Principles of Geology Part 68

It was the opinion of the German naturalist Forster, in 1780, after his voyage round the world with Captain Cook, that coral animals had the power of building up steep and almost perpendicular walls from great depths in the sea, a notion afterwards adopted by Captain Flinders and others; but it is now very generally believed that these zoophytes cannot live in water of great depths.

Mr. Darwin has come to the conclusion, that those species which are most effective in the construction of reefs, rarely flourish at a greater depth than 20 fathoms, or 120 feet. In some lagoons, however, where the water is but little agitated, there are, according to Kotzebue, beds of living coral in 25 fathoms of water, or 150 feet; but these may perhaps have begun to live in shallower water, and may have been carried downwards by the subsidence of the reef. There are also various species of zoophytes, and among them some which are provided with calcareous as well as h.o.r.n.y stems, which live in much deeper water, even in some cases to a depth of 180 fathoms; but these do not appear to give origin to stony reefs.

There is every variety of form in coral reefs, but the most remarkable and numerous in the Pacific consist of circular or oval strips of dry land, enclosing a shallow lake or lagoon of still water, in which zoophytes and mollusca abound. These annular reefs just raise themselves above the level of the sea, and are surrounded by a deep and often unfathomable ocean.

In the annexed cut (fig. 114), one of these circular islands is represented, just rising above the waves, covered with the cocoa-nut and other trees, and inclosing within a lagoon of tranquil water.

[Ill.u.s.tration: Fig. 114.

View of Whitsunday Island. (Capt. Beechey.)[1116]]

The accompanying section will enable the reader to comprehend the usual form of such islands. (Fig. 115.)

[Ill.u.s.tration: Fig. 115.

Section of a Coral Island.

_a_, _a_, Habitable part of the island, consisting of a strip of coral, inclosing the lagoon.

_b_, _b_, The lagoon.

The subjoined cut (fig. 116.) exhibits a small part of the section of a coral island on a larger scale.

[Ill.u.s.tration: Fig. 116.

Section of part of a Coral Island.

_a_, _b_, Habitable part of the island.

_b_, _c_, Slope of the side of the island, plunging at an angle of forty-five to the depth of fifteen hundred feet.

_c_, _c_, Part of the lagoon.

_d_, _d_, Knolls of coral in the lagoon, with overhanging ma.s.ses of coral resembling the capitals of columns.]

Of thirty-two of these coral islands visited by Beechey in his voyage to the Pacific, twenty-nine had lagoons in their centres. The largest was 30 miles in diameter, and the smallest less than a mile. All were increasing their dimensions by the active operations of the lithophytes, which appeared to be gradually extending and bringing the immersed parts of their structure to the surface. The scene presented by these annular reefs is equally striking for its singularity and beauty. A strip of land a few hundred yards wide is covered by lofty cocoa-nut trees, above which is the blue vault of heaven. This band of verdure is bounded by a beach of glittering white sand, the outer margin of which is encircled with a ring of snow-white breakers, beyond which are the dark heaving waters of the ocean. The inner beach incloses the still clear water of the lagoon, resting in its greater part on white sand, and when illuminated by a vertical sun, of a most vivid green.[1117] Certain species of zoophytes abound most in the lagoon, others on the exterior margin, where there is a great surf. "The ocean," says Mr. Darwin, "throwing its breakers on these outer sh.o.r.es, appears an invincible enemy, yet we see it resisted and even conquered by means which at first seem most weak and inefficient. No periods of repose are granted, and the long swell caused by the steady action of the trade wind never ceases. The breakers exceed in violence those of our temperate regions, and it is impossible to behold them without feeling a conviction that rocks of granite or quartz would ultimately yield and be demolished by such irresistible forces. Yet these low insignificant coral islets stand and are victorious, for here another power, as antagonist to the former, takes part in the contest. The organic forces separate the atoms of carbonate of lime one by one from the foaming breakers, and unite them into a symmetrical structure; myriads of architects are at work night and day, month after month, and we see their soft and gelatinous bodies through the agency of the vital laws conquering the great mechanical power of the waves of an ocean, which neither the art of man, nor the inanimate works of nature could successfully resist."[1118]

