GEOLOGICAL HISTORY OF EGYPT AND THE NILE VALLEY. 309 
ing of the Nile Valley at Cairo to the shores of the Mediter- 
ranean, and from the plateau of the Libyan Desert on the 
West to the Isthmus of Suez on the Hast. This western 
margin of the Delta is formed of horizontal beds of Nummu- 
lite limestone, which break off in an escarpment, rising from 
200 to 250 feet, above the surface of the Delta, and thus 
producing a commanding site for the Pyramids, the Sphinx, 
and other structures of ancient Egyptian art. 
The range of hills forming the margin of the Badiet et Tih, 
or Desert of the Wanderings, east of the Isthmus of Suez, is 
formed of similar limestone strata, and these are again met 
with along the range of hills, extending from Jebel Mokattam, 
near Cairo, to Jebel Attaka, which rises above the Gulf of 
Suez in a terraced escarpment of limestone rock, strikingly 
imposing when viewed from the waters of the gulf. 
The general structure, therefore, of the tracts bordering 
the Delta to the south, east, and west leads us to infer that 
the region of Lower Egypt has been formed by the denuda- 
tion or erosion of the limestone strata, which once extended 
over its whole surface, almost horizontally (Fig. 3). The 
eroded portion has been subsequently to some extent filled in 
by sediment brought down by the Nile, and annually distri- 
buted during flood-time over its surface, so that it consists of 
a vast plain of alluvial land with scarcely any natural eleva- 
tions except the sandhills near the coast. These sandhills 
rest upon a reef which forms a powerful dam against the 
encroachments of the Mediterranean, and which Russegger 
describes as being in a constant state of formation and 
waste. It consists of a calcareous stone of a dull grey 
colour, composed of sand mixed with worn fragments of 
marine shells, sometimes minute or microscopic. A few of 
these are of land or fresh-water origin, brought down by the 
Nile and thrown up bythe sea and mingled with marine shells.* 
2. The Libyan Desert.—This second natural division con- 
sists of a vast slightly elevated plain composed of limestone, 
which breaks off in an escarpment ranging along the western 
margin of the Nile Valley and the plain of Lower Egypt, te 
the shore of the Mediterranean near Alexandria. 
The average level of this plateau may be taken at 1,000 
feet; but towards the south, in the latitude of Siut, the lime- 
stone plateau gradually rises to levels of about 1,500 feet, 
and below the First Cataract breaks off in lofty rocky escarp- 
* Resin in Europa, Asien und Africa, Stuttgart, 1843. 
