88 ZOOLOGY. 
the sea. The reefs are often of great thickness, for, as 
Dana says, ‘‘ could we raise one of these coral-bound islands 
from the waves, we should find that the reefs stand upon 
[ AN. 
Fig. 58.—Coral island or atoll.—From Dana. 
Fig. 59.—Section of a reef.—From Dana, 
the submarine slopes, like massy structures of artificial 
_masonry ; some forming a broad flat platform or shelf 
ranging around the land, and others encircling it like vast 
