348 CORALS AND CORAL ISLANDS. 



CHAPTER VI. 

 GEOLOOIOAL CONCLUSIONS. 



The geological bearing of the facts that have been de- 

 tailed in the preceding pages, may have been already perceived 

 by our readers. A brief review of the points of more special 

 interest, may serve as a convenient recapitulation of the sub- 

 ject. 



I. FORMATION OF LIMESTONES. 



Coral reefs are beds of limestone made of corals, with the 

 help of shells. The mode of formation is essentially the same, 

 whichever of the two kinds of organic products, corals, or 

 shells, predominate ; although in one case the bed would be 

 called coral limestone, and in the other, shell limestone. 



The reefs illustrate two different modes of origin of such 

 beds : (1), by undisturbed growth, with only additions of fine 

 material to fill up the intervals ; (2), by the grinding of the 

 corals, etc., to fragments, sand, or mud, through the agency 

 of the waves. 



Beds made by the former method, have many open spaces 

 between the grouped masses or branches, and could not be 

 turned into a solid layer of limestone, if situated too deep in 

 the ocean to feel sensibly the movement of the waves, — unless 

 Rhizopods, or minute shells of some kinds, multiplied so rap- 

 idly over the same sea-bottom, as to fill up the interstices. 

 There is no reason to believe that such aid from shells or 



