Colony and British protectorate of West Africa. The colony is formed by the city of Lagos and the territory that immediately surrounds it (surface: 3535 sq. Km. With 325.020 residents in 1931); the protectorates include Southern Nigeria (sup.: 229,158 sq km; 8,168,227 residents in 1931) and Northern Nigeria (sup.: 721,351 sq km; 11,434,924 residents in 1931). For administrative reasons, the territory that constitutes the mandate of Cameroon was annexed to Nigeria (sup.: 87,644 sq km; estimated population 774,585 residents).
According to VAULTEDWATCHES.COM, Nigeria (with Cameroon) includes the coastline from some distance W of Lagos to the eastern side of Mount Cameroon and the lower Niger basin together with most of the basins of its left tributaries: Sokoto (or Gulbin Kebi), Kaduna and Benué. Towards the NE. Nigeria reaches the western shore of Lake Chad.
The deltas of Niger and minor watercourses form almost the entire coast, except to the west and east, where some estuaries flow, including the main one of Calabar which forms the mouth of the Cross River. The Niger Delta is a low-lying region, all with water channels, covered by an immense mangrove forest, sometimes 350 km wide.
The structure of West Africa, like most of the continent, is formed by a large ancient base that contains gneiss of an age comparable to that of the most ancient European rocks, uncertain, but perhaps also of the pre-Paleozoic, and large deposits of unaltered sandstone, also generally attributed to the pre-Paleozoic. The absence of rocks referable to the beginning of the Paleozoic has given rise to the hypothesis that in that ancient era Africa was already part of a large continent, and everything reveals the long period of denudation it suffered. The most ancient datable rocks belong to the upper Cretaceous and show that this period was for this region, as for many other parts of Africa and for Europe, a period of submersion. In the Tertiary sector, Africa, both eastern and western, it suffered major fractures; Recent research in East Africa has shown that such fractures are, for a considerable part, very recent (Pleistocene) and have produced major changes in hydrography. In the case of Nigeria it is well known that the heights ending towards S. with the Cameroon volcano (4075 meters) and the islands of Fernando Poo, S. Thomé and Principe, indicate widely extended fracture lines in Nigeria and S. with outflow of lava which in some cases still continues. It is probable that this fracture determined the main morphological aspects and produced a deeply depressed area in the West of Cameroon; this would have determined the orientation on this side of the higher courses of Niger and Benué. For a considerable stretch of their course these two rivers flow through regions formed by layers of the Cretaceous or Tertiary, which are sometimes displaced and sometimes cover it. These layers are sometimes crossed by igneous rocks and covered by lava currents, such as, p. eg, on the Bauchi plateau in Nigeria of the lower Benué. This fact also indicates the recent age of the fractures, which have contributed so much to the current topography.
Upper Niger, which has a direction from the SW. NE. in French West Africa, it flows, as soon as it leaves the hills from which it originates, in a wide valley; then to E. and SE. and downstream of Timbuktu, the bed narrows and is interrupted by a series of rapids, between Ansongo and Say, in French territory, and between Bussa and Rabba in northern Nigeria. Such rapids, or at least the latter, are connected with fairly recent fiery intrusions. The river continues in a rather sharply cut valley in the sandstone; valley that extends towards S. almost to the border between northern and southern Nigeria. Just before reaching that border, the Niger receives the Benué and the confluence forms a lake-like expansion before the river narrows again. In southern Nigeria it flows through flat land and downstream from Onitsha, 130 km. to S. of the border, immense regions remain submerged during the floods.
This virtually constitutes the start of the delta. The change in appearance and direction that can be seen between the SW-oriented part of the course. NE. and the next part oriented from NO. to SE. with long series of rapids, it is the result of the capture of the upper course by the lower one; it was able to prevail over orogenic changes which determined a topography that considerably modified the primitive surface.
The Benué, mostly navigable, flows W through northern Nigeria in a wide valley; but its lower course is approached by heights of tertiary rocks superimposed on the cretaceous deposits and the ancient base.
It is clear from this that the Benué-lower Niger basin as a whole forms a vast depression with some regions rising to more than 200 msm A Nigeria of Benué and on both sides of the Niger upstream of the confluence, most of the territory it exceeds 200 meters above sea level, except in the vicinity of the river; but the Niger valley no longer reaches that height after passing Ansongo. At Nigeria del Benué and E. of the Gulbin Kebi valley is the vast plateau of Bauchi with recent igneous intrusions and lava currents that make their way through the ancient terrains. They penetrate into the Cretaceous period in some points close to Benué and thus demonstrate their relatively recent age. The plateau of Bauchi, mainly due to such intrusions and lava currents, reaches a height of over 1300 m. in the vicinity of Bukuru and in the remainder it is for the most part more than 500 meters above sea level. In several points of the plateau there are residual hills. Towards the Nigeria the surface is partly covered with sand.