es; of Burma; and of many other places. They are all
related to the earlier stages of the metamorphic cycle and occur in
close genetic association with igneous activity. They include deposits
in the body of igneous rocks,--in the form of well-defined veins,
replacements along zones of fissuring and shearing, and disseminated
masses,--as well as veins and replacements in the rocks, particularly in
limestones, adjoining igneous intrusions. The deposits present a wide
variety of shapes depending on the courses of the solutions by which
they were formed. The materials of the ore minerals are believed to have
been derived from the igneous rocks and to have been deposited by hot
solutions. The source of the solutions--whether magmatic or
meteoric--presents the same problems which have been discussed elsewhere
(pp. 41-42). The ores are frequently mined to great depths. Because of
their complexity they require involved processes of treatment to
separate out the values.

Ores of this nature have already been referred to in the discussion of
the copper ores of Bingham and Butte, and will be referred to in
connection with the zinc-lead-silver ores of Leadville, Colorado.
Special reference may be made here to the Coeur d'Alene district of
Idaho, which is the second largest producer of lead in the United
States.

The Coeur d'Alene deposits are almost unique in that they contain
galena as vein-fillings and replacements in quartzite, with a gangue of
siderite (iron carbonate). Quartzite (instead of limestone) is an
unusual locus of replacement ores, and siderite is an unusual gangue.
These ores are believed to owe their origin to acid igneous intrusives,
because of the close association of the ores with some of these
intrusives, and because of the content of high-temperature minerals.
Some of the ore bodies are found far from intrusives, but it is supposed
that in such cases further underground development may disclose the
intrusives below the surface. Secondary concentration has been
insignificant.

In general, weathering of lead ores at the surface and secondary
sulphide enrichment below are not so extensive as in the case of copper
and zinc. Galena is fairly stable in the oxide zone, and even in moist
climates it is found in the outcrop of many veins. Weathering removes
the more soluble materials and concentrates the lead sulphide with the
residual clay and other gangue. In some districts cerussite and a little
anglesite are al