BORING AND SAND-FEEDING ORGANISMS. 333 



outside atolls is certainly many times greater than within atolls, and in my opinion does not 

 materially decrease until a depth of over 20 fathoms is attained. Until a surface reef be 

 formed, or until the reef nearly reaches the surface, there is probably no outward sweeping 

 of talus to spread the foundations of the whole. Interstices are filled in largely by nullipores 

 and Foraminifera, or else bridged over. All the corals keep pace in their growth, and the 

 rate of the whole reef would be the average rate of its corals. 



Considering all the various factors, and particularly remembering the necessarily slower 

 growth at the initiation of a reef and as it approaches close to the surface, it yet seems 

 to me to be probable that an oceanic shoal at a depth of 25 fathoms might well in 1000 

 years, or even less, be covered with a perfect surface reef, built up by nullipores and reef 

 corals. In effect, if Falcon Island, erupted to a height of about 250 feet in 1885 and now 

 a mere shoal, be cut down to 25 fathoms by the end of this century — a by no means unlikely 

 proposition — its place might well be marked by surface reefs, perhaps even by a perfect atoll 

 considerably before the year 3000 A.D. Should these deductions be, as I believe, fairly accurate, 

 a natural explanation is at once afforded of the rarity of submerged banks of all sorts in 

 coral-reef areas as compared with surface atolls and reefs. 



Section V. The Action of Boring and Sand- Feeding Organisms. 



1. General. 



The skeletons of the corals and other organisms of a reef are either built up into 

 a coral limestone by the animals themselves, or are worn down into mud and sand. A third 

 fate may, however, at any time befall them in being dissolved by the sea-water. It is obvious 

 that as a rule the smaller the skeleton the greater must be the amount of surfixce that 

 will be exposed to solution. Hence sand should be more acted upon, and should suffer more 

 loss in weight, than corals, shells or nullipores. Now the ground-down skeletons of these 

 organisms form the greater part of the sand, and it is most necessary to examine the 

 means by which their massive skeletons are broken up. 



The main destructive agents of reefs are, undoubtedly, marine animals and to a lesser 

 degree plants, aided to some extent by the solvent action of the water and erosion by the 

 currents. The destructive organisms have two modes of procedure. First they bore into corals 

 and other skeletons, and so weaken them that they break either by their own weight or 

 the motion of the sea. Indirectly they probably by their organic matter attract other animals 

 to ingest the fragments and further break them down. The latter or second action, that of 

 sand trituration, is of no small significance. 



In actual number, the animals that depend on sand for their food in any atoll are 

 probably more numerous than the boring or free-living forms, as undoubtedly they are the 

 most difficult to get any adequate knowledge of. Besides breaking down the fragments of 

 rock into sand, they are the main factors in the creation of mud. The deeper layers of 

 the sand of a flat pass through their bodies again and again, being in all cases ejected 

 in a still finer state on the surface of the bottom. With the ebb and flow of the tide, 

 with storms, etc. much of the finest matter must pass into suspension in the sea-water, to 

 be, perhaps, deposited outside the atoll. The influence of this outwash on the character of 

 the bottom deposits outside reefs has been thoroughly established by the Challenger expedition, 

 and there is no call for further comment here. 



