likewise a nearly continuous
progressive process, contemporaneous with the ore deposition, and
perhaps developing under a single great shear which caused more or less
simultaneous and overlapping systems of fractures in the various
directions.

="Porphyry coppers."= Another type of copper deposits in igneous rocks
is the disseminated or "porphyry" deposits. The term "porphyry" as
commonly used includes true porphyries, monzonites, granites, and other
igneous rocks. Ores of this type are represented by the great deposits
of Bingham, Utah; Ray, Miami, and the New Cornelia mine of Arizona; Ely,
Nevada; Santa Rita, New Mexico; Cananea, Sonora, Mexico; northern Chile;
and many other districts of importance. They form the greatest known
reserves of copper ore. These deposits contain copper minerals, usually
in the marginal portions of acid porphyries, in many irregular, closely
spaced veins, and in minute seams and spots disseminated through the
mass of the rock. In the Ray and Miami and other districts the
mineralization has spread largely through adjacent schists, but these
deposits are included with the porphyry copper deposits in commercial
parlance. The porphyry deposits are of an undulating blanket form of
considerable areal extent and shallow depth. At the surface is a leached
and weathered zone, often containing more or less of the oxides,
carbonates, and silicates of copper, ranging in thickness up to 1,000
feet, but averaging 200 feet or less. Below this is a zone carrying
copper in the form of chalcopyrite, enriched by chalcocite deposition
from above, ranging in thickness up to 400 feet. The ore in this zone
varies from one-half of 1 per cent to 6 per cent of copper and
ordinarily averages between 1 and 2 per cent. The use of ore of this
grade is made possible by the large quantities and by the cheap and
efficient mining and metallurgical practices. The ore body grades below
into a zone characterized by lean chalcopyrite, which is supposed to
represent original or primary deposition from hot waters associated with
the porphyry intrusion. This primary ore, or protore, was clearly formed
after the solidification of the igneous rocks, though soon after, by
solutions from igneous sources which followed fractured and shattered
zones.

=Copper in limestone near igneous contacts.= Another great group of
copper deposits occurs as replacements of limestone adjacent to porphyry
or granitic intrusives. This type is illust