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Африка (Eng.) - (реферат)Африка (Eng.) - (реферат)Дата добавления: март 2006г. AFRICA AFRICA, the name of a continent representing the largest of the three great southward projections from the main mass of the earth's surface. It includes within its remarkably regular outline an area, according to the most recent computations, of 11, 262, 000 sq. m. , excluding the islands. 1 Separated from Europe by the Mediterranean Sea, it is joined to Asia at its N. E. extremity by the Isthmus of Suez, 80 m. wide. From the most northerly point, Ras ben Sakka, a little west of Cape Blanc, in 37 deg. 21' N. , to the most southerly point, Cape Agulhas, 34 deg. 51' 15'' S. , is a distance approximately of 5000 m. ; from Cape Verde, 17 deg. 33' 22'' W. , the westernmost point, to Ras Hafun, 51 deg. 27' 52'' E. , the most easterly projection, is a distance (also approximately) of 4600 m. The length of coast-line is 16, 100 m. and the absence of deep indentations of the shore is shown by the fact that Europe, which covers only 3, 760, 000 sq. m. , has a coast-line of 19, 800 m. The main structural lines of the continent show both the east-to-west direction characteristic, at least in the eastern hemisphere, of the more northern parts of the world, and the north-to-south direction seen in the southern peninsulas. Africa is thus composed of two segments at right angles, the northern running from east to west, the southern from north to south, the subordinate lines corresponding in the main to these two directions. Main Geographical Features. —The mean elevation of the continent approximates closely to 2000 ft. , which is roughly the elevation of both North and South America, but is considerably less than that of Asia (3117 ft. ). In contrast with the other continents it is marked by the comparatively small area both of very high and of very low ground, lands under 600 ft. occupying an unusually small part of the surface; while not only are the highest elevations inferior to those of Asia and South America, but the area of land over 10, 000 ft. is also quite insignificant, being represented almost entirely by individual peaks and mountain ranges. Moderately elevated tablelands are thus the characteristic feature of the continent, though the surface of these is broken by higher peaks and ridges. (So prevalent are these isolated peaks and ridges that a special term [Inselberg-landschaft] has been adopted in Germany to describe this kind of country, which is thought to be in great part the result of wind action. ) As a general rule, the higher tablelands lie to the east and south, while a progressive diminution in altitude towards the west and north is observable. Apart from the lowlands and the Atlas range, the continent may be divided into two regions of higher and lower plateaus, the dividing line (somewhat concave to the north-west) running from the middle of the Red Sea to about 6 deg. S. on the west coast. We thus obtain the following four main divisions of the continent: —-(1) The coast plains—-often fringed seawards by mangrove swamps—never stretching far from the coast, except on the lower courses of streams. Recent alluvial flats are found chiefly in the delta of the more important rivers. Elsewhere the coast lowlands merely form the lowest steps of the system of terraces which constitutes the ascent to the inner plateaus. (2) The Atlas range, which, orographically, is distinct from the rest of the continent, being unconnected with any other area of high ground, and separated from the rest of the continent on the south by a depressed and desert area (the Sahara), in places below sea-level. (3) The high southern and eastern plateaus, rarely falling below 2000 ft. , and having a mean elevation of about 3500 ft. (4) The north and west African plains, bordered and traversed by bands of higher ground, but generally below 2000 ft. This division includes the great desert of the Sahara. The third and fourth divisions may be again subdivided. Thus the high plateaus include: —(a) The South African plateau as far as about 12 deg. S. , bounded east, west and south by bands of high ground which fall steeply to the coasts. On this account South Africa has a general resemblance to an inverted saucer. Due south the plateau rim is formed by three parallel steps with level ground between them. The largest of these level areas, the Great Karroo, is a dry, barren region, and a large tract of the plateau proper is of a still more arid character and is known as the Kalahari Desert. The South African plateau is connected towards the north-east with (b) the East African plateau, with probably a slightly greater average elevation, and marked by some distinct features. It is formed by a widening out of the eastern axis of high ground, which becomes subdivided into a number of zones running north and south and consisting in turn of ranges, tablelands and depressions. The most striking