7. =The Geology of the Nile Valley.=--South of Gondokoro along the Victoria and Albert Niles, and at the lakes, the rocks are generally granites, crystalline schists and quartzites. The hills of Uganda are covered with red clay and marl on the higher lands, while the valleys consist of a rich black loam. All the cataracts are granites and granitic rocks or diorites. The Ruenzori range consists of lofty volcanoes. The surface of the ground is covered with a fine Kankar (nodulated limestone) in many places. North of Gondokoro the plains are formed of sandy deposits mixed with coarse peat in places. The hills of the Bahr-el-Gazelle and Arab river are all crystalline. Abyssinia is a volcanic plateau. It is the detritus of this rich volcanic soil swept down by the Blue Nile and Atbara which constitutes the richness of the soil of Egypt and of the water of the Nile. Those parts of the eastern Sudan south of Khartoum and El-Damer and at Kassala, which are the deltas of the Blue Nile, the Dinder, the Rahad, the Atbara, and the Gaäsh, are possessed of a soil in every sense similar to that of Egypt itself. At Khartoum and in the bed of the Blue Nile at Kamlin are extensive deposits of nodular limestone corresponding to the Kankars of India. The main Nile from Khartoum to Assuân flows between low hills and tables of Nubian sandstone overlying crystalline rocks of gneiss, mica schists, hornblendic granite and red granite. Where the crystalline rocks come to the surface we have cataracts; where the Nubian sandstone is at the surface we have reaches of unbroken water. From Assuân[4] to near Edfu the Nile flows between hills of Nubian sandstone, the best known of which is Gebel Silsila. From Edfu to near Luxor, the Nubian sandstone which overlies the crystalline rocks dips under the Nile and its place along the Nile Valley is taken by green and grey clays containing nitrate and phosphate deposits. The former are inexhaustible and have constituted the manure of this part of the valley for thousands of years. With these deposits are thick banks of soft white limestone. [4] Condensed from a description of the geology of the Nile Valley in Egypt written by Capt. Lyons for the second edition of “Egyptian Irrigation”. From Luxor northwards the clays dip under the Nile and the Nile Valley is bounded by the superposed white eocene limestone up to Cairo. The Nubian sandstone is always soft and porous. The limestone is generally soft, though hard siliceous beds are sometimes met with. North of Cairo there is no building stone of any value except the siliceous sandstone of Gebel Ahmar near Cairo and the basalt of Abu Zabel, a recent outcrop furnishing a black rock of great durability. The area covered by this rock is small.