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FORI! OF MIXERALS. 45
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plex
combination of five forms is represented in the crystal of calc-spar,
fig. 46, its sign with, the letters on the faces being
combinations
are seen most distinctly in pure quartz or rock-crystal, the pyramids
of the first kind appearing as rhom-bohedrons, those of the second kind
as trigonal pyramids, the dihexahedral prisms as ditrigonal prisms, and
the prism oo P2 as a trigonal prism. Most of these forms, however,
occupy but a very subordinate place in the combinations which consist
essentially of the prism
bohedron
IV. Jthombie System.—The
rhombic system is characterized by three axes, all unequal, but at
right angles to each other. One of these is assumed as the chief axis,
when the others are named subordinate. The plane passing through the
secondary axes, or the basis, forms a rhomb, and from this the name is
derived. This system comprises only a few varieties of forms that are
essentially distinct, and its relations are consequently very simple.
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The closed forms are,—(1st.) The rhombic pyramids (figs. 47, 48), bounded by eight scalene triangles, whose
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