INFLUENCE OF OCEAN CURRENTS.

THE ZOOLOGICAL PROVINCE OF AUSTRALIA.

Although there are few places on the Australian coast where one can spend a more enjoyable summer holiday than the Abrolhos, both on account of the free and “simple life” they afford, and the great historic interest they possess as being the scene of the final catastrophe which befel the Dutch attempt to colonise Australia in 1629 under the unfortunate Pelsart, the islands themselves are of peculiar interest, and have given rise to much speculation as to their origin.

They are all composed entirely of dead coral, here and there partially covered by wind-blown sand, and for the most part not rising more than ten or twelve feet above the water, while many of them do not attain an elevation of more than three or four feet. They are situated on the 29th parallel of south latitude, and are probably further removed from the Equator than any other coral islands in the world, while the indigenous vegetation, such as it is, belongs to a latitude well within the tropics. It is a curious speculation therefore, how these islands came to exist in their present position.

When we “lay to” on our recent trip our boat drifted considerably northward, while closer to Geraldton the northerly current is still more defined. These northerly flowing waters are the cold waters of the Southern Ocean travelling into the equatorial regions, where they are warmed and supply the current which passes westerly and southerly through the Indian Ocean, attaining its greatest velocity as it passes through the Mozambique Channel, thence round the Cape of Good Hope, and north-westerly through the Carribean Sea to form the Gulf stream, which takes a north-easterly course to impart their warm, genial climate to the British islands.

The equatorial current of the Pacific is broken up and delayed in its course as it passes through the Indian Archipelago and Sunda Islands, and its waters become warmer than those of any other oceanic waters on the globe, while the current itself is split up and diverted into innumerable directions, which make navigation extremely intricate and dangerous. It finally emerges from this intricate maze of islands at about latitude 15 south and longitude 115 east, where portion of its waters join the easterly current to Mozambique, and the remainder flows about south by west, being kept from the Australian shores by the stream of cool water from the Southern Ocean, which hugs the Westralian coast line till it reaches well into the tropics, and merges into the warmer water of Oceania between Sharks Bay and Java, as a lower current. A glance at the map of the world will show exactly how the islands of the Indian Archipelago would operate in forming the intricacies of the currents along the north-west coast.

Owing to the rotation of the earth being from west to east, the tendency of all currents in the ocean is to flow westward, unless some local cause deflects them from that course. The most notable example of this deflection is the Gulf stream, while the next most important is the Pacific current in its ramifications through the islands of the Indian Archipelago.

Polar waters always flow towards the equator, still partaking, however, of the westerly direction unless deflected by local causes; and, being cold, flow beneath the warmer currents, which have been expanded by equatorial heat, until, being warmed, they gradually rise and mingle with the equatorial currents, which, in addition to their westerly motion, also flow towards the Poles, till, becoming cooled, they sink and merge with the Polar waters, thus maintaining a continued circulation.

Thus the polar current which flows northerly past Geraldton has come up from the south of Australia, and being deflected by the southern currents from the Pacific and Indian Ocean, has been pressed, as it were, against the coast line, attaining its narrowest part as it passes beyond Sharks Bay.

The longitude of the Abrolhos Islands is about 113-30E., and here we may refer to a bottle which was picked up on December 9th—two or three weeks before our visit—on East Wallaby Island, by Mr. F. Burton, which has an important bearing on the question under discussion. This bottle was thrown over from the German ship Innsbruck on the 2nd June in longitude 111-41 E. and latitude 21 S., and, as mentioned in the “Guardian” of 15th December, which quoted the written paper it contained, was for the purpose of determining the course of the current from that point. We do not know how long this bottle was on the beach before it was discovered, but we may be pretty certain that the longest portion of its period afloat would be after it arrived at the Abrolhos, in battling with the local counter currents which the islands themselves would produce. During the six months which elapsed from the time it left the Innsbruck, it had travelled through 8 degrees of latitude southward and 1-11 of longitude westward—a distance of about 600 miles—in nautical phrase, about south by quarter west. The position 111-41 E. and 21 south would be about 80 miles west of Flaming Head, and although we have no data on which to estimate the rate of the current, the bottle gives us absolute information as to its direction—south by quarter west.Australia, and the whole of what is known as the zoological province of Australia, stands on an ocean plateau which rarely exceeds 100 fathoms in depth; while immediately beyond it, the ocean suddenly attains a depth of from 1000 to 1,200 fathoms. The edge of this plateau is at Lombock Strait, between Lombock and Sambawa Islands, to the east of Java, and passes northerly through the Strait of Macassar, between Celebes and Borneo, thence easterly, embracing the Molucca Islands and New Guinea, and southerly between Australia and New Zealand and embracing Tasmania. The Abrolhos also stand on this plateau.

