hem in an area of granite. For every mineral resource there are
broad geologic conditions of this sort, particularly the genetic,
structural, and metamorphic conditions, which make it possible to
eliminate vast areas from consideration and to concentrate on relatively
small areas.

After the elimination of unfavorable areas, there comes the hunt for
positively favorable geologic conditions--for a definite kind of
sediment or igneous rock, for a definite structure, for the right kind
of mineralogic and metamorphic conditions, or for the right combination
of these and other geologic elements. The geologic considerations used
in exploration for the various mineral deposits are so many and so
diverse, and they require so much adjustment and interpretation in their
local application, that one would be rash indeed to attempt anything in
the nature of an exhaustive discussion. It is hardly practicable to do
more than to outline, for illustrative purposes, a few of the geologic
factors most commonly used in exploration.

MINERAL PROVINCES AND EPOCHS

Mineral deposits may be similar in their mineralogic and geologic
characters and relations over a considerable area. They may give
evidence of having developed under the same general conditions of
origin; perhaps they may even be of the same geologic age. The
gold-silver deposits of Goldfield, of Tonopah, the Comstock Lode of
Virginia City, and many other deposits through the Great Basin area of
the southwestern United States and Mexico have group characteristics
which have led geologists to refer to this area as a "metallogenic" or
"metallographic" province. The gold-silver ores on the west slope of the
Sierra Nevadas, for nearly the entire length of California, likewise
constitute a metallogenic province. The Lake Superior copper ores on the
south shore of Lake Superior, the silver ores on the north shore,
miscellaneous small deposits of copper, silver, and gold ores to the
east of Lake Superior, the nickel ores of Sudbury, and the
silver-nickel-cobalt ores of the Cobalt district are all characterized
by similar groups of minerals (though in highly differing proportions),
by similar geologic associations, by similar age, and probably by
similar conditions of origin. This area is a metallogenic province. The
lead and zinc ores of the Mississippi Valley constitute another such
province. The oil pools of the principal fields are characterized by
common geologic conditions over