GUIDE TO THE MINERALOGIC COLLECTIONS 



23 



Fig. 81 



Pyramidal group 



The general symmetry of this group is shown in fig. 81; as in 

 the normal group the vertical axis is an axis of tetragonal sym- 

 metry; a single plane of symmetry passes 

 through the horizontal axes, which are not 

 axes of binary symmetry as is the case in 

 the normal group. 



The forms which have been described 

 under the normal group occur also in the 

 pyramidal group with the exception of the ditetragonal prism 

 and pyramid. 



The relation between the symmetry of this 

 group and that of the preceding one may be best 

 studied by referring to fig. 82 which shows the ter- 

 mination of a pyramidal crystal. The absence of 

 vertical planes of symmetry, characteristic of this 

 group should be noted. 

 Two new forms occur, namely: prism of the third order, repre- 

 sented in fig. 84; pyramid of the third order, represented in 

 fig. 83 by the faces marked x. 



The relations of the pyramid and prism of the third order 



Fig. 82 



Fig. 83 Fig. 84 



to the corresponding forms of the first and second order are 

 shown in fig. 84. Fig. 83 represents some combinations in this 

 group. 



Sphenoidal group 



The general symmetry of this group, which is shown in fig. 

 85, is somewhat analogous to that of the tetrahedral group of 

 the isometric system. The crystallographic axes are axes of 



