NATURE 



{_Nov. 29, 1883 



careous organisms in the surface waters and of the com- 

 parative rapidity with which these remains might be 

 accumulated on the sea-bottom. 



Reef-builders starting on a submarine bank, whether 

 prepared for them by erosion, by subsidence, or 1)y the 

 upward growth of organic deposits, would form reefs that 

 must necessarily tend to assume the atoll form The 

 central portions of the colony or clump of coral will 

 gradually be placed at a disadvantage as compared with 

 the peripheral parts of the mass in being further removed 

 from the food supply, and will consequently dwindle and 

 die. In proportion as the reef approaches the sea-level 

 these central parts are brought into increasingly uncon- 

 genial conditions, until at last an outer ring of vigorous, 

 growing coral-reef encircles an inside lagoon overlying 

 the central stunted and dead portions. The possibility 

 of such a sequence of events was likewise recognised by 

 Darwin. " If a bank, either of rock or of hardened 

 sediment," he says, "lay a few fathoms submerged, the 

 simple growth of the coral, without the aid of subsi- 

 dence, would produce a structure scarcely to be distin- 

 guished from a true atoll." ' 



As the atoll increases in size the lagoon becomes pro- 

 portionately larger, partly from its waters being less sup- 

 plied vfith pelagic food and therefore less favourable to 

 the growth of the more massive kinds of coral, partly 

 from the injurious effects of calcareous sediment upon 

 coral-growth there, and partly also from the solvent 

 action of the carbonic acid of the sea-water upon the 

 dead coral. The solution of dead calcareous organisms 

 by sea-water is undoubtedly one of the most interesting 

 facts brought to light by the naturalists of the Cliatlenger 

 Expedition. 



Moreover, a connected chain of atolls might be formed 

 on a long, submarine bank, and similar conditions of 

 growth would then be displayed as in the case of a single 

 atoll. The marginal atolls having a better supply of 

 food would grow more vigorously than those towards 

 the centre, and would tend to assume elongated forms, 

 according to the shape of the bank beneath them. Many 

 of them might coalesce, and might even ultimately give 

 rise to one large atoll. Such a chain of atolls as that of 

 the great Maldive group may be thus explained without 

 the necessity for any disseverment by oceanic currents 

 as Darwin supposed. Un the other hand, the submerged 

 coral-banks of the Lakadivh, Caroline, and Chagos archi- 

 pelagos may be regarded as representing various stages in 

 the growth of coral-reefs, some of them being still too deep 

 for reef-builders, others with coral-reefs which have not yet 

 quite grown up to the surface. But scattered among these 

 banks are some of the most completely formed atolls. 

 Mr. Murray contends that it is difficult to conceive how 

 such banks can have been due to subsidence, when their 

 situation with respect to each other and to the perfect 

 atolls is considered. He reverses the order of growth 

 as given by Darwin, who cited the great Chagos 

 bank as probably an example of an atoll which had been 

 carried down by a subsidence more rapid than the rate at 

 which the corals could build upwards. 



From a careful study of barrier-reefs Mr. Murray 

 concludes that, in their case also, all the phenomena can 

 be explained without having recourse to subsidence. He 

 found from personal observation and a comparison of the 

 Admiralty charts that most exaggerated notions prevail 

 regarding the depth of water immediately outside the 

 reef, which is usually supposed to be very great. After 

 minutely exploring the barrier-reef of Tahiti, and sounding 

 the water both inside and outside the reefs, he found that 

 the slopes are just such as might be looked for on the 

 su;)position that the corals have grown up without any 

 sinking of the bottom. The accompanying section (Fig. 

 l), drawn to a true scale will show that there is 

 nothing abnormal in the declivities. Beginning near the 



' op. cit. p. 134. 



shore or wherever the bottom whether of rock or sediment 

 comes within the range of the reef-builders, a barrier reef 

 grows vigorously along its outer face, while its inner parts, 

 as in the case of an atoll and for the same reason, are 

 enfeebled and die. The force of the breakers tears off huge 

 masses, sometimes 20 or 30 feet long, from the face of the 

 reef, especially where from the borings of mollusks, 

 sponges, &c., the coral-rock has been weakened. These 

 blocks tumble down the seaward face of the reef, forming 

 a remarkably steep talus. It is this precipitous part of 

 the reef which has probably given rise to the notion that the 

 water outside suddenly descends to a profound depth. 

 The steep front of fallen blocks is succeeded by a de- 

 clivity covered with coral sand, beyond which the bottom 

 slopes away at an angle of no more than 6""", and is covered 

 chiefly with volcanic detritus. Mr. Murray insists that any 

 seaward extension of the reef mu^t be on the summit of 

 the talus of broken coral. The reef will gradually recede 

 from the shore of the island or continent, and will leave 

 behind here and there a remnant to form an island in the 

 slowly broadening lagoon-channel. 



The very general occurrence of proofs of elevation 

 among the regions of barrier-reefs and atolls is in harmony 

 with the volcanic origin of the ground on which these 

 coral-formations have grown, but is, as Mr. Murray con- 

 tends, most difticult of explanation on the theory of 

 widespread subsidence. He affirms that all the chief fea- 

 tures of coral-reefs and islandsnot only do not necessarily 

 demand the hypothesis of subsidence, but may be 

 satisfactorily accounted for, even in areas where the 

 movement is an upward one, by the vigorous outward 

 growth of the corals on the external faces of the reef in 

 presence of abundant food, by their deaih, disintegration, 

 and removal by the mechanical and chemical action of 

 the sea in the inner parts, and by the influence of subaerial 

 agencies and breaker-action in lowering the level of the 

 upraised areas of coral-rock. Arch. Geikie 



{To be contitiued.\ 



NOTES 

 It will be seen from our Diary that the meeting of the 

 Linueaii Society on December 6 is to be exclusively devoted to 

 the reading of a posihumous essiy on In.stinct by the late Mr. 

 Darwin. We are informed that this essay is full of important 

 and hitherto unpulilished matter with regard to the facts of 

 animal instinct considered in the light of the theory of natural 

 selection ; and as the exi^tence of the es>ay has o ily now been 

 divulged, we doubt not that the next meeting of the Linnean 

 Society will be of an unusually interesting character. 



The death is announced, at the age of seventy-six, of Mr. 

 John Eliot Howard, F. R.S., well known as a chemist and 

 quinologist. He was the son of Mr. Luke Howard, F.R.S., a 

 well-known meteorologist in his day. 



We announced some time ago that the Finnish Senate had 

 vjted a sum of 37,000 marks to Prof. Lems'roji for the con- 

 tinuation of his experinents with the aurora borealis at SoJan- 

 kylii in the Finnish Lappmark during 18S2-83, of which he gave 

 an account in Nature (vol. xxvii, p. 3S9). The plan to be 

 followed during the present winter at this station is to make 

 obervations three times in every twenty-four hours, with the 

 exception only of the first and fifteenth of every month, when 

 they are made every five minutes throughout the twenty-tour 

 hours, and three days of the month when they will be effected 

 every half minute during two hours. In order partly to obtain 

 the necessary data for the control of the variation of the current 

 from the atmosphere h ith the latitude, and partly to reduce the 

 eff.ct of probable influences a branch station will be temporarily 

 e tablidied during the months of November, December, January, 

 February, and part of March at the huildmgs of the Kuliala gold 



