they carry commercial minerals, such as the precious stones, mica,
feldspar, cassiterite (tin ore), and others. They show a complete
gradation from dikes of definitely igneous characteristics to veins
consisting largely of quartz in which evidence of igneous origin is not
so clear. The pegmatites thus afford a connecting link between ores of
direct igneous sources and ores formed as "igneous after-effects," which
are discussed in the next paragraph. Aplites are fine-grained acid
igneous rocks of somewhat the same composition as the pegmatites and
often show the same general relations to ores.

MINERAL DEPOSITS WITHIN AND ADJACENT TO IGNEOUS ROCKS WHICH WERE FORMED
IMMEDIATELY AFTER THE COOLING AND CRYSTALLIZATION OF THE MAGMAS THROUGH
THE AGENCY OF HOT MAGMATIC SOLUTIONS.

These deposits are closely associated in place and age with igneous
rocks, either intrusive or extrusive, and are usually considered to have
come from approximately the same source; and yet they afford distinct
evidence of having been deposited after the adjacent igneous rocks were
completely crystallized and fractured. They are thus _epigenetic_
deposits. They are not themselves igneous rocks and they do not
constitute pegmatites, but they often grade into pegmatites and belong
to the same general stage in the sequence of events. They include
deposits formed by contact metamorphism. They are sometimes designated
by the general term "igneous after-effects"--a term also applied in some
cases to pegmatites. Some geologists discriminate between "deep vein"
deposits (p. 43) and "contact-metamorphic" deposits, but the two are so
closely related in place and origin that for our purposes they will be
considered together.

The ores of this class are clearly deposited from vagrant solutions
which wander through openings of all kinds in the igneous rock and
outward into the adjacent country rocks. They also replace the wall
rocks; limestone is especially susceptible. This is a phase of contact
metamorphism. Some of the most important metalliferous deposits belong
in this class, including most of the gold, silver, copper, iron, lead
and zinc ores of the western United States and the copper deposits of
Lake Superior.

In general, ores of this class are more abundant about intrusive igneous
rocks, that is about igneous rocks which have stopped and cooled before
reaching the surface,--than in association with extrusive igneous rocks
which have poured over th