 Chapter 50 Part 1 of Principles of Geology This is a LibriVox recording. All LibriVox recordings are in the public domain. For more information or to volunteer, please visit LibriVox.org. Principles of Geology by Charles Lyle Formation of Coral Reefs The powers of the organic creation in modifying the form and structure of the Earth's crust are most conspicuously displayed in the labours of the coral animals. We may compare the operation of these zoophytes in the ocean to the effects produced on a smaller scale upon the land by the plants which generate peat. In the case of the sphagnum, the upper part vegetates while the lower part is entering into a mineral mass in which the traces of organization remain when life has entirely ceased. In corals, in like manner, the more durable materials of the generation that has passed away serve as the foundation on which the living animals continue to rear a similar structure. The stony part of the lamelliform zoophyte may be likened to an internal skeleton, for it is always more or less surrounded by a soft animal substance capable of expanding itself. Yet, when alarmed, it has the power of contracting and drawing itself almost entirely into the cells and hollows of the hard coral. Although oftentimes beautifully coloured in their own element, the soft parts become, when taken from the sea, nothing more in appearance than a brown slime spread over the stony nucleus. The growth of these corals, which form reefs of solid stone, is entirely confined to the warmer regions of the globe, rarely extending beyond the tropics above two or three degrees, except under peculiar circumstances, as in the Bermuda Islands in latitude 32 degrees north, where the Atlantic is warmed by the Gulf Stream. The Pacific Ocean, throughout the space comprehended between the 30s parallels of latitude on each side of the equator, is extremely productive of coral, as also are the Arabian and Persian Gulfs. Coral is also abundant in the sea between the coast of Malabar and the island of Madagascar. Flinders describes a reef of coral on the east coast of New Holland as having a length of nearly 1000 miles, and as being in one part unbroken for a distance of 350 miles. Some groups of coral islands in the Pacific are from 1100 to 1200 miles in length by 300 or 400 in breadth, as the dangerous archipelago, for example, and that called Radak by Kotsabu. But the islands within these spaces are always small points, and often very thinly sewn. Of the numerous species of zoophytes, which are engaged in the production of coral banks, some of the most common belong to the Lamarckian genera, astrea, porites, madrepor, millipore, cariofilia, and meandrina. Rate of the growth of coral Very different opinions have been entertained in regard to the rate at which coral reefs increase. In Captain Beachy's late expedition to the Pacific, no positive information could be obtained of any channel having been filled up within a given period. And it seems established that several reefs had remained for more than half a century at about the same depth from the surface. Ehrenberg also questions the fact of channels and harbors having been closed up in the Red Sea by the rapid increase of coral limestone. He supposes the notion to have arisen from the circumstance of havens having been occasionally filled up in some places with coral sand, in others with large quantities of ballast of coral rock thrown down from vessels. The natives of the Bermuda Islands point out certain corals now growing in the sea, which, according to tradition, have been living in the same spots for centuries. It is supposed that some of them may be in age with the most ancient trees of Europe. Ehrenberg also observed single corals of the genera meandrina anfavia having a global form, from 6 to 9 feet in diameter, which must, he says, be of immense antiquity, probably several thousand years old, so that a farro may have looked upon these same individuals in the Red Sea. They certainly imply, as he remarks, that the reef on which they grow has increased at a very slow rate. After collecting more than 100 species, he found none of them covered with parasitic zoophytes, nor any instance of a living coral growing on another living coral. To this repulsive power, which they exert whilst living, against all others of their own glass, we owe the beautiful symmetry of some large meandrinae and other species, which adorn our museums. Yet Balani and Cipulae can attach themselves to living corals, and holes are excavated in them by Saksikovus Moluska. At the island called Tapoto, in the South Pacific, the anchor of a ship, wrecked about 50 years before, was observed in seven fathoms' water, still preserving its original form, but entirely encrusted by coral. This fact would seem to imply a slow rate of augmentation. But to form a correct estimate of the average rate must be very difficult, since it must vary not only according to the species of coral, but according to the circumstances under which each species may be placed. Such, for example, as the depths from the surface, the quantity of light, the temperature of the water, its freedom from sand and mud, or as the absence or presence of breakers, which is favorable to the growth of some kinds, and is fatal to that of others. It should also be observed that the apparent stationary condition of some coral reefs, which, according to Bici, have remained for centuries at the same depth under water, may be due to subsidence, the upward growth of the coral having been just sufficient to keep pace with the sinking of the solid foundation on which the sauophytes have built. We shall afterward see how far this hypothesis is borne out by other evidence in the regions of annular reefs or atolls. In one of the Maldives islands, a coral reef, which, within a few years, existed on an islet bearing coconut trees, was found by Leutnant Prentice, quote, entirely covered with live coral and madripur, end quote. The natives stated that the islet had been washed away by a change in the currents, and it is clear that a coating of growing coral had been formed in a short time. Experiments also of Dr. Allen on the east coast of Madagascar prove the possibility of coral growing to a thickness of three feet in about half a year, so that the rate of increase may, under favorable circumstances, be very far from slow. It must not be supposed that the calcerous masses, termed coral reefs, are exclusively the work of sauophytes. A great variety of shells, and among them some of the largest and heaviest of known species, contribute to augment the mass. In the South Pacific, great beds of oysters, mussels, pinnaya marinae, kamae or tridaknae, and other shells cover in profusion almost every reef. And on the beach of coral islands are seen the shells of achini and broken fragments of crustaceous animals. Large shells of fish are also discernible through the clear blue water, and their teeth and hard pellets cannot fail to be often preserved, although their soft cartilaginous bones may decay. It was the opinion of the German naturalist Forster in 1780, after his voyeur drowned 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 sauophytes 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 phasms or 120 feet. In some lagoons, however, where the water is but little agitated, there are, according to Kotzebu, beds of living coral in 25 phasms 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 sauophytes, and among them some, which are provided with calcerios as well as horny stems, which live in much deeper water, even in some cases to a depth of 180 phasms. 