70 A POPULAR TREATISE ON GEMS.
observed
in triclinohedric felspars, particularly the albite and labradorite, is
that the twin axis corresponds with that normal of the brachydiagonal
which is situated in the plane of the base. In pericline, a variety of
albite, these twins appear as in fig. 93, where the two crystals are
united by a face of the basal pinacoid P, while the faces of the two brachypinacoids (M and M') form
edges with very obtuse angles (173° 22'), reentering on the one side
and salient on the other. These edges, or the line of junction between M and M\ are also parallel to the edges formed by these faces and the base, or those between M and P. In this case also the macles are occasionally several times repeated when the faces appear covered with fine striae.
Irregular Aggregation of Crystals.
Besides
the regular unions now described, crystals are often aggregated in
peculiar ways, to which no fixed law can be assigned. Thus some
crystals, apparently simple, are composed of concentric crusts or
shells, which may be removed one after the other, always leaving a
smaller crystal like a kernel, with smooth distinct faces. Some
specimens of quartz from Beeralston in Devonshire consist apparently
of hollow hexagonal pyramids placed one within another. Other minerals,
as fluor spar, apatite, heavy spar, and calc-spar, exhibited a similar
structure by bands of different colors.
Many
large crystals, again, appear like an aggregate of numerous small
crystals, partly of the same, partly of different forms. Thus some
octahedrons of fluor spar from Schlaggenwald are made up of small dark
violet-blue cubes,
