only
would a substance of a moderately high density such as the Diamond be
difficult to move by any but more rapidly running water, but also
transportation over many miles of country would mean a considerable
reduction in size even to the Diamond. As in South Africa, the
jjot-holes in the river beds are often found to yield a large number of
the stones.
"What
are known as " valley deposits " are very similar to the river
deposits, except that they represent much older detrital matter carried
down by the stream when its bed was at a higher level than now. This
detritus is now left along the banks in the old river terraces. In some
cases material of this kind is found to have been re-cemented into a
conglomerate.
The
"plateau deposits" near Diamantina consist of large broken fragments of
the rocks from the district around, embedded in a red earthy ground
mass. This mass is known locally as " gurgulho." A noteworthy point is
that the rock fragments are angular, or only very slightly rounded,
suggesting an origin similar to that of the " brockram " of the New
lied Series in England.
Some
distance to the west of Diamantina, at Sao Joao da Chapada, the
Diamonds occur in a very peculiar plateau deposit, lying in great
trenches of over 100 feet in depth, and twice that in width, and
several hundred yards long. These trenches are filled with alternating
beds of clay and itacolumite; the beds are well marked and have a
pronounced inclination to the east. Penetrating these beds are
*
"veins"
containing Quartz, Haematite, and Rutile. Other minerals in association
with the Diamond are Tourmaline, Anatase, and oxides of iron. All the
minerals are found in
i 2
