Dec. 6, 1883] 



NATURE 



sist wholly of solid coral.' Prof, Agassiz has followed 

 the growth of a reef upon a platform of calcareous organic 

 debris, and he has found elevated coral-reefs which rest 

 on such a platform. Mr. Murray's observations explain 

 how a reef may grow outward on a talus of its own debris. 

 There appears to be no reason, indeed, why a calcareous 

 mass of almost indefinite thickness might not be formed 

 without the aid of subsidence. Its upper zone might be 

 directly due to coral growth, while the larger part of the 

 mass might be composed of an aggregate of coral debris 

 mi.xed with the remains of mollusks, echinoderms, and 

 other calcareous organisms. So rapid is the destruction 

 of organic structure through the solution and redeposit 

 of carbonate of lime by infiltrating water, that a special 

 and careful search might be required to detetmine the 

 actual limits of the true reef and of its calcareous plat- 

 form, and even such a search might not be successful. 



After a full consideration of this second difficulty I feel 

 compelled to admit that no valid argument in favour of 

 subsidence can be based on the steepness of the seaward 

 face of a reef and the thickness of the calcareous mass 

 of the reef itself. 



3. The depth of some lagoons and lagoon-channels 

 furnishes probably the strongest argument in favour of 

 Darwin's views. Occasionally a depth of forty fathoms 

 is reached, and as this is beyond the depth at which reef- 

 builders ordinarily live, it has been regarded as a proof 

 that subsidence has taken place. 



This third difficulty is thus met by the opponents of 

 subsidence. We must remember, they say, that from the 

 very conditions of their growth, patches of coral tend to 

 assume an annular or atoll-like form, because the outer 

 parts grow vigorously, while the central portions eventually 

 die. Where the coral-patches coalesce and e.xtend along a 

 bank or shore, it is their outer or seaward faces that flourish. 

 The inner parts, as they are more and more cut off from 

 the food-supply, gradually die. While the outer face of 

 the reef grows seaward, the inner margin is attacked 

 partly by the solvent action of the carbonic acid of sea- 

 water, partly by wind-waves, and the tidal scour sweeps 

 away much fine detritus through gaps in this reef. In 

 this way the lagoon-channel is widened and deepened. 

 In a perfect atoll, that is, an unbroken annular reef of 

 coral, the lagoon could not be deepened by any mere 

 abrasion of the dead coral and removal of the detritus in 

 suspension, but solution by carbonic acid would still come 

 into play. It is further to be borne in mind that small 

 lagoons are shallow and are being filled up, and that it is 

 only the large ones, encircled by nearly continuous reefs, 

 where the corals in the lagoon and along the margin are 

 dead, and where the effects of solution may be conceived 

 to have been longest in operation, that the depth of the 

 lagoon descends below the limits at which reef-builders 

 live. 



I do not regard this solution of the difficulty as wholly 

 satisfactory. Of the fact that dead calcareous organisms 

 are attacked and carried away in solution by the carbonic 

 acid of sea-water there cannot be any question, and this 

 process must be of great geological importance. Whether 

 the solvent action is sufficient to account for the excep- 

 tional depth of some lagoons, is still, I think, open to 

 inquiry. It seems to me not improbable that these com- 

 paratively few deep lagoons may owe their depth partly 

 to subsidence. But if this be the case it wculd lend, I 

 am afraid, but slender support to a theory of wide oceanic 

 depression. That there must be some areas of subsidence 

 over the coral regions is almost certain, and the few 

 scattered deep lagoons may possibly indicate some of these 

 areas. 



Having thus fully examined the arguments on both 

 sides of this interesting and important question, I feel 



* Prof. Dana (0/. cit.) cites examples of raised coral-reefs 250 to 300 feet 

 above sea-level ; but we do not yet know how much of the rock is sohd coral 

 and how much may be formed of aggregated organic debris. 



myself reluctantly compelled to admit that Darwin's 

 theory can no longer be accepted as a complete solution 

 of the problem of coral-reefs. No one could be more 

 impressed than myself with the simplicity of this theory, 

 the brilliancy of its generalisation, its remarkable fitness 

 in geological theory, and the grandeur of the conceptions 

 of geographical revolution to which it leads. I am fully 

 alive to the serious changes which its abandonment 

 will make in some departments of geological speculation. 

 But in the face of the evidence which has now been 

 accumulated, I can no longer regard the accepted theory 

 as generally applicable. That it may possibly be true 

 in some instances may be readily granted. There may 

 be areas of subsidence, as there certainly are areas of 

 elevation, over the vast regions where coral-reefs occur. 

 It may be conceded that subsidence may sometimes have 

 provided the platform whereon coral-reefs have sprung 

 up, and may have contributed to heighten some reefs and 

 to deepen some lagoons and lagoon-channels. But I do 

 not believe that we are now justified in assunring sub- 

 sidence to have taken place, from the mere existence ot 

 atolls and barrier-reefs. Its occurren'^e at any locality 

 must be proved by evidence of special local movement. 

 It may have gone on at many localities where atolls and 

 barrier-reefs are found, but the existence of such reefs 

 is no more necessarily dependent upon subsidence than 

 upon elevation. These subterranean movements must 

 be looked upon as mere accidents in a general process of 

 coral growth which is wholly independent of them. 



I may in conclusion refer to one or two difficulties 

 which have long been felt to be serious drawbacks to the 

 theory of subsidence, but which disappear when the 

 newer views of the origin of coral-reefs are accepted. If, 

 as Darwin supposed, the coral-islands of the Pacific and 

 Indian Oceans represent the last peaks of submerged 

 continents, it is incredible that continental rocks should 

 not be found among them. The oceanic islands (except 

 of course those composed of coral-rock) are of volcanic 

 origin and show none of the granites, schists, and other 

 rocks which might have been looked for on such elevated 

 summits. They have been piled up by the accumulation 

 of lavas and tuffs discharged from the earth's interior, and, 

 where they occur, point to upheaval rather than sub 

 sidence. Again, as Mr. Murray has shown, the inorganic 

 deposits of the ocean-floor are composed of volcanic 

 debris with a singular absence of the minerals that con- 

 stitute the usual crystalline rocks of our continents. 



No satisfactory proofs of a general subsidence have 

 been obtained from the region of coral reefs, except 

 from the structure of the reefs themselves, and this is an 

 inference only, which is now disputed. From the nature 

 of the case, indeed, traces of subsidence can hardly be 

 expected. A few examples have been cited, such as the 

 occurrence of trunks of cedar-trees in a layer of red soil 

 at Bermuda, lying between the calcareous deposits and 

 at a depth of 42 feet below low-water mark. This indi- 

 cates a recent subsidence of that tract ; but it may be 

 merely local, and may be due to the sinking down of the 

 roof of one of the caverns into which the limestone is so 

 abundantly honeycombed. Occisionally along the mar- 

 gins of lagoons trees are found at the water edge, in a 

 position suggestive of subsidence. But the removal of 

 the calcareous rock by solution or wave-action might 

 equally account for their condition. 



Of elevation in the region of atolls and barrier- 

 reefs, there is almost everywhere more or lees distinct 

 evidence. Prof. Dana has collected the facts which 

 prove that recent elevatory movements of unequal and 

 local extent have occurred in all parts of the ocean.' 

 Upheaval has taken place even in areas where barrier- 

 reefs and atolls are in vigorous growth. Such an associa- 

 tion of upheaval with an assumed general subsidence 

 requires, on the subsidence theory, a cumbrous and 



■ Corals and Coral Islands," 2nd edit. p. 2S4. 



