STONE CORALS. 



373' 



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). 



Stone Corala. 



Many of the high rocky islands of the Pacific lie, like a 

 picture in its frame, ii the middle of a lagoon encircled by 

 a reef. 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 



c c 



