of the total. The production is
mainly from the Seward Peninsula of northwestern Alaska. For American
tin smelters, Bolivia is about the only available source of supplies;
metallic tin can be obtained from British possessions, but no ore,
except by paying a 33-1/3 per cent export tax. The United States exports
tin-plate in large amounts, and in this trade has met strong competition
from English and German tin-plate makers.

A world shortage of tin during the war required a division of available
supplies through a central international committee. Somewhat later, with
the removal of certain restrictions on the distribution of tin,
considerable quantities which had accumulated in the Orient found their
way into Europe and precipitated a sensational slump in the tin market.

GEOLOGIC FEATURES

The principal mineral of tin is cassiterite (tin oxide). Stannite, a
sulphide of copper, iron, and tin, is found in some of the Bolivian
deposits but is rare elsewhere.

About two-thirds of the world's tin is obtained from placers and
one-third from vein or "lode" deposits. Over 90 per cent of the tin of
southeastern Asia and Oceania is obtained from placers. Tin placers,
like placers of gold, platinum, and tungsten, represent concentrations
in stream beds and ocean beaches of heavy, insoluble minerals--in this
case chiefly cassiterite--which were present in the parent rocks in
much smaller quantities, but which have been sorted out by the
classifying action of running water.

The original home of cassiterite is in veins closely related to granitic
rocks. It is occasionally found in pegmatites, as in certain small
deposits of the Southern Appalachians and the Black Hills of South
Dakota, or is present in a typical contact-metamorphic silicated zone in
limestone, as in some of the deposits of the Seward Peninsula of Alaska.
In general, however, it is found in well-defined fissure veins in the
outer parts of granitic intrusions and extending out into the
surrounding rocks. With the cassiterite are often found minerals of
tungsten, molybdenum, and bismuth, as well as sulphides of iron, copper,
lead, and zinc, and in some cases there is evidence of a rough zonal
arrangement. The deposits of Cornwall and of Saxony show transitions
from cassiterite veins close to the intrusions into lead-silver veins at
a greater distance. The gangue is usually quartz, containing smaller
amounts of a number of less common minerals--including lithium