Page:Popular Science Monthly Volume 23.djvu/344

330 I am afraid, however, that any such modern estimate has not a much surer foundation than the ancient guess. For, in the first place, there are many reasons for believing that the action of the Nile has not been uniform throughout the whole period represented by the deposit of alluvium; and, in the second place, if it had been, the simple process of division of the total thickness of the alluvium by that of the annual deposit does not by any means necessarily give the age of the whole mass of alluvium in the delta, or, in other words, the time which elapsed during the filling of the delta, as it is sometimes supposed to do.

According to Figari Bey, the deepest, and therefore earliest, alluvium in the delta contains gravel and even bowlders; so that, if these are fluviatile beds, which is, perhaps, not quite certain, they indicate that, at the time when they were deposited, the current of the Nile in this region was much more powerful than it is now, and, consequently, that its annual additions were much more considerable.

If the flow of the Nile in these ancient times was more rapid, the probabilities are that the volume of its waters was greater, and sundry observations have been adduced as evidence that such was the case. Thus, at Semneh, above the second cataract, Lepsius, many years ago, discovered inscriptions of a Pharaoh of the twelfth dynasty, Amenemhat III, who reigned between 2,000 and 3,000 b. c, which registered the level of the highest rise of the Nile at that time. And this level is nearly twenty-four feet higher than that of high Nile at the same place now. Another fact has been connected with this. Between the narrow gorge of the Nile at Selsileh and the first cataract, alluvial deposits, containing shells of animals now living in the river, lie on the flanks of the valley, twenty to thirty feet above the point which high Nile reaches at the present day. It has been suggested that, before the Nile cut the gorge, the sandstone bar at Selsileh, as it were, dammed up the Nile, and caused it to stand at a higher level all the way back to Semneh. But, as the late Dr. Leith Adams long ago argued, the sandstone strata of Selsileh could hardly have played the part thus assigned to them. The deposits in question indicate that the supposed barrier at Selsileh was about thirty feet high; while Semneh is at least one hundred and thirty feet higher than Selsileh.

The cause of the difference of level of the Nile at Semneh, between the days of Amenemhat and now, is surely rather to be sought in the progressive erosion of the Nubian valley. If four thousand years have elapsed since Amenemhat reigned, the removal of one thirteenth of an inch per annum from the bed of the river will be more than enough to account for its present depression. Considering the extraordinary activity of the denuding forces at work in Nubia, I see nothing improbable in this estimate. But, if it is correct, there is no need to suppose that the Nile conveyed a greater body of water four thousand years ago than it does now. Nor is there anything in