1843.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



419 



Admitting then, that in by far the greater number of cases an atoll consists 

 of a simple elongated ring, we are still as far as ever from embracing the 

 idea of these atolls being formed on the summits of volcanos, as is presumed 

 by Mr. Lyell ; that they cap submarine mountains is very evident, that vast 

 numbers have one general base is equally evident, thus in Marshall group 

 many atolls are united together by linear reefs : and in the shallows of the 

 Red Sea most of them are thus united in groups or in a linear direction, the 

 barrier reef polypii'ers being pre-eminently the architects of the atolls and 

 the circular reefs, formed in entirely by the one particular family. Upon 

 examining the early development of some of these encircling and barrier 

 reefs, we find that the polypifer fixes itself almost indifferently to the level 

 and to the slope, always affecting growth perpendicular to the plane of its 

 position ; this circumstance alone must have some influence on its extension : 

 thus, for instance, if it take root on the side of the sand bank, it fixes that 

 portion of the sand to which it is attached, and in growth takes the form of 

 the upper portion of the bank, fringing it more or less all round : advancing 

 thus, and radiating towards the surface of the waters, and outwards it con- 

 tinues to advance while, the interior or apex of the sandbank experiences in 

 many instances little or no alteration, the tide passing through it by con- 

 stant disturbance, preventing the polypifers from extending in that direction : 

 that this is one cause of the hollow in the centre in the reef or island is 

 proved by the nature of the material which forms the bottom of the basin, 

 which is ocean marl, sand, or such matters as are cast by the waters upon 

 the reef. On the other hand many of the islets and atolls covering vast 

 areas spring up as table land, within an equal surface which have no lagoons, 

 and other lagoons when open to the sea rapidly fill up with the living coral. 

 Wherever soundings were obtained off Egmont Island and the neighbouring 

 atolls, the bottom is found to be invariably sandy, and the currents run with 

 great force around them ; this alone is sufficient to account for the precipi- 

 tous appearance of these formations, for, in the live or moving sands, it is 

 impossible for the polypifers to extend their edifice over the area thus dis- 

 turbed, and consequently their labours are confined to building upwards. 



Again, there are other causes which tend to give form to the reef, thus the 

 polypifers spread over the submarine plain, covering a large area ; they 

 build upwards and attain a considerable height, uhen a sudden evolution of 

 sands or calcareous matters partially or wholly destroys them : if partially 

 destroyed, the disturbing causes having ceased, they again spring forth in 

 detached groups upon the parent bases, forming fields of lesser extent, and 

 continue to advance towards the surface until a second catastrophe has the 

 like effect: thus the building arises from vast depths by a succession of steps. 

 Wherever sands can accumulate upon a bed of coral, then the polypifers are 

 necessarily subject to partial or entire destruction. That the tidal currents 

 have much influence in forming the reefs, is manifest by its form and growth ; 

 within tranquil waters the reefs ha\e a gentle inclination from summit to 

 base, the natural consequence of uninterrupted and expanding growth, and 

 in a chain of reefs they may be often seen overhanging until their upper parts 

 meet, vast caverns or arches are formed, which, if not filled up with deposits 

 from the waters, may continue thus, age upon age; for no sooner is light and 

 heat excluded than the polypifer ceases to perform its functions. It appears 

 evident that ihe law of forces is the law of growth of coral, that the poly- 

 pifer taking root within a quiet area, will enlarge its parts on every side 

 until it fill th it area, embracing in its rising structure all the inequalities of 

 that plain : that if a tidal current bounds that plain at one or more of the 

 cardinal points, and brings with it sands or other matters, and thus forms 

 accumulating beds, such formations must prove natural barriers to the ad- 

 vance of the coral in these directions, so long as this action continues ; the 

 general steepness of the Chagos and Maldive atolls is produced by these 

 causes : the uniform distribution of the reefs on the leeward side of the Mau- 

 ritius and in the extensive shallows of the Red Sea, demonstrating that qui- 

 etude is essential to their expansion. Again, that where the sands or ocean 

 marl accumulate over a given space of a reef, then without consideration of 

 the depth, the polypifers cease to build, so long as the disturbance continues ; 

 it is indeed an admitted fact, that the greater part of the bottom of most la- 

 goons is formed of sediment, varying in its nature, but having the one effect 

 the destruction of the artificers of the reef. 



Mr. Darwin says the islets placed to leeward are liable to be occasionally 

 swept entirely away by gales, equalling hurricanes in violence, and therefore 

 their absence is a comparatively unimportant fact : it is true that the action 

 of the waves does often wash away so much of new islets as appears above 

 the surface, but beyond this the most tempestuous seas can have no influ- 

 ence, consequently if islets did exist in this direction, as he would have it 

 understood, they would still present themselves to the view as submerged 

 reefs, but in reality the polypifers do not extend themselves in the disturbed 

 line, or if they do, they are very small compared with those to windward. 

