it was introduced into its present situation by an artesian circulation,
for which the structural conditions are favorable. If the ore was
derived from overlying horizons, downward moving solutions accompanying
erosion did the work. If the primary source was within the horizon of
present occurrence of the ores, both upward and downward moving waters
may have modified and transported them locally. For each of these
hypotheses a plausible case can be made; but much of the evidence can be
used interchangeably for any one of them. In spite of the wealth of data
available, it is astonishingly difficult to arrive at a conclusion which
is exclusive of other possibilities. Without attempting to argue the
matter in detail the writer merely records his view, based on some
familiarity with these districts, that, on the whole, the evidence
favors the accumulation of these deposits by downward moving meteoric
solutions during the weathering of overlying strata; but that it is by
no means certain that a part of the ores has not been derived from lower
horizons. The great area of the producing districts in comparison with
their depth, the uniform association of the ore-bearing zone with the
surface regardless of geologic horizon uncovered by erosion, the failure
of the ores to extend in quantity under cappings of later formations,
and the known efficacy of oxidizing waters in local downward transfers
of zinc and lead, seem to suggest concentrating agencies which are
clearly related to surface conditions.

It is of interest to note that in many places in the limestones of
Missouri and Virginia, and elsewhere in the Paleozoic rocks, there are
sinks of limonite and clay near the surface, which are likewise believed
to have originated through downward movement of waters deriving their
mineral contents from the erosion and stripping of overlying sediments.
Still further, the primary deposition of Clinton iron ores in many parts
of the Mississippi Valley and eastward to the Appalachians took place in
stratigraphic horizons not far removed from the horizons of lead and
zinc deposition. When the peculiar conditions controlling the deposition
of the Clinton ores are understood (see pp. 52-53) it is entirely
possible that they may throw some light on the genesis of the lead and
zinc ores.

Since the ores were introduced into essentially their present locations,
secondary concentration has produced an oxide zone of clay, chert, and
iron oxide,