LEAD AND ZINC ORES.


but between them the following physical differences exist: blende
is much harder, and not more than half as heavy, and its luster
inclines to resinous or adamantine, while that of galenite is always
metallic.
It appears from the writings of Pliny, who speaks of it as gaencta,
that this ore has been known from ancient times under substantially
the same name, the only change being one recently introduced in
the termination ite, for the sake of uniformity in mineralogical
names.
The impurities which are known to occur in galenite are antimony,
iron, copper and zinc. Of these, the only ones occurring in the
galenite of the Lead Region are iron and zinc, the former of which
is found as iron pyrites, and the latter as blende or sphalerite, and
smithsonite. As these impurities never exist in chemical combina-
tion, but alwavs as a mechanical mixture, their removal by mechanical
methods is easy, and they do not interfere with the chemical proc-
esses of reduction.
The workable deposits of lead ore in the Lead Region occur in
the Trenton division of the Lower Silurian formation, and mainly
within a vertical range of 125 feet above the base of the formation.
Of this metalliferous stratum, the lower fifty feet include the Tren-
ton limestone proper, which is locally subdivided into the Buff and
Blue limestones, and above the Blue limestone there remains about
75 feet of Galena limestone, especially productive of lead ore, from
which mineral this part of the formation derives its name.
The Galena limestone is characterized by fissures and crevices of
various kinds, of which the greater number have either an east and
west, or a north and south direction. Both kinds exist in every
mining district, but the east and west crevices are by far the largest,
longest and most productive of ore. The length of these crevices
varies from a few yards to a mile or more, and their width from one
inch to several feet. They are usually nearly vertical, but are some-
times slightly inclined in passing from one bed to another.
An examination of the fissures indicates that they were at one
time courses of an extensive underground drainage, which is, to
some extent, going on at the present time. In those portions of the
crevices where softer rocks existed, erosion went on more rapidly,
and a local enlargement of the crevice, which in mining language is
termed an " opening," was the result. Similarly the flat openings,
whose width is much greater than their height, appear to have been
formed by the partial removal of a softer stratum of rock, contained
between two harder ores.


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