XV.] CORAL LAND. 257 



on which the polypes lived and died. The details of 

 so simple, yet complete an explanation, deserve closer 

 examination. 



It has already been shown that coral-polypes can multiply 

 by processes of budding and splitting ; but it should be 

 added, that they can also multiply by means of germs, which 

 are thrown off from the parent as free-swimming bodies. 

 Suppose that some of these embryonic corals settle upon a 

 sloping shore, in shallow water, where the conditions of life 

 are favourable ; there they may go on multiplying, until they 

 form masses of considerable extent, skirting the land, but 

 never extending seawards to a depth of more than 20 or 30 

 fathoms. Let the land, with its little fringing-reef, now 

 slowly sink ; that part which is carried down lower than 

 about 30 fathoms will consist of nothing but dead coral; but 

 the upper part of the reef will continue to grow, and, if the 

 subsidence be not more rapid than the upward growth, the 

 level of the reef will appear to remain stationary, at about the 

 sea-level. It has been said that the coral-polype flourishes 

 best at the outer margin of the reef, where it is bathed 

 by the surf. For this and other reasons, the reef is 

 highest at this edge ; while, between the outer margin of the 

 reef and the shore, there is a channel formed by the entry 

 of sea-water during subsidence. In fact, the fringing-reef, 

 as it has been slowly carried down, has become converted 

 into a barrier-reef This will be easily understood by re- 

 ference to the sections in Figs. 73 and 74. In Fig. 73 an 

 island, A, is skirted by a fringing-reef, B B : on the sinking 

 of the land to a lower level, as in Fig. 74, the bank of 

 coral, B B, becomes thicker by upward growth, and a 

 channel, C C, comes to be formed between the barrier 

 and the shore. 



Outside the barrier, on its seaward edge, there may be a 



s 



