Africa by Élisée Reclus/Volume 3/Chapter 1
THE EARTH AND ITS INHABITANTS.
WEST AFRICA.
CHAPTER I.
THE CENTRAL AND SOUTH ATLANTIC.
The Fabled Atlantis.
HE term "Atlantis" should properly be restricted to the Atlas region-that is, to Barbary, and more especially to Marocco, which is dominated by the Atlas Range, the Idraren Daren, or "Mountain of Mountains." But by long-established usage this name has been applied to lands which have no existence, and which have probably been submerged for long geological periods. A mythical tradition, referred by Plato to Solon, who was said to have received it from the Egyptian priests of Saïs, has been the main source of the endless conjectures advanced by the learned regarding the identification and locality of some great islands and of a continent supposed to lie beyond the Columns of Hercules. Yet the Greek philosopher's relation contains not a single detail in accordance either with known history or with the vague memories of the oldest peoples of antiquity. When speaking of the Athenians as a civilised nation contending some "nine thousand years" previously with Atlas, son of Neptune, for the supremacy over the Mediterranean world, Plato enters the domain of pure fiction. This Atlantis, which he describes as "larger than Libya and Asia," was for him doubtless an ideal land, a region belonging to the golden age. Its inhabitants were assumed to have long flourished, according to his political ideas, under the sway of ten kings, absolute in their respective territories, but deliberating together for the common weal; and it was the neglect of this model constitution that was supposed to have provoked the angry intervention of the gods, followed by the submergence of this fabled Atlantis. Nevertheless, Plato may well have heard of some shadowy tradition on the 2 WEST AFEIOA.
existence of a land to the west of the Mediterranean, the site of which it is now
difficult to determine. Assuming that it still exists, this Atlantis might possibly
be the New World, which, after having been discovered by the Phoenician sea-
farers, was again forgotten, to be rediscovered two thousand years afterwards by
the Norman and Genoese navigators. Or is it to be identified with the seaboard of
tropical Africa, coasted far beyond the Mediterranean skies by some daring
adventurers in remote times? Or else was this Atlantic region nothing more
than an insiJar group, or perhaps a solitar}^ island, enlarged by human fancy to
the proportions of a continent ? Some writers, such as the Swede, Rudbeck, have
even identified it with the polar lands, or with Scandinavia, although, according
to the more general hj^othesis, it was simply another name for the " Hesperides,"
the "Fortunate Islands," or " Isles of the Blest," expressions current in ancient
legend and tradition.
Others again accept Plato's statement in all its essential features, believing
that a distinct continental mass, filling a great part of the oceanic basin west of
the Atlas, was really engulfed during the present geological epoch, at a time
when some civilised peoples were struggling for the dominion of the Mediter-
ranean lands. Such a conjecture, however, which became a sort of literary com-
monplace in classic poetry, rests on no solid foundation of fact. A convulsion,
which " in a single night " shifted the equilibrium of land and water, changing a
continent to an oceanic basin, could not have occurred without causing a tremendous
reaction, especially if, as KrummeFs investigations seem to show, the weight of the
continents, from their submerged roots to their summits, is precisely the same as that
of the oceans and inland seas.
Changes in the Relief of the Atlantic.
But if there has been no Atlantis, as a distinct region, in the present geological
epoch, there can be no doubt that in previous ages, over " nine thousand years "
before Plato, there existed a continent in this section of the terrestrial surface.
Doubtless, no means are yet available for directly studying the rocks of the oceanic
bed, whence the soundings have brought up only some specimens detached from the
deposits of chalky mud. But the disposition of the opposite shores of the Atlantic,
and the fossils embedded in their strata, offer a ready solution of many geological
problems. Where the corresponding formations on either side of the Atlantic,
although belonging to the same geological age, present considerable differences in
their respective faunas, it is inferred that the marine laboratories where they were
deposited must have been separated by upheaved lands. On the other hand, a
close resemblance and even identity of organic forms in the two now distant
regions shows that the corresponding strata were at one time connected by dry land
of some sort.
Thanks to these comparative studies, it may be asserted that w^hen the Jurassic
sedimentary rocks were constituted, the waters of the Old and New World were not continuously connected as they now are by the deep depression of the Atlantic. A
continent occupied the waste of waters on whose bed the Transatlantic submarine cables have been deposited. In the same way the existence of an identical organic life in the stratified Miocene rocks of Nebraska and Europe shows that, notwithstanding their present distinct faunas and floras, these two regions at one time formed continuous land. How often during the physical history of the globe has the relief of the continents thus been modified, mere passing forms which arise and vanish like the clouds in the heavens!
Yet who shall relate all the vicissitudes of land and water in the valley of the tropical Atlantic even since Jurassic times? The Azores, Madeira, the Canaries, the Cape Verd archipelago, may themselves possibly be surviving fragments of the continental mass that once filled this oceanic region. They are at all events disposed like a border range skirting a semicircular shore, describing a regular arc, in the same direction as the Central Andes of Peru and Bolivia, and the volcanic system of North America, from Mount St. Elias to the Californian Shasta. These Atlantic groups consist almost entirely of igneous rocks and volcanic cones, like those American border ranges. Hence, if the conjecture be true that craters occur along the lines of fracture from the marine shores, all these archipelagoes would indicate the outlines of the ancient coast of a geological Atlantis. They also greatly resemble each other in their general constitution, forming altogether a distinct group amongst the physical regions of the globe.
