HEXAGONAL SECTION OF HEXAGONAL SYSTEM. AD 
of cleavage) is 90°; and in oligoclase and the other triclinic 
feldspars it is 3° to 5° from 90°, being in oligoclase 93° 50’, and 
in anorthite 94° 10’. This difference in angle involves oblique 
intersections between the axes 6 and c, and c¢ and a, which are 
yectangular in monoclinic forms. There is a similarly close re- 
lation between the triclinic form of rhodonite and that of pyrox- 
ene, and a resemblance also in composition. 
The diametral prism in this system is similar to fig. 7 on 
page 41, under the monoclinic system, but differs in having the 
planes all rhomboidal instead of part rectangular. The form 
corresponding to the oblique rhombic prism of the monoclinic 
system (fig. 8, p. 41) also has rhonboidal instead of rhombic 
planes; moreover, the two prismatic planes have unequal in- 
clinations to the vertical diametral section, and are therefore 
dissimilar planes. The prism, consequently, is made of two 
hemiprisms, and the basal plane is another, making in all three 
hemiprisms. 
2, Cleavage.—Cleavage takes place independently in differ- 
ent diametral or diagonal directions. In the triclinic feldspars 
it conforms to the directions in orthoclase, with only the excep- 
tion arising from the obliquity above explained. 
VI. HEXAGONAL SYSTEM. 
This system is distinguished from the others by the charac- 
ter of its symmetry—the number of planes of a kind around 
the vertical axis being a multiple of 3: The number of lateral 
axes 1s hence 3. It is related to the dimetric system in having 
the lateral axes at right angles to the vertical and equal, and is 
hence like it also in the optical characters of its crystals. Its 
hexagonal prismatic form approaches trimetric crystals in the 
obtuse angle (120°) of the prism, some trimetric crystals having 
an angle of nearly 120°. 
Under this system there are two sections : 
1. The HexaGgonaL sEcTION, in which the number of planes 
of a kind around each vertical axis above or below the basal 
nection is 6 or 12. 
2. The RHOMBOHEDRAL SECTION, In which the number of 
planes of a kind around each half of the vertical axis, above or 
below the basal section, is 3 or 6; and, in addition, the planes 
above are alternate in position with those below. ‘The forms 
are mathematically hemihedral to the hexagonal, but not so 
in their real nature. 
