310 PROFESSOR E. HULL, LL.D., F.R.S., F.G.8., ON THE 
ments, along the Nile Valley and the northern boundaries of 
the Oases of Chargeh and Dachel. 
From the Oasis of Siuah, in lat. 29° 10’ N., a well- 
pronounced escarpment trends in a N.N.W. direction for a 
distance of 200 miles, as far as Moghara, then bends sharply 
round to the north-west, and stretches away towards the 
coast of the Mediterranean, where it terminates in Ras el 
Kanais. The elevated table-land thus enclosed is represented 
by Zittel as formed of marine strata, consisting of limestones, 
marls, and shales of Miocene age.* To the south of the 
Oasis of Siuah at the “ Echo-Thal,” lacustrine beds set in, 
overlying those of marine age, and apparently extending 
under the vast tract of sand dunes which continues south- 
wards for several hundred miles. Zittel arranges the Libyan 
Desert under three forms: (1) The plateau desert tracts, 
formed of hard stony ground, destitute of vegetation, and 
covered by blocks and fragments of limestone, due to the 
decomposition of the rock; the process of decomposition being 
accelerated by the changes in temperature from day to 
night, which cause the stone to crack and disintegrate in all 
directions. (2) The hollows (Daga) in the horizontal beds 
of limestone, containing small lakes or salt marshes, and 
bounded by mural terraces and escarpments. Of these, the 
most remarkable is the Birket-el-Quertin (or Lake of the 
Horn), lying in the depression of the Fajtim (Faytim) at a 
distance of about 20 miles to the west of the Nile above 
Cairo.t It is about 30 miles in length from west to east, 
bounded on the north by a terraced escarpment of limestone 
cliffs, and its surface, according to Mr. Cope Whitehouse, 
about 150 below the level of the Mediterranean.t Inside 
this basin are recent terraces composed of estuarine materials. 
The lake is fed by the Bahr Jussuf (or Joseph’s Canal), 
which takes its supply from the Ibrahmic Canal, fed by the 
Nile not far from Assiut, and serves to both irrigate and 
* First described by Ehrenberg in 1820, see further on, p. 13. 
+ Schweinfurth, ‘Karte des Depressionsgebetes in Umkreise des 
Fajim,” Zedtschrift d. Gesells. f. Erdkunde, Berlin, 1882. See also Cope 
Whitehouse, Zhe Baar Jusuf, London, 1886. 
{ Schweinfurth makes the surface 40071 métres ; Mr. Cope White- 
house makes it 150 feet below the level of the sea, and the Rayan de- 
pression still deeper, viz., 200 feet below. At the meeting of the British 
Association at Leeds a paper was read by Mr. C. Whitehouse on the 
great works of irrigation and water supply being carried out by utilising 
this great natural depression as a reservoir for the Nile waters when in 
flood. 
