234 CORALS AND CORAL ISLANDS. 
surface. The layer of dead coral rock which makes the body 
of the reef, has its border of growing corals, and is thus un- 
dergoing extension at its margin, both through the increase in 
the corals, and the débris dropped among them. 
But besides the small fragments, larger masses will be 
thrown on the reefs by the more violent waves, and commence 
to raise them above the sea. The clinker fields of coral by 
this means produced, constitute the first step in the formation 
of dry land. Afterward, by further contributions of the | 
coarse and fine coral material, the islets are completed, and 
raised as far out of the water as the waves can reach—that is, 
about ten feet, with a tide of three feet; and sixteen to 
eighteen feet with a tide of six or seven. 
The Ocean is thus the architect, while the coral polyps af 
ford the material for the structure; and, when all is ready, it 
sows the land with seed brought from distant shores, covering 
it with verdure and flowers. 
The growth of the reefs and islands around high lands is 
the same as here described for the atoll. The reef-rock is 
mainly a result of accumulations of coral and shell débris. 
There are reefs where the corals retain the position of growth, 
as has been described on a former page. But with these the 
débris comes in to fill up the intervening spaces or cavities, and 
make a compact bed for consolidation. There are other parts, 
especially portions of the outer reef along the line of break- 
ers, which are formed by the gradual growth of layer upon 
layer of incrusting Nullipores; but such formations are of 
small extent, and only add to the results from other sources. 
II. ORIGIN OF THE SHORE PLATFORM. 
Among the peculiarities of coral islands, the shore plat- 
form appears to be one of the most singular, and its origin 
