al deposits on the basis of origin is more or
less arbitrary. The sharp lines implied by the use of class names do not
exist in nature. Mineral deposits are so complex and so interrelated in
origin, that a classification according to genesis indicates only the
essential and central class features; it does not sharply define the
limits of the classes.

It is practically impossible for any geologist to present a
classification which will be accepted without qualification by other
geologists, although there may be agreement on essential features.
Difficulties in reaching agreement are increased by the inheritance from
the past of names, definitions, and classifications which do not exactly
fit present conceptions based on fuller information,--but which,
nevertheless, have become so firmly established in the literature that
it is difficult to avoid their use. In the progress of investigation
many new names are coined to fit more precisely the particular situation
in hand, but only in fortunate cases do these new names stand up against
the traditional currency and authority of old names. The geologist is
often in despair in his attempt to express his ideas clearly and
precisely, and at the same time to use terms which will be
understandable by his readers and will not arouse needless controversy.

As illustrative of the above remarks reference may be made to a few
terms commonly used in economic geology, such as _primary_, _secondary_,
_syngenetic_, _epigenetic_, _supergene_, _hypogene_, _protore_, etc.

The most commonly used of these terms are _primary_ and _secondary_. It
is almost impossible to define them in a way which will cover all the
conceptions for which they have been used, and yet in their context they
have been very useful in conveying essential ideas. An ore formed by
direct processes of sedimentation has sometimes been called primary,
whereas an ore formed by later enrichment of these sediments has been
called secondary. An ore formed directly by igneous processes has been
called primary, while an ore formed by enrichment of such primary ore by
later processes has been called secondary. It is clear, however, that
these terms are merely relative, with application to a specific
sequence, and that they do not fix the absolute position of the ore in a
sequence applying to all ores. For instance, ores deposited directly as
sediments or placers may be derived from the erosion of preexisting ore
bodies,--in which