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FORM OF MINERALS.
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renders many triclinohedric crystals very unsymmetrical in appearance.
Triclinohedric
pyramids (fig. 63) are bounded by eight triangles, whose lateral edges
lie in one plane. They are equal and parallel two and two to each other
; each pair forming, as just stated, a tetartopyramid or open form,
only limited by combination with other forms, or, as we may suppose,
by the chief sections. The prisms are again either vertical or
inclined ; the latter named domes, and their section is always
rhomboidal. In deriving the forms, the fundamental pyramid is placed
upright with its brachy-diagonal axis to the spectator, and the
.partial forms designated, the two upper by 'P and P', the two lower
by ,P and P„ as in the figure. The further derivation now follows as in
the rhombic system, with the modifications already mentioned, so that
we need not delay on it longer, especially as the minerals
crystallizing in these forms ai-e not numerous.
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Some
combinations of this system, as the series exhibited by most of the
felspars, approach very near to the mono-clinohedric system; while
others, as the blue copper, oi
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