s 4.0, and the hardness 4. Crystals
exhibit pyroelectrical characters, since they possess four uniterminal
triad axes of symmetry.

[Illustration: FIG. 2.]

[Illustration: FIG. 3.]

Crystals of blende are of very common occurrence, but owing to twinning
and distortion and curvature of the faces, they are often rather complex
and difficult to decipher. For this reason the mineral is not always
readily recognized by inspection, though the perfect dodecahedral
cleavage, the adamantine lustre, and the brown streak are characters
which may be relied upon. The mineral is also frequently found massive,
with a coarse or fine granular structure and a crystalline fracture;
sometimes it occurs as a soft, white, amorphous deposit resembling
artificially precipitated zinc sulphide. A compact variety of a pale
liver-brown colour and forming concentric layers with a reniform surface
is known in Germany as _Schalenblende_ or _Leberblende_.

A few varieties of blende are distinguished by special names, these
varieties depending on differences in colour and chemical composition. A
pure white blende from Franklin in New Jersey is known as cleiophane;
snow-white crystals are also found at Nordmark in Vermland, Sweden.
Black blende containing ferrous sulphide, in amounts up to 15 or 20%
isomorphously replacing zinc sulphide, is known as marmatite (from
Marmato near Guayabal in Colombia, South America) and christophite (from
St Christophe mine at Breitenbrunn near Eibenstock in Saxony).
Transparent blende of a red or reddish-brown colour, such as that found
near Holywell in Flintshire, is known as "ruby-blende" or "ruby-zinc."
Pribramite is the name given to a cadmiferous blende from Pribram in
Bohemia. Other varieties contain small amounts of mercury, tin,
manganese or thallium. The elements gallium and indium were discovered
in blende.

Blende occurs in metalliferous veins, often in association with galena,
also with chalcopyrite, barytes, fluorspar, &c. In ore-deposits
containing both lead and zinc, such as those filling cavities in the
limestones of the north of England and of Missouri, the galena is
usually found in the upper part of the deposit, the blende not being
reached until the deeper parts are worked. Blende is also found
sporadically in sedimentary rocks; for example, in nodules of
clay-ironstone in the Coal Measures, in the cement-doggers of the Lias,
and in the casts of fossil shells. It has occasionally been found on t