cious than those of the primary sediments, as would
be expected from the fact that their concentration has taken place
through the agency of percolating waters from the surface, which worked
along devious channels determined by a vast variety of structural and
lithological conditions. The working out of the structural conditions
for the different mines and districts constitutes one of the principal
geologic problems in exploration. These conditions have been fully
discussed in the United States Geological Survey reports, and are so
various that no attempt will be made to summarize them here.

One of the interesting features of the concentration of Lake Superior
iron ores is the fact that it took place long ago in the Keweenawan
period, preceding the deposition of the flat-lying Cambrian formations,
at a time when the topography was mountainous and the climate was arid
or semi-arid. These conditions made it possible for the oxidizing and
leaching solutions to penetrate very deeply, how deeply is not yet
known, but certainly to a depth below the present surface of 2,500 feet.
At present the water level is ordinarily within 100 feet of the surface,
and oxidizing solutions are not going much below this depth. This
region, therefore, furnishes a good illustration of the intermittent and
cyclic character of ore concentration which is now coming to be
recognized in many ore deposits.

Subsequent changes far beneath the surface have folded, faulted, and
metamorphosed some of the Lake Superior iron ores but have not enriched
them. The same processes have recrystallized and locked together the
minerals of some of the lean iron formations, making them hard and
resistant, so that subsequent exposure and weathering have had little
effect in enriching them to form commercial ores.

The weathering of limestones containing minor percentages of iron
minerals originally deposited with the limestones may result in the
residual concentration of bodies of limonite or "brown ores" associated
with clays near the surface. This process is similar in all essential
respects to the concentration of the Lake Superior ores. Such limonitic
ores are found rather widely distributed through the Appalachian region
and in many other parts of the world. Because of the ease with which
they can be mined and smelted on a small scale they have been used since
early times, but have furnished only a very small fraction of the
world's iron.

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