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 sauophytes 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, one of these circular islands is represented, just rising above the waves, covered with the coconut and other trees, and enclosing within a lagoon of tranquil water. This accompanying section will enable the reader to comprehend the usual form of such islands. Of 32 of these coral islands visited by beachy in his voyage to the Pacific, 29 had lagoons in their centers. 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 coconut 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 encloses the still clear water of the lagoon, resting in its greater part on white sand, and when illuminated by a vertical sun of the most vivid green. 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 shores 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 though 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. As the coral animals require to be continually immersed in salt water, they cannot rise themselves by their own efforts above the level of the lowest guides. The manner in which the reefs are converted into islands above the level of the sea is thus described by Camiso, a naturalist, who accompanied Kotsibu in his voyages. When the reef, says he, is of such a height that it reminds almost dry at low water the corals leave off building. Above this line a continuous mass of solid stone is seen composed of the shells of mollusks anihini with their broken off bricles and fragments of coral united by calcerous sand produced by the pulverization of shells. The heat of the sun often penetrates the mass 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 calcerous 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 seabird's 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 in the above description the solid stone is stated to consist of shell and coral united by sand but masses of very compact limestone are also found even the uppermost and newest parts of the reef such as could only have been produced by chemical precipitation. Professor Agassiz 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 Camiso imagined but by innumerable perforations of lithodomy and other boring testesia the carbonate of lime may have been principally derived from the decomposition of corals and testesia for when the animal matter undergoes putrefaction the calciferous residue must be set free under circumstances very favorable to precipitation especially when there are other calciferous substances such as shells and corals on which it may be deposited thus organic bodies may be enclosed in a solid cement and become portions of rocky masses the widths of the circular strip of dead coral forming the islands explored by captain beachy 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 the depth of the lagoons is various in some entered by captain beachy it was from 20 to 38 phasems the two other peculiarities which are most characteristic of the annual 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 passage often of considerable depths from the sea into the lagoon mildive and locative aisles the chain of reefs and islets called the Maldives situated in the indian ocean to the southwest of malabar forms a chain 470 geographical miles in length running due north and south with an average breath of about 50 miles it is composed throughout of a series of circular assemblages of islets all formed of coral the larger groups being from 40 to 90 miles in the longest diameter captain horsberg whose chart of these islands is subjoined states that outside of each circle of 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 center of each atoll there is a lagoon from 15 to 49 phasems deep in the channels between the atolls no soundings can usually be obtained at the depths of 150 or even 250 phasems but during captain morrisby's survey soundings were struck at 150 and 200 phasems the only instances as yet known of the bottom having been reached either in the indian or pacific oceans in a space 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 darvin supposes the larger annual 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 a principal 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 shells and corals such as maybe 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 shells corals pieces of wood and shells of the coconut the lackative islands run in the same line with the Maldives on the north as do the aisles of the kagos 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 lagooned in the center and being surrounded on all sides by a deep ocean naturally suggested the idea that there 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 formally embraced in the first place it had been remarked that there were many active volcanoes in the coral regions of the pacific and that in some places as in gumbiers 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 comeonies have made their appearance in the times of history within the circular gulf of san toriin it was also observed that as in south shetland barren island and others of volcanic origin there is one narrow breach in the walls of the outer cone by which ships may enter a circular gulf so in like manner there is often a single deep passage 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 kalapagos 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 encrusted with coral they would resemble true atolls in shape another argument which i adduced when formerly defending this doctrine was derived from erenberg'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 janira 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 remain ignorant of the fact that the reef building species do not grow at greater depths than 25 phasms end of chapter 50 part 1 chapter 50 part 2 of principles of geology this is a LibriVox recording all LibriVox recordings are in the public domain for more information or to volunteer please visit LibriVox.org principles of geology by Charles