222 



NA TURE 



{Jtdy 4, 1889 



a beginner would draw from such a paragraph would be 

 that the predominant effect of the moon in causing tides 

 was simply due to its proximity to the earth, whereas the 

 fact cannot be too strongly insisted upon, that it is the 

 differential attraction of the sun and moon upon the 

 earth's surface and centre that causes tidal action. The 

 atlas is complete and trustworthy. 



LETTERS TO THE EDITOR. 



[ The Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications. ] 



Coral Reefs. 



I BELIEVE all the questions asked by Captain W. Usborne 

 Moore in Nature of June 27 (p. 203) have been already 

 answered in my papers on this subject. However, as I have 

 examined the reefs at Kandavu, Matuku, Ovalau, and other 

 places at the Fiji Islands, I may attempt to answer his questions 

 with reference to these special cases. 



According to Mr. Darwin's view, an island equal in extent to 

 the lagoon of an atoll has sunk from view and the circular reef 

 that has grown up marks approximately the position of the 

 ancient coast line. In the case of a barrier reef, land equal in 

 area to the lagoon channels has been submerged, the barrier 

 reef here, too, marking the position of the ancient coast line. 

 The opponents of Mr. Darwin's theory hold that there is no 

 sufficient evidence that any such land once occupied the site of 

 the lagoons and lagoon channels. I believe they might go 

 further, and hold that, had such extensive submergence taken 

 place, these lagoons and reefs would necessarily present features 

 other than those by which they are everywhere characterized in 

 our seas at the present time. 



We hold that all the phenomena presented by these coral 

 reefs and islands can be better explained on other principles 

 without calling in subsidence ; that slow subsidence or elevation 

 or rest would only modify in a minor way the general features of 

 a reef ; that any one of the three kinds of reefs may be formed 

 indifferently in a rising, sinking, or stationary area. In a sinking 

 area the lagoons would probably be relatively deeper, the reefs 

 narrower, and the islands on them small or absent ; in a rising 

 area the lagoons would be relatively shallower, the reefs broader, 

 and the islands more numerous or united into a continuous band 

 of land. 



In surveying the coasts of any volcanic islands situated in the 

 ocean where there are no coral reefs, Captain Moore must know 

 that these are sometimes surrounded by banks extending much 

 further seawards in one direction than in another. These banks 

 are doubtless formed of the loose materials of the island, which 

 are spread out by wave action. The position of these banks 

 depends on the nature of the rocks in the different parts of the 

 islands ; in the case of Graham Island, the whole island was 

 spread out, forming a submerged bank. In addition to the 

 nature of the materials, the depth over the banks depends also 

 on the extent and depth of the surrounding seas. The 20, 50, 

 or 100 fathom line follows sometimes the shape of the coast 

 within, sometimes differs widely from the shape of the coast, 

 just the same as barrier reefs do. 



The Fijis are, in my opinion, such a volcanic group, where 

 the shallower waters have now become the home of myriads of 

 lime-secreting organisms. These have built up the wholly sub- 

 merged banks into atolls, and the banks around islands into 

 barrier reefs, the depth and distance from land most favourable 

 for vigorous coral growth being determined by a variety of local 

 circumstances. When the reef reaches the surface, it spreads 

 seawards. There is, of course, plenty of living coral in the 

 lagoons, but this becomes less and less as the reef becomes mere 

 continuous and less oceanic water is admitted. The growth of 

 corals is always, however, much less vigorous and much less 

 rapid in lagoons than on the seaward faces of reefs. The posi- 

 tion of the opening in the reef is determined by local conditions, 

 such as the mud from rivers in the case of barrier reefs. I 

 have elsewhere fully explained my view as to the removal of 

 dead coral, and even coral heads and islands, from the lagoons, 

 through solution of the lime by sea-water. 



