157 



M'hich may be defined in this way, and which 

 seem sufficient for use, are, the plumbaginous, 

 the silky, the resinous, the vitreous, the flinty 

 and the waxy. The four first have been already 

 used by mineralogists, and are easily understood 

 from the very definite character of the well 

 known objects from which they are derived. 

 The lustre of common flint is inferior to that of 

 glass, and is a convenient point of comparison 

 for some rocks. The lustre of wax that has 

 been melted, is still inferior, and occurs in many 

 substances. When broken, it offers another con- 

 venient object of comparison. Intermediate, or 

 complicated kinds of lustre, are easily described 

 by the joint use of some of these terms. 



It must be recollected, in estimating the 

 lustre of rocks, that it is frequently complicated, 

 or modified by the minute surfaces of the smaller 

 varieties of fracture. 



TRANSPARENCY. 



In the strict sense of the word, this property 

 is not found in rocks, although it occurs in some 