Lyle formation of coral reefs part 2 maybe 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 pre-existing 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 on the latter kind is Vani quoro celebrated on account of the shipwreck of La Peruse where the coral reef runs at the distance of two or three miles from the shore the channel between it and the land having a general depth of between 200 and 300 feet this channel therefore is analogous 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 around 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 homing so also new caledonia along 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 encompasses 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 northeast 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 20 to 70 miles and the depth of the great arm of the sea thus enclosed is usually between 10 and 20 fathoms but towards one end from 40 to 60 this great reef would extend much further according to mr juke's if the growths of coral were not prevented off the shores of new guinea by a muddy bottom caused by rivers charged with sediment which flow from the southern coast of that great island two classes 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 class of reefs called by mr darving fringing reefs which approach much nearer the land than those of the encircling or barrier class and which indeed so nearly touched the coast as to leave nothing in the intervening space resembling a lagoon that these reefs are not attached quite close to the shore 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 it will at once be conceded that there is so much analogy 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 the ampere that high land and deep seas go together in other words steep mountains coming down abruptly to the seashore are generally continued with the same slope beneath the water but where the reef as at b and c at figure 118 is distant several miles from the steep coast a line drawn perpendicularly downwards from its outer edges bc 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 128 deep that the original route immediately beneath the points bc is actually as far from the surface as d e is not merely inferred from the ampere's rule but confirmed by the fact that immediately outside the reef soundings are either not met with at all or only at enormous steps in short the ocean is as deep there as might have been anticipated 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 my new detail all the phenomena above described mr. Darwin has offered in explanation a theory now very generally adopted the coral forming polypy 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 mass 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 food by food on the shore 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 our encircling reefs and autos produced and in confirmation of his views mr. darman has pointed out examples which illustrate every intermediate state from that of lofty islands such as otahiti and circled by coral to that of come here's group where a few peaks only of land rise out of a lagoon and lastly to the perfect at all having a lagoon several hundred feet deep surrounded by a reef rising steeply from an unfathomed ocean 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 shores of a continent thus suppose a in figure 119 to represent the northeast portion of australia and bc 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 meantime having been enlarged and raised to the point g the distance between the shore f and barrier reef g is now much greater than originally between the shore c and the reef d and the longer the subsidence continues this further 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 at all 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 passage referred to as published by me in 1831 it is a remarkable circumstance that there should be so vast an area in eastern oceania studded with my new islands without one single spot where there is a wider extent of land then belongs to such islands as otaheti of high and a few others which either have been or are still the seeds 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 suppose a show 600 miles in length to sink 15 feet and then to remain unmoved for a thousand years during that interval the growing coral may again approach the surface then let the moss be re elevated 15 feet so that the original reef is restored to its former position in this case the new coral formed since the first subsidence will constitute an island 600 miles long an analogous result would have occurred if a lava current 15 feet thick had overflowed the submerged reef the absence therefore of more extensive tracks of land in the pacific seems to show that the amount of subsidence of by earthquakes exceeds in that quarter of the globe at present the elevation due to the same cause another proof also of subsidence derived from the structure of otto's was pointed out by me in the following passage in all former additions the low coral islands of the pacific says captain beachy follow one general rule in having their windward side higher and more perfect than the other at gambia and matilda islands this inequality is very conspicuous the weather side of both being wooded and of the former inhabited while the other sides are from 20 to 30 feet underwater where however they may be perceived to be equally narrow and well defined it is on the leeward side also that the entrances into the lagoons occur and although they may sometimes be situated on a side that runs in the direction of the wind as at bow island yet there are none to windward these observations of captain beachy accord with those which captain horsebook and other hydrographers have made in regard to the coral islands of other seas from this fortunate circumstance ships can enter and sail out with ease whereas if the narrow inlets were to windward vessels which once entered might not succeed for months in making their way out again the well-known security of many of these harbors depends entirely on this fortunate peculiarity in their structure in what manner is the singular conformation to be accounted for the action of the waves is seen to be the cause of the superior elevation of some reefs on the windward sides where sand and large masses of coral rock are thrown up by the breakers but there is a variety of cases where this cause alone is inadequate to solve the problem for reefs submerged at considerable depth where the movements of the sea cannot exert much power have nevertheless the same conformation the leeward being much lower than the windward side i am informed by captain king that on examining the reefs called roley shawls which lie off the northwest coast of australia where the east and west monsoons prevail