106 



MINERALOGY 



FIG. 202. Diagram of the Equatorial Plane, showing the Relation of the Trigonal 

 Prisms and Pyramids to the Lateral Axes. 



2 a : 2 a : a : me 



II. Trigonal pyramids of the second order, 

 (hh2hl), (2hhhl). 



When the poles in Fig. 201 are moved so as to lie on 



dotted line representing 



the 

 the 



FIG. 203. The Minus Left Trigonal Pyra- 

 mid of the Third Order. 



lateral axes, a new form, the 

 trigonal pyramid of the second 

 order, is the result, Fig. 204 ; a 

 form which will not differ from 

 the trigonal pyramids of the 

 third or first orders in appear- 

 ance, but differs in its relation 

 to the lateral axes, which ter- 

 minate in the central point of 

 each equatorial edge and in 

 the opposite angle, as is shown 

 in Fig. 202. 



III. Trigonal prisms of the 

 third order, 



r/l 



na: 



a:a: oo;c 



n i 



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



When the poles in Fig. 201 lie on the primitive circle, between the 

 points a, the terminations of the lateral axes and the points p, the 



