38 TERMINOLOGY. . 170. 



itron, the striae produced upon the faces of the latter, are 

 parallel to perpendicular lines, which may be drawn from 

 the single plane angle of the faces towards the opposite 

 edge ; upon the faces of the icositetrahedron, the striae are 

 parallel to the'longer one of those edges, which join two 

 different solid angles formed by four faces. It would be 

 superfluous to mention here any more examples ; for every 

 case referring to this subject may be explained with as 

 great facility as the preceding combinations of rhombohe- 

 dral Quartz, and of hexahedral Iron-pyrites. 



The curvature of the faces (. 1,59.) sometimes is pro- 

 duced by streaking. Thus, in rhombohedral Tourmaline, 

 the three-sided prisms, with convex faces, are produced by 



numerous striae between the faces of . * ?(E)andP-f so (-?). 



A r ol. II. Fig. 138. 



The streaking of the faces may be very useful in rinding 

 out those which are homologous, since homologous faces al- 

 ways shew similar occurrences of this phenomenon ; a fact 

 proved by numerous examples in rhombohedral Lime-ha- 

 loide, in dodecahedral Garnet, &c. 



. 170. DIVERSE QUALITIES OF THE FACES OF 

 CRYSTALLISATION. 



The property of the surface of crystalline forms, 

 designated by the terms rough and drusy> arises 

 from elevations projecting from the faces of crys- 

 tals. They differ from each other only in the size 

 of the elevated particles. 



In the species of octahedral Fluor-haloide, octahedral 

 crystals, sometimes of considerable size, seem entirely to 

 consist of minute hexahedrons. The faces of such octahe- 

 drons cannot be planes ; but they consist of the faces of 

 the hexahedrons, which are perpendicular upon each other ; 

 in such situations, that a plane passing through their solid 



