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.