nward concentration from overlying sources which have been removed by
erosion, although this conclusion fails to explain why certain sulphide
deposits give so little evidence of important downward transfer from
their present position. This matter is further discussed on pages
216-219. The choice of the various alternatives has some practical
bearing on exploration.

Since these ores were brought into approximately their present
position, they have undergone considerable oxidation near the surface
and secondary sulphide enrichment below. The chemical and mineralogical
changes are pretty well understood, but the quantitative range of these
changes and their relative importance in determining the net result are
far from known. Undoubted evidence of secondary sulphide enrichment has
led in some quarters to an assumption of effectiveness in producing
values which is apparently not borne out by quantitative tests.

A group of mineral deposits in sandstones in Utah is regarded as due to
chemical concentration of material originally disseminated in the rock.
They include silver, copper, manganese, uranium, and radium deposits.
The Silver Reef deposits, including silver, copper, uranium, and
vanadium, are commercially the most important of this type.[8] The ore
minerals are commonly associated with carbonized material representing
plant remains, and have replaced the calcareous and cementing material
of the rock, and also some of the quartz grains. The deposits are
regarded as having been formed by circulating waters which collected the
minerals disseminated through the sedimentary rocks, and deposited them
on contact with carbonaceous matter, earlier sulphides, or other
precipitating agents. The circulation in some places is believed to have
been of artesian character and to have been controlled to a large extent
by structural features. The Silver Reef deposits are near the crest of a
prominent anticline. Most of the minerals have been later altered by
surface solutions.

Another great group of ores to be considered under this head are the
iron ores of Lake Superior,--which were originally deposited as
sediments, called jaspers or iron formations, with too low a percentage
of iron to be of use, and which have required a secondary concentration
by surficial agencies to render them valuable. The process of
concentration has been a simple one. The iron minerals have been
oxidized in place and the non-ferrous minerals have been l