J. S. Gardiner — Origin of Coral Reefs. 211 



edlj serve not only to prevent the lagoons from being filled 

 up but also to enlarge them. Both to some degree increase 

 as the atoll-form, i. e. the ring of reef round a bank, becomes 

 more perfect. Neither action can proceed through the bodies 

 of living organisms, and on an open bank the whole slope of 

 any reef is often practically completely covered by fixed and 

 living animal and plant life, so that both this action and solu- 

 tion are impossible. The movement of tine mud tends to 

 choke and kill the organisms on the lower slope of any reef 

 on a bank or within a lagoon. Since more mud is deposited 

 as a bank becomes enclosed, sedentary life in this position 

 must be seriously affected, and then the boring animals enter, 

 weakening the coral masses so that they topple' over, giving 

 fresh openings for the entrance of a new series of boring 

 forms. The effect is well seen in the practically perpendicular 

 lagoon slopes of the reefs, that fringe or lie within the lagoons 

 of the Maldive atolls, from three fathoms or so to within a 

 few fathoms of the bottom." 



With this gradual but sure increase in the size of any 

 lagoon it is necessary for the life of any atoll that its rim reefs 

 should be growing outwards at least as rapidly as destruction 

 is taking place inside. All conditions point clearly to this 

 being the case. From the outer edge of an encircling reef -flat 

 there is generally to seaward a gradual slope to 30-50 fathoms 

 in 200-400 yards, succeeded by the steep already referred to. 

 This slope is essentially the growing area, being covered almost 

 completely by living organisms, great spreading and branching 

 colonies of all kinds of corals, which are being bound together 

 and fixed by Lithothamnion^ Polytreraa and many other kinds 

 of encrusting organisms. At the edge great masses are con- 

 stantly reaching the surface, and at the same time being bound 

 by the same organisms on to the reef flat behind. Fissures 

 remain for some time in the edge of the reef for the escape of 

 the water, but these subsequently become bridged across, and 

 finally built into the solid reef. At the same time the outwash 

 of detritus, largely due to the under-currents, causes a raining 

 down of coral masses and sand over the edge of the steep, 

 carrying it out and allowing the extension of the whole out- 

 wards as a fairy ring. 



The whole mode of growth sounds most simple, while in 

 reality it is produced by a series of nicely correlated action'^, 

 which result in the atoll shape, undoubtedly the dominant and 

 characteristic form of oceanic reefs. Should the encircling 

 reef become too narrow, the conditions within it will become 

 more open, and more of its lagoon will be covered by coral 



* This perpendicular slope is not consistent with, the possibility of the 

 lagoons having been filled in by detritus washed over their encircling reefs. 



