d with running water. The process is most effective for minerals
which are resistant to abrasion and to solution, and of such density as
to differentiate them from the other minerals of the parent rock.

The origin of deposits of this kind is fairly obvious where they are of
recent age and have not been subsequently altered or buried. A
considerable amount of experimental work has brought out clearly the
main elements of the processes. Physiographic and climatic conditions
play an important part, and cannot be safely overlooked by anyone
studying such deposits.

Extensive copper deposits exist as sediments (pp. 205-206). It is not
clear to what extent they are mechanically or to what extent chemically
deposited. For the most part the concentration of copper in this manner
has not been sufficient to yield deposits of large commercial value; the
mineral is too much dispersed. Relatively small amounts are mined in the
Mansfield shales of Germany and the Nonesuch shales and sandstones of
the Lake Superior country.

The Clinton and similar iron ores of the United States and Newfoundland,
the pre-Cambrian iron ores of Brazil, and the Jurassic iron ores of
England and western Europe (pp. 166-167) are now commonly agreed to be
direct sedimentary deposits in which mechanical agencies of sorting and
deposition played a considerable part. How far chemical and bacterial
agencies have also been effective is not clear. The climatic,
topographic, and other physiographic and sedimentary conditions which
cause the deposition of this great group of ores present one of the
great unsolved problems of economic geology. The study of present-day
conditions of deposition affords little clue as to the peculiar
combination of conditions which was necessary to accomplish such
remarkable results in the past.

On the whole, minerals of this mechanically deposited group are not
greatly affected by later surficial alteration and concentration,
because, having already been subjected to weathering, they are in a
condition to resist such influences.

CHEMICALLY AND ORGANICALLY DEPOSITED MINERALS

The products of surface weathering and erosion are in part carried away
in chemical solution and redeposited as sediments. Sediments thus formed
include limestone and dolomite, siderite, salt, gypsum, potash, sulphur,
phosphates, nitrates, and other minerals. Precipitation may be caused by
chemical reactions, by organic secretion, or by evaporation of t