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An elevation is produced in the soil of Egypt, at the same time that this extension of its surface takes place, and the bed of the river rises in the same proportion as the adjacent plains, which makes the inundations of every succeeding century pass far beyond the marks which it had left during the preceding ones. According to Herodotus, a period of nine hundred years was sufficient to establish a difference of level amounting to ten or twelve feet. At Elephantia[94], the inundation at present exceeds by seven feet the greatest heights which it attained under Septimus Severus, at the commencement of the third century. At Cairo, before it is judged sufficient for the purpose of irrigation, it must exceed, by three feet and a half, the height which was necessary in the ninth century. The ancient monuments of this celebrated land have all their bases more or less buried in the soil. The mud left by the river even covers, to a depth of several feet, the artificial mounds on which the ancient towns were built[95].