These Atlantic archipelagoes are not physical dependencies of the African continent, as might be supposed from a cursory view of the maps. Doubtless most of them lie relatively close to the mainland; but the intervening oceanic depths, hitherto supposed to be inconsiderable, are, on the contrary, now found greatly to exceed 3,000 feet, while a complete separation is established by the contrasts in the respective faunas and floras. In many respects these archipelagoes form an inter- mediate domain between three worlds. In climate and products the Azores, Madeira, and even the Canaries, belong rather to Europe than to the neighbouring African mainland. Through their first known inhabitants the Canaries formed part of the Berber world, that is, of North Africa; lastly, many of their vegetable species have been brought by the Gulf Stream from the American continent. Historically, also, these groups formed natural zones of transition, serving as links in the discovery of the New World. Even still, Saint Vincent, a member of the Cape Yerd group, is the chief shipping station between Europe and Brazil, while the more densely peopled islands in the Azores and Canaries are so many gardens of acclimatisation for the plants introduced across the Atlantic from the surrounding continents.
The Azorian Waters.
The oceanic tract above which rises the Azores archipelago, should be more specially named the Atlantic, for these are the waters which, stretching due west from the- Atlas and Pillars of Hercules, were frequented by the seafarers of antiquity. But this expression, Atlantic, that is, "Sea of the Atlas," has gradually been extended to the whole depression separating the Old and New Worlds, from the Frozen Ocean to the Antarctic lands. If no clear natural division can be THE AZOEIAN WATERS. 5 drawn between the continental masses owing to llie gradual transition of relief, geology, and climate, still less can any well-defined lines of demarcation be traced across the oceanic basin. Following the motion of the sun along the ecliptic, the system of aerial and marine currents is subject to incessant modifications. The seasons are alternately displaced from north to s6uth, and from south to north, while the ever-restless floods, setting now in one, now in another direction, inter- mingle the climatic zones in their ceaseless changes. It must suffice to indicate in a general way as the dividing zone the relatively narrow section of the Atlantic comprised between the submarine plateau of Western Europe and the Bank of Newfoundland. This is the " telegraphic " bed, the first part of the ocean that has been sj^stematically explored for the purpose of laying the cables between Europe and America. The bed of this region, which has a mean depth of over 2,000 fathoms, presents the greatest regularity, the most gradual slopes and uniform depths for vast tracts in the whole marine basin. The section which stretches south of the telegraphic plateau, and which may be called the Azorian Atlantic, from the archipelago lying nearest to the centre, is sufficiently, well defined southwards by the narrower zone comprised between Africa and South America. A line drawn from the Bissagos Archipelago through the islets of Saint Paul and Fernam de Xoronha to Cape Saint Roque, has a length of less than 1,750 miles, and in this dividing zone the waters are somewhat shallower than in the sections lying to the north and south. In its general outline the Azorian Atlantic forms a flattened crescent with its convex side facing westwards, and bounded by the United States, the Antilles, the Guiana and Brazilian coasts. The inner concave line is indicated by the African seaboard from the Strait of Gibraltar to Cape Palm as. The bed of this section of the Atlantic is much more irregular than that of the North Atlantic to the south of Greenland and Iceland. The oceanic depths between Africa and the Antilles are not only interrupted by several insular groups and the plateaux supporting them, as well as by the surrounding banks, but in this region there also occur numerous submarine mountains, which, like the upheaved archipelagoes themselves, are very probably due to lava formations. These submarine masses are met chiefly to the west of the Azores, where a sudden subsidence of 3,000 feet would reveal the presence of numerous islands, some disposed in scattered groups between the telegraphic bed and the Azores, others stretching from this archipelago for some fifteen degrees of longitude westwards in the direction of the Bank of Newfoundland. Most of the banks, however, indicated on the old charts — those, for instance, of Sainte-Marie and Kutusov — south of the Azores, have not been met during more recent soundings. Earthquakes, waterspouts, schools of cetaceans, or floating masses of pumice, have often led to the assumed existence of reefs in places where the plummet descends to depths of 14,000 to 16,000 feet without touching the bottom. Sadden changes of colour, from green to blue, from blue to black, usually correspond in the Azorian Atlantic to varying depths. Such at least is the inference drawn from these coincidences by the members of the Tallyman expedition in a part of the Azorian Atlantic presenting great inequalities of depth, and where four soundings revealed 500, 95, 150, and 830 fathoms in rapid succession. Nevertheless, the same naturalists observed that the pate was of a sea-green colour between the Canaries and Cape Verd group, where, instead of meeting with comparatively shallow water, they determined a mean depth of no
less than 1,500 fathoms. In fact, this phenomenon of shifting colours in seawater has been explained by physicists in the most diverse ways. While Toynbee asserts that a green tint prevails when the surface is cool and the atmosphere moist, the observations of the Gazelle would seem to show that the contrast between blue and greenish hues is due to the different degrees of salt held in solution, blue water being the most dense. EELl'Er, GEOLOGY, AND TEMPEEATUEE OF THE AZOEIAN BASIN. 