y to
over-emphasize the persistence of primary ores in depth. The very use of
the terms "secondary" and "primary" has suggested antithesis between
surficial and deep ores. Progress in investigation, as indicated on
previous pages, seems to indicate that the primary ores are not
uniformly deep and that in many cases they are distinctly limited to a
given set of formations or conditions comparatively near the surface.

In general the processes of oxidation and secondary sulphide enrichment
have been studied mainly by qualitative methods with the aid of the
microscope and by considerations of possible chemical processes. These
methods have disclosed the nature but not the quantitative range and
relations of the different processes. Much remains to be done in the way
of large scale quantitative analysis of ores at different depths, as a
check to inferences drawn by other methods. One may know, for instance,
that a mineral is soluble and is actually removed from the oxide zone
and redeposited below. The natural inference, therefore, is that the
mineral will be found to be depleted above and enriched below. In many
cases its actual distribution is the reverse,--indicating that this
process has been only one of the factors in the net result, the more
rapid solution and deposition of other materials being another factor.
If one were to approach the study of the concentration of iron ores with
the fixed idea of insolubility of quartz from a chemical standpoint, and
were to draw conclusions accordingly, he would fail to present a true
picture of the situation. While quartz is insoluble as compared with
most minerals, it is nevertheless more soluble than iron oxide, and
therefore the net result of concentration at the surface is to
accumulate the iron rather than the silica. Descriptions of enrichment
processes as published in many reports are often misleading in this
regard. They may be correct in indicating the actual existence of a
process, but may lead the reader to assumptions as to net results which
are incorrect.

RESIDUAL MINERAL DEPOSITS FORMED BY THE WEATHERING OF IGNEOUS ROCKS IN
PLACE

Igneous rocks not containing mineral deposits may on weathering change
to mineral deposits. The lateritic iron ores such as those of Cuba (p.
172), many bauxite deposits, many residual clays, and certain chromite
and nickel deposits are conspicuous representatives of this class. The
chemical and mineralogical changes involved in the