alternately he found the open side of one crescent shape reefed the imperious turned to the east and of other the mermaid turned to the west while a third oval reef of the same group was entirely submerged this want of conformity is exactly what we should expect where the winds vary periodically it seems impossible to refer the phenomenon now under consideration to any original uniformity in the configuration of submarine volcanoes on the summits of which we may suppose the coral reefs to grow for although it is very common for craters to be broken down on one side only on one side only we cannot imagine any cause that should breach them all in the same direction but the difficulty will perhaps be removed if we call in another part of the volcanic agency subsidence by earthquakes suppose the windward barrier to have been raised by the mechanical action of the waves to the height of two or three yards above the wall on the leeward side and then the whole island to sink down a few fathoms the appearance is described would then be presented by the submerged reef a repetition of such operations by the alternate elevation and depression of the same mass and hypothesis strictly conformable to analogy might produce still greater inequality in the two sides especially as the violent efflux of the tide has probably a strong tendency to check the accumulation of the more tender corals on the leeward reef while the action of the breakers contributes to raise the windward barrier previously to my adverting to the science above enumerated of a downward movement in the bed of the ocean dr. McCulloch captain beachy and many other writers had shown that masses of recent coral had been laid to try at various heights above the sea level both in the red sea the islands of the pacific and in the east and west indies after describing 32 coral islands in the pacific captain beachy mentioned that they were all formed of living coral except one which although of coral formation was raised about 70 or 80 feet above the level of the sea and was encompassed by a reef of living coral it is called elizabeth or henderson's island and is five miles in length by one in breadth it has a flat surface and on all sides except the north is bounded by perpendicular cliffs about 50 feet high composed entirely of dead coral more or less porous honeycombed at the surface unhardening into a compact calcerous mass which possesses the fracture of secondary limestone and has a species of millipore interspersed through it these cliffs are considerably undermined by the action of the waves and some of them appear on the eve of precipitating their separate incumbent weight into the sea those which are less injured in this way present no alternate ridges or indication of the different levels which the sea might have occupied at different periods but a smooth surface as if the island which has probably been raised by volcanic agency had been forced up by a one great subterraneous convulsion at the distance of a few hundred yards from this island no bottom could be gained with 200 phasms of line it will be seen from the annexed sketch communicated to me by loyton smith of the blossom that the trees come down to the beach towards the center of the island a break at first sight resembling the openings which usually lead into lagoons but the trees stand on a steep slope and no hollow of an ancient lagoon was perceived beachy also remarks that the surface of henderson's island is flat and that in queen charlotte's island one of the same group but underwater there was no lagoon the coral having grown up everywhere to one level the probable cause of this obliteration of the central basin or lagoon will be considered in the sequel that the bed of the pacific and indian oceans where atolls are frequent must have been sinking for ages might be inferred says mr darvin from simply reflecting on two facts first that the efficient coral building saw fights do not flourish in the ocean at a greater depth than 120 feet and secondly that there are spaces occupying areas of many hundred thousand square miles where all the islands consist of coral and yet none of which rise to a greater height than may be accounted for by the action of the winds and waves on broken and triterated coral where we to take for granted that the floor of the ocean had remained stationary from the time when the coral began to grow we should be compelled to assume that an incredible number of submarine mountains was height for the ocean is always deep and often unfathomable between the different atolls had all come to within 120 feet of the surface and yet no one mountain had risen above water but no sooner do we admit the theory of subsidence than this great difficulty vanishes however varied may have been the altitude of different islands or the separate peaks of particular mountain chains all may have been reduced to one uniform level by the gradual submergence of the loftiest points and the additions made to the calcerios cappings of the less elevated summits as they subsided to great depths openings into the lagoons in the general description of atolls and encircling reefs it was mentioned there is almost always a deep narrow passage opening into the lagoon or into the steel water between the reef and the shore which is kept open by the efflux of the sea as the tide goes down the origin of this channel must according to this theory of subsidence before explained be traced back to causes which were in action during the existence of the encircling reef and when an island or mountaintop rose within it for such a reef precedes the atoll in the order of formation now in those islands in the pacific which are large enough to feed small rivers there is generally an opening or channel in the surrounding coral reef at the point where the stream of fresh water enters the sea the depth of these channels rarely exceeds 25 feet and they may be attributed says captain beachy to the aversion of the leesophytes to fresh water and to the probable absence of the mineral matter of which they construct their habitations mr darwin however has shown that mud at the bottom of river courses is far more influential than the freshness of the water in preventing the growth of the polypy for the walls which enclose the openings are perpendicular and do not slant off gradually as would be the case if the nature of the element presented the only obstacle to the increase of the coral the increase of the coral building animals when a breach has just been made in the reef it will be prevented from closing up by the efflux of the sea at low tides for it is sufficient that a reef should rise a few feet above low watermark to cause the waters to collect in the lagoon at high tide and when the sea falls to rush out at one or more points where the reef