72 



THE FLOOR OF THE OCEAN 



BANKS 



BANKS WITH 

 INSET REEFS 



BARRIER 

 Reef f REEFS 



> ATOLLS 



FIGURE 40. SECTIONS ILLUSTRATING FLATNESS OF THE FLOORS OF REEF 

 LAGOONS AND THE CLOSE SIMILARITY OF WATER DEPTHS WITH THE DEPTH 



ON OLD BANKS. 



along the edges of those flats. There they grew up as walls, 

 to be destroyed during the next Glacial stage. 



The sections of Figure 41 illustrate the postulated history, 

 so far as it relates to the time since the last inter-Glacial stage. 

 The last world-wide rise of sealevel permitted the colonizing 

 corals to build the wall-like reefs living in the tropical belt. 



Space cannot be taken for a full statement of this Glacial- 

 control theory, but two tests of its validity may be mentioned. 



First, we note that the 300-foot upward shift of sealevel is 



