70 TERMINOLOGY. 



The directions of cleavage are somewhat various in crystals be- 

 longing to different species. In some crystals, but one is visible, 

 several possess two, Galena and Calcareous spar, have three, Fluor 

 four, anJ Blende six, while others sliil, possess cleavages in more di- 

 rections than six. 



As the planes produced by cleavage are parallel, a form of cleav- 

 age cannot result from less than three sets of cleavages. Never- 

 theless, if cleavage takes place in but two directions, provided these 

 lead to the formation of a quadrangular prism, it may be said to pro* 

 duce a form of cleavage ; since, if we arrive at a knowledge of the 

 lateral faces of a prism, we possess independently of the cleavage, 

 means for determining the base, whether it be horizontal or oblique. 



It has been remarked, ( 60,) that there exist differences among the 

 cleavage planes of a crystal, by which one cleavage or one set of 

 cleavages, are capable of being distinguished from others. The 

 importance of this fact is very great. If we attempted to consider 

 the forms of cleavage, or cleavage crystals, as they are formed 

 from the whole of their cleavages, we should sometimes have forms 

 too complicated to be understood. We therefore describe separately 

 the solids resulting fiom the meeting of cleavage planes of thesamo 

 kind, which will lead us to point out in a few instances, more than 

 one form of cleavage as belonging to the same crystal. 



The following arc the different forms of cleavage which have 

 hitherto been observed. 



1. The Cube, (Fig. 33.) Three cleavages, with equally brilliant 

 -and even planes, perpendicular to one another. Examples Galena, 



Grey Cobalt- and Leucite. This last mineral presents, in addition to 

 the foregoing, six other cleavages, vvnich afford planes parallel to 

 the sides of a regular rhombic Dodecahedron. 



2. The right square Prism, (Fig. 40.) Three cleavages, two 

 producing similar planes, and all perpendicular to each other. Ex- 

 amples Idocrase and Scapolite. 



3. The right rectangular Prism, (Fig. 41.) Three cleavages, 

 perpendicular to one another; but no two of which are similar, 

 Examples Olivine and Wolfram. 



4. The right rhombic Prism, (Fig. 42.) Two cleavages of equal 

 degrees of distinctness, not perpendicular to one another; and a third, 

 perpendicular or at right angles to the first. Examples Sulphate of 

 Barytes and Staurotide. 



5. The right oblique angled Prism, (Fig. 43.) Two cleavages, 

 pot at right angles to each other, unequally distinct; and a third, per- 

 pendicular to the ftrst. Examples Epidote and Sulphate of Lin^o, 