happens to be the lowest or weakest this event is strictly analogous to that witnessed in our estuaries where a body of salt water accumulated during the flow issues with great velocity at the above the tide and scores out or keeps open a deep passage through the bar which is almost always formed at the mouth of the river at first there are probably many openings but the growth of the coral tends to obstruct all those which do not serve as the principal channels of discharge so that their number is gradually reduced to a few and often finally to one the fact observed universally that the principal opening fronts a considerable volley in the encircled island between the shores of which and the outer reef there is often deep water scarcely leaves any doubt as to the real origin of the channel in all those countless outdoors where the nucleus of land has vanished end of chapter 50 part 2 chapter 50 part 3 of principles of geology this is a LibriVox recording all LibriVox recordings are in the public domain for more information or to volunteer please visit LibriVox.org principles of geology by Charles Lyle chapter 50 formation of coral reefs part 3 size of atolls and barrier reefs in regard to the dimensions of atolls it was stated that some of the smallest observed by beachy in the pacific were only a mile in diameter if their external slope underwater equals upon an average an angle of 45 degrees then would such an atoll at the depth of half a mile or 2,640 feet have a diameter of two miles hence it would appear that there must be a tendency in every atoll to grow smaller except in those cases where oscillations of level enlarge the base on which the coral grows by throwing down a talus of detrital matter around the original cone of limestone bow island is described by captain beachy as 70 miles in circumference and 30 in its greatest diameter but we have seen that some of the Maldives are much larger as you're sure of an island or continent which is subsiding will recede from a coral reef at a slow or rapid rate according as the surface of the land has a steep or gentle slope we cannot measure the thickness of a coral by its distance from the coast yet as a general rule those reefs which are farthest from the land imply the greatest amount of subsidence we learn from flinders that the barrier reef of northeastern australia is in some places 70 miles from the mainland and it should seem that the cal-serious formation is there in progress 1000 miles long from north to south with a breadth varying from 20 to 70 miles it may not indeed be continuous over this vast area for dopedless innumerable islands have been submerged one after another between the reef and mainland like some which still remain as for example mary's island latitude nine degrees and 54 minutes south we are also told that some parts of the gulf enclosed with a barrier are 400 feet deep so that the efficient rock building corals cannot be growing there and in other parts of it islands appear encircled by reefs it will follow as one of the consequences of the theory already explained that provided the bottom of the sea does not sink too fast to allow the zoophiles to build upwards at the same pace the thickness of coral will be great in proportion to the rapidity of subsidence so that if one area sinks two feet while another sinks one the mass of coral in the first area will be double that in the second but the downward movement must in general have been very slow and uniform or where intermittent must have consisted of a great number of depressions each of slight amount otherwise the bottom of the sea would have been carried down faster than the corals could build upwards and the island or continent would be permanently submerged having reached a depth of 120 or 150 feet at which the effective reef constructing zoophiles cease to live if then the subsidence required to account for all the existing atolls must have amounted to three or four thousand feet or even sometimes more we are brought to the conclusion that there has been a slow and gradual thinking to this enormous extent such an inference is perfectly in harmony with views which the grand scale of denudation everywhere observable in the older rocks has led geologists to adopt in reference to upward movements they must also have been gradual and continuous throughout indefinite ages to allow the waves and currents of the ocean to operate with adequate power the map constructed by mr darwin to display at one view the geographical position of all the coral reefs throughout the globe is of the highest geological interest leading to splendid generalizations when we have once embraced the theory that all atolls and barrier reefs indicate recent subsidence while the presence of fringing reefs proves the land to be stationary or rising these two classes of coral formations are depicted by different colors and one of the striking facts brought to light by the same classification of coral formations is the absence of active volcanoes in the areas of subsidence and their frequent presence in the areas of elevation the only supposed exception to this remarkable coincidence at the time when mr darwin wrote in 1842 was the volcano of torus straight at the northern point of australia placed on the borders of an area of subsidence but it has been since proved that this volcano has no existence we see therefore an evident connection first between the bursting force every now and then of volcanic matter through rands and fissures and the expansion or forcing outwards of the earth's crust and secondly between a dormant and less energetic development of subterranean heat and an amount of subsidence sufficiently great to cause mountains to disappear over the broad face of the ocean leaving only small and scattered lagoon islands or groups of atolls to indicate the spots where those mountains once stood on a review of the differently colored reefs of the map alluded to it will be seen that there are large spaces in which upheaval and others in which depression prevails and there are placed alternately while there are a few smaller areas where movements of oscillation occur thus if we commence with the western shores of south america between the summit of the andes and the pacific a region of earthquakes and active volcanoes we find signs of recent elevation not attested indeed by coral formations which are wanting there but by upraised banks of marine shells then proceeding westward we traverse a deep ocean without islands until we come to a band of atolls and uncircled islands including the dangerous and society archipelagos and constituting an area of subsidence more than 4 000 miles long and 600 broad still further in the same direction we reach the chain of islands to which the new hybrides salamon a new island belong where