CLEAVAGE. fift 



are parallel to the edges, and to the diagonals, of 

 the square base, being at the same time perpendicular 

 to the plane of that base. 



If all the planes of any primary form be similar, as 

 those are of the cube, rhomboid, and some other 

 forms, the primary cleavages will generally be ef- 

 ected with equal facility in the direction of each of 

 those planes, and the new planes developed by this 

 cleavage will be similar in lustre and general cha- 

 racter. This may be illustrated by cleaving galena 

 and carbonate of lime. 



Where the planes of a primary form are not all 

 similar, as in all prisms, and some octahedrons, the .. 

 primary cleavage is not effected with equal facility in 

 all directions, nor do the new planes all agree in 

 their general characters. Hence the cleavage planes 

 of a mineral will frequently enable us to determine 

 what is not its primary form, by their similarity or 

 dissimilarity; but, as will be seen in the section on 

 primary forms, the cleavage is not sufficient to deter- 

 mine what the primary form really is. 



Felspar, cyanite, and sulphate of lime, afford in- 

 stances of the greater facility with which a cleavage 

 takes place in one or two directions than in any 

 other. 



The Abbe Haiiy has supposed that these unequal 

 cleavages are occasioned by the unequal extension of 

 the different primary planes. The broader planes, 

 presenting more points of contact than the narrower 

 ones, may, he imagines, be held together with greater 

 force than the narrower ones are. This may possibly 

 be the cause of the observed inequality of cleavage, 

 or possibly where the planes are unequal, the degree 

 of attraction between point and point is unequal 

 also. 



