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38
A PRACTICAL TREATISE ON GEMS.
and with the signsv'hich is the case when
the lateral axes are supposed infinite, then it becomes a pinacoid, consisting properly of two basal faces, open to­wards the lateral axes, and designated by the sign OP. The ditetragonal pyramids are produced by taking in each lateral axis distances n greater than 1, and drawing two planes to these points from each of the intermediate polar edges. The parameters of these planes are thereforeand
the general sign of the formthe most common values
of n beingWhena tetragonal
pyramid of the second kind arises, designated generally by
, the most common in the mineral kingdom being andThe relation of these to pyramids of the first
kind is shown in fig. 31, where ABBBX is the first, and ACCCX the second kind of pyramid. In like manner from the prismthe ditetragonal prismsare derived,
and finally whenthe tetragonal prism of the sec-
ond kind, whose sign is
The combinations of the tetragonal system are either holohedric or hemihedric ; but the latter are rare. Prisms and pinacoids must always be terminated on the open sides by other forms. Thus in fig. 32 a square prism of the first kind is terminated by the primary pyramid, and has its