fringing reefs and masses of elevated coral indicate another area of upheaval again to the westward of the new hybrides we meet with the uncircling reef of new caledonia and the great australian barrier implying a second area of subsidence the only objection deserving attention which has hitherto been advanced against the theory of atolls as before explained is that proposed by mr mclaren on the outside he observes of coral reefs very highly inclined no bottom is sometimes found with a line of 2000 or 3000 feet and this is by no means a rare case it follows that the reef ought to have this thickness and mr darvin's diagrams show that he understood it so now if such masses of coral exist under the sea they ought somewhere to be found on terra firma for there is evidence that all the lands yet visited by geologists have been at one time submerged but neither in the great volcanic chain extending from sumatra to japan nor in the west indies nor in any other region yet explored has a bed or formation of coral even 500 feet thick been discovered so far as we know when considering this objection it is evident that the first question we have to deal with is whether geologists have not already discovered called serious masses of the required thickness and structure or precisely such as the upheaval of atolls might be expected to expose to view we are called upon in short to make up our minds both as to the internal composition of the rocks that must result from the growth of corals whether in lagoon islands or barrier reefs and the external shape which the reefs would retain when upraised gradually to a vast height a task by no means so easy as sam may imagine if the reader has pictured to himself large masses of entire corals piled one upon another for a thickness of several thousand feet he unquestionably mistakes altogether the nature of the accumulations now in progress in the first place the strata at present forming very extensively over the bottom of the ocean within such barrier reefs as those of australia and new caledonia are known to consist chiefly of horizontal layers of calsarius sediment while here and there an intermixture must occur of the detritus of granitic and other rocks brought down by rivers from the adjoining lands or washed from sea cliffs by the waves and currents secondly in regard to atolls the stone making polypifers grow most luxuriously on the outer edge of the island to a thickness of a few feet only beyond this margin broken pieces of coral and calsarius sand are strewed by the breakers over a steep seaward slope and as the subsidence continues the next coating of live coral does not grow vertically over the first layer but on a narrow annular space within it the reef as was before stated constantly contracting its dimensions as it sinks thirdly within the lagoon the accumulation of calsarius matter is chiefly sedimentary a kind of chalky mud derived from the decay of the softer coralines with a mixture of calsarius sand swept by the winds and waves from the surrounding circular reef here and there but only in partial clumps are found living corals which grow in the middle of lagoon and mixed with fine mud and sand a great variety of shells and fragments of destacea and echinoderms we owe to loyton and nelson the discovery that in the bermudas the calsarius mud resulting from the decomposition of the softer coralines is absolutely undistinguishable when dried from the ordinary white chalk of europe and this mud is carried to great distances by currents and spread far and wide over the floor of the ocean we also have opportunities of seeing in appraised atolls such as elizabeth island tonga and hapai which rise above the level of the sea to heights varying from 10 to 80 feet that the rocks of which they consist do not differ in structure or in the state of preservation of their included zoophytes and shells from some of the oldest limestones known to the geologist captain beachy remarks that the dead coral in elizabeth island is more or less porous and honeycombed at the surface and hardening into a compact rock which has the fracture of secondary limestone the island of pulonias of sumatra which is about 3000 feet high is described by dr jack as being overspread by coral and large shells of the chama tridacna gigas which rest on quartz oes and aeronaceous rocks at various levels from the sea coast to the summit of the highest hills the cliffs of the island of timor in the indian ocean are composed says mr juke's of a raised coral reef abounding in astraya meandrina and porites with shells of strombus conus nerita arca pectin venus and lucina on a ledge about 150 feet above the sea a tridacna or large clam shell two feet across was found bedded in the rock with closed walls just as they are often seen in barrier reefs this formation in the islands of sandalwood sumbava madura and java where it is exposed in sick lifts was found to be from 200 to 300 feet thick and is believed to ascend to much greater heights in the interior it has usually the form of a chalk like rock white when broken but in the weathered surface turning nearly black it appears therefore premature to assert that there are no recent coral formations uplifted to great heights for we are only beginning to be acquainted with the geological structure of the rocks of equatorial regions some of the upraised islands such as elizabeth and queen charlotte in the pacific although placed in regions of atolls are described by captain beachy and others as flat topped and exhibiting no traces of lagoons in explanation of the fact we may presume that after they had been sinking for ages the descending movement was relaxed and while it was in the course of being converted into an ascending one the ground remained for a long season almost stationary in which case the corals within the lagoon would build up to the surface and reach the level already attained by those on the margin of the reef in this manner the lagoon would be effaced and the island acquire a flat summit it may have ever be thought strange that many examples have not been noticed of fringing reefs uplifted above the level of the sea mr darvin indeed cites one instance where the reef preserved on dry land into Mauritius is peculiar moat like structure but they ought he says to be of rare occurrence for in the case of atolls or of barrier or fringing reefs the characteristic outline must usually be destroyed by denudation as soon as the reef begins to rise since it is immediately exposed to the action of the breakers and the large and conspicuous corals on the outer rim of the atoll or barrier are the first to be destroyed and to fall to the bottom of vertical and undermined cliffs after slow and continued upheaval a wreck alone can remain of the original reef if therefore says mr darvin at some