"It is, however, conceivable that, at the time when the present low land of the Nubian Valley was formed, the cataracts at Assuan were in a totally different state; one that would, in some degree, justify the overcharged descriptions of the ancients, according to whom they made so great a noise that the dwellers near them became deaf. The damming up of the inundation at Assuan could have no material influence on Egypt, any more than that at Semne, or the land from thence to Assuan." It appears therefore, from the above statements, that at the time mentioned, the Nile, during the inundations, stood 26 feet 8 inches higher than the highest level to which it now rises in years of the greatest floods; and that, to account for this, Professor Lepsius conceives that, between the time of Moesis and the present day, the bed of the Nile, from a considerable distance above Semne to Assuan, must have been worn down to that extent. In the index to the volume of the Berlin Monatsbericht, in which the letters of Professor Lepsius are inserted, there is the following line:-- "NIL, _senkung seines Bettes um 25 Fuss seit 4000 Jahren_." "_Nile_, sinking of its bed about 25 feet (Paris) within the last 4000 years." Rivers are, undoubtedly, among the most active agents of change that are operating on the earth's surface; the solid matter which renders their waters turbid, and which they unceasingly carry to the sea, afford indisputable proof of this agency. But the power of rivers to abrade and wear down the rocks over which they flow, and to form and deepen their own bed, depends upon a variety of circumstances not always taken into account; and although the great extent of that power, in both respects, is shewn in the case of many rivers, to conclude, as some have done, from these instances, that all rivers have excavated the channels in which they flow, is a generalization that cannot be safely assented to. The excavation of the bed of a river is one of those problems in geological dynamics which can only be rightly solved by each particular case being subjected to the rigorous examination of the mathematician and the physicist. The solid matter which rivers carry forward is in part only the produce of their own abrading power; and the amount of it must be proportional to that power, which is mainly dependent on their velocity; they are the recipients of the waste of the adjoining lands by other combined agencies, and the carriers of it to the lower districts and to the sea. They often afford the strongest evidence of the vast lapse of time that must be included between the beginning and close of a geological period; and, when they flow through countries whose remote political history is known to us, they supply a scale by which we may measure and estimate that lapse of time. This is especially so in the case of the Nile.