228 TERMINOLOGY. . 168. 



relative to the quality of the faces of cleavage in different 

 varieties. One of the most striking examples of this oc- 

 curs in hexahedral Iron-pyrites. Several varieties of this 

 species cleave very readily in faces which are pretty even, 

 shining, &c. ; others only with the greatest difficulty in 

 faces very much interrupted by asperities. In both cases, 

 however, the resultant form of cleavage is the hexahe- 

 dron ; and the faces under which it is contained are of the 

 same perfection and quality, at least in each individual ta- 

 ken separately, as it must be on account of their belonging 

 to one and the same simple form. 



-The circumstance that two or more faces of cleavage 

 possess exactly the same properties, cannot be consider- 

 ed as a proof of their belonging to one and the same 

 simple form. An example of this may be taken from pris- 

 matic Gypsum-haloide. The form of cleavage of this 

 species is represented by the combination P OD. Pr + oc. 

 Pr + os : the two latter, although faces of two different 

 forms, shew almost exactly the same quality ; and yet the 

 same two forms in prismatoidal Gypsum-haloide, where 

 they likewise appear as forms of cleavage, differ very much 

 in their aspect. 



On the contrary, in respect to the second part of the 

 proposition, the difference of the simple forms to which they 

 belong, follows in every instance from the different quali- 

 ty of the faces of cleavage, or from their not being homo- 

 logous. In the above mentioned compound form of cleav- 

 age of pyramidal Feld-spar, the face perpendicular to the 

 axis appears very different from those parallel to it, being 

 less even, &c. : in a similar way in rhombohedral Emerald, 

 the face perpendicular to the axis is more perfect and even 

 than those parallel to it. As a very remarkable example, 

 the form of cleavage of prismatoidal Gypsum-haloide de- 

 serves our particular attention. The form considered by 

 itself appears as a right oblique-angular four-sided prism. 

 It is more natural, however, to consider it in reference 

 to its ci'ystalline forms, as the hemi-prismatic oblique-rect- 

 angular four-sided prism (. 153.), similar to Vol. II. 



