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XXXIV.
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THE CHAPADA.
Peculiarities of Brazilian Stones—A Diamond-Bearing Rock —A Notable Gem, named after the District where it was found.
N a recent number of the American Journal of Science, Mr.
A. 0. Derby, in a paper on the geology of the diamond, gives some
interesting results of his researches. It has been generally stated
that the Brazilian diamond has its matrix in itacolumite, which is a
granular quartzose rock sometimes flexible. Mr. Derby, however, shows
that, under the name of itacolumite rocks, two distinct geological
series have hitherto been confounded. The diamond-bearing rock of Grao
Mogul probably belongs to the newer of the two series; but the stones
have not been formed in these rocks, and occur there only as
derivative bodies like the associated pebbles. At Sao Jao de Chapada
the diamond is found in a deposit of clay; and its original matrix is
described as a vein of quartz accompanying a rock of unknown nature,
but containing iron and crystals of tourmaline, traversing a series of
schists and itacolumite. Mr. Derby is of opinion that the original
diamond formation in Brazil is probably of Cambrian age. A notable
diamond of
87-1/2 carats
was found in 1851, in the rich mineral district of Chapada, or
Santa-Cruz-da-Chapada, three leagues North-East of Fanado, in the
province of Minas-Geraes, Brazil It has been named after the district
that produced it.
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