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PRECIOUS STONES.
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ground
around Diamantina at a height of 4,000 feet above sea level. A mountain
chain, the Serra do Espinhaço, running generally north and south, ends
in this high ground of the plateau ; the surface rock is a thinly
laminated sandstone containing flakes of a green Mica. It has the
peculiar property of flexibility, just as some of the Indian sandstones
have, and much interest would attach to a microscopic examination of
this rock to determine the shape of the sand grains. In places the rock
is of a coarser nature, more approaching a conglomerate. The age of the
stratum is not definitely known, but is almost certainly of
considerable geological antiquity ; from its extensive occurrence in
the Serra Itacolumi it is generally spoken of as itacolumite ; it is
interbedded with schists in which Hornblende, Haematite, and Mica are
developed. This is a point of great importance, for it points to the
rocks having undergone considerable metamorphism. Underlying these
rocks are others described as gneiss ; the itacolumite and the schists
are penetrated by " veins" containing a good deal of Quartz.
Where
the rivers have cut through layers of Diamond-bearing rock the gem is
found in thin beds, often for a considerable distance ; thus the Rio
Jequetinhonha and its tributaries have Diamond-bearing gravels as far
down as Mendanha, though the tributaries coming in from the plateau to
the west are most productive. Other streams which do not cut these
rocks yield no Diamonds, as, for instance, the Rio Doce.
The
gravels worked in the present river beds are the " river deposits." It
is found that the larger Diamonds occur in the upper part of the
stream, while lower down the stones are smaller. This is as one would
expect, for not
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