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, 
