CLEAVAGE. 33 



Includes the square prism and square octahedron, (figs. 4, 

 5, 6.) 



3. Trimetric system, (from tris, three times, and metron, 

 alluding to the three axes being unequal.) Includes the right 

 rhombic prism, right rectangular pi ism and rhombic octane- 

 dron, (figs. 7, 8, 9.) 



4. Monoclinic system, (from monos, one, and klino, to 

 incline, one axis being inclined to the other two which are 

 at right angles.) Includes the right rhomboidal prism and 

 oblique rhombic prism, (figs. 10, 11, 12, 13.) 



5. Triclinic system, (from tris and klino, the three axes 

 being oblique to one another.) Includes the oblique rhom- 

 boidal prism, (figs. 14, 15.) 



6. Hexagonal system. Includes the rhombohedron ana 

 hexagonal prism, (figs. 16, 17, 18.) 



CLEAVAGE. 



It has already been stated that crystals of calcareous spar 

 may be chipped off easily in three directions, and by this 

 means, the fundamental form, a rhombohedron, may be ob- 

 tained. In all other directions only an irregular fracture 

 takes place. This property of separating into natural layers, 

 is called cleavage, and the planes along which it takes place, 

 cleavage joints. 



Cubes of fluor spar may be cleaved on the angles, with a 

 slight pressure of the knife, and the process continued affords 

 successively the forms represented in figures 20, 20a, and 

 finally the completed octahedron, as already explained. A 

 lead ore, called galena, yields cubes by cleavage. Mica — 

 often improperly called isinglass — may be torn by the fingers 

 into elastic leaves more delicate than the thinnest paper. 



In many species cleavage is obtained with difficulty, and in 

 others none can be detected. Quartz is an instance of the 

 latter ; yet it may sometimes be effected with this mineral by 

 heating it and plunging it while hot into cold water. 



The following are the more important laws with respect to 

 this property : 



Cleavage is uniform in all varieties of the same mineral. 



It occurs parallel to the faces of a fundamental form or 

 along the diagonals. 



It is always the same in character parallel to similar faces 



What is cleavage 1 How does it differ in different minerals 1 Wha 

 are the laws relating to cleavage. 



