GEOLOGICAL CONCLUSIONS. 299 



CHAPTER YI. 



' GEOLOGICAL CONCLUSIONS. 



The geological bearing of the facts that have been detailed 

 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 

 subject. 



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 hmestone. 



The reefs illustrate two different modes of origin of such 

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

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

 corals, &c., 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 

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



