86 CORALS AND CORAL ISLANDS. 
Od 
Rhizopods is consistent with the grouping of living corals . 
thickly enough to form reefs. 
The other kind of limestone beds referred to, where un- 
mixed with the former, grow up in compact layers to the 
surface, as a necessary consequence of wave-action ; and lime- 
stones are made in such regions, instead of sandstones and 
shales, because the material exposed to degradation is corals 
and shells, instead of common rocks. 
The facts show that there are formed about coral reefs, 
in indefinite amount, all the ordinary products of degrada- 
tion by wave-action—fragments large and small, down to 
sand, and even mud. With such an agent as the ocean’s 
waves, driven often by the storm, so powerful and so per- 
sistent at lifting, rending, 
little account, at least about outer reefs, that some coral 
grinding, and transporting, it is of 
stems or masses are first weakened below by the boring 
sponge or mollusk; and neither fish, nor holothurian, nor alcy- 
onoid is needed, in order to keep up the supply of particles 
for sand or mud-beds. In accordance with these facts, the reef- 
formations illustrate that not only coral conglomerates, or coral 
rag, may be made of corals, but also the very finest and most 
compact unfossiliferous limestones; that fine compact lime- 
stone, as flint-like in fracture as any of Silurian time, is one of 
the most common of coral-reef rocks, and is nothing but con- 
solidated mud, or fine sand, of coral origin. 
The elevated portion of the island of Metia, which con- 
sists largely of this kind of white, compact, coral-made lime- 
stone, appears to correspond to the interior of the original la 
goon of the island; it exemplifies the kind of rock-making © 
which is going forward in most coral island lagoons. In 
archipelagos like that of the Feejees, where the reef chan- 
nels are very broad, there is an opportunity for the formation 



