y "bonanzas."

In gold deposits, the greater resistance to solubility of this
metal than most of the others, renders the phenomena of migration to
depth less marked. Further than this, migration is often interfered
with by the more impervious quartz matrix of many gold deposits.
Where gold is associated with large quantities of base metals,
however, the leaching of the latter in the oxidized zone leaves the
ore differentially richer, and as gold is also slightly soluble,
in such cases the migration of the base metals does carry some of
the gold. In the instance especially of impregnation or replacement
deposits, where the matrix is easily permeable, the upper sulphide
zone is distinctly richer than lower down, and this enrichment is
accompanied by a considerable increase in sulphides and tellurides.
The predominant characteristic of alteration in gold deposits is,
however, enrichment in the oxidized zone with the maximum values
near the surface. The reasons for this appear to be that gold in its
resistance to oxidation and wholesale migration gives opportunities
to a sort of combined mechanical and chemical enrichment.

In dry climates, especially, the gentleness of erosion allows of
more thorough decomposition of the outcroppings, and a mechanical
separation of the gold from the detritus. It remains on or near
the deposit, ready to be carried below, mechanically or otherwise.
In wet climates this is less pronounced, for erosion bears away
the croppings before such an extensive decomposition and freeing
of the gold particles. The West Australian gold fields present an
especially prominent example of this type of superficial enrichment.
During the last fifteen years nearly eight hundred companies have
been formed for working mines in this region. Although from four
hundred of these high-grade ore has been produced, some thirty-three
only have ever paid dividends. The great majority have been unpayable
below oxidation,--a distance of one or two hundred feet. The writer's
unvarying experience with gold is that it is richer in the oxidized
zone than at any point below. While cases do occur of gold deposits
richer in the upper sulphide zone than below, even the upper sulphides
are usually poorer than the oxidized region. In quartz veins
preeminently, evidence of enrichment in the third zone is likely
to be practically absent.

Tin ores present an anomaly among the base metals under discussion,
in that the primary form of