7 The systematic exploration of the Azorian Atlantic is still far from complete, some of the recorded soundings occurring only at intervals of several hundred miles. The only section of the ocean whose relief has been accurately determined is the plateau on which have been laid the telegraphic cables between West Europe and the United States. Further south, ^he reports of vessels specially equipped for scientific expeditions are disconnected, and separated from each other by extensive unexplored spaces. The Challenger^ the Magenta^ and long before them, the Venus, traversed the waters between the Azores and the coast of Brazil in an oblique direction ; the Gazelle, the Saratoga, and the Dolphin visited the eastern section between Madeira and the Cape Verd Islands ; the Talisman and the Gettysburg confined their operations mainly to the vicinity of the archipelagoes ; while the soundings of the Silvertown were made only for the purpose of laying the cable between the Cape Yerd group and the Portuguese possessions on the neighbouring mainland. Off the American seaboard soundings have also been recorded by the Blake and several other vessels between Newfoundland and the Bermudas, and thence to Florida and the Bahamas. But from these isolated records it is impossible to prepare a complete oceanic chart, most of the bathymetric curves having still to be filled in on more or less plausible conjectures. Hence the great discrepancies in the published charts, which are, nevertheless, all based on the materials supplied by the same soundings. Fresh researches will be needed to gradually remove the unknown elements, and at some points new observations have already been begun, for the purpose of verifying or correcting former records. Thus the section between the Cape Yerd and Bissagos groups has been twice explored, the more careful soundings, made with improved appliances, revealing greater depths than those previously registered. In the same way the Talisman has corrected several of the figures supplied by the Challenger. Before the introduction of the new registering plummet, there was always a danger of the line running out indefinitely without indicating the bottom ; hence the exag- gerated depths reported, amongst others, by Denham and Parker in the Brazilian waters. At the same time the more sensitive modern apparatus is liable to the opposite danger of under-estimating the real depth, by recording the shocks pro- duced, not by contact with the bed of the sea, but by casual friction, the lurching of the vesselj a passing fish, and the like. Relief, GeologYj and Temperature of the Azorian Basin. The mean depth, calculated by Frummel for the whole depression of the Atlantic, would appear to be about 2,000 fathoms, which is probably somewhat less than that of the Azorian basin. If the Azores with their western submarine con- tinuation constitute a transverse ridge in mid- Atlantic, the prolonged axis of these partly upheaved partly still flooded elevated lands will indicate one of the deepest abysses hitherto discovered in the Atlantic. This abyss lies to the south of the New- foundland bank, where a sudden subsidence of considerably over 3,000 fathoms would still leave a vast marine basin filled with water. Another great cavity occurs in the 8 WEST AFEICA. almost immediate vicinity of the West India Islands, where, about 90 miles north of St. Thomas, the Challenger reported nearly 4,000 fathoms, supposed to be the greatest depth till the Blake recorded 4,350 fathoms some sixty miles farther west. In the Cape Verde waters also, and even between that archipelago and the African mainland, extensive tracts occur with 2,700 fathoms and upwards. Altogether the Azorian Atlantic presents the form of a double valley, one skirt- ing the African the other the American seaboard, with a long dividing ridge, which runs to the south-west of the Azores in the direction of Guiana. This " Dolphin's Back," as it is called by English geographers, would appear to be prolonged in the southern Atlantic by the so-called " Junction Back," in the direction of a third ridge which takes the name of the Challenger. But the recorded soundings are not yet sufficiently numerous to determine this point, although the connecting ridge is already indicated on most bathymetric charts of the ocean. Except near the islands, where coral beds occur, the matter brought up from the bottom during the sounding operations consists mainly of mud presenting little diversity of character. In the shallower sections it contains the remains of globi- gerines and other animalculae ; but in the abysses of over 2,000 fathoms the frag- ments of shells are so minutely ground and altered by the enormous pressure, that it becomes impossible to detect the mingled remains of organisms in this impalpable substance, whose composition is similar to that of chalk. At still lower depths the characteristic deposit is a sort of red clay. The naturalists of the Talisman have noticed three perfectly distinct colours : a reddish yellow on the Morocco coast, a green in the neighbourhood of Senegal, and a white mud round about the Azores. Thus are being formed strata analogous to those of the upheaved rocks belonging to the successive geological epochs of the earth's crust. Most of these muddy de- posits on the bed of the Azorian Atlantic contain volcanic elements, especially pumice, which must have come from the centres of explosion in the archipelagos, as they are met in larger quantity round about the islands containing active craters. Besides these products of eruptive origin, the Talisman has fished up from depths of 2,000 or 3,000 fathoms specimens of other rocks, such as granite, gneiss, schists, sandstones and limestones. The cavities of these rocks were for the most part filled with a bluish mud composed chiefly of globigerines. In these tropical seas, as in the northern oceanic waters, the temperature falls with great uniformity. On the surface the water, exposed to the incessantly changing influence of the seasons and atmospheric currents, undergoes corresponding changes of temperature, being alternately cooled by the north-eastern trade-winds and warmed by the land breezes. In the Azorian Atlantic the mean annual tem- perature oscillates within a range of 33° F., although at times rising to 38° or even 42°, and falling to 27° and under. But the action of external climatic influences diminishes rapidly under the surface, and at a depth of 400 feet the water ceases to be affected by the alternating hot and cold atmospheric changes. Within this thin surface layer the temperature falls with the greatest rapidity, so that 200 miles to the south of the Cape Verd Islands the thermometer indicating 77° F. at the surface falls to 53° at a depth of 300 feet. Lower down the fall is extremely gradual, a difference of scarcely more than the fraction of a degree being observed in a layer several hundred fathoms thick. The result of two hundred and twelve soundings taken by the Talisman shows for depths of 500 fathoms a temperature of rather less than 50° F., or 27° degrees less than that of the surface waters; at 1,000 fathoms it oscillates round 39° F., at 2,000 fathoms it falls to 87°, at the bottom approaching 32°, which, however, for salt water is not the freezing point. In the region lying between the Azores and the Cape Verd group, the temperature on the bed of the ocean remains at 84° F. In the Bay of Biscay it is somewhat lower, and lower still towards the west, near the Antilles and Bermudas, and especially under the equator, whee the lowest in the Atlantic basin (32°5 F.) has been recorded.
