ion.

A physiographic problem of another sort is the determination of the
conditions surrounding the origin of sedimentary ores. Certain mineral
deposits, like the "Clinton" iron ores, the copper ores in the "Red
Beds" of southwestern United States and in the Mansfield slates of
Germany, many salt deposits, and almost the entire group of placer
deposits of gold, tin, and other metals, are the result of
sedimentation, from waters which derived their materials from the
erosion of the land surface. It is sometimes possible from the study of
these deposits to discover the position and configuration of the shore
line, the depth of water, and the probable continuity and extent of the
deposits. Similar questions are met in the study of coal and oil.

This general problem is one of the phases of geology which is now
receiving a large amount of attention, not only from the standpoint of
ore deposition, but from a broader geologic standpoint. In spite of the
fact that sedimentary processes of great variety can be observed in
operation today, it is yet extremely difficult to infer from a given
sedimentary deposit the precise conditions which determined its
deposition and limited its distribution. For instance, sedimentary iron
formations furnish a large part of the world's iron ore. The surface
distribution, the structure, the features of secondary enrichment, are
all pretty well understood; likewise the general conditions of
sedimentation are reasonably clear,--but the close interpretation of
these conditions, to enable us to predict the extent of one of these
deposits, or to explain its presence in one place and absence in
another, is in an early and sketchy stage.

An understanding of the principles and methods of physiography is also
vital to an intelligent application of geology to water resources, to
soils, to dam and reservoir construction, and to a great variety of
engineering undertakings, but as these subjects involve the application
of many other phases of geology, they are considered in separate
chapters. (Chapters V, VI, and XX.)

ROCK ALTERATIONS OR METAMORPHISM

This is one of the newer special phases of geology which for a long time
was regarded as the plaything of the petrographer or student of rocks.
With the systematic development of the subject, however, it was found
that the extremely numerous and complex alterations of rocks and
minerals may be definitely grouped, and that they are controlled by
broad