great- 

 est utility, since it allows the forms of cleavage to be con- 

 sidered in the right point of view, even though only a few 

 of their faces should present themselves to the observer. 



As to the first, we find that, all the faces of cleavage in 

 the individuals of rhombohedral Lime-haloide, of octahe- 

 dral Diamond, of dodecahedral Garnet-blende, of prismatic 

 Topaz, and of many others, are of exactly the same qua- 

 lity, so that they can by no means whatever be distinguish- 

 ed from each other ; these faces being found to possess the 

 same properties throughout the whole individual. A very 

 remarkable difference, however, is found in several species, 



