392 



COELENTERATA ANTHOZOA 



the substratum of the coral reefs and islands is coral -formed 

 limestone. To test the truth of this assumption an expedition 

 was sent out to obtain, by boring, evidence of the character of 

 the substratum of a typical atoll. The island of Funafuti in the 

 Ellice group of the Pacific Ocean was selected, and after several 

 attempts a successful boring was made to a depth of 1114 feet. 

 The material from the boring was found to consist of rocks or 

 sands entirely derived from the calcareous skeletons of marine 

 Invertebrate animals and calcareous Algae. 1 Moreover, in the 

 cores from various depths down to the lowermost the fossilised 



if 



Mmm> 



<<4miNtf 



Fig. 173. Section of the outer edge of one of the Maldive Atolls. A, foundation of 

 primitive rock cut down by the currents ; B, upgrowth of the rim by the deep-sea-, 

 intermediate depth- and (B') reef-organisms ; C, extension outwards by means of 

 the talus slope ; D, lagoon. Scale in fathoms. (After Stanley Gardiner.) 



skeletons of the common genera of recent corals, and very few or 

 no representatives of genera of corals now extinct were discovered. 

 These facts, therefore, prove the justice of Darwin's assumption 

 as to the nature of the substratum and give support to the 

 subsidence-theory as applied to this particular island. A strong 

 opinion has, however, been expressed by several authors of recent 

 years that the subsidence-theory cannot account for the formation 

 of all the atolls and barrier reefs that have now been investigated, 

 and alternate hypotheses have been put forward to account for 

 particular cases. The main chain of the Maldive Archipelago 

 in the Indian Ocean, for example, presents special difficulties to 

 the acceptance of the subsidence -theory as one of general applica- 

 tion. 2 The main chain of these islands is more than 300 miles 

 long, and lies at right angles to the monsoon currents of the 



1 For the details of these borings, see " The Atoll of Funafuti," Royal Societ 

 of London, 1904. 



2 For further information, see J. Stanley Gardiner, The Fauna and Geograpl 

 of the Maldive and Laccadive Archipelagoes, vol. i. pt. ii. 1902, p. 172. 



