
RIHOMBOILEDRAL SECTION OF TLEXAGONAL SYSTEM. 49 
sided pyramid. Sometimes there are traces of cleavage parallel 
to the faces of a six-sided pyramid. 
5. Irregularities of Crystals.— 
Distortions sometimes disguise 14. 
greatly the real forms of hexagonal 
crystals by enlarging some planes 
at the expense of others. This is 
illustrated in fig. 14, representing 
the actual form presented by a 
crystal having the planes shown 
in fig. 13. Whenever in a prism 
the prismatic angle is exactly 120° 
or 150°, the form is almost always 
of the hexagonal system. 

2. RHOMBOHEDRAL SECTION. 
1. Descriptions of Forms.—The following figures, 1 to 17, 
represent rhombohedral crystals, and all are of one mineral, cal- 
cite. They show that the planes of either end of the crystal 
are in threes, or multiples of threes, and that those above are 
alternate in position with those below. There is one exception 
& e Caer A 9. 12. 
aden 
A A. 
D5 ee 10. 
8. 
LVS 
me 
to this remark, that of the horizontal or basal plane OQ, in figs. 
8, 11, 13. Thesimple rhombohedral forms include: 
1. Lhombohedrons, figs. 1 to 6. These forms are included 
under six equal planes, like the cube, but these planes are 

Hos 

FIGURES OF CRYSTALS OF CALCITE. 
