FORMS OF CLEAVAGE. 71 



6. The oblique rhombic Prism, (Fig. 4-1.) Tv/o cleavages, equal- 

 ly distinct, not perpendicular to one another; and a third, forming an 

 equal oblique angle with each of the first. Examples Hornblende 

 arid Pyroxene. 



7. The doubly oblique Prism, (Fig. 45.) Two cleavages, une- 

 qually distinct, not perpendicular to one another, and a third, in- 

 clining differently to each of the first. Examples Feldspar and 

 Sulphate of Copper. 



8. The regular Octahedron, (Fig. 35.) Four cleavages, equally 

 distinct, and equally inclined to an axis, (under the angle of 35 15' 

 51",) so that any two of their opposite intersections are perpendicu- 

 lar to each other, or so that the three sections through the edges are 

 squares, and that the faces are equilateral triangles. Examples 

 Fluor and Diamond. 



9. The Octahedron with a square base, (Fig. 37.) Four cleav- 

 cges, equally distinct, and equally inclined to an axis, so as to form 

 equal isosceles triangles, and so that the three sections through the 

 edges are in one instance a square, and in the others, rhombs. Ex- 

 amples Zircon and Tungsten. 



10. The Octahedron ivith a rectangular base, (Fig. 38.) Four 

 cleavages, disposed about an axis under two different inclinations, 

 the two on opposite sides being equally inclined and equally dis- 

 tinct ; fiom whence it follows, that the triangles are all isosceles, 

 but of two different kinds. Example Arseniate of Copper. 



11. The Octahedron with a rhombic base, (Fig. 39.) Four cleav- 

 ages, equally distinct, equally inclined to an axis, but so as to form 

 scalene triangles, and that the sections through the edges are rhombs. 

 Example Sulphur. 



12. The Rhomboid, (Fig. 46.) Three cleavages, equally distinct, 

 and equally inclined among one another. Examples Carbonate of 

 Lime and Corundum. 



13. The regular Dodecahedron, (Fig. 3G.) Six cleavages, equal- 

 ly distinct, and uniting two and two upon an edge, under an angle 

 of 120. Example Blende. 



14. The regular six sided Prism, (Fig 47.) Three cleavages, 

 equally distinct, parallel to the axis of the crystals, intersecting 

 each Other, under angles of 120; and a fourth, perpendicular to 

 the first. Examples Emerald and Phosphate of Lead. 



The last substance has cleavages also, equally distinct in six di- 

 rections ; viz. parallel to the terminal edges of the prism, which re- 

 sult in a Dodecahedron with equal isosceles triangles. 



