78 



NA TURE 



\_Nov. 25, 1886 



Prof. Joseph Le Conte in the instance of the Florida 

 reefs. He then pointed out that since corals will not 

 grow on muddy shores or in water upon the" bottom of 

 which sediment is collected, the favourable conditions 

 can only be obtained at some distance from the shore, 

 where a barrier-reef would ultimately be formed Ihiiitai 

 on one side by the iniiddiness and on the other by t/te depth 

 0/ the water. 



The foregoing conditions may be described as the 

 determining causes of a barrier-reef. After the reef has 

 been formed, the lagoon-channel will be kept open 

 by such agencies as solution, diminished food-supply, 

 tidal scour, organic degradation, and other influences 

 The circumstance that barrier reefs are frequently 

 situated at or near the borders of submarine plateaus 

 receives a ready explanation in the view first advanced 

 by Prof Le Conte, since in such situations the necessary 

 conditions of depth and clearness would be found. 



Anomalous Depths of some Atolls and Barrier-Reefs. — 

 One of the principal arguments in favour of the theory of 

 subsidence lies in the assertion that lagoons and lagoon- 

 channels are sometimes deeper than the reef-coral zone. 

 I will, however, endeavour to show that this assertion is 

 founded on a misconception of the conditions that limit 

 the depth of this zone. The extent to which the depth 

 may vary is demonstrated in the great divergence be- 

 tween the estimates of different observers in every region 

 •of coral reefs. Those of Ouoy and Gaimard, Ehrenberg, 

 Dar'vin, Dana, Murray, A. Agassiz, and others, range from 

 5 to 40 fathoms, liut this variation may also be found in 

 the saiTie region of coral reefs. Thus, in the Solomon 

 Islands, I found that the depths at which reef-corals 

 flourished ranged in different localities from 12 to 

 40 fathoms and beyond, the variatioi being due to 

 differences of local conditions, such as the degree of in- 

 clination of the submarine slope, the presence and posi- 

 tion of submarine declivities, the amount of sediment 

 held in suspension, the force of the breakers, and other 

 influences. The main determining condition, as Prof 

 A. Agassiz points out, is to be found in the injurious effect 

 of sand and sediment rather than in the general influence 

 of depth ; and the distribution of these materials is de- 

 pendent on the local conditions above referred to. Local 

 conditions will usually restrict the reef-coral zone to depths 

 less than 30 fathoms ; but, where there is a gradual sub- 

 marine slope, reef-corals are to be found in depths beyond 

 the sand and gravel. Inasmuch as most observers have 

 regarded these materials as necessarily limiting the zone, 

 they did not push their inquiries beyond. Under favour- 

 able conditions, however, retf-corals may thrive in depths 

 of 50 or 6d fathoms ; and thus we can readily explain 

 the apparently abnormal depths inside some atolls and 

 barrier-reefs. 



An apparent objection here presents itself If reefs 

 begin to build their foundations in depths greater than 

 those which are generally assigned to them, the thickness 

 of the elevated reef-formations discovered bv me in the 

 Solomon Group should have been much greater than 

 150 feet, the actual limit of their thickness. It will, how- 

 ever, have been gathered from the previous remarks that 

 local conditions will usually confine reef-corals to depths 

 less than 25 or 30 fathoms, and that it will be only under 

 occasional circumstances that reefs will commence to be 

 formed in deeper water. Fringing-reefs themselves are 

 at first res:ricted to shallow waters around the coast, and 

 their seaward extension in localities where the submarine 

 slope is at all steep, as is generally the case, must be ex- 

 tremely slow. Again, in an area of elevation, such as 

 that in which the Solomon Islands are included, barrier- 

 reefs, which may have begun to grow in depths not less 

 than 50 fathoms, might owe their approach towards the 

 surface as much to the elevating movement as to the very 

 slow upward growth of the corals. It should also be 

 borne in mind that the rapid subaorial denudation, to 



which these regions of heavy rainfall are subjected, would 

 be an important agency in the thinningaway of the raised 

 coral formations. 



In the latter part of my paper I refer, amongst other 

 subjects, to the extensive character of the degradation of 

 coral reefs by multitudes of organisms. I also give proofs 

 of the outward growth of reefs on their own talus (as 

 described by Murray) — (i) in the circumstance that mas- 

 six'e corals may be commonly observed to increase in size 

 as one approaches the lagoon from the outer margin of 

 the reef-flat ; (2) in the presence of old lines of erosion 

 evidently produced at the existing sea-level, but which 

 have been cut oft" from the action of the waves by the 

 advancing edge of the reef-flat ; (3) in the characters and 

 position of the wooded islets situated on reefs, which in 

 course of time would cover the whole reef-flat, were it not 

 for one counteracting circumstance, the seaward growth 

 of the reef. 



Lastly, I refer to the deposits at present forming on the 

 outer slopes of reefs in depths down to 100 fathoms. 

 Reef-debris, foraminiferous tests, especially oi Orbitolites, 

 joints of the calcareous alga Halimedd opuntia, portions 

 of Niillipora:, and the small detached corals of the genus 

 Heteropsammia, enter largely into the composition of these 

 deposits. I should add that a rock of this composition is 

 one of the commonest types of the so-called coral lime- 

 stones in the Solomon Group. 



In this short abstract of a long paper I have not been 

 able to do much more than indicate the general bearing 

 of my conclusions. The facts and data are given at 

 length in the original paper. 



THE BRITISH ASSOCIATION AND LOCAL 

 SCIENTIFIC SOC/ETI£:S 

 '"pHE second annual Conference of Delegates held 

 J. under the new rules of the British Association met 

 at Birmingham on September 2 and 7, in the library of 

 the Medical Institute. Forty-nine local Societies carry- 

 ing on work in various parts of the L'nited Kingdom have 

 been enrolled this year as " Corresponding Societies " of 

 the Association, and of these thirty-two were represented 

 by Delegates at the Birmingham meeting. The following 

 report of the proceedings of the Conference, signed by 

 Mr. Francis Galton and Prof R. Meldola, the Chairman 

 and Secretary of the Committee, has just been circulated 

 among the Corresponding Societies, and it will be seen 

 that this new branch of the work of the Association pro- 

 mises to be of mutual advantage both to the Societies and 

 the Association : — 



At the first Conference the chair was taken by Dr. 

 A. W. "U'illiamson, F.R.S., General Treasurer of the 

 British Association, the Corresponding Societies Com- 

 mittee being represented by Captain Douglas Galton, 

 F.R.S., General Secretary of the Association, Dr. Garson, 

 Mr. John Hopkinson, F.L.S., and Pj of. R. Meldola, F.R.S , 

 Secretary. 



The Secretary read the Report of the Corresponding 

 Societies Committee which had been presented to the 

 Council of the Association. 



The Chairman made some remarks explanatory of the 

 objects of the Conference of Delegates, and suggested 

 that among other subjects of investigation in which it 

 might be useful to secure the co-operation of the local 

 Societies was that of injurious insects, already so much 

 studied by iVIiss E. A. Ormerod. 



The Secretary also made some observations in explana- 

 tion of the constitution of the Corresponding Societies 

 Committee and the relations existing between the Con- 

 ference of Delegates and the British Association. 



Some remarks were made by Mr. J. W. Davis and 

 others with reference to the advisability of securing the 

 co-operation of the local Societies for the purpose of in- 