As the coral animals require to be continually immersed in salt water, they cannot raise themselves by their own efforts, above the level of the lowest tides. The manner in which the reefs are converted into islands above the level of the sea is thus described by Chamisso, a naturalist, who accompanied Kotzebue in his voyages:--"When the reef,"

says he, "is of such a height that it remains almost dry at low water the corals leave off building. Above this line a continuous ma.s.s of solid stone is seen composed of the sh.e.l.ls of mollusks and echini, with their broken-off p.r.i.c.kles and fragments of coral, united by calcareous sand, produced by the pulverization of sh.e.l.ls. The heat of the sun often penetrates the ma.s.s of stone when it is dry, so that it splits in many places, and the force of the waves is thereby enabled to separate and lift blocks of coral, frequently six feet long and three or four in thickness, and throw them upon the reef, by which means the ridge becomes at length so high that it is covered only during some seasons of the year by the spring tides. After this the calcareous sand lies undisturbed, and offers to the seeds of trees and plants cast upon it by the waves a soil upon which they rapidly grow, to overshadow its dazzling white surface. Entire trunks of trees, which are carried by the rivers from other countries and islands, find here, at length, a resting-place after their long wanderings: with these come some small animals such as insects and lizards, as the first inhabitants. Even before the trees form a wood, the sea-birds nestle here; stray land-birds take refuge in the bushes; and, at a much later period, when the work has been long since completed, man appears and builds his hut on the fruitful soil."[1119]

In the above description the solid stone is stated to consist of sh.e.l.l and coral, united by sand; but ma.s.ses of very compact limestone are also found even in the uppermost and newest parts of the reef, such as could only have been produced by chemical precipitation. Professor Aga.s.siz also informs me that his observations on the Florida reefs (which confirm Darwin's theory of atolls to be mentioned in the sequel) have convinced him, that large blocks are loosened, not by shrinkage in the sun's heat, as Chamisso imagined, but by innumerable perforations of lithodomi and other boring testacea.

The carbonate of lime may have been princ.i.p.ally derived from the decomposition of corals and testacea; for when the animal matter undergoes putrefaction, the calcareous residuum must be set free under circ.u.mstances very favorable to precipitation, especially when there are other calcareous substances, such as sh.e.l.ls and corals, on which it may be deposited. Thus organic bodies may be inclosed in a solid cement, and become portions of rocky ma.s.ses.[1120]

The width of the circular strip of dead coral forming the islands explored by Captain Beechey, exceeded in no instance half a mile from the usual wash of the sea to the edge of the lagoon, and, in general, was only about three or four hundred yards.[1121] The depth of the lagoons is various; in some, entered by Captain Beechey, it was from twenty to thirty-eight fathoms.

The two other peculiarities which are most characteristic of the annular reef or atoll are first, that the strip of dead coral is invariably highest on the windward side, and secondly, that there is very generally an opening at some point in the reef affording a narrow pa.s.sage, often of considerable depth, from the sea into the lagoon.

[Ill.u.s.tration: Fig. 117.]

_Maldive and Laccadive Isles._--The chain of reefs and islets called the Maldives (see fig. 117.), situated in the Indian Ocean, to the south-west of Malabar, forms a chain 470 geographical miles in length, running due north and south, with an average breadth of about 50 miles.

It is composed throughout of a series of circular a.s.semblages of islets, all formed of coral, the larger groups being from forty to ninety miles in their longest diameter. Captain Horsburgh, whose chart of these islands is subjoined, states, that outside of each circle or _atoll_, as it is termed, there are coral reefs sometimes extending to the distance of two or three miles, beyond which there are no soundings at immense depths. But in the centre of each atoll there is a lagoon from fifteen to forty-nine fathoms deep. In the channels between the atolls no soundings can usually be obtained at the depth of 150 or even 250 fathoms, but during Captain Moresby's survey, soundings were struck at 150 and 200 fathoms, the only instances as yet known of the bottom having been reached, either in the Indian or Pacific oceans, in a s.p.a.ce intervening between two separate and well characterized atolls.

The singularity in the form of the atolls of this archipelago consists in their being made up, not of one continuous circular reef but of a ring of small coral islets sometimes more than a hundred in number, each of which is a miniature atoll in itself; in other words, a ring-shaped strip of coral surrounding a lagoon of salt water. To account for the origin of these, Mr. Darwin supposes the larger annular reef to have been broken up into a number of fragments, each of which acquired its peculiar configurations under the influence of causes similar to those to which the structure of the parent atoll has been due. Many of the minor rings are no less than three, and even five miles in diameter, and some are situated in the midst of the princ.i.p.al lagoon; but this happens only in cases where the sea can enter freely through breaches in the outer or marginal reef.

The rocks of the Maldives are composed of sandstone formed of broken sh.e.l.ls and corals, such as may be obtained in a loose state from the beach, and which is seen when exposed for a few days to the air to become hardened. The sandstone is sometimes observed to be an aggregate of broken sh.e.l.ls, corals, pieces of wood, and sh.e.l.ls of the cocoa-nut.[1122]

The Laccadive islands run in the same line with the Maldives, on the north, as do the isles of the Chagos Archipelago, on the south; so that these may be continuations of the same chain of submerged mountains, crested in a similar manner by coral limestones.

