Part 10 (1/2)

When the structure of a fringing reef is investigated, the bottooon is found to be covered with fine whitishup of the dead corals Upon thiscorals, or occasionally great blocks of dead coral, which have been torn by storoon Shellfish and worms of various kinds abound; and fish, some of which prey upon the coral, sport in the deeper pools

But the corals which are to be seen growing in the shalloaters of the lagoon are of a different kind froe of the reef, and of which the reef is built up Close to the seaward edge of the reef, over which, even in calm weather, a surf almost always breaks, the coral rock is encrusted with a thick coat of a singular vegetable organisreat deal of lime--the so-called Nullipora Beyond this, in the part of the edge of the reef which is always covered by the breaking waves, the living, true, reef-polypes make their appearance; and, in different forms, coat the steep seaward face of the reef to a depth of one hundred or even one hundred and fifty feet Beyond this depth the sounding-lead rests, not upon the wall-like face of the reef, but on the ordinary shelving sea-botto reef extends from the land corresponds with that at which the sea has a depth of twenty or five-and-twenty fathoms

If, as we have supposed, the sea could be suddenly withdrawn fro reef, such as the Mauritius,[122]

the reef would present the aspect of a terrace, its seaward face, one hundred feet orwith the animal flowers of the coral, while its surface would be hollowed out into a shallow and irregular moat-like excavation

The coral oon, and hich all the interstices of the coral skeletons which accumulate to form the reef are filled up, does not proceed fro action of the waves alone; innumerable fishes, and other creatures which prey upon the coral, add a very important contribution of finely-triturated calcareous ether, gradually harden and give rise to a sort of liood deal in texture Sometimes it remains friable and chalky, but, ed with carbonic acid, dissolves some of the calcareous matter, and deposits it elsewhere in the interstices of the nascent rock, thus glueing and ceether into a hard mass; or it may even dissolve the carbonate of lime more extensively, and re-deposit it in a crystalline foroon, where the coral sand is washed into layers by the action of the waves, its grains becoether into strata of a li when struck with a ha with that of the surface of the beach The hard parts of the many animals which live upon the reef become imbedded in this coral limestone, so that a block may be full of shells of bivalves and univalves, or of sea urchins; and even sos of turtles in a state of petrification

The active and vigorous growth of the reef goes on only at the seaward ins, where the polypes are exposed to the wash of the surf, and are thereby provided with an abundant supply of air and of food

The interior portion of the reef arded as almost wholly an accumulation of dead skeletons Where a river comes down from the land there is a break in the reef, for the reasons which have been already ing reef, such as that just described, are plain enough The embryos of the coral polypes have fixed theed shore of the island, as far out as they could live, naeneration has succeeded another, building itself up upon the dead skeletons of its predecessor The mass has been consolidated by the infiltration of coral mud, and hardened by partial solution and redeposition, until a great rampart of coral rock one hundred or one hundred and fifty feet high on its seaward face has been foraps as result from the outflow of rivers, in the place of sally-ports

The structure of the rocky accu reefs and in the atolls is essentially the sa reef But, in addition to the differences of depth inside and out, they present some other peculiarities These reefs, and especially the atolls, are usually interrupted at one part of their circumference, and this part is always situated on the leeward side of the reef, or that which is the more sheltered side Now, as all these reefs are situated within the region in which the tradewinds prevail, it follows that, on the north side of the equator, where the trade-wind is a northeasterly wind, the opening of the reef is on the southwest side: while in the southern hemisphere, where the trade-winds blow fro lies to the northwest The curious practical result follows frooons to these reefs really foret inside the reefs and the atolls, on the one hand, and the fringing reefs on the other, lies in the fact of the reater depth of water on the seaward faces of the former As a consequence of this fact, the whole of this face is not, as it is in the case of the fringing reef, covered with living coral polypes For, as we have seen, these polypes cannot live at a greater depth than about twenty-five fathoms; and actual observation has shown that while, down to this depth, the sounding-lead will bring up branches of live coral froreater depth it fetches to the surface nothing but dead coral and coral sand We must, therefore, picture to ourselves an atoll, or an encircling reef, as fringed for one hundred feet, or ed in fabricating coral; while, below this comparatively narrow belt, its surface is a bare and smooth expanse of coral sand, supported upon and within a core of coral limestone Thus, if the bed of the Pacific were suddenly laid bare, as was just now supposed, the appearance of the reef-mountains would be exactly the reverse of that presented by h mountains on land

