80 



PRIMARY FORMS, 



Thus a hexagonal prism of carbonate oflime, which 

 is one of the secondary forms of that mineral, may 

 be cleaved in a direction parallel to the planes of a 

 rhomboid, which is its primary form. 



Fig. 110. 



When the prism results from a decrement pn the 

 inferior angle of the rhomboid, as shewn in the table 



O ' 



of modifications of the rhomboid class e, this cleavage 

 will take place parallel to the three alternate terminal 

 edges of the prism, as shewn in fig. 110. 



Fig. 111. 



But when the prism results from a modification 

 on the inferior edges of the rhomboid, corresponding 

 with modification class n, the cleavage will take place 

 on the alternate solid angles of the prism as in fig. 111. 



A similar prism of phosphate of lime, zMch is the 

 primary form of that substance, cannot be cleaved in 

 any other direction than parallel to its own planes. 



But cleavage alone cannot be relied on for deter- 

 mining the primary form of a mineral. For if, as it 



