226 TERMINOLOGY. . 167. 



combinations. Both may be either perfect (finite) forms, 

 or imperfect (infinite) forms. 



If we continue cleaving an individual of octahedral Fluor- 

 haloide, till it is comprehended on all sides within faces of 

 cleavage, the result will be the perfect form of cleavage 

 belonging to the species ; it will be a simple form, because 

 the faces of cleavage homologous to each other, are parallel 

 only to the faces of the octahedron, and not to those of any 

 other simple form. It is exactly the same with the forms 

 of cleavage of hexahedral Lead-glance, of hexahedral 

 Rock-salt, of octahedral Corundum, of dodecahedral Gar- 

 net-blende, and many other species. 



Peritomous Ruby-blende cleaves in the direction of the 

 regular six-sided prism R + co. The form of cleavage is 

 therefore a simple one; but it is incomplete or infinite, 

 and terminated in the direction of the axis, either by faces 

 of separation, which are not faces of cleavage, or by faces of 

 crystallisation. 



Prismatic Topaz cleaves parallel to the faces of P ~ co. 



The form of cleavage is a simple one, but imperfect ; and 



is contained in the directions parallel to the axis under 



faces of crystallisation, or under such faces of separation, 



as do not owe their existence to cleavage. 



Rhombohedral Fluor-haloide cleaves parallel to the faces 

 of P + co ; but at the same time, also to those of R co. 

 The form of cleavage, R co. P + co, is therefore a com- 

 bination. But it is a perfect form, since by these faces the 

 space is limited on all sides. 



Pyramidal Feld-spar and pyramidal Garnet are cleav- 

 able in the direction of two rectangular four-sided prisms ; 

 and at the same time perpendicularly to their axis. Their 

 form of cleavage therefore, P co. P + cc. [P + cc], is a 

 combination and at the same time a perfect form of cleavage. 



Paratomous Augite-spar, and in most cases also hemi- 

 prismatic Augite-spar, cleave in the direction of vertical 

 oblique-angular four-sided prisms ; and at the same time in 

 the direction of planes, passing through the axes and the 

 diagonals of the prisms. Their forms of cleavage are ex- 