period as far in futurity as the secondary rocks are in the past the bed of the pacific with its atolls and barrier reefs should be converted into a continent we may conceive that scarcely any or none or the existing reefs would be preserved but only widely spread strata of calcerius matter derived from their wear and tear when it is urged in support of the objection before stated that the theory of atolls by subsidence implies the accumulation of calcerius formations two thousand or three thousand feet thick it must be conceded that this estimate of the minimum density of the deposits is by no means exaggerated on the contrary when we consider that the space over which atolls are scattered in polynesia and the indian oceans may be compared to the whole continent of asia we cannot but infer from analogy that the differences in level in so vast an area have amounted antecedently to subsidence to five thousand or even a greater number of feet whatever was the difference in height between the loftiest and lowest of the original mountains or mountainous islands on which the different atolls are based that difference must represent the thickness of coral which has now reduced all of them to one level flinders therefore by no means exaggerated the volume of the limestone which he conceived have been the work of coral animals he was merely mistaken as to the manner in which they were enabled to build reefs in an unfathomed ocean but is it reasonable to expect after the waste caused by denudation that calcerius masses gradually upheaved in an open sea should retain such vast thicknesses or may not the limestones of the cretaceous and oolithic epochs which attain in the Alps and Pyrenees a density of three thousand or four thousand feet and are in great part made up of coralline and Shelley matter present us with a true geological counterpart of the recent coral reefs of equatorial seas before we attach serious importance to arguments founded on negative evidence and opposed to a theory which so admirably explains a great variety of complicated phenomena we ought to remember that the upheaval to a height of four thousand feet of atolls in which the coralline limestone would be four thousand feet thick implies first a slow subsidence of four thousand feet and secondly an elevation of the same amount even if the reverse or ascending movement began the instant the donward one ceased we must allow a great lapse of ages for the accomplishment of the whole operation we must also assume that at the commencement of the period in question the equatorial regions were as fitted as now for the support of reef building zoophytes this postulate would demand the continuance of a complicated variety of conditions throughout a much longer period than they are usually persistent in one place to show the difficulty of speculating on the permanence of the geographical and climatic circumstances requisite for the growth of reef building corals we have only to state the fact that there are no reefs in the atlantic of the west coast of africa nor among the islands of the gulf of guinea nor in st. Helena ascension the capyverdes or st. paul with the exception of bermuda there is not a single coral reef in the central expanse of the atlantic although in some parts the waves as at ascension are charged to access with calceros matter this capricious distribution of coral reefs is probably owing to the absence of fit stations for the reef building polypifers other organic beings in those regions obtaining in the great struggle for existence a mastery over them their absence in whatever manner it be accounted for should put us on our guard against expecting upraised reefs at all former geological epochs similar to those now in progress lime whence derived dr mokoloch in his system of geology volume one page 219 expressed himself in favor of the theory of some of the earlier geologists that all limestones have originated in organized substances if we examine he says the quantity of limestone in the primary strata it will be found to bear a much smaller proportion to the silly seos and argillac seos rocks than in the secondary and this may have some connection with the rarity of the stasis animals in the ancient ocean he further infers that in consequence of the operations of animals the quantity of calceros earth deposited in the form of mud or stone is always increasing and that as the secondary series far exceeds the primary in this respect so a third series may hereafter arise from the depths of the sea which may exceed the last in the proportion of its calceros strata if these propositions went no further than to suggest that every particle of lime that now enters into the crust of the globe may possibly in its turn have been subservient to the purposes of life by entering into the composition of organized bodies i should not deem the speculation improbable but when it is hinted that lime may be an animal product combined by the powers of vitality from some simple elements i can discover no sufficient grounds for such a hypothesis and many facts militate against it if a large pond be made in almost any soil and filled with rainwater it may usually become tenanted by the stacia for carbonate of lime is almost universally diffused in small quantities but if no calceros matter be supplied by water's flowing from the surrounding high grounds or by springs no tufa or shell marl are formed the thin shells of one generation of mollusks decompose so that their elements afford nutriment to the succeeding races and it is only where a stream enters a lake which may introduce a fresh supply of calceros matter or where the lake is fed by springs that shells accumulate and form marl all the lakes in forfer shire which have produced deposits of shell marl have been the sites of springs which still evolve much carbonic acid and a small quantity of carbonate of lime but there is no marl in lochvithi near forfer where there are no springs although that lake is surrounded by these calceros deposits and although in every other respect the site is favorable to the accumulation of aquatic testesia we find those carrier which secrete the largest quantity of calceros matter in their stems to abound near springs impregnated with carbonate of lime we know that if the common hen be deprived altogether of calceros nutriment the shells of her eggs will become of too slight a consistency to protect the contents and some birds eat chalk greedily during the breeding season if on the other hand we turn to the phenomena of inorganic nature we observe that in volcanic countries there is an enormous evolution of carbonic acid either free in a gaseous form or mixed with water and the springs of such districts are usually impregnated with carbonate of lime in great abundance no one who has traveled