STRUCl'URE OF CORAL ISLANDS. 141 



apparent by a review of their characters. The description will 

 be found to be a simple recapitulation of a former paragraph. 



The reef of the coral atoll, as it lies at the surface still 

 uncovered with vegetation, is a platform of coral rock, 

 usually two to four hundred, yards wide, and situated so low 

 as to be swept by the waves at high tide. The outer edge, 

 directly exposed to the surf, is generally broken into points 

 and jagged indentations, along which the waters of the 

 resurging wave drive with great force. Though in the midst 

 of the breakers, the edge stands a few inches, and sometimes 

 a foot, above other parts of the platform ; the incrusting 

 NuUipores cover it with varied tints, and afford protection from 

 the abrading action of the waves. There are usually three to 

 five fathoms water near the margin ; and below, over the 

 bottom, which gradually deepens outward, beds of corals are 

 growing profusely among extensive patches of coral sand and 

 fragments. Generally the barren areas much exceed those 

 flourishing with zoophytes, and not unfrequently the clusters 

 are scattered like tufts of vegetation in a sandy plain. The 

 growing corals extend up the sloping edge of the reef, nearly 

 to low-tide level. For ten to twenty yards from the margin, 

 the reef is usually very cavernous or pierced with holes or 

 sinuous recesses, a hiding-place for crabs and shrimps, or a re- 

 treat for the Echini, asterias, sea-anemones and inullusks ; and 

 over this portion of the platform, the gigantic Tridacna, some 

 times over two feet long and 500 pounds in weight, is often 

 found lying more than half buried in the solid rock, with barely 

 room to gape a little its ponderous shell, and expose to the waters 

 a gorgeously coloured mantle. Further in are occasional pools 

 and basins, ahve with all that lives in these strange coral seas. 



The reef-rock, when broken, shows commonly its detritus 

 origin. Parts are of compact homogeneous texture, a solid 

 white limestone, without a piece of coral distinguishable, and 

 rarely an imbedded shell. But generally the rock is a breccia 

 or conglomerate, made up of corals cemented into a compact 

 mass, and the fragments of which it consists are sometimes 

 many cubic feet in size. 



