The systematic examination of the geological structure of Egypt, carried out by the Geological Survey during the last thirteen years, has, in conjunction with previous studies, revealed the fact that the surface of the country is composed of very varied materials: limestones, sands, clays, sandstones, granites, schists, etc. The co-ordination of view as to the distribution of these rocks[1] presented by a geological map has further shown that they are not arranged in a random manner, but that certain very definite relations exist between them. If, for instance, the Moqattam hills behind the Citadel be ascended, the lower part of the scarp is found to be composed of white limestone, which is capped by a series of sandy limestones and clays differing alike in colour and in the rapidity with which they are worn away by the streams, due to the rare but destructive winter rains. Finally, the summit is crowned by beds in which boulders of flint and quartz play the most important part, the matrix of sand which bound them together having long been removed by the action of the wind. Similarly, if we were to go southward towards Aswan, a change would be noted in the character and composition of the rocks which form the cliffs bordering the Nile Valley. From Cairo to Qena, limestones predominate; but from thence southward to Esna, clays play a large part at the base of the limestones, and the slipping of the latter over these softer members has given rise to the “tumbled” country which is so conspicuous a feature between Armant and Matana. South of Esna the clays in their turn disappear, while the sandstone which is seen underlying them near Mahamid becomes the dominant constituent of the hills from near Edfu to the neighbourhood of Aswan; at this locality the sandstone itself vanishes, except in so far as it forms isolated caps on the granite, which is the principal rock in the well-known district of the First Cataract. The same lesson as to the order of succession of the rocks in Egypt is forced home if we move from Qena eastward to the Red Sea hills, or south-westward to Kharga Oasis. East of Qena the clays are a conspicuous feature at the base of the outlying limestone hills (Abu Had, etc.), and in their turn rest on sandstone, which forms striking plateaus seamed by deep ravines giving entry to the heart of the Red Sea hills. On traversing these gorges, a confused hill-country of granite (worn into boulders on the surface), or dark-green schists is entered, on whose summits the sandstone occurs as isolated outliers near the main sandstone mass, but to the east disappears altogether.