SCIENTIFIC JOTTINGS IN EGYPT. 827 



indeed that camels make no impress on it with their broad feet. 

 At some places the surface pebbles are of many shades of brown, 

 intermingled with black and white, and these are so closely laid 

 and regularly distributed as to resemble a mosaic pavement, but 

 of course a patternless one. The surface particles are generally 

 coarser than those immediately beneath ; they are chiefly lime- 

 stone, sometimes of coralline limestone, intermingled with flint 

 and other varieties of amorphous quartz. Many of the pebbles 

 show on their surface beautifully regular pittings and furrows 

 carved out by the wind -driven sand. The fine-grained sand has 

 all been lifted high in air by the powerful winds, whirled away, 

 and dropped into depressions or on the lee sides of hills. Hun- 

 dreds of acres have no surface stones larger than an ostrich-egg ; 

 no water whatever is found in this region, much less any signs 

 of vegetable or animal life, rarely even a passing bird. 



On this desolate plain, when overtaken by night, one place 

 is as good (or bad) as another for pitching the tents, unless 

 perhaps a small hillock is reached, which may serve as a partial 

 shelter from the gales that sometimes threaten to overturn the 

 canvas. 



In the region of extensive plains, the wadis, or dried-up water- 

 courses, being depressed but little, closely resemble them. The 

 floor of the wadi hardly differs from that of the plain, except 

 when a torrent has swept before it large bowlders and deposited 

 them irregularly in its bed. The sorting power of the water, 

 however, is noticeable, as also the well-defined vertical walls, per- 

 haps only a few inches deep, excavated at the point of lowest level. 

 On the margins, too, of the wadis of the plain, and at points pro- 

 tected from the full force of the winter floods, several varieties of 

 green shrubs grow in widely separated tufts. I often remarked 

 mud-cracks, apparently of recent date ; but these indications of 

 water probably remain undisturbed in this desolate region for a 

 considerable period, perhaps for several seasons. 



In the limestone hills these wadis take the form of canons, hav- 

 ing nearly vertical walls, sometimes hundreds of feet high as in 

 Wadi Tayyibeh. The regular erosion on their sides produces, 

 often, picturesque effects, as at Ras Abu Zanimeh. 



In the granitic district the wadis form V-shaped valleys, 

 broken by narrower ones entering at right angles, and bounded 

 by bold peaks many thousand feet above the beholder. In the 

 beds of these wadis are scattered specimens of the rocks of the 

 surrounding country ; often bowlders of great size testify to the 

 violence of the torrents during the winter months, especially in 

 Wadi Feiran. 



The absolute dependence of the population of Egypt upon the 

 Nile is a familiar fact, discussed from the time of Herodotus to 



