414 



PRINCIPLES OF STRATIGRAPHY 



reef-area remained unconsolidated in the southern part, where the 

 temperature was lower. 



A characteristic feature of the closing stages of reef formation 

 at any locality seems to be the production of coarse oolitic sands. 

 Dana (20:1^6) points out that such oolitic beds "appear to be con- 

 fined to the superficial formation of a reef, that is, to the beach 

 and wind-drift accumulations." The same thing was shown by 

 Agassiz. 



Destruction of the Coral Reef. 



Formation of Coral and Other Organic Lime Sand and Mud. 

 The agents at work destroying the reef rock are both inorganic and 

 organic. (74:140.) The most effective inorganic agent is the 

 surf which constantly beats against" the outer edge of the reef. 

 Alone, however, it is of little effect, unless it succeeds in breaking 

 off branches of the corals, which may then be hurled up onto the 

 reef, or over it into the lagoon. Coral polyps seem to flourish best 



Fig. 84. Organic limestone of corals, corallines, serpula tubes, echinoderm 

 fragments, etc. Pourtales plateau. (After A. Agassiz.) 



where the surf is strongest, so that we have the anomaly of a rock 

 mass growing into the very teeth of one of the most powerful 

 agents of rock destruction. Wherever the surf encounters loose 

 blocks of stone, such as volcanic bombs, or coral heads torn from 

 their foundations, it at once assumes all of its force as an agent of 

 destruction, for it will roll about these rock masses and so grind the 

 coral rock beneath them to powder. On the Florida reefs boulders 



