NEW YORK STATE MUSEUM 



Prism of the second order, having four faces, represented in 

 fig. 73 by the. faces marked a. 



Fig. 72 Fig. 73 



Ditetragonal prism, having eight faces. 



The basal plane or base consists of a pair of planes parallel 

 to the horizontal or basal axes. 



The symmetry of this group can be best observed by consid- 

 ering what is called the termination of the crystal, that is, the 



\ 























/ 



\ 



Fig. 74 



Fig. 76 



Fig. 77 Fig. 78 Fig. 79 Fig. 80 



way in which the planes are grouped about the extremity of the 

 vertical axis; two such terminations are shown in plan in fig. 

 74 and 75. Some of the combinations in the normal group are 

 shown in fig. 72-80. 



