224) TERMINOLOGY. . 166. 



Several minerals may be cleaved into exceedingly delicate 

 laminae, others do not admit of cleavage to such an extent. 

 Among the first, several species of the genus Talc-mica are 

 particularly remarkable. Prismatoidal Gypsum-haloide may 

 also be cleaved into uncommonly thin laminae ; and we 

 might succeed in attenuating them still more, if instru- 

 ments could be found of sufficient delicacy. Cleavage may 

 be continued so far in these cases, because, except that single 

 cleavage, there are no other directions in which the minerals 

 cleave with the same facility, or, what is the same thing, be- 

 cause it is very difficult to separate their particles at all in 

 other directions. The other class of cleavable minerals com- 

 prehends the individuals of such species as present more than 

 one direction of cleavage, or whose particles may be more 

 easily separated in uneven irregular faces. The facility with 

 which the particles may be separated from each other in 

 more than one direction of cleavage, or in irregular faces, 

 prevents the cleavage from being more apparent, and ob- 

 tained with greater facility in one of the directions. Hexa- 

 hedral Lead-glance, rhombohedral Lime-haloide, &c. may 

 be quoted as examples of minerals, which cleave with equal 

 facility in more than one direction. 



. 166. FACES OF CLEAVAGE PARALLEL TO FACES 

 OF CRYSTALLISATION. 



Every direction of cleavage (. 164.) is parallel 

 to the face of a form of the Series of* Crystallisa- 

 tion of that species, to which the cleavable indivi- 

 dual belongs. 



In the species of octahedral Fluor-haloide, the solid 

 angles of the hexahedron may be broken off by cleavage 

 with the greatest facility ; in the place of every one of 

 those solid angles, there will appear an equilateral triangle, 

 or an equiangular hexagon, which is the face of cleavage. On 

 account of its figure, it is situated perpendicularly to a rhom- 

 bohedral axis, that is to say, like a face of the octahedron, a 



