. 170. OF THE STllUCTURE OF MINERALS. 231 



mere accident of the exterior form, but it is a property in- 

 timately related to the existence of the species itself; and 

 every one of the forms which it is capable of assuming, is 

 deeply founded in its interior, or in the regular structure 

 of the mineral ; so that a face of crystallisation is parallel 

 to every direction of cleavage, and a direction of cleav- 

 age, more or less distinct, parallel to every face of crystal- 

 lisation of the species. The first of these results has al- 

 ready been perfectly confirmed by observation. More ac- 

 curate information with respect to the second will be ob- 

 tained by future investigations of this interesting subject. 



. 170. DESIGNATION AND NOMENCLATURE OF THE 

 FOE MS OF CLEAVAGE. 



The forms of cleavage are designated like those 

 of crystallisation ; several particular cases of its oc- 

 currence have been provided with appropriate verb- 

 al expressions, for the purpose of the Systematical 

 Nomenclature and of the Characteristic. 



Since the crystallographic designation of forms of crys- 

 tallisation and of forms of cleavage is exactly identical ; it 

 becomes necessary to indicate whether the sign refers to 

 cleavage or to crystallisation. 



In the systems of variable dimension, the cleavage is said 

 to be A.rotomous*, if it consists of a single face perpendicular 

 to the axis, or parallel to the base of the fundamental form. 

 The same expression may be employed, although, beside 

 this single cleavage, others should appear parallel to the 

 axis, or including an angle with it, yet it is always required, 

 that such faces should be less distinct, and thus form a con- 

 trast with the single one* This observation extends also 

 to the following expressions relative to some other pecu* 

 liarities of cleavage. 



* From, gy, the axis, and ri/ttw, I cut or cleave. 



