Atolls, or Coral Islands--Their strange Appearance--Their
Connexion with Volcanoes--Their Mode of Formation--Antarctic
Volcanoes--Diatomaceous Deposits
To the southward of the Sandwich Islands, on the other side of the
equator, there is a large group of islands in the Pacific, which
have a very peculiar appearance. They are called Atolls or Coral
Islands. Although not exactly of volcanic origin, yet the manner in
which they are formed has some connexion with submarine volcanic
action.
An atoll consists essentially of a ring of coral rocks but little
elevated above the level of the sea, and having in its centre a
lagoon or salt-water lake, which generally communicates by a deep
narrow channel with the sea. The ring of rocks is flat on the
surface, which is composed of friable soil, and sustains a
luxuriant vegetation, chiefly of cocoa-nut palms. It is seldom more
than half a mile in breadth between the sea and lagoon, sometimes
only three or four hundred yards. The outer margin of the ring is
the highest, and it slopes gradually down towards the lagoon; but
on the outside of the ledge of rocks is a beach of dazzling
whiteness, composed of powdered and broken coral and shells. The
appearance they present is thus not less beautiful than singular.
Some of these islands are of large size, from thirty to fifty miles
long, and from twenty to thirty broad, but they are in general
considerably smaller. Their most frequent form is either round or
oval. The rocks composing them are all formed by different species
of coral. The animal which constructs them is of the polyp tribe,
and so small that it can be seen only under the higher powers of
the microscope. It multiplies by means of buds like those of a
tree, the individuals all combining to form a composite stony mass,
which is called a polypidom. A number of such polypidoms growing
close together form a coral reef. See woodcuts.
[Illustration: Coral]
[Illustration: Coral Polyp]
It was at one time supposed that these coral reefs were erected on
the edges of the craters of submarine volcanoes, an opinion to
which their annular form, and the lagoon in the centre, lent some
countenance; but the vast size of some of them, united to several
other particulars connected with them, threw great doubts over this
supposition.
More recently it has been shown by Mr. Darwin that, while volcanic
agency does perform a part in their formation, it is different from
what had been formerly imagined. His supposition is, that these
coral reefs were built round the coasts of islands which had once
stood very much higher above water than they do now. He conceives
that the bottom of the sea under them being very volcanic, and
containing large collections of molten lava beneath a thin solid
crust, the islands have gradually sunk down into the lava, until
their central parts have become covered with a considerable depth
of water. The central parts thus submerged, he imagines, form the
lagoons in the middle of the islands, while the ring of coral reefs
has gradually grown upwards, as the ground on which it rested sank
downwards.
[Illustration: Coral Reef.]
The corals thus rise to near the surface, but immediately on their
being uncovered by the water they die, and the reef ceases to grow.
Then the waves by their action break the upper part of it into
pieces, which thus become heaped up by degrees on the remainder,
until the mass attain so great a height that the sea can no longer
wash over it. Thus the curious ring of land is gradually formed,
and affords a nutritive soil, in which cocoa-nuts, on being cast
ashore, germinate and grow to be large trees. Other seeds, wafted
by the waves or carried by birds, also begin to grow, until the
whole surface becomes covered with vegetation. Then comes man and
builds his habitation upon those fertile spots, and finds in them
an agreeable and convenient abode, well suited to those who are
accustomed to live by fishing and other simple means.
You will thus perceive that the connexion between the atoll and the
volcano consists in this--that while the coral builds up the reef,
the volcano beneath ingulfs the island and causes it to sink down.
In some instances, however, the volcano, after a while, reverses
its action, and raises up the island with the reef upon it. In such
cases, the coral reefs are seen standing out of the water, forming
perpendicular cliffs several hundred feet in height. Then also the
interior of the island becomes once more dry land, and that, too,
of great fertility.
[Illustration: Mount Erebus.]
Almost due south of that region, in the Pacific, where the coral
islands abound, but at a great distance from them, and considerably
within the limits of the Antarctic zone, lies South Victoria. Here,
in lat. 76 degrees S., Captain Ross discovered, in 1841, two
volcanoes, which he called Erebus and Terror, after the names of
his two ships. Of the former, which is the higher of the two, a
view is given in the annexed woodcut. It is covered with perpetual
snow from the bottom even to the tip of the summit. Nevertheless,
it is continually sending forth vast columns of vapour, which glow
with the reflection of the white hot lava beneath. These vapours
ascend to a great height, more than two thousand feet above the top
of the cone, which is itself twelve thousand feet above the level
of the sea.
There is found in these frozen regions a remarkable botanical
curiosity, having a certain connexion with volcanoes. The waters of
the ocean, all along the borders of the icy barrier, produce in
amazing abundance the family of water-plants named Diatomaceae. The
Diatoms are so called from their faculty of multiplying themselves
indefinitely by splitting into two; and so rapidly is this process
performed, that in a month a single diatom may produce a thousand
millions. The quantity found in the Antarctic regions is so immense
that, between the parallels of 60 degrees and 80 degrees of south
latitude, they stain the whole surface of the sea of a pale olive-brown
tint. These plants, which are so minute as to be individually
invisible, save under the higher powers of the microscope, have the
curious property of encrusting themselves with a sheath, or shell,
of pure silica. These shells remain after the death of the plant,
and are as indestructible as flint. They are marvellous objects,
both as respects the elegance of their forms and the beauty of
their markings. So great is the accumulation of these shells at the
bottom of the sea, that they have formed an immense bank 400 miles
in length by 120 in breadth, between the 76th and 78th degrees of
south latitude. One portion of this bank rests on the coast at the
foot of Mount Erebus.
Now, it is remarkable that these microscopic shells of Diatoms are
not unfrequently found in the ejections of volcanoes; while it is
generally supposed that, in the case of those situated near the
sea, eruptions are caused by the formation of explosive steam
consequent on the access of sea-water to the reservoirs of molten
lava lying underground. The proximity of this Diatomaceous bed to
Mount Erebus would easily explain how these minute shells might be
found abundant in the fine dust ejected from that volcano.