The
chief deposits were found, at first, in river beds and ravines in a
breccia of clay, quartz pebbles, and sand, charged with oxide of iron.
Some of the richest beds were opened along the rivers Jequetinhonha and
Pardo in the valley of Sejues, and on the line of the rivers Aboite,
Andaja, da Serreno, da Prata, and San Francisco in the province of
Minas Geraes.1
The
diamond-bearing ground was worked under government agents or leased to
contractors. Quick returns were the first object. So gangs of slaves
were put on the grounds, regardless of loss, if only the cream of the
fields was skimmed. In the dry season the beds of the smaller sierran
streams were nearly or wholly dry. Underlying the surface wash of sand
in the bed was the formacao or cascalho, heavy diamond-bearing gravel
intermixed with boulders. The alluvial soil was generally from eight
to twenty feet thick, a silicious sand chiefly, deep colored by
ferruginous clay. The diamonds and other minerals of high specific
gravity were held in the bottom layer of this alluvium, usually
cemented in a coarse pudding-stone of quartz and itacolumite — the
cascalho. The sand was rudely scraped away or carried off in
pans, the boulders pried out, and the cascalho exposed. Then the gravel
was collected laboriously in pans and piled in heaps to await the
rainy season, when the streams filled the dry courses and there was
water at hand for washing the gravel.
Bacus
or shallow pits were sunk in the sand along the brink of the streams,
and in these pits a few panfuls of gravel were thrown. The bottom of
the bacu was made to slope so that the dashing of water on the gravel
heap would readily wash away the clinging sand and the lighter and
larger stones. The expert slaves washed the heaps in the bacus with
splashes of water cast
1
"The Diamond Fields of Brazil," Report of United States Minister Bryan,
March 12, 1899, conveying report of American Secretary of Legation,
Dawson. "A Treatise on Gems," Lewis Feuchtwanger, M.D., 1867. "An
Account of Diamonds found in Brazil," James Castro de Sarmente, M.D.
"Genuine Account of the Present State of the Diamond Trade in the
Dominions of Portugal," a Lisbon merchant, London, 1785. "Travels in
South America," J. J. von Tschudi.
