. 162. 163. OF THE STRUCTURE OF MINERALS. 

 . 162. CLEAVAGE. 



An individual is said to be cleavable, or to admit 

 of Cleavage, if by a mechanical separation of its 

 particles, the regular structure can be rendered vi- 

 sible. 



Certain individuals may be cleaved with great facility ; 

 and with only the blow of a hammer, they will divide 

 into fragments contained under even faces, like those men- 

 tioned in the preceding examples. This is not the case 

 with others, in which the mere percussion yields only irre- 

 gular faces. In these, however, we are not yet forced to 

 assume the non-existence of cleavage, but by the help of 

 delicate chisels, or of other appropriate instruments, and by 

 a careful examination of the resulting faces, we have to de- 

 termine whether or not cleavage occurs in the individual. 

 It is very useful to expose such faces to an intense light, 

 the reflection of which will very soon decide which of these 

 is the case. A certain degree of skill is required in cleav- 

 ing minerals, which, however, a little practice will teach 

 more accurately than could be done by many words ; and 

 therefore we may omit here, as superfluous, all farther 

 particulars. 



. 163. FACES OF CLEAVAGE. 



The faces obtained in cleaving a mineral, are 

 termed its Faces of Cleavage. 



The faces of cleavage are distinguished from each other 

 in respect to their properties or their relative aspect. 



These properties depend upon the perfection of the 

 faces, in as far as they may be compared to mathematical 

 planes, and upon the degree of lustre which they possess. 

 It is very easy to tell, from the mere ocular inspection of 

 faces, whether their quality be the same, or whether they 

 differ more or less from each other. These differences, 



