the works have become free, fraud has greatly diminished. Under the old rule it overtook half the diamonds in the gravels."
This
method of washing is not confined to the river sands, but is also used
to separate the diamonds in the upland deposits. These upland deposits
include strata of considerable extent, composed of clay derived from
the decomposition of a coarse conglomerate. The strata are divisible
into three distinct layers. The first, a soil cap, is somewhat
diamondiferous; the next lower, a clay called secunclina, is regarded sterile, probably on account of its tenacity, which makes it almost unwash-able; while the third, called tenia, is the diamond layer par excellence. Large
areas of this sort have been and are still being worked with more or
less profitable returns, an illustration of the latter being given by
Gorceix, who states that he knows of miners who have washed the cascalhos of Bagagem for twenty years without finding a single diamond of value.
79 '4
