102 



MINERALOGY 



a : oo a : a : me 



(hohl), 



II. Trigonal pyramid of the first order, 

 (ohhl). 



When the poles of Fig. 190 are moved into the vertical planes of 

 symmetry, two adjacent faces of the ditrigonal pyramid will fall 

 in one plane, producing a form, the trigonal pyramid of the first 

 order, bounded by six isosceles triangles, Fig. 193 ; here the rela- 



FIG. 192. The Negative Di- 

 trigonal Pyramid. 



FIG. 193. Trigonal Pyra- 

 mid of the First Order, 

 (0111). 



tion of the faces to the hexagonal pyramid of the first order is also 

 shown. The lateral crystallographical axes terminate, two in each 

 equatorial edge, dividing it into three equal parts. 



III. Ditrigonal prism, 



na: 



n i 



a:a: ooc 



; (hkio), (khio). 



If the poles are moved to the equator between the a axes and the 

 intermediate axes, the ditrigonal prism results, Fig. 194, a form 



4; --- 



t" 



FIG. 194. Ditrigonal 

 Prism, (hkll). 



FIG. 195. The Trigonal 

 Prism of the First Order, 

 (ohho). 



