ss of sedimentary ores, the iron minerals are supposed
to have been introduced as replacements of limestones subsequent to
sedimentation. Such ores are not always easy to discriminate from ores
resulting primarily from sedimentation. This class is represented by the
high-grade deposits of Bilbao, Spain, Austrian deposits, and by smaller
deposits in other countries. The Bilbao ores consist mainly of siderite,
which near the surface has altered to large bodies of oxide minerals.
They occur in limestones and shales and are not associated with igneous
rocks. The deposits are believed to have been formed by ordinary surface
waters carrying iron in solution, and depositing it in the form of iron
carbonate as replacements of the limestones. The original source of the
iron is believed to have been small quantities of iron minerals
disseminated through the ordinary country rocks of the district. The
action of surface waters, in thus concentrating the iron in certain
localities which are favorable for precipitation, is similar to the
formation of the lead and zinc ores of the Mississippi valley, referred
to in the next chapter. Deposits formed in this manner may be roughly
tabular and resemble bedded deposits, or they may be of very irregular
shapes.

The sedimentary iron ores in general evidently represent an advanced
stage of katamorphism, and illustrate the tendency of this phase of the
metamorphic cycle toward simplification and segregation of certain
materials. The exact conditions of original sedimentation present one
of the great unsolved problems of geology, referred to in Chapter III.

=Iron ores associated with igneous rocks.= About five per cent of the
world's production of iron ore is from bodies of magnetite formed in
association with igneous rocks. These are dense, highly crystalline
ores, in which the iron minerals are tightly locked up with silicates,
quartz, and other minerals, suggestive of high temperature origin. The
largest of these deposits is at Kiruna in northern Sweden; in fact this
is the largest single deposit of high-grade ore of any kind yet known in
the world. Here the magnetite forms a great tabular vertical body lying
between porphyry and syenite. In the Adirondack Mountains of New York
and in the highlands of New Jersey, magnetites are interbedded and
infolded with gneisses, granites, and metamorphic limestones. In the
western United States there are many magnetite deposits, not yet mined,
at con