SECTION VIII. 

 SECONDARY FORMS. 



THE secondary forms of crystals are either simple 

 or compound. The simple consist of modifications 

 of the primary forms, produced by single decrements. 

 The compound consist of several modifications occur- 

 ing together on one crystal, produced by as many 

 decrements operating simultaneously upon it. The 

 cube with the solid angles truncated or replaced by 

 three or six planes, is an instance of a simple second- 

 ary form produced by a single modification ; but if 

 the edges be also truncated, or bevilled, or the solid 

 angle be both truncated and replaced by three or six 

 planes, it will aftbrd an example of a compound 

 secondary form. 



The secondary planes frequently obliterate entirely 

 the primary ones, and produce a new form apparently 

 belonging to another class of primary forms, as in 

 the instance of the rhomboid being converted into a 

 six-sided prism by the truncation of all its solid 

 angles, or of its terminal solid angles and its lateral 

 edges. 



Particular secondary forms sometimes predominate 

 in particular species of minerals, as the cube in fluate 

 of lime, whose primary form is an octahedron. Par- 

 ticular modifications of primary forms are also found 

 to affect particular districts of country. 



