STONE CORALS. 3738 
with a hemispherical living shell, 
about half an inch thick ; and in some 
porites of the same size the whole 
mass is lifeless, except the exterior 
for a sixth of an inch in depth. 
We are astonished when travellers 
tell us of the vast extent of certain 
ancient ruins; but how utterly insig- 
nificant are the greatest of these when compared with the piles of 
stone accumulated in the course of ages by these minute, and in- 
dividually so puny architects! The history of the formation of 
coral-reefs is no less wonderful than their extent. They have been 
divided, according to their geological character, into three classes. 
The first fringes the shores of continents or islands (shore-reefs) ; 
the second, rising from a deep ocean, at a greater distance from 
the land, encircles an island, or stretches like a barrier along 
the coast (encircling-reefs, barrier-reefs); the third, enclosing a 
lagoon, forms a ring or annular break-water round an interior 
lake (atolls, or lagoon-islands). 
Astrea. 
BS irnate 
Sesh 
SAN 
Stone Corals. 
Many of the high rocky islands of the Pacific lie, like a 
picture in its frame, in the middle of a lagoon encircled by 
areef. A fringe of low alluvial land in these cases generally 
surrounds the base of the mountains; a girdle of palm-trees, backed 
by abrupt heights, and fronted by a lake of smooth water, only 
separated from the deep blue ocean by the breakers roaring 
against the encircling reef; such, for instance, is the scenery of 
OMe 
