308 ALEXANDER AGASSIZ 



what is now the coast line of the mainland. On these 

 hars and flats corals began to grow in post-Pliocene 

 times, and as the thickness of this reef is not over fifty 

 feet, the greatest dejith at which the corals began to 

 grow was probably considerably less than the greatest 

 depth at which corals are known to thrive. 



The reef was slightly raised and then eroded, leaving 

 patches of elevated reef and coral sand beaches; this 

 sand has been blown up to form the seolian rock of the 

 keys, which has solidified into hard ringing limestone 

 by the action of rain or sea spray, while the coast line of 

 the mainland he believes to have been formed in a 

 similar way. He believed that the sounds that separated 

 the keys from the mainland are due to the mechanical 

 and solvent action of the sea, while the ship channel 

 separating the outer line of reef patches from the main 

 line of keys probably represents a sink of greater ex- 

 tension which the currents have swept clean and subse- 

 quently deepened. " Finally, it is upon the remnants of 

 the old elevated reef that the present growing reef 

 flourishes, forming, as it does in the Bahamas and Ber- 

 mudas, a comparatively thin crust upon the underlying 

 foundation rocks, which are now known to be Pliocene, 

 and which occur at a depth considerably less than that 

 at which reef corals are known to grow." 



Griswold and others are of the opinion that the 

 oolites of the mainland were laid down under water. 

 Agassiz, on the other hand, considers them seolian, and 

 explains their stratification by the action of rainwater 

 containing carbonic acid on successive layers of coral 

 sand, more or less mixed with quartz sand. The rain 

 would take up a little lime, and on evaporation would 