feature is the existence of two great lines of depression, due largely to the subsidence of whole segments of the earth's crust, the lowest parts of which are occupied by vast lakes. Towards the south the two lines converge and give place to one great valley (occupied by Lake Nyasa), the southern part of which is less distinctly due to rifting and subsidence than the rest of the system. Farther north the western depression, sometimes known as the Central African trough or Albertine rift-valley, is occupied for more than half its length by water, forming the four lakes of Tanganyika, Kivu, Albert Edward and Albert, the first-named over 400 m. long and the longest freshwater lake in the world. Associated with these great valleys are a number of volcanic peaks, the greatest of which occur on a meridional line east of the eastern trough. The eastern depression, known as the East African trough or rift-valley, contains much smaller lakes, many of them brackish and without outlet, the only one comparable to those of the western trough being Lake Rudolf or Basso Norok. At no great distance east of this rift-valley are Kilimanjaro—with its two peaks Kibo and Mawenzi, the former 19, 321 ft. , and the culminating point of the whole continent—and Kenya (17, 007 ft. ). Hardly less important is the Ruwenzori range (over 16, 600 ft. ), which lies east of the western trough. Other volcanic peaks rise from the floor of the valleys, some of the Kirunga (Mfumbiro) group, north of Lake Kivu, being still partially active. (c) The third division of the higher region of Africa is formed by the Abyssinian highlands, a rugged mass of mountains forming the largest continuous area of its altitude in the whole continent, little of its surface falling below 5000 ft. , while the summits reach heights of 15, 000 to 16, 000 ft. This block of country lies just west of the line of the great East African trough, the northern continuation of which passes along its eastern escarpment as it runs up to join the Red Sea. There is, however, in the centre a circular basin occupied by Lake Tsana. Both in the east and west of the continent the bordering highlands are continued as strips of plateau parallel to the coast, the Abyssinian mountains being continued northwards along the Red Sea coast by a series of ridges reaching in places a height of 7000 ft. In the west the zone of high land is broader but somewhat lower. The most mountainous districts lie inland from the head of the Gulf of Guinea (Adamawa, &c. ), where heights of 6000 to 8000 ft. are reached. Exactly at the head of the gulf the great peak of the Cameroon, on a line of Volcanic action continued by the islands to the south-west, has a height of 13, 370 ft. , while Clarence Peak, in Fernando Po, the first of the line of islands, rises to over 9000. Towards the extreme west the Futa Jallon highlands form an important diverging point of rivers, but beyond this, as far as the Atlas chain, the elevated rim of the continent is almost wanting. The area between the east and west coast highlands, which north of 17 deg. N. is mainly desert, is divided into separate basins by other bands of high ground, one of which runs nearly centrally through North Africa in a line corresponding roughly with the curved axis of the continent as a whole. The best marked of the basins so formed (the Congo basin) occupies a circular area bisected by the equator, once probably the site of an inland sea. The arid region, the Sahara—the largest desert in the world, covering 3, 500, 000 sq. m. —extends from the Atlantic to the Red Sea. Though generally of slight elevation it contains mountain ranges with peaks rising to 8000 ft. Bordered N. W. by the Atlas range, to the N. E. a rocky plateau separates it from the Mediterranean; this plateau gives place at the extreme east to the delta of the Nile. That river (see below) pierces the desert without modifying its character. The Atlas range, the north-westerly part of the continent, between its seaward and landward heights encloses elevated steppes in places 100 m. broad. From the inner slopes of the plateau numerous wadis take a direction towards the Sahara. The greater part of that now desert region is, indeed, furrowed by old water-channels. The Hydrographic Systems. —-From the outer margin of the African plateaus a large number of streams run to the sea with comparatively short courses, while the larger rivers flow for long distances on the interior highlands before breaking through the outer ranges. The main drainage of the continent is to the north and west, or towards the basin of the Atlantic Ocean. The high lake plateau of East Africa contains the head-waters of the Nile and Congo: the former the longest, the latter the largest river of the continent. The upper Nile receives its chief supplies from the mountainous region adjoining the Central African trough in the neighbourhood of the equator. Thence streams pour east to the Victoria Nyanza, the