80 TERMINOLOGY. 



2. Even is applied to such faces as are nearly flat. It is doubtful, 

 however, whether this fracture can be said to occur except among 

 compound minerals. 



3. Uneven, when the surface is irregular, without presenting any 

 shell-like concavities or elevations. 



4. Hackly, which results rather from the tearing than from the 

 breaking of a mineral, is applied to surfaces covered with little hook 

 shaped filaments, very perceptible to the touch. 



. 70. SURFACE. 



The kinds of surface presented by minerals may be 

 considered under four heads; viz. 1, faces of crystalliza- 

 tion, 2. faces of cleavage, 3. faces of fracture, 4. faces 

 of composition. 



Of all these kinds of faces, the most important are those which 

 are even, since, in the mineral kingdom, the uneven faces are not 

 subject to any constant law. The even faces are confined to the 

 faces of crystallization and the faces of cleavage. 



1. Faces of crystallization. The differentjqualities of the faces 

 of crystallization depend upon their being smooth, without any reg- 

 ular elevations or depressions; upon their being striated, rough, or 

 drusy. 



Smooth faces may be said to be the most abundant among crys- 

 tals. We include, however, under this term such as sometimes 

 have slight elevations and depressions, provided they are so faint 

 that the general appearance of eveness and continuity of the faces 

 is not effected by their occurrence. 



Striated faces are those upon which we observe parallel grooves 

 or striae. These are frequent among minerals ; and their observa- 

 tion is of great importance, since they are confined to particular 

 planes and assume a constant direction. For example, in Quartz;, 

 the alternate lateral faces are striated horizontally; in cubic crystals 

 of Iron Pyrites, all the faces are striated with this remarkable pe- 

 culiarity, that the striae are parallel to each other upon parallel fa- 

 " ces, and perpendicular to each other upon such faces as are not 

 parallel ; in Titanite and Beryl, the prismatic faces are striated lon- 

 gitudinally, &c. 



