156 



they may contain ; and that, when fresh from 

 the interior of a mass, they are often exceedingly 

 brittle, but become tough after a few days of 

 exposure to the air. 



LUSTRE. 



Under this term is necessarily comprised 

 even the absence of all lustre. 



The highest degree of lustre found in rocks, 

 is the plumbaginous ; which occurs in some ar- 

 gillaceous schists : it is a term easily understood, 

 from the familiar nature of the object of com- 

 parison. Where there is an absence of all lustre, 

 as in chalk, the appearance is called dull, or 

 earthy. The intermediate degrees, as far as it 

 is necessary to discriminate them, are not very 

 numerous ; although, from passing into each 

 other, they are not very definite. As the techni- 

 cal terms commonly used for this purpose are 

 unintelligible without the examples, it seems more 

 convenient to use the examples themselves in 

 the place of terms, or to use adjective denomina- 

 tions derived from these. The degrees of lustre 



