4 J. D, Dana on the Feldspar group of minerals. 407 
In order to appreciate the following comparisons, the reader 
should have before him a dodecahedral crystal, (of garnet, for 
)  example,) or a model of the form, placed with a trihedral angle 
. _ at top, and with one face of the trihedral pyramid sloping to 
) the left ; in which position it is a 6-sided prism with trihedral 
| summits, with its planes parallel to those of the figure. 
The prismatic angle of the feldspars, as in many other min- 
erals, is very closely the angle of the dodecahedron (120°) ; and 
the four planes J enclosing it correspond therefore to four of 
the vertical faces of this dodecahedron ; while the two planes 7-2 
which make angles with J of 120° 26’, are the other two. 
The basal plane O is another dodecahedric plane ; for O: I 
= 122° 16’; and O : 7-i=90°, the exact dodecahedral angle. This 
makes eight out of the twelve. 
42’, i-) : 1=116° 53’, the mean of which is nearly 120°. The 
divergence from 120° is due to the 8° of obliquity which the 
2-3 are faces of the cube (or hexahedron), and are 