Thus by a remarkable contrast the waters of the Azorian are found to be warmer than those of the equatorial Atlantic. In both regions the mean difference in corresponding
liquid volumes of 1,660 fathoms is about 8° 5' in favour of the northern section as far as 40° N. lat. This phenomenon, which seems opposed to the physical laws of the globe, must be attributed to the influence of the oceanic currents. While the region lying between the Antilles, the Canaries, and Cape Verd group is comparatively tranquil, and subject to the broiling heat of the sun, the equatorial waters are to a great extent constantly renewed on the surface by currents from the North Atlantic, which skirt the African seaboard along its whole length from north to south. At lower depths cold waters set steadily from the Antarctic regions along the bed of the West Atlantic to the north-east of the Antilles. According to the observations of the Challenger and Gazelle, these deep Antarctic currents meet in the zone to the south-west of the Azores, between 36° — 37° N. lat. The thermic equator of the oceanic bed, as indicated by warmer layers than those to the north and south, is thus deflected far beyond the geometric equator of the globe. It crosses the Azorian Atlantic obliquely, 1,200 miles to the north of the equator, so that on water as well as on dry land the zone of equilibrium between the northern and southern climates falls within the northern hemisphere.
But whatever be the local differences of temperature, the gradual normal fall from the surface towards the bottom down to 2° or 8° above zero, or even lower, is constant for every part of the oceanic basin. The case is different for the nearly landlocked basin of the Mediterranean, which from the neighbouring ocean receives _ only surface waters at a temperature always above 58° or 54°F. M. Faye's well-known theory regarding the density of the terrestrial crust is largely based on the fact that the lower oceanic waters are nearly always icy cold. Being exposed for long geological ages to this cooling influence, the rocky bed itself has become cooler down to a certain depth. It has thus become contracted, with a corresponding increase of thickness and density, so that, bulk for bulk, the submarine are heavier
than the continental masses, the latter consequently exercising a less relative influence on the vibrations of the pendulum.
Atmospheric Currents of the Azorian Basin.
As regards its aërial currents, the Azorian Atlantic partakes of two different zones, in the north coming within the European zone of westerly winds, in the south within that of the trade winds, that is, the oceanic zone properly so called. The African waters are further distinguished by a special atmospheric system, the neighbourhood of the mainland reversing the normal disposition in the development of the local daily breezes and the periodical monsoons. The Azores lie nearly on the limit between the trade-winds and the opposing currents, which descending from the upper regions, take a normal south-westerly or westerly direction. The winds which set towards the coasts of Iberia, France, and the British Isles have their origin in this central part of the oceanic basin. M. Brault's exhaustive studies on the direction and intensity of the North Atlantic winds show that in summer, Flores, one of the western Azores, forms the focus of a regular aërial rotation. The waters around this island are the only Atlantic region where the northern blow as frequently as the southern gales, and where the western are balanced by the eastern currents. East of this point the prevailing breezes are northerly, westwards southerly, northwards mainly westerly, southwards easterly. Hence round this central region revolves the great atmospheric ocean of the Azorian Atlantic, a fact which will add greatly to the importance of the submarine cable about to connect the Azores with all the European meteorological stations. The chief station will be established at the point of intersection of the great aerial currents, whence more or less trustworthy weather forecasts can be announced some days in advance for the west of Europe.
