108 MINERALOGY 



Plus and minus trigonal pyramid of the first order, (hohl), (ohhl). 



Right and left plus and minus trigonal prisms of the third order, 

 (hkio), (khio), (ihko), (ikho). 



Plus and minus trigonal prisms of the second order, (hhaho), 

 (2hhho). 



Plus and minus trigonal prisms of the first order, (hoho), (ohho). 



Base, (0001). 



Examples. As yet there are no representatives of this type. 



TYPE 10, TRIGONAL HOLOAXIAL 



Symmetry. Crystals of this type are symmetrical in regard to 

 one trigonal axis of symmetry, the c axis, and three digonal axes, 

 the a axes. Fig. 207 illustrates the symmetry of the type. 



It may also be considered as a tetartohedral class, and the forms 

 are derived by superimposing the rhombohedral hemihedrons, 

 type 19, upon the trapezohedral hemihedrons, type 16, and extend- 

 ing the faces not thus marked, as below : 



2 6 10 



J _ 2 . j^ J _ 6 . 



15 19 23 



In the rhombohedral method of selection, alternating dodecants 

 above and below the equator are crossed out and suppressed ; in the 



trapezohedral method every other 



.*' '">. face above and below the equator is 



x / crossed out, as suppressed. There 



will remain of the 24 faces of the 



/ o \ / \ dihexagonal pyramid, represented 



| ................. #- ............... \ above, 6 faces, 2, 6, and 10 above, 



x / \ / and 15, 19, and 23 below the equator. 



'' x / When this method of selection is 



O \ / compared with the rhombohedral 



"V, f ,^'' tetartohedral method, page 97, it 



will be seen that there the faces be- 



FIG. 207. Type 10, Trigonal 



Hoioaxiai. low lie symmetrically between those 



above, but here they are not sym- 



metrically located, as 15 is nearer 2 than to 6, which in the new 

 form will produce a long inclined edge between 6 and 15 and a 

 short edge between 2 and 15. These corresponding edges are equal 



