Was This Effect Produced Beneath
The Depths Of A Profound Ocean?
Or did a covering of strata
formerly extend over it, which has since been removed?
Can we believe that any power, acting for a time short of
infinity, could have denuded the granite over so many thousand
square leagues?
On a point not far from the city, where a rivulet entered
the sea, I observed a fact connected with a subject discussed
by Humboldt. [7] 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.
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