The normal movement of the winds in the Azorian Atlantic has been well known since the early navigators began to frequent these waters. All were struck by the regularity of the currents blowing off the coasts of Madeira and the Canaries, to which they gave names betraying their knowledge of the law regulating the circulation of the winds in this region. For the Portuguese these currents setting regidarly from the north-east to the south-west are the "geraes" or "general;" for the French the "alizes," that is, "uniform" or "regular;" while for the English they are at first the " tread winds," that is the "steady," or " constant," afterwards by an unconscious but easily understood play of words, changed to the "trade winds." But notwithstanding their general regularity, these sea breezes are subject to certain changes of velocity from season to season, as well as to deflections to right and left of the normal direction. The main features of this atmospheric system may be studied in Maury's pilot-charts, in those of Brault and Toynbee, which give the results of many hundred thousand observations, and which continue the labours of previous meteorologists in this field. During the summer of the northern hemisphere the whole space stretching from the Azores southwards to the fourteenth degree N. ]at. is swept by the trade winds, which in winter are deflected much farther south. Thus, while the Azorian waters are temporarily brought within the influence of the variable western breezes, the Central Atlantic as far as 3° or 4° S. lat. is exposed to the action of the trade winds.
Seafarers have also to study the zones of calm or less intense aerial currents, one of which lies about the equator, the other to the south of the Azores, both forming elliptical spaces round which are developed the curves of equal force first described by Brault, and by him named "isanemonic curves." Lastly there remains to be considered the thickness of the aerial curves constituting the trade winds, above which set the counter-winds which, after rising vertically into the zone of equatorial calms turn northward in the direction of the pole, gradually falling towards the surface of the earth. At the Peak of Teyde, in the Canaries, the intervening zone between the trade and counter-winds rises in summer and descends in winter on the upper slopes of the mountain, and Piazzi Smith has been able several times to measure the exact thickness of the lower current blowing in the direction from north-east to south-west. But the Teyde Peak is a mere islet in this atmospheric ocean, and there still remain to be studied in the same systematic way the heights of Madeira and the Cape Verd Islands, as well as the general movement of all the counter-winds.
12 WEST AFEIOA. Marine Currents of the Azorian Basin. The more salient features of the marine, like those of the aerial currents, in the Azorian Atlantic are already known ; but many obscure and doubtful points still remain to be cleared up. It is aU the more difficult to follow the course of the circulating waters, that certain currents move too slowly to be directly measured. They canlbe detected only by means of the thermometer, when their temperature differs from that of the circumambient liquid. In this way has been determined the existence of a deep stream flowing from the Antarctic seas to the equatorial waters and even to the neighbourhood of the Azores ; by means of the thermometer the presence of corresponding cool currents from the Arctic Ocean has been revealed in the same region. But as a rule the waters occupying nearly the whole of the Central Atlantic basin have a very perceptible velocity, in some places reaching one or two miles per hour. Altogether the section of the Atlantic comprised between the telegraph plateau and the equator, between the west coast of Africa and the Antilles, is filled by a vast vortex incessantly rotating, and constantly influenced by the same forces. The current, deflected from the Senegambian coast, bends across the ocean in the direction of the West Indies. Here it ramifies into two branches, one penetrating into the Caribbean Sea, the other skirting the east side of the Bahamas, beyond which it joins the American Gulf Stream, flowing thence east and north-east. The current returning from America towards the Old AVorld traverses the Azorian Atlantic, and in the neighbourhood of the Portuguese and Maroccan coasts bends southwards, thus completing the vast circuit. These oceanic streams flow nearly parallel with those of the atmosphere above them, from which they differ only in their more sluggish motion, and in the deflec- tions imposed upon them by the sudden obstacles of insular and continental barriers. The surface waters being directly exposed to the action of the wind, necessarily move in the same direction, lashed into crested billows under high gales, gently rippled beneath the soft zephyrs. The casual winds produce only a passing effect, their action never reaching far below the surface. But regular currents, such as the trade winds, acting from century to century throughout countless ages, have gradually penetrated to great depths, thus largely contributing to determine their general movement. Till recently physicists supposed that the chief cause of the equatorial current flowing westwards in the contrary direction to the globe itself, was the terrestial rotation, a movement necessarily outstripping that of the encircling oceanic waters. The transverse currents would then be explained in the same way by the greater velocity of planetary rotation acquired by the waters under the equa- torial latitudes. According to Miihry, the centrifugal force of the globe, being greater on the equator than elsewhere, is the primary cause of the general oceanic movement. But in any case the varying degrees of salinity and heat between the liquid layers must also tend to produce these currents, although the effects produced by them cannot be determined with the most delicate observations con- tinued for many years by skilled observers. The mean velocity of the chief currents in the Azorian Atlantic can scarcely be estimated at much more than two-thirds of a mile per hour, or one-fortieth of that of the winds above them. In the eastern section of the main eastern current the movement varies from 15 to 18 miles in the twenty-four hours. In the central parts of the basin, and especially in the Sargasso Sea, the speed slackens, but again increases near the coast and in the straits between the archipelagoes. Although weak, the movement of the "trade waters," aided by the corresponding winds, is
none the less of great aid to vessels bound for the New World, and but for these favourable conditions Columbus would certainly never have reached Guanahani. Thanks also to these currents, sailing vessels have often reached America when their crews were endeavouring to gain the islands or penetrate into the South Atlantic waters. Nothing is more probable than that in remote times ships were thus turned from their course, and that Phœnicians, for instance, or other involuntary immigrants from the Old World, founded colonies in America. At the same time these conjectures regarding early epochs are confirmed by no direct evidence, whereas in modern times many instances have been recorded of ships driven westwards by the trade winds and corresponding marine currents. Thus it was that, in the year 1500, Alvarez Cabral discovered Brazil when bound for the East Indies. Yiera y Clavijo relates that a vessel sailing from the village of Lanzarote, in the Canaries, stranded on the coast of Venezuela. In 1731 another ship with a cargo of wine setting sail from Teneriffe for another island in the Canaries, was driven westwards by a storm, at last reaching Port of Spain, in Trinidad. Being provisioned only for five or six days, the crew had been reduced to live exclusively on wine after the supplies were exhausted. On another occasion a magistrate belonging to Terceira, while endeavouring to reach this island from the neighbouring San-Jorge, was driven all the way to Brazil, whence he returned by the Lisbon route.
