Jan. 3, 1889] 



NATURE 



237 



gives a very misleading idea of the real nature of this class of 

 reef. A section of Keeling Atoll, drawn from the io<X)-fathom 

 line on a true scale of an inch to the mile, and intended to 

 illustrate a breadth of six miles, and a depth in the lagoon of 

 9 or 10 fathoms, would represent to the naked eye a flat-topped 

 mountain, the depth of the so-called basin on the summit being 

 merely represented by a slight central depression of about l/ioo 

 of an inch. If the lagoon possessed a depth of 30 fathoms, the 

 inclosed basin so-called would only Be indicated in this section 

 by a central depression of about 3/100 of an inch. So trifling 

 a proportion does the depth of an atoll of ordinary size bear to 

 the breadth, that such a reef can only be accurately described 

 as possessing a broad level surface, with very slightly raised 

 margins. A correct model of Keeling Atoll would at once 

 convey a just idea of the true relative dimensions of a reef 

 of this class. The lagoon would be there only represented by 

 a film of water occupying a slight hollow in the level mountain- 

 top. By thus grasping these facts, we at once perceive that by 

 reason of our failing to view an atoll in relation to its surround- 

 mgs, and through our misconceptions of its dimensions, we have 

 been led to introduce a great cause to explain a very small 

 effect. The slightly raised margins can be easily explained by 

 causes dwelt upon by Murray, Agassiz, and others. No move- 

 ment of the earth's crust is necessary for this purpose. The 

 mode of growth of corals, the action of the waves, and the 

 influence of the currents, afford agencies quite sufficient to 

 produce the slightly raised margins of an atoll. 



The development of the islands of an atoll into horse-shoe or 

 crescentic islands, as in the instance of Keeling Atoll, or into 

 perfect small atolls or atoUons, as in the Maldive Group, is a 

 subsequent process to be shortly explained. These small atolls 

 and horse-shoe islands only -assume their characteristic forms 

 after the island has been thrown up by the waves. Such was the 

 conclusion I arrived at concerning small atolls and crescent- 

 shaped coral islands in the Solomon Islands (Proc. Roy. Soc. 

 Edin., 1885-86, p. 900) ; and as just stated I have formed the 

 same opinion concerning the islands of Keeling Atoll. There is 

 in the first place the island from which "lateral extensions grow 

 out on either side so as to ultimately form a horse-shoe reef," 

 which itself under [favourable conditions may develop into a 

 small atoll. In the Solomon Islands I imperfectly grasped the 

 method by which these changes in form are effected. In Keel- 

 ing Atoll I saw the process in operation, and I arrived at the 

 conclusion that wherever a coral island stems a constant surface- 

 current, the sand produced by the breakers on the outer edge of 

 the reef will mostly be deposited by the current on each side of 

 the island in the form of two lateral banks or extensions, giving 

 the island ultimately a horse-shoe form, with the convexity pre- 

 sented against the current. The process may be aptly compared 

 to the formation of a V-shaped ridge of sand when a stake or 

 some other obstacle is placed in a river-bed. The stake repre- 

 sents 4he original small island thrown up by the waves. The 

 V-shaped ridge of sand represents the arms of the horse-shoe 

 island which are subsequently formed. The back-wash or eddy 

 may in the river-bed join the arms of the V'shaped ridge of 

 sand. In a similar manner a horse-shoe island may have a bank 

 thrown up across the mouth, and thus a small atoll is formed. 

 Such is the process, imperfectly disclosed to me in the Solomon 

 Islands, that I found illustrated in all its stages in Keeling 

 Atoll. In the Keeling Islands, however, it was necessary to 

 satisfy myself of the reality of the agencies chiefly concerned in 

 this process. For instance, I had to ascertain how and to what 

 extent the surface-currents acted, and to discover the source of 

 the sand. It was also necessary to observe what changes in the 

 form and extent of the islands had occurred in the experience of 

 the residents during the half-century of their occupation. 



The westerly equatorial drift or south-east trade current, 

 sinking the south-east angle of the atoll, there divides and 

 sweeps around the coasts, the two branches meeting and form- 

 ing an eddy off" the north-west island, a spot where drift timbers 

 are often detained and stranded after having been swept around 

 half the circumference of the atoll. Advantage of this current is 

 taken by the proprietor of the islands, who directs his men to 

 mark any logs of valuable timber thrown up on the weather or 

 south-east coast, and then to launch them again outside the 

 breakers. In this way huge logs are transported by the current 

 to any particular island. If left alone, the logs, whether drifted 

 around the north or south side of the atoll, arrive finally in the 

 eddy off" the north-west angle. This current finds its way into 

 the lagoon through the several passages between the islands its 



rate there varying usually from half a knot to two knots in the 

 hour. Only rarely is there any check to the inflow of water 

 through the passages, as, for instance, during north-wcbt gales. 



