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.
(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.