largest African lake (covering over 26, 000 sq. m. ), and west and north to the Albert Edward and Albert Nyanzas, to the latter of which the effluents of the other two lakes add their waters. Issuing from it the Nile flows north, and between 7 deg. and 10 deg. N. traverses a vast marshy level during which its course is liable to blocking by floating vegetation. After receiving the Bahr-el-Ghazal from the west and the Sobat, Blue Nile and Atbara from the Abyssinian highlands (the chief gathering ground of the flood-water), it crosses the great desert and enters the Mediterranean by a vast delta. The most remote head-stream of the Congo is the Chambezi, which flows south-west into the marshy Lake Bangweulu. From this lake issues the Congo, known in its upper course by various names. Flowing first south, it afterwards turns north through Lake Mweru and descends to the forest-clad basin of west equatorial Africa. Traversing this in a majestic northward curve and receiving vast supplies of water from many great tributaries, it finally turns south-west and cuts a way to the Atlantic Ocean through the western highlands. North of the Congo basin and separated from it by a broad undulation of the surface is the basin of Lake Chad—-a flat-shored, shallow lake filled principally by the Shad coming from the south-east. West of this is the basin of the Niger, the third river of Africa, which, though flowing to the Atlantic, has its principal source in the far west, and reverses the direction of flow exhibited by the Nile and Congo. An important branch, however—the Benue—comes from the south-east. These four river-basins occupy the greater part of the lower plateaus of North and West Africa, the remainder consisting of arid regions watered only by intermittent streams which do not reach the sea. Of the remaining rivers of the Atlantic basin the Orange, in the extreme south, brings the drainage from the Drakensberg on the opposite side of the continent, while the Kunene, Kwanza, Ogowe and Sanaga drain the west corst highlands of the southern limb; the Volta, Komoe, Bandama, Gambia and Senegal the highlands of the western limb. North of the Senegal for over 1000 m. of coast the arid region reaches to the Atlantic. Farther north are the streams, with comparatively short courses, which reach the Atlantic and Mediterranean from the Atlas mountains. Of the rivers flowing to the Indian Ocean the only one draining any large part of the interior plateaus is the Zambezi, whose western branches rise in the west coast highlands. The main stream has its rise in 11 deg. 21' 3'' S. 24 deg. 22' E. at an elevation of 5000 ft. It flows west and south for a considerable distance before turning to the east. All the largest tributaries, including the Shire, the outflow of Lake Nyasa, flow down the southern slopes of the band of high ground which stretches across the conbnent in 10 deg. to 12 deg. S. In the south-west the Zambezi system interlaces with that of the Taukhe (or Tioghe), from which it at times receives surplus water. The rest of the water of the Taukhe, known in its middle course as the Okavango, is lost in a system of swamps and saltpans which formerly centred in Lake Ngami, now dried up. Farther south the Limpopo drains a portion of the interior plateau but breaks through the bounding highlands on the side of the continent nearest its source. The Rovuma, Rufiji, Tana, Juba and Webi Shebeli principally drain the outer slopes of the East African highlands, the last named losing itself in the sands in close proximity to the sea. Another large stream, the Hawash, rising in the Abyssinian mountains, is lost in a saline depression near the Gulf of Aden. Lastly, between the basins of the Atlantic and Indian Oceans there is an area of inland drainage along the centre of the East African plateau, directed chiefly into the lakes in the great rift-valley. The largest river is the Omo, which, fed by the rains of the Abyssinian highlands, carries down a large body of water into Lake Rudolf. The rivers of Africa are generally obstructed either by bars at their mouths or by cataracts at no great distance up-stream. But when these obstacles have been overcome the rivers and lakes afford a network of navigable waters of vast extent. The area of the Congo basin is greater than that of any other river except the Amazon, while the African inland drainage area is greater than that of any continent but Asia, in which the corresponding area is 4, 000, 000 sq. m. The principal African lakes have been mentioned in the description of the East African plateau, but some of the phenomena connected with them may be spoken of more particularly here. As a rule the lakes which occupy portions of the great rift-valleys have steep sides and are very deep. This is the case with the two largest of the type, Tanganyika and Nyasa, the latter of which has depths of 430 fathoms. Others, however, are shallow, and hardly, reach the steep sides of the valleys in