land are all of
these types. Their principal iron mineral is hematite, although the
English ores also contain considerable iron carbonate or siderite. The
cementing or gangue materials are chiefly calcite and quartz, in
variable proportions.

The large reserves of high-grade hematite in the Minas Geraes district
of Brazil are also original sediments, but lack the oolitic texture.

An insignificant proportion of the world's iron is obtained from "bog
ores," which are sedimentary deposits of hydrated iron oxide in swamps
and lakes. These ores have been used only on a small scale and chiefly
in relatively undeveloped countries. They are of particular interest
from a genetic standpoint in that they show the nature of some of the
processes of iron ore deposition as it is actually going on today.

None of the ores of this class, with the exception of the iron
carbonates, have undergone any considerable surface enrichment since
their primary deposition. Neither, with the exception of the Brazilian
ores, have they undergone any deep-seated metamorphism. The shapes,
sizes, and distribution of the deposits may be traced back to the
conditions of original deposition. In England and western Europe the
principal deposits have been only slightly tilted by folding. In the
United States the Clinton ores have partaken in the Appalachian folding.
In Brazil, the ores have undergone close folding and anamorphism.

2. Deposits of the second class, which owe much of their value to
further enrichment since deposition, are represented by the hematite
ores of the Lake Superior district. These may be thought of as the
locally rusted and leached portions of extensive "iron formations," in
which oxidation of the iron, and the leaching of silica and other
substances by circulating waters, have left the less soluble iron
minerals concentrated as ores. The Lake Superior iron formations now
consist near the surface mainly of interbanded quartz (or chert) and
hematite, called _jasper_ or _ferruginous chert_ or _taconite_. These
are similar in composition to the leaner iron ores of Brazil, called
_itabirite_, but differ in that the silica is in the form of chemically
deposited chert, rather than fragmental quartz grains.

[Illustration: FIG. 11. Alteration of Lake Superior iron
formation to iron ore by the leaching of silica.]

When originally deposited the iron was partly hematite (perhaps some
magnetite) and largely in the form of iron carbonate