Africa

Africa

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.

I. PHYSICAL GEOGRAPHY

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 following table gives the approximate altitudes of the chief

mountains and lakes of the continent:—

Mountains. Ft. Lakes. Ft.

Rungwe (Nyasa) . 10,400 Chad . . . . 8502

Drakensberg . . 10,7002 Leopold II . . 1100

Lereko or Sattima . 13,2143 Rudolf . . . 1250

(Aberdare Range) Nyasa . . . 16453

Cameroon . . 13,370 Albert Nyanza . 20282

Elgon . . . 14,1523 Tanganyika . . 26243

Karissimbi . . Ngami . . . . 2950

(Mfumbiro) . 14,6833 Mweru . . . . 3000

Meru . . . 14,9553 Albert Edward . 30043

Taggharat (Atlas) . 15,0002 Bangweulu. . . 3700

Simen Mountains, . 15,1602 Victoria Nyanza. 37203

Abyssinia Abai . . . . 4200

Ruwenzori . . 16,6193 Kivu . . . . 48293

Kenya . . . 17,0073 Tsana . . . . 5690

Kilimanjaro . . 19,3213 Naivasha . . . 61353

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 calculation of the areas of African drainage systems, made by Dr A.

Bludau (Petermanns Mitteilungen, 43, 1897, pp. 184-186) gives the following

general results:—

Basin of the Atlantic . . . . . 4,070,000 sq. m.

'' '' Mediterranean . . . 1,680,000 ''

'' '' Indian Ocean . . . . 2,086,000 ''

Inland drainage area . . . . . 3,452,000 ''

The areas of individual river-basins are:—

Congo (length over 3000 m.) . . 1,425,000 sq. m.

Nile ( '' fully 4000 m.) . . 1,082,0004 ''

Niger ( '' about 2600 m.) . . 808,0005 ''

Zambezi ( '' '' 2000 m.) . . 513,500 ''

Lake Chad . . . . . . . . . 394,000 ''

Orange (length about 1300 m.) . . 370,505 ''

'' (actual drainage area) . . 172,500 ''

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,

Страницы: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17