All
of the diamonds are to be cut in Germany, and it is estimated that the
wages paid for the work will amount to $500,000 annually at the present
rate of production of the field. The diamonds are to be taxed at such a
rate that the miners will secure about one-half the profit. Mining
consists in simple sieving, washing, and picking out of diamonds.
Almost no capital for machinery has been required so far and the work
is done by natives with white overseers.
The
extent of the diamond field is being found to be greater than at first
thought. Diamonds have been found along the coast to the south about
150 miles from Luderitz Bay and 75 miles from Orange River.a That
the deposits have formed from alluvial deposits is thought probable,
and Orange River is pointed out as a possible original source.
SOUTH AMERICA.
Brazil.—The
geology of the diamond-bearing highlands of Bahia— Chapada Diamantina,
as a portion of it is called—has been described by J. C. Branner.6
This region lies northwest of Bahia and south and southeast of Rio Sao
Francisco. About it comparatively little is known. Two railroads
approach but do not penetrate this area, and travel must be
accomplished with mules. The region is semiarid, and the climate hot,
though healthful. Much of the country is covered with catinga forests,
a tough, scrubby growth of timber. The country is fairly well watered
along the streams, though subject to droughts away from them.
The
following is an outline of the geology with probable ages of the
formations: Along the coast to the north of the city of Bahia is a
series of Cretaceous and Tertiary strata resting on pre-Cambrian
schists, gneisses, and granites. The latter extend from the
sediment-aries along the coast westward to the highlands and form a
nearly level plain with a few scattered hills and peaks over it. The
Serra de Jacobina is the first mountain range of the highlands and is
com-
p
osed of the
conglomerates, shales, schists, and quartzites of the Minas series,
1,000 meters thick, and of Cambrian age. The bedding of this series
strikes nearly north with the range and dips steeply to the east or is
vertical. The crystalline rocks also appear on the west of the Jacobina
Range. To the west of this is the great Tombador Range, composed of 400
meters of nearly horizontal Cambrian sandstones and quartzites resting
directly on the crystallines. The edges of these beds of the Tombador
formation, which dip gently westward, form great walls or scarps on the
east. The Tombador beds can be traced northward toward Rio Sao
Francisco, where they cap the flat-topped mountains or form the
monoclinal ridges of that region. Above the Tombador beds are the
Jacuipe flints, about 100 meters thick. Above these are 500 meters of
the Caboclo formation, of Devonian age, composed of gray, red, yellow,
black, and cream-colored shales. The upper part of this formation is in
contrast with the overlying false-bedded pinkish sandstones,
conglomerates, and quartzites of the Lavras series, of Carboniferous
age. This series carries the diamonds and is about 700 meters thick.
The Lavras series is overlain by 350 meters of red Triassic sandstones,
very like the Triassic sandstones of New Jersey and Connecticut. These
have been called the Estancia red beds. The last series exposed is the
oMin. Jour., London, March 20,1909. 6 Eng. and Min. Jour., May, 15,1909.
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