Note 46 — Chapter X · ASSUAN AND ELEPHANTINE.

“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.... 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.”—“Journal of Researches,” by Charles Darwin, chap. i, p. 12, ed. 1845.