Thus all the Pacific waters which pass to the eastward of Celebes and Lombock pass over a considerable portion of this plateau, where the water is only 100 fathoms deep, and therefore absorb a much larger amount of equatorial heat than the waters to the west of that line do, where they attain a depth of 1000 to 1,200 fathoms.

This eastern water, of the Australian province, heated by passing over the Australian plateau, is the water into which the captain of the Innsbruck threw his bottle. Thus, while the climate of Geraldton is tempered by the cool waters of the northern current from the Antarctic Ocean, the Abrolhos have an undue share of warmth transmitted to them by the heated waters of the Pacific as they enter the Indian Ocean. It is this undue share of heat which places these islands in the unique position of being the most southerly coral formations of the world. The coral insect can live and work in the temperature which the waters of the Pacific convey to them to a latitude beyond which the same temperature is denied elsewhere.

Thus most of the flora and fauna of these islands are distinctly tropical in their character, although their habitat is situated six degrees outside the tropics.

The Australian plateau is not a mere geographical phenomenon. It is more, and forms the boundary between the zoological provinces of Australia and Malay. To the east of Lombock Strait everything—birds, animals, fish and even molluscs—are distinctly Australian in their types. To the west they are as distinctly Malayan, and although Lombock Strait is less than twenty miles in width, yet the flora and fauna on either side of it are as distinct as though they were separated by half the diameter of the globe. The birds, which could easily bridge the distance in their flights, or the fish, which could traverse the waters in an hour, remain distinct and faithful to the type of their own zoological province. In the flora, too, the trees and shrubs, the grasses and the herbs, true to their province, retain their botanical individuality, with as much persistence as though separated from their neighbors across the strait, by the broad waters of a mighty ocean and give a contradiction to the doctrines of Darwinism that migration has anything to do with the origin of species, or that natural selection plays any part in permanency of races.

The whole surroundings go to prove the truth of the observations of Agassis—that each zoological province has brought forth life best suited to its own chemical and physical environment, without reference to the life already existing in the neighboring provinces.

Another point which forcibly strikes an observant visitor to the Abrolhos, is that the mass of most of the islands is composed of dead coral with scare a trace of soil except when the guano deposits have, in a few localities, become mixed with the wind-blown sand, which has enriched such patches so that they now grow stunted shrubs, and on the richer of these patches a species of wild oats, and silver grass, both of which have evidently been introduced. This is specially noticeable on Rat Island, where particularly rich patches aggregating probably 100 acres, grow a most luxuriant and dense crop of these grasses. But Rat Island was for a long time worked for its guano deposits, tramways were built and horses were stationed there, so that the seeds brought over with their fodder have germinated in the better portions of the island. These portions would thus at once become the favorite feeding grounds for the horses, whose droppings have further enriched the land, till now it would be capable of cultivation with good results.

But it is the islands in their natural state, such as the Wallabies, Pigeon Island, Woody Island, and the numerous islets which rise from three or four to ten or twelve feet above the sea—all coming under the description of “dead coral”—which attract attention. The coral insect does not build above water, nor does it build at all in water beyond a moderate depth. How, then, do these coral islands come to exist in their present position?

There are two possible explanations for the existence of these islands. Either the plateau on which they stand has been upheaved, and thus pushed them up from below, or the ocean itself has receded, and left them exposed to the atmosphere. Either of these would have caused the destruction of the insects who build them. A review of the evidence in support of these causes will be of interest.

For land to be upheaved it must necessarily be removed to a greater distance from the earth’s centre than it was previously, and there are only two kinds of forces which can effect such a removal. First, the recoil of volcanic action at a distance. When we discharge a gun, half the force of the charge is expended in driving the bullet, and the other half in its reaction against the shoulder. If we push a weight, half the strength exerted is in moving the weight, while the other half is spent in holding our feet in position. And so with all forces—the amount spent in accomplishing its object is equalled by the resistance against which the force is acting. Thus, if a volcanic effort were being made at, say Karakatoa, which is the nearest volcanic vent to the Abrolhos, the force expended in the outbreak would be equalled by a recoil acting on some part of the viscid interior of the earth, in the opposite direction to the seat of force. So that if the seat of force were to lie between Karakatoa and the Abrolhos, one portion of the force would have its visible effect at Karakatoa, while the other might drive the molten interior backward beneath those islands, and thus elevate them. But in any case, by the displacement of matter at the seat of force, a vacuum would be caused which, in its turn, would be compensated for by the subsidence of a portion of the earth’s surface, or ocean bed, to make up for it.