in tuscany through the region of extinct volcanoes and its confines or who has seen the map constructed by targione 1827 to show the principal sites of mineral springs condoped for a moment that if this territory was submerged beneath the sea it might supply materials for the most extensive coral reefs the importance of these springs is not to be estimated by the magnitude of the rocks which they have thrown down on the slanting sides of hills although of these alone large cities might be built nor by a coating of travertine that covers the soil in some districts for miles in length the greater part of the calceros matter passes down in a state of solution to the sea and in all countries the rivers which flow from chalk and other marley and calceros rocks carry down vast quantities of lime into the ocean lime is also one of the component parts of ogid and other volcanic and hypo gene minerals and when these decompose is set free and may then find its way in a state of solution to the sea the lime therefore contained generally in seawater and secreted so plentifully by the testesia and corals of the pacific may have been derived either from springs rising up in the bed of the ocean or from rivers fed by calceros springs or impregnated with lime derived from disintegrated rocks both volcanic and hypo gene if this be admitted the greater proportion of limestone in the more modern formations as compared to the most ancient will be explained for springs in general hold no arduous and but a small quantity of silly shells matter in solution but they are continually subtracting calceros matter from the inferior rocks the constant transfer therefore of carbonate of lime from the lower or older portions of the earth's crust to the surface must cause at all periods and throughout an indefinite succession of geological epochs a preponderance of calceros matter in the newer as contrasted with the older formations end of chapter 50 concluding remarks of principles of geology this is a libravox recording all libravox recordings are in the public domain for more information on to volunteer please visit libravox.org principles of geology by charles lile concluding remarks in the concluding chapters of the first book i examined in detail a great variety of arguments which have been induced to prove the distinctness of the state of the earth's crust at remote and recent epics among other supposed proofs of this distinctness the dearth of calceros matter in the ancient rocks above adverted to might have been considered but it would have been endless to enumerate all the objections urged against those geologists who represent the course of nature at the earliest periods as resembling in all essential circumstances the state of things now established we have seen that in opposition to this doctrine a strong desire has been manifested to discover in the ancient rocks the signs of an epic when the planet was uninhabited and when its surface was in a chaotic condition and uninhabitable the opposite opinion indeed that the oldest of the rocks now visible may be the last monuments of an antecedent era in which living beings may already have peopled the land and water has been declared to be equivalent to the assumption that there never was a beginning to the present order of things with equal justice might an astronomer be accused of asserting that the works of creation extended throughout infinite space because he refuses to take for granted that the remotest stars now seen in the heavens are on the utmost verge of the material universe every improvement of the telescope has brought thousands of new worlds into view and it would therefore be rash and unphilosophical to imagine that we already survey the whole extent of the vast scheme or that it will ever be brought within the sphere of human observation but no argument can be drawn from such premises in favor of the infinity of the space that has been filled with worlds and if the material universe has any limits it then follows that it must occupy a minute and infinitesimal point in infinite space so if in tracing back the earth's history we arrive at the monuments of events which may have happened millions of ages before our times and if we still find no decided evidence of the commencement yet the arguments from analogy and support of the probability of a beginning remain unshaken and if the past duration of the earth be finite then the aggregate of geological apex however numerous must constitute a mere moment of the past a mere infinitesimal portion of eternity it has been argued that as the different states of the earth's surface and the different species by which it has been inhabited have all had their origin and many of them their termination so the entire series may have commenced at a certain period it has also been urged that as we admit the creation of man to have occurred at a comparatively modern epic as we can see the astonishing fact of the first introduction of a moral and intellectual being so also we may conceive the first creation of the planet itself i am far from denying the weight of this reasoning from analogy but although it may strengthen our conviction that the present system of change has not gone on from eternity it cannot warrant us in presuming that we shall be permitted to behold the signs of the earth's origin or the evidences of the first introduction into it of organic beings we aspire in vain to assign limits to the works of creation and space whether we examine the starry heavens or that world of minute animacules which is revealed to us by the microscope we are prepared therefore to find that in time also the confines of the universe lie beyond the reach of mortal ken but in whatever direction we pursue our researches whether in time or space we discover everywhere the clear proofs of a creative intelligence and of his foresight wisdom and power as geologists we learn that it is not only the present condition of the globe which has been suited to the accommodation of myriads of living creatures but that many former states also have been adapted to the organization and habits of prior races of beings the disposition of the seas continents and islands and the climates have varied the species likewise have been changed in yet they all have been so modeled on types analogous to those of existing plants and animals as to indicate throughout a perfect harmony of design and unity of purpose to assume that the evidence of the beginning or end of so vast a scheme lies within the reach of our philosophical inquiries or even of our speculations appears to be inconsistent with a just estimate of the relations which subsist between the finite powers of man and the attributes of an infinite and eternal being end of concluding remarks and end of principles of geology by charles lyle