 The circumstance of there being no living coral on some submerged reef, is 

 anything but proof that those reefs were once elevated above the surface 

 of the waters, but is rather demonstrative of the destroying effects of sands 

 when thrown by occasional disturbances upon the living barriers, for where- 

 ever these depositions lie, there the polypifers cease to build, until new gene- 



rations spring from the sandy base, hiving no relationship in many in- 

 stances to those who have gone before them : however thin the coating of 

 sand may be, a sufficient cause exists as in the case of the Great Chagos 

 Bank, Speakers Bank and numerous others, for the appearance ot dead rock 

 in quantities more or less. 



That tidal action as well as accidental disturbances has much ;o do with 

 the form of reefs, is manifest in the numerous openings of reef's, and in their 

 geographical distribution ; the fringing reefs encircling the Mauritius havea 

 straight passage open in front of every river and streamlet running into the 

 ocean, and at Great Port there is a channel like that within the barrier reef 

 extending parallel to the shore, each end being entered by a river, the two 

 streams bending towards each other. The Australian barrier extending 

 nearly 1000 miles, bending to the sinuosities of the coast, would appear to 

 have its basis .on a submarine mountain range on either side; its boundary 

 of growth from the bottom appears to lie defined by sands ; it has few open- 

 ings sufficiently large for vessels to pass through, and from its general dis- 

 positions it has the appearance of being one vast submarine mountain chain 

 varying in its composition beneath but capped throughout with coral reefs 

 the polypifers being unable to extend their breadth in consequence of tidal 

 action and shifting sand, throughout this vast range there is no evidence of 

 extensive dislocation such as might be expected had it been elevated by vol- 

 canic action, but its sinuosities and genera] character give it a striking simi- 

 litude to many of the calcareous formations on the main land, all of which 

 bear evidence of their elevation above the present sea ; tidal action is a so 

 manifest in the fringing reefs of Eastern Africa, and in fact all other places 

 where those formations abound. 



The coral formation appearing above the waters is violently a;:acked by 

 the element which gave it birth, and before the repeated attacks of tempests 

 it falls piecemeal, and the upper portion being washed away, ii once more 

 becomes a sunken reef, and the polypifers may be once more observed busily 

 building up the edifice ; again, from the effects of currents, many islets are 

 washed away as is recorded of the Maldives, and as I have repeatedly ob- 

 served in the Red Sea : but this destruction is far from being general, for the 

 increase of coral reefs, coral banks, and calcareous beds, in adding to the 

 earth, is infinitely beyond the decrease occasioned by the destroying powers 

 of flood or fire, and every island and every barrier reef standing above the 

 waters, from its unbroken appearance, and the simplicity of its material 

 demonstrates the vast increase the earth constantly receives principally from 

 the labours of these minute forms of life. 



Of the largest lakes disposed within coral groups, such as the Atoll Suadiva 

 which is 44 miles in length, and 34 in breadth, and encloses a great expanse 

 of water from 250 to 300 feet deep, other and more extensive views may be 

 taken of their mode of formation such as the peculiar form of their submarine 

 bases, the effects of the currents, and of depositions, and when above the 

 surface, the effects of storms. It is observed as a peculiarity and exception 

 to the general rule, that many of them have a greater number of openings on 

 the leeward than on the windward side, thus on the near sides of Ari, and 

 the two Nilandoo atolls which face S. Male, Phaleedoo and Mologue Atolls, 

 there are 73 deep water channels, and only 25 on their outer sides, this dif- 

 ference is attributable to the action taking place while the reef was far be- 

 neath the surface of the waters, the more numerous lodgments of sands, 

 marls, &c, inimical to polypius life, being deposited on the extreme edge of 

 the reef in the tidal line, which occasioned more numerous separations or 

 openings on the less exposed side of the reef. 



From whence then, are derived the vast quantities of calx requisite to supply 

 the building of these enormous formations, constituting mountain groups and 

 chains, and filling the ocean valleys and the valleys of all seas, with calca- 

 reous beds many hundred feet in thickness, and many thousand miles in area 

 constituting also, so great a portion of the surface of terrestrial earth? 

 Dr. Buckland says " It is difficult to account for the source of the enormous 

 masses of carbonate of lime that compose nearly one eighth part of the 

 superficial crust of the globe; but until it can be shown that these animals 

 have the power of forming lime from the elements, we must suppose thai 

 they have been derived from the sea, either directly or through the medium 

 of its plants ; " — this is a very unphilosophical way of disposing of the ques- 

 tion, for, admitting that it cannot be proved that the lime is elaborated within 

 the living body, we can still demonstrate that it cannot be derived from 

 springs or through the medium of plants : the quantity of lime held in sus- 

 pension by the ocean waters is very trifling, and as nothing compared to the 

 chlorides of sodium and magnesia— and to furnish these three earths in the 

 quantities required, the whole interior of the globe would be insufficient: 

 again, was it derived from springs, what an enormous supply would be re- 

 quired to fill up the bed of the Mediterranean alone, which, according to 

 Uonati. is 900 feet in thickness in calcareous matter : again, fuci are not to 

 be found in the lowest depths ; they follow animal species in the order of de- 

 velopement, and abound principally in those regions where eoral formations 

 are wholly unknown ; they do notsecrete lime, although in the shallows they 

 sometimes become coated with this material by polypifers. 



The sraa 1 portion of lime held in suspension by the waters of different seas 