_Origin of the circular form--not volcanic._--The circular and oval shape of so many reefs, each having a lagoon in the centre, and being surrounded on all sides by a deep ocean, naturally suggested the idea that they were nothing more than the crests of submarine volcanic craters overgrown by coral; and this theory I myself advocated in the earlier editions of this work. Although I am now about to show that it must be abandoned, it may still be instructive to point out the grounds on which it was formerly embraced. In the first place, it had been remarked that there were many active volcanoes in the coral region of the Pacific, and that in some places, as in Gambier's group, rocks composed of porous lava rise up in a lagoon bordered by a circular reef, just as the two cones of eruption called the Kamenis have made their appearance in the times of history within the circular gulf of Santorin.[1123] It was also observed that, as in S. Shetland, Barren Island, and others of volcanic origin, there is one narrow breach in the walls of the outer cone by which s.h.i.+ps may enter a circular gulf, so in like manner there is often a single deep pa.s.sage leading into the lagoon of a coral island, the lagoon itself seeming to represent the hollow or gulf just as the ring of dry coral recalls to our minds the rim of a volcanic crater. More lately, indeed, Mr. Darwin has shown that the numerous volcanic craters of the Galapagos Archipelago in the Pacific have all of them their southern sides the lowest, or in many cases quite broken down, so that if they were submerged and incrusted with coral, they would resemble true atolls in shape.[1124]

Another argument which I adduced when formerly defending this doctrine was derived from Ehrenberg's statement, that some banks of coral in the Red Sea were square, while many others were ribbon-like strips, with flat tops, and without lagoons. Since, therefore, all the genera and many of the species of zoophytes in the Red Sea agreed with those which elsewhere construct lagoon islands, it followed that the stone-making zoophytes are not guided by their own instinct in the formation of annular reefs, but that this peculiar shape and the position of such reefs in the midst of a deep ocean must depend on the outline of the submarine bottom, which resembles nothing else in nature but the crater of a lofty submerged volcanic cone. The enormous size, it is true, of some atolls, made it necessary for me to ascribe to the craters of many submarine volcanoes a magnitude which was startling, and which had often been appealed to as a serious objection to the volcanic theory. That so many of them were of the same height, or just level with the water, did not present a difficulty so long as we remained ignorant of the fact that the reef-building species do not grow at greater depths than twenty-five fathoms.

_May be explained by subsidence._--Mr. Darwin, after examining a variety of coral formations in different parts of the globe, was induced to reject the opinion that their shape represented the form of the original bottom. Instead of admitting that the ring of dead coral rested on a circular or oval ridge of rock, or that the lagoon corresponded to a preexisting cavity, he advanced a new opinion, which must, at first sight, seem paradoxical in the extreme; namely, that the lagoon is precisely in the place once occupied by the highest part of a mountainous island, or, in other cases, by the top of a shoal.

The following is a brief sketch of the facts and arguments in favor of this new view:--Besides those rings of dry coral which enclose lagoons, there are others having a similar form and structure which encircle lofty islands. Of the latter kind is Vanikoro, (see map, fig. 39, p.

351,) celebrated on account of the s.h.i.+pwreck of La Peyrouse, where the coral reef runs at the distance of two or three miles from the sh.o.r.e, the channel between it and the land having a general depth of between 200 and 300 feet. This channel, therefore, is a.n.a.logous to a lagoon, but with an island standing in the middle like a picture in its frame. In like manner in Tahiti we see a mountainous land, with everywhere round its margin a lake or zone of smooth salt water, separated from the ocean by an encircling reef of coral, on which a line of breakers is always foaming. So also New Caledonia, a long narrow island east of New Holland, in which the rocks are granitic, is surrounded by a reef which runs for a length of 400 miles. This reef encompa.s.ses not only the island itself, but a ridge of rocks which are prolonged in the same direction beneath the sea. No one, therefore, will contend for a moment that in this case the corals are based upon the rim of a volcanic crater, in the middle of which stands a mountain or island of granite.

The great barrier reef, already mentioned as running parallel to the north-east coast of Australia for nearly 1000 miles, is another most remarkable example of a long strip of coral running parallel to a coast.

Its distance from the mainland varies from twenty to seventy miles, and the depth of the great arm of the sea thus enclosed is usually between ten and twenty fathoms, but towards one end from forty to sixty. This great reef would extend much farther, according to Mr. Jukes, if the growth of coral were not prevented off the sh.o.r.es of New Guinea by a muddy bottom, caused by rivers charged with sediment which flow from the southern coast of that great island.[1125]

Two cla.s.ses of reefs, therefore, have now been considered; first, the atoll, and, secondly, the encircling and barrier reef, all agreeing perfectly in structure, and the sole difference lying in the absence in the case of the atoll of all land, and in the others the presence of land bounded either by an encircling or a barrier reef. But there is still a third cla.s.s of reefs, called by Mr. Darwin "fringing reefs,"

which approach much nearer the land than those of the encircling and barrier cla.s.s, and which indeed so nearly touched the coast as to leave nothing in the intervening s.p.a.ce resembling a lagoon. "That these reefs are not attached quite close to the sh.o.r.e appears to be the result of two causes; first, that the water immediately adjoining the beach is rendered turbid by the surf, and therefore injurious to all zoophytes; and, secondly, that the larger and efficient kinds only flourish on the outer edge amidst the breakers of the open sea."[1126]

[Ill.u.s.tration: Fig. 118.

