eached out,
leaving iron ores. This process contrasts with the concentration
described above, in that there is little evidence of collection of iron
minerals from disseminated sources. The Lake Superior iron ores are
essentially residual concentrations in place. The outstanding problems
of secondary concentration relate to the structural features which
determined the channels through which the oxidizing and leaching waters
worked, and to the topographic and climatic conditions which existed at
the time the work was done. As with many other classes of ores, it was
first assumed that these processes were related to the present erosion
surface; but it is now known that concentration happened long ago under
conditions far different from those now existing. These deposits
contribute to the rapidly accumulating evidence of the _cyclic_ nature
of ore concentration.

Our least satisfactory knowledge of the Lake Superior ores relates to
the peculiar conditions which determined the initial stage of
sedimentation of the so-called iron formation. As in the case of the
Clinton iron ores, no present-day sedimentation gives an adequate clue.
Students of the problem have fallen back on the association of the iron
formation with contemporaneous volcanic rocks, as affording a possible
explanation of the wide departure from ordinary conditions of
sedimentation evidenced by these formations.[9]

Coal deposits are direct results of sedimentation of organic material.
They are mainly accumulations of vegetable matter in place. To make them
available for use, however, they undergo a long period of condensation
and distillation. Conditions of primary deposition may be inferred from
modern swamps and bogs; but, as in the case of sediments described under
the preceding heading, we are sometimes at a loss to explain the
magnitude of the process, and especially to explain the maintenance of
proper surface conditions of plant growth and accumulation for the long
periods during which subsidence of land areas and encroachment of seas
are believed to have been taking place. The processes of secondary
concentration are also understood qualitatively, but much remains to be
learned about the influences of pressure and heat, the effect of
impervious capping rocks, and other factors.

Various oil shales and asphaltic deposits are essentially original
sediments which have subsequently undergone more or less decay and
distillation. The migration of the dis