334 J. STANLEY GARDINER. 



To quote Sir John MuiTay's Summary of his Report on Deep-Sea Deposits, " Coral 

 Muds and Sands cover a large area in all coral-reef regions, estimated at about 2,700,000 

 square miles, including those from shallow water and also the area of the islands and of 

 the lagoons and lagoon-channels. The coral-reef region of the Pacific is by far the most 

 extensive, and there Coral Muds and Sands attain their maximum development, being estimated 

 to occupy about 1,500,000 square miles; in the Atlantic they cover about 800,000 square 

 miles, and in the Indian Ocean about 400,000 square miles '." The area in the slightest 

 degree covered or enclosed by actual surface reefs in the Indian Ocean is certainly less than 

 25,000 square miles, so that the space actually covered by deposits, mainly formed from the 

 same, is at least sixteen times as great. As to the rate of formation of the deposit Sir John 

 Murray remarked : — " Around some coral reefs the accumulation must be rapid, for, although 

 pelagic species with calcareous shells may be numerous in the surface waters, it is often 

 impossible to detect more than an occasional pelagic shell among the other calcareous debris 

 of the deposits ^" 



The main cause of the formation of lagoons is, assuredly, the solution of the calcium 

 carbonate by the water, but the outwash of mud is an important and direct subsidiary cause. 

 At the same time the mud has an indirect action as well, i.e. by preventing the growth 

 of coral and other sedentary organisms on the floor of the atolls, and by constantly checking 

 and restricting all such growth. This was an action repeatedly observed by us, but from 

 its nature one almost incapable of direct estimation. I am here, however, more expressly 

 concerned with the organisms which are the cause in their individual capacity, rather than 

 the effect which they ultimately produce. 



Although it is scarcely necessary, one is here impelled to refer to Darwin's work on 

 "Vegetable Mould and Earthworms" in which he estimates that 10 tons of earth per acre 

 annually pass through the bodies of earthworms, and are brought to the surface. The action 

 of the reef organisms takes in not only the deeper sand but the surface material as well. 

 It is independent of summer and winter, drought and frost, and in the smallest estimate 

 could not be placed at less than 50 times as much as that of earthworms, with an erosive 

 action by the average marine animal on the sand many times as strong. 



The phenomena, however, are not confined to reefs, and have been demonstrated by 

 Buchanan in the deposits of the ocean' : — " As the result I was led to believe that the 

 principal agent in the comminution of the mineral matter found at the bottom of both 

 deep and shallow seas and oceans is the ground fauiia of the sea, which depends for its 

 subsistence on the organic matter which it can extract from the mud." With this conclusion 

 I cordially agree, but I do not think that the sand-feeding organisms of coral reefs have 

 any considerable chemical action on the sand. The latter is always free from smell, even 

 on mud flats, as off Huludu, Addu atoll, and, whether dry or wet, sulphuretted hydrogen (H^S) 

 is not given off, when acted upon by acid. The sand consists of almost pure carbonate of 

 lime, and, although I sought carefully for any digestive action on the sand — the reaction of 

 the contents of the gut was usually neutral, rarely alkaline — I failed to find any trace of 

 such in the true, sand-feeding organisms'*. The action of these forms, then, is purely one 

 of trituration. 



' p. 247. •• I proved experimentally that Lobophytum and massive 



- p. 411. Astraeid corals will dissolve sand grains that may be taken 



^ " On the Occurrence of Sulphur in Marine Muds and into their coelentera. I have also found Operculina in Fla- 



Nodules, and its bearing on their Mode of Formation." Proc. helium and other Foraminifera in Coenopsammia, with their 



R. S. Edin. vol. xviii. pp. 17 — 39 (1890). shells completely or largely dissolved. 



