pyrites
are frequently striated, and in three directions at right angles to
each other. In calc-spar the faces of the rhombohedron,—1/2R (g in
fig. 43 above) are almost never without striœ parallel to the oblique
diagonal. The stria-tion is said to be simple when only one series of
parallel lines appears on each face, or feathered when two systems
diverge from a common line. In other crystals the faces, then said to
be drusy, are covered by numerous projecting angles of smaller
crystals; an imperfection often seen in fluor spar. The faces of
crystals occasionally appear curved either, as in tourmaline and beryl,
from the peculiar oscillatory combination mentioned, or by the union
of several crystals at obtuse angles, like stones in a vault, as in
stilbite and prehnite. A true curvature of the faces probably occurs
in the saddle-shaped rhombohedrons of brown spar and siderite, in the
lens-like crystals of gypsum, and in the curved faces so common on
diamond crystals. In chabasite similar curved faces occur, but concave.
In galena and augite the crystals are often rounded on the eorners, as
if by an incipient state of fusion. On other crystals the faces are
rendered uneven from inequalities following no certain rule. These
imperfections furnish valuable assistance in developing very complex
combinations, all the faces of each individual form being
distinguished by the same peculiarity of surface.
Irregularities
in the forms of crystals are produced when the corresponding faces are
placed at unequal distances from the centre, and consequently differ in
form and size. Thus the cubes and octahedrons of iron pyrites, galena,
and fluor spar, are often lengthened along one axis. Quartz is subject
to many such irregularities, which are seen in a very remarkable manner
on the beautiful transparent and sharply angular crystals from
Dauphiné. In such irregular forms, instead of one line, the axes are
then represented by an
