STRUCTURE OF CORAL ISLANDS. DS 
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 1s 
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 