the dry season. Such are Lake Rukwa, in a subsidiary depression north of Nyasa, and Eiassi and Manyara in the system of the eastern rift-valley. Lakes of the broad type are of moderate depth, the deepest sounding in Victoria Nyanza being under 50 fathoms. Apart from the seasonal variations of level, most of the lakes show periodic fluctuations, while a progressive desiccation of the whole region is said to be traceable, tending to the ultimate disappearance of the lakes. Such a drying up has been in progress during long geologic ages, but doubt exists as to its practical importance at the present time. The periodic fluctuations in the level of Lake Tanganyika are such that its outllow is intermittent. Besides the East African lakes the principal are: —-Lake Chad, in the northern area of inland drainage; Bangweulu and Mweru, traversed by the head-stream of the Congo; and Leopold II. and Ntomba (Mantumba), within the great bend of that river. All, exceot possibly Mweru, are more or less shallow, and Chad appears to by drying up. The altitudes of the African lakes have already been stated. Divergent opinions have been beld as to the mode of origin of the East African lakes, especially Tanganyika, which some geologists have considered to represent an old arm of the sea, dating from a time when the whole central Congo basin was under water; others holding that the lake water has accumulated in a depression caused by subsidence. The former view is based on the existence in the lake of organisms of a decidedly marine type. They include a jelly-fish, molluscs, prawns, crabs, &c. , and were at first considered to form an isolated group found in no other of the African lakes; but this supposition has been proved to be erroneous. Islands. —With one exception—-Madagascar—the African islands are small. Madagascar, with an area of 229, 820 sq. m. , is, after New Guinea and Borneo, the largest island of the world. It lies off the S. E. coast of the continent, from which it is separated by the deep Mozambique channel, 250 m. wide at its narrowest point. Madagascar in its general structure, as in flora and fauna, forms a connecting link between Africa and southern Asia. East of Madagascar are the small islands of Mauritius and Reunion. Sokotra lies E. N. E. of Cape Guardafui. Off the north-west coast are the Canary and Cape Verde archipelagoes. which, like some small islands in the Gulf of Guinea, are of volcanic origin. Climate and Health. —-Lying almost entirely within the tropics, and equally to north and south of the equator, Africa does not show excessive variations of temperature. Great heat is experienced in the lower plains and desert regions of North Africa, removed by the great width of the continent from the influence of the ocean, and here, too, the contrast between day and night, and between summer and winter, is greatest. (The rarity of the air and the great radiation during the night cause the temperature in the Sahara to fall occasionally to freezing point. ) Farther south, the heat is to some extent modified by the moisture brought from the ocean, and by the greater elevation of a large part of the surface, especially in East Africa, where the range of temperature is wider than in the Congo basin or on the Guinea coast. In the extreme north and south the climate is a warm temperate one, the northern countries being on the whole hotter and drier than those in the southern zone; the south of the continent being narrower than the north, the influence of the surrounding ocean is more felt. The most important climatic differences are due to variations in the amount of rainfall. The wide heated plains of the Sahara, and in a lesser degree the corresponding zone of the Kalahari in the south, have an exceedingly scanty rainfall, the winds which blow over them from the ocean losing part of their moisture as they pass over the outer highlands, and becoming constantly drier owing to the heating effects of the burning soil of the interior; while the scarcity of mountain ranges in the more central parts likewise tends to prevent condensation. In the inter-tropical zone of summer precipitation, the rainfall is greatest when the sun is vertical or soon after. It is therefore greatest of all near the equator, where the sun is twice vertical, and less in the direction of both tropics. The rainfall zones are, however, somewhat deflected from a due west-to-east direction, the drier northern conditions extending southwards along the east coast, and those of the south northwards along the west. Within the equatorial zone certain areas, especially on the shores of the Gulf of Guinea and in the upper Nile basin, have an intensified rainfall, but this rarely approaches that of the rainiest regions of the world. The rainiest district in all Africa is a strip of coastland west of Mount Cameroon, where there is a mean annual rainfall of about 390 in. as compared with a mean of 458 in. at Cherrapunji, in Assam. The two