. 154. OF COMBINATIONS. 201 



inclination upon the axis, shew the hemi-prismatic character 

 of the combination. 



Supposing again the inclination of the axis to be = 0, 

 the triple combination P as. Pr + co. Pr -f cs will be a 

 right rectangular four-sided prism, its base an oblong rect- 

 angular face, and the combination itself a prismatic one, as it 

 appears from what has been stated above, . 152. The same 

 triple combination P co. Pr + oo. Pr -^ cs , upon the sup- 

 position of the axis being inclined in the plane of the 

 longer, or the shorter diagonal, or the triple combinations 



?Lf. Pr + c=. Pr + co or L_ n . Pr + co. Pr + co, 

 2 2 



appear as oblique rectangular four-sided prisms, two faces 

 of which are perpendicular to the rectangular base, while 

 the two others produce with it horizontal edges of combi- 

 nation, supplemental to each other. 



The axis is not always inclined to the base at the same 

 angle, but varies according to the different species in 

 which it occurs. 



Prismatoidal Gypsum-haloide, prismatic Azure-mala- 

 chite, paratomous, hemi-prismatic and prismatoidal Augite- 

 spar, may be quoted as examples of hemi-prismatic combi- 

 nations. 



. 154. TETAIITO-PEISMATIC COMBINATIONS. 



The Tetar to-prismatic Character of a combina- 

 tion requires, that of the forms which constitute it, 

 the scalene four-sided pyramids shew only one- 

 fourth, and the prisms, both horizontal and ver- 

 tical, only one-half the number of their faces, or 

 that these faces are distinguished from each other 

 by their angles, which refer to an axis inclined in a 

 plane perpendicular upon the base, and passing 

 through neither of its diagonals. 



