he
solutions. The processes are qualitatively understood and it is usually
possible to ascertain with reasonable accuracy the conditions of depth
of water, relation to shore line, climate, nature of erosion, and other
similar factors; yet the vast scale of some of these deposits, and their
erratic areal and stratigraphic distribution, present unsolved problems
as to the precise combinations of factors which have made such results
possible.

Chemically and organically deposited minerals of this class are usually
susceptible to further alteration by surface weathering, and some of
them, for instance the phosphates and siderites, are thus secondarily
concentrated. These processes are discussed under the next heading.

In general the great unsolved problem of the origin of the entire group
of mineral deposits in placers and sediments relates to the scale of the
results. Observation of present-day processes and conditions of
deposition of these minerals affords satisfactory evidence of their
nature, but fails to give us a clear idea of the precise combinations of
agencies and conditions necessary to produce such vast results as are
represented by the mineral deposits. For example, solution of iron on a
land surface and redeposition in bogs and lagoons (as actually observed
to be taking place today) show how some iron-ore sediments may be
formed; but these processes are entirely inadequate to explain the
deposition of iron ores in thick masses over broad areas without
intermingling of other sediments--as represented by the Clinton iron
ores of North America, the Jurassic ores of Europe and England, and the
ancient iron ores of Brazil. The Paleozoic seas in northern and eastern
United States encroached over land areas to the north and east and
deposited ordinary sediments such as sandstone, shale, and limestone.
Suddenly, without, so far as known, tapping any new sources of supply on
the ancient land areas, and without any yet ascertainable change in
topographic or climatic conditions, they deposited enormous masses of
iron ore. There is clearly some cyclic factor in the situation which we
do not yet understand.

The various deposits of salt, gypsum, potash, sulphur, and other
minerals are known to be the result of evaporation, and the deposition
of each of these minerals is known to be related to the degree of
evaporation as well as to temperature, pressure, and factors such as
mass action and crystallization of double sal