(siderite) and
iron silicate (greenalite), interbanded with chert. The original
condition is indicated by the facts that deep below the surface, in
zones protected from weathering solutions, siderite and greenalite are
abundant, and that they show complete gradation to hematite in
approaching the surface. The ore has been concentrated in the iron
formation almost solely by the process of leaching of silica by surface
or meteoric waters, leaving the hematite in a porous mass. Figure 11
illustrates this change as calculated from analyses and measurements of
pore space. During this process a very minor amount of iron has been
transported and redeposited. In short, the Lake Superior iron ores are
residual deposits formed by exactly the same weathering processes as
cause the accumulation of clays, bauxites, and the oxide zones of
sulphide deposits. The development of an iron ore rather than of other
materials as an end-product is due merely to the peculiar composition of
the parent rock. The solution of silica on such an immense scale as is
indicated by these deposits has sometimes been questioned on the general
ground that silica minerals are insoluble. However, there is plenty of
evidence that such minerals _are_ soluble in nature; and the assumption
of insolubility, so often made in geologic discussions, is based on the
fact that most other minerals are _more_ soluble than silica minerals,
and that in the end-products of weathering silica minerals therefore
usually remain as important constituents. Iron oxide, on the other hand,
is _less_ soluble even than silica,--with the result that when the two
occur together, the evidence of leaching of silica from the mixture
becomes conspicuous.

The fact that these deposits are almost exclusively residual deposits
formed by the leaching of silica has an important bearing on
exploration. If they have been formed by the transportation and
deposition of iron from the surrounding rocks, there is no reason why
they should not occasionally be found in veins and dikes outside of the
iron formation. As a matter of fact they do not transgress a foot beyond
the limits of the iron formation. Failure to recognize the true nature
of the concentration of these ores has sometimes led to their erroneous
classification as ores derived from the leaching and redeposition of
iron from the surrounding rocks.

The distribution and shapes of ore deposits of this class are far more
irregular and capri