The delta region is swampy, and forms, for a distance of from 40 to 70 m. inland, a network of interlacing creeks and broad sluggish channels fringed with monotonous mangrove forests. The main rivers are navigable for ocean-going steamers for a distance of from 15 to 40 m. from their mouths. Beyond the delta firm ground takes the place of mud and the mangroves disappear. The land rises gradually at first, becoming, however, in many districts very hilly, and is covered with dense forests. The Niger at its confluence with the Benue is not more than 250 ft. above the sea. North of this point are hills forming the walls of the plateau which extends over the centre of the protectorate and is part of the great plateau of North Africa. This plateau, broken only by the valleys of the rivers, does not attain an elevation approaching that of the plateaus of the southern half of the continent, the culminating point (apart from particular mountain districts), situated in about 10° N., reaching a height of 3000 ft. only. The valleys of the Niger and Benue, especially the latter, are very much lower, the town of Yola on the Benue, some 400 m. inland, lying at an altitude of little over 600 ft. The surface is generally undulating, with isolated “table mountains” of granite and sandstone often rising abruptly from the plain. It is clothed largely with thin forest, but becomes more open to the north until, near the French frontier, the arid steppes bordering the Sahara are reached. Much of the country north of Zaria (11° N.) is covered with heavy loose sand. The most mountainous districts are northern Bauchi (a little north of 10°), where heights of 6000 to 7000 ft. occur; parts of Muri, along the north bank of the Benue; and the southern border of the Benue basin, where the hills (consisting of ironstone, quartz and granite) appear rich in minerals. The mountainous area covers some 50,000 sq. m. On the east the plateau sinks to the plains of Bornu (q.v.), which extend to Lake Chad. Tributaries of the Niger traverse the western portion of the country, the most noteworthy being the Gulbin Kebbi or Sokoto river and the Kaduna, which flows through a valley not more than 500 ft. above the sea. The north-eastern part of the country drains to Lake Chad by the Waube or Yo, an intermittent stream, which in its lower course forms the Anglo-French boundary. The western portion of Lake Chad (q.v.) belongs to the protectorate, which contains no other large lake. The water parting between the Chad and Niger systems runs N.W. and S.E. from about Katsena in 13° N. to the Bauchi hills. Of the tributaries of the Benue the most important is the Gongola. During the dry season most of the small rivers cease running and the water in the larger streams is low. The great rise of the Niger within the protectorate takes place in August and September and there is a second rise about the beginning of the year.
Geology.—The fundamental formation consists of crystalline rocks. From the edge of the coast belt to near the confluence of the Benue and Niger they are overlain by unfossiliferous sandstones, lying undisturbed and possibly of the age of the sandstones of the Congo basin. Limestones, with fossils indicating a Tertiary age, have been found near Sokoto. Superficial deposits occupy the coast belt. Recent alluvium and a thick deposit of black earth border the upper reaches of the Benue and cover wide areas, around Lake Chad.
Climate.—The country lies wholly within the tropics. The climate of the coast-lands is moist and hot, and extremely unhealthy, malarial fever being prevalent and deadly. The annual rainfall in the delta regions varies between 100 and 140 or more inches; the mean temperature is over 80° F. The heat does not vary greatly, rarely
