104 



MINERALOGY 



Fig. 198 represents that portion of the di trigonal pyramid above 

 the equatorial plane. In this hemipyramid the lateral axes 



-Type 12, Ditrigonal 

 Polar. 



FIG. 198. The Upper Plus 

 Ditrigonal Hemipyramid 

 (hkil). 



hold the same relation to the edges as in the ditrigonal equatorial 

 types. 



II. Trigonal hemipyramids of the first order, 



u/l 



a : oo a : a : me 



(hohl), (ohhl), (hohl), (ohhl). 



The trigonal hemipyramid appears here as a new form ; Fig. 199 

 represents the upper minus trigonal hemipyramid of the first 

 order. 



Other forms in 

 the type are simi- 

 lar to the forms 

 of the ditrigonal 

 equatorial, except 

 the hexagonal 



FIG. 199. Upper Negative pyramid of the 

 Trigonal Hemipyramid of second Order and 

 the First Order, (ohEl). 



the base are upper 



and lower forms which have appeared in 

 the dihexagonal polar type. 



Combinations. The possible forms to 

 combine in this type are : 



Upper and lower, plus_and minus ditrig- 

 onal hemipyramid, (hkil), (khil), (hkil), 

 (khU). 



Upper and lower, plus and minus 



FIG. 200. Tourmaline, a 

 Combination of the Upper 

 and Lower Trigonal Pyra- 

 mids r ; Hexagonal Prism, 

 Second Order a; the Lower 

 Base c, and the Trigonal 

 Prism, First Order m. 



