160 FORMATION OF CORAL REEFS. 



depth of many hundreds, or even thousands, of 

 feet, and it was taken for granted that they must 

 have had' their home where they were found; 

 but the facts recently ascertained respecting the 

 subsidence of ocean-bottoms have shown that the 

 foundation of a Coral wall may have sunk far 

 below the place where it was laid. And it is now 

 proved, beyond a doubt, that no Reef-Building 

 Coral can thrive at a depth of more than fifteen 

 fathoms, though Corals of other kinds occur far 

 lower, and that the dead Reef-Corals, sometimes 

 brought to the surface from much greater depths, 

 are only broken fragments of some Reef that has 

 subsided with the bottom on which it was grow- 

 ing. But though fifteen fathoms is the maximum 

 depth at which any Reef-Builder can prosper, 

 there are many which will not sustain even that 

 degree of pressure ; and this fact has, as we shall 

 see, an important influence on the structure of 

 the Reef. 



Imagine now a sloping shore on some tropical 

 coast descending gradually below the surface of 

 the sea. Upon that slope, at a depth of from ten 

 to twelve or fifteen fathoms, and two or three or 

 more miles from the main land, according to the 

 shelving of the shore, we will suppose that one of 

 those little Coral animals, to whom a home in 

 such deep waters is genial, has established itself. 

 How it happens that such a being, which we 