Fauna of the Azorian Basin. — The Sargasso Sea.
The Challenger, the Talisman, the Magenta, and other vessels recently engaged in exploring the Atlantic, have not only brought back valuable information regarding the temperature, currents, and other features of the marine depths, but the naturalists accompanying them have paid special attention to the organisms inhabiting these waters. The Azorian Atlantic having a higher temperature than the equatorial seas, is extremely rich in animal life. Certain tracts especially in the vicinity of the Canaries seem to be alive with myriads of creatures of every form and colour, some opaque and almost invisible, other transparent and bright with the most varied tints. Cetaceans, sharks preceded by their " pilots " (the pilot-fish or Naucrates ductor), and hundreds of other species, animate these waters. Flying-fish are often seen darting from the crest of one wave to another, where they fall a prey to their enemies. The nautilus moves along like a tiny ship studded with white sails ; while below this upper fauna, which migrates northwards in summer, southwards in winter, naturalists are now studying a second fauna which has a far wider range, thanks to the greater uniformity of temperature at lower depths.
As remarked nearly a century ago by Humboldt, the sea is above all a centre of animal life, few plants growing except on the rocky cliffs of the islands and encircling continents. Thus even these have their roots embedded on the terra firma. Nevertheless the Azorian Atlantic has also its deep-sea flora, the so-called sargasso (sargassum), formerly supposed to be a survival of the vanished Atlantis, a boundless plain of seaweed floating above the engulfed continent. With their branching stems, their lateral membranes resembling indented foliage, their floats almost like berries, these algae, or "grapes of the tropics" (Fucus natans, Sargassum bacciferum), might easily be taken for plants organised like those of the dry land. Nevertheless they are mere weeds like those of the surrounding shores, in which no trace of reproductive organs has ever been detected. Nor are they so much flotsam, as was once supposed, torn by the waves from the West Indian and THE GUINEA WATERS. 15 American coasts, and sent drifting in tlie everlasting vortex of the tropical waters. It was first shown by Meyen in 1830, and afterwards fully confirmed by Leps, that the berry-bearing sargasso is a true oceanic plant, produced in the seas where it is found covering thousands of square miles. A fissure near the middle of the mature plant marks the point where the parent stem has thrown off a younger branch, which will in due course multiply itself in the same way. Thus are developed, not vast " praderias," or meadows, as hyperbolically described by the early navigators, but strings of tufted weeds following in islands and archipelagoes some yards long, at times some acres in extent, constantly changing their outlines under the action of the waves. They are easily separated by the prows of passing vessels, for they form only a surface layer, nowhere superimposed in thick masses. They disappear altogether to the east of the Azores, abounding mostly in the regions west and south-west of this archipelago, where they stretch across a space of over fifteen degrees of latitude and longitude, covering altogether an area of about 1,200,000 square miles. Farther west near the Antilles there occurs another less extensive Sargasso Sea, consisting of more open herbacious islets, with long broken lines of floating algi3D penetrating between the West Indian Islands into the Caribbean Sea. Like those of dry land, these islands have also their proper fauna, all the sargasso berries being thickly incrusted with white polyzoa. The fishes lurking in their shade or amid their tufted foliage have become assimilated in colour to the protecting environment ; hence they are not easily detected even by the naturalist among these algao, whose prevailing olive-green hue is mingled with white and yellow tints. The Antennarius marmoratus, one of these fishes, which was at first taken for a shapeless spray of fucus, from two to four inches long, seems better adapted for walking than for swimming. Bj^ a strange coincidence its fins, already suggesting the extremities of quadrupeds, terminate in real toes, the front fins also taking the form of arms, with elbow, fore-arm, and fingered hands. By means of adhesive threads this curious creature builds itself nests in the seaweed. The sargasso, fauna comprises altogether sixty species, including fish, crustaceans, and molluscs. The inhabitants of the Azores might establish profitable fisheries in these fields of floating wrack, where they would also find inexhaustible supplies of manure to increase the fertility of their gardens. This growth might also yield large quantities of iodine, bromine, and other valuable chemical substances. The Guinea Waters. The waters which bathe the west coast of South Africa may be regarded as a distinct basin, at least in the form of its bed, its system of currents, and the insular groups rising above its surface. Thanks to the numerous soundings that have been taken in the neighbourhood of the mainland and islands, and less frequently in the high southern latitudes towards the Antarctic regions, the relief of the marine bed may now be figured on our charts, if not with absolute precision, with sufficient accuracy to reproduce its most salient features. The submerged ridge running obliquely from north-east to south-west across the section between Liberia and Brazil, abruptly changes its direction under the latitude of Cape Palmas, some five degrees from the coast. Here the higher grounds, still however flooded to
depths of 1,400 and 1,700 fathoms, trend due north and south between the oceanic depression near the African coast and the still more profound abysses on the American side. This parting line, above which rise the peak of Ascension and the two insular groups of Tristam da Cunha and Goncalo Alvarez, forms the median limit between the two sections of the South Atlantic. A straight line drawn along the meridian from Sierra Leone to Tristam da Cunha indicates exactly the "great divide " between the Guinea and Brazilian basins.