I cannot have seen as much of Kandavu as Captain Moore,. 

 but what I did see in no way led me to believe that it was a 

 sinking island ; ^ indeed it was here that, not being able to apply 

 Mr. Darwin's theory in explanrition of the phenomena of the 

 Kandavu reefs, I commenced to doubt it altogether. The coast 

 around Cape Washington (the western end) appears to be par- 

 ticularly unfavourable for the formation of extensive reefs, 

 because of the high land (2750 feet) and deep water, hence we 

 have only narrow fringing reefs or no reefs. At the north end 

 the extensive banks surrounding the islets are, on the other 

 hand, specially favourable for coral growth, owing to the almost 

 complete absence of rivers and of detritus from the land ; here 

 we have what might be called an atoll, except for the presence 

 of the small rocks, which, after reaching the surface, has extended 

 seawards. 



If Captain Moore thinks the rock, Solo, was once an island 

 about four miles in diameter and of considerable height, he must 

 explain how in sinking it has left a lagoon around the remaining 

 rock with an average depth of not more that 12 fathoms. He 

 must also explain the nature of the bank extending west fronn 

 One, where there is no continuous barrier. 



It is in every way desirable that practical surveyors like Captain. 

 Moore should take an interest in these theoretical views, and their 

 observations will be none the less welcome and appreciated what- 

 ever side they may take in the controversy. The more observations 

 accumulate the more does it seem to me probable that there 

 never was a barrier reef or atoll formed after the manner required 

 by Mr. Darwin's theory. If Profs. Dana, Bonney, Huxley, and 

 Judd, would mention any one barrier reef and any one atoll 

 that they believe undoubtedly to have been formed in accordance 

 with the subsidence theory, then I think it might be possible 

 within the next few years to undertake a thorough examination 

 of these with the view of testing the rival theories. 



Challenger Office, Edinburgh. John Murray. 



I have been immersed in examinations, and away from 

 London for a few days, so that I did not see Mr. Guppy's letter 

 in Nature of June 20 (p. 173) till it was loo late to reply. I 

 will now only ask space for a few last words. To me the matter 

 does not appear to have " resolved itself into an affair of out- 

 posts." The position which I was led to take up from the 

 study of the recent literature on coral reefs, and which I had 

 hoped that I had made clear in my last letter, is this — that till 

 Mr. Guppy can produce cases of growing reefs at depths well 

 exceeding 25 fathoms, isolated instances of ttie occurrence, at 

 such depths, of living corals which are among the reef-builders 

 do not really help him ; and that till he can do this he is only 

 supporting hypothesis by hypothesis. For example, I have not 

 seldom, in the Alps, gathered phanerogamous plants, flourishing 

 and in full bloom, at elevations of eleven or even twelve thousand 

 feet above the sea ; but I should not direct anyone to this 

 mountain zone who desired to pick a posy of Alpine flowers. 



Into the remainder of Mr. Guppy's letter it would be waste oi 

 time to enter, or to continue this controversy, for two reasons : 

 one, that evidently Mr. Guppy and myself differ widely as to 

 the nature of hypothesis, and the difference between assumption 

 and proof ; the other, that I trust, before another number of 

 Nature appears, to be among the Alps (with strict orders 

 against forwarding letters or papers), so that any return shots 

 will fail to reach me, until, in this rapidly- moving world, they 

 have passed into the region of ancient history. 



June 28. T. G. Bonney. 



Before replying to Captain Moore, I should bear witness to 

 the searching character of his questions, which quite makes up 

 for the fact that during his valuable services of many years abroad 

 he has been, fortunately perhap-, beyond the reach of recent 

 coral reef literature. 



However, I would first point out that when he speaks of the 

 second edition of Mr. Darwin's " Coral Reefs," which was pub- 

 lished fifteen years ago (1874), as given to the world "after forty 

 years of deep research into various problems of Nature," he surely 

 forgets that after the first edition of 1842 the author had no fur- 

 ther acquaintance with coral reefs. Captain Moore also omits to 

 reflect that during the interval between the two editions Dana 

 and Jukes were the only two principal observers that stood up 



See "Narrative of the Challenger Expedition," vol. i. r. 507. 



