e rock back to exactly the same condition from which it
started. More sediments are formed than are changed to schists and
gneisses, and more schists and gneisses are formed than are changed back
to igneous rocks. Salts in the ocean continuously accumulate. The net
result of the metamorphic cycle, is, therefore, the accumulation of
materials of the same kinds. Incidental to these accumulations is the
segregation of commercial mineral products.

The metamorphic cycle becomes a logical and convenient geologic basis
for correlating, interpreting, and classifying mineral products. Because
of the great variety of materials and conditions represented in mineral
deposits, prodigious efforts are required to remember them as
independent entities; but as incidents or stages in the well-known
progress of the metamorphic cycle, their essential characteristics may
be easily remembered and kept in some perspective.

Ores of certain metals, such as iron, occur in almost every phase of the
metamorphic cycle,--as igneous after-effects, as weathered products, as
sediments, and as schists. The ores of each of these several phases have
group characteristics which serve to distinguish them in important
particulars from ores belonging to other phases of the cycle. Having
established the position of any particular ore in the metamorphic cycle,
a number of safe inferences are possible as to mineralogical
composition, shape, extent, and other conditions, knowledge of which is
necessary for an estimate of commercial possibilities.

FOOTNOTES:

[1] Clarke, F. W., Data of geochemistry: _Bull. 695, U. S. Geol.
Survey_, 1920, p. 35.

[2] Clarke, F. W., Data of geochemistry: _Bull. 695, U. S. Geol.
Survey_, 1920, p. 33.

[3] Clarke, F. W., Data of geochemistry: _Bull. 695, U. S. Geol.
Survey_, 1920, pp. 22-23.

CHAPTER III

SOME SALIENT FEATURES OF THE GEOLOGY AND CLASSIFICATION OF MINERAL
DEPOSITS

VARIOUS METHODS OF CLASSIFICATION

Mineral products may be classified according to use, commercial
importance, geographic distribution, form and structure, mineralogical
and chemical composition, or origin. Each of these classifications is
useful for some purposes. The geologist usually prefers a classification
based on origin or genesis. In the following chapters on mineral
resources, however, such a classification is not the primary one,
because of the desire to emphasize economic features. The mineral
commodities are treated as un