distinct rainy seasons of the equatorial zone, where the sun is vertical at half-yearly intervals, become gradually merged into one in the direction of the tropics, where the sun is overhead but once. Snow falls on all the higher mountain ranges, and on the highest the climate is thoroughly Alpine. The countries bordering the Sahara are much exposed to a very dry wind, full of fine particles of sand, blowing from the desert towards the sea. Known in Egypt as the khamsin, on the Mediterranean as the sirocco, it is called on the Guinea coast the harmattan. This wind is not invariably hot; its great dryness causes so much evaporation that cold is not infrequently the result. Similar dry winds blow from the Kalahari in the south. On the eastern coast the monsoons of the Indian Ocean are regularly felt, and on the south-east hurricanes are occasionally experienced. While the climate of the north and south, especially the south, is eminently healthy, and even the intensely heated Sahara is salubrious by reason of its dryness, the tropical zone as a whole is, for European races, the most unhealthy portion of the world. This is especially the case in the lower and moister regions, such as the west coast, where malarial fever is very prevalent and deadly; the most unfavourable factors being humidity with absence of climatic variation (daily or seasonal). The higher plateaus, where not only is the average temperature lower, but such variations are more extensive, are more healthy; and in certain localities (e. g. Abyssinia and parts of British East Africa) Europeans find the climate suitable for permanent residence. On tablelands over 6500 ft. above the sea, frost is not uncommon at night, even in places directly under the equator. The acclimatization of white men in tropical Africa generally is dependent largely on the successful treatment of tropical diseases. Districts which had been notoriously deadly to Europeans were rendered comparatively healthy after the discovery, in 1899, of the species of mosquito which propagates malarial fever, and the measures thereafter taken for its destruction and the filling up of swamps. The rate of mortality among the natives from tropical diseases is also high, one of the most fatal being that known as sleeping sickness. (The ravages of this disease, which also attacks Europeans, reached alarming proportions between 1893 and 1907, and in the last-named year an international conference was held in London to consider measures to combat it. ) When removed to colder regions natives of the equatorial districts suffer greatly from chest complaints. Smallpox also makes great ravages among the negro population. Flora. —The vegetation of Africa follows very closely the distribution of heat and moisture. The northern and southern temperate zones have a flora distinct from that of the continent generally, which is tropical. In the countries bordering the Mediterranean are groves of oranges and olive trees, evergreen oaks, cork trees and pines, intermixed with cypresses, myrtles, arbutus and fragrant tree-heaths. South of the Atlas range the conditions alter. The zones of minimum rainfall have a very scanty flora, consisting of plants adapted to resist the great dryness. Characteristic of the Sahara is the date-palm, which flourishes where other vegetation can scarcely maintain existence, while in the semidesert regions the acacia (whence is obtained gum-arabic) is abundant. The more humid regions have a richer vegetation—dense forest where the rainfall is greatest and variations of temperature least, conditions found chiefly on the tropical coasts, and in the west African equatorial basin with its extension towards the upper Nile; and savanna interspersed with trees on the greater part of the plateaus, passing as the desert regions are approached into a scrub vegetation consisting of thorny acacias, &c. Forests also occur on the humid slopes of mountain ranges up to a certain elevation. In the coast regions the typical tree is the mangrove, which flourishes wherever the soil is of a swamp character. The dense forests of West Africa contain, in addition to a great variety of dicotyledonous trees, two palms, the Elaeis guincensis (oil-palm) and Raphia vinifera (bamboo-palm), not found, generally speaking, in the savanna regions. The bombax or silk-cotton tree attains gigantic proportions in the forests, which are the home of the indiarubber-producing plants and of many valuable kinds of timber trees, such as odum (Chlorophora excelsa), ebony, mahogany (Khaya senegalensis), African teak or oak (Oldfieldia africana) and camwood (Baphia nitida. ) The climbing plants in the tropical forests are exceedingly luxuriant and the undergrowth or ``bush'' is extremely dense. In the savannas the most characteristic trees are the monkey bread tree or baobab (Adanisonia digitata), doom palm (Hyphaene) and euphorbias. The coffee plant grows wild in such widely separated places as Liberia and