+e 
ow 
Pea eee ee 
_ plane O. 

HEXAGONAL SECTION OF HEXAGONAL SYSTEM. 4 
lateral axes connect the centres of the opposite lateral faces. 
This prism is represented in fig. 5. 
The lateral edges of the hexagonal prisms occur sometimes 
with two similar planes on each edge, and these planes, when 
extended to the obliteration of the hexagonal prism, make 
a twelve-sided prism. These two 
planes are seen in fig. 8, along 
with the planes Z of the hexago- 
nal prism, and 1 of a double six- 
sided pyramid, besides the basal 

Double pyramids. The double 
pyramids are of three kinds: (1) 
A-series of six-sided, whose planes belong to the same verti- 
cal zone with the planes Z| The planes of two such pyramids 
(lettered 1, 2) are shown in figs. | and 2, three of them in fig. 
3 (lettered 4, 1, 2), and one in fig. 7, and one such double 
pyramid, without combination with other planes, in fig. 6. 
(2) A series of six-sided double pyramids, whose planes are in 
the same vertical zone with 2-2, examples of which occur on fig. 
2 (plane 2-2)—on fig. 3 (planes 1-2, 2-2, 4-2). The form of this 
double pyramid is like that represented in fig. 6, but the lateral 
axes connect the centres of the basal edges. The double six- 
sided pyramid is sometimes called a quartzoid, because it occurs 
in quartz. (3) Twelve-sided double pyramids. Two planes of 
such a pyramid are shown ona hexagonal prism in fig. 9, also in 

fig. 2 (the planes 3-3), and the simple form consisting of such 
planes in fig. 10—a form called a berylloid, as the planes are 
common in beryl. In fig. 11 the planes 1 belong to a double 
six-sided. pyramid; and those next below (of which three are 
lettered W) to a double twelve-sided pyramid. 
