508 FORMATION OF CORAL ISLANDS. 



the progressive changes, which have been altering the surface of 

 the Globe since it has been tenanted by Man. To it is due the 

 existence of a large proportion of the Islands of the Polynesian 

 Archipelago, as well as many of those in the Indian Ocean ; and 

 the extent of these islands is far less than that of the reefs, which 

 are not yet raised above the level of the sea, some presenting 

 themselves at a distance from any upraised land, others fringing 

 the shores of continents and islands, composed of other formations. 

 It is not correct, however, to affirm (as has been frequently done) 

 that these islands and reefs have been upreared by the Coral- 

 polypes from the depths of the ocean. It is now satisfactorily 

 ascertained that no known species can build from a greater depth 

 than twenty fathoms ; and a large proportion seem to prefer a 

 depth of from twenty to thirty feet. As very deep water is 

 found in the immediate neighbourhood of many of these reefs, 

 the question arises, upon what basis they are constructed ; and 

 to solve this, it is necessary to look at the forms which these 

 massive structures present. 



1076. A large proportion of the Coral Islands of the Poly- 

 nesian Archipelago are shaped like a crescent, sometimes like a 

 complete ring ; and these islands never rise many feet above the 

 surface of the ocean. The highest part is always on the windward 

 side,* against which the waves are almost constantly dashing. 

 Within the crescent or ring is a basin, termed a lagoon ; and 

 this usually communicates with the open sea, by a channel, 

 sometimes of considerable width, on the leeward side of the 

 island. Occasionally this channel is completely filled up by the 

 growth of the Coral ; and the lake, thus inclosed, only commu- 

 nicates with the sea by filtration through the Coral rock. The 

 Coral-polypes never build above low-water mark ; and they are 

 not, therefore, immediately concerned in the elevation of the surface 

 from beneath the waves. This is principally accomplished by the 

 action of the sea itself. Large masses are often detached, by the 

 violence of the waves, from the lower part of the structure ; and 



* The prevalence of easterly winds in the tropical regions, causes this name to 

 he given to the eastern side^of islands situated there ; the western shore being 

 known as the leeward. 



