. 167. OF THE STRUCTURE OF MINERALS. 225 



form which in fact belongs to the series of crystallisation 

 of octahedral Fluor-haloide. The solid angles of the oc- 

 tahedrons cannot in this species be broken off by faces of 

 cleavage, but this form may be cleaved parallel to its own 

 faces in four directions, intersecting each other at angles of 

 109 28' 16" and 70 31' 44". 



The directions of the faces of cleavage in which the pyra- 

 mid (P) 3 of rhombohedral Lime-haloide may be cleaved, 

 are parallel to those of faces passing through every two con- 

 secutive lateral edges of the pyramid. But this is the di- 

 rection peculiar to the faces of the rhombohedron 11. The 

 faces of cleavage are therefore parallel to the faces of the 

 rhombohedron R, which is the fundamental form in the 

 series of crystallisation of the species. In the rhombohe- 

 dron, the cleavage may be continued parallel to its own 

 faces, in three directions, intersecting each other at angles 

 of 105 5' and 74 55'. 



llhombohedrai Talc-mica, pyramidal Kouphone-spar, 

 prismatic Topaz, &c. admit of a cleavage perpendicular to 

 their axes. The faces of cleavage will be parallel either to 

 It os or to P oo, both of them being limits of their 

 respective series of crystallisation. 



It appears, from the given examples, that not every 

 crystalline form is cleavable parallel to some one or the 

 other of its faces. Several of them, however, shew this 

 property. Simple forms of finite dimensions, if cleavable 

 parallel to their own faces, have on that account by prefer- 

 ence been chosen for fundamental forms, even though such 

 forms should not as yet have been produced by nature 

 among the crystalline forms of the species. 



. 167. FORM OF CLEAVAGE. 



A form contained only under faces of cleavage, is 

 termed a Form of Cleavage. 



The forms of cleavage may be either simple forms or 



VOL. I. p 



