tacts between igneous intrusives and sedimentary rocks,
particularly limestones (so-called "contact-metamorphic" deposits). The
ores of the Cornwall district of Pennsylvania and some of the Chilean,
Chinese, and Japanese ores are of the same type.

Magnetites containing titanium, which prevents their use at the present
time, are known in many parts of the world as segregations in basic
igneous rocks. They are actually parts of the igneous rock itself (p.
34). Among the large deposits of this nature are certain titaniferous
ores of the Adirondacks, of Wyoming, and of the Scandinavian peninsula.

In all of these cases, it is clear that the origin of the ores is in
some way related to igneous processes, and presumably most of the ores
are deposited from the primary hot solutions accompanying and following
the intrusion of the igneous rocks; but thus far it has been difficult
to find definite and positive evidence as to the precise processes
involved. None of these deposits have undergone any important secondary
enrichment at the surface. Their sizes, shapes, and distribution are
governed by conditions of igneous intrusion, more or less modified, as
in the Adirondacks, by later deformation.

=Iron ores due to weathering of igneous rocks.= A small part of the
world's iron ores, less than 1 per cent of the total production, are the
result of surface alteration of serpentine rocks. These ores are mined
principally in Cuba (Fig. 12). Here they have been developed on a
plateau-like area on which erosion is sluggish. The process of formation
has been one of oxidation of the iron minerals and leaching of most of
the other constituents, leaving the iron concentrated near the surface
in blanket-like deposits. The minerals of the original rock contained
alumina, which, like the iron, is insoluble under weathering conditions,
and hence the Cuban iron ores are high in alumina. They also contain
small quantities of nickel and chromium which have been concentrated
with the iron. A large part of the iron minerals, especially where close
to the surface, have been gathered into small shot-like nodules called
_pisolites_. It is thought that the solution and redeposition of the
iron by organic acids from plant roots may be at least a contributing
cause in the formation of this pisolitic texture.

[Illustration: FIG. 12. Representing in terms of weight the
mineralogical changes in the katamorphism of serpentine rock to iron
ore, on the assu