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, sometimés 
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 
