n
foreign influence was practically destroyed.

In the United States about a third of the production of lead comes from
southeastern Missouri and about a fourth from the Coeur d'Alene
district of Idaho. The five states, Missouri, Idaho, Utah, Colorado and
Oklahoma, produce about nine-tenths of the country's total output.
Reserves of lead ore are not large in proportion to demand, contrasting
in this regard with zinc ore.

GEOLOGIC FEATURES

The principal lead mineral is the sulphide, galena, from which the great
bulk of the world's lead is derived. Cerussite (lead carbonate) and
anglesite (lead sulphate) are mined in some places in the upper part of
sulphide deposits, and supply a small fraction of the world's output.

The ores of lead are of two general classes:

The first class, the so-called "soft" lead ores, nearly free from copper
and precious metals, and commonly associated with zinc ores, are found
in sedimentary beds independent of igneous intrusion. They are of
world-wide distribution, were the first to be extensively exploited,
were at one time the dominant factor in world production of lead, and at
present produce about 30 per cent of the world's total. They are
represented by the deposits of the Mississippi Valley, of Silesia, and
some of the Spanish deposits. The general description of the origin of
the zinc ores of the Mississippi Valley on pp. 216-218 applies to this
class of lead ores. It should be noted, however, that in the principal
United States lead-producing district, that of southeastern Missouri,
the lead ores occur almost to the exclusion of the zinc ores, and are
more disseminated through the limestone than is characteristic of the
zinc ores. Ores of this type have been found extending only to shallow
depths (not over a few hundred feet), and because of the absence of
precious metals their treatment is comparatively simple.

The second class consists of ores more complex in nature, which are
found in association with igneous rocks, and which usually contain some
or all of the metals, zinc, silver, gold, copper, iron, manganese,
antimony, bismuth, and rare metals, with various gangue minerals among
which quartz, siderite, and silicates are important. Today these ores
are the source of about 70 per cent of the world's lead. They are
represented by the lead deposits of the Rocky Mountain region (Coeur
d'Alene, Idaho; Leadville, Colorado; Bingham, Utah; etc.); of Broken
Hill, New South Wal