. 169. OF THE STRUCTURE OF MINERALS. 229 



Fig. 4G., of the dimensions given in the Characteristic. 

 From this point of view it becomes at once evident, 

 that only those pairs of parallel faces which are of 

 the same quality, can belong to the same simple forms, 

 which could not agree with the hypothesis of a right 

 oblique-angular four-sided prism. The crystalline forms 

 being always supposed to have been previously brought 

 into an upright position, the most even, smooth, and shin- 

 ing face of cleavage will correspond to the form Pr + oo. 

 The other two forms, likewise very different from each 

 other, though less so than from Pr 4- cc, correspond to 



Pr + co and to il Many individuals, containing only 



one very apparent face of cleavage, as, for instance, the one 

 perpendicular to the axis in pyramidal Euchlore-mica, in 

 prismatic Topaz, &c. besides shew traces of other forms 

 of cleavage, which here likewise may serve as examples. 



But the most interesting of all the consequences to 

 be drawn from these and similar examples, is the full 

 confirmation of the theory of crystalline forms, as it has 

 been given in the preceding chapter. According to that 

 theory, there exist forms which, although they appear in 

 a single face, or in two or more, in such directions that 

 they cannot include the space from all sides, yet must be 

 considered as peculiar simple forms. The physical quality 

 of the faces of cleavage in the individuals incontestibly 

 proves the correctness of that method of considering forms, 

 Avhich nevertheless has been merely the consequence of geo- 

 metrical inquiries. 



. 169. FORMS OF CLEAVAGE MEMBERS OF THE 

 SERIES OF CRYSTALLISATION. 



The forms of cleavage represent members of the 

 series of crystallisation of those species, from the 

 individuals of which they have been extracted. 



The demonstration of this proposition follows immedi- 



