utte district of Montana; about 12 to 15 per
cent from Keweenaw Point, Michigan; and about 12 per cent from Bingham,
Utah. From 3 to 5 per cent of the country's output comes from each of
the states of New Mexico, Nevada, Alaska, and California. All other
states together produce only a little over 2 per cent of the total.

The so-called "porphyry" coppers in Utah, Arizona, Nevada, and New
Mexico, described below, are the source of about 35 per cent of the
present production of the United States. The deep mines of Butte and
Michigan are responsible for about 30 per cent of the production, and
the ore bodies of Arizona (other than porphyry) and of Alaska produce
about 25 per cent.

Reserves of copper ore are such as to give no immediate concern about
shortage, nor to indicate any large shift in the distribution of
production in the near future. Development is on the whole considerably
in advance of present demands. The principal measured reserves are in
the so-called porphyry coppers of the United States and Chile. In the
United States the life of these reserves now estimated is approximately
25 years. The reserves of the Chile Copper Company are the largest of
any known copper deposit in the world, and the Braden copper reserve
(also in Chile) is among the largest. For the deep mines of the United
States, the developed reserves have a life of perhaps only five years,
but for most of these mines the life will be greatly extended by further
and deeper development. The porphyry coppers, because of their
occurrence near the surface and the ease with which they may be explored
by drilling, disclose their reserves far in advance. The deep mines are
ordinarily developed for only a few years in advance of production.

GEOLOGIC FEATURES

The principal copper minerals may be classified into the sulphide group,
the oxide group, and native copper. Native copper, mined in the Lake
Superior region, is the source of 8 to 10 per cent of the world's copper
supply. The oxide group of minerals--including the copper carbonates,
azurite and malachite; the silicate, chrysocolla; the oxide, cuprite;
the sulphates, chalcanthite and brochantite; and some native copper
associated with these minerals--probably supplies another 5 per cent.
The remaining 85 per cent is derived from the sulphide group. Of the
sulphide group by far the most important mineral is chalcocite (cuprous
sulphide), which supplies the bulk of the values in the majority of