466 
would be 

DYNAMICAL GEOLOGY. 
~ 
[Boor UL 
formed upon it, which, by successive accumulations of — 
SCALE OF FEET 
¢ 
oe 
Fia. 176.—Srction or A Cora REEF. 
A B, Portion above tide mark (a 6), covered with vegetation and habitable; oo, edge of lagoon, with two insular masses of coral (D D); 
the open ocean lies to the right of the slope a h. 
materials thrown up by the breakers 
or brought by winds, would remain 
permanently above water. On these 
islets palms and other plants, whose 
seeds might be drifted trom the ad- 
joining land, would take root and 
flourish. Inside the reef there would 
be a shallow channel of water, com- 
municating, through gaps in the reef, 
with the main ocean outside. Fring- 
ing reefs of this character are. of 
common occurrence at the present 
time. In the case of a continent they 
front its coast for a long distance, but 
they may entirely surround an island. 
If the site of a fringing reef under- 
goes depression at a rate sufficiently 
slow to allow the corals to keep pace 
with it, the reef may grow upward as 
fast as the bottom sinks downward. 
The lagoon channel inside will become 
deeper and wider, while, at the same 
time, the depth of water outside will 
increase until a Barrier Reef (A’ B, 
Fig. 177) is formed. In Fig. 178, for 
example, the Gambier Islands (1248 
feet high) are shown to be entirely 
surrounded by an interrupted barrier 
reef, 1uside of which les the lagoon. 
Prolonged slow depression must con- 
tinually diminish the area of the land 
thus encircled, while the reef will retain 
much the same size and position. At 
last the final peak of the original island 
may disappear under the lagoon (o Fig. 
177), and an Atoll, or true coral island, 
will be formed (A” A” Fig. 177, and Figs. 
174and 175). Should any more rapid or 
sudden downward movement take place, 
it might carry the atoll down beneath 
the surface, as seems to have happened 
at the Great Chagos bank in the Indian 
Ocean, which is a submerged atoll. 
In this simple and luminous ex- 
planation of the history of coral reefs 
every stage in the progress of the — 
changes is open to observation, from 




