derable change.

Of the mine production of silver in the United States, about two-thirds
of the total comes from the states of Montana, Utah, Idaho, and Nevada.
Other considerable producers are Colorado, Arizona, California, Alaska,
and New Mexico. All the other states together produce less than 5 per
cent of the total. The most important single districts are the Butte
district of Montana, the Coeur d'Alene district of Idaho, and the
Tonopah district of Nevada, supplying respectively about one-fifth,
one-eighth, and one-tenth of the country's total silver output.

GEOLOGIC FEATURES

The most important mineral of silver is the sulphide, argentite or
"silver glance." Other minerals which yield a minor percentage of the
total silver produced are the silver-antimony sulphides, pyrargyrite or
"ruby silver," stephanite or "black silver," and polybasite; the
silver-arsenic sulphides, proustite or "light ruby silver" and pearcite;
and the silver antimonide, dyscrasite. In the oxide zone the most
abundant minerals are cerargyrite (silver chloride) and native or "horn"
silver. In addition to these definite mineral forms, silver is present
in many ores in an undetermined form in other sulphides, notably in
galena, sphalerite, and pyrite. Silver differs from gold in that it is
chemically active and forms many stable compounds, of which only the
more important have been mentioned.

The fact that half the world's silver is obtained as a by-product in the
mining of other metals has been referred to. In the United States about
a third of the production comes from dry or siliceous ores, over a third
from lead and zinc ores, and a fourth to a third from copper ores. A
fraction of 1 per cent of the total is obtained as a by-product of gold
placers, and all the remainder is won from lode or hard-rock deposits.

The general geologic features of the silver-bearing copper and lead
ores, and of the dry or siliceous gold and silver ores, have been
described on previous pages. The Philipsburg district has been referred
to in connection with manganese ores, and the Bolivian tin-silver ores
will be described in connection with tin. We shall consider here only a
few of the more prominent districts which have been primarily silver
producers.

The Cobalt district of northern Ontario is the most productive silver
district in North America. The ores are found in numerous short, narrow
veins, principally in pre-Cambrian sediments near a thick qu