IO 



THE ROCK-FORMING MINERALS 



III. Hexagonal System. 



Fig. 3. — Forms of the Hexagonal 

 System : Hexagonal Prism ; Rhombo- 

 hedron; Scalenohedron. 



Here four axes are employed, three 

 equal lateral axes intersecting at 

 angles of 60 degrees, and a ver- 

 tical axis, which is perpendicular 

 to and longer or shorter than the 

 laterals. Includes the rhom- 

 bohedron, hexagonal prism, and 

 scalenohedron. 



IV. Orthorhombic System 

 The three axes intersect at right angles 



FlG. 4. — Forms of the Orthorhom- 

 bic System : Rhombic Octahedrons. 



(rhombic, trimetric). 

 and are all of different lengths ; 

 rectangular and rhombic prisms, 

 and rhombic octahedron. 



V. Monoclinic System (mono- 

 symmetric, oblique) . — All three 

 axes are of different lengths ; two of the axes, usually the laterals, 



are at right angles to each other, 

 while the third is oblique : right 

 rhomboidal and oblique rhom- 

 bic prisms. 



VI. Triclinic System (anor- 



thic, asymmetric). — Three 



axes of unequal lengths and 



oblique rhomboidal prism, doubly 



Fig. 5. — Forms of the Monoclinic 

 System : Monoclinic Pyramid and Prism. 



oblique to one another 

 oblique octahedron. 



It is important to bear in mind the relations which the funda- 

 mental forms sustain toward one another. For example, a regular 

 octahedron may be derived from a cube by evenly paring off the 

 eight solid angles, until the planes thus produced intersect one 

 another, the centres of the faces of the cube becoming the apices 

 of the solid angles of the octahedron. Conversely, a cube may be 

 formed from an octahedron by symmetrically truncating the angles, 

 until the planes thus formed intersect. By slicing away the twelve 

 edges of a cube or an octahedron a dodecahedron will result. 

 These crystalline forms are, therefore, so related as to be all de- 

 rivable one from another, and the relations of their axes remain 



