LIMESTONE BANKS 



73 



banks the luxuriance and fulness of life are astonishing, myriads 

 of animals flourishing in the warm waters, and abundantly supplied 

 with food by the great ocean currents which sweep over the banks. 

 Innumerable molluscs, echinoderms, and calcareous worms are 

 continually dying and adding their hard parts to the sea-floor ; 

 the waves and tides sweep calcareous sand and mud from the coral 

 reefs over the flats, and all of these masses are rapidly consolidated 

 into rock. 



An example of a limestone bank in moderately deep water is 

 the Pourtales plateau, which extends southward from the Florida 



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FIG. 71. — Rock from Pourtales plateau. (A. Agassiz.) 



Keys, and is covered by 90 to 300 fathoms of water. "The bot- 

 tom is rocky, rather rough, and consists of a recent limestone, 

 continually, though slowly increasing from the accumulation of the 

 calcareous debris of the numerous small corals, echinoderms, and 

 molluscs, living on its surface. These debris are consolidated by 

 tubes of serpulse ; the interstices are filled up by Foraminifera and 

 further smoothed over by nullipores. — The region of this recent 

 limestone ceases at a depth varying from 250 to 350 fathoms, 

 and beyond it comes the trough of the straits." (A. Agassiz.) 

 It is not known how thick these modern limestone banks are, 