southern Abyssinia. The higher mountains have a special flora showing close agreement over wide intervals of space, as well as affinities with the mountain flora of the eastern Mediterranean, the Himalayas and Indo-China (cf. A. Engler, Uber die Hochgebirgsflora des tropischen Afrika, 1892). In the swamp regions of north-east Africa the papyrus and associated plants, including the soft-wooded ambach, flourish in immense quantities—-and little else is found in the way of vegetation. South Africa is largely destitute of forest save in the lower valleys and coast regions. Tropical flora disappears, and in the semi-desert plains the fleshy, leafless, contorted species of kapsias, mesembryanthemums, aloes and other succulent plants make their appearance. There are, too, valuable timber trees, such as the yellow pine (Podocarpus elongatus), stinkwood (Ocotea), sneezewood or Cape ebony (Pteroxylon utile) and ironwood. Extensive miniature woods of heaths are found in almost endless variety and covered throughout the greater part of the year with innumerable blossoms in which red is very prevalent. Of the grasses of Africa alfa is very abundant in the plateaus of the Atlas range. Fauna. —The fauna again shows the effect of the characteristics of the vegetation. The open savannas are the home of large ungulates, especially antelopes, the giraffe (peculiar to Africa), zebra, buffalo, wild ass and four species of rhinoceros; and of carnivores, such as the lion, leopard, hyaena, &c. The okapi (a genus restricted to Africa) is found only in the dense forests of the Congo basin. Bears are confined to the Atlas region, wolves and foxes to North Africa. The elephant (though its range has become restricted through the attacks of hunters) is found both in the savannas and forest regions, the latter being otherwise poor in large game, though the special habitat of the chimpanzee and gorilla. Baboons and mandrills, with few exceptions, are peculiar to Africa. The single-humped camel—as a domestic animal—is especially characteristic of the northern deserts and steppes. The rivers in the tropical zone abound with hippopotami and crocodiles, the former entirely confined to Africa. The vast herds of game, formerly so characteristic of many parts of Africa, have much diminished with the increase of intercourse with the interior. Game reserves have, however, been established in South Africa, British Central Africa, British East Africa, Somahland, &c. , while measures for the protection of wild animals were laid down in an international convention signed in May 1900. The ornithology of northern Affica presents a close resemblance to that of southern Europe, scarcely a species being found which does not also occur in the other countries bordering the Mediterranean. Among the birds most characteristic of Africa are the ostrich and the secretary-bird. The ostrich is widely dispersed, but is found chiefly in the desert and steppe regions. The secretary-bird is common in the south. The weaver birds and their allies, including the long-tailed whydahs, are abundant, as are, among game-birds, the francolin and guinea-fowl. Nany of the smaller birds, such as the sun-birds, bee-eaters, the parrots and halcyons, as well as the larger plantain-eaters, are noted for the brilliance of their plumage. Of reptiles the lizard and chameleon are common, and there are a number of venomous serpents, though these are not so numerous as in other tropical countries. The scorpion is abundant. Of insects Africa has many thousand different kinds; of these the locust is the proverbial scourge of the continent, and the ravages of the termites or white ants are almost incredible. The spread of malaria by means of mosquitoes has already been mentioned. The tsetse fly, whose bite is fatal to all domestic animals, is common in many districts of South and East Africa. Fortunately it is found nowhere outside Africa. (E. HE. ; F. R. C. ) 3 See the calculations of Capt. T. T. Behrens, Geog. Journal, vol. xxix. (1907). 4 The estimate of Capt. H. G. Lyons in 1905 was 1, 107, 227 sq. mi. 5 including waterless tracts naturally belonging to the river-basin. In shape and general geological structure Africa bears a close resemblance to India. Both possess a meridional extension with a broad east and west folded region in the north. In both a successive series of continental deposits, ranging from the Carboniferous to the Rhaetic, rests on an older base of crystalline rocks. In the words of Professor Suess, ``India and Africa are true plateau countries. '' Of the primitive axes of Africa few traces remain. Both on the east and west a broad zone of crystalline rochs extends parallel with the coast-line to form the margin of the elevated plateau of the interior. Occasionally the crystalline belt comes to the coast, but it is usually reached by two steps known as the coastal belt and foot-plateau. On the flanks of the primitive western axis certain ancient sedimentary strata