AFRICA - GEOLOGY Africa has a simpler geological structure than any other continent, although it has many unique features and forma tions. It consists essentially of a plateau of which the rocks have not been bent into steep folds by compression since primaeval times, except for a strip in Cape Colony, which was part of an ancient southern continent, and for the Atlas Mountains, which are geologically and biologically European. The continent be tween these remote chains of fold mountains has undergone no close folding, and most of it has remained above sea level since the time of the oldest known fossils. The margins have been re peatedly reached by the sea, which has submerged the coastal plains and occasionally extended inland up the valleys. The pla teau has been fractured and cleft by earth movements which oc casioned widespread volcanic eruptions; but the plateau surface has remained a land area on which plants and animals have evolved continuously, subject to the stimulus of changing climate, but never interrupted by submergence. Africa is part of an an cient continent which included most of South America, India and Australia. This southern continent, Gondwanaland, was separated from the contemporary northern land of Eurasia, except for an occasional isthmus, by a sea, the Tethys, which extended from central America through the Mediterranean and across southern Asia to the Eastern Archipelago. The range of this southern con tinent is best established in Carboniferous times; by the Jurassic its disruption had begun by subsidences which formed the Atlantic and Indian oceans, introduced great changes in its biological rela tions, and repeatedly caused widespread volcanic eruptions.
The foundation of the continent consists of a vast slab of rocks belonging to the Primaeval or Pampalaeozoic era. They are exposed over about one-third of the continent. They outcrop from Cape Town to southern Egypt, and from the Gold Coast on the west to Somaliland, the eastern horn of Africa. Most prog ress in the classification of these rocks has been made in South Africa, where they have great mineral wealth. They are divided into three main divisions. The lowest, the Swaziland system, con sists of gneisses and coarse schists; in addition to the typical areas in South Africa, it forms most of Africa, including Nubia, the highlands along the eastern and Atlantic coasts, and the Sudan from Abyssinia to Nigeria and the Gold Coast. This division corresponds to the old gneiss of other lands, such as the Lauren tian of Canada and the Lewisian of Britain.
The second division consists of sedimentary rocks, including quartzites, limestones, dolomites and slaty schists, with vast sheets of volcanic rocks. They are classified in South Africa as the Wit watersrand, Ventersdorp and Transvaal systems. Some of the rocks are little altered that they have been expected to yield fossils ; but none has been found and the Orthoceras, said to prove the Ordovician age of the Otavi dolomite, was based on cylindrical concretions. Some of these beds were regarded as Lower Palaeo zoic ; but they are probably earlier owing to the absence of fossils, and their resemblance to the pre-Cambrian rocks of other conti nents. The representatives of this division include important cop per, lead and zinc mining fields in northern Rhodesia and the Congo basin, the goldfield of Kilo in the eastern Belgian Congo, the goldfields of the Gold Coast Colony and the less important gold occurrences that have been discovered in Tanganyika Terri tory and Kenya Colony.
The third division includes sandstones that are so little altered that they have been persistently regarded as the inland continua tion of the Devonian red sandstones of the coast at Cape Town. The chief member is the Waterberg sandstone of the northern Transvaal, which is probably represented in Uganda by part of the Karagwe series (most of which is referable to the second division), and in Angola by the Oendolongo series. These sand stones are probably the equivalent of the Torridonian of Britain and the Keweenawan of the United States and Canada.
The Palaeozoic era began with apparently the whole of Africa as part of a continent, for the only Cambrian beds known in it are in Morocco near Tetuan, though the sea came close to the eastern coast in Sinai. No certain Ordovician rocks are known, though they have been doubtfully recorded from the Atlas moun tains. Many beds in Africa have been called Ordovician and Silu rian on inadequate evidence, but some Silurian beds with grapto lites occur in the central Sahara.
The Devonian is the oldest African system with widespread fossiliferous beds. It includes the Bokkeveld beds in Cape Colony, and has a wide extent in the Sahara, Libya and the western Sudan, and it reaches the Atlantic on the Gold Coast ; marine representa tives occur of the lower, middle and upper divisions. Marine Car boniferous rocks occur to the north of the Devonian beds in Cape Colony and limestones with crinoids and Productus show that a contemporary sea covered part of Egypt. The Lower and Middle Carboniferous are widely developed in the northern and north western Sahara, and in south-western Tripoli ; and there are small outcrops by the Gulf of Suez. The Carboniferous land deposits are extensive. The lowest part of the Karroo system of southern Africa is Upper Carboniferous and includes the glacial Dwyka Conglomerate and rich coal seams. Coal is also found in southern Rhodesia, Tanganyika Territory and Lake Nyasa. The glacial deposits of this period extend northward into the Congo basin and southern Madagascar. The later divisions of the Karroo sys tem range from the Permian to the Lower Lias and are represented in equatorial Africa they have yielded many remarkable fossil reptiles. The Permian beds (Lower Beaufort) are represented by shales of freshwater origin with Palaeanodonta on the Sabaki in Kenya Colony, and the Trias (Upper Beaufort) by the Tanga beds and part of the Buruma sandstone, of which the fossil Dadoxylon forest may be the equivalent of that of the Molteno beds of South Africa. In the Congo there are large areas of sand stones, the Lualaba and Lubilache series, which, judging from their sparse fossils, are Permian and Trias.
