92 FUNAFUTI 



75 fathoms deeper in the sea. No fact is better estab- 

 lished than the rise and fall of islands situated in mid- 

 ocean, and thus there is nothing antecedently improbable 

 in this supposition. But once grant it and Darwin's 

 explanation of atolls naturally follows. Thus, let a be 

 an island with its summit rising 100 fathoms above the 

 sea ; let its shores become peopled with corals, which 

 extend seawards down to the limit of 25 fathoms, beyond 

 which, as we admit, they cannot proceed : a reef is thus 

 started, which will continue to grow, rising upwards till 

 it reaches the level of low tides : when this is attained 



OAR WIN 



FRINGING REEF 



FIG. 20. First 



upward growth will cease, and the reef will begin to pass 

 into decay, from the shore-edge outwards. So long as 

 the island remains stationary, neither rising nor falling 

 with respect to the sea-level, this is practically all that 

 will happen, and the final result is a reef not much 

 exceeding 25 fathoms in thickness (Fig. 20, first stage). 

 But let us next suppose that the island begins slowly to 

 sink into the sea, carrying the reef with it ; the upward 

 limit to the growth of the corals will be displaced ; they 

 w r ill commence to flourish afresh, and the reef will 

 continue to extend upwards till the level of the low 



