to a certain depth in mud, as at Memphis and Heliopolis, is the era when the city fell into such decay that the ancient embankments were neglected, and the river allowed to inundate the site of the temple, obelisk, or statue.
Even if we knew the date of the abandonment of such embankments, the enclosed areas would not afford a favourable opportunity for ascertaining the average rate of deposit in the alluvial plain; for Herodotus tells us that in his time those spots from which the Nile waters had been shut out for centuries appeared sunk, and could be looked down into from the surrounding grounds, which had been raised by the gradual accumulation over them of sediment annually thrown down. If the waters at length should break into such depressions, they must at first carry with them into the enclosure much mud washed from the steep surrounding banks, so that a greater quantity would be deposited in a few years than perhaps in as many centuries on the great plain outside the depressed area, where no such disturbing causes intervened.
Ancient Mounds of the Valley of the Ohio.
As I have already given several European examples of monuments of pre-historic date belonging to the recent period, I will now turn to the American continent. Before the scientific investigation by Messrs. Squier and Davis of the 'Ancient Monuments of the Mississippi Valley,'[1] no one suspected that the plains of that river had been occupied, for ages before the French and British colonists settled there, by a nation of older date, and more advanced in the arts than the Red Indians whom the Europeans found there. There are hundreds of large mounds in the basin of the Mississippi, and especially in the valleys of the Ohio and its tributaries, which have served, some of them for temples, others for out-
- ↑ Smithsonian Contributions, vol. i., 1847.