There is no established marine Trias in Africa, except for a small development in southern Tunis and along the Atlas of beds like those in Andalusia; though the sea lay a little north in the Mediterranean, most of the Triassic beds of Algeria and Tunis are of the continental type, like those of England and Germany. The breaking up of Gondwanaland had nevertheless probably be gun in the Trias, and in the next period, the Jurassic, marine beds were deposited along parts of the east African coast. An arm of the sea then ran up the Mozambique channel between Mada gascar and the mainland. The eastern part of Madagascar consists of the primaeval gneisses and schists, on which rest some Karroo beds; the western slope consists of marine Jurassic, Cretaceous and Kainozoic beds, with some volcanic rocks. The Jurassic beds of Madagascar and Tanganyika Territory range from the Lias to the end of the system ; further north, in Kenya Colony and Somal iland, the horizons established represent from the Bathonian to the Tithonian. In Tanganyika Territory the Upper Jurassic beds (Corallian to Tithonian) in Tenduguru are famous for their fossil reptiles, of which Gigantosaurus is the largest known animal.
The advance of the sea during the Jurassic received in East Africa a check in the Cretaceous; for though a Neocomian gulf ran northward from Uitenhage in Cape Colony past Madagascar, the lowest Cretaceous series is barely represented in Kenya and its occurrence in Abyssinia is probably due to a gulf from the Mediterranean. The widespread Cretaceous rock of eastern Af rica is the Nubian sandstone (possibly partly Upper Jurassic), a land deposit which extends from Egypt to Abyssinia and Somali land. In Egypt an arm of the Mediterranean sea deposited Creta ceous beds, which represent all the series from the Lower Ceno manian to the Danian.
A Cretaceous strait in the Upper Albian and Senonian extended from the Mediterranean across the Sahara, past Lake Chad and along the Benue valley as far south as Angola. The Cretaceous in Africa is best represented in Tunis, Algeria and Morocco. At the beginning of the period the sea covered a narrow strip of land from the coast to just south of Tunis and Oran; but by the Middle and Upper Cretaceous compact limestones were deposited over much of northern Algeria, and the Cenomanian lagoons lay along the front of the Sahara.
In the same region the Cretaceous sea was succeeded directly by that of the Lower Eocene, wherein were deposited the phos phatic limestones that are the most valuable mineral deposit of northern Africa. A still wider submergence in the Middle Eocene formed the Nummulitic limestone, which ranges across northern Africa from Cairo through Cyrenaica and along the Atlas Moun tains to Morocco; and a gulf southward extended on to some of the lower parts of the Sahara. A strait from this gulf continued to the Niger valley and covered the district of Sokoto, and reached the Gulf of Guinea in southern Nigeria and the Cameroons. The Oligocene saw a general retreat of the sea from northern Africa, but beds with small Nummulites were deposited in bays. The freshwater lakes of the Oligocene have yielded at Fayum the f os sils which revealed to C. W. Andrews the story of the origin of the elephants from the primitive Moeritherium.
The Miocene sea along the Atlas was restricted to a strait from Oran to central Tunis, which was one of the passages that connected the Atlantic and the western Mediterranean. A renewed uplift at the end of the Miocene of the channel reduced these straits, though that south of the Riff in Morocco was apparently still occupied by the sea. In other parts of Africa the marine Cainozoic deposits are scanty. The sea occasionally reached the tropical coasts. A full series of Eocene limestones occur in Somaliland, with also the Upper Oligocene. Eocene and Miocene deposits occur both on the coasts of Kenya Colony and in Tanganyika Territory at Lindi; Miocene limestone occurs in the island of Pemba. The marine Pliocene of the Mediterranean is represented in Algeria and Egypt, while marine Pliocene deposits allied to those of the Persian Gulf also occur, notably in Zanzibar island and at Mombasa.
Africa, therefore, has a long stable development, which was occasionally interrupted by volcanic outbreaks connected with the fractures made by the foundering of the adjacent ocean basins. The eastern half of Africa appears to have been slowly upraised during the Mesozoic into a broad, flat arch contemporary with the formation of a great sea-filled trough along the East African coast. The wider subsidence that formed the Indian Ocean led to the giving way of the eastern side of the arch, and the Infall of its roof formed the Great Rift valley, which extends from Mo zambique throughout eastern Africa and along the Red Sea to the Jordan valley. The fractures connected with its formation were accompanied by great volcanic eruptions, and the Kapitan phono lites of Kenya Colony are probably contemporary with the Dec can traps of north-western India. The earth movements con nected with the formation of the Great Rift valley lasted from the Oligocene to the Pleistocene, and they were accompanied by repeated volcanic eruptions. They built up the two highest of African mountains—Kilima Njaro, which still has a crater upon its summit, and Kenya, which is the denuded neck of an old volcano. Subsidences around volcanic centres formed great caul drons, such as Ngorongoro in Tanganyika and Menengai in Kenya Colony. Volcanoes along the fractures connected with the forma tion of the Atlantic produced the shonkinite vent at Chieuca in Angola, the volcano of the Cameroons, which is still active, while eruptions along a fracture parallel to the coast formed the islands of the Guinea Gulf, Fernando Po, San Thome, Principe and Annobon. The deepest subsidences along the Rift valley in Africa formed the lake basins of Tanganyika, Nyassa, the Albert Nyanza and Lake Rudolf.
The economic geology of Africa is connected mainly with its rich mineral fields, especially of gold and diamonds in South Africa, southern Rhodesia and the Gold Coast ; of gold at Kilo ; of diamonds, copper, zinc and lead in Northen Rhodesia and Katanga. Tin and platinum are found in the Transvaal, and chromium in southern Rhodesia. The most important phosphate deposits are along the Atlas Mountains. The only productive African oil-field is on the coast of the Gulf of Suez; oil-shale is known in South Africa, Nigeria and elsewhere; rich coal-fields are worked in the Transvaal, Natal and Southern Rhodesia; and coal also occurs in Tanganyika Territory, Nyassaland and Nigeria. Iron ores are widespread, as are also deposits of bauxite and manganese, which are most extensively worked on the Gold Coast. The chief African mica mines are in Tanganyika Territory.