370 TRANSACTIONS OF SECTION C. 
(E) In the Gulf of Suez area another factor has come into play. Here sea- 
arms project far inland between land-surfaces subject to desert conditions, and 
their waters become centres of far-reaching chemical activity. Thus coral-reefs 
are changed to dolomites, sea-shells of carbonate of lime to gypsum, hydro- 
carbons are in quantities of economic importance, and mineralised areas of lead 
and zinc ores, of manganese oxide, of iron pyrites, and of sulphur are present 
in the young Tertiary beds which fill these Red Sea depressions. From Suez 
to beyond Halaib, that is, throughout the length of Egypt, gypsum forms a 
conspicuous fringe between the ancient hills and the sea, generally dipping 
gently seaward on the borders of the Red Sea itself. Further north, in the 
Gulf of Suez area, the conditions are more complicated. Dyapir, or piercing 
folds such as have been described by Professor Mrazec in Rumania, are of 
common occurrence, and there is remarkable interplay between the hard and 
soft members of the folded series. 
The surface structure of an arid land is not only the direct reflex of its 
geological structure, but also of former climatic change. Many factors in Egypt 
point to great rainfall in the past, such as gravels of igneous material in the 
Nile Valley far from their source of origin, masses of travertine in the Oases, 
the varying terraces of the Nile Valley itself, the evidence of expansive lakes 
at Kom Ombo, &c. 
Though the main features of a desert land depend on the geological structure 
and in part on past climatic conditions, there are characteristics which are typical 
of all arid regions. These are far removed from the great marine areas and 
from the zone of rainfall dependent upon solar activity in lands beneath the 
tropics. 
These typical desert features have already been referred to, and include :— 
1. The sweeping of all fine material from the surfaces of the plains by the 
action of the wind, and formation of plateau summits. 
2. Intense scouring of these surfaces by wind-driven sand. 
3. The breaking up of the most solid rocks by temperature-variation. 
4. The formation of sand-dunes behind obstructions or where the relief of 
the ground favours their development. 
5. The formation of mushroom-shaped pillars, or standing-out of harder 
materials on bases undercut by the sand. 
6. The formation of sand-worn pebbles of typical angular outlines, the well- 
known Dreikante. 
7. Vermicular markings on limestones, due it may be to etching during the 
movement of evaporating saline solutions. 
8. Formation of desert-crusts by leaching out of the soluble materials con- 
tained in the rocks, with evaporation at the surface, resulting in deposition of - 
the oxides of iron and manganese. Mr. Lucas, Director of the Survey Depart- 
ment Laboratory, has made a special study of these desert and river films, the 
latter probably only differing from desert ones in degree. 
9. Flaking off of surfaces in the surface zone affected by temperature varia- 
tion. Also fracture due to the same cause. Fragments of porphyry, limestone, 
&e., are often split into a series of parallel flakes standing vertically, their 
original connection to one another being clearly indicated by their close juxta- 
position. 
In the half-desert where rain, though brief, is intensely active while it 
lasts, a series of interesting phenomena are presented : deep cafion-like valleys, 
boulder-strewn gullies, saw-back ridges, parallel-dyke country, saline marshes, 
dry waterfalls or steep precipices in the valley-floors, and great talus-slopes. 
Mr. Ferran, in reply to a question asked by Professor A. P. Coleman, 
explained that the slope of the wadis from the watershed towards the Nile was 
about 1 to 1,000 and towards the Red Sea 1 to 200 or 1 to 300, but that the 
slope was of little moment, owing to the sudden rush of storm-water from its 
gathering-ground on the bare mountain-sides. He had not actually observed 
scratchings on rocks because they had not been sought, but he had seen great 
heaps of boulders in unstable equilibrium, which, if overbalanced, could not 
avoid being mutually scratched. He was aware that the scratches on some of 
