. 176. OF SURFACE. 239 



angles is parallel to the face of the octahedron. The more 

 the size of these hexahedrons diminishes, and consequently 

 the more their number increases, the more the faces them- 

 * selves will assume the appearance of exact planes. They 

 are said to he drusy, if the asperities upon the faces are still 

 easily distinguishable ; they are termed rough, if they may 

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

 asperities merely can be inferred from the want of lustre of 

 the resultant faces. 



The faces of the hexahedron exhibit in the same species 

 a phenomenon connected with the former, which in many 

 respects is very remarkable. They are someti mes covered as 

 it were with small very flat four-sided pyramids, whose late- 

 ral edges are parallel to the edges of the hexahedron ; and 

 of which only the upper part is visible. The faces of these 

 pyramids are the faces of the hexahedral trigonal-icosite- 

 trahedron, a form not uncommon in this species. If they 

 become very minute, they may render the faces drusy or 

 rough, or they may produce striae parallel to the edges of 

 the hexahedron. 



Another very remarkable fact, which in particular is 

 very often met with in rhombohedral Quartz, must be 

 classed along with the preceding peculiarities. Many crys- 

 tals of the same form, P. P + co, appear as if grouped pa- 

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

 form. Exact parallelism is here understood, as it is in 

 every occurrence of this kind. Sometimes, however, the 

 aggregation of crystals of rhombohedral Quartz depends 

 upon regular composition (. 179.). The preceding obser- 

 vations, in respect to striated surfaces^ as to the quality of 

 homologous faces, applies likewise to such as are rough or 

 drusy. 



Those particles which project from the faces of the 

 crystals, must not be considered as single individuals ; and 

 crystals with drusy faces therefore are not compound mi- 

 nerals. They indicate rather the gradual progress of the 

 formation of crystals, from the interruption in which they 

 arise. If we suppose in the octahedrons of octahedral Fluor- 



