326 THE POPULAR SCIENCE MONTHLY. 



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 denud- 

 ing 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 debris 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 ap- 

 plication 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 Selsi- 

 leh, 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 



