Personal Narrative Of Travels To The Equinoctial Regions Of America During The Years 1799-1804 - Volume 2 - By Alexander Von Humboldt And Aime Bonpland.
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The Waters Of Great Rivers Contain Carbonic Acid;
And, Were They Even Entirely Pure, They Would Still Be Capable, In
Very Great Volumes, Of Dissolving Some Portions Of Oxide, Or Those
Metallic Hydrates Which Are Regarded As The Least Soluble.
The mud of
the Nile, which is the sediment of the matters which the river holds
suspended, is destitute
Of manganese; but it contains, according to
the analysis of M. Regnault, six parts in a hundred of oxide of iron;
and its colour, at first black, changes to yellowish brown by
desiccation and the contact of air. The mud consequently is not the
cause of the black crusts on the rocks of Syene. Berzelius, who, at my
request, examined these crusts, recognized in them, as in those of the
granites of the Orinoco and River Congo, the union of iron and
manganese. That celebrated chemist was of opinion that the rivers do
not take up these oxides from the soil over which they flow, but that
they derive them from their subterranean sources, and deposit them on
the rocks in the manner of cementation, by the action of particular
affinities, perhaps by that of the potash of the feldspar. A long
residence at the cataracts of the Orinoco, the Nile, and the Rio
Congo, and an examination of the circumstances attendant on this
phenomenon of coloration, could alone lead to the complete solution of
the problem we have discussed. Is this phenomenon independent of the
nature of the rocks? I shall content myself with observing, in
general, that neither the granitic masses remote from the ancient bed
of the Orinoco, but exposed during the rainy season to the
alternations of heat and moisture, nor the granitic rocks bathed by
the brownish waters of the Rio Negro, assume the appearance of
meteoric stones.
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