Island and Reef Builders. 



the atolls exist to-day, the land, according to his theory, 

 gradually subsided and its area diminished ; but the 

 area enclosed by the coral reefs did not diminish in a 

 corresponding degree, and the young corals, growing 

 on the debris of the older ones as they sank, con- 

 tinued the growth of the reef in a direction nearly 

 vertical to the sea bottom. In this way the fringing 

 reefs gradually became barrier reefs, and were separated 

 from the mainland by a lagoon of considerable depth. 

 Finally, when the mountain peaks disappeared beneath 

 the waves, a ring-shaped reef or atoll was all that remained 

 to mark the position of the former land. 



The fundamental assumption of Darwin's subsidence 

 theory is that the substratum of the coral reefs and 

 islands is coral-formed limestone. Borings recently 

 made on the island of Funafuti, in the Pacific Ocean, 

 to test the truth of this assumption, were successfully 

 carried out to a depth of 1,114 f ee ** The result abso- 

 lutely proved and confirmed the justice of Darwin's 

 assumption as to the nature of the substratum, for in 

 the cores from various depths down to the very lowest 

 obtained by the boring, the fossilized skeletons of the 

 common genera of recent corals, and very few or no 

 representatives of genera of corals now extinct, were 

 discovered, giving the fullest possible support to the 

 subsidence theory as applied to this particular island. 

 As I have stated elsewhere,* that Darwin's " theory of 

 gradual subsidence may not be applicable in a few 

 cases is quite possible. Other natural causes, such as 

 the abundant deposition of the remains of calcareous 

 organisms, may have been, under favourable condi- 

 * Cassell's Natural History. 



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