SURFACE. 81 



Rough or Drusy. This property of the surface of crystalline 

 forms arises from elevations projecting from the faces of the crys- 

 tals: the only difference in the application of the terms arises out 

 of the size of the elevated particles. Thus, among those octahedral 

 crystals of Fluor which consist apparently of minute cubes, we 

 have frequent instances of these varieties of faces. The faces of 

 such octahedrons cannot be planes; but they consist of the faces of 

 cubes, which are perpendicular upon each other, and so situated, 

 that a plane passing through their solid angles would be parallel to the 

 faces of the octahedron. The smaller these cubes are, the more the 

 general faces of the Octahedron will assume the appearance of exact 

 planes. They are said to be drusy, if the asperities upon the faces 

 are still easily distinguishable; they are termed rough, if they are 

 only perceived with difficulty, or if the existence of such asperities 

 can merely be inferred from the want of lustre. Instances of drusy 

 faces often occur in like manner among the crystals of Quart/. 

 A great number of prismatic crystals appear as if grouped parallel 

 to each other, or round a larger crystal of the same kind. 



In the above instances those particles which project from the 

 faces of the crystals must not be considered as single individuals ; 

 and crystals with drusy faces are therefore, not compound minerals. 

 They indicate rather the gradual progress of the formation of crys- 

 tals from the interruption in which they arise. 



2. Faces of cleavage, present us with very little remarkable dif- 

 ferences in their quality. Those which are smooth are frequently 

 denominated perfect. Notice is sometimes taken of striated appear- 

 ances, or parallel lines traversing cleavage faces, as in those of 

 Feldspar, Corundum, &c. They are not attended, however, as is 

 the case with striae upon faces of crystallization, by elevations and 

 depressions upon the faces, but originate in the flow of light, through 

 natural joints of the crystals, in which this property is observable 

 perpendicularly, or nearly so, to the planes in which it occurs. Its 

 observation may occasionally be employed to advantage, in discov- 

 ering the similar cleavages of a mineral. 



3. Faces of fracture, differ in the greater or less smoothness of their 

 inequalities, and according to this measure in particular, the differ- 

 ent kinds of fracture are said to be more or less perfect. 



4. Faces of composition, which are those in which several individ- 

 uals touch one another, are sometimes even, yet this is not common, 

 and when they are so, there is no danger of confounding them along 

 with faces of cleavage ; because those particles which are contained 



