94 



MINERALOGY 



metry, the equatorial plane, and a center, Fig. 173. The forms are 

 parallel-faced hemihedrons, and may be considered as derived from 

 the holohedrons of type 20 by extending alternate pairs of faces 

 which intersect in the equatorial edge, as the shaded faces in Fig. 

 174, which will produce a minus hemihedron. 



Forms 

 I. Hexagonal pyramid of the third order, 



na: 



a: a: me 



n i 



ir(hkil), ir(khil). 



The faces represented by the poles of Fig. 173 bound a pyramid, 

 which in shape does not differ from the hexagonal pyramid of the 

 first or second order except here the lateral axes do not terminate 

 either in the center of the faces or bisect the edges, but on the line 



\ 



FIG. 175. The Hexagonal Pyra- 

 mid of the Third Order. 



FIG. 176. 



connecting these two points, Fig. 175, also Fig. 176, which is a plan 

 of the first, second, and third order pyramids and prisms drawn on 

 the equatorial plane. 



II. Hexagonal prism of the third order, 



na: a:a: ooc 



n i 



; ir(hkio), ir(khio). 



If the poles in Fig. 173 are moved so as to lie upon the equator, 

 between the a axes and the intermediate axes, then two faces of the 