are thrown into folds which were completed before the commencement of the mesozoic period. In the south, the later palaeozoic rocks are also thrown into acute folds by a movement acting from the south, and which ceased towards the close of the mesozoic period. In northern Africa the folded region of the Atlas belongs to the comparatively recent date of the Alpine system. None of these earth movements affected the interior, for here the continental mesozoic deposits rest, undisturbed by folding, on the primary sedimentary and crystalline rocks. The crystalline massif, therefore, presents a solid block which has remained elevated since early palaeozoic times, and against which earth waves of several geological periods have broken. The formations older than the mesozoic are remarkably unfossiliferous, so that the determination of their age is frequently a matter of speculation, and in the following table the European equivalents of the pre-Karroo formations in many regions must be regarded as subject to considerable revision. Rocks of Archean age cover wide areas in the interior, in West and East Africa and across the Sahara. Along the coastal margins they underlie the newer formations and appear in the deep valleys and kloofs wherever denudation has laid them bare. The prevailing types are granites, gneisses and schists. In the central regions the predominant strike of the fohae is north and south. The rocks, for convenience classed as pre-Cambrian, occur as several unconformable groups, chiefly developed in the south where alone their stratigraphy has been determined. They are unfossiliferous, and in the absence of undoubted Cambrian, Ordovician and Silurian strata in Africa they may be regarded as of older date than any of these formations. The general occurrence of jasper-bearing rocks is of interest, as these are always present in the ancient pressure-altered sedimentary formations of America and Europe. Some unfossiliferous conglomerates, sandstones and dolomites in South Africa and on the west coast are considered to belong to the Cambrian, Ordovician and Silurian formations, but merely from their occurrence beneath strata yielding Devonian fossils. In Cape Colony the Silurian age of the Table Mountain Sandstone is based on such evidence. The Devonian and Carboniferous formations are well represented in the north and south and in northern Angola. Up to the close of the palaeozoic period the relative positions of the ancient land masses and oceans remain unsolved; but the absence of marine strata of early palaeozoic age from Central Africa points to there being land in this direction. In late Carboniferous times Africa and India were undoubtedly united to form a large continent, called by Suess Gondwana Land. In each country the same succession of the rocks is met with; over both the same specialized orders of reptiles roamed and were entombed. The interior of the African portion of Gondwana Land was occupied by several large lakes in which an immense thickness—amounting to over 18, 000 ft. in South Africa—-of sandstones and marls, forming the Karroo system, was laid down. This is par excellence the African formation, and covers immense areas in South Africa and the Congo basin, with detached portions in East Africa. During the whole of the time—-Carboniferous to Rhaetic—that this great accumulation of freshwater beds was taking place, the interior of the continent must have been undergoing depression. The commencement of the period was marked by one of the most wonderful episodes in the geological history of Africa. Preserved in the formation known as the Dwyka Conglomerate, are evidences that at this time the greater portion of South Africa was undergoing extreme glaciation, while the same conditions appear to have prevailed in India and Australia. At the close of the Karroo period there was a remarkable manifestation of volcanic activity which again has its parallel in the Deccan traps of India. How far the Karroo formation extended beyond its present confines has not been determined. To the east it reached India. In the south all that can be said is that it extended to the south of Worcester in Cape Colony. The Crystal Mountains of Angola may represent its western boundary; while the absence of mesozoic strata beneath the Cretaceous rocks of the mid-Sahara indicates that the system of Karroo lakeland had here reached its most northerly extension. Towards the close of the Karroo period, possibly about the middle, the southern rim of the great central depression became ridged up to form the folded regions of the Zwaarteberg, Cedarberg and Langeberg mountains in Cape Colony. This folded belt gives Africa its abrupt southern termination, and may be regarded as an embryonic indication of its present outline. The exact date of the maximum development of this folding is unknown, but it had done its work and some 10, 000 ft. of strata had been removed before the commencement of the Cretaceous