The existence of alluvial soil, apparently of the same kind as that deposited by the Nile, in situations above the Cataract of Assuan, at a level considerably above the highest point which the inundations of the river have reached in modern times, to which allusion is made by Professor Lepsius, has been noticed by other travellers, and even at still higher levels than those he mentions. Whether that alluvial soil be identical with, or only resembles the Nile deposit, would require to be determined by a close examination, and especially with regard to organic remains, if any can be found in it. There is no evidence to shew that it was deposited during the historical period, and it may be an evidence of a depression and subsequent elevation of the land antecedent to that period. It may not be of fresh-water origin, but the clay and sand, or till, left by a drift while the land was under the sea. For remote as is the antiquity of Nubia and Egypt, in relation to the existence of the human race, it appears to be of very modern formation in geological time. The greater part of Lower Egypt, probably all the Delta, is of post-pliocene age, and even late in that age; and the very granite of the Cataract of Assuan, that of which the oldest monuments in Egypt are formed, and which, in the earlier days of geology, was looked upon as the very type of the rock on which the oldest strata of the earth were founded, is said to have burst forth during the later tertiary period. We learn from Russegger, that the low land which lies between the Mediterranean and the range of hills that extends from Cairo to the Red Sea at Suez, and of which hills a nummulite limestone constitutes a great part, is composed of a sandstone which he calls a “Meeresdiluvium,” a marine diluvial formation, and considers to be of an age younger than that of the sub-appennines.[83] This sandstone he found associated with the granite above Assuan, and covering the cretaceous sandstone far into Nubia. It appears, therefore, that, in the later ages of the tertiary period, this north-eastern part of Africa must have been submerged, and that very energetic plutonic action was going forward in the then bed of the sea. The remarkable fact of the granite bursting through this modern sandstone is thus described by Russegger:—
“We arrived at a plateau of the Arabian Chain south-east of Assuan. It is about 200 feet above the bed of the Nile, and consists of the lower and upper sandstone, which are penetrated by innumerable granite cones from 20 to 100 feet in height, arranged over the plateau in parallel lines, very much resembling volcanic cones rising from a great cleft. The sandstone is totally altered in texture near the granite, and has all the appearance as if it had been exposed to a great heat. 'I cannot refrain,' he says, 'from supposing that the granite must have burst, like a volcanic product, through long wide rents in the sandstone, and that, in this way, the conical hills were formed.'”[84]
An eruption of a true granite during the period of the sub-appennine formations, one possessing the same mineral structure as that we know to have been erupted during the period of the palæozoic rocks, would be a fact of so extraordinary a kind, that its age would require to be established on the clearest evidence, and especially by that of organic remains in the sandstone.
Having thus ventured—I trust without any want of the respect due to so eminent a person—to reject the hypothesis proposed by Professor Lepsius for the high levels of the Nile at Semne, indicated by the sculptured marks he discovered, it may perhaps be expected that I should offer another more probable explanation. If in some narrow gorge of the river below Semne, a place had been described by any traveller, where, from the nature of the banks, a great landslip, or even an artificial dam, could have raised the bed to an adequate height; that is, proportionate to the fall of the river, as it was more distant from Semne, a bar that, in the course of a few centuries, might have been gradually washed away, I might have ventured to suggest such a solution of the problem. But without any information of the existence of such a contraction of the river's channel, or any exact knowledge of the natural outlets and dams to running water along the 250 miles of the Nile Valley, from Semne to Assuan, it would be idle to offer even a conjecture. These marks are unquestionably very difficult to account for, in the present imperfect state of our knowledge of the structure of that portion of the Nile Valley; and any competent geologist, well versed in the questions of physical structure involved, who may hereafter visit Nubia, would have a very interesting occupation in endeavouring to solve the difficulty.
7th April 1850.
[71] Russegger, Bd. ii., 1 Thl. 569 to 584.
[72] Rennie, Report cited above, p. 422.
[73] Parthey, 318.
[74] Russegger, Reison, Bd. ii. 300 and 320. Lancrot, Description de l'Égypte, Memoire sur l'île de Philæ, 15-58. Rossellini, I Monumenti dell'Egitto e della Nubia. Monumenti del Culto, 187. Wilkinson's Thebes and General View of Egypt, 466. Smith's Dictionary of Greek and Roman Biography, Arts. Ptolemy, Ph. and Nectanabis.