FRACTURE. 79 



this will enable him to fix upon the primary form, agreeably to the 

 rule by which that form has been selected, (.66.) If, however, 

 all the crystals within his reach have the same form, a knowledge 

 of the relations of primary and secondary forms, and their transi- 

 tions into one another and into new forms, will enable him to decide 

 within one or two of the solid in question, which will often be suffi- 

 cient for his purpose. Thus, if the crystal is a Cube, the primary 

 form must either be identical with that of the crystal, or it must be 

 a regular Tetrahedron, a regular Octahedron or a rhombic Dodeca- 

 hedron, the only solids with which it comes into connexion. Is it 

 an Octahedron with a square base ? this form must be its primary 3 

 or the right square Prism, &c. 



. 68. FRACTURE. 



A mineral when broken in a direction so as to make its 

 irregular structure appear, or contrary to its cleavage, ex- 

 hibits its fracture. 



Cleavage relates to the smooth and even surfaces produced by 

 breaking a mineral : fracture to those which arc uneven. The lat- 

 ter property, though in general more easily observed in a mineral 

 than the former, is nevertheless of considerably inferior conse- 

 quence on account of its want of constancy. 



Fracture is considered here as a property of individuals, or of sim- 

 ple minerals in general ; accordingly, several varieties of fracture 

 frequently alluded to in works on mineralogy, will be treated of 

 under compound minerals, to which they relate. 



. G9. KINDS OF FRACTURE. 



The kinds of fracture are determined according to the 

 quality of the faces produced : this may be said to be of four 

 sorts; viz. 1. conchoidal, 2. even, 3. uneven, 4. hackly. 



1. The Conchoidal represents that kind of irregular structure 

 where the faces resemble the inside or the outside of a common bi- 

 valve shell, as a clam; and the terms perfectly, imperfectly, large, 

 small, and flat, are sometimes appended to point out more minutely 

 the size and concavity of the surface, though in general they are 

 of but little importance. 



