STRUCTURE OF CORAL ISLANDS. 175 



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 retreat for 

 the echini, asterias, sea-anemones and mollusks ; and over this 

 portion of the platform, the gigantic Tridacna, sometimes 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 colored mantle. Further in are occasional pools 

 and basins, alive 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. 



It is apparent that we are describing a second time an 

 outer reef. Without dwelling further upon its characters, we 

 may pass to the features of the reef when raised above the 

 waters and covered with vegetation. 



Sections of coral islands and their lagoons have been given 

 by Captain Beechey and Mr. Darwin. We add another, by 

 way of illustration, although little may be presented that is 

 novel after the excellent descriptions of these authors. Sketch- 

 es of several of these islands, showing the general relation of 

 the rim of land to the reef and the lagoon within, are given 

 in the maps of islands on pages 165, 168. The following sketch 

 represents a section of the rim of land from the sea on one 

 side (the left), to the lagoon on the other. In the view, the 

 part m a represents the shallow sea bordering an island, and 

 abruptly deepening one to six hundred feet from the line of 



