312 PROFESSOR E. HULL, LL.D., F.R.S., F.G.8., ON THE 
The great tracts of desert above described lying to the 
north of lat. 25° terminate along the south in the escarpment 
of Jebel Djefata, which reaches a height of 1,550 feet above 
the sea level. To the south of this escarpment the beds of the 
Cretaceous formation set in, with a slight northerly dip, to be 
succeeded still further south by those of the Nubian sand- 
stone, which first makes its appearance in the Nile Valley at 
Esbeh, near Edfu, and which in turn reposes on the crystal- 
line rocks of the First Cataract at Assouan, the ancient 
Syene. 
3. The Region of the Arabian Mountains.—This region, lying 
between the Nile Valley and the Red Sea, presents a striking 
contrast to that we have just described lying to the west of 
the river. Its western portion consists of the same strata as 
those which form the Libyan Desert, but it is generally more 
diversified by high plateaux bounded by terraced escarp- 
ments and deep valleys, and is also intersected by a range of 
mountains formed of crystalline rocks of vast geological 
antiquity, which gives a special character to this region, and 
which may be designated “the Archean protaxis.” In a 
word, while the region west of the Nile is an elevated table- 
land, that we are now considering is, properly speaking, a 
mountainous tract. (See Fig. 1, p. 14.) 
At Cairo, the lofty terraced banks which form the eastern 
margin of the Nile Valley abruptly change their direction and 
bend round to the east at Jebel Mokattam and thence extend 
in a broken escarpment to Jebel Attaka, already referred to 
as overlooking the Gulf of Suez. These escarpments form 
the northern margin of the Arabian Mountains, which stretch 
southwards, till they ultimately merge into the vast Nubian 
plateau. 
The protaxis, or culminating ridge, of this tract is formed 
by the line of crystalline rocks which ranges from the Wady 
el Arabah in a southerly direction, and therefore parallel to 
the shore of the Gulf of Suez, by Jebel Ghareb (Mt. Agrib), 
by Doukhan (Mons Porphyrites) to Fateereh, and Zobara, 
containing quarries of green breccia marble, quarried by the 
ancient Egyptians. The higher elevations of this range 
attain a height of 6,000 to 7,000 feet above the sea. The 
ridge presents its steepest flank to the eastward, and sends 
off numerous valleys on either side ; those running to the east 
opening out on the sea coast, those im an opposite direction 
opening into the Nile Valley, between lofty terraces or lime- 
stone. The most remarkable of these valleys are the Wady 
Qeneh, which runs along the western base of the ridge for a 