The somewhat quadrilateral section comprised between this divide and the African seaboard, and stretching north and south from Cape Palmas to the Cape of Good Hope, is by no means of uniform depth. It may in fact be subdivided into two secondary basins with cavities of over 2,800 fathoms, one extending west and east parallel with the Gold and Slave Coasts, the other of nearly oval form, with its greatest depression to the south-east of St. Helena. The greatest depth hitherto revealed in this section of the African waters is 3,250 fathoms; and the whole basin, presenting a general depth of over 2,200 fathoms off the south-west coast of Africa, has an area of about 2,800,000 square miles—that is, over twice that of the Mediterranean Sea. South of a line running from the mouth of the Orange River to Tristam da Cunha there stretches a second basin also of 2,200 fathoms, limited southwards by the submarine heights on which stands the island of Bouvet, and which slope gently towards the coasts of the Antarctic lands.
Currents of the Guinea Basin.
In this vast cauldron of the African seas the waters are in continual motion, the mean result of all the shifting and ever opposing currents being a general movement running parallel with the coast from the Cape of Good Hope to Cape Lopez, then trending westwards in the direction of the New World, and returning by the south and east to complete this vast circuit. Thus this southern vortex corresponds with that of the North Atlantic, of which the Gulf Stream forms the western branch. But its general movement is reversed, while also presenting more uniform outlines, thanks to the greater regularity of its basin. Its mean diameter may be estimated at 2,400 miles, with a varying velocity which, however, is never very great except under the influence of high winds. During her voyage from the Cape Verd Islands by Ascension to the mouth of the Congo, the Gazelle found a part of the equatorial current south of the equator moving westwards with a velocity of 1| mile per hour, whereas most other observations had recorded a speed of little over 1¼ half a mile, and in some cases not more than 500 feet. In many parts of these oceanic regions there is in fact no perceptible motion at all, the whole mass accomplishing its vast circuit by a slow movement of translation, while here and there the obstruction of the coastline or the local winds produce secondary currents running in the opposite direction to the main drift.
The Guinea Stream.
The most powerful of these backwaters is that which skirts the continental seaboard between Cape Palmas and the Bight of Biafra, and which sets from west to east with a mean velocity of a little over two-thirds of a mile an hour. But off Cape Palmas it attains an occasional speed of 3½ miles, or nearly 90 miles a day. This "Guinea Stream," as sailors call it, intervenes between the two sections of the equatorial current, which flow from the Old towards the New World, so that a vessel sailing either east or west parallel with the equator may take advantage either of the main or the counter current to accelerate its speed. The Guinea Stream shifts with the seasons, in September occupying more than half the breadth of the Atlantic to the south of the Cape Verd Islands.
The cause of this movement from west to east in the same direction as that of the globe itself, is a question that cannot be discussed apart or independently of the still unsolved general problem of the circulation of the oceanic waters. The part played in these movements by the rotation of the globe, by the winds, the varying temperature from the surface downwards, the varying degrees of salinity
in the intermingling waters, cannot yet be determined. Certainly none of the different theories suffice to explain all the phenomena observed by the few meteorologists who have themselves visited these oceanic regions. In general the Guinea Stream is regarded as a lateral backwater, a "compensating current" produced by the reflux of the equatorial waters. It cannot in any case be attributed to the direct action of the winds, for it flows in the opposite direction to the trade winds and monsoons prevailing in these waters. Even off the Niger delta and the Cameroons, where the Guinea Stream trends south-eastwards and then southwards till it merges in the equatorial current, the movement is still opposite to the normal direction of the winds. To this phenomenon of the Guinea Stream running counter to the winds and laterally pressed upon by another marine current flowing in the opposite direction, is perhaps to be attributed the tremendous surf, forming the so-called "bar," which renders the approach to the Guinea coast so difficult and at times so dangerous between Cape Palmas and the Cameroons. A little "Sargasso Sea" like that near the Antilles occurs also off the mouth of the Congo in the secondary vortex produced by the collision between the Guinea Stream and the other current flowing from the south along the coast of Benguela and Angola.
Atmospheric Currents — Rainfall — Salinity.
