58 



CLEAVAGE. 



There are among minerals some substances which 

 yield readily to mechanical division in one or two 

 directions, but do not admit of distinct cleavage in a 

 third direction, so as to produce a regular solid. 



This circumstance has introduced into mineralogy 

 the terms single cleavage, or double, triple, fourfold, 

 &c. cleavage, which are sometimes perplexing to a 

 learner, as they may be confounded with the different 

 sets of cleavages before spoken of. 



But these terms single, double, triple cleavage, &c. 

 are intended to refer strictly to the sets of primary 

 cleavage only. 



When a mineral can be split in only one direction, 

 the cleavage is said to be single ; when in two direc- 

 tions, which may be conceived to give four sides of a 

 prism, it has a double cleavage. 



When there is a cleavage in . three directions, such 

 as to produce either the lateral planes of the hex- 

 agonal prism, or a solid bounded by six planes which 

 are parallel when taken two and two, it is termed a 

 triple cleavage. 



A four-fold cleavage, or one in four directions, will 

 produce a tetrahedron, an octahedron, or a perfect 

 hexagonal prism ; the two latter solids consisting of 

 four pairs of parallel planes, lying in as many dif- 

 ferent directions. 



The rhombic dodecahedron possesses six pairs of 

 parallel planes lying in different directions, and may 

 be said therefore to have a sixfold cleavage. 



Sometimes the natural joints of a crystal may be 

 perceived by turning it round in a strong light, al- 

 though it cannot be cleaved in the direction of those 

 joints. 



Different specimens of the same substance will also 

 yield to the knife or hammer with unequal degrees of 

 facility; and even carbonate of lime, which splits 



