96 DYNAMICAL GEOLOGY. 



case of a tree, so in corals, life passes continually outward 

 and upward, leaving the middle parts dead, and, in fact, 

 wholly composed of mineral matter (lime carbonate), re- 



FIG. 49. Astrea, a head-coral. 



taining, however, the peculiar structure given it while 

 permeated with living matter. 



Coral Forests. Corals, however, reproduce also by 

 eggs. These are formed within, below the stomach, ex- 

 truded through the mouth, and having, like the eggs of 

 many lower animals, the power of locomotion, swim away 

 and settle to the bottom, where, if conditions are favor- 

 able, they form single corals, which, by budding, soon form 

 coral-trees or coral-heads. In this way a coral forest or 

 grove is formed, and spreads in all directions as far as 

 favoring conditions allow (Fig. 50). 



Coral Reefs. But coral forests are not yet coral reefs. 

 These are formed by the growth and decay on the same 

 spot of countless generations of coral forests. Each 

 generation in its death leaves its limestone behind ; and 

 thus the coral ground rises or is built up without limit 

 except by reaching the sea-level. As a peat-bog is formed 

 by the accumulated remains of successive generations of 



