Page:Popular Science Monthly Volume 23.djvu/346

332 of evaporation, may have been immense. Under such circumstances, it is easily conceivable that a swift and voluminous torrent, periodically swollen by the contributions of the great southern affluents, covered the delta with a permanent inundation, and swept down gravel and bowlders into the lowest part of its course.

That the outflow of the Nile once extended far beyond its present limits appears to be certain, for a long, deep, dry valley so like an ancient river-bed that the Arabs call it the Bahr-bela-Ma, or waterless river—runs from south to north in the Libyan desert along the western edge of the delta, and ends in the Mediterranean shore beyond Taposiris, far to the west of the Canopic mouth, the most westerly of the outlets of the Nile known during the historical period. And, in the extreme east, far beyond the most easterly arm known to the ancients—in fact, in the middle of the Isthmus of Suez, about Lake Timseh—alluvial deposits, containing Nile shells and hippopotamus bones, show that the Nile once extended into this region, and perhaps poured some portion of its waters into the Red Sea, by way of anticipating the engineering operations of more modern days.

These facts tend to show that any calculation of the age of the delta, based upon the present action of the Nile in the way indicated, may need to be abbreviated. But, on the other hand, there are many obvious considerations which tend the other way.

It is easy to see that the time required for the deposition of a certain thickness of alluvial soil, in any one part of the delta, can only be a measure of the time required to fill up the whole, if the annual sediment is deposited in a layer of even thickness over the entire area. But this is not what takes place. When the river first spread out from the southern end of the delta, it must have deposited the great mass of its solid contents near that end; and this upper portion of the delta must have been filled up when the lower portion was still covered with water. And, since the area to be covered grew wider, the farther north the process of filling was carried, it is obvious that the northern part of the delta must have taken much longer to fill than the southern. If we suppose that the alluvium about Memphis was deposited at the rate of one twentieth of an inch per annum, and that there are fifty feet of it, ten thousand years may be the minimum age of that particular part of the delta; but the age of the alluvium of the delta as a whole must be very considerably greater. And indeed there are some indications that the shore-line of the nascent delta remained, for a long time, in the parallel of Athribis, five-and-twenty miles north of Cairo, where the remains of a line of ancient sand-dunes are said to attest the fact. Hence, all attempts to arrive at any definite estimate of the number of years since the alluvial plain of the delta began to be formed, are frustrated. But, the more one thinks of the matter, the more does the impression of the antiquity of the plain grow; and I, for my part, have no doubt that the extreme term imagined by