Appendix Il: Phytogeography and Geology. 1095 
herein dealt with — common salt or sodium chloride, of which there 
are abundant quantities in allmost all strata of the tertiary (eocene) 
formation, constituting the eastern desert, where it occurs both in 
the solid limestone beds, and in the alternative beds of clayey and 
calcareous marl. ‘The winter rains are insufficient to wash away 
the salt from all the outcropping strata: all it can do is to remove 
it from the valley bottoms and gullies by which it runs oft into 
the Nile. It is for this reason that vegetation occours in the district 
only in strips along the dry water-beds. 
Perennial plants are just about half as numerous as the delicate 
annuals. Their existence is in dependent of the fluctuating and variable 
annual winter rains. They shoot anew and blossom even after a 
rainless or all but rainless winter. In marked contrast are the annual 
herbs which depend absolutely upon the rainfall; nor is all rain of 
equal value in promoting their development. For a rich spring 
vegetation of annuals, the rain should fall about the end of February 
and the early part of March, at which time the growing heat of 
the sun is capable of promoting germination. Trees are hardly met 
with im the district. Acacia torsilis, Retama Raeam and some 
Tamarniks occur as trees, with well-formed trunks. 
[. Libyan Desert. No more striking contrast can be imagined 
than that between the intensely cultivated Valley of the Nile and. 
the barren deserts. There are arid wastes in many parts of the 
world — in Australia, in the western States of America, in Asia — 
but in point of desolateness, in the absence of animal and vegetable 
lif, there is probably nothing to rival the greater portion of the 
Libyan desert, on the west side of the Nile. Its barreness is aggressive; 
it is not necessary to travel far to make its acquaintance'). So 
sharp is the junction that, in a single step, one may pass from the 
richly cultivated alluvial soile of the Nile to the bare sandy plains 
which skirt the more rocky interior of the desert. Geographically 
the Libyan Desert is the eastern and most inhospitable portion of 
the Sahara, or Great Desert of Africa. On the north and east its 
boundaries are clearly defined by the Mediterranean Sea and the 
Nile Valley; on the south it is bounded by the Darfur and Kord- 
ofan regions of the Heyptian Sudan. With the exception of a narrow 
belt fringmg the Mediterranean, the region is, to all intents and 
purposes, rainless, the occasional thunderstorms being extremely local, 
and seldom breaking over the same district in two consecutive years. 
The Eeyptian portion of the Libyan desert, is itself divisible 
into three areas having essentially different characters, the northern 
being an undulating nolling country of sandstones, grits, and gravels; 
1) Beadnell: An Egyptian Oasis. — London 1909. 
