this metal in most workable deposits
is an oxide. Tin in this form is most difficult of solution from
ground agencies, as witness the great alluvial deposits, often of
considerable geologic age. In consequence the phenomena of migration
and enrichment are almost wholly absent, except such as are due
to mechanical penetration of tin from surface decomposition of
the matrix akin to that described in gold deposits.

In general, three or four essential facts from secondary alteration
must be kept in view when prognosticating extensions.

Oxidation usually alters treatment problems, and oxidized ore
of the same grade as sulphides can often be treated more cheaply.
This is not universal. Low-grade ores of lead, copper, and zinc
may be treatable by concentration when in the form of sulphides,
and may be valueless when oxidized, even though of the same grade.

Copper ores generally show violent enrichment at the base of the
oxidized, and at the top of the sulphide zone.

Lead-zinc ores show lead enrichment and zinc impoverishment in
the oxidized zone but have usually less pronounced enrichment
below water level than copper. The rearrangement of the metals
by the deeper migration of the zinc, also renders them
metallurgically of less value with depth.

Silver deposits are often differentially enriched in the oxidized
zone, and at times tend to concentrate in the upper sulphide zone.

Gold deposits usually decrease in value from the surface through
the whole of the three alteration zones.

SIZE OF DEPOSITS.--The proverb of a relation between extension
in depth and size of ore-bodies expresses one of the oldest of
miners' beliefs. It has some basis in experience, especially in
fissure veins, but has little foundation in theory and is applicable
over but limited areas and under limited conditions.

From a structural view, the depth of fissuring is likely to be more
or less in proportion to its length and breadth and therefore the
volume of vein filling with depth is likely to be proportional to
length and width of the fissure. As to the distribution of values,
if we eliminate the influence of changing wall rocks, or other
precipitating agencies which often cause the values to arrange
themselves in "floors," and of secondary alteration, there may be
some reason to assume distribution of values of an extent equal
vertically to that displayed horizontally. There is, as said, more
reason in ex