GROWTH OF COKAL KNOLLS. 225 



cut (see fig. 69, a). If the growth were to continue equal on 

 all parts of the surface, the polyps at the summit would reach 

 the normal low-water mark sooner than those at the sides ; 

 thus, too, they would be the first exposed to the action of the 







Fig. 69.— Biagram of the growth of the colonies of Porites. a, first stage, in which the 

 summit of the coral touches the surface of the sea ; 6, the second, in wliich it gj-ows 

 only in circumference ; c, the third, during which the surface, intact at b has 

 died away and been hollowed out. ' 



air and rain at low ebbs, and so die the sooner. The polyps 

 at the sides continue .to grow, tending not merely to raise the 

 mass of the coral, but also to extend it horizontally ; and thus 

 the upper and rapidly dying surface spi'eads laterally so much 

 that sand, stones, plants, and various forms of animal life find 

 room upon it. At first this level remains at very miich the 

 height to which such a block may normally grow (6) ; but when 

 it has reached a considerable size —from 6 to 8 feet in diameter or 

 even more — the centre plateau is large enough to afford room for 

 the collection of a considerable body of water, with sand, plants, 

 and animals. In consequence of this the upper surface must 

 perish, and then is easily hollowed out by the waves washing 

 over it (c). If moreover, in the rainy season, any considerable 

 amount of water falls into this basin at low tides — and that this 

 is possible cannot be disputed — the fresh water collected in it 

 will soon overflow the margin for lack of room. Now the whole 

 coral is very porous, and is always traversed already, as we 

 have seen, by more or less deep furrows. The water natu- 

 rally seeks these ready-formed channels and widens them, 

 working through the margin in favourable spots where the 

 formation is softest, and thus slowly but surely eating channels 

 through the raised ring round the coral-block. In the cut a 

 diagram of these processes of growth is given. 



11 



