498 Mr Gardiner, The Coral Reefs of [Mar. 7, 



of carbonic acid gas, than to the more abundant food supply. 

 Undoubtedly the better food supply would be advantageous to 

 the corals so that these would nourish also, but my experiments 

 have shown that possibly the fresh supplies of carbonic acid gas 

 are here too the more important factor. 



My reasons have already been given for thinking that reefs are 

 growing outwards at about the low tide level, and the deep 

 dredgings off Funafuti showed that stones and fragments of coral 

 rest on the steep slope, while there is an area of about 250 yards 

 in width outside the reef before the steep commences. The uni- 

 formity in width of this gives evidence, I think, of the depth to 

 which the greatest effective wave action extends, and also gives 

 approximately the depth at which nullipores, the chief reef-builders, 

 flourish. I found at Rotuma that there is over this area, when the 

 tide commences to rise as well as when it is falling, a distinct 

 undertow from the edge of the reef. I found that if I threw a 

 fishing line with a heavy weight attached from the edge of the 

 reef it was not swept back again to me. Making a further experi- 

 ment by substituting a large round pebble for the lead, the whole 

 line of 60 fathoms was quickly drawn off its stick by the strain ; it 

 was necessary, however, to throw the pebble a distance of about 

 30 yards out, as otherwise it was simply washed in at my feet. I 

 never repeated the experiment, but the absence of pieces of reef 

 debris in my dredgings from this area leads me to believe that 

 there is really such a backwash, which sweeps the slope clean, and 

 piles up the talus to form the steep. 



The precipitous character of the reef towards the lagoon, and 

 the mushroom-shaped heads of coral are, no doubt, due to solution. 

 This theory implies that the reef, as fast as it is formed outside, is 

 dissolved away inside as these barrier and atoll-reefs are fairly 

 uniform in breadth. The solution forms the cliff, and undermines 

 the shoals ; the sand at the base of the cliff represents the debris 

 thrown over the reef, and is in this position reformed as quickly as 

 it is dissolved. 



The rate of growth of the reef outwards depends on many con- 

 siderations, foremost amongst which must be placed the depth of 

 the ocean floor. The reef, as it grows, has to build up its foundation, 

 and, until it has built up this to the requisite height, it cannot grow 

 outwards ; hence naturally the reef should extend out fastest where 

 the slope is smallest. Again, deep currents running along the 

 shore would naturally clean the steep of its talus, where it projects, 

 and so the growth outwards should be slowest here. Thus currents 

 would act in such a case in opposition to the slope, and the result 

 would depend upon which was the stronger. The reef also should 

 grow faster on the side on which the current runs, as it would 

 constantly change the surrounding water. The waves on the 



