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144 THE DIAMOND MINES OF SOUTH AFRICA
diamond-bearing gravel were buried under ground too deep for profitable working, or covered by the waste of flooded rivers.
As
the mines advanced up the hillsides, following the course of the
mountain streams, it was seen that there were gupiaras or deposits of
diamond-bearing gravel along the steep slopes of the ravines, and these
were worked by carrying the gravel to the banks of streams, or by
cutting sluiceways to the deposits. Finally, on the sierran ridges and
plateaus the conglomerate beds were reached, from which the deposits in
the river beds had been washed by the mountain streams. This
conglomerate was chiefly itacolumite, a micaceous sandstone,
accompanied by mica-schist and penetrated irregularly by quartz veins.
This was the prevailing composite in the Serro de San Antonio, in which
the Jequetinhonha rises in the Serro de Matta de Corda, the fountain
head of the Rio Francisco.
Here
the diamonds were not as thickly sprinkled as they were in the cascalho
concentrate, but the quantity was sufficient to make extraction
profitable, if the conglomerate could be disintegrated and w7ashed.
This was effected by collecting rain water in pools at points above the
conglomerate and carrying down the water through ditches into gullies
cut in the beds. By the flow of the water, the formacao was separated
from the mass of rocks and sand. This device worked well, but owing to
the scarcity of water, the washing could only be continued for a few
weeks, at most, in the course of a year. In 1832 mining in these fields
was opened to the public, but the most accessible and prolific beds had
been worked, and there was little apparent encouragement for the
investment of capital in any large undertaking which might have
advanced the science of diamond winning. It is said that more than half
of the diamonds produced in Brazil were stolen by the workmen and sold
to contraband dealers, by whom they were secretly sent out of the
country.
Outside
of the Indian and Brazilian fields no considerable source of supply had
been discovered anywhere. Some diamond-bearing ground had been found
in Borneo, which yielded for many years a dribbling return, and in 1829
the first-known
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