MUSEUM OF COMPAEATIVE ZOOLOGY. 143 



reef, through which there are several large and deep channels. Egmont, 

 or Six Islands, is an instance of an atoll in which the encircling reef is 

 perfect, and unbroken by any channels. There are several submerged 

 banks, nearly all of which have an atoll form. The great Chagos Bank 

 is a huge submerged atoll ; so are the Pitts, Ganges, and Centurion banks. 

 Darwin considered that the Great Chagos Bank afforded particularly good 

 evidence of the truth of the subsidence theory, yet Mr. Bourne considers 

 that a more intimate knowledge of the Great Chagos Bank, and of the 

 relations of it and other submerged banks of existing land, shows this 

 view to be untenable. For as the rim of the Great Chagos Bank is on 

 an average only six fathoms below the surface, and in the most favorable 

 depth for growth of corals, there are actually six islets on the north- 

 western edge rising above high water. Bourne has also noticed the 

 great and rapid destruction of parts of Diego Garcia, both inside and out- 

 side of the lagoon, and has called attention to the transfer of material 

 due to storms and tides, showing that the normal action of tides and 

 winds and waves is constantly tending to lower the sea level, and thus 

 lay bare dry land that may have been formed by elevation or otherwise. 

 It does not seem surprising, therefore, that the majority of atolls and 

 barrier reefs are under such circumstances only just able to maintain 

 their surfaces above the sea level. He gives an explanation of atollons 

 in the centre of large lagoons, based upon the production of oceanic 

 conditions in the interior of a large lagoon, as in Tilla-dou-Matte, where 

 he thinks the atollons have been formed before any land reached the 

 surface, in which the islets forming the large lagoon were few in number 

 and distant from one another, so that the atollon would practically have 

 an oceanic character, and be swept by currents, establishing all the con- 

 ditions for a new atoll. The corals thus flourishing on the circumferen- 

 tial parts of the reef surrounding the islet, new atolls with shallow 

 lagoons would be formed as long as the deep channels between the outer 

 distant islets were swept by strong currents, becoming wider and deeper 

 because corals could not thrive in them. 



Bourne emphasizes the favorable conditions under which corals flour- 

 ish as occurring in localities where there is a moderate current flowing 

 over them, not so strong as to dash them to pieces, but strong enough to 

 prevent the deposition of sand, these conditions being found everywhere 

 in external slopes. He lays greater stress on currents than on food sup- 

 ply, as he considers that to be at variance with the existence of thriving 

 coral patches within a lagoon. While we do not deny the fact, yet the 

 lagoon patches do not spread as vigorously as the corals growing on the 



