e surface as lava flows--but the latter are by
no means insignificant, including as they do such deposits as the Lake
Superior copper ores, the Kennecott copper ores of Alaska, some of the
gold-silver deposits of Goldfield and other Nevada camps, and many
others.

There is general similarity in the succession of events shown by study
of ore bodies related to intrusives. First, the invasion of the magma,
resulting in contact metamorphism of the adjacent rocks, sometimes with,
and often without conspicuous crowding effects on the invaded rocks;
second, the cooling, crystallization, and cracking of both the igneous
rock and the adjacent rock; third, the introduction of ore-bearing
solutions into these cracks,--sometimes as a single episode, sometimes
as a long continued and complex process forming various types of
minerals at successive stages. This order may in some cases be repeated
in cycles, and overlapping of the successive events is a common feature.

One of the interesting facts is the way in which the igneous mass has
invaded and extensively altered the country rocks in some mineral
districts,--in some cases by crowding and crumpling them, and in others
without greatly disturbing their structural attitudes. The latter is
illustrated in the Bingham district of Utah and the Philipsburg district
of Montana. In such cases there is so little evidence of crowding of the
country rocks as to raise the question how such large masses of
intrusives could be introduced without greater disturbing structural
effect. This leads naturally to consideration of the general problem of
the manner of progress of magmas through adjacent rocks,--a subject
which is still largely in the realm of speculation, but which is not
thereby eliminated from the field of controversy. Facts of this kind
seem to favor the position of certain geologists that magmas may
assimilate the rocks they invade.

EVIDENCE OF IGNEOUS SOURCE

No one ever saw one of these deposits in the process of formation; the
conclusion, therefore, that they originated from hot solutions, either
aqueous or gaseous, or both, which were essentially "after-effects" of
igneous activity and came from the same primary source as the associated
igneous rocks, is an inference based on circumstantial evidence of the
kind below summarized:

(1) The close association both in place and age with igneous rocks. This
applies not only to individual deposits, but to certain groups of
depo