It seems not impossible that these phenomena may be of some geologic aid
in the future, but experimental work is yet in a very early stage.

THE USE OF STRUCTURE AND METAMORPHISM IN EXPLORATION

The necessity for careful use of structural data in exploration scarcely
requires discussion. References have been made to structural features in
connection with coal, oil, iron ore, and other minerals. This phase of
study can scarcely be too intensively followed. The tracing of a folded
or faulted vein, in a particularly complex system of veins, requires
application of all of the methods and principles of structural geology.

Similarly, the importance of applying the principles of metamorphism,
embodied in the _metamorphic cycle_ (pp. 27-28) is almost self-evident.
Certain kinds of metamorphism are suggestive of the nature of the
mineral deposits with which they are associated. One would not look for
minerals known to be caused mainly by surficial processes in rocks which
have been altered mainly by deep-seated processes. The presence of
metamorphism indicating high temperatures and pressures to some extent
limits the kinds of minerals which one may expect to find. On the other
hand, minerals known to be primarily formed at great depths, providing
they are resistant to surface weathering, may be found in deposits which
are the result of surficial alterations or katamorphic processes; that
is, they may become concentrated as residual materials in weathered
zones or as placers.

DRILLING IN EXPLORATION

In the absence of distinctive outcrops, as well as when outcrops are
found, drilling is a widely used method of underground exploration in
advance of the sinking of shafts or the driving of tunnels. Drilling is
more useful in the locating and proving of mineral deposits of large
bulk, like deposits of coal, iron, and oil, than mineral deposits of
small bulk and high value, like gold and silver deposits. However, it is
not always used in the exploration of the first class of deposits and is
not always eliminated in the exploration of the second class. With the
development of better mechanical devices, better methods of controlling
and ascertaining the direction of the drill hole, and more skillful
interpretation of drill samples, the use of drilling is rapidly
extending into mineral fields where it was formerly thought not
applicable.

The geologist takes an active part in drilling operations by locating
the drill holes,