46 



CRYSTALLOGRAPHY. 



L HEXAGONAL SECTION, 



1. Description of Forms. — Figs. 1 to 3 represent some of the 

 forms under this section. Figs. 2 and 3 show only one ex- 



MIMETITE. 



BERYL. 



APATITE. 



tremity; and such crystals are seldom perfect at both. All 

 exhibit well the symmetry by sixes which characterizes this 

 section of the hexagonal system. 





4. 









5. 



: c 





iFPl 



k 



-~- 



A 





1 v 



; 



J-^fl 



\ 



!i 



! 12 



In * 



\12 12 



k "'" 



— 



'^-.. 





h- 



--> 



4Z^ 



^rby 



I*risms. Under this system there are two hexagonal prisma 

 and a number of occurring twelve-sided prisms. Fig. 4 repre- 

 sents one of the hexagonal prisms, with its axes — the three 

 lateral connecting the centres of the opposite edges. The 

 lateral angles of the prism are 120°. If the lateral edges of 

 this prism are truncated, as in the figure of apatite (fig. 3), the 

 truncating planes, i-2, are the lateral faces of another similar 

 hexagonal prism, in which, as the relations of the two show, the 



