April s. 1888] 



NA TURE 



549 



this, it seems, Mr. Murray has not taken sufficient account, and 

 has, therefore, overstated the destructive agency of the sea. 

 Secondly, the growth of corals, and the consequent formation 

 of coral rock within the lagoon, is generally overlooked. 



Whilst diving for corals at Diego Garcia, I had abundant 

 opportunities of studying the formation of coral rock within the 

 lagoon, in depths under 2 fathoms. The layers of tolerably 

 compact rock thus formed are of no mean extent or thickness ; 

 they soon become covered with sand, and are thus protected 

 from the solvent action of the water. I have found it impos- 

 sible to reconcile Mr. Murray's views with what I saw of coral 

 growth within a lagoon. Not only do the more delicate branch- 

 ing species of the Madreporaria flourish in considerable numbers, 

 but ti-ue reef-building species— Porites, Maeandrina, Pocillopora, 

 and various stout species of Madrepora — are found there. It is 

 a mistake to suppose that certain species of corals are restricted 

 to the external shores, others to the lagoon. My collections 

 proved that many of the species growing in the lagoon at dis- 

 tances of 5 miles and upwards from its outlet are identical with 

 those growing on the outer reef. In addition to them are 

 numerous species, such as Seriatopora stricta, Mussa corymbosa, 

 Favia lobata, Fungia dentata, and many others that are not 

 found on the outside. The reason is that the last-named are 

 either free forms, such as Fungia, or are attached by such slen- 

 der and fragile stems to their supports that they could not pos- 

 sibly obtain a foothold and maintain themselves among the 

 powerful currents and waves of the open ocean. 



These various species, numbers of which grow close together, 

 form knolls and patches within the lagoon, and it cannot be 

 doubted that their tendency is to fill it up. 



These considerations have led me to discredit the solution 

 theory as an explanation of lagoons and lagoon channels, and 

 other objections have been lately urged with great force by 

 Captain Wharton. The conclusion which I reached, after care- 

 fully considering the conditions of submerged lakes of atoll 

 form, is that the ring-shape of the outer reef is to be explained 

 by the peculiarly favourable conditions for coral growth found 

 on the external slopes. Although corals may, and do, flourish 

 in lagoons, they are only found in knolls and patches, and are 

 always liable to be smothered, when, by a change in the tidal 

 currents, sand is thrown down upon the place where they are 

 growing. On the external slopes, however, corals grow in ex- 

 traordinary abundance, and chiefly those massive forms whose 

 skeletons take so conspicuous a share in the formation of coral 

 rock. If once it is admitted that the periphery of the reef offers 

 peculiarly favourable conditions to the growth of reef-forming 

 corals, it follows that, as the reef rises to the surface, its external 

 parts will outstrip the more internal, and will reach the surface 

 first, forming a rim around a central depression or lagoon. This 

 elevated ring will be as marked a feature in submerged as in 

 complete atolls. 



Corals are always thickest along the slopes around a coral reef, 

 and the reef tends to increase at its periphery, growing upwards 

 there, whilst it tends at the same time to spread outwards. 

 These principles hold good in the case of a submerged bank as 

 well as in the case of a reef that is awash, and a submerged bank 

 must tend in the course of time to reach the surface in its circum- 

 ferential portions, and form an atoll-shaped reef, on the rim of 

 which detritus may be heaped from place to place, forming 

 shingle cays or islets which may temporarily form dry land. 

 In atolls where storms are of frequent occurrence, regular storm- 

 beaches may be formed, till the fragments piled high upon one 

 another may form low islets standing some 6 or 10 feet above 

 high-water mark, upon which vegetation may subsequently find 

 a footing. Atolls are often formed in this way, without any 

 elevation taking place, and such has undoubtedly been the case 

 in the Florida reefs, where atolls (the Tortugas) and barrier reefs 

 and islands have been formed in an area of complete rest. No 

 one who has read the admirable work of Alex. Agassiz on the 

 Florida reefs can fail to agree with the author's conclusion that 

 the islets there have been formed by the action of the wind and 

 waves alone, without any assistance from the upheaval of the bed 

 of the sea. But I am not satisfied that this has been the case in 

 the Chagos Group. Storms are of very infrequent occurrence 

 there, and the horizontal masses of reef rock standing above high- 

 water mark cannot be attributed to the normal action of the 

 prevailing winds and currents. 



In the Florida reefs the nature of the soil betrays its origin — 

 its strata slope towards the sea on every side, and the lamination 

 of the rocks attests the long-continued action of waves and spray. 

