(1/7. "Personal Narrative" volume 5 part 1 page 18.) At the
cataracts of the great rivers Orinoco, Nile, and Congo, the
syenitic rocks are coated by a black substance, appearing as if
they had been polished with plumbago. The layer is of extreme
thinness; and on analysis by Berzelius it was found to consist of
the oxides of manganese and iron. In the Orinoco it occurs on the
rocks periodically washed by the floods, and in those parts alone
where the stream is rapid; or, as the Indians say, "the rocks are
black where the waters are white." Here the coating is of a rich
brown instead of a black colour, and seems to be composed of
ferruginous matter alone. Hand specimens fail to give a just idea
of these brown burnished stones which glitter in the sun's rays.
They occur only within the limits of the tidal waves; and as the
rivulet slowly trickles down, the surf must supply the polishing
power of the cataracts in the great rivers. In like manner, the
rise and fall of the tide probably answer to the periodical
inundations; and thus the same effects are produced under
apparently different but really similar circumstances. The origin,
however, of these coatings of metallic oxides, which seem as if
cemented to the rocks, is not understood; and no reason, I believe,
can be assigned for their thickness remaining the same.
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