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Brazilian Diamonds.
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Itacolumite, a
rock which was named by Eschwege, from the Serra do Itacolumi. The true
Itacolumite of penologists is a sandstone, remarkable for possessing
flexibility, so that a thin slab admits of being readily bent to and
fro. This pecular rock is, however, only a rare variety of the
Itacolumite, most of which is a granular schistose quartzite, or
metamorphic sandstone, destitute of flexibility.
The
Brazilian Itacolumite long figured in works on mineralogy as the
original matrix—the true parent-rock .—of the Diamond ; and the
occurrence of a somewhat similar rock with Diamonds in India and in
North Carolina led to premature generalizations as to the origin of the
gem.
In
the geological section under description, the Itaco-lumites are
associated with a group of hydro-mica schists and Itaberites, or
schists containing specular iron-ore. Traversing these rocks are
certain more or less defined veins of clayey matter containing
Diamonds. The mineral is here supposed to occur in its primitive
position, the clayey material being probably its decaying matrix.
Diamonds are also found in the quartzites of an overlying series, but
here they are to be regarded as pebbles washed out of their original
home in the lower group of rocks. They are likewise distributed through
the gravels of the Brazilian Highlands, where they find a resting-place
after having been set free from their enclosing matrix. It is possible
then that a Diamond, born originally in the lower metamorphic series,
may have been transported among the materials which enter into the
constitution of the upper series, and then on the wearing down of these
upper rocks, may have been once more disturbed, and finally deposited
in the gravels of the present river valleys. Such appears to be the
geological history of many a Brazilian Diamond.
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