Christmas on the Briny, The Innocents Abroad / Or, A Holiday Trip to the Abrolhos Islands

by W. Bede Christie

THEIR PHYSICAL CONDITION.

A SHRINKING WORLD.

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.