period. It appears to approximate in time to the similar earth movement and denudation at the close of the palaeozoic period in Europe. It was doubtless connected with the disruption of Gondwana Land, since it is known that this great alteration of geographical outline commenced in Jurassic times. The breaking up of Gondwana Land is usually considered to have been caused by a series of blocks of country being let down by faulting with the consequent formation of the Indian Ocean. Other blocks, termed horsts, remained unmoved, the island of Madagascar affording a striking example. In the African portion Ruwenzori is regarded by some geologists to be a block mountain or horst. In Jurassic times 1he sea gained access to East Africa north of Mozambique, but does not appear to have reached far beyond the foot-plateau except in Abyssinia. The Cretaceous seas appear to have extended into the central Saharan regions, for fossils of this age have been discovered in the interior. On the west coast Cretaceous rocks extend continuously from Mogador to Cape Blanco. From here they are absent up to the Gabun river, where they commence to form a narrow fringe as far as the Kunene river, though often overlain by recent deposits. They are again absent up to the Sunday river in Cape Colony, where Lower Cretaceous rocks (for long considered to be of Oolitic age) of an inshore character are met with. Strata of Upper Cretaceous age occur in Pondoland and Natal, and are of exceptional interest since the fossils show an intermingling of Pacific types with other forms having European affinities. In Mozambique and in German East Africa, Cretaceous rocks extend from the coast to a distance inland of over 100 m. Except in northern Africa, the Tertiary formations only occur in a few isolated patches on the east and west coasts. In northern Africa they are well developed and of much interest. They contain the well-known nummulitic limestone of Eocene age, which has been traced from Egypt across Asia to China. The Upper Eocene rocks of Egypt have also yielded primeval types of the Proboscidea and other mammalia. Evidences for the greater extension of the Eocene seas than was formerly considered to be the case have been discovered around Sokoto. During Miocene times Passarge considers that the region of the Zambezi underwent extreme desiccation. The effect of the Glacial epoch in Europe is shown in northern Africa by the moraines of the higher Atlas, and the wider extension of the glaciers on Kilimanjaro, Kenya and Ruwenzori, and by the extensive accumulations of gravel over the Sahara. The earliest signs of igneous activity in Africa are to be found in the granites, intrusive into the older rocks of the Cape peninsula, into those of the Transvaal, and into the gneisses and schists of Central Africa. The Ventersdorp boulder beds of the Transvaal may be of early palaeozoic age; but as a whole the palaeozoic period in Africa was remarkably free from volcanic and igneous disturbances. The close of the Stormberg period (Rhaetic) was one of great volcanic activity in South Africa. Whilst the later Secondary and Tertiary formations were being laid down in North Africa and around the margins of the rest of the continent, Africa received its last great accumulation of strata and at the same time underwent a consecutive series of earth-movements. The additional strata consist of the immense quantities of volcanic material on the plateau of East Africa, the basalt flows of West Africa and possibly those of the Zambezi basin. The exact period of the commencement of volcanic activity is unknown. In Abyssinia the Ashangi traps are certainly post-Oolitic. In East Africa the fissure eruptions are considered to belong to the Cretaceous. These early eruptions were followed by those of Kenya, Mawenzi, Elgon, Chibcharagnani, and these by the eruptions of Kibo, Longonot, Suswa and the Kyulu Mountains. The last phase of vulcanicity took place along the great meridional rifts of East Africa, and though feebly manifested has not entirely passed away. In northern Africa a continuous sequence of volcanic events has taken place from Eocene times to latest Tertiary; but in South Africa it is doubtful if there have been any intrusions later then Cretaceous. During this long continuance of vulcanicity, earth-movements were in progress. In the north the chief movements gave rise to the system of latitudinal folding and faulting of the Moroccan and Algerian Atlas, the last stages being represented by the formation of the Algerian and Moroccan coast-outline and the sundering of Europe from Africa at the Straits of Gibraltar. Whilst northern Africa was being folded, the East African plateau was broken up by a series of longitudinal rifts extending from Nyasaland to Egypt. The depressed areas contain the long, narrow, precipitously walled lakes of East Africa. The Red Sea also occupies a meridional trough. |
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