Page:Philosophical Transactions - Volume 145.djvu/124

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learned, by an examination of, and quotations from, ancient writers; whereas the problem to be solved was more one of physical geography, and belonged more to the geologist than to the man of letters. Admitting the theory of Herodotus as to the mode of formation of the Delta, he combats that part of it in which he refers the operation to historical time.

That there has been an annual inundation of the Nile, of greater or less amount, from the earliest period to which history or tradition reaches, does not admit of a doubt; and it is equally certain that the river so flooded was loaded with solid materials, for the fertilizing effect of a sediment left upon the ground is recorded equally with the fact of an annual inundation[1]. Unless therefore this same addition was wholly washed away again from the surface between the fall of the water of one year and the rise of the next, there must have been an accumulation from year to year. That the fertilizing effect of the inundation is exhausted, or nearly so, is true; but in a country where there are no streams tributary to the main river, where rain is almost unknown in the greater portion of it, that is in Upper Egypt, and with an inclination so slight as that of the land over which the inundation spreads, the solid insoluble matter must in great part remain where it is deposited. That the fertilizing effect of the inundation is increased in proportion to the depth of the water, is shown by the unequivocal proof that the taxes, from time immemorial, have been levied upon the land according to the height to which the river rises[2]. It was to regulate this impost that the Nilometer on the island of Elephantine near the First Cataract was erected in ancient times, and that a similar instrument was set up in the island of Rhoda, near Cairo, a thousand years ago.

To investigate the formation of the alluvial land in the valley of the Nile in Upper and Lower Egypt is therefore an object of the highest interest to the geologist and the historian. Nowhere else on the face of the earth can we hope to find such a link connecting the earliest historical with the latest geological time; for in Egypt we have accurate records of the earliest periods of the human race, in which any trace of civilization has been discovered, combined with records, of scarcely less accuracy, of geological changes contemporaneous with history, and these last having such a degree of uniformity as to warrant us in carrying back the dates of changes of a like nature beyond that of the earliest historical documents.

Having been long impressed with a conviction that this geological problem could only be solved by having shafts and borings made in the alluvial deposits to the greatest practicable depth, and concurring in the opinion, long ago expressed by Cuvier, that it was a matter of regret that the depth of these deposits between the surface and

  1. How much the fertilizing effect of the inundation belongs to the solid matter held in suspension and deposited on the land, and how much belongs to the matter held in solution in the water, is a question that, so far as I know, has not been solved.
  2. Beyond a certain height, it is disadvantageous, as the water has not drained off sufficiently to leave the land in a proper state at the right sowing time of the following year.