For these are white with snow at the top, while their bases are clothed with an abundant and gaudily-coloured vegetation But the coral cones would look grey and barren belohile their suay with a richly-coloured parterre of flowerlike coral polypes

The practical difficulties of sounding upon, and of bringing up portions of, the seaward face of an atoll or of an encircling reef, are so great, in consequence of the constant and dangerous shich sets towards it, that no exact infor the depth to which the reefs are composed of coral has yet been obtained There is no reason to doubt, however, that the reef-cone has the same structure frohout mainly composed of dead coral

And now arises a serious difficulty If the coral polypes cannot live at a greater depth than one hundred or one hundred and fifty feet, how can they have built up the base of the reef-cone, which may be two thousand feet, or et over this objection, it was at one ti polypes had settled upon the summits of a chain of subraphy to justify the assu for one thousand ht, that none should rise above the level of the sea, nor fall one hundred and fifty feet below that level?

How, again, on this hypothesis, are atolls to be accounted for, unless, as soe in the wild supposition that every atoll corresponds with the crater of a submarine volcano? And what explanation does it afford of the fact that, in so reefs occur, while others present none but fringing reefs?

These and other puzzling facts remained insoluble until the publication, in the year 1840, of Mr Darwin's faiven to all the difficult problems connected with the subject, and every difficulty was shown to be capable of solution by deductive reasoning froical and biological truths Mr Darwin, in fact, showed that, so long as the level of the sea re formed, or if the level of the sea relatively to that of the land is falling, the only reefs which can be for reefs While if, on the contrary, the level of the sea is rising relatively to that of the land, at a rate not faster than that at which the upward growth of the coral can keep pace with it, the reef will gradually pass fro or barrier reef And, finally, that if the relative level of the sea rise so ed, the reef must necessarily pass into the condition of an atoll

For, suppose the relative level of the sea to re reef has reached that distance from the land at which the depth of water amounts to one hundred and fifty feet Then the reef cannot extend seaward by the erms would find the bottom of the sea to be too deep for them to live in And the only manner in which the reef could extend outwards, would be by the gradual accumulation, at the foot of its seaward face, of a talus of coral fraght, in course of ti its upper surface within the lirowth, and in that manner provide a sort of factitious sea-bottoht perch

If, on the other hand, the level of the sea were slowly and gradually lowered, it is clear that the parts of its bottoradually be brought within the required distance of the surface, and thus the reef ive rise neither to an encircling reef nor to an atoll, but to a broad belt of upheaved coral rock, increasing the dimensions of the dry land, and continuous seawards with the fresh fringing reef

Suppose, however, that the sea-level rose instead of falling, at the sa in some parts of the world,--not more, in fact, than a few inches, or, at most, a foot or two, in a hundred years Then, while the reef would be unable to extend itself seaward, the sea-bottoradually more and more removed from the depth at which the life of the coral polypes is possible, it would be able to groards as fast as the sea rose But the groould take place almost exclusively around the circuion in which the coral polypes would find the conditions favourable for their existence The bottooon would be raised, in the main, only by the coral debris and coral mud, forins of the reef would rise faster than the bottooon would constantly becoradually increase in breadth; as the rising sea, covering more of the land, would occupy a wider space between the edge of the reef and what re sea would eventually convert a large island with a fringing reef into a s reef And it will be obvious that when the rising of the sea has gone so far as cohest points of the island, the reef will have passed into the condition of an atoll

But how is it possible that the relative level of the land and sea should be altered to this extent? Clearly, only in one of tays: either the sea must have risen over those areas which are now covered by atolls and encircling reefs; or, the land upon which the sea restsextent

If the sea has risen, its rise must have taken place over the whole world siht over all parts of the coral zone Grounds have been shown for the belief that the general level of the sea ested, for exa one of the cold periods of the earth's history necessarily implies a diminution in the volume of the sea proportioned to the amount of its water thus permanently locked up in the Arctic and Antarctic ice-cellars; while, in the warreater or less disappearance of the polar ice-cap i addition of water to the ocean And no doubt this reasoning h it is very hard to say what practical effect the additions and subtractions thus made have had on the level of the ocean; inasht be either intensified or nullified, by contees in the level of the land And no one has yet shown that any such greatof the level of the water of the ocean, has taken place since the existing atolls began to be formed

