Page:Popular Science Monthly Volume 23.djvu/340

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expansion and contraction which they undergo, as if they had been broken by a road-maker's hammer; and the fragments collect in great heaps at the bottom of every steep incline. Not a blade of grass, not a drop of water, is to be seen anywhere; and yet the form and arrangement of the ravines are such that it is impossible to doubt that they have been formed, like other valleys, by the scoring and denuding action of rapid streams.

I remember that one day, wandering in the desert, northeast of Cairo, in the direction of the petrified forest, I was exceedingly struck with the resemblance of the superficial scorings of the surface of the rocky soil to those which are ordinarily made by rain. I was puzzling myself to account for the fact, when a sudden storm, accompanied by very heavy rain, came up, drenched me to the skin and vanished, all in less than an hour. Immediately after the rain began to fall, every one of the ramified scorings which had interested me was filled by a stream of water, rushing to join with its fellows and flow down some bigger groove to a lower level. It was obvious that the resemblance which had struck me was not deceptive, and that all these ramified scorings were miniature "wadys"—the dry beds of minute rivulets produced by former sudden showers of the same sort as that which I had experienced.

It was a capital lesson in physiography, and I did not forget it when I looked down on the great wadys of the Libyan desert at Thebes. Twelve hours' heavy rain would send a roaring torrent, sweeping before it all the accumulated débris of years, down every one of them. And if we suppose the process repeated only every twenty years; still, since the Libyan Hills have been where they are for the last ten thousand years, five hundred repetitions of the application of this most efficient rain-plow would have cut some pretty deep furrows, even if, during all this time, rain has never been more frequent or more abundant than it is now.

Still farther to the south, about El Kab, close upon the twenty-fifth parallel of latitude, the fringe of cultivable land which borders the Nile becomes narrower and narrower, and the limestones in which the valley has hitherto been excavated are replaced by sandstones as far as Assouan. The low hills of such rock (rarely more than two hundred feet high) which lie on each side of the river at Gebel Selsileh, about forty miles north of Assouan, approach one another so closely that the gorge through which the stream passes is little more than one thousand feet wide. There is every reason to believe that the opposite walls of this gorge were once continuous, and that the river swept as a rapid over the correspondingly elevated margin of the sandstone plateau, through which it has since cut its channel back to Assouan, until, at present, its bed, for the forty miles to that place, has no greater inclination than elsewhere.

Near Assouan, under the twenty-fourth parallel, on the frontier of