The sources of water comprise _galts_ (rock basins forming rain water reservoirs in the mountains), springs and wells. Of these, _galts_ yield the purest water, and form the principal supply of the pastoral Arabs, except in years of no rainfall, while springs and wells, the latter usually rude excavations in the alluvia of wadis, are most used by travellers from their easier accessibility. Small _galts_ are called _megal_ (Ababda) or _megwel_ (Bisharin); they are frequently accompanied by small springs. The small scale map on Plate III, which shows all sources known to exist within the area, gives a good idea of the distribution of water supplies. Water is scarce in the western parts of the country, Bir Abu Hashim being the only source within a radius of about sixty kilometres of itself, but _galts_ and wells are fairly frequent among the mountainous tracts further east. The springs of Abraq and Abu Saafa are the most important sources in the central area. The portion of the country under the Sudan Administration is far richer in water that the Egyptian part, containing numerous fine wells and springs, such as Birs Meneiga, Abu Hodeid, Akwamtra, and Frukit. The coast plain is waterless, except for salty wells near the sea. Water can usually be obtained at intervals of about two or three days when on the march, and in some parts much more frequently. The water of certain wells and springs has a purgative effect due to the absorption of magnesium salts, especially after a long interval without rain. That of others, again, such as Muelih and Shalatein, is so salty as to be only drinkable by camels. =Geology.[45]= The rocks composing this part of Egypt are principally igneous and metamorphic deposits of very ancient origin. Granite is most prominent in the Nugrus, Faraid, and Elba areas, schists and diorites cover a large portion of the remaining country, and huge masses of serpentines form the mountains of Abu Dahr, Korabkansi, and Gerf. Of sedimentary rocks, plateaux of Nubian sandstone (Cretaceous) cover large areas round Bir Abraq and westward of it, while a narrow belt of the same rock also occurs along the sea coast in the north part of the district. Gypseous limestones (Miocene?) form the hills of Ras Benas and occur along the coast north of Wadi Lahami, as well as in small areas near the coast further south in the neighbourhood of Halaib. PLATE III. [Illustration: SKETCH-MAP OF SOUTH-EASTERN EGYPT. Showing the Water-sources and the Roads connecting them. Ball. Geography & Geology of South-Eastern Egypt. Photo-Metal-Process. Survey Dept. Cairo 1910. (60-190) Scale 1:2,000,000.]