The current in these passages carries daily a large amount of 

 sand into the lagoon. I discovered this accidentally whilst 

 using the tow-net for catching the pelagic animals brought in by 

 the current. The source of this sand is the weather edge of the 

 reef on the outer side of the islands, where the breakers are 

 unceasingly at work in keeping up the supply. After several 

 measurements under varying conditions of current, tide, and 

 depth, I estimated that during every day of ordinary weather at 

 least 10 tons of sand are carried through the passages into the 

 lagoon. During gales and cyclones this amount is greatly in- 

 creased ; and probably the estimate for an ordinary year would 

 not be less than 5000 tons. The bulk of this sand is deposited 

 by the current near the inner mouths of the passages and on the 

 margins of the lagoon, where it goes to extend the islands in the 

 form of banks stretching into the lagoon. In this manner an 

 island obtains a horse-shoe shape, just as the V-shaped ridge is 

 formed by placing a stake in a river-bed. The first stage is 

 represented by an island with two sand-banks extending into the 

 lagoon, one from each extremity. The second stage is that in 

 which the island has attained a semi-crescentic shape by the 

 encroachment of its vegetation on the newly formed banks. In 

 the course of time, when the vegetation of the island has entirely 

 occupied the banks, the third stage, that of the horse-shoe 

 island, is reached. In some instances, there is yet a further stage, 

 when during a long continuance of westerly winds another bank 

 is thrown up across the mouth of the horse-shoe, and a small 

 atoll with a shallow lagoonlet is produced. Thus the currents 

 are the principal agencies in forming the horse-shoe islands of 

 Keeling Atoll. In large atolls, where more open-sea conditions 

 prevail in the lagoon, and especially where, as in the Maldives, 

 there are two opposite sets of winds and surface-currents, each 

 prevailing in its own half of the year, we should expect to find 

 the horse-shoe island replaced by an atollon. Keeling Atoll, 

 however, lies for eleven months out of the twelve within the 

 region of the constant trade-wind and westerly drift current, so 

 that the situation is only one favouring the formation of horse- 

 shoe islands facing to the southward and eastward. The pro- 

 tected character of the lagoon, also, is not a condition that would 

 assist the growth of a circular island or atollon. 



Another important feature in this atoll is to be found in the 

 existence outside the seaward edge of the present reef of a series 

 of submerged lines of growing corals separated from each other 

 by sandy intervals. Unfortunately, I was not able to examine 

 these to the extent I desired, since it can only be satisfactorily 

 done later in the year, when the sea is sufficiently smooth to allow 

 boats to approach the breaker edge of the reef. This feature, 

 however, is familiar to the residents, who have supplied me with 

 information on the subject. It would seem that all around the 

 circumference of this atoll there is a space outside the present 

 edge of the reef varying from 200 to 500 or 600 yards in width, 

 where ships have anchored, and where boats in the calm season 

 go with fishing parties. Here the submarine slope slopes 

 gradually down to 20 or 30 fathoms ; but beyond this the descent 

 is precipitous. It is on this gradual- slope that the lines of grow- 

 ing coral occur, separated by sandy intervals from each other. 

 There may be two or three of these lines, the innermost covered 

 by 4 or 5 fathoms, and the outer by from 20 to 30 fathoms. 



We are thus able to perceive that the outward extension of the 

 reef is effected, not so much by the seaward growth of the pre- 

 sent edge of the reef, as by the formation outside it of a line of 

 growing corals, which when it reaches the surface reclaims, so to 

 speak, the space inside it, which is soon filled up with sand and 

 retidebris. The evidence, in fact, goes to show that a reef 

 grows seaward rather by jumps than by a gradual outward growth. 

 This inference is of considerable importance, since it connects all 

 classes of reefs together in the matter of their seaward growth, 

 the degree of inclination of the submarine slope being the chief 

 determining factor. 



Following Le Conte, I have previously shown (Proc. Roy. 

 Soc. Edin. 1885-86, p. 884) that where there is a very gradual 

 submarine slope the deposition of sand and the presence of 

 much sediment in the water will prevent the growih of corals 

 in the shallow water outside the seaward edge of the reef, and 

 that in consequence a line of living corals will spring up in the 

 clearer and deeper waters a considerable distance beyond. The 

 appearance of this line of coral at the surface will result in the 

 production of a barrier-reef with a lagoon-channel inside. In 



