PRIMARY FORMS. 



Fig. 113. 



But this circumstance cannot be relied upon in all 

 instances for determining the primary form of a mi- 

 neral. For the cubic planes of iron pyrites, which 

 are found by cleavage to be the primary planes, are 

 striated in such directions, as might lead us to sup- 

 pose them secondary planes, and its primary form to 

 be a pentagonal dodecahedron. And the cubic crys- 

 tals of blende are frequently striated parallel to the 

 alternate diagonals, as shewn in fig. 113, which would 

 indicate a tetrahedral primary form. But the cleavage 

 is parallel, as we have seen, to all the diagonal planes 

 of the cube, producing a rhombic dodecahedron, and 

 not parallel to the planes a b /, as it should be if the 

 tetrahedron were the primary form and consequently 

 were to be produced by cleavage. 



Fig. 114. 





To illustrate the relation of these stria? to the sup- 

 posed tetrahedron, we shall derive that figure from a 

 cube by cleavage, and to do this we may again have 



