Appendix Il: Phytogeography and Geology. 1089 
mud, and now if meanders on the flood-plain which it has formed. 
In earlier times side channels followed the lower margins of the 
valleys, and lagoons and swamps existed in the same part of the 
valley, but now owing to controlled irrigation such parts have been 
reclaimed and former water channels such as the Sohagia Canal, 
and the Bahr Yusuf are to-day supply-canals which irrigate the 
marginal portions of the valley. For the past fifty centuries at 
least the Nile has been depositing in this reach, and the average 
rise of the bed due to this is about 0,10 metre per century, so that 
some 5 metres of alluvial mud have been laid down in historical 
times. The needs of agriculture, and the requirements of a dense 
population have produced a strict control of the river so that the | 
water of the low stage supply may be used as economically as 
possible, and the turbid water of the flood spread as widely as 
possible in order to deposit its sediment on the cultivated lands. 
The river therefore is more of the nature of a great supply canal 
than a stream free to meander through its flood-plain. Similar 
control of the water and consequent reclamation of the land have 
diminished the lake which once occupied the depression of the Faytim, 
until now a small and rapidly shrinking lake alone remains. 
Only a few species are characteristic to this subregion, some 
of them are from Tropical Africa and Asia (+). 
+* Nasturtium niloticum. + Ammania attenuata. 
Brassica bracteolata. + Vahlia viscosa. 
+ Polygala erioptera. | ++ Campanula dimorphantha. 
Spergularia atheniensis. + Leptadenia heterophylla. 
+ Bergia ammanioides. Cuscuta monogyna. 
Hig »  suffruticosa. + Heliotropium pallens. 
+ Hibiscus verrucosus. +* Striga hermonthica. 
+ Corchorus tridens. Plantago exigua. 
+ Cissus digitata. + Panicum Petiveri. 
Lupinus angustifolius. + Schoenefeldia gracilis. 
Astragalus falcinellus. + Eragrostis nutans. 
+ Acacia laeta. 
III. Oases of the Libyan Desert. 
The chief oases of the Libyan desert Siwa, Little Oasis, Dakhel, 
Farafra, Great Oasis, occupy extensive depressions cut down through 
the horizontal Hocene strata (with the exception that Dakhel is almost 
entirely cut in Cretaceous strata) to the underlying saddle of Creta- 
ceous rocks; some of the more porous beds of the latter are water- 
bearing and from them, either through natural passages or through 
artificial borings, the water rises to the surface, often under con- 
siderable pressure. The floor level varies considerably but the culti- 
Muschler, Manual Flora of Egypt. 69 
