ts. The nature of the
processes is fairly well understood; but again, observation of the
present-day operation of these processes fails to give us much clue to
the enormous accumulations at certain times and places in the past. It
is difficult to say just what conditions of climate, in combination with
particular physiographic factors, could have preserved uniformity of
conditions for the long periods necessary to account for some of the
enormously thick salt deposits. Again some cyclic factor in the
situation remains to be worked out.

SEDIMENTARY MINERAL DEPOSITS WHICH HAVE REQUIRED FURTHER CONCENTRATION
TO MAKE THEM COMMERCIALLY AVAILABLE

The conditions for the direct deposition of sedimentary mineral deposits
of the foregoing class are also responsible for the deposition of
minerals in more dispersed or disseminated form, requiring further
concentration through surface agencies to render them commercially
available. Some of these deposits are discussed below.

The lead and zinc ores of the Mississippi Valley, Virginia, Tennessee,
Silesia, Belgium, and Germany (pp. 211-212, 216-219) are in sedimentary
rocks far removed from igneous sources. Lead and zinc were deposited in
more or less dispersed form with the enclosing sediments. It is supposed
that deposition was originally chemical and was favored by the presence
of organic material, which is a rather common accompaniment of the
sediments. It is supposed further that these organic participants were
originally localized during sedimentation in so-called estuarine
channels and shore-line embayments. When subsequently exposed to
weathering, the lead and zinc minerals were dissolved and redeposited in
more concentrated form in fissures and as replacements of limestone.

Agreement as to origin of these deposits, so far as it exists, does not
go beyond these broad generalizations. There is controversy as to
whether the original sources of the ore minerals were the sediments
directly above, from which the mineral solutions have been transferred
downward during weathering and erosion, or whether the original minerals
were below and have been transferred upward by artesian circulation, or
whether they were situated laterally and have been brought to their
present position by movement along the beds, or whether there has been
some combination of these processes. It is the writer's view that the
evidence thus far gathered favors on the whole the conclusion of direct
dow