In the absence of any evidence that the sea has ever risen to the extent required to give rise to the encircling reefs and the atolls, Mr Darwin adopted the opposite hypothesis, viz, that the land has undergone extensive and slow depression in those localities in which these structures exist

It see paradox, to suppose that the land is less fixed than the sea; but that such is the case is the unifory Beds of sandstone or limestone, thousands of feet thick, and all full of marine remains, occur in various parts of the earth's surface, and prove, beyond a doubt, that when these beds were formed, that portion of the sea-bottoradual depression to a distance which cannot have been less than the thickness of those beds, and , therefore, that the great areas of the Pacific and of the Indian Ocean, over which atolls and encircling reefs are found scattered, have undergone a depression of some hundreds, or, it may be, thousands of feet, Mr Darwinforced or improbable, but was entirely in accordance e know to have taken place over similarly extensive areas, in other periods of the world's history But Mr Darwin subjected his hypothesis to an ingenious indirect test If his view be correct, it is clear that neither atolls, nor encircling reefs, should be found in those portions of the ocean in which we have reason to believe, on independent grounds, that the sea-botto

Now it is known that, as a general rule, the level of the land is either stationary, or is undergoing a slow upheaval, in the neighborhood of active volcanoes; and, therefore, neither atolls nor encircling reefs ought to be found in regions in which volcanoes are numerous and active

And this turns out to be the case Appended to Mr Darwin's great work on coral reefs, there is areefs are indicated by one colour, fringing reefs by another, and active volcanoes by a third And it is at once obvious that the lines of active volcanoes lie around thereefs It is exactly as if the upheaving volcanic agencies had lifted up the edges of these great areas, while their centres had undergone a corresponding depression An atoll area ins of which have been pushed up by the subterranean forces, to which the craters of the volcanoes have, at intervals, given vent

Thus we ine the area of the Pacific now covered by the Polynesian Archipelago, as having been, at soreat continent, with the ordinarily diversified surface of plain, and hill, and reat land were doubtless fringed by coral reefs; and, as it slowly underwent depression, the hilly regions, converted into islands, beca reefs, and then, as depression went on, these beca reefs, and these, finally, into atolls, until a irdled islets took the place of the original landreefs furnish us with clear, though indirect, evidence of changes in the physical geography of large parts of the earth's surface; and even, as ested, give us indications of thefacts connected with the distribution of aniht about For example, Australia and New Guinea are separated by Torres Straits, a broad belt of sea one hundred or one hundred and twenty miles wide Nevertheless, there is in many respects a curious resemblance between the land animals which inhabit New Guinea and the land animals which inhabit Australia But, at the same time, the marine shellfish which are found in the shalloaters of the shores of New Guinea are quite different from those which are met with upon the coasts of Australia Now, the eastern end of Torres Straits is full of atolls, which, in fact, form the northern termination of the Great Barrier Reef which skirts the eastern coast of Australia

It follows, therefore, that the eastern end of Torres Straits is an area of depression, and it is very possible, and on hly probable, that, in former tiether, and that Torres Straits did not exist If this were the case, the existence of cassowaries and of marsupial quadrupeds, both in New Guinea and in Australia, becoible; while the difference between the littoral molluscs of the north and the south shores of Torres Straits is readily explained by the great probability that, when the depression in question took place, and as, at first, an ar Australia from New Guinea, the northern shore of this new sea became tenanted with marine animals frorants fro rowth of the reef depends upon that of successive generations of coral polypes, and as each generation takes a certain tirow to its full size, and can only separate its calcareous skeleton from the water in which it lives at a certain rate, it is clear that the reefs are records not only of changes in physical geography, but of the lapse of time It is by no means easy, however, to estiy, and the attempts which have been rows vertically have yielded anything but precise results A cautious writer, Mr Dana,[125] whose extensive study of corals and coral reefs e, states his conclusion in the following ter madrepore is not over one and a half inches a year As the branches are open, this would not be equivalent to ht of solid coral for the whole surface covered by the hths of an inch of solid lie bare patches without corals, and the coral sands are widely distributed by currents, part of the corals; not ion is, in fact, covered with growing species This reduces the three-eighths to ONE-SIXTEENTH Shells and other organic relics may contribute one-fourth as e upward increase of the whole reef-ground per year would not exceed ONE-EIGHTH of an inch