58 CORAL FORMATIONS 
generally the rock is a breccia or conglomerate, made up of all kinds 
of corals cemented together 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 farther 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 illus- 
tration, although little may be presented that is novel after the excel- 
lent descriptions of these authors. Sketches of several of these islands, 
showing the general relation of the rim of land to the reef and the 
lagoon within, are given on the preceding plate. The following 
sketch represents a section of the rim of land from the sea on one 
SECTION OF A CORAL ISLAND REEF. 
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 breakers. In these shallow 
waters are the growing corals; yet, as before stated, a large part is 
barren sand or coral rock. 
From a to 6 is the shore platform of reef-rock, nearly at low tide 
level, with the margin (a) slightly elevated, and much incrusted at 
the top with Nullipores. Irom the platform there is a rise by a 
steep beach (0 c,) of six or eight feet, to the wooded part of the coral 
belt represented between c and d. From d to e there is a gently 
sloping beach bordering the lagoon. Beyond e, the waters of the 
lagoon at first deepen gradually, and then fall off more or less abruptly. 
In the Paumotus, the shore platform, the steep beach, and the more 
gently sloping shore of the lagoon are almost constant characteristics. 
The width of the whole rim of land, when the island gives no 
evidence of late elevation, varies from three hundred yards to one-third 
of a mile, excepting certain prominent points, more exposed to the 
