The same material, with the same name, is also found deposited at all
heights upon small terraces at the sides of the valleys through which the
rivers flow. It is coarser and less rolled, and has very likely been
deposited by risings of the rivers during the period when the valleys were
being formed. These deposits bear the name of _gupiarras_. Finally, it is
found in a still coarser state, mixed with red earth and deposited in
horizontal strata upon the upper plateau. It is then called _gorgulho_.
Of these different deposits, the most important are those of the river
beds, the material here having undergone a true mechanical preparation and
being richer. These are the deposits that have been the object of the most
important exploitations.
The year is divided into two distinct seasons--the dry, from May to
September, during which rain is exceptional, and the rainy, from October
to April. As water is necessary for all the operations, no work can be
done upon the high plateaux except through rain water stored up in large
reservoirs. These beds form what are called the "rainy season washings."
In the rivers the working of the beds requires a preliminary drying, which
is effected by diverting the river's course.
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