98 DYNAMICAL GEOLOGY. 



will form islands on any kind of submarine bank when 

 the water is shallow enough for the waves to touch and 

 chafe the bottom. When, therefore, the reef rises nearly 

 to the surface, the beating waves will break off coral- 

 trees, coral-heads, and even masses of the reef-rock. 

 Great masses are thus rolled up on the inner side of the 

 reef, and form a nucleus about which other masses gather. 

 Among these larger masses smaller masses are thrown, 

 then finely comminuted coral limestone (coral sand) is 

 sifted among these, and the whole is cemented into a solid 

 rock by carbonate of lime in the sea-water. The island 

 rock, therefore, is a breccia of coral limestone, as shown 

 in Fig. 51. The island thus formed is at first barren 



PIQ. 51. Ideal section across a coral island ; I, I, sea-level ; R, living reef ; C.R.B., 

 coral reef-rock. 



rock j but, slowly, seeds are brought by waves and wind ; 

 it becomes covered with vegetation, and inhabited by 

 animals and by man. 



Thus we have traced the whole process, and find no 

 evidence of purpose or will, much less the admirable vir- 

 tues of perseverance and industry, often attributed to 

 them. It is a pity to spoil a moral ; but truth is the best 

 moral. 



Conditions of Growth. Reef-building corals do not 

 grow over the whole sea-bottom, nor in all oceans. They 

 are strictly limited by certain conditions : 



1. They will not grow where the mean winter tempera- 

 ture of the ocean is less than 68 Fahr. This condition 

 confines them mostly to the tropics. The most notable 



