Stony Corals. 601 



her decorations. They are studded with small shells, 

 and beautifully marked with outlines expressive of their 

 origin. The edges of the reefs, particularly those ex- 

 posed to the waves, partake of a considerable degree of 

 lightness, and form small coves and caverns, the resort 

 of live corals, sponges, sea-eggs, and trefangs, or sea 

 traces, (valued in China, for their invigorating quality,) 

 and enormous cockles, which are scarcely to be distin- 

 guished from the rock, excepting when they suddenly 

 close their shells, and discharge living fountains, which 

 rise to the height of four or five feet. 



With regard to the formation of coral reefs, it has been 

 conjectured, from the appearance of the low islands in 

 some parts of the South Sea and Indian Ocean (where 

 they occur in rows or groups, while they are totally absent 

 in other parts of the same seas), that Coral animals rear 

 their habitations on marine shoals, or, to speak more 

 properly, at or near the top of sub-marine mountains. 

 As it is known, however, that the polypes can only build 

 their coral within a small distance of the surface of the 

 sea, and the water is often of immense depth close to the 

 coral reefs, it has been supposed that in the Pacific 

 Ocean, where the greater part of the Coral reefs and 

 islands are met with, the bottom of the sea has been 

 gradually undergoing changes, deepening in some places 

 and becoming shallower in others, and by this supposi- 

 tion most of the peculiarities of the Coral reefs and 

 islands may easily be accounted for. "Where reefs are 

 formed the bottom is generally sinking ; islands indicate 

 that the bottom is stationary or rising. In the latter 

 case, when the Corals approach close to the surface, 

 floating substances of every kind are caught by their 

 stony tree-like fabrics, till at length a solid mass of rock 

 is formed, which gradually advances to the surface of the 

 water. The deposits of the ocean no longer tenaciously 

 adhere, but remain in a loose state, and form what is 

 termed by mariners a key upon the summit of the reef; 

 while the sea, by throwing up sand and mud on the top 

 of these animal rocks, progressively raises them above 

 its level. The new island, for such it may now be called, 

 is soon visited by sea-birds ; plants successively appear, 



