surface outcrops and with the aid
of magnetic surveys, iron formations are found which are the mother rock
of the ores. In Michigan, it has been possible to use certain percentage
expectations in the areal location of iron formations within certain
series of rocks extending over wide areas. Such percentage coefficients
have been useful, not only in exploration, but also in the valuation of
lands which are so covered with drift that no one knows whether they
carry an iron formation or not.

Examination of the iron formations results in elimination of large parts
of them, because their metamorphic condition is not favorable to ore
concentration. In the remaining areas more intensive methods are
followed. It is scarcely possible to summarize briefly all of the
structural and stratigraphic methods used in locating the ore bodies.
These have often been described in print.[41] Comparatively recent
advances in this phase of exploration work have been in the more
detailed application of stratigraphic methods to the iron formation. The
group characteristics of the iron formation are fairly uniform and
distinctive as compared with all other rocks; yet within the iron
formation there are so many different kinds of layers represented that
it is possible to use these variations with great effectiveness, in
correlating favorable horizons for ore deposition, in interpreting drill
records, and in other ways. Another method of approach, employed chiefly
on the Mesabi Range, relates to the slumping of the ore layers which
results from the leaching of silica during the concentration of the ore.
This slumping can be measured quantitatively, and has been used to much
advantage in exploration, in correlation of ore horizons, in preparation
of sections and ore estimates, etc.

Early geologic explorations in the Lake Superior country were based on
the assumption that the ores were concentrated by waters working down
from the present erosion surface; but recognition of the fact that the
waters which did the work were related to a far older and different
erosion surface, under conditions which allowed of a far deeper
penetration, has modified exploration plans for certain of the districts
like the Marquette and Gogebic.

Notwithstanding the complexity of the geologic factors involved, their
net result has been to concentrate iron ores in a surprisingly uniform
ratio to the mass of the formation in different parts of the
region,--with the resu