principles. It became apparent also that these principles apply
both to the economic and non-economic minerals and rocks,--in other
words, that the segregation of economic minerals is a mere incident in
pervasive cycles of the alterations which affect all rocks. Metamorphic
geology, therefore, for some geologists becomes a convenient approach to
the subject of economic geology. It has the great advantage that it
tends to keep all minerals and all processes of ore deposition in proper
perspective with relation to rocks and rock processes in general. It is
not argued that this is the only approach or that it is the best for all
purposes. A brief account of this phase of geology is given in Chapter
II.

APPLICATION OF OTHER SCIENCES

Geology is sometimes defined as the application of other sciences to the
earth. Considered broadly, there is no phase of science which is not
involved in economic geology. In other chapters in this book many
references are made to applications of engineering, mathematics,
physics, chemistry, metallurgy, biology, and economics.

At different times and places the requirements for earth materials are
quite different. In the Stone Age there was little use for metals; in
later ages the use of metals broadened. The multiplicity of demands of
modern civilization, the increasing knowledge of processes of
metallurgy, chemistry and physics, better transportation, better
organization of commercial life, and many other factors, tend to bring
new earth materials into use,--and, therefore, into the field of
economic geology. A comparatively few years ago alumina, one of the most
common and abundant substances of the earth's crust, was in no general
demand except for very limited use as an ornament. Little attention was
paid to it by economic geologists as a commercial product; now, however,
aluminum is in great demand, and the raw materials which produce it have
become the subjects of intensive study by economic geologists.

In short, economic geology includes the consideration of man in reaction
to his physical environment. There are some earth materials and some
conditions of the earth environment which do not yet come within the
field of economic geology. But so large a proportion of them do, that
the "complete economic geologist" should indeed be almost omniscient.
When one considers what an insignificantly small portion of this field
can be covered by any individual, it is apparent that the title