case it may sometimes be convenient to refer to the
sedimentary ores or placers as secondary and the earlier ores as
primary. Or a sulphide deposit originating through igneous agencies may
undergo two or three successive enrichments, each successive one
secondary to the preceding, but primary to the one following. In spite
of these obvious difficulties, the terms primary and secondary may be
entirely intelligible as indicating relative order of development under
a given set of conditions.

The term _syngenetic_ has been used for mineral deposits formed by
processes similar to those which have formed the enclosing rocks and in
general simultaneously with them, and _epigenetic_ for those introduced
into preexisting rocks. In certain cases _syngenetic_ may be roughly
synonymous with _primary_, and _epigenetic_ with _secondary_, and yet a
primary ore may be epigenetic. For instance, zinc sulphides in the
Mississippi valley limestones (pp. 54-55) are epigenetic, and yet are
primary with reference to a later enrichment. The two sets of terms are
meant to convey somewhat different ideas and are not interchangeable.

Ransome[4] has suggested, especially for vein and contact deposits, a
series of names which has the considerable advantage of
definiteness:--_hypogene ores_, formed in general by ascending
non-oxidizing solutions, perhaps hot; _supergene ores_, formed in
general by oxidizing and surface solutions, initially cold and downward
moving; and _protores_, or metallized rock or vein substances which are
too low in tenor to be classed as ores, but which would have been
converted into ores had the enriching process been carried far enough.
In this connection Ransome defines primary ore as unenriched material
that can be profitably mined. In view of the general use of the terms
primary and secondary as expressing a sequential relation of ore
development, it is doubtful whether this more precise definition will
supersede the older usage. Also it may be noted that commercial
conditions might require, under these definitions, the designation of an
ore as a protore at one time or place and as a primary ore at another.
Hypogene ores are dominantly primary, and supergene ores are dominantly
secondary, but either may include both primary and secondary ores.

The terms of these several classifications overlap, and seek to express
different aspects of the same situation. While almost synonymous in
certain applications they are not i