 But the alternate horizontal layers of sand and rock occurring so 



abundantly at Diego Garcia are quite different ; they do not dip 

 seawards, their composition differs from the rocks of the Florida 

 reefs, and their edges, instead of showing signs of accumulation 

 of fresh material, are often bluff, and show that the sea is 

 gradually eating them away. It is difficult to explain these 

 appearances except on the hypothesis of slight elevation. It 

 might be objected that if any upheaval had taken place, the 

 banks lying at various depths below the surface would have been 

 raised to different heights, and that it would be in the highest 

 degree unlikely that so many would be raised some 4 or 5 feet 

 above high-watermark and no more, throughout so large areas as 

 the Laccadive, Maldive, and Chagos Islands, and the various low 

 groups in the Pacific. The force of the objection must be 

 admitted, but it may be observed that atolls raised from 10 to 40 

 feet above the waves are not so uncommon as has been hitherto 

 supposed, and that the numerous submerged banks lying at 

 very various depths show that all the reefs have not been raised 

 to one height in a single area of elevation. The uniform level 

 of many atolls and barrier reefs admits of a further explanation. 

 A reef raised some few feet above the sea-level is at once 

 attacked by the waves, and as the rim is very narrow, it must 

 soon be worn away till the whole of the land is eaten away, 

 and its surface is brought awash once more. Thus every 

 slight movement of elevation would soon be compensated by 

 the denuding action of the waves. The island of St. Pierre, 

 already described, is a good instance of this process of erosion. 

 It cannot be doubted that this island, which has recently been 

 raised 40 feet, is undergoing rapid waste, and must soon be 

 reduced to the level of the sea. At Diego Garcia I was 

 astonished at the rapid destruction of dry land which is in pro- 

 gress, on the outside as well as the inside of the lagoon. The 

 destruction is not so great on the outside as on the inside as a 

 rule, for in the former case the rampart of coral boulders thrown 

 up by the waves compensates in many places for their erosive 

 action. But in the bay above Horsburgh Point, exposed to the 

 full strength of the south-east trades, the destruction is very 

 great. M. Spurs, an old resident of the island, writes to me on 

 this subject : "Cette destruction est tres rapide ; Diego perd en 

 moyenne un pied de terrain par an, tant interieurement qu'ex- 

 terieurement, excepte aux pointes nord-est et nord-ouest, oil une 

 partie des sables, entraines du fond de la bale par les vents de 

 sud-est, conservent a ces deux pointes leur largeur premiere." 



M. Spurs has over-estimated the rate of destruction, but there 

 can be no doubt that it is very considerable. It is most conspi- 

 cuous along the shores bordering the lagoon. The stumps of 

 cocoa-nut palms, the newly-made breaches into the land, forming 

 shallow inland lagoons, the vertical faces of old banks of half- 

 consolidated sand, all attest it. J^ist above Point Marianne is a 

 road running along the lagoonward shore, which when I left the 

 island had been narrowed by the action of the sea to a mere 

 path, and was in some places almost impassable, as the sea had 

 made clean breaches across it, and found its way into some 

 shallow fresh-water lagoons lying on the other side of the road. 

 I was assured that this road had been over 12 feet wide some 

 years previously, and that it was formerly separated from the 

 lagoon by a narrow strip of land of an equal width. Perhaps 

 the best evidence of the destruction of land is afforded by the 

 "barachois" at the southern extremity of the island. These 

 barachois are inland lagoons connected with the main lagoon by 

 a narrow outlet some 2 fathoms deep or more. They are filled 

 and emptied every tide, and their floor is intersected by numerous 

 small channels running in every direction. No corals grow 

 within the barachois, and a slight study convinces the observer 

 that the daily scour of the tides is denuding their shores and 

 floors very considerably. 



Barachois are formed in the following way : — During unusually 

 high tides, when the waters of the lagoon are dammed back by 

 a north-westerly wind of unusual violence, the water rises to 

 great heights and invades the land in several places. In some 

 instances it actually makes a breach in the lagoonward shore, 

 and fills up the shallow depressions which are often found in the 

 middle of the strip of land. A pool of salt water is thus formed, 

 which kills the cocoa palms and other vegetation growing in its 

 bed, and, as this process is repeated again and again, in the course 

 of a few years a channel is cut out between the pool and the 

 lagoon, which finally becomes so deep that spring tides, and 

 finally even neap tides, run. in and out of the pool regularly. As 

 soon as these conditions are established, the channel is scoured 

 out and deepened, and the daily tides scour out the bed of the 

 pool, forming a complete barachois. 



It is not easy for one who has not seen it to understand how 



