342 ON THE FORMATION OF CORAL ISLANDS. 



contrivance than all else which appertain to these 

 wonderful structures. 



It must now be observed, that though the wind- 

 ward side is vertical, it cannot become an effectual 

 protection against the sea, for the leeward parts, 

 without a further set of operations acting in aid of 

 the original process. The real dam, or pier, under 

 the protection of which the whole mass extends, is 

 produced by the fragments of the corals. As the 

 animals cannot live out of the water, they cease to 

 grow whenever they have arrived at the high water 

 mark, and the proper animal structure is therefore 

 terminated at that point, for ever ; but at low water, 

 the reef is above the sea, and thus becomes exposed 

 to the force of the waves, which breaks off the upper 

 parts and washes them to leeward, where they col- 

 lect ; while the animals, on the windward side, keep 

 up a constant supply of materials. In this way, the 

 fragments of coral, and the sand produced by their 

 attrition, become a species of alluvium on the sur- 

 face of the living coral rock, which is gradually ce- 

 mented through the action of the water on carbonat 

 of lime, so as to produce a species of oolithic lime- 

 stone, resembling that so well known in the West 

 India Islands. Thus does the whole platform gra- 

 dually become elevated above high water mark, as 

 the outer dam had at first been ; until it forms a con- 

 tinuous surface, aided in the regularity of its le- 

 velling, for a considerable time, by the breach of the 

 sea. And the effect also of this process is to produce 

 regular strata : while fragments of these, being often 

 large blocks of stone, are frequently piled up on the 

 outer ridge, and even further, so as to add to the 

 total elevation, and consequently, to secure addi- 