The anemometric charts of Brault and other observers show that in the South African Atlantic the mean annual direction of the winds is marked by great
regularity. Storms properly so called are extremely rare, and the "general" winds — that is to say, the south-east trade winds — blow with such uniformity that, especially at the time of the solstices, seafarers in these waters are able to calculate with great probability the length of their passage. But this regularity prevails only on the high sea, as near the coast the aërial currents are deflected inland. Above the English, German, and Portuguese possessions in South Africa, as well as about the coastlands on the Lower Congo and Ogoway, the winds blow from the south-west or else directly from the west, whereas on the coasts to the west of the Cameroons they come from the south. These are the vapour-charged atmospheric currents which bring the rains to the coastlands, and which deluge tke Cameroons uplands throughout the whole year. The other elevated lands on this seaboard also receive a large share of the rainfall, which is nowhere heavier in any part of the Atlantic than in this oceanic region exposed to the influence of the north-east and south-east trade winds between the projecting coasts of West Africa and South America. Tere the still air arrests the rain-clouds brought by both trade winds, the vapours are condensed and precipitated in tremendous downpours on the subjacent waters. In many places this rain water, owing to its less specific
gravity, spreads over the surface in sufficiently thick layers to enable passing vessels to replenish their supply of fresh water.
Although incessantly intermingled by the aërial and marine currents, the waters of the Atlantic basin differ none the less in their degree of salinity not only on the surface, but also in the deeper strata. The most saline is that encircling St. Helena, the specific gravity of which is 1⋅0285. Owing to the heavy rains a the region of calms the proportion is less in the Guinea Stream, the difference being as much as two or even three thousandths in the north-east part of the TEMPERATURE— FAUNA. 21 Guinea waters, with which is mingled the discharge of the Niger and Congo, the two African rivers which have the greatest volume. In the South Atlantic regions also the water is less salt than in the neighbourhood of St. Helena, in consequence of the melting of the icebergs and floating ice brought by the oceanic currents from the Antarctic lands. These frozen masses penetrate farthest north in the months of June, July and August, that is, in the Austral winter season, when these fantastic glittering forms — domes, towers, obelisks — continually changing their outlines with the displacement of the centre of gravity, are met in the Cape waters, and even as far as 35° S. latitude. Farther south the ocean is strewn with myriads of floating fragments, which to vessels rounding the African continent present the appearance of an endless panorama of gorgeous palaces, temples, colonnades all aglow in the fiery rays of the setting sun. Temperature — Fauka. While diminishing the salinity, these icy crystals also considerably lower the temperature of the liquid masses flowing from the equatorial regions. Between the Cameroons and the Cape there is noticed on the surface a gradual decrease of heat, corresponding to that which also takes place in the atmospheric strata. The isothermal lines follow with considerable regularity from 82° F. on the Slave Coast to 59° towards the southern extremity of the continent. But in the deeper layers the vertical decrease down to the bed of the ocean presents some remark- able contrasts, due to the inflow from the broad Antarctic seas to the gradually narrowing Atlantic basin. Of these contrasts the most striking is the relatively low temperature of the equatorial waters. Taking the mean of the liquid mass lying under the equator between Africa and America, the average for the tepid surface and cool deep waters is found to be about 41° F., that is to say, con- siderably less than a degree higher than that of the tracts stretching to '63° S. latitude. On the other hand these same equatorial waters are fully four degrees colder than those of the north temperate zone under 33° N. latitude. This surprising contrast, attesting the great preponderance of the Antarctic over the Arctic current, occurs regularly in each of the isothermal zones between the two sections of the Atlantic tying north and south of the equator. At equal depths the greatest differences of temperature are recorded. Thus, under 33^ N. latitude, a sounding-line 500 fathoms long records a mean of about 50^ F., while at the same distance to the south of the equator the average is found to be only 39° F., showing a difference of eleven degrees between the two corresponding latitudes. The temperature falls slightly in the neighbourhood of the coast, owing to the steady influx of deep sea currents. In some places a difference of three degrees has been observed between the in-shore and outer waters within a distance of a few miles. The abrupt changes of temperature in the South Atlantic serve to limit the range of animal life, and to modify its outlines with the seasons. The deep-sea species living at. great depths in a uniform cold medium, can, of course, extend their domain from the Arctic to the Antarctic Ocean, but it is very different with the cetaceans and other animals confined more to the surface waters. Thus the southern whales, till recently very numerous in the tracts stretching west and south-west of the Cape of Good Hope as far as the small Tristam da Cunha Arychipelago, never reach the latitude of St. Helena. According to Maury's expression, they are arrested by the tepid tropical waters as by a "wall of flames." The Lusitanian and Mediterranean species in the same way diminish gradually southwards, as do also those of the West Indies. Nevertheless, a large number of the latter are still met in the neighbourhood of Ascension, in the very centre of the South Atlantic. About the river mouths again animal life is much more abundant than in the open sea. As we approach the Congo estuary the number of fishes inhabiting the surface waters steadily increases, causing a corresponding increase of the phosphorescence visible at night, notwithstanding the diminution of salinity caused by the great volume of fresh water discharged by the Congo.