As a rule, these subsidences are sudden, and, if they take place on land, are always disastrous. Thus the great earthquake at Lisbon in 1775, was so sudden that in less than six minutes the land had sunk 600ft., and of 60,000 persons assembled on the new marble pier, not one of the bodies ever rose to the surface. At the same time a town in Morocco, with 10,000 inhabitants was swallowed, and not one escaped. At Kingston harbor, in Jamaica, a similar subsidence took place with equal suddenness, and for nearly a century afterwards the remains of warehouses could be seen a hundred feet or so below the shipping. And so on. Instances may be multiplied. The Runn of Cutch in India, the sinking of the South Island of New Zealand, the disaster at San Francisco, and the catastrophe of Messina, all are the effects of the same forces which result in the sudden alteration of relative levels.

There is another force which also causes upheaval. If two subsidences take place, causing what are known as “deeps” in the ocean, in proximity to each other, they act as wedges on the ocean bed intervening, and force it upward, often folding and distorting stratified deposits, in a remarkable manner, but always forcing it into ridges and mountains which frequently are volcanic. One of the most notable instances of this kind of upheaval is New Zealand. On each side of it is a “deep,” caused by subsidence of the ocean bed. These acting against each other, forced New Zealand up into high ranges, which at once became the theatre of volcanic action. One of these “deeps” lies to the east of New South Wales.

Another instance is the “deep” which lies to the south of, and is the cause of, the Australian Bight. This “deep,” acting against the “deep” to the east of New South Wales, forced up the south-eastern corner of Australia, and, if any part of Australia should be volcanic, that is the part. It is only in this part of our island continent that we find extinct craters, such as Mount Gambier in South Australia, and many undoubted volcanic vents in Victoria, while much of New South Wales is also of undoubted volcanic origin, the cause of which can only be sought in the action of these two “deeps,” one against the other.

The Abrolhos group extends for a distance of about sixty miles north and south, and if they had been upheaved by any such action as that above referred to, the area on which they stand must have been broken up into ridges and contorted, and its original levels, therefore, must have been altered relatively one to the other, while portions which were not of coral formation would also have been forced above water. As these islands, however, are all of that formation, and all of nearly uniform elevation, the evidence is pretty conclusive that upheaval has had no share in the cause of their appearance.

But subsidence of the ocean bed has another and different effect, which causes the appearance of dry land without upheaval. The world is continually shrinking in diameter at the rate of about four feet in a year. The sun’s present rate of contraction, as determined by astronomers, is four miles in a century, that of the earth is one mile in about fourteen centuries.Every volcanic outburst, every earth tremor, is the effect of this shrinking of the earth’s diameter, and there is no outpouring of volcanic production that is not accompanied by a compensating subsidence at some part of the earth’s surface. But as the area of the oceans is two and a half times the area of dry land, it follows that five-sevenths of the subsidences which take place are in the ocean, and, therefore, not visible to us, while much of that which takes place even on the land is in inaccessible and uninhabited parts, of which we have no cognisance. Thus we know of but a very small proportion of the subsidences which are continually taking place, although the seismograph, an invention of recent years, places on record very many earth tremors of which we otherwise would have no knowledge. All these tremors are the result of the settlement of the earth’s crust, to accommodate itself to the continually altering strain of which volcanic outbursts are the visible effect.

These subsidences of the ocean bed, drain the waters off the shallower portions, or oceanic plateaux, such as that on which the Abrolhos stand. If the highest of these coral islands had been built up to the water level, and a series of subsidences in various parts of the ocean had taken place which lowered the surface of the waters, say, twelve feet, that would leave those islands standing twelve feet out of the water; while those portions of the plateau on which the coral insects had only built to within eight feet of the surface, would be left four feet out of the water, and the relative levels of the islands would thus remain unaltered.

One cannot sail amongst the Abrolhos group without being struck with the unaltered relative positions in level which they occupy, and, therefore, that their existence to-day is due to subsidences in the ocean’s bed which have taken place in various parts of the world, probably at various periods, to tell us of seismic disturbances of which we otherwise have no record.