Supposed section of an island with an encircling reef of coral.

A, The island.

_b_, _c_, Highest points of the encircling reef between which and the coast is seen a s.p.a.ce occupied by still water.]

It will at once be conceded that there is so much a.n.a.logy between the form and position of the strip of coral in the atoll, and in the encircling and barrier reef, that no explanation can be satisfactory which does not include the whole. If we turn in the first place to the encircling and barrier reefs, and endeavor to explain how the zoophytes could have found a bottom on which to begin to build, we are met at once with a great difficulty. It is a general fact, long since remarked by Dampier, that high land and deep seas go together. In other words, steep mountains coming down abruptly to the sea-sh.o.r.e are generally continued with the same slope beneath the water. But where the reef, as at _b_ and _c_ (fig. 118), is distant several miles from a steep coast, a line drawn perpendicularly downwards from its outer edges _b c_ to the fundamental rock _d e_, must descend to a depth exceeding by several thousand feet the limits at which the efficient stone-building corals can exist, for we have seen that they cease to grow in water which is more than 120 feet deep. That the original root immediately beneath the points _b c_ is actually as far from the surface as _d e_, is not merely inferred from Dampier's rule, but confirmed by the fact, that, immediately outside the reef, soundings are either not met with at all, or only at enormous depths. In short, the ocean is as deep there as might have been antic.i.p.ated in the neighborhood of a bold coast; and it is obviously the presence of the coral alone which has given rise to the anomalous existence of shallow water on the reef and between it and the land.

After studying in minute detail all the phenomena above described, Mr.

Darwin has offered in explanation a theory now very generally adopted.

The coral-forming polypi, he states, begin to build in water of a moderate depth, and while they are yet at work, the bottom of the sea subsides gradually, so that the foundation of their edifice is carried downwards at the same time that they are raising the superstructure. If, therefore, the rate of subsidence be not too rapid, the growing coral will continue to build up to the surface; the ma.s.s always gaining in height above its original base, but remaining in other respects in the same position. Not so with the land; each inch lost is irreclaimably gone; as it sinks, the water gains foot by foot on the sh.o.r.e, till in many cases the highest peak of the original island disappears. What was before land is then occupied by the lagoon, the position of the encircling coral remaining unaltered, with the exception of a slight contraction of its dimensions.

In this manner are encircling reefs and atolls produced; and in confirmation of his views Mr. Darwin has pointed out examples which ill.u.s.trate every intermediate state, from that of lofty islands, such as Otaheite, encircled by coral, to that of Gambier's group, where a few peaks only of land rise out of a lagoon, and lastly, to the perfect atoll, having a lagoon several hundred feet deep, surrounded by a reef rising steeply from an unfathomed ocean.

[Ill.u.s.tration: Fig. 119.]

If we embrace these views, it is clear, that in regions of growing coral a similar subsidence must give rise to barrier reefs along the sh.o.r.es of a continent. Thus suppose A (fig. 119), to represent the north-east portion of Australia, and _b c_ the ancient level of the sea, when the coral reef _d_ was formed. If the land sink so that it is submerged more and more, the sea must at length stand at the level _e f_, the reef in the mean time having been enlarged and raised to the point _g_. The distance between the sh.o.r.e _f_, and the barrier reef _g_, is now much greater than originally between the sh.o.r.e _c_ and the reef _d_, and the longer the subsidence continues the farther will the coast of the mainland recede.

When the first edition of this work appeared in 1831, several years before Mr. Darwin had investigated the facts on which his theory is founded, I had come to the opinion that the land was subsiding at the bottom of those parts of the Pacific where atolls are numerous, although I failed to perceive that such a subsidence, if conceded, would equally solve the enigma as to the form both of annular and barrier reefs.

I shall cite the pa.s.sage referred to, as published by me in 1831:--"It is a remarkable circ.u.mstance that there should be so vast an area in Eastern Oceanica, studded with minute islands, without one single spot where there is a wider extent of land than belongs to such islands as Otaheite, Owhyhee, and a few others, which either have been or are still the seats of active volcanoes. If an equilibrium only were maintained between the upheaving and depressing force of earthquakes, large islands would very soon be formed in the Pacific; for, in that case, the growth of limestone, the flowing of lava, and the ejection of volcanic ashes, would combine with the upheaving force to form new land.