re in
veins and replacements in sedimentary rocks at shallow depths,
independent of igneous association, and are supposed to have been formed
by cold solutions. They are found in the Mississippi Valley, in Silesia,
and in many of the smaller European deposits. They were formerly the
leading zinc-producers, and now produce about 45 per cent of the world's
total. Zinc ores of the second type consist of veins and replacements
related to intrusive rocks, sometimes extending to considerable depths,
and of more complex composition. They include most of the deposits of
the American Cordilleran region (Butte, Coeur d'Alene, Leadville,
etc.), of Franklin Furnace, of Australia, of Burma, and of many other
places.

The zinc-lead ores of the type found in the Mississippi Valley are of
special interest, in that they are sulphide ores of an origin apparently
independent of igneous agencies. These ores occur as fissure-fillings
and replacements, mainly in nearly flat-lying Paleozoic limestones and
dolomites--the Bonne Terre dolomitic limestone of southeastern Missouri,
the Boone formation of southwestern Missouri and Oklahoma, the Galena
dolomite of Wisconsin and Illinois. They are variously associated with a
gangue of dolomite, calcite, quartz, iron pyrite, barite, and chert. Not
infrequently they are spread out both in sheets and in disseminated form
along carbonaceous layers within or at the base of the limestone.

The source of the primary sulphides has been a subject of much
discussion. All are agreed that they were first deposited with the
sediments in minutely dispersed form, through the agency of the organic
contents of the sediments, and that such deposition was somewhat
generally localized by estuarine conditions which favored the
accumulation of organic remains. Many years ago, before the evidence of
estuarine deposition was recognized, Chamberlin suggested an ingenious
hypothesis for the northern Mississippi Valley,--that the organic
material had been localized by ocean's currents forming something in the
nature of a Sargasso sea. Differences of opinion become acute, however,
when the attempt is made to name the precise sedimentary horizon, out of
several available horizons, in which for the most part this primary
concentration occurred. Judging from the organic contents of the several
beds, the primary source may have been below, within, or above the
present ore-bearing horizons. If the ore came from the lower horizons,