Appendix IL: Phytogeography and Geology. 1097 
lat to the Southern limits has been crossed at various points by 
many travellers during the present century'). The constitution of 
the surface of this desert-district naturally depends upon the geological 
formation. In the west, towards the Nile valley, we have tertiary 
nummulite limestone; here the desert has the character of a plateau- 
desert, like that of the Libyan desert, of which it forms merely a 
continuation, separated by the Nile valley. This formation is follo- 
wed on the east by a sandstone, which appears to belong to the 
‘Nubian sandstone’, one of the latest members of the. tertiary for- 
mation. The middle, the heart of the mountain system, is occupied 
mainly by dullooking primary rock, consisting of diorites (greenstones), 
diorite-breccias, and black or green-stone porphyries; with these are 
often intermingled very beautiful red-coloured granites and porphyries, 
and massive highly-coloured veins and lodes everywhere permeate the 
dark rock. The chief masses, those on which the others, so to speak, rest, 
are mainly composed of such granite, gneiss being less common. They 
rise to a height of 400 feet. These rocks are nowhere covered, as in other 
countries, with a layer of humus; but the geologist is not allowed to 
behold Earth in all her nakedness, since the superficial layer is 
generally traversed to such an extent with fissures, often of consi- 
derable depth, that it is not easy to break off a fragment the size 
of the fist showing a fresh fracture on all sides, while in ascending 
a mountain, from the crumbling of the surface, a firm footing cannot 
be obtained. In other districts, where much rain falls, this disinte- 
grated rough-casting is washed away; here it remains, and the 
whole of the mountains look as if burned by the sun. The rocks 
in some ravines, where there are permanent waterfalls, do not show 
these fissures; they are firm, hard, and smooth as marble, since the 
water can take effect here. 
About 80 species (given in the following list) belongs only to 
the northern part (D. a. sept.) of the Arabic desert and 10 of these 
are endemic. Those which are known from Sinai are marked by 
a +; an asterisk means the species is common in the district. 
+ Papaver Decaisnei. Helianthemum Sancti Antoni. 
Hypecoum pendulum. Dianthus Guessfeldtianus. 
Sisymbrium erysimoides. * Gypsophila Rokejeka. 
Leptaleum: filifolium. Silene Hussoni. 
+ Isatis microcarpa. Alsine picta. 
+ Schimpera arabica. + Paronychia sinaica. 
+ Moricandia sinaica. + Telephium sphaerospermum, 
Reseda Boissieri. * Reaumuria hirtella. 
+ + pruinosa. Fagonia latifolia. 
Helianthemum niloticum. Rhus Oxyacantha. 
1) Barron and Hume: Topography and romney of the Hastern Desert 
of Egypt. — Cairo 1902. 
