onglomerate known as _cascalho_; (2) in terraces
(gupiarras) in a similar conglomerate occupying higher levels in the
present valleys; (3) in plateau deposits in a coarse surface
conglomerate known as _gurgulho_, the diamond and other heavy minerals
being embedded in the red clay which cements the larger blocks. Under
all these three conditions the diamond is associated with fragments of
the rocks of the country and the minerals derived from them,
especially quartz, hornstone, jasper, the polymorphous oxide of
titanium (rutile, anatase and brookite), oxides and hydrates of iron
(magnetite, ilmenite, haematite, limonite), oxide of tin, iron
pyrites, tourmaline, garnet, xenotime, monazite, kyanite, diaspore,
sphene, topaz, and several phosphates, and also gold. Since the heavy
minerals of the _cascalho_ in the river beds are more worn than those
of the terraces, it is highly probable that they have been derived by
the cutting down of the older river gravels represented by the
terraces; and since in both deposits the heavy minerals are more
abundant near the heads of the valleys in the plateau, it is also
highly probable that both have really been derived from the plateau
deposit. In the latter, especially at Sao Joao da Chapada, the
minerals accompanying the diamond are scarcely worn at all; in the
terraces and the river beds they are more worn and more abundant; the
terraces, therefore, are to be regarded as a first concentration of
the plateau material by the old rivers; and the _cascalho_ as a second
concentration by the modern rivers. The mining is carried on by
negroes under the supervision of overseers; the _cascalho_ is dug out
in the dry season and removed to a higher level, and is afterwards
washed out by hand in running water in shallow wooden basins
(_bateas_). The terraces can be worked at all seasons, and the
material is partly washed out by leading streams on to it. The washing
of the plateau material is effected in reservoirs of rain water.

It is difficult to obtain an estimate of the actual production of the
Minas Geraes mines, for no official returns have been published, but
in recent years it has certainly been rivalled by the yield in Bahia.
The diamond here occurs in river gravels and sands associated with
the same minerals as in Minas Geraes; since 1844 the richest mines
have been worked